EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

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                NOTICE
The mention  of  trade names or  commercial
products  in this  publication  is for
illustrative  purposes only  and does not
constitute endorsement  or recommendation
for use  by  the USEPA.
                   11

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                           FOREWORD
The  formation of  the United  States Environmental Protection
Agency marked  a  new era of  environmental  awareness  in America.
This  Agency's  goals are national  in scope and  encompass  broad
responsibility in  the areas of  air and water  pollution,  solid
wastes,  pesticides,  and  radiation.   A  vital part  of  EPA's
national pollution  control  effort  is  the  constant  development
and dissemination of new  technology.

It is now clear that only the most effective design and operation
of  pollution  control facilities  using  the  latest  available
techniques  will  be  adequate to  ensure  continued  protection  of
the nation's  waters.  It  is essential that this new technology be
incorporated   into  the  contemporary  design of  pollution  control
facilities  to achieve  maximum benefit of our expenditures.

The  purpose  of   this  manual  is  to provide  the engineering
community and  related industry with  a  new  source of  information
to be  used  in the  planning, design, and  operation of  present
and  future   wastewater  pollution  control  facilities.    It  is
recognized  that there are a number of design manuals  and manuals
of  standard  practice,   such  as  those  published  by  the  Water
Pollution  Control  Federation,  available in   the  field,  and
that  each of  these adequately describes and  interprets  current
engineering   practices  as  related  to traditional  plant  design.
It  is  the  intent  of  this  manual  to supplement this existing
body  of  knowledge  by describing new treatment methods and by
discussing  the application  of new techniques for more effectively
removing a  broad  spectrum of contaminants from wastewater.

Much of  the  information presented is based on the evaluation and
operation of  pilot, demonstration,  and full-scale plants.   The
design criteria  thus  generated represent  typical  values.   These
values  should be used  as  a guide  and  should be tempered with
sound  engineering judgment  based  on a complete  analysis of the
specific application.

This manual  is one  of several available from Technology Transfer
to describe  technological  advances  and  new  information.   Future
editions will  be  issued  as warranted by advancing state-of-the-
art  to  include new  data as  they become available  and  to revise
design criteria as  additional full-scale operational information
as generated.
                               111

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                           ABSTRACT
The purpose of  this  manual  is  to present a contemporary review
of sludge  processing technology,  with particular emphasis  on
design methodology.   This  is  a revision of a manual originally
published in October  1974.

The revised  edition  incorporates chapters on  design approach,
disinfection,   composting,  transport,  storage,   sidestream
treatment,  and  instrumentation.    Other sections  have  been
considerably expanded.

Design examples are used throughout the manual to  illustrate
design principles.
                               iv

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                       TABLE OF CONTENTS
                                                             Page
FOREWORD
ABSTRACT .......

LIST OF TABLES .

LIST OF FIGURES

ACKNOWLEDGEMENTS
CHAPTER 1.   PURPOSE AND SCOPE
1.1  Purpose ................
1.2  Scope ..................
1.3  Process Classification .
1.4  References .,	
CHAPTER 2.  GENERAL CONSIDERATIONS 	
2.1  Introduction and Scope ............
     Legal and Regulatory Considerations
    2.2.1
2.2
2.3
           Effect of Effluent Discharge Limitations
           on Wastewater Solids Management ..........
    2.2.2  Restrictions on Wastewater
           Solids Treatment ,..,.....	,
                 Air Emissions Limits .....,..,...,.,
                 Nuisances	
                 State and Local Requirements .......
                and Regulations Governing Wastewater
                s Utilization and Disposal ..,..,,,..
    2.2
2.
2.
2.
3
2.
2.
2.
2.
2.
2.
2
2
2
.2
.2
.2
.1
.2
.3
Laws
2
2
2
2
2
2
ou
.3
.3
.3
.3
.3
.3
_L JL
.1
.2
.3
.4
.5
.6
                 Federal Water Pollution Control Act
                 Resource Conservation and
                 Recovery Act	
                 Toxic Substances Control Act	
                 Marine Protection,  Research and
                 Sanctuaries Act ................,..,
                 Environmental Policy Acts *.........
                 State and Local Reuse and
                 Disposal Requirements	,
    2.2.4  The Comprehensive Nature of Section 405
           of the Clean Water Act	
     Other Non-Technical Factors Affecting
     Wastewater Solids Management ......,...,.,.,...,
    2.3.1  Availability of Construction Funds .......
    2.3.2  Special Funding Requirements .............
                                                              111
                                                               IV
                                                              XXV
                                                            XXXVI
                                                             xlix
1-
1-
1-
1-
1-

2-
2-
2-

2_
1
1
2
2
2

1
1
1
2-
2-
2-
2-
2-
2-
2-
2-
2-
2-
2
2
3
3
3
3
4
3
5
5
2-  5

2-  5

2-  6
2-  6
2-  7
                                v

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                  TABLE OF CONTENTS (continued
    2.3.3  Time Span of Decisions 	.,.,,.»,,,,,.,,..   2-  7
    2.3.4  Uncertainties ..................	....   2-  8
    2.3.5  The Design Team	   2-  8
    2.3.6  Public Involvement	   2-  9
    2.3.7  Social and Political Factors Affecting
           Waste Export . ....	   2- 10
2.4  References	....	   2- 11

CHAPTER 3.  DESIGN APPROACH	   3-  1
3.1  Introduction ..........................	.»   3-  1
3.2  Systems Approach ....................I..............   3-  1
3.3  The Logic of Process Selection	. .   3-  2
    3.3.1  Identification of Relevant Criteria	   3-  2
    3.3.2  Identification of System Options	   3-  4
    3.3.3  System Selection Procedure	   3-  6
        3.3.3.1  Base and Secondary Alternatives ........   3-  6
        3.3.3.2  Choosing a Base Alternative:
                 First Cut	   3-  7
        3.3.3.3  Choosing a Base Alternative:
                 Second Cut	   3- 10
        3.3.3.4  Third Cut	   3- 11
        3.3.3.5  Subsequent Cuts	   3- 12
    3.3.4  Parallel Elements	   3- 12
    3.3.5  Process Selection at Eugene, Oregon ..........   3- 13
3.4  The Quantitative Flow Diagram	   3- 18
    3.4.1  Example:  QFD for a Chemically Assisted
           Primary Treatment Plant	   3- 18
    3.4.2  Example:  QFD for Secondary Plant
           With Filtration		   3- 24
3.5  Sizing of Equipment ................................   3- 27
3.6  Contingency Planning ..................................   3- 29
    3.6.1  Example of Contingency Planning
           for Breakdowns ,,..,......,...,,....,..,,.....   3- 29
3.7  Other General Design Considerations ................   3- 34
    3.7.1  Site Variations	   3- 34
    3.7.2  Energy Conservation ..........................   3- 35
    3.7.3  Cost-Effective Analyses ......................   3- 36
    3.7.4  Checklists ........	   3- 38
3.8  References	   3- 39

CHAPTER 4.  WASTEWATER SOLIDS PRODUCTION AND
            CHARACTERIZATION ...		   4-  1
4.1  Introduction .....................I*................   4-  1
4.2  Primary Sludge	   4-  1
    4.2.1  Primary Sludge Production ....................   4-  1
        4.2.1.1  Basic Procedures for Estimating
                 Primary Sludge Production	»»,»   4-  1
        4.2.1.2  Industrial Waste Effect	   4-  2
        4.2.1.3  Ground Garbage Effect	   4-  3
        4.2.1.4  Other Sludges and Sidestreams	   4-  3
                               VI

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                  TABLE OF CONTENTS (continued)

                                                             Page

        4.2.1.5  Chemical Precipitation and
                 Coagulation .,	...,....,.,..,,.....   4-  3
        4.2.1.6  Peak Loads	   4-  3
    4.2.2  Concentration Properties .....................   4-  6
    4.2.3  Composition and Characteristics ..............   4-  7
4.3  Biological Sludges .................................   4-  9
    4.3.1  General Characteristics ......................   4-  9
    4.3.2  Activated Sludge ....			   4-  9
        4.3.2.1  Processes Included ..........I..........   4-  9
        4.3.2.2  Computing Activated Sludge
                 Production - Dry Weight Basis ..........   4-  9
        4.3.2.3  Example:  Determination of
                 Biological Sludge Production	   4- 19
        4.3.2.4  Interaction of Yield  Calculations and
                 the Quantitative Flow Diagram (QFD) ....   4-24
        4.3.2.5  Concentration of Waste-Activated
                 Sludge	   4- 25
        4.3.2.6  Other Properties of Activated Sludge ...   4-27
    4.3.3  Trickling Filters	   4- 29
        4.3.3.1  Computing Trickling Filter Sludge
                 Production - Dry Weight Basis ..........   4- 29
        4.3.3.2  Concentration of Trickling Filter
                 Sludge	4....*..**..*......   4- 33
        4.3.3.3  Properties - Trickling Filter Sludge ...   4-34
    4.3.4  Sludge from Rotating Biological Reactors , ....   4-34
    4.3.5  Coupled Attached-Suspended  Growth Sludges ....   4-35
    4.3.6  Denitrification Sludge	   4- 36
4.4  Chemical Sludges ...................................   4- 36
    4.4.1  Introduction	   4- 36
    4.4.2  Computing Chemical Sludge
           Production - Dry Weight Basis ................   4- 37
    4.4.3  Properties of Chemical Sludges	   4- 38
    4.4.4  Handling Chemical Sludges ....................   4- 38
        4.4.4.1  Stabilization	   4- 39
        4.4.4.2  Chemical and By-Product Recovery  	   4-39
4.5 Elemental Analysis of Various Sludges	   4- 39
    4.5.1  Controlling Trace Elements	   4- 39
    4.5.2  Site-Specific Analysis .......................   4- 41
    4.5.3  Cadmium	   4- 42
    4.5.4  Increased Concentration During Processing ....   4-43
4.6  Trace Organic Compounds in Sludge ..................   4- 44
4.7  Miscellaneous Wastewater Solids	   4- 45
    4.7.1  Screenings	   4- 46
        4.7.1.1  Quantity of Coarse Screenings	   4- 46
        4.7.1.2  Quantity of Fine Screenings ............   4- 48
        4.7.1.3  Properties of Screenings ...............   4- 48
        4.7.1.4  Handling Screenings	   4- 48
        4.7.1.5  Screenings From Miscellaneous
                 Locations	   4- 49
    4.7.2  Grit .		   4- 50
                               VII

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                  TABLE OF CONTENTS  (continued)
        4.7.2.1  Quantity of Grit	
        4.7.2.2  Properties of Grit 	...	
        4.7.2.3  Handling Grit	
    4.7.3  Scum	
        4.7.3.1  Quantities of Scum ..I..................
        4.7.3.2  Properties of Scum	,
        4.7.3.3  Handling Scum ..........................
    4.7.4  Sept age	
        4.7.4.1  Quantities of Septage	
        4.7.4.2  Properties of Septage	
        4.7.4.3  Treating Septage in Wastewater
                 Treatment Plants ..,.,....,.,..	   4- 60
    4.7.5  Backwash	   4- 61
    4.7.6  Solids From Treatment of Combined
           Sewer Overflows	   4- 62
4.8  References ..........,,.,..,.,.......*.,,,,,,,......   4- 63

CHAPTER 5 .   THICKENING . „	 .   5-  1
5.1  Introduction	,,	   5-  1
    5.1.1  Definition	   5-  1
    5.1.2  Purpose ..,,...,,,,..,.............,.,........   5-  1
    5.1.3  Process Evaluation	   5-  1
    5.1.4  Types and Occurrence of Thickening
           Processes .	,	   5-  2
5.2  Sedimentation Basins .........	   5-  2
    5.2.1  Primary Sedimentation ........................   5-  2
    5.2.2  Secondary Sedimentation	   5-  3
5.3  Gravity Thickeners	 •  5-  3
    5.3.1  Introduction	•.**..................*   5-  3
    5.3.2  Theory	.	   5-  3
    5.3.3  System Design Considerations	   5-  5
        5.3.3.1  Minimum Surface Area Requirements ......   5-  6
        5.3.3.2  Hydraulic Loading	   5-  8
        5.3.3.3  Drive Torque Requirements	   5-  8
        5.3.3.4  Total Tank Depth	   5-  9
        5.3.3.5  Floor Slope	   5- 10
        5.3.3.6  Other Considerations .,.,,..,	   5- 11
    5.3.4  Design Example	   5- 12
    5.3.5  Cost	   5- 15
        5.3.5.1  Capital Cost	   5- 15
        5.3.5.2  Operating and Maintenance Cost .........   5- 15
5.4  Flotation Thickening	   5- 16
    5.4.1  Dissolved Air Flotation (DAF)	.,   5 -18
        5.4.1.1  Theory 	».,.,.,.	   5- 19
        5.4.1.2  System Design Considerations ...........   5- 19
    5.4.2  Design Example	   5- 33
    5.4.3  Cost	   5- 35
        5.4.3.1  Capital Cost	   5- 35
        5.4.3.2  Operating and Maintenance Costs 	.»   5-36
5.5  Centrifugal Thickening	   5- 36
                              VI11

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                  TABLE OF CONTENTS (continued)
    5.5.1  Introduction	   5- 36
    5.5.2  Theory	,   5- 38
    5.5.3  System Design Considerations	   5- 39
        5.5.3.1  Disc Nozzles	   5- 39
        5.5.3.2  Imperforate Basket	   5- 45
        5.5.3.3  Solid Bowl Decanter	   5- 49
    5.5.4  Case History	 ...   5- 53
    5.5.5  Cost .......			..   5- 55
        5.5.5.1  Capital Cost	   5- 55
        5.5.5.2  Operating and Maintenance Cost .........   5- 56
5.6  Miscellaneous Thickening Methods ...................   5- 59
    5.6.1  Elutriation Basin	   5- 59
    5.6.2  Secondary Anaerobic Digesters	   5- 60
    5.6.3  Facultative Sludge Lagoons ...................   5- 60
    5.6.4  Ultrafiltration .,	,.   5 -60
5.7  References ...	..,..,.,.,.,...,.,.,.,.,,,.,.,   5 -60

CHAPTER 6.  STABILIZATION	   6-  1
6.1  Introduction .......................................   6-  1
6.2  Anaerobic Digestion ................................   6-  2
    6.2.1  Process Description 	.,.,.,..,,..   6-  2
        6.2.1.1  History and Current Status	   6-  2
        6.2.1.2  Applicability	   6-  3
        6.2.1.3  Advantages and Disadvantages ...........   6-  4
        6.2.1.4  Microbiology	   6-  5
    6.2.2  Process Variations ...........................   6-  7
        6.2.2.1  Low-Rate Digestion	   6-  7
        6.2.2.2  High-Rate Digestion	   6-  7
        6.2.2.3  Anaerobic Contact Process	.,   6- 15
        6.2.2.4  Phase Separation	   6- 16
    6.2.3  Sizing of Anaerobic Digesters	   6- 18
        6.2.3.1  Loading Criteria  ....,.»...».,...,.,...   6- 18
        6.2.3.2  Solids Retention Time	   6- 18
        6.2.3.3  Recommended Sizing Procedure ...........   6- 20
    6.2.4  Process Performance	   6- 23
        6.2.4.1  Solids Reduction	   6- 26
        6.2.4.2  Gas Production	   6- 29
        6.2.4.3  Supernatant Quality	   6- 31
    6.2.5  Operational Considerations	   6- 34
        6.2.5.1  pH	   6- 34
        6.2.5.2  Toxicity	   6- 36
    6.2.6  System Component Design	   6- 42
        6.2.6.1  Tank Design	   6- 42
        6.2.6.2  Heating	   6- 46
        6.2.6.3  Mixing			   6- 52
        6.2.6.4  Covers	   6- 62
        6.2.6.5  Piping	   6- 66
        6.2.6.6  Cleaning	   6- 67
    6.2.7  Energy Usage	   6- 72
    6.2.8  Costs	   6- 74
                               IX

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                  TABLE OF CONTENTS (continued)
    6.2.9  Design Example ...............................   6- 74
        6.2.9.1  Design Loadings ..,..»»	,....,,»   6- 74
        6.2.9.2  System Description .....................   6- 75
        6.2.9.3  Component Sizing ,	   6- 77
6.3  Aerobic Digestion .........,.,..,,.,....«,........».   6- 82
    6.3.1  Process Description ..........................   6- 82
        6.3.1.1 History		...................   6- 82
        6.3.1.2  Current Status	   6- 82
        6.3.1.3  Applicability	   6- 82
        6.3.1.4  Advantages and Disadvantages ...........   6- 82
        6.3.1.5  Microbiology 	......».,,.,,.,,.,.,..   6- 83
    6.3.2  Process Variations 	.,,.....	   6- 84
        6.3.2.1  Conventional Semi-Batch Operation ......   6-84
        6.3.2.2  Conventional Continuous Operation ......   6-84
        6.3.2.3  Auto-Heated Mode of  Operation ..,.,..,..   6- 85
    6.3.3  Design Considerations	   6- 86
        6.3.3.1  Temperature	   6- 86
        6.3.3.2  Solids Reduction	   6- 86
        6.3.3.3  Oxygen Requirements  ....................   6- 88
        6.3.3.4  Mixing	   6- 89
        6.3.3.5  pH Reduction		   6- 90
        6.3.3.6  Dewatering	..........	   6- 91
    6.3.4  Process Performance	   6- 92
        6.3.4.1  Total Volatile Solids Reduction	   6- 92
        6.3.4.2  Supernatant Quality	   6- 93
    6.3.5  Design Example	   6- 93
    6.3.6  Cost	   6- 99
        6.3.6.1  Capital Cost	   6- 99
        6.3.6.2  Operation and Maintenance  Cost .........   6- 99
6.4  Lime Stabilization	   6-100
    6.4.1  Process Description	   6-101
        6.4.1.1  History	   6-101
        6.4.1.2  Current Status	   6-102
        6.4.1.3  Applicability	   6-102
        6.4.1.4  Theory of the Process 	.»	   6-103
    6.4.2  Design Criteria ...".....	   6-103
        6.4.2.1  pH and Contact Time	   6-104
        6.4.2.2  Lime Dosage	   6-104
    6.4.3  Process Performance ..........................   6-107
        6.4.3.1  Odor Control	   6-108
        6.4.3.2  Pathogen Reduction	   6-109
        6.4.3.3  Dewatering and Settling
                 Characteristics	   6-110
        6.4.3.4  Chemical Characteristics ...............   6-110
    6.4.4  Process Design .........	   6-112
        6.4.4.1  Design of Lime Handling Facilities 	   6-112
        6.4.4.2  Mixing Tank Design	,	   6-118
    6.4.5  Costs and Energy Usage	   6-121
        6.4.5.1  Capital and Operating Costs 	   6-121
        6.4.5.2  Energy Usage	   6-122

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                  TABLE OF CONTENTS (continued)

                                                             Page

    6.4.6  Design Example ...............................   6-124
        6.4.6.1  Design Loading	   6-124
        6.4.6.2  System Description......,.,,,,...,..»,,   6-124
        6.4.6.3  Component Sizing	..........,,,   6-126
6.5  Chlorine Stabilization	   6-127
    6.5.1  Process Description .,,,,,.,,....«..,,,»....,,   6-128
    6.5.2  Uses, Advantages, and Disadvantages ..........   6-131
    6.5.3  Chlorine Requirements	   6-132
    6.5.4  Characteristics of Chlorine-Stabilized
           Materials	   6-133
        6.5.4.1  Stabilized Slude	»	   6-133
        6.5.4.2  Supernatant/FiItrate/Subnatant
                 Quality	   6-134
    6.5.5  Costs .....,..,..,.,..	   6-134
        6.5.5.1  Operating Costs	,.•,....   6-135
        6.5.5.2  Capital Costs	   6-136
6.6  References ......	.........,..,....,,,.....   6-138

CHAPTER?.  DISINFECTION	   7-  1
7.1  Introduction ,.,.,.....,,.........«..*..............   7-  1
7 . 2  Pathogenic Organisms	   7-  1
    7.2.1  Pathogen Sources	   7-  2
    7.2.2  Pathogen Characteristics	   7-  2
        7.2.2.1  Viruses	   7-  2
        7.2.2.2  Bacteria	   7-  3
        7.2.2.3  Parasites	   7-  4
        7.2.2.4  Fungi	   7-  6
    7.2.3  Pathogen Occurrence in the United States .....   7-  6
7.3  Pathogen Survival During Sludge Stabilization
     Processes	.......,..,.,.,,,,,..,.,.,,	   7-  7
    7.3.1  Pathogen Reduction During Digestion ..........   7-  7
        7.3.1.1  Viruses	   7-  7
        7.3.1.2  Bacteria	   7-  8
        7.3.1.3  Parasites .... ,v	   7-  9
    7.3.2  Long Term Storage	..»,...,   7- 10
    7.3.3  Chemical Disinfection	   7- 10
        7.3.3.1  Lime	   7- 10
        7.3.3.2  Chlorine		   7- 10
        7.3.3.3  Other Chemicals	   7- 11
7.4  Pathogen Survival in the Soil .........,.,..,.,....,   7- 11
    7.4.1  Viruses	,.	   7- 11
    7.4.2  Bacteria		   7- 11
    7.4.3  Parasites	   7- 12
7.5  Potential Human Exposure to Pathogens	   7- 12
7.6  Heat Disinfection Processes	   7- 13
    7.6.1  Sludge Pasteurization ..,.,,,..............,*.   7- 14
        7.6.1.1  Process Description ,.....,,..,	   7- 15
        7.6.1.2  Current Status .........................   7- 16
        7.6.1.3  Design Criteria ........................   7- 16
                               XI

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                  TABLE OF CONTENTS  (continued)

                                                             Page

        7.6.1.4  Instrumentation and Operational
                 Considerations	,,   7- 17
        7.6.1.5  Energy Impacts .........................   7- 17
        7.6.1.6  Cost Information	   7- 17
        7.6.1.7  Design Example	   7- 20
    7.6.2  Other Heat Processes	   7- 24
        7.6.2.1  Heat-Conditioning 	»	   7- 25
        7.6.2.2  Heat-Drying .............. i	   7- 25
        7.6.2.3  High Temperature Processes 	   7- 25
        7.6.2.4  Composting	   7- 25
7.7  Pathogen Reduction With High-Energy Radiation ......   7-26
    7.7.1  Reduction of Pathogens in Sludge With
           Electron Irradiation	   7- 26
        7.7.1.1  Process Descritpion	   7- 27
        7.7.1.2  Status	   7- 28
        7.7.1.3  Design Considerations	   7- 28
        7.7.1.4  Instrumentation and Operational
                 Considerations	   7- 30
        7.7.1.5  Energy Impacts	   7- 30
        7.7.1.6  Performance Data .	   7- 30
        7.7.1.7  Production Production and Properties ...   7-31
        7.7.1.8  Cost Information ...,........*.,...,....   7- 31
    7.7.2  Disinfection With Gammer Irradiation	   7- 32
        7.7.2.1  Process Description	   7- 33
        7.7.2.2  Current Status - Liquid Sludge .........   7- 33
        7.7.2.3  Current Status - Dried or Composted
                 Sludge	   7- 34
        7.7.2.4  Design Criteria	   7- 35
        7.7.2.5  Instrumentation and Operational
                 Considerations ................*..-...*..   7- 35
        7.7.2.6  Energy Impacts	 .•	   7- 36
        7.7.2.7  Performance Data	   7- 37
        7.7.2.8  Cost Information	   7- 38
7.8  References	   7- 44

CHAPTER 8.  CONDITIONING	   8-  1
8.1  Introduction	......,..»	   8-  1
8.2  Selecting a Conditioning Process ...................   8-  1
8.3  Factors Affecting Wastewater Solids Conditioning ...   8-  1
    8.3.1  General Wastewater Solids Properties .........   8-  1
        8.3.1.1  Particle Size and Distribution .........   8-  3
        8.3.1.2  Surface Charge and Degree of
                 Hydration		   8-  4
        8.3.1.3  Particle Interaction	   8-  4
    8.3.2  Physical Factors 		   8-  4
        8.3.2.1  Effect of Processing Prior to
                 Conditioning  ...........................   8-  5
        8.3.2.2  Conditioner Application  ................   8-  5
8.4  Inorganic Chemical Conditioning	   8-  6
    8.4.1  Introduction	   8-  6
                                Xll

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                  TABLE OF CONTENTS (continued)
    8.4.2  Dosage Requirements	   8-  6
    8.4.3  Availability ..............	   8-  7
    8.4.4  Storage, Preparation,  and Application
           Equipment ........I..,,..,...,......,..,.,....   8-  8
    8.4.5  Design Example ...,.,,,.........	,.,..*.   8-  8
    8.4.6  Cost	   8-  9
        8.4.6.1  Capital Cost	   8-  9
        8.4.6.2  Operation and Maintenance Cost	   8- 10
8.5  Chemical Conditioning With Polyelectrolytes ,,,»,...   8-14
    8.5.1  Introduction	   8- 14
    8.5.2  Background on Polyelectrolytes ...............   8- 14
        8.5.2.1  Composition and  Physical Form ..........   8- 14
        8.5.2.2  Structure in Solution 	,	   8- 17
        8.5.2.3  How Polyelectrolyte Conditioning
                 Works	   8- 17
    8.5.3  Conditioning for Thickening	   8- 18
        8.5.3.1  Gravity Thickening .....................   8- 18
        8.5.3.2  Dissolved Air Flotation Thickening ,.,,.   8-18
        8.5.3.3  Centrifugal Thickening	   8- 20
    8.5.4  Conditioning for Dewatering	   8- 20
        8.5.4.1  Drying Beds	   8- 21
        8.5.4.2  Vacuum Filters	   8- 21
        8.5.4.3  Recessed Plate Pressure Filters	   8- 22
        8.5.4.4  Belt Filter Presses	   8- 23
        8.5.4.5  Centrifuges .,	   8- 24
    8.5.5  Storage, Preparation,  and Application
           Equipment	   8- 25
    8.5.6  Case History	   8- 25
    8.5.7  Cost	   8- 27
        8.5.7.1  Capital Cost	   8- 27
        8.5.7.2  Operation and Maintenance Cost .........   8- 29
8.6  Non-Chemical Additions		...   8- 29
8.7  Thermal Conditioning ...............................   8- 31
    8.7.1  Advantages and Disadvantages	   8- 33
    8.7.2  Process Sidestreams	   8- 34
        8.7.2.1  Gaseous Sidestreams .,	   8- 34
        8.7.2.2  Liquid Sidestreams		   8- 35
    8.7.3  Operations and Cost ...		   8- 36
        8.7.3.1  General Considerations .................   8- 36
        8.7.3.2  USEPA Survey Results	   8- 38
8.8  Elutriation	   8- 39
8.9  Freeze-Thaw	   8- 40
    8.9.1  Indirect Mechanical Freezing .....,,,,...,....   8- 40
    8.9.2  Direct Mechanical Freezing 	   8- 41
    8.9.3  Natural Freezing .............	   8- 41
8.10  Mechanical Screening and Grinding	   8- 41
8.11  Miscellaneous Processes	   8- 42
    8.11.1  Bacteria	   8- 42
    8.11.2  Electricity	   8- 42
    8.11.3  Solvent Extraction	   8- 43
                              XI 11

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                  TABLE OF CONTENTS  (continued)

                                                             Pag_e

    8.11.4  Ultrasonic	».	   8- 43
8.12  References .......	..............	......   8- 43

CHAPTERS.  DEWATERING	..•........,.,	   9-  1
9.1  Introduction ,,..,..........,....»,,,..,.......,.,,.   9-  i
    9.1.1  Process Evaluation	.,   9-  1
    9.1.2  Methods of Dewatering	. ....   9-  3
9.2  Natural Sludge Dewatering Systems ,,,.,.,,»,,»	   9-  3
    9.2.1  Drying Beds	,   9-  3
        9.2.1.1  Basic Components  and Operation .,..,.,».   9-  4
        9.2.1.2  Types of Drying Beds	   9-  5
        9.2.1.3  Process Design Criteria ................   9-  9
        9.2.1.4  Costs	.......		   9- 12
    9.2.2  Drying Lagoons	..........	   9- 14
        9.2.2.1  Basic Concept		   9- 15
        9.2.2.2  Design Criteria ....................	   9- 15
        9.2.2.3  Costs	   9- 16
9.3  Centrifugal Dewatering Systems  .....................   9- 17
    9.3.1  Introduction	   9- 17
    9.3.2  Imperforate Basket	   9- 18
        9.3.2.1  Principles of Operation	   9- 18
        9.3.2.2  Application	   9- 19
        9.3.2.3  Performance		   9- 19
        9.3.2.4  Case History	   9- 19
    9.3.3  Solid Bowl Decanters	   9- 23
        9.3.3.1  Application	   9- 23
        9.3.3.2  Performance	   9- 24
        9.3.3.3  Other Considerations	   9- 24
9.4  Filtration Dewatering Systems	   9- 25
    9.4.1  Introduction	   9- 25
    9.4.2  Basic Theory	   9- 26
    9.4.3  Filter Aids	   9- 26
    9.4.4  Vacuum Filters	   9- 27
        9.4.4.1  Principles of Operation	   9- 28
        9.4.4.2  Application .	   9- 32
        9.4.4.3  Performance	   9- 33
        9.4.4.4  Other Considerations ...................   9- 33
        9.4.4.5  Case History	   9- 39
        9.4.4.6  Costs	   9- 41
    9.4.5  Belt Filter Press	   9- 43
        9.4.5.1  Principles of Operation	   9- 45
        9.4.5.2  Application	   9- 46
        9.4.5.3  Performance	   9- 46
        9.4.5.4  Other Considerations	   9- 47
        9.4.5.5  Design Example	   9- 49
        9.4.5.6  Costs	   9- 51
    9.4.6  Recessed Plate Pressure Filters ..............   9- 52
        9.4.6.1  Principles of Operation	   9- 52
        9.4.6.2  Application	   9- 55
        9.4.6.3  Performance ............................   9- 56
                               xiv

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                  TABLE OF CONTENTS (continued)
        9.4.6.4  Other Considerations	   9- 56
        9.4.6.5  Case History	.,   9- 59
        9.4.6.6  Cost 	..	.,.,..,.....«,,....»   9- 60
    9.4.7  Screw and Roll Press ,,.,,,*...,	,»,,,,...   9- 63
        9.4.7.1  Screw Press	   9- 63
        9.4.7.2  Twin-Roll Press	   9- 66
    9.4.8  Dual Cell Gravity (DCG)  Filter ..,,,	   9- 67
    9.4.9  Tube Filters	   9- 68
        9.4.9.1  Pressure Type	   9- 68
        9.4.9.2  Gravity Type	»	   9- 68
9.5  Other Dewatering Systems	   9- 69
    9.5.1  Cyclones	   9- 69
    9.5.2  Screens	   9- 70
    9.5.3  Electro-Osmosis	   9- 70
9.6  References	   9- 70

CHAPTER 10.  HEAT DRYING		  10-  1
10.1  Introduction	  10-  1
10.2  Heat-Drying Principles	.,,.,.........,  10-  1
    10.2.1  Drying Periods	  10-  1
    10.2.2  Humidity and Mass Transfer	  10-  2
    10.2.3  Temperature and Heat Transfer	  10-  3
10.3  Energy Impacts	*	.	  10-  5
    10.3.1  Design Example	  10-  6
    10.3.2  Energy Cost of Heat-Dried Sludges Used
            for Fertilizers 	.,.,.,	  10- 11
10.4  Environmental Impacts	  10- 12
    10.4.1  Air Pollution	...»  10- 12
    10.4.2  Safety	  10- 13
    10.4.3  Sidestream Production	  10- 13
10.5  General Design Criteria	  10- 13
    10.5.1  Drying Capacity	 .  10- 13
    10.5.2  Storage Requirements	  10- 14
    10.5.3  Heat Source	  10- 14
    10.5.4  Air Flow	  10- 14
    10.5.5  Equipment Maintenance .......................  10- 15
    10.5.6  Special Considerations ......................  10- 15
10.6  Conventional Heat Dryers 	,	  10- 15
    10.6.1  Flash-Drying	  10- 16
        10.6.1.1  Process Description ...................  10- 16
        10.6.1.2  Case Study:  Houston, Texas 	  10- 18
    10.6.2  Rotary Dryers	  10- 19
        10.6.2.1  Direct Rotary Dryers .....'	  10- 19
        10.6.2.2  Indirect Drying ..		  10- 22
        10.6.2.3  Direct-Indirect Rotary  Dryers	  10- 24
    10.6.3  Incinerators	  10- 25
    10.6.4  Toroidal Dryer	  10- 25
        10.6.4.1  Process Description ...................  10- 25
        10.6.4.2  Current Status	  10- 27
    10.6.5  Spray-Drying	  10- 27
                               xv

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                  TABLE OF CONTENTS  (continued)
        10.6.5.1  Process Description	...........  10- 27
        10.6.5.2  Current Status .,	  10- 27
10.7  Other Heat-Drying Systems	  10- 28
    10.7.1  Solvent Extraction--BEST Process ...»	.,  10- 28
        10.7.1.1  Process Description	  10- 28
        10.7.1.2  Current Status ...»	  10- 29
        10.7.1.3  Operating Experience ..................  10- 30
    10.7.2  Multiple-Effect Evaporation—Carver
            Greenfield Process	  10- 30
        10.7.2.1  Process Description ............ .......  10- 31
        10.7.2.2  Current Status	,,	...»  10- 31
10.8  References	  10- 32

CHAPTER 11.  HIGH TEMPERATURE PROCESSES	  11-  1
11.1  Introduction ,,..»...*...,......,,....,.....»,»,,..  11-  1
11.2  Principles of High Temperature Operations .........  11-  2
    11.2.1  Combustion Factors ..........................  11-  3
        11.2.1.1  Sludge Fuel Values	»».....  11-  3
        11.2.1.2  Oxygen Requirements for Complete
                  Combustion	  11-  6
        11.2.1.3  Factors Affecting the Heat Balance ....  11-  7
    11.2.2  Incineration Design Example	  11- 10
        11.2.2.1  Problem Statement	  11- 10
        11.2.2.2  Approximate Calculation Method 	  11- 13
        11.2.2.3  Theoretical Calculation Method	  11- 20
        11.2.2.4  Comparison of Approximate and
                  Theoretical Calculation Methods ....,,,  11- 24
    11.2.3  Pyrolysis and Starved-Air Combustion
            Calculations ...»		  11- 25
    11.2.4  Heat and Material Balances 	.	  11- 28
11.3  Incineration	  11- 29
    11.3.1  Multiple-Hearth Furnace	  11- 31
    11.3.2  Fluid Bed Furnace		  11- 42
    11.3.3  Electric Furnace	  11- 49
    11.3.4  Single Hearth Cyclonic Furnace	...*  11- 55
    11.3.5  Design Example:  New Sludge Incineration
            Process	.,,.,.,,..,.,.,...  11- 59
        11.3.5.1  Approach	  11- 61
        11.3.5.2  Preliminary Design	  11- 62
11.4  Starved-Air Combustion	  11- 65
    11.4.1  Development and Application	  11- 68
    11.4.2  Advantages and Disadvantages of SAC  	  11- 71
    11.4.3  Conversion of Existing Multiple-Hearth
            Incineration Units to SAC ...................  11- 75
    11.4.4  Design Example:  Retrofit of an Existing
            Multiple-Hearth Sludge Incinerator to a
            Starved-Air Combustion Reactor	  11- 76
        11.4.4.1  Approach	  11- 77
        11.4.4.2  Preliminary Design	  11- 78
                               xvi

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                  TABLE OF CONTENTS (continued)
11.4  Starved-Air Combustion ...»	  n_ 55
    11.4.1  Development and Application »	  11- 68
    11.4.2  Advantages and Disadvantages of SAC .........  11- 71
    11.4.3  Conversion of Existing Multiple-Hearth
            Incineration Units to SAC	  n- 75
    11.4.4  Design Example:  Retrofit of an Existing
            Multiple-Hearth Sludge Incinerator to a
            Starved-Air Combustion Reactor	  11- 76
        11.4.4.1  Approach	  11- 77
        11.4.4.2  Preliminary Design	  11- 78
11.5  Co-Combustion of Sludge and Other Material ........  11- 81
    11.5.1  Co-Combustion with Coal and Other
            Residuals ..	...»  11- 81
    11.5.2  Co-Combustion with Mixed Municipal Refuse
            (MMR)	  11- 83
        11.5.2.1  Refuse Combustion Technology ..........  11- 84
        11.5.2.2  Sludge Combustion Technology	  11- 87
    11.5.3  Institutional Constraints ...,....,,,,,.,.,,.  11- 92
    11.5.4  Conclusions  about Co-Combustion 	  11- 94
11.6  Related Combustion Processes Used in Wastewater
      Treatment	,	  11- 94
    11.6.1  Screenings,  Grit, and Scum Reduction ........  11- 94
    11.6.2  Lime Recalcination ...,..,...,.,,.,.,........  11- 96
    11.6.3  Activated Carbon Regeneration	  11- 98
        11.6.3.1  Granular Carbon Systems  (GAC) 	  11- 99
        11.6.3.2  Powdered Activated Carbon (PAC)  .......  11-100
        11.6.3.3  Jet Propulsion Laboratory Activated-
                  Carbon Treatment System  (JPL-ACTS) ....  11-100
11.7  Other High Temperature Processes	  11-102
    11.7.1  High Pressure/High Temperature Wet Air
            Oxidation	  11-102
    11.7.2  REACT-O-THERMtm	  11-109
    11.7.3  Modular Starved-Air Incinerators	  11-110
    11.7.4  Pyro-Soltm Process	  11-110
    11.7.5  Bailie Process	  11-113
    11.7.6  Wright-Malta Process	  11-113
    11.7.7  Molten Salt Pyrolysis	  11-115
11.8  Air Pollution Considerations	  11-115
    11.8.1  National Ambient Air Quality Standards
            (NAAQS)-State Implementation Plans (SIP) ....  11-116
    11.8.2  National Emission Standards for Hazardous
            Air Pollutants (NESHAPS) .....................  11-177
    11.8.3  Standards of Performance for New
            Stationary Sources (NSPS)	  11-118
    11.8.4  New Source Review Standards (NSR)	  11-119
    11.8.5  Prevention of Significant Deterioration
            (PSD)	  11-119
    11.8.6  The Permit Process	  11-120
    11.8.7  Air Emissions Test Procedures	  11-120
    11.8.8  Design Example	  11-120
                              xvi i

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TABLE OF CONTENTS  (continued)




                                            Page
11.8.8.1 Identify Applicable State and Local

11.8.8.2 Establish Air Pollution Abatement

11.9 Residue Disposal 	 ..»,......,,*....»»»,,,

CHAPTER 1 2 . COMPOSTING .... 	 , 	


12.2.1 Moisture 	 	 	 	 	 	 ....,..,

12.2.3 pH 	 	 	 	 	 	 .,
12.2.4 Nutrient Concentration 	 	





12.3.1.2 Public Health and Environmental


12.3.2 Aerated Static Pile Process 	 	
12.3.2.1 Individual Aerated Piles ..............
12.3.2.2 Extended Aerated Piles 	 	
12.3.2.3 Current Status 	 	 	 	

12.3.2.5 Bulking Agent 	 	 	 	 	 	 	

12.3.2.7 Public Health and Environmental


12.3.3 Case Studies (Unconfined Systems) ...........
12.3.3.1 Joint Water Pollution Control Plant,
Carson, California ...................
12.3.3.2 Beltsville, Maryland 	 	 	 	


12.3.3.5 Cost Analysis 	 	 	
12.4 Confined Composting System .......................

12.4.2 Metro-Waste Aerobic Thermophilic
Bio-Reactor 	 	 	 , , 	 	
12.4.3 Dano Bio-Stabilizer Plant 	
12.4.4 BAV Bio-Reactor 	 	 	 	 	


CHAPTER 13. MI SCELLANEOUS PROCESSES 	 	


. 11-121

, 11-123
. 11-132
. 11-136
. 12- 1
. 12- 1
. 12 2
, 12- 3
. 12 4
. 12- 5
, 12- 5
, 12- 5
12- 5
. 12- 11
. 12- 12
. 12- 16

12--16
. 12- 18
. 12- 22
, 12- 23
, 12- 24
. 12- 25
. 12- 26
. 12- 26
12- 27

12- 27
. 12- 29
. 12- 36

12- 36
. 12- 38
. 12- 42
12- 46
12- 49
. 12- 51
12- 51

. 12- 51
. 12- 51
12- 53
12- 53
12- 57
. 13- 1
. 13- 1
             XVI 11

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                  TABLE OF CONTENTS (continued)
                                                             Paqe
13.2  Chemical Fixation Process .....	  13-  l
13.3  Encapsulation Process .,.,...	  13-  3
    13.3.1  Polyethylene Process ........,,..........,,,,  13-  3
    13 . 3 . 2  Asphalt Process .....,,,..,,..,»,»,.,,»	,  13-  4
13.4  Earthworm Conversion Process ......................  13-  4
    13.4.1  Process Arrangement	  13-  4
    13.4.2  Advantages of the Earthworm Conversion
            Process	  13-  6
    13.4.3  Possible Operating Difficulties	  13-  7
    13.4.4  Limitations	  13-  7
13.5  References ........................................  13-  9

CHAPTER 14.  TRANSPORTATION	  14-  1
14.1  Pumping and Pipelines ..,..,...,.,.....,......	  14-  1
    14.1.1  Simplified Head-Loss Calculations ...........  14-  1
    14.1.2  Application of Rheology to Sludge
            Pumping Problems	  14-  3
        14.1.2.1  Solution of Pressure Drop Equation ....  14-  4
        14.1.2.2  Design Example	  14-  8
        14.1.2.3  Thixotropy and Other
                  Time-Dependent Effects	  14- 12
        14.1.2.4  Obtaining the Coefficients	  14- 14
        14.1.2.5  Additional Information	  14- 17
    14.1.3  Types of Sludge Pumps	  14- 17
        14.1.3.1  Centrifugal Pumps	  14- 17
        14.1.3.2  Torque Flow Pumps		..  14- 18
        14.1.3.3  Plunger Pumps	  14- 19
        14.1.3.4  Piston Pumps ...,..,,.»	  14- 21
        14.1.3.5  Progressive Cavity Pumps	  14- 22
        14.1.3.6  Diaphragm Pump	  14- 24
        14.1.3.7  Rotary Pumps	,»,,...	  14- 26
        14.1.3.8  Ejector Pumps .			  14- 27
        14.1.3.9  Gas Lift Pumps	  14- 27
        14.1.3.10 Water Eductors	  14- 28
    14.1.4  Application of Sludge Pumps		....  14- 29
    14.1.5  Pipe, Fittings, and Valves .»	  14- 29
    14.1.6  Long Distance Pumping  	,.	  14- 31
        14.1.6.1  Experience	  14- 31
        14.1.6.2  Design Guidance	  14- 32
    14.1.7  In-Line Grinding	...»  14- 36
14.2  Dewatered Wastewater Solids Conveyance	  14- 37
    14.2.1  Manual Transport of Screenings and Grit .....  14- 37
    14.2.2  Belt Conveyors	  14- 37
    14.2.3  Screw Conveyors	  14- 40
    14.2.4  Positive Displacement Type Conveyors  ........  14- 43
    14.2.5  Pneumatic Conveyors ...,..............,,.*...  14- 43
    14.2.6  Chutes and Inclined Planes .,,»,..	  14- 44
    14.2.7  Odors	  14- 46
14.3  Long Distance Wastewater Solids Hauling .,	  14- 46
    14.3.1  Truck Transportation	  14- 47
                               xix

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                  TABLE OF CONTENTS  (continued)
        14.3.1.1  Types of Trucks	 ,  14- 47
        14.3.1.2  Owned Equipment vs. Contract
                  Hauling	  14- 48
        14.3.1.3  Haul Scheduling	  14- 49
        14.3.1.4  Trucking Costs	  14- 49
    14.3.2  Rail Transport 	..,,»,  14- 49
        14.3.2.1  Advantages and Disadvantages
                  of Rail Transport .....................  14- 49
        14.3.2.2  Routes ..........I..............	,,  14- 50
        14.3.2.3  Haul Contracts	....	  14- 50
        14.3.2.4  Railcar Supply ..	  14- 51
        14.3.2.5  Ancillary Facilities	........  14- 53
        14.3.2.6  Manpower and Energy Requirements ......  14- 53
    14.3.3  Barge Transportation	  14- 54
        14.3.3.1  Routes and Transit Times	,  14- 54
        14.3.3.2  Haul or System Contracting	  14- 55
        14.3.3.3  Barge Selection and Acquisition	  14- 56
        14.3.3.4  Ancillary Facilities ,.,,,..,,	  14- 57
        14.3.3.5  Spill Prevention and Cleanup	  14- 57
14.4  References	 ..  14- 57

CHAPTER 15.   STORAGE		.	  15-  1
15.1  Introduction	  15-  1
    15.1.1  Need for Storage	  15-  1
    15.1.2  Risks and Benefits of Solids Storage Within
            Wastewater Treatment System .................  15-  1
    15.1.3  Storage Within Wastewater Sludge Treatment
            Processes	  15-  2
    15.1.4  Effects of Storage on Wastewater Solids 	  15-  2
    15.1.5  Types of Storage	  15-  4
15.2  Wastewater Treatment Storage 	 	  15-  5
    15.2.1  Storage Within Wastewater Treatment
            Processes	  15-  5
        15.2.1.1  Grit Removal	  15-  6
        15.2.1.2  Primary Sedimentation	  15-  7
        15.2.1.3  Aeration Reactors  and Secondary
                  Sedimentation . „	  15-  9
        15.2.1.4  Imhoff and Community Septic Tanks 	  15- 10
        15.2.1.5  Wastewater Stabilization Ponds ........  15- 11
    15.2.2  Storage Within Wastewater Sludge Treatment
            Processes	  15- 11
        15.2.2.1  Gravity Thickeners	  15- 12
        15.2.2.2  Anaerobic Digesters	  15- 12
        15.2.2.3  Aerobic Digesters	  15- 18
        15.2.2.4  Composting ............................  15- 18
        15.2.2.5  Drying Beds	  15- 18
15.3  Dedicated Storage Facilities	  15- 18
    15.3.1  Facilities Provided Primarily for Storage
            of Liquid Sludge .	 .,	  15- 19
        15.3.1.1  Holding Tanks	  15- 19
                               xx

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                  TABLE OF CONTENTS (continued)
        15.3.1.2  Facultative Sludge Lagoons ............  15- 23
        15.3.1.3  Anaerobic Liquid Sludge Lagoons 	  15- 41
        15.3.1.4  Aerated Storage Basins ..,.,..,....,...  15- 43
    15.3.2  Facilities Provided Primarily for Storage
            of Dewatered Sludge 	,	.	  15- 45
        15.3.2.1  Drying Sludge Lagoons ...,,,,,,.....,,.  15- 47
        15.3.2.2  Confined Hoppers or Bins ..,........,,,  15- 51
        15.3.2.3  Unconfined Stockpiles	  15- 56
15.4  References	..,.....,.,»,.	  15- 58

CHAPTER 16.  SIDESTREAMS FROM SOLIDS TREATMENT
             PROCESSES	  16-  1
16.1  Sidestream Production .	..,....,...,...,,,,  16-  1
16.2  Sidestream Quality and Potential Problems	  16-  2
16.3  General Approaches to Sidestream Problems 	  16-  3
    16.3.1  Elimination of Sidestream	  16-  4
    16.3.2  Modification of Upstream Solids Processing
            Steps	  16-  4
    16.3.3  Change in Timing, Return Rate,  or Return
            Point	  16-  5
    16.3.4  Modification of Wastewater Treatment
            Facilities ...	  16-  5
    16.3.5  Separate Treatment of Sidestreams ...........  16-  7
        16.3.5.1  Anaerobic Digester Supernatant ........  16-  8
        16.3.5.2  Thermal Conditioning Liquor ...........  16- 10
16.4  References	  16- 17

CHAPTER 17.  INSTRUMENTATION	  17-  1
17.1  Introduction , ,.	  17-  1
    17.1.1  Purposes of Instrumentation .................  17-  1
    17.1.2  Instrumentation Justification and
            Design Considerations	  17-  1
17.2  Measurements ,......,.,....,.,....,..,,,»	»,..  17- 41
    17.2.1  Level Measurements .,..,,,..,..,..,	  17- 41
      .  17.2.1.1  Bubblers	  17- 41
        17.2.1.2  Diaphragms	  17- 41
        17.2.1.3  Capacitance Transmitters	  17- 44
        17.2.1.4  Ultrasonic Transmitters	  17- 44
        17.2.1.5  Tape-Supported Floats .................  17- 45
    17.2.2  Flow Measurements 	...,	  17- 45
        17.2.2.1  Venturi Tubes ,,,.,.	  17- 46
        17.2.2.2  Nozzles	  17- 46
        17.2.2.3  Magnetic Meters	  17- 46
        17.2.2.4  Ultrasonic Meters	  17- 47
        17.2.2.5  Doppler Meters	  17- 47
        17.2.2.6  Rotameters	  17- 48
        17.2.2.7  Propeller Meters	  17- 48
        17.2.2.8  Pitot Tubes	  17- 48
        17.2.2.9  Weirs and Flumes	  17- 49
        17.2.2.10 Orifice Plates	  17- 49
                               xxi

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17
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17
17.2.2.11 Turbine Meters 	 	 	 	
17.2.2.12 Vortex Meters 	 	

17.2.2.14 Pump and Transport Displacement





17.2.4.1 Resistance Temperature
Detectors (RTDs ) 	 	 	 	


17.2.5.1 Static 	 ................ 	 	
17.2.5.2 Mass Flow 	 	 	 	 	
17.2.6 Density and Suspended Solids Measurements ...
17.2.6.1 Density , 	 	 	 , 	










17.2.14.2 Calorimeter 	 	 	 	



17.2.18 Blanket Level Measurements .,....., 	
17.2.19 Hydrocarbons and Flammable Gas Detectors ....


17.2.21.1 Empty Pipe Detectors ».,,...... 	
17.2.21.2 Vibration - Acceleration and

17.2.21.3 Flow Loss Monitors 	 	
17.2.21.4 Overload Devices 	 	 	






CHAPTER 18 . UTILIZATION 	 	 	 	
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            XXI 1

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TABLE OF CONTENTS (continued)










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18.2.2 Principles and Design Criteria for


18.2.2.2 Site Selection 	 	 	

18.2.2.4 Facilities Design ....................
18.2.2.5 Facility Management, Operations,






18.3.3.3 Generators ...... 	

18.3.3.1 Energy Recovery From Digester Gas ....
18.3.3.2 Recovery of Energy From Incinerator
Flue Gas . ., 	 	 	 	 	

.4 Other Uses of Wastewater Solids and Solid


CHAPTER 19 . DISPOSAL TO LAND 	 	 	
19

19
























.2 Sludge Landfill 	 	 	 	 	
19.2.1 Definition 	 	 	 	
19.2.2 Sludge Landfill Methods 	 	 	
19.2.2.1 Sludge-Only Trench Fill ..............
19.2.2.2 Sludge-Only Area Fill 	 	 	
19.2.2.3 Co-Disposal with Refuse 	 ,,.,,
19.2.2.4 Suitability of Sludge for


19.2.3.1 Sludge Characterization ..............
19.2.3.2 Selection of a Landfilling Method ....
19.2.3.3 Site Selection 	 	 	 	 	
19.2.4 Facility Design .,,» 	 	 	 	

19.2.4.2 Site Characteristics 	 	 	
19.2.4.3 Landfill Type and Design .............
19.2.4.4 Ancillary Facilities .................
19.2.4.5 Landfill Equipment 	 	
19.2.4.6 Flexibility and Reliability 	 	
19.2.4.7 Expected Performance .................


19.2.5.1 Operations Plan 	 	 	 	
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            XXI 11

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                  TABLE OF CONTENTS (continued)
        19.2.5.2  Operating Schedule		   19- 21
        19.2.5.3  Equipment Selection and Maintenance  ...   19- 21
        19.2.5.4  Management and Reporting	.   19- 22
        19.2.5.5  Safety .......	   19- 22
        19.2.5.6  Environmental Controls	   19- 23
    19.2.6  Site Closure .,	,	   19- 24
        19.2.6.1  Ultimate Use .»	   19- 24
        19.2.6.2  Grading at Completion of Filling ......   19- 24
        19.2.6.3  Final Grading	   19- 25
        19.2.6.4  Landscaping .	   19- 25
        19.2.6.5  Continued Leachate and Gas Control ....   19- 25
    19.2.7  Landfilling of Screenings,  Grit, and Ash ....   19- 25
19.3  Dedicated Land Disposal	   19- 25
    19.3.1  Defintion	   19- 25
    19.3.2  Background ............	   19- 26
    19.3.3  Site Selection	   19- 27
        19.3.3.1  Ownership by Wastewater Treamtent
                  Authority ............	.............   19- 27
        19.3.3.2  Groundwater Patterns  ..................   19- 27
        19.3.3.3  Topography	   19- 28
        19.3.3.4  Soil Types	   19- 28
        19.3.3.5  Availability of  Suffient Land .........   19- 28
    19.3.4  Storage	.,,,...   19- 28
        19.3.4.1  Climatic Influences ........	....	   19- 28
        19.3.4.2  Operational Storage			   19- 29
    19.3.5  Operational Methods and  Equipment ...........   19- 29
        19.3.5.1  Liquid Sludge	   19- 29
        19.3.5.2  Dewatered Sludge	   19- 34
        19.3.5.3  Sludge Application Rates	   19- 35
    19.3.6  Environmental Controls and  Monitoring	   19- 37
        19.3.6.1 - Site Layout	., . ,.   19- 37
        19.3.6.2. Groundwater Controls	   19- 38
        19.3.6.3  Surface Water Runoff  Controls .........   19- 38
        19.3.6.4  Air Pollution Control	   19- 39
        19.3.6.5  Site Monitoring	   19- 39
    19.3.7  Costs	   19- 39
    19.3.8  Case Examples	   19- 39
        19.3.8.1  Colorado Springs,  Colorado	   19- 40
        19.3.8.2  Sacramento, California	   19- 50
19.4  References	,	   19- 58
                              xxiv

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                         LIST OF TABLES
Number
                            CHAPTER 3

 3- 1   Example of Initial Screening Matrix for Base
        Sludge Disposal Options	,	   3-  8
 3- 2   Example of Process Compatibility Matrix ,,,.,»»,,   3-  9
 3- 3   Example of Treatment/Disposal Compatibility
        Matrix	   3-  9
 3- 4   Example of Numerical Rating System for
        Alternatives Analysis	   3- 10
 3- 5   Estimated Costs of Alternatives for
        Eugene-Springfield	*..	   3- 17
 3- 6   Mass Balance Equations for Flowsheet of
        Figure 3-7 . ...	   3- 21
 3- 7   Mass Balance Equations for Flowsheet of
        Figure 3-9		   3- 26
 3- 8   Solid Properties Checklist	   3- 37
 3- 9   Process Design Checklist ........................   3- 37
 3-10   Public Health and Environmental
        Impact Checklist	   3- 38

                            CHAPTER 4

 4- 1   Predicted Quantities of Suspended Solids and
        Chemical Solids Removed in a Hypothetical
        Primary Sedimentation Tank	   4-  4
 4- 2   Primary Sludge Characteristics	   4-  8
 4- 3   Alternate Names and Symbols for
        Equation (4-1) 	,.,.............,,.	   4- 11
 4- 4   Values of Yield and Decay Coefficients for
        Computing Waste-Activated Sludge	»   4- 12
 4- 5   Design Data for Sludge Production Example	   4-21
 4- 6   Activated Sludge Characteristics ................   4- 28
 4- 7   Trickling Filter Solids Production 	   4- 30
 4- 8   Daily Variations in Trickling Filter Effluent,
        Stockton, California ..	   4- 33
 4- 9   Description of Sloughing Events .,....,,,,..,,..,   4- 33
 4-10   Concentration of Trickling Filter Sludge
        Withdrawn from Final Clarifiers 	   4- 34
 4-11   Trickling Filter Sludge Composition .............   4- 35
 4-12   Sludge from Combined Attached-Suspended
        Growth Processes	,   4- 36
 4-13   Metals in Ferric Chloride Solutions .............   4- 40
                               xxv

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                    LIST OF TABLES (continued)

Number                                                       Page

 4-14   Progress in Source Control of Toxic
        Pollutants ..,.,...,.....	   4- 41
 4-15   Cadmium in Sludge .,..».,...,...,»	   4- 42
 4-16   Increased Metals Concentration
        During Processing . , . . ,	,,.......	   4- 43
 4-17   Aroclor (PCB)  1254 Measurements in Sludge .,.,,,.   4-44
 4-18   Chlorinated Hydrocarbon  Pesticides in
        Sludge 	....,.,.......•	   4- 45
 4-19   Screening Experience  .	,.,   4- 47
 4-20   Analyses of Screenings	   4- 49
 4-21   Methods of Handling Screenings .	   4- 50
 4-22   Grit Quantities ...»	   4- 52
 4-23   Sieve Analysis of Grit		   4- 53
 4-24   Scum Production and Properties ..................   4- 57
 4-25   Methods of Handling Scum , ,.,,	   4- 58
 4-26   Characteristics of Domestic Septage ....,.,..,.*.   4- 60
 4-27   Metals Concentrations  in Solids From Treatment
        of Combined Sewer Overflows	   4- 62

                            CHAPTER 5

 5- 1   Advantages and Disadvantages  of Gravity
        Thickeners	   5-  3
 5- 2   Typical Gravity Thickener Surface Area
        Design Criteria	   5-  7
 5- 3   Reported Operating Results at Various Overflow
        Rates for Gravity Thickeners  ,.„..»»....,»..,...,.   5-  8
 5- 4   Typical Uniform Load  (W)  Values	   5-  9
 5- 5   Definition of  Torques Applicable  to Circular
        Gravity Thickeners ..............................   5- 10
 5- 6   Types of Municipal Wastewater Sludges Being
        Thickened by DAF Thickeners .....................   5- 18
 5- 7   Advantages and Disadvantages  of DAF
        Thickening 	»	»,,	,	 .   5- 19
 5- 8   Typical DAF Thickener Solids  Loading Rates
        Necessary to Produce  a Minimum 4  Percent
        Solids Concentration	   5- 23
 5- 9   Field Operation Results  From Rectangular  DAF
        Thickeners	   5- 24
 5-10   Reported DAF Thickener Hydraulic Loading
        Rates	.	   5- 27
 5-11   Advantages and Disadvantages  of Disc Nozzle
        Centrifuges	.......*..-.   5- 40
 5-12   Typical Performance of Disc' Nozzle Centrifuge ...   5-43
 5-13   Advantages and Disadvantages  of Imperforate
        Basket Centrifuge	   5- 45
 5-14   Typical Thickening Results Using Imperforate
        Basket Centrifuge	, .   5- 47
 5-15   Advantages and Disadvantages  of Solid Bowl
        Decanter Centrifuges	   5- 50
                               xxvi

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                    LIST OF TABLES (continued)

Nurnbejr                                                       Page

 5-16   Typical Characteristics of the New Type
        Thickening Decanter Centrifuge WAS 	,    5- 52
 5-17   Estimated Capital and O&M Cost for Various
        Centrifuges for Thickening of Waste-Activated
        Sludge at Village Creek -For-. Worth, Texas 	    5-55
                                     U

                            CHAPTER 6


 6- 1   Type and Reference of Full-Scale Studies on
        High Rate Anaerobic Digestion of Municipal
        Wastewater Sludge ,,.,....,,...........,,,...,,,»    6-  3
 6- 2   Results of Recirculating Digested Sludge to
        the Thickener at Bowery Bay Plant, New York , ....    6-11
 6- 3   Operating and Performance Characteristics for
        the Bench-Scale, Two-Phase Anaerobic Digestion
        of Waste-Activated Sludge	    6- 17
 6- 4   Typical Design Criteria for Sizing
        Mesophilic Anaerobic Sludge Digesters ...,.•,.,..    6- 19
 6- 5   Solids Retention Time Design Criteria for
        High Rate Digestion	».,.,...	    6- 24
 6- 6   Average Physical and Chemical Characteristics
        of Sludge From Two-Stage Digester System .,..,,..    6-25
 6- 7   Materials Entering and Leaving Two-Stage
        Digester System .....*.......................,,.*    6- 25
 6- 8   Gas Production for Several Compounds in
        Sewage Sludge	..........	    6- 29
 6- 9   Characteristics of Sludge Gas ...................    6- 31
 6-10   Supernatant, Characteristics of High-Rate,
        Two-Stage, Mesophilic, Anaerobic Digestion
        at Various Plants ...............................    6- 33
 6-11   Effect of Ammonia Nitrogen on Anaerobic
        Digestion	    6- 37
 6-12   Influent Concentrations and Expected
        Removals of Some Heavy Metals in Wastewater
        Treatment Systems	    6- 38
 6-13   Total Concentration of Individual Metals
        Required to Severely Inhibit Anaerobic
        Digestion	    6- 39
 6-14   Total and Soluble Heavy Metal Content of
        Digesters	    6- 40
 6-15   Stimulating and Inhibitory Concentrations
        of Light Metal Cations	    6- 40
 6-16   Synergistic and Antagonistic Cation
        Combinations	    6- 41
 6-17   Heat Transfer Coefficients for Hot Water
        Coils in Anaerobic Digesters ....................    6- 47
 6-18   Heat Transfer Coefficients for Various
        Anaerobic Digestion Tank Materials ..............    6- 52
                              xxvi i

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                    LIST OF TABLES (continued)

Number                                                       Page

 6-19   Relationship Between the Velocity Gradient
        and Unit Gas Flow	,......,.,..    6- 63
 6-20   Design Loading Assumptions	    6- 79
 6-21   Selected Aerobic Digestion Studies on
        Various Municipal Wastewater Sludges ,....,,..,..    6- 83
 6-22   Characteristics of Mesophilic Aerobic
        Digester Supernatant ,......'....,	    6- 93
 6-23   Summary of Current Aerobic Digester
        Design Criteria	,,.,»,.    6- 94
 6-24   Aerobic Digestion Labor Requirements	    6-100
 6-25   Lime Requirement to Attain pH 12 for
        30 Minutes at Lebanon,  Ohio	    6-105
 6-26   Lime Doses Required to Keep pH Above
        11.0 at Least 14 Days	    6-105
 6-27   Bacteria in Raw, Anaerobically Digested,  and
        Lime Stabilized Sludges at Lebanon, Ohio	    6-109
 6-28   Chemical Composition of Sludges at Lebanon,
        Ohio, Before and After Lime Stabilization ..,,...    6-111
 6-29   Chemical Composition of Sludge and
        Supernatant Before and After Lime
        Stabilization	    6-113
 6-30   Characteristics of Quicklime and
        Hydrated Lime ...................................    6-114
 6-31   Mechanical Mixer Specifications for
        Sludge Slurries	    6-122
 6-32   Estimated Average Annual Costs for Lime
        Stabilization Facilities	•..	    6-123
 6-33   Estimated Chlorine Requirements for
        Sludge and Sidestream Processing ,,,,..,.,	,.    6-133
 6-34   Actual Operating Costs for Chlorine
        Stabilization System	    6-135
 6-35   Chlorine Stabilization Capital Costs, 1979 ......    6-137


                            CHAPTER 7:


 7- 1   Pathogenic Human Viruses Potentially in
        Wastewater Sludge ...............................    7-  3
 7- 2   Pathogenic Human Bacteria Potentially in
        Wastewater Sludge	    7-  4
 7- 3   Pathogenic Human and Animal Parasites
        Potentially in Wastewater Sludge	    7-  5
 7- 4   Pathogenic Fungi Potentially in
        Wastewater Sludge ,	    7-  6
 7- 5   Pathogen Occurrence in Liquid Wastewater
        Sludges	,	    7-  8
 7- 6   Pathogen Survival in Soils ......................    7- 12
 7- 7   Time and Temperature Tolerance for
        Pathogens in Sludge .....,...,,»,,,«».,,,..,.,...    7- 14
                              XXVlll

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                    LIST OF TABLES (continued)

Number^                                                       Page

                            CHAPTER 8

 8- 1   Effects of Either Polyelectrolyte Conditioning
        or Thermal Conditioning Versus No Conditioning
        on a Mixture of Primary and Waste-Activated
        Sludge Prior to Gravity Thickening ,.,*..	, .    8-  2
 8- 2   Typical Conditioning Dosages of Ferric
        Chloride (FeCl3) and Lime (CaO) for
        Municipal Wastewater Sludges	    8-  7
 8- 3   Suppliers of Polyelectrolytes 	    8- 14
 8- 4   Representative Dry Powder Cationic
        Polyelectrolytes .................	    8- 16
 8- 5   Representative Liquid Cationic
        Polyelectrolytes ...,.......*................,.,,    8- 16
 8- 6   Typical Polyelectrolyte Additions for
        Various Sludges	..	    8- 21
 8- 7   Typical Levels of Dry Polyelectrolyte Addition
        for Belt Filter Presses	,    8- 23
 8- 8   Typical Levels of Dry Polyelectrolyte Addition
        for Solid Bowl Decanter Centrifuges
        Conditioning Various Sludges ....................    8- 25
 8- 9   Performance of Solids Handling System at
        Bissell Point, St. Louis STP 1972-1976  ..........    8- 26
 8-10   Performance of Solids Handling System at
        Bissell Point, St. Louis STP 1977-1978  .,	    8- 28
 8-11   Advantages and Disadvantages of Ash Addition
        to Sludge for Conditioning	    8- 33
 8-12   General Characteristics of Separated  Liquor
        From Thermal Conditioned Sludge .................    8- 36
 8-13   Filtrate and/or Centrate Characteristics From
        Dewatering Thermal Conditioned Sludge ...........    8- 38
 8-14   USEPA July 1979 Survey of Existing Municipal
        Wastewater Thermal Conditioning .......,..,,,,..»    8- 39
 8-15   Comparison of Sewage Sludge Handling  and
        Conditioning Processes	    8- 41

                            CHAPTER 9

 9- 1   Pilot-Scale Sludge Dewatering Studies	    9-  2
 9- 2   Advantages and Disadvantages of Using
        Sludge Drying Beds			 .    9-  3
 9- 3   Advantages of a Wedge-Wire Drying Bed ...........    9-  7
 9- 4   Characterization of Sand Bed Drainage ...........    9-  9
 9- 5A  Summary of Recognized Published Sand  Bed
        Sizing Criteria for Anaerobically Digested,
        Non-Conditioned Sludge ...,,.,,....,....	    9- 10
 9- 5B  Summary of Recognized Published State Bed
        Sizing Criteria for Sand Beds by USEPA
        Regions Square Feet/Capita	    9- 11
 9- 6   Wedge-Wire System Performance Data ..............    9- 12
                               XXIX

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                    LIST OF TABLES (continued)

Number                                                       Page

 9- 7   Sludge Drying Beds, Labor Requirements	    9- 13
 9- 8   Advantages and Disadvantages of Using Sludge
        Drying Lagoons ....,,,.,.....	,.»,.,.    9- 14
 9- 9   Sludge Drying Lagoons, Labor
        Requirements 	,,,,,,.,»...,.	«.    9- 17
 9-10   Advantages and Disadvantages of Basket
        Centrifuges	,	    9- 18
 9-11   Typical Performance Data for an Imperforate
        Basket Centrifuge	    9- 20
 9-12   Specific Operating Results for  Imperforate
        Basket  .		    9- 20
 9-13   Operating Results for Basket Centrifuge
        Dewatering of Aerobically Digested
        Sludge at Burlington, Wisconsin	    9- 22
 9-14   Advantages and Disadvantages of Solid Bowl
        Decanter Centrifuges	    9- 23
 9-15   Typical Performance Data for a  Solid Bowl
        Decanter Centrifuge	„    9- 24
 9-16   Precoat Process Performance on  Fine
        Particulate Sludges	    9- 27
 9-17   Advantages and Disadvantages of Using Rotary
        Drum Vacuum Filters	.........    9- 28
 9-18   Typical Dewatering Performance  Data for
        Rotary Vacuum Filters - Cloth Media	    9- 34
 9-19   Typical Dewatering Performance  Data for
        Rotary Vacuum Filters - Coil Media	    9- 35
 9-20   Specific Operating Results of Rotary Vacuum
        Filters - Cloth Media	    9- 35
 9-21   Specific Operating Results of Rotary Vacuum
        Filters - Coil Media	    9- 36
 9-22   Operational Cost for Lakewood,  Ohio Vacuum
        Filter Operations	    9- 41
 9-23   Advantages and Disadvantages of Belt
        Filter Presses	    9- 45
 9-24   Typical Dewatering Performance  of  Belt Filter
        Presses	......,.....,.,,..    9- 48
 9-25   Labor Requirements for Belt Filter
        Presses	    9- 51
 9-26   Advantages and Disadvantages of Recessed  Plate
        Pressure Filters	    9- 52
 9-27   Expected Dewatering Performance for a Typical
        Fixed Volume Recessed Plate Pressure Filter . ....    9-56
 9-28   Specific Operating Results of Fixed Volume
        Recessed Plate Pressure Filters .................    9- 57
 9-29   Typical Dewatering Performance  of  a Variable
        Volume Recessed Plate Pressure  Filter ...........    9- 57
 9-30   Pressure Filtration and Incineration
        Operational Cost ................................    9- 61
 9-31   Performance Results From a Screw Press ..,,....,.    9- 65
                               XXX

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                    LIST OF TABLES (continued)

Numbe_r                                                       Page

 9-32   Summary of Performance Results For a Dual
        Cell Gravity Filter - Mentor, Ohio 	»	.    9- 68

                           CHAPTER 10

10- 1   Estimated 1977 Costs for Dewatering, Drying
        and Bagging at Largo, Florida ..*»*,*.*..	,.   10- 22

                           CHAPTER 11

11- 1   Chemical Reactions Occurring During
        Combustion ,..»,..,.,,,..,»......,.„...,	. .   11-  4
11- 2   Representative Heating Values of Some
        Sludges  ., .	.........................   11-  5
11- 3   Theoretical Air and Oxygen Requirements
        for Complete Combustion . »	. ....   11-  7
11- 4   Approximate Combustion Calculation -
        Supplemental Fuel Requirements	,,,.   11- 18
11- 5   Combustion Calculations-Molal Basis	   11- 21
11- 6   Combustion Calculations-Molal Basis ..,....,,,.,.   11- 23
11- 7   Comparison Between an Approximate and a
        Theoretical Calculation of Furnace
        Combustion	 *	   11- 25
11- 8   Hypothetical Wastewater Treatment Plant
        Design Data	..............	   11- 31
11- 9   Heat and Material Balance for Sludge
        Incineration in a Multiple-Hearth Furnace .......   11- 39
11-10   Typical Hearth Loading Rates for a
        Multiple-Hearth Furnace 	.	   11- 48
11-11   Heat and Material Balance for Sludge
        Incineration in a Fluid Bed Furnace	. .   11- 51
11-12   Heat and Material Balance for Sludge
        Incineration in an Electric Infrared
        Furnace	   11- 57
11-13   Heat and Material Balance for Sludge
        Incineation in a Cyclonic Furnace ...............   11- 60
11-14   Design Example:  Wastewater Treatment Plant
        Operating Data	   11- 61
11-15   Design Example:  Sludge Furnace Design
        Criteria		.   11- 62
11-16   Design Example:  Heat and Material Balance
        for a Fluid Bed Furnace	   11- 63
11-17   Heat and Material Balance for Starved-Air
        Combustion of Sludge in a Multiple-Hearth
        Furnace	   11- 70
11-18   Heat and Material Balance Comparison of
        Starved-Air Combustion and Incineration ...,,.,,.   11- 72
11-19   Design Example:  Wastewater Treatment Plant
        Operating Data ............I........,,,,....,....   11- 77
                              XXXI

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                    LIST OF TABLES (continued)

Number                                                       Page

11-20   Design Example:  Heat and Material Balances
        for Multiple-Hearth Furnaces ....................   11- 79
11-21   Conventional Approaches to Co-Combustion
        of Wastewater Sludge and Mixed Municipal
        Refuse	   11- 83
11-22   Heat and Material Balance for Co-Combustion
        by Starved-Air Combustion in a Multiple-Hearth
        Furnace	   11- 91
11-23   Carbon Regeneration Methods .I...................   11- 99
11-24   Basic Types of Pyrolysis, Thermal Gasification,
        and Liquefaction Reactors - New, Demonstrated,
        or Under Development	   11-103
11-25   Health Effects of Air Pollutants		....   11-118
11-26   San Francisco Bay Area - Maximum Allowable
        Pollutant Concentrations ........................   11-124
11-27   Uncontrolled Emission Rates from
        Multiple-Hearth Furnaces	   11-125
11-28   Design Example:  Exhaust Gas Data from a
        Multiple-Hearth Furnace	   11-130
11-29   Design Example:  Auxiliary Fuel Correction
        for a Multiple-Hearth Furnace ,,,	   11-130
11-30   Design Example:  Multiple-Hearth Furnace
        Pollutant Concentrations After Scrubbing ........   11-131
11-31   Description of Solid and Liquid Waste
        Classifications	   11-133
11-32   Classification of Waste Disposal Sites ..........   11-134

                           CHAPTER 12

12- 1   Suggested Monitoring Program for a Municipal
        Wastewater Sludge Composting Facility ,...*.,..,.   12-  6
12- 2   Densities of Various Compost Bulking
        Agents	   12- 12
12- 3   Beltsville Equipment	   12- 41
12- 4   Beltsville Actual and Projected Operating
        Costs	   12- 43
12- 5   Estimated Annual Labor and Equipment
        Requirements, Bangor, Maine	   12- 45
12- 6   Bangor Equipment	   12- 46
12- 7   Bangor Materials Requirements for 2,170 Wet Ton
        Annual Sludge Input	......,...,..,..,.»».....   12- 46
12- 8   Facility Processing 10 Dry Tons of Sludge per
        Day ......			   12- 50
12- 9   European Wastewater Sludge Composting
        Processes .............I......,...,,,...,..,.,...   12- 57

                           CHAPTER 13

13- 1   Partial List of Fixation Processes	   13-  2
13- 2   Parameters for Earthworm Conversion .............   13-  6
                              xxxi i

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                    LIST OF TABLES (continued)

Number                                                       Page

13- 3   Possible Operating Difficulties in
        Earthworm Conversion	.,.,.,.,.,,,,.,,.,,.»  13-  8

                           CHAPTER 14

14- 1   Summarized Calculations for Non-Newtonian
        Flow Example Problem ..........I.................  14- 11
14- 2   Pressure Required to Exceed Yield Stress -
        Example Problem	  14- 12
14- 3   Applications for Sludge Pumps	 ....  14- 30
14- 4   Typical Long Pipelines Carrying Unstabilized
        Sludged	  14- 32
14- 5   Typical Long Pipelines Carrying Digested
        Sludge ....		  14- 32
14- 6   Long Pipelines for Unstabilized Sludge:
        Additional Locations .,..,..,.,.,...,,.......,...  14- 34
14- 7   Long Pipelines for Digested Sludge:
        Additional Locations	  14- 34
14- 8   Typical Minimum Tank Car Requirements  ...........  14- 52
14- 9   Typical Transit Times for Railroad
        Transportation .,,..,.,..................I.......  14- 53
14-10   Manpower Requirements for Railroad
        Transport	,	  14- 54
14-11   Tug Costs for Various Barge Capacities	  14- 55
14-12   Typical Barge Sizes and Costs	  14- 56

                           CHAPTER 15

15- 1   Wastewater Solids Storage Applicability	  15-  5
15- 2   Calculations for Digester Effluent Mass
        Flow Rate from Equation 15-1	  15- 16
15- 3   Advantages and Limitations of Using Facultative
        Sludge Lagoons for Long-Term Storage  ............  15- 24
15- 4   Sacramento Central Wastewater Treatment
        Plant Volatile Reductions, Digested Sludge
        Quantities and FSL Area Loadings	  15- 33
15- 5   Sacramento Central Wastewater Treatment
        Plant FSL Design Data	  15- 35
15- 6   Sacramento Central Wastewater Treatment
        Plant FSL Sludge Inventory, Dry Tons	  15- 35
15- 7   Sacramento Central Wastewater Treatment
        Plant Recycled FSL Supernatant Quality	  15- 36
15- 8   Sacramento Central Wastewater Treatment
        Plant Comparison of Digested FSL and
        Removed Sludge Analytical Data		  15- 37
15- 9   Sacramento Central Wastewater Treatment
        Plant Odor Risk for 40 Acres of FSLs, Annual
        Events (Days)	  15- 39
15-10   Sacramento Regional Wastewater Treatment
        Plant Ultimate Odor Risk for 124 Acres
        of FSL, Annual Events (Days)	  15- 41
                             XXXI11

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                    LIST OF TABLES (continued)

Number                                                       Page

15-11   1978 Removed Sludge-Prairie Plan Land
        Reclamation Project, The Metropolitan Sanitary
        District of Greater Chicago	   15- 44
15-12   1973/1974 Supernatant-Praire Plan Reclamation
        Project, The Metropolitan Sanitary District of
        Greater Chicago	   15- 45

                           CHAPTER 16

16- 1   Effect of Polymer on Elutriation	,»   16-  5
16- 2   Effect of Supernatant Return ,	   16-  6
16- 3   Estimated Increase in Wastewater Stream
        Biological Treatment Capacity Required to
        Handle Sidestreams From Various Solids
        Treatment Processes »..,,,,,,,,,,,,,,,.,,...,.,,.   16-  7
16- 4   Possible Digester Supernatant Treatment
        Processes .......................................   16- 10
16- 5   Chlorine Treatment of Digester Supernatant ......   16- 11
16- 6   Aerobic Digestion of Heat Treatment Liquor	   16- 13
16- 7   Activated Sludge Treatment of Thermal
        Conditioning Liquor .............................   16- 14
16- 8   Aerobic Biological Filtration of Thermal
        Condition Liquor ................................   16- 15
16- 9   Chlorine Oxidation Treatment of Thermal
        Conditioning Liquor .............................   16- 16

                           CHAPTER 17

17- 1   Thickening ........,..,,,	   17-  3
17- 2   Stabilization	   17-  5
17- 3   Disinfection	   17-  8
17- 4   Conditioning	,.   17- 10
17- 5   Dewatering	   17- 13
17- 6   Heat Drying	   17- 17
17- 7   High Temperature Process	   17- 22
17- 8   Composting	   17- 27
17- 9   Miscellaneous Conversion Processes ...,,,.,......   17- 30
17-10   Transportation	.,,,.,,.,   17- 31
17-11   Storage			   17- 32
17-12   Sidestreams .....................................   17- 36

                           CHAPTER 18

18- 1   Comparison of Current and Potential Sludge
        Utilization to Commercial Fertilizer
        Consumption in the United States ................   18-  3
18- 2   Examples of Communities Practicing Land
        Utilization	   18-  4
                              XXXIV

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                    LIST OF TABLES (continued)

Number                                                       Page

                           CHAPTER 19

19- 1   Suitability of Sludges for Landfilling	   19-  7
19- 2   Sludge and Site Conditions	.	   19-  9
19- 3   Landfill Design Criteria 	.................   19- 15
19- 4   Leachate Quality From Sludge-Only Landfill  ......   19- 16
19- 5   Landfill Equipment Performance
        Characteristics ,	.	   19- 19
19- 6   Typical Equipment Type and Number as a Function
        of Landfill Method and Site Loading 	..,,,,..   19- 21
19- 7   Potential Environmental Problems and Control
        Practices	   19- 23
19- 8   Surface Application Methods and Equipment  for
        Liquid Sludges	   19- 30
19- 9   Subsurface Application Methods and Equipment
        for Liquid Sludges	   19- 31
19-10   Furrow Slope Evaluation	   19- 33
19-11   Methods and Equipment for Application of
        Dewatered Sludges ...............................   19- 35
19-12   Colorado Springs Population and Wastewater  Flow
        Projections	   19- 40
19-13   Colorado Springs Projected Cost of Sludge
        Management System ...............................   19- 43
19-14   Colorado Springs Climatic Conditions Affecting
        Sludge Disposal	.,	,............,,,,.   19- 44
19-15   Colorado Springs Dedicated Land Disposal/
        Subsurface Injection System Design Data ....»..,.   19- 49
19-16   Sacramento Regional Wastewater Treatment Plant
        Projected 1985 Wastewater Flow and Loadings .....   19- 51
19-17   Sacramento Regional Wastewater Treatment Plant
        Projected Digested Sludge Production	   19- 54
19-18   Sacramento Test OLD Runoff Water Analysis	   19- 55
19-19   Sacramento Regional Wastewater Treatment Plant
        Projected 1985 OLD Staffing Requirements ,	   19- 58
19-20   Sacramento Regional Wastewater Treatment Plant
        Projected Costs of Sludge Management System
        Following Anaerobic Digestion ,,,.,.,,	, ....   19- 59
                              XXXV

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                         LIST OF FIGURES
Number
                                                             Page
                            CHAPTER 1

 1- 1   Classification of Treatment Disposal Options ....   1-  3

                            CHAPTER 3

 3- 1   Criteria for System Selection	,.   3-  3
 3- 2   Components for System Synthesis	   3-  4
 3- 3   Flowsheet Developed From Components for
        System Synthesis	...,,..,«...,....   3-  5
 3- 4   Parallel Elements .......		.,	   3- 12
 3- 5   Candidate Base Alternatives for Eugene-
        Springfield	   3- 14
 3- 6   Flowsheet for the Eugene-Springfield Sludge
        Management System	   3- 17
 3- 7   Blank QFD for Chemically-Assisted Primary
        Plant	,..,.,..   3- 19
 3- 8   QFD for Chemically-Assisted Primary Plant .......   3-22
 3- 9   QFD for Secondary Plant with Filtration	   3-25
 3-10   Contingency Planning Example .,...«»...,,,,,	   3- 30

                            CHAPTER 4

 4- 1   Typical Relationship Between Peak Solids
        Loading and Duration of Peak for Some Large
        American Cities .,,..,»...,,..,.,...,.....,.	   4-  5
 4- 2   Peak Sludge Loads,  St.  Louis Study	   4-  6
 4- 3   Net Growth Rate Curves	   4- 18
 4- 4   Schematic for Sludge Quantity Example ...........   4- 20
 4- 5   Sludge Wasting Methods	   4- 26
 4- 6   VSS Production Data for Three Trickling Media
        Designs	   4- 32

                            CHAPTER 5

 5- 1   Typical Concentration Profile of Muncipal
        Wastewater Sludge in a Continuously Operating
        Gravity Thickener	   5-  4
 5- 2   Typical Gravity Thickener Installation ..........   5-  5
 5- 3   Cross Sectional View of a Typical Circular
        Gravity Thickener	....,.,,.,..,.«..   5-  6
 5- 4   Annual O&M Man-Hour Requirements - Gravity
        Thickeners 	...,..,,,	   5- 16
 5- 5   Annual Power Consumption - Continuous
        Operating Gravity Thickeners	,...   5- 17
                              XXXVI

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                   LIST OF FIGURES (continued)

Number                                                       Page

 5- 6   Estimated June 1975 Maintenance Material Cost
        For Circular Gravity Thickeners	    5- 18
 5- 7   Typical Rectangular, Steel Tank, Recycle
        Pressurization Dissolved Air Flotation
        Thickener , .	.......	    5- 21
 5- 8   Float Concentration and Subnatant Suspended
        Solids Versus Solids Loading of a Waste-Activated
        Sludge - Without Polymers ,.	,	    5- 25
 5- 9   Float Concentration and Subnatant Suspended
        Solids Versus Solids Loading of a Waste-Activated
        Sludge - With Polymers  	»	. ....    5- 26
 5-10   Effect of Hydraulic Loading on Performance in
        Thickening Waste-Activated Sludge	    5- 28
 5-11   Float Concentration and Subnatant Suspended
        Solids Versus Air-Solids Ratio With Polymer For
        a Waste-Activated Sludge	    5- 29
 5-12   Float Concentration and Subnatant Suspended
        Solids Versus Air-Solids Ratio Without Polymer
        For a Waste-Activated Sludge ....................    5- 30
 5-13   Annual O&M Man-Hour Requirements - DAF
        Thickeners	..,,....,.,.,......,,    5- 37
 5-14   Annual Power Consumption - Continuous Operating
        DAF Thickeners	    5- 38
 5-15   Estimated June 1975 Maintenance Material Cost For
        DAF Thickeners ,	,..,.,    5- 39
 5-16   Typical Disc Nozzle Centrifuge in the Field .....    5-40
 5-17   Schematic of a Disc Nozzle Centrifuge	    5- 41
 5-18   Typical Disc Nozzle Pretreatment System	    5- 43
 5-19   Effect of Activated Sludge Settleability on
        Capture and Thickening  .............I,.,.,.....,,    5- 44
 5-20   General Schematic of Imperforate Basket
        Centrifuge	.	    5- 46
 5-21   Relative Influence of One Pocess Variable as a
        Function of Feed Solids Content for Imperforate
        Basket Centrifuge Holding All Other Process
        Variables Constant	    5- 48
 5-22   Schematic of Typical Solid Bowl Decanter
        Centrifuge	    5- 50
 5-23   Solid Bowl Decanter Centrifuge Installation .....    5-51
 5-24   Estimated June 1975 Solid Bowl Decanter
        Installation Capital Cost	 ....    5- 56
 5-25   Annual O&M Requirements - Solid Bowl Decanter
        Centrifuge	    5- 58
 5-26   Estimated June 1975 Maintenance Material Cost for
        Solid Bowl Decanter Centrifuge	    5- 59

                            CHAPTER 6

 6- 1   Summary of the Anaerobic Digestion Process ......    6-  5
 6- 2   Low-Rate Anaerobic Digestion System .............    6-  8
                             XXXVI1

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                   LIST OF FIGURES (continued)

Njjmbejr                                                       Page

 6- 3   Single-stage, High-Rate Anaerobic Digestion
        System	    6-  9
 6- 4   Flow Diagram for the Torpey Process .............    6- 10
 6- 5   Two-Stage, High-Rate Anaerobic Digester
        System .	    6- 12
 6- 6   Carbon and Nitrogen Balance for a Two-Stage,
        High-Rate Digestion System	    6- 14
 6- 7   Effect of Recycling Digester Supernatant on the
        Suspended Solids Flow Through an Activated
        Sludge Plant	    6- 15
 6- 8   Anaerobic Contact Process .......................    6- 15
 6- 9   Two-Phase Anaerobic Digestion Process ...........    6- 16
 6-10   Effect of SRT on the Relative Breakdown of
        Degradable Waste Components and Methane
        Production ...,..,,,,..,,,.......,..,.,.,........    6- 21
 6-11   Effect of Temperature and SRT on the Pattern
        of Methane Production and Volatile Solids
        Breakdown	    6- 22
 6-12   Effect of Solids Retention Time and Temperature
        on Volatile Solids Reduction in a Laboratory-
        Scale Anaerobic Digester	,,.,...,    6- 27
 6-13   Volatile  Solids Reduction vs Temperature x SRT
        for Three Types of Feed Sludges .................    6- 28
 6-14   Effect of Temperature on Gas Production .........    6- 30
 6-15   Relationship Between pH and Bicarbonate
        Concentration Near 95°F (35°C)	    6- 35
 6-16   Cylindrical Anaerobic Digestion Tanks ...........    6- 43
 6-17   Rectangular Anaerobic Digestion Tank ............    6- 44
 6-18   Egg-Shaped Anaerobic Digestion Tank at Terminal
        Island Treatment Plant, Los Angeles ,,,,,...,....    6- 45
 6-19   Schematic of the Heat Reservoir System for a
        Jacketed  Pipe or Spiral Heat Exchanger ..........    6- 48
 6-20   Spiral Heat Exchanger Operating Off Secondary
        Heat Loop at Sunnyvale, California	    6- 49
 6-21   Effect of Solids Concentration on the Raw Sludge
        Heating Requirement ,.,,,	    6- 51
 6-22   Circulation Patterns Produced by Draft Tube and
        Free Gas  Lift Mixers	    6- 56
 6-23   Draft Tube and Free Gas Lift Pumping Rate	    6-57
 6-24   Comparison of Lance and Draft Tube Mixing in
        Clean Water	 .    6- 58
 6-25   Effect of Temperature on the Viscosity of
        Water	    6- 60
 6-26   Effect of Solids Concentration and Volatile
        Content on the Viscosity of Digesting Sludge ....    6-61
 6-27   Types of Digester Covers ...*.*.......*»*........    6- 64
 6-28   Overall View of Four Digesters With Downes
        Floating Covers at Sunnyvale, California ........    6- 65
 6-29   Typical Digester Supernatant Collection
        System	    6- 68
                             XXXVI11

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                   LIST OF FIGURES (continued)

Number                                                       Page

 6-30   Digester Drain System ............................   6- 71
 6-31   Digester Washwater Cleaning by Cyclonic
        Separators, Grit Dewaterers, and Static Screens
        at Los Angeles County Carson Plant ..............   6- 72
 6-32   Energy Flow Through an Anaerobic Sludge Digestion
        System ......................	,.,..,......   6- 73
 6-33   Construction Costs for Anaerobic Digestion
        Systems 	..	   6- 75
 6-34   Operating, Maintenance, and Energy Costs for
        Anaerobic Sludge Digestion Systems ..............   6- 76
 6-35   Conceptual Design of an Anaerobic Sludge
        Digestion System	   6- 80
 6-36   Process Flow Diagram for a Conventional
        Continuously Operated Aerobic Digester ..........   6- 85
 6-37   Reaction Rate K
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                   LIST OF FIGURES (continued)

Number                                                       Page

 7- 3   Energy Requirements for Sludge ..................
        Pasteurization Systems 	.	    7- 18
 7- 4   Construction Costs for Sludge Pasteurization
        Systems Without Heat Recovery ,...,»»,.,.».,.....    7- 19
 7- 5   Construction Costs for Sludge Pasteurization
        Systems With Heat Recovery	    7- 20
 7- 6   Labor Requirements for Sludge
        Pasteurization Systems	.....,,.»,    7- 21
 7- 7   Maintenance Material Costs for Sludge
        With Heat Recovery ..,....*......,..*....	    7- 24
 7- 9   Equipment Layout for Electron Beam Facility .....    7-28
 7-10   Electron Beam Scanner and Sludge  Spreader  ....,,.    7-29
 7-11   Schematic Representation of Cobalt-60 Irradiation
        Facility at Geiselbullach, West Germany	    7- 34
 7-12   Gamma Radiation Treatment of Liquid Sludge  Power
        Requirements ..........................	«    7- 36
 7-13   Radiation Treatment of Dewatered  Sludge  - Power
        Requirements ......,.,,.,.,*.,.,.,...............    7- 37
 7-14   Gamma Radiation Treatment of Liquid Sludge  -
        Capital Costs	    7- 39
 7-15   Gamma Radiation Treatment of Liquid Sludge  Labor
        Requirements	    7- 40
 7-16   Gamma Radiation Treatment of Liquid Sludge
        Maintenance Material Supplies and Costs  .........    7- 41
 7-17   Gamma Radiation Treatment Facility for Handling
        25 Tons per Day or More of Dewatered Sludge .....    7-41
 7-18   Gamma Radiation Treatment of Dewatered Sludge
        Capital Cost	    7- 42
 7-19   Gamma Radiation Treatment of Dewatered Sludge-
        Labor Requirements ..............................    7- 43
 7-20   Gamma Radiation Treatment of Dewatered
        Maintenance Materials and Supplies Cost	    7-44

                            CHAPTER 8

 8- 1   Basic Parameters for Evaluation of a Sludge.
        Conditioning System ,	    8-  2
 8- 2   Particle Size Distribution of Common
        Materials	    8-  3
 8- 3   Typical Concentration Profile of  Municipal
        Wastewater Sludge in a Continuously Operating
        Gravity Thickener ......,..,....,.,.,..,..*...,..    8-  5
 8- 4   Capital Cost of Ferric Chloride Storage  and
        Feeding Facilities	    8- 10
 8- 5   Capital Cost of Lime Storage and  Feeding
        Facilities	'..............    8- 11
 8- 6   Ferric Chloride Storage and Feeding Operating  and
        Maintenance Work-Hour Requirements ..............    8- 11
 8- 7   Electrical Energy Requirements for a Ferric
        Chloride Chemical Feed System ..,,.	    8- 12


                               xl

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                   LIST OF FIGURES (continued)

Number                                                       Page

 8- 8   Lime Storage and Feeding Operation and
        Maintenance Work-Hour Requirements .,,,.,,,,.,,*.    8- 12
 8- 9   Electrical Energy Requirements for a. Lime Feed
        System	...	    8- 13
 8-10   Polyacrylamide Molecule - Backbone of the
        Synthetic Organic Polyelectrolytes 	    8- 15
 8-11   Typical Configuration of a Cationic
        Polyelectrolyte in Solution ..	    8- 17
 8-12   Schematic Representation of the Bridging Model
        for the Destabilization of Colloids by
        Polymers . »	..    8- 19
 8-13   Effect of Biological Solids on Polymer
        Requirements in Belt Press Dewatering ...........    8- 24
 8-14   Relative Influence of Polymer Addition on
        Imperforate Basket Centrifuge Process
        Variables	    8- 28
 8-15   Polymer Storage and Feeding Operation and
        Maintenance Work-Hour Requirements »...,.»...,,.,    8- 29
 8-16   Electrical Energy Requirements for a Polymer
        Feed System	    8- 30
 8-17   General Thermal Sludge Conditioning Flow Scheme
        for a Non-Oxidative System	    8- 32
 8-18   General Thermal Sludge Conditioning Flow Scheme
        for an Oxidative System .,...,,.,,.....	    8- 33

                            CHAPTER 9

 9- 1   Schematic of Sludge Dewatering in a Drying Bed
        System	    9-  4
 9- 2   Typical Sand Drying Bed Construction	    9-  6
 9- 3   Typical Paved Drying Bed Construction	    9-  6
 9- 4   Cross Section of a Wedge-Wire Drying Bed ........    9-  7
 9- 5   Estimated June 1975 Maintenance Material Cost
        for Open Sand Drying Beds  ....	    9- 13
 9- 6   1977 Flow Diagram of Burlington, Wisconsin
        Wastewater Treatment Plant	    9- 21
 9- 7   Flow Diagram of a Filtration System	    9- 25
 9- 8   Cutaway View of a Drum or Scraper-Type Rotary
        Vacuum Filter	.	    9- 29
 9- 9   Operating Zones of a Rotary Vacuum Filter »,..,,,    9-29
 9-10   Cross Sectional View of a Coil Spring - Belt
        Type - Rotary Vacuum Filter	    9- 30
 9-11   Typical Coil Spring - Belt Type - Rotary Vacuum
        Filter Installation	    9- 31
 9-12   Cross Sectional View of a Fiber Cloth - Belt
        Type - Rotary Vacuum Filter	    9- 32
 9-13   Typical Fiber Cloth - Belt Type - Rotary Vacuum
        Filter	    9- 33
 9-14   Rotary Vacuum Filter System ..,.»,,.»,,.»..,...,.    9- 36
                               xli

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                   LIST OF FIGURES (continued)

Number                                                       Page

 9-15   Rotary Vacuum Filter Productivity as a Function
        of Feed Sludge Suspended Solids Concentration ...    9-38
 9-16   Sludge Cake Total Solids Concentration as a
        Function of the Feed Sludge Suspended Solids
        Concentration .........I.........................    9- 39
 9-17   Lakewood, Ohio Wastewater Treatment Plant Flow
        Diagram ..............	,	........    9- 40
 9-18   Estimated June 1975 Capital Cost for Rotary Drum
        Vacuum Filters ......	    9- 42
 9-19   Annual O&M Man-Hour Requirements - Rotary Drum
        Vacuum Filters ..................................    9- 42
 9-20   Power Consumed by Rotary Drum Vacuum Filtration
        Process ,.,,,...»	,,	    9- 43
 9-21   Estimated June 1975 Annual Maintenance Material
        Cost - Rotary Drum Vacuum Filter	    9- 44
 9-22   The Three Basic Stages of a Belt Press ,	    9- 46
 9-23   Alternative Designs for Obtaining Water Releases
        with Belt Filter Presses	    9- 47
 9-24   Typical Dewatering Performance of Belt Filter
        Presses ......................	    9- 48
 9-25   Schematic Side View of a Recessed Plate Pressure
        Filter	......    9- 53
 9-26   Cross Section of a Fixed-Volume Recessed Plate
        Filter Assembly	    9- 53
 9-27   Typical Recessed Plate Pressure Filter
        Installation at Wassau, Wisconsin ...............    9- 54
 9-28   Cross Section of a Variable Volume Recessed Plate
        Filter Assembly	    9- 55
 9-29   Schematic of an In-Line Conditioning System For
        Recessed Plate Pressure Filter	    9- 58
 9-30   Brookfield, Wisconsin Wastewater Treatment Plant
        Flow Diagram	....*    9- 60
 9-31   Performance Data for a Pressure Filter
        Brookfield, Wisconsin ...,....,....*....,......»,    9- 61
 9-32   Estimated June 1975 Costs for Fixed Volume
        Recessed Plate Pressure Filters	    9- 62
 9-33   Annual O&M Man-Hour Requirements - Fixed Volume
        Recessed Plate Pressure Filter	    9- 63
 9-34   Fixed Volume Recessed Plate Pressure Filter
        Power Consumption	    9- 64
 9-35   Estimated June 1975 Annual Maintenance Material
        Cost-Fixed Volume, Recessed Plate Pressure
        Filter	    9- 64
 9-36   System Schematic for One Type of Screw Press
        System	"	 ....    9- 65
 9-37   Cross Section View of a Twin-Roll Vari-Nip
        Press		...	....    9- 66
 9-38   Cross Section View of a Dual Cell Gravity
        Filter	».,,,.»...	    9- 67
                              xlii

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                   LIST OF FIGURES (continued)

Number                                                       page

10- 1   Estimate of Energy Required to Dry Wastewater
        Sludge as a Function of Dryer Feed Solids
        Content ..,.,,...,...»...,...»,,,,...,	......  10-  6
10- 2   Schematic for Sludge Drying Example	  10-  7
10- 3   Flash Dryer System (Courtesy of C.E. Raymond) ...  10- 17
10- 4   Schematic for a Rotary Dryer 	  10- 20
10- 5   Jacketed Hollow-Flight Dryer (Courtesy Bethlehem
        Corporation) ,	......	  10- 23
10- 6   Toroidal Drying System	,	  10- 26
10- 7   Schematic of BEST Process	  10- 29

                           CHAPTER 11

11- 1   Basic Elements of High Temperature Processes ....  11-  3
11- 2   Effect of Excess Air and Excess Temperature
        on Supplemental Fuel Requirements	....,,,.  11-  8
11- 3   Effect of Dry Solids Heating Value and Sludge
        Moisture on Capability for Autogenous
        Combustion	..,..,,,,,.........,	  11- 11
11- 4   Effect of Sludge Moisture Content and
        Combustible Solids Content on Supplemental
        Fuel Consumption ,..,»»,,,........	  11- 12
11- 5   Hypothetical Wastewater Treatment Plant
        Flowsheet	,	  11- 30
11- 6   Cross Section of a Multiple-Hearth Furnace  	  11- 33
11- 7   Shaft Cooling Air Arrangement in a
        Multiple-Hearth Furnace .........................  11- 34
11- 8   Process Zones in a Multiple-Hearth Furnace  .,,,,,  11- 35
11- 9   Flowsheet for Sludge Incineration in a
        Multiple-Hearth Furnace ,,.„,,,...,.,,,»....,....  11- 37
11-10   Multiple-Hearth Furnace Start-Up Fuel
        Requirements ................,.,,....,,,...,,.,..  11- 41
11-11   Multiple-Hearth Furnace Construction
        Cost	  11- 42
11-12   Multiple-Hearth Furnace Operating and
        Maintenance Labor Requirements	  11- 44
11-13   Multiple-Hearth Furnace Fuel
        Requirements .............................,....*.  11- 45
11-14   Multiple-Hearth Furnace Electrical Power
        Requirments .....................................  11- 46
11-15   Multiple-Hearth Furnace Maintenance Material
        Costs	.,.,.»..  11- 47
11-16   Heat Balance for the Recycle Concept
        in a Multiple-Hearth Furnace ....................  11- 49
11-17   Cross Section of a Fluid,Bed Furnace	  11- 50
11-18   Flowsheet for Sludge Incineration in a Fluid
        Bed Furnace	,	  11- 52
11-19   Fluid Bed Furnace Fuel Requirements .............  11- 53
11-20   Fluid Bed Furnace Electrical Power
        Requirements	..........................  11- 54


                              xliii

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                   LIST OF FIGURES (continued)

Number                                                       Page

11-21   Cross Section of an Electric Infrared Furnace ...   11- 55
11-22   Flowsheet for Sludge Incineration in an
        Electric Infrared Furnace	......	. .   11- 56
11-23   Cross Section of a Cyclonic Furnace ..	...	   11- 58
11-24   Flowsheet for Sludge Incineration in a
        Cyclonic Furnace	.	   11- 59
11-25   Design Example:   Heat and Material Balance
        in a Fluid Bed Furnace	....,,,,,	   11- 64
11-26   Comparison of Excess Air Requirements:
        Incineration vs. Starved-Air Combustion .........   11- 66
11-27   Flowsheet for Starved-Air Combustion in a
        Multiple-Hearth Furnace	   11- 69
11-28   Design Example:   Starved-Air Combustion
        in a Multiple-Hearth Furnace	   11- 82
11-29   Typical Grate-Fired Waterwalled Combustion
        Unit	   11- 84
11-30   Vertical Shaft Reactors	   11- 87
11-31   Autogenous Combustion Requirements for
        Co-Disposal	,	   11- 88
11-32   Flowsheet for Co-Combustion Full Scale Test
        at the Central Contra Costa Sanitary District,
        California .		   11- 90
11-33   Flowsheet for Co-Combustion at  the Western
        Lake Superior Sanitary District, Duluth,
        Minnesota	   11- 93
11-34   Cross Section of the Watergate  Furnace for
        Scum Incineration	   11- 97
11-35   JPL Activated Carbon Treatment  System	   11-101
11-36   Volatile Solids and COD Content of Heat
        Treated Sludge	   11-105
11-37   Flowsheet for High Pressure/High Temperature
        Wet Air Oxidation	 . . .	   11-106
11-38   Wet Air Oxidation - Electrical  Energy
        Requirements ...........	......,»...*.»..   11-108
11-39   React-0-Thermtm on Sludge/Liquid Waste
        Destruction		   11-109
11-40   Modular Controlled-Air Incinerator
        Configurations .,....,........,............*.....   11-111
11-41   Pyro-Sol Limited Pyrolysis System	   11-112
11-42   Bailie Process Flowsheet	   11-113
11-43   Wright-Malta Process Flowshee	   11-114
11-44   Air Emissions	   11-115
11-45   San Francisco Bay Area Quality  Management
        District:  Auxiliary Fuel and Oxygen
        Correction	... *	   11-126

                           CHAPTER 12

12- 1   Effect of Solids Content on the Ratio of Wood
        Chips to Sludge by Volume	   12- 4


                              xliv

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                   LIST OF FIGURES (continued)

Number
                                                             Page
14- 2   Comparison of Behaviors of Wastewater Sludge and
        Water Flowing in Circular Pipelines	  14-  6
14- 3   Friction Factor for Sludge, Analyzed as a Bingham
        Plastic ••,,..*,.......,,.,,,......,,.....»...„,,  14-  7
14- 4   Friction Factors for Example Problem ............  14- 12
14- 5   Pressure Drops for Example Problem	  14- 13
14- 6   Viscometer Test of Sewage Sludge	  14- 16
14- 7   Centrifugal Pump ..,,,.........,..,..,.,...	  14- 18
14- 8   Torque Flow Pump ,	  14- 19
14-12   Progressive Cavity Pump .........................  14- 23
14-13   Diaphragm Pump ..................................  14- 25
14-14   Rotary Pump	  14- 27
14-15   Ejector Pump	 ....  14- 28
14-16   Belt Conveyor	  14- 38
14-17   Inclined Belt Conveyor Features .................  14- 41
14-18   Flexible Flat Belt Conveyor	  14- 42
14-19   Screw Conveyor	  14- 42
14-20   Tabular Conveyor	  14- 43
14-21   Bucket Elevator	  14- 44
14-22   Pneumatic Ejector ...............................  14- 45
14-23   Pneumatic Conveyor	  14- 45

                           CHAPTER 15

15- 1   Solids Balance and Flow Diagram-Design Example
        Single-Phase Concentration and Displacement
        Storage	  15- 13
15- 2   Effect of Various Operating Strategies on
        Dewatering Unit Feed Rates ......................  15- 17
15- 3   Proposed Design for Blending Digester—Sacramento
        Regional Wastewater Treatment Plant	  15- 21
15- 4   26,000 Gallon Sludge Equalization Tank (Typical
        of Two) Aliso Solids Stabilization Facility .....  15- 22
15- 5   Schematic Representation of a Facultative Sludge
        Lagoon (FSL)	  15- 25
15- 6   Typical Brush-Type Surface Mixer, Sacramento,
        California	  15- 27
15- 7   Typical FSL Layout	  15- 29
15- 8   Typical FSL Cross Section	  15- 29
15- 9   Layout for 124 Acres of FSLs--Sacramento Regional
        Wastewater Treatment Plant	  15- 30
15-10   Sacramento Central Wastewater Treatment Plant
        Surface Layer Monitoring Data for FSLs 5 to 8 ...  15 -34
15-11   Sacramento Central Wastewater Treatment Plant
        1977 Fecal Coliform Populations for Various
        Locations in the Solids Treatment-Disposal
        Process	  15- 38
15-12   Typical Wind Machines and Barriers Sacramento,
        California	  15- 40
                              xlvi

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                   LIST OF FIGURES (continued;

Number
                                                             Page
12- 2   Locations for Temperature and Oxygen
        Monitoring at One End of a Windrow or
        Individual Aerated Pile . »	,.	  12- 7
12- 3   Sludge Composting Mass Balance Diagram 	  12- 9
12- 4   Temperature Profile of a Typical Compost
        Windrow ....»».,,».........,,,.,,,,,...,.,«.,....,  12-14
12- 5   Turning a Windrow at Los Angeles Compost Site ....  12-15
12- 6   Destruction of Pathogenic Organisms as a
        Function of Time and Temperature During
        Composting of Digested Sludge by the Windrow
        Method .,,	,...,»»,.,,........,.»,,,	  12-17
12- 7   Process Flow Diagram - Windrow Composting
        Sludge - 10 MGD Activated Sludge Plant	  12-21
12- 8   Configuration of Individual Aerated Piles ......,,  12-22
12- 9   Aeration Pipe Set-Up for Individual Aerated
        Pile			  12-23
12-10   Configuration of Extended Aerated Pile ,.	  12-25
12-11   Destruction of Pathogenic Organisms as a
        Function of Time and Temperature During
        Composting of Undigested Sludge by the Aerated
        Pile Method	,	,	  12-28
12-12   Odor Filter Piles at Beltsville	  12-29
12-13   Process Flow Diagram for the Extended Pile
        Compost Sludge Facility - 10 MGD Activated Sludge
        Plant	  12-32
12-14   Design Example Extended Aerated Pile
        Cons true t ion .......,..,».......,,,.........,	  12-34
12-15   Compost Piles Being Taken Down	  12-35
12-16   Finished Screened Compost	  12-37
12-17   Composting/Drying System - County Sanitary
        Districts - Los Angeles ,	...................  12-39
12-18   Composting Site Layout - Bangor, Maine ...........  12-44
12-19   Cross Section of Aeration Pipe Trench Durham
        Compost Pad Design	  12-48
12-20   Typical Process Flow Schematic Confined
        Composting System	, . ,	 . . . ,	  12-52
12-21   Partial Diagram Metro - Waste System -
        Resource Conversion Systems, Inc.	  12-53
12-22   Typical Layout of a Dano Bio-Stabilizer Plant ....  12-54
12-23   BAV Bioreactor	  12-56

                           CHAPTER 13

13- 1   Diagram of an Earthworm Conversion Process ......  13-  5

                           CHAPTER 14

14- 1   Approximate Friction Head-Loss for Laminar Flow
        of Sludge	  14-  3
                               xlv

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                   LIST OF FIGURES (continued)

Number                                                       Paqe

14- 2   Comparison of Behaviors of Wastewater Sludge and
        Water Flowing in Circular Pipelines .............  14-  6
14- 3   Friction Factor for Sludge, Analyzed as a Bingham
        Plastic .....,,,..,,.,.»	  14-  7
14- 4   Friction Factors for Example Problem	  14- 12
14- 5   Pressure Drops for Example Problem	  14- 13
14- 6   Viscometer Test of Sewage Sludge ................  14- 16
14- 7   Centrifugal Pump ................................  14- 18
14- 8   Torque Flow Pump	  14- 19
14-12   Progressive Cavity Pump . ,.	  14- 23
14-13   Diaphragm Pump	,  14- 25
14-14   Rotary Pump »*.»,.,..	  14- 27
14-15   Ejector Pump	,..,,..........	  14- 28
14-16   Belt Conveyor	.,	  14- 38
14-17   Inclined Belt Conveyor Features	  14- 41
14-18   Flexible Flat Belt Conveyor	  14- 42
14-19   Screw Conveyor	..........  14- 42
14-20   Tabular Conveyor	  14- 43
14-21   Bucket Elevator	  14- 44
14-22   Pneumatic Ejector	 ....  14- 45
14-23   Pneumatic Conveyor	  14- 45

                           CHAPTER 15

15- 1   Solids Balance and Flow Diagram-Design Example
        Single-Phase Concentration and Displacement
        Storage	  15- 13
15- 2   Effect of Various Operating Strategies on
        Dewatering Unit Feed Rates	  15- 17
15- 3   Proposed Design for Blending Digester—Sacramento
        Regional Wastewater Treatment Plant 	  15- 21
15- 4   26,000 Gallon Sludge Equalization Tank (Typical
        of Two) Aliso Solids Stabilization Facility .....  15- 22
15- 5   Schematic Representation of a Facultative Sludge
        Lagoon (FSL)			  15- 25
15- 6   Typical Brush-Type Surface Mixer, Sacramento,
        California	  15- 27
15- 7   Typical FSL Layout	  15- 29
15- 8   Typical FSL Cross Section	,	  15- 29
15- 9   Layout for 124 Acres of FSLs—Sacramento Regional
        Wastewater Treatment Plant 	  15- 30
15-10   Sacramento Central Wastewater Treatment Plant
        Surface Layer Monitoring Data for FSLs 5 to 8 ...  15 -34
15-11   Sacramento Central Wastewater Treatment Plant
        1977 Fecal Coliform Populations for Various
        Locations in the Solids Treatment-Disposal
        Process	  15- 38
15-12   Typical Wind Machines and Barriers Sacramento,
        California	,.,.....	  15- 40
                              xlvi

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                   LIST OF FIGURES (continued)

Number                                                       Page

15-13   Anaerobic Liquid Sludge Lagoons,  Prairie Plan
        Land Reclamation Project, the Metropolitan
        Sanitary District of Greater Chicago .,	   15-  42
15-14   Plan View of Drying Sludge Lagoon Near West-
        Southwest Sewage Treatment Works, Chicago .......   15-  49
15-15   Cross Section of Draw-Off Box Area Drying Sludge
        Lagoon Near West-Southwest Sewage Treatment
        Works, Chicago ,»,,	   15-  50
15-16   Cross Section of Drying Sludge Lagoon With
        Slackline Cable Near West-Southwest Treatment
        Works, Chicago	.......	   15-  50
15-17   Isometric of Sludge Storage and Truck Loading
        Station, Joint Water Pollution Control  Plant,
        Los Angeles County, California 	   15-  55
15-18   Storage Bin Discharge Control System, Joint Water
        Pollution Control Plant,  Los Angeles County,
        California	. .   15-  57

                           CHAPTER 16

16- 1   Example of Sidestream Production ................   16-  2
16- 2   Possible Treatment Scheme for Anaerobic
        Digester Supernatant	   16-  9
16- 3   Aerobic Digestion of Heat Treatment, Batch
        Tests	   16-12
16- 4   Flow Diagram, Anaerobic Filtration of Heat
        Treatment Liquor .... .»,.,.».,,,.,,,.....,»...»,   16-14
16- 5   Schematic Diagram of Plant for Processing Heat
        Treatment Liquor	...,*,.,,,.	   16-16
16- 6   Chlorine Treatment of Heat Treatment
        Liquor .,..,,.,.,*,,,...,..,,.,,,,,	   16-17

                           CHAPTER 17

17- 1   Typical Bubbler Schematic With Air Purge
        Capabilities	   17-  42
17- 2   Typical Bubbler Schematic With Diaphragm
        Element	,	   17-  43
17- 3   Cylindrical Chemical Seal for Sludge Pressure
        Measurement ...........I...,,,.,.,,..,,..	   17-  52
17- 4   Direct Reading Olfactometer (DRO) ,.,.,....,.....   17-  62
17- 5   Aeration Control Graphic Panel and Console
        Lights Set Manually on-Graphic Panel ............   17-  69
17- 6   Incinerator-Digester Control Graphic Panel Lights
        Controlled by Remote Valve Limit Switches	   17-  70

                           CHAPTER 18

18- 1   The Release, Conversion,  Forms and Uses  of
        Energy From Sludge	   18-  10


                              xlvii

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                   LIST OF FIGURES (continued)

Numb_er_                                                       Page

18- 2   Schematic of Combined Boiler/Condenser System
        for Hot Water Production	   18- 12
18- 3   Process Schematic for Example of Energy Recovery
        From Digester Gas	   18- 16
18- 4   Energy Flowsheet for Example of Energy Recovery
        From Digester Gas ...............................   18- 18
18- 5   Mean Molal Heat Capacities of Gases at Constant
        Pressure (Mean Values From 77° to T°F)	   18- 23
18- 6   Flowsheet for Example of Energy Recovery From
        Incinerator Flue Gas	   18- 27
18- 7   Steam Conditions for Example of Recovery of
        Energy From Incinerator of Flue Gas .............   18- 28
18- 8   Energy Flowsheet for Example of Energy Recovery
        From Incinerator Flue Gas .......................   18- 32

                           CHAPTER 19

19- 1   Wide Trenching Operation, North Shore
        Sanitary District ...............................   19-  4
19- 2   Dewatered Sludge Landspreading, Metropolitan
        Denver Sewage Disposal District No. 1,
        Denver, Colorado .,«.*..*««*..<...,,,,,,,.,,,,.,,   19- 36
19- 3   Flow Diagram Sludge Management System, Colorado
        Springs, Colorado	   19- 41
19- 4   Overall Sludge Disposal Site Layout,  Colorado
        Springs, Colorado	   19- 42
19- 5   Sludge Application Rate-DLD System Colorado
        Springs, Colorado	   19- 45
19- 6   Estimated Net DLD Area Requirements Sludge
        Applied at 5 Percent Solids Concentration,
        Colorado Springs, Colorado	   19- 46
19- 7   Estimated Net DLD Area Requirements at Various
        Sludge Concentrations, Colorado Springs,
        Colorado ...		   19- 47
19- 8   Sludge Application Rates by Subsurface Injection,
        Colorado Springs, Colorado	   19- 48
19- 9   Prototype Dredging Operation, Sacramento Regional
        County Sanitation District	   19- 50
19-10   Prototype Subsurface Injection Operations
        Sacramento Regional County Sanitation
        District	,,,,	   19- 52
19-11   Flow Diagram - Projected 1992 Normal Solids
        Treatment and Disposal Operation, Sacramento
        Regional Wastewater Treatment Plant	   19- 53
                             xlvi ii

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                       ACKNOWLEDGEMENTS
This  design manual  was  prepared  as  part  of  the Technology
Transfer  Series  of  the  Center  for  Environmental   Research
Information,  U.S.  Environmental  Protection  Agency,  Cincinnati,
Ohio.    Development,  coordination and  preparation  were carried
out by  Brown  and Caldwell, Consulting  Engineers,  Walnut Creek,
California,  with  the  assistance of Environmental Technology
Consultants,  Inc., of  Springfield,  Virginia.   Technical review
and coordination  were  provided  by  the Office  of  Water Program
Operations,  USEPA,  Washington,  D.C.    Additional  technical
review and  contributions  were provided  by Regions V  and  IX of
the USEPA,  by the Metropolitan  Sanitary District of  Greater
Chicago,  and  by  the Technical Practice  Committee of  the  Water
Pollution Control  Federation.   USEPA project  officers  on  this
manual  were Dr.  Joseph  B.  Farrell,  Municipal Environmental
Research  Center,  and  Dr.  James  E.  Smith, Jr.,  Center  for
Environmental  Research  Information, Cincinnati, Ohio.
                             xlix

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EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
            Chapter 1.  Purpose and Scope
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

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                          CHAPTER 1

                      PURPOSE AND SCOPE


1.1  Purpose

The purpose of  this  manual  is to present  an  up-to-date  review
of design  information on  all  applicable  technologies  available
for  treatment and  disposal  of  municipal wastewater solids.
Wastewater  solids  include  grit,  scum,  screenings,  primary
sludges,  biological sludges,  chemical  sludges, and septage.

The  Federal  Water  Pollution Control  Act Amendments  of 1972
(Public Law 92-500)  and the Clean Water Act Amendments of 1977
(Public  Law  95-217)  require levels of  municipal  wastewater
treatment  sufficient  to  meet  the  congressional  mandate  of
cleaning  up  the  nation's waterways.   Through  the  USEPA
Construction  Grants Program,  financial  incentives have been
provided to assist  publicly owned  treatment  works  (POTWs)  in
meeting  these  requirements.   Federal and  state  requirements
impact  both  effluent  quality  and  treatment  alternatives,
utilization, and disposal of wastewater solids.

The  tasks  associated with  managing  municipal  wastewater  solids
are  neither  simple  nor  cheap.   In providing higher levels
or additional treatment  of  wastewaters, greater  volumes  of
wastewater solids are  produced.   The  combination  of greater
volumes  of sludges,  mixtures  of various  sludges,  and more
restrictive management  requirements  have complicated the  solids
management  options available to the design  engineer.  These facts
require both  the  design engineer and the  operations  personnel
to give  serious  consideration to the  interdependence of both
the liquid  and solids portions of the  treatment facility.  The
need for sound wastewater  solids management  is  significant.
Typically,  solids  processing  and  disposal  costs  can  account for
20 to 40  percent of the total operating and maintenance  cost of a
treatment  facility  (1).   Thus,  there  is  strong  incentive to
utilize  the most  appropriate and cost-effective  alternatives
available.

This  manual supersedes the  USEPA  Process  Design Manual for  Sludge
Treatment and  Disposal,  EPA  11-74-006, published  in 1974.   Since
1974, new wastewater solids processing techniques have developed,
existing  techniques  have matured, and operating experience and
data are available.  Current legislation, solids  management
requirements,  and advances  in  sludge treatment  and  disposal
technologies warrant this revision.
                              1-1

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1.2  Scope

This manual has been prepared  for  use  by professionals engaged in
the design and approval of municipal wastewater solids treatment
and disposal systems.   Design  information presented includes:

     •  Origins,   quantities,  and  characteristics  of  municipal
        wastewater treatment plant  solids;

     •  Process descriptions,  including  theory  and  appropriate
        design criteria;

     •  Energy requirements;

     *  Public health  and  environmental considerations;

     •  Cost and performance data;  and

     •  Design examples.

Some  material is  not included  because it  has  been presented
elsewhere.   A section on sanitary landfills has been omitted
because an EPA manual  of this  subject has been published recently
(2).  The treatment of land utilization is abbreviated because an
EPA Design Seminar publication is available (3).


1.3  Process Classification

The  manual  is divided   into 19  chapters,  with  15 chapters
devoted to sludge  processes.  Additional  chapters  cover  general
considerations,   design  approach,  and  sludge properties.
Figure  1-1 depicts  the  basic  classification  by  process.   It
should  be  noted   that  processes within  classifications  overlap
to  some  extent.   As  an example,   stabilization, disinfection,
and disposal also take place  during high temperature processing.
The processes, as  they appear on  Figure 1-1, should  be  read in
left-to-right sequence; they  do  not, however, necessarily appear
in a treatment system  in the order  shown.  Figure 1-1 is arranged
to  display sludge  treatment and  disposal  options rather  than to
suggest any particular order of  operations.


1.4  References

1.  USEPA.  Construction Costs for  Municipal Wastewater Treatment
    Plants  1973-1977.   Office  of Water  Program Operations.
    Washington, D.C.   January  1978.

2.  USEPA.    Process  Design Manual;  Municipal Sludge Landfills.
    Environmental   Research Information  Center,  Office of  Solid
    Waste,  Cincinnati,  Ohio   45268.    EPA-625/1-78-010,  SW-705.
    October 1978.

3.  USEPA.   "Principals   and  Design Criteria for  Sewage  Sludge
    Application on Land."  Sludge  Treatment and  Disposal Part 2.
    Technology Transfer,   Cincinnati,  Ohio  45268.   EPA-625/4-78-
    012.  October  1978.


                              1-2

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                                          GENERAL CONSIDERATIONS - DESIGN APPROACH
PRODUCTION    .                                               HIGH TEMPERATURE                    , MISCELLANEOUS          . DISPOSAL
AND PROPERTIES   THICKENING I STABILIZATION   DISINFECTION [^CONDITIONING __ [ j)EWATERING^ PROCESSES    _|  DRYING _|  COMPOSTING   1^ PROCESSES     UTILIZATION  TO LAND

                  I          I          i          I        !            |       ' |^"—    " | -         Y      =l*~    1
                                                 DRYING BEDS
                                                    TRANSPORTATION
                                                   I INSTRUMENTATION
                                                    INTERFACING
                                                 FIGURE 1-1
                                 CLASSIFICATION OF TREATMENT
                                        AND  DISPOSAL OPTIONS
                                                     1-3

-------
EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
           Chapter 2.  General Considerations
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

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                            CHAPTER  2

                     GENERAL CONSIDERATIONS
2.1  Introduction and Scope

Non-technical factors can  heavily  influence  the planning, design,
construction,  and  operation of  solids management  systems,  and
these non-technical  considerations must be  faced  from  the day a
project  is  conceived.   Non-technical  factors  include  legal  and
regulatory  considerations, as well as other  issues,  such as
public participation.
2.2  Legal and Regulatory Considerations

The thrust of this section is to describe the intent and effects
of federal legislation  and  to provide  a  reference list  which
features  the most  current  criteria.   Where  state and  local
requirements may be  involved,  they  are  so noted.
    2.2.1  Effect of Effluent  Discharge Limitations
           on Wastewater Solids  Management

The Federal Water  Pollution Control Act  of  1972 (PL  92-500)
established levels  of  treatment, deadlines for meeting  these
levels, and penalties for  violators.   For plants discharging to
surface waters,  effluent  requirements are  expressed  in  permits
issued  by  the  National  Pollutant Discharge  Elimination System
(NPDES).  NPDES permits have generally mandated the upgrading of
existing treatment  plants  or  the construction of  new  plants to
provide higher levels of treatment and reliability.

The law, while  providing direction  toward  the  goal  of  a cleaner
environment, has created problems for designers and operators of
wastewater  treatment  plants.   Higher level  treatment  generally
means  a greater mass and volume  of solids to be managed.   Solids
treatment systems not only  must  handle more material but must do
so more effectively.  Solids not  captured therein are returned to
the wastewater treatment  process and can potentially  degrade
effluent quality and  defeat the  very  purpose  of  the law.  Thus,
stricter discharge  limits  have  had  the  effect of  making solids
treatment and disposal  more important, more  difficult,  and more
expensive.
                               2-1

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    2.2.2  Restrictions on Wastewater Solids Treatment

Wastewater solids must be  managed  so that laws and regulations
are not  violated.   Air  emissions'  limits and nuisance prohibi-
tions are of  particular importance.


        2.2.2.1   Air Emissions Limits

The  Clean Air  Act  Amendments  of  1970   (PL  91-604)  and  1977
(PL  95-95)  contain provisions for regulating  point  source
emissions,  for  example,  emissions  from incinerators.   USEPA
has  promulgated several  regulations   in  response to  this
legislation.  The most restrictive are  the New Source  Review
regulations   (40  CFR  51-18)  and  Prevention  of  Significant
Deterioration (40 CFR 52-21) regulations.  New Source  Review
(NSR)  regulations apply  in areas where allowable levels  for any
pollutant are exceeded.    The  regulations  affect  any  new  source
which,  after  installation  of  an air  pollution  control  device,
could emit >50 tons per year (45 t/yr) of  the  offending pollutant
(controlled  emission)  or which  could emit >100 tons per year
(91 t/yr)  of the offending pollutant were there no  pollution
control device or were  the existing  device  to  fail (uncontrolled
emission).  These sources are prohibited  unless their emissions
can be compensated for by the reduction of emissions  from other
sources within the same area.   This  compensation clause is known
as the Emissions Offset Policy.   Relaxation of the  Emissions
Offset Policy is being  considered  for  certain categories  of
resource  recovery  projects.  Presently, few urban areas exceeding
200,000  population meet  all  national air quality standards.
Therefore, NSR regulations will apply to almost all urban plants,
particularly  larger ones.

Prevention of Significant Deterioration  (PSD) regulations apply
primarily to areas  which  are  presently  meeting  air quality
standards.   They  affect  28 major  stationary  source   categories
with potential uncontrolled emissions exceeding  100 tons per year
(91  t/yr) and  any  other  source with potential uncontrolled
emissions of  over  250 tons per year (227 t/yr).   Such sources are
allowed  provided they use Best Available Control Technology
(BACT)  to treat  gaseous  discharges  and provided  the emissions of
specified pollutants do not  increase  at  rates  greater than set
forth by  regulatory  schedules.

The Clean Air Act also requires "state   implementation  plans"
(SIPs)  to  regulate all significant point   sources, including new
sources.    SIPs  generally  limit  emissions,  establish emissions
offset policies,  require   reporting,  and  establish penalties and
administrative  procedures.   State or regional boards  usually
administer the permit system.

Historically, air  emissions  limits  have affected  incinerators
more than  other wastewater solids treatment processes.  However,
air  emission limits  can affect any solids treatment  system.


                              2-2

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Examples  include sludge  drying processes  and the  burning of
gases from anaerobic digesters either by flaring or  in internal
combustion  engines.    USEPA  has  already  issued  New  Source
Performance  Standards for sludge incineration  (40 CFR -6.Q-150) and
"Amendments  to National  Emission Standards"  (40 CFR^61-52).
These establish particulate  air pollution emission standards and
limit mercury  emissions  from incinerators and dryers of wastewater
treatment plant solids.   Chapter 11  contains further  information
on air pollution  regulations.


        2.2.2.2  Nuisances

Courts  have ordered  municipalities to pay  damages or  cease
operation when wastewater solids  treatment  processes have been
proven  to be  the source of nuisances such as noise and  odor.
In some cases, judgments have  resulted  in the permanent shutdown
of plants  containing expensive equipment.   Since  most  NPDES
permits specify that treatment plant  operations  be nuisance free,
this  is  a  goal  which  designers and operators must  strive to
achieve.
        2.2.2.3   State and Local Requirements

When  state  and  local  requirements  are more  stringent  than
federal regulations,  the state  and  local  conditions govern.   Air
Pollution  Criteria  are  the  most  striking example  of  this.
The  criteria  are  particularly  restrictive  in California,  where
local  nitrogen oxide  (NOX)  regulations  may require  that  new
stationary  reciprocating engines above a certain  size be equipped
with  catalytic converters  (1).   As another instance  of  local
controls,  deed restrictions and  local ordinances effectively
prevent  sludges  produced at the  Easterly Plant  in Cleveland,
Ohio, from  being  processed on the plant site  (2).


    2.2.3  Laws and Regulations Governing Wastewater
           Solids  Utilization and Disposal


        2.2.3.1   Federal Water Pollution Control  Act

The  Clean  Water  Act of  1977  (PL  95-217)  contains two  major
provisions  for  wastewater  solids  utilization and disposal.
Section  405  requires  USEPA  to  issue guidelines  and regulations
for the disposal  and  reuse of wastewater  solids.  Guidelines and
regulations to be  issued  in  the  next few years  are expected to
limit  the  quantity and kinds  of  toxic materials  reaching  the
general  public by setting  limits  on the quantity and  quality
of sludge  distributed for public  use or applied to lands where
crops  are  grown  for human  consumption.  The methods  by  which
sludge is applied to land are expected to be controlled to meet
aesthetic requirements,  and  groundwater protection will probably


                              2-3

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be required  at wastewater  solids  disposal sites.   The  degree
of stabilization  or  disinfection  for sludge  is  expected  to be
specified,  along with  monitoring  and  reporting requirements
and  design  criteria.    The guidelines and  regulations  will
probably rely  on  the fact  that wastewater  solids  may endanger
the  public  and  the environment  if  not  properly managed,
and  that  requisites for  use must  be  stricter than  those for
disposal.


The  other  major  provision  is intended  to  encourage  sludge
utilization.   This provision, Section  307, requires pretreatment
of industrial wastes  if  such wastes  inhibit wastewater treatment
or sludge utilization.   This should  increase  the  potential for
sludge reuse.
        2.2.3.2  Resource Conservation  and  Recovery Act

The  Resource Conservation and  Recovery  Act  (RCRA)  of  1976
(PL 94-580)  requires that solid wastes  be utilized or disposed of
in a safe and  environmentally acceptable manner.  Wastewater
solids  are included by definition  in provisions relating to solid
waste  management.   USEPA  is  currently developing guidelines
and  criteria  to implement the provisions of this act.   These
guidelines and  criteria  will fall  into three general categories:
(a)  treatment  and  disposal  of  potentially  hazardous  solid
wastes  (wastewater  solids  are  expected  to  be excluded  from this
category  in  most if  not  all  cases);  (b)  criteria  and  standards
for  solid waste disposal  facilities;  and  (c)  criteria defining
the  limits  for  solid waste application to  agricultural  lands.
USEPA  will  issue  the guidelines  and criteria that  relate to
municipal sludge management under  joint  authority of  RCRA  and
Section 405  of  the Clean Water  Act.
        2.2.3.3  Toxic Substances Control  Act

The Toxic Substances Control Act of 1976  (PL 94-469) authorizes
USEPA  to obtain  production  and  test  data  from  industry  on
selected chemical  substances and  to  regulate  them where
they  pose  an  unreasonable risk  to the environment.    This
act,  in  combination  with  other  federal  legislation  cited
(PL 95-217  and PL  94-580), should help reduce the amount  of
pollutants discharged  to  the municipal system  from manufacturing
processes.   Of particular significance to wastewater  solids
utilization  is  the fact  that the act prohibits the production of
polychlorinated  biphenyls  (PCBs)  after January 1979  and the
commercial distribution  of  PCBs  after  July 1979.   PCBs  can  be
concentrated in wastewater sludges and  are a chemical constituent
of concern in meeting proposed  utilization criteria.  Sludge PCB
levels  should  decrease  once  PCBs no longer  enter the  waste
treatment system.


                               2-4

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        2.2.3.4  Marine Protection, Research and
                Sanctuaries Act

Several  large  cities,  including New  York and Philadelphia,
as well as some  smaller cities in the New York  -  New Jersey area,
dispose  of wastewater  solids  by barging  them to the  ocean.
The  1977  amendments  to  the  Marine  Protection,  Research,
and  Sanctuaries  Act  of  1972,  as  well  as other  laws  and
regulations  prohibit disposal  of  "sewage sludge" by  barging
after December  31, 1981.   In addition,  no  federal construction
funds are available for  wastewater solids  treatment and disposal
systems that  include  any  type of ocean disposal, either by barge
or pipeline.   Therefore,  no further  coverage  of ocean disposal
will  be made  in  this manual.
        2.2.3.5   Environmental Policy Acts

The National  Environmental  Policy Act  of 1969 requires that
the federal  government  consider environmental  effects  of many
actions.   Municipal  wastewater treatment systems,  including
solids treatment, utilization,  and disposal systems are covered
by this act because of their  potential  effect  on  the environment
and because they  are funded by federal construction  grants.  Most
states have similar policy acts.  The acts,  which  require reports
and hearings,  assure  that  the environmental consequences  of
proposed operations are considered,  and also provide the designer
with  a  useful forum  to  develop public  response  (see  Section
2.3.6).   They  do,  however, usually lengthen  the facility planning
and design process.


        2.2.3.6   State and Local Reuse and Disposal
                 Requirements

While  most  states  and  municipalities  follow federal guidelines,
many  may  formulate more  restrictive  measures.   For  example,
localities that apply sludge  to  land on which  food  crops are
grown may wish  to analyze their  sludges more  frequently than
required by federal guidelines  or  limit sludge application rates
more  severely.   Many state  and local regulatory  agencies are
presently awaiting the issuance  of federal  guidelines before
finalizing their requirements.


    2.2.4  The Comprehensive Nature of Section  405
           of  the Clean Water Act

As  indicated,  Section  405  of  the  Clean Water  Act  of 1977
(PL 95-217)  requires  USEPA  to promulgate  regulations governing
the  issuance of permits  for  the  disposal  of sewage  sludge
relative to Section 402 NPDES permits  and to develop and publish
from  time  to  time regulations providing guidelines  for the
disposal  and  utilization  of  sludge.   These regulations  are  to


                               2-5

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identify  uses  for  sludge,  specify  factors  to  be  taken  into
account in  determining the  measures  and  practices  applicable  to
each such use  or  disposal  (including publication  of  information
on  costs)  and identify  concentrations  of  pollutants which
interfere with each  such  use  or disposal.

This broad  authority to issue  regulations covering different
sludge  management practices  has  been  viewed  as  a mechanism  to
allow USEPA  to  bring  together all of  the  regulations  that  have
been or will be issued under various legislative  authorities  for
controlling  municipal  sludge management at a  single  location  in
the  Code  of Federal Regulations,  under  the  joint authority  of
Section 405.   Therefore,  regulations  on  air  emission  controls
will be  issued  under  the joint  authority of  Section 405 of  the
Clean  Water Act  and  various  sections  of the Clean  Air Act;
regulations  on  land  disposal and  land application under joint
authority  with the  Resource  Conservation  and  Recovery Act;
regulations on ocean disposal  under  joint authority  with  the
Marine Protection,  Research   and  Sanctuaries  Act,  and  so
forth.    Regulations  covering practices not influenced  by other
authorities  (for example, home use,  give-away or  sale  of  sludge
derived  products)  could  be  issued  solely  under the  broad
authority  of Section 405.


Thus,  all  regulations  related  to management  of  municipal
wastewater  solids will  be  issued, administered,  and  enforced
under the umbrella of Section 405.   Sludge management facilities
and  practices  will  therefore be  approved  or  disapproved along
with NPDES permits.


2.3  Other Non-Technical  Factors  Affecting
     Wastewater Solids Management


    2.3.1   Availability of  Construction Funds

Construction  of  municipal  wastewater  solids   treatment   and
disposal  facilities  is  usually  financed with  public money.
Currently,  federal  funds are  used to  pay  for  75  percent  of
grant-eligible  construction  costs.   State contributions vary.
In  addition,  PL  95-217  gives   projects  using  innovative   and
alternative   technologies,  for example,  sludge  utilization   and
energy  recovery,  a  15  percent   advantage  in  cost-effectiveness
comparisons   over  projects  using  conventional  technology.    They
are  also  given a 10 percent bonus  (to  85 percent)  on federal
construction grants  (3).    Innovative  technologies  can also  be
replaced  with  100  percent  funding  if  they fail  within  two
years.   Thus,  federal  and  state  grant  fund  requirements  may
influence to  a  considerable  degree the sludge  management  system
chosen and  the way a system  is designed. .Cost-effective  design,
careful and  conservative  cost estimating, and  clear explanations
                               2-6

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to  decision-makers  of  the rationale for  selected  treatment  and
disposal  systems  will  assist  greatly  in obtaining federal  and
state construction funds.

Design  engineers  should  refer to the  USEPA  Construction  Grants
Manual  for federal grant  requirements  (4).    In  many  states
these requirements are  supplemented by  state regulations.

Occasionally,  a governmental agency  may declare certain features
of  a  design  to  be  ineligible for  grant funding.   Sometimes
these declarations  are  in direct contradiction with  the  design
engineer's  opinion  regarding their  necessity.   The  designer
should  be aware  of these potential conflicts  of opinion  and
submit  full  documentation  and  justification along  with  the
request for funding.   The  design  year  for full loading, special
loading allowances,  system  reliability  requirements, and facility
flexibility allowances  are  important parts of this documentation.


    2.3.2   Special Funding  Requirements

The designer  must  be  aware  of  special  conditions associated
with  federal  and state grant funding,  such  as  "buy American"
provisions,  "or equal"  clauses, affirmative action in employment,
and special auditing and cost  control requirements.

Competitive bidding  is required  for  public works  construction
contracts.  Equipment specifications  for these  contracts must be
carefully written  to  assure  that  the resulting  installation
satisfies  the  treatment and disposal requirements  at minimum life
cycle costs.  Where the designer  knows  of no  equal  to  a specific
needed  item,  he  should document the  need  for  such  equipment  and
assure  compliance with funding  restrictions  prior to putting
the specification out  to  public  bid.    USEPA has  recognized  the
designer's  need to  achieve  better control  over the equipment
to  be used  for  wastewater  treatment  systems  and  is  proposing to
issue  regulations  which  allow prequalification  of   critical
equipment  items.                                           -


    2.3.3   Time Span of Decisions

Frequently,  several  years  elapse  from  the choice of a  specific
process to  the operation of  that process.  This  time  is usually
spent in the necessary  work of  completing environmental  hearings,
detailed  designs  and  regulatory reviews,  and  arranging  for
funding,   construction,  and start-up.   Furthermore,  most
facilities must be operated for close to life expectancy to avoid
waste of  construction  funds.   During  this extended time  span,
technology may improve, new  laws  may  be passed,  new regulations
may be  issued,  and  economic  factors  may change.    The  engineer
must consider these possibilities  for change  in decision making.
He  should favor  processes  that  are  sufficiently flexible  to
remain  useful  in the face of  changing technology,  regulations,
economics, and sludge characteristics.


                               2-7

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    2.3.4   Uncertainties


The  selection of  a  specific process normally  hinges on  its
cost  in  comparison  with   the  cost  of  competing processes.
Uncertainties  make cost comparisons  difficult.   For  example,
consider  two  competing processes,  one labor-intensive,  the
other requiring expensive chemicals.   There are uncertainties as
to how  many  man-hours  will be  needed per  ton of  sludge,  what
chemical additions  will  be  required, and what future cost trends
will be.   It  is often  difficult to  predict whether  labor or
chemicals  will  be  more  severely affected by  inflation.   Labor
productivity  also  must  be predicted.   Given these uncertainties,
it may  be necessary to say that "Process  A is probably  more
cost-effective," rather  than  "Process  A is more cost-effective."
Cost uncertainties  are usually greater  for processes that are not
widely  used.   There  are also uncertainties in the quality of
solids  that will be produced.    For instance,  if incineration is
selected  on  the basis  of previous  dewatering unit production
of a cake with 35 percent solids,  but  only  20 percent  solids is
actually  obtained, then the  cost of  incineration may  become
excess ive.

Experience  at  Kenosha, Wisconsin,  where  one of  the  first
filter   presses  for  sludge  in the United States  was  installed,
illustrates the difficulties of making accurate cost  estimates.
Pilot testing  indicated  that an  optimal lime  dose would be equal
to 12  percent  of  the  sewage solids  fed.    In addition  a ferric
chloride  dose  equal  to  3 percent of  the sewage  solids fed was
required.    Full-scale  operating experience,  however,   indicates
a  17  percent   lime dose is  required;  therefore,  lime  costs are
40 percent greater than anticipated.   Also,   in this plant, only
part-time operator  attention  was  anticipated,  because  the
units were fully automated.   In  practice, full  time operation is
required.   Maintenance costs, assumed  to  be  nominal,  have
instead been  significant,  averaging  about  $3 per ton  of dry
solids  (5).

Whenever  possible,  the engineer  should  investigate  full-scale
working systems to determine  actual  operating  conditions and
operating and  maintenance  costs.    If  there is  insufficient
full-scale operating experience  to estimate  these  conditions
and  costs with  confidence,  the  design  engineer  must make  liberal
allowances for uncertainties.
     2.3.5  The Design Team

 Many factors are  important in selecting  and  designing sludge
 treatment  and disposal processes,  for  example,  capital  costs,
 operating strategies,  and  environmental effects.   Different
 individuals  have  different perspectives  on wastewater solids
                               2-8

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management.   These  individuals should be heard.   Therefore,
a "design team"  concept is helpful.   The design  team should
include:

     •  Those involved in the day-to-day design  effort;  that  is,
        the design staff.

     •  An advisory  committee  composed  of  those  who  are  not
        involved  in  the  day-to-day design effort  but  who must
        operate and administer the  wastewater solids management
        system or  whose  services  are required to  implement  the
        design; for  example,  treatment  plant operators,  public
        works  directors, grant  administrators, regulatory
        officials,  engineering  reviewers including  value
        engineers,  and  special  consultants.   The advisory
        committee  serves  in a policy-making and review role.

The  advisory  committee  should  be  made  aware,  through clear
and  accurate  reporting, of all  aspects of  sludge management
alternatives,   including the  design staff's evaluations  and
recommendations.

The  design  staff  should  expect  criticism and guidance   from  the
advisory  committee.   If  a proposal  or  criticism  appears  to have
merit,  it  should  be  evaluated with  respect to its  effect  on  the
solids  treatment  and disposal  scheme.   If it  does  not,  the
consequences  of  incorporating  it  into  the design  should  be
clearly explained.

A better  project will be achieved  by an early exchange  of views.
While responding  to criticism may cause delays early  in  the
project, delays are small in  terms  of both time and cost compared
with those that  would  be experienced  were  dissatisfaction to
surface late in the project.
    2.3.6  Public Involvement

 Public involvement in  environmental  decision  making is  not
 only  wise,  it is mandatory.   The  National  Environmental  Policy
 Act of 1969,  the Clean  Water  Act  of 1977  (PL  95-217),  and  the
 Resource  Conservation  and  Recovery Act of  1976  (PL  94-580)  all
 require public involvement mechanisms and  activities.  Acceptance
 of  the project  by  residents of  the community and a working
 relationship between the public and the design team is essential.
 Experience has  shown that  programs are more  easily  accepted
 if  the public understands what they are.

 The relationship  between  the design staff  and the public is
 similar  in many ways  to that between the  design  staff, and  the
 advisory  committee.   The public also  serves  in a policy-making
 and review  role;   it  should  be made aware  of all aspects of
 sludge management  alternatives and should provide criticism  and
 guidance  to the design  staff.  A  means  of  educating the public


                               2-9

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and  creating  a  dialog between  the  public and the  design  st
must be  established.  Mechanisms  for  accomplishing  this  are  the
mass media,  bulletins,  public hearings,  and presentations
interested groups.
staff
   :he
   to
Special efforts should be made to involve groups and individuals
who,  from past experience, have  demonstrated  an interest  in
environmental affairs or  those who are  likely to be directly
affected  by  the  proposed project.   Developing a  list  of
interested persons and  organizations for  formal and informal
notifications and  contacts  is a  good  way  to  ensure public
participation. The  group might  consist of:

     •  Local  elected  officials.

     •  State  and local  government  agencies,  including  planning
        commissions,  councils  of  government,  and individual
        agencies.

     •  State  and  local public works personnel.

     •  Conservation/environmental groups.

     •  Business  and  industrial groups,  including Chambers  of
        Commerce and selected trade and industrial associations.

     •  Property  owners  and  users  of proposed sites and
        neighboring  areas.

     •  Service clubs and  civic organizations,  including the
        League of  Women Voters.

     •  Media, including  newspapers, radio, and television.

Public  participation programs  are discussed in detail in two
recent publications  (6,7).


    2.3.7  Social  and  Political  Factors Affecting
           Waste  Export

For  metropolitan   areas,   potential   sludge  disposal  studies
generally  include  land  disposal in some  form by  export to  low
population open space.   Even  if  these  spaces  are  located  in  the
same  political jurisdiction,  local  opposition towards  accepting
the wastes of "others" is often intense.   If  the proposed  export
is  to another political  jurisdiction,  the  opposing forces  are
generally so great as  to  effectively preclude  this option.

It  is often hoped that such opposition can be overcome  by  public
participation and education.  However,  the  social and  political
factors at work have been demonstrated  to  be remarkably immune  to
such  efforts.
                              2-10

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These comments are not intended  to preclude export options but as
a caution  to  designers  not  to be so  swayed  by  the  economic and
technical advantages  of  such  plans  that  inadequate  attention is
given to alternatives which  have  a  greater possibility of being
implemented.


2.4  References

 1.  Rule  213.   South  Coast Air Quality  Management  District.
     9420 Telstar Avenue, El  Monte,  California, 91731.  Effective
     October,  1976.

 2.  Ungar, A.T.  and D. Patrick.   "Cleveland  Pushes  to  Meet
     Strict Effluent  Limitations."    Water & Wastes Engineering.
     Vol. 15,  p.  57, (February 1978).

 3.  USEPA.   Innovative  and Alternative  Technology  Assessment
     Manual,   Draft  Copy.  Office  of  Water  Program Operations.
     Washington,  D.C., 20460.   EPA 4-30/9-78-009.

 4.  USEPA.   Municipal  Wastewater  Treatment  Works  Construction
     Grants Program  References,  (with  updating supplements).
     Office of Water Program  Operations.  Washington B.C., 20460.

 5.  Nelson,   O.F.   "Operational  Experiences with   Filter
     Pressing."  Deeds and Data.  Water Pollution  Control  Federa-
     tion,   p. 5, March  1978.

 6.  Water Pollution Control Federation.   Public Information
     Handbpgjc.  Washington, D.C.,  1977.

 7.  USEPA.   Process  Design  Manual, Municipal Sludge Landfills.
     Technology  Transfer.    Cincinnati,  Ohio,  45268.   EPA-625/
     1-78-010.  October  1978.
                              2-11

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EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
              Chapter 3. Design Approach
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

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                          CHAPTER 3

                       DESIGN APPROACH
3.1  Introduction and Scope

This chapter presents a methodology for the  design  of wastewater
solids  management systems.   Topics discussed  include systems
approach,   process  selection  logic,  mass  balance  calculations,
concept of sizing equipment, contingency  planning,  and other
general design  considerations such  as  energy  conservation and
cost-effective analysis.


3.2  Systems Approach

Overall wastewater treatment plant performance is the sum of the
combined  performances of  the plant's linked components.   The
actions of one  component  affect the performance of  all the
others.  For example:

     •  Materials  not  captured  in solids  treatment  processes
        will  be  returned  in  the  sidestreams  to the wastewater
        treatment system  as a recirculating  load.   This  load may
        cause a  degradation in effluent quality, an increase  in
        wastewater treatment costs, and process upsets.

     •  Failure  to remove  and to treat solids at  the same rate
        as  they  are  produced within the wastewater  treatment
        system will  eventually  cause  effluent  degradation and
        may increase wastewater treatment operating  costs.

     •  Hydraulic  overloads  resulting from  inadequate solids
        thickening  can cause  downstream solids treatment
        processes (such as,  anaerobic digestion)  to operate less
        effectively.

     •  The  addition  of  chemicals  to  the  wastewater  treatment
        process  for  purposes  of  nutrient  and  suspended solids
        removal  will  increase  the quantity  and alter the
        characteristics  of  solids which must  be  treated and
        disposed.

It  is  important  to understand the  relationship  between process
parameters  and  the performance  of  processes,  for  example, how
thickener feed rate affects  thickener performance.   It  is equally
important to understand  how  individual  processes affect one
another when combined into  a system, for instance,  how the


                            -.  3-1

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performance of  the  thickener affects digestion  and  dewatering.
Interactions between the processes  in a  system are described in
this chapter.


3.3  The Logic of Process Selection

Wastewater treatment and wastewater solids  and disposal  systems
must  be  put  together so as  to  assure the most  efficient
utilization of resources such as,  money,  materials,  energy,  and
work  force  in meeting  treatment  requirements.   Logic  dictates
what the process elements  must be and the order in which  they go
together.

A methodical process of selection must be followed in choosing a
resource-efficient and environmentally  sound system from  the
myriad of  treatment  and  disposal options available.   The  basic
selection mechanism used  in this  manual is the  "principle of
successive  elimination,"  an  iterative procedure   in which  less
effective options  are progressively  culled  from the  list of
candidate systems until, only  the  most suitable system or systems
for the particular site remain.

The concept of  a  "treatment train" has been propounded as a
result  of a  systems  approach  to  problem  solving.   However,
this  concept  is  useful only if  all  components of  the  train  are
considered. This includes  not only  sludge treatment and disposal
components, but  wastewater  treatment options and  other  critical
linkages such  as sludge transportation,  storage,  and side stream
treatment.  The  successful  devlopment of  a  treatment train from
a  collection  of  individual  components   depends  on a rigorous
system selection procedure, or  logic.  For  large plants, system
selection is  complex  and  a  methodical  approach is required.
Progressive and concurrent documentation  of the procedure is
mandatory  in  that  it  prevents  a cursory dismissal  of  options.
For smaller plants (that is, <1  MGD)  the system choices are often
necessarily more obvious and  the  selection  procedure  is  usually
shorter and less complex.

The general sequence of events in  system selection  is:

    1.  Selecting relevant  criteria.

    2.  Identifying options.

    3.  Narrowing the list  of  candidate systems.

    4.  Selecting a system.


    3.3.1  Identification  of Relevant Criteria

Criteria  for  system selection must  be pinpointed prior to system
synthesis.  A  listing  of potential  criteria  for consideration is
shown  on Figure 3-1.   The  list is  not  necessarily complete  and


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FLEXIBILITY

  • ABILITY TO RESPOND TO:
       • NEW TECHNOLOGY
       • CHANGING REGULATIONS
       • CHANGING LOADS
  • EXPAND IN GRADUAL INCREMENTS
RELIABILITY

  • VULNERABILITY  TO DISASTERS
  • PROBABLE FAIL RATE
  • BACK UP  REQUIREMENTS
  • REQUIRED OPERATOR ATTENTION
  • VULNERABILITY  TO STRIKES

SIDESTREAM EFFECTS
                                  I
  • PRODUCES SIGNIFICANT SIDESTREAMS  s

TRACK RECORD

  • NUMBER OF U.S.  INSTALLATIONS
  • PERFORMANCE DATA
                                                          IMPLEMENTABILITY
 I SYSTEM LEGAL
 I FINANCING ASSURED
 » TECHNOLOGY AVAILABLE
                COMPATIBILITY
         • WITH EXISTING LAND USE PLANS
         • WITH AREA WIDE WASTEWATER.
          SOLID WASTE AND AIR POLLUTION
          PROGAMS
         • WITH EXISTING TREATMENT
          FACILITIES
                               	
                                                                CRITERIA
DIRECT ENERGY DEMANDS

  • OPERATION DEMANDS
  • CONSTRUCTION DEMANDS
  • OFFSETTING ENERGY RECOVERY

INDIRECT ENERGY DEMANDS

  • ENERGY TO PRODUCE CHEMICALS
  • ENERGY TO TRANSPORT MATERIALS
  • CREDITS FOR USE OF PRODUCTS
   WHICH NEED LESS ENERGY TO
   PRODUCE
DIRECT COSTS AND BENEFITS
  • CAPITAL COSTS
  • OPERATING AND MAINTENANCE COSTS
  • REVENUES
  • BENEFIT/COST RATIO

INDIRECT COSTS AND BENEFITS

  • CHANGE IN LAND VALUES
  • REMOVAL OF LAND FROM TAX ROLLS
  • CHANGE IN CROP PRODUCTIVITY
                                                                 FIGURE 3-1

                                              CRITERIA  FOR SYSTEM SELECTION
PUBLIC HEALTH

  • PATHOGENIC ORGANISMS
  • TOXIC ORGANICS
  • HEAVY METALS

EFFECTS ON SOIL
  • DRAINAGE
  • STABILITY
  • PRODUCTIVITY

EFFECTS ON WATER  QUALITY

  • GROUNDWATER
  • SURFACE WATER
  • MARINE ENVIROMENT

EFFECTS ON AIR QUALITY

  • AEROSOLS
  • ODORS

EFFECTS ON BIOTA

  • TERRESTIAL
  • FRESH WATER
  • MARINE

EFFECTS ON NATURAL RESOURCES
  • DEPLETION OF RESOURCES
  • RESTORATION OF RESOURCES

SOCIAL EFFECTS

  • DISPLACE HOUSING AND BUSINESSES
  • ENHANCEMENT OR  DEGRADATION
    OF CONTIGUOUS AREAS
  • GROWTH INDUCEMENT
                                                                                                                   • EFFECT OF CONSTRUCTION
                                                                                                                    AND OPERATION ON PUBLIC
                                                                                                                   • EFFECT OF SLUDGE TRANSPORTATION
                                                                                                                    ON HIGHWAYS
           ADMINISTRATIVE BURDENS
         • LEVEL OF EFFORT
         • MARKETING RESPONSIBILITIES
         • RESOLUTION OF JURISD1CTIONAL
           DISPUTES
         • PUBLIC RELATIONS
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-------
 THICKENING
                          SECONDARY TREATMENT
                                       TERTIARY TREATMENT
           STABILIZATION
                     DISINFECTION
                              CONDITIONING
                                        DEWATERINQ
                                        VACUUM FILTER
                                        FILTER PRESS
                                        CENTRIFUGE
                                        DRYING BEDS


                                        DRYING LAGOONS


                                         STRAINERS
                                                   DISINFECTION
                                                                     EFFLUiNT RECtlVEH

                                                                    JI
                                                                    RIVER
                                                                    LAKE
                                                                    OCEAN
                                                                    RECLAMATION PROJECT
   FLASH

   ROTARY
I SOLVENT EXTRACTION
 MULTIPLE EFFECT
                 HIGH
                4PERATURE
                PROCESSES
        MISCELLANEOUS
         CONVERSION
         PROCESSES
 RESOURCE RECOVER1
 LAND RECLAMATION I
                 DISPOSAL ON LAND
 FINAL INSTITUTION

\       1
 FOREST INDUSTRY
 OTHER INDUSTRY
 FARMS
 HOUSEHOLD
 METRO
 LOCAL
 STATE
 FEDERAL
 OTHERS
                                 FIGURE 3-2

                    COMPONENTS FOR SYSTEM SYNTHESIS
     3.3.2   Identification of System Options

Candidate   systems   are  synthesized  from  an  array  of  components,
such  as  these  shown  on  Figure  3-2.    Wastewater  and  solids
management  components  are   listed  as  a  reminder  that  all
components  of  the  train  must  be   considered.    Figure  3-3
illustrates how Figure  3-2 can be  used  to develop  a  specific  flow
sheet.   Process  streams can  be drawn on  copies of the  master
                                     3-4

-------
drawing.   Relevant  information such as  solids  concentrations  and
mass  flow  rates  can be  entered  directly  on  the  flow  sheet,  if
desired.   The advantages of  using arrays  such  as Figure  3-2  are
that nearly  all potential options are identified and process
streams  are clearly  displayed.
                            SECONDARY TREATMENT
                                        TERTIARY TREATMENT
                                                                      EFFLUENT RECEIVER
                                 __     	„   .	
                                 THERMAL r~_'._   VACUUM FILTER  !         I	
                                 	fl  \	——L-J
                                 CHEMICAL I  L^j BELT FILTER  g
   SOLVENT EXTRACTION

    MULTIPLE EFFECT
     UTILIZATION
   RESOURCE RECOVERY
                   DISPOSAL ON LAND
    AGRICULTURE  [

     FOREST   j
                  DEDICATED LAND DISPOSAL
                   PERMANENT LAGOONS
   LAND RECLAMATION
 FINAL INSTITUTION

r~~~~~~
 FOREST INDUSTRY
 OTHER INDUSTRY
 FARMS
 HOUSEHOLD
/METRO
 i.«M
 STATE
 FEDERAL
 OTHERS
1— 	 -" ™™l
CONFINED
L, .,-.. - _J
UNCONFINED

1 CHEMICAL FIXATION
ENCAPSULATION ;
1 EARTH WORM '•
1 CONVERSION |
ASH ^
KEY:
—	WASTEWATER

^^^^ SLUDGE

—	 SIDESTREAMS
                                 FIGURE 3-3

               FLOWSHEET DEVELOPED FROM COMPONENTS FOR
                             SYSTEM SYNTHESIS
                                     3-5

-------
    3.3.3  System  Selection Procedure

The  process selection procedure consists  of  (1)  developing
treatment/disposal systems which  are  compatible  with  one  another
and appear  to satisfy  local  relevant  criteria,  and  (2) choosing
the best system or  systems by  progressive  elimination  of  weaker
candidates.   Related  to these  are  the  concepts of  base  and
secondary alternatives.


        3.3.3.1  Base and Secondary Alternatives

A base alternative  is  defined  as  a wastewater  solids management
system which,  during evaluation, appears able to provide reliable
treatment and disposal  at all  times  under  all  circumstances  for
sludges.   It therefore  meets  the prime criterion of reliability.

It must also satisfy the following seven conditions:

    1.  It must  be legally acceptable.

    2.  Sites for processing  and disposal operations must  be
        readily  available.

    3.  Environmental and  health  risks must be  sufficiently  low
        to  satisfy the public and all agencies having
        jurisdiction.

    4.  It must  be competitive  with cost to other alternatives on
        a first-round analysis.

    5.  The necessary  equipment  and material  must  be readily
        available.

    6.  The contractor  must  be able  to begin construction
        immediately following  design  and  have   the  system
        operational  almost immediately after construction.

    7.  Financing  of   the system must  be straightforward  and
        assured.

A  secondary  alternative  is  defined as  a wastewater   solids
management system  which does  not meet the prime criterion of
reliability, that is,  the system cannot accept  all of the  sludge
under  all  circumstances all of  the  time.  This does not mean
secondary  alternatives  ^are  without value;  they  may  in fact  be
used  to  great advantage in  tandem with  base  alternatives  and
may  in fact accept a  greater  quantity  of sludge than the  base
alternative.  As an example,  a  city's  horticultural market may be
insufficiently developed  to  accept all of  the  city's sludge  all
of  the time; therefore, horticulture cannot be  considered  a  base
disposal alternative.   However,  it may  cost less to  release  the
sludge to  horticulture than  to dispose of  it by means  of  city's
base  disposal  alternative,  for example,  landfilling.    The
                               3-6

-------
city should  therefore   make every effort to dispose of  as  many
solids as  possible  via  horticulture,  the secondary alternative.
However,  should the secondary alternative fail  or be interrupted
for any reason, the sludge going to the secondary system  must be
readily and quickly diverted back to the base alternative,  which
must  remain  fully operational  and  thus immediately capable  of
receiving the entire sludge  flow.


        3.3.3.2  Choosing  a  Base Alternative:  First Cut

The purpose of  the first  cut  is to rapidly  and with  minimum
effort  produce  a list  of  candidate  base  alternatives which
are technically  feasible and reasonably  cost-effective.   The
alternatives must be environmentally acceptable and implementable
in  the  time  frame of the project.  Analyses are qualitative at
this stage.  The first  cut involves determination of:

    1.  Practical base  disposal  options.

    2.  Practical base  solids  treatment  systems.

    3.  Practical treatment/disposal combinations.

Determi_nat.ion_of_ Practical Base  Disposal Options

The method of  solids disposal  usually  controls  the selection of
solids treatment  systems  and  not  vice  versa.  Thus,  the system
selection  procedure normally begins when  the solids disposal
option is specified.

In  the first  cut,  feasible  base disposal  alternatives  and
relevant criteria are set  up in  matrix form.  An example is shown
in Table 3-1.  Feasible  alternatives are those which appear to be
suitable  for  the situation  at  hand.  Obviously inapplicable
alternatives would  not  be included in  this matrix.   Only  those
criteria which  the planner see's as critical for the site at hand
should be  considered  in  this  first  cut.    Other,  less critical
criteria can be  considered  in  subsequent  iterations,  where  more
in-depth  investigation  is  needed for each of  the  candidate
processes.

For the  hypothetical  situation  described  in Table  3-1,  nine
utilization/disposal options are considered  feasible  and  are
set up for  evaluation.  The criteria most  important  to the  site
are  judged  to  be  reliability,  environmental  impacts,  site
availability and cost.   Base disposal alternatives are judged to
be practical only  if they  satisfy all  the relevant  criteria.
In Table 3-1,  utilization of sludge on private  agricultural  land
is an  unacceptable  base  disposal  alternative.  Reasons for  this
might  be  insufficient  acreage or  a  lack of assurance that  the
farmers would accept all  of  the  sludge.  Alternatives which would
seem to satisfy relevant  criteria for base disposal alternatives
are utilization  on public agricultural  land,  landfill,  and


                              3-7

-------
dedicated  land disposal.   Before  considering these,  however, one
must  determine  what  combinations  of  solids  treatment processes
make sense  for the site in question.     . .      .
                              TABLE 3-1

               EXAMPLE OF INITIAL SCREENING MATRIX FOR
                    BASE SLUDGE DISPOSAL OPTIONS

                                    Relevant criteria
                                         '"  ' '     •     -      Acceptable
 Utilization/disposal             Environmental     Site            for base
      options       Reliability     impacts     availability   Cost  alternative
Bag-market as
  fertilizer             0           X          X        X       0
Agricultural land
  (private)              O           X          X        X       O

Agricultural land
  (public)              X           X          XXX

Forested land (pri-
  vate)                 0           X          0        0       O

Forested land (public)     X           X          O        0       O

Give to citizens
  (horticulture)          0           X          X        X       O
Combine with commer-
  cial topsoil           O           X          X        X       O

Dedicated land dis-
  posal                 X           X          XXX

Landfill                X           X          XXX
 0 = unacceptable.
 X = acceptable.


Determine  Practical Base Treatment Systems

Table  3-2  illustrates  process compatibility matrix  for treatment
alternatives.  Incompatible  processes  and processes  which are  not
applicable in given locations  are eliminated.   The combination of
drying beds  and mechanical  dewatering, for example,  is considered
incompatible because both dewatering and drying take place on  the
drying  bed;  mechanical dewatering  is  not needed.  On  the other
hand,  the combination of  incineration  and mechanical dewatering
of  unstabilized  sludge  is generally  compatible,  but  for  the
hypothetical   case investigated  is  ruled out  because  of   air
pollution  considerations.    After first-cut  analysis,  seven base
treatment  options  are  considered  feasible  and  are  further
evaluated.

Dgj^£giJJlg_P£agt_i_cal_gase_ Treatment/Disposal Combinations

Practical  base   treatment  and  disposal  combinations   are
then  combined  in  a  matrix,  which  is  subjected  to  further
                                 3-8

-------
culling.  Table 3-3  shows  the  matrix  of base  treatment/disposal
combinations  made  by  bringing forward  the base  disposal and
treatment  options   from  Tables  3-1   and  3-2.    Incompatible
combinations  and  systems  ruled  out by local  constraints are then
eliminated.   For example,  undewatered  wastewater solids are
not  generally disposed of  in  landfills.  An  example  of  local
constraints  is the  ruling out  of applying lime  stabilized sludge
on agricultural lands because of already high soil pH.

                             TABLE 3-2

              EXAMPLE OF PROCESS COMPATIBILITY MATRIX
                Digestion options
                                        Undigested sludge options
Anaerobically or
aerobically digested
Final processing
step
No further processing
Drying beds
Heat dry
Pyrolysis
Incineration
Compost
Mechanically
dewatered
xa
0
X
0
0
X
Not
dewatered
X
X
o
0
0
o
Not stabilized
Mechanically
dewatered
ob
O
0
0
0
X
Not
dewatered
O
O
O
0
O
O
Lime
stabilized
Mechanically
dewatered
X
0
0
0
0
o
Thermally
conditioned
Mechanical ly
dewatered
0C
0
0
0
0
0
Wet air
oxidation
Mechanically
dewatered
0
0
0
O
0
0
X = generally compatible .
                              TABLE 3-3

        EXAMPLE OF TREATMENT/DISPOSAL COMPATIBILITY MATRIX
                                    Treatment options
Digested sludge options

Viable
disposal
Agricultural
(public)
Landfill

local
options
land

Dedicated land disposal

Mechanically
dewatered
xa
X
X

Mechanically
dewatered,
heat dry
X
X
X

Mechanically
dewatered,
compost
X
oc
o

Not
mechanically
dewatered
X
o
X
Not
mechanically
dewatered,
drying beds
X
X
X
Undigested sludge options

Mechanically
dewatered ,
composted
X
X
X
Lime
stabilized,
mechanically
dewatered
f
X
0
 X - generally compatible.
b
 0 - generally compatible, but ruled out by local considerations.

 0 - generally not compatible.
The  number of candidate base  treatment/disposal  systems  is  thus
reduced.   For the hypothetical case  of  Table 3-3, sixteen systems
remain for further evaluation.
                                3-9

-------
          3.3.3.3  Choosing a Base Alternative:   Second Cut

The  purpose  of  second-cut  analyses  is to  further reduce  the  list
of  candidate  systems.  Analyses are  more  quantitative than in the
first  cut,  but  the level of  effort used  to  investigate   each
option  is  not yet  intensive.    Information  used  in  the  second cut
is  general  and  readily   available,  for  instance,   equipment  cost
curves which  are not site-specific,  areawide  evaluation of soils,
geology,  hydrology,   topography  and  land  use, and  general energy
costs.

One approach  is to set up  a  numerical  rating system  for  the
remaining  candidate  systems,   such  as  that   shown  in  Table   3-4.
The list  of  criteria  to be  considered  may be expanded  beyond
those  critical  criteria  used  in  first  cut  analyses  to encompass
the  full range  of  criteria listed on  Figure  3-1, or  any  fraction
of  it.   This  follows the principle  that as  the  list  of candidate
process  narrows, each will be analyzed in greater detail.


                                  TABLE 3-4

                EXAMPLE OF NUMERICAL RATING SYSTEM FOR
                          ALTERNATIVES ANALYSIS
                                            Ratings of alternatives


Categories and criteria
Effectiveness
Flexibility
- Reliability
Sidestream effects
Track record

Relative
weight3

3
S
3
2


ARb

4
3
10
5


WRC

12
15
30
10


AR

6
5
9
7

WR

18
25
27
14


AR

9
5
5
4
stive 3 Alternative 4

WR AR WR

27 5 15 ..
25 2 10 ..
15 6 18 ..
8 9 18 ..


AR

6
2
7
6


WR

18
10
21
12
Compatibility
   With existing land use
    plans
   With areawide wastewater,
    solid waste and air
    pollution programs
   With existing treatment
    facilities

Economic impacts
 - Net direct costs
 - Net indirect
    costs
3
4
4
1
3
5
7
8
9
20
28
8
fi
5
8
9
18- 3
20 6
32 0
9 6
3
24 ,
32
6
5
8
9
3
15
32
36
33
                                    21

                                    12



                                    28


                                     8
Environmental impacts
 - Public health
Administrative burdens
   Level of effort
   Marketing respons-
    ibilities
  - Resolution of juris-
    dictional disputes
   Public relations

Total weighted alternative
  ratingd
                             1,576
 6

10
1,430
                   7


                   14
                   4
                   10
4

18
                                   1,317
 3Relative importance of criteria as perceived by reviewer; scale, 0 to 5; no importance rated zero, most important rated 5.

 Alternative rating. Rates the alternatives according to their anticipated performance with respect to the various criteria;
 scale 0 to 10; least favorable rated zero, most favorable rated 10.

 CWeighted rating. Relative weight for each criteria multiplied by alternative rating.

 Sum of weighted ratings for each alternative.
                                     3-10

-------
In  the  second  cut,  subjective  judgments  are combined  with
technical  measurements.    Numerical  values are  assigned  to all
criteria for all alternative systems.  The planner's  perception
of  the  relative  importance  of  each  criterion is indicated on a
rating  scale,  say  of 0  to 5, with  highest ratings  given to
criteria  the  planner considers  to  be of greatest  importance,
and  the lowest  to those of  least important.   For  example,
if  reliability  is highly  valued  for the  site in  question,
reliability  may  be  assigned  a relative weight  of  5.

Next,  each  alternative  system  is  rated  according  to its
anticipated  performance  with  respect  to  the various  criteria,
again by using  a  rating scale,  say 0 to 10.  An alternative  which
rates  favorably is given  high  scores;  one which  rates less
favorably  is given  lesser scores.   For example, an  alternative
which  is  not  dependable  may  be  rated  at  2 with  respect to
reliability.

The relative weight is then multiplied by  the alternative  rating
to  produce  a  weighted  rating for each criteria/alternative
combination.   For  the examples  described in the previous two
paragraphs,  the weighted  rating for  the alternative  in question
with respect to reliability  is  5 x 2 = 10.

Finally, the weighted ratings are  summed for  each alternative to
produce a total  or  overall  rating.   Systems  with lowest  overall
ratings are  eliminated, with higher  rated systems carried  forward
for  further evaluations.   I.n  the example shown in Table 3-4,
Alternatives 3 and  4 are eliminated  and Alternatives  1, 2,  and n
are carried  forward.
        3.3.3.4  Third  Cut

The  third cut  uses the same methodology  as  the  second, but
the number of  alternatives remaining  is  more  limited;  typically
to  a  maximum  of 3 to 5--and the  analysis  is  more detailed.
Information may include:

     •  Analyses  of potential sludge  disposal  sites  (soils,
        geology, and groundwater).

     •  Local surveys  to  determine  marketability of sludge and
        sludge by-products.

     •  Possible  effects  of  industrial  source  control/
        pretreatment programs  on process viability and quality  of
        sludge for disposal.

     •  Data oriented  literature search.

     •  Detailed analysis  of  effect of candidate  systems on the
        environment (air, water, land).
                              3-11

-------
     •  Information developed from site-specific pilot work.

     •  Mass balances.

     •  Energy analyses.

     •  Detailed cost analyses.


        3.3.3.5  Subsequent Cuts

Subsequent  cuts  are even more  detailed.   Analyses  are repeated
until the optimum base treatment/disposal alternative is defined.


    3.3.4  Parallel Elements

By means of  the  procedure discussed  above,  a base alternative is
selected.  However, the optimum system may include more than just
this  base  alternative.  A number of parallel elements  may be
involved which provide  flexibility,  reliability,  and  operating
advantages.   For  example,  the base  alternative for  the  system
depicted  on  Figure  3-4 is  thickening,  anaerobic digestion,
storage  in  facultative  sludge  lagoons,  and spreading  of  liquid
sludge  on  agricultural land.   Parallel elements cosnsist of
the  application of  liquid sludge  on  forest  land and  drying
beds  followed by  distribution  for  horticultural purposes.   If
horticultural  and  forest land  outlets were  each large  enough to
accept  all of  the sludge  under  all circumstances and  at all
times, three  base  alternatives are then available.   If not, the
forest  land  and drying  beds/horticulture applications  would be
considered  secondary alternatives.
                            BASE ALTERNATIVE
      r
        THICKEN
ANAEROBIC
DIGESTERS
FACULTATIVE
  LAGOON
                                     PARALLEL
                                     SYSTEMS
 SPREAD ON
AGRICULTURAL
   LAND
                                                    APPLICATION
                                                       ON
                                                      FOREST
                                                       LAND
                             FIGURE 3-4

                         PARALLEL ELEMENTS
                               3-12

-------
The concept  of  providing for more than one  base  alternative  may
at first  seem contradictory but  a  given base  alternative  might
not always be  reliable  because  unpredictable events might occur.
For example,  new  owners of farmland may decide they  do  not wish
to accept  sludge, or  a disaster or  strike could  interrupt  one
method  of  transporting  sludge to  its  ultimate destination.   To
minimize risks, therefore municipalities may wish to provide more
than one base  alternative.   The  selection  procedure presented in
Section 3.3.3 has the advantage of clearly depicting which is the
second or even third most desirable base alternative.

Parallel  base alternatives are  more  common  in  large  systems,
which  are  generally  located in urban areas  where  land  is scarce
than  in small plants,  which  are  usually located  in rural  areas
where  land  is more plentiful and  temporary  storage and  disposal
options therefore  more  numerous.   Large plants  may maintain two
or three base alternatives to ensure solids disposal.  Since this
may increase the  cost of operation, it  leads  to  the observation
that very large systems do not necessarily benefit from economies
of scale when it comes to wastewater solids disposal.


    3.3.5  Process Selection at Eugene,  Oregon

Eugene, a  city  of  100,000  people,  is  located at the southern end
of  the agricultural  Willamette  Valley  in  Western Oregon.   The
Metropolitan  Wastewater Management  Commission  (MWMC) was formed
in 1977  to  implement the findings of a  facility  planning effort
which  called for  the construction of  a  regional sewage  treatment
plant.  The  plant,  to be constructed on  the site  of the existing
Eugene  plant,  will  serve the whole  metropolitan area.  This area
is composed  of the  cities  of  Eugene, Springfield,  and  urbanized
portions of  Lane County.

Regionalization and  upgrading of the plant to meet a 10/10 summer
effluent   standard  for  BOD5  and suspended  solids  prior  to
discharge  to the  Willamette River, means  that sludge quantities
are dramatically  increased.   The plant is  to  serve a population
of 277,000 by  the  year  2000.   Design  average dry  weather flow is
49 MGD (2.15 m3/s) ,  wet weather flow is 70  MGD (3.07 m3/s),  and
peak wet weather flow is 175 MGD (7.67 m3/s).

The plant  will  use an activated sludge  process, with flexibility
for operation  in  plug, step, contact stabilization,  or  complete
mix modes.   Provision is also made for the addition of mechanical
flocculators  in the  secondary  clarifiers  and tertiary filtration
if either or both prove desirable at a later date.

It was  decided  early that sludge  thickening would be economical,
regardless of the sludge management system which would eventually
be used.   Consequently,  two existing  thickeners,  one gravity and
one flotation,  will  be  retained  for thickening  primary sludge,
waste-activated sludge, or a combination of the two.
                               3-13

-------
A key provision in the selection of a suitable sludge management
system was that the system  be  fully  operational  by  the  time the
wastewater  treatment  system  is started  up.   This seemingly
straightforward condition was  complicated  by  the fact  that
planning for the sludge system did not start until design of the
wastewater treatment plant  was  already  under way.    This  meant
that  the  sludge  management system would  be forced  to  fit  into
an already developed plan  for  the wastewater  treatment  facility
(which is by  no means unusual).

As  a  first  cut,  sludge   disposal   options  were   immediately
developed  and screened for  acceptability as part of a  base
alternative,   using  a  matrix  similar  to that  developed  in
Table 3-1.   Practical  treatment systems were  identified  from  a
process  compatibility  matrix  similar to  Table  3-2.   Practical
disposal/processing combinations were then developed in  a matrix
form  (as  in Table 3-3).    Physically incompatible  or otherwise
unsuitable  combinations were  eliminated  in this  matrix.   A
flowsheet  was then  prepared  for the remaining options,  with
necessary intermediate storage  and  transport  requirements  added
in.  The flowsheet  of alternatives  for Eugene  second  cut analysis
is shown on Figure  3-5.
•OLD * DEDICATED LANO DISPOSAL
                            FIGURE 3-5

         CANDIDATE BASE ALTERNATIVES FOR EUGENE-SPRINGFIELD
                               3-14

-------
It is worth  noting  that utilization on agricultural land  could
not  be  considered  as  a base  alternative  despite  the large
agricultural  acreage north of Eugene  and  the fact that  the new
regional plant is on the north side of  the city.   It would have
been a  requisite that  MWMC  own  sufficient  farmland  (2,000 to
3,000 acres)  to  accept all  of  the  sludge generated.   The cost
of  purchasing  such  acreage  was  deemed  unacceptably high;
furthermore,  there was  opposition to  converting private  land to
public  land.   Thus  agricultural utilization was  not  considered
further in the search  for a base alternative.

The second cut analysis was more quantitative.   Information used
was general and readily available.  For example,  costs  were  taken
from  current cost  curves,  and  certain  environmental  impacts
were  assessed  from  projects with  similar disposal systems and
soil/groundwater   conditions.   With  numerical  data  established
for each  criterion,  a  rating table  was  produced similar  to that
of Table  3-4.  The data were developed by the project  engineers,
but  the  ratings were  analyzed  extensively  by  the  Citizens
Participation Committee (CPC) on sludge management which had been
recruited from the population at large  at the very beginning of
the  project.  The  committee  was  composed  of various vested
interest  groups, representatives  of government agencies and
private unaffiliated citizens who were interested in the project.

Systems with  the lowest total  ratings  were  then eliminated.
Incineration was found to  be  unacceptable primarily  because
it would  impact  the  already  limited  dilution capacity  available
during  the  summer  in  the  trapped  valley airshed  of  Eugene;
pyrolysis was eliminated  primarily  because of  its  perceived
inability to  meet the  construction  deadline  for plant  start-up;
and  lime  stabilization  with  disposal  to landfill  was  eliminated
primarily on  a cost-effective  basis.   At the end  of  the second
cut  analysis,  all  alternatives which could  accommodate raw
sludges were  eliminated,  since,  as  indicated,  most raw sludge
options  (incineration,  pyrolysis,  lime  stabilization)  were not
viable  and there was  a strong desire  to make  use of existing
digesters.  A  decision  was made to combine primary and secondary
sludge  in order  to  avoid the cost  and  problems  of constructing
and operating separate systems for each.

The  same  methodology used  in  the  second cut  was used in the
third;  however,  data  used  in  the  analysis  were  more site
specific, so that economic  and environmental comparisons could be
better  refined.  As  examples:

     •  Actual  routes  were  selected to off-site facilities;
        river crossings were defined,  and decisions were  made on
        routing pipes  under  bridges or jacking under freeways.

     •  For  disposal  at the local  sanitary  landfill,  estimates
        were  made of  (1)  the  contribution  of  the  sludge to
        landfill  leachate production and subsequent marginal
        leachate  treatment  costs  to  be  passed  back from the
                               3-15

-------
        Lane  County Solid  Waste  Division to  MWMC,  and (2)  the
        actual  net volume of  landfill required for  sludge
        disposal,  allowing  for sludge consolidation.


     •  For dedicated land  disposal,  seasonal water tables  and
        detailed  groundwater migration  patterns, as  well as
        private  well locations and depths were determined.


     •  Estimates  were made  of comparative nitrate loadings  which
        would  eventually  reach the  Willamette  River  from  treated
        landfill leachate discharge; from groundwater  migration
        from  dedicated  land disposal;   and  from filtrates  from
        mechanical  sludge  dewatering  (which  is  subsequently
        discharged with the  effluent).


     •  Transportation  modes were analyzed in detail and  costed
        for various sludge  solids  concentrations and  transport
        routes and distances.


These  detailed  analyses still  left a  number  of  viable  base
alternatives.   At this  point,  other  less  tangible factors
were  considered.    These  were  (1)  that  the  chosen   base
alternative(s) be  compatible with desired secondary alternatives,
and (2) that flexibility and reliability be  provided through the
use of  parallel  systems.   After  intensive  screening,  it  was
decided that two  base alternatives would  be used:   spreading of
liquid  sludge  on  dedicated land and open-air  drying  followed
by landfill  disposal.   Both  alternatives  included force  main
transport  of  digested  sludge from the regional treatment plant to
a  remote  sludge  management site,  where the sludge  was  to be
stored  in  facultative  sludge  lagoons.    Liquid  sludge  would be
spread on  dedicated  land  at the sludge  management  site.   Dried
sludge would be  trucked to landfill.  Operations associated with
disposal  (spreading,  drying,  and  landfilling) would be  carried
out during dry weather.   These systems provide  the desired
flexibility and  reliability and  are  compatible with  preferred
secondary  alternatives.


Several variations  of  sludge  utilization on  land  were  adopted
as secondary alternatives, since  there was  a strong feeling
that  sludge  should be used beneficially.  The  alternatives of
particular interest  to  the Eugene-Springfield area  included
agricultural  use  on private  farm land,  use  for ornamental
horticulture,  in nurseries  and public parks,  and use in  a mixture
with  commercial  topsoils  in. landscaping.   Sludge  would be
provided to these  outlets as the market demands.  Variable  demand
is particularly important  in  Oregon's Willamette  Valley,  where
prolonged  winter rainfall and summer harvesting schedules control
the timing of agricultural  sludge use.


The flowsheet for  the  Eugene system  is shown on Figure 3-6.


                              3-16

-------
PRIMARY
SLUDGE

WASTE -
ACTIVATED
SLUDGE
GRAVITY
THICKEN

FLOTATION
THICKENER
                     ANAEROBIC
                     DIGESTERS
                             LONG
                             DISTANCE
                             PIPELINE
                                    FACULTATIVE
                                     STORAGE
                                     LAGOONS*
    AT SLUDGE MANAGEMENT SITE

    ON PRIVATE FARMLAND

    CITIZEN PICK-UP AT SLUDGE
    MANAGEMENT SITE
DEDICATED
 LAND
DISPOSAL*
                                                 DRYING
                                                  BEDS*
                                                        TRUCK
                                                 TRUCK
                                                             LANDFILL
                                                            AGRICULTURE
                             FIGURE 3-6

                FLOWSHEET FOR THE EUGENE-SPRINGFIELD
                     SLUDGE MANAGEMENT SYSTEM
The  ability  to use  base facilities  and equipment for desired
secondary  alternatives was a major  consideration in selecting  the
                            the  force  main, sludge lagoons,  and
                            be  used for  dedicated land  disposal
                            also required for  agricultural use.
                            sludge from the sludge management site
                             however,  be  an additional expense  for
base  system.   In Eugene,
application  equipment  to
of  the liquid  sludge  are
Trucks to  transport  liquid
to agricultural  sites will,
the secondary  alternatives.
It  is  hoped  that  eventually all  sludge  can be  utilized  on
land.   As  indicated,  however,  in Table  3-5,  full  agricultural
utilization  of sludge  is  estimated to be  more costly than  either
of the  pure  disposal options.   This is because more  equipment  is
needed  to transport  sludge  to and spread it  on  the agricultral
sites than  is  needed for  the pure disposal options.   Thus, as  of
1979, any system  which even partially incorporates  agricultural
utilization  will  be more costly than pure  disposal options.   This
could change  if  the  farmers can be persuaded  to  pay  for the
sludge.
                              TABLE 3-5

      ESTIMATED COSTS OF ALTERNATIVES FOR EUGENE-SPRINGFIELD
  Sludge form
                         Alternative
                                                 Total annual cost,
                                                  million dollars
     Liquid
     Dried
                  Dedicated land disposal only
                  Agricultural utilization only

                  Landfill only
                  Agricultural utilization only
1.03
1.53

1.14
1. 32
                                3-17

-------
At the time this manual was written (1979), MWMC was involved in
public hearings aimed  at  selecting  a  suitable sludge management
site.
3.4  The Quantitative Flow  Diagram

Overall  system  performance  is  the  sum  of  the  combined
performances of the  system's  linked processes.   This is nowhere
more clearly expressed than on a Quantitative Flow Diagram (QFD).
The QFD  is  used to  estimate  loadings  to  the  various wastewater
treatment, solids  treatment, and solids disposal processes.   The
QFD is the  starting  point  for understanding process  interactions
and is nothing more  than a materials balance.   Although balances
can be struck  for  components like  nitrogen,  phosphorus  and
chemical  oxygen  demand,  the most useful balances are  those
for suspended solids.   The QFDs to be presented here  are  for
suspended solids.   Each  flowsheet has its  own unique  set of
balance equations.   In  the following  pages, mass balances for a
specific, rather simple flowsheet are derived, thus  illustrating
the technique.  The mass  balance equations are  then summarized
in tabular  form.  Mass  balance  equations  for  a  more  complex  and
more  common  flowsheet are  later presented, without  derivation.
Two worked  QFDs are  presented as examples.   The intent is to
demonstrate  the usefulness  of  the method.
    3.4.1  Example:   QFD for a  Chemically Assisted
           Primary Treatment Plant

The  flowsheet  for a  chemically  assisted  primary wastewater
treatment  plant  with  anaerobic digestion  and  mechanical
dewatering  of  the sludge  is  shown  on  Figure  3-7.    In  this
example chemicals are added  to  enhance the  sedimentation process.
Sidestreams from  the  digester  and  dewatering units are recycled
to the primary  sedimentation  basin.   The calculation is carried
out in a step-by-step procedure:


    1.  Draw the flowsheet (as  on Figure  3-7).


    2.  Identify all streams.   For  example,  stream  A  contains raw
        sewage  solids  plus  chemical  solids  generated  by dosing
        the sewage  with chemicals.   Let the mass  flow rate of
        solids in Stream A be equal to A  Ib per  day.


    3.  For each processing unit,   identify  the relationship of
        entering  and  leaving streamsto  one another in terms of
        mass.   For example,  for  the primary sedimentation tank,
        let the ratio  of  solids  in  the tank underflow (E) to
        entering  solids  (A •+ M)  be equal to XE.  XE is  actually
                               3-18

-------
an  indicator  of  solids  separation  efficiency.   The
general   form  in  which  such  relationships  are
expressed is:

         mass of solids in stream 6
      mass of~~solids entering the unit
                    P         J
For example, Xp = =— - — 5-, Xj = p-  .  The processing unit's
performance is specified when a  value is assigned to XQ.
            DEGRITTED SEWAGE
                 SOLIDS
                                        SOLIDS
                                        GENERATED
                                        BY CHEMICAL
                                        ADDITION
M

PRIMARY
SEDIMENTATION
B

 SUPERNATANT
 FILTRATE
                DIGESTION
                   1
               DEWATERING
                                        EFFLUENT
    SOLIDS
    DESTROYED
•^ (CONVERTED
    TO GAS AND
    WATER)
   CONDITIONING
   'CHEMICALS
               TO ULTIMATE
                 DISPOSAL

                     FIGURE 3-7

         BLANK QFD FOR CHEMICALLY-ASSISTED
                   PRIMARY PLANT
                       3-19

-------
Combine  the  mass balance  relationships^  so as to  reduce
them  to one equation describing a specific stream  in
terms of  given  or known quantities.   In  the  calculation
to be presented, expressions will be manipulated  until  E,
the primary  solids  underflow  rate,  can  be expressed  in
terms of A, XE ,  X j , X^ /  Xp,  and  X$ ,  quantities which  the
designer would know  or assume from plant  influent surveys,
knowledge of water  chemistry  and an understanding  of  the
general solids separation/destruction efficiencies  of  the
processing involved.   The  calculation is  carried  out  as
follows:

a.  Define M  by solids  balances on streams  around the
    primary sedimentation tank:
                                                    (3-D
    Therefore,
    M =  -- A                                      (3-2)
        XE

b.  Define M by balances on recycle streams:

    M = N + P                                       (3-3)

    N = XNE                                         (3-4)

    P = XP(S + K)                                   (3-5)

    S = XSK                                         (3-6)

    Therefore,

    P = XP(1 + XS)K                                 (3-7)

    K + J + N = E                                   (3-8)

    Therefore,

    K=E-J-N=E- XjE - XNE = E(l  - Xj  -  XN )   (3-9)

    and

    P  =  XPE(1 -  Xj - XN)(1  + XS)                (3-10)

    Therefore,

    M  =  E[XN  +  XP(1 - Xj - XN)(1 + XS)]           (3-110
                       3-20

-------
        c.   Equate  equations  (3-2) and  (3-11) to eliminate M:
             - -  A =  E[XN  +  Xp(l  - Xj - XN)(1 + Xs)j
                          _
                             -  XN  -  xp(i- Xj - xN)(i + xs)
            E is  now expressed in  terms of  assumed  or  known
            influent  solids  loadings  and  solids  separation/
            destruction efficiencies.

Once the  equation  for  E is derived,  equations for other streams
follow rapidly;  in  fact,  most have already been derived.   These
are summarized in  Table 3-6.
                            TABLE 3-6

         MASS BALANCE EQUATIONS FOR FLOWSHEET OF FIGURE 3-7

                                  A
                E  =
                    *  -  XN  -  XP  (1-XJ-V(1 + V
                M =  ^ - A
                     E

                B -  d-X) (A  + M)
                J - XTE
                     J
                N = XNE
                K - E(1-X -X  )
                S  =  XSK
                p = xp (1 + XS)K
                L - K (1 + Xg)(l-Xp)
                               3-21

-------
                 DEGRITTED SEWAGE
                      SOLIDS
                           299,000
                          409,000
                                  110,000
                             SOLIDS
                             GENERATED
                             BY CHEMICAL
                             ADDITION
       M
     56,030
       0
     56,030
   PRIMARY
SEDIMENTATION
   XE = 0.90
                            E
                          418,527
DIGESTION
Xj  = 0.25
XN = 0.0
                         I   K
                         313,895
                        i
 DEWATERING
   XP = 0.15
   X  = 0.19
                          317,505

                        T
                   TO ULTIMATE
                     DISPOSAL

                46,503
                  104,632
                                           -^ EFFLUENT
                 59,640
SOLIDS
DESTROYED
(CONVERTED
 TO GAS AND
 WATER)
CONDITIONING
CHEMICALS
ALL QUANTITIES ARE
EXPRESSED IN POUNDS
PER DAY

1  Ib/day = 0.454 kg/day
                          FIGURE 3-8
                QFD FOR CHEMICALLY-ASSISTED
                        PRIMARY PLANT
                              3-22

-------
Figure 3-8  is  a worked  example  in which  all  solids  flow rates
are calculated.  For  this  example the following information was
provided:

        a.   Based on  estimates  from  facility  planning  studies,
            average  influent  suspended solids loading  is
            299,000  pounds  per day (136 t/day).  Alum is added to
            the degritted  raw  sewage  to  increase  capture.   The
            chemical  solids generated  as the  result of  alum
            addition  is estimated at  110,000  pounds per  day
            (50 t/day).   The latter figure is derived from pilot
            work at Seattle, Washington,  where  the ratio  of new
            solids  generated/solids in untreated  raw  sewage was
            0.37/1  when alum  ( A12 ( 804 ) 3 • 1 4H20 )  additions  of
            110 to  125  mg/1 were  added  to raw wastewater  (1).
            Therefore,  A =  299,000 (1 +  0.37)  =  409,000  pounds
            per day  (185 t/day).

        b.   Primary  sedimentation  solids  capture is 90 percent of
            the sum  of  sewage solids,  chemical solids and recycle
            solids  which  enter  the  basin.   Note that  solids
            capture  as  usually  computed  (sewage solids  basis
            only)  is only 84.4 percent, i.e.,

            , -,  _ effluent suspended solids. -,„,,
                  influent  sewage  solids


                            10° =  84-4 percent
        c.   Twenty-five  percent of  the suspended solids  fed to
            the digestion system  are  destroyed,  i.e., converted
            to gas or water  (Xj = 0.25).   The number assumed is
            somewhat  less than the usual value used  (0.30-0.40),
            since the biodegradable fraction of digester feed in
            this instance is low  because of the large proportion
            of chemical  solids  present.

        d.   Digesters are not supernated  (X^ =  0.0).

        e.   Solids capture in the dewatering units is 85 percent
            (XP = 0.15) .

        f.   Conditioning chemicals   are  19 percent by  weight
            of  digested  sludge  fed   to  the  dewatering  units
            (XS = 0.19).

When all loadings are expressed quantitatively  and superimposed on
the flowsheet,  the designer  can begin to  develop  a feel for the
process.  The  effects  of recycle  loading and  individual process
efficiencies on  overall  process  performance can  be  assessed by
manipulation of  the  variables.   Calculations  can be done  very


                               3-23

-------
rapidly when the mass balance equations (presented in Table 3-6)
are  set  up for  solution on  a computer  or a programmable
calculator.

The investigator must exercise  judgment  in estimating the various
process  efficiencies ( X@ ) .   For  example, one  should  assume
reduced efficiencies for primary sedimentation if recyle streams
contribute large quantities of solids to the sedimentation tank,
since recycled solids tend  to  be less easily removed than fresh
solids from the sewer system.  Their  mere presence in the recycle
stream is an indication of  the difficulties  in  separating them.


    3.4.2  Example:   QFD  for Secondary Plant with Filtration

The example just worked was relatively simple.   Figure 3-9 shows
a jno_r_e_ comp 1 e x  sy s tern— second ary ____ ae r obi c _ biologies It re a t me n t
followed by filtration.   Mass  balance equations  for this system
are summarized in Table  3-7.   For  this flowsheet  the following
information must  be  specified.

        a.   Influent solids  (A).

        b.   Effluent solids  (Q),  that is, overall suspended solids
            removal  must  be  specified.
        c.   XE ,  XQ ,  X j ,  XN ,  XR,  and  Xg  are  straightforward
            assumptions  about  the  degree  of  solids  removal,
            addition  or  destruction.

        d.   XD,  which  describes  the  net  solids  destruction
            reduction  or  the  net  solids synthesis  in  the
            biological  system,  must  be  estimated  from  yield
            data (see Section 4.3.2.4).  A positive XD signifies
            net  solids destruction.   A negative XD signifies net
            solids growth.    In  this example  8 percent of the
            solids  entering  the  biological  process  are assumed
            destroyed,  i.e., converted  to  gas or liquified.

Note that alternative processing schemes can be evaluated simply
by manipulating  appropriate  variables.   For  example:

        a.   Filtration can be  eliminated by  setting XR to zero.

        b.   Thickening can be  eliminated by  setting XG to zero.

        c.   Digestion can be eliminated by  setting Xj to zero.

        d.   Dewatering can be  eliminated by  setting Xp to zero.

        e.   A system without  primary sedimentation can be
            simulated   by   setting  XE  equal  to approximately
            zero,  e.g.  ,   1  x  10~8.   x^  cannot  be set  equal
            to  exactly  zero,  since  division  by  Xg  produces
            indeterminate solutions when computing E.


                               3-24

-------
         DEGRITTED SEWAGE
             SOLIDS
A
299,000
f M
77,794 "
4 N
r 15,415
P
22,012
i
_ TREATMENT 0

CHEM1CALS D SOLIDS DESTROYED
10,831 OR SYNTHESIZED
— 40,367 ^ |R
PRIMARY
SEDIMENTATION
XE = 0.65
1
251,438
' 1
DIGESTION
XN = 0.05
Xj = 0.35
\
DEWA1
V
Xp

K
184,978
p
"ERING
0.10
L
198,111
SECONDARY
8 » -cn^MT^TinN c » FILTRATION Q fc
135,390*" ^D"VlCArJKATION 57,667*" XR = 0.70 17,300*
**"-• XD = 0.08
G F
10,034 66,892
1 '

THICKENING
X_ = 0.15
b
H
56,858
j SOLIDS DESTROYED
1U/,9U4 GAS AND WATER)
S CONDITIONING
35,146 "CHEMICALS
AM HI 1AM
                                                            EFFLUENT
          TO ULTIMATE
            DISPOSAL
EXPRESSED IN POUNDS
         PER DAY
 1 Ib/day = 0.454 kg/day
                             FIGURE 3-9

              QFD FOR SECONDARY PLANT WITH FILTRATION
A  set  of  different  mass  balance  equations must  be  derived
if flow  paths  between processing units  are  altered.   For example
the  equations  of  Table  3-7  do  not  describe  operations   in
which  the  dilute  stream  from thickener  (stream G)  is returned
to  the biological  system  instead  of  the  primary  sedimentation
tank.
                                3-25

-------
                     TABLE 3-7


MASS BALANCE EQUATIONS FOR FLOWSHEET OF FIGURE 3-9
    E =
A -  (r^)(Y - V

   ?  -  a  - 3 (Y)
   XE
    Where   a = Xp  (1 - Xj - XN)(1  +.Xg)  + XN
                 (1  -  X_). (1 - X_)
             D _   	^    .  _^_
             P -   	v	~—
                        XE
            Y - XG  + a (1 - XQ)
    B = (1 - XE)E

           XE
        1 - XR

    D = XDB


    F - 3 E - T-2-
    G = XQF


    H = (i - XG)F


    J = XT (E + H)
         J

    K = (1 - Xj -  XN) (E|-+" H)


    L = K  (1 + Xg) (1 - Xp)
    M = ^   -  G - A

         E


    N = X.T  (E  +  H)
    P = Xp  (1  +•Xg)K



        XR

    R =l^Q
       1 XR
    S = XSK
                        3-26

-------
3.5  Sizing of Equipment

The QFD described in the  previous  section can be an important aid
to a designer in predicting long-term  (i.e.,  average) solids
loadings  on  sludge  treatment components.   This  allows  the
designer  to  establish  such  factors as  operating  costs  and
quantities of sludge for  ultimate  disposal.  However,  it does not
establish the  solids  loading which each equipment item  must  be
capable of processing.  A particular component should be sized to
handle   the  most rigorous  loading  conditions  it is  expected  to
encounter.   This  loading is usually  not  determined  by applying
steady-state models  (e.g.,  QFD  calculations) to peak plant loads.
Because of storage and plant scheduling considerations, the rate
of solids  reaching  any  particular  piece  of equipment does  not
usually rise and fall in direct proportion to the rate of solids
arriving at  the  plant headworks.   Consider  a system  similar  in
configuration  to  that  shown on Figure  3-9.   If  maximum solids
loads at  the headworks   (Stream A) are  twice  the  average value,
it does not  necessarily  follow  that at  that  instant  maximum
dewatering  loads (Stream K) are twice  the  average dewatering
load.

To pursue this  further,  consider the design  of  a centrifuge
intended to dewater anaerobically digested primary and secondary
sludge  at a small treatment plant.  The flow scheme is similar to
that shown on Figure 3-9.  The  plant is staffed on only one shift
per  day,  seven days per week.   The  digesters  are complete-mix
units equipped  with floating  covers.   Because  of  the floating
covers,  digester  volume  can  vary.   Secondary sludge  is wasted
from the  activated  sludge systems to a  dissolved  air flotation
thickener prior to digestion whenever operators  are available  to
operate the thickener.

As indicated, the  average  loadings  to  the  centrifuge can  be
defined by the QFD,  but  computation  of the necessary centrifuge
capacity requires analysis  of  both  the  load  dampening effect  of
the  storage  in the  digesters and  the  plant  operating schedule.
During  periods  of  peak  plant solids  loadings,  loads  to  the
dewatering units  may  be  attenuated  by  storing  portions of  the
peak loadings  within  the digester.   This  can be done  by either
mechanism 1  or mechanism 2 below, acting either singly  or  in
concert.

    1.   Digester  volume  is  increased  by   allowing  the digester
        floating cover to rise.

    2.   Solids are  allowed  to  concentrate and  thus  accumulate
        within the digester (See Chapter 15, Section 15.2.2.2 for
        example of storage by mechanism 2).

The  effect of both mechanisms  1 and 2  is  storage within  the
digester  of  part of  the load which  would otherwise  go to  the
centrifuge.    Thus peak  dewatering  loads  will not be  2.5  times
the  average  when peak solids mass withdrawn from primary  and
                               3-27

-------
secondary  sedimentation  tanks are 2.5 times  the average, but
something less, for  example,  only  1.4 times the  average value.
The degree of load  dampening is a direct function of the  size and
operating configuration  of  the digester.

Since  the  centrifuge  will  only  operate  when  attended,  the
"design"  loading must account  for  this factor.   The  centrifuge
must be  either  capable  of  processing,  during one  shift, all the
sludge which must  be extracted from the digester during  the peak
day (for  example,   1.4  times  average quantity)  or  the operators
must  dewater  sludge for  longer  than one  shift per  day.   A
judgment  would be needed  at this  point  whether to  pay for
increased equipment capacity or operator overtime  to handle the
peak loads.   With  no operator  overtime, the "design"  centrifuge
capacity  would have to be  1.4  x 24/8 =4.2 times the  average
daily  digested sludge production to account  for  both the effect
of sludge peaking,  storage volume  and  only  one operations  shift
per day.

Note that the dissolved  air flotation  thickener would need  to be
designed  for 24/8  x  2.5 = 7.5  times  the  average daily  rate of
waste  activated sludge  production  if  it is assumed no  upstream
storage  is   available   for  dampening  thickener  loadings, the
thickener itself has no storage  capacity,  and  the thickener is
only operated one shift  per day.

The foregoing  example  shows  the influence  of solids peaking,
storage  volume and operating strategy  on  the  selection  of
design  loadings  for a particular  sludge handling  process.
Several other  factors are  important in  selecting  the capacity a
unit must have, including:

     •  Uncertainties.   When  systems   are  designed without the
        benefits of  pilot  or  full-scale testing,  actual sludge
        quantities  and characteristics as well as efficiencies of
        the   sludge  handling system components  may not be  known
        with certainty.    The  degree  and potential  significance
        of  the  uncertainties must  be  considered  when  developing
        design criteria.   This  usually  has  the effect of
        introducing  a   safety  factor  into  the  design  so that
        reliable   performance can be obtained  no matter what
        conditions  are encountered  in  the full-scale application.
        The magnitude of the safety factor  must be determined by
        the designer, based on his  judgement and experience.

     •  Equipment  reliability.   Greater capacity or parallel
        units  must  be   specified if  there  is  reason to believe
        that downtime for  any  particular units will be  high.

     •  Sensitivity  of downstream components.   If  losses in
        efficiency of   a particular  sludge  handling  component
        at  peak loading  conditions  would cause  problems for
        downstream  processes,   this   upstream  process should


                               3-28

-------
        be  designed  conservatively.    Conversely,  if  reduced
        efficiency  could  be  tolerated,  design  need not  be so
        conservative.
3.6  Contingency  Planning

As  indicated previously,  flexibility  to  cope with  unforeseen
problems is  highly  desirable  in any wastewater  solids  management
system.   Such problems  and possible solutions include:

     •  Equipment  breakdowns.    Downtime  may  be minimized by
        having maintenance people on call, by advance purchase of
        key  spare  parts,  by  providing  parallel processing  units
        and by making use  of storage.

     •  Solids disposal problems.   These may include closures of
        landfills,  unwillingness  of  current  users to  further
        utilize  sludge, failure  of  a  process to provide a  sludge
        suitable  for utilization, strikes by  sludge  transporters,
        and  inability  to dispose  of  sludge  due  to  inclement
        weather.    Disposal  problems  can  be  reduced  by providing
        long-term storage  and/or  more  than  one  disposal
        alternative.

     •  Sludge production  greater than expected.   In some
        instances  this may be  dealt  with by operating for more
        hours per week  than  normal  or by  using  chemicals to
        modify sludge  characteristics,  thus  increasing  solids
        processing capacity.

Because  of  these  factors, it  is desirable  to have more than  one
process  for  sludge  treatment  and disposal.   Often it is possible
to  add  considerable  flexibility with  modest investment.    Backup
or  alternative  wastewater solids  treatment  processes  often  have
higher  operating  costs  per  ton  of  sludge processed  than  the
primary  processes.  This  is  acceptable  if the alternative  process
is  not  frequently  needed  and  can  be  provided  at  minimum  capital
cost.
    3.6.1  Example of  Contingency Planning for Breakdowns
Assume  the plant  is  a  10  MGD
sludge  thickening,  anaerobic
dewatering as  shown on Figure
include:
 activated  sludge facility  with
digestion,  and  digested  sludge
 3-10.  Pertinent design  details
    !•  The waste activated  sludge  (WAS) thickener can be
        operated with or without polymers.   If  polymers  are  used,
        a more  concentrated  sludge can be  produced.   WAS can  be
        diverted to the headworks  if  the  WAS thickener  is  removed
        from service.
                               3-29

-------
WASTE ACTIVATED SLUDGE WHEN THICKENER IS INOPERATIVE
r
PRELIMINARY ^
PRIMARY . .. ACTIVATED _ DISINFECTION
p TREATMENT *^^ SEDIMENTATION " 	 "" SLUDGE ' " DISCHARGE
GRIT, ETC.
SIDESTREAM
800 Ib/day
1
SIDESTREAM
1,000 Ib/day
1
THICKENER FEED
9,000 Ib/day
1.0% SOLIDS
r 108,000 gpd
I SLUDGE
THICKENER
PRIMARY 1 1 N PI F R F 1 OW
PRIMARY SLUDGE ONLY
10,000 Ib/day 	
"5.0% SOLIDS
24,000 gpd
PRIMARY SLUDGE + W.A.S.
18,000 Ib/day
2.5% SOLIDS
86,000 gpd
*
f I I

DIGESTER DIGESTER
1 2

THICKENED W.A.Sr(8,000 Ib/day)
NO POLYMER
3.5% SOLIDS
27,000 gpd
WITH POLYMER
4.5% SOLIDS
2 1,000 gpd


""^ DIGESTED SLUDGE (11,000 Ib/day)


SLUDGE
STOCKPILE
1 1
DEWATERING DEWATERING
UNIT UNIT
1 2
* *

DEWATERED CAK
33.9 yd3 @ 17
^ _ OR
""* 26.2 yd 3@ 22
1 i
D
E (10.200 Ib/dav)
% SOLIDS
% SOLIDS
                              TO LANDFILL
                             VIA 16 yd3 TRUCK
    1 Ib/day = 0.454 Kg /day
    1 gpd  = 0.00378 m3/day
              ,3
                          FIGURE 3-10

             CONTINGENCY PLANNING EXAMPLE
                              3-30

-------
    2.   Two complete-mix  digesters with  floating  covers are
        provided.    Each digester  has  a  net  volume  of
        610,000  gallons  (2,310  m3 ) at minimum  cover height.
        Net volume  at  maximum cover  height  is 740,000  gallons
        (2,803  m3), thus total digester storage volume is
        2   (740,000-610,000)  = 260,000 gallons  (984 m3).   The
        digesters are not  supernated.

    3.   Two dewatering  units  are provided.   Each  unit,  when fed
        at 90 gpm  (40.8  m3/hr)  can produce a  22  percent solids
        cake.    When the  dewatering units  are fed  at 110 gpm
        (49.9  m3/hr) a 17  percent  solids cake  is  produced.  The
        units  are fed  at  90  gpm  (40.8  m3/hr)  unless  conditions
        dictate  otherwise.  The  bulk  density  of each cake is
        65.5 pounds  per cubic foot  (1,050 kg/m3).

    4.   The cake  is trucked  to  ultimate  disposal.   Each  truck
        holds  16 cubic  yards (12 m3) of cake.

    5.   A   dewatered sludge storage area of  capacity 750  cubic
        yards  (574 m3)  is  available.

    6.   Weekends are 2.7  days  long (from 5  p.m.  Friday  to 8  a.m.
        Monday).

CjiS_e__A.  All units  available:


    1.   Digester detention time = ( ^QOQ^^OOof gpd = 24  days'


    2.   Dewatering  operation:

        a. Weekly  sludge  feed =  7  (24,000 + 27,000 gpd)
           =  357,000 gallons  (1,350 m3).

        b. Hourly  throughput = 2  x  (90 gpm) (60 min/hr)
           =  10,800 gal  per hr (40.8 m3/hr).
                             .  ,    t        357,000 gal
        c. Operation  is  carried  out over  10,800 gai/hr
           =  33 hours  per week.

        d.  26.2 cubic yards  (20.0 m3)  of   22  percent solids
            sludge  cake is produced  each day.

    3.   If dewatering  is  not  operated over  the weekend,  then
        51,000 gpd  (2.7 days)  = 138,000 gal (522 m3)  of digested
        sludge  must be   stored  in the  digesters  during  this
        period.   Available  storage  which  can be  obtained
        by letting  the floating  cover  rise   is  260,000  gallons
        (983  m3 ) .  Therefore digester storage capacity is
        adequate for weekend  storage,  including  long  (3.7  day)
        weekends.
                               3-31

-------
    4.   Truckloads required to haul dewatered  cake = 26>2 yd3/day
          ,  ,  ,   , n   ,      ,   ,,,                16 yd3/truck
        =1.6  truckloads per day (11  per week).

In summary,  the dewatering operation can  be  carried out  in
a normal  5-day, 8-hour-per-day week.   Time is available  for
start-up  and  shutdown and  for  providing  good supervision.
Digester detention time is more than  adequate  for good digestion.

Case  B.   Thickener  is  out of  service.   All other  units  are
available.   Waste activated  sludge  is  diverted to the  plant
headworks  and  is  subsequently  removed in  the  primary
sedimentation  tank.
    1.   Digester  detention  time  =  -  (86°ooo° pd^  =  14 days;
        short, but tolerable.

    2.   Dewatering operation:

        a.  Weekly  sludge feed  =  7  (86,000 gpd) = 602,000  gal
            (2280 m3).

        b.  Hourly  throughput.  At  90 gallons per  minute,
           throughput is 10,800 gallons  per hr (40.8 m3/hr).   At
           110 gallons per  minute,  throughput is 13,200 gallons
           per hr (49.9 m3/hr).

        c.  Operating hours  required.  At  90  gallons  per minute
             „« „  T /, ,      •   „     ^ •     u       602,000  gal
            (40.8 mj/hr), required operating   hours =  IQ 800  qph
           =  56  hours  per week.   This  requires substantial
           overtime or  a  second shift.   At  110 gallons  per
           minute   (49.9  m3/hr),  required  operating  hours  =

                    gau  = 46 hours  per  week.  This reduces  the
              on   u
             13,200 gph
            amount of overtime required.

        d.   If the  dewatering  units operate at 90  gallons per
            minute  (40.8  m3/hr),  26.2  cubic yards per  day
            (20.0 m3/day)  of 22  percent  cake  is produced.
            Operation at  110 gallons per minute  (49.9  m3/hr)
            produces  33.9  cubic yards per day  (25.9 m3/day ) of
            a 17  percent solids sludge cake.

    3.  If dewatering  units are not rurt  on weekends, 86,000 gal/
        day x 2.7 days = 232,000  gallons  (878 m3 )  must be stored
        in the digesters.   Digester storage  capacity is adequate
        for normal weekends,  but not long weekends.
    4.
For 22 percent cake, 11 truckloads per week are required.
For 17 percent cake, 15 truckloads per week are required.
                               3-32

-------
In summary,  loss of the  thickener reduces  digester detention
time,  increases required  dewatering unit  operating  time  and  the
amount of trucking required for disposal of cake.   The operation
can be  managed, but with more difficulty.   This example also
illustrates  the value of  the  thickener.

Case C.   One digester  is  out of  service.   All other units  are
operating:

    1.  Digester detention time  = 24,000°;°2?, 0^ gpd =  12 d^s«
        This is only marginally adequate.  By using polymers  in
        the  thickener,   assume  waste activated  sludge  thickness
        is  increased from 3.5 to 4.5 percent.   Detention  time  is

        increased   to 24,000°|Q21,000 gpd = 14 da^s' sti11 short'
        but  an improvement.

    2.  Dewatering  operation.   This is not greatly  affected  by
        loss of the digester.   It can still  be operated  with  a
        single shift and  a 22 percent cake  can can be produced.

    3.  Weekend  storage.   Without polymer   addition   to
        the   thickener,  required storage volume  is  2.7 days
        x 51,000 gpd = 138,000  gallons (522  m3) .   One  digester
        (130,000  gallons  or 492 m3 )  has inadequate   storage
        and  a dewatering  machine must be  run part  of  the
        weekend.   If  polymer  is  used, required  storage  =  2.7
        x 45,000 =  122,000 gallons  (462  m3) .  One  digester  is
        marginally  adequate for  storage.

    4.  Eleven (11)  truckloads per week are required to transport
        the  sludge  cake.

In  summary  loss of a  digester  can  be compensated  for  by using
polymer in the thickener.

Case D.   One  dewatering  machine  is out  of service.  All other
units are available.

    1.  Digestion is not affected.

    2.  Dewatering  operation.   Try the  following alternatives:

        a.   Feed  rate 90 gallons per minute  (40.8  m3/hr).
                                            51,000  gpd     _ Q 4
            Required  operating   time   = 90 gpm  (60  min/hrr ~ 9>4
            hours  per  day,  every day, excluding  start-up  and
            shutdown time.

        b.  Feed  rate  is 110  gallons  per   minute.    Required

            operating time =     gpm° (SO^in/hr) =  7'8 hours/day,
            every day,  excluding start-up  and  shutdown time.


                               3-33

-------
        c.  Try  adding  polymers  to  thickener  and  maintaining a
            110  gallons  per  minute  feed  rate  to the dewatering
            units.   Required  operating  time = ,,A 45/OQ0 9Pd	
                                             110 gpm  (60 min/hr)
            =  6.8  hours  per day,  every  day,  excluding start-up
            and shutdown times.

    3.  Weekend  digester  storage  is not  an  issue  as dewatering
        units must  be run seven days a  week.

    4.  Eleven  (11)  truckloads  are  required to  transport
        22  percent  cake,   15  truckloads  are  required  for
        17 percent  cake.

In summary, loss of one  dewatering  unit will require operation of
the remaining unit  for seven  days a week.  overtime costs will be
high.

Case E.   Truck strike lasting a month.  Assuming 22 percent cake,
sludge,  accumulates at about 25 cubic yards  (19  m3)  a day.   The
sludge storage  area  stockpile must,  therefore, be  able  to store
about 25 (30) = 750 cubic yards (570 m3 ) of sludge to avoid major
problems due  to the  strike.   Odors  from  the  stockpile  could be
a problem.

Conclusion:   The  system as  designed  should be  able to  handle
contingencies.


3.7  Other General  Design Considerations


    3.7.1  Site Variations

Characteristics such as size  and  location  of  the plant  and
solids disposal  sites strongly influence  the  nature  and cost of
treatment and disposal systems.

     •  Disposal  may often be  accomplished  on  land,  thus
        eliminating  expensive  dewatering,  provided  adequately
        sized  sites are  within  reasonable  distances  from the
        treatment plant.   However,  dewatering is  usually required
        if  the  amount of land available  for  sludge  disposal is
        limited  or  if the sludge  must be trucked long  distances
        for  disposal.    Sufficient land  also  permits  long-term
        storage  in faculative lagoons,  which  can  also provide
        some inexpensive disinfection.

      •  Zoning regulations are  different  for  different sites.

      •  Locations   near  waterways  and  railroads  provide
        opportunities  for   barge   and  rail  transportation of
        sludges  and supplies.


                               3-34

-------
     •  Structures are  less  costly if foundation conditions are
        good.  Quite  often,  however,  wastewater  treatment plants
        are  located  in valley  bottoms,  tidelands,  or  reclaimed
        landfills where expensive foundations  are required.

     •  Costs for  labor,  electricity, freight on chemicals, and
        trucking can vary  markedly from one  region to another.

Because of these variations,  the best  alternative for one site is
often not  the  best at another site.   Also,  reported capital and
operating costs  from  one  site must be carefully  adjusted before
being used at another site.


    3.7.2  Energy Conservation

As fossil  fuel  supplies become more  scarce  and  more expensive,
energy conservation becomes  increasingly  important.  The designer
should employ energy-efficient processes and  recover energy from
sludges and sludge by-products,  where  practical.

The  following points  should be  considered  in the design of
energy-utilization processes:

     •  Energy  from  high  temperature   sources  is generally  more
        useful  than energy derived from  low temperature sources,
        since it can be put  to a wider variety of uses.

     •  The evaporation of water in dryers and furnaces, consumes
        large amounts of energy.   Such processes  should therefore
        be provided with  a we 11-dewatered sludge.   Inert
        materials  such  as,  chemicals   or ash  used  to condition
        sludge  for dewatering are,  however,  also  energy consumers.

     •  Energy  required for  digestion  and thermal conditioning is
        minimized where thickening  is used to reduce  the water
        content of process feed  sludges.

     •  Trucking energy  can  be reduced if haul distances are
        short and the sludge is  well-dewatered.

     •  Energy  is required for the manufacture and transportation
        of chemicals.   Therefore, chemicals  should  be  added in
        minimum amounts that  are  consistent with good operation.
        Whenever possible  chemicals  should  be  employed which
        require the least  energy to produce  and transport.

     •  Costs  saved  by  reducing peak  energy  demands  can be
        subtantial.    In  some  instances,  a  treatment  plant's
        electrical bills  are largely   determined by  peak energy
        loadings,  as  opposed to  total energy   consumed.   The
        designer  should actively  seek solutions to  reduce
        peak energy  demand.    Energy   recovered  from  sludge and
        sludge-derived fuel  can be used for  this  purpose.


                               3-35

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     •  Motors  should  be  accurately  sized.   Motors are  most
        efficient when  operated  near  capacity.   However,  motors
        in  wastewater  treatment  plants are  frequently  operated
        far below capacity.

     •  Where anaerobic  digester gas  is  utilized,  gas  storage
        should be provided  to minimize wastage.

     •  Recycle  loads from  solids  treatment  processes should  be
        minimized.   Recycled  loads  increase the  power and
        chemical requirements of  wastewater treatment processes.

The  designer should  always keep  in mind,  however, that  true
economy is  not  found  by minimizing  specific  uses  of  energy, but
by minimizing overall  costs.

Energy recovery  is  discussed  in Chapter  18.    Energy costs for
many of the  sludge  treatment and disposal  options  are contained
in  chapters  describing  those  options.   A 1978 publication (2)
contains  more  detailed guidance  on  making  energy-effective
analyses  as well as  a  great  deal  of  information on primary  energy
consumption, the  electricity and fuel consumed directly  at the
treatment  plant and  secondary  energy consumption,  the  energy
required  to manufacture  chemicals used in sludge treatment.


    3.7.3   Cost-Effective Analyses

One of the  decisive factors in process selection  is  cost.   Cost
analyses  must  be carried  out  so that  all alternatives are
evaluated  on an equivalent  basis.  EPA has issued  guidelines for
cost-effective analyses  (3).  Monetary costs may be calculated  in
terms of present worth values  or equivalent annual values  over a
defined planning period.  Capital  and  operating and  maintenance
costs  must be  considered   in the  evaluation.   Indirect costs
should be  included  such as  loss of property  taxes when  private
land  is  acquired  and incremental  costs which the wastewater
treatment  facility  must bear  when  sidestreams  are  returned  to
them.   Credits  for  such items  as crops  and  recovered  energy
should be taken where appropriate.   The discount rate to  be  used
in  the  analysis is established  annually  by  the Water Resources
Council.   All construction  cost data is referenced to a  specific
location  and year using cost  indices such  as the Engineering
News-Record  Construction Cost  Index,  the  EPA  Sewage  Treatment
Plant Index, or the EPA  Sewer Construction Cost  Index.  Inflation
in  costs and  wages  are  not  considered  in  the analyses,  since  it
is  assumed  all  prices will  tend  to  change  over  time  by  the  same
percentage.

Cost-effective  analysis  for   sludge treatment  and disposal
systems  has  been discussed  in somewhat  greater  detail  in a
1979  publication (4).   Present  worth  and  equivalent  annual
value  calculations  are  discussed  in  References 5 and 6, among
others.
                               3-36

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                                  TABLE 3-8

                        SOLID PROPERTIES CHECKLIST

 1.    Origin  and  type

 2.    Quantity

 3.    Concentrations

 4.    Chemical composition  and  biological properties  including
         biodegradability

 5.    Specific gravity

 6.    Rheological  properties  (e.g.,  viscosity)

 7.    Settling properties

 8.    Dewatering properties

 9.    Fuel  value

10.    Suitability  for utilization  or  disposal without  further
         processing
                                  TABLE 3-9


                         PROCESS DESIGN CHECKLIST


      1.  Description of process
           Details of works, schematic drawing, logical  location in overall
            sludge treatment flowsheet.

      2.  Process Theory

      3.  Current status
           Number of suppliers; usage  in USA; good and bad experience and potential
            for avoiding problems; advantages and disadvantages with respect to
            competing processes.

      4.  Design criteria
           Process loadings (solids and hydraulic); pilot scale investigations
             (when to make them, methods, costs, limitations); special considerations
             (solids origin).

      5.  Instrumentation specific to the process.

      6.  Operational considerations:  Flexibility.

      7.  Energy impacts
           Primary and secondary requirements.
           Potential for energy recovery.

      8.  Actual performance data and case histories.

      9.  Public health and environmental impacts.

     10.  Solids production and properties.

     11.  Sidestream production and properties.

     12.  Cost information
           Construction/operation  (tie to ENR and  EPA Construction Cost Indexes);
            constraints (site-specific).  Break down costs by category (labor,
            electricity, etc.) so that adjustments can  be made for different
            conditions.
                                     3-37

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     3.7.4   Checklists

The following checklists provide information a  designer  must
have  to design wastewater  solids treatment and disposal  systems.
Three  checklists  are provided.

     1.   A  Solids  Properties Checklist appears  in Table  3-8.   This
         checklist summarizes  required  information  concerning raw
         solids entering  the  solids  treatment  system and  solids
         produced  in the various processes and  operations.

     2.   A  Process  Design  Checklist appears  on Table  3-9.    This
         checklist describes  information necessary  to select and
         design sludge treatment and disposal processes.

     3.   A  Public  Health  and  Environmental  Impact  Checklist
         appears  in  Table  3-10.    This  checklist  summarizes  key
         interactions  that must  be resolved between  proposed
         process and the surrounding environment.
                             TABLE 3-10

                  PUBLIC HEALTH AND ENVIRONMENTAL
                          IMPACT CHECKLIST


 1.  Control of  vectors (bacteria, parasites, virus, flies,  rats)

 2.  Odor

 3.  Air pollution

 4.  Groundwater contamination

 5.  Surface water contamination (by run-off)

 6.  Soils contamination

 7.  Land use

 8.  Social-economic

 9.  Utilization (sludge or byproducts used beneficially)

10.  Occupational safety

11.  Risk of accidents involving the public

12.  Control of potentially hazardous  substances

13.  Effects on biota  including transfer and accumulation of pollutants
    in  the food chain

14.  Use of material resources
                                3-3!

-------
Designers should refer frequently  to  these
that all  relevant  topics are  given  proper
planning  stages and system  design.   An
checklists dealing  with  wastewater  solids
been prepared for EPA (4).   The checklists
as aids  for  the review  of  facility  plans
designs  and  specifications and the  writi
maintenance manuals.
                                        checklists  to assure
                                        consideration during
                                       extensive series  of
                                        management  has  also
                                       are intended to serve
                                       ,  for preparation  of
                                       ng of operations  and
3.8 References
1.  Brown and  Caldwell.   	
    IV,  Chemical  Treatment.
    Metropolitan Seattle.
    1978.
                      West Point Pilot  Plant  Study:   Volume
                          Prepared  for the  Municipality  of
                        Seattle,  Washington  98101.   December
2.  USEPA. Energy Conservation  in Municipal Wastewater Treatment.
    Office of Water Program Operations.  Washington, D.C., 20460.
    EPA 4-30/9-77-011.   March 1978.

3.  Federal Register.   "Cost-Effectiveness Analyses."  40 CFR 35-
    Appendix A.   September 1975.

4.  USEPA.  Evaluation of  Sludge Management Systems:  Evaluation
    Checklist and Supporting Commentary.   (in  draft).   Office of
    Water Program Operations.   Washington, D.C.  20460.  August 1,
    1978.
5.
Grant,  E
Economy.
1964
L.  and  Ireson, W.G.   Principles of  Engineering
 Fourth  edition.   New  York;  Ronald Publishing Co.
    Peters, M.S.  and Timmerhaus,  K.D.  Plant  Design and Economics
    for Chemical Engineers.   New  York.  McGraw-Hill Book  Co.
    1962.
                               3-39

-------
EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
         Chapter 4. Wastewater Solids Production
                      and Solids
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                           CHAPTER 4

                  WASTEWATER  SOLIDS PRODUCTION
                      AND  CHARACTERIZATION
4.1  Introduction

This chapter principally discusses the quantities and properties
of sludges produced  by  primary biological and chemical wastewater
treatment processes.   Screenings,  grit,  scum,  septage,  and
other  miscellaneous  wastewater  solids, including the  sludge
produced  in the treatment  of combined sewer  overflows,  are
discussed  briefly.


4.2  Primary Sludge

Most wastewater  treatment  plants  use primary  sedimentation  to
remove  readily  settleable solids  from raw  wastewater.   In  a
typical plant with  primary sedimentation and  a conventional
activated  sludge process for secondary treatment, the dry weight
of primary  sludge  solids  is  roughly 50 percent  of  that  for the
total  sludge solids.   For several reasons,  primary sludge  is
usually easier  to  manage   than  biological and  chemical  sludges.
First,  primary  sludge   is  readily  thickened by  gravity,  either
within a primary sedimentation tank or within a separate gravity
thickener.   In  comparison with  biological  and many chemical
sludges,  primary sludge with low conditioning requirements can be
mechanically dewatered  rapidly.   Further,  the dewatering device
will produce a drier cake and give better solids capture than it
would for  most biological and chemical sludges.


    4.2.1   Primary  Sludge Production


        4.2.1.1  Basic  Procedures  for  Estimating
                 Primary Sludge  Production

Primary sludge  production is typically within the range of 800 to
2,500 pounds per million gallons (100  to 300 mg/1) of wastewater.
A  basic approach to estimating primary sludge  production  for  a
particular plant is by  computing the quantity of total suspended
solids (TSS) entering  the primary  sedimentation tank and assuming
an efficiency  of  removal.   When  site-specific  data  are  not
available  for  influent  TSS, estimates of 0.15  to 0.24 pound per
capita per  day  (0.07  to  0.11  kg/capita/day)  are  commonly  used
(1).   Removal efficiency of TSS  in the primary sedimentation tank


                               4-1

-------
is usually in the 50 to 65 percent range  (2).   An  efficiency of
60 percent is  frequently used for estimating purposes,  subject to
the following  conditions:

     •   That  the sludge is produced  in  treatment  of a domestic
        wastewater without major industrial loads.

     •   That   the  sludge  contains  no chemical coagulants or
        flocculents.

     •   That  no other  sludges—for  example, trickling  filter
        sludge—have been added to the influent wastewater.

     •   That  the sludge contains no major  sidestreams  from sludge
        processing.

As  an  example,   if  a  designer  estimates the  TSS entering
the  primary  clarifier  as  0.20 pound per capita per  day
(0.09 kg/capita/day,  and  the  removal  efficiency  of  the clarifier
as  60  percent,  the estimated primary   sludge production is
0.12 pound per capita per day (0.054 kg/capita/day).

If relevant data are available on  influent  wastewater suspended
solids  concentrations,  such  data should,  of course, be used for
design  purposes.   Estimates  of  TSS removal efficiency  in primary
sedimentation  tanks  may be refined  by use of operating records
from in-service  tanks or  by  laboratory  testing.   The "Standard
Methods"  dry  weight  test  for settleable  matter estimates under
ideal  conditions the  amount of  sludge  produced in an  ideal
sedimentation  tank (3).  Sludge production will be  slightly lower
in actual  sedimentation tanks.
        4.2.1.2   Industrial Waste Effect

Suspended  solids removal efficiency in primary  sedimentation
depends  to a large  extent on  the nature  of the  solids.  It
is difficult to generalize about  the effect  that industrial
suspended solids can have on removal  efficiency,  but  an  example
illustrates  that  the  effect can  sometimes  be  dramatic.  At
North Kansas  City,  Missouri, a municipal plant serves residential
customers and  numerous major  industries,  including  food
processing,  paint manufacturing,  soft-drink  bottling,  paper
manufacturing,  and grain  storage  and  milling.    Raw  wastewater
entering  the  plant  had a 15-day  average  suspended  solids
concentration  of  1,140  mg/1  that  was attributable  to  the
industries.   Primary  sedimentation  removed  90 percent of  these
solids.   The quantity of primary  sludge was,  therefore,  about
8,000  pounds per  million gallons  (1,000 mg/1)  of wastewater
treated.   This value  is  several  times the  normal  one  for
domestic wastewater.    On two of  the 15 days,  removal  exceeded
14,000 pounds per million  gallons (1,700 mg/1)  (4).
                              4-2

-------
        4.2.1.3   Ground Garbage Effect

Home garbage  grinders  can significantly increase the  suspended
solids  load on  a wastewater  treatment plant.   These solids are
largely settleable.   Estimates  of the  increased primary  sludge
resulting  from the use of garbage grinders  range  from 25  percent
to over 50 percent (1,5,6).
        4.2.1.4   Other  Sludges and Sidestreams

Operating  experience  shows  clearly that the  amount of sludge
withdrawn  from  the  primary  sedimentation  tank   is  greatly
increased  when  sludge treatment process sidestreams  such as
digester  supernatant,  elutriate,  and  filtrates or centrates
and  other sludges  like  waste-activated are  recycled  to the
primary sedimentation tank.   Quantifying  the  solids  entering and
leaving the primary  clarifier by all streams  is  an important tool
for  estimating primary sludge  production when recycled  sludges
and  sludge  process   sidestreams contribute  large quantities of
solids.
        4.2.1.5   Chemical Precipitation and Coagulation

When  chemicals  are  added  to  the raw  wastewater  for  removal
of  phosphorus or  coagulation of  nonsettleable  solids,  large
quantities of chemical precipitates are  formed.   The  quantity of
chemical  solids produced  in  chemical treatment of  wastewater
depends  upon  the type  and  amount of  chemical(s)  added,
chemical constituents  in the wastewater, and performance of the
coagulation and clarification processes.   It  is difficult to
predict  accurately the quantity of chemical solids that will
be  produced.  Classical jar  tests are favored  as a means for
estimating   chemical  sludge  quantities.    The quantities  of
suspended solids  and  chemical  solids removed in  a  hypothetical
primary  sedimentation tank  that  is processing wastewater which
has been  treated by  lime,  aluminum  sulfate  or   ferric  chloride
addition are estimated in Table 4-1.
        4.2.1.6  Peak  Loads

Peak rates of primary sludge production  can  be  several  times the
average.  Peak solids production levels  also vary  from  one plant
to another.   Four studies of primary sludge  production  rates are
summarized and  presented  here.

At Ames,  Iowa,  (9)  the  wastewater  is  basically  of  domestic
origin.   A  university  contributes  about  30 percent  of  the
volumetric and mass  loads.   Storm .runoff is collected and kept
separate from  the domestic  wastewater.   For  21 years of  record,
the  suspended  solids  loads  in  the peak  month of each  year were
divided by  the  yearly  average.   The average of  these  ratios
                              4-3

-------
was  1.37.   The  average  for  comparison of  peak days and  peak
months  over ten  years  of  record  was 1.59.   Thus,  in  a  typical
year,  the  maximum daily  flow  would  be about 1.37  x 1.59,  or
2.2  times  the average.   The maximum day's sludge  production
was,  therefore,    expected  to  follow  a  similar  pattern  and was
estimated  to be 2.2 times the  average value.
                              TABLE 4-1

        PREDICTED QUANTITIES OF SUSPENDED SOLIDS AND CHEMICAL
  SOLIDS REMOVED IN A HYPOTHETICAL PRIMARY SEDIMENTATION TANK (7,8)

                                              Chemical  addition3
Sludge type
Suspended solids, Ib/mg
Chemical solids, Ib/mg
Total sludge production, Ib/mg
(kg/cu m)
No chemical
addition
1,041
-
1,041
(0. 13)
Lime
1, 562
2,082
3,644
(0.44)
Alumd
1,562
362
1,924
(0.23)
Iron6
1, 562
462
2,024
(0.24)
 Assumes 10 mg/1 influent phosphorus concentration (as P) with
 80 percent removed by chemical precipitation.

 Assumes 50 percent removal of 250 mg/1 influent TSS  in primary
 sedimentation.

°125 mg/1 Ca(OH)2 added to raise pH to 9.5.

d!54 mg/1 A12(SO4)3 • 14 H20 added.

S84 mg/1 FeCl3 added.

Note:  Assumes no recycle streams (for example, recycle of waste-activated
      sludge to primary sedimentation, digester supernatant, etc.).

      Secondary solids production would be cut from  833 Ib/mg without
      chemical addition to 312 Ib/mg with chemical addition in this
      hypothetical plant.
A study conducted  in 1936  used data from  Chicago,  Cleveland,
Columbus,  Syracuse,  Rochester,  and several  other large  American
cities (10)  to  show  a  typical relationship between peak  raw
sewage  solids  loads entering  a plant and duration of  time that
these peaks  persist.   This relationship  is shown  graphically
on Figure  4-1.   The  curve is appropriate  for  large cities with a
number  of   combined  sewers  on  flat.grades.    The  peaks  occur at
least  partly  because  solids  deposited in  the  sewers  at low flows
are flushed out by storm  flows.

Data  were  collected over a  five-year period from the  West Point
plant  at Seattle, Washington and used in a 1977  study (11).  Peak
primary  sludge loads  of  four- to ten-day  durations were compared
with  average  loads.  The  duration  of four days was  selected
because  it  appeared  to  be  highly significant  to  digester
operations  at  this  plant, and because loads  tended to  drop after
about  four days  of heavy  loading.    The highest four-day primary
                                 4-4

-------
sludge production was more than four times the normal production
from  the  plant's  service area.   Main  contributors  to  the peak
load were:

     •  Solids  deposits  in  the  sewers.  These deposits were
        resuspended  during  high  flows and  carried to  the
        treatment plant.  The  computer-operated  storage system,
        which minimizes  combined sewer overflows,  apparently
        contributed to solids deposition/reentrainment.
        Storm  inflow.   Measurements of  TSS
        fluctuate widely but often  show  over
        solids.  A  large  portion of the West
        contains combined  sewers.
                                      in storm  drainage
                                      200 mg/1 suspended
                                      Point service area
        Sludge conditioning  and  dewatering.   Problems  in  these
        processes  have  caused  the  sidestreams  to  contain more
        solids than usual.
Q
<
O
            500
            400
        Q
        LLJ
cc
LLJ
        LU
        O
        DC
        LU
        0.
            300
            200
    100
                            10
                           15
20
25
30
                        DURATION OF PEAK LOAD, days

                            FIGURE 4-1

          TYPICAL RELATIONSHIP BETWEEN PEAK SOLIDS LOADING
      AND DURATION OF PEAK FOR SOME LARGE AMERICAN CITIES (10)
The  fourth study,  done  in  1974,  discussed  two plants in
St.  Louis,  Missouri  (12).   The graphs shown  on  Figure 4-2
illustrate the variation in daily waste primary sludge production
as a fraction of the average waste primary sludge  production with
duration of  that  production rate for the eight months  that data
                               4-5

-------
were  taken.    Both  of
loads, and  both serve
sewers.
                 these plants  have  significant  industrial
                large areas of  combined storm and sanitary
     5.0
     4.0
     3.0
 5   2.0
     1.0
                  BISSELL POINT
        MAXIMUM
        MINIMUM
                              5.0 i-
                              4.0 -
                              3.0 -
                              2.0 -
                              1.0
                                              LEMAY
                                        0246

                                              CONSECUTIVE DAYS


                                        C - C RESULTS FROM
                                            THE LEAST EXTREME
                                            OF THE EIGHT MONTHS
0246

      CONSECUTIVE DAYS

KEY: A - A AVERAGE OF RESULTS
         OF EIGHT MONTHS
    B - B MOST EXTREME
         RESULTS RECORDED
         IN ALL OF THE
         EIGHT MONTHS
                             FIGURE 4-2

               PEAK SLUDGE LOADS, ST. LOUIS STUDY (12)



    4.2.2   Concentration Properties

Most  primary  sludges can  be concentrated  readily  within  the
primary  sedimentation tanks.   Several  authors claim that  a five
to  six  percent solids concentration  is attainable when  sludge is
pumped  from well-designed  primary sedimentation tanks  (2,10,13*
14).   However,  values  both  higher and  lower than the five to
                                4-6

-------
six percent range are common.  Conditions that influence  primary
sludge concentration  include:

     •  If wastewater is  not  degritted before  it  enters  the
        sedimentation tanks,  the grit may be  removed  by  passing
        the raw  primary  sludge  through cyclonic  separators.
        However,  these separators do  not function properly  with
        sludge concentrations above one percent (15).

     •  If the sludge contains  large amounts of fine  nonvolatile
        solids,  such  as silt, from storm inflow,  a concentration
        of well  over  six percent may  sometimes  be attained
        (11,16).

     •  Industrial  loads  may  strongly  affect  primary sludge
        concentration.   For  example,  at a  plant  receiving  soil
        discharged  from  a tomato  canning  operation,  a  primary
        sludge with a  17  percent  solids  concentration, of  which
        40 percent  is volatile, was  recorded.   Normal  primary
        sludge  at  this  plant  had a  solids  concentration  of
        from  five to six percent solids  (60  to 70  percent
        volatile)  (17).

     •  Primary sludge may float  when buoyed up by gas  bubbles
        generated under anaerobic  conditions.  Conditions
        favoring  gas  formation  include:    warm temperatures;
        solids deposits within sewers; strong  septic  wastes;
        long  detention times  for wastewater  solids  in  the
        sedimentation  tanks;  lack of  adequate prechlorination;
        and recirculating  sludge  liquors (18) .  To prevent  the
        septic conditions  that  favor gas  formation,  it may  be
        necessary to  strictly limit  the storage time of sludge in
        the sedimentation tanks.  This is done by increasing the
        frequency and rate of primary  sludge pumping (19).

     •  If biological  sludges  are mixed with  the  wastewater,  a
        lower primary sludge  concentration will generally result.
    4.2.3  Composition and  Characteristics


Table  4-2 lists  a number  of  primary  sludge characteristics.
In many cases, ranges and/or "typical" values are given.   In the
absence of recirculating sludge process sidestreams, the percent
of volatile  solids  in the  primary sludge  should approximate the
percent  volatile  suspended solids  in  the  influent wastewater.
A  volatile  solids  content  below  about  70  percent  usually
indicates the presence  of  storm water inflow, sludge processing
sidestreams, a  large  amount  of  grit,  sludge from  a  water
filtration plant that was  discharged  to the sanitary sewer, low
volatile solids from  industrial waste, or wastewater solids that
have a long detention time  in  the  sewers.


                               4-7

-------
                                          TABLE 4-2

                          PRIMARY SLUDGE CHARACTERISTICS
       Characteristic
PH
Volatile acids, mg/1  as ace-
  tic acid

Heating value, Btu/lb (kJ/kg)
Specific gravity of individ-
  ual  solid particles

Bulk specific  gravity  (wet)
BOD5/VSS ratio

COD/VSS ratio

Organic N/VSS ratio

Volatile content, percent by
  weight of dry  solids
 Cellulose, percent by weight
  of dry  solids

 Hemicellulose, percent by
  weight  of dry  solids

 Lignin, percent  by weight of
  dry solids

 Grease and fat,  percent by
  weight  of dry  solids

 Protein,  percent by weight
  of dry  solids

 Nitrogen, percent by weight
  of dry  solids

 Phosphorus, percent by weight
  of dry  solids
 Potash, percent by weight of
Range of values

     5-8

   200  - 2,000


 6,800  - 10,000
                                                  Typical
                                                  value
                                                                     Comments
                                                                                          Reference
   0.5 -  1.1

   1.2 -  1.6

  0.05 -  0.06

    64 -  93


    60 -  80
                                    - 15
     6-30
     7-35
    20 - 30
    22 - 28
    1.5 - 4
    0.8 - 2.£
                                                  10,285
                                                   7,600
                      1.02
                                                    1.07
                                                      65
                                                      40
                                                      40
                         10
                        3.8
                        1.6
Depends upon volatile content,
  and sludge composition,  re-
  ported values are on a dry
  weight basis.
Sludge 74 percent volatile.
Sludge 65 percent volatile.

Increases with increased grit,
  silt, etc.

Increases with sludge thickness
  and with specific gravity of
  solids.
Strong sewage from a system of
  combined storm and sanitary
  sewers.
Value  obtained with no sludge re-
  cycle, good degritting; 42
  samples, standard deviation 5.

Low value caused by severe storm
  inflow.
Low value caused by industrial
  waste.
Ether soluble
Ether extract
                               Expressed as N
                               Expressed as P205-  Divide
                                values as P2C>5  by 2.29  to
                                obtain values as P.

                               Expressed as K20.  Divide
                                values as K2
-------
the fragmented screenings appear in the primary sludge.   Smaller
plastic and rubber items  that pass  through screens also appear in
the primary sludge.

Primary  sludge  typically contains  over 100  different  anaerobic
and facultative  species of bacteria (24).   Sulfate-reducing
and oxidizing  bacteria,  worm  and  fly  eggs,  and  pathogenic
microorganisms are typically present.


4.3  Biological Sludges


    4.3.1  General Characteristics

Biological sludges  are  produced by treatment  processes  such  as
activated sludge,  trickling  filters, and  rotating biological
contactors.  Quantities  and characteristics of biological sludges
vary with  the metabolic  and  growth rates of  the various micro-
organisms  present  in  the sludge.   The quantity  and  quality  of
sludge produced by the biological process is intermediate between
that  produced  in  no-primary  systems  and  that  produced  in
full-primary systems  in  cases when  fine  screens  or  primary
sedimentation tanks  with  high  overflow  rates  are used.
Biological sludge  containing  debris such  as grit, plastics,
paper,  and fibers  will  be produced  at plants lacking  primary
treatment.   Plants with primary  sedimentation normally  produce
a  fairly  pure biological sludge.    The concentrations  and,
therefore, the  volumes   of waste  biological  sludge  are  greatly
affected  by  the  method of   operation  of  the clarifiers.
Biological sludges  are  generally more difficult  to  thicken and
dewater than primary sludge and  most chemical sludges.


    4.3.2  Activated Sludge


        4.3.2.1  Processes  Included

Activated  sludge  has numerous  variations:    extended  aeration;
oxidation ditch;  pure   oxygen, mechanical  aeration, diffused
aeration;  plug  flow;  contact stabilization, complete  mix,  step
feed,  nitrifying activated  sludge;  etc  (2).  This manual does not
discuss  lagoons  in which  algal growth is  important  or  lagoons
that tend  to accumulate  wastewater solids  or biological  solids.
These  methods, however, can be used  for predicting activated
sludge  production  in highly loaded  aerated lagoons where the
bacteria are maintained  in  solution.


        4.3.2.2  Computing  Activated Sludge Production -
                 Dry Weight Basis

The quantity  of waste-activated sludge  (WAS)  is affected by two
parameters:   the  dry  weight  of  the sludge and the concentration
of  the  sludge.   This section  describes  how the dry weight of
activated  sludge production may  be  predicted.


                               4-9

-------
Basic J?r_ed ictive Equations

The most important variables  in  predicting waste-activated sludge
production are  the  amounts  of organics removed  in  the process,
the mass of microorganisms in the system,  the biologically inert
suspended solids  in  the  influent  to the biological  process,  and
the loss of suspended solids  to  the  effluent.

These  variables  can be assembled  into two  simple and  useful
equations:


    Px = (Y)(sr)  - (kd)(M)                                  (4-1)


    WAST = Px  + INV - ET                                    (4-2)


where:

    Px   = net growth of  biological  solids (expressed as volatile
           suspended solids  [VSS]),  Ib/day or kg/day;

    Y    = gross yield coefficient,  Ib/lb or kg/kg;

    sr   = substrate  (for example,  6005)  removed,  Ib/day  or
           kg/day;

    kd   = decay coefficient,  day~l;

    M    = system  inventory of  microbial solids (VSS)  micro-
           organisms, Ib  or kg;

    WAST = waste-activated  sludge production, Ib/day  or kg/day;


     NV  = non-volatile  suspended  solids fed  to the  process,
           Ib/day or kg/day;

    ET   = effluent suspended  solids,  Ib/day or kg/day.

These equations,  as  stated or with  slight variations,  have been
widely  used.   Equation  4-1  dates  back to 1951  (25).   However,
different terms and  symbols have been used by various authors in
expressing Equations  4-1 and  4-2.   Table  4-3 summarizes some of
the  terminology  that has evolved.   The  technical literature
reflects  some  inconsistency  in  terminology  with the  term "M."
Test  results  reported  by various  authors and  presented  in
Table  4-3 were  derived on  the basis of  "M"  defined  as mixed
liquor VSS only.

To use  Equation  4-1,  it is necessary  to obtain  values  of Y and
kd.  While Table 4-4 summarizes  several  reported  values  for these
parameters, it  is best  to determine  Y and k^  on an individual
waste stream whenever possible.


                               4-10

-------
                               TABLE 4-3

           ALTERNATE NAMES AND SYMBOLS FOR EQUATION  (4-1)

      As used in this chapter
                                 Other symbols for    Other common names for
   ,  ,       „         _. •   .    similar quantities     similar quantities
 Symbol       Name       Dimensions         M                    ^

  Px   Biological solids    Mass    AX , dX/dt, A, S,  Accumulation, net growth,
         production        Time      dM/dt, Rg         excess microorganisms
                                                  production

  Y    Gross yield         Mass    a, Ks, c          Yield coefficient, synthesis
         coefficient3       Mass                      coefficient,  growth-yield
                                                  coefficient

  s    Substrate removal    Mass    dF/dt, S, B, Fi, R Food, utilization, load
   r                     Time

  kd   Decay constant        1      b, K^, Ke         Endogenous  respiration.
                         Time                      maintenance energy,
                                                  auto-oxidation

  M    Microbial solids     Mass    S, X, Xv          Microbial mass, solids under
         inventory                                  aeration, solids inventory,
                                                  mixed liquor solids



 SThe letter Y has also been  used  for the net yield coefficient Px/sr.  The net yield
  coefficient is quite different from the gross yield coefficient.
To  use  Equation  4-2,  it   is   necessary  to  estimate
non-volatile  influent  solids,  and Eip,  effluent suspended solids.
The following  are generally included within the term
      •   Non-volatile solids in  influent  sewage, including recycle
         sludge  liquors.


      •   Chemical precipitates--for example,  aluminum  phosphates--
         when alum is added to the  activated  sludge  process.

      •   Stormwater  solids that  are  not  removed  in  previous
         processes (36).


      •   Normal  non-volatile  content  of  the  activated  sludge.
         In  the  absence of  sludge  liquors, chemical precipitates,
         and stormwater,  activated  sludge will be about 80 percent
         volatile  (less  in extended  aeration)  at  most municipal
         treatment plants.

To  compute  ET,  a small  value such  as  10 mg/1  TSS should  be
used.

The  following sections  discuss  several factors that can influence
the  production  of  waste-activated  sludge.    Section  4.3.2.3  is a
detailed example of how sludge  quantities  should be computed.


                                   4-11

-------
                                      TABLE 4-4

                 VALUES OF YIELD AND DECAY COEFFICIENTS FOR
                      COMPUTING WASTE-ACTIVATED SLUDGE
Gross yield
Reference coefficient3
25 0.5
26 0.70

26, 27 0.67

28, 29 0.73

30 0.94


31 0.73

32 0.5

12 0.74


30 1.57


33 1.825



34 0.65


34 0.70


34 0.54

35 1.1

Decay
. . D
coefficient
0.055
0.04

0.06

0.075

0.14


0.06

Not calculated
(negligible)
0.04


0.07


0.20



0.043


0.048


0.014

0.09

Type of
wastewater
Primary effluent
Primary effluent

Primary effluent

Primary effluent

Primary effluent {wastewater
includes dewatering
liquors)
Primary effluent

Primary effluent (military
base)
Primary effluent (much in-
dustry)

Raw degritted including de-
watering liquors

Raw degritted



Raw degritted


Raw degritted


Raw degritted

Raw

Scale of
plant
Bench
Pilot-

Pull

Pilot

Pilot


Pilot

Pilot

Pilot


Pilot


Bench



Bench


Bench


Full

Full


Aeration
Air
Oxygen

Air

Air

Air


Oxygen

Air

Oxygen


Air


Air



Air


Air


Air

Air

Temperature,
Or-

19
Not

18

10

15


18

0

17


15


4



20


20


Not

Not

^
- 22
stated

- 27

- 16

- 20


- 22

- 7

- 25


- 20


- 20



- 21


- 21


stated

stated

Sludqe age.
days
2.8 - 22
1-4

1,2 - 8

1-12

0.5 - 8


2.5 - 17

Long

2.1 - 5


0.6 - 3


1-3



d
11 and up

d
Long

d
Long

1.1 -2.4

BOD removal
1 1 1- '

Influent
Influent minus
effluent
Influent minus
effluent
Influent minus
effluent
Influent minus
soluble ef-
fluent
Influent minus
effluent
Influent

Influent minus
soluble ef-
fluent
Influent minus
soluble ef-
fluent
Soluble in-
fluent minus
soluble ef-
fluent
Influent minus
effluent

Influent minus
effluent

Influent minus
effluent
Influent minus
effluent
3Gross yield coefficient Y, Ib (kg) VSS/lb (kg) BOD5.

 Decay coefficient k , days

CMean cell residence time or sludge age 9^, measured as mass of mixed liquor
 VSS divided by biological solids production P^. Note that coefficients may be
 somewhat different if total system inventory of VSS (mixed liquor VSS plus VSS
 in clarifiers) is used rather than just mixed liquor VSS.

 Extended aeration.

 Note: All values in this table are for an equation of the type px = Ysr - kdM (equation 4-1).
Effect of_Sludg_e  Age and  F/M Ratio

Equation   (4-1)   can  be   rearranged  to  show   the  effect  of  the
sludge age  (9m).
                   (sr)
                      r                                                           ( 4 -
                                                                                  (4
                  (kd)(em)
where Qm =  •=?- =  sludge' age,  days.
                                          4-12

-------
Similarly,  Equation 4-1 can be rearranged to show the effect of
the food-to-microorganism ratio (F/M):
       = (Y)(sr)  -
where:
    C2  = coefficient  to match  units  of  sr  and  "F"  in  F/M;  if  sr
          is 8005 removed (influent minus  effluent),  then  C2  is
          BOD5 removal  efficiency, about 0.9;

    F/M = food-to-microorganism ratio;

           BODs applied daily
          VSS (mass)  in system

As  6m  increases  and  F/M  decreases,   the biological  solids
production Px decreases.   Sludge handling is expensive, and  costs
can be  reduced by using  high values  of  Qm  or  low  values  of F/M.
However, there are offsetting cost factors, such as increases  in
the aeration tank volume needed, oxygen  requirements for the
aerobic biological system,  etc.   Also, as seasons change,  so
may the optimum  Om  and  F/M for. maximum  wastewater treatment
efficiency.   Therefore,  it  is desirable to be able  to  operate
across  a  range  of  conditions.    Obviously,  trial-and-error
calculations are required to determine the  least costly system.

Effect of Nitrif i cat ion

Nitrification  is  the  bio-oxidation of  ammonia  nitrogen and
organic nitrogen to the nitrite and nitrate  forms.   Compared with
processes  that  are designed for carbonaceous  (8005,  COD)  oxida-
tion only,  stable nitrification processes operate  at long sludge
ages  (Sm)  and low food-to-microorganism  ratios  (F/M).   Also,
nitrification  processes  are  often  preceded by other processes
that  remove much of  the 8005  and SS.   As a  result, activated
sludge  in  a  nitrification mode  generally   produces  less  waste-
activated  sludge  than conventional  activated  sludge  processes.
However,  there  is an  additional  component  to  nitrification
sludge,  the  net yield of nitrifying bacteria,  YN.  This may  be
estimated at  0.15 pounds  SS per pound of total Kjeldahl nitrogen
(organic plus ammonia)  removed  (37).  YN varies with temperature,
pH, dissolved oxygen,  and cell  residence time.   However, detailed
measurements  of  Y^  are not  ordinarily  required  for sludge
facility design  because the yield of  nitrifying bacteria  is
small.  For example,  if YN is 0.15 and if  the  nitrifying process
removes an ammonia  nitrogen  concentration of  20 mg/1 and  an
organic nitrogen concentration of  10  mg/1 then nitrification
would add  0.15 x  (20+10)  = 4.5 milligrams of nitrifying bacteria
per liter  of  wastewater  (38  pounds  per  million gallons).   These


                               4-13

-------
quantities are small compared to other sludges.   In single-stage
nitrification processes,  the sludge production  figures  must  also
include  the  solids produced from the carbonaceous oxidation,
computed at the 0^ and  F/M of  the nitrifying system.


Effect of Feed Composition


The  type of  wastewater that is  fed  to the  activated sludge
process  has  a major  influence  on  the gross  yield  (Y)  and
decay  (k^)  coefficients.    Many  industrial  wastes  contain
large  amounts of soluble BOD5  but small  amounts of suspended
or  colloidal  solids.   These  wastes  normally  have  lower Y
coefficients   than  are  obtained  with domestic  primary  effluent.
On the other hand, wastes with large amounts of solids,  relative
to BOD^,  either have higher  Y  coefficients or require adjustments
to reflect the influent inert  solids.  Even among soluble wastes,
different compositions  will  cause different yields.


Effect ofDissolved Oxygen Concentration


Various  dissolved oxygen  (DO)  levels have  been maintained  in
investigations of  activated sludge processes.   Very low  DO
concentrations—for example, 0.5 mg/1—in conventional  activated
sludge systems  do appear to  cause  increased solids production,
even  when other  factors   are  held constant  (38).   However,
there  is  vigorous disagreement  concerning solids  production  at
higher  DO levels.  Some  investigators  state  that use of  pure
oxygen  instead  of air  reduces  sludge  production.   This  is
attributed to the high  DO  levels attained through the use of  pure
oxygen  (39,40).   Other investigators  in  recent  well-controlled
investigations have concluded that if at  least  2.0 mg/1  DO  is
maintained in  air-activated sludge systems,  then  air and  oxygen
systems produce the same yield at equivalent conditions  (such  as
food-to-microorganism  ratio) (41, 42).


Efj_ect_of Temperature

The  coefficients  Y (gross  yield)  and k^  (decay)  are related  to
biological activity and, therefore, may  vary due to temperature
of  the wastewater.  This  variation has  not been well documented
in pilot  studies  and process  investigations.  One study obtained
no  significant  difference due  to temperature  over the range
39°  to 68°F  (4°to 20°C)(33).   However,  others have  observed
significant   differences within  the  same  temperature  range.
Sometimes a simple exponential ("Arrhenius") equation is used for
temperature corrections to  Y  and k^.   For instance,  it  has  been
stated that  chemical  and  biochemical rates double  with an  18°F
(10°C)  rise in temperature.  Exponential  equations have  been
found to be accurate for pure cultures of bacteria, but are quite
inaccurate when  applied to  Y  and  k^  for  the  mixed  cultures
found in real activated sludges  (43,  44).


                              4-14

-------
For the design engineer,  the  following  guidelines are recommended
until such time as process  investigations  and research efforts in
this area provide more consistent  and reliable  information:

     •  Wastewater temperatures in the range of from 59° to 72°F
        (15° to 22°C) may be considered to be a base case.  Most
        of the available data  are from this range.   Within this
        range, there  is no need to make  temperature corrections.
        Any  variations  in  process  coefficients  across  this
        temperature range are likely  to be small  in comparison to
        uncertainties caused  by other  factors.

     •  If wastewater  temperatures are in the  range of from 50°
        to 59°F  (10°  to  15°C), the same  kd  value  as  for 59° to
        75°F (15° to 22°C)  should  be  used, but  the Y value should
        be increased by 26  percent.   This  is based on experiments
        that compared systems at 52°F  (11°C)  and  70°F (21°C).  In
        these  tests,  kd was  the same,  but  Y  was 26  percent
        higher.   (On  a COD basis, Y was found  to be 0.48 at 38°F
        [11°C] and 0.38 at 56°F [21°C])  (45).

     •  If  wastewater temperatures are  below  50°F  (10°C),
        increased sludge production should  be  expected  (46), but
        the  amount  of  increase cannot  be  accurately  predicted
        from available data.   Under  such conditions,  there is  a
        need for pilot-scale  process  investigations.

     •  If wastewater  temperatures are above 72°F  (22°C) , values
        of  the  process coefficients  from the  range  59°  to 72°F
        (15°  to  22°C) may be used for  design.  The  resulting
        design may be somewhat conservative.

Effect of^Feed^Pattern

Various  feed  patterns for the activated sludge process  include
contact stabilization, step  feeding, conventional plug-flow, and
complete-mix.   For design purposes,  it appears  to  be best to
ignore the feed pattern when estimating solids  production.

Computing Peak Rate of Waste-Activatec3
Sludge Production

Peak  solids production occurs because of  unfavorable combinations
of  the  elements  in  Equations  4-1,  4-3,  and 4-4,  presented
previously:
    PX = (Y)(sr) - (kd)(M)                                  (4-1)



    PX = ____r__.                                       (4-3)



                               4-15

-------
                   (kd)(sr)
    PX = (Y)
-------
To accomplish  the  desired inventory  reduction,  solids handling
facilities must  have  the  capacity to accept  the  wasted solids.
For wastewater treatment  plants  without  major known BOD5  and SS
loading variations, allowance should be made  in designing solids
processing facilities  for  the  wasting  of  an additional  two percent
of M per  day  and lasting  up  to  two  weeks.   Such plants include
those  serving stable  domestic populations.   Industrial  loads
would be either small  or unusually stable.


For  plants  with major  seasonal  variations in  loads,  allowance
should  be made for wasting an additional  five  percent of  M per
day and lasting  for  up to two weeks.   Such  plants serve resort
areas,   college  towns,  etc.    A  similar  allowance  should be made
for  plants that practice  nitrification during  only part of
the  year.  Lastly,  for  plants with major  weekday-to-weekend
variations  of  over  2  to 1   in  8005  load, and medium or  high
food-to-microorganism  ratios   of  over  0.3  during the high loads,
allowance should be  made  for a  one-day  sludge  wasting of  up to
25 percent of  M.  The  plant should also  be  able  to handle wasting
of five percent  of M  per  day  and lasting for two weeks.  Plants
in this category  serve major industrial  systems,  large  office
complexes, schools, and ski areas.


Since  inventory  reductions are  not  generally  practiced  during
peak loading  periods,  these   above-discussed  capacity  allowances
should  be added  to average  solids production.  The maximum  rate
of waste-activated sludge production  is determined by whichever
is greater:   production during peak loading or the sum  of average
production plus inventory  reduction allowances.


Occasionally,   sludge  is wasted  in a pattern  so that M  increases
at some times and decreases at  others.   An example  of  such  a
pattern is  the withdrawal of WAS  only  during  the  daytime.   The
Tapia,   California, Water  Reclamation  Plant uses this  pattern to
obtain  good process control (51).  Use of  such  patterns will, of
course, increase the maximum  rate at which  WAS must  be  removed.


Measurements of Sludge Yield  Coeffijciejrts


Pilot  studies and full-scale  operating records  can  provide
better  data  for establishing  sludge  production design criteria
than  any general  compilation  of data  from other  locations.
Measurements of  the  sludge  yield  coefficients  are  of  two  basic
types.    First, both  the  gross  yield  Y and the decay  k^ may be
determined.   Second,  observed net yields  alone may be used.


Equations  4-1,  4-3,  and  4-4  are used when  the  food-to-
microorganism  ratio F/M and  the sludge  age,  6m, may be expected
to vary in  the prototype  plant.   To  use these equations,   it is
necessary to  determine  the  two  sludge  yield coefficients,  Y
                               4-17

-------
and k^j.   To  establish  these two
must  be  measured  under at  least
                          coefficients,
                          two different
solids  production
conditions  of F/M
and Q
     ITT
Equation 4-1  can be rearranged  slightly to Equation  4-5:
    M
                                                               (4-5)
where :
    sr/M =
         =  net growth rate  =  l/6m days"

            lb(kg)  BOD5 removed  per day
                    Ib (kg) VSS
This equation provides a basic  straight-line relationship between
PX/M  and sr/M.   For  each  condition  of operation,  PX/M  and  sr/M
are calculated and plotted,  and a  straight line is  drawn through
the points.   The slope  of  the line is  the yield coefficient (Y),
and the intercept  represents  the  decay coefficient (kd) .   (See
Figure  4-3.)
    1.00
     .80
     .60
         BOD BASIS —

      £*- = 0.67 (2L) - 0.06
      M     M
       Y = 0.67, kd = 0.06
  on  .40
     .20
    -.20
                                       COD BASIS

                                     £*. = 0.34 (-!!) - 0.06
                                     M     M

                                      Y = 0.34, kd = 0.06
                   .50
                               1.00
                                           1.50
                                                        2.00
                                                                    2.50
                             sr _ LB SUBSTRATE REMOVED/day
                             M ~
                                     LB MLVSS
                              FIGURE 4-3

                     NET GROWTH RATE CURVES (27)
                                 4-18

-------
If the design  conditions  of  sr/M or 6m are known  and  if solids
production can be  measured under  these conditions, then it is not
necessary to determine  both Y and  k^ .  Instead, a simple observed
net yield  may  be  calculated.  Equations 4-1  and  4-3  are easily
rearranged to show:
    Yobs =    = * - kd/(sr/M)  = T___                (4-6)


where :

         = ne*: yield coefficient,

             _ _ lb(kg)  VSS  produced __
           lb(kg)  substrate  (for  example, 8005) removed

Net yield  coefficients  are often reported in the literature.
They are directly  applicable  only under the  conditions  of  sr/M
and 6m that occurred during  the experiments; they are meaningless
unless  sr/M  or  6m  are measured  also.    For gathering  data  from
pilot plants  or  existing plants  for use  in establishing sludge
yield coefficients,  several  precautions  should be exercised.
Either automatic dissolved oxygen (DO)  control should  be used in
the test or ample  air or  oxygen should be provided to ensure that
the mixed  liquor  DO concentration is over  2  mg/1  at  all times.
Data from widely differing temperatures should not be  plotted on
the same graph  to  determine  Y and k^.   Instead,  data  from  each
temperature range should  be used  to  determine  Y  and k^ for  that
range.   Each  condition  of sr/M or Qm  should  be  maintained  long
enough to  obtain  stable  operation.   To assure system  stability,
a period of  time  equal   to three times the  sludge age  should
elapse between  tests.   The  designer should use  the term I^v in
Equation 4-2  to correct  the  effect  of  sidestreams.   The percent
volatile content of the solids produced  should  be recorded.  This
will be  useful in  computing  the total solids in the sludge.


        4.3.2.3   Example: Determination  of  Biological
                 Sludge Production

This  example illustrates the use  of yield  factors   and decay
factors.   Figure 4-4 shows a flow diagram for a hypothetical
plant.   The problem  is  to  prepare an  initial estimate  of the
loading  to the waste-activated  sludge thickener.    Table 4-5
contains  information  required for  this  calculation,  including
average  and maximum  day  loadings  and  activated sludge  operating
characteristics.    It is assumed that  the  thickener  in  this
example  will have  to handle the maximum-day waste-activated
sludge production.   Peak loadings of shorter  duration  than the
maximum  day production  will be  handled  by  storing   the added
suspended  solids  in the   aeration basins.   For  the purposes  of
this example, the  sludge  treatment  processes  such as  digestion,
                               4-19

-------
dewatering,  disinfection,  thermal conditioning,  and chemical
conditioning  have  not been  identified.   Depending  upon  the
selection  and design  of  the sludge  treatment  processes,  the
recycle loads  from  such processes could have a significant effect
upon  the  quantities  of  waste-activated  sludge  and  primary
sludge  that  must  be processed.     When  they are  known,  the
degradable organics  (BOD)  and non-volatile  fractions  of  the
recycle streams  should  be added to  the substrate removal  (sr)
and non-volatile suspended  solids (I^y)  factors.   Subsequent
calculations  in  Equations  4-1  and  4-2  are for the purposes of
obtaining   a  sludge mass balance,   which  includes  the effect of
recycle streams.
COMMERCIAL WASTE

^ PRELIMINARY
TREATMENT
!
GRIT
RECYCLE

PRIMARY
TATION
PRIMARY SLUDGE



\ 1
DISINFECTION
AERATION fc FINAL AND DISCHARGE
^ TANKS CLARIFIERS
RECYCLE

1
RETURN ACTIVATED SLUDGE
WASTE -ACTIVATED SLUDGE -
TO BE CALCULATED
1


THICKENED SLUDGE


%
b
_i
u_
cc
UJ
Q
z
=>
oc
LU
U_
CC
5
(J
r
HICKENER

TREATMENT
                        SLUDGE FOR REUSE
                          OR DISPOSAL.
                           FIGURE 4-4

              SCHEMATIC FOR SLUDGE QUANTITY EXAMPLE


Step 1.  Determine 8005 load  to the activated sludge process


    Average day 6005 load:
5 .0 MGD x
                   Ib/MG
                1 rag/1
                                /±  x  (1 - 0.35) = 5,150 Ib/day
                               4-20

-------
     Maximum  day  6005 load  (similar  calculation):
     9.5  MGD x 8'i4m1/;(MG x  160  mg/1  x  (1 -  0.25)  =  9,510  Ib/day
                                   TABLE 4-5

                DESIGN DATA FOR SLUDGE PRODUCTION EXAMPLE
       Description
                         Value
                                           Description
Influent flow,  mgd  (m^/day)
  Average day                5.0  (18,900)
  Maximum day                9.5  (36,000)

Influent 8005,  mg/1
  Average day                    190
  Maximum day                    160

Influent suspended solids,
  mg/1
    Average day                  240
    Maximum day                  190

8005 removal in primary
  sedimentation,  percent
    Average day                  35
    Maximum day                  25

Suspended solids removal in
  primary sedimentation
    Average day                  65
    Maximum day                  50
 Ib (kg) BOD5 applied daily
  Ib(kg)mixed liquor VSS

 Data  from other plants must be  used.

1 mgd  =  3,785 m /day

Note:  Maximum day  influent BOD,- and suspended solids concentrations
      reflect a dilution from average day data due to the  higher
      flow.
Value
                                    Sludge  thickener capture
                                      efficiency
                                        Average, percent              95
                                        Maximum day, percent          85

                                    Food-to-microorganism
                                      ratio3
                                        Average                     0.3
                                        Maximum                     0.5

                                    Temperature of wastewater
                                      Average, degrees  F
                                        (degrees C)               65 (18)
                                      Minimum, degrees  F
                                        (degrees C)               50 (10)

                                    Dissolved oxygen in aera-
                                      tion  tanks
                                        Average, mg/1               2 . 5
                                        Minimum, mg/1               2.0
                                        Control:  automatic

                                    Effluent limitations,  30-
                                      day average
                                        BOD5, mg/1                   30
                                        Suspended solids,  mg/1        30

                                    Usable  test data for               ,
                                      solids production            None
Step  2.   Determine M,  the  mass  of  microorganisms

                         8005 applied/day
     Average:   F/M  =
                           VSS  in system
                                         =  0.3
M =
                  = 17,170 pounds  VSS
                                       4-21

-------
    Maximum day:   F/M = 0.5


    M = 9/.5c°  =  19,020 pounds VSS
         U • D

Step  3.    Determine Y,  the  gross  yield coefficient,  and k^ ,
the  decay coefficient.   No test  data are available for this
waste,  so estimates must be made from tests  on other wastes.
For average conditions, use Los Angeles data from Table 4-4  (27):
Y =  0.67 pound  (kg)  VSS formed  per pound (kg)  6005 removed;
kd = 0.06 day  -1.

For maximum conditions,  use minimum  temperature  of  36°F  (10°C),
which  produces  the maximum Y  value.  Use  the correction from
Section 4.3.2.2,  which increases Y by 26 percent.


    Ymax = 0-67  x  1.26 =  0.84; do not adjust k^


Step 4.   Determine  sr  (substrate  removal)  in  units to match Y.

    Average daily  substrate removal:

    BOD5 applied                        5,150 Ib/day
    Effluent BOD5  (assume 10 mg/1*      - 420 Ib/day
      BOD5 in  effluent)                 4,730 Ib BOD5 removed/day

    Maximum daily  substrate removal:

    BOD5 applied                        9,510 Ib/day
    Effluent BOD5  (assume 10 mg/1*      - 790 Ib/day
      BOD5 in  effluent)                 8,720 lb/BOD5 removed/day

Step 5.   Determine Px,  the  biological solids production.   Use
Equation 4-1 from  4.3.2.2:


    PX = (Y)(sr)  - (kd)(M)

    Average :

         Ib VSS  produced  , 7^n jj BOD5 removed
    n ,_
    U'b/ Ib BOD5 removed    '          day

    - (0.06 day-1)  (17,170  Ib VSS) = 2,140 Ib VSS produced/day
*Allow  10 mg/1  for effluent  BOD5,  even though  the plant  is
 permitted to  discharge  30 mg/1.   Activated sludge  plants  can
 often attain  10 mg/1  effluent  BOD5.   Sludge capacity should  be
 provided for the sludge  produced under such conditions.
                              4-22

-------
    Maximum day, similar calculation:


    (0 .84) (8,720) - (0 .06) (19,020) = 6,184 Ib VSS produced/day


Step  6.   Compute  INv-  (non-volatile suspended solids  fed  to the
activated sludge process).

    Average daily input of non-volatile suspended solids:



    5.0 MGD x 8 'i^1/!^ x 24° m<3/1 x (1 ~ 0.65)(0.25*)



    = 880 Ib/day


    Maximum daily input of non-volatile suspended solids:



    9.5 MGD x B'i4m1/{MG x 190 mg/1 x (1 - 0.50)(0.25*)



    = 1,800 Ib/day


Step  7.  Compute Erj, (effluent suspended solids).

    Average :
    5.0 MGD x -'i"     * 10 mg/1 = 420 Ib/day



    Maximum day:



    9.5 MGD x —         x 10 mg/1 = 790 Ib/day
Step 8.  Compute waste-activated sludge  (WAST) production

    From Equation (4-2);


    WAST = Px + INV ~ ET


    WAST = 2,140 + 880 - 420 = 2,600 Ib TSS/day
                               (1,180 kg/day)
                               4-23

-------
Maximum day:


    WAST 6,184 +  1,880  -  790 =  7,274 Ib TSS/day
                               (3,302 kg/day)


Step 9.  Compute  inventory  reduction allowance.

    Inventory reduction allowance = ( 0.02) (17,170) = 343 Ib/day
                                                   (156 kg/day)

In  the  present case,  the  inventory  reduction allowance  can  be
small.  Allow two percent of M per day.   The 343 Ib/day computed
here is much smaller than  the difference between the average and
maximum waste-activated sludge  production  (Step 8); therefore,  if
capacity is  provided  for  maximum solids  production,  then there
will  be ample capacity for  inventory  reduction.   It  is  not
necessary to reduce inventory during peak  loads.
        4.3.2.4  Interaction of  Yield Calculations and
                 the Quantitative  Flow Diagram  (QFD)

The  example just presented  demonstrates  a technique  for
calculating solids  production on  a  once-through basis;  that is,
any solids associated with recycle streams were  not considered in
the  calculation.   The  QFD considers  the  effects of  recycle
streams.   Before the QFD can be  constructed for biological
treatment  processes,  an   estimate of  net solids  destruction or
synthesis  must  first be  made.  The relationship between solids
entering and  leaving  the biological unit is established via the
parameter XD, which is  defined  as net  solids destruction per
unit  of solids  entering the  biological unit.   The  data and
calculations  from  the previous design example  allow an initial
estimate of XD to be made.

For the average flow:

    1.  Solids  leaving  the biological unit  =  Px +  INy =   2,140
        +  880 = 3,020 pounds per day '

    2.  Solids entering  the biological unit  are equal to solids
        in the primary effluent, which  can be calculated  from the
        data  on Table 4-4.   Primary effluent solids  = (1 -  0.65)
        (240) (8.34) (5.0)  = 3,503 pounds per  day.

     3.  Net  solids  destruction  =  solids  in - solids out =  3,503
        -  3,020 = 483 pounds per day (219 kg/day).
              483
             3,503
XD = ^m = 0.138
                               4-24

-------
For maximum day flows:

    1.  Solids  leaving  the biological unit  = 6,184  + 1,880
        = 8,064 pounds  per  day  (3,661 kg/day).

    2.  Solids entering  the biological unit =  (1 -  0.50)(190)
        (8.34)(9.5)  =  7,527 pounds per  day (3,147 kg/day).

    3.  Net solids destruction = 8,064  - 7,527  =  537  pounds  per
        day (244 kg/day).


    4.  XD    = - 1*1  = 0.07
         umax   7,527

Once XD  is  known,  the  QFD  calculation  can be undertaken.   After
the QFD  calculation is  completed, the  designer  may wish  to  make
new estimates  of  Px and I^y  based  on information derived  from
the QFD  calculation.   For  example,  if  the  QFD  calculation shows
that recycle loads are  substantial, then the designer  may wish to
modify estimates  of  sr and  I^v  anc^  calculate  new values of  Px
and INV,  as indicated  in Section  3.4.
        4.3.2.5  Concentration of Waste-Activated Sludge

The  volume  of  sludge produced  by  the  process  is directly
proportional to the dry weight and inversely proportional  to the
thickness or  solids concentration in  the waste  sludge  stream.
Values  for  waste-activated sludge concentration can vary,  in
practice, across  a  range  from 1,000  to 30,000 mg/1  SS  (0.1  to
3 percent SS) .

An  important variable  that can affect waste-activated  sludge
concentration is  the  method of  sludge wasting.   A number  of
different methods are  illustrated in  Figure  4-5.   Sludge  solids
may be  wasted  from  the  clarifier  underflow.   It has been  argued
that wasting solids from the mixed liquor should improve  control
of  the  process (2,35).   In this  case,  waste  sludge  is  removed
from the  activated  sludge  process at the  same concentration  as
the mixed liquor suspended solids, about 0.1 to 0.4 percent.
This low concentration can be  a disadvantage because a  large
volume  of mixed liquor must be removed to obtain a given  wastage
on  a dry weight basis.   The most common  arrangement involves
sludge  wasting   from   the  clarifier  underflow,  because  the
concentration of  sludge there  is higher  than  in  the mixed  liquor.
Subsequent  discussions in  this  section  are  based  on  sludge
wasting from the  clarifier  underflow.

Estimating Waste-Activated  Sludge  Concentration

The  two  primary factors  that affect  waste-activated  sludge
concentration are the settleability  of the sludge and  the solids
loading rate to  the sedimentation tank.   These two factors have


                              4-25

-------
    (a). WASTING FROM CLARIFIER UNDERFLOW
                                                         (b). WASTING FROM REAERATION TANK
FEED
 Z
 a:
 uu
       Ai RATION

         TANK
                                                    FEED
                     WASTE SLUDGE
                                                          WASTE SLUDGE
                                  (c). WASTING BY BATCH SETTLING
               DURING FEED:
   FEED
                       NO EFFLUENT

                       NO SLUDGE REMOVAL
NO FEED
                                                                DURING WITHDRAWAL:
TANK NOT
AERATED,
OPERATED
AS BATCH
CLARIFIER
*
WASTE SLUDGE
PROCESS EFFLUENT^

                                 (d). WASTING FROM MIXED LIQUOR
FEED 	 ^
RETURN ACTIVATED
I SLUDGE 1
1 T '
AERATION TANK
r i
1 REAERATION TANK |
MIXED LIQUOR _£, .„,,_,,_ J\. PROCESS EFFLUENT __

] (IF USED) j ^
I 	 1
1
EC
LU
LL
E
5
o
!
O
LL
oc
LU
Q
Z
D
                                         WASTE SLUDGE
                                         FIGURE 4-5


                               SLUDGE WASTING METHODS
                                             4-26

-------
been  considered  in  detail  in  the development of  solids flux
procedures for predicting  the clarifier underflow concentration
of activated  sludge (52).


Factors Af_f_ec ting__Unde_rf_low_ C o n c en t r ation

Various   factors   that affect   sludge   settleability  and  the
clarifier sludge  loading rate include:

     •  Biological  characteristics  of  the  sludge.    These
        characteristics may  be  partially  controlled  by  mainte-
        nance  of  a particular mean  sludge  age  or F/M.   High
        concentrations  of   filamentous  organisms  can  sometimes
        occur in  activated  sludge.  Reduction of these organisms
        through sludge  age  or F/M control  helps to produce more
        concentrated  clarifier underflow.

     •  Temperature.   As wastewater temperatures are  reduced, the
        maximum attainable  clarifier underflow sludge concentra-
        tion  (cu) is  also  reduced   as a result  of  increased
        water density.  Also, temperature  can affect the setting
        properties of the  sludge.

     •  Solids flux.   The  solids  flux  is the  solids load from the
        mixed liquor divided  by the clarifier area (for example,
        pounds per day  per  square  foot).   Higher rates of  solids
        flux require that clarifiers be operated at  lower  solids
        concentration.

     •  Limits of sludge  collection  equipment.   Because  of the
        pseudo-plastic and  viscous  nature  of waste-activated
        sludge, some  of the available  sludge  collectors and pumps
        are  not  capable of  smooth,  reliable operation  when cu
        exceeds about 5,000 mg/1.

     •  Heavy suspended solids in the  sludge.  If raw wastewater,
        instead of primary sedimentation tank effluent, is  fed to
        the  activated  sludge process,  higher  cu  values usually
        result.  Chemicals added  to the  wastewater  for phosphorus
        and  suspended  solids removal  may similarly  affect cu.
        However,   such  additional   solids will  also increase the
        solids load to the clarifiers.
        4.3.2.6  Other Properties of Activated  Sludge

Table 4-6  contains  several  reported  measurements  of the
composition and properties of activated sludge  solids.   Comparing
Table  4-6  with that of  Table  4-2 for primary  sludge,  activated
sludge contains  higher  amounts of  nitrogen, phosphorus, and
protein;  the  grease,  fats,  and  cellulose amounts, and  specific
gravity are lower.


                               4-27

-------
                                           TABLE 4-6

                         ACTIVATED SLUDGE CHARACTERISTICS


pH
Heating value, Btu/lb (kJ/kg)
Range of

6.5 - 8


6,
(15,


5.5
540
200)


Can be less in high puritv oxygen
systems or if anaerobic decom-
position begins.
Baltimore , Maryland
Increases with percent volatile
content


53, 54
55
56
 Specific gravity of individ-
   ual solid particles

 Bulk specific gravity


 Color




 COD/VSS ratio

 Carbon/nitrogen ratio
 Organic carbon,  percent by
   weight of dry  solids

 Nitrogen, percent by weight
   of dry solids  (expressed
   as N)
 17 - 41
 23 - 44
4.7 - 6.7

2.4 - 5.0
 Phosphorus, percent by weight  3.0 -  3.7
  "of dry solids  as P2°5
   (divide by 2.29 to obtain    2.8 -  11
  phosphorus as  P)

 Potassium, percent by weight   0.5 -  0.7
  of dry solids  as I^O
   (divide by 1.20 to obtain
  potassium as K)

 Volatile solids, percent by     61 -  75
  weight of dry  solids (per-
  cent ash is 100 minus        62 -  75
  percent volatile)            59 -  70
 Volatile solids (continued)
 Grease and fat,  percent by       5-12
   weight of dry  solids

 Cellulose, percent by weight
   of dry solids

 Protein, percent by weight      32 -  41
   of dry solids
                                                1.08
            1.0 + 7 x 10"
                    2. 17

                    12.9
                     6.6
                    14 .6
                     5.7
                     3.5
5.6

6.0


7.0

4.0
                    0.56
                    0.41
                     63


                     76


                     88
                         x C   C is  suspended solids concentra-
                                tion, in mg/1.

                              Some  grayish sludge has  been
                                noted.  Activated sludge becomes
                                black upon anaerobic decomposi-
                                tion.
Baltimore,  Maryland
Jasper, Indiana
Richmond, Indiana
Southwest plant, Chicago,  Illinois
Milwaukee,  Wisconsin  (heat dried)

Zurich, Switzerland
Four plants

Zurich, Switzerland
Chicago, Illinois
Four plants
Milwaukee,  Wisconsin

Zurich, Switzerland
Chicago, Illinois
Pour plants
Milwaukee,  Wisconsin

Zurich, Switzerland
Chicago, Illinois
Milwaukee,  Wisconsin
                              Zurich, Switzerland
         Four plants
         Renton, Washington (Seattle Metro),
           1976 average

         San Ramon, California  (Valley Com-
           munity Services District), 1975
           average

         Central plant, Sacramento County,
           California, July 1977 - June
           1978 average

         Ether extract
                              Includes lignin
                                                                 57
58

55
55
55
55
55

28
55

28
59
55
59

28
59
55
59

28
59
59
                                            28
                                            58
                                            60
                                            55
                                                                 60


                                                                 61
Several  types  of  microorganisms  are  present  in  large  numbers
in  activated   sludge.     Floe-forming  (zoogleal)   bacteria   include
species  of  Zoogloea,  Pseudomonas,  Arthrobacter,   and  Alcaligenes.
                                               4-28

-------
Activated sludge also contains  filamentous microorganisms such as
Sphaerotilus, Thiothrix, Bacillus,  and  Beggiatoa  (62).   Various
protozoa are present, including ciliates and flagellates.


    4.3.3  Trickling Filters

Trickling   filters   are  widely  used  in  municipal wastewater
treatment.    This  section  covers  trickling  filters  that are used
with clarifiers.   When a  clarifier is not  used,  the  trickling
filter effluent  is  usually fed to  an  activated  sludge process.
Refer to Section 4.3.5 for  such combinations.
        4.3.3.1  Computing Trickling  Filter Sludge
                 Production - Dry  Weight Basis

Trickling filter microorganisms are  biochemically similar to
microorganisms that  predominate in activated sludge  systems.
Consequently, solids  production  from trickling filters  and from
activated sludge systems is roughly  similar when compared on
the  basis  of pounds  of  solids  produced per  pound  of  substrate
removed.  There are  differences  between the two systems, however,
with respect to solids production prediction methodology and the
pattern of  sludge wasting.    Attempts  have  been made  to develop
solids  production  models  consistent with  biological  theory
(47,63,64).   However, presently  (1979),  empirical methods  are
usually used for design purposes.  Table  4-7 presents  sludge
yields  observed  at  several  treatment  plants  and  from  one
long-term pilot study. These data are  primarily based on heavily
loaded  filters.

Equations  that relate  the production of  suspended material in a
trickling  filter can be developed in a form similar to that used
in predicting  activated  sludge  production.   The main difference
lies  in the term used to define  the quantity of microorganisms
in  the  system.    In  long-term  studies  of  trickling  filter
performance, Merrill  (64)  assumed that the  total  mass  of micro-
organisms  present  in the  system  was  proportional  to  the media
surface area.   The  resulting  equation  for volatile  solids
production was:


    Px  = Y1(sr)-Ka(Am)                                     (4-7)


where:

    Px  = net growth of biological  solids  (VSS), pounds per day or
         kg per day;

    Y1  = gross yield coefficient,  pound per pound or kg/kg;

    k<3  = decay coefficient,  day"-'-;


                               4-29

-------
      Sv  =
       substrate   (for  example,   BOD5)   removed,  pounds  per   day
       or  kg/day  =  BOD5  in  minus  soluble effluent BOD5;
          =  total  media  surface  area  in  reactor,   square  feet  or
             sq m.
                                         TABLE 4-7


                       TRICKLING FILTER SOLIDS PRODUCTION


                              Unit solids production3


Plant
Stockton, California
Average of 13 months
Highest month


Lowest month

Sacramento, California
9 rioncanning months
Average
Highest month
3 canning months
Average
Dallas, Texas
Dallas, Texas
Livermore, California
San Pablo, California
Seattle , Washington-^
Total
BOD5b
basis

0.74
1.01
(5/76)

0.49
(1/77)


-
-

-
0.42
0.65.
i.io1
-
IT-ES IT-ES
BOD5o COD
basis basis

0.67 0.43
0.92 0.60
(5/76, (7/76)
7/76)
0.48 0.30
(1/77) (1/77)


-
-

-
-
_
_
0.8-0.9

SS
basis

1.00
1.17
(6/76,
1/77)
0.61
(3/76)


1.01
1.09

1.20
_
-
1.39
1.39
1.0

vss
basis

0.94
1.08
(10/76)

0.60
(3/77)


1.00
1.09

1.24
_
-
1.51
-

Solids percent
volatile

77
86
(8/76, 11/76)

64
(3/76, 6/76)


78
B3

76
_
-
84
'-

BOD5
Ioad9 Media Reference
Plastic, 27 ft2/ft3 65
27
73
(8/76)

15
16/76)
Plastic 66

_
-

-
Rock 67
Rock 67
57 Rock 2 to 4 in. 68
199 Plastic, 29 ft2/ft 37
30-250 Plastic, various 64
  Solids production includes both waste sludge (clarifier underflow) and clarifier effluent solids.

  Pounds volatile suspended solids (VSS) per pound 3005 removed {same as kg/kg). BOD- removal based
  on total (suspended plus dissolved) measurements.

  Pounds VSS per pound 6005 removed. BOD5 removal based on influent total minus effluent soluble (IT-ES)
  measurements.

  Pounds VSS per pound chemical oxygen demand (COD) removed.  COD removal based on influent total
  minus effluent soluble measurements.

  Pounds total suspended solids (SS) produced per pounds SS applied.

  Pounds VSS produced per pound VSS applied.

  Pounds total 6005 applied per day per 1,000 cubic feet of media.

  Stockton and Sacramento plants have heavy industrial loads about August to October from fruit and
  vegetable canneries.

  Roughing filter.  For BODc, basis, BOD^ removal was computed by 6005,^ minus (0.5 times unsettled
  BOD5(OUt).  1971 average data.

  Pilot studies.  SS basis was found to describe data well over a wide range of loadings.  Wastewater
  included some industrial load and recycle liquors from dewatering digested sludqe.
The   production  of   trickling   filter   sludge  requiring  subsequent
sludge  handling  may  be  expressed:
WTFS =  Px + INV  -  ET
                                                                                       (4-8)
where:

      WTFS
          waste  trickling  filter  sludge  production,   pounds   per
          day  or  kg/day;
                                             4-30

-------
    INV  = non-volatile  suspended solids  fed to  the  process,
           pounds  per day or kg/day;

    ET   = effluent suspended solids,  pounds per day or  kg/day.

The coefficients Y'  and k^  for Equation  4-7  are  obtained for
a particular  system  by  computing the  slope  and  intercept of a
                                                 P      s
line of best fit through plotted data points for  j-*- vs  •=-£.   VSS
                                                 "m    "m
production data for three different trickling filter media  designs
are given on Figure 4-6.

Nitrification  in trickling  filters  causes  a  synthesis  of
nitrifying bacteria.   As  in  activated  sludge,  however,  the
quantity  is small.   A value of 25 pounds per million gallons
(3  mg/1)  has  been  suggested  for  design purposes (67).   This
quantity  must  be added to  the other  solids produced  by the
trickling filter.

It  is  known  that temperature  and  loading  rate affect sludge
production:   "The  quantity of  excess sludge  produced  in a
low-rate  trickling filter  is much  lower than  that  reported for
high-rate filters  or  for  the  activated sludge process.   The lower
rate of  solids accumulation  may be attributable to the grazing
activities of  protozoa.   The activity of the  protozoa  is reduced
considerably at low temperatures (47)."  However,  there are few
data to quantify these variations.

Peak sludge  loads are produced  by  trickling'  filters.   These may
be  due  to variations  in influent load, rapid  climatic  changes,
and/or biochemical factors  that cause unusually large  amounts of
biomass to peel off  from  the  media.  The term "sloughing" is used
by  some  authorities  to include  steady state  as  well as peak
solids  discharges.    Others restrict the term  "sloughing" to
unusually large discharges.  In any case,  peak solids  loads must
be  considered.   Table  4-8  shows  some variations due to both
unusual biomass  discharges  and to variations  in influent  load.
Table 4-9,  on  the other hand, shows the biomass discharge  alone.
Each of the three events  in Table 4-9  "occurred during  periods of
light  organic loadings  (30  to 50 pounds  BOD5 per 1,000 cubic
feet per  day  [0.49  to 0.81  kg/m3/day] ) which  had  been preceded
by  periods   in  which  exceptionally heavy organic  loadings
(215 to  235 pounds  BOD5 per 1,000 cubic  feet per  day  [3.48 to
3.81 kg/m3/day]) had been applied on a sustained basis  (4-14
days)"  (64).  Table 4-9 shows that  effluent solids  were much
greater  than  influent  solids.   This is  quite  different from
average conditions,  under which effluent solids were about equal
to  the influent solids.

In  low-rate  filters  especially, there are seasonal  variations in
solids production.   "Slime  tends  to  accumulate in  the  trickling
filter  during  winter operation and  the  filter tends   to  unload
the slime  in the  spring  when the activity  of the microorganisms
is  once again  increased"  (47).


                               4-31

-------
Q
HI
w  ->.
Q  £

2  sr
o --.
O %
o >
 LU
 O
       4

       3

       2

        1

       0

       5
Q  **
O  o>
cc  ^
.§    4
tO  T
>
        3

        2

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         1
                                                            MEDIA TYPE - PLASTIC SHEET
                                                            MEDIA SURFACE DENSITY - 27 sq ft/cu ft
                                                            MEDIA DEPTH - 22 ft
                    Y' = 0.80
                    k'd = 0.03
                                                            MEDIA TYPE - PLASTIC SHEET
                                                            MEDIA SURFACE DENSITY - 27 sq ft/cu ft
                                                            MEDIA DEPTH - 11  ft
                    Y' = 0.89
                    k 'd - 0.32
                                                                  MEDIA TYPE - PLASTIC SURFACE
                                                                  MEDIA SURFACE DENSITY -
                                                                     4 ft - 25 iq ft/cu ft
                                                                     4 ft - 31 iq ft/cu ft
                                                                     4 ft - 37 sq ft/cu ft
                                                                     4 ft - 40 sq ft/cu ft
                                                                     5 ft - 43 sq ft/cu ft
                                                                  MEDIA DEPTH - 21 ft
                   ORGANIC  REMOVAL, POUNDS BOD5  REMOVED  / 1000 sq ft/day
                      (1.00  Ib BODB / 1000 sq ft/day =  4.88 kg BOD6  / 1000 m2/day)
 (  1.00 ft = 0.30m )
 (  1.00 sq ft /cu ft = 3.28 m2/m3 )
                                          FIGURE 4-6
                     VSS PRODUCTION DATA FOR THREE TRICKLING
                                     MEDIA DESIGNS  (64)
                                              4-32

-------
                                  TABLE 4-8


             DAILY VARIATIONS IN TRICKLING FILTER EFFLUENT,

                        STOCKTON, CALIFORNIA (65)


                                                                       Five
                       Number of      Average TSS ,       Coefficient,      percent
       Period           samples3          mg/1         of variation       ratioc


March-July 1976      ,        57             144              0.28           1.5
August-September 1976         26             187              0.33           1.6
November 1976 - March
  1977                      51             149              0.31           1.7
 Samples are  trickling filter effluent (before sedimentation),
 total suspended solids, 24-hour refrigerated composites.  Flow
 variations within each sample population were small;  that is,
 ratios in this table represent mass variations as well as con-
 centration variations.

 Standard deviation divided by average.
Q
 Ratio of individual sample concentration to average concentration
 that is exceeded by 5 percent of the samples.

 Heavy industrial load in August and September from fruit and
 vegetable canneries.
                                 TABLE 4-9


                  DESCRIPTION OF SLOUGHING EVENTS  (65)


Period
October 22-26, 1976
August 5-6, 1977
July 31-August 5,
1977


days
5
2

6
Suspended solids,
mg/1

Influent
114
132

147

Effluent
256
289

222
Flow, gpm/sq ft

Influent
0.44
0.63

0.63

Recycle
2.06
1.56

1.56
Appliedc
loading,

cu ft/day
33
50

50
Media

sq ft/cu ft
"d
27a

Graded6
^Influent wastewater flow divided by plan area of filter.

 Recycle flow (from trickling filter effluent) divided by plan
 area of filter.

 Based on influent flow.
 Plastic sheet media, 22 ft deep.

ePlastic sheet media, 22 ft deep; specific surface ranged from
 25 sq ft/cu ft at the top of the filter to 43 sq ft/cu ft at
 the bottom.
               T    2
 1 gpm/sq ft = 2.46 mj/hr/m
 1 Ib BOD5/1,000 cu ft/day = 0.0162 kg/mVday




The  amount  of  solids  requiring  sludge  treatment  depends  on

sedimentation   performance,  which  is  usually  50  to  90  percent

removal  of  suspended  solids.    Sedimentation  performance  is

improved  by  careful  design,  light  loads,  tube  settlers,  and

coagulation and flocculation  (19,64).




          4.3.3.2  Concentration of  Trickling  Filter  Sludge


Trickling   filter  sludge  loadings  on  the  secondary  sedimentation

tank  are typically low—5  to  10 percent  of observed solids  loads




                                     4-33

-------
to activated sludge  sedimentation tanks.   Trickling filter sludge
also  has better  thickening properties  than  activated  sludge.
Consequently,  trickling  filter sludge can  be withdrawn at a much
higher concentration than waste-activated  sludge.   Concentration
data are summarized  in Table 4-10.
                            TABLE 4-10

             CONCENTRATION OF TRICKLING FILTER SLUDGE
                  WITHDRAWN FROM FINAL CLARIFIERS
     Type of sludge
  Trickling filter,
   alone
 Trickling filter, com-
   bined with raw primary
Percent dry
  solids
         Comments
                                                           Reference
  5-10
    7
    7
    3
  3-4
  4-7

  3-6
Depends on solids residence time
  in trickling filter
Low-rate trickling filter
High-rate trickling filter
 69
 13
 70
 70
 71
  2

2,69
The solids flux method  for predicting  sludge  concentration may be
used  with trickling  filter  sludge  (52).   This method  requires
measurement  of initial solids  settling  velocity  versus  solids
concentration.   Such  relationships  have been  reported  for at
least one trickling filter process  (64).
        4.3.3.3  Properties -  Trickling  Filter Sludge

Table  4-11 contains  a few  analyses of  trickling  filter  sludge
properties.   The microbial population  that inhabits  a trickling
filter  is  complex  and  includes  many species  of  algae,
fungi,  protozoa,  worms,  snails,  and insects.   Filter
their  larvae  are often present in large  numbers  around
filters.
                                    bacteria,
                                    flies  and
                                    trickling
    4.3.4  Sludge from Rotating  Biological  Reactors

Rotating  biological  reactors  (RBRs)  are  used for  the  same basic
purposes  as  activated sludge  and trickling  filters:    to remove
BODc;  and suspended  solids and,  where  necessary,  to nitrify.
The  RBR  process uses  a  tank  in  which  wastewater,   typically
primary  effluent,  contacts  plastic  media  in  the shape of large
discs.   Bacteria grow on  the  discs.   The discs  rotate slowly on
horizontal shafts;  the  bacteria are  alternately  submerged in the
wastewater and  exposed to  air.   Excess bacteria  slough from the
discs  into  the  wastewater.    After  contacting the bacteria,  the
wastewater  flows  to a  sedimentation tank, where  the  excess
bacteria  and  other  wastewater solids  are  removed.   These  removed
                                4-34

-------
solids are RBR  sludge.   RBR sludge is roughly  similar in quantity
by  dry  weight,  nutrient  content, and  other characteristics,  to
trickling filter sludge.

                             TABLE 4-11

                 TRICKLING FILTER SLUDGE COMPOSITION
         Property

 Volatile content, percent of
   total solids

 Nitrogen, percent of total
   solids
 Phosphorus as P2C>5,  percent
   of total solids

 Fats, percent of total solids

 Grease, percent of total
   solids

 Specific gravity of individ-
   ual solid particles

 Bulk specific gravity (wet)
 Color
   Value

   64 - 86


  1.5 -  5

    2.9
    2.0

    2.8
    1.2

      6

    0.03
    1.52
    1.33

    1.02
   1.025

Grayish brown
Black
                                             Comments
                                                            Reference
See Table 4-7
Depends on length of storage
  of sludge in filter.
Ether soluble.

Test slime grown in primary
  effluent.
69

71
13

71
13

13

72
                          73
                           2

                          13
                           2

                          13
                          64
A  small  body  of published  data  is  available  on  RBR  sludge
production  rate from  full-scale municipal installations.   At
Peewaukee,  Wisconsin, total  suspended solids production has been
reported to be 0.62  to 0.82  pounds of  total suspended  solids
per  pound  BOD5  (0.62  to  0.82 kg  TSS/kg)  removed.   The  final
sedimentation tank removed 70  to 83 percent  of these  solids as
sludge.   The biological sludge alone  had a concentration  of 1.5
to  5.0 percent solids.    Other investigations  of municipal  and
industrial  waste applications  have  concluded that  sludge produc-
tion  for the RBR process  amounts  to  0.4 to 0.5  pound  of  total
suspended  solids per  pound of  BOD5 (0.4  to 0.5 kg  TSS/kg  BOD5)
removed  (74,75,76).


    4.3.5   Coupled  Attached-Suspended Growth Sludges

There  are several installations of  coupled attached and suspended
growth processes in  the United  States.   These  dual processes
are  usually  installed  where  nitrification  is  required  or where
strong  wastes must  be  treated.   The  attached  growth  reactor is
a  trickling  filter  or  a  rotating  biological  reactor.   Its role
is  to reduce  the  load  on the suspended growth  process.   The
suspended  growth  process uses  an aeration  tank  and a  final
clarifier.   Flow  recirculation  is  usually  practiced  around
the  attached growth  reactor.   Several  reports  describe  these
                                4-35

-------
processes  and note that the  sludge  is  similar  to activated
sludge, both in quantity  and in characteristics (5,67,68,77,78).
The  sludge  characterized  in  Table 4-12  contains  some  particles
of  dense  solids  from the  attached  growth  reactor.    These
particles may improve the  thickening  characteristics  of the
sludge (78).
                             TABLE 4-12


     SLUDGE FROM COMBINED ATTACHED-SUSPENDED GROWTH PROCESSES


                                                      Primary sludge mixed
                                                     with biological sludge
                                  Solids production          ™	~~"   *	™	" """"
                                  Ib TSS produced/   Percent    Percent   Percent
      Process            Location       Ib BODg removed   volatile    solids    volatile
Roughing filter plus     Livermore, California (68)      0.98     Not stated    3.3       84
 nitrifying activated
 sludge
Roughing filter plus     San Pablo, California (37)      1.47       78.2    Not stated  Not stated
 nitrifying activated
 sludge


    4.3.6   Denitrification Sludge

Denitrification  is  a  biological  process for  the  removal  of
nitrate  from wastewater.   An  electron donor,  carbon in primary
effluent or  methanol,  is added to the nitrate-bearing wastewater.
Denitrifying  bacteria  extract energy for growth  from the reaction
of nitrate  with  the  electron donor:
    Nitrate + Electron donor (reduced state)
    Nitrogen gas  +  Oxidized electron donor  +  Energy


Denitrification  has  been  extensively  studied,  and  a  few
denitrification  processes have  been built  into  municipal plants.
Denitrifying  bacteria  can  grow  either in  a  suspended  growth
system similar  to  activated sludge  or in  an  attached  growth
system similar  to  a  trickling  filter.   Sludge production for
ordinary nitrified  domestic waste  is roughly  300  pounds per
million  gallons  (30 mg/1)  of wastewater  treated  (37).


4.4  Chemical  Sludges


    4.4.1   Introduction

Chemicals  are  widely  used  in wastewater treatment to  precipitate
and  remove phosphorus,  and  in  some cases,  to  improve  suspended
solids  removal.   At  all such  facilities,  chemical  sludges are
formed.   A few  plants apply  chemicals to  secondary effluent and


                                4-36

-------
use tertiary clarifiers to remove the chemical precipitates.   An
example of  this  arrangement is  the  plant at  South  Lake  Tahoe,
California.   However,  it  is  more  common  to  add the chemicals to
the raw wastewater  or to a  biological process.   Thus,  chemical
precipitates are usually mixed with either primary sludge  solids
or biological sludge solids.

The discussion  below  is  brief  because  the subject  of  chemical
sludges  and  their  characteristics  is discussed  in  detail
elsewhere  (79-82).   A 1979 publication  provides considerable
background  information  on theoretical  rates  of  chemical  sludge
production, as well  as  actual operating data from wastewater
treatment plants  employing  chemicals  for removal  of phosphorus
(7).  Also, production of chemical sludges in primary sedimenta-
tion is discussed in Section 4.2.2.5.
    4.4.2  Computing Chemical  Sludge Production -
           Dry Weight Basis

Chemicals can greatly increase sludge production.   The amount of
increase depends on the  chemicals used and  the  addition rates.
There is no simple  relationship between the mass of the chemical
added and the mass  of sludge produced.  It  is beyond the scope of
this manual to  describe  in  detail the chemistry associated with
the  chemicals  used  in  treating  wastewater,  and the  various
solids-producing reactions  that  can  occur.   However,  several
types of precipitates  that  are produced and  must  be  considered
in measuring the total sludge  production are listed below:

     •  Phosphate   precipitates.    Examples   are A1PC>4  or
        Al (H2P04~) (OH) 2 with  aluminum  salts,  FePC>4  with  iron
        salts, and  Ca3(P04)2 with  lime  (79,82,83).

     •  Carbonate precipitates.   This  is  significant  with lime,
        which  forms  calcium  carbonate,  CaCC>3.   If  two-stage
        recarbonation is used,  a  recarbonation  sludge of nearly
        pure CaCC>3  is formed  (84).

     •  Hydroxide precipitates.   With iron  and  aluminum salts,
        excess  salt  forms  a  hydroxide,  Fe(OH)3  or  Al(OH)3.
        With  lime,  magnesium  hydroxide,  Mg(OH)2f   may  form;  the
        magnesium comes  from  the  influent wastewater,  from the
        lime,  or from magnesium salts.

     *  Inert  solids from  the chemicals.  This  item  is  most
        significant with  lime.   If  a quicklime is 92 percent CaO,
        the remaining  eight  percent  may be  mostly inert solids
        that  appear  in  the  sludge..   Many  chemicals  supplied in
        dry form may contain significant amounts of inert solids.

     •  Polymer solids.   Polymers  may  be  used as primary
        coagulants  and" to  improve   the  performance of  other
        coagulants.    The polymers themselves  contribute little


                               4-37

-------
        to total  mass,  but  they  can greatly  improve  clarifier
        efficiency  with  a concomitant  increase in  sludge
        production.

     •  Suspended  solids  from  the wastewater.   Addition of  any
        chemical  to  a  wastewater  treatment  process affects
        process efficiency.   The change in sludge production must
        be considered.

Quantities of  the various precipitates in chemical  sludges  are
determined by such conditions as pH, mixing, reaction time,  water
composition,  and opportunity  for flocculation.

Chemical sludge production,  like the production of other sludges,
varies  from  day  to day.   The variation depends  strongly  on
chemical dosage and on wastewater  flows.   If  the chemical  dosage
is about constant  in terms of milligrams per liter of wastewater,
chemical solids production will still vary, since flows  fluctuate
from  day to day.  Changes in wastewater chemistry  may also
affect   the  production  of chemical  sludge.   For  example,
stormwater inflow  typically  has a  lower alkalinity than ordinary
wastewater.    During storms, the  production  of chemical  sludge
will be  different  from production  in dry weather.
    4.4.3  Properties  of  Chemical Sludges

Chemical sludge  properties  are  affected mainly by  the  precipi-
tated compounds and by the other wastewater solids.  For example,
a lime  primary  sludge will probably dewater better  than  a  lime
sludge  containing  substantial amounts  of  waste-activated  sludge
solids  (80).   Generally  speaking, lime  addition results  in  a
sludge  that  thickens  and  dewaters  better  than the  same  sludge
without chemicals.   When iron or aluminum  salts are  added  to raw
wastewater, the  primary  sludge  does  not  thicken or  dewater  as
well as non-chemical  sludge.  Iron sludges dewater slightly  more
easily than aluminum sludges  (79).  When aluminum salts are added
to  activated  sludge,  the  sludge may  thicken  much better  than
non-chemical activated sludge  (85,86).  Anionic polymers can  often
improve the thickening  and dewatering properties of chemical
sludges.

For  efficient chemical  usage,  feed rates  must be adjusted  to
match changes  in wastewater flow and composition.
    4.4.4  Handling Chemical  Sludges

Most  of  the  common sludge treatment processes  can  be  used with
chemical sludges:    thickening,  stabilization  by digestion,
incineration, etc.    This  section  summarizes  information  on
stabilization and also on recovery of chemicals and by-products.
                               4-38

-------
        4.4.4.1   Stabilization


Lime sludges may be stabilized by a small  additional dose of
lime.  Lime stabilization may also be  used  for  aluminum and  iron
sludges.   The lime  improves dewatering of these  sludges  by acting
as a conditioning agent.  Chapter 6 discusses lime  stabilization
of  chemical  sludges.   Dewatered  lime-stabilized sludges  can
usually be buried in  sanitary landfills.

Digestion of mixed  biological-chemical sludges  is  generally
feasible.    Pure  chemical  sludge will  not digest.  Studies  done
in  1974   and  1978,  however,  note significant reductions in
digestibility as chemicals  were added  to sludge;  the studies
investigated the  addition  of aluminum,  iron, and polymer (87,88).
        4.4.4.2   Chemical and By-product Recovery

Where lime  use  results in  calcium  carbonate  formation,  it may
be feasible to  recover lime by  recalcination.  Tertiary lime
treatment,  as  practiced at  the  South Lake Tahoe,  California,
plant is well suited  to lime recovery; a  recalcination process
has been operated there for several years.  Where lime is  added
to raw  wastewater, lime recovery is more difficult but still
possible.    Lime  recovery  does not  reclaim  all  of the calcium,
as some  is  always lost with  the  phosphate,  silica, and other
materials  that must  be removed from  the  system.  Lime recovery
reduces  but does not  eliminate the  amount  of residue for
disposal.    Feasibility  of  lime  recovery  depends on  plant  size,
amount of  calcium carbonate formed, cost of new lime,  and cost of
sludge disposal  (81,82).
4.5  Elemental  Analysis  of Various Sludges

As a rule, almost anything  can be found  in sludge.   This  section
describes trace  elements  in all  types of  sludge.   Data on
concentrations  of the 74 elements found  in wastewater sludge  are
included in References 89-95.
    4.5.1  Controlling  Trace Elements


It  is  a basic  principle  of  chemistry  that elements  are not
created or destroyed but chemically recombined.   Therefore, the
mass of  each  element  entering  a treatment plant  fixes  the  mass
that either accumulates  within the  plant or  leaves it.  The
mass leaving  the  plant does  so in  gaseous  emissions, effluent,
a special  concentrated  stream, or sludge.   Extracting toxic
elements from sludge appears to  be impractical; source control  is
the most practical way  to reduce toxicants.


                              4-39

-------
Trace  elements  are present  in industrial  process waste,
industrial waste spills, domestic water supply,  feces and urine,
and detergents.   Additional  trace  elements are derived from:


     •   Chemicals  in  photographic  solutions,   paints,  hobby
        plating  supplies, dyes,  and pesticides used in households
        and commercial enterprises.


     •   Storm inflow  (this  is  particularly  true  for lead  from
        gasoline anti-knock  compounds).


     •   Corrosion  of water  piping,   which contributes  zinc,
        cadmium, copper, and lead  (96).


     •   Chemicals  used  in  wastewater  treatment,   sludge
        conditioning,  etc.    Table 4-13  shows an  analysis  of
        ferric  chloride, which  is  an  industrial  by-product
        (pickle  liquor)  of wastewater solids treatment.
                            TABLE 4-13

              METALS IN FERRIC CHLORIDE SOLUTIONS (97)

              Constituent       Concentration, mg/1
                Cadmium                 2-3.5

                Chromium               10-70

                Copper                 44  -  14,200

                Iron              146,000  -  188,000

                Nickel            .     92  -  6,200

                Lead        ;            6  -  90

                Silver                    2
                Zinc                  400  -  2,150
          aThree different liquid sources  were
           analyzed (43 percent FeCl3).
The  quantity  of  toxic  pollutants  may  be  significantly reduced
by  source control.   At  Los  Angeles County,  metal  finishing
industries were  a major   source of  cadmium,  chromium, copper,
lead,  nickel,  and  zinc.   A source control program was  developed
in cooperation with the  local Metal  Finisher's Association.  This
program was quite successful, as  shown in Table 4-14,  by the
general downward trend in wastewater concentrations over time.
                               4-40

-------
                             TABLE 4-14

         PROGRESS IN SOURCE CONTROL OF TOXIC POLLUTANTS (98)
                         Concentration in mg/1 in influent wastewater
 Wastewater
 pollutant

  Cadmium
  Chromium
  Copper
  Lead
  Nickel
  Zinc
January-June
  1975
July-December
   1975
January-June
  1976
July-September
   1976
October-December
    1976
0.037
0.70
0.45
0.40
0.31
1.55
0.031
0.73
0.45
0.31
0.33
1.48
January
 1977
0.029
0.78
0.45
0. 34
0. 35
1.37
0.033
0.61
0.33
0.28
0.34
1.41
0.027
0.47
0. 34
0.32
0.27
1.29
                                                    0.019
                                                     0.43
                                                     0.30
                                                     0. 34
                                                     0.21
                                                     1.17
    Data for Joint Water Pollution Control Plant, Los Angeles County,
    California; weekly composite samples. (13).
Occasionally,  elements can be  converted from a highly  toxic  form
to a less toxic  form  in wastewater treatment.  Chromium is a  good
example of this.   In  its hexavalent form,  it  is highly  toxic,  but
may be  converted  to  the  less  toxic trivalent  form in  secondary
treatment.


    4.5.2  Site-Specific Analysis

The elemental compositions of  various  sludges differ from  one
another.   If  sludges are  to be reused,  they should be  analyzed
for a  number of  elements.    The  importance  of  site-spec ific
analysis of  sludges  varies  with the  size  of  the  project,
regulatory  requirements,  industrial  activity,  and the   type  of
reuse desired.   A  sampling program should  recognize  that:

    •   One  plant's   sludge may have  100  times  or  more of  a
        certain  element than another plant's.

    •   There  may  be  major variations between samples at  the  same
        plant.  A single grab sample may  produce  misleading
        results.   Careful  attention to sampling and statistical
        procedures  will  tend  to  reduce  the  uncertainty.   A
        detailed report on such procedures is available (99).

    •   Estimates  of  trace  element  sludge  contamination  based
        on  wastewater  analysis  are  usually  less useful  than
        estimates  based  on  sludge  testing.    However, if  an
        element  can be measured in the  influent wastewater and if
        flow  rates are known,  then a mass  load  (Ib  or kg  per  day)
        may  be  computed.   For purposes of estimating  sludge
        contamination, it  is  reasonable to assume  that large
        trace  amounts of cadmium,  copper, and zinc  appear in the
        sludge.   Analyses  of sludge and supernatant samples  from
        a facultative  sludge  lagoon have shown  that  there  is  a
        tendency for  nickel and  lead  to be gradually  released
        from  the sludge to the  liquid phase  (97).

    •   Sludge  samples should be  analyzed  for percent  solids
        and  percent volatile as well as  for trace elements.
                                4-41

-------
     4.5.3   Cadmium

Because  it is often found  in amounts that  limit sludge reuse  as  a
soil conditioner,  cadmium  is  a critical  element.   If  sludge
containing  cadmium is applied  to  agricultural  cropland,   some
cadmium may enter the  food  chain.   It  has been  argued,   with
much controversy, that the  normal human dietary intake of cadmium
is  already  high  in comparison to  human  tolerance  limits and
that  sources  of  additional  cadmium  should  be strictly  limited
(100,101).   Table  4-15 summarizes reports on  cadmium in sludge.

Chapter 18   includes  a  discussion  of  the  control  of  sludge
application  rates  for  the  purpose of  limiting cadmium  levels in
soil  and  crops.    Additional  information  on this  subject is
provided in  reference 90.
                              TABLE 4-15

                          CADMIUM IN SLUDGE
                                   Concentration, mg/dry kg
Type of sludge
Digested
Heat dried
Anaerobic
"Other"
Not stated

Incinerator ash
Digested
Digested waste-acti-
vated
Dewatered digested
primary
Digested
Raw
Digested
Raw primary
Mesophilic digested
Thermophilic digested
Waste- activated
Anaerobically digested
chemical and waste-
activated (3.9 per-
cent average solids)
Anaerobically digested
chemical and waste-
activated (3.2 per-
cent)
Anaerobically digested
chemical and waste-
activated (4.2 per-
cent)
Raw primary

Raw primary

Raw primary
Raw primary and bio-
filter
Raw primary and bio-
filter
Raw primary and bio-
filter
Location
12 U.S. cities
4 U.S. cities
Various U.S.
Various U.S.
42 cities in England,
Wales
Palo Alto, California
Chicago (Calumet)
Chicago (West-Southwest)

Seattle (West Point)

Cincinnati (Millcreek)
Several U.S. cities
About 25 U.S. cities c
Los Angeles (Hyperion)
Los Angeles (Hyperion)0,
Los Angeles (Hyperion)
Los Angeles (Hyperion)
Chatham, Ontario0



Simcoe, Ontario



Tillsonburg, Ontario



Sacramento, California
(Northeast)
Sacramento (Rancho
Cordova)
Sacramento (Natomas)
Sacramento (Highland
Estates)
Sacramento (County Sani-
tation District 6)
Sacramento (Meadowview)

Standard
Mean deviation
89
150
106
70
-

84
-
340

48
A
130
30
75
39
140
120
110
2.6



78



9



2.8

3.0

3.5
4.1

3.6

3.1

72
200
-
-
-

-
-
-

-
K
1.51°
15
104
-
-
-
-
1.4



5



1



1.1

1.4

1.1
1.3

3.3

1.0

Median
65
67
16
14
<200

-
-
-

-

-
20
31
-
-
-
-
1.8



72



9'



, 2.6

2.6

3.6
3.8

2.5

2.6

Range
6.8
15
3
4
<200
(7
68
10






9




0



66



7



1.4

1.2

2.2
2.8

1.0

2.3

- 200
- 440
- 3,410
- 520
- 1,500
>200)
- 99
- 35
-

-

-
-
- 550
-
-
-
-
- 10



- 110



- 12



- 4.2

- 4.5

- 5.1
- 5.9

- 9.1

- 4.4

Number of
samples
12
4
98
57
42

2
-
43

100
approximate
25
20
80
-
-
"
-
225



198



40



. 5d
,
5d
,
5
5

5d
J
5d

                                                               Reference

                                                                 89
                                                                 89
                                                                 90
                                                                 90
                                                                 91
                                                                 92
                                                                 93
                                                                 102

                                                                 94

                                                                 95

                                                                 95
                                                                 95
                                                                 103
                                                                 103
                                                                 103

                                                                 103
                                                                 99
                                                                 99
                                                                  97

                                                                  97
                                                                  97
                                                                  97
                                                                  97

                                                                  97
 Geometric mean.
 b
 Spread factor for use with geometric mean.
 cConcentrations reported on wet weight basis and converted
 to dry weight basis.
 Weekly composites of daily samples.
                                 4-42

-------
                               TABLE 4-15

                     CADMIUM IN SLUDGE (CONTINUED)

                                     Concentration, mg/dry kg

Type of sludge
Raw primary and bio-
filter
Waste activated
Raw primary and waste-
activated
Raw primary

Anaerobically digested
ferric chloride
Anaerobically digested
chemical (mostly alum)
Anaerobically digested
lime
Anaerobically digested
ferric chloride
Geometric mean.
bc

Location
Sacramento (City Main)

Sacramento (Arderi)
Sacramento (Rio Linda)

Sacramento (County
Central)
North Toronto , Ontario

Point Edward, Ontario

Newmarket, Ontario

Sarnia, Ontario


.. .

Mean
10.5

5.4
9.7

29

29

8.5

7.5

76



Standard
deviation Median Range
2.0 11 7.6 - 13

2.6 6.7 2.3 - 7.7
2.9 9.1 6.2 - 14

28 12 8.3 - 72

9 - -

1.9

4.2

21



Number of
samples
5d •

5d
5

. 5d
1
f 60

61

59

40




Reference
97

97
97

97

104

104

104

104



Concentrations reported on wet weight basis and converted
 to dry weight basis.

 Weekly composites of daily samples.
     4.5.4  Increased  Concentration During Processing

Toxic  elements  often  are  non-volatile  solids  that  remain in
sludge  after volatile solids  have  been removed.   Removal of
volatile solids  such as  organic matter increases  the  concentra-
tion of non-volatile  components, expressed on a  dry weight basis.
Table 4-16 shows this effect  for  four metals at one plant.   This
increased  concentration may be  important  if sludge  reuse is
desired and  if  regulations  limit  reuse for  sludge that  contains
contaminants  that exceed  certain concentrations.
                                TABLE 4-16

          INCREASED METALS CONCENTRATION DURING PROCESSING

                                    Concentration, mg/kg dry weight
       Element

   Chromium
   Copper
   Nickel
   Zinc
   Number of samples


Note: 1977 data, Sacramento County Central treatment plant, California.  Anaerobic
    digesters also receive thickened waste-activated sludge (metals content not
    measured).
                                   4-43


Raw primary sludge
(79





percent volatile)
110
200
46
620
(5)
Anaerobically digested
sludge
(68 percent volatile)
160
340
63
930
(2)

Lagooned sludge
(56 percent volatile)
220
450
65
1, 400
(30)

-------
4.6   Trace Organic Compounds  in Sludge

Several  of  the trace  organic compounds  found in  sludge,  for
example,   polychlorinated  biphenyls  (PCBs),  are toxic,  slow  to
decompose and  widely distributed  in the environment.   Table  4-17
quantifies  the amount   of  Aroclor 1254,  a  common  PCB,  found  in
sludge.    Three other PCBs,  Aroclors 1242,  1248,  and  1260,  have
also  been found in sludge  (105,107,108).   In 1970,  the production
of  PCBs  for several end  uses was halted in  the  United  States
and  was  completely  phased out in  1977.   As of  1979,  imports  of
PCBs  are  prohibited except  for  a  few  special purposes.   It  is
anticipated that these  measures will help  to reduce PCB levels  in
sludge.   However,  products containing  PCBs  are still  in use,  and
these  chemicals are widely distributed,  so  that  several years  may
elapse  before  PCBs become  undetectable  in  sludge.


                              TABLE 4-17

              AROCLOR (PCB) 1254 MEASUREMENTS IN SLUDGE
Average
concentration of
samples with
compound detected
Location
Hamilton, Ontario
Kitchener , Ontario
Newmarket, Ontario
North Toronto, Ontario
Wet
basis,
ug/1
81
110
74
120
Dry
basis, Number of Samples with Year of sample
mg/kg samples compound detected collection
- - 1976
- - - 1976
- - 1976
- - - 1976
Reference
105
    Sludge type


Undigested
Undigested (with Al)
Undigested (with Ca)
Undigested (with Fe)

Raw primary        Sacramento, CA (North-     50
               east)

              Sacramento, (Natomas)     60

              Sacramento (County       80
               Central)

Ra« pr imary and biofilter Sacramento, {City Main)     30

              Sacramento (County Sani-    50
               tation District 6)

              Sacramento (Headowview)     50


              Sacramento (Rio Linda)     90
Raw primary and waste
 activated
Lagooned digested primary Sacramento (County
 and waste activated    Central)
Digested

Heat dried
10 U.S. cities

4 U.S. cities
                    1.5

                    1.8
                    3.8

                    2.0
                    2.4


                    3.5
3.9


9.3
10

4
 1977

 1977



 1977

 1977


 1977


 1977



 1977



1971-1972

1971-1972
Weekly composite of daily samples.
Because  of  their  fat-soluble  nature,  PCBs  tend  to  concentrate
in  skimmings  and  scum  at  wastewater  treatment  plants.   The
conventional  procedure   of  introducing  skimmings   into  the
digester can  cause  higher  concentrations of  PCBs  in  the final
sludge.   Alternative  disposal  procedures  for  skimmings,  such  as
incineration, can  reduce this  problem.

Table  4-18  presents  data  on  three  chlorinated  hydrocarbon
pesticides found  in  sludge  from several treatment plants.
                                  4-44

-------
                             TABLE U-18

       CHLORINATED HYDROCARBON PESTICIDES IN SLUDGE (97, 106)
Compound
Hexachlorobenzpnp
Hexachlorobenzene
Lindane

Technical -qradp rhlordane



Sludge type
Waste- activated
Raw primary
Waste- activated
Raw primary
Raw primary
Raw primary
Raw primary
Lagooned anaerobically di-
Plant
Arden
County Central
Arden
Northeast
Northeast
Natomas
County Central
County Central
samples with compound
detected, mg/dry. kg
0.8
0.4
1.0
0.6
2.6
2.3
2.8
4.2
Total
samples
5a
5a
5a
5a
5a
a
5
30
Samples with
compound detected
1
2
1
1
1
2
5
3C
              gested primary and waste-
              activated
             Waste-activated
             Raw primary and waste-
              activated
             Raw primary and biofilter
             Raw primary and biofilter
Arden
Rio Linda
Meadowview
City Main
4.4
5.5
• 0.6
 19
All plants in Sacramento County, California.


 Weekly composites of daily samples.
4.7  Miscellaneous  Wastewater Solids

In  addition  to the primary,  biological,  and chemical  sludges
discussed in  previous  sections,  there  are  several other
wastewater  solids  that  must be properly handled  to  achieve  good
effluent,  general environmental  protection,   and  reasonable
treatment  plant  operations.   These  solids  include  screenings,
grit, scum, septage,  and filter backwash.

When mixed with  primary or secondary  sludges, screenings,  scum,
grit, and  septage  can interfere  with the processing  and reuse of
the  sludge.   Before  mixing these wastewater solids  with primary
and  secondary  sludges, design engineers  should  consider  the
following:

     •  Screenings  and  scum detract  from  the final  appearance,
        and marketability,  and  utilization of  sludges.   They can
        also  clog piping,  pumps,  and mixers,  and  occupy valuable
        space in  digesters and other tankage.

     •  Scum  presents  a special  problem  when  mixed with  other
        solids  and  subjected to gravity  thickening,  decanting, or
        centrifugation.    Under  these  conditions,  scum  tends to
        concentrate in  the  sidestream and  to be  recycled to the
        wastewater  processes.   Eventually  some of  this recycled
        scum  is discharged to the effluent.

     •  Grit  can  block pipelines,  occupy  valuable  space in
        digesters and other  tankage,  and cause excessive wear to
        solids  piping  and processing equipment.
                                4-45

-------
    4.7.1  Screenings

Screenings are materials  that  can  be removed from wastewater by
screens or racks with openings of 0.01 inch (0.25 mm) or larger.
Coarse  screens or  racks have openings  larger.than 0.25  inch
(6 mm), whereas fine screens  have openings  from 0.01 to 0.25 inch
(0.25 to 6 mm).  If  openings are larger than 1.5 inches (38 mm),
the screens are often called  trash  racks.

Racks and  screens are  usually  installed  to treat the wastewater
as  it  enters  the  treatment plant.   Racks  and  coarse screens
prevent debris from  interfering with  other  plant equipment.   Fine
screens remove a significant fraction  of  the influent suspended
solids and 8005, thus  reducing the load  on subsequent treatment
processes.   In this regard,  fine  screens may act  like  primary
sedimentation  tanks, although  they do not ordinarily  remove  as
much of  the  solids  as  do sedimentation tanks.   Fine screens are
usually protected  by upstream coarse  screens or racks.


        4.7.1.1 Quantity of  Coarse Screenings

Coarse  screenings   are   basically  debris.    Items  typically
collected  on  coarse screens  include rags,  pieces of string,
pieces of  lumber, rocks,  tree  roots,  leaves,  branches,  diapers,
and plastics.

The quantity  of coarse screenings  is highly  variable, but  most
plants report  0.5  to 5 cubic  feet per million gallons  (4  ml/m3
to  40 ml/m3)  on average  flows.   Table 4-19 shows the quantities
of screenings reported  for a  number of communities.  The quantity
of screenings depends on:

     •  Screen opening  size.    Generally,   greater  quantities are
        collected  with smaller screen openings.   This was  seen
        most  clearly at  Grand  Island, Nebraska,   where  a  change
        from 0.5-inch to 1.25-inch (13 to 32 mm)  openings
        caused  screenings production to  drop  from about  7  to
        about  3  cubic  feet  per  day   (0.2  to  0.08 m3/day) (114)  .
        Tests  at  Chicago,  Illinois, and  Adelaide, Australia,
        showed this  tendency  also  (13).

     •  Shape  of  openings.    For  example, bar  racks may  have
        openings 0.75  inch  (19 mm)  wide  and over  2  feet   (over
        600  mm) long.    Such  a rack  will  pass  twigs, ballpoint
        pens, and  other debris, that  would  be  captured on a mesh-
        type screen with  square openings of 0.75  inch  (19 mm).

     •  Type of sewer  system.    Combined storm and sanitary
        sewers  produce  more  screenings  than  separate  sanitary
        sewers.  This  effect is especially pronounced where much
        or all  of  the  combined wastewater is treated during and
        after storms, rather than  being bypassed.
                               4-46

-------
          TABLE 4-19




SCREENING EXPERIENCE (109,  110)
Rack or screen
opening, in.
3-3/8
3
3
3
1-3/8
1-1/2
1-1/2
1-1/2
1-1/2
1-1/2
1-1/2
1-1/4
1-1/4
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
7/8
7/8
3/4
3/4
3/4
3/4
3/4
3/4
3/4
3/4
1/2
1/2
1 in. = 2.54 cm.
1 mgd = 3,785 m3/day
1 cu ft/mil gal = 7 .

City
Norwalk, Connecticut
New Haven, Connecticut
East Hartford, Connecticut
San Jose, California
New York, New York, Jamaica
Philadelphia, Pennsylvania, North
Oklahoma City, Oklahoma, Southside
Cranston, Rhode Island
Taunton, Massachusetts
Meadville, Pennsylvania
Grove City, Pennsylvania
Uniontown, Pennsylvania
Fargo, North Dakota
New York, Wards Island
New York, Owls Head
Minneapolis-St . Paul, Minnesota
New York, Hunts Point
East Bay, Oakland, California
New York, Coney Island
New York, 26th Ward
New York, Tallmans Island
Bridgeport, Connecticut, West Side
New York, Rockaway
Waterbury, Connecticut
Bridgeport, Connecticut, East Side
Duluth, Minnesota
Austin, Minnesota
Fond du Lac, Wisconsin
Findlay, Ohio
Massillon, Ohio
York, Nebraska
Marion, Ohio
Gainesville, Florida
Marshalltown, Iowa
East Lansing, Michigan
Birmingham, Michigan
Boston, Massachusetts, Nut Island
Richmond, Indiana
Detroit, Michigan
New York, Bowery Bay
Hartford, Connecticut
Portsmouth, Virginia
Sheboygan, Wisconsin
Aurora, Illinois
Topeka , Kansas
Oshkosh, Wisconsin
Green Bay, Wisconsin
Manteca, California

f
48 m3/! x 106 m3.
Flow,
mgd
11.75
8
4.0
-
65
48.2
25.0
8.32
3.5
2.5
0.8
3.0
2.7
180
160
134
120
98
70
60
40
17
15
15
14
12
9
7.2
7
5.2
5
5.0
5
4.0
3.8
1.5
125
6.2
450
40
39.0
9.7
8.0
8.0
7. 5
6.0
10.0
1.5



Screenings,
cu ft/mil gal
0. 17
1.0
1. 33
0.25
0.6
2.20
2.1
0.65
1.0
0.6
0. 1
0. 9
4.55
1.0
0.6
0. 9
0.7
1.6
1.4
1.1
0.7
0.93
1.0
2.35
2.04
0.56
1.1
5
0. 39
1.5
1. 5
2.5
3.5
0.25
0.4
1. 2
1.2
1. 2
0.47
1.1
1.6
0.82
0.25
1.42
1.30
1.7
1.2
5.2



             4-47

-------
     •   Operating  practices.   Where manual  cleaning is used,
        operatorssometimes  pass some  screenings  through  or
        around the screens.  Where automatic  equipment  is  used,
        the operating pattern can greatly affect removals  (112).

     •   Length of  sewer  system.  The volume of screenings removed
        may double with a  short,  as  opposed  to  lengthy,  inter-
        ceptor system.  This  condition  may be explained by  the
        fact that  solids are more subject to disintegration  with
        a lengthy  collection  system (5).  Wastewater pumping  will
        also tend  to  disintegrate  large solids.

Screenings loads may  increase dramatically during peak flows.   It
is  estimated  that,  at  the  East Bay Municipal  Utility  District
plant  in Oakland, California,  the  screenings  load was about
10 times the average  during peak flows.   For the most part,  this
plant services separate sanitary sewers,  but the screenings  load
is concentrated (110).


        4.7.1.2 Quantity of  Fine  Screenings

Fine screens  are  usually used  as  an  alternative to conventional
primary  sedimentation  to  remove suspended solids.   Screens
with 0.09  to  0.25 inch (2  to  6 mm)  openings remove  about  5  to,
10  percent of suspended  solids  from  typical  municipal  raw
wastewater.   If 0.03  to 0.06 inch (0.8 to  1.5  mm)  openings  are
used instead,   about  25  to  35  percent of  suspended solids may be
removed  (5).   Higher removals  increase  the dry  weight and  the
moisture content of  the screenings.   For example, consider  fine
screens  that  remove 25 percent of  suspended  solids  from  an
influent concentration of 300 mg/1.  In this case, screenings are
630 dry  pounds per million  gallons (75 mg/1)  of  wastewater.   If
the screenings are ten percent  solids and weigh 60 wet pounds per
cubic  foot  (961  kg/m3), then  the  volume  is  105  cubic  feet  per
million gallons (14.04 m3/lx!06 m3).   This  is over 25 times  more
than a  typical value for  coarse screenings of  4  cubic  feet  per
million gallons (0.53 m3/lx!06  m3).

        4.7.1.3 Properties  of  Screenings

If  screenings  have   not  been  incinerated,  they  may  contain
pathogenic  microorganisms.    They are  also odorous  and tend  to
attract  rodents  and insects.   Screenings  have been  analyzed
for  solids content, volatile content,  fuel  value, and  bulk
wet  weight.   Some  of  the reported  values  are summarized  in
Table 4-20.


        4.7.1.4 Handling Screenings

Screenings  may be  ground and handled with other  sludges;  direct
landfilled; and incinerated, with  the  ash  disposed in landfill.
Table 4-21  summarizes the advantages  and  disadvantages of various
methods.
                               4-4!

-------
                               TABLE 4-20

                        ANALYSES OF SCREENINGS
  Solids content,
   percent dry
     solids
Volatile content,
   percent
Fuel value,
Btu/lb dry
  solids
Bulk wet weight,
  Ib/cu ft
                                                    Comments
                                                                References
      20
    10 - 20

     B - 23
      6.1
      17
                            5,4003
   80 - 90

   68 - 94
                  96
                  86
                            7,820
                     Coarse screenings. Fine
                      screenings may have
                      lower solids content.

                     Common values

                     Various plants, fine
                      screens, 0.03 to 0.12
                      inch openings

                     Thickened ground screen-
                      ings from 0.75-inch
                      racks; after grinding,
                      screenings were thick-
                      ened on a static screen
                      with 0.06-inch openings.

                     Dewatered ground screen-
                      ings from 0.75-inch
                      racks; after grinding,
                      screenings were de-
                      watered on a rotating
                      drum screen with 0.03-
                      inch openings.

                     Fine screenings
                                                                  13
                                                                  113
                                                                  113
 Computed.

1 Btu/lb dry solids = 2,32 kJ/ dry solids.
1 Ib/cu ft = 16.03 kg/m3.
1 in. = 2.54 cm.
 Some fecal  solids accompany the larger materials  such as  rags and
 twigs.   For this  reason, as well  as  to save labor time and cost,
 it  is  desirable  to  mechanize  screenings handling.   Also,  where
 coarse screenings  are landfilled  or  incinerated,  it is desirable
 to  use the  largest rack opening that will adequately  protect
 downstream  processes.    This  will  minimize  the  quantity  of
 screenings  that must  be  handled separately.

 Screenings  may be transported pneumatically  (116), in sluiceways,
 on  conveyors,  and  in  cans,  dumpsters,  or  covered  trucks.
 Screenings-water  mixtures  that  are  ground  may  be pumped.   For
 thickening  and  dewatering,  fine  static  screens,  drum  screens,
 centrifuges   (113),  and  drum  or  screw  presses  may be  used.
 Chemical conditioning  is not required.


          4.7.1.5  Screenings from Miscellaneous  Locations

 Screens  are  occasionally used on streams  other than influent
 wastewater.   For  instance, when it is  fed to a  trickling  filter,
 primary  effluent  may be  screened  to  prevent clogging of  orifices
 in the distributor on  the trickling  filter (109).  At one  heavily
 loaded plant where  regular  influent  screening  equipment  was
 partially  bypassed,  screens  installed  in aeration basin  effluent
 channels,   chlorine contact  tank outlets,  and  other  locations
 prevented  coarse,  floating  objects  from being   discharged  with
                                  4-49

-------
the  effluent   (117).     Another  example  occurred  at  a  plant  where
digested   sludge  was  discharged  to  the   ocean.    Fine  screens  were
used  to  prevent  floatable  materials   from  being  discharged  (118).
Other   examples  of   the   use  of   screens  on   streams  other   than
influent  wastewater   are  the  screening of  overflow  water  from  grit
separators  and   the   screening  of   feed  sludge   to   disc-nozzle
centrifuges  to  prevent  clogging  (113,119).
                                            TABLE 4-21

                            METHODS OF HANDLING SCREENINGS
               Method
                                               Advantages
                                                                                Disadvantages
1.  Comminution within main wastewater stream
    handle comminuted screenings with
    other wastewater solids, e.g., primary
    sludge
2.  Removal  from main stream, grinding or
     maceration, and return to main stream
3.  Removal3 from main stream, draining or
     dewatering, landfill
   Removal3 from main stream, dewatering,
     incineration, landfill of ash.
   Anaerobic digestion of fine screenings
     alone  (not mixed with other
     solids)

   Anaerobic digestion of screenings
     together with scum but separate
     from other sludges
Highly mech.ykized, low operating labor re-
 quiremen,*.

Minimizes number of unit operations
Usually free of nuisance from flies and
 odors
Widely used, familiar to plant operators
Similar to Method 1, except more complex
  mechanical ly
Keeps screenings out of other sludges;
  avoids disadvantages of Methods 1 and 2.
Can be fairly well mechanized.
Keeps screenings out of other sludges;
  avoids disadvantages of Methods 1 and 2.
Ash is very small in volume and easy to
  transport and dispose of.
If incineration is used for other sludges
  and/or grit, then screenings can be
  added at modest cost*
Pathogen kill
Sludge contains screenings,  which may inter-
  fer with public acceptance for reuse of
  sludge as a soil amendment.
Sludge probably needs further maceration or
  screening if it is to be pumped or
  thickened in a disc centrifuge.
If sludge is to be digested, digesters must
  be cleaned more often.  Plastics and
  synthetic fabrics do not decompose in
  digesters. Aggravates digester scum pro-
  blems . Ground screenings tend to
  agglomerate in digesters.
Not appropriate if suspended solids removal
  is required (fine screens).
Not appropriate for very  large screenings
  loads, especially if high grit loads are
  also present  (large plants, combined
  sewers)

Similar to  Method 1, except Method 2 can be
  designed  for very large flows and screen-
  ings loads.  Method 2 is more expensive
  than Method 1 for small screenings loads.

Transport of screenings may be difficult.
Unless carefully designed and operated,
  causes fly and odor nuisances and health
  hazards.
Regulations for landfill  disposal may strongly
  affect operations.

High cost if an incinerator is required for
  screenings alone.
Unless  incinerator  is properly designed and
  operated, air pollution  (odor and partic-
  ulates) will be serious.
Not well adapted to wide  fluctuations in
  screenings quantities,  unless screenings
  are only  a small  part of  the total in-
  cinerator load.

Digestion was tested at large scale at
  Milwaukee, Wisconsin, but found to be
  impractical..  (115)

Tested  at Malabar plant,  Sydney, Australia,
  but found to  be inoperable.  Material
  handling  was  the  chief difficulty.
 Mechanical removal is usually practiced at large plants.  Manual removal is  frequently used at small plants.  The advantages
 of manual removal are simplicity and low capital cost; the disadvantages are high operating labor requirements and fly and odor
 problems. A common arrangement at small plants is to install a single comminutor with a manually cleaned bar  rack as a standby unit.
       4.7.2    Grit

 Grit   is  composed   of   heavy,  coarse  solids  associated  with  raw
 wastewater.      It   may   be  removed  from   wastewater  before   primary
 sedimentation  or  other  major  processes.   Alternatively,  it may  be
                                                 4-50

-------
removed from primary  sludge  after  the primary sludge is removed
from the  wastewater.    Typical  ingredients of grit  are gravel,
sand,  cinders,  nails,  grains of corn, coffee grounds, seeds, and
bottle  caps.


        4.7.2.1  Quantity of  Grit

The amount of  grit  that is removed varies tremendously from one
plant to  another.   Table 4-22 shows grit quantities measured at
several plants.  Additional  values  have been published  elsewhere
(5,109).  The quantity of grit depends on:

     •   Type of collection system.   If a system is combined, then
        street sanding,  catch basin  maintenance, and  amount of
        combined  sewer overflow become important.

     •   Degree of  sewer system corrosion.   Grit may include
        products  of hydrogen sulfide corrosion  derived from the
        pipes ( 122) .

     •   Scouring  velocities  in   the  sewers.   If   scouring
        velocities  are not regularly  maintained, grit will  build
        up  in  the  sewers.   During peak flows,  the  grit  may be
        resuspended,   and  the treatment  plant may  receive  heavy
        loads during peak flows.

     *   Presence of  open  joints and cracks_in _the__sew_e_r system .
        These permit  soil  around the "pipe's to~e~nter "the sewers .
        This effect  also  depends  upon  soil characteristics and
        groundwater levels.

     •   Structural  failure of sewers.  Such failures can deliver
        enormous  amounts of grit to the  wastewater system.

     *   Quantities  of  industrial wastes .
     *  S££££llJ^=iWJ^                     grinders  are  used.

     •  Efficiency of grit removal at the  treatment plant  ( 5 ) .

     •  Amount of septage.

     •  Occurrence of construction in the service  area or at the
        treatment plant.

It  is not possible  to develop a  formula which allows  for all
these factors.    Cautious  use of  available  information is,
therefore, recommended.   It  is  important  to recognize  that
extreme variations occur in grit volume and quantity.   A generous
safety factor  should  be used  in  calculations  involving the
storage,  handling,  or disposal  of  grit  (5).   In a  new system
where  there are separate sanitary  sewers and favorable  conditions
                               4-51

-------
such  as  adequate  scouring  velocities,  an  allowance  of  15  cubic
feet  per  million  gallons   (2  m3/lxl06  m3 )  should  suffice  for
maximum  flows.    On  the  average,  the  quantity  of  grit  in  waste-
water  will  usually  be  less  than 4  cubic  feet  per  million  gallons
(0.53  m3/lxl()6 m3)  for separate  sewer  systems,  (5)  but
higher values  have been observed  (see  Table  4-22).
                                    TABLE 4-22

                                GRIT QUANTITIES
          Plant
  Quanti ty,
cu ft/mil gal
                                                   Comments
Santa Rosa, California (College
 Avenue)
San Jose, California
Manteca, California
Santa Rosa, California (Laguna)
Seattle, Washington (West Point)
Dublin-San Ramon, California

Los Angeles, California
  (Hyperion)

Livermore, California
Gary, Indiana


Renton, Washington
    O.HH
    0.3
    1.4

    2.5
     5.2
     3.2
     9.5

     5.0
     2.1
    10.7

     2.6

    11.2

      7

      2


     1.0

     0.3
     2.4

     8.6
      89

     1.7


     4. 1

     7.0
Average.  Separate sewers.
Minimum month
Maximum month

Separate sewers.  Older removal
  .systems removed less yrit (0.3
  and 1.4 cu ft/mil gal)

Average.  Separate sewers.
Minimum month
Maximum month

Average.  Separate sewers.
Minimum month
Maximum month

Average.  Combined storm and sani-
  tary sewers.
Maximum day

Average.  Separate sewers.

1973 average.  Separate sewers.
Average over 24 months.  Separate
  sewers.
Lowest month
Highest month

Annual average. Combined sewers.
Highest value on test  runs.

Average over 19 months before im-
  provements to grit removal equip-
  ment. Separate sewers.
Average over 12 months after im-
  provements.
Maximum month, following improve-
  ments .
References

   110



   110



   110



   110



   110



   120

    99


    68
   110
   121
   119
1 cu ft/mil gal = 7.48 m /.I x 10  m
           4.7.2.2   Properties  of  Grit
 Grit has  been  analyzed  for  moisture,  volatiles  content,  specific
 gravity,  putrescibility,  (123)  particle  size,  and  heating  value.
 All  of   these  depend  on the  kind  of  sewer  system  and the  method
 of grit  removal  and  washing.
                                        4-52

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The  moisture  content  of  grit  is  reported  as ranging from 13  to
65 percent, and  the volatiles content  from 1 to 56  percent  (109).
Specific  gravity  of grit particles  varies;  values  from  1.3  to
2.7  have been  reported  (109).  The range  for volatile  solids was
8 to  46  percent  (123).   Particle  size   for grit removed from  five
plants  is shown  in Table  4-23, along with  an analysis of digester
bottom  deposits. .
                               TABLE 4-23

                         SIEVE ANALYSIS OF GRIT
                                        Percentage retained
Sieve size


    4
    e
   10
     12
     20
     28b

     40
     50
     60

     65b
     80
    100

    150b
    200
          Sieve opening,
 4.76
 2.38
 2.08

 1.41
 0.84
 0.6C

 0.42
 0.30
 0.25

 0.21
 0.18
0.149

0.105
0.074
                   Green Bay,
                   Wisconsin
                    3.7


                    9.1


                   19.8
                   29.6


                   51.7


                   78.2


                   96.1


                   (109)
              Kenosha,
              Wisconsin
                                Tampa,
                                Florida
St. Paul,
Minnesota
                                        1-7
                                        5-20
                                  99.5


                                  (109)
 Renton,
Washington


 2.5 - 13.5

19.5 - 34.5


 50 - 74.5


 71 - 88.5

90.5 - 94



  97.5


  99.5

  (119)
 Renton,
Washington


 0 - 0.5


 2-11


 10 - 41


 27 - 62

 60 - 76.5
                                         95 - 98

                                          (119)
Digester deposits,
  Los Angeles,
  California
                           7.3

                          28.3
                          77.6
                          84.9
                                                                 (118)
 U.S. series, except as noted.

 Tyler series sieve.

 Dried at 103°C. Four tests. Volatile contents 34 to 55 percent.

 Same samples as previous column, ashed at 550°C and resieved.
Grit  quality can  be  varied  to some  extent.    If  a  "clean"  grit
with  very  low  putrescibility  is  desired,  it  may be  obtained  by
grit  washing  and operational  adjustments  to the  grit  removal
system.   However,  such operations  may make it  impossible  to
remove  fine  sand  (of less  than 0.08 inch [0.2  mm]).   For example,
if  a  separate grit  washer  is used, fine sand  may be  recycled' in
the  wash  water.   If  it  is essential,   fine  sand  can  be removed
with  high  efficiency.    However,  the  sand  will be accompanied  by
large  amounts of  putrescible  solids.   A compromise  between
cleanliness  of grit  and  high  removals  of  fine  particles  is
necessary  (124).    If  good washing equipment  is  used,  operators
can often remove  significant  quantities of fine materials without
sacrificing  cleanliness.   Grit should  be  regarded  as containing
pathogens unless  it has  been  incinerated.
         4.7.2.3   Handling  Grit

The  first  step  in  grit handling  is
from the  main  stream  of  wastewater.
                             the separation  of the  grit
                               Grit  may be  removed  from
                                  4-53

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    4.7.3  Scum
Scum  is  the material  that floats on wastewater, except  where
flotation is involved.   In a  flotation unit, scum  is incorporated
into  the  float.   Scum  may  be  removed  from  many  treatment
units  including  preaeration  tanks,  skimming  tanks,  primary
and  secondary  sedimentation  tanks,  chlorine  contact  tanks,
gravity thickeners, and digesters.   The term  "skimmings" refers
specifically to scum that  has been removed.
        4.7.3.1  Quantities  of  Scum


Quantities of scum are generally small compared to those of such
wastewater solids  as  primary  sludge and waste-activated sludge.
Table 4-24  lists some properties  and  quantities of  scum.   The
data in this  table  are based  on scum from primary sedimentation
tanks.   Scum  is often removed from secondary clarifiers  and
chlorine contact tanks, but there is almost no available data on
the quantities removed.


Although  there  is  some data  on the quantity  of grease removed
during wastewater treatment, grease  loads  are  not indicators of
scum quantities.  As  shown in Tables  4-2 and 4-3,  the  grease
content of primary sludge  can exceed  25  percent of  the  total
solids.   In  biological sludges,  it can  be  over ten  percent.
Since the quantities of these sludges are  usually large compared
to the amount of scum, it can be assumed that most of the grease
is in the  sludge, not  in the scum.   Typically, the grease content
of  raw  domestic wastewater  is 100 mg/1  (2),  but  the largest
amount of  scum  indicated  in Table 4-24 is 17  mg/1,  and in many
instances, the amounts are  lower.   At one plant,  it was  estimated
that only  five  percent of  grease was removed  in  the 'scum.   The
remainder was in the primary sludge  (131).


Scum production is influenced  by:

     •  Wastewater temperature,dissolved sol^s^^jand pH.

     •  Design  and operation  of grease  traps  at  c ojm m ejr c i a 1
        kitchens, gas  stations^ and  industries.

     •  Amount  and  character  of^septage that Jj=jmi_xed with the
        wastewater.

     •  Habits  of  residential   population  and  small  bus^ine^^ses.
        (Spent motor^oir and coo¥Tng"~Tat are likely  to be"r^moved
        as scum  if they reach  the sewers.)

     •  Preaeration and prechlorination.
                               4-55

-------
     •  Efficiency of  upstream  processes  in  removing colloidal
        grease.    This  Is  true  for chlorine  contact  tank scum,'
        since  chlorine  breaks  emulsions,  allowing  grease
        particles  to coalesce  and  float.   Chlorine dose  and
        mixing  may also affect contact  tank scum.

     •  Scum that  is  returned  from sludge handling.   Anaerobic
        digesters  usually have a  scum  layer.   Recycled digester
        supernatant may carry portions of  this  scum  back  to the
        influent wastewater.   Similarly,  scum may be returned in
        sidestreams from  gravity thickening and centrifugation.

     •  Scum removal  equipment effectiveness.   Some arrangements
        produce  better removal efficiencies  than  others.   Also,
        some arrangements produce a  scum with  a high  solids
        content  and,  therefore, a  small volume.

     •  Tendency of sludge  solids  to float _in _sed_ime_ntation tanks
        due to  formation  of gas bubbles.

     •  Process unit  from which  scum is  removed.  If primary
        sedimentation  is  used,   most of  the scum  is usually
        removed  there.  Amounts of  scum from secondary clarifiers
        and  chlorine  contact tanks are normally  small  in
        comparison.

     e  Actinomycete  growths  in  activated sludge  (50).   These
        growths  may cause large amounts of solidsto float in the
        clarifers.

At existing  treatment plants,  it  is often possible  to estimate
scum  quantities from  such  data as scum  pump  operating hours or
the  frequency  with  which scum pits must be emptied.   Design
calculations should always  allow for large variations in quantity
of scum.

        4.7.3.2   Properties of  Scum

Table 4-24  contains  information on  the solids content, volatile
content, fuel value,  and grease  content of  scum.   Scum usually
has a specific  gravity  of about 0.95  (110).

Varying quantities of vegetable and  mineral  oils,  grease, hair,
rubber  goods, animal fats, waxes,  free fatty acids,  calcium and
magnesium  soaps,  seeds,  skins, bits  of cellulosic material such
as wood,  paper or cotton,  cigarette  tips,  plastic  and  pieces
of garbage  may comprise scum (110).  When gases are entrained in
particles of primary and  secondary sludge,  these particles become
components of  scum  (126).   At one plant, a variation in scum
consistency was noted.  At 36°F  (10°C), the scum was a  congealed,
clotty  mass.   At 54°F  (20°C),  it flowed  freely, in a  manner
similar to  that of four percent combined  thickened  sludges (126).
Scum  should not  be stored for more than  a few days because the
grease will begin to decompose, with  a resulting odor  production.


                               4-56

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                                       TABLE 1-2H

                         SCUM PRODUCTION AND PROPERTIES
                  Quantity
                  (volume),
                           Quantity (dry weight)
                 gal/mil gal   Ib/roil gal
     Treatment plant  of wastewater of wastewater  wastewater   percent of total solid:
Dublin-San Ramon, Calif-
 ornia

Lower Allen Township, nea
 Harrisburg, Pa.


Northwest Bergen County,
 Oakland, New Jersey

Wichita, Kansas
Minneapolis-St. Paul,
 Minnesota

East Bay, Oakland, Calif-
 ornia
West Point, Seattle,
 Washington
Not stated

San Mateo, California

Salisbury, Maryland



Three New York City plants



Jamaica, New York City


County Sanitation Districts,
 Los Angeles County, CA

Albany, Georgia
                              Volatile     Fuel value,
                mg/1 of   Solids, solids, percent   Btu/lb dry
                                         solid3
250


 14
                           . 110
                           • 60
                    29

                    50
                         64
                         43
                         51
                 2,9

                 2.3
 Two samples. About 58 percent of nonvolatile solids was calcium carbonate.

 91 percent of total solids were oil and grease. Scum from primary sedimentation, measured after decanting
 in a heated unit.

 CSludge was tending to float in the sedimentation tanks.  Amount shown is estimate of pumpage.  Skimming
 system was unable to keep up with scum production under these poor conditions.

 1 gal/mil gal =- m3/! x 106 m3
 1 lb/rail gal - 0.12 kg/1 x 103 m
 1 Btu/ lb dry solids =2.32 kJAg        F •
        From primary sedimentation,   120
         domestic waste

 6,900a    From low lime primary sedi-   125
 3.100      mentation (pH 9.4 to 9.S),


        From gravity thickener     126



         Grease is 30 percent of
         skimmings after decant-
         ing.
        Grease balls from preaer-    127
         ation tanks.

13,000     From pri.mary sedimentation   128


   -     Averaqe, July "isto^ - Jusw   129
         1970b

   -     Maximum month          129
        Minimum month          129
14,000     1965 -  1966 data         130

        As pumped from primary     131
         sedimentation tanks
   <     As above, after decanting;   13].
         6.4 percent grease
   -     From sedimentation tanks    131
         under poor conditions0

16,750            --          114

                          110

        From primary clarifiers.    133
         Heavy grease load from
         industry

        From primary clarifiers;    133
         about 80 percent of


   -     From secondary clarifiers;   133
         (no primary)

   —     Primary sedimentation     i 34
                                                                     Heavy industrial load.
Ordinarily,   scum  should   be   seen  as   containing   pathogens.
However,  some   scum  handling  processes  may  disinfect.     If   scum
has  been  heated   to  176°F  (80°F)   for  decanting,   incinerated,  or
treated  with  a  dose of  caustic  soda  sufficient  to produce a pH of
12,  few  pathogens  are  likely to remain.


           4.7.3.3   Handling  Scum

Table  4-25  lists  the  advantages  and  disadvantages  of  various
approaches   to  scum  disposal.    Progressive  cavity-type  pumps  have
been  found   suitable for pumping scum, although  they  are  unable to
handle  large  grease  balls  (125)  unless  some   sort  of  rack  or
disintegrator  is   provided.    Pneumatic  ejectors  are  suitable  if
grease  does  not   interfere  with  the  .controls.     Piping   should  be
glass-lined  and   kept  reasonably  warm  to minimize  blockages.
                                           4-57

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                                                          TABLE 4-25

                                          METHODS OF  HANDLING SCUM
                                                                Advantages
                                                                                                            Disadvantages
      Mix with other sludges,  digest
        aerobically.
      Mix with other sludges,  digest
        anaerobically.
     Landfill separately
 Partial decomposition occurs  (134), so some
   of  the scum does not require further
   handling.
 Avoids complexity of separate handling.
 Widely used
                                               Similar to Method 1,  above.
                                               Low capital cost
 May cause  grease balls to form, which must be
   manually removed and disposed of, and which
   may  increase odors.
 May cause  petroleum contamination of sludge,
   which will  interfere with reuse.
 Degrades appearance of sludge  if to be re-
   used.
 May cause  scum buildup due to  return of scum-
   containing  liquors from sludge handling to
   influent wastewater.

 If  digester is not strongly mixed, greatly
   increases cleaning requirements.  (119)  Di-
  gester cleaning is expensive and
  odorous; also material still requires dis-
  posal .
 Even if digester is well mixed, a scum blan-
  ket will form to some extent; therefore,
  digester must be physically larger (136)
 Degrades appearance of sludge if to be re-
  used; may cause petroleum contamination.
 May cause scum buildup, like Method 1.
 Requires good decanting to avoid pumping
  excess water to the digester.

 May have very high operating cost.
 Possible odors during storage.
 Requires good decanting to minimize volume
  and fluidity of scum.
 4.  Burn in open lagoon
                                               Very low cost.
                                                                                            Severe air pollution (128); illegal under
                                                                                              present laws.
 5.  Incinerate in separate "Watergrate"
       furnace (Nichols)a
 6.  Incinerate in separate single purpose
       multiple-hearth furnace3
 7.  Incinerate in multiple-hearth furnace
       with other wastewater solids3
                                               Very small amount of ash in slurry.
                                               Very small  amount  of  ash
Low incrernenta1 cost
Fuel value of scum can be used to offset
  fuel requirements of other solids
High capital cost,  especially for small  plants
Despite low emissions, may not be acceptable
  to air pollution regulators.
Problems with feed systems.

High capital cost (130)
High maintenance cost
Despite low emissions, may not be acceptable
  to air pollution regulators
Requires good decanting

Requires good decanting
Requires very careful feed to the furnace,-
  otherwise causes  high maintenance  and
  severe smoke problems.   These problems can
  be avoided.  (137)
     Incinerate  in  fluidized  bed  furnace
       with other wastewater  solidsa
                                               Similar to Method 7.
                                             Unless well decanted,  can tax furnace
                                               capacity. (126)
                                             If scum is mixed with  sludge before injec-
                                               tion into furnace,  unstable operation  is
                                               likely.   (126)
 9.   Reuse  for cattle  feed
                                              Provides reuse, not disposal (however, do
                                                not expect revenue
                                              Low capital cost
                                             Toxic organic materials (e.g.,  DDT)  tend to
                                               concentrate in grease
                                             Erratic market demand for waste grease  (133),
                                               it may be impossible to find  anyone
                                               that wants it.
                                             Treatment for reuse must begin  within a  few
                                               days; otherwise grease begins to decompose.
                                                                                            Requires good decanting  because  of  long dis-
                                                                                              tance transportation.
                                                                                            Subject to interference  from actinomycete
                                                                                              growths in activated sludge, which  increase
                                                                                              the amount of solids that  are  not grease
                                                                                              but are in the scum.
10.   Reuse  for low  grade  soap  manufacture
                                              Same as Method 9.
                                                                                            Similar to Method 9, but  less serious.
                                                                                              Caustic  soda  could be added at the treat-
                                                                                              ment plant, preventing  decomposition and
                                                                                              probably making the  material more usable
                                                                                              to grease reclaimers, but raising operating
                                                                                              costs.
11.   Return to influent  wastewater
                                              Almost zero direct cost

                                              Highly suitable for scum from chlorine con-
                                                tact tanks, secondary clarifiers, etc.
                                                when scum is removed from primary sedi-
                                                mentation tanks
                                             Slight increase in hydraulic  load  on  the
                                               treatment plant
                                             Inapplicable to the main source  of scum
                                               (primary sedimentation tanks if  used,
                                               secondary clarifiers if primary  tanks are
                                               not used).
3For further information on scum incineration, see Chapter 11, High Temperature Processes.
                                                               4-58

-------
Piping  should be heated to  a  minimum of 60°F  (15°C).   Higher
temperatures  are preferred,  especially if pipe  sizes  of less
than four-inch diameter  (100  mm)  are  used or if  pipe  lengths are
substantial.   Flushing connections and  cleanouts  should  be
liberally  provided.  When scum  is to be incinerated,  a small
amount  of  fuel  oil  should be added  as  a  convenient means  of
ensuring that the scum can be pumped  (137).  An in-line  grinder
should be provided if decanting or  incinerating  is to  take place
(125,137).

Decanting (simple thickening  by  flotation)  is occasionally used
to increase the  solids content of  the scum.  Decanting requires
some care in design,  in order to reduce the  effects of  unpleasant
odor and high  grease  and  solids  content in the decanted water.
At  least two  manufacturers  market  a  heated  decanting unit.
Heating scum to  about 180°F  (80°C) greatly  improves  the
separation  of solids from water.    Thus, the decanted  water will
have a  lower  solids and grease  content, whereas  the thickened
scum will contain less moisture.


    4.7.4  Septage

Domestic  septic  tank wastes  (septage) may be  defined  as  a
partially  digested mixture  of liquid and  solid  material that
originates  as  waterborne domestic wastes.  Septage  accumulates in
a septic  tank or cesspool over  a  period of several months  or
years.   Normally, household wastes derive from  the toilet, bath
or  shower,  sink,  garbage disposal, dishwasher, and  washing
machine.    Septage  may  also  include  the pumpings from  the
septic  tanks  of schools,  motels,   restaurants,  and  similar
establishments.   Septage is frequently discharged  into municipal
wastewater  systems.   With careful design and operation, municipal
systems can handle  septage adequately  (138-140).


        4.7.4.1   Quantities of Septage

For  Connecticut,  Kolega  and  others  (138) estimated  residential
septage  at  66 gallons per capita  per year (250  1/capita/yr).
Some tanks  were  pumped  only  after  many years of service; others
were pumped more than three times  a  year.  Frequent  pumping was
associated   with  seasonally  high  groundwater  levels.    Based  on
the  detailed  observations  of  three  tanks,  Brandes  recommended
designing  for a  septage  volume of 53 gallons per  capita per
year  (200  1/capita/yr)  (141).   Others have recommended 50  to
360 gallons per  capita per year (189 to 146  I/capita/year).


        4.7.4.2   Properties of Septage

Table 4-26  contains a wide  range  of data on various constituents
of  septage.   Septage  may  foam  and generally  has a  highly
offensive odor (140).   Settling  properties  are  highly variable.
Some  samples  settle  readily to  about 20  to  50   percent  of
their  original  volume,  whereas others  show little settling.
                              4-59

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Significant   amounts  of  grit  may   be  present   (140).    Large
concentrations  of  total   coliforms,   fecal  coliforms,   and  fecal
streptococci  have  been found in  septage (140,141).
                                TABLE 4-26
               CHARACTERISTICS OF DOMESTIC SEPTACE  (140)
         Parameter

Total solids (TS)
Total volatile solids (TVS),
  percent of total solids
Suspended solids  (SS)
Volatile suspended solids
  (VSS), percent  of suspended
  solids
5-day biochemical oxygen de-
  mand (8005)
Total chemical oxygen demand
  (CODT)
Soluble chemical  oxygen de-
  mand (CODs)
Total organic carbon (TOC)
Total Kjeldahl nitrogen  (TKN)
Ammonia nitrogen  (NH3~N)
Total phosphorus  (Total P)
pH (units)
Grease
Linear alkyl sulfonate (LAS)
Iron  (Fe)
Zinc  (Zn)
Aluminum (Al)
Lead  (Pb)
Copper  (Cu)
Manganese (Mn)

Chromium (Cr)
Nickel  (Ni)
Cadmium  (Cd)
Mercury  (Hg)

Arsenic  (As)
Selenium (Se)
Mean3


38,800

  65.1
13,014


  67.0

 5, 000
42,850

 2,570b
 9,930

   677
   157
   253
   6.9C
 9,090
   157
   205
  49.0
   48
   8.4
   6 .4
  5.02
 Standard
deviation3
    Range'
    07
    90
  0.71
  0 .28
  0. 16
 0.076
 23,700

   11.3
  6,020
    9.3


  4 , 570

 36,950


  6,990

    427
    120
    178


  6, 530
     45
    184
   40.2

     61
   12.7
    8.3
   6.25

   0.64
   0.59
   2.17
   0.79

   0.18
  0 .074
3,600 - 106,000

   32 - 81
1,770 - 22,600
Number of
 samples

   25

   22
   15
51
1,460
2,200

1,316
66
6
24
6.0
604
110
3
4.5
2
1.5
0.3
0.5
0.3
0.2
<.05
< .0002
0.03
<0.02
- 85
- 18,600
- 190,000
_
- 18,400
- 1,560
- 385
- 760
- 8.8
- 23,468
- 200
- 750
- 153
- 200
- 31
- 38
- 32
- 2.2
- 3.7
- 10.8
- 4.0
- 0.5
- 0.3
15
13
37
21
9
37
25
37
25
17
3
37
38
9
5
19
38
12
34
24
35
12
13
 aValues are concentrations in mg/1, unless otherwise noted.
 bSoluble COD is 6 percent of total COD.
 °Median.

          4.7.4.3   Treating Septage in  Wastewater
                    Treatment Plants

 When  treated  at wastewater  treatment plants,  septage  is  often
 mixed with the influent wastewater.   In some situations,  however,
 it  is  treated  or  pre-treated  separately.    Septage  may  also  be
 added  directly  to  the  wastewater  sludge.   Septage  is  delivered
 from   tank   trucks,   loaded  into  the   system  immediately,   or
 temporarily  stored   and  added gradually  to  the  wastewater  or
 sludge.    Holding  tanks for  septage  are  therefore  recommended  in
 many cases.
                                    4-60

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If  septage  is added  to  wastewater,  the  quantities  of  all
wastewater  solids  in the treatment plant  increase  for  the
following reasons:

     •  Septage contributes to grit, scum, and screenings.

     •  The  suspended solids   in  the septage may  be largely
        removed in primary  sedimentation,  increasing  the  amount
        of primary sludge.  One pilot study found  55  to  65 per-
        cent removals  of  septage  suspended  solids  (140),  but
        very different  values might occur under other conditions.

     •  In  biological processes,  septage increases the  8005
        load and,  therefore, the sludge production.  Furthermore,
        septage may  produce as much as twice the amount of  sludge
        per unit BOD5  removed as ordinary  wastewater,  since the
        septage has  a  high ratio  of  suspended  solids to -8005
        (140).

     •  Addition of  septage  increases the phosphorus load  at  a
        treatment  plant.    For   plants  which  must  meet  effluent
        limits  on phosphorus,  the   addition  of  septage  will
        increase the necessary  chemical  dose.   Thus,  costs and
        the amount of chemical sludge  will increase.

At some plants, sludge thickening and dewatering properties have
been degraded by septage,  but there are few data available on the
extent  of the problem,  and different results are obtained at
different  locations.   At  Shrewsbury,  Massachusetts,  dewatering
difficulties were encountered when  the  septage/sewage hydraulic
ratio  exceeded  about  0.0033  (140).   Furthermore, problems
associated with  bulking  activated  sludge  may  be related to
septage  (140).   Bulking  sludge  has  very poor thickening and
dewatering properties.  The metals content of septage may also be
high.


    4.7.5  Backwash

Wastewater  is  sometimes filtered to remove suspended solids.  As
used  in  this  section,  the term  "filters" includes sand filters,
dual  and  mixed-media  filters,  and  microstrainers.    Solids
accumulate  in  filters  as   they  are removed  from  the wastewater.
They  are  subsequently  removed  from the  filters  by backwashing.
The  volume  of  backwash water  is great,  often  several percent of
the  total  wastewater flow.  However,  the  quantity of suspended
solids in  backwash  is  normally  about  300  to  1,500 mg/1 (0.03 to
0.15 percent).  The  dry weight load is usually small compared to
those from primary,  biological,  and chemical sludges.

Backwash  is normally  returned  to  the influent  wastewaters and
its  suspended  solids are  removed in wastewater processes such as
primary sedimentation and  activated sludge.


                              4-61

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When  designing a  plant with  filters,  the  following  measures
should be taken to allow for  backwash  and associated solids:

     •  If the backwash  is  produced  intermittently, as is usually
        the case,   then  a spent backwash holding  tank  should  be
        provided.   Thus,  the  backwash  need  not drastically
        increase the flows to  be  treated.   Solids may  settle  to
        some  extent in   the  holding tank.   Therefore, washout
        facilities or  possibly  air agitation should be  provided.

     •  An allowance for increased  flow  due to recycle  of
        backwash  should  be  included  when sizing  wastewater
        treatment  processes.

     •  Filter  solids  should be  allowed  for when  primary,
        biological,  and  chemical  sludge quantities are  computed.


    4.7.6  Solids  from Treatment of  Combined Sewer
           Overflows

Solids generated  in the treatment  of  combined  sewer  overflows
(CSOs) may be  treated separately, or  discharged under  non-storm
conditions  to the  dry-weather sludge treatment and   disposal
facilities.   The  volumes and characteristics of solids  produced
from CSO treatment  vary  widely.   The  volume  of solids  residuals
evaluated in a recent  study ranged from less than one percent  to
six percent of the raw volume treated  and contained 0.12 percent
to 11 percent  suspended  solids (142).  The  volatile content  of
these sludges varied between  25  percent  and  63 percent,  with
biological  treatment residuals  showing  the  highest   volatile
content  (about 60  percent).

Pesticides and PCB  concentrations  in  the CSO  sludges  have  been
found to be high at some locations  (142).   PCB concentrations  as
high as  6,570  ug/kg dry  solids  have  been measured.

Heavy metal concentrations  in the  CSO  sludges have been found  to
vary widely.   The  range of  heavy  metal  concentrations  for the
sites studied  in reference  142  are given in Table 4-27.

                            TABLE 4-27

                     METALS CONCENTRATIONS IN
                     SOLIDS FROM TREATMENT OF
                   COMBINED SEWER OVERFLOWS (142)

                                  Concentration,
                 Metal           mg/kg dry solids
Zinc
Lead
Copper
Nickel
Chromium
Mercury
697
164
200
83
52
0.01
- 7,154
- 2,448
- 2,454
- 995
- 2,471
- 100.5
                               4-62

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4.8  References

  1.   Water  Pollution Control Federation.    MOP 8 Wastewater
      Treatment  Plant  Design.    Water  Pollution  Control
      Federation.  1977.

  2.   Metcalf and Eddy, Inc.   Wastewater Engineering;  Treatment
      Disposal, Reuse.   2nd  Edition.   McGraw-Hill Book Company.
      1979.

  3.   American Public  Health Association,  American  Water  Works
      Association,  Water  Pollution Control  Federation.   Standard
      Methods  for  the Exami n at ion__of__W a t e r	and W a s t e w a t e r .
      14th Edition.   1975.

  4.   Schmidt,  O.J.  "Wastewater Treatment  Problems  at  North
      Kansas  City,   Missouri.     Journal Water Pollution Control
      Federation.   Vol. 50,  p.  635.   (1978).             	

  5.   Babbitt,  H.E.  and  E.R.  Baumann.   Sewe
      Treatment.   Eighth  Edition, Wiley, 1958.

  6.   Brisbin,  S.G.   "Flow  of Concentrated Raw Sludges in Pipes."
      Journal Sanitary Engineering  Diyj_s_ion_,___ASCE_.   Volume  83,
      SA3,  1957.

  7.   USEPA Review  of  Techniques  for Treatment  and  Disposal  of
      Phosphorus-Laden  Chemical  Sludges.  Office  of  Research  and
      Development,Cincinnati,Ohio,45268.   EPA-600/2-79-083,
      February  1979.

  8.   Knight,  C.H.,  R.G.  Mondox,  and  B.  Hambley.    "Thickening
      and Dewatering Sludges  Produced in  Phosphate Removal."
      Paper presented at  Phosphorous Removal  Design  Seminar,
      May 28-29,  1973,  Toronto.

  9.   Young,  J.C.,  J.L.  Cleasby,   and  E.R.  Baumann.    "Flow
      and Load Variations  in  Treatment Plant Design."   Journal
      Environmental  Engineering  Pivis^io^n_AS_CE.   Vol.  104,  (EE2),
      p.  289,  (1978).

 10.   Fischer,  A.J.   "The Economics  of Various Methods of Sludge
      Disposal."   Sewage  Works Journal.  Vol. 9  (2).  March 1936.

 11.   Anderson, C.N.  "Peak Sludge Loads at  a Municipal Treatment
      Plant."    Presented at   the  44th  Annual  Meeting  of  the
      Pacific Northwest Pollution Control Association.  Portland,
      Oregon.  November  2-4,  1977.

 12.   USEPA.  Cost-Effective  Design of  Wastewater Treatment
      Facilities  Based  on Field  Derived Parameters.    Office  of
      Research  an Development,Cincinnati, Ohio,45268.  EPA-670/
      2-74-062. July 1974.
                               4-63

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13.  Babbitt, H.E.  Sewerage and Sewage Treatment.  6th Edition
     Wiley.  1947.              '  — -
"
14.  Smith,  J.E.,  Jr.    "Ultimate  Disposal of  Sludges
     Technical  Workshop  on  Advanced  Waste Treatment,  Chapel
     Hill,  North Carolina.  February  9-10,  1971.

15.  Prazink,  J.A.    "Process  Control in the Real World."  Deeds
     and_ Data  (WPCF).  July 1978.                          ~" -

16.  Burd,  R.S.   A Study  of Sludge  Handling and  Disposal.
     Federal Water  Pollution Control  Administration.   Report
     WP-20-4.   1968.

17.  Dewante and Stowell  and  Brown and Caldwell.   1973 Study,
     Central Treatment  Plant,  County  of Sacramento Final Report,
     County of  Sacramento,   Sacramento,  California.    February
     1974.

18.  Metropolitan Engineers.   West Point Waste  Activated Sludge
     Withholding  Experiment .   Report  to  Municipality  of
     Metropolitan Seattle, Washington.  October  1977.

19.  USEPA.   Process Design  Manual for Suspended Solids Removal
     Removal .    Technology Transfer,  Cincinnati, Ohio,  45268.
     EPA-625/l-75-003a.  January 1975.

20.  Owen,  M.B.   "Sludge  Incineration."   Journal  Sanitary
     Engineering Division _ASCE .  Vol. 83, p. 1177.  1957.

21.  Metcalf,  L.  and  H.P.  Eddy.   American  Sewage  Practice
     Vol.  Ill,  Disposal of Sewage.  Third  Edition, McGraw-Hill,
     1935.

22.  Municipality  of  Metropolitan Seattle,  Washington.   Data
     from Renton Treatment Plant operations reports, 1976.

23.  Teletzke,  G.    "Wet Air Oxidation  of  Sewage  Sludges".
     Process Biochemistry .  1966.

24.  Kersch,  G.T.   "Ecology  of  the Intentinal Tract."  Natural
     History.   November 1973.

25.  Heukelekian, H., H.E. Orford, and R.  Manganelli.   "Factors
     Affecting  the Quantity  of Sludge  Production  in  the
     Activated  Sludge  Process."    Sewage and  Industrial Wastes.
     Vol.  23,  (8),  1951.

26.  Colbaugh,  J.E.  and A. Liu.   "Pure Oxygen  and Diffused Air
     Activated Sludge Studies at  Hyperion."  Presented at 48th
     Annual Conference  of  the California  Water  Pollution Control
     Association,  South  Lake Tahoe,  California.   April 14-16,
     1976.
                              4-64

-------
27.  USEPA.     Characterization  of  the Activated Sludge Process
     Office  of  Research  and  Development.   Cincinnati^  Ohio,'
     45268.   EPA-R2-73/224,  April 1973.

28.  Wuhrmann,  K.  "High-Rate Activated Sludge Aeration and Its
     Relation  to Stream  Sanitation."  -Sewage_an_d_I ndustr ial
     Wastes.   Vol.  26,  (1),  (1954).          ™~  — —      —

29.  Eckenfelder, W.W.   Water  Quality Engineering for Practicing
     Engineers.   Barnes  &  Noble,  1970.                  ~~~

30.  Brown and  Caldwell.   West Point Pilot Plant Study,  Vol. II,
     Activated  Sludge  Report  for Municipality  of  Metropolitan
     Seattle.  Seattle,  Washington.  December 1978.

31.  Beere,  C.W., and  J.F.  Malina.    Application  of Oxygen
     to Treat  Waste From Military  Field  Installations':   An
     Evaluation  of  an  Activated  Sludge  Process  Employing'
     Down flow. Bubble  Contact Aeratioru  Part  II ,  Final ReportV
     University  ol Texas,  Austin, ~ Texas 78712  EHE-740-01,
     July  1974.

32.  Sletten, R.S. and  A.  Viga,  Wastewater Treatment  in Cold
     Climates ,  Report to  U.S. Army Cold  Regions Research  and
     Engineering Laboratory,  Hanover, New Hampshire, 03755.
     1975.

33.  Sayigh,  B.A.,  and J.F.  Malina.  "Temperature Effects on the
     Activated  Sludge Process."   Journal Water Pollution Control
     Federation.   Vol. 50,  p.  678, 1978.

34.  Middlebrooks,  E.J., and C.F. Garland.   "Kinetics  of  Model
     and  Field  Extended-Aeration Wastewater  Treatment Units."
     Jour_na_l  Water Pollution Control Federation.  Vol. 40,  p 586
     1968.
"Hydraulic Control of Activated Sludge
e and Indus_tria_]L_W_a_s^es_.  Vol.  30,  (3),
35.   Garrett,  M.T., Jr.
     Growth Rate."   Sewage
     1958.

36.   Obayashi,  A.W.,  B.  Washington,  and C.  Lue-Hing.   "Net
     Sludge Yields  Obtained During  Single-stage  Nitrification
     Studies  at Chicago's West-Southwest  Treatment  Plant."
     Proceedings of 32nd Industrial Waste  Conference.   May 10-
     12,  1977,  Purdue  University, Ann  Arbor Science,  p.  759,
     1978.

37.   USEPA.    Process  Design  Manual  for  Nitrogen  Control.
     Technology Transfer, Cincinnati, Ohio, 45268, EPA-625/1-75-
     007,  October 1975.

38.   Hopwood,   A. P.  and A.L. Downing.    "Factors  Affecting the
     Rate   of  Production  and Properties  of Activated  Sludge  in
     Plants Treating Domestic Sewage."  Journal of the Institute^
     of Sewage Purification.  Part 5, 1965.
                              4-65

-------
39.  Chapman,  T.D.,  L.C.  Matsch  and  E.H.  Zander.   "Effect of
     High  Dissolved  Oxygen  Concentration  in  Activated  Sludge
     Systems."  Journal  Water  Pollution^  Control  Federation.
     Vol. 49,  p.  2486,  (1976).   ~~  ———————

40.  Miller,  M.A.  "Two  Activated Sludge  Systems  Compared."
     ^ter_an_d__Wastes Engineering.  Vol.  15,  (4),  (1978).

41.  Kalinske,  A. A.  "Comparison of Air  and Oxygen."   Journal
     Water Pollution Control  Federation.  Vol.  48,  (11), (1976) ."

42.  Parker,  D.S., and M.S. Merrill.   "Oxygen  and Air Activated
     Sludge:   Another  View."   Journal  Water Pollution Control
     Federation.   Vol.  48,  p.  2511, (1976).

43.  Muck, R.E.,  and C.P.L.   Grady,  Jr.    "Temperature Effects
     on  Microbial Growth  in CSTRs."   Journal Environmental
     Engineering Division ASCE.   Vol  101,""(EE5), p.  1147,
     October  1974.

44.  Randall,  C.W., Jr.  Discussion  of "Temperature  Effects
     on  Microbial Growth  in CSTRs."   Journal Environmental
     Engineering Division ASCE.   Vol  101,  (EE3), p.  1458,'
     June 1975.                *"  "

45.  Gujer, W.  and  D.  Jenkins.   "The  Contact Stabilization
     Activated   Sludge  Process—Oxygen Utilization,  Sludge
     Production,  and Efficiency."   Water ^Res e arch .  Vol. 9, 516,
     (1975).

46.  USEPA. Extended Aeration  Sewage Treatment  in Cold Climates.
     Office of Research and Development,  Cincinnati, Ohio 45268.
     EPA-660/2-74-070,  December  1974.

47.  USEPA.   Design  Guidelines  for  Biological  Wastewater
     Treatment Processes.   Office  of  Research  and Development,
     Cincinnati,  Ohio 45268.   EPA  Report  11010  ESQ, August 1971.

48.  Young, J.C., J.L.  Cleasby,  and E.R.  Bauman.   "Flow and Load
     Variations  in Treatment  Plant  Design."   Journal Environmen-
     tal Engineering Division  ASCE .  Vol.  104,  (~EE2 ) ,  April
     1978.

49.  USEPA.   Flow Equalization.   Municipal Seminar Publication.
     Municipal  Seminar  Publication.    Cincinnati,  Ohio  45268,
     EPA 625/4-74-006.  May 1974.

50.  Pipes, W.O.    "Actinomycete  Scum Production  in Activated
     Sludge  Processes."    Journal   Water  Pollut ion  Cp_n_t_r_o_l
     Federation.   Vol.  50,  p.  628,  (1978).

51.  Strokes,  H.W.,  and  L.D. Hedenland.   "Tertiary Treatment:
     Wrong Solution  to a  Non-Problem?"  Civil Engineering-ASCE
     Vol. 44,  (9), (1974)
                              4-66

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52.
53.
54.
Dick, R.I. "Folklore in the Design of Final Settling
Tanks." Journal Water Pollution Control Federation.
Vol. 48, p. 633, (1976).
Mueller, J.A., T.J. Mulligan, and D . M . DiToro. "Gas
Transfer Kinetics of Pure Oxygen System." Journal Environ-
mental Engineering Division - ASCE . Vol. 99, (EE3),
p. 269, (June 1973).
USEPA. Status of Oxygen/Activated Sludge Wastewater
     Treatment.   Municipal  Seminar  Publication,  Cincinnati,
     Ohio 45268.   EPA  625/4-77-003.  October 1977.

55.   Anderson,  M.S.  "Comparative  Analysis of  Sewage  Sludges."
     Sewage and  Industrial Wastes.  Vol. 28, (2), (1965).

56.   Zimmermann,  F.J.  Chemical  Engineering.    August  25,  1958.
     pp 117-120,  as cited by  Committee on Sanitary Engineering
     Research,  "Sludge Treatment and Disposal  by the Zimmermann
     Process."    Journal  Sanitary  Engineering   Division  -  ASCE.
57.
58.
59.
60.
61.
Vol. 85, (SA4), July 1959.
Dick, R.I. "Sludge Treatment." Phys iochemical Processes
for Water Quality Control. W.J. Webe
Interscience, 1972.
Hurwitz, E., G.H. Telezke, and W.B.
Oxidation of Sewage Sludge." Water
r , Ed i tor . Wi ley
Gitchel "Wet Air
and Sewage Works.
August 1965.
Anderson, M.S. "Fertilizing Characteristics of
Sludge." Sewage and Industrial Wastes. Vol. 31,
(1959). ^
Rudolfs, W. "Fertilizer and Fertility
Sludge." Water and Sewage Works. 1949.
Values of
Ford, D.L. "General Sludge Characteristics." Water
Improvement by Physical and Chem
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p. 678,
Sewage
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     Eckenfelder and Gloyna  Editors.  1970.

62.   Sezgin,  M.   The Effect  of  Dissolved Oxygen Concentration
     on Activated  Sludge Process  Performance.    Ph.D.  Thesis,
     University of California,  Berkeley California, 1977.

63.   Bentley,  T.L. ,  and  D.F.  Kincannon.    "Application  of
     Activated  Sludge Design  and Operation."   Water  and Sewage
     Works.   Reference  Issue,  A-pril  30, 1978.

64.  Brown  and Caldwell.   West  Point  Pilot  Plant  Study,
     Vol. Ill,  Fixed Growth Reactors.    Report for Municipality
     of  Metropolitan  Seattle.    Seattle,  Washington  98101.
     December 1978.
                              4-67

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65.  USEPA.  Converting Rock Trickling  Filters  to Plastic Media;
     Design and Performance.    Office  of Research  and  Develop-
     ment, Cincinnati, Ohio  45268.   Contract 68-03-2349, Draft
     Report,  April 1978.

66.  Derived   from  E.  Herr.   Special  Solids  Balance  Operating
     Reports,  Sacramento City  Main  Treatment Plant,  Sacramento
     County   Regional  Sanitation   District,   Sacramento,
     California 95814.   1977-1978.

67.  USEPA,     Attached  Growth Biological Wastewater Treatment;
     Estimating Performance and Construction Costs and Operation
     and Maintenance  Requirements.    Office  of  Researchand
     Development,  Cincinnati,  Ohio 45268.   Contract 68-03-2186,
     January  1977.

68.  USEPA.   _T_he  Coupled Trickling_  Filter-Activated  Sludge
     Process:   Design  and Performance.   Office~3fResearch  and"
     Development,  Cincinnati,  Ohio  45268.    EPA-6001  2-78-116,
     July 1978.

69.  Fair, G.M.,  and J.C.  Geyer Water Supply and Wastewater
     D_jLsp_os_ a 1.  Wiley,  1954.                "        ——       .

70.  Smith, J.E.,  Jr.  Ultimate Disposal of Sludges.   Technical
     Seminar/Workshop  on Advanced Waste Treatment  Chapel Hill,
     North Carolina, February  9-10, 1971.

71.  Vesilind,  P. A.  Treatment  and   Disposal  of  Wastewater
     S_liad_g_es_.   Ann Arbor  Science Publishers Inc., 1974.

72.  Heukelekain,  H.,  and E.S. Crosby.   "Slime Formation  in
     Sewage.   III.  Nature and Composition of  Slimes."   Sewage
     and Industrial Wastes.  Vol.  28, (28), (1956).

73.  Stanley,  W.E.  Personal  Communication,  1967,  as cited  in
     1974 edition  of this manual.

74.  Marki, E.  "Results  of Experiments by EWAG with  Rotating
     Biological  Filter."    Eidg  Technische  Hochschule,
     Zurich-Fortbildungskurs der EWAG,  (1964).

75.  Kolbe, F.F.   "A  Promising New Unit  for  Sewage  Treatment"
     Die Sivielle__In_genieur (S.  Africa).  December 1965.

76.  Gillespie,  W.J., D.W.  Marshall,  and  A.M.  Springer,  "A
     Pilot Scale Evaluation  of Rotating Biological Surface
     Treatment of Pulp and  Paper Mill  Wastes."  Proceedings of
     29th  Industrial  Waste Conference May  7-9,  1974,  Purdue
     University, p.  1026, 1975.

77.  Stenquist,  R.J.,  D.S.  Parker, W.E. Loftin and R.C.  Brenner.
     "Long-Term Performance  of  a  Coupled  Trickling Filter-
     Activated  Sludge  Plant."    Journal Water Pollution Control
     Federation.  Vol.  49,  (11), (1977).
                              4-68

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78.   Reimer,  R.E.,  E.E. Hursley, and R.F. Wukasch  "Pilot Plant
     Studies  and Process  Selection  for Advanced Wastewater
     Treatment,  City  of  Indianapolis,  Indiana."   Presented
     at  National  Conference  on  Environmental  Engineering,
     July 12-14,  1976,  Seattle, Washington.

79.   USEPA.   Process  Design Manual  for Phosphorus Removal.
     Technology   Transfer,  Cincinnati^Ohio45268.EPA-625/1-
     76-001.   April  1976.

80.   Brown  and  Caldwell.   West  Point Pilot  Plant  Study,
     Vol. IV,  Chemical  Treatment.    Report for Municipality of
     Metropolitan  Seattle,  Seattle,   Washington,  98101,
     December  1978.

81.   Gulp,  R.L. ,  G. Wesner,  and G.  Gulp.   Handbook of Advanced
     Wastewater  Treatment,  Second Edition.    Van  Nostrand
     Reinhold,  1978.

82.   USEPA Lime  Use  in Wastewater  Treatment:   Design and  Cost
     Data_.  Office of Research  and Development,  Cincinnati,
     Ohio 45268.  EPA-600/2-75-038, October 1975.

83.   Scott,  D.S.,  and  H.  Harling.   Removal  of  Phosphates  and
     Metals  from  Sewage   Sludge.    Environmental Protection
     Service  (Canada) , Research" Report No.   28,  about 1975.

84.   Merrill,  D.T. and  R.M. Jorden  "Lime-induced Reactions
     in Municipal Wastewaters."  Journal Water Pollution Control
     Federation.  Vol.  49,  (12),  (1975).

85.   Baillod,  C.R., G.M.  Cressey,  and  R.T. Baupre'.  "Influence
     of  Phosphorus  Removal  on Solids  Budget."   Journal Water
     Pollution  Control  Federation.   Vol.  49,   (1),  (1977).

86.   Finger.  R.E.  "Solids  Control  in  Activated  Slduge  Plants
     with  Alum."    Journal  Water Pollut i q n _C_qn trol^ Feder a t i o n.
     Vol. 45,  (8),  (.1973) .

87.   Gossett,  J.M., P.L. McCarty, J.C. Wilson, and D.S.  Evans,
     "Anaerobic  Digestion  of .Sludge from Chemical Treatment."
     Journal  Water Pollution  Control  Federation.   Vol.  50,
     pT	513V (1978V.	"	"  ——          —

88.   Parker,  D.S.,  D.G.  Niles  and  F.J. Zadick  "Processing
     of  Combined Physical-Chemical-Biological  Sludge."   Journal
     Water  Pollution^ _C_g_ n t_r o j^_ Federation .   Vol.  46 p. 2281,
     (1974).

89.   Furr,  A.K.  "Multielement  and  Chlorinated  Hydrocarbon
     Analysis  of Municipal  Sewage  Sludges of  American Cities."
     E n v i r o nm e n t^a 1 S c i e_n_ce_ a nd  Technology;.    Vol.  10,  (7),
     (1976).


                              4-69

-------
 90.  Sommers, L.E.  "Chemical  Composition  of Sewage Sludges and
     Analysis of Their  Potential  Use  as Fertilizers.   "Journal
     of_Ejwironmen_tal Quality.   Vol. 6,  (2),  (1977).     ~~~   ~~


 91.  Berrow,  M.L.,  and  J.  Webber.    "Trace Elements  in Sewage
     Sludges."   Journal of  Science of  Food and  Agriculture
     Vol.  23, January 1972.       "™~      ~	~~"	~	~—~~

 92.  Gulbrandsen,  R.A.,  N.   Rait,  O.J.  Krier,  P.A. Baedecker,
     and  A. Childress.   "Gold,  Silver,  and Other Resources
     in  the Ash of Incinerated  Sewage  Sludge  at Palo  Alto,
     California, a  Preliminary  Report."   U.S. Geological Survey
     Circular 784,  1978.                                 ~	

 93.  Bernard, H.  "Everything You  Wanted  to  Know  About  Sludge
     But Were Afraid  to  Ask."   Proceedings  of the  1975 National
     Conference  on Municipal  Sludge  Management  andDisposal.
     Information Transfer,  Inc.   1975.

 94.  Metropolitan Engineers.   Draft Facility  Plan  for Upgrading
     Metro Puget Sound Plants-System Wide  Volume, Part 1,  Basis
     of  Planning.    Report to  Municipality  of  Metropolitan
     Seattle, Seattle, Washington  98101.   1976.

 95.  USEPA "Elemental   Analysis  of  Wastewater  Sludges  from
     33  Wastewater  Treatment  Plants in  the United States."
     Pretreatment  and Ultimate Disposal  of Wastewater  Solids.
     Office of Research and  Development^Cincinnati,Ohio 45268.
     EPA-902/9-74-002, May  1974.

 96.  Carun,  G.F.,  and L.J.  McCabe.   "Problems  Associated with
     Metals  in Drinking Water."    Journal  AmericanJWater Works
     Association.  Vol. 67,  (11),  (1975).

 97.  Sacramento Area Consultants.   Sewage Sludge Management
     Program Final  Report-Vol.  I  SSMP Final Report, Work Plans,
     Source SurvVy.   Sacramento Reg ional County Sani tat ion
     District, Sacramento,  California  95814.  September 1979.

 98.  Eason, J.E., J.G. Kremer, and F.D.  Dryden "Industrial Waste
     Control  in Los  Angeles  County."  Journal Water Pollution
     Control Federation.  Vol. 50, p.  672, (1978).

 99.  Monteith,  H.D.,  and J.P.  Stephenson.    Development  of  an
     Efficient  Sampling  Strategy  to Chara cterize  Digested
     Sludges"^Environmental  Protection Service  (Canada) ,
     T'raining and  Technology  Transfer Division, Research Report
     No. 71, 1978.

100.  Epstein,  E.,  and R.L. Chaney.   "Land  Disposal  of  Toxic
     Substances and  Water-Related Problems."  Journal  Water
     Pollution Control Federation.  Vol. 59, (8),  (1978).
                               4-70

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101.   Stern,  L.  "The Great Cadium Controversy."   Sludge Magazine.
      Vol.  1,  (2), March/April 1978.                          "

102.   Peterson, J.R.,  C. Lue-Hing,  and D.R.  Zenz.   "Chemical
      and Biological  Quality  of  Municipal Sludge"  Recycling
      Treated Municipal Wastewater  and Sludge  Through Forest
      and Cropland^W. V .   Sopper  iTndTTr^  Kardos  Editors.
      Pennsylvania State  University Press.   p.  26, (1973).


103.   Ohara,  G.T.,  S.K. Raksit,  and D.R.  Olson.    "Sludge
      Dewatering  Studies at  Hyperion Treatment Plant."  Journal
      Water Pollution Control  Federation.   Vol. 50,  p~912,
      (1978) ."'     ~~~   '     ——   -

104.   Chawla,  V.K.,  J.P.  Stephenson,  and  D.  Liu.    "Biological
      Characteristics  of Digested Chemical  Sewage  Sludges."
      Proceedings, Sludge Handling and Disposal  Seminar,  Toronto^
      Ontario, 1974.EnvironmentalProtection  Service(Canada)7
      Conference Proceedings No. 2.

105.   Lawrence,  J. ,  and  H.M. Tosine.   "Polychlorinated  Biphenyl
      Concentrations  in Sewage  and  Sludges  of  Some Waste
      Treatment  Plants  in   Southern  Ontario."   Buileti n  o f
      Environmental Contamination and Toxicology.   Vol18,(1) ,
      (1977)."

106.   Sacramento Area Consultants.   Sewage Sludge  Management
      P r o g r a m Final  Rep o r t  —  V q 1 _.  2,   SSB_ Operation	and'
      P e r f o rma nee.

107.   Dube,  D.F.f  G.D.  Veith,  and G.F. Lee.    "Polychlorinated
      Biphenyls  in Treatment  Plant Effluents."   Journal  Water
      P o 11 u t i o n C o rvtr ql_ _Federati on.  Vol. 46, p.  966,  (1974).

108.   Michigan Water Resources  Commission.   "Monitoring  for
      Polychlorinated  Biphenyls  in  the Aquatic Environment."
      1973.

109.   American Society  of  Civil  Engineers  and Water Pollution
      Control  Federation.  Manual of Practice  8 Sewage Treajtmejrt
      Plant  Design.  1959 Edition,  (1972 printing).

110.   Sacramento  Area  Consultants.    Study  of  Wastewater Solids
      Processing and Disposal.    Report to  Sacramento  Regional
      County  Sanitation  District.   Sacramento,  California 95814,
      June  1975.

111.   Merz,  R.C.  "Operation  of  Screens,  Grit Chambers  and
      Sedimentation  Tanks."  Sewage  Works Journal.   Vol.  17,
      (4) ,  July  1945.

112.   Fisichelli,  A.P. "Raw Sludge  Pumping--Problems  and
      Interdisciplinary Solutions."   Journal Water  Pollution
      Con t ro1  Federation.  Vol. 42, (11), (1970).

                              4-71

-------
113.  Volpe, G.J.  "Static Screen and Rotostrainer Screenings
      Dewatering  Test."   Printed  as Appendix  B  of  Study to
      Wastewater  Sol ids  Processing and  Disposal.   Report
      Fo Sacrame nto Reg ional  County  Sanitation Di s tr ict.
      Sacramento,  California, 95814, June 1975.

114.  Carnes,  B.A.,  and  J.M.   Eller.    "Characterization of
      Wastewater  Solids."   Journal Water  Pollutio n Control
      Federation.   Vol.  44, (8),  (1972).

115.  Townsend,  D.W.   Water Works and  Sewerage.   1933.   As  cited
      by Metcalf,  L.  and H.P.  Eddy.   American Sewage  Practice,
      Vol.  Ill,  Disposal  of Sewage.  Third Edition,  McGraw-Hill,
      1935.


116.  Blanchard,  C.T.  "Pressurized  Air Simplifies  Conveying
      Sewage Solids."   Water and Sewage Works.    Vol.  123,  (11),
      (1976).

117.  Sacramento  Area  Consultants.    Northeast Treatment  Plant
      Interim Improvements.  Report to Sacramento Regional County
      SanitationDistrict,   Sacramento,   California,  95814.
      September 1976.

118.  Garber,  W.F., and  G.T. Ohara.   "Operation  and Maintenance
      Experience  in  Screening  Digested  Sludge."   Journal  Water
      Po11u t ion C on t r oj^ Fe^d e r a t ion.  Vol. 44, (8), (1972).

119.  Finger,  R.E.,  and J.  Parrick.   "Optimization of  Grit
      Removal at  a Wastewater  Treatment  Plant."   Presented at
      51st  Annual Conference, Water Pollution Control Federation,
      Anaheim,  California,  October 3,  1974.
120.
121.
Questionnaire for San Francisco Bay Region Wastewater
Solids Study, Oakland, California 94611, 1977.
Mathews, W.W. 1957 Operators' Forum, W.D. Itatfield,
Leader. Sewage and Industrial Wastes. Vol. 30, April
1958.
122.  USE PA.     Process Design Manual for  Sulfide Control  in
      Sanitary Sewerage Systems.     Technology  Transfer,
      Cincinnati, Ohio.45268.EPA 625/1-74-005.   October  1974.

123.  Fischer.   Sewage  Works  Journal  1930.    As  cite  by
      Metcalf,  L.  and  H.P.  E ddy.   American  Sewage Practice,
      Vol.  Ill, Disposal of Sewage.   Third Edition,McGraw-Hill,
      1935.

124.  Cooper,  T.W.  1963 Operators' Forum,  A.J.  Wahl,  President.
      Journal  Water Pollution  Control Fedejrati^qn.   Vol. 36,  (4),
      (1964).
                               4-72

-------
125.   Drago,  J.A.  Internal  Memorandum on Site  Visit  to Hampton
      Roads Sanitary District,  Virginia,  Brown and  Caldwell,
      Walnut  Creek, California  94596.   1976.

126.   Baer, G.T.,  Jr.   "Wastewater Skimmings:   Handling  and
      Incineration."   WPCF  Deeds  and  Data, September 1977.

127.   Lee,  R.D.  "Removal and Disposal of Grease and Skimmings at
      Wichita,  Kansas."   S_ewag_e _ajnd^ Industrial Wastes.   Vol.  31,
      (6),  (1959).

128.   Mick, K.L. "Removal  and  Disposal  of  Grease and Skimmings
      at Minneapolis-St.  Paul,  Minnesota."   Sewage and  Industrial
      Washes.   Vol. 31,(6),  (1959).

129.   East Bay  Municipal  Utility  District, Special District No. 1
      Annual  Report Supplement, 1970.   East  Bay  Municipal Utility
      District,  Oakland,  California  94623.   1971.

130.   Ross, E.E. "Scum Incineration  Experiences."  Journal Water
      Pollution Control  Federation.   Vol. 42,  (5),  (1970).

131.   Burwell,  H.W.  Memoranda  on  West  Point Treatment Plant
      Operations,  Municipality  of  Metropolitan  Seattle,
      Washington 98101.   1976.

132.   Banerji,   S.K.,  C.M.  Robson,  and B.S. Hyatt,  Jr.   "Grease
      Problems   in Municipal  Wastewater  Treatment  Systems."
      Proceedings  of  29th  Industrial  Waste  Conference^_May7-9,
      19J7J, Purdue University,  p. 768, 1975.

133.   Donaldson,  W.  "Utilization  of  Sewage  Grease."   Sjsjv^ajgjg
      Works Journal.  Vol.  16,  (3),  May 1944.


134.   Harris,   R.H.  Skimmings   Removal  and  Disposal Studies.
      A report submitted to the Research  and Development  Section,
      County Sanitation Districts of Los  Angeles County,  Whittier,
      California 90607.   September 28, 1964.

135.   Brown  and Caldwell Study  of Operations, Phase 1 Report of
      City of Albany, Georgia 31702.   March  1979.

136.  Metropolitan  Engineers.   Sludge  Handling and Disposal
      Interim Measures.   Report  toMunicipalityofMetropolitan
      Seattle,  Seattle,  Washington 98101. November  1975.

137.  Lyon, S.L. "Incineration of Raw Sludges  and Greases."   WPCF
      Deeds and Data, April 1973.

138.  USEPA.  Treatment and Disposal of Wastes Pumped  from Septic
      Tanks.    Office  of Research  and  Development,  Cincinnati,
      Ohio 45268.  EPA-600/2-77-198,  September 1977.

                                4-73

-------
139.   USEPA.   Septage Treatment and Disposal.   Prepared for 1977
      Technology  Transfer Seminar Program.   Technology Transfer,
      Cincinnati,  Ohio 45268.   1977.

140.   USEPA.     Treatment  and  Disposal of Septic Tank Sludges, A
      Status  Report.    Design  Seminar Handout,  Small  Wastewater
      Treatment  Facilities.    Technology Transfer,  Cincinnati,
      Ohio 45268.   January  1978.

141.   Brandes,  M.  "Accumulation  Rate  of  Septic  Tank  Sludge
      and Septage."   Journal Water
                                                    ^
      Vol.  50,  (5),  (1978).

142.   USEPA.  Handling  and  Disposal of Sludges from Combined Sewer
      Overflow  Treatment.    Office  of Research  and  Development,
      Cincinnati,  Ohio 45268.   EPA-600/2-77-053a,  b,  and  c.
      May 1977.
                               4-74

-------
EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
                Chapters.  Thickening
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                           CHAPTER 5

                          THICKENING


5.1  Introduction

The  purpose of  this  chapter is  to provide  the  reader with
rational  design  and operating  information  on which  to base
decisions about cost-effective thickening  processes.   Thickening
is only one part of the wastewater solids  treatment and  disposal
system and must be  integrated  into the overall treatment  process,
so that  performance  for both liquid and solids  treatment  is
optimized and  total  cost  is minimized  (1-3).


    5.1.1  Definition

Thickening is  defined  in  this chapter as  removal  of  water  from
sludge to achieve a volume reduction.  The resulting material  is
still fluid.


    5.1.2  Purpose

Sludges  are thickened  primarily  to decrease  the capital and
operating costs  of  subsequent  sludge   processing  steps  by
substantially reducing  the  volume.   Thickening  from one  to
two percent solids concentration, for example,  halves the sludge
volume.  Further concentration to five percent solids reduces  the
volume to one-fifth  its original  volume.

Depending  on  the  process  selected,  thickening may also
provide  the  following benefits:    sludge  blending,  sludge  flow
equalization,   sludge  storage,  grit removal,  gas  stripping,  and
clarification.


    5.1.3  Process Evaluation

Although it is good  design practice to pilot thickening equipment
before designing  a  facility,   pilot  testing  does  not guarantee
a  successful full-scale  system.   Designers must  be cognizant  of
the difficulties involved in  scale-up and the changing character
of wastewater  sludge and  allow for  them in design.

The main design variables of any  thickening process are:

     •  Solids  concentration and  volumetric  flow rate  of  the
        feed stream;


                              5-1

-------
     •  Chemical demand and  cost  if  chemicals are employed;

     •  Suspended and dissolved  solids concentrations  and
        volumetric flow rate of the  clarified stream;

     •  Solids  concentration  and volumetric  flow rate  of  the
        thickened sludge.

Specific design  criteria  for selection of a  thickening  process
can also be dependent on the chosen  downstream process train.

Another important consideration is the operation and maintenance
(0/M)  cost and the  variables affecting  it.   In the past,  0/M
costs have not  been  given  enough  attention.   This  should change
as USEPA begins  to  implement its new  Operations Check  List  (4)
in all phases of the  Construction  Grants Program.

Finally, thickening reliability is important for successful plant
operation.    A  reliable thickening system is  needed  to  maintain
the desired concentration and relatively uninterrupted removal of
sludge from a continuously operated  treatment plant.  Sludges  are
being generated constantly,  and if they are allowed to accumulate
for  a  long time,  the  performance of  the entire plant will be
degraded.


    5.1.4   Types and  Occurrence of Thickening Processes

Thickening is  accomplished  in  sedimenation  basins; gravity,
flotation  and  centrifugal  thickeners;  and  in  miscellaneous
facilities  such  as  secondary  anaerobic  digesters,  elutriation
basins, and sludge lagoons.
5.2  Sedimentation Basins
    5.2.1  Primary Sedimentation

A primary  clarifier can  be  used as a  thickener under certain
conditions.  Primary  sludge thickens well, provided the sludge is
reasonably fresh, solids of biological origin (for example,
waste-activated sludge)  are kept  to a minimum, and the wastewater
is reasonably cool.  If  sludge of five  to  six percent  solids
content is to  be  recovered  from a primary sedimentation  system,
it is  essential that the sludge  transport facilities be designed
to move  those  solids.   This will  require  short suction  piping,
adequate net positive suction heads on  the  primary  sludge pump,
suction-sight glass  inspection  piping,  and a positive means  of
ascertaining  the  quantity pumped and  the concentration   of  the
slurry (5).
                              5-2

-------
    5.2.2  Secondary  Sedimentation

Thickening  in secondary  or  intermediate  clarifiers  has  not
been  successful  in  the  past  because  biological  sludges  are
difficult  to thicken  by  gravity.  Thickening has been improved by
using side water depths  of from 14 to 16 feet (4 to 5 m), suction
sludge  withdrawal  mechanisms  rather than  plow mechanisms,  and
gentle  floor  slopes, for example,  1:12.   Although thickening
within  a sedimentation basin  can  be beneficial  under certain
conditions,  separate  thickening is usually recommended.


5.3  Gravity Thickeners


    5.3.1  Introduction

Separate,  continuously operating gravity thickening for  municipal
wastewater  sludges was  conceptualized  in the  early  1950's  (6) .
Until  that  time,  thickening  had  been carried  out within the
primary  clarifier.   Operating  problems  such  as floating sludge,
odors,  dilute sludge,  and  poor  primary  effluent  led  to the
development  of the separate  thickening  tank.  Gravity thickeners
became  the most commonly  used   sludge concentrating  device;  now,
however,  their use  is being  challenged  by  other thickening
processes.

Table  5-1  lists  advantages and  disadvantages  of gravity
thickeners compared to other  thickeners.
                             TABLE 5-1

        ADVANTAGES AND DISADVANTAGES OF GRAVITY THICKENERS

             Advantages                         Disadvantages
Provides greatest sludge storage          Requires largest land area
  capabilities

Requires the least operational skill       Contributes to the production of odors

Provides lowest operation (especially      For some sludges,
  power) and maintenance cost             - solid/liquid separation can be erratic
                                   - can produce the thinnest least
                                      concentrated sludge
     5.3.2   Theory

Since  the  early  work  of Coe and  Clevenger (7), understanding of
gravity thickening  has  slowly  improved  (8-11).   The  key to
understanding  the  continuous  gravity  thickening process is
recognition  of  the  behavior  of materials  during  thickening.


                               5-3

-------
Coarse  minerals  thicken  as  particulate   (nonf1occulent)
suspensions.   Municipal wastewater sludges, however,  are  usually
flocculent suspensions  that  behave  differently  (12).

Detailed,  comprehensive  analysis  of  current  gravity  thickening
theory for  municipal  wastewater  sludges  is  beyond the  scope
of  this  manual;  those  desiring  such  detail  should  consult
Design and  Operational  Criteria  for Thickening  of Biological
Sludges
follows
 (13)
(12) .
A short  descriptive  summary of  current  theory
OVERFLOW
                     INFLOW
     ZONE OF CLEAR LIQUID

     SEDIMENTATION ZONE
                ..^jj
     THICKENING ZONE  T
               UNDERFLOW
                                     ZONE OF CLEAR LIQUID

                                     SEDIMENTATION ZONE
                                     	h
                                   THICKENING ZONE
                                         Cj
                                         Cb

                                        SOLIDS CONCENTRATION
                                               IN THICKENER
Cj - INFLOW SOLIDS CONCENTRATION
Cb - LOWEST CONCENTRATION AT WHICH FLOCCULANT SUSPENSION IS IN
   THE FORM OF POROUS MEDIUM
Cu - UNDERFLOW CONCENTRATION FROM GRAVITY THICKENER

                             FIGURE 5-1

             TYPICAL CONCENTRATION PROFILE OF MUNICIPAL
           WASTEWATER SLUDGE IN A CONTINUOUSLY OPERATING
                         GRAVITY THICKENER

Figure 5-1  shows  a  typical  solids  concentration  profile  for
municipal wastewater sludges within a continuously operating
gravity  thickener.    Sludge moving  into the thickener  partially
disperses  in water  in the sedimentation  zone and  partially flows
as  a density  current to  the  bottom  of the sedimentation zone.
The  solid  phase of  the sludge, both dispersed and  in  the density
current, creates  floes that  settle  on top of  the  thickening
zone.   Floes  in the thickening  zone  lose  their  individual
character.   They have mutual  contacts and thus become  a part of
                               5-4

-------
the matrix of solids compressed by the pressure of the overlying
solids.   The  displaced water  flows upward  through channels  in
the solids matrix.

Generally, in decision making  about thickener size, the settling
process in  the  sedimentation  zone as well  as  the consolidation
process in  the  thickening  zone  should  be  evaluated;  whichever
process (sedimentation or  thickening) requires  greater  surface
area  dictates  the size  of   the thickener.   For  municipal
wastewater sludges, the thickening zone area  required  is  almost
always greater than that  for the  sedimentation zone.
    5.3.3  System Design  Considerations

Circular  concrete  tanks  are  the  most common  configuration  for
continuously operating gravity thickeners,  though circular steel
tanks  and  rectangular  concrete  tanks  have  also  been  used.
Figure  5-2  shows a  typical gravity thickener installation;
Figure  5-3  is  a cross-sectional view  of a  typical circular
gravity thickener (14).
                           FIGURE 5-2

              TYPICAL GRAVITY THICKENER INSTALLATION
                              5-5

-------
          HANDRAILING
 1"GROUT
                    INFLUENT
                    PIPE
1
1

BRIDGE
1



/ J/
/ r-P/
                 BAFFLE
                 SUPPORTS
                /
                  TURNTABLE
                  L
EFFLUENT
   WEIR
                                      MAX. WATER SURFACE
                                       1'3"MIN.

                                       I
                                        EFFLUENT
                                        LAUNDER
       1V4" BLADE        /
       CLEARANCE    SLUf"'*
                 PIPE!
                                                  ADJUSTABLE
                                           "SCRAPER  SQUEEGEES
                                           BLADES
1 ft = 0.305 m
1 in = 2.54 cm
HOPPER
SCRAPERS
                             FIGURE 5-3

             CROSS SECTIONAL VIEW OF A TYPICAL CIRCULAR
                         GRAVITY THICKENER

 At  minimum, the  following  should be  evaluated for every  gravity
 thickener:   minimum surface area requirements, hydraulic  loading,
 drive  torque requirements,  and total tank depth.   Floor slope  and
 several  other  considerations will also influence the  final  design
 of  the gravity thickener.
         5.3.3.1  Minimum Surface Area Requirements

 If  sludge from the  particular  facility is available for  testing,
 the required  surface  area can  be  found by  using  a  settling
 column,  developing  a  settling  flux  curve,  and  calculating  the
 critical flux  (mass loading,  Ibs/sq ft/hr)  for  that particular
 sludge  (4,  13,  15).   In most  cases,  however, the  sludge to  be
 thickened is not  available, and the designer must resort  to  other
 methods.

 Table  5-2 provides criteria for calculating required surface  area
 when  test  data are  not available and pilot plant work is  not
 reasonable.   The  designer must  specify the sludge  type  (for
 mixtures, the approximate proportions should be known), the  range
 of  solids  concentrations  that are  expected in the  thickener
 inflow,  and  the  underflow  concentration  required for downstream
 processing.   Part  A  of  the  design  example  (Section 5.3.4)
 illustrates the use of Table 5-2 in sizing gravity thickeners.
                               5-6

-------
                                     TABLE 5-2

        TYPICAL GRAVITY THICKENER SURFACE AREA DESIGN CRITERIA2
concentration ,
Type of sludge percent solids
Separate sludges:
Primary (PRI)
Trickling filter (TF)
Rotating biological
contactor (RBC)
Waste activated sludge
(WAS)
WAS - air
WAS - oxygen
WAS - (extended
aeration)
Anaerobically digested
sludge from primary
digester
Thermally conditioned
sludge :
PRI only
PRI + WAS
WAS only
Tertiary sludge:
High lime
Low 1 ime
Alum
Iron
Other sludges :
PRI 4 WAS

PRI 4 TF
PRI 4- RBC
PRI + 'iron
PRI 4- low lime
PRI + high lime
PRI + (WAS + iron)
PRI -f (WAS + alum)
(PRI + iron) 4 TF
(PRI + iron) + WAS
WAS 4 TF
Anaerobically digested
PRI + WAS
Anaerobically digested
PRI 4 (WAS 4 iron)

2
1

1


0.5
0.5

0.2





3
3
0.5

3
3

0.5

0.5
2.5
2
2




0.2
0.4

0.5





- 7
- 4

- 3.5


- 1.5
- 1.5

- 1.0


8


- 6
- 6
- 1.5

- 4.5
- 4.5
-
- 1.5

- 1.5
- 4.0
- 6
- 6
2
5
7.5
1.5
- 0.4
- 0.6
1.8
- 2.5

4

4
concentration,
percent solids

S
3

2


2
2

2





12
8
6

12
10

3

4
4
5
5




4.5
6.5

2





- 10
- 6

- 5


- 3
- 3

- 3


12


- 15
- 15
- 10

- 15
- 12
-
- 4

- 6
- 7
- 9
- 8
4
7
12
3
- 6.5
-8.5
3.6
- 4

8

6
Mass loading,
Ib/sq ft/hrb

0.
0.

0.


0.
0.

0.





1.
1.
0.

1.
0.

0.

0.
0.
0.
0.




0.
0.

0.





8
3

3


1
1

2





6
2
9

0
4

1

2
3
5
4




5
6

1





- 1.
- 0.

- 0.


- 0.
- 0.

- 0.


1.0


- 2.
- 1.
- 1.

- 2.
- 1.
-
- 0.

- 0.
- 0.
- 0.
- 0.
0.25
0.8
1.0
0.25
- 0.
- 0.
0.25
- 0.

0.6

0.6

2
4

4


3
3

3





1
8
2

5
25

4

6
7
8
7




7
8

3




Reference

16
16

16


16 •*>:•
17

16


18


19
19
19

18, 20
18, 20
-
20

20
16
16
16
18
18
18
18
•20 '•••• '•':.
20
18
16

18

18
 Data on supernatant characteristics is  covered later in this section.

 Typically, this term is given in Ib/sg  ft/day.  Since wasting to the
 thickener is not always continuous over 24 hours,  it is a more
 realistic approach to use Ib/sq ft/hr.

1 Ib/sq ft/hr =4.9 kg/m2/hr
                                       5-7

-------
        5.3.3.2  Hydraulic  Loading

Hydraulic loading  is  important for  two  reasons.   First,  it
is related to mass loading.   The  quantity  of  solids  entering
the  thickener  is  equal to  the  product  of  the  flow  rate and
solids concentration.   Since there  are  definite  upper limits for
mass  loading,  there  will  therefore  be  some upper limit for
hydraulic loading.   Secondly,  high hydraulic  loading  causes
excessive carryover  of  solids  in  the thickener effluent.

Typical maximum  hydraulic  loading  rates of 25 to  33 gallons per
square foot  per  hour  (1,200 to  1,600 l/m^/hr) have  been used in
the  past  but mainly for primary sludges.   For  sludges such as
waste-activated  or  similar types,  much  lower hydraulic loading
rates, 4  to  8 gallons  per square foot per  hour (200  to  400  l/m^/
hr)  are  more  applicable  (16).   Table  5-3  gives some  typical
operating results.   Note  that  the hydraulic  loading  rate in
gallons per  square foot per hour can be converted to an average
upward tank velocity in  feet per  hour by dividing by  7.48.

                             TABLE 5-3
     REPORTED OPERATING RESULTS AT VARIOUS OVERFLOW RATES FOR
                    GRAVITY THICKENERS (20,21)

Location
Port Huron, MI
Sheboygan , WI
Grand Rapids, MI
Lakewood, OH

Influent
solids
Thickened
solids
concentration , Hydraulic
Sludge percent loading ,
type*3 solids gal/sq ft/hr
P4WAS
P+TF
P+ (TF+A1)
WAS
P+(WAS4A1)
0 6
0
0
0
3
5
2
3
8
18.
19.
4.
25.

6
0
1 •
8
Mass
loading ,
Ib/sq ft/hr
0
0
0
0
0
.34
.46
.73
.42
.6
concentration
percent
solids
4
8
7
5
5
.7
.6
.8
.6
.6
Overflow
suspended
solids ,
mg/1
2,500
400
2,000
140
1,400
   Values shown are average values only.  For example, at Port Huron, MI the hydraulic
   loading varies between 7 to 9 gal/sq ft/hr (300-400 l/m2/hr) ,  the thickened solids in
   the underflow between 4.0 and 6.0 percent solids; and the suspended solids in the
   overflow, from 100 to 10,000 mg/1.
   P = Primary sludge
   TF = Trickling filter sludge
   WAS = Waste-activated sludge                      1 gal/sq ft/hr =40.8 1/m /hr
   Al = Alum sludge                              1 Ib/sq ft/hr =4.9 kg/m2/hr


Using  the typical maximum  hydraulic  loading  rates mentioned
above,  maximum velocities for primary sludges  are 3.3 to 4.4 feet
per  hour (1.0 to  1.3m/hr)  and for  waste-activated sludge  are
0.5 to 1.1 feet/hour  (0.2 to  0.3 m/hr).

Several researchers  have  related overflow rates to odor control,
but  odor is due to  excessive  retention of solids and  can be
better  controlled by  removing  the  thickened  sludge from  the
thickener at an increased frequency.
        5.3.3.3  Drive Torque  Requirements

Sludge on  the  floor of a circular  thickener resists
of  the  solids  rake  and  thus produces  torque.
the movement
Calculation
                               5-8

-------
of  torque for  a circular drive  unit is  based on  the simple
cantilevered beam equation represented by  Equation  5-1:

    T = WR2                                                 (5-1)

where:
           T = torque, ft/lb
           W = uniform load—this is sludge  specific,
               Ib/ft (see Table 5-4)
           R = tank radius, ft

                            TABLE 5-4
                 TYPICAL UNIFORM LOAD  (W) VALUES
                                        Truss  arm W,
                  Sludge type              lb/fta
          Primary only (little grit)   :       30
          Primary only (with grit)            40
          Primary + lime                  40 to 60
          Waste-activated sludge (WAS)
            Air                              20
            Oxygen                           20
          Trickling filter                   20
          Thermal conditioned                80
          Primary + WAS                   20 to 30
          Primary + trickling filter      20 to 30

           Rake arms typically have a  tip  speed
           between 10 to 20 ft/min (3  to 6 m/min).
          1 Ib/ft = 1.49 kg/m
Note  that  there are  several  levels of  torque which must  be
specified for a circular gravity thickener (22).   Table  5-5  lists
and defines the various torque conditions.

        5.3.3.4  Total Tank Depth
The total vertical depth of a gravity thickener is based on  three
considerations:   tank free board,  settling  zone (zone  of  clear
                              5-9

-------
liquid and  sedimentation zone),  and compression and  storage zone
(thickening zone).
                             TABLE 5-5


DEFINITION OF TORQUES APPLICABLE TO CIRCULAR GRAVITY THICKENERS (22)


Running torque - this is the torque value calculated from equation 5-1
Alarm torque - torque setting,  normally 120 percent of running, which tells the operator
  that there is something wrong
Shut-off torque - torque setting, normally 140 percent of running, which would shut off
  the mechanism              . .   .
Peak torque - torque value, determined by the supplier of the drive unit.  This torque
  is provided only for an instant and is normally 200 percent of the running torque
Free
Tank  free board is the  vertical  distance  between  tank liquid
surface and  top  of vertical tank wall.   It is a  function  of tank
diameter,  type  of bridge  structure--half or full bridge—type of
influent  piping arrangement,  and whether  or  not  skimming  is
provided.   It will usually  be at least  2  to 3 feet  (.6 to  .9 m)
although  free-board  distances up to 7  to 10 feet (2  to  3  m) have
been used by  some  designers.

Settling  Zone

This  zone encompasses  the  theoretical  zone of  clear liquid  and
sedimentation  zone  as  shown on  Figure  5-1.   Typically 4  to
6 feet  (1.2  to  1.8 m)  is  necessary, with the greater depth  being
for  typically  difficult sludges, such as  waste-activated  or
nitrified sludge.

Compression  and  Storage Zone

Sufficient  tank  volume must be provided  so  that the  solids will
be retained  for  the  period of time required to thicken the slurry
to the  required concentration.   In addition, sufficient  storage
is  necessary to compensate for fluctuations in  solids  loading
rate.

Another consideration is  that  gas may  be  produced because of
anaerobic  conditions  or  denitrif ication.    Development of  these
conditions  depends  on the  type of  sludge,  liquid  temperature,
and  the  length  of  time sludge  is  kept  in  the  thickener.   Plant
operating experience  has  indicated  that  the  total volume in
this zone should not  exceed 24 hours of maximum  sludge wasting.
        5.3.3.5   Floor Slope

The floor  slopes  of  thickeners are normally greater  than 2 inches
of vertical  distance per foot  of  tank radius  (17/cm/m).   This is
steeper than  the floor slopes  for standard clarifiers.   The
                               5-10

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steeper  slope maximizes  the depth  of solids  over the sludge
hopper,  allowing the thickest sludge  to be removed.   The steeper
slope also reduces sludge raking problems  by allowing gravity  to
do a greater part of  the work  in moving the settled solids to the
center of the thickener.
        5.3.3.6  Other Considerations

L j. ft ing De v i ce s

Optimum  functioning  of a  thickener  mechanism can be  inhibited
by  heavy  accumulation  of solids due  to  power outages  or
inconsistent accumulations  of  heavy  or  viscous  sludges.
Thickeners can be provided  with  either  a  manual  or an automatic
lifting  device  that  will  raise  the mechanism  above  these
accumulations.  This device has not been considered necessary  in
the majority of municipal wastewater  treatment plants except
in  applications  involving  very dense sludges  (for example,
thermally-conditioned sludge or primary  plus lime sludge).

Skimmers

Several  years ago,  it was  rare for skimmers  to be  installed
on  gravity  thickeners.   Today  it is common  practice  to  specify
skimming and baffling for new plants.   The reason for the change
is  the  increased processing  of  biological sludges and the
inherent floating scum layer associated  with those sludges.

Polymer Addition

Addition of polymer  to gravity thickener feed has been practiced
at  several  plants  (23,24).   Results indicate that  the addition
of  polymers  improves  solids  capture  but has  little or no effect
on  increasing solids  underflow concentration.  (See Chapter 8 for
further discussion).

Thjjckejier Sjjpejcnatant

Thickener supernatant or overflow is normally returned to either
the primary or secondary  treatment process.  As indicated  in
Table  5-3,  the strength of  the  overflow, as measured by  total
solids,  can vary significantly.  The  liquid treatment  system
must  be sized  to handle  the  strongest  recycled load.   (See
Chapter 16 for further discussion).

Pi£ke_ts_

Stirring with  pickets  in  gravity thickeners   is  thought  to
help  consolidate  sludge  in the thickening zone  (25).   However,
the support rake mechanism  usually can  provide sufficient sludge
mixing to make special pickets unnecessary.
                              5-11

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jjeed Pump and Piping

The following guidelines are applicable for feed  pump  and  piping:


     •  Use  positive  displacement  feed  pumps with  variable
        speed  drives  for variable  head  conditions  and positive
        feed control.

     •  Provide as nearly continuous pumpage  as possible.


     •  Design piping for operational flexibility.

Th i eke ne r Und er flow; JPump __and_Pi£JLn£

For  variable  head conditions  and  typical abrasiveness  of many
sludges, a  positive  displacement  pump with variable speed drive
should  be  used  and its  operations  should  be controlled by some
type of  solids  sensor,  for  example,  either  by  a sludge  blanket
level indicator or solids concentration indicator.  Pumps should
be  located  directly adjacent to  the thickener for  shortest
possible suction  line.   A  positive or pressure  head  should  be
provided on the suction  side  of  the pump.   A minimum of  10 feet
(3 m)  should be  provided  for primary sludges and  a  minimum  of
6 feet  (2  m)  for  all other  sludges.   It  is critical to  provide
adequate clean-outs and  flushing  connections  on both the pressure
and suction  sides  of  the  pump.   Clean-outs should be brought  to
an elevation greater  than that of the water  surface so that the
line may be rodded without  emptying  the thickener.


    5.3.4  Design  Example

A designer  has calculated  that  it  is  necessary to  thicken
a maximum  of  2,700 pounds  (1,225 kg) per day of  waste  sludge,
(dry weight).   The sludge consists of 1,080  pounds  (490  kg)  of
primary  at 4.0 percent solids   and  1,620  pounds  (735  kg)  of
activated  at 0.8  percent  solids.   Wasting from  the  primary
clarifier  will be initiated  by a time  clock  and terminated
by  a  sludge density  meter  when   the  sludge  concentration drops
below a given value.   Waste-activated sludge will be pumped from
the  final  clarifier  24  hours  per day at  17  gallons  per minute
(64 1/min).

Thickener Surface  Area

Since this  is a new  facility and  pilot testing  is not possible,
the designer must  utilize Table 5-2.

There  are   two  possible  thickening  alternatives.   The  first
alternative is thickening  of straight waste-activated  sludge
with a  maximum influent solids  concentration  of 0.8  percent
                              5-12

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solids.   At maximum conditions,  the designer has selected a mass
loading  of 0.2  pounds  per  square  foot per hour (1.47 kg/m2/hr)
and will design  for a 2.0 percent solids in the  underflow.
              i'620 lb/day	 = 337.5  sq  ft  (31.4
     (0.2 Ib/sq  ft/day)  (24 hrs/day)
The second  alternative is thickening a combination  of  waste-
activated  sludge and  primary  sludge.   The  density meter  on
the primary  clarifier  will  be set to allow the  sludge  pump  to
continue as  long as  the solids  concentration is greater  than  or
equal  to  4.0 percent solids.    The primary  sludge  pump  will  be
equipped with a  variable speed controller and has  a maximum rated
pumping  capacity of  10 gallons per minute (38 1/min).

On a mass loading basis, the designer's past  experience indicates
that  surface  area  required  for  the  combination  of  primary  and
waste-activated sludge is less than that required  for  waste-
activated  alone.    However,  to  assure system reliability,
sufficient  surface  area  should be  provided  to thicken  only
waste-activated.   With  the  addition  of primary sludge,  the
expected underflow solids concentration is 4.0  percent.

Hydraulic Loading

The maximum possible hydraulic flow to the gravity  thickener
would be  17 gallons per minute  (1.0 I/sec)  of  waste-activated
and  10  gallons per minute   (0.63  I/sec)  of  primary  sludge.
The designer is cognizant of the solids recycle problem  from
the thickener overflow and has selected a value of  6  gallons
per square  feet  per hour  (250 l/m2/hr)  as the maximum overflow
rate.
    ((17 + 10)  gal/min) x  (60 min/hr))
             6  gal/sq  ft/hr
The  area  required for  hydraulic  loading is  less  than  that
required for mass  loading.

Since  continuous  operation  of the  sludge handling  system is
essential, two gravity thickeners,  each capable of handling the
sludge  flow,  will be  provided.   The minimum required  area
is  337.5  square   feet  (31.4  m2),   which  is  equivalent  to  a
20.7-foot  (6.2 m)   diameter unit.  In  this  size  range, equipment
manufacturers  have  standardized  on   1-foot  (0.3  m) increments;
therefore, a 21-foot (6.3 m)  diameter, 346-square-foot (32.2 m2)
unit will be specified.
                             5-13

-------
Torque Requirements

The 30 pounds per foot (45 kg/m)  value will  be  used  for  the  truss
arm loading  (Table  5-4).   From Equation 5-1,  the running torque
required is:



    3°foot"ds x (10-5 feefc)2  = 3'307  ft-lb  <465 m~k9)
The  designer will  specify  a  minimum  running torque  capacity
of  3,307 foot  pounds (465  m-kg ) .   The  other torques  (alarm,
shut-off, and peak)  would be  specified  as  in  Table  5-5.

     Depth
Because both  the  full and the  half  bridge systems work equally
well  and  the  full bridge  is  less  expensive  to install,  the
designer  will  use  a  full   bridge  thickener  mechanism  that
will  rest  atop  the gravity  thickener and will  have  a skimming
mechanism attached.

In order to  accommodate  the  skimming arm beneath the bridge and
allow room  to perform  maintenance  work,  the designer has selected
24 inches  (0.61  m)  for the freeboard  in  the thickener.

From past experience,  the  designer  has  selected a typical depth
of 5 feet  (1.54  m)  for the settling zone.

To calculate the depth of the  thickening  zone, it  is assumed that
the average  solids  concentration in the  zone would be 1.4 percent
solids and  that  one-day storage  would be  utilized.

The following assumptions  were  made  in  order to  arrive  at this
percentage :

     •  Only waste-activated sludge would  be thickened.

     •  The  top  of the  thickening  zone would  hold  0.8  percent
        solids .

     •  The  bottom  of the thickening zone would  hold 2.0 percent
        solids .

     •  The  average  concentration  would be  equal to 0.8  plus
        2.0  quantity divided by  2.


       1,620 Ib  of  waste-activated sludge      _ _., f.  ,,  ,,
    (0.014){8.34) (7.48 gal/cu  ft)(346  sq  ft)   ^'Jb rt u
                              5-14

-------
The total vertical  side-wall  depth  of the gravity  thickener  is
the sum of the free  board,  settling zone, and required thickening
zone.    In this  case,  it would be 12.36 feet  (3.77  m) .   At  this
time,  no allowance has  been made  for the depth of the cone height
of the  thickener which  would  reduce slightly  (21  inches  [.27  m]
the  vertical  side  wall  depth  of  the  thickening  zone  when
subtracted from the  thickening  zone depth.


    5.3.5  Cost
        5.3.5.1  Capital  Cost

Several recent  publications  have developed capital  cost  curves
for  gravity  thickeners  (26-28).    Probably the  most factual
is the  reference based  on  actual USEPA  bid  documents for  the
years 1973-1977 (27).

According  to  a  USEPA  Municipal  Wastewater  Treatment  Plant
Construction  Cost  Index  -  2nd  quarter  1977  (27),  although  the
data were  scattered,  a  regression  analysis  indicated  that  the
capital cost could  be  approximated by Equation 5-2.


    C = 3.28 x 104Q1-10                                     (5-2)
where:

           C = capital  cost  of  process in dollars

           Q = plant design  flow  in million gallons of
               wastewater  flow  per day

The  associated  costs   include those  for excavation,  process
piping, equipment, concrete, and steel.   In addition, such costs
as those  for  administrating  and  engineering  are  equal  to 0.2264
times Equation 5-2 (27).


        5.3.5.2  Operating and  Maintenance Cost

Staffing

Figure  5-4  indicates annual  man-hour  requirements  for  operation
and maintenance.  As an example, for a gravity thickener surface
area  of 1,000  square  feet  (93  m^),  a designer would  include
350 man-hours of operation and  maintenance in the cost analysis.

Power
Figure  5-5  shows annual  power consumption  for a continuously
operating gravity  thickener  as a  function of  gravity thickener
surface  area.   As an  example,  for a  gravity  thickener  surface


                              5-15

-------
area  of  1,000  square  feet (93 m2) ,  a  designer would  include  a
yearly power  usage  of  4,500 kwhr  (16.2  GJ)  in the  cost analysis.
Figure 5-5  does not include accessories such  as pumps  or polymer
feed  systems.
DC
O
LJ_
to
DC
ID
O
I
OC
O
2
2
<
    100
                 4 56789 1,000  2  3  456789 10.000  2

                   THICKENER AREA, sq ft (1 sq ft = 0.093 m2)
                                                       4 56789
                           FIGURE 5-4

           ANNUAL O&M MAN-HOUR REQUIREMENTS - GRAVITY
                          THICKENERS
Maintenance Material Costs
 Figure .5-6  shows  a curve  developed  for estimating circular
 gravity  thickener maintenance  material  costs as  a function  of
         thickener surface area.   As  an  example,  for  a  gravity
                           1,000 square feet  (93 m2),  a designer
                           materials  cost of $375.   Since  this
                           1975 cost,  it  must be  adjusted to the
gravity
thickener surface
would  estimate  a
number is based  on  a  June
current design period.
area of
yearly
 5.4  Flotation Thickening

 Flotation is  a  process for  separating solid  particles  from a
 liquid phase.   Flotation of  solids is usually created  by  the
 introduction  of  air into the  system.   Fine bubbles either adhere
 to,  or  are   absorbed by,  the  solids, which   are  then  lifted
 to the surface.    Particles with a greater density  than  that of
 the  liquids can  be  separated by  flotation  (24,29).
                              5-16

-------
In one  flotation method,  dissolved  air flotation,  small  gas
bubbles (50-100 ym) are generated as  a  result  of  the  precipita-
tion of  a gas  from a solution  supersaturated with  that gas.
Supersaturation occurs when air  is dispersed through  the  sludge
in a closed,  high  pressure  tank.  When the sludge is removed  from
the tank  and exposed to atmospheric pressure,  the previously
dissolved air leaves solution in  the form of fine bubbles.
       1,000
          100
                  3 4 56789 1,000   2  3  456789 10,000  2  3  466788 100,000

                        THICKENER AREA, sq ft (1 sq ft = 0.093 m2)
                            FIGURE 5-5

         ANNUAL POWER CONSUMPTION - CONTINUOUS OPERATING
                        GRAVITY THICKENERS

In a second method,  dispersed air  flotation, relatively large  gas
bubbles  (500-1000  urn)  are  generated  when gas  is introduced
through a revolving  impeller or through porous media (30,31).
In biological flotation, the gases  formed
activity are used  to  float  solids  (32-34).
by natural biological
In vacuum flotation,  Supersaturation  occurs when  the sludge
is subjected initially  at atmospheric pressure,  to a vacuum
of approximately  9  inches  ( 2'30  mm)  of  mercury in  a closed
tank  (35,36).

Although  all  four  methods have  been  used in wastewater  sludge
treatment systems,  the   dissolved air  flotation  process  has
been  the dominant method  used in  the United States.
                              5-17

-------
      100       2   3456789 1,000   2    3  456789 10,000  2   3  456789

                       THICKENER AREA, sq ft (1 sq ft = 0.093 m2)

                              FIGURE 5-6

         ESTIMATED JUNE 1975 MAINTENANCE MATERIAL COST FOR
                    CIRCULAR GRAVITY THICKENERS
                              TABLE 5-6

      TYPES OF MUNICIPAL WASTEWATER SLUDGES BEING THICKENED
                          BY DAF THICKENERS
   Primary only
   Waste activated sludge (WAS)
   WAS (oxygen) only
   Trickling filter only
   Primary plus WAS (air)
          Primary plus trickling filter
- air only  Aerobically digested WAS
          Aerobically digested primary plus WAS  (air)
          Alum and ferrous sludge from phosphorus
            removal
    5.4.1   Dissolved  Air Flotation (DAF)

Since  the  1957  installation of the  first municipal DAF  thickener
in the  Bay Park Sewage  treatment  plant, Nassau  County,  New York,
about  300  U.S. municipal installations  (over 700 units)  have been
installed.   Although  the principal use of   the  DAF thickener has
been  to thicken waste-activated  sludge,  about  20  percent  of the
installations  handle  other sludge types (37).   Table  5-6  lists
                                 5-18

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the  types  of  municipal wastewater  sludges  currently  being
thickened by DAF thickeners.

Table  5-7  lists advantages  and  disadvantages  of DAF  thickeners
compared  to other major  thickening equipment.
                              TABLE 5-7


          ADVANTAGES AND DISADVANTAGES OF DAF THICKENING


           Advantages                            Disadvantages

Provides better solids-liquid separation      Operating cost of a DAF thickener is
  than a gravity thickener                  higher than for a gravity thickener
For many sludges, yields higher solids        Thickened sludge concentration is less
  concentration than gravity thickener         than in a centrifuge
Requires less land than a gravity thickener    Requires more land than a centrifuge
Offers excellent sludge equalization control   Has very little sludge storage capacity
Has less chance of odor problems than a
  gravity thickener
Can remove grit from sludge processing
  system
Removes grease
         5.4.1.1  Theory

In  the  DAF thickening  process,  air  is added  at pressures  in
excess of  atmospheric pressure either to  the  incoming sludge
stream  or to a separate  liquid stream.   When pressure  is  reduced
and  turbulence  is created,  air  in excess  of that required  for
saturation  at  atmospheric  pressure leaves  the  solution  as  very
small bubbles  of 50 to 100 ym  in diameter.   The bubbles adhere to
the  suspended  particles  or become enmeshed  in the  solids  matrix.
Since  the  average density of  the  solids-air aggregate  is  less
than  that of water,  the agglomerate floats  to the surface.   The
floated  solids  build  to a depth of  several  inches at  the  water
surface.    Water  drains  from the  float  and   affects  solids
concentration.    Float  is  continuously removed by  skimmers  (35).
Good  solids flotation occurs with  a solids-air aggregate specific
gravity  of  0.6  to 0.7.


         5.4.1.2  System Design  Considerations

DAF  thickeners  can  be  utilized  either to  thicken  wastewater
solids  prior  to  dewatering   or  stabilization  or  to  thicken
aerobically digested  or  other  solids prior   to disposal  or
dewatering.

DAF  thickeners  can  be rectangular   or circular,  constructed
of  concrete or  steel,  and can operate in the full,  partial,  or
recycle  pressurization modes.
                                5-19

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Full,  Partiaj^a^nd^Recycle  Pressurization

There  are three  ways in which  a  DAF system  can be operated.
The first method is  called "full  or total  pressurization."
With this  design,  the entire sludge flow  is pumped  through  the
pressure  retention  tank, where the sludge is  saturated with
air and  then passed  through  a  pressure reduction valve  before
entering the  flotation chamber.  A distribution device is used  to
dissipate inlet energy and thus  to  prevent turbulence and  limit
short  circuiting.  The primary advantage of  pressurizing the
total  flow  is  that  it  minimizes  the size  of  the flotation
chamber,  a significant part  of  the capital cost.  However, the
advantage of  a  smaller chamber may  be partially offset by the
cost of a higher head  feed pump, larger pressure vessel,  and more
expensive operation.   Operational problems may result from floe
shearing and  clogging when sludge is passed through  the  pressure
regulating valve.

The second method of  operation  is  called  "partial  pressuriza-
tion."   With  this design  only part of  the  sludge  flow is  pumped
through the  pressure  retention  tank.  After pressurization the
unpressurized and pressurized  streams are combined and mixed
before  they  enter the flotation chamber.   In this  arrangement
the pressurizing  pump and pressure  vessel are smaller  and the
process  is not as susceptible  to  flow variations  as is  total
pressurization;  this  is the case when the necessary pump  controls
are included  in  the  design.   The size of  the  flotation  chamber
would  be the  same  as  that  for  a total pressurization  system.

The third  method  is  called  "recycle pressurization."   Here,  a
portion  of the clarified liquor  (subnatant) or an alternate
source containing  relatively little  suspended  matter   is
pressurized.   Once saturated  with air,  it  is combined and  mixed
with the  unthickened sludge  before  it  is released  into the
flotation chamber.

The major  advantage  of this  system  over   the   total  and  partial
pressurization system is that  it minimizes  high  shear  conditions,
an  important parameter  when  dealing  with  flocculent-type
sludges.   Another  advantage  arises  when wastewater sludge
streams containing stringy materials are thickened.   The recycle
pressurization system  eliminates  clogging problems  with the
pressurization pump,  retention tank, and pressure release  valve.
For the  above reasons, recycle  pressurization systems  are  the
most commonly used units in the United States.   Figure 5-7  shows
a typical rectangular steel tank  installation.

In  this  system,  the pressure  retention tank   may be either
unpacked  or  packed   (meaning  that  the  tank   is  filled with  a
packing material  to  create  turbulence).   The  use of either  is
dependent principally  on  the source of the pressurized  recycle
f low.

The pressurized  recyle  flow can  be obtained either from the
subnatant stream or,  typically,  from  the secondary effluent. The
advantages of using  secondary effluent  are that  it results in  a


                              5-20

-------
much cleaner stream (low suspended solids and low grease content)
and allows the use of a packed pressure retention tank.  A packed
tank  is smaller  than  a packless tank,  has  lower  associated
capital cost, and provides for a more efficient saturation of- tne
liquid stream.  In this case, less air is required to achieve tne
same  level of liquid  saturation as a packless tank and  power
requirements  are  lower.    Packed  tanks may,  however,  eventuallY
require cleaning,  and  the use  of secondary plant  effluent will
significantly  increase  the flow  through  the  secondary treatment
system, thereby  increasing pumping costs  and  possibly affecting
the performance of the secondary clarifier.




                             FIGURE 5-7

      TYPICAL RECTANGULAR, STEEL TANK, RECYCLE PRESSURIZATION
                  DISSOLVED AIR FLOTATION THICKENER
Rectangular or .C^ir_c_ul_ar

The use of  rectangular  DAF  thickeners  has  a  number of
over circular units in float removal.  First, skimmers  can ea
be  closely  spaced; secondly,  they  can  be  designed to  skim
entire surface.  Because of the side-walls, float does  not ea;
                                                                ily
                                                                the
                                                                ily
                              5-21

-------
move around the  end  of  the  skimmers.   Bottom sludge flights are
usually driven  by  a  separate  unit  and, hence,  can be operated
independently  of the  skimmer  flights.   Water  level in the tank
can be changed  readily  by adjusting  the end  weir.   This permits
changing  the depth  of water  and  flight  submergence to accommodate
changes  in float  weight  and  displacement,  which affect  the
ability to remove this material  from  the unit.

The main  advantage  of  circular  units is their lower cost  in
terms  of  both  structural concrete and mechanical equipment.  For
example,  two  60-foot (18 m)  diameter circular units are  the
equivalent of  three 20-foot  by 90-foot  (6 m by 27 m) rectangular
units.    The rectangular units  require  approximately  11 percent
more structural  concrete,  as well  as  more  drives  and controls
which  increase maintenance requirements.

Concrete  or Steel

Steel  tanks come completely  assembled and only require a concrete
foundation pad and  piping  and  wiring  hookups.  Although equipment
purchase  price is much higher  for  steel  tanks, considerable field
labor  and  expensive equipment  installation  are  eliminated.
Structural  and  shipping problems limit  steel  DAF  units  to  the
smaller sizes  (450  square feet  [40.5 m2] or less for rectangular
units  and 100  square  feet  [9 m2]  for  circular units).

For a  large  installation  requiring  multiple  tanks or  large
tanks,  concrete  tanks are  more economical.

Pilot-  or Bench-Scale Testing

If  sludge  is available, the designer should,  as a  minimum,
perform  bench-scale  testing  (38,39).   If money is  available,
consideration  should  be  given to renting  a  pilot  DAF thickener
and conducting  a  four- to  six-week test program to  evaluate
the effects of  such  parameters  as  recycle ratio,   air-to-solids
ratio,  solids  and hydraulic  loading,  and polymer  type and dosage.
If  sludge  is  not available,  then  a  detailed  review  must be made
of experience  at installations where  a  similiar type of sludge is
being  thickened  by  DAF thickeners.
The first  step  in designing a DAF  thickener  is  to evaluate the
characteristics  of the  feed  stream.   The designer must evaluate
the  type  of  sludge(s)  to  be thickened and  the approximate
quantities of each  under  various plant loadings and  modes of
operation.   If  waste-activated sludge  is to  be  thickened, the
expected  range  of  sludge ages must  be  determined,  since sludge
age  can significantly  affect  DAF  thickening  performance   (40).
Information is  needed  about the  source  of  waste  sludge and the
range of solids  concentrations  that  can  be expected.  Also,  there
should  be  an  evaluation  of  any characteristic of the feed stream
that  may  affect  air solubility--for example,  concentration of
dissolved salts, and  range  of liquid  temperatures.
                              5-22

-------
Surface Area

To  calculate the  effective  surface  area  of a  DAF thickener,  a
designer  must know  the  net  solids  load,  solids surface  loading
rate, and hydraulic  surface loading rate.

Net Solids Load

Since a DAF  thickener  is  not entirely efficient, more  sludge  must
be pumped into the thickener than the actual amount removed.   The
actual  amount  removed is the net  solids  load.   From a design
standpoint,  the net  load  is  the amount  of solids  that must  be
removed  from the liquid  processing  train each  day.   This value
divided by the appropriate solids loading rate gives the  required
effective surface area.

The  gross solids  load is calculated by  dividing the net  load
by  the  expected  solids   capture  efficiency  of   the  system.    The
gross solids  load is  important in sizing system  hydraulic piping.
The  allowable  solids  loading  rate  is related  to  the minimum
solids  flux  that  will occur  within  the  range  of  sludge
concentrations  found  in the  thickener (41).   This  flux is a
function of  the  type  of  sludge  processed, the float concentration
desired, and polymer used.  Pounds  of  dry solids per square  foot
per day or  pounds of dry solids per square foot per  hour  are  tne
units used  to express this rate.

The  effect   of  sludge type  on  the  solids loading  rate is shown
in Table  5-8.   The  loading rates  indicated  will normally  result
in a  minimum of  four percent solids concentration in  the  float.
Actual operating data are listed  in Table 5-9.
                             TABLE 5-8

  TYPICAL DAF THICKENER SOLIDS LOADING RATES NECESSARY TO PRODUCE
              A MINIMUM H PERCENT SOLIDS CONCENTRATION

                                 Solids loading rate,  Ib/sq ft/hr
       Type of sludge          No chemical addition     Optimum chemical addition
 Primary only                    0.83 - 1.25              up to 2.5

 Waste activated sludge (WAS)
   Air                            0.42                up to 2.0
   Oxygen                        0.6-.0.8               up to 2.2

 Trickling filter                  0.6-0.8               up to 2.0

 Primary + WAS (air)                0.6 - 1.25              up to 2.0

 Primary + trickling filter         0.83-1.25              up to 2. 5
1 Ib/sq ft/hr =4.9 kg/m2/hr


                               5-23

-------
                             TABLE 5-9

      FIELD OPERATION RESULTS FROM RECTANGULAR DAF THICKENERS
Installation
Eugene , OR
Springdale, AR
Athol, MA
Westgate Fairfax, VA
Warren, MI
Frankenmuth, MI

Cinnaminso, NJ
San Jose, CA

Boise, ID


Levittown, PA

Xenia, OH
Indianapolis, IN
Columbus, OH
(Jackson Pike)
Wayne County, MI
Dalton, GA
Middletown, NJ
Sludge
typea
P+TF
P+TF
A.
Ab
Ac
AC
AC
A
P+Ad
P+AS
A
A
A
A
P+A
A
P+A

A
A
P+A
A
Solids
leading
rate,
Ib/sq f t/hr
1.25
2.5
3.2
7.0

0.58

2.0
1.9
1.6
1.0
1.17
1.13
0.54
1.00




0.83
0.75
2.0
Feed
solids
concentration ,
mg/1
5,000
20,000
8,000
14,000
11,000
5,000
8,000
5,000
23,000
17,000
4,600
5,000
5,000
8,000
6,400
4,000
10,000

6,000
4,500
12,900
10,000
Polymer
dosage ,
Ib per dry
ton solids
0
7
2
1-4
40
0
26
5
0
0
0
3
6
0
0
30
30

0
0
0
5-6
Float
concentration ,
percent
solids
4.5-5.0
6.5
4.0
7.3
5.0
3.0
3.5-5.5
4.0
7.1
5.3
4.0
3.8
4.0
6.5
8.6
2.5-3.0
3.5-4.2

3.2
4.6
6.1
4.0
Subnatant
suspended
solids,
ng/1
500
200
50
20
200
750
90
250



500
500


100
100-1,000

800


500
Ref
43
43
43
43
16
14

16
14
14
14
14
14
14
14
16
16

16
14
14
14
a P = Primary sludge
 A = Waste-activated a]-",*^.,^*
 TF = Trickling filter sludge

 Oxygen plant

°Considerable brewery waste

 Non-canning season

eCanning season

1 Ib/sq ft/hr =4.9 kg/m2/hr
1 Ib/ton =0.5 kg/t
In  general,  increasing  the solids  loading  rate decreases the
float  concentration.  Figure 5-8  illustrates this  phenomenon
without polymer addition,  and Figure 5-9 with polymer addition.
The addition  of
loading rate.
polyelectrolyte.will  usually increase the solids
Hydraulic Loading

The  hydraulic loading rate  for  a DAF  thickener is  normally
expressed as  gallons  per  minute  per  square  foot.  When like
units  are  cancelled,   the hydraulic  loading rate  becomes  a
velocity  equivalent to the  average  downward velocity of water  as
it flows  through  the  thickening tank.  The maximum  hydraulic rate
must always  be less than the minimum rise rate of  the sludge/air
particles  to ensure that all the  particles will reach the  sludge
float before the  particle  reaches the effluent  end  of the tank.
                               5-24

-------
1°
1
a?
z~
g
<
o
§  3
o
                                          FLOAT
                                    CONCENTRATION
                           SUBNATANT
                           SUSPENDED
                           SOLIDS
                                                             800
                                                             700
                                                             600
                                                             500
                                                             400
                                                             300
                                                             200
                                                             100
                                                                 in
                                                                 Q
                                                                 o
                                                                 LU
                                                                 O
                                                                 z
                                                                 LU
                                                                 o_
                                                                 
                                                                 CD

                                                                 00
    01234567

                  SOLIDS LOADING, Ib/sq ft/hr (1 Ib/sq ft/hr = 4.9 kg/m2/hr

                            FIGURE 5-8

          FLOAT CONCENTRATION AND SUBNATANT SUSPENDED
              SOLIDS VERSUS SOLIDS LOADING OF A WASTE
             ACTIVATED SLUDGE - WITHOUT POLYMERS (16)

Reported values  for hydraulic  loading rates range  from 0.79  to
4.0 gallons  per  minute  per square  foot  (0.54-2  to. 7  1/min/m2)
(32,42-46).    This  wide  range  probably  indicates a  lack  of
understanding of  the  term.   In some cases, the hydraulic  loading
refers simply  to  the influent  sludge  flow,  while in others,  the
recycle  flow is  included.  In most  sources,  no  definition  of
the term was  given.  Table  5-10  indicates the hydraulic  loading
rates  found in the literature.

Since  the total flow through the thickener affects  the particles,
the hydraulic  loading  rate should be  based  on the total  flow
(influent plus  recycle).   Extensive  research  on waste-activated
sludge  (48)  has resulted in  the conclusion that  a peak rate  of
2.5 gallons  per minute  per square  foot  (1.7  I/sec/ m^ )  should
be employed.   This value  is based  on use of polymers.    When
polymers are not used, this value  is expected to be  lower,  but no
design criterion  has been  suggested  at this  time.   Figure  5-10
shows  the  effects of polymer  and hydraulic loading rate  on DAF
thickener subnatant chemical oxygen demand  (COD)  (48).

Air- to- Sol ids-
Another design  parameter to  be  considered  in  DAF thickening  is
that  of  the  air- to-solids  (A/S)  ratio.   Theoretically,  the
quantity  of  air  required  to  achieve satisfactory  flotation  is
                              5-25

-------
directly proportional to the  quantity of  solids entering the
thickener (defined as gross solids load in the previous section).
For  domestic  wastewater  sludges,  reported ratios range  from
0.01 to 0.4, with most systems operating at a value under 0.1.
I  6
8
a?
Z  5
LU
u
§  3
o
                                           FLOAT
                                     CONCENTRATION
                          SUBNATANT
                          SUSPENDED
                          SOLIDS
                                                             800
                                                             700
                                            600
                                                             500
                                                             400
                                            300
                                                             200
                                                             100
                                                CO
                                                Q
                                                _J
                                                O
                                                co
                                                Q
                                                LU
                                                Q
                                                I
                                                CO
                                                h-
                                                                CO

                                                                CO
             1       23456

                  SOLIDS LOADING, Ib/sq ft/hr (1 Ib/sq ft/hr = 4.9 kg/m2/hr)

                           FIGURE 5-9

          FLOAT CONCENTRATION AND SUBNATANT SUSPENDED
              SOLIDS VERSUS SOLIDS LOADING OF A WASTE
               ACTIVATED SLUDGE - WITH POLYMERS (16)
The  appropriate  A/S ratio  for a particular application  is  a
function  of the  characteristics of  the sludge,  principally,
the  sludge volume  index (40),  the  pressurization systems  air
dissolving  efficiency,  and  the distribution  of the  gas-liquid
mixture into the thickening tank.  Figures 5-11 and  5-12 show the
effects of  A/S of  float  concentration  and subnatant  suspended
solids, with and without polymer addition.
Polymer Usage

Polymers have a
a designer  must
performance with
marked effect on  DAF thickener performance, and
 therefore be  careful  to  differentiate  between
and without polymer use.
Polyelectrolytes  may  improve  flotation by  substantially
increasing the size  of  the particles present in the  waste.   The
particles  in  a given waste may not be amenable  to  the  flotation
process because their small size will not allow proper air bubble
                              5-26

-------
attachment.   Doubling the  diameter  or size of the particle  can
result  in  a fourfold  increase  in  the rise rate  provided  the
previous A/S ratio is maintained.   The surface  properties  of  the
solids  may  have to be altered  before effective flotation  can
occur.   Sludge  particles  can  be  surrounded by  electrically
charged layers  that disperse these particles in the liquid  phase.
Polyelectrolytes can  neutralize the charge, causing the particles
to coagulate so that  air bubbles can attach to them for effective
flotation.   Thus, with use  of polymers,  the  following operating
advantages  may occur:   the size  of  the DAF  thickener may be
reduced;  solids  capture may be  improved,  thus  reducing  the
amount of solids recycled back  to the  liquid  handling  system; an
existing, overloaded  facility  in which  polymers  are  not  being
utilized may be  upgraded.  They  also act  a  surfactant, thus
allowing better attachment of air bubbles.
                           TABLE 5-10

          REPORTED DAF THICKENER HYDRAULIC LOADING RATES3

                  Hydraulic loading rate (gpm/sq ft)

Influent only           Influent plus recycle           Reference

                              1.5-2.5                      44
                                2.5                        45
                              1.0-4.0                      46
                                0.79                       47
                             1.25-1.5                      48
     0.9                        3.0                        49
aAll values reported  are  associated with polymer usage.   Values
 for systems not using  polymer could not be found in the literature,

1 gpm/sq ft = 40.8  1/min/m


The  major  disadvantage  of polymers  is  cost  (polymer  cost,
operation  and  maintenance of  polymer  feed  equipment)  when
calculated  over  the  useful  lifetime of  the  plant.   In  addition,
the  actual  amount  required  is  very  difficult to determine until
flotation studies can be  run on the actual installation.   If
polymers  are  to be  used,  it is best  to  design conservatively,
so  that  the  possibility   of  the  exceptionally  high polymer
demand needed to keep  marginal operation at capacity is avoided.
Table  5-9 lists current operating results of plants with and
without polymer addition.
Pressurization System
The  air dissolution  equipment, which consists of the pressuriza-
.tion pump,  air  dissolution  tank, and other .mechanical equipment,


                              5-27

-------
is the  heart of a DAF  thickener system.
tion  system, the  designer  must decide
and a quantity of  pressurized  flow  and
affecting  the performance of the system.
  In sizing a  pressuriza-
on  an  operating  pressure
must  be aware of  factors
   300  i-
   200  —
Q
O
CJ
DO

CO
   100
               O
                                O
      LEGEND

 O  WITHOUT POLYMER

 D  WITH POLYMER
             HYDRAULIC LOADING (INFLUENT + RECYCLE) RATE (gpm/sq ft)
                          (1 gpm/sq ft = 40.8 l/min/m2)

                             FIGURE 5-10

            EFFECT OF HYDRAULIC LOADING ON PERFORMANCE IN
               THICKENING WASTE ACTIVATED SLUDGE (48)
Operating Pressure

Most  commercial available pressurization systems operate at  40  to
80  psig  (276  to  522 kN/m2).    For a  given A/S  ratio,  the air
                                5-28

-------
required  to float the  sludge can  be obtained by  increasing
the operating  pressure  of the  system  to dissolve  more  air,  or
holding a lower  operating  pressure  and increasing the volume of
pressurized  flow.
 g
 i-
 DC
 I-
 01
 U
 Z
 o
 CJ
o
7

6


5



4


^
2

1


^

_


-



-



_
o
o
0

0 °


Q _^r O
<^!^0
a ,4 &
W v FLOAT
* °1 , — rONCFNTRATION
^

-
I
r-^-T 	
bP
JO SUBNATANT
SUSPENDED
SOLIDS
-
.
bUU
700

600 E
1/5"
a
500 8
Q
Llj
Q
400 g
a.
D
in
Z
H
200 z
m
D
100

Q - _1 .K-H-X. ™*L . j_ .. . 1. _H 	 L . _ .... _ _L . 1 .1 ..1.1 [ Q
0 .02 .04 .06 .08 .10 .12 .14 .16 .18 .20 .22 .24 .26 .28
                           AIR SOLIDS RATIO

                           FIGURE 5-11

        FLOAT CONCENTRATION AND SUBNATANT SUSPENDED SOLIDS
          VERSUS AIR-SOLIDS RATIO WITH POLYMER FOR A WASTE
                      ACTIVATED SLUDGE (16)


In one  study  (40),  it  was  shown that  the  higher the operating
pressure of a flotation  thickener system,  the  lower the rise rate
of the  sludge.  The  reason  for  a  higher rise rate  at  40 psig
(276  kN/m2)  than  at  60 or  80 psig  (414  or 552  kN/m2)  is that
the optimum  bubble size  is predominant at  this  lower operating
pressure.  This study concludes that attempting to raise the A/S
ratio by  increasing the operating pressure  is detrimental to the
thickening process.  These results are important  in that it will
be in the  user's best interest to operate at  the  lowest pressure
possible.  The  requirement for  higher head  pumps,  larger air
compressors,  and higher  pressure  rated retention tanks raises the
initial cost of the process as well  as operating costs.

Quantity of Pressurized  Flow

For a DAF thickener  to work  effectively,  the  proper amount of
air must  be  present for each  pound  of solids  to  be  handled
(A/S  ratio).  The  design pressurized  flow should be based on
the maximum gross  solids  load that the DAF  thickener  is designed
                              5-29

-------
to receive.   For multiple  units,  each basin should have  its  own
independent pressurization  system.  This  is  especially  important
to remember if  the  thickening system  is designed  to operate over
a wide range of  influent solids  concentrations  and flows.
   7 I-
   6
<
a:
o  4
z
o
o
                FLOAT
                CONCENTRATION
                        SUBNATANT
                        SUSPENDED
                        SOLIDS
                                                             900
                                                             800
                                 700  =g,
                                     E
                                     co~
                                 600  §
                                     o
                                     CO
                                     Q
                                 500  yj
                                                             400
                                                             300
                                                             200
                                                             100
                                     m
        .02
            .04
                .06
                    .08
                        .10
.12  .14  .16  .18

AIR SOLIDS RATIO
                                                .22
                                                    .24   .26
                                                            .28
                            FIGURE 5-12

        FLOAT CONCENTRATION AND SUBNATANT SUSPENDED SOLIDS
           VERSUS AIR-SOLIDS RATIO WITHOUT POLYMER FOR A
                      WASTE ACTIVATED SLUDGE


Factors Affecting Performance

The  designer should be  aware  of  two physical  factors,  air
saturation and  turbulence,  which can  affect the performance
of the pressurizing system.

Air Saturation.   The   basic  mechanism  that  makes  flotation
possible is  the  increase  in the  amount of gas  dissolved  when
pressure  is   increased.   The  relationship  between pressure  and
quantity  dissolved  is shown in Henry's Law,  which  states that if
no  reaction  prevails  between  the gas  and liquid phases,  the
solubility of the gas  is directly  proportional to  the  absolute
pressure  of  the  gas  at equilibrium  with the liquid  at  constant
temperature.
                               5-30

-------
In practice,  the  actual  amount  of air dissolved for a given  air
input  depends  on the  efficiency of  the  pressurization  device,
liquid  temperature  and  concentration  of  solutes  in  the  liquid
stream being pressurized.

Normally  a pressure retention  tank  is  used  to optimize  the
air-water  interface  for  efficient air  transfer in the  shortest
detention time.   Depending on tank design (packed  tank,  packless
tank, tanks with mechanical  mixers, etc.), efficiencies can  range
from  as  low as 50 percent  to over  90  percent.   It  is  current
design  practice  in  the  United States  to specify a  minimum of
85 to 90 percent efficiency.

The equilibrium concentration of  a gas  in a liquid is inversely
related to the temperature of the liquid phase.  The temperature
effect  is  substantial.   For  example,  the  saturation  of air in
water at 140°F (60°C) is about one half less than  the saturation
of air in water at 66°F (18.8°C)  at one  atmosphere.

The presence of salts such as  chloride will  normally decrease  the
air solubility at a  given  temperature and pressure.   The  effect
of salt concentration  on  air  dissolving  efficiency  is best
evaluated by conducting bench-scale treatability tests or  a  pilot
unit  test program.

Turbulence.  The  proper  amount  of  turbulence  must be  present
~at the  po~int  of  pressure  reduction  to  cause  bubble formation.
Without the necessary  turbulence, the  rate  at which air  bubbles
form   is  slow  and  may occur  too  late  in  the process.   Excessive
turbulence can result  in increased bubble agglomeration  and  floe
shear.  Under this condition,  the majority of bubbles formed  will
be considerably larger than  the  50 to 100  ym needed for effective
flotation.

Number^of Units to_b_£JJsed

The number of DAF thickeners to  be provided  at  a facility depends
on the following factors:

     •  The availability and configuration of available land.


     •  The operating cycle  that will be used,  for  example,  seven
        days  per  week,  24  hours per  day;  five days  per week;
        eight hours  per day; etc.

     •  Seasonal  variability;  for example,  the  operation of  a
        food  processor  six  months of the  year,  the waste  flow
        from which will go to the municipal  facility.

     •  The variance  in  average-to-peak hourly  solids load  that
        can be expected on a day-to-day basis.
                              5-31

-------
Adequate capacity to thicken peak hourly waste sludge  production
is  necessary.  In addition,  provision must  be  made  to handle
the sludge  flow  if  a unit must  be  taken  out of service.   (See
discussion in Chapter  2).

0_therConsiderations

In  addition  to  the system  design  considerations  previously
discussed,  the designer must  also give consideration to feed
sludge  line  sizing,  thickened  sludge  removal,  bottom draw-off
piping, subnatant piping,  pressurized  flow  piping,  and  controls.
Each of these items  is briefly discussed below.

Feed _JSjLu d g e  L i n e

Feed sludge  flow  rate must be controlled to  stay  within  allowable
limits.   This requires a flow  meter that accurately measures
a high  solids  stream  and  piping large enough to handle maximum
flow.

Thickened Sludge  Removal

The  surface  skimmer  brings  the  thickened sludge   over  the
dewatering  beach and  deposits  it in  a   sludge  hopper.   The
thickened sludge must  then  be  pumped to the  next  phase  of the
solids handling system.    In  pump selection,  it  is important to
remember  that air  has  been entrained in this sludge  by the
flotation thickening process.   Pumps that can  air lock should
not be  used; positive  displacement  pumps are common in this
application.

For pipe  sizing and  final  pump  selection,   consider  that the
thickened sludge can  reach  concentrations  in the  range  of ten
percent.  (See Chapter 14 for further discussion).

Bottom Sludge ..Draw_0ff_

In a rectangular  DAF tank, the bottom collector moves the settled
solids to the  influent  end of  the basin.   Here  it is  deposited
into  either multiple  hoppers or  a cross-screw  conveyor that
delivers  it to a hopper.  The  bottom collector  in  a circular
DAF tank  delivers the  settled  solids directly  to a  hopper in
the center of the tank.   Once the solids are  in  the hopper,  they
must  be  removed from the tank.   Depending on  where  this flow
goes,  it can be handled  by either gravity or pumps.

One major  consideration  that  applies  to either removal  system,
but  particularly  to  gravity  removal,  is the  static  head
available.   Since the  draw-off point is  at  the  bottom of the
flotation basin,  the entire depth of the liquid in the  basin  must
be considered as available static head.  Although fine  control is
not required,  this head must be  dissipated in order to restrict
the flow.  A positive  displacement pump with variable  speed  drive
will assure control  of bottom sludge withdrawal.
                              5-32

-------
This draw-off  is  at the lowest point in the basin and  therefore
could also be used as a basin drain.   If a  tee  and  drain valve is
installed on this line  at the  outside of the tank  wall, draining
can  take place.  The  line  from  the  drain valve  can  go  to the
plant's drain system.

Subnatant Line

Pipe sizing  should  be  such  that it can handle  the maximum total
flow (influent  plus  recycle)  without  any appreciable head loss.

Pressurized Flow Piping

Because  of the high pressure requirements of this  flow,  the
pressurization liquor  is usually  delivered  to  the pressure tank
by a high-speed,  closed impeller  centrifugal pump.  Piping must
be sized to handle  the maximum  liquid throughput  rate  of  the
pressure tank selected.

£ont£0l_s_

The controls for  a  DAF thickener  are dependent upon  the system,
the  degree  of   automation required,  and the equipment manufac-
turer's design.   They  usually  include,  at  a minimum, a pressure
controller for  the  pressure  vessel  and  flow meters for the feed
and thickened sludge flows.


    5.4.2  Design  Example

A designer has calculated that it will be necessary to thicken a
maximum  of  2,700  pounds (1,225 kg)  per day of  waste  sludge  at
0.5  to  0.8  percent  solids  from  a contact stabilization  plant
employing no  primary  clarification.    The  facility  will  have  a
sludge  handling system consisting of  a DAF thickener for  the
waste activated sludge,  mechanical dewatering  by belt  press  and
composting.   The  treatment  plant  will  be manned eight hours  per
day, seven days per week but dewatering  operations  will only
take place  six hours  per day, five  days  per  week.   Thickening
operation would take place 7.5  hours  per day, five  days per week.
Waste sludge  flow  from the  final  clarifier  would  be continuous
during  the thickening operation—that  is, 7.5 hours per day, five
days per week.

The  designer has  decided to provide  polymer feed  equipment  for
the  DAF thickener  to  be  used in  emergency  situations  only.
Polymers are not used in normal operation.

The  designer has also decided to use  a packed pressurization
tank,  which  requires  a  relatively  clean  source  of  pressurized
flow. Secondary effluent will  be utilized.

Effective Surfac_e_Are_a

The  maximum  daily waste sludge production  expected was given as
2,700 pounds  (1,225  kg)  of  waste-activated sludge with a  solids
concentration of 5,000 to 8,000 mg/1.
                              5-33

-------
The maximum net hourly load (actual  amount  of  solids  that  must  be
captured and removed per hour by the thickener)  is:


        (2,700 Ib/day) (7 days/wk)        _„.  . ...
    TT75 hrs/day)(5 day/wk operation)  =  504  Ib/hr  (228.6 kg/hr)


The sludge  being  thickened is  considered  to  be equivalent to a
straight waste-activated  sludge even though  primary solids are
mixed  with  it.  From Table 5-8,  a  value  of  0.42  pounds per  square
foot per hour (2.1 kg/m2/hr)  is  selected.


       504  Ib/hr max.  net load       ,  „._.    ...  . . nn  9,
    0.42 Ib/sq ft/hr loading  rate  =  1 '20° Sc< ft  (108  m2)


Based  on the solids loading rate (hydraulic  loading rate needs  to
be  checked), the  maximum effective surface area  required  is
1,200  square feet (108  m2).
Feed,  pressurized recycle,  thickened  sludge,  and subnatant must
be calculated to determine  pump  size and piping requirements.

Feed Flow Rate - Both  the  gross solids load  and minimum solids
concentration must be  known to calculate the feed flow rate.

The gross solids load  is  the amount of  solids  that must be fed to
the thickener in order for the system to capture and thicken the
required  net solids  load.   The maximum net  hourly  load has
already been calculated to be 504 pounds per  hour (228.6 kg/hr).
Since  polymers are not  to be used during normal operation, a
capture  efficiency  of 85  percent is used  (standard  for the
industry) .    The  maximum  gross  solids  load is then calculated as
follows:
                            =  593  Ib/hr  (269  kg/hr)
    n
    0.85 efficiency factor
The minimum  solids concentration  expected  is 5,000  mg/1 .   The
maximum feed flow rate can now be  calculated  as follows:
                             ' 237 «*" <897
           (34M60 min/hr)


Pressurized recycle flow rate - The design  pressurized  flow
should be  based  on the  maximum  gross  solids load expected  from
the  DAF  thickener.   For  this  example,  the maximum hourly  gross
solids load used was 593 pounds per hour (269 kg/hr).


                              5-34

-------
After  discussing  the operating conditions  with  several  DAP
thickener  equipment  suppliers,  the engineer designed  for  a
maximum of 237 gallons per minute  (14.95 I/sec).

Thickened sludge flow  rate  - The  maximum hourly net  solids  load
was  504  pounds  per  hour (228.6  kg/hr).   At  the  minimum  four
percent  solids  concentration,  the  expected  flow  rate can be
calculated as follows:


           504 Ib/hr          „ r   '    . ..  _,. , .
    T0)4)(8.34)(60 min/hr)  = 25'2 9pm (1'59 1/sec>


Subnatant flow rate  -  This  rate  is  equal  to  the maximum  total
flow into  the tank — 237 gallons  per  minute  (14.95 I/sec)  feed
plus 237  gallons per minute  (14.95 I/sec)  recycle.

Hydraulic Surface Loading Rate

Based  on solids  loading,  the  minimum  thickener  surface  area
was  calculated  to be 1,200 square  feet  (108 m2).   The total
maximum  flow  rate  (influent  plus recycle)  was  calculated to be
474  gallons  per minute  (1,794 1/min).   The maximum hydraulic
surface loading rate would be:
                =  2'53 9Pm/S(3 ft
The 2.53 gallon per minute per square  foot  (105 1/min/m2)  is on
the high side for  a  system that does not employ polymer  addition.
Under maximum conditions, polymer usage would be required.

Number of Units

Only  one  unit  will  be used,  with  an  adequate  spare   parts
inventory to minimize  down time.

Manufacturers
Several reputable manufacturers  of DAF thickeners  were  contacted
for  their  comments  on the designer's  calculations  and  proposed
application.


    5.4.3  Cost
        5.4.3.1  Capital  Cost

Several  recent  publications  have  developed capital  cost  curves
for  DAF  thickeners (26-28).    As  discussed in  Section  5.3.5.1,
the  most factual  is  the reference based on  actual USEPA  bid
                              5-35

-------
documents for the years  1973-1977  (27).   Although  the  data were
scattered, a regression  analysis  indicated the capital cost could
be approximated  by Equation  5-3:
    C = 2.99 x 104Q1-14
(5-3)
where:

    C = capital cost  of  process  in dollars;

    Q = plant design  flow  in mil gal wastewater flow per day.

The associated costs include,  those for,  excavation, process
piping, equipment,  concrete and  steel.  In addition, such cost as
those  for  administrating  and  engineering  are  equal to  0.2264
times Equation 5-3  (27).
        5.4.3.2   Operating and Maintenance Costs

Staffing

Figure 5-13 indicates  annual man-hour requirements for operations
and maintenance.   As an example, for a DAF thickener surface area
of 1,000 square  feet (93  m2)  a designer would include 2,700 man-
hours of operation and maintenance in the cost analysis.

Power

Figure  5-14  shows annual power  consumption  for  a  continuously
operating DAF thickener  as  a  function of DAF  thickener  surface
area.   As  an  example,  for  a  DAF thickener surface  area  of
1,000  square feet (93 m^),  a designer would  include a yearly
power  usage of  720,000  kWhr  (2,592 GJ)  in  the  cost analysis.
Figure 5-14 does  not include accessories such as pumps or  polymer
feed systems.

Maintenance Material Cost

Figure 5-15  shows a curve developed for estimating DAF thickener
maintenance material cost   as  a  function  of   DAF thickener
surface area.  As  an  example,  for  DAF  thickener  surface  area  of
1,000  square  feet (93 m^), a designer would  estimate a  yearly
materials cost of  $275.   Since this  number  is based on a June
1975 cost,  it must be  adjusted to the current design period.


5.5  Centrifugal  Thickening


    5.5.1  Introduction

The  concept  of   using   centrifuges  for  thickening municipal
wastewater sludges (waste-activated  sludge)  was first considered
in the United States in the  late 1930's  (49).  At that time, disc
                              5-36

-------
 nozzle  centrifuges  were used.  Early installations used machines
 developed  for  industrial processing.    Equipment manufacturers
 did  not appreciate that  the  composition  of municipal wastewater
 sludges is extremely variable  from plant  to  plant  and  within a
 plant,  and  that  most  wastewater  treatment  facilities  provided
 little,  if  any,  of  the  preventive  maintenance  common in
 industrial applications.
oc
O
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CO
cc.
D
o
X
 I
^
cc
o
z
z
<
       10
                3  456789 100   2  34567 891,000   2


                    THICKENER AREA, sq ft (1 sq ft = 0.093 m2)


                            FIGURE 5-13

                ANNUAL O&M MAN-HOUR REQUIREMENTS -
                        DAF THICKENERS (28)
                                                          4  56789
               early installations developed numerous operational
                     Dlems;  thus,  for a  period  of time designers
 and  users  did  not  favor  the centrifuge
Consequently, early  installations  developed  nt
and maintenance  problems;  thus,  for  a  period
 By the late  1960's, equipment manufacturers had  designed new
 machines  specifically  for  wastewater  sludge  applications,  and
 centrifuges  began to be  used once  again.   Considerable  experience
 resulted  in  improved application of  centrifuges and  centrifuge
                               5-37

-------
support systems (chemical  conditioning and chemical feed systems,
pumps,  and  electrical controls).   Today,  more sophisticatd
machines are  being  built  that require less  power and attention
and produce less noise.
o
CD
co
n
i_
_c
g
t
5
D
CO
Z
O
o
cc
LU

O
a.
D
Z
Z
3 456789 100   2   3456789 1,000   2

    THICKENER AREA, sq ft (1 sq ft = 0.093 m2)
                                                     3456789
                           FIGURE 5-14

         ANNUAL POWER CONSUMPTION - CONTINUOUS OPERATING
                       DAF THICKENERS  (28)


At present,  disc  nozzle, imperforate basket  and scroll-type
decanter  centrifuges  are used in municipal wastewater  sludge
thickening.


    5.5.2  Theory

Centrifugation  is  an  acceleration  of sedimentation through
the use of centrifugal force.   In a settling  tank, solids sink to
the bottom and the liquid  remains at  the  top.   In a centrifuge,
the rotating bowl  acts  as  a  highly  effective settling  tank.
Space  limitations  within  this manual make it  impossible  to
discuss  the  theory and mathematics  involved in centrifugation.
Complete discussions  can be found in other references (50-52).
                              5-38

-------
    10,000

       7
       6
       5
       4
_ro
"5
to
C/3
O
u
1,000
  g
  8
  7
  6
  5-
       3  -
       2  -
     100
         2   34 567891,000  234  5678910,000  2   3  456789

                 THICKENER AREA, sq ft (1 sq ft = 0.093 m2)

                       FIGURE 5-15

       ESTIMATED JUNE 1975 MAINTENANCE MATERIAL COST
                 FOR DAF THICKENERS (28)
One  aspect that  should be mentioned about centrifuge  theory,
because of  its misapplication  by the wastewater  design profes-
sion,  is  the use  of  a  Sigma  factor   to  evaluate bids  from
different centrifuge manufacturers. First developed in 1952  (53),
the  Sigma  concept  is an  established me thbd derve lcTb!f3tTcT
predict  the  sedimentation  performance  of  centrifuges  that are
geometrically and hydrodynamically  similar.   It cannot, however,
be used  in engineering  bid specifications  to  compare different
units when the two  basic assumptions, geometric and  hydrodynamic
similarity,   are  not valid.    This  is normally  the case in
scroll-type decanters.
    5.5.3  System Design Considerations
        5.5.3.1  Disc Nozzles

Disc nozzles  were  first used in  the  United  States in 1937  (49).
To  date,  approximately 90  machines  have been  installed at over
                              5-39

-------
50  municipalities  (37).   Table 5-11 lists  the  advantages  and
disadvantages  of  a  disc nozzle  as  compared  to other  thickening
systems.   Figure  5-16  shows  a  typical disc  nozzle  centrifuge.
                                TABLE 5-11

      ADVANTAGES AND DISADVANTAGES OF DISC NOZZLE CENTRIFUGES
             Advantages
           Disadvantages
  Yields  highly clarified centrate without
   the use of chemicals
  Has large liquid and  solids handling
   capacity in a very  small snace
  Produces little or no odor
Can only be used on sludges with particle
  sizes of 400 vm or less
Requires extensive prescreening and grit
  removal
Requires relatively high maintenance if
                                         designed
                                        Requires skilled maintenance personnel
                                FIGURE 5-16

              TYPICAL DISC NOZZLE CENTRIFUGE IN THE FIELD
                                   5-40

-------
Principles of Operation
Figure 5-17 features a cut away view of a disc  nozzle  centrifuge.
The  feed  normally enters  through  the top  (bottom  feed is  also
possible)  and passes down through a feedwell in the  center  of the
rotor.  An  impeller within  the  rotor .accelerates  and  distributes
the  feed slurry,  filling  the rotor  interior.   The heavier  solids
settle  outward  toward  the circumference  of the rotor  under
increasingly greater centrifugal force.   The  liquid and  the
lighter solids  flow inward through  the  cone-shaped disc  stack.
These lighter particles  are settled out  on the underside  of  the
discs, where they agglomerate,  slide down  the  discs,  and migrate
out  to  the nozzle  region.   The gap  of  0.050  inches (1.27  mm)
between the discs means  that the particles  have a short distance
to travel  before settling  on  the  disc  surface.   The  clarified
liquid passes on through the disc stack into the overflow chamber
and is then di-sefetrge.' tk- .  ^Vthe  *£ £i««rrir i
                    FEED
                 EFFLUENT
                 DISCHARGE
      CONCENTRATING
      CHAMBER
      SLUDGE
      DISCHARGE
 FEED
 EFFLUENT
DISCHARGE
                                                      ROTOR
                                                       BOWL
                                                       ROTOR
                                                     NOZZLES
              SLUDGE
           DISCHARGE
                           RECYCLE FLOW

                            FIGURE 5-17

               SCHEMATIC OF A DISC NOZZLE CENTRIFUGE
                              5-41

-------
The centrifugal action causes the solids to concentrate as they
settle outward.   At  the  outer rim of  the  rotor  bowl,  the high
energy imparted  to  the fluid  forces  the concentrated  material
through  the  rotor  nozzles.   One  part of  this  concentrated
sludge is  drawn  off  as  the thickened product and another  is
recycled  back to the base of the rotor and pumped back  into the
concentrating chamber;  there,  it is  subjected  to additional
centrifugal  force  and is  further  concentrated  before  it  is
once again discharged  through  the  nozzles.   This  recirculation
is  advantageous  because  it  increases  the  overall underflow
concentration;  minimizes  particle  accumulation  inside  the  rotor
by  flushing  action;  allows the  use   of larger  nozzles,  thus
decreasing  the potential  for  nozzle plugging;  and  helps  to
achieve  a  stable  separation equilibrium that lends itself  to
precise adjustment and  control.
Disc nozzle  centrifuges can be applied  only  to sludges consisting
of  smaller  particles  (less  than  400   m [54])  and  void  of
fibrous  material.  In early installations,  severe operating
and maintenance problems  occurred  from  pluggage (24,49,55,56).
For wastewater treatment,  then,  only those  systems  that provide
primary  treatment  and  separate  the  primary  sludge  from  the
waste-activated  sludge  can  be  equipped  with  a  disc  nozzle
centrifuge  and only  activated  sludge  can  be thickened  in  this
way.  Even for those systems  that  keep the  necessary separation,
designers  have  frequently  forgotten  the amount  of   fibrous
material  that can be  recycled back  into  the  aeration  system
from a dirty  anaerobic digester supernatant stream.  This  also
eventually causes  severe pluggage.

Pretreatment

To further reduce  operation and maintenance  requirements, current
design recommendations provide  for pretreatment  of  the  disc
nozzle feed  stream.  Figure 5-18 shows  a  disc nozzle pretreatment
system.

Raw WAS  is pumped to a strainer in order to remove  large solids
and fibrous  material.   Strainers should be made of stainless
steel, should be  self-cleaning,  and  should  be easily accessible.
Approximately one percent  of the  inlet  flow will  be  rejected.
The reject  stream  should go  to  the primary  sludge  handling
system.

After screening,  the flow goes to  a  degritter; however,  even
after aerated grit removal  and  primary treatment, some  grit
gets into the aeration  basin.  Under the  velocities generated in
a  disc  nozzle, this grit becomes  abrasive and causes  nozzle
deterioration.   The degritter  does not eliminate  the problem
completely but it does increase the running time between nozzle
replacements.   Approximately  10 percent  of the degritter inlet
flow  is  rejected, and  this  rejected stream is  usually  combined
with the screen flow.
                             5-42

-------
         STRAINER
RAW WASTE
                                         NOZZLE
                                         SEPARATOR
              REJECT FLOW GOES
              BACK TO PRIMARY
              SLUDGE HANDLING SYSTEM
                                                        p) RECIRCULATION
                                                      -V-> PUMP
                     THICKENED
                       SLUDGE
                             FIGURE 5-18

              TYPICAL DISC NOZZLE PRETREATMENT SYSTEM
Performance

Table  5-12  lists typical performance  that can  be expected of
disc  nozzle  centrifuges.    In  addition  to  the standard  process
variables,  the disc nozzle  machine variables considered  are bowl
diameter,  bowl  speed,  operation of  recycle,  disc spacing,  and
nozzle configuration.   Possibly the most important consideration,
however,  is the nature  of the  sludge.   As with other  centrifuge
applications,  an increasing  sludge volume index  (SVI)  influences
machine  performance.   Figure
capture  and  thickening (57).
5-19 shows  the effect  of SVI's  on
                             TABLE 5-12

           TYPICAL PERFORMANCE OF DISC NOZZLE CENTRIFUGE
Ref
5
5
5
5
24
60
Capacity,
gallons
per
minute
150
400
50-80
60-270
66
200
Feed
solids ,
percent
solids
0. 75-1.0
-?
0.7
0.7
1. 5
0.75
Underflow
solids ,
percent
solids
5-5.5
4.0
5-7
6.1
6.5-7.5
5.0
Solids
recovery,
percent
90 +
80
93-87
97-80
87-97
90
Polymer,
pounds per
dry ton
of solids
None
None
None
None
None
None
    1 gpm = 3.78 1/min
    1 Ib/ton = 0.5Aa/t
                                5-43

-------
    DC
    LLJ
    >
    o
    u
    LU
    QC
100


 90


 80


 70


 60


 50


 40


 30
                                               2V/*
                   2.Q
                     3.0
4.0
5.0
6.0
                       THICKENED SLUDGE SOLIDS, %

                            FIGURE 5-19

            EFFECT OF ACTIVATED SLUDGE SETTLEABILITY ON
                    CAPTURE AND THICKENING (57)
In general,  it  can be said of  disc  nozzle performance that the
concentration of  the  thickened sludge  tends to  increase with
increasing solids concentration  in  the  inlet.  Depending on  inlet
solids concentration, thickened sludge will be five to ten  times
more concentrated  than the  feed.   The capability  to concentrate
will  decrease as  the inlet  solids become more  concentrated.
Solids capture of 90 percent or better for  the material fed into
the disc  nozzle (after  screening and grit removal)  should be
obtainable without  the use of  polymers.

Other Considerations

As  noted  in  the  discussion  of  pretreatment  requirements,
approximately 11  percent  of the  flow  to the disc  nozzle system is
rejected.   The reject stream contains  two  to three  percent solids
and is usually pumped to  the primary  sludge handling system.

The centrate  stream is normally returned  to  the  aeration  tank.
This  line should  be  designed to  handle  the entire flow being
pumped to the pretreatment system.
                              5-44

-------
Typically,  equipment  suppliers  furnish  disc  nozzle  systems
complete,  including  all necessary  pumps.   The system  must  be
assembled  in the field.
         5.5.3.2   Imperforate Basket

Imperforate  basket  centrifuges  were  first used  in  the  U.S.  in
1920,  and to  date,  approximately  100  municipal installations
(over 300 machines)  have been  installed  (37).   About one half  are
used  for thickening.   In  fact,  the largest  centrifuge facility
in  the  world, the  Joint  Water  Pollution Plant of  the  County
Sanitation Districts of  Los Angeles County,  California, utilizes
48  imperforate  basket centrifuges.    Table  5-13  lists   the
advantacjos and disadvantages  of an  imperforate 'OH sky I: centrifuge
compared to other thickening systems.
                             TABLE 5-13

  ADVANTAGES AND DISADVANTAGES OF IMPERFORATE BASKET CENTRIFUGE

             Advantages                         Disadvantages
Facility can be designed so that same      Unit is not continuous feed and discharged
 machine can be used both for thickening   Requires special structural support
 and dewatering
          ...  .     , .                Has the highest ratio of capital cost to
Is very flexible in meeting process              .,
 requirements

Is not affected by grit

Of all the centrifuges, has the lowest
 operation and maintenance requirements

Compared to gravity and DAF thickener
 installations, is  clean looking and has
 little to no odor problems

Is an excellent thickener for hard-to-handle
 sludges
             f  Operation

Figure 5-20  is  a  schematic of  a top  feed  imperforate basket
centrifuge  illustrating general location of sludge inlet, polymer
feed,  and centrate  piping and location of cake discharge.

The  following describes  one complete  batch operating cycle of  a
basket centrifuge.   When  the "cycle start"  button is pushed,  the
centrifuge  begins to accelerate.   After  approximately 30  seconds,
the  feed pump  is  started through a timer relay.   Depending  on
the  feed pump  rate, it  will take one  to  three  minutes for  the
bowl  to reach operating  speed.   Sludge  enters the unit  through  a
stationary  feed  pipe  mounted  through  the  curb  cap.   This pipe
extends  to  the bottom portion  of  the basket and  ends at  an  angle
just  above  the floor in  order  to  impart a  tangential velocity  to
the  input stream.  The  duration of the  feed time  is controlled  by


                               5-45

-------
either a pre-set timer or a centrate  monitor that shuts the feed
pump off when a certain  level of  suspended  solids  appears  in the
centrate.  The centrate  is  normally  returned  to  the  inlet  of the
secondary treatment system.
                            FEED
                POLYMER
      SKIMMINGS
                                                       KNIFE
                     CAKE
                               CAKE
                           FIGURE 5-20

         GENERAL SCHEMATIC OF IMPERFORATE BASKET CENTRIFUGE
Deterioration in the centrate  indicates  that the centrifuge bowl
is filled with  solids,  and separation can no  longer  take place.
At this point, the sludge feed pump is turned off.

Turning off or diverting the feed pump decelerates the centrifuge.
When the centrifuge has  decelerated  to 70  rpvn's, a plow (located
by the  center spindle shaft)  is activated  and  starts to travel
horizontally  into  the bowl  where  the  solids  have  accumulated.
                              5-46

-------
When  the plow blade reaches the  bowl  wall,  a  dwell  timer  is
activated to  keep the plow in the same  position for approximately
5 to 15  seconds until all  the solids have  been discharged.   When
the plow retracts,  a  cycle  has  been  completed and  the machine
will automatically begin to accelerate,  starting a new cycle.

Application

Basket centrifuge is  a  good  application for  small plants  (under
1 to 2  MGD [44 to 88 I/sec]) pumping capacity.   The appropriate
plant  would  provide  neither  primary  clarification  nor grit
removal  (that  is,  extended  aeration,   aerated lagoons, contact
stabilization), but  would require:

     •   Thickening before aerobic or  anaerobic digestion,

     •   Solids  content of  less  than ten  percent to minimize  cost
        of hauling liquid sludge for  land disposal, and

     •   A machine that  can thicken sludge  part of  the  time and
        dewater sludge part of the  time.

Performance

Table 5-14 lists  typical basket centrifuge performance on several
types of  sludges.   Figure  5-21 shows  the relative  influence  of
one process  variable as  a  function  of  feed solids  content,
holding all other process variables constant.
                             TABLE 5-14

            TYPICAL THICKENING RESULTS USING IMPERFORATE
                         BASKET CENTRIFUGE
   Sludge type
                       Feed solids
                      concentration,
                      percent solids
Raw waste-activated
  sludge              '. ,
Aerobically
  digested sludge
Raw trickling filter sludge
  (rock & plastic media)
Anaerobically digested
  sludge, primary and rock
  trickling filter sludge
  (70:30)
0.5-1.5

  1-3

  2-3

  2-3
  Average
 cake solids
concentration,
percent solids

    8-10

    8-10

    8-9
    9-11
    8-10
    7-9
Polymer
required,
pounds dry
per ton dry
feed solids
0
1.0-3.0
0
1.0-3.0
0
1.5-3.0
0
1.5-3.0
Recovery
based on
centrate ,
percent
85-90
90-95
80-90
90-95
90-95
95-97
95-97
94-97
 1 Ib/ton =0.5 kg/t
                                5-47

-------
   FEED, % TOTAL SOLIDS
                                                       POSSIBLE
                                         FEED, % TOTAL SOLIDS
c
1
a
o>
                                      O)
   FEED, % TOTAL SOLIDS
                                         FEED, % TOTAL SOLIDS
o
LU

O
a.
FEED, % TOTAL SOLIDS
                                             FEED, % TOTAL SOLIDS
                           FIGURE 5-21

         RELATIVE INFLUENCE OF ONE PROCESS VARIABLE AS A
         FUNCTION OF FEED SOLIDS CONTENT FOR IMPERFORATE
          BASKET CENTRIFUGE HOLDING ALL OTHER PROCESS
                       VARIABLES CONSTANT
                              5-4!

-------
Othg_r_C£n_si_derations

In discussions of  hydraulic flow rate,  a  distinction must be
made between instantaneous feed  rate and  average feed  rate.
Instantaneous feed rate  is the actual hydraulic pump rateto
the basket.   The  average  feed rate  includes  the  period  of time
during  a  cycle when sludge is not  being pumped to the basket
(acceleration,  deceleration,  discharge).     Therefore,  dividing
total gallons pumped per  cycle by  total cycle time gives  the
average  feed rate.

Basket  centrifuge  performance is affected by  the solids feed rate
to the  machine.   As the  solids concentration  changes,  the flow
rate must be  adjusted.  Every  effort should  be made  to  minimize
floe shear.   For  this  reason,  positive displacement  cavity feed
pumps with 4 to 1  speed  variation are recommended.
Cake solids concentration  can only be discussed as average solids
concentration.   The solids  concentration  in  a  basket centrifuge
is maximum at the  bowl  wall  and decreases  toward the center.   The
solids  concentration  discharged will  be the  average  for  the
mixture.

The centrate stream should be returned to  the secondary system.


    5.5.3.3  Solid Bowl Decanter

The first  solid bowl decanter  centrifuge  in  the  U.S.  to operate
successfully on municipal  wastewater sludge was installed in  the
mid-1930's (58).   Since  then there  have  been approximately
150 installations  (over 400 machines) (37).  Few of  these units
were used for thickening because the rotating scroll  created
disturbances in the thickening  sludge, and the gravity force  that
had to  be  overcome  in  climbing the  beach  made  it more difficult
for the liquid  thickened sludge to be discharged.

Technological   advances  have  made  solid  bowl  decanters  for
thickening  waste-activated  sludge available.   Table  5-15 lists
the current advantages  and  disadvantages  of  solid  bowl decanter
centrifuges in waste-activated  sludge thickening.

Principles of Operation

Figure  5-22 is  a  schematic  of  a  solid  bowl decanter centrifuge.
The sludge  stream enters the bowl through  a feed pipe mounted at
one end of the centrifuge.

As soon as  the  sludge  particles are exposed to the gravitational
field,  they start  to  settle  out  on the  inner surface  of  the
rotating bowl.  The lighter liquid,  or centrate, pools above  the
sludge  layer and  flows  towards the centrate outlet ports  located
at the large end of the machine.
                              5-49

-------
                               TABLE 5-15
 ADVANTAGES AND DISADVANTAGES OF SOLID BOWL DECANTER CENTRIFUGES
              Advantages
            Disadvantages
Yields high throughput in a small area
Is easy to install
Is quiet
Causes no odor problems
Has low capital cost for installation
Is a clean looking installation
Has ability to constantly achieve four  to
  six percent solids in the thickened sludge
Is potentially a high maintenance item
May require polymers in order to operate
  successfully
Requires grit removal in feed stream
Requires skilled maintenance personnel
                                                                      FEED
         \
                                                           .
                                                          .';••.'; DEWATERED
                                                          ' "•'.'.    SOLIDS
                               FIGURE 5-22
               SCHEMATIC OF TYPICAL SOLID BOWL DECANTER
                               CENTRIFUGE
The  settled  sludge on  the  inner  surface  of the rotating  bowl  is
transported  by the rotating  conveyor  towards  the  conical  section
(small end)  of the bowl.   In  a  decanter designed  for dewatering,
the  sludge,  having  reached  the  conical  section,  is  normally
conveyed up  an  incline to the  sludge outlet. Waste-activated
                                  5-50

-------
sludge is too  "slimy"  to  be  conveyed  upward  without  large  doses
of polyelectrolyte.   In  the newly designed  machines, maximum
pool   depths  are  maintained;   in  addition,  a  specially  designed
baffle is located at the beginning  of  the conical  section.   This
baffle,  working in conjunction with the deep liquid  pool,  allows
hydrostatic  pressure  to  force the  thickened  sludge  out of  the
machine  independent  of  the  rotating  conveyor.    This design
eliminates  the need for  polymer addition  to aid in conveying
thickened sludge  up the incline towards the  sludge discharge  and
allows only  the  thickest  cake at  the  bowl  wall  to  be  removed.
Figure 5-23  shows a  typical installation of a centrifuge designed
for thickening.
                           FIGURE 5-23

            SOLID BOWL DECANTER CENTRIFUGE INSTALLATION
Application

Because  of  the  specially  designed  baffle,  the new  type  of
thickening decanter  centrifuge  can be  used to  thicken  only
straight waste-activated or aerobically digested  waste-activated
                              5-51

-------
sludge.   A thickening-type decanter centrifuge  cannot  be  used  for
primary  or  equivalent  sludges, whereas  the  old style decanters
without  baffle can be used to thicken any sludge.

Performance

Operating data on  the  newly designed machines are  very  limited.
Table 5-16 shows typical  operating results  supplied  by  one
equipment manufacturer.


                            TABLE 5-16

        TYPICAL CHARACTERISTICS OF THE NEW TYPE THICKENING
                 DECANTER CENTRIFUGE ON WAS (63)

   	—	-	—	                .	      	 	    	-Solid bowi
        Parameter                                      conveyor

 Operating  method                                     Continuous
 Bowl  diameters, inches                                 14-40
 Normal  G range                                      1,400-2,100
 WAS feed solids, percent                              0.5-1.5
 Thickened  WAS solids,  percent                           5-8
 Recovery,  percent                                      85-95
 Polymer range, Ib/ton                                   0-6


 1  inch  = 2.54 cm
 1  Ib/ton =0.5 kg/t
Other Considerations

Pumps should  provide positive  displacement  and variable  speed.
There should be no rigid piping connections  to  the  centrifuge.

Several  points of  polymer  addition should be provided.  This  is
necessary  because  of  differences  in  polymer  charge  densities,
effect of  polymer  reaction  times with the sludge,  and  variances
in sludge  characteristics.   Polymer  can  be  added at  the  sludge
feed line,  just  before  either  the  junction of the feed pipe  and
the  centrifuge or the  inlet  side  of the sludge  feed pump,  or
immediately downstream  from  the outlet side  of  the sludge feed
pump.

There should  be  a  moveable  overhead hoist  for  removing  and
replacing the internal conveyor.

A washwater connection must be  provided in the  feed  line to wash
the  decanter  internally  if the unit is to be shut  down  for more
than several  hours.   It is  important that the material not  dry
out within the machine, as  it can cause a  load imbalance.
                              5-52

-------
    5.5.4  Case History

The  following  is  a summary  of  a three-year  project in  which
a  disc  nozzle, imperforate basket  and  solid  bowl decanter
centrifuge were  evaluated  for their  ability  to  thicken  waste-
activated sludge.   The  study  was  concluded  at  the Village  Creek
Plant,  Fort  Worth, Texas  (59),  where wastewater temperatures
reach 86°F (30°C).   The plant had been unable  to gravity  thicken
waste-activated sludge over  a  maximum of  2.5 percent.   In
addition,  sludge  blanket  turnovers  and other  process upsets
proved troublesome.  Use of polymers,  dilution  water,  and  mixing
with primary  sludge did not resolve the problems associated with
gravity thickening  waste-activated sludge.

After  some pilot  testing,  two  disc-nozzle   centrifuges  were
installed  to   concentrate  waste-activated   sludge   prior  to
anaer~oblcditjestiorr,and ~an equipment  testing" prograirr was~
undertaken on other centrifuges.   An  expansion  from 45 to  96 MGD
(2 to 4 mVs) was anticipated without  an  increase in  the  plant's
existing digester capacity.  This meant that sludge would  have  to
be concentrated to  at least five percent total  solids.

Over  a three-year period,  the  existing  disc  nozzle centri-
fuge system  was redesigned  and  optimized and  other  centrifuges
(imperforate  basket and  solid bowl decanter)  were tested.

The test  program at Village  Creek graphically  illustrated  that
the thickening  characteristics  of waste-activated sludges  vary
markedly depending  on  the  design  and  operating  criteria of the
activated sludge process  and on  the  storage  conditions  of the
sludges.  These variations can be reduced considerably by  the use
of polyelectrolyte  conditioners.   The  effect of polyelectrolytes
on unit process costs varies; the advisability  of using them must
be determined for each  individual case.

Dis^c Nozzle

Testing  was   conducted on  a 24-inch (61  cm)  diameter  unit,
operating at  4,290  rpm and having  a  0.07-inch  (1.7  mm)  nozzle
opening.   The optimum design  for  obtaining  a  five percent
sludge and 90  percent  recovery was at  200 gpm  (12.62  I/sec) and
750 pounds per hour (340 kg/hr) of solids.

In operation,  the  nozzles on  a  disc-nozzle  machine will  plug  up
in minutes if prescreening  is  not provided.  For activated sludge
the  screen must be chosen with  care.  Vibrating  screens can
become  coated  with grease  and fiber.   They may  coat  over even
when provided with  spray nozzles, or they may tear from abrasion.
A  rotating  drum  wedge wire  screen with either   0.010-inch
(0.25  mm)  or  0.020-inch  (0.51  mm)  openings  offered the  best
results.  The rejects from  this screen were  about 5 to 15  percent
of  the feed   solids.   These  rejects  consisted  of approximately
60 percent hexane extractables and 30 percent fiber.


                              5-53

-------
Even  with  prescreening,  the centrifuge  nozzles  eventually  plug
up,  and  performance  deteriorates.    An  examination  of  the
centrifuge after performance deterioration  revealed  that  grease
had built  up  and had begun  to  back  up  into  the  disc  stack  and
interfere  with clarification.   The centrifuge then had to be
disassembled  and cleaned,  about  an  eight-hour  (16 man-hour)
operation.   With only drum  screening,  runs  of about  three  days'
duration were  experienced,  but  when an in-line wedge-wire  backup
screen was installed,  the runs were of  seven to ten days duration.

Other  installations  have  had  success  removing  this grease
in-line by periodically flushing the centrifuges  with  hot  water
introduced into the feed pipe;  the run is thereby lengthened to
more than thirty days.

In addition,  the nozzles, their  holders,  and  recycle  tubes  also
underwent extreme wear.   Erosion due to  fine  grit in  the  sludge
(despite primary treatment)  was  ruining one  nozzle or its  holder
about every three days until cyclone degritters  were  installed.
These reduced  the pressure to 100 gallons per minute (6.31  I/sec)
at 45  psi  (310 kN/m^)  and  removed grit down  to 75 mm.   They
also  reduced wear,  so that  the  nozzle had  to be  replaced  only
once every six  months.

^perforate Basket

Testing  was conducted on  a  40-inch  (102-cm)  diameter unit
operating at 1,500  rpm.   Polymer usage was not evaluated.   The
optimum design  was  for a six  percent cake and 80 percent recovery
at an average  feed  rate  of  40 gpm (252  I/sec)  and  150 pounds  per
hour (68 kg/ hr) of  solids.

Solid Bowl JDe_£ a n t.e r

Testing  was  conducted and  scaled up  for a 2.5-inch  (63.5-cm)
diameter  bowl  unit.   Polymer  usage  was  not  evaluated.   The
optimum design  was  for a 7.5  percent cake and 90 percent recovery
at 150  gpm  (9.46 I/sec)  and 562 pounds per hour  (255  kg/hr) of
solids.

Analysis of ResjjTts

Since all  three types of  centrifuges  were capable of producing,
without polymer,  cake  solids  content  and  solids  capture
considered adequate at Fort Worth, the issues of reliability  and
cost  came into  question.   Cost  is based on  73,300 pounds per  day
(33,275  kg/  day),   the maximum  expected  solids to  be  generated
over  an  entire month.    This  would  require 20  basket,  six
horizontal scroll, and  four   disc  nozzle centrifuges.    The
horizontal scroll  and  disc nozzle  centrifuges  would require
cycloning grit  removal equipment  and  the disc nozzle prescreen-
ing.  Comparative capital  and operation and  maintenance costs are
listed  in Table 5-17.   Estimated operating costs consist of power
and  additional  head for  cyclones.   Maintenance costs were known


                             5-54

-------
from plant data for  disc nozzle  machines and other manufacturer's
horizontal bowls and considered  minimal for the  basket.  Cleaning
costs were included  for the disc  nozzle.
                              TABLE 5-17

       ESTIMATED CAPITAL AND O/M COST FOR VARIOUS CENTRIFUGES
              FOR THICKENING OF WASTE-ACTIVATED SLUDGE
                AT VILLAGE CREEK - FORT WORTH, TEXAS
                                             Dollars
           Item
                           Six solid bowl
                             decanters
Capital cost
  Centrifuges
  Associated equipment

Annual cost
  20-yr life, 7 percent interest

Operating cost
  Maintenance parts
  Manpower ($10/hr)
  Electricity  (1.75 cents/kWhr)

Total cost of thickening

Cost/ton of sludge processed
900,000
300,000
113,280
             Five disc
              nozzle
600,000
300,000
 84,960
               Twenty
             imperforate
               basket
1,400, 000
1,500,000
  179,360
17, 800
12,480
55, 188
180,948
12.33
20,000
41,600
51,739
198,299
13.40
10,000
20,800
144,975
325,135
21.97
 Building, piping, and pumping to and from facility not included.

1 kWhr =  3.6 MJ
    5.5.5   Cost
         5.5.5.1  Capital Cost

Disc Nozz1e Centrifuge

Capital  cost  data  for disc  nozzle  systems  are  not  readily
available.   In  one study  (59) the  1978  cost  for five  200-gpm
(12.62  I/sec)   units   with  pretreatment  equipment  would  be
$900,000.   This cost figure was restricted to  equipment.

Basket Centrifuge

Capital  cost  curves for basket centrifuge installations  are  not
available  at  this  time.    In June 1979,  a  typical  top  feed,
48-inch  by 30-inch (122 cm  by 76  cm)  imperforate basket  (the
size  most  commonly  used)  with drive,  electrical  control  panel,
                                 5-55

-------
flexible  connectors, necessary spare  parts,  air compressor,
sludge feed  pump,  polymer feed system and  start-up services was
$160,000 to $170,000.

SolidBowl Decanter Centrifuge

Figure 5-24  shows  estimated June  1975  capital cost for  a solid
bowl  decanter  installation.   The  cost  includes  centrifuge
equipment, pumps,  hoist,  electrical facilities, and building.
co
CO
o
o
g
h-
o
3
cc
CO
z
o
CJ
               2   345678910    2  3  456789100    2   3456789

                SINGLE UNIT INSTALLED  CAPACITY, gpm (1gpm = 40.8 l/min)


                            FIGURE  5-24

              ESTIMATED JUNE 1975 SOLID BOWL DECANTER
                   INSTALLATION CAPITAL COST (28)
        5.5.5.2  Operating and Maintenance Cost

Disc No z z 1 e Cgntr^f_u_g_e

Operating and  maintenance  cost data for  disc  nozzle  systems are
not readily available.   In one  study (59)  the following  1978
costs were  given  for operating four 200-gpm  (12.62  I/sec)  units
24 hours per day:

     •  Maintenance parts - $20,000/year

     •  Manpower cost at $10/hr - $41,600

     •  Electricity at 1.75 cents/kWhr - $51,739
                              5-56

-------
The  following  provides a  rough  guide for  operating  and
maintenance requirements for  a  disc nozzle  centrifuge:

     •  It is  best to  run  a disc  nozzle  24  hours per  day to
        prevent  shutdowns  from  materials  that will dry  out
        between stacked plates  when machine  is not operating.

     •  At least once  every  eight  hours,  each machine  should be
        inspected  for  general   machine  operation,   product,  and
        amperage draw--l/2 man-hour per unit.

     •  At least once  a week,  each machine should  be  shut down
        to be  given  a  thorough flushing,  and  nozzles  should be
        removed and cleaned—two man-hours  per unit.

     •  If grease  is present  in the system,  the  machine  should
        be flushed with  hot water  at least  once  every  other
        day--three  man-hours  per unit.

     •  Depending on sludge  characteristics,  the  length  of time
        before  a machine  has to be  completely  disassembled and
        cleaned  is quite variable.   A  complete cleaning  will
        take  approximately 16 man-hours.

     •  Even  with good  pretreatment, nozzles, holders and recycle
        tubes will  have to be replaced.

     •  Other  parts that will  need replacing are drive  belts
        and pumps.

Lmper_£cirate Basket  Centrifug£

For  a  well  designed system,  operation and maintenance  for one
48-inch  by 30-inch  (122  cm x  76 cm)  basket  using a  hydraulic
drive can be  approximated  as  follows:

     •  Normal  start-up and  shutdown -  0.5  man-hour.

     •  Observation time per  eight-hour shift--!.0 man-hour.

     •  Basket  oil  change (1 quart  SAE 10-40  motor  oil [0.95 1]
        10-40 motor oil)  is  required every  200 operating hours--
        0.5 man-hour.

     •  General machine lubrication  is  needed  every  200 operating
        hours--0.5 man-hour.

     •  Air  compressor  should  be  serviced every 1,000 operating
        hours--!.0 man-hour.

     •  Hydraulic  oil  change  (65  gallons   [246  1])  is required
        every  3,500  operating  hours or  once  per year—3.0  man-
        hours .
                              5-57

-------
     •  High pressure  oil  filter should  be  changed  every 1,000
        operating hours—0.5  man-hour.

     •  If the  machine  is to  be  shut down  for more than 24 hours,
        the  basket  should  be  cleaned  with water  (tap  water
        pressure).    This can be  provided as an  automatic  or a
        manual  operation--0.5 man-hour for manual operation.

     •  Basket bearings  should  be replaced  every  100,000
        operating hours—40 man-hours.
        Standard materials  repair cost per  1,000
        operating hours  is  $300  to  $350  (June 1979).
  machine
     •  Specific  power draw  for  this  size  basket centrifuge
        ranges from 1.1 to  1.3  horsepower  per gallon per minute
        (13 to 15  kW/l/sec)  flow rate.

SoJL_ij_Bowl DecajT,ter Centr i fuge

Figure 5-25 indicates annual man-hour requirements for operation
and maintenance.   Included  in the curve are  labor requirements
directly  related   to the centrifuge, sludge  conditioning,  and
other associated equipment.
oc
O
LL
o
X
 I
^
cc
O
     1,000
              2  345678910     2   3  456789100    2

                      AVERAGE  FLOW, gpm (1 gpm = 40.8 l/min)

                            FIGURE 5-25
           ANNUAL O&M REQUIREMENTS - SOLID BOWL DECANTER
                          CENTRIFUGE (28)
3 456789
                              5-58

-------
Power


Power  is  dependent on  machine design, but  it should range  from
0.28 to  0.37  horsepower per gallon  per  minute flow rate  (3.3  to
4.4 kW/l/sec) .


Maintenance Material Costs


Figure  5-26  shows  a  curve  developed for estimating  solid  bowl
decanter centrifuge maintenance material cost.
 o
•a
C/3
O
O
    100,000
       9
       8
       7
       6
       5
       3


       2
    10,000


       7
       6
       5

       4
     1,000
                  I
                                 I
I
                                                       3 456789
        1      2  34567 89 10    2   34567 89100    2


                      AVERAGE FLOW, gpm (1 gpm = 40.8 l/min)


                            FIGURE 5-26


           ESTIMATED JUNE 1975 MAINTENANCE MATERIAL COST
                FOR SOLID BOWL DECANTER CENTRIFUGE





    5.6  Miscellaneous Thickening Methods



        5.6.1  Elutriation Basin


Elutriation is a satisfactory  process  for washing and thickening
digested  primary  sludges.  Elutriation  is  also effective for
mixtures  of  primary and  biological sludges  as long  as a  small
                               5-59

-------
dosage of  flocculent  is  used  to coflocculate the mixed sludges.
This prevents excessive loss of fines in the overflow elutriate.
(See Chapter 8 for further discussion.)


    5.6.2  Secondary Anaerobic  Digesters

Gravity thickening of biologically produced sludges in secondary
anaerobic  digesters  does  not  work  well as  presently designed.
Digesters  should  not be  relied upon  to function as  gravity
thickeners.  They may, however, be used to generate more methane
(five  to  ten percent) and  to  function as  sludge  holding tanks
(only if equipped with floating-type  covers).  (See Chapter 6 for
further discussion.)


    5.6.3  Facultative Sludge Lagoons

Although sludge  lagoons  are out  of  favor with  many  designers,
properly  designed  facultative  sludge  lagoons  can provide  an
effective means for further concentrating anaerobically digested
sludge (60).   (See Chapter 15 for  further discussion.)


    5.6.4  Ultraf iltration

Thickening waste-activated sludge from one to six percent solids
by ultraf iltration has  been  studied  (61).   Minimum estimated
membrane area required to  concentrate one  ton  (0.9  t) of waste-
activated  sludge  per day  from  one  to six  percent  solids  was
260 square feet  (23.4 m2).  High  pressure  drops  of  25 to 75 psi
(17  to 52  N/cm2)  had  to  be  used.   Power  requirements  were
approximately 540 kWhr per ton  (595 kWh/t) of dry feed solids.
5.7  References
 1.
2.
3.
    Craig,  E.W.,  D.D.  Meredith,  and A.C. Middleton.  "Algorithm
    for Optimal  Activated Sludge Design."   Journal of  the
    Environmental  Engineering
    p.  1101 (1978).
                                                 Vol.   104,  EE6 ,
Dick,  R.I.  and  D.L.  Simmons.
Process  for  Sludge  Management."
Conference on  Sludge Management  Disposal
                      12/14-16/76,
                                       "Optimal  Integration of
                                         Proceedings  3rd National
                                                and  Utilization,
    Miami  Beach,
    NSF and  ITI,
                 Florida,
                 p.  20.
                                        sponsored  by ERDA, USEPA,
     Anderson,  R.K.,  B.R.  Weddle,  T.  Hillmer,  and  A. Geswein.
     Cost  of  Landspreading  and Hauling  Sludge  from _ Muji ij^jjpjj.,
                          pTarvtsT   USEPA" "Office  of S oYi'cT Wa~steT
     Washington, DC 20460.  EPA 530/SW-619.   October  1977.
4.   USEPA Operations  Check  Lists
     Washington, DC 10460.
                                     Water
                           December  1978.
                                             Program  Operations
                              5-60

-------
 5.  USEPA   Prj3cesj3_Design  Manual  for  Upgrading  Existing Waste-
     water Treatment Plants.   Technology  Transfer^  Cincinnati,'
     Ohio 45268   EPA 625/l-71-004a.   October  1974.

 6.  Torpey, W.N.   "Concentration of  Combined Primary  and Acti-
     vated  Sludges  in Separate Thickening  Tanks."    Journal
     Sanitary  Engineering Division, Proceedings American "Society
     ojf_Cj"yi 1__E ngineers .   Vol.  80, p.  443  (1954).    '

 7.  Coe, H.S.,  and G.H.  Clevenger.    "Methods  for Determining
     the Capacities  of Slime  -  Settling Tanks."   Transactions
     American Inst.i.tute^of^ Mining Engineers.    Vo 1.  55,  p^356
     (1916).

 8.  Kynch,  G.J.   "A  Theory of Sedimentation."  T^ransactions
     of  Faraday Society.   Vol.  48, p.  166,  (1952).  ~

 9.  Talmage, W.P. and E.B.  Fitch.   "Determining Thickener Unit
     Areas."    Industrial and Engineering Chemistry.   Vol.  37,
     p.  38 (195 5~T

10.  Shannon, P.T., R.D.   Dechass, E.P.  Stroupe,  and E.M. Torry.
     "Batch  and  Continuous  Thickening."  Industrial  and
     Engineering   Che mist ry^ Fm]i^ajTi ejvt aj.s^.    Vol. 2,  p.  203,
     (August 1963).

11.  Dick,  R.I.   "Thickening."   Water Quality Engineering;
     New  Concepts  and   Developments.   E.L.  Thackston  and
     W.W.Eckenfelder,Jr.Editors,Jenkins Publishing Co.,
     Austin and New York,  1972.

12.  Kos, P.    Continuous  Gravity  Thickening  of  Sludges.   Dorr
     Oliver Technical Reprint 705, 1978.~"

13.  Keinath, T.M., M.D.  Ryckman, C.H. Dana, Jr.,  and D.A. Hofer.
     Design and Operational Criteria for Thickening of Biological
     Sludges  Parts I, II,  III, IV.  Water Resources  Research
     Institute, Clemson University.  September 1976.

14.  Courtesy of Envirex,  Inc.,  Waukesha,  Wisconsin  53187.

15.  "Gravity   Thickening."     Process  Design  Techniques  for
     Industrial Waste  Treatment.   Edited  by   Carl  E.  Adams and
     W.W. Eckenfelder, Enviro Press, Inc.   1974.

16.  Noland, R.F.  and R.B.  Dickerson.   "Thickening of  Sludge."
     USEPA  Technology Transfer Seminar on Sludge  Treatment
     and  Disposal.Vol. 1, USEPA-MERL, Technology  Transfer,
     Cincinnati, Ohio 45268,  October 1978.

17.  Benefield, L.D.  and  C.W. Randall.    "Air or 02 Activation:
     Verdict Still Undecided on Best  System  for  Settleability."
     Water and Sewage Works.   p. 44,  (April  1979).
                              5-61

-------
18.  USEPA Sludge  Handling and  Conditioning.   Office  of Water
     Program Operations.   Washington,  DC  20460.   EPA 430/9-78-
     022.  February 1978.

19.  Summary of reported operating data for  various U.S.  thermal
     conditioning facilities.

20.  Review  of  Techniques  for  Treatment  and Disposal  of
     Phosphorus-La den   Chemical  Sludges.    USEPA-MERL
     contract 68-03-2432 to be  published in the summer of  1979.

21.  Voshel,  D.    "Sludge  Handling  At Grand  Rapids, Michigan
     Wastewater  Treatment Plant."    Journal  Water Pollution
     Control  Federation.  Vol.  38,  p. 1506  (1966).             "~~~"

22.  Boyle,  W.H.    "Ensuring  Clarity and Accuracy  in  Torque
     Determinations."   Water andSewage Works.   (March 1978).

23.  Jordan, V.J.  and  C.H.  Scherer.   "Gravity  Thickening
     Techniques  at  a   Water  Reclamation  Plant."   Journal Water
     Pollution  Control  Federation.  Vol. 42, p. 180 (1970).~~~

24.  Ettelt, G.A.  and T.  Kennedy.  "Research and Operational
     Experience  in Sludge Dewatering  at Chicago."  Journal Water
     Pollution  Control  Federation.  Vol. 38, p. 248 (1966).

25.  Dick,  R.I.  and B.B. Ewing.  "Evaluation of Activated Sludge
     Thickening  Theories."   Journal  of  the Environmental
     Engineering  Division,  ASCE.  Vol. 93, EE 4, p. 9 (1967).

26.  USEPA.   Areawide  Assessment Procedures Manual - Volume III.
     Municipal  Environmental  Research  Laboratory.   Cincinnati,
     Ohio 45268.   EPA  600/9-76-014.  July 1976.

27.  USEPA.   Construction Costs for  Municipal  Wastewater
     Treatment  Plants.OfficeofWaterProgram Operations.
     Washington,  DC 20460.   MCD 37.  January, 1978.

28.  Culp/Wesner/Culp.  Cost  and  Performance Handbook  Sludge
     Handling Processes.    Prepared for Wastewater  Treatment and
     Reuse Seminar, South Lake  Tahoe.  October  26-27, 1977.

29.  Svarovsky,  L.   "Introduction to  Solid-Liquid Separation."
     Solid-Liquid Separation,  Butterworths,  Inc., 1977.  p. 1.

30.  Taylor, . R.W.   "Dispersed Air  Flotation."   Po 11 u t i o n
     E_ncp. nee ring.   p.  26,  (January  1973).

31.  Vrablik,  E.R.  "Fundamental Principles  of  Dissolved-Air
     Flotation  of   Industrial  Wastes."    14th  Purdue ^industrial
     Waste Conference,  1959.
32.  Laboon, J.F.   "Experimental Studies on  the  Concentration
     Raw Sludge."  Sewage and Industrial  Wastes.  Vol.  24, p. '
     (1952) .
                                                         of
                                                        423
(1952)
                              5-62

-------
33.  Laboon,  J.F.   "Pittsburgh  Plans Unique  Project  to  Abate
     Stream  Pollution."   Civil Engineering.    Vol.  24,  p.  44
     (January 1954).

34.  Burd,  R.S.   A Study of Sludge  Handling and  Disposal.   U.S.
     Department of  Interior  WP-20-4, May 1968.          ~~

35.  Logan,  R.P.    "Scum Removal  by  Vacuator  at  Palo Alto."
     Sewage and industrial Wastes.  Vol. 21, p.  799 (May 1949).

36.  Mays,  T.J.   "Vacuator  Operations  at  Santa  Maria."   Sewage
     and Industrial^Wastes.  Vol. 25, p. 1228,  (October 1953Ti

37.  Taken  from equipment manufacturer's installation lists.

38.  Wood,  R.F.  and  R.I.   Dick.    "Factors Influencing  Batch
     Flotation Tests."    Journal  Water  P o11u t ioni_C cm t r o1
     Federation.  Vol.  45, p. 304 (1973).           -.-..—_

39.  Eckenfelder,  W.W.  and  D.L.  Ford.   Water Pollution Control  -
     Experimental  Procedures for Process Design,   Pemberton  Press
     Jenkins  Publishing Co., 1970.  p.  75.

40.  Gulas,  V., L.  Benefield and  C.  Randall.   "Factors Affecting
     the Design of  Dissolved  Air  Flotation  Systems."   Journal
     Water  Pollution Control Federation, Vol. 50,  p.  1835 (1978).

41.  Weber, W.J.   Physiochemical  Processes for Water Quality
     Con_t r o 1.   Wiley-Interscience New York, 1972.   p.  555.

42.  Komline,  T.R.    "Dissolved Air  Flotation  Tackles Sludge
     Thickening."  Wate_rjrg_nd__Was_tes Engineering.   p.  63 (February
     1978).

43.  Bare,  W.F.R., N.B.  Jones,  and E.J.  Middlebrooks.    "Algae
     Removal  Using  Dissolved  Air  Flotation."   Journal  Water
     Pollution Control  Federation.  Vol. 47, p.  153 (1975).

44.  Water  Pollution  Control Federation.   MOP 8  Wastewater
     Treatment Plant Design.  Water Pollution Control Federation,
     1977.

45.  Mulbarger, M.C.  and D.D.  Huffman.    "Mixed Liquor Solids
     Separation by  Flotation."   Journal of the Environmental
     Engineering Division, ASCE.   Vol.  96, SA 4,  p.  861 (1970).

46.  Walker Process.    Dissolved Air Flotation,  Theory and Desigji
     Calculations.   Standard C120-0-1, February  1977.

47.  Komline   Sanderson.   K-S_ Dissolved Air  Flotation.    Latest
     bulletin, 1978.      ——    ______„         —

48.  Brown  and Caldwell.  Pilot Plant Study for Wastewater Solids
     Process ing.   Report prepared for Selma-Kingsburg-Fowler
     County Sanitation District.  February 27,  1978.


                              5-63

-------
49.   Kraus,  L.S.  and J.R.  Longley.   "Concentrating  Activated
     Sludge  with  a  Continuous  Feed  Centrifuge."   jSewage Works
     Journal.  Vol.  11,  p.  9,  (1939).

50.   Keith,  F.W.  and T.H.  Little.   "Centrifuges  in  Water and
     Waste Treatment."  Chemical Engineeri.nq[_Pr_o_gres_s_.   Vol. 65,
     (November 1969).

51.   Svarovsky,  L.    "Separation  by Centrifugal Sedimentation."
     Solid-Liquid   Separation,  Butterworths,   Inc.,   Ladislav
     Svarovsky,  editor,  1977.   p.  125.

52.   Vesilind, P.A.   Treatment  and  Disposal_of_W_aj3te water Sludge.
     Ann Arbor Science Publishers,  1974.

53.   Ambler,   C.M.   "The Evaluation  of  Centrifuge  Performance."
     Chemical Engi n e e_r i n g_ Pr ogre s s .   Vol.  48  #3  (1952).

54.   Landis,  D.M.    "Centrifuge   Applications."   Filtration
     Engendering.   p. 27,  (September 1969).

55.   Bradney, L. and  R.E. Bragstad.   "Concentration of Activated
     Sludge  by  Centrifuge."    S_gw_a_g_e	gJ!J_^IjJJJ5_!-.JLJ:Al __Waste *
     Vol. 27, p. 404, (1955).

56.   Reefer,  C.E.   and  H.  Krotz .   "Experiments on Dewatering
     Sewage  Sludge  with a Centrifuge."   j3ewag_eWorks Journal,
     Vol. 1,  p.  120, (1929) .

57.   Vaughn,  D.R.  and G.A.  Reitwiesner.   "Disc-Nozzle Centrifuges
     for  Sludge  Thickening."    Journal   Water  Pollution Control
     Federation.  Vol. 44,  p.  1789,  (1972).

58.   Albertson,  O.E. and  E.E.  Guidi,  Jr.   "Centrifugation of
     Waste Sludges."  Journal  WaterPollution ControlFederation.
     Vol. 41, p. 607, (1969).

59.   McKnight, M.D.,  "Centrifugal  Thickening of Excess Activated
     Sludge."  Unpublished  article  on experience  at Village Creek
     Plant, Ft.  Worth, Texas.   1978.

60.   Earnshaw,  G.    "A  Storage  System  That  Works."   Sludge
     Magazine.  March-April,  p.  29/  (1979).

61.   NCASI.  A Pilot Plant  Study of Mechanical Dewatering Devices
     Operated on Waste Activated Sludge.    Prepared  for National
     Council of the Paper Industry  for Air and Stream Improvement
     Technical Bulletin  288,  November 1976.
                              5-64

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EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
               Chapters.  Stabilization
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                          CHAPTER 6

                        STABILIZATION
6.1  Introduction

The principal purposes of stabilization are to make  the  treated
sludge less odorous and putrescible  and to  reduce  the pathogenic
organism  content.   Some procedures  used  to accomplish these
objectives can also result  in  other  basic changes  in  the  sludge.
The selection of  a  certain method hinges primarily on the final
disposal  procedure planned.  If the sludge  is  to  be dewatered and
incinerated,  frequently no stabilization procedure is employed.
Most  stabilization  methods, particularly  anaerobic  and   aerobic
digestion,  result in a  substantial decrease in  the amount of
suspended  sludge  solids.   Hence,  the corollary function of
conversion is included  in the description of these  processes.

This  chapter provides  detailed discussion  of four processes
that have  the  primary  function  of  sludge  stabilization.   These
processes  are  anaerobic digestion,   aerobic  digestion,  lime
stabilization,  and  chlorine  oxidation.    Both  anaerobic  and
aerobic digestion  are  currently  increasing  in  popularity.   The
former is  receiving  revived attention  from  some  cities  and new
attention  from  others  for  several   reasons.   The   production of
methane in anaerobic digestion  is  attractive  in  view of energy
shortages,  as  is the  suitability of anaerobically digested
sludges  to disposal  on land.   Also,  it  is being  recognized
that  problems  experienced  previously  with  anaerobic digestion
were  actually  due to  other  wastewater process  considerations.
Interest  in  aerobic  digestion of  excess  activated sludge is
growing  because  it has  the  potential for  providing  a  good
quality liquid process  stream and can produce  exothermic reaction
conditions.  A major impetus for processes  such  as anaerobic and
aerobic digestion and lime treatment is the growing  emphasis on
utilization  of  sludge rather  than mere  disposal.   Chlorine
oxidation  is of  limited use  for  special  situations or where
septic tank wastes are  involved.

Several other sludge treatment processes provide varying  degrees
of stabilization,  although  this  is  not their  principal function.
Composting is practiced  in  several  United States  cities and
is being  actively  investigated for  others.  This  process is
considered important enough,   with  the emphasis  on recycling of
sludge to  the land, that it  alone  is discussed   in  Chapter 12.
Heat  treatment,  discussed  in  Chapter  8, has  been installed in
several new  United  States plants to  improve  sludge conditioning
and dewatering economics.   Some  processes used to disinfect
                              6-1

-------
sludge,  such  as heat drying  and  pasteurization,  also provide
limited  stabilization.    These  processes  are  discussed  in
Chapters  7  and 10.

Selection of  the optimum stabilization method for  any  treatment
and  disposal  system requires an  in-depth  understanding  of
the method  and its limitations.   The designer must recognize
these  limitations and  accommodate them  in the design of the
subsequent  processing  and disposal steps.


6.2  Anaerobic Digestion


    6.2.1  Process  Description

Anaerobic  digestion  is the biological degradation  of complex
organic substances  in the absence of  free oxygen.   During  these
reactions,  energy is  released  and much of the organic matter is
converted to  methane, carbon  dioxide,  and  water.   Since  little
carbon and  energy remain available to  sustain further biological
activity, the  remaining solids are rendered  stable.


        6.2.1.1  History and Current Status

Anaerobic  digestion  is  among the  oldest  forms of  biological
wastewater  treatment.   It was  first  used a  century  ago  to  reduce
both  the quantity  and  odor  of  sewage  sludges.   Originally,
anaerobic  digestion was1  carried  out in  the same  tank  as
sedimentation, but the  two-story  tanks developed  in  England by
Travis and  in  Germany by Imhoff began  a trend toward separating
the two processes.  Separate sludge digestion tanks  came  into use
in  the first  decades of  this century.   At first,  these were
little more than simple holding  tanks, but they  provided the
opportunity to control environmental  conditions during  anaerobic
digestion and,  thereby,  improve  process  performance.  With the
development  of  digester  heating and,  subsequently,  mixing,
anaerobic digestion became  the most common  method of  stabilizing
sludge.

As  both industrial  waste loads and  the  general   degree  of
wastewater   treatment  increased, the sludges generated  by  treat-
ment  plants  became more varied and  complex.   Digester  systems
failed because  their design and operation were empirically
developed  under simpler  conditions.   As  a result, anaerobic
sludge digestion  fell  into  disfavor.   However,   interest in
anaerobic  digestion  of  dilute wastes  stimulated a new wave of
research into  the  process.   The  resulting  development  of  steady
state  models  in the  1960s  (1,2,3),  dynamic models  in  the  1970s
(4,5,6),  and  increasing  research  into the basic biochemical
processes  (7,8,9,10) led  to  significant  improvements in both
reliability and performance of  anaerobic digesters.
                               6-2

-------
Currently,  sludge  stabilization  by  anaerobic  digestion  is  used
extensively.   A  1977  survey  (11)  of  98  municipal  wastewater
treatment  plants   in   the  United  States  found  that  73  used
anaerobic digestion to stabilize  and reduce the volume  of sludge.
Because of  emphasis on  energy  conservation  and  recovery  and
environmental  pressure  to  use wasteewater  sludges on land,  it is
expected  that anaerobic digestion  will  continue  to play  a  major
role in municipal  sludge processing.


        6.2.1.2   Applicability

A  wide variety  of  sludges  from  municipal wastewater  treatment
plants  can  be stabilized  through  anaerobic  digestion.   Table  6-1
lists  some  types of  sludge  that  have been  anaerobically digested
in full-scale, high rate digesters.
                              TABLE 6-1

             TYPE AND REFERENCE OF FULL-SCALE STUDIES ON
              HIGH RATE ANAEROBIC DIGESTION OF MUNICIPAL
                  WASTEWATER SLUDGE (13,14,15,16-34)

                                               Reference
                                    Mesophilic           Thermophilic
             Sludge type                diqestion             digestion
   Primary and lime                    16, 17                     -
   Primary and ferric chloride            18
   Primary and alum                    19
   Primary and trickling filter           20, 21
   Primary, trickling filter, and alum     22                        -

   Primary and waste activated            23, 24, 25, 26         25, 27, 23, 29
   Primary, waste activated, and lime      30, 31
   Primary, waste activated, and alum      30, 32, 33                 -
   Primary, waste activated, and ferric
     chloride                         30
   Primary, waste activated, and sodium
     aluminate                        32, 33
   Waste activated only (pilot plant
     only)                           13, 14, 15,.34         13, 14, 15
Solids-liquid separation of digested primary sludge  is  downgraded
by  even  small  additions  of  biological  sludge,  particularly
activated  sludge.   Although  mixtures  of primary and  biological
sludge  will  break down  readily  under anaerobic conditions,  the
net  rate  of  the  reaction  is slowed  slightly  (12).   Experience
with  full-scale anaerobic  digestion  of  straight  activated sludge
is  limited,  although  laboratory  (13,14) and pilot-scale  studies
(15)  demonstrate  that separate digestion of  activated  sludge is
feasible.

Chemical  sludges  have been  successfully  digested  anaerobically,
although  in  several  cases,  volatile solids  reduction  and  gas
                                 6-3

-------
production were  low,  compared with  conventional  sewage sludges
(35,36).   Decreased  performance  appears  to  result  from reduced
biodegradability, rather  than  from  toxic  inhibition of  the
anaerobic microorganisms  (35).

Anaerobic digestion  is  a feasible  stabilizing  method  for
wastewater sludges  that  have  low  concentrations  of  toxins
and a  volatile  solids  content above  50  percent.   The obligate
anaerobic  microorganisms are  sensitive  and do  not  thrive  under
fluctuating operating conditions.   Consequently, the process must
be carefully  considered  for use  at treatment  plants  where wide
variations in  sludge  quantity  and quality  are  common.


        6.2.1.3   Advantages and Disadvantages

Anaerobic digestion offers several  advantages over other methods
of sludge stabilization;  specifically, the process:

     •   Produces  methane,  a usable source  of energy.   In most
        cases,  the process  is  a  net  energy  producer,  since
        the energy content of digester  gas  exceeds  the  energy
        demand  for  mixing  and   heating.    Surplus  methane
        is frequently  used  for  heating  buildings,  running
        engines,  or generating electricity (37,38,39).   (Refer to
        Chapter  18.)

     •   Reduces  total  sludge  mass  through  the  conversion  of
        organic  matter  to primarily methane, carbon dioxide,  and
        water.   Commonly,  25 to  45 percent  of the  raw  sludge
        solids are destroyed  during  anaerobic  digestion.   This
        can substantially reduce the cost  of sludge disposal.

     •   Yields  a  solids  residue  suitable  for use  as a  soil
        conditioner.   Anaerobically digested sludge  contains
        n"utr~ients and  organic  matter  that can  improve  the
        fertility and texture of  soils.    Odor levels are greatly
        reduced  by anaerobic digestion.

     •   Inactivates pathogens.    Disease-producing   organisms  in
        sludge die off  during  the relatively long detention times
        used in anaerobic digestion.   The high  temperatures used
        in thermophilic  digestion  (122 to  140°F, 50  to 60°C) have
        an additional  bactericidal effect.   Pathogen  reduction
        during anaerobic  digestion  is  discussed  in Chapter 7.

Principal  disadvantages  of anaerobic sludge  digestion  are that
it:

        Has a high capital  cost.    Very  large,  closed  digestion
        tanks are required, which must be  fitted with  systems for
        feedina.  heatinq, and  mixinq  the  sludqe.
Udiirio  a i. d  L. c ^ i-i J- L. <^ *_i , wii-L^ii iiitaou. *-"^ a- j. »- v- «— ^*
feeding, heating,  and mixing the sludge.
                               6-4

-------
        Is  susceptible  to  upsets.    Microorganisms  involved  in
        anaerobic decomposition are  sensitive  to  small  changes  in
        their environment.   Monitoring  of performance and  close
        process  control  are required to  prevent upsets.

        Produces a poor  quality sidestream.    Supernatants   from
        anaerobic digesters often have  a  high oxygen demand and
        high  concentrations  of  nitrogen  and  suspended  solids.
        Recycling of digester  supernatant to  the plant influent
        may upset the liquid process stream or produce  a build-up
        of fine  particles  in the treatment plant.  In plants  that
        are required to remove  nitrogen from  the wastewater, the
        soluble   nitrogen  in the supernatant  can cause problems
        and/or increased costs  of treatment.

        Keeps me thane-producing bacteria growth at a  slow  rate.
        Large reactors are required to hold the sludge for  15  to
        30 days  to stabilize  the   organic  solids effectively.
        This slow  growth  rate  also limits the  speed  with  which
        the  process  can   adjust to  changes in waste  loads,
        temperature, and other  environmental conditions (40).
        6.2.1.4  Microbiology

Anaerobic  digestion   involves  several  successive   fermentations
carried out  by a mixed culture  of  microorganisms  (7,10).   This
web of  interactions  compromises  two general degradation phases:
acid  formation and methane production.   Figure  6-1  shows, in
simplified form,  the  reactions  involved in anaerobic  digestion.

r
\

COMPLEX
SUBSTRATE
CARBOHYDRATES,
FATS AND
PROTEINS
ACID
FORMATION
A
r
MICRO-
ORGANISMS STABLE AND 1
*" , DEGRADATIO
METHANE
PRODUCTION
A
A
MICRO-
NTERMEDIATE ORGANISMS
N PRODUCTS
PRINCIPALLY ORGANIC ACIDS, CO,, METHANE
ACID FORMERS HjO, AND CELLS ^ BACTERIA

^
PH + CO + OTHER END
UH4 + UU2 + PRODUCTS
H20, H2S
CELLS AND STABLE
DEGRADATION
PRODUCTS
                            FIGURE 6-1
            SUMMARY OF THE ANAEROBIC DIGESTION PROCESS
In  the first  phase  of  digestion,  facultative bacteria  convert
complex organic  substrates  to  short-chain  organic  acids--
primarily  acetic,  propionic,  and  lactic  acids.   These  volatile
                               6-5

-------
organic acids tend  to  reduce  the  pH,  although  alkaline buffering
materials are also  produced.  Organic matter is converted  into a
form suitable for breakdown by the second  group of  bacteria.

In  the  second  phase,  strictly anaerobic  bacteria  (called
methanogens),  convert the volatile acids  to  methane  (CH4),  carbon
dioxide (€02), and  other trace  gases.   There  are several  groups
of methanogenic  bacteria,  each  with  specific  substrate require-
ments,  that  work in  concert to  reduce  complex wastes  such as
sewage sludge.  Tracer studies indicate that there are two major
pathways of  methane  formation:


     •   The  cleavage of  acetic  acid  to  form methane  and  carbon
        dioxide.
        CH3COOH	*-CH4  +  C02
        The reduction of  carbon  dioxide, by  use of  hydrogen
        gas or  formate  produced  by  other bacteria,  to  form
        methane.
                           2H20


When an anaerobic digester  is  working  properly,  the two phases of
degradation are  in dynamic equilibrium;  that  is,  the volatile
organic acids  are converted to methane at  the  same rate that they
are formed from the more  complex  organic molecules.  As a result,
volatile  acid  levels  are  low in a working  digester.   However,
methane formers are inherently slow-growing,  with doubling times
measured  in days.  In addition, methanogenic bacteria  can be
adversely  affected  by  even small  fluctuations  in  pH,  substrate
concentrations, and temperature.   In  contrast,  the  acid formers
can function  over  a wide  range  of  environmental  conditions and
have doubling  times normally  measured  in hours.    As  a result,
when an anaerobic  digester is stressed by shock loads, tempera-
ture fluctuations,  or  an  inhibitory  material,  methane bacteria
activity  begins  to lag  behind  that  of the  acid  formers.   When
this happens,  organic  acids  cannot  be converted to  methane as
rapidly as  they  form.    Once  the balance  is  upset,  intermediate
organic  acids accumulate  and the pH drops.   As a result, the
methanogens are  further  inhibited,  and  the  process   eventually
fails unless corrective action is taken.

The anaerobic  process  is  essentially controlled  by the methane
bacteria  because  of their  slow  growth rate  and  sensitivity to
environmental  change.   Therefore, all  successful designs must be
based  around  the  special limiting  characteristics  of  these
microorganisms.
                               6-6

-------
    6.2.2  Process Variations

Experimentation over the years has  yielded  four basic variations
in  anaerobic  sludge digestion:    low-rate  digestion,  high-rate
digestion, anaerobic contact, and  phase  separation.

High-rate  digestion  is  obviously  an improvement  over  low-rate
digestion, and its features  have been incorporated into standard
practice.   The anaerobic contact  process  and phase separation,
while offering  some specific  benefits,  have not  been  used for
sludge digestion  in full-scale facilities.


        6.2.2.1  Low-Rate Digestion

The simplest  and  oldest type  of  anaerobic  sludge stabilization
process  is low-rate digestion.    The  basic  features  of  this
process  layout are shown on  Figure  6-2.   Essentially, a low-rate
digester  is a  large  storage  tank.   With the possible  exception
of heating,  no attempt is  made  to accelerate the process by
controlling the environment.   Raw  sludge  is fed  into  the  tank
intermittently.  Bubbles of  sludge  gas  are  generated soon after
sludge  is fed to  the  digester,  and their  rise to  the surface
provides the only  mixing.  As a result,  the contents of  the tank
stratify,  forming three distinct  zones:   a floating layer of
scum,  a  middle level of  supernatant, and a lower zone of sludge.
Essentially,  all  decomposition is  restricted  to  the  lower zone.
Stabilized sludge, which accumulates and  thickens  at the  bottom
of the  tank,  is   periodically  drawn off from the  center  of the
floor.   Supernatant is  removed  from the  side of the tank and
recycled back to the treatment plant.  Sludge gas collects above
the liquid surface and  is drawn off  through  the cover.


        6.2.2.2  High-Rate  Digestion

In the  1950s,  research  was  directed  toward improving  anaerobic
digestion.  Various studies  (24,41,42,43,44)  documented the value
of heating,  auxiliary mixing, thickening  the raw  sludge, and
uniform  feeding.    These  four  features, the essential elements of
high-rate digestion, act together to create a steady and uniform
environment,  the  best conditions for the  biological process.  The
net result  is  that volume  requirements  are  reduced  and  process
stability is enhanced.   Figure 6-3  shows  the  basic  layout of this
process .
The contents of  a  high-rate digester are heated and  consistently
maintained  to  within  1°F  (0.6°C)  of  design  temperature.
Heating  is beneficial  because  the rate of microbial growth and,
therefore,  the  rate  of  digestion,  increases  with  temperature.
Anaerobic organisms,  particularly  methanogens,  are  easily
inhibited by even small changes in temperature.  Therefore, close


                               6-7

-------
control  of the  temperature  in a digester helps  maintain the
microbial  balance  and improves  the balance of  the  digestion
process.
                                      DIGESTER GAS
                               GAS
          RAW SLUDGE
                              SCUM
                      \\\x\\\\\\\\\\\\\
                          SUPERNATANT
                      \\\\\\\\\\\\\\\\\\\.\\\
                            ACTIVELY
                        DIGESTING SLUDGE
                      \\\\\\\\\\\\\\\\v
                           STABILIZED
                             SLUDGE
SUPERNATANT
                                     DIGESTED SLUDGE
          UNHEATED
          UNMIXED
          INTERMITTENT FEEDING AND WITHDRAWAL
          DETENTION TIME: 30-60 DAYS
          LOADING RATE: 0.03-0.10 Ib VSS/cu ft/day
                      (0.4-1.6 kg VSS/m3/day)

                            FIGURE 6-2

               LOW-RATE ANAEROBIC DIGESTION SYSTEM
Methane production has  been  reported at temperatures ranging from
32°F to as high as  140°F  (0 to  60°C).   Most commonly, high-rate
digesters  are operated between  86 and  100°F  (30 and  38°C).
The organisms that grow  in this temperature range  are  called
mesophilic.    Another  group of  microorganisms,  the thermophilic
bacteria,  grow at  temperatures between  122 and  140°F  (50 and
60°C).   Thermophilic  anaerobic  digestion has  been  studied
since  the  1930s,  both  at laboratory scale  (13,45,46)  and plant
scale  (27,28,29).   This   research  was  recently reviewed  by
Buhr and Andrews  (47).   In  general,  the  advantages claimed for
thermophilic over  mesophilic digestion  are:  faster  reaction
rates  that permit  lower  detention  times,  improved dewatering of
the digested  sludge, and  increased destruction of pathogens.

Disadvantages of thermophilic digestion include  their  higher
energy requirements  for heating;  lower  quality  supernatant,
containing   larger quantities  of  dissolved  materials  (29);
                               6-8

-------
and  poorer  process  stability.    Thermophi1ic  organisms  are
particularly sensitive to temperature fluctuation.  More detailed
information on  the  effects  of  temperature   on  digestion is
included in Section 6.2.4.   Design of digester  heating systems is
discussed in Section 6.2.6.2.
                                          DIGESTER GAS
                                        DIGESTED SLUDGE
      HEATED TO CONSTANT TEMPERATURE
      MIXED
      CONTINUOUS FEEDING AND WITHDRAWAL
      DETENTION TIME: 10-15 DAY MINIMUM
      LOADING RATE: 0.10-0.50 Ib VSS/cu ft/day
                   (1.6-8.0 kg VSS/m3/day)

                           FIGURE 6-3

                SINGLE-STAGE, HIGH-RATE ANAEROBIC
                        DIGESTION SYSTEM
Auxiliary Mixing

Sludge  in  high-rate digesters  is  mixed  continuously  to  create
a  homogeneous  environment  throughout  the reactor.   When
stratification  is  prevented,  the  entire  digester  is  available
for  active  decomposition,  thereby  increasing  the  effective
detention  time.   Furthermore,  mixing  quickly  brings the
raw  sludge  into contact with  the  microorganisms  and evenly
distributes  metabolic waste  products  and  toxic  substances.
Methods of mixing  and mixing system  designs are  described  in
Sect ion 6.2.6.3.
                               6-9

-------
Pre-thickening

The  benefits of  thickening  raw sludge  before  digestion were
first  demonstrated  by  Torpey  in  the  early  1950s  (24).   By
gravity thickening a combination of primary  and  excess  secondary
sludge  before  digestion, he  was able  to achieve  stabilization
equivalent to digestion  without  thickening in one quarter of the
digester  volume.   In addition,  liquid  that  had previously been
removed as  digester  supernatant  was instead  removed  in the
preceding  thickener.   Since thickener supernatant  is of far
better  quality  than  digester  supernatant,  it had  significantly
less adverse  impact  when  returned  to  the  wastewater   treatment
stream.   &lso, heating requirements were  considerably reduced by
pre-thickening,  since smaller  volumes  of  raw sludge entered the
digesters.                                 •     ,,/

Later full-scale studies  by  Torpey and  Melbinger  (48) showed that
thickening of digester  feed  sludge could  be improved by  recycling
a portion of  the  digested  sludge back to the gravity thickener.
This variation  of high-rate digestion,  often  called  the Torpey
process,  is  shown schematically  on  Figure 6-4.   The  results of
            Melbinger's  studies are   summarized in Table  6-2.
            initial  effect  of  recirculation   was  to  improve
            further benefits were obtained.   Improved thickening
            sludge increased the  detention time  (solids  retention
time)  in  the digesters  and,  thereby,  enhanced   solids   reduction
during  digestion.  The result  was that the volume of  sludge for
final  disposal  was reduced by  43  percent.   These  results were
obtained with the  same overall plant  treatment  efficiencies and
wastewater aeration  requirements as had  been  achieved  prior to
the recycling of digested sludge.
Torpey  and
While  the
thickening,
of the feed
  PRIMARY
                          RECIRCULATING
SLUDGE
MODIFIED
AERATION
SLUDGE
i



DIGESTED SLUDGE
/^~~\ S~\
! I GRAVITY \ THICKENED /ANAEROBIC\ DIGESTED
h-HICKENEfd MIXED SLUDGE 1 DIGESTER I SLUDGE

^ TO WASTEWATER
1



                                                            TO
                                                          DISPOSAL
                     SUPERNATANT  TREATMENT STREAM
                            FIGURE 6-4

               FLOW DIAGRAM FOR THE TORPEY PROCESS
There  is,  however,  a  point  beyond which  further  thickening
of  feed  sludge  has  a  detrimental  effect  on digestion.   Two
problems  can result from over-concentration of  feed  sludge.
                               6-10

-------
         Good  mixing  becomes difficult  to  maintain.    The  solids
         concentration in  the digester  affects  the viscosity,
         which,  in  turn,  affects  mixing.    Sawyer and  Grumbling
         (49)  experienced  difficulty  in  mixing when  the  solids
         content  in  the digester  exceeded  six  percent.   Because
         of  the  reduction  of volatile solids occurring during
         digestion,  the solids  concentration  within  the  digester
         is  less  than  the  feed solids  concentration.   Therefore,
         feed  solids  concentrations  may  reach  eight  to  nine
         percent  before mixing is impaired.

         Chemical  concentrations can reach levels that can inhibit
         microbial  activity.    A  highly  thickened  feed sludge
         means that  the contents of  the digester will be  very
         concentrated.   Compounds  entering  the  digester, such  as
         salts  and heavy  metals,  and  end products of  digestion,
         such  as  volatile  acids and  ammonium  salts,  may reach
         concentrations toxic  to the  bacteria in  the digester
         (50).   For  example,  in  one  case,  digester  failure
         followed  a  three-month period  during  which  feed  solids
         concentrations ranged  from  8.2 to  9.0  percent  (51).   It
         is  believed that  this caused  ammonium alkaline  products
         to  reach  toxic concentrations.
                             TABLE 6-2
            RESULTS OF RECIRCULATINC DIGESTED SLUDGE TO
          THE THICKENER AT BOWERY BAY PLANT, NEW YORK (48)

                                    Without                  With
                                 recirculation3           recirculation
Raw sludge
  Dry weight,  Ib/day                    108,000                 101,500

Digester  feed  (includes recircula-
  tion)
   Dry weight, Ib/day                  108,000                 144,300
   Solids concentration,  percent            8.2                     9.9

Digested  sludge to disposal
  Dry weight,  Ib/day                     60,000                  47,500
  Solids  concentration, percent             4.6                     6.1
  Volume, cu ft/day                     20,700                  12,300
aAverages  for operation in 1961.  Average treatment plant flow = 105 MGD.
bAverages  for 15 months of operation with 33,  50, or 67 percent recirculation
 of digested sludge.  Average treatment flow = 101 MGD.

 1 Ib/day  = 0.454 kg/day
 1 cu ft/day = 0.0283 rtH/day



Uniform  Feeding

Feed   is  introduced into  a   high-rate  digester at  frequent
intervals  to help  maintain  constant conditions in the reactor.



                                 6-11

-------
In the past, many digesters were fed only once a day or even less
frequently.  These  slug  loadings  placed an unnecessary stress on
the  biological  system and destabilized  the  process.   Although
continuous  feeding  is  ideal,  it  is  acceptable  to charge a
digester intermittently, as  long  as  this is done frequently (for
example,  every  two  hours).   Methods  of automating digester
feeding are described in Section 6.2.6.5.


Two- S_ t a g e _ D i c[e s t_i q n

Frequently, a  high-rate  digester  is  coupled  in  series  with
a  second  digestion  tank  (Figure 6-5).   Traditionally,  this
secondary digester  is  similar  in  design to the primary digester,
except that it is neither heated nor mixed.  Its main function is
to allow gravity  concentration  of  digested  sludge  solids and
decanting of supernatant liquor.   This reduces the volume of the
sludge  requiring  further  processing  and disposal.   Very little
solids  reduction  and  gas  production  takes  place in  the second
stage (23).
                                                        DIGESTER
   RAW
          HEAT
 SLUDGE
        EXCHANGER
ACTIVE
 ZONE
                      MIXING
                                TRANSFER
                                           SUPERNATANT
                  l\\\\\\\\\\\\\\\\\\\\

                      DIGESTED
                       SLUDGE
                                                       SUPERNATANT
                                                        DIGESTED
                                                         SLUDGE
                  PRIMARY DIGESTER
                                        SECONDARY DIGESTER
                             FIGURE 6-5

           TWO-STAGE, HIGH-RATE ANAEROBIC DIGESTER SYSTEM
Unfortunately,  many  secondary  digesters  have  performed  poorly
as  thickeners,  producing  dilute sludge and  a high  strength
supernatant.   The  basic  cause of  the problem  is  that,  in most
cases,  anaerobically digested sludges  do not  settle  readily.
Basically,  two factors contribute  to  this  phenomenon  (52).
                                6-12

-------
    1.  Flotation of  solids.   The  contents  of the  primary
        digestion  tank may  become  supersaturated with  digester
        gas.   When this sludge is transferred  into the  secondary
        digestion  tank,   the  gas  will come  out of  solution,
        forming  small bubbles.   These bubbles attach to  sludge
        particles  and provide  a  buoyant  force that  hinders
        settling.

    2.  High proportion  of fine-sized particles.   Fine-sized
        solids  are produced  during  digestion by  both mixing  (53)
        and the  natural  breakdown  of particle size  through
        biological decomposition (54).  These fines settle  poorly
        and enter  the  supernatant.    The  problem is compounded
        when secondary and tertiary sludges are  fed  into the
        digesters.   The solids in  these sludges  have quite often
        been  flocculated  and,  thus,  are more  easily  broken up
        during digestion than primary sludge solids.

The return to  the head of  a  plant  of poor quality supernatant
from  two-stage  digestion  often has an adverse  impact on the
performance of  other  treatment processes.    Supernatant  commonly
contains larger  quantities  of  dissolved and suspended materials.
(See  Section  6.2.4.3 for a more  detailed  description of
supernatant quality).   For  example, Figure  6-6 shows that  at one
secondary  treatment  plant,  most  of  the  carbon and  nitrogen
leaving the secondary digester was  found in the  supernatant  and,
consequently,  was returned  to the liquid  process  stream.   The
impact  of  high recycle  loads on  treatment at  one  midwestern.
plant  is shown  on Figure 6-7.   When digester  supernatant was
recycled,  solids  built up in  the plant, and the  total amount of
suspended  solids  in the final effluent  increased  by 22  percent.

Suggestions for improving liquid-solids separation in secondary
digesters  have  included vacuum degassing  (56),  elutriation  (57),
and enlarging  the  secondary  digester.   However, in many  cases,
particularly  when biological sludges  are digested, it is  better
to eliminate the  secondary  digester  altogether  (52).    Digested
sludge  is  then taken directly  to either  a facultative sludge
lagoon  (see Chapter 15)  or mechanical  dewatering  equipment
(see  Chapter 9).   Since  solids  capture is better in the  units,
their  sidestreams are  of  relatively  high   quality compared  with
supernatant from  secondary digesters.

A  secondary  digester may  successfully  serve  the  following
functions:

      e  ^Thickening digested  primary sludge.

      •  Providing  standby digester capacity.   If  the  secondary
        digesterIsequippedwithadequateheating, mixing, and
        intake  piping.

      •  Storing digested  sludge.   A  secondary  digester  fitted
        with a floating cover  can provide storage for sludge.


                               6-13

-------
 Assuring against short-circuiting of raw  sludges through
 digestion.    This may  be  important  for  odor  control  if
 digested  sludge is  transferred  to open basins  or lagoons
 (see Chapter 15).    It  also  provides a margin  of safety
 for pathogen  reduction.
   120
_  100
D5
    80
•a
c
o
.c
    60
 _•   40
    20
           12
           10
                               01
         c
         to
         en
         O
         -C
         FEED  1ST   2ND
             STAGE STAGE

            CARBON
         20,000 = 9.1kg x 103
LEGEND
      GAS

      FEED SLUDGE

      TRANSFER SLUDGE
                FEED   1ST   2ND
                     STAGE STAGE

                   NITROGEN
                2000lb = .91kg x 103
                4000lb = 1.82
                GOOOIb = 2.72
                SOOOIb = 3.63
               10,000lb = 4.54
         SUPERNATANT

         STABILIZED SLUDGE

         UNACCOUNTABLE

FIGURE 6-6
    CARBON AND NITROGEN BALANCE FOR A TWO-STAGE,
            HIGH-RATE DIGESTION SYSTEM (23)
                          6-14

-------
RAW SEWAGE
16,035 /
(10,520)
1 15,969
(36,801)
PRIMARY
CLARIFIER



;
PRIMARY
SUPERNATANT
L
AERATION
TANK

9,501
(15.306)
SECONDARY FINAL EFFLUENT
CLARIFIER 2;836
._ 	 . 13 4R71
RETURN SLUDGE
SLUDGE
13,249
(19,626)
0
(30,172)
i
WASTE SLUDGE
r
ANAEROBIC-
DIGESTER
9,593
(14,645)


DIGESTED SLUDGE

 DATA IN PARENTHESES WERE OBTAINED WHEN UNTREATED
 SUPERNATANT WAS RETURNED TO THE HEAD OF THE PLANT.
 (AVERAGE OF THREE GRAB SAMPLES). DATA NOT IN
 PARENTHESES WERE OBTAINED WHEN NO SUPERNATANT
 WAS RECYCLED.  (AVERAGE OF THIRTEEN GRAB SAMPLES).
 SOLIDS FLOWS DO NOT BALANCE BECAUSE OF GRAB
 SAMPLING. ALL VALUES EXPRESSED AS Ib SS/day (1 Ib day =
 0.454 kg/day).
                             FIGURE 6-7

             EFFECT OF RECYCLING DIGESTER SUPERNATANT
              ON THE SUSPENDED SOLIDS FLOW THROUGH AN
                    ACTIVATED SLUDGE PLANT (55)
        6.2.2.3   Anaerobic Contact  Process

The  anaerobic contact process is  the anaerobic equivalent  of
the  activated sludge  process.    As  shown  on  Figure  6-8,  the
unique  feature  of  this variation is  that  a  portion  of  the
active  biomass  leaving the  digester  is concentrated  and  then
mixed  with  the  raw  sludge  feed.   This recycling  allows  for
adequate  cell retention  to  meet  kinetic  requirements while
operating  at  a significantly reduced hydraulic detention  time.
                                      POSITIVE 1 CLAfllFIED
                                     LIQUID-SOLIDS 1	, .  . .
                                     SEPARATION  LIQUID
                              FIGURE 6-8

                     ANAEROBIC CONTACT PROCESS

Positive  solids-liquid  separation  is essential  to the  operation
of  the anaerobic  contact  process.   To  gain any  of the  benefits
from  recycling, the  return stream must  be more concentrated than
                                 6-15

-------
the  contents  of  the  digester.   The difficulties  in thickening
anaerobically digested  sludge  have  been discussed above.  Vacuum
degasifiers have been used in anaerobic contact systems to reduce
the  buoyancy effect  of entrapped  gas,  thereby improving  cell
settling (56).

The  anaerobic  contact  process  has found application  in  the
treatment of  high  strength industrial wastes  (56,58,59),  and it
has been operated successfully at a laboratory scale to stabilize
primary sludge  (60).   Nevertheless,  this system configuration is
rarely considered in municipal anaerobic sludge digestion because
of the difficulty in achieving the necessary concentration within
the return stream.

        6.2.2.4  Phase Separation

As  discussed  in  Section  6.2.1.4,  anaerobic  digestion  involves
two general phases:   acid formation  and  methane  production.   In
the  three  preceding anaerobic  digestion processes,  both  phases
take  place in  a single -reactor.    The potential benefits of
dividing  these  two phases into  separate  tanks were discussed
as early as 1958 (61).

Subsequent research (62,63) has shown that two-phase digestion is
feasible for the treatment of sewage sludges.  Figure 6-9 shows a
schematic of  this  multi-stage  system as  conceived  by Ghosh,  and
othe'rs (63).
                                                          DIGESTER

                                                           GAS
HEAT
RAW [ — ^ — )
SLUDGE ,
EXCHANGER
BlOf
c
MIXING
vlASS RECYC

f" POSH
| SEPAR
1
LE 	 -, .
                                 HEAT
EXCHANGER

	,	4	
                                        1 MIXING


                                        ^ --- '
 POSITIVE   CLARIFIED
LIQUID-SOLIDS !	*-
 SEPARATION  I
                                                           LIQUID
                                     BIOMASS RECYCLE
         DIGESTED
                                                          SLUDGE
           ACID DIGESTER
                                     METHANE DIGESTER
                            FIGURE 6-9

              TWO-PHASE ANAEROBIC DIGESTION PROCESS

An effective  means  of separating  the  two  phases  is essential to
the  operation of  anaerobic digestion in  this mode.   Possible
separation techniques include dialysis (62), addition of chemical
inhibitors, adjustment  of  the  redox potential  (64),  and kinetic
control  by  regulating the  detention  time  and  recycle  ratio for
each  reactor  (63).   The  latter  approach  is  the  most practical
and  has been  developed  into a  patented  process (U.S.  Patent
4,022,665) .
                               6-16

-------
Operating data for a bench-scale system, summarized in Table 6-3,
show the differences  between the reactors in a two-phase system.
The  acid digester has  a  very short  detention  time  (0.47  to
1.20  days),  low pH  (5.66  to 5.86),  and  produces negligible
amounts  of methane.   Conditions  in the methane digester are
similar  to those  found  in  a  conventional  high-rate digester,
which  is  operated  to  maintain  the  optimum environment  for the
methanogenic  bacteria.   The detention  time  listed  in  Table 6-3
for  the  methane digester  (6.46  days) is  significantly  lower
than  the  detention time  in a conventional  high-rate  digester.
However,  this  is  probably because the  two-phase  system was
operated  in a bench-scale  system rather than in a full-scale
system where  conditions  are not ideal.   The main  advantage of a
two-phase system is that  it  allows  the  creation  of  an optimum
environment for  the acid  fermenters.  As  of  1979,  a  two-phase
system has never been operated at a plant scale.

                             TABLE 6-3
          OPERATING AND PERFORMANCE CHARACTERISTICS FOR
          THE BENCH-SCALE, TWO-PHASE ANAEROBIC DIGESTION
                   OF WASTE ACTIVATED SLUDGE  (63)
         Parameter
  Acid
digester
Methane
digester
Temperature, °C
Detention time, day
Loading,

  Ib VS/day/cu ft
pH
Ammonia nitrogen, mg/1

Averaqe alkalinity,
  mg/1 CaCC>3
Gas composition, mole percent
    CH4

    C02
    N2
Gas yield,  standard cu ft/lb
  VS reduced
Methane yield, standard
  cu ft/lb VS  reduced
VS reduction,  percent
Effluent volatile  acid,
  mg/1 HAc
     37

0.47-1.20


1.54-2.67

5.66-5.86

 490-600

    790
    37

  6.46


  0.18

  7.12

   766

 4,127
19-44
73-33
8-23
0.2-0.9
0.1-0.3
8.5-31.1
3,717
69.7
29.0
1.3
17.7
11.9
29. 3
134
Combined
two-phase
 system

    37

6.86-7.66


  0.20

  7.12

   766

 4,127


  65.9

  32.3

   1.8

  15.7


  10.7

  40.2

   134
 1 Ib/day/cu ft =  16.0 kg/day/nT
 1 cu ft/lb = .0623 m3/kg
                                6-17

-------
    6.2.3  Sizing of Anaerobic  Digesters

Determination of  digestion  tank  volume  is  a  critical  step
in the design of an anaerobic digestion system.   First,  and  most
important,  digester volume  must be sufficient to prevent  the
process  from failing  under  a11 expected  conditions.  Process
failure  is  defined  as  the  accumulation  of  volatile  acids
(volatile  acids/alkalinity ratio  greater  than  0.5)  and  the
cessation of  methane  production.   Once  a digester turns  sour,
it usually  takes  at least  a  month  to  return  it to service.
Meanwhile,  raw  sludge  must  be  diverted to  the  remaining
digesters,  which  may  become overloaded  in turn.    Furthermore,
sludge  from a sour digester  has  a strong, noxious  odor,  and
therefore,  its storage  and disposal are a great  nuisance.

Digester capacity must  also  be large enough to ensure that  raw
sludge is adequately stabilized.  "Sufficient stabilization"  must
be defined on a  case-by-case basis,  depending on  the  processing
and disposal  after  digestion.   In the past,  digested sludge
quality has  been acceptable as  long  as  the digester remained  in
a balanced  condition and produced  methane.   However, higher
levels of stabilization may be  required  after the  1970s because
wastewater  sludges  increasingly  are being  applied  to land  and
coming into  closer  contact with  the public.


        6.2.3.1   Loading Criteria

Traditionally,  volume requirements for anaerobic digestion  have
been  determined  from empirical loading criteria.  The oldest
and simplest  of  these  criteria is per capita volume  allowance.
Table  6-4 lists typical  design  values.  This crude loading  factor
should  be  used  only for  initial  sizing  estimates,  since  it
implicitly  assumes a value for  such  important parameters  as  per
capita waste  load,  solids  removal efficiency in  treatment,  and
digestibility  of  the sludge.   These  parameters  vary widely  from
one area  to  the  next  and  cannot accurately be  lumped into  one
parameter.

A more  direct  loading  criterion is the  volatile  solids loading
rate,  which  specifies  a certain reactor volume  requirement  for
each   unit of volatile dry solids in  the  sludge  feed per unit  of
time.    This  criterion  has  been  commonly used to  size  anaerobic
digesters.    However,  as early  as  1948,  Rankin  recognized  that
process  performance  is  not  always correlated with  the  volatile
solids loading rate.  The problem  stems  from the  fact  that  this
parameter is  not  directly  tied to  the fundamental  component  in
anaerobic digestion, the microorganisms  actually  performing  the
stabilization.


        6.2.3.2   Solids  Retention Time

The  most  important consideration  in sizing  an  anaerobic
digester is  that  the  bacteria  must be given sufficient time  to


                               6-18

-------
reproduce  so that  they  can  (1) replace cells  lost with  the
withdrawn sludge,  and (2) adjust their population  size to  follow
fluctuations in  organic  loading.

In  a  completely  mixed  anaerobic digester,  cells  are evenly
distributed throughout  the tank.   As a result,  a  portion  of
the bacterial population is removed with each  withdrawal  of
digested  sludge.    To  maintain  the  system  in  steady state,
the rate of cell  growth must at  least match  the  rate  at  which
cells  are removed.  Otherwise,  the population  of bacteria  in  the
digester declines  and the  process eventually fails.
                            TABLE 6-4

            TYPICAL DESIGN CRITERIA FOR SIZING MESOPHILIC
                 ANAEROBIC SLUDGE DIGESTERS (65,66)

                                    Low-rate         High-rate
           Parameter                 digestion         digestion
Volume criteria,
  cu ft/capita

    Primary  sludge                      2-3             1.3

    Primary  sludge  +

    Trickling filter humus              4-5            2,7-3.3

    Primary  sludge +

    Activated sludge                    4-6            2,7 - 4
      c                             0.04-0.1         0.15 - 0.40
  Ib VSS/day/cu ft

Solids retention time, days            30-60            10 - 20
1 cu ft/capita - ,028 m /capita
1 Ib/day/cu  ft = 16.0 kg/day/m3


To ensure that  the  process  will not fail, then, it is  critical  to
know the growth rate of the bacteria  in the  digester.   It  is not
practical to measure  directly  the rate  at  which  the  anaerobic
bacteria multiply.   However,  as  these  bacteria  grow and
reproduce,  they metabolize  the  waste and produce  end  products.
As  a  result, the  bacterial growth  rate can  be  determined  by
monitoring  the  rate  at  which  substrate  is reduced  and end
products are produced.  Studies of these rates of change began  in
the  late 1950s and have  led to an  understanding of digester
process kinetics (9,10,67).

The  key design parameter for  anaerobic biological treatment
is  the biological  solids retention time  (SRT),  which is the
                               6-19

-------
average time a unit of microbial mass is retained in the  system
(68).   SRT can  be  operationally  defined  as the total  solids
mass in the  treatment system divided by the quantity of  solids
withdrawn  daily.   In anaerobic digesters  without  recycle, the SRT
is  equivalent to  the  hydraulic  detention  time.   Recycling of a
concentrated stream  back  to  the  head  of  the  system, which  is the
unique feature of  the anaerobic contact process, increases the
SRT relative to the hydraulic detention time.

Figure  6-10  illustrates  the  relationship  between  SRT  and
the  performance   of  a lab-scale  anaerobic  digester fed with
raw  primary sludge.   Specifically,  the  figure shows how the
production of methane,  as  well  as  the  reduction of degradable
proteins,  carbohydrates, lipids,  chemical oxygen demand, and
volatile  solids,  are related to  the SRT.  As the  SRT is reduced,
the  concentration of  each  component  in  the  effluent gradually
increases  until   the   SRT  reaches  a value beyond  which  the
concentration  rapidly  increases.    This  breakpoint  indicates
the SRT at  which  washout of microorganisms begins--that  is, the
point where the  rate  at which  the organisms leave  the  system
exceeds their rate of  reproduction.  Figure 6-10 shows that the
lipid-metabolizing  bacteria  have  the slowest growth  rate and,
therefore,  are  the  first  to washout.   As  the  SRT  is shortened
beyond the  first   breakpoint (occurring  at  an  SRT between  eight
to  ten days  at 95°F  [35°C]),  more  types of bacteria are  washed
out  and performance  is  increasingly inhibited.  The SRT can
be  lowered  to  a critical  point  (SRTC)  beyond  which  the
process will fail completely.  Calculations based  on process
kinetics  predict an  SRTC  of  4.2 days for  the digestion  of
wastewater sludge at  95°F  (35°C)   (69), which  corresponds with
Torpey's  pilot-scale  study  (70),  in  which  anaerobic  sludge
digesters  operating  at 99°F  (37°C)  failed at an SRT of 2.6 days.
Performance began deteriorating  sharply as  the  SRT  was  reduced
below five days.

Temperature has  an  important  effect on bacterial  growth  rates
and,  accordingly,  changes  the  relationship  between SRT and
digester  performance.   The effect  of  temperature  on methane
production and volatile solids reduction  is  shown  on Figure 6-11.
The  significance   of this  relationship  is  that stabilization is
slowed at  lower  temperatures,  with  68°F (20°C)  appearing  to be
the  minimum temperature at which  sludge  stabilization  can be
accomplished within  a  practical  solids retention  time (69).  The
critical  minimum   solids  retention  time  (SRTC)  is also affected
by  temperature.   O'Rourke  (69)  found  that  the SRTC for the
digestion  of a primary  sewage sludge in a bench-scale digester
was  4.2 days at  95°F  (35°C),  7.0 days  at  77°F (25°C),  and 10.1
days at 50°F (10°C) .


        6.2.3.3   Recommended Sizing  Procedure

The  size  of  an  anaerobic digester  should  be adequate to  ensure
that the solids  retention time in the system never falls below a
certain  critical value.   This design solids  retention time
                              6-20

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BENCH-SCALE DIGESTION OF
PRIMARY SLUDGE AT 95°F
              10       20        30        40
                     SOLIDS RETENTION TIME iSRT}, days

                            FIGURE 6-10
             EFFECT OF SRT ON THE RELATIVE BREAKDOWN
              OF DECRADABLE WASTE COMPONENTS AND
                     METHANE PRODUCTION (69)
                                6-21

-------
  •


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                                   RAW SLUDGE:  11.8 gm/l DEGRADABLE
                                                6.6 gm/I NQNDEGRADABLE
                                                18,4     TOTAL
                                              DEGRADABLE
                                            VOLATILE SOLIDS
                   10         20         30        40         50

                           SOLIDS RETENTION TIME (SRT), days

                                FIGURE 6-11

             EFFECT OF TEMPERATURE AND SRT ON THE PATTERN
                   OF METHANE PRODUCTION AND VOLATILE
                          SOLIDS BREAKDOWN  (69)
                                                                 60
                                    6-22

-------
       and  the  conditions  under  which  it must be met should be
selected  with care.  A margin of  safety  must  be  provided,  since
SRTC  was determined on  the basis of  bench-scale  digesters
maintained at such ideal conditions as complete  mixing,  uniform
feeding  and  withdrawal  rates,  and  closely  controlled digestion
temperature.    However,  in  a  full-scale  facility,  the ideal
condition of complete mixing  is  not achieved.   Both  the quantity
and the  chemical  characteristics  of the  feed sludge  vary over
time,  and sludge  temperature may  fluctuate.   All these  actual-
system characteristics tend  to  slow the  rate of the microbial
digestion  process.   As  a result, SRT^  must be  considerably
greater  than SRTC.  McCarty  (71) recommends  a  minimum  safety
factor of 2.5.

Several  researchers  (43,49,57,72,73,74,75)  have  recommended ten
days as  a minimum acceptable  solids retention  time  for high-rate
digesters  operating  near  95°F  (35°C).    (Values  for  systems
operated  at  other  temperatures  are shown in  Table  6-5.)   This
sizing criterion is  reasonable,  since  it   corresponds with the
replication  time of the  slowest  growing bacteria.   However, this
criterion must be met under all  expected  conditions,  including:

        Peak hydraulic loading.   This  value  should be estimated
        by combining  poor thickener performance with the  maximum
        plant loading  expected  during  seven continuous  days
        during the  design period.

        Maximum  grit and  scum  accumulations.    Considerable
        amounts  of  grit and scum may accumulate before  a digester
        is cleaned.   This reduces  the active volume of the tank.

        Liquid level  below highest level.   Several feet of liquid
        level variability (two  to three,  usually) must  be
        retained  to allow for  differences  in the  rate  of  feeding
        and  withdrawal  and to  provide reasonable  operational
        flexibility.

These  conditions  may  very  well  occur  simultaneously  and,
therefore,  the  designer  should  compound  them when  applying
the ten-day SRT^  sizing  criterion.   In  the past,  "liberal"
detention  time  criteria  have  been  applied  at  the  average
conditions.    However,  problems  arise  during  critical periods,
not when conditions  are  average.   For  this reason,  the  most
rational  approach  to  sizing  a  full-scale facility  is to apply
experimentally  based  design criteria (increased  by a  reasonable
margin of safety)  to the  actual  set of  expected  peak conditions.
(An example  is included in Section 6.2.9.3).


    6.2.4  Process  Performance

The primary  result  of anaerobic sludge digestion is  the reduction
of  both  volatile  solids  and pathogenic organisms.   Volatile
solids are  degraded into  smaller molecules,  and  eventually a


                              6-23

-------
large  portion  are converted  into gas,  primarily  methane  (CH4)
and  carbon  dioxide  (CC>2) .    Pathogens  are  reduced  through
natural die-off  because  the anaerobic environment is  unsuitable
for their  survival.   (Refer to Chapter 7).  Many other  chemical
and physical  changes occur during  anaerobic  sludge  digestion,
some of which are described later  in  this  section.
                            TABLE 6-5

             SOLIDS RETENTION TIME DESIGN CRITERIA FOR
                      HIGH RATE DIGESTION (71)

                              Solids retention time, days
Operating
temperature ,
OF
65
75
85
95
105
Minimum
(SRTC)
11
8
6
4
4
Suggested
for design
(SRTd)
28
20
14
10
10
It is not possible to predict precisely the nature and extent of
all  changes  occurring during  anaerobic digestion.   Wastewater
sludges have  a complex,  variable  character and  there  are many
reactions  that occur  during  digestion  within the mixed culture of
anaerobic  microorganisms.   This section describes general  trends
of digester  performance  and identifies the  major influences on
anaerobic  digestion.

To  provide  an overview of anaerobic digester performance,
operating  data for a  full-scale  digestion  facility are shown in
Tables 6-6 and  6-7.   These  data  are  for  a two-stage, high-rate
digester system in which  only the  primary digester was heated and
mixed  (23).   The  second tank provided  a  quiescent  zone  for the
gravity separation of digested solids  from supernatant  liquor.
Operating  temperature in  the first  stage  was maintained  at 94°F
(34°C),  and   detention time in each tank was  39 days.   Feed
sludge  consisted  of approximately  equal amounts  of  primary
sludge and waste-activated sludge.

Essentially,  all stabilization occurred in  the primary digester.
In this  first  stage, 57 percent of  the  volatile  solids were
converted  to  liquid  or gas.   Only 2.8 percent  of the volatile
solids in the raw  sludge were reduced  in the secondary digester.
A  similar pattern of performance  is shown  in Table  6-7 for
carbohydrate, lipid,   and protein reduction.  While data indicate
                               6-24

-------
that  fixed solids  also  decreased  during digestion,  this  is a
little understood  phenomenon, and research  on  the subject is
continuing (76).
                               TABLE 6-6

     AVERAGE PHYSICAL AND CHEMICAL CHARACTERISTICS OF SLUDGES
                 FROM TWO-STAGE DIGESTER SYSTEM (23)

                                          Concentration, mg/1
Component
PH
Alkalinity
Volatile acids
Total solids
Fixed solids
Carbohydrates
Lipids
Carbon
Proteins, as gelatin
Ammonia nitrogen, as NH,
Organic nitrogen, as NH^
Total nitrogen, as NH-}
Feed
sludge


1,
35,
9,
9,
8,
15,
18,

1,
1,
5.7
758
285
600
000
680
310
450
280
213
346
559
Transfer
sludge

2,

18,
6,
1,
2,
6,
11,


1,
7.7
318
172
200
600
550
075
950
200
546
879
425
Supernatant

2,

12,
3,
1,
1,
4,
6,


1,
7.8
630
211
100
310
020
321
440
580
618
564
182
Stabilized
sludge

2,

32,
12,
3,
3,
10,
17,

1,
2,
7.8
760
185
800
300
100
490
910
200
691
455
146
   Except pH.
                                TABLE 6-7

   MATERIALS ENTERING AND LEAVING TWO-STAGE DIGESTER SYSTEM3 (23)
Volatile solids
Fixed solids
Carbohydrates (as
  glucose)
Lipids
 Feed
sludge

 79.9
 26.9
 28.9

 24.8
                                        Quantity, tons
Transfer
 sludge
                                   Supernatant
       Stabilized
         sludge
  34.1
  19.4
  4.55

  6.09
Carbon
Ammonia nitrogen
Organic nitrogen
Proteins (as gelatin)
Total nitrogen (as NH3>
46.2
0.64
4.02
54.6
4.66
20.4
1.61
2.58
32.9
4.20
23.4
 8.8
2.71

2.40
 8.5
 5.1
1.28

1.44
                                                              Gas
       1st stage  2nd  stage
11.8
1.64
1.50
17. 1
3.25
4. 5
0.28
0. 60
7. 1
0.89
22.1
-
-
-
0.47
                                                                   2.7
                                                                   0.04
 Period of analysis = 33 days.
 1 ton = .907 t
                                   6-25

-------
Reduction of solids during digestion has the effect of producing
a  more dilute  sludge.   For  example,  in this case,  the  raw
sludge  fed  to the system  had a total  solids  concentration  of
3.56  percent,  yet the  solids  concentration  was  reduced  to
1.86 percent in  the first  stage  of  digestion.   Although  gravity
concentration did  occur  in  the  second-stage  tank, the  largest
portion of the digested solids was contained in the supernatant.
At  this plant,  the  supernatant was  recycled to the primary
clarifiers and then the  solids it contained either returned  to
the primary  digester  or  left  the  plant in the final  effluent.

The preceding example illustrates the general performance  of
anaerobic digesters.    In  the  remainder of this section,  three
topics  are   discussed in  more detail:  solids reduction, gas
production and  supernatant quality.


        6.2.4.1   Solids Reduction

Solids  reduction  is  one of  the  main objectives  of  anaerobic
digestion.   It  not only makes  the  sludge  less putrescible  but
also  reduces  the amount  of solids  for ultimate  disposal.
It is usually assumed  that  this reduction takes  place only  in the
volatile  portion  of  the sludge  solids.   Therefore,  a common
measure of digester performance  is  the percent of  the  volatile
solids  destroyed.    Volatile  solids  reduction   in  anaerobic
digesters usually ranges  between 35 to 60 percent.   The degree  of
volatile solids  reduction achieved in any particular application
depends on both  the  character of  the sludge  and   the  operating
parameters of the digestion system.

The character of  the sludge  determines  the  upper  limit for
volatile  solids  reduction.    Not all  of  the volatile solids
can be  converted by   the  anaerobic  bacteria.   Limited  research
(77 to  80) suggests that  only  60 to 80 percent of  the  volatile
solids  in municipal wastewater sludge  is  readily  biodegradable.
The remaining fraction  consists chiefly of inert organics  such  as
lignins and   tannins.   These  complex  organic molecules may  even-
tually be degraded when held for several months  in  a facultative
sludge  lagoon,  but can  be  considered  indigestible within the
contact times normally  associated  with anaerobic digestion.

The most important operating  parameters affecting volatile  solids
reduction are  solids  retention time  and  digestion- temperature.
As shown on  Figure 6-12,  volatile solids reduction  climbs  rapidly
to 50 to 60  percent as the SRT is increased.   Beyond this  point,
further reduction  is  minimal even with  substantial  increases  in
the SRT.  Similar curves  have been produced by  other researchers
(43,60,81).   The shape of the  response curve  and  the point  at
which it levels  out are influenced strongly by  the  temperature  of
the digester.  Figure 6-12 shows  that  at  any given SRT,  raising
the  operating  temperature  to 95°F  (35°C)  will  increase the
proportion of volatile solids  destroyed during digestion.   This
response  to  temperature  change  is  not instantaneous  but  would
                              6-26

-------
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                              J	L
                                               * PILOT PLANT REF. (82]
                                               * PILOT PLANT REF, (S3)
                                J	_L
                                              ACTIVATED SLUDGE ONLY
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                                  *  PILOT PLANT REF, (13)
                                  «  PILOT PLANT REF. (14)
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                    TEMP. ("C) x SOLIDS RETENTION TIME (DAYS)

                          FIGURE 6-13

  VOLATILE SOLIDS REDUCTION VS TEMPERATURE X SRT
        FOR THREE TYPES OF FEED SLUDGES (82-85)
                               6-28

-------
        6.2.4.2  Gas  Production

A particular advantage of anaerobic digestion over other  methods
of sludge  stabilization is that  it produces  a medium-energy
gas as a by-product.   Digester  gas  can be burned to provide  heat
and  generate  electricity  for  the  treatment  plant.   Several
off-site  uses of digester gas  are  also feasible,  including:
blending with  the  domestic gas  supply,  generation of steam  or
electricity for  sale  to  adjacent industries, bottling  for  use  as
a portable fuel,  and  production  of  chemicals such as ammonia and
methanol.   Utilization of  digester  gas  is  described  further  in
Sections 6.2.6.2,  6.2.7,  and Chapter  18.   Before any utilization
program can be  established,  the quantity and  quality of available
digester gas  must be determined.

The  generation  of  digester gas  is  a  direct result  of  the
destruction  of solids.  The microbiology  and  biochemistry  of
this   conversion  are  described  in  Section 6.2.1.4.    Because  of
this  close relation  between gas production and solids  retention,
gas production is best expressed in  terms of the volume of gas
produced per unit of  solids destroyed.    This parameter,  termed
specific gas production,  is commonly  expressed  as  cubic feet  of
gas per pound  of  volatile  solids (VS) destroyed.   Specific gas
production values for the  anaerobic digestion of some of the
principal  components of sludge are presented  in Table 6-8.  Fatty
substances have  a higher  energy content per unit weight than
other forms of organic matter.   Thus, the breakdown of a  sludge
with  a high proportion of fats, oils, and greases can be expected
to yield a greater quantity  of gas per unit of solids destroyed.
                            TABLE 6-8

              GAS PRODUCTION FOR SEVERAL COMPOUNDS
                       IN SEWAGE SLUDGE (86)

                           Specific gas
                        production, cu ft/lb       CH4 content,
      Material               destroyed                percent	
Fats                           18 - 23               62 - 72

Scum                           14 - 16               70 - 75

Grease                             17                    68

Crude Fibers                       13               45 - 50

Protein                            12                    73
1 cu ft/lb = .0623 m3/kg


Specific gas production for anaerobically digested municipal
sludges generally ranges  between  12  to  17  cu  ft  per  Ib  of
                               6-29

-------
occur after a  period  of acclimatization.
out  that at  higher SRTs,  the  effect of
pronounced.
                   The  graph also points
                   temperature  is  less


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50
40

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10
                                      RAW SLUDGE CHARACTERISTICS
                                      TYPE
                                      SOLIDS CONC,
                                      VOLATILE CONTENT
                             PRIMARY
                             2.3%
               10
20        30        40

SOLIDS RETENTION TIME, days
50
60
                           FIGURE 6-12

          EFFECT OF SOLIDS RETENTION TIME AND TEMPERATURE
           ON VOLATILE SOLIDS REDUCTION IN A LABORATORY-
                   SCALE ANAEROBIC DIGESTER (69)
The  combined  effect of  SRT  and temperature  on  volatile solids
reduction  for  three common  sludges  is plotted  on  Figure 6-13.
Although  the  data  points  are  somewhat scattered,  they  suggest
that primary sludge degrades  faster  than a mixture of primary and
waste-activated  sludge, which  in  turn  degrades  faster  than
straight activated  sludge (12).  The empirical correlation term,
temperature times  SRT,  has  been  found  useful when the spread of
temperatures in a set of data is  not  great.

A.  1978  laboratory  study  (34)  found  that  thermal  treatment
of  activated sludge  (347°F [175°C])  for  a half hour prior to
anaerobic digestion increased  volatile solids reduction  and
resultant gas  production.    Dewaterability of  the  digested
sludge was also improved by  thermal  pretreatment.
                               6-27

-------
volatile  solids destroyed
shows how  specific  gas
Conversion of  volatile
(35°C)  and 130°F  (54°C).
no effect on  specific
exceeded.   Lengthening
         (0.75  to 1.1 mVkg) •
                                                    Figure 6-14
                        production  is  affected by  temperature.
                        solids  is most  efficient  at about 95°F
                         Detention time, or SRT,  has essentially
                        gas  production  so  long  as  the  SRT is
                        the  SRT,  however,  increases the total
quantity of  gas  produced  because volatile  solids  reduction  is
increased.   As  discussed earlier, the mix of organic  compounds in
the feed  sludge strongly  influences  specific gas production
values.
          20 i-
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          10 -
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                                BASED ON DATA FROM 23 STUDIES
            80
90
                                                 130
                100     110      120

                  TEMPERATURE, °F

                FIGURE 6-14

EFFECT OF TEMPERATURE ON CAS PRODUCTION (87)
140
Instantaneous  rates of  gas production  can  vary widely because
of  fluctuations  in the  feed  rate,  sludge  composition,  and
bacterial activity.   These momentary peaks  must  be  considered
in  sizing gas piping and  storage facilities.  Generally, gas
production increases soon  after  sludge  is  fed to the  digester.
Therefore,  continuous  feeding aids  in providing  uniform gas
production.
                              6-30

-------
The characteristics  of sludge  gas  from  several  digester
installations  are  shown  in Table 6-9.   A  healthy  digestion
process produces  a  digester gas  with about 65  to  70 percent
methane,  30  to 35 percent  carbon  dioxide,  and very  low  levels  of
nitrogen,  hydrogen,  and  hydrogen sulfide.    The  carbon  dioxide
concentration  of  digester gas has been found to  increase with the
loading rate  (60,88).


                             TABLE 6-9

                CHARACTERISTICS OF SLUDGE CASa (85)

	Constituent	Values for various plants, percent by volume
 Methane (CH^)           42.5  61.0  62.0  67.0     70.0    73.7 75.0   73 - 75
 Carbon dioxide (C02)      47.7  32.8  38.0  30.0     30.0    17.7 22.0   21 - 24
 Hydrogen  (H2)            1.7  3.3    -C    -       -     2.10.2    1-2

 Nitrogen  (N2)            8.12.9    -c  3.0       -     6.5  2.7    1-2
 Hydrogen  sulfide  (H2S)      -    -  0.15    -  0.01 - 0.02  0.06  0.1    1 - 1.5
 Heat value, Btu/cu ft      459  667   660  624      728     791  716   739 - 750
 Specific  gravity  (air = 1) 1.04  0.87  0.92  0.86     0.85    0.74 0.78  0.70 - 0.80
 Data from 1966 studies by Herpers and Herpers.
 Except as noted.
 CTrace.
The  hydrogen  sulfide content of  the  gas is  affected  by  the
chemical  composition  of  the  sludge   (84).    Sulfur-bearing
industrial  wastes  and saltwater  infiltration  tend  to increase
H2S  levels  in sludge  gas.   However, metal  wastes and metal  ions
added  during chemical  treatment  or conditioning  can reduce  the
amount of f^S  in the  sludge by  forming  insoluble  salts. B^S,
a major source of  odors in digested  sludge, can also  be  corrosive
in the presence  of  moisture,  by forming sulfuric  acid.

Although  the  hydrogen  content  has  some  effect  on  the heat
value, methane  is  the  chief  combustible  constituent in digester
gas.   The  high  heat value for digester gas ranges between  500 to
700  Btu  per cu  ft  (4.5  to 6.2 kg-kcal/m3),  with  an average  of
about  640  Btu per cu  ft  (5.7 kg-kcal/m3)  (84).   The high  heat
value  is the heat  released  during  combustion  as  measured in  a
calorimeter.  However, gas engine efficiencies  are usually based
on  the low heat value, which  is  the heat  value of gas when none
of  the water vapor  formed by  combustion  has  been condensed.   By
way  of comparison,  sludge gas containing  70  percent methane  and
no  other  combustibles  has  a  low heat  value  of  640 Btu per cu ft
(5.7  kg-kcal/m3)  and a  high  heat  value of 703 Btu  per  cu  ft
(6.26  kg-kcal/m3)  (84).


         6.2.4.3   Supernatant Quality

Supernatant  from an  anaerobic digestion  system can  contain high
concentrations  of  organic  material, dissolved and  suspended


                                6-31

-------
solids,  nitrogen, phosphorus,  and other  materials that,  when
returned to the plant, may impose extra loads on other treatment
processes  and  effluent  receiving waters.-  Mignone  (89)  has
reviewed  the  literature on  anaerobic  digester  supernatant
quality.  Methods of  treating digester supernatant are described
in Chapter  16  and in other  references  (90,91,92).   However,  in
most cases  it  is preferable to minimize or eliminate, rather than
treat, highly  polluted digester supernatant  (52).

It is  very  difficult to  generalize about  supernatant quality
because it  can  vary widely,  even  at  a single  treatment  plant.
Table  6-10 presents  reported  characteristics of  anaerobic
digester supernatant  for three common types of feed sludge.   Many
factors contribute to the  wide  range of variation in supernatant
quality (90,91,97,98) .

The  suspended  solids,   biochemical  oxygen  demand,  soluble
phosphorus,  phenols,  and ammonia in the supernatant can all  cause
problems in  a  treatment plant.  If the anaerobic supernatant must
be returned to  the  plant  flow  for  treatment, it  should  be
recycled continuously to spread the loading.

Suspended Solids

Supernatants may  contain  high  concentrations of  finely  divided
suspended  solids  because,   as  discussed  in Section 6.2.2.2,
anaerobically  digested sludges  settle poorly, particularly  when
biological  sludge  is  fed into the digestion system.  Unless  these
fine-sized  particles are removed with the  digested  sludge,  they
will  build  up  in  the plant, causing process  overloading  and
eventually,  degradation of the plant effluent.

Biqchemi^cal  Oxygen Demand

Because suspended  and dissolved solids from an anaerobic digester
are  in  a  chemically reduced  state,  they  impose  a  large  oxygen
demand when returned to the  liquid process  stream.  The aeration
requirement  for aerobic biological  treatment  is  often  increased
substantially  by the recycling  of high BOD digester  supernatant.

Soluble Phosphorus

The recent  emphasis on removal of phosphorus from wastewaters has
created sludges  that contain high proportions  of this  element.
In biological  phosphorus removal,  phosphorus  is  taken  up by
the   growing cell mass and is removed from the wastewater stream
in the wasted  biological sludge  (99,100).   Chemical methods
of phosphorus removal  entail  the precipitation of phosphates
with  metal  ions—predominantly   ferrous,  ferric, aluminum,  and
calcium.  The  fate of phosphorus during the anaerobic digestion
of phosphorus-laden  biological  and chemical sludges  has been the
subject of  several  studies  (55,101-104).    The  results  of  these
studies  are not  entirely consistent.   In  some  cases  (99,101),
bound  phosphorus  was  resolubilized  during  anaerobic  digestion


                              6-32

-------
and  released  to the  digester  supernatant.    The  return  of  this
phosphorus-laden supernatant  to  the liquid treatment  stream  can
substantially  reduce  the  net phosphorus removal efficiency of  the
plant  (101)  and/or increase  chemical  demand.   However,  in most
studies  (55,102-104), release of  soluble  phosphorus  into digester
supernatant  was minimal.
                               TABLE 6-10

             SUPERNATANT, CHARACTERISTICS OF HIGH-RATE,
             TWO-STATE, MESOPHILIC, ANAEROBIC DIGESTION
                   AT VARIOUS PLANTS (90,93,94,95,96)

                                       Concentration3, mg/1
Parameter
Reference
Total solids
Total volatile solids
Primary
(95)
9,400
4,900
Primary and trickling
sludge filter sludge Primary and activated sludge
(90)b (94)
4,545
2,930
(951 (901° (94)
1,475
814
(94) (95) (96) (90)b
2,160 -
983 -
Suspended solids
 Average
 Maximum
 Minimum

Volatile suspended solids
 Average               •  -
 Maximum
 Minimum

BOD
 Average
 Maximum
 Minimum

COD

TOC

Total (P04)-P

MH3-N

Organic N
PH                     8.0

Volatile acids
Alkalinity (as CaCC>3)        2,555

Phenols
 Average     .            -   0.23     -     -    0.23     -                   0.35
 Maximum                 -   0.80     -     -    0.50     -                   1.00
 Minimum                 -   0.06     -     -    0.06     -                   0.08
4,
17,
2,
10,
1,








277
300
660
645
850
420
713
880
200
-
-
-
-
-
-
-
-
2,205
1,660
-
4,565
1, 242
143
853
291
7.3
264
3,780
1,518
-
-
2,230
-
85
-
678
7.2
-
-
7,772
32,400
100
4,403
17,750
60
1,238
6,000
135
-
-
-
-
-
-
-
-
383
299
-
1, 384
443
63
253
53
7 .0
322
1,349
143
118
-
1,310
320
87
559
91
7.8
250
1,434
740 1,075
750
515
1,230
-
100
480
360 560
7.0 7.3
-
-
4,408
14,650
100
3,176
10,650
75
667
2,700
100
-
.
-
-
-

-
-
aUnless noted, all values are average for the sampling period studied.

bvalues indicated are a composite from seven treatment plants.

cValues indicated are a composite from six treatment plants.
Phenols

Phenols  have  been  found  in digester  supernatants  in  concentra-
tions  sufficient  to  inhibit  biological  activity  (56).   Typical
phenol  concentrations  are  included  in  Table  6-10.    The source
of  phenols   is  not  usually  industrial  waste  discharges  but
putrefaction of  proteins, which  begins in  the human  body  and
continues  in  the  sewage  system.   Phenols are  very toxic and  are
used   commercially as  an  antiseptic.    In dilute concentrations,
phenols  do  not  necessarily  kill  bacteria  but  slow  their growth


                                   6-33

-------
and  inhibit  their  normal metabolic activity.   As  a result,  the
phenols contained in digester  supernatant, combined with phenolic
compounds  already  in the sewage,  may  be an  important  cause  of
sludge bulking  (90).   In addition, the  recycling  of  phenols  in
supernatant may contribute  to  odor  problems.

Ammonia

As shown in Table 6-10, high levels of ammonia are often found  in
digester supernatant.  In plants that  are nitrifying,  the  super-
natant return will  provide a large  portion of the ammonia feed  to
the  wastewater  process.  The  conversion  of  this  ammonia  to
nitrate will therefore result in increased  costs  to  provide the
required oxygen  for  treatment.   In plants  that must  achieve  a
nitrogen  limitation in  their  effluent,  the  recycle ammonia
loadings must be carefully evaluated as  to  their  overall  effect
in meeting  the  standards.
    6.2.5  Operational  Considerations


        6.2.5.1   pH

As was noted in Section 6.2.1, anaerobic digestion  is  a  two-step
process consisting of "acid-forming" and "methane-forming"  steps.
During the  first step, the  production  of volatile acids  tends
to  reduce the  pH.   The  reduction is  normally  countered by
destruction of  volatile acids by methanogenic bacteria and  the
subsequent production of bicarbonate.

Close pH control is necessary because methane-producing  bacteria
are extremely sensitive to slight changes  in  pH.   Early  research
(105-107)  showed  that the  optimum pH for  methane-producing
bacteria  is  in  the  range of  6.4-7.5  and that  these bacteria
are very  sensitive to  pH  change.   A  1970 study (108) seems to
indicate  that the pH  tolerance of methane-producing bacteria
is  greater  than previously thought.    The  bacteria  are  not
necessarily killed by  high  and  low  pH  levels;  their growth is
merely stopped.   Because  of  the  importance of these  findings to
system control,  more  research is needed  to verify these  results.

Several  different acid-base  chemical  equilibria are  related to
pH. In the anaerobic digestion process,  the  pH range  of  interest
is  6.0  to  8.0, which makes the  carbon dioxide-bicarbonate
relationship the most important.   As Figure  6-15 indicates,
system  pH is controlled  by  the CC>2 concentration  of the  gas
phase  and the bicarbonate  alkalinity  of the liquid phase.   A
digester  with  a given  gas-phase CC-2 concentration and  liquid-
phase  bicarbonate alkalinity  can exist at  only one  pH.  If
bicarbonate  alkalinity  is added to  the  digester  and  the
proportion of CC<2  in  the gas phase remains  the  same,  digester
                               6-34

-------
pH must  increase.   For any fixed  gas-phase  CC>2 composition,  the
amount of  sodium  bicarbonate  required to  achieve  the  desired pH
change is given by the following equation:
    D  = 0.60 (BA at initial pH - BA at final pH)
                                                      (6-1)
where:

    D  = sodium bicarbonate dose, mg/1

    BA = digester bicarbonate alkalinity as mg/1 CaCC>3
The  pH increase  is less  important,  however,  than the  effect
on system buffering  capacity,  (that is,  the  system's  ability
to resist pH  changes).   If bicarbonate  alkalinity is  added,
buffering  capacity  is  increased,  system  pH  is  stabilized,  and
the  system becomes less susceptible to  upset.  The effect of
buffering  capacity  on  anaerobic  digester operations is discussed
elsewhere  (110,111).
          DU
          40
o
oc   30
LU
          20
       CM
      o
      o
          10
                                                   to    s

                                                    OPERATING
                                                  'TEMPERATURE
                                                    96°F (35°C)
                              LIMITS OF
                               NORMAL
                              ANAEROBIC
                             TREATMENT
                   I
            260    500    1000      2500   5000  10,000

                   BICARBONATE ALKALINITY AS CaC03, mg/1

                            FIGURE 6-15

              RELATIONSHIP BETWEEN pH AND BICARBONATE
                CONCENTRATION NEAR 95°F  (35°C) (109)
                                                 25,000
                                6-35

-------
Bicarbonate alkalinity  can be calculated from total alkalinity  by
the following equation:


    BA   = TA - 0.71  (VA)                                   (6-2)


where:


    BA = bicarbonate  alkalinity as mg/1 CaCC>3

    TA = total alkalinity as mg/1 CaCC>3 determined  by  titration
         to pH 4.0

    VA = volatile acids measured as mg/1 acetic acid


0.71 is obtained by the multiplication of  two  factors,  (0.83 and
0.85).  0.83  converts  volatile acids  as acetic  acid  to  volatile
acid alkalinity as CaCC>3.   0.85  is  used because  in  a  titration
to pH 4.0, about 85 percent  of  the acetate  has been converted  to
the acid form.

It  has  been  suggested  (110)  that  the only sensible  way  to
increase digester pH  and  buffering capacity is  by the addition  of
sodium bicarbonate.   Other materials,  such  as  caustic soda,  soda
ash,  and  lime, cannot  increase  bicarbonate  alkalinity  without
reacting  with soluble carbon  dioxide, which causes a partial
vacuum  within the system.   Above pH 6.3,  lime  may react with
bicarbonate to form insoluble calcium carbonate,  promoting  scale
formation or encrustation.  Ammonia gas  (NH3)  could be used
without causing vacuum problems,  but  control  of pH  with  sodium
bicarbonate is preferred  because  it provides  good buffering
capacity  without raising  the pH as  much as  NH3  would.  Both
sodium and  ammonia can inhibit anaerobic bacteria;  care  must  be
taken during pH control to avoid reaching toxic concentrations  of
these chemicals.


        6.2.5.2  Toxicity

Much  of  the  published data  on  toxicity in anaerobic  digestion
systems  are  erroneous  and  misleading  because  of inadequate
experimental techniques and a general lack  of understanding
(112).   Therefore,  before any discussion  of  toxicity  can  take
place, a review of  several fundamentals is needed.

First, for  any material  to  be biologically  toxic,  it must  be  in
solution.   If a substance  is  not in solution,  it cannot pass
through the cell  wall  and therefore cannot affect the organism.

Second,  toxicity is a relative  term.  There are many organic
and  inorganic  materials  which,  if  soluble,  can be  either


                              6-36

-------
stimulatory or  toxic.   A  good  example is the  effect,  shown in
Table 6-11, of ammonia  nitrogen  on anaerobic digestion.


                           TABLE 6-11

     EFFECT OF AMMONIA NITROGEN ON ANAEROBIC DIGESTION (113,114)

                  Ammonia
            concentration,  as  N,
                   mg/1                 Effect
                  50 - 200        Beneficial

                 200 - 1,000      No adverse effects

               1,500 - 3,000      Inhibitory at pH
                                    over 7.4 - 7.6

                Above 3,000       Toxic
Acclimatization is the  third consideration.   When the  levels
of  potentially  toxic materials are  slowly  increased  within
the  environment,  many organisms  can rearrange  their  metabolic
resources and overcome the metabolic block produced by the toxic
material.  Under shock load  conditions,  there  is not sufficient
time  for this  rearrangement  to take place  and the digestion
process fails.

Finally, there  is  the possibility  of  antagonism and synergism.
Antagonism is defined as  a reduction of  the  toxic effect  of one
substance by  the presence of  another.  Synergism  is defined as an
increase in the toxic effect  of one substance by the presence of
another.  These are important relationships in cation toxicity.

Though  there are many potentially  toxic  materials,  this section
concerns itself only with the following:

        Volatile acids
        Heavy metals
        Light metal cations
        Oxygen
        Sulfides
        Ammonia
Vo1a t i1e Acids

Until  the  1960s, it was  commonly believed that volatile  acid
concentrations over  2,000  mg/1 were toxic to anaerobic digestion.
There  was  also considerable controversy  about whether  or not
alkaline substances  should be added  to maintain adequate  buffer
capacity.


                              6-37

-------
In the early  1960s,  McCarty  and  his coworkers published results
from carefully controlled  studies  (113,115,116).   Their results
showed :

     •   That volatile  acids,  at least  up to  6,000-8,000  mg/1,
        were not  toxic to methanogenic bacteria as long as there
        was adequate  buffer capacity to maintain the system pH in
        the range  of  6.6-7.4.

     •   That pH control by the  addition  of  an alkaline material
        was a valid  procedure  as  long as the  cation  associated
        with  the  alkaline  material  did  not  cause   toxicity.
        It was found  that  alkaline sodium, potassium, or ammonium
        compounds  were  detrimental but that alkaline magnesium or
        calcium compounds  were not.

Heavy Metals

Heavy metal toxicity  has frequently been  cited as  the  cause
of anaerobic digestion  failures.   Even though  trace amounts
of most  heavy  metals  are  necessary  for   maximum biological
development (117), the concentrations in  raw wastewater sludges
could be problematic.

Heavy metals  tend  to attach themselves to  sludge  particles
(118,119).  Heavy  metals which cannot be detected  in the influent
wastewater can be  concentrated  to  measurable  levels  in  the
sludge.  Table 6-12 gives the range of influent concentrations of
some heavy metals.  The  range is quite  wide, with the  higher
values normally attributed  to  a local  industrial polluter.
                            TABLE 6-12

           INFLUENT CONCENTRATIONS AND EXPECTED REMOVALS
   OF SOME HEAVY METALS IN WASTEWATER TREATMENT SYSTEMS (120,121)

                                        Removal efficiency,  percent
Heavy metal
Cadium
Chromium
+ 3
+ 6
Copper
Mercury
Nickel
Lead
Zinc
Arsenic
Iron
Manganese
Silver
Cobalt
Barium
Selenium
Influent concentration,
mg/1
< .008

< .020
< .020
< .020
< .0001
< .1
< .05
< .02
.002
< .1
.02
< .05
- 1.142

-5.8
- 5.8
- 9.6
- .068
- 880
- 12.2
- 18.00
- .0034
- 13
- .95
- .6
Secondary
treatment
20 -

40 -
0 -
0 -
20 -
15 -
50 -
35 -
28 -
72
25
-
45

80
10
70
75
40
90
80
73



Alum treatment
60

90
-
90
65
35
85
85
-
-
—
-
Below detection

*
-
-
47
79


-
"
                               6-38

-------
Table  6-12  gives  the  typical  range  of  removal  that can  be
expected from standard  secondary  treatment.   Published data seem
to indicate  that  the  percent  removal,  without chemical addition,
is a function of  influent  concentration:  the higher the influent
concentration, the higher the percent removal.

The  last column  of Table 6-12 shows removals  of heavy metals
achieved with  additions  of  alum.   In treatment systems  that
add  chemical coagulants for phosphate removal,  a significant
amount of influent heavy metals will also be removed (122).

Soluble  and  total  heavy  metal  concentrations are  often greatly
different because anions such as carbonate and sulfide can remove
heavy  metals from  solution  by  precipitation and  sequestering.
Consequently, it  is not possible  to define  precise  total  toxic
concentrations for any heavy metal (123).   Total  individual  metal
concentrations  that have  caused  severe inhibition  of anaerobic
digestion are shown in  Table 6-13.   However,  only the dissolved
fraction of these metals caused the inhibition.  Table 6-14  shows
the total and soluble concentrations of heavy metals in anaerobic
digesters.   Inhibition  of anaerobic digestion occurs  at soluble
concentrations  of  approximately 3  mg/1 for  Cr,  2 mg/1  for  Ni,
1 mg/1 for Zn, and 0.5 mg/1 for Cu (129).
                            TABLE 6-13

         TOTAL CONCENTRATION OF INDIVIDUAL METALS REQUIRED
           TO SEVERELY INHIBIT ANAEROBIC DIGESTION (123,124)
     Metal
                          Concentration in digester contents
Metal as percent
 of  dry solids
Millimoles metal per
kilogram dry solids
Soluable metal,
    mg/1
Copper
Cadmium
Zinc
Iron
Chromium
+ 6
+ 3
Nickel
0
1
0
9

2
2

93
08
97
56

20
60
-



1,



150
100
150
710
420
500
-
0,

1.

3.

2.
.5
-
.0
-
.0
-
.0
Except for  chromium,  heavy  metal toxicity in anaerobic digesters
can  be  prevented  or  eliminated  by  precipitation  with  sulfides
(124-127).   Hexavalent  chromium is usually  reduced to trivalent
chromium, which,  under  normal  anaerobic  digester  pH conditions,
is relatively insoluble and not very toxic (128).

Sulfide  precipitation  is  used  because  heavy  metal sulfides
are  extremely  insoluble (129).   If sufficient sulfide  is  not
available from  natural  sources, it must be  added  in the  form of
sulfate,  which is  then  reduced  to  sulfide  under anaerobic
conditions.
                               6-39

-------
                           TABLE 6-14

              TOTAL AND SOLUBLE HEAVY METAL CONTENT
                        OF DIGESTERS (124)
       Metal
Total concentration,
      mg/1
Soluble concentration,
      mg/1
Chromium +6
Copper
Nickel
Zinc
88
27
2
11
- 386
- 196
- 97
- 390
0.03
0.1
0
0.1
- 3.0
- 1.0
- 5
- 0.7
One potential drawback of using the sulfide saturation method is
the possible production of hydrogen sulfide gas or sulfuric acid
from excess dissolved sulfide in the digester.  Because of this,
it  is  recommended  that ferrous  sulfate  be used as  a  source of
sulfide  (112).   Sulfides will  be produced  from  the biological
breakdown of sulfate,  and  the  excess will  be held out of solution
by  the  iron  in  the  sulfide form.   However,  if heavy  metals
enter  the  digester,   they will  draw  the  sulfide  preferentially
from the  iron because  iron  sulfide  is  the most soluble heavy
metal  sulfide.  Excess  sulfide additions can be monitored by
either  analysing digester  gas for sulfide or by the  use of a
silver-silver electrode  located  within  the digester (126,130).

Light Metal Cations

The  importance  of  the  light  metal cations  (sodium, magnesium,
potassium,  calcium)  in anaerobic  digestion was  shown in the mid
1960s  (112,131,132).  Domestic  wastewater  sludges  have  low
concentrations of light  metal cations.   However,  significant
contributions,  enough  to  cause toxicity,  can come from industrial
operations and the addition of  alkaline material for pH control.
Not  only  can  each  of these  cations be  either stimulatory or
toxic,  depending on  concentration  (Table  6-15),   but  certain
combinations of  them will  form  either  an  antagonistic  or a
synergistic relationship  (Table 6-16).   Inhibition  caused by an
excess  of a certain  cation can be  counteracted by the addition of
one or  more of the  antagonist  cations  listed in  Table 6-16.
                            TABLE 6-15

            STIMULATING AND INHIBITORY CONCENTRATIONS
                   OF LIGHT METAL CATIONS (133)


                                  Concentration, mg/1
Cation
Calcium
Magnesium
Potassium
Sodium
Stimulatory
100 - 200
75 - 150
200 - 400
100 - 200
Moderately
inhibitory
2,500 - 4,500
1,000 - 1,500
2,500 - 4, 500
3, 500 - 5, 500
Strongly
inhibitory
8,000
3,000
12,000
8, 000
                               6-40

-------
                            TABLE 6-16

     SYNERGISTIC AND ANTAGONISTIC CATION COMBINATIONS (112, 132)
     Toxic
    cations

   Ammonium

   Calcium

   Magnesium

   Potassium

   Sodium
       Synergistic
         cations
Calcium, magnesium, potassium

Ammonium, magnesium

Ammonium, calcium



Ammonium, calcium, magnesium
        Antagonistic
          cations
Sodium

Potassium, sodium

Potassium, sodium

Ammonium, calcium, magnesium,
  sodium
Potassium
Oxygen

Many  engineers  have  expressed  concern over  the  possibility
of  oxygen  toxicity  caused  by using  dissolved air  flotation
thickeners  for  sludge  thickening.   Fields  and Agardy (134)
injected oxygen into  a bench-scale  digester  at  the  rate of
0.1 ml  02 per liter  per hour  (equivalent to one volume of  air
per 2,100 volumes  of digester contents per hour).   Total  gas
production fell  36.5 percent after 19 hours and ceased  completely
after  69 hours.   However,  this rate of oxygen injection is
significantly  higher  than would  be  produced by  a dissolved  air
flotation thickening system.   Consequently,  no problems  are
expected under normal circumstances.
Soluble sulfide  concentrations  over  200  mg/1  are  toxic  to
anaerobic digestion systems (125,135).   The soluble sulfide
concentration within  the digester is  a  function of the  incoming
source  of  sulfur,  the  pH, the  rate of  gas  production, and  the
amount  of  heavy metals  available  to  act  as precipitants .    High
levels  of  soluble  sulfide  can be reduced by the addition of  iron
salts  (136)  to  the liquid, or scrubbing of  the  recirculated  gas.

Ammonia

Ammonia,  produced during  the anaerobic  degradation of  proteins
and  urea,  may reach   toxic levels  in highly  concentrated
sludges (113,114,133).    Two  forms  of  ammonia  are  found  in
anaerobic  digestion:   ammonium  ion  (NH4+)  and  dissolved ammonia
gas  (NH3 ) .   Both forms  can inhibit  anaerobic  digestion,  although
ammonia gas has  a  toxic  effect at a  much  lower concentration  than
ammonium ion.

The  two forms  of  ammonia  are in equilibrium  and  the relative
concentration  of  each depends  on  pH , as   indicated by.   the
following equilibrium equation:
                               6-41

-------
At low pH levels,  the  equilibrium  shifts  to  the left and ammonium
ion  toxicity  is  more likely to be  a problem.   At higher pH
levels,  the equilibrium shifts to  the  right  so that inhibition is
related  to the ammonia gas  concentration.

Ammonia   toxicity  is  evaluated  by analyzing the  total  ammonia-
nitrogen  concentrations.    If  the  total  ammonia-nitrogen
concentration  is  from 1,500  to  3,000  mg/1  and the  pH  is above
7.4-7.6,  inhibition may  result from  ammonia  gas.   This  can be
controlled by the  addition  of  enough  HC1 to maintain  the pH
between   7.0  and 7.2.   If total ammonia-nitrogen  levels  are over
3,000 mg/1,  then the  NH4 +  ion  will  become toxic no  matter
what the  pH  level.  The  only  solution is to dilute the incoming
waste sludge.
    6.2.6  System Component Design


        6.2.6.1  Tank Design

Anaerobic digestion tanks are  either  cylindrical, rectangular, or
egg-shaped.   A simplified sketch of  each tank design  type is
shown on Figures 6-16,  6-17, and  6-18.

The most common  tank design is a low, vertical cylinder ranging
in diameter  from  20  to 125 feet (6  to 38  m) ,  with a side water
depth between 20 to 40  feet (6 to 12  m).  Gas-lift  mixing is most
effective when the ratio of tank  radius to  water depth is between
0.7 and 2.0  (137).   The tanks  are usually made of  concrete, with
either  internal  reinforcing or  post-tensioning  rods  or  straps.
The latter  design is  the  least  expensive  of  the  two  for tanks
with diameters greater  than 65 feet.  Some steel  tank digesters
have been constructed to diameters of 70  feet.

The floor  of a cylindrical digester  is  usually  conical,  with a
minimum slope of 1:6.   Sludge  is withdrawn from the low point in
the center  of  the tank.   Digestion  tanks  with "waffle bottoms"
have been  put  into  operation  at the East  Bay Municipal Utility
District Plant in  Oakland,  California  (138,139).    Digesters with
similar  bottoms  have been  designed  for  Tacoma,  Washington,  and
Portland, Oregon.

The principal objective of the waffle floor design  is to minimize
grit  accumulation and, to practically  eliminate  the  need  for
cleaning.  As shown  on  Figure  6-16,  the  tank floor  is subdivided
into pie-shaped hoppers,  each sloping  toward  a separate drawoff
port along  the  outside  edge of tank.  Subdivision of the bottom
area and use of multiple drawoff  ports  allow steeper floor slopes
and reduce  the  distance that  settled  solids must  travel.   As a
result,  less grit  is likely  to  accumulate.   Construction costs


                               6-42

-------
are higher for  this  type  of bottom because  it requires more
complex  excavation,  form work,  and  piping  than  a  conventional
bottom.   It has  been estimated that the incremental construction
cost for  waffle  bottoms  on  the 90-foot (27 m)  diameter digesters
in Oakland was  estimated to be  $120,000  per  tank (1978 dollars)
(139).    However, it  is  expected  that savings  will  be realized
during   operation because cleaning  requirements will  be  greatly
reduced .
 WITHDRAWAL
 PIPE
                              BOTTOM PLAN
                                                  SECTION
 WITHDRAWAL-1
 PORTS
                                  CONICAL BOTTOM TYPE
BOTTOM
 PLAN
SECTION
                                  WA F FjyysOTTOMj[YPE


                            FIGURE 6-16

               CYLINDRICAL ANAEROBIC DIGESTION TANKS
The  primary  advantages  of  rectangular  digestion  tanks  are
simplified construction  and efficient  use  of a limited plant
site.   However,  it  is more difficult to  keep the contents of
                               6-43

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a rectangular digester uniformly mixed because "dead  spots"  tend
to form at the corners.   Figure 6-17 shows a plan and section  of
a rectangular digestion  tank.
                                             INLET
 OUTLET
             t,
               -cr
                               PLAN
                                                       GAS OUTLET
                              SECTION


                           FIGURE 6-17

              RECTANGULAR ANAEROBIC DIGESTION TANK
Although  egg-shaped  digesters have  been used  extensively  in
Europe, originating in Germany over 20  years  ago,  they  are only
now entering American practice.   The  first egg-shaped digesters
in the  United  States  were built in Kansas City,  Kansas,  in  the
mid-1970s,  and  four more  are  now under  construction  (1979)  in
Los  Angeles,  California,  at  the  Terminal  Island Wastewater
Treatment Plant.   Each  of the Terminal  Island  digesters wil1
have  a  capacity of 184,000 cubic feet  (5,200 m3) and measure
100  feet  (30 m) from top to bottom,  with a  maximum  horizontal
diameter of 68  feet (21 m)  (140).
                               6-44

-------
                                          MIXER
  PLATFORM TO
  OTHER
  DIGESTERS
 SCUM DOOR
 HOPPER
 SUPERNATANT
- WITHDRAWAL
                                                          PIPING TO
                                                          WITHDRAW
                                                          BOTTOM
                                                          SLUDGE
                            FIGURE 6-18

              EGG-SHAPED ANAEROBIC DIGESTION TANK AT
           TERMINAL ISLAND TREATMENT PLANT, LOS ANGELES
The purpose  of forming  an  egg-shaped  tank  is to  eliminate  the
need for  cleaning.   The digester  sides  form a cone  so  steep at
the bottom that grit cannot accumulate (Figure 6-18).  The top of
the digester  is  small,  so that scum contained  there  can be kept
fluid  with  a  mixer  and removed  through  special  scum  doors.
Mixing in  the  Los Angeles digesters  is  promoted by gas evolution
during digestion combined with  pumped  circulation of sludge from
the bottom to  the top of  the  tank.   A  60 horsepower (45 kW) pump
is used at the rate of  500 gpm (32  1/s).   Gas spargers ring  the
inside wall  of  each tank and  can be used to detach  any material
adhering  to  the  walls  or to  increase  mixing,  if necessary.
Construction of egg-shaped  tanks  requires complex  form  work  and
special  building techniques.  Accordingly,  capital costs  are
                               6-45

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higher  than for  other tank  designs.   The  1976  construction
cost estimate  for  the  four digesters in  Los  Angeles  was about
$5,000,000.
        6.2.6.2   Heating

A heating  system  is  an important feature of a modern anaerobic
digester.   Raising  the  temperature of  the  digesting  sludge
increases  the metabolic  rate of the  anaerobic organisms  and
reduces digestion time.   Maintenance of  the temperature consis-
tently  within ±1°F  (0.6°C)  of  design  temperatures  improves
process stability by  preventing thermal  shock.
Heating  equipment must  be capable  of  delivering enough  heat
to raise the temperature of incoming sludge to operating levels
and  to  offset  losses  of  heat  through  the  walls, floor,  and
cover of the digester.   Methods used to  transfer heat  to sludge
include:

     •   Heat exchanger coils placed inside the  tank

     •   Steam injection directly into the sludge

     •   External  heat  exchanger  through   which sludge  is
        circulated

     •   Direct  flame  heating  in which hot  combustion  gases
        are passed through  the sludge (141)


External heat  exchangers  are  the  most commonly  used  heating
method.  Internal  heat  exchanger  coils  were   used  in  early
digesters;  however, they are difficult to inspect  and clean. This
is a  serious disadvantage  because  the  coils  become  encrusted,
reducing  the rate of heat transfer.  To  minimize caking of sludge
on the  coils, water circulating through  the  coils  is kept between
120 to 130°F (49 to  55°C)  (84).   Typical values of  heat-transfer
coefficients for hot-water  coils are  listed  in  Table 6-17.

Steam  injection heating   requires  very little  equipment  but
dilutes  the digesting sludge and requires 100  percent  boiler
makeup water.   The  cost  of  this  water may  be considerable,
particularly if hardness  must be  removed before addition to
the boiler.

Three types  of  external  heat exchangers are  .commonly  used  for
sludge  heating:   water bath,  jacketed  pipe,  and  spiral.  In the
water bath  exchanger,  boiler  tubes and  sludge  piping are located
in a common  water-filled container.   Gravity  circulation of hot
water across the sludge   pipes  is  augmented with a pump,  to
                              6-46

-------
increase  heat transfer.   The  heat exchanger and  boiler are
combined  in a single  unit,  a  feature which  can  increase the
explosion  hazard  in  the digester area.   In a jacketed pipe
exchanger,  hot water is pumped counter-current to the sludge
flow, through a concentric pipe surrounding the sludge pipe.  The
spiral  exchanger is also a  counter-flow design;  however, the
sludge and water passageways  are cast in a spiral.   One side of
the  heat  exchanger  is  liquid,  providing ready access to the
interior of  the  sludge  passageway  for  cleaning.   Heat  transfer
coefficients for  design  of external heat exchangers  range between
150 to  275  Btu/hr/sq ft/degree  F  (740  to 1,350 kg-cal/hr/m2/°C)
depending  on  heat  exchanger  construction and  fluid  turbulence.
To minimize clogging with rags  and debris,  sludge passageways in
a heat exchanger  should  be as large as possible. The interior of
these  passageways  should be  easily accessible  to  allow the
operator to quickly  locate and clear a blockage.


                           TABLE 6-17

             HEAT TRANSFER COEFFICIENTS FOR HOT WATER
                 COILS IN ANAEROBIC DIGESTERS (84)

                                         Transfer
             Material surrounding   coefficient (u) ,
               hot  water coils       Btu/hr/sq ft/°F
            Thin supernatant              60  - 80

            Thin sludge                     30

            Thick sludge                   8-15
            1 Btu/hr/sq ft/°F -  4.9  kg-cal/hr/mV C.

A piping arrangement  used  to  control  hot  water supply  to a
jacketed pipe or spiral heat  exchanger is shown on Figure 6-19.
Hot water  is  pumped through  the  heat exchanger and  circulated
through  the secondary  heat  loop.    When  the  temperature of the
sludge  leaving  the heat  exchanger falls below  the  set point,
some hot water from the primary heat loop is  introduced through a
modulating  valve into the  secondary heat loop,  displacing an
equal  volume  of  cooler water back  into  the  primary  heat loop.
Balancing valves are required to  assure  that the secondary loop
will  not be  bypassed  altogether and to  allow adjustment  of
circulation pump capacity.   Supply water temperature is kept
below  155°F (68°C).  Although higher  temperatures will  increase
the rate of heat  transfer,  caking  of  sludge will  occur when
the flow of sludge  is stopped.  This  system allows the heat
source  to be -remote from  the  heat  exchanger.  This assures
maximum  safety  and  supports the  recovery  of waste  heat (see
Chapter 18).  Figure  6-20  shows a spiral heat  exchanger operating
off a  secondary  heat  loop.


                              6-47

-------
Each  digester  should  have a  separate  heat exchanger and  in
larger plants,  addition of a  single  heat exchanger  for warming
raw sludge should be considered.  Cold raw sludge should never be
added  directly  to the  digester.   The  thermal  shock will  be
detrimental  to  the anaerobic  bacteria,  and  isolated  pockets  of
cold  sludge  may form.   Raw sludge should be  preheated  or  mixed
with large quantities of warm circulating sludge before being fed
to the digester.
              20G°F
                  PRIMARY LOOP
                  CIRCULATION PUMP
    PRIMARY
     HEAT
     LOOP

                   MODULATING VALVE
                     r
SECONDARY LOOP
CIRCULATION PUMP
              SLUDGE OUTLET
                                                  HEAT
                                                  EXCHANGER
                                                     SLUDGE INLET
                            FIGURE 6-19

                      SCHEMATIC OF THE HEAT
                      RESERVOIR SYSTEM FOR A
                      JACKETED PIPE OR SPIRAL
                          HEAT EXCHANGER
The hot  water or steam  used  to heat digesters  is  most commonly
generated in  a boiler  fueled  by sludge  gas.   Up to 80 percent of
the heat value  of  sludge gas can  be  recovered  in a  boiler.
Provisions  for  burning  an alternate  fuel  source  (natural  gas,
propane, or fuel oil) must be included to maintain heating during
periods  of  low  digester  gas  production or high  heating demand.
                               6-48

-------
Natural gas is the most compatible alternate fuel because it has
a low  heat  content  and,  consequently,  can be blended and burned
in a boiler with  minimal  equipment  adjustment.
                            FIGURE 6-20
                SPIRAL HEAT EXCHANGER OPERATING OFF
           SECONDARY HEAT LOOP AT SUNNYVALE, CALIFORNIA

Often,  waste  heat  from  sludge  gas-powered  engines  used  to
generate electricity or directly drive equipment is sufficient to
meet digester heating  requirements.   Typically,  18 to 20  percent
of  the  low heating  value  of  engine  fuel  can be  recovered  from
the  engine  cooling  system  (38).   Engines  can  be  cooled  by
either a forced draft system in which water is pumped through the
engine or  a natural draft  system  (termed  ebullient  cooling)  in
which water is  vaporized  and circulates  without  pumping.    The
latter method yields a  higher  temperature  (and  thus  more  useful)
source of  heat  and  also increases  engine  life.   A combination
exhaust silencer  and  heat-recovery unit can  be used  to  extract
from the exhaust an additional ten  to thirteen percent of  the low
                               6-49

-------
heating  value of the engine fuel  (38).   To  prevent  formation of
corrosive acids,  exhaust gases should not be  cooled  below  400°F
(200°C).

Solar energy has  been  successfully  used  to heat  anaerobic
digesters  (142),  freeing  sludge gas  for  higher  grade uses.
Heat is  transferred to the raw sludge feed  by  passing  the  sludge
piping  through a  tank  of solar-heated  water.   The  optimal  size
solar-heating system can  supply  82  to  97  percent of the  total
annual  heat  load  from  solar  energy, depending on  geographical
location  (142).    The  economic  attractiveness of  using solar
heating  for  digesters,  however, is strongly dependent on the
economic value of  the  sludge gas  saved.

A  unique method  of  generating  heat  is to  precede  anaerobic
digestion with pure oxygen aerobic digestion (143).  Biologically
generated heat released  in  the Vaerobic  reactor  is sufficient to
warm the sludge  to as high as  125 to 140°F  (52 to  60°C),  as  long
as  the solids concentration of the  feed  to  the  digestion  system
is  kept  above about  3.5  percent  and  the  tank  is well  insulated.
Heat balance  calculations  indicate  that these  temperatures are
only attainable  when  pure oxygen  is used  because the low  gas  flow
does not cool the reactor (144).   The warm  sludge  is transferred
to  the  anaerobic  digester,  where  the  bulk of  stabilization
occurs.   In  pilot tests (143),  the contents of  the anaerobic
digester were maintained at 95°F (35°C)  without the  addition of
supplemental  energy,  other  than  the power  required  to  generate
the pure oxygen.  Temperature  is  controlled  by changing  the  flow
of  oxygen  to the  aerobic digester.  As  yet there have been no
full-scale installations of  this  method of heating  digesters.

Heat Required for Raw Sludge.   It is  necessary to raise the
temperature  of  the   incoming sludge  stream.    The  amount of
heat required is:
    Qs =•  ai_o_suae     -        
-------
significance  of this  graph is  that a seemingly small  change
in  feed  sludge  concentration  can  have  a  substantial  effect
on the raw sludge  heating  requirement.
         60  r~
     £•   GO
     ^
     en
  So
    .
  Ei S.
    a
    * -
         40
         30
  Pif   20
  T  i-
         10
                               1
                      24          i         8        10

                        TOTAL SOLIDS CONCENTRATION, %

                            FIGURE 6-21

               EFFECT OF SOLIDS CONCENTRATION ON THE
                  RAW SLUDGE HEATING REQUIREMENT
Heat Required to Make up for Heat  Losses.    The  amount  of  heat
lost to  the  air and  soil  surrounding  a  digester  depends on the
tank shape,  construction materials, and  the  difference between
internal and  external  temperatures.   The general  expression for
heat flow through compound structures  is:
    Q = (U)(A)(T2 - T3)
                                                    (6-4)
where:

    Q

    A
= heat-loss  rate,  Btu/hr

= area of  material  normal to  direction  of heat  flow,
  sq ft
    T2 = temperature within the digestion  tank,  °F
                               •6-51

-------
    T3 = temperature  outside the digestion  tank,  °F

    U  = heat   transfer  coefficient,  Btu/hr/sq  ft/°F,   which
         is  directly  affected  by  the  film coefficient  for
         interior  surface of  tank, and the film coefficient  for
         exterior  surface of  tank, and inversely affected  by the
         thickness  of  individual  wall  material,  and  the  thermal
         conductivity of  individual wall material.

Several other  factors may affect the  heat transfer  coefficient U;
however,  they  may  be considered  negligible  for  the  purposes of
digester design.   Further discussion  of heat  transfer principles,
along  with lists of values  for  film  coefficients  and thermal
conductivities,  is available  (84,145,146).   Various  values  of  U
for  different digester  covers,   wall construction, and  floor
conditions are given  in Table 6-18.


                             TABLE  6-18

               HEAT TRANSFER COEFFICIENTS FOR VARIOUS
               ANAEROBIC DIGESTION TANK MATERIALS (147)

                                       Heat transfer coefficient (u),
             Material                        Btu/hr/sq ft/°F
Fixed steel cover (1/4  in. plate)                        0.91

Fixed concrete cover (9 in. thick)                        0.58

Floating cover (Dowries-type with wood
  composition roof)                                   0.33

Concrete wall (12 in. thick) exposed to air                0.86

Concrete wall (12 in. thick), 1  in. air
  space and 4 in. brick                                0.27

Concrete wall or floor  (12 in. thick)
  exposed to wet earth  (10 ft thick)                      0.11

Concrete wall or floor  (12 in. thick
  exposed to dry earth  (10 ft thick)                      0.06
1 Btu/hr/sq ft/°F =4.9 kg-cal/hr/m2/°C.
1 in. = 2.54 cm
1 ft = 0.304 m
Heat  losses  can  be reduced  by insulating  the  cover and  the
exposed walls  of  the  digester.   Common  insulating materials
are glass wool,  insulation board, urethane foam,  lightweight
insulating concrete and dead air  space.   A facing is  placed
over  the  insulation for  protection  and to  improve  aesthetics.
Common  facing  materials are  brick,  metal siding,  stucco, precast
concrete panels,  and sprayed-on mastic.


         6.2.6.3   Mixing

Digester  mixing  is  considered  to have  the following  beneficial
effects:
                                6-52

-------
     o  Maintaining  intimate  contact between the active biomass
        and  the  feed sludge.

     •  Creating physical, chemical,  and biological uniformity
        throughout the digester.

     •  Rapidly  dispersing  metabolic  end  products  produced
        during  digestion and  any toxic materials entering
        the  system, thereby minimizing their inhibiting effect on
        microbial activity.

     •  Preventing formation of  a  surface scum layer  and the
        deposition of  suspended  matter  on the bottom  of the
        tank.   Scum and  grit  accumulations adversely  affect
        digester  performance  by  consuming  active volume  in
        the  tank.

While  the benefits  of digester mixing  are widely  accepted,
controversy  and confusion arise in attempting  to answer such
questions  as how much  mixing  is  adequate, and what  the most
effective and efficient method is for mixing digesting sludge.

Although  general theory of  slurry mixing is well  developed
(148,149),  little research has been focused  on  mixing of sludge.
Studies of mixing in full-scale  digesters  have  been made of both
dye  (150)  and  radioactive (105,151) tracers.   These  and  other
studies  have shown that  the  contents of the  digester are not
completely  mixed and  that the degree  of  mixing  attained  is
closely  related to the total power actually  delivered to the
contents of  the tank,  irrespective  of  the  actual  mixing  method
used.

A  certain amount  of  natural  mixing  occurs  in  an  anaerobic
digester, caused by both  the  rise of sludge gas bubbles and the
thermal  convection currents  created by  the addition  of  heated
sludges.  The effect of natural mixing  is significant  (150,152),
particularly  in  digesters  fed continuously  and at  high loading
rates.  However, natural mixing  does not maximize the benefits of
mixing and  is insufficient  to ensure stable performance of the
digestion process.   Therefore, mixers are an essential  component
in a high-rate digestion system.  Methods used for mixing include
external  pumped circulation, internal mechanical mixing, and
internal  gas mixing.  A  review of digester mixing methods is
available (57).

Ext erna 1 _Pu_mped_ CjLrcu 1 a t ion

Pumped  circulation, while relatively simple,  is  limited in  a
physical sense  because large  flow  rates  are necessary for  high-
rate digester  mixing.   However,  this  method can  effect
substantial mixing,  provided  that sufficient energy (0.2 to 0.3
hp per thousand cu ft of  reactor  (5  to  8  W/m3)  is  dissipated in
the  tank  (75).   Greater  pump  power will be required if  piping


                              6-53

-------
losses  are  significant.   Pumped  circulation  is  used  most
advantageously  in combination with other mixing systems.  Besides
augmenting  agitation,  circulation allows  external  exchangers  to
be used  for  heating the digester and  uniform blending  of  raw
sludge with heated  circulating  sludge  prior to the  raw sludge's
entering  the  digester.

A pumped circulation mixing system was recently  installed in an
80-foot  (24 m) diameter, fixed  cover  anaerobic digester  at  the
Las Vegas Street  Plant  in Colorado Springs.  Sludge is withdrawn
from  the top-center  of the  tank and  pumped with  a 16-inch
(41 cm)  horizontal, solids  handling centrifugal  pump  to  two
discharge  nozzles.   These nozzles are  located at the base of
the sidewall,  on opposite sides of the  tank,  and direct  the
sludge flow tangentially, inducing an  upward spiral motion in the
tank.  Return  flow  from the pump can  be  directed to a  single
scum-breaker  nozzle  mounted near the liquid surface.  The  pump
capacity is  rated  at  6,800 gallons  per  minute  (429 1/s)  at
21 feet  total  dynamic head (6.4 m)  and  is sufficient to  pump
the entire  digester  contents in 3.5 hours.   The new mixing system
has  successfully eliminated  temperature  stratification  and
scum buildup.  Another  type of  pumped  circulation  system using
sequential  pumping  through  multiple  pipes  strapped  to  the floor
of the digester is described in Reference 153.

Internal  MechanicalMixing

Mixing by means of  propellers,  flat-bladed turbines,  or  similar
devices  is widely  practiced  in the process  industries.   Its
usefulness,  when applied  to  wastewater  sludge  digesters,  is
limited by the nature of non-homogeneous wastewater sludge.   The
large amounts of  raggy and  relatively  inert, nonfluid material in
wastewater  sludge  results in  fouling  of  the  propellers  and
subsequent  failure  of the  mechanisms.   The practice of grinding
screenings  within the wastewater flow  will  accelerate ragging.

Mechanical  mixers  can be  installed  through  the  cover  or
walls  of the tank.   In one design,  a  propeller drives  sludge
through  a  draft  tube  to promote  vertical  mixing.    Wall
installations  restrict  maintenance  and  repair  to  the  time
when  the digester  has  been emptied  (usually every three  to
five  years  in well maintained  plants).   Strong mechanical
mixing  can  be  effected  with   about  0.25 hp/thousand  cubic
feet of reactor (6.6  W/m3)  (75).

Internal  Gas  Mixing

Several  variations  of gas  mixing  have  been used  for digesters,
including:
        The injection  of  a  large  sludge gas bubble at the bottom
        of a  12-inch  (30  cm)  diameter  tube  to create  piston
        pumping  action and periodic surface  agitation.
                              6-54

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     •   The injection  of  sludge gas sequentially through a series
        of lances suspended  from the digester cover to as great a
        depth as possible, depending on cover travel.

     •   The  free  or unconfined release of gas  from a  ring  of
        spargers mounted  on  the floor of the digester.

        The  confined release  of  gas within  a draft tube
•rne  conrinea  release  o
positioned inside the  tank.
The  first method  generally has  a low power requirement,  and
consequently,  produces only a low level of mixing.   As a result,
the major benefit derived  from  its  use  is  in scum control.   Lance
free  gas lift,  and  draft  tube  gas  mixing, however,  can  be
scaled to  induce strong mixing  of  the digester contents.   The
circulation patterns produced by  these  two mixing methods differ.
As shown on Figure  6-22  in the free  gas lift system, the  gas
bubble velocity  at  the  bottom  of the tank is zero,  accelerating
to a maximum as the bubble  reaches  the  liquid surface.  Since  the
pumping action of the gas is directly related to the velocity of
the bubble, there is no pumping from the bottom of  the tank with
a free gas lift system.  In contrast, a draft tube  acts as a  gas
lift pump  which,  by  the  law of continuity,  causes  the flow .of
sludge entering  the  bottom  of  the  draft tube to be  the same as
that  exiting at  the top.   Thus,  the pumping  rate is largely
independent of height, as  shown on  Figure  6-23.  The significance
of  this  difference  is that draft  tube  mixers  induce  bottom
currents to prevent  or  at  least reduce accumulations of settle-
able  material.   Velocityj profiles shown on Figure 6-24  (see
page  6-58)  indicate  that 'lance type  mixers induce comparable
bottom velocities.  Another  difference among internal gas  mixing
systems  is that the  gas  injection devices  in a free gas lift
system are fixed on the bottom  of the digester and thus cannot be
removed  for  cleaning without  draining  the tank.   To  reduce
clogging problems,  provisions should  be made for flushing the  gas
lines and diffusers with high pressure  water.  With the lance  and
draft tube systems, the gas diffusers are  inserted  from the roof
and,  therefore,  can be withdrawn  for  cleaning  without removing
the contents  of  the  tank.   A drawback  of  these systems,  though,
is that  the  draft tube and  gas  lines  suspended  inside  the tank
may foul with rags and  debris contained in the digesting sludge.

Basis for Sizing Gas Lift Mixers.   Three basic criteria have been
used to determine the size  of gas lift  mixing systems:

     •  Unit power (power  per unit  volume)

     •  Velocity gradient  (G value)

     •  Unit gas flow (gas  flow per  unit volume)

Each  of  these  criteria  is interrelated so that  one can  be
calculated from  the  other  once  a few assumptions are made about
gas discharge  pressure  and  sludge  viscosity.   The  size  of  new


                               6-55

-------
                            il"
                               POINT OF
                               GAS INJECTION
          DRAFT TUBE MIXER
          FREE GAS LI FT _M IXE R

             FIGURE 6-22
                             POINTS OF
                             GAS INJECTION
CIRCULATION PATTERNS PRODUCED BY DRAFT
     TUBE AND FREE GAS LIFT MIXERS
                6-56

-------
mixing  systems has  tended to increase  in recent  years as the
importance  of  strong mixing  in  anaerobic  digesters  has become
more widely recognized.    However,  oversizing  of mixing systems
not only results in excess equipment and  operating  costs  but also
may aggravate foaming problems.
                 O
                 m
                 o
                 m
                 h-
                 X
                 g
                 LU
                 I
                     LIQUID SURFACE
                      FREE GAS
                      LIFT
\	
  DRAFT TUBE
                     ».*,***•**!
       ±
                            PUMPING RATE

                           FIGURE 6-23

             DRAFT TUBE AND FREE GAS LIFT PUMPING RATE
Unit Power.   The  use  of the unit power criterion stems  from  the
observation that  the  relative effectiveness of mixing  is closely
related to the total power expended (137,152).  Generally,  strong
mixing  can be  achieved  if  0.2  to 0.3 hp is used  to mix each
thousand cubic  feet  (5  to 8 W/m^)  of digester volume.   The unit
power  criterion is  expressed  in terms of  the motor  horsepower
used to  drive the compressor.   Less power is  actually delivered
to  the liquid  because of  losses  in  the mixing   system (for
example, friction losses, compressor inefficiency).

Velocity Gradient.   Camp and Stein  (154)  have suggested use of
the  root-mean-square  velocity gradient (G) as a  measure of
mixing intensity expressed mathematically:
        w
                                                            (6-5)
where:
                                            f t / S & C!       n
    G = root-mean-square velocity gradient,  —=  sec"1
                               6-57

-------
    W = power dissipated  per unit volume

        ft-lbforce/sec
             cu ft
                        =  lbf/sq ft/sec
    w.f
                                                      (6-6.
where:

    E =

    V =

    M =
rate of work  on energy transfer (power),  ft-lbf/sec, and

volume of  reactor, cu ft

absolute viscosity of the  liquid,  lbf-sec/sq  ft
The  velocity gradient  is  a more  refined design criterion  for
mixing than the  unit power criterion  in  that  it takes  into
account  the power  actually  transferred  to  the  liquid  (E),
and  the  viscosity  of the  liquid  (  ).   Determination  of these
w 3 1 n e> c fnr  <"i^S  lift-  mivinrr  in rlinAel-pva  is
1 **l ^ *» ** tb* *_-*!•**   *™ ^  *~ ™ »* *  .r1™*«^».   ™ M « ~I™-^**™M«*^.  _ **r
 Q
 £
 HI
 a
             0.5
                      1.0
                          10
                                    0,6       1.0

                          VELOCITY, fps (1fps = 0.30m/sec)
                                                 10
                                                              30 ft FROM
                                                              AIR SOURCE
                                                 o.s
                                                          IjO
                      MIXER TYPE
                                   DEPTH OF AIR RELEASE, ft (m)

                                         17.0 (5.1)

                                         9.5 (2.9)

                                         12.5 (3.8)
                            DEPTH OF TANK = 20 ft (6.1m)
                            AIR FLOW RATE = 300 scfm (8.5 m3/min)

                             FIGURE 6-21

                 COMPARISON OF LANCE AND DRAFT TUBE
                     MIXING IN  CLEAN WATER (147)
                                 6-58

-------
When gas  is discharged into a  digester, liquid  flow results
from the  transfer  of  energy from  the  gas  to the  liquid as
the gas  isothermally expands  and rises  to  the  surface.   If
the liquid  vapor pressure and  the  kinetic energy  of the gas
are ignored, the  power transferred  from the gas to the liquid may
be expressed as (155):
    E = 2.40 P1(Q)ln                                        (6-7
where :

    E  = rate of work  or  energy transfer (power),  ft-lbf/sec

    Q  = gas flow,  cfm

    P^  = absolute pressure at the liquid surface,  psi

    ?2  = absolute pressure at the depth of gas injection,  psi


Therefore,  given  a  gas flow  through  a  mixer  system and the
depth  of  the diffuser,  Equation 6-7 can be  used to calculate
the  power transferred to the  digester liquid  (E).   The power
dissipated  per  unit  volume  (W)  can  then  be   calculated  by
dividing  the  rate  of  energy transfer  (E) by  the volume of the
digester (V) .

There  is  little information on  the  rheology (flow  properties)
of unstabilized wastewater  sludges  although  some  data does  exist
on the  rheology of  anaerobically digested sludge  (156,157).  This
is partly because  it  is  extremely difficult to  do such  studies
correctly (158).  In general,  digesting sludge seems to be  a
pseudoplast ic  material   exhibiting  only  slight  thixotropic
properties  (156).  Pseudoplastic liquids become  less  viscous at
higher  shearing  rates.   Thixotropic  liquids become less  viscous
with time at a constant sharing  rate.  Chapter 14 has additional
information  on sludge  rheology.

Three parameters--temperature, solids concentration,  and volatile
content appear to affect  sludge viscosity.  As the temperature of
sludge  is increased,  its viscosity is reduced.   The  relationship
between  temperature  and viscosity  for water is presented on
Figure   6-25.    (A  similar  relationship between   temperature and
sludge  viscosity exists,  although this has  not been  documented.)
The  viscosity  of sludge increases exponentially  as   the  solids
concentration  increases  (159),  as  shown on  Figure  6-26.   This
graph  also  shows  that viscosity  increases with the volatile
content of  the  sludge;  however, the effect is  only  noticeable
when the  solids  content  of  the  digesting sludge  is greater than
three percent.   The entrapment of gas bubbles in  digesting  sludge
may  also  affect  viscosity,  although th.e magnitude of  this  effect


                              6-59

-------
has not  been measured.   In general,  then, it is  not  possible
to pinpoint the viscosity of digesting  sludge although major
influences can be identified.
        4.0 i-
        3,0
t o
O x
« c
> g-
Ly "tl
t- I
3 U

CQ
        2,0 -
        1.0 -
          50
                  70
90        110
                              TEMPER ATURE,°F
130
150
           (1 centipoise = 2.08 x 10'5 Ib-sec/ft 2 )

                            FIGURE 6-25

          EFFECT OF TEMPERATURE ON THE VISCOSITY OF WATER
The appropriate  "G  value"  to  use for  design is difficult to
determine.   In  general,  the "G value"  should  be  between 50 and
80 sec"1.   Walker (75) recommends  a  "G value" of  85 sec"1 for
substantial  auxiliary  mixing.   A  design value at  the high end
of the range should be selected for a large digester with only a
single mixer,  or in  a  case where grit  or  scum problems appear
likely.  A lower  "G value"  is appropriate in cases where several
mixers  are distributed  through  the tank  or where  sufficient
detention time  has  been  provided  to  allow  a slower  rate of
                               6-60

-------
ffl
.2
o
9-
V
g
u
 %
a,
o
ra
O
a
LU
cc
a.
a.
    2000
    1750
1500
1250
    1QQQ
     ISO
 500
     250
       0
MEASUREMENTS MADE
WITH BROOKF1ELD
LVF VISCQMETER-
SPINDLE 2a
                                                   8
                       TOTAL SOLIDS, %
 THE BROOKFIELD VISCOMETER OPERATES IN A VERY
 LOW SHEAR STRESS RANGE, SO APPARENT VISCOSITIES
 ARE VERY HIGH. THIS DATA SHOULD NOT BE USED TO
 CALCULATE SLUDGE FLOW IN PIPES IN THE LAMINAR
 FLOW REGIME.

                     FIGURE 6-26

    EFFECT OF SOLIDS CONCENTRATION AND VOLATILE
 CONTENT ON THE VISCOSITY OF DIGESTING SLUDGE (156)
                        6-61

-------
digestion.   The  use  of a  two-speed  compressor provides  the
capability to match mixing intensity with variations  in operating
conditions.

An example of gas  mixer sizing is found in Section 6.2.9.3.

Unit Gas Flow.   As  described in  the  preceding  paragraphs,  gas
flow through a mixing  system can be related  to the mixing energy
delivered to the liquid.   Therefore,  a simple way  to  size a
gas lift  mixer is to specify a  unit gas  flow.   For  a draft-tube
system,  5  to  7   scf m/thousand  cubic  foot  of  digester  (5  to
7 m-^/min/km3) at about  6  psig  (41.4  kN/m^) is  sufficient  to
produce  strong mixing.  Less gas is  required  for  a  free-lift
system,  4.5  to 5  cfm  per thousand cubic feet (4.5 to 5 m^/min/
km3) of reactor;   however, the pressure must  be  higher since the
gas  is  discharged  at the  bottom  of  the  tank (75).    1.5  to
2.0 cfm  per foot  (0.14  to  0.19 m3/min/m)  of diameter  (0.14  to
0.19 m^/min/m) has  also been  recommended  for free gas  lift
mixers  (137 ) .

The unit  gas flow can be related to  the velocity  gradient  by
combining equations  6-5,  6-6  and  6-7  and  solving  for  ^,  the
unit gas flow:
                                                           (6-8
The values in Table  6-19 were calculated from this equation.
        6.2.6.4  Covers

Anaerobic sludge  digestion  tanks  are covered to  contain odors,
maintain  operating temperature,  keep out oxygen,  and collect
digester  gas.   Digester covers  can be  classified  as  either
fixed or  floating.   Cross  sections  of  both  types are shown on
Figure  6-27.   Floating  covers  are more  expensive  but allow
independent additions  and  withdrawals  of sludge,  reduce gas
hazards, and can be  designed to control formation of  a  scum-mat.

Fixed  digester  covers  are  fabricated  from steel,  reinforced
concrete  and,  since  the  mid-1970s,  corrosion-proof  fiberglass
reinforced  polyester  (FRP).   In most cases,  fixed covers are
dome-shaped, although  conical  and flat  concrete  covers  have  been
built.   Concrete roofs are susceptible to  cracking  caused by
rapid temperature changes.  Consequently,  gas  leakage  has been a
frequent problem with  reinforced concrete covers (75).

Generally, fixed-cover  digesters are operated  so as to  maintain a
constant water surface level  in  the  tank.  Rapid  withdrawals of
digested  sludge  (without  compensating  additions of  raw  sludge)


                              6-62

-------
can draw  air into the  tank,  producing an  explosive  mixture  of
sludge gas and oxygen.  The explosive range of sludge  gas in air
is  5  to 20  percent  by volume  (52).  In  addition, there  have
been  cases  in  which the  liquid  level  under  the fixed cover
has been  allowed to  increase  sufficiently to damage  the  cover
structurally.  Usually, this involves a tightly clogged overflow
system and a forgotten feed  valve.
                            TABLE 6-19

            RELATIONSHIP BETWEEN THE VELOCITY GRADIENT
                        AND UNIT CAS FLOW

                     G                     Q/v
              Velocity gradient,      Unit  gas flow  ,
                        1            cfm/1,000 cu ft
                     40                    2.1

                     50       .             3.3

                     60                    4.4

                     70                    6.4
            Calculated assuming depth of gas release
            is  13  ft and that absolute viscosity of
            sludge is the same as for water at 95°F.


           1 cfm/1,000 cu ft = 1 m3/min/l,000  m3


Traditionally,  floating  covers  have followed  one of two designs:
the pontoon or  Wiggins  type and the  Downes  type  (Figure  6-27).
Both types  of  covers  float directly on  the  liquid and commonly
have a maximum vertical  travel  of 6 to 8 feet (2 to 3 m).   These
cover designs  differ  primarily  in  the  method used  to maintain
buoyancy, which,  in turn,  determines  the  degree  of submergence.
In  the  Wiggins  design,  the  bottom  of the cover  slopes steeply
along the  outer edge.  This outer portion  of the cover  forms
an  annular pontoon  or float  that  results   in  a large liquid
displacement  for  a  small degree  of cover-plate submergence.
Therefore,  Wiggins covers have only  a portion of the annular area
submerged,  with the largest  portion  of  the cover  exposed  to the
gas above the  liquid surface.  However, for the Downes design,  as
shown on Figure  6-27,  the  bottom of  the  cover slopes gradually
throughout the entire radius,  thereby providing only a  small
liquid  displacement  for  a greater degree  of  ceiling  plate
submergence.  Typically, the  outer one-third of the radius of the
Downes  cover  is  in  contact with  the liquid.   However,  it  is
desirable to increase the degree  of  submergence by adding ballast
to  the  cover, thus keeping  the  liquid level  a few inches  within
                              6-63

-------
the central  gas  dome.  This keeps  floating  matter  submerged  and
subject to mixing  action,  reduces  the area exposed to corrosive
sludge  gas,  and  adds  to  cover  stability.    The fundamental
principle  used  to calculate  ballast requirements is  that  at
equilibrium,  a floating cover displaces a volume of liquid equal
in weight to the  total  weight  of  the  cover.  Ballast can be added
as concrete  blocks or  as a layer  of  concrete  spread  across  the
upper surface of  the cover.
                           FIXED COVERS

    WIGGINS TYPE
                           DOWNES TYPE
                          FLOATING COVERS
GAS HOLDER
                           FIGURE 6-27

                    TYPES OF DIGESTER COVERS
A variation  of  the floating  cover  is the  floating  gas holder,
shown on  Figure 6-27.   Basically, a gas  holder is a  floating
cover with an extended skirt (up to 10 feet [3 m] high) to allow
storage  of gas during  periods  when gas production exceeds demand.
However, storage pressure  in  a gas holder  is  low--a  maximum of
15 inches  water column  (3.7  kN/m2).  Therefore, this  type of
cover will store up  to  three to six hours of gas production,
based on about six  feet (2m)  of net travel.  Greater storage is
achieved  by compressing  the  gas for high  pressure storage .in
spheres  or horizontal cylinders, or  by  providing a separate low
pressure displacement  storage  tank.
                              6-64

-------
Gas-holding covers  are  less  stable  than  conventional  floating
covers  because  they  are supported  entirely  by a  cushion of
compressible gas  rather  than  incompressible  liquid  and  because
they expose a large side  area  to lateral  wind loads.   To  prevent
tipping or  binding, ballast at  the bottom  of the  extended skirt
and spiral guides must be provided.

Typical  appurtenances for  a digester  cover  include sampling
ports;  manholes  for  access,  ventilation, and  debris removal
during cleaning; a liquid overflow system;  and  a vacuum-pressure
relief  system  equipped  with  a flame trap.    The  permissible
range of  gas  pressure  under a digester  cover is  typically 0 to
15  inches  of  water (0 to 3.7  kN/m2).   Figure   6-28  provides an
overview  of four floating  covered digesters  with  appurtenant
equipment.
                           FIGURE 6-28

            OVERALL VIEW OF FOUR DIGESTERS WITH DOWNES
             FLOATING COVERS AT SUNNYVALE, CALIFORNIA
                               6-65

-------
        6.2.6.5   Piping

The piping  system  for  an anaerobic  digester is  an  important
component  of the design.   Many  activities take  place  during
the operation of a digester:   feeding  of  raw sludge, circulation
of sludge  through the heat exchanger, withdrawal  of digested
sludge and  supernatant, and collection of  sludge gas.   The piping
system  should be designed to  allow these  activities to occur
concurrently, yet independently.   Flexibility  should also be
built  into the piping system to allow operation in a variety of
modes  and to ensure  that  digestion  can  be  continued  in  the event
of equipment  breakdown or pipe clogging.

Feeding  of  incoming  sludge  into anaerobic  digesters  can be
automated to load the tanks frequently  and  uniformly.   Switching
feeds  between  several  tanks  can be  controlled  based  on either
time,  hydraulic  flow,  or solids flow.   A  time-controlled  feed
system  uses  a  repeat-cycle  timer  to sequentially   open  and
close  the  feed  valve  for each digester.   Switching between
digesters  can occur  every thirty minutes  to  four  hours.  A
flow-controlled  feed system uses a  flowmeter  on  the raw sludge
pipeline, in  combination with a totalizer,  to load  preset volumes
to each digester.  These may or may not be equal depending on the
individual  characteristics of  each digester.   A feed control
system based  on solids  flow requires  the  measurement of both raw
sludge flow  and  density.   Since density  is correlated with the
concentration of solids  in  sludge,  these two  signals  can be
combined to yield a measure of the solids mass being fed to the
digesters.   Selection of flowmeters and density meters  for sludge
is discussed  in  Chapter 17.

Raw sludge  should  enter  the digester  in the zone of intense
mixing  to  disperse  the  undigested  organics  quickly.   Raw
sludge,  before entering  the digester, should be mixed  with  warm
circulating sludge to seed the  incoming sludge and  avoid thermal
shock.   The  introduction of cold  feed  sludge  into  regions where
there  is no  local mixing  results  in the  feed sludge sinking to
the digester  bottom  and becoming an isolated mass.

Digested sludge is usually drawn off the  bottom of  the tank,
although  means  to  withdraw  sludge  from  at  least  one  other
point  should  be  provided  in case the main  line becomes plugged.
A  supernatant collection  system, when  required,  should  have
drawoff points at three  or more elevations  to  allow  the operator
to remove the clearest  supernatant.   An example of  a supernatant
collection system is shown on  Figure  6-29.   The telescopic valve
is used  to adjust the water  surface  level  in the digester.   An
unvalved overflow with  a  vent  as a jsiphon  breaker  is provided to
ensure that the  tank cannot be overfilled.

Special consideration  should  be given  in  the  design of
sludge-piping systems to  prevent  the deposition  of  grease  and
clogging  with debris.  Sludge  piping generally  has  a minimum
diameter of  6  inches  (150  mm),  except for  pump discharge lines


                              6-66

-------
in small  plants, where  four-inch (100 mm) diameter pipes may
be acceptable.   Where possible, considering these minimum pipe
size  recommendations,  velocities  in  sludge pipelines should
be maintained above  four  feet per second  (1.2  m/sec) to keep
sludge solids in suspension.   The hydraulics of  sludge  piping  is
described  in  detail  elsewhere  (84,160).    Glass  lining of cast
iron and  steel  pipe  will  prevent  the  buildup  of grease and  is
recommended  for  all  pipes  conveying scum  and  raw sludge.   The
grease content of  sludge  is  typically  reduced  by 50 percent  or
more during  digestion, so that glass lining is  not warranted for
pipes carrying digested or circulating  sludge.   Sludge  piping  is
generally  kept as short as practicable, with a  minimum  number  of
bends.   Long radius  elbows  and  sweep tees are  preferred for
changes in direction.   Provisions are commonly  made  for cleaning
sludge  lines with steam,  high  pressure  water,  or  mechanical
devices.  These  provisions  should include blind  flanges, flushing
cocks, and accommodation  for thermal expansion.

A problem  unique  to anaerobic  digestion systems  is the buildup  of
crystalline  inorganic  phosphate  deposits on the  interior walls  of
the tank  and  downstream piping.  This encrustation will increase
pipeline  friction, displace  volume in the digestion  tank, and
foul  downstream  mechanical  equipment  (102).    This  chemical
scale has formed  not  only  in  digested  sludge lines,  but also  on
mechanical aerators for facultative sludge  lagoons and  in  pipes
carrying  either  digester  supernatant  or filtrate/cent rate.
Laboratory analyses have  identified  this  material as  magnesium
ammonium  phosphate  (MgNH4P04  •  6  1^0),  more  commonly   known
as guanite  or  struvite.   It has  a  specific gravity of 1.7,
decomposes  when  heated,  and  is  readily  soluble only in acid
solutions.   Methods  successfully used to  prevent this buildup
include (161):

     •  Aerobic  digestion of  the sludge  stream with  the highest
        phosphate content

     •  Dilution  of   digested sludge  flows to  prevent super-
        saturation and to  raise  pipeline velocities

     •  Limiting  magnesium   ion  concentration   in  the stream

     •  Substitution   of  PVC  pipe  for  cast-iron  pipe  to  reduce
        interior roughness


        6.2.6.6   Cleaning

Anaerobic digestion  tanks can become partially filled with  a
bottom  layer  of  settled  grit  and a  top  layer  of  floating  scum.
These  accumulations  reduce  the volume  available  for active
digestion and thereby  degrade  the  performance of  the digesters.
Periodically, the  digestion tank must  be drained and   these
                               6-67

-------
deposits removed.   This cleaning process is usually  expensive and
unpleasant.    Furthermore,   it  can  disrupt normal  processing  of
sludge  for  as long  as  several  months.   Therefore,  attention
should  be  given during  design  to  (1)  reducing the rate  at  which
grit and scum  can  accumulate, and  (2) making  it  easy  to clean the
digester when  it becomes necessary.
                                            TOP OF DIGESTER
               VENT
            DIGESTER
            OVERFLOW
SUPERNATANT
COLLECTION BOX

TELESCOPIC VALVE

MAX, W,a ELEVATION


GROUT

M)N. W.S. ELEVATION
                                                   SUPERNATANT
                                                   DRAWOFF PORTS
 TO PLANT HEADWORKS
                                            TO SECONDARY DIGESTER,
                                            HEADWORKS, OR SUPERNATANT
                                            TREATMENT
                             FIGURE 6-29

          TYPICAL DIGESTER SUPERNATANT COLLECTION SYSTEM
                                6-68

-------
              Grit  and  Scum
The most sensible approach to minimizing  digester  cleaning is to
prevent  grit and scum from  entering the system.   This can be
accomplished through effective grit removal  in the  headworks of
the plant coupled with separate processing of  scum (for  example,
incineration or hauling  to a  rendering  plant).  A  second  mitiga-
tion measure,  which is almost as effective,  is  to maintain a
homogeneous  mixture  within the digester so that the grit  and  scum
cannot separate out.  This is best achieved  by strong  mixing and
positive submergence of the liquid surface under a  floating cover
(refer to the preceding sections on mixers and  covers).

Provisions  can also be made  to remove grit  and scum easily  from
the digester while normal digestion continues.   Grit removal  from
the digester  can be  improved  by providing  multiple  withdrawal
points, or  steep  floor  slopes  (as in  a waffle bottom or
egg-shaped  digester).   An  access  hatch  in the  digester cover, or
pipes  extending  into  the  upper levels  of the  digesting  sludge,
can be  used  to  remove floating  material  in the  tank before it
forms a  mat.  Strong  mixing  in  the tank will carry floating
material down into the zone of  active digestion, where  it  will be
broken  down.  Other methods of  scum control  in  digesters are
described in References 41 and  162.

Facilities  for  Digester Cleaning

Traditionally, digester  cleaning has been a  difficult, dirty
task.   As a  result,  it is  often postponed until tank capacity is
severely reduced.   Cleaning  then  becomes even  more  onerous
because  of the increased  urgency  and  scope of  the  operation.  If
a digester  can be cleaned  easily,  it  is much more  likely  that it
will be  cleaned regularly.

To  ensure   that  the digesters can  be  easily cleaned,  it is
important for the designer to consider the following questions:  ;

    •   What will be done with  the  raw  sludge while the  tank
        is out of  service?   Ty p i ca 1 ly ,  r aw sludge  f 1 ow Ts~
        distributed to  the  remaining tanks  as  long as there
        is adequate capacity.   The  problem,  however,  becomes
        much more serious in  a  plant  with only  one  digester.
        Possibly, a  temporary  aerobic  digester or  an  anaerobic
        lagoon  can  be devised,  although  odors may be a  problem
        with the latter.   Lime may be added  to the raw sludge to
        disinfect it and control odors (see Section 6.4).

    •   How  will the  tank  be drained?  There  is  a  risk of
        explosion during  the  period  in which  the  tank  is being
        emptied,  making  it important to  speed this step in the
        cleaning  process.   Addition of  a  separate  digester drain
        pump  in  the  Sunnyvale treatment  plant  in California
        allows  each  tank to be  emptied in less  than two days.  As
        shown  on  Figure  6-30,  the intake of  the  drain  pump is


                               6-69

-------
located below the  low  point  on the digester floor, from
where the pump draws.  As a result, the pump also serves
to remove the slurry of grit and washwater rapidly.  The
volume of washwater required has been greatly reduced by
the addition of the drain pump.   Four  to  5 feet (1.2 to
1.5 m)  of sand  on the bottom can be  washed  from the
tank with washwater  amounting  to  less  than a quarter of
the total tank volume.

Traditionally, the  volume  of  washwater  is two  to four
times  the tank  volume.   Once drained,  the  Sunnyvale
digesters can be  scrubbed down in one  day, and start-up
can  begin the  next day.    Before the drain  pump was
installed, all material  removed from the  tank  had been
lifted out  through  the manholes  in the  sidewalls.
Consequently, it  took  30  to 60 days to  drain and  clean
a digester.    In either case, an additional month will be
required  to  restore the  biological  process completely,
unless  it is  seeded  from  other  "healthy"  digesters.
Ten  to  fifteen   percent  of  the  digester's  volume  is
usually required  for adequate  seeding.  A seeded digester
can be brought back into  full  biological activity in less
than a week.

Where  will  the contents  of  the tank and the washwater be
         Placing  these  materials  on a sand-drying bed
o~F~Tn~ an existing sludge lagoon are  two simple solutions
to the problem.   Construction  of  a  small earthen basin,
specifically  for  use during  digester cleaning,  may be
warranted.    Hauling  material  in  tank  trucks  to  another
treatment plant or to a suitable dispoal site is  another
option.   Mechanical  dewatering  equipment  may  be used to
reduce the  volume for hauling,  but the large proportion
of abrasive material  (grit)  contained  in  the  sludge and
wash water may produce excessive wear.

At the Joint  Water  Pollution Control Plant, operated by
the  County  Sanitation Districts of  Los  Angeles  County,
all washwater is  treated in  a  separate digester cleaning
facility.   The  washwater   is  first  passed  through  sieve
bend type (static) strainers and then pumped to cyclonic
grit  separators.    The  removed  grit is  cleaned  in a
helical  screw grit washer and,  along  with the screenings,
is  transported  by conveyor  to storage hoppers.    These
hoppers  are emptied  daily  and  the  material trucked to a
sanitary landfill.   Figure 6-31 shows the cyclonic grit
separator and static  screens  at this plant.  The liquid
discharged  from the  cyclonic grit  separators  is  further
processed   in dissolved air  flotation  tanks.   Liquid
underflow from  these  flotation  tanks is  diverted  to the
primary  sedimentation  tanks,  while float and  settled
material  are combined  with  digested  sludge  flow and
fed  to  the plant's sludge dewatering  system.   The
digester cleaning  facility  now  serves 33 digesters with a
                       6-70

-------
        combined capacity of  5.7  million cu ft  (21,200  m3).   A
        full-time seven-man crew  is required  for digester
        cleaning,  allowing  a five-year cleaning cycle.   New
        digester  additions  under  construction  in  1979  will
        lenghten this period  to  seven years.   In  1973,  the bid
        for construction of  the  digester  cleaning facility was
        approximately $3,000,000.

        How will  access be  provided into  the  tank?  Manholes
        should  be provided  through both  the cover and  the
        sidewalls  of  the tank to allow for ventilation, entrance
        of equipment  and  personnel,   and  removal  of  organic and
        inorganic  debris.  Often  in the past, the number and size
        of these  openings  has  not  been sufficient for  easy
        cleaning.

        Is there  a  source  of water  for washing  the tank and
        refilling  it  for start-up?  Washdown  water  should be air-
        gapped  and capable of supplying  a pressure in excess of
        60 psi  (414  kN/m2)  through  a  hose  of  at  least one-inch
        (2.5  cm)  diameter.   Larger  capacities  are  required for
        digesters  greater than 55 feet (17 m) in diameter.  Once
        the tank has  been cleaned, start-up begins  by  filling the
        tank  with  either raw  wastewater, primary effluent,  or
        unchlorinated secondary effluent,  and bringing the entire
        contents up  to  operating  temperature.  If seed sludge is
        to be  used,  it should  be  fed  into  the digester as soon as
        its liquid contents  have achieved operating temperature.
Additional discussions of digester
found in references 164  and  165.
cleaning and start-up can  be
                     ANAEROBIC
                     DIGESTER
               SLUDGE __,
               LAGOON  7
                                 RECESSED IMPELLER
                                 DRAIN PUMP
                            FIGURE 6-30

                      DIGESTER DRAIN SYSTEM
                               6-71

-------
                            FIGURE 6-31

              DIGESTER WASH WATER CLEANING BY CYCLONIC
              SEPARATORS, GRIT DEWATERERS, AND STATIC
            SCREENS AT LOS ANGELES COUNTY CARSON PLANT
    6.2.7  Energy Usage

The flow of energy through a typical  anaerobic  digester  system  is
displayed on  Figure  6-32.   In this simple  system,  a hot  water
boiler,  fueled  with  sludge gas,  is  used  to heat the digesters.
The digestion  system  shown on  Figure  6-32  produces more  energy
than it requires in the form of digester gas. The energy  required
for digestion  is mainly to  heat the  sludge.  The energy  consumed
in mixing  the digester  contents  is very small in  comparison.
Surplus  digester  gas  can  be  (1)  burned  in  a  boiler to  produce
heat for buildings in  the plant,  (2) used to power an engine  to
generate electricity or  directly  drive a  pump,  (3)  sold  to the
local utility  for  use  in  the domestic  gas supply, or (4)  flared
                               6-72

-------
   MIXING*
                           TOTAL GAS PRODUCTION a

                                        7.3
                                                                         • SURPLUS GAS
ANAEROBIC
DIGESTER

""-•^. ^-"-""^
1
CIRCULATING* \
SLUDGE HEATING . t /"
K 1,75 f
V** ns X- -*— [BOI
J

^~^~^^^ c

ALL VALUES \H UNITS OF
10B BTU/TON DRV SOLIDS

r

ij^y— 	 ^
F ' " ' \


RAW SLUOQEb ^




V
J
.75

'
HEATING HEAT
LOSSES
     FED TO THE DIGESTER
(1 Btu/lh = 0,56
                           RAW
                          SLUDGE
 Raw sludge  volatile solids contents, percent               75
 Volatile  solids reduction during digester, percent         50
 Specific  gas production, cu ft/lb VS reduced               15
 Heat value  of gas, Btu/cu ft                             650
2,000  Ib/ton  (.75) (.50)  (15
                            cu  ft1
                             Ib  '
                     (650
                                             )  = 7.3  x  10  Btu/ton
 Feed solids concentration,  percent
 Specific  heat of sludge, Btu/lb/°F
 Rise in temperature, °F
(1
                        (25°F)  =  1.3 x 106 Btu/ton
c  Makeup  Heat Requirement   =  .
 Raw Sludge  Heat Requirement
 Net boiler  and heating system efficiency, percent          70

 F'eed solids concentration,  percent                          4
 Detention time, days                                      20
 Mixing  requirement, bhp/1,000 cu  ft                      0.25
  2.000 Ib/ton
'(.04)  62.4  Ib/cu ft
                    ,,,„
                    (2°
                ,,,.
                (24
                                   hr
                         ,,. _,-
                         (0'25
                                                 bhp
                                             1,000 cu  ft

                                      FIGURE 6-32
                                                          ,^  _ . _
                                                          (2'547
                                                                  Btu
                                                                        = 2-4
                                                                                   Btu/ton
                       ENERGY FLOW THROUGH AN ANAEROBIC
                             SLUDGE DIGESTION SYSTEM
                                           6-73

-------
in a  waste-gas  burner.   The energy flow through a  more  complex
gas utilization system, in which gas  is used  to  fuel  an  engine-
generator, is described  in Chapter 18.

The  energy  flow  diagram  shown  on Figure  6-32  conveys very
effectively  the  relative magnitude and  direction  of  energy
exchanges in  an anaerobic digestion  system.   This type  of  diagram
is helpful in  the design of  a  gas  utilization system.  However,
more  detail  must  be  added  and  the  full  range  of expected
conditions  must  be evaluated,  rather  than just  the average
conditions depicted  for  this  case.

More complete discussions of  digester gas utilization systems can
be found in Chapter  18  and elsewhere (38,39).


    6.2.8  Costs

Cost curves  have  been  compiled  that plot construction costs for
anaerobic digestion  systems  versus  either  digester  volume  (166),
sludge  solids  loading   (167,168,169),  or  total  treatment  plant
flow (170,171,172).   However, these curves  differ significantly,
even when converted to a common  cost  index and  plotted in  terms
of  a single sizing  parameter  (Figure  6-33).    Cost  curves
are  generally  constructed  to allow  comparison of equivalent
alternatives and consequently do  not  always  describe  actual
costs.

Estimated annual  costs  for  operation and maintenance  are  shown
in Figure 6-34.  No credit has been given  in  this  graph  for the
value of  surplus sludge gas.   In most cases,  use of this gas
requires, construction  of additional facilities for  conditioning,
compressing, and burning the  gas.  The cost for  construction
and  operation of  these  systems  (38) must  be  included in
calculations  of the  net  value of surplus sludge gas.
    6.2.9  Design Example

This  section illustrates the basic  layout and  sizing of the
major components  in  an  anaerobic  sludge digestion system.   For
this  example,  it is  assumed  that  the  treatment  plant  provides
activated  sludge secondary   treatment  to  a  typical municipal
wastewater.    A mixture of  primary  sludge  and  thickened
waste-activated  sludge is  to be anaerobically digested, held  in  a
facultative sludge lagoon, and ultimately spread as  a stabilized
liquid onto land.


        6.2.9.1   Design  Loadings

Sludge  production estimates  for two  flow  conditions, average
and peak day, are listed in  Table 6-20  (see  page  6-79).   The
peak  loading  is  listed  because several components must  be  sized


                              6-74

-------
to  meet this  critical condition.   Refer  to Chapter 4 for  a
discussion of  the procedures  to determine sludge  production
values.   Sludge  solids concentrations  and  the resulting  sludge
volumes are also  included  in Table 6-20.
cc
z
UJ

I
-s
a
I
* .
H
O
U
GC
te
I
   10.0  I—
     9
     8
     7
     8
     S

     4
    1.0
     9
     8
     7
     6
     5
     4  -
     3  -
      _  REFERENCE 16?
                                                     REFERENCE 171
                                       'CONSTRUCTION COST ONLY,
                                        DOES NOT INCLUDE ENGINEERING
                                        OR CONTINGENCIES.
                                   I
                                                            J__L_J
      10
                 34B678S1CMJ     2    346678 91,000   2

                  DIGESTER TANK VOLUME, 1,000 cu ft (1 cu ft = .028m3|
                                                               4  5
                              FIGURE 6-33

                   CONSTRUCTION COSTS FOR ANAEROBIC
                      DIGESTION SYSTEMS  (111-168,171)
         6.2.9.2   System Description

The  conceptual  design  for  a  high-rate  anaerobic  digestion
system  is  presented  on Figure 6-35  (see page  6-80).   At the
heart  of the system are  two cylindrical  single-stage,  high-rate
digestion  tanks operated  in  parallel.   The  contents  of  both
digesters  are  heated  to  95°F   (35°C)  and vigorously mixed  with
draft-tube  gas  mixers.   Floating covers are used on both tanks to
keep  floating material soft and submerged,  and to allow  in-line
storage  of  sludge in the  digestion tanks.
                                6-75

-------
er
O
03
5
j
<
§
e
4!
_
o
z
                                                            .001
                                                            .OOC1
oc
111

o
CL

E
5
_
•
c
o
                                                               O
                                                               O
                                                               12


                                                               <
                                                          •*-" .00001
    0.1
                      1.0                 10

                 AVERAGE PLANT FLOW, MGD flMGD = 3,785m3/day)
                           FIGURE 6-34

            OPERATING, MAINTENANCE, AND ENERGY COSTS
            FOR ANAEROBIC SLUDGE DIGESTION SYSTEMS (171)
Raw primary  and secondary  sludges  are first  combined  and then
heated  to  95°F  (35°C)   in a  jacketed  pipe  heat  exchanger.
The rate  of the  raw sludge  flow is measured with  a  magnetic
flowmeter.  The signal from this meter is  integrated  to  indicate
the hydraulic  loading to  digestion.   This  information  is also
used to indicate equal  volumes  of raw sludge for even distribu-
tion  to  each  digester.    The  controls  are  set  so that each
digester  is  fed approximately  ten  times  each day.   Raw  sludge
is  mixed  with  circulating  sludge  and  added  to the  digester
through the gas dome  in  the center  of the cover.  The operating
temperature  in  the digester  is maintained  by circulating a
                               6-76

-------
small volume  of  sludge through an external spiral  heat exchanger.
Digested  sludge  is  withdrawn  daily  from the  bottom  of  the
tank and  transferred  by  gravity to facultative  sludge lagoons.
For monitoring purposes,  a flowmeter  is  included  in  the digested
sludge  withdrawal  line.   This  provides a  means for  evenly
distributing  the  sludge  to  several  lagoons.   Both  tanks
are  operated as  completely  mixed primary  digesters  without
supernatant removal.


        6.2.9.3   Component Sizing

Digestion T a n k s

Sizing criter a:

     •   >_IQ days solids  retention time during the most critical
        expected  condition  to   prevent  process failure  (See
        Section  6.2.3.3).

     •   2.50  percent  volatile   solids  reduction  at average
        conditions  to  minimize odors  from the  facultative sludge
        lagoons.

Tank volume:

    Raw sludge flow at peak conditions (Qp)

    — Assume  peak  day  conditions (this   is conservatively large
      but provides  a margin of safety) .


    Qp = 6,010 + 3,430 =   9,440  cu ft per day    (267  m3/day)


Active volume (Va)
    Va =  f____    (1Q das) = 4? >2^ cu  ft
    Correction for volume  displaced  by grit and scum  accumula
    tions and  floating cover level.

    Assume:

    4-ft grit  deposit

    2-ft scum  blanket

    2- ft cover below maximum

    8-ft total displaced height
                              6-77

-------
Therefore, if  original  sidewater depth of  the  tank is 30 feet,
                      O Q __ p
active volume is only —^Q—  =  °-73  of  the  total  tanks  volume.


    Tank volume (Vt)


        v  = 47,200 cu ft  /  1  \
         fc       tank      \'73/

           = 64,700 cu ft per tank
             Say 65,000  cu ft per tank  =  (1,800 m3/tank)


    Solids retention time at  average conditions  (SRTa)


        q   _   	65,000 cu  ft per tank  (2  tanks)	
           a    3,200 cu ft per day  + 2,000  cu ft per day


            = 25.0  days,  based  on total  volume,  50  percent
              volatile  solids  reduction  can be expected with
              this solids  retention  time  (see Section 6.2.4.1).


Tank dimensions:

    Diameter (D)

        Assuming  initially,  a  30-foot  sidewater height  and
        neglecting the volume in the bottom  cone:
            ^  J4(65,000 cu ft)    co  c  ...    ,,,  .   .
            D =\—	J3Q £t)	L = 52.5  ft  =  (16.0 m)
    Sidewater height (h)

    Since  floating  covers  come  in  5-foot  diameter  increments,
    enlarge diameter and adjust sidewater height:
        h = 4(65,000 cu ft)  =
                (55 ft)2
    Note:  This  adjustment  increases  displacement  volume
           effect and reduces  active  volume  to   —•=-=—j—  or 0.71.
           This  is  ignored  in  this  example because  of previous
           conservative assumptions.
                               6-78

-------
                     TABLE 6-20

            DESIGN LOADING ASSUMPTIONS

                               Flow condition
                                         Peak
           Parameter          Average    day
   Sludge production, Ib dry
     solids/day
       Primary sludge         10,000  '  15,000
       Waste activated
         sludge                5,000     7,500

   Solids concentration,
     percent
       Primary sludge            5.0       4.0
       Waste activated
         sludge                  4.0       3.5

   Sludge volume ,  cu ft/day
     Primary sludge            3,200     6,010
     Waste activated sludge    2,000     3,430
    Sludge volume =
                  sludge production
      (solids concentration)  (density of sludge)

      e.g.,      10,000  Ib/day       _ onri     ... ,,
            ' t n c \   i r ->—-A—4\r~r^—jnrr - 3,200 cu  ft/day
             (.05)   (62.4 Ib/cu ft)                  J
    I Ib/day = .454 kg/day
    1 cu ft/day =  .0283 m3
                ft/day


Heat_Exchangers - (See Section 6.2.6.2)

Raw sludge heat exchanger capacity  (Qs)

    Assume:

     •  Peak day sludge loading

     •  Minimum temperature of raw  sludge =  55°F
- /9,440 cu ftV (62.4 lb\ / 1 day\  /    Btu   \  (95oF_55o
-                ~5lTTE~  V24 hrs   V- Ib - °F   (^^
= 982,000 Btu/hr = (247,000 kg-cal/hr;
                        6-79

-------
Makeup heat exchanger capacity  (Qm)

    Assume:

     •  Tank completely buried  but above water table,  U = 0.06

     •  Bottom exposed to wet  soil,  U = 0.11

     •  Cover insulated, U  =  0.16

     •  Minimum soil temperature = 40°F

     •  Minimum air temperature = 10°F
Qm =
          heat loss through  walls + bottom + top
        =  (0.06 Btu/sf/°F/hr) ( [2    ft]55 ft/4[27.4  f t] ) (95°F-40°F )
        +  (0.11)(  [55 ft]2/4] (95°F-40°F)
        +  (0.16)(  [55 ft]2/4] (95°F-10°F)
        =  76,029 Btu/hr =  (19.2  kg-kcal/hr)
The  above calculated  values are used  for  sizing  equipment.
Average  heat  requirements  would be substantially  less.
 PRIMARY
 SLUDGE
 WASTE
 ACTIVATED
 SLUDGI
                  RAW SLUDGE
                  HEAT EXCHANGER
                                        RAW SLUDGE FLOW METER


                                         FEED CONTROL VALVE

                                               MAKE-UP HEAT EXCHANGER
                                                        		 CIRCULATING
                                                            SLUDGE PUMP
            DRAFT TUBE
            GAS MIXER
                                         DIGESTED SLUDGE
                                          FLOW M6TER
                                                        DIGESTED SLUDGE
                                                        CTO FACULTATIVE
                                                        SLUDGE LAGOONSI
                             FIGURE 6-35

     CONCEPTUAL DESIGN OF AN ANAEROBIC SLUDGE DIGESTION SYSTEM
                                 6-80

-------
Mj-xjing (See Section 6.2.6.3)

Sizing criterion:

    Assumptions:

         •  Velocity gradient (G) = 60 sec"1

         •  Plant located at sea level PI = 14.7 psi

         •  Gas  released  13  ft  below  the water surface ?2 = 14.7
            + 0.434 (13) = 20.3 psi

         •  Viscosity of  the digesting  sludge is the same as for
            water at 95°F or 1.5 x 10""^  lbf-sec/sq ft


Rate of energy transfer (E)

    Combining Equations 6-5  and 6-6 and  solving for E:


    E = V M G2

      = 65,000 cu ft/tank  (1.5 x 10~5 lbf - sec/sq ft)(60 sec"1)2

      = 3,510 ft lbf/sec/tank =  (4.8 kW/tank).


This  is the power  delivered  to the digester contents.   Motor
horsepower for the  compressor will be substantially higher.

Gas Flow (Q) solving Equation 6-7 for Q.
2.4 (Pl) |ln |^J



  3,510 ft-lb/sec/tank
                                                             6_8)
         2.
       =  308  cfm/tank  (0.145 mj/sec/tank)
                                6-81

-------
6.3  Aerobic Digestion

Aerobic digestion is the biochemical oxidative stabilization  of
wastewater sludge in open or  closed  tanks  that are separate from
the liquid process system.

    6.3.1   Process Description

        6.3.1.1   History

Studies on aerobic  digestion of municipal wastewater  sludge
have been conducted since the  early 1950's  (175,176).   Early
studies (177,178) indicated that aerobic digestion performed  as
well as,  if not better than, anaerobic digestion  in  reducing
volatile  solids in sludge.   Aerobic digestion processes were
economical to  construct,  had  fewer operating  problems  than
anaerobic  processes,  and  produced  a  digested sludge that drained
well.  By  1963,  at  least one  major  equipment supplier (179) had
approximately 130 installations  in plants  with flow from 10,000
to 100,000  gallons  per day  (37.8  to 378 m3/day) .    By  the late
1960's  and early 1970's,  consulting  engineers  across the country
were specifying aerobic digestion  facilities for  many  of the
plants  they were designing.

        6.3.1.2   Current Status

As of  early 1979,  numerous plants  use  aerobic digestion, and
several of  them are quite  large  (11).   Because  of significant
improvements in design  and  control of  anaerobic processes,
coupled with the significant mid-1970  jump  in energy  costs,
the  continued use  of  aerobic digestion,  except in the  small
facility,  is much in  doubt.

        6.3.1.3   Applicability

Although  numerous lab and pilot-scale studies have been conducted
on a variety of municipal wastewater sludges, very few  docu-
mented, full-scale  studies  have  been reported  in the literature.
Table  6-21 lists some of these  aerobic digestion  studies and
provides  information on the type of  sludge studied, temperature
of digestion,  scale  of study,  and literature  reference.

        6.3.1.4   Advantages and Disadvantages

Various  advantages  have  been  claimed   (66,197)  for  aerobic
digestion  over  other  stabilization  techniques,  particularly
anaerobic  digestion.   Based  on all  current  knowledge,  the
following  advantages can  be  cited  for  properly designed and
operated  aerobic digestion processes:

     •  Have capital costs  generally lower than  for  anaerobic
        systems  for plants under  5 MGD (220  1/s) (170).

     •  Are relatively easy  to operate  compared to anaerobic
        systems.


                              6-82

-------
        Do  not generate  nuisance odors  (199,200).

        Will  produce  a supernatant  low in  BOD5,  suspended
        solids, and ammonia nitrogen  (199,200).

        Reduce the  quantity  of grease  or  hexane solubles  in the
        sludge mass.

        Reduce  the  number of  pathogens  to a  low level  under
        normal design.   Under  auto-heated design,  many  systems
        provide 100 percent pathogen  destruction  (187).
                              TABLE 6-21

            SELECTED AEROBIC DIGESTION STUDIES ON VARIOUS
                    MUNICIPAL WASTEWATER SLUDGES
     Sludge tyoe
 Primary sludge
 Primary sludge plus
  waste-activated
  lime
  iron
  alum
  waste-activated + iron
  .trickling filter
  waste paper
 Contact stabilization sludge
 Contact stabilization sludge plus
  iron
  a 1 urn
 Waste-activated sludge
 Trickling filter sludge
   )indicates full-scale study results.
                                  Studies
                                   under
                                   50°F
        180

        187
                   Studies
                   'between
                   50°- 86°F
                Studies
                 over
        192
 181,  (182 )

 187
 188
 189
 189,(190)
 190
 131
 191
 192,  197, (199)

(190)
(190), (194)
 195
 181,  196
133, 184

(185) (186) (187)
As  with  any  process,  there  are  also  certain  disadvantages.
In aerobic digestion processes, the  disadvantages  are:
         Usually  produce  a  digested  sludge
         mechanical dewatering characteristics.
                        with  very poor
         Have  high power
         small plants.
costs  to supply oxygen,  even  for  very
         Are  significantly   influenced  in
         temperature,  location, and  type of tank
                        performance   by
                        material.
         6.3.1.5  Microbiology

Aerobic  digestion  of  municipal wastewater  sludges  is  based on
the  principle  that,  when  there  is  inadequate external substrate
available,  microorganisms metabolize their  own celluar mass.  In
                                 6-83

-------
actual operation,  aerobic  digestion  involves the direct oxidation
of  any biodegradable matter  and  the  oxidation  of microbial
cellular material by organisms.  These two steps are illustrated
by the following reactions:
Organic

matter
02
Bacteria
                              Cellular
                              material
                        C02 + H20
                                                           (6-9)
Cellular

material
           o2
                Digested

                sludge
                        C02 + H20
                                                          (6-10)
The  process  described  by  Equation  6-10  is  referred  to  as
"endogenous  respiration";  this  is  normally  the  predominant
reaction in aerobic digestion.
    6.3.2  Process  Variations
        6.3.2.1   Conventional Semi-Batch Operation

Originally,  aerobic  digestion  was designed as  a semi-batch
process, and  this concept  is still functional at many facilities.
Solids are pumped directly from  the  clarifiers  into the aerobic
digester.   The time required  for filling the digester depends  on
available  tank volume,  volume of  waste sludge, precipitation,  and
evaporation.    During   the  filling  operation,  sludge  undergoing
digestion  is  continually aerated.   When  the tank  is  full,
aeration  continues for two  to three weeks  to assure  that  the
solids are thoroughly  stabilized.  Aeration is then discontinued
and the stabilized solids  settled.  Clarified liquid is decanted,
and  the thickened  solids are removed  at a  concentration  of
between  two  and  four percent.   When  a sufficient amount  of
stabilized sludge and/or supernatant have been removed, the cycle
is repeated.  Between cycles,  it  is  customary to leave some
stabilized   sludge  in  the  aerator  to  provide  the necessary
microbial  population  for  degrading  the  wastewater  solids.   The
aeration device  need  not  operate for several  days,  provided  no
raw sludge is added.

Many  engineers  have  tried to  make  the   semi-batch  process  more
continuous by installing  stilling wells  to  act as  clarifiers  in
part of the digester.   This has not proven effective (200-202).


        6.3.2.2   Conventional Continuous Operation

The  conventional  continuous  aerobic  digestion  process  closely
resembles  the activated sludge  process as shown on (Figure 6-36).
As  in the semi-batch  process,  solids  are pumped  directly  from
                               6-84

-------
  UNSTABILIZED
    SOLIDS
AEROBIC
DIGESTER
CLAR1FIER
THICKENER
SUPERNATANT
                                   STABILIZED SOLIDS

                           FIGURE 6-36

             PROCESS FLOW DIAGRAM FOR A CONVENTIONAL
             CONTINUOUSLY OPERATED AEROBIC DIGESTER
clarifiers  into the  aerobic digester.  The  aerator operates
at a fixed  level,  with  the  overflow going  to a solids-liquid
separator.   Thickened and  stabilized  solids  are  either  recycled
back to the  digestion  tank  or removed  for further processing.


        6.3.2.3   Auto-Heated Mode of Operation

A  new  concept that  is  receiving  considerable  attention  in  the
United  States is the  auto-heated  thermophilic  aerobic digestion
process (187,203).   In  this  process,  sludge  from the clarifiers
is usually  thickened  to  provide a digester feed solids concentra-
tion of  greater  than four percent.   The heat liberated  in  the
biological  degradation of  the  organic solids  is sufficient
to raise  the liquid temperature in  the  digester to as high
as 140°F  (60°C) (187).   Advantages  claimed  for this mode  of
operation are higher  rates of organic solids destruction,  hence
smaller volume requirements;  production of  a pasteurized sludge;
destruction of  all  weed seeds;  30  to  40 percent less  oxygen
requirement  than for  the mesophilic process, since  few,  if any/
nitrifying  bacteria exist in this temperature range; and improved
solids-liquid separation  through  decreased  liquid viscosity
(187,203,204).

Disadvantages  cited for  this process are  that it must incorporate
a  thickening  operation, that mixing requirements are  higher
because of  the higher  solids content,  and that non-oxygen aerated
systems require  extremely efficient aeration  and insulated tanks.
                              6-85

-------
    6.3.3  Design Considerations


        6.3.3.1  Temperature

Since the majority of aerobic digesters are open tanks, digester
liquid  temperatures  are dependent on weather conditions and
can fluctuate extensively.  As with  all  biological systems,  lower
temperatures  retard  the  process   while  higher   temperatures
speed  it up.   Table 6-21 lists studies  on aerobic  digestion
of  municipal  sludges  as  a  function  of  liquid   temperature.
When  considering  temperature  effects  in  system  design, one
should design a system to minimize heat losses by using concrete
instead of steel tanks,  placing  the  tanks  below rather  than  above
grade, and using sub-surface  instead  of  surface aeration.  Design
should allow for the necessary degree of sludge stabilization at
the lowest expected liquid  operating  temperature, and should meet
maximum  oxygen  requirements  at the  maximum  expected  liquid
operating temperature.


        6.3.3.2  Solids  Reduction

A major  objective  of  aerobic digestion  is  to reduce the mass of
solids for  disposal.   This  reduction  is  assumed to  take  place
only with  the  biodegradable  content of  the sludge, though some
studies  (205,206)  have shown  that there  may be destruction of the
non-organics as well.  In this discussion,  solids reduction will
pertain only to the biodegradable content  of the sludge.

The change in biodegradable volatile  solids can be represented by
a first order biochemical reaction:
where:

    rlM                                •
    -fir = rate of change of biodegradable  volatile  solids
         per unit of time - (Amass/time)

    K^ = reaction rate constant - (time   ~1)

     M = concentration of biodegradable  volatile solids
         remaining at time t in the aerobic  digester  -
         (mass/volume).


The time t  in Equation 6-11 is actually the sludge age or  solids
residence  time  in the  aerobic  digester.    Depending  on  how  the
aerobic digester  is being  operated,  time  t can  be  equal  to or


                               6-86

-------
considerably  greater  than  the  theoretical hydraulic  residence
time.   The reaction  rate  term K^  is  a function  of sludge  type,
temperature,  and  solids concentration.   It  is  a pseudoconstant,
since the  term's  value is the  average  result of many  influences.
Figure  6-37  shows  a plot  of  various  reported  K^ values as a
function of  the digestion temperature.   The data  shown are  for
several different  types  of waste  sludge, which partially explains
the  scatter.   Furthermore,  there  has  been  no  adjustment   in  the
value of K,3  for sludge  age.   At this   time, not  enough data  are
available  to  allow segregation of K^ by  sludge type;  therefore,
the  line drawn  through  the data  points  represents  an overall
average K^ value.  Little  research  has  been  conducted  on  the
effect  of  solids  concentration on  reaction rate K^.  The results
of one  study  with waste-activated  sludge  at  a temperature of
68°F  (20°C)  are  shown on  Figure  6-38,  which  indicates that Kd
decreases with increasing  solids  concentration.
 !B
 •a
 •6
 ^
 LJJ

 5
 BE
 z
 o
Ul
cc
   ,40 i-
   ,35 —
   ,30 —
.25 -
.20 -
   .15
,10 —
   .05
X
0 -
D -

A -
         -PILOT PLANT RiF
         - PI LOT PLANT REF
         - FULL SCALE HEF
         - PILOT PLANT REF
         - PILOT PLANT REF
         - Pi LOT PLANT REF
          PI LOT PLANT REF
          PILOT PLANT REF
(2071
(208)
(185!
(208)
(209)
(196)
{210}
               10        20        30       40        50

                   TEMPERATURE OF LIQUID IN AEROBIC DIGESTER, °C
                             FIGURE 6-37

              REACTION RATE K
-------
    .7
 *  .6

LU

<
QC

Z
g

o
<
HI
DC
        	1__	_L	_	L	_.  {	1      1.1     I      i     I

           6000        10,000       14,000       18,000       22,000

      TOTAL SUSPENDED  SOLIDS CONCENTRATION IN AEROBIC DIGESTER , mg/1

                            FIGURE 6-38

                 EFFECT OF SOLIDS CONCENTRATION ON
                      REACTION RATE Kd( 194)
        6.3.3.3  Oxygen Requirements

Activated sludge  biomass  is  most  often  represented  by  the
empirical equation  C5H7NO2.   Under  the prolonged  periods  of
aeration  typical of  the  aerobic  digestion process,  Equation 6-10
can be written as follows:
              7O2
5CO2 + 3H20 + H+
(6-12)
Hypothetically,  this  equation  indicates  that  1.98  pounds
(0.898 kg) of oxygen are required  to  oxidize  one  pound  (0.45  kg)
of cell mass.   From pilot  and full-scale studies, however,  the
pounds  of  oxygen  required  to  degrade  a  pound of  volatile
solids were  found  to  be 1.74  to  2.07 (0.789  to  0.939 kg).   For
mesophilic systems, a  design value of  2.0 is recommended.   For
auto-thermal systems,  which  have temperatures  above 113°F  (45°C),
nitrification does  not  occur  and a value  of  1.45  is  recommended
(187,203,204).
                               6-88

-------
The actual  specific oxygen  utilization  rate,  pounds oxygen  per
1,000  pounds  volatile  solids per  hour,  is  a  function of  total
sludge age  and  liquid  temperature  (192,199,205).   In one  study,
Ahlberg and  Boyko (199)  visited several operating  installations
and developed  the relationship  shown on Figure  6-39.   Specific
oxygen utilization  is  seen to  decrease  with increase  in  sludge
age and decrease in digestion temperature.
   uj £   8l°


   t >   6,0
   D ™
> C

-------
the process  for a  particular tank geometry.   Figure 6-40  shows
the  chart  developed  by  Envirex  Incorporated  for low  speed
mechanical aerators in noncircular  basins.   The  use  of this  chart
is explained in the design example  in Section  6.3.5.
SURFACE AREA
A^fjqus* £Mt|
(1 h "O.D8Q mj|
     BOO

     MO
                                    Pl¥OT
                                                           PtDuctP SHAFT
          i
                            FIGURE 6-40

  DESIGN CHART FOR LOW SPEED MECHANICAL AERATORS IN NON-CIRCULAR
       AERATION BASINS TO CALCULATE ENERGY REQUIREMENTS FOR
                   MEETING OXYGEN REQUIREMENTS
        6.3.3.5  pH Reduction

The effect  of increasing  detention time on  pH  of sludge  in  the
aerobic digester during mesophilic  temperature range  operation is
shown on Figure 6-41.

The drop  in pH and  alkalinity is  caused  by acid  formation  that
occurs during nitrification.   Although  at one,time the low pH  was
considered  inhibitory  to  the process, it has been shown  that  the
                                6-90

-------
system will  acclimate  and perform just as well  at the lower pH
values (186,192,213).   It should  be noted that  if  nitrification
does not take place, pH  will  drop little  if  at  all.  This could
happen at  low liquid  temperatures  and short  sludge  ages or in
thermophilic operation (203).   Nitrifying bacteria  are  sensitive
to  heat  and do not  survive in temperatures over  113°F  (45°C)
(214).
     B.O
     7.0
     6,0
  I
  a.
     5.0
     4.0
     3.0
                                    LIQUID TEMP 40°? (5°C»
             LIQUID TEMP 67°F {2Q°C}
     ,10            30            50            70

                     SLUDGE AGE IN AEROBIC DIGESTERS - DAYS

                            FIGURE 6-41

         EFFECT OF SLUDGE AGE ON pH DURING AEROBIC DIGESTION


        6.3.3.6  Dewatering

Although  there  are published  reports  of  excellent operating
systems  (193)  much  of  the literature  on full-scale  operations
has indicated  that  mechanical dewatering  of  aerobically  digested
sludge  is  very  difficult  (182,189,215).    Furthermore,  in  most
recent investigations,  it  is agreed that  the dewatering
properties of  aerobically digested sludge  deteriorate  with
increasing sludge age  (181,11,189,216).   Unless  pilot  plant  data
indicate otherwise,  it  is  recommended  that  conservative  criteria
be used for designing mechanical sludge dewatering  facilities for
aerobically digested sludge.    As  an  example,  a designer would
probably   consider  designing  a  rotary vacuum  filter  for  a
                               6-91

-------
production rate of 1.5 pounds of  dry  solids  per square  foot per
hour (7.4 kg/m^/hr), a cake  solids  concentration of 16  percent,
with a  FeCl3  dose of 140  pounds  (63.5  kg),  and  a lime  dose
(CaO) of  240 pounds (109 kg).   This assumes  an aerobic  solids
concentration of  2.5  percent  solids.    For more  detailed
information  on  results  of various  types of  dewatering  systems,
see Chapter 9.
    6.3.4  Process Performance
        6.3.4.1  Total  Volatile  Solids Reduction

Solids  destruction  has been  shown to  be primarily a  direct
function  of both  basin liquid temperature  and the length  of
time during  which  the  sludge was in the  digester.   Figure 6-42
is  a  plot of  volatile solids  reduction  versus the parameter
degree-days.  Data were  taken from both pilot and  full-scale
studies  on  several  types  of  municipal wastewater sludges.
Figure 6-42  indicates  that,  for  these  sludges,  volatile  solids
reductions of  40  to  50  percent are  obtainable  under  normal
aeration conditions.
  ui
  z
  o
  D
  O
  LU
  cc
  w
  D
  -1
  O
  w
  UJ
  O

  H

  UJ
  O
  IT
  UJ
  O-
50
40 -
30 -
      20  -
10
X
•
D
A
+
A
o
*
— PILOT
- FULL
- PILOT
- FULL
— PILOT
- PILOT
— PILOT
- FULL
PLANT
SCALE
SCALE
SCALE
PLANT
PLANT
PLANT
SCALE
REF
REF
REF
REF
REF
REF
REF
REF
(18B)
(1941
(1781
(185)
(208)
(21 It
(192)
(196!
        0    200   400   600   800  1000  1200  1400  1600  1800  20OO

                    TEMPERATURE °C x SLUDGE AGE, days

                           FIGURE 6-42

        VOLATILE SOLIDS REDUCTION AS A FUNCTION OF DIGESTER
            LIQUID TEMPERATURE AND DIGESTER SLUDGE AGE
                               6-92

-------
        6.3.4.2  Supernatant Quality

The  supernatant  from  aerobic  digesters  is normally returned
to  the head end  of  the treatment  plant.   Table  6-22 gives
supernatant  characteristics  from several  full-scale  facilities
operating  in  the  mesophilic  temperature  range.   Table  6-23
summarizes  the current  design criteria for  aerobic  digesters.
                             TABLE 6-22

                   CHARACTERISTICS OF MESOPHILIC
                   AEROBIC DIGESTER SUPERNATANT
                         Reference 196
Reference 199'
Reference 213
Turbidity - JTU
NO -N - mg/1
TKN - mg/1
COD - mg/1
PO.-P - mg/1
Filtered P - mg/1
BODc - mg/1
Filtered BODj - mg/1
Suspended solids - mg/1
Alkalinity - mq/1 CaCC>3
SO - mg/1
Silica - ing/1
PH
120
40
115
700
70
-
50
-
300
-
-
-
6.8
_
_
2.9-1, 350
24-25,500
2.1-930
0.4-120
5-6, 350
3-280
9-41,800
-
-
-
5.7-8.0
_
30
_
-
35
-
2-5
_
6.8
150
70
26
6.8
  Average of 7 months of data.

 DRange taken from 7 operating facilities.

 "Average values.
    6.3.5  Design Example

Given

Using  the  information provided  in Chapter  4,  a design  engineer
has determined  that  the  following quantities  of sludge  will  be
produced at a 0.5-MGD (22  1/s) contact  stabilization plant:
    Total daily solids generation

      Amount due to chemical  sludge
      Amount that will be  volatile
      Amount that will be  non-volatile
    1,262  pounds  (572 kg)

             0
      985  pounds  (447 kg)
      277  pounds  (125 kg)
In addition, the designer  has  the  following  information:

     •  Estimated minimum  liquid  temperature (winter)  in  digester
        is 50°F  (10°C).

     •  Estimated maximum  liquid  temperature (summer)  in  digester
        is 77°F  (25°C).
                                6-93

-------
         System  must  achieve greater than  40 percent volatile
         solids reduction  during  the winter.

         A  minimum  of two  continuously operated  tanks are required
         (see  Figure 6-36).   (This  is  a  state  requirement for
         plants under 1' MGD [44 1/s]).

         Expected waste sludge solids concentration to  the  aerobic
         digester is  8,000  mg/1.

         Expected   thickened  solids   concentration  for  the
         stabilized sludge  is  .three percent  ,(30,000  mg/1),  based
         on designer's experience.

                                TABLE 6-23

         SUMMARY OF CURRENT AEROBIC DIGESTER DESIGN CRITERIA
                                             Days
Solids residence time required to achieve
  40 percent volatile solids reduction
  55 percent volatile solids reduction
Oxygen requirements
Oxygen residual

Expected maximum solids  concentration
  achievable with decanting

Mixing horsepower
                       108
                        31
                        18
                       386
                       109
                        64
                                                         Liquid
                                                       temperature
40°F
60°F
80°F
40°F
60°F
80°F
               2.0  pounds of oxygen per pound of volatile
                 solids destroyed when liquid temperature
                 113°F or less
               1.45 pounds of oxygen per pound of volatile
                 solids destroyed when liquid temperature
                 greater than 113°F
               1.0  mg/1 of oxygen at worst design
                 conditions
               2.5  to 3.5 percent solids when dealing with
                 a  degritted sludge or one in which no
                 chemicals have been added
               Function of tank geometry and type of
                 aeration equioment utilized.  Should
                 consult equipment manufacturer.
                 Historical values have ranged from 0.5
                 to 4.0 horseoower per 1,000 cubic feet
                 of tank volume
 1 Ib = 0.454 kg                ,
 1 hp/1,000 cu ft - 26.6 kw/1,000 rri

Sludge  Age Required

Figure  6-42  (presented previously)  offers  a quick  method for
calculating  the   number  of  degree  days  required  to  achieve the
40  percent   volatile  solids  reduction  required.    The result  is
475  degree-days.    At  a basin temperature of 50°F  (10°C)  then:
     475 degree-days
        10 degrees
= 47.5 d ay s
                                   6-94

-------
Therefore,  the  volume of  the  aerobic digester must  be adequate
to provide  47.5  days sludge age  to  meet  minimum  volatile solids
reduction during the winter.

During  the  summer,  the  basin  temperature  will be  77°F  (25°C):
25°C x 47.5 day sludge age = 1,175 degree-days.

From Figure 6-42, at 1,175 degree-days, there would be 49 percent
volatile solids reduction.

Volatile Solids Reduction

For winter conditions,  there would be a  40  percent volatile
solids  (VS)  reduction.   The actual pounds  of  solids  reduced
are:

    985 Ib VS x Q>4 = 394 Ib VS reduced {179 kg/day)



For summer conditions,  there would be a  49  percent volatile
solids reduction.  The actual pounds of solids reduced are:

    985 Ib VS x o 49 = 403 Ib VS reduced (219 kg/day)
       day       '              day


Oxygen Requirements

Since  nitrification is  expected,  provisions must be  made  to
supply  2.0  pounds  of  oxygen  per  pound  of volatile solids
destroyed (2 kg 02/kg volatile solids destroyed).


    Winter conditions: 394 Ib VS dest x 2.0 Ibs 02  = 788 Ibs 02   (358 kg/day)
                           day      Ib VS dest.     day

    Summer conditions: 483 1*> VS dest   2.0 Ibs 02 = 966 Ibs O2       /
                           day      Ibs VS dest.     day    v    •*/  -r'


During  summer  conditions,  a minimum  of  1.0 mg/1  oxygen residual
must be provided.

CalculatingJTank Volume

Sludge age  in an aerobic digester  can be defined as follows:

                 	total Ib  SS  aerobic digester   	^
    Sludge  age - total lb ss iost  per day from aerobic digester


where SS =  suspended solids.
                               6-95

-------
The suspended  solids concentration  in  the digester  will  range
from the value of the influent suspended solids concentration or
8,000  mg/1  to  the  maximum value  of the thickened and stabilized
solids  concentration  of  30,000  mg/1.   On  the  average,  the
suspended solids concentration  within  the digester  is  equal to
70 percent of the thickened  solids  concentration, or 21,000 mg/1.

An  average  poundage of  suspended  solids in the supernatant
can be approximated  by  the following  equation.

    (SS concentration in  supernatant)(1-f)(8.34)(influent flow)

where  f is  the fraction of  influent  flow  into the  aerobic
digester that  is  retained,  and 1-f  is  the fraction that leaves
as supernatant.  The term f can be approximated by the following
equation.
    f _ influent SS concentration     fraction of solids
        thickened SS concentration  x   not destroyed


For winter  conditions,  the  fraction  of  solids  not destroyed is:


    1,262 Ib total solids  -  394  Ib  of solids reduced = 0 59
                 1,262 Ib  total  solids


Then, the term f for this  example,  is:
      8'000      x 0.69 = 0.18
     30,000 mg/1

Therefore,  18  percent of the  influent  flow into the  aerobic
digester  will  be  retained,  and 82  percent will  leave  as
supernatant.

For  a  properly designed  sol id s- 1 iqu id  separator   (under
200 gallons  per day per  sq  ft  [8.16  m3/day/m2]  overflow rate),
the  suspended   solids concentration  would  be approximately
300 mg/1.

The influent  flow  can be  found  by dividing the influent solids
load  (1,262 pounds  per day  [572  kg/day]  by the  influent
solids concentration [8,000 mg/1]).  The  result  is 18,914 gallons
per day (71.5 m3/day).

The pounds  of suspended solids  intentionally wasted per day from
the aerobic digestion system can  now be  approximated  from the
following expression.

(SS concentration in thickened sludge )( f )( 8 . 34) ( influent flow).

All the terms in the above equation have  been  previously defined.


                              6-96

-------
It is now possible to solve for the required tank volume for any
given sludge  age.    In  this example,  winter  conditions govern,
and  it  was  previously  calculated  that  a 47.5-day minimum was
required.  From the values previously  discussed:


    47 5 davs =      (21,000 mg/1)(8.34)(tank volume-million gallons)
          y    ((300 mg/1)(1-0.18)+(30,000)(0.18))(8.34)(0.018915 mil gal)'

    Tank volume = 0.233  million  gallons (881 m3 )


Theoretical  hydraulic detention  time:

        233,000 gallons       ,„'   ,
     	3	 = 12.3  days
     18,915  gallons per  day

This is  the minimum  volume, to  which  must be  added capacity for
weekend  storage and precipitation  requirements.  For this design,
two  tanks will  be provided, each  to have a  volume  capacity of
233,000   gallons  (881 m3)(100  percent stand-by capacity  as  per
state requirements).

The  actual  dimensions  of the  tanks depend on  the  aeration
equipment utilized  and are discussed in the following section.

Power Requirements

The  designer  has  decided  to  use  low-speed  mechanical  aerators
for mixing and oxygen transfer in  the  aerobic digester.

Previous calculations  have indicated that  the maximum  oxygen
requirement  was  966  pounds oxygen  per day  (438  kg/day).   After
making  corrections for plant elevation,  alpha  and  beta factors,
water   temperature,   and  minimum  residual  requirements,  the
engineer calculated  an overall  mass transfer  coefficient  Kj^a of
3.53 hr"1.   From  this  value,  in  conjunction with Figure  6-7,
power requirements  will  be calculated  as  follows.

Initially,  a depth  of  12 feet  (3.65  m) is  selected.   Since
each tank  is  to be  233,000  gallons  (881 m3) ,  the  surface  area
with  a   12-foot (3.65  m) liquid  depth  would  be  2,596 sq ft
(241 m2).   A pivot  point P is  located  by  placing a  straight-
edge across scales  D  and KLa of  Figure 6-40.  Then  a line is
drawn through  pivot point  p  connecting   scale As,  tank surface
area, to the  required  reducer  shaft  horsepower  scale.   The
required  shaft  horsepower for  one tank   would be  19  horsepower
(14.1 kW).   Assuming a  motor reducer efficiency  of  92 percent,
total motor horsepower  would equal  19  4 0.92, or 20.6  horse-
power (15.4 kW).  The  aerator  manufacturer  recommends  that a
minimum   10  horsepower unit (7.5 kW) will be  required  to mix the
12-foot   (3.65 m) liquid depth.   Each  10 horsepower unit (7.5 kW)
                               6-97

-------
could  mix an area  40 feet  by 40  feet (12.1 m  by 12.1 m) .   After
making  some  calculations,  the   designer  decides  to  use  two
10-horsepower  (7.5 kW)  units  in each tank,  each  tank  being
36 feet  (10.9 m) wide by  72 feet  (24.5 m)  long and having a total
tank  depth  of  14  feet  (4.2  m)  allowing 2  feet  (0.61 m)  of
free board.   Figure 6-43  shows  a view of the plan.


SUMMER CONDITIONS; 483 it» vs REDUCED/DAY - see itw o,Mav
WINTER CONDITIONS: 394 Ibn VS REDUCED/DAY - 788 Its 0,/day
EACH TANK; 72 ft LONG BY 36 fi WIDE x 13 It LIQUID DEPTH PLUS
        2ft OF FREEBOARD
                    AEROBIC DIGESTER #1
  18,915 §pd
 8,000 ma/I §s
             •**•      •

                   TANK VOLUME = 233,000
                    AEROBIC DIGESTER #2
                   TANK VOLUME <= 233,000 gal
               - 10 HP LOW SPEED MECHANICAL
                AERATOR
                                    is-
1 gpd = O.QO378 m3/day
1 cu ft = 0,0283 m3
I ft - 0.304 m
1 Ib = 0,454 kg
                    RECYCLE 30,000 mg/l as
                            iACK TO SECONDARY
                            TREATMENT
                            ' 15,51 0 gpd
                            300 mg/l ss
               WASTE STABILIZED
                  SLUDGE
                 30,000 mg/l »
                              FIGURE 6-«

       SUMMARY OF RESULTS FOR AEROBIC DIGESTION DESIGN EXAMPLE
Clarifier Surface Area

Surface  area  was  based on  an overflow rate of  200 gallons per
square  foot  per  day (8.16  m3/day/m2) .    At an  influent  flow of
18,915  gallons  per  day  (71.5 m3/day),  the  required  surface area
is  95 square feet  (8.8  m2).  The designer selected  a  12-foot
(3.7  m)  diameter  clarifier.
Supernatant Flow

It  was previously calculated  that  82  percent of  the
to  the aerobic  digester would leave as  supernatant,.
                                 influent
                                 Based  on
an  influent  of  18,915
supernatant flow  will  be
plus  any  precipitation.
gallons  per  day  (71.5
 15,510 gallons  per  day
 nH/day),   the
(58.6 m3/day),
                                 6-98

-------
    6."3.6  Cost
        6.3.6.1   Capital Cost

A  regression  analysis  of construction  bids from  1973-1977
indicated  that,  on the  basis of USEPA Municipal  Wastewater
Treatment Plant Construction Cost Index -  2nd  quarter 1977, the
capital cost could be approximated by Equation 6-13  (198).


    C = 1.47 x  105 Q1-14                                   (6-13)


where:

        C = capital cost of process in dollars

        Q = plant design  flow  in  million gallons  of  wastewater
            flow per day

The  associated costs  included those  for excavation,  process
piping,  equipment,  concrete,  and  steel.   In addition,  such
costs as  those  for administrating  and  engineering  are equal to
0.2264  times Equation 6-13  (198).


        6.3.6.2   Operation and Maintenance Cost

Although  there  are many  items that  contribute  to  operation
and maintenance cost,  in most  aerobic  digestion systems, the two
most prevalent  are staffing requirements and power  usage.

SJbaffjLng Requirements

Table  6-24  lists labor  requirements  for both operation and
maintenance.  The  labor indicated includes:   checking  mechanical
equipment,  taking  dissolved  oxygen  and solids  analyses, and
general maintenance around the clarifier.

Power Requirement^

In 1979,  the cost of power for operating  aeration  equipment has
become   a  significant factor.   It is possible  to minimize  power
consumption through two  developments in environmental  science.

     •  Make sure  that the  tank geometry- and  aeration equipment
        are compatible  (212).    The  difference  between optimized
        and unoptimized  design can mean as much as a  50 percent
        difference in power consumption.

     •  Pace devices to  control oxygen (power)  input (218).
        Because  of temperature effects,  oxygen  requirements
        for  any  given  aerobic  digestion  system can  vary as


                               6-99

-------
        much  as 20  to 30 percent between summer  and winter.
        One must  design to  meet  the  worst  conditions  (summer),
        for without some type  of  oxygen controller,  considerable
        power is wasted during other times of the year.


                            TABLE 6-24

             AEROBIC DIGESTION LABOR REQUIREMENTS (217)
Plant design flow,
     MGD
                                            Labor,
                                        man hours per year
                          Operation
Maintenance
0.5
1
2
5
10
25
100
160
260
500
800
1,500
20
30
50
100
160
300
                                                             Total

                                                              120
                                                              190
                                                              310
                                                              600
                                                              960
                                                             1,800
1 MGD = 3,786 ni /day
Other Requirements

Besides  manpower  and power  cost,  the  designer  must consider
lubrication requirements.   If  mechanical  aerators  are being
used, each unit needs to have  an  oil change  once,  and  preferably
twice, a year.  Depending  on horsepower  size,  this  could  be  5  to
40 gallons per  unit per  change (19-152  I/unit/change).   Further,
the designer must make sure an adequate  inventory  of  spare parts
are available.
6.4  Lime Stabilization

Lime  stabilization  is a  very simple  process.   Its principal
advantages over  other stabilization processes  are low cost  and
simplicity of  operation.   Evaluation of  studies  where lime
stabilization was  accomplished  at  pH ranges of 10-11,  has  shown
that odors return  during  storage  due to pH decay.   To  eliminate
this problem  and  reduce  pathogen  levels, addition  of  sufficient
quantities of  lime to raise and maintain  the  sludge pH  to  12.0
for two  hours is  required.   The  lime-stabilized  sludge  readily
dewaters with mechanical equipment  and  is  generally suitable  for
application  onto  agricultural land or disposal in a sanitary
landfill.

No direct  reduction  of organic matter  occurs  in  lime  treatment.
This has  two  important impacts.   First, lime addition does  not
make  sludges chemically  stable;   if  the  pH drops below 11.0,
biological decomposition  will   resume,  producing   noxious  odors.
Second, the quantity of sludge for disposal is not
is  by  biological  stabilization  methods.    On the
        reduced,  as  it
        contrary, the
                              6-100

-------
mass of dry  sludge  solids  is  increased  by  the lime added and by
the chemical precipitates that derive from this addition.  Thus,
because of  the  increased  volumes,  the  costs  for  transport  and
ultimate disposal are  often greater for lime-stabilized sludges
than for sludge stabilized  by  other  methods.


    6.4.1   Process Description


        6.4.1.1  History

Lime has been  traditionally used to reduce  odor  nuisances  from
open pit privies  and the graves of domestic  animals.   Lime  has
been used  commonly  in  wastewater sludge treatment  to  raise  the
pH in stressed anaerobic digesters and to condition sludge prior
to vacuum  filtration.   The original objective of lime condi-
tioning was to improve  sludge  dewaterability but, in time, it was
observed that  odors  and pathogen levels were  also  reduced.   In
1954,  T.R.  Komline filed a  patent (No. 2,852,584) for a method of
processing  raw  sludge   in  which  heavy  dosages  of  hydrated  lime
(6 to  12  percent of  total dry  solids)  were  added specifically
to  cancel  or  inhibit odors.   However, only recently  has
lime  addition  been considered a  major  sludge stabilization
alternative.

Many  studies describe the effectiveness of lime in  reducing
microbiological  hazards  in  water  and  wastewater,  but  the
bactericidal value of adding lime to sludge has been noted  only
recently (219-222).   A report  of operations  at  the  Allentown,
Pennsylvania  wastewater  treatment  plant   states  that  lime
conditioning an  anaerobically digested  sludge  to a pH of 10.2 to
11, and then vacuum filtering  and storing the  cake, destroyed all
odors  and  pathogenic enteric bacteria  (233).   Kampelmacher  and
Jansen reported similar experiences  (224).  Evans noted that lime
addition  to  sludge released ammonia  and  destroyed  coliform
bacteria and that the  sludge  cake was  a good source of nitrogen
and lime to the land (225).

Lime  stabilization  of raw sludges has been conducted  in  the
laboratory  and  in  full-scale  plants.    Farrell  and  others (226)
reported  that lime  stabilization of a primary sludge  reduced
bacterial   hazard  to a  negligible value,  improved  vacuum filter
performance,  and  provided   a  satisfactory  means  of stabilizing
sludge  prior to ultimate  disposal.  Paulsrud and Eikum (227)
determined  the  lime dosage required to prevent odors occurring
during  storage  of  sewage  sludges.   Primary  biological sludges,
septic  tank sludges, and different chemical sludges were used in
the study.   An important  finding was  that lime dosages greater
than those  sufficient  to initially  raise  the  pH  of the sludges
were required  to prevent pH decay and the  return of odors during
storage.   Laboratory and pilot scale work by Counts and Shuckrow
(228)  on  lime stabilization showed significant reductions in
pathogen populations and obnoxious  odors when the  sludge pH was


                             6-101

-------
greater than 12.   Counts  conducted  growth studies on greenhouse
and outdoor plots which  indicated that  the disposal of  lime-
stabilized domestic sludge on cropland would have no detrimental
effect  on  plant  growth  and soil  characteristics.   Disposal
of  the  lime-stabilized  domestic sludge  at  loading rates  up  to
100 tons dry solids per acre (224 t/ha) on green-house plots and
40  tons dry solids per acre  (90  t/ha) on outdoor plots had  no
detrimentatal effect  on  plant growth.-and soil characteristics.

A  full-scale  lime stabilization facility  was  built as  part
of  a  1-MGD (43.8 1/s )  wastewater treatment plant in Lebanon,
Ohio.   Operation began  in 1976.  'A  case  study  of lime treatment
and land  application of  sludge from  this  plant, along with a
general economic comparison  of lime stabilization with anaerobic
digestion,  is available  (229).
        6.4.1.2  Current  Status

As of  May 1978,  lime  treatment  is being used  to  stabilize  the
sludge from at  least 27 municipal wastewater treatment plants in
Connecticut.    Average  wastewater flows  treated  at  these  plants
vary  from 0.1  to 31 MGD  (4.4  to 1358  1/s).   In  most  of  the
plants,  incinerators  have  been  either  wholly  or partially
abandoned.  While  few  chemical  or  bacterial data are available,'
qualitative observations  indicate that  treatment  is satisfactory.
Most of  the  communities  have indicated  that  they  will  continue
with lime stabilization.

Landfill  burial  is  the  most   common  means  of  disposal  for
lime-stabilized sludge.   However, lime-treated sludge from eight
of the plants  in  Connecticut  is  applied  onto  land.   At  Enfield,
Connecticut,  dewatered  sludge  is  stockpiled  in large mounds.  The
sludge is  spread  onto  cornfields when  application  is compatible
with  crop cycles  and  weather  conditions.   Few nuisances  are
caused by the practice. Odors have  not been a problem, even when
piles  have  been opened  for  spreading of  the sludge. ..  In
Willimantic,  Connecticut,  lime-stabilized  sludge is  mixed with
leaves  and grasses.  After stockpiling,  a portion  of mixture
is screened and distributed to local  nurseries.   The remainder is
used as final cover for  landfill.
        6.4.1.3  Applicability

Lime stabilization can be an  effective alternative when there is
a need to provide:

     o  Backup for existing stabilization facilities.     A  lime
        "stabilization system can be  started  (or stopped), quickly.
        Therefore, it can be  used  to supplement  existing sludge
        processing  facilities when  sludge  quantities  exceed
        design  levels,  or  to replace  incineration  during  fuel
                             6-102

-------
        shortages.   Full  sludge  flows can be  lime-treated  when
        existing facilities are  out  of service for  cleaning  or
        repair.

     •  Interim  sludge  handling.  Lime stabilization systems  have
        a comparatively low capital cost and,  therefore, may  be
        cost effective  if  there are plans to abandon the plant  or
        process  within  a few years.

     •  Expansion of existing facilities or_c_ons^tructioa of new
        facilities to improve  odor and pa'thog^n~cc)ntrol.     Lime
        stabilization  is particularly  applicable  in small plants
        or when  the plant  will be loaded only seasonally.

In all  cases, a  suitable  site for  disposal or  use  of stabilized
sludge is required.


        6.4.1.4   Theory of  the Process

Lime addition to sludge  reduces odors and  pathogen levels  by
creating  a  high  pH  environment  hostile to  biological  activity.
Gases  containing nitrogen and  sulfur that  are  evolved during
anaerobic decomposition  of organic  matter are  the principal
source  of odors  in sludge (228).   When  lime  is  added,  the
microorganisms   involved   in  this decomposition  are strongly
inhibited or destroyed   in  the  highly  alkaline  environment.
Similarly,  pathogens  are  inactivated  or  destroyed  by   lime
addition.

High lime dosing of sludge  also affects the chemical and physical
characteristics of  sludge.    Although  the  complex chemical
reactions between lime  and sludge are  not well  understood,  it
is likely that  mild reactions,  such  as  the  splitting of  complex
molecules by hydrolysis, saponification, and acid neutralization,
occur  in  the  high pH  environments  created  in lime stabilization
(228).   These  reactions   reduce the  fertilizer  value  of the
stabilized  sludge,  improve its  dewaterability,  and  change the
character of  liquid sidestreams.   The nature  of  these  chemical
changes is described in Section 6.4.3.4.


    6.4.2  Design Criteria

Three  fundamental design  parameters  must  be considered in  the
design of  a  lime  stabilization  system:   pH,  contact time,
and lime  dosage.  At this  early  stage  in  the development of the
process, the selection  of  the levels of these parameters has  been
largely  empirical.  The  results  of  earlier  studies now can  be
used  as  a  starting  point,  but  because  of the  complexity  of
chemical  interactions  that apparently occur in lime treatment  of
sludge, bench-scale  and pilot  studies are  recommended as  part  of
designing a  large-scale  system,  particularly  if  substantial
departures from  these conditions  are contemplated.


                            6-103

-------
        6.4.2.1   pH  and  Contact Time


The  primary  objective  of  lime  stabilization  is  to inhibit
bacterial decomposition  and  inactivate pathogenic organisms.   The
extensive  use of  lime  is  medicinal;  the masking  of noxious
odors from  decaying  substances permits  uncritical  acceptance  of
its use  for  sludge  treatment.   Nevertheless,  evidence  is  needed
of its value  and of  the necessary  dose  levels  and  contact  times
for effective treatment.

The  effective factor   in lime  treatment  is evidently the  pH
level and not just the  dose of lime.   As  with  most disinfection
processes,  the time  of  exposure (the extensive factor) is equally
as important as  the  pH  (the  intensive factor).  Investigations  by
Farrell and others  (226), Counts and  Shuckrow  (228),  and  Noland
and  others  (229),  have established  time,  pH,  and processing
conditions   for  producing  satisfactory  lime  stabilization.
Process  performance  is discussed  in a  subsequent section
(Section 6.4.3).

The  design  objective  is to maintain  pH above  12  for  about  two
hours  to ensure pathogen destruction, and  to  provide enough
residual alkalinity  so  that the pH does  not drop below 11  for
several  days,  allowing sufficient  time  for disposal or  use
without the  possibility  of renewed putrefaction.  The recommended
design criteria  for  accomplishing these  objectives are:

     •  Treat sludge  in  the  liquid state.

     •  Bring the sludge to  pH 12.5  by lime addition and maintain
        pH above 12.5 for 30 minutes (which keeps pH >12 for  two
        hours).

Farrell  and others  (226)  attempted  to determine whether  the
additions of lime that would occur in conditioning of sludge  for
dewatering  would produce adequate stabilization.  They mixed
liquid sludge with lime for two  minutes,  and then dewatered  the
sludge on a  Buchner  funnel.   Their results indicated inadequate
bacteriological destruction.   Later  results  by Strauch  and
others  (230)  in  Denmark and  unpublished  results  at Downington,
Pennsylvania (See Section 6.4.4.1)  indicate that special reaction
conditions or intense mixing of sludge cake with lime can produce
satisfactory results.


        6.4.2.2   Lime Dosage

The amount of lime required  to stabilize sludge is determined by
the  type of sludge,  its chemical  composition,  and the  solids
concentration.  Table 6-25 summarizes the results of plant-scale
tests  at Lebanon,  Ohio,  and  shows that  lime  additions ranging
from 6 to 51  percent  of the total  dry solids in the sludge  were
required to  raise  the  pH to the levels indicated  in the  table.
These lime   dosages  were sufficient to  keep  the  sludge pH above


                            6-104

-------
12.0  for  30 minutes.   Primary sludges  required  the  lowest
dosages, while the highest average dosages were required to raise
the pH  level  of  waste-activated sludges.   The  results  of studies
conducted  by  Paulsrud and  Eikum (227)  agree  generally  with  the
Lebanon  tests and are displayed in  Table  6-26.   Iron  and  alum
sludges  required  the  highest dosages.   Farrell,  and  others (226)
also found  that  alum additionally increased the  lime requirement
and suggested that  part of the  lime  added  to alum sludge  may be
bound as a calcium-aluminum compound.
                            TABLE 6-25

           LIME REQUIREMENT TO ATTAIN pH 12 FOR 30 MINUTES
                       AT LEBANON, OHIO (228)
Solids
concentration,
percent
Sludge type
Primary sludge
Waste activated sludge
Anaerobically digested mixed
sludge
Septage
Range
3-6
1-1.5

6-7
1-4.5
Average
4.
1.

5.
2.
3
3

5
7
0
0

0
0
Lime
Ib
dosage,
Ca(OH) 2
pH, average
Ib dry solids
Range
.06-0.
.21-0.

.14-0.
.09-0.
Average
17
43

25
51
0.
0.

0.
0.
12
30

19
20
Initial
6
7

7
7
.7
.1

. 2
.3
Final
12.7
12.6

12.4
12.7
 Includes some portion of waste activated-sludge.

 1 Ib/lb = 1 kg/kg.


                            TABLE 6-26
              LIME DOSES REQUIRED TO KEEP pH ABOVE 11.0
                       AT LEAST 14 DAYS (226)
                Type of sludge
                              Lime  dose,
                            Ib Ca(OH}2/lb
                           suspended solids
Primary sludge
Activated sludge
Septage
Alum-sludge
                      '
                        jlus  pri-
  mary sludge1
Iron-sludge^1
0.10
0 ,30
0.10
0.40
- 0.15
- 0.50
- 0.30
- 0.60
                                        0.25 - 0,40
                                        0.35 - 0.60
           aPrecipitation of primary treated effluent,

            Equal  proportions by weight of each type
            of  sludge.
           1
   b

b = 1 kg/kg.
                              6-105

-------
Figure 6-44 displays the general relationship between  lime dosage
and pH  for  a typical municipal  sludge  at several solids concen-
trations.  Table  6-26  calculated from data on Figure  6-44,  shows
that  the lime dose  per unit  mass  of sludge  solids  required  to
attain  a particular pH  level  is relatively  constant.   That  is,
lime  requirements  are  more closely related  to  the total mass  of
sludge  solids, rather than the sludge  volume.  Consequently,
reduction in  volume  by thickening may have  little  or no effect
on the amount of lime required, because the mass of sludge solids
is not changed.
       12


       11


       10


        9


        8
                    o  1.0% SOLIDS
                    A  2.0% SOLIDS
                    o  3.0% SOLIDS
                    «  3.5% SOLIDS
                    •  4.4% SOLIDS
                  2,000
4,000
6,000
8,000
10,000
                            Ca(OHl2 DOSE, rng/l

                            FIGURE 6-44

    LIME DOSES REQUIRED TO RAISE pH OF A MIXTURE OF PRIMARY SLUDGE
 AND TRICKLING FILTER HUMUS AT DIFFERENT SOLIDS CONCENTRATIONS (228)

Lime additions must be sufficient to ensure  that  the pH of  sludge
does not  drop below  the desired level  after  prolonged storage.
If  insufficient  lime is added, the pH  will  decay as the treated
sludge ages  (227-229).    This  phenomenon  is displayed on
Figure 6-45.  Notice  that  higher lime dosages not only raise  the
initial pH but, more  importantly, prevent, or at  least delay,  the
drop in pH levels.  Consequently, in practice, lime doses must be
greater  than  that sufficient to  raise the  pH to the  desired
value.   In  most  cases, significant  pH decay will  not  occur if
enough lime  is added  to raise the  sludge pH to 12.5 and maintain
that value for at least  30 minutes  (229).
                             6-106

-------
                                       02BlbLlME DOSAGE Ca{QH)2/!bSS
      12
      10
              SS=5.07%,VSS/S£=74.6%
              TEMP,=6S°F!20SCS
        1lfa/lb= 1 kg/kg
  12      16

 DAYS OF STORAGE



FIGURE 6-45
                                             20
24
           CHANCE IN pH DURING STORAGE OF PRIMARY SLUDGE
                    USING DIFFERENT LIME DOSAGES
Several mechanisms  of  pH decay have  been  proposed -tnd some have
been documented  (227,228).   The  initial pH drop results from the
uptake  of  atmospheric  CC>2  and slow  reactions of  hydroxyl ions
with sludge solids.  The rate of pH reduction  is accelerated once
the  pH reaches  a point at which  bacterial  action  can  resume
production  of  organic acids  through  anaerobic  microbial
degradation.

The  foregoing discussion makes  it clear that a dose level  cannot
be defined without reference  to the  specific sludge.   Actual
dose  levels  will  have  to  be determined  in bench-scale  tests.
Approximate levels  can  be  selected  from the information above in
order to establish size  of equipment and to  estimate costs.


    6.4.3  Process Peformance

Lime stabilization reduces odors and odor production potential in
sludge, reduces pathogen levels, and alters  dewatering, settling,
and  chemical characteristics  of  the  sludge.   The  nature and
extent  of  the  effects -produced  are  described in  the following
paragraphs.
                             6-107

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        6.4.3.1  Odor Control
Lime  treatment  deodorizes  sludge by  creating  a high pH
environment  in  the  sludge,  thus  eliminating  or suppressing  the
growth of  microorganisms  that  produce malodorous gases.  In  one
laboratory study  (228),  the  threshhold odor number of  raw mixed
primary and  trickling filter  sludges was  8,000,  while that of
lime-treated sludges ranged  between 800 and 1300.  The  threshold
odor  number defines  the greatest  dilution of  the  sample with
odor-free  water  to  yield  the  least definitely perceptible  odor
(231).  Sufficient lime must be added  to retard  pH decay because
odor generation will  generally  resume once the pH of  the sludge
falls below pH 11.0  (220,228).

Hydrogen  sulfide  (H2S),   a  malodorous gas  present  in  dissolved
form  in sludge,  is  a major  cause of  sludge odors.   Figure  6-46
shows that, as the pH of  sludge is  raised,  the  fraction of total
sulfide  in  the  H2S  form decreases  from  about 50  percent at
pH 7  to essentially  zero  at  pH 9.  Consequently, above this  pH,
there is no longer any H2S odor.
  HI
  O
  LL.
  *J
  3
          LL
          O
          I-
          z
          CE
          UJ
          a.
            100
             80
             60
     40
     20
              0
                                 MOLAR SOLUTION
                                      10
        6789

                  pH

                    FIGURE 6-4S

EFFECT OF pH ON HYDROGEN SULFIDE-SULFIDE EQUILIBRIUM
During full-scale operations at the Lebanon plant (229),
intense  when  septic raw  sludge was  first pumped  to
stabilization  mixing  tank.     Odor
diffused  air  was applied for  mixing.
sludge odor was masked by the odor of
from  the sludge  by  the air bubbled
                                                 odor was
                                                the  lime
                               intensity  increased  when
                                When lime was  added,  the
                              ammonia,  which was stripped
                              through  the mixture.   The
                             6-108

-------
ammonia  odor was  most intense with anaerobically digested sludge
and  was  strong  enough  to  cause  nasal  irritation.   As  mixing
proceeded,  the treated sludge acquired a  musty, mucus-like odor.


         6.4.3.2  Pathogen  Reduction

Significant pathogen reductions  can  be  achieved  in sludges  that
have  been  lime-treated  to pH  12.0  (228,229).   Table  6-27 lists
bacteria  levels   measured   during  the   full-scale  studies  at
Lebanon  and shows  that lime  stabilization of raw sludges reduced
total  coliform, fecal  coliform, and fecal  streptococci concentra-
tions  by more  than 99.9 percent.   The numbers  of Salmonella and
Pseud omonas   aerug inosa were  reduced below  the level  of
detection.   Table  6-27 also  shows  that pathogen concentrations in
lime-stabilized sludges  ranged from  10  to 1,000  times less  than
those  in anaerobically digested sludge from the same plant.
                             TABLE 6-27

            BACTERIA IN RAW, ANAEROBICALLY DIGESTED, AND
            LIME STABILIZED SLUDGES AT LEBANON, OHIO (228)
                                   Bacterial density,  number/100 ml
     Sludge type
 Total
coliform9
 Raw
  Primary
  Waste-activated
  Septage

 Anaerobically digested
  Mixed primary and
    waste-activated

 Lime stabilized
  Primary
  Waste-activated
  Septage
  Anaerobically
    digested
2.9 x 10
8.3 x 10
2.9 x 10C
8
 2.8 x 10
1.2 x 10
2.2 x 10
2.1 x 10

   18
       7
    Fecal
   coliform
8.3 x 10s
2.7 x 10J
1.5 x 10'
          1.5 x 10
          5.9 x 10-
          1.6 x 10
            265

             18
           Fecal
         streptococci
3.9 x 10!
1.0 x ' a.
             6.7
                  10-
                    2.7 x 10-
             1.6 x lOJ
             6.8 x 10:
               665
             1.6 x 10-
                                                Salmonella
                        Ps.
                     aeruginasa
                                 62
                                  6
                                  6
                        <3
                        <3
                        <3

                        <3
  195  .
5.5 x 10-
  754
                                            42
                        <3
                        13
                        <3

                        <3
 aMillipore filter technigue used for waste-activated sludge and
  septage.  MPN technique used for other sludges.
  To pH equal to or greater than 12.0.
 CDetection limit = 3.
Information on  virus destruction in  sludge by lime  stabilization
is  scant.    There  are  numerous  investigations  on  removal of
viruses  from  wastewater  by  lime  flocculation  but  little on
destruction  of  viruses  by  elevated  pH.    A study  by  Berg  (233)
measured the structure  of a polio virus in water by  pH  adjustment
alone,  and indicate very rapid destruction above pH  11.   Similar
effects would be expected for other  animal viruses.
                               6-109

-------
Qualitative observation under a microscope has shown substantial
survival of higher  organisms,  such  as hookworms,  amoebic cysts,
and  Ascaris  ova,  after  contact  times of  24  hours  at  high
pH  (226).   It  is  not known  whether long-term contact  would
eventually destroy  these  organisms.   A more complete discussion
of sludge disinfection is  contained  in Chapter 7.


        6.4.3.3   Dewatering and Settling  Characteristics

Lime has been  used extensively  as  a  conditioning agent to improve
the  dewaterability of  sludge.   Trubnick  and  Mueller  (234)
presented  detailed procedures to  be followed in conditioning
sludge  for filtration,  using  lime  with  and  without  ferric
chloride.  Sontheimer  (235) described  the improvements in sludge
filterability produced  by  lime  addition.   A  more  detailed
discussion of  lime conditioning is  contained in Chapter 8.

The addition of lime has  been  shown  to improve the filterability
of alum  and iron  primary  sludges  (226).  Specific resistance was
reduced  by a  factor of approximately four,  and  filter yield was
increased  by  a  factor  of  two  when  lime  conditioning  was used.
Counts and Shuckrow (228)  studied  the effect of lime  treatment on
the filterability of  primary sludge and  trickling filter sludge
but could not  detect any consistent  trend.

The  impact of lime stabilization  on sand bed  drying  of sludge
has  been examined  by  several  researchers  (226,228,229).   Lime
additions  to  raw  sludge  increased  the rate of  drying  at least
initially  and,  in one  study,  produced  a drier   final  cake.
However, lime-treated primary  sludge  did  not dry as  fast as
either  lime-treated  or untreated  anaerobically  digested sludge
(229).

The  settling  of   lime-stabilized  primary and mixed  sludges  was
enhanced in one  study (228),  indicating  that gravity thickening
after lime treatment  may  be  used  to reduce the volume of sludge
to be dewatered.
        6.4.3.4  Chemical Characteristics

Lime  stabilization  causes  chemical changes in  the  sludge.   The
nature of  these  changes  is illustrated in Tables 6-28 and  6-29,
which compile chemical data from two studies.  The general effect
of lime addition is a reduction in component concentration.  This
is caused by both  dilution with  lime slurry  and  loss  of some
volatile sludge components to the atmosphere.

Lime-stabilized sludges have  lower  concentrations  of  soluble
phosphate,  ammonia  nitrogen,  and  total  Kjeldahl  nitrogen than
anaerobically digested sludge  from the same  plant,  as shown  in
Table 6-28.   These  lower nutrient  levels  reduce the agricultural
value of  the sludge  but,  assuming nitrogen  limits  the  rate-  at


                             6-110

-------
                                                    reaction
                                            form  calcium-phosphate
                                            phosphate  in the  super-
                                             believed  to be  largely
which sludge  can  be applied,  would  allow more sludge to  be
applied  per  acre of  land.   A reduction  in  the  soluble  (filter-
able)  phosphate  concentration  is caused  by  the  reaction  between
lime  and  dissolved  orthophosphate to
precipitate.    For this  reason,  residual
natant/filtrate  after lime  treatment is
organic  in nature (228).   Nitrogen  levels can  be reduced  during
lime  stabilization if  gaseous  ammonia  is  stripped  during  air
mixing of  the  treated sludge.  As  the pH  of  the sludge  increases
from  near  neutral to  12,  the  predominant form  of ammonia  shifts
from  the  ammonium  ion  (NH4+) to  dissolved  ammonia  gas  (NH3).
Some  of  this  gas is  carried off by  the  air  bubbled through  the
sludge for mixing.
                              TABLE 6-28

    CHEMICAL COMPOSITION OF SLUDGES AT LEBANON, OHIO, BEFORE AND
                    AFTER LIME STABILIZATION (228)
                                            s, mg/1
    Sludge type

Primary
 Before lime addition
 After lime addition
Waste activated
 Before lime addition
 After lime addition

Anaerobically digested
 mixed sludge
  Before lime addition
  After lime addition

Septage
 Before lime addition
 After lime addition
              Alkalinity
                1,885
                4,313
                1,265
                5,0'JO
                     Total.
                      COD
54,146
41,180
12,310
14,700
                     (.6,372
                     IB,670
                1,897
                3,475
                          Soluble
                           COD
3,046
J,55b
1,043
1,618
      1,011
      1,809
      1,223
      1,537
            Total
           phosphate
       Total
Soluble  Kjeldahl
phosphate  nitrogen
           Ammonia
           nitrogen
                        Total
                       suspended
                        solids
                                    Volatile
                                    suspended
                                     solids
35U
283
218
263
       580
       381
       172
       134
69
36
85
25
       15
       2.9
       25
       i.4
     1,656
     1,374
        711
       1,034
      2,731
      1.78U
              223
              145
            51
            64
              70S
              494
            VI
            110
48,700
38,370
12,350
10,700
                 61,140
                 66,350
                   21,120
                   23,1'K
36,100
23,480
10,000
 7,136
      33,316
      26,375
                        12,600
                        11,390
A  direct  result of  adding lime to  sludge is  that  the  total
alkalinity  will rise  to  a high value.   This  can  affect  the
suitability  of  the treated sludge for land  application.   The
input  can be  positive  or  negative,  depending  on soil  conditions
at  the  application  site.   Data in  Table 6-28  indicates  the
magnitude  of  change in  alkalinity.

Biochemical  oxygen demand, chemical  oxygen  demand,  and  total
organic  carbon  concentrations  increase  in  the  liquid  fraction of
wastewater sludges  when lime is  added  (228,229).  Organic matter
is  dissolved in the high pH  environment.  Possible reactions
involved include  saponification  of fats  arid oils, hydrolysis and
dissolution  of  proteins,  and  decomposition of  proteins  to form
methanol (228).

Lime  stabilization usually does  not  produce  the  substantial
reductions  in  volatile matter  associated with anaerobic  and
aerobic  sludge  digestion.    However,  volatile  solids  concentra-
tions  decreased  by  10  to  35 percent  after  lime  additions  in the
                               6-111

-------
plant-scale studies  at Lebanon  (229),  as shown  in  Table 6-28.
Reductions in total solids  concentration  after lime stabilization
were measured by  Counts  and  Shunckrow (228).   These reductions,
displayed  in Table  6-29,  are greater  than  can  be accounted for
simply by  dilution with lime slurry.   It may  be simply that the
lime  interfered with  the  volatile solids  analysis.   However,
reactions between lime  and  nitrogenous  organic matter may cause a
loss of sludge  solids.   Hydrolysis  of  proteins and destruction of
amino  acids  are  known to  occur  by reaction  with strong bases.
Volatile  substances  such  as ammonia,  water,  and  low  molecular
weight amines or  other  volatile  organics may  possibly  be formed
and lost to the  atmosphere.
        6.4.4  Process  Design

A  lime  stabilization  operation is  divided  into  two operations:
lime  handling  and sludge mixing.   Lime  handling  comprises
facilities  for receiving  storing,  transporting,  feeding,  and
"slurrying"  of the lime.  The  sludge mixing operation consists of
a  holding  tank provided with  mixing. A  discussion  of  design
considerations for these  two operations  follows.
        6.4.4.1  Design of  Lime  Handling  Facilities

Lime,  in  its various  forms,  is the principal and  lowest cost
alkali used in industry and wastewater  treatment.  As a result, a
substantial  body  of knowledge  has  evolved concerning  the most
efficient  handling of  lime.   Only the  basic elements  of lime
system design are described in  this manual.  Detailed information
is contained  in  several  references  that  focus on the selection,
handling,  and use of lime (236-239).

Lime Character is ti_cs_

Lime is a general term applied  to several chemical compounds that
share  the  common characteristic of being  highly  alkaline.   The
two forms commercially available are quicklime  (CaO)  and hydrated
lime  (Ca(OH)2).   The characteristics  of  these two  chemicals
are summarized in Table 6-30.   Lime  is  a  caustic material and can
cause  severe  injury  to tissue,  particularly to eyes.  Equipment
must  be  designed with safe handling in  mind;  eyewash fountains
and safety  showers  should  be  provided, and operating procedures
should mandate  use  of  proper  handling  procedures and protective
clothing.

Quicklime   is  derived  from limestone  by a  high  temperature
calcination process.   It  consists primarily of the  oxides of
calcium and magnesium.   The grade of quicklime  most commonly used
in wastewater treatment contains 85  to  90 percent CaO.
                             6-112

-------
                                    TABLE 6-29

             CHEMICAL COMPOSITION OF SLUDGE AND SUPERNATANT
                 BEFORE AND AFTER LIME STABILIZATION3 (227)
           Parameter
        Primary
        sludge
Trickling filter
     humus
Mixed
sludge
Whole sludge
  PH
    Before lime addition
    After lime addition

  Total solids (wt percent)
    Before lime treatment
    After lime treatment

  Total alkalinity (mg/1 as
    CaCO3)
      Before  lime addition
      After lime addition

  Ammonia nitrogen (mg N/l)
    Before lime addition
    After lime addition

  Organic nitrogen (mg N/l)
    Before lime addition
    After lime addition

  Nitrate nitrogen (mg N/l)
    Before lime addition
    After lime addition

  Total phosphate (mg P/l)
    Before lime addition
    After lime addition

  Filterable  phosphate (mg P/l)
    Before lime addition
    After lime addition

Supernatant
  TOC  (mg/1)
    Before lime addition
    After lime addition

  BOD  (mg/1)
    Before lime addition
    After lime addition

  Threshhold  odor number
    Before lime addition
    After lime addition

  Total solids (wt percent)
    Before lime addition
    After lime addition
           6.0
          12.1
           3.6
           3.2
         1,141
         6,920


           211
            91


         1,066
         1,146


             3
            25


           342
           302


            92
            32
         1,000
         2,083


         1,120
         1,875


         4 ,889
           467
           0 .1
           0.6
       6.3
      12.3


       3.0
       2.7
     1,151
     6,240


       274
       148

     1,179
       995
         7
        22


       305
       235


        96
        17
       917
     1,883


       964
     1,981


     5,333
       333


       0. 1
       0. 5
  6. 1
 12.0


  3.6
  3.3
1,213
5,760


  192
   87


1,231
1,099


   16
   31


  468
  337


   80
   31
 1,175
 2,250


 1,137
 2, 102


  933
   67
  0.2
  0.7
 Values in this table are averages of three  tests for each  sludge type.

 °The greatest  dilution with  odor-free water  to yield the least perceptible  odor.
Quicklime  is  rarely  applied  directly  (that  is,   in  a   dry
condition)  to  the  sludge.   First  it  is  concerted  to hydrated  lime
by  reaction  with water  in  an  exothermic  reaction  called  slaking.
     CaO 4-  H2O
Ca(OH\2 +  Heat
                                     6-113

-------
During slaking, the generally coarse CaO particles are  ruptured,
splitting  into  mi croparticles of  Ca(OH)2«   These  smaller par-
ticles have a  large  total surface area and are highly  reactive.
The slaking reaction is carried  out  under closely  controlled
conditions to promote  maximum lime reactivity.

                           TABLE 6-30

          CHARACTERISTICS OF QUICKLIME AND HYDRATED LIME
Common name/
formula
Quicklime/
CaO









Available
forms
Pebble
Crushed
Lump
Ground
Pulverized






Containers and
requirements
80-100 Ib moisture-
proof bags, wooden
barrels , and car-
loads. Store dry,-
maximum 60 days in
tight container -
3 months in mois-
ture-proof bag.



Appearance and
properties
White (light grey,
tan) lumps to
powder . Unstable ,
caustic irritant.
Slakes to hydrox-
ide slurry evolving
heat (490 Btu/lb) .
Air slakes to
CaC03. Sat. sol.
approximately pH
12.5
Weight, Ib/cu ft
(bulk density)
55 to 75; to calcu-
late hopper capa-
city - use 55; Sp.
G. , 3.2-3.4.







Commercial
strength
70 to 96 percent CaO
(Below 88 percent
can be poor quality)








Solubility
in water
Reacts to form Ca(OH)2
each Ib of quicklime
will form 1.16 to
1.32 Ib of Ca(OH)2,
with 2 to 12 percent
grit, depending on
purity.




Hydrated lime/
Ca(OH)2






Powder 50 Ib bags, 100 Ib White, 200-400 mesh;
(Passes 200 barrels, and car- powder free of
mesh) loads. Store dry; lumps; caustic,
sorbs «2° and CO2
from air to form
Ca(HC03)2. Sat.
sol. approximately
pH 12.4.
25-40; to calculate
hopper capacity -
use 30; Sp. G. ,





Ca(OH)2 - 82 to 98
percent; CaO - 62
to 74 percent (Std.
percen




10 lb/1.
70°F
5.6 lb/1





000 gal at

,000 gal at





1 Ib - 0.454 kg
100 Btu/lb = 55 kg-cal/kg
1 Ib/cu ft = 16 kg/m3
1 lb/1,000 gal = 0.120 g/1

If  slaking  is done  by  the lime  manufacturer,  hydrated lime  is
delivered  to  the wastewater  treatment  plant.   The  manufacturer
adds only enough water  for hydration,  producing  a dry Ca(OH)2
powder.   At  the  waste treatment plant,  the powder  is then  slur-
ried with more water prior to  mixing with  sludge.   Alternatively,
slaking may be carried out at  the wastewater treatment plant; the
delivered product  is,  therefore,  quicklime.   In  this  case, the
lime is  slaked, then diluted  (if  necessary) prior to process
application.

Direct  addition  of  dry quicklime to  sludge and  without the use
of  a  separate slaker,  is practiced  in Denmark  in  at   least
ten Swedish  treatment plants.   Potential advantages are the
elimination  of  slaking  equipment  and  the generation  of  heat,
which  can  improve  pathogen  reduction  and  speed dewatering
through  evaporation.   In one  case (230), direct additions  of
dry quicklime  were  made  to raise sludge pH above  13.0  and  bring
the temperature  to 176°F  (80°C).    Salmonella and intestinal
parasites were  killed within two hours.   Heat  generated  by
slaking  of  quicklime  does not  raise temperature  significantly
unless  the sludge is  dewatered  and  the lime  dose is high—on the
order of 400 to 800 Ib per ton dry solids  (200-400  kg/t).

The decision  whether to  purchase quicklime or  hydrated lime
in  a particular  situation  is  influenced  by a number of  factors
such as  size  of treatment  facility,  material  cost, and storage
                             6-114

-------
requirements.   The  cost  of hydrated  lime  is about 30  percent
greater  than  the cost  of  quicklime with an  equivalent  calcium
oxide  content.   The difference is  due  to the higher production
and  transportation costs  for hydrated lime.   Nevertheless,
hydrated lime is preferred for small-scale  operations mainly
because  its use  eliminates  the labor and equipment required for
slaking.   Hydrated  lime  is also more  stable and therefore  is
easier to  handle  and to store.  When  lime  use  exceeds  three  to
four  tons per day   (3,000-4,000  kg/day),  quicklime should  be
considered because  of  its  inherent economy  (236).  Selection
of the type of lime to  be  used should  be  based on a detailed
economic analysis, taking into account all the unique factors  of
the particular application.

Both  quicklime  and  hydrated  lime  react  spontaneously  with
atmospheric CC>2.


    CaO + C02   	>   CaCC>3

    Ca(OH)2 +  C02  	>   CaC03  + H20


In addition, quicklime  can  be slaked by  the  water  vapor  in the
air.

These reactions cause two  problems:

     •  Lime quality  is  degraded  because the  reaction  product,
        CaCC>3,  is ineffective  in raising  pH.

     •  The partial  reaction  with  C02,  and  in  the  case  of  CaO,
        with water  vapor,  causes  caking.   This  interferes  with
        lime slaking and  feeding.

Thus,  lime storage,  slaking, and  feeding  equipment should  be
sealed to  as  great  a  degree  as possible to  prevent  contact  of
lime with atmospheric C02  and  water  vapor.

D.e^i.vg.ry__and__S_tg_rag_e _qf Jjime

Lime  can be delivered  either in bags  or in  bulk.  The choice
depends  mostly  on  the rate  of  chemical use at the treatment
plant.   Bagged lime  costs about 20  percent  more than bulk  lime,
but it is  generally  preferred where daily requirements  are   less
than 1000  to  1500  pounds  of lime  per day (236).   At this  small
scale, handling and  storage of bagged lime is relatively simple,
involving manual labor or simple  mechanical aids.   As  the  scale
of operation increases, it  becomes  more  efficient and economical
to use  bulk lime,  which  can  be delivered  in large quantities,
transported in  mechanical  or  pneumatic  conveying  systems,  and
stored in weather-tight  bins or silos.

Bagged  lime must  be stored  under cover to  prevent rain  from
wetting  the bags.   Proper  handling  is especially important  when
quicklime  is  used,  because it is  highly reactive with water,


                             6-115

-------
producing heat  and  swelling that  can  cause the  bags  to burst.
Because  heat  can be  generated  during accidental slaking  of
quicklime,  bags  should never  be stored  close to  combustible
materials.

Hydrated lime may be  stored under dry  conditions for periods  up
to a year without serious  deterioration by reaction with atmos-
pheric CC>2  (recarbonation) .   Quicklime deteriorates more rapidly.
Under  good  storage  conditions,  with  multiwall moisture-proofed
bags, quicklime may  be held  as  long as six  months, but in general
should not  be stored for more than three months  (236).

Bulk quicklime  and  hydrated lime can be  stored in  conventional
steel or concrete silos or bins.  The storage facilities must  be
airtight to prevent slaking  and  recarbonation.   Pebble quicklime
is free-flowing and will discharge  readily from storage bins  if
the  hopper  bottoms  have a  minimum slope of  60  degrees  from the
horizontal.    Pulverized quicklime and  especially  hydrated  lime
have  a tendency  to  arch  and   therefore  require  some  type  of
mechanical  or aeration agitation to ensure  uniform discharge from
storage  bins.  Detailed descriptions  of  the  various  types  of
flow-aiding  devices  can be  found elsewhere  (236,240).

Storage facilities should be sized on daily lime demand, type and
reliability   of  delivery,  future  chemical  requirements,  and  an
allowance for flexibility and  expansion.   As a minimum, storage
should be provided  to supply a  seven-day  lime  demand;  however,
sufficient  storage  to supply  lime  for two  to three  weeks  is
desirable.  In  any  case,  the   total  storage  volume should  be
at least 50 percent  greater than the  capacity  of  the delivery
railcar or truck  to  ensure  adequate  lime  supply between
shipments (236 ) .

Lime Feeding

Lime is nearly always delivered to the sludge mixing vessel as a
Ca(OH)2  slurry  (milk-of-lime).    This   facilitates  transport  to
the  point   of application  and  improves  lime  dispersion  and
reaction efficiency.   The exact  series of steps  through which dry
lime is wetted  and  introduced  to the  sludge varies according  to
such  factors as  the  scale  of   the operation,  the  type of  lime
purchased,   and  the  method  of storage.   The following paragraphs
outline  the  basic  lime-feeding  schemes.  The discussion  is
largely derived from  a bulletin  published  by  the  National  Lime
Association  (236), which should  be referred to for more detail.

Feeding of  Hydrated  Lime -  In small treatment plants  where bagged
hydrated lime is purchased,  the  dry chemical  is  simply mixed with
water  in a  batch  tank and  metered to the  sludge mixing tank  as
required.    Solutions  of  lime  are  not corrosive, so  that  an
unlined steel  tank  is sufficient for mixing and storage of the
slurry.   Hydrated  lime is fed  as  a  6 to  18  percent  Ca(OH)2
slurry by weight,  the  percentage depending  on the application and
                             6-116

-------
on operator preference.   The  milk-of-lime  can be discharged  to
the sludge  in one batch  or metered continuously to the basin
through a solution feeder.

In larger operations,  where hydrated  lime  is stored in bulk, a
more  automated mixing  and feeding scheme is  appropriate.  A  dry
chemical feeder  is  used  for  continuous  delivery  of a  measured
amount  of  dry lime  to a dilution  tank.  The feeder  is often
positioned  directly  at the  base  of  the  bulk storage bin  to
minimize dry lime  transport distance.

Two general  types  of  automated dry feeders are available:

     •  Volumetric feeders,  which  deliver  a  constant, preset
        volume of chemical in  a unit  of  time,  regardless  of
        changes  in material density.


     •  Gravimetric feeders, which discharge a constant  weight  of
        chemical  in a  unit of time.

Gravimetric feeders  cost roughly  twice as  much  as volumetric
feeders with an  equivalent capacity and require more  maintenance,
but they are  more accurate.   Most manufacturers  of  gravimetric
feeders will guarantee a minimum accuracy of  within  one  percent,
by weight,  of  the set  rate.   Volumetric feeders, on the other
hand,  may  have an error of 30 percent by weight,  due to  the
varying bulk density  of  hydrated lime.   Gravimetric  feeders  are
preferred because of  their  greater  accuracy  and  dependability,
but the  less  expensive  volumetric type  may  be sufficient when
limited  funds are  available,  when  greater  chemical feeding
accuracy is  not  required,  or when a reduced  degree  of maintenance
is desirable.

Dry hydrated lime is  delivered to a dilution  tank that  is often
fitted directly  onto  the feeder.  The  tank  is agitated  by  either
compressed   air,  water  jets, or  impeller type mixers.   The  lime
slurry  is then  transferred  to the  sludge  mixing  basins.   This
transfer operation is  the most troublesome  single  operation  in
the  lime handling  process.    The  milk-of-lime  reacts with
atmospheric CC>2  or  carbonates in  the dilution  water to form
hard,  tenacious CaC03  scales,  which, with  time, can plug  the
transfer  line.    Because the magnitude of  this  problem is  in
direct proportion to  the  distance over  which  the  slurry must  be
transferred, lime  feeder facilities should be located as close as
possible to the lime/  sludge mixing tanks.    Pumping  of  the  lime
slurry should  be  avoided (if possible, gravity transfer  should be
used),  and  all apparatus should be  accessible  for cleaning.
Scaling  in   lime  slurry systems  has  been  prevented through
the use of  a  chemical  additive that  interfers with crystal
formation.    Design  features  and  operating techniques used
successfully for  milk-of-lime  transfer are described  in  detail in
reference 236.
                            6-117

-------
Direct  addition of  dry hydrated  lime to  centrifuge cake was
tested in a  pilot-scale  study  at  the wastewater treatment  plant
in Downington,  Pennsylvania.   An undigested mixture  of  primary
and secondary sludges was dewatered  to a solids concentration  of
20 percent,  and  then  blended with  powdered  Ca(OH)2  for ten
minutes  in  a  twin-paddle  mixer.    Addition of  200  pounds  of
hydrated lime  per  ton  dry  (100 kg/t) raised sludge pH to  11.8,
reduced  pathogen  levels  to  below  the  detection  limit, and
controlled odor and fly  problems.

Slaking  and  Feeding of  Quicklime - Feeding  of  quicklime  is
similar  to  that  for hydrated lime,  except that  there  is  an
additional  step, slaking, in which the quicklime reacts spontan-
eously with water  to form hydrated lime.  Bagged quicklime can  be
slaked in batches  by simply mixing one part quicklime with two  to
three parts water  in a steel  trough while  blending with a hoe.
Proportions  should be adjusted so that  the  heat of  the reaction
maintains the temperature of  the reacting mass near 200°F  (93°C).
The  resulting thin  paste  should be held for  30 minutes  after
mixing to complete  hydration.   Manually operated batch  slaking  is
a  potentially  hazardous operation  and  should  be  avoided  if
possible.  Uneven  distribution of  water  can  produce  explosive
boiling  and  splattering of  lime slurry.   Use  of  protective
equipment should be  mandatory.  For  small plants,  the potential
gain in using the  lower-priced  quicklime is smaller,  because lime
consumption  is  smaller.   Use of slaked lime  is safer, simpler,
and requires  less  labor.

Continuous  slaking  is   accomplished  in  automated machines  that
also  dilute   and  degrit  the  lime  slurry.   Several  types  of
continuous  slakers  are available.   They  vary  mainly  in the
proportion  of  lime to water  mixed  initially.   A  volumetric  or
gravimetric  dry chemical feeder is used to  measure  quicklime  as
it is moved  from bulk storage to the  slaker.  Since quicklime  is
available in a  wide  range of  particle  sizes,  it is  important  to
match the dry feeder with the  type of quicklime to be used in the
particular  application.
        6.4.4.2  Mixing  Tank Design

A tank  must  be provided for mixing raw sludge  with  lime  slurry
and then holding the  mixture for a minimum contact time.   Many of
the currently operating  lime stabilization facilities do not have
tanks with  sufficient  capacity  to hold sludge  for more  than  a
few  minutes.   Although these  operations generally have  been
successful,  the acceptability  of very  short  detention  times  has
not  been conclusively demonstrated.  Because  of  the  uncertainty
surrounding  this  practice,  it is recommended  that  all  lime
stabilization facilities include a tank large enough  to  hold the
lime  sludge mixture for  30  minutes.  The pH  of the reacted
mixture should exceed 12.5  during this period.
                            6-118

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The  following paragraphs discuss  two aspects  of mixing  tank
design  -  tank sizing and mixing.   To determine tank  size,  a
designer must  first  select  a flow mode.   The following section
on tank sizing describes  flow  modes.   The subsequent section on
tank  mixing covers  the general  types of  mixers and  suggests
criteria for sizing mixing systems.

Tank Sizing Considerations

Mixing  tanks  can be  operated as  either  a batch  process  or
continuous flow process.  In the  batch  mode,  the tank is filled
with sludge, and  then sufficient lime  is added to maintain the pH
of the  sludge-lime  mixture  above  12.5  for  the  next  30 minutes.
After this  minimum contact  time,  the stabilized sludge  can  be
transferred to dewatering facilities  or to either tank trucks or
a pipeline for  land  application.    Once  the  holding  tank  is
emptied, the cycle begins  again.

In the  continuous flow mode, the  pH and  volume  of sludge in the
holding  tank are  held constant.  Entering  raw sludge displaces an
equal volume of  treated  sludge.   Lime is added continuously,  in
proportion to the flow  of  incoming  raw  sludge, and  thus,  the
holding  time  would  vary.   The  lime  dose must  be sufficient  to
keep the contents of  the  tank  at  a pH of 12.5.   Often the daily
cycle of  sludge  production  does not match the pattern of sludge
disposal.    In  this  case,  a  system could  be  operated  on a semi-
continuous basis,  where the  quantity  of sludge  in  the  tank
fluctuates  through  the day.    Here  the treatment  tank would  be
used as  a  buffer  between sludge production and disposal.

It is most common to  operate  lime stabilization systems in the
batch flow  mode.   Batch  operations are  very simple  and are well
suited for small-scale, manually operated  systems.  When adequate
capacity  is provided, the  mixing tanks can  also  be  used  to
gravity  thicken the lime-treated sludge  before disposal.  In very
small treatment  plants,  tank capacity  should  be adequate  to
treat the  maximum-day  sludge  production  in one  batch.   This  is
because  small plants are generally operated only during the day,
and  it  is  usually desirable to stabilize  the entire day's sludge
in one batch.   Larger plants are more  likely to be manned round-
the-clock.  Because  sludge  can  be processed over the whole day,
stabilization  tanks can be relatively  smaller.

Continuous-flow stabilization systems  require  automated control
of lime feeding and  therefore are usually not cost-effective for
small-scale operations.  The  primary advantage of  continuous-flow
systems  over  batch  systems  is that a smaller tank  size may  be
possible.    Capacity  does not have to  be provided for storage of
sludge  between batches.   Instead, the mixing tank must only  be
large  enough  to ensure  that  all sludge particles are  held
at high pH for a contact  time sufficient toadestroy  odor and
disease-producing organisms.                   ^
                             6-119

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The  most  important design  parameter  for  a  continuous  flow,
well-mixed reactor is the nominal  detention time  (defined as tank
volume divided  by  volumetric  input flow rate).   Unlike  a batch
tank,  where contact time of  all particles is the same,  some
particles  in  a well-mixed, continuously  fed tank  escape  after
relatively short contact.  Thus,  30 minutes  of  pH  at  12.5  in a
batch mixer might  not  be the  same  as  30  minutes residence time
in a well-mixed, continuously  fed  reactor.

In making  a recommendation for detention time,  the nature of the
treatment  that  occurs must be  considered.   Unlike  some
treatments,  such as irradiation,  the  treatment does not stop
after the  treated  sludge leaves  the vessel.   If pH is  12.5  as
the  sludge  leaves  the  mixing  tank, it  remains  at  this  pH after
leaving.   Consequently,  a  30-minute  detention time  in a
continuously fed,  well-mixed reactor is  adequate, provided the pH
is measured in  an exit  line.   If  pH of the  limed sludge appears
to fall too rapidly upon standing,  it is a simple matter to move
the pH sensor and  to control lime  feed rate to a  position further
downstream.

Thickening of  raw  sludge  before  lime   addition  will reduce the
mixing  tank capacity  requirement  in direct proportion to the
reduction  in sludge  volume.   However,  the lime  requirement will
be reduced only slightly by prethickening, since  most of  the 1ime
demand is associated with the  solids (227), and  total solids mass
is not changed by  thickening.

Tank Mixing

Lime/sludge mixtures  can be  mixed  with either  diffused air  or
mechanical mixers.  The  agitation should be  great enough to keep
sludge solids suspended  and to distribute the lime slurry evenly
and  rapidly.   Both diffused air  and  mechanical systems  can
provide  adequate  mixing,  although the  former has been  more
commonly  used  in  pilot  studies  and full  scale  operations.   In
addition  to  their  mixing  function, sparger air systems supply
oxygen and, thereby, can be  used  for sludge aeration before the
sludge  is dosed  with  lime.   If  storage of unlimed sludge  is
contemplated,  the designer should check that the air requirement
for mixing is sufficient to meet the oxygen demand of the sludge.
Oxygen  requirements  are discussed in the  section on  aerobic
digestion.

There  are  disadvantages to both types  of  mixing   systems.
Mechanical mixers are  subject  to  fouling  with rags, string, and
other  debris  in  the sludge.  Although air  spargers may clog,
fouling problems are greatly reduced by  mixing with  air.  Ammonia
will  be  stripped  from the  sludge when  mixing  is done  with
diffused  air,  producing  odors  and reducing  the  fertilizer value
of  the  treated sludge.    However,  if  nitrogen  levels limit land
application rates,  this  stripping  of  ammonia  will  reduce land
requirements for disposal.  A further,  although probably minor,
                             6-120

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problem with air mixing  is  that  CC>2 is absorbed by the sludge/
lime mixture,  tending to raise the  quantity of  lime >required to
reach  the  desired pH.  The  selection of the method  of mixing
should be based on  the  factors described above, coupled with an
economic evaluation.

With air mixing, coarse bubble diffusers  should  be  used, mounted
along  one  of  the  tank walls to induce  a spiral-roll 'mixing
pattern.  An  air  supply of 20 to  30  scfm per 1,000 cubic feet
(20-30 m3/min /I,000 m3) is  required  for adequate  mixing  (241).
If the mixing tank is enclosed,  ventilation  should  be sufficient
to remove odorous gases stripped from  the sludge during mixing.
In many cases,  these gases should be  treated in an  odor control
unit before  being discharged into the atmosphere.

Mechanical  mixer  specifications  for various  tank sizes are
presented  in Table  6-31.    Sizing is  based on two  criteria:
maintaining the bulk  fluid  velocity (defined as  the turbine
agitator pumping capacity  divided by the  cross sectional area of
the mixing  vessel) above  26  feet  per minute  (8.5 m/min), and
using  an  impeller  Reynolds  number greater  than   1,000.   The
tank/mixer   combinations in  Table 6-31 are  adequate for mixing
sludges  with  up  to 10  percent dry  solids  and  viscosity  of
1,000 cp.    Impellers on mechanical  mixers should be designed to
minimize fouling with debris  in the sludge.


6.4.5  Costs and Energy  Usage

Engineering decisions are commonly based on a comparison of  costs
for  feasible solutions.   Energy  considerations  are  now also
becoming important  in the decision-making process.   This section
discusses  costs and  energy usage  for lime stabilization  systems.


        6.4.5.1 Capital and Operating Costs

Cost  estimates for the construction  and  operation  of  three
different  size lime  stabilization systems are summarized in
Table  6-32.  A comparison  of these costs  shows that  there is
a  large economy of  scale,  especially for the  capital costs.
Operation and maintenance  expenses,  particularly  those  for  lime,
are more closely related to the quantity of  sludge treated.

Comparisons  of   the  cost  of  lime treatment   with  other
stabilization methods must  take  into  account that   the  addition
of  lime increases the  quantity  of solids  to be handled after
stabilization.  In contrast, s.ludge solids actually  decrease
during anaerobic and aerobic digestion.   This  difference between
stabilization methods  can have an important  effect  on costs
for  final   disposal of  sludge.    The magnitude  of this cost
differential is site-specific and depends on such factors  as the
method of  disposal  and  the distance to the disposal  site.
                             6-121

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                             TABLE 6-31

      MECHANICAL MIXER SPECIFICATIONS FOR SLUDGE SLURRIES (228)
   Tank size,
      gal
      5,000
     15,000
     30,000
     75,000
    100,000
 ft
                    9.5
13.7
                   17.2
                   23.4
25.7
ter, Motor size,
hp
7 .5
5
3
20
15
10
7.5
40
30
25
20
100
75
60
50
125
100
75
Shaft speed,
rpm
125
84
56
100
63
45
37
84
68
56
37
100
68
56
45
84
68
45
Turbine
diameter, ft
2.7
3.2
3. 6
3.7
4.4
5.3
5.6
4.8
5.1
5.5
6.8
5.2
6.2
6.6
7.3
6.0
6. 5
7.8
Assumptions :
  Bulk fluid velocity >26 ft/min. (8.5 m/rain.).
  Impeller Reynolds number >1,000.
  Mixing tank configuration.
   Liquid depth equals tank diameter.
   Baffles with a width of 1/12 the tank diameter,
     placed at 90 degrees spacing.

Mixing theory and equations after References 155 and 242.

1 gal = 3.785 1
I ft = 0.305 m
1 hp = 0.746 kW
         6.4.5.2  Energy Usage

Energy  is  required  during  both the  construction and  operation
of  a lime  stabilization system.   During  operation of  a lime
stabilization  facility,   the principal  direct  use  of energy  is
electricity for mixing the  lime/sludge mixture.   A rough estimate
of  the  annual  energy requirement for  mixing with  diffused air  is
290  kWh  per year per cfm of blower  capacity (based  on continuous
duty).   This  estimate was  made  assuming a  six psig  (0.4 kg/m^ )
pressure  boost,   standard  inlet   conditions,  and  an  overall
compressor/motor efficiency of   60  percent.    One horsepower  of
mechanical  mixing  requires  about 6,500  kWhr  of  electricity per
year.  These mixing energy  demands  can be expressed  in terms of  a
primary  fuel requirement (that  is,  fuel  oil,  coal,  natural gas)
by  applying a  conversion factor  of  10,700 Btu  (2,700 kg-cal) per
kWh  of  electricity.   This factor  assumes  a fuel conversion
efficiency  of  35  percent  at the power plant  and a  transmission
efficiency  of 91 percent.
                               6-122

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                             TABLE 6-32

              ESTIMATED AVERAGE ANNUAL COSTS FOR LIME
                    STABILIZATION FACILITIESa (228)

                                   Treatment  plant
                                    capacity,  mgd
                  Item           1
           Capitalb             10,500   30,100    87,200
           Operation  and  main-
             tenance0          12,600   35,900   257,400

               Total            23,100   66,000   344,600

           Unit cost  ,
             (dollars/ton dry
             sludge solids)      54.17    39.27     20.51
            All costs expressed in 1978 dollars.

            Amortized over 30 years at 7 percent.
            Includes cost of all buildings, equip-
            ment, and piping for lime storage and
            funding, and for sludge mixing and
            lagoon storage, except as noted.
           °Average lime dose of 0.2 Ib Ca(OH)2/lb
            dry solids.  Hydrated lime (47 percent
            CaO, $44.50/ton) used in the 1-mgd
            system, otherwise, quicklime (85 per-
            cent CaO, $40/ton).  All labor at
            $6.50/hr.  Does not include cost for
            transport and disposal.

            Primary plus waste-activated sludge,
            2,300 Ib dry sludge solids produced/
            mil gal of wastewater treated  (1.20
            x 10~4 Kg/m3).
           g
            Includes sludge thickening but not
            lagoon storage.

Large amounts of  energy  are  used  in the  production of quicklime.
Quicklime (CaO)  is produced by  burning limestone (CaC03)  an
kilns.   This  process,  termed calcination,  is  illustrated  in the
following reaction:
    CaC03  	>  CaO + C02

The current national average energy consumption for all quicklime
production is  about  7.0  million Btu per  ton  of  quicklime  (1.9 x
10^ kg-cal/metric  ton)  (243).   This  figure  is  decreasing  since


                             6-123

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modern plants, using  large  and more efficient kilns,  should  be
able to produce one  ton  of  quicklime with  about  5.5  million Btu
(1.5 x 106 kg-cal/metric  ton).


    6.4.6   Design  Example

This section illustrates  the  layout  and sizing  of the major
components in  a  lime stabilization system.   For  this  example,
it  is  assumed that  the  treatment plant has  a capacity   of
approximately 8 MGD  (350  1/s) and provides secondary treatment  to
typical municipal  wastewater.   A  mixture of primary sludge and
thickened  waste-activated sludge is to be  stabilized with  lime,
then mechanically  dewatered,  and ultimately spread onto  land.


        6.4.6.1  Design Loading

Sludge production  estimates  for two flow  conditions,  average and
peak day,   are  listed  in  Table  6-20 (provided previously in the
anaerobic digestion  section).    The  peak-loading is listed
because critical  components must be sized  to meet  this  critical
condition.   Chapter  4 provides a discussion of the procedure
to determine  sludge  production values.   Sludge  solids concentra-
tions  and the resulting sludge  volumes are also included  in
Table 6-20.
        6.4.6.2   System Description

The  conceptual  design for' the lime  stabilization  system  is
presented on Figure 6-47.   Prior to stabilization,  all  sludge  is
passed through  an  in-line grinder.  This  conditioning  improves
sludge  mixing  and  flow  characteristics, protects  downstream
pumping  and  dewatering   equipment, and  eliminates  unsightly
conditions (such as rags,  sticks,  plastic)  at the disposal  site.
Two batch mixing  tanks are  provided,  each with  the capacity  to
treat the total   sludge  produced  in an eight-hour shift  during
peak  day conditions.   While  one  tank  is  filling,  sludge  in
the other is dosed with  lime,  mixed for  30 minutes,  and then
discharged to the dewatering equipment.   Since  the mixing  tanks
are sized for peak  conditions,  they can provide  some  short-term
storage  for treated  sludge during periods  of  lower loading.
Design of an  actual  facility  should take  into consideration  the
operating schedule  for dewatering and disposal.

In  this  example,  it is  assumed  that  dewatering  is  operated
continuously and  therefore  only  minimal  inline storage  is
required.  However, if dewatering equipment was  operated  for  two
shifts,   and  serviced  during  the third,  at least eight hours  of
storage  would be required.

Air discharged through  coarse bubble diffusers is used  to  mix  the
sludge with  the  lime  slurry.  Air mixing is started as  raw sludge


                            6-124

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 is  first  added  to  the  tank
 and  producing  odors.   When
 mixing  is  continued for  at
                 to keep the sludge  from  turning  septic
                  the  tank  is filled, lime is added and
                 least 30 minutes.
                   VOLUMETRIC—\— li
                   FEEDERS
                                                    PROGRESS! KG
                                                    CAVITY PUMP
ACTIVATED
SLUDQE
                                              TO
                                              EQUIPMENT
           COARSE AIR
           BU68LE D1FFUSERS —•
                            FIGURE 6-17

         CONCEPTUAL DESIGN FORA LIME STABILIZATION FACILITY
To reduce
from
unit
odors, the
                     mixing tanks are covered, and gases stripped
      the sludge  during mixing are removed in an odor control
         This  unit  is  a  packed  bed  scrubber.   The  scrubbing
solution is dilute sulfuric acid.  Ammonia gas is absorbed  by  the
sulfuric acid solution.  All  wetted parts are constructed of
acid-resistant materials.

Quicklime  is  used in  this installation.    A  bulk  storage  silo,
with  capacity  to  hold a  30-day  lime requirement  under average
conditions, supplies lime to two volumetric  feeders.  Each  feeder
measures out quicklime to a slaker,  where  the lime is hydrated,
slurried, and discharged  into  the  mixing tank.  The lime dose is
sufficient to maintain  the sludge above pH 12.5 for 30 minutes.
                              6-125

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        6.4.6.3  Component Sizing

Mixing Tank

Sizing criterion:

Conditions.

Volume requirement (V):

    Peak-day sludge production shown in Table 6-21.

    V  =   8 hr/tank (6010 cu ft/day + 3430 cu ft/day)
           24 hr/day
        =  3,150 cu ft/tank .
        =  (89 m3/tank)

Tank surface area (A) :
(Assume 10 feet liquid depth)

    A  =  3,150 cu ft   =  315 ft2
             10 ft
                       =  39.3 m2

Tank dimensions:
(Assume 2 feet freeboard)
     18 ft  x 18 ft x  12 ft
    (5.4 m x 5.4 m x 3.7 m)
Sizing criterion:

    30 cfm/1,000 cu ft

Blower capacity  (Q):
(One blower per tank)

    Q  =  (3,150 cu ft) {3o cfm/1,000 cu ft)
              tank
       =  95 cfm/blower
       =  (2.6 m3/min/blower)

Lime Storage

Sizing criterion:

    30-day storage during average loading.

Quicklime characteristics:

    Purity -            90 percent CaO
    Bulk density -      55 Ib/cu ft
                             6-126

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Lime dosage:

    Primary sludge   - 0.12 Ib Ca(OH)2/lb dry solids
    Activated sludge - 0.30 Ib Ca(OH)2/lb dry solids

Average daily lime requirement (W):

    Expressed as hydrated lime -

    wCaOH2  =  (10,000 Ib/day) /. 12 i^\ + (5,000 Ib/day) /-30  lb\
                              I    lb/                 \  Ib  /
            =  2,700 Ib Ca(OH)2/day

            =  (1,230 kg/day)

    Expressed as purchased quicklime (90 percent purity)  -


    T7_ _   /0 _An , ,  _ ,_TT,   ,  .  /  56 Ib CaO/Mole  \  /100\
    WCaO = (2,700 Ib Ca(OH)2 day)  ^74 lb Ca (OH) 2/molej  (w)

          =  2,270 lb CaO/day
          =  (1,030  kg/day)

Storage requirement  (Vs):

    VQ  =  2,270 Ib/day (30 days)
     s     55 Ib/cu  ft
        =  1,240 cu  ft
        =  (35 m3)

Slaker:

Sizing criterion:

    Ability to dose  one batch  in 15 minutes.

Slaker capacity  (C):

    c  _  2,270 lb CaO/day  (1 batch)
            3 batchs/day    (15 min)

       =  50 lb CaO/min
          (23 kg/min)


6.5  Chlorine Stabilization

Stabilization by chlorine addition was developed as a proprietary
process and is marketed under the registered trademark "Purifax."
The  chlorine  stabilization  process  is  applied  to wastewater
treatment plant  sludges and sidestreams to reduce putrescibility
and  pathogen  concentration.   The  process  has also  been  used  to
improve  the  dewaterability  of digested sludge  and  to  reduce  the
impact of recycled  digester supernatant on the wastewater treat-
ment  systems.   Because  chlorine  reactions with sludge  are very


                             6-127

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rapid, reactor volumes are relatively small, reduced system size
and  initial  costs.   The process  results in  no appreciable
destruction of volatile  solids,  and unlike anaerobic  digestion,
yields no  methane  gas  for  energy generation and  little  sludge
mass reduction.

Chlorine-stabilized  sludges are  buff-colored,  weak in  odor,
sterile,  and generally easy to dewater, either mechanically or on
drying beds.   The  stabilized  sludge has  been  used  as  a  soil
conditioner.  However,  there  is  concern about its use on cropland
and its disposal  in landfills because  of  its high acidity,  high
chloride  content,  and  potential  for  releasing chlorinated
hydrocarbons  and  heavy metals.   The stabilized sludges  are
corrosive  unless  pH has  been adjusted.   Process  equipment  that
comes into  contact  with  sludges that  have  not  been neutralized
must be constructed  of acid-resistant materials or be coated with
protective films.


    6.5.1   Process Description

Chlorine  treatment  stabilizes sludge by both reducing  the number
of organisms available to create unpleasant or malodorous condi-
tions and making  organic substrates  less  suitable for bacterial
metabolism  and growth.   Some of  the mechanisms  responsible  are
oxidation,  addition of chlorine  to unsaturated  compounds,  and
displacement of  hydrogen  by chlorine.

The immediate reaction from addition  of gaseous chlorine to water
is shown  below:


                    HOC1 + H+ Cl~                         (6-14)


In  the chlorine  stabilization process,  sufficient  acid  is
produced   to reduce  the pH  of the  sludge  to a range of  2  to 3.
Dissociation of  HOC1 to H* and OC1~ is  suppressed by  low pH
and  therefore  is  not significant.   C12  and  HOC1 are  highly
reactive  and powerful  bactericides  and  viricides.   The chloride
ion has no disinfection capability.

The process stream immediately   following  the  chlorine  addition
is substantially a chlorine solution  containing  sludge.   The
solution contains  (in  molecular  form)  as  much  as ten  percent of
the total  chlorine  species present.   The predominant  species in
solution  is undissociated HOC1.   HOC1  and C12  react with sludge
to oxidize ammonia to chloramines and organic nitrogen to organic
chloramines.   Other  reduced ions, such  as Fe+2  and S~2,  are
oxidized  at the  same  time.   Some  of the  oxidized end products,
such  as chloramines and  organic chloramines,  are germicidal and
viricidal  (244).

The chlorine  stabilization  unit consists  of  a  disintegrator,  a
recirculation pump,  two reaction tanks, a  chlorine  eductor, and a
pressure  control  pump.    A   chlorine  evaporator  and/or  a


                             6-128

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chlorinator,  feed  pump,  and  inlet  flow meter  can  be purchased
with the unit or  separately.   The  unit is  often supplied by the
manufacturer as  a  complete  package mounted on  a  skid plate and
ready  for  installation.   A detailed diagram  of  the  unit  is
shown on Figure  6-48.
             EDUCTOR
 QD
 RECIRCULATION
 PUMP
                                    SECOND
                                    REACTOR
             CONDITIONED
             SLUDGE
PRESSURE
CONTROL
PUMP
              CHLORINE
              SUPPLY
                                   CHLORINATOR
    EVAPORATOR
           DISINTEGRATOR
           RAW
         SLUDGE
                       SUPPLY
                       PUMP
                            FIGURE 6-48

         SCHEMATIC DIAGRAM OF A CHLORINE OXIDATION SYSTEM

In  the  first operating  step,  sludge is  pumped  through  a
disintegrator which  reduces  particle  size and  therefore, provides
greater  sludge  surface area  for  contact with  the  chlorine.
Chlorinated sludge  from the  first reactor  is mixed with  raw
sludge  just prior  to  reaching the  recirculation  pump.   The
combined flow  then passes through  the first reaction  tank.
Chlorine is  added via  an  eductor located  in the recirculating
loop.   Recirculation  aids  mixing and  efficient chlorine use.
The ratio of recirculated reacted product to raw sludge at design
capacity is about 7  to  1.   System  pressure  is maintained in the
30  to  35 psi (210 to  240  kN/cm2)  range, by  a pressure  control
pump  located at the  discharge  of  the 'second  reactor.   The
pressure provides  a  driving  force  to ensure penetration  of
chlorine into the sludge  particles.   The  second reactor tank
increases  system detention time,  allowing  a  more complete
reaction between the  sludge  and  the chlorine.

Flow patterns within the two reactor tanks are high,  in the form
of  velocity spirals, with  tangential  discharges.   The tanks are
                             6-129

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oriented with the spiral axis of the first in a horizontal plane
and the  second  in a vertical plane.   Solids  that  settle  'during
periods of non-operation are easily resuspended when the process
is started  again.  The  system is neither  drained nor cleaned
between operating periods.   A holding tank should be provided for
feed storage  and for flow  equalization.   Blending done  in the
tank also helps  to maintain  feed uniformity,  thus providing
sludge  of uniform  chlorine demand and minimizing the need  to
frequently adjust chlorine dose.  Sludge blending is particularly
valuable for processing  of primary  sludges, which tend to be more
concentrated when  initially  pumped from  the  sedimentation  tank
than at  the  end  of  the  pumping  cycle.   Similarly,  where primary
and secondary sludges  are  treated together,  blending  can  be
accomplished in  the holding  tank.  Continuously wasted activated
sludge,   however,   may   be   adequately treated without   prior
blending, provided  that  solids  concentration  is  nearly  constant
with  time.   Mixing is usually  done  by mechanical or air
agitation.  Air mixing  is preferable, because it enhances aerobic
conditions,   reduces  odors,  and  averts  problems with  fouling  of
the impellers by rags and strings.  Odor can be controlled  in the
holding tank if a portion of  the  filtrate or supernatant  from the
dewatering process is returned to it.

If the  chlorine  demand  of  the liquid  fraction of  the sludge  is
high,  separation of some of  the liquid  from solids  by thickening
prior  to chlorination  may  substantially reduce  total  sludge
chlorine demand.   If,   however,  the  chlorine demand  is  low,
thickening will  not be beneficial.   For more detailed discussion
of  chlorine demands   exerted  by sludge solids and liquid
fractions,   see  Section 6.5.3.    Solids  concentrations  above
certain  defined  limits should not be  exceeded,  because  the
diffusion rate  of  chlorine  through  the  sludge is  hindered and
processing rates must be reduced  to provide  additional  time for
the chlorine to reach reaction sites.   Normally, processing rates
are not affected if solids concentrations are below the following
values:

     •  Primary  sludge  or primary  plus trickling filter humus  -
        four percent.

     •  Primary  plus waste-activated  sludge  -  four  percent.

     •  Waste-activated  sludge -  1.5 percent.

Processing rates for higher  concentrations must be  determined  on
a case-by-case basis.

Use of  a holding  tank  downstream  of the  chlorine oxidation
process  allows subsequent processes to run  independently  and  at
their own best rate. Solids  settling may occur in the tank after
an initial period of flotation.   The tank can, therefore, be used
to separate the solid  and  liquid fractions  of the stabilized
product.


                             6-130

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     6.5.2  Uses, Advantages,  and Disadvantages

 Chlorine oxidation has  been used  to  treat  raw  and digested
 primary sludge, raw  and digested secondary sludges, septage,
-digester supernatants, and sidestreams from dewatering processes.

 The  chlorine   stabilization  process  has   several  attractive
 features.    It  can  be  operated  intermittently,   so  long  as
 sufficient storage volume is  available prior  to and following the
 unit.   Unlike  biological sludge processing  systems,  the  process
 can  be started up,  run for a  few  hours,   and  turned  off.   A
 constant supply of process feed is not required.  As  a  result,
 operating costs are directly  dependent upon production rates, and
 costs attributable to overcapacity  are eliminated.

 Chlorine oxidation is  a  chemical process  and  is  thus opera-
 tionally insensitive  to  factors such as toxic materials in the
 sludge, which  adversely  affect  biological stabilization  systems.
 It  can also process  feed streams of widely varying character,
 such as digested sludge and digester  supernatant,   within a  short
 period of  time.    This  flexibility  is not characteristic  of
 anaerobic or aerobic digestion processes.

 Disadvantages  of   the  chlorine  stabilization  process center
 on  chemical,  operational, and  environmental factors.  From a
 chemical  standpoint,  the  low pH  of  chlorine-stabilized  sludge
 may  require the  sludge  to  be partially neutralized prior  to
 mechanical dewatering  or before  being  applied  to acid soils.
 Costs of neutralization are in addition  to chlorine costs.   These
 are  discussed  in Section 6.5.5.1.  As mentioned earlier,  chlorine
 stabilization  does not reduce sludge  mass nor produce methane gas
 as  a by-product for  energy  generation.  The process consumes
 relatively  large   amounts of  chlorine.    Special  safety  and
 handling precautions  must be  used when handling this gas.   If
 high  alkalinity wastes—for   example, digested sludge,  digester
 supernatants--are  processed,  CC>2  generated  during  chlorination
 may  promote cavitation in downstream  pumps.

 There  is concern  that  chlorine oxidation of  sludges,  septage,
 and  sidestreams from sludge' treatment processes could result
 in   increased   levels of  toxic chlorinated  organics  in  the
 treated materials (245).    Data  available are inconclusive.
 Investigations  are  underway that will  help clarify  this
 issue.   In the meantime,  measures   should  be  taken  to mitigate
 environmental  concerns when  the chlorine oxidation  processes  is
 used.   These are:

        Provisions should be made to  deal  with the filtrate,
        centrate,  or  decant   from  the process,  including  return
         to  the wastewater treatment  plant,  unless  this  practice
         leads   to  wastewater  treatment  plant  upset  or  to
        violations of  effluent  standards; or  to  treat  by
        activated  carbon absorption or other  means.
                             6-131

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     •  If the treated  sludge  leaving  the pressurized chlorinator
        is discharged to a tank sparged with air, the gases from
        the tank should be  vented  away  from workers.

     •  Treated solids  should  be disposed of with care.  Consid-
        eration should  be given  to:

        1.  Using  secured  landfills  or  landfills  located  at
            hydrogeographically  isolated sites.

        2.  Treating  leachates from secured  landfills to prevent
            contamination of surface or groundwaters.

        3.  Directly  incorporating  the  solids into soils at rates
            sufficiently  low  to minimize  leachate  production.
            Direct incorporation as opposed  to surface spreading
            should be  used  to prevent consumption of  solids  by
            grazing animals.

        4.  Using  erosion control measures  to  prevent  runoff
            contaminated with  toxic  chlorinated  compounds  from
            entering  surface waters.

        5.  Providing adequate monitoring of facilities to assure
            detection of unexpected problems.


    6.5.3  Chlorine Requirements

Chlorine  demand  varies with  the  characteristics of  each  waste
stream.   Demand can  be estimated  from Table 6-33 for  cases  in
which  a  combination  of sludges  and/or sidestreams makes  up the
process feed.   The demand  of  a  sludge produced  by combining two
streams is the weighted average of the demands of the individual
streams.   For example, using  Table 6-33 one  estimates  that the
demand of a  sludge  composed of  five volumes  of  0.7  percent
waste-activated sludge  and one  volume  of  four  percent  primary
sludge  is  about (17  +  5[7])/6  =  9 pounds  per  thousand  gallons
(1 kg/1,000 1) .

If  a  chlorine  residual is desired to provide  added protection
against  septicity, then additional chlorine should  be  added  in
an  amount equal  to  the required residual.   For instance,  if
a residual of  200 mg/1  chlorine  is required  for the  waste-
activated/primary  sludge  combination  just  discussed,  then
an additional (200 mg/1)(0.00834  Ib/thousand gal) = 1.7 pounds of
                                      mg/1
chlorine  per  1,000 gallons of  sludge (0.20 kg/1,000  1)  should
be added, bringing the  total chlorine  addition to 10  to 11 pounds
per 1,000 gallons (1.2  to  1.3  kg/1,000  1) of sludge.

BIF Division of General  Signal  (246) states that,  for  solids
concentrations other   than  those shown,  the  chlorine  demand
per gallon varies in  proportion  to the  solids concentration.  For
example,  if  the solids concentration  were to  double,  chlorine
demand would also double.
                             6-132

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                            TABLE 6-33
               ESTIMATED CHLORINE REQUIREMENTS FOR
                SLUDGE AND SIDESTREAM PROCESSING3
          Feed stream
Primary sludge

Waste-activated sludge
  With prior primary treatment
  No primary treatment
  From contact stabilization

Sludge from low and high rate
  trickling filters

Digester supernatant
Septage
Suspended solids,
    percent

     4.0


     0. 7
     0.7
     0.7


     1.0
     0.3

     1.2
Chlorine requirement,
   lb/1,000 gal
        17


        7
        7
        7


        10

      2-10

        6
 Information obtained from R. C. Neal of BIF.
1 lb/1,000 gal = 0.12 kg/1


    6.5.4  Characteristics  of  Chlorine-Stabilized  Materials


        6.5.4.1  Stabilized  Sludge

Characteristics of  freshly  treated  sludge are a pH of  2  to 3  and
a chlorine  residual  of  approximately 200  mg/1.   Retention  in  a
downstream holding  tank allows  the  chlorine residual  to drop to
zero  and the pH  to rise to between  4.5 and 6.5.   Normally,  a
slight  medicinal  odor is  present.   After adequate  addition
of  chlorine,  the  color of  the sludge  changes from black  to
light brown.

Chlorine oxidation  generally  improves  the sand  bed dewaterability
of  many sludges and  septages.   If  properly chlorinated,  the
sludges are stable and do not undergo  anaerobic  activity  for
at  least 20 days.   When properly disposed in  landfills  or on
the  soil, the  chlorinated sludge  does not exhibit septicity
during  handling  and  disposal.    If stored  in lagoons,  the
sludge-liquid mixture must  be sufficiently  aerated to avoid odor
and septicity problems,  especially in  warm weather.

Production  of  chlorinated  hydrocarbons  by  the  chlorine
stabilization  process has  been the  subject of research efforts
since the process was  conceived.  Early  studies  (1971) by Metcalf
and  Eddy,  for  the BIF  Company by  then-current  technology  were
aimed  at  the  detection of  specific objectionable compounds.
This  work  indicated  that,  rather than  producing the  compounds,
chlorine stabilization actually seemed to  lower  their concentra-
tions in  most   instances.   Later work (1978) using more  advanced
gas  chromatograph-mass spectrometry  techniques  has  revealed  the
production of  0.9 to 1 percent by weight organic  chlorine in
several sludges  stabilized by the  chlorine oxidation process
                              6-133

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(245).  These results  indicate  that as much as 10 to 20 percent  of
initial sludge solids had  chemically  reacted with  the chlorine.
Additional studies by ASTRE  for the BIF  Company  suggested  that
total  identifiable chlorinated  organic  compounds  nearly  doubled
when  the particular raw  sludges  studied  were  treated by  a
chlorine oxidation process.  A six-fold  increase was found in the
amount of chlorinated organic Consent  Decree toxics  (see 43  PR
4109,  January 31,  1978  for  ConsentDecree  list  of toxic
substances)  following  chlorine  oxidation  and  an eight-fold
increase in  the amount of total  organic  Consent Decree toxics.
        6.5.4.2   Supernatant/Filtrate/Subnatant Quality

These  process   streams  are produced  by thickening  and/or
dewatering operations after  chlorine  treatment.   Filtrates  from
sandbed dewatering  are  typically clear  and  colorless.   The  pH
varies  from 4 to 6,  and  no residual  chlorine  remains.   Filtrate
from chlorine-treated sludge generally contains  lower suspended
solids  and 8005 than  the  filtrate  produced  when filtering
digested  sludges.   Typical filtrate  composition  is  50-150  mg/1
suspended  solids  and 100-300 mg/1  6005  with  low  turbidity  and
color.

In bench-scale studies simulating the  chlorine oxidation process,
Olver,  and others  found  that  acidic conditions enhanced  the
release of heavy metals  from  sludges  (247).

Sukenik  and  others  (248) noted  an  increase  in supernatant
chemical  oxygen  demand  (COD) after  sludge  treatment  by  chlorine
oxidation. Though the  reason for the  increase  is undetermined,
the  suggestion was  made  that  chlorine  may  solubilize  the
oxygen-demanding material  rather  than oxidize it.   Biochemical
oxygen demand of the supernatant is generally  comparable  to that
of raw  wastewater.   Data collected  at Alma,  Michigan, indicates
that chemically   precipitated phosphate is not  redissolved by the
chlorine oxidation process (246).

A 1978  report indicated  that  chlorinated  organics were present in
the  centrate from  several  chlorinated  sludge  samples  (245).
Although  less  than one  half  of  one percent  of  the  organic
compounds assumed  to  be present  could be  identified,  eight
chlorinated   compounds   on the  Consent  Decree   list of  toxic
substances were detected, including  three known or suspected
carcinogens.
    6.5.5  Costs

Data  reported  herein  were  derived  by  Purifax  from  actual
installations.
                             6-134

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         6.5.5.1  Operating  Costs

Because  the  chlorine  stabilization  process  can  be  operated
intermittently,  annual  operating costs are  proportional  to  the
quantity  of  material processed.   Table  6-34 displays operating
cost  data.  Chlorine,  the major expense factor,  historically  has
cost  between  9 and  14  cents  per pound (19.8  to 30.9  cents  per
kg).   Chlorine  unit costs  vary with annual usage,  method of
transportation and transportation distances,  and  competition.  In
the last  few years, prices  have decreased  because of an increased
demand  for sodium hydroxide  (chlorine  is  a  by-product of sodium
hydroxide production).


                              TABLE 6-34

      ACTUAL OPERATING COSTS FOR CHLORINE STABILIZATION SYSTEM3

                                                 Cost, dollars/ton of
                              Chlorine                dry solids

    Process stream and     Dosage, Ib/ton    Cost,               ,   Chlorine
      year reported       of dry solids  cents/lb   Chlorine  Power   and  power

Primary and waste-activated
  sludge
    Ravena-Coeymans, NY,
      1974                   167        11.35    18.95     1.90    20.85
    Plainfield, CT, 1973        148        14.00    20.72     2.07    22.79

Extended aeration
  Plainfield, CT, 1975         180        14.00    25.20     2.52    27.72

Waste-activated sludge only
  Fair Lawn, NJ               211         9.85    20.78     2.08    22.86
 Information obtained from D. L. Moffat of BIF.
 Estimated at 10 percent of chlorine cost.
Note:  Estimated operation and maintenance (6).
      Operation - 2 hr/shift.
      Maintenance - $200/yr.
1 Ib/ton = 0.504 kg/tonne
1 cent/lb =2.20 cents/kg
1 dollar/ton = 1.10 dollars/tonne

Although  it  is  not related to  the cost of  chlorine stabilization
of  sludge,   additional  chemical  costs  can result  if  chemical
conditioning  is  necessary  prior   to   mechanical  dewatering.
Chemical  conditioning  of chlorine-stabilized sludge  consists
of  adding  sodium  hydroxide  or lime  to  raise  the  pH to  between
4.5  and  5.5  and then  adding the  proper dosage  of an appropriate
coagulant. Although  more  expensive,  sodum  hydroxide  is  generally
preferred to  lime  because  it  reacts  faster.   Neutralizing
can  be done  in-line,  without  need of  an  intermediate  detention
tank.   Sodium  hydroxide  requirements  range from  20 to  30  pounds
per  ton  of dry solids  (10 to 15 kg/t)  for primary sludge to 10 to
20  pounds per  ton of dry solids (5  to  10 kg/t)  for  secondary
sludge.    At  a  1976  cost  of  eight cents  per pound  (18  cents/kg)


                               6-135

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this  is  equivalent  to a cost  $0.80  to $2.40 per  ton  ($0.88 to
$2.65/t) of  dry  solids.   Polymer  costs  are equivalent to those
required for dewatering of sludges stabilized by other means and
are generally greater than  the cost of  pH adjustment.   (See
Chapter 8).

Costs  for  neutralizing  chlorine-stabilized  sludge  prior to
spreading it on acid soils  are about  $0.60  to $0.90/ton ($0.66 to
$0.99/t), assuming  that 20  to 30 pounds  of Ca(OH)2 are required
per ton  (10  to 15  kg/t)  of stabilized sludge solids and Ca(OH)2
costs are $0.03 per pound  ($0.07/kg).

Power costs  of  operating  the  stabilization system are  estimated
at  ten  percent of  chlorine  costs.    Additional  power  costs are
incurred if mixing  is used in the holding  tank upstream from the
stabilization process.

Labor  costs are  incurred only  for  daily start-up,  shutdown,
periodic checks, and  maintenance, and are  small  in comparison to
other operating costs.


        6.5.5.2  Capital  Costs

Capital costs for  chlorine  stabilization systems tend  to be
less  than for  conventional  anaerobic digestion  systems of equal
capacity.  Normally,  the system  is furnished  by  the manufacturer
on  a  skid-plate and  in a ready-to-install  condition.  Table 6-35
shows actual 1979  capital  costs  for systems of specified capacity
for two different  feed sludges.
                             6-136

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                 TABLE 6-35

  CHLORINE STABILIZATION CAPITAL COSTS, 1979a

       Capacity, gal/hr
Primary and
waste -
activated
sludge*3
660
1,320
2,940
5,880
13,080

Waste -
activated
sludge onlyc
960
1,800
4,200
8,520
18,300

Budgetary
cost ,d
dollars
82,000
137,000
175,000
228,000
307,000e
 Information obtained from R. C. Neal of BIF,

 Solids concentration 3 percent by weight.
c:
 Solids concentration 1.5 percent by weight.
 Budgetary costs based on an ENR 20 cities
 average construction cost index of 2869 for
 December 1978.  Costs include chemical
 oxidizer, sludge macerator, sludge feed
 pump, motor starters, vacuum-type
 chlorinator, freight and start-up service.

 Budgetary cost includes chlorine
 evaporator.

1 gal/hr = 3.79 1/hr
                  6-137

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6.6  References

  1.   Stewart,  M.J.-   "Reaction Kinetics and  Operational  Param-
      eters of Continuous Flow  Anaerobic  Fermentation Process."
      Sanitary Engineering  Research  Laboratory  Publication  4.
      IER Series  90.   University  of  California,  Berkeley  94720.
      1958.

  2.   Agardy,  F.J., R.D.  Cole,  and E.A. Pearson.   "Kinetic  and
      Activity Parameters  of  Anaerobic Fermentation  Systems."
      Sanitary Engineering  Research  Laboratory  Report  63—2.
      University  of California,  Berkeley 94720.  1963.

  3.   Lawrence,  A.W.,  and P.L. McCarty.   "Kinetics  of  Methane
      Fermentation in  Anaerobic  Treatment."    Journal  Water
      Pollution Control Federation Research Supplement.   p.   Rl.
      February 1969.

  4.   Andrews, J.F.  "Dynamic Modeling of the Anaerobic Digestion
      Process."     Journal Sanitary E n g i ne e r ing p i y i s j.on_-_ASCjS .
      Vol. 95, SA1.  p.  95.  1969.

  5.   Andrews,  J.F.  and  S.P.  Graef.   "Dynamic  Modeling  and
      Simulation  of the Anaerobic  Digestion  Process."   Advances
      in Chemistry  Series  No. 105.    American Chemical  Society.
      1971.

  6.   Collins, A.S.  and  B.E.  Gilliland.   "Control  of Anaerobic
      Digestion  Process."  Journal Environmental Engi neering
      Division -  ASCE.   Vol. 100, EE2, p. 487.  1974".""

  7.   Toerien, D.F.   "Anaerobic Digestion  -  The  Microbiology of
      Anaerobic  Digestion."   Water Research.  Vol.  3, p.  385.
      1969.

  8.   Kotze,  J.P.    "Anaerobic  Digestion  -  Characteristics  and
      Control of  Anaerobic Digestion."  Water Research.   Vol. 3,
      p. 459.   1969.

  9.   Pretorius,   W.A.    "Anaerobic  Digestion  -  Kinetics  of
      Anaerobic Fermentation."   Water Research.   Vol. 3,  p. 545.
      1969.

 10.   Kirsch,  E.J. and  R.M.   Sykes.    "Anaerobic Digestion in
      Biological   Waste  Treatment."    Progress  in  Industrial
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 11.   USEPA.   Sludge Handling  and  Disposal Practices at Selected
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      April 1977.
                              6-138

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12.   USEPA.    "Sludge  Digestion  of  Municipal Wastewater
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13.   Malina,  J.F.,  Jr.   "The Effect of Temperature on  High
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     16th Industrial Waste Conference.   Purdue University,
     Lafayette,  Indiana  47907.  p. 232.  1961.

14.   Malina,  J.F.    "Thermal  Effects on  Completely Mixed
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     January 1964.

15.   Garrison,  W.E.,  J.F.  Stahl, L. Tortorici,  R.P.   Miele.
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     Journal  Water  Pollution  Control  Federation.   Vol.  50,
     p. 2374.   1978.

16.   USEPA.   "Sludge Processing For Combined Physical-Chemical-
     Biological  Sludges."   Environmental Protection Technology
     Series.  Cincinnati,  Ohio 45268.EPA-R2,73  -  250.July
     1973.

17.   Black,  S.A.  "Anaerobic Digestion  of  Lime  Sewage Sludge."
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18.   Water Pollution Control Federation.   "Phosphorus Removal by
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     1/71.

19.   Wilson, T.E.,  R.E.  Bizzarri,  T.  Burke,  P.E.  Langdon,  Jr.,
     and  C.M.  Courson.    "Upgrading  Primary Treatment  with
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20.   Water Pollution Control Federation.  "Ultimate Disposal of
     Phosphate  From  Wastewater  by  Recovery  as Fertilizer."
     Water Pol 1 u t i o n Cpntr^JResearch^Seri^ .   17070  ESJ 01/70.

21.   Water  Pollution  Control  Federation.    "Development  of
     a Pilot  Plant to  Demonstrate  Removal  of  Carbonaceous,
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22.   Earth,  E.F.  "Phosphorus Removal from Wastewater by Direct
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     Pollution Control Federation.  Vol. 41, p. 1932.   1969.
                             6-139

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23.   Woods,  C.E.  and V.F.  Malina.   "Stage Digestion  of  Waste-
     water Sludge."  Journal Water Pollution Co n t:r_ o 1^ F_ederat ip n .
     Vol.  37,  p.  1495.   1965.

24.   Torpey,  W.N.   "High-Rate  Digestion of Concentrated Primary
     and  Activated Sludge."    Sewage and  Industrial Wastes.
     Vol.  26,   p.  479.   1954.

25.   Ohara,  G.T.  and J.E. Colbaugh.   "A Summary of Observations
     on Thermophilic Digester  Operations."   Proceedings of the
     1975  National  Conference  on  Municipal  Sludge  Management
     and Disposal.   Information Transfer,  Inc.,  Rockville,
     Maryland  20852.  August 1975.

26.   Graef,  S.P.    "Anaerobic  Digester  Operation  at the
     Metropolitan  Sanitary  Districts  of  Greater Chicago."
     Proceedings  of  The  National  Conference  of Municipal
     Sludge  Management.   Information  Transfer,  Inc., Rockville,
     Maryland  20852.  June  1974.

27.   Garber,  W.F.   "Plant-Scale Studies Of Thermophilic
     Digestion  at Los  Angeles".   Sewage	Industrial Wastes.
     Vol.  26,  p.  1202    1954.           "

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                             6-148

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138.   Davis,  G.H.   "New  Twist  in Digester Design."  American City
      and County.   p.  68.  May 1976.

139.   Boarer,  J.    "Unusual  Waffle  Bottom  Digester Design."
      Paper  presented  at  California Water  Pollution Control
      Association,  Northern  Regional  Conference.   Redding,
      California.   October 20, 1978.

140.   Water  Pollution  Control Federation.   Operation of  Waste-
      water  Treatmen t	Plants.    Manual o€  Pra c_t i ce  No.  11.
      Washington,  D.C."  1976.                       '"

141.   Guarino,  C.F.    "Sludge   Digestion  Experiences  in
      Philadelphia."   Journal  Water Pollution Control Federation.
      Vol. 35,  p.  626.   1963.           "

142.   USEPA.    Use  of  Solar  Energy to Heat Anaerobic Digesters.
      iMunicipal Environmental  Research  Laboratory.   Cincinnati,
      Ohio 45268.   EPA-600/2-78-114.   July 1978.

143.   Drnevich, R.F.  and  L.C. Match.   "A New  Sludge  Digestion
      Process."     Proceedings of  the 5th National Conference on
      Acceptable  Sludge  Disposal  Techniques,  Orlando,  Florida.
      Janauary  31-February 2,  1978.    Information Transfer  Inc. ,
      Rockville, Maryland 20852.

144.   Gould,  M.S.   and  Drnevich, R.R.   "Autothermal Thermophilic
      Aerobic  Digestion."  Journal  Environmental Engineering
      Division  -ASCE.   Vol. 104. No.  EE2, p.  259.  1978.

145.   Perry,  R.H.  and C.H. Chilton  Chemical Engineers' Handbook.
      McGraw-Hill.  1973.

146.   Baumeister, T.  and  L.W. Marks.   Standard Handbook for
      Mechanical Engineers.  McGraw Hill.  1967.
147.  Courtesy of Envirex.

148.  Holland  and  Chapman.   Liquid  Mixing and Processing  in
      Stij:red_ Tanks.   Reinhold,  New  York.   1966.

149.  Uhl and Gray.   Mixing Theory  and Practice.  Vol. I and II.
      Academic Press.   New  York,  New York.  1966; 1967.

150.  Verhoff,  F.H., M.W. Tenney, and  W.F. Echelberger.  "Mixing
      in Anaerobic Digestion."   Biotechnology and Bioengineering.
      Vol. XVI,  p. 757.  1974.

151.  Burgess,  S.G.   "The  Determination of Flow Characteristics
      in  Sewage  Work  Plant."    Journal Institute  of  Sewag_e
      Purification.    Part  3.   p.  206.   (Great Britain)  1957.

152.  Zoltak,  J. and  A.L.  Gram.   "High-Rate Digester  Mixing
      Study Using Radioisotope Tracer."   Journal Water Pollution
      Control Federation.  Vol.  47,  p.  79.  1975.
                              6-149

-------
153.   "Peripheral  Mixing  Turns Sludge  Into  Fuel Gas."   The
      AmericanCity  and^County.  p. 58.  July 1977.           "™

154.   Camp,  T.R.,   and  P.C.   Stein.    "Velocity  Gradients
      and Internal  Work in Fluid Motion."  Journal  of  the Boston
      Society of  Civil  Engineers.  p. 203.  October  1943.   ——

155.   Fair,  G.M., U.C.,  Geyer  and D.A.  Okun.   Water  and Waste-
      water Engineering.  Vol.  2  John Wiley and Sons.   New
      York,  New York 1968.

156.   Buzzell, J.C.   and  C.M.  Sawyer.   "Biochemical vs  Physical
      Factors  in Digester Failure."   Journal  Water  Pollution
      Contrgd _F^eder a t ic*n .  Vol. 35, pg . 205." 1963.

157.   Vesilind,  P.A.     Treatment and  Disposal  of Wastewatej:
      Sludges.    Ann Arbor Press.   Ann Arbor, Michigan 48106.
      1974.

158.   Dick, R.I.  and B.B. Ewing.   "The Rheology  of  Activated
      Sludge"   Journal  WPCF.  Vol. 39,  p.  543.  1967.

159.   Hatsfield,  W.D.   "The  Viscocity  or  Pseudo-Plastic
      Properties  of Sewage  Sludges."   Sewage Works Jjgjjrnal.
      Vol. 10, p.  3. 1938.

160.   Sparr,  A.E.   "Pumping Sludge Long Distances."  Journal
      Water Pollution  Control Federation.   Vol.  43, p.  1702.
      T9"7T.

161.   Borgerding,  J.   "Phosphate Deposits in Digestion Systems."
      Journal  Water Pollution  Control  Federation.   Vol. 44,
      p. 813.""1972.

162.   Rankin.  R.S.   "Scum Control in  Digesters."   Sewage  Works
      Journal. Vol. 19,  p. 405.   1947.

163.   Ghosh, S. and  D.L.  Klass.   "Two Phase Anaerobic Digestion."
      U_. _§_._ P'a t e n t 4,022,665.  May  10, 1977.

164.   Water  Pollution  Control  Federation.    Manual of Practice
      No. 16 Anaerobic  Sludge Digestion.  Water Pollution Control
      Federation, 1968."

165.   USEPA.    Anaerobic Sludge Digestion Operations Manual.
      Office  of Water  Program  Operations.   Cincinnati,
      Ohio 45268.  EPA  430/9-76-001.  February 1976.

166.   USEPA.   Estimating Costs  and Manpower  Requirements for
      Conventional Wastewater Treatment Facilities.     Office   of
      Research  and Development.    Cincinnati,  Ohio 45268.
      17090 DAN 10/71.   1971.
                              6-150

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167.   CH2M-H111,  Inc.  and Environmental Imact  Planning Corp.
      Wastewater  Solids  Process, Transport and  Disposal/Use
      Sy s terns.    Task Report  for San  Francisco Bay  Regi on
      Wastewater  Solids Study.   Oakland, California.    November
      1977.

168.   Brown  and  Caldwell.   SantaCruz  Wastewater Facilities
      Planning  Study.   May 1978.

169.   Metcalf and  Eddy, Inc. Water Pollution Abatement  Technology
      --Capabilities  and  Co_sts--Publicly_0wned_ Treatment Works.
      Appendix F .  N a "tio n a 1  C o m m I s s ~io~n  o n w"a~t e r  Quality.
      Washington,  D.C.  PB-250-690-03.  March 1976.

170.   USEPA.    Construction  Costs  for Municipal Wastewater
      Treatment Plants:  1973-1977.   Office of Water  Program
      Operations.   Washington,  D.C.   EPA 430/9-77-13.   MCD-37.
      January 1978.

171.   USEPA.  Areawide  Assessment  Procedures Manual.  Appendix H,
      Point  Source Control Alternatives^;	Performance and Cost.'
      Municipal  Environmental  Research La^Qrat^^—--—^YnTTnnaTT,
      Ohio 45268.   1976.

172.   USEPA.    A  Guide to the Selection  of Cost-Effective Waste-
      water  Treatment  Systems.   Office  of  Water  Program
      Operations^Washington,  D.C.   20460.   EPA-430/9-75-002.
      July 1975.

173.   Herpers,  H.   and  Herpers,  E.   "Importance, Production and
      Utilization  of  Sewage Gases."   KWG-Kohlenwosse-Stoffgase.
      Vol.  72,  p.  18.   1966.

174.   Rankin, R.S.   "Digester  Capacity  Requirements."   Sewage
      Works Journal.  Vol. 20, p.  478.  1948.

175.   Coackley,  P.   "Research  on  Sewage Sludge  Carried  Out  in
      the C.E.  Department  of University College London."  Journal
      L"£t_itute_ Sewage  Purification Vol.  59.   (England) 1955.

176.   Coackley, P.   "Laboratory Scale  Filtration Experiments
      and  Their  Application   to  Sewage  Sludge  Dewatering."
      Biological  Treatmen t  of  Sewage  and  Industrial  Waste.
      Vol.  2, p.  287,  Reinh'dld Publishing Co., N.Y.   1958.

177.   Eckenfelder,  W.W.,  Jr.   "Studies on the Oxidation Kinetics
      of Biological  Sludges."   Sewage  and  Industrial  Wastes.
      Vol.  28,  8,  p.  983.  1956.

178.   Lawton, G.W. and J.D.  Norman.    "Aerobic  Sludge Digestion
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      Vol.  36,  4,  p.  495.  1964.
                              6-151

-------
179.   Drier,  D.E.   "Aerobic Digestion  of  Solids."   Proceedings
      18th Purdue Industrial Waste Conference,  Purdue  University,
      Lafayette,  Indiana 47902.   1963.

180.   The  Effect of  Temperature on the  Aerobic  Digestionjpf
      Wastewater Sludge.  NTIS-PB-245-280,  June 1974.   "~~

181.   USEPA.     Aerobic Digestion of  Organic  Waste^Jaludge^     EPA
      17070-DAV-12/71, NTIS-PB-211-024.   1971.

182.   Burton,  H.N. and  J.F.  Malina,  Jr.   "Aerobic  Stabilization
      of  Primary  Wastewater Sludge."   Proceedings  19th  Purdue
      Industrial Waste Conference.  Purdue  University"Lafayette,
      Indiana 47902.   1964.

183.   Raw  Sewage  Coagulation _and_ Aerobic  Sludge  Digestion.
      NTIS-PB-249-107. 1975.  "                   '           ~~~

184.   Coulthard,  T.L.  and P.M. Townsley.  Thermophilic Processing
      of Municipal Waste."  Paper No.  74.219,  Canadian Society of
      Agricultural Engineers.  1974.

185.   Gay, D.W.,  R.F.  Drnevich,  E.J.  Breider,  and  K.W.  Young.
      "High  Purity  Oxygen  Aerobic  Digestion Experiences  at
      Speedway Indiana."   Proceedings  of the National Conference
      on Municipal Sludge Management.   Information Transfer  Inc.
      Rockville,  Maryland.  June 1974.

186.   Full-Scale Conversion  of  Anaerobic Digesters to Heated
      Aerobic Digesters.  EPA R2-72-050, NTIS PB-211-448.   1972.

187.   Jewell,  W.J. and R.M.  Kabrick.   "Autoheated Aerobic Thermo-
      philic  Digestion  with  Air Aeration."   Presented at  the
      51st Annual  Water Pollution Control  Conference.   Anaheim,
      California.  October 1978.

188.   Hamoda,  M.F. and  K.J.  Ganczarczyk.   "Aerobic Digestion of
      Sludges  Precipitated  from  Wastewater   by Lime  Addition."
      Journal Water Pollution _C_p n t. r ql JF_ed_e_r a t i_on.   Vol.  49,  #3,
      p. 375.   1977.

189.   Ganczarczyk, K.J.  and  M.F.  Hamoda.   "Aerobic Digestion of
      Organic  Sludges  Containing Inorganic  Phosphorus  Precip-
      itates,  Phase  I."   Research Report  #3, Canada-Ontar io_
      Agreement on Great Lakes Water Quality,  Environment Canada.
      Ottawa.  1973.

190.   USEPA.    Review of Techniques for Treatment and  Disposal of
      Phosphorus Laden Chemical  Sludges.   Office of Research  and
      Development, Cincinnati,  Ohio, 45268.   EPA Contract  68-03-
      2432.  To Be Published in  1979.

191.   Tarquin, A.J. and  R.  Zaltzman.   "Influence of  Waste  Paper
      on Aerobic  Sludge  Digestion."  Public  Works,  Vol.  101,  #3,
      p. 80.   1970.


                              6-152

-------
192.   Koers,  D.A.  and D.S.  Mavinic.   "Aerobic Digestion  of  Waste
      Activated  Sludge at  Low Temperatures."   Journal v\fat_er
      Pollution Control  Federation.  Vol. 50, |3, p.  460.l9lTT~

193.   Reynolds,  T.D.   "Aerobic Digestion of  Waste Activated
      Sludge."   Water and  Sewage Works.   Vol.  114, p.  37.   1967.

194.   Reynolds,  T.D.   "Aerobic Digestion of  Thickened Waste
      Activated  Sludge."   Proceedings 28th  Purdue Industrial
      Waste Conference.   Purdue University,  Lafayette,  Indiana,
      49707.   1973.

195.   Baillod,  C.R.,  G.M.  Cressey,  and R.T. Beaupre.   "Influence
      of Phosphorus  Removal  on Solids  Budget."   Journal  Water
      Pollution Control Federation.   Vol.  49,  p.  131"!  T9T7Y

196.   Aerobic  Stabilization  of Waste Activated Sludge  - An
      Experimental  Investigation.  NTIS-PB-246-59~3/AS."  1975.

197.   Randall, C.E.,  W.S.  Young  and  P.H.  King.   "Aerobic
      Digestion  of  Trickling  Filter Humus."   Proceedings  4th
      Environmental Engineering   and  Science  Conference.
      University of  Louisville,  Louisville, Kentucky.   1974.

198.   Smith,  A.R.   "Aerobic  Digestion Gains  Favor."   Water and
      Waste Engineering. Vol.  8, (2), p. 24.  1971.

199.   Ahlberg,  N.R.   and  B.I.   Boyko.   "Evaluation and Design
      of Aerobic  Digesters."    Journal  Water  Pollution  Control
      Federation.   Vol.  44, p.  634.   1972.

200.   Ritter,  L.E.    "Design  and Operating  Experiences Using
      Diffused Aeration for  Sludge  Digestion."   Journal  Water
      Pollution Control  Federation.   Vol. 42, p. 1982.  1970.

201.   Folk, G.   "Aerobic  Digestion  of  Waste  Activated  Sludge."
      Journal Water Pollution Control Federation Deeds and  Data.
      July 1976."''"

202.   Parades,  M.   "Supernatant Decanting of Aerobically Digested
      Waste Activated  Sludge."   Journal Water Pollution Control
      Federation Deedsand  Data.  October 1976.

203.   Matsch,  L.C.  and R.F.  Drnevich.   "Autothermal Aerobic
      Digestion."   Journal  Water Pollution  Control  Federation.
      Vol.  49,  p.  296"!  19TT:

204.   Surucu,  G.A.,  E.S.K.  Chain, and  R.S.  Engelbrecht.  "Aerobic
      Thermophilic Treatment  of High Strength Waste^aters."
      Journal Water Pollution Control Federation.   Vol.  48,  #4,
      p. 669.   1976. ~   .  '
                              6-153

-------
205.   Randall,  C.W., J.B. Richards, and P.H. King.   "Temperature
      Effects on  Aerobic  Digestion Kinetics."   Journal Environ-
      mental  Engi n e e ri_ng	Dj.vision  ASCE .   Vo 1 ~  101, p~.T95".
      October 1975".	

206.   Benefield,  L.D. and  C.W. Randall.   "Design Relationships
      For Aerobic Digestion."   Journal Water  Pollution Control
      Federation.  Vol.  50,  p.  518.   1978.          "'

207.   Popal,  F.V. and  C.  Ohnmacht.    "Thermophilic  Bacterial
      Oxidation  of  Highly  Concentrated Substrates."   Wat e r
      Research.   Vol.  6,p.807.   1972.

208.   Smith, J.E., Jr.   "Biological Oxidation and  Disinfection of
      Sludge."   Water  Research.  Vol.  9, p.  17.  1975.

209.   Jaworski,  N., G.W.  Lauton  and G.A.  Rohlick.   "Aerobic
      Sludge Digestion."   3rd Conference  on  Biological  Waste
      Treatment.   Manhattan  College,  New York, New  York  100207
      April  1960.

210.   USEPA.     Thermophilic  Aerobic Digestion of Organic Solid
      Wastes.   Office  Research  and  Development, Cincinnati, Ohio,
      45268."  EPA 620/2-73-061, NTIS-PB-222-396.   1978.

211.   "Aerobic Sewage  Digestion Process."  U.S. Patent 4,026,793.
      1977.                                ——.  ______

212.   Rooney,  T.C.   and N.A.  Mignone.   "Influence of  Basin
      Geometry  on Different  Generic Types  of Aeration Equipment."
      Proceedings 33rd Purdue Industrial Waste Conference.     Ann
      Arbor  Science,  Ann Arbor, Michigan,  48106.   1978.

213.   Stankewich,  M.J.,  Jr.   "Biological  Nitrification with the
      High Purity Oxygenation Process."   Proceedings  27th Purdue
      Industrial  Waste Conference.  Purdue University, Lafayette,
      Indiana,  47907.  p,"l.  1972.

214.   Brock,  T.D.  and   G.K.  Darland.    "Limits of.  Mi crobial
      Existence,  Temperature  and pH. "    ^c^e_n_c_e,  Vol.  169,
      p. 1316.   1970.

215.   Hagstrom,  L.G.  and  N.A.  Mignone.   "Operating  Experiences
      with  a  Basket  Centrifuge on Aerobic  Sludges."   Water and
                           February 1978.

216.   Bisogni,  J.J.  and A.W.  Laurence,   "Relationship Between
      Biological  Solids  Retention  Time and  Settling Character-
      istics of  Activated  Sludge."   Water Research.  Vol.  5,
      p. 753.   1971.

217.   USEPA.  Sludge Handling and  Conditioning.   Office of Water
      Program Operations.   Washington  D.C".  20460.    EPA  430/9-
      78-002.   February  1978.
                              6-154

-------
218.   Design Procedures for Dissolved  Oxygen Control  of  Activated
      Sludge Processes, NTIS P8-270  960/8BE."  April 1977.

219.   Riehl,  M.L.   "Effect of  Lime-Treated  Water  on  Survival
      of  Bacteria."    Journal American Water Works  Association.
      Vol. 44, p.  466.   1952.      '

220.   Buzzell,  J.C.,  Jr.  and C.N. Sawyer.   "Removal  of Algal
      Nutrients Prom  Raw Wastewater  with  Lime."   Journal Water
      Pollution Control Federation.  Vol.  39,  p. R16.  19~67.

221.   Grabow,  W.0.K.    "The  Bactericidal  Effect  of  Lime
      Flocculation Flotation as  a Primary  Unit Process  in a
      Multiple  System  for  the  Advanced  Purification  of Sewage
      Works Effluent."   Water Resources.  Vol. 3,  p.  943.  1969.

222.   USEPA.    Lime Disinfection  of  Sewage  Bacteria  at  Low
      Temperature.   Environmental Protection  Technology  Series.
      CincinnatiT Ohio 45268.   EPA-660/2-73-017.   Sept.  1973,

223.   "How Safe is Sludge?"  Compost Science.   March-April 1970.

224.   Kampelmacher,   E.H.  and  N.  Van Noorle  Jansen,  L.M.
      "Reduction of Bacteria in Sludge Treatment." Journal Water
      Pollution Contrql_Federation.  Vol 44, p. 309.   1972.

225.   Evans, S.C., "Sludge Treatment at Luton" Journal Industrial
      Sewage Purification.  Vol. 5 p.  381.   1961.

226.   Farrell,  J.R., J.E.  Smith, Jr.   and  S.W. Hathaway.   "Lime
      Stabilization of Primary Sludges."  Journal  Water  Pollution
      ControlFederation.  Vol. 46,  p. 113.   1974.

227.   Paulsrud, B. and A.S. Eikum.  "Lime Stabilization  of Sewage
      Sludges."  Water Research.  Vol. 9, p. 297.   1975.

228.   US E PA.   Lime Stabilized Sludge:   Its Stability and Effect
      on Agricultural Land.  National  Environmental  Research
      Center.   Washington,  D.C.  20460.   EPA-670/2-75-012.   April
      1975.

229.   USEPA.    Full  Scale  Demonstration  of  Lime  Stabilization.
      Environmental  Protect "ion "Technology  Series.    Cincinnati,
      Ohio  45268.   EPA-600/2-77-214.  November 1977.

230.   Stravch,  D., H.  Schwab, T. Berg, and W. Konig.   "Vorlaufige
      Mitleilung  for Frage Der  Entseuchenden  Wirkung Von
      KalkstickstoHf: Und Kalk In Der  Abwassertechnik."   Korres-
      pondenz Abwasser.  Vol 25.  p. 387.  1978.

231.   Standard Methods  for  the Examination  of Water __a_n_d
      Waste wTt'eTV "TTt h  e d i t i o n~   American  Public Health
      Association, Washington, D.C. 1975.

232.   Sawyer,  C.N.  and McCarty,  P.L.  Chemistry  for   Sanit_ar_y
      Engineers.   McGraw-Hill.  New York.  1967.


                              6-155

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233.   Berg,  G., R.B. Dean, and  D.R.  Dahling.   "Removal  of  Polio
      Virus  1  from  Secondary  Effluents by Lime  Floculation  and
      Rapid Sand  Filtration."   Journal of  the  American  Water
      Works  Association.   Vol. 60, p. 193.  1968.

234.   Trubnick,  E.H.  and P.K.  Mueller.    "Sludge Dewatering
      Practice."    Sewage and   rndustrial  Wastes.   Vol.  30,
      p.  1364.   1967.

235.   Sontheimer,  H.   "Effects  of Sludge Conditioning  with Lime
      on Dewatering."   Advances in Water Pollution Research,
      Proceedings  and  I"te^"ational^_^CjC^J^ejrerice^^W_ater  Pollution
      Research. Munich.   1967.

236.   Lime;   Handling,  Application  and Storage  in Treatment
      Processes.    National Lime  Association,  Washington,  D.C.
      1977.   Bulletin 213.

237.   Lime for Water and Wastewater  Treatments.   B.I.E.   Unit of
      General  SignalProvidence,RhodeIsland.   02901   Ref.
      No.  1.21-24.   June  1969.

238.   USEPA.    Lime Use  in Wastewater Treatment;   Design and Cost
      Data.    Municipal  Environmental  Research  Laboratory.
      Cincinnati,  Ohio  45268.    EPA-600/2-75-038.   October  19,75.

239.   USEPA.   Process  Design Manual  for Suspended Solids
      Removal.    Technology Transfer.   Cincinnati, Ohio  45268.
      EPA 625/l-75-003a.   January 1975.

240.   Kraus,  M.N.    "Pneumatic  Carrying-General  Considerations,
      Equipment  and Controls," Chemical  Engineering.   April
      1965.

241.   Water Pollution  Control  Federation.    Manual of  Practice
      No.  8.   Wastewater  Treatment   Plant Design  Water  Pollution
      Control Federation.   Washington,  D.C. 1977.

242.   Hicks,  R.W.,  J.R.  Martoa, and  J.G.  Felic  "How  to  Design
      Agitators  for  Desired  Process Response.   Chemical
      Engineering,  April  1976.

243.   USEPA.    Energy Requirements   for Municipal Control  Facil-
      ities.   Office of  Water Program Operations.    Washington,
      D.C. 20460.   March  1977.

244.   Saunier,  B.M.   Kinetics  of  Breakpoint  Chlorination  and
      Disinfect ion.    Ph.D.  Thesis.   Department  of   Civil
      Engineering,  University  of California,  Berkeley,  California
      94720.   1976.

245.   USEPA.   Partial Characterization  of Chlorinated Organics in
      Superchlorinated  Septages  and  Mixed Sludges~  Office  o f
      Research and  Development.    Cincinnati,  Ohio,  45628.
      EPA-600/2-78-020.  March 1978.
                              6-156

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246.   Williams,  T.C.   "Phosphorous Removed  at  Low Cost."   Water
      and Wastes Engi nee r i ng.  Vol. 13.  1975.

247.   Oliver, J.W., W.C.  Kreye,  and P.H.  King.  "Heavy  Metal
      Release by Chlorine  Oxidation of Sludges."  Journal  of the
      Water  Pollution Control  Federation.  Vol. 47, p.  2490.
      1975.

248.   Sukenik,  W.H., P.H.  King, and  J.W.  Oliver.  "Chlorine and
      Acid Conditioning of Sludge."   Journal of  the Environmental
      Engineering Division-ASCE.  Vol. 6, p. 1013.  1977.
                              6-157

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EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
                Chapter 7. Disinfection
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                           CHAPTER 7

                          DISINFECTION
7.1  Introduction

Wastewater sludge disinfection, the  destruction  or  inactivation
of pathogenic organisms  in  the  sludge, is carried out principally
to minimize public health concerns.   Destruction is  the physical
disruption or disintegration of  a pathogenic organism, while
inactivation, as used  here,  is  the  removal of  a pathogen's
ability to infect.  An important but secondary concern  may be  to
minimize  the  exposure  of  domestic  animals  to pathogens in  the
sludge.   At  the  present time  in  the United States, the use  of
procedures to reduce the  number  of pathogenic organisms is  a
requirement before sale  of  sludge or sludge-containing  products
to the  public as a  soil amendment, or  before  recycling  sludge
directly to croplands, forests,  or parks.  Since the final use  or
disposal  of  sludge may  differ greatly  with  respect to public
health concerns,   and  since  a great  number  of  treatment  options
effecting   various degrees  of pathogen  reduction are  available,
the system chosen for reduction of  pathogens  should be  tailored
to the demands of the  particular situation.

This  chapter  identifies  the  major  pathogenic organisms
found  in  wastewater  sludges;  briefly describes  the  pathogen
characteristics,  including  size,  life and  reproductive
requirements, occurrence in sludge,  and survival under  different
environmental conditions;  and discusses methods  for reducing
the number of pathogenic  organisms in  sludge.   The effect  of
conventional  sludge  treatment  processes  on  pathogen  reduction
will be reviewed.  Two  types of processes designed  specifically
for the reduction of pathogenic  organisms in sludge are heat
pasteurization and high-energy irradiation,  and  they will  be
developed   in  detail.   Other  processes  such  as long-term  storage
and composting will also be discussed.


7.2  Pathogenic Organisms

A pathogen or pathogenic agent  is  any biological species that  can
cause  disease in the host organism.   The  discussions  in this
chapter will be confined to pathogens that produce disease in  man
and complete their life  cycles  in  North America.  These  organisms
or agents  fall into  four broad categories:    viruses,  bacteria,
parasites, and fungi.   Within  the  parasite category,  there  are
protozoa,  nematodes,  and helminths.   Viruses,  bacteria, and
parasites  are primary  pathogens  that are present at some level  in


                               7-1

-------
sludge as a result  of  human activity upstream from the wastewater
treatment  plant.  Fungi  are  secondary pathogens and are only
numerous in sludge  when  given the opportunity to grow during  some
treatment or storage process.


    7.2.1  Pathogen Sources

Pathogens  enter  wastewater  treatment  systems  from  a number  of
sources:

     •  Human  wastes,  including  feces,  urine,  and  oral  and
        nasal discharges.

     •  Food wastes from homes and  commercial establishments.

     •  Industrial wastes from  food  processing, particularly
        meat packing plants.

     •  Domestic  pet feces and urine.

     •  Biological  laboratory  wastes  such as  those from
        hospitals.

In  addition,  where  combined  sewer  systems  are used,   ground
surface  and  street runoff materials,  especially  animal  wastes,
may enter  the  sewers  as storm  flow.   Vectors  such  as rats  that
inhabit some sewer systems may also add a  substantial  number of
pathogens.


    7.2.2  Pathogen Characteristics

Viruses, bacteria,  parasites,  and fungi  differ in size,  physical
composition, reproductive  requirements,  occurrence in the United
States population,  and prevalence  in wastewater.


        7.2.2.1  Viruses

Viruses  are  obligate  parasites  and  can  only  reproduce  by
dominating  the  internal processes of host  cells  and using  the
host's resources  to produce more  viruses.  Viruses are very small
particles whose  protein surface  charge  changes  in magnitude  and
sign with pH.  In the  natural pH  range of wastewater and  sludges,
most  viruses  have  a   negative surface charge.   Thus, they  will
adsorb  to  a  variety of  material  under  appropriate chemical
conditions.   Different  viruses  show varying  resistance  to
environmental factors  such as heat  and moisture.  Enteric viruses
are  acid-resistant and  many show  tolerance to  temperatures  as
high as 140 °F (60° C).

Many  of  the viruses  that cause  disease  in  man  enter  the sewers
with  feces or other  discharges  and  have  been identified,  or
are  suspected of  being,  in  sludge.  The major virus subtypes
                               7-2

-------
transmitted in feces  are  listed in Table 7-1  together  with  the
disease  they  cause.   Viruses  are excreted  by  man in numbers
several  orders  of magnitude  lower  than  bacteria.   Typical
total  virus  concentrations  in  untreated  wastewaters are
1,000 to 10,000  plaque-forming  units  (PFU)  per 100  ml;  effluent
concentrations   are  10  to 300 PFU  per 100  ml.   Wastewater
treatment,  particularly  chemical coagulation  or biological
processes  followed   by  sedimentation,  concentrates  viruses
in sludge.   Raw primary  and  waste-activated sludges contain
10,000  to 100,000 PFU  per 100 ml.


                            TABLE 7-1

             PATHOGENIC HUMAN VIRUSES  POTENTIALLY IN
                       WASTEWATER SLUDGE
                  Name
       Disease
          Adenoviruses
          Coxsackie virus,
            Group A
          Coxsackie virus,
            Group B
          ECHO virus, (30
            types)

          Poliovirus (3 types)
          Reoviruses

          Hepatitis virus A
          Norwalk agent


          Rotavirus
Adenovirus infection

Coxsackie infection;
  viral meningitis;
  AFRIa,  hand,  foot,
  and mouth disease

Coxsackie infection,
  viral meningitis;
  viral carditis,  end-
  emic pleurodynia,
  AFRIa

ECHO virus infection;
  aseptic meningitis;
  AFRIa

Poliomyelitis

Reovirus infection

Viral hepatitis

Sporadic v.iral  gastro-
  enteritis

Winter vomiting dis-
  ease
           iAFRI" is acute febrile respiratory  illness.
        7.2.2.2  Bacteria

Bacteria  are single-celled  organisms that  range in size from
slightly  less than one micron (^)  in diameter to SM  wide by
15M   long.   Among  the  primary  pathogens,  only  bacteria  are able
to  reproduce  outside  the host organism.   They  can grow and
                              7-3

-------
reproduce  under a  variety  of  environmental conditions.    Low
temperatures  cause  dormancy,  often for  long  periods.   High
temperatures are more  effective  for  inact ivation,  although  some
species  form heat-resistant spores.   Pathogenic  bacterial species
are he terotrophic and  generally grow best at a pH between 6.5  and
7.5.  The ability of  bacteria to  reproduce  outside  a  host  is an
important factor.   Although  sludge may  be disinfected, it can be
reinoculated and recontaminated .
Bacteria are numerous in  the human  digestive tract;  man excretes
up  to 10^3  coliform  and -lO-^ other bacteria  in his  feces  every
day.  The most important of the pathogenic bacteria are listed  in
Table 7-2,  together with the diseases they cause.
                              TABLE 7-2

               PATHOGENIC HUMAN BACTERIA POTENTIALLY
                        IN WASTEWATER SLUDGE
               Species
                                                 Disease
      Arizona hinshawii
      Bacillus cerejus
      Vibrio cho'lerae
      Clostridium perfringens

      Clostridium tetarri

      Esc_her ich_i.a coli

      Leptospira sp
      Mycobacterium tuberculosis
      Salmonella paratyphi,  A, B,
      Sa_lmonella sendai
      Salmonella sp   (over  1,500
        serotypes)
      Salmonella typhi
      Shigella sp

      Yersinia enterocolitica
      Yersinia pseudotuberculosis
Arizona infection
B. cereus gastroenteritis; food poisoning
Cholera
C. perfringens gastroenteritis; food
 poisoning
Tetanus

Enteropathogenic E. coli infection; acute
 diarrhea
Leptospirosis; Swineherd's disease
Tuberculosis
Paratyphoid fever
Paratyphoid fever

Salmonellosis; acute diarrhea

Typhoid fever
Shigellosis; bacillary dysentery; acute
 diarrhea
Yersinia gastroenteritis
Mesenteric lymphadenopathy
         7.2.2.3  Parasites

Parasites include protozoa, nematodes, and helminths.  Pathogenic
protozoa are single-celled animals  that range  in size from  8n  to
25/u  •   Protozoa are transmitted  by cysts,  the  nonactive  and
environmentally  insensitive  form  of  the  organism.   Their  life
cycles  require  that  a  cyst  be  ingested  by  man  or  another
host.    The cyst  is  transformed  into  an active organism  in  the
intestines, where  it matures and  reproduces,  releasing  cysts  in
the  feces.   Pathogenic  protozoa are  listed in  Table 7-3,  together
with  the diseases they  cause.

Nematodes are roundworms and hookworms that may  reach sizes up to
14  inches  (36  cm)  in the human  intestines  (1).   The more  common
roundworms  found in  man and the diseases they  cause are listed in
                                 7-4

-------
Table  7-3.    They  may  invade  tissues  other than the  intestine.
This situation  is especially  common when man  ingests the  ova
of  a roundworm  common  to  another species  such as  the  dog.   The
nematode  does  not  stay  in  the  intestine  but  migrates  to  other
body tissue such as  the  eye  and encysts.   The cyst,  similar to
that formed  by  protozoa,  causes inflammation  and fibrosis in
the  host tissue.    Pathogenic  nematodes  cannot  spread  directly
from man  to  man.   The  ova  discharged   in   feces  must  first
embryonate  at  ambient  temperature,  usually  in the  soil,   for at
least  two weeks.
                                TABLE 7-3

                PATHOGENIC HUMAN AND ANIMAL PARASITES
                   POTENTIALLY IN WASTEWATER SLUDGE
                Species
    Protozoa
     Acanthamoeba sp
     Balantidium coli
     Dientamoeba fragilis
     Entamoeba histolytica
     Giardia lamblia
     Isospora bella
     Naegleria fowleri
     Toxoplasma gordii

    Nematodes
     Ancyclostoma dirodenale
     Ancyclostoma sp
     Ascaris lumbricoides

     Enterobius verraicularis
     Necator americanus
     Strongyloides steTcoralis
     Toxocara cani_s

     Toxocara cati
     Trichusis trich_iura

    Helminths
     Diphyllobothrium laturn
     Echinococcus granuTbsis
     EchTnococcus multilocularis
     Hymenolepis diminuta
     Tymenolepis nana
     Taenia saginata
    • Taenia solium
               Disease
Amoebic meningoencephalitis
Balantidiasis,  Balantidial dysentery
Dientamoeba infection
Amoebiasis; amoebic dysentery
Giardiasis
Coccidiosis
Amoebic meningoencephalitis
Toxoplasmosis
Ancylostomiasis; hookworm disease
Cutaneous  larva migrans
Ascariasis; roundworm disease; Ascaris
  pneumonia
Oxyuriasis; pinworm disease
Necatoriasis;  hookworm disease
Strongyloidiasis; hookworm disease
Dog roundworm disease, visceral larva
  migrans
Cat roundworm disease; visceral larva
  migrans
Trichuriasis;  whipworm disease
Fish tapeworm disease
Hydated disease
Aleveolar hydatid disease
Rat tapeworm disease
Dwarf tapeworm disease
Taeniasis; beef tapeworm disease
Cysticercosis; pork tapeworm disease
Helminths  are  flatworms,  such as  tapeworms, that may be more
than 12  inches  (30  cm)   in  length.   -The most  common types  in  the
United  States   (listed  in  Table  7-3)  are  associated  with  beef,
pork,  and  rats.   Transmission occurs  when  man  ingests raw or
inadequately cooked  meat  or  the  eggs  of  the  tapeworm.    In  the
less  serious  form,  the  tapeworm  develops   in  the  intestine,
maturing  and  releasing eggs.    In  the more  serious  form,   it
localizes  in the  ear, eye,  heart,  or central nervous system.
                                   7-5

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        7.2.2.4  Fungi


Fungi  are  single-celled  non-photosynthesizing  plants  that
reproduce  by developing  spores, which  form new colonies  when
released.   Spores range  in size  from 10 to 100  .   They are
secondary pathogens in wastewater sludge, and large numbers have
been found growing in compost (2).  The pathogenic fungi, listed
in Table 7-4,  are  most  dangerous  when  the spores are  inhaled by
people  whose systems  are  already stressed  by a disease  such
as  diabetes,  or by  immunosuppressive  drugs.   Fungi  spores,
especially those of Aspergillus fumigatus, are ubiquitous in the
environment  and  have  been  found  in pasture  lands, hay  stacks,
manure piles, and the  basements  of  most homes  (2).
                            TABLE 7-4

                 PATHOGENIC FUNG! POTENTIALLY IN
                       WASTEWATER SLUDGE

                  Species
                                Actinomycosis

              Aspergillus sp      Aspergillosis; Asper-
                                  gillus pneumonia
                                  otomycosis

              Candida albicans    Moniliasis;  candidiasis
              __^_ _,___^_^      oral thrush
    7.2.3  Pathogen Occurrence  in  the United States


Information on pathogen  occurrence  and  associated morbidity and
mortality data vary greatly  with  pathogenic  species.   Available
data, complied by  the Center  for Disease Control  (CDC)  of the
United  States Public Health  Service,   indicates that  enteric
viral, bacterial,  and  parasitic  infections  annually  affect tens
of  thousands of people in  the  United  States  (3-7).   Data on
the  occurrence of  bacterial disease in the United States are
scarce.   However,  the  frequent  detection  of enteropathic bacteria
(bacteria which affect  the intestinal  tract), such as  E.  coli,
Salmonella,  fecal  streptococci, Shigella,  and  others in untreated
wastewater and wastewater sludges indicates that these pathogens
and their associated diseases are-endemic to the United States.


As recently as 1977, over 12 percent of  stool samples  checked by
state and  territorial  public health laboratories  were  positive
                               7-6

-------
for one  or more pathogenic  parasites.   A. lumbricoides, which
produces a resistant  ova, was  found  in over two percent of the
samples (6,7).

The frequent  occurrence of enteric pathogens in the  United States
population indicates  that  pathogens  should be  expected  in all
wastewaters and sludges.


7.3  Pathogen Survival During Sludge Stabilization Processes

Sludge  stabilization  processes are ideally  intended  to
reduce  putrescibility,  decrease  mass, and  improve  treatment
characteristics such as dewaterability.   Many stabilization
processes  also accomplish  substantial  reductions in pathogen
concentrations.
    7.3.1  Pathogen  Reduction During Digestion

Sludge digestion  is one  of  the  major  methods  for  sludge
stabilization in the United States.   Well-operated digesters can
substantially reduce  virus and bacteria  levels but are less
effective against parasitic  cysts.
        7.3.1.1  Viruses

Viruses  are  removed  most  readily  in  wastewater  treatment
processes when attached to larger particles such as chemical or
biological  floes.   Sagik has  reported  primary treatment  virus
removal  from  three percent  to  extensive  (8).   Metcalf has
measured primary  treatment removals of  60 to 95 percent with a
one-hour detention time  (9).  Sagik and Moore have  reported  70 to
99 percent removals with activated sludge  (8,10).

Virus  concentration  ranges  for  raw  and  anaerobically  digested
sludges are given in Table 7-5.  The large difference between the
high  and  low  value for  the number  of  viruses  in  untreated
sludge results from several factors, including variation  in  virus
occurrence  in  the  human  population,  differing treatment  plant
removal efficiencies, and disparity in viral preconcentration and
assay  techniques.   Anaerobic digestion has  been shown to reduce
the concentration of detectable  viruses  by one  to  several orders
of magnitude.  Moore and others reported  a reduction by four
orders  of  magnitude for poliovirus by  anaerobic  digestion for
30 days  at  85°F  (30°C)  (10).  Ward and  Ashley  reported  four log
inactivation of poliovirus  in  four days  at 82°F  (28°C)  (17).
Ward  also found that naturally occurring  ammonia (NH3) was a
viricidal  agent  for  poliovirus,  Coxsackie,  and ECHO  (18).
However,  it was less effective against  reoviruses.   Digester
detention  time,  operating  temperature,  and method of operation
                              7-7

-------
are  apparently  the  most  important  factors  affecting  virus
removal.   Stern and  Farrell  report  almost 50  percent  virus
inactivation with  sludge  storage  at 67°F  (20°C)  for  two  weeks
under  laboratory  conditions  (11).   Reduction continued  with
longer  storage.   Increased  operating  temperature  also improves
reduction.                        .-....•:
                            TABLE 7-5

                     PATHOGEN OCCURRENCE IN
                    LIQUID WASTEWATER SLUDGES

                               Concentration, number/100 ml
Pathogen
Virus
Bacteria
Bacteria
Bacteria
Bacteria
Bacteria
Bacteria
Parasites
Parasites
Name or species
Various
Clostridia sp
Fecal coliform
Salmonella sp .
Streptococcus faecalis
Total coliforms
Mycobacterium tubercu-
losis
Ascaris lumbricoides
Helminth eggs
Unstabilized raw
sludge3
2.5 x 103
6
8
3
5
200
200
- 7 x 104
x 106
1Q93
x 10J
x 107
x 109
10?
- 1,000
- 700
Digested sludge3'
100
2
3 x 104
BDL
4 x 104
6 x 104
0
30
- 103
x 107
- 6 x 106
- 62
- 2 x 10°
- 7 x 107
106
- 1,000
- 70
Reference
9, 10,
12
13,
11
11
11
14
15
16
11
12
 Type of sludge usually unspecified.

3Anaerobic digestion; temperature and detention times
 varied.

"BDL is below detection limits, <3/100 ml.
Thermophilic anaerobic digestion of  sludge  at a temperature
of  121°F (50°C)  with  a  20-day retention time  at the  City of
Los  Angeles  Hyperion  Treatment Plant  showed  a  two  log greater
virus  reduction than  for  comparable mesophilic  digestion  at a
temperature of  94°F  (35°C)  and the same  time  period  (19).   Half
the  thermophilic samples,  however,  still  showed measurable
viruses, which  was unexpected; this  may be due  to  the  way that
digesters  are  operated.    Plant-scale  digesters  are  usually
operated on  a  fill-and-draw  basis.   If  the digesters  are mixed
continuously, the  daily fraction  of  sludge which  is  removed to
make room for the addition of raw  sludge will contain sludge that
has been in the process for only  a short time.  Considering this
fact,  the  appearance of viable pathogens  in  digested  sludge is
not surprising.


        7.3.1.2  Bacteria

Most  bacteria  in  wastewater are  readily  sampled  and  measured.
Commonly found  concentrations and  types of bacteria are shown in
Table  7-5.    The  sensitivity of  assay   techniques  for  different
                               7-8

-------
bacterial species do vary,  from  3  MPN  per 100 ml for Salmonella
to 1,000 MPN per  100 ml  for  total coliform,  fecal coliform,
and  fecal  streptococcus.    In general,  anaerobic digestion
reduces  bacterial  counts  by one  to four logs.   Work conducted
at Hyperion,  in parallel with the  virus  studies discussed
previously,   showed  thermophilic anaerobic  digestion of  sludge
decreased bacterial counts  by  two  to three logs over mesophilic
digestion  (19).    Increasing   both  the  temperature  and  the
detention time  increases  bacterial  inactivation.   Fill-and-draw
operation, however, prevents digestion  from removing  as  large  a
fraction of  the  bacteria as it  might in  another operating mode.

Farrell and  Stern reported the  following  bacterial concentrations
in an aerobically digested waste-activated sludge  (13):

    fecal coliform           7  x  107 MPN  per 100 ml

    Salmonella               1.5  x  104 MPN per 100 ml

The Salmonella values  are  higher  than  the upper  end of  the
typical  range of  values  given  for  anaerobically digested sludge
in Table 7-5.

For  thermophilic  oxygen-aerobic digestion,  Ornevich and  Smith
reported that increasing  temperature decreased the time required
for bacteria  inactivation  (20).   At 113°F  (45°C) Salmonella and
Pseudomonas  were reduced to below detectable limits in 24 hours;
at 140°F(TO°C), the time  was reduced to  30  minutes.
        7.3.1.3  Parasites

There  is a wide  variation  in  the apparent  level of  parasite
infestation from region to region in the United States  (6,7,21).
Protozoa cysts  should  not  survive  anaerobic  digestion,  but
helminth ova  definitely do  and  should be  expected  in digested
wastewater sludge unless testing  proves  the  contrary.

The  data  for  parasite  occurrence  and  persistence during
wastewater treatment  are  much  more  limited than  those  for
bacteria.  Cysts of the protozoa  Entamoeba histolytica,  have been
reported at about  four per  liter  in  untreated wastewater (16).
Protozoan  cysts  have  a low  specific gravity  and  are  not likely
to be  removed  to  any  great degree in primary sedimentation.
Secondary  treatment by  the activated  sludge process is reported
to  incompletely   remove  all  cysts.    Trickling  filters  can
remove up  to 75 percent of cysts  (8).  E. histolytica are easily
inactivated by well-operated  mesophilic  sludge  digestion.

Data  for helminths are  also sparse;  limited  data  for sludges,
reported in Table 7-5, indicate  that digestion  can cause some ova
reduction.   Stern  and  Farrell reported that Ascaris ova survived
thermophilic (121°F,  [50°C])  digestion at the  Hyperion Treatment
Plant (11).
                               7-9

-------
        7.3.2  Long Term Storage

Pathogen  reduction  has been  recognized  for years as  a  side
benefit of sludge  storage  in lagoons.   Hinesley and others have
reported 99.9 percent  reduction  in  fecal coliform density after
30-days  storage  (22).  For an  anaerobically digested  sludge
stored in anaerobic conditions for 24 weeks at 39°F  (4°C), Stern
and Farrell  reported  major  reductions  in  fecal  coliform, total
coliform,  and Salmonella bacteria (11).   In similar tests  at 68°F
(20°C), the  same  bacteria  could  not  be measured after 24 weeks.
Viruses were  reduced  by 67  percent  at  39°F  (4°C)  and  to below
detectable limits at 68°F (20°C)  in the  same  time period.  Recent
work by Storm and others showed  fecal coliform reductions of one
to  three  orders  of  magnitude  during  long-term storage of  an
anaerobically digested  mixture  of  primary  and  waste-activated
sludge in facultative  lagoons (23).


        7.3.3  Chemical Disinfection

A number of  chemicals  used for wastewater sludge stabilization,
including lime and chlorine,  also reduce the  number of pathogenic
organisms in sludge.
        7.3.3.1  Lime

Lime  treatment  of wastewater  sludge  is discussed  in  detail in
Chapter 6.   Plant-scale liming  of  wastewater  sludge was evaluated
at Lebanon,  Ohio  (24).  Two chemical-primary  sludges,  one with
alum  and  one with ferric  chloride,  were  limed to  pH  11.5 and
placed  on  drying  beds.  After one  month,  galmonella  sp^ and
Pseudomonas aeruginosa were  undetectable.   Bench  testing was also
conducted on ferric  chloride-treated  wastewater  raw  sludges that
were limed to pH 10.5,  11.5 and 12.5; these  sludges  were sampled
after 0.5 hours and  24  hours and  bacterial tests performed  (24).
Pathogenic   bacteria  reduction  improved  with time  and  was
substantially better at pH values of  11.5  and 12.5.  Qualitative
checks for higher  life forms  such as Ascaris ova indicated that
they survived 24 hours at a pH greater than  11.0.  Virus studies
on limed sludges have  not been reported, but a pH    in excess of
11.5 should inactivate known viruses  (11).
        7.3.3.2  Chlorine

Chlorine  is  a strong  oxidizing  chemical  used  for disinfecting
drinking  water  and wastewater effluents.   It  is  effective for
bacteria and virus inactivation  if  applied  in sufficient quantity
to  develop  a free  chlorine  residual  in the  solution - be ing
treated.   Chlorine is less  effective  in disinfecting solutions
with a  high  suspended solids  concentration.   Cysts  and  ova of
parasites are  very resistant to chlorine.  The  use of chlorine
for wastewater sludge  treatment  is presented  in  Chapter  6.   Few


                               7-10

-------
data are available on the potential of  chlorine  for  reducing  the
number of pathogenic  organisms in sludge.  Some samples  of  sludge
treated  with  large  doses of  chlorine  in South Miami,  Florida,
and Hartland, Wisconsin, showed large  reductions  in bacteria
and coliphages  (25).   Chlorine doses  of 1,000  mg/1 applied  to
waste-activated  sludge  (WAS) with a 0.5 percent solids  concentra-
tion reduced  total bacteria  counts by  four  to seven logs  and
coliform bacteria  and coliphage  to  below detection limits.
Primary  sludge with  a  0.5  to 0.85 percent  solids  concentration
was  treated  with 1,000  mg/1  chlorine, and  total  and  fecal
coliform counts  were  reduced below detectable limits.
        7.3.3.3   Other Chemicals

Other strong oxidizing chemicals such as ozone are sometimes  used
for drinking water and wastewater disinfection.  While  they may
prove useful for sludge disinfection, they are as yet untried.


7.4  Pathogen Survival in  the Soil

An objective of  reducing  the  number of pathogens  in  wastewater
treatment  plant  sludge   is to  produce  a  product  that may be
beneficially utilized.  As such, the behavior of sludge pathogens
in the  soil is  important.  Sludge is returned to the soil by
spray irrigation,  surface  flooding, wet or dry surface spreading,
or subsurface injection.    These techniques  expose  the  sludge to
the sun, air, water,  and soil in different ways that may strongly
affect pathogen  survival.
    7.4.1  Viruses

Data for the survival  of viruses, bacteria, and parasites  in  soil
are summarized  in  Table  7-6.   Factors  that have been found  to
affect survival include soil  temperature, pH, clay concentration,
cation  exchange capacity,  specific surface  area,  and organic
content.    Virus adsorption to  soil particles is  the chief
mechanism for  their retention when  applied to the land.  Virus
adsorption  in  soil  is reversible.  Viruses survive  best  at
slightly  alkaline  pH's.    Cooler  temperatures  prolong virus
infectiveness,  as  does  a  moisture  content  between  15  and
25 percent (8).
    7.4.2  Bacteria

Maximum recorded bacterial  survival times, vary with species,  from
a little over one month to almost a year,  as shown in Table  7-6.
The  important  variables  in  bacteria  survival  are moisture
content, moisture  holding  capacity,  temperature,  pH,  sunlight,
organic matter, and competition or predation  (26).  Moisture
content is most  important, since  desiccation  often leads  to


                               7-11

-------
cellular death.  Lower  temperatures prolong survival, and a lower
pH increases  the rate of inactivation.   The  presence of organics
may promote survival  or even regrowth.
                             TABLE 7-6

                     PATHOGEN SURVIVAL IN SOILS
   Pathogen
    type
Name or species
Length of survival,
     days
Reference
   Virus
   Virus
   Virus
   Virus
   Virus

   Bacteria
   Bacteria
   Bacteria
   Bacteria
   Bacteria
   Bacteria
   Bacteria
   Bacteria

   Parasite
   Parasite
   Parasite
Poliovirus
Poliovirus 1
ECHO 7
ECHO 9
Coxsackie B3
Clostridium sp
Leptospira sp
Mycobacterium tuberculosis
Salmonella sp.
Salmonella typhi
Shigella sp.
Streotococcus faecalis
Total colilorm
Entamoeba histolytica
Ascaris lumbricoides
Hookworm larvae
Up
Up
Up
Up
Up
Up
Up
to
to
to
to
to
to
to
More
Up
Up
Up
Up
Up
Up
Up
Up
to
to
to
to
to
to
to
to
84
170
170
170
170
210
43
than 180
570
120
210 ,
210
210
8
2,550
42
8
10
10
10
10
15
27
27
28
27
15
15

27
27
27
Burge reported  that sludge applied  by  subsurface injection tends
to maintain its  identity  in clumps  (29).  Since bacteria  and
viruses  in  sludge  are  associated  with the  solids, they  may be
protected from  natural  predation and other environmental factors
in the  sludge.   Burge also stated  that ammonia  in sludge  may be
bactericidal.

If sludge  is  applied  by a  surface method and  allowed  to  dry
before  incorporation  into  the  soil,  considerable  bacterial
reduction can  be  achieved.   This potential  advantage  of surface
applications must be weighed  against the associated odor risk and
the cost of subsurface injection.


    7.4.3  Parasites

Protozoa  cysts  are  reported  to  be destroyed in  eight  days after
land application.   Helminth  ova, however,  are very durable  and
may survive up  to seven years.   Hookworm larvae may be viable for
over a month.


7.5  Potential  Human Exposure to Pathogens

Man may be exposed  to pathogens  in  wastewater sludge in a variety
of ways  and at greatly varying  concentrations.   Figure  7-1 lays
out in  simplified form  some of  the potential pathways.   There is
                                7-12

-------
no  firm  scientific evidence  to  document a single confirmed  case
where  human  disease is directly  linked  to exposure to pathogens
from  wastewater sludge.   Viable  pathogens have,  however,  been
isolated from intermediate  points  in the  sludge  management
system,  such as from surface  runoff from sludge treated  fields.
These  factors  should be considered  in  the selection and design of
a process for  reducing the number of pathogenic organisms.
HUMAN
 OB
ANIMAL
SOURCES
 SLUDGE
TREATMENT
PROCESSES
(NGESTJON, DIRECT CONTACT,INHALATION
                                         DIRECT CONTACT/INHALATION
                             FIGURE 7-1

                POTENTIAL PATHOGEN PATHWAYS TO MAN
 7.6   Heat  Disinfection  Processes

 The  number of  pathogenic  organisms in  wastewater  sludge can  be
 effectively  reduced  by applying  heat  to  untreated  or  digested
 sludges.   Heat may  be used  solely for  pathogen reduction  as
 in  pasteurization or  as one  step in  a  processes  designed  to
 stabilize  sludge,  improve  treatability  or  reduce  mass.   The  focus
 of  this section will be on sludge  pasteurization.  Other heating
 processes,  such as  thermal processing  and incineration, are
 developed  in Chapters 8 and 11 and will only be  reviewed briefly
 here.
                                7-13

-------
    7.6.1  Sludge Pasteurization

Man has  recognized  for many  years that heat  will inactivate
microorganisms as  well as  the  eggs  and cysts  of parasites.
Different species and  their  subspecies  show  different  sensitiv-
ities  to  elevated  temperatures  and  duration  of  exposure.
Roediger,  Stern,  and  Ward  and  Brandon  have  determined  the
time-temperature relationships for  disinfection of wet  sludges
with heat  (30-32).   Their results,  summarized  for a number  of
microorganisms in Table  7-7,  indicate that pasteurization  at
158°F (70°C)  for  30  minutes  inactivates parasite  ova and  cysts
and reduces population of measurable pathogenic viruses  and
bacteria below detectable  levels.  For bacteria, Ward and Brandon
found that fecal streptococci were most heat-resistant,  followed
by coliforms and then Salmonella  (32).   Nicholson indicates  that
a  higher temperature for  a  shorter time period  (195°F  [91°C],
10 minutes)  also destroys  all pathogens  (33).

                            TABLE 7-7

                TIME AND TEMPERATURE TOLERANCE FOR
                  PATHOGENS IN SLUDGE (30, 31, 32)

                                    Exposure  time  for
                                        organism
                                    inactivation, min
                   Species
                                     Temperature, C

                                   50  55  60  65  70
           Viruses                                  25

           Mycobacterium tubercu-
             losis                                  20
           Micrococcus pygogenes                    20
           Escherichi coll                  60       5
           Salmonella typhi                 30       4

           Fecal  streptococci                       60
           Fecal  coliforms                          60
           Corynebacteriuin dipth-
             eriae                      45     ,      4

           Brucella  abortus             60       3
           Cysts  of  EntamoeJDa his-
             tolytica                5
           Eggs of Ascaris lumbri-
             coides                 60   7
           Aspergillus flavus
             conidia                       60
           °F  =  1.8 °C +  32
                               7-14

-------
        7.6.1.1   Process  Description

The critical requirement  for pasteurization  is  that  all  sludge be
held above a predetermined temperature for a minimum time  period.
Heat  transfer  can be  accomplished by steam injection or with
external  or  internal heat  exchangers.    Steam   injection  is
preferred because heat  transfer through the  sludge slurry  is  slow
and undependable.   Incomplete mixing will either  increase  heating
time,  reduce process  effectiveness, or both.  Overheating  or
extra  detention are not  desirable, however,  because trace metal
mobilization may  be increased, odor problems will be exacerbated,
and  unneeded  energy  will  be  expended.    Batch processing  is
preferable to avoid reinoculations if short  circuiting occurs.
 PREHEATER 1.45 psi.
   CONCENTRATED
   DIGESTED SLUDGE
    \          64°F
     V
      -\^=--
                              113°F
                                       1,45 psi
                                       BLOW-OFF
                                       TANKS
                         TO THE VACUUM PUMPS


                         TO THE VACUUM PUMPS

               ,RECUPERATQR|. «,—,_.
                                 CONDENSER


                                COOLING WATER


                                      INLET_
                                     OUTLET
                                  PASTEURIZED
                                  SLUDGE
                                    TO TANKER
   STORAGE BASIN
                PUMPS
                                       PUMP
                       STORAGE BASIN PUMP
        SLUDGE
        HEATING STEAM
        VAPORS
        VACUUM {AIR)
        WATER
TO CONVERT DEGREES FAHRENHEIT TO DEGREES CENTIGRADE,
SUBTRACT 32 AND THEN DIVIDE BY 1.8.
TO CONVERT LB PER SQUARE INCH TO kN/m2,
MULTIPLY BY 6.9.
                             FIGURE 7-2

             FLOW SCHEME FOR SLUDGE PASTEURIZATION WITH
                SINGLE-STAGE HEAT RECUPERATION (11)


The  flow  scheme for  a typical  European  sludge  pasteurization
system with  a one-stage heat  recuperation system  is shown  on
Figure  7-2.   Principal  system components include  a steam  boiler,
a  preheater,  a  sludge  heater,  a  high-temperature holding  tank,
blowoff  tanks, and  storage basins  for  the untreated  and  treated
sludge.   Sludge  for pasteurization enters  the  preheater where the
                                7-15

-------
temperature is  raised  from 64 to  100°F  (18  to 38°C) by  vapors
from the  blow-off  tank;  30 to 40 percent of  the  total  required
heat is thus provided by recovery.   Next,  direct steam  injection
raises  the temperature to  157°F  (70°C)  in  the pasteurizer  where
the sludge resides for at  least  30  minutes.   Finally the  sludge
is transferred to the  blow-off tanks, where it is cooled  first to
113°F (45°C)  at  1.45 pounds per  square inch  (10 kN/m2)  and  then
to 98°F (35°C) at 0.73  pounds per square inch (5 kN/m2)  (31).

For sludge flows of 0.05  to 0.07 MGD (2 to 3 1/s),  a single-stage
heat recuperation system is  considered  economical.   In the
0.11 to 0.13  MGD (4.8 to  5.7  1/s)  flow  range a  two-stage  heat
recuperator is  considered  economical.   For flows  over  0.26 MGD
(11  1/s),  a  three-stage heat  recuperation  is considered
economically attractive.


        7.6.1.2   Current  Status

There  is  only  one operating municipal sludge  pasteurization
facility in the  United States  today,  a  heat  conditioning  system
converted  for pasteurization.   Pasteurization  is  often  used
in  Europe and  is  required in Germany  and  Switzerland before
application of sludge, to pasture lands during  the spring-summer
growing season.  • Based on  European  experience,  heat  pasteuriza-
tion is a proven technology,  requiring skills such as boiler
operation and understanding  of  high temperature and pressure
processes.   Pasteurization can be  applied  to  either  untreated
or  digested sludge with minimal pretreatment.   Digester  gas,
available  in many plants,  is  an  ideal  fuel  and  is  usually
produced in sufficient quantities  to  disinfect  locally  produced
sludge.   Potential  disadvantages include  odor  problems and
the need  for  storage facilities following  the process—where
bacterial  pathogens may regrow if sludge is reinoculated.


        7.6.1-.3   Design Criteria

A pasteurization system should be designed to provide a uniform
minimum temperature of  157°F  (70°C) for  at  least 30  minutes.
Batch processing  is  necessary. to  prevent  short  circuiting and
recontamination,  especially by bacteria.   In-line mixing  of  steam
and sludge  should  be  considered  as  a  possible  aid to  increase
heat transfer  efficiency  and  assure  uniform  heating.   In-line
mixing  will also eliminate  the need to mix the sludge  while  it is
held at  the pasteurization temperature.    The system should be
sized  to  handle peak  flows or  sludge  storage should  be  used
to  reduce peak  flows.   Sizing of storage  capacity  and the
pasteurization system  will  depend on the  type of sludge  treated,
the average sludge flow,  and the  end  use of  the  sludge.  If
digested  sludge is to be  pasteurized,  the  digesters  may  have
sufficient  volume to  hold  sludge  during  minor  mechanical
breakdowns or  when inclement weather prevents  an end use such as
land application-.  If  sludge is  to  be stored  after treatment and


                              7-16

-------
prior to pasteurization, a minimum  storage  volume  should  be two
days average flow.  Storage facilities must be equipped for odor
control or with  aeration  capacity  to prevent septic conditions.
Storage capacity  for  pasteurized sludge  should  be  adequate  to
hold at least  four days'  amount of  processed  sludge at average
flow.  Odor control must be provided,  and pilot-scale testing may
be  needed  to determine the  best odor  control process design.
Sludge thickening  prior to pasteurization  may be cost-effective
for  increasing  overall  energy efficiency, but  the value  of
thickening  should be  determined  on  a  case-by-case  basis.
Piping, pumps,  valves,  heat  exchangers, flow meters,  and other
mechanical  equipment should, at  a minimum, be comparable to those
for  thermophilic digesters.   The tanks  for holding sludge  during
pasteurization should  be corrosion-resistant.


        7.6.1.4  Instrumentation and  Operational Considerations

Temperature monitoring  at several  points in each pasteurization
system  is  a  minimum  requirement.    Flow metering  devices,
boiler  controls,  emergency  pressure  relief valves,  and level
sensors  in tanks  should  also  be  considered  (see  Chapter 17,
Instrumentation).

Heat  pasteurization  has flexibility  to  respond  to  variable
solids concentrations and  flow  rates,  provided  there  is  enough
basic  system  capacity.    Expansion  of  facilities  with parallel
modules should  work well; multiple  modules  also improve  system
reliability.


        7.6.1.5  Energy  Impacts

Pasteurization requires  both electricity for  pumping  and fuel for
heating  the sludge.    Energy  requirements  for  pasteurization
processes,  with  and without  heat  recovery,  have been estimated
for  secondary  activated  sludge  plants  where  either  raw  or
digested sludge  is pasteurized  (34).   A combination of primary
and waste-activated sludge with  4,800 gallons of  untreated sludge
per 1,000,000 gallons  (4.8 1/m3 ) raw  sewage or with 3,100 gallons
of  digested  sludge per  1,000,000 gallons (3.1 l/m-*)  raw sewage,
with a solids content of five percent and a specific heat  of one
Btu  per  °F  (1900  J/°C) were  assumed.   The  process  allowed for
10  percent heat loss and a  100 to  125 pounds per  square  inch
(690 to 860  kN/m2) boiler with  an  80 percent efficiency.   Steam
injection heats  the sludge to 157°F  (70°C), where it is held for
45  minutes  with steam  reinjection  to maintain  the  temperature.
The  energy requirements for processes with a range of wastewater
flows are summarized on  Figure 7-3.


        7.6.1.6  Cost  Information

The  only  sludge pasteurization  process  operating  in the  United
States was  not  initially  designed  for  pasteurization.  Thus  no
actual cost  data are  available.   Costs have been  estimated for
                               7-17

-------
the processes discussed under  "Energy Impacts" (34).   It was
assumed  that  the  processes would  have parallel pasteurization
reactors and  four-day  storage  volume for the pasteurized sludge.
The use  (volume  of throughput per given size)  for the processes
increases with increasing system size.
  1,000
     9
     8
     7
     6
     5

     4
                                                         100
"3
o

i_
-5*
as
 8
 G
 D
 u_
     3  —
 i TOO
 S   3
     8
                     FUEL

             WITH HEAT RECOVERY
                                                               to
                                                             £  H
                                                              cc
                                                              b
                                                              UJ
                                                              _J
    10

      1
           PLANT CAPACITY, MGD of wastewater (1 MGD = 0,044 m3/i)

                            FIGURE 7-3

     ENERGY REQUIREMENTS FOR SLUDGE PASTEURIZATION SYSTEMS (34)
Cost estimates were made in June 1977 for construction materials,
labor,  equipment,  normal excavation, contractor overhead and
profit, operating and  maintenance  labor,  materials and supplies,
and  energy.  Summary graphs  for  these  estimates are given on
Figures 7-4 through 7-7.
                               7-1!

-------
_ra
"o
O
u
cc
                              UNTREATED SLUDGE
     5             10            20       30     40   50 . 60  70 SO 90100

                PLANT CAPACITY, MGD of wisiewater {1 MOD = 0,044 m3/s)

                             FIGURE 7-4

           CONSTRUCTION COSTS FOR SLUDGE PASTEURIZATION
                SYSTEMS WITHOUT HEAT RECOVERY (34)

These  graphs were  used to  estimate unit  pasteurization costs
for  a  50-MGD  (2.2-m^/s)   secondary  wastewater  treatment plant.
Additional assumptions made  were  that  yard  piping for the system
would cost 15 percent of the total construction cost, electricity
would  cost  three  cents  per kilowatt hour,  fuel would  cost
$3.00 per  million  Btu's  ($2.84/GJ), labor  would  cost $10.00 per
hour, and  capital  was amortized over  20 years  at seven percent.
The  resulting pasteurization cost was  $15.00 per ton ($16.50/t)
of dry solids with heat recovery.  A similar calculation was made
for  a  10-MGD (0.44-m-Vs)  secondary plant with  no heat recovery,
a cost of  $33.00 per ton  ($36.40/t)  of dry  solids was estimated.
                               7-19

-------
                               UNTREATED SLUDGE
                  10             20       30    40   60  60  70 80 90100

                PLANT CAPACITY, MGD of wastfwatef (1 MGO = 0.044 m3/s|

                             FIGURE 7-5

           CONSTRUCTION COSTS FOR SLUDGE PASTEURIZATION
                  SYSTEMS WITH HEAT RECOVERY (34)
        7.6.1.7  Design Example

To establish  the  equipment requirements and layout  for a  typical
pasteurization  system,  digested  combined primary  and  waste-
activated sludge  from  a 50-MGD (2.2 m^/s) activated sludge  plant
are to be pasteurized prior to reuse by direct  injection.   If  the
sludge  is produced at a  rate  of  2,000 pounds of  solids  per
million gallons  (0.24  kg/m^), and  40 percent  of the solids  are
                               7-20

-------
destroyed  during
2.4 percent  solids.   The sludge
per million  gallons  (4.8  1/m3).
the  flow  rate is  0 . 3-MGD  " "
facility
rate  is 0.42
(18.9 1/s).
         digestion,  the  resulting  digested
                        flow rate is about  4,800  gallons
                        For the  50-MGD  (2.2 m3/s)  plant,
                   (13.0  1/s).   If  the pasteurization
is run  24  hours per  day, five
     MGD  (18.9  1/s)  or  about
              sludge has
days per  week,  the flow
300 gallons per  minute
   12 -
                               WITH HEAT
                               RECOVERY
                                           WITHOUT HEAT
                                           RECOVERY
                   10             20       30   • 40   50   60  70  SO 90100

                  PLANT CAPACITY, MGD of wastewatw ?1 MGD = Q.Q44 m3/^

                             FIGURE 7-6

                   LABOR REQUIREMENTS FOR SLUDGE
                     PASTEURIZATION SYSTEMS (34)
                                7-21

-------
   16 i-
                           UNTREATED SLUDGE
                    10       ,      20      30    40   SO  60  70 80 90100

                  PLANT CAPACITY, MGD of wastewater (1 MGD = 0,044 m3/sS

                              FIGURE 7-7

               MAINTENANCE MATERIAL COSTS FOR SLUDGE
                     PASTEURIZATION SYSTEMS  (34)
To  select the  reactor size,  assume  that there  are  two parallel
units  and each can  be charged,  held, and emptied,  in 1.5  hours.
Determining  the volume per reactor:
    V =
        NH
where:

    S =

    C ••

    N '
total sludge  volume per week, gallons;

cycle time, hours;

number of  reactors/cycle;
                                7-22

-------
    H = total operating hours.

For this example,


    v- (2.1 x 106 gallons) (1.5 hr/cycle)  ., , __   ..    ,.n _,  ,%
    V	(2 reactors/cycle)(120 hr)    = 13'125 ^allons <49'7 m3>


Assume  a 13,500  gallon (51  m3 )  storage tank will  be used  to
store  this  sludge.   Set prepasteurization  storage at  2.5  times
the  average daily  flow,  or  at one  million gallons  (3780 m3 ) .
Set post pasteurization storage at four  times  the  average  daily
flow or  1.7 million  gallons  (6350 m3) .   Three heat  exchangers
in  series  heat the  digested  sludge from 68°F  to  131°F  (20°  to
55°C);  the boiler  supplies  steam to  raise the temperature  to
157°F (70°C).  The heat exchangers can  be either sludge to sludge
or sludge to water to sludge.   Sludge-to-sludge exchangers should
be carefully specified as they have a history of fouling.

The  sludge pumps should be  sized and piped  either  to fill  or
empty a  13,500  gallon  tank (51 m3) in  30 minutes,  equivalent  to
450  gallons per minute  (28  1/s).   At  least three  pumps are
needed; providing one pump  on standby.

The required boiler capacity is calculated with the equation:


    „ _ AT h W
where:

    E  = energy required in Btu per hour

    AT = the temperature difference between  sludge  from the heat
         exchanger and sludge in the reactor;

    h  = heat capacity of the sludge,  Btu/lb°F;

    W  = wet sludge weight, lb;

    t  = time for heating;

    e  = boiler conversion efficiency.

If h is one Btu per lb °F (864 J/kg°C); e = 80 percent;    T = 63°F
(35°C);   W = 112,600 lb (51,200  kg); and    t = 0.5 hr;  then,
    E =    /;:o     = 17,700,000 Btu/hr (3.9 GJ/hr)
           ( U . b ) ( I) . o )
                               7-23

-------
An  additional  allowance  of  ten  percent  should  be  added  to
maintain  the  reactor temperature for 30  minutes,  giving a  total
of 19.5 million  Btu/hr  (4.3 GJ/hr) or about 600 horsepower.


Figure  7-8 provides a  schematic layout  for the major process
components.
                                      PASTEURIZATION
                                       REACTORS
UNTREATED
OR
DIGESTED
                                               15T°F
SLUDGEo
FROM
STORAGE
FEED
PUMPS


HE U HI ZED
.4
0 u
a <;
5"

^/X^X
S HEAT
' EXCHANGER
^
1



f


t
PASTi
iZf
SLUt
PUM



:UR-
D
)GE
PS
PAStEuftiZEO
 SLUDGE
 STORAGE
                                   PREHEATED
                                SLUDGE 131°F (56°C1
                                              u  -r


                                                                FUEL
                                                 125 psi
                                               (175°C
                                               860 kN/nf*
                                                               WATER
              SLUDGE
               FOR
            UTILIZATION
                             FIGURE 7-8

                SYSTEM COMPONENT LAYOUT FOR SLUDGE
                 PASTEURIZATION WITH HEAT RECOVERY
    7.6.2  Other  Heat Processes


The reduction of pathogenic  organisms in  sludge  may be  an  added
benefit  of  other sludge  treatment processes.    In this  chapter
heat  processes  are  subdivided  into heat-conditioning,  heat-
drying, high temperature combustion, and composting.
                                7-24

-------
        7.6.2.1   Heat-Conditioning

Heat-conditioning includes  processes  where  wet  wastewater  sludge
is pressurized  with or without oxygen and  the  temperature  is
raised  to 350°   to  400°F  (177° to  240  °C) and  held  for 15  to
40 minutes.   These processes destroy all pathogens in sludge, and
are discussed in detail in Chapter 8.


        7.6.2.2   Heat-Drying

Heat-drying  is  generally  done with a flash drier or a rotary
kiln.  Limited data  from analyses on Milwaukee,  Wisconsin's dried
sludge,  Milorganite,  produced with  a direct-indirect rotary
counterflow   kiln type  dryer,  indicates it  is  bacteriologically
sterile  (13).   Data on samples  of  flash-dried  sludge  taken
in Houston,  Chicago,  Baltimore, and Galveston, showed no coliform
bacteria  in  the Houston sludge and no greater  than 17 MPN/gm
dry  sludge in the  other  sludges.   Total  non-confirming  lactose
fermenters (spore formers)  ranged  from  14  MPN to  240,000 MPN per
gm (35).  No tests were made for viruses  or  parasites;  other
pathogens may also survive  if  some bacteria do.

Data for the  Carver-Greenfield process gathered  during  testing  by
LA/OMA showed a  seven order of magnitude  reduction  for  total and
fecal coliform,   to a detectable level of  less  than  one  organism
per  gram  (36).   Fecal streptococci  were  reduced six orders  of
magnitude to  two MPN  per gram  and Salmonella from  50,000 MPN per
gram to less than 0.2 MPN  per gram.   Ascaris ova  were  reduced  to
less than 0.2 ova per gram.


        7.6.2.3   High Temperature Processes

High temperature processes  include incineration,  pyrolysis, or a
combination   thereof  (starved-air  combustion).    These  processes
raise  the sludge temperature  above  930°F  (500°C) destroying
the  physical  structure of  all sludge pathogens  and effectively
sterilizing  the sludge.   The product  of a  high  temperature
process  is  sterile  unless shortcircuiting occurs within the
process.


        7.6.2.4   Composting

Composting is considered  here  as  a  heat  process  because a major
aim of sludge composting operations is to produce  a pathogen-free
compost  by  achieving and  holding a thermophilic temperature.
Available data  indicate   that a well-run  composting process
greatly  reduces  the  numbers  of primary pathogens   (37-40).
However, windrow or  aerated pile  operations  have not  achieved a
sufficiently uniform  internal temperature to  inactivate all
pathogens.  Adverse environmental  conditions, particularly heavy
rains,  can  significantly  lower  composting temperatures.    An


                               7-25

-------
additional problem with  composting  is  the potential regrowth of
bacteria.  This  is  particularly true with windrows where mixing
moves material from the outside of the mound to the center  (40).
However, storage of compost  for several months following windrow
or pile composting  helps to further  reduce pathogen  levels.

Secondary  pathogens,  particularly heat-resistant fungi  such
as Aspergillus,  have been  found to propagate rapidly during the
composting of  wastewater sludges.   Aspergillus  apparently  will
die out during storage  of several  months or more  (22).

Enclosed  mechanical  composting systems  may  achieve  sufficient
temperature,  157°F  (70°C) or greater, for  an adequate time;  more
research can verify the efficiency of mechanical systems for
pathogen reduction.


7.7  Pathogen Reduction With High-Energy Radiation

The  use of  high-energy  radiation  for  wastewater sludge
disinfection has been  considered  for over 25 years.  Two energy
sources,  beta  and  gamma  rays,  offer the  best  potential system
performance.   Beta  rays are high-energy electrons, generated with
an accelerator  for use in disinfection, while gamma  rays are
high-energy photons emitted  from  atomic   nuclei.    Both  types of
rays  induce  secondary  ionizations  in sludge as  they penetrate.
Secondary  ionizations  directly  inactivate pathogens and produce
oxidizing and reducing  compounds that in turn attack pathogens.


    7.7.1  Reduction of Pathogens  in Sludge With
           Electron Irradiation

High-energy  electrons,  projected  through wastewater  sludge  by
an appropriate generator,  are  being pilot tested as a means for
inactivating or destroying pathogens in sludge at the Deer Island
Wastewater Treatment Plant in  Boston, Massachusetts  (41).   The
electrons  produce  both biological  and chemical  effects as they
scatter  off  material  in  the  sludge.   Direct  ionization by the
electrons may damage molecules  of  the pathogen, particularly the
DNA  in  bacteria  cell  nuclei and the DNA  or  RNA  of the  viruses.
The  electrons  also cause  indirect action by  producing e^q
(hydrated electrons) and  H and OH free  radicals that react with
oxygen  and other molecules to produce ozone and hydroperoxides.
These  compounds  then  attack organics in the sludge--including
pathogens--promoting  oxidation,  reduction, dissociation, and
other forms of degradation.

The pathogen-reducing  power of the electron beam  (e-beam) depends
on the  number  and  the  energy of electrons impacting the sludge.
E-beam  dose  rates are  measured  in rads; one rad  is equal to the
absorption of 4.3 x 10~6 Btu per pound  (100 ergs/gin) of  material.
Since the radiation distributes  energy   throughout  the volume
of  material  regardless  of  the material  penetrated,  the degree


                               7-26

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of  disinfection  with  an  irradiation  system  is essentially
independent of the sludge  solids concentration within the maximum
effective penetration depth  of the radiation.   The penetrating
power of electrons is limited,  with a maximum range of 0.2 inches
(0.5 cm)  in water or  sludge slurries,  when  the  electrons  have
been accelerated by a potential of one million volts (MeV).


For  e-beam disinfection  to be effective,  some minimum  dosage
must be achieved  for all  sludge  being treated.   This  effect
is  attained  by  dosing above  the average dosage desired  for
disinfection.    One method used to ensure  adequate disinfection
is  to  limit the thickness of the sludge layer  radiated  so  that
ionization  intensity  of electrons exiting  the  treated  sludge  is
about  50 percent  of the maximum  initial  intensity.   For  the
0.85 MeV electrons used  in the existing facility, this constraint
limits  sludge  layer thickness to about 0.08  inches  (0.2  cm).


Accelerated electrons  can  induce radioactivity  in substances
which  they impact.   However,  the  electron  energy levels  for
sludge   irradiation, up to about 2  MeV,  are  well below the  10  MeV
needed  to induce significant  radioactivity with electrons.
        7.7.1.1  Process  Description

Disinfection with  an  e-beam has been  proposed  for use  on  both
untreated and  digested sludges.  The  major system components  of
the Deer  Island  facility  shown  on  Figure  7-9  include  the sludge
screener,  sludge grinder,  sludge feed pump,  sludge spreader,
electron beam  power  supply,  electron  accelerator,  electron  beam
scanner, and sludge  removal  pump.   A  concrete  vault  houses  the
electron  beam,  providing  shielding for  the workers  from  stray
irradiation,  especially  x-rays.   X-rays  are  produced by  the
interaction of  the  electrons  with  the  nucleus  of atoms  in
the mechanical  equipment  and  in the sludge.  The  pumps  must  be
progressive cavity  or  similar  types to  assure smooth sludge
feed.    Screening  and  grinding  of sludge  prior  to irradiation  is
necessary to assure  that a uniform layer of  sludge is passed
under  the e-beam.

At Deer  Island,  sludge  from  the feed pump discharges  into  the
constant head  tank  (see  Figure  7-10),  which is  equipped  with  an
underflow discharge weir.   Sludge  is discharged  under  the  weir
in a thin  stream and  then flows down an  inclined  ramp.   At  the
bottom  of the  ramp,  it  moves by free-fall into the receiving
tank.

The electrons  are first accelerated.  They leave the accelerator
in a continuous beam that is scanned back and  forth at 400  times
per second across the  sludge  as  it  falls free in a thin film from
the end of the inclined  ramp.  The dosage is varied by adjusting
the height of the underflow  weir  and hence the sludge flow rate.


                               7-27

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                          HIGH VOLTAGE CABLE
                                                       ELECTRON
                                                    ACCELERATOR
FEED
SLUDGE






          SLUDGE SLUDGE
          SCREEN GRINDER
                          ELECTRON
                          BEAM
                          SCANNER
SLUDGE
 FEED
 PUMP
                                                          CONCRETE
                                                          SHIELDING
                                           T
                                        SLUDGE
                                       SPREADER
SLUDGE
REMOVAL
 PUMP
                            FIGURE 7-9

          EQUIPMENT LAYOUT FOR ELECTRON BEAM FACILITY (41)




        7.7.1.2  Status

E-beam  sludge  irradiation  must  be considered  a developing
technology.   The  Deer   Island   irradiation  facility,  as  of
August  1979, is  the  only e-beam facility now operated  in the
United  States  for sludge disinfection.    This pilot project  is
designed to  treat  0.1  MGD (4  1/s)  sludge  at  up to eight percent
solids  with  a dosage  of  400,000  rads.   According  to  Shah,  the
facility has  been  operated about  700  hours since  it was brought
on line  in  1976,  with the longest continuous on-line time being
eight hours  (42).


        7.7.1.3  Design Considerations

Design  criteria  for  an e-beam  sludge facility are  difficult  to
establish because  operational data  are  available  from  only one
pilot facility.   However, the work  at Deer Island provides  good
baseline  information.   A minimum level of electron irradiation
should be 400,000 rads, which can best be  supplied with a one  to
two MeV  electron  accelerator.   This energy  level  provides  good
                               7-28

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penetration for  0.2-inch  (0.5-cm). thick sludge layers, making  the
achievement  of a  uniform sludge 'layer-'-less  important  than with
lower  energy  electrons.  However,  screening  and  grinding of
sludge  before  disinfection are  still  necessary to ensure uniform
spreading  by  this feed  mechanism.   The  high-energy electrons,
combined with  a  short  spacing of about 2.75-inches (7 cm) between
the  scanner  window and  the  sludge  film,  ensure  efficient energy
transfer in the  system.
    INPUT
(UNTREATED Oft
DIGESTED SLUDGE)
                                     ELECTRON
                                       BEAM
   CONSTANT
   HEAD
   TANK

   UNDERFLOW
   WEIR
   INCLINED
   FEED RAMP
                                                        ELECTRON BEAM
                                                             SCANNER
HIGH ENERGY
DISINFECTION
      ZONE
    SLUDGE
  RECEIVING
      TANK
                                                            .OUTPUT
                                                          (DISINFECTED
                                                          SLUDGES
                             FIGURE 7-10

             ELECTRON BEAM SCANNER AND SLUDGE SPREADER
Only  digested  sludge  has  been  irradiated  at  Deer  Island.
Nonstabilized  sludge  disinfection  by  e-beam  irradiation  still
requires  pilot-scale testing before any design  is considered.
                                7-29

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Owing to the limited penetrating power of high energy electrons,
this method  of treatment  is  probably only feasible  for  liquid
sludge.    Piping   pumps,   valves,   and  flow  meters  should  be
specified as equal  to  those  used  for anaerobic sludge digestion
systems.
systems.

        7.7.1.4  Instrumentation  and Operational
                 Considerations

Instrumentation needs  for an e-beam facility  should  include
flow  measurement  of  and  temperature  probes  in  the  sludge
streams entering and leaving  the irradiator.   Alarms  as well  as
monitoring should be used  to indicate variation in  sludge  flow
and high or low radiation doses.

Sludge disinfection by  e-beam  irradiation has  large  inherent
flexibility.    The radiation  source  (the  e-beam)  can  be switched
on and off as  easily as  an electric motor.   The  unit can be run
as needed, up  to  its  maximum  throughput  capacity.   Electron
accelerators have a proven  record  for reliability  over at least
20 years  in  industrial  applications and  should prove dependable
in wastewater  treatment  applications.   According  to  Haas,  the
reliability  of  the electron  beam  generator  and  associated
electronics presently  used  for  medical  and  industrial applica-
tions is  comparable  to that  for  the  microwave radar  systems  at
major airports (43).  Accelerators for sludge disinfection would
use the same basic components  and would have similar reliability.
Other system components--pumps, screens,  and grinders--are all  in
common  use in waste treatment  plants.   Cooling  air  for  the
scanner must be provided at several hundred cfm (about 10 m^/s).
This constant introduction  of  cooling  air leads to the generation
of ozone  in  the  shielding  vault  around the  accelerator.   If the
ozone were vented into  the  plant  or into  the atmosphere, some air
pollution would result.  At Deer.Island,  this problem is avoided
by venting the cooling  air  through  the sludge, where the ozone  is
consumed by chemical reduction.  These reactions  provide a small
amount of additional disinfection and  COD reduction.


        7.7.1.5  Energy Impacts

Energy  use for  e-beam facilities has  been  estimated  for  the
equipment used at Deer  Island.  A facility with a 50-kW  (50-kJ/s)
beam  would require  about  100  kW  (lOOkJ/s) of  total  electrical
power  including 25 kW  (25 kj/s) for screening, grinding,  and
pumping,  10 kW for (10  kJ/s) window cooling, and  12 kW  (12 kJ/s)
for  electrical  conversion  losses.    Energy  requirements  for
0.1  MGD  (4 1/m3) are  6 kWhr  per ton (24  MJ/t)  of wet  sludge
at five percent solids  or 120  kWhr per dry ton  (480 MJ/t)  (41).

        7.7.1.6  Performance  Data

Data  for e-beam disinfection of both untreated  and  digested
sludges  are available  as  a result of laboratory testing  done
prior  to  the operation  of  the  Deer  Island facility.   For
                               7-30

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untreated primary  sludge,  a dose  of  400 kilorads  (krads)  with
3 MeV electrons reduced total bacteria count by five logs,  total
coliform by more than  six  logs, below  detectable limits, and
total  Salmonella  by over  four  logs,  also  below detectable
limits.    Fecal  streptococci  were only reduced by  two  logs  with
data  indicating  that some  fecal  streptococci  are  sensitive  to
radiation while others are resistant.

For samples of  anaerobically digested  sludge  irradiated at  Deer
Island with 0.85  MeV electrons,  total bacteria were reduced  by
four logs at a  dose  of  280  krads,  total  coliform  by five  to six
logs at  a dose  of  150  to  200  krads;  a dose  of 400 krads reduced
fecal streptococci by 3.6  logs.

Virus inactivation has also been measured.   A  dose  of  400  krads
will  apparently  reduce the  total virus measured  as  plaque
forming  units  (PFU) by  one to  two  logs.    Laboratory  batch
irradiation of  five enteric viruses  showed about  two  logs
reduction at a  dose of 400 krads;  Coxsackie virus were  most
resistant while Adeno virus were least resistant.   These results
correlate directly with virus  size.   Larger viruses are  larger
targets and  hence more  susceptible  to  electron  "hits" (41).

Data  for parasite  reduction are scarce  but 400  krads  will
apparently destroy all Ascaris ova  (41).  Comparing these perfor-
mance data  with information from  Table  7-5 on the  quantity  of
pathogens in sludge  indicate  that a dose  of  400 krads may  be
adequate to disinfect  anaerobically digested sludge, but raw
sludge or aerobic sludge may require higher doses.


        1.1.I.I  Product Production and Properties

Odor  problems  are dramatically  lower for  irradiated  sludge  as
compared with  pasteurized sludge  (41).   Irradiation of digested
sludge  with an e-beam  may  also improve  sludge  dewaterability
and destroy some  synthetic  organic chemicals,  as  well  as  reduce
pathogen levels.  Irradiation has  reduced specific resistance  of
sludge by up  to 50 percent  at a  dose of  400 krads  (41).   Since
specific resistance is normally measured  on a  log  scale,   a
50  percent  reduction may  indicate  minimal  improvement  in  sludge
dewaterability.


        7.7.1.8  Cost Information

The  only cost estimates  available on e-beam sludge  treatment
process  result  from  work  done  at Deer Island.   The hypothetical
facility  used  for  the  cost  estimate  had the  following
characteristics:

     •  Electron beam power  of  75 kW  (75  kJ/s).

     •  Accelerator voltage  of  1.5  MeV.


                               7-31

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     ©   Disinfection dose  of  400 krad.

     •   Yearly throughput  of  50 million gallons (190,000 m3)  with
        process operating  300 days per year.  This throughput is
        equivalent to  the raw sludge  from a  25-MGD (I.l-ra3/s)
        activated  sludge plant or the digested sludge  from a
        35-MGD (1.5-mVs)  activated sludge plant.

The  total  capital  cost was  $600,000.   The  cost included  the
following:    accelerator component  with  scanner--$350,000;
automatic controls--$30,000;  sludge handling equipment—$100,000;
and  building  construction and  facility  installation--$120,000.
Annual  costs  were as  follows:  capital  (20  years at 10 percent)
$30,000;  depreciation--$30,000 ;  operation  and ma intenance--
$40,000; electric power  at  three  cents per kWhr (.83  cents  per
mJ)  $28,000;  and  water—$2,000.   This  cost estimate was carried
out  in  Boston in  late 1977.   At  that  time  the ENR construction
cost index was  about 2,650.   The net  cost  was  $2.53  per
1,000 gallons ($0.67/m3) of liquid  sludge treated.

The energy requirements  (fuel  and  electricity)  for an irradiation
system are  estimated  to be 90  to  98  percent less than those  for
heat pasteurization.
    7.7.2  Disinfection With  Gamma  Irradiation


Gamma  irradiation  produces effects  similar  to those  from an
electron beam.  However, gamma rays differ from electrons in two
major ways.   First,  they  are very  penetrating;  a layer of water
25  inches  (64  cm)  thick  is  required  to stop 90  percent  of the
rays  from  a cobalt-60  (Co-60) source;  in comparison,  a 1-MeV
electron can  only  penetrate about  0.4  inches   (1  cm)  of water.
Second, gamma  rays  result from decay of  a radioactive isotope.
Decay from a source  is  continuous and uncontrolled; it cannot be
turned off and on.   The energy level  (or levels) of the typical
gamma  ray  from a given radioactive isotope  are also relatively
constant.   Once  an  isotope  is  chosen  for use  as  a source, the
applied energy can only be varied with exposure  time.


Two  isotopes,  Cs-137 and  Co-60, have been  considered  as "fuel"
sources  for  sludge irradiators.   Cs-137 has  a half  life of
30  years and  emits  a 0.660 MeV gamma ray.   In the late 1970's,
it  was  available  in the United States  as  a  by-product from the
processing of nuclear weapons  wastes.   If  the United  States
establishes  a nuclear  reactor   spent-fuel  rod  reprocessing
program, it would also  be  available at  a rate  of about 2 pounds
per  ton  (1 kg/t)  of fuel.   Co-60  has  a half  life of five years
and  emits  two  gamma  rays  with  an  average energy of 1.2 MeV.  It
is made by bombarding normal  cobalt metal,  which  is  stable cobalt
isotope 59, with neutrons.


                               7-32

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        7.7.2.1  Process Description

Two  general  types  of  gamma  systems  have  been proposed  for
wastewater sludge disinfection.   The first  is  a  batch-type system
for  liquid  sludge,  where  the  sludge is circulated  in a closed
vessel surrounding the gamma ray source.  Dosage  is regulated by
detention and source  strength.   The  second  system  is for dried or
composted sludge.  A  special  hopper  conveyor  is  used to carry the
material for irradiation to the  gamma ray source.  Conveyor speed
is used to control the dosage.
        7.7.2.2  Current Status  -  Liquid  Sludge

The only gamma ray system in active operation is a liquid sludge
facility  at  Geiselbullach  (near  Munich)  in  West  Germany.
Sludge  has  been  treated  in   a  demonstration-scale  facility
since 1973.   The design capacity  is 0.04 MGD  (2.0  1/s)  but the
initial Co-60 charge  only  provided  radiation to treat 0.008 MGD
(0.3  1/s).    The basic  flow scheme  is shown  on Figure  7-11.
Digested sludge  is pumped or otherwise moved into the vault with
the Co source and circulated until the desired dosage  is reached.
The chamber is then completely emptied and recharged.


Wizigmann and Wuersching  (45) reported  on  the efficiency of the
Geiselbullach facility  when the  applied  dose was 260  krads  in
210 minutes.   Bacterial  tests were  made on samples of processed
sludge and  showed  a  two-log reduction  in total bacterial count,
an Enterococcus  reduction of two logs, and an Enterobacteriaceae
reduction of four to five logs.   Two of 40 samples were positive
for Salmonella.   Bacterial  regrowth  was measured in sludge-drying
beds where the sludge was placed after irradiation.


Plastic  encapsulated  bacteria   samples  were  also  irradiated  in
the system to a dosage of 260 krads.  Two of  nine E. coli strains
were  radiation-resistant  and  reduced  five  to six logs;  three
strains were totally  inactivated,  and  four strains  were
reduced six to eight  logs.   Tests  on ten  strains of Salmonella in
170 samples showed four  to  seven  log  reduction,  with 85 percent
of  the  samples  over five  logs and 61  percent over six  logs.
Klebsiella  were reduced  six   to eight  logs.   Gram-negative
species were more sensitive  to gamma radiation than gram-positive
ones, and  spores were more  resistant than vegetative forms.   A
comparison of the disinfection results of the real sludge samples
and the plastic  encapsulated cultures indicates that  circulation
in the sludge system apparently  did not result in a very uniform
dose exposure.


Parasite ova (Ascaris suum)  circulated  through  the  system  in
plastic  capsules failed   to  develop  during  three weeks  of
incubation.   This  observation period was not adequate,  however,
to assure that long-term recovery  would not take place.
                               7-33

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               SLUDGE
                INLET
VENT
      GROUND
       LEVEL


                                               COBALT
                                                RODS

       CONCRETE
       SHIELDING
                                                        SLUDGE
                                                        OUTLET
                            FIGURE 7-11

              SCHEMATIC REPRESENTATION OF COBALT-60
      IRRADIATION FACILITY AT GE1SELBULLACH, WEST GERMANY (44)
According to the latest available reports, land spreading of the
sludges  treated  at GeiseIbullach  has been  well received by
local  farmers and  the general  public.   No radiation hazards
have  resulted  and  the  treated  sludges  satisfy  disinfection
requirements.   The  competing system  in  Germany, heat pasteuriza-
tion, requires more energy  and produces an odorous product  that
is more difficult to handle.
        7.7.2.3  Current Status -  Dried  or  Composted Sludge

A dry  sludge irradiation  system  using  a  gamma  source is being
developed by Sandia Laboratories in Albuquerque, New Mexico.  The
eight-ton-per-day  (7.2  t/day)  demonstration facility,  containing
about one  million  Curie of Cs-137,  underwent  final  testing and
start-up in  June 1979.   The  facility will be  used to  irradiate
bagged composted sludge for agricultural experiments and bagged
dried raw primary sludge for testing as  a cattle-feed supplement.
                               7-34

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Owing to  the  high cost  of  Co-60,  the overall viability  of  any
sludge  irradiation  facility in  the  United  States depends  on
Cs-137 supplies.   Cs-137 will be available in quantity  only  if
the  political  and  technical difficulties associated with power
plant fuel  rod  reprocessing can  be  resolved.   About  200 mega-
curies  of  Cs-137  could  be available  from  processing  wastes
from weapons manufacture  and could be used for further testing.
        7.7.2.4  Design  Criteria


The design  criteria  for gamma irradiation facilities  depend  on
the  type of  wastewater  sludge  treated.   Current literature
discussions  suggest a dose of  400 krads  but this  level does  not
ensure  complete  virus  removal  (41).   The  dose  level should
probably be  varied in  relation'  to  other treatments the  sludge
receives.  A composted,  bagged product with  an 80  percent  solids
content  needs a lower  dose  than a mixture of  raw primary  and
waste-activated sludge  because the  dried product already  has  a
reduced  pathogen'  level  owing  to  the drying process.   Data from
the demonstration  facility at  Sandia  Laboratory for design of  a
dry facility should  be available by late 1979.   For  a liquid
sludge  facility,  data on  dose-response  and  pathogen levels
(Table 7-5 and Section 7.7.2.2) can be combined  with information
from  Geiselbullach to  set  the required radiation doses.   The
storage  capacity for both untreated and irradiated sludge  should
be equal  to  that  for a pasteurization  facility of  similar size
(see Section 7.6.1.7).


When  a dry  system radiation  source  is not  in use,  it  should  be
shielded in  a  steel-lined  concrete  vault.   'The vault  should  be
designed to  be  flooded  with water during loading  and  unloading
of  the  radiation source,  to shield  workers  from radiation.
Provision must be made for pool water treatment in the event that
the radiation source  leaks.   Cooling air  is circulated around the
source both  during system operation and down times.    This  air
must be filtered to prevent  a radioactive air release.   Since the
dried sludge is a  flammable material,  there  must be smoke  and/or
heat  detection and  a fire  suppression system.   For  a liquid
storage system the treatment   vessel serves as a radiation  source
storage vault.
        7.7.2.5  Instrumentation  and Operational
                 Considerations


Instrumentation  should include radiation  detectors and  flow
metering  for the wet  sludge system.   When either facility is
operating,  arrangements must be  made for  periodic radiation
safety  inspection.   The disinfection  effectiveness  should  also
be  tested  by periodic  sampling  of the sludge  before  and  after
disinfection.
                               7-35

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        7.7.2.6  Energy Impacts

In May  1977,  Ahlstrora  and  McGuire  (46)  projected  annual energy
requirements for  both wet and  dry  gamma irradiation facilities,
using a  dose  rate of 1,000 krads.   Their results are summarized
on  Figures  7-12  and 7-13.    For  a  0.1-MGD  (4-1/s)  facility
treating sludge with five percent solids, 300 days per year, the
unit energy use is  about  5.2  kWhr per 1,000 gallons (5 MJ/m3) or
25 kWhr  per ton  (100 MJ/ton)  dry solids.   For  a plant  treating
35 tons  per day  (32 t/day)  at  60  percent  solids,  300  days per
year  (equivalent  to  the  solids from  the  previous  example), the
energy  use is  5.6  kWhr  per ton dry (22  MJ/t) solids, almost
80 percent  less than the  facility  treating five percent solids.
These  energy uses should be  compared  to  120 kWhr per  dry ton
(450 MJ/t)  for an e-beam  system.
  1,080
    fl
    8
1

   100
     I
     ;
    te
                                               .D-"
                                                   _L
      10
                            e. 7 a
                                 8100
                                                         6739
1,000
               SLUDGE TREATMENT CAPACITY, 0,001 MGO (4,4 x ItT5 m3/s)

                            FIGURE 7-12

            GAMMA RADIATION TREATMENT OF LIQUID SLUDGE
                      POWER REQUIREMENTS  (46)
                                7-36

-------
         100

           9

      •£•  '  8

      ^  ,  7
      I
           4  -
      a
      LU
      cc
      01
      LU
      I
      Q,
      Z
Q
          10
                       2      3   4   5  S  7 8 9
            10                                  100        200

             SLUDGE TREATMENT CAPACITY, dry tons/day (0,907 tonne/day)

                            FIGURE 7-13

                RADIATION TREATMENT OF DEWATERED
                 SLUDGE - POWER REQUIREMENTS (46)
It is  important to note  that the liquid system  would  require a
much  larger Cs-137 charge since  it would  be treating  almost
12 times the volume of material at the same dose level.  However,
the  rod configuration for a dry  facility  would be much less
efficient in terms of radiation transfer than a liquid one.


        7.7.2.7  Performance Data

In June 1979 no performance data for the Sandia facility were yet
available.  Data for the Geiselbullach facility are summarized in
Section 7.7.2.2.
                               7-37

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        7.7.2.8  Cost Information

Cost estimates for both liquid and dry  facilities were developed
together with  the  energy data  of  Section 7.7.2.6.   The liquid
facility included the following  components:

     •   Insulated concrete building with 25-foot  (7.6-m) ceiling.

     •   Equalization  sludge  storage tank.

     •   Emergency water dump tank (for  source shielding water).

     •   Irradiating capsules (radiation source).

     •   Steel-lined source handling pool.

     •   Deionizer.

     •   Data aquisition and  control system.

     •   Oxygen injection facility.

     •   Pumps, piping,  and flow  meters.

     •   Radiation alarm.

     •   Fire suppression system.

A  capital  cost  graph  for the wet  facility  is  given  on
Figure  7-14;  the estimates were  made  in May  1977.   Graphs  for
labor hours per year  and operations and maintenance materials  and
supplies are  given on Figure 7-15 and 7-16,  respectively.    The
additional  operating  cost is $2.00  per 1000  gallons  ($0.53/m3)
for the Cs-137 (the irradiator).

The dry system uses a bucket conveyor to move the sludge past  the
radiation source  (see Figure  7-17).   This  dry system would
include the following:

     •  Loading and unloading conveyors.

     •  Concrete shielding.

     •  Source-handling pool.

     •  Holder for the Cs-137 capsules.

     •  Holder moving mechanism.

     •  Steel building.

     •  Pumps.

     •  Ventilators.
                               7-38

-------
        Filters.

        Hoists.

        Radiation alarm system.

        Pool water testing tank.

        Fire suppression system.
  10,000
     8
     6
     7
     6
     5

     4
 o
1,000
  fl
  i
  ?
  %
  5
    100
      10
                          _L__J_J
                                                    I
                          5676
                                100
6  6739
       1,000
                SLUDGE TREATMENT CAPACITY, 0,001 MGD 14.4 x 10"? m3/i|


                            FIGURE 7-1U

           GAMMA RADIATION TREATMENT OF LIQUID SLUDGE -
                         CAPITAL COSTS (46)
The  capital  costs  for  the  dry  system are  summarized  on
Figure  7-18;  these  costs were also  calculated in  May 1977.
Figure  7-19   and  7-20  present  labor  hours,  materials  ,and
operations  and maintenance supplies,  respectively.   The  Cs-137
source is estimated to cost $1.55 per ton  ($1.70/t)  for a  10-ton-
per-day (9.1-t/d)  capacity facility and  $1.22 per ton  ($1.35/t)
for facilities of 50  ton per day  (45 t/d)  and  larger.
                               7-39

-------
  10,000
     a

     8

     7

     6
  ID

  flC
  O
  (0
  z
  z
  1,000

                 'O"
                   J_
      J   I
                                      _L
-LJ.
     10
   3   45878 S          2     3   4667SS
                 100                           1,000
SLUDGE TREATMENT CAPACITY, 0.001 MGO (4,4 x 1C"5 m3/sj
                            FIGURE 7-15

                GAMMA RADIATION TREATMENT OF LIQUID
                   SLUDGE LABOR REQUIREMENTS (46)
If labor  plus  overhead is $20.00  per hour, power  is  three cents
per kWhr,  ($0.33/GJ) and  capital is  amortized  over  20  years  at
8 percent, the  cost  for a   0.1-MGD  (4-1/s) liquid system  is
$38.50  per  ton  ($42.40/t)  dry  solids.   A  dry  system  costs
$24.00  per ton  ($26.50/t)  dry  solids.   Both these costs  are
considerably higher  than  those for e-beam irradiation and similar
to those for heat pasteurization.              -
                               7-40

-------
 12
 £
 i
   100
    9
    B
    7
    6

    5
8
2
10
 9
 8
 7
 8
    2 —
    10
                                            _L
                       4   S  6 7 8 9
                                  100
                                         2     346


            SLUDGE TREATMENT CAPACITY, 0.001 MGD (4.4 * 1(T5 m3/s)

                         FIGURE 7-1S

        GAMMA RADIATION TREATMENT OF LIQUID SLUDGE
         MAINTENANCE MATERIAL SUPPLIES COSTS (46)
                                                            7 s
                                                                iroeo


                           FIGURE 7-17

        GAMMA RADIATION TREATMENT FACILITY FOR HANDLING
          25 TONS PER DAY OR MORE OF DEWATERED SLUDGE
                               7-41

-------
   10,000
       i
       8
       7
       s
       5
j!
O
<
_l
<
7
6
5
4
     100
                 O'
                     I
                    I
                     2       3    456780
        10                                     100         200

           SLUDGE TREATMENT CAPACITY, ton/day (0.907 tonne/day)

                          FIGURE 7-18

      GAMMA RADIATION TREATMENT OF DEWATERED SLUDGE
                       CAPITAL COST  (46)
                             7-42

-------







fc*
J5
"5
-o
£
te
o
o
_l
t
_
o


9
8
7
6
5
4
3
2
1,OOO
9
8
7
:<
4
3
2
100
— .
—
—
„,,
—
—
_,
-•*
O^^
^**
^ ^ **
?*"
„
—
i i i i i i i i I
2 3 4S6789
   10                                    100         200

      SLUDGE TREATMENT CAPACITY, ton/day (0.907 tonne/day)

                     FIGURE 7-19

GAMMA RADIATION TREATMENT OF DEWATERED SLUDGE -
              LABOR REQUIREMENTS  (46)
                        7-43

-------
            1,000

              9

              i

              7

              6
          O
          o
            100
                _	-°-
                        2     3   45fl7S9
               10                             100       200

                 SLUDGE TREATMENT CAPACITY, ton/day (0.907 tonne/day)


                            FIGURE 7-20

             GAMMA RADIATION TREATMENT OF DEWATERED
            MAINTENANCE MATERIALS AND SUPPLIES COST (46)
7.8  References
 1.
 2.
 3.
Branden,  J
Proceed ings
 R.  "Parasites  in  Soil/Sludge  Systems."
of Fifth National Conference  on Acceptable
     Sludge Disposal Techniques,  Orlando,  Florida, January 31  to
February  2,  1978.
Maryland 20852, p.
        Information
      130.
                                       Transfer,  Inc.  Rockville,
Oliver, W. M. "The Life and Times of Aspergillus  Fumigatus,"
Compost  Science/Land  Utilization.   Vol. 20,  No. 2,  March/
April 1979.

U.S. Public  Health Service.   Enteric  and Neurotropic Viral
Diseases  Surveillance,  1971-1975.   Center for  Disease
Control,Atlanta,Georgia 30333.  Issued  January  1977.
                               7-44

-------
 4.   U.S.  Public Health Service.   "Shigella Surveillance,  Annual
     Summary 1976."   Center for Disease Control, Atlanta,  Georgia
     30333.   Issued  October 1977.

 5.   U.S.   Public Health Service.    Salmonella  Surveillance,
     Annual Summary  1977.  Center for  Disease  Control,  Atlanta,
     Georgia 30333.   Issued March 1979.

 6.   U.S.  Public  Health  Service.    Intestinal  Parasite
     Surveillance,  Annual Summary  1976.   Center  for Disease
     Control,  Atlanta,  Georgia 30333.  Issued August 1977.

 7.   U.S.   Public  Health  Service.     " Intestinal   Parasite
     Surveillance,  Annual Summary  1977 ."  Center for Disease
     Control,  Atlanta,  Georgia 30333, Issued September  1978.

 8.   Sagik,  B.P.   "Survival  of Pathogens in Soils."   Proceedings
     of Williamsburg  Conference on  Management  of  Wastewater
     R_e SJL d_u a. l._s _,	Williamsburg, Virginia,	November 13-14,  1975 .
     OTsT  Science  Foundation^^i"hTngTon  DTc~.  2"OlT50lRANN-AEN
     74-08082,  p.  30.

 9.   Metcalf,  T.G.   "Role of  Viruses  in Management of  Environ-
     mental  Risks."     Proceedings of Williamsburg Conference  on
     ^anagement  of Wastewater  Residuals, Williamsburg,  Virginia,
     N'ovember" 1975.   U.S. National Science Foundation,  Washington
     D.C.  20550.RANN-AEN 74-08082 p.  53.

10.   Moore,  B.F.,  B.P.  Sagik, and C.A.  Sorber.   "An Assessment  of
     Potential  Health Risks Associated with  Land Disposal  of
     Residual  Sludges."   Proceedings of Third National  Conference
     on Sludge Management, Disposal and Utilization,  Miami  Beach,
     Florida.   December 14-16, 1976.   Information  Transfer,  Trie.
     Rockville,  Maryland 20852.  p. 108.

11.   Stern,   G.,  and  J.B.   Farrell,   "Sludge  Disinfection
     Techniques."  Proceedings of National Conference on Compost-
     ing of Municipal Residues and Sludges.     Washington,   D.C.
     August  1977 .    Information  Transfer,  Inc.,  Rockville,
     Maryland  20852.   p.  142.

12.   Fenger,  B.,  0.  Krogh,  K.  Krongaard, and  E.  Lund.    "A
     Chemical,  Bacteriological,  and  Virological Study of Two
     Small  Biological  Treatment  Plant. "   Fifth Meeting  of  the
     North Wesrt_ European Microbiological  Group.     Bergen,  Norway
     T97TT"

13.   Farrell, J.B., and G. Stern.   "Methods  for Reducing  the
     Infection Hazard  of Wastewater  Sludge."    Radiation for a
     Clean  Environment, Symposium  Proceeding.   International
     Atomic Energy Agency, Vienna.  1975.

14.   Lund, E.  "Public  Health  Aspects  of Wastewater  Treatment."
     Radiation  for  a  Clean  Environment,  Symposium  Proceeding.
     International Atomic Energy Agency.Vienna. 1975.
                               7-45

-------
15.   Hyde,  H.C.   "Utilization of  Wastewater  Sludge  for Agricul-
     tural  Soil  Enrichment."    Journal  Water  Pollution  Control
     Federation.   Vol.  48, p. 77.  1976 .      "

16.   Hays,  B.D.   "Potential for  Parasitic  Disease  Transmission
     With  Land  Application of  Sewage  Plant  Effluents  and
     Sludges."   Water Research.   Vol.  11, p.  583.  Pergamon
     Press.   1977.

17.   Ward,  R.L. and C.S.  Ashley.  "Inactivation of Poliovirus in
     Digested Sludge."   Applied and  Environmental Microbiology.
     Vol.  31, p.  921.   1976.     "                   - —-

18.   Ward,  R.L.  "Inactivation  of  Enteric Viruses  in Wastewater
     Sludge."    Proceedings of  Third  National  Conference  on
     S_ludge  Management,	Disposal, and Utilizatioru^Miami Beach,
     Florida. December  14-16, 1976.   InformatTon~~TrransfeFT" Inc.
     Rockville,  Maryland,20852.  p. 138.

19.   Ohara,  G.T. and J.E. Colbaugh.   "A Summary  of  Observations
     in Thermophilic Digester  Operations."   Proceedings  of  the
     191_5 National Conference  on Municipal  Sludge Management
     and Disposal, Anaheim,  California,  August 18-20,  1975.
     Information Transfer,  Inc., Rockville, Maryland  20852.
     p. 218.

20.   Ornevich,  R.F. and J.E. Smith, Jr.   "Pathogen  Reduction in
     the Thermophilic  Aerobic  Digestion Process."   Proceedings
     of the  48th  Wat_er Pollution  Control  Federation ^Conference,
     Miami  Beach,  Florida.   October 1975.

21.   Theis,  J.H.,  V. Bolton,  and  D.R.  Storm.   "Helminth  Ova in
     Soil and  Sludge  from  Twelve U.S. Urban  Areas."  Journal
     Water  Pollati_on  Control  Federation.   Vol.  50,  p.  2485
     1978.

22.   USEPA.   Agricultural  Benefits  and Environmental Changes
     Resulting from the Use of  Digested  Sludges  on  Field Crops.
     emInterim  Report on  a Solid Waste  Demonstration Project.
     Office of Research and Development, Cincinnati, Ohio 45268.
     Report SW-30d.  1971.

23.   Sacramento  Area  Consultants.   Sewage Sludge Management
     Program Final Report, Volume  6 Miscellaneous Use Determina-
     tions .   Sacramento  Regional County  Sanitation  District,
     SacFamento,  California  95814.  September 1979.

24.   Farrell, J.B.,  J.E.  Smith,  S.W.  Hathaway,   and  R.B.  Dean.
     "Lime  Stabilization  of  Primary  Sludge."   Journal  Water
     Pollution Control  Federajti_g_n.  Vol. 46, p. 113.   1974.

25.   Data from B.E.F. Unit of General Signal, West Warwick, Rhode
     Island  02893.  Personal  Communication  from D.L. Moffat.
     January 2, 1979.
                              7-46

-------
26.   Gerba,  C.P.,  C. Wallis,  and J.L.  Melnick.   "Fate  of
     Wastewater  Bacteria and  Viruses  in Soil."   Journal
     Irrigation and  Drainage Division.  ASCE.  p. 152.  September
     1975.                   ___

21.   Parsons,  D.,  C. Brownlee,  D. Welter, A. Mauer,  E.  Haughton,
     L.  Kornder,  and M.  Selzak.   Health  Aspects  of Sewage
     Effluent  Irrigation.  Pollution Control  Branch, British
     Columbia  Water  Resource  Service,  Department  of Lands,
     Forests  and  Water  Resources.   Victoria, British  Columbia.
     1975.

28.   Hess,  E.,  and C.  Breer.  "Epidemiology of Salmonella and the
     Fertilizing of  Grassland  with Sewage Sludge."   Zentrabblatt
     Bakterioliogie  Parasitenkunde.   Infektious  Krankheitenand
     Hygeine,  Abeilung I.   Orig.  B-161  54.  1975.

29.   Burge,  W.D.   "Bacteria and Viruses  in Soil/Sludge  Systems."
     Proceedings  of  Fifth National  Conference on Acceptable
     Sludge Disposal Techniques,  Orlando, Florida.  January  31 to
     February  2,  1978.   Information Transfer,  Inc.   Rockville,
     Maryland  20852.   p.  125.

30.   Roediger,  H.  "The  Techniques of Sewage  Sludge  Pasteuriza-
     tion;  Actual  Results  Obtained  in  Existing  Plarxts;
     Economy."   International  Research  Group on  Refuse  Disposal,
     Informational Bulletin Number 21-3JL.   p. 325.   August 1974
     to  December 1976.

31.   Stern, G. "Pasteurization of Liquid Digested  Sludge."
     Proceedings  of  National  Conference on Municipal Sludge
     Mangement,  Pittsburgh.   June  1974.    Information  Transfer
     Inc.,  Rockville,  Maryland 20852.  p. 163.

32.   Ward,  R.L. and  J.R.  Brandon.  "Effect on Heat  on Pathogenic
     Organisms  Found  In  Wastewater Sludge.    Proceedings  of
     National  Conference  on  Composting  of  Muncipal  Residue and
     Sludges,  Washington,  D.C.    August 23-25, 1977.Information
     Transfer  Inc.,  Rockville, Maryland 20852.p. 122.

33.   Data from Zimpro  Corporation, Rothchild, Wisconsin.  Personal
     communication from J.R. Nicholson.  July 1979.

34.   Wesner,  G.M.    "Sludge Pasteurization  System Costs."
     Prepared  for  Battelle Northwest, Richland, Washington 99352.
     June 1977.

35.   Connell,  C.H. and M.T. Garrett, Jr.  "Disinfection Effective-
     ness  of  Heat  Drying  at Sludge."   Journal Water Poljhjtion
     Control  Federation.   Vol. 35,  (10).   1963.

36.   Regional   Wastewater  Solids   Management  Program.    "Carver-
     Greenfield Process  Evaluation."   Los  Angeles/Orange County
     Metropolitan  Area (LA/OMA Project).     Whittier,  California
     90607  December  1978.
                               7-47

-------
37.   Burge,  W.D.,  P.B. Marsh, and P.O.  Millner.   "Occurrence of
     Pathogens  and Microbial  Allergens  in the  Sewage Sludge
     Composting  Environment."  Proceedings of National  Composting
     Conference  on  Municipal Residue and Sludges,  Washington  D.C.
     August  23-25,  1977.   Information Transfer,  Inc.,  Rockville,
     Maryland  20852.   p. 128.

38.   Kawata,  K.,  W.N.  Cramer,  and W.D.  Burge.   "Composting
     Destroys Pathogens in Sewage  Sludge."  Water and Sewage
     Works.  Vol. 124, p. 76.  1977.

39.   County  Sanitation  Districts  of  Los Angeles  County.
     "Pathogen  Inactivation   During  Sludge  Composting."
     Unpublished Report to USEPA.    Whittier,  California  90607.
     September 1977.

40.   Cooper,  R.C.  and  C.  G.  Colueke.   "Survival of Enteric
     Bacteria  and  Viruses  in Compost and Its Leachate."   Compost
     Science/Land Utilization.  March/April 1979.                ~~

41.   Massachusetts  Institute of Technology.  High Energy Electron-
     Irradiation of Wastewater Liquid Residuals.    Report  to  U.S.
     National  Science  Foundation,  Washington  D.C.,  20550.
     December  31, 1977.

42.   Massachusetts  Institute of Technology. Boston, Massachusetts
     02139.  Personal  communication  from D.N. Shah.  May 1979.

43.   Siemens  Medical  Laboratory  Inc.   Walnut Creek,  California
     94596.    Personal  communicaiton from Werner  Haas.   January
     1979.

44.   Farrell,  J. B. "High  Energy Radiation in Sludge Treatment--
     Status  and Prospects."   Proceedings of  the  National
     Conference on Municipal  Sludge Management  and  Disposal,
     Anaheim,  August  18-20,  1975.   Information  Transfer  Inc.,
     Rockville,  Maryland 20852.

45.   Wizigmann,  I.  and F.  Wuersching.   "Experience  With  a  Pilot
     Plant  for the  Irradiation of  Sewage Sludge:   Bacteriological
     and Parasitological Studies  After  Irradiation."   Radiation
     for a Clean  Environment,  Symposium Proceedings.  Inter-
     national  Atomic Energy Agency.  Vienna.  1975.

46.   Ahlstrom, S.B. and H.E.  McGuire.  An Economic Comparison of
     Sludge Irradiation and Alternative Methods of  Municipal
     Sludge  Treatment.   Battelle Northwest  Laboratories.
     Rlchland, Washington 99352.   PNL-2432/UC-23.   November 1977.
                               7-48

-------
EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
               Chapters.  Conditioning
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                           CHAPTER 8

                          CONDITIONING
8.1  Introduction

Conditioning involves the biological,  chemical,  and/or  physical
treatment  of a  sludge stream  to enhance  water  removal.   In
addition, some conditioning processes  also  disinfect  wastewater
solids,  affect  wastewater  solids odors, alter  the wastewater
solids  physically,   provide  limited  solids  destruction  or
addition, and improve solids recovery.


8.2  Selecting a  Conditioning Process

Conditioning always  has  an effect  on  the efficiency  of the
thickening  or dewatering  process  that follows   (1-3).   Any
evaluation of the conditioning process must  therefore take into
consideration  capital,  operating  and  maintenance  costs for
the entire  system and the  impact of sidestreams on other  plant
processes,  the plant  effluent, and resultant air quality.

Figure 8-1  shows  how the  evaluation  would  look  in  a  quantified
flow diagram.

This type of analysis is necessary because conditioning processes
differ and,  therefore,  produce  differing  consequences  for the
total  system.    For  instance, Table 8-1 compares the  effects
expected  with no  conditioning as  opposed  to those  expected with
polyelectrolyte   conditioning or   thermal  conditioning  prior  to
gravity thickening.


8.3  Factors Affecting Wastewater  Solids Conditioning


    8.3.1  General Wastewater Solids Properties

Wastewater  solids are composed  of screenings,  grit, scum and
wastewater  sludges.   Wastewater  sludges consist of primary,
secondary,  and/or  chemical solids  with  various  organic and
inorganic particles  of mixed sizes; the sludges each have  various
internal  water  contents,   degrees of  hydration,  and  surface
chemistry.   Sludge  characteristics  that affect thickening or
dewatering and for  which  conditioning  is employed  are  particle
size  and  distribution,  surface  charge  and  degree  of  hydration,
and particle interaction.


                              8-1

-------
                                GASEOUS EFFLUENT

                                    PLOW RAT!
                                    AMMONIA
                             VOLATILE ORGANIC SUBSTANCES
        SOLIDS FEED

         FLOW RATE
       SUSPENDED SOLIDS
                                FILTRATE  OR
                            CONCENTRATE  STREAM
                                 FLOW HATE
                                                              iOD
                                                                t     t    t
                                                                SUSPENDED SOLIDS
                                                               REFRACTORY GHGAN1CS
                                     THICKENED OR DEWATERED
                                             SOLIDS
                                            FLOW RAT!
                                           SOLIDS OQNTiMT

                                      FIGURE 8-1

                     BASIC PARAMETERS FOR EVALUATION OF A
                           SLUDGE CONDITIONING SYSTEM


                                      TABLE 8-1

EFFECTS OF EITHER POLYELECTROLYTE CONDITIONING OR THERMAL CONDITIONING
  VERSUS NO CONDITIONING ON A MIXTURE OF PRIMARY AND WASTE-ACTIVATED
                      SLUDGE PRIOR TO GRAVITY THICKENINGS
                                Polyelectrolyte
                                 conditioning
                        Thermal  conditioning
   Conditioning mechanism
   Effect on allowable solids
    and hydraulic  loading rates
   Effects of supernatant stream
   Effects on underflow
    concentration
   Effects on manpower
Flocculation
Will increase

Will improve
 suspended
 solids  capture

May increase

Little to none
Alters  surface properties and
  ruptures biomass cells, releases
  chemical - water bonds -
  hydrolysis
Will significantly increase

Will cause significant increase in
  color,  suspended solids, soluble
  BOD_, COD and NH,-N.  Improve
  suspended solids capture
Will significantly increase

Requires  higher skilled operators
  and strong preventive maintenance
  program
    It is assumed  that the processes involved  will work well.
                                          8-2

-------
        8.3.1.1   Particle Size and Distribution

Particle  size is  considered to  be the single  most important
factor influencing sludge dewaterability (4-7).  As  the  average
particle  size decreases, primarily from mixing or shear,  the
surface/volume  ratio  increases exponentially (8).  Increased
surface area means greater hydration, higher chemical demand,  and
increased resistance to  dewatering.   Figure  8-2 shows  relative
particle sizes of common  sludge  materials.
01 0.01 0.1 1.0 10 102 103 104
MICRON 0,001 in. mm. cm.
10

ANGSTROM

UNITS



















£
15
1 O
ITl

1
||
I
I -
8
1 £
11
1












COLLOIDS

FINE

MEDIUM

COARSE

LARGE

CLAY

SILT

FINE
SAND

COARSE
SAND

GRAVEL

                           FIGURE 8-2

          PARTICLE SIZE DISTRIBUTION OF COMMON MATERIALS
Raw municipal wastewaters contain  significant  quantities of
colloids  and  fines,  which,  because  of their  size  (1  to 10
microns),  will almost  all escape capture in  primary  clarifiers
if coagulation and flocculation are not employed.   Secondary
biological  processes,  in addition to removing  dissolved BOD,
also partially remove these colloids and  fines  from  wastewater.
Because of  this,  biological  sludges,  especially  waste-activated
sludges are difficult to thicken or  dewater  and  also  have  a  high
demand for conditioning  chemicals.
A primary objective of conaitioi
by combining the small  particles into larger
cond itioning
is to increase  particle  size
        aggregates.
                              8-3

-------
        8.3.1.2  Surface  Charge  and Degree of Hydration

For the  most  part,  sludge particles repel,  rather  than attract
one another.    This  repulsion,  or  stability,  may  be due  to
hydration  or  electrical  effects.  With  hydration, a  layer  or
layers of water bind  to the particle surface, providing a buffer,
which  prevents close  particle approach.   In addition, sewage
solids  are negatively charged and thus tend  to  be  mutually
repulsive.   Conditioning is  used  to overcome  the effects  of
hydration and  electrostatic repulsion.

Conditioning is a two-step process consisting of destabilization
and flocculation.   In destabilization,  the surface  characteris-
tics of the particles are altered  so that  they will adhere to one
another.  This desirable  change is brought about through the use
of  natural  polymeric material  excreted by  the  activated sludge
organism, synthetic  organic  polymer,  or  inorganic  metal salts.
Flocculation is  the  process  of providing  contact opportunities,
by  means  of mild  agitation,  so  the destabilized particles  may
come together.

Destabilization either with synthetic organic polyelectrolyte  or
with  inorganic metal  salt  is  readily available to the  plant
operator,  but  it  represents  an increase  in  operating  cost.
The  degree  to  which natural  flocculation  is available  is
difficult  to  predict since it  is  dependent  on  the   type  of
activated sludge or  the  attached-growth  biological  process that
has been designed into the plant.


        8.3.1.3  Particle Interaction

Municipal wastewater  sludges  contain large  numbers  of colloidal
and agglomerated  particles,   which  have   large  specific surface
areas.   Initially  these particles  behave in a  discrete manner
with  little interaction.    As  the  concentration of sludge  is
increased by the separational process, interaction increases.  As
shown   on  Figure 8-3  this  flocculant  behavior results  in  three
distinct zones for  a  gravity  thickener.

Conditioning can increase the rate of settling in the sedimenta-
tion zone,  and compression thickening in the thickening  zone;  it
can also improve the  quality  of  the overflow.  These improvements
result  from the ability of  the  conditioner  to neutralize  or
overcome the surface  charge,  which  in  turn allows the particles
to  adhere  to  one  another,  thus preserving  the  dimensional
integrity of the sludge matrix in  the thickening zone.


    8.3.2  Physical  Factors

The amount of  conditioning required  for  sludges  is  dependent  on
the processing conditions to which the sludge has been subjected
and on the  mechanics  of the conditioning process available.
                               5-4

-------
              OVERFLOW
                                       INFLOW
                  ZONE OF CLEAR LIQUfD
                INFLOW SOLIDS CONCENTRATION
                • LOWEST CONCENTRATION AT WHICH FLOCCUtANT SUSPENSION IB IN
                THE FOBM OF POftOUS MEDIUM
                . IJNOERPL0W CGNClNTftATIDN FROM GRAVITY
                            FIGURE 8-3

            TYPICAL CONCENTRATION PROFILE OF MUNICIPAL
          WASTEWATER SLUDGE IN A CONTINUOUSLY OPERATING
                      GRAVITY THICKENER (12)
        8.3.2.1  Effect of Processing  Prior to Conditioning


Both the degree of hydration  and  fines content of a sludge stream
can  be materially  increased  by exposure  to shear,  heat,  or
storage.   For example,  pipeline transport  of sludge  to  central
processing  facilities,  weekend  storage  of  sludge  prior  to
mechanical  dewatering,  and  storage of  sludges for  long  periods
of time  have been shown  to  increase  the demand  for conditioning
chemicals  in  all  types  of dewatering  and should  be accounted for
in the design of the dewatering  facility (10-13).
        8.3.2.2  Conditioner  Application
The  optimum sequence  for adding  conditioner is  best  determined
by  trial and  error,  when  two  or more  conditioners  are  used.
With  ferric chloride  and  lime,  the  ferric chloride is  normally
added first.   In addition,  it has been shown  that deterioration
of  the  floe after conditioning  (due  to both time  and  high shear
mixing)  can be  a major determinant of  chemical  requirement (13).
When  a   combination  of anionic  and  cationic  polymer  is  needed,
anionic  polymer  is added  first.

-------
In order  to  minimize  floe shearing, mixing  should  provide  just
enough energy to disperse the conditioner throughout the sludge.
In dewatering applications,  consideration should be  given to
providing individual conditioning  for each dewatering unit, since
it is  not always economical to provide  one common conditioning
unit  for  several   dewatering  units.    Problems  can   arise  in
balancing  the  flow  rates of  the  various streams  when  starting
up or shutting  down individual  units.    The  location  of  the
conditioning  unit  relative  to  each dewatering  device  requires
optimization.

Many  types  of conditioning units  are  available.   Recent USEPA
publications  (14,15)  describe  the  more common  designs,  design
layouts,  and  operating problems.   Additional information  can be
obtained from thickening  and dewatering equipment suppliers.


8.4  Inorganic Chemical  Conditioning
    8.4.1  Introduction

Inorganic  chemical  conditioning  is  associated  principally  with
mechanical sludge  dewatering,  and  vacuum  filtration  is  the
most common application.   The chemicals  normally used  in  the
conditioning of municipal wastewater sludges are lime and ferric
chloride, although ferrous  sulfate  has also  been used.

Ferric chloride is added first.  It  hydroylzes in water, forming
positively  charged  soluble  iron complexes  which  neutralize  the
negatively charge sludge solids, thus causing them to aggregate.
Ferric chloride  also reacts with  the  bicarbonate  alkalinity in
the  sludge to  form hydroxides  that act  as  flocculants.   The
following  equation  shows the  reaction  of   ferric  chloride  with
bicarbonate alkalinity:
    2FeCl3 + 3Ca(HCC>3)2  - *•  2Fe(OH)3 + 3CaCl2 + 6CC>2


Hydrated  lime  is usually  used  in conjunction with  ferric iron
salts.   Although lime  has some slight  dehydration  effects  on
colloids,  it  is  chosen  for  conditioning principally  because  it
provides  pH  control,  odor  reduction  and disinfection.   CaCC>3 ,
formed  by the  reaction  of  lime  and bicarbonate,  provides  a
granular  structure  which increases sludge  porosity  and reduces
sludge compressibility.


    8.4.2  Dosage Requirements

Iron salts are usually added  at  a  dosage  rate of 40 to 125 pounds
per  ton  (20  to 63  kg/t) of  dry solids   in  the  sludge  feed,
whether  or  not  lime  is  used.   Lime  dosage  usually  varies


                               8-6

-------
from  150 to  550 pounds  per  ton  (75  to 277 kg/t)  of dry  sludge
solids  fed.   Table  8-2  lists  typical  ferric  chloride and  lime
dosages  for various sludges.
                             TABLE 8-2

          TYPICAL CONDITIONING DOSAGES OF FERRIC CHLORIDE
           (FeCl ) AND LIMt (CaO) FOR MUNICIPAL WASTEWATER
                           SLUDGES3  (16)
         Sludge type
 Raw primary
 Raw waste-activated sludge (WAS)-air
 Raw (primary + trickling filter)
 Raw (primary + WAS)
 Raw (primary + WAS + septic)
 Raw (primary + WAS + lime)
 Elutriated anaerobically digested
  primary
  primary + WAS (air)
 Thermal conditioned sludges
 Anaerobically digested sludges
  primary
  primary + trickling filter
  primary + WAS (air)
                                   Vacuum filter
                   Recessed plate
                  pressure filters
FeCl-
                                           CaO
                                                   FeCl.
                                                             CaO
40-80
120-200
40-80
50-120
50-80
30-50
50-80
60-120
none
60-100
80-120
60-120
160-200
0-320
180-240
180-320
240-300
none
0-100
0-150
none
200-260
250-350
300-420
80-120 220-280
140-200 400-500






none none


80-200 220-600
  All values shown are for pounds of either FeCl or CaO per ton of dry solids pumped
  to the dewatering unit.
 1 Ib/ton =0.5 kg/t
Inorganic  chemical  conditioning  increases  sludge  mass.    A
designer  should expect  one pound  of additional sludge for  every
pound  of lime  and  ferric  chloride  added  (13). This  increases
the  amount  of  sludge for  disposal  and lowers the   fuel  value
for  incineration.   Nevertheless,  the presence of  lime can be
beneficial  because  of its  sludge  stabilization effects.  The  use
of polyvalent metal  salts  and  lime offers  advantages over  other
methods, because the combination  can better condition sludge
which has extreme  variations  in  quality.
    8.4.3   Availability

Ferric chloride,  the most widely  used polyvalent  metal  salt
conditioner,  is available  in dry or  liquid  form, with  the liquid
form  being the  most  common.   In  the past,  most ferric  chloride
has  been  made  from  scrap metal and  chlorine, but  during the
past  decade,  much  larger  quantities  have  been made available
through  conversion  of waste  acids  from  large industrial  pigment
producers.   It is supplied  as  either a  30 or  40  percent by weight
solution.
                                 8-7

-------
Liquid  ferrous  sulfate,  a by-product of  certain  industrial
processes, is  not  generally available  in  large  quantities.   If
availability is  not  at  issue  and testing proves  it capable of
conditioning the sludge, liquid ferrous sulfate can be used  like
ferric chloride.

Lime  is  purchased in  dry form.  It is readily available  and
comes in  many  forms.    Pebble  quicklime (CaO)  and hydrated  lime
(Ca(OH)2)  are most often used  for  sludge conditioning.
    8.4.4  Storage,  Preparation,  and Application Equipment

There have been numerous problems such as lime scaling and FeCl3
corrosion with in-plant storage, preparation, and application of
both  lime  and ferric  chloride.   Two  excellent  references deal
with lime problems and how  to solve  them  (17,18).  Information on
ferric chloride can  be found in USEPA's Process Design Manual for
Suspended Solids  Removal (15).
    8.4.5  Design Example

A designer has  calculated  that the  rotary drum,  cloth belt,
vacuum filter that will be utilized  at the  plant,  must  be  capable
of  dewatering  a maximum  of 600 pounds  (272  kg) per  hour of
sludge.  The  sludge  will be  a mixture of 40 percent primary and
60  percent  waste-activated sludge,  which  will  be  anaerobically
digested.   The  vacuum filter will  operate seven hours per day,
five days per week.

To design for a  margin of  safety  in the  chemical  feed  equipment,
the  designer has  used  the  higher values shown  in  Table 8-2.
Chemical feeders  should  be capable  of adding 120 pounds  per ton
(60 kg/t) of FeCl3 and 420 pounds per  ton (210 kg/t)  of CaO.

Maximum daily amount of sludge to be dewatered is:
     600 Ib sludge  x 7_hr = 4^QO lb sludge per day (1^905 kg/day)
         hr         day
Maximum amount of FeCl3 required per day is

     4-200
The  FeCl3 is available at a  40  percent solution (4.72 pounds
      per gallon (0.567 kg/1)  of solution).

-------
252 Ib FeCl3   ± gallon of product
~day4.72 Ib FeClo	 =.53.4 gallons of solution per day
        day         4.72 Ib FeClo


    (202 I/day)


Maximum amount of  CaO  required  per  day  is:
                                         CaO per day ,400 Kg/day)
The pebble quicklime  is available  at  90 percent  CaO:
    882 Ib CaO  Ib pebble quicklime   _on lu  . , .     . _ . .
    - - -  - x - c — 0.9 Ib CaO - = 980 Ib pebble quicklime per day
     (45 kg/day)


The  amount  of  extra sludge  produced  due to chemical addition  is
estimated  at  one  pound  (0.45 kg)  for every pound  of  FeCl3 and
pebble  quicklime  added.   Therefore, total  maximum  daily dry
solids to be disposed of are:
    4,200 Ib sludge + 252 Ib FeCl3 + 980  Ib quicklime


which are  equal  to 5,432 pounds  (2,464 kg) of  solids.   This  is
the equivalent  of 27,160 pounds  (12,320 kg) of  wet sludge at a
minimum of 20 percent solids.


    8.4.6  Cost


        8.4.6.1  Capital Cost

Figure  8-4 shows  the relationship between construction costs
of  ferric chloride  storage  and  feed  facilities and  installed
capacity.   For  example,  if a designer  needed  to feed 100  pounds
(45.4 kg) per hour of ferric chloride the estimated  cost would  be
$330,000.   Since cost are  given  in June  1975  dollars,  the cost
must be adjusted to the proper time period.  Costs for Figure 8-4
are  estimated  on  the  basis of  liquid ferric chloride use.
Chemical feed equipment  was sized for  a  peak  feed rate of twice
the average.   At  least  15  days of  storage was  provided  at the
average  feed  rate.    Piping  and  buildings  provided  to house the
feeding equipment are included.


                               8-9

-------
in
r-.
o>

0)
c
3
J5
"5
co~
CO
o
o
O

O
D
DC

CO
2
O
O
1,000,000
     9
     8
     7
     6
     5
 100,000
    9
    8
    7
    6
    5
    4

    3
    2 -
     10,000
                   UJLLLL
         10
                 3  4 5 6 7 89100
                                   3 456789 1,000
                                                 L_U_U_I_U
                                              1  3 456789 10,000
            INSTALLED CAPACITY, pounds Ferric Chloride Fed/Hour (1 Ib = 0.454 kg)

                            FIGURE 8-4

            CAPITAL COST OF FERRIC CHLORIDE STORAGE AND
                       FEEDING FACILITIES (22)

Figure 8-5  gives  construction costs of  lime storage and feeding
facilities  as  a  function of  installed  capacity.  Cost estimates
shown  on Figure  8-5  are based  on the use  of hydrated  lime in
small  plants  (50 pounds per  hour  [22.1  kg/hr]  or less)  and
pebble quicklime  in larger  plants.   Allowances for peak rates of
twice the average  are  built into the lime  feed  rates.   At least
15 days of  storage  is  provided  for at  the average rate.   Storage
time  varies from installation to  installation  because  it is
dependent upon  the relative  distance  to and  reliability  of the
chemical supply.   Piping  and  buildings to  house the  feeding
equipment are included  in the  estimates.   Estimated  costs of
steel bins  with  dust  collector  vents and filling accessories are
also included.
        8.4.6.2  Operation and Maintenance Cost

Figure  8-6 indicates  the  relationship  between  man-hours spent
annually   for  operation  and  maintenance  and pounds of FeCl3
fed per hour.   The labor  includes  unloading the  ferric chloride
and the operation and maintenance of the chemical feed equipment.
Unloading requirements  are  as  follows:   for  a 4,000-gallon
(15.1 m3)  truck—1
72  per  truck — 9
                ,5 man-hours;
                man-hours.
for 50-gallon (0.19 m-
These  requirements
)  barrels,
are  shown
                               8-10

-------
08

o

cc
o
Li-

en
cc
D
O
I
D
Z
                                        3456 7891,000
3456 78910,000
             INSTALLED CAPACITY, pounds Lime/Hour (as CaO) (1 Ib = 0.454 kg)


                                FIGURE 8-5


        CAPITAL COST OF LIME STORAGE AND FEEDING FACILITIES (22)
     100
        10     2   3 456789100    2   3456789 1,000   2   3456 78910,000



                     Pounds Ferric Cloride Fed/Hour (1 Ib = 0.454 kg)


                                FIGURE 8-6


            FERRIC CHLORIDE STORAGE AND FEEDING OPERATING

            AND MAINTENANCE WORK-HOUR REQUIREMENTS (22)
                                    8-11

-------
as man-hours  per pound  of chemicals fed  to the process.  Metering
pump  operations and  maintenance  is  estimated at  five minutes  per
pump  per shift.

o
IT
UJ

LLI
y
DC
u
UJ
     1,000
               345 0, 7 8 8 10
                              3 4  56789 100
                                              3  4 66789 1,000
                                                             3 4 6 6 7 8 9 10.0OO
                             FEEDING RATE, Ib (1 Ib = 0.454 kg)/hr

                                  FIGURE 8-7


               ELECTRICAL ENERGY REQUIREMENTS FOR A FERRIC
                     CHLORIDE CHEMICAL FEED SYSTEM (23)
00
o
OC
o
LL
CO
QC
=>
O
I
<
z
    10,000
    1,000
       T
        100
              2   34 567891,000   2   34  5678910,000  2   34 56789100,000


                        Pounds Lime Fed/Hour (I Ib = 0.454 kg)
                               FIGURE 8-8

                LIME STORAGE AND FEEDING OPERATION AND
               MAINTENANCE WORK-HOUR REQUIREMENTS (22)
                                   I-12

-------
Figure  8-7 indicates  annual  electric  power requirements  for  a
ferric chloride chemical  feed system.

Annual maintenance  material  costs are typically 3 to 5 percent  of
the total  chemical  feed  system equipment cost.

Figure 8-8  indicates man-hours for operation and maintenance  as a
function of pounds  of lime  fed  per hour.   The  curve consists  of
lime  unloading  requirements  and labor  related  to  operation and
maintenance of  the  slaking  and  feeding equipment. These  require-
ments are  summarized as  follows:  slaker--one hour per eight-hour
shift per  slaker  in use;  feeder--ten minutes per hour per feeder;
slurry pot-feed line  (for slaked lime)--four hours per week.
                                  PUMPED FEED OF
                                  SLAKED LIME

                                  GRAVITY FEED OF
                                  QUICKLIME
                                                    GRAVITY FEED
                                                    OF SLAKED LIME
                            PUMPED FEED OF
                            QUICKLIME
     1,000
        100
 3  4 567891,000  2   34  5678910,000 2

      Pounds Lime Fed/Hour (1 Ib = 0.454 kg)


             FIGURE 8-9

ELECTRICAL ENERGY REQUIREMENTS FOR
      A LIME FEED SYSTEM (22)
                                                     3 4567 89100,000
                                8-13

-------
              curves
Figure
a  1 ime
used  in  the
1,000 pounds  (454 kg)
activators--2.7 to  0
collection  fans—0.04
slurry feed pumps--2.2
8-9  shows  annual  electric power  requirements  for
feed  system.   The  major  components  and the  values
               all  expressed kilowatts  per  hour  per
              of lime fed are: slakers--1.6 to 0.8;  bin
              36;  grit conveyors--0.45 to 0.06;  dust
              to  0.02;  slurry mixers--0.027  to 0.020;
              to 1.4.
Annual  maintenance  material  costs  are  typically
1.5 percent of  the  total  lime feed system equipment cost.
                                                        0.5  to
8.5  Chemical Conditioning With Polyelectrolytes


    8.5.1  Introduction

During the past decade,  important advances  have  been  made  in  the
manufacture  of polyelectrolytes for use  in wastewater sludge
treatment.    Polyelectrolytes  are now  widely used  in sludge
conditioning  and as indicated in Table 8-3, a  large  variety  are
available.   It  is  important to understand that these  materials
differ greatly in chemical composition, functional  effectiveness,
and cost-effectiveness.
                            TABLE 8-3

                  SUPPLIERS OF POLYELECTROLYTES

Company
American Cyanamid
Allied Colloids
Betz
Calgon
Number
of grades
and tynes
40
34
7
18


Company
Dow
Drew
Hercules
Nalco
Rohm & Hass
Number
of grades
and types
33
8
29
43
4
Selection of  the  correct  polyelectrolyte  requires  that  the
designer  work  with polyelectrolyte  suppliers, equipment
suppliers, and plant operating personnel.   Evaluations  should be
made on site  and with the sludges  to be  conditioned.   Since  new
types and  grades of polymers are  continually  being introduced,
the evaluation process  is  an  ongoing one.


    8.5.2  Background on Polyelectrolytes


        8.5.2.1  Composition  and Physical Form

Polyelectrolytes  are   long   chain,  water  soluble,  specialty
chemicals.   They  can  be either  completely  synthesized from
individual monomers,   or  they  can be  made  by  the  chemical
                               1-14

-------
addition  of  functional  monomers,  or  groups,  to  naturally
occurring  polymers.  A monomer is the subunit  from which polymers
are made  through various  types of  polymerization  reactions.
The backbone monomer  most widely  used  in  synthetic  organic
polyelectrolytes is  acrylamide.   As  of  1979  the  completely
synthesized  polymers are  most widely  used.   Polyaerylamide,
created when  the  monomers  combine  to  form  a  long,   thread-like
molecule with a  molecular  weight in the  millions,  is  shown  on
Figure  8-10.   In the form  shown polyacrylamide  is   essentially
non-ionic.   That  is to say it carries no net electrical charge in
aqueous  solutions.   However,   under  certain  conditions  and
with  some  solids,  the polyacrylamide  can be  sufficiently
surface-active to perform as a flocculant.
— 	 	 .L Pll



pt-i _„ -„.
Vrf PI — imm™
1
C = 0
,
,lr-T-_T..1 pu pi_i _,,

1
C = 0
f
\
OLJ r>Lj

•
C = 0
*




              NH2               NH2               NH2
                           FIGURE 8-10

            POLYACRYLAMIDE MOLECULE - BACKBONE OF THE
               SYNTHETIC ORGANIC POLYELECTROLYTES
Anionic-type   polyacrylamide  flocculants   carry   a   negative
electrical  charge  in  aqueous  solutions  and  are made  by  either
hydrolyzing  the  amide  group  (NH2)  or  combining  the acrylamide
polymer  with   an  anionic  monomer.    Cationic polyaerylamides
carry  a  positive  electrical  charge  in  aqueous solutions  and
can  be prepared  by  chemical modification of  essentially
non-ionic-polyacrylamide  or by  combining  the  cationic monomer
with acrylamide.   When cationic monomers  are copolymerized with
acrylamide in varying proportions,  a family  of  cationic
polyelectrolytes with varying  degrees  of charge  is produced.
These  polyelectrolytes are the most widely used  polymers  for
sludge conditioning, since most sludge solids  carry a negative
charge.  The  characteristics of the  sludge  to be  processed  and
the type of thickening or  dewatering device used will  determine
which  of the  cationic  polye lectroly tes  will work   best  and
still  be  cost-effective.   For example,  an increasing  degree
of  charge  is  required  when  sludge particles  become  finer,
when  hydration  increases, and when relative surface  charge
increases.
                              8-15

-------
Cationic  polyelectrolytes  are  available  as  dry powders  or
liquids.   The liquids come as  water solutions or emulsions.   The
shelf  life of the  dry powders  is  usually several years,  whereas
most  of  the  liquids  have  shelf lives  of two  to six months  and
must  be  protected  from  wide  ambient   temperature  variations  in
storage.   Representative dry cationic polyelectrolytes  are
described  in  Table 8-4.   This  table does not list the myriad of
available  types  but  does show some of  the differences in  the
materials.   The  original  dry  materials  introduced  in the  1960s
were  of  relatively  low  cationic functionality  or  positive
charge  and high  molecular weight.   They were  produced  for  the
conditioning  of  primary  sludges  or  easy-to-condition mixed
sludges.   The incentive to  produce polymers  of higher positive
charge resulted  largely  from efforts to  cope with mixed  sludges
containing  large  quantities of  biomass.


                             TABLE 8-4

       REPRESENTATIVE DRY POWDER CATIONIC  POLYELECTROLYTES

                      Relative cationic     Molecular    Approximate  dosage,
         Type             density          weight       Ib/ton dry  solids
 Polyacrylamide copolymer      Low            Very high         0.5 - 10
 Polyacrylamide copolymer      Medium          High               2-10
 Polyacrylamide copolymer      High           Medium high         2-10
 Polyamine homopolymer        Complete        High               2-10
Relatively  low  molecular weight  liquid  cationics with  a 30  to
50  percent  solids  content  were  also available  in  the 1960s.
They  were,  however,  largely displaced  by the higher cationic
functionality,  high  molecular weight and  newer, 'less costly
liquid cationics.    The various liquid  cationics,  in either
dissolved or  emulsion  form,  are  described in  representative
fashion only,  in Table 8-5.   These liquid cationics eliminate the
dustiness  inherent in some dry powders  but  also  require much  more
storage space.   The selection of  a dry, liquid, or emulsion  form
material  usually  depends on a  comparison  of  cost-effectiveness,
ease of handling,  and storage requirements.
                             TABLE 8-5

          REPRESENTATIVE LIQUID CATIONIC POLYELECTROLYTES

            Type            -    Molecular weight          Percent solic3s_
      Mannich product              Low                      20
      Tertiary polyamine            Low                      30
      Quaternary polyamine           Very low                  50
      Cationic homopolymer           Low to medium             16 - 20
      Emulsion copolymer            Low to medium             25 - 35
                                 5-16

-------
        8.5.2.2  Structure in Solution

Organic polyelectrolytes dissolve in water  to  form solutions of
varying  viscosity.  The  resulting viscosity  depends  on  their
molecular weight  and  degree  of  ionic  charge.   At  infinite
dilution,  the molecule  assumes the  form of  an extended  rod
because of  the repulsive  effect of the  adjacent-charged  sites
along the length of the polymer  chain.   At normal  concentrations
the long thread-like  charged  cationic polyelectrolyte assumes the
shape of a random coil,  as shown on Figure 8-11.  This simplified
drawing,  however, neither shows the  tremendous  length of  the
polymeric molecular chain  nor  does  it  illustrate the very  large
number of active polymer  chains that are  available in a polymer
solution.    It  has  been estimated that  a  dosage of  0.2  mg/1 of
polyelectrolyte having a molecular  weight  of 100,000  would
provide 120  trillion  active chains per  liter of water treated.
               0
                                                  0
                                                         ©
                     ©
                           FIGURE 8-11
                TYPICAL CONFIGURATION OF A CATIONIC
                   POLYELECTROLYTE IN SOLUTION
        8.5.2.3  How Polyelectrolyte Conditioning Works

Thickening and dewatering are  inhibited by  the sludge particles,
chemical  characteristics,  and  physical  configurations.   Poly-
electrolytes  in  solution  act  by adhering  to the sludge particle
surfaces thus causing:

     •  Desorption of bound surface water.
                               8-17

-------
     •  Charge neutralization.


     •  Agglomerization of small  particulates by bridging between
        particles.

The result  is the  formation  of  a  permeable  sludge  cake matrix
which  is  able to release water.   Figure 8-12 illustrates the
polyelectrolyte-solid  attachment  mechanisn.    The first two
reactions  noted  on  Figure  8-12  are  the desirable ones and
represent  what  occurs  in normal  practice.   The  other  four
reactions represent what  can  occur  from over-dosage  or too much
shear of flocculated sludge.   The problems  reflected  in reactions
three through six rarely occur with  a well-designed process.
    8.5.3  Conditioning for Thickening

The various methods for thickening  sludge  are discussed in detail
in Chapter 5.
        8.5.3.1  Gravity Thickening

Normally, the  addition  of polyelectrolyte  is  not considered in
the original  design  because  of operating  cost,  but  it has been
used to upgrade  existing  facilities  (21,22).  Experience to date
has indicated  that  the  addition of polyelectrolyte to a gravity
thickener will:

     •  Give a  higher  solids capture than  a  unit not receiving
        polymer addition.

     •  Allow  a  solids   loading  rate two  to  four times greater
        than a unit not  receiving  polymer  addition.

     •  Maintain  the  same underflow  solids  concentration  as a
        unit not receiving polymer addition.

When polyelectrolyte  is used  to  condition sludge  for gravity
thickening,   it  should  be added into  the  sludge  feed line.  The
point  of addition should  provide  good  mixing and  not  cause
excessive shear before  the conditioned sludge  discharges  into  the
sludge feed  well.
        8.5.3.2  Dissolved Air Flotation Thickening

The effects of  polyelectrolyte  addition  on  solids capture, float
concentration,  solids  loading rate,  and hydraulic  loading rate
are covered in detail in Chapter 5.
                               8-18

-------
                      REACTION 1
    INITIAL ADSORPTION AT THE OPTIMUM POLYMER DOSAGE
   POLYMER
  o
PARTICLE
                                     DESTABILIZED PARTICLE
                      REACTION 2
                    FLOG FORMATION

                          FLOCCULATION
DESTABILIZED PARTICLES
       (PERIKINETIC OR
       ORTHOKINETIC)
FLOG PARTICLE
                      REACTION 3
  ^s     SECONDARY ADSORPTION OF POLYMER

  xj^/       NO CONTACT WITH VACANT SITES
      ^—
-------
        8.5.3.3  Centrifugal Thickening

Centrifugal  thickening  includes  thickening  by disc  nozzle,
imperforate  basket,   and   solid  bowl  decanter  centrifuges.
The disc  nozzle unit does  not  utilize poly electrolyte sludge
conditioning,  as  it depends  solely  on centrifugal  force
(G =  3,000  to  5,000)  to achieve solids-liquid separation.   The
imperforate  basket centrifuge  may or may not use  polyelectrolyte
addition.   If  polymer is added, it is in  the range of one to
three pounds of  dry  polymer  per ton  of  feed solids  (0.5 to
1.5 kg/t).  This addition allows higher hydraulic  feed rates and
sometimes  gives better solids recovery.  It does  not change the
thickened  solids  concentration.

Solid bowl  decanter  centrifuges  normally require  as  much as
20 pounds of dry  polymer per ton  of  feed  solids  (10 kg/t) for
thickening of a sludge,  especially a waste-activated sludge.  A
new solid bowl  unit  has been  developed  for both  thickening
waste-activated sludge and  obtaining an 85  to 95  percent solids
capture  with only 0 to  6 pounds of dry polymer per  ton of  feed
solids (0  to 3  kg/t).

When  polyelectrolyte conditioning  is  used  with  centrifugal
thickening  of  sludge,   several points  of addition  should be
provided.    The  optimum point  of addition  is   influenced by
differences  in  polymer charge  densities,  required  polymer
sludge reaction times, and  sludge  characteristics.  . Recommended
points of  addition are:


     •  Directly  before the  inlet side of the sludge  feed pump.


     •  Immediately  downstream of the sludge feed  pump.


     •  To the  centrifuges'  sludge  feed line and  just before its
        connection to  the centrifuge.


    8.5.4   Conditioning for  Dewatering

The  various dewatering methods  are  discussed  in  detail in
Chapter 9.   Polyelectrolytes  were originally  used  to  condition
primary  sludges  and  easy-to-dewater  mixtures of  primary and
secondary sludges  for  dewatering  by  rotary vacuum  filters
or  solid  bowl  decanter  centrifuges.   Improvement  in the
effectiveness of  polyelectrolytes has led to their increasing use
with  all  types  of dewatering processes.    Reasons  for  selecting
polyelectrolytes  over  inorganic  chemical conditioners are:


     •  Little  additional sludge mass  is  produced.    Inorganic
        chemical  conditioners typically increase   sludge mass by
        15 to 30  percent.
                               8-20

-------
     •  If  dewatered sludge  is  to  be used  as  a  fuel  for
        incineration,  polyelectrolytes  do  not lower  the fuel
        value.

     •  They allow  for cleaner material-handling operations.

     •  They reduce operation and maintenance problems.


        8.5.4.1  Drying Beds

Polyelectrolyte  conditioning  is  not  widely  practiced.   Indica-
tions are,  however, that adding 0.5  to  2.0  pounds of dry polymer
per  ton  of dry  solids (0.25  to  1  kg/t)  can  increase dewatering
rates by two to  four times  (23,24).


        8.5.4.2  Vacuum Filters

The  majority  of municipal vacuum  filtration  processes in the
United  States  still  dewater sludge  conditioned with ferric
chloride  and  lime.   Several facilities have, however,  begun
using polyelectrolytes  for conditioning  and have  realized cost
savings  (4) due  to  less  equipment maintenance,  fewer materials
handling  problems,   and reduction  of cost  in  downstream sludge
processing  operations  (1,2,4).   Table 8-6 shows addition levels
of  dry  polyelectrolyte  used in conditioning  different  types
of  sludge  for vacuum  filtration.   When using polyelectrolyte
conditioning prior  to  vacuum filtration,  the designer should be
aware  that sludge  formation  properties  can  be  quite different
from  those of  inorganic  chemical  conditioners.   More   operator
attention  may  be required  to  obtain good cake  release   from the
cloth.  Cake dryness will probably  be 10  to  15  percent lower and
the  volatile  content of  the dry  cake  will  be  significantly
higher  than  if  the sludge   had  been  conditioned with ferric
chloride and lime.


                            TABLE 8-6

                TYPICAL POLYELECTROLYTE ADDITIONS
                       FOR VARIOUS SLUDGES3
              Sludge type
Raw primary
Waste-activated
Anaerobically digested primary
Primary plus trickling filter
Primary plus air waste-activated
Primary plus oxygen waste-activated
Anaerobically digested (primary plus air
  waste-activated)
Pounds of dry polymer added per
     ton of dry sol_ids_
0

1
2



.5
8
.5
.5
4
4
5
- 1.0
- 15
- 4
- 5
- 10
- 8
- 12
 Data supplied by equipment manufacturers.

 1 Ib/ton =0.5 kg/t
                                8-21

-------
        8.5.4.3  Recessed Plate  Pressure Filters

No published  information  could  be  found  on operating experience
in  the  United  States  with  polyelectrolyte  conditioning  of
municipal  wastewater  sludge  prior  to  pressure   filtration.
Several  English  studies  indicated  that  polyelectrolyte
conditioning  can  be  effectively  used with  pressure filtration
if done with  care.    Dosage  must  be optimized  and carefully
controlled for optimum cake  solids concentration, solids capture,
and ability to release the cake (25).   A comprehensive  study
on filter press  operating experience  in  the North American pulp
and paper industry was  recently published and gives some insight
to the  use  of polymers for conditioning  (26).  Excerpts  from
the study are given below.

"Many existing  pulp  and paper  industry  installations  have  been
conducting polyelectrolyte  evaluations on their own  with,  what
initially appeared  to be, very encouraging results.  The polymers
that have met with  greatest  success  are those which form what can
be best described as  strong  'pin-floe.1 An array of low molecular
weight cationic polymers have been cited  as providing acceptable
press  performance.    The  reasons  for  adopting polymer  as  a
conditioning  agent   have  included  (a)  reduced  conditioning
costs;  (b) reduced quantities of  solids  for handling due  to the
avoidance of large  amounts of  inorganics;  and,  (c) elimination of
those  problems  in final  disposal  operations  that have  been
associated with inorganic conditioning agents.  Projected polymer
requirements vary  from 3  to 30  pounds  of  polymer per  ton of
sludge solids. "

Several  mills  have  identified  special considerations associated
with polyelectrolyte conditioning.   In one instance,  the polymer
conditioned   cakes  are discharging  less readily  than those with
inorganic conditioning.   However, several  other  mills  report no
noticeable  difference  in  discharge  characteristics.    It  is
generally observed  that both cake consistencies and densities are
lower  when  using  polymer  conditioning.   However,  in several
instances,  the  difference  is  felt to be  associated with the
bulk  of  the  inorganic  conditioning  added as  dry  solids  before
pressing."

"The  handling  of  polymer-conditioned  sludge prior  to  pressing
has been identified   as  important.   Complete  initial  mixing of
the sludge and polymer  is crucial and subsequent handling  should
involve  a minimum  of  shear.   It has been  proposed  that  mixing
be accomplished  by  injecting  the polymer  into  the  suction side
of a  positive  displacement  pump or  the discharge side  of  a
centrifugal pump.   Mills  have indicated  the existence  of an
optimum flocculation  time between conditioning and pressing.  One
mill  reports  that  at the discharge of the  press  feed  pump, the
floe  is sufficiently sheared  to  render  it very difficult to
dewater  but  that in  the  remaining 30  feet  of pipe to the  press,
virtually complete reflocculation occurs.   At the other extreme,
several  instances  of  intermittent  sludge  septicity  demonstrated


                              8-22

-------
that extended  sludge storage can be detrimental."   Caution should
be  exercised  in  extrapolating  paper  mill  data  to municipal
sludge.
        8.5.4.4   Belt Filter Presses

Operating experience  indicates  that  all belt  presses require
the  use  of  polyelectrolyte  conditioning to  make  them  work.
Compared  to other  mechanical  dewatering  processes, belt  presses
seem  to  have the greatest  need for optimizing the  polymer  dosage
as a  function  of  the  incoming  sludge's  characteristics  (27).
Underconditioning results in inadequate dewatering  in  the  initial
drainage  section(s),  causing  either  extrusion  of  inadequately
drained  solids  from  the  press  section(s), or in  extreme
instances,  an uncontrolled  overflow of  sludge from the  drainage
section(s).   Underconditioned  biological solids  can also blind or
clog  the  fine mesh filter  media.   Overconditioning  can  also be a
problem.   Too  much polyelectrolyte can cause  cake doctoring or
removal  difficulties and aggravate media-blinding  problems.  The
type  of polymer also influences the tendency of  a media  to blind.
In  addition, overflocculated  sludge  may  drain  so rapidly that
the solids  are  not distributed across the  media.

Table 8-7  lists typical levels of dry polyelectrolyte  addition' to
condition  sludges  for  dewatering on  belt  presses.   The  big
spread  in polymer addition  requirements  is  attributable  to  the
percentage  of biological solids present  in the total waste sludge
stream.   Figure 8-13  is the  result  of one  study  and  indicates
that  as the  percent  of biological solids increases so  do  the
polymer requirements (27).


                             TABLE 8-7

          TYPICAL LEVELS OF DRY  POLYELECTROLYTE ADDITION
                      FOR BELT FILTER PRESSES3

                                      Pounds  of dry polymer added per
             Sludge type                       ton of dry solids
Raw primary                                        4-8
Primary plus trickling filter                          3-10
Primary plus waste-activated (air)                       4-10
Waste-activated (air)                                8-12
Waste-activated (oxygen)                              8-12
Aerobically digested (primary plus waste-
  activated {air})                                   4-10
Anaerobically digested primary                         2-6
Anaerobically digested (primary plus waste-
  activated {air})                                   3-9
 Data supplied by equipment manufacturers.

1 Ib/ton = 0.5 kg/t
                                 i-23

-------
    60
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                = ONE OPERATING FACILITY
    20
    10
                 20
                                                             100
                             40         60



                       PERCENT BIOLOGICAL SOLIDS



                             FIGURE 8-13



               EFFECT OF BIOLOGICAL SOLIDS ON POLYMER

             REQUIREMENTS IN BELT PRESS DEWATERING (32)




        8.5.4.5   Centrifuges


As was  noted in  the  detailed discussion  in  Chapter 9, two  types

of centrifuges  can be  used for  dewatering:   imperforate  baskets
                               8-24

-------
and  solid  bowl decanters.   Although  many  imperforate basket
centrifuges  do not  use polyelectrolytes  for  sludge conditioning
prior to dewatering,  the addition of 1 to 3 pounds of dry polymer
per  ton  of dry feed  solids  (0.5 to 1.5  kg/t)  can greatly reduce
overall  operating cost.   The  reason  for this  reduction is  that
basket  centrifuges  are used  for  dilute,  difficult-to-dewater
sludges such  as  aerobically  digested,  extended aeration, and
nitrification  sludges.   Since  the  cost of polymer  is offset  by
the  reduction  in operating  time, a decision  is normally made  in
favor of adding polymer.

Solid bowl decanter  centrifuges usually require polyelectrolytes
to  obtain  good performance  on municipal wastewater  sludges.
Table 8-8  lists typical levels of dry polyelectrolyte addition  to
various  sludges  for  conditioning  prior  to dewatering  by  solid
bowl decanter  centrifugation.


                             TABLE 8-8

         TYPICAL LEVELS OF DRY POLYELECTROLYTE ADDITION FOR
                  SOLID BOWL DECANTER CENTRIFUGES
                   CONDITIONING VARIOUS SLUDGES3
            Sludge type
Raw primary
Raw primary plus WAS (air)
  Thermal conditioned (primary plus WAS
   {air})
  Thermal conditioned (primary plus
   trickling filter)
Anaerobically digested
  Primary
  Primary plus WAS  (air)
Pounds of dry polymer added per
     ton of dry solids

          2-5
          4-10
          6-10
          7-10
 Data supplied by equipment manufacturers.
 1 Ib/ton =0.5 kg/t


    8.5.5   Storage,  Preparation, and Application Equipment

 Storage,  preparation, and application  equipment for both dry  and
 liquid polymers are  discussed  in great  detail  in  two current
 USEPA publications  (15,28).


    8.5.6   Case History

 The  following  summarizes  the  conversion  of  the  sludge
 conditioning  process for the vacuum filters at  the  Bissell  Point,
 St.  Louis  treatment plant from  an  inorganic  chemical  to  an
 organic  chemical process  (29).   The Bissell Point  plant dewaters
 and incinerates 35,000 dry tons  (31,745 dry)  of  raw primary
                                 5-25

-------
sludge  per year.   Conditioning  of  the  sludge  before vacuum
filtration was  with  ferric  chloride  and  lime until  July  1976.
Table 8-9 summarizes  the solids  handling systems performance
from  1972-1976.
                              TABLE 8-9


              PERFORMANCE OF SOLIDS HANDLING SYSTEM AT
               BISSELL POINT, ST. LOUIS STP 1972-1976 (29)


                                                      Cost,
 _           Item	Usage	dollars/dry ton '

Lime dosage, Ib/dry ton        ,         352                  6.90
Ferric chloride dosage, Ib/dry ton         64                  5.09
Auxiliary fuel (natural gas), therms/
  dry tonb                           62                 12.75

   Total annual cost                    -                 34.74

Yield (average), Ib/sq ft/hr              7.1
Solids content, percent                 30
Volatile solids fraction, percent          42
 All costs are adjusted to a July 1978 value.
 All tons (tonnes) are net dry tons (tonnes).  This is
 defined as the dry tons (tonnes) of filter cake produced
 less the dry quantities of chemicals required to produce
 the cake.

1 Ib = 0.454 kg
1 therm = 0.116 GJ
1 Ib/sq ft/hr =4.9 kg/m /hr
1 ton = 0.907 t


 Since   plant   startup in  1970,  numerous  problems  have  developed
 from the  use of  ferric  chloride  and lime.   The  major  problems
 were:


      •  Lime  coating of  filter cloths  and grid work.


      •  Scale buildup in filtrate and  plant drainage  lines.


      •  Constant cleanup of lime spills.

 In  July 1976, after six months  of planning  and experimentation,
 the conditioning process was converted  from  ferric chloride
 and lime  to   a  dual polymer  process  utilizing  either  anionic or
 cationic polymers.  Several  equipment modifications  and  operator
 training  programs  had  to be  undertaken  in order  to make  the
 system work properly.

     Grease Separation.   The  mixing of  primary  tank  skimmings
     with  the raw sludge  caused  blinding of  the  filter cloth.
     The large volume of  skimmings  also  influenced the solids
     concentration.    The  skimmings  did  not  upset  the ferric


                                  8-26

-------
    chloride-  and lime-conditioned sludge filters  as  much  as  they
    did  the  polymer-conditioned  sludge  filters.    The solution
    employed  was to separate the skimmings and sludge and treat
    each  separately.  Skimmings were dewatered by  a modified  grit
    dewatering screw and then fed directly into the incinerator.

    Cloth-Washing  Equipment.   For polyeleetrolytes  to be
    effective, it  is mandatory that the filter cloth be  cleaned
    continuously.    The  original  filter spray water  system
    included  one  spray nozzle strainer.   When this  strainer
    had  to be  cleaned,  the  unit  had to  be stopped.   To  correct
    the problem, the one strainer was replaced with a duplex-type
    strainer  which allowed switching  of  the  strainers  with
    no change  in the filter  operation.

    Miscellaneous Filter Improvements.  Several  modifications
    were  necessary  to improve cake  removal from  the media.  The
    doctor blades  were modified to  fit  together  and  against the
    cloth media.  Operating with polymers was  found best at low
    vat levels.   To avoid  loss  of vacuum when  running  at low
    levels,  bridge  blocks  in the vacuum valve  were  installed to
    modify the pickup zone.

    Operator  Education.   It was necessary to  convince the plant
    operators   that  po~lymer  usage would be  beneficial  to them.
    An extensive  educating process was conducted  for several
    months informing the  operators of  the  benefits they would
    obtain using polyelectrolytes.

The  conversion was  considered  very successful.    Table  8-10
summarizes  performance  information for  the solids  handling
processes  after  implementation  of  the   polyeleetrolytes
conditioning process for 1977-1978.   Comparison  of  the
performance data in  Tables  8-9 and  8-10  shows that the  use of
organic  polymers   in  place of  inorganic conditioners  reduced
auxiliary fuel  requirements  by  26 percent and conditioner  cost
by 53 percent.  Overall annual cost per  dry ton of solids was
reduced by 56  percent.
     .5.7  Cost
        8.5.7.1   Capital Cost

Figure  8-14 gives construction costs for polymer storage and
feed  facilities  as  a function of  installed capacity.   Cost
estimates were based  on  the  use of dry polymer.   Chemical feed
equipment  was  chosen specifically for  a  0.25  percent stock
solution.   Piping  and buildings to house the  feeding  equipment
and store the bags were included.   For example,  for an  installed
capacity of 10  pounds (4.5 kg)  of dry polymer  per hour, the
approximate June 1975 cost was  $110,000.   The  cost would  need  to
be adjusted to the current  design period.


                              8-27

-------
                                     TABLE 8-10

                  PERFORMANCE OF SOLIDS HANDLING SYSTEM AT
                   BISSELL POINT, ST. LOUIS STP 1977-1978  (29)
                Item
Anionic  dosage, Ib/dry ton  ,
Cationic dosage, Ib/dry ton
Auxiliary,  fuel  (natural gas), therms/
  dry ton

    Total  annual cost

Yield (.average) , Ib/sq ft/hr
Solids content, percent
Volatile solids fraction, percent
Usage_

 0 .34
   65

   46
  7. 8
   28
   56
      Cost,      ,
dollars/dry ton '
        0.42
        5.25

        9.52

       15.19
 All costs are adjusted to  a July 1978 value.

 All tons  (tonnes) are net  dry tons (tonnes).  This is defined
 as the  dry tons  (tonnes) of filter cake produced less the  dry
 quantities of chemicals required to produce the cake.

1 Ib = 0.454 kg
1 therm  =  0.116 GJ
1 Ib/sq  ft/hr = 4.9 kg/in /hr
1 ton =  0. 907 t
      UJ

      D
      Q
      a
      g
      u
      it
      v>
      Q
          100,000

              a
              7
              6
              5

              4
           lO.QOU
                              5 t 7 S » 1     2   345S7B91Q      3

                              INSTALLED CAPACITY, Ib Polymer/hr (1 Ib = 0.454 kg)


                                     FIGURE 8-m
                                                                    3  4  § « 7 B
                  RELATIVE INFLUENCE OF POLYMER ADDITION ON
           IMPERFORATE BASKET CENTRIFUGE PROCESS VARIABLES (22)

-------
        8.5.7.2  Operation and Maintenance Cost

Figure  8-15  gives man-hours  for operation and  maintenance of
a dry polymer  feed  system  as  a function of pounds of chemicals
fed per  hour.   Unloading requirements are 16 minutes  for 10-
to 50-pound (4.5 to  22.6 kg) bags.  Mixing labor was estimated at
ten man-hours  per 1,000 pounds  (453.5 kg) of polymer  under a
wastewater flow  of  10 MGD  (26.2  m3/s ) and  three hours per
1,000 pounds (453.5 kg) of polymer for wastewater  flows over
10 MGD  (26.2 m^/s).   Operation  and maintenance  requirements
were taken as 385  man-hours per year per feeder.
 O
 LL
 O
 I
 z
 z
      100
              2   34 BB7891    234  S 678910    234  56188100

                        POLYMER FED, Ib/hr  (I Ib - 0,454 kg)

                           FIGURE 8-15

             POLYMER STORAGE AND FEEDING OPERATION AND
              MAINTENANCE WORK-HOUR REQUIREMENTS (22)
Figure  8-16  gives annual  electrical  power  requirements for
a  polymer  feed  system.   The  graph was  based on  the  use of
plunger metering pumps and 6.4  hp  hour  (4.7  kWhr)  for  mixing of
100 pounds (45.4  kg)  of polymer.
Annual  maintenance  material  costs  are  typically  0.5
1.5 percent of the total polymer feed system equipment cost.
to
8.6  Non-Chemical Additions

Power plant or sludge  incinerator ash has been used successfully
to  improve  mechanical  dewatering  performance  on full-scale
vacuum  filters and  filter presses (30).  The  properties of  ash
                               8-29

-------
that improve dewatering  of  sludge include  the  solubilization  of
its  metallic  constituents,  its  sorptive  capabilities,  and  its
irregular particle  size  (31).   The  advantages  and  disadvantages
of  adding ash for  sludge dewatering are  given in Table  8-11.
Major  advantages  are  lower chemical  requirements   and  improved
cake release.  Major  disadvantages are  the  addition of a sizable
quantity  of  inerts  to  the sludge  cake and  additional  material
handling.  For  installations where  landfilling  of  sludge follows
mechanical dewatering  by vacuum  filters  or  filter  presses,  the
use of ash to improve the total solids content of the cake  should
be  evaluated.  If  incineration is  to follow the dewatering  step,
other  additives such as pulverized coal  or waste pulp should
receive  preferential  considerations   (32-34).   In  the design  of
incineration facilities, one of  the
or  eliminate auxiliary fuel demand.
the  driest  solids  cake  possible
by  enhancing the  fuel value of the
of ash to the sludge assists the dewatering device in producing a
dry  cake,  but  it does  nothing for  the  fuel value  of the  cake.
Ash  has  no  heating value and,  in  fact,  requires additional heat
input to raise its temperature.
                                     main  objectives  is  to reduce
                                     This  can  be  done by feeding
                                     to the  incinerator  and/or
                                     sludge solids.   The addition
I
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                                                      3456 739100
          0     234 567891     1   34  S678910    1

                        POLYMER FED Ib/hr (1 Ib = 0.454 kg)

                            FIGURE 8-16

               ELECTRICAL ENERGY REQUIREMENTS FOR A
                     POLYMER FEED SYSTEM (22)

A pilot-scale  vacuum  filtration study has found  pulverized  coal
to  be  an excellent  sludge conditioner  for  improved  dewatering
(32).    The coal  contributed  the   same benefits as  ash  and
increased  the  Btu  content  of the  sludge  solids.   Economic
                               8-30

-------
analysis showed it to  be cost-effective  when compared to  the
addition of  other  supplemental fuels such  as natural gas  or
#2  fuel  oil.   A  full-scale solids  handling study  at St.  Paul,
Minnesota,  demonstrated that an  existing seven-hearth  wastewater
sludge cake incinerator could be  fed  coal  or wood chips  with  the
sludge  cake  to  reduce  consumption of  natural gas  or fuel  oil
(35).  The process  was  found to  be  economically justifiable  and
practical  only  when a  large quantity of natural gas or  fuel  oil
is required  for  sludge  cake incineration.
                            TABLE 8-11

           ADVANTAGES AND DISADVANTAGES OF ASH ADDITION
                    TO SLUDGE FOR CONDITIONING

             Advantages                         Disadvantages
 Substantial increase in total cake        Ash handling generates considerable dust
  solids
 Significant improvement in filtrate       Ash fines build-up
  quality
 Excellent cake discharge               Possible equipment abrasion problems
 Elimination or significant reduction      Increase in materials handling problems
  in use of other conditioning agents      For those installations with incineration,
                                   the addition of ash lowers the percent
                                   volatile solids in the feed.  Fuel usage
                                   can therefore increase.
The  use of  waste  paper as  a conditioner  for sludge has  also
been  studied  in the  laboratory  and on  a  plant  scale (33,34).
Some  paper-conditioned sludge was  dewatered on  full-scale vacuum
filters  (34).   Results were excellent, indicating  that the use of
waste  paper  and polymer  were significantly  more  economical  than
ferric chloride and lime.
8.7  Thermal Conditioning

This  process  involves .heating  of  wastewater  sludge  to
temperatures  of 350°  to  400°F  (177°  to  240°C)  in  a reaction
vessel  under pressures of 250  to  400 psig  (1,723  to 2,758  kn/m^)
for  periods  of  15  to 40  minutes.  One modification  of  the
process  involves  the  addition  of a  small amount of  air.
Figures  8-17   and  8-18  show   a  general   thermal  conditioning
flow scheme for plants  without  and with  the addition of  air,
respectively.

Thermal  conditioning of  sludge was  first  studied by William K.
Porteous  in England  in the  mid-1930s (36).  Thermal  conditioning
in  the  United States  was first  studied  in  the  mid-1960s,
and  the  first facility having  no  air addition  was installed
at  Colorado  Springs,  Colorado,  in  1969  (37-39).    The
first  plant  with  air addition  was  installed   at  Levittown,
                                8-31

-------
Pennsylvania,  in 1967 (40).  Since  then,  over one hundred thermal
sludge  conditioning  installations have  been built  in the United
States.
       RAW SLUDGE
                          SLUDGE -WATER
                           SLUDGE HEAT
                           EXCHANGER
POSITIVE
DISPLACEMENT
PUMP
DECANT
LIOUOR
                                                            CAKE
                             FIGURE 8-17

          GENERAL THERMAL SLUDGE CONDITIONING FLOW SCHEME
                     FOR A NON-OXIDATIVE SYSTEM
                                 8-32

-------
       RAW SLUDGE
                              COMPRESSED  AIR
       POSITIVE
       DISPLACEMENT
       PUMP
          i	IXH
SLUDGE -
SLUDGE HEAT
EXCHANGER
STEAM
                                                 BOILER   _  TREATED
DECANT
LIQUOR
                                                           CAKE
                            FIGURE 8-18

          GENERAL THERMAL SLUDGE CONDITIONING FLOW SCHEME
                      FOR AN OXI DATIVE SYSTEM
    8.7.1  Advantages and  Disadvantages

Thermal  conditioning of  wastewater  sludges has  the following
advantages:

     •  Except  for straight waste-activated  sludge,  the  process
        will produce   a  sludge  with  excellent  dewatering
        characteristics.    Cake  solids  concentrations  of
        30 to 50  percent are  obtained with mechanical dewatering
        equipment.
                                !-33

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     •  Processed  sludge  does not  normally  require chemical
        conditioning  to  dewater well on mechanical equipment.

     •  Process  sterilizes  the  sludge,  rendering  it free  of
        pathogenic  organisms.

     •  If done prior to incineration, the process will provide a
        sludge with  a heat  value  of 12,000  to 13,000 Btu per
        pound of volatile solids  (28 to 30 kJ/g).

     •  Process is  suitable for many types of sludges that cannot
        be stabilized biologically  because the presence of toxic
        materials.

     •  Process is  insensitive  to changes  in sludge composition.

     •  No length or  elaborate  start-up procedures are required.

The disadvantages of  thermal conditioning  include:

     •  The  process  has  high  capital  cost due  to the  use  of
        corrosion-resistant materials  such as  stainless steel in
        the heat exchangers.  Other support equipment is required
        for odor collection  and  control  and high pressure fluid
        transport.

     •  Process  requires  supervision,  skilled operators,  and a
        strong preventative maintenance program.

     •  Process  produces  an odorous gas stream  that must  be
        collected and treated before release.

     •  Process produces sidestreams  with high concentrations of
        organics, ammonia nitrogen, and color.

     •  Scale  formation  in heat  exchangers,  pipes,  and  reactor
        requires acid washing.


    8.7.2  Process  Sidestreams

Thermal  sludge  conditioning produces both gaseous and  liquid
sidestreams that must be considered in design.


        8.7.2.1  Gaseous Sidestreams

A thermal sludge conditioning  process produces odorous materials
in:

     •  Vapors from  treated sludge in  the decant  or  thickener
        tanks.
                               8-34

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     •  Vacuum  filter  pump exhaust  and vacuum  filter  hood
        exhaust.    ,                     • .  • .

     •  Air  exhausted  from the  operations  and hopper  areas
        of any  enclosed  mechanical dewatering system.

These odors  must be treated by  processing all  exhaust air in
some  type of  odor  control  system.   Methods of odor  control
include  combustion,  adsorption,  scrubbing,  masking, dilution, and
surface  evaporation  (41).


        8.7.2.2  Liquid  Sidestreams

Thermal  sludge  conditioning  sidestreams  originate from the
conditioned sludge  when  it  is  decented, thickened or lagooned,
or  when  it  is  mechanically  dewatered.    The  composition of
thermally conditioned  sludge  liquor  is  difficult  to  assess.
In  one  study  of thermal conditioning with  no air  addition,
several  types  of  sludges  were  treated  and it  was noted  that in
general  (42):

     •  The  concentration  of  the  individual components  in a
        heat-treatment  sidestream  increased  in proportion to the
        feed-solids  concentration.


     •  The COD  of  heat-treatment  liquor was proportional to
        the dissolved solids for  all sludges  under all process
        conditions.

     •  The  organic N  content  of   heat-treatment  liquor was
        proportional  to the  dissolved  solids,  there  being
        one relationship for  activated sludge  and  others for
        trickling filter, primary plus activated,  and  digested
        sludges.


     •  The breakdown  of  organic N to ammonia in activated sludge
        heat  treatment liquor was  a  time-temperature  phenomenon.

In  general,  therefore,  the   composition of  the  liquor  is a
function  of  the   type of  sludge,  feed   volatile  solids  content,
reaction  time,  and  temperature.   Without  a  pilot  scale
investigation of process feasibilityV "it is  difficult  to specify
design data.    Table 8-12 gives ranges  for  various constituents
that have  been  reported  for both the  process with air  addition
and  the  process  without  air addition  that conditioned  sludges
having 3 to 6 percent  feed solids concentrations (41-50).

Table 8-13 summarizes data from  the literature on filtrate
or  centrate  composition.   Except   for suspended solids, the
parameters of  filtrate  are  similar   if  not  equal 'to   the decant
tank supernatant.


                               8-35

-------
                            TABLE 8-12

         GENERAL CHARACTERISTICS OF SEPARATED LIQUOR FROM
                   THERMAL CONDITIONED SLUDGE3
Parameter
Suspended solids, mg/1
Dissolved solids, mg/1
COD, mg/1
BOD5, mg/1
Phosphorus0, mg/1
Total N, mg/1
Organic N, mg/1
Ammonia N, rag/1
pH
Color
Metals
Oxidative
100-20,000 .

10,000-30,000
5,000-15,000
150-200
650-1,000

400-1,700
5.0-6.5
1,000-6,000 units
~d
Non-ox idative
300-12 ,000
. 1,700-12,000
2,500-22,000
1,600-12,000
70-100 ,
700-1,700
100-1,000
30-700
5.0-6.4
2,000-8,000
_e
    Mixture of 50 Dercent primary and 50 percent waste-activated at a feed solids
    concentration between 3 to 6 percent.

    Less than 20 percent of the COD is non-biodegradable.

   "Depends on P of influent sludge.

    See Reference 43.

    See Reference 44.
Many methods  have been used to treat  the  liquid sidestreams, and
they are discussed  in  Chapter  16.
    8.7.3  Operations  and  Cost

Analysis of  the  cost of  installing and  operating a  thermal
conditioning process  should  be comprehensive, as it impacts other
parts of the liquid  and  sludge handling  system.   The discussion
in this  section is general;  for those interested in more detail,
two recent reports  are available  (41,53).
        8.7.3.1  General  Considerations

Thermal sludge  conditioning has  been operating in  the United
States  for about  ten years.   During  that time,  over a hundred
facilities  have  been built  and much has  been learned  from
past  mistakes.   Following are current design guidelines that
must be considered  in the cost determinations for  a basic thermal
sludge-conditioning  system:

     •  If  there is  a chance  of  high  chloride  content  (greater
        than 400  mg/1)  in  the  sewage  or sludge  metal  with
        corrosion-resistant  properties  greater   than  stainless
        steel  must  be used  in the  hot heat  exchanger  (nearest
        reactor).
                                $-36

-------
     •   All  potential sources  of  odor  (decant  tank,  dewatering
        area,  vacuum  filter  exhaust must be  enclosed.  In
        addition,  an  air collection and treatment  system  must  be
        provided.


     •   Strength of  the  recycle  streams  depends on  many
        variables.   The worst possible  conditions  should  be
        used  as the design basis for the recycle liquor system.

     •   Good  grit removal from  the  sludge is essential to  prevent
        abrasion of  metal  piping.   The provision of grit  removal
        at the  plant influent  does not imply that  grit  will  be
        absent in the sludge stream. Large quantities of material
        can  blow  into clarifiers  and  aeration  tanks;  therefore,
        separate  grit  removal  before  the  thermal-conditioning
        system should be considered.


     •   Only  the most  rugged  types  of sludge  handling pumps
        should be used.


     •   Present-day  energy economics  dictate  careful  review  of
        heat  recovery systems.


        8.7.3.2  USEPA Survey Results

In~May  and June of  1979,  USEPA Technology Transfer, Cincinnati,
Ohio, conducted a survey of operation  and maintenance problems  at
76 thermal  conditioning process  facilities.   Table  8-14  lists
suppliers,  number  of  plants  involved  in  survey,  and  sum  of
operating experience.

Nearly  all the plants contacted indicated high costs of operation
and maintenance.  The high  operating  costs  resulted mainly  from
the cost of fuel  for steam generation, the addition of chemicals
for  boiler  water treatment,   and in  some cases  (Lexington,
Kentucky; Haverhill, Massachusetts;  Poughkeepsie, New York),
the  addition  of  chemicals to  improve dewatering.    Plants  that
utilize waste heat from sludge cake incineration are able to cut
considerably  both fuel  usage  and   the  volume of  sludge  (as  ash)
that must be  hauled.  .

Maintenance  costs involve replacing  various parts  on a somewhat
regular basis,  washing  the heat exchanger  and  reactor with  acid
to remove scaling,  and the  costs  of the manpower needed  to
perform these  tasks.   Plants that  have operating experience
express requirements  for  highly  trained personnel,  regular
preventive maintenance,  and a  good surveillance program.   These
practices  can  substantially  reduce maintenance  costs  due  to
excessive shutdown time or replacement  of major components  that
do not  normally wear  out.
                               8-37

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                                    TABLE 8-13

             FILTRATE AND/OR CENTRATE CHARACTERISTICS FROM
                  DEWATERINC THERMAL CONDITIONED SLUDGE
        Sludge type

Raw primary plus trickling
  filter sludge (heavy
  industrial load)
Anaerobically digested (primary
  plus waste-activated) plus.
  raw primary
Raw primary plus waste-
  activated
Raw primary plus waste-
  activated (high tannery load)

Anaerobically digested (primary
  plus waste-activated) plus
  raw sludge
Anaerobically digested primary
  plus oxygen waste-activated
Raw primary plus trickling
  filter sludge

Raw primary plus waste-
  activated
Raw primary plus waste-    ,  ~ /
  activated
Raw primary plus waste-
  activated
Raw primary plus waste-
  activated
Raw primary plus waste-
 Average values.
    Dewatering process

Recessed plate pressure
  filter
Rotary vacuum filter cloth
 media
Rotary vacuum filter cloth
 media
Rotary vacuum filter cloth
 media
Sand dry beds
Recessed diaphragm plate
  and frame pressure filter
Rotary vacuum filter cloth
  media
Centrifuge

Centrifuge
Rotary vacuum filter cloth
  media
Rotary vacuum filter cloth
  media
                         Coil vacuum filters
      Characteristics

Feed solids, percent = 9.0
Filtrate3
  Total solids, mg/1 = 8,000
  SS, mg/1 =150
  BOD5, mg/1 = 6,500
  COD, mg/1 = 12,000
  Total N, mg/1 = 1,075
  pH, units = 6.4
Feed solids, percent =10-15
Filtrate SSa, mg/1 = 5,000
BOD5a, mg/1 = 10,000
Feed solids, percent =6-10
Filtrate SS, mg/1 1,000
Feed solids, percent =8-13
Filtrate SS, mg/1 2,000
BOD5, mg/1 = 7,900 - 9,600
Soluble BOD5 of drainage does not ex-
  ceed 6,000 mg/1
Reference

   49
                                                       51
Feed solids, percent  14
Filtrate SS, mg/1 1,400

Feed solidsa, percent = 18
Filtrate SS, mg/1 9,000
BOD5, mg/1  6,800

Feed solids, percent = 6-7
Filtrate SS = 3,000 mg/1

Feed solids, oercent =6-7
Filtrate SS = 6,000-9,000 mg/1
Soluble BOD5, mg/1 = 4,200

BOD5, rag/1 = 7,300 - 9,100
Feed solids, percent 10 - 20
Filtrate, percent = 2 - 2.5
Soluble BOD5, mg/1 = 6,000 - 7,000
Feed solids, percent  13
Filtrate, percent solids =6-7
The  buildup  of   scale  in  the  heat  exchanger,  reactor,  or  pipes
occurs  in  most  plants  that  have  hard water  or  industrial  wastes
in the  influent.    Regular washing  with  acid  is  practiced  in all
plants  with  this problem.    The  length  of  operating  time  between
washes  varies  from  as  much   as  1,500  hours  to  as  little   as
200 hours.   Many plants  acid-wash on  a regular  basis,  about  every
month,  not  only to  remove  scale,  but  to  prevent  its  initial
buildup.

Many   operators, of   the  non-air  thermal  conditioned  systems
indicated  that  an  important  factor  in  a  good maintenance  program
is the  upkeep of a parts  inventory.   This eliminates  the  chance
of the  system  being shut down over  an extended  period  while  parts
are ordered.
                                         8-38

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                            TABLE 8-U

       USEPA JULY 1979 SURVEY OF EXISTING MUNICIPAL WASTEWATER
                       THERMAL CONDITIONING

Total installations
Number of installations
contacted in survey
Operating more than 120 hr/
week
Operating less than 120 hr/
week
Not operating
Period of operation
Less than 1 year
Between 1 to 2 years
Between 3 to 5 years
Over 5 years
Zimpro
83

57

27

20
10

11
15
11
20
Envirotech Nichols
30 '• 6

19 0

7

6
6

1
6
9
3
Zurn
1

0










 Formally called the Porteous process.  Porteous process was
 licensed by Envirotech in the mid-1960's.

 Formally known as the Dorr Oliver Farrer System. Purchased
 by Nichols in the early 1970's.
The  concensus  of the  operators  is that  after the  "bugs"  are
worked  out of  the  system,  after  the  personnel  have been
familiarized and trained, and after  a  routine  maintenance program
is established, the process performs satisfactorily.
8.8  Elutriation

Elutriation  is  the  term commonly used to refer to the  washing  of
anaerobically digested  sludge before vacuum filtration.   Washing
causes a dilution of the bicarbonate  alkalinity  in the  sludge and
therefore reduces  the  demand for acidic metal salt by  as  much  as
50 percent (54).

The process itself was  patented  by Center  in 1941  (55).   Although
it  typically  employs  one  or  two  tanks, any  number of  tanks
can be  used.    Two  to  six  volumes  of washwater, typically  plant
effluent,  flow  countercurrent to one  volume of anaerobically
digested  sludge.   Elutriation  tanks are designed  to act
as  gravity  thickeners,  with  a mass  solids  loading  of  8  to
10 pounds per square foot per day (39  to 48.8  kg/m2/day).

At  this  time the process   is not used  as extensively  as it  had
been because, in addition to reducing alkalinity, it also washed
out 10  to 45  percent  of  the  solids from  the incoming sludge
stream  (56-60).   Elutriate was recycled  back to the  main  plant
and eventually degraded the plant effluent  (57,58,60).
                                8-39

-------
Full-scale research  (60-62)  has shown  that  the  solids  problem
can  be  solved,  and 90  to  92  percent capture  achieved, with
the  use  of  polymers.  Recommended current  elutriation design
considerations are  listed below:

     •  Tanks  should be  loaded at  hydraulic  loadings  (total
        of both sludge and washwater flow)  of 200 to 300 gallons
        per  day per  square  foot (69  to 104  1/day/m2)  and  solids
        loading of  8 to 15 pounds per day  per square foot  (39  to

     •  Tanks should  have  the best possible  inlet  structure  to
        minimize inlet momentum.

     •  Baffling should be used  to prevent tank currents.

     •  Tanks should be provided with scum collection.


     •  Polymer addition  should  be provided.


8.9  Freeze-Thaw

In 1929, Babbit and  Schlenz demonstrated the benefit of freezing
wastewater sludge (63).   They  noted that, after sludge was frozen
on a sand drying bed during the winter and thawed in the spring,
its  drainage  qualities  were  improved  and  it dried to  a  higher
solids content.

Research  has since been  conducted  in  three areas  of  freeze
conditioning:  indirect and  direct mechanical systems and natural
freezing.


    8.9.1  Indirect Mechanical Freezing

Until  recently, all mechanical freeze-conditioning  research
has  been  oriented   toward  indirect  freezing methods.    Indirect
freezing  involves  the  separation  of the  refrigerant and the
sludge  by some type  of partition.    The  studies (64-66)  on
wastewater sludges  indicate  that freezing:

     •  Causes  cellular  dehydration and  thus  allows  better
        flocculation.

     •  Destroys the slirainess of biological sludges.

     •  Improves  dewatering  characteristics  as  measured  by
        sandbed and vacuum filter dewatering rates.


     •  Must occur  slowly to be  effective.
                               8-40

-------
Although freeze  conditioning has been shown  to  be  beneficial,  it
is  expensive  to  implement.    This  is because  the system  cannot
utilize  the heat generated by the fusion of  the  frozen sludge  to
cool the refrigerant.
    8.9.2  Direct Mechanical Freezing

To overcome  the  above-mentioned problem, pilot  work has  been
conducted on  direct  freezing  (67).    In  direct  freezing,  the
liquefied refrigerant  is  vaporized  and dispersed through the
sludge  slurry  at  a  controlled rate.   In  Table  8-15, slurry
freezing  (direct mechanical method)  is compared to  solid freezing
(indirect freezing) and  several  other  treatment processes.
                             TABLE 8-15

COMPARISON OF SEWAGE SLUDGE HANDLING AND CONDITIONING PROCESSES (67)
     Process
 Reduction
 in sludge
COD percent
                               Sludge
                             solubilization
                                          Supernatant and
                                          filtrate quality
 Slurry freezing
 Solid freezing
 Anaerobic digestion
 Aerobic digestion
 Chemical addition
     35
  50-70
  60-70
  30-70
  20-40
Low
High
High
Low
Low

_pH_
7-8
7-7.5
6-7
4-7
6-6.5

Quality
Good
Poor
Poor
Good
Moderate
Cost/ton
dry solid
6-20
5-35
15-20
15-30
10-25
 1 ton = 0.907 t

    8.9.3  Natural Freezing

In  this  method,  the  freezing  is  done  by  the  environment.
At least  one  facility (68) is operating  in Canada,  and extensive
full-scale research is  being conducted in facility design  in
order to  improve this method  of  conditioning  (69).


8.10  Mechanical Screening and Grinding

In some  applications,  screening  or  grinding  can  be  considered
part  of  the  sludge  conditioning process.   A good example  of
screening for conditioning is in the  application  of a disc nozzle
centrifuge.   A  stainless  steel,  self-cleaning  screen is required
to remove large solids  and  fibrous material that  would  clog the
disc nozzle machine.

Grinding  of  primary sludge  is an important step for some sludge
handling  processes.   It has   also  been indicated  that grinding  of
a thick  (over 8 percent solids)  sludge stream  reduces viscosity,
thus making  the slurry  easier to pump.   One outstanding example
of this is in the municipal  system at Glen  Cove,  New York.
                                1-41

-------
8.11  Miscellaneous  Processes

In  addition to  the more commonly  known conditioning methods
previously discussed, research  has  also been conducted on more
novel methods,  such  as bacteria, electricity, solvent extraction,
and ultrasonic.
    8.11.1  Bacteria

Autotrophic sulfur  bacteria  may  provide  cond itioning" irf  added
to digested sludge  prior to  dewatering (54).   Under aerobic
conditions, sulfur-oxidizing bacteria stimulate the production of
sulfuric  acid, which,  in turn,  lowers  the pH  of  the  sludge
and enhances the dewatering process as  measured  by  the specific
resistance test.   In another study  (70),  it  was  shown that
filtration  rates  of  waste-activated sludge  could be  increased
under anaerobic conditions with  the use of the enzyme lysozyme.
    8.11.2  Electricity

In  extensive  laboratory  and  pilot plant work  studies,  graphite
anodes  and iron  cathodes have  been  used  to  conditon sludge
(71-76).

These studies  indicate  that:

     •  At  pH  values  lower  than 4.0  electrical  current can
        condition  sludge  for filtration  without  the  use of
        chemicals.

     •  The quantity  of  water  removed  during  dewatering
        (vacuum filtration) was  proportional  to  the  amount of
        electricity  used.   Thinner sludges required less  current.


     •  Sludges  electrically conditioned  seemed  to  produce
        drier  cakes  than  chemically conditioned sludges.

The disadvantages  are that:

     •  Anodes had  to  be  replaced frequently  because  a dried
        crust  continually formed on them.

     •  The system  uses  a great  deal of electricity;  optimum
        current density  was  approximately  0.3  amp  per sq ft
        (3.3 amp/m2) of anode surface, with a  potential  drop of
        4 volts between the electrodes.

     •  No  full-scale  facilities have ever  -been tested to
        evaluate operating  problems.


                              8-42

-------
    8.11.3   Solvent Extraction

In 1957,  research was  conducted at  Rockford, Illinois,  with
carbon  tetrachlorethylene  as the  solvent,  with  distillation
end products being dried  oils, fats,  and  greases  (77).   It was
not considered  to  be very economical  at that  time.

Although  solvent extraction  is  becoming popular  in  industry
(78),  only recently  has there  been promotional  activity  in
the municipal  field (79).   To date, no municipal  installations
are using  the process.


    8.11.4   Ultrasonic

Conditioning of sewage  sludges by ultra- or supersonic vibration
has been explored (54).  Ultrasonic vibrations degasify sludge,
which  is  beneficial,  but the vibrations also tend to  destroy
sludge  floes,  resulting  in fine solids  that are  difficult to
dewater.
 3.12  References

 1.  Schillinger,  G.R.   "Conversion of  Sludge  Conditioning
     Chemicals."    Water Pollution Con'trg_jL_Fe_djera.t.ion Deeds and
     Data.   Vol.  16.  April 1979.

 2.  Nelson,  J.K.  and  A.H. Tavery.   "Chemical  Conditioning
     Alternatives  and  Operational  Control for Vacuum  Filtra-
     tion. "   Journal Water Pol 1 u t i o n  Con t_r_o_l_ F e d e r a t i o n .
     Vol.  50,  p.  507  (1978).

 3.  Carry,  C.W., R.P.  Miele,  and J.F.  Stahl.    "Sludge
     Dewatering."   Proceedings  of  the National  Conference on
     Municipal  Sludge Management.  I nf orma t ion  Transfer  Inc.
     Rockville,  MD  (1974).

 4.  Bargman,  R.D.,  W.F.  Garber,  and  J.  Nagano.   "Sludge
     Filtration  and Use  of Synthetic Organic Coagulants at
     Hyperion."   Sewage and Industrial Waste.   Vol.  30,  p.  1079
     1958.

 5.  Coackley,  P.  and R. Allos.   "The Drying  Characteristics
     of Some Sewage  Sludges."   Institute  of Sewage  Purificat.ion
     Journal Proceedings.  Pt. 6, p. 557.   1962.

 6.  Lapple,  C.E.    "Particle-Size  Analysis  and  Analyzers."
     Chemical  Engineering.  p. 149.  May 20, 1968.

 7.  Karr,  P.R.  and  T.M.  Keinath.   "Influence  of  Particle
     Size  on Sludge  Dewaterability."   Presented at  49th Annual
     Conference Water Pollution Control Federation.   Minneapolis,
     Minnesota.   10/3-8/76.


                              8-43

-------
 8.   Heukelekian,  H.  and E. Weisberg.  "Sewage Colloids."  Water
     and Sewage  Works.   Vol.  105,  p.  428.  October 1958.   . .71

 9.   Kos,  P.    Continuous Gravity Thickening of  Sludges.    Dorr
     Oliver Technical  Reprint 705.   1978.

10.   Tenney,  M.W., W.F.   Echelberger,  Jr., J.J.  Coffey,  and
     T.J. McAloon.  "Chemical Conditioning of Biological Sludges
     for  Vacuum  Filtration."     Journal Water Pollu t^on_ _Cqntrg_l
     Fede_ration.   Vol.  42,  p.  Rl .   1970.

11.   Hagstrom,  L.G.  and  N . A .  Mignone.    "Whatto Consider
     in Basket Centrifuge  Design."   Water_ anc3_Was_te_Eng_ineer ing .
     p. 58.  March 1978.

12.   Zacharias, D.R.  and  K.A.  Pietila.   "Full-Scale  Study of
     Sludge Processing  and  Land Disposal Utilizing Centrif ugation
     For Dewatering."    Presented  at the 50th Annual  Meeting of
     the  Central  States  Water  Pollution Control Federation,
     Milwaukee,  Wisconsin.    May  18-20, 1977.

13.   USEPA.  Evaluation of Dewatering  Devices for Producing High
     Sludge Solids Cake.    Office  of Research and  Development.
     Cincinnati, Ohio  45268.    Contract 68-03-2455.  1979.

14.   USEPA.   Performance  Evaluation  and  Troubleshooting  at
     Municipal Wastewater Treatment Facilities.   Office  of Water
     Program Operations.   Washington D.C.   20460.  USEPA 430/9-
     78-001.  January  1978.

15.   USEPA.   Process  Design  Manual for Suspended Solids Removal.
     Technology  Transfer,   Cincinnati,  Ohio 45268.   USEPA 625/1-
     75-003a.  January  1975.

16.   USEPA.  Energy  Conservation  in  Municipal Wastewater
     Treatment .   Office  of Water Programs.   Washington  D.C
17
1 8 .

     Washington D.C.   Second  edition.  May  1971

19.   Culp/Wesner/Culp .   Cost  and Performance Handbook  Sludge
     Handling Processes.   Prepared for  Wastewater Treatment and
     Reuse Seminar,  South Lake  Tahoe.  10/26-27/75.

20.   Ruehrwein,  R.A.  and T. Ward.   "Mechanism of Clay Aggregation
     by Polyelect roly tes . "   Soil Science.   Vole 73.   p.  485.
     January-June 1952.


                               8-44
20460. USEPA 68-03-2186, March 1977.
USEPA. Lime Use In Wastewater Treatment:
Data. Office of Research and Development,
45268. EPA 600/ 2-75-038. October 1975.
National Lime Association - Handling
Storage. Published by the National L
Design and Cost
Cincinnati, Ohio
Application and
ime Association,

-------
21.  Jordan,  V.J.  and  C.H.  Scherer.    "Gravity  Thickening
     Techniques  at  a Water  Reclamation Plant."   Journal Water
     Pollution Control Federation.  Vol.  42, p.  180.  1970.     ~~

22.  Ettelt,  G.A.  and T.  Kennedy.   "Research and  Operational
     Experience  in Sludge Dewatering at  Chicago."  Journal Water
     Po 11 ution Cpntrpj._Federation.  Vol.  38, p.  248.  1966.     ~

23.  Beardsley,  J.A.  "Sludge  Drying Beds Are Practical."  Water
     and Sewage  Works.  Part  1, p.  82.  July 1976.  Part 2, p. 42
     August 1976.

24.  Jennett,  J.C.  and  I.W.  Santry.   "Characteristics  of Sludge
     Drying."   Journal of the Sanitary^Jr^gJ.jT£e^J.j^g_DJLvision ASCE.
     SA 5, p.  849.   1969.

25.  Harrison,  J.R.   "Developments in Dewatering Wastewater
     Sludges."  Vol. 1 Sludge Treatment and Disposal.  USEPA-ERIC
     Technology  Transfer, Cincinnati, Ohio 45268.   October 1978.

26.  NCASI.   Full-Scale  Operational  Experience  with  Filter
     Presses for Sludge  Dewatering  in the North  American Pulp and
     Paper Industry.  Prepared  for National Council of the Paper
     Industry  for  Air and Stream Improvement.  Technical Bulletin
     299.  October 1977.

27.  NCASI.   A Review  of the  Operational Experience  With  Belt
     Filter Presses  for  Sludge Dewatering  in  the  North American
     Pulp and  Paper  Industry.   Prepared  for National Council of
     the  Paper Industry for Air  and Stream  Improvement.
     Technical Bulletin  315.   October 1978.

28.  USEPA.   Sludge Handling and Conditioning.  Office  of Water
     Program Operations.   Washington  D.C.  20460.    USEPA 430/9-
     78-002.  February 1978.

29.  Schillinger,   G.R.    "Conversion   of  Sludge-Conditioning
     Chemicals."   Journal Water Pollution  Co_n_tjrojl Federation
     Deeds andData^Vol. 16.April 1979.

30.  USEPA.     Pressure Filtration of Wastewater Sludge with Ash
     Filter Aid.  Office of Research  and  Development, Cincinnati,
     Ohio 45268.  EPA-R2-73-231. 1973.

31.  Smith, J.E.,   Jr.,  S.W. Hathaway,  J.B.  Farrell,  and
     R.B.  Dean.   "Sludge Conditioning  with  Incinerator Ash."
     Presented  2J7jth_JPurdue  Industrial  Waste Conference.   May
     1972.'

32.  Hathaway, S.W.  and  R.A.  Olexsey.  "Improving Sludge Inciner-
     ation and Vacuum Filtration with Pulverized Coal."  Journal
     Water PollutionControlfederation.  Vol. 49, pp. 2419-2430.
     1977.
                               8-45

-------
33.   Cargen,  C.A.  and  J.F.  Malina.   "Effect  of Waste  Paper
     Additions on  Sludge  Filtration Characteristics."   Center
34.
for Water Research #24. University of Texas, Austin.
Campbell, H.W., R.W. Kuzyk, and G.R. Robertson.
Use of Pulped Newsprint As A Conditioning Aid
Vacuum Filtration of A Municipal Sludge." Progress i
1968.
"The
in the
n Water
Technology . Vol. 10, pp. 79-88. 1978.
USEPA. Draft Copy Coincineration of Sewage Sludge with Coal
or Wood Chips. Office of Research and Development,
35.

     Cincinnati,  Ohio 45268.   EPA grant

36.  Porteous,  I.K.   "Mechanical Treatment  of  Sewage Sludge by
     the Steam Injection Method."  Municipal Engineer.  Vol. 16.
     December 1966.

37.  Harrison,  J.  and H.R.  Bungay.   "Heat  Syneresis of Sewage
     Sludges."    Water and  Sewage Works.   Vol.  115,  #5,  Part I,
     p. 217 and #6, Part II,  p.  268.   1968.

38.  Teletzke,  G.H.    "Low Pressure  Wet  Air Oxidation of
     Sewage  Sludge."    Proceedings  20th Purdue Industrial Waste
     Conference .   p. 40.   May 1965.

39.  Sherwood,  R.  and  J. Phillips.   "Heat Treatment  Process
     Improves Economics of Sludge Handling and  Disposal."  Water
     and Wastes Engineering .   p.  42.   November  1970.

40.  Blattler,  P.X.  "Wet Air Oxidation  at Levittown."  Water and
     Sewage Works.  Vol.  117, p.  32.   1970.

41.  USEPA.   Effects of  Thermal Treatment  of  Sludge On Munic-
     ipal Wastewater Treatment Costs'^    Municipal  Environmental
     Research Laboratory,  Cincinnati, Ohio 45268.   USEPA 600/2-
     78-073, June 1978.

42.  Brooks, R.B.   "Heat Treatment  of Sewage Sludges."  Journal
     of  the Instituteof Water Pgllu t j._g_n_^£on^ro 1 .  Vol. 69,
     p. 221 (1970) .

43.  Sommers, L.E.  and E.H.  Curtin.   "Wet  Air Oxidation:  Effect
     on  Sludge  Composition."    Journal Wate_r_Pol_lution Control
     Federa t ion .   Vol. 49,  p. 2219 (1977).

44.  Everett,  J.G.   "The  Effect  of  Heat Treatment on the
     Solubilization  of  Heavy Metals, Metals  and Organic Matter
     from  Digested  Sludge."    Journal  of the  Institute of Water
     Pollution  Control.   Vol. 73, p.  207  (1974).

45.  Brooks, R.B.   "Heat  Treatment of  Sewage Sludge."   Third
     National Chemical Engineering Conference .   Mildura Victoria,
     Australia.  August 1975.


                               8-46

-------
46.   Sarfert,  F.    "Composition  of the  Filtrate  From  Thermally
     Conditioned  Sludges."   Water  Research.   Vol.  6,  p. 521
     (1972).                           '   ~~~"

47.   Corrie,  K.D.    "Use  of Activated Carbon in the Treatment of
     Heat Treatment  Plant  Liquor."   Journal of  the  Institute of
     Water Pollution Control.   Vol. 71,  p.  629  (1972).

48.   Whitehead,  C.R.  and  E.J.  Smith.    "Sludge  Heat  Treatment:
     Operation  and  Management."  Journal of the  Institute of
     Water Pollution Control.   Vol. 71,  p.  31  (1976).~~

49.   Hirst,  G.,  K.G. Mulhall,  and M.L.  Hemming.   "The  Sludge Heat
     Treatment  and Pressing Plant  at Pudsey:  Design  and  Initial
     Operating   Experiences."    Journal  of  the Institute of Water
     Pollution  Control.   Vol.  71, p.  455 (1972) .       __-___—_

50.   Erickson,  A.H.  and  P.V.   Knopp.    "Biological  Treatment
     of Thermally-Conditioned  Sludge  Liquors."  Advances in Water
     Pollution  Research.   Vol.   II edited  by   S.M.  Jenkins.
     Published  by  Pergamon Press, London,  1972.

51.   Jones,  E.E.   "Finding A Better  Way To Dispose of Sludge."
     Publ ic  Works  Magaz j._ne .  March 1975.

52.   SCS  Engineers.   Review  of Techniques  for Treatment and
     Disposal  of Phosphorus-Laden Chemical  Sludges.USEPA-MERL
     Contract  68-03-2432  to be published in the summer  of  1979.

53.   Thermal Conditioning  Cost  Effect ivene^^_Repg^rt.    Zimpro,
     Inc., November 1978.       "  	~" "  "~™~

54.   Burd, R.S.   A Study of Sludge Handling and  Disposal.  U.S.
     Department  of Interior WP-20-4,  May 1968^~~~~~

55.   Center,  A.L.   U.S.  Patent 2,259,688,  October  21,  1941.

56.   Garrity,  L.V.   "Sludge Disposal Practices  at Detroit-
     Discussion."   Sewage Works  Journal.  Vol. 18, p.  215  (1946).

57.   Sparr,  A.E.   "Elutriation Experience  At the Bay  Park Sewage
     Treatment  Plant."   Sewage  and Industrial Wastes.   Vol. 26,
     p. 1443 (1954).

58.   Taylor,  D.    "Sludge  Conditioning and  Filtration  at
     Cincinnati's  Little  Miami  Sewage  Works."   Sewage and
     Industrial  Waste.  Vol.  29,  p. 1333 (1957).

59.   Chasick,  A.H.  and   R.T.  Dewling.   "Interstage  Elutriation
     of Digested Sludge."  Journal Wate r _JP_g_l _Tut_io_n _^iPJ} JLE2A_ F^ .9*e r ~
     at ion.   Vol.  34, p.  390  (1962).

60.   Dahl, B.W.,  J.W. Zelinski,  and  O.W. Taylor.   "Polymer Aids
     in Dewatering Elutriation."   Journal  Water ^Pollution  Control^
     Federation.  Vol.  44,  p.  201 (1972).


                               8-47

-------
61.   Goodman,  B.L.   "Chemical Conditioning of Sludges:   Six Case
     Histories."   Water and Wastes Engineering.    Vol.  3,  p.  62
     (1966).

62.   Goodman,  B.L.  and  C.P.  Witcher.    "Polymer-Aided Sludge
     Elutriation  and  Filtration."   .Journal Wa ter TPollu t ion
     Control  Federation.  Vol. 37, p. 1643  (1965).~~~~

63.   Babbitt,  H.E. and  H.E. Schlenz.  The Effect of Freeze Drying
     on  Sludges.    Illinois  Engineering Experiment  Station,
     Bulletin No.  198,  p. 48,  1929.

64.   Sewerage  Commission  City  of Milwaukee.    Evaluation  of
     Conditioning  and Dewatering Sewage Sludge by Freezing.
     Water Pollution Control Research Series  11010 EVE 01/71.

65.   Clements,  G.S.,  R.J.  Stephenson,  and C.J.   Regan.   "Sludge
     Dewatering by Freezing with  Added  Chemicals."   Journal and
     Proceedings  Institute   of  _ S_e_w_ag_ e  P u r i f i c a t i on  Journal.
     Part 4,  p. 318  (1950) .

66.   Cheng,  C.,  D.M.  Updegroff,  and  L.W. Ross.   "Sludge
     Dewatering  by High Rate  Freezing  at  Small  Temperature
     Differences."     Environmental  Science  and  Technology.
     Vol. 4,  p. 1145  (1970)."

67.   Randall, C.W.    "Butane Is Nearly 'Ideal'   For Direct
     Slurry Freezing."   Water and Wastes  Engineering.    p.  43,
     March 1978.

68.   Penman,  A.  and  D.W.  Vanes.   "Winnipeg Freezes  Sludge,
     Slashes   Disposal  Cost  10  Fold."   Civil Engineering-ASCE.
     Vol. 43,  p.  65  (1973).

69.   Rush,  R.J.  and  A.R.  Stickney.   Natural Freeze-Thaw Sewage
     Sludge Conditioning  and  Dewatering.    Canada  Environmental
     Protection Service Report EPS 4-WP-79-1, January 1979.

70.   Envirogenics  Co.    Biological Methods of Sludge Dewatering.
     FWQA-W-72-05838.   NTIS PB 207-480.   FWQA-14-12-427,  p. 147,
     August 1971.

71.   Slagle,  E.A.  and  L.M. Roberts.   "Treatment of Sewage  and
     Sewage Sludge  by Electrodialysis. "   Sewage Works JjQijrjna^.
     Vol. 14,  p.  1021  (1942).

72.   Beaudoin,  R.E.   "Reduction  o'f Moisture  in  Activated Sludge
     Filter Cake  by  Electro-Osmosis."   S_ewage  Works Journal.
     Vol. 15,  p.  1153  (1943).

73.   Hicks,  R.  "Disposal of Sewage  Sludge."   The Surveyor.
     pp. 105,  303.  April 19,  1946.


                              8-48

-------
74.  Cooling,  L.F.    "Dewatering of  Sewage Sludge  by Electro-
     Osmosis."  H^i^£_£Il5_^Iliia^X_J^Il£jjl£eiiB3,'   Vol.  3, p.  246
     (1952).

75.  Spohr,  G.   "Electrical Stimulation of Bacteria."  Water
     Works _and__ _Wa_s _t_es_ _E ng_ i_n e e r jLng   Ap r i 1 1 9 6 4 .

76.  Spohr,  G.    "Electrical  Stimulation  of  Bacteria."  U.S.
     patent 3,166,501.

77.  Stallery,  R.H.  and  E.H.  Eauth.   "Treatment  of Sewage
     Sludge  by  the  McDonald  Process."   Pjublic  Works.    p.  Ill,
     March 1957.

78.  Hanson,   C.   "Solvent  Extraction -  An Economically  Competi-
     tive Process."   £^l^]ILi£2^__^£i££££ill3.'   ?•  ^/  May  1979.
79.  Olson,  R.L. ,  R.K. Ames,  H.H.  Peters,  E.A.  Gustan,  and
     G.W. Bannon.   "Sludge Dewatering With  Solvent  Extraction."
     Proceedings  of  the  National  Conference Management  and
     Disposal  of  Residues  From  the Treatment  of Industrial
     Wastewaters .  Washington,  D.C. p. 175,  February  3-5,  1975.
                               8-49

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EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
                Chapters. Dewatering
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                           CHAPTER 9

                           DEWATERING
9.1 Introduction

Dewatering  is the removal  of water from  wastewater treatment
plant  solids to achieve  a  volume reduction greater than  that
achieved by thickening.  Dewatering is done primarily to decrease
the capital  and operating costs  of  the subsequent direct sludge
disposal or  conversion and  disposal  process.   Dewatering sludge
from a  5  to  a 20  percent  solids  concentration reduces  volume by
three-fourths and  results  in a  non-fluid material.  Dewatering is
only one component of  the  wastewater  solids treatment process and
must be  integrated into the overall  wastewater treatment system
so  that performance  of  both  the liquid  and solids treatment
schemes is optimized and total  costs  are minimized (1-3).
    9.1.1  Process  Evaluation

Several pilot-scale studies have been published that compare the
performance of  various  dewatering  devices  or  techniques  on
different sludge types  (4-9).  Table  9-1 summarizes equipment and
sludge types evaluated.   One  conclusion that  can  be  drawn from
these  studies  is  that selecting sludge  dewatering  processes  is
still  very  much  an art rather  than  a science.  Bench  or pilot
scale  testing  is always  recommended  before  final  design.   This,
however,  does  not  always guarantee  successful  operation  of the
full-scale  system.    As will  be shown,  there  are  many  problems
involved in the scale-up of  dewatering equipment,  and  this,
combined with  the changing  character  of municipal wastewater
sludges,  can  cause  significant problems.    Designers must  be
cognizant of these problems and allow for them in  the design  of
full-scale installations.

The main  variables  in any  dewatering  process  are:


     •  Solids  concentration  and  volumetric  flow rate  of the
        feed stream.


     •  Chemical demand and cost.


     •  Suspended and dissolved solids  concentrations and
        volumetric  flow rate of  the sidestream.
                               9-1

-------
         Solids  concentration
         dewatered  sludge.
                 and  volumetric flow  rate  of  the
                                 TABLE 9-1

                 PILOT-SCALE SLUDGE DEWATERING STUDIES
Reference
Sludge type
Type of equipment
            Mesophilic, anaerobically
             digested primary sludge
             from a publicly owned
             treatment work  (POTW)
            Mesophilic and thermophilic
             anaerobically digested
             sludge  (3/4 by weight
             primary plus 1/4 by weight
             waste-activated sludge)
             from a POTW

            Waste-activated sludge from
             a pulp and paper activated
             sludge plant
   7,8       Raw primary sludge  (1/3  by
             weight) plus waste-
             activated sludge  (2/3  by
             weight  from a POTW)

     9       Mesophilic, anaerobically
             digested primary  sludge
              (1/3 by weight) plus
             waste-activated sludge
              (2/3 by weight) from a
             POTW
                   Combination of a horizontal, solid bowl,
                    decanter centrifuge and a imperforate
                    basket centrifuge
                   Rotary drum, cloth-belt, vacuum filter
                   Rotary drum, coil-belt,  vacuum filter
                   Recessed plate pressure  filters

                   Horizontal, solid bowl,  decanter centrifuge
                   Imperforate basket centrifuge
                   Rotary drum, cloth-belt, vacuum filter
                   Recessed plate pressure  filters
                   Drying beds
                   Horizontal, solid bowl decanter centrifuge
                   Rotary drum, precoat vacuum filter
                   Recessed plate pressure filters
                   Belt filter press
                   Capillary suction
                   Ultraflitration
                   Dual cell gravity filter with multiple roll

                   Rotary drum, cloth-belt vacuum filter
                   Recessed plate pressure filters
                   Diaphragm recessed plate pressure filters
                   Belt filter press

                   Horizontal, solid bowl, decanter centrifuge
                   Rotary drum, cloth-belt, vacuum filter
                   Belt filter press
Specific  design criteria  for  selection  of  a  dewatering  process
can  also be  dependent upon subsequent  processing steps.  Both the
sludge  composting  and  the   incineration  process  require  sludge
with a relatively  low solids concentration.


Another  important  consideration  is  the  operation and maintenance
(0/M)  cost  and   the  variables  affecting  it.    In  the past,
O/M  costs  have  been  given little attention.    This  should  change
as  USEPA  implements   its   new Operations  Check  List  (10)   in all
phases of  the Construction Grants Program.


Finally,  dewatering  device  reliability  is  important  for
successful  plant  operation.   A  reliable  dewatering system is
needed to  maintain relatively uninterrupted removal  of wastewater
solids from  a continuously operated wastewater treatment process.
                                    9-2

-------
Sludges are  generated constantly,  and if  they are  allowed  to
accumulate  for  a  long  time,  the  performance  of  the  entire
wastewater  treatment plant will  be  impaired.
    9.1.2   Methods of Dewatering

While  numerous  techniques  fulfill  the  basic  functional
definition  of dewatering,  they do  so to widely varying  degrees.
It  is  important  to  note  these  circumstances  when
different  devices.   For example, drying beds
only  to  dewater  a  sludge,  but  also  to dry
concentration of greater than  50  to 60 percent.
circumstances and  particular  device  involved,
from  a  mechanical device  may  vary from  a wet,
form, to a  harder  and more friable  form.
                                              comparing
                                       can be used  not
                                       it to  a  solids
                                       Depending on the
                                       dewatered  sludge
                                        almost  flowable
9.2  Natural  Sludge Dewatering  Systems

When  land  is  available,  sludge  dewatering  by  nature  can  be
extremely  attractive  from both a  capital  and an  operating  cost
viewpoint.    Considering escalating  electrical  power  costs,
this method  is even more attractive.  Two types of systems  can  be
categorized  as natural:  drying beds  and  drying lagoons.
    9.2.1
Drying  beds
dewatering
two-thirds
Drying Beds

      the
             are  the  most widely  used method  of  municipal  sludge
             in the  United  States  (11).   At  the present time,
             of all  United  States  wastewater  treatment plants
utilize drying  beds  and  one-half  of  all  the  United  States
municipal  sludge is  dewatered by  this  method.   Although  the  use
of  drying  beds  might  be expected  in smaller plants  and in  the
warmer sunny  regions,  they  are  also  used in  several large
facilities in northern climates (12).  Table 9-2 lists advantages
and disadvantages of the drying bed method.
                              TABLE 9-2

                  ADVANTAGES AND DISADVANTAGES OF
                      USING SLUDGE DRYING BEDS
               Advantages
                                    Disadvantages
  When land is readily available, this is
   normally the lowest capital cost


  Small amount of operator attention and
   skill is required
  Low energy consumption

  Less sensitive to sludge variability

  Low to no chemical consumption

  Higher dry cake solids contents than fully
   mechanical methods
                        Lack of a rational engineering design
                          approach allowing sound engineering
                          economic analysis

                        Requires more land than fully mechanical
                          methods

                        Requires a stabilized sludge
                        Must be designed with careful concern for
                          climatic effects

                        May be more visible to the general public

                        Removal usually labor intensive
                                 9-3

-------
Research into  the  dewatering  of sludge  by  drying beds has been
conducted since the early 1900s, when it was noted that digested
sludge dewatered more rapidly than raw sludge  (13). Design data,
however,  are  still  very empirical, and  only recently  has an
effort  been  made to  develop  a  rational  engineering  design
approach  (14-16).   An  excellent  review of  past  work, detailed
theoretical  analysis,  and  current  understanding  of  the  sludge
drying process  is  given  by  Adrian  (14).   Sludge dewatering on a
drying bed  is  a multi-phase process  and is shown pictorially on
Figure 9-1.
                       RAIN IF BED \$ UNCOVERED

                                               I
               EVAPORATION DUE TO RADIATION AND CONVECTION

  t        t          f         t         t         t         I
                              SLUDGE

                POROUS MEDIUM - SLUDGE SUPPORT STRUCTION
               *         \
                DRAINAGE OF WATER THROUGH POROUS MEDIUM

                            FIGURE 9-1

                SCHEMATIC OF SLUDGE DEWATERING IN A
                        DRYING BED SYSTEM


        9.2.1.1  Basic Components and Operation

Drying beds generally  consist of  a one-  to  three-foot  (0.3-1.Om)
high  retaining wall  enclosing  a  porous drainage  media.  This
drainage  media  may be made up  of  various sandwiched layers
of sand and  gravel,  combinations of  sand and gravel with cement
strips,  slotted  metal  media,  or  a  permanent  porous media.
Appurtenant equipment  includes:   sludge  feed pipelines  and  flow
meters;  possible  chemical  application tanks, pipelines, and
metering  pumps;  filtrate  drainage and  recirculat ion lines;
possible  mechanical  sludge removal equipment; and  a possible
cover or enclosure.

Operational procedures common to all  types of drying beds involve;

     •  Pump 8  to  12  inches (20  to  30  cm)  of  stabilized liquid
        sludge onto the drying  bed surface.


                               9-4

-------
     •   Add chemical conditioners  continuously,  if conditioners
        are used, by injection  into the sludge as  it  is pumped
        onto the  bed.

     •   Permit,  when the bed is filled to the desired level,  the
        sludge to dry to the desired final solids concentration.
        This  concentration  can  vary  from  between  18   to
        60 percent,  depending on  the  type  of sludge,  processing
        rate needed,  degree  of  dryness required for lifting,  etc.

     •   Remove  the   dewatered  sludge  either mechanically  or
        manually.

     •   Repeat the cycle.


        9.2.1.2   Types  of Drying Beds

Drying  beds may be  classified as  either  conventional,  paved,
wedgewire, or vacuum-assisted.

Conventional Sand Drying Beds

Sand drying beds are  the  oldest, most commonly used  type  of
drying  bed.   Many  design  variations  are  possible  including
the  layout  of drainage  piping,  thickness  and type of gravel  and
sand layers, and  construction materials.

Current United  States  practice (17-19)  is to make  drying  beds
rectangular with dimensions  of 15  to  60  feet (4.5 to 18 m)  wide
by  50  to  150 feet  (15  to  47 m)  long with vertical  side walls.
Usually 4  to 9 inches  (10 to  23  cm) of  sand is placed over  8  to
18  inches (20-46 cm)  of graded gravel  or stone.  The sand  is
usually  0.012 to 0.05  inches  (0.3  to  1.2 mm)  in  effective
diameter and has a uniformity coefficient less than 5.0.  Gravel
is  normally  graded  from 1/8 to 1.0 inches (0.3   to  2.5 cm),  in
effective diameter.   Underdrain  piping  has  normally  been  of
vitrified   clay,  but plastic pipe   is also  becoming  acceptable.
The  pipes should be no less than  4  inches (10   cm),  should  be
spaced 8 to 20 feet  (2.4 to 6 m) apart,  and have  a minimum slope
of one percent.

Figure  9-2  shows a  typical sand   drying  bed  construction.  Sand
drying beds can  be built with or without provision for mechanical
sludge removal,  and  with or  without a  roof.

Paved Drying Beds

Paved drying beds have  had limited  use since  1954  (20).  The  beds
are  normally  rectangular in shape  and are  20  to 50  feet (6  to
15  m)  wide by 70 to 150 feet (21  to 46  m)  long with  vertical
side walls.  Current  practice is  to use  either a concrete  or
asphalt lining.    Normally,  the  lining rests  on an 8- to 12-inch
(20- to 30-cm) built-up sand or gravel  base.   The lining should
have a minimum 1.5 percent slope to the drainage area.  A minimum
four-inch   (10-cm) diameter  pipe would convey drainage  away.   An


                               9-5

-------
unpaved  area, 2  to  3 feet  (0.6  to 1  m)  wide  is  placed along
either side or down  the  middle for drainage.  Paved  drying  beds
can be built with or  without a roof.
                                              GATE
                  COLLECTION  ^
                  SYSTEM -^   >J;
                  DRAINAGE
                   —£
>
t *
                                :^£ v.':':/" s A NO ;.. V-'-VyVV.;.;
iUZ,—.^
*_P_ **'.!••*• „*_*_»***_•••* __.«_» Aj
 D'«.,
 ••::
. _:*x
                     >..-
                     "f
                             FIGURE 9-2

             TYPICAL SAND DRYING BED CONSTRUCTION  (18)
For a given amount of sludge, paved drying beds require more area
than sand beds.  Their main advantages are that front-end loaders
can be  used  for  sludge  removal  and reduced bed maintenance (21).
Figure 9-3 shows typical paved drying bed construction.
 MIKJIMIJM 1_K%

 SLOPE
                      - ASPHALT OR
                      CONCRETE LINING
                 f - -, - - ^ >
               SAND
                 ,7^'irV-,*,
                  SAND
                    \  \  \
          \   \   \   \   \
       DRAINAGE
                             FIGURE 9-3

               TYPICAL PAVED DRYING BED CONSTRUCTION

                               9-6

-------
Wedge-Wire Drying  Beds

Wedge-wire drying bed  systems have  been successfully  used in
England for  over 20 years  to dewater both  municipal  (22)  and
industrial (23,24) wastewater sludges.   Used in the United States
since   the  early 1970s,  there  are  presently  18  wedge-wire
installations.   Ten of  these installations  are for municipal
wastewater sludge.

In  a  wedge-wire  drying  bed,  sludge  slurry is  introduced onto a
horizontal, relatively open-drainage  media in a way that yields a
clean  filtrate  and provides a reasonable drainage rate  (25).
Table 9-3 lists  reported  advantages for this type of drying bed.


                             TABLE 9-3

             ADVANTAGES OF A WEDGE-WIRE DRYING BED (26)
 No clogging of the media
 Constant and rapid drainage
 Higher throughput rate than sand beds
             Easy bed maintenance
             Difficult-to-dewater sludges, for example,
              aerobically digested can be dried
             Compared to sand beds dewatered sludge is
              easier to remove
Figure  9-4 shows  a typical  cross section  of  a  wedge-wire bed.
The bed consists  of a  shallow rectangular watertight basin  fitted
with  a false floor of wedgewater  panels.   These panels have
slotted openings  of 0.01  inches  (0.25 mm).   This false
made  watertight with  caulking where  the
An  outlet  valve  to control  the  rate
underneath  the  false floor.
                     panels abut
                    of dra inag e
   floor  is
 the  walls.
is located
   CONTROLLED DIFFERENTIAL HEAD IN VENT
   BY RESTRICTING RATE OF DRAINAGE
        VENT
           t
PARTITION TO FORM VENT

                                                       ±r
     WEDGEWIRE SEPTUM
     OUTLET VALVE TO CONTROL TO CONTROL
     RATE OF DRAINAGE  	—
                              FIGURE 9-U

               CROSS SECTION OF A WEDGE-WIRE DRYING BED
                                9-7

-------
The  procedure  used  for  dewatering  sludge  begins with  the
movement  of  water or plant  effluent into the  wedgewater unit
until  a depth  of approximately one  inch  (2.5  cm) over  the
wedge-wire septum  is  attained.  This water serves  as a  cushion
that permits  the  added  sludge  to  float  without causing  upward
or  downward  pressure  across  the  wedge-wire  surface.   The
water  further prevents  compression  or  other  disturbance of  the
colloidal particles.   After  the bed is filled with  sludge,  the
initially  separate  water  layer and  the drainage water  are
allowed  to percolate  away at  a  controlled  rate,  through  the
outlet valve. After the  free water  has been  drained, the  sludge
further concentrates by  drainage and evaporation until there is a
requirement for  sludge removal.

Vacuum-Assisted  Drying Beds

The  only  operating vacuum-assisted drying  beds  at this time
are  two 20 feet  (6 m)  by 40 feet  (12 m) units built  in 1976
at Sunrise City,  Florida.   They dewater a  two  percent solids
concentration,  aerobically  digested   sludge  from a  contact
stabilization wastewater treatment plant (27).

The principal components of the  Sunrise  facility are:

     •  A bottom ground  slab  consisting of reinforced concrete.

     •  A layer of  stabilized  aggregate  several  inches thick
        which provides   support  for  the rigid multi-media  filter
        top.    This space is also  the vacuum chamber  and  is
        connected  to a vacuum pump.

     •  A rigid  multi-media filter top is placed on  the aggregate
        support.   Sludge  is then  applied  to the  surface  of
        this  media.

The operating sequence is  as follows:

     •  Sludge is  introduced  onto the filter  surface by  gravity
        flow  at  a  rate of  150 gallons per minute (9.4  1/s)  and to
        a depth  of  12 to 18 inches (30 to 46 cm).

     •  Filtrate drains  through  the multi-media filter and into
        the space  containing the  aggregate and  then  to  a sump,
        from which  it  is pumped  back  to  the  plant  by  a self-
        actuated submersible pump.

     •  As soon  as the  entire surface  of  the multi-media  filter
        is covered with sludge,  the vacuum system  is  started
        and  vacuum  is  maintained  at  1   to  10 inches  mercury
        (3 to 34 kN/m2).

Under  favorable weather  conditions,  this system  dewaters  the
dilute  aerobically  digested sludge  to  a  12 percent  solids
concentration in  24  hours  without polymer addition,  and  to
the  same   level  in eight hours  if polymer  is  added.   This


                              9-8

-------
particular sludge  of 12 percent  solids  concentration  is  capable
of being  lifted from the bed  by  a fork or mechanical  equipment.
The  sludge  will  further dewater to  about  20 percent solids
concentration in 48  hours.

        9.2.1.3  Process Design Criteria
Covered Beds

Whenever  there  is  the possibility of long periods of rain,  snow,
or cold weather;  potential  odor or insect problems; or a problem
with esthetics; consideration  should  be given  to employing covers
for  the  drying  beds.  When  properly  ventilated, so that  air  can
flow  over the  surface of  the bed,  covered  sand beds can  be
employed  and require 25  to  33 percent less  area than  open  sand
beds  (17,26).    Although  covers  can  be provided for paved,
wedge-wire,  and vacuum beds,  no  documentation could be  found  on
how covers affect  or improve bed loading  rates.
Sludge Conditioning

Sludge  conditioning  can  dramatically improve  drying bed
throughput (28) and should  be  considered as  part of the design.
See Chapter  8 for  further discussion  on conditioning.

Sludge Removal

The  majority of United States  facilities employ manual  labor  to
remove dried sludge  from drying beds.  With this type of removal,
a  30 to  40  percent solids conoentrat ion  is required.  With
mechanical sludge  removal systems  (21,29,30),  solids  concentra-
tion between 20 and 30  percent can  be  handled (31).    Depending
on the  bed  size,  a tiltable  unit  similar to  the  lift  and  dump
mechanism  of  a  dump truck is available for the  wedge-wire drying
bed.

S ides t reams

The  only  sidestream  from a drying  bed  operation   is under
drainage  liquor.   While little  is  known about  the  characteristics
of this  sidestream, Table  9-4 shows the  results from  one  pilot
study.   This flow  is not normally  treated  separately, but  is
typically  returned to the plant headworks.

                             TABLE 9-H

             CHARACTERIZATION OF SAND BED DRAINAGE (32)

Sludge type - Anaerobically digested mixture of primary and trickling  filter sludge
Bed media   - 6 inches of sand
Color      - clear, dark amber
COD       - 300-400 mg/1
BOD,-       - 6-66  mg/1
          - 1,900-2,360 mg/1 (over 90 percent nitrogenous)

 1 inch = 2.54 cm
                               9-9

-------
                               TABLE 9-5A

       SUMMARY OF RECOGNIZED PUBLISHED SAND BED SIZING CRITERIA
         FOR ANAEROBICALLY DIGESTED, NON- CONDITIONED SLUDGE

                                  Uncovered beds,
                            Area,           Loading,        Covered beds area,
   Initial sludge source    sq ft/capita    Ib solids/sq ft/yr      sq ft/capita3

 Primary
  Reference 33                  1.0             27.5
  Reference 34               1.0 - 1.5                           0.75 - 1.0

  Reference 36
    N45° N latitude            1.25                                0.93
    Between 40-45° N            1.0                                0.75
    S40° N latitude            0.75                                0.56

 Primary plus chemicals
  Reference 33                  2.0               22
  Reference 34               2.0 - 2.25                            1.0 - 1.25
  Reference 36
    N45° N latitude             2.5                                1.87
    Between 40-45° N            2.0                                1.50
    S40° N latitude             1.5                                1.12

 Primary plus low rate
  trickling filter
    Reference 33                1.6               22
    Reference 34            1.25 - 1.75                            1.0 - 1.25
    Reference 36
      N45° N latitude           1.87                                1.56
      Between 40-45°N           1.50                                1.25
      S40° N latitude           1.12                                0.93

 Primary plus waste-
  activated sludge
    Reference 33                3.0               15
    Reference 34            1.75 - 2.5                           1.25 - 1.5
    Reference 36
      N45° N latitude           2.18                                1.68
      Between 40-45°N           1.75                                1.35
      S40° N latitude           1.31                                1.01
aOnly area loading rates available  for covered beds.
 1 Ib/sq ft/yr =4,9 kg/m /yr
 1 sq ft = 0.093 m2
BedSizing  Criteria

Despite  the  number  of  drying beds  in use today,  the  lack  of
published  bed  sizing  criteria  have  limited applicability.   The
majority  of  published  and  professionally  utilized  design  data
(33-36)  are  based  on  operations  during  the  1940s and  1950s.
Tables 9-5A and 9-5B  summarize the  data for sand drying beds.   At
that  time,   sludges   applied  to sand  beds were  anaerobically
digested.  They  originated predominantly in  primary,  primary
plus  low rate trickling  filter,  or  primary plus  conventional


                                   9-10

-------
waste-activated  sludge  wastewater treatment  processes.   Many of

the  sludges presently  generated  do not  readily fall  within these

categories.    (                               -




                                TABLE 9-5B


           SUMMARY OF RECOGNIZED PUBLISHED STATE BED SIZING

     CRITERIA FOR SAND BEDS BY USEPA REGIONS3 SQUARE FEET/CAPITA



 EPA Region      _I	   II     III      IV   	V°    	Yi___ _VII     VIII   	IX  	Xf	

           uacduc   uc    ucuc     uc   u   c   ucuc    uc

Anaerobically
 digested
Primary only     1.5 1.0  1.5 0.75        0.5-1.0         1.0      1.0               1.5   1.0
Prirrary + low rate
 trickling filter  1.75 1.25  1.5 0.75        0.75-1.2         0.5-1.0 0.25 1.5     1.0 1.0  1.0   1.5-2.0 1.0-1.25
Primary + sand
 filter                         1.0           1.0                  0.5
Primary + high rate
 trickling filter                    1.0           1.0             1.251.25 1.0   2.0   1.25
Priirarv + waste
 activated sludge  2.5 1.5  2.01.0         1.5-2.5         1.0-1.51.0         1.351.35 1.0   1.5-2.51.0-1.5
Prijnary +
 chemical            2.0 1.0         1.0-1.33         1.0             1.5 1.3       3.0   2.0
Imhoff              1.5 0.75        0.66-1.0         1.0
Imhoff + low rate
 trickling filter                    1.0-1.2         1.0


 aTaken from individual State design criteria that do not use 10 State Standards.
 The states encompassed in USEPA Regions III and V do not have published reguirements at this time.
 GState of Idaho: Values shown are for rainfall of 30-45 inches (76-114 cm); for rainfall between
 10-30 inches (25-76 cm), reduce these values by 25 percent; for rainfall of less than 10 inches
 (25 cm), reduce these values by 50 percent.
 U = uncovered sand beds
 C = covered sand beds

 1 sq ft/capita = 0.093 m /capita
Also,  most data  are  given  in  square  feet  of  bed  surface
area  required  for  dewatering  on  a  per  capita  basis.    This
criterion  is  only  valid  for  the  characteristics  of  a particular
wastewater and has no rational  design  basis.   A better  criterion
for sizing  sand  drying  beds  is  the  pounds  of  solids  per  square
foot  of  bed  surface  area  per year.   The  best criteria would
take into  consideration  climatic conditions  (such as  temperature,
wind  velocity  and   precipitation),   sludge  characteristics,
(grit,   grease,  fiber,  and   biological  content),   and   solids
concentration.


No  generalized bed   sizing criteria  could  be  found  for paved
beds.  Also  very  little  information  is  available  from full-scale
facilities on  bed  sizing  criteria  for  wedge-wire  units.   In one
United States,  wedge-wire facility,  150  gallons per day  (568  1/d)
of  excess  biological  sludge at  a two  percent solids  concentration
is  conditioned  with a  polyelectrolyte and dewatered  to a  liftable
eight percent  solids  concentration  in  two  to  three  hours  (27).
Table 9-6  contains data on  the  performance  of wedge-wire  systems
with several different sludges.
                                    9-11

-------
                            TABLE 9-6
             WEDGE-WIRE SYSTEM PERFORMANCE DATA (25)
Sludge type
Primary
Trickling filter humus
Digested primary +
waste activated sludge
(WAS)
Fresh WAS
Fresh WAS
Thickened WAS
Feed solids,
percent
8.5
2.9
3.0
0.7
1.1
2.5
Sludge solids
concentration,
percent
25.0
8.8
10.0
6.2
9.9
8.1
Dewatering
time
14 days
20 hours
12 days
12 hours
8 days
41 hours
Solids capture,
percent
99
85
86
94
87
100
 All sludges were chemically conditioned.
        9.2.1.4  Costs

Capital _C_ost_s

Several  recent  publications  have developed  capital  cost curves
for open  sand  beds (37-39).   Probably  the  most accurate is the
reference based  on  actual USEPA bid  documents for  the years
1973-1977 (38).

Although  the  data   were scattered,  a regression  analysis
indicated,  that,  on   the  basis  of a  USEPA  Municipal Wastewater
Treatment Plant Construction Cost Index for the  2nd quarter 1977
(38),  the capital cost could be  approximated  by  Equation  9-1.
    C = 9.89 x 104 Ql-35
(9-1)
where :

    C = capital cost of process  in  dollars.

    Q = plant design flow  in  million  gallons of wastewater flow
        per day.

The  associated   costs  include  excavation,   process  piping,
equipment,  concrete,  and steel.   In  addition, such  costs as
those  for administrating and engineering are  equal  to 0.2264
times Equation 9-1 (38).

Operating and Maintenance
Table  9-7  indicates open  sand  bed  labor  requirements  for both
operation  and  maintenance.    The  labor  indicated  includes:
removal  of dried sludge  from the  beds,  sand maintenance, and
weeding as  necessary.
                               9-12

-------
                                  TABLE 9-7


              SLUDGE DRYING BEDS, LABOR REQUIREMENTS (18)



                                                Labor,  hours per year
>tal bed area,
sq fta
1,000
5,000
10,000
50,000
100,000
Operation
300
400
500
1,500
2,900
Maintenance
100
180
220
710
1,500
Total
400
580
720
2,210
4,400
  Assumes dry solids loading rate of 20 Ib/sq ft/yr of bed area.


  1 sq ft = 0.093 m2      0

  1 Ib/sq ft/yr = 4.9 kg/mVyr
       3


       2
    10,000
       B
       e
       7
       6
in
O
u
z
z
       4  —


       3  "^
       2  —
1,000
   9
   8
   7


   S

   4
       3  -
       2  —
                234 6678910,000   2    34  56789100,000 2


                           DRYING BED AREA, sq ft (1 sq ft = 0,093 m2)


                                 FIGURE 9-5


             ESTIMATED JUNE 1975 MAINTENANCE  MATERIAL COST

                      FOR OPEN SAND DRYING BEDS (39)
                                                           3  4 i 6 789
                                     9-13

-------
Figure 9-5 shows  a  curve  developed  for  estimating  open  sand
bed  maintenance  material  cost  as  a function of  sand  bed  surface
area.   As an  example, for a  sand bed surface  area  of 10,000
square  feet   (930  m2) ,  a  designer  would estimate  a  yearly
materials cost of  $400.    Since this number is  based  on a  June
1975 cost, it must  be adjusted  to  the current design period.
    9.2.2  Drying  Lagoons


Sludge  drying  lagoons are  another  method  (12)  of  sludge
dewatering   when  sufficient,   economical  land  is  available.
Sludge  drying  lagoons  are similar to drying beds.   However,  the
sludge  is placed  at depths  three  to four  times greater  than  it
would be  in  a  drying bed.     Generally,  sludge  is  allowed  to
dewater  and  dry  to some predetermined solids concentration before
removal  and   this might require  one to  three years.   The  cycle  is
then  repeated.  Sludge  should be  stabilized prior  to  addition  to
the lagoon to minimize odor problems.  Large areas  of  lagoons  can
produce  nuisance odors as they  go through a  series  of  wet and  dry
conditions.    See  Chapter 15  for  further discussion.   Table  9-8
lists present  advantages and  disadvantages  for  sludge drying
lagoons.


                               TABLE 9-8

    ADVANTAGES AND DISADVANTAGES OF USING SLUDGE DRYING LAGOONS
              Advantages
           Disadvantages
 Lagoons are low energy consumers
 Lagoons consume no chemicals
 Lagoons are not sensitive to sludge
  variability

 The lagoons can serve as a buffer in the
  sludge handling flow stream.  Shock
  loadings due to treatment plant upsets
  can be discharged to the lagoons with
  minimal impact

 Organic matter is further stabilized
 Of all the dewatering systems available,
  lagoons require the least amount of
  operation attention and skill

 If land is available, lagoons have a very
  low capital cost
Lagoons may be a source of periodic odor
  problems, and these odors may be difficult
  to control

There is a potential for pollution of
  groundwater or nearby surface water

Lagoons can create vector problems (for
  example, flies and mosquitos)
Lagoons are more visible to the general
  public

Lagoons are more land-intensive than fully
  mechanical methods

Rational engineering design data are
  lacking to allow sound engineering
  economic analysis
Very  little  research  has  been  conducted  concerning  sludge
drying  lagoons.   Dewatering  occurs in  three  ways:   drainage,
evaporation, and transpiration.    Research  seems to  indicate
that dewatering  by  drainage  is independent  of  lagoon depth.
Dewatering by drainage alone cannot  produce  a  sludge sufficiently
dry  for easy removal  (40 , 41). These  studies  further indicate  that
evaporation is the most important dewatering  factor.
                                  9-14

-------
        •9.2.2.1  Basic Concept

Sludge  drying  lagoons  consist  of  retaining  walls  which are
normally  earthen dikes  2 to 4 feet  (0.7 to 1.4  m)  high.  The
earthen  dikes  normally  enclose  a  rectangular  space  with  a
permeable surface.  Appurtenant equipment  includes:   sludge feed
lines and metering pumps,  supernatant  decant  lines,  and some type
of mechanical  sludge  removal  equipment.    The  removal equipment
can include a bulldozer,  drag  line or  front-end  loader.   In areas
where permeable soils  are unavailable,  underdrains  and associated
piping may be required.
Operating procedures
involve:
common  to all  types of  drying  lagoons
        Pumping liquid sludge,  over a period  of  several months or
        more,  into  the lagoon.   The pumped  sludge  is  normally
        stabilized prior to  application.   The sludge is usually
        applied until  a lagoon  depth of  24 to 48 inches (0.7 to
        1.4m) is achieved.

        Decanting supernatant,  either continuously  or intermit-
        tently, from  the  lagoon surface  and  returning it to the
        wastewater treatment plant.

        Filling the  lagoon  to  a desired  sludge  depth  and then
        permitting  it  to  dewater.  Depending  on  the  climate
        and  the  depth of  applied  sludge,  the  time  involved for
        dewatering to a final  solids content of between  20 to
        40 percent solids  may be 3  to 12  months.

        Removing  the  dewatered   sludge  with  some  type  of
        mechanical removal equipment.

        Resting (adding no new  sludge) to the lagoon for  three to
        six months.

        Repeating the  cycle.
        9.2.2.2  Design Criteria

Proper design  of  sludge  drying lagoons requires a  consideration
of the  following  factors:  climate, subsoil permeability,  sludge
characteristics, lagoon depth,  and area management  practices.  A
detailed discussion of these factors  follows.

Climate
After dewatering by drainage and supernating, drying in a  sludge
lagoon  depends  primarily  on  evaporation.   Proper size of a
lagoon,  therefore,  requires climatic  information  concerning:

     •  Precipitation  rate (annual  and  seasonal distribution).
                               9-15

-------
     •  Evaporation  rate  (annual  average,  range,  and  seasonal
        fluctuations).

     •  Temperature extremes.

Subsoil Permeability

The subsoil should have a moderate permeability of 1.6 x 10~4 to
5.5 x  10~4  inches per second  (4.2 x 10~4  to  1.4 x 10~3 cm/s),
and the  bottom  of the lagoon  should be a  minimum  of  18 inches
(46 cm)  above  the  maximum  groundwater  table,  unless  otherwise
directed by local authorities.

SJLudge Characteristics

The type  of  sludge  to  be  placed in  the  lagoon can significantly
affect the amount  and  type  of  odor and vector problems that can
be produced.    It  is  recommended  that  only those sludges which
have  been anaerobically  digested be  used in drying  lagoons.

Lagoon.Depth  arid Area

The actual depth  and area requirements  for  sludge drying lagoons
depend  on several  factors  such  as  precipitation,  evaporation,
type of sludge,  volume and  solids  concentration.  Solids loading
criteria have been given as 2.2  to 2.4 pounds of  solids per year
per cubic foot  (36  to 39 kg/m3)  of capacity (46).   A  minimum
of two separate  lagoons are  provided  to ensure  availability
of storage space  during cleaning,  maintenance, or  emergency
conditions.

General Guidance

Lagoons may be  of  any  shape, but a rectangular shape facilitates
rapid  sludge  removal.   Lagoon dikes should have  a  slope of 1:3,
vertical  to  horizontal,  and should be of a shape and  size to
facilitate maintenance, mowing,  passage of maintenance vehicles
atop the  dike,  and  access for  the entry of trucks and front-end
loaders into  the  lagoon.   Surrounding areas should be graded to
prevent  surface water from entering  the  lagoon.  Return must
exist for removing the  surface  liquid and piping  to  the treatment
plant.  Provisions  must  also be made for  limiting public access
to the  sludge lagoons.  Chapter  15  provides  a description of a
successful sludge  drying  lagoon  operation  for the Metropolitan
Sanitary District of Greater Chicago.


        9.2.2.3  Costs

Current published  information  on  capital  cost  of constructing
sludge  lagoons  is  almost  nonexistent.   Some   information is
available from a  recent  USEPA  publication  (38),  and  from
Chapter 15.   Table 9-9 indicates  labor requirements  for sludge
                               9-16

-------
drying lagoons.   The requirements  include: i application of sludge
to the lagoon; periodic removal of supernatant; periodic removal
of solids;  and  minor maintenance requirements,  such  as  dike
repair  and  weed control.   No information  could be found  on
maintenance  material costs.
                           TABLE 9-9

         SLUDGE DRYING LAGOONS, LABOR REQUIREMENTS (18)

                                        Labor, hours per year
 Dry solids applied,                	
    tons/year                    Operation      Maintenance      Total

         100                        30             55          85
        1,000                        55             90          145
       10,000                       120 .  ,         300          420
       50,000                       450           1,500        1,950


 1 ton = 0.9 t
9.3  Centrifugal Dewatering Systems


    9.3.1  Introduction

Centrifuges  were  first  employed  in  the  United  States  for
dewatering  municipal  wastewater treatment plant  sludges during
the year 1920,  in Milwaukee, Wisconsin, and  during  1921 in
Baltimore, Maryland (42).  Early centrifuges were  not designed to
process  extremely  variable slurries such  as  those of municipal
wastewater  treatment plants.    In addition,  most  wastewater
treatment  facilities  provided  little,   if  any,   preventive
maintenance. Consequently,  early installations developed  numerous
operational  and   maintenance  problems,  and  this  led to an
anti-centrifuge reaction among environmental engineers.

By  the  late   1960s,   equipment  manufacturers  were  designing
and building  new  machines specifically for wastewater sludge
applications,   and  the use  of  centrifuges for  municipal sludge
dewatering  increased.    In  the  past  ten  years,   continuous
improvements in design and  materials have led  to better machines.
The  machines  now available  (1979)  require  less  power  and
attention and  produce less  noise.

Two categories of  centrifuges are used  for municipal wastewater
sludge dewatering:  imperforate basket and scroll-type decanter.
A  detailed  discussion of  each follows.   The  basic  theory of
thickening and process costs are presented  in  Chapter  5.
                               9-17

-------
    9.3.2   Imperforate  Basket

Basket  centrifuges  for dewatering municipal wastewater treatment
plant  sludges were  first used  in the United States  in 1920  (42).
Since  the  mid  1960s approximately 300  machines were installed  in
100 municipal  treatment plant  applications (43).   About one-half
of the  installed machines are used  for  dewatering;  the other  half
and  used  for thickening.   The largest centrifuge facility  in
the  world is  located  at  the  County  Sanitation  Districts  of
Los Angeles County  Carson Plant  in  California,  and  uses 48 basket
centrifuges (44).   Table 9-10 lists  the advantages  and disadvant-
ages  of  a  basket  centrifuge  compared  with  other dewatering
systems.
                              TABLE 9-10

        ADVANTAGES AND DISADVANTAGES OF BASKET CENTRIFUGES

              Advantages                          Disadvantages
Same machine can be used for both dewatering  Requires special structural support
  and thickening                             ,.        .. . , ,
                                     Except for vacuum filter, consumes more
It may not require chemical conditioning      direct horsepower  per unit of product
Centrifuges have clean appearance, little-     processe
  to-no odor problems, and fast start-up     Skimming stream could produce significant
  and shut-down capabilities                recycle load

Basket centrifuge is very flexible in        Limited size capacity
  meeting process requirements                  .   ,      ,  n ,    ,    ,
                                     For easily dewatered sludges, has the
It is not affected by grit                 highest capital cost versus capacity

It is an excellent dewatering machine for
  hard-to-handle sludge                   For most sludges, gives the lowest cake
,.  ,   ,   .  . i     . .     ,   ..          solids concentration
It has low total operation and maintenance
  costs.

Does not require continuous operator
  attention
         9.3.2.1  Principles of Operation

The  operation  of  an  imperforate  basket  centrifuge  is described
in Chapter 5.   There  is,  however, one  additional  operation  to be
added  to that discussion.

After  the centrifuge  bowl  is  filled with solids,  the unit  starts
to decelerate.    In the  thickening  mode,  deceleration  was  to  a
speed  of 70 rpm  or lower  before commencement of plowing.   In the
dewatering  mode,  another  step  called  "skimming"  takes place
before  the  initiation of  plowing.   Skimming  is  the  removal  of
soft sludge from  the inner wall of  sludge within the basket
centrifuge.  The  skimmer moves  from its  position in  the center of
the  basket towards  the  bowl wall.   The  amount  of horizontal
travel  is set  at  the  time  of  installation, and start-up  depends
                                 9-18

-------
on sludge type  and downstream processing  requirements.   The
skimming  volume  is  normally 5  to  15  percent  of the bowl volume
per cycle.   After  the  skimmer  retracts,  the  centrifuge further
decelerates to the 70 rpm level  for plowing. Skimming streams are
typically 6  to  18 gallons  (22  to  66  1)  per  cycle with a solids
content of almost zero to eight  percent.  Treatment  of this stream
is typically  by ,returning it either to the primary or  secondary
wastewater treatment system, or  to  some  other  pre-sludge handling
step such as a thickener.


        9.3.2.2  Application

A basket  centrifuge is  well suited for small  plants that do not
provide   either   primary clarification  or   grit  removal  (for
example,  wastewater plants  that use  extended aeration, aerated
lagoons,  and  contact stabilization).  These small  plants require
a piece  of  equipment  that  can, at  different times,  dewater or
thicken  conventional as well as biological sludges  with  a long
sludge  age.  Also  low  overall operation and maintenance, and
low operating costs, are associated with basket  centrifuges.


        9.3.2.3  Performance

Table 9-11 lists typical performance data for  a  basket  centrifuge
in a  number of different applications.   These data are expected
values  and are based  on the performance of several  different
installations.   Table  9-12  lists  the  average  results from two
specific  operating  facilities.


        9.3.2.4  Case History

In 1973,  a dewatering study  was  made in Burlington, Wisconsin, on
the wastewater treatment  facility  located there  (46).   The  plant
treats a  combination  of domestic-industrial  wastewater flow  of
1.5 MGD  (66  1/s)  during dry weather and 2.0  MGD (88 1/s) during
wet weather. The treatment plant has no primary  clarification and
uses  the contact stabilization process  with  aerobic  digestion.
Approximately  150,000  gallons  (568 m^)  per  week  of aerobically
digested  sludge with a  1.4  percent solids  concentration requires
disposal.

As the plant is  located on  a low  flood plain,  it  was  originally
necessary  to truck  the  dilute  sludge to the  lagoon.   In  1972,
the Wisconsin Department  of  Natural  Resource's ordered  Burlington
to  discontinue  use of  the  lagoons.   Since the only options
available  were  landfilling   or  cropland application,  dewatering
was required.   In  1973,  an  engineering  evaluation was  performed
to  select the optimum  dewatering  unit.  The  equipment  evaluated
included:   an  imperforate  basket   centrifuge,  a  recessed  plate
filter,  a horizontal belt filter press,  and a rotary drum vacuum
                               9-19

-------
                                   TABLE 9-11
   TYPICAL PERFORMANCE DATA FOR AN IMPERFORATE BASKET CENTRIFUGE
Sludge type
Raw primary
Raw trickling filter
(rock or plastic media)
Raw waste activated

Raw primary plus rock
trickling filter (70-30)
Raw primary plus waste
activated (50-50)
Raw primary plus rotating
biological contactor (60-40) „
Anaerobically digested
primary plus waste
activated (50-50)
Aerobically digested

Combined sewer overflow
treatment sludge
Centrate from decanter
dewatering lime sludge
Polymer
Average required. Recovery
Feed solids cake solids pounds dry based on
concentration, concentration, per ton dry centrate,'
percent solids percent solids feed solids percent
4-5
2-3

0.5-1.5

2-3

2-3

2-3

1-2


1-3

Extremely

1-2

25-30
9-10
10-12
3-10
12-14
9-11
7-9
12-14

20-24
17-20
12-14
10-12
8-10
8-11
12-14
variable -

10-13

2-3
0
1.5-3.0
0
1.0-3.0
0
1.5-3.0
1-3

0
4-6
0
1.5-3.0
4-6
0
1-3
see study by

0

95-97
90-95
95-97
85-90
90-95
95-97
94-97
93-95

85-90
98 +
75-80
85-90
93-95
80-95
90-95
EPA (4'5)

95-98

 Skimming losses, if any,  have not been used in calculating recovery.
 1 Ib/ton =0.50 kg/t
                                   TABLE 9-12
           SPECIFIC OPERATING RESULTS FOR IMPERFORATE BASKET
                             County sanitation district
                              of Los Angeles,  CA  (44)
Burlington, WI (4j5J
Type of sludge
Instantaneous flow rate, gpm
Feed solids concentration,
  mg/1
Polymer requirement
Cake solids content, percent
Centrate, mg/1
Skimmed volume,  percent of
  total basket volume
Centrate from solid bowl
decanter dewatering
anaerobically digested
primary sludge
e, gpm 50
ion,
29,000
4a
ercent 20
1,500
t of
Not given
Aerobically digested,
activated sludge from a
plant without primary
clarification
23

14,000
0
6-8
100

50
88

14,000
30b
13-15
100

14
 Dry polymer at 4 Ib/ton  (2.0 kg/t) of dry solids.
 Combination anionic-cationic system.  Thirty dollars/ton
 ($33/t) of dry solids.
 1 gpm  = 0.063 1/s
                                       9-20

-------
filter.  The recessed plate pressure  filter option was  ruled  out
as too  expensive  for Burlington's  small  plant.   The horizontal
belt filter  press  produced a  low  cake solids concentration  and
required high levels of  polymer addition at a  cost of  $40  per  ton
($44.44/t).  The  vacuum filter was not selected  because  of high
capital cost.   The imperforate  basket  was selected as the most
cost-effective unit.   Figure 9-6 shows a flow  scheme  of  the
Burlington wastewater  treatment  plant as it was  operating  in
1977.

The original design,  as a result  of  the  engineering  evaluation,
called  for  one basket  centrifuge  to  operate  40 hours  per week.
This centrifuge was  to  dewater 96,000 gallons (370 up) per week
of sludge at a 1.8 percent solids  concentration to a  nine to  ten
percent solids concentration  without  the use  of  polymers.  This
was  all based on several  days of pilot plant  work conducted
several months before  equipment selection  was made.  At  the
time  of centrifuge  start-up,  the actual sludge  volume to  be
dewatered was  150,000  gallons  (568 m^)  per week at  1.4  percent
solids  concentration.   The column labeled "Without Polymer"  in
Table  9-13  shows performance  results  under this  condition.
Because of  the  50 percent  greater  sludge  volume  and  poorer
operating results  than  had been indicated by pilot testing,  the
basket centrifuge  had to operate 24 hours per  day, seven days  per
week.   This type  of  operation was prohibitive  for  a  plant  the
size of the Burlington facility.
                                         wFLUENT
                                                    SKlMMIhGS HAULED
                                                    AS LIQUID TO LAND
                                                    APPLICATION
                             CENTHATE TO HJAp QF PIAMT
                            FIGURE 9-6

            1977 FLOW DIAGRAM OF BURLINGTON, WISCONSIN
                  WASTEWATER TREATMENT PLANT

                               9-21

-------
                            TABLE 9-13

              OPERATING RESULTS FOR BASKET CENTRIFUGE
                DEWATERING OF AEROBICALLY DIGESTED
                  SLUDGE AT BURLINGTON, WISCONSIN

                                      Without   With
                                      polymer  polymer

           Gal/week of sludge to
             dewater                  150,000  150,000

           Lb/week of sludge to
             dewater                   17,500   17,500

           Instantaneous feed rate,
             gpm                           23       88

           Feed solids concentration,
             mg/1                      14,000   14,000

           Hr/week operation required     168       44

           Labor and trucking cost
              (dollars)/week at 45
             percent of the time          378       99
           Electricity utilized/week,
             kWhr                       4,888    1,584

           Electricity cost at
             $0.03/kWhr                146.63    47.52

           Chemical cost, dollars/ton       0       30
           Cake solids, percent           6-8    13-15
           Skimming volume of basket,
             percent of total              50       14

           Cost/ton, dollars            59.96    46.74
            Material was untruckable.

            Material was truckable.

            1 gpm = 3.78 1/min
            1 gal = 3.78 1
            1 Ib  = 0.454 kg
            1 ton = 0.907 t
            1 kWhr= 3.6 MJ

The  plant  superintendent  instituted a  polymer testing  program
and  evaluated  several  hundred  polyelectrolytes.    The  final
selection resulted in  the  addition of an anionic  polymer  to the
sludge feed line  at  a  point several feet upstream of  the  sludge
entry to the  basket  and then the addition of  a cationic  polymer
at  the  bowl.    The  results of  using polyelectrolytes  are  given


                               9-22

-------
in the  column labeled "With  Polymer"  in Table 9-13.   The results
show that  operating costs  were  $13.22 per ton  ($14.69/t) cheaper
with  polymer addition  than without.  :The  savings  occurred in
reduced labor and  power  requirements.


    9.3.3  Solid Bowl  Decanters

Decanter  centrifuges   for  dewatering  municipal  wastewater
treatment  plant sludges  were first used  in  the United  States in
the  mid 1930s.   Since  then, approximately  500  machines  have
been placed  in 175  municipal installations  (43) .   Most of these
installations  were for dewatering applications.  Table 9-14 lists
the  advantages and  disadvantages  of  a  solid   bowl   decanter
centrifuge compared  with other dewatering processes.
                             TABLE 9-14


  ADVANTAGES AND DISADVANTAGES OF SOLID BOWL DECANTER CENTRIFUGES


             Advantages                          Disadvantages

 Centrifuges have clean appearance, little-   Scroll wear potentially a high maintenance
  to-no-odor problems, and fast start-up   •  item
  and shut-down capabilities             Requires  grit removal or possibly a grinder
 It is easy to install                    in the  feed stream
 Provides high throughput in a small surface  Requires  skilled maintenance personnel
  area
 Gives for many sludges a cake as dry as any
  other mechanical dewatering process
  except for pressure filtration systems
 Has one of the lowest total capital cost
  versus capacity ratios
 Does not require continuous operator
  attention
        9.3.3.1   Application

Early applications of solid  bowl centrifuges were  for dewatering
coarse  easily dewaterable municipal wastewater  treatment  plant
sludges.   These included raw  primary,  anaerobically digested
primary,  and  lime  sludges,  to name  a  few.   The   application
of  centrifuges   to  dewatering  mixtures of  sludges containing
greater than 50  percent by weight  of waste-activated sludges was
limited because  of  very  poor centrate  quality.  Advancements in
design,  especially  in the  entrance configuration,  had  reduced
floe  shear. The development of new polyelectrolytes has  also
contributed  to  greatly  improving  centrate  quality.   These
developments  have  made  the   solid bowl  decanter centrifuge
applicable  to a  much wider range of  sludge types.   Further
available capacities  range  from  6  gallons  per minute  (22 to
38  1/min) to  over  400 gallons per minute (1,514  1/min).   The
decanter  can successfully  operate with a  highly variable feed.
                                9-23

-------
         9.3.3.2   Performance

Table 9-15 lists  operating  results that can  be expected when
dewatering the  sludges  indicated  with a solid bowl decanter.  The
data  in  this  table  can  be  used  for  conducting  engineering
evaluations when  actual  test results are not  available.


                               TABLE 9-15

  TYPICAL PERFORMANCE DATA FOR A SOLID BOWL DECANTER CENTRIFUGE
    Sludge type
 Feed solids
concentration,
percent solids

     5-8

     2-5
     9-12
                                 2-5
                               0.5-3

                                 1-3

                                 9-14
                                13-15
                                 7-10

                                10-12
                                 4-5
                                 2-4
                                 4-7
                               1.5-2.5
  Average
 cake solids
concentration,
percent solids

    25-36
    28-36
    28-35
    30-35
    25-30
                 28-35
                  8-12

                  8-10

                 35-40
                 29-35
                 35-40
                 30-35
                 30-50
                 18-25
                 15-18
                 17-21
                 18-23
                 14-16
  Polymer
 required,
pounds dry
per ton dry
feed solids

   1-5
    0
   6-10
    0
   1-3
                6-10
               10-15

                3-6

                 0
                1-4
                 0
                2-4
                 0
                3-7
                7-10
                4-8
                2-5
               12-15
Recovery
based on
centrate,
 percent

  90-95 .
  70-90
  98 +
  65-80
  82-92
            95 +
            85-90

            90-95

            75-85
            90-95
            60-70
            98 +
            90-95
            90-95
            90-95
            90-95
            85-90
            85-90
                                 Extremely variable -  see study by USEPA 45
Raw primary

Anaerobically digested
  primary

Anaerobically diaested
  primary irradiated at
  400 kilorads
Waste-activated
Aerobically digested
  waste-activated
Thermally conditioned
  primary + waste-activated

  primary + trickling filter

High lime
Raw primary + waste-activated
Anaerobically digested
  (primary + waste-activated)
Anaerobically digested
  (primary + waste-activated)
  + trickling filter)
Combined sewer overflow
  treatment sludge
 1 Ib/ton =0.50 kg/t
         9.3.3.3  Other  Considerations

Solid  bowl  decanter  centrifuges  are  available  in  either
countercurrent or  concurrent  flow  design and  either  "high speed"
or  "low  speed"  design.  In the  countercurrent  design,  the sludge
feed enters  through  the  small  diameter  end  of  the bowl,  and
solids  are  conveyed towards the same end.   In  the  concurrent  flow
design, the sludge  feed enters  through  the  large  diameter end of
the  bowl  and  solids  are conveyed towards  the  opposite  end.
Concurrent  flow units  have only been  in use for about ten years.
The  reasons for  conveying solids  away  from the sludge  inlet  are
to  reduce  inlet  turbulence conditions  and therefore  reduce  floe
shear  and  to provide  a longer residence time for  the  solids.
Though  there  are  reports  from  Europe  (47)  indicating advantages
of  concurrent designs  over countercurrent  designs, United States
experience is limited.   One  extensive  comparative study  (48)
                                   9-24

-------
 showed  the  countercurrent design to perform  best  on aerobically
 digested waste-activated  sludge and the concurrent one to perform
 best  on  raw waste-activated sludge.

 There is considerable  controversy  over  the  benefits associated
 with  "high speed"  or  "low speed" solid bowl decanter centrifuges.
 One  aspect  of this controversy is  the definition of "high speed"
 and  "low speed."   In a publication  by one of the major suppliers
 of  "low speed" machines  (49),  "low  speed"  was generally defined
 as a  bowl speed of 1,400  rpm or less.

 Manufacturers  indicate  that  "low speed"  decanter centrifuges
 consume  less  energy;  require less polymer addition to the sludge;
 have  a  lower  noise  level;  and  require less  maintenance  than a
 comparable  "high speed" machine to satisfy the same requirements.
 This  combination  should  therefore  give  "low speed"  machines a
 significant  economical advantage  on  a total cost per unit weight
 of  solids dewatered.   European work seems  to substantiate  this
 (29), but this has  not  been  the case  in the United States.   In
 very  extensive  side-by-side  studies  conducted at the Dallas-Fort
 Worth,  Texas  (50), Chicago-Calumet,  Illinois, (9), Chicago-West-
^Southwest,  Illinois  (50),   Milwaukee,  Wisconsin  (48),   and
 'Columbus,  Ohio-Southerly wastewater treatment plants  (50),  "low
 speed"  machines were  not  overall clearly advantageous compared to
 the  high speed  ones.  In fact,  in most cases, they were  more
 expensive on  a total  cost basis than the  "high speed" machines.

 Additional  information on solid bowl decanter centrifuges can be
 found in Chapter 5.

 9.4   Filtration Dewatering Systems

     9.4.1   Introduction

 Filtration  can be  defined as  the removal of solids from a liquid
 stream  by  passing  the  stream through  a  porous medium  which
 retains  the  solids.   Figure 9-7  shows  a  flow  diagram  of a
 filtration  system.


                                SUSPENSION
REMOVED SOLIDS L
FILRATION J^-^*^*'
HARDWARE
POROUS
MiOIA
\

t
PRIVING FORCE
(PRESSURE DROP I
I

                                FILTRATE


                             FIGURE 9-7

               FLOW DIAGRAM OF A FILTRATION SYSTEM (51)



                                9-25

-------
As indicated on Figure 9-7, a pressure drop is required in order
for liquid to flow through  the porous medium.  This pressure drop
can be achieved in four ways:   by  creating a  vacuum on one side
of the porous  medium,  by  raising the pressure above atmoshperic
pressure  on  one  side  of  the  medium, by  creating  a centrifugal
force on  an  area  of  the porous  medium,  and by designing  to make
use of gravitational  force  on the medium.

Sludge filtration-dewatering processes use one or  more  of these
driving forces and fall under the general filtration category  of
surface  filters.   "Surface  filters are the general type  of
filtration in which solids  are deposited  in the form of a cake  on
the upstream side  of  a relatively thin filter medium" (54).

    9.4.2  Basic Theory

All  filtration  theory stems from  Darcy's original work  in  the
mid-1850s  (52).  Darcy found that the flow rate  Q  of  a filtrate
of viscosity p. through a  bed of thickness L and  face  area A was
related to  the driving pressure  AP.   This relationship is shown
in Equation 9-2.

    Q = KAAp                                               (9-2


where K is a constant referred to as  the permeability of the bed.
Many times, Equation  9-2 is written:

    O - AAp
    Q " MR

where R is called the medium resistance  and is equal to L/K,  the
medium thickness divided by the  bed permeability.

Extensive research  has  been,   and  continues  to be,  conducted
in defining the  factors  involved  and  level of  influence  in
dewatering both  compressible  or  incompressible  sludges.  A
comprehensive discussion  on  filtration has  recently  been
published  (51).   This  discussion, through examples,   shows
the  effects of  constant  pressure  filtration;  constant  rate
filtration;  constant rate-constant  pressure   filtration;
and  variable  pressure and variable rate filtration on  both
compressible and non-compressible sludges.

    9.4.3   Filter  Aids

Filter aid is material such as  diatomite, perlite,  cellulose,  or
carbon (50)  that  serves  to improve,  or  increase  the  filtration
rate  by physical  means only.  Filter  aids are  not added directly
to the sludge body,  as a  conditioning  agent is,  but they are
added in  fixed amounts to the  porous  medium of  the  particular
dewatering equipment.   The amount of   filter  aid  added  is
independent of sludge solids   concentration.   The filter aid
literally becomes the "filtering surface"  that  achieves the
                              9-26

-------
liquid/solids  separation, and the  equipment functions  as  a filter
holder.    In  order  to perform  its  function  satisfactorily,  the
filter  aid's  particles  should  be  inert,  insoluble,  incompress-
ible, and  irregularly shaped, porous,  and small  (53).

Filter  aids  normally  assist  in  dewatering  difficult-to-handle
industrial  sludges by  either  vacuum  filtration or  pressure
filtration  (54).   In  the  past  ten years,  research  has  been
performed  on  the  use of  filter aids  for  improved dewatering of
municipal  wastewater  treatment  plant sludges  (55).   Table  9-16
lists results  obtained  from several test studies  in which either
a  rotary  drum  vacuum filter  or  a  recessed  plate pressure filter
were used.
                             TABLE 9-16

                  PRECOAT3 PROCESS PERFORMANCE ON
                      FINE PARTICULATE SLUDGES

                        Sludge properties                 Performance
  Mixture alum and
   KASb - RVPFC

  WAS - RVPF      ,
   conditioned HAS-FP

  WAS - RVPF
   conditioned WAS-FP

  WAS - RVPF
   conditioned WAS-RVPF

  Alum RVPF

  Alum RVPF
 Diatomite.
 Waste-activated sludge.
cRotary vacuum precoat filter.
 Filter press.
 Fly ash conditioning and precoat.

 1 Ib/sq ft/hr =4.9 kg/m2/hr
 1 ton = 0.907 t
 1 Ib = 0.454 g
 1 Ib/ton =0.5 kg/t
Feed solids Particle
concentration , size,
percent micron
0.5 4
5.0 2
2.2 10
11. 4e
1.0 - 2.0
1.0 - 2.0
1.5
1.5
0.4 - 0.8
8.0 15
Diatomite
Specific 7 Solids Cake used,
resistance x 10 , loading, solids, Ib/ton
sec^/gm Ib/sq ft/hr percent dry solids
354 0.28 26
1.00 23
3.2 2.20 25 - 30
D.30 40 - 45
40 - 790 0.55-2.09 26-33
2 - 317 0.23 - 1.44 26 - 40
53 0.88 29
16.8 2.51 25
0.3 25 - 30
118 1.37 25
820
280
160
140
200
280
120
800
120
Solids
capture ,
percent
99
99.
99.
98.
99,
98,
99.
99.
99.
99
.9 +
.9 +
.9 +
.5
.9 +
.0
.9 +
.9 +
.9 +
.9 +
    9.4.4   Vacuum Filters

In  vacuum  filtration,  atmospheric pressure,  due  to a  vacuum
applied downstream of  the  media,  is  the  driving  force  on the
liquid  phase that moves  it through the  porous media.

Vacuum  filters  were patented  in  England in  1872 by William and
James Hart.   The  first United States  application  of a  vacuum
filter  in  dewatering  municipal  wastewater  treatment  plant
sludge  was in  the  mid-1920s (56).   Until the 1960s, the  drum or


                                 9-27

-------
scraper-type  rotary vacuum  filter  was predominant.   Since then,
the belt-type filter with  natural or synthetic fiber cloth, woven
stainless  steel  mesh,  or coil springs  media has become dominant.
Recently,  dewatering of municipal sludges  by a top  feed vacuum
filter has been  studied  on  a pilot scale (57).  Results indicated
that  yiel.ds  could be improved  by  15 to 20  percent.  The  full
scale  operation  is expected  to begin  in the  summer of 1979.
Table  9-17  lists  the  advantages and  disadvantages of vacuum
filtration when  it is compared  to other dewatering processes.
                             TABLE 9-17

               ADVANTAGES AND DISADVANTAGES OF USING
                     ROTARY DRUM VACUUM FILTERS

             Advantages                         Disadvantages
Does not require skilled personnel          Consumes the largest amount of energy per
Has low maintenance requirements for         unit of sludge dewatered in most
              . .      .   .             applications
  continuous operating equipment
Provides a filtrate with a low suspended     Requires continuous operator attention
  solids concentration                   Auxiliary equipment (vacuum pumps) are very
                                    loud
        9.4.4.1   Principles of Operation

Figure 9-8  shows the  cutaway view  of a drum or scraper-type,
rotary  vacuum filter.   The unit consists mainly  of  a horizontal
cylindrical  drum that  rotates,  partially submerged,  in  a vat of
conditioned  sludge.    The drum surface is divided  into sections
around its  circumference.   Each  section  is sealed  from its
adjacent  section and  the ends  of  the drum.  A separate drain  line
connects  each section to a rotary valve  at  the axis  of  the drum.
The valve has "blocks"  that divide it into zones corresponding to
the  parts  of the filtering  cycle.   These zones are  for cake
forming,  cake drying, and cake discharging.   A vacuum is applied
to  certain   zones  of  the  valve and  subsequently  to  each  of the
drum  sections  through   the  drainlines as  they pass  through the
different zones  in the  valve.

Figure  9-9   illustrates the various  operating  zones encountered
during  a  complete  revolution of the drum.

About  10  to 40 percent  of the  drum  surface  is  submerged in a vat
containing  the sludge slurry.    This  portion  of  the drum is
referred to as  the  cake  forming  zone.   Vacuum applied  to  a
submerged drum section  causes  filtrate to pass through  the media
and  cake  to  be  formed  on the  media.   As  the  drum rotates,  each
section  is  successively carried through  the  cake  forming zone to
the  cake drying or  dewatering zone.  This  zone is  also under
vacuum  and   begins where and  when  a drum  section carries formed


                                9-28

-------
            CLOTH CAULKJNG
                    STRIPS
   AUTOMATIC VALVE
AIR AND
FILTRATE
LINE
DRUM



  FILTRATE PIPING


   CAKE SCRAPER
                                            SLURRY AGITATOR
                                           VAT
        AIR BLOW-BACK LINE
                               SLURRY FEED
                         FIGURE 9-8

          CUTAWAY VIEW OF A DRUM OR SCRAPER-TYPE
                   ROTARY VACUUM FILTER
                       PICK-UP OR FORM
                            ZONE
                         FIGURE 9-9

         OPERATING ZONES OF A ROTARY VACUUM FILTER
                            9-29

-------
cake out  of the sludge  vat.  The  cake  drying zone  represents
from 40 to  60  percent  of the drum surface and terminates at the
point where  vacuum  is  shut  off  to  each  successive section.   At
this point, the  sludge  cake  and drum  section enter  the cake
discharge zone.   In this final  zone, cake  is  removed  from the
media.   Belt-type rotary vacuum  filters  differ from the drum or
scraper-type units,  because  the drum  covering  or  media-belt
leaves  the  drum.    There are basically  two  coverings used with
belt-type units:  coil  springs  or fiber cloth.
                         WASH WATER
                         SPRAY PIPING
                     INTERNAL
                        PIPING
                                                 VACUUM
                                                  GAUGES
                                                   DRUM
CAKE
DISCHARGE
COIL SPRING
FILTER MEDIA
                                                      VACUUM AND
                                                         FILTRATE
                                                          OUTLETS
                                                         AGITATOR
                                                            DRIVE

                                                        VAT DRAIN
                            FIGURE g-10

         CROSS SECTIONAL VIEW OF A COIL SPRING - BELT TYPE -
                       ROTARY VACUUM FILTER
Figure  9-10  shows  a  cross  sectional  view of  a  coil  filter
spring-belt type  rotary  vacuum.   This filter uses two  layers of
stainless steel  coils  arranged around the drum.    After the
dewatering cycle,  the  two layers of  springs leave the drum and
are separated from  each  other.   In  this way, the cake  is  lifted
off the  lower layer of  springs  and  can be discharged from the
upper layer.   Cake release is essentially never  a  problem.   After
cake discharge,  the coils are washed  and returned to  the  drum.
                               9-30

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The coil  filter has been and  is  widely used for  all  types of
sludge.   However, sludges with  particles  that  are both extremely
fine  and  resistant to  flocculation  dewater  poorly  on  coil
filters.   Figure 9-11 shows a typical installation.
                            FIGURE 9-11

                  TYPICAL COIL SPRING - BELT TYPE -
                 ROTARY VACUUM FILTER INSTALLATION
 Figure  9-12  shows  a  schematic  cross  section  of  a  fiber
 cloth-belt, rotary vacuum filter.   Media on this type unit  leaves
 the drum surface at the end of the drying zone and passes  over a
 small-diameter discharge roll  to facilitate  cake discharge.
 Washing of the media occurs after  discharge and before it returns
 to the drum for another cycle.  This type of filter normally  has
 a  small-diameter  curved bar between  the  point where the belt
 leaves  the drum   and  the discharge  roll.   This  bar  aids in
 maintaining belt  dimensional stability.    In  practice,  it is
 frequently used to ensure adequate cake  discharge.  Remedial
 measures,  such as addition  of  scraper blades,  use of  excess
 chemical conditioner,  or addition of fly ash,  are sometimes
 required to  obtain cake release  from  the  cloth media.  This is
 particularly  true  at  wastewater  treatment plants  which  produce
 sludges  that  are greasy,  sticky,  and/or contain a large  quantity
 of activated sludge.   Figure  9-13  shows a typical installation.
                               9-31

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                           FIGURE 9-12

            CROSS SECTIONAL VIEW OF A FIBER CLOTH - BELT
                   TYPE - ROTARY VACUUM FILTER
        9.4.4.2   Application

Vacuum  filters  have probably  been used to dewater more  types
of  municipal  wastewater  treatment  plant  sludges  than  any
other mechanical dewatering equipment.  Since the mid-1920s,
more  than 1,700  vacuum filters  have  been  installed  in  over
800  United States  municipalities (43).   The  era of vacuum
filtration may be declining.   Improvements  in other dewatering
devices, as well  as  the development  of  new dewatering devices,
have  permitted municipalities  to  dewater their sludge  as  well
as they could with  vacuum filters but  at lower operation and
maintenance costs.
                              9-32

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                           FIGURE 9-13

                 TYPICAL FIBER CLOTH - BELT TYPE
                      ROTARY VACUUM  FILTER
        9.4.4.3  Performance

As with  all types  of  mechanical  dewatering  equipment,  optimum
performance is dependent upon  the  type  of  sludge and its solids
concentration, type and  quality  of  conditioning,  and  how the
                      Selection of  vacuum  level,  degree  of drum
                      media,  and cycle  time  are  all critical to
                        Tables  9-18 and  9-19  contain expected
                      cloth  and  coil media rotary vacuum filters
                       indicated.   Tables  9-20  and  9-21  contain
specific operating data  for several wastewater treatment plants
using cloth media and  coil media.
filter is  operated.
submergence,  type of
optimum  performance.
performance data for
for the  sludge  types
        9.4.4.4  Other Considerations

Auxiliary Equipment

Rotary  vacuum  filters  are  normally  supplied  with  auxiliary
equipment including vacuum pump, filtrate receiver and pump, and
sludge  conditioning  apparatus.   Figure 9-14  shows  a  typical
                               9-33

-------
complete  rotary  vacuum  filter process.   Usually,  one vacuum pump
is  provided  for each  vacuum  filter,  although  some larger  plants
use less  than  one pump per  filter and  the pumps connect  to a
common  header.   Until  the  1960s,  reciprocating type  dry  vacuum
pumps  were  generally  specified,  but since  the early  1970s  wet
type  vacuum  pumps are  universally used.   The wet  type pumps are
more  easily  maintained  and provide sufficient  vacuum.   Wet  type
pumps utilize seal water, and  it is essential that a satisfactory
water be  used.   If the water is  hard and unstable,  it may be
necessary  to  prevent  carbonate  buildup on the  seals  through the
use  of  a  sequestering  agent.    The vacuum pump  requirements are
normally  1.5  to 2.0  adiabatic cubic  feet  per minute  of air per
square  foot  of  drum surface area  at  20  inches  of  mercury  vacuum
(1.5  m3/min/m2  at 69  kN/m2) .   This is true unless  the  expected
yield  is  greater than  40 to  50 pounds  per square  foot per  hour
(20 to  25  kg/m2/hr)   and  extensive sludge  cake  cracking  is
expected.    In  the  latter  case,  an  air  flow  2.5 times higher
should  be  used.
                              TABLE 9-18

              TYPICAL DEWATERING PERFORMANCE DATA FOR
                 ROTARY VACUUM FILTERS - CLOTH MEDIA
     Type of sludge
 Raw primary  (P)
 Waste-activated sludge
   (WAS)
 P plus WAS
 P plus trickling filter
   (TF)

 Anaerobically digested
  P
  P plus WAS
  P plus TF
 Aerobically digested no
  primary clarification

 Elutriated anaerobic
  digested
    P
    P plus WAS

 Thermally conditioned
  P plus WAS
 Feed solids
concentration,
   percent
                                  Chemical dosage,
  4.5 - 9.0

  2.5 - 4.5
    3-7

    4-8


    4-8
    3-7
    5-10


  2.5 - 6
    5-10
  4.5-8


    6-15
lb/ton dry
FeCl3
40-80
120-200
50-80
solids
CaO
160-200
240-360
180-240
Yield,
Ib dry solids/
sq ft/hr
3.5 - 8.0
1.0 - 3.0
2.5 - 6.0
Cake,
percent
solids
27-35
13-20
18-25
40-80
60-140
50-80
60-120
       180-240
60-100  200-260
80-120  300-400
80-120  250-350
       150-240
0-100
0-150
          3-7


          3-7
          2-5
        3.5-8

        1.5 - 4.0
4-8
3-6

4-8
           23-30


           25-32
           18-25
           20-27

           16-23
27-35
18-25
                              35-45
 aAll values shown are for pure FeCl3 and CaO.  They must be adjusted for anything
 else.
 Filter yields depend to some extent on feed solids concentrations.  Increasing
 the concentration normally gives a higher yield.

 1 lb/ton =0.5 kg/t
 I Ib/sq ft/hr =4.9 kg/m /hr
                                  9-34

-------
                                   TABLE 9-19

                TYPICAL DEWATERING PERFORMANCE DATA FOR
                     ROTARY VACUUM FILTERS  - COIL MEDIA
      Type of sludge

 Raw primary (P)
 Trickling filter (TF)
 P  plus waste-activated
   sludge  (WAS)
 Anaerobically digested
                                          Chemical dosage,"
Feed solids
concentration,
percent
8
) 4
- 10
- 6
Ib/ton dry solids
FeCl3
40-80
40-60
CaO
, .160-240
100-140
Yield,
Ib dry solids/
sq ft/hr
6 .5
6
- 8.0
- 8
Cake,
percent
solids
28-32
20-28
3-5
             20-60
                    180-220
                               2.5 - 4.0
                                               23-27
P plus FT i
P plus WAS
Elutriated anaerobically
digested primary
5 -
4 -

8 -
8
6

10
50-80
50-80

20-50
240-320
200-300

30-120
4
3.5

4
- 6
- 4.5

- 8
27-33
20-25

28-32
  All values shown  are for pure FeCl3  and CaO.  This must be
  adjusted for anything else.
  Filter yields depend to some extent  on feed solids concentration.
  Increasing the solids concentration  normally gives a higher yield.
  1 Ib/ton =0.5 kg/t
                        2
  I Ib/sq ft/hr =4.9 kg/m*/hr
                                   TABLE 9-20

                        SPECIFIC OPERATING RESULTS OF
                   ROTARY VACUUM FILTERS - CLOTH MEDIA
Location
willoughby Eastlake, OH
Tamaqua, PA
Grand Rapids, MI
Fort Atkinson, WI
Frankenmuth, MI
Oconomowoc , WI
Genessee City, MI
Feed solids
concentration ,
Sludge type percent
P plus (WAS) plus septic 4-6
Anaerobically digested 6
(P plus WAS)
Thermally conditioned 10 - 15
(P plus WAS)
WAS 3-4
WAS 3 . 7
Anaerobically digested 2.3
(P plus WAS)
P plus WAS 8
Cake, Yield,
Conditioner used, percent Ib dry solids/ Filtrate,
percent by weight^5 solids sq ft/hr mg/1
FeCl3-
Lime -
FeCl3-
Lime -
None
FeCl3-
Lime -
FeCl3-
Lime -
Fed -
Lime -
FeCl3
Lime -
3 20 2.8 - 4.8
14
3 18 3 SS 20 - 30
26
50 6 SS 5,000
BOD 10,000
6 19 3.0 - 3.5
16
8 15 3.2
14
6 18 2.-S - 3.0 SS 500 - 1,100
20 BOD5 10
27 5.6
16
WAS = waste-activated sludge
 P = primary sludge

Numbers shown are based on pure Fed, and pure CaO.

1 Ib/sq ft/hr - 4.9 kg/m2/hr
                                        9-35

-------
      Location
Blytheville,  AR


York,  PA


Wyomissing Valley, PA

Bayonne, NJ


Woodbridge, NJ


Shadyside, OH


Arlington, TX
                                         TABLE 9-21

                      SPECIFIC OPERATING  RESULTS OF ROTARY
                             VACUUM FILTERS - COIL MEDIA
                             _Sludge  type
TF


Anaerobically digested
  (P plus WAS)

Anaerobically digested TF


Anaerobically digested P
Anaerobically digested
  (P plus WAS)
Conditioner used.
percent by weight
FeCl3
CaO
Fed,
CaO
TF FeCl3
CaO
P FeCl,
CaO
FeCl-3
CaO
FeCl3
CaO
FeCl3
CaO
- 36
- 94
- 80
- 250
- 62
- 272
- 28
- 62
- 40
- 240
- 64
- 310
- 64
- 174
Cake, Yield,
percent Ib dry solids/
solids sq ft/hr
33.1 10.4
21.1 4.7
18.2 6.0
30.9 7.8
29.7 8.0
29 4.2
25.2 8.8
 WAS =  waste-activated sludge; P = primary sludge.  No data available for feed solids and filtrate
 concentrations.
 Numbers shown are based on pure Fed3  and pure CaO.

 1 Ib/sq ft/hr =4.9 kg/m2/hr
                             FERfllC CHLfiaiOE
                                                                                      AIR TO
                                                                                    ATMQEFHEflE
                                                                                            SILENCER
                                                                              WATSR
                                                                         FILTRATE
                                                                         PUMP
                                                               \
                                                                                       VACUUM
                                                                                         PUMP
                                         FIGURE 9-14

                            ROTARY VACUUM FILTER SYSTEM
                                               9-36

-------
Each  vacuum filter  must  be  supplied  with a  vacuum receiver
located  between  the filter  valve and  the vacuum  pump.   The
principal purpose of  the  receiver  is to separate the air from the
liquid.   Each  receiver  can  be equipped with  a vacuum-limiting
device  to admit air  flow  if  the design  vacuum is exceeded  (a
condition  that  could  cause the vacuum  pump to overload).   The
receiver also  functions as  a reservoir  for  the  filtrate  pump
suction.  The filtrate pump must be sized to carry  away the water
separated  in the vacuum receiver, and  it  is  normally sized  to
provide a capacity two to four times  the design sludge feed rate
to the filter.

The  filtrate pump  should be able to  pump against a minimum
total  dynamic head  of  between  40 and  50 feet  (12  to 15  m),
which  includes  a minimum suction  head  of  25-feet (7.5  m).
Centrifugal-type  pumps are  commonly used but can become air bound
unless they have  a balance  or  equalizing line connecting the high
point  of the  receiver  to the pump.   Typically,  nonclogging
centrifugal style pumps  are used  with  coil filters  because they
permit a  somewhat higher  solids  concentration  in  the filtrate.
Self-priming centrifugal pumps  are used most  frequently,  since
they  are relatively maintenance free.   Check  valves  on  the
discharge side  of the pumps are usually provided to minimize air
leakage  through the  filtrate pump and receiver  to  the  vacuum
pump.

Sludge conditioning  tanks are  discussed in  Chapter  8.

FijLter Media

A major  process  variable is  the  filter media.  The  ideal  media
performs the desired  liquid/solid  separation and gives a filtrate
of acceptable clarity  (58).  Further,  the  filter cake discharges
readily  from it, and  it is  mechanically   strong  enough  to give
a long life.  The  media  must be chemically  resistant  to the
materials  being handled  and  provide  minimal  resistance  to
filtrate flow.   A  further characteristic to be minimized  is
"blinding" or  clogging.   All  the  characteristics mentioned
above  need  to  be evaluated during the  selection procedure.   One
must,  therefore, through  experience,  or  bench or pilot-scale
rotary vacuum  filter  testing,  select the  best  media  in  terms of
porosity,  type  of weave,  material  of  construction,   etc.  for  a
particular  sludge.   This selection  is  normally  made  at  the time
of  equipment  start-up by  the equipment  supplier   (15,59).   The
trend over the  past few years  is  to select  a monofilament fabric,
as they seem the most resistant to blinding.

So 1 i d s Fe d Co n t e n t
The higher the feed suspended  solids  concentration of the sludge,
the  greater will  be  the  production  rate  of the rotary  vacuum
filter  (Figure 9-15)  and  the  cake suspended  solids concentration
(Figure  9-16).   Generally,  municipal wastewater treatment plant
                               9-37

-------
sludges  are not concentrated beyond  about 10 percent  solids,
since above this concentration,  the  sludge  becomes  difficult to
pump, mix with chemicals,  and  to  distribute  after conditioning to
the  filter.   In addition,  to increased production rates, higher
sludge feed concentrations result in lower  chemical dosage rates
and lower cake  moistures.   Both of these consequences affect the
cost of sludge dewatering  and  ultimate  disposal.
             12

             11

             10

             9

             8

             7

             6

             5

             4

             3

             2

             1
Q
_i
UJ
                  a
                     o
D DIGESTED
» PRIMARY
o BLENDED
A ACTIVATED
                                                    J
                  1   2   3   4   5  .6   7   8 . 9   10  11

                            FEED SOLIDS {%)

                            FIGURE 9-15

              ROTARY VACUUM FILTER PRODUCTIVITY AS A
                FUNCTION OF FEED SLUDGE SUSPENDED
                    SOLIDS CONCENTRATION (60)
The  lowest  feed  sludge  suspended  solids  concentration  for
successful vacuum filtration  is  generally  considered  to be
3.0 percent.   Below this  concentration  it  becomes  difficult to
produce  sludge  filter  cakes  thick  enough  or  dry enough  for
adequate discharge.   For this  reason, it is  extremely important
that the design  and operation  of the preceding sludge processes
take  into consideration  the  need for  an  optimal  solids
concentration when dewatering on vacuum filters.
                               9-38

-------
          3B
          30
          2B
OJ
Q   20
_j
O

Hi   15
          1.0
                                            MCCARTY
                                      PRIMARY
                               ACTIVATED
                     11 gm/L CaO, 3,7 gm/L FeCI3
                 1    23456789   10

                           FEED SOLIDS (%)

                           FIGURE 9-16

      SLUDGE CAKE TOTAL SOLIDS CONCENTRATION AS A FUNCTION OF
        THE FEED SLUDGE SUSPENDED SOLIDS CONCENTRATION (60)


        9.4.4.5   Case History

This  study  is summarized  from  a  USEPA-sponsored  investigation
(61).    Figure 9-17 shows  the 1977  flow  diagram for the 13-MGD
(34 m-^/sec) Lakewood, Ohio,  wastewater treatment plant.   The
sludge  being  handled at  this plant has changed  several times
since  the  facility was  built  in 1938.    At  that time,  the
plant  was designed  for  primary  treatment, with  sludge being
anaerobica1ly digested   and  dewatered  on  sand   drying beds.
Secondary treatment was added in  1966.   Gravity thickeners, two
new anaerobic digesters,   two  vacuum  filters,  and  a flash dryer
were  installed to  handle  additional  sludge.   In 1974 and 1975,
the plant was further upgraded.   Alum  (aluminum  sulfate) was
added  to the aeration  basin effluent  channel for  phosphorus
removal,  and  the  sludge  handling  system  (filters  and dryer)
operating schedule  was extended to two shifts.  Finally,  in 1977,
the plant was returned  to single  shift sludge handling,  and
excess liquid  sludge was hauled to land disposal.

The Lakewood plant  has two polyethylene  cloth belt  rotary vacuum
filters.   Only  one  can  be operated  at  a time because of the
                              9-39

-------
limited  capacity  of  the
effective  area of  376
at a drum  speed of  one revolution per eight
submergence between  30 to 36 inches (0.76 to
flash ; dryer.  _^Each  filter has  an
                  and operates best
                  minutes  and a drum
                  0.91 m).   A  filter
square  feet  (35 m
is operated  five days per  week in either one  or two 6.5-hour
shifts per day.  Conditioning chemical dosages  are approximately
275 pounds of dry lime (pebble  lime -  72 percent CaO)  per  ton of
dry feed  solids (137 kg/t)  and 30 pounds of  FeCl3 (liquid at
40 percent FeCl3) per  ton of  dry feed solids (15 kg/t).
                                COMMUNI TOflS
                           FIGURE 9-17

     LAKEWOOD, OHIO WASTEWATER TREATMENT PLANT FLOW DIAGRAM


Prior  to  1975,  before  alum was  added for  phosphorus removal
(63 mg/1 alum  added), the  average  total  solids concentration  of
the digested sludge (vacuum filter solids feed) was 4.45 percent.
On the average,  the sludge was dewatered  to 23.8 percent solids.
After  alum  addition,  the  feed  sludge  solids  concentration
increased  to 6.5 percent, but the dewatered cake percent dropped
to 21.4.

Table  9-22  indicates  operational  costs  for 6.5-hour and
13-hour-per-day operations  based  on before and  after  alum
addition for phosphorus  removal.   Because  of the increase  in the
number of tons  from 650  dry  tons per year  (590 t/yr)  in 1974  to
1,820 dry tons per year  (1,651 t/yr) in 1976, the treatment  cost
per ton of dry  total  solids  was not much  greater  than  it  was  in
1974.
                              9-40

-------
                           TABLE 9-22

   OPERATIONAL COST OF LAKEWOOD, OHIO VACUUM FILTER OPERATIONS
                                  Single shift
                                operation - 1974
                                 dollars per ton
                                  dry solids
  Double shift
operation -  1976
 dollars per ton
  dry solids
Ferric chloride and lime
Electricity
Maintenance supplies
Maintenance and repair labor
Operational labor
Overhead
Total
8.90
1.98
1.11
3.65
3.46
2.25
21.35
8.90
1.29
1.10
3.60
6.25
3. 11
24.25
 1 ton = 0.907 t
        9.4.4.6  Costs

Figure 9-18  gives  the 1975 capital cost as  a  function  of filter
area for rotary vacuum filters.  As an example, a 400-square-foot
(37.2 m2)  area filter  would  cost  400,000  dollars.   Since  this
number is based on  a  June 1975 cost,  it must  be  adjusted to the
current design period.  Costs include those for filter,  auxiliary
equipment, piping,  and building.

The  labor  requirements  indicated  in  Figure  9-19 are given  as  a
function of  average  area  in use and  include:   start-up time and
clean-up after the- filter run, operation of filter,  and  operation
of sludge pumping and conditioning facilities prior to treatment.
As an example,  a  vacuum filter having 400 square  feet  (37.2 m2)
of filter area  would  require 550  man-hours  of  operation and
maintenance per year  and  would  be  included in  the  cost  analysis.

Figure  9-20 gives  power  consumption  as  a function of  filter
area.   As an example,  a vacuum  filtration area  of 400  square
feet (37.2  m2)  would require  330,000 kilowatt-hours  per
year  (1,200 GJ/yr )  of  electrical energy.  If power  costs are
0.05 dollars per kilowatt-hour  (0.014 dollars/MJ),  the cost would
be 33,000 dollars annually.  Operating parameters used were based
on two  adiabatic cubic  feet  of air  per  minute per  square  foot
(10  1/s/m2), 20 inches  of  vacuum  (68  KN/m2), and  a  total dynamic
head  of  50  ft  (15 m)  for the filtrate  pump.   Power  required
includes that for drum drive,  discharge roller, and vat  agitator,
but  does  not  include  other accessory  items, such  as  sludge  feed
pump or chemical feed system.

Figure 9-21  shows  a  curve  developed  for   estimating  rotary  drum
vacuum filter  maintenance  material cost as  a  function  of filter
area.  As  an example,  for  a  filtration  area of 400  square  feet
                               9-41

-------
t/J

EC
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o

tc.
o
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IT

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i
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-z.
<
    100,000
                  2    34  56788100    234 S8789lrtXX»   234 56789




                        SINGLE VACUUM FILTER AREA, sq ft  (1 sq ft - 0,093 rn2)




                                  FIGURE 9-18



            ESTIMATED JUNE 1975 CAPITAL COST FOR ROTARY DRUM

                            VACUUM FILTERS  (39)
        2  -
     1,000
                 2   3  4B67831QQ    2    3   4^67891,000   2   3   456789



                         , AVERAGE AREA IN USE, sq ft {1 sq ft = 0,093 m2}




                                  FIGURE 9-19



           ANNUAL OSM MAN-HOUR REQUIREMENTS - ROTARY DRUM

                             VACUUM FILTERS  (39)
                                      9-42

-------
(37.2 m^) ,  a  designer would  estimate  a yearly materials  cost  of
4,000 dollars.   Since this number  is  based on a June  1975  cost,
it must be adjusted to the current  design  period.
  to
  £

  S
  SI

  X
  O
  ut
  DC

  >-
  O
  c
  UJ

  uu
  -i

  o
  fie

  o
  01
       10,000
           10    234  56 789100   2   34 567Bi1,QQQ   234 56 78910,000


                      VACUUM FJLTRATION AREA, iq ft (1 iq ft - 0.093 m2}


                            FIGURE 9-20


              POWER CONSUMED BY ROTARY DRUM VACUUM

                      FILTRATION PROCESS (39)
    9.4.5  Belt Filter  Press


Belt  filter  presses  employ  single or  double  moving belts  to
dewater sludges continuously.


The  early belt presses used in the  United States were  those
developed by Klein  and  by  Smith  and  Loveless  in  the  1960s
(62,63).  Belt  filter presses  are currently very popular not only
                                9-43

-------
in the United States (64) but in other parts of the world as well
(65).   At  least 20 equipment suppliers can  furnish  some  type  of
belt press.   This popularity has  led to many units  being  sold,
with very  little operational experience  to support  the  claimed
advantages.    One detailed report  that  evaluated  belt  press
operating experience found  that there were  many  operational  and
maintenance problems  that still  needed  to  be  solved  (66).   As
was  pointed  out by  Austin  (65), significant  developmental work
is  still being conducted.   Table  9-23  lists  advantages  and
disadvantages of belt filter presses.
o
u
Z
              2   34B678S1QG    2   34 667891,000  2   3  4567B9

                      AVERAGE AREA IN USE, sq ft (1 sq ft = Q.Q93 m2)


                            FIGURE 9-21

          ESTIMATED JUNE 1975 ANNUAL MAINTENANCE MATERIAL
              COST - ROTARY DRUM VACUUM FILTER (39)
                               9-44

-------
                            TABLE 9-23

        ADVANTAGES AND DISADVANTAGES OF BELT FILTER PRESSES

          Advantages                          Disadvantages
 High pressure machines are capable of      Very sensitive to incoming feed
   producing very dry cake                characteristics
 Low power requirements                 Machines hydraulically limited
                                    in throughput
                                   Short media life as compared with other
                                    devices using cloth media
        9.4.5.1  Principles of Operation

Any belt  filtration  process  includes three  basic operational
stages:    chemical conditioning  of  the  feed  slurry, gravity
drainage   to  a nonfluid  consistency,  and  compaction  of  the
predewatered sludge (6).

Figure 9-22  depicts  a  simple  belt press and  shows  the location
of  the three  stages.   Although  present-day  belt presses  are
more  complex,  they  follow  the  same  principles  indicated  in
Figure 9-22.

Good  chemical  conditioning is  the key to  successful  and
consistent performance  of  the  belt  filter  press,  as   it  is
for other dewatering  processes.    This is fully discussed  in
Chapter 8.

After conditioning, the readily drainable water is separated from
the slurry by discharge of  the  conditioned  material onto  the
moving belt  in the gravity drainage section.   Typically,  one or
two minutes  are  required for  drainage.   Following drainage,  the
sludge will  have  been reduced in volume by  about  50 percent  and
will  have a  solids  concentration of 6  to 10  percent.    "The
formulation of an even surface cake at this point is essential to
the successful operation  of subsequent stages  of  the dewatering
cycle.    The  even  surface prevents  uneven  belt  tension  and
distortion while  the relative  rigidity of  the mass  of  sludge
allows further manipulation and  gives maximum  speed through  the
machine"  (65 ) .

The third  stage  of the belt  press  begins  as soon  as  the  sludge
is  subjected  to  an  increase  in  pressure,  due  to either  the
compression  of the sludge  between  the  carrying belt  and  cover
belt  or  the  application  of  a vacuum on  the  carrying  belt.

Pressure   can  be  widely  varied  by  design,   as  shown  by  the
Pressure  can  be  widely  v
alternatives on Figure 9-23.
                               9-45

-------
       CHEMICAL
      CONDITIONAL
        STAGE
      POLYILECTROUTE
        SOLUTION
 GRAVITY
'DRAINAGE
 STAGE
    COMPRESSION
	. DE WATER ING
      STAGE
SLUQGi
    CONDITIONED
    sty ocs
                                               WASH WATER
                            FIGURE 9-22

               THE THREE BASIC STAGES OF A BELT PRESS
During  pressure  application, the  sludge cake,  squeezed  between
the two belts, is  subjected  to  flexing  in opposite directions as
it passes over the various rollers.  This action causes increased
water release and allows greater compaction of the sludge.

Figure 9-24 shows a typical belt press installation.


        9.4.5.2  Application

Belt  filter  presses  are  being  installed  in many  United States
municipalities to  dewater many  types  of sludge.   At  this  time,
there  is  not enough  operational  data available to  indicate  any
sludges to which a belt filter press could not be applied.


        9.4.5.3  Performance

It  is  difficult  to  generalize  about the  operating  performance
of belt presses  because results depend  on  many  factors:   method
of  conditioning,  maximum pressure, number of  rollers,  etc.
Table 9-24 was developed from minimum and maximum values given in
all published data.

Published material  on operating  belt  press  installations is
very  limited.   Medford,  New  Jersey  (67)  reported  on a  belt
press  dewatering  aerobically  digested sludge  from a  contact
stabilization  system.   Feed  sludge of  a  3  to  4  percent solids
concentration was dewatered  to  a cake of 17 to 19 percent solids
(67).   Polymer was added  for  conditioning  at 7 to  10  pounds of
dry  polymer per  ton  of dry  feed  solids  (3.5  to 5 kg/t).   The
solids  concentration  in  the  combination washwater  and  filtrate
was 100 mg/1 for an overall solids capture of 99 percent.
                               9-46

-------
GRAVITY
DRAINAGE

[o;o;o;o;o;o;oj
******
COMPRESSION
DEWATERING
LOW PRESSURE
SECTION
J^oWalo
p;o;oio!b~**
* * * t
HIGH PRESSURE
SECTION
V * \ f f
$Lf\fi*
///y K\
/ / 1 * „
/ ^ t
     III!
     t Mtt tttt
                                                   4   *   *
  VACUUM ASSISTED

.'»>	,...:-, r	-*»
 P  LLJ LLJ  LfJ Q
 If**!
                            FIGURE 9-23

              ALTERNATIVE DESIGNS FOR OBTAINING WATER
                RELEASES WITH BELT FILTER PRESSES (66)
        9.4.5.4   Other  Considerations

Failure  of  the chemical  conditioning  process  to  adjust  to
changing  sludge   characteristics   can  cause   operational
problems  (66).    If it  is  underconditioned,  sludge does  not
                               9-47

-------
                          FIGURE 9-24



            TYPICAL BELT FILTER PRESS INSTALLATION
                          TABLE 9-24




    TYPICAL DEWATERINC PERFORMANCE OF BELT FILTER PRESSES
Type of sludge
Raw primary (P)
Waste activated sludge (WAS)

P + WAS
P + trickling filter (TF)
Anaerobically digested
P
WAS
P + WAS
Aerobically digested
P + WAS

Thermal conditioned
P + WAS
Feed
solids ,
percent
3-10
1-3
0.5-1.5
3-6
3-6

4-10
3-4
3-9

1-3
6-8

4-8
Cake ,
percent
solids
28-44
16-32
12-28
20-35
20-40

26-36
18-22
18-44

12-18
20-30

38-50
Polymer,
pounds dry
per ton
dry solids
2-9
2-4
4-12
2-10
3-10

2-6
4-8
3-9

4-8
2-5

0
1  Ib/ton =0.5 kg/t
                             9-48

-------
drain  well  in  the  gravity  drainage section,  and  the  result
is either  extrusion  of  inadequately  drained solids  from the
compression  section,  or uncontrolled  overflow  of sludge from the
drainage section.  Both underconditioned and overconditioned
sludges can  blind  the  filter media.  In addition, overconditioned
sludge drains  so rapidly that solids cannot distribute across the
media.  Inclusion  of a sludge blending  tank step before the belt
press reduces  this problem.   See Chapter 15 for a discussion of
blending tanks.

The combined filtrate and belt washwater  flow is normally about
one and  one-half  times the  incoming flow.   Some  belt presses
recirculate  washwater  from  the  filtrate collection system,  but
normally,  secondary effluent  or potable water is  used.  This
total  flow  contains  between 100  and  1,000 mg/1  of  suspended
solids  and is  typically  returned  either to  the primary  or
secondary treatment  system.

Belt  presses  have  numerous  moving  parts,  and  spare parts
should  be  kept  available  to  prevent  prolonged  unit down-time.
Belts,  bearings,  and  rollers  deteriorate quickly,  especially
in  municipal  wastewater  treatment  plants  where  preventive
maintenance  is  not normally practiced.


        9.4.5.5  Design Example

The  designer  for  an  existing wastewater treatment  plant has
calculated  that the plant needs  to dewater 5,000  dry pounds of
sludge  (2,268  kg)  per day,  five days  per  week.   The  sludge
to be  dewatered is  a  mixture of  one  part primary  and two parts
waste-activated, stabilized  by  a  two-stage, high-rate,  anaerobic
digestion process.  Total feed solids  concentration to the belt
filter  press  was 2.8 percent.    Pilot  plant  testing  with a
one-meter-wide  belt  filter press produced the  following  results.

     •  Total  solids in the  dewatered  sludge  ranged  from 23 to
        30  percent,  averaging 25 percent.

     •  Optimum polymer dosage was 6 to  8 pounds of dry  polymer
        per ton (3 to  4  kg/t)  of dry  feed  solids,  or 80 to
        100  pounds of liquid polymer per ton  (40 to 50 kg/t) of
        dry  feed solids.

     •  At  the  optimum  polymer  dosage, the  total  solids  in
        the  filtrate  plus  washwater  flow was 2,000  mg/1.   The
        suspended  solids averaged 900 mg/1.

     •  Optimum hydraulic  feed rate at  2.8 percent solids for a
        one-meter-wide  belt was 47  gallons per minute  (3 1/s).

     •  Washwater requirements were  25 gallons per  minute
        (1.6 1/s).
                              9-49

-------
On the  basis  of  pilot plant data, the engineer decided  that one
1-meter-wide  belt  filter press  could dewater  the 5,000  pounds
(2,268 kg) of  sludge  in 7.6 hours.   Since it was  important  that
the wastewater treatment plant always  be  able to  dewater sludge,
two 1-meter-wide belt filter presses  would be purchased.

The current  cost of  dry polymer  in  50 pound (22.7 kg)  bags was
$1.85  per pound  ($0.84/kg); for liquid polymer in  55 gallon,
650 pound  (208  1-295  kg)  drums,  the  cost  was  $0.13 per  pound.
Daily cost for dry polymer at 8 pounds per ton (4  kg/t)  would be:


    5,000 Ib solids      8 Ib poly       $1.85    ,,__  nn
    -"	da7	 x 2,000 Ib solids X Ib poly  =  $37'00 per


Daily  cost  for  liquid  polymer  at 100 pounds per ton  (50 kg/t)
would be:


    5,000 Ib solids     100 Ib poly      $0.13  _  $32  5Q     ,
          day       x 2,000 Ib solids x Ib poly  "  $^-50 Per  day


Because sludge  characteristics  can  change with  time, a dual
polymer  system capable  of  utilizing  either  liquid or  dry polymer
will  be  installed.    Since  liquid  polymer is  currently less
expensive, it will be used initially.

To allow subsequent  computation  of  solids capture, the  filtrate
flow  is  calculated,  using  a suspended solids balance and  a  flow
balance.   The specific  gravity of  the feed, dewatered  cake and
filtrate  are assumed  to be 1.02, 1.07  and  1.01,  respectively.
The suspended solids balance is:


    47 gal feed  8.34 x 1.02 Ib feed    0.028 Ib  solids
         min           gal feed             Ib feed


      Q  gal filtrate  8.34 x 1.01  Ib  filtrate    900 Ib solids
           min             gal filtrate         106 Ib filtrate


      M  gal cake  8.34 x 1.07 Ib sludge   0.25 Ib  solids
         min            gal cake             Ib  cake


The flow balance is:


    47 gal feed   25 gal washwater
         min       ™   """"TnTrr


      Q  gal filtrate + M gal cake
           min            min


                               9-50

-------
The suspended solids and flow balances  are solved simultaneously.

The flow  of filtrate  (Q)  is 67.2 gallons per minute  (254  1/m).


    Solids capture


      Solids in feed - solids in  filtrate
                Solids  in feed
                                           x  100
                                                     900
      47  (8.34 x 1.02)  (0.028) -  67.2  (8.34  x  1.01)  TTg-
    = _	_ 							..-  .-.-  - .  .   ^--'   v  inn
                47  (8.34 x 1.02)  (0.028)


    = 95 percent


All filtrate  is returned to  the secondary treatment  process.


        9.4.5.6  Costs

Current  published information  on capital cost of belt filter
presses  is  almost  nonexistent.    Some  information  is  available
from  a  recent  USEPA  publication  (68).    According  to this
publication,  construction  costs  for  a  belt  filter  press,
sludge  feed  pump,  polymer  pump, and control panel  to dewater
1,000 pounds  (454 kg) of sludge per hour was $97,000.  To  dewater
2,500 pounds  (1,134 kg) per  hour,  the  cost  would  be  $120,000.

Table  9-25   lists  labor  requirements   for  the  operation and
maintenance of belt filter presses.  The  labor indicated includes
periodic  operational  adjustments  and  minor routine maintenance.
No information is available  on maintenance  material  cost.
                             TABLE 9-25

            LABOR REQUIREMENTS FOR BELT FILTER PRESSES (19)


       Number                         Labor, hours per year
         of                     	
       units               Operation       Maintenance       Total
         1                    265            100            365
         2                    530            200            730
         3                    795            300          1,095
         4                   1,060            400          1,460
         5                   1,325            500          1,825
                                9-51

-------
    9.4.6  Recessed Plate Pressure Filters

Pressure filtration for  sludge  dewatering  evolved  from  the
similar practice in sugar manufacturing of forcing juices  through
cloth.   The  first United  States municipal sludge  dewatering
installations, which  were  also the first  large-scale  mechanical
dewatering  applications   in  this  country,  were  located  in
Worcester,  Massachusetts,  and  Providence,  Rhode  Island,  in  the
early  1920s  (56).   Fixed- and variable-volume  recessed plate
pressure filters are discussed  in  this  section.

Fluid  pressure generated  by pumping  slurry  into  the   unit
provides  the  driving  force  for recessed plate pressure  filters.
Performance  reliability  is  increased  by modern design  concepts,
such as  use  of  new construction materials  to  resist attack  by
acids  and alkalis; mechanization  of   the  operating  sequence  to
reduce manpower  requirements;  and  the  use  of membrane  diaphragms
for  variable  volume filtration  (69).    Table   9-26  lists  the
advantages  and  disadvantages  of  pressure  filters compared  with
other dewatering methods.
                            TABLE 9-26

          ADVANTAGES AND DISADVANTAGES OF RECESSED PLATE
                         PRESSURE FILTERS
            Advantages
         Disadvantages
 Highest cake solids concentration
Batch operation
High labor cost
High capital cost
Special support structure requirements
Large area requirement
        9.4.6.1  Principles of Operation

Fixed-volume,  recessed  plate  pressure  filters,  illustrated
on  Figure  9-25,  are  constructed  from  a  series  of  recesssed
plates.   As  shown on Figure 9-26,  volume is  provided by  the
depressions on the sides of the plates.

The  surfaces  of both sides  of  the  filter plate are designed so
that  the filtrate  drains from  the filter  cloth  and from  each
plate.

A  filter  cloth is mounted over  the two surfaces of each  filter
plate.  Conditioned sludge is pumped into the pressure  filter and
passes  through  feed  holes in the filter  plates  along  the  length
of the filter and into the recessed chambers.  As the sludge  cake
forms  and  builds up  in the  chamber,  the pressure  gradually
increases  to a point at which  further sludge  injection would
                               9-52

-------
FIXED OR
FEED HEAD
PLATES
MOVEABLE
HEAD

•idh
aodaj
: *
dcij
o
ri
i
: \
ho \
                                                    CLOSING
                                                    HEAD
                                     HYDRAULIC
                                     CLOSURE
                             FIGURE 9-25

       SCHEMATIC SIDE VIEW OF A RECESSED  PLATE PRISSURI FILTER
                            CAKE
           SLURRY
           INLET
                               FILTRATE OUTLETS


                             FIGURE 9-26

                   CROSS SECTION OF A FIXED-VOLUME
                   RECESSED PLATE FILTER ASSEMBLY
                                 9-53

-------
be counter-productive.  Pressure filters
100 pounds per  square  inch  (690 kN/m2)
square inch (1,550 to 1,730 kN/m2).
 operate at a pressure of
or 225  to  250  pounds  per
A typical  pressure  filtration cycle  begins  with the  closing  of
the press  to  the position shown  on  Figure 9-25.  Sludge  is  fed
for a 20- to 30-minute period until the press is effectively full
of cake.   The pressure  at  this  point is  generally  the designed
maximum and is maintained for  a  one- to  four-hour period,  during
which  more filtrate  is removed  and the desired  cake  solids
content is achieved.  The filter is then mechanically opened,  and
the dewatered cake dropped from the chambers onto a conveyor belt
for removal.  Cake  breakers  are  usually  required to  break  up  the
rigid  cake  into conveyable  form.    Figure 9-27  shows  a typical
pressure filter installation.
                            FIGURE 9-27

                  TYPICAL RECESSED PLATE PRESSURE
              FILTER INSTALLATION AT WASSAU, WISCONSIN
Construction of  a  variable-volume recessed plate pressure filter
is  similar  to  the fixed-volume  filters,  except that a diaphragm
is placed behind the media as shown on Figure 9-28.  A dewatering
cycle  begins as  conditioned  sludge is fed into each chamber from
a  slurry  inlet pipe  located  in  the top  or bottom of each plate.
Generally,  about 10  to 20 minutes are required to fill the press
                               9-54

-------
and  reach an  end  point determined  by either  instantaneous  feed
rate,  filtrate  rate,  or time.   When  the  end point  is  reached,
the  sludge  feed pump is automatically  turned  off.   Water or air,
under  high  pressure,  is then pumped  into the space  between the
diaphragm  and  plate  body  squeezing  the  already
partially  dewatered  cake.    Typically,  15  to 30
constant  pressure  are  required to dewater the cake to
solids  content.  At the end  of the cycle, the water
to  a reservoir, plates  are automatically  opened,  and
is  discharged.
                            formed and
                            minutes of
                            the desired
                            is returned
                            sludge  cake
 SLURRY,
 INLET
 (TOP OR
 BOTTOM)
    CAKE
FILTRATE
OUTLET
(TOP OR
BOTTOM)
                      CLOTH
                      SOFT RUBBER
                      MEMBRANE
HIGH PRESSURE
WATER

     FILTRATE
                             CAKE UNDER
                             COMPRESSION
                             •MOULDED
                             RUBBER BODY
     SHAPE OF FILTER CHAMBER
       DURING FILTRATION
           SHAPE OF FILTER CHAMBER
           DURING CAKE COMPRESSION
               BY DIAPHRAGM
                             FIGURE 9-28

                 CROSS SECTION OF A VARIABLE VOLUME
                   RECESSED PLATE FILTER ASSEMBLY
         9.4.6.2  Application

 Pressure filtration is an advantageous choice for sludges of poor
 dewaterability, such as  waste-activated  sludges,  or for cases in
 which it  is  desirable  to dewater a  sludge to a  solids content
                                9-55

-------
higher than  30  percent.   If  sludge  characteristics are expected
to change drastically  over  a  normal  operating period, or if less
chemical conditioning is desired, the variable-volume units would
probably be selected rather than the fixed-volume units.
                            TABLE 9-27

        EXPECTED DEWATERINC PERFORMANCE FOR A TYPICAL FIXED
               VOLUME RECESSED PLATE PRESSURE FILTER




Conditioning


Type of sludge
Raw primary (P)

Raw P with less than
50 percent waste
activated sludge (WAS)
Raw P with more than
50 percent WAS
Anaerobically digested
mixture of P and WAS
Less than 50 percent WAS

More than 50 percent WAS

WAS

Feed
solids,
percent
5-10

3-6


1-4



6-10

2-6

1-5

Ibs/ton

FeCl3a
100

100


120



100

150

150

dry

CaOa
200

200


240



200

300

300


dosage ,
solids

Ash

2,000

3,000


4,000



2,000

4,000

5,000
Cake with
conditioning
material ,
percent
solids
45
50
45
50

45
50


45
50
45
50
45
50
Cake without
conditioning
material,
percent
solids
39
25
39
20

38
17


39
25
37
17
37
14


Cycle
time,
hours
2.0
1.5
2.5
2.0

2.5
2.0


2.0
1.5
2.5
1.5
2.5
2.0
 All values shown are for pure FeCl, and CaO.  Must be adjusted for anything else.

1 Ib/ton =0.5 kg/t
1 Ib/sq ft/hr =4.9 kg/nT/hr
        9.4.6.3  Performance

As  of  1979,   very few  fixed-volume  recessed  plate  pressure
filters are operating in the  United  States,  and there are no
variable-volume  installations  operating.    Table  9-27   contains
expected  performance  data  for  typical fixed-volume  units,  and
Table  9-28  lists  actual data from operating installations.
Table 9-29 lists a performance  from a large variable-volume pilot
unit  (62.4 square  feet [5.8 m2] of filtering area).


        9.4.6.4  Other Considerations

Sludge Conditioning Process

Most  systems  are designed   so that ferric chloride  and  lime
are  added in  batches  to sludge  contained in  an agitated tank,
and  the  conditioned sludge  is pumped from  the tank  into the
pressure  filter  as required.  However,  experience  indicates
                               9-56

-------
                                        TABLE 9-28

                  SPECIFIC OPERATING RESULTS OF FIXED VOLUME
                         RECESSED PLATE PRESSURE FILTERS
                                                         Percent solids
    Location
Kenosha, WI
Wausau,  WI
Cedar Rapids,  IA
Brookfield, WI
   Sludge type

Anaerobically di-
  gested mixture
  (P plus WAS)
Water plant plus
  thermal conditioned
  mixture of anaer-
  obically digested
  (P plus WAS)
Anaerobically di-
  gested mixture
  (P plus TF)
                 WAS plus raw P
Feed
solids ,
percent
3.5 - 5
Conditioner ,
Ib/ton dry
solidsb
FeCl3 - 54
Lime -340
Cake with
conditioning
material
41.5
Cake without
conditioning
material
35
Year and
total cost/
dollars/ton
dry solids
1975 - 61
Reference
70
                                    2 -
                                                       34 - 45
                                                                  35 - 45    Not given     71
li- 3.5 - 7
re
jested 4
P

Fly ash at 6Q
about
2,500
FeCl3 - 143 43
Ash - 1,200
Lime - 346
27 1972 - 30

25 Not given


72

61


 P = primary sludge;  WAS = waste-activated sludge; TF = trickling filter sludge.
 All values shown for FeCl^ and CaO are for pure chemicals.   Must be adjusted for
 anything else.

1 Ib/ton =0.5 kg/t
1 ton = 0.907 t
                                        TABLE 9-29

           TYPICAL DEWATERINC PERFORMANCE OF A VARIABLE VOLUME
                          RECESSED PLATE PRESSURE FILTER
Site
1
2
3
4
5
6
7
8
9

Type of sludge
Anaerobically digested
60 P: 40 WAS
60 P: 40 WAS
40 P: 60 WAS
40 P: 60 WAS
50 P: 50 WAS
60 P: 40 WAS
Raw WAS
Raw (60 plus 40 WAS)
Thermal conditioned
50 P: 50 WAS
Feed
solids,
percent
3.8
3.2
3.8
2.5
6.4
3.6
4. 3
4.0
14.0
Chemical
dosage,3
Ib/ton dry
solids
FeCl3
120
180
120
180
80
160
180
100
0
CaO
320
580
340
500
220
320
460
300
0
                                                                          Percent solids
Yield,
Ib/sq ft/hr
1.
0.
0.
0.
2.
0.
0.
0.
2.
0
7
6
6
0 .
8
6
9
5
Cake with
chemicals
37
36
40
42
45
50
34
40
60
Cake without
chemicals
30
25
32
30
39
40
25
33
60
  All values shown are for pure FeCl3 and CaO.  Must be adjusted for
  any-thing else.

  P = primary sludge; WAS = waste-activated sludge.

 1 Ib/ton =0.5 kg/t
 1 Ib/sq ft/hr =4.9 kg/m /hr
                                            9-57

-------
 that  the  prolonged  agitation  and  tank  storage  time  associated
 with  batch  conditioning can  result in  a  feed of  varying and
 deteriorating  dewaterability.  For  this reason,  conditioning
 processes  are  now  frequently designed to  provide  "in-line"
 conditioning.   This can  be  accomplished by either  the  continuous
 pumping  of sludge  into a small  tank  and addition of  chemicals,
 or  directly injecting  conditioning  chemicals  into the sludge on
 its way  into  the  filter.    In-line conditioning  diminishes
 the deleterious effects of  storage and prolonged  agitation.
 Figure  9-29  shows a  schematic  for  in-line  conditioning.
    IPOLYELECTRQUTE
     MIXING TANK
ALUMINUM
CHLQHOHYDRATE
SILO
            LEVEL SWITCHES CQNTRQUNG SLUDGE FEED
            AND DILUTE CHEMICAL FEED PUMPS
                                                          FILTRATE TO
                                                          HEAD OF
                                                          WORKS
SLUDGE HOLDING
   TANK
                             FIGURE 9-29

           SCHEMATIC OF AN IN-LINE CONDITIONING SYSTEM FOR
                 RECESSED PLATE PRESSURE FILTER (73)
 Feed Pump System

 One major problem with pressure  filters has  been the need  to
 design  a system  that will  pump from  30 to 2,000 gallons per
 minute  (1.9  to  126 1/s) of  a  viscous,  abrasive slurry  at  pres-
 sures of 40 to 225 pounds per square inch (276 to 1,551 kN/m^).

 Ideally,  the  feed system  should  inject  conditioned sludge  into
 the chamber  as  rapidly as possible but  slowly  enough  to  permit
 sufficiently prompt formation of a  uniform and  thick enough cake
 to prevent any incursion of sludge particles  into the filter
 cloth. Imbalance  of the sludge  feed and  cake  formation rates  can
 result in nonuniform,  high resistance cake, or  in  cloth blinding
 and/or  initial  poor  filtrate quality.   If  a nonuniform cake  is
 formed or excessive fines migrate,  then  a long filter cycle  or  an
 inordinate amount of  cloth plugging will result.
                                9-58

-------
The filter feed method used for some pressure filters  involves a
combination of pumps  and pressure vessels.   These  combinations
are used  to  obtain a high  initial feed rate of  approximately
2,000  gallons per  minute  (126  1/s)  via  the pressure vessel,
followed by the use of  reciprocating  ram high pressure  pumps  to
pump at  a pressure of  225  pounds per sqare  inch  (1,551  kN/m2)
at feed rates  of  100 to  200  gallons per minute (6.3 to  12.6 1/s).
In some  cases, a combination of progressive cavity  pumps and
pressure  vessels  is  used  for  the  lower pressure,  high-rate
chamber filling phase.

Cloth Washing  and  Cleaning

Because recessed  plate pressure filters operate at  high pressures
and because many  units  use lime  for  conditioning,  the  designer
must assume that  cloths will  require routine washing with  high
pressure water, as well  as  periodic washing with acid.   Practices
vary according to the particular sludge and proprietary  process.
Designers should  ask for recommendations from equipment suppliers
on frequency  of washing.

Dewatered Cake Breakers

Design of suitable breakers  is a function  of the structural
properties of the  dewatered  cake.   Pressure filter  cake  is
usually  friable  enough that  use of  breaker wires,  bars,  or
cables beneath the filter will be sufficient.   If, however,
polyelectrolyte  conditioning is contemplated,  consideration
should be given to the resulting  changes in cake structure.
        9.4.6.5  Case History

This  information is  summarized from a  recent sludge handling
investigation by USEPA (61).   The 1978 flow diagram for  the
5-MGD  (13-m^/sec)  Brookfield,  Wisconsin,  wastewater  treatment
plant  is shown  on  Figure 9-30.   In January  1974, Brookfield
commenced treatment  by the  contact stabilization activated  sludge
process.   Addition  of  ferrous sulfate  from pickle liquor  for
phosphorus  removal  in  the aeration  tank was  initiated in  June
1976.   The  plant has one  fixed-volume,  recessed  plate  pressure
filter with  a  design  capacity  of  530  pounds  dry solids  per  hour
(241 kg/hr).

Performance

The  pressure  filter   is  generally operated  four days per  week,
16 hours per day, 45  weeks  per  year.  The other 7 weeks per year,
the  sludge  is  applied to land.  Figure 9-31 summarizes operating
performance before (letter B)  and after  (letter A)  the  addition
of  ferrous  sulfate.    Figure  9-31  also presents  a mass  flow
diagram of an operating  recessed plate pressure filter.
                               9-59

-------
                               RETURN ACTIVATED SLUDGE
                          PRIMARY SLUDGE
SCRUBBER
WATER
                  DIGESTER SUPERNATANT
                                                 SECONDARY
                                                 CWASTE ACTIVATED!
                                                 SLUDGE
                               SLUDGi
                             CONDITIONING
                               TANK
ALTERNATE
DISPOSAL Of
LIQUID SLUDGE
BY TANK TRUCK
    LANDFILL
                            FIGURE 9-30

            BROQKFIELD, WISCONSIN WASTEWATER TREATMENT
                        PLANT FLOW DIAGRAM
The 1976  operating  and maintenance costs for  the  pressure  filter
are combined with the  incinerator  operational  cost in Table 9-30.
With the  initiation of chemical addition for  phosphorus  removal,
the  cost  of  treating and  disposing  of  a  ton  of  dry  solids
decreased  by  approximately $1.33, as  shown in Table  9-30.   This
reduction  was due  to decreases in the amounts of  chemical  condi-
tioners  and  electricity  required  by  the  plate pressure  filter.
These decreases were,  however, partially offset  by an increase in
the amount of  auxiliary fuel  used by  the  incinerator.   This  was
the result of decreased incinerator  volatile solids  feed  rates.


        9.4.6.6  Cost

Figure  9-32  gives  fixed-volume,   recessed  plate pressure  filter
capital cost  as a  function  of  press  volume.  Costs  include
those  for filter  auxiliary  equipment, piping,  and building.
As an  example,  a pressure filter  having 100  cubic  feet  (2.8  nH)
capacity  would  cost about $700,000.   Since this  number  is based
on June 1975 cost,  it must  be  adjusted  to the  current  design
year.
                                9-60

-------
                                                                FILTER CAKE


SLUDGE TO
PRESSURE FILTER
QA = 328,000 gal/mo
TSA = 131,000 Ib/mo
%TSA = 4.77
QB •= 395,000 gal/mo
TSB = 116,000 Ib/mo
%TSB = 3.54
CONDITIONING ADMIX
ASH FECL3 LIME
79,000 Ib/mo 1,810 gal/mo 32,400 gal/mo
0.60 Ib ASH/ 8,840 Ib/mo 22,600 Ib/mo
Ib DRY SOLIDS 135 Ib FECLj/ 346 Ib LIME/
I TON DRY SOLIDS TON DRY SOLIDS
\ 1 i
98,000 Ib/mo 1,770 gal/mo 28,800 gal/mo
0.85 Ib ASH/ 8,280 Ib/mo 20,100 Ib/mo
Ib DRY SOLIDS 152 Ib FECL3/ 345 Ib LIME/
TON DRY SOLIDS TON DRY SOLIDS
Q= FLOW
TS = TOTAL SOLIDS
VS = VOLATILE SOLIDS
FS = FIXED (NONVOLATILE) SOLIDS —
%TS
%VS
%FS
= PERCENT DRY TS BY WEIGHT ""
= PERCENT DRY VS BY WEIGHT
= PERCENT DRY FS BY WEIGHT
TOTAL:
SLUDGE PLUS
ADMIX TO FILTER
WET CAKEA = 506,000 Ib/mo
TSA = 219,000 Ib/mo
VSA = 71,000 (32.6% OF Ib TS]
FSA = 148,000
%TSA > 43.4
%VSA= 14.1
%FSA = 91%
WET CAKEB * 421,000 Ib/mo
TSB = 182,000 Ib/mo
VSB = 61,000 (33.6% of OF Ib TS)
FSB = 121,000
%TSB - 43.2
%VSB = 14.5
%FSB = 28.7
QA = 362,000 gal/mo PRESSURE
TSA - 240,000 Ib/mo _.J.'t_3 	
%TSA = 7.95 AFTER
90 runs/mo
1.73 hrs/run
1 55 hrs/mo
QB = 426.000 gal/mo BEFORE
TSB = 243,000 Ib/mo ~}g run s/n7o^
%TSB = 6'85 2.83 hrs/run
232 hrs/mo
FILTRATE
QA = 328,000 gal/mo
TSA = 21,000 Ib/mo

QB = 397,000 gal/mo
TSB - 62.0OO Ib/mo
1 Ib = 0.454 kg
1 gallon = 3.78 I
                                FIGURE 9-31

               PERFORMANCE DATA FOR A PRESSURE FILTER
                         BROOKFIELD, WISCONSIN
                                 TABLE 9-30

        PRESSURE FILTRATION AND INCINERATION OPERATIONAL COST
                                                      1976 Dollar cost
                                                     per  ton  dry solids
   Item

 FeCl3
 Lime
 Natural gas0
 Electricity
 Labor

 Total'
                     Unit cost,
                   1976  dollars
                               Before
                                            After
0.0305
0.001786
0.04
6.00
  Includes incinerator warm-up.

 1 ton = 0.907 t
9
10
ri
10
20
$62
.61
.52
.73
.40
.00
.26
8.69
10.55
12. 29
9.60
20.00
$61.13
                                    9-61

-------
-g
U)
U>
u
E
CCS
Z
o
   10,000,000
        9
        a
        7
        6
        5
1,000,000
     §
     8
     ?
     6
     i
     4
                                  I
                                                I
                   3  4  56789 TOO   2   3  4  S 6 7891,000   2

                       SINGLE PRESS VOLUME, cu ft fl cu ft = 0.02B m3)
                                                   3 4 5 6 7B9
                            FIGURE 9-32

             ESTIMATED JUNE 1975 COSTS FOR FIXED VOLUME
                RECESSED PLATE PRESSURE FILTERS  (39)
Figure  9-33  indicates fixed-volume,  recessed plate  pressure
filter  labor requirements.   Labor requirements  are  based on
continuous, seven-day-per-week operation with  two-hour cycles  and
include  operation  and maintenance  for both press and related
auxiliaries  (chemical  feed system  and  pumps).    As an  example,
a pressure filter  having 100 cubic feet (2.8 m^) of  capacity
would require  8,000 man-hours of  operation  and maintenance  per
year and would be included in the cost  analysis.


Figure 9-34 gives power consumption  as  a  function of feed  solids
concentration and  operating  volume.   The graph  is  based on a
filter  that operates continuously, seven  days per week,  and
has  a  2-hour cycle  time.   Power  consumption  includes   that  for
the  feed pump,  open  and  close mechanisms,  and moveable head
mechanism.


Figure  9-35 presents  a graph developed for estimating annual
material and maintenance costs for a  fixed-volume, recessed  plate
                               9-62

-------
pressure filter.   The graph is based on  unit  operation of seven
days per week with a two-hour cycle time.
 DC
 O
 LL
 Vt
 (E

 O
 X
 O
 z
       2 -
              2   3456789100    2   34 567891,000   2   3 456789

                AVERAGE FILTER PRESS VOLUME m USE, cu ft (1 cu ft = 0,028 m3}

                            FIGURE 9-33

          ANNUAL O£M MAN-HOUR REQUIREMENTS - FIXED VOLUME
                 RECESSED PLATE PRESSURE FILTIR (39)



    9.4.7  Screw and Roll Press
        9.4.7.1  Screw Press

This dewatering device employs a screw surrounded by a perforated
steel (screen) cylinder.   Sludge  is  pumped  inside  the screen and
is deposited against  the screen wall  by  the rotating  screw.   The
cake that forms acts as a continuous filter.  The screw moves the
progressively dewatered  sludge  against  a containment at  the
outlet and  further  dewaters the sludge by
action  against the  restriction.   Figure
layout from one screw press manufacturer.
municipal   wastewater  treatment  plants
operation,   large-scale  studies  have  been
lists typical results.
 pressure of  the  screw
 9-36 shows  a typical
 Although  no  full-scale
 are  known  to  be  in
conducted.  Table  9-31
                               9-63

-------
«
II
^.
i
5
1
Q
t


I
«
UJ
1
z
z
te
8
_j
D
Z
z
    100,00?
        §
        8
        7
        6
        S
        4
        3
                 2   3  45678B100    2   3  4 5 6 7 89 1,000   2  3  4 5 6 7BS

                   AVERAGE FILTER PflESS VOLUME IN USE, cu ft (1 cu ft = 0.028 m3)

                                FIGURE 9-34

                 FIXED VOLUME RECESSED PLATE PRESSURE
                     FILTER POWER CONSUMPTION (39)
                I
                2   34 56789100    2   34 667891,000   1   34 B67S9

                  AVERAGE FILTER PRESS VOLUME IN USE, cu ft (1 cu ft = 0.028 m3)

                                FIGURE 9-35

            ESTIMATED JUNE 1975 ANNUAL MAINTENANCE MATERIAL
        COST-FIXED VOLUME, RECESSED PLATE PRESSURE FILTER (39)
                                   9-64

-------
 SLUDGE FEED
                                                                    WASH WATER


/MER '
CTOR
5SEL
f.
r
i
i
i
i
i
1 i
1
1
1
1


C
(.
I
(OPTIONAL)
»_•— "->!
N
:
!>
-— %_
i
'SLUDGE
CAKE
*^~^~- SCREW
FILTRATED

9 1
1
FILTRATE x-M I
PUMP (^J ,
                                       L	
                                    FIGURE 9-36

         SYSTEM SCHEMATIC FOR ONE TYPE OF SCREW PRESS SYSTEM
    Location
                                    TABLE  9-31

                  PERFORMANCE RESULTS FROM A SCREW PRESS
                    Sludge type
 Stratford, CT   Primary only
Feed
solids,
percent
3-5
Polymer,
Ib dry/ton
dry solids
0
Cake
solids,
percent
25-31
Filtrate,
percent
solids
0.9-1.4
Reference
74
 Norwich, CT
Primary plus waste-
•  activated
    50:50 mixture
    67:33 mixture

Anaerobically digested
  mixture 60 percent
  primary plus 40 percent
  waste-activated
                                      3-3.3
                                     2.7-4.0
                                     5.5-9.8
                                               3.9-5.6
13-17     0.7-2.0
20-27     0.7-2.0
                                                        18.6-22.6
                                                                   0.2-1.0
                                                                                75
1 Ib/ton =0.5 kg/t
                                         9-65

-------
        9.4.7.2  Twin-Roll Press

Figure 9-37 shows a cross section of a twin-roll, vari-nip press.
Developed  in  1970  by modifying  a  fixed  nip twin-roll press,  the
vari-nip press was  installed  in  17  plants by 1976.   One of  these
plants is municipal (76).
  HOOD
  CAKE DOCTOR
  AND SEAL
                                                       SHREDDER
                                                       CONVEYOR
  MOVEABLE
  ROLL

  PRESSATE
  CHANNELS
  PRE-THtCKENlNG
  MODE
                                                    ROLL CLEANING
                                                        SHOWERS
                                                           PRESS
                                                           ROLLS
       FIXED
        ROLL
VARIABLE SPEED
VAT AGITATOR
                                     VAT
                      SLUDGE FEED

                            FIGURE 9-37

          CROSS SECTION VIEW OF A TWIN-ROLL VARI-NIP PRESS
The unit  consists  of a pair of  perforated  rolls,  one roll fixed
and the  other moveable,  so  that the nip  (or  space)  between the
rolls  can be  varied.   The horizontal rolls  are mounted in a
sealed  vat.   Sludge is  pumped  into  the vat  under  a  slight
pressure of two  to  four  pounds  per square inch (14 to 28 kN/m^).
This low  vat pressure  moves the  sludge into the  nip,  where  it
is further dewatered  by  a nip  pressure  load of 200 to 400 pounds
per lineal inch  (36 to 72 kg/lineal cm)  of  roll length.  Filtrate
passes from the   sludge  through the perforated  rolls  and
discharges by gravity.   The  compressed  cake is then doctored off
the rolls and discharged  into a shredder and conveyor.

The  "Pig's Eye  Plant"  at  St.  Paul,  Minnesota  has evaluated
the  dewatering  of mixtures  of  primary  and  waste-activated
sludge  (76).   Results showed that  on raw  primary  sludge,  a cake
                               9-66

-------
of 35  percent was  obtainable  after  sludge conditioning with
approximately seven pounds of dry  polymer per ton (3.5 kg/t) of
dry feed solids.   When biological  sludge was added, performance
decreased  and polymer requirements  increased.   At  a  mixture
of 50:50,  cake  solids dropped to  28 percent,  while
requirements increased to  17  pounds of dry  polymer
(8.5  kg/t)  of dry feed  solids.  The  conclusion was that
an excellent dewatering unit  for primary  sludge.
                                  polymer
                                  per  ton
                                  this  was
    9.4.8  Dual Cell Gravity (DCG)  Filter

The  DCG unit  consists  of  two  independent  cells formed  by a
nylon filter  cloth.   The cloth  travels  continuously  over guide
wheels and is  rotated  by  a  drive roll and sprocket assembly.  A
cross section of a  typical DCG unit is  shown  in  Figure 9-38.
Dewatering occurs in  the  first cell,  and  cake formation, in  the
second cell.
          GUIDE
          WMEEL
DRIVE ROLL AND
SPROCKET ASSEMBLY
NYLON
FILTER CLOTH
                                                    GUIDE WHEEL
CAKE FORMING
CELL
                                                         DEWATERING
                                                         CELL
                                                            SLUDGE
                                                            fNLET
                            FIGURE 9-38

          CROSS SECTION VIEW OF A DUAL CELL GRAVITY FILTER
Sludge  is  introduced  in the  dewatering  cell,  where  initial
liquid/solids separation takes place.  The  dewatering solids are
then carried over the drive roll separator  into the second cell.
Here,  they are continuously rolled  and formed  into  a  cake of
relatively low moisture content.   The weight  of this sludge
cake  presses  additional  water from the  partially  dewatered
sludge carried over  from  the  dewatering  cell.   When the cake of
dewatered  solids grows  to a certain size,  excess quantities are
discharged  over  the  rim  of  the second  cell to  a conveyor belt
that moves the material  out of the  machine.
                               9-67

-------
Table  9-32  summarizes  the  operating  results from  Mentor,  Ohio,
which  has three  units to dewater an aerobically  digested mixture
of primary, waste-activated sludge  and a mixture of  primary,
waste-activated,  and  alum  sludge  generated   from  phosphorus
removal.
                             TABLE 9-32

          SUMMARY OF PERFORMANCE RESULTS FOR A DUAL CELL
                 GRAVITY FILTER - MENTOR, OHIO (61)
                                        Primary
                                       olus waste
                                     activated sludge
               Primary plus
              waste activated
             plus alum sludge
 Feed - percent total solids
 Cake - percent total solids
 Polymer usage
   Cationic - liquid Ibs per ton solid
   Anionic - dry Ibs per ton solids
 Filtrate characteristics
2.1-2.7
8.8-9.2

  143
  0.4
2.5-3.1
8.2-9. 1

 136
 0.04
                                               Not given
 1 Ib/ton =0.5 kq/t
    9.4.9  Tube Filters

Tube  filters  can  be either  of  the  pressure  type  or of  the
gravity type.


        9.4.9.1  Pressure Type

Commonly known as tube filter presses,  pressure type tube filters
have been used in industry  (77).   However,  there are no municipal
installations.   Typically,  this  type  of  device  consists of  an
outer  cylinder,  an  internal  rubber  bladder, and an  internal
perforated  cylinder which  is  covered  with a filter media.   The
whole assembly is mounted vertically.

Slurry  is  pumped  into  the  annular space between the bladder
and media-covered  wall.   When this area is  full, the bladder
is  filled  with  liquid,  and the  slurry is compressed against
the  filter  media.   Filtrate  flows  through  the  media and  is
discharged.  When the desired  cake solids  concentration has  been
obtained, liquid  pressure is released  and  the  cake  is  discharged
with a blast of air.

        9.4.9.2  Gravity  Type

In  this  application,  sludge  is mixed with polymer  and  then  held
in  suspended porous bags.   The weight  of  the sludge forces  water
out  of  the  bag  sides and  bottom.   Sludge  is retained for  a
maximum of  24  hours, depending upon  the desired dryness, and  is
then released through a bottom  opening.
                                9-68

-------
Following is a  description  of  the 0.5-MGD (21.9 1/s)  dewatering
facility at Half Moon Bay, California.

This  facility  consists of four  bags, each  3  feet  (0.9  m)  in
diameter  and 9  feet (2.7  m)  long with  a ring  at the top  to
support the polyester media bag and a  ring at the bottom,  which
is engaged circumferentially by a motor-driven chain.   The  chain
twists the ring  about 360  degrees,  thereby  closing off the bottom
so that the bag can be filled.   Suspended down the center of the
bag is a  polyester  tube about  6  inches (15 cm)  in diameter with
the end  extending  about 12  inches (0.3 m)  beyond  the bottom  of
the closed ends.  All four  bags  are mounted  outdoors  on a  steel
framework over  a  concrete pad  containing  the  drainline  and
chemical conditioning system.   The sludge fills the annular core,
and the filtrate seeps through the outer polyester media surface
and the inner core  tube.

The batch  operation  practiced  at  Half  Moon Bay is  on a 24-hour
cycle  consisting of  a  four-hour fill period (waste-activated
sludge from a complete  mix  aeration plant) and  a  20-hour drain.
With a 1.5 percent  solids  feed,  a  16 percent solids cake has been
obtained.


9.5  Other Dewatering Systems

Several other types  of  dewatering devices  are available that  do
not readily fall into  any  of  the previously discussed units.
These include cyclones,  screens, and electro-osmosis.


    9.5.1  Cyclones

In the  municipal wastewater  field, cyclones or hydrocyclones
(name given to  cyclones specifically designed for liquids) have
been  used  for  cleaning  and  dewatering grit  from  grit chambers,
primary  clarifiers,  and  anaerobic  digesters  since  the  early
1950s.  Since then,  over 1,400  units have been installed (43).

When  a  liquid   stream  enters a  cyclone,  the  particles  are
separated by centrifugal  acceleration.   Unlike centrifuges,
cyclones have no moving  parts.   The liquid  motion inside the unit
causes  the  necessary acceleration.  The  theory of cyclones  is
thoroughly covered  in a  recent  discussion by Svarovsky (78).

By itself,  a  cyclone does not dewater.  The  underflow  from the
cyclone discharges  into  a  type  of  dewatering device.  This device
may be  as  simple  as a steel  bin with  drainage  holes,  or  as
complex as  a  rotating screen  screw  or rake  classifier.   These
dewatering devices will produce  a grit with  a  moisture content
ranging from 20  to  35 percent.

The degritted liquid stream (overflow)  from a cyclone  degritting
raw sludge normally goes  to  a  gravity  thickener.   When  the
cyclone is degritting the flow from  grit chambers,  the  overflow


                               9-69

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is usually  recycled  to the grit  chamber.   Some designers have
found it  necessary  to  screen  this overflow to keep debris from
overwhelming the  system.  The drainage from the dewatering  device
is collected and typically  returned to the  head  of  the  treatment
plant.


    9.5.2  Screens

"Screening  is  the process  of  separating  grains,  fragments or
lumps of  a  variety of  sizes into  groups, each of which  contains
only particles  in  the size range between  definite maximum and
minimum  size  limits"  (79).    In  addition to  being  used  in
dewatering   (26),  screens  have  also  been  used  for  primary
treatment  (80),  thickening   (81,82),  and  conditioning (see
Chapter 8).

The  primary use  of  screens  in  dewatering  would be  with bar
screenings or the underflow  from grit cyclones.   In  one  extensive
study (83),  the following results  were found:

     •  Ground  bar screenings  could be dewatered to six percent
        solids  with  a static type  screen.

     •  Ground  bar  screening  could  be dewatered to  sixteen
        percent solids  with  a revolving drum screen.

     •  Underflow from a  grit cyclone  could be  dewatered to
        25  percent  solids  with either the static  or  revolving
        screen.

The  popularity of screens  is  slowly increasing  in  the United
States because in certain applications they offer  advantages in
both capital cost and operating cost.
    9.5.3  Electro-Osmosis

The use  of electro-osmosis  for dewatering municipal  wastewater
sludge has been studied on a pilot-plant  scale  (84).   The  system
consists  of  a vertical-mounted,  endless moving  belt which is
drawn over vertical plate-mounted, stainless  steel cathodes,
submerged  in a tank of waste  sludge.  Results indicated that
cakes of  over  20 percent  solids  could be  obtained  from an
anerobically  digested sludge having 2.6 percent  feed solids.


9.6  References

 1.  Craig, E.W.,  D.D.  Meredith,"and A.C.  Middleton.   "Algorithm
     for Optimal  Activated Sludge  Design."
     Environmental Engineering  Division, ASCE.
     p.  1101.   1978.
                              9-70

-------
     Dick, R.I.  and  D.L.  Simmons.    "Optimal  Integration  of
     Process  for Sludge  Management."   .Proceedings  3rd  National
     Conference  on  Sludge  Management  Disposal  and  Utilization,
     Miami Beach,  FlT^ 12/14-16/76 ,  sponsored  by ERDA,  USEPA,
     NSF and ITI,  p.  20,  Information  Transfer  Inc.,  Rockville,
     Maryland 20852.

     USEPA.   Cost  of Landspread ing   and  Hauling  Sludge  from
     Municipal Wastewater Treatment  Plants.   Office o~f  Sol id
                         ~~    ~~
     WasTte .   Wa¥hTngTol^~lxrTbTiro~  E~PA~530/SW-619 .  October
     1977.

 4.   Carry,  C.W.,   R.P.  Miele,  and  J.F.  Stahl.    "Sludge
     Dewatering."   Proceedings  of the National Conference  on
     Municipal  Sludge Management.    Pittsburgh ,   PA,  6/11-13/74 .
     Sponsored  by  Allegheny  County,  PA,  p.  67,  Information
     Transfer  Inc.,  Rockville,  Maryland  20852.

 5.   Ohara,  G.T.,  S.K.  Raksit,  and D.R. Olson.   "Sludge
     Dewatering Studies  at  Hyperion Treatment Plant."   Journal
     Water Pollution Control  Federation.  Vol. 50, p. 912 ( 1978) .

 6.   USEPA.   j?ilot  Investigation  of Secondary  Sludge Dewatering
     Alternatives^   Industrial  En vi ronmen tal  Re se arch   Lab ,
     Cincinnati,  Ohio 45268.    NTIS PB-280-982,  February  1978.

 7.   USEPA.    Evaluation  of  Dewatering Devices for Producing High
     Solids  Sludge Cake.    Office  of  Research   and  Development.
     Cincinnati, Ohio 45268.   EPA  600/2-79-123.   February 1979.

 8.   Cassel,  A.F.  and  B.P.  Johnson.   "Evaluation  of  Dewatering
     Units  to  Produce  High  Sludge  Solids Cake."   Presented
     at  the  51st  Annual  Conference  Water Pollution Control
     Federation.   Anaheim, California.   October 2, 1978.

 9.   Zenz,  D.R.,  B. Sawyer,  R. Watkins, C.  Lue-Hing, and
     G.  Richardson.   "Evaluation of  Unit Processes for Dewatering
     of  Anaerobically Digested Sludge at Metro Chicago's Calumet
     Sewage Treatment Plant."   Presented at  the 49th Annual
     Conference Water Pollution Control  Federation.  Minneapolis,
     Minnesota. October  1976.

10.   USEPA.    Operations  Check Lists.    Office  of  Water  Program
     Operations.   Washington,  DC  20460.   MCD  48B.   February 10,
     1977.

11.   USEPA.      Cost Estimates  for  Construction of Publicly  Owned
     Wastewater Treatment  Facilities  -  Summaries of  Technical
     Data.   Office  of Water  Program Operations .   Washington,  DC
     20460.  MCD 48B.  February 10,  1977.
                              9-71

-------
12.   USEPA.     Sludge  Handling  and Disposal Practices at Selected
     Municipal  Wastewater Treatment Plants.   Office of Water
     Program Operations.   Washington, DC  20460.   MCD  36.   April
     1977.

13.   Spillner,  F.   "The Drying of  Sludge."   J3gw_a_g e Sludge.
     London, England,  1912.

14.   USEPA.    Sludge Dewatering and Drying on Sand Beds.   Office
     of Research  and  Development,  Cincinnati,  Ohio 45268.   EPA
     600/2-78-141, August 1978.

15.   Eckenfelder,  W.W. and D.L.  Ford.   Water Pollution Control.
     Pemberton Press,  Austin, TX and New York, NY 1970.

16.   Walski,  T.M.  "Mathematical Model Simplifies  Design  of
     Sludge  Drying Beds."   Water  and  Sewage __Wg_rk_s_.   p.  64.
     April  1976.

17.   Water  Pollution  Control  Federation.   MOP  8 Wastewater
     Treatment Plant Design.  Water Pollution Control Federation.
     1977.

18.   USEPA.    Performance Evaluation  and  Troubleshooting  at
     Municipal Wastewater Treatment Facilities.   Office  of  Water
     Program  Operations.    Washington,  DC  20460.   EPA  430/
     9-78-002.  February  1978.

19.   USEPA.    Sludge Handling and Conditioning.    Office  of  Water
     Program  Operations,  Washington,  DC  20460.    EPA  430/  9-78-
     112.   February 1978.

20.   South,  W.T.   "Asphalt Paved Sludge Beds."  Water and Sewage
     Works.   Vol.  106,  p.  R396.   1959.

21.   Lynd,  E.R.   "Asphalt-Paved Sludge Drying Beds."  Sewage and
     IjTdjJS trjial^Was tes.   Vol. 28, p. 697.  1956.

22.   Lewing, V.H.   "Survey of Some Methods of Sludge Dewatering."
     The Surveyor.  Vol.  121, #3680, p. 1521.  1962.

23.   Swanwick,  J.D.  and Baskerville,  R.C.    "Dewatering  and
     Industrial Sludges on Drying Beds."   Chemistry and Industry.
     p. 338, February  20,  1965.

24.   Stokes,  F.E.   and J.M. Harwood.   "Aluminum  Chlorohydrate  in
     Sludge  Treatment."   Effluent  and Water  Treatment  Journal.
     Vol.  4, p.  329.   1964.

25.   Crockford, J.B.  and  V.R. Sparham.  "Developments  to Upgrade
     Settlement  Tank  Performance,  Screening,  and  Sludge
     Dewatering Associated with Industrial Wastewater Treatment."
     Proceedings  of 27th Purdue Industrial  Waste Conference,
     Purdue  University, Lafayette, Indiana 47907.  1972.


                               9-72

-------
26.   U.S.  Department  of  Interior.  A Study of Sludge Handling and
     Disposal.   Federal  Water Pollution  Control  Administration,
     Office of  Research  and Development No. WP-20-4.  May 1968.

27.   USEPA.   "Developments   in  Dewatering Wastewater  Sludges."
     Technology  Transfer  Seminar on  Sludge  Treatment  and
     Disposal.   Vol.  1.   Technology  Transfer.  Cincinnati,
     Ohio 45268.  October  1978.

28.   Beardsley,  J.A.   "Sludge Drying Beds Are Practical."  Water
     and Sewage Works.   Part 1, p.  82, July;  Part 2, p.42.
     August (1976).

29.   Thompson,  L.H.   "Mechanized  Sludge  Drying Beds."    The
     Engineer.   July  1966.

30.   Kershaw, M.A.   "Development  in Sludge Treatment and Disposal
     at the Maple Lodge  Works, England."  Journal Water Poj-jjjJbj.C)n
     Cgrvy^j1_Federati_on_.   Vol. 37, p. 674.  1965.

31.   Water Pollution  Control  Federation.    MOP   20  Sludge
     Dewatering.   Water  Pollution  Control  Federation.   1969.

32.   Jeffrey, E.A. and P.F.  Morgan.   "Oxygen Demand  of Digested
     Sludge  Liquor."   Sewage and Industrial Wastes.   Vol.  31,
     p. 20.  1959.

33.   Imhoff, K. and  G.M.  Fair.   Sewage Treatment.  John  Wiley  &
     Sons,  New  York,  New York.  1956.

34.   Water Pollution Control Federation.   MOP 8 Sewage Treatment
     Plant Design.   Water  Pollution Control Federation.  1959.

35.   Haseltine,  T.R.    "Measurement  of  Sludge Drying  Bed
     Performance."    Sewage  and Industrial Wastes.   Vol.  23,
     p. 1065.  1951.

36.   Recommended Standards for Sewage Works.   Great  Lakes/Upper
     Mississippi River Board  of State Sanitary Engineers,  1971.

37.   USEPA.   Areawide Assessment Procedures  Manual - Volume III.
     Municipal   Environmental  Research  Laboratory.   Cincinnati,
     Ohio 45268.  EPA 600/9-76-014.  July 1976.

38.   USEPA.  Construction Costs  for  Municipal Wastewater
     Treatment  Plants.    Office of  Water  Program Operations.
     Washington, DC  20460.  MCD 37.  January  1978.

39.   Culp/Wesner/Culp.    Cost  and  Performance  Handbook Sludge
     Handling Processes.   Prepared for Wastewater Treatment and
     Reuse Seminar,  South  Lake Tahoe, California.  October 1977.
                               9-73

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40.   Jeffrey,  E.A.    "Laboratory  Study  of  Dewatering  Rates
     for Digested  Sludge  in  Lagoons."  Proceedings of 14th Purdue
     Industrial Waste  Conference^   Purdue University,  Lafayette,
     Indiana 47907.   1959.

41.   Jeffrey,  E.A.   "Dewatering Rates  for  Digested Sludge  in
     Lagoons."   Journal  Water _Polluj^io_n_^Cgntrol Federaticm.
     Vol. 32,  p.  1153.   1960.

42.   Reefer,  C.E.  and  H.  Krotz.   "Experiments on Dewatering
     Sewage Sludge With  a  Centrifuge."   Sewage  Works  Journal,
     Vol. 1, p.  120.   1929.                          """"  "~

43.   Taken from equipment manufacturers installation lists.

44.   Hansen, B.E., D.L.  Smith,  and  W.F.  Garrison.    "Start-up
     Problems  of Sludge  Dewatering  Facility."   Presented at the
     51st  Annual  Conference Water  Pollut i onL_Cgn t .EX)jL_Fe_de. r;a t i.on .
     Anaheim,  California.  October 1978.     _____

45.   USEPA.  Handling  and Disposal of Sludges From Combined  Sewer
     Overflow  Treatment  Phase  III  -  Treatability  Studies.
     Environmental  Protection  Technology  Series,  Office  of
     Research  and Development,  Cincinnati,  Ohio 45268.   EPA-600/
     2-77-053C,  December  1977.

46.   Zacharias,  D.R.  and K.A. Pietila.   "Full-Scale Study  of
     Sludge Processing  and Land Disposal Utilizing Centrifugation
     For Dewatering."   Presented at the 50th  Annual Meeting  of
     the  Central States  Water  Pollution Control Federation,
     Milwaukee,  Winconsin.   May 18-20, 1977.

47.   Albertson,  O.E.  and E.E. Guidi, Jr.   "Centrifugation  of
     Waste Sludges."   Journal Water  Pollution Control Federation.
     Vol. 41,  p.  607.   1969.

48.   Camp, Dresser & McKee,  Inc.  Centrifugal Dewatering of  Waste
     Activated Sludge.   Report  on testing and equipment proposals
     for Jones  Island Wastewater  Treatment Plant, Milwaukee,
     Winconsin.   October  1977.

49.   Guidi, E.J.  "Growth and  Benefits  of  Low Speed Centrifuga-
     tion."  Water and  Sewage Works.  June 1977.

50.   Personal  communication with Mr.  F.W.  Keith,  Jr.,  Director
     of  Environmental  Technology,   Sharpies-Stokes,  Warminster,
     Pennsylvania.  April 1979.

51.   Svarovsky,  L.   "Filtration Fundamentals."  Solid-Liquid
     Separation,  Butterworths,  Inc., Ladislav Svarovsky,  editor,
     1977.
                               9-74

-------
52.  Darcy,  H.P.G.   "Les Fontaines Publiques de la Ville de Dijon
     (The Public Wells  of  the City of Dijon)."  V. Dalmont Paris,
     1856.   English translation by  J.J.  Fried.   Water Resources
     Bulletin  American  Wate^^Res^ou^rcqsAjssqciationTVo 1.  1,  p. 4 .
     1965.

53.  Masters,  A.L.   "Filter  Aids."  Solid-Liquid Separation,
     Butterworths,  Inc.    Ladislav Svarovsky,  editor, 1977.

54.  Basso,  A.J.   "Getting  the  Most  Out  of  Filter  Aids."
     Chemical  Engineering.   p.  185.  September 12, 1977.

55.  NCASI.    A Pilot  Plant  Study of  Mechanical  Dewatering
     Devices Operated  on  Waste Activated Sludge.  Prepared  for
     National  Council  of  the  Paper  Industry for  Air  and Stream
     Improvement.  TechnicalBulletin 288, November 1976.

56.  Flynn,  E.O.  "The Mechanical Dewatering of Sewage Sludge  on
     Vacuum Filters."  Sewage Works Journal.   Vol.  5,  p.  957
     1933.

57.  Leary,  R.D.,  L.A.  Ernest, G.R. Douglas,  A.  Geinopolos  and
     D.G.  Mason.   "Top-feed  Vacuum  Filtration  of  Activated
     Sludge."    Journal  Water Pollution  Control  Federation.
     Vol. 46.  p. 1761.   1974.

58.  Purchas,  D.B.   "Filtration in  the Chemical  and  Process
     Industries - 1,"   Filtration.   p. 256.  1964.

59.  Vesilind,   P.A.    Treatment  and  Disposal  of  Wastewater
     Sludges.   Ann Arbor  Science.   Ann  Arbor, Michigan 48106.
     1974.

60.  Bennett,  E.R., D.A.  Rein,  and K.D.  Linstedt.    "Economic
     Aspects of  Sludge Dewatering and Disposal."  Journal of the
     Environmental Engineering Division  ASCE.   Vol.   99,  p.  55.
     1973.

61.  USEPA.   Review of Techniques  for  Treatment  and  Disposal  of
     Phosphorus-Laden  Chemical Sludges.    Office  of Research  and
     Development.   Cincinnati, Ohio 45268.   EPA 600/2-79-083.
     February  1979.

62.  Goodman,  B.L.  and   R.B. Higgins.   "A  New  Device  for
     Wastewater Treatment Sludge  Concentration."   Water  and
     Wastes Engineering.   August  1970.

63.  Goodman,  B.L. and R.B. Higgins.   "Concentration of  Sludges
     by Gravity and Pressure."   Proceedings  of 25th Purdue Indus-
     trial Waste Conference.  Purdue University,  West Lafayette,
     Indiana 47907.   1970.

64.  Dembitz,  A.E.   "Belt  Filter Press:    A New  Solution
     to  Dewatering?"   Water  and Wasjtes Engineering.   p.  36.
     February  1978.                     ~


                               9-75

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65.   Austin, .E.P.    "The  Filter  Belt Press  -  Application  and
     Design."   Filtration and  Separation.   p.  320.   July/August
     1978.

66.   NCASI.    A Review  of  the Operational  Experience  with  Belt
     Filter  Presses for Sludge Dewatering  in 'the North American
     Pulp and  Paper Industry.   Prepared  for National  Council
     of  the  Paper  Industry  for Air  and  Stream Improvement.
     Te£hnJ£a^Bulletin  315.   October  1978.                   ~

67.   Eichman,  B.W.   "Dewatering Machine  Solves Sludge  Drying
     Problems."  Water  and Sewage^Jgp_rks .    p.  99 (October 1977).

68.   USEPA.    Innovative  and  Alternative  Technology  Assessment
     Manual  -  Draft.   Office  of  Water  Programs.   Washington,  DC
     20460.  MCD  53.   1979.

69.   Wake man, R.J.    "Pressure Filtration."  Sol id-Liquid
     Separation.  Butterworths, Inc.,  Ladislav Svarovsky, editor,
     1977.

70.   Nelson, O.F.   "Operational Experience  with  Filter Pressing."
     Water  Pollution  Controjl^^ede^ration - Deeds  and Data.   March
     1978.

71.   Bizjak,  G.J.  and  A.E.  Becker,  Jr.   "Wausau  Solves  Dual
     Problem   by  Using  Filter  Press."   W_a t e r__ a n d  Wastes
     Engineering .   p.  28.  February  1978.

72.   USEPA.    Pressure  Filtration of Wastewater  Sludge  With  Ash
     Filter  Aid.  Office of Research and  Development, Cincinnati,
     Ohio 45268.  EPA-R2-73-231 .   1973.

73.   Farnham  Pollution  Control  Works,  Thames  Water  Authority,
     England.   1977.

74.   Bechir,  M.H.  and  W.A.  Herbert.   "Sludge  Processing  Using
     Som-A-Press. "   Presented  at the New England Water Pollution
     Control Association.  October 1976.

75.   Taylor,   J.A.     Evaluation  of  Somat Som-A-System Dewatering
     Method for  the Norwich Water  Pollution  Control  Plant,
     Norwich, Connecticut.    Somat  Corporation.   Pomeroy,
     Pennsylvania.   July 1978.

76.   Bergstedt,  D.C.  and  G.J.  Swanson.   "Evaluation of  A
     Twin-Roll Continuous  Press For  Municipal Sludge Dewatering."
     Presented at  the  49th Annual Conference  Water P
     Control Federation,  Minneapolis, MN .  October 1976.

77.  Gwilliam,  R.D.   "The E.C.C. Tube Filter Press."  Filtration
     and Separation.  March/April  1971.

78.  Svarovsky, L.   "Hydrocyclones . "   Solid-Liquid Separation,
     Butterworths,  Inc.   Ladislav  Svarovsky,  editor.   1977.


                               9-76

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79.   Osborne,  D.G.   "Screening."    Solid-Liquid  Separation,
     Butterworths,  Inc.  Ladislav Svarovsky, editor.   1977.

80.   Phillips,  T.G.   "Screening ... A Novel Approach to  Primary
     Treatment."   Presented at  Ontario Pollution Control
     Association annual meeting, Toronto, Canada.   April 1977.

81.   Fernbach,  E.  and G. Tchobanoglous.    "Centrifugal  Screen
     Concentration for Activated  Sludge  Process."   Water and
     Sewage Works.   Part I.  January; Part  II.   February 1975.

82.   Syal,  R.K.  "Compare Sludge Handling  Alternatives."   Water
     and Wastes Engineering.  Vol. 16, p. 60.  March  1979.

83.   Brown and  Caldwell Consulting  Engineers.   Study of
     Wastewater Solids  Processing and Disposal.  Prepared  for the
     Sacramento Regional  County  Sanitation  District.   June 1975.

84.   Dewatering Sewage  Sludge  by Electro Osmosis.  Part I  -
     Basic  Studies,  Part  II  - Scale Up Data.   Prepared  by the
     Electricity Council  Research  Centre,   Capenhurst, England.
     NTIS PB-276 and NTIS  PB-276 412, 1975  and 1976.
                              9-77

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EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
               Chapter 10.  Heat-Drying
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                           CHAPTER 10

                           HEAT-DRYING


Heat-drying is  the  process of evaporating water  from  sludge  by
thermal means.   Ambient  air-drying  of  sludges is  discussed  in
Chapter 9,  and composting,  in  Chapter 12.


10.1  Introduction

In the United States, dry  waste-activated sludges  and those from
Imhoff tanks have been  heat  dried to produce  a soil conditioner
and nutrient source  since  the  early 1920s.  Historically, the use
of heat-drying has  been  justified based  on the expectation that
sales of  the  dried  material  would  substantially  offset process
costs.  However,  demand  for the product has generally been low in
the  fertilizer  market.    Milwaukee,  Wisconsin; Houston,  Texas;
Chicago,  Illinois;  and  Largo,  Florida,  are  notable  exceptions
where  marketing has  been successful.   Because  revenues  have
generally  been low and  because heat-drying  is expensive,  net
costs have  often  been high,  and the process  has  not  found wide
application.    The  use  of  heat-drying  must  be  evaluated  in
the context of overall sludge  management at a given facility.


10.2  Heat-Drying Principles

Sludge is  heat  dried  at temperatures  too  low  to destroy organic
matter.  Water vapor is  carried away by  a moist gas  (usually
air).   The  designer  establishes  the  actual  conditions  of
drying — for example,  temperature,  humidity,  detention  time,
velocity,  and direction  of  flow of the  gas  stream across the
drying surface.


    10.2.1   Drying Periods

The following are the  three well-defined stages  in heat-drying:

    1.  Initial Drying.    During  this  stage,  the  sludge
        "temperature  and  the  drying   rates  are  increased  to the
        steady state conditions of  the  second stage.   Stage one
        is  usually short;  little drying occurs during this time.

    2.  Steady State Drying.   The time that the  sludge is in this
        stage is generally the  longest  of all  the  stages.   The
        surfaces of  the  sludge particles are completely saturated
        with water.    Surface  water  is replaced with  water from
                              10-1

-------
        the interior of the  solid  as fast as it  is  evaporated.
        Drying  proceeds as if  the  water were evaporated from a
        pool  of liquid.  The  solid  itself does not significantly
        influence  the drying  rate.    For  this drying  period,  the
        temperature  at  the   sludge/gas  interface  is  ordinarily
        kept  at the  wet-bulb  temperature  of the gas.   As  long as
        unbound  surface  moisture  is  present,  the  solid is
        heated only to the  wet-bulb  temperature  of the gas;
        solids may  therefore  be dried  with fairly hot gases
        and not  themselves  attain  elevated  temperatures.    For
        example,  the wet-bulb temperature is  133°F  (56°C) for a
        gas stream that has  an  absolute  humidity  of  0.01  pounds
        water  per pound  dry air  and a  temperature  of 600°F
        (316°C).

    3.   Final  Drying.   The  final stage occurs when sufficient
        water  has evaporated  that  the  solid  surface is only
        partially  saturated.   Surface water  is  evaporated
        more  rapidly than it can be  replaced by  water from  the
        interior  of  the solid.   As  a  consequence,  overall  drying
        rates  are markedly  lower  in stage 3 than  in stage 2.
        During this period,  the temperature of  the solid/gas
        interface  increases  because  latent  heat  cannot be
        transferred  from the  sludge  to the gas phase as  rapidly
        as sensible  heat is received  from the  heating medium.

Sludge  moisture  content  is  normally  expressed   in percent
moisture,  percent solids,  or pounds  water per pound  dry  sludge.
The minimum sludge  moisture  content  practically  attainable with
heat drying depends upon the design  and  operation of the  dryer,
moisture content  of  the sludge feed,  and the chemical composition
of the sludge.   For ordinary  domestic wastewater  sludges,  sludge
moisture  contents  as  low  as  five  percent  may be achieved.
Chemical  bonding of water within  the sludge,  which can occur
through chemical  addition for  sludge conditioning,  or chemicals
present in industrial  sludges  can  increase  the  amount of water
retained in the dried  products  beyond  the  five  percent moisture
level.
    10.2.2  Humidity  and Mass Transfer


Humidity  is a  measure  of  the moisture content of the gas  phase
at  a  given temperature  and  is  important  to consider when
determining drying rates.   Absolute humidity  is  a measure  of  the
weight of water per  unit weight of  dry gas  (for example,  pounds
water per pound dry air).


In  heat-drying  of   sludge,  water  is transferred  to  the gas
phase.    The  driving  force  for  transfer  is  the  difference
between  absolute humidity  at the  wetted solid/gas  interface
                             10-2

-------
and  the absolute humidity in  the gas  phase.   The  transfer
rate — that is,  the drying rate—can be  described  by the  following
equation:


    W = KyA (Ys - Ya)                                      (10-1)


where:

    W  = rate  of drying,  pounds  water per  hour  (kg/hr);

    Ky = mass  transfer coefficient of the  gas phase, pounds water
         per hour per square foot per unit of humidity difference
         (kg/hr/m2/unit of humidity difference);

    A  = area  of wetted  surface  exposed  to  drying medium,
         square feet  (m2);

    Ys = humidity at  the sludge/gas interface temperature, pounds
         water per pounds dry gas (kg/kg);

    Ya = humidity  of  the  gas phase,  pounds  water per  pounds
         dry gas (kg/kg).


    10.2.3  Temperature and Heat  Transfer

In  heat-drying,  the  temperature  difference  between  the heating
medium  and  the sludge/gas  interface provides  the driving force
for heat transfer.

Dryers  are  commonly  classified  on  the  basis  of the predominant
method  of  transferring heat to  the wet solids being dried. (1).
These methods  include:

Convection (direct drying) .  Heat transfer  is  accomplished
by  direct contact between the  wet sludge  and  hot  gases.  The
sensible heat  of  the  inlet  gas provides the latent heat required
for evaporating  the  water.   The vaporized liquid is carried off
by the hot gases.  Direct dryers  are the most common  type used in
heat-drying of  sludge.   Flash dryers,  direct rotary dryers, and
fluid bed dryers employ this method.  Convective  heat transfer is
described by Equation 10-2.


    tJconv = hcA (tg - ts)                                  (10-2)


where:

    qconv = convective heat transfer, Btu  per hour  (kJ/hr);

    hc    = convective heat  transfer  coefficient, Btu  per  hour
            per square foot per  °F (kJ/hr/m2/°C);


                              10-3

-------
    A
          = area of  wetted  surface exposed to  gas,  square  feet
          = gas temperature,  °.F  (°C);

          = temperature  at  sludge/gas  interface, °F (°C).
Conduction (indirect drying).   Heat  transfer is  accomplished  by
contact  of the wet solids with hot surfaces  (for example,  a
retaining wall separates  the  wet solid  and the  heating medium).
The vaporized liquid  is  removed independently of the  heating
                  film dryer .employs this principle.  Conductive
                           by  Equation 10-3.
          wall
    vaporized
medium.   The  thin
heat transfer is described
                                                          (10-3)
where:
    ^cond = conductive heat  transfer, Btu per hour (kJ/hr);

    ncond
     m
          = conductive  heat  transfer  coefficient,  Btu  per  hour
            per°F (kJ/hr/°C);

          = area of heat  transfer  surface, square feet (m^);

          = temperature  of drying medium--for example,  steam,
            O Ui / O (~* \ .
               V ^ / /                            '

    ts    = temperature of  sludge  at drying surface, °F  (°C).

The conductive  heat  transfer  coefficient  (hconc3)  is  a composite
term that  includes  the effects  of the heat transfer  surface and
sludge-side and  medium-side films.   Descriptions  of  methods for
computing  hcon(~| are available  in   textbooks and  from dryer
manufacturers  (1-4).

Radiation (infrared or radiant heat-drying).    Heat  transfer  is
accomplishedbyradiant  energy  supplied  by  electric resistance
elements,  by gas-heated  incandescent refractories that  also
provide  the  advantage  of convective heating,  or by  infrared
lamps.   The Shirco Company furnace and multiple-hearth furnaces
are  examples  of drying  equipment  that  use  radiant heat.
Radiation heat transfer is  described by Equation 10-4.
    qrad = e s A a   (t
                        .- t
(10-4)
where:

    qrad = radiation heat transfer,  Btu/per hour  (kJ/hr);

    es   = emissivity of the  drying  surface, dimensionless;
                              10-4

-------
    A    = sludge  surface area  exposed to  radiant  source,
           square feet  (m^);

    a    = Stefan - Boltzman constant, 1.73  x  10~9  Btu/per  hour
           per square  foot  per  °R  (4.88 x  10~8 k cal/m2/hr/°k);

    tr   = absolute temperature of  the radiant source, °R;

    ts    =  absolute  temperature  of  the  sludge  drying  surface,
             °R;

This discussion  of  heat  drying is necessarily  brief;  the  reader
is referred elsewhere for more information (1-5).  Equations for
mass and heat transfer rates and for associated drying times for
specific dryer types are  discussed  in detail  in these references.
It is often difficult  to  determine  appropriate values of mass and
heat transfer coefficients to be used in  these equations.   Thus,
results  predicted  by the equations  and results  obtained  in
practice may  be  divergent,  perhaps critically so.   Most  usable
design  information  is  obtained by  testing  with  actual  process
feeds under  conditions closely simulating prototype operations.
Many dryer manufacturers  provide such  testing services.


10.3  Energy Impacts.

Thermal  evaporation  of  water  from  sludge requires  considerable
energy.   The  amount of  fuel required   to  dry  sludge depends
upon  the  amount  of water  evaporated.   It is  imperative  that  a
dewatering step precede  heat-drying  so that  overall  energy
requirements can be minimized.  Figure 10-1  shows a relationship
between the solids content of  the sludge  and the energy required
to produce a product containing ten percent moisture.  The energy
estimates for heat-drying of sludge must be considered  rough
approximations,  since values can  vary  considerably depending
upon the type of dryer, whether or not energy recovery is  a part
of  the  process,  the flow  sheet,  and  the characteristics  of the
sludge.

The  heat  required  to  evaporate  water  from  the  wet  sludge
is composed of:

     •  Heat to  raise  the  sludge  solids  and  associated residual
        water to the temperature of the sludge product  as  it
        leaves the dryer.

     •  Heat to raise the water temperature to  the point where it
        can  evaporate and  then  to vaporize the water  (latent
        heat).

     •  Heat  to raise the  temperature  of the exhaust  gas,
        including water vapor,  to  the  exhaust temperature.

     •  Heat to offset heat losses.


                              10-5

-------
The above-mentioned heat must be supplied by the  heating medium,
for example, hot air or steam.
         80
     c
 0
 4aJ

 •H1
 CD
tO
 O

  %
 Q
 LU
   yj
   UJ J*~l
   x "5
     +j
   LU CQ
   < c
   X
   O
   CC
   Q.
   O.
         60
         40
         20
          0
                           ASSUMPTIONS;
                             -10 PERCENT MOISTURE IN DRIED SLUDGE
                             -2000 Btu ARE REQUIRED TO EVAPORATE
                              ONE POUND OF WATER
                     10   15   20   25   30    35    40    45

                        PERCENT SOLIDS IN DRYER FEED
                                                          50
                            FIGURE 10-1

                ESTIMATE OF ENERGY REQUIRED TO DRY
                WASTEWATER SLUDGE AS A FUNCTION OF
                    DRYER FEED SOLIDS CONTENT
    10.3.1  Design Example

Ten thousand pounds  per  hour  (4,540  kg/hr)  of  a  dewatered  sludge
containing  20  percent solids  is  to be  dried  by direct  contact
with  hot air.   The sludge  temperature  is  60°F  (17°C).   The
temperature of  the air  prior  to  heating  is 70°F (22°C) and  its
absolute  humidity  is 0.008 pounds  water per  pound of dry  air.
The temperature  of the  dried  sludge is 140°F  (60°C).   The dried
sludge  is  91  percent solids and 9 percent water.   The dryer
exhaust gas  temperature is  240°F  (116°C),  and   it contains
0.12  pounds  of water  per  pound  of  dry air.    Radiant heat
losses  from the dryer  structure  are 1,000,000  Btu per hour
(1,054,000  kj/hr).    A preheater is used to  heat  the air prior to
                              10-6

-------
 its  entering  the  dryer.   Figure  10-2  is  a schematic  diagram for
 this example.    The  required  air  flow   (G),  the  required  air
 inlet  temperature  to  the  dryer  (t2),  and  the  dryer  evaporative
 efficiency must be calculated.
                                         WET SLUDGE
                                     LOADING = 1(5.000 Ib/hr
                                     SOLIDS CONTENT = 20%
                                     TEMP = 60°F
    lNLET_AiB
VOLUME - (COMPUTE)
Y = MOISTURE CONTENT
 - 0:008 Ib water/
      Ib dry air
TEMP - 7Q°F
         	—*-
                                             (T)
                                            r
DRYER INLET AIR
TEMP •= t* (COMPUTE t
(1)
SLUDGE
DRYER
X
/"%
X
          HEAT SUPPLIED TO PROCESS
              HA - (COMPUTE)
      I
t Ifa/hr = 0,454 k^'hr
     = 1.054 kJ/hr
   DRIED SLUDGE
SOLIDS CONTENT - 91%
TtHP'-
                         EXHAUST GAS
                      Y = MOISTURE CONTEMT
                        - 0.12 Ib wiltsf
                           Ib dry air
                      TEMP = 240°F
 RADIATION I OSS

HR - IxlO6 I
                                FIGURE 10-2

                  SCHEMATIC FOR SLUDGE DRYING EXAMPLE
 The  following heat  capacity  information  is  known  or assumed:

                             Heat Capacity,
Substance
Dry air
Dry solids
Water
Water vapor
Step 1 - Determine
Btu/lb/°F
0.24
0.25
1.0
0.45
the required air flow,





(G)
                                                             Calculate  a
 moisture balance of  substances entering~and leaving the  dryer.

      1.  Moisture in:

             ....    .    ,  ,     /, n ,,,.„ Ib sludge\ / n 0 Ib water \  Q ,,,,.-, -,,
          a.  Moisture in sludge = 110,000 	^—^-110.8 ij-, QiU(jqe I = 8'000 lb
             ^,« ^- U«, ,^/TC4-/Ui^\'               ' *            '
             per hour (3.6 t/hr).
                                   10-7

-------
   b.  Moisture  in inlet air = (G lb d^ airWo.008 J-b water \ =
                            \     hr    / V     Ib dry air /
       Ib per hour.

2.  Moisture out:

   a.  Moisture  in sludge
       = (lO,000 lb^ludgeW   lb dry flidsV   9 lb water   \  200 lb
        \  '        hr  /y      lb sludge /y 91 lb dry solidsy
       per hour (91 kg/hr).

    b.  Moisture in air = (G lb d^ air)(o.l2   lb/ater }
                       y     hr   /y      lb dry airy
     3.  Equate moisture in and moisture out 8,000 + 0.008 G  =  200  + 0.12 G.

     4.  Solve for inlet air flow (G):

        G = 69,600 pounds per hour (31.6 t/hr).

Step 2  -  Determine the  required  air  inlet  temperature  (t2).
C a 1 c u 1 a te~"a" "Ii'e'a^t  baTan~ce for  the  d ry e r .   A~~s "ub~s~t~an c e T s  he a t
content with  respect  to  a given base  temperature  can  be
calculated by  assuming the heat  required to  bring the  substances
from the  base  temperature to the  temperature  being  considered.
For  this example,  a base temperature of  32°F  (0°C) is  arbitrarily
selected,  and heat content  (also known as  enthalpy)  is  calculated
with respect  to  it.  At  steady state,  heat in  must  equal heat
out.   Consider the  heat content of  streams  entering and  leaving
the  dryer:

     1.  Heat into the dryer is the sum of

         a.  Heat content of sludge (H4)

            (1)  Heat content of dry  solids

                      000 l^M   o.20            o.25
            . (10,
            = 14,000 Btu per hour (14.8 GJ/hr).

        (2)  Heat content of  water

            = /      lb sludge] [Q  0 lb water  \ /     Btu ]
              I  '       hr    y I     lb sludgey y    lb°/F /

            = 224,000 Btu per hour (233 GJ/hr).

        (3)  Summing,

            H4 = 14,000 + 224,000  =  238,000 Btu per hour
                 (251 GJ/hr).


                            10-8

-------
   b.  Heat content of air entering the dryer (H2)

       (1)  Heat content of dry air

           = ^69,600 j^Yo.24

           = 16,700 (t2-32) Btu/hr.

       (2) Determine  the  heat' content  of  the moisture
           associated  with  the  air.    This  includes  heat
           required  to  raise  the moisture  temperature  from
           32°F (0°C)  to the  dewpoint,  vaporize the moisture,
           and  finally  increase  the  vapor  temperature  to
           t2.    From  psychrometric  charts  (1),  the  dewpoint
           (the temperature  at  which  the air  in  question  is
           saturated)  of air  containing 0.008 pounds  of water
           per pound  of dry  air  is  50°F (10°C).   From steam
           tables  (6),  the  latent  heat of  vaporization  at
           50°F (10°C)  is 1,065 Btu  per pound (2.5 GJ/kg).

           Heat  content of   moisture  associated with  air
  / 'an cnn Ik dry air\ /_  nrio  Ib water
=  69,600 - -   0.008
                          lb dry air


                                                          i sn -
                                                    lb/°FPu
                                                               °
               1,065 ~+(0.45
                            Btu
                               =  603,000 + 250.7 (t2-50)
             Btu per hour.

       (3)   Summing,  H2 = 16,714  (t2 - 32) + 603,000 + 250.7  (t2 - 50)

                        = 16,960 t2 + 55,600 Btu per hour.

2.   Heat out of the dryer is the sum of:

    a.  Heat content of the "dried" sludge

       ( 1 )   Heat content of the dry solids
            - fin nnn Ik sludge \ /_  . lb solids \ /.  -,  Btu W
            -  10,000 — — ^ - J ^0.20 lb sludge; ^°'25    0
            = 54,000 Btu per hour  (57 GJ/hr).

       (2)  Heat content of residual water

             /,. nnn lb sludge \L  on lb solids \ / 9 lb water V. . Btu
            ^10,000 - HF^J^20 lb sludge] (si lb solidsA1'0

            x (140-32°F) = 21,400  Btu per hour (23 GJ/hr).


       ( 3 )  Summing ,

            H3 = 54,000 + 21,400  =  75,400 Btu per hour (80 GJ/hr).
                            10-9

-------
       b.  Heat content of the exhausted air

           (1)  Heat content of the dry air

                       lb dr  airVo.24 Btu\/240-32°F
                                  lb/op

                 per hour (3.7 TJ/hr).
                                                    _ ,_. nnn  OJ_
                                                  = 3,474,000  Btu
           (2)   Determine  the heat content  of  the  moisture
                associated  with   the   exhausted  air.   From
                psychrome tr ic charts  (1), the  dewpoint of  air
                containing  0.12  pounds water  per  pound  of
                dry air  is 135°F  (58°C).   The latent heat  of
                vaporization  at  135°F  (58°C)  is  1017 Btu  per
                pound  (2.4 GJ/kg) .

                Heat  content  of  moisture  associated  with
                exhausted air
— I fiQ fion
+ 1017 +l(
lb dry air\ /-
hr J\ '
J 45 Btu ^ f- 40
J'45 lb/°F/^4U
, n lb water \
lb d
-135 "FJ
ry airy
= 9,7
j( 0 Btu \
^'U lb/°FJ
50,000 Btu
( ^
1 ^S ^?°P
V
per hour
                  (10.3 TJ/hr).

           (3)  Summing,

               H5 = 3,474,000 + 9,750,000 = 13,224,000 Btu per hour

                   (13.9 TJ/hr).

        c.  Radiant  heat  loss,  Hr =  1,000,000   Btu  per  hour
            (1.05 TJ/hr).

    3.   Calculate an overall  heat balance around  the  dryer.   At
        steady state, heat into the dryer equals  heat  out,  that
        is  H4  + H2  =  H3  + H5  + Hr.    Therefore,  238,000
        + 16,960 t2 + 55,600 = 75,400 + 13,224,000 + 1,000,000.

    4.   Solve for dryer inlet air temperature (t2)

        t2 = 826°F (441°C).

Step 3  - Determine the evaporative efficiency.  In  this  example,
evaporative efficiency  is  defined  as  the  heat  supplied  to
evaporate one  pound  of water,  in comparison to  the theoretical
heat of vaporization:

    1.   Determine heat  supplied  to the  process  (;HA).   By  an
        overall  heat  balance  around the  process (including
        the air preheater), HA = H3 + H5 + HR -  H4- HI.


                              10-10

-------
        a.   From previous calculations,  H3 + H5  + HR =  "heat  out"

            = 75,400 +  13,224,000  +  1,000,000  = 14,299,000  Btu
              per hour  (15.0  TJ/hr).

        b.   From previous calculations,  H4 =  238,000 Btu  per
            hour (251 GJ/hr).

        c.   Determine H]_, the heat  content of the inlet air

            (1)   Heat content of dry  air
                 = (69,600 £p)(0.24 jg^pH 70-32 °F)  = 635,000  Btu

                    per hour (669 GJ/hr).

            (2)   Heat  content of  moisture associated with  dry
                 inlet air
                                                                \
                                    0.008
                             hr    II--— ib dry air
1.0
                                                       Btu
                 +  1065 +(0.45 1^/oJ|70-50°F) =  608,401  Btu per hour
                              Ib/ FJ \     j

                    (641 GJ/hr).

            (3)  Summing,

                 HI = 635,000 + 608,000  =  1,243,000  Btu per hour
                      (1.3 TJ/hr).

        d.  HA  = 14,290,000-238,000  -  1,243,000  =  12,809,000 Btu

            per hour (13.5 TJ/hr).

    2.  Heat supplied to evaporate  1 pound of water.

        = 12 809 000 Btu                       of water
          7,800 Ib water    r     .    r   r

          (1.8 GJ/kg).

    3.  Heat  of vaporization of water  at  the  inlet sludge
        temperature = 1060 Btu per  pound (2.5 GJ/kg):

        Evaporative efficiency = -,  ' ft A0 (100)  = 64 percent.
                                 -L  f O T •£
    10.3.2  Energy. Cost of Heat-Dried Sludges Used for
            Fertilizers

A simple analysis shows that heat-dried sludge is not competitive
with  commercial  fertilizers when  the two  are  compared on  the
basis of  energy  required  per  unit  of  nutrient  produced.   From


                              10-11

-------
Figure 10-1,  the  energy required to  flash-dry  a well-dewatered
sewage sludge (40 percent solids concentration)  is approximately
5.6 x 106 Btu per ton  (7.3  x  106 kJ/t)  of dry solids.   Assuming
that the  solids  are  four percent  nitrogen by weight and that half
of the nitrogen  is  in  plant-available form,  the energy required
to produce 1.0  ton,   (0.9  t) of  plant-available  nitrogen  is
5.5 x 1Q6 Btu      100 ton dry solids        2 ton N     _        6
ton dry solids  x      4 ton N       x   ton available N    -bu x ±u
(295 x 106 kJ).


The  energy  required  to produce  and  distribute  one  ton  of
commercial  nitrogen  is estimated  to be 49  x  10^  Btu  per ton
of nitrogen  (57  x  lO^.kJ/t) (7).   Assuming  all  nitrogen  in
commercial  fertilizers  is  plant-available  and  that  94  percent
of the energy consumed is  for production  and  six percent for
distribution of  raw   materials  and finished  product,   then
approximately 46  x  10^  (49 kJ)  is required  to  produce  one ton
(0.9 t) of  nitrogen on  a commercial  basis  (7).   This  is approx-
imately 16  percent  of  the energy required  to  produce  one ton of
available nitrogen from flash-dried  sludge.

By  similar  calculations,  it  can  be  shown  that one  ton  of
phosphorus from flash-dried  sludge requires about 15 to 20  times
as  much  energy  to produce  as one  ton of  phosphorus  from
commercial fertilizers.
10.4  Environmental Impacts

Heat-drying of  sludge  produces  a  material  that usually contains
10 percent or  less moisture,  a  moist gas stream that is ejected
to the  atmosphere,  and in  some  cases,  a liquid sidestream.   The
impacts of all of these  products  must be  considered  in  the
design  of  the  heat-drying  facilities.    Some  data on pathogenic
organism survival  through  heat-drying processes are presented in
Chapter  7.   Heat-dried  sludge  should  not be  allowed  to become
rewetted,  since moisture  creates  an environment  favorable  for
regrowth of organisms.   Once sludge  is  rewetted,  anaerobic
decomposition  can begin  with the  concomitant  generation of
noxious  odors.   This  is particularly a problem for sludges that
have not been previously stabilized.

Potential users  of dried sludge prefer a granular or pelletized
product.  A product which is dusty,  odorous, or contaminated
with materials  such  as plastics,  strings,  Or cigarette butts is
difficult to sell or give away.


    10.4.1  Air Pollution

The  gas stream  exhausted  from  the  dryers may  be  the  source of
odors and visible emissions.   These  appear to  be most significant
in high-gas  velocity  processes  where the product  is subject to


                              10-12

-------
abrasion and dusting occurs.  The most effective control measure
for  these problems is  afterburning.   However,  afterburning
requires  supplementary  fuel  and may  be  prohibitively  expensive
for many  installations.   Cyclones,  wet scrubbers,  electrostatic
precipitators,  and baghouses have been used with varying degrees
of success.

Wet-scrubbing,   electrostatic  precipitators,  and baghouses  were
tested  for the control  of odors and  visible emissions  from a
Toroidal  dryer  located  at the Blue Plains  plant  in Washington,
D.C.   The  electrostatic precipitator  and  wet  scrubber  were
unable to  reduce  emissions sufficiently  to satisfy Washington's
stringent  air  pollution  requirements.   Baghouses  were effective
when operating, but they  persistently caught fire  as a result of
ignited grease  deposits  and thus  were  not reliable.
    10.4.2  Safety


Drying systems are exposed to heavy  dusting and have had problems
with  fires.  The combination  of combustible particles,  warm
temperatures, sufficient  oxygen, and  high-gas  velocities  make
these systems susceptible  to fires.
    10.4.3  Sidestream Production


Liquid sidestreams  are  produced by certain ancilliary equipment
in heat-drying  (for  example,  wet scrubbers).   These sidestreams
frequently can be  recycled to  the headworks of the  treatment
plant but may require separate treatment.


10.5  General Design Criteria

There are  several common  features  of  heat-drying  processes for
which general design criteria can be developed.
    10.5.1  Drying Capacity


The number  and  size  of the dryers depend  on  the type of drying
operation contemplated.  If the dryers  are  operated continuously,
extra dryer capacity  is needed so that  all  sludge produced can be
dried  while maintenance and repairs  are  being performed.   In
cases where  non-continuous  operation  (for  example,  40 hours  per
week)  is  envisioned  or  where  only one  dryer  is installed,  the
dryer(s) must have sufficient  evaporative  capacity to handle all
the  sludge, including that generated when the dryers  are  not
on line.  In the latter case,  wet sludge storage  requirements may
be significant.


                              10-13

-------
    10.5.2  Storage  Requirements


The design engineer  should  consider storage requirements for both
the wet  sludge  feed and  the dried  product.   Sufficient wet
sludge storage should be  provided  to allow orderly  shutdown  of
continuously operated drying processes (approximately three day's
production at a  peak rate).   Storage  for  the  dried product
depends on the final disposal  arrangement.   Sales of the product
are likely to  be seasonal,  and considerable  storage may  be
necessary unless  bulk  buyers  provide  off-site  storage.   If
the dried product  is burned  as a  fuel or  undergoes further
processing,  storage  requirements are indicated by subsequent
steps  in the sludge-processing system.  Dust can become a problem
if the dried product is  stored in bulk and is not pelletized.   In
some cases, the material should be appropriately contained.
    10.5.3  Heat Source


The large amounts of energy  required for heat-drying dictate that
close attention  be  given  to the source used to  heat  the  drying
medium.    Natural  gas and  fuel oil are most  frequently  used  but
are becoming more expensive,  and  shortages  have  occurred  in  the
past few  years.   Energy recovery within  the  heat-drying  system
itself provides  one  way of reducing energy  usage;  for  example,
heat exchangers can be used  to recover  heat  from the exhaust
gases.   Recovery of heat from a power source within the  plant is
another method;  for  example,  Milwaukee recovers  waste heat from
gas turbine exhausts.   The  dried  sludge  itself has a  fuel value
and may be used as a heat  source  for the drying medium.
    10.5.4  Air Flow


Air flow  is  an important consideration in  the  design  of  direct
dryers.  Air flow  may be cocurrent, countercurrent, or crossflow.
In direct drying,  cocurrent  flow  offers  the advantage  of  higher
thermal efficiency  due to rapid  cooling  of the  heating  medium
near the  feed  end with concomitant  reduced  heat  losses  through
the dryer structure.    In  addition,  the dried  sludge  is  not
subjected  to  high-gas   temperatures  near  the discharge end,  as
it would  be  in counterflow  operation.  This  is advantageous
because  it minimizes  distillation  of  odorous  materials  and
increases  thermal  efficiency  somewhat by reducing heat  lost with
the dried  sludge.


The  rates of  air  flow are  a function  of the  dryer design.
However,  turbulent conditions  must  be  maintained  to  ensure
intimate contact  between  the  warm air and  wet  sludge.   Dusting
problems may limit air  flow rate.
                              10-14

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   10.5.5  Equipment Maintenance

A  major  maintenance  problem  in  some dryers  is  erosion  of
conveying equipment  and drying  shells by  the  abrasive  dried
sludge.  This is particularly  a problem  for dryers processing WAS
from activated sludge plants which have  only coarse screening for
grit removal.   The  use of  ferric  chloride  as a dewatering aid
may also create corrosive conditions  that exacerbate the problem.
Worn conveying equipment can  lead to dusting problems.   Abrasive
sludge  may result  in  replacement of rotary dryer drum  shells
every few years.


    10.5.6  Special  Considerations

Special equipment may  be needed when dried material is produced.
For  example,  the  value  of  dried  sludge may be increased  by
nutrient  supplements such as  nitrogen,  phosphorus, or potassium.
Also,  the  dried  product  may  require finishing before sales; for
example, pelletizing or bagging operations may be needed.

In  the United States,  the  Organiform process,  developed  by
Orgonics, Inc., has  been used  to  increase the  nitrogen content of
the  dried sludge.   This process,   based  on urea-formaldehyde
technology,  was used  in an existing heat-drying operation at
Winston-Salem,  North Carolina,  from  1973  to  1975,  and  the
prototype  system is  still  used  at a leather tanning facility in
Slatersville,  Rhode  Island  (8).    The  heat-drying  operation  at
Winston-Salem  was  abandoned,  however,   because railroad   siding
and  terminal  facilities  for bulk  storage  and shipment could not
be  funded.   The Basel  County  Thermal Sludge Drying  Plant in
Switzerland  has  provisions  for  adding  nitrogen,  phosphorus, and
potassium  to  the dried sludge for improvement of its fertilizing
properties.


10.6  Conventional  Heat Dryers

Conventional  heat-drying  is usually  preceded by mechanical
dewatering and  may  be followed  by air  pollution control devices
and systems which alter the  form  of  the  dried  material.

Mechanical dewatering  is discussed in detail  in Chapter 9.  It is
an  important pretreatment step  since   it  reduces  the  volume of
water  that must be removed in  the  dryer.    In  the dryer, water
that  has  not  been  mechanically  separated  is evaporated without
decomposing  the  organic  matter in the sludge  solids.  This means
that  the  solids temperature  must be  kept  between  140  and 200°F
(60°  and  93°C).  A  large  portion of the dried  sludge  is often
blended with  the sludge feed to the dryer, making the  drying
operation  more efficient by  reducing agglomeration (large balls
of  sludge),  thus exposing a  greater  solids  surface  area  to the
drying  medium.   Dried sludge  and exhaust  gases are separated in
the dryer  itself and/or in a cyclone.  The gas stream can go to a


                              10-15

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pollution control system for  removal  of  odors and particulates.
The  dried sludge  is  then sent  to  a  finishing  step  such  as
palletizing or bagging, or it is stored in bulk for marketing or
use in the next portion of  the sludge  management scheme.


    10.6.1  Flash-Drying

Flash-drying is the  rapid removal of moisture by  spraying or
injecting  the  solids  into a  hot  gas  stream.   This  process was
first applied  in 1932  to  the  drying of wastewater sludge at the
Chicago Sanitary District.


        10.6.1.1  Process  Description

The Combustion Engineering-Raymond Flash Drying and  Incineration
Process  shown  on  Figure 10-3,  is  typical  of flash-drying  units
used in the United  States.

The  flash-drying process  is  based  on three distinct  components
that  can be combined  in  different arrangements.    In the  first
component, the wet  sludge  cake is blended with previously  dried
sludge  in  a  mixer  to improve pneumatic conveyance.  The  blended
sludge  and the hot gases from the  furnace at  1,300°F  (704°C) are
mixed  ahead  of the  cage  mill, and flashing  of  the  water  vapor
begins.  Gas velocities on the order of 65  to 100  feet  per second
(20  to  30  m/sec)  are used.   The cage mill mechanically  agitates
the  sludge-gas mixture, and  drying is virtually complete by the
time the sludge leaves the cage mill.   The  mean  residence  time is
a matter of  seconds.   The  sludge,  at  this stage,  has  a  moisture
content of only 8 to 10 percent and is considered  dry.  The  dried
sludge  is  then separated from  the  spent drying  gases  in   a
cyclone.   Temperature  of the  dried  sludge  is  about 160°F  (71°C),
and  the exhaust gas temperature is  about 220°F to  300°F  (104° to
149°C).  The dried  sludge  can be sent  either  to  storage or to the
furnace for  incineration.

The  second  component is  the  incineration process.   Gas,  oil,
coal,  or  partially dried  sludge  is  burned in the  furnace to
provide  heat needed  to  dry  the  sludge.   Combustion air,  provided
by  the  combustion  air  fan,   is preheated  and  injected  into the
furnace  at  high velocity  to  promote  complete  fuel  combustion.
Any  ash that accumulates   in  the  furnace bottom is  periodically
removed.

The  third  component  is  the effluent  gas  treatment facility
or  induced  draft  facility.   This  consists of the  deodorizing
preheater, the combustion  air heater,  the  induced  draft  fan, and
a gas  scrubber.  Odors  are  destroyed  when  the temperature of the
gas  from the cyclone  is  elevated  in  the deodorizing  preheater.
Part of the heat  absorbed is  recovered in the  combustion air
preheater.   The  gas  then  passes  through a  dust  collector
(generally a scrubber) and is discharged  to the  atmosphere.


                              10-16

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                               EXHAUST
                               GAS
CYCLONE
                                                                     VAPOR FAN
                                    AUTOMATIC
                                    DAMPERS
                                          -*— INDUCED
                                             DRAFT FAN
     EXPANSION
     JOINT
                                                       EXPANSION
                                                       JOINT
                                          EXPANSION
                                          JOINT
DOUBLE
FLAP VALVE
       MANUAL
       DRY
       DIVIDER
                                              COMBUSTION
                                              AIR PREHEATER
                      PRY PRODUCT
                      CONVEYOR
     WET SLUDGE
     CONVEYOR
                               DEODORIZING
                               PREHEATER
        DISCHARGE SPOUT
                                    AUTOMATIC
                                    DAMPERS
                           COMBUSTION AIR FAN
REMOTE
MANUAL
DAMPERS
CAGE MILL
                                  H OT GAS OU CT  *-—™

                                  FIGURE 10-3

            FLASH DRYER SYSTEM  (COURTESY OF C.E. RAYMOND)
                                    10-17

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        10.6.1.2   Case Study:  Houston, Texas

The  flash-drying  operations  at  Houston,  Texas,  'illustrate  the
operating experience and  performance of  the  C-E Raymond  Flash
Drying  process.    There are  four  flash  dryers  at  the 45-MGD
(1.97-m3/s)  Sims  Bayou  plant and five flash dryers at  the 75-MGD
(3.29-m3/s)  Northside  plant,  with  two additional units  under
construction.    The  liquid  process  stream  consists  of  bar
screening and  activated sludge.   Sludge  treatment consists  of
degritting,  vacuum filtration with ferric  chloride addition,  and
flash-drying.

After  gravity thickening,  the sludge  solids  concentration is
about two percent at the Sims Bayou plant and about three percent
at  the  Northside plant.   The  cake  from  the  vacuum filters is
about 15  percent  solids.   The  ferric  chloride  additions amount
to  about 75  pounds per ton (37 kg/t)  of  dry  solids, or  about
3.8 percent.

Dewatered sludge  is transported to the dryers  by belt  conveyors.
Each flash dryer,  with cage mill  and 14-foot  (4.3 m)  diameter
cyclone,  is rated  at 12,000  pounds  of  water per  hour  (5,448  kg/
hr) but  is operated at  9,000 to 10,000  pounds  of  water  per hour
(4,086  to 4,540 kg/hr).    Heat exchangers  are  provided  for
high temperature  deodorization  and for preheating the  combustion
air.   The cage  mill  inlet  temperature  is  900°F  to 1,150°F
(482°C  to 621°C) ,  and   the  temperature  at the cyclone  is  about
220°F  (104°C).   The  deodorization temperature  is controlled
around 1,200°F (649°C),  and  the  stack gas temperature is 500°F to
600°F  (260°C  to  316°C)  after  heat  recovery.   The fuel used is
natural gas,  and  the heat  input is about 22 million Btu per hour
(23.2 million kJ/hr) or  2,200 to 2,400 Btu per pound (5,100  kJ/kg
to 5,600 kJ/kg) water evaporated.

Moisture content  of  the  dried product  is about 5.5 percent.
About  nine times  as much solids  on  a  dry  weight  basis  are
recycled  to the  predryer  double paddle mixer as  are  removed as
product.  The  product  is conveyed  to a storage  area  or directly
to railroad cars  for shipment.

The  process  is  automated  and  panel boards  are provided  that
indicate  and record variables  such  as  air flow,  temperatures at
critical  points,   and amperage  on fan motors.   The  controls  are
enclosed in  air-conditioned cubicles.  Horn  alarms  indicate
unsuitable temperature conditions.

The controls  for  the  ferric chloride  feeding have proven  to be
inadequate and have led  to operational problems.

Dust is  also a major problem at the Sims Bayou plant.   The  dried
sludge dust is extremely abrasive, causing wear on all  mechanical
equipment.   Wet sludge has also  overflowed  the top of  the
conveyors at  times,  creating  housecleaning problems.


                             10-18

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No specific cost  data  are available for the Houston facilities.
The dried  product, Hou-actinite,  is  sold through  a  broker by
yearly contract.


    10.6.2  Rotary Dryers

Rotary dryers use a sloped rotating cylinder to move  the material
being  dried  from one  end to the other  by gravity.   Direct,
indirect, and direct-indirect rotary dryers have  been used to dry
sludge.


        10.6.2.1  Direct Rotary Dryers

Direct rotary dryers have been used in the United States and in
Europe for drying  sludge.   These  include  installations at Largo,
Florida, and  Stamford,  Connecticut  (in  conjunction with a refuse
incinerator)  and  in Basel,  Switzerland.    Manufacturers include
the Heil Company,  Combustion Engineering, Bartlett-Snow,  and
Euranica, Inc.

Process Description

The  features  of  a typical  direct  rotary drying system  are
illustrated on  Figure   10-4.   Mechanically  dewatered  sludge is
added  to a mixer  and  blended with previously  dried  sludge to
provide  a  low moisture dryer-feed.   Hot  gas at temperatures of
1,200°F  (649°C)  is added  to  the  dryer,  usually in a cocurrent
flow pattern.   After  the sludge  has been  held  in the dryer for
20 to 60 minutes, the  dried sludge is discharged  at  a temperature
of 180°F to 200°F  (82 to  93°C).   Exhaust gases are  conveyed  to a
cyclone where entrained solids are separated from the gases.  The
spent gases exit at about  300°F (149°C).   A portion of the dried
product is recycled, and the balance goes  to a  finishing step, to
further processing, or  to disposal.  Gaseous discharge from the
cyclone goes to an air  pollution  control system for  deodorization
and particulate removal  as  necessary.   Figure 10-4  shows several
alternatives for handling the exhaust gas.   A long residence  time
in the dryer may minimize deodorization requirements.

Design Considerations

The rotary  drum  usually  consists of  a cylindrical steel  shell
that revolves at  5 to  8  rpm.   One end of  the dryer is slightly
higher than the other,  and  the wet  sludge  is fed into  the higher
end.   Flights  projecting from  the inside wall of  the  shell
continually raise  the  material and  shower it  through the dryer
gas, moving the material toward the outlet.

Gas flow  through  the  drum may be  either  cocurrent or counter-
current to the  sludge flow.   Gas  velocities must be limited  to 4
to 12  feet  per  second  (1.2 m/sec  to 3.7  m/sec)  to prevent  dust
from being entrained with the exhaust gas.


                             10-19

-------
                                                      a.. DIRECT DISCHARGE
                                                        TO ATMOSPHERE
                                                      *• ATMOSPHERE
Alfi
BURNER '
IBCKPF
1


SCRUBBER
                                                       • ATMOSPHERE
                 FEED SLUPOE
                                                v
                                                V
                                ALTERNATIVES AVAILABLE FOR EXHAUST GAS DEO DO R IZ AT I ON
                                         AND ("ARTICULATE REMOVAL
                            FIGURE 10-4

                   SCHEMATIC FOR A ROTARY DRYER
Case Study :_ _L_argo, Floric[a

The  Largo,  Florida,  Wastewater Treatment Plant  has  a  rated
capacity of  9  MGD (0.39 m^/s)  with  average  summer flow of 6 MGD
(0.26 ITH/S) and winter  flows greater than 9 MGD  (0.39 m3/s).  The
liquid process stream consists of coarse screening, grit  removal,
contact  stabilization activated  sludge,  chlorination,  and
dual  media   filtration.   Waste-activated  sludge  is  aerobically
digested, batch gravity decanted, and thickened.   Since 1976, the
thickened sludge  has  been  dewatered  by belt  filter  presses and
heat-dried   in  a  rotary  dryer.   This system  was supplied by
Ecological Services Products, Inc. (ESP).

Approximately  1.6 dry tons  (1.45  t) of  digested  sludge is
produced daily and is processed at a rate of 2.2 tons (2.0 t) per
day  for  a  five-day  week.    Typical  thickened aerobic  sludge is
1  to  1.1 percent solids.   The  belt  filter  presses produce a
sludge cake  that  is  typically  10 to 12 percent  solids.   Polymer
is used to condition the sludge prior to filtration.
                             10-20

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The  rotary dryer,  manufactured  by the  Heil Company,  has an
evaporative capacity  of  approximately 5,400 pounds water  per  hour
(2,450  kg/hr).   The Heil  dryer employs  a  3-in-l drum design.
Sludge  moves forward  through  the  center  cylinder,  then back
through the intermediate cylinder,  and  forward again  through  the
outer  cylinder toward  a fan located  at the  discharge of  the
machine.  The three  cylinders are concentric and are mechanically
interlocked so  that  they rotate at  the  same  speed.   Internal-
external flights on each cylinder repeatedly raise the  sludge to
the  top of  the  drum.   This design  is claimed  to  provide  better
heat utilization by minimizing  radiation  losses,  but  maintenance
on the drums is  more  complex than with a single shell.


The  facilities were  designed  assuming  1,000 pounds  per hour
(454 kg/hr) of  dry  solids  throughput, based  on feeding  a  sludge
cake  of about 20 percent  solids.   The dryer is water-limited
because the cake produced by  the  belt  presses  is  only 10 to
12 percent  solids.   Actual throughput is about   600 pounds  per
hour (272 kg/hr) of  dry  solids.


Heated air is provided  by a natural  gas burning furnace.   Typical
dryer  inlet air  temperature is  about 800°F  (427°C),  and  the
outlet  temperature  is  about  180°F (82°C).   The  average  gas
temperature in  the dryer is estimated to  be about 250°F  (121°C).
Off-gases from the cyclone  separator are typically 120°F  (49°C).


The  dried  product,  Lar Grow, is  a  relatively  fine pellet
produced naturally by the rotating drum.  Product  bulk density is
45 to  55  pounds per  cubic  feet (720 to 880  kg/m3).  The  bagged
product moisture content  is  about five  percent.  The product
is screened before bagging  to remove cigarette filters  and  other
nondegradable materials such  as plastics.   In  1978,  a garden
products  wholesaler  contracted  to  purchase the  sludge  produced
for one year (approximately 570  dry  tons,[517  t])  at  $54  per  ton
($59/t).   Because the  wholesaler's markets  are seasonal,  the
bagged  product  is  stored  on-site  for a portion of the year.


The Ecological Services Products,  Inc.  (ESP) sludge drying  plant
was installed in 1975-76 at a contract price of $850,000  (cost of
the building not included).  The approximate capital cost for the
facility  can  be broken  down  as follows:   41  percent for  sludge
and  polymer pumping system,  belt  filter presses, and polymer
preparation and feed system;  32 percent for the dryer,  ductwork,
fan, cyclones, and scrubber system;  and 27 percent for mechanical
conveyors,  recycle  bin,  production storage bin  and  bagging
facility.   According   to  ESP  personnel,  the 1978  cost for  a
similar plant would  be  between  $1.2 to $1.3  million,  including
installation and startup.

Typical operating and  maintenance costs  for dewatering,  drying,
and bagging during 1977  are shown in Table 10-1.
                            10-21

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                            TABLE 10-1

               ESTIMATED 1977 COSTS FOR DEWATERINC,
              DRYING AND BAGGING AT LARGO, FLORIDA (7)
                  Item
                 Polymer
                 Gas
                 Labor
                 Power
Annual cost,
dollars
13,000
26 ,000
21,000
11,000
Cost/ton,
dollars
23
45
36
20
                   Total       77,000         134
These  costs are  based  on unit  costs  at Largo  of $2.60  per
pound  ($5.72/kg)  of polymer,  $1.62  per  1,000  cubic  feet
($57.20/1000 m3) of natural gas,  3.4  cents per kWhr of electri-
city,  and  $0.24  per  bag.   Hence,  approximately 9.9  pounds
(4.5 kg) of  polymer,  27,800 cubic  feet  (790  m3)  of natural gas,
590 kWhr  of electricity,  and 42 bags are used per dry  ton of
product.

Although a  specific deodorization  system  has  not been included,
odor problems   have  been minimal.   There are occasional  odor
problems when  sludge  that is  too  wet enters  the  dryer.   There
have been some  problems with wear  in the conveying  facilities due
to the dried sludge material being more abrasive than originally
estimated.    The pug mill  blades  and  screw conveyor to the dryer
have been  replaced.   Replacement  parts  have  been specified to
include heat treatment of the screw conveyor and the addition of
cellite  or carborundum  plates on the wearing surfaces.    The
system supplier, ESP, has  indicated  that  these  changes  will be
considered for future  equipment.   There have been  few  other
operating and maintenance  problems.
        10.6.2.2  Indirect Drying

Indirect rotary dryers  have  not been  used  in the United States
for drying  sludge.   Vertical  thin  film dryers are  used at the
Dieppe,  France,  coincineration facility  (9,10).   The  two  LUWA
Double-Wall Dryers  installed at  Dieppe  operate  on  140  psi
(966 kN/m2) steam at a  temperature  of  about 355°F (180°C).   The
evaporaters are  vertical, with  top  inlet and bottom  outlet.
Steam generated from  refuse incineration is  forced  into the dryer
and heats  a "jacket" surrounding  the incoming dewatered sludge.
The sludge  is  spread  over the  inner cylindrical  surface of the
dryer  by  a rotor carrying self-adjusting vanes,  at  a top speed
of about 25 feet per  second (7.6 m/sec).   The  water vapor travels
upward,  counter  to  the  sludge  flow,  and  is blown into  the
incinerator, where it is deodorized.  The dried sludge falls onto
a conveyor belt and  is  incinerated with the  refuse.
                             10-22

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Another  type  of indirect  sludge dryer  is  the jacketed and/or
hollow-flight dryer and  conveyor.  A schematic of a  jacketed
hollow-flight  dryer is  presented  on Figure 10-5.  These units can
perform the dual function  of  heat transfer  and solids conveying
in one piece  of equipment--generally a horizontal, semi-circular
trough  with  a  jacket or  coil  to  provide  heat  (10) .    This
equipment has  one  or more  agitation devices (for example,  screw,
flight, disc,  paddle) rotating on the axis through the center of
the trough.   A significant degree of  agitation  is  necessary to
maintain  reasonable heat  transfer.   Simple  screw conveyors
are notably  poor   in  this  regard, because  increasing  the  speed
reduces  the residence  time in the dryer by  moving the sludge
rapidly through the  system.  Heat transfer coefficients for this
type of  equipment  range from 15  to  75 Btu per  hour  per  square
foot per  °F  (18.6  to  93  cal/sq cm/°C),  depending on  moisture
content and degree  of agitation.
                   -IN! FT
                       BREAKER
                         BARS
 JACKETED
  VESSEL
            AGITATOR
                                                         ROTARY
                                                          JOINT
                          DISCHARGE
                            FIGURE 10-5

                  JACKETED HOLLOW-FLIGHT DRYER
                 (COURTESY BETHLEHEM CORPORATION)
The  agitators,  paddles, or  flights  should also  be  designed  to
minimize build-up on  the walls  of  the  dryer and on the agitator
itself.  Generally,  baffles or ploughs  should be provided between
                             10-23

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the flights  to improve  mixing and to  break up any  lumps that
form.   The rotating  flights  are often fitted with small paddles
or similar projections  to improve  agitation and reduce fouling of
the shell surface.

Significant increases  in  heat  transfer can  also  be  obtained if
the rotor is  hollow and fitted  for steam  heating.   A hollow
heated rotor  often  provides  one  to two  times  the heat  transfer
area available in the shell.
        10.6.2.3  Direct-Indirect  Rotary  Dryers

The direct-indirect  rotary  dryer is  similar  to indirect dryers
employing hot  air or gases  as  the heating medium.   In direct-
indirect drying,  however, the heating  medium  is recirculated to
flow  in direct contact with the drying sludge in  addition to
heating the metal drying surfaces.

Case Study:   Milwaukee,  Wisconsin

The drying  operation at  Milwaukee's  200-MGD  (8.76-m3/s)  Jones
Island  Plant  employs  ten direct-indirect  rotary,   counterflow,
kiln-type dryers  for treating waste-activated sludge.  The plant
is designed for  continuous operation.   To achieve this,  nine
dryers  must always  be  in operation.   The drying system  produced
over 74,000 tons  (67,300  t) of dried product in 1976.  Thickened
waste-activated  sludge  is conditioned with  ferric  chloride and
filtered on vacuum filters.   Wet  filter cake (approximately
14 percent  solids)  is mixed  with an approximate equal weight of
previously  dried .material  in a  screw conveyor and fed to the
direct-indirect  dryers.   The  ten custom-built dryers  are  each
8 feet  (2.4 m)  in diameter and  57  feet (17.4 m)  long.  Each dryer
can evaporate  approximately  10,000 pounds  (4,540  kg)  water per
hour  (at 90 percent capacity)  with  an inlet  air temperature of
1,200°F  (649°C).   The  rotating  drum, with lifting angles, picks
up the  wet  mixture  that  is dropped subsequently to the bottom as
a  shower of particles.    The sludge  is  continuously lifted and
dropped  through  the hot  gases,  progressing as  a  moving curtain
through the length of  the  dryer during the 45-minute drying
cycle.   The granular dried sludge  (Milorganite) has  been sold as
a  fertilizer  since 1925.  Rejected  dust and  fine particles are
pelletized, and  the pellets are  reground to produce  granular
saleable material.

The dryer air inlet temperature  is controlled  at 1,200°F  (649°C).
The exhausted gas leaves the dryer at 250°F (121°C) and is passed
through  cyclone  separators to remove  fine  particles.  Each dryer
has its  own furnace.  Originally, coal was used as a fuel, then
coke oven gas  (after furance modification), and then natural gas
with  standby  fuel oil.   In the mid-1970s,  gas turbines  were
installed,  and  the  gas  from these turbines, at a temperature of
approximately 900°F  (482°C), is now  fed  to  the  modified  furnaces
and two waste heat recovery boilers.   The gas  burners are used to


                             10-24

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provide the additional  heat  necessary to maintain the dryer inlet
temperature at  1,200°F  (649°C).   The  recovered  turbine  exhaust
heat  supplies 70 percent  of the heat required for the  sludge
drying operation.

The dried  sludge  product  is  abrasive,  and  the wet sludge  is
corrosive because of the ferric chloride  used.  Internals  of the
drum  must  be replaced about  every three  years.   The present
dryers are  over  20 years old, and  plans  are being made  to add
three  direct,  cocurrent rotary  dryers  and to  rehabilitate  the
existing dryers.


    10.6.3   Incinerators

In sludge  incineration,  the  temperature of  the  sludge  is  raised
to 212°F  (100°C),  and  the  water is evaporated  from  the  sludge
before it  is  ignited; that  is,  the sludge  is dried prior  to
ignition.   Several  options  are available  with incinerators.   If
heat  inputs are  reduced,  the  incinerator  can  be used  as  a dryer
alone.   Alternatively, a  portion of the  dried sludge  can  be
removed  at an  intermediate  point  in the  incinerator, with the
remainder  proceeding onward  to be burned.   Finally,  all  sludge
may be incinerated.

Modifications may  be required  if  these units  are to be  used for
drying  alone;  for  example,  modifications  to  a multiple-hearth
furnace would include fuel burners at the top and bottom hearths
plus  down-draft  of  the gases.   If the sludge  is to be  disposed
of,  incineration  provides  greater volume  reduction than  drying
alone.

Incineration is  discussed  in Chapter 11.   Processes include
multiple-hearth, fluid-bed,  and electric furnaces.


    10.6.4   Toroidal Dryer

The Toroidal  (doughnut-shaped)  dryer is  a  relatively  new dryer
that  is employed  in the  UOP, Inc. ORGANO-SYSTEMR for  sludge
processing.  The dryer works on a jet mill principle and contains
no moving  parts.   Transport of solid material within the  drying
zone  is accomplished entirely by  high-velocity air movement.


        10.6.4.1  Process Description

A  simplified  process  flow diagram of  the  UOP ORGANO-SYSTEMR is
shown on  Figure  10-6.  The  system is composed of wet  sludge
storage, mechanical  dewatering, sludge  drying, air pollution
control, final product finishing,  and storage.

The  mechanical  dewatering step  is designed  to  deliver  the
dewatered  sludge  to  the dryer at about 35 percent  to 40 percent
solids.   The dewatered sludge  is mixed  with previously dried
sludge to  reduce the moisture concentration  of the dryer feed.
                             10-25

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  INCOMING
                   MECHANICAL   SECONDARY
                   DE WATERING   STORAGE
                                              AIR
                                                       AIR EMISSIONS
                                                         CONTROL
                                                         PRODUCT TO
                                                          CUSTOMER
                                TOROIDAL DRYER
                                              PRODUCT FINISHING
                            FIGURE 10-6

                      TOROIDAL DRYING SYSTEM


Heated process  air  is  distributed through  three  manifold jets to
the lower  segment  of the toroidal drying zone chamber.   The  air
from  one of  the three  jets  is  directed  in such  a  way as  to
impinge  upon  the  incoming  wet  feed  material  and  propel this
material into  the  drying  zone,  where particle size reduction  and
drying begins.   Additional  jets  in the  drying  zone  convey  the
material  into  the  toroid  for  additional  drying,  grinding,  and
classifying.

Process  air and  solids  within  the toroid move  at a  velocity
of approximately  100  feet per second (30 meters per  second).
The  high-velocity  gas  stream  reduces  the  size  of  lumps  or
agglomerated  feed  material by  impingement  against  the  interior
walls of  the  drying  chamber  and  by collision  with other
particles.   Wetter  and heavier  particles travel  a  path along  the
internal  periphery of  the dryer,  whereas  drier  and  lighter
particles are  swept  out with the  gas stream and  are  removed  from
the drying  zone.   Heavy,  wet  particles stay  in the dryer  until
they are broken up and dried.

The inlet termperature is  usually controlled  within  the  range of
500°F to 1,400°F (260°C to 760°C).  There is  a sharp drop in  the
gas temperature  within  the dryer when the  hot  inlet  gas stream
meets the incoming wet sludge.  The dryer exhaust  temperature is
usually controlled  at a  specific setpoint within the  range  of
190°F to 300°F (90°C to 150°C).  The product temperature normally
does not exceed 150°F (66°C).
                             10-26

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The dried  sludge particles exiting  the toroid  are sent to  a
cyclone where they are separated from  the gas stream.   A portion
of the  dried sludge  is  back-mixed with  the wet feed, and  the
remainder  is  transferred  to  the  product  finishing section.
There,   the  dried  product may be  extruded (at a  temperature of
140°F   [60°C]),  cut  into  pellets,  and  bagged,  if desired.
Otherwise,  the product is routed  to subsequent  sludge processes
including codisposal/energy  recovery or land  application.   Gases
from the cyclone  are treated  by  processes that  may include
wet  scrubbers,  electrostatic  precipitators,   and  baghouses.
Deodorizing chemicals  may be  required.


        10.6.4.2   Current Status

The toroidal dryer has been  demonstrated  on  a full-scale  basis.
A 240-tons-water-per-day evaporative capacity  ORGANO-SYSTEMR
was  operated  by UOP Organic  Recycling at  the  Blue  Plains
wastewater treatment plant  in  Washington,  DC,  for over  three
years.   Raw  sludge, digested primary sludge,  and waste-activated
sludge,  as  well  as mixtures of these sludges,  were  processed.
This system  is no  longer in operation.  A 24-tons-water-per-day
evaporative  capacity  unit is installed  at UOP's West  Chester,
Pennsyvania, research  and development facility.


    10.6.5   Spray-Drying

Spray-drying systems  are  similar to flash-drying  systems in that
almost  instantaneous drying occurs  in both.

        10.6.5.1   Process Description

Spray-drying involves  three  fundamental  steps:   liquid  atomiza-
tion,   gas/droplet mixing,  and  drying   from liquid droplets (1).
Atomizers  are  usually  high-pressure  nozzles,  or high-speed
centrifugal dishes or bowls.  The atomized droplets  are usually
sprayed downward .into a  vertical  tower  through which  hot gases
pass downward.  Drying  is  complete within  a few seconds;  the
product is  removed   from the   bottom,  and  the gas  stream is
exhausted through a cyclonic  dust  separator.

Abrasive materials can cause  problems with the atomizing devices.
Centrifugal  bowls or  discs  apparently require  less  maintenance
because they are  less  likely  to  become  plugged.
        10.6.5.2  Current  Status

A  Nichols  Spray  Dryer  was  installed  and  operated  at  the
wastewater  treatment  plant  at Ansonia,  Connecticut,  to  dry
sludge.  Dewatered  sludge  was  sprayed into the top of a cone-like
apparatus containing rotating  "wheels."  The  heating  medium  was
hot flue  gases  (1,300°F   [705°C])  from the stack of  a municipal


                            10-27

-------
refuse  incinerator.   Operation of  the  incinerator  has been
limited  to  about  five  hours  per  day because  of  state  air
pollution  control requirements; the drying  time was likewise
limited.   A burnable, dried product with greater  than  90  percent
solids has been produced  with this system.   The dried  sludge has
been given away as  a  soil  conditioner rather  than burned  in the
refuse incinerator.


10.7  Other Heat-Drying Systems

Two  are  currently available that  differ  somewhat  from  conven-
tional heat-drying systems.   They  are the Basic Extractive Sludge
Treatment  (BEST)  process, which employs  solvent  extraction,
and  the  Carver-Greenfield   process,  which   uses  multiple-effect
evaporation.   Both of these  systems employ an externally supplied
liquid to assist in the removal of water from wet  sludge.


    10.7.1  Solvent Extraction:—BEST Process

The  BEST process is based on the  use of an organic  solvent
to  reduce the  amount of  water  that must be evaporated in a
conventional drying  step.   The  process was developed by and
is  available  from Resource Conservation Company  of  Renton,
Washington.


        10.7.1.1  Process Description

The  BEST process, shown schematically  on  Figure  10-7,  uses  an
aliphatic amine solvent  (triethylamine or TEA)  to  separate sludge
solids and water.   The  key to this  process is the  temperature-
sensitive miscibility properties  of TEA.  Below 65°F (18°C), TEA
and water solutions of any  concentration are completely miscible
and  form a  single-phase,  homogeneous  solution.   Above this
temperature,  the mixture separates into  two distinct layers, the
top  layer  being  nearly  all TEA and  the bottom layer  nearly all
water.

As  shown  in  the  diagram,  incoming sludge  is mixed with chilled,
recycled solvent.    The cooled mixture  is then fed  into a
conventional dewatering  unit,  such as  a  vacuum   filter,  press,
or  centrifuge.   After dewatering,  the wet  cake  is  fed to a
continuous dryer operated  between 250°F  and 290°F (120°C and
140°C).   The liquid in  the wet  cake  contains a  high  percentage
of  TEA.   The latent  heat  of TEA is approximately 133  Btu per
pound  (309 kJ/kg)  compared  to  approximately 1,000  Btu  per pound
(2320  kJ/kg) of  water.   Because of  this,  the drying  process  is
faster and uses less direct energy for drying  than if  the liquid
were  only  water.  Vapors  coming from  the  dryer are condensed
(condenser not shown) and combined with  the liquid left from the
dewatering step.   This  solvent/water mixture  is  then  heated and
collected in a  decanter, where  the  components separate  into two
distinct layers.


                             10-28

-------
1
*

-*•
so
LVENT r
DEC
140°F V
* \
k
         r~\
  HEAT
EXCHANGER
                     STILL
Y
   MIX
 JUNCTION
      20°F|       OIL
            BEPF
                           50" F
  HEAT
EXCHANGER
SLUDGE
  HEAT
EXCHANGER
                 LIQUID/SOLID
                 SEPARATOR
                                                 140" F
WATER
STILL
                              DRYER
                                        T
                                 DRY   PRODUCT
                                SOLIDS   WATER
                            FIGURE 10-7

                   SCHEMATIC OF B.E.S.T. PROCESS
The  solvent  is drained off the top of  the  decanter  and recycled
(after chilling) to mix with new incoming sludge.   Meanwhile,  the
water  is  decanted  to a distillation  column  to be steam-stripped
of  residual  solvent, which  also  is recycled.   Oils and fats
extracted  from the sludge  by the  solvent  are recovered  in  the
solvent still.  The product water is returned to the  headworks of
the  treatment plant.

Resource  Conservation  Company  claims  that the  system is entirely
closed, except for a small gas vent, and creates no environmental
problems.    Air  pollution  and  odor  control  equipment,  if
specified, would  be required  to  handle only a relatively  small
volume of exhaust gas.


        10.7.1.2  Current Status

A full-scale BEST system has yet to be operated.  A 1-gallon-per-
minute (4 1/m)  demonstration test  unit known  as  "mini-BEST"
was  evaluated  by Metropolitan  Engineers  in 1975  as part  of
Municipality  of  Metropolitan  Seattle's  research  program.
Combinations  of settled  primary  and  thickened  waste-activated
sludges  were  treated in  the pilot  facility.   The study team
concluded that  the  BEST process was  not cost-effective  for
                             10-29

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Seattle Metro (12).   The process was also compared by the  LA/OMA
project with several other candidate sludge disposal  systems  and
found  to be one  of the  more  expensive alternatives for  the
Los Angeles area (13).


        10.7.1.3  Operating Experience

Operating  experience  is limited to  laboratory  and  pilot  plant
tests.   Dried  solids  (about  5  percent water) and  product  water-
are  disinfected  as a  result  of  the  high  temperature  (250°F
[121°C] )  in  the dryer  and the high pH of  the  solvent solution.
Sodium hydroxide  (NaOH)  is  added  to maintain an alkaline
condition,  since  TEA  precipitates  an acidic environment.   NaOH
also  conditions the  sludge  to improve dewatering  and  the  dryer
performance.   The dried product is easy to handle and transport;
however,  pelletizing may be  necessary  to prevent dusting  and to
enhance product marketability.

Primary sludge  from Seattle Metro's West Point plant, containing
3.4 percent  solids  and  pretreated  with  2  to 5 g  NaOH/1 (100 to
300  pounds  per  ton  dry  solids),  was  blended  with  TEA  and
centrifuged.   A  cake  of approximately 30  percent  solids  was
produced.  A solvent-to-sludge ratio of 6:1 was  maintained.   The
liquid fraction  contained 60  percent solvent  and  40 percent
water, which reduced the energy  required to evaporate  the liquid,
compared to  drying  of  30 percent cake with  a 100  percent  water
fraction.   The dried product averaged 86  percent  solids with
1.6  percent  solvent  by  weight.   Product water, following
decanting  and  solvent  extraction  in  the  water  still,  averaged
280  mg/1 suspended  solids,  contained less  than 0.01 percent
solvent,  and had a pH  of 10.6.

This  high-technology process  is quite  complex and  may require a
competent  chemical  engineer  to  ensure  efficient  operation  (12).
There  are  a  relatively  large  number  of  components  in the  system
and,  hence,  maintenance  costs may be high.   Unpleasant  odors
(ammonia-like) existed in the exhaust gas during  the Seattle
study.  A  deodorization system may be required (12).   Full-scale
data  on  chemical  and  energy  requirements,  as well as operating
reliability,  are not currently  available on the BEST  system.


    10.7.2   Multiple-Effect Evaporation--Carver Greenfield
            Process

Multiple-effect evaporation is  another technique  that can be used
to  remove  water  from sludge.   The Carver-Greenfield process,
offered  jointly   by  Foster  Wheeler  Energy  Corporation  and
Dehydro-Tech Corporation,  uses  this technology.

The  basis  of  economy  for multiple-effect  evaporation  is  steam
reuse.   Steam  generated  in  the first evaporator (by  evaporation
of water from sludge)  is used as the heating fluid in the  second
evaporator.  The  method  is  feasible  if  the second evaporator is
operated at a lower pressure  than the first.


                             10-30

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        10.7.2.1  Process Description

The Carver-Greenfield  process  uses  a  multiple-effect evaporation
process to  extract  water  from sludge.   The  major  steps  in the
process are  oil  mixing,  multiple-effect evaporation,  oil-solid
separation,  and condensate-oil separation.

The applied sludge is mixed  with  a petroleum hydrocarbon oil
(Number  2  fuel oil  and  Isoparl,  an Exxon product,  have  been
used).   The use of  oil  maintains  fluidity in all evaporator
effects  and minimizes scale formation and corrosion of  heat
exchange  surfaces.   The  sludge-oil  slurry is pumped through
a  grinder  to  the  multiple-effect  evaporator.   The grinder
reduces the size  of slurry  solids  to  prevent  obstructions  in the
evaporator  tubes,  to optimize  evaporation,  and to  simplify
control.

Falling-film evaporation  is used;  that  is,  the water to be
evaporated  is  removed  as  the slurry  rolls down  the evaporator
tubes  in film flow.   Steam  and vapor flow is countercurrent
to the slurry  flow.  Vapors  flow  from high temperature (high
pressure) to  low  temperature (low pressure),  while  the  slurry
flows  from  low temperature  (low pressure)  to  high temperature
(high  pressure).   Steam is  applied,  at  pressures as  low as
50 psig  (345  kN/m^), to  the shell side  of the  first  effect
(last  stage) and its condensate  returned to  the  boiler.   The
water vapors removed  from the  tube side in that  stage provide the
steam  for the next (second)  effect  shell side.  The water vapors
condensed in the  second effect are  drained  to the hot well.   The
steam  energy,  thus,  is  used many  times.   In each subsequent
effect, the vapor temperature is  lower.   The  vapor  from the  last
effect (first stage) is condensed in a surface  condenser and
drained to the  hot  well.

Oil remaining  after  evaporation of  water  is separated from the
solids by  centrifuging.    Oil  is reused  in the  process, and
the dried  sludge  product  is subjected  to  further processing or
disposal.   The condensate   from  the  evaporation  system  results
in  a   sidestream  containing  ammonia  and  dissolved  organics,
but  few  inorganics.   This  sidestream may require subsequent
treatment.   Gaseous emissions from the system must be  sent   to a
boiler or incinerator  for odor destruction.
        10.7.2.2  Current  Status

According  to  the  manufacturer,  over  65 Carver-Greenfield
installations are in operation worldwide.   Many  of  these systems
have  operated at industrial  facilities  in  the United States,
including  a  four-effect  system at  the  Adolph  Coors  Brewery in
Golden,  Colorado.   This  system's  water evaporative  capacity
is  60,000 pounds  per hour (27,240 kg/hr) which  allows  it to
process  approximately  180,000  gallons  per day  (682  m3/day) of
a  4 percent waste-activated  sludge feed  (8,10).  Two systems


                            10-31

-------
are also  operating at sewage  treatment plants  in  Japan.   The
first,  installed  at  Fukuchiyama,  is a  three-effect unit which
processes  combined primary and  secondary sludge  at  rates up to
43,000  gallons per day (170 m-^/d) of 4.5 percent feed material.
The second,   installed at Hiroshima,   is  a four-effect unit,
which  can process up to 264,000 gallons per day (998 m3/d)  of a
2 percent  feed solids.  The product at both  facilities is  used as
boiler  fuel.

A 200-pound-per-hour  (91 kg/hr)  evaporative  capacity  single-
effect  pilot  unit was  evaluated  at  the Hyperion plant in
Los Angeles  by LA/OMA (14).   LA/OMA engineers  concluded  that
the Carver-Greenfield system appeared  to be a viable sludge
drying  process that offered considerable energy efficiency when
compared  to  conventional direct  and indirect contact  dryers.
However,   it  was  recommended  that  a large-scale facility should
be  built  and operated  to  conclusively  demonstrate  process
reliability and economics.

Energy  requirements for  a four-effect  Carver-Greenfield system
with hydroextraction were projected to  be  about  0.44 pounds of
steam per  pound  of water evaporated.   This value  was  based on
data supplied by the manufacturer, data determined  for  the
Coors facility,  and supported by theoretical analysis of  the
system.   The  energy requirement, including  steam  production,  was
estimated at about 675  Btu per  pound (1,568 kJ/kg)  of water
evaporated.  This compares favorably with the  1,200  to 2,000 Btu
per  pound  (2,790  to 4,650  kJ/kg)  water required   in most
conventional  heat dryers.
10.8  References

 1.  Perry, R.H. and C.H. Chilton,  editors.   Chemical Engineers'
     Handbook,  Fifth Edition.  New York.  McGraw-Hill, 1973.

 2.  McCabe,  W.L.  and  Julian  C.  Smith.   Unit  Operations of
     Chemical Engineering, Third Edition. New York.   McGraw-Hill,
     1976.

 3.  Faust, A.S., Wenzel, L.A., Clump,  C.W.,  Maus,  L.  and L.B.
     Anderson.   Principles of Unit  Operations.   Corrected Second
     Printing.   New  York.  John Wiley & Sons, Inc.  July 1962.

 4.  Treybal, R.E.   Mass-Transfer Operations.  New  York.  McGraw-
     Hill Book Company,  Inc..  1955.

 5.  Rich, L.G.   Unit Operations of Sanitary Engineering.  Photo-
     Offset.   Linvil G.  Rich.  Clemson, South Carolina.  1971.

 6.  Combustion  Engineering,  Incorporated.     Steam Tables.
     Available  from  Combustion  Engineering,  Inc.,  Windsor,
     Connecticut 06095.
                            10-32

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 7.   USEPA.     Current  and  Potential Utilization of Nutrients in
     Municipal  Wastewater  and  Sludge.    First  Draft.    Office  of
     WaterProgram Operations.   Washington,  D.C.  20460.
     July,  1978.

 8.   Yamamota,  J.H.,  Schnelle,  J.F.,  Jr.,  and  J.M.   O'Donnell.
     "High-Nitrogen Synthetic  Fertilizer Produced  from Organic
     Wastes."   Public __Works_.   January,  1975.

 9.   Krzeminski,  J. "Sludge  Drying  Processes:  More Flexibility—
     Higher Costs."  Sludge  Magazine.   p 32.   May-June,  1978.

10.
USEPA. A Review of Techniques for Incineration of Sewage
Sludge with
Development.
December, 1976
Solid Wastes. Office
Cincinnati, Ohio 45268.
of Research and
EPA 600/2-76-288.
11.   Regional Wastewater Solids Management Program., Los Angeles
     Orange  County  Metropolitan  Area  (LA/OMA  Project).   Sludge
     Processing  and  Disposal.    A  State  of  the Art  Review.
     Whittier, California.   April,  1977.

12.   Metropolitan Engineers.    BEST  Process Feasibility Study.
     Prepared for Municipality of Metropolitan  Seattle.  October,
     1975.

13.   Davis. W. and R.T. Haug.  "Los  Angeles  Faces  Several Sludge
     Management Problems."   Water and Wastes  Engineering.  April,
     1978.

14.   Regional Wastewater Solids Management Program, Los Angeles/
     Orange  County   Metroipo1itan  Area   (LA/OMA  Project,
     Carver-Greenfield Process Evaluation.  Whittier, California.
     December,1978.
                             10-33

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EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
        Chapter 11.  High Temperature Processes
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                          CHAPTER 11

                   HIGH  TEMPERATURE PROCESSES
11.1  Introduction

High  temperature processes have  been used  for  combustion  of
municipal wastewater  solids  since the early 1900s.   Popularity  of
these processes  has  fluctuated  greatly  since  their  adaptation
from the industrial combustion field.   In the past, combustion  of
wastewater solids was  both practical  and  inexpensive.    Solids
were easily dewatered  and  the  fuel required  for  combustion was
cheap and  plentiful.   In addition, air emission  standards  were
virtually non-existent.

In  today's environment,  wastewater solids  are more  complex and
include  sludges  from  secondary and  advanced waste treatment (AWT)
processes.   These sludges are  more  difficult to dewater and
thereby  increase fuel  requirements   for  combustion.   Due  to
environmental  concerns with  air  quality and  the  energy  crisis,
the use  of high  temperature  processes for combustion of municipal
solids is being  scrutinized.

However, recent  developments in more efficient solids dewatering
processes and advances in  combustion technology have  renewed  an
interest in the  use  of high temperature processes  for  specific
applications.    High  temperature  processes  should  be  considered
where available land  is  scarce,  stringent  requirements for
land disposal exist,  destruction  of toxic materials is required,
or  the  potential exists for recovery of energy, either  with
wastewater solids  alone  or  combined  with  municipal refuse.
High  temperature processes  have several  potential advantag
over other methods  (1):
es
     •  Maximum volume  reduction.   Reduces volume and weight  of
        wet sludge  cake by  approximately 95  percent,  thereby
        reducing disposal requirements.

     •  Detoxification.   Destroys  or reduces  toxics that may
        otherwise create adverse environmental impacts (2).


     •  Energy recovery.    Potentially  recovers  energy   through
        the combustion of  waste  products, thereby  reducing  the
       .overall expenditure of energy.
                              11-1

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Disadvantages  of high temperature processes  include (1):


     •  Cost.   Both  capital  and  operation and maintenance costs,
        including costs  for supplemental  fuel,  are generally
        higher than for other disposal  alternatives.


     •  Operating problems.   High  temperature  operations  create
        high  maintenance  requirements  and  can  reduce  equipment
        reliability.


     •  Staffings.  Highly skilled and experienced operators  are
        required for  high  temperature  processes.   Municipal
        salaries  and  operator status  may  have  to be  raised  in
        many locations to attract the proper personnel.


     •  Environmental  impacts.   Discharges  to  atmosphere
        (particulates and other toxic  or  noxious emissions),
        surface waters  (scrubbing  water), and  land   (furnace
        residues) may  require  extensive  treatment to assure
        protection of the environment  (3).


This  chapter  describes  both proven high  temperature  processes
and  those  having  high  probability  of  success,  as indicated
by  current research.   Multiple-hearth and fluid  bed  furnaces,
the  most  commonly  used  sludge combustion  equipment  in  the
United  States, Europe,  and Great Britain,  are  discussed,  as
well  as newer  furnace  types  such  as  the electric  furnace,
the  single  hearth  cyclonic furnace, and modular  combustion
units.   New  thermal  processes   for wastewater  solids  reduction
are  also described.   These   processes   include  starved-air
combustion and co-combustion  of  sludges  and  other residues.
Also  presented in  the  chapter  are   examples  that  illustrate
the  methodology used  in  selecting  and  designing  processes  and
equipment.
11.2  Principles of High Temperature  Operations

Combustion  is  the rapid  exothermic oxidation  of combustible
elements in  fuel.   Incineration  is complete combustion.
Classical  pyrolysis  is  the  destructive distillation,  reduction,
or thermal cracking and condensation  of  organic matter under heat
and/or pressure in  the  absence of oxygen.   Partial  pyrolysis,
or  starved-air combustion,  is  incomplete  combustion  and  occurs
when  insufficient  oxygen  is  provided to  satisfy  the  combustion
requirements.   The basic elements of each  process are  shown
on  Figure 11-1.   Combustion of wastewater solids,  a two-step
process,  involves drying followed by  burning.
                             11-2

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SLUDGE
FEED

COMBUSTIBLE
ELEMENTS, INERTS,
MOISTURE
AIR
(OXYGENI —

EXCESS
AIR SUPPLEMENTAL
-1 i r— FUEL (IF REQUIRED)
FURNACE


MOISTURE, EXCESS AIR,
PARTICULATES,
NOX, SOX, HC, C02,
OTHER PRODUCTS OF
COMPLETE COMBUSTION
                                                                  STACK GASES
                                                                  {NOT
                                                                  COMBUSTIBLE)
                          ASH (RELATIVELY INERT)

                     (A) INCINERATION (COMPLETE COMBUSTION)




                                HEAT
SLUDGE
FEED
COMBUSTIBLE
ELEMENTS, INERTS,
MOISTURE


FURNACE


                                                MOISTURE, PARTICULATES,
                                                NOX, SOX, HC, CO, C02,
                                                CONDENSATES (TAR & OIL),
                                                OTHER HIGHER
                                                HYDROCARBONS
COMBUSTIBLE
OFF-GASES
(UP TO 600
Btu/cf)
                     RESIDUE (COMBUSTIBLE CHARACTERISTICS)

                          (B) PYROLYSIS (NO OXYGEN)
AIR
(OXYGEN)
i
SLUDGE
FEED
COMBUSTIBLE
MOISTURE




SUPPLEMENTAL
FUEL (IF REQUIRE
1




                                                MOISTURE,
                                                PARTICULATES,
                                                NOX, SOX,

                                                CONDENSATES (TAR & OIL),
                                                OTHER PRODUCTS OF
                                                INCOMPLETE COMBUSTION
                                 f
COMBUSTIBLE
OFF-GASES
(UP TO 400
Btu/cf
                     RESIDUE (CAN BE UP TO 30% COMBUSTIBLE)
                   (Cl STARVED-AIR COMBUSTION (OXYGEN DEFICIENT)


                                FIGURE 11-1


             BASIC ELEMENTS OF HIGH TEMPERATURE PROCESSES
11.2.1  Combustion  Factors
     11.2.1.1   Sludge Fuel  Values

A  value  commonly used in sludge  incineration  calculations  is
10,000 Btu per pound  of  combustibles  (see  Table  11-1).    It  is
important  to  clearly understand the meaning of  combustibles.   For
combustion processes, solid  fuels  are  analyzed  for volatile
solids and total   combustibles.   The difference  between  the  two
measurements  is the fixed carbon.   Volatile solids  is  determined
by  heating the fuel in  the  absence of  air.   Total  combustibles
is  determined  by  ignition  at  1,336°F   (725°C).   By  definition,
                                     11-3

-------
the  difference  in weight  loss is  the fixed -carbon. - In  the
volatile  solids  determination  used  in sanitary  engineering  (see
Standard Methods,  Reference 5), sludge is heated  in  the  presence
of air  at 1,021°F (550°C).   This  measurement  is higher  than  the
volatile solids  measurement for fuels  and includes the fixed
carbon.   Numerically,  it  is  nearly  the same as  the  combustibles
measurement.  In the following,  if volatile  solids  is used in  the
sense of  the  fuels engineer, it will  be  followed paranthetically
by the  designation "fuels usage."   If  the term "volatile solids"
or "volatiles"  is used  without designation,  it  will indicate
sanitary  engineering  usage  and will  be used  synonymously with
"combustibles."
                            TABLE 11-1

          CHEMICAL REACTIONS OCCURRING DURING COMBUSTION

                                 High heat value
„_» _
c +
c +
CO +
H2 +
CH,
2H,S
C +
Reaction
o2 — -~
1/2 02 __
1/2 02 _^
1/2 02 	 	
+ 202 _
+ 302 _
H20 (gas) — — -
Sludge combustibles —
of reaction3'13
CO,
CO
CO,
H20
C02 + 2H2°
2SO2 + 2H2O
CO + H
— CO2 + H20
-14,
-4',
-4,
-61,
-23,
-7,
+4,
100
000
400
100
900
100
700
Btu/lb
Btu/lb
Btu/lb
Btu/lb
Btu/lb
Btu/lb
Btu/lb
-10,000 Btu/lb
combustibles
of C
of C
of CO
of H2
of CH.
of H2S
of C
of
Reference
4
4
4
4
4
4
Calculated
Estimated
aNegative sign convention indicates an exothermic reaction.

 High heat value assumes the latent heat of water generated
 is available for use:  conversely, low heat values assumes
 the latent heat of water is not available hence no water
 is condensed.

1 Btu/lb = 2,324 J/kg
The amount  of  heat released from a given sludge  is  a  function of
the  amounts  and  types  of  combustible  elements present.   The
primary combustible elements in  sludge and  in most available
supplemental  fuels  are  fixed  carbon,  hydrogen,  and  sulfur.
Because  free sulfur  is  rarely  present  in  sewage sludge  to any
significant  extent and because sulfur  is being limited in fuels,
the  contributions  of sulfur to  the combustion  reaction  can
be  neglected  in  calculations  without  compromising accuracy.
Similarly,  the oxidation of metals  contributes little to the heat
balance and  can  be ignored.
                               11-4

-------
Solids with a high fraction  of  combustible material;  for example,
grease and  scum,  have high  fuel  values (see Table 11-2).   Those
which contain  a large fraction of inert  materials;  for example,
grit  or  chemical  precipitates,  have low  fuel  values.   Chemical
precipitates  may  also  exert  appreciable heat demands  when
undergoing high temperature  decomposition.   This  further reduces
their effective fuel value.
                            TABLE 11-2

         REPRESENTATIVE HEATING VALUES OF SOME SLUDGES (6)
           Material
Combustibles,
  percent
High heating value,
Btu/lb of dry solids
    Grease and scum

    Raw wastewater solids

    Fine screenings

    Ground garbage

    Digested sludge

    Chemical precip

    Grit
;olids



.tated solids

74
86
85
60
57
33
                      16,700

                      10,300

                       9,000

                       8,200

                       5,300

                       7,500

                       4,000
1 Btu/lb = 2,324 MJ/kg
The following  are  experimental methods  from  which sludge heating
value may be estimated or  computed:

     •  Ultimate  analysis—an analysis  to determine  the amounts
        of  basic  feed  constituents.    These constituents  are
        moisture, oxygen,  carbon,  hydrogen,  sulfur, nitrogen, and
        ash.   In addition,  it is  typical to  determine  chloride
        and  other  elements that  may contribute  to air emissions
        or ash disposal  problems.   Once  the  ultimate analysis has
        been  completed,  Dulong's  formula (Equation  11-1)  can be
        used to  estimate the  heating  value  of  the  sludge.
        Dulong's formula is:
                                          °2
        Btu/lb =  14,544  C  +  62,208 (H2 - -)  + 4,050 S     (11-1)
        where C, H2,  02, and  S represent the  weight  fraction
        of  each element  determined  by  ultimate analysis.   This
        formula  does not  take  into  account  endothermic chemical
        reactions  that  occur with chemically  conditioned  or
        physical-chemical sludges.

        The  ultimate analysis is used  principally for developing
        the  material balance,  from which  a  heat balance  can  be
        made.
                               11-5

-------
     *  Proximate   analysis—a  relatively  low-cost  analysis  in
        in which moisture content,  volatile  combustible  matter,
        fixed carbon,  and ash  are determined.   The fuel value  of
        the sludge is calculated as  the  weighted  average  of  the
        fuel  values of its individual components.

     •  Calorimetry—this  is  a  direct method  in which  heating
        value is  determined experimentally  with a  bomb  calori-
        meter.   Approximately  1 gram of  material  is burned  in  a
        sealed,  submerged container.   The heat  of  combustion  is
        determined  by noting  the  temperature  rise of  the  water
        bath.  Several  samples must be taken and then  composited
        to obtain  a representative  1 gram sample.   Several  tests
        should  be  run,  and the results must be  interpreted  by  an
        experienced  analyst.   New  bomb calorimeters can use
        samples  up to 25  grams  and this  type of  unit  should  be
        used  where  possible.


The above tests give approximate fuel  values  for sludges and
allow the  designer to  proceed with  calculations  which simulate
operations of  an  incinerator.    If a  unique  sludge will  be
processed, or unusual operating  conditions will be used,  pilot
testing  is  advised.   Many manufacturers have  test furnaces
especially suited  for pilot  testing.


        11.2.1.2  Oxygen  Requirements for Complete Combustion

Air  is  the  normal source  of  oxygen for  combustion, although
pure  oxygen  feed   systems are  sometimes  used.    Theoretical  air
and oxygen requirements  for the combustion reactions  are  shown
in  Table  11-1.   For rigorous  analyses,  the constants given  in
Table 11-3 should  be used.    For general  applications in  which
fuel  oil,  methane,  and/or sludge  are  used,  a  rule of thumb  is
that it requires 7.5  pounds  (3.4 kg)  of air to release  10,000 Btu
(10.55 MJ) from  sludge or supplemental fuel (7).

In  practice, incinerator operations require  air in  excess  of
theoretical  requirements  for  complete combustion.    Excess  air
added  to  the   combustion  chamber  increases   the  opportunity
for contact  between the  fuel and oxygen.   To  ensure complete
combustion,  it  is  necessary  to maintain 50 to 150 percent excess
air  over  the stoichiometric  amount  required  in  the  combustion
zone.  When  the amount of excess air is  inadequate,  only  partial
combustion of  carbon  occurs,  and carbon monoxide,   soot,  and
odorous hydrocarbons  are  produced.

The excess air  required for  complete  combustion adversely  affects
the  cost of  operation,  because  additional heat is   needed  to
raise the  excess  air  temperature to that  of the  exhaust gases.
Supplemental  fuel  may be needed to furnish this  additional heat.
Thermal economy therefore demands that excess air be held to the
minimum value required to effect  complete combustion.  The amount


                              11-6

-------
of excess  air required varies  with the  type of  incineration
equipment,  the nature of  the  sludges to be incinerated, and the
disposition of the stack .gases. , The impact of excess air use on
the cost of fuel in sludge incineration is shown on Figure 11-2.
                            TABLE 11-3

             THEORETICAL AIR AND OXYGEN REQUIREMENTS
                   FOR COMPLETE COMBUSTION (H)

                                   Ib/lb of substance

                  Substance           Air      Oxygen
Carbon
Carbon monoxide
Hydrogen
Sulfur
Hydrogen sulfide
Methane
Ethane
Ammonia
11
2
34
4
6
17
16
6
.53
.47
.34
.29
.10
.27
.12
.10
2
0
7
1
1
3
3
1
.66
.57
.94
.00
.41
.99
.73
.41
           1 Ib/lb = 1 kg/kg.
        11.2.1.3  Factors  Affecting  the  Heat Balance

The  heat released  by  burning the  wastewater  solids must be
sufficient to raise  the temperatures  of all entering substances
from ambient  levels  to those  of  the exhaust  and solid residue
streams.  Also, any  radiant heat loss from the combustion struc-
ture must be included.   If the  heat  is sufficient, the process is
termed  autogenous.   If it  is  not  sufficient,  supplemental   fuel
must be burned to make  up  for  the heat deficit.

A number  of  variables  influence  the amount of supplemental  fuel
required.   As shown on Figure 11-2, the amount of  excess air
required to produce  complete combustion has an important effect.
Water associated with the  sludge also  exerts significant demands.
For example, it takes almost 2,000 Btu per pound  (4.64 MJ/kg) to
vaporize water  and  raise  the  temperature  of  the water vapor to
exhaust  temperatures.   When  allowances  are made  for radiation
losses  and for heating  of gas  streams and  sludge feed solids, it
is found  that approximately 3,500 Btu (3.69 MJ)  are required for
every pound  (0.45  kg)   of water evaporated in a multiple-hearth
furnace (8).

The  following  example  illustrates  how   the  feed  solids
concentration required  for autogenous  combustion  is determined.


                              11-7

-------
       io r
  D I
  LU
  E

  o
  LU
  K
           ASSUMPTIONS
            FEED: 30% SOLIDS
            COMBUSTIBLES: 70% OF THE DRY SOLIDS
            COMBUSTIBLE HEAT VALVE; 10,OOO Btu/lb
            SUPPLEMENTAL FUEL; NATURAL GAS
     i-o
                                                     DIFFERENCE DUE TO
                                                     SUPPLEMENTAL
                                                     EXCESS AIH FOR
                                                     FUEL
                              _L
                              GD      TO      100

                                 EXCESS AIR, portent
                                                    120
                                                           140
                                                                  Tea
                             FIGURE 11-2

           EFFECT OF EXCESS AIR AND EXCESS TEMPERATURE ON
                   SUPPLEMENTAL FUEL REQUIREMENTS
Example

A designer uses a proximate analysis  to derive  the following
values  for  a  given  sludge:    volatile  solids  content(fuels
usage--66  percent,  fixed carbon  content—11  percent, and  inert
content—23 percent.   The sludge is to be  dewatered and burned in
a multiple-hearth  incinerator.   The solids  concentration  required
for autogenous  combustion in a  multiple-hearth  incinerator can be
determined.

The  sludge  heating  value  can   be  estimated  by  multiplying
the  approximate  fuel value of  sludge--! 0,00 0  Btu  per  pound
(23.2 MJ/kg) by the combustible fraction in the sludge.   In this
                               11-8

-------
example,  the combustible  fraction is  the  sum of the  volatile
solids (fuels usage)  and  fixed  carbon, or 77 percent.  Therefore,
sludge heating value  is:


    10,000 Btu/lb x 0.77  =  7,700 Btu per pound  (17.89 MJ/kg)


The minimum percent sludge  solids required to maintain autogenous
combustion can  be determined  by  equating  the heat  released  by
combustion to the heat  required by the water.  Therefore:


    (P)(Q) = (100 - P)(W)
where:

    P = Minimum percent dry  solids  in sludge required  for
        autogenous  combustion

    Q = Fuel value  of  sludge, Btu per pound of dry solids

    W = Heat  required  to  evaporate  one pound of  water  in  a
        multiple-hearth  furnace, Btu


The above equation  is  solved for P:


    P = JL. (ioo)                                          (11-2)
        Q+W


For this example:



    p = i 7nn'!°7  Rnn  (10°)  = 31'3  Percent
        7,700+3,500


If  the  solids  could be  dewatered  to 31.3 percent,  they would be
combusted autogenously.   However,  feed solids concentrations of
this magnitude are  seldom achieved  without chemical conditioning.
Allowances  for the  effect of chemical  conditioning  should
therefore  be   made.    Assume  conditioning  requirements  are
25  percent  lime and 3  percent  ferric  chloride  by  weight of dry
solids  fed.  Therefore, for every  100 pounds  (45.,4 kg) of sludge
dewatered,  28  pounds  (12.7 kg)  of  chemicals are added.  Assuming
there  is  no heating value in  the  lime  and  ferric chloride, the
                                                       100
combustible fraction of the  feed solids  is reduced to  y^} x °-77
=  60 percent and the  sludge heating value is  6,000 Btu per pound
(13.9 MJ/kg).   Using  Equation 11-2, the  dewatered sludge must be
36.8 percent solids to be autogenous.


                             11-9

-------
Figure  11-3  shows  a  family  of  curves  that  can  be  used  to
calculate  the  minimum percent  solids  required at various dry
solids heating values.  This method of estimating takes  into
account the effect  of moisture  content,  inerts,  and combustibles
on the combustion process and can be used for basic sizing  prior
to detailed analysis.

For example,  in the  above  analysis,  a  sludge  heating  value  of
6,000  Btu per pound  of  solids (13.9 MJ/kg) was calculated.   From
Figure 11-3,  the 6,000 Btu per  pound (13.9 MJ/kg)  curve crosses
the break-even point at approximately 36 percent dry solids.
The importance of  dewatering  the sludge  is  illustrated  on
Figure 11-4.  The amount of supplemental fuel required is plotted
as a  function  of  feed moisture content  and  combustible  solids
concentration.

The amount of supplemental  fuel  can  be reduced  if  heat can
be recovered from  the process exhaust gases  and  reused.   As
 an example, heat may be transferred from the  furnace  flue
gas to  incoming combustion  air by  means of  heat exchangers
(recuperators).   Although energy recovery  can significantly
improve  thermal efficiency,  heat  recovery equipment can  be
expensive and  can  only be recommended  after  complete  economic
evaluation.
    11.2.2   Incineration Design Example

To evaluate combustion processes, a  designer  must  determine  if
the sludge  will burn  autogenously.   He  must also assess  the
effects of  different excess air rates,  the effects  of  different
types   and  quantities  of  supplemental fuel,  and  combustion  air
requirements.

Approximate  and theoretical methods  for  calculating  combustion
requirements are presented  in  the following examples   A summary
is then  provided  that compares the results  of  each method.
Either method  provides  the  information necessary for preliminary
evaluation  and conceptual design  of  a sludge   incinerator.
When an  ultimate analysis  of  the sludge  is  available  or  a  good
estimate of sludge  constituents  can  be made,  a  theoretical
analysis  is  preferred.


        11.2.2.1  Problem Statement

The dewatered   sludge production  rate expected  for  a  wastewater
treatment  plant   is  14,000  pounds  (6,350  kg)   per hour  at
20 percent  solids.   The  dewatered  material  is  a mixture  of
undigested  primary  and waste-activated  sludges,  with  a volatile
(combustible)  content of  77  percent.  The  sludge temperature
is 60°F  (16°C).  To  limit  hydrocarbon emissions, an afterburner
is  used  to heat   furnace  exhaust gases to  1,400°F   (760°C).
The design  is  based on 100 percent  excess  air (two  times  the
theoretical requirement).   If supplemental fuel  is  required,


                             11-10

-------
20 i-
        10
               20
                                       30
                         30      40

                     DRV SOLIDS m SLUDGE, %

                      FIGURE 11-3

EFFECT OF DRY SOLIDS HEATING VALUE AND SLUDGE MOISTURE
       ON CAPABILITY FOR AUTOGENOUS COMBUSTION
                                                       TO
                       11-11

-------
30   29    28
27
                  SOLIDS CONTENT OF FEED SLUDGE, %
                   26   25   24    23    22    21
 r

70
 20
T~
                                                     1g    18    17
     71
                 MOISTURE CONTENT OF FEED SLUDGE,
                   14   75   76    77   78    79
                                       81   82   S3
 60   61
               63   64    65    66   67   68   69
                  SOLIDS CONTENT OF FEED SLUDGE, %
                                                                 Z&OG
    ASSUMPTIONS
    SLUDGE HEAT VALUE - 10,000 Btu/lb COMBUSTIBLE
    FURNACE EXCESS AIR - 75%
    AUXILIARY FUEL EXCESS AIR - 10%
    EXHAUST GAS TEMPERATURE = 1200°F
               37   36   35   34    33   32   31   30
                 MOISTURE CONTENT OF FEED SLUDGE, %
                                                                73
                          FIGURE 11-4

EFFECT OF SLUDGE MOISTURE CONTENT AND COMBUSTIBLE SOLIDS
         CONTENT ON SUPPLEMENTAL FUEL CONSUMPTION
                             11-12

-------
No. 2 fuel oil will be used.   Twenty-five percent excess air will
be used for combustion of  the  fuel  oil.   The air temperature is
60°F (16°C); the absolute humidity of the air is 0.013 pounds of
water per pound  of dry air.    Heat  capacities  of  dry air, water
vapor,  dry  sludge  solids, and water  are  0.256, 0.5,  0.25 and
1.0  Btu per pound  per °F, respectively,  (1.07, 2.1,  1.0, and
4.2  kJ/kg/°C.   The latent heat of water is  970.3  Btu per pound
(2,253 kJ/kg).


        11.2.2.2  Approximate Calculation Method

Assuming  10,000  Btu per pound  (23.2 MJ/kg)  of sludge,  the heat
content of the sludge is:
    10,000 y^- x 0.77 = 7,700 Btu per pound  (17.9  MJ/kg)



From Figure  11-3,  a  value of approximately 32 percent solids  in
the  dewatered  sludge is  required  for  autogenous  combustion.
Therefore, supplemental fuel is required  and  its quantity must  be
determined.   The  demand  for supplemental fuel equals  the heat
required minus the heat value of the sludge.

    Step 1.  Sludge Heating Value

    The heating value of the sludge


      14,000 Ib sludge   0.2 Ib solids   0.77  Ib VS    10,000 Btu
             hr        x   Ib sludge     Ib  solids     Ib VS


    = 21.56 x 106 Btu per hour (22.75 KJ/hr)

    Step 2.  Combustion Air Requirements

    Therefore, combustion air requirements


    - 21.56 x 1Q6 Btu   7.5 Ib dry air   2fexcess  air  factor)
    	_	 x   10/000 |tu  .x  ^(excess  air  tactor)


    = 32,340 pounds dry air per hour (14.68  t/hr)


    Step 3.  Heat Required to Raise Ambient  Air Temperature


    The basic formula for determining the heat required is:


    Q = Mass x heat capacity x temperature change            (11-3;


                              11-13

-------
Heat required to  raise  dry  air from 60°F (15.6°C) to  1,400°F
(760°C)
  32,340 lb dry air x O.        x  (lf4000p .  6()0p)
= 11.09 x 106 Btu per hour  (11.70 GJ/hr)
Heat required  to  raise  the temperature of water vapor  in  air
from 60°F (15.6°C) to 1,400°F  (760°C)
= 32,340 lb dry air   0.013 lb water   0.5 Btu          _
        hr        x    lb air    x  lb-°F  x (1'400 F   60 F)
= 0.28 x 106 Btu per hour  (0.30 GJ/hr)
Step 4.  Heat Required to Raise Solids Temperature

Heat  required  to  raise the temperature  of the  volatile
(combustible) material  from  60°F  (15.6°C) to  1,400°F  (760°C)
  14,000 lb sludge   0.2 lb solids   0.77 lb VS   0.25  Btu
         hr        x   lb sludge   x  lb solid  x   lb-°F


x (1,4QO°F - 60°F) = 0.72 x 106 Btu per hour (0.76  GJ/hr)


Heat required  to raise the temperature  of  inerts  (ash)  from
60°F  (15.6°C)  to the  ash discharge temperature  of  200°F
(93.3°C)
_ 14,000 lb sludge   0.2 lb solids   (1-0.77) lb inerts)   0.25 Btu
        hr       x   lb sludge   x      lb solids        lb-°F


x (200°F - 60°F) = 0.02 x 10^ Btu per hour  (0.02  GJ/hr)
Step 5.  Heat Required to Raise Temperature of Water
Associated with the Feed Sludge

This  calculation  does  not  include  water formed  during the
combustion reaction.
                          11-14

-------
Heat  required  to raise  the  water  temperature  from 60°F
(15.6°C)  to 212°F (100°C)


= 14,000 lb sludge   0.8 lb water   10 Btu x 212oF _ 6QO
       hr          lb sludge    lb-°F              '


= 1.70 x  106 Btu per hour  (1.79 GJ/hr)


Heat required to evaporate water
  14,000 lb sludge   0.8 lb water   970.3  Btu
         hr        x   lb sludge  x    lb
» 10.87 x 106 Btu per hr (11.46 GJ/hr)


Heat  required to  raise the  temperature of  water vapor  to
1,400°F (760°C)
  14,000 lb sludge   0.8 lb water  0.5 Btu
        hr       x lb sludge  x  lb-°F
= 6.65 x 106 Btu per hour (7.02 GJ/hr)
Step 6.  Heat Required to Raise Temperature of
Water_Formed During ,the__Cp_mbustion Reaction

Assume  water  formed during  the  combustion reaction  to
be  0.5 pound  per  10,000  Btu  (21.5 g/MJ)  of  sludge  and
supplemental  fuel  burned (9).  The  heat value  of  the
sludge  burned  and supplemental  fuel  are  equal  to the  heat
demands.   Therefore,  water  formed during  combustion must  be
calculated on the basis of heat demands.   Heat demands  may be
approximated by summing the calculations  thus  far:


Heat requ_ir_ed for               Btu/hr x  106

Air
  Dry air                           11.09
  Water vapor in air                 0.28

Sludge
  Volatile solids                    0.72
  Inerts                             0.02
                        11-15

-------
Heat required for               Btu/hr x106

Sludge (continued)
  Free water
    Water                            1.70
    Evaporation                      10.87
    Water vapor                      6.65

Total                                31.33  (33.05 GJ/hr)


Water formed due to the combustion reaction


-   0-5 Ib    31.33 x 106 Btu =                          .
— ^ x /\/\/\ T-. i  A      i          JL f ~J\J f t-JUUl H-4O k-"Cl_ 11VJ\J,1_  I / _L _L JVM/ ill. I
  10,000 Btu        hr         »    t~    r~       \     ^/  /


The  heat of combustion  given  is  the "high  heat of  combus-
tion,"  which assumes  all  water  formed  is  condensed.    Heat
must  be  provided to evaporate  this  water  and bring it up  to
exhaust  temperature.

Heat required to evaporate the water


- 1.567 Ib water   970.3 Btu _         6               Q   ,
—      .       A    i ,     •" X • ~/^> A J.U OL.U L/tTl. llwUl.  I X . \J\J ww / 111. /
       nr          ID


Heat  required  to raise the temperature of  water vapor  to
1,400°F  (760°C)


- 1.567  Ib water   0.5 Btu    M 4nnop _ oioopx
	j—       x  lb_0p  x  (1,4UU  i   m  t)


= 0.93 x  106 Btu per hour (0.98 GJ/hr)


Step  7.	Heat Required to  Compensate for  Radiation Losses

Assume   a  radiation  loss of  5 percent  of  the  total   heat
demand.   Total heat demand is


Heat required for               Btu/hr x 106

Total from Step 6                    31.33
Water formed during combustion
  reaction
    Evaporation                       1.52
    Water Vapor                       0.93      ;

Total                                33.78 • (3,5.;64  GJ/hr)


                          11-16

-------
Heat to compensate .for radiation losses



= 33'78 ^r10  BtU x 0.05 = 1.69 x 106 Btu per hour (1.78 GJ/hr)



Step 8.  Determine Supplemental Fuel Required

Total heat requirements (from Step 7)


= 33.78 x 106 Btu/hr + 1.69 x 10^ Btu/hr


=35.47 x 106 Btu per hour  (37.42 GJ/hr)


Total supplemental heat demand


= Heat demand minus heating value of sludge


= (35.47 x 106 - 21.56 x 10^) Btu/hr


= 13.91 x 106 Btu per hour  (14.68 GJ/hr)
Therefore,  supplemental  fuel  (No.  2  fuel  oil)  must  be
supplied  to  provide 13.91 x  106 Btu per  hour  (14.68  GJ/hr)
of heat.

Supplemental fuel also  requires  air for combustion,  and this
air exerts a heat demand.   The air required for supplemental
fuel  is  1.25  times   the  theoretical value needed  for
supplemental fuel.

Air required for supplemental fuel


- 13.91 x 1Q6 Btu   7.5 Ib dry air  ,  „  ,excess air factor)
	_      x  10fOOQ Btu   x l.2b  (excess air tactor)
= 13,000 pounds dry air per hour (5,920 kg/hr)


The  13,041 pounds (5,920  kg/hr)  dry air  (plus any  water
formed by  its  reaction  with the supplemental fuel)  must also
be  raised  to  1,400°F  (760°C).   By calculations similar  to
those  presented  in Steps 1  through 8, it can  be  shown that
heat  required  to do this  (and  to account  for additional
                          11-17

-------
radiation losses)  is  20.24 x 106 Btu per hour  (21.35  GJ/hr).
Since  only  13.91  x  106 Btu  per  hour  (14.68  GJ/hr)  was
released by  burning  supplemental  fuel,   there  is  a  heat
deficit  of  20.24 x  106  - 13.91  x 106 =  6.33  x  105  Btu per
hour  (6.67  GJ/hr).   Thus,  the  effect of adding  supplemental
fuel was  to reduce but  not  eliminate the  initial deficit of
13.91 x 106 Btu per hour  (14.68 GJ/hr).
To  make  up for this deficit,
equivalent  to  6.33  x  10^ Btu per
If  25 percent  excess  air is used
per hour  (2,694 kg/hr) of  excess
                                   more  supplemental fuel,
                                  hour  (6.68  GJ/hr)  is added.
                                  for this  fuel,  5,934 pounds
                                   air will be  required.   The
heat  released is  again  insufficient  to raise  the air  plus
water  vapor  formed to  1,400°F  (760°C) and  to make up  for
additional radiation  losses.  The deficit  for this iteration
is 2.88 x 106 Btu per hour  (3.04  GJ/hr).

The  calculation can  be carried forward  for  several more
steps.   Table  11-4 shows  that  progressively  smaller  addi-
tions  of supplemental  fuel and  air  are  required for  each
iteration and  that the  amount  of  air and  fuel needed  for
each  iteration  is a  fixed  fraction  (0.45)  of  the  fuel
and  air needed  for  the previous  iteration.   In general,
if  fuel required for  each iteration is  r  percent of  that
required for  the previous iteration, then  total fuel required
=  (initial  deficit)(l  +  r +  r2  +  r3  +  ...  +  rn).    The
term  in the  second  bracket is  an  infinite  geometric  series

equal to   rn.   The series  converges to

value of r is less  than  one (10).
                                               if the absolute
                         TABLE 11-4

            APPROXIMATE COMBUSTION CALCULATION -
               SUPPLEMENTAL FUEL REQUIREMENTS
      Heat input
      —-"' • ™
Unit
Sludge
Supplemental fuel
Supplemental fuel
Supplemental fuel
Supplemental fuel
Heat value,
106 Btu/hr
21. 56
13.91
6.33
2.88
1.31
Heat value.
Unit 106 Btu/hr
Slndqe and
excess i
Supplemen a
excess i

Supplemen a
excess i
Supplemen a
excess ai
35.47
fuel and 20.24
fuel and 9.21
fuel and 4.19
fuel and 1.91
Supplemental
fuel requirements,
10& Btu/hr
13.91
6.33
2.88
1.31
0.60
                                            .46


                                            .45


                                            .45


                                            .46
 Combustion
air requirements,
   Ib/hr


   32,340


   13,041


   5,934


   2,700


   1,228
                                                         .46


                                                         .46
 aRatio of supplemental fuel to that in the previous iteration.

 Ratio of air to air in the previous iteration.

 CRatio in this case is not applicable since sludge is included (100 percent
  1 x 10 Btu/hr = 1,055 MJ/hr
  1 Ib/hr = 0.45 kg/hr
                           11-18

-------
The total  supplemental  fuel  requirements can be derived from
Equation 11-4.
Total supplemental fuel = Initial deficit x  ^—         (11-4)
Total supplemental fuel



= 13.91 x 106
              hr    1-0.45


= 25.32 x 106 Btu per hour (26.6 GJ/hr)


Step 9.  Total Air Requirements

The air  requirements  for the  supplemental  fuel alone can be
found from Equation 11-5, an analog to Equation 11-4.

Total supplemental air requirements
= excess air for initial supplemental fuel addition x T^—        (11-5)
Total supplemental air rquirements


= 13,041 Ib air     1
       hr       x 1-0.45


= 23,735 pounds dry air per hour (10,766 kg dry air/hr)


Total dry air requirements


= air for sludge plus air for supplemental fuel


  (32,340 -I- 23,735) Ib dry air/hr


= 56,075 pounds dry air per hour (25,458 kg/hr)


Assuming an  air  density  of 0.0749 pounds  per cubic  feet
(1.2 kg/m3):


                          11-19

-------
    Air flow rate
      56,075 Ib/hr    hr     -n /no  u-  ^  x.     •  *.  /r n  •*/   x
    = —n n7AQ— x £n mi r. = 12/478 cubic feet per minute (5.9 nH/sec)
        0.0749     60 min
    Assume that  No.  2 oil has  heating  value of 141,000  Btu  per
    gallon


    Supplemental fuel rate


      25.32 x 106 Btu/hr     hr
      141,000 Btu/gallon x 60 min


    = 3.0 gallons per minute  (0.18 1/s)



        11.2.2.3  Theoretical Calculation Method

The  method  presented herein  is based  on the  actual  combustion
reactions  and  the method  of approach  used  in steam  generation
calculations (9).  Table 11-5  is  to  be  used  for steam  generation
calculations.  A blank form  is  provided  at the  end  of  Chapter 11
for the reader's own use in making the calculations.

    Step 1.  (Line b) .  Determine the  fuel  analysis and  include
    on the right hand side of the table (ultimate  analysis).

    Step 2.  (Lines 1 through 12).  Determine the pounds  of
    component,  moles of component, theoretical oxygen requirement
    and moles  of material contributed to  the flue gas by  the
    fuel,  based on  100  pounds  of  fuel feed.  Assume complete
    combustion and no loss of combustibles to the  ash.

    Step 3.  (Lines 13,  14, and 15).   Assume  the amount of excess
    02 to be used (100 percent) and  calculate the  moles of excess
    02 required.

    Step 4.  (Line 16) .   Calculate  the amount  of  N2 added  from
    the air from the total 02 (theoretical plus excess).

    Step 5.  (Lines 17,  18, 19, and  21).  Calculate the amount of
    dry air, water  in the air,  the amount  of  wet air from  the
    total dry air (02 + N2).

    Step 6.  (Lines 20 and s).   Calculate  the  moles   of   all
    components in the flue gas  and the  moles  of wet and dry  flue
    gas.


                              11-20

-------
                                            TABLE 11-5
                  COMBUSTION CALCULATION - MOLAL BASIS
                                             Flu*
                                              Malr= pnr Full Unrt (AF)
                                                                       Fu* Anil, » ffcwi (AFX % by W « VM
       CtoCOi
       CtaCO
       oo io cos-
                                                                      TrtM * (T A) issuing ar b^QRSAT HI %
                                                                          Ur*s I, g,
                                                                      WL IIMI ynft -
                                                                      Wol,
                                                                      D«i,ilti..:.TriJ«»K!FiKlrv -
                                                                     Fuel hen! value. Btu/lti IH1
                                                                     Catnbuitibti in refute. % "C"
                                                                     Carbon unbumiiii,"ib/lin Ifc liud
                                                                       • % ash in fuel x
           Os *nd Air, Mslti for Tow Air - If i "it
                 {see lined at rlgM)
        0) (tilTO) re»1 - Os, llnf 12
        Oi (ucera) - --ifla   X 0|. tin* I?
        0] (toW)wppli«i - lines 13 + H
        Ns SUpplltd - j.?$ X Oi, Urn IS
        Air (dry) supplied -  0;
                                                                     t«it timp »f flu* pt*. !.-
                                                                     Dry bulb {ambient) temp, f i
                                                                     Rei huntid,
                                                                     6*, &sre»™trk: preMur*. in, Hf
                                                                     Stt pr«i Htp »< imB tw|Wi "
                                                                     A*, pf«s«, HjO HI air. iiwi (» x q), ta. MI
                           ^ __ .
        Air (twt) nipplM -fcleil? -IB
        Not« - fur air it 30 T and MO* rMttlv* lummlKj,   1 »  IWB7 is gtt«» cacrf a= itsndird
                         D*t«r™n«Hwi of Fhw 6w wid &-ibu.hble L«l»l In BH [Hf Fwl Urrt e»fb9ninrwfus» - line kx 14.100                     i««i I
        Du« Is tiBbumed CO In f\tm ill - nw!« C to CO x « x 9,755      •*•• Z6 -I- 291- JRH- rtlttMkn ttt
                          100 x in* i far striH
                         • 3,4 „,,„,, x lOuv
                        fuol »irt - lin* 33 - Ii04 31
t Ply* s« tmlyt 1* by ORSAT.  If CO I* prmtmjt In nun B»«», * cirbon
 b»l«r>tt It UMd IP d«t*«-n«lri«dlili-lbiJtl«i of C, thux:
 All C Ml y •mlytlt^U,
 MelMCInr*iy**'llrMh^i-  Molw C In CO. » (n»t« C In fuel  -iMl«C
 InrrtlM) x ICO,  byCMSAT^I (CO,. CO) by OMHT.
 M«l« In C In CO -  mJM C In fu.l  - mala C In rrfu» - mh. C In CO],
                                                                                CtMtRAJ. HOTCS:
                                                                                *
                                                                                *   M*AtH, u UHd In
 tt By Dueeno famub (tl-H «• by «lflctawiry,
ttt R*3l*0on **w«wd to t* t flx*d pwc«r* =f Iliw U. iwtul
   Copyrbjhl 1471 by th. BtbCdCk Hid WlkOX CoBKMny, Mlrw
                                                                                1 It>-(,l9kf
                                                                                I In. - 1,U«
                                                                               i it/cu n • it
   mid* |» tnli ofeta to "Ik™ Iv
                                of yu wl»
                                                      , T»W«w«r h«
                                                 11-21

-------
Step 7.   (Lines  22  through 26)
content  of
used.   The
Reference  4
           the  gas.   A  base
            values for mean specific
  Determine the  sensible  heat
temperature of 60°F  (15°C)  is
        heat  can be found  in
Note:   mean  molar specific  heat  =  mean specific heat  x
molecular weight.

Step 8.  (Line 27) .   Determine the  latent heat  of water  in
the flue gas.

Step 9.   (Line 28).  Sum  all heat in flue gas.

Step 10.  (Lines  29, 30,  and 31).  Calculate  heat  losses  due
to carbon in  refuse  (residue),  unburned  CO in  the flue  gas,
and  radiation (assumed  to  be  5 percent).    Sum all heat
losses.

Step 11.  (Line 32).   Determine  heat value  of the  sludge  per
100 pounds,  wet basis.
Step 12.   (Line 33).   Determine  if the  sludge  is  autogenous
or  requires  supplemental  fuel by  subtracting  line 32  from
line 31.   A zero or positive  number indicates that the  sludge
is  autogenous,  supplemental  fuel is  not  required, and  the
computation  is  complete.   A n<
Disputation  is  complete.   A negative  number  shows that
supplemental  fuel is  necessary.  The method used to determine
the amount of fuel required is shown in steps  13  through  15.

Step 13.   If  Step  12 indicates  that supplemental fuel  is
required,   proceed  through  another   theoretical  calculation
method table  for the supplemental  fuel  in  the  same manner  as
Steps 1 through  12  (lines  1 through 33).  This  determines  the
amount  of excess  heat  in the fuel after the  combustion
reaction.   Table  11-6 illustrates the  supplemental fuel
calculation for  this  example.
Step 14.   Determine  the  amount
100 Ib (45 kg)  of wet  sludge.
                                 of  supplemental  fuel per
Ib supplemental fuel  required
 100 Ib of sludge,  wet  basis
  heat required from fuel  (line 33, Table 11-5)
  available heat from fuel  (line 33, Table 11-6;
   91,139 Btu/100  Ib sludge
  1,165,443 Btu/100  Ib  fuel
= 7.82 Ib fuel/100  Ib  sludge, wet basis
                          11-22

-------
                                               TABLE 11-6
                    COMBUSTION CALCULATION - MOLAL BASIS
          Tabte «-•  Comburton Cakutotloni-Molal Bwrit
32
             Fu«, Q.,, »nd Air [W Unit of Fuel
           fyst
        CISCO:
        CtoCO
        C unturned,
         fmk
        Hj
        s
        COi
                    Fwl
                    Unit,
                         Oiui.
                11.7
                 1.6
                 IJ
                 1,1
                 1
                 *
                              tJ*
Lit
M1
 I
 I
 I
 1
                                       Htqd
                                   TJ1
                                     I
                                     I
 1.11
 Ul
-1.11
  D
  O
  o
  o
                                                Moles per Furl Unit (*F)
                                              SO;
                                              IJI
                                        It.ll
            O-i tnii Aw, Moles for Total Air -
                  £JM line -J al
        02 i  f in, IIL
                                                                           MIDCITT HIPMIIY
                                                                             100 '"' *8|M ^^^ lutl1
                                                                   00
                                                                          Fun An»t, « Fired (ftf), % 6y W at Vrt
                                                                      MOO
                                           OQ2
                                                          CO
                                                                             arr (TJL) asa^ntti or by fflRSAT III %
    br*s f, 5, h Far Sas
Wt (ml -Jral - I {mates E
Mai. ••« of fud - lira f , 'l
                                                                                               s Fuels
                                                                                             cd x ma. wi) Ib
                                           Fuel 1«t »Blg«, Sty/113  21,441
                                           Crb-:r un Burned, ib/tbo It hd
                                             . % ish in fuel x rm—5T-TF::
                                           E«lt tsmp at nys us, %
                                                      ent) temp, f i
                                                                                            !^i —
                                                                                             3^4 6y tt
                                                                                                   tt
                                                                                                    1
                                                                                                Hll
                                                                                               :   II f
                                           Rfrl hjrrid
                                           8* Sarginrtnc SJfSSSMfS, In. Mg    |§j
                                           S*t PTOSS. N^O 3t arnb t«mpf in. Hg
                                           A*. fftBi, Htc in v, mm {a x q). w. HI
                                                                      Tet»i
                                                                      Metes
                                                                                   Wrt
                                                                                      IIJ1
                                                                 Clry f IPB Oas
                                                                   tIJl
    *Npt» - for aira« 80 f and HXni relative humidilv  -7^- (1037 ii OHSI U9«l « BtBndartl,
                     O*»«™lMli«i af FliH fits J(id Ccmlmrtibl* Unm in Mu pv Fu*l IMI (AF>
    Flye |^as constituents                                      COi+SO;
    Off, main, !i Hj f i (fgr Tj -                                 llJl-U
    In dry Me mi - mofcs MCU, »«ii M x Wep x^f" - fi'l            IMfnim
    tn N20 in air - mdas HjO, Sn« IB x Mc» X Hi — f"j)
        in wni iwtrt. H?0 In  n &-*"]
        (n btent heit, HjO in fu«» - mot*s, llnss {! + 1ft) x 1 (WO * IS
        T«ftl In w»t fly* g»6
        Cue ta carton in titui* - line k X 14,1W
        CUB tn .jnburnafi CO in HUE gas - tnntas C ta CO .:•: 12 .•: 9,755
                                                                        9s
                                                                        "i.j"
                                                                               u
                                                                             *7J.7H
                                                                                  71.731
                                                                                 111,171
         I«B« ftu* BIS ^8l«6 + unb*ra
                                            - lines 2B I- 2i f 30 4- f*SHU« ttt
                           100 X line i )w solid and liquid ruet
                           M4 y ||ne , ,x 1
         T«ai ««Hi !»n pff fud unit - line 32 - line 31
                                                                                              OQ
                                                                                                      Tntii
                                                                                                      II Jit
                                                                                                 »1 1,171
                                                                                             UI.711
                                                                                               I
                                                                                               i
                                                                                            M*M«
  t Flue gtt *n*ly*» t)j OHSAT ,  If CO it pr*i*rt In flui D^HI, i carbtxi
   balanci I* uud ID d*t*rmlo» dlttrlbMlten of C. ltn«:
   All C In fwl * C in Tlus gif ««fUbJwiti - C in r**utt  Mahn C in
   full = X C by anilylil -i-t2,
   Mal« C In rj - [mol»t C in to* - nwln C
   tn r«fti»«) x I C»i by (MSAT^ t tco j + CO) br OWSAT,
         lo c In CO = i»ol«i C In fuel - raolkm C m rofu» - mai« C in CO].
 tt By D«*w«j f=™tiU (11-1) or far l
ttt RMtlitikin itiuflMd to b* • 1*«d iwriunt of llm tt, nwmtly ; •> $ p*r«*.
                                                                                   IU*j»r *> uud In Mi
                                                                                   ttM*, It Ow r**k»u* (lift)
                                                                                   from tlvt prsc»M .
                                                                               1 ID • «.«• 19
                                                                               1 In. -3. Ma
                                                                               1 Btu/lb -
                                                                               1 Ib/cu ft . !•
   Copyright 1J75 fay ttiB B^KDck end Wlleanc CdO^Mny. Mlnar clkiiijw* nw* b**n
   Tnilfl t« t*il« t»b(« to allnw fur Mia of un Kltfl HMtgi lludp*. T*Di* mff tM
       t wltinxit pernlulon. Hlnovir. cr«ll 1 In Bibcock «1
                                                    11-23

-------
Step 15.  Calculate  the  total fuel  demand  for 14,000  pounds
per hour  of wet sludge (6,356 kg/hr):
Total fuel

  7.82 Ib fuel
                x 14,000 Ib sludge/hr
  100 Ib sludge


= 1,095 pound fuel per hour (497 kg/hr)


From line i, Table 11-6, Btu value


= 1,095 Ib fuel/hr x 20,440 Btu/lb


= 22.38 x 106 Btu per hour (23.61 GJ/hr)


Step J.6.  Calculate  the total  combustion  air  requirements:

From Table 11-5, line 17 combustion air  required for  sludge  =
8.47 moles/100 Ib sludge.

From Table 11-6,  line 17  combustion air  required for
supplemental fuel = 61.83 moles/100 Ib fuel.

Total dry air


   '8.47 moles air
      ^100 Ib sludge / \

                     \ /             \~
                                        29  Ib  air
  ^61.83 moles air\/n nQt, Ib fuel\
  \  100 Ib fuel  y^1'050   hr   )
                                       Ib  mole  air


    = 54,040 pounds per hour (24,534  kg/hr)


        11.2.2.4  Comparison of Approximate  and
                  Theoretical Calculation  Methods

Table 11-7  shows  that  the  approximate method requires  slightly
more fuel and air  than the theoretical  method,  but  the values are
close.   This comparison shows that the  approximate  method is
suitable for preliminary  evaluations.   More  detailed information
and combustion theory  can be  found in  the literature (1,4,6,7,9,
and 11-16).
                          11-24

-------
                              TABLE 11-7


        COMPARISON BETWEEN AN APPROXIMATE AND A THEORETICAL

                 CALCULATION OF FURNACE COMBUSTION


                    Approximate method (AM)        Theoretical method (TM)

                                                     Calculation   Difference
                                                      reference   AM-TM  . nn
                                            Value         (TM)     TM  * 1UU

                                          1,661 Btu3     Table 11-4    -7.28
                                          Ib asfed        line i

                                          91,139 Btub     Table 11-4     9.01
Item
Sludge heating value
Furnace heat deficit
Supplemental fuel heating
value
Supplemental fuel required
Total combustion air
Value
10,000 Btua
Ib VS
13.91 x 106 Btu
~hr~
141,000 Btuc
gal
25. 32 x 106 Btu
hr
56,075 Ib
Calculation
reference
(AM)
Assumed
Step 8
Step 8

Step 8
Step 9
                                        100 Ib wet sludge     line 33

                                          20,440 Btu     Table 11-5     -4.19
                                             Ib         line i

                                         22.38 x 106 Btu     Step 15     13.14
                                                hr

                                           54,040 Ib       Step 16     3.77
 required                  h~r"                     Hr"
 10,000 Btu/lb VS at 77 percent VS = 7,700 Btu/lb dry solids,
 1,661 Btu/lb as fed v 20 percent solids = 8,305 Btu/lb dry solids.
b91,139 Btu/100 Ib wet x 14,000 Ib wet/hr = 12.76 x 106 Btu/hr.

C141,000 Btu/gal ; 7.2 Ib/gal = 19,583 Btu/lb.

1 Btu/lb = 2,324 J/kg
1 Btu/hr = 1,055 J/hr ,
1 Btu/gal = 279 kj/m3
1 Ib/hr = 0.45/hr
    11.2.3   Pyrolysis and  Starved-Air Combustion
             Calculations

Pyrolysis  and  starved-air  combustion  have  received considerable
attention  recently.   The   yield and  composition  of  the gas  and
residue  depend upon several variables.   The actual  interrelation-
ships  are  so  complex that final  product characteristics must be
determined  empirically.

Currently,  data  are insufficient  to  provide  information  for
designing  pyrolysis  equipment.   Several  large pyrolysis  projects
have been  proposed,  and some  are in start-up  or early operation.
However, most work to 1979 has been at  laboratory scale.  At this
writing,  there  are  no  full-scale  pyrolysis projects proposed or
under  development that  use sludge  alone; all  are for solid  waste
or specific industrial  wastes.

Starved-air combustion,  a partial pyrolysis  process, has  had
a  number   of  successful   tests,  such  as those  conducted  at
the  Central Contra Costa Sanitary  District  (17,18),  and  the
Interstate  Sanitation Commission   (19),  and  several  modular
combustion  units have  used municipal solid waste,  sewage sludge,
and/or  agricultural  wastes.   Starved-air combustion has  also had
some  failures  such  as  at  the  Baltimore  plant,  which  used  only
solid  waste.  The  furnace  at  Baltimore  is  now being modified for
further testing and use.   Multiple-hearth  furnaces  have  been
                                11-25

-------
tested for  both  sludge and  co-disposal  starved-air  combustion.
This work on  starved-air  combustion  by multiple-hearth  furnaces
has been  sufficient to allow development  of empirical  design
criteria (17-20).

Some  engineers  and manufacturers use  a hearth  loading  rate
of  10  to 14  total pounds  per square foot  per  hour (48.8  to
68.3  kg/m2/hr)  over  the whole  effective  hearth  area while
assuming that  up  to 15 percent  of the  input  energy  remains  in
the ash  as  a  char.   Other engineers  and manufacturers  use  the
following design  criteria which  assumes a  lower  hearth  loading
rate and an  additional  hearth area  to gasify the fixed  carbon.
This  design results in a very low combustible content  in  the
ash (20):

    •  15  percent  of  the  combustible  matter  becomes fixed
        carbon.

    •  Fixed carbon is gasified at a  rate  of 0.5 to 0.8 pounds
        per square foot per  hour  (2.4 to  3.9 kg/m2/hr).

    •  Wet sludge feed rate (hearth loading rate) varies between
        8 and 12 total pounds.per square foot per hour (39.0 and
        58.6 kg/m2/hr).
        Assuming afterburning,  85  percent of the total feed energy
        remains in the afterburner gases.
ExjampJLe
Estimate the required hearth  area  of  a multiple-hearth furnace to
burn  the  sludge  generated from a  20  MGD (0.88 m3/s) wastewater
treatment plant by starved-air  combustion and the heat content of
the  hot  gas from  the  afterburner.   Assume the  furnace  feed is
40,000 pounds  per day dry solids  (18,140  kg/day).   Assume that
the  furnace  feed is  40  percent  solids  and that  the solids  are
65  percent combustibles.   Afterburning  to  1400°F  (760°C) is
required.

    Wet sludge feed rate


      40,000 Ib dry solids      1  Ib  sludge      1 day
              day          X  0.4  Ib dry  solids  X 24 hr


    = 4,167 pounds wet sludge per  hour (1,890 kg/hr)


    Fixed carbon rate


      40,000 Ib dry solids   0.65  combustible solids
              day                 Ib  dry solids


                             11-26

-------
      0.15 Ib fixed carbon    1  day
      Ib combustible solids   24 hr
    = 163 Ib fixed carbon per hour (73.9  kg/hr)


Estimate hearth  area  and multiple-hearth  furnace  size.    Hearth
area is considered as  the sum of the area required  to  convert  the
wet sludge to the fixed carbon  stage and the area  needed  to burn
out the fixed carbon.


    Hearth area
      wet sludge feed - fixed carbon feed   fixed  carbon  feed
         allowable hearth loading rate      gasification  rate


      4,167 Ib/hr - 163 Ib/hr      163 Ib/hr
          10 Ib/sq ft/hr        0.5 Ib/sq ft/hr


       726 square feet (67.44 m2)


After  discussions  with the  furnace  manufacturers,  a  14-foot
3-inch (4.34 m)  diameter,  8  hearth  unit  with  an effective  hearth
area of 760 square feet (70.6 m2) is selected.

Estimate the heat  content  of hot gases  leaving the  afterburner:
    Heat content
      40,000 Ib dry solids   0.65 Ib combustible  solids
              day          x       Ib dry solids


      	10,000 Btu	   1 day   n ftc-
    x Ib combustible solids x 24 hr x u*tti


    = 9.2 x 106 Btu per hour (9.72 GJ/hr)


Portions of this heat  can  be  recovered  and  used  benefically,  for
example, to generate steam or hot water (see Chapter  18).

BSP Division  of  Envirotech Corporation,  and Nichols  Engineering
and Research  (now  part of  Wheelabrator)  have  developed a  large
data  base  for evaluation  of  starved-air combustion  operations.
                              11-27

-------
Even with  the  amount of work  that  has been completed  to  date,
however,  calculations  for  starved-air  combustion  are  still
empirical.   Because  starved-air  combustion  is  extremely complex
and not  completely  understood,  it  is desirable  to pilot  any
starved-air  combustion  process  and,  where possible,  test  at
full-scale.  There  are  several excellent  texts and  articles  on
combustion,  but  none  deal  to any great degree  with oxygen-
deficient combustion.  Starved-air combustion is discussed  in  a
number of publications  (17-30).


    11.2.4  Heat and Material Balances

Analysis  of  high temperature  processes  must  include  heat  and
material balances.   Once provided, equipment  can  be sized  and
operating  costs estimated.   Throughout  the  remainder of  this
chapter, heat  and material  balances  are  displayed for several
alternative  combustion  processes,  all  being  fed  the   same
hypothetical  sludge.  A  flowsheet  for  a hypothetical  wastewater
treatment plant  is  depicted  on Figure  11-5. Design data for  5,
15, and  50 MGD  (0.22,  0.66, and  2.19  m3/sec)  wastewater  treat-
ment  plants  using this  configuration  are shown in Table  11-8.
The "A" and "B" alternatives  vary  only in  the percent solids feed
(20 percent  and 40  percent,  respectively)  and the  addition  of
conditioning  chemicals  to obtain  a dewatered  cake  of  40 percent
solids.    Use  of conditioning chemicals reduces  the percent
combustibles  of the  "B"  alternatives.

In  Section 11.3,  detailed  heat and material balance  tables  are
presented  for  each  furnace type.   The tables  also  display  the
amount of  fuel and power each type of  furnace requires,  for each
different  treatment  plant  alternative.    Balances given  are  for
yearly  average  conditions.   Operational  costs  can  be estimated
from  the  requirements  for  supplemental  fuel and  connected
horsepower.  General sizes and types of support facilities, such
as  ash  handling equipment,  water  supply  for the  air pollution
control  equipment,  and  operating  fuel  requirements can  also  be
estimated on the basis  of the data shown in the heat and material
balance tables.

In  any  steady-state  balance, all  inputs must  equal  all  outputs.
The following  is a representative example  of a heat and material
balance for the Alternative  IA  in  Section  11.3.1.
Alternative IA—Heat Balance

           Inputs

   Combustibles in sludge
   Supplemental fuel

   Total
106 Btu/hr
    13.91
     2.64

    16.55  (17.46 GJ/hr)
                              11-28

-------
          OutjDirbs

    Furnace exhaust
    Ash
    Radiation
    Shaft cooling air  (unrecovered
      portion)

    Total
106  Btu/hr

   15.96
    0.04
    0.32

    0.22

   16.54  (17.45 GJ/hr)
Values  are  essentially equal;  the balance  checks.   Note that
shaft cooling air  is  an  internal loop in the system.   Since  it  is
neither an input or output,  only the unrecovered  portion  need  be
considered in the  heat balance.
           jAj-j-M a t e r i a lBalance
          Inputs

    Dry solids  in  the  sludge
    Water in the sludge
    Supplemental fuel
    Combustion  air

    Total
      Ib/hr

      1,806
      7,224
        143
     22,060

     31,233  (14,180 kg/hr;
         Outputs

    Ash
    Furnace exhaust

    Total
      Ib/hr

        415
     30^817

     31,232  (14,179 kg/hr;
Again,  values  are  essentially equal; the balance checks.

Reference 23 contains valuable information on heat and  material
balances.
11.3  Incineration

Incineration  is a two-step  oxidation process  involving first
drying  and then  combustion.    Drying and  combustion  may be
accomplished  in  separate units or  successively in  the same
unit,  depending  upon  temperature  constraints  and  control
parameters.    The drying step  should  not  be confused with
preliminary dewatering,  which is  usually  done mechanically prior
to  incineration.   In all  furnaces, the drying and  combustion
processes follow the  same  phases:  raising the temperature  of the
feed sludge to  212°F (100°C), evaporating water  from  the  sludge,
increasing  the temperature  of  the water  vapor and  air,  and
                             11-29

-------
increasing the  temperature of  the  dried  sludge volatiles  to
the  ignition  point.   Although  presented  in  simplified form,
incineration is  a  complex process  involving thermal and  chemical
reactions  which  occur  at  varying  times, temperatures,  and
locations in the furnace.
        BAR    GRIT      PRIMAH'Y
      SCREENS  REMOVAL  SEDIMENTATION
  AERATION
(CAK1QNACEQLS    SECONDARY   CHL08INE
 OXIDATlONf    SEplMiNTATION  CONTACT
                        -*- SCUM TO LANDFILL

                                      ASH TO LANDFILL

                             FIGURE 11-5

        HYPOTHETICAL WASTEWATER TREATMENT PLANT FLOWSHEET
Manufacturers  have developed a  variety of  equipment,  each of
which has  advantages  and  disadvantages  (19,  31-34).   There  are
two major  wastewater sludge  incinerator equipment  types  used in
the United  States:   the multiple-hearth and  the  fluid bed.   The
electric furnace, which is relatively new,  has  been used and, as
                               11-30

-------
of  1979,  is  planned for  use  in several  wastewater  treatment
plants.    A fourth type  is  the  single  hearth  cyclonic  furnace.
This  furnace  has  been  used  in Great  Britain,   but  its  only
application in  the United  States has  been  in  industrial  service.
These four systems  are described  in  detail  in  this section.   Heat
and material  balances  are  included  for  each  type,  assuming  each
is used  in the hypothetical wastewater treatment plants  described
in Figure 11-5 and  Table 11-8.
                              TABLE 11-8

        HYPOTHETICAL WASTEWATER TREATMENT PLANT DESIGN DATA
      Alternative
 Sewage flow, MGD

 Sludge solids,
  Ib/day dry basis
  of dry solids
  Ib/hr, dry basis

 Conditioning chemicals,
  Ib/hr, dry basis

 Total feed to furnace,
  Ib/hr, dry basis
 Furnace loading rate
  Ib/hr, wet basis

 Volatile content of fur-
  nace feed, percent of
  total solids              77       65       77       65       77
I (5-MGD
:a A
5
10,320
lercent
77
hr/week 40
urnace,
; 1,806
.cals,
ib 0
lace,
; 1,806 .
furnace
• weight 20
ite,
; 9,030
flow)
B
5
10, 320
77
40
1,806
325
2,131
40
5,327
II (15-MGD
A
15
31,000
77
80
2,713
0
2,713
20
13,565
flow)
B
15
31,000
77
80
2,713
488
3, 201
40
8,003
III (50-MGD
A
50
flow)
B
50
103,000 103,000
77
168
4,292
0
4,292
20
21,460
77
168
4, 292
772
5,064
40
12,660
 a
 The A alternatives have a 20 percent solid feed sludge while
 the B alternatives have a 40 percent solids feed sludge including
 conditioning chemicals.
 b!5 percent lime (CaO) and 3 percent ferric chloride (FeCl^), dry
 weight basis for the 40 percent cake only.
 1 MGD = 0.04 m3/s
 1 Ib/day =  0.45 kg/day
 1 Ib/hr = 0.45 kg/hr
     11.3.1  Multiple-Hearth Furnace

The  multiple-hearth furnace  (MHF)  is  the most  widely used  sludge
incinerator  in  the United  States.   As  of 1977,  approximately
340  units  had  been  installed  for  wastewater  sludge  combustion
(35).  The  MHF is  durable, relatively  simple to  operate, and  can
handle  wide  fluctuations  in  feed quality and loading rates.   The


                                 11-31

-------
MHF is designed  for  continuous operation.  Start-up fuel  require-
ments  and  the extended  time needed  to bring  the  hearths and
internal  equipment up  to temperature  from a  completely cold
condition normally preclude  intermittent operations.  The MHF is
a  vertically  oriented,  cylindrically   shaped,   refractory-lined
steel shell containing  a series of horizontal refractory  hearths,
one above  the other.   MHFs  are  available  with  diameters  ranging
from 4 feet-6  inches to 29 feet  (1.4 to  8.8  m)  and can have from
4 to  14  hearths.   A cross section of a typical MHF is  shown on
Figure  11-6.   A central  shaft  extends from the  bottom of the
furnace to  the  top  and supports rabble arms above  each  hearth.
There are  either  two or four  rabble arms  per hearth.   Each arm
contains several  rabble teeth,  or  plows,  which rake the sludge
across  the hearth in a spiral  pattern.  Sludge  is fed at the
periphery  of  the top hearth  (see Figure 11-6)  and is  rabbled
toward the  center, where  it drops  to  the  hearth below.   On the
second  hearth,  the sludge  is rabbled outward  to holes at the
periphery of the bed.   Here  the  sludge drops  to  the  next  hearth.
The  alternating  drop  hole locations   on each hearth  and the
counter-current flow  of  rising exhaust  gases and descending
sludge provide contact  between the  hot  combustion gases  and the
sludge feed.  Good contact ensures complete combustion.   The drop
holes on  the  "out"  hearths  distribute  the sludge evenly around
the  periphery of the  hearth  beneath.   The  drop holes  also
regulate gas velocities.

Sludge is  constantly turned  and  broken  into  smaller  particles by
the  rotating  rabble  arms.    Thus, a large sludge surface  is
exposed to  the hot  furnace  gases.   This procedure  induces  rapid
and complete drying, as well  as burning.   The rabble arms also
form spiral ridges of sludge on each hearth.   The surface  area of
these ridges varies with the angle  of repose of the sludge, and
the  angle  varies with  the  moisture  content of  the  material.
Because of the ridges,  the actual  surface  area  of  sludge  exposed
to the  hot gases  is considerably  greater than the  hearth  area.
An effective  area of up  to 130 percent of  the hearth  area is
available.  Two access doors are generally provided  at  each
hearth.   They  have fitted  cast-iron  frames  and machined  faces
to provide reasonably tight  closure.   An observation  port is
provided in each door.

Figure  11-7 shows an interior  cut-away view of  the  MHF.   The
central  shaft of the  furnace  is  a  hollow  iron  column  cast
in sections;   shaft speeds are  adjustable from  about 1/2  to
1-1/2  revolutions  per minute.    The   hollow  rabble arms are
connected to machined arm sockets  in  the  shaft.   The shaft and
rabble arms are air-cooled and normally  are  insulated.    A cold
air tube runs  up the center of the shaft.  Air lances extend from
the cold air tube out  to  the  ends of each rabble arm.   Ambient
air of  regulated  pressure and volume is forced  through the cold
air tube and  lances  by means  of a  blower.   The cold air  exits
from the tips of  the lances,  flowing backward through the  space
between the lances and  the rabble  arm walls  to  the  annular  space
in the central shaft known as the hot air compartment.  This flow


                             11-32

-------
              COOLING AIR
              DISCHARGE
SLUDGE CAKE,
SCREENINGS,
AND GRIT-
                                    OUT_HEA_RTK
AUXILIARY
AIR PORTS

RABBLE ARM
2 OR 4 PER
HEARTH

  GAS FLOW
    CLINKER
    BREAKER
                                                        BURNERS
                                                        SUPPLEMENTAL
                                                        FUEL

                                                        COMBUSTION AIR
                                                        SHAFT COOLING
                                                        AIR RETURN
                                                        SOLIDS FLOW
          DROP HOLES
            ASH
      DISCHARGE
                              FIGURE 11-6

             CROSS SECTION OF A MULTIPLE-HEARTH FURNACE
                                 11-33

-------
     HOT AIR
     COMPARTMENT-
     COLD AIR
     TUBE
                                      IR LANCE
                               RABBLE ARM TEETH
                            CENTER SHAFT
                            GEAR DRIVE
                                                      SHAFT COOLING
                                                      AIR FAN
                                                                         STEEL SHELL
AIR HOUSING
     COURTESY ••? DIVISION Of ENVlflQNTECH CORPORATION


                                   FIGURE 11-7

       SHAFT COOLING AIR ARRANGEMENT IN A MULTIPLE-HEARTH FURNACE
                                      11-34

-------
of air cools the arms.  The  air  is  conducted  through  the hot air
compartment, cooling the shaft.   The  air  is  either  discharged to
the  atmosphere  via the exhaust gas  stack or  returned to  the
bottom hearth  of  the  furnace as  preheated  air  for  combustion.
Cooling air  vented to the  atmosphere  represents a heat  loss  of
roughly the same magnitude  as the radiation loss from  the furnace
structure.

The MHF can be divided into  four  zones, as  shown on Figure  11-8.
The  first  zone,  which consists of  the  upper  hearths, is  the
drying zone.  Most of the  water is evaporated in the drying  zone.
The second  zone, generally  consisting  of  the  central  hearths,  is
the combustion zone.   In this  zone,  the majority of combustibles
are burned  and  temperatures reach  1,400°F  to 1,700°F  (760°C  to
927°C).  The  third zone  is the fixed  carbon  burning  zone,  where
the remaining carbon  is  oxidized to carbon dioxide.   The fourth
zone  includes  the   lowest  hearths and is  the  cooling zone.   In
this  zone,  ash   is  cooled  by the  incoming  combustion air.    The
sequence of  these   zones is  always the same,  but  the  number  of
hearths in each zone is dependent on the  quality of  the feed,  the
design of the furnace,  and  the operational conditions.
        NORMAL
       SLUDGE/ASH
     TEMPERATURES
                         NORMAL
                           AIR
                      TEMPERATURES
                           DRYING ZONE
     \\V\\
     v  1400° to
        1700°F
COMBUSTION ZONE
                           FIXED CARBON
                           BURNING ZONE
        100° to
        400°F
        \\\\\
  ASH COOLING
     ZONE
                   SLUDGE
                    FLOW
                 AIR
                FLOW
                            FIGURE 11-8

             PROCESS ZONES IN A MULTIPLE-HEARTH FURNACE
                              11-35

-------
When the heating value  of  the  sludge  is insufficient  to sustain
autogenous  combustion,  the  additional  heat  required  is  supplied
by adding supplemental fuel to burners located at various points
in  the  furnace.  Burners  may operate either  continuously  or
intermittently and  on  all or selected hearths.

A measure  of  the quantity  of  water evaporated  from  the sludge
during burning  is  the drop in  temperature  of the hot  gases  as
they  pass  between the  combustion zone  and the  gas   outlet.
In a MHF,  gas temperatures in the combustion zone may exceed
1,700°F  (927°C).  These  gases sweep  over the cold, wet sludge  fed
to the drying  zone,  giving  up considerable portions of their heat
in evaporating the  water.   While the temperature of the solids  is
only marginally increased in the drying zone, the gas temperature
is drastically reduced, typically  to  the  range  of  600  to  900°F
(316 to  482°C).   Exhaust gas temperatures should be maintained  at
less  than  900°F  (482°C)   by  controlling  air flow  to   prevent
distillation  of odorous greases and tars from the drying solids.
If temperatures are so controlled,  it  may  be possible  to operate
MHFs without  devices such as  afterburners,  which are  used  to
reduce odors  and  concentrations of unburned hydrocarbons.

However,  afterburning  MHF,  exhaust  gases will probably be needed
in areas with very stringent carbonyl  and  unburned  hydrocarbon
emission  limitations.    In  afterburning,  furnace  exhaust  gases
are conveyed  to  a  chamber  where their  temperature  is raised  by
direct contact with  ignited  supplemental  fuel; the offending
pollutants  are  oxidized   to  CC>2  and water.   Afterburning,
however,  requires  supplemental fuel, which  raises operating
costs significantly.  In this  respect,  the  MHF is at a disadvan-
tage relative to FBF  and single  hearth cyclonic  furnaces,  which
do not require  afterburning.   The  reason  may be  seen  when  the
air-sludge  contact patterns  in these  furnaces  are contrasted
against  the  pattern  in the  MHF.   In the MHF,  warm  air and
unburned  solids  are contacted  at the  top  of the  furnace.   Any
compounds distilled from the solids are  immediately  vented from
the furnace at temperatures too low to effect their destruction.
In contrast,  temperatures in FBF  and single hearth  cyclonic
furnaces  are  high  (1,200  to 1,600°F  [649 to  760°C])  and nearly
uniform  throughout  the furnace.  Sludge and  air are injected into
the lower portion of the furnace, and any objectionable compounds
distilled  from the solids must traverse  the entire length  of
the hot  furnace before being vented.  In  the  FBF and single
hearth cyclonic  furnaces,  therefore,  the  volume  of  the furnace
above the  sludge injection zone is in effect an afterburner,
supplying ample contact time and temperature for the destruction
of pollutants.  A  flowsheet for  the MHF  process is  shown  on
Figure 11-9.

The MHF  can be  provided with  instrumentation to  convey  critical
operating data to a central control panel.   Temperature  data  can
be monitored for each  hearth and for other  points in the exhaust
gas system,  such as  the  furnace  exhaust,   heat  recovery device
                             11-36

-------
outlet,  and scrubber exhaust.   The  temperature can be controlled
on each  hearth  to within +  40°F  (22°C).   Instrumentation such
as CC>2 or  02  meters can be  used to  control  the  flow  of excess
air,  thereby conserving fuel and reducing the overall  operating
cost.   Malfunctions  such  as  burner  shutdown,  furnace  over-
temperature,  draft loss, and  feed shutdown can be monitored.
In the  event  of  power  or  fuel  failure, the  furnace  should
be shut  down  automatically  and  the  shaft cooling  air  fan
automatically  transferred  to  a  standby power  source.   This
procedure  will provide continued  cooling and prevent  serious
deformations  of  the  shaft  and the  rabble  arms  due  to  high
temperature.   Further  details  on instrumentation are provided in
Chapter  17.
      COOLING AIR
                            FIGURE 11-9

               FLOWSHEET FOR SLUDGE INCINERATION IN A
                     MULTIPLE-HEARTH FURNACE
                             11-37

-------
Problems encountered  with multiple-hearth  furnaces  have  included
(a) failure of  rabble  arms and teeth, (b) failure of hearths, and
(c) failure of refractories.  Improvements  in materials used in
constructing the  rabble  arms and  teeth  have  reduced the  first
problem, increasing their ability to withstand  high  temperatures.
Many  refractory  problems   result because furnaces  are not
carefully  heated  and  cooled  during   start-up and  shutdown.
Twenty-four hours  or more are required to bring  the  furnace  up to
temperature or to cool  it.   This  is an  operational disadvantage
since start-up  fuel costs can be significant.  However, there are
several  installations that  do  operate intermittently  without
significant refractory problems.   The  normal procedures  at  these
installations  is  to  fire  supplemental  fuel  to  maintain the
temperature of the  furnaces during  the  hours when  they are
not  in  use,  thus  reducing  long  reheat  times.    This  procedure,
known as "hot  standby"  is  not generally economical.  MHFs should
not be operated at temperatures  above  1,800°F  (982°C)  due to the
metals exposed to the  temperature.  Thus  with high energy  fuels
(for  example,  sewage  scum), there may be problems with  high
temperatures in the combustion zones.

Heat and material balances  for  the hypothetical  treatment  plant
alternatives listed in Table.11-8 are presented  in Table  11-9 and
should  be  used with  the flowsheet presented  in  Figure  11-9.
Figure  11-9 is  the   flowsheet  for  a  typical  multiple-hearth
furnace.   Figures  11-10 through  11-15  are  generalized curves for
capital and operating  and maintenance costs for  multiple-hearth
furnaces.    Table  11-10 gives typical  hearth  loading rates for
multiple-hearth furnaces.

As expected, there are important differences between Alternatives
"A"  (20  percent  solids feed) and  "B"  (40  percent  solids  feed)
in  terms of equipment size,  capital costs, and operation and
maintenance costs.   This  illustrates  the value  of preparing
comparative cost  tables  for all options.    Specific  discussions
of  the  MHF can  be found  in the  literature  (6,15,16,31, and
37-52).

The recycle concept is  relatively  new in  MHF  applications  (53).
This  concept  (54)  is  a  modification  of  the  multiple-hearth
designed "....to  control sludge  combustion  to  burn where  it is
designed to burn,  rather than to  let  it burn  where  it  wants to
go"  (55).   Recycle includes three  control  loops:   an  exit gas
loop, a drying rate  control  loop,  and a  furnace  combustion loop
(see  Figure 11-16).   The  exit gas loop allows  hot gases to
be  exhausted  from  either  or  both the  top-drying  hearth and
the  combustion zone.  For  wet  sludge, most or  all of  the air
would be exhausted from the  drying hearth, ensuring minimal fuel
consumption (conventional  MHF).   For hot or dry  sludges, most of
the air would be drawn from  the  combustion zone  so  as to prevent
uncontrolled burning on the  upper hearths.

The  drying  rate  control  loop takes  the  air exhausted  from the
drying  hearth  and heats this  air with exhaust  gases from the
combustion zone via  an  air heater (recuperator).  The heated


                             11-38

-------
exhaust from  the  drying  zone is returned as preheated combustion
air to the furnace.  This reduces the overall excess air require-
ments.   The  gas  from the  combustion  zone exits  from the first
recuperator and enters a second,  which serves as a preheater for
makeup combustion air.  Additional heat can be withdrawn from the
combustion zone gas it passes through a scrubber and is vented by
means of a heat recovery boiler.


                            TABLE 11-9

         HEAT AND MATERIAL BALANCE FOR SLUDGE INCINERATION
                   IN A MULTIPLE-HEARTH  FURNACE3

                                      Alternatives
Stream
Furnace design
Diameter, ft- in.
Number of hearths
Hearth loading rate, Ib
wet solids/sq ft/hr
Sludge feed
Lb dry solids/hr
Heat value, 106 Btu/hr
Volatile content, percent
dry solids
Supplemental fuel
No. 2 fuel oil, Ib/hr
Heat value, 106 Btu/hr
Combustion air
Mass at 60° F, Ib/hr
Shaft cooling air
Mass, Ib/hr
Shaft cooling air return
Mass at 325°F, Ib/hr
Heat value, 10° Btu/hr
Shaft cooling air not
recovered
Heat loss, 10 Btu/hr
Ash
Mass at 500°F, Ib/hr "
Heat value, 106 Btu/hr
Radiation
Heat loss, 10 Btu/hr
Furnace exhaust
Mass, Ib/hr
Heat value, 10 Btu/hr
Boiler exhaust
Heat value at 500°F,
106 Btu/hr
Recoverable heat
70 percent efficiency,
106 Btu/hr
Precooler and Venturi water-
feed
Flow at 70°F, gpm
IA
•5' MGD
20 percent
solids

18-9
7

7.3

1,806
13.91

77

143
2.64

., . 22,060

19,273

16,560
1.26


0.22

415
0.04

0.32

30,817d
15.96


13.26


1.89


90
IB
5 MGD
40 percent
solids

14-3
6

9.3
b
2,131D
13.91

65

0
0

27,531

9,178

0
0


0.71

740
0.07

0.21

32,123e
12.94


9.6.4


2. 31


86
IIA
15 MGD
20 percent
solids

'22-3
7

.7.4

.2,173
20.89

77

205
3.79

32,959

24,321

20,880
1.59


0.28

624
0.06

0.41

46,102d
23.93


19.73


2.94


135
IIB
15 MGD
40 percent
solids

16-9
6

9.5
K
3,201°
20.89

65

, 0
0

41,544

13,766

0
0


1.06

1,110
0.10

0.26

48,434e
.19.48


12.28


5.04


130
IIIA
50 MGD
20 percent
solids

22-3
10

8.4

4,292
33.06

77

312
5.77

51,945

34,416

29,520
2.25


0.40

987
0.09

0.53

72,735d
37.81


31.11


4.69


215
IIIB
50 MGD
40 percent
solids

18-9
7

10.3
K
5,064S
33.06

65

0
0

66,740

19,273

0
0


1.48

1,757
0.15

0.33

77,643e
31.11


19.61


8.05


' 209
                              11-39

-------
                                 TABLE 11-9

          HEAT AND MATERIAL BALANCE FOR SLUDGE INCINERATION
               IN A MULTIPLE-HEARTH FURNACE3 (CONTINUED)
                                            Alternatives



Stream
IA
5 MGD
20 percent
solids
IB
5 MGD
40 percent
solids
IIA
15 MGD
20 percent
solids
IIB
15 MGD
40 percent
solids
I IIA
50 MGD
20 percent
solids
IIIB
50 MGD
40 percent
solids
Scrubber water feed
  Flow at 70°F, gpm            182       174        273       260       429        41S

Scrubber drain
  Flow, gpm                  296       264        428       398       676        638
  Temperature, °F             98        98        98        98        98        98

Gas exhaust
  Mass, Ib/hr              26,667     38,938     44,278     58,646     61,116     91,393
  Temperature, °|             142       170        139       168       138        166
  Heat value, 10 Btu/hr       9.44      6.00      14.01      6.80      22.09      10.82

Connected power
  Horsepower                 238        93        305       178       305        238

Installed cost,  thousand
  dollars                 2,000      1,600      2,200      2,000      2,400      2,000


Footnotes for Table 11-8.
 All data supplied by the manufacturer.

 Solids for B alternatives (40 percent solids feed),  larger than A
 alternatives (20 percent solids feed), due to conditioning chemicals.
 See Table 11-7.

 Afterburner not included.

 At 800 °F.

6At 1,000 °F.

 Costs as of early 1978.

1 Ib/sq ft/hr =4.9 kg/m2/hr
1 Ib/hr = 0.45 kg/hr
1 x 106 Btu/hr = 1,055 MJ/hr
1 gpm =0.06 1/s
1 ft = 0.31 m
1 in.  = 0.02 m
1 MGD =0.04 m3/s
The   furnace  combustion   control  process  allows  the  furnace
to  operate with  sludges  with  a  very high volatile  content  (for
example,  large  amounts of  scum)  or those  requiring  supplemental
fuel.    This  loop integrates  the  functions  of  the exit gas  loop
and  the  drying  rate  loop,  providing  for automatic  control  of  the
process without  regard  to  feed  quality.

The  manufacturer of  the  furnace  which uses  the recycle  concept
claims that  strict  limits  on  gaseous  emissions  can  be met  without
use  of  an afterburner.   The  air  that  is  exhausted  has not  con-
tacted wet sludge  (the  sludge  in  the  drying  zone)  and thus  has
not  distilled off odors or  excess  hydrocarbons from  the sludge  .
Figure 11-16  is  a flowsheet for a 50  MGD  (2.2 m3/s) plant.
                                   11-40

-------
    5.0 i—
5   4.0

o

n
k_
.c
*-•
EC

^   3.0
a
*-<
m

o
=  2,0

c
LL
DC

O
lit
cr:
_t
                                   ASSUMPTIONS
                                 EFFECTIVE HEARTH AREA
                                      SQJFT
                                    LESS THAN 400
                                      4O&BQO
                                     BOO-MOO
                                     1,400-2,000
                                  GREATER THAN 2,000
HEATOP TIME TO
REACH 1,400* F
    HR
    18
    27
    36
    H
   108
                                 FREQUENCY OF STAflTt* (5 A F UNCTION OF INDIVIDUAL UNIT

                                 FUEL li NATURAL GAS OR FUEL OIL
                               I
                  §00         1,000         1,500        2,000

                     EFFECTIVE HEARTH AREA, iq ft (1 sq ft - 0,093 m^j


                              FIGURE 11-10

       MULTIPLE-HEARTH FURNACE START-UP FUEL REQUIREMENTS (36)
               2,500
 Disadvantages  of  the recycle concept  include  those  inherent
 in  the MHF  construction, as  well as  problems  associated with
 ducting  hot  gases  and with  recuperators.   Additional instruments
 and  equipment  add to operating  and maintenance costs.   These
 costs  may  be offset  by  a reduction in supplemental  fuel demand.
 One  municipal sludge  installation  similar to  that   depicted on
 Figure 11-16  is  under construction  in San Mateo,  California.  The
 recycle  concept has  been  used  in the  MHFs for  many  years to
 produce  bone char  (a  "hot"  feed material)  in  the sugar  industry.
                                11-41

-------
-5
o
g
t-
D
EE
C/3

I
    100
     1
     7
     6
     E
     4
    10
     B
     6
     7
     6
     S
    1,0
     S
     S
     7
     €
     5
    0,1
                   PLANT CAPACITY, MGD (1 MGD = 0.04 m3/sj

                    1                  10
                                                        100
                              ASSUMPTIONS;
                              LOADING RATE = 6 Ibfa ft/hr
                              SLUDGE: PRIMARY + WAS SLUDGE
                                  AT 16% SOLIDS
                              COSTS: JUNE 1978
       100   234 E 6 7891,000  234  5678910,000  234 56786100,000

                  WET SLUDGE FEED, Ib/hr (1 Ib/hr = 0.46 k§/hrj

                            FIGURE 11-11

          MULTIPLE-HEARTH FURNACE CONSTRUCTION COST (36)


    11.3.2  Fluid Bed  Furnace

The first  fluid  bed  wastewater  sludge  furnace was  installed  in
1962.    There  are approximately 60 operating  units  in the United
States (35)  and many more  in Europe.   The  fluid bed  furnace
(FBF)   is a vertically  oriented, cylindrically shaped, refractory-
lined  steel  shell that contains  a  sand  bed  and  fluidizing air
diffusers.   The FBF  is  normally  available  in sizes  from 9  to
25  feet (2.7  to  7.6 m)  in diameter.   However,  there  is one
                               11-42

-------
industrial unit with  a  diameter of 53  feet*( 16.2 m) .    A cross
section of the fluid bed furnace is shown  on Figure  11-17.   The
sand bed  is approximately  2.5  feet (0.8 m) thick and  sits  on a
refractory-lined  grid.   This grid contains tuyeres through which
air is  injected  into  the furnace  at  a pressure of  3  to 5  psig
(21 to  34  kN/m2  gage)  to fluidize  the  bed.  The  bed expands to
approximately  100 percent of its at rest volume.  Temperature of
the bed is controlled  between 1,400°F and  1,500°F (760°C and
816°C)   by  auxiliary burners located  either  above or  below the
sand bed.   In  some  installations,  a water  spray or  heat removal
system  above   the  bed  controls the  furnace  temperature.    In
essence,  the   reactor  is a single chamber  unit  in which  both
drying   and combustion occur in either  the dense or dilute phases
in  the  sand bed.   All of the  combustion gases pass  through the
combustion zone  with  residence  times of several  seconds at
1,400°F to 1,500°F (760°C to 816°C).   Ash is carried  out the top
of  the  furnace and  is  removed  by air  pollution control devices,
usually venturi  scrubbers.    Sand  carried  out with  the  ash
must be replaced.   Sand  losses  are  approximately 5 percent of the
bed volume for every 300 hours  of operation.  Feed to the furnace
is  introduced  either above  or directly  into the  bed.


Air  flow  in   the  furnace  is  determined  by  several  factors.
Fluidizing and combustion  air  must be  sufficient to expand the
bed to  a proper density  yet  low enough  to prevent the sludge  from
rising   to  and  floating  on  top  of  the  bed.  Too  much  air blows
sand and  products  of incomplete combustion  into  the off-gases.
This depletes  stored heat  energy  and  increases fuel  consumption
unnecessarily.   Minimum  oxygen  requirements must be met to assure
complete  oxidation  of  all  volatile  solids in  the sludge cake.
Temperatures   must  be  sufficiently  high  to  assure  complete
deodorizing but  low enough  to protect  the  refractory,  heat
exchanger, and flue  gas ducting.   The  quantities of excess air
are maintained at  20  to 45 percent to  minimize effects on   fuel
costs (see Figure 11-3).   The fluid bed  furnace  operates at lower
excess   air  rates  than  typically experienced  in MHF  operations.
This accounts  for  the  greater  heat efficiency  of  the  fluid bed
system   at  similar  exit  temperatures   .   The  intense  and violent
mixing   of  the  solids  and gases within  the  fluid  bed results in
uniform conditions of  temperature,  composition,  and particle  size
distribution throughout  the  bed.  Heat  transfer  between the gases
and the solids is extremely rapid because  of  the large surface
area available.  .


There  are two basic  process  configurations  for the  FBF.   In
the  first process,  the fluidizing  air passes through  a  heat
exchanger, or  recuperator,  prior to injection into the combustion
chamber.  This arrangement is known as  a hot windbox  design.  In
the second process, the fluidizing air  is  injected directly  into
the furnace.  This arrangement  is known  as  a cold windbox design.
The  first  arrangement   increases the  thermal  efficiency  of the
process by  using  the  higher temperature of the exhaust gases to
preheat the incoming combustion air.                  .


                             11-43

-------
 cc
 a
 CO


 s
 m
 Q
    100,000
        0
        8

        6
        e

        4
                    PLANT CAPACITY, MGO ( 1 MGO * ttQ4 m3/s}


                      1                 10
                                                         100
     10,000
        §
        1
        7
        6
        e
     1,000
        B
        8
        7
        6
        5

        4
      100
ASSUMPTIONS
LOADING RAT« • e Ih/K) ft/hr
SLUDC6: PRIMARY + WAS SLUDGE AT 18% SOLIDS
                                J_
         100    234 iB7*t 1,000   234  §678910,000  2  3  456789100,000


                    WET SLUDGE FEED, Ib/hr (1 Ib/hr - 0.45 kg/hrl


                             FIGURE 11-12


        MULTIPLE-HEARTH FURNACE OPERATING AND MAINTENANCE
                       LABOR REQUIREMENTS (36)
Preheating  the  incoming combustion  air from 70°F to 1,000°F  (21°C
to  538°C)  can  yield a  reduction in  fuel  costs  of approximately
61  percent  per  unit  wet  sludge  (39).   Air  preheating costs
can represent  15  percent  of  the  fluid  bed  furnace  cost;
therefore,  a  careful  economic analysis  is  needed  to determine
cost-effectiveness for a given situation.
                               11-44

-------
     1,000,000
             CURVE NO.  SLUGSE TYPE
i

o_
1
w

"a
 cp

 1
 d
 ul
 £
 5
 o
  LLJ

  Li.
                      PRIM, +• FERRIC CHLORIDE {F(CI_)
                      PRIM, + LOW LIME
                      PRIM, + WASTE ACTIVATED SLUDGE (W.A.8.!
                      PRIM, + |WAS + F*C13!
                      (PRIM, + FKS_) * WAS
                      DIGESTED PRIMARY
                                        ASSUMPTIONS;
                                        HEAT VALUE OF VOLATILE SOLIDS 10,000 BtuflU
                                        LOADING BATES, fe/iq
                                           CURVE NO,      RATE
                                                         14
                                            2.4A7J         6,B
                                              3           18
                                              6           M
                                        SEE TABLE 11*10 FOR FEED SLUDGE DATA
                                        COMBUSTION TEMPERATURE I/WO* F
                                        DOWNTIME IS A FUNCTION OF
                                          INDIVIDUAL SYSTEM
                                        40% EXCESS AW, NO PR EM EATER
                                        5TABT-UI1 FUEL NOT INCLUDED; 73,000 9wtw ft
                                          FOP) STARTUP
                                        FUEL IS NATURAL GAS on PUEL O»L
         too
           10    33 456769100  2  3 4667691,000  2  3 4 6 6 78910,000 a  3 466768100,000

                       DRY SLUDGE FEED, Ib/hr {1 Ib/hf - 0,45 kg^hr|


                              FIGURE 11-13

           MULTIPLE-HEARTH FURNACE FUEL REQUIREMENTS (36)
Violent  mixing  in  the  fluidized  bed  assures  rapid and  uniform
distribution  of  fuel  and  air,  and  consequently,  good  heat
transfer and   combustion.   The  bed  itself  provides  substantial
heat  storage  capacity.   This  helps  to  reduce short-term
temperature  fluctuations  that  may  result' from  varying  feed
heating  values.   This  heat storage capacity also  enables quicker
start-up,   if   the  shutdown period has  been  short  (for  example,
overnight).   Organic  particles  remain  in the sandbed  until
they  are reduced to  mineral ash.  The violent  motion  of  the  bed
comminutes  the ash material,  preventing  the buildup of clinkers.
The  resulting  fine  ash is constantly stripped from the  bed by  the
upflowing  gases.
                                 11-45

-------
I
Dt
a:
UJ

o
Q.
-I
<
u
oc

o
UJ
_i
UJ
                  ASSUMPTIONS:

                  SOLIDS CONCENTHATIOK, *

                       14-17
                       tfrSS
                       23-30
                        31
      100,000
           100
                1  34  667891,000  234 §878910,000 234 66789100,000

                   EFFECTIVE HEARTH AREA, iq ft (1 iq ft = ^093 m2)

                           FIGURE 11-14

    MULTIPLE-HEARTH FURNACE ELECTRICAL POWER REQUIREMENTS (36)
An  oxygen analyzer  in  the  stack controls air  flow  into the
reactor.  This type of control has limited application, since air
flow ranges  have upper  and  lower rates  required  for proper bed
fluidization.   The rate of  use of auxiliary  fuel is controlled
by  furnace  exhaust gas  temperature.    Shutdown  controls  must
be  provided  for  emergency  situations.   Further  details  on
instrumentation are provided in Chapter 17.

Heat  and  material  balances   for the  hypothetical treatment
plant  alternatives  (Table  11-8)   are  presented  in Table  11-11.
Figure  11-18  is the  flowsheet for a typical FBF  system.
Figures 11-19 and 11-20 are generalized curves depicting fuel and
power required for FBF systems.
                              11-46

-------
i
   1,000,000
        9
        S
        7
        6
        6
        4

        3
    100,000
        e
        s
        6
        5
LU
o
     10,000
        9
        8
        7
        i
        5
        4
      1,000
                     PLANT CAPACITY, MGD (1 MGD - 0,04

                      I                 10
               I
                        ASSUMPTIONS;
                        LOADING RATE = 6 Ib/sq ftAr
                        SLUDGE: PRIMARY + WAS SLUDGE
                           AT 16% SOLIDS
                        COSTS: JUNE 1f78
100
               234 667691.000  1  3 4 5678910,000  234 BB 780100.000

                     WET SLUDGE FEED, !b/hr (1 tt»/br = 0.4S kf/hr)

                            FIGURE 11-15

      MULTIPLE-HEARTH FURNACE MAINTENANCE MATERIAL COSTS (36)
The  FBF  is relatively  simple  to  operate,  has  a  minimum of
mechanical  components,  and  typically has a slightly lower capital
cost than  the  MHF.   Normal operation  of  the  FBF produces exhaust
temperature  in excess  of  1,400°F  (760°C).   Because  the exhaust
gases  are  exposed  to  this temperature  for  several  seconds,
carbonyl and  unburned  hydrocarbon emissions are minimal,  and
strict hydrocarbon  emission regulations  are met  without the use
of  an  afterburner.   However,  it  is important  that  operating
conditions  be  optimum  to assure  this emission level at all times.
                               11-47

-------
                            TABLE 11-10

    TYPICAL HEARTH LOADING RATES FOR A MULTIPLE-HEARTH FURNACE3
    Type of sludge
                    Percent
                    solids
Primary                 30

Primary plus ferric
  chloride (FeCl3)        16

Primary plus low lime      35

Primary plus waste-
  activated sludge (WAS)    16

Primary plus (WAS plus
  FeCl3)                20
  Percent
combustibles
   Chemical  ,
concentration,
    mg/1
    60


    47

    45


    69


    54
    N/AU


     20

    298


    N/A


     20
plus WAS
WAS
WAS plus Fed 3
Anaerobically digested
primary
16
16
16
30
53
80
50
43
20
N/A
20
N/A
 Typical wet
sludge loading
   rate,c
 Ib/sq ft/hr
  7.0 - 12.0


  6.0 - IX). 0

  8.0 - 12.0


  6.0 - 10.0


  6.5 - 11.0


  6.0 - 10.0

  6.0 - 10.0

  6.0 - 10.0


  7.0 - 12.0
 Data supplied by the manufacturer.
 Assumes no dewatering chemicals.

 Low number is applicable to small plants, high
 number is applicable to large plants.
 N/A - not applicable.

 1 Ib/sq ft/hr =4.9 kg/m2/hr
Problems  with  the  FBF  have  occurred  primarily  with  feed
equipment and  temperature  controls.   When  sludge is  injected
directly into  the  bed,  screw feeders may  jam  if the  sludge
has  been overdried or  if  it  solidifies at  the  point  of
injection.   When  spray nozzles have  been used,  thermocouples
have  occasionally burned out.  These problems have  generally
been  solved by the  use  of  different construction materials.
There  have  been some  problems  with  preheaters  and  with sand
scaling  on  the venturi  scrubber.    In  some  installations, there
have  been  serious  erosion problems  in  the   scrubber  due  to
the  excessive  carryover  of  bed  material  and  the  resulting
sandblasting effect.   The fluid bed  furnace can  be operated at
2,200°F  (1,204°C)  with  appropriate design modifications  and  is
suitable  for high  energy  sludges.    Combustion at  temperatures
over  2,000°F  (1,093°C)  can create many side  effects  such as
ash  fusion, high  temperature  corrosion,  scaling, and  clinker
formation.   Since a minimal amount  of air  is  always  required
for bed fluidizing, energy savings  from turndown (feed reduction)
are  minor.   More  detailed information  can  be   found  in  the
literature (39,40,41,43,48,49,50,  and  56-63).
                               11-48

-------
   SLUOCi FEED
   H,*UO as/to
   6 2§» SOUDS"
   < 10,000 BtuJIb
   VOLATlLES,
   30% A5IH)
                                               GAS EXH4UST
 SUPPLEMENTAL
   fuEL
                           FIGURE 11-16

             HEAT BALANCE FOR THE RECYCLE CONCEPT IN A
                   MULTIPLE-HEARTH FURNACE (55)

    11.3.3  Electric Furnace

The first electric furnace was installed in Richardson,  Texas,  in
1975.  The electric, or  infrared,  furnace  (EF)  is  a  horizontally
oriented, rectangular,  steel  shell containing a moving horizontal
woven-wire belt.   The  unit  is  lined with  ceramic-fiber  blanket
insulation.   Electric furnaces are  available  in a  range of  sizes
from  4  feet  (1.2 m) wide by 20  feet (6.1 m)  long  to 9.5  feet
(2.9  m)   wide  by  96 feet  (29.3  m)  long.   Larger  sizes  are
currently being  developed.   A typical cross  section  is shown  on
Figure 11-21.

Sludge is fed  into  the  EF through a  feed  hopper  that discharges
onto the  woven-wire belt.  Shortly  after  the  sludge  is  deposited
                                         an  internal  roller  to  a
                                         cm),  across  the  width of
                                         on several new installa-
                                         sludge  layer to afford
                                         moves under the  infrared
on the  belt,  it  is leveled by  means  of
layer approximately one inch thick  (2.5
the belt.  A rabbling device is provided
tions  to break  up the surface of the
better combustion.   This layer  of sludge
heating  elements,  which provide supplemental energy for  the
incineration process,  if  required.    Ash  is discharged  from  the
end of the belt to  the  ash  handling system.   Combustion air  flow
is countercurrent to the sludge flow,, with most of  the combustion
air being introduced  into the  ash discharge  end of the unit.
Excess air rates for  the  EF vary from 20  to  70  percent.   The EF
is divided into  a feed  zone,  a drying and combustion zone,
and an ash discharge zone.  The  feed and discharge  zones are  each
8  feet  (2.4  m) long.   The length of the drying  and  combustion
zone varies with  the design.
                              11-49

-------
                                      *- EXHAUST AND ASH
  THERMOCOUPLE
             C
   SLUDGE
   INLET
FLUIDIZING
AIR INLET
FLU I CM ZED A
SAND BED •
                        REFRACTER
                          ARCH
                                            PRESSURE TAP
                                           ASiGHT
                                           9 GLASS
                                               BURNER
  TUYERES

  FUEL
  GUN
PRESSURE TAP
                   STARTUP
                 -i PREHEAT
                  HBURNER
                 _T FOR HOT
                   WINDBOX
                       FIGURE 11-17

           CROSS SECTION OF A FLUID BED FURNACE
                          11-50

-------
                            TABLE 11-11

              HEAT AND MATERIAL BALANCE FOR SLUDGE
                INCINERATION IN A FLUID BED FURNACE3
                                       Alternatives


IA
5 MGD
20 percent
Stream
Furnace design
Inside diameter, ft
Loading rate, Ib wet
solids/sq ft/hr
Sludge feed
Lb dry solids/hr
Heat value, 106 Btu/hr
Volatile solids, percent
dry solids
Supplemental fuel
Mass, Ib/hr
Heat value, 10 Btu/hr
Combustion air
Mass, Ib/hr
Heat value, 10 Btu/hr
Ash
Mass, Ib dry solids/hr
Heat value, 10^ Btu/hr
Water flow, gpm
Radiation
Heat loss, 10 Btu/hr
Recoverable heat
70 percent efficiency,
106 Btu/hr
Recuperator
Venturi water
Recycle water, gpm
Makeup water at 70°F, gpm
Scrubber water feed
Flow at 70°F, gpm
Scrubber drain
Flow at 130°F, gpm
Gas exhaust
Volume, cfm
Temperature, F
Connected power
Horsepower
Installed cost, thousand
dollars
solids

14

56.9

1,806
13.91

77

151
2.80

19,353
4.4

416
0.12
20

0.42

3.5d
Yes

83
10

365

391

5,042
120

218

1,100
IB
5 MGD
40 percent
solids

12

47.0
H
2,131
13.91

65

0
0

16,250
0

746
0.14
32

0.29

6.26
No

68
12

345

359

3,972
120

162

1,000
IIA
15 MGD
20 percent 40
solids

18

53.3

2,713
20.89

77

224
4.14

28,976
6.7

623
0.18
30

0.63

5.3d
Yes

124
15

548

582

7,524
120

320

1,400
IIB
15 MGD
percent
solids

14

47.0
K
3.201
20.89

65

0
0

23,576
0

1,117
. 0.26
43

0.44

9.46
No

102
19

565

600

5,949
120

234

1,100
IIIA
50 MGD
20 percent
solids

22

56.5

4,293
33.06

77

353
6.52

45,978
10.6

959
0.29
40

1.00

8.4d
Yes

197
24

863

924

12,007
120

425

1,600
IIIB
50 MGD
40 percent
solids

18

45.0
K
5,064b
33.06

65

0
0

38,620
0

1,772
0.42
70

0.71

12. 7e
No

161
30

824

900

9,459
120

350

1,500
All data provided by Dorr-Oliver, Inc.
Solids for B alternatives (40
percent solids feed) ,
alternatives (20 percent solids feed) ,
See Table 11-7.
Afterburner not required.
dAt 1,400°F.
6At 1,650°F.
Costs as of early 1978.
1 ft = 0.31 m
1 Ib/sq ft/hr =4.9 kg/m /hr
1 Ib/hr =0.45 kg/hr





1 gpm =
1 cfm =
1 MGD =
larger than A



due to conditioning chemicals.





0.06 1/s
4.72 x 10-4 m
0.04 m /s






3/s

























1 x 10° Btu/hr = 1,055 MJ/hr
                               11-51

-------
                    FURNACE EXHAUST
                                            GAS EXHAUST
                           FIGURE 11-18

      FLOWSHEET FOR SLUDGE INCINERATION IN A FLUID BED FURNACE
A  flowsheet  for  the typical  electric  furnace  is  shown  on
Figure 11-22.   Heat and material  balances  for  the hypothetical
treatment  plant  alternatives  (Table  11-8) are presented  in
Table 11-12.   In  addition  to  the alternative cases I,  II,  and
III,  balances  for a 1  MGD (0.04  m^/s)  treatment plant  have
been  included.   The EF  is  suited  to small wastewater  treatment
plants.
The effective belt loading  rate  of  a  large EF is slightly greater
than the hearth  loading  rate  of a multiple-hearth furnace.   The
supplemental energy  requirements  of the  EF  are  lower  than  the
requirements of  the  MHF, FBF, or  the cyclonic  furnace.   Because
electricity is used  to provide  the supplemental energy,  no fuel
is burned,  and  consequently,  no excess air  for this  purpose is
required.   However,  when  the generation efficiency of electricity
is included, the  supplemental  energy  requirements are similar for
                              rather than fossil fuel,  is  the
                              Electricity is  generally  a  more
                              the  fossil fuel  used  by  the other
                              the  energy cost  differential,  the
all furnaces.   Electricity,
energy  source for  the EF.
expensive energy source than
unit types.   Depending upon
                              11-52

-------
advantage  of  low excess air may  be reduced.   when autogenous
sludge  is available,  the only difference  between the  EF and other
processes with low  excess air rates would be the motive power.
    1,000,000
in
IA
O
—-,
 3
 4^
CD
 3
 4-"
 m
.2
E

Q*
LU
x

a
LU
(E

LU

LL
                     S-LUDGE TYPE
                     PRIMARY
                     PRIM. + FERRIC CHLORIDE (F»CU
                     PRIM, + LOW LIME
                     PRIM, * WASTE ACTIVATED SLU&st
                     PRIM. + {WAS +
                     (PRIM. * F«CI3J + WAS
                     DIGESTED PRIMARY
                                        ASSUMPTIONS:

                                        LOADING HATES PER TABLE 11-10
                                        INCOMING SLUDGE TEMPERATURE IS 57 F
                                        COMBUSTION TEMPERATURE ; 1,400* F
                                              FOB COOL-OOWN EQUALS
                                          STARTW TIME
                                        FREQUENCY FOR STARTUPS IS A FUNCTION
                                          OF INDIVIDUAL SYSTEMS
                                        EXCESS AIR IS 106%
                                        FUEL IS NATURAL GAS OR FUEL OIL
                                        MO STARTUP FUiL IS INCLUOEO
                                          (SEE FIGURE 11-10)
       100
          10   2  3 4 56789100  2  34 667881,000 3  34 6678910,000 214 5671»TOO.OOC


                     DRY SLUDGE fEf D, pounds per hour (1 Ib/hr = 0.46 kg/hrj

                              FIGURE 11-19

               FLUID BED FURNACE FUEL REQUIREMENTS (36)
Low  capital  cost combined with  modular construction  makes the  EF
attractive,  especially  for small treatment systems.    Because  of
the  use  of ceramic-fiber  blanket insulation instead  of solid
refractories, the  electric  furnace may  be shut down  and heated  up
without  the  refractory  problems  that  can  occur  in the other
furnaces.  This  makes the  EF  suitable for intermittent operation.
However,  each restart requires supplemental energy  (electricity),
                                 11-53

-------
since there  is no heat  sink  similar to  the  sandbed in the  FBF.
Currently, no  EF  units are  installed  with a capacity of  over
1,200 pounds per hour.
    1,000,01X1,000

          I

«        ;
"        3
 tl
 k^
-?   100,000,000

i        !
_        6
—        4
I
•c        *
LU
E
D
a
Ul
IE
o
a.
O
cc
o
LU
_i
UJ
10.000,000
    i
       100,000
                                             ASSUMPTIONS:
                                             FULL TIME
                                             OPERATION
                                                     i	i  i M mi
           10   234 587*9100  3  | < »67t»1,OQO 2  3 41078810,0002  3 4 6 S 788100,000

                         BED AREA, *q ft {1 sq ft- 0.093 m2)


                            FIGURE 11-20

       FLUID BED FURNACE ELECTRICAL POWER REQUIREMENTS (36)
The EF  appears to be  a feasible alternative  for both small  and
large  systems  due  to its  inherent  simplicity and  low cost.
However,  the  EF  requires  considerably more floor  space than
furnaces which  are vertically oriented.   Another concern  is  the
replacement  of various  components   such  as the  woven-wire  belt
(3   to  5-year life)  and the  infrared  heaters  (3-year  life).
These  items represent a sizable portion  of  the capital cost.
Replacement  costs  must be considered  in  any  overall  evaluation.
Connected power, whether  for heating or motive power,  may create
                              11-54

-------
a large  electric  demand charge in  some  areas.   This may  be the
case  whether  the energy  is  used or  not.   Also,  time-of-day
charges  could  be  significant.   One concern  is  the  high voltage,
240 to  480  V, required  for  the  furnace infrared heaters.   This
may create  safety problems  in small  plants,  where workers  are
unaccustomed to high  voltage equipment.
                   1 flOLLIR
                   r LiVELER
RADIANT
INFRARED
HEAfING
ELEMENTS !T¥?i
WOVEN WIDE
CONTINUOUS BELT
                                                            COMBUSTION
                           FIGURE 11-21

           CROSS SECTION OF AN ELECTRIC INFRARED FURNACE
Because the gas  flow  in an EF runs countercurrent  to  the  sludge
flow, the furnace will  probably  require  an  afterburner to  comply
with strict carbonyl  and  hydrocarbon  emission  regulations.   This
would increase the supplemental energy requirement,  the amount of
equipment, and the  capital  and operating  costs to levels greater
than those shown in Table 11-12.   Allowing for  the low excess air
requirements  and  the  countercurrent  flow pattern,   air  emission
control  equipment  would  generally  be smaller  than control
equipment on MHF or FBF units of similar feed capacity.


    11.3.4  Single Hearth Cyclonic Furnace

Cyclonic furnaces were developed by the British (64), and several
units are operating in Great Britain.   However, as of 1979,  there
are no units  processing wastewater sludge  in  the United States.
The cyclonic  furnace  is sometimes called a  single-rotary  hearth
furnace.   It  is  a vertical, cylindrical,  refractory-lined,  steel
shell,  normally  provided  with  a  domed cover.    There is  one
rotating  hearth  and  a  fixed plow  that moves  the combustible
material from  the  outer edge of  the  hearth to the  center.   The
furnaces are  currently  available  with  hearths  to  30 feet (9.1 m)
in  diameter,   but larger sizes  can  be  built.  The  sludge  is
fed  by  a screw  feeder  and deposited  near  the periphery of  the
rotating hearth.   A  sectional  view of  the furnace is  given on
Figure 11-23.
                              11-55

-------
                                             6AS EXHAUST

      SLUDGE   SUPPLEMENTAL   COOLING
       FEfO    ENERGY       AIR

      J	L_l
           ELECTHIC FURftACE
               1      T
               RADIATION    ASH
                              AIR
                           FIGURE 11-22

 FLOWSHEET FOR SLUDGE INCINERATION IN AN ELECTRIC INFRARED FURNACE
The cyclonic furnace design differs  from  the multiple-hearth
and fluid bed  designs  in that it does  not  allow  the combustion
air to  pass  upward through  the  feed material.   Combustion  air
and supplemental  fuel,   if  required,  are  injected  tangentially
into the  combustion chamber above the  rotating hearth.   This
creates a  swirling (cyclonic) action  that  mixes the  gases  and
allows  adequate contact between the oxygen and the furnace feed.
The gases from  the combustion  process spiral  upward to  the
outlet.  The  furnace exhaust  temperature  is approximately 1,500°F
(816°C).  Heat  could  be recovered from  the  exhaust  with  a heat
recovery boiler followed by  a  recuperator.   The  ash  is moved to
the middle of the  hearth, where it drops  through to a quench tank
for final disposal.  The rotating hearth is  sealed  at  the edges
by a water bath.
                             11-56

-------
                       TABLE 11-12

HEAT AND MATERIAL BALANCE FOR SLUDGE INCINERATION
           IN AN ELECTRIC INFRARED FURNACE3
                                Alternatives
IA IB IIA IIB IIIA
5 MGD 5 MGD 15 MGD 15 MGD 50 MGD
20 percent 40 percent 20 percent 40 percent 20 percent
Furnace design
Number of units 21213
Overall width, ft 8.5 8.5 9.5 9.5 9.5
Overall length, ft 72 72 88 88 96
Belt area/furnace,
sq ft 382.6 3B2.6 560.5 560.5 616.8
solids/sq ft/hrb 11.8 13.9 12.1 14.3 11.6
Sludge feed
Heat value, 10 Btu/
hr 13.91 13.91 20.89 20.89 33.06
percent dry solids 77 65 77 65 77
Water, Ib/hr 7,224 3,200 10,582 4,800 17,172
Heat value, 10
Btu/hr 0.28 0. 12 0.41 0.18 0.65
Supplemental power
kW 280.8 0 402.5 0 643.8
Heat value, 10 Btu/ 2.98& 4.2?S 6.826
hr 00
Combustion air
Heat value, 106 Btu/
hr . 0.26 0. 36 0.38 0.54 0.61
Ash
Mass at 500°F, Ib/hr 415 747 624 1,120 987
hr 0.10 0.18 0,16 0. 28 0.24
Radiation
Heat loss, 10 Btu/
hr .36 .18 .47 .24 .77
Mass, Ib/hr 26,351f 29,3729 39,616f 44, 064 9 62,628f
Heat value, 10 Btu/
Boiler exhaust
106 Btu/hr 13.00 8.53 19.49 12.79 31.33
Recoverable heat
70 percent efficiency,
106 Btu/hr 1.37 3.85 2.05 5.81 2.25
Flow, at 70°F, gpm " 397 201' 584 314 1,049
Scrubber drain
Flow, gpm 390 196 606 306 1,081
Tempera ture,°F 120 120 120 120 120
Gas exhaust
Mass, Ib/hr 29,538 35,811 39,616 53,838 54,744
Temperature, °F 120 120 120 120 120
Heat value, 106 Btu/
' hr ' 1.98 2.77 . 2.96 4.18 4.71
Total connected power
Horsepower 22 25 30 40 50
Total installed cost j
thousand dollars 1,000 700 1,300 900 1,500
All data supplied by Shirco, Inc.
b
CSolids for B alternatives (40 percent solids feed), larger than A
See Table 11-7.
Afterburner not included.
Autogenous with combustion air preheated to 500 °F. kw =* 10,600
Btu/hr to allow for generation efficiency.
fAt 750 °F.
9At 1,200 °F.
Does not include supplemental power requirements for infrared heaters.
jCosts as of early 1978.
IIIB
50 MGD 1 MGD
40 percent 40 percent
2 1
8.5 6
88 32
479.5 94.5
13.2 11.3
33.06 2.79
65 65
7,596 641
0.29 0.02
0 0
0 0
0.85 0.07
1,772 149
0.44 0.04
.43 .07
69,732g 5,8809

20.23 1.71
9.18 0.69
498 201
485 196
120 150
85,186 7,183
120 120
6.57 0.55
60 7
1,200 300



  1 ft * 0.31 in
  1 sq ft = 0.093 m
  1 Ib/sq ft/hr =4.9 kg/m2/hr
  1 Ib/hr -0.45 kg/hr
  1 x 106 Btu/hr - 1,055 MJAr
  1 gpm = 0.06 1/s
  1 MGD = 0.04 mVs
                          11-57

-------
              EXHAUST
COMBUSTION AIR
                                                     CYCLONIC ACTION

                                                     ROTATING HEARTH

                                                     FfXED PLOW
        TANGENTIAL
        AIR PORTS
                                                 SLUDGE
                                                  INLET
            BURNER ITVP)
                                      ASH DISCHARGE IN
                                     CENTER OF FURNACE
                            FIGURE tl-23

                CROSS SECTION OF A CYCLONIC FURNACE
A general  flowsheet  for  the furnace  is given  on Figure  11-24.
Heat and material  balances  for  the  hypothetical treatment plant
alternative (Table 11-8) are presented in Table  11-13.

The  rotary hearth furnace has  a  relatively  low capital cost
and  is mechanically  simple,  since  it  has only one  rotating
hearth.   However,  the  feed  mechanism is similar  to  that of  the
fluid  bed  furnace and  has  the  same plugging  problem.   Because
exhaust  temperatures  are high, afterburners  or supplemental
heaters are  generally  not  required  to  achieve  compliance with
strict carbonyl or hydrocarbon air emission regulations.  As with
the  FBF,  good operating  conditions must  be maintained if low
gaseous emission  limitations are to be  met.   The rotary hearth
furnace requ:'res 30 to 80  percent excess air.
                              11-58

-------
                                              6AS EXHAUST
                           FIGURE 11-24

      FLOWSHEET FOR SLUDGE INCINERATION IN A CYCLONIC FURNACE
    11.3.5  Design Example:
            Process
                             New Sludge  Incineration
To  minimize  increasing  disposal  costs,  a  municipal wastewater
treatment plant with  an  average  daily  flow  of 5 MGD  (0.22 m^/s)
must modify its present solids  handling  and  disposal  system.  The
plant uses a conventional activated  sludge process with anaerobic
digestion of combined primary sludge, waste-activated  sludge, and
scum.   Table 11-14  shows the basic plant  data.  The  digested
sludge  is  vacuum  filtered and is  hauled  to the local landfill.
This landfill  is  scheduled  to  close.   The new landfill site has
                  capacity and  is located  several miles from the
                   Projected  disposal  costs  for the new site are
                 treatment plant  site has very little unoccupied
                 surrounding the  plant  has been heavily developed
                  and rendering  operations.   These  industries
                  amounts  of  animal greases and  oils  to  the
                  Naturally, they are concerned about industrial
somewhat limited
treatment plant.
very high.   The
space.   The area
by  meat packing
discharge  large
treatment plant.
sewer service  charges resulting  from  any action  by  the plant.
                              11-59

-------
                                      TABLE 11-13

                HEAT AND MATERIAL BALANCE FOR SLUDGE
                  INCINERATION IN A CYCLONIC FURNACE3
                                                      Alternatives
IA
5 MGD
percent
solids
19.50
30.4
1,806
14.27b
77
132
2.48
19,665
1,100
2,280
415
0.19
0.90
NO
Yes
30,692
1,420
19.90
960
15.66
0
12
292
319
120
23,468
120
1.79
175
1,300
IB
5 MGD
40 percent
solids
13.75
30.9
1,806
14.27
77b
0
0
19,665
60
0
415
0.19
0.60
Vfes
No
23,765
1,411
13.48
500
6.87
4.63
5
197
207
110
21,209
110
1.62
125
1,000
IIA
15 MGD
20 percent
solids
24.00
30.1
2,713
21.43b
77
184
3.46
29,519
1,100
3,178
624
0.29
1.17
No
Yes
45,817
1,420
29.75
960
23.43
0
19
437
477
120
34,969
120
2.67
260
1,600
IIB
15 MGD
40 percent
solids
17.00
30.1
2,712b
21.43b
77b
0
0
29,519
60
0
624
0.29
0.80
Yes
No
35,675
1,421
20.34
500
10.32
7.01
7
296
311
110
31,838
110
2.43
190
1,100
I IIA
50 MGD
20 percent
solids
30.25
29.9
4,292
33.91b
77
546
10.28
46,694
1,100
9,430
987
0.46
2.00
No
Yes
77,143
1,420
50.10
960
39.45
0
30
699
763
120
62,225
120
4.75
460
N/A
IIIB
50 MGD
40 percent
solids
21.50
29.7
4,292b
33.91b
77b
0
0
46,694
60
0
987
0.46
1.00
Yes
No
57,424
1,420
32.38
500
16.51
11.11
15
507
535
110
49,002
110
3.74
290
1,500
Sludge feed
  Lb dry solids/hr
  Heat value,  106 Btu/hr
  Volatile solids, percent
   dry solids

Supplemental fuel
  Mass, Ib/hr
  Heat value,  10  Btu/hr

Primary air
  Mass, Ib/hr
  Temperature, F

Burner air
  Mass at 60%, Ib/hr

Ash
  Mass at 260°F, Ib/hr
  Heat value,  106 Btu/hr

Radiation
  Heat loss, 10  Btu/hr

Waste heat boiler

Recuperator

Furnace exhaust
  Mass, Ib/hr
  Temperature, r
  Heat value,  10  Btu/hr

Boiler/recuperator exhaust
  Temperature, °F,
  Heat value,  10  Btu/hr

Recoverable heat - boiler
  70 percent efficiency,
    106 Btu/hr

Precooler water feed
  Flow at 60°F, gpm
Scrubber water feed
  Flow at 60°F, gpm

Scrubber drain
  Flow, gpm
  Temperature, F
Gas exhaust
  Mass, Ib/hr
  Temperature,°F
  Heat value,  10  Btu/hr

Connected power
  Horsepower

Installed cost , thousand
  dollars


 All data provided by AFB Engineers/Contractors sole U.S. distributors of the Lucas Cyclonic Furnace.

 Data used by manufacturer is slightly different from that developed in Table 11-7.

 Afterburners not required.

 Not available.

CCosts as of early 1978.

1 ft = 0.31 m
1 Ib/sq ft/hr =4.9 kg/m /hr
1 Ib/hr =0.45 kg/hr
1 x 106 Btu/hr =  1.055 MJ/hr
1 gpm =  0.06 I/;
1 MGD =  0.04 nr3,
              /s
                                           11-60

-------
Most of the industrial  wastes  discharged to the plant are removed
in  the  primary  tanks, and the  result is a  combined  scum  and
sludge with an extremely  high  heating value.
                           TABLE 11-14

    DESIGN EXAMPLE:  WASTEWATER TREATMENT PLANT OPERATING DATA

          	Parameter               Value

          Plant flow, MGD                        5

          Sludge to disposal,  Ib/day dry
            basis                           10,320

          Solids heat value, Btu/lb dry
            basis                           11,000

          Volatile solids to digester,
            percent of dry solids               77

          Sludge solids content,  percent''
            solids by weight            ^*"       20

          Vacuum filter operation,  hr/
            week                                40
          1 MGD = 0.04 m /s
          1 Ib/day = 0.45 kg/day
          1 Btu/lb = 2,324 MJ/kg
        11.3.5.1  Approach

A consultant was hired  to evaluate several  disposal methods,
including land disposal,  composting,  heat treatment,  combustion,
and continuation of  landfill disposal.  Combustion was identified
as the most  cost-effective solution.   The high energy content of
the  sludge  and the  limited  available  land for  sludge  disposal
influenced this decision.   The digestion step was eliminated from
the design  so  that  the full heat  value  of the  sludge  could be
used in combustion.   It was expected  that this would  obviate the
need for any supplemental fuel.   The  existing  digesters  would be
converted to  sludge  thickening/storage  units, and the  existing
vacuum filters would provide an  incinerator feed solids  content
of approximately 20  percent.

At present, the  vacuum filter  operates  6  to  8  hours a  day,
5 days per  week.   Because of  the  limited  plant area,  no  space
is available for filter cake  holding facilities.  Therefore,
the  furnace will be designed to  operate in conjunction  with
the vacuum  filters.   A  review  of the various furnace  systems
indicated  that  because  of the  high  heating value  of  the


                             11-61

-------
sludge,  the  intermittent operation  requirements,  and  the space
limitations,  a fluid  bed  system would  be the most  cost-  and
energy-effective solution...


        11.3.5.2  Preliminary Design

Fluid  bed furnace  manufacturers  were  provided  the data  in
Table 11-15  for  analysis and development  of  heat and material
balances.   Table 11-16  and Figure 11-25 show all  sizing criteria,
as well as  the requirements for peripheral equipment.   On  the
basis of  this  and  additional data, a  15-foot  (4.6 m) diameter
fluid bed  furnace was  specified.   A recuperator to recover  the
heat in the  exhaust gas  and  return  it to the  furnace  (hot wind
box design) was included.
                          TABLE 11-15

          DESIGN EXAMPLE:  SLUDGE FURNACE DESIGN CRITERIA

                    Parameter              Value
          Sludge feed
            Solids  content, percent by
              weight                            20
            Volatile solids content,
              percent  of dry solids             77
            Heat value, Btu/lb of dry
              solids                        11,000

          Furnace operation, hr/week            40

          Average solids loading rate,
            Ib/hr,  dry basis                 1,810
          1 Btu/lb  =  2,324 MJ/kg
          1 Ib/hr = 0.45 kg/hr
Detailed  design of  the  complete  system actually  begins with
the  data  provided  by  the furnace  manufacturer.   More than
one  manufacturer should  be consulted for  design data.   Air
emissions  must  be  estimated  and  these  estimates  submitted
to local,  state,  and  federal  authorities in  order  to  obtain a
permit  to construct.   Because  of the  small orifices  in the
venturi scrubber,  potable  makeup  water at  5 gpm  (0.3  1/s)  is
required.   The  impingement  scrubber  water flow  of  397 gpm
(24  1/s),  0.6 MGD   (0.03 m^/s),  will be secondary  effluent.
Note that the  scrubber water flow  is  12 percent of the average
plant flow and  approximately 25  percent  of  the  plant's minimum
flow.   Because this return flow is  expected to be of  low BOD
and  of  high  SS,  it will  be  returned  to  a  point  upstream  of


                             11-62

-------
                            TABLE 11-16

       DESIGN EXAMPLE:  HEAT AND MATERIAL BALANCE
                    FOR A FLUID BED FURNACE3
         Stream, unit
Connected power,  hp

Startup fuel requirements
  Weekday operation,  16-hr
    shutdown,  106 Btu/hr
  Monday morning  operation,
    64-hr shutdown,  106
    Btu/hr
                                 Value
Furnace design
  Inside diameter,  ft               15.0
  Loading rate,  Ib  wet solids/
    sq ft/hr                        51.2

Sludge feed
  Lb dry solids/hr                 1,810
  Heat value,  106 Btu/hr           19.91
  Volatile solids,  percent of
    dry solids                        77

Supplemental  fuel                      0

Combustion air
  Mass, Ib/hr                     22,950
  Heat value,  10 Btu/hr            5.40

Ash
  Mass, Ib dry  solids/hr             416
  Heat value,  10b Btu/hr            0.12
  Water flow,  gpm                     20
Radiation
  Heat loss,  10  Btu/hr             1.27

Furnace exhaust
  Temperature,  °F                  1,400

Recoverable heat          ,
  70 percent efficiency,  10
    Btu/hr                          4 . 2

Recuperator                         Yes

Venturi water
  Recycle water, gpm                  94
  Makeup water  at 70 °F,  gpm           5

Scrubber water  feed
  Flow at 70 °F, gpm                 397

Scrubber water  drain
  Flow at 130  °F, gpm                410

Gas exhaust
  Volume, cfm at 120°F             6,162
                                    240
0.42
 Data supplied by Dorr-Oliver, Inc.

 At 1,400 °F.

 Fuel required:
  1 hr on Saturday.
  1 hr on Sunday.
  1/2 hr on Monday morning.
        1  ft  =  0.30 m            ,
        1  Ib/sq ft/hr =4.89 kg/m /hr
        1  Ib/hr =  0.45 kg/hr
        1  x 106 Btu/hr = 1,055 MJ/hr
        1  cfm = 4.7 x ID"4 m3/s
        1  gpro = 0.06 1/s
                                11-63

-------
the  aeration  tank (see  Chapter  16).   The  temperature  of the
sidestream, 130°F  (54°C),  was not  considered  to have an adverse
effect on the secondary process.
                                                  ~f~, o» i on c
                                                 'GAS* EXHAUST
                                                     IMDUCED DRAFT FAN
 SUPPLEMENTAL FUEL s

   19.91 k 10EBlu/*i[
                                     CONNECTED POWEH
 t x 10°Bai/»= 106b MJ.tir
 1 apro = o.oe i/s
 1 Ihflif = QM Kj/hr
 1 cfm - 4.? k 1CT4 m3A

                            FIGURE 11-25

            DESIGN EXAMPLE: HEAT AND MATERIAL BALANCE IN
                        A FLUID BED FURNACE

Contracts for  disposing  of the wet ash  must be established.
Methods  for  transporting  the ash slurry,  conveying it to trucks
at the  plant site,  and discharging it  from the  trucks at the
disposal site must be investigated and designed.


Options  for  using  available  excess  heat should also be examined.
As shown in Table 11-16,  4.2 x 106 Btu  per hour  (4.4 GJ/hr)
are  available  for  use.   However,  heat  is available  only
intermittently and not necessarily at the time  it is most needed.
Another  approach  is  to  transfer the  heat  to  hot  water tanks and
use the  heated  water  for  space heating.  Alternatively, the heat
can be  utilized in an  absorption refrigeration  unit  to produce
chilled  water.  This water  can  be stored and used  to  satisfy
subsequent space cooling demands.
                              11-64

-------
Other design  considerations  to  be investigated include but are
not limited to:

     •   Ash dewatering methods

     •   Ash hauling  by owner  or by separate contractor

     •   Type  of  auxiliary  equipment such  as   sludge  conveyors,
        fans,  and feed equipment

     •   Heat recovery methods

     •   Electrical distribution

     •   Control  philosophy

     •   Sophistication of instrumentation and control

     •   Supplemental fuel availability  and  storage  (for  start-up
        and problem  periods)

     •   Area clearance,  including access platforms

     •   Furnace  housing  requirements

     •   Structural   requirements,  for example,  seismic and wind
        factors

     •   Noise levels and other safety requirements

     •   Heating,  ventilating,  and cooling  the area  near the
        furnace

     •   Spare parts

     •   Level and quality of  staffing

These points  relate only to  the  installed  system.  An  important
consideration is the  interfacing  of  the existing  plant with
the  construction  of the  furnace.   All  systems and  details
relating  to the furnace should be discussed  with the furnace
manufacturer and,  in  some  cases,  made the responsibility of
the  manufacturer.  For example, the combustion  air fans, the
recuperator  (if  used), the heat  recovery  boiler  (if used),
scrubbers, and  some or all  of  the controls should  be part of
the total contract.


11.4  Starved-Air Combustion

Starved-air combustion (SAC) has  been demonstrated  to be an
effective  method  for  burning sludge in  a  furnace  (17-20,22,24,
26,29,30).  Strict air quality standards can be  met with  SAC, and
large amounts of supplemental fuel are not required.


                              11-65

-------
The  key to  SAC  is the  use of  less  than  theoretical  quantities
of  air  in  the  furnace--30  to  90  percent  of  stoichiome trie
requirements.   This  makes SAC more  fuel-efficient than incinera-
tion  in an MHF.   This is  shown  on  Figure  11-26.   When a SAC-MHF
is  combined with an  afterburner, an overall  excess air  rate of
25  to  50 percent  can  be  maintained,  as  compared to an  excess air
rate of 75 to  200 percent  for  multiple-hearth  incinerator with an
afterburner.
               EXHAUST
                                                   EXHAUST
      AFTERBURNER
      (1,200° F)
                     SUPPLEMENTAL
                        FUEL

                     10 TO 25% EXCESS
 AIR FOR FUEL
100% THEORETICAL
AFTERBURNER
{1,200° Ft  .
                     AIR FOR FUEL
   50 TO 160%
   EXCESS AiR
     100%
 THEORETICAL AIR
                                       0% EXCESS AIR
                   .JOTOJOK	
                 THEORETICAL AIR
                                       25 TO 50%
                                                           EXCESS AIR
                                                            10 TO 70%
                                     THEORETICAL AIR
           75% TO 200% OVER All
            EXCESS AIR RATE
                            26% TO 50% OVERALL
                             excess AIR RATE
             INCINERATION
                          STARVED-AIR COMBUSTION
ASSUMPTION: AUTOGENOUS SLUDGE FEED
                             FIGURE 11-26
      COMPARISON OF EXCESS AIR REQUIREMENTS: INCINERATION IN A
        MULTIPLE-HEARTH FURNACE VS. STARVED-AIR COMBUSTION
SAC  is,  in effect,  incomplete  combustion.  The  reaction products
are  combustible gases,  tars, and  oils,  and a solid  char that  can
have an  appreciable heating  value.   The relative proportion
of  each  varies  with the amount of  heat  applied  and  the feed
moisture.   Generally,  higher reaction temperatures  yield simpler
products  and greater quantities  of low  heating value  gas.   This
is at  the expense of combustible  solid products  (25).
                                11-66

-------
The low heating value gases may be burned,  and  the  heat generated
can be  recovered and used  beneficially.   Alternatively,  the gas
may be  cooled  and  stored for subsequent off-site use.  The most
effective  utilization appears  to be the burning  of the total
gas stream,  with subsequent  recovery of portions of  the heat
generated.   Off-site use appears to be impractical  because:

     •  The  gas fuel  value is  low.  Thus,  delivery  of  any
        significant quantity of  energy  requires  the transport of
        very large  volumes  of gas.

     •  Cooling  of the gas  for off-site use would  result in
        permanent loss  of much of its heat content.

     • .The  condensates  (tars,  oils)  produced when  the  gas is
        cooled  are high  strength and  corrosive.   Containing
        the  condensates  and  disposing of  them  are  significant
        problems.

     •  The  condensates  themselves have  significant  heat
        values.  The heating  values  of  the gas  is  diminshed when
        condensates are removed.

In  full-scale  test work  (17),  the  SAC combustible exhaust gas
was found  to have  a heating  value of 90  Btu/standard dry cubic
foot  (3.4  MJ/m^).  The gas contained  hydrogen,  carbon monoxide,
carbon dioxide, methane,  ethylene, butane,  nitrogen,  oxygen,
water, and some higher  hydrocarbons.

SAC ash may contain combustible material; the amount depends upon
furnace operation.   SAC reduces sludge to an ash containing from
3  to  30 percent combustibles,  including  up to  20  percent fixed
(elemental)  carbon.   More combustibles can be  released  to the
gas stream by adding  more air,  oxygen,  or steam to the lower
part of the furnace.  This  has the advantage of transferring part
of  the heat in  the  residue to  the gas stream.   However,  the
transfer  leaves the residue  depleted in  heat  value.   In some
circumstances,  it may be better  not to burn  out the  residue
completely.  Conceivably,  char could be used  as  an adsorbent or
as a filter aid for sludge  conditioning prior to dewatering.

The  operating  temperature of  the  furnace  can  be controlled
within  a  wide  range.   The lower temperature limit  is the point
when  the  rate  of decomposition  of  high  molecular  weight  organic
compounds  becomes too low,  about 1,300°F  (704°C).  The upper
temperature  limit  is defined by  the point  at  which there  is ash
melting or damage  to  refractories, about  1,800°F  (982°C)  (22).
One  temperature  consideration  is  that vaporization  of heavy
metals  must  be minimized,  since  it  is  difficult  to remove heavy
metals from the gas stream  with conventional scrubbing equipment.
It  is  therefore preferable  to  burn the  sludge  at as low  a
temperature  as possible.   Full-scale  test  work (17) and other
published data  (18,19,  and  20)  indicate  that  1,500°F  (816°C)
appears to be a reasonable  operating temperature for minimum
heavy metal vaporization.                                       :


                             11-67

-------
Fluid bed,  electric, and cyclonic furnaces could also  be  operated
in a SAC mode.   To date,  none has been  operated  in  this manner
with a sludge  feed.  Operation in a SAC mode is particularly well
suited  to  the MHF.   There appears  to be little incentive to
operate the FBF  in  this mode because (1) excess air rates for SAC
and the FBF are about the same, and  (2)  an afterburner  would be
required for a converted MHF  whereas  afterburning  is not needed
where the  FBF  is  used  in the incineration mode.  Several  types of
furnaces,  including  an  FBF  (21),  have  been operated in the
starved-air combustion mode on wood wastes to  produce charcoal.


    11.4.1   Development and Application

Starved-air combustion of sludge, and/or refuse-derived fuel, was
successfully demonstrated  in  a full-scale test  at the   Central
Contra  Costa  Sanitary District's  wastewater  treatment   plant in
Concord, California  (17).  The  use of  refuse-derived  fuel is
discussed  in Section  11-5.   The furnace  and  an afterburner were
operated at 1,400°F  (760°C) without  supplemental  fuel addition.
The feed was  primary  and  trickling  filter sludge  from  a mostly
domestic wastewater.   The  combined sludge had a  heating  value
of 9,000 Btu  per pound (20.9 MJ/kg)  of  combustible solids, a
combustible solids content  of 75  percent,   and  a feed sludge
solids concentration of 24 percent (17).  The  Concord  SAC reactor
was a  converted six-hearth,  16-foot 9-inch  (5.1 m)  diameter
MHF.    Dewatered  sludge  was burned by  using  approximately
50 percent  of the  theoretical air  requirement,  and an  exhaust
gas was produced with a heating  value  of 90  Btu per  standard
dry cubic  foot (3,353  MJ/m^).   All  of  the exhaust  gas was burned
in an  afterburner at 1,400°F (760°C).  The  resulting  SAC ash
contained  30  percent combustibles,  of  which  20 percent  were
fixed carbon.   Other  important  results  and  conclusions of this
two-month  SAC  test  program were:

    •   Starved-air  combustion  was  easier  to  control  than
        incineration (the furnace was also run in an incineration
        mode).

    •   Hearth temperature  could be  used  to control the  furnace,
        with air  addition as the manipulated variable.

     •   Air addition  to the  furnace  should  be  automatically
        controlled.

    •   Particulate  production per pound  of solids fed was  about
        50  percent  lower than  conventional incineration.

    •   The completeness of  the reaction  depends  upon the amount
        of  air fed,  not on temperature.

    •   The most  corrosion  resistant  alloys for high  temperature
        conditions  were Type HK stainless steel and Inconel 690.
        For low  temperature  conditions the  most corrosion
        resistant alloys were  Hastelloy C-176  and Inconel 625.


                              11-68

-------
                                                 GAS EXHAUST
SHAFT COOLING
AIR RETURNED
  SLUOGI
       TO FURRACE
  FEED
                C7
                    AFTERBURNER
CDMBUSTJON
  AlP
                                SHAFT COOLING AIR NOT RETURNED
                    FURNACE
                     EXHAUST
                       AFTtfiiUftNER
                        EXHAUST
                                  -SHAFT COO LINO Ain
                                   RETURNED TO AFTIRiURNifl
             AFTERBURNER


                     PRECOOLER-
                     AND VENTURI


               BOILER IX.HAUST   /
                                  INOUCiD
                                  DRAFT FAN
             MULTIPLE
             HEAfltH
             STARVED
            AIR REACTOR
                       COMBUSTJQNAIR
       SHAFT
                         VENTURI WATER

                      CONNECTED
     COOLING AIR
                             FIGURE 11-27

              FLOWSHEET FOR STARVED-AIR COMBUSTION IN A
                      MULTIPLE-HEARTH FURNACE
A flowsheet for  an MHF operated as  a  SAC reactor is provided in
Figure  11-27.   Comparison  with Figure 11-9  shows the  difference
to  be  the  addition  of  an  afterburner.    Heat  and  material
balances   for  the  hypothetical  treatment  plant  alternatives
(Table  11-8)  are  presented  in Table  11-17.    Table 11-18  takes
selected  data from the  heat  and material  balances  previously
presented  to  permit  direct  comparison  of SAC  with  incineration
options.   Direct comparisons are  made for  an autogenous  sludge,
and  feed  rates   to  all  systems  are  identical except for  that to
the  cyclonic  furnace.  SAC appears  to have  an  advantage  overall
                                11-69

-------
                                TABLE 11-17

     HEAT AND MATERIAL BALANCE FOR STARVED-AIR COMBUSTION
              OF SLUDGE IN A MULTIPLE-HEARTH FURNACEa

                                      Alternative (all 40 percent solids)
Stream
Furnace design
Diameter, ft-in.
Number of hearths
Hearth loading rate, Ib wet
solids/sq ft/hr
Sludge feed
Lb dry solids/hr
Heat value, 106 Btu/hr
Volatile solids, percent dry
solids
Supplemental fuel
Combustion air
Mass, Ib/hr
Temperature, °F
Shaft cooling air
Mass, Ib/hr
Shaft cooling air return
Mass at 350 °F, Ib/hr
Shaft cooling air to stack
Mass at 325 °F, Ib/hr
Shaft cooling air to afterburner
Mass at 350 °F, Ib/hr
Ash
Mass, Ib/hr
Temperature, °F
Heat value, 106 Btu/hr
Radiation .
Heat loss, 10 Btu/hr
Furnace exhaust
Mass at 800 °F, Ib/hr
Heat value, 10° Btu/hr
Afterburner combustion air
Mass at 60 °F, Ib/hr
Afterburner exhaust •
'• Mass, Ib/hr
Temperature, °F
Heat value, 1.0 6 Btu/hr
IB
5 MGD

12-9
6

12.1

2,131
7.35

65
0
h
0D
0

9,178

6,480

0

2,698

787
500
0.23

0.44

11,010
6.82

4,382C

17,368
1,495
12.76
IIB
15 MGD

14-3
7

12.0

3,201
10.73

65
0
h
,780C
60

10,095

8,640

0

1,455

1,181
500
0.34

0.62

16,250
10.16

8,805C

26,537
1,495
19.18
IIIB
50 MGD

16-9
8

11.4

5,064
16.90

65
0
h
1,500
60

15,602

13,380

0

2,222

1,869
500
0.54

0.94

25,658
16.05

14,098C

42, 041
1,495
30.04
Boiler exhaust           ,
  Heat value at 500  F, 10  Btu/hr
Recoverable heat
  70 percent efficiency, 10
   Btu/hr
6.76
 4.2
                9.18
                 7.0
                               13.04
                                11.9
 All data supplied by the manufacturer.
 In addition to shaft cooling air returned to  furnace.
cln addition to shaft cooling air returned to  afterburner.,-

 Costs as of early 1978.
1 ft = 0.30  m
1 in. = 0.02 m         ,
1 Ib/sq ft/hr =4.9 kg/m /hr
      1 Ib/hr = 0.45 kg/hr
      1 x 106 Btu/hr = 1,055 MJ/hr
      1 gpm = 0.06 1/s
      1 MGD = 0.044 m3/s
                                    11-70

-------
                              TABLE 11-17

        HEAT AND MATERIAL BALANCE FOR STARVED-AIR COMBUSTION
          OF SLUDGE IN A MULTIPLE-HEARTH FURNACE3 (Continued)

                                   Alternative  (all 40 percent solids)
                                  IB             IIB            IIIB
           Stream           .      5 MGD          15 MGD          50 MGD
Precooler and Venturi water feed
  Flow at 70 °F, gpm                   51             77            121

Scrubber water feed
  Flow at 70 °F, gpm                   102  .          153        .    243

Scrubber drain
  Flow, gpm                          160            240            380
  Temperature,  F                     98             98             98

Gas exhaust
  Mass, Ib/hr                      14,280         21,480          34,080
  Temperature, °F                     120            120            120
  Heat value, 106 Btu/hr               4.62           5.96           7.94

Connected power
  Horsepower                         78            123            218

Installed cost, thousand dollars'3       1,400          1,600           2,300
aAll data supplied by.the manufacturer.

 In addition to shaft cooling air returned to furnace.

CIn addition to shaft cooling air returned to afterburner.

dCosts as of early 1978.

1 ft = 0.30 m                        1 * 106 Btu/hr = 1,055 MJ/hr
1 in. = 0.02 m        -,               1 9P™ = Q-06 1/s
1 Ib/sq ft/hr =4.9 kg/m /hr             1 MGD = 0.044 m3/s
1 Ib/hr = 0.45 kg/hr
but  the  FBF in  terms  of  air required,  as  indicated by  lesser
exhaust  flow  rates.    SAC has less connected horsepower than
the  other  options,  arid  except  for  the  FBF,  higher exhaust
temperatures and  thus,  greater  potential  for energy  recovery.

Additional  details  of  the  test  work and SAC application can
be  found in  the literature  (8,17-30,65,66,67).    Additional
information can also  be  gained  by  working  with  the furnace
manufacturers.
     11.4.2  Advantages and Disadvantages  of SAC

Test  work, much  of which  is  still  underway,  shows that  SAC in  an
MHF  using  sludge  alone has  many advantages  over incineration  or
other combustion  processes.


                                 11-71

-------
                             TABLE 11-18
HEAT AND MATERIAL BALANCE COMPARISON OF STARVED-AIR
                 COMBUSTION AND INCINERATION



Multiple-hearth Fluid bejl
Item :
Alternative IA
1 Sludge feed, Ib dry
solids/hr
Supplemental fuel,
106 Btu/hr
Furnace exhaust
Mass, Ib/hr
Temperature, °F
Recoverable heat
70 percent effi-
ciency, 10° Btu/
hr
Connected power
Horsepower
Alternative IB
Sludge feed, Ib dry
solids/hr
Supplemental fuel,
106 Btu/hr
Furnace exhaust
Mass, Ib/hr
Temperature, °F
Recoverable heat
70 percent effi-
ciency, 1Q6 Btu/
hr
Connected power
Horsepower
Alternative IIB
Sludge feed, Ib dry
solids/hr
Supplemental fuel,
106 Btu/hr
Furnace exhaust
Mass, Ib/hr
Temperature, F
Recoverable heat
70 percent effi-
ciency, 10^ Btu/
hr
Connected power
Horsepower
Alternative IIIB
Sludge feed, Ib dry
solids/hr
Supplemental fuel.
106 Btu/hr
Furnace exhaust
Mass, Ib/hr
Temperature, °F
Recoverable heat
70 percent effi-
ciency, 'lo6 Btu/
hr
Connected power
Horsepower
aSee Table 11-9.
bSee Table 11-11.
cSee Table 11-12.
dSee Table 11-13.
incinerator0


1,806

2.64

30,817
800



1.89

238


2,131

0

32,123
1,000

_

2.31

93


3,201

0

48,434
1,000



5.04

178


5,064

0

77,643
1,000



8.05

238




furnace0


1,806

2.80

19,353
1,400



3.50

218


2,131

0

16,250
1,650



6.2

162


3,201

0

23,576
1,650



9.40

234


5,064

0

38,620
1,650



12.7

350





Electric
furnace0


1,806

2.98f

26,351
750



1.37

22h


2,131

0

29,372
1,200



3.85

25h


3,201

0

44,064
1,200



5.81

40h


5,064

0

69,732
1,200



9.18
U
60"





Cyclonic
furnace


1,806

2.48

30,692
1,420



2.97g

175


1 , 806 1

0

23,765
1,411



4.63

125


2,712

0

35,675
1,421



7.01

190


4,292

0

57,424
1,420



11.11

290




Starved-air
combustion -
multiple hearth6


_

-

-
-



-

-


2,131

0

17,638
1,495



4.20

78


3,201

0

26,537
1,495



7.00

123


5,064

Q

42,041
1,495



11.90

218




  See Table 11-17.
  Infrared heaters  (kw = 10,600 Btu to allow for generating efficiency).
 ^Recuperator only.
  Does not include power requirements for infrared heaters.
  Based data used by manufacturer is different from that for other furnaces.
 1 Ib/hr =0.45 kg/hr
 1 x 10° Btu/hr = 1,055 MJ/hr
                                 11-72

-------
The SAC process provides a  greater  solids  throughput  because  of
the higher  allowable  hearth  loading  rates.    (This  assumes  that
a portion  of the combustibles remain in  the ash.)  Operation
of a multiple-hearth furnace  with SAC permits  hearth loading
rates  30  to  50  percent higher than  an optimum  incineration
mode.    This  can  be explained  in  terms  of  heat release and  gas
velocity,  although other factors  also  affect  loading  rate.    In
incineration, the heat liberated in the furnace by  combustion  of
the feed  solids  must be limited  to prevent high-temperature
damage  to the  furnace refractories.  Under  SAC,  heat  liberation
is minimized in the  furnace  by air control  with  combustibles
passing out in a gas form to an auxiliary combustion chamber,  or
afterburner.   The afterburner which has  no  moving mechanical
parts,  can  be designed  for the  high  temperatures.    Thus,  with
the two-stage  combustion  process which  occurs  under  SAC,  high
furnace temperature  is not a limiting condition.  Gas velocity  is
another factor which  affects hearth loading rate.   An excessive
gas velocity  entrains large quantities  of  solids   particles  in
the furnace, leading to gas  cleanup difficulties.  With  SAC,
considerably less air is used in the furnace than with incinera-
tion,  and this can be traded-off against  the increased volume  of
combustible  gases  created  by  higher  hearth loading  rates.
Therefore,   for  a  fixed maximum  gas  velocity, a greater  hearth
loading rate can  be  applied with SAC than with incineration.


A second  advantage offered  by SAC is reduced fuel usage  when
afterburning  is required.   Even when it is possible  to dewater
the sludge  feed  to  an  autogenous  state,   (eliminating the  need
for supplemental  fuel  to  the  furnace), a  considerable quantity
of fossil  fuel  is  still  required for  the  afterburner  in  an
incineration mode.   Essentially  no  fuel  is  required for  the
afterburner in an  SAC  mode.

SAC offers more  stable operation and  ease of control,  with
minimal furnace  response  to feed  changes.    With  incineration,
increasing  or decreasing the feed rate results in a  corresponding
rise  or fall  in  hearth  temperature since  the  solids  combustion
rate increases  or  decreases.   With  SAC,  the  extent  of  the
heat-generating combustion reactions are limited by  the available
oxygen  supply.   Fluctuations  in  feed  rate  will  not  change  the
temperature level  because it  does not  change the  amount  of
combustion occurring,  provided  air rate does not change.

A fourth advantage of SAC  over incineration is  that it produces
fewer air emissions.  SAC's  lower  furnace  gas velocity, for the
same  solids  loading  rate,  results in  less  particle entrainment
and reduced  particulate  emissions.   During full-scale tests  at
Concord, particulate  production  with  SAC  was about  50 percent
less  than  incineration  under  equal solids  feed  conditions  (17,
18).    Furthermore, particulates  leaving the  furnace were  larger
than  those from incineration.   These particulates are more  easily
removed by  cyclones  or  other simple gas cleanup equipment.   At
Concord, nitrogen oxide  and sulfur oxide emissions  also appeared


                             11-73

-------
to be lower with SAC.   It is probable that  when  organic  nitrogen
is oxidized,  one  of the  reaction  products is  nitrogen  oxides.
The reaction is  illustrated  for NO:
    Organic-N +  02	»- NO + H20
Thus,  incineration of  sludge, which  contains  a  fairly large
organic  nitrogen  fraction, produces  nitrogen oxides.   When
organic nitrogen is subjected  to SAC,  however,  little  organic-N
is  converted  to  nitrogen oxide  because little oxygen is
available.   The  organic  nitrogen is instead converted to ammonia.
The ammonia, when oxidized  (for example,  in  the  afterburner) is
converted to nitrogen gas and water.
    4NH3 + 302	»~2N2 + 6H20
As  long  as  afterburner  temperatures  are  maintained  below
approximately 1,600°F  (871°C), conversion of N2  to nitrogen
oxide is also minimized.  Thus, the key  to  lower  nitrogen oxide
production with SAC appears to  be  its  ability  to  direct organic
nitrogen destruction  toward  ammonia formation,  rather  than  to
oxide formation.


Data supporting the observation of  low  sulfur  oxide  emissions  in
the Concord tests  are limited.   Measurements  indicate that  much
of the sulfur in the feed solids ends  up in the  ash  (17).   With
incineration,  most  of  the feed sulfur  is  delivered to  the stack.


Other  advantages  of  SAC  include  the  fact  that  essentially
all  equipment needed  is currently available  and has  a   long
performance history, and that  most  existing  MHFs can be easily
retrofitted to operate in a SAC mode.


Disadvantages  of SAC should  also  be considered in design.   The
afterburner  requirement can  limit   use  of  SAC  in  existing
installations  for several reasons.   The  afterburner is  normally
a large chamber,  and space  may not  be  available.   Floor  loadings
of  existing  buildings can easily   be  exceeded  by  a  large
refractory-lined device.   Supplemental  fuel and  air  must  be
supplied  to the afterburner,  requiring additional  space for
piping and equipment.

A second disadvantage is that  SAC  requires  more  instrumentation
than does incineration.   Proper  control is essential  for  good SAC
operation;  therefore, temperature controllers must be included  on
each hearth  to  control  air feed rate.   Draft and  other common
incinerator instrumentation must also be provided  and maintained.


                              11-74

-------
If for some  reason  the  furnace  exhaust  gases have to bypass the
afterburner, they  .may  create  emission violations.   Furnace
exhaust gases are  high  in pollutants,  such  as hydrocarbons and
other  noxious products  of incomplete  combustion.   They could
flare in the atmosphere,  causing  stack damage.  Also,  these gases
are corrosive.  All construction  materials in the  gas  stream must
be properly selected,  as  described  previously.  Corrosion  results
found at the full-scale  test  in  Concord,  California, are found in
the literature (17).

Additionally, combustibles  in the  SAC  ash  may  create ultimate
disposal problems.  For  example,  in a  landfill,  they may not be
as inert as incinerator  ash.
    11.4.3  Conversion of Existing  Multiple-Hearth
            Incineration Units to SAC

One of the greatest advantages of SAC  is  that  most  existing units
can be  converted  to operate as SAC reactors.   -This  retrofitting
involves  relatively  few changes.  The  costs and  benefits are
site-specific.   One  definite  incentive  for  conversion  is  that
the  existing  unit  may  be  able  to  handle  increased  sludge
loads without the  addition  of  more incinerators.   This incentive
is demonstrated  in a  design example presented  later  in  this
section.  Assuming an increase in solids  loading  of approximately
30 percent, the basic changes necessary are:
        CHANGE

Add an afterburner (if existing
system has an afterburner,  its
size may have to be increased).
If furnace is large enough, top
hearth may be used as an after-
burner; however, refractories
must be examined.

Add combustion air flow
control and temperature
controllers.

Possibly replace combustion
air fan.

Modify induced draft fan--may
need change only in speed or
damper position.
            REASON

Required to burn combustible
fuel gas prior to exhaust.
Requ i red
process.
to  control  SAC
May  be  required to  control
reduced air flow rate.

Required because total  unit,
including  afterburner,  uses
approximately 50  percent
excess  air,  while  an
                                     MHF  incinerator  uses,
                                     including afterburner,  75  to
                                     200 percent excess  air.  See
                                     Figure  11-26.
                              11-75

-------
        CHANGE
                          REASON
Review and modify venturi
and wet scrubber.
Add additional emission control
equipment.
Review furnace system and
and replace remote
instrumentation.
Generally "tighten up1
system.
furnace
              Required  to maintain high
              performance  with  lower air
              flows.   Also  may  need
              precooler  section if  boiler
              is not in  process  train.

              Required   depending  upon
              local  air  emission   control
              regulations  regarding
              applicability  of new  source
              performance  standards.
              Modification of process may
              change applicable  standards.

              Good practice for any  major
              process  revision.
SAC process depends on  good
air control.   Peak  and  poke
holes must be  modified  along
with  other  openings  into
the   furnace  to  reduce
uncontrolled air  leakage
into the furnace.
With these  modifications  and any  others  found  necessary during
the  review of  the  site-specific system,  the  retrofitted  MHF
system will be suitable  for  SAC  operation.
    11.4.4  Design Example:   Retrofit  of  an Existing
            Multiple-Hearth  Sludge  Incinerator to a
            Starved-Air Combustion  Reactor


A 20-MGD  (0.88-m3/s) domestic  wastewater treatment  plant  in the
Midwest has been  incinerating primary and waste-activated sludge
in two multiple-hearth  furnaces.   All sludge is thickened prior
to dewatering on vacuum filters that produce a 25 percent solids
feed cake.   Polymers are used  in  the vacuum filters.   The ash
from  the furnaces  is  sluiced to  ash   holding  ponds, and the
supernatant is recovered and  returned to  the plant  influent
sewer.   Stabilized ash is removed from the ponds at least once a
year and hauled to the  local  landfill.


One furnace is  normally  required  for  sludge  reduction; however,
the original design  provided  100  percent redundancy.   The plant
is  currently  overloaded  and  both multiple-hearth  furnaces are
used simultaneously  about three  months of the year.
                              11-76

-------
Substantial growth  in  wastewater flows  are  anticipated  in  the
next four  years.   Planning for  an  10-MGD  (0.44-m3/s)  expansion
is currently underway.   The design will handle  projected  flows
through 1988.   In  addition,  new air emission  regulations  were
recently promulgated limiting hydrocarbon,  carbonyl, and  carbon
monoxide  emissions  to  about half  of the current incinerator
emissions.   The  city has  been given  notice  to  correct  this
situation or be subject to fines levied  by  the  local air  quality
management district (AQMD).  A time extension to review  and
correct this problem has  been granted  to  the  city.   Data  for the
existing plant  are shown  in  Table 11-19
                           TABLE 11-19

    DESIGN EXAMPLE: WASTEWATER TREATMENT PLANT OPERATING DATA

                     Parameter               Value

          Plant operating conditions
            Design flow, MGD                    20
            Total solids, Ib/day dry
              basis                         40,800
            Volatile solids, percent of
              dry solids                        75

          Furnace operating conditions
            Operating hours/week               168
            Loading rate, Ib/hr dry basis    1,700
            Solids content of feed, per-
              cent dry weight           • .       25
            Loading rate, Ib/hr wet basis    6,800


          1 MGD = 0.044 m3/s
          1 Ib/day = 0.45 kg/kg
          1 Ib/hr = 0.45 kg/hr
        11.4.4.1  Approach

The  city  retained a consultant to prepare  a facilities plan/
project report to obtain Construction Grants  funding  for a  plant
expansion to  30-MGD  (1.32-m3/s).   Because  of  the  urgency of  the
air  emissions problem,  the city authorized  the  hiring of  air
pollution experts to  assist  the  consultant  in  developing  an
interim plan consistent with the  goals of the expansion.

Following several detailed design estimates,  afterburning  at
1,200°F  (649°C) for one-half second was  determined to be  the
most  cost-effective  solution.    This  approach  was also felt  to
guarantee a continuous and  dependable operation while satisfying
all regulations.


                              11-77

-------
Since afterburning  was  proposed,  it was  also decided to  study
SAC.  SAC could possibly increase existing furnace capacity and/
or reduce the equipment  to be added.   Prior to review  of  SAC,  it
was determined that,  in  the  incineration mode, each furnace  would
require  an  afterburner,   and  that  a new furnace  and  afterburner
would be required for the plant expansion  to 30-MGD (1.32-m3/s).


        11.4.4.2  Preliminary Design

Two  experienced  multiple-hearth furnace manufacturers  were
provided the  data  in Table  11-19.   Detailed heat and material
balances  were  developed  for  incineration  and  starved-air
combustion—both followed by external  afterburning.   The  schemes
used  the existing  vacuum  filters to  provide  a  feed  cake  of
25  percent solids.   Also,  each manufacturer  was  to  analyze
two  additional  cases that  entailed  use of  improved  dewatering
equipment to  produce  a  feed cake solids  content  of  35 percent.
Both of  these cases  used SAC, but  one had an external  afterburner
and the  second used the  top hearth of  the  present furnace as  the
afterburner.  The  manufacturers  were  asked to use the existing
furnace  to  determine  the capacity of each  of  the  four systems.
The cases considered were as  follows:

Case I    Add an external afterburner and heat recovery boiler to
          each  furnace.    One additional furnace  is required  to
          satisfy  future loading.

Case II    Convert  existing furnaces  to SAC.   Add an external
          afterburner and heat recovery boiler  to  each furnace.
          One additional  furnace  is  required to  satisfy  future
          loading.

Case III  Convert  existing furnaces  to SAC.   Add an external
          afterburner and heat recovery boiler  to  each furnace.
          Sludge feed rate  calculated using  allowable rates  for
          SAC with  improved dewatering equipment (35  percent
          solids).   Note that afterburner  temperature  is  1,430°F
          (777°C).    No  additional furnaces  required  for  future
          loading.

Case IV   Convert  existing  furnaces  to  SAC  and  use   top  hearth
          as  an afterburner.   Add a  heat  recovery  boiler  to
          each  furnace.  Sludge feed rate  calculated  from
          allowable  rates  for  SAC  with   improved  dewatering
          equipment   (35   percent solids) and desired  afterburner
          temperature of 1,200°F  (649°C).  No additional  furnaces
          are required for future  loading.

Table 11-20  shows  a  summary of  the  manufacturer's calculations
for  the  four  cases  and   the  existing condition.   An  interesting
comparison  can be  made  between Cases  I and  II.   Both  cases
use  an   afterburner  and  recover heat but  Case II utilizes
SAC.   Heat recovery gains by using  SAC  are  impressive.    The
city would  save 1.33 x   10^  Btu  per  hour   (1.40  GJ/hr) by  using
SAC,  which would  produce an  annual  fuel  savings of slightly


                             11-78

-------
                                   TABLE 11-20

            DESIGN EXAMPLE:   HEAT AND MATERIAL BALANCES
                      FOR MULTIPLE-HEARTH FURNACES
   Type of operation
Furnace design
  Number of  furnaces
  Diameter,  ft-in.
  Number of  hearths '
  Hearth loading rate,
    Ib wet solids/sq
    f t/hr
  Afterburner
Sludge feed
  Lb dry  solids/hr
  Heat value, 106 Btu/

  Volatile solids, per-
    cent  dry solids
  Feed solids, percent

Afterburner supple-
  mental  fuel
    Mass, Ib/hr
    Heat  value, 10
      Btu/hr

Furnace combustion air
  Mass at 60 °F, Ib/hr

Shaft cooling air
  Mass, Ib/hr

Shaft cooling air return
  to furnace
    Mass  at 350 °F,
      Ib/hr

Shaft cooling air to
  stack
    Mass  at
      Ib/hr
    Heat  va
      Btu/hr
  afterburner
    Mass at
      Btu/hr

Ash
  Mass,  Ib/hr
  Heat val
    Btu/hr

Radiation
  Heat loss, 10
N.A. - Not applicable.
Existing
condition
Incinerator
2
16-9
7
7.0
..None

1,700
12.75
75
25
N.A.
N.A.
17,833
15,939
Case I
Modified
incinerator
3C
16-9
7
7.0
External

•1,700
12.75
75
25
189
3.77
17,833
15,939
Case II
SACb
3C
16-9
7
7.0
External

1,700
12.75
75
25
128
2.44
9,822
15,939
Case III
SACb
2C
16-9
7
10.2
External

3,473
26.05
75
35
0
0
12,507
15,939
Case IV
SACb
2C
16-9
7d
10.2
Internal
(top hearth)
2,957
22.18
75
35
0
0
9,867
15,939
                          13,548
                                       13,548
                                                     9,840
                                                              12,480
                                                                           9,867
350 UF,
2,391
.ue, 10
0.35
ig air to
:r
350 °F,
N.A.
ir , , 425
:, 106
0.04
106 Btu/hr 0.29

2,391

0.35



0
425

0.04
0.29

5,919

1.09



180
478e

0.14
0.29

0

0



3,660
975e

0.28
0.29

0

0



6,072
866e

0.25
0.29
 All data supplied by the manufacturer.
 SAC - Starved-air combustion.
^Number of furnaces required in  1988  (30 MGD),  for
 increased sludge quantities with one furnace on
 standby.

 Note, top hearth is afterburner, therefore,  not
 included in hearth loading  calculations.
"Includes combustible heat content.
 Existing system does not include boiler.
                                                              1  ft = 0.30 m
                                                              1  in. = 0.02 m
                                                              1  Ib/sq ft/hr =4.9  kg/m2/hr
                                                              1  Ib/hr =0.45 kg/hr
                                                              1  x 106 Btu/hr = 1,055 MJ/hr

                                                              1  gpm = 0.06 1/s
                                                              1  MGD = 0.044 m3/s
                                       11-79

-------
                                   TABLE 11-20

             DESIGN EXAMPLE:  HEAT AND MATERIAL BALANCES FOR
                    MULTIPLE-HEARTH FURNACES  (Continued)
   Type of operation
Furnace exhaust
  Mass at 800 °F,  Ib/hr
  Heat value, 106  Btu/
    hr

Afterburner combustion
  air
    Mass at 60°F,  Ib/hr

Afterburner exhaust
  Mass, Ib/hr
  Temperature, °F
  Heat value, 106  Btu/
    hr

Boiler exhaust
  Heat value at 400  F,
    106 Btu/hr

Recoverable heat
  70 percent efficiency,
    106 Btu/hr

Precooler and Venturi
  water feed
    Flow at 70 °F, gpm

Scrubber water feed
  Flow at 70° F, gpm

Scrubber drain
  Flow, gpm

Gas exhaust
  Mass, Ib/hr
  Temperature,  F
  Heat value, 106  Btu/hr
                        Existing
                        condition
Incinerator


  24,209

   12.07



   N.A.
   N.A.
   N.A.
   N.A.
   N.A.
    2.24"
      51
     243
     306
  21,368
     120
    6.35
                                      Case I

                                     Modified
                                    incinerator
                                                 Case II   Case III
24 ,209

 12.07



 2,555
26,764
 1,200

 15.84
                 9.51
                 4.43
                   51
                  243
                  306
20,759
   120
  7.53
             SAC
                            4,296
19,366
 1,200

 14 .05
                             3.69'
                               45
                              282
                              338
15,480
   120
  4.11
                       SAC
16,145     21,454

 11.23      14.87
          11,534
32,987
 1,430

 24.24
                                       13.92
                                       7.22
                                         65
                                        456
                                         536
26,220
   120
  8.52
                                                 Case IV
                                   SAC
            17,448

             10.55



            10,989
28,437
 1,200

 21.64
                                                   12.89
                                                    6.13
                                                      65
                                                     456
                                                     532
27,837
   120
  2.64
N.A.  - Not applicable.
 All data  supplied by  the manufacturer.

3SAC - Starved-air combustion.
'Number of furnaces required in 1988  (30 MGD),  for
 increased sludge quantities with one  furnace on
 standby.

 Note, top hearth is afterburner, therefore, not
 included  in hearth loading calculations.
2Includes  combustible  heat content.

 Existing  system does  not include boiler.
     1  ft = 0.30 m

     1  in. = 0.02 m
     1  lb/sq'ft/hr =4.9 kg/m2/hr

     1  Ib/hr = "0.45 kg/hr

     1
     1
                                    106  Btu/hr = 1,055 MJ/hr
                                  gpm
                        0..06 1/s
                              :3
                                1 MGD  =  0.044 m /s
over  $30,000  at  $2.70  per  106  Btu  ($2.60  per  GJ).    However,
it   appears  that  this  savings  would  not  justify  the  conversion
when compared  with the large  capital  expenditure.
                                      11-80

-------
Cases III and  IV  both use SAC,  but  start with a cake  that  has
been dewatered  to  35  percent solids.  Recoverable heat quantities
are far higher  than for Cases  I and II.   Case IV,  which does  not
use  an  external  afterburner  has  a capital  cost  advantage over
Case III.   Also,   the  system  could  easily handle  the  expected
sludge  loads through  the design year.   The fuel savings  would  be
almost  $90,000 per  year, and the energy  generated  would  be
sufficient to save another $250,000 per  year (at 10,600  Btu/kWhr
[11.18  MJ/kWhr]  and  $0.05/kWhr).   These  savings  alone would
justify  capital  expenditures  of over $3,600,000 (20 years  at
7 percent per  year).   In addition,  there would be a capital
savings because a  third  furnace would not be required.


After receiving detailed cost  estimates,  the city  authorized  the
Case IV design.  The  flowsheet  is given on Figure 11-28.
11.5  Co-Combustion  of  Sludge and Other Material


The net fuel  value of sludge depends on the fraction of  its  total
combustible solids,  the  fuel  value of those  combustible  solids
(generally about 10,000  Btu per pound  [23.24 MJ/kg]),  and  the
amount  of water present.   Wastewater treatment  plant sludge
generally has  a  high  water content  and,  in  some  cases,  fairly
high levels of inert materials.   As a result,  its  net fuel  value
is often low.  Autogenous  combustion of wastewater treatment
plant sludge  is  generally  only  possible  when  the  sludge  solids
content  is 30  to 35 percent or greater.   These  solids  contents
are  often difficult  to achieve  by  conventional  dewatering
techniques;  consequently,  supplemental  fuel is  required  for
the  combustion  operation.   If  sludge is combined with other
combustible materials  in a  co-combustion scheme, a  furnace  feed
can be created that  has  both a low water concentration and  a  heat
value high  enough  to  sustain  autogenous  combustion and may  be
cost-effective.
    11.5.1  Co-Combustion with Coal and Other Residuals


Many materials  can  be combined with  sewage  sludge to  create  a
furnace  feed with  a  higher heat  value than  sludge.  Some  of
these materials  are coal;  municipal  solid  waste;  wood  wastes;
sawdust;   textile wastes; and  agricultural  wastes,  such  as  corn
stalks,  rice husks,  and bagasse  (68, 69).  Virtually any material
that can be burned  can be combined with sludge in a  co-combustion
process.   An advantage  of co-combustion is  that a municipal
or  industrial waste  material  can  often be disposed of while
providing an autogenous sludge feed, thereby solving two disposal
problems.
                              11-81

-------
                                                    3,64 a 10° Btv/hr
                                                    GAS IXHAUST
                                                   27,837 Ifefhr * 12BPF
          9,867 Ib/hf
G1
23,lS*MJ*Bttt/hr
SLUPGi FiiD
2.967 Ih/hr DRY
M% SOLIDS FEED
  a
         16,438 to/ht
                      ASH 88B !Mw
                                            1Btu/hr= 10S5 J/hr
                                            llb/hrTOKShg/
                                            Igpm - 0,08 I/I
AIR
                             FIGURE 11-28

            DESIGN EXAMPLE:  STARVED-AIR COMBUSTION IN A
                      MULTIPLE-HEARTH FURNACE
Recent  studies have shown that  the  addition of pulverized  coal  to
liquid  sludge prior  to  dewatering  can  markedly increase the  cake
solids  content  (70-73).   A  drier filter  cake  is produced;  thus,
the net heat value of the sludge-coal  mix is much greater  than  if
the  coal were added  to the  filter cake following  filtration.
Also,  the sludge-coal  mix  is homogenous,  which  leads  to  better
combustion.    It  may  be  possible  to  reduce   the  amounts  of
inorganic  filter  aids  (lime,   ferric  chloride)  required and
produce an  autogenous feed.   This approach may become appropriate
                                11-82

-------
in many plants  that  are  close  to coal mines or coal-fired power
stations.   Coal, however,  is  not  a  waste material, and its use to
improve filtration  and  increase the  fuel value  of  the furnace
feed is not  as  desirable  as  using  a combustible waste for these
purposes.   Two  plants, one  in  Rochester,  New York,  and  an other
in Vancouver,  Washington,  are  experimenting  with sawdust  as  a
filter aid prior  to  combustion.    The  results  to  date have been
very good, but  detailed data are not available.  Minneapolis has
tried using woodchips as  a  supplemental fuel  (70).
    11.5.2  Co-Combustion  with  Mixed Municipal Refuse (MMR)


Currently there are more  than  twenty  sludge and mixed municipal
refuse  (MMR)  co-combustion systems,  including incineration,
pyrolysis, and  starved-air  combustion,  that are being operated,
tested,  or  demonstrated  in full-scale plants  (74-77).   The
systems  described  in  this  section  have been operated  at full-
scale  and have been developed sufficiently to be  implemented
whenever they prove cost-effective.


There  are  two basic  approaches  to  co-combustion of  sludge
with MMR:  (a) use of  refuse  combustion  technology by  adding
dewatered or dried sludge to an MMR combustion unit, and  (b)  use
of sludge  combustion  technology  by adding  raw  or  processed  MMR
as a  supplemental fuel  to the sludge furnace.    Table 11-21
illustrates the commonly used approaches  to  co-combustion.
                           TABLE 11-21

     CONVENTIONAL APPROACHES TO CO-COMBUSTION OF WASTEWATER
                 SLUDGE AND MIXED MUNICIPAL REFUSE

          Mixed municipal refuse technology

            Grate-fired  (refractory or waterwalled)
              Sludge dried via flue gases
              Sludge dried via steam from furnace
              Sludge added directly to furnace

            Vertical packed bed reactors  (sludge added
              to  bed)
               Air  (Andco-Torrax)
               Oxygen  (PUROX™, a Union Carbide System)

          Sludge  technology

            Multiple-hearth
            •  Incineration
              Starved-air combustion

            Fluid bed                   .  ,
                              11-83

-------
                                                           GAB
                                                          EXHAUST
                                                              STACK
                            FIGURE 11-29
SP""
          TYPICAL GRATE-FIRED WATERWALLED COMBUSTION UNIT



         11.5.2.1  Refuse Combustion Technology

 Historically,  grate-fired  refractory and  waterwalled combustion
 units  have been  used  to  burn  raw mixed municipal  refuse.
 Figure  11-29  illustrates  this approach.   This practice is common
 throughout  Europe, where there are several hundred  installations.
 When  sludge disposal became a problem,  the first approach was to
 burn  the sludge with the  refuse.   The quantity  of  sludge was
 normally small compared to the  refuse.   This  was attempted in
 several  locations,  but efforts were generally unsuccessful, with
 failures due to  the  following problems:

      •   Uniform mixing  of sludge  and refuse  was difficult to
         accomplish on  a large scale.  Poorly mixed  feeds produced
         alternate  "hot"  and  "cold"  feeds,  resulting  in  erratic
         furnace operation.

      •   Biodegradation of  materials  in  the refuse/sludge  holding
         bins caused  unacceptable  odors.  Detention  times in these
         bins are often several days long, which  is  sufficient for
         biological action  to  be established.
                              11-84

-------
     •   High moisture content  of  the sludge  and  inadequate
        furnace  detention times sometimes caused  non-autogenous
        combustion  and wet residues.

However,  as previously  stated, several  systems currently  in
operation have  been designed specifically  to  incinerate MMR with
sewage sludge (78).  A number of these are  described below.

Sludge Drying via Steam Generated by Furnace

Several grate-fired,  waterwalled combustion  units in  Europe are
presently  incinerating  refuse  and sewage sludge.   At  Dieppe,
France, 54 tons  (49 t)  of MMR and  21  tons  (19  t)  of dried sludge
are  incinerated daily  (79).   Digested  sludge  with a  solids
content of  four percent  is  pumped  from  the wastewater treatment
plant and dried  with 350°F (177°C)  process  steam in two thin-film
evaporators to a  solids content  of 55  percent.   The  vapors
generated  are  returned  to  the furnace.   The dried  sludge  is
conveyed  to the charging  chutes  of  the  furnace and is  mixed
with the solid waste  from the receiving pit.   A  small plant  at
Brive,  France (80), is similar to  that  at  Dieppe,  except  that  it
uses raw sludge.

Sludge Drying via Fornace Flue _Gas_e_s

A waterwalled  combustion  unit  at the Krefeld plant  near
Dusseldorf,  Germany, processes 600  tons (544  t)  of MMR and
45 tons (41  t)  of  dry wastewater solids daily (75-77,81  ).   The
facility generates  electricity for the wastewater  treatment plant
and  incineration facility and exports  hot water  for  use  in the
community.   Raw sludge,  with  a solids content  of  5 percent,
is  pumped  from  the wastewater  treatment  plant to  the disposal
facility.   The sludge  is centrifuged  to a  solids  content  of
25 percent and  then flash-dried  in a  vertical-shaft flash-drying
chamber  with 1,500°F  (816°C)  flue  gases at  from  the  refuse
combustion  unit.   The  powdered sludge is  then  injected  into
the  furnace  immediately  above the  top  of  the flame  (suspension
firing).  The facility has been  in operation for four  years.

Two  plants  in  the United States  use  flue  gases generated  in
grate-fired, refractory-walled  combustion units  to  dry  waste-
water  solids prior to combustion  with MMR  (74).  In Ansonia,
Connecticut,  sludge  with  four  percent  solids  is dried  in
a  disk-type,  co-current  spray  dryer  with  1,200°?  (649°C)
incinerator   flue gases.   Dried sludge  and vapors  are injected
into the incinerator  for suspension burning.  The plant capacity
is  200  tons (181  t)  per day of solid  waste.   Presently, the
sludge is not being incinerated but used as  a soil conditioner.
Holyoke, Massachusetts,  uses a  similar  incinerator and averages
250 tons (227 t) per week of refuse and 19 tons  (17  t) per week
of dry sludge throughput.  However, the sludge is  dewatered to 28
percent solids  prior  to  drying  in  a  rotary  unit  using hot flue
gases.   Dried sludge  and vapors are  added to the furnace above
the combustion zone.
                             11-85

-------
/Sludge Added JDirectly to Furnace

 Recently  at  Norwalk,  Connecticut,  a process was tested in which
 a  stoker-fired  incinerator was used to co-combust  sludge and
 refuse  (82,83).   In  this  project,  sludge with a solids  content
 of  five  percent  was sprayed onto the front wall of the charging
 chute to form a  thin sludge layer on top of  the  refuse.  The
 sludge layer  dries  and  burns  during the  30-minute residence time
 in  the  combustion  unit.   This process  has been  incorporated into
 the  design  of a plant  at  Glen  Cove,  New York, that will burn a
 mixture  of  25 tons (23 t)  per  day  of sludge (20 percent  solids
 content)  and  175  tons  (159  t) per day  of  mixed  municipal  refuse.
 The  plant  is  designed  to  produce  2.2 megawatts  of  power,
 sufficient to meet  the  demands  of the  wastewater treatment plant
 and  the   incineration  facility.   Construction  of  the  Glen Cove
 facility  is scheduled to be completed in  1982.

 Vertical  Packed Bed

 There are two  vertical  packed bed,  solid waste, starved-air
 combustion systems currently  available in the United States:
 Andco-Torrax and PUROXtm (see Figure 11-30).

 The  Andco-Torrax  system (84,  85) is  a vertical shaft,  slagging
 type furnace  in  which unprocessed municipal solid  waste  is
 charged   into the  unit from the top.   The refuse  is burned at
 the  bottom   of the ram at  3,000°F  (1,649°C)  by the addition of
 small  quantities  of  air  heated by  countercurrent  heat exchange
 with the afterburner exhaust.  The combustible off-gases are
 afterburned  at  2,000°F (1,093°C)  and processed by  electrostatic
 precipitators.   Wet sludge has  been added to an existing 75 ton
 (68  t)  per  day system, but detailed test data  are  not  presently
 available.

 The  PUROX system,  a  trademark  of  Union  Carbide,  is  a vertical
 furnace  for  combustion  of a processed refuse (86,  87).    Proces-
 sing  includes  shredding and  ferrous metal separation.    The PUROX
 system uses pure oxygen rather than  air.   The refuse is  burned at
 3,000°F  (1,649°C),  and a  fuel  gas  is produced  that  has  a heat
 value  of 385 Btu  per standard  dry  cubic foot  14.3 MJ/m3 dry).
 The  molten  slag produced at  the  high combustion temperature is
 primarily  inert  silica. j A  processed  refuse and sludge  mixture
 was  successfully  run through  the  test   unit for  two months at
 South Charleston,  West Virginia  (87).   Average wet  test feed
 rates were 90  tons  (82  t)  per day.   Test  data  indicated that the
 refuse-to-sludge ratio  was  4.26:1.   Lower ratios were not tested
 because  the  availability  of  sludge  was limited.  The pure oxygen
 feed  rate was approximately  0.2  tons of  oxygen per  ton  (0.2 t
 C>2/t) of  feed.    Fuel gas production   and  quality,  and slag
 production  and  quality from  mixed  refuse-sludge  feeds,  did not
 differ radically from that of pure  refuse combustion in  the PUROX
 reactor.  Heavy metals in the sludge were trapped in the slag and
 were  not  discharged with the exhaust gases.


                              11-86

-------
             Off GAS
 SHREDDED
 MIXED
 MUNICIPAL
 REFINE
4
                                                            MIXED
                                                           MUNICIPAL
                                                            REFUSE
                                        REFUSE .?
                                         PLUG 'i
                                       DRYING /
                                         ZONE '
                                   COMBUSTION
                                        AIR

                                   PRIMARY
                                 COMBUSTION
                                      AND
                                     ZONE
                                        PVROLYSIS I
                                                           I DROPOFF
                                                            AND
                                                            QUENCH
       PUR0X  REACTOR
  (CWRTESY OF UNION CARBIDE
                                         AN0CQ-TQRRAX REACTOR

                                       JCOUHTESV Of ANDCO 1HCQ HPORATED}
                            FIGURE 11-30

                      VERTICAL SHAFT REACTORS

        11.5.2.2  Sludge Combustion Technology

The most widely  used sludge combustion methods  are  the multiple-
hearth and fluid  bed furnace.  Both  types  of units  have
successfully  burned  refuse.   Although  the  electric  furnace
and  the cyclonic furnace appear to be  capable of refuse and
sludge  combustion,  no full-scale work  has  been done  to  date.
Figure  11-31  presents requirements  for sustaining autogenous
combustion when sludge  is  mixed  with  refuse.

Multiple-Hearth Incineration

Several plants in Great Britain and  Europe  have  been  practicing
co-incineration  in   multiple-hearth furnaces  for several  years.
However, serious  problems  such  as  severe  erosion  of the hearths,
poor temperature  control,  refractory  failures, and air  pollution,
                               11-87

-------
50 50
40 60
30 70
    LU
    O
    Q
    -i
20 80 -
10 90
 0100
                     ASSUMPTIONS:
                         MULTIPLE HEARTH FURNACE
                         AFTERBURNER AT 1400°F
                         HEAT REQUlRED/lb WATER = 3500 Btu/lb
                         SLUDGE COMBUSTIBLES = 10,000 Btu/lb
                         MWIR DRY SOLIDS - 6500 Btu/lb
                         MMR MOISTURE CONTENT = 25%
                                                                   v>
                                                                   £
                                                                   g

                                                                   <
                                                                   oc
                                                                   m
                                                                   ui
                                                                   LL
                                                                   IU
                                                                   a:
                                                               3   y
                                                                   z
                                                                   3
                                                                   Q
                                                                   LU
                                                                   X
                                                               6

                                                               7
                                                               8
                                                               9
                                                               10
                                                                   LU
                                                                   o
                                                                   Q
                                                               50
                                                               100
      0
             10
60
               20      30      40      50

                SLUDGE SOLIDS CONTENT, %

                      FIGURE 11-31

AUTOGENOUS COMBUSTION REQUIREMENTS FOR CO-DISPOSAL
                               11-88

-------
have been experienced (88).  All of  these  problems  appear to  be
a direct result of poor solid waste processing  prior to  addition
into the furnace.  Poor pre-processing causes extreme variations
in feed heat value, which in turn causes wide and uncontrollable
temperature  fluctuations  in  the  furnace.   These  result  in
refractory failures and  air  emission problems.

These  problems,  were  resolved  in  test  work conducted at the
Central  Contra  Costa  Sanitary  District  wastewater treatment
plant  at  Concord, California  (17).  All  refuse was shredded,
air-classified,  and screened  prior  to use.   This provided a
feed which was  relatively free  of  metals  and  had  a  reasonably
consistent heating value,  as well as a consistent particle size.
When the  furnace  was  operated  in an incineration mode, none
of  the problems  encountered  in Great Britain  or  Europe were
experienced,  but temperature control was still difficult.  This
was corrected by operating the  furnace  in a SAC  mode.  This work
and the  European  experience  indicates that  for MHF furnaces
pre-processing  of municipal  refuse  is required  and SAC of  refuse
and sludge is preferred  over MHF  incineration.

MuJJ^iple-Hearth ,=iS_taryed-Air Combustion

Co-combustion of sludge  with processed municipal  solid waste was
first  successfully performed by SAC in a multiple-hearth furnace
during  a small-scale   test in  November  1974  at  Burlingame,
California  (89).   A   full-scale prototype  test was  later
implemented  at  Concord,  California  (17).   A  flow  diagram of the
test system  is  given  on  Figure  11-32.   The  test  SAC-MHF  burned a
combination  of  wastewater  sludge and refuse-derived fuel  (RDF)  in
several ratios  varying  from 100 percent  sludge  to  100  percent
RDF.

Municipal  refuse  was  shredded, a ir-class i f iedi,  and screened
to  produce  a  refuse-derived   fuel.    The  RDF  had  a  heating
value   of  7,500  Btu per  pound  (17.4  MJ/kg)  of  dry solids and a
moisture content of 25  percent.  A combined feed rate of up  to
10,000  pounds per hour  (4,540 kg/hr) was applied to the 6-hearth,
16-foot 9-inch  (5.1-m) diameter SAC-MHF.  Because of the  addition
of  RDF,  the heat  value of the  feed was  greatly increased  as
compared to  sludge alone.   This produced a  fuel  gas  heat value
averaging 136 Btu  per  standard dry cubic  foot  (5.07  MJ/m^ dry)
and afterburner  temperatures  up to  2,500°F  (1,371°C).    Stable
furnace control  was achieved by regulating the addition of  air  to
maintain hearth .temperature.

Results of the  test indicate that to maximize energy conversion,
RDF should be fed to  a  mid-furnace  hearth,  and  sludge  to the top
or second hearth.  In other words,  the  point of  sludge  addition
remains  as   in  conventional systems,  and the  RDF  is  treated
like any other fuel and added  to the combustion  zone.  The ash
handling  system must be  capable of handling  small amounts  of
metal.   Test;'results  indicate  that autogenous combustion  of
a  16 percent solids  sludge cake  can be  accomplished  with  an
RDF-to-sludge wet ratio  of 1:2.


                             11-89

-------
                                                          REFUSE
                                                          DERIVED
                                                          FU6L
                     ASH
                                    HEAVY MATERIAL
                                     TO LAHDFILL

                           FIGURE 11-32

        FLOWSHEET FOR CO-COMBUSTION FULL SCALE TEST AT THE
        CENTRAL CONTRA COSTA SANITARY DISTRICT, CALIFORNIA

This type  of  system  is  being  reviewed for several plants, with
implementation expected  for  the Central  Contra  Costa Sanitary
District and the  City of Memphis, Tennessee.

The flow sheet for  a  multiple-hearth furnace  used for  combustion
of sludge and solid waste is similar to Figure 11-27, except that
a  refuse-derived  fuel is  added  to the  middle  hearth(s).   Heat
and  material  balances  for the hypothetical  treatment  plant
alternatives (Table  11-8) are  presented in Table  11-22.  The
effect  of  a 20  percent  sludge  solids feed  versus  a  40 percent
sludge  solids feed  is  again exhibited.  An important item  in
this table  is  the  recoverable  heat, which is four times greater
than  that  for  other  sludge-only  combustion  processes (see
Table 11-18).  This  shows  the  effect of  the  addition  of refuse-
derived fuel.  Also,  in  Case  IIB,  note the effect of  excess air
on the  afterburner  temperature.   With 40  percent excess  air,  a
temperature of  2,450°F   (1,343°C) would  be expected  (consistent
with Cases IB and IIB);  however,  a  temperature of  1,800°F (982°C)
occurs with an excess air rate  of 150  percent.

Specific information specifically concerning  co-combustion  by SAC
in a MHF can be  found in the  literature  (8,17,18,35,69,74-77,81,
89-96).
                              11-90

-------
                                      TABLE 11-22

   HEAT AND MATERIAL BALANCE FOR CO-COMBUSTION  BY STARVED-AIR
                 COMBUSTION IN A MULTIPLE-HEARTH  FURNACE3
                                                 Alternative
Stream
Furnace design
Diameter, ft-in.
Number of hearths
Hearth loading rate, Ib
wet solids/sq ft/hr
Sludge feed . . ' '
Lb dry solids/hr
Percent of total furnace
feed
Volatile content, percent
RDF feed
Lb dry solids/hr
Percent of total furnace
feed
Volatile content, percent?
Percent moisture
Combined feed rate
Total Ib wet solids/hr
Heat value, 106 Btu/hr
RDF to sludge ratio, wet
basis
Furnace combustion air.
Ib/hr
d
Excess air rate, percent
Ash
Mass, Ib/hr
Heat value, 10 Btu/hr
Afterburner combustion air
Mass, Ib/hr
Afterburner exhaust
Mass, Ib/hr
Heat value, 10 Btu/hr
Temperature , °F
Radiation
Heat loss, 10 Btu/hr
Recoverable heat
70 percent efficiency.
106 Btu/hr •
Connected power
Horsepower
Installed cost, thousand
dollars6
IA
5 MGD
20 percent
solids

22-3
6

11.4

1,806

50
77

7,224

50
84
20

18,060
20.28

1:1

12,753
' 40

1,749
,0.50

34,123

63,186
63.27
2,290

1.62


23

555

. 2,800
IB
5 MGD
40 percent
solids

16-9
7

10.8

2,131

50
65

4,267

• 50
'. ' ' 84
20

10,664
11.12

1:1

7,320
'. ' 40 '

1,589
.0.46

. 25,867

42,260
42.80
2,457

"i.12'.


20
'
343

5,200
IIA.,
15 MGD
20 percent
solids

25-9
6

11.8

2,713

50
77

10,850

50
84
20

27,126
30.71

1:1
': •' ,
19,316
40

2,627
0.76
' r
51,049

94,861
95.27
2,294

" 2.33


42

725

3,000 •
IIB
15 MGD
40 percent
solids

: 18-9
8

11.3

3,201

50
65

6,400

50
84
20

16,000
16.38

1:1

10,782
40

2,384
0.69

39,065

63,461
64.29
2,458

1.61


26

418

2,400
IIIA
50 MGD
20 percent
solids

25-9
9

12.5

4,292

50
77

17,172

50
84
20

42,930
47.29

1:1

29,747
40

4,158
1.20

81,838

150,355
150.9
2,294

3.57


70

725

3 , 500
IIIB
50 MGD
40 percent
solids

22-3
8

12.1

5,064

50
65

10,128

50
84
20

25,320
25.53

1:1

16,806
150

3,772
1.09

112,888

151,240
101.8
1,800

2.45


42

555

3,000
 All data supplied by the Eimco BSP Division of Envirotech-Corporation.
b
 Solids for B alternatives (40 percent solids feed), larger than A alternatives
 (20 percent solids feed).           •

 Sludge volatiles heat value  10,000 Btu/lb:  RDF volatiles heat value 8,500 Btu/lb.
d
 For total system - furnace And afterburner. ,-.'""     ,   .    .

 Costs as of early 1978.     •             • „,?
1 MOD = 0.04 m /s
1 ft = 0.3 m
1 in. = 0.02 m
1 Ib/sq ft/hr =4.9 kg/m /hr
1 Ib/hr =0.45 kg/hr
1 x 106 Btu/hr = 1,055 MJ/hr

-------
EVluid Bed

Municipal  solid  waste  and  wastewater  sludge  have been  co-
incinerated in a fluid bed furnace  in  Franklin, Ohio, since 1971
(97).   In the  solid waste  separation  process, a  wet  pulper
removes  ferrous metal and heavy  solids from 150 tons  (136 t)
per day  of shredded refuse.  Fiber is recovered  from the pulper
effluent by selective  screening and elutriation.   All  unrecovered
residuals from the fiber-recovery  step are conveyed  to a barrel
thickener.   Sludge  from a  2.5-MGD (O.ll-m^/s)  secondary wastewater
treatment  plant is added  to the  thickened residuals,  and the
combined stream is dewatered  in  a  cone press  to a solids content
of 45  percent  before injection  into the  furnace.   The furnace
feed  is  blown into  the  bed about one  foot over the  tuyeres.
Because heavy inert materials accumulate  within  the bed, there is
buildup in bed  volume,  and  a small amount of bed material must be
removed  periodically  from  the  furnace.    The preparation  steps
reduce the amount of noncombustible material  in the furnace feed
to between three and  six  percent  and  the  feed  size  to 1/2 inch
(1.27 cm) or  less  (97).

In a  conventional  dry shredding  and  separation  operation,  the
feed  stock would  not  be as uniform  as  it is at the  Franklin
facility.  If the  feed  to  the fluid bed furnace  is not uniform in
both  size  and  density,  heavy material  tends to sift  downward
through  the  bed.   This material must be  removed quickly  or it
could  upset  the air flow  through the bed.  Systems have been
developed to  remove settled, noncombustible material continuously
from the bed.

An FBF system  using  sludge  cake and  RDF  produced by  a  dry
processing approach was constructed in Duluth, Minnesota, and the
shakedown operations  began in 1979.  A process flowsheet of the
system is presented on  Figure 11-33.


    11.5.3  Institutional  Constraints

Co-combustion of sewage  sludge with municipal  solid  waste is a
viable  and  socially beneficial  approach  to  solids  disposal
problems.  Not  only  are  both  wastes  disposed of  in  an environ-
mentally  acceptable  manner,  but  benefits  can  be  accrued by
utilizing  the  waste  heat or  combustible  exhaust  gases  for
energy  conservation.    Cost-effectiveness,  however,  is very
site-specific,   and   in   general,  co-combustion  systems  are
not  economically  feasible without federal  and state  funding
(81,98,99).   This  is  due to the relative costs  of disposal and
relative quantities of the feed  material  involved.  For example,
solid  waste  quantities,  dry  basis,  are  approximately ten times
that  of  sludge  quantities and  can be disposed of  at one tenth
the  cost  of  sludge.   Therefore,  sludge disposal costs have a
significant impact on solid waste  operations, yet solid waste is
too  costly on  a unit  energy basis  to supplement fossil  fuels.
To  assist  in   funding,   the  federal  government  has  adopted


                             11-92

-------
guidelines  for  allocating costs  for co-combustion  systems (100).
As  solid  waste  disposal  and  fossil  fuel  costs  increase  and
funding becomes  more  available,  co-disposal  economics  will
become  favorable  in  more  applications.  Although   the  technical
feasibility  of  co-combustion  of sludge  and solid waste  has  been
demonstrated, there remain a  number of institutional  constraints
that may  have to  be  resolved prior  to implementation of  a large
scale  co-combustion project.   Because full-scale operations are
limited  and the  technology is  growing, risk analyses  should  be
conducted.   These analyses  would provide authorities with a basis
for making a decision and  with   an  understanding of  the  impacts
of that decision  (101).
                  TO If (HJWWiHf
                  PROCCfS

»V. i.tH I ILUuTt EMIMION
          ftSJGK/IWCF
              	- is*
                                  OgHATEIHP tLUPKel
                                 -tr	"":;"	:
                               FLUID BED
                               FURNACE
HIAT IMXJT ILLMXK UN -II)' BbAi
HEAT WPVr REFUIE T1.32 • 10 « Bm.^,
                                    ASH
                    OPERATIC*! OF I
                   STEAM POWERED!
                     EQUIfMENT J
 m-a.Mm
 1r/l«ct • 2.3 lid. DRY p'ti|3
 1 IU,'a4ii - P.45
 llb/h •
        1056 «Jli|»
                             FIGURE 11-33

      FLOWSHEET FOR CO-COMBUSTION AT THE WESTERN LAKE SUPERIOR
                SANITARY DISTRICT, DULUTH, MINNESOTA
In many  localities, wastewater treatment  and solid waste disposal
are   controlled   by   different  governmental  agencies.    Many
communities  have  contracts with private  firms for refuse handling
and  disposal  that  release ownership  of  the  refuse  to  the
contractor.   In such cases,  the municipality is not able to do as
it wishes  with the refuse.   Some contracts  are long term,  lasting
15  to 20  years.    Although  there  have  been legal  opinions that
these  agreements  can  be modified  for the benefit  of  the  public,
these  opinions have  not been  tested  in  court.   In  recent waste
                               11-93

-------
disposal contract  negotiations,  local  governments have attempted
to retain ownership of the refuse, with the private firm acting
strictly as a collector and hauler.   Retaining ownership of the
waste material would simplify resource recovery operations.

Consolidation  of  the governmental  agencies responsible  for
solid and  liquid  waste disposal  would also  simplify  disposal
operations  as they  relate to  co-disposal.   With more emphasis
on co-combustion techniques by both federal and  state agencies,
serious   institutional  problems  may  be resolved  by  governmental
interaction with local agencies.


    11.5.4   Conclusions about Co-Combustion

Of all areas of  technological growth in combustion,  co-combustion
may  have the greatest  potential  (81,102, 103) for use.   Co-
combustion  is  a  relatively   new  venture,  and  its  use  must
be  thoroughly researched and  tested,  and project  economics
evaluated.   Many  solid  waste  projects have failed  for economic
reasons.    Additionally,   institutional  requirements must be
satisfied before the project can reach fruition.


11.6  Related  Combustion Processes Used in Wastewater
      Treatment

Several  high  temperature  processes  are  used  in wastewater
treatment  plants  for purposes other  than wastewater  sludge
reduction.   These  processes include reduction of other wastewater
solids such as screenings,  grit, and scum,  and also  the regenera-
tion  of  chemicals such as lime and carbon.   High temperature
equipment   configurations are basically  the  same  as  those
discussed  in Sections  11.3  and  11.4, but some  new  types of
furnaces are introduced in  the sections that follow.
    11.6.1  Screenings, Grit, and Scum Reduction

Besides sludge, other solids produced in a  wastewater  treatment
plant  (screenings,  grit,  and scum)  can be processed in high
temperature systems.   Some of  the  unique operating  problems
presented by these  materials are described below:

     •   Screenings  tend to  clog  feed mechanisms  and  should be
        shredded  before  being  fed to  the  incinerator.   Bulky and
        non-combustible materials  should  be  removed  and  disposed
        of in a landfill.

     •   Grit  is  often odorous,   extremely abrasive,  normally
        contains fairly  large quantities  of  organics, and is
        relatively  dry,  thus making  it  autogenously combustible
        in many cases.  Because  of the odors, high temperature
        disposal  tends to be the desirable stabilization method.


                             11-94

-------
     •   Scum and grease are very  difficult  to  handle  because  of
        their adhesive  properties; however, they have a very high
        heating  value up to 16,700 Btu per pound (37.8 MJ/kg)  of
        dry solids  (Table  11-2)  (104).  Air flow must be adequate
        to assure that  the scum is totally burned;  if  it is not,
        the furnace will smoke.   To  provide  thorough  mixing and
        thus proper burning,  the scum and air should be injected
        into the furnace  at  the same point.   Scum  has  been fed
        through  atomizers,  but  this  feed  system was not totally
        effective because  the   resulting  vapors  and  smoke  have
        been difficult  to  control  (105).

A separate furnace  may  be  difficult to justify for any one of the
above  materials because  their quantities, as  compared  with
sludge, are  small.   In some cases,  the  material  can  be blended
with feeds and disposed of in existing sludge furnaces.   Burning
of  the residues will  not  usually  cause   capacity  problems.
Although scum can provide  considerable heating value, it can also
create  significant problems  with smoking  and  hot spots.   The
latter may damage refractory material.   Screenings  and  grit can
also create hot spots, but  they generally  cause considerably
fewer problems than  scum.

Complete  mixing  of  feeds  can  eliminate hot  spots due  to
nonhomogeneity,  but mixing is  often  difficult  to achieve.   The
location  of the mixing step  is also a  serious concern.   When
combined with sludge before dewatering, screenings, grit, or scum
can  cause dewatering  equipment  problems.   These can  include
excessive wear,  filter  blocking, and  poor dewatered cake release.
On  the other hand, it  is difficult  and costly  to produce  a
homogeneous mixture  when materials are combined after dewatering.

Since the materials are removed separately and  require different
dewatering techniques,   they  may  in  many instances be  disposed
of  more  appropriately by  means other  than  high temperature
processing.  Several plants have  provided digestion for sludge,
and incineration for screenings, grit, and scum, with sludge gas
used as  the  fuel for  the  furnace.   Other plants  have  provided
separate  furnaces for  scum reduction.   In one  plant,  a separate
furnace was provided for screenings only.

Furnaces  for  screenings,  grit, and  scum in small  plants  (less
than  10  MGD  [0.44  m^/s]),  tend to  be single-chamber  batch
operations with little or  no air  emission  control  devices.
However, high excess air  rates  and  large quantities of  fuel are
used to make  the burning  relatively clean and  odor-free.   Such
an operation is  costly.  For large plants, the furnaces described
in  Sections 11.3 and  11.4 are used.   However,  while  several
multiple-hearth  furnaces are  used successfully for  scum (106),
a starved-air combustion  operation  is desirable  to  control  the
combustion process  and  prevent serious  temperature excursions,
localized  hot spots, and smoking--all typical problems when scum
is burned.
                             11-95

-------
To address the problem of  scum  burning,  Nichols  Engineering  and
Research Corp.  has developed a furnace specially  suited  for high
energy  liquids that  are lighter  than water, such as grease,
waste  oils,  and  scum.   Their  WATERGRATEtm furnace  is shown
on Figure 11-34.   It  is  a  two-chamber, refractory-lined  furnace
that uses water as  the  feed grate.   As  the  material is  burned,
the ash sinks  and  is  removed.   The lower chamber  is a  reducing
furnace (starved  air),  and  the  resulting combustible gases  are
burned in the  upper chamber,  which  functions as  an afterburner,
thereby permitting better  control of the process.  More than
ten units have been  installed and are operating.   Some have
experienced  severe problems  with scum transport and feed  systems
external to  the furnace.


Other small,  modular  furnaces (see  Section 11.7)  have  consider-
able potential  for screening,  grit,  and scum  reduction,  provided
that pollution control devices  are  adequate  to meet strict  air
emission  codes.   USEPA  and the State of California have been
conducting  several  tests  on  modular  furnaces  to  determine
expected air  emission  levels (107).
    11.6.2  Lime Recalcination


Lime is  often  used  to remove phosphorus,  suspended  solids,  and
trace metals  from  wastewater.    It  is generally added prior  to
primary clarification  (108,109) or following a biological  process
(108,110).   Often,  energy  and  economic  analyses  indicate  lime
recovery and reuse to be  viable,  since net  lime  requirements
are  lower and  the  mass  of solids  for  disposal  is less  when
recovery is practiced.  There is  considerable experience with the
recalcining and reuse  of  lime from water treatment plants.   These
techniques, with suitable  modifications, are also used to  recover
lime in wastewater  applications.


In  the  liquid  process,   the  bulk of  the  lime  reacts to  form
calcium  carbonate  (CaC03).   The  resultant  slurry,  commonly
called lime  sludge, can  be thermally processed  for  recovery  of
calcium oxide  (quicklime  or CaO), while simultaneously  oxidizing
any entrapped organic  solids.  The recalcining reaction  is:
    CaC03 + heat -^ CaO  +  C02                               11-17
The  economics  of  lime  recalcination  as a  chemical recovery
process  depend  upon  a  number  of variables:    efficiency  of
rejection of  inert material, moisture  content of  feed material,
thermal efficiency  of the drying and recalcining system,  capture
of CaO  as  a usable product, and reactivity  (capture  of  CaO)  in
the product (111) .


                             11-96

-------
     THERMOCOUPLE
       ACCESS DOOR
     COMBUSTION AIR
     INLETS (4)
      THERMOCOUPLE
                            COMBUSTION TEMPERATURE
                             BETWEEN 1600 AND16QQ°F
                            COMBUSTION TEMPERATURE
                             iETWEEN 1400 AND 1600°F
                                                          CASTABLE
                                                          REFRACTORY

                                                          INSULATION

                                                          CIRCULAR STEEL
                                                          SHELL
                                                          REFRACTORY
                                                          BAFFLE
PACKAGE
AUXILIARY
FUEL SYSTEM
                                                          IGNITION AIR
                                                          INLET
                                                          MECHANICAL CRUST
                                                          BREAKER (RAKE)
                                                          FEED INLET
                                                          MAKE-UP WATER
                            COURTESY NICHOLS ENGINEERING AND RESEARCH CORPORATION

                              FIGURE 11-34

CROSS SECTION OF THE WATERCRATEtm FURNACE FOR SCUM INCINERATION
                                 11-97

-------
Economies are realized when lime  is  recovered  and  reused,  since
net lime requirements and the amount of  material to  be  disposed
of are drastically reduced.   However,  lime recovery is  expensive
and always energy-intensive because recalcining  is  endothermic.
Generally,  wastewater lime  sludges  are  low in organic  material
(volatiles)  that can contribute  to the heat value of  the  sludge,
so supplemental  fuel requirements  to calcine the  wet  sludge  cake
are substantial.   The major operating  cost of recalcination  is
supplemental fuel.  Fuel cost  can be minimized by control of
excess  air  at a rate no greater  than that required to assure
complete  combustion and completion of  the chemical reaction.
Fuel costs may also be lowered by  reducing  the water content of
the feed.  An overall economic balance must be  made to  determine
if the fuel  savings exceed the added cost of dewatering.

Complete recovery  of  spent  lime  cannot  be  expected  for  several
reasons.   Lime  sludge  contains  inert  materials  that must be
wasted from  the  system  or  the  quantity  of  sludge to be  handled
will  build-up  infinitely.   Magnesium  hydroxide  and calcium
phosphate  are  precipitated along with CaCC>3  and should be
removed  prior to  recalcination  to  reduce recycle  of inerts.
Complete rejection of Mg(OH)2 and other  inerts,  such as  silica,
can never be  achieved.   However,  wet and dry classification  steps
can limit recycle  of  inerts,  thus providing a relatively  clean
product.  These classification  steps necessarily reject  some
CaCC>3  and CaO, so that the  recovery of  available  lime is  limited
to 60  to 77  percent (108,112).

Three high   temperature  systems have  been used for  lime
recalcination:    the  multiple-hearth furnace,  the  fluid bed
furnace (pellet  bed and  sand bed), and  the rotary kiln  calciner.
It  has  also  been claimed  that   the  electric furnace has the
capability  to  recalcine,   but  no  installations  exist.   The
multiple-hearth   furnace   is  most  frequently used  in wastewater
treatment plant   sludge recalcining, while  the   fluid  bed  furnace
is  typically  used  in water treatment plants.    Both the  rotary
kiln  and  the fluid bed  are widely used  on industrial sludge,
primarily by  the pulp and  paper  industry.  As with other  high
temperature  processes,   opportunities for  energy  conservation
and heat recovery are  available.   A  detailed discussion of
lime  recalcination  is beyond  the  scope  of this  chapter.   More
information  is available  in  the literature  (108-119).
    11.6.3  Activated  Carbon Regeneration

The use of  activated  carbon for  removal  of  organic  contaminants
from water  and  wastewater  is  an established practice.  In most
applications,  regeneration  and reuse of spent carbon  are  required
for overall  cost-effectiveness.  Most carbon absorption processes
use granular carbon  in packed  columns.   There is  a  growing
interest  in  the addition  of  powdered  carbon  to unit processes
such as  activated  sludge  systems.   Table 11-23 summarizes  the
methods available for  carbon regeneration  (reactivation).


                              11-98

-------
                          TABLE 11-23

                CARBON REGENERATION METHODS (120)

                                   Granular  Powdered
Thermal
Multiple -hearth
Fluid bed
Transport reactor
Rotary kiln
Indirect heated
vertical moving bed
Radiant heated belt
reactor
Chemical
Wet air oxidation
Chemical oxidation
Solvent extraction

X
L
, NA
X

X

X

NA
X
X

X
X
X
L

NA

X

X
NA
NA
             Acid or base extrac-
              tion       .              X        NA
             Biological regenera-
              tion                     L         L
           X = has been done on pilot or full-scale.
           L = limited success
           NA =  not attempted.
Typical  granular  and  powdered  carbon  systems  are  briefly
summarized below.   Also,  the JPL process for carbon  reactivation
in a wastewater treatment plant  is discussed.


        11,6.3.1 Granular Carbon Systems (GAG)

Regeneration of granulated  carbon  (121,122)  is  usually  conducted
in  a  multiple-hearth  furnace  in  five steps:   dewatering the
slurry to about 50 percent solids,  drying  the carbon, pyrolyzing
the   absorbed  organics, oxidizing the  pyrolysis  residue  (carbon
reactivation),  and  quenching the reactivated carbon  in  water and
washing it to remove  fines.

In  a  multiple-hearth  furnace, about  30 minutes  are  required
for regeneration,  with dwell  times of 15  minutes  for drying,
5  minutes  for pyrolysis,  and  10  minutes for  reactivation.
Loading  rates  for multiple-hearth furnaces  must  be  adjusted
to  provide  about  1  square foot  (0.09 m2)   of hearth  area per
40 pounds (18  kg)  of  spent  carbon  per  day.   The off-gases from


                              11-.99

-------
a carbon  regeneration  furnace are  relatively  high  in carbon
particles  and unburned organics.   Afterburning  and  wet  scrubbing
are suggested.  The  injection  of  steam  at 1 pound per pound  of
carbon (1 kg/kg) reduces the apparent density of the carbon  and
increases  the iodine number.   Heat required  for  the  process,
including  steam but excluding any  afterburner fuel  requirements,
is  approximately  4,250  Btu per  pound  (9.88 MJ/kg)  of carbon
regenerated.   Further  details on the MHF regeneration process  can
be found  in the literature  (123).

The electric  furnace  is also becoming an alternative for granular
carbon regeneration, with several units either under construction
or  in  the  planning stages.   A  test  unit  is being installed  in
Pomona, California, to develop detailed long-term data.


    11.6.3.2   Powdered Activated Carbon (PAC)

During  the  regeneration  of  powdered  carbon, organics  must
be  removed from  the  micropores,  and  since PAC  is  generally
associated  with  excess  waste  biomass,  these solids  must  be
incinerated simultaneously  (120,124).  Also,  PAC is much  smaller
in  particle size  than GAC  and  must be handled with care during
combustion to prevent excessive losses and excessive loadings  of
particulates  on emission control systems.

Multiple-hearth systems have been used successfully  to regenerate
PAC (123,125).  MHF-regenerated carbon  appears  to  be  of virgin
quality  and  has  been reused  in  a  40-MGD  (l.VS-m^/s)  plant.
Available  data on a 50-gpm (30-1/s)  pilot  plant  indicate  similar
results with  fluid  bed technology  (126).

Use of the  transport reactor  has been demonstrated  on a
10-ton-per-day (9-t/day),  full-scale  facility with  a recovery  of
80  to  90  percent  of   the spent  carbon.   This reactor  is a  fast
co-current  thermal plug  flow system (127,128).  The unit  is
operated  for  regeneration  of spent  carbon  from corn syrup
manufacturing.


        11.6.3.3  Jet  Propulsion Laboratory Activated-Carbon
                  Treatment System (JPL-ACTS)

Extensive  laboratory and  pilot testing  by  the Jet  Propulsion
Laboratory in  Pasadena, California, has led to the development  of
an  activated  carbon  treatment  system for  wastewater  (129,130).
The system is  based on starved-air combustion of sludge.   All  PAC
used in this  process  is  produced by  the  SAC of  sewage sludge  and
lignite  coal.  The   system  was tested  for the Orange County
Sanitary  District  in  a  1-MGD  (0.04 m3/s )  pilot  plant  at
Huntington Beach,  California.

The  flowsheet  for  the  Orange  County plant  is  shown  on
Figure 11-35.   Sludge from  the primary  sedimentation tank  is
dewatered  in  a filter press to 35  percent solids.  The sludge
cake is flash-dried to 90  percent  solids before  passing  into  the


                              11-100

-------
rotary  kiln.   Activated  carbon and ash are  generated by starved-
air combustion of the carbon-sludge solids.   Activated carbon-ash
mixture  is fed  back to  the secondary  clarifier  to  complete the
carbon  cycle.   A  portion of  the  carbon-ash  is purged  from the
kiln to prevent build-up of inert materials.   The energy value of
the purged  carbon can be recovered in a separate furnace.
               CARBON + SEWAGE SOLIDS
RAW
SEWAGE
                   ( ~ 5% SOLIDSf
(DEGRITTED)
PRIMARY
CLARtFIER
                                               GRAVY FILTER
                                               (MIXED MEDIA)
                                                EFFLUENT
                                                     SEWAGE
                                                     SOLIDS -i-
                                                     CARBON
                                                     FINES
                                    ACTIVAT1D CARBON
                                      (10% SLURRY)
               CARBON •*• SEWAGE SO LIDS
               !~,5%SQL!DSt
          FilTER PRESS
          | DEBATE RING)
                     35-40%
                     SOLIDS
                 FLASH
                 DRYER
                         90%
                        SO Li OS
  THERMAL
REGENERATION
   UNIT
                                                              *

                                                              I
                                 LIGNITE COAL
                                (MAKEUP ENERGY
                                  + CARBON)
                                                  ASH
                                                  PURGE
                             FIGURE 11-35

            JPL ACTIVATED CARBON TREATMENT SYSTEM (129)
Various  practical  problems     (primarily  corrosion  at  high
temperatures)  associated with  the  kiln  and flash-dryer  have
caused  the  developers to substitute a multiple-hearth furnace for
these two  system elements.   No  actual test work  with the MHF has
been done  to  date.

Activated  carbon makeup  requirements  are dependent on adsorption
characteristics.   Under some circumstances,  the carbonized sludge
can satisfy the  makeup requirements.  Otherwise,  activated carbon
makeup  is  necessary.  Lignite  coal is a source of  low ash carbon
with  an adsorptive capacity  comparable  to commercial  activated
carbons.   Lignite  coal  also  provides, at low  cost, the necessary
makeup  energy to the system.

Preliminary economic studies  by the developers  indicate that the
JPL-ACTS  process  for  wastewater  treatment  is  competitive  with
activated  sludge for plant flows  exceeding  175 MGD (7.67 m3/s).
                                11-101

-------
11.7  Other High  Temperature Processes

There are a number of high temperature conversion processes  that
differ substantially  .from those previously discussed.  Some  are
presently  being  used  for  combustion  or co-combustion of
wastewater  sludge, and  others  are  claimed  suitable for  sludge
processing.  These processes include:

     •  High pressure/high temperature wet air oxidation

     •  REACTO-THERMtm   (Met-Pro  Corporation,  Systems  Division)

     •  Modular  controlled-air  incinerators  for  co-disposal
        (Consumat, Kelly, and others)

Also,  numerous thermal  processes are being developed,  mainly
of  the pyrolysis or  starved-air  combustion  type;  which  are
applicable  to wastewater sludge or  mixtures of sludge  and  solid
waste  (Table 11-24).    These processes  are  potentially  important
because they produce a  high  heating value  fuel gas  that may be
directly usable  in existing  furnaces  and  burners.   Of the  true
pyrolytic  processes (thermal  decomposition in  the absence of
air),  only the  Pyro-Sol process  appears  to be sufficiently
developed  to be  considered here  for co-disposal  and perhaps
sludge  disposal.  Some  of  the processes shown  in Table 11-24
have  been  discussed  previously (PUROX and  Andco-Torrax).  Other
developing  processes with potential for sludge  burning  include
the  Bailie  process,  the Wright-Malta  process,  and  Molten  Salt
pyrolysis.


    11.7.1  High  Pressure/High  Temperature Wet Air Oxidation

Any burnable substance may be  oxidized  in  the  presence of  water
at  a sufficiently  high temperature  (flameless combustion).
Therefore,   this  process  can be  an alternative  to  incineration
while providing a similar ash residue (134).

The  high  pressure/high  temperature wet air oxidation process
(HPO)  is  similar to thermal  conditioning, except that higher
temperatures and  pressures and much more air are  used  to  effect
complete oxidation.  Figure 11-36  is  a composite representation
of results  of wet oxidation  for a typical  sewage  sludge,  showing
volatile solids  content  or  COD content  in  the solid  phase  and
the total sludge  as a function  of total oxidation in both  phases.
The  vertical distance  between the two curves  is the content
in  the  liquid  phase.   Up  to  about 50  percent  total oxidation,
reduction  in the  volatile solids or COD in  the  liquid  phase  are
minimal; above 50 percent,  the volatile solids and  COD of  both
phases are reduced to low values.  At 80 percent total oxidation,
about  5  percent  of  the  original total volatile solids  in  the
sludge  is  in the solid  phase  and 15 percent  is in the liquid
phase.


                               11-102

-------
                                           TABLE 11-24

              BASIC TYPES OF PYROLYS1S, THERMAL GASIFICATION,  AND
            LIQUEFACTION REACTORS - NEW, DEMONSTRATED,  OR UNDER
                                 DEVELOPMENT (131,132,133)
                                                                                 Main products
     Solids  flow and
     bed conditions
     Examples of processes,
    developers, R&D programs
Vertical-flow reactors
  Moving packed bed       Forest Fuels Mfg., Inc.  (Antrim,
    (gravity solids flow;    N.H.)
    also called fixed bed) Battelle Northwest (Richland, WA)
                         American Thermogen (location un-
                           known)
                         Andco/Torrax Process (Buffalo, NY)
                         H.F. Funk Process^ (Murray Hill,
                           NJ)
                         Tech-Air Crop/Georgia Inst. Tech.
                           (Atlanta, GA)
                         Union Carbide Purox Process
                           (Tonawanda, NY)
                         Motala Pyrogas  (Sweden)
                         Urban Research  & Development
                           (E. Granby, CT)
                         Wilwardco,  Inc.  (San Jose, CA)
                         U. of California  (Davis,-CA)
                         Foster Wheeler  Power Products
                           (London,  England)
                         Destrugas Process  (Denmark)
                         Koppelman Process  (Encino, CA)

  Moving stirred bed      BSP/Envirotech  (Belmont,  CA)
    (gravity solids flow)  Nichols Research & Engr.  (Belle
                           Mead, NJ)
                         Garrett Energy  Research & Engr.
                           (Claremont, CA)
                         Hercules/Black, Crow & Eidsness
                           (Gainesville, FL)
  Moving entrained bed
    (may include
    mechanical bed trans-
    port)

Fluidized reactors
Horizontal  and inclined
  flow reactors
    Tumbling  solids bed
Occidental Petroleum Co./Garrett
  Flash Pyrolysis Process  (La
  Verne, CA) •
Copeland Systems Inc.  (Oak Brook,
  IL)
Coors  Brewing Co./U.  Of Missouri
  (Rolla, MO)
Energy 'Resources Co.  (ERCO)
  (Cambridge, MA)
Hercules/Black Grow & Eidsness
  (Gainesville, FL)
Bailie Process/Wheelabrator
  Incin. Inc.  (Pittsburgh, PA)
A.D.  Little  Inc./Combustion
  Equipment  Assoc.  (Cambridge,
  MA/New York, NY)

Devco  Management Inc.  (New York,
 •NY)
Monsanto Landgard/City of
  Baltimore, MD Watson Energy
  Systems  (Los Angeles, CA)
                                                              Feedstock
                 Fuels or char
                  materials
                                                                                             Steam
                                  FAR
Refuse
Refuse
Refuse
Refuse
FAR
Refuse, FAR
Refuse
Refuse
FAR, sludge
FAR
Refuse, tires
Refuse
FAR
Sludge, refuse
Sludge, wood
Manure
Refuse
X
-
_
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
-
-
-
_
-
-
-
-
-
-
X
X
-
-
Refuse




Sludges

Refuse,  FAR

Refuse,  FAR

Refuse

Refuse

Refuse



Refuse

Refuse
                                               11-103

-------
                                        TABLE 11-24

             BASIC TYPES OF PYROLYSIS,  THERMAL GASIFICATION, AND
           LIQUEFACTION REACTORS -  NEW, DEMONSTRATED, OR UNDER
                        DEVELOPMENT  (131,132,133)  (Continued)
                                                                            Main products
Solids flow and
bed conditions
Horizontal and inclined
flow reactors (con-
tinued)











Agitated solids bed


Examples of processes,
developers, R&D programs

Ecology Recycling Unlimited, Inc.
(Santa Fe Springs, CA)
Pyrolenergy System/Arcalon
(Amsterdam)
Pan American Resources, Inc.
(West Covina, CA)
Kobe Steel (Japan)
JPL/Orange County, CA (Fountain
Valley, CA)
Rust Engineering (Birmingham, AL)
Tosco Corp/Goodyear Tire and
Rubber (Los Angeles, CA/Akron,
OH)
Deco Energy Co. (Irvine, CA)
Enterprise Co. (Santa Ana, CA)
Kemp Reduction Corp. (Santa
Feedstock

Refuse

Refuse, FAR

Refuse, FAR

Tires
Sludge

Refuse, sludge
Tires


Tires
Refuse
Refuse, FAR
Fuels or char
materials

X

X

X

X
X

X
X


X
X
X
Steam

-

-

-

-
-

-
-


_
-
-
  Static solids bed

Molten metal or salt
  beds
    Floating solids bed
      (horiztonal flow)

    Mixed molten-salt
     bed (various
     possible flow
     schemes)
                         Barbara, CA)
                       PyroSol  (Redwood City,  CA)
Thermex, Inc.  (Hayward, CA)
Michigan Tech. U. (Houghton, MI)
  (Puretech Pyrolysis System)

Battelle Northwest (Richland, WA)
Anti-Pollution Systems, Inc.
  (Pleasantville, NJ)
Multiple-reactor systems
  Combined entrained-    U. of California  (Berkeley, CA)
    bed/static-bed
    reactor system
  Combined moving        Battelle Columbus Laboratories
    packed-bed/entrained-   (Columbus, OH)
    bed reactor
  Combined mechanically
    conveyed static-
    solids-bed/moving
    packed-bed  reactor
Mansfield Carbon Products, Inc.
  (Gallatin,  TN)
                                 Fluff from
                                   scrapped autos
Refuse, FAR


Refuse
Refuse, sludge
                                 Pulping liquor
                                 Paper, biomass
                                                        Refuse
 Forestry and/or agricultural residues.

 Pressure above atmospheric.
                                             11-104

-------
         100
               10   20   30   40   50   60   70  80   90   100
                            OXIDATION- %

                                         COURTESY ZIMPRO INC.

                           FIGURE 11-36

      VOLATILE SOLIDS AND COD CONTENT OF HEAT TREATED SLUDGE
The  degree  to which organic materials  are  oxidized  is a
function  of  temperature,  reaction  time,  and quantity of air
(or  oxygen)  supplied.   The process may be applied  to dilute
suspensions of sludge  requiring  only thickening.  However, the
solids content should be  four to six  percent  to minimize  reactor
volume requirements and  to  maintain  a  thermally  self-sustaining
reaction.   Solids  concentrations  greater than  about 10  percent
create  problems  with  mixing  and  consequent  mass  transfer
of the  oxygen.   There  is  insufficient data  to  indicate any
advantage  from use of pure  oxygen  rather  than air as the  oxidant
source.
                              11-105

-------
The  high pressure/high temperature wet  air oxidation  process  is
shown schematically  in Figure  11-37.   Thickened  sludge,  at about
six  percent  solids,  passes  through a  grinder to reduce  the size
of all  feed solids to  less  than 1/4 inch  (0.64  cm).
SLUDGE
                   GROUMD
                   SLUOGi
                                         HEAT
                                       EXCHANGER
        GRINDER
           AIR
siui-
TA
c
A tit
SLUDGE
j I 	 |_ \
SLUDGE HIGH
FEtO




PUMP PRESSURE
SLUDGE PUMP
(POSITIVE
OlSPLACEMEMTJ
Alft


D
of—1
3| t
-I 1
" I
s
                                                 ot
                                                X
                                                       REACTOR
                                                         STEAM
                                              SLUDGE
                                                        INJECTION
                                                AIR
      AIR COMPRESSOR
   STERILE
NGN -PUTS ESC IB Li
    SOLIDS
               ALTERNATE METHODS
                 OF DEWATERING
            FILTER PRESS
          VACUUM FILTER
            CENTRIFUGE
                                                        BOILER
                                                       (START-UP
                                                        STEAM*
               DRAINAGE iEDS

                   LAGOONS
 LiViL
CONTROL
 VALVE
                                OXIDIZED
                                 SLUDGE
                                 SLURRY
                  SUPERNATANT
                                         (1) WET SCRUBBING, CARBON
                                         AiSOHPTION, OR AFTERBURNING


                                               COURTESY ZIMPRQ INC.
                             FIGURE 11-37
   FLOWSHEET FOR HIGH PRESSURE/HIGH TEMPERATURE WET AIR OXIDATION
The slurry is then pressurized.   The air quantity  supplied is the
stoichiometric amount  required    for  complete oxidation of  the
combustible sludge solids  (about 7.5  Ib per 10,000 Btu)  (2  g/J).
The pressure applied must be  sufficient to prevent  the  water from
vaporizing at the temperature  selected for the reaction.

The sludge-air mixture  is then  passed  through a heat  exchanger,
where  it  is  heated  to close  to the  desired reaction temperature
by  the reactor  effluent  stream and  introduced  into the  reactor
for oxidation.   Temperatures  and pressures up  to 500°F  (260°C)
and  1,000  to 1,800  psig (6,895  to  12,411  kN/m2)  are  used  with
detention  times  of 40 to 60 minutes.   The oxidized  slurry is then
cooled  in  the heat exchanger,  gases  are removed in  a vapor-liquid
                                 11-106

-------
separator,  and the gases  are  reduced to  atmospheric pressure
through a  pressure  control valve.   The  gases are  processed  to
eliminate  odors.   They  consist mainly of oxygen, nitrogen,  carbon
dioxide,  and water  vapor.   Nitrogen oxides are  formed  from  the
organic nitrogen  present  in  the  feed,  but no  nitrogen  is  fixed
from the  air.   Elemental  sulfur, hydrogen sulfide,  and  organic
sulfur compounds  are  oxidized  to  sulfate  (S04).   Gas  clean-up
methods have included wet scrubbing, activated carbon absorption,
afterburning with  fossil  fuel,   and  catalytic  oxidation.    With
the last two methods,  energy recovery is  possible through  use  of
heat recovery  boilers,  gas-liquid heat exchangers,  and  similar
methods (135-137).

Slurry  from  the  gas-liquid separator  is  removed  through  a
liquid-level control valve and  dewatered  for  final disposal.   At
high degrees of oxidation, the  residual solids resemble  ash from
thermal incineration and  are easily dewatered to a  high  solids
content  by conventional  means  (settling, centrifugation,  or
vacuum filtration).

The liquid phase is recycled  to the  treatment  plant or  given
separate   treatment for  reduction of  the  residual  soluble
organics.   Treatment  and  effects of  this  liquid stream  are
discussed  in Chapter 16.

High  pressure/high temperature wet  air  oxidation processes
generate  excess  heat  when  they operate  with  a high heating
value  sludge  and  an  adequate  solids content   (approximately
six percent).  Still,  a  source  of high pressure  steam (separate
boiler  or  an existing  plant   system)  must  be  provided  for
start-up.

There are  over ten  HPO systems  in operation on  sewage sludge  in
the United States.  The most  notable of these are Rockland  County
and Rensselaer, New York,  and Akron, Ohio.   These units operate
at approximately  500°F  (260°C) at pressures of 1,000 to 2,000 psi
(6,895 to  13,790  kN/m2).   The  capacities  of  the units as well
as the sludge oxidized  are  very different in each of these  plants
Rockland County processes  12.4   tons per day  (11.3  t/day)  of  a
mixed digested primary  plus waste-activated  sludge.  The  Akron
facility  (Botzum  Plant)  oxidizes 50 tons per day (45 t/day)  of
waste-activated sludge.   The Rensselaer facility  oxidizes  a more
conventional mixture of primary plus waste-activated sludge.

Shutdowns  with HPO systems  are associated with the high pressures
involved,  heat exchanger  scaling and corrosion, and required
supernatant liquid treatment.  The HPO  process may provide a good
system for oxidation of  toxic and hazardous waste materials,  and
research in this  area is  under way  (138).

Lack of extensive operating data prevents reliable estimation of
the cost of HPO as  a means to  sludge disposal.   It appears that
if  equipment  maintenance and replacement  costs  are reasonable,
the costs  would be competitive with  thermal processing.  The only


                              11-107

-------
additional element  of  cost  is  treatment  of  the recycle  stream.
Electrical  energy  requirements   are  shown   in  Figure  11-38.
Additional information can be  found in  the literature  (134-139).
E
ID
fj
It
^

^_
£
g
•o
o
o
UJ
a
LU
CE
|£
LU

§
a.
y
DC

Q
111
_l
ut
                                   PRIMARY +
                                   WASTE
                                   ACTIVATED
                                   SLUDGE
                                WASTE
                                ACTIVATED
                                SLUDGE
                                          ASSUMPTIONS:
                                           SLUDGE FEED
                                            PRIMARY + WAS = 4,6% SQUOS,
                                              «9* VQLATILES
                                            WAS -"3.5* SOUDS, §0% VOLATILES
                                            VOLATILiS - 10,000 Btu/lb
                                           REACTOR PRESSURE
                                            PRIMARY + WAS « 1,700 pi%
                                                 1,800
                                           CONTINUOUS OPERATION
                                           INCLUDES:
                                             PRiSSURlZATtON PUMPS
                                             SLUDGE GRINDERS
                                             DECANT TANK
                                             BOILER FEED PUMPS
                                             AIR COMPRESSORS
                                           TYPE OF ENERGY REQU RED; ELECTRICAL
                                           NOTE; FUEL IS REQUIRED
                                               ONLY AT START-UP
         1,0
                   3  4 G §7 6910
                                      3456 7891QO
3456 7891,000
                      TREATMENT CAPACITY, gpm (1 gpm = 0.06 l/s)
                              FIGURE 11-38

       WET AIR OXIDATION - ELECTRICAL ENERGY REQUIREMENTS (36)
Another  HPO  unit  presently  being  tested  for  feasibility with  a
feed  of  sewage sludge is  the Vertical Tube  Reactor  (VTR).   This
is  a  deep-well type of  process  in  which  the required pressure is
obtained simply by the depth  of the well.   The Municipal  Environ-
mental  Research  Laboratory of  USEPA is conducting  test work  with
a VTR system in Colorado.  Data should be available in 1980.
                                  11-108

-------
     11.7.2  REACT-0-THERMtm

This is a three-stage combustion device with  SAC  in  the  first two
stages followed  by  complete  combustion in the third stage.   This
proprietary system developed  by Met-Pro Corporation, Systems
Division, is unique  in  that  auxiliary fuel and air  are  burned  in
the primary  combustion  chamber  (first stage)  and the  resulting
gases pass  into  the rotary  chamber,  where the sludge is  burned.
The interior design  of  the rotary kiln second stage recovers the
heat generated in the first  stage and  transmits this heat  through
a  stainless  steel  helix and chains  to the sludge.  The residue,
which  contains  some combustibles,  is deposited  into  a fixed,
cylindrical  ash  chamber,  where  it  is removed by an auger.   The
gases from  the  rotary  chamber flow into the  secondary combustion
chamber  (third stage),  and air  and  fuel are  added as required  to
complete  the  combustion of  the  gases and  destroy odors prior  to
discharge  to  atmosphere.  The  unit  is  available as a  complete,
skid-mounted package (see Figure 11-39).   The unit is  primarily
designed  for  low-volume applications  (50  to  300  gallons per hour
of  wet  sludge  [0.05 to 0.30 1/s]).  Two  units are  presently
operating  on  a  physical-chemical  sewage  sludge  in Prudhoe  Bay,
Alaska.
SECONDARY
COMBUSTION
CHAMBER
                  PfliMAHY
               COMBUSTION CHAMBER
                 ASH
                 CONVEYER
HiAT TH&NSfiR
   MEDIA
ROTARY CHA«6£R
   DRIVE
                                           COURTESV METJPRQ COITORATIOM, SYSTEMS DIVISION
                            FIGURE 11-39
                      _tm
         REACT-O-THERM  SLUDGE/LIQUID WASTE DESTRUCTION
                                11-109

-------
Detailed  heat and  material  balances  are  available  from the
manufacturer for specific applications. Emission test data from
the  manufacturer  indicate  that  the  unit,  operated  at rated
conditions,  can  meet  USEPA's New Source  Performance Standards.
However,  the  New   Source  Review  Rule  may  be applicable  in
some areas  and Best Available  Control Technology (BACT)  may be
required.


   11.7.3   Modular  Starved-Air Incinerators

Modular  controlled-air  incinerators  are static,  and contain
two-chambers.   The  first  chamber  is operated  by  starved-air
combustion,  and  the gaseous  products  of  combustion are passed
to  the  second  chamber where  combustion  is  completed and  odors
are  destroyed (see Figure  11-40)  (107).   A  number  of these
incinerators have  been installed for  municipal and industrial
solid waste.  There  are also  units under study for co-disposal of
municipal  refuse and  sewage  sludge  (141).   There  are  no  known
installations  (or  test data)  for sludge alone.   However, the
unit appears  to be suitable for sludge  reduction.  The units
are  available  in modules from  60  pounds per hour (27 kg/hr) to
250  tons  per day  (227  t/d).   Equipment manufacturers  (Consuroat,
Kelley,  and others)  state  that  USEPA New  Source  Performance
Standards  can  be met  without  additional  air pollution  control
equipment; however,  the New Source Review  Rule may  be applicable
in  some areas  and  BACT may  be  required.   A test program  being
funded  jointly  by  the  EPA  and the  State  of California is
currently underway  at  Little  Rock, Arkansas,  to obtain definitive
air emission data on municipal solid waste incineration.   Further
information on  controlled-air  incinerators   is  included  in the
literature (107,140-145).


    11.7.4  Pyro-Soltm Process

The  Pyro-Sol process  is a  pyrolysis  project  presently  operating
on  solid  waste.  In the Pyro-Sol process,  waste  is  fed  to  a
pyrolysis unit  which, in  the absence of  oxygen and  in the
presence  of  heat,   causes  chemical  decomposition  of the waste.
Products of the process are a gas and  char/ash residue.   A  50 to'
75-ton  per day  (45 to 68  t/d)  (MMR), full-scale  plant is in
operation in Redwood  City,  California.   A  flowsheet of that
system is presented  in Figure 11-41.

The  process is autogenous,  but heat-up  and  standby energy is
provided by natural  gas.  A portion of  the produced gas  is burned
in  eight  radiant heat  tubes  to  provide heat  for the endothermic
pyrolysis process.    The  solids  are  fed by an airlock  and  moved
through the furnace  by means  of a vibrating conveyor.

The  resulting gas  (largely  hydrogen  [H2J  and  carbon monoxide
[CO]) exits from the  pyrolyzer  at  approximately 1,100°F  (543°C)
and  less  than  0.5  inches  of  water column (125  N/m2) and enters
                               11-110

-------
                                                                   SEE NOTE
                                                                   BELOW
                      AFTERBURNER
                       SECONDARY
                       AIR SUPPLY
     COMBUSTION AIR SUPPLIED
     AT HIGH VELOCITIES
                                                                         CONTROL
 Csl Fim Mijor Cwiflgutatkkn
                                                   (b) Second M*|&r

                             -SEE NOTE
                             BELOW
                          SELF-SUSTAINING
                          DIRECT-FLAME AFTERBURNER
                    FORCED AIR
                              Third Moiii- Cuil1l«unilijn

                 NOTE: STACK TO ENERGY RECOVERY EQUIPMENT
                      AND/OR EMiSSfON CONTROL DEVICE
                      OF NECESSARY)


                             FIGURE 11-40


MODULAR CONTROLLED-A1R INCINERATOR CONFIGURATIONS  (140)
                                   11-111

-------
a dry cyclone where the particulate matter larger than 10 microns
is  removed.   The hot gas  is  pulled  through  a  wet scrubber/
quencher  where  the  remaining  particulates  are  removed.   The
small amount  of  water that  circulates  to- the scrubber/quencher,
receives primary  and  secondary treatment,  including filtration,
before  disposal  to the  plant sewer  or to  an on-site treatment
plant.  The scrubbed gas has a heating value of 400-500 Btu/cu ft
(14.9 to 18.6 MJ/m3).
                                                  MAKE-UP WATER
                                                  TRFATMFMT
                  ENCLOSURE
                      Tr TFFIf B MklU;      ICVrirvSlE SCSUaBE3
                              roN,'FY-!=l I          BLLWLJ
             VIE RSI ING BtD
          COURTtET FYBO SQL l«CORPOm4TEC-
                            FIGURE 11-41

                 PYRO-SOL LIMITED PYROLYSIS SYSTEM
The gas  is  transferred to a  surge tank and  fed  from there to a
steam boiler.  The steam can be used as process steam or to drive
a turbo-generator.


Pyro-Sol, with  feeds of up  to 50 percent  moisture,  can achieve
a net  energy production  of  60 percent  of the  input heat value
in the  fuel  gas.   Due  to  the high recovery  of  input heat value
with relatively wet  cakes,  as compared  with  normal solid waste,
this process should be  amenable  to co-disposal and  possible
sludge combustion.
                               11-112

-------
    11.7.5  Bailie Process

The Bailie Process  integrates  a combustion  fluid bed  furnace
with a pyrolysis fluid bed reactor (146-147).  The process, shown
on Figure  11-42,  involves  feeding solid waste  into the pyrolysis
fluid  bed  reactor.   The endothermic  pyrolysis  reaction  is
maintained in the 1,300 to 1,500°F (704-816°C)  range by recycling
hot fluidized sand from the combustion reactor.  The fuel for the
combustion  reaction  is  contained in  the  same recycle  from  the
pyrolytic  reactor  and  from char  collected  in  the combustion and
pyrolysis gas cyclones.  Some of  the pyrolysis gas is returned to
the pyrolytic reactor  to control reaction kinetics.   Both excess
pyrolysis gas and char may be recovered.
COMBUSTION
 PRODUCTS
 TO STACK
                   OQMSUSTION
                    FLUID
                    1ED
                   REACTOR
  A|R BLOWER
PYflOLVSlS
GAS
PRODUCT
                                          PVflOLVSlS GAS
                                          RECYCLE BLOWER
                           FIGURE 1.1-42

                   BAILIE PROCESS FLOWSHEET (146)
The  Bailie  Process is  a  potentially important method  of sewage
sludge pyrolysis.  Less auxiliary fuel is needed for incineration
of  the sludge,  and  a number  of  energy recovery options  are
available.    Heat from the  off gases  can be  recovered  and a
combustible fuel gas is generated.

The  Bailie  Process   is  patented  and  has  been  piloted.   No
full-scale test  has  been conducted,  but  the  manufacturer states
that one is planned in  the near future.


     11.7.6  Wright-Malta Process

The  Wright-Malta  Corporation (W-M)  is  developing  a pressurizied
rotary kiln  gasifier-gas turbine system  for  generating electric
                               11-113

-------
power from municipal solid waste and wastewater sludge (148,149).
Figure 11-43  shows the  process in terms  of  energy  flows.   The
pressurized gasifier produces a hot, low heat value fuel gas that
is combusted  and  fed directly  to  the  gas turbine.   The  turbine
drives an electrical generator and the  associated air compressor.
The hot exhaust is  used  to  preheat  the  sludge  and  to raise steam
temperature in a  heat recovery  boiler.   The  steam  is superheated
and  passed back  to  the kiln,  where  it cools and condenses,
supplying heat for the gasification process.
                      3310
                                             COMBUSTION
                                              CHAMiER
                                                            10 MW
                                                           GENfiRATOR
        NOTE: ALL UNITS IN 10* Biu/day |106B MJ/d»y)
               UNLESS NdfiP
                           FIGURE 11-43

               WRIGHT-MALTA PROCESS FLOWSHEET (150)
Wastewater sludge  contributes  about four percent  of  the organic
fuel  to  the  system.    At  the  pressures  involved,  the  water
evaporated from  the  sludge  provides motive  force for the turbine
in  addition  to  the products of  combustion  from the  fuel  gas
produced.  The  cycle is comparable  to  the  combined cycle system
used  in  electrical  power generation,  where  hot gas  turbine
exhaust  flows to a  boiler  to  produce steam.   The turbine exhaust
                               11-114

-------
in the W-M  process generates  steam in the kiln.   This steam,
along  with  the burned  fuel  gas,  drives the  turbine.   The
resulting  fuel  efficiency  .is  close  to  the  combined  cycle
efficiency.   This process appears ideal for very  moist fuels, and
the  high  moisture  content of  the sludge is beneficial.   The
Wright-Malta process has been operated in a batch mode on a bench
scale.  Further progress depends on  develpment of  a  rotary kiln
that can be operated at high pressures and temperatures.


    11.7.7  Molten  Salt Pyrolysis

Bench-scale studies were  conducted  by Battelle-Pacific Northwest
Laboratories  on  the  pyrolysis   of  refuse  in molten sodium
carbonate  (150).   The  products of reaction were studied for
different conditions with steam, air, and oxygen as the gasifica-
tion  agents.   While  the processing of  municipal  refuse  in
the  molten  salt  (sodium  carbonate) reactor was  found to  be
technically feasible, the lack of  a cost-effective method of ash
removal and  the problems of refractory degradation have hindered
further development.  This  type of  process is  not new.  However,
no  information  is  available  as  to  the  applicability of the
process to sludge disposal.


11.8  Air Pollution Considerations

In any combustion process,  air emissions  are a major concern and
may  be the  most  difficult  and  costly environmental consideration
to satisfy.  On the  federal  level, the  USEPA  has established
standards of performance  for municipal incinerators (solid waste)
and  wastewater sludge incinerators.   In  co-combustion schemes
involving municipal  solid waste  and wastewater sludge, both
standards  will probably  apply,  with allowable  emissions  being
prorated according  to the fractions of energy  in the solid  waste
and  in the  sludge.  In September, 1978,  the  USEPA published
proposed  emission  standards  for new, modified,  or reconstructed
electric utility steam generating  units that burn fossil fuel  or
a  combination  of  fossil  fuels and other  fuels such  as solid
wastes.  These  guidelines offer some indication of air pollution
requirements in co-combustion  schemes.

Generally,  these  guidelines   indicate  that new  sludge  furnaces
will have to comply with  the  following standards:

      •  National Ambient  Air Quality  Standards (State Implementa-
        tion Plans).

      •  National Emission Standards for Hazardous Air Pollutants,
        subparts A and E.

      •  Standards  of Performance  for New Stationary  Sources,
        parts A, 0, and  probably E, if co-combustion is proposed.

      •  New Source Review Rule.
                               11-115

-------
     •  Regulations  Pertaining  to Prevention of  Significant
        Deterioration of Air Quality.

In  all  cases,  the minimum  standards  are  set  by  the  USEPA.
However,  state  and local  jurisdictions  may promugate stricter
standards.

A basic problem in evaluating  any emission is predicting the
effect  on  the  overall air  basin.   Projecting  emissions and
estimating  resulting  air quality is,  at  best,   an imperfect
science.   Air  basins  in  which  critical  air quality levels are
consistently  exceeded  have been  studied  in  depth  and have  been
the  object  of  mathematical modeling.   The results  of these
efforts  have  been mixed.
    11.8.1  National Ambient Air Quality Standards (NAAQS)-
            State  Implementation Plans (SIP)

Federal air quality regulations  are  derived  from the Clean Air
Act Amendments of  1970,  the Energy Supply  and Environmental
Coordination Act of 1974,  and  most  recently, the Clean Air Act
Amendments of  1977  (151).  The NAAQS  established threshold  levels
of  air  pollutants  below which  no  adverse effects would  occur.
These  levels  were  designed to  provide an  adequate margin of
safety so as to protect  the public health.

Air pollutants are  classified into two groups: primary pollutants
and secondary pollutants.   Primary pollutants are  those emitted
directly from sources, while secondary pollutants are formed by
chemical and  photochemical reactions of  primary  pollutants  with
the atmosphere,  as shown  on Figure  11-44.   Primary  pollutants
include  carbon monoxide  (CO),  hydrocarbons  (organic gases),
oxides of  nitrogen  (NOX),  sulfur dioxide  (SC^),  total  suspended
particulates  (TSP)  and  lead (Pb).   Photochemical oxidants and
nitrogen  dioxide  (NC>2)  are  the  principal  secondary  pollutants.
These  form  a  visible  brown-yellow haze.    The quantity of
secondary pollutants is  dependent on  the availability  of  sunlight
as  much  as on  the  availability  of  primary  pollutants.    Health
effects of contaminants  are summarized in Table 11-25.

The 1970  Amendments to  the Clean Air Act  required  each  state to
develop  its own State  Implementation Plans (SIP)  to meet the
federal  standards by 1975  or  1977,  the  date dependent on the
severity of the state  air  quality problems.  The  1977 Amendments
extended  the  attainment deadlines and  detail some  appropriate
control measures.   For  those  areas  which  have not yet  attained
NAAQS, states  must  have  approved  implementation plan revisions by
July 1, 1979, which provide for attainment by December  31,  1982.
If  a  state  demonstrates that such attainment  is  not possible, it
must  submit a second  plan revision  by December  31,  1982,  which
provides for  attainment by December 31,  1987.  For areas already
meeting  NAAQS standards,  implementation  plans  must include  a
program  to prevent significant deterioration  of air quality.


                              11-116

-------
The  USEPA guidelines  require  the  SIPs  to  provide  for  emission
controls,  transportation  controls,  source  monitoring,  ambient
air  quality  monitoring,  and procedure  for  review  and  approval
of new  sources  of  air pollution prior to construction.   The
USEPA has the authority  to approve  or disapprove these  plans
and  to  promulgate  an  acceptable  plan if  the  submitted  plan is
disapproved.  The USEPA, state  air  resources boards and local air
quality management  districts also have  the  authority to restrict
issuance  of  permits  for  construction of  stationary  sources  if
emissions  from  that source  would  cause a  violation of  any air
quality  standards.   This  is accomplished  by an  emission offset
policy.    In both  nonattainment  and  nondegradation  areas,  major
stationary sources  may be  constructed only  by permit and must at
least meet applicable  new  source  performance standards.
                             REGULATED VIA STATE
                             IMPLEMENTATION PLANS
                             (LIM«T POLLUTANTS TO
                             PROTECT PUBLIC HEALTH!
        PRIMARY
       POLLUTANTS
                     SUNLIGHT
SECONDARY
POLLUTANTS
    11.8.2
                          NATIONAL AMBIENT AfR QUALITY
                                STANDARDS
                         { CLEAN AiR AMENDMENTS OF 1977 I

                            FIGURE 11-44

                           AIR EMISSIONS
National Emission Standards  for  Hazardous
Air Pollutants  (NESHAPS)
Subpart  A of  NESHAPS  (40  CFR 61)  comprises general  provisions
covering definitions,  applications, reporting,  and waivers.
Subpart  E deals  with mercury emissions and  applies  to  all opera-
tions that  burn  or  dry  wastewater sludge.   The  NESHAPS  standard
(Federal  Register,  Vol. 40, No.  199,  Tuesday, October  14,  1975)
is currently  seven  pounds  of mercury  (3.2 kg) per 24-hour period
for any  source.
                                11-117

-------
                                     TABLE 11-25

                    HEALTH EFFECTS OF AIR POLLUTANTS (152)
                      Pollutant lev
  Air quality
   level
  Significant
    harm
            TSP
          (24-hour),
(24-hour),  (8-hour),
 Ug/m3    mg/m3
                   2,100
                   1,600
(1-hour),
 Ug/m3
  N02
(1-hour)
 ug/m3
 Health
 effect
descriptor
               1,200
                                1,000
                      3,750
                                       3,000
                                       2,260
                                       1,130
                                               very
                                             unhealthful
                                             Unhealthful
                                                      General health effects
                                                                       Cautionary statements
    NAAQS
                                    Premature death of ill
                                    and elderly. Healthy
                                    people will experience -
                                    adverse symptoms that
                                    affect their normal
                                    activity.

                                    Premature onset of cer-
                                    tain diseases in addition
                                    to significant aggrava-
                                    tion of symptoms and
                                    decreased exercise toler-
                                    ance 'in healthy persons.

                                    Significant aggravation
                                    of symptoms and decreased
                                    exercise tolerance in
                                    persons with heart or
                                    lung disease, with wide-
                                    spread symptoms in the
                                    healthy population.

                                    Mild aggravation of
                                    symptoms in susceptible
                                    persons, with irritation
                                    symptoms in the healthy
                                    population.
                                      All persons should remain
                                      indoors, keeping windows
                                      and doors closed.  All
                                      persons should minimize
                                      physical exertion and avoid
                                      traffic.

                                      Elderly and persons with
                                      existing diseases should
                                      stay indoors and avoid
                                      physical exertion. General
                                      population should avoid out-
                                      door activity.

                                      Elderly and persons with
                                      existing heart or lung
                                      disease should stay indoors
                                      and reduce physical activity.
                                      Persons with existing heart
                                      or respiratory ailments
                                      should reduce physical
                                      exertion and outdoor activity.
  50 percent
   of NAAQS
  No index values reported at concentration levels below those specified by "Alert Level" criteria.
 b                            .-                           •
  Annual primary NAAQS.

 C400 Mg/m  was used instead of the 03 Alert Level of 200 Ug/m .
      11.8.3   Standards  of  Performance  for  New Stationary
                Sources  (NSPS)
Subpart  A   of   NSPS   ( 40
covering   definitions,
monitoring  requirements.
that  burn  municipal
particulates  discharged
ton  (0.65   kg/t)  of  dry
shall   not  have  more
co-combustion,  Subpart  E
charging   rate   greater
                  CFR  60)   involves  general  provisions
                  performance  tests,   authority,  and
                  Subpart 0 is applicable  to  incinerators
               wastewater  sludge  and  requires  that
                cannot  be  in   excess  of  1.30  pounds  per
                sludge  feed  and  that  the  gas  discharged
               than  20 percent  opacity  (154).    For
                 is  applicable to  all incinerators  with  a
                 than   50   tons  per  day   (45  t/d)  with
municipal  refuse  comprising   50  percent  or  more  of   the   charge.
Subpart   E  requires   that  particulates  discharged  be  no   greater
than  0.08  grains  per  standard  dry  cubic  foot  (0.18  g/m3  dry)
corrected  to  12  percent carbon  dioxide.

-------
    11.8.4  New Source  Review  Standards  (NSR)

This regulation, ,40 CFR 51.18, requires a preconstruction review
of  all  new or  modified  stationary  sources  to  determine  if the
source  will meet all  applicable emission  requirements  of the
State Implementation Plans  and the USEPA's Emission Offset Policy
(44 CFR 3274,  January 16,  1979).

The reviewing authority is usually a state agency that can apply
stricter  emission  standards  than the  USEPA regulations.   The
state also  sets  emission  offset  required  for stationery sources
affected by the  NSR.   Federal law requires  emissions offsets in
areas where NAAQS are violated for a particular pollutant if:

    1.  The new  source could,  after  installation  of  a pollutant
        control device, emit > 50 tons per year (45 t/yr)  of the
        offending pollutant; or

    2.  Could emit  >100 tons per year (91 t/yr) of the offending
        pollutant were there no pollution control device or were
        the existing device to fail.


State and  local authorities may mandate  a stricter criterion.  In
addition,  the lowest achievable emission rate  is required for any
regulated  source that mandates Best Available Control Technology
(BACT).

The  present definition  of the  term "potential emissions" is
uncontrolled emissions  or,  those anticipated if the  emission
control  device  is  bypassed   or  nonfunctional.   This use of
potential  emissions  in the regulations  has  a  serious effect  on
which  sources  come  under  the  perview  of this  regulation.   For
example, if only one ton per year (0.9  t/yr) of actual emissions
were expected  and  the  control device was  98  percent efficient,
the  "potential  emissions"  would  be  50  tons  per  year (45  t/yr).
The definition of "potential emissions"  is the subject of pending
court action, and this action is  expected  to be settled in  late
1979.
     11.8.5  Prevention of  Significant  Deterioration (PSD)

Regulation 40 CFR  52.21  limits  increases  in particulate and
sulfur dioxide concentrations to specified increments above base
levels measured  in  attainment  areas.   Data  on  total emissions
for  the  entire  air basin are required in  order to  evaluate
incremental increases  in specific emissions  due  to operation of
any new or modified furnaces.  If the potential emission rate of
a regulated pollutant(s)  exceeds 250  tons per  year  (227 t/yr) and
the allowable emission rate  exceeds  50 tons  per  year (45 t/yr),
then this regulation must be  used and public notice is required.
                               11-119

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    11.8.6  The Permit Process

Permits  for construction  and/or operation of processes that
discharge  gases to  the atmosphere  are the primary means for
control of air emissions by a state and, in some cases,  the  local
jurisdictions.   Regulations applicable to a  specific plant site
must be thoroughly reviewed to determine if  permits  are required
for  the  proposed  project.    Generally,  sludge  incineration and
most other  combustion operations require permits.    The  form and
stages  of  permit requirements  will vary considerably between
state  and  local agencies  and  must  be explored at  that level.
Federal  permits for PSD regulations may be required.  In the
San Francisco Bay Area of  California,  for  example, two  stages of
permits are required  (154).  These are:

     •  Permit to  construct—to be applied for and  granted  before
        construction  of  a facility may proceed.

     •  Permit  to operate--to  be  issued after construction and
        generally  after  point  sources  have  passed  stack emission
        tests.
    11.8.7  Air Emissions Test Procedures

The criteria pollutants as  defined  in the  Clean  Air  Act  of  1977,
are particulate matter,  SC>2,  NOX,  CO,  hydrocarbons, and  ozone.
The USEPA has  promulgated  stack emission sampling  and test
procedures  for these pollutants.    However,  state  and  local
agency procedures  may differ somewhat  from  those  of USEPA  and
from each other.  For example, some agencies define  particulates
as  filterable  particulate  matter while others  count the  total
catch   (including  condensible  pollutants).    For  this reason,  a
measurement  made  under  one  jurisdiction may  not  be directly
applicable to another.


   11.8.8  Design  Example

There  are many regional  and local  variations in  the rules,
test procedures,  and methodologies used  to attain the NAAQS.
Therefore, firm guidelines  for procedures  cannot be  provided  to
encompass all areas  of  the  nation.  Designers must determine
federal,   state, and local requirements at  an early  project  stage
and meet  with  USEPA Regional  officers and as well as state  and
local  officials to negotiate changes or additions to the present
regulations  based  on the  project  design, construction,  and
initial  operation.   This  is just  the start;  contact  must  be
continued with  the  USEPA  Regional  Offices, and  state and  local
air quality management districts throughout  the  project.   Also,
the Federal  Register and  national  and statewide  newsletters
should  be monitored  because  they provide  a  good  source  for
proposed  changes in requirements.


                              11-120

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The  following  design  example provides  a framework  for  project
analysis.   It is  based  upon  experience with the San Francisco Bay
Area Air  Quality Management District  (BAAQMD)  which governs  a
nonattainment area.   The BAAQMD generally promulgates rules  more
restrictive than  federal requirements.   This  particular  area was
selected for the example  since  the  local authority (BAAQMD)  has
developed  a  complex  set of  regulations  that  many areas may  be
using as guidelines.

The first  step  is to  identify the applicable emission regulations
(154-156)  and then  establish  the  requirements  for emission
control devices.    These requirements are  reviewed  with  several
manufacturers to  determine feasibility and cost before the device
is  incorporated  into the  design.   The last step  is  the  startup
and  testing  of  the control  device  and  the receipt  of a  permit
to  operate.   To  maintain  the operating  permit,  good plant
monitoring,  operations,  and  maintenance procedures are required.


        H.8.8,.1   Identify Applicable State
                  and Local  Regulations

New Source Review (NSR)

Combustion processes  are subject to  the  NSR rule  adopted  by the
California  Air Resources Board (CARB)  for application by  the
BAAQMD.   NSR is  required by the USEPA  in the  Bay Area and  in
other  regions where  clean  air standards  are violated.    NSR
governs the issuance  of  permits to construct  new or modified
stationary sources  of air  pollution.

The  requirements  apply only  to  facilities  that  would emit  large
amounts of pollutants.   These requirements are that:

     •  The  facilities  must employ  "best available  (emission)
        control technology"  (BACT), Section 1308(a)(154).

     •  The applicant must  meet  current  air  quality regulations
        regarding all  sources of  emission  that  it  owns  or
        operates  in the  Bay  Area.  Section 1307.1 (154).

     •  The applicant  must offset proposed emission increases in
        NOX, CO,  and  HC with more  than  equivalent restrictions
        at other  sources  in  the  region.   Section 1309(a)  (154).

The  NSR rule  is  probably  the most  difficult  environmental
regulation  facing the  designer.   The  NSR rule  requires  that
new  stationary sources which  emit pollutants  above a certain
criterion  level  be approved if they use  BACT.   The criterion
levels are: 150  pound  per day  (68, kg/d)  each for  NOX, SOX,  HC,
and TSP; and 1,500  pound per day (681 kg/d)  for  CO.  Below  these
levels, a  permit  may be  granted  without regard to NAAQS,  and  BACT
need not be applied.   A permit  can  be  issued  where BACT  is  used
and  the criterion  is  not met;  however,  the  NSR rule allows  no
exemption  from  BACT.


                              11-121

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Another requirement  is  that existing facilities owned or operated
by the  applicant  must  meet  all  air pollution  regulations.   Any
wastewater treatment  plant,  or  other  facility  operating  under
common ownership,  must be upgraded  to  meet existing regulations
before a new  source  can be added.  Recent rulings make exemptions
to this doubtful.

The  third requirement of the  NSR  rule applies to stationary
sources that will emit more  than 250  pounds per  day  (114  kg/d)
of NOX,  SOX, HC,  or  TSP and more than 2,500  pounds per  day
(1135 kg/d) of CO.   This  requirement is  intended to prevent  the
plant from contributing to violations  or increased violations of
the  clean  air standards.   Since some  standards  in  the  Bay  Area
are already being  violated,  no sources  with controlled emissions
above this level  can be  built  unless  project  proponents  reduce
emissions  from another source,  thus offsetting  the  air quality
effects of the project. In other words,  if BACT is employed,  and
if the emission  level is  above 250  pound  per day (114 kg/d),  the
project cannot  be  built unless offsets  are  applied.   The project
proponents can  offset  the  project's emissions by modifying  other
facilities to  reduce  emissions or by shutting down polluting
facilities.

In the past, the  BAAQMD has  required that  the  offset facilities
be in the vicinity of the proposed project so that the portion of
the  air  basin  surrounding the  project  receives the  benefit  of
the offset.  The  rule  also requires that the amount of emission
reduction be  slightly higher than the amount of emission increase
anticipated  from the  project.  The  current  offset amount  is
1.2  times  the emission.   For example,  an  industry  can purchase
a paint shop presently discharging  500  pounds per day (227  kg/d)
of  hydrocarbons,  close  the  shop,  and  credit  the  industry
with:  500 T  1.2 = 417  pounds  per day (184 kg/d).

The  feasibility  of offsets  depends  on  the  availability  of
suitable  existing  polluting  plants,  the   cost  of purchase  or
modification,  and  the public  acceptability of  the offset.
If suitable  plants  are found,  purchase of additional control
devices to  reduce emissions   will  probably be  more politically
acceptable than  purchasing  a  privately  owned  facility  and
closing it down.   The  cost of  any of these  alternatives would be
extremely high.

The alternative  route  for  a  large-scale plant would be to  obtain
an exemption from the offset portion  of the  new  source  review
rule.  The rule provides  exemptions for  a  new stationary  source
that  "represents   a  significant  advance  in   the  development of  a
technology that  appears to offer extraordinary environmental  or
public health benefits  or other  benefits of overriding importance
to the  public health or  welfare."   An exemption  granted  by  the
BAAQMD would  require concurrence  of CARB and the USEPA.  While an
exemption may be provided, the likelihood that one would be given
at the  present  time is slight.   Facilities  that potentially
represent an advance in technology  are  normally reviewed  at  the
USEPA  headquarters  in Washington,  D.C.,  rather than  locally.


                              11-122

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The BAAQMD is seriously considering adoption of an NSR rule that
would apply  the  offset requirement only  to CO and  HC ,  but not
to NOX .   This is important  to  any combustion process proposed,
because  NOX  control  is unproven and very costly.  BACT  would
continue to apply as  previously  stated.

Prevention of Significant  Deterioration (PSD)

The  USEPA prevention  of  significant  deterioration  rule  is
designed  to  prevent   increases  in air  pollutant  concentrations
that are below the national health standards in a particular air
basin.   This  is in  contrast  to the  New  Source  Review  Rule
designed to  prevent  increases  in levels  of air  pollutants that
already exceed standards.   In the San Francisco Bay Area, levels
for two pollutants, particulates and sulfur  dioxide, are below or
better than  standards.  If  BACT is applied, as required by NSR,
and  if  controlled emissions of SC>2  or  TSP do  not  exceed the
50-ton  per  year  (45-t/yr)  criterion  level, PSD  will add  no
additional constraints.

                                 ( NSPS )
Sludge incinerators will  be  subject  to BAAQMD NSPS regulations.
These limits  are  1.30 pound per  ton  (0.65-kg/t)  of dry solids,
with gas discharge of  not  more  than 20  percent opacity.

Limitation on Pollutant Concentrations

The  BAAQMD  requires,   as  do  many other  jurisdictions,  that the
concentration of  major pollutants in  the gas  stream  (NOX,  SOX,
HC ,  TSP,  and CO)  be  limited  to some maximum  value.   The
limits established  for the  San Francisco Bay  Area  are  shown in
Table 11-26.   If  supplemental  fuel  is  used  in an incinerator,  a
correction is required to  remove  the product of combustion of the
fuel from the calculation.  Note  that the  concentrations shown in
Table 11-26 are based  upon concentrations per standard dry  cubic
foot  (m^  dry) corrected  to a standard  of  six percent  oxygen.
This correction is applied in the  design  portion of  this example,
11.8.6.3.    Regulatory  agencies vary  in their treatment  of  these
corrections,  but  generally,  all  require  the  gas volumes  to be
corrected  to some   standard  concentration  of  C02  (usually
12 percent) or ©2 (usually six or nine percent).   Some require a
supplemental fuel  correction, which can have a significant effect
on the allowable  emissions.


        11.8.8.2   Establish  Air Pollution  Abatement  Procedures

Requirements

The  designer  of  an   incineration facility  must  develop the
following  information  about  the  flue  gas characteristics before
control devices can be designed:   total flue gas flow rate, flue
gas temperature,  particle  size  distribution, chemical  composition
of emissions, corrosiveness of gas over the operating range, and
moisture content.
                               11-123

-------
                           TABLE 11-26

            SAN FRANCISCO BAY AREA - MAXIMUM ALLOWABLE
                 POLLUTANT CONCENTRATIONS (155)


                Component3          Concentration13
               Particulates              0.05°

               SOX                        300d

               NOX                        175d'e

               HCf                         25d
            BAAQMD Regulation 2.

            All concentrations per dry standard  cubic
            foot corrected to 6 percent 02.

           °Grains/sdcf  (2.3 std g/m3).
           d
            ppm.
           e
            Fuel oil fired - there is no BAAQMD
            standard for solid fuel.

            Nonmethane hydrocarbons.
Until  recently,  municipal  sewage sludge  furnaces  have been
subject  only  to  particulate  emission  controls.   Therefore,
limited basic  data  are available on  emission  rates  of SOX and
NOX  from  sewage  sludge  furnaces.  Table  11-27 presents the
available  data  on  uncontrolled  emissions   from  multiple-hearth
furnaces.


The  following calculations  and  discussions  are  based  on
Alternative  IIIA   (50-MGD  [2.2-m3/s]  plant  flow with  a
sludge  solids concentration of  20  percent), as developed in
Section  11.2.4 (Table  11-9).    The  incinerator  considered is
the  multiple-hearth  furnace  (MHF)  operated  in the  incineration
mode.   Auxiliary  fuel is  assumed to be  natural gas.   Where
local  regulations  apply,  the  BAAQMD  rules  are  used (see
Section 11.8.8.1 and  Table 11-26 and Figure 11-45).  Figure 11-45
is excerpted  from  the  BAAQMD rules.   Installations  under  other
jurisdictions  will presumably have different  regulations:


Step 1  - Calculate Uncontrolled EmjLs_sions  of
         Criteria  Pollutants


    a.   Quantity of  dry sludge solids = 51.5  ton/day  (46.7  t/d) .
                              11-124

-------
    b.   From Table 11-27, daily emissions are:

        Particulates: 51.5 tons dry solids      33  pound
                             day             ton dry solids

                      = 1,700 pound/day  (771.8 kg/d)
        so •  51.5 tons dry solids    	1 pound
          2'         day              ton dry solids

              =51.5 pound/day (23.4 kg/d)
         n    51.5 tons dry solids   	5 pound
          *:        day            x ton dry solids

              = 257.5 pound/day (116.9 kg/d)
              51.5 tons dry solids   	1 pound
                     day           x ton dry solids

              =51.5 pound/day (23.4 kg/d)
                            TABLE 11-27

            UNCONTROLLED EMISSION RATES FROM MULTIPLE-
                       HEARTH FURNACES (157)

                                    Emission factor,
                                   Ib/ton dry sludge
                 Pollutant               solids
Particulates
sox
NOX
Hydrocarbons
CO
33
1
5
1
0
.0
.0
.0
.0
.0
           1 Ib/ton = 0.50 kg/tonne



Ste'p_2_j^j:aj1c:ul:ate Deg^ee o£ Control Required  to  Meet  NSPS

NSPS deals  only  with particulate emissions  (other pollutants  are
covered by NSR).

    a.  NSPS = 1.3 Ib particulates/ton  (0.65 kg/t)

        „-,-,   ui     u •   T *.      1-3 pound   51.5 ton  solids
    b.  Allowable particulates :   	^^	  x 	—	—

            =  67 pound/day  (30.4 kg/d)


                                11-125

-------
     c.   Required  particulate removal  efficiency:

            ,       67 pound  day
              ~
                 1,700  pound day
                                        ,„„               ....  ..
                                     X  10°  ?ercent =  96-!  Percent
 DIVISION 8 —CALCULATION METHODS AND GENERAL
        SAMPLING PROCEDURES
       CHAPTER 1—CALCULATIONS
$8100 Calculation ol
plished by the calcula:
method* which yield ei
eifitally prescribed in i
ihods prescribed in ihii Chapter I. or by
                 (a) Sloklnometric Combu»rion of Auxiliary Fuel

                      6.000   12.000    6,000   12,000
                      CH. +  20,	 CO, + 2H,0

                      12.000 >und«<] cubic (CMof oxygen required
{ 8! 10 Correction (or the use ol auxiliary fuel shall be as specified in
(Bill, and cor ret lion to a basis of 6^ oxygen by dry volume that! be u
specified in j BII2. For the purposes of \\ Bill and 8112 ihe term "meaj-
ured volume" ihall mean the emined. or meiered volume to be corrected,
ncpreued in tundird cubk feet.

J6ll! AUXILIARY FUEL CORRECTION. This calculation is in-


exitied if me luxiliary fuel had not been introduced, and result* obtained
by (hit procedure shall be deemed to represent such correction. The
method coniisu of four siepv

(») Calculate the amount of oxygen required (or iioichiometric combus-
tion of the •luilUrr fuel, at the rate of combustion occurring during the
period of test.
                   6,000 iund>rd tubii ftecCOj. 12,000 iund>rd cubic f«i H2O


                 (c} -100,000 + 12,000 = 412,000


                 (d) 412,000- 18.000 = 394.000 .undird cubic fe«

                     TABULATION OF VOLUME CH\NGE (SCF)
Componr,.,
CO;,
CO
o,
N,
H,O
Total
M«tu,rtl
40.1)00
8.000
21,600
281.200
49,200
400.000
Con re i, on
- 6.000
+ 12.000
- 12,000
- 6.000
F,nil
34.000
B.OOO
33.600
281.200
37,200
394,000
rected (or •uulury fud u
UUry fuel loial 400.000 itindird cubic feet during a if it period, a
                                                            atmospheric oxyRcn content) jub-
                                                            obuimil in step (b) .
                                                          (d) Diviik i hi- rr*nli «l stq> (.) l» ll.H'iii. (This is n.2095 - 0.06.)
(e) Multiply the dry volume obtained in Mcp (a) by the quotient ob-
tained in \if[) (d) IK |five 'hf (oifcned dry volume on a 6r/r oxygen basis
                                                          (0 Divide the weight <>f jir
                                                          urne obtained in sn-p (c) to j
                                                                           ni. by the torrefied vol-
Component
CO,
CO
o,
Nj
H,O
Total
<"„ iVol., *«>
8-64
2-0)
8.53
71 36
9.44
10000
<•; i\ol dr%)
953
2.24
9.42
78.81
0.00
100.00
SCF
S4.000
R.OOO
SJ.600
281,200
37,200
394.000
                                              Also
(b) Calculate the composit
Chiomeiric combuation in ox
(c) Add. to (he measured
step (»)
(d) Subtract, from the re
on and quantity of the product! of such ttoi-
gen.
vo ume, ihe amount o oxygen a cu at in
ult of step (c) . the volume of combiutioo
{ 8112 OXYGEN COR
reel the measured tone
lion fnr the use of iiutilu
measured volume for pu
Rl-CTK
if fuel is
rptiics o
)N Th.j calculauon n

this section 8112. The
mended to cor- 35b,800
to that which ^e) 02(W5

(n (7 9 ibi
                                                  : the weight of air contaminant is 7.9 pounds
                                              (a) 394.000 - 37,200 = 356.800 SCF. dry volume

                                              (b) .IS.fiOfl
                                                B3HJ- =0.0942. volume fract.on of ox;
                                                                        oxygen
                                              (e) (0.782){35(i.ll<«) = 275.WM) SDCf. at fi^{ oxygen, the torretti


                                              (f) (7 9 II)) ("OOOgr/lb) = ().20|(r/SD(;F. the corrected lonccmraiin:
                                               775.H(M) slx;i
                                              Where a concentration subject to trm
                                              volume, the c
                                              the ratio of th
                                              (e) above
        n subject to trm correction ii based on a meat
        n shall conim of multiplying the concentratio
        red volume to the corrected volume obtained in
                                   FIGURE 11-45

        SAN FRANCISCO BAY AREA AIR QUALITY MANAGEMENT DISTRICT:
                AUXILIARY FUEL AND OXYGEN  CORRECTION  (155)
Step  3 -  Select  The  Control Device for Satisfying NSPS


The  actual  selection  of  an  emission control  device  is  beyond  the
scope of  this  manual.    Equipment  selection can  be quite involved
and  complex.   Several   excellent  publications  are  listed   in  the
references  to  provide  a   detailed  understanding  of  emission
control   equipment  (158-161).     A  number of  publications  are
available   in  the   literature   for  further   detail  on  theory,
specific  furnaces,   and  combustion  (1,4,8,9,11,12,14,17,23,38,46,
58,61,74,98,105,141,158-181).     Additional  sources   for  detailed
information  include   furnace  manufacturers,  emission  control
device  manufacturers,   operating   installations,  and  air  quality
control consultants.
 In  this   design  example,   a  venturi   followed   by  a
 wet  scrubber   is  selected.    BAAQMD  considers  this
 BACT.
                                                            tray-type
                                                            equipment
                                        11-126

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Step 4 - Check  Conformance with the New jaource Review
         Rule  (NSR)

    a.  The BAAQMD  requires  that  all  pollutants  be  below
        150 pound per  day  (68.1 kg/d), except CO  which  is
        1,500  pound  per  day (681  kg/d)  (unless BACT is  applied).
        As per  Step  1,  S02,  HC,  and CO  meet  this requirement,
        even as uncontrolled emissions and need not be considered
        further under  NSR.

        Particulates and NOX require BACT.  Since the venturi  and
        wet scrubber  combination   is  considered  BACT  for  partic-
        ulates, the  particulates criterion is satisfied.

        The venturi-scrubber  combination  will  also reduce  NOX to
        a certain extent.  The  N0-N02  distribution  in flue  gas
        for sewage sludge incinerators  is not well  known.   For
        general combustion,  N02  content  represents  10  to
        20 percent  of the NOX and  can be effectively removed
        by the  wet  scrubber.   Assuming  that  a ten  percent  N02
        component of NOX is  removed  by scrubbing,  the  NOX
        emission rate  drops to 232 pounds  per day (105 kg/d).

        At present,  very few  control  processes  are effective
        in reducing NOX  emissions.   However, major  research
        efforts are  being made to  solve the problem.  The  process
        with  the best  potential has  been developed and  tested in
        Japan  only.  It  is a patented catalytic ammonia  injection
        process which  reduces  NOX by  90 percent.   Current
        research in  the  United  States has been conducted  only on
        a small scale. Therefore,  in effect,  there are no fully
        developed,  available  NOX  control  devices.    Until  full-
        scale   systems  for NOX  control  are tested,  an  exemption
        or variance  will probably  be granted.

        Another potential way  to  reduce  NOX  is via  combustion-
        controlled  processes  such as SAC,  reduction  of  excess
        air,  and staged  combustion.   Firm data are not  available
        with  sewage  sludge feed.   Presently  it is  not known if
        the 150 pounds per day  (68.1 kg/day) criterion for NOX can
        be met  by  combustion control.

    b.  Check  to see  if emissions exceed  the  250  pounds  per  day
        (114  kg/d)  level at which offsets must  be  obtained.   In
        this  example,  the  levels  are below  250 pounds per  day
        (114 kg/d);  thus offsets are not required.

Step 5 - Check  that  Concentrations of Criteria Pollutants
         Pg__Not__Exceed_Regulatory  Standards

The objective  is  to  calculate pollutant  concentrations at  some
standard  condition,  so  that the particulate  emission  can be
compared  with  emissions  from other sources  on  an equivalent
basis.   This  correction  is  made   by  first calculating  pollutant


                              11-127

-------
gas flow  (under standard conditions),  then calculating  total
standard exhaust gas flow and correcting  the  latter for auxiliary
fuel.   Finally, the  pollutant concentrations are  calculated
to a  six percent  oxygen basis.   A  detailed  calculation  for
hydrocarbons  (HC)  is presented below.   It is assumed  that HC
are not removed  in  the wet scrubbing system.

    a.  Calculate  the  volumetric HC  flow  at  standard  temperature
        and  pressure  (STP).   The  volumetric flow  rate  of  HC is
        calculated  as:

        51.5 pound  HC   pound mole  HC
              day     x  28 pound HC

                 359 cu ft          day
           x  pound mole at STP x 1440 min."

        =  0.46  standard cfm (2.17  x 10~4  std m3/s)

        It is assumed  HC  are  ethylene with a  molecular weight
        of 28.

    b.  Calculate  exhaust  gas  flow at  STP.   The  data  in
        Table  11-28  are  available.   Off  gas  temperature
        and pressure are  800°F  (427°C)  and one  atmosphere
        respectively.   The   pollutant  (NOX,  S02,  HC,  CO,
        particulate)  masses  are small compared  to  the masses of
        the  constituents  in  Table  11-28 and  thus were ignored in
        calculating exhaust gas volume.

        Total  volumetric flow  rate  of the  exhaust  stream,  wet
        basis; reduced to standard  conditions:
          ..  ,._   _     60°F + 460°F
        = 44'403 scfm x 800°F + 460°F

        = 18,325 scfm  (8.65 std m3/s)

        Note:  standard conditions are taken to be 60°F (16°C) and
        one  atmosphere.

    c.  Correct for  auxiliary  fuel (See  Figure  11-45  and
        Table 11-29).   The  intent  of  this calculation  is to
        correct  the measured  exhaust gas volume  to  the volume
        that would have existed  had auxiliary fuel  not  been
        introduced.  Assume  here that 100 scfm (4.72 x  10~2 std
        m3/s) of   natural  gas  was  used.   The combustion of
        100 scfm   (4.72  x  10~2 std  m3/s)  of natural gas is
        depicted by the following equation:


           CH4  +     202   ——>    C02   +  2H20
        100  scfm   200 scfm      100 scfm   200  scfm
                              11-128

-------
The auxiliary  fuel  correction procedure is:

1.  Calculate  the  amount  of  oxygen,  200  scfm  (0.09  std
    m3/s)  for s to ichiome tr ic  combustion of auxiliary
    fuel.

2.  Calculate  the quantity  of combustion  products
    300 scfm (0.14  std m3/s) .

3.  Add  the oxygen calculated,  200  scfm  (0.09  std  m3s)
    to the  measured gas  volume 18,325  scfm (8.65  std
    m3/s),  then  subtract the  volume  of  combustion
    products calculated  in Step  5c2, 300  scfm  (0.14  std
    m3/s).  The sum, 18,225  scfm  (8.60  std m3/s),  is
    gas  volume  corrected  for  auxiliary fuel   (see
    Table 11-29) .

Correct  for oxygen (see Figure  11-45).   The  intent  of
this calculation is to correct the measured concentration
of  contaminant  to  that  which  would  exist were  the  same
quantity of  contaminant  contained  in   a  dry volume,
corrected to  six  percent  oxygen.  All  calculations  are
based on the final  flow  rate at STP per Table 11-28.   The
procedure is as  follows:

1.  Subtract  the  volume  of  water  vapor,  7,056  scfm
    (3.33 std  m3/s)  from  the final volume,  18,225  scfm
    (8.60 std  m3/s), to give the dry volume, 11,169  scfm
    (5.27 std  m3/s) .

2.  Calculate  the  oxygen content  as a decimal fraction of
    the dry volume:

           Scfm  =  0.1035 02
    11,169 scfm

3.  Subtract the decimal fraction calculated  in Step 5d2
    from the 0.2095  (average atmospheric oxygen content):
    0.2095 - 0.1035  =  0.1060.

4.  Divide  the result  of  Step  5d3  by  0.1495  (this  is
    0.2095 - 0.06) :


    °-1060 - 0.709
    0.1495

5.  Multiply the dry volume obtained in 5dl,  11,169 scfm
    (5.27 std  m3/s),  by the quotient obtained in Step
    5d4,  0.7090, to  get  the  corrected  dry volume  on
    a six percent oxygen basis:

    0.7090  x  11,169 scfm  = 7,919  scfm (3.74  std  m3/s)
                       11-129

-------
        6.  Divide the  volumetric  HC flow  of Step  5a,  0.46  scfm
            (2.17  x  10~4 std  m^/s)  by  the  corrected  dry volume
            on  a  six  percent  basis  to  obtain concentration  on  a
            six  percent basis:
            0.46  scfm
            7,919 scfm
            106 = 58
                             TABLE 11-28

              DESIGN EXAMPLE: EXHAUST CAS DATA FROM A
                      MULTIPLE-HEARTH FURNACE
Constituent
C02
N2
°2
Water vapor
Total
Ib/hr
7
39
4
20
72
,749
,623
,813
,551
,736
Percent of
total gas
volume
6
49
5
39
100
.1
.1
.2
.6
.0
Actual
• CFM
2,
21,
2,
' 17,
44,
712
793
315
583
403
            1 Ib/hr =0.45  kg/hr
            1 cfm = 0.028 m3/min
                             TABLE 11-29

          DESIGN EXAMPLE:  AUXILIARY FUEL CORRECTION FOR A
                     MULTIPLE-HEARTH FURNACE3
   Component
Percent of total
  gas volume
C00
2
NT
2
02
Water vapor
Total
6

49

5
39
100
.1

.1

. 2
.6
.0
CFM
at
STP
1,
8,

7,
18,
119
994
956
256
325
Final CFM at
Correction
-100

+200
-200.,
-100 : ,
ST:
i,
8,
i,
7,
18,
pb
019
994
156
056
225
 See Step 5(c) .
bSTP = Standard  temperature and pressure = 60°F (15.6°C) at one atmosphere.

1 cfm = 0.028 m3/min
                                 11-130

-------
Step 6 - Compare Calculated Pollutant Concentrations
         Against Emission Standards (Table 11-30)

The emissions standard  is  25  ppm.   The HC limit  is exceeded  and
afterburning will be required.

From similar calculations,  the  concentrations  in  Table  11-30  are
obtained  (corrected  to  six  percent oxygen  and  auxiliary  fuel,
prior to afterburning).

None  of  the other  pollutants  (particulates,  NOX, SOX) are  in
violation of concentration standards.
                           TABLE 11-30

        DESIGN EXAMPLE:  MULTIPLE-HEARTH FURNACE POLLUTANT
                 CONCENTRATIONS AFTER SCRUBBING3

              Pollutant     Concentration   Standard
Particulates,
grains/sdcf
HC , ppm
NOX , ppm
S0xe, ppm

0.04
5&c
147
17

0.05
25
175
300
           aCorrected  for  auxiliary  fuel  and  to  6
            percent  oxygen.

            As  ethylene.

           CDoes  not include  afterburning.

           dAs  N02.

           SAs  S02.

           1 grain/sdcf  =2.3 std  g/m
    Step 7 - Summary

A venturi,  wet  tray-type  scrubber and afterburning will  satisfy
all emission  requirements  except NSR  requirements  for NOX.   An
exemption is  expected  for  NOX,  since  technology  for  NOX  removal
is not  sufficiently developed for  field  applications.   Note that
not all  jurisdictions  require auxiliary  fuel and oxygen  correc-
tions.  As shown, the corrections can have profound  impacts.  The
type  of control  scheme  required may  hinge  upon the  regulatory
agency's decision  as  to  whether such  corrections are  necessary.
The procedures used in Step 5 were taken  directly from Regulation
2 of the BAAQMD Regulations (155).


                              11-131

-------
11.9  Residue Disposal

The residues remaining  after  sludge combustion (ash, particulates
from dry scrubbing,  etc.)  must be disposed of.   Due to the drain
of  natural  resources,  the  constructive  utilization of residues,
particularly ash, is undergoing  considerable  research.   Because
the ash concentrates the setteable material in wastewater, there
is an interest in recovering  valuable scarce metals such as gold.
In Palo Alto, California, a firm  is working on methods to recover
such metals  from  the ash  (182).   In this  case,  recovery  may  be
cost-effective,  since the treatment plant  receives the wastewater
from many electronics firms and the scarce metal content is high.
In  general,  however, there is  no economical  process  to use ash;
consequently, it is  typically disposed of  to a landfill.

Residues (ash)  from  the combustion of municipal wastewater solids
generally contain high  concentrations of trace metals.  Leachate
from sites where incinerator  ash  is landfilled must be controlled
to  prevent metal contamination  of  groundwater.   Many  states
are  beginning  to classify  disposal sites  according to their
relationship to  nearby groundwater  and  the  material   to  be
landfilled.  Tables  11-31 and 11-32 describe methods  used by the
State of California  for classifying waste materials and disposal
sites.   Typically,  wastewater  sludge furnace  ash  requires  a
"protected" Class II-l  site and municipal  refuse incinerator
ash  requires a  hazardous  fill  site.   These  are described  on
Table  11-32.   Outside  these broad classifications, the  ash
will require sampling  and analysis,  including detailed review by
state and  local health  agencies.   A serious problem,  however,  is
that no  standard  analyses or procedures are  presently available
that allow a particular ash to  be  classified  (leachability  of
certain contaminants at various  pH' s  and  over  different  times).
This type of analysis is  expensive, and  the results are difficult
to  interpret.   No data base  is available  to  compare  the  labora-
tory results with actual  field  conditions.  Work is being done in
this area  and hopefully proper procedures and guidelines will be
developed.

Detailed design and operating  data are  beyond  the  scope of this
manual.    More  detailed  discussions  on landfilling ash  and
sludge  landfilling procedures can  be  found in the  literature
(184,185,186) .
                              11-132

-------
                                     TABLE 11-31

       DESCRIPTION OF SOLID AND LIQUID WASTE CLASSIFICATIONS (183)
           Group 1
          Group  2
          Group  3
Consist of or contain  toxic
substances and substances
which could significantly
impair the quality  of  usable
waters.

Examples include:

  •  Saline fluids  from water
     or waste treatment pro-
     cesses

  •  Community incinerator
     ashes

  •  Toxic chemical toilet
     waste
     Industrial brines

     Toxic and hazardous
     fluids

     Pesticides or  chemical
     fertilizers or their
     discarded containers

     Other toxic wastes
Consist of or contain
chemically or biologically
decomposable material which
does not include  toxic  sub-
stances nor those capable of
significantly impairing the
quality of usable waters.

Examples include:
  •  Garbage

  •  Rubbish

  •  Construction debris
     such as paper,  card-
     board, rubber,  etc.

  •  Refuse such  as  yard
     clippings,  litter,
     glass, etc.

  •  Dead animals

  •  Abandoned vehicles
  •  Sewage treatment resi-
     due such as  solids from
     screenings and  grit
     chambers, dewatered
     sludge, and  septic tank
     pumpings

  •  Infectious materials
     from hospitals  or
     laboratories
Consist entirely  of  nonwater
soluble,  nondecomposable
inert solids.

Examples  include:

  •  Construction and
     demolition debris,
     asphalt paving, inert
     plastics,  etc.

  •  Vehicle tires

  •  Industrial wastes  such
     as clay products,  glass,
     slags,  tailings, etc
                                       11-133

-------
                                                        TABLE  11-32

                        CLASSIFICATION  OF WASTE  DISPOSAL SITES  (1&3)
                   Class I
Class I disposal sites are those at which
complete protection  is provided for all
time for the quality of ground and surface
waters from all wastes deposited therein and
against hazard to public health and wildlife
resources.   The following criteria must be
met to qualify a site as Class I:

(a)  Geological conditions are naturally
     capable of preventing vertical
     hydraulic continuity between liquids
     and gases emanating from the waste in
     the site and usable surface or ground-
     waters.

(b)  Geological conditions are naturally
     capable of preventing lateral hydraulic
     continuity between liquids and gases
     emanating from  wastes in the site and
     usable surface  or groundwaters,  or the
     disposal area has been modified to
     achieve such capability.
(c)  Underlying geological formations which
     contain rock fractures or fissures of
     questionable permeability must be
     permanently sealed to provide a com-
     petent barrier  to the movement of
     liquids or gases from the disposal site
     to usable waters.

(d)  Inundation of disposal areas shall not
     occur  until the site is closed in
     accordance with requirements of the
     regional board.
(e)  Disposal areas  shall not be subject to
     washout.
(f)  Leachate and subsurface flow into the
     disposal area shall be contained within
     the site unless other disposition is
     made in accordance with requirements of
     the regional board.

(g)  Sites  shall not be located over zones
     of active faulting or where other
     forms  of geological change would impair
     the competence  of natural features or
     artifical barriers which prevent con-
     tinuity with usable waters.

(h)  Sites  made suitable for us» by man-made
     physical barriers shall not be located
     where  improper  operation or maintenance
     of such structures could permit  the
     waste, leachate, or gases to contact
     usable ground or surface water.
(i)  Sites  which comply with a, b, c, e, f,
     g, and h,  but would be subject to
     inundation by a tide or a flood of
     greater than 100-year frequency may be
     considered by the regional board as a
     limited Class I  disposal site.
Class II disposal  sites are those at which    Class  III disposal sites are those  at which
protection is  provided to water quality from  protection  is provided to water quality  from
Group 2 and Group  3 wastes.  The types of
physical features  and the extent of pro-
tection of groundwater quality divides
Class II sites  into the two following
categories:

Class II-l sites are those overlying usable
groundwater and geologic conditions are
either naturally capable of preventing
lateral and vertical hydraulic continuity
between liquids and gases emanating from the
waste in the site  and usable surface or
groundwaters, or the disposal area has been
modified to achieve such capability.

Class II-2 sites are those having vertical
and lateral hydraulic continuity with usable
groundwater but for which geological and
hydraulic features such as soil type, arti-
ficial barriers, depth to groundwater, and
other factors will assure protection of the
quality of usable  groundwater underneath or
adjacent to the site.

The following criteria must be met to qualify
a site as Class II:

(a)   Disposal areas shall be protected by
     natural or artificial features so as
     to assure  protection from any washout
     and from inundation which could occur
     as a result of tides or floods having
     a predicted frequency of once in 100
     years.

(b)   Surface drainage from tributary areas
     shall not  contact Group 2 waters in the
     site during disposal operations and for
     the active life of the site.

(c)   Gases and  leachate emanating from waste
     in the site shall not unreasonably
     affect groundwater during the active
     life of the site.

(d)   Subsurface flow into the site and the
     depth at which water soluble materials
     are placed shall be controlled during
     construction  and operation of the site
     to minimize leachate production and
     assure that the Group 2 waste material
     will be above the highest anticipated
     elevation  of  the capillary fringe of
     the groundwater.  Discharge from the
     site shall be subject to waste dis-
     charge requirements.
Group 3 wastes by  location, construction,  and
operation which prevent erosion of deposited
material.
                                                             11-134

-------
                                        Calcu»4tkin*-n«Ql«»
                                                                                             ind
                                                                                 0«t«
    13

    1*

    15

    16

    17


    IS

    19

    20


   (21
               Fysl, Os, a«f Air pw Unit af Fuel
             Ftwl
          Gertstltueftt
         CtoCO

         CO to CO:

         Cynbvrned,
           line K
         0; (deduct)

         Nj

         COs

         H:O

         *sh

         Sum
              Psr
              Fuel
              Unit,
               Ib
                      100.0
Mul.
 m
Dhri.
 sor
                                 Motes
                                 Fuel
                                  rtt-
                                       ptiw
                                   Mote
                                   Theo
                                   Rtq.J
                                    0
                                    o
                                    o
                                    0
    Os- ind Air, Moles for Tatal Air -
           (§se line dai righl)

O; (th«o) rend - Q?l line 12

Oi (eicsw)  T'Aim°° x Oa, line 12

Oj (total) supplied - lines 13 I- 14

NE 4u|5ptteJ - 3.76 x. Oj, iivc IS

Air (sry; supplied ~ Qs + Nj

                        —*
H3Q in air

Air (wetj supplied - lines 1? i  IB

Flue g«5 comrtitutnts - lines 1 to J 8, iotal

•Ncrri: —for air ?it 30 F anrtKW* relative humidity i
                                                     .
                                             Moles per Fuel Unit fAf,
                                                                                 Fuel
                                                                                 Sourc*
OOs
 -I-
SO,
                                                                                        .
                                                                                 HOP males, gaitcuj fi»(i
Fual Anal, as Flrwl (AF), % By Wl a1 Vpt
                              Cfo      Oa      CO      Nj      %t

                              Total an (TJ&.) assBTOd « &/ORSAT    %

                                  yn« f, i, h For Qiseais Fuels

                              Wl ftjel yftit - J (moles *ai^i x md, Wt) Ib

                              Mol" wt'irf'fuei"-line r : 103
                                                                           ! heat value, 8tu/lb
                                                                                     Xzfl unburned. lb/100 Ib f««l

                                                                                     % ash In fya y
                                               100 -
                              Exit t«ip of Hue gaj, S,
                              Dry-bulb {ambient J temp, f i

                                    ''
                              Rei humid. (p5^Hrgrnttr*C chart)  .

                              S*, barameirir: prc$$yr^, m Ng   ;

                              Sat. preai. HjO at amS temp, in. HE

                              A*, press. HjO In sir, lines {o x' q), In.
                               Total   '  Wet Flye Qas
                              .Moles_[	
                                                                                                Drj Flue 4ii
                                               •  ft.03?  is oftsn used as standard.

                                                                     CO; + SOi
                                   Flue G*s «nd

Fly^ £35 ^^nstJtys^ts

Me>, mean, iz to t'i ftof ft -

lo dry flue gas « males each, line 20 ,x Men X (^ — f 3)

In HjO in air   malfls HjO, lint 18 x Me? x C?j— d)

I n s*ns heal, HiQ In fuel - moles, II n es (5110) x *te? M (b — r

In. ta[tnt heat  MK) in fvsl  - mt«BS. lines (5 + 10) x. ]D*0 x IS

Total in wet flue gis

EKje to CafbDn in refuse ™  line 1( x I^.IDO

fc« to ynbumsd CO in flue gas - nwtss C to CO x 12 x 9.75S

Total riue gas laaaes • unburttied cambuaUMe - llnaa 2S129 +• 3d t ndiotlon ttt

Heat value of fuel  unit -

Tsui tiota l*n! k* fwl unit » Itn* 32 - I In* 31
                                                                           in Btu per Fuel Unit g formula C It -t) ar by cjlorlnMlry,

ttfPtudlition «((umwi ID t» « flxecl ptrcenl of line li, rvjrnnally 1 to 5 percent,

   Copyright 197S by th« Babcock and Wllcox Company.  Minor chang«> have  b««n
   made to this table to allow for ease of use  with sewage iludge.   Table may be
   uied  without permission.  However, credit to Babcock and Wllcox Company
   should  be given.
                                                                                   CEMERAL W7TIS;

                                                                                   *   Sue taul for uie trf ubl« -
                                                                                   *   IWut*, >• UMd in UMl
                                                                                       uble,  li the r«kiu* (iih)
                                                                                       from [h«prot«j.
                                                                                   1 in. =
                                                                                   1 Btu/lb « J,
                                                                                   1 Ib/cu ft » 1*
                                                         11-135

-------
11.10  References

  1.   Niessen,  W.R.   Combustion  a n d Incineration P r o c e_s£e_s_.
      Marcel Dekker,  Inc.   1978.

  2.   Shen,  T.T.,  M.  Chen,  and J. Lauber.  "Incineration of  Toxic
      Chemical Wastes."  Pollution  Engineering.  October 1978.

  3.   Dunn,  K.S.   "Incinerations  Role  in Ultimate Disposal  of
      Process  Waste."   Chemical Engineering.  82:21.  October  6,
      1975.

  4.   Perry,  R.H.,  and  C.H.  Chilton.   Chemical Engineers'
      Handbook.  McGraw-Hill  Inc.   5th Ed.  1973.

  5.   Standard Methods  for  the  Examination  of Water and
      Wastewater.   American  Public  Health  Association.   14th  Ed.'
      1976.

  6.   Owen,  M.B.    "Sludge  Incineration."    Journal  Sanitary
      Engineering  Division  -  ASCE.  February 1957.

  7.   Combustion Fundamentals for Waste Incineration.    American
      Society of Mechanical  Engineers. .1974.

  8.   Howard,  F.S., T.D.  Allen,  and G.F.  Kroneberger.   "Energy
      Production through Sludge/Refuse Pyrolysis."  Journal  Water
      Pollution Control  Federation.   Vol.  51.  April 1979.

  9.   Babcock  and  Wilcox.   Steam,  Its  Ge n_e ra_t i^oji ,_g_nd,_U_s_e_.
      Babcock and  Wilcox.   38th Ed.   1975.

 10.   Smail, L.L.   Calculus.  Appleton-Century-Crofts,   Inc.
      1949.

 11.   Baumeister,  T.,  and  L.S.  Marks.    Standard Handbook  for
      Mechanical Engineers.   McGraw-Hill Inc.  1967.

 12.   Corey,  R.C.   Principles and Practices  of Incineration.
      Wiley Interscience.   1969.

 13.   Hicks, T.G.   Standard  Handbook  of Engineering Calculations.
      McGraw-Hill  Inc.   1972.

 14.   North  American  Mfg.   Co.    Combust ion Hand book.    North
      American Mfg.  Co.  2nd  Ed.  1978.

 15.   USEPA.    Computerized  Design  and  Cost  Estimation  for
      Multiple-Hearth Sludge  Incinerators.    Office of  Research
      and Monitoring.   Cincinnati 45268.   17070 EBP.  1971.

 16.   Unterberg,   W.,   G.R.  Schneider,  and  R.J.  Sherwood.
      "Computerized Predesign and Costing  of Multiple-Hearth
      Furnace  Sewage  Sludge Incinerators."   AIC_h_E  _Syinp_o_s_i_uni
      Series.   1972.
                              11-136

-------
17.   Brown and  Caldwell.   Solid  Waste  Resource Recovery  Full
     Scale Test  Report.   Prepared for the  Central  Contra  Costa
     Sanitary  District.  March 1977.

18.   Bracken, B.D.,  R.B.  Sieger,  J.R.  Coe, and  T.D. Allen.
     "Energy  from Solid  Waste  for  Wastewater  Treatment—A
     Demonstration Project."   Proceeds of the Annual Conference
     of the Water  Pollution 'Control__Fed_eration.  1977.

19.   Camp  Dresser  McKee  and  Alexander  Potter  Associates.
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     Program.  Prepared  for   the  Interstate   Sanitation
     Commission-New York-New Jersey-Connecticut.   June  1976.

20.   Galandak, J., and M.  Racstain.   "Design Considerations  for
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21.   Barrett,  D.   "Pyrolysis of  Organic Wastes."  Proceedings of
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22.   Kashiwaya,  M.    "Studies  on  Sewage Sludge  Pyrolysis."
     Proceedings  on  the  Fifth   U.S./Japan  Conference  on  Sewage
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23.   Lewis, F.M.   "Sludge Pyrolysis for  Energy  Recovery  and
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24.   Majima,  T.,  T.  Kaskara, M. Naruse,  and  M.  Hiraoka.
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25.   Olexsey,  R.A.   "Pyrolysis  of Sewage  Sludge."   Proceedings
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26.   Shelton, R.D.    "Stage  Wise Gasefication in a  Multiple
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27.   Sieger, R.B.   "Sludge Pyrolysis:  How  Big a Future?"  Civil
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28.   Srinivasaraghavan,   R.,  T.E.  Wilsqn, and K.R.   DeLisle.
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29.   Takeda, N., and  M. Hiraoka.  "Combined Process of Pyrolysis
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30.   vonDreusche,  C.,  and  J.S.  Negra.    "Pyrolyses  Design
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31.   Jacobs,  A.,  J.  Anderson,  B. Pickart, and  D.   Brailey.
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32.   Jones, J.L.,  D.C.  Bomberger,  F.M. Lewis, and  J.  Jacknow.
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33.   Jones,  J.L.,  D.C.  Bomberger,  and F.M.  Lewis.    "The
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34.   Los Angeles/Orange  County Metropolitan Area  (LA/OMA),
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35.   USEPA.   Assessment  of the  Use  of  Refuse-Derived Fuel  in
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36.   USEPA.    Energy  Conservation   in  Municipal  Wastewater
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37.   Camp,  I.C.  "Examples of Sewage Sludge  Incineration  in the
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38.   Cardinal,  P.J.,   and  F.P. Sebastian.   "Operation, Control
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39.   Federal Water Pollution Control Administration.  A Study of
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40.  Federal Water  Quality  Administration.    State of  t_he^
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                              11-138

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41.   Ferrel,  J.F.   "Sludge  Incineration."  Pollution Engineerinq.
     5:3.   March  1973.                        	      ~™—'	

42.   Grieve,  A.   "Sludge Incineration with Particular Reference
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43.   Hescheles,  C.A.,  and  S.L.  Zeid.   "Investigation  of  Three
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44.   Klein, J.O., and  B.C.  Bergstedt.   "Unusual  Experiences  in
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45.   Petura,  R.C.   "Heat Recovery from Multiple Hearth Furnaces
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46.   Petura,  R.C.   "Operating  Characteristics  and  Emission
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47.   Reeve,  D.A.D., and N.  Harkness.    "Some Aspects  of Sludge
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48.   Schroeder,  W.H.   "Principles   and Practices  of  Sludge
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49.   USEPA.   Sludge  Treatment and  Disposal.  Technology
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50.   USEPA.   S_ta_te_of Art Review  on Sludge  Incineration
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     1970.

51.   Warwick,  M.G.    "Incineration  of  Sewage  Solids on  the
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52.   Zasada,  A.S.    "Operating  Experience  with  Waste  Heat
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53.   Anderson, R.J.   Combustion.    Wheelabrator  Incineration,
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                             11-139

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54.  Anderson,   R.J.    System for Controlling the Operation pfa
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55.  Anderson,   R.J.    Multiple  Hearth  Furnace  Recycle  Process
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56.  Albertson,  O.E.   "Low Cost Combustion  of  Sewage  Sludges."
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57.  Becker, K.P.,  and  C.J. Wall.   "Fluid  Bed  Incineration  of
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58.  Copeland,  B.J.   "A Study of  Heavy Metal  Emissions  from
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59.  Copeland,  G.G., and  I.G. Lutes.   "Fluidized Bed  Combustion
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60.   Federal Water Quality Control Administration.   Mathematical
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61.   Liao,  P.B.,  and  M.  J. Pilat.   "Air Pollutant Emissions
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62.   Noon,   T.A.   "Fluosolids-Operational   Experiences."
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63.   Wall, C.J.   Fluosolids Incineration of  Biological Sludges.
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64.   Stribling,  J.B.   "Sludge  Incineration by  Cyclone Furnace."
     Ef_fl_uent Water Treatment  Journal.   August  1972.

65.   Beeckmans,  J.M.,  and  P.C.  Ng .   "Pyrolysed Sewage  Sludge:
     Its  Production   and  Possible  Utility."    En v i r onm enJLal
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66.   Lombana, L.A.,  and J.G.   Campos,    Incineration  Method  and
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67.   vonDreusche,   C.F.   Method  and  Apparatus  for  Incinerating
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68.   U S E P A .  Pyrolysis  of  Industrial Wastes  for  Oil a n_d_
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69.   Bracken, B.D.,  and  T.U.  Lawson.     "Alternative  Fuels  for
     Multiple  Hearth  Furnaces."    Presented  at  the  Annual
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                             11-140

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70.   Hathaway,  S.W.,  and R.A. Olexsey.  "Improving Incineration
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71.   Pitzer,  R.L., R.F.  Wukash, and  D.B.  Wells.   "The  Vacuum
     Filtration and Incineration of Sewage Sludge Using Crushed,
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     University Industrial_W_as_te_Con_fe_re_nce.  1977.

72.   Swanson, G.J., and D.C.  Bergstedt.   "Coal as a Supplement
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73.   USEPA.   Coincineration of  Sewage  Sludge  with Coal or Wood
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74.   USEPA.     A Review of  Techniques  for Incineration of Sewage
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     1976.

75.   Sussman, D.B., and H.W.  Gershman.   "Thermal Methods for the
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76.   Sussman,  D.B.    "More  Disposal  Operations  Mixing
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77.  Sussman,  D.B.   "Municipal  Solid  Waste  and Sewage Sludge—
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78.  Reardon,  F.X.  "Economics  and  the  Selection of Incineration
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79.  Krings,  J.   "French Experience with Facilities for Combined
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80.  Department  of Energy.  Case Study  of the Thermal Complex of
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81.  General  Accounting Office.   Report to Congress;  Codisposal
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     Problems.CED-79-59.May 16, 1979.
                              11-141

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82.   Cosulich,  W.F.   "Co-Burning of Sludge  and  Refuse  with  Heat
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83.   Dvirka, M.,  and N.  Bartilucci*    "Co-Disposal of Sewage
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84.   Davidson,  P.E., and  T.W.  Lucas.    "The Andco-Torrox  High
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85.   Legille,  E., F.A.  Berczynski,  and K.G.  Heiss.   "A Slagging
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86.   Moses, C.T.,  K.W. Young, G. Stern , and  J.B.  Farrell.
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87.   USEPA.    The Codisposal of Sewage Sludge and Refuse in the
     PUROX  System.   Office   of Research  and  Development.
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88.   Grant, R.A., and  N.A.  Gardner.    "Operating Experience  on
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89.   Eimco-BSP  Envirotech.    Solid  Waste Furnace Tests  for  Brown
     and Caldwell.   Prepared  for  the  Central  Contra Costa
     Sanitary  District.  January 1975.

90.   Aberley,  R.C.,  R.B.  Sieger, and  B.D.  Bracken.   "Pyrolysis
     Gas from Solid Waste Will Provide Total Power  Demand  for a
     Major  Wastewater Reclamation Plant."  Proceeds of  the
     International   Conference  on^._Aj1tjer native  Energy  Source^s.
     TTTT."    "	"           "       "  ~~     !  '  ~™  ~

91.   Bracken,  B.D., J.R.  Coe , and  T.D. Allen.   "Full Scale
     Testing  of  Energy Production  from  Solid  Waste."
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92.   Brovko, N.   "Combined Disposal of  Sewage  Sludge  and  Solid
     Wastes by  Pyrolytic  Process."  Progress in Water Technology
     (England).   9.   1977.


                             11-142

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 93.   Frazer,  J.M.   "Simultaneous Incineration of  Refuse
      and Sewage  Sludge:   The Principles and  Application  at
      Bowhouse, Alloa,  Scotland."   Public Health  Engineering
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 94.   Kimbrough, W.C., and L.E. Dye.  "Pyrolysis of Sewage Sludge
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 95.   Nichols Engineering and Research Corporation.  Pyrolysis  of
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 96.   Sieger,   R.B.,  and B.D.  Bracken.    "Combined Processing
      of  Wastewater  and  Solid  Waste."    AIChE Solids  Symposium
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 97.   USEPA.    A  Technical,  Environmental,   and   Economic
      Evaluation  of the  Wet  Processing  System for  the  Recovery
      and Disposal of  Municipal  Solid Waste.   Office  of  Solid
      Waste Management.   Washington,  DC  20460.   68-01-2211.
      1975.

 98.   USEPA.    Engineering  and  Economic Analyses  of  Waste  to
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      1978.

 99.   Jones, J.L.   "The  Costs for  Processing  of Municipal Refuse
      and Sludge."   Proceedings of the ConferenceonAcceptable
      Sludge Disposal Techniques.  1978.

100.   USEPA.    Cost Allocations for  Multiple  Purpose  Projects.
      Office of Water Programs Operations.  Washington,  DC 20460.
      Construction Grants Program  Requirements,   Memorandum No.
      PRM 77-4.  1976.

101.   USEPA.    Risks  and Contracts.   Office  of  Solid Waste
      Management.   Washington, DC 20460.   Resource Recovery Plant
      Implementation:   Guides for  Municipal Officials,  SW-157.7.
      1976.

102.   Krzeminski,  J.  "Codisposal of Sludge with Refuse."   Sludge
      Magazine.  March-April  1978.

103.   Smith,  E.M., A.R.  Daly.  "The  Past, Present, and Future
      of   Burning  Municipal  Sewage  Sludg-e  Along with Mixed
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      on_Municipal Sludge Management and Disposal.  1975.

104.   Petura,  R.C., C.R.  Brunner,  and R.F.  Bonner.  "Utilization
      of  Sewage  Skimmings  as Fuel  to Generate  Process  Steam."
      Proceedings  of  the National Waste  Processing  Conference_-
      ASME.  1978.
                              11-143

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105.  Guillory,  J.L.   "Particulate  Emissions  Resulting  from
      Combustion of  Municipal  Sewage  Skimmings."    Proceeds  of
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      Engineers. 1977.       ,    ""    —              —

106.  Ross,  E.E. "Scum  Incineration Experiences."  Journal Water
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107.  USEPA.    Evaluation  of  Small   Modular Incinerators  in
      Municipal  Plants.   Office  of  Solid  Waste  Management.
      Washington, DC  20460.   Contract  68-01-3171.   1976.

108.  USEPA.   Lime Use in  Wastewater;   Design  and Cost  Data.
      Municipal  Environmental  Research .Laboratory.    Cincinnati
      45268.   EPA 600/2-75-038.  October 1975.

109.  USEPA.     Sludge Processing for  Combined Physical-Chemical-
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110.  USEPA.   Advanced Wastewater Treatment as  Practiced  at
      South_Tahoe.  Project  17010 ELQ.  August 1971.

111.  Evans,  R.R.    Sludge Disposal  and  Chemi_caJ^_R_e_c_o_y_e.j:y.
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112.  Brown  and Caldwell.   Lime Sludge Recycling  Study.   Central
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113.  Cardinal, P.J., and R.J. Sherwood.   Plural Purpose Sludge
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114.  USEPA.    Solids Handling_and  Reuse of Lime Sludge.   Project
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115.  Hotz,  H.J.,  P.   Hinkley,  and  A.  Er dm'a n.    "Fluidized
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116.  Gulp,  R.C., and G.L. Gulp.   Advanced Wastewater^ Treatment.
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117.  Moran,  J.S.,  and C.J. Wall.    "Operating Parameters  of
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118.  Kroneberger,  G.F.    "Lime  Recalcination in the  U.S.  Sugar
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119.  Wall,  C.J.   Fluosolids Reburning  of  Lime  Sludges.  Dorr-
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                              11-144

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120.   Stadnik,  J.G.,  and  B.P. Hynn.  "State of the Art:   Powdered
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121.   USEPA.   Process  Design  Manual  for Carbon  Absorption.
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122.   Kittredge,  D.   "The Economics  of Carbon Regeneration--State
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123.   vonDreusche,  C.  "Process Aspects  of  Regeneration  in  a
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124.   Shell,  G.L., and  D.E. Burns.    "Powdered  Activated  Carbon
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125.   Rudolfs, W.,  and  E.H.  Trubnick.    "Activated Carbon  in
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126.   Wallace,  W.N.,  and  D.E. Burns.   "Factors Affecting Powdered
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127.   Koches,  C.R.,  and  S.B.   Smith.     "Reactivate  Powdered
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128.   Westvaco  Corporation.  U.S. Patent  3,647,716.

129.   Lewis,  R.E.,  J.J.  Kalvinskas,  and W.  Howard.  JPL  Activated
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130.   Sigler,  J.E.   "Activated Carbon and  Fuel  from  Sewage
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131.   Jones,  J.L. "Converting Solid  Wastes  and Residues  to Fuel."
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132.   Jones,  J.L.,  R.C.  Phillips,   S.  Takaoka,  and  P.M.  Lewis.
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                             11-145

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133.   Klass,  D.L.    "Energy  from Biomass and  Wastes:   1978
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134.   Wilhelmi,  A.R.,  and  P.V.  Knopp.   "Wet Oxidation  as an
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      Meeting  of the  American  Institute of Chemical  Engineers.
      1978.

135.   Flynn,  B.L.  "Energy  Aspects of  CPI Wastewater Treatment
      by Wet Air Oxidation."  Zimpro Technical  Publication.

136.   Pradt, L.A.   "Wet Oxidation of Coal for Energy."  Presented
      at the K en.tucky Coal Conference.  May 1979.

137.   Pradt,  L.A.     "Wet  Oxidation  Boiler-Incinerator."
      Proceedings  of  the  National  Waste  Processing  Conference-
      Energy  Conservation  Through Waste Utilization  -  ASME.
      1978.

138.   USEPA.  Destroying Chemical Wastes in  Commercial  Scale
      Incinerators.  Facility Report No. 4 - Zimpro, Inc.   Office
      of Solid Waste.   Washington,  DC  20460.   NTIS:   PB-267  987.
      December 1976.

139.   Pradt, L.A.   "Developments in Wet Air Oxidation."   Chemical
      Engineering Progress.   68:12.  1972 (Updated  1976).

140.   Hathaway,  S.A.   Design  Features of Pa c k a g e In c in ejc a to r
      Systems.   U.S.  Army  Construction Engineering  Research
      Laboratory.   NTIS:  AD/A-040 743.  May 1977.

141.   Niessen, W.R.,  A.A.  Kalotkin,  F.C. Sapienza, and P. Nese.
      "Air  Pollution  from  Refuse -  Sludge  Coincineration in
      Modular  Combustion  Units."   Presented at the Mid-Atlantic
      States  Section  of  the  Air Pollution	Control  Association.
      Newark.   April 1979.

142.   Hofmann,  R.E.   "Controlled-Air  Incineration  -  Key to
      Practical  Production  of Energy  from Wastes."  Publj1cmWgrk_s
      Ma_g_a_z_irie_.   April  1976.

143.   Martin, T.L.   "A Total  Package Concept for Solid Waste
      Management."  Public Works Magazine.  April  1975.

144.   "City Finds Disposal Solution."   So1id  Wastes  Managemenit.
      March 1978.

145.   Titlow,  E.  "Refuse Incineration - Waste to  Energy Systems
      for the Smaller  Community."   Presented  at the Governmental
      Refuse  Collection and  Disposal  Association  Meeting.   San
      Leandro,California.March 1978.

146.   Bailie,  R.C.   Production  of High Energy  Fue1  G a.s^
      Municipal  Wastes.  U.S. Patent No. 3,853,498.


                             11-146

-------
Association. Vol. 27. October 1977.
USEPA. Environmental News. August 23, 1976.
USEPA. Inspection Manual for Enforcement of New
Performance Standards: Sewage Sludge Incinerators.
of Enforcement. Washington, DC 20460. Stationary
Source
Office
Source
147.   Fluidized  Bed Gasification  Project,  Department  of  Chemical
      Engineering,  West Virginia University.   "Solid Waste;   A
      New Natural  Resource."  Morgantown, WV.  May 1971.

148.   Coffman,  J.A.,  and R.H. Hooverman.   "Power from Wastes
      via Steam Gasification."  ACS Symposium  Series No. 76,
      Solid  Waste  and  Residue;   Conversion  by Advanced  Thermal
      Processes.  American Chemical Society.  1978.

149.   Hooverman,  R.H.,  and  J.A.  Coffman.    "Rotary  Kiln
      Gasification of Solid and Liquid Wastes."  Presented at the
      Annual  Meeting  of  the  American  Institute  of  Chemical
      Engineers.  1977.

150.   USEPA.    Feasibility Study of Use of Molten  Salt Technology
      for Pyrolysis of  Solid Waste.   Office  of and  Development.
      Cincinnati 45268.  EPA-670/2-75-014.  January 1975.

151.   Easton,  E.B., and F.J.  O'Donnell.    "The  Clean Air Act
      Amendments of  1977,  Refining  the  National  Air  Pollution
      Control  Strategy."    Journal of the Air Pollution Control


152.

153.
      Enforcement  Series, EPA 340/1-75-004.  February 1975.

154.  San Francisco Bay Area  Air  Pollution  Control District.
      New Source Review  Rules.  Sections  1304 through  1311.2.
      December 20,  1977.

155.  San Francisco Bay Area  Air  Pollution  Control District.
      Regulation 2.   Adopted May 4, 1960, and amended thereafter.

156.  San Francisco Bay Area  Air  Pollution  Control District.
      Regulation 8.   December 1976.

157.  USEPA.   Supplement  for Compilation  of  Air Pollutant
      Emission Factors.   Research  Triangle  Park, North  Carolina
      27711.   AP-42.   1975.

158.  Bethea,  R.M.   Air  Pollution  Control  Technology;   An
      Engineering  Analysis Point of View.   Van Nostrand-Reinhold
      __1978^

159.  USEPA.   Air  Pollution  Engineering Manual.   Office of  Air
      and Water Programs.  Research Triangle Park, North  Carolina
      27711.   AP-40.   2nd Ed.   May 1973.

160.  USEPA.   Industrial  Guide   for  Air Pollution  Control.
      Environmental  Research  Information  Center.   Cincinnati
      45268.   EPA-625/6-78-004.  June 1978.
                             11-147

-------
161
162
163
164
165,


166,

167
169.


170.


171.


172.



173.



174.


175.
USEPA.   Scrubber Handbook.
Carolina  27711
                      NTIS:
             	   Research Triangle Park, North
             PB 213-016.   1972.
Calvert,  S.    "How to  Choose  a  Particulate  Scrubber."
Chemical  Engineering.  84:18.  August 29, 1977.

Calvert,  S.   "Upgrading Existing  Particulate  Scrubbers."
Chemical  Engineering.  84:23.  October 24, 1977.
Farrell,
Pollution
Report.
J.B.,  H.O.  Wall,  and B.A.  Kerdolff.   "Air
from  Sewage Sludge  Incinerators:   A  Progress
  Presented at  the  U.S./Japan  Conference"^
Cincinnati.   October  30, 1978.

Gilbert,  W.   "Troubleshooting Wet  Scrubbers."    Chemical
Engineering.   84:23.  October  24, 1977.

Classman,  I.   C^mb^u_sj^iori.  Academic Press.  1977.

Jacknow,  J.   "Environmental  Aspects  of  Municipal  Sludge
Incineration."    Presented  at the  Fifth  Conference  on
Acceptable  Sludge  Disposal  Techniques.    Orlando  Florida.
January IT  to February  T~,1978.   I~n formation Transfer,
Inc.,  Rockville,  Maryland 20852.
168.   Jackn ow
                 Environmental  Impacts from  S 1 u d g e
      Incineration-Present State  of the  Art.
      Furnace  Technology Committee.  1976 .
                                             WWEMA Sludge
Kirchner,  R.W.   "Corrosion of Pollution Control Equipment."
Chemical Engineering  Progress.  71:3.  March 1975.

Marchello,  J.M., and J.J.  Kelly.  Gas  Cleaning for  Air
Quality Corvtrol.   Marcel Dekker, Inc.  1975.
Parker,  Albert.    	
McGraw-Hill Inc.   197~8~7
          Industrial  Air  Pollution  Handbook.
Semrau,  K.T
Scrubbers."
1977.
     "Practical Process
   Chemical Engineering.
Design of  Particulate
 84:20.   September 26,
Shen,   T.T.
Incineration."
Division-ASCE.
     "Air Pollutants  from Sewage  Sludge
        Journal of the Environmental
     105:1.  February 1979.
Stern,  A.   Air Pollution.   Vol.  Ill,  3rd Ed.  Academic
Press.  1977.

Sugiki, A.   "Survey of  Economical and Technical Performance
for  Emission  Control  Equipment  Installed  with Sludge
Incinerators . "    Presented  at  the  Fifth  U.S./Japan
Conference  on  Sewage Treatment Technology.  1977.
                             11-148

-------
              .  D.G.  Jones,  L.P.  Papay,  S.  Calvert,  and  S.  Yung.
      "Factors  Influencing Plume Opacity."  E nv ironmenta1 Sci e n ce
               ology.  Vol.  10.  1976.      ~~~~——
176.   Weir,  A.,
      "Factors  ^.^^.^
      and Technology
177.   USEPA.   Afterburner Systems Study.  Office of Air Programs.
      Research Triangle Park, North Carolina  27711.   EPA-R2-72-
      062.   1972.

178.   USEPA.   Air Pollution Aspects  of Sludge  Incineration.
      Technology  Transfer.    Cincinnati,  Ohio  45268.   EPA  625/
      4-75-009.  June  1975.

179.   USEPA.    Air  Pollution:  Control Techniques for Hydrocarbon
      and Organic Solvent  Emission  from Stationary Sources.
      Office  of Air  and Waste  Management.   Research Triangle
      Park,  North  Carolina 27711.   NTIS:   PB-240  577.   October
      1973.

180.   USEPA.    Air Pollution, Control Techniques for Particulate
      Air Pollutants.   Office  of Air and  Waste Management.
      Research Triangle  Park,  North Carolina  27711.   NTIS:
      PB-240-573.   1973.

181.   USEPA.   Capital and  Operating  Costs  of  Selected  Air
      Pollution Control Systems.  Office of  Air  and  Waste
      Management  and  Office  of  Air Quality  Planning  and
      Standards.   Research Triangle Park, North Carolina  27711.
      EPA-450/3-76-014.  1976.

182.   Gabler,  R.C.,  and D.L.  Neyland.    "Incinerated  Municipal
      Sewage  Sludge   as  a  Secondary Source  for Metals  and
      Phosphorus."    Proceedings  of the  National  Conference  on
      Sludge Management, Disposal and Utilization.    Information
      Transfer, Inc.,  Rockville, Maryland 20852.  1977.

183.   California  Administrative  Code:   Title  23;  Chapter  3,
      State  Water  Resources  Control  Board:   Subchapter 15,  Waste,
      Disposal to  Land.

184.   USEPA.   The  Sanitary  Landfilling of  Sludge and/or  Ash.
      Presented at  the USEPA  Technology Transfer Sludge Treatment
      and Disposal Seminar.   Boston,  Massachusetts.   September
      1977.

185.   USEPA.    Process Design Manual, Municipal Sludge Landfills.
      Technology  Transfer.    Cincinnati, Ohio  45268.   EPA-625/1-
      78-010,  SW-705.   October 1978.

186.   Reinhardt,  J.J., and D.F.  Kolberg.    Pulp  and  Paper  Mill
      Sludge Disposal  Practices  in Wisconsin.   April 5, 1978.
                             11-149

-------
EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
               Chapter 12.  Composting
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                           CHAPTER  12

                           COMPOSTING


12.1  Introduction

Although sludges  have  been composted  as a  minor  constituent  of
refuse  in  many  countries  since the early  1900s,  only  since  the
early seventies has major  attention been  directed to composting
of municipal wastewater sludges  in  the United States.

A  major study  of  the composting of  wastewater  sludges  was
conducted  at  Salt Lake City from  1967  to  1969 (1).   This  work
was  followed in  1972  by  research  at pilot-scale wastewater
sludge  composting  facilities  at the USDA  Agricultural  Research
Center at Beltsville,  Maryland  (2-4) and full-scale operations at
County  Sanitation  Districts  of   Los  Angeles  County  plant  at
Carson,  California.   Based on the operating experiences  and
developments  at  these plants,   new projects were  undertaken  at
Bangor,  Maine  (5);  Durham,  New  Hampshire (6);  and  Windsor,
Ontario  (7).  A number  of  other plants  are  in  various  phases  of
planning or development.

Sludge  composting  is  the   aerobic  thermophilic  decomposition  of
organic  constituents  to a  relatively  stable humus-like  material
(8).   Environmental  factors  influence the activities of  the
bacteria,  fungi,  and  actinomycetes in  this oxidation decomposi-
tion  process and  affects the  speed  and  course  of composting
cycles.   The  volatility and  type  of material,  moisture  content,
oxygen  concentration,  carbon/nitrogen  ratio,   temperature,  and
pH  are  key  determinants  in  the  process   (9).   Sludge is  not
rendered totally  inert by  composting.   The composting process is
considered  complete when the product can be  stored without giving
rise  to  nuisances such as odors,   and when  pathogenic  organisms
have  been   reduced  to  a  level such  that  the  material can  be
handled with minimum risk.

Compost produced  from municipal wastewater sludges can  provide a
portion  of  the  nutrient requirements  for growth  of  crops.   The
organic matter  in compost  is  particularly  beneficial as a  soil
conditioner, because  it has  been  stabilized,  decomposes  slowly,
and remains  effective  for  a  longer time  than  the  organic matter
in uncomposted wastes.  Composted   sludge can improve the quality
of soils containing excessive  amounts  of  sand  or clay as well  as
already  more balanced soils.   Improved  physical properties
include:'

     •  Increased  water content  for sandy soils
     •  Increased  water retention for sandy  soils


                             12-1

-------
        Enhanced aggregation
        Increased aeration  for  clay soils
        Increased permeability  for clay soils
        Increased water  infiltration for clay soils
        Greater root  depth
        Increased microbial population
        -Decreased surface crusting (10)

The persistence  of  organic chemicals, pathogenic  organisms,  or
heavy metals  in  some composted sludges may restrict  the  use  of
the material  for application  to  crops  for  human consumption
(8,11).   The composting  process  results in a significant nitrogen
reduction within the wastewater sludge and,  therefore, a reduced
amount of nitrogen available to  the soil and plants.

Processes for  composting wastewater  sludge differ from those for
composting  refuse.   There  are  several principal  advantages  of
sludge  composting as  compared  to  refuse  composting.   Sludge
composting does not  require the complex materials management and
separation  techniques  necessary  for  most  refuse  composting
operations.   Municipal wastewater  sludge is  more  uniform  in
composition causing  less  operating  difficulties.   The  final
composted mixture utilizing sludge is more suitable for marketing
because   it  generally  does  not  contain the  plastics,  metal,  and
glass commonly found in refuse  compost.    Sludge  composting  is
often viewed  as  an  alternative  disposal  method and does  not
have to  be  evaluated on profit-making potential  as  some  refuse
composting operations have  been.

Classical and  new solid waste  composting techniques  have  been
modified for sludge  composting.  These can be classified as:

     •  Unconfined processes
         •  Windrow
         •  Aerated  static  pile
                Individual  pile
                Extended pile
     •  Confined processes

Unconfined  processes  are  not enclosed,  although  a roof  may  be
provided  to protect  the compost  from  precipitation.   Unconfined
processes make  use  of  portable  mechanical  equipment such  as
front-end loaders or  mixers  for  compost  mixing and  turning.
Confined systems utilize  a stationary-enclosed container  or
reactor  for composting.


12.2  The Composting  Process

Although  each  composting   technique  is unique,  the   fundamental
process  is similar.   The basic  process  steps are as follows:

     •  If  required,  bulking  agents  for  porosity  and  moisture
        control (for  example, recycled  compost, wood chips, etc.)
        or  feed  amendments for  a  source  of  limiting nutrients
                              12-2

-------
        such as carbon  (for example, sawdust, rice hulls, etc.)
        are added  to  the  dewatered sludge to  provide  a mixture
        suitable for  composting.   ,The  mixture must  be porous,
        structurally stable,  and  capable  of  self-sustaining  the
        decomposition  reaction.

     •  A temperature  in the  range  of 130° to 150°F (55° to 65°C)
        is attained to ensure destruction of pathogenic organisms
        and provide the driving  force for  evaporation,  which
        reduces the moisture  content.

     •  The compost is stored  for extended  periods after  the
        primary composting  operation to  further  stabilize  the
        mixture at  lower temperatures.

     •  Additional  air drying  (for example,  windrowing)  may  be
        required if the cured  compost is too wet for  further
        processing.

     •  When bulking agents are  reused, a separation operation is
        required.

Composting represents1- the  combined activity  of  a  succession  of
mixed  populations  of  bacteria,  actinomycetes,  and  other  fungi
associated with a ^diverse succession of environments.  Moisture,
temperature,  pH,  nutrient concentration,  and  availability  and
concentration of oxygen supply are  principal factors which affect
the biology of  composting  (12).


    12.2.1  Moisture

Decomposition of organic matter is dependent  upon moisture.  The
lowest moisture content at which  bacterial activity  takes place
is from 12 to  15 percent;  however, less than  40 percent moisture
may  limit the  rate  of decomposition.   The  optimum  moisture
content is in  the  range of 50  to  60  percent.   If the mixture is
over 60 percent water,  the  proper structural  integrity will  not
be obtained.

Dewatered municipal  sludges  are usually  too  wet  to  satisfy
optimum  composting  conditions.   The moisture content  can  be
reduced by  blending the sludge  with  a  dry bulking  material or a
recycled product,  and  dewatering the  sludge to  as great an extent
as  economically  possible.    The best approach  for  a particular
site  can  be determined  from a mass balance  of the  particular
composting facility  and  by  a site-specific  economic  analysis
based  on  the mass  balance  results.  Figure  12-1 illustrates  the
effect of  the  solids  content of dewatered sludge on the required
mixing ratio of  wood  chips to  sludge by  volume  for  one compost
operation.  The amount of  wood chips  needed for a  40  percent
filter cake  would  be about one-fifth the amount required  for a
20  percent  solids  cake.   In addition to  savings on  wood chips,
there  would  be a  substantial  reduction  in  material management
costs  and site sizes (13).


                              12-3

-------
  LU
  O

  LU
  O
  Q
  D
  _i
  e/i
  yj
  O
  x
  O
  O
  O
  O
  cc
  O
  z
  X
  5
                                   NOTE: THIS CURVi IS SITE-SPECIFIC FOR
                                        ONE COMPOST OPERATION, THIS
                                        CURVE WILL SHIFT DEPENDING ON
                                        THE RELATIVE VOLATILITY AND
                                        SOLIDS CONTENT OF THE WOOD
                                        CHIPS AND SLUDGE.
                  10         20          30

                        PERCENT SOLIDS IN SLUDGE
                                                   40
50
                             FIGURE 12-1

            EFFECT OF SOLIDS CONTENT ON THE RATIO OF WOOD
                    CHIPS TO SLUDGE BY VOLUME (14)
     12.2.2  Temperature

For most efficient operation,  composting processes depend  on
temperatures of  from 130°  to  150°F (55°  to  65°C) but not  above
176°F  (80°C).   High  temperatures are  also  required  for  the
                      pathogens  in  the  sludge.   Moisture  content,
                      and shape  of  pile,  atmospheric  conditions,
                      the  temperature  distribution in a  compost
                      temperature elevation will be  less  for a
inactivation of human
aeration  rates,  size
and nutrients  affect
pile.   For example,
given quantity of heat released  if  excessive  moisture is  present,
                               12-4

-------
as heat will be carried off  by  evaporation.   On the other hand,
low moisture content will  decrease  the  rate of microbial activity
and thus reduce the rate of heat evolution.
    12.2.3  pH                                           ;

The optimum pH range for growth  of most bacteria is between 6 and
7.5  and  between  5.5 and  8.0  for  f ungi .•( 14) .   The pH  varies
throughout the pile, and throughout  the composting operation, but
it is  essentially  self  regulating.   A high initial pH resulting
from the  use  of  lime for dewatering will solubilize nitrogen in
the  compost  and  contribute to  the  loss   of  nitrogen  by ammonia
volatilization.  It is difficult to alter the pH in the pile for
optimum biological  growth, and  this: has  not  been  found  to be an
effective operation control.


    12.2.4  Nutrient Concentration
                            *'
Both  carbon and  nitrogen are  required  as  energy sources  for
organism  growth.   Thirty parts by  weight  of  carbon  (C)  are
used by microorganisms for each  part of nitrogen (N); a C/N ratio
of 30  is,  therefore,  most desirable for efficient composting,
and  C/N  ratios between  25 and  35  provide  the best conditions.
The  carbon considered  in this ratio is biodegradable  carbon.
Lower C/N  ratios  increase  the loss  of nitrogen by volatilization
as  ammonia, ..and  higher values  lead to  progressively  longer
composting  times  as nitrogen  becomes  growth-rate  limiting (12).
No other macro-nutrients or trace  nutrients have been found to be
rate limiting in composting municipal  wastewater sludge.


    12.2.5  Oxygen Supply

Optimum oxygen  concentrations  in a  composting  mass are between
5  and 15 percent by  volume  (15).   Increasing  the oxygen
concentration beyond  15  percent by  air addition will result in a
temperature decrease  because  of the greater  air flow.  Although
oxygen concentrations  as low  as :0.5  percent  have''teen observed
inside windrows  without anaerobic -symptoms^ at  least 5 percent
oxygen is generally required  for aerobic  conditions  (12).


    12.2.6  Design Criteria and  Procedures

The  basic  criteria  for  successful composting  are that  the
material  to be composted  be  porous  and  'structurally  stable and
contain  sufficient  degradable material   so  that  the degradation
reaction  is  self-sustaining (that is, heat released by oxidation
of  volatile  material  is  sufficient ' to ' raise  the mixture to
reaction  temperature  and to  bring  it to required dryness).   In
this  section,  a  procedure to meet these" criteria  of porosity,
structural  stability,  'and   sufficient   biodegradability  will


                              12-5

-------
be  discussed.    An  equally  important  design  consideration  is
flexibility.    A  compost  operation  must  be  able  to   operate
continuously  even  with  changes  in  sludge  solids  content  and
volume.    Changes  in bulking  agent  supply  and  equipment  failure
must also be  anticipated,  and  the  design must  be  flexible  to  deal
with these changes.

To  obtain minimal  assurance  that  the  composting  activity  is
proceeding  properly,  the   temperature  and  oxygen content within
the  pile  are  constantly monitored.   Equipment  required  to  conduct
this monitoring  includes   a  portable,  0  to  25  percent,  dry-gas
oxygen  analyzer  which   is   used  to  measure  the  oxygen   content;  a
probe-thermistor-type temperature  indicator, with  at  least  a
6-foot  probe and  scale  reading  from  32°  to  212°F (0° to 100°C)
is  also needed.   Additionally,  monitoring of heavy metals, patho-
gens, and environmental parameters such  as air  and water  quality
ensures  a  safe  and  acceptable  compost  and  composting   operation.
A comprehensive  monitoring program is outlined  in  Table  12-1.
                                  TABLE 12-1

                   SUGGESTED MONITORING PROGRAM FOR A
                       MUNICIPAL WASTEWATER SLUDGE
                          COMPOSTING FACILITY (17)

     Activity/time           Component             Analysis              Frequency
 Before composting        Sludqc and bulking       Heavy metals and PCB's      Monthly
                     material

 During composting        Aerated pile or windrows  Acceptable time, temperature, Temperature and oxygen con-
                                       dissolved oxygen relation-   tent measurements taken at
                                       ships, that is, 131°F (55°C)  least 6 days during first
                                       and 5 to 15 percent oxygen   2 weeks. (Additional
                                       content for 3 to 5 days.     measurements sometimes
                                                            required to get true
                                                            average!.

 After composting         Compost (prior to       Certain selected indicator   Monthly or bimonthly depending
                     marketing)            heavy metals and pathogens.   on use of compost.

 Site monitoring during     Personnel             Physical examination prior   Annually-
  entire ope ration                          to employment and periodi-
                                       cally thereafter.
                                      Protective equipment and    Continuously
                                       clothing as needed.

                    Odors               Odor strength           Continuously, but especially
                                                            during wet periods with
                                                            •temperature inversions and
                                                            little to no wind.

                                      Odor filter pile effective-  Continuously
                                       ness .

                                      Log of odor complaints.     Continuously

                    Dust                Assessment of particulate    Continuously but especially
                                       concentrations.           during dry period under
                                                            windy conditions

                    Leachate and runoff      BOD and suspended solids.    Monthly, downwind at locations
                                                            critical to public hea1th
                                                            concerns.
Airborne spores
Mi crometeoro logical


Numbers generated and
transported .
Temperature at 5 ft (1.5m)
and 25 ft (7.6 ml
Wind speed
Wind direction
Monthly
Continuously
Continuously
Cont inuously
                                    12-6

-------
Pour locations  for temperature and  oxygen  measurements at both
ends of each pile are  shown  on Figure 12-2,
                                            _L
                                                y- B
                 JL
                 2
                      BASE "B"
                            FIGURE 12-2

               LOCATIONS FOR TEMPERATURE AND OXYGEN
               MONITORING AT ONE END OF A WINDROW OR
                      INDIVIDUAL AERATED PILE

Haug  and  Haug  (17)  have shown the compost  reaction  is  self-
sustaining  when  the  ratio W  is  <_ 10.   This ratio  is  defined
as:


      _ 	mass of water  in the initial compost mixture	
        mass of organics degraded  under compostingconditions


In  windrow  and  mechanical  composting, porosity  and structural
stability are  provided  when  the  sludge  is mixed  with  recycled
compost product or bulking agent to  obtain a solids concentration
of approximately 40 to 60  percent.   With aerated pile  composting,
a bulking agent  such  as wood  chips  is used to provide porosity
and  structural  stability.    When   the  composting  process  is
complete,  the  bulking  agents  are generally screened  out  of the
compost and  recycled back to  the  mix point for reuse.  The fine
portion of the bulking agent is usually retained with  the compost
product because  it  passes through the  screen  with  the finished
compost.  Fresh  bulking agent  must  be  added at the  mix  point to
compensate for this material loss.

Mixture  degradability can  be  adjusted  by the  addition  of
materials that contain  high concentrations  of  degradable organic
material.  These materials are usually  dry and  reduce  the ratio W
by  increasing  the  volatile  fraction  and decreasing  the  moisture
fraction of  the mixture.
                              12-7

-------
Figure  12-3 shows a generalized mass balance diagram for  the
compost process.   The  recycle stream could  consist  of  finished
compost  only  (typical  for  windrow  and mechanical methods),
bulking  agent  only  (typical  for aerated  pile  methods)  or  a
combination of bulking  agent  and  finished compost.  Amendment  may
also be  added  with bulking agent.  The  exact  quantities of  the
various streams are dependent  on  the mass balance equations  (12-1
and 12-2)  derived  from  Figure 12-3  and  the type  of composting
process utilized.

A  set  of equations  can be developed from  an analysis of  the
mass balance diagram.   Two general  equations have been  arranged
that apply to all composting methods.   Equation 12-1 is used
to  determine  the  recycled compost  or wood  chip quantity  and
Equation 12-2 is used to determine the ratio W  (17):


         XC(SM-SC)  +  XA(SM-SA)  +  xB(sM-sB)
    XR = -   (SR-SM) -- - -                 <12-1)
      =                     + XB(1-SB) + XR(1-SR)
        XCSCVCkc + XASAVAkA  + XBSBVBkB + XRSRVRkR          (12-2)


Compost Processes With  No  External Bulking Agent

To design a compost facility employing no external bulking agent,
the parameters  Xc,So v"okC' SR' vR'kR'  and  SM must  be  determined
analytically,  assumed,  or  calculated.  The wet weight of recycled
compost  (XR)  is  calculated, assuming  no  amendment or  external
bulking  agent  addition (XA=XB=0),  to  provide  a  desired  solids
content of the mixture  (S^)  in the 0.40 to 0.50 range:


         XC(SM-SC)
                                                          (12-3)
           e  q
          (SR-SM)


Once  XR is determined for  these  conditions,  the  ratio W  is
calculated :
        XC(1-SC)  + XR(1-SR)
              ~
If  the  ratio  W  is  less  than  ten,  the  compost  mixture  has
sufficient energy available  for  temperature  elevation  and  water
evaporation.    The  ratio number  of  ten  is not  absolute  because
climatic conditions affect  the thermodynamic energy requirements.
In a  hot,  arid climate, W may be higher  because  evaporation  of
water from the  compost mass is increased  by  a  high humidity


                             12-8

-------
   AMMENDMENT
                     EXTERNAL   !
                     8ULKJNG AGENT
Note:  RECYCLE is defined as  finished  compost  for  the windrow and
       mechanical systems and as  recycled wood  chips for the
       aerated pile system.
       The exact value for these  parameters must be determined
       from samples of the sludge,  external bulking agent,
       amendment, and estimated  for the  recycle values unless
       otherwise known.
  Process Variables  and Range  of  Average Values  (in Parenthesis)
X  = Total wet weight of  sludge
     cake produced/day.

X  = Total wet weight of
     amendment/day.

X  = Total wet weight of
     recycle/day.

X  = Total wet weight of  external
     bulking agent/day.

X  = Total wet weight of  mixture/
 M   day.

S  = Fractional solids content of
 L   sludge cake (0.20 to 0.55).

S  = Fractional solids content of
     amendment (0.50 to 0.95).

S  = Fractional solids content of
     recycle (0.60 to 0.75).

S  = Fractional solids content of
     external bulking agent  (0.50
     to 0.85).

S  = Fractional solids content of
     mixture (0.40 to 0.50).

V  = Volatile solids content  of
     sludge cake,  fraction of
     dry solids (0.40 to  0.60) -
     Digested;  (0.60 to 0.80) -
     Raw.

V  = Volatile solids content  of
     amendment, fraction  of dry
     solids  (0.80  to 0.95).
V  = Volatile solids content of
     recycle, fraction of dry
     solids  (0.00 to 0.90).

V  = Volatile solids content of
     external bulking agent,
     fraction of dry solids
     (0.55 to 0.90).

VM = Volatile solids content of
     mixture, fraction of dry
     solids  (0.40 to 0.80).

k_ = Fraction of sludge cake
     volatile solids degradable
     under composting conditions
     (0.33 to 0.56) .

kft = Fraction of amendment
     volatile solids degradable
     under composting conditions
     (0.40 to 0.60).

k  = Fraction of recycle volatile
     solids degradable under
     composting conditions (0.00
     to 0.20) .

k  = Fraction of external bulking
     agent volatile  solids de-
     gradable under  composting
     conditions (0.00 to 0.40).
k  = Fraction of mixture volatile
     solids degradable under com-
     posting conditions (0.20 to
     0.60) .
                               FIGURE 12-3

           SLUDGE COMPOSTING MASS BALANCE DIAGRAM
                                  12-9

-------
driving force  and  higher  initial pile  temperatures.   In a cold
climate,  more  biological energy  is  required  to heat the pile to
normal operating temperatures  and thus  W may  have  to be as low as
seven to  ten ( 17 ) .

The ratio W  can be  reduced  by adding amendment.  The parameters
SA'^A' an<3  kA  are  known.    The  amendment  dry weight is  assumed,
and a new recycle compost mass (XR)  is  calculated:


         xc (SM-SC)  + XA (SM-SA)
The ratio W is also recalculated:

         xc d-sc)  + XR (i-sR)  + XA

                     XRSRVRkR 4-  XASAVAkA
If  W  is  still not  below  ten,  the  quantity  of amendment is
increased and  XR  and  W are recalculated until the W requirement
is satisfied.

If  these guidelines  are  followed, a  mixture with  sufficient
energy to  compost will be produced.   The  actual  values for the
process  parameters are  site-specific and the most  economical
design is dependent on accurate  information about the composting
characteristics that  affect  the mass and  thermodynamic balance.

Compost Processes _Using__^ternal__B^]LkJLng__Ag_e_n_t

Design criteria  for  processes  using  external  bulking  agent are
similar  to  those  just described except that the recycle rate is
calculated  in  a  different manner.   In  the  former  process, the
ratio  of total  bulking  agent  to  sludge  is  specified  without
regard  to the mixture's moisture  content, since  it  is  not as
important as  the  structural  integrity  of the  pile.   The recycle
rate,  XR,  and  makeup  supply are  calculated using Equations  12-7
and 12-8.
    XR = (l-f2) fiXG                                       (12-7)

    XB = fi XC - XR                                        (12-8)


where  f j_  is  defined  as  the  ratio of  external bulking  agent
(recycle and makeup) to sludge


         XR + XB
    fi = --
                              12-10

-------
and £2  represents  the  fraction of total  external  bulking  agent
lost  from  the process  by  volatilization  or because  it  remains
with the finished  compost.
    f      XB
    f2 =
         XB + XR


The  values for  f^  and £2 must  be assumed  based on operating
experience at an existing facility.   The  range  of values  for  f±
are  0.75 to  1.25,  and for  f2 are  0.20  to 0.40.   Once  these
values  are chosen, the amount of recycled  bulking agent  (XR)
and  new external  bulking agent  ( Xg) can be  calculated  using
Equations 12-7 and  12-8.

The value of  the ratio W is then calculated using Equation 12-2,
indicating no amendment is used (X^  =0).   If  W is less than  or
equal to  ten, then the mixture has sufficient energy to compost.
If W is greater than ten,  two options for  reducing the  ratio are
possible.  More  external bulking agent can be  used  (that  is,  f]_
is increased).   If the bulking agent is  more  volatile  than the
sludge, W  should be reduced.   The  recycle and  makeup quantities
of bulking agent must  be recalculated and W determined again.  If
the bulking agent is of low volatile  fraction, this approach will
not work because W will be reduced  only slightly.  In this case,
amendment must be added.

For  any amount  of  amendment  addition,   the ratio W can  again
be calculated using  Equation 12-2.  Increasing the amount  of
amendment until W is below ten will result in the proper compost
energy balance.

The operation at Bangor,  Maine,  successfully composts  sludge  by
the aerated pile method in winter months.   No amendment is used,
and  the ratio  of  external bulking agent  (bark) to sludge  by
volume  is 2.5:1.   The value for  W ranges from seven  to ten
at this operation (17).

The best  means  to determining the quantities of external bulking
agent and  amendment used  will  be  a careful  economic analysis  of
the  process  and accurate estimation  of   the process variables.
Table 12-2 lists some  of  the  density ranges  for various compost
materials as experienced at various  compost facilities.


12.3  Unconfined Composting Systems

In  the  United   States,  the  windrow and aerated  static  pile
processes have  been   used  almost  exclusively  for composting
dewatered  municipal wastewater sludges.   The basic  steps  to  be
followed  in  these  two  processes  are similar,  but the processing
technology for  the composting stage differs  appreciably.  In
                              12-11

-------
the windrow method, oxygen is drawn  into the pile  by  natural
convection  and  turning,  whereas  in  the static pile  method,
aeration is  induced  by  forced  air  circulation.


                           TABLE 12-2

                  DENSITIES OF VARIOUS COMPOST
                       BULKING AGENTS (13)

                                               Density,
            Material                           Ib/cu yd

       Digested sludge             .        1,500 to 1,700
       Raw sludge                          1,300 to 1,700
       New wood chips                        445 to 560        .  .
       Recycled wood chips                   590 to 620
       Finished compost                      930 to 1,040


       1 Ib/cu yd = 0.595 kg/m3


    12.3.1   Windrow  Process

The windrow process is normally conducted in uncovered areas and
relies on  natural  ventilation  with frequent  mechanical mixing
of  the  piles  to maintain aerobic conditions.    In areas  of
significant rainfall,   it may  be  desirable  for operational
reasons  to provide  a  roofed  structure  to cover the windrows for
composting  sludge.  The  largest operating windrow process in the
United States  is located  at  the  Joint  Water  Pollution  Control
Plant of the County  Sanitation Districts  of Los Angeles County in
Carson,  California.

In the windrow  composting process,  the mixture  to be composted is
stacked  in long parallel rows or windrows.  The cross section of
the windrows may  be trapezoidal or triangular, depending largely
on  the  characteristics  of the mobile equipment used  for mixing
and turning  the piles.   The width  of  a typical  windrow is 15 feet
(4.5 m)  and  the height  is 3 to 7 feet (1  to 2 m).

Based on processing 20  percent  solids  sludge,  land requirements
for the  windrow  process  are  greater  than for  the aerated pile
process.   Colacicco estimates an extra 25 percent land usage for
the windrow  process based on windrows  5 feet  (1.5 m)  high and
7 feet (2 m) wide with  a two-week composting period (18).   Even
more  land  would  be necessary  for the  longer composting time
experienced  in  the Los  Angeles operations.

The mixing  of  a bulking  agent  with  the wet sludge cake has
enabled  the windrow process  to  be  used  for  composting  digested
dewatered  sludge.   Bulking  agents may include the recycled
composted sludge  itself  or external agents  such  as  wood chips,


                              12-12

-------
sawdust, straw, rice hulls,  or  licorice  root.   The quantity of
bulking agent  is adjusted to obtain a mixture  solids content of
40 to 50 percent.   The  use of a bulking  agent  also  increases the
structural  integrity of  the mixture and thus,  its  ability to
maintain  a properly  shaped windrow.    Porosity of the mixed
material is greatly  improved, which in turn improves the  aeration
characteristics.   External  bulking agents can  also provide a
source  of  carbon for  the composting  process.  The carbon to
nitrogen (C/N)  ratio of digested activated sludge  is in the range
of 9 to 15:1.   If wood chips are  used  as the bulking agent, the
C/N  ratio  will be  raised to approximately 20 to 30:1 in the
composting  mixture.

Convective  air  movement within windrows is  essential  for
providing  oxygen for the  microorganisms.   The aerobic  reaction
provides heat  for warming  the  windrows.  This  causes  the air
to rise, producing  a  natural chimney effect.   The  rate of air
exchange  can  be  regulated  by  controlling  the porosity  and
size  of the windrow  (2).   The  turning of  the windrow  also
introduces  oxygen to the microorganisms.   This method of  aeration
can  be  expensive if   used excessively  to obtain high oxygen
concentrations  and may reduce the  temperature within the  windrow.

As a result of the biological decay process,  temperatures in the
central portion of  the  windrow reach as  high  as 150°F  (65°C).
Operating  temperatures of  about 140°F (60°C)  may be maintained in
the  central portion of  the windrow for as long as ten days.
Temperatures in the  outer layers are considerably cooler  and may
approach atmospheric conditions.   During  wet periods and winter
conditions,  maximum  temperatures may only be 130° to 140°F  (55°
to 60°C).   A high temperature maintained  throughout the  pile for
a  sufficient  period  of   time  is important to the control of
pathogens  (see  Chapter 7).   A  satisfactory  degree of stabiliza-
tion is indicated by a decline  in temperature, usually  to about
113° to 122°F (45° to 50°C).  These variations in  temperature are
illustrated in  Figure 12-4.

Large-scale,  270 dry  tons per  day (243 t/day) processing of
digested primary sludge  (23 percent solids)  using  the   windrow
process, with recycled composted sludge  as the bulking agent, has
proven a viable method of  sludge stabilization by  the Los Angeles
County Sanitation Districts.  Successful operation of the windrow
process using bulking  agents such as wood chips and sawdust  with
digested primary and secondary sludge has also been achieved at
Beltsville.   This process has not proven  suitable for composting
unstabilized primary or secondary sludges.   At  Beltsville during
early tests with windrows, undigested primary and  waste-activated
sludges  were  found to  produce  offensive  odors  (3).    Also,
composting  of  digested sludge did not kill  all seeds, and these
were present in the  final  product.

The  Los Angeles  .County Sanitation Districts  are  currently
composting  digested,  centrifuged  primary  sludge  (23  percent
solids)  in  windrows  mixed  with  recycled  composted  sludge


                             12-13

-------
(60 percent  solids) in  a 1:2.2  ratio  (dry weight).   A compost
mixing machine is used  to turn the mixture.  Recycled compost is
added  to the sludge  before  the windrow  is  constructed.   Each
windrow  must  be  turned  two  or three times a day  for  the  first
five  days  to mix  the  material  completely, minimize  odors,  and
ensure  sufficient  oxygen  transfer.   The  sludge is  then turned
once  a  day for about  30 days, depending  on  weather conditions.
Figure 12-5 shows a windrow being turned at Los Angeles.
 o
 o
 EC
 D

 <
 tr
 LU
 O.
 S
 LU
71.1

60.0


48.9


37.9


26.7


15,5


 4,5


-7.8
                                   I
I
                                   o  TEMPERATURE AT 18-INCH
                                     DEPTH IN WINDROWS

                                   D  AMBIENT ATMOSPHERIC
                                     TEMPERATURES
                      D
160


140


120
     LL
     O
100   Ly
     QC
                  80


                  60


                  40


                  20


                  0
     EC
     LU
     CL
     ^
     LU
         0   20   40  60   80  100  120  140  160  180  200  220
         1  in = 2.54 cm
                               DAYS
                            FIGURE 12-4
             TEMPERATURE PROFILE OF A TYPICAL COMPOST
                           WINDROW (12)

Large, portable,  heavy materials  handling  equipment  is required
for the windrow  system.   The  Los Angeles operation requires four
windrow mixing-turning machines  capable of  turning  3,400  tons
per hour  (3,084  t/hr) of a  density of 1,890  pounds  per  cubic
yard  (1,120 kg/m^).   This is  equivalent  to  a volume  capacity
of 3,600  cubic  yards  per hour  (2,752 m3/hr).  Three  machines
operate  continuously  for two  shifts a day.   A fourth machine
is required to  provide  backup whenever any  of  the  others  is
being repaired.   In case of  rain  all  four  machines must operate
continuously.
                              12-14

-------
                           FIGURE 12-5

          TURNING A WINDROW AT LOS ANGELES COMPOST SITE


Sawdust, shredded paper,  and wood chips were  the external bulking
agents  used in  the Beltsville  windrow  tests.   Only  shredded
paper was  found  to be unsatisfactory (2).   The  windrow area at
Beltsville  was  paved  with  18  inches (0.46  m) of crushed stone
to support  heavy  equipment  and  the windrow composter.   The area
was  later  paved  with  asphalt  and then with  concrete  to assure
positive leachate  collection  and  to  eliminate rock  pickup from
the  collection equipment  and  damage  to  the  screening equipment.
To start  the windrow,  a  layer of  wood chips 15  inches  (0.38 m)
deep  and  15 feet  (4.5 m)  wide  was placed  on the paved  area.
Sludge  (20  to  25  percent  solids)  was distributed to  the chips
at a 1:3 volume ratio.   The compost  machine  then mixed  the
sludge  and  chips.  After several turnings, the two materials
were  thoroughly  mixed.   The  windrow was  turned five  times a
week, flattened  after  two weeks  to a 12-inch (0.30  m)  layer and
harrowed for further drying,  generally to  greater  than 65 percent
solids.   The material was then removed from  the windrow area and
stockpiled for  an additional 30 days  for curing purposes.  Curing
was  required to  improve  compost quality  and to  further control
pathogens.   After  curing,  the  composted  mixture  was distributed
to local government agencies as  screened  or  unscreened material.
Wood  chips separated during  the screening  operation were recycled
and  reused  as  bulking agent.    The  use of a bulking agent  may
substantially increase the cost  of the composting process unless
the bulking agent is itself  a waste material  (7).   At Beltsville,
                              12-15

-------
a fresh  supply of wood  chips was required  to make up  for  the
estimated 25 to 30 percent lost in the composting process.   Some
of  the  bulking agent  was  consumed in  the  biological  oxidation
processes during composting,  and a large portion was lost in  the
screening process.


        12.3.1.1  Energy  Requirements

Thermodynamic considerations in the composting of sludge  are
discussed in a recent article by Haug  &  Haug  (17).   As  indicated
previously,  the reaction is  self-sustaining when the ratio W is
less  than ten.  Over  80 percent  of  the heat  released by  the
biological reaction  is  used to evaporate moisture associated with
the sludge.

In  the  windrow process,  the  only external  energy  requirements
are gasoline  for  transportation,  diesel  fuel  for  operation  of
composting machines, and electricity  for  leachate  treatment  and
site  services,  including  lighting.   In the Beltsville  windrow
tests, which  used wood chips as a bulking agent,  the  following
energy consumption figures have  been estimated (18).


                     Operating Requirements
              per  dry ton per  day  (0.9 t/day)  for a
       10 to 50 dry  ton per day  (9  to  45 t/day)  operation

    Labor           1.8 to 3.0 hours
    Gasoline        1.1 gallons  (4.5 1)
    Diesel Fuel     3.3 to 4.0 gallons (13.5 to 16.5 1)
    Electricity     3.0 to 8.0 kWhr (12 to 32 MJ)


Where  finished  compost  is   used as the  bulking  agent,   and
increased windrow  turning frequency is practiced, a higher diesel
fuel consumption should be expected.


        12.3.1.2  Public  Health  and Environmental Impacts

Numerous  studies  have  indicated  that a  community's  wastewater
contains organisms  which reflect the  local prevalent  endemic
diseases (19).   The  pathogens  borne  by  wastewater  are  not
entirely  inactivated  during   conventional  sludge digestion  and
drying  techniques  and may  persist  in  the  soil  for  extended
periods  of time.    Figure  12-6  shows  this  time-temperature-
destruction relationship  of pathogens  for windrows (20,21).

Intensive studies conducted  by the Los Angeles  County Sanitation
Districts indicate that total coliform and Salmonella concentra-
tions are rapidly reduced in  the first ten days of  composting in
the interior  of  windrows.   For interior  samples,  final  compost
coliform concentrations  of   less  than  one per gram have been
                              12-16

-------
attained,  but  higher  values  for  exterior  samples  have  been

measured  consistently.    Very low levels  of  virus, parasitic ova,

and Salmonella  have been assayed in  the majority of final compost

samples.
    8.0
 a,

i$~
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UJ
o
z

o
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UU

b
    6.0 -
    4,0 ~
    2.0 -
                                      SALMONELLA
   -2,0
I
3 10
1
20
1
30
1
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                                                              70
60
50
    o
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-------
Recycling large quantities  of finished compost as bulking agent
provides  good odor  control  for digested sludges, as  long  as
process upsets are kept under control.   Interruption of regular
turning  of  the sludge may cause  odor problems,  since  compost
windrows  quickly become anaerobic under these  circumstances.
Unpleasant odors  may  also  be generated  during  periods  of  high
rainfall,  as  well as by poor mixture control  and  inefficient
mixing.  In dry and windy  areas, wetting  of  the  compost windrows
should be practiced to prevent excessive dust generation.

A drainage  and collection  system  is  required  for stormwater
runoff  from the site because the  contaminated  water  requires
treatment.    The  runoff  may  be   returned   to  the wastewater
treatment plant.  At  Beltsville,  a wooded area adjacent to the
site was spray irrigated (2).

Workers at a  compost site should avoid inhaling dust.   Respira-
tory  protection,  such  as  breathing  masks, should be  worn  in
dusty areas, and  the area should be sprinkled with water during
dry  periods.    Although  recent  experiments  have  shown  high
concentrations of the fungus Aspergillus  fumigatus,  a secondary
pathogen, to be airborne at  sludge  composting sites, preliminary
data  indicate  that  these higher  spore  levels are generally
restricted to  the  immediate composting  area and should not pose a
significant health threat to surrounding residential, commercial,
or industrial  areas  (22).  However,  individuals with a history of
lung ailments  should  not work in composting operations.  Research
is continuing on potential  health effects  of  exposure to the
fungus A. fumigatus  (23 to 27).  For  additional  discussion, see
Chapter"?.


        12.3.1.3  Design Example

This  design example  illustrates  the procedure  for  a  10 MGD
(0.45  m^/s) municipal  wastewater  secondary treatment  plant.
The dewatered, digested  primary  and secondary sludge (20 percent
solids)  is generated  at  the rate  of  one dry  ton per  million
gallons  (.00024 t/m3).  The compost  facility  will handle ten
dry tons per day  (9  t/day)  at 20 percent  solids, seven  days per
week.  The values  for the process design variables are similar to
those  reported for Beltsville.   The  availability  and  cost  of
amendments  and suitable land for  the operation  will  strongly
influence  the economic analysis  of the  project.   This design
example, however,  does  not  consider these site-specific economic
parameters.

The design  of  this  windrow  composting  facility is based on the
following assumptions:

     •  The water  content and total weight of the compost mixture
        will  be reduced  by approximately  40  to  50  percent
        and volatile solids content  will be reduced  by about
        20 to  40  percent.   The density  will decrease  by 15  to
        25 percent because of evaporation.


                             12-18

-------
     •  The values  for  the process variables  defined  previously
        are assumed to be as follows:
        Sc = 0.20
        VC = 0.50
        kc = 0.45
             SR = 0.70
             VR = 0.35
             kR = 0.15
           SA = 0.90
           VA = 0.90
           kA = 0.50
SM = 0.40
VM = 0.50
     •  If the  mixture  has a  high  ratio of water  to degradable
        organics by weight  (W  ratio  greater than ten),  amendment
        will be added to reduce W.
The amount of  finished compost to be recycled  can  be calculated
using Equation 12-3.
    XR =
 Xc (SM - S
  (SR - sMT
50 (0.04 - 0.20)
 (0.70 - 0.40)
       = 33.3 tons per day (30.3 t/day)
This indicates  that  if a mixture moisture  content  of  40 percent
is to  be  obtained,  0.67 tons (.67 t/t) of  finished compost must
be added to each ton  (0.9  tonne)  of  sludge  cake to be  composted.

The ratio W is  checked  using  Equation  12-4  in order to determine
whether to compost.
    W =
XC(1-SC) + XR(1-SR)

Xcscvckc + XRSRVRkR
                   50(1-0.20) + 33.3(1-0.70)
        50(0.20)(0.50)(0.45) + 33.3 (0.70)(0.35)(0.15)
      = 14.4
The calculated value for W is too high, indicating that amendment
addition  is  required.    Increasing  the recycle rate  to  create a
mixture of  50  percent  solids (XR =  50 tons per  day  [45 t/day])
would only  lower W  to  13.5,  because  the proportion of degradable
organics does not increase significantly in the mixture.

Assuming  that  1.0  ton  (0.9  t)  amendment per  ten tons  (9  t)  of
sludge  cake  are added  to the mixture,  the recycle  rate  can  be
calculated using Equation 12-5:
                              12-19

-------
    XR =
                     xA(sM-sA)
                (SR-SM)
         50 (0.40 - 0.20)  + 5 (0.40 -  0.90)
                   (0.70 - 0.40)
       = 25.0 tons per day (22.7 t/day)
The amount of recycled compost has dropped  from  0.67  tons per ton
(0.61 t/t)  to 0.5 tons  per  ton  (0.5 t/t)  of  sludge cake.   The
ratio W is calculated using Equation 12-6:
    W =
        XC
XR (1
XRSRVRkR
                                XA  (1
                               XASAVAkA
        	50(1-0.20) + 25(1-0.70) + 5(1-0.90)	
        50(0.20)(0.50)(0.45) + 25(0.70)(0.35)(0.15) + 5(0.90)(0.90)(0.50)
      = 9.2
This mixture of sludge  cake, recycled compost,  and  amendment
is  self-sustaining  and  will degrade  properly.   Figure  12-7
                 process  and shows  the materials  balance.
illustrates this
A 7-foot  (2  m)  high,  65-foot  (20  m)  long,  windrow  with  a base
of 15  feet (4.6 m)  is constructed each day.  Longer  windrows
can be made if the windrow is extended each  day with the mixture
to be  composted.   The final volume of composting  at the  end of
six weeks  of turning  is approximately 65 percent of  the original
volume.    In  continuous  operation  there  would  be   about  11
windrows, 250-feet (76 m)  long.

Each windrow must  be turned  at least two  times  per day for the
first  five days to  mix  the materials completely,  to  minimize
odors,  and to  insure sufficient  oxygen  transfer.  After the
initial  five-day period,  the  windrows  must be turned frequently
enough to maintain the proper  oxygen level  and  temperature in the
composting material.   This is  dependent on  weather  conditions.

Other site operations must include  a mixing  area, maintenance and
operations building,  a  curing area to  stockpile  the  finished
compost,  and  enough  land area  for  handling  all other  site
operations and  for future  expansion.

Equipment required for  the  operation  includes  a windrow turning
machine;  a front-end  loader  for  site  preparation, dismantling


                              12-20

-------
piles  and  loading  transfer trucks;  and  transfer  trucks  to  haul
the  sludge  and  amendment to the compost  facility  and to  haul  the
finished compost  away.
   DIGESTED
  DEW ATERED
   SLUDGE
    MIXING
                                      OFF-fiASES
 WINDROW
COMPOSTING

 42 DAYS
 RETENTION
  DRYJNG
(IF REQUIRED)

  6 DAYS
 RETENTION
    6
  AMENDMENT
 COMPOST
CURING AND
 STORAGE
 60 DAYS
 CAPACITY
                   7 DAY PER WEEK OPERATION
      LOCATION

         1
         2
         3
         4
         5
         6
         7
WET
TONS
50
5
80
41
39
25
14
PERCENT
SOLIDS
20
90
40
70
70
70
DRY
TONS
10.0
4.5
32.0
5.0
27.0
17.5
9.5
DENSITY
(Ib/cu yd)
1,600
1,000
1,300
1,000
1,000
1,000
VOLUME
(cu yd)
63
10
123
78
50
28
PERCENT
VOLATILE
SOLIDS
50
90
50
35
35
35
                                  1 ton = 0.907 tonne
                                  1 Ib/cu yd = 0.6 kg/m3
                                  1 cu yd = 0.76 m3
                               FIGURE 12-7

              PROCESS FLOW DIAGRAM - WINDROW COMPOSTING
                SLUDGE - 10 MGD ACTIVATED SLUDGE PLANT
                                  12-21

-------
Optimum windrow compost design will do the  following:

     9  Minimize hauling and handling cost.

     •  Maximize  use of  existing equipment in  the  compost
        operation.

     •  Minimize the  use  of amendment  which adds to the  cost  and
        is not recoverable.

     •  Maximize  the solids  content  of the  dewatered  digested
        sludge  cake  to minimize  the amount  of recycled  compost
        used  for  moisture control and also reduce the amount  of
        amendment  required.   The cost  of  dewatering  should  not
        exceed the savings  at the compost facility.


     12.3.2  Aerated Static Pile  Process

An aerated static pile system was developed in  order to eliminate
many of the  land  requirements and other problems associated with
the  windrow  composting  process   and to  allow  composting of  raw
sludge.   This  system consists of  the following steps:  mixing  of
sludge with a bulking agent; construction of  the composting  pile;
composting;   screening of  the composted  mixture; curing;  and
storage.   A  diagram of an  aerated  pile  for composting sludge  is
shown in  Figure 12-8.
         AIR
SCREENED Oft-	
UNSCREENED
COMPOST
         SLUDGE AND
         BULKING
         AGENT
                 PERFORATED
                 PIPE
                           DRAIN FOR
                           CONOEN5ATES
                                        — EXHAUST FAN
                                                         FILTER PILE
                            FIGURE 12-8

             CONFIGURATION OF INDIVIDUAL AERATED PILES
                               12-22

-------
 The  forced  air method  provides for  more  flexible operation  and
 more  precise  control of  oxygen and  temperature  conditions  in
 the  pile  than  would  be obtained  with a  windrow system.   Since
 composting  times  tend  to  be  slightly  shorter  and  anaerobic
 conditions  can be  more  readily prevented,  the  risk of  odors  is
 reduced.

 Two  distinct  aerated  static  pile  methods  have  been  developed,
 the individual  aerated pile  and  the  extended aerated  pile.


         12.3.2.1   Individual  Aerated  Piles

 An individual  aerated pile may be  constructed in  a  manner similar
 to  the Beltsville  method,  in which  loop of perforated  plastic
 pipe,  4  to 6  inches  (10 to  15  cm)  in diameter is placed  on  the
 composting  pad,  oriented longitudinally,  and  centered  under  the
 ridge  of  the  pile  under construction.   In  order to avoid  short
 circuiting  of  air, the  perforated  pipe terminated at least 8  to
 10 feet (2  to  3 m)  inside  the ends  of  the  pile.   A  non-perforated
 pipe  that extends  beyond  the  pile  base  is  used to connect  the
 loop of perforated  pipe  to the blower.   (See Figure 12-9).
                  AIR
SCREENED OR
UNSCH
COVER
        BULKING ACjENT
        AND SLUDGE  '   /
            BULKING
            AGENT BASE
                      NON PiHFOflATiD FIFE
                                              L- F(LTiR PILE
                                                SCREENED
                                                COMPOST
                             FIGURE 12-9

           AERATION PIPE SET-UP FOR INDIVIDUAL AERATED PILE
 A  6-  to 8-inch  (15 to  20  cm)  layer  of  bulking agent  is  placed
 over  both  the pipes  and the  area to be  covered  by the pile.
 This  base  facilitates the movement and even  the distribution  of
 air  during composting  and  absorbs excessive  moisture that  may
 otherwise  condense  and drain from  the  pile  (19).
                                12-23

-------
At Beltsville a mixer or  front-end loader  is used to mix  one
volume  of sludge  cake containing 22  percent  solids  and  two
volumes  of  bulking  agent.    The  resulting  mixture  contains
40 percent solids and  is placed  loosely upon the prepared  base by
the  front-end loader to  form  a  pile with  a  triangular  cross
section 15 feet (4.6 m) wide  by  7.5  feet (2.3 m) high.

The  pile is  then  completely  covered  with a  12-inch  (0.3  m)
layer of  cured,  screened compost  or an  18-inch  (0.4  m) layer of
unscreened  compost.   This  outer blanket of  compost provides
insulation  and  prevents  escape  of  odors during composting.
Unstabilized sludge can generate odors during dumping and  initial
pile construction.   Conditioning with lime during dewatering will
minimize  this, however.   The non-perforated pipe is connected to
a 1/3  horsepower  (0.25 kW),  335 cubic  feet  per minute  (158 1/s)
blower that  is  controlled  by a timer  (28).   Aerobic composting
conditions are maintained  if air  is intermittently drawn  through
the pile.  The timing  sequence for the blower is 5 minutes  on and
15 minutes off  for a  56-foot (17  m) long pile  containing  up to
80 wet tons  (73  t)  of sludge.   If the aeration rate is  too high
or the blower  remains on  too long,  the pile will  cool, and  the
thermophilic process will be  inhibited  (12).

The effluent air from  the compost  pile is conducted into  a  small,
cone-shaped filter pile of cured,  screened compost approximately
4  feet  (1.2  m)  high and  8 feet  (2.5  m)  in  diameter  where
malodorous gases  are  absorbed.    The odor  retention  capacity of
these  piles  is  inhibited  if their  moisture content is  greater
than 50  percent.   The odor  filter pile  should  contain one cubic
yard (0.76 m^)  of screened compost for each four dry tons  (3.6 t)
of  sludge in  the  compost  pile.    Filter piles  are sometimes
constructed with  a 4-inch  (10  cm)  base  layer  of  wood chips to
prevent high back pressures on the blower.

Land area requirements are estimated at one acre  per 3 to 5  dry
tons (1.0 ha/6.7 to 11.2  t)  of sludge  treated.   The lower  figure
includes  space  for runoff collection,  administration,  parking,
and  general  storage.   The actual composting area  (mixing area,
aerated  piles,  screening  area,  drying  area,  and  storage  area)
is estimated  to  be one acre per  5  dry tons (1.0  ha/11.2  t)  of
sludge (19) .


        12.3.2.2  Extended Aerated Piles

To  make  more  effective use  of  available  space,  another  static
pile  configuration called  the  extended  aerated pile has been
developed.   An  initial pile is  constructed  with a triangular
cross  section  utilizing  one  day's  sludge production.    Only
one  side and the  ends of  this  pile are  blanketed with  cured,
screened  compost.   The remaining  side  is  dusted with only about
an  inch  (0.5  cm)  of   compost  for overnight odor  control.   The
next day, additional  aeration pipe is  placed on the pad  parallel
to the dusted side of  the  initial  pile.  The pile bed is  extended
                              12-24

-------
by covering the  additional pipe  with more  bulking agent  and
sludge-bulking agent  mixture  so  as  to  form a  continuous  or
extended  pile.   This  process  is  repeated daily for 28 days.
The first section  is  removed after  21 days.  After seven sections
are removed in sequence, there is sufficient space for operating
the  equipment  so that,  a  new  extended  pile can  be started.
Figure 12-10  shows  such a  system.   The area  requirement of  an
extended  pile system  is about  50 percent less  than that  for
individual piles.   The amount of recycled bulking  agent required
for covering the pile and bulking agent used in the construction
of the base is also reduced by about 50 percent.   At Beltsville,
research  into  extending  aerated  piles  in both the  vertical  and
horizontal directions is  ongoing.
                                        EXJUPC.25T
                                        HtHPVJn
                                        MERE
                                                " MSXTUfll ~U
                           FIGURE 12-10

              CONFIGURATION OF EXTENDED AERATED PILE



        12.3.2.3  Current Status

The  aerated pile system has proven effective on a full-scale
basis  at  Beltsville,  Maryland;  Bangor,   Maine;  Durham,
New  Hampshire;  Detroit,  Michigan;  and Windsor,  Ontario.   After
start-up, mean  temperatures  in aerated piles  are  176°F (70°C);
and  after stable conditions  are  achieved,  minimum temperatures
are  usually   130°F  (55°C).   When  the piles are constructed
properly, neither excessive rainfall nor low ambient temperature
adversely affect the composting process  (28).

Currently most  of  the  interest  in composting of wastewater
sludges is centered  on this technique.  The applicability of this
system for the treatment of undigested  sludges  provides  it with a
significant advantage over the windrow method.   Other advantages
are  superior  odor  control,  greater inactivation  of  pathogenic
organisms, and use of less site area.   The aerated pile  technique
exposes  all  sludge  to more uniform  temperature.   Capital  costs
are  also  lower  for  the aerated pile system,  but operating  costs
tend  to be higher  because of the cost of  the bulking agent.
Comparisons of  capital  and operating costs using  wood  chips  as
bulking agent in  aerated piles, as well as in windrows,  are made
by Colacicco (18).   In experiments at  Los  Angeles County,  it
has  been found  necessary to follow this  technique by windrow
                              12-25

-------
composting for  2  to 3 days  to  dry off the moisture.   At  other
locations, the  air flow  is  reversed  without disruption of  the
pile as another means  to  reducing moisture content.


        12.3.2.4  Oxygen  Supply

Centrifugal fans  efficiently  provide  the necessary  pressure  to
move air through the compost  and odor filter piles.  Variation in
the blower pressure is a necessity for optimum  conditions  and  a
site-specific  operating  parameter.   The  oxygen  concentration  in
the pile should be maintained between  5 and  15 percent;  this  can
be  achieved with  an  aeration rate  of  about 500  cubic  feet  per
hour per  ton  (15.6 m^/hr/t)  dry sludge.   If the pile  cools  at
this air  rate,  the air  flow  must  be   reduced.   Aeration  cycles
of  20  to  30  minutes  with  the  fan  operating 1/10 to  1/2 of  the
cycle  have proven satisfactory  (19).   While  the  fan is  not
operating,  the natural  convective chimney effect,  typical  of
windrows,  takes place.   In the  absence of forced aeration,  this
effect   causes  warming of  the outer edges,  destroying pathogens
more effectively.

Moist  air drawn  through  the  pile condenses  in the  slightly
cooler  sections.    When  enough  condensate   accumulates,   it
will drain from the  pile  and  leach material  from the sludge.
Condensed moisture which collects in  the aeration pipes  is
removed by a  water trap.   This material must  be collected  and
treated along with the  contaminated  rainfall  runoff  from  the
site,  because it   can become  a source of  odors if  allowed  to
accumulate in  puddles  around  the piles.  Data is not available on
combined  leachate  and  condensate water characteristics;  the
quantity may,  however, vary from 6 to  20  gallons  per  day  (22  to
75  I/day)   per  pile containing  50  cubic yards (38 m3)  of  sludge
during  dry weather  (29).   (Refer to Chapter  16  for further
information. )


        12.3.2.5  Bulking Agent

While bulking  agents  are in  the aerated  pile composting system,
they serve primarily  to  maintain  the  structural  integrity  and
porosity  of the pile.   The  quantity  of  external  bulking  agent
required  is determined   by the  need  for  structural  support  and
porosity.   The requirements for  moisture  control are not  as
critical  as adequate  porosity;  thus,   sludge  moisture  can  vary
considerably  as long as sufficient bulking  agent is  added  to
assure  adequate porosity.  The  design  factors discussed  for
windrows do not apply  here  (17).

Wood chips and  other  bulking agents also increase  the  volatile
solids   content of  the  composting mixture; volatility  of new  and
recycled  wood chips  has  been  reported as  90  and 86  percent,
respectively  (18).   The  actual  contribution  of  the wood  chips  to
the compost mixture is limited  because  their  composting rate  is
slower.
                             12-26

-------
 hen wood  chips are  mixed  with unstabilized  sludge  an average
volatility of about 75 percent  results; this is well in excess of
the  40  to  50  percent  volatility  achieved  in  the mixture  of
digested  sludge and  recycled  compost.   Volatility content  is
therefore  not  a limiting factor  in aerated  pile  composting  of
unstabilized sludge, as it can be in the digested sludge windrow
system.
        12.3.2.6  Energy Requirements
Energy costs for  aerated  pile  composting  are  a small portion of
the overall operating  costs.  The  bulk  of  the overall  energy
requirement of the process is provided by the volatile solids in
the composting  mixture.   A range of resources for labor, external
bulking agent, gasoline  for  small vehicles,  diesel  fuel  for the
front-end  loaders, and  electricity usage  for leachate treatment
is listed below (18).
                     Operating  Requirements
              per dry ton  per day  (0.9 t/day) for a
             10 to 50 dry  ton per  day  (9 to 45 t/day;
             	operation  (20  percent sludge)	
    Labor
    Wood Chips
    Gasoline
    Diesel Fuel
    Electricity
1.5 to 2.8  hours
2 to 8 cubic  yards  (2.1 m3)
1.1 gallon  (4.1  1)
2.7 to 3.5  gallon  (10.2 to  13.2 1)
7.5 to 17.5 kWhr  (29.7 to 69.3 MJ)
        12.3.2.7  Public Health  and  Environmental Impacts

Extensive studies have been made on the destruction of pathogens
in  aerated  piles (30).   Although  Salmonella,  fecal  coliforms,
and  total  coliforms  initially  increased  in numbers,  they  were
reduced  to  essentially  undetectable  levels  by the  tenth  day  of
composting.   Studies  using  "F" bacteriophage and virus  as  an
indicator  showed that  the  virus was  essentially destroyed  by
the  thirteenth  day.   However,  survival  of  the virus  did  occur
for  some time  in the  blanket-compost interface where  lower
temperatures prevailed.   Storage in a curing  pile  for  30  days
will complete the  destruction  of viruses or  reduce  the  numbers
to  an  extremely low level  (19).  Studies  have shown that  the
composting  process in  an  aerated  pile  is essentially unaffected
by  low ambient  temperatures or rainfall, which makes this system
particularly well  suited  to operation  under  difficult  climatic
conditions  (31).   Figure  12-11  shows the  time-temperature-
destruction relationship of pathogens for aerated piles (20).

Odor control  is the  primary environmental  consideration  in  the
operation  of an  aerated pile  composting  system.   Good odor
control  results  from  prompt  mixing  of sludge  and  bulking  agent
                              12-27

-------
and  formation  of the  aerated  pile.   In  addition,  lumps  of
material or  puddles  of liquid must  not  be allowed to  remain  in
the mixing  area.   No  thin  spots or  holes  should be present  in
the compost blanket.   There should be leakproof transport  of
aeration  air between blower  and odor  filter pile.   Moisture
content within  odor  filter piles  (Figure  12-12)  should be  kept
below  50  percent.   Condensate,  leachates,  and  runoff  from  the
piles must be collected and treated  as quickly as possible.   The
compost should  be  adequately cured before  it  is removed from the
area, and any unstabilized material  should  be  recycled  back  into
the composting process for further treatment.
             E
             E
             J3>
             LU
             o
                4
             cc
             to
                0
                            TOTAL COL1 FORMS
                                              80
                                              60
                                              40
                     u
                     o
                     UJ
                     
-------
                           FIGURE 12-12

                  ODOR FILTER PILES AT BELTSVILLE
        12.3.2.8  Design  Example

This  design  example  is  based  on a  Beltsville-type  sludge
composting  system  utilizing  existing  technology and  available
design criteria.   The  example p r^o vided  is  specific to  a
10  million gallon per  day
secondary treatment plant.
(0.45 m-^/s)  municipal  wastewater
The weight and volume  of  sludge and -bulking  agent at various
points in  the  process must  be  known  so that the volumetric flow
capacity of a  composting  facility  can  be  determined.   The basic
design decisions  include  the  bulking agent to  sludge  ratio  and
the ratio of new to recycled bulking  agent.

The materials  balance in  this  example  is  based  on the following
assumptions:

     «  Sludge to  be  composted  is  50 wet  tons  per  day (45 t/d)
        of  undigested  sludge,  seven  days  per  week, with  no
        digestion.

     •  Wood chips  are  added  to the wet  sludge at the  rate  of
        2  cubic  yards  of chips  per  cubic yard  (2.0/m3)  of  wet
        sludge.

     •  Three-fourths  of the chips  are  recovered by screening  and
        reuse.
                              12-29

-------
     •  The water content  and total  weight of  the compost
        mixture  is  reduced  by  approximately  30 to 40 percent
        and volatile  solids  content  is  reduced  by about  10  to
        15 percent.    The  density  decreases  15 to 20 percent
        because of evaporation.

     •  The extended  aerated  pile system  will be  used.

Information on the bulk density  of sludge  is surprisingly scarce.
Tests  conducted  at  Beltsville  for an  engineering study  of a
large-scale  composting facility  provide  some  basic data  on
the bulk  density of  sludge  and wood chip bulking  agents.   The
following bulk densities are  used in  this  design  example (20):


                                           Bulk  Density	
                                        Pounds per
    Constituent                         cubic yard
    Dewatered Sludge                      1,600         960
    (20% solids)

    New Wood Chips                          500         300

    Recycled Wood Chips                     600         360

    Screened Compost                        865         519

    Unscreened Compost                    1,000         600


It is  also  assumed  that the process variables have the following
values :


     Sc=0.20          SB =0.70          SR=0.70
     VC=0.75          VB=0.90          VR=0.80
     kc = 0.45          kB =  0.10          kR  =  0.10


Sludge  composting will  operate  5 days  per week, 8 hours per day
using  the extended  aerated static pile method.  The volume to be
composted per work day is  as  follows:


    50 wet tons   7 week-days  =                 k d   (63   t/work d  }
      week day   5 work-days              ^        J              J


It is  assumed  that the dewatered  sludge  arrives  on-site 5 days
per week from the dewatering  operation  which runs  only  5  days per
week.   Equalization  storage  to cover weekend  operation of the
plant  is  provided for sludge in the  liquid state upstream from
the dewatering process.


                              12-30

-------
The amount of recycled and new wood chips can be calculated using
Equations 12-7 and 12-8 and assuming f^=0.75 and f2=0.25;


    XR = (1-0.25)(0.75)70 = 39.4 tons per day (35.7 t/day).


    XB = (0.75)70 - 39.4 = 13.1 tons per day (11.9 t/day).


The ratio W can be calculated using Equation 12-2:
        	70(1-0.2) + 39.4(1-0.7)  + 13.1(1-0.7)	
        70(0.2)(0.75)(0.45) + 39.4(0.7)(0.9)(0.1) + 13.1(0.7)(0.8)(0.1)
      = 9.0
Since W is less than 10, no amendment addition  is required.

The daily  volume  of  the compost material is calculated using  the
assumed values previously stated:
                             Mass                 Volume
  Constituent              tons/day           cubic yards/day

Dewatered sludge             70                     87.5
New wood chips               13.1                   52.4
Recycled wood chips          39.4                  131.3

Total                       122.5                  271.2
                           (111.1 t/day)          (206.8 mVday)


The  pile  will be  8  feet  (2.4  m)  high and  50  feet  (15 m)  long.
Each  day,  the  pile will  be extended 18.5  feet (5.6  m) .   The
amount of new wood chips required to  construct  a  one-foot  (0.3 m)
thick pad for the compost  is as follows:


     (50 ft)(18 5)(1 ft) =  34i3  cubic    ds        m3)/day
      27 cu ft/cu yd                  J                  J


Unscreened compost is  required  each  day to cover  the pile.
This layer  will be 18 inches (0.46 m)  thick:
                                                 ,39
                               12-31

-------
Figure  12-13 is  the process  flow diagram for the  extended  aerated
pile  compost   facility  and  summarizes  the  design  materials
balance.
NiW WOOD
CHIPS


WOOD CHIP
PAO
                      5 DAY PER WEEK OPERATION

                                           PERCENT
                  WST  PERCENT  DRY DENSITY VOLUME  VOLATILE
           LOCATION  TONS  SOLIDS  TONS (Ib/cu yd)  (cu yd)  SOLIDS
                  70
                  13.1
                  39.4
                  122.5
                   8.6
                  58.7
                  18.6
                  90
                  32
20
70
70
41
70

65
65
60
14
 9.2
27.6
50.8
 6
10.3
12
58.5
18.9
1,600
 500
 600
 900
 500

 725
 725
 975
 87.5
 52.4
131.3
271.2
 34.3

 51.4
248.3
 64.6
75
90
30
80
90

65
65
45
1 too - 0.907 toon*
1 Ib/cu yd - 0.6 KB/HI*
1 cu yd - 0.76 m5
                              FIGURE 12-13

              PROCESS FLOW DIAGRAM FOR THE EXTENDED PILE
              COMPOST  SLUDGE FACILITY -  10 MGD  (0.44m3/s)
                        ACTIVATED SLUDGE PLANT
Approximately  250  feet
perforated aeration pipe,
three  4-inch  (10-cm)   tee
with  weather protection
      (76  m)  of   4-inch   (10-cm)  diameter
      50 feet  (15  m) of  non-perforated pipe,
      connectors,   and  one  blower/timer unit
     and  condensate  collection system are.
required for each  daily pile.   Only one  blower rated at 335  cubic
                                 12-32

-------
feet per minute (158 1/s)  will  be  used  to draw air into the pile.
In general, the blower should be rated at a minimum of 150 cubic
feet  per  hour per  wet  ton (1.3 1/s/t)  of  sludge in  the  daily
pile.  Non-perforated pipe should  be  used to connect the aeration
pipe loop to the blower.   The exhausted air will be filtered in a
pile of screened compost.   The filter pile will contain at least
one  cubic yard of  material per  30  wet tons  (1  m3/35.5  t)  of
sludge  in  the daily  pile or  4  cubic  yards  (3  m3)  for  this
design.   Figure  12-14 illustrates  this design  example.   The
minimal area requirements  for various composting site components
is as follows:
              MINIMAL COMPOSTING  AREA REQUIREMENTS
                 50 wet tons  per  day  (45 t/day)
                 10 dry tons  per  day  (9 t/day)
                                                 Area Required
                                               Square
                                                feet

                                                5,000
    Function

Truck unloading and  mixing

Composting
  (28 days)(50)(18.5)(1.15  excess)              30,000

Unscreened compost                             10,000

Drying and screening                           20,000

Compost curing and storage
  (60 day)(200 cu yd/day)(27  wet  tons)
  (10 ft deep) + excess                         33,000

New wood chip storage
  (60 day)(87 cu yd/day)(27 wet tons)
  (12 ft deep) + excess                         15,000

    Subtotal                                  113,000

Maintenance building,  operations
  building and laboratory,  Lunch
  room and locker room                          4,000

Employee and visitor parking                     5,000

Miscellaneous storage                            1,000

    Subtotal                                   10,000

    Total                                     123,000
Square
meters

   465
                                                           2,792

                                                             931

                                                           1,862



                                                           3,071



                                                           1,396

                                                          10,517
NOTE:  123,000  square  feet (11,447  m2)  =  3  acres  (1.14
       Land Utilization = 6.6  dry tons per acre  (14.8  t/ha).
                                                            ha)
                              12-33

-------
- PAD AND PIPE BEFORE ADDITION
, Of 11*000 CHIPS AND COMPOST
 Pt-PfGPATED-
 *tHA
 PIPt
 1 in - 2.54 cm

                           FIGURE 12-T4

                 DESIGN EXAMPLE EXTENDED AERATED
                        PILE CONSTRUCTION

The  overall  space required  is about 3  acres  (1.2 ha)  which  is
0.15 acres per  ton per day (0.07 ha/t/day) of  dry sludge solids
composted.   Reducing  the bulking  agent  would decrease  the  area
required.

Although porosity is the key factor for the aerated pile, control
of moisture is  important  for a  successful  sludge composting
system.   The sludge should be  dewatered  or mixed with sufficient
bulking  agent to  obtain enough porosity  in  the composting piles
for  optimum  composting conditions.   For  optimum  composting  the
composted mixture  should  have  a solids  content  of not  less  than
40 percent or more than 50 percent.  Figure 12-15 shows a compost
pile as it is being taken down.

Approximately 8.5  cubic feet per minute  (4 1/s/t)  of air per  ton
of dry  sludge  solids  in  the  pile  is required.   At Beltsville,
this was delivered by  a  centrifugal  fan operating  at  5 inches
differential water  pressure  (1.25  kN/m2)  (18).   The  Bangor,
Maine  system uses  a  1/3 horsepower (0.25 Kw)  blower  rated  at
335  cubic  feet  per minute  (158  1/s)  at  5  inches  water pressure
(1.25  kN/m2)   for each pile consisting  of  50 cubic yards
(38  m3)  sludge  and 150 cubic  yards (114  m3)  bulking  agent  (7).

The  blowers  are operated  intermittently to maintain  the oxygen
level  in the 5 to  15  percent range  and to obtain  as  uniform a
temperature as possible.
                              12-34

-------
                           FIGURE 12-15

                  COMPOST PILES BEING TAKEN DOWN
For large composting  systems,  a  permanent  central  blower system
may be  considered.   A header  pipe  could be  utilized  to provide
the necessary suction for each  pile.  Only  one  or  two  large
blowers located  in  a covered  area  would be  required.   Although
capital  cost would  be high  because of the  needed piping  and
control  devices, the  operation and maintenance costs  of  many
individual blowers  would  be  eliminated.   On the other  hand,  a
central blower system  is  not  especially flexible.    Since  it is
important to maintain the proper aeration rates in  each pile, an
air flow  metering  device will  be  required  for each pile.   A
decision  for or against  a  permanent  system would be  based on
economic analysis and  the need
changing composting  conditions.
for system flexibility to handle
The composting area should  be  paved.  Probably the most efficient
design in a permanent facility involves the use of fixed aeration
and drainage  systems.   The aeration piping  and  drainage system
could  be placed in trenches in the  composting pad and the blowers
placed in permanent protected structures and equipped with water
traps  and controls.   The  disadvantages  of  this  type of combined
system are  the  high  initial  cost  and  the reduced flexibility of
operation.   Possible elimination  of  the one-foot  (0.3  m)  wood
chip pad  and  the  disposable plastic pipe  processed  through  the
screens  is a  potential advantage of  fixed trenches  for  the
                              12-35

-------
aeration pipes.   Special  precautions  would  be necessary to keep
the centralized aeration  piping  and  pile  drainage trenches from
clogging and to provide for  condensate water drainage.

Odor filter  piles  should  be replaced periodically.   The filter
piles  are  replaced  every  other month at  Bangor; during  cool
weather the system has  operated without  significant odor problems
and with no filter piles.  At  Beltsville,  the  odor  filter pile is
replaced each time the  compost pile is dismantled.

After  the  piles are  formed, they should be  covered with a layer
of compost  or  wood  chips  for  insulation and to prevent the dust
which  is caused by excessive drying of  the outer pile edges from
blowing.

Most composting facilities  use a base  layer of bulking agent or
unscreened   compost  to  cover the  aeration piping.   However,  the
piles  are  now  constructed at  Bangor  with  no special  base layer;
the  sludge-bulking  agent  mixture  is  placed  directly on  the
aeration piping.

Rotary or  vibrating  screens are  commonly used  to separate wood
chips  for  reuse.   Compost containing  wood chips with a moisture
content  of greater  than  40 to  50 percent  can be difficult to
screen; the  operation  is  therefore not  conducted  on  rainy days.
Allowance  should  be  made  for drying  the compost  if  the solids
content  is  less  than  50 percent,  and  the  screens  should be
sized  to handle  a large  volume  of compost  during fair weather.
Figure 12-16 is a  photograph of the finished screened compost.


    12.3.3   Case  Studies (Unconfined  Systems)

The  four  case  studies  chosen involve  Los Angeles County
Sanitation   District,  California; Beltsville,  Maryland;  Bangor,
Maine; and  Durham, New  Hampshire.  The Los Angeles  system handles
80  to 120  dry tons per day  (73 to  109  t/day).  Beltsville
composts approximately  14 dry tons per  day (12.6  t/day), Bangor
about  2  dry tons  per  day  (1.8  t/day),  and Durham around  3  dry
tons per day (2.7  t/day).


        12.3.3.1   Joint Water  Pollution  Control  Plant,
                  Carson,  California

A  large-scale  windrow  composting system was established in 1974
at  the Joint  Water Pollution  Control Plant.  This  operation
is  currently  composting  400 to 600 wet tons per day  (364 to
545  t/day)  of  anaerobically-digested,  polymer-conditioned,
centrifugally dewatered, primary sludge with  a  25  percent solids
content  (19).   The  sludge  is transported to  the  nearby compost
site in  fifteen ton  (13.5 t) sludge hauling trucks equipped with
end  discharge and  conveyor bottom  trailers, provided  to make
windrow  construction relatively  easy.    Approximately  15  cubic
                              12-36

-------
yards  (11  m3)  of finished  compost  are  added to  the  truck  along
with  the  dewatered  sludge.   The wet  and  the dry  materials  are
initially  mixed  in  the truck.   Complete mixing  is subsequently
provided by a compost  turning machine  in  the windrow.   Given  the
current consistency of the sludge and the type of equipment  used,
the  windrows  that can  be constructed are  about 3 feet  (0.9  m)
high and 10 feet  (3.0  m)  wide.   Typically,  each  windrow is  about
500  feet  (451  m) long and  is  constructed with  eight to  ten
truckloads of material.  The windrows  are  placed  on  sixteen  to
eighteen foot (14.6  to 16.5 m) centers,  leaving  a clear aisle  for
the wheels of the turning machine.
                            FIGURE 12-16

                     FINISHED SCREENED COMPOST


When  a  windrow is  first  placed,  it  is  turned twice  to  mix  the
wet  cake with  the dry  compost.   Thereafter,  each windrow is
turned once  per day to  maintain sufficient voids for the natural
passage  of  air and to  promote  drying.   The  process  can produce
objectionable odors,  particularly  in  the early part of the cycle
and is likely to generate dust under moderately windy conditions.

In addition  to  being  equipped with conveyor bottom trailers,  the
composting trucks have also been modified with extended sidewalls
                              12-37

-------
to increase their capacity,  and sealed bottoms,  so that  they  may
be used to haul wet  cake  on  public  roads.   At a production rate
of 500 tons per  day  (450  t/day)  of wet  cake, about  25  hours  of
truck time  are  required each  day  to  construct  windrows.  Four
turning machines, each  with  a rated  capacity of  3,400  tons  per
hour (3,084 t/hour),  are  available  for  the  operation.   They  are
relatively high  maintenance  items,  and  generally,  only two  or
three are  operated.   With the current  sludge production and  a
composting time of three  weeks,  about ten hours of  machine time
are required  to turn  all the  windrows  each day (32).

In addition,  two  loaders are  used for  loading dry sludge  into  the
trucks;  one  crawler  tractor   is  used  for pushing windrows into
stockpiles; one grader   is  used for road  maintenance  and  cleaning
between the windrows; and  a  water truck  is  used to  control dust
on the plant  roads.

The composting operation takes place  over a  10-hour  day, 7 days
per  week  and employs approximately  twenty  operators  and
mechanics, excluding  the sludge haulers.

Kellogg Supply Company  currently uses earth  movers  to  transfer
composted, dried  sludge  to a  neighboring site.  Kellogg  has been
highly successful in distributing and selling the compost as  an
organic soil  conditioner.

The  composting operation of  the  County Sanitation Districts
of  Los  Angeles  County provides a good demonstration of   the
feasibility  of  sewage sludge  composting   on  a large scale.
Figure 12-17  illustrates the  process flow for this  operation.


        12.3.3.2   Beltsville,  Maryland

Many methods  and concepts   have  been developed  at Beltsville
through the  integration of   experimental research and  practical
operation.   The first  method attempted at  Beltsville was  the
windrow process.  The  windrows  performed  well  when digested
sludge  was  used,  but odors  developed  when  unstabi1ized,
dewatered/combined  primary and secondary sludge  was  composted  by
this method.   The  individual aerated  pile method was  developed
by the Beltsville researchers to eliminate the  odor problems
associated with the windrow process.

The  research  programs  demonstrated  that  either  digested  or
undigested  sludge  can  be   composted  in   the  aerated  pile.
Destruction  of pathogens was much  greater  with aerated pile
composting than  with windrow composting (33,34).    The  extended
aerated pile method was also  developed at Beltsville to  minimize
composting area requirements.

The  extended  aerated  static   pile process  is currently used  in
continuous, 5  day  per   week   operation to compost  60 to 120  wet
tons  per   day  (54 to   109 t/day) of dewatered,  unstabilized


                              12-38

-------
sludge  (approximately  20 to  22  percent solids).   The sludge  is
conditioned with  lime  and ferric  chloride,  dewatered and  loaded
into  tractor-trailer  dump trucks  at the  treatment plant  during
                                                  3  t) and  has  a
                                                  of  sludge to  be
                                                   sludge  to  the
                                                  is delivered  at
the  night.   Each  truck holds 20  wet  tons (1!
watertight rear door.   Depending  on the quantity
composted, three  to  six trucks  transport the
compost site  in  the  morning.  All  of  the sludge
once, which facilitates pile construction.
DRY
SLUDGE
LOADING
STATION


LANDFILL
STORAGE
AREA


KELLOGG
SUPPLY
COMPANY
                                       OFF-GASES
     LOCATION

       1
       2
       3
       4
       5
       6

WET TONS
PER DAY
1,600
2,960
1,140
1,820
1,360
460

PERCENT
SOLIDS
23
40
—
60
60
60

DRY TONS
PER DAY
368
1,184
92
1,092
816
276

DENSITY
(Ib/cu yd)
1,890
1,510
_
1,215
1,215
1,215

VOLUME
(cu yd)
1,690
3,930
—
3,000
2,240
757
PERCENT
VOLATILE
SOLIDS
55
50
—
40
40
40
                                                 1 ton = 0.907 tonne
                                                 1 Ib/cu yd = 0.6 Kg/m3
                                                 1 cu yd = 0.76 m3
                            FIGURE 12-17

            COMPOSTING/DRYING SYSTEM - COUNTY SANITARY
                    DISTRICTS - LOS ANGELES (18)
The extended  pile is constructed on a concrete  pad  approximately
100 feet  (30 m) wide  and  about  400  feet  (122  m)  long.   The  sludge
is dumped onto  the wood chips and mixed  by  a  front-end  loader,  in
a 2.5:1 chip to sludge volumetric ratio.
                               12-39

-------
The composting area for each daily mixture is prepared by laying
out aeration piping on  the  concrete  composting  pad and covering
it with a 12-inch  (0.3  m)  layer  of wood chips using a front-end
loader.  The compost mixture  is then  placed on the wood chip base
using a front-end  loader.   The  mixture is piled  to  a height of
8 feet  (2.5 m), and  the  top and  ends  are  then capped  with an
18-inch (0.5 m)  layer  of unscreened finished compost or a 12-inch
(0.3 m) layer of screened, finished compost.   At the end of each
day's  operation,  the  side  of  the  pile (which  will have  new
material added to  it  the  next day)  is  covered with a thin layer
of compost.   A  pile containing 60  wet  tons  (54  t) of sludge and
wood chips is approximately 8 feet (2.5 m) high, 12 feet (3.6 m)
wide and 75  feet (23 m)  long.

To  insure  proper  aeration,  a  1/3 horsepower (0.25  kW)  blower,
rated  at 335  cubic  feet  per  minute  (158  1/s)  at 5  inches
differential water pressure (1.2 kN/m^) is connected  to  the
piping.   The exhausted air  is  filtered through a  5  cubic yard
(3.8 m^)  filter  pile  of screened  compost  for deodorization.
The blower's is  operation  is  controlled by  a timer.  Currently,
blowers  are operated  for 8  minutes out of  every 20 minutes.

At  Beltsville,  one  blower  is  used  to aerate  120 wet  tons
(109 wet  t)  of  primary undigested  sludge  mixed  with wood chips.
One blower has proven  sufficient  for  two piles when the sludge is
brought to the site at  a rate of 60  wet tons  per day  (54 t/day).
Thus,  only  approximately 10  blowers and  odor  filter  piles  are
required (excluding spare equipment)  to  operate  a  21-day extended
aerated pile facility.

Composted material is removed  from the piles after 21  days.
The compost  pile is dismantled by  a front-end  loader and moved to
the curing  stockpile.   The  compost stays  in  the curing pile for
at least  30  days and  is not mixed before screening and off-site
use.   A mobile  rotary drum screen separates  the cured material,
which  must  be at  least 60  percent  solids  to screen  properly.
Finished, cured  compost that is  too wet  to  screen is placed in
windrows  and  turned as  frequently as  possible  for 2  to 3 days
until it is  sufficiently dry.

Leachate, condensate,   and  stormwater runoff  are  collected  in a
holding pond  at  the far end of the  compost  facility.   When the
level of  the pond  rises to a maximum allowable height, the water
is  pumped  to a  forested  site and sprayed on the forest floor.
Test wells  at  the  compost site  and at the land application site
have indicated  no  groundwater contamination with the use of this
system.

Additional   research   is  being  conducted  at  Beltsville  on  a
modification of  the aerated, extended  pile  process,  called the
extended  high pile method.   Since land area  requirements can be
reduced  by  increasing  pile  height,  one pile in  the  shape  of a
pyramid has been  constructed to  a  height  of  18  feet  (5.5 m).
Aeration pipes  are installed  at three elevations  in the pile—at
the base, at  6  feet (2 m) ,  and  at 12 feet (4 m) above the base.


                              12-40

-------
Those at the base and  at  12  feet  (4 m) levels operate at negative
pressure while  the  pipe  at  the  6-foot  (2 ra)  level  operates  at
positive pressure.   Tests are presently underway to determine the
maximum  height  at  which piles  can  be  built  and effectively
aerated with pipes  placed only at the  base (35).

The Beltsville staff  consists of eight  full-time people,  two
administrative personnel  and six  operators,  excluding the sludge
transfer truck drivers.  This number  is more than actually needed
for  normal  operations.  The additional  personnel  are  used  for
special  operations and  to  support  the research demonstration
program.   Each member of the operation  staff is qualified  on
each  piece  of equipment and the staff is  able to perform  all
preventive maintenance and  much  of  the  repair work.   A list  of
equipment is  shown  on Table  12-3.    All  equipment  has enclosed
operator cabs so that  dust and moisture do not interfere with the
equipment operators.
                            TABLE 12-3

                     BELTSVILLE EQUIPMENT (15)

    3  Terex rubber-tired  front  loaders, 4.5 cubic yards
    5  Dumr> trucks,  20  ton
    1  Rotary drum screen  with power  unit
    1  Sweco screen
    1  Fixed Toledo truck  scale
    1  Mobile office
    1  Storage building
    1  Covered building -  compost  test,  concrete floor with
         aeration pipe  in  floor
    1  Portable oxygen  analyzer  and temperature indicator and
         probe
    1 cu yd = 0.76 m
    1 ton =   0.907 t


Some  of  the finished  compost  is used by  the USDA at  its
agricultural research  center  for  other test programs.   Most of
the  compost is provided free of charge  to local public  works
departments who pick up the material  at  the Beltsville site.

The  approximate  material  quantities  used in  the  Beltsville
operation are based  on the following:   annual  undigested sludge
cake  (with  a solids  concentration  of  approximately  23  percent)
input  of  15,000  wet  tons  (13,605't);  ratio of  2.5:1 wood chip
bulking  agent  to   sludge cake  by  volume;  and  5/8-inch  (1.5  cm)
screening of  all  compost for wood  chip  recovery and  recycle of
75 to 80 percent;  the wood  chip  loss/attrition rate at Beltsville
is  currently  about  41  percent  (36).   In this  example,  the
materials loss through composting  and curing is estimated.


                              12-41

-------
The building used for test purposes at Beltsville has a concrete
floor  with aeration  piping  built in.   Channels  6 inches  by
6  inches (15  cm  x  15  cm)  are recessed  in the  floor and  the
aeration piping is placed into them.   The  channels run the  width
of the  building  and  are spaced  6 feet (2 m)  apart along  the
length  of  the building.  One  end of the  piping is connected
to a  header  system and  the  other is  closed  off.   One  large
blower draws air  through  the  header system.   Limited tests have
suggested  this arrangement will  be  proven  successful,  and  a
refined  version of  this system is being constructed  at  Durham,
New Hampshire.   The  Beltsville  structure is approximately 80 feet
(24 m) wide and 240  feet (73 m) long.  Composting is conducted in
an 80  by 200  foot  (24  by 61  m)  area and  the  remainder  is  an
enclosed and heated  equipment  storage and maintenance area.

The estimated and  projected  costs  for this  extended aerated
pile  operation are listed  in Table 12-4.   The cost for  early
composting  operations  included  extensive testing, monitoring,  and
optimization.   The  cost per  ton for this operation would  be
reduced  if  a  facility were  designed  to  operate  continuously  and
to use the  process as  it has been  optimized at Beltsville.


          12.3.3.3  Bangor,  Maine

In August 1975, composting operations  began in Bangor to dispose
of the  sludge  generated by the wastewater  treatment  plant.   An
average wastewater flow of 7 MGD  (307  1/s)  receives only primary
treatment.    The  plant  produces 2,500 wet  tons  per  year  (2,268
t/year) of  lime conditioned vacuum-filtered sludge  cake with  an
average  solids content  of  20  percent  (5).  The composting site
selected by the City of Bangor is  located  about  3 miles  (4.8  km)
from the wastewater  treatment  plant.

Initially,  the sludge  was  dumped onto a  bed  of bulking  agent
(wood  bark)  in the  mixing area,  mixed with  a  front-end  loader,
and formed  into a compost pile.   Currently, no  base material is
used;  the sludge bulking agent mixture is  placed directly  on  the
pad and  aeration  pipes.   Generally,  one composting  pile  is
constructed per week  and typically consists  of 40  to  60  cubic
yards   (30  to  46  m^) of  undigested primary  and  secondary  sludge
cake which  is  mixed  in  1:2.5  ratio with about  120  to 180  cubic
yards   (91  to  137  m^)  of  bulking  agent.   Bark  with  a less than
50 percent  moisture  content  is  used as the bulking agent.

The total area required  for  composting 3,000 cubic yards per year
(2,280  m-Vyear)   of  dewatered  sludge at  20  percent solids  is
about  1.7 acres (0.7 ha).  Precipitation,  runoff, and condensate
from the composting  operation  are  channeled into a drainage ditch
leading to  the sanitary  sewer  line  (Figure 12-18).

The base for the compost  pile is prepared  using 7-foot  (2  m)
lengths of perforated schedule 40  steel  pipe,  joined together by
short  pieces of plastic pipe.  The city  found that the  short
                              12-42

-------
lengths  of  steel  pipe  can  be  removed  from  the  pile  without
significant  damage  and  reused many times.  Longer pipes were used
previously but were easily bent  when pulled  from the pile.
                                TABLE 12-4

                    BELTSVILLE ACTUAL AND PROJECTED
                   OPERATING COSTS -  1977 DOLLARS (15)
                                       Estimated  October  1977 to  September 1978
                                                  costs, dollars
Total, excluding off-site

Dry tons sludge/yr (23
  percent solids)

Annual cost,  dollars/dry
  ton sludge  solids
                           Actual
                            1976
On-site operations
  Telephone and travel
  Utilities
  Fuel and oil
  Sludge hauling
  Labor including fringes
  Miscellaneous contract
    services
  Wood chips
  Supplies and materials
  Equipment insurance
  3,971
   426
 13,036
120,000
152,919

112,942C
 73,145
 32,176
  3,955

512,570
  3,450
    149
           15,000 wet
             tons/yr
  1, 300
  2,211
 10,500
132,000
125,750

 27,540
144,000
 22,250
  4, 000

469,551
  3,450
                 136
            18,200 wet
            tons/yra'b
  1,300
  2,211
 10,500

 80,000

 27,540
144,000
 22,250
  4,000

291,801
  4, 200
                                69
            45,500 wet
            tons/yra'b
  1,300
  3,000
 25,000

125,750

 37,000
350,000
 35,000
  4,000

581,050
 10,500
                                              55
 Excluding requirements of research work.
 Assume 50 percent of compost marketed unscreened and 70
 percent recovery of bulking agent after screening finished
 compost.
CIncludes screening performed by outside contract, screening
 now performed on site.
 When this analysis was conducted a wood chip attrition rate of
 20 percent was used. 1979 analysis indicates that an actual value
 of 41 percent should be used for wood chip attrition.  (36)

1 ton = 0.907 t
The  city  has  used  unscreened  compost  as the  bulking  agent  in  a
number of  piles.   This has  dramatically reduced requirements for
new  bulking  material, and  the  city plans further tests.

The  compost  piles   are constructed  as  high   as  the  front-end
loader can reach and capped with  1  to  2 feet  (0.3  to 0.6  m)  of
unscreened  compost.   The  finished  pile is  10  to 12  feet  (3  to
4  m)  high.    Each  pair of  compost  piles  is provided  with  one
mechanical  blower.   Blowers are  operated by  timers.
                                   12-43

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                   DRAINAGE DITCH
                                                 FENCE
                                COMPOST PILES
                                UNSCREENED COMPOST STORAGE
                                 SCREENED COMPOST STORAGE   SCREEN
BARK STORAGE
           ©UTILITY POLE
^- BLOWER HOUSING
WATER TRAP
                                             MANHOLE TO SANITARY SEWER
                            FIGURE 12-18

             COMPOSTING SITE LAYOUT - BANGOR, MAINE (5)
During  cold weather,  all available heat  must be conserved to
bring the  piles  up to temperature.   Recycled unscreened compost
provides  a  warm  bulking agent.    The  interiors  of the  wood
bark  storage  piles  are  also  sources  of warm  materials  for
mixing.   Generally,  if the compost pile mixture can initially be
maintained  at  39°F (4°C), the interior of  the  pile will  warm up
to normal  composting  temperatures  much more  readily  than if the
mixture falls  below  39°F  (4°C).   Warm  exhaust  air recycled from
an older  composting  pile  into the  new pile also  helps  for the
first few  days,  but  recycling should  be  discontinued after this
period  because  it causes  high  moisture  levels  in  the new pile.
Increasing  the  unscreened  compost blanket from  1  to  2  feet
(0.3 to 0.6  m) during  the winter also helped to retain more heat
within  the  composting pile.   The city purchased an air heater to
provide initial heat to the piles.
The piles  are composted  for  at  least 21 days.   Temperature and
oxygen  levels are monitored  every two  to  five  days  during the
compost cycle.   Blower operating  cycle  is  adjusted according to
the performance  of the pile.   The aeration  pipes, blowers, and
moisture traps are checked for freezing during cold weather.

At the  end  of the composting cycle,  the  pile is dismantled, and
another pile  is  usually constructed.   The  material removed from
the pile is either used as  the  bulking agent for the new pile or
transferred to curing.

Unstabilized  sludge  is  not  stored  at  the  compost  site.
Generally, operations  are scheduled so that  sludge is dewatered
and a compost pile is  constructed  once a  week.   The exact day of
pile construction is varied depending on weather  conditions.  The
                              12-44

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city has been able to compost nearly all of  the  sludge  produced,
because it is prepared to construct the compost  pile during good
weather.

A Lindig rotary drum screen is used to separate  compost prior  to
distribution.  The  drum is  presently  fitted  with  a  one-inch
(2.5 cm) mesh  screen.   City personnel are planning to  construct
a 5/8-inch (1.6 cm)  screen  assembly so that  either
can be  produced.   Tests  performed at Bangor
screen is capable  of  handling  about 20  to  25
(15 to 19 mVhr)
put in the screen with a front loader.   One
laborer are required during screening  operations
                                  size
                              indicate
                           cubic yards
 material
 that  the
 per  hour
of feed under the best  conditions.   Compost is
                          loader operator and a
Currently,  operations at  Bangor  are performed by  treatment
plant  personnel  under  the direction of  the treatment plant
superintendent.  There  are no full-time composting  personnel
because of the  cyclical nature of  the  operations.  Approximately
11 man-hours per week  are required  for  a  truck driver  to  deliver
and  unload sludge  at the  site.    Sampling and  monitoring for
temperature and  oxygen content  require  10  man-hours  per week.
Pathogen and heavy metals monitoring  is performed  under  contract
with  the  University  of  Maine. Supervision and  administration
require about 15 man-hours  per week.   Annual equipment and labor
requirements are  shown in  Table  12-5.   The equipment used for
composting operations  is shown in  Table 12-6.  This  equipment  is
provided by the  city  motor  pool  and is available  for  composting
when needed.
                            TABLE 12-5

               ESTIMATED ANNUAL LABOR AND EQUIPMENT
                 REQUIREMENTS, BANGOR, MAINE (5)
        Operation
                          Labor,
                          hours
Equipment,
  hours
 Composting
   labor
   front  loader
 Sludge hauling
   labor
   truck
 Monitoring
   labor
   pickup
 Administration
   labor
 Screening  (8,000 cubic yards)
   labor
   screen
   front  loader
 Maintenance
   labor
                            572


                            468


                            520


                            780

                          1,040



                            100
   468
   468
   520
   520
   520
 1 cu yd =  0.76 m~
                              12-45

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                              TABLE 12-6

                         BANGOR EQUIPMENT (5)

         1  Case W24B rubber-tired  front  loader,  4 cubic yard
         1  Rubber-tired front  loader,  1.5 cubic  yard
         1  Truck,  sludge hauling
         1  Mobile  screen,  Lindig
            Small tools,  as required
            Miscellaneous vehicles  as needed  from motor uool


         1 cu.yd =  0.76  m3


Approximate  materials  quantities  for  1976 are  shown  in
Table  12-7.    This is  based  on an  annual  sludge  input  of
3,000 cubic yards (2,280 m3)  and a  mixture of three parts bulking
agent to one part sludge.
                              TABLE 12-7

                  BANCOR MATERIALS REQUIREMENTS FOR
              2,170 WET TON  (1,968t) ANNUAL SLUDGE INPUT (5)


              Limed raw primary sludge,  wet
                tons                              2,170
                  Solids, percent                    23
                  Cubic yards,  cu yd              3,000
                  Density,  Ib/cu yd               1,450
                  Dry tons                          500

              Static pile construction
                Sludge,  cu yd                     3,000
                Bulking agent,  cu yd              9,000
                Pile cover, cu yd                 1,560
              1 ton = 0.907 t 3
              lcuyd=0.76m
              1 Ib/cu yd - 0.6 kg/m
            12.3.3.4  Durham, New Hampshire

Durham,  New Hampshire,  provides  primary  treatment to
approximately  1  MGD (44 1/s) of  wastewater.   About 15 wet  tons
(13.6  t)  of  unstabilzed,  dewatered,  primary sludge (20  percent
solids)  is produced  each week.   The treatment  plant is being
upgraded to secondary  treatment  capability,  and  this is expected
to double the quantity of sludge generated.
                              12-46

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In an effort to cope with current sludge production and to solve
the  problem of  future  sludge  disposal,  Durham investigated
several  sludge utilization and disposal  alternatives.   Land-
filling had  to  be  terminated because  the  landfill  was reaching
its  maximum  capacity  and no  other  suitable land  was  available
within  the  town  limits.   It  was  considered too  costly  and
time consuming  to  obtain additional  land  in  an  adjacent  town.
Incineration was  considered  but  rejected  because  previous
experience with  burning  solid  waste had been unsatisfactory.

A permanent  composting  facility was chosen  (after  an  extensive
pilot-scale  investigation)  as  the sludge  disposal  alternative.
It was  determined that this  facility would best meet the needs of
the community for the following  reasons (31):

     •  Estimated  cost  of  the  compost  facility  was 658,000
        dollars,  of which Durham, by virtue of  state and  Federal
        funding,  would pay approximately 33,000 dollars.

     •  The  compost facility  would  be  an  integral part of  the
        wastewater  treatment  plant,  and  plant personnel  could
        operate  the facility.

     •  Sale of finished compost would help defray the  operating
        costs.

     •  Composting  returns a viable product to the land  at a cost
        competitive with  landfilling and incineration.

The  new  composting  facility  incorporates  many innovations
that reduce  operation  and  maintenance  problems.   It  should  be
recognized that since there are  many  innovations  in this  design
that they are  not a proven technology.    The  composting  and
all  other  outdoor  operations  will take place  on  a concrete  pad
which is easier to clean than a gravel base, prevents rocks from
mixing  with the compost,  and  is a better  year-round working
surface.   The pad  is sloped to  allow  runoff collection from  the
compost piles.   The runoff  is recycled to the treatment plant to
provide protection  for the surrounding  land and streams.  The  pad
is 250  x  152 feet (76  x 46 m) , and is  spacious  enough  for  the
screening operation.

The  aeration pipes are  placed in triangular  troughs 6  inches
(15 cm) deep which  are recessed  below the pad surface and covered
with an aluminum grating, flush with the pad.  Once the aeration
pipe is  in  place,  wood  chips  are used to  fill  up the  remaining
space  in  the trough under  the  grates.   It  is anticipated that
chips directly  under  the grating will  be  changed occasionally,
but  the pipe will  be  used  for an extended  period  of time.   The
sludge-wood  chip mixture will then be placed directly on  the
concrete  pad  over the  grates without  any  wood  chip  base.
Figure 12-19 shows  a cross section of an aeration trough with  the
aeration pipe.


                              12-47

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A  4-foot retaining  wall will  be  built along  the  edge of  the
composting area  of  the pad.   This wall will be  constructed  to
protect  the  blowers which will  be  located on  the  side  away  from
the composting operation and  to provide  a  positive  backstop  for
front loader operations.
   ALUMINUM
    GRATE
                        3 TO 8 ft
                       (0.9 TO 2,4mJ
      CONCRETE PAD
                        WOOD CHIPS

                          FIGURE 12-19
AERATION PIPE
              CROSS SECTION OF AERATION PIPE TRENCH
                   DURHAM COMPOST PAD DESIGN
The sludge  processing  building  of the  new secondary treatment
plant  will  be  placed  adjacent to  the  composting  pad.   Primary
and waste-activated sludges  will be  mixed together prior to
coil vacuum filter dewatering  to provide  for more  consistent
operation.   The mixing  tanks for both primary  and  secondary
sludge  will  be  located  in this  building  along  with  the condi-
tioning  chemicals,  chemical  feed  equipment  and coil filters.  To
provide  flexibility in  operation, the  new plant will  have a
one-week  liquid storage capacity  for  both  activated  sludge
and primary sludge.

After  the sludge is dewatered, a pug  mill will mix it with wood
chips fed  from a hopper.  A  conveyor belt will  transport the
compost  mixture from  the building for pick-up  by  the  loader
and placement  on the pad.  The  mixing operation is  conducted
inside  the  building to  protect  the  operation  from the weather.
Coil filter personnel  will  operate  the mixing  process,  thus
minimizing  personnel requirements.

Screening  will  be executed  using a Lindig  Rotary  Screener
with a material  throughput  capacity  of  280 to  400 cubic yards
per day  (213  to  304   m3/day).    Screening  capacity  exceeds
production  requirements,  so  that  the screen  needs  to be run only
part of  the time.   This  frees  the  screen and loader operators
to undertake other  tasks.

Storage  bins  for  the  composted  material and  chips will  be
placed  directly against the composting pad such that the top of
the bins  are even with  the pad.   There will be four bins with a
                             12-48

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capacity of 1,200  yards  (912  m3)  each.   Three  of the bins will
be used for storing compost and one  bin for the storage of wood
chips.  As  the  composted material is screened on the  composting
pad,  the compost will drop into the bins  for storage  and curing.
A conveyor  will  collect  the  wood chips  and return  them to the
fourth bin  for storage.   The  screen can  be shifted  to link the
compost pile being  dismantled  with a  storage bin  (31).

The storage  bins  will be  used  for curing  the  compost and will
have   sufficient  capacity for  storage  of  all  production during
winter months  when no distribution  is  planned.   The bins will
be unloaded after  the  sludge is  cured  for  about  four weeks.  This
two-yard (1.4 m3)   front-end loader will also build and dismantle
the piles, feed the screen, load  the chip  hopper and  trucks with
the finished product,  keep  the  pad  free of ice and snow, and
provide a  backup  for  the mixing  operation.   A wood   chipper and
a seven-yard  (5  m3)  dump  truck  for  hauling  purposes are  other
equipment to be used.


            12.3.3.5  Cost Analysis

Comparing  the cost of  composting at  different facilities is
extremely difficult because  local factors  such  as  the  weather,
labor, and  equipment  are highly  variable.   Operations in  warm,
dry climates  will require   less bulking agent  and  probably
be more successful  with  the screening process than operations in
cold  winter  areas.    Labor and  bulking agent  costs  are  a  large
portion of  composting  expenditures  and  vary widely according to
geographic area.

A generalized annual  operating  cost  analysis has been performed
for an  extended aerated  pile  system for  an operation  processing
ten dry tons per day  (9  t/day) of sludge  based on the  operations
at the Beltsville  facility (18, 34).  This  analysis is presented
in Table  12-8.  A 10 dry ton  per day  (9 t/day) compost site
should handle the  sludge generated by  a  secondary  treatment  plant
from  a  community of 100,000  people.   The site  is assumed  to be
operating eight hours  per day,  seven  days  per week.

In 1976, when the  original analysis was done, the operating cost
was calculated to  be 40 dollars  per  dry  ton ($44.44/t).   Although
all prices have  increased since then,  the one item which is
significantly more  expensive  is wood  chips.  In the  analysis in
Table  12-8, wood  chip  attrition  had  been estimated at  20  percent.
Analysis done  in  1979  indicates  that 41  percent  is   the actual
value.  Wood  chip  costs  have  increased from the $3.50 per  cubic
yard   ($4.61/m3),  the  value indicated  in  Table 12-8,  to a 1979
value  of  approximately $7.00  per cubic yard ($9.21/m3).  In
addition cost for  transporting  sludge  to the compost side must be
included.

An analysis of the  capital cost  is not presented,  because capital
costs  are site specific.   The  development  cost  of  the  site cannot


                              12-49

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be generalized,  and the  type  of composting  systems used, aerated
pile  or  windrow,  will largely  influence  the capital  cost.   The
replacement  cost of the  equipment  can  be a  large  portion of the
capital  cost.    The largest  capital  cost is usually  the compost
pad.  The capital  cost for all equipment  and structures at Durham
is estimated to be  about $600,000.   Durham's  annualized cost  is
anticipated  to be  $80  per dry ton  ($88/t)  for capital and $60 per
dry ton  ($66/t)  for operating.   This makes  the total annual cost
for  sludge   composting at  Durham  $140.00 per  dry  ton ($154/t).
This  facility  is highly  mechanized  and may  represent  one of the
most  capital-intensive composting  operations.   The  operation  at
Bangor,  however, utilizes a portion of an abondoned taxi way and
uses  the individual  pile composting method.   The  capital  costs
for this facility are  estimated at  about $10 to $15  per dry ton
($11  to  $16/t).

Except for wood  chips  and labor, the  best approach  for estimating
annual   operating  costs  for  design  purposes is  to determine
the  costs  from  a  similar compost  facility  operating in  the
same  geographic  area.    Wood  chip  and  labor  costs  must   be
determined  for  the  specific  site.   Capital  costs are  best
developed  and annualized  for the  specific  site  chosen  for  the
facility.
                             TABLE 12-8
                FACILITY PROCESSING 10 DRY TONS (9 t) OF
                SLUDGE PER DAY a (1976 DOLLARS) (19,31)
                             Dollars/yr
Operations ^
Wood chips at $3.50/cu yd
Plastic pipe
Gasoline
Diesel
Electricity
Equipment maintenance
Equipment insurance
Pad, road maintenance
Water/sewer
Labor
Miscellaneous supplies

35,000
12,200
2,300
5,300
1,500
8,400
1,400
1,200
500
77,500
4,400
 Total
          Dollars/dry ton
                                              9.60
                                              3.34
                                              0.63
                                              1.45
                                              0 .41
149,700
                                               30
                                               30
                                             0.33
                                             0. 14
                                             21.23
                                             1.20

                                             41.01
  Percent of
operating cost
                               23
                                8
                                1
                                4
                                1
                                6
                                6
                                0.
                                0.
                               52
                                3
                                                            100
  Based on the Beltsville operation and assumed to operate
  eight hours per day, seven days per week.

 bln 1979 wood chips cost $6.50/cu yd at Detroit and ?7.92/cu yd
  at Blue Plains.  In addition the wood chip attrition rate went
  from an assumed 20 percent to a confirmed 41 percent. (36)

 1 ton = 0.907 t ,
 1 cu yd = 0.76 m
                               12-50

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12.4  Confined Composting  System

Mechanical   composting  is  accomplished   inside  an  enclosed
container or basin.  Mechanical systems are designed to minimize
odors  and  process time by  controlling  environmental conditions
such as air flow,  temperature,  and  oxygen concentration.


    12.4.1  Description of  Process

The primary  differences  among  mechanical  composting systems are
in  the  methods of process  control.  Some  provide  aeration by
tumbling or  dropping  the  material  from  one floor  to  the next.
Others use devices which stir the composting mass.  Tumbling the
compost in a rotating  cylinder  is another approach.   In addition,
an  endless  belt  is used  to combine  forced  bottom  aeration and
stirring.   Water  is added  to the  composting  mass  at  critical
times to increase  biological activity  in some mechanical systems.
Also,  some  mechanical composters can  introduce  heat  to the
composting mass to keep the  composting reaction continuing at the
optimum rate during cool weather.

The brief  detention  times  which  equipment  manufacturers specify
for mechanical composters do not allow adequate stabilization of
the sludge.   If shorter detention  times  are provided,  a two- to
three-month  maturation  period  will be  necessary  to  reduce the
remaining volatile matter.   Thus,   the amount  of  time  and total
area  required  for mechanical processes   approaches  that for
unconfined  processes.   Mechanical  processes  are  more  capital-
intensive  than unconfined  processes.   Currently only a few
mechanical  composting  processes  are operating  in the  United
States  and these  are  generally  used to compost  a mixture of
refuse and wastewater sludge.  A schematic of a typical confined
composting process is  shown  on  Figure  12-20.


    12.4.2  Metro-Waste Aerobic Thermophilic
            Bio-Reactor

The Metro-Waste process utilizes a  compost chamber and an endless
belt to  achieve adequate  aeration.   The endless  belt  lifts the
composting material to a height of  three  feet (0.9 m), and drops
it  behind  as  it moves  from one end of the  bin to the  other.  A
large fan  introduces  air  into the  mixture  through  a perforated
floor in the  compost chamber.  A  partial  diagram  of this system
is shown on Figure 12-21.

This process,  including environmentally controlled  buildings, is
available in module units of 10 dry tons per day  (9 t/day)  with
retention capacities  of 7  to 21 days (37).


        12.4.3  Dano  Bio-Stabilizer  Plant

Figure  12-22 shows a  typical layout of a  Dano Bio-Stabilizer
plant.   The  process  makes use  of  a  large,   slowly  rotating


                              12-51

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drum,  the  interior of  which
Material  is  injected  into
slowly for one  to  three days,
Aeration  is  acccomplished by
                    is equipped  with
                    one  end of  the
                     and  ejected from
                     tumbling  action.
into  the
oxygen.
vanes or  baffles.
machine,  rotated
the  opposite  end.
  Air is  injected
interior  of the drum to insure  a constant  supply of
 WASTE WATER
    SLUDGE
                         MIXING
   BULKING
    AGENT
                          HEAT
                       {IF REQUIRED)
                                        MECHANICAL
                                        COMPOSTER
                                        {REACTOR)
                                                           AIR
              SCREENING
               CURING
               FINISHED
               PRODUCT
                            FIGURE 12-20

                        TYPICAL PROCESS FLOW
                SCHEMATIC CONFINED COMPOSTING SYSTEM
                               12-52

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The  "maturation"  or "curing"  period  for a  Dano  Bio Stabilizer
can  be  reduced  to  one  month  if  the  material  is turned
occasionally  (9).   The  Dano  process  is  generally  designed  for
refuse composting  with  sludge  addition.
         TRIPPER
                                           AG1 LOADER
                                                  T\  PERFORATED
                                                     FLOOR
   PLENUM
                                     AERATION
                           FIGURE 12-21

                         PARTIAL DIAGRAM
                     METRO - WASTE SYSTEM -
                RESOURCE CONVERSION SYSTEMS, INC.
        12.4.4  BAV Bio-Reactor

The BAV system composts  municipal wastewater sludge in an upright
cylindrical reactor.  Sludge  is  mixed  with finished  compost,  or
other  bulking agent  such as  sawdust, and  the mixture  is  fed
through  the  top  of the  cylindrical  reactor.   The composted
mixture is  drawn  off  the bottom of  the  reactor as  new  material
enters  from  the  top.    The  detention time  in the  reactor  is
between ten and fourteen days.  Air  is fed evenly throughout the
reactor and the oxygen  concentration is monitored by an electric
measuring  and regulating system.  Municipal solid waste  can also
be composted with the sludge, but then the compost would require
further processing  to   remove  nonmagnetic  metals and pieces  of
wood, glass,  plastic, and  other  non-organic  materials  before  it
is ready  for  landscaping  use.   Figure  12-23  illustrates the BAV
process.


12.5  European Composting Experience

Of the seven  European countries recently surveyed for wastewater
sludge  composting  practice,  West  Germany  is  the  center  of
activity,  with more  than 30 operating  plants  (38).    Sweden
follows with 20,  which  are  either in  operation or  in planning and
                              12-53

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design stages; Switzerland has nine; France has five; the United
Kingdom  has one; Italy  and  the Netherlands have  none.   These
systems  are  located  where wastewater  sludge  is the predominant
waste  component  usually  mixed  with municipal solid  waste.   The
number of sludge-only composting  systems  are  few.
              CONVEYOR FOR
              REJECTS
       EXHAUST AIR TO
       SOIL FILTER
    CONVEYOR FOR
    PULVERIZED
  \ MATEftfALS
X  %
  \
   \  \
       \
                ADDITION OF SEWAGE
                SLUDGE OR WATER
                       INDUSTRIAL AND
                       SULKY WASTES
                                                      — -^ HAUL AWAY
                                                   HAUL AWAY
                                                   HAUL AWAY
                                   INCINERATOR
                           FIGURE 12-22

           TYPICAL LAYOUT OF A DANO BIO-STABILIZER PLANT
The  feasibility  of composting wastewater  sludge mixed  with
a bulking agent  is established in Europe,  but the  future of
general composting technology in Europe appears to depend on the
market economics and continued public acceptance, rather than on
technological improvements.

The  predominant  experience  in  Europe has been with enclosed
mechanical  systems.   This  is  primarily  a result  of attempts
to minimize  compost facility area requirements.  Table  12-9  lists
the  various operating European wastewater  sludge  composting
processes.

Although numerous attempts were  made from  1930  until  the present,
wastewater  sludge  is  no  longer composted in windrows  without
additives (sawdust, straw,  bark) in Germany.   Dewatered sludge,
                              12-54

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without  additives, has  a low  porosity which  impedes natural
aeration.  Strong, objectionable odors developed,  and  caused  all
attempts  to  be  abandoned.  The  following  illustrates recent
composting experiences,  in Germany,  the  United  Kingdom,  Sweden,
and Switzerland (38).

    West Germany

    In the last  three  to  five  years in  the  Federal Republic  of
    Germany,  about  30  plants for  the  composting  of  wastewater
    sludge only have been  built  or are under  construction.  When
    all of these  plants  are in  service,  they together will  be
    capable of  managing  the sludge from an equivalent  population
    of 800,000.   The fact that  these 30 confined  facilities
    can only together  service  the  sludge  from  a population  of
    800,000, contrasts sharply  with the  fact  that  unconfined
    processes,  such as the windrow  operation  in Los Angeles,  or
    aerated static  pile process  in Washington,  singularly each
    process equal to or greater than 800,000  population.   In
    most  cases,  composting  of  wastewater  sludge occurs with
    the help of bulking  materials  such as  sawdust  or straw.
    Currently,  a research program  is  being conducted by  the
    German Umweltbundesamt  to determine  whether these  processes
    do indeed  produce  a pasteurized  and  pathogen-parasite-free
    product.    Preliminary  results  of this research program,  as
    yet  unpublished, show that  in some  of these  processes,
    pasteurization is  incomplete  and indeed some  final  composted
    products do contain both human and plant pathogens.

    United Kingdom

    As of 1978, only one operating plant located  at Wanlip, near
    Leicester,  is composting wastewater  sludge  in  the United
    Kingdom.   Although 10  to 15  years ago,  municipal solid  waste
    (MSW)  composting in Dano rotating drums  was common, most  of
    the plants  using these have shut down.   In  1974,  the  Wanlip
    plant reopened, and  it now  processes 1,100 tons (1,000  t)  of
    MSW mixed  with  551 tons  (500  tonnes)  of  digested  wastewater
    sludge  (five  percent  solids)  each  week.   The  product,
    packaged  under  the  brand-name,  "Lescost," is marketed with
    some success throughout Great Britain.

    Sweden

    In 1975, the Swedish   parliament  passed  a resolution  which
    emphasizes  recycling  through better  solid waste management.
    With this  resolution,   20 Swedish  communities  or regions  are
    planning,  or are in the  process of constructing,  composting
    plants.  At present,  less  than  one percent of  the  total
    MSW  and wastewater   sludge  produced  is  recycled  by  a
    composting  technique.   According to  recent estimates by  the
    Swedish National Protection Board, in  the  next  two  years
    approximately seven  percent  of  the  total  MSW  and  wastewater
    sludge produced will be recycled by composting methods.
                              12-55

-------
Switzerland

Currently, there  are  nine composting plants  in  Switzerland,
the newest of which  went into  operation  in  1975 in Biel.
All but one,  in  Uzwel,  mix  sewage  sludge with MSW.   In most
cases, incineration and composting  equipment are  located side
by side.   The composting operation is used  to dispose of
sewage sludge.   The  incinerator  burns most of the municipal
waste  and the  rejects  from the  composting  installation.
Nearly all  the  plants  use  the  Dano  system for  composting.
The auxiliary mechanical machinery, such as hammermi11s,
conveyors and screens,  is usually produced  by  Buehler.

The construction  of  composting  plants  has almost ceased in
European  countries  other  than Sweden.   Apparently  most
operating plants  have  difficulties in marketing  the  compost
at a  satisfactory  price.   It may  well be,  however,  that
careful operation  of  the  plant  and  better marketing  could
improve sales of the compost.  It  appears  very unlikely that
a number  of  combined MSW/wastewater  sludge  composting plants
will be built in the near future.  One of the  reasons  is that
the rejects  of composting must be  burnt  (landfilling  is,  for
reasons  of  space,  not  feasible  in  most  relatively small
European  countries); therefore,  an incinerator  is necessary
in any case.  Building a larger  incinerator  instead of  a
combined system seems,  in many cases,  the simpler solution.

                       MIXTURE TO BE COWPOSTEO
                     1 .1 i, |. i_fi_*i£-,/i / f< £_
                     I ' T IT  yvyjZj/ ycT
'  /
. /t A I
                                          -SCREW-TYPE
                                          CONVEYER
                                              1 AIR
                         COMPOST DISCHARGE


                         FIGURE 12-23

                       BAV BIOREACTOR
                          12-56

-------
                            TABLE 12-9

       EUROPEAN WASTEWATER SLUDGE COMPOSTING PROCESSES (38)

                                                     Number of
     Category	 	Process               operating plants
Within vessel           BAV                               19
                       Carel Fouche Languenin              1
                       Roediger/Fermenttechnik             1
                       Schnorr Valve Cell                  2
                       Societe General
                         D1assainissement et de
                         Distribution  (SGDA)              '1
                       Triga                              2
                       Weiss                              3
Windrow                 BIO-Manure                         1
                       Hazemag           .                 -
                       PLM
Rotating drum           Buehler                            9
                       Dano                               9
                       HKS                                2
Pressed brick           Brikollare                         2
Fermentation cells       Prat                               1
12.6  References

 1.  Satriana,  M.J.   Large  Scale  Comp o s; t i r\g .   Noyes Data
     Corporation, Park Ridge,  NJ.1974.

 2.  Willson, G.B. and  J.M.  Walker.    "Composting  Sewage  Sludge,
     How?"   Compost  Science  Journal  of Waste  Recycling.    p.  30.
     September-October (1973).

 3.  Epstein,  E. and G.B.  Willson.   "Composting Raw Sludge."
     p r o c.   1975 National  Conference on  Municipal  Sludge
     Management and DisposjQ.Information Transfer Inc.   p~.2457
     August 1974.

 4.  Epstein,  E.,G.B.  Willson,  W.D. Burge, B.C.  Mullen,  and
     N.K.  Enkiri.   "A Forced  Aeration System  for Composting
     Wastewater  Sludge."    Journal  Water  Pollution  ContjrgJL
     Federation.  p.  688,  Vol.  48, No. 4.April 1976.

 5.  USEPA.   "Composting Sewage Sludge by High-Rate Suction
     Aeration Techniques."   Office of Solid  Waste.   Washington,
     DC 10460.  Interim report SW-614d.  1977.

 6.  Wolf,  R.   "Mechanized  Sludge  Composting  at  Durham,  New
     Hampshire."   Compost Science Journal  of Waste  Recycling,
     p. 25.  November-December 1977.

 7.  Heaman, J.   "Windrow Composting  -  A Commercial Possibility
     for  Sewage  Sludge  Disposal."   Water andPollution Control.
     p. 14.  January 1975.


                              12-57

-------
     Ehreth,  D.J.  and  J.M. Walker.   "The Role  of Composting
     and  Other  Beneficial  Use  Options  in  Municipal  Sludge
     Management."   Proc.  National  Conference  on Composting
9.
10.
11.
12.
13.
of Mun ic
Transfer,
Golueke,
Rodale Pr
Epstein,
Municipal
of Munic
Transfer,
J e lenek ,
Perspect
National
Sludges .
August 19
Poincelot
National
Sludges .
ipal Residues and Sludges. p. 6. Information
Inc., Rockville, MD. August 1977.
C.G. Biological Reclamation of Solid Wastes.
ess, Emmaus, PA. 1977.
E. and J.F. Parr. "Utilization of Composted
Wastes." Proc. National Conference on Composting
ipal Residues and Sludges. p. 49. Information
Inc., Rockville, MD. August 1977.
C.F., F.B. Read, and G.L. Braude. "Health
ive, Use of Municipal Sludge on Land." Proc.
Conference on Composting of Municipal Residues and
p. 27. Information Transfer, Inc., Rockville, MD.
77.
, R.P. "The Biochemistry of Composting." Proc.
Conference on Composting of Municipal Residues and
p. 33. Information Transfer, Inc., Rockville, MD.
August 1977.
Willson, G.B. "Equipment for Composting Sewage Sludge in
Windrows and in Piles". Proc. National Conference on
Composting Municipal Residues and Sludges. p. 56. Informa-
14.
tion Transfer, Inc., Rockville, MD. August 1977.
Golueke, C.G. Composting - A Study of the Process and Its
     Principles.  Rodale Press,  Emmaus.  PA.  1972.

15.   Wesner, G.M.    "Sewage Sludge  Composting."   Technology
     Transfer  Seminar  Publication  on  Sludge  Treatment and
     Disposal.  Cincinnati, OH 45628.   September 1978.

16.   Parr,  J.F.,  G.B. Willson,  R.L.  Chaney,  L.J.  Sikora and
     C.F.  Tester.   "Effect  of Certain  Chemical and Physical
     Factors on the  Composting Process  and  Product Quality."
     Proceedings   of  Design  of  Municipal   Sludge  Compost
     Facilities^p~.  130.   Chicago, IL~.   Information  Transfer,
     Inc., Rockville, MD.  August 1978.

17.   Haug,  R.T.,  and  L.A. Haug.    "Sludge  Composting:    A
     Discussion of Engineering Principles," Parts  1 & 2.   Compost
     Science/Land  Utilization Journal  of  Waste  Recycling.
     November-December  (1977)and January-February.1978.

18.   Colacicco, D.   "A Cost  Comparison with the Aerated  Pile and
     Windrow Methods."   Proc.  National Conference on Composting
     Municipal Residues  and  Sludges.   p.  154.   Information
     Transfer,Inc., Rockville,  MD.August 1977.
                             12-58

-------
19.   Shuval,  H.I.   "Nightsoil  Composting  State  of  the Art
     and  Research  -  Pilot  Study  Needs."   Research  Working
     Paper Series,  P.U. report  RES12, International Bank for
     Reconstruction  and Development,  Washington,  DC.    November
     1977.

20.   Smith, D.  and  M.W. Selna.   Pathogen  Inactivation During
     Sludge Composting.  Internal  Reports,  County Sanitation
     Districts of  Los Angeles.  September 1976, February  1977.

21.   Burge,  W.D.   "Occurrence,of Pathogens  and  Microbial
     Allergens  in the  Sewage  Composting Environment."  Proc.
     National  Conference on Composting of Municipal  Residues and
     STu'dg~es~.   Information Transfer, Inc. , Rockville, MD.  August
     1977.

22.   Olver,  W.M.  Jr., "The  Life  and  Times  of As pe rg il1 us
     fumigatus." .  Compost  Science/Land  Utilization.   March-April
     1979.

23.   Burge,  W.D.,  P.B.  March,  and P.O.  Millner.   "Occurrence of
     Pathogens  and  Microbial  Allergens  in  the  Sewage Sludge
     Composting Environment."   Proc. 1977 National Conference on
     Composting of Municipal Residues and Sludges.     Information
     Transfer, Inc., Rockville, MD.  1978.

24.   Slueski,  S.   "Building  Public Support  for a Compost Plant."
     Compost  S^c^enc^e/Land Util i zation.  Vol.  19,  p. 10.  1978.

25.   Solomon,  W.R., H.P.  Burge,  and  J.R.  Boise.   "Airborne
     Aspergillus  fumigatus Levels  Outside and  Within a Large
     (JliHical  Center."  Journal  Allergy Clinical  Immunology.
     Vol.  62,  p.  56.  1978.

26.   Schwartz,  H.J.,   K.M.  Citron, E.H.  Chester,  J.  Kaimal,
     P.  Barlow, G.L. Baum, and M.R. Schuyler.   "A Comparison of
     the Prevalence of Sens itization  to Aspergillus Antigens
     Among Asthmatics  in Cleveland  and  London."Journal Allergy
     Clinical  Immunology.  Vol. 62,  p. 9.  1978.

27.   Slavin,   R.G.   "What  Does A  Fungus Among Us Really Mean?"
     Journal_Allergy Clinical Immunology.  Vol. 62, p.  7. 1978.

28.   Epstein,  E.    "Composting  Sewage  Sludge  at  Beltsville,
     Maryland".   Proc.  of Land Application of Residual Materials
     Engineering,  Foundation  Conference.  Publishing  ASCE.   New
     York, NY.  October  1976.

29.   USEPA.    Sludge Handling and Conditioning.  Office  of  Water
     Program   Operations.  Washington,  DC 10460.   EPA 430/9-78-
     002.   February  1978.

30.   Kalinske, A.A.   "Study of Sludge  Disposal  Alternatives for
     the New  York-New Jersey Metropolitan Area."   Paper presented
     at 48th  Water Pollution Control Federation Conference,  Miami
     Beach,  Florida.  October 1975.
                             12-59

-------
31.   Crombie, G.   "Mechanized  Forced Aeration  Composting for
     Durham,  New  Hampshire".  Town of Durham.   1978.

32.   Horvath,  R.W.    "Operating  and  Design Criteria for Windrow
     Composting  of  Sludge."   Proc.  National  Conference on Design
     of  Municipal  Sludge  Compost  Facilities.Information
     Transfer,  Inc.,  Rockville, MD.   August 1978.

33.   Camp,  Dresser  and McKee,  Inc.   Alternative  Sludge Disposal
     Systems for the District of Columbia Water  Pollution PTant
     at Blue Plains,  District  of Columbia.   Unpublished report
     prepared  for  the  Department  of  Environmental  Services
     District of  Columbia, December 1975.

34.   Colacicco,  D.,  E.  Epstein, G.B. Willson,  J.F.  Parr, and
     L.A. Christensen.    "Cost of  Sludge Composting".   USDA,
     Agricultural  Research Service,  ARS-NE-79.    Washington, DC.
     February 1977.

35.   Wilson, G.B.,  J.F.   Parr,  and  D.C.  Basey.   "Criteria for
     Effective  Composting  of Sewage  Sludge in Aerated Piles and
     for Maximum  Efficiency of  Site  Utilization."   Design of
     Municipal  Sludge Compost Facilities Conference.   Information
     Transfer,Inc.,Rockville, MD.August 1978.

36.   Sikora,  L.   "Materials Balance  in  the Beltsville Aerated
     Pile Method of  Sewage  Sludge  Composting."    Proc. National
     Conference  and  Exhibition on Municipal and  Industrial Sludge
     Management.   Information  Transfer, Inc.,  Rockville, MD.
     November 1979.

37.   Resource Conversion Systems, Inc., Company  Process Brochure,
     Houston, Texas.   December 1977.

38.   USEPA.   Evaluation of "Within  Vessel"  Sewage  Sludge
     Composting  Systems   in  Europe.  Draft  Report.   Municipal
     Environmental  Research  Laboratory.   Cincinnati,  Ohio 45268.
     Contract 68-03-2662.  1978.
                             12-60

-------
EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
          Chapter 13. Miscellaneous Processes
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
 structure  of the  solids; a monolithic solid is much  less subject
 to leaching  than is a granular solid.   However, monolithic solids
 may deteriorate if exposed  to wet-dry  or freeze-thaw  cycles  (11).
 Leaching  tests  to  estimate  long-term  weathering  resistance
 of the  fixed solids  are  still being  formulated  (12).   It should
 be emphasized  that the   information presented  in this  paragraph
 was derived  from experience with sludges of an industrial origin.
 Experience  with  municipal  sludges may  be  similar  to  that  with
 some  industrial ones.
                              TABLE 13-1

                  PARTIAL LIST OF FIXATION PROCESSES


                                                       Additive
                                                       quantity,
       Vendor             Process             Additives         percent    References
Dravo Corporation          Synearth3           Calcilox                7^      3, 4
                                      Thiosorbic lime

IU Conversion Systems, Inc.    Poz-O-Teca           Lime                  4       3, 4, 5
                     Poz-0-Soila


Chemfix, Inc.             Chemfix3             Portland cement           7,       3, 4
                                      Sodium silicate           2


TRW, Systems Group0            -             1,  2 - polybutadiene      3 - 4d     6

(nonproprietary)           Flyash-limestone        Fly ash                -       4
                                      Limestone
Registered trademarks.  Process is in full-scale use on hazardous
 industrial sludge or flue gas desulfurization sludge or both.
 Additive as percent by weight of dry sludge solids for flue gas
 desulfurization sludge and fly ash at coal-burning power plants.
cBench scale tests.
 Additive as a percent of dry sludge solids.


 The cost  of utilizing the  chemical  fixation  process  is affected
 by the degree  of  dewatering  required,  the  type  of  fixation
 chemical(s)  employed,  and  the  method  of mixing  the  chemical(s)
 and  sludge.    In  addition  chemical  fixation  processes  are
 generally  proprietary  and  require  royalty payments.   Therefore,
 schemes including  chemical fixation are  generally more expensive
 than  conventional  systems  for processing  municipal  sludges.
 Consequently,  applications  of  the  chemical   fixation  process  to
 municipal  sludges  will  probably  remain uncommon except when  such
 sludges  contain  significant  concentrations   of heavy  metals  or
 other  toxicants.    Variables  affecting  the  cost  of  fixation
 include:

      •  Availability of  fly  ash.   Some processes  use  fly ash
         to reduce the need  for other chemicals.

      •  Sludge dewaterability.   Fixation  costs increase with the
         amount of water present.
                                13-2

-------
     •  Volume and mass  of  sludge  to be treated.

     •  Physical  properties  required  for the  fixed  sludge.   A
        granular  product tends  to cost  less than a  monolithic
        product,  for  example.

     •  The degree  to  which  the fixed product must resist
        leaching.

     •  Reactivity of the sludge with the fixation chemicals.

     •  Unit prices of treatment chemicals.   In some  cases,  this
        factor is complicated by the  fact that the chemicals are
        proprietary.


13.3  Encapsulation Process

Encapsulation  is  the  encasing of sludge in an impervious, durable
material.    Encapsulation processes  are expensive to  employ but
are a  useful  treatment alternative  when  the  sludge  contains
significant concentrations  of leachable toxic materials.   As  with
fixation processes,  there is little reported  experience for the
system with municipal  sludges.    The  information  presented  here
has been  obtained from experience  with industrially  derived
sludges.   Two examples  of  encapsulation  processes  are discussed
below.
    13.3.1  Polyethylene  Process

Encapsulation of a sludge with polyethylene has been investigated
in  the  laboratory (6,13,14).   This  process  involves  putting  a
block, a  55-gallon  (208   1)  drum,  or other  container  of  sludge
that  has  been treated by  the  chemical  fixation process  into  a
bed of polyethylene powder.   The  polyethylene is  then  heated to
350°F (180°C) so  that it  melts  and fuses  into a  1/4 inch  (6 mm)
thick seamless  layer.    The  approximate  amount  of  polyethylene
required is 4 percent  by  weight  of the sludge  to be encapsulated^
Polyethylene is  tough  and  may be  severely  deformed without
rupture.   Leaching tests of several materials treated  by the
polyethylene process  showed virtually no release of the chemical
constituent.

The extremely high system temperatures  cause  water  to  evaporate
at  pressures up  to  about 130 psig (900 kN/m2).   Therefore,
one of  the following three  stringent  conditions must be  met:

      •  The process must  be carried out  under  pressure.

      •  The  sludge must  be  sealed in  vessels that  are able to
        withstand an  internal working  pressure  of  130  psig
        (900  kN/m2)  before  the sludge is  delivered to  the
        encapsulation  process.


                             13-3

-------
     •  The sludge  must be  in a thoroughly dry  form such  as
        either  sludge incinerator  ash  or heat-dried sludge.


    13.3.2  Asphalt  Process

Asphalt may be used to encapsulate wastes.   In this process,
the waste is mixed  with  asphalt  at  300°F (150°C) in such a way
that each individual particle  is  coated  with  asphalt.   Moisture
is  removed  as  steam.   The  coated particles  are then  placed  in
55-gallon (208 1)  drums  or other  containers  where they cool
and form  a  solid,  nonporous  mass.  The  encapsulated product  is
highly resistant  to leaching,  mechanical damage,  and  bacterial
attack.   About  one pound of asphalt is required for each pound  of
dry solids (15).

Asphalt  encapsulation has  been used  in  Europe  on  medium-level
radioactive  wastes  since  1965.   There  is  little  United  States
operating  experience,  but European experience makes  it possible
to  estimate costs  for  wastewater  sludge  applications.    An
installation with a capacity of  five  hundred 55-gallon (208  1)
drums  per year  could  handle  about 84  tons  (76 t) of dry  sludge
solids per  year.   Capital  and  operating costs  are  estimated  at
$1.45  million  and $62,000  per  year,  respectively, at  1977 U.S.
price  levels.   Amortizing capital  over twenty years at 7 percent,
the total cost  is  about $2,400  per  ton dry  solids processed
($2,600/t).  This  cost  includes encapsulation machinery and
associated  building  space,  drums, drum  storage,  asphalt,  steam,
cooling  water,  and  operating  labor.    It  does  not include
engineering  (except for  engineering  performed by  the  equipment
supplier),  sludge  dewatering  which  precedes  the  encapsulation
process,   transportation  and  disposal  of  the  finished  product,
treatment of  contaminated  steam that  might  be  produced,   or
maintenance.    Possibly,  cost  savings  can  be  obtained from
economies  of  scale  and less rigorous  conditions than  those  at
nuclear  power  plants.


13.4  Earthworm Conversion Process

A  novel   municipal  wastewater sludge  treatment  process uses
earthworms  (Oligochaete  annelids) .  This system  is  often  called
"earthworm conversion," vermicomposting, or annelidic consumption
(16).   Vermicomposting  is different  from the  conventional
composting  of  wastewater   treatment plant  sludge.    In the
earthworm conversion process,  the worms  are  provided  an optimum
environment to  consume or metabolize the sludge and produce feces
or  castings.   These castings may  be used as  a soil conditioner.


    13.4.1  Process  Arrangement

Earthworm  conversion is  basically  a  simple   process,  and  a
schematic diagram of  it is  shown on  Figure  13-1.   The process
requires  worm  beds  and  a supply  of worms.   Generally, digested


                             13-4

-------
and dewatered  sludge is put  into  the beds, although  experiments
are underway, where raw liquid sludge  is placed  in beds.   If
anaerobic digestion  is  used  prior  to  earthworm  conversion,
additional pretreatment  may be  needed.   A bulking agent such  as
wood chips may be useful in some cases for  keeping the  bed porous
and aerobic, especially if moisture  is high.   Sludge  is,  however,
generally applied  without any bulking  agent.  A  worm bed may
take the  form of a  simple  tray.  Windrows  similar  to those for
composting may  also be used.    After the  worms have  consumed the
sludge, they  must be  separated from the  castings.   This may  be
done with  an  earthworm harvester, a drum screen that  rotates  on
a  nearly  horizontal  axis.   Castings fall  through  the screen
openings  while  worms  tumble  through the  length  of  the  drum.
Table 13-2 contains  some  critical operational parameters for the
earthworm conversion process.
                                    MAKE-UP
                                   IABTHWORMS
AIR06ICALLV
DIGESTED SLUDGE _


DEWATEfllfiS
BULKING A6EHT
IF REQUIRED

1
Mixtn
                      BULKING AGENT
ANAERQUCALLY
DIGESTED
SLUDGE __

fIF REQ
&EVM ATE RING


PBETREAHMENT
|AEBATION,ETCj
uineOi

~l
MIXER




ftSC

WORM
BIDS


hAHIH'lYlV'PJ
HABVESTEfi
YCLED EARTHWORMS,
a
3
E
Q
                                      UNEATEN SLUW5E PARTICLES.
                                      AND |IF USED) BULKING AGENT
             CASTINGS
             FOR LAND
             UTILISATION
             AS SOIL
             AMENDMENT
                                                     I!
                                                     isi ^
                                  BULKING AGENT
                                    FOR RECYCLE
j SEPARATIQW Of   I
J EARTHWOflWS FROM I
*] tULKING AGENT   )
 IIP RESlUIfliOl   1
                                               SURPLUS EAHTHWOBMS
                                                  FOR SALB
                            FIGURE 13-1

            DIAGRAM OF AN EARTHWORM CONVERSION PROCESS
The  main product of  the earthworm conversion process is  the
worm's castings.   In some process  arrangements  there  may be  a  net
earthworm production.   The excess  earthworms  may  then be sold  for
                              13-5

-------
fish  bait or animal  protein supplement.   Earthworm marketing is a
complex  problem.   For  municipal sludge applications, surplus
earthworms may  be  considered  a by-product;  the principal  product
is the  castings,  which  can be a  resource.


                              TABLE 13-2

                PARAMETERS FOR EARTHWORM CONVERSION

              Parameter                            Values
 Detention time of sludge in worm beds       2 days (19)
                                    32 days (18)

 Worm reproductive cycle                  1 to 2 months

 Rate of worm feeding (15°C)               0.17 to 1.7 grams dry sludge per gram dry
                                     worm weight per day (17)

 Optimum temperature                     15°C to 20°C (17)

 Dry matter content of worms               20 to 25 percent (Eise_rria. foetida)  (20)

 Minimum solids content of the worm bed      20 percent solids
  mixture



 Species of worm being tested: Eisenia foetida (redworm, hybrid redworm,
 tiger worm, dung worm)  (17), Lumbricus rubellus (red manure worm, red
 wiggler worm) (18), and Lumbricus terrestris (nightcrawler).  (17).

 Actual minimum solids content depends on such factors as porosity, type of
 sludge, ability to keep aerobic.  Experiments are being conducted to better
 define these parameters.

 1°F = 32 + 1.8°C
    13.4.2   Advantages  of  the Earthworm  Conversion Process

When dry,  earthworm castings are essentially odorless;  when damp,
they  have  a  mild  odor like  a good  quality topsoil.   Also,  the
castings  have a  favorable appearance.  When sifted and dry,  they
are granular, about  0.02  to  0.1 inches  (0.5 to  3 mm)  in  maximum
dimension  (with  some  fines);  color  is brownish  gray.   In  a study
where  municipal  sludge was applied  to a wheat  crop,   it was found
that  when  earthworms  were added  to  the  sludge,  the germination
rate  of the  wheat was  improved (21).   The  odor, appearance,  and
soil  supplementation  advantages  of the   earthworm  conversion
process may help in  the acceptance of  sludge by  farmers  and
householders.

Earthworm  conversion affects several  other sludge char-
acteristics.    The   oxygen   uptake  rate   increases  (17);   the
acid-extractable  fraction  of  various nutrients  increases  (21).
The volatile content  of the  solids  drops slightly and  humic  acid
concentrations  fluctuate  (17).   While  these  effects  may  be
beneficial,  there are no data  to  show how the results  affect
design or   operation  of  earthworm  conversion installations.


                               13-6

-------
The earthworm conversion process would appear to be low in cost,
although this cannot be  said  with  certainty,  since no cost data
are available for  full-scale  operations  on  sludge.   The process
does not require  chemicals, high  temperatures,  or  large amounts
of electricity.  Only  a  small amount  of  low-speed  mechanical
equipment is needed.   Significant expenditures may be required to
offset the  potential  operating  difficulties discussed below.


    13.4.3   Possible  Operating  Difficulties

A number of potential operating difficulties and their solutions
are  listed in  Table  13-3.    None of  these  difficulties  are
insurmountable.    Probably  it   is most  difficult to economically
pretreat anaerobically digested sludge so that it is nontoxic to
the worms.
    13.4.4  Limitations

Limitations are:

      •  Earthworm  conversion decreases  the total  nitrogen
        values in  the sludge  because  ammonia nitrogen  will  be
        lost to the atmosphere.

      •  Published  information  to  date  (1979)  is  almost
        nonexistent on full-scale municipal wastewater treatment
        plant sludge operations.   Consequently,  costs  are
        unpredictable.

      •  TWO common  ions in municipal wastewater sludge, ammonium
        and copper, may  be toxic to worms.   Studies  have  found
        that  these ions  were  lethal  at  additions equivalent
        to  180  mg  NH4-N  and 2,500 mg  Cu  per kilogram of  wet
        substrate  (26,27).   Safe limits  for  these  elements  are
        not known.

      •  Cadmium  accumulates  in  the  worm  Eisenia  foetida.   Zinc
        apparently  does  not  accumulate  in Eisenia  foetida  but
        does  accumulate in  other  species  (27,28).   If the  worms
        are  to be  used as  animal  feed,  the system must  be
        operated  such that  cadmium  and  zinc  concentrations  in
        the worms  do not  exceed  recommended  levels  for animal
        consumption.

      «  Space  requirements  may rule out earthworm  conversion
        at some treatment plants.

      •  The earthworm business has been  afflicted  with unsound
        investments and  excessive  claims.  For  example,  it  has
        been  claimed that  earthworm processing  is able  to
        reduce  concentrations  of  heavy metals (29).    Any  such


                             13-7

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          reduction  could  only  be  caused  by  simple  dilution
          with  uncontaminated    waste  or  by  concentration   of   the
          contaminants  in  the  earthworms.
                                     TABLE 13-3

       POSSIBLE OPERATING DIFFICULTIES IN  EARTHWORM CONVERSION
            Possible difficulty
                Comments
Worm drowning

Predation by birds and animals


Worm loss due to migration from  the
  process



Toxicity of sludge to worms
Toxicity or unpalatable nature  of
  dewatering chemicals
Worm shortage in the process,  so  that
  worm additions are required
Shortage of worms for initial  inventory
  or restart
Temperature  extremes
Shortage of  enzymes
Worms must  be protected from flooding.

Not a problem at San Jose - Santa Clara,
  California experiments (22).

Caused by flooding, toxic sludge, unpalat-
  able sludge, adjoining areas  attractive
  to worms, lack of artificial  lighting on
  rainy nights.

Significant for anaerobically digested
  sludge.   However, toxicity is eliminated
  by exposing the sludge to air for two
  months (17) or wetting sun-dried sludge
  daily for 14 days (21).  Stabilization by
  lime or chlorine is not recommended for
  sludge that will be fed to earthworms.
  Toxicants such as copper salts might also
  cause problems.  Aerobic digestion is best
  suited for sludge to be converted by
  earthworms.

Avoided at  Hagerstown, Md., by  use of food-
  grade polymer  (19).  Drying beds may be
  used;  drying beds do not usually require
  chemicals.

Worms reproduce via egg capsules.  These
  capsules may be lost from the process in
  the castings.  Also, toxic conditions,
  drowning, and other problems  will cause
  worm populations to drop.  At Hagerstown,
  Md. , a worm raising operation has beer.
  proposed  to supply the necessary make-up
  worms to  the sludge conversion process  (19)

To begin operation, a large worm inventory
  may be needed, so large that  local worm
  suppliers may be unable to fill it.
  Gradual start-up is therefore desirable,
  especially for large plants.   Also, earth-
  worm exchanges may become available
  natiB«wide so that sludge operations can
  draw on larger numbers of earthworm
  suppliers.

Worm feed most rapidly at 15 to 20 degrees C;
  about 5 degrees C, feeding is quite slow
  (17).  Freezing will kill worms.  High
  temperatures can also cause problems.  It
  may be necessary to stockpile sludge dur-
  ing the winter or provide a heated
  building  for the conversion process.

Not a problem, despite claims by marketers
  of enzyme preparations that these prepara-
  tions are valuable to the process  (23).
                                       13-8

-------
                               TABLE 13-3

 POSSIBLE OPERATING DIFFICULTIES IN EARTHWORM CONVERSION  (CONTINUED)
          Possible difficulty
                                                  Comments
Exposure to light
Dehydration
Salinity in castings
Contamination of castings by heavy metals,
  motor oil, rags, and similar materials
Odors
Worms avoid bright light.  Some sort of
  cover or shade should be provided so that
  worms will convert the top layer of the
  sludge .

There is a minimum moisture content for the
  worm bed (23).

Under some conditions,  castings may have
  sufficient dissolved salts to inhibit
  plant growth.  This problem may be elim-
  inated by leaching or by mixing the
  castings with other materials with lower
  dissolved salts (24,  25).

Source control may be used, where feasible,
  as for other processes aimed at reuse of
  sludge as a soil conditioner.  See
  Chapter 2 for regulations on sludge pro-
  ducts .

The most likely source is raw or
  aerobically digested sludge, which has
  been stockpiled to await earthworm con-
  version .
3C = 0.555 (°F -32) .
       •   If  a  particular  sludge  is  suitable  for  earthworm
          conversion,   that  sludge  should  also be  suitable  for
          reuse  as  a soil  conditioner  without being processed  by
          earthworms.   However,  earthworm  conversion reduces  odor,
          improves  texture,  and may  increase germination rate.

 These limitations  may be  significant  but not  overwhelming.   There
 is  considerable research  and  development underway.    It appears
 that  earthworm  conversion may have  a  role  in  municipal wastewater
 treatment plant  sludge processing.
 13.5   References

  1.  R.K.  Salas.    "Disposal  of  Liquid  Wastes  by  Chemical
      Fixation/Stabilization  -  The  Chemfix  (R)   Process."
      Toxic and  Hazardous  Waste Disposal, Volume 1.   R.B. Pojasek,
ed. Ann Arbor Science, 1979.
J.T. Schofield. "Sealosafe (SM)." Toxic and Hazardous
Waste Disposal, Volume 1.
Science ,
1979.
R.B. Pojasek, ed. Ann Arbor
  3.  Francis   O'Donnell.     "Scrubber  Sludge:    Nightmare   for
      Utilities."   Sludge Magazine.   Vol.  1 no.  2, p.  26.  March-
      April, 1978  .

                                 13-9

-------
 4.   J.W.  Barrier,  H.L.  Fawcett,  and L.J. Henson.   "Economic
     Assessment  of FGD Sludge  Disposal  Alternatives."   Journal
     Environmental Engineering Division_ ASCE.    Vol.  104 ~~p~.9^T,
      Oct.,  1978  .

 5.   Hugh  Mullen, Louis  Ruggiano,  and S.I.  Taub.   "Concerting
     Scrubber  Sludge and  Flyash into  Landfill  Material."
     Pollution Engineering.  Vol. 10,  no.  5, p.  71, May, 1978 .

 6.   USEPA.    Development of a Polymeric  Cementing and Encapsul-
     ating Process  for Managing Hazardous Wastes.   Office of
     Research and Development, Cincinnati,  Ohio 45268.   EPA-
     600/2-77-045.  August 1977.

 7.   Raymond Swan.    "Indianapolis Project:   From Lagoons to
     Landspreading  in  Three  Not-so-Easy Lessons."    Sludge
     Magaz ine.  Vol. 1, no. 3, p. 16.   May-June,  1978.

 8.   USEPA.    Field Evaluation of Chemically  Stabilized Sludges.
     Land  Disposal  of Hazardous Wastes.   Proceedings  of  the
     Fourth Annual  Research  Symposium.   San Antonio,  Texas.
     March  6-8,  1978.   Office  of Research  and Development,
     Cincinnati,  Ohio  45268.  EPA-600/9-78-016.   1978.

 9.   USEPA.   Laboratory Assessment of Fixation  and Encapsulation
     Processes for  Arsenic-Laden  Wastes.    Land  Disposal  of
     Haz ardous   W astes,  Proceedings  of  the Fourth Annual
     Research  Symposium.   San Antonio, Texas.  March 6-8, 1978.
     EPA-600/9-78-016.

10.   USEPA.   Pollutant Potential  of Raw  and  Chemically Fixed
     Hazardous iTTdiTstrial Wastes  and Flue Gas Desulfurization
     sTucfges.   Interim report.   Office of  Research and Develop-
     ment,  Cincinnati, Ohio 45268.  EPA-600/2-76-182.  July 1976.

11.   R.  E.  Landreth and J.L.  Mahloch.   "Chemical  Fixation of
     Wastes."    Industrial Water Engineering.    Vol.  14,  no. 4,
     p.  16.   July-August 1977.

12.   Robert Pojasek.   "Stabilization,  Solidification  of Hazardous
     Wastes."    Environmental Science  and Technology.    Vol.  12,
     p.  382. April  1978.

13.   USEPA.    Encapsulation Techniques for Control  of Hazardous
     Materials.   Land Disposal  of  Hazardous  Wastes, Proceedings
     ofFourth Annual Research  Symposium.   San Antonio, Texas,
     March 6-8, 1978.  EPA-600/9-78-016.   1978.

14.   H.R.   Lubowitz  and  C.C. Wiles,  "Encapsulation  Technique
     for  Control of  Hazardous Wastes."   Toxic  and  Hazardous
     Waste Disposal, Volume 1.   R.B.   Pojasek,  ed.   Ann  Arbor
     Science,  Ann Arbor, Michigan 48106.   1979.
                             13-10

-------
15.  R.D. Doyle,  "Use of an  Extruder/Evaporator  to Stabilize
     and Solidify Hazardous Wastes."   Toxic and Hazardous Waste
     Disposal, Vo1 ume  1.  Ann Arbor Science,  1979.   R.B. Pojasek,
     ed. p.  65.

16.  Frank  Carmody,   "Practical  Problems  in Application of
     Earthworms  to Waste Conversion Processes."  Utilization of
     Soil  Organisms  in  Sludge  Management ,  proceedings o~f
     conference,  Syracuse,  New  York: 6/25-17/78.    National
     Technical Information Service PB-286932. ed. R. Hartenstein.

17.  M.J.  Mitchell,   R.M.  Mulligan, Roy Hartenstein,  and
     E.F. Neuhauser.  "Conversion of Sludges into  "Topsoils1 by
     Earthworms."  C ompost Scie n ce.  Vol. 18, p.  28. July-August,
     1977 .          :	

18.  David Newman.   "Earthworm and Electrons:  Technology's Outer
     Limits."   Sludge Magazine,  Vol. 1, no.  1,  p.  30 • January-
     February  1978  .

19.  Cathy  Dombrowski,  "Postscript:    Earthworms."   Sludge
     Ma^gazine.  Vol. 1, no. 5, p. 10  September-October, 1978 .

20.  J.R. Sabine.   "The  Nutritive Valve  of Earthworm  Meal."
     Utilization  of  Soil  Organisms  in  Sludge  Management,
     proceedings  of  conference,  Syracuse, New  York:   6/15-17/78.
     National Technical  Information  Service  PB-286932.   ed.
     R. Hartenstein.

21.  M.3.  Kirkham.   "Availability to Wheat  of  Elements in
     Sludge-Treated  Soil with Earthworms."   Utilization  of  Soil
     Organisms in  Sludge Management,  proceedings of  conference,
     SyracuslT;New  York:   6/15-17-78.    National  Technical
     Information  Service PB-286932.  ed. R.  Hartenstein.

22.  J.E.  Collier.    "Use of  Earthworms  in Sludge  Lagoons."
     U t i1i z ation  of  Soil  Organisms  in  Sludge  Management,
     proceedings  of  conference.Syracuse^New York:6/15-17-78.
     National Technical  Information  Service  PB-286932.   ed.
     R. Hartenstein.

23.  Linda Theoret,  Roy Hartenstein, and M.J. Mitchell. "A Study
     on  the Interactions  of  Enzymes with Manures and  Sludges."
     Cgmpost_Sc ience.  Vol. 19, p. 29.   January-February, 1978  .

24.  Soil  and  Plant  Laboratory,  Inc.  Soil Fertility Analysis  -
     Earthworm Castings.   Report  on sludge-derived  castings  from
     San Jose  -  Santa  Clara, Calif,, experiments. May  17,  1977.

25.  N.  Stark,   P.  Pawlowski,  and S. Bodmer.   "Quality of
     Earthworm Castings and the  Use of Compost on Arid  Soils."
     Utilization  of  Soil  Organisms  in  Sludge  Management,
     proceedings  of conference.   Syracuse,NewYork:6/15/78.
     National Technical  Information  Service  PB-286932.   ed.
     R. Hartenstein.  p.  87.

                            13-11

-------
26.   E.F. Neuhauser,  "The Utilization of  Earthworms  in  Solid
     Waste   Management",  Utilization of Soil Organisms in Sludge
     Management,  proceedings  of conference.   Syracuse, New York:
     6/15-17/78 .   National  Technical  Information Service
     PB-286932.   ed.  R.  Hartenstein.  p. 138.   (Value converted
     from ammonium  acetate basis  to ammonia nitrogen basis.)

27.   R.  Hartenstein  et al.,  "Heavy Metals,  Sludges,  and
     the Earthworm  Eisenia   foetida."   Journal of Environmental
     Qual i ty .   In review, 1971TI

28.   R.I. Van Hook,  "Cadmium,  Lead,  and  Zinc  Distributions
     Between  Earthworms  and  Soils:   Potentials  for  Biological
     Accumulation."    Bulletin  of Environmental Contami^nat^qn.^ajrvd
     Toxicology.  Vol.  12, p. 509, 1974  .

29.   AnProS,  An  Ecologically,  Environmentally,   &  Economically
     Sound  Ap~proach to Sewage Sludge  Management:.   GTA,  Inc.,
     Wilmington,  Delaware,1978,  pamphlet.
                             13-12

-------
EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
              Chapter 14.  Transportation
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                          CHAPTER 14

                        TRANSPORTATION
The fundamental  objective  of  all  wastewater  treatment operations
is to  remove undesirable constituents  present in  wastewater
and  consolidate  these  materials  for further  processing  and
disposal.   Solids removed  by wastewater  treatment  processes
include screenings and  grit,  naturally  floating  materials called
scumf  and  the  remainder  of  the  removed  solids called  sludge.
This  chapter  discusses the transportation  of  solids,  or
the movement of  sludge,  scum, or  other miscellaneous  solids
from  point  to   point  for  treatment,  storage,  or  disposal.
Transportation includes movement of solids by pumping, conveyors,
or hauling  equipment.


14.1   Pumping and Pipelines

Unless  a  sludge has  been  dewatered,  it  can  be  transported
most  efficiently  and economically by pumping through pipelines.
Sludge  is  subject to the same physical  laws as other  fluids.
Simply stated,  work put into a fluid by a pump  alters velocity,
elevation,  and pressure, and overcomes friction loss.  The unique
flow  characteristics  of  sludge  create special  problems  and
constraints.   Nevertheless,  sludge has been  successfully pumped
through short pipelines at up to  20  percent  solids by weight, as
well  as  in pipelines of  over 10 miles  (16 km) long at  up to
8 percent solids  concentrations.

Most  of the  following information  is  related  to  sludge, although
screenings, grit, and scum may also  be transported by pipeline.
Mention  is  made of these  miscellaneous  solids  when  special
considerations are involved.
   14.1.1  Simplified  Head Loss Calculations

Head losses must  be  estimated  for sludge pumping;  they are not
available in  standard  tables.   Head requirements for  elevation
change  and velocity  are  the  same as  for  water.    However,
friction losses may be much  higher than  friction  losses in water
pipelines.   Relatively simple procedures are often used  in design
work; such  a procedure is described below.  The accuracy of these
procedures  is often  adequate, especially at solids  contents
below 3 percent by weight.   However,  as  the  pipe  length, percent
total solids,  and percent volatile solids increase, these simple


                             14-1

-------
procedures  may give  imprecise or misleading results.  A more
elaborate method  for  situations  demanding  greater  accuracy  is
given in Section 14.1.2.

In water piping,  flow is almost always turbulent.  Formulas
for  friction  loss  with  water,  such  as  Hazen-Wi 11 iams  and
Darcy-Weisbach,  are  based  on  turbulent  flow.   Sludge also  may
flow turbulently,  in which case the friction loss  may be  roughly
that of water.  Sludge, however, is unlike water in that  laminar
flow also is common.   When  laminar  flow occurs, the friction loss
may be  much greater  than  for  water.   Furthermore,  laminar  flow
laws for  ordinary  "Newtonian"  fluids,  such  as water,  cannot  be
used for laminar flow of  sludge because sludge is not a Newtonian
fluid;  it follows  different flow laws.

Figure  14-1  may  be used to  provide  rough estimates  of friction
loss under  laminar flow  conditions.  This figure  should  be  used
when:

     •  Velocities are at  least 2.5  feet per second  (0.8  m/s) .
        At  lower  velocities,  the difference between  sludge  and
        water may  greatly  increase.

     •  Velocities do  not  exceed  8  feet per second  (2.4  m/s).
        Higher velocities are  not  commonly  used because  of  high
        friction loss and  abrasion  problems.

     •  Thixotropic behavior  is not considered.   Friction losses
        may  be  much  higher  in suction piping.   Also,  when
        starting a pipeline  that  has been shut down  for over a
        day, unusually high pressures may be needed.

     •  The pipe  is  not  seriously  obstructed  by grease or  other
        materials.

As an  example, consider  a pipe carrying unstabilized primary
sludge.  The pipe is 500 feet  (152 m) long and 6 inches (150  mm)
in diameter;  flow rate  is  300 gallons per minute  (19  1/s) .
Assume  that  the  sludge  solids  concentration  may  be  up  to
7 percent solids  on  occasion.   Using the Hazen-Williams  formula
with a  "C"  of 100,  a  friction loss of  6.5 feet  (2.0 m) would
apply.   If  laminar sludge  flow  occurs,  Figure  14-1 gives a
multiplication factor of 5.8,  so a friction  loss of 38 feet
(12 m)  might occur.    The  friction loss could easily  vary  from
6.5 to  38 feet  (2.0  to 12  m)  in actual operation  due to  changes
in sludge properties  and  factors not  considered on Figure  14-1.

Grit slurries  are usually dilute;  also,  grit particles do  not
stick to  each  other.   Therefore,  ordinary  friction formulas  for
water  are usually  adequate.   A velocity  of about 5 feet  per
second  (1.5  m/s)  is  typically  used.   Low  velocities  may  cause
deposition  of  grit within the pipe; high  velocities  may  cause
erosion.
                              14-2

-------
      14
      12

   x

   g  10
   o
       8
   O
u   6

G,

5   4
                                                 DIGESTED
                                                  SLUDGE
               UNTREATED PRIMARY AND'
               CONCENTRATED SLUDGES
        01234     56789    10

                  SLUDGE CONCENTRATION, % solids by weight

   NOTE: MULTIPLY LOSS WITH CLEAN WATER BY K TO
        ESTIMATE FRICTION LOSS UNDER LAMINAR
        CONDITIONS (SEE TEXT).

                            FIGURE 14-1

             APPROXIMATE FRICTION HEAD-LOSS FOR LAMINAR
                          FLOW OF SLUDGE
    14.1.2  Application of Rheology to
            Sludge Pumping Problems

Water, oil, and  most  other common fluids are  "Newtonian."   This
means  that the  pressure drop is  directly proportional to  the
velocity  and  viscosity  under  laminar flow conditions.   As  the
velocity  increases  past  a critical value,  the  flow becomes
turbulent.  The transition from laminar to turbulent flow depends
on  the Reynolds number,  which  is  inversely proportional  to
the fluid's viscosity.   The viscosity is  a constant  for  the  fluid
at  any given  temperature.  Formulas for Newtonian fluids  are
available in fluid mechanics textbooks.

Wastewater sludge,  however,  is  a  non-Newtonian   fluid.    The
pressure drop  under laminar conditions is not  simply proportional
                              14-3

-------
to flow, so the viscosity is not a constant.   Special  procedures
may be  used, however,  to  determine  head  loss  under  laminar  flow
conditions,  and the velocity at which  turbulent flow begins.
These procedures use  at least  two constants to describe the  fluid
instead of a  single  constant  (the viscosity) which is  used  for
Newtonian fluids.

The behavior of wastewater  sludge is  compared with  the  behavior
of water on  Figure  14-2.   This figure is based on  steady  state
behavior, after  thixptropic  breakdown.    (Thixotropic  breakdown
will  be discussed in  a  subsequent  paragraph.)   The  following
features  are  notable  concerning the  behavior  of wastewater
sludge :


      •  Essentially  no   flow occurs  unless the pressure is  high
        enough  to  exceed  a yield stress TO.


      •  Turbulent  flow  may occur, but a  much  higher velocity  is
        needed  for sludge  than for water.


      •  In fully developed turbulent flow, the pressure drop  is
        roughly that  of water.


      •  For the laminar plastic  flow region, sludge approximately
        obeys the  laws  of  a  "Bingham plastic."  A Bingham plastic
        is described  by two  constants, which are  the yield  stress
        T0 and  the coefficient of rigidity, rj .


      •  It  is  also possible   to  consider  sludge  to  be  a
        "pseudoplastic" material.    In  that  case,  two  other
        constants  are used,  and the  formulas  are  different.   The
        following  discussion uses  the Bingham plastic approach.
        14.1.2.1  Solution  of  Pressure Drop Equation

If  the  two constants  ro  and  ?? can be determined, it is quite
easy  to  determine  pressure  drop over the  entire  range  of
velocities with the aid of  Figure  14-3 and ordinary equations for
water.   To use  this  figure,  calculate  the  two dimensionless
numbers  (Reynolds  and Hedstrom)  by reading the  graph.    The
only  real difficulty  is  in  obtaining  the  two  constants;  see
Section 14.1.2.4.

The two  dimensionless  numbers are a Reynolds number,  given by:
    Re =  .P                                               (14_1}
                              14-4

-------
where:

    Re  = Reynolds number,  dimensionless

     f  = density of sludge,  Ib (mass)/ft3f  (g/cm3)

    V  = average velocity, ft (cm/s)

    D  = diameter of pipe, ft (cm)

    n  = coefficient of rigidity,  Ib  (mass)/ft-sec, poise  (same
         as dyne-s/cm2 and g/cm-s);


and the Hedstrom number,  given by:
         DT0 gcp
    He =       °                                           (14-2)
where:

    He = Hedstrom number,  dimensionless

    ro = yield stress,  lb(foree)/ft2

    gc = units conversion factor:
         32.2 lb(mass)-ft/lb(force)-sec2  for English units
         1.0 for metric units
           2fPLV2                                          . , .  -..
           	—                                          (14-3)
where:

   Ap   =  pressure  drop  due  to  friction,   1b(force)/ft2,
           (dyne/cm2)

    f   =  Fanning friction  factor  from Figure  14-3,  dimension-
           less

    L   = length of pipeline, ft (cm)


There are a few subtleties in the correct use of these  equations.
First, the  Reynolds  number  in  Equation  14-1  is not the same  as
a  Reynolds number based  on viscosity.   In plastic flow,  an
effective viscosity may be defined,  but it is variable  and  it can
be much greater  than  the  coefficient  of  rigidity.   Consequently,
the two  Reynolds  numbers  can differ by factors of more  than ten
under  some  conditions.   Second, many  textbooks  use  a  somewhat


                              14-5

-------
different definition  of f,  which  is  four  times the  value  as
used  in  Equation 14-3  and  Figure 14-3.'  Third,  care is  required
with  units.    For  English  units,  it   is  not  possible  to use
pounds  (mass)  in density at  the  same time as  pounds (force)  in
stress  without  introducing the  conversion factor  (gc)  into
Equations 14-2  and  14-3.   Alternatively, the  "slug,"  English mass
unit could be used.
     r
          BINGHAM PLASTIC, e.* SLUDGE AFTSR
          THIXQTRQPIC BREAKDOWN
          NEWTONIAN FLUID, e.g. WATER
  o
  u
  5
  mtM
  DZ
  Is
i
Ul Z -a
(c o ~,
uj Q —
X tr. H
EH a. 3
CURVATURE IN
THIS REGION
DUE TO PLUG
FLOW IN THE
MIDDLE Of
THE PIPE
SLOPE s; COEFFICIENT
OF RIGIDITY
T
                                            X
                               BAfl Of SHEA?!
                         PHQPQRTIO-NAL TO VELOCITY IN PIPELINE
                         UNITS: SECONDS''
                             FIGURE 14-2

            COMPARISON OF BEHAVIORS OF WASTEWATER SLUDGE
               AND WATER FLOWING IN CIRCULAR PIPELINES
These  equations  apply  to  the entire  range  from virtually  zero
velocity  to the  fully turbulent  range,  except  that Figure  14-3
does  not  allow for pipe  roughness.   To allow for  pipe  roughness,
ordinary  water  formulas  may be  used.   If,  for  example,  the
Hazen-Williams  formula  gives   a   higher  pressure   drop  than
Equation  14-3,  then pipe  roughness  is  dominant,  the flow  is
                               14-6

-------
fully  turbulent, and  the pressure  drop will be  given by the
ordinary water formula to a  sufficiently good  approximation for
engineering purposes  (3).
                                              He, HEDSTROM NUMBER
cc
O
cj
<
u.
Z
g

u
CE
U
   0.001
0.01
                        Re, REYNOLDS NUMBER (DV^/,,)

                           FIGURE U-3

            FRICTION FACTOR FOR SLUDGE, ANALYZED AS A
                        BINGHAM PLASTIC
Figure  14-3 also shows  whether  flow  is laminar  or  tubulent.
The friction factor  f  is located  by  the intersection of the
Reynolds  and Hedstrom numbers  (Re  and He).   If  this point is
above  the dashed line on Figure 14-3, or  if  the Reynolds number
Re is  less  than 2,000,  the flow is  laminar; otherwise it is
turbulent.   For example, at  Re  = 10^,  a  Hedstrom  number of
1Q4 gives turbulent  flow,  while  a Hedstrom  number of 106 gives
laminar flow.


Interpolation  on logarithmic  graphs  such  as Figure  14-3 is
somewhat difficult.   This  is particularly "true for the  Hedstrom
number curves on Figure  14-3.  If the logarithm (base 10) of He
is calculated,   interpolation  between  lines  will be  linear.
Alternatively,  if flow is laminar,  the  Buckingham  equation  (3,4)
may be used.  Figure  14-3  incorporates the Buckingham equation in
the laminar  region.   The  Bingham pressure  loss  equation is an
approximate  solution  of the Buckingham equation (5,3).
                             14-7

-------
        14.1.2.2  Design  Example

The  designer  wishes  to  transport  anaerobically  digested  sludge
6  miles  from  one  plant  to  another  plant  where  there  are
dewatering facilities.    If  transported  at  5  percent solids,  the
sludge quantity  is  100,000  gallons per  day  (378 m3/day).   The
sludge  may be  diluted  or  thickened,  if  desired,  to improve
economics.  All of the sludge  must  be  pumped  in  a 4-hour  period
each  day  to  accommodate dewatering  schedules  at  the  receiving
plant.

It  is  assumed that the  sludge can be  considered as a Bingham
plastic using  the following  data from Canton, Ohio (2):
Ca_se

  1
  2
  3
    Solids
concentration,
   percent	

    7.12
    5.34
    3.56
                           Yield stress
                              dyne/cm^

                                100
                               30.5
                                5.8
                                             O'
Coefficient
of rigidity
y ,  g/cm-s

   0.40
   0.24
   0.13
For  a  comparison,  water  has  a yield  stress  of  zero and  a
coefficient  of  rigidity of  about 0.01  g/cm-s.   The pipe  is
assumed to be unlined  steel pipe,  schedule 40; nominal pipe sizes
of 4 to 10 inches (100 to 250  mm)  in diameter will be considered.

The calculation is illustrated in detail for the 8-inch (200-mm)
pipe and  7.12  percent sludge.  First, the  flow  rate is needed.
If the sludge were at 5 percent solids, 100,000 gallons (378 m3)
of sludge  would  be  transported  daily.   Since the  sludge  is  at
7.12 percent, the volume  is:
    100,000 x ?512 =  70,224  gallons  (266
and the flow rate is:
    	70,224 gallons/day	
    4 hours flow/day x  60  min/hr
                      =  292.6 gpm  (18.46 1/s)
Calculations of Reynolds  and  Hedstrom numbers will be carried out
in the centimeter-gram-second (cgs) system because T Q  and  77  are
given in cgs units.   The  flow rate  in cgs units is:
292.6 gpm x  3.785  1/gal  x  1,000 cm3/! =
             60  sec/min
                                       Cm
                                                    3/sec
                              14-8

-------
The internal  diameter of an  8-inch (200 mm) Schedule  40 pipe is
7.981  inches  (20.27 cm)  and the  cross sectional area is
322.7  cm2.   The  velocity  V is  the flow  rate  divided  by the
area:
    .,   18,460  cm^/sec   '  _  „    ,
    V = 	'-	£5	  = 57.2  cm/sec
           322.7  cm2


The Reynolds number  is  obtained  from Equation 14-1:
          VD   1.0  x  57.2  x  20.27    nooo .,.        ,
    Re = P— =        	n An	  = 2898 (dimensionless )
           i)             U • ft U
The Hedstrom number  is  obtained  from Equation 14-2:


   He . 5?!32fiSc =  (20.27)2 x 100 x 10 x 1.0 = 25     (dimensionless)
          T,2               (0.40)2
Refering  to  Figure 14-3, f  is  about 0.08.  The  flow is laminar,
not turbulent.

The length L  is needed  in  cgs  units:


    L = 6 miles x  5,280 ft/mile x 30.48 cm/ft = 965,600 cm


Now  Equation 14-3  is  used  to calculate pressure drop  due to
friction:


    Ap = 2£ P Lv2 = 2 x °-08 *l-°* 9f -60Q x (57'2)2 = 24,940,000 dyne/cm2
Convert this  value  to pounds per square inch:
     24,940,000  dyne/cm2  =
                           24.94 x 106 dyne x 2.248 x HT* Pounds (force)
                                                          dyne
                                   cm2 x 0.1550 in.2/cm2

                        = 362 psi (2.49 MN/m2)
                               14-9

-------
This value may be compared to  the  value  for water for the  same
conditions,  calculated from the Hazen-Williams equation:


    V = 1.318  C R0.63 S0.54                                (14-4)


where:

    V = average velocity, ft/sec,

    C = friction  coefficient,

    R = hydraulic radius = -j of diameter,  ft,

    S = hydraulic gradient, ft/ft.


This equation may be  rearranged  and  solved on  a  calculator,
or  tables or nomographs  may  be used.   In the  present case,
V =  57.2  cm/sec  =  1.88 ft/sec  and R = 0.166 ft.   With  a C of
100, S  is 0.00310,  indicating  a  pressure drop  of  98.2 ft or
42 psi.   The  drop with  this  sludge is  362 psi or about  9 times
higher  than  the drop  for water.

For the various cases, calculations are  summarized  in  Table 14-1.
Friction factor plots from Figure 14-3 are shown  on Figure 14-4.

A precaution that is  useful  for detection of computational error
is  to  check  to  see  whether  the  pressure drop  across the pipe
calculated by the  above procedure  produces  a sufficient shear
stress  at the  pipe wall  to exceed the yield stress  of  the  sludge.
If  the  yield  stress  is  not  exceeded,  the sludge will not flow.
The pressure drop needed is  calculated  by setting  the  calculated
shear stress  at the wall equal to yield  stress:
                                                          n,  c
           =  To9c                                         (14-5
      4L
where :

    A po = pressure  drop needed to exceed yield stress.


Results of  the  calculation  are  shown for Case  I  and  Case 2 in
Table  14-2.   Equation  14-5  is also  useful  as a  screening
test.   If  TO, D,  and  L  are known,  it is possible  to quickly
calculate the minimum pressure  drop  that could occur,  regardless
of  velocity or flow rate.   If  Apo  is excessive,  the diameter
D  should  be increased.   Impractical  pipe  sizes could be
quickly  eliminated as requiring too high a  pressure drop for
consideration.
                             14-10

-------
Values  from  Table  14-1 and  14-2 are  plotted  on Figure 14-5.
Selection  of the  optimum pipe diameter  and  solids  content
requires  an economic  analysis.    However,  it  is  evident that  at
the more  reasonable  pressure  drops (below 200 psi or 1400 kN/m2),
the  7.12 percent solids  has a  much higher  pressure  drop at  a
given pipe  diameter even though  the  volumetric flow rate is much
lower than  for the  other  two cases.   At 8  inches  (200 mm), the
pressure  drops are  about  the same for  the 5.34  percent and the
3.56 percent  sludges.   However,  as noted  in  Table 14-1, the flow
is not in the  turbulent  regime  for the 5.34 percent sludge.  This
is  a disadvantage  because  small  changes  in  the rheological
constants   To and  77  could  cause  changes  in  f.   The 3.56 percent
solids  content  is  probably  a  better selection  based  on the
likelihood  of  more  stable  operation.   At  10  inches  (250 mm), the
value of  f  is considerably  higher  for  the   5.34  percent sludge
than for  the  3.56  percent  sludge.    The  choices  between 8-inch
and  10-inch  (200  and  250  mm)  diameter  and 3.56 percent and
5.34 percent sludge would have  to be  made  on  the basis of minimum
overall  cost.   The  5.34 percent sludge  will be  more   expensive
to  transport,  but  this cost  increase  may be  offset  by  more
economical dewatering  at the plant receiving  the sludge.
                            TABLE 14-1

            SUMMARIZED CALCULATIONS FOR NON-NEWTONIAN
                       FLOW EXAMPLE PROBLEM
                                                      Pressure drop.
Diameter

Case in
1 4.
5.
6.
7.
10.
2 4.
5.
6.
7 .
10.
3 4.
5.
6.
7.
10.



cm
03
05
06
98
02
03
05
06
98
02
03
05
06
98
2
10
12
15
20
26
10
12
15
20
25
10
12
15
20
25
.2
.8
.4
. 3
.4
.2
.8
.4
. 3
.4
.2
.8
.4
. 3
.4
Average
velocity,
cm/sec
225
143
99.1
57.2
36.3
300
190
132
76.3
48.4
450
286
198
114
72.6
Reynolds
number ,
Re
5
4
3
2
2
12
10
8
6
5
35
28
23
17
14
,750
,580
,820
,900
,310
,780
,150
,480
,440
,130
,400
,200
,500
,800
,200
Hedstrom
number ,
He
65
103
148
257
405
55
87
126
218
343
36
56
82
141
222
,000
,000
,000
,000
,000
,300
,000
,000
,000
,000
,000
,000
,000
,000
,000
Fanning
friction
factor ,
f
•010b
•019b
•°3b
•08b
.20°
.0083
.0085
.0090
.019°
.035b
.0066
.0070
.0072
.0075
.0080
psi

sludge
1,380
775
534
362
290
2,038
673
285
152
90C
3,650°
("*
1,250=
513 =
135C
f
46C

water3
1,190
394
162
42
14
2,020
667
275
72
24
4,280
1,423
582
152
50
 aCalculated from Hazen-Williams equation With a friction coefficient (C) of 100.

 Flow is not in the turbulent region.
 GNote that pressure drop for sludge, by equation 14-3,  is less than the pressure
 drop for water if C = 100.  The pressure drops would be about the same if C=110.

            2             2
 1 psi = 6.9 kN/m  = 69,000 dyne/cm
                               14-11

-------
Note  that the  pressure  drop for  Cases 1  and 2  is greater  in all
cases  than the minimum drop Apo  (see Figure  14-5).

                              FIGURE 14-4
                FRICTION FACTORS FOR EXAMPLE PROBLEM
                1,0 11
                                             US, HtDSTHOM HUM BE R

               o.ooi
                              He, REYNOLDS NUMBER
                              TABLE 14-2
                  PRESSURE REQUIRED TO EXCEED YIELD
                      STRESS - EXAMPLE PROBLEM
      Diameter,
         in.

         4.03
         5.05
         6.06
         7.98
        10.02
                                    Pressure drop Apo, psi
                             Case 1
                                                       Case 2
= 100 dyne/cm^
To = 30.5 dyne/cm
    548
    437
    363
    276
    220
      167
      133
      111
       84
       67
 Pressure drop to cause the shear, stress at pipe wall to
 exceed the yield stress To-  Higher pressures may be
 needed to start the pipeline due to thixotropic effects
 not considered in Figure 14-3.

 1 in. = 2.54 cm _              2
 1 psi = 6.9 kN/m  = 69,000 dyne/cm


         14.1.2.3   Thixotropy  and Other Time-Dependent
                    Effects

Besides  possibly  being  dependent  on  the  shearing  rate,  the
flow  resistance  of  liquids  can depend  on  the length  of  time of
shearing or on some  function of  both the  time and  intensity of
shearing.   The most  commonly  encountered  time-dependent  change in
viscosity  is  a drop which occurs  with time of shearing,  followed
by  a gradual recovery  when shearing is stopped.   This behavior is
called  thixotropy.   A familiar  example is an  ice cream milkshake,
                                 14-12

-------
which "sets up"  in its container and will  only  flow out when the
container is  rapped  or jarred several  times.   The  structure
rebuilds when  the rapping is stopped.   Paints  typically not only
are Bingham plastics  but are  thixotropic as well.  They will flow
for a short time  after being  "worked" by the paint brush so brush
lines tend to  disappear.  Their "plastic"  characteristics rebuild
quickly after  shearing stops  so the paint  does  not flow downwards
on vertical surfaces.
   2,000 r-
D_
o
cc
o
UJ
a:

w)
w
UJ
DC
   1,000
    6QQ
400
     200
     100
     40
           1 inch = 2,54 cm
           1 psi = i.9 kN/m2
                                                             10
                             PJPE DtABJETER,


                             FIGURE 14-5

                PRESSURE DROPS FOR EXAMPLE PROBLEM
                               14-13

-------
Wastewater  sludge   is  also thixotropic.    The  effect  is
increasingly   important  as  the  percent  solids  and  percent
volatile solids increase.  Thixotropy has three major  effects:

     9  It complicates the measurement of constants such as  the
        yield  stress  rQ.

     •  It makes pump suction conditions  very  important.  In one
        case,  a centrifugal  pump  produced ample pressure  to  move
        the sludge through a hose.  The  pump was suspended  in  a
        lagoon  but  the  sludge would not  flow into  the   pump
        suction.   It was  found  that  mixers   next to  the   pump
        caused thixotropic breakdown sufficient for satisfactory
        pumping (5,6).

     •  It raises  the pressure needed  to start a pipeline  that
        has been shut down.   At one  installation, this effect was
        found  to be  significant for shutdowns  exceeding one  day.
        An operating  procedure is used to prevent this problem;
        that is, if shutdowns over 8 hours are  expected, the  line
        is purged of  sludge  (5,6).

Permanent degradation of yield  stress  can occur  with time  of
shearing.    Intense  shearing  produces  this  result in   high
polymers.  This phenomena can be  expected in wastewater sludges,
when shear levels  are sufficiently  high  to  physically disrupt  a
portion of the  particles making up  the sludge.   If this  occurs,
it may be difficult to later thicken or dewater the  sludge.

Sometimes the  reduction in viscosity  that   occurs with   time
of  shearing  is actually  the effect of  a temperature  increase
produced  by  the energy  delivered to the liquid.   The general
effect  of an  increase  in  temperature with both Newtonian and
non-Newtonian  liquids is a reduction in viscosity.  However, for
sludge, the main  effect  of  temperature  is that low  temperatures
may  cause  the grease fraction of  the  sludge  to harden.   Other
temperature  effects  appear to be unimportant, at least up  to
160°F (70°C)  (5,7).

There  is  another unusual   effect  that  occurs in  wastewater
sludge pipelines:  slippage and  seepage   (6).    Essentially,  the
sludge is riding on a thin film of water  next  to the  wall of the
pipe.   This  effect  is  noticeable at very  low velocities  when
starting a sludge  pipeline;   it partially  offsets the  thixotropic
effect.   Seepage  and slippage  are hard to  calculate but are
useful when starting  pipelines  flows (6).


        14.1.2.4  Obtaining  the Coefficients

Figure 14-3 cannot be used   unless  the yield  stress   o and  the
coefficient of rigidity    can be obtained.  There is  a reasonable
amount of  data on anaerobically  digested sludges  (3,5,7,8)  but
very little data on sludge that has  not been digested.


                              14-14

-------
Several  types  of  instruments  are  available  for  viscosity
measurements.   However, only  two  of these types  are suitable
for  sludge:   test  pipes  and  rotational  viscometers.   Some
instruments, such  as capillary  viscometers,  are  unable  to handle
the relatively large particles  in sludge; other  instruments,
such  as  ball-drop viscometers,   are  not  suited   to  strongly
non-Newtonian fluids such as sludge.

Flow curves  from  test pipes are directly  scalable  to  full-scale
pipes provided  flow is laminar.  However, the onset  of  turbulence
in a  large  pipe cannot be predicted directly  from small pipe
tests.  It  is  necessary to use the yield  stress  and  coefficient
of rigidity, compute Reynolds and  Hedstrom numbers, and use
Figure 14-3 to predict the  onset of turbulence.   The  flow curves
obtained  with test  pipes do not provide  fundamental  rheological
data,  because  at a given  flow rate, shear stress and rate of
shear vary across the radius of  the  pipe.   By using the
Rabinowitsch equation,  the  flow  curve  can  be  transformed  into
a  rheologically  correct  shear  stress  versus  rate  of  shear
curve  (9).   An offsetting  disadvantage  of test  pipes  is that  a
high  degree of experimental  skill  is  required  to  get  reliable
data.  Also these  installations  are relatively expensive and
cumbersome and  require large sample volumes.

For  sludge, the  best  instrument  appears  to be  a  rotational
viscometer.   In this  type of machine, the test  liquid  is placed
between  two concentric cylinders, one of  which rotates.   The
torque on  a cylinder is measured  as a function of rotational
speed.   Such machines  can  produce approximately  uniform  shear
rates at  given  shear stresses, provided the space between the bob
(inner cylinder)  and  cup  (outer  cylinder)  is  small compared to
the bob  radius.   Viscometers  in which  the  bob  rotates and the
twisting  force on  the  cup  is  measured are relatively easy to
design mechanically but  turbulence  occurs  at low shear  rates for
low viscosity materials.  Turbulence onset  does  not  occur  until
much higher  shear rates  for viscometers  in which the  cup rotates
and the twisting force on  the  bob  is measured.   In  both types of
viscometers, end  effects become substantial if  the bob and cup
are not long relative  to the clearance.

There  are  a  number  of  viscometers  which  feature  rotational
movement,  but either do not have  constant clearances  between an
inner  and  an  outer  cylinder, or do not  control  or measure
shearing  rate or shear stress.  These devices are of little  value
for  obtaining  consistency curves for  non-Newtonian liquids.

The  nearly uniform shear  rate  achievable   in rotational
viscometers  allows  direct  measurement  of  the fundamental  shear
stress-rate of  shear  curve, which  is a major advantage when it
comes  to application to complex flow  relationships.   Rotational
viscometers are simple to operate.   Their primary disadvantage is
that  close clearances  between outer  and  inner cylinders are
needed  to  give uniform  shear rates across  the  gap between
cylinders.   Obviously  too  small  a   clearance  will give  erroneous
results for sewage sludges.  Gap size should  not  be  reduced  below


                             14-15

-------
1.0 mm  (0.025 inch).   Sludge  must  be screened  to  remove large
particles.    This  creates  no  substantial  error because a  few
large particles do not strongly affect the coefficients.

A representative test  curve adapted from  Rimkus  and Heil (5) is
shown on  Figure  14-6.   In  this  test, the  viscometer  speed  was
gradually increased from  zero to 100 rpm  and then decreased.
Torque  was measured and converted to shear stress,  providing
"consistency  curves."   The  upper  curve   (increasing  speed)
shows  thixotropy;   the  lower  curve  (decreasing  speed)  shows
behavior of the fluidized sample.  The lower  curve is appropriate
for pipeline  design because  the  sludge  is  fluidized  by  passing
through a pump.  In this case,  the shear stress projected  to  zero
rpm (232  dynes/cm^)  is  the  yield stress   To; the coefficient of
rigidity  rj is  the slope  of  the straight  part of the lower curve.
Even when  fluidized,  sludge is not exactly a Bingham plastic, as
shown by curvature in the lower curve  at low  rpm.  This departure
from  Bingham plastic  conditions  can  be  used to refine  the
pressure  drop  calculations.    The viscometer for this test was a
Haake Model RV-3 Rotoviso with  sensor  head MV-1.
              10
             20
                          RPM OF VISCOMETER HEAD

                        30   40    50    60   70
                                             BO
90    100
  ,u
  iA
  m
  c

  "
  Hi
  cc
  te
  £E
  <
  yj
  X
  V)
      700
      600
      500
      400
300
200
      100
           /^
           /   v'fr
          /   V*
         \v
          \\
            \*
             \
              \
            - T0 = 232 dyne/cm2
                                                 235
                                                 SEC.-1  ,
                                                 100 RPM-1-
                      J_
                             SAMPLE; LAGOONED ANAEROBICALLY
                                   DIGESTED SLUDGE
                                   13% SOLIDS, 40% VOLATILE

                               I    I     i    I    I.. ..
       0     20   40   60   SO   100   120  140  160  • 180  200   220

                           SHEAR RATE, sec.-1


                            FIGURE 14-6

                VISCOMETER TEST OF SEWAGE SLUDGE (5)
                               14-16

-------
        14.1.2.5   Additional Information

Sludge  has  been  successfully  and reliably  pumped  in the
laminar  flow range.   Some  of the  installations  describedin
Section  14.1.6,  Long  Distance Pumping,  operate  in  this  range.
That section  also contains several design recommendations.

Several  researchers have investigated sludge pumping,  rheology,
and related subjects  (10 through 24).


    14.1.3 Types of Sludge Pumps

Sewage sludges can  range  in  consistency  from  a  watery scum to
a thick  paste-like  slurry.   A different type  of  pump may be
required  for each type of sludge.   Pumps which are currently
utilized for  sludge transport  include  centrifugal,  torque  flow,
plunger,  piston,  piston/hydraulic  diaphragm,  progressive  cavity,
rotary,  diaphragm, ejector and  air lift pumps.  Water eductor
pumps are  sometimes used to pump  grit  from aerated  grit  removal
tanks.
        14.1.3.1   Centrifugal Pumps

A centrifugal pump  (Figure  14-7)  consists  of a set of  rotating
vanes in a  housing  or  casing.   The vanes may be either open or
enclosed.   The  vanes impart energy to a fluid through  centrifugal
force.   The  non-clog centrifugal pump for sewage or sludges, in
comparison  to a centrifugal pump designed to handle clean water,
has  fewer but larger and less obstructed  vane passageways in  the
impeller;  has  greater  clearances  between  impeller  and casing;
and has  sturdier bearings,  shafts,  and seals.   Such non-clog
centrifugal  pumps may be used to circulate  digester contents  and
transfer sludges  with lower solids  concentrations, such  as  waste
activated sludge.   The.larger passageways and greater  clearances
result  in increased  reliability at a cost of lower  efficiency.

The  basic   problem  with  using  any  form  of centrifugal  pump
on sludges  is  choosing  the  correct size.   At any given speed,
centrifugal  pumps operate well only if pumping head is  within a
relatively  narrow range;  the variable  nature of  sludge,  however,
causes  pumping  heads to vary.  The selected  pumps must  be  large
enough  to pass solids without  clogging of the impellers and  yet
small enough to  avoid  the problem of  diluting  the  sludge by
drawing  in  large quantities of overlying  sewage.   Throttling
the  discharge  to  reduce  the capacity of a  centrifugal pump is
impractical both because  of energy  inefficiency and because
frequent clogging of the  throttling  valve  will  occur.   It is
recommended  that  centrifugal  pumps  requiring capacity  adjustment
be equipped with variable-speed  drives.   Fixed capacity in
multiple pump  applications  is  achieved  by  equipping  each  pump
with a  discharge flow  meter and  using  the flow meter signal
in conjunction  with  the variable speed  drive to control the  speed


                             14-17

-------
                                              on pv
of the pump.  Seals last  longer  if  back  suction "pumps  are  used,
Utilizing the back of  the  impeller  for  suction removes  areas  of
high pressure  inside  the  pump casing  from  the  location of  the
seal and prolongs  seal  life.
                                   DISCHARGE
                BEARINGS
            X
                 SHAFT
                        SEAL

            NON-CLOG IMPELLER
                                                   SUCTION
                                         CASING
                           FIGURE 14-7

                        CENTRIFUGAL PUMP

Propeller  or mixed flow  centrifugal pumps  are  sometimes used
for  low  head applications because  of higher efficiencies,  a
typical application  is  return activated  sludge  pumping.   When
being considered  for  this  type of application, such pumps  must  be
of sufficient size (usually at least  12 inch  [300  mm]  in  suction
diameter)  to provide internal clearances  capable  of passing the
type of debris  normally  found within the activated sludge  system.
Such pumps should not be  used  in activated sludge systems  which
are not preceded  with primary sedimentation facilities.


        14.1.3.2   Torque Flow Pumps

A  torque  flow pump  (Figure  14-8),  also known  as  a recessed
impeller  or  vortex  pump, is a  centrifugal pump in  which the
impeller  is  open faced and recessed  well  back  into the pump
casing.   The size  of  particles that  can  be "handled by this
type of pump is  limited only  by  the  diameter of  the  suction  or
discharge  openings.   The  rotating impeller imparts a  spiralling
motion  to the fluid passing  through  the  pump.  Most  of the
fluid does not actually pass  through the  vanes of the  impeller,
thereby minimizing abrasive  contact with it  and reducing the
chance  of clogging.   Because  there  are no  close  tolerances
                             14-18

-------
between  the  impeller and  casing,  the chances  for  abrasive
wear within  the  pump  are further reduced.    The  price  paid for
increased pump longevity and  reliability is  that  the pumps are
relatively  inefficient compared with other non-clog centrifugals;
45 versus  65  percent  efficiency  is  typical.   Torque flow pumps
for sludge  service should always have nickel or chrome abrasion
resistant  volute  and  impellers.    The pumps  must be  sized
accurately  so that excessive  recirculation  does not occur at any
condition at operating head.   Capacity  adjustment and control is
achieved in the same manner as for other centrifugal pumps.
                            DISCHARGE
                               j i
    OPEN
    IMPELLER
                                                     SUCTION
                           FIGURE 14-8

                        TORQUE FLOW PUMP
        14.1.3.3   Plunger Pumps

Plunger  pumps (Figure  14-9)  consist  of pistons  driven by an
exposed  drive crank.  The  eccentricity of the  drive crank is
adjustable,  offering  a variable  stroke  length  and  hence a
variable positive  displacement pumping action.   The  check valves,
ball  or  flap, are  usually paired  in  tandem before  and after
the  pump.    Plunger pumps  have  constant  capacity  regardless
of  large variations  in  pumping  head,  and can  handle  sludges
up  to 15  percent  solids   if  designed  specifically for  such
service.   Plunger  pumps are  cost-effective where  the installation
requirements  do not exceed  500  gpm  (32  1/s),  a  200 feet (61 m)
                             14-19

-------
discharge  head,  or 15 percent sludge  solids.  Plunger pumps
require  daily  routine servicing  by the  operator,  but  overhaul
maintenance effort and cost are low.
          DESURGING
           CHAMBER
                                          PACKING
                                               DESURGING
                                                CHAMBER
 DISCHARGE
                                                           SUCTION
                            FIGURE 14-9

                           PLUNGER PUMP
The  plunger  pump's  internal  mechanism  is visible.   The pump's
connecting rod attaches to the piston inside its hollow  interior
and this "bowl" is filled  with oil  for lubrication  of the  journal
bearing.   Either  the  piston  exterior or  the  cylinder  interior
houses the packing, which  must be kept moist  at all times.  Water
for this purpose is usually  supplied from  an  annular pool  located
above the packing; the pool receives a constant trickle of clean
water.   If the packing  fails, sludge  may be sprayed over the
surrounding area.

Plunger pumps  may operate with up  to  10  feet (3m)  of  suction
lift; however, suction lifts  may reduce the  solids concentration
that  can be  pumped.   The use  of  the  pump with the  suction
pressure higher than the  discharge  is not practical because flow
will  be forced  past  the check  valves.    The  use of  special
intake and  discharge  air  chambers  will  reduce  noise  and
vibration.    These  chambers also smooth  out  pulsations  of
intermittent  flow.   Pulsation dampening  air chambers,  if used,
should be  glass  lined  to avoid destruction  by  hydrogen  sulfide
corrosion.   If  the pump  is  operated when the discharge pipeline
                              14-20

-------
is obstructed, serious damage  may  occur to the pump,
pipeline;  this problem  can be  avoided by a simple
arrangement.
motor, or
shear pin
        14.1.3.4   Piston Pumps

Piston  pumps are similar  in  action to the plunger pumps, but
consist  of  a  guide piston and  a fluid  power piston.   (See
Figure  14-10).   Piston  pumps  are capable of  generating high
pressures  at low flows.   These pumps are more expensive than
other  types  of  positive  displacement  sludge pumps  and are
usually used in special applications such  as  feed pumps  for heat
treatment systems.  As for  other  types  of  positive  displacement
pumps,  shear pins  or other  devices  must be used to prevent  damage
due to obstructed  pipelines.
                           DISCHARGE
                               t
                              J~L
       DIAPHRAGM
         (TYP)
                           HYDRAULIC
                             SYSTEM
                                                  SUCTION
                                         POWER
                                         PISTON
                        GUIDE
                        PISTON
                           FIGURE 14-10

                           PISTON PUMP
                             14-21

-------
A variation of the piston pump has been developed for use where
reliability and close control  are  needed.   The pump utilizes a
fluid  power piston  driving  an  intermediate hydraulic  fluid
(clean water),  which in turn  pumps  the sludge  in  a  diaphragm
chamber (Figure 14-11).   The speed of  the hydraulic fluid drive
piston can  be  controlled to  provide  pump discharge conditions
ranging from constant flow  rate to  constant pressure.  This pump
is  used  primarily  as a feed  pump for  filter presses.   This
special pump has  the  greatest  initial  cost  of any piston pump,
but  the  cost  is  usually offset by low  maintenance  and high
reliability.
                                              FLUID
                                              POWER
                                              PISTON
                                           y


                                        SUCTION
                                       SUCTION

                FIGURE 14-11

COMBINATION PISTON/HYDRAULIC DIAPHRAGM PUMP
        14.1.3.5  Progressive Cavity Pumps

The  progressive  cavity  pump  (Figure  14-12)  has  been  used
successfully on almost all types of sludge.   This  pump comprises
a  single-threaded  rotor that  operates  with an  interference
clearance  in a double-threaded helix  stator
A  volume or  "cavity"  moves  "progressively"
discharge when the rotor is rotating, hence the
cavity."    The progressive  cavity pump  may
discharge heads of  450  feet  (137  m) on sludge.
available  to  1,200  gpm  (75 1/s).  Some  progressive  cavity
pumps will pass solids  up to 1.125  inches  (2.9 cm)  in diameter.
                                    made of rubber.
                                    from suction  to
                                   name "progressive
                                    be  operated  at
                                      Capacities  are
                             14-22

-------
Rags or stringy material  should be ground up before entering this
pump.   The  rotor  is  inherently  self-locking  in  the  stator
housing when not in operation,  and will  act  as a check  valve
for  the sludge pumping line.  An  auxiliary motor brake  may  be
specified  to enhance  this  operational feature.

     UNIVERSAL
     JOINT
                                           SUCTION
 DISCHARGE
                                                         DRIVE
                             CAVITIES

                           FIGURE 14-12

                     PROGRESSIVE CAVITY PUMP
The total head produced  by  the  progressive cavity pump is divided
equally  between  the  number of  cavities created  by  the  threaded
rotor  and helix stator.   The  differential  pressure between
cavities  directly  relates  to  the wear  of  the  rotor  and  stator
because  of  the  slight  "blow  by" caused  by this  pressure
difference.    Because  wear  on the rotor and  stator  is  high,  the
maintenance  cost for this type of pump  is  the highest  of  any
sludge  pump.   Maintenance  costs  are  reduced  by  specifying  the
pump  for one class higher pressure service (one extra  stage)
than would  be used for  clean  fluids.   This  creates  many extra
cavities, reduces the  differential pressure between cavities,  and
consequently  reduces rotor and stator wear.   Also, speeds  should
not exceed  325  rpm  in  sludge  service, and  grit concentrations
should be minimized.

Since  the rotor  shaft has  an eccentric motion, universal  joints
are  required  between  the  motor  shaft  and  the  rotor.    The
design  of the universal  joint varies  greatly among different
manufacturers.  Continuous duty, trouble-free operation  of these
universal joints is best achieved by using the best quality (and
usually  most  expensive)  universal gear  joint  design.   Discharge
pressure safety  shutdown  devices  are required  on  the pump
                              14-23

-------
discharge to prevent  rupture  of  blocked discharge lines.  No-flow
safety shutdown devices are  often  used  to prevent  the rotor and
stator from becoming fused due  to  dry operation.   As previously
mentioned,  these pumps are expensive to maintain.   However, flow
rates are easily controlled,  pulsation is minimal,  and operation
is clean.   Therefore,  progressive  cavity pumps are  widely used
for pumping sludge.


        14.1.3.6  Diaphragm Pumps

Diaphragm pumps (Figure 14-13)  utilize  a flexible  membrane that
is pushed  or  pulled  to contract or  enlarge  an enclosed cavity.
Flow  is  directed  through  this  cavity by  check valves, which may
be either ball or  flap  type.  The capacity of  a diaphragm pump is
altered  by  changing  either  the length  of the  diaphragm stroke
or  the  number  of strokes  per minute.   Pump  capacity  can  be
increased  and  flow  pulsations smoothed out by  providing two
pump  chambers  and utilizing both  strokes of  the  diaphragm for
pumping.    Diaphragm  pumps  are  relatively  low  head  and low
capacity units;  the  largest available air-operated diaphragm pump
delivers 220 gpm  (14  1/s)  against  50 feet (15 m)  of head.   The
distinct advantage of  the diaphragm  pumps  is their  simplicity.
Their needs for operator  attention  and  maintenance are minimal.
There  are no  seals,  shafts,   rotors,  stators,  or  packing  in
contact with  the  fluid;  also,   diaphragm pumps can  run  in a dry
condition indefinitely.

Flexure of  the  diaphragm  may be accomplished mechanically  (push
rod or  spring)  or  hydraulically (air or  water).   Diaphragm life
is more a function of  the discharge head and  the total number of
flexures than the abrasiveness  or viscosity of the pumped fluid.
Power to  drive air  driven  diaphragm pumps  is  typically double
that  required  to  operate  a  mechanically  driven pump of similar
capacity.    However,  hydraulically operated (air or  water)
diaphragms generally  outwear  mechanically driven diaphragms by a
considerable amount.    Hydraulically  driven  diaphragm  pumps
are  suitable  for  operation  in  hazardous explosion-prone areas;
also  a  pressure release  means  in  the  hydraulic  system provides
protection  against obstructed  pipelines.   Typical  repairs  to a
diaphragm pump  usually cost  less than $75 (1978 basis) for  parts
and require approximately  two hours  of labor.   In some locations,
high humidity intake air will cause  icing problems to develop at
the  air release valve and  muffler  on  an air driven diaphragm
pump.   A  compressed  air  dryer   should be  used in  -the air supply
system when such a condition  exists.

The  overall construction of some diaphragm pumps,  the common
"trash  pump,"  is  such that  abrasion may  cause  the  lightweight
casings  to  fail before the  diaphragms,   since the  pumps are not
designed for  continuous  service.   For wastewater  treatment
applications the  mechanical   diaphragm "walking  beam"  pumps
are  more  appropriate.   These  pumps  are  dependable,  have quick
                              14-24

-------
cleanout  ball  or flap check  valves and are presently used to
handle scum and  sludge  at numerous small plants  throughout  the
country.
                                         DIAPHRAGM
                                                   CHECK
                                                   VALVE
                           FIGURE 14-13

                         DIAPHRAGM PUMP

One  air-driven diaphragm  pump is sold  in  a  package expressly
intended  for pumping  sludge  from primary  sedimentation tanks
and  gravity  thickeners.   The  basic  pump package consists of  a
single-chambered,  spring return diaphragm pump,  an  air  pressure
regulator, a solenoid  valve, a  gage, a muffler, and an electronic
transistorized timer.   This  unit pumps  a single  3.8 gallon
(14.4  1)  stroke  after an interval  of time.   The  interval  is
readily  adjusted  to   match  the  pumping rate  to the  rate  of
formation of the sludge  blanket in the sedimentation tank  or
thickener.   The  large single  stroke  capacity  of this  pump  has
several maintenance advantages.  Not only is total flexure  count
reduced,  but ball valve flushing is improved,  so large particles
cause  less  difficulty.   The  maximum recommended  solids size  is
7/8  inch  (2.2  cm).  Pump  stroke speed  is constant regardless  of
the  selected pump  flow so that minimum  scouring  velocities  are
always maintained in  the  discharge piping  during  the pumping
surge.
                              14-25

-------
The traditional sequence  of intermittent  pumping for  primary
sedimentation tanks has been  to  thicken  for an interval without
pumping  and then draw  the sludge blanket  down.  A relatively
long interval is  required  by pump motors,  since  frequent motor
starts  can cause  over heating.   Theoretically if the  sludge
concentration is  10  percent  on the  bottom  and decreases  to
8 percent at  the top  of  the  pumped  sludge zone,  then  the pumped
average is  9  percent.   However,  by  using air drive,  a diaphragm
pump can operate with  starts  every  few seconds instead of every
several minutes or  longer.   The  manufacturer  claims  its system
will draw  single intermittent pulses  from the 10 percent bottom
layer since the  sludge blanket depth  is maintained at a virtually
constant height.   Downstream sludge treatment processes can have
greater solids capacity because more concentrated sludges can be
obtained.

The City of  San  Francisco  ran independent pump evaluation tests
in  1975  (25).   They concluded  that proper use of air-driven
diaphragm  pumps  will  increase the  sedimentation  tanks'  ability
to  concentrate sludges.   The sludge  collection system in  the
sedimentation tanks and  the  sludge  pumping  equipment  had  to be
controlled together  to give optimum thickening.   Savings  in
operations  and  maintenance  as well  as  improved  thickening were
accomplished by lowering the  overall  average  rate  of  sludge
withdrawal and making the  sludge  collectors  work  continuously at
a reduced  rate instead of  intermittently.  When considering such
a  pump  installation,  the  capacity  requirement is based  on  the
maximum  rate  at  which the sludge blanket forms  in  the tank  and
not the capacity required  to  maintain minimum pipe velocities.


        14.1.3.7  Rotary Pumps

Rotary pumps  (Figure  14-14)  are  positive displacement pumps  in
which two  rotating  synchronous  lobes  essentially  push  the fluid
through the pump.   Because  rotary  pump  lobe  configurations can be
designed for  a  specific application,  rotary pumps  are suitable
for  jobs  ranging  from  air  compressor  duty  to sewage  sludge
pumping.  Rotational speed  and shearing stresses are low.  Sewage
pumping  lobes are  noncontact  and  clearances are  factory changed
according  to the  abrasive  content  of  the  slurry.    It  is  not
recommended that the  pumps be considered  self-priming or suction
lift pumps  although  they  are advertised  as  such.   Experience at
one plant  indicates that  the  pump  operates best with a bottom
suction  and  top  -discharge.   Only very  limited operational data
are available for rotary pumps used on  sludge.  Two manufacturers
now advertise hard  metal two-lobed pumps  for sludge usage.  Lobe
replacement  for  these pumps  appears  to be  less  costly  than
rotor and  stator  replacement on  progressive cavity pumps.   One
manufacturer  is  offering  hard rubber  three-lobed rotary pumps,
which are  used  successfully  for  sludge  pumping in Europe.   Test
units of  this pump are presently being  evaluated in  the United
States.   To  date   these  tests have been  unsuccessful  due  to
                              14-26

-------
the  failure of  the lobe  liners,
positive displacement pumps, must be
obstructions.
Rotary  pumps,  like  other
protected against pipeline
                           DISCHARGE
                            SUCTION


                          FIGURE 14-14

                          ROTARY PUMP

        14.1.3.8   Ejector Pumps

Sewage  ejectors  use  a charging pot  which is  intermittently
discharged  by  a  compressed  air  supply  (See  Figure  14-15).
Ejectors  are  most applicable  for incoming average  flow rates
less  than 150 gpm  (9  1/s).   These pumps require  a  positive
suction and usually  discharge  to a vented  holding  tank or basin.
Scum  and  sludge  can  incapacitate  the standard  mechanical or
electronic probe-type  level sensors offered by  most manufacturers
to  sequence  pot  discharge;  custom  instrumentation  may  be
necessary.  Large flushing  and  cleanout  connections  should be
provided.   If  ejectors are to be used  to  discharge sludge to an
anaerobic digester where  the air  could produce  an explosive
mixture,  special  precautions  should be  taken  to see  that the
units cannot bleed excessive quantities  of air  into the digester.
Ejector pumps have  been used  in some  installations  to  pump
thickened  waste-activated  sludge produced  by  the dissolved air
flotation  process.


        14.1.3.9   Gas  Lift Pumps

Gas  lift  pumps  use  low pressure gas released  within a  confined
riser pipe with  an open  top and bottom.   The released  gas bubbles
rise, dragging the liquid  up and out of the riser pipe.   Air is
commonly used,  in  which  case the pump is called an air lift pump.
                             14-27

-------
Air lift pumps  are  used  for  return  activated  sludge  and  similar
applications;  gas  lift  pumps using  digester  gas  are used  to
circulate  the  contents  of  anaerobic  digesters.    The main
advantage of these  relatively inefficient pumps  is  the  complete
absence  of  moving  parts.   Gas  lift  sludge  pumps  are usually
limited to lifts of  less  than 10 feet.   The  capacity  of  a  lift
pump can be varied by changing its bouyant  gas supply.   Reliable
gas  lift pumping   requires   the  gas  supply  to  be  completely
independent  of outside  flow  or  pressure  variables.  Gas lift
pumps with  an  external gas supply  and  circumferential  diffuser
can pass solids of a size equivalent to  the internal  diameter  of
the  confining  riser  pipe without  clogging.   When  the gas  is
supplied by  a  separate  inserted  pipe,  the obstruction  created
negates  this  non-clog feature.   Gas  lift pumps,  because  of
their low lifting capability, are very  sensitive  to  suction and
discharge  head variations,  and  to variations  in the depth  of
bouyant  gas  release.   Special  discharge  heads  are usually
required to enhance the complete  separation of diffused air  once
the discharge elevation has been  reached.
                                                       DISCHARGE
            AIR CHARGE
            CONNECTION
 SUCTION
ISOLATION
 VALVE
        CHECK
        VALVE
 CHECK
 VALVE
                           FIGURE 14-15

                          EJECTOR PUMP


        14.1.3.10  Water Eductors

Water eductors use the suction  force  (vacuum) created when a high
pressure water  stream  is  passed  through  a streamlined confining
tube  (venturi).   Like  the  air  lift  pump,  water eductors  have  no
                              14-28

-------
moving  parts.   When water  is required to  transport a  solid
material,  the water  eductor  becomes a very convenient  pump.
Most water  eductors  with  reasonable  water  demands  cannot  pump
solids of golf ball size.  They have, however, been successfully
used to remove grit from aerated grit  removal  tanks and discharge
the grit into dewatering classifiers.


    14.1.4  Application  of  Sludge Pumps

The previous  section  describes the  types of pumps available for
sludge pumping.  This section  describes  appropriate applications
for these pumps  and identifies some limitations and constraints.
This section covers screenings, grit,  and scum as well as sludge.

Suction conditions  require  special attention when pumping sludge.
When pumping water  or  other Newtonian  fluids,  calculations of net
positive suction head  (NPSH)  can be  used  to determine permissible
suction piping arrangements.   However, sludge is a non-Newtonian
fluid, especially  at  high  solids  concentrations.   This behavior
may drastically  reduce  the available NPSH.   Consequently,  long
suction pipelines should be avoided  and the sludge pump should be
several feet  below the  liquid level  in  the  tank  from which the
sludge is to be pumped.   If these conditions are not met,  a pump
will not be able to handle  sludge at high concentrations.

Special  precautions  are  usually  required  to  reliably  pump
screenings and grit.  Screenings  should  be  ground up and pumped
by  pumps  with the  ability  to pass  large material.   Torque flow
pumps  are ideal for  this  application.  Grit pumping  requires
special  abrasion and  non-clogging  considerations.    Both
screenings  and grit  pumps should  be easy  to disassemble  with
quick access to the volute  and impeller.

Table  14-3  presents   an  application matrix  that  identifies  the
various  types  of  sludges or solids normally  encountered  in
wastewater applications, and  provides  a guide  for the suitability
of each type of pump  in  that  service.


    14.1.5  Pipe, Fittings, and Valves

Materials for wastewater solids pipelines include steel; cast and
ductile iron; pretensioned  concrete  cylinder pipe; thermoplastic;
fiberglass reinforced plastic;  and  other materials.   Steel and
iron are most common.  With steel or iron, external corrosion may
occur  in  unprotected  buried   lines;  corrosion  may  be adequately
controlled under most conditions by coatings  and,  where needed,
cathodic  protection.   Inside' the pipe,  a  lining of  cement,
plastic, or  glass  may be used to  protect the pipe from internal
corrosion and abrasion.   With  raw sludges  and scum,  linings
have  an additional  function:   they provide a smooth  surface
that  greatly  retards  accumulations  of  grease on the  pipe  wall
(26,  27).   With  anaerobically  digested  sludge, linings may  be


                              14-29

-------
useful  to  prevent  crystals  of  struvite  from  growing  on the  pipe
wall.   (Refer  to the anaerobic  digestion  portion  of Chapter 6 for
control of  struvite).   Smooth linings are  especially valuable in
pump  suction  piping  and  in key portions of  piping  (header pipes
and  the  like)  where maintenance  shutdowns  would  cause  process
difficulties.
                               TABLE U-3
                     APPLICATIONS FOR SLUDGE PUMPS
           Misce3laneous solids
                         Primary sludge
                                       Secondary sludge



Centrifugal 0
Torque flow 5
Plunger 0



000
4 J 5
044


Settled
3
4
4


Thickened
2
3
4


Trickling
filter
4
4
4
Thickened
sludge
Activated — ——-•••- •
4 Oa 3
4 Oa 4
1 1C 4
                                                             Lagooned
                                                    Digested sludge,  sludge,
                                                      percent    percent
                                                    Mixed Thickened Wet Dry
                                                                 - ; low efficiency

                                                                 Daily attention
                                                                  required
Progressive
 cavity

Piston/hydraulic
 diaphragm

Diaphragm

Rotary


Pneumatic ejector

Air lift

Water eductor
Float may cause air binding.

Varying quality and head conditions requires positive flow control.

Restricted to low flows.

Maximum li percent solid .

High discharge pressure nly.

            nding.
 Should be preceded by gr

 Large bore pumps may be
 Batch Pneumatic Ejector

 DShort distance only.
             sed with m-line grinding.
                                                                 High
                                                                 Low lift

                                                                 Low lift
                                              Key:
                                               0 - Unsuitable
                                               1 - Use only under special circumstance.
                                               2 - Use with caution
                                               3 - Suitable with limitations
                                               4 - suitable
                                               5 - Best type to use
Fittings  and  appurtenances  must  be  compatible  with  sludge  and
pipe.   Long sweep  elbows  are preferred  over short  radius  elbows.
Grit  piping may be  provided  with  elbows  and tees made of  special
erosion resistant materials.

Valves  of  the  nonlubricated  eccentric plug  type have proven
reliable  in  sludge  pipeline service.   Care  must  be  taken  if  a
cleaning  tool is to pass  through  the valves.   Grit pipelines are
usually equipped with  tapered  lubricated plug valves.

Wastewater  solids  piping  should  be   designed  for  reasonably
convenient  maintenance.    Even  under  good  conditions,  pipe  may
occasionally have  erosive  wear,   grease  deposits,   or  other
difficulties.    Pipe  in tunnels  or  galleries  is  more accessible
than  buried  pipe.     An   adequate  number  of  flanged  joints,
mechanical  couplings,  and  take-down fittings  should be  provided.
It  is recommended  that 4  to  6 inches (10  to 15 cm) be considered
the  minimum diameter for  wastewater solids  pipelines to  minimize
                                  14-30

-------
grease clogging or particle blockage and facilitate maintenance.
Blind flanges and  cleanouts should  be  provided  for ease of line
maintenance.   Gas  formation  by wastewater solids  left  for long
periods  in  confined pipe  or equipment can  create explosive
pressures;  therefore, provision should be made  for flushing and
draining all  pipes,  pumps,  and  equipment.  The pressure rating of
wastewater  solids  pipelines  should  be  adequate for  unusual  as
well  as routine  operating  pressures.    Unusual  pressures
will  occasionally  occur  due  to  high  solids  concentrations,
pipe  obstructions,  gas  formation,  water hammer, and  cleaning
operations.

Temperature changes may  cause  stress in  the  pipe.   Temperatures
are changed by heated material as it enters  cold pipe; flushing;
and the use of hot  fluids during cleaning to remove grease.  Pipe
should be designed  to accommodate such stresses.
    14.1.6  Long Distance  Pumping

Sludge  may be  pumped  for miles.   A  pipeline  is frequently
less  expensive  than  the  alternatives  of  trucks,  rail  cars,  or
barging (see Section 14.1.3 and reference 28), especially if,  by
pipelining, mechanical  dewatering can be avoided.   Pipelines may
have less  environmental  impact  along  their routes than trucks.


        14.1.6.1  Experience

Tables  14-4  and  14-5 describe  some  typical  pipelines  for
unstabilized  and  digested  sludges.    There  is   considerable
additional U.S.  experience;  see  Tables  14-6 and  14-7.   An
examination of  these  tables shows  that:

     •  Centrifugal pumps   are  widely used,  even on  unstabilized
        sludge.

     •  Operating pressures are usually  below  125 psig (860 kN/m2
        gage).

     ®  Velocities are  usually  below  3.5 ft/sec  (1.1 m/s).

     *  If the  volatile  solids  content of  the sludge  is low,
        the  sludge  can  be pumped  at  a  high  total solids
        concentration.   This  is  well  illustrated by  the lagoon
        sludge  pipelines,  which have  operated  at up  to 18 percent
        solids;  lagooned sludge has  a very low volatile content.

In some cases,  sludge  thickening at the  receiving  location was
adversely  affected by the  shearing or the septicity  that occurred
in the pipelines.  Special flushing practices after  pipeline use
or use of  a pipe cleaning  device were not used in several cases.
Need  for  these  techniques seem  to  depend  on the nature  of the
sludge being  pumped, although  experience  is not conclusive  on
this point.


                             14-31

-------
                                TABLE 14-4

                     TYPICAL LONG PIPELINES CARRYING
                           UNSTABILIZED SLUDGE



Length, mi
Diameter, in.
Pipe material

Percent solids
Flow rate, gpm
Velocity, ft/sec
Total pressure, psl g
Pump type

Operating schedule

Use of cleaning tool
Septicity of sludge
Commen s




Cleveland, OH

13.2
12
Cast iron, un lined
activated13
3- 3.5C
350C
1.0°
150 - 175°
Centrifugal, three in
series
Continuous

Every 4-6 weeks
-
D.,fficulty with
at receiving plant



Indianapolis , IN

7.5
Twin 14
Ductile iron
activated
0.75 - 1.75
1,000 minimum
2 minimum
90 normal
Centrifugal

Continuous

None
YPS

sludge that has
been pumped from
Southport
Jacksonville , FL
District II to

7
8
-
activated
3
500 normal
3
90 normal
Centrifugal , two in
series
30 - 60 minutes every
two hours
Possible, not needed
Yes
Heat treatment de
ering ess >


Kansas City, MO
West Side to Big

6.6
12
Ducti le iron
rimary
0.4 - 1.0
1,000
2.8
65
Centrifugal

Continuous

Weekly
Some; chlorine used

receiving plant



Philadelphia, PA
Southeast to Southwest
5
8a
Ductile iron
scum
2.5 - 5
500
3
90 normal
Centrifugal

Continuous

Every 1 to 2 weeks
Not much odor




 Two ductile iron lines will replace a single line. The old lines is subject to external corrosion and
 will be abandoned over most of its length. The new lines have polyethylene wrap and cathodic protection.

 Pickle 1iquor is added to primary treatment for phosphorus removal. Skimmings are handled separately.

 Data from Reference 10. Later, sludge thickness was decreased to 1-2 percent solids to reduce operating
 pressures and line breaks.

 There is a heavy grease buildup in the pipe, especially in winter.

 1 mi = 1.6 km
 1 in. = 25.4 mm
 1 gpm = 0.063 1/s
 1 ft/sec = 0. 30 m/s
 1 psig = 6.9 kN/m2 qage
         14.1.6.2  Design Guidance


Proper  pre-planning  of  a  pipeline  installation  is  of  great
importance.   For example,  a pump  breakdown or a  plugged  pipeline
has  a great  impact  on plant operation, and its likelihood  can be
greatly minimized by  good initial design  and equipment selection.


If  digestion  is  to  be  part  of the system,  the  digesters  may
be  located  either before  or  after the  long sludge pipeline.
However,   sludge   is   much  easier  to  pump  after   it  has  been
digested.  In  addition,  raw sludges may  cause  problems related to
thickening,  odors,  and   corrosion  at  the  receiving  point,  since
septic conditions may develop in the pipeline.   If  raw  sludge is
to  be pumped long  distances,  the  least environmental impact will
result  if the pipeline contents  are  discharged  directly  into
anaerobic digesters.
                                  14-32

-------
                                  TABLE 14-5

                      TYPICAL LONG PIPELINES CARRYING
                               DIGESTED SLUDGE


Length, mi
Diameter, in.
Material
Sludge type

Percent solids

Percent volatile
Flow rate, gpm
Velocity, ft/sec
Pumps


Operating schedule


- e o ^ go
Chicago , IL .
agoon no.
1.7
16
Steel
Lagooned

13 average
15 maximum
40
1 , 300
2.1
Centrifugal with
mixers

Intermittent


None
Denver, CO
Northside to Metro
'- 2
Twin 8.
Cast iron
Anaerobically-digested
primary
4-7

49
700
2
40 — 60
Centrifugal


1-2 hr/day, not
flushed

None
Fort Wayne,

3
12, some 10
Unlined cast iron
Digested

5 maximum

35 - '40
600
1.6
20 - 30
Centrifugal


3 hr/day , can flush
but not needed

None
Rahway Valley Sanitary
Authority , NJ
3 •
8
_
Anaerobically di-
gested primary and
3-4

-
500
3
80
Two-stage centrifugal ,
formerly recipro-
cating
4 hr/day, not flushed

h
Not needed
San Diego, CA
Point Lcma
7.5
8
Fiber reinforced plastic
Anaerobically digested
primary
Up to 7.56

57
550 - 60C
3.5
155
Torque flow


5 times/week, flushed
before and after use

None
Temporary pipeline to clean Lawndale lagoon no. 28 (5,6). No longer in service.

Also, a 25-mi pipeline has been designed but not yet constructed, as of early 1979.

Fiber reinforced plastic replaced a lined and coated steel pipe that corroded.

Anaerobically digested primary and waste-activated sludges with phosphorus-precipitating chemicals.

Dilution water is needed sometimes to get the sludge started. Once it is moving, the dilution water
may be shut off, depending on pressure.

Non-clog centrifugal pumps are suitable for ordinary digested sludge. A nickel-alloy torque flow pump
is being added for digester cleaning and septic tank waste.

Three pumps in series, two of which have variable speed drives.

In the past, a novel ice bag tool was used (26).


1 mi = 1.6 km
1 in. = 25.4 mm
1 gpm = 0.063 1/s
1 ft/sec = 0. 30 m/s
1 psig =6.9 kN/m gage
 Sludge  that  has  been piped  for  a long  distance  may  experience
 floe  breakdown.   If this occurs,  thickening and  dewatering  may  be
 impaired.    Chemical  conditioning  may  require  a  higher  chemical
 dose;  thermal  conditioning  may  produce  a  sludge  with poorer
 dewatering  properties.

 The  following  special  design  features  should  be  considered for
 long  distance pipelines:

     1.  Provide  two  pipes   unless  a single  pipe can  be  shut down
          for several  days without  causing problems  in  wastewater
          treatment system.             ;

     2.  Consider  external  corrosion   and  pipe  loads   just   as
          for  any  other   utility   pipeline,  for   example,   water
          or  natural gas.   External  corrosion has  been  a  problem  on
          some long sludge  pipelines.   Electrical  return  currents,
                                    14-33

-------
             TABLE 14-6

LONG PIPELINES FOR UNSTABILJZED SLUDGE
        ADDITIONAL LOCATIONS
City
Austin, TX
Houston, TX
Jersey City, NJ

Knoxville, TN

Linden-Roselle, NJ


Miami, FL
San Francisco, CA



There are additional
Two 16-in. pipes over
CTwo pipes.
1 mi = 1.6 km
1 in. - 25.4 mm
Length,
Treatment plants mi
Walnut Creek v "•"
Southwest
Simms Bayou to Northside 6.8
Eastside to Westside 2.5

Loves Creek to Third 3.2
Creok system
Linden-Roselle Sewerage 1
Authority
.
North Point to Southeast 6

__ .
system
pipelines in Houston {26} .
most of the route .



Diameter of
pipe , in. Sludge type
12 Primary, waste-activated
"a- Waste-activated
zi ' Primary

6 Primary, trickling
filter
24 Primary

b . .
10 Primary with ferric
chloride
C -






Percent
solids Pump
' 1-1.2 Positive di
. 0.5-1 Centrifugal
4 Plunger, 3
(maximum)
1-3 Centrifugal

2-4 Centrifugal

i
1 Centrifugal


variable





type
splacement
, 2 in series
speed






, 2 -speed


speed





              TABLE 14-7

  LONG PIPELINES FOR DIGESTED SLUDGE
        ADDITIONAL LOCATIONS
Length,
Location mi
Austin, TX - Govalle plant 7
Boston, MA - Nut Island 4.5
plant
Chicago, IL - West-Southwest 5.5
plant to Lawndale lagoons
Chicago, IL - 1970 rail 3.5
loading3
Chicago, IL - barge loading 1 . 0
Chicago, IL - Calumet 1
lagoons
East Rockaway, NY - Bay 1. 5
Park plant
Evansville, .IN 3.5
Fulton County, IL 10.8
Los Angeles, CA - Hyperion 7
Morgantown , WV ""4.5
Philadelphia, PA - Southwest 1
Wantagh, NY - Cedar Creek 11
Temporary pipeline, now out of service.
1 mi = 1.6 km
1 in. = 25.4 mm
Diameter of
pipe, in..
10
12

16

12
16
18
16

8
20
20
2"
-
10



Type of digestion
Aerobic
Anaerobic

-Anaerobic

Anaerobic, lagoon
Anaerobic , lagoon
Anaerobic, lagoon
,); Anaerobic

Anaerobic
Anaerobic, lagoon
Anaerobic, aerobic, diluted
~"1 " Anaerobic
Anaerobic, laqoon




Percent solids Pump type
0.8 , Positive displacement
„ . 3 Centrifugal, reciprocating

3.5-4.5 ' Centrifugal

4-15
9.2 average
,8 - 18 Centrifugal with mixers
12 ' - Centrifugal
3 . 7 Variable speed

1-9 Torque flow, plunger
4-8 Centrifugal
0.9 -
"Reciprocal ing
10-12 normal
15 average
2 . 5 Centrifugal , 3 stage



                14-34

-------
    acid soils,  saline  groundwater,  and  other  factors  may
    cause serious  difficulty unless  special corrosion control
    measures .are'used. -Advice of specialists on the need for
    cathodic protection  is  advised.  ,  :

3.  Provide  for adding controlled amounts  of water to dilute
    the sludge or  flush  the  line.   Primary  effluent  may  be
    used in raw sludge pipelines; disinfected final effluent
    may  be preferred for  digested sludge  pipelines.   The
    water connection should have a  flow rate indicator.   The
    flushing water should flow at about 3 fps (0.9 m/s) .

4.  Provide  for  inserting and removing  a  cleaning  tool
    ("pig,"  "go-devil")  which can be sent through the line if
    needed (10, 28a,  28b).  Such cleaning  may be frequently
    required  if unstabilized sludge is pumped,  even  if  scum
    is handled separately.   If  tool cleaning  is to be used,
    some additional recommendations  apply:

    a.  Valves must provide an unobstructed waterway to  pass
        the tool.

    b.  Flushing water pressure  should be  sufficient to  push
        the tool through the full length of pipeline.

    c.  Pipe  bend  fittings  should  be  45-degree or,  if
        possible,   22-1/2-degree.   Some cleaning  tools  will
        pass  90-degree bends, but such bends are likely to be
        trouble  spots.   Length/radius of  bends  should  be
        checked with the tool supplier.

    d.  A recording  or totalizing   flowmeter  should  be
        provided. ' '" (See  Chapter  17,  Instrumentation.)  If the
        tool  gets  stuck  in the  line,  the  flow record can be
        used   to  compute  the  number of  gallons  pumped since
        the tool was inserted.   Thus,  the tool can be located
        and retrieved.

5.  The  pipeline route  should  be  selected  for ease  of
    maintenance.

6.  At  high  points,  air  or  gas  relief   valves should  be
    provided.   With  care,  automatic relief valves  can be
    made  reliable  on digested  sludge lines;  however,  in
    unstabilized  sludge  lines,  grease and  debris generally
    cause automatic valves to be  unreliable.   Simple manual
    blowoff valves are  generally  better  for  unstabilized
    sludge.   Air  and gases_  from   sludge  pipelines  may be
    odorous.   In confined spaces, the  air  or gas may also be
    toxic, flammable,  explosive,  and corrosive.

7.  If  sludge  is  to be  pumped at more  than  about 3 percent
    solids, the  pumps and pipeline  should be  designed  for
    high and variable friction head losses.  Sludge may  flow
                          14-35

-------
       •more  like a  Bingham  plastic than an  ordinary  Newtonian
        fluid.   A multiplication factor,  such as  those on
        Figure  14-1,  should  not  be  used.   A  more accurate  design
        method, such  as the one  in Section  14.1.2,  should be
        used.

    8.   If centrifugal pumps  are used, flow rates  will be
        somewhat  unpredictable  because  of  the  varying  flow
        resistance properties of  the sludge.   Storage  provisions
        should  be made  for  these  variations.   Pumps  should be
        capable of  operating at shutoff head  with  very  low  flow
        during  pipeline  startup.

    9.   Positive  displacement  pumps  may  experience  difficulty
        when  starting a  long  sludge pipeline.   The thixotropic
        nature  of sludge may cause  very high  resistance to  flow
        during  start-up.  Consequently excessive pressures  may be
        generated  by  positive displacement pumps.   To  avoid  this
        problem,  variable speed drives should be provided and the
        pumps  should  be started  at low speeds.  An air chamber
        (see  Section  14.1.3.3)  may  be installed on the discharge
        side  of the pumps;  the  chamber will  assist in start-up,
        as well as dampen pulsations.   With  digested  sludge,  a
        relief  valve  piped back to  the digesters may be used  near
        the pumps.

   10.   For very  long  lines,  a  booster  pumping station may be
        required.    If  positive displacement booster  pumps
        are used,  a  holding  tank  should be provided.   It is
        practically impossible  to match booster pumping  rates to
        the sludge  flow reaching  the  booster  station unless
        centrifugal pumps are used.

   11.   Waterhammer  is  best controlled  by  limiting  velocity.
        Unless  a  special evaluation is made,  velocities  should
        not exceed about 3  fps  (0.9 m/s).   Even lower  velocities
        may be  required  in some cases.


    14.1.7 In-Line Grinding

In-line grinders  are  used to reduce the size of sludge solids to
prevent  problems with  the  operation of  downstream processes.
Grinders require  high maintenance;   therefore  they  should  not be
installed unless  shown to be absolutely necessary.  For locations
where a  grinder may  be  installed  in the  future, removable  spool
pieces   should  be  inserted   into  the pipeline to  facilitate  the
later  installation  of  a grinder.  Grinders  may be applicable
to  streams carrying debris,  rags or  stringy  materials, but
are  usually  not  needed for  streams  carrying  only  secondary
(biological)   sludge.    Grinders   have   often  been  installed
preceding equipment with ball  or flapper  check valves.   However,
utilizing  dual  check   valving,  proper  stroke  seating can be
                             14-36

-------
obtained  and  the grinders  can  often be eliminated.   Grinders
remain a  necessity  upstream from small diameter, high  pressure
positive  displacement pumps.

Sophisticated,  slow speed,  hydraulic  or electric grinders  that
can sense blockages  and  clear themselves by reverse operation are
now  available.    Special  combination  centrifugal  pump-grinders
are  available for use  as  digester circulation  pumps, and  are
effective  in  preventing rag .balls.   .Experience   indicates  such
pumps require  as  much maintenance as grinders.


14.2  Dewatered Wastewater  Solids Conveyance

Dewatered or  dried sludges,   screenings,  ash,  and  grit  can
be  conveyed  by  most forms of  industrial  materials  handling
equipment, including belt,  tubular,  and  screw  conveyors;  slides
and inclines;, elevators;  and pneumatic systems.   Each may be used
to  advantage  in  certain applications.  Because  the consistency
of wastewater  solids is  highly  variable,  and because the  solids
are  often difficult to move and may  tend to  flow, the design
of  this   equipment  must  consider  the most severe conditions
that may be expected.


    14.2.1  Manual Transport of  Screenings  and Grit

A common method of handling screenings  or grit is simply to place
a  mobile  container (29)  beneath  the  discharge point and  to
periodically empty  the  mobile container  into a  larger  container
to  be hauled  away to  a landfill.   The  mobile  container  may
have  wheels for  ease of movement or it may  be maneuvered  by an
overhead  crane.   The principal  disadvantage  of  this approach is
the  amount of manual  labor required.  However, for  small or
isolated operations  this may be  the  most appropriate method.
    14.2.2  Belt Conveyors

Troughed  belt  conveyors  are  simple  and reliable  (Figure  14-16).
They  may be  equipped  with  load-cell  weigh-bridge  sections  for
totalization  of  conveyed  solids  weight.   (See  Chapter  17,
Instrumentation).   Totalization  is useful when an accurate solids
balance must be calculated  for a dewatering facility or treatment
plant.   Sludge  concentrated enough  to maintain a semi-solid
shape  (15  percent)  can be conveyed  at about  18  degrees  maximum
inclination  on  troughed  belt  conveyors.  Sludges with a higher
solids content can be moved up steeper  slopes.  Where wash sprays
are utilized,  splash pans should be provided on the underside of
belts  to direct the used washwater  to a  proper  disposal point.
Such  splash  protection will  assist  in  keeping  the area  dry
and preventing head  and tail pulley  slippage.   Head and  tail
pulley lagging -(grooving),  crowning, and other auxiliary  ways of
maintaining ;belt guidance  should  be  thoroughly reviewed  with
conveyor  manufacturers  before  specifying   a   troughed  belt


                              14-37

-------
installation.    Most troughed belt  installations for  sludge
currently  utilize  steel  idlers  and  pulleys  with  lubricated
anti-friction bearings.   The fisheries industry, which also uses
conveyors in constantly  wet applications,  is successfully using
lubricated  thermoplastic  (TFE,   Delrin)  idler  bearings  with
Schedule 80 PVC  pipe  rollers;  these provide longer service life
than is achieved with all steel construction.
                               HEAD
                               PULLEY
 FEED CHUTE/
 LOADING SKIRTS
 TAIL
 PULLEY
                    TROUGHED BELT
                    CARRYING IDLERS
 TAIL TAKEUP
 FRAME
                                            DRIVE
                                             UNIT
                                             DISCHARGE
                                                CHUTE
                                                    CROSS SECTION
                                                     TROUGHED
                                                     BELT IDLERS
                                    SUPPORT
                                     BENTS
                                                   CROSS SECTION
                                                  FLAT BELT IDLERS
                           FIGURE 14-16

                          BELT CONVEYOR
In sludge applications, belt failures usually occur first at the
zipper-like mechanical  belt seams.   Endless belts with field
vulcanized  seams  may  be specified to  eliminate this  mode of
failure.   Belt material  must be resistant to dilute  sulfuric
acid,  formed  by  the reaction of  hydrogen  sulfide and moisture.
Material selection must also consider oil,  grease  and a multitude
of other elements found in sludge.

Belt conveyors  have been successfully  used  to transport coarse
solids  removed  from mechanically  cleaned bar  racks,  and can be
used to transport grit.   Special consideration  should be  given to
the type of  belt  design,   construction  materials,  bearings,
type of  drive  and controls.   Since  screenings are heavily laden
                             be  designed to  contain and direct
                             disposal.   A means of changing belt
                             so that a  range of loads  can be
with water,  the  belt must
draining water to a point of
speeds  should  be provided
accommodated.
The handbook on belt  conveyors  for  bulk materials by the Conveyor
Equipment Manufacturers  Association (30)  is a good reference for
general  design of belt conveyors.   However,  there  is  little
                              14-38

-------
specific information available  relating  to  the special problems
associated with the  cohesive,  non-uniform properties of dewatered
sludge.   Experience at  existing  facilities  using this  type  of
conveying   equipment  and  transporting  sludge   with  similar
characteristics provides  the most useful design information.

The experience of the County Sanitation Districts  of  Los Angeles
County  in the  first three years  of operation  of a  two-stage
digested  sludge  dewatering  station provides useful  guidance
for  conveying  centrifuge-dewatered  digested  sludge  (31),    The
facility includes solid bowl centrifuges as a first stage,  after
which  the  centrate  is  screened and  then dewatered using basket
centrifuges.    The   system uses  belt  conveyors  to  transport
dewatered sludge between production,  storage, and  truck loading.

The system has  44 belt conveyors totaling approximately one-half
mile in length.   Troughed  conveyor  belts  carry both  first  stage
centrifuge cake at  32 percent solids and second stage centrifuge
cake at 17 percent solids.  Dewatered sludge  is usually stored in
the  twelve storage bins at  22  to  24 percent solids  and  then
transported to trucks by  additional belt conveyors.

Helpful  guidelines  resulting  from  start-up  of  this  facility
include the following:

    1.   Reduction of splashing  at  transfer points:   The  dump
        point should be  enclosed and  the drop distance minimized.
        Skirtboards (stationary  sidewalls  at edges  of belts)
        should be  used  at  critical areas and covered  if
        necessary. Rubber gaskets from hoppers  to skirtboards and
        on the  bottom  of skirtboards may be  required  to reduce
        splashing  or  spillage.   Where  long drops  cannot  be
        avoided  transfer  chutes should have  interior impact
        baffles to dissipate the momentum of  falling sludge.

    2.   Removal of sludge from returning belts:  Counter-weighted
        rubber-bladed scrapers at head pulleys are not effective
        in scraping  sludge  off return belts and are a maintenance
        problem.    The  use of  adjustable  tension finger-type
        scrapers is  recommended.   To avoid  problems  with  idler
        roller  vibration  and  irregularities,  and  to ensure
        continuous  contact, scrapers should  be installed beyond
        the idler on the  flattened portion of the belt.

    3.   Assuring  minimum  pulley slippage:    Appurtenances  that
        contact the dirty  side of  the belt  should  be avoided.
        Figure  14-17  illustrates  both  the   undesirable  and  the
        recommended  design features  of  inclined  belt conveyors.
        Snubber pulleys  and   trippers  (devices that  remove  the
        moving  material  from   the  belt)  cannot  be successfully
        used for sludges.  Gravity counterweight take-ups should
        be  avoided,   and  screw  take-ups should be  used instead.
        Where  long   lifts  are  required,  multiple short  belts
        should be used  instead of one  long belt to avoid the need
        for gravity  take-ups.


                              14-39

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    4.  Importance  of  housekeeping  facilities:   Notwithstanding
        the care  taken to  avoid  spillage or  splashing,  sludge
        handling facilities are  dirty,  and must  be  designed  to
        facilitate  cleanup.   Non-skid  cover  plates,  rather than
        grating, should be  used  for  all access areas except those
        immediately  over  storage  hoppers.    Convenient  hose
        stations should be  located  to serve all  areas.   Floors
        and slabs  should  be  provided with  exaggerated drainage
        slopes  (up to one  inch per foot  [8  cm/m]) and  should
        drain  to liberally  distributed drain sumps.  Special care
        should be  used  at  all transfer  points,  take-up pulleys,
        and dump points to  minimize  sludge spillage or splashing,
        or to  provide  surroundings that are easily  cleaned.

Flexible  conveyors are now available in styles  with  integral
pockets,  sidewalls  and cleats  that allow steep,  high capacity
operations on  almost  all materials  (Figure 14-18).  The belts may
change inclination  at  several  points in their run.  They are best
cleaned by  a  combination   brush  and spray cleaner.   Except for
belt pockets,  sidewalls, and  cleats, their mechanical components
are similar  to those  on  troughed  belts; maintenance  costs for
mechanical drives and  rollers  are  also similar.

There  are  patented flexible conveyors  that can  not  only  change
inclination but also  change direction  or even spiral vertically
upwards.   One  unit  may replace several straight line  belts.
These  units  are  not  actually belts  but segmental chain and
sprocket-driven mechanisms  with   interlocked,  pleated  rubber
trough  sections.   Drive mechanism wear  and  corrosion  is high  in
comparison with  flat   belt  conveyors.   These conveyors  are not
recommended where  there is  sufficient room to allow installation
of multiple conventional troughed  or pocketed conveyors.
    14.2.3  Screw Conveyors

Screw  conveyors  (Figure  14-19)  are  silent,  reliable,  and
economical  (32).   They  are  used for horizontal movement of grit
or sludge, or may be used to convey  dewatered sludge up  inclines.
(The  degree of  incline  depends upon  sludge  moisture content
and  consistency).    Conservative  sizing,  abrasion  resistant
construction materials,  and  integral  wash down  systems within
enclosed housings are recommended for  solids handling facilities.
All enclosed housings should  have numerous quick opening access
plates  for maintenance  and observation.   Screw conveyors for
dewatered  sludge  should  not have internal intermediate  bearings
because sludge can pile  up on the bearing  and restrict or prevent
flow.  For  this  reason,  screw  conveyor  lengths should be limited
to 20  feet.   Screw conveyors with reversible direction, or with
several slide gate controlled discharge  openings  in the  bottom of
the conveyor housing, allow  the point  of conveyor discharge to be
changed as appropriate,  providing flexibility of  operation.
                              14-40

-------
                                        "TRIPPER WITH
                                         SHUTTLE BELT FOR
                                         TRANSFER TO MULTIPLE
                                         BINS
                                                ._-  SNUBBER
                                                O-*	PULLEY
•GRAVITY COUNTERWEIGHT TAKEUP
 FOR CONVEYOR BELT TENSION
                      UNDESIRABLE LAYOUT
              'NOT RECOMMENDED FOR USE WITH SLUDGE
TRANSFER OF CONVEYOR MATERIALS
             HEAD
             PULLEY
MOVEABLE RUBBER
BLADED PLOWS
                       HEAD
                     PULLEY
                                TAIL
                                PULLEY
                                     DUMP INTO
                                     TANK AT HEAD
                                     OF CONVEYOR
                                        SCREW TAKEUP
                                        FOR CONVEYOR
                                         BELT TENSION
                     RECOMMENDED LAYOUT

                          FIGURE 14-17

              INCLINED BELT CONVEYOR FEATURES (31)
                             14-41

-------
                         FLEX IBLY CLEATED AND
                           ' SIDE WALLED
                         FLAT BELT CONVEYOR
                            FIGURE 14-18

                    FLEXIBLE FLAT BELT CONVEYOR
                                               INLET
                DISCHARGE
                            FIGURE 14-19

                          SCREW CONVEYOR
Screw conveyors have  been  successfully used for transporting grit
but their  application to  screenings  is  questionable because rags
may become  entangled on the  conveyor shaft.   Oversized objects,
such as  sticks,  can  jam  the  screw or fall out  of  the conveyor,
creating housekeeping problems.   To  reduce wear, ;_open or ribbon
type screw conveyors  are sometimes used  for grit.
                               14-42

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    14.2.4  Positive  Displacement Type Conveyors

Positive  displacement  type  conveyors include  tubular  conveyors
and bucket  elevators.   Tubular  conveyors  (Figure 14-20)  are
tubular conduits  through  which circular  flights  are pulled  by
chains.   They  may be used for  the  horizontal  transportation  of
dry solids  such  as  incinerator  ash  or semi-dry grit.   They  are
several times  as  expensive  as  flat  belts per linear foot,  but
require much less  room,  are fully enclosed and air tight, and  can
be routed anywhere a  conduit will fit.  Maintenance  is high.
Most plants  utilizing  these conveyors routinely replace the chain
elements at  least  once  per month.
                           FIGURE 14-20

                        TUBULAR CONVEYOR

Bucket  elevators  (Figure  14-21)  incorporate  chain  and sprocket
driven  buckets  in  a manner  similar to  the  tubular  conveyors
except  that  the  chain flights  are  not in  continual  contact
with  the  product.   As a result,  mechanical  longevity is greatly
increased.   They  are usually restricted  to  vertical lifts with
limited horizontal displacement.


    14.2.5  Pneumatic Conveyors

Pneumatic  conveyors  are  usually  not appropriate  for  dewatered
sludge,  but can  effectively handle screenings, grit,  and dry
finely divided materials such as  incinerator ash.  Screenings and
                              1.4-43

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grit can be easily transported,  even  over  long distances, through
the use of a  batch pneumatic  ejector system (Figure  14-22).
Such pneumatic  ejector systems  have  provided good  service  for
distances up to  one-half  mile  and up to  100  feet  of lift.   The
transport system between  the  points of loading and discharge is a
totally enclosed  pipe, which is  clean and  odor-free  and can be
easily  routed  along  available  passages.    The  entire  system
utilizes a minimum of moving parts.   Consideration must be given
to  the  use  of  abrasion resistant materials,  especially  at  pipe
bends,   and  an  air pressure  system consistent with  the distance
and lift to be  traversed.
                   MATERIAL
                     OUT
                                     MATERIAL
                           FIGURE 1U-21

                         BUCKET ELEVATOR
Continuous  pneumatic  conveying  systems  (Figure  14-23),  either
pressure or vacuum  type,  are  widely used where dry, particulate
materials are to  be  transported.   Their use in sludge transport
is limited  to materials  such as  incinerator ash.  Where  long
distances or  complex routings  are  involved  pneumatic conveyor
systems are  especially well  suited  to ash transport.

Ash is an extremely  abrasive material and rotary valves and elbow
segments  in  particular  must  be  carefully   specified  to  provide
maximum  abrasion resistance.   The blowers  may require  noise
shielding.


    14.2.6  Chutes and Inclined Planes

Chutes  and  inclined  planes  for sludge,  screenings, ash,  and
grit  should  be  tested  for  minimum  inclination on  the specific
                              14-44

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transported product whenever possible.   In general, inclinations
for dewatered sludge  should be  greater than  60 degrees  from
the horizontal.   For  dry bulk  materials,  such  as  ash,  the
inclinations  should  at least  be  greater  than the material's
natural angle  of  repose.
   DISCHARGE
   HOPPER
                         SCREENINGS
                           OR GRIT
                            INLET
                                     CONTROLS
                     GATE
                                                     AIR  ^^>
                                                  COMPRESSOR
                                                   RECEIVER   J
                                                          u
                            FlGURt Tl-22

                        PNEUMATIC EJECTOR
            MATERIAL
            AND AIR
                                                AIR OUT
                              MATERIAL
                                OUT

                            FIGURE 14-23

                        PNEUMATIC CONVEYOR
                              14-45

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    14.2.7  Odors

Open sludge conveyance  can  be a source of  odors.   All solids
transporting  facilities  should be  well  ventilated  and,   if
necessary, provided with odor  control for the vented air.  Even
with stabilized sludges, if  large holding or ' equalization  tanks
are required for the pumping  system,  floating covers or  special
odor control facilities  for  venting tank air should  be provided
when the  detention  time  is  greater  than several  hours.   See
Chapter 15 for  more detailed  information on sludge storage.


14.3  Long Distance Wastewater Solids Hauling

It is  often necessary to  transport the wastewater solids  for long
distances,  that is, beyond  the  boundaries of  the wastewater
treatment plant site.   This may  be done  by  pumping  if the
material  is sludge or  scum  (covered in Section 14.1.6)  or  by
other  methods,  which shall  be termed long  distance hauling.  For
this chapter,  long  distance hauling  is limited to trucking, rail
transport, and  barging.

Ettlich  (28),   in  developing cost formulas for transport  of
wastewater sludge,  makes  the  following general observations  about
the comparative  economics  of  the  long  distance sludge  hauling
methods:

    1.   Transportation of dewatered sludge

         «  Total annual  cost  for railroad is less than  truck for
            all annu'al  sludge volumes (7,500 to 750,000  cu  yd
            [5730 to 573,450  m3]  and  distances  (20  to 320  miles
            [32 to  515  km])  studied with and without terminal
            facilities  for loading  and  unloading sludge to the
            transport vehicle.

         •  Railroad facilities are more capital  intensive than
            truck facilities.

         •  Transport equipment can  be leased for both  truck and
            railroad transport.

    2.   Transportation  of liquid  sludge

         «  Truck  is the  least expensive mode  for  one way
            distances  of 20  miles  (30 km)  or  less and sludge
            volumes less than 10  to  15 million gallons (38,000  to
            57,000  m3)  per  year.

          •  Pipeline is  the  least expensive mode for all  cases
            when  the annual  sludge  volume is greater than
            approximately  30  to  70  million  gallons  (110,000  to
            260,000 m3)•


                              14-46

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         «  Pipeline is  not  economically attractive  for  annual
            sludge volumes of 10 million gallons  (38,000 m3)  or
            less because  of the high capital investment.

         •  Pipeline is capital intensive and the terminal  points
            are not easily changed.   Pipeline  is ideal for  large
            volumes  of   sludge  transported  between  two   fixed
            points.

         •  Rail  and  barge  are  comparable  over  the  7   to
            700 million  gallons (30,000  to  2,600,000  m3)  volume
            range for long haul distances.

         •  Barge  is more economical than  rail for short  to
            medium distances  for  annual  sludge volumes greater
            than 30  million gallons (110,000 m3).

While  much information  is available on costs of transporting
sludge in  specific  situations  (33,  34,  35,  36) there is a wide
disparity in reported costs  since  there  are  so many  variables  in
each situation.   Consequently it is much more accurate to utilize
an approach such  as  Ettlich's, than to rely upon  cost estimates
from  other treatment plants  where  conditions  may  be  quite
different (28).
    14.3.1  Truck  Transportation

For most  small  plants  and  some large -plants, the use of  trucks
is the  best approach.  Trucking provides a  viable option for
transport of both  liquid and dewatered sludge.  Trucking  provides
flexibility not found in other modes of transport since  terminal
points  and  route can  be  changed  readily  at  low  cost (35) .
Provided trucks are  leased rather than purchased,  a truck hauling
option is not capital intensive and allows more  flexibility  than
pumping or  other  transport modes.   This  flexibility  is  valuable
since locations of reuse or disposal may change.
        14.3.1.1  Types  of Trucks

Sludge  hauling trucks  are  similar to standard  highway trucks
because both  types  of  trucks must  use  public roads and  comply
with  their  overall  vehicle width,  height  and  gross weight
restrictions.   Most  of the  variability can  be  seen in sludge
containment body configuration.  For the majority  of cases,  which
involve comparatively short  distances with one-way  travel  times
less  than one  hour,  ease and  speed  of loading  and unloading
are of  paramount importance.   The larger trucks are the most
economical except for one-way haul distances  less  than  ten  miles
and  annual  sludge  volumes  less  than 3,000 cubic  yards  for
dewatered sludge and  for less than one  million  gallons  per


                             14-47

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year for  liquid  sludge.   Generally, diesel engines  are  used  in
the larger trucks and are the economical choice for small trucks
that are operated at high  annual mileage  (35).

Where  it  is  determined  that  economic,  environmental,   and
institutional  considerations  allow  direct  land application
of  liquid digested  sludge,  special tank  trucks are available
equipped with specially  designed spreaders, auger beaters, and/or
special  application  apparatus.   Some  manufacturers  equip  their
trucks with subsoil  injectors for sub-surface treatment.   Use  of
such dual purpose  trucks  allows  transport  and  ultimate  disposal
without  an  intermediate  storage/pumping step.   Specialized
tanks  or trucking equipment can be custom  built for specific
applications.   One company produces flexible  tanks  designed  to
fit on a flatbed  truck  (37).

Spillage  or  leakage  from  sludge  hauling  operations  are
unacceptable  because of  aesthetic  and  health considerations.
This has meant a shift away from belly-dump vehicles,  even  for a
very well dewatered sludge cake.   There is increased  concern for
covering  the top of the  sludge to  minimize  both odor  release
during transit and the chance of spillage due to sudden  stops  or
accidents.  Consequently,  tank^-type  bodies  are gradually  becoming
the most  common,  even for mechanically dewatered sludges.  These
vehicles  require  unusually large  hatch  openings for  loading
purposes,  and well  designed water-tight  hatches or tailgates
for unloading.   Tanks  for  liquid  sludge  transport are  of  more
standard  design,  but  the provision of  internal  baffles  to
minimize load shifting  is  recommended for highway transport.


        14.3.1.2   Owned  Equipment vs. Contract Hauling

The  foregoing  concerns  apply  equally whether or  not  the
wastewater treatment management agency  contracts  out  its sludge
hauling or uses  its  own vehicles.   The choice between utilizing
agency personnel  or contracting  for private  companies  to  drive
sludge trucks  is often  decided not  on  the  basis of cost, but  on
the size of  the  plant.   Smaller plants  favor the use  of  both
their own vehicles and  staff.

The choice of contract  hauling  can be limited to the provision of
tractor units and driver services, with  the trailers owned by the
agency.   This  has  two  major benefits.   First,  treatment  plant
staff, assigned  to sludge handling  and/or dewatering operations
are working  in the  immediate vicinity of  the  trailers,  and can
therefore re-spot  the trailers under a conveyor belt  at  the  best
times.  Second, with most  contracts  awarded for  only one  to three
year terras, the  contractor would otherwise need to figure in his
bid price a very  rapid  amortization  of  custom trailers, which may
be of  no  further use to him if he is not re-awarded the contract
at  a  later date,  even  though  they may  have a useful  life far in
excess of the contract  period.  Since it  is economically  sensible
to  operate with  more trailers than  tractor  units,  trailer  cost
depreciation can  be a significant overall cost  factor.


                              14-48

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       •14.3.1.3  Haul  Scheduling

A common problem,  usually not recognized, is the need to properly
schedule  trucking  operations.    In  general,  the  total  cost
of  truck  transport  will be  decreased  (per unit  of  material
hauled)  if  the daily  period  of truck operation  is increased,
because capital intensive equipment is better utilized.   However,
restrictions may be placed on  any  significant  truck operations,
such  as  requiring  specific routes  or limiting  operations to
daylight  hours (35).   Such  haul scheduling may  require the
provision  of  some  form  of  temporary sludge  storage  at the
plant.  See Chapter 15 for sludge storage  information.   Whenever
intermittent  operations  are  possible,  however,  mechanically
dewatered  sludge  is  usually  loaded  directly  from a  conveyor
belt.   Using  trucks  or trailer bodies as  temporary  storage may
not be  the  most  economical method  when drivers'  work hours,
overtime  pay,  and the  cost of  re-spotting  trailers  under a belt
are considered.

In  designing  sludge  handling  facilities, it  is  desirable to
provide several dump points  with the  capability  to  quickly  shift
from one to another.   If trailers  are  used,  the ability to fill
several units  before  the tractor unit returns adds  flexibility to
scheduling  and  reduces  storage requirements.   If  the  receiving
vessel  for  dewatered  sludge  is  not  self-powered  (such  as a
trailer),  consideration should  be given to movable  dump  conveyors
to allow the load  to be distributed uniformly within the vessel.
Dewatered  sludge  will mound  when loaded  from  a  single point.
This may prevent effective utilization of the transport  vessel.


        14.3.1.4  Trucking Costs

When  considering  sludge  trucking,  it  is worthwhile  to  remember
that pumping equipment can handle  digested  sludge  at least  up to
20  percent  solids concentration, and to  note  that the layout
and design  of   loading  and  unloading  facilities can  contribute
markedly to cost savings.   A more detailed  breakdown of relative
costs associated with  truck transportation is available  (28).
    14.3.2  Rail  Transport

Rail  transport  is suitable  for  transporting sludges of  any
solids concentration.   It  is,  however,  not  a common method  of
transporting sludge in  the  United States.


        14.3.2.1   Advantages and Disadvantages of
                  Rail  Transport

Rail transport has a lower  energy cost per unit volume  of sludge
than pipelining and truck hauling,  and once found  to be  feasible
has a  right-of-way already  established, which is  not usually  the


                              14-49

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case with a pipeline.   Rail  transport can suffer from many of the
same problems as pipelines,  such  as  large  unrecoverable  capital
expenditures and fixed  terminal points.  In addition, it has some
of the same problems  associated with trucking, such as an ongoing
administrative   burden,  vulnerability to  labor  disputes  and
strikes,  risk of spills, and because  of  the  labor requirements,
an operational cost  that will rise continually.  However,  special
circumstances may favor rail hauling.   For  example,  if sludge is
to  be  used to  rehabilitate  strip-mined lands,  a rail  line  may
have been built  for  hauling  out the  coal.   That line would still
be available for the  transport of sludge.


        14.3.2.2 Routes

The construction of  a new railroad line may not be cost-effective
or even possible for the sole purpose of transporting wastewater
sludge.    New construction  is normally  limited  to a  short  spur
from  a  mainline railroad  or the  provision  and/or expansion
of  small switching  yards  on a  large treatment  plant site  in
conjunction with chemical  delivery  facilities.   Any  attempt  at
longer new  lines is  impractical.  This  limits  the  overall route
selection,  generally  between  the  treatment  plant  site  and
the final  sludge disposal  point, to  railroad lines  already  in
existence.   In  turn,  this  will limit either the selection of
rail  for sludge transport  or severely limit the choice of or
subsequent change in  disposal site location.
        14.3.2.3   Haul  Contracts

Railroad  cars must  be hauled  by  a  railroad  company, except
possibly for  switching.   Therefore  a contract must  be  obtained
with the railroad.  Since  this  contract  hauling  is  a major cost
element, and  since  the railroad often cannot provide rapid  and
realistic cost  estimates,  some  time  and consideration  will  be
required.

Railroads  are a regulated utility; this  complicates  the rate
quotation process.   Rates are  of  two  general types:   a  "class
rate"  and  a  "special commodity  rate."   The  class rates  are
readily obtained,  but are  usually  prohibitively expensive  for
sludge.   To  obtain a special  commodity rate,  the following
procedure is necessary:

    1.   An  application is  submitted  to the railroad, including a
        complete  description of  what is  to be shipped; how  it is
        to  be shipped (type of  material,   liquid  or  solid);
        precisely where it is  to  be  shipped; the  frequency  of
        shipping  (how  much  per  day, per  week);  the approximate
        loading and  unloading  time;  what other types of materials
        are  similar  in  form,  concentration,  and  makeup  to
        the  material  being  shipped  (for example,  Code  5630,
        North Coast Freight Bureau,  "tankage"—a  commodity used


                              14-50

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        in production  of  fertilizers);  and  a  statement of  the
        price the shipper would  be '.willing to pay  in  cents  per
        100 pounds net  weight  (45.4 kg).

    2.  The  local   railroad—-the .. carrier—reviews  the  load,
        distance,   terrain,  switching   requirements,   and
        competition  and calculates a rate.

    3.  The rate is published  by the local  freight bureau  (for
        example,  for Seattle, Washington, the North Coast Freight
        Bureau)  for a  notice  period of 30  days  for  review by
        other,  possibly competing, carriers,  and  by one of  the
        five regional   freight bureaus:   Western,  Southwest,
        Central,  Southern, or Northeastern.   The regional freight
        bureaus  are  conglomerations  of  the local  ones  and  they
        regulate  and control prices between bureau  jurisdictions.

    4.  Comments and  appeals of  rates  can be made to  the
        Interstate  Commerce  Commission  (ICC).   An  appeal  of  a
        proposed  rate will cause  that rate  to be suspended  for  a
        seven-month  period  for  the  case   to  be  heard by  the
        suspension  board of ICC  and  for the carrier  to justify
        that rate.    Historically,  appeals  have caused  proposed
        rates to be  eliminated from the carriers'  tariffs.   This
        effectively  eliminates the option  of rail  transport of
        sludge  for this locality.

Generally speaking,  railroads  are interested in providing  sludge
transportation.   However,  many  railroads  are unfamiliar with
sludge  hauling; similarly, many  environmental  engineers  are
unfamiliar with  railroad procedures (38).


        14.3.2.4  Railcar  Supply

There are  three  methods of  ensuring  railcar equipment  adequacy:
by  leasing,  by  outright  purchase, or  through  placement of  the
required  number of  cars  in "assigned service"  by the carrier
under the  terms  of  the haul  contract.   Generally, an  assigned
service option is only  available  for  a solid (dry) or  semi-solid
(mechanically  dewatered)  sludge which can be transported in
hopper cars.  A liquid  sludge must be carried in  tank  cars  which
are  not normally available  "free"  from  the railroad.   As  a
generalization,  the  amortization  of  the purchase  of either
type  of  car  (at  approximately $90,000 to  $120,000  new) will be
at  considerably  higher  cost than  the rental   or lease  fee.
Consequently, it  is expected  that  the  assigned   service  option
would  be selected  for hopper cars,  and  a lease  arrangement
negotiated with a private  tank car rental company  for  tank  cars.

Railroad hopper car use is  subject to  minimum  shipment  fees  per
car and certain demurrage  criteria.  For example,  a single  hopper
car minimum shipment is 180,000 pounds (82,000  kg) and demurrage
criteria  are that  the  car  must  be loaded  within 48  hours  and
                             14-51

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unloaded within 24 hours.  Reference  time is 7 a.m.   If  a car is
delivered  between midnight and  8  a.m.,  the time begins -at 7 a.m.
the same day.   If a  car is delivered between  8  a.m.  and  midnight,
the time begins at 7 a.m.  the  following day.   Typical hopper car
capacities  are 2,600,  3,215, and  4,000  cubic  feet (75,  91,  and^
113 m3 ) , with the smallest size being typically the most readily
available .                                                         .
Tank cars  are normally rented  by  the month from private  tank car
rental  companies with  a minimum  five-year commitment.   A  large
non-insulated  coiled car  (coiled  to  prevent  freezing during the
winter months)  will  rent  for  approximately $450 per  month
(1978  prices).  Tank car  capacities are  typically 10,000  to
20,000 gallons  (37,850 to  75,700  1).   The selection of  rail
transport, with its  high transit times, for more putrescible
sludges without special gas venting and control equipment,  should
be avoided.   Typical minimum tank and hopper  car requirements are
shown  in Table  14-8.
                             TABLE 14-8

             TYPICAL MINIMUM TANK CAR REQUIREMENTS (28)
                                                Car loads'
  Approximate secondary
  treatment plant size,
         MGD
           10
           50
          100
Annual sludge One-way
volume, MG distance, mi
7.5 20
40
80
160
320
15 20
40
80
160
320
75 20
40
80
160
320
150 20
40
80
160
320
Per
year
375
375
375
375
375
750
750
750
750
750
3,750
3,750
3,750
3,750
3,750
7,500
7,500
7,500
7,500
7,500
Per
day
1
1
1
1
1
2
2
2
2
2
10
10
10
10
10
21
21
21
21
21
Cars j
required
5
5
7
8
9
9
9
13
15
17
47
47
68
78
89
97
97
139
160
181
 aCar size 20,000 gal (76 m3).
 Estimate assumes that ample storage is available so
 that extra cars are not required for peak sludge
 production or scheduling problems.

 1 MGD = 0.044 m3/s
 1 MG = 3,785 m3
 1 mi = 1.6 km
 1 gal = 3.8 1
                                14-52

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The exact calculation of car  requirements  is very site-  and
area-specific and  should  be  checked  directly  for any  given
situation.   It  should  be recognized that  the  speed  of  railroad
transport will depend in part on the track conditions and on the
railroad's normal traffic schedule;  the  track conditions may also
limit the loads  carried per car, and hence  the size and number of
cars required.  As a guide only, typical transit times are shown
in Table 14-9.
                            TABLE 14-9

                    TYPICAL TRANSIT TIMES FOR
                    RAILROAD TRANSPORTATION

      One-way distance,            Round-trip transit  time,3
          miles                             days
            20                                 4

            40                                 4

            80                                 6

           160                                 7

           320                                 8
      aFor  estimating rail car demand, an allowance  of  25
       to 50 percent should be added to accommodate
       scheduling and car holdup problems.  Also,  the
       transit  time does not include time for loading
       and  unloading, which must be estimated separately.
        14.3.2.5  Ancillary  Facilities

Railroad  transport of  sludge  requires  loading  storage  and
equipment (tanks,  pumps,  and piping  for  liquid sludge and hoppers
and  conveyors  for dewatered  sludge), railroad  sidings,  and
unloading equipment.   Unloading  is ordinarily  accomplished  by
gravity.   Car  maintenance  and  storage will  be undertaken  by
the  owner of the cars—not normally the  wastewater  treatment
authority—but car cleaning  and  washdown facilities may  be
required.

        14.3.2.6  Manpower  and  Energy Requirements

The wastewater authority  will have labor requirements for loading
and  unloading  railroad  cars  and  for  associated maintenance;


                             14-53

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estimates  are given  in Table 14-10.   Data on  energy demands
associated with  railroad  transport are  not readily  available,
but  energy demands are  relatively  low  compared with  other
transportation modes.  The  fuel consumed  in  transporting the
sludge  should nevertheless  be  estimated  for  inclusion in the
sludge management  program's energy effectiveness analysis.


                           TABLE 14-10

        MANPOWER REQUIREMENTS FOR RAILROAD TRANSPORT (28)

           Liquid sludge	                Dewatered sludge

Annual volume,
mil gal
7 . 5
15
150
750
Labor, manhours/yr

Operation
4, 124
4,124
10,500
28,500

Maintenance
130
260
500
1,200

Annual volume,
thousand cu yd
7.5
15
150
750
Labor, manhours/yr

Operation
1,650
3, 300
4, 125
10, 000

Maintenance
130
260
500
1,200
1 cu yd = 0.76 m
1 mil gal = 3,785 mj
    14.3.3 Barge  Transportation

Barge  transport  for  the  ocean  dumping  of  sludge has  been
practiced for many  decades around the world.   Recent  decisions to
limit ocean dumping, combined with rapidly escalating costs for
dewatering  or drying sludges,  have led to more  consideration
of barge transport  of  liquid  sludges  between the wastewater
treatment plant or plants and  land disposal  sites many miles
distant.   Barge transportation of  sludges is generally only
feasible for liquid sludges (to  the solids concentration limit at
which it may be pumped)  and over longer distances,  generally over
30 miles.  Additional  information is available (28,36,39).
        14.3.3.1   Routes and Transit Times

It is  evident that the key  feature in consideration of barge
transportation is  the  proximity to a suitable waterway.  However,
in planning a barge  transport system, the transit time  also plays
a critical role.   The traffic on the waterway;  physical features
such as  drawbridges,  locks,  and height  limitations,and  natural
characteristics such  as currents,  tides,  and even  wave  heights
will all affect  the  transit  time.   Local  operators familiar
with the waterway  should be  contacted for information and a
conservative  safety  factor should be  applied.   Loading  and
unloading  times  then must  be added  to  estimate  the  overall
turnaround time — the key  feature when contracting for  towing
service.   Towing  speeds and  cost  estimates  are given in
Table 14-11.
                             14-54

-------
                            TABLE U-11

              TUG COSTS FOR VARIOUS BARGE CAPACITIES3


                      Average velocity,  knotsb
  Barge Capacity,                                   Tug  costs,C
     barrels	Loaded     Unloaded          dollars/hour


      25,000              6           7                120

      50,000              7           8                150

     100,000              8           10                195
   Source:   Foss  Tug,  Seattle, Washington, a division
   of Dillingham  Corporation, various personal inter-
   views with Metropolitan  Engineers/Brown and
   Caldwell  staff members,  1975  through  1976.

   Velocities in  open  water.  Waterway restrictions
   reduce average speeds.

   Costs are for  late  1975  and early 1976.  Inflation
   has been  at about  15  percent  per annum compounded
   since 1976.

   1 barrel  = 159 1
   1 knot =  0.51  m/s  = 1.85 km/hr
        14.3.3.2  Haul or System Contracting

Only for very  large  plants should ownership of the motive power
unit(s)   (tug  or powered  barge)  be  considered.    Self-propelled
barges  are  no  longer generally  considered  cost-effective when
initiating  a new system,  although the specifics of  any particular
case could  modify  this  conclusion.   This means  the  choice  for
most wastewater treatment authorities narrows down  to contracting
for either  complete  barge  transport services  or for tug  service
alone.   Full  service contracts may prove  the best  for small
operations  with intermittent transport requirements.  Moderate to
large plants will generally  favor contract towing  only, with  the
barge(s)  owned  by  the  authority  (although  Chicago's  barging
system  is  a full  service  contract).   Contractual  agreements
should  clearly  define  in detail all services  to be provided  and
include a barging schedule.  In  certain cases it may be possible
for two or more wastewater  treatment authorities  to  join in  a
common  contractual  agreement whereby  sludge from two  or more
plants  is picked up in tandem by the one  haul contractor.
                              14-55

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         14.3.3.3  Barge  Selection and  Acquisition

Both the useful life and salvage value of barges tend  to be high.
This  will  often lead  to  a  decision to  purchase rather  than
lease  equipment.    Size  and  number  of  barges  will  depend on
plant size  and the specific sludge processing system.

Some  data  on  typical  barge sizes  and  costs  are  given in
Table 14-12.   Physical dimensions of barges are not  standardized,
since they  are usually custom built within certain  dimensions set
by some  waterway constriction, such as lockage limitations.  Lead
times on construction  are about two years.   Barge proportions are
commonly length  to  breadth  4  or  5 to 1, and  breadth to  depth
3 or  4  to  1.   For  inland waterways,   about  two  feet   (0.6  m) of
freeboard under the maximum loaded condition is usually adequate.
Barges  are  very  common  in  the 20,000  to 25,000  barrel  (3,200 to
4,000 m3) capacity range.  Construction  costs  in 1976  were about
$6 per  cubic foot ($212/m3) for a 25,000  barrel (4,000  m3)  barge,
with only a slight reduction  in  unit  costs  as  size  increases, to
about  $5.50  per cubic  foot  ($194/m3)  at  the  100,000   barrel
(16,000  m3)  size.  Greater  flexibility in operations will  usually
dictate  the choice of  smaller barges,   unless distances are about
200 miles  (330  km)  or  more  and number  of  waterway restrictions
low.   Then  the  increased  speed  offered by  a  larger tug/barge
combination will  substantially cut transit time  and thus  reduce
towing  fees.
                              TABLE 14-12

                                                 .a
                    TYPICAL BARGE SIZES AND COSTS'
                                                          Cost,C
    Capacity,
     barrels

Length

240
240
_
_
286
320
Dimensions,
Breadth

52
60'
_
_
62
70
ft
Depth

15
13.5
_
_
18
20

Draft
_
13.5
-
-
-
16
18
thousand
Newd'e
_
1,100
-
-
-
1,750
2,300
dollars
Usedf
225
-
-
650
625
-
~
     14,000
     20,000
     23,000
     27,000
     33,000
     35,000
     50,000



 aExamples are for barges custom built for liquid sludges but do not include
 pumps necessary for unloading.

 One barrel equals 42 gallons  (159 1).

 °Costs are for 1976.  Inflation in new and used barges has been about 15
 percent per annum compounded 1976 through 1979.
 dSource: L. R. Gloston and Associates, Naval Architects, Seattle, Washington.

 Construction costs were approximately 50 cents/lb of steel in the barge  ($1.10/kg) in
 1976 and are about 80 cents/lb ($1.80/kg) in 1979.

 Source: William Drury Company, Seattle, Washington, communication to
        Metropolitan Engineers/Brown and Caldwell, September 30, 1976.

 1 barrel = 0.16 m3
 1 ft = 0.30 m
 1 cent/lb = $0.022/kg


                                14-56

-------
        14.3.3.4  Ancillary  Facilities

A critical factor  in determining the  feasibility of  barging
sludge lies in the cost of  facilities for  loading and offloading,
and receiving the sludge.   If  the  treatment plant is not close to
the waterway, it may  be desirable to  locate a sludge  storage
tank or lagooon near the barge loading  dock.   For a tank, design
would  need to  be  similar  to an  unheated digester because of
continued anaerobic  decomposition.  Lagoons should be operated as
facultative sludge lagoons.  In either case, costs of the tank or
lagoon should be included in the barge system costs.

In most cases, it is desirable to load  and meter the flow from a
fixed pumping  station  located on  a fixed wharf.   Offloading is
often  accomplished by  a   pump  mounted  on  the  barge  itself.
The disposal  site  should  be located  near a  dock capable of
mooring a  suitably  sized barge.  Floating docks are usually more
expensive  in  both marine and  freshwater environments  than fixed
wharfs, due  to  the  complexity of anchoring devices  capable of
sustaining the  loads imposed by a large barge.   In  certain
instances, however,  a  floating  dock may be  more acceptable from
an environmental standpoint.

Unloading to a land  pipeline typically takes about 6 hours.  If a
tug must remain with the  barge during the  unloading period, rapid
unloading becomes economically important.


        14.3.3.5  Spill Prevention  and Cleanup

One  important element of  a  barge transportation system  is a
well  developed spill  prevention and cleanup program.   Spills
resulting  from accidents during transport can  result  in serious
water  pollution  and associated health  problems.   Sludge spills
should be  contained immediately and transferred to storage tanks
or  another barge as  quickly  as possible to reduce risks.   The
risk of  spills  during loading and unloading  can be minimized by
careful attention to design and operator training.
14.4  References

 1.  Metcalf  &  Eddy,  Inc.   Wastewater  Engineering;  Treatment^
               Reuse.   McGraw-Hill.   1979 (second  edition).
 2.  Hanks, R.W.  and  B.H.  Dadia.   "Theoretical  Analysis of the
     Turbulent  Flow  of Non-Newtonian Slurries  in  Pipes."
     American Institute of  Chemicjil^Enginee rs  Journal .   Vol. 17,
     p. 554.  May 1971.

 3.  Caldwell,  D.H.  and  H.E.   Babbitt.   "The  Flow of  Muds,
     Sludges, and Suspensions in Circular Pipe."   Transactions of
     American Institute of  Chemical Engineers^.   Vol.  37,  p. 237.
     April 25, 1941.


                              14-57

-------
 4.   Buckingham,  E.   "On Plastic Flow  Through  Capillary  Tubes."
     Proceedings  of  the  American  Society  of _Testing  and
     Material's^Vol. 21, p. 1154.1921.~~

 5.   Rimkus,  R.R. and R.W. Heil.  "The Rheology  of Plastic Sewage
     -Sludge."     Proceedings of the Second National Conference on
     Complete  Water  Reuse.    Chicago,   Illinois:    5/4-8/75.
     American  Institute of  Chemical Engineers.    L.K. Cecil, ed.
     p.  722.

 6.   Rimkus,  R.R.   and R.W.  Heil.    "Breaking   the  Viscosity
     Barrier."    Proceedings of the Second National Conference on
     Complete Water  Reuse.   Chicago,  Illinois;   5/4-8/75.
     American  Institute of  Chemical Engineers.    L.K. Cecil, ed.
     p.  716.

 7.   Kenny,  J.P.   Bulk Transport of Waste Slurries to Inland and
     Ocean  Disposal  Sites.   Volume  III.   Bechtel  Corporation.
     1969.   Published  by National  Technical  Information  Service
     as  PB  189759/BE.

 8.   Babbitt,  H.E.  and D.H. Caldwell.   "Laminar  Flow of  Sludges
     in  Pipes  with  Particular  Reference  to  Sewage  Sludge."
     University  of  Illinois  Engineering Experiment  Station,
     Bulletin  Series, No. 319.1939.

 9.   Rabinowitsch,  B.   Z. Physical Chemistry.   Vol. 145A, p. 1.
     1929.

10.   Wolfs,  J.R.  "Factors Affecting Sludge Force  Mains."   Sewage
     and Industrial  Wastes.  Vol. 22, p. 1.  January 1950.

11.   Holland,  F.A.   Fluid Flow for ChemicalEngineers.   Chemical
     Publishing Company.  1973.

12.   Bourke,  J.D.   "Sludge Handling Characteristics in Piped
     Systems."    Proceedings of  the Northern Regional Conference
     of  the  California  Water Pollution Control Association.
     Monterey,  California:  10/19-20/73.

13.   Babbitt,  H.E.  and  D.H. Caldwell.   "Turbulent  Flow of  Sludges
     in   Pipes."     University of Illinois Engineering Experiment
     Station,  Bulletin  Series, No.  323.  1940.

14.   Hedstrom, B.O.A.   "Flow  of Plastic Materials in  Pipes."
     Industrial Engineering Chemistry.  Vol. 44, p. 651.   1952.

15.   Behn,  V.C.  and R.M.  Shane.   "Capillary vs.  Pipeline  in
     Determining  Sludge Flow  Behavior."   Wa_te r  &  Sewage Works.
     Vol.  110,  p. 272.   July 1963.

16.   Alves, G.E.,  D.F. Boucher, and  R.L.  Pigford.  "Pipeline
     Design  for  Non-Newton ion  Solutions  and  Suspensions."
     Chemical Engineering Progress.  Vol.  48, p. 385.  1952.
                               14-58

-------
17.   Hanks,  R.W.   "The Laminar-Turbulent Transition for  Fluids
     With a  Yield  Stress."   American  Institute  of  Chemical
     Engineers  Journal.  Vol. 9, No. 3, p. 306.  1964.    "~

18.   Hanks,  R.W.  and  D.R. Pratt.  "On the Flow of Bingham Plastic
     Slurries  on Pipes and  Between  Parallel  Plates."  Society  of
     Petroleum  Engineers Journal.  p. 342.  December 1967.

19.   Kenny,  J.P., E.J. Wasp,  and T.L.  Thompson.   "A Design iModel
     for Pipeline Flow of Solid Wastes."  Water-1970.   Chemical
     Engineering  Progress  Symposium Series.    American  Institute
     of Chemical  Engineers.  Vol. 67, no. 107, p. 364.   1971.

20.   Dick, R.I.  and  B.B.  Ewing.    "The Rheology  of  Activated
     Sludge."    Journal  Water  Pollution  Control  Federation.
     Vol.  39, p.  543.   1967.

21.   Bingham, B.C.   Fluidity and Plasticity.   McGraw-Hill.  1922.

22.   Brisbin,  S.G.  "Flow of  Concentrated Raw Sewage Sludges  in
     Pipes."   Journal of  the Sanitary  Engineering Division ASCE.
     Vol.  83, no.  SA3,  p. 1274.  June 1957.

23.   Chou, T.L.   "Flow of Concentrated Raw Sewage Sludges  in
     Pipes."    Journal of the Sanitary Engineering Division ASCE.
     Vol.  84, no.  SAl,  p. 1557.  February 1958.

24.   Vesilind,  P.A.   "Treatment  and  Disposal  of  Wastewater
     Sludges."   Ann Arbor Science.   Chapter 4.  1979 (Second
     Edition).

25.   City of San Francisco.   "Primary  Sludge  Pump Evaluation."
     Prepared   by  the City's  Division  of Sanitary  Engineering.
     October  1975.

26.   Sparr,  A.E.   "Pumping Sludge Long Distances."  Journal Water
     Pollution Control Federation.   Vol.  43,  p.  1702.  August
     1971.

27.   Williams,  M.L.   "A Guide to the Specification of Glass Lined
     Pipe."  Water  &  Sewage Works.   Vol 124,  no.  10,  p.  76.
     October 1977.

28.   USEPA.  Transport of Sewage  Sludge.    U.S. Environmental
     Protection Agency report EPA-600/2-77-216.  December 1977.

28a. Weller,  L.W.  "Pipeline Transport and  Incineration."  Water
     Works  and  Wastes Engineering.    Kansas  City,  Missouri,
     installation.   September 1965.

28b. Wirts,  J.J.  "Tips and Quips—Contribution from Cleveland."
     Sewage  Works J qu rna_l.  Vol. 20, No.  3,  p. 571.  May 1948.

29.   Tchobanoglous,  G.> H.  Theisen,  and  R.  Eliassen.  S o_li_d
     Wastes.   McGraw-Hill.  Chapter 5.   1977.

30.   Conveyor  Equipment  Manufacturers  Association.   Belt
     Conveyors  for Bulk^Materials.   Cahners  Publishing  Company.
     1966.

                             14-59

-------
31.   Hansen, B.E.,  D.L. Smith,  and W.E. Garrison.   "Start-up
     Problems of  Sludge  Dewatering Facility."  Proceedings  of  the
     51st Annual  Water Pollution Control Federation  Conference.
     October 1978.   Anaheim, California.

32.   Conveyor  Equipment  Manufacturers  Association.    S_c_re_w
     Conveyers.   Book  No. 350.  1971.

33.   Dallon,  F.E.  and R.R.  Murphy.    "Land  Disposal   IV:
     Reclamation  and  Recycle."   Journal Water Pollution Control
     Federation.   Vol.  45, no. 7, p. 1489 (July 1973).

34.   USEPA.   Cost  of Landspreading   and  Hauling Sludge from
     Municipal Wastewater Treatment Plants.    U.S.  Environmental
     Protection  Agency, Office  of Solid  Wastes,  Cincinnati,
     Ohio 45268.   EPA/530/SW-619, Oct. 1977.

35.   Ettlich, William F.    "Economics of Transport  Methods  of
     Sludge."   Proceedings  of  the Third National Conference  on
     Sludge Management;  Disposal and Utilization.  Miami  Beach,
     Florida.   December 14-16,  1976.  Information Transfer Inc.,
     p.  7.

36.   Guarino,  C.F.,  M.D. Nelson,  S.A.  Townsend,  T.E.  Wilson,  and
     E.F. Ballotti.  "Land and Sea Solids Management  Alternatives
     in  Philadelphia."    Journal  Water  Pollution  Control
     Federation.   Vol.  47, noT 11, p. 2551.   November  1975.

37.   Billings,  C.H., S.H.  Conner, J.R.  Kircher,  and G.M. Scales.
     1979 Public  Works Manual.  Public Works Journal  Corp.   1979.
;     p.  D-49.

38.   Heller, N.   "Working With the Railroad."  Proceedings  of  the
     Third  National Conference  on  Sludge  Management:   Disposal
     ~an d~~uFiTTz a t i o n^    Miami  Beach,  Florida.    December  14-16,
     1976.   Information Transfer  Inc.  p. 50.

39.   USEPA.  "Evaluation of Sludge  Management Systems:   Evalua-
     tion  Checklist  and  Supporting   Commentary."   Technical
     bulletin prepared  by  Culp/Wesner/Culp.   April  1979  draft.
     To be published.
                              14-60

-------
EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
                Chapter 15. Storage
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                          CHAPTER 15

                           STORAGE


15.1  Introduction

Storage is an integral part  of  every wastewater solids treatment
and disposal system,  since  it is  necessary to the assurance
that the system will be used to full capacity.   Recent  emphasis
on  the control  of  wastewater  solids treatment  and disposal
mandates that  effective  storage  be  provided.  Storage  that  is
compatible  with the  objectives  of a system must be incorporated
into its design to  enhance both  the  system's  reliability  and its
efficiency.


    15.1.1   Need  for Storage

Storage allows  different  processes to operate on schedules which
best fit overall system objectives and  precludes the  need  to
force  all  processes  to operate  on  the  same  schedule.   For
example,  solids  are  generated from the  wastewater treatment
system  24  hours per day, but it may be  most  convenient to operate
the solids processing  system only on the day  shift.   Solids must
therefore  be stored  during  off-hours.   Storage  must  also  be
provided between  adjacent treatment  or disposal processes which
operate at  different rates--for  example,  between centrifuges
(which  discharge solids  at  100 tons  per  hour [91 t/hr])  and
incinerators (which  must  be  fed at 50  tons per hour [45  t/hr]).
In addition,  it must be provided upstream  from virtually any land
disposal system,  since sludge can usually  be  applied to land only
part of  the year,  whereas the  waste  treatment plant  generates
solids  all  year around.


    15.1.2   Risks and  Benefits of Solids Storage
            Within Wastewater Treatment System

Stored  solids can  be  washed  from the  wastewater treatment
system,  thereby degrading  effluent  quality.  They may  also
become  septic,  with  the same effect.  As  a general  rule, solids
should  not  be stored  in wastewater  treatment systems  unless
storage  provides  benefits  that clearly  outweigh  the  risks
involved.   For many small plants,  if sludge processing units
are operated only  on the day  shift,  the benefits do outweigh
the risks.   These  plants frequently  store solids within the
wastewater treatment  process  for  periods  as long   as  24 hours.
Large  plants,  which  typically  process sludge  around-the-clock,


                             15-1

-------
make less frequent  use of  storage within the wastewater treatment
system.  The main exception to this rule is the storage of  solids
within  wastewater stabilization ponds, where  solids and dead
algae  settle  to  the  bottom of  the  ponds  and  anaerobi cal ly
decompose.   These solids are  seldom removed and  often  accumulate
for many years with  no deleterious effect.


    15.1.3   Storage Within Wastewater Sludge
            Treatment Processes

Solids  can be stored  within sludge treatment  processes with
fewer adverse effects  than if they were stored within  the  waste-
water treatment system.   Whereas  the  processes  of  disinfection,
conditioning,  mechanical dewatering, high-temperature conversion,
and  heat-drying  do  not  provide  storage,   those of  gravity
thickening,  anaerobic and aerobic digestion,  air drying, and
composting  do.  Used  judiciously,  these processes  can store
enough solids  to enable necessary adjustments  to  be  made  in rates
of  flow  between  processes.   One  or  two of these  processes can
provide cost-effective  storage for periods exceeding  one  month.
However, because of process limitations, some  cannot provide
storage for minimum periods  of  three  to  four days even  though
they can store for periods of three to four weeks and longer.


    15.1.4   Effects  of Storage on Wastewater Solids

If  wastewater  solids  are  to  be  stored  for any  extended  period
of  time, they must  be stable.   Stable liquid sludge  with less
than  ten percent solids  can be  stored in facultative sludge
lagoons, anaerobic sludge  lagoons,  or aerated basins.   When
it  is air dried to greater than 30  to  40  percent solids,  stable
sludge  can be stored  safely and  without odors  in  relatively
small, confined structures or in  unconfined  stockpiles.   It is
impractical to store  unstabilized  dewatered  or partially dried
sludge  (sludge containing more  than 10 percent and  less than
30  percent solids)  for much  longer  than three  to  four days
because septic conditions  and problems associated with septicity
(odors, poor solids  transport properties) can develop.

Wastewater solids  are  usually  stored  in concentrated  form.
If  these  solids  are  biodegradable,  indigenous  oxygen  supplies
can  rapidly be depleted  and anaerobic decomposition begins.
Anaerobic  decomposition is  often,  but  not always  ,accompanied
by  the  production  of  undesirable  odors.  However,  anaerobic
decomposition will not occur  if:

     •  Biodegradable  materials  present  are  insufficient to
        support biological activity.    For example,   screenings
        and grit are relatively non-odorous, provided they have
        been  processed  and   transported hydraulically  prior to
        final  dewaterings.    The washing  action  which occurs


                             15-2

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        during  these operations  reduces  the  concentration  of
        putrescible  organic material.   Conversely,  if  processed
        and transported mechanically  (that is,  without  washing),
        they may be the source of strong odors  when subsequently
        stored.

     •  Oxidizing conditions  can be maintained.  Agents  such  as
        oxygen,  chlorine, and  hydrogen  peroxide  can be used
        to this  end  if the sludge  is  in  liquid form.  Forced
        aeration or physical manipulation can be used to maintain
        the aerobic  condition if  solids  are dewatered and
        managed,  as is done in composting.

     •  Moisture is  reduced  to discourage  biological  activity.
        For example,   air  dried  stabilized sludge with  a  solids
        content  greater than  40 to 50  percent  and  unstabilized
        heat-dried  sludges  can  be  stored  indefinitely  without
        nuisance, provided rewetting does not occur.

     •  pH  is  adjusted to  values above  approximately 12 and
        below  approximately 4 by  adding chemicals  like  lime  or
        chlorine.  Note that pH  extremes must be maintained.
        These  treatments  do  not destroy  putrescible materials,
        and the  biocidal effects caused  by extreme pH are lost  as
        the pH  drifts toward neutral values  as the result  of
        interaction with atmospheric carbon dioxide.

The  fact  that  anaerobic  digesters   and  facultative sludge
lagoons have  operated without nuisance odors  clearly  indicates
that  storage  can be  accomplished under  anaerobic conditions
without adverse effects.   Work  on  facultative  sludge lagoons  in
Sacramento documents  these conclusions  (1).

Nuisance  odors  will  not  develop  in  anaerobic  storage when
sufficient methane bacteria are  present.  If the methane bacteria
are destroyed, however, serious  odor  problems may result.   As  an
example, consider anaerobically digested sludge  which  is  placed
on a  drying bed or in a drying  lagoon.   The top layer  of  sludge
is dewatered,  and methane  bacteria die as the sludge aerates and
dries.  Odor  levels  are  extremely low, since the sludge  is too
dry to support anaerobic biological activity.  Should the surface
of the  sludge be re-wetted  (for example,  by  rainfall or surface
flooding), however,  anaerobic  activity would resume,  the organic
acid  concentration  would rapidly   increase,   and  odors would
increase  to nuisance  levels.   Odor problems  experienced with
approximately  580 acres (235 ha) of  drying lagoons  at  San  Jose,
California, immediately following a rainstorm,   is an example  of
this type of problem  (2).

Not all the effects  of solids  storage are  negative.   Storage
of anaerobically digested sludge in the liquid  state can  be
beneficial for its ultimate disposal.   If  such  sludge  is  stored
                              15-3

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for several years without being contaminated by freshly digested
sludge,  its organics content  (40  to 50 percent)  and its content
of pathogenic  bacteria,  viruses,  and  parasites  will  be greatly
reduced  (1,3).
    15.1.5  Types of Storage

Wastewater  solids  may  be  stored  in  facilities  within  the
treatment system,  within  the sludge  treatment and  disposal
system, and within  tanks,  lagoons,  bins,  or stockpiles provided
primarily for  storage.   This  latter  group  is  divided  into two
divisions, those provided for  either liquid  or dewatered sludge.
The use of wastewater and sludge treatment facilities for solids
storage must not adversely  affect their  treatment  capability.  If
this  potential  exists,   then  facilities dedicated  primarily to
storage must be provided.

Three methods  of storage  are described as follows:

     •  Single-Phase  Concentration.    Solids  accumulate  in  a
        completely-mixedvesselas  a result  of  increasing
        concentration.   The   solids  concentration is  uniform
        throughout and vessel  volume is constant.  For example,
        solids buildup within  the aeration   reactor of an
        activated sludge  system if  solids are not  wasted.

     •  Two-Phase Concentration.    Storage  is within  the solids
        layer of  a  liquid/solids  separation device.   Volume of
        the solids layer increases; however, total system volume
        remains  constant.   For example,  solids   are accumulated
        in a  gravity  thickener by  terminating sludge withdrawal
        from  the  thickener and allowing  the sludge  blanket to
        build up.

     •  Displacement.   Solids  are stored as  a result of changing
        total system volume.    For example, solids can accumulate
        within  digesters with  floating covers  by  displacement
        storage, since the  covers can rise  to accommodate greater
        volumes of sludge.

Storage  may  be  accomplished  by  two or three  methods  operating
in  concert.   For example,  solids  can  accumulate in a  floating
cover equipped  secondary digester by simultaneous  two-phase
concentration and displacement.

Storage may be further categorized  as follows by  detention time:

     •  Equalization Storage    Solids  detention  time  should not
        ——————exceed three to four days.

     •  Short-Te rm_S to rage      Solids  detention  time  should not
                  ~"exceed three to four weeks.
                              15-4

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           Long-Term Storage
                           Solids  detention
                           than  one  month.
               time   is  greater
Table  15-1   lists  wastewater  solids
method,  and  detention  time  category.
                                    storage   by   type,   facility,
                                       TABLE 15-1

                   WASTEWATER SOLIDS STORAGE APPLICABILITY
                                          Detention tim
       Type
                                Equalizing
                                 (3 to 4
                                  days)
                           Short term
                           (3-4 weeks)
 Long term
  (Greater
than 1 month)
                                                                         Comments
Storage within waste-
  water treatment
  processes

   Grit removal


   Primary sedi-
    mentation

   Aeration reactors
Two-phase concentra-
 tion
Two-phase concentra-
 tion
                 Single-phase concen-
                  tration
         Use of wastewater treatment processes for
           storage must not adversely affect treat-
           ment efficiency.
         Storage time depends on sewer system grit
           loading to plant.
                                                              Storage for over
   Secondary sedi-    Two-phase concentra-
    mentation        tion

   Imhoff tanks      Two-phase concentra-
                  tion


   Community septic    Two-phase concentra-
    tanks          tion

   Wastewater sta-    Single and two-phase
    bilization ponds   concentration
Storage within sludge
  treatment processes


   Gravity thickeners  Two-phase concentra-
                  tion


   Anaerobic digesters  Single and two-phase
                  concentration and
                  displacement
         Temperature sensitive.
           24 hours.

         Storage within extended aeration systems,
           for example, oxidation ditches , can
           exceed 3 weeks if accomplished in con-
           junction with secondary sedimentation
           concentration.

         Highly temperature sensitive. Storage for
           over 8 hours requires chemicals.

         Lightly loaded systems can store for over
           6 months.  Most systems will require
           solids removal every 4 to 6 weeks.

         Sludge from many septic tanks is removed
           only once in several years.

         Aerated ponds operate like aeration
           reactors.  Other ponds use two-phase
           concentration and can store solids for
           many years.

         Use of sludge treatment processes for
           storage must not adversely affect sludge
           treatment efficiency.

         Temperature sensitive .  Usually not used
           with WAS.  Storage for over 24 hours re-
           quires chemicals.
         Floating covers allow for displacement
           storage.  Two-phase concentration stor-
           age impracticable if WAS present.
           Single-phase concentration storage pos-
           sible if digesters operated in conjunc-
           tion with primary sedimentation
           concentration changes.
15.2   Wastewater  Treatment  Storage

Influent  variability  and  fixed   effluent  requirements  make
operational   flexibility   a   necessity    for  every   wastewater
treatment   plant.     One  of  the   most  cost-effective   means  of
providing   flexibility   for   small  plants   is   to   assure  that
treatment  processes  contain storage  within  themselves.


      15.2.1    Storage  Within  Wastewater  Treatment  Processes

Listed  in Table   15-1  are several  wastewater  treatment   processes
that  can  provide  solids storage.   The  following  sections  describe
ways  in which this storage  can be used effectively.
                                          15-5

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                                       TABLE 15-1

            WASTEWATER SOLIDS STORAGE APPLICABILITY (Continued)
                                         Detention time
      Type
                                Equalizing
                                 (3 to 4
                                 days)
                          Short term
                          (3-4 weeks)
 Long term
  {Greater
than 1 month)
Storage within sludge
 treatment processes
 (continued)

   Aerobic digesters
   Composting
   Drying beds
Facilities provided
 primarily for stor-
 age of liquid sludge
                Single and two-phase
                 concentration and
                 displacement
Two-phase concentra-
 tion and displace-
 ment

Two-phase concentra-
 tion and displace-
 ment
Holding tanks
Facultative sludge
lagoons
Single and two-phase
concentration and
displacement
Two-phase concentra-
tion
   Anaerobic liquid
    sludge lagoons
Two-phase concentra-
 tion
         Decanting can be limiting. Short-term
           storage possible if digesters operated
           in conjunction with sedimentation con-
           centration.  Displacement storage
           requires aeration systems which will
           operate with variable level.

         Evaporation with process accomplishes two-
           phase concentration.  Processed solids
           not removable for 3 to 4 weeks.

         Initial settling accomplishes two-phase
           concentration.  Processed solids not
           normally removable for 3 to 4 weeks.
         Storage limited to equalizing by high
          costs of detention and continuous
          mixing.

         Time required for initial settling limits
          storage to short or long term.
          Mechanics of sludge removal makes short-
          term storage very expensive. Odor free
          operation requires anaerobically di-
          gested solids.  Organic loadings must be
          restricted and surface agitation pro-
          vided. Odor mitigation required when
          surface area exceeds 30 to 40 acres.

         Time required for initial settling limits
          storage to short or long term.
          Mechanics of sludge removal makes short-
          term storage very expensive. Odor
          minimization requires anaerobic digested
          solids.  Usually operated without organic
          loading restriction. No surface agita-
          tion provided,  potential odor risk
          high, although no quantifying data
          available.
           15.2.1.1   Grit  Removal

Grit  removal  basins   and  channels  may be  used  to  store  unusually
heavy  grit  loadings  which,   when   combined  sewer  systems   are
involved,  generally   arrive  at  the   treatment  plant   after  a   dry
spell  and during  the  first  flush  of  a   storm.    Storage  must  be
provided  to  contain all  of  the grit  which  could  accumulate  during
the  storm.    The  required  storage   volume   is  a  function  of  grit
loading  and  the  rate  at which  the grit  can  be  transferred  out of
the  basin or  channel.     Where  grit   is  transferred  manually   (for
example,  in  small  plants  with  duplicate  channels),  the  designer
may  wish  to  provide  storage  sufficient  to   hold  grit  during
periods  when  the plant  may  be  unattended   (long weekends).    Grit
production figures  are  shown  in  Chapter  4.          ..  ,•
                                         15-6

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                                  TABLE 15-1

           WASTEWATER SOLIDS STORAGE APPLICABILITY (Continued)
                                    Detention time
     Type
                            Equalizing
                             (3 to 4
                             days)
                       Short term
                       {3-4 weeks)
 Long term
  (Greater
than 1 month)
Facilities provided
 primarily for stor-
 age of liquid sludge
 (continued)

  Aerated storage
Facilities provided
 primarily for stor-
 age of dewatered
 sludge

  Sludge drying
   lagoons
  Confined hoppers
   or bins
  Unconfined stock-
   piles
              Single and two-phase
               concentration and
               displacement
Two-phase concentra-
 tion and displace-
 ment
              Displacement
              Displacement
                                        High energy demand usually restricts
                                         detention time.  Same limits as
                                         aerobic digesters.
        Initial settling accomplishes two-phase
         concentration. Process solids not
         normally removable for one to two
         months. Odor minimization requires
         anaerobically digested solids. Can be
         odorous if aerobically stabilized sur-
         face layers begin to decompose
         anaerobically when rewetted.

        Moist (15 to 30 percent solids) dewatered
         sludge can present major material manage-
         ment and odor production problems if
         storage time exceeds 3 to 4 days.
         Structures usually too expensive for
         long-term storage. Short-term storage
         can be successful with dry (greater than
         30 to 40 percent solids) stabilized
         sludges.

        Requires stabilized dry (greater than 30
         to 40 percent solids) sludge.  Stock-
         piles are usually covered in very wet
         climates. Natural freeze drying is
         possible.
Special  techniques  or  equipment  may be  needed  to  transfer  heavy
grit  accumulations.   If  grit is transferred  mechanically  (by
flight,  bucket,  and  screw conveyors),  the  equipment must be able
to  start  while the entire  basin   or  channel  is  filled with  grit.
If  grit  is  transferred   hydraulically,  air  agitation  should  be
used  to  loosen  up  the  accumulated  solids  during  the  removal
operation.     Hydraulic removal  can  be accomplished by  eductors,
air-lift  pumps,   or  special  centrifugal  pumps.   When  special
centrifugal  (torque-flow  or  vortex) pumps are  used,   the  grit
should  be  loosened  up  in  the  immediate   vicinity  of   the  pump
suction  by  a  high-velocity  water   jet.   More design information on
grit removal  facilities is  available (4,5).


          15.2.1.2   Primary  Sedimentation

If   storage  is  provided   in  primary  sedimentation,  solids
processing  systems  can  operate  at  rates   independent  of
the  rate  at   which  solids  are  removed  from  the  wastewater.
This   is  especially  useful  for small  plants  which   are  not
manned  continuously  and for  any   size  plant  that  experiences
large  diurnal  or  seasonal  fluctuations  in settleable  solids.
                                     15-7

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Concentration of  sludge  removed from the  primary  sedimentation
tank may be controlled to some degree if the depth  of the sludge
layer in the sludge removal hoppers is controlled.   Hopper sides
should  be  sloped  at least  60 degrees off  the  horizontal  so
that solids  can  flow by gravity  to  the pump  suction.   Primary
sedimentation tank storage  capacity should be sufficient to allow
suitably sized primary  sludge  pumps  to remove the  peak  sludge
loadings.   Otherwise the solids may interfere with  the gathering
function of  the   longitudinal  sludge collectors in  rectangular
tanks or the  main collector in  circular  clarifiers.

Efficient  use of  primary sedimentation  storage  requires  the  use
of a control  timer,  density,  and  blanket level instrumentation.
Ideally,  all three devices  can  control  primary  sludge pump
operations.   Blanket  level sets the time when  the  pump  starts;
control  timers  set  the  cyclical  period  when the pumps  can
share the discharge  piping (if necessary)  and  the  minimum pump
operating  period  if  the density of  the pumped sludge is  below
the required concentration; and  density sets the time when  the
pump shuts down.   Chapter  17  provides  more  information  on this
instrumentation.

More design  information on  primary sedimentation tank design
is available  (4,5).

Design Example

The  designer of a  7 . 5-MGD  (0.33-m3/s)  average  design flow
wastewater  treatment plant  wishes to  determine the available
sludge storage volume in three  rectangular primary  sedimentation
tanks,  the  tanks are  designed to  treat  a peak  wet weather
flow of  20  MGD  (0.88 m3/s).    Available storage will  determine
the maximum  time  allowed  between sludge pumping cycles  and  the
maximum capacity  of  the  sludge  pumps.

Tank  design  is   based  on  conservative experience involving
overflow rates  and mean  velocities at average design flows.  Each
tank is 110 feet (33.5 m)  long and 19 feet (5.8 m)  wide,  with an
average  sidewater  depth  of  ten  feet  (3.05  m).   Longitudinal
collectors  operating continuously bring  the settled  sludge to  the
head end of the tank, where it is conveyed to the sludge  removal
hopper by cross-collectors.   The sludge is  then pumped from  the
removal hopper on  a timed  cycle with density  and  blanket  level
instrumentation.    Cross  collection channels  and sludge  removal
hoppers have been laid out to  aid in the concentration,  storage,
and removal  of  the collected sludge  by providing steep side
slopes,  ample depths,  and short suction pipelines.  Combined
storage volume of the cross collector channel and removal hopper
of  the  selected  tank design  is approximately 350 cubic feet
(9.9 m3) for  each tank.

It  is  assumed that peak and  wet  weather  flows will be of  at
least eight  hours duration and  will have  an  average  suspended
solids content of  200 mg/1.   Primary sedimentation  tank  removal


                              15-8

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efficiency is  assumed  to  be only 50 percent at peak  wet  weather
flow, down  from 60  percent at  average  design flow, because  of
higher  overflow rate and  higher mean  velocity.   Using these
assumptions,  the solids  collected in each  primary  sedimentation
tank during the storm can  be calculated  as follows:


   J20_MGD) (200 rog/1) (0.50)  (8.33 Ib/gal)
   — - (3 tanks) (24  hr/day) - = 231 lb/hr(105 kg/hr)


Primary  sludge solids  concentration  and wet  bulk  specific
gravity  are  assumed to be six  percent  and 1.07,  respectively.
Using these  assumptions,   the  volume  produced  in each  tank  can
be calculated as follows:
                                 = 58 ft3/hr (1.6  m3/hr)
    (0.06) (1.07) (62.4 lbs/ft3)
By dividing  this production  into  the storage volume  available,
the  designer  finds the  maximum  period of  time  between pump
cycles to be slightly greater than six hours.

The  primary  sludge  piping  to the digester  is  arranged so  that
only one primary  sludge  pump can  operate at  a  time.    To  assure
sufficient pumping capacity to  handle the  peak wet weather
sludge, it  is  necessary that  each  pump operate only one-third  of
the time.   Each pump,, therefore,  must have the capacity to  remove
all of the sludge stored  during  the  six-hour cycle  in  two  hours.
This capacity is calculated as follows:
    ,n nooK/       n-  /. .  = 21.6 gpm (1.36  1/s)
    (0.06) (8.92 Ib/gal) (60 min/hr)         ^


As  an additional  safety  factor,  to  assure maximum  reliability
and operational  flexibility, this  pumping rate  is doubled  and
rounded  off  to 50  gallons  per minute  (3.2  1/s).   The pump
selected  (a diaphragm pump,  see Chapter  14) can  be  adjusted down
to 25 gpm (1.6 1/s) if higher flow rates  are found to pull  liquid
instead of concentrating solids.


        15.2.1.3  Aeration Reactors  and Secondary
                  Sedimentation

Solids  are  stored  in  aeration reactors  and  secondary  sedimen-
tation  tanks  whenever there  is  an  increase  in  the solids
concentration of the mixed liquor.  This  solids increase requires
the two processes  to be operated as  one,  with  the  sedimentation
tank  providing  the  two-phase concentration  necessary   to  fully


                              15-9

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utilize the  single-phase  concentration storage  capabilities  of
the reactors.  Reactors should  be  designed  with the flexibility
to operate  either in the  plug  flow,  step  feed, reaeration  or
contact stabilization  modes or any combination of these.   Given a
fixed reactor size, maximum solids storage capability is  provided
when the process operates  in a combination of the reaeration  and
contact stabilization  modes.  Often the ability to switch between
complete plug flow and partial reaeration modes allows the solids
to be  removed  from the  hydraulic flow  stream and prevents  their
loss when that stream  receives a shock  loading.  Operation in  the
step feed mode also minimizes  the solid loading rates to  the
secondary sedimentation  tanks.   This solids  storage flexibility
should  be provided regardless of whether  the source of  aeration
comes  from dissolved  air  or  pure  oxygen.  Plug  flow nitrifying
aeration  systems, which  are often  required  to retain solids
for  two  to  three  weeks,  operate  at  maximum  efficiency when
the  hydraulic  and  organic  loadings  have  the  least  diurnal
fluctuation.   This uniformity is often  achieved in smaller plants
through  upstream flow  equalization.  Oxidation ditches are  a
simple  type  of aeration reactor found in many  small  treatment
plants.  More  design  information on aeration reactors  and flow
equalization is available  (4-8).

Secondary sedimentation  tank two-phase concentration  storage  is
vital to the successful  operation of  an aeration system.  Design
of  secondary  sed invent at ion  facilities  usually  involves  the
use of  the  solids flux theory, which is discussed briefly  in
Chapter 5 and in detail  in references 9 and  10.   To take maximum
advantage of the  concentration capabilities,  secondary sedimenta-
tion tanks   are  usually from  150  to  200 percent deeper than
primary  sedimentation  tanks (14  to 20  feet  [4.3 to  6.1 m]-)«
Blanket  level instrumentation  is  commonly  used to keep track
of sludge  storage levels within  the  secondary sedimentation
tanks.    Instrumentation  for  this  determination  is  discussed  in
Chapter 17.    More design  information on  secondary sedimentation
tanks is available  (4,5,7).


        15.2.1.4   Imhoff and Community  Septic Tanks

Both the  Imhoff  and  the community  septic tank were  in  use long
before  most  of the sludge  treatment  processes discussed in this
manual.  For this reason,  it is not surprising that their design
includes significant   sludge storage  capabilities.   Imhoff  tanks
are  still  in  use  in  many of  the  older  treatment plants,  and
therefore,  still provide those plants  with  extensive solids
storage  capacity  in  what  are essentially  unheated  low rate
anaerobic digesters  (see  Chapter  6).   The   storage  capacity  of
Imhoff  and septic tanks  is part of  the empirical design  criteria
for  these  facilities.  While  their future  use may be  limited
because of  today's  secondary treatment mandate,  both processes
offer  low  cost primary  treatment for  upgrading  small community
wastewater stabilization pond facilities.  In Newman, California,
existing community septic  tanks are  being   upgraded  to provide


                             15-10

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primary  treatment  for  a 0.76-MGD  (33.3-1/s)  complete  treatment
plant  with  pond  stabilization  for  secondary  treatment  and
overland flow for  tertiary  treatment  (11).   More information on
Imhoff and community  septic  tank design is available (4,12,13).
        15.2.1.5   Wastewater Stabilization Ponds


Wastewater  stabilization  ponds  are  cost-effective  because
of  their ability  to store  solids.   Pure aerobic wastewater
stabilization  ponds provide only single-phase  concentration  type
storage,  whereas  the  more  commonly used anaerobic and facultative
ponds,  can  provide for long-term,  two-phase  concentration  type
storage  of  removed settleable and created biological solids.
When debris  is thoroughly removed from their influent,  secondary
facultative  ponds can  store most of  the wastewater  solids  from a
large secondary  treatment  plant  for many years.   In  Sunnyvale,
California,  secondary treatment  facultative stabilization ponds
covering  425 acres (172 ha) have  been  receiving the majority  of
sewage solids from a 15-MGD  (657-1/s)  plant for the past ten
years  with  no  ill effects.    Sunnyvale's tertiary  treatment
facilities  for  algae  and nitrogen removal  return all solids
removed by dissolved  air flotation and gravity filtration  to the
ponds (13).   Primary  sludge is removed from the plant  before the
primary  effluent  is  discharged into the pond  and anaerobically
stabilized  in  complete-mix digesters.   Supernatant from these
digesters is discharged  daily  into the plant's  influent.  Most  of
the  solids  eventually  find their way  to the  facultative pond.
Bottom sludge is  withdrawn  every  week  or  ten  days  from the
complete-mix  digesters  and   discharged  to  anaerobic sludge
lagoons.   The primary sedimentation effluent,  and the  uncaptured
and  unrecycled  contents  of   the  supernatant  merge  with  the
anaerobic bottom  layers  in the  secondary  treatment facultative
stabilization  ponds.

Primary  wastewater (usually anaerobic  stabilization) ponds  that
receive  raw sewage must  be  drained and cleaned approximately
every  five  to ten years,  depending  on loadings.   Secondary
wastewater  (usually  facultative stabilization)  ponds  that  are
sufficiently deep  (6 to 8  feet [1.8 - 2.4 m]  )  and  that receive
only those  solids generated  by  biological  activity probably
never require  cleaning.   More  design  information  on  wastewater
stabilization  ponds is  available  (14).
    15.2.2  Storage Within Wastewater Sludge
            Treatment  Processes

Table  15-1 lists  wastewater sludge  treatment  processes  that
provide some degree of solids storage.   The following paragraphs
discuss how much of this storage  capability can  be  used  and how
its use can be  made as effective as possible.


                             15-11

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        15.2,2.1   Gravity Thickeners

Gravity thickeners separate  liquid  from  primary  and  fixed-growth
biological  secondary  solids.   In this  sense, they  function
like  primary and  secondary  sedimentation  facilities.   Cool
temperatures  and  chemicals   which  retard  septicity  enable
gravity thickeners to store sludge  for .several  days.  Equipment
precautions  recommended for  primary  sedimentation facilities
apply to gravity thickeners.   Using the  same  type of  calculation
indicated  in the  primary sedimentation design examples, storage
capacity can be  increased  by  providing  extra depth.   For more
design information  on gravity thickeners  see Chapter  5.


        15.2.2.2   Anaerobic Digesters

Anaerobic  digesters provide all  three types  of  storage.   Those
with floating covers have  the  flexibility  to store  about 20 to
25 percent of the digester's volume.   The  cover  movement is used
to provide displacement storage.    Fixed cover digesters must be
protected  from excessive vacuum  or  pressure  conditions whenever
an attempt is made  to achieve displacement  storage.

Secondary  digesters can be used  for two-phase concentration
storage by  means of liquid-solids separation  as long  as they
are  not  treating  stabilized  biological  suspended growth
(waste-activated) secondary  sludge.   Biological fixed  growth
secondary  sludge  normally does not use  secondary  digester,
two-phase  concentration  storage.   More and  more  treatment plants
are finding that the stabilization of  waste-activated sludge has
a  major  impact  on digester operation.  Without waste-activated
sludge,  the  liquid-solids  separation  process  in  secondary
digesters  can concentrate  and  store solids for considerable
periods of  time.   These  time periods  usually equal  the time
required to fill  the  secondary digester at  design flow rates and,
depending  on the quality of acceptable supernatant,  can often be
extended.

All  digesters can be  used  to  provide  equalization  storage.
Digesters  may be used to equalize peak loadings  and  thereby make
downstream  dewatering  more  cost-effective  as the  following
example illustrates.

Design Example

This example  illustrates  how  the  digester  storage  can  be used
to  "damp-out"  solids  surges  and  thus  prevent overloading of
downstream dewatering units.

Consider  a primary  wastewater  treatment plant with  the flow
scheme  and average  loads depicted  on Figure  15-1.   Average
loading  to  the  dewatering  units  is 103,313  pounds per  day
(46,904  kg/d).    Dewatering  unit  capacity  is 200,000  pounds


                              15-12

-------
                           FEED

                           270,100 = RAW INFLUENT SOLIDS, ibs/day
                   GRIT CHAMBER
                                       31,500
RECIRCULATED
SOLIDS
      259r262
                      PRIMARY
                   SEDIMENTATION
                    65% CAPTURE
                  90,742
                           168,520
SUPERNATANT

   (ZERO)
                     DIGESTERS
                     (39% SOLIDS
                    DESTRUCTION)
                  65,723
20,663
      102,797

     ««	
                                       516
                           103,313
           FILTER
        (80% CAPTURE)
  FILTRATE I     f
 	T    <
                                   I
          CENTRIFUGES
          (80% CAPTURE)
            I     f
                                     GRIT
                                 PRIMARY
                                EFFLUENT
             SOLIDS
         CONVERTED
         TO GAS AND
             WATER
                                                      POLYMER
CENTRATE   I
82.650  ^ SOLIDS TO
       ^    WASTE
Ibs/day - 0.454 kg/day
                        FIGURE 15-1
    SOLIDS BALANCE AND FLOW DIAGRAM-DESIGN EXAMPLE
SINGLE-PHASE CONCENTRATION AND DISPLACEMENT STORAGE
                           15-13

-------
per day  (90,800 kg/d);  under  average loading conditions,  the
dewatering  units are  clearly  not  stressed.  The treatment plant,
however, receives flow  from a combined sanitary/storm  sewer
network.  During storms,  hydraulic loadings  increase dramatically
as a  result of  infiltration and  inflow to  the sewer  system.
Plant solids  loadings  also  increase sharply as the  result of
solids being carried  into the sewer by run-off and the scouring
of previously  accumulated materials from the sewer system.


From historical records,  the  peak 5-day  solids loading (average
load  for^ the  most  heavily  loaded five consecutive days)  is
433,000  pounds per  day  (196,582  kg/d).  This  is 2.57  times
greater  than the average digester load.  If  the storage available
upstream of the dewatering  units  is not utilized, dewatering
unit  loading  would  also be 2.57  times the  average value  or
265,000  pounds per day  (120,310  kg/d).   The dewatering  units
would therefore  be  overloaded.   Overload  can  be prevented,
however,  if  digester  storage is  properly utilized.  Solids can be
stored within  the digester  so that,  during  peak loading periods,
dewatering  capacity  is not  exceeded.   The accumulated solids can
be released after the storm has passed and the dewatering units
are no longer  stressed.


                             the  digesters by  either  of  two
                                or in concert.
Solids  may  be  stored  in the
mechanisms,  acting either singly
    •   The  digester  working  volume  is  increased by allowing the
        floating  covers to rise (displacement  storage).


    •   The  digester  feed is thickened  to a greater degree than
        previously.   As a result,  the solids concentration of the
        digested  material increases (single-phase concentration).


The following analysis  examines how  one  of several  possible
operating strategies can be  implemented.   It is  assumed that
the system  is at average conditions (see Figure  15-1)  when a
large  storm occurs and  for  the next five days average digester
loadings increase to 433,000 pounds per day (196,582  kg/d).  At
the onset of  the storm, the  plant operator decides  to  ease a
potentially  serious dewatering overload  situation by (1) allowing
the floating  covers to  rise at the rate  of one  foot per day
(0.305  m/d)  and  (2)  by  thickening the raw sludge withdrawn from
the primary sedimentation  basin from  the normal  five percent
concentration  to  seven  percent  concentration.    The  additional
thickening  is accomplished by  allowing  sludge to  accumulate
to greater  depths in  the  primary  sedimentation  tanks  cross-
collection  trough and sludge  hoppers.   The  intent  of these two
operations  is to maintain digested solids mass flow rates  below
200,000  pounds per day  (90,800 kd/d) to prevent dewatering unit
overload.
                             15-14

-------
The  effects  of  these operations  can be  estimated  from  an
unsteady  state  analysis  of  digester  operations.   The  basic
predictive equation is  derived  by an  unsteady state mass balance:

    1.  Solids  in -  solids out  -  solids destroyed  =  solids
        accumulated.

        a.  Solids in  = QCi

        b.  Solids out  = (Q-k)C

        c.  Solids destroyed =  QCi  X

        d.  Solids accumulated  = —  -..  •
                                   dt

    Where:

        Q  = digester  feed  rate, volume  per time;

        Ci = digester  feed  solids concentration, mass per volume;

        C  = digester  sludge  solids  concentration,  mass  per
             volume;

        k  = rate of liquid accumulation in the digesters due to
             rise of floating covers, volume per time;

        X  = fraction   of digester  feed destroyed  by digestion,
             dimens ionless;

        V  = digester  liquid volume;

        t  = time.


    2.  Summing the terms:


        QCi - (Q-k)C - QCiX =


    3.  The right-hand  side of the above equation can be further
        developed:


              - V    * C    - V    + CK
    4.  Simplifying:


        QCi (1-X) - QC = V  |
                              15-15

-------
    5.   Make the simplifying assumptions that digester feed  flow,
        feed concentration,  and liquid volume are constant for the
        period t.   The above  equation  is integrated  and  solved
        for C.
C = Ci(l-X)  - [Ci(l-X)-C0]  exp  - £
                                                            15-1
        Equation  15-1  predicts  digested  sludge  solids  concen-
        tration at  any  time beyond  initiation of the  operating
        strategy.   Co is defined as digested  sludge concentration
        at the time the  operating strategy  is put  into  operation.
        Note  that  the  product  of  digested   sludge concentration
        (C)  and digester  effluent  liquid  flow (Q-k)  is  the  load
        which the  dewatering units  must process.
                            TABLE 15-2

             CALCULATIONS FOR DIGESTER EFFLUENT MASS
                   FLOW RATE FROM EQUATION 15-1
                                  Digester
                                  volume
Time after
start of
Operating strategy days
A. Floating cover rise = 0
1 ft/day; digester feed 0*
thickened to 7 percent 1
2
3
4
5
remains at 5 percent 1

4
5
C. Floating covers are not Q~
allowed to rise; di- 0
gester feed thickened 1
to 7 percent 2
3
4
5
1 Ib/day = 0.454 kg/d
1 gpd = 0.00378 m3/d
1 gal = 0.00378 m3

Dige


Ib/day percent
168
433
433
43:
433
433
433
168
433
433

433
433
168
433
433
433
433
433
433



,520 5
,000 7
,000 7
,000 7
,000 7
,000 7
,000 7
,520 5
,000 5
,000 5
,000 5
,000 5
5
,520
,000
,000
,000
,000
,000
,000




feed
gpd
396,051
72 ,875
72 ,875
72 ,S75
72 ,875
72 ,875
72 ,875
396.051
1,017,626
1,017,626
1,017,626
1,017,626
1,017,626
396,051
26,875
26,875
26,875
26,875
26,875
26,875




due to rise
covers , grid
0
,j
176 2 ].
176
176
176
176
176
176,
176
176
2 >
2 3
2 3
2 •
A
0
24 .

24 •
24 ^
>:•
i:
C=
£
I,:
0
il





Digester

5.97 x
5.97 x
6.05 X
6.23 x
6.41 x
6.58 x
6.76 X
5.97 x
5.97 x
6.05 X

6.53 x
6.76 x
5.97 x
5.97 x
5.97 x
5.97 x
5.97 x
5.97 x
5.97 x




'«?
iob
10"
10°
10o
10o
10°
wl
106
$
10
in6
10b
10*
10G
10
10
wf:
wl
iob




Fractional

0. 39
0.20
0.20
0.20
0.20
0.20
0.20
0. 39
0.20
0.20
0.20
0.20
0.20
0. 39
0.20
0.20
0.20
0.20
0.20
0.20



Digester
effluent
gpd
396,051
550,632
550,632
550,632
590,632
590,632
550,632
396,051
841, 383
841,383

841, 383
841,383
396,051
726,875
726,875
726,875
726,875
726,875
726,875



Digested
sludge solids
percent
3.05
3.05
3.38
3.58
i.78
3.96
4.11
3.05
3. as
3.20

3.49
3.56
3.05
3.05
3.34
3.6O
3.83
4.03
4.21



Dewatering
unit
Ib/day
102,797
142,919
156,429
167,772
177,373
185,561
192,594
102, 97
218, 85
228, 03

249, 40
254, 89
102, 97
188, 64
206, 45
222, 55
236, 28
249, 78
260, 88



Calculations related to the operating strategy just described  are
summarized  in Table  15-2  part  A.   The digested solids mass  flow
rates are calculated just  before the  storm  (t  = 0~),  immediately
after  the  storm  begins  (t  = 0+)  and  for  each of  the next
five consecutive days.   It is  assumed  digester loading  increases
in  one  step from 168,520 pounds  per  day  (76,508  kg/d)   to
433,000 pounds per day (196,582 kg/d) at  t  =  0.   Digested  sludge
liquid volume at the beginning  of  the storm  is 5.97 x  10^  gallons
(22,600 m3).  Each 1 foot  (0.305 m)  of cover  rise increases  tank
volume by  176,243  gallons  (667 m3) .   Solids  destruction  within
the  digesters is  assumed to be 39  percent  ( X  = 0.39)  during
                              15-16

-------
average  conditions,  dropping  to 20  percent  (  X  = 0.20)  during
the storm due  to decreased digester  retention time.  The  calcula-
tion  shows  that  dewatering capacity  (200,000  pounds  per  day
[90,800 kg/d] )  is  not  exceeded  during  the storm,  thus  the
operating  strategy  has  been successful.    Calculations  for  two
other  strategies  which  were not  successful  are  also  included.
The results  are  shown graphically on Figure 15-2.
   300,000 i-
BJ
~a
a
•3   200,000	
>
TJ
S

LLI"
Q
LU
    100,000 -
Z
ec
LJJ
H

3
LLI
Q
                 STRATEGY 8.
                 FEED SOLIDS CONCENTRATION
                 5%; ALLOW COVERS TO RISE 1 ft/day
                                             STRATEGY C.
                                             FEED SOLIDS CONCENTRATION
                                             7%; COVERS ARE STATIONARY
                     CAPACITY OF
                     DEWATERING UNITS
                                         STRATEGY A.
                                         FEED SOLIDS CONCENTRATION
                                         7%; ALLOW COVERS TO RISE 1 ft/dav
              1 ft/day = 0.305 iti/day
                    0         \         1        3

                          TIME AFTER START OF STORM, days

                              FIGURE 15-2

                EFFECT OF VARIOUS OPERATING STRATEGIES
                    ON DEWATERING UNIT FEED RATES
                                15-17

-------
        15.2.2.3  Aerobic  Digesters

To use  an aerobic  digester  for two-phase  concentration  type
storage,  the normally  highly  agitated contents must  be  made
quiescent and the  solids  made to settle from  the liquor before
the whole mass  becomes anaerobic and  starts  to decompose and
create nuisance  odors.  Chemical treatment can facilitate solids
settling.    Without  successful decanting, only single-phase
concentration type storage and  displacement  type  storage can be
used   by  aerobic digesters.   When  displacement type  storage is
used  with a  fixed  surface  area,  the  liquid  surface  must rise or
fall.   Under such  conditions,  the aeration and mixing source must
automatically adjust to such changes.  Floating mechanical units
and fixed-bottom  mounted  diffusers  are both adaptable  to these
requirements;  fixed  mechanical aerators  are  not.   Long-term
storage in aerobic digesters  will have  a relatively  low capital
cost  and a very  high  operating  (energy) cost.  Often, evaporation
can account  for significant  concentration of  the  stored solids.
As long  as  the solids  remain aerobic  throughout  the  digester,
the odor impact  of  such  storage is  very minimal.    For  more
information on  aerobic digesters, see Chapter 6.


        15.2.2.4  Composting

Composting is one of  the  two wastewater solids  processes  with
storage  capabilities  that  are effective for  long-term  storage.
Once   the  wastewater  solids have been  stabilized  by  composting,
the curing step can  be  extended  as long  as  storage  is required.
This  curing  step  usually  involves nothing  more than the placing
of the composted material  in  unconfined stockpiles exposed to the
atmosphere.   As long as  there are no  site restrictions,  this
method of storage  can be very  economical, for it is actually just
another use  of  time  needed for curing  and  removing  the  material
to its  point of final disposal.   For more  design information on
composting,  see  Chapter  12.


        15.2.2.5  Drying Beds

Drying  beds are  used  extensively  by  many smaller plants in
conjunction with  anaerobic and  aerobic digestion.   They are
operated on  a fill and  draw  basis and  are  often used to provide
two-phase  concentration  and  displacement  type  storage  between
production and disposal.   To assure adequate storage capability,
the designer should allow  for up to 50 percent excess drying bed
area.   More design information on sludge drying beds can be  found
in Chapter 9.


15.3   Dedicated  Storage  Facilities

When  solids  storage  within  wastewater treatment processes and
sludge solids treatment processes cannot provide the operational
flexibility necessary to maintain cost-effective solids treatment


                              15-18

-------
and disposal,  these within-process storage  capabilities must
be augmented with  special  dedicated storage facilities.   These
dedicated storage  facilities  can  provide  storage for sludge in
either  the  liquid  or  dewatered  state,  and may,  depending on
design considerations and upstream treatment, be utilized  for any
of the three detention  times listed in Table 15-1.
    15.3.1   Facilities Provided Primarily for
            Storage of Liquid Sludge

Usually, dedicated liquid  storage  facilities consist  of one
of the  three  types listed  in  Table 15-1.  Although listed as
primarily for  storage  of liquid sludge, any of  these  facilities
that are used  for  anything  other  than equalizing storage  (3 to
4 days)  will  also  provide  some  degree  of solids  treatment.
Holding  tanks,  without air  agitation, and facultative  sludge
lagoons  usually  continue  anaerobic  stabilization.   Holding
tanks,  with air agitation,  and aeration basins  continue  aerobic
stabilization.   As  these  are side  benefits  to the main design
functions of  these  facilities,  they have  been  ignored  for the
purpose of  these classifications.   However, if  the  storage is for
a long  term,  then the  additional  treatment afforded certainly
must be taken  into account in setting final disposal  criteria.


        15.3.1.1  Holding Tanks

Holding tanks  are commonly provided as an  integral part of most
conditioning processes and many stabilization processes.   Holding
tanks may be used  for blending different materials  as  well as for
equalizing  storage,  thereby  assuring that  the  downstream sludge
treatment  process  is  uniformly  loaded,  both in quality and
quantity.   Holding  tanks  also  often provide  the  decanting
facilities  for sludge  treatment  processes,  such  as  thermal
conditioning,  which  create  products  that support  two-phase
concentration.

Holding tanks  that are to be  used for blending  must be maintained
in a  homogeneous  condition.   Such  tanks  can  thus provide only
single-phase  concentration  type  storage  or displacement type
storage. Usually  such tanks  are relatively small,  with detention
times measured in hours  instead of  days.   Most of the storage,
therefore,  is  provided by volume adjustments.  Holding tanks that
involve  blending and provide equalizing storage  are  usually
limited  to a  batch,  or  a  near-batch,   type  of operation or
continuous  level adjustments.  Tank contents  can  be mixed by
mechanical  impellers,  hydraulic  recirculation,  or  gas  agitation.
Each method's applicability may  be restricted by  the  type of
material  requiring  the  blending.    For example,  mechanical
impellers  are not  applicable  when unground   sludge  is  being
stored.  The  use of gas  agitation  and recirculation mixing is
normally only  limited by the  volume which must be blended.


                             15-19

-------
If the holding tank is located  downstream from a  sludge treatment
process, special  precautions may  be required.    For example, if
downstream from anaerobic  digestion  and  planned  for more than a
few hours of storage,  the blending  tank should be designed with a
cover and be  equipped  to  collect and remove combustible digester
gas.    If downstream  from chlorine stabilization,   it  should be
designed  to  function  in a  very  low  pH  (acid)   environment.
Whatever its function, however,  the  holding  tank  must be designed
to eliminate  the  production  of malodorous  gaseous discharges.
This   elimination  is  made  especially difficult when the holding
tank  must provide  equalizing  storage  and  operate on  a  batch
basis.   Unless  the  solids  supplied  to the  holding  tank are
completely  stabilized  (a condition  seldom  encountered  with
wastewater  sludge),  the  tank's use  for  extended periods of
storage will result in the creation  of nuisance odors.

Even  short  periods  of   storage  of  unstabilized  primary and
secondary sludges  in a holding  tank  can produce nuisance odors if
no form of temporary inhibiting  treatment  has been  applied.
Decant  tanks  following thermal conditioning often  create   major
odor   problems.  There  are many  ways of  dealing  with the odorous
gases created by these holding  tanks—for example, by passing the
gases back through the aeration  system, activated carbon filters,
chemical scrubbers, and  incinerators. The best  design solution,
however, is to minimize their creation.

Design Examples

The  Sacramento  California Regional Wastewater  Treatment Plant,
now  under construction,  is to  be  provided with  a holding and/or
blending  tank  that  will be  capable  of receiving the  daily
anaerobically digested sludge  discharged from  nine complete-mix
digesters  (15).  This  digested  sludge  discharge  will  vary   from
0.56  to 0.94 MGD  per day  (24.5   to 41.2  1/s )  over  the  next
20 years.   The  blending tank  will be 110 feet  (33.5 ra) in
diameter and  will have a  38.5-foot  (11.7 m)  sidewater depth.  It
will   be provided with  a Downes  type  floating cover that will  have
a vertical movement of at least  14  feet  (4.3  m) .   This floating
cover  movement  will  allow the  blending  tank to  mix the entire
daily discharge from  all  the nine digesters prior to discharging
its  daily accumulation to  downstream facultative sludge lagoons.
This   blending tank will provide  a  complete separation between the
operational control of  the complete-mix  tank  and the controlled
feeding of  the 20 lagoons.  Total  solids  retention time of the
blended sludges will  be  at least  three  days,  and  approximately
one-third of  the  liquid  contents  of  the blending  tank  will be
displaced each day.   Except  for the provision for  the  extra
floating  cover  travel  and  the  use  of  bottorn  mounted gas
diffusers,  the  blending  tank will  have  the same  design as the
four   complete-mix  tanks  now  under  construction.   This method of
blending  and  containment  will  minimize  the release  of odorous
gases  and  maintain a  safe control  on the production and use of
the  digester gas  during the blending  operation.   Figure  15-3
shows a sectional  sketch  of this proposed blending  digester.  In


                             15-20

-------
Aliso,  California,  two  26,000-gallon  blending  tanks  are being
proposed  to  blend and  equalize  unstabilized  sludge  flows  from
several  sources  at  the  Aliso  Regional  Solids  Stabilization
Facility  (16).   These  tanks  are being  provided  with hydraulic
mixing  and  fixed  covers.  The gas  cap above  the  varying  liquid
level  within the fixed covered tanks will be  maintained at  a
constant  pressure by  an intertie  with the  low pressure digester
gas  system.    This intertie  will  eliminate the  need  for special
odor  control  equipment, minimize  the  danger  from the possible
production  of an  explosive gas-air  mixture, and negate the  need
for  some highly  complicated  pressure  control  system  to protect
against  a rapid  drawdown that might  pull  a  vacuum or air into the
blending  tank.  Figure  15-4 shows a sectional elevation of  this
raw sludge blending  tank.
WATEB SURFACE IW.SVl
WO"
                     GAS DOME
                TYPE
            FLO AT INC COVER
                           6" PMSSU PS-VACUUM
                           FtEUif ANO FLAME
                           THAT
                                        14™ AND W" 0(A
                                        GAS COLLECTION ANP
                                        SLUDGE SUWLY
r                                        PIPELINES
                                        SLOPf TO GAS DOME
                                                      HOSES FQfi
                                                    COVER TflAVEL
                                                 ,_J1	,„„.
                   '7—^
j|	l_igjgT !'°" ABOVE ipTTQM QF GA$ DOME
                        STABIOTY_CONCR:ETE^ BALLA5TJjlMG_

                           __^ 1W DIA 	
                                     •COVER SUPPORT CORBEL |TYP OF "t*\
                         \
                                                             PROVIDE «S*
                                                                SPIRAL
                                                                GUIDES
1 ft = 0,305 m
1 in - 2.54 cm
                           8" OIA Cl HC SLUDGE
                          - SUCTION AND
                         f SUPPLY PIPING
                           (TYPICAL OF 4J
                         L—
                      GAS DIFFUSES
                        ASSEMBLY
                        (TYP Of
                                                           DlflESIER
                                                            PVMP STATION
                             FIGURE 15-3

         PROPOSED DESIGN FOR BLENDING DIGESTER—SACRAMENTO
               REGIONAL WASTEWATER TREATMENT PLANT
General  Comm^enj:s_

While very  little  specific  design  guidance  is  provided  in
the  literature  for  sludge  holding  tanks,  the major  issue  that
must  be  dealt  with is  the same  as  for most  sludge  treatment
processes—material  management.    Wastewater  sludge  can contain
almost  anything.   If  a holding tank design  is to  incorporate
mechanical   mixing,  which  can  be  incapacitated  by  stringy
                               15-21

-------
material,   the  designer  must make  sure  that  material  is  either
removed  or  cut  up  before  reaching  the  blending  tank.   Likewise,
hydraulic  mixing  pumps must  be  of the  non-clog  type  or  the
material  reduced  in particle size by  grinding  so that it can pass
through  the minimum clearances  of the type of  pump  used.
               ULTRASONIC LEVEL
               TRANSMITTER
6" DIA DIGESTED
SLUDGE
         24" OfA M,H.
                                                              6" PRESSURE VACUUM
                                                           RELIEF AND FLAME TRAP
                                                                  6" FLAME TRAP
                                                     6" DIA
                                               LOW PRESSURE
                                                 SLUDGE GAS
                                                 CONNECTION
                                                TO DIGESTERS
                             MAX W.L. E LEV 214.3
                                          42  DIA
                                          ACCESS M.H.
                                 ELEV 209,5

                         x GROUND E- =v 207,5
      EQUIPMENT PIT

     EQUALIZING
     CIRCULATING
     PUMP
   6" DIA CIRC
   PUMP
   DISCHARGE
                                       MIN W.L, ELEV 196-0
                      DISCHARGE NOZZLE
                      TO ASSURE MIXING
                       t ELEV 195.5
                       6" DIA
                       RAW
                       SLUDGE
                       SUPPLY


                                  DIG ESTER SUPPLY
                                   PUMP (T'YP OF 5
                                  FOR TWO TANKS}
               S6" DIACiRC PUMP
                SUCTION
sftSiPatfSas?
 1
  iNV ELEV 191.5
i" DIA DIGESTER
SUPPLY SUCTION
          1 ft = 0.305 m
          1 in •» 2.56 ui*
                                               6" DIA SUCTION
                                                   TO OTHER
                                                   *•
                                                   DIGESTER
                                                     SUPPLY
                                                     PUWIPS
                                                  AND DRAIN
                                                      SUMP
                                 TABLE 15-4

        26,000 GALLON SLUDGE EQUALIZATION TANK (TYPICAL OF TWO)
                    ALISO SOLIDS STABILIZATION FACILITY
                                    15-22

-------
The other major design problem  involves the control of odors that
are so often an integral  part of  any type of sludge holding tank.
The Sacramento  and  Aliso holding  tank design  examples  indicate
two very  successful  means  of . dealing with such  odors  (that  is,
containing  and  incorporating  them with  the low  pressure
digester gas system).   In  many locations stabilized material  is
held  within the  holding tank only  a few hours.   Under  these
circumstances,  their  design  depends on minimum odor generation,  a
reasonable assumption  given  the  short retention period.   Often
decant  tanks  and conditioning  blending  tanks  cannot depend  on
either of these methods of odor control.   The designer  should  be
very aware that when such a situation exists  it will be  expected
that odors will be confined and treated to the point where their
discharge ceases  to  create  a  nuisance.   Odor  control is  a very
complicated  subject.   Designers  are referred  to a Manual  of
Practice soon to be  released by a Joint Committee of the ASCE  and
Water Pollution Control Federation.


        15.3.1.2  Facultative Sludge Lagoons

Introduction

Sludge  lagoons  have  been used  for  years to  store wastewater
solids.   Unfortunately, most of  this  use has  been  with  complete
disregard to the aesthetic  impact on the  surrounding environment.
Such  misuse has become  so widespread  that  just  the use  of
the term "sludge lagoon" is  often  enough  to eliminate  their
consideration in present-day alternatives analyses.

Recent studies  in Sacramento, California,  based on the successful
operation of facultative  sludge lagoons in Auckland, New  Zealand,
indicate that  sludge  lagoons can be  designed  to be environmen-
tally  acceptable  and  still  remain  extremely  cost-effective
(17,18).  The facility studied  in Sacramento provides storage  for
at  least  five  years  of sludge  production. The  sludge stored  in
the facultative sludge  lagoon continues  to  stabilize without
creating an odor  level unacceptable to the surrounding  neighbor-
hood.    Table 15-3 lists  the advantages and limitations  of using
facultative sludge lagoons  for  long-term  storage.
Facultative sludge  lagoons  (FSLs)  are designed  to  maintain
an  aerobic surface  layer free  of scum  or  membrane-type  film
build-up.  The aerobic layer is maintained by keeping the annual
organic loading to the lagoon at  or  below  a critical area loading
rate and by using surface  mixers  to  provide agitation and  mixing
of  the  aerobic surface  layer.  The  aerobic  surface  layer of the
FSLs is usually from one to three feet (0.30 to 0.91 m) in depth
and  supports a very  dense population of between 50  x  10-^ and
6 x  10" organisms/ml  of  algae  (usually  Chorella ) .   Dissolved
oxygen  is supplied  to  this  layer  by algal  photosynthesis,  by
direct  surface transfer from the atmosphere,  and  by the surface
mixers.   The  oxygen  is  used  by  the bacteria  in the  aerobic


                             15-23

-------
degradation  of   colloidal  and  soluble  organic  matter  in  the
digested  sludge  liquor, while  the  digested sludge  solids  settle
    the  bottom  of    the  basins  and  continue   their  anaerobic
                    Sludge  liquor  or supernatant is  periodically
to
decomposition.
returned to  the plant's  liquid  process  stream.

                                 TABLE 15-3

        ADVANTAGES AND LIMITATIONS OF USING FACULTATIVE SLUDGE
                     LAGOONS FOR LONG-TERM STORAGE
              Advantages
                                                   Limitations
Provides  long-term storage with
  acceptable environmental impacts
  (odor and groundwater contamination
  risks are minimized).

Continues anaerobic stabilization, with up
  to 45 percent VS reduction in first year.

Decanting ability assures minimum solids
  recycle with supernatant (usually less
  than 500 mg/1) and maximum concentration
  for storage and efficient harvesting
  (>6 percent solids)  starting with digested
  sludge  of <2 percent solids.

Long-term liquid storage is one of few
  natural (no external energy input) means
  of reducing pathogen content of sludges.

Energy and operational effort requirements
  are very minimum.

Once established, buffering capacity is
  almost  impossible to upset.

Allows for all tributary digesters to
  operate as primary complete-mix units
  (one blending unit may be required for
  large installations).

Provides  environmentally acceptable place
  for disposal of digester contents during
  periodic cleaning operations.

Sludge harvesting is completely independent
  from sludge production.
                                         Can only be used  following anaerobic
                                           stabilization.  If acid phase of
                                           digestion takes place in lagoons they
                                           will stink.

                                         Large acreages require special odor
                                           mitigation measures.

                                         Requires large areas of land, for
                                           example, 15 to  20 gross acres (6 to
                                           8 ha) for 10 MGD, (438 1/s) 200
                                           gross acres (80 ha) for 136 MGD
                                           (6,000 1) carbonaceous activated
                                           sludge plants.

                                         Must be protected from flooding.

                                         Supernatant will contain 300-600 mg/1
                                           of TKN, mostly  ammonia.

                                         Magnesium ammonia phosphate (struvite)
                                           deposition requires special supernat-
                                           ant design.
The  nutrient and carbon dioxide  released  in both the aerobic and
anaerobic  degradation of the remaining  organic  matter within
the  digested  sludge  are,  in turn,  used by  the  algae in the
cyclic-symbiotic  relationship.    This  vigorous  relationship
maintains  the  pH  of  the   FSL   surface  layer  between  7.5  and 8.5,
which   effectively  minimizes  any  hydrogen  sulfide   (r^S)  release
and  is  believed  to be one  of the  major  keys  to  the  successful
operation  of this sludge  storage  process.

Facultative sludge  lagoons   must  operate  in  conjunction  with
anaerobic  digesters.  They  cannot  function  properly  (without
major   environmental  impacts)  when  supplied  with  either
                                   15-24

-------
unstabilized or  aerobically  digested  sludge.   If the  acid  phase
of anaerobic stabilization becomes predominant,  the  lagoons  will
stink.   Figure  15-5 provides  a  schematic representation of  the
reactions in a typical  FSL.
in

§1
ui
LU
H
     in
     D

     1
     o
II
UJ Q
^ a.
<
       V.
                            FIGURE 15-5

            SCHEMATIC REPRESENTATION OF A FACULTATIVE
                       SLUDGE LAGOON (FSL)
Current Status

Facultative sludge  lagoons  were installed initially  in 1960
in  the  Auckland,  New  Zealand, Manukau sewage  treatment plant
to  provide for  the  storage  and  disposal of  that  plant's
anaerobically  digested  primary  sludge.    Although  lagoons were
installed  at  Dublin-San Ramon,  California in  1965,  Medford,
Oregon  in  1971,  and  other sites in the United States since 1960
in  an attempt  to  duplicate  the  successful  Auckland  installation,
it  was  not until  1974  that the area  loading  became the  critical
criterion  for  their  success.   Studies at  Sacramento since  1974,
with approximately  40  acres  (16.2 ha)  of  FSLs,   have determined
that the standard annual  loading rate can be doubled during  the
                              15-25

-------
warm, long, sunny days of July,  August,  and  September.   Reduced
algae activity  during  the colder winter months indicates  that  the
standard loading  rate  should not be exceeded.

Since  1974,  additional  FSLs  have been  placed in  service at
Corvallis,  Oregon -  4.5 acres  (1.82 ha) and Salinas,  California -
6.0  acres  (2.43  ha).   Other  FSLs  are  being  built  or are under
design  for Eugene-Springfield,  Oregon - 25 acres,  (10.1 ha);
Red  Bluff, California -  0.93  acres  (0.38  ha);  Sacramento,
California -  84  acres  (34  ha);  Flagstaff,  Arizona 7.3 acres
(2.95 ha);  and Colorado Springs,  Colorado - 60  acres  (24.3 ha).
Successful operation was  experienced  this  past winter under
freezing conditions at  Corvallis, Oregon.   Experience  to date
indicates  the  design  criteria established  at Sacramento  are
applicable  under  other climatic conditions.

Design Criteria

Design  considerations for  the FSLs  include the area  loading
rate,  surface  agitation requirements, dimensional  and layout
limitations,  and physical  factors.   All have been developed
during  the studies conducted over the past five years at  the
Sacramento  lagoons.

Area  Loading   Rate.   To  maintain  an aerobic  top   layer,  the
annual  organic  loading  rate to  that  FSL must be  at or below
20 pounds  of volatile solids  (VS) per 1,000 square feet per day
(1.0  t  VS/ha-d).   Lagoons  have   been  found to be  capable of
receiving the  equivalent  of  the daily  organic loading  rate every
second,  third, or fourth day without experiencing  any upset.
That  is, lagoons  have assimilated up to  four times normal daily
loadings as  long as they have had three days  of rest  between
loadings.  Loadings  as high as 40 pounds VS per  1,000  square feet
per  day  (1.0  t VS/ha-d)  have been successfully assimilated  for
several months  during the warm summer and fall.  Experiments on
small basins loaded to failure indicate  that  peak loadings up to
90 pounds VS per  1,000 square  feet per day (4.4  t VS/ha-d)  can be
tolerated during  the summer and fall as long as  they  do not occur
for more than  one week.

Surface Agitation Requirements.    Experiments  on FSLs that were
continuously  loaded at  the  standard  rate  indicate  FSLs  cannot
function in an environmentally  acceptable manner  without daily
operation of surface agitation equipment.   Observations  indicate
the brush-type  mixer is required to breakup the  surface film that
forms  during  the feeding of  the lagoon.  If  this  film is  not
dissipated,  a  major source  of  oxygen transfer to  the  surface
layer  is eliminated.   FSLs  with surface  areas  of  from 4 to
7 acres  (1.6  to 2.8 ha) require the operation of  two  surface
mixers from 6  to 12 hours per day to successfully maintain scum-
free  surface  conditions.   All  of the  successful installations
to date  have used  brush-type  floating surface mixers  to  achieve
the  necessary  surface  agitation.   Figure  15-6  shows a  typical
brush-type surface mixer.    Recent  experiments indicate that


                              15-26

-------
two brush-type mixers with  8-foot-long  (2.4-rn)  rotors  turning  at
approximately 70 rpm and driven by 15 horsepower (11.2  kW)  motors
are required  for  a  4  to 7 acre  (1.6  to 2.8  ha)  lagoon.  The
mixers  need  to operate  12  hours per day.  Lagoons  of much  less
             (1.62 ha) should be able to achieve the same  results
                with  6-foot  (1.8-m)  long  rotors and 5-horsepower
                  Operation time is expected  to be about  the  same
                per day.   FSLs  of  larger than 7 acres  (2.8 ha)
                     to  be cost-effective because of the  need  to
                     of  service during sludge removal operations.
than 4 acres
with two mixers
(3.7 kW) motors.
number of  hours
have not been  found
take the lagoons out
                            FIGURE 15-6

                 TYPICAL BRUSH-TYPE SURFACE MIXER,
                      SACRAMENTO, CALIFORNIA
Brush  type  mixers have been  used  to limit the agitation  to  the
surface layer  of  the FSLs.   So  far this has been  an  acceptable
application;  however,  there  is  some  question  as  to  their
applicability  for very cold  climates.   Several  submerged pump-
type  floating aerators have been reviewed, and they could  be
                              15-27

-------
adapted  to provide the  necessary  surface  agitation  if  the
brush-type could not function  under  severe  freezing  conditions.
Two mixers are  used per FSL to assure maximum scum  break-up  in
those areas of the lagoon where the prevailing  wind  deposits  the
daily loading of  scum.   The agitation and mixing action of  the
two mixers located  at opposite ends or sides of  the  lagoon  also
act to maintain  equal distribution of the anaerobic solids.

Dimensional  and  Layout  Limitations.    FSL  size  is  usually
determined by the number of  lagoons  required to  assure  adequate
surface  area,  while sludge  is removed from a  lagoon.   If  the
removed  sludge  is  to  be  reused,  several  spare  lagoons  are
required to keep full lagoons out of service for the 2- to 3-year
pathogen  die-off period  (3).   The  maximum area  for  a single
lagoon  area  is  somewhat arbitrary  but  is  based  on  the  most
practical  size  for loading,  surface  agitation,  mixing,  and
removal requirements.  Large, 4 to 7 acre (1.6-2.8 ha) individual
lagoons would be  applicable  only  to  plants  with over 70  acres
(28 ha) of FSLs.   FSLs  as  small as 150 feet (45.7 m) on a  side
have been operated successfully.

Lagoon  depth was established by  the practical limitation  of
commercially  available dredges  with a  proven  capability  of
removing  wastewater  solids  from  beneath  liquid  surfaces.
Equipment that  meets  this  requirement  is  available to  extract
sludge  from  FSLs up to 11-1/2 and 15 feet  (3.5 and 4.7 m)  of
depth.   For  plants  <10  MGD  (440  1/s) ,  the  11-1/2 foot  (3.5  m)
depth dredge should  be  adequate.   For plants >10 MGD (440  1/s)
the 15-foot  (4.7-m)  depth  should  be  used to provide additional
storage  flexibility.    If surface  agitation  must be  maintained
by  submerged pump  type  aerators,  it  may be  necessary to employ
the deepest lagoon possible to  assure adequate separation between
the aerobic  zone and the anaerobic settling  zone of the FSL.
Contractors can  supply dredge equipment for a lagoon,  either with
or without the manpower  to operate it.

FSLs  are usually   best designed  to  have  a  long  and  a  short
dimension with  the  shortest dimension oriented  parallel to  the
direction of  the  maximum prevailing  wind.   The longer side  is
made  conducive  to  efficient dredge  operation,  while the  short
side's  parallel orientation to the prevailing  wind direction
helps  to minimize  wave  erosion  on  the  surrounding  levees.
Figure  15-7  is  a  typical FSL  layout, while  Figure 15-8  is  a
typical FSL cross section.

When  the  area of  FSLs  exceeds  40  acres (16.2  ha),  the potential
cumulative effect of  large  odor emission areas  to  the  vicinity
must  be  considered.    Figure 15-9  shows  the  layout  for  the
124 acres  (50.2 ha) of Sacramento FSLs  that were sited on  the
basis of the  least odor  risk  to surrounding areas.

Work  at Sacramento  has  also  determined  that batteries  of  FSLs
totalling 50  to 60  acres (20  to 24 ha)  are about  the maximum size
for most effectively reducing the  transport of odors.


                             15-28

-------
                                             PREVAILING WIND DIRECTION

! '' 1
\ I
/OVERFLOW r
I * 	 AUTOMAT 1C /
/ / PHNTRDI VAl UP -/
/ - J IT
j- -y |
t
- SLUDGE REMOVAL 
-------

LAYOUT FOR 124 ACRES OF FSLs—SACRAMENTO
 REGIONAL WASTEWATER TREATMENT PLANT
                15-30

-------
located  upstream from  the prevailing winds  to minimize  scum
build-up  in its  vicinity.  FSL  supernatant  will precipitate
magnesium  ammonia phosphate  (struvite)  on any rough surface
that  is  not completely  submerged.  It has  also been found  to
precipitate inside cavitating  pumps.   This  crystalline  material
can completely clog cast  iron  fittings and pump  valves  when the
surface goes through a fill-and-draw cycle or when its operation
results  in the presence  of  diffused air.   The  most practical
approach to successful  elimination  of  this  problem has been  to
use PVC  piping throughout the FSL supernatant  process and  to
design the  process for gravity  return to the plant influent,  with
a minimum of critical depth conditions.   If  pumping is required,
submerged slow-speed non-clog centrifugal  pumps with low suction
and discharge  velocities   (to  minimize cavitation)  will be  the
most  trouble-free.   All equipment  that  cannot be PVC  or  other
smooth  non-metallic material  should be  coated  with a smooth,
impervious  surface.

Two digested  sludge  feed  lines  are  provided,   each with  its
own automatic  valve,  to assure adequate  distribution of  solids
over the whole volume of the FSL.   Surface mixers are downstream
of the  prevailing winds.   The  harvested  sludge dredge hookup  is
centrally  located.   Lagoon dike  slopes  are  conservative--three
horizontal  to one  vertical—with adequate rip-rap provided  in the
working  zone  of  the  surface  level.   Sufficient freeboard  is
provided to  protect  against any  conceivable overtopping of  the
dikes.   Digested sludge  feed  pipelines are  located directly
below  the  bottom  of  the  lagoons,  with  the  inlet  surrounded  by  a
protective concrete surface.   All piping within the basin  is
out of the  way of  future dredging  operations.

Many  of the  physical  considerations  for  the basins  have
been  required by the  State Dam  Safety  Agency.   Larger  FSLs
most  probably will come  under  some  regulatory agency  whose
responsibility involves  seeing  that earthen  structures used  to
confine  large  quantities   of liquid  a  significant  height  above
the existing ground  surface are safe.   It is  wise to check early
to ascertain what, where,  and how  these agencies will be involved
in FSL design.

Operational Cqnisidera,t_ions_

Operational considerations can  be divided into three categories:
the loading or placement  of  sludge into  the  FSLs; their routine
operation; and the  removal of  their solids.  Considerations
listed below were  developed during the five years of study on the
Sacramento lagoons.

§^££^.^H£_aJl^_t2a.^AD.S•    FSLs  should  be  initially filled  with
eTFriTervtfi  I d~eaTry, "that effluent  should then have about three to
six weeks  for development of  an  aerobic  surface  layer  prior  to
the  introduction of  digested  sludge.   All FSLs  should  be
loaded daily,  with the  loading distributed equally between FSLs.
Loadings should be held below 20 pounds VS per 1,000 square feet


                             15-31

-------
per day (1.0 t VS/ha-d)  on an average  annual  basis.  As  indicated
earlier,  considerable  flexibility  does  exist.    Loads  can vary
from day to day, and  batch or intermittent loading of once every
four days  or  less  is acceptable.  Shock  loadings,  such as with
digester cleanings, should be distributed to all operating FSLs
in proportion to  the  quantity of sludge  inventory they possess.
FSLs should  be loaded  during periods  of favorable atmospheric
conditions, particularly  just above  ground surface,  to maximize
odor dispersion.   The fixed  and volatile sludge  solids loadings
to  the FSLs  and  their  volatile contents  should be  monitored
quarterly.


Daily  Routine.    Surface  mixers should operate for  a  period
of  between  6  and  12  hours.   Operation should not coincide with
FSL  loading and should always  be during the hours of minimum
human exposure (usually midnight to 5  a.m.) and during periods of
favorable atmospheric conditions.  FSL  supernatant return  to the
wastewater treatment process should be regulated  to  minimize
shock  loadings of  high  ammonia.   Supernatant  return flows  should
be  monitored  so   that   their  potential impact  on the  liquid
treatment  process  can  be discerned.   The sludge blanket in a
lagoon should not  be  allowed to rise  higher  than two feet below
the operating water surface.


SJ1u_d_g_e__R_emo_v_a_!.   FSLs  that  are to  be emptied  of  accumulated
solids should be removed  from routine  operation at least 30 days
prior to the removal of any solids.  Pathogen safe reuse requires
removal  from operations  for two  to  three  years  (3).   Sludge
removal should  be  limited to those FSLs that are concentrating
the  sludge solids  to six to eight percent. During FSL  sludge
removal operations, the water surface  level should not be allowed
to drop more  than  12  to  18 inches (30  to 46  cm) below its  normal
operating level.

Energy Impacts


Energy requirements of FSLs are  relatively small because FSLs use
solar  energy.   The sun supplies the  needed energy for the algal
photosynthesis.  In turn,  the algal  cells supply the   dissolved
oxygen  to  support  the  aerobic   bacterial action  in  the surface
layer.  The only outside power used  in  normal  FSL operation  is for
surface  agitation,  supernatant   pumping and   treatment,  and  the
supply and  removal of  the sludge.    For the 124-acre  (50.2  ha)
Sacramento  installation,  it  was recently calculated  that these
energy requirements could  equal  31,700'  x 10^ Btu  per year
(33,400 GJ/yr)  when the FSLs became  fully  loaded in 1990 (19).
As  loading  is based on area, the energy impact  of  FSLs will be
255  x  10^  Btu/yr/acre  (670  GJ/yr/ha).  With  maximum odor  source
control and transport reduction   measures,  this  energy use will
increase  to  294  x 10^  Btu  per year  per acre (765 GJ/yr/ha).
As no  chemicals or  major structures are involved, all FSL  energy
impacts are direct.  There are no secondary impacts.


                              15-32

-------
Actual Performance Data

The following figures and  tables  report the actual performance of
the eight  FSLs  in operation at the  Sacramento Central Wastewater
Treatment Plant.   Although  the plant is designed as a  24-MGD
(I.l-m3/s)  carbonaceous  activated  sludge  secondary  wastewater
treatment  plant  -with   anaerobic  digestion  for  solids
stabilization,  it treats  the  total  solids  from  three  upstream
secondary  treatment  plants,  the  total annual  flow of which  is
considerably  greater than  its  own.    Solids from  those  upstream
plants  are transported to the Central plant  by its tributary
sewer  collection system.   The  Central  plant  also receives a
substantial  solids  loading (up  to  35  percent  daily  surcharge)
from seasonal  canning operations.   Table  15-4  indicates  the FSL
loadings for the  four years from  1975  through 1978.
                            TABLE 15-4

            SACRAMENTO CENTRAL WASTEWATER TREATMENT
                PLANT VOLATILE REDUCTIONS, DIGESTED
              SLUDGE QUANTITIES  AND FSL AREA LOADINGS
 Year
Digester volatile
reduction, percent
     Digested solids to FSLs          FSL loading

Annual average
.total solids,   Percent   Percent
 103 lb/daya    volatile    solids
  Annual average
Ib volatile solids,
  103 sq ft/daya
1975
1976
1977
1978
52
50
51
45
44 .1
35.9
44.0
52.7
63
67
68
66
1.7
1.6
1.6
1.6
22.5
15.9
17.1
20.7
 Dry weight.

Source:  Treatment plant records.

Ib = 0.4536 kg.
sq ft = 0.0929 sq m.
Figure  15-10 summarizes  typical  surface  layer  data for  four  of
the  FSLs  for July  1977 through  June 1978.   Unfortunately,  some
turbidity and  algae count  data are missing,  but the seasonal
trend  is  quite  apparent.  Table  15-5  summarizes the FSL's design
data and  provides the necessary  background  to understand  the FSL
solids  inventory  in  Table  15-6.   Data from Table 15-6 was  used to
calculate a  volatile  solids reduction of 42  percent.   Solids
profiles  are  taken quarterly  in  all FSLs.

Recycled  FSL  supernatant quality  for 1978  is given in Table 15-7,
and  complete mineral, 'heavy  metals,  and  chlorinated hydrocarbon
data for  digested, FSL,  and harvested solids for 1977 is provided
in Table  15-8.  While the  specific  conductance in the supernatant
remains high  (2,500  to 4,300  mhos/cm),  the supernatant contains
very little of  the heavy metals.  Rainfall increases the quantity
                               15-33

-------
of supernatant and decreases its  strength.   Winter-specific
conductivity  always dropped in Sacramento following significant
rainfall.   The  only solution to this problem would seem to be to
reduce  the heavy  metals  concentrations  in  the unstabilized
sludge.
                                         ^f:
                 r
                                 j A 3 0 N 0 J  * tt * M ;
                          FIGURE 15-10

           SACRAMENTO CENTRAL WASTEWATER TREATMENT
              PLANT SURFACE LAYER MONITORING DATA
                         FOR FSLs 5 TO 8
                             15-34

-------
Public  Health  and Environmental Impact

FSLs   have  been  found  to  have   the  following   insignificant
environmental   impagts  at  Sacramento  during  five  years  of  study:

      •   No vector impacts

      •   No groundwater  impacts

      •   Controlled pathogen impacts

      •   Acceptable odor  impacts

                                    TABLE  15-5

               SACRAMENTO CENTRAL WASTEWATER TREATMENT
                             PLANT FSL DESIGN DATA

                           Depth from
              Date placed   water surface
         FSL  in operation  to  bottom, ft
                 7/73


                 8/73


                 9/74


                 11/74


                 8/76


                 8/76


                 11/75


                 11/75
Area at water
bh from
surface
ttom, ft
11

11

14

14

15

15

15

15

'otal

surface
1,000 ft2
(acres)
164.
(3.
164.
(3.
244.
(5.
229.
(5.
204 .
(4.
204.
(4.
270.
(6.
270.
(6.
1,749.
(40.

0
8)
0
8)
2
6)
0
3}
2
7)
2
7)
0
2)
0
2)
6
1)
Volume below
sludge blanket,
1,000
1,

1,

2,

1,

1,

1,

2,

2,

15,

cu
030.

030.

137.

983.

851.

850.

689.

689.

259.

ft
4

4

0

0

0

0

0

0

8

Loading capacity
of basin,3
1,000 Ib
3.

3.

4 .

4.

4.

4.

5.

5.

31.

VS/day
28

28

88

58

08

08

40

40

80

        aCapacity of lagoon based on a design volatile solid (VS) loading
         of  20 lb/1,000 ft2 of water surface area per day.

        1 ft = .3048 m .
        1 ft2 = .0929 m .
        1 Ib = 0.4536 kg.
        1 cu ft = 28.32 1.


                                    TABLE 15-6


               SACRAMENTO CENTRAL WASTEWATER TREATMENT

                   PLANT FSL SLUDGE INVENTORY,  DRY TONS
   Parameter


 Digested sludge
                VS  TS
                               VS   TS
           3,925  2,6^0  4,580  2,995  5,398  3,416 3,801 3,5^ 2,222 1.4C.1 2,211 1,454 3,4Sf> 2,31? 3,275 2,177 30,898  20,106
 Stored sludge   1,973   B&O  1,009  l,62l)  2,y50  1,721  3,845 I ,W2 1 ,45'J  Hl<> 1, 173  719 3,792 2,214 3,208 1,076 21,399  11,727
 Quantities account for sludyc that has been (1) added to the SSB.s, (2) applied to land
 (1,256 dry tons in 1974, l,f>B8 in 1975, 976 in 197C, and 1,930 in 1977) and (3) transferred
 between basins since beginning of operations.
 b
 Quantities calculated based on data obtained from sludge samples collected July 12, 1978.
                                       15-35

-------
                            TABLE 15-7

            SACRAMENTO CENTRAL WASTEWATER TREATMENT
             PLANT RECYCLED FSL SUPERNATANT QUALITY

            10/5/78   10/6/78   10/7/78  10/11/78  10/30/78  12/20/78  Average
BOD
TP04
Sulf ides
COD
TKN
pH
SS
NH3-N
140
51
0
-
-
-
-
-
140
50
0
-
-
-
-
-
140
66
0
-
-
-
-
-
96
' 120
0
910
220
7.7
470
-
200
.'• ! no
0
960
360
7.7
420
300
                                                    110      143
                                                     80,       79
                                                     0
                                                    874      935

                                                    394      290
                                                    7.8      7.7
                                                    728      445
                                                    335      300
 In mg/1 except for pH.
Vector  Impacts.   Rodents and  flies have  apparently  not bred
around  the  FSLs  for  the  last  five  years.  Scum control  is
obviously the key to elimination of this problem.

Groundwater Impacts.   Groundwater  contamination is  nonexistent.
Monitoring wells  surrounding  the  40 acres (16.2 ha) of existing
FSLs in Sacramento have been  sampled monthly  and have never shown
any  indication of  groundwater contamination  traceable  to the
lagoons.   Tests  show  that sludge  which settles  to the  bottom
quickly and  effectively seals off  the  lagoon contents  from the
surrounding  soils.  Undisturbed  soil samples  taken directly from
the  bottom  of  a  lagoon with  a  limited history  (one  to two
years)  and a lagoon  with a  long  history (four to  five  years)
confirm that the FSL contents have  a limited  penetration into the
surrounding  soils.   These studies  indicate  that  the sealing of
FSLs  is a combination of soil pore plugging  by  suspended and
colloidal materials  within  the  sludge and  the formation of
mucus-like materials that create an impermeable membrane  between
the  stored sludge  and the underlying  soil.   Sandy soils take
longer  to  seal  than silty clay soils, but both achieve complete
sealing in two to three months.

The  two-  to six-inch  (5.08  to 15.24 cm) engineered  fill seal
provided  over the  natural bottom and side slopes of the  typical
FSL  cross-section  on  Figure   15-8  assures that none of  the FSL
start-up  sewage  or diluted  sludge content escapes during the
natural sealing  process.

Pathogen Impacts.    It  has been recognized  for many years that
long-term  liquid  storage  signif i.cantly  reduces  the pathogenic
microorganism  content in sludge   (3).   Studies  at Sacramento
confirm  this  for  the  most common bacteria.   Figure 15-11
indicates  that  the fecal  coliform  population  decreases  as the
sludge  passes through  the sludge management  system.  Studies of
parasitic  protozoans  and their  cysts,  helminths  and their eggs
(ova),  and virus were  inconclusive either  because  insufficient
                              15-36

-------
numbers  were  found  or  the techniques  required  for  reasonable
reproducibility were  unavailable  to  the project.   The  system of
disposal  selected,  that  of dedicated  land disposal,  made further
investigatory  work  unnecessary.

                                  TABLE 15-8

           SACRAMENTO  CENTRAL WASTEWATER TREATMENT PLANT
                COMPARISON OF DIGESTED FSL AND REMOVED
                          SLUDGE ANALYTICAL DATA
                                           Stored sludge
      Constituent
   Alkalinity
   Chloride0
   Ammonia0
   Soluble phosphorus (P)c
   Sulfate

 Percent dry weight
   Total phosphorus (P)
   Total nitrogen (N)

 pprc^dry weight
   Calcium
   Magnesium
   Potassium
   Sodium
   Arsenic
   Beryl li-um
   Cadmium
   Chromium
   Copper
   Lead
   Mercury
   Molybdenum
   Nickel
   Selenium
   Silver
   Zinc
   PCB 1242
   PCB 1254
   Tech chlordane  ,
   Other pesticidals

 Units as noted
   Cd/Zn ratio, percent
   Total solids, percent
   Volatile solids, per-
    cent of total
   pH            c
   Specific conductance ,
    umhos/cm
  1.8
  8.7
21,000
 5,800
 5,500
 9,200
   47
 <2.2
   12
  165
  340
  185
  3.7
  <22
   63
  1.6
   28

  930
  1.3
  1.7
   68
  7.5
                    4,742
  2.0
  5.1
27,000
 8,200
 3,200
 3,100
   75

   24
  218
  410
  134
  5.3
 <13.4
   58
  1.7
   26

 1,700
 <2.8
  5.5
  3.8
 0.30
  1.4
  7.0
   55
  7.3
                          5,109
  1.9
  5.2
25,000
 7,900
 3,900
 3,450
   72

 <1.1
   26
  245
  398
  123
  5.1
  <16
   72
  1.4
   26

 1,500
 <3.1
  5.3
  4.0
 0.27
  1.7
  6.3
   55
  7.3
  1.7
  5.2
21,000
 7,900
 3,800
 3, 500
   89

   19
  224
  385
   96
   70
  1.6
   26

 1, 300
 <2.9
  4.0
  3.6
 0.25
  1.5
  6.1
   53
  7.3
                                      5,743
28,000
 6,300
 2,900
 3,300
  101


   16
  243
  721
  134

  5.2
 <12.5
  115
  1.4
   23

 1,325
 <2.6
  4.8
  4.0
 0.22
  1.0
  7.6
  52
  7.3
                                            4,914
        1.4
        5.4
28,000
 5,500
 2,600
 4,100
   22


   14
  173
  400
  116
  5.0
 <13.7
   46
  4.1
   34
 1,207
 <2.3
  4.7
  3.9
 0.25
  1:1
  4.7
   60
  7.2
                                                  4,434

FSL 6
1,687
166
452
50
77
1.6
6.2
24,000
5,300
3,000
5,600
28
13
220
477
183
5.8
<15.4
48
3.2
38
1,400
<2.6
3.8
4.2
0.25
1.3
3.4
62
7.4
4,093

FSL 7
2,239
171
613
51
68
1.6
5.8
26,000
6,300
3,100
4,600
82
21
278
456
153
5.8
<12.2
60
2.6
35
1,400
<3.0
6.6
5.9
0.27
1.5
4.8
61
7.3
5,061

FSL 8
2,175
186
600
49
49
1.4
5.1
21,000
3,500
3,200
4,200
62
17
188
353
121
4.2
<11 . 8
53
1.4
27
1,090
<3.0
3.3
3.8
0.23
1.5
5.7
52
7.3
4,760
Removed
sludge3
2,069
171
573
45
151
1.9
5.9
24,000
8,600
4,500
5,400
15.4
19
181
384
159
5.6
< 13
77
5.6
28
1,200
< 2 . 1
4.6
5.0
<0.7
1.5
4.1
54
7.4
4,731
 aValues are averages from samples collected during 1977.
  As CaCO-, determined by potentiometrie titration of supernatant.
  As CaCO-, determined by potentiometrie titration of supernatant.
 CDetermined on supernatant; other determinations run on solution resulting from acid digestion of whole sample.
  Other pesticidals include residues such as DDT, DDE, dieldrin, etc.
  Analysis not performed.
Odor  Impacts.    Odor  impacts  change  in  direct  proportion  to
theFSL1ssurface  area.    In  most  small plants  (those  requiring
<40 acres  [16.2  ha]  of FSLs),  controlling the loading rate,  using
adequate  surface  agitation,  providing  sufficient  buffering  area
and  carefully  selecting   the  best  time  periods  for  feeding  and
surface agitation  operation are  sufficient  to  achieve acceptable
levels  of  odor  risk.    Table  15-9  shows  the  annual odor  risk
analysis developed  for the existing  40  acres (16.2 ha) of FSLs at
the Sacramento site  before  the  installation  of  the  barriers  and
wind  machines  (1).    No  high   technology  mitigation  has been
                                     15-37

-------
required  to maintain  this acceptable  risk level.   For larger
areas of FSLs,  additional odor control measures would probably be
required.   These might  include the  installation of  a blender
digester to  keep  raw sludge  from  short  circuiting  to the FSLs,
vacuum vaporization  to remove entrained  odors  from the digested
sludge  prior  to  its discharge into  the  FSLs,  separation of
batteries of FSLs,  construction  of special 12-foot (3.7 m) high
barriers  around  the FSLs,  to ensure maximum odor  dispersion
at low  wind speeds, and the use  of wind machines to aid odor
dispersion  when  the atmosphere is calm.   Figure 15-12 shows
typical wind machines and barriers  at  the  Sacramento FSLs.
o
i
I
c
d
LU

OJ

Q
w
(3
BE
o
IQ10 i
108
10*
107
106
10B
104
103
10*
10°
Iff1
102


-
—
_,
—
_
-
-
-
^
—






























JAN APR JUL OCT































JAM APR JUL OCT
































JAN APB JUL OCT













1














MAY JUL StP
JUN AUG OCT

-
-
-
-
-
-
-
_
_,
-
jyt oct
           RAW
          SLUDGE
DIGESTED
 SLUDGE
 FSL
STORED
SLUDGE
  FSL
REMOVED
 SLUDGE
TREATED
 SOILS
                           FIGURE 15-11

            SACRAMENTO CENTRAL WASTEWATER TREATMENT
             PLANT 1977 FECAL COLIFORM POPULATIONS FOR
            VARIOUS LOCATIONS IN THE SOLIDS TREATMENT-
                        DISPOSAL PROCESS
                              15-38

-------
                             TABLE 15-9

    SACRAMENTO CENTRAL WASTEWATER TREATMENT PLANT ODOR RISK
             FOR 40 ACRES OF FSLa, ANNUAL EVENTS (DAYS)

  Downwind odor              Direction towards which wind is blowing
  concentration,   "
       C         N     NE      E     SE     S      SW     W     NW   Total
*
5d
iod
2.8
0.3
0.08
• 2.1
0.3
0.06
3.2
0.5 .
0.10
7.3
1.2
0.20
11.5
1.6
0.3
6.7
0.8
0.2
4.1
0.4
0.1
3.1
0.3
0.1
38.9
5.4
1.1
 Includes source control mitigation - controlled organic surface loading rate,
 adequate surface mixers, and controlled feeding and mixer operating times -
 and odor transport mitigation - 2,000 to 5,000 feet of buffer.

 2 ou/cf barely detectable ambient odor criteria.

 5 ou/cf threshold complaint conditions.

 10 ou/cf consistent complaint conditions.

 1 AC = .4047 ha.
 foot = 0.3048 m.
 1 cf = 0.02832 m3.
The  odors from 40 acres (16.2  ha) of  FSLs at  Sacramento  have
proven  to  be completely  acceptable.  An analysis  of  the expected
annual  odor risks  for  the  124 acres  (50.2  ha)  of  FSLs  to be
constructed  for  the  new  regional  treatment  plant   (see
Figure  15-9) is  shown in Table  15-10  (1).   This  analysis  shows
that  with the  installation of  complete  control measures,  the
incidence  of  threshold complaint  odor  levels  at  the  plant
boundary  (2,000 to 5,000 feet  [610  to 1,520 m] downwind)  will be
less than  once  every  two years, regardless  of  wind direction, and
once  every  seven years  for the  worst  specific   wind  direction.
This level of odor risk  was  found  to be acceptable in  the public
environmental impact  hearings.


Cost Information

The major  elements  involved in determining  FSL costs  are land and
earth moving.   Both  are usually quite site specific.   Normally,
land  costs   vary  less  predictably  than  construction  costs.
A  typical  FSL  storage facility  for a 10-MGD  (438-1/s)  secondary
carbonaceous activated  sludge  treatment plant with  primary
sedimentation,  anaerobic digestion, and norma^l strength domestic
and  industrial  sewage  will  cost about $1.5 million  to  construct
and $25,000  per year  to operate.  Construction costs  are based on
a  3500  Engineering News Record Construction Cost Index  and do
not  include  the cost of  land.   Operation costs are based on 1978
wage  rates  and  do not  include dredge operators or any  other
removal costs. ;s                              .  •   .
                               15-39

-------
                 . • V,:*&v#. '.-:•-.•                   ^
                              '  f 3ti"'J-'"'>"'':"»^'"->^-'; -'  ;- •>"..
                          '"  '   ••
                           FIGURE 15-12

                TYPICAL WIND MACHINES AND BARRIERS
                      SACRAMENTO, CALIFORNIA
Construction  costs  include  the  installation  of  three  complete
four-acre  (1.62  ha)  FSLs.   This is  assumed  to  be the  capacity
needed to  meet  the annual  digested  sludge  loading  rate  criterion
of 20  pounds  VS per  1,000 square feet  per day (1.0 t  VS/ha-d).
It is  based on a conservative  unstabilized  sludge  production
rate and  a nominal  50  percent  volatile solids reduction in  the
anaerobic digesters.   The three  lagoons  will provide capacity  for
daily loading, digester cleaning, and  maintenance and  storage  for
intermittent removal  to  dedicated  land  disposal.    FSLs  are
assumed to  be 15 feet  (4.6  m)   in  depth  and have  3:1  dike side
slopes.  If they are  required, purchase  of  the dredge  and  booster
pump would  add  another $150,000  to $180,000  to the construction
costs.

Odor  control  costs,  including  blending  digester,  vacuum
vaporizer,   barriers,  and  wind  machine  could  increase   the
construction costs  another $250,000  and  the operation costs
                              15-40

-------
another $25,000  per year.    As indicated  by the  odor  impact
assessment,  sufficient area  to ensure  maintenance of  loading
criteria, together  with surface agitators and  proper  buffer,
would  make it  possible  to  avoid the  cost of  the  aforementioned
more extensive  odor mitigation facilities.

                            TABLE 15-10

     SACRAMENTO REGIONAL WASTEWATER TREATMENT PLANT ULTIMATE
        ODOR RISK FOR 124 ACRES OF FSLa, ANNUAL EVENTS (DAYS)

  Downwind odor    	Direction towardg_which wind is blowing
  concentration,                         '          :   """	
       C         N     NE     E     SE      S      SW     W     NW   Total
2b
^
10d
0
0
0
.44
.08
.02
0.15
0.02
<0.01
0.18
0.03
<0.01
0.41
0.06
0.01
0
0
0
.85
.13
.02
0.31
0.04
0.01
0.22
0.03
0.00
0.33
0.05
0.01
2.9
0.44
<0.09
 Includes source control mitigation - controlled organic surface loading rate,
 adequate surface mixers , blending digester, vacuum vaporization and controlled
 feeding and mixer operation times, and odor transport mitigation - 2,000 to
 5,000 feet of buffer and, separation of groups of FSLs, barriers and wind
 machines.
 2 ou/cf barely detectable ambient odor criteria.
Q
 5 ou/cf threshold complaint conditions.
 10 ou/cf consistent complaint conditions.
1 AC = .40407 ha.
foot = 0.3048 m.
1 cf = 0.02832 m3.


Construction  costs  for the 124  acres  (50.2 ha)  of FSLs  with
complete odor mitigation  facilities  for the  Sacramento  Regional
Wastewater  Treatment  Plant  are  estimated  to  be  $28.7  million.
This  includes almost $3.3  million for  the existing  40  acres
(16.2  ha)  of  FSLs with barrier wall and  wind machines.   This
acreage  will  store the  solids from  a 136-MGD  (5,960-1/s)  secon-
dary  carbonaceous  activated  sludge  treatment  plant.  Operation
costs are estimated  to be $650,000 per year.


         15.3.1.3   Anaerobic Liquid Sludge Lagoons

Many  such  lagoons  are  being  operated throughout  the  United
States.   One  system   that  has collected  some meaningful data  is
the  220.2  acres  (89.1  ha)  in  operation  at the  Metropolitan
Sanitary District of  Greater  Chicago (MSDGC)  Prairie Plan land
reclamation  project  in  Fulton County,  Illinois.   In  a  personal
communication  R.R. Rimkus,  Chief  of Maintenance  and  Operations
MSDGC  provided the  layout  shown  on Figure 15-13  of  the  four
lagoons  at  this site.  He  reports  that Lagoons 1  and  2 have been
in service  for eight  and seven  years,  respectively,  and Lagoons
3a and  3b  for six years.   Lagoons  1 and 2 have an  average depth
of  35 feet (10.7 m),  plus  or  minus  one  foot  (0.3 m),  while
Lagoons  3a  and  3b  are about 18 feet  (5.5  m)  deep.   Lagoons 3a and
3b  are  utilized  more for supernatant  treatment  and  storage.
                               15-41

-------
                3 a    TRANSFER
                                          HOLDING BASINS
  1 acre = 0,405 ha
                    FIGURE 15-13
ANAEROBIC LIQUID SLUDGE LAGOONS, PRAIRIE PLAN LAND
 RECLAMATION PROJECT, THE METROPOLITAN SANITARY
           DISTRICT OF GREATER CHICAGO
                      15-42

-------
Rimkus  further  indicates  barged  anaerobica1ly  digested
waste-activated sludge  from Chicago  is  discharged  into Fulton
County Lagoons 1  and  2  throughout the year,  when river shipment
conditions permit,  at  a frequency  of about  20  days  per month.
Solids  loading  varies  between 65,000 to  95,000  dry  tons
(59,000  to 86,200  t) per  year.   Based on  the total  loading
received  by Lagoons  1  and 2 and  the  volatile  solids  content of
the digested sludge  equaling  57  percent,  the organic loading rate
to the Fulton  County  Lagoons varies  between  36  and 50 pounds VS
per  1000 square  feet per  day  (1.7  to 2.4 t/ha-d).   This  is
considerably above the 20 pounds VS per 1000  square feet per day
(1.0  t/ha-d)  established  at Sacramento  to  maintain facultative
conditions within the lagoons.    If  the area  of  all four lagoons
is considered,  this  organic  loading  rate  drops to 21 to 29 pounds
VS per 1,000  square  feet  per day (1.0  to 1.4  t/ha-d),  which is
close to the facultative  sludge  lagoon concept.

Rimkus reports  that the  solids concentration  of  sludge pumped
from the  barge to the lagoons varies from four to six percent by
weight.   Further,  the sludge  pumped  from lagoons  to  fields  in
1978  varied  from  3.57 to 5.93  percent by weight.   The average
annual quantity of removed sludge is 60,000  dry tons (54,400 t).
Mean  value  for volatile  solids content of  1978  removed sludge
was  47.5 percent.   If the  barged sludge  volatile  content is
57 percent,  then  the  lagoons  are reducing  the  volatile solids
by 17  percent.  Data  for sludge removed in  1978 are  given in
Table  15-11.   Sludge  removal  is usually accomplished  in  about
115 days, between  May 1  and  November  15.

According to Rimkus, Fulton County supernatant is disposed  of on
1,320  acres (534.2  ha)  of  alfalfa-brome hay  fields.   Average
annual quantity to  dispose  equals 700,000 wet  tons  (634,900 t)
with an  average ammonia content of 109.9 mg/1 and an average TKN
content of 156.4  mg/1.   Table 15-12  provides other data on lagoon
supernatant.   Dissolved oxygen (D.O.)  measurements  taken in
the  summer  and fall of 1977 in Lagoons 3a  and  3b indicate the
surface  D.O.   ranged between 0.9 and 8.5 mg/1,while  the bottom
D.O.  ranged  between  0.4  and   2.6  mg/1.   The  lowest  lagoon
temperature during this period  was  40.6°F (15.5°C).   The lagoon
surface  is frozen between  45  and  60  days  per year, with  scum
build-up experienced only  during periods  of new sludge input.  No
surface  agitation equipment is  used  on any  of  the lagoons.   The
nearest  residence  to the  lagoon is  approximately  6,000  feet
(1,800 m) from the perimeter of  the  installation.  No  information
is available regarding odors  or  odor  complaints.


        15.3.1.4   Aerated  Storage Basins

To use aerated storage basins successfully for wastewater solids,
a design must meet the following criteria:

     •  Basin  contents must be  sufficiently  mixed to assure
        uniformity  of  solids  concentration  and  complete
        dissemination of  oxygen.


                             15-43

-------
     •  Sufficient oxygen  must  be
        conditions  throughout  the
        solids  concentration.
available  to maintain  aerobic
 basin  at  maximum  attainable
     •  Liquid  level  variation  must  be  sufficient  to
        accommodate maximum  storage  needs  under  anticipated
        rainfall.
                             TABLE 15-11

                1978 REMOVED SLUDGE-PRAIRIE PLAN LAND
               RECLAMATION PROJECT, THE METROPOLITAN
               SANITARY DISTRICT OF GREATER CHICAGO3
Constituent
pH, units
EC, urahos/cm
Total phosphorus
Kjeldahl nitrogen - N
Nitrogen as ammonia - N-NHj
Alkalinity as CaC03
Cloride - Cl
Iron - Fe
Zinc - Zn
Copper - Cu
Nickel - Ni
Magnesium - Mn
Potassium - K
Sodium - Na
Manganese, Mg
Calcium - Ca
Lead - Pb
Chromium - Cr
Cadmium - Cd
Aluminum - Al
Mercury - Hg
Total solids, percent
Total volatile solids, percent
Minimum,
mg/lb
7.2
2,500
900
1,276
772
1,640
228
1,000
87
44.8
9
8.5
80
30
80
710
25.9
90. 6
7.5
340
0.132
3.57
43.5
Maximum,
mg/lb
7.9
6,800
2,960
2,905
1,338
5,750
752
2,900
231
124
28
28.3
200
120
810
1, 800
54.5
513
20.2
900
1.920
5.93
50.0
Mean,
mg/lb
_
4, 675
1,416
2,329
1,046
3, 630
388
1,938
171
81.6
18
18. 0
166
88
450
1,185
42.1
175
13.2
679
0.417
4.75
47.5
Mean content
Ib/dry ton
_
-
59.6
98. 1
44. 0
153
16. 3
81.6
7. 2
3.44
0.76
0. 758
6.99
3. 7
18. 9
49. 9
1.77
7.37
0. 556
28. 6
0. 018
2, 000
950
aLiquid fertilizer applied to fields from May 23, 1978 to November 18,  1978.
 Results are based on 24 weekly composite samples.  Data supplied by Metropolitan
 Sanitary District of Greater Chicago.

 mg/1 unless otherwise noted.
1 Ib = 0.4536 kg
1 ton = .907 t
Mixing  Requirements

Equipment required  for aerated storage basins  is  similar to that
for  aerobic  digestion  (see  Chapter  6).    Unfortunately  for the
designer,  mixing  capability  for  various  types of  static  or
mechanical aeration devices  varies  greatly.   Fixed or  floating
                                15-44

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turbine or  propeller-type  aerators  are  often affected  by  very
limited side boundaries,  while  brush-type aerators and aspirating
pumps  often have almost unlimited  side  boundaries but rather
restricted  vertical  mixing  capabilities.    Submerged  static
aeration devices are excellent  for  vertical mixing but are always
limited by  very  confined side boundaries.    The  designer should
rely on a performance-type specification  to achieve  desired
results.   The equipment supplier should  be given  information
about the configuration of the basin, its liquid level operating
range,   the maximum  solids  concentration expected,  and  the level
of dissolved oxygen to be  maintained.   The designer is expected
to have established  the  most cost-effective  basin configuration
based on loading,  site-specific conditions and  available aeration
equipment requirements.   A maximum  horsepower limit  should  be
established, and the specifications should include a bonus to be
added to  the  bid  price  and a penalty to be  subtracted  from the
bid price based on  the energy  costs  involved when the equipment
meets the  required  performance.   A  guarantee  should  be  used  to
assure  that the final installation will  meet the performance
requirement.
                           TABLE 15-12

          1973/1974 SUPERNATANT-PRAIRIE PLAN RECLAMATION
               PROJECT, THE METROPOLITAN SANITARY
                   DISTRICT OF GREATER CHICAGO*
Mean value,
Constituent mg/1
BOD -
BOD -
COD -
COD -
TSS
total
soluble
total
soluble

170
62
951
695
276
Range ,
mg/1
28 -
20 -
325 -
328 -
52 -
466
114
2,120
1,026
1,041
            Data supplied by  The  Metropolitan Sanitary
            District of Greater Chicago.
Oxygen Requirements

Oxygen  requirements to  maintain aerobic  conditions within an
aerobic  storage  basin  will  be  considerably less  than  that
required for aerobic digesters  if  the  material being  stored has
been stabilized prior to its introduction to the basin.   Minimum


                             15-45

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measurable dissolved oxygen  levels  of  about 0.5 mg/1  are  quite
adequate  to  maintain  a  basin  free  from anaerobic  activity,  as
long  as it  is  provided  with adequate  mixing.   If  the  basin
influent  is not  sufficiently stabilized  to  minimize oxygen
requirements,  then  the  aerobic  storage basin must be designed for
oxygen requirements similar  to  aerobic digesters (see Chapter 6).
Oxygen  transfer capabilities  are  similar  to mixing capabilities
for the various  types of  applicable equipment.  The design should
therefore  include  oxygen transfer requirements as  part of  the
performance  requirement  indicated  in  the  preceding section  on
mixing specifications.

Level Variability

Often,  aerated  storage  basins  cannot  be decanted,   because
solids  settle when  the  aerator  is turned  off,  and anaerobic
decomposition  may  also  occur, resulting  in  odor production.
Attempts  at  in-basin decanting  without  aeration  and  mixer
shutdown will usually result  in the recycling of the concentrated
solids back  to  the  liquid process.  Separate continuous  decanting
is  usually  possible  either  by  sedimentation   or  dissolved
air  flotation.   Evaporation  will  also quite  often result  in
significant liquid removal.  Aerobic storage  basins  that  do
not  have  separate decanting  facilities  must be  operated  on
single-phase concentration or displacement storage concepts.

The single-phase concentration  concept will function as  described
for  aerobic digesters.    The  displacement  concept,  however,  will
require liquid level variability and make  aerated  storage  basin
equipment  installation  quite  complicated.  Under such conditions,
this  equipment  must be  capable  of  maintaining adequate  mixing
and  oxygen  transfer over the complete range  of  liquid  level
variation.   This   requirement  may  cause  this equipment  to have
varying mixing and aeration  capabilities,  depending on  the  basin
depth.  Variable speed drives,  multi-speed  drives,  or  variation
in  the  quantity  of diffused air should be  investigated.   At  no
time should the equipment be operated under conditions  that will
waste energy.   Mixing and aeration design requirements and layout
details can  be  found in Chapter 6.


    15.3.2  Facilities  Provided Primarily for Storage of
            Dewatered Sludge

Dedicated dewatered sludge storage  of wastewater solids  can
include the  storage of  easily managed  dry solids  (>60  percent
solids) or  hard to manage wet  solids  (15  to 60 percent solids).
Dry  solids  are  usually the product  of  heat-drying,   high
temperature  conversion, or air-drying processes and can be stored
by  standard  dry material storage  techniques.   Descriptions  of
these  techniques  are  readily  available in  materials  processing
textbooks,  and, if  desired,  more  detailed  data is available
(20,21).  The storage  of wet solids is another matter,  however.
The  successful application of  common storage  techniques  to this


                              15-46

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normally  unstable  organic  material  is  practically  impossible.
The most commonly accepted methods  of providing dedicated storage
for wet organic  material  involves the use  of drying  sludge
lagoons, placing the material in some type of confined structure
or placing  it  in unconfined stockpiles.   All  three methods can
involve special'design considerations.
        15 . 3 . 2 .1 •• Drying Sludge Lagoons

Drying sludge  lagoons are probably the most  universally practiced
method of storing of wet organic  sludge.  Actually, the material
arrives at  the  lagoons  in  a liquid form, but as described under
Chicago's actual performance data, most  of the storage capability
is derived while the material  is  in a partially dewatered state.
Unfortunately,   many  existing   applications  of  this  method
of storage  are  being operated with sludge that has  not  been
anaerobically  stabilized prior  to its  discharge  to the lagoons.
In some  cases,  drying   sludge lagoons are used  after aerobic
digestion, and  in  other cases  they have been used as digesters
with no  upstream  stabilization.   In these  instances, odors that
are quite unacceptable to the surrounding community are produced.
When such lagoons  are considered  a means  of ultimate disposal,
they are  called "permanent  lagoons."   Because  permanent sludge
lagoons have  sometimes been  the source  of strong odors, they are
often rejected  as  a  means  to store sludge,  either in the liquid
or  semisolid  state  (22).   A detailed discussion of  design
criteria for drying sludge  lagoons can be found in  Chapter 9.


Performan c e__Dat. a


Several reasonably successful  drying sludge  lagoon operations do
exist.   An  investigation  of their actual   performance, however,
indicates that  these lagoons  are  acceptable because they receive
adequately  stabilized  anaerobically  digested sludge  and  do not
normally  generate  the odors  associated with the  acid  phase of
anaerobic stabilization.


San Jose, California.   The San Jose/Santa   Clara Water Pollution
Control Plant in  San Jose, California,   is  a  secondary  treatment
plant that  operates  on the  Kraus modification  of the  activated
sludge  process  during  its  seasonal   canning  loading  period.
The  plant  stores  its  anaerobically digested primary  and
waste-activated sludge in 73 sludge lagoons  on 580  acres  (235 ha)
of land immediately adjacent to the plant (2).  In  1978  the plant
operated  both anaerobic  liquid sludge  lagoons and drying sludge
lagoons  with 35  either filled  or more than half  filled  with
liquid sludge and 32 containing 2  feet  (0.60 m)  or less of dried
sludge.  Three  lagoons have  never been  used,'and three have been
dredged and  are now empty.   The drying  sludge lagoons  were filled
in layers of  approximately one foot  (0.3 m), and  each layer was
allowed  to  dry  by  evaporation prior to  the  addition of the next


                              15-47

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layer.    The  drying  lagoon operation took  place  from  1974  until
1976,  when operational  limitations  and  odor production resulted
in the return to anaerobic liquid sludge lagoon storage.   Liquid
sludge lagoon storage  had  been practiced prior to 1974.

As a result of existing operations, the present storage capacity
of the lagoons will  last until  1986.  Because the plant does  not
have existing dewatering facilities,  it  will  not be able  to
dispose  of over  900,000 gallons  per  day   (3,400 1/d)  of  liquid
sludge without providing  additional  sludge treatment  facilities
by 1986.  Studies  are now  under  way evaluating alternative
dewatering and drying processes  and  facilities  for the disposal
and use of dewatered and dried sludge.

Residents  living  in  areas near  the sludge  lagoons have  become
increasingly concerned about odors  produced  by  the lagoons.
During  1976,  several complaints were  registered with the  Air
Pollution Control  Board.   The area most  affected is a residential
community just southeast of the plant.   Correlation of complaints
with atmospheric  conditions indicates that  the greatest odor risk
occurs with a northwest wind  and when dry  weather is followed by
heavy rain.   This points to  the danger of  rewetting  the  dried
surface  layers and anerobically   stabilized material and confirms
that this can create  strong odors.

Chic a go, Illinois

The Metropolitan  Sanitary District  of  Greater  Chicago  (MSDGC)
operates 30 drying sludge lagoons,  each with an average  storage
capacity of 200,000  cubic  yards   (153,000 m3) and a storage  depth
of 16 feet (4.9 m) (23).  Figure 15-14 provides a plan view of a
typical  lagoon.   Anaerobically digested sludge  is  pumped  to  the
MSDGC lagoons at  a solids content  of  about 4 percent.   Volatile
content  of this material is approximately  57 percent.   Sludge is
usually  applied  to  each  available lagoon  in 6-inch (152-mm)
layers in rotation.  Rotations are  repeated.

Supernatant appears  on the lagoon  surface  approximately  five to
seven  days after each fresh sludge application.  It is then
drained  from the surface and  returned to the West-Southwest
Sewage Treatment  Works by  removing  one or more stop logs from the
draw-off box.   Once   the  supernatant  is decanted,  the  eight to
ten percent solids sludge is further concentrated by evaporaion.
Evaporation  tapers off,  however,  as  an  aerobic sludge  crust
develops.   Supply sludge concentration  (4 percent  solids)  is
beneficial, as it covers  the entire lagoon  surface with  only a
slight  gradient  from the point of  application.   Any higher
concentration would   inhibit  this  coverage  and  reduce  the
evaporative surface  area per  unit volume.   Lagoons  that have been
filled to  capacity by this method have  an  average solids content
of 18 to 22  percent by  weight.   Volatile  solids content  of this
material  is  in the range of  35 to  40  percent,  indicating that
the lagoons  are  producing  about a  34  percent volatile solids
reduction.
                              15-48

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                                               DRAW - OFF BOX & TRUSS

                                               CRESCENT SCRAPER
                                               AND CARRIER

                                               SLACKLINE CRANE

                                               SLUDGE INFLUENT

                                               TAIL ANCHORAGE
                                               (BULLDOZER!

                                               DRAGLINE (LOADING
                                               PARTIAL OEWATERED SLUDGE)

                                               FIVE AXLE DUMP TRUCK

                                               LAGOON PERIMETER

                                               ADJACENT LAGQQMS
                            FIGURE 15-14

               PLAN VIEW OF DRYING SLUDGE LAGOON NEAR
          WEST-SOUTHWEST SEWAGE TREATMENT WORKS, CHICAGO

Once the  drying  sludge  lagoons are filled, they are  taken  out  of
service  and  preconditioned  to  provide an  improved  drainage
gradient.   For  this  purpose,  the  sludge is  excavated from  the
area adjacent to the draw-off  box and  the  slope  within  the  lagoon
is  allowed  to stabilize  to  the point at  which  the  area  remains
            free  of solids.    Excavation  is by  pump with  nearby
            additional  water, if  necessary, to  assure  sludge
            Figure 15-15  illustrates a  cross  section of this
            preconditioning  is complete.  When  the sludge has
              the  lagoon is  left  dormant through  the  following
reasonably
mixers  and
f lu id i ty .
area  after
s tabilized,
winter and
drained by
            early  spring.
           gravity to the
 Trapped
draw-off
water  and
structure.
rainfall runoff are
Once relatively  dry  weather returns, a slackline  cable  system  is
utilized with  a  dragline crane  to further condition  the  sludge.
The slackline  system,  which is shown on Figure 15-16, is  used  to
improve the  lagoon  surface  drainage and to scrape as  much  of  the
dried crust  as possible to the  side  of the lagoon.    This  system
provides the following  four operational benefits:

     •  Drier  sludge  is scraped  to the  side, where it  can  be
        reached by portable dragline  or clamshell  and  loaded onto
        dump trucks.

     •  Piling sludge  along  sides  improves  lagoon  drainage
        pattern and profile.

     •  Removal of  crust exposes wetter sludge to atmosphere  for
        optimum evaporation.
                               15-49

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     •  Some  of  dried  crust mixes  with wetter  material during
        removal and increases the wet sludge solids content.
                                         MONORAIL BEAM
   WALKWAY
                  STEEL TRUSS
                                                 MONORAIL HOIST
   SEWER
                           Vi%£S?FZ£^*^^
                    DRAW-OFF
                      BOX
LAGOON BOTTOM
                            FIGURE 15-15

             CROSS SECTION OF DRAW-OFF BOX AREA DRYING
             SLUDGE LAGOON NEAR WEST-SOUTHWEST SEWAGE
                     TREATMENT WORKS, CHICAGO
Figure 15-16  shows the  location  of the  equipment  during lagoon
partial dewatering and removal operation.
  CRANE
                             CRESCENT
                             SCRAPER
                    TAIL
                 ANCHORAGE
                 (BULLDOZER}
                        LAGOON BOTTOM

                            FIGURE 15-16

             CROSS SECTION OF DRYING SLUDGE LAGOON WITH
                SLACKLINE CABLE NEAR WEST-SOUTHWEST
                     TREATMENT WORKS, CHICAGO
                              15-50

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Once  the  sludge  crust is scraped  to the side  of the lagoon,
it is  removed by portable  dragline or  clamshell, loaded onto
watertight five-axle dump  trucks,  and delivered to  the  general
public for reuse.   This  lagoon  sludge, at  its  time of  delivery,
usually has  an average  solids  content  of 30  to 35 percent  by
weight.  Tree nurseries, sod farms,  landfills,  and  stripped  land
are among  the major users of this material.  In  1977,  the MSDGC
disposed of  69,362 dry  tons  (62,925 t) of drying lagoon  sludge  at
an average  cost  of $16.75 per  dry  ton ($18.47/t).   In 1978,
production was expected to exceed 100,000 dry  tons  (90,700 t)  at
a cost of $17.76  per  dry ton  ($19.58/t).  Preconditioning costs
are approximately $3.00  per  dry ton ($3.31/t), which  makes  the
cost for the  whole operation about $21.00 per dry ton ($23.15/t).
Preconditioning is accomplished  by MSDGC  manpower and equipment,
and the  services  of  the slackline,  dragline,  and trucks are
contracted out.   The overall operation  requires little  capital
investment,  minimal  lead  time,  and  limited  effort.   Natural
processes  are optimized  and odors minimized.    The  level of  odor
involved has  not  been  qualified.


        15.3.2.2   Confined Hoppers or Bins

A designer is  often tempted to  take  advantage  of  the volumetric
reduction  in material provided  by the dewatering  process  and  lay
out his sludge disposal system based on  short   and  long-term
storage (3 weeks  to >6 months)  of the dewatered product.  If  the
product  is  too  wet  (<30  percent  solids),   several  problems
may arise with this type  of  storage.   These  problems include
continuing decomposition,  liquefaction,   and   concentration   and
consolidation.   Although each  may have its  own  result,  all three
problems  are  interrelated  and  combine  to limit the use of  this
type of storage to  equalization storage and then only if  special
attention is given to  controlling the  difficulties.   A brief
description  of  some  of these  difficulties  is  given  in  the
following  paragraphs.

Continuing Decomposition

Unless it is  stabilized to non-reactive levels  (<50 percent
by weight),   the  biodegradable volatile  organic material  of
wastewater  solids  will  continue to decompose if the moisture
content  is  too  high  (solids  content <30  percent).   This
decomposition will  reduce  organic material and generate  gaseous
byproducts.    Depending  on the  stage  and sometimes  the  type  of
stabilization employed  prior   to  dewatering,  the  method  of
conditioning  for  dewatering,  and the  dewatering  method  itself,
gaseous byproducts  may  or may  not  be  odorous.  For example,  a
biodegradable  volatile  content  of <50  percent would  result  in
strong  odors;  aerobically  stabilized  dewatered sludge would  be
more  subject  to  strong odors  than anaerobically stabilized
dewatered  sludge; polymer-conditioned  dewatered sludge would  be
more  subject-to   strong  odors  than  lime and  ferric  conditioned
dewatered  sludge;  and centrifuged dewatered sludge would  be  more
subject to strong odors  than vacuum filtered dewatered sludge.


                             15-51

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Enclosed structures are  often used  in  this  type of  storage  to
assure  odor-free  operation.   Such  structures  may  be  extremely
hazardous  if  the designer  fails to recognize the potentially
explosive nature of some  of  these  gaseous byproducts  and assure
that  they are  never  mixed  with  air  within  the  combustible
range.   If such  protection  involves  the replacement of  the
displaced  volume,  it  may become  the limiting feature of  the
storage structure's  ability to manage the sludge.

One solution  to this  problem is  to  treat the  volume  above  the
solids as part of  the digester gas storage system.  However,  this
is only  practical  if  the  overall  solids  treatment system  uses
anaerobic  digestion for  stabilization  and  the gas collection
system  has sufficient capacity  to fill  the  void created  by
storage  discharge within  the required  period of time.  Major
problems of such  a  system are the sealing of  sludge  supply  and
discharge and  the  assurance of accessibility for maintenance.

To eliminate  the discharge and  supply  problems  and assure
convenient access to the storage loading equipment,  the enclosed
area  of the storage  structures should be  sufficiently ventilated.
The area must be ventilated with about  20  to  30 air changes  per
hour.   Air movement should be felt by  the operators who work  in
the area.  To assure ventilation of all areas,  regardless of  any
continuously   or  intermittently  operating openings, both  supply
and exhaust air should be managed  by  powered  fans.   All exhaust
air should pass    through an  odor  removal system.   The quantity
of exhaust ventilation  air  should  be slightly  greater  than  the
quantity of supply ventilation air to assure a negative pressure
within the area  and  minimize  leakage  that might bypass the  odor
removal  system.   The  atmosphere  of enclosed areas  should  be
monitored with hydrocarbon detectors  (see Chapter 17)  to provide
ample  warning if the  gas  release begins to develop  dangerous
mixtures of methane and air.

Liquefaction

When the reduction of putrescible organic material is carried out
within a confined structure  used  for  short  or  long-range storage
(three to four weeks to more  than one month), the liquefaction of
dewatered  solids  occurs.    Liquefaction  is negligible  when  the
storage  is  limited  to equalization  (three  to  four days).   The
designer must  be  aware  of   the effects of  this  liquefaction
and  realize  that as  the liquid  or  moisture  content of  the
sludge  increases,  the  difficulties of  transport  also  increase.
An example of  this liquefaction,  in which  no evaporation  or
additional moisture is  assumed to be  added  during storage, can be
seen in the following  calculation:

    Typical Liquefaction Calculation

    Dewatered digested  sludge (polymer  conditioner used)

    Solids to be stored,  dry  wt,  tons            1,000     (907 t)


                              15-52

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    Total solids (TS)  content,  percent             20

    Volatile solids,  percent                      65

    Assumed reduction of VS  during
      6 months storage,  percent                   20

    Water content of  dewatered  sludge,  tons     5,000   (4,535 t)

    VS,. dry tons at start of storage              650     (590 t)

    VS, dry tons at end  of storage                520     (472 t)

    Fixed solids, dry  tons (unchanged)            350     (317 t)

    TS, dry tons at end  of storage                870     (789 t)

    Total solids content at  end of storage,
      percent                                   14.8

The example  indicates  how  a reasonably  dry,  dewatered  digested
sludge  (20 percent solids) can be liquefied to a fairly  wet,
digested sludge  (14.8 percent  solids)  if the putrescible organic
material continues to  be reduced.  The  speed of this reduction is
greatly  affected  by temperature and  organic  content in  the
dewatered sludge.  Thus,  liquefaction  will  be a  greater problem
in warm climates or during  the hot summer  seasons.   If  lime and
ferric chemicals  are  used to  condition  the digested  sludge  for
dewatering,  liquefaction  will  be greatly reduced,  both  because
of the  lower overall organic  content of  the material  and  the
inhibiting effects of  the chemicals on  the bacterial reduction of
the putrescible organic  matter.

Concentration and Consolidation

The material handling  properties  of the dewatered sludge  entering
the  storage  facilities  often  do  not  resemble those  of  the
material  discharged  from  the same  facility.  The  method  of
controlling  the  discharge must  be flexible  enough  to  adapt  to
these changes in properties  at  any time.  A live bottom discharge
for variable  positive  control  and back-up  isolating  valves  for
positive  shut-off if  the  live  bottom  equipment  fails or  the
material starts  to run  like water  are  mandatory  when  the volume
of  storage  greatly  exceeds  the  volumetric capacity  of  the
transport  system  receiving the discharge.     As  long as  the
storage structure's  volume  does not exceed the  capacity of the
transport  system receiving the discharge, and  that transport
system is  of the bulk  handling  type   (for  example,  truck,  rail
car,  or barge)  the discharge control can be a simple  open-close
valve.   Water collecting,  tracked,  hopper valves with remote
motor or  air cylinder operation can be  used   for  this  control.
Facilities whose storage  volume  exceeds  the discharge transport
system capacity  or whose transport  system is  of  the  continuous
rate type  (for example,  conveyor belts, screw  conveyors,  and


                             15-53

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pipelines)  must be  provided  with a discharge system  capable  of
infinite variability  under  all  degrees  of  moisture  content  or
concentration.    Such  systems  must  be  provided  with remote
controls  that  are  capable  of  detecting  overloads  prior  to
their overwhelming  the  transport system.   The controls must  be
capable of automatically  closing  the  discharge  control  system's
back-up, open-close  isolating valve.  , Sonic  level  detectors  and
capacitance  probes  can  be used  for  this function.   Chapter  17
provides additional  information on this  type  of  level detection
instrumentation.

The use of polymers to  condition the  sludge  prior  to  dewatering
can have a major effect on its ability to be stored conveniently
in  the dewatered  state.    Hansen reports  that  high polymer
doses used experimentally  (testing  a  belt filter press) at  the
Los Angeles  County plant  created a dewatered  sludge that  was
quite viscous.   This  material tended to act like glue and  was
extremely  difficult  to  remove  from  conveyors  especially  at
transfer  points  and  the head  point above the  hoppers.    The
material  could  be  stored,  but required  a positive  type  of
unloading  system at  the  storage discharge  to  assure that  the
lumps were pushed  onto the discharge conveyor.

When exceptionally dry dewatered  sludge  (greater  than  30 percent
solids)  is  stored,  bridging can be a very  difficult problem.
None of the facilities  investigated had  successfully solved  this
problem.  It  is  suggested  that any  large system which anticipates
storing dewatered  sludge much dryer than 30 percent solids  set up
a test  facility to develop a reliable system for  overcoming  this
difficulty.

Performanc e  Data

Probably one  of  the  most successful confined bin dewatered  sludge
storage facilities  is  located at the County Sanitation Districts
of Los  Angeles County  Joint Water Pollution Control Plant in
Carson,  California.  The  Joint  Water Pollution Control Plant
(JWPCP) provides advanced  primary wastewater treatment for  about
350 MGD  (15.3 m3/s) of  wastewater.   The  JWPCP  also receives  the
sludge  from five  tertiary treatment plants that employ activated
sludge  followed by multimedia filtration  and  have a combined
capacity  of  120  MGD  (5.2 m3/s),   Sludge from  all six plants
is  treated  at  the JWPCP using  the  anaerobic  stabilization
(digestion),   dewatering   (centrifugation),   and  composting
(windrow)  processes.

In June  1979,  Mischeri reported  the  centrifuges  were producing
about 400 to  600  wet  tons (360  to 540 t)  of dewatered  digested
sludge each day  with a 25  percent average solids content.   Twelve
storage bins, each  capable   of holding 550  wet tons  (500 t)  of
dewatered  sludge,  are provided  to  equalize  24-hour-per-day
centrifuge production  with 10-hour-per-day windrow construction.
The  storage  bins also  provide the five-day storage needed to
assure continuous dewatering  when both the composting  and  backup


                              15-54

-------
sanitary fill  disposal  options are  unavailable  due to excessive
rainfall.   The facilities have been  in service about three
years, and  according to Hansen,  the maximum  period of disposal
unavailability  has  not  exceeded two days to date, although there
have been times when all twelve of the bins have been filled with
dewatered sludge.   An  isometric  sketch  of  the JWPCP storage and
truck loading station is shown on Figure 15-17.
                                            END OF CONVEYOR
                                            ANfy TAKFUVS OVFB
                                            frW SiOHAUt BINS
        SCKtW
        cOhvEVOrt
                S4J=lGE
                BINS
                                                          ;• RUSTIC SOOA
                                                          A ', SCflUBBtHS
                                                            ASSEMBLY
                                      CONTROL
                                      SYSTEM
                                      !TVP Oi- 12:
      *z—^f ~£—«2C

  FROM DEWAT6R
-------
content is greater than 18 percent and the sludge is not left in
storage more than a few days.   Bubbles, which can be observed in
the standing water  on top of  the stored sludge, attest  to  the
fact that decomposition is  continuing  in  the bins.

Each storage bin is fabricated of steel,  is 30 feet  in diameter,
and tapers  at  the bottom to  a  five-foot-square  discharge.   The
taper is  at 30  degrees off  the  vertical.  Hansen indicates this
taper  seems to eliminate  bridging,  except  during  the storage
of  extremely  dry  (greater  than 30  percent  solids) sludge.
The five-foot-square  (1.5-m  square)  discharge  is equipped with
five  12-inch  (30.5 cm)  diameter  continuous  screw  conveyors
(live-bottom system) that can be  operated  in any combination or
number to positively control  the  stored  sludge  discharge  to  the
discharge conveyor belt.   Normal operation requires only  the
three middle screw  conveyors  to  be  in service.   A cylinder-type
plug  valve with  five  ten-inch  (25.4-cm)  long  by eight-inch
(20.3-cm)  wide openings  has  been provided  to  assure positive
isolation between  the  live-bottom  system  and  the  discharge
conveyor.   The  plug valve  is  fabricated  of  0.406-inch  (1.03  cm)
steel  wall, 12-inch  (30.5  cm)  O.D.  steel pipe, approximately
five feet  (1.5  m)  long and is actuated  by a pneumatic cylinder
that positively rotates the valve 90 degrees  from a  full open to
a  tight  shut-off  position.   An isolating bull  gate  that  can be
hydraulically  forced between  the  bottom  of the  storage bin  and
the top of the live-bottom assembly  is also provided.   It  can be
used to cut off sludge discharge  should  the live-bottom assembly
fail with  a load  in the  hopper.   It  has been  suggested  that a
hydraulically  operated gate valve or knife-gate  valve could also
be  used  to provide this  isolation.   An  isometric  view of this
discharge control  system  is shown  on Figure 15-18.

Hansen reports  the  storage facilities were  built in  1973 at a
cost of  $3  million.   Sludge  variability  during  start-up created
several  problems  that  have now  been  successfully  solved  (24).
Solutions included:   simplifying  the  supply  to  the storage
tanks by  equipping  each  with  a plow  and moving the end  of  the
supply  belts  over  the end  hoppers;  providing  the live-bottom
discharge system with  a  positive  discharge isolation  valve;  and
increasing the ventilation level  in the  supply and storage areas
to achieve  the  "breeze" atmosphere necessary  to satisfy operator
safety concerns.


        15.3.2.3  Unconfined Stockpiles

Unconfined stockpiles  are a major method of  providing  long-term
storage for dewatered  sludge.   This  method is used primarily for
the storage of  air-dried, anaerobic or aerobic stabilized  sludge
at  thousands of  small  plants  across  the country.   Probably  the
largest  storage  and weathering  installation  is  operated  by  the
Metropolitan  Sanitary District  of Greater  Chicago (MSDGC)  at
their West-Southwest  Sewage  Treatment Works  (WSW-STW).   All  of
the air-dried  Imhoff sludge at WSW-STW is  stored  and  aged up to


                              15-56

-------
several years
land  and  then
"Nu-Earth"  (23).   The  air-dried  material weathers to
50 percent  moisture after one  to two years of  aging.
                on between 50  and  100 acres (20  and 40  ha) of
                 made  available  for delivery  to  the public as
                                                           less  than
BULL GATE
HYDRAULIC
OPERATOR
                            80" DiA DISCHARGE
                         FflQM 550 Wit ION STOftAGfc
                                BIN
    LtVf BOTTOM
    DRIVE ASSEMBLY
                                                        1?" DIA
                                                       5=0" LONG
                                                     live BOTTOM
                                                      DISCHARGE
                                                       CONTROL
                         CONrnoi
                         VALVE
                         PNEUMATIC
                         OPFBflTQR
MAINTENANCE
TRACK
        EMERGENCY
        LIVE BOTTOM
        ISOLATING BULL GATE
                  BULL GATE
                  OPERATING TRACK
LIVi BOTTOM OVERLOAD
COUPLING-DISENGAGED
TD CONTROL NUMBER OF
SCREW CONVENORS IN OPERATION
          1 f1 - 0.105 m
          1 in m 2.54 on
          1 ton - 0.80? I
                                      'CONVEYOR LOAD
                                      MONITORING
                                      PHOBFS
                             FIGURE 15-18

             STORAGE BIN DISCHARGE CONTROL SYSTEM, JOINT
             WATER POLLUTION CONTROL PLANT, LOS ANGELES
                          COUNTY, CALIFORNIA


Unconfined  stockpiles  of  mechanically dewatered  stabilized
sludge, which  has less  than 25  percent  solids,  usually are
destroyed  (loose  all semblance of stability)  when exposed  to
extensive  rainfall.   While  it  is  possible to  maintain such  a
stockpile  for  equalizing  or  short-term storage,  especially  in
very  dry  climates  like  the southwest, long-term  storage  is
usually  quite  impossible.  Stabilized sludges  with a high chemical
content  (greater than 40 percent  lime plus some  ferric)  or a  very
low  organic content  (less  than  50  percent  volatile  solids)
sometimes  prove  to be  exceptions.  Highly  stabilized  lagooned
                                15-57

-------
sludges can  also  be one  of these  exceptions.   Such open
stockpiles  usually  quickly  absorb atmospheric  moisture  and
rapidly  deteriorate  in  climates  with  intense  or  frequent
rainfall.

Covered stockpiles  are often used in those areas where rainfall
is intense or frequent to assure  the dewatered sludge integrity
during  periods  of  equalizing  storage.    Such  stockpiling  is
usually limited because of the  expense  of developing covered
areas  of  sufficient  size  to  provide adequate storage  area  and
equipment  accessibility.   The North  Shore  Sanitary  District
(NSSD)  (25),  north  of  Chicago,  Illinois,  disposes of  their
anaerobically stabilized (digested)  dewatered sludge in deep
trenches  on  a  300 acre  (121  ha) site.   During  10 to 20 days
per year,   the NSSD  disposal operation  is  abandoned  due  to  wet
conditions,  and  the  dewatered  sludge is stored in  a covered  and
enclosed building  for  disposal within  a few days.   The building
is  enclosed to maintain  odor  control.   The  District also
frequently liberally  sprinkles  the  dewatered sludge  with  lime
during transport and storage to maintain odor  control.

Unfortunately, no  quantitative  work  has been published regarding
the odor risk of stockpiling dewatered  sludge.  Drying lagoons,
like those operated at San Jose, California, do create malodorous
conditions  in  surrounding  urban areas  during  or  immediately
after being wetted  by  rainfall.   Work in  Sacramento, California,
however,  indicates  that odors are generated cumulatively  in
direct relationship  to  the  area  covered   by  the  odor producing
sludge  (1).    Good  housekeeping  around such  stockpiles  is
mandatory  to assure proper rodent control.
15.4  References

 1.  Sacramento  Area Consultants.   Sewage  Sludge Management
     Program  Final  Report,  Volume  4,  SSBs  and  Odors,  1978.
     Sacramento Regional County Sanitation  District.  Sacramento,
     California 95814.  September 1979.

 2.  San  Francisco  Bay  Region  Wastewater  Solids  Study,
     San Francisco  Bay Region Sludge Management Plan.  Volume V,
     San Jose/Santa  Clara Project  and  Environmental  Impact
     Report.   p.  2-4.  Oakland, California  94620.  December 1978.

 3.  USEPA.    Communication  to J.B.  Farrell,  Ultimate  Disposal
     Section  to Office of  Solids  Waste.   Best  Management
     Technology  Definitions for  (a)  Sludge Stabilization  and
     (b) Additional  Pathogen Reducing  Processes.   MERLE.
     Cincinnati,  Ohio  45268.  November 1978.

 4.  Metcalf  and  Eddy,  Inc.   Wastewater Engineering:  Treatment
     Disposal, Reuse - Second Edition.  McGraw-Hill Book Company.
     p.  322 and 353.   1979.
                             15-58

-------
 5.   Water Pollution  Control Federation.  Manual  of Practice
     No. 8,  Wastewate r  P1 an _t_D e s i g n .   WPCA Washington,  D.C.
     p.  57.   1977.

 6.   USEPA.   MERL  Publication  Series.   ^Evaluation  of  Flow
     Equalization  in Municipal Wastewater Treatment.   Cincinnati^
     Ohio,  45268.  EPA-600/2-79-096.   May 1979.

 7.   USEPA.    Technology  Transfer  Upgrading Existing  Wastewater
     Treatment Plants.   USEPA Cincinnati,  Ohio  45268.  October
     1974.

 8.   Berk,  W.L.    The  Design,  Construction and  Operation of the
     Oxidation Ditch.   RAD-211,  Lakeside  Equipment  Corporation,
     1022 E.  Devon Avenue, Bartlett,  Illinois  60103.

 9.   Dick,  R.I.,  E.L.  Thakston,  and  W.W.  Eckenfelder, Jr., Ed.
     Water  Quality  Engineering  New  Concepts  and  Developments.
     Jenkins  Publishing Co., Austin and New York.   1972.

10.   Keinath, T.M,  M.D.  Ryckman, C.H.  Dana, Jr.,  D.A. Hofer.
     Design and  Operational Criteria for Thickening of Biological
     Sludges,  Parts  I,  II, III, IV.  Water Resources Research
     Institute,  Clemson University.  September 1976.

11.   Tucker,  D.L.,  N.D.  Vivado.   "Design of an  Overland Flow
     System at Newman, California."   Proceeds of.  the 51st Annual
     Water  Pollution  Control  Federation  Conference.    Anaheim,
     California.   October 1978.

12.   Liptak,  B.C.,   Ed.    Environmental  Engineers  Handbook,
     Volume I  Water  Pollution.   Chilton  Book  Comany.   p.  807.
     Radnor,  Pennsylvania.  1974.

13.   State  of California Water  Resources Control  Board.   Final
     Report-Phase I,  Rural Wastewater Disposal  Al ternatjLyjss.
     Sacramento, California,  p.  12.   September  1977.

14.   USEPA.   Upgrading Lagoons.  Technology Transfer  Seminar
     Publication.    Cincinnati,  Ohio  45268.   EPA-626/4-73-0016.
     Revised  June  1977.

15.   Sacramento Area Consultants.    Sewage  Sludge Management
     Program  Final  Report, Volume 7,  Environmental Impact Report
     and Advanced Site Planning.   Sacramento Regional County
     Sanitation  District.   Sacramento, California  95814.
     September 1979.

16.   Brown  and  Caldwell.    Joint Regional Wastewater and Solids
     Treatment Facility Project  Design Report.     Moulton-Miguel
     Water  District-Aliso Water Management Agency.   March 1978.

17.   Sacramento Area  Consultants.    Sewage Sludge Management
     Program  Final  Report,  Volume  2, SSB  Operation  and
     Performance.  Sacramento  Regional County  Sanitation
     District.   Sacramento, California 95814.   September  1979.


                             15-59                    *

-------
18.   Sacramento  Area   Consultants.    Study of Wastewater Solids
     Processing and  Disposal, Appendix C.
     County Sanitation
     June 1975.
                  District,
                             Sacramento  Regional
                    Sacramento,  California  95814.
19
Sacramento
Technology
  Area  Consultants.     Innovative and Alternative
  Documentation Sacramento  Regional Wastewater
20,
     Treatment  Plant - Solids  Project.
     County Sanitation
     April 1979.
                  	    Sacramento  Regional
                  District.   Sacramento,  California 95814.
Hawk, B.C.
Pitsburg,
1971.
,  Ed.    Bulk Materials  Handling.
School  of  Engineering.   Pittsburg,
                           University of
                           Pennsylvania.
21.  National  Lime  Association.    Lime Handling Application and
     Storage in Treatment  Processes,  Bulletin 213
     D.C.   Second Edition.   1971.
                                                Washington,
22.
Ve s ilind,
Sludges.
Michigan.
 P.A,
Treatment  and  Disposal  of  Wastewater
                Ann-Arbor
                1974.
           Science  Publishers,  Inc.    Ann Arbor,
23.
24.
25.
Rimkus, R.R.,  J.M.  Ryan,  R.W.  Dring.   "A  New  Approach to
Dewatering  and  Disposal  of  Lagooned Digested  Sludge."
Proceeds  of  the Annual  Convention,  ASCE, Chicago,  IljLino^s.
October 1978.

B.E.  Hansen,   D.L.  Smith,    W.E.  Garrison.   "  Start-Up
Problems  of  Sludge  Dewatering  Facility."   Proceeds of the
51st Annual Water  Pollution  Control Federation Conference,
A n a h e i m, C a 1 if or_n_ia..   October 1978.

Lukasik, G.D., J. W.  Cormack.   Development  and Operation of
a Sanitary  Landfill for  Sludge  Disposal - North Shore
ganitary District.   North  Shore Sanitary District.  1976.
                              15-60

-------
EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
      Chater 16.  Sidestreams from Solids Treatment
                      Processes
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                          CHAPTER 16

                    SIDESTREAMS  FROM SOLIDS
                      TREATMENT  PROCESSES


Sidestreams are a major reason why solids treatment and disposal
facilities often  become trouble  spots  at  wastewater  treatment
plants.   Failure  to  account  for these sludge processing liquors
in the wastewater treatment  design  can  result in overloading of
the  treatment  facility.   It has been  conventional  practice to
return sludge sidestreams to the treatment plant at a convenient
point, usually  at  the headworks, with  no  pretreatment  and with
little concern for its pollutant  loadings.  These sidestreams can
increase the  organic loading  by  5 to 50  percent, depending on the
type and number of solids  treatment processes  used.

The  major objectives  of  this chapter  are to  describe  the
sidestreams produced by sludge treatment processes, factors that
affect sidestream quality,  and options available to designers in
managing the  sidestreams.  Information on the pollutant loads of
the  sidestream  produced by  a particular process is presented in
the chapter dealing  with that process.


16.1  Sidestream Production

Sidestreams are produced when wastewater  solids are concentrated,
and  when  water,  usually  plant effluent,  is used to remove odors
or particulate  matter  from  flue  gases,  or to wash and transport
debris  from  structures and   equipment.  Some  sidestreams require
special  attention  because  of  their  impact on  a wastewater
treatment plant's efficiency.

Usually several  sidestreams  are  produced  at  a particular plant.
Figure  16-1  is  a  flow diagram  showing  eight wastewater solids
sidestreams:    (1)  screenings  centrate,   (2)  grit  separator
overflow,  (3)  gravity thickener supernatant,  (4)  dissolved air
flotation subnatant,  (5)  decantate  following heat treatment,
(6) vacuum filter filtrate and washwater,  (7)  scrubber water  from
furnace flue  gas  cleanup,  and (8)  overflow from biological odor
removal system.

This  chapter devotes  special  attention to  the  most pronounced
examples  of   the problem--anaerobic digester supernatant and
thermal  conditioning liquor.   For additional information on
production  and treatment   of  wastewater  solids sidestreams,
several  publications are  available.     Municipal Wastewater
Treatment Plant Sludge and Liquid Sidestreams   deals  with   side-
streams from  severalsolids  handlingandtreatment  processes  (1).
                              16-1

-------
Effects  of  Thermal Treatment  of Sludge  on  Municipal Wastewater
Treatment Costs  describestheincreased wastewatertreatment
capacity required by use of thermal conditioning  (2).
                                                      WA5TEIVATER

                                                      WAST LWATEH SOLIDS
                                                    i —-. NON-CHLOBJNATFO EF
                                                   miTfi'l GASLLhJEDISCHAIGEE

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    SCREENINGS
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                                                    "  ~"|	~
                                                           1  FINAL
                                                     INC;NE!I4TOR i~		n •-.—*•
                                                           i  DISPOSAL
                            FIGURE 16-1

                 EXAMPLE OF SIDESTREAM PRODUCTION


16.2  Sidestream Quality and  Potential  Problems

The  interrelationship  between a wastewater treatment plant's
effluent  quality  requirements and  the  processes used  for  solids
treatment  and  disposal must  be  carefully scrutinized during
planning and design to avoid problems  caused  by  sidestreams.
Generally,  more sophisticated wastewater  treatment plants produce
greater  quantities  of   more  difficult-to-manage  biological  and
chemical  sludges,   When processed,  these sludges may  indirectly
cause  the production of sidestreams containing  large  quantities
of soluble  and colloidal materials  including nutrients.

Sidestream  quality from a  specific  process is  strongly  affected
by  upstream solids  handling  processes.   Vacuum filter  filtrate
and  washwater  quality,  for  example,  are determined  by  the
upstream  conditioning or stabilization  process.
                               16-2

-------
Sidestreams  should  be returned  to points  in the  wastewater
treatment process  which  will -result in  treatment  of the  side-
stream  and  prevent  nuisances and  operational problems.  The
return points shown on Figure 16-1 comply with  this requirement.

Runoff  from  sludge composting  areas and  leachate from sludge
landfilling  areas may pose  a unique problem,  since it may be
difficult and  costly  to  return them to  the  treatment plant if
the  landfill or composting  site  is remote  from  the treatment
facility.    Chian and DeWalle  extensively investigated the
composition   and   treatment  of   sanitary  landfill  leachate,
including anaerobic  biological  filtration,  chemical  precipita-
tion,  chemical  oxidation,  and activated  carbon treatment  (3).
Data  are  also available  on  groundwater  monitoring near  sewage
sludge or combined solid waste sewage  sludge landfills  (4,5,6).
In  addition, USEPA's  Process Design Manual, Municipal Sludge
Landf ills,  discusses  methods  of handling  leachate  (7).   In dry
climates, leachate can  often  be recycled to the landfill  site.

At  Beltsville, Maryland,  runoff from a  composting  site is  stored
in a pond and periodically  sprayed on a forest floor.   Monitoring
wells have been  installed,  and no groundwater  contamination has
been  detected. At Durham,  New Hampshire,  and Bangor, Maine,
runoff is recycled back to the treatment works without pretreat-
ment. At  Sacramento,  California,  runoff has been  returned  from
a  dedicated  land  disposal  site to  the plant headworks and has
been  monitored  for several parameters  (8) .   It was  found  that
runoff  is  polluted with constituents,  particularly the  first
runoff following the spreading of  sludge.  The concentrations of
insoluble  constituents  such  as  heavy  metals,  however, were
1 ow.


16.3  General Approaches  to Sidestream Problems

Several general approaches  to preventing or solving problems that
may result from sidestreams can be identified:

     •  Modification  of  solids treatment  and   disposal  systems
        to eliminate  particular sidestreams.

     •  Modification of previous  solids processing steps to
        improve  sidestream  quality from a particular  solids
        treatment process.

     •  Changing  the  timing, return rate, or  return point  for
        reintroducing sidestreams  into  the wastewater treatment
        process.

     •  Modification  of  wastewater  treatment  facilities to
        accommodate sidestream loadings.

     •  Provision of  separate sidestream  treatment prior to
        return.

Potential applications  for  each of these  are  described.


                              16-3

-------
    16.3.1  Elimination  of  Sidestream

Although  not  generally  practical,  specific situations  arise  in
which it  is possible to modify the solids treatment and disposal
system and eliminate  a  troublesome sidestream.  A particular case
involves  anaerobic  digester  supernatant,  which  has  often  been
identified as a source  of problems when a mixture of primary and
waste-activated sludges  is digested.    Mignone  has pointed  out
that where mechanical dewatering follows anaerobic digestion,  it
would be beneficial  to  eliminate  the secondary (unmixed) digester
by converting it to a primary mode (9,10,11).   There would be  no
variable  supernatant  stream, only  a predictable filtrate  or
centrate stream of low solids content which would be amenable  to
biological treatment.


    16.3.2  Modification of Upstream
            Solids Processing  Steps

Thickening of  sludge prior to  anaerobic  digestion  by  the  use  of
gravity,  flotation,  or  centrifugal  thickeners  can  improve  the
quality and  reduce  the  quantity of   digester  supernatant  (12).
Residence time in  the digesters  is  increased  and/or smaller
digesters   can  be  constructed.   Liquor that would  otherwise  be
produced  by  the  secondary digester  as  supernatant  is  produced
instead in the thickening step.  Its  quality will be better, and
it will have a lesser  impact when returned  to  the  wastewater
treatment  facility.

Other digester operating parameters such  as  organic loading and
temperature  also affect supernatant  quality.   An increase  in
organic  loading  will  generally result  in  poorer supernatant
quality (13).  Thermophilic digestion  produces poorer supernatant
quality than mesophilic  digestion.

Substitution  of  an equivalent solids  treatment process  for
another  may  also reduce  sidestream  problems.    For  example,
substitution  of  chemical  conditioning  for elutriation or  heat
treatment   can  reduce the  level  of contaminants  in sidestreams
from subsequent dewatering  steps.

The  high  colloidal  content  of  elutriate  has  been successfully
reduced  in   several  instances  by   addition  of  chemicals,
particularly polymer, to  the  elutriation process.   In  1973 the
sludge treatment system at  the  District of  Columbia's Blue  Plains
plant  (a  253-MGD  [11.l-m3/sec]  facility)  consisted  of gravity
thickening,  single-stage  anaerobic  digestion,  elutriation  of
digested sludge,  chemical sludge  conditioning, and vacuum filtra-
tion.   Large quantities of  fines and  activated sludge  solids
were recycled with the elutriate, and the primary clarifiers and
aeration process could not accommodate them.  Solids accumulated
in the  plant;  upsets occurred  in both the wastewater  and  sludge
treatment systems;  and  it  became   necessary  to temporarily
discharge  elutriate .directly  to  the plant effluent.  Eventually,
addition  of  polyelectrolyte  to  the  elutriation  process, coupled
                              16-4

-------
with  intensive  effort on  the  part of  plant staff  to improve
elutriation and vacuum filtration  performance,  resulted in a
90 percent solids capture through the two processes.

The Metropolitan Toronto main plant and  the  Richmond,  California,
facility experienced  the  same  results  as the Blue Plains  plant.
An example of successful use of  polymer  to improve  elutriation is
shown in Table 16-1 (14).
                            TABLE 16-1

               EFFECT OF POLYMER ON ELUTRIATION (14)
                   Parameter
                         Before
                         polymer
                          use
 After
polymer
  use
            Elutriate suspended
              solids,, mg/1             3,385      365

            Solids capture, percent     65.1     95.3

            Underflow solids con-
              centration, percent        3.5      4.3
    16.3.3
Sidestreams
facilities
sidestreams
rather than
overloads.
fluctuations
Change in Timing,  Return  Rate,
or Return Point

 are  normally  returned  to  the  wastewater treatment
 at  the  plant  headworks.   In  general,  return  of
 to plant headworks  should be  at  a low, steady rate
in slugs, since these  are  likely  to  cause upsets and
  In  instances  where there are  high diurnal  load
 and  the plant is  approaching capacity,  consideration
should  be  given  to returning sidestreams during off-peak  hours,
thus  equalizing  wastewater  loadings.  Adverse effects on  primary
treatment facilities,  such as  septicity, odors,  and  floating
sludge  can be  avoided by  returning  sidestreams  to  the biological
treatment  process  influent.   Alternatively,  mixing  supernatant
with  waste-activated sludge  before  returning  it  to  the  headworks
may also  aid  in  reducing odors  because of the  adsorptive  nature
of the  activated sludge  particles.
    16.3.4  Modification of Wastewater
            Treatment Facilities

Liquid treatment  facilities  should  be  designed with  the  capacity
to  treat  recycled  sidestreams  whenever  the  sidestream will
contain significant  concentrations  of  pollutants or  have a  large
hydraulic  impact.   Table  16-2  shows an example of the effect of
supernatant return on suspended solids and phosphorus loadings at
                              16-5

-------
an  activated  sludge  plant  using  two-stage  anaerobic digestion
(15).   Table  16-3  shows  estimated increases  in 6005 treatment
capacity required  by sidestreams  from  several  sludge treatment
processes (16) .
                            TABLE 16-2
                 EFFECT OF SUPERNATANT RETURN (15)
                       Suspended solids, Ib/day
Phosphorus, Ib/day

Point of measurement
Raw wastewater
To primary clarifiers
To secondary clarifiers
Final effluent
Primary sludge
Waste activated sludge
With
supernatant
return3
10,520
36,801
15,306
3,467
19,626
14,645
Without
supernatant
return
16,035
15,969
9,501
2,836
13,249
9, 593
With
supernatant
return3
756
1,304
991
435
299
453
Without
supernatant
return
857
914
803
500
156
287
 Returned ahead of primary clarifiers.
The  Central Contra  Costa Sanitary  District Water  Reclamation
Plant,  an  advanced  waste  treatment  facility,  removes
nutrients  through  chemical-primary  treatment and  nitrification-
denitrification.   Recycled sidestreams were  taken  into  account  in
plant design  by  allowing  for additional loads of  12 percent for
8005  and  21  percent for  suspended  solids.  Recycled streams
include gravity  thickener  overflow,  centrate  from  a  two-stage
dewatering centrifuge, and drainwater from a wet  scrubber.

Sidestreams  may  contain compounds that  are difficult  to  remove
in  wastewater treatment  facilities.   For  example,  the  nonbio-
degradable COD in  heat  treatment  liquor  will  pass  through  normal
secondary treatment unchanged.    Digester  and  sludge  lagoon
supernatant  may  contain  high  concentrations of  nutrients.    In
some  instances  separate  treatment  may be  appropriate.   The
Metropolitan  Sanitary  District  of Greater  Chicago has  conducted
several  investigations  involving  nitrification and nitrogen
removal  from  sludge lagoon  supernatant,,using  both attached
growth and suspended growth biological processes  (17,18,19).

In  evaluating  solids  treatment  and  disposal processes, both the
direct costs of the solids treatment  and disposal  systems  and the
indirect  costs  associated  with  return of sidestreams to the
wastewater treatment  facilities should  be  included in  the  cost-
effectiveness analysis.    The  cost  of  handling  the  increased
sidestream flows  may or may  not  be  negligible,  but capital and
operating expenses will  surely  increase as a result of  the 8005
and  suspended  solids  load  of  the  returned  stream.   Major
                              16-6

-------
components of such indirect  costs  include  increased aeration tank
size  and  blower  capacity  (for  diffused  air-activated  sludge
systems),   increased  sludge  treatment  capacity,  increased power
requirements for  blowers, and  increased  labor for operating and
maintaining more  heavily  loaded  secondary treatment facilities.
Additional costs will also be  incurred  if  odor control facilities
are required.


                            TABLE 16-3

             ESTIMATED INCREASE IN WASTEWATER STREAM
            BIOLOGICAL TREATMENT CAPACITY REQUIRED TO
              HANDLE SIDESTREAMS FROM VARIOUS SOLIDS
                     TREATMENT PROCESSES (16)

                                     Required  capacity
               Treatment process      increase, percent
           Liquid sludge to land              0
           Raw sludge to drying beds           7
           Chemical conditioning and        6-11
             filter pressing
           Rotoplug dewaterer             10  - 30

           Digestion and drying beds         0.6
           Digestion, chemical con-            5
             ditioning,  and filter
             pressing
           Digestion, chemical con-            4
             ditioning,  and vacuum
             filtration

           Heat treatment of raw             30
             sludge
           Heat treatment of di-              7
             gested sludge
Indirect  solids  treatment  costs  for  handling  sidestreams will
vary  significantly.   The indirect  costs associated  with heat
treatment have been estimated as 20  percent of  the  direct  thermal
treatment costs.   A report has been prepared describing the
effects of sludge  heat  treatment  on  overall  wastewater  treatment
costs (2).


    16.3.5  Separate Treatment of  Sidestreams

Most  sidestreams   from  properly operating  solids  treatment and
disposal  systems  can  be recycled  to the wastewater  treatment
facilities without significant problems.  In many  cases  two-stage,
anaerobic digester supernatant return to the wastewater  treatment
                              16-7

-------
^facilities causes operating difficulties.   Heat  treatment  is less
widely used, but  it  results  in conversion  of some of the COD to
the  soluble form.   Furthermore,  a portion of the  COD  can be
nonbiodegradable .


        16.3.5.1  Anaerobic Digester Supernatant

In most cases,  6005 and suspended solids are  of  concern, although
under certain  circumstances, nitrogen  and  phosphorus  removal may
also  be  desirable.   Anaerobic  digester  supernatant  characteris-
tics are summarized in Chapter 6, and typical values  are given as
a part of the example on Figure 16-2.   Table 16-4  lists possible
treatment  processes  for each  major  constituent (20).  Chemical
treatment of digester supernatant has been  studied  for many years
(21,22,23).   Rudolfs and  Gehm studied coagulation using ferric
chloride,  lime,  caustic  soda,  sulfuric acid,  chlorine, bentonite
clay, and zeolite (21).  It was found that  a  lime/ferric chloride
combination  gave  the best results  and 150 mg/1 ferric chloride
and  1,200 mg/1 lime reduced turbidity from  420 to 110  units.

The  carbon dioxide in digester  supernatant  will react with the
lime  to  form  calcium  carbonate precipitate.   Lime  requirements
and  the  quantity  of  lime  sludge produced  can  be reduced
significantly by  first air stripping carbon  dioxide from the
supernatant.   This  may also release odors,  and  for  this  reason,
its  use  should be approached with caution.   Because  lime raises
the  pH of  the  supernatant and under  high pH conditions the
ammonia molecule  tends  to  be  in the  nondissociated form,  ammonia
stripping can be affected after coagulation.   The relatively high
temperature  of  digester supernatant also  aids  ammonia stripping
for  the same reason.

Figure  16-2  shows  a  possible  treatment scheme for  digester
supernatant based principally on chemical coagulation (20).  Also
shown are probable  removals  and  common  influent and expected
effluent  concentrations.   Straight  aeration of digester super-
natant  at  plant  scale  has   also  been  attempted  (12,24,25) .
Even  where the supernatant after aeration was  not settled prior
to return  and  no discernible  improvement in  quality  resulted,  it
was  found  that wastewater  treatment  operation improved, probably
as a  result of better settling in the primary clarifiers.

Biological  filters,  either  aerobic  or  anaerobic,  appear to  be
feasible  methods  of biologically treating digester  supernatant.
The  Greater London Council studied aerobic  biofilter  treatment  of
supernatant  liquor  using coke  as the  filter medium   (26).  At  a
1:1  dilution  with  clarified  plant effluent,  85  to 90  percent
      removal and 60 percent ammonia removal were obtained.
 Howe  suggested  storage  of  digester supernatant  in  lagoons
 for  long periods to  reduce contaminant levels  (22) .   In one
 experiment,  a  detention time of 60 days  reduced  6005, suspended
 solids,  color,  and  ammonia by  about  85 percent;  hydrogen  sulfide


                              16-8

-------
was  reduced  by approximately  95  percent.   Facultative  sludge
lagoons designed for  long-term  storage have  been found to reduce
levels  of all  contaminants except  ammonia;(see Chapter 15).
                     SUPERNATANT
                     BQDg = 7,&QQ
                       SS^ 5,000
                    ORG.N =  400
                        P *  ISO
                      NMg»  goo
                      C02 - 1,000
                     C02 STRIPPING
                                          REDUCTIONS
95-98 PERCENT CO2
                         LIME/
                    FERRIC CHLORIDE
                     COAGULATION
                     PLUS SETTLING
                                      1
70-85 PERCENT BQD5
80-90 PERCENT SS
65-70 PERCENT ORG.N
85-95 PERCENT P
                       AMMONIA
                       STRJPPING
85-90 PERCENT NH3
                  j    • TREATED
                  i    SUPERNATANT
                     BODg * 1,750 mg/1
                       SS=  750
                    ORG.N =' 150
                        P -  15
                      NH3 =  75
                      CO2 =  50'
                               FIGURE 16-2

                      POSSIBLE TREATMENT SCHEME
               :FOR ANAEROBIC  DIGESTER SUPERNATANT  (20)
                                  16-9

-------
                           TABLE 16-4

                  POSSIBLE DIGESTER SUPERNATANT
                    TREATMENT PROCESSES (20)
              Constituent

            Suspended
              solids
           BOD:
        Processes
Coagulation, settling,
  microstraining
Removal with suspended
  solids, stripping of
  volatile acids,  bio-
  logical .treatment,  ad-
  sorption on activated
  carbon
           Phosphorus
           Nitrogen
           CO,
Removal with suspended
  solids, chemical pre-
  cipitation,  ion exchange

Removal with suspended
  solids (limited),
  ammonia stripping,  ion
  exchange

Lime addition,  air strip-
  ping
The chlorine stabilization process (see Chapter 6)  has also been
used to  treat  digester  supernatant before  it  is returned to the
treatment  plant  (Table  16-5).   Low  chlorine doses  (100  to
300  mg/1)   have  little  effect on  8005  and  COD  levels,  but
according to the  manufacturer,  they  may be used to reduce odors
and improve treatability  of  the supernatant.   Very high dosages
(1,500  to  2,000 mg/1)  are  required  to appreciably  reduce the
levels of oxygen demanding materials  in  the supernatant liquor.


        16.3.5.2  Thermal  Conditioning Liquor

Heat  treated  sludge liquor,  which  is received  as  decantate
and  filtrate  or centrate,  contains  high  levels  of  soluble
pollutants   and  a significant fraction  of  nonbiodegradable COD.
The  color   level of  the  liquor may  dlso be  high,  affecting
the color of the  final  effluent  (27).   Furthermore, chlorination
of effluent containing  recycled  heat  treatment  liquor may cause
taste  and  odor  problems  if the receiving stream is  used for
drinking water  supply (28).

Loll has cited average BOD5  loading increases  of 7  to 15 percent
and COD  increases  of 10  to  20  percent  at wastewater facilities
recycling  untreated  liquor (29).   Recycle  of heat  treatment
                              16-10

-------
liquor  at Colorado Springs,  Colorado,  caused  the  6005  loading
to be  increased  by 20 percent  and the  suspended  solids  load by
30 percent (27).


                             TABLE 16-5

            CHLORINE TREATMENT OF DIGESTER SUPERNATANT

                                        Value3
                                    Supernatant treated at indicated
                                         chlorine dose, mg/1
                    Untreated
     Parameter       supernatant     500     1,500    1,800    1,900    2,000

Suspended solids,
  percent                 1.9       1.7      1.8      1.7      1.7     2.0

Chlorine residual,  mg/1        0        0       0       10      80     190
pH                      6.8       5.8      5.5      4.8      4.4     2.7

Specific conductance,
  micromhos
Alkalinity, mg/1

BOD5, mg/1

COD, mg/1

Total nitrogen, mg/1

Total phosphate
  phosphorus, mg/1           510       430      440      400      380     260
1,950
1,100
2,600
43,900
2, 100
2,750
170
2,600
43,100
2,200
2,380
83
2,600
40,800
1,900
2,500
60
2,200
32,000
1,600
2,600
32
2,000
31,200
1,400
4,500
0
1,500
20,200
1, 100
SBased on results obtained with Purifax laboratory unit.
Trickling  filters,  the activated  sludge  process,  anaerobic
biological  filtration,  and  aerobic digestion  have  been  used  to
treat  the liquor.  To  reduce the nonbiodegradable COD, activated
carbon has  been used.  Ozonation or chlorination can also be  used
to reduce COD  levels.

Loll  has  described  experiments  using  autothermal  therraophilic
aerobic  digestion  of  heat  treatment liquors  (29).   Because
the  reactions  are  exothermic,   the  process  is  thermally
self-supporting.

Presented  on  Figure  16-3  are  the  results  of  batch  aerobic
digestion  tests.   Note  that  the  temperature  rose  during the
period of most rapid degradation.   The results  of continuous  flow
tests  are  presented  in Table  16-6 at residence times  of  five
and  ten  days.   The  COD reduction  is significantly  less than the
BOD5  reduction,  reflecting  the nonbiodegradable character of  a
portion of  the waste.

Erickson  and  Knopp   used  the activated sludge  process  for  heat
treatment  liquor   (30).    They  reported  a  COD  reduction  of
83 percent  and  a BOD5  reduction of 98 percent  with an aeration
time of 41  hours.   Results are  shown in Table 16-7,  (page 16-14).
                               16-11

-------
          15
                        o  COD

                        X  BOD5

                        D  VOLATILE ACIDS

                           TOC

                           TEMPERATURE
                                                      60
                                          50
                                                      40
      8
0
                                                      30
                                                          o
                                                           *.
                                                          LJJ
                                                          ac
                                                          DC
                                                          LU
                                                          0.
                                                          s
                                                          LU
                                                          I-
                                          20
                                                      10
                      5         10

                         TEST LENGTH, days

                            FIGURE  16-3
                       AEROBIC DIGESTION OF
                 HEAT TREATMENT, BATCH TESTS (29)
Anaerobic biological filtration of heat treatment  liquor has  been
tested  for use  at  the City  of  Los Angeles Hyperion  treatment
plant (31).  The waste-activated sludge treatment  scheme is shown
on Figure  16-4.   The  anaerobic filter,  originally developed by
Young and  McCarty  is similar  to  the conventional  aerobic trick-
ling filter  in that organisms are  attached  to the media surface
and  a  short  hydraulic  detention  time results  (32).   Advantages
                              16-12

-------
are that the production of methane can result in energy recovery
and that  no  power is  required  for  oxygen addition.    Care  must
be taken,  however,  to avoid any plugging  from periodic  high
suspended solids  loadings.  Results of a two-month test are  shown
in Table 16-8 (31).  At a  hydraulic  detention  time of  two  days,
8005  and COD removals  averaged  85 and 76  percent,  respectively.
This study  concluded  that detention time  could be  reduced  to
about  0.5 to  1.0 days  without  significant  deterioration  in
performance.   Other pilot scale  tests on anaerobic filtration  of
heat  treatment liquor  have been conducted.   One  study reported
COD removals of  approximately 65  percent  at detention  times  of
3.5 days and organic  loadings  of 125  Ib  COD  per 1,000  cubic  feet
per day (2.0  kg/m3/day)(33) .
                           TABLE 16-6

          AEROBIC DIGESTION OF HEAT TREATMENT LIQUOR (29)

                                    Residence time,
                                         days
                  Parameter            5         10

             Temperature,  °c            38        34

             COD
               Influent,  mg/1       13,500     12,400
               Effluent,  mg/1        4,100     3,800
               Reduction,  percent       66        71

             BOD5
               Influent,  mg/1        6,900     6,100
               Effluent,  mg/1          510       250
               Reduction,  percent       94        96
Figure 16-5  illustrates  the   AS pilot treatment  scheme  used  in
a pilot  study  in  Great Britain (28).   The  purpose  of  the  study
was  to  reduce the  quantity of  refractory  organics entering
the  Thames  River from  treatment  plants  conditioning sludge
with heat treatment.   The study  was  prompted by the fact that the
Thames  is used for  water supply,  and possible taste and  odor
problems would  result  from  chlorinating  the water;  in addition,
there was uncertainity about  the exact composition and effects of
the  organics  in  the liquor.  The  process  can reduce COD  from
20,000 mg/1  to  about  100 mg/1, or by approximately 99.5 percent.

The  chlorine oxidation  process can also  be used for treating
liquor  from  thermal   sludge  conditioning.   6005  and  COD levels
are  reduced by approximately  25  to 35 percent.  The  odor  is
changed  from noxious to  chlorinous or medicinal.   The  color  is


                              16-13

-------
changed  from dark  brown to yellow or tan  which may  allow the
liquor  to go  undetected  when  diluted  in  the  liquid  stream.
Results  of  a pilot  test on Zimpro  process  liquor  are  shown in
Table 16-9.   A flow diagram indicating  sampling  point locations
is shown on Figure 16-6.
                            TABLE 16-7

              ACTIVATED SLUDGE TREATMENT OF THERMAL
                     CONDITIONING LIQUOR (30)

                                     Aeration  time,
                                          hours
                  Parameter
           21.8
         40 .9
             Temperature,
°C
             COD
                Influent, mg/1
                Effluent, mg/1
                Reduction, percent

             BOD5
                Influent, mg/1
                Effluent, mg/1
                Reduction, percent
33.4
          10,600
           4,300
              59
           4,700
             400
              91
31.7
        11,900
         2,000
            83
         5,900
           110
            98
                                                   METHANE
                                                 CAR BOM DIQXiDE
WASTE
ACTIVATED


CONCENTRATION
» 1%
i r
UNDERFLOW
TO TREATMENT
PLANT

\THICKENI NG/
HEAT r. _A /
IEATMEMT , 5,^ \ ~~ '/
Y
m
1 1
DEWATERING f-

1
i * p
! *
i
HEAT
TREATMENT
LIQUOR
EFFLUENT
u^r RECYCLED TO
TREATMENT
PLANT
ANAEROBIC
FILTRATION
                                      CAK.E

                            FIGURE 16-4

               FLOW DIAGRAM, ANAEROBIC FILTRATION OF
                    HEAT TREATMENT LIQUOR (31)
                              16-14

-------
                 TABLE 16-8

   AEROBIC BIOLOGICAL FILTRATION OF THERMAL
            CONDITION LIQUOR (31)
           Parameter
Value
Hydraulic detention time, days
Temperature, °C

COD
  Influent, mg/1
  Effluent, mg/1
  Reduction, percent

BOD5
  Influent, mg/1
  Effluent, mg/1
  Reduction, percent

Suspended solids
  Influent, mg/1
  Effluent, mg/1

Total solids
  Influent, mg/1
  Effluent, mg/1

Volatile acids
  Influent, mg/1
  Effluent, mg/1

Alkalinity, as CaCO3
  Influent, mg/la
  Effluent, mg/1
PH
  Influent3
  Effluent
   2.0
    32
 9,500
 2,300
    76
 3,000
   450
    85
   110
   100
 8,800
 4,900
   520
   300
 2,200
 3,500
   7.1'
   7.1
 Decant liquor.

 pH following thermal conditioning was
 approximately 5.5; 1,600 mg/1 alkalinity
 added to influent for pH adjustment.
                   16-15

-------
  HEAT TREATMENT LIQUOR
COD 20,000 mg/1 APPROXIMATELY
            I
         ROUGHING
           FILTER
     (COD 3,000 mg/|)
1
—1
ACTIVATED
CARBON
COLUMN
l_


"I
ACT IV
CARE
con

AERATION
  TANKS
  49 MRS.
DETENTION
                                                                   I
                                            ACTIVATED
                                              CARBON
                                              COLUMN
                                                               I
                                                               I
                                                             *** EFFLUENT TO SEWAGE
                                                                 TREATMENT WORKS
                                                                    (COD 10Qmg/i}
                                                            (COD 900 mg/|)
                                    FIGURE 16-5
                 SCHEMATIC DIAGRAM OF PLANT FOR PROCESSING
                          HEAT TREATMENT LIQUOR (2)
                                    TABLE 16-9
                        CHLORINE OXIDATION TREATMENT
                        OF THERMAL CONDITIONING LIQUOR
             Parameter

  COD, mg/1
  Suspended solids, mg/1
  Total  solids,  mg/1
  Total  volatile solids, percent
  Ammonia, mg/1
  Chlorine dose, mg/1
  Chlorine residual after three
    hours, mg/1
  pH


40
19
24
t





1
,664
,300
,500
63 .1
225
0
0
5.1
Value
2
31,280
15,400
16,800
65.5
209
1, 000
0
3.7
a , b
3
3,910
172
5,700
66.4
209
1,000
0
3 .5

4
70, 380
51,600
52,000
56.1
269
1,000
0
3.9
   For location of sampling  point, see  Figure 16-6.
  ""Data taken at Canton Water Pollution Control Center,
   May 10 and 11, 1977.
                                       16-16

-------
FROM
Z1MPRO
PROCESS


DECANTING
1


PURIFAX
TREATMENT


rT1
DECANTING
1


                 TO
              DEWATERING
NOTE: CIRCLED NUMBERS DESIGNATE SAMPLING
     POINTS; SEE TABLE 16-9 FOR QUALITY DATA.


                            FIGURE 16-6

           CHLORINE TREATMENT OF HEAT TREATMENT LIQUOR
16.4  References

 1.   Municipal  Wastewater  Treatment Plant  Sludge  and Liquid
 2.
 3.
     Sidestreams.   USEPA Report  No.  EPA
     Water  Program  Operations.  p. 119.
                                   430/9-76-007.
                                   June 1976.
                                                  Office  of
Ewing ,  L . J ,
of  Thermal
            ,  Jr., H.H.  Almgren,
            Treatment  of Sludge
and  R.L.  Gulp.  Effects
on Municipal  Wastewater
 7.
     Treatment  Costs
        102
         June 1978
                     USEPA  Report  No.  EPA-600/2-78-073 .
Chian,  E.S.K.,  and  F.B.  DeWalle.   "Sanitary  Landfill
Leachates and  Their  Treatment."   Proceedings ASCE,  Journal
of the Environmental  Engineering Division. Vol. 102,  p.  411.
1976.
                  H .T
                   Phung,  R.P. Stearns,  and  J.J. Walsh.
            Disposal  of  Municipal  Wastewater  Treatment
Lof y ,  R . J .
Subsurface _
Sludge, Environmental  Assessment.   USEPA  Office  of
     Waste,  prepublication issue, Contract No.  68-01-4166.
                                                      Solid
                                                      1978.
     Sikora,  L.J.,  C.M.  Murray,  N.H.  Frankos,  and J.M.  Walker.
     "Water Quality at  a  Sludge Entrenchment Site." Groundwater.
     Vol.  16.   1978.
     Walker, J.M. ,  L
     Kaminski.  USEPA.
     by  Small Communities
                  Ely,  P
                   Sewage
                           Hundenmann,  N.  Frankos, and A.
                         Sludge Entrenchment System for  Use
                       EPA-600/2-78-018.  February 1978
USEPA,  Technology  Transfer
Municipal  Sludge  Landfills.  p.
                                  Process  Design  Manual
                               195.  October 1978.
                             16-17

-------
 8.  Sacramento  Area Consultants.   Sewage Sludge  Management
     Program,  Final  Report,  Volume  5,  Dedicated Land  Disposal
     Study.   Sacramento Regional  County Sanitation District.
     Sacramento,  California 95814.  September 1979.

 9.  Mignone,  N.A.    "Digester Supernatant Does Not Have To Be  a
     Problem." Water&  Sewage Works.  p. 57.  December  1976.

10.  Mignone,  N.A.    "Survey  of Anaerobic Digestion  Supernatant
     Treatment Alternatives."  Water & __Sew_age  Works.   p. 42.
     January 1977.

11.  Mignone, N.A.    "Elimination  of Anaerobic Digester
     Supernatant."   Water & Sew_age_Works;.  p. 48.   February  1977.

12.  Kappe,  S.E.    "Digester  Supernatant:    Problems, Character-
     istics,  and Treatment."   Sewage  and I ndustrial  Wastes.
     Vol.  30,  p.  937.  1958.

13.  Mueller,  L., E.  Hindin,  J.V.  Lundsford,  and G.H.  Dunstan.
     "Some Characteristics  of  Anaerobic Sludge  Digestion  -  I,
     Effect of Loading."  Sewage and Industrial  Wastes.   Vol. 31,
     p.  669.   1959.

14.  Burd, R.S.  "Use  of  New Polyelectrolytes  in Sewage Sludge
     Conditioning."    Proceedings of the 2nd yande_rbilt_ Sanitary
     Engineering  Conference.  May 1963.

15.  Geinopolos, A., and  F.I. Vilen.   "Process Evaluation  -
     Phosphorus  Removal."   Journal Water Pollution  Control
     Federation.  Vol. 43, pp. 1975-1990.  1971.

16.  Clough,  G.F.G.    "The  Effect  of  Sludge Treatment  Processes
     on  the  Design  and Operation  of  Sewage Treatment  Plants."
     Water Pollution Control.  Vol. 76., p.  452.   1977.

17.  Lue-Hing, C.,  A.W.  Obayashi,  D.R. Zenz, B.  Washington, and
     B.M.   Sawyer.    "Nitrification  of  a High Ammonia Content
     Sludge Supernatant by  Use of  Rotating Discs."    Presented
     at  the  29th Annual  Purdue  Industrial Waste  Conference.
     West  Lafayette,  Indiana.  p. 245.  May 1974.

18.  Lue-Hing, C.,  A.W.  Obayashi,  D.R. Zenz, B.  Washington, and
     B.M.  Sawyer.   "Biological  Nitrification  of  a High  Ammonia
     Content Sludge  Supernatant.  Under Ambient  Winter and Summer
     Conditions  by  Use of Rotating Discs."   Presented at the
     47th  Annual New X°rk Water  Pollut_ion__Control  Conference.
     January 1975.

19.  Prakasam, T.B.S., W.E. Robinson, and C.  Lue-Hing.  "Nitrogen
     Removal  From  Digested Sludge  Supernatant Liquor Using
     Attached  and   Suspended Growth Systems."   Presented  at
     the  32nd Annual  Purdue  Industrial  Waste  Confere nee.
     West  Lafayette,  Indiana.  p. 745.  May 1977.


                             16-18

-------
20.   Malina,  J.F.,  and J. DiFilippo.   "Treatment of  Supernatant
     and Liquids Associated with  Sludge Treatment."   Water &
     Sewage Works, Reference Number, p. R-30.   1971.      ~"

21.   Rudolfs, W.,  and L.S.  Fontenelli.   "Supernatant Liquor
     Treatment with  Chemicals."   Sewage Works Journal.   Vol. 17,
     p.  538.  1945 .

22.   Howe,  R.H.   "What To  Do with Supernatant."    Wa __s t e s
                  Vol. 30, p. 12. 1959 .                 —
23.   Reefer,  C.E.,  and H. Kratz, Jr.   "Treatment  of  Supernatant
     Sludge  Liquor  By Coagulation  and  Sedimentation."   Sewage
     Works Journal.  Vol. 12, p. 738. 1940 .                     "

24.   Erickson,  C.V.   "Treatment  and Disposal of  Digestion Tank
     Supernatant  Liquor."   Sewage  Works Journal.    Vol.  17,
     p.  889.  1945  .

25.   "The PFT Supernatant Liquor Treater."  Sewage  Works Journal.
     Vol. 15,  p. 1018. 1943  .(Author anonymous).

26.   Brown,  B.R.,   L.B.  Wood,  and H.J.  Finch.   "Experiments on
     the Dewatering of  Digested  and  Activated  Sludge."   Wa_tejr
     Pollution Control .  Vol. 71, p. 61. 1972.

27.   Boyce,  J.D.  and  D.D.  Gruenwald.   "Recycle  of Liquor from
     Heat Treatment of Sludge."  Journal Water  Pollution Control
     Fe_der_ati£n.  Vol.  47, pp. 2482-2489, 1975 .

28.   Corrie,  K.D.    "Use of  Activated  Carbon  in  the Treatment of
     Heat Treatment Plant Liquor."  Water  Pollution Contro^L.
     Vol. 71,  p. 629 .  1972 .

29.   Loll,  U.   "Treatment of  Thermally  Conditioned Sludge
     Liquors."  Water  Research.  Vol. 11, pp.  869-872. 1977  .

30.   Erickson, A.H. and  P.V.  Knopp.   "Biological Treatment of
     Thermally  Conditioned  Sludge  Liquors."   Proceedings of
     the  5th International Water  Pollution Control Research
     Conference, San Francisco.  Vol. II, p.  30.  1970  .

31.   Haug,  R.T., S.K.  Raksit, and G.G.  Wong.   "Anaerobic Filter
     Treats  Waste  Activated  Sludge."   Water  &  Sewage Works.
     p.  40.   February  1977.

32.   Young,  J.C.   and P.L.  McCarty.    "The Anaerobic Filter
     for Waste Treatment."   Journal Water Pollution Control
     Federation.   Vol. 41,  Research  Supplement, p. R160. 1969  .

33.   USEPA  Pilot  Scale  Anaerobic  Filter  Treatment  of  High
     Strength Heat  Treatment  Liquors; .   MERL, Cincinnati,
     Ohio 45268.   Draft, Undated Contract No.  68-03-2484.
                              16-19

-------
EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
             Chapter 17.  Instrumentation
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                           CHAPTER  17

                        INSTRUMENTATION
17.1  Introduction

Wastewater  solids  treatment  and disposal  systems  are generally
under-instrumented  in  comparison to  other treatment  systems,
such as those in water  supply or chemical processing plants (1).
While  the  economics  and operating  efficiencies  of  various
measuring devices,  control  equipment,  and operator  interface
displays  should  be carefully evaluated  by  the treatment system
designer,  increased use of instrumentation  is  recommended.  This
chapter examines  instruments  suitable  for  sludge  treatment and
disposal facilities.


    17.1.1  Purposes  of  Instrumentation

Most  of the measuring devices described  in this  chapter are
"on-line"  equipment  designed  for  essentially  unattended
operation.  However,  some  critical  data  can be obtained only by
the use of  portable  test or  laboratory  equipment  that  requires
manual  operation  or  attention.    On-line instrumentation serves
the following purposes in a wastewater  solids  treatment system:

        Reduces  labor
        Reduces  chemical consumption
        Reduces  energy consumption
        Improves treatment process  efficiency  and reliability
        Provides information  for planning
        Verifies compliance with discharge  requirements
        Assures  personnel safety


    17.1.2  Instrumentation Justification
            and  Design Considerations

Some  uses of instrumentation—for example,  to reduce labor,
chemical  consumption,  or energy  consumption--will  be justified
primarily  from  an  economics  viewpoint.   Economics  may,  however,
be a secondary consideration  in  decisions to install  instrumenta-
tion for  any  of the  other purposes listed above.   For instance,
instrumentation   for  providing  planning  information  and/or for
verifying  compliance  with discharge  requirements may  be justified
on  non-economic  grounds.    The  information provided  may  be
essential  for planning  new  facilities  and/or  improving  existing
facilities.   Such information may  also  be required for monitoring


                              17-1

-------
treatment results  for  reports  to  various .government  agencies.
Economic considerations  will  also be secondary for those  systems
requiring continuous monitoring  to  protect  operating  personnel.

Economic  analyses  of  instrumentation,  when required,  must
include both capital and operation  and  maintenance  (0/M)  costs.
0/M costs can be  high, especially in sludge management,  where  the
materials being measured are  usually  debris-laden  and  sometimes
corrosive.    A 1976  USEPA  study  found  that  many wastewater
treatment instruments are  not  properly operated  or  maintained
and quickly fall into disuse (1).  This is  particularly true
in small  plants  where the maintenance staff usually does  not
include full-time  instrumentation specialists, and  where contract
instrumentation   specialists  are  unavailable.   The  designer
must consider  whether proper operation and maintenance  will  be
available before  incorporating  instrumentation into  a  plant's
design.   In larger plants  (20 to 30 MGD  [0.88 to 1.3 m3/s]),
0/M staffs  should include full-time instrumentation specialists.

Aside  from  the cost evaluations and  0/M  requirements  discussed
above,  several  factors  will influence  the  selection  of
instruments  for a  specific application.  These include:

     •  Characteristics  of  the  process  fluid,  particularly  the
        water,  grease,  grit,   and gas  content  and  the  degree  of
        variability  in the influent material from day to day.

     o  Configuration  of process  piping,  channels, or  vessels.

     •  Requirements relating  to  instrument measurement range  and
        accuracy.

     •  Utility  availability  (instrument   air,   purge  water,
        electricity, etc.).

The instrumentation information presented  in Tables 17-1  through
17-12  is  applicable to  a wide  variety  of  sludge  treatment  and
disposal  processes.  The tables list  the  process and  process
variables,  the  measurements,  and  the suggested instruments
for  individual process  steps in treatment  and disposal.   The
specific instruments listed  in these tables  should be  considered
as candidates,  not as  specific recommendations.

More  detailed information  about   the  various instruments  is
available including  illustrations,  descriptions,   and  lists  of
manufacturers  (2).   Note,  however, that  although  many specific
instruments   are  used  in both sludge  processing and  in  conven-
tional industrial  processes,  some manufacturers are not active in
the wastewater field.   The  suitability  of their instruments  for
sludge applications  has  not been  established.
                              17-2

-------
                                     TABLE 17-1

                                     THICKENING
    Process arid process
	variables

 Gravity Thickener

   Feed  sludge
   Dilution water
   Tank sludge depth

   Supernatant
   Collection equipment

   Thickened sludge
  Measurements
     Flow
                                    F.I ow
                                    Blanket  level
   Polymer or chemicals
 Flotation Thickener

   Feed sludge
   Thickened float or
     sludge
   Subnatant
   polymer or chemicals
(See  Table  17-12,
     Torque or  power
       draw
     Flow
     Pressure

     Density


     Level


     Flow


     Weight



     Flow


     Pipe  empty

     Flow
                              Suggested  instruments
        Venturi with diaphragm sensors
        Magnetic
        Doppler
        We ir
        Pump displacement
        Venturi
        Magnetic
        Ultrasonic
        Propeller
        Orifice
        Optical
        Ultrasonic
Sidestreams)
        Shearpin
        Ammeter
        Magnetic
        Doppler
        Pump displacement
        Bourdon with cylindrical seal
        Diaphragm
        Nuclear
        Optical
        Ultrasonic
        Tape and float
        Capacitance
        Ultrasonic
        Magnetic
        Rotameter
        Pump displacement
        Static
        Venturi with diaphragm sensors
        Magnetic
        Doppler
        Capacitance
        Nuclear
        Venturi with diaphragm sensors
        Magnetic
        Doppler
        Pump displacement
(See Table 17-12,  Sidestreams)
     Level                Capacitance
                          Ultrasonic
                          Tape  and  float
     Flow                 Magnetic
                          Rotameter
                          Pump'' displacement
                                    Weight
                                                         Static
                                           17-3

-------
                                     TABLE 17-1

                             THICKENING (Continued)

   Process and process
        variables              Measurements                 Suggested instruments
Flotation Thickener  (continued)

  Dissolution system (assuming      Flow                Venturi
    subnatant recycle                                  Magnetic
    or full make-up)                                   Ultrasonic
                                                      Propeller
                                                      Orifice
                                   Pressure            Bourdon
                                                      Diaphragm
  Air supply                       Flow                Rotameter
                                                      Pitot tube
                                   Pressure            Bourdon
                                                      Diaphragm

Centrifuge

  Feed sludge                      Flow                Magnetic
                                                      Pump displacement
                                   Pipe empty          Capacitance
                                                      Nuclear
  Centrate                   (See  Table 17-12, Sidestreams)
  Thickened sludge                 Level               Ultrasonic
                                   Flow                Pump displacement
                                   Pressure            Bourdon with cylindrical  seal
                                   Density             Nuclear
                                                      Optical
                                                      Ultrasonic
  Centrifuge operation             Vibration           Accelerometer
                                                      Displacement probes
                                   Torque or power     Ammeter
                                     draw
  Polymers or chemicals            Level               Capacitance
                                                      Ultrasonic
                                                      Tape and float
                                   Flow                Magnetic
                                                      Rotameter
                                                      Propeller
                                                      Pump displacement
                                   Weight              Static
                                         17-4

-------
                                     TABLE 17-2
                                  'STABILIZATION
   Process and process
        variables
                                  Measurements
                             Suggested instruments
Anaerobic Digesters
  Feed sludge
  Digester liquid surface
    Floating cover
    Fixed cover
    Gas holding cover
  Digester contents

  Circulating sludge
  Digested sludge
  Supernatant
  Digester gas
  Hot water heating system

  Atmospheric monitoring
    Flow

    Pressure
    Density
    Level
    Level
    Level
    Temperature
    pH and ORP
    Pressure
    Temperature
    pH and ORP
    Flow
                                    Pressure

                                    Density
     pH  and  ORP
(See  Table  17-12,
     Flow
     Pressure
     Compos ition
     Heat  value
     Pressure
     Temperature
     Hydrocarbons
     Odors
       Venturi wjth diaphragm sensors
       Magnetic
       Doppler
       Bourdon with cylindrical seal
       Nuclear
   •  •  Optical
       Ultrasonic

       Tape  (attach to cover)
       Bubbler with nitrogen purge
       Diaphragm
       Capacitance
       Ultrasonic
       Diaphragm  (differential pressure)
       RTD
       Portable selective-ion
       Bourdon with cylindrical seal
       RTD (pad type)
       Selective-ion  (pipeline mtg)   "
       Venturi with diaphragm sensors
       Magnetic
       Doppler
       Pump  displacement
       Bourdon with cylindrical seal
       RTD (pad type)         '
       Nuclear
       Optical
       Ultrasonic
       Portable selective-ion
Sidestreams)
       Orifice
       Turbine
       Vortex
       Diaphragm
       Chromatograph
       Calorimeter
       Bourdon
       RTD
       Catalytic
       Portable olefactometer
                                          17-5

-------
                                     TABLE 17-2

                            STABILIZATION  (Continued)
   Process and process
        variables
                                Measurements
                                                             Suggested  instruments
Aerobic Digesters
  Feed sludge
  Digester liquid surface
  Digester contents
  Sedimentation tank

  Supernatant
  Recycled sludge
  Digested sludge
Lime Treatment
  Feed Sludge
        Flow


        Pressure
        Density

        Level
        Temperature
        Suspended  solids
        Dissolved  oxygen
        pH  or  ORP
        Blanket  level

(See  Table  17-12,  Sidestreams)
        Flow
                                  Density
                                  Flow
        Pressure
        Temperature
        Density


        pH and ORP


        Flow
  Treated sludge
        Pressure
        Density


        pH and  ORP
        Flow

        Pressure
        Temperature
        Density


        pH and  ORP
 Venturi  with  diaphragm  sensors
 Magnetic
 Doppler
 Bourdon  with  cylindrical  seal
 Nuclear
 Optical
 Bubbler
 Diaphragm
 Capacitance
 Ultrasonic
 RTD
 Optical
 Polarographic
 Galvanic
 Thallium
 Portable selective-ion
 Optical
 Ultrasonic
)
 Venturi  with  diaphragm  sensors
 Magnetic
 Doppler
 Nuclear
 Venturi  with  diaphragm  sensors
 Magnetic
 Doppler
 Pump displacement
 Bourdon  with  cylindrical  seal
 RTD (pad type)
 Nuclear
 Optical
 Ultrasonic
 Selective-ion (pipeline mtg)
 Magnetic
 Doppler
 Venturi with  diaphragm  seal
 Pump displacement
 Bourdon with  cylindrical  seal
 Nuclear
 Optical
 Ultrasonic
 Portable selective-ion
 Magnetic
 Pump displacement
 Bourdon with  cylindrical  seal
 RTD (pad type)
 Nuclear
 Optical
 Ultrasonic
 Selective-ion (pipeline mtg)
                                          17-6

-------
                                     TABLE 17-2

                            STABILIZATION (Continued)
   Process and  process
        variables
                               Measurements
                                                            Suggested  instruments
Lime Treatment (continued)
  Chemicals
Chlorine Treatment
  Feed sludge
  Treated sludge
  Chemicals
 Level
 Flow

 Weight
 Flow
 Pressure
 Density


 Flow

 Pressure
 Temperature
 Density

 PH
. Flow

 Pressure
 Weight
Ultrasonic
Magnetic
Pump displacement
Static
Venturi with diaphragm  sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical  seal
Nuclear
Optical
Ultrasonic
Magnetic
Doppler
Bourdon with cylindrical  seal
RTD
Nuclear
Optical
Selective-ion (pipeline mtg)
Rotaraeter
Orifice
Bourdon with diaphragm  seal
Static
                                         17-7

-------
                                      TABLE 17-3

                                    DISINFECTION
   Process and process
        variables
                                Measurements
                           Suggested instruments
Pasteurization

  Feed sludge
  Pasteurization system





  Pasteurized sludge


  Steam supply






Electron _lrr_adiation

  Feed Sludge
  Irradiation system E-beam
    monitoring
  Irradiated sludge
Level
                                  Flow
Pressure
Density


Pressure

Temperature
Time

Level
Flow
Pressure
Flow

Pressure

Temperature



Level
                                  Flow
                                  Pressure
                                  Temperature
                                  Density
Power draw
Flow
                                  Pressure
                                  Temperature
Bubbler
Diaphragm
Capacitance
Ultrasonic
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Bourdon with flush diaphragm
  seal
RTD (pad type)
Digital
Synchronous motor
Ultrasonic
Pump displacement
Bourdon with cylindrical seal
Nozzle
Orifice
Bourdon with steam service
  siphon
RTD
Bubbler
Diaphragm
Capacitance
Ultrasonic
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Ultrasonic

Ammeter
Venturi with diaphragm seal
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
                                          17-8

-------
                                     TABLE 17-3

                             DISINFECTION  (Continued)
   Process and process
        variables
                                Measurements
                           Suggested  instruments
Electron Irradiaticm
  (continued)
    Cooling air
Gamma Irradiation
  Feed sludge
  Irradiation system
    Radiation
  Irradiated sludge
Flow
Flow loss
Level
                                  Flow
Pressure
Density


Dosage
Safety


Flow
                                  Pressure
                                  Temperature
                                  Radiation
Pitot tube
Vane
Differential pressure
Thermal

Bubbler
Diaphragm
Capacitance
Ultrasonic
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical  seal
Nuclear
Optical
Ultrasonic
Geiger counter
Geiger counter
Dosimeter
Badge
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Transport displacement
Bourdon with cylindrical  seal
RTD  (pad type)
Geiger counter
                                          17-9

-------
                                      TABLE 17-4

                                    CONDITIONING
   Process and process
        variables
                                Measurements
                                                             Suggested instruments
Inorganic Chemical Conditioning
  Feed sludge                     Flow
  Chemicals
    (aluminum sulfate,
      aluminum chloride,
      lime ferric chloride,
      ferrous sulfate)
Organic Chemical Conditioning
  Feed sludge
  Polymers
Non-Chemical Additions
  Feed sludge
  Miscellaneous materials
    (ash, pulverized coal,
    sawdust, wastepaper
                                  Pressure
                                  Density
Level

Flow


Pressure
Weight



Flow



Pressure

Density


Level


Flow


Pressure
Weight


Flow



Pressure
Density



Level

Weight
                      Venturi with diaphragm sensors
                      Magnetic
                      Doppler
                      Pump displacement
                      Bourdon with cylindrical seal
                      Nuclear
                      Optical
                      Ultrasonic
Ultrasonic
Tape and float
Magnetic
Doppler
Pump displacement
Bourdon with chemical seal
Static
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal

Nuclear
Optical
Ultrasonic
Capacitance
Ultrasonic
Tape and float
Magnetic
Rotameter
Pump displacement
Bourdon with cylindrical seal
Static
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic

Capacitance
Ultrasonic
Static
Mass flow
                                        17-10

-------
                                      TABLE 17-4

                             CONDITIONING (Continued)
   Process and process
        variables
                                Measurements
                                                             Suggested  instruments
Thermal Conditioning
  Feed sludge
  Conditioning
  Solids separation
   Flow
   Pressure
   Temperature
   Density


   Pipe empty

   Pressure
   Temperature

   Level
   Blanket  level
       Venturi  with  diaphragm sensors
       Magnetic
       Doppler
       Pump displacement
       Bourdon  with  cylindrical  seal
       RTD (pad type)
       Nuclear
       Optical
       Sonic
       Capacitance
       Nuclear
       Bourdon with  cylindrical
       RTD (pad type)
       Thermocouple
       Ultrasonic
       Optical
       Ultrasonic
                                                                                 seal
  Atmospheric monitoring

  Decant liquor
  Conditioned sludge
  Steam supply
  Air supply
 Elutriation
   Feed  sludge
   Odors

(See  Table  17-12,
   Flow
   Pressure
   Temperature
   Flow

   Pressure
   Temperature
   Flow
                                  Pressure
   Flow

   Pressure
   Density
       Portable olefacttometer
       Panel
Sidestreams)
       Venturi with diaphragm sensors
       Magnetic
       Doppler
       Pump displacement
       Bourdon with cylindrical seal
       'RTD (pad type)
       Nozzle
       Orifice
       Bourdon with steam siphon
       RTD
       Venturi
       Rotometer
       Orifice
       Bellows
       Diaphragm
       Venturi with diaphragm sensors
       Magnetic
       Bourdon with cylindrical seal
       Nuclear
       Optical
       Ultrasonic
                                        17-11

-------
                                    TABLE 17-4

                            CONDITIONING  (Continued)
   Process  and  process
        variables
     Measurements
Suggested  instruments
Elutriation  (continued)

  Solids separation
  Elutriate
  Conditioned  sludge
  Wash water
       Level
                                 Blanket level
                            Bubbler
                            Diaphragm
                            Ultrasonic
                            Optical
                            Ultrasonic
(See  Table 17-12, Sidestreams)
                            Venturi  with diaphragm sensors
                            Magnetic
                            Doppler
                            Pump displacement
       Pressure              Bourdon  with cylindrical seal
       Flow                  Venturi
                            Magnetic
                            Rotameter
                            Propeller
                                       17-12

-------
                                      TABLE  17-5

                                     DEWATERINC
   Process and process
        variables
                                 Measurements
                                 Suggested  instruments
Drying beds

  Feed sludge
  Bed contents

  Dewatered sludge
  Drainage and surface
    runoff

  Weather
  Atmospheric monitoring

Drying Lagoons

  Feed sludge
Flow
Pressure
Density


Moisture content

Flow (volume)
Weight
Moisture content
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Portable ohmmeter
Lab test
Transport displacement
Static
Portable ohmmeter
Lab test
  (See Table 17-12, Sidestreams)
                             Anamometer
Wind speed (15 ft (4.6 m))
  above ground

Wind direction (15 ft
  (4.6 m)) above
  ground

Temperature,  dry bulb
  (5 and 25 ft (1.5 and
  and 7.6 m))  above
  ground

Relative humidity
Rainfall

Solar radiation
Odors



Flow
                             Pressure
                             Density
                                                          Wind  vane
                                                          RTD with  solar  shield
                                                          Thermistor  with solar  shield


                                                          RTD with  lithium chloride
                                                            cloth (wet bulb tempera-
                                                            ture)
                                                          Tipping bucket

                                                          Thermopile
                                                          Portable  olefactometer
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
                                         17-13

-------
   Process and process
        variables
                                      TABLE 17-5

                              DEWATERING (Continued)
                                 Measurements
                                 Suggested  instruments
Drying Lagoons (Continued)

  Lagoon contents

  Harvested sludge

  Supernatant and
    surface runoff

  Weather
  Atmospheric
    monitoring

Centrifugal Dewatering

  Feed sludge
  Centrate
  Centrifuge operation


  Dewatered sludge
  Polymers or chemicals
Moisture content

Flow (volume)
We ight

  (See Table 17-12,
         Portable  ohmmeter
         Lab test
         Transport displacement
         Static
Wind speed (15 ft
  (4.6 m)) above ground

Wind direction (15 ft
  (4.6 m above ground

Temperature (5 and 25
  ft (1.5 and 7.6 m))
  ground

Relative humidity
Sidestreams)

         Anemometer
Rainfall
Solar radiation
Odors
Flow



Pressure
Density


Pipe empty
                                                         Wind vane


                                                  ft     RTD with solar shield
                                                  above  Thermistor with solar shield


                                                         RTD with lithium chloride
                                                           cloth (wet bulb temperature)
                                                         Tipping bucket
                                                         Thermopile
                                                         Portable olefactometer
         Venturi with diaphragm  sensors
         Magnetic
         Doppler
         Pump displacement
         Bourdon with cylindrical  seal
         Nuclear
         Optical
         Ultrasonic
         Capacitance
         Nuclear
  (See Table 17-12, Sidestreams]
Torque of power draw
Vibration

Flow (volume)

Weight

Moisture content

Level



Flow
                             Pressure
                             We ight
         Ammeter
         Accelerometer
         Displacement probes
         Pump displacement
         Transport displacement
         Static
         Mass flow
         Portable ohmmeter
         Lab test
         Capacitance
         Ultrasonic
         Tape and float
         Magnetic
         Rotameter
         Propeller
         Pump displacement
         Bourdon with chemical  seal
         Static
                                        17-14

-------
                                     TABLE 17-5

                              DEWATERING (Continued)
   Process and process
        variables
                                Measurements
                                                             Suggested instruments
Filtration Dewatering

  Feed sludge
  Vacuum filter
    Operation
    Filtrate
    Spent wastewater
      and rejected
      feed sludge
    Washwater
  Belt filter presses
    Operation
    Filtrate
    Spent wastewater
      and rejected
      feed sludge
    Washwater
  Recessed plate filter
    presses
      Operation
      Filtrate
      Spent washwater
        and reject
        feed sludge
Flow



Pressure
Density


Pipe empty



Level

Pressure
Speed
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical  seal
Nuclear
Optical
Ultrasonic
Capacitance
Nuclear
Capacitance
Ultrasonic
Bourdon with chemical  seal
Reluctance pick-up
  (See Table 17-12,  Sidestreams)

  (See Table 17-12,  Sidestreams)
Flow
                             Pressure
Pressure
                             Speed
Venturi
Rotameter
Propeller
Orifice
Bourdon
                             Bourdon or bellows with
                              chemical seal
                             Diaphragm
                             Reluctance
  (See Table 17-12,  Sidestreams)

  (See Table 17-12,  Sidestreams)

Flow
                             Pressure
Pressure
Venturi
Rotameter
Propeller
Orifice
Bourdon
Bourdon with cylindrical seal
  (See Table 17-12,  Sidestreams)

  (See Table 17-12,  Sidestreams)
                                        17-15

-------
                                             17-5

                             DEWATERINQ  (Continued)
   Process and process
        variables

Filtration Dewatering
  (Continued)

    Dewatered  sludge
    Polymers or chemicals
Cyclonic Separation
  Feed wastewater solids
  Overflow
  Underflow

Screening

  Feed wastewater
      Measurements



   Flow

   Height

   Moisture  content

   Level


   Flow
   Pressure
   Weight

   Flow

   Pressure
   Density
    Suggested instruments
Pump displacement
Transport displacement
Static
Mass flow
Portable ohmeter
Lab test
Capacitance
Ultrasonic
Tape and float
Magnetic
Rotameter
Propeller
Pump displacement
Bourdon with chemical seal
Static


Magnetic
Doppler   •-
Bourdon with cylindrical  seal
Nuclear (sludge system only)
Ultrasonic
     (See  Table  17-12, Sidestreams)
   Flow  (volume)                Transport displacement
  Feed wastewater solids
  Automatic bar screens

  Hydraulic sieve bends

  Moving screens



  Screened liquid
   Level

   Flow



   Level


   Flow



   Pipe  empty

   Torque  or power  draw

   Level

   Level

   Speed

(See Table 17-12, Sidestreams)
Bubbler
Diaphragm
Venturi
Magnetic
Doppler
Weirs and flumes
Bubbler
Diaphragm
Ultrasonic
Venturi with diaphragm sensors
Magnetic  ....'.
Doppler
Pump displacement
Capacitance
Nuclear
Shear pin
Ammeter
Bubbler
Diaphragm
Bubbler
Diaphragm
Bubbler
Diaphragm
                                       17-16

-------
                                      TABLE 17-6
                                    HEAT DRYING
 Process and process variables
                                      Measurements
Flash drying
  Feed sludge
  Drying operation
  Dried sludge
  Hot air furnace
    Burner operation
    Fuel
    Combustion air
    Heated air
    Fan monitoring
    Scrubber water
Direct rotary dryer
  Feed sludge
  Drying operation
  Dried sludge
Flow, volume

Weight

Moisture content

Pipe empty

Temperature
Flow, volume
Temperature
Weight

Moisture content

Flame monitoring
Flow

Flow

Pressure

Temperature
Temperature
Flow loss
                              Suggested instruments
            Pump displacement
            Transport displacement
            Static
            Mass flow
            Portable ohmmeter
            Lab test
            Capacitance
            Nuclear
            RTD  (pad type)
            Transport displacement
            RTD  (pad type)
            Static
            Mass flow
            Portable ohmmeter
            Lab test

            Ultravilot scanner
            Pitot tube
            Orifice
            Positive displacement
            Pitot tube
            Orifice plate
            Diaphragm
            Bellows
            RTD
            Thermocouple
            Vane
            Differential pressure
            Thermal
                                Vibration
Flow, volume
Weight
             Accelerometer
(See  Table  17-12,  Sidestreams)

             Transport displacement
Moisture content

Temperature
Speed
Torque or power draw

Flow, volume
Temperature
Weight

Moisture content
             Static
             Mass  flow
             Portable ohmmeter
             Lab test
             RTD  (pad type)
             Reluctance
             Shearpin
             Ammeter
             Transport displacement
             RTD  (pad type)
             Static
             Mass  flow
             Portable ohmmeter
             Lab test
                                         17-17

-------
                                      TABLE 17-6
                              HEAT DRYING  (Continued)
 Process and process variables
Direct rotary dryer (continued)
  Hot air furnace
    Burner operation
    Fuel
    Combustion air
    Heated air
  Fan monitoring
  Scrubber water
Indirect and direct-indirect
  rotary dryers
    Feed sludge
    Drying operation
    Dried sludge
    Hot air furnace
      Burner operation
      Fuel
      Combustion air
      Heated air
    Fan monitoring
    Scrubber water
      Measurements
Flame monitoring
Flow
Flow

Pressure
Temperature
Temperature
Flow loss

Vibration
         (See Table 17-12,


Flow,  volume

Weight

Moisture content

Tempera ture
Speed
Torque or power draw

Flow,  volume
Temperature
Weight

Moisture content

Flame monitoring
Flow
Flow

Pressure

Temperature
Temperature
Flow loss
                              Suggested instruments
                                Vibration
Ultravilot scanner
Pitot tube
Orifice
Vortex
Positive displacement
Pitot tube
Orifice
Diaphragm
RTD
Thermocouple
Vane
Differential pressure
Thermal
Accelerometer
Sidestreams)
Pump displacement
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
RTD (pad type)
Reluctance
Shearpin
Ammeter
Transport displacement
RTD (pad type)
Static
Mass flow
Portable ohmmeter
Ultraviolet scanner
Pitot tube
Orifice
Vortex
Positive displacement
Pitot tube
Orifice
Bellows
Diaphragm
RTD
Thermocouple
Vane
Differential pressure
Thermal
Accelerometer
         (See Table 17-12,  Sidestreams)
 See Table 17-12, Sidestreams.
                                         17-18

-------
                                     TABLE 17-6
                             HEAT DRYING  (Continued)
 Process and process variables
Incinerators
Torodial_dry_ers_
  Liquid or dewatered solids
    storage
  Dewatering
  Feed sludge
  Drying operation
  Dried sludge
                                      Measurements
                              Suggested instruments
  Hot air furnace
    Burner operation
    Fuel
    Combustion air
    Heated air
  Fan monitoring
  Scrubber water
Spray drying
  Feed sludge
  Drying operation
  Dried sludge
   Hot air supply
         (See Table 17-7,  High Temperature Processes)
         (See Table 17-11,  Storage)
         (See Table
Flow, volume
Weight

Moisture content

Temperature
Flow, volume
Temperature
Weight

Moisture content

Flame monitoring
Flow
Flow

Pressure

Temperature
Temperature
Vibration
Flow loss
17-5,  Dewatering)
       Transport displacement
       Static
       Mass flow
       Portable ohmmeter
       Lab test
       RTD (pad type)
       Transport displacement
       RTD (pad type)
       Static
       Mass flow
       Portable ohmmeter
       Lab test

       Ultraviolet scanner
       Pitot tube
       Orifice
       Vortex
       Positive displacement
       Pitot tube
       Orifice
       Bellows
       Diaphragm
       RTD
       Thermocouple
       Accelerometer
       Vane
       Differential pressure
       Thermal
         (See Table 17-12,  Sidestreams)
Flow
Pressure
Density

Temperature
Flow, volume
Temperature
Weight

Moisture content
Temperature
       Pump displacement
       Bourdon with cylindrical seal
       Nuclear
       Optical
       Ultrasonic
       RTD (pad type)
       Transport displacement
       RTD (pad type)
       Static
       Mass flow
       Portable ohmmeter
       Thermocouple
 See Table 17-12, Sidestreams.
 "'see Table 17-7, High Temperature Processes.
               'See Table  17-11, Storage
               See Table  17-5, Dewatering.
                                        17-19

-------
                                    TABLE 17-6
                            HEAT DRYING (Continued)
 Process  and process  variables
      Measurements
                              Suggested instruments
        extraction
  Feed  sludge
  Cooled sludge
  Extraction system

  Dried sludge
  Product water
  Hot air

  Chilled water
Multiple-effect evaporator
  Feed sludge
  Fluidizing system
    Fluidizing tank
Flow
Pressure
Temperature
Density

Temperature
Pressure
Temperature
Flow,  volume
Temperature
Weight

Moisture content

Flow

Pressure
Temperature
Suspended solids
Chemical oxygen demand
Temperature

Flow

Pressure
Temperature

Flow
                                Pressure
                                Temperature
                                Density
Level
Temperature
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Ultrasonic
RTD (pad type)
Bourdon with chemical seal
RTD
Transport displacement
RTD (pad type)
Static
Mass flow
Portable ohmmeter
Lab test
Venturi
Magnetic
Pump displacement
Bourdon with chemical seal
RTD
Optical
TOC analyzer
RTD
Thermocouple
Rotameter
Propeller
Orifice
Bourdon
RTD
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD  (pad type)
Nuclear
Optical
Ultrasonic

Ultrasonic
RTD  (pad type)
                                       17-20

-------
                                    TABLE 17-6
                            HEAT DRYING (Continued)
 Process and process variables
Multiple-effect evaporator
  (continued)  ,           T~
  Fluidizing system (continued)
    Fluidizing pump
      Measurements
                              Suggested instruments
    Feed tank
    Feed pump
  Evaporation system
  Dried sludge
  Condensate
  Recycled oil
  Steam supply
Pressure
Temperature
Level
Temperature
Flow
Pressure
Temperature
Pressure
Temperature
Flow, volume
Temperature
Weight

Moisture content

Flow

Pressure
Temperature
Level
Flow

Pressure
Temperature
Flow

Pressure
Temperature
Bourdon with cylindrical  seal
RTD (pad type)
Ultrasonic
RTD (pad type)
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical  seal
RTD (pad type)
Bourdon with chemical seal
RTD (pad type)
Transport displacement
RTD (pad type)
Static
Mass flow
Portable ohmmeter
Lab test
Rotameter
Orifice
Bourdon with chemical seal
RTD (pad type)
Ultrasonic
Orifice
Pump displacement
Bourdon with diaphragm seal
RTD
Nozzle
Orifice
Bourdon with steam siphon
RTD
                                        17-21

-------
                                      TABLE 17-7
                            HIGH TEMPERATURE PROCESS
   Process  and  process
        variables
                                   Measurements
                                                         Suggested instruments
Incineration
  Feed  sludge
  Furnace  operation
    Multiple-hearth
  Fluid-bed
  Electric
  Single-hearth  cyclonic
  Ash
  Combustion  air
  Recycled  flue  gas
  Afterburner
    Multiple-hearth  furnace

    Electric  furnace

  Exhaust  (stack gas)
Flow (volume)

Temperature
Weight

Moisture content
Temperature
Speed
Torque of power
  draw
Flame monitoring
Pressure
Temperature
Flame monitoring
Temperature
Speed
Power draw
Temperature
Speed
Torque or power
  draw
Flame monitoring
Flow (volume)
Temperature
Weight

Flow loss
Pressure

Temperature
Temperature
Temperature
Flame monitoring
Temperature
Power draw
Pressure

Temperature
Oxygen content


Opacity
Other measurements
  as required by
  local air quality
  management dis-
  tricts
Pump displacement
Transport displacement
RTD
Static
Mass flow
Portable ohmmeter
Lab test

Thermocouple
Reluctance
Shear pin
Ammeter
Ultraviolet scanner
Bourdon with diaphragm  seal
Thermocouple
Ultraviolet scanner
Thermocouple
Reluctance
Ammeter
Thermocouple
Reluctance
Shear pin
Ammeter
Ultraviolet scanner
Transport displacement
Thermocouple
Static
Mass flow
Vane
Differential pressure
Thermal
Diaphragm
Bellows
Thermocouple
Thermocouple
Thermocouple
Ultraviolet scanner
Thermocouple
Ammeter
Bellows
Diaphragm
Thermocouple
Paramagnetic
Catalytic
Ceramic
Optical
As required
                                        17-22

-------
                                     TABLE 17-7

                    HIGH TEMPERATURE PROCESS (Continued)
   Process  and  process
        variables
                                   Measurements
                                                         Suggested  instruments
Incineration (continued)
  Heat recovery  system
    Flue gas
    Boiler


  Steam produced
  Scrubbing water
  Fuel
    Electric furnace
    Other furnaces
   Temperature
   Level
   Pressure
   Temperature
   Flow
Thermocouple
Float (cage mounted)
Bourdon
Thermocouple
Nozzle
Orifice
   Pressure              Bourdon with steam siphon
   Temperature           Thermocouple
(See  Table 17-12, Sidestreams)
Starved Air Combustion
  Feed sludge
  Furnace operation
  Ash
  Combustion air
  Afterburner
   Power  draw
   Level

   Flow

   Pressure
   Flow  (volume)

   Temperature
   Weight

   Moisture  content

   Pressure

   Temperature

   Flow  (volume)
   Temperature
   Weight

   Flow  loss


   Pressure
   Temperature
   Temperature
   Flame  monitoring
Ammeter
Diaphragm
Tape and float
Positive displacement
Orifice
Bourdon
Bellows
Diaphragm
Pump displacement
Transport displacement
RTD
Static
Mass flow
Portable ohmmeter
Lab test
Bellows
Diaphragm
Thermocouple


Transport displacement
Thermocouple
Static
Mass flow

Vane
Differential pressure
Thermal
Bellows
Diaphragm
RTD
Thermocouple
Ultraviolet scanner
                                       17-23

-------
                                     TABLE  17-7

                     HIGH TEMPERATURE PROCESS (Continued)
   Process and process
        variables
Starved Air__Cgmbus_tion (continued)
  Exhaust (stack gas)
                                    Measurements
                                                         Suggested instruments
  Heat recovery system
    Flue gas
    Boiler
    Steam produced
  Scrubbing water
  Fuel
    Flue gas for after-
      burner
    Supplemental fuel
   Pressure

   Temperature
   Oxygen  content
                                  Opacity

                                  Other measurements
                                    as rquired by
                                    local  air quality
                                    management dis-
                                    tricts
   Temperature
   Level
   Pressure
   Temperature
   Flow

   Pressure
   Temperature
Bellows
Diaphragm
Thermocouple
Paramagnetic
Catalytic
Ceramic
Optical

As required
Thermocouple
Float (cage mounted)
Bourdon
Thermocouple
Nozzle
Orifice
Bourdon
RTD
(See  Table  17-12,  Sidestreams)
Watergate Furnace
  Feed scum
  Furnace operation
  Exhaust (stack gas)
   Pressure

   Level

   Flow

   Pressure





   Flow
   Pressure
   Density


   Level

   Temperature
   Flame monitoring
   Pressure

   Temperature
Bellows
Diaphragm
Diaphragm
Tape and float
Orifice
Positive displacemeent
Bourdon
Bellows
Diaphragm
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Diaphragm
Ultrasonic
Thermocouple
Ultraviolet scanner
Bellows
Diaphragm
Thermocouple
                                       17-24

-------
                                     TABLE 17-7

                     HIGH TEMPERATURE PROCESS (Continued)
   Process and process
        variables
                                    Measurements
                                                         Suggested instruments
Watergate Furnace (Continued)

  Exhaust (stack gas)
    (continued)
   Oxygen  content
Paramagnetic
Catalytic
Ceramic
Optical

As required
  Scrubbing water
  Fuel
   Opacity

   Other  measurements
     as required by
     local  air quality
     management dis-
     tricts

(See  Table  17-12, Sidestreams)
   Level                 Diaphragm
                        Tape and float
   Flow                  Orifice
                        Positive displacement
   pressure              Bellows
                        Diaphragm
Co-Combustion with Hunicipal  Refuse

  Feed sludge

    Liquid state                  Flow
    Dewatered state
  Municipal refuse
  Furnace operation
    Grate fired

    Multiple-hearth
   Pressure
   Density



   Flow (volume)
   Weight

   Moisture  content


   Flow (volume)
   Weight

   Moisture  content
   Temperature
   Flame monitoring
   Temperature
   Speed
   Torque or power
     draw
   Flame monitoring
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical  seal
Nuclear
Optical
Ultrasonic

Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test

Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
Thermocouple
Ultraviolet scanner
Thermocouple
Reluctance
Shear pin
Ammeter
Ultraviolet scanner
                                        17-25

-------
                                     TABLE 17-7

                    HIGH TEMPERATURE PROCESS  (Continued)
   Process and process
        variables
                                   Measurements
                                                         Suggested instruments
Co-Combustion with  Municipal  Refuse
  (continued)                 ™ ™"
  Furnace operation (continued)
    Fluid-bed
  Ash
  Combustion air
  Recycled flue gas
  Afterburner
    Multiple hearth

  Exhaust (stack gas)
  Heat recovery system
    Flue gas
    Boiler
    Steam produced
  Scrubber water
Pressure

Temperature
Flame monitoring
Flow (volume)
Temperature
Weight

Flow loss
Pressure

Temperature
Temperature

Temperature
Flame monitoring
Pressure

Temperature
Oxygen content

Opacity
Other measurements
  as required by
  local air quality
  management dis-
  tricts
   Temperature
   Level
   Pressure

   Temperature
   Flow

   Pressure
(See  Table 17-12,
                        Bellows
                        Diaphragm
                        Thermocouple
                        Ultraviolet scanner
                        Transport displacement
                        Thermocouple
                        Static
                        Mass flow
                        Vane
                        Differential pressure
                        Thermal
                        Diaphragm
                        Bellows
                        RTD
                        Thermocouple
                        Thermocouple
                        Ultraviolet scanner
                        Bellows
                        Diaphragm
                        Thermocouple
                        Paramagnetic
                        Catalytic
                        Ceramic
                        Optical
                        As required
                      Thermocouple
                      Float  (cage mounted)
                      Bellows
                      Diaphragm
                      Thermocouple
                      Nozzle
                      Orifice
                      Bourdon  with  steam siphon
               Sidestreams)
                                        17-26

-------
                                      TABLE 17-8

                                      COMPOSTING
   Process and process
        variables
                                    Measurements
                                                          Suggested  instruments
Unconfined
  Windrow
    Feed sludge
    Composting
    Composted sludge
    Amendment or bulking
      agent
    Leachate and surface
      runoff
   Level

   Flow  (volume)
   Weight
   Moisture  content
   Temperature
   Moisture  content

   Aerobic condition

   Flow  (volume)
   Weight
   Moisture  content
   Flow  (volume)
   Weight
   Moisture  content
Capacitance
Ultrasonic
Transport displacement
Static
Portable ohmmeter
Portable thermometer
Portable ohmmeter
Lab test
Portable galvanic cell
Portable polarographic cell
Truck displacement
Static
Portable ohmmeter
Lab test

Transport displacement
Static
Portable ohmmeter
Lab test
(See  Table  17-12,  Sidestreams)
      Weather
  Atmosperic monitoring
  Aerated pile
    Feed sludge
   Wind  speed  (15  ft
    (4.6 m ) ) above
    ground
   Wind  direction
     (15 ft  (4.6 m))
     above  ground
   Temperature  (5  and
     25  ft  (1.5 and
     7.6 m)) above
     ground
   Relative  humidity

   Solar radiation
   Odors
   Level
   Flow (volume)
   Weight
   Moisture content
                                                        Anemometer
                                                        Wind  vane
                                                        RTD
                                                        Thermistor
           with solar shield
                                                        RTD with  lithium  chloride cloth
                                                            (wet  bulb  temperature)
                                                        Thermophile
                                                        Portable  olefactometer
Capacitance
Ultrasonic
Transport displacement
Portable ohmmeter
Lab test
                                          17-27

-------
                                     TABLE 17-8
                              COMPOSTING (Continued)
   Process and process
        variables
                                   Measurements
                                                         Suggested instruments
Unconfined (continued)
  Aerated pile
    Feed sludge
    Composting
    Composted sludge
    Aeration air
    Amendment or bulking
      agent
    Leachate and surface
      runoff
    Weather
   Level

   Flow  (volume)
   Weight

   Moisture  content

   Temperature

   Moisture  content

   Aerobic condition

   Flow  (volume)
   Weight
   Moisture  content

   Flow
   Flow  (volume)
   Weight

   Moisture  content
Capacitance
Ultrasonic
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
RTD
Portable thermometer
Portable ohmmeter
Lab test
Portable galvanic cell
Portable polarographic  cell
Transport displacement
Static
Portable ohmmeter
Lab test
Venturi
Pitot tube
Orifice

Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
(See  Table  17-12,  Sidestreams)
   Wind  speed  (15  ft     Anemometer
    Atmospheric monitoring
                                    (4.6 m)) above
                                    ground
                                  Wind  direction
                                    (15 ft  (4.6 m))
                                    above ground
                                  Temperature  (5 and
                                    25  ft (1.5 and
                                    7.6 m)) above
                                    ground
                                  Relative  humidity

                                  Rainfall
                                  Solar radiation
                                  Odors
                        Wind vane
                        RTD
                                  )
                        Thermistor)
                                    with solar shield
                        RTD with lithium chloride cloth
                           (wet bulb temperature)
                        Tipping bucket
                        Thermopile
                        Portable olefactometer
                                        17-28

-------
                                     TABLE 17-8

                              COMPOSTING  (Continued)
   Process and process
        variables
                                   Measurements
                                                         Suggested instruments
Confined Systems
  Feed sludge
  Composting
  Composed sludge
  Amendment or bulking
    agent
  Atmospheric monitoring
Level

Flow (volume)
Weight

Moisture content

Temperature
Moisture content
Aerobic condition

Level

Flow (volume)
Weight

Moisture content
Level

Flow (volume)
Weight

Moisture content

Odors
Capacitance
Ultrasonic
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test

RTD
Portable ohmmeter
Portable galvanic cell
Portable polarographic  cell

Capacitance
Ultrasonic
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test


Capacitance
Ultrasonic
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test

Portable olefactometer
                                        17-29

-------
                                  TABLE 17-9

                  MISCELLANEOUS CONVERSION PROCESSES
   Process  and  process
        variables
Fixation

  Feed sludge
  Fixed sludge
Encapsulation
  Feed sludge


  Polyethlene  system


  Asphalt system

  Encapsulated sludge

Earthworm Conversion

  Feed sludge




  Castings (egesta)
                                   Measurements
Flow (volume)
Moisture content


Moisture content
Flow (volume)
Moisture content

Pressure
Temperature

Temperature

Flow (volume)
Flow (volume)
Temperature
Moisture content
Flow (volume)
Moisture content
                       Suggested instruments
Transport displacement
Portable ohmmeter
Lab test

Portable ohmmeter
Lab test
Transport displacement
Portable ohmmeter
Lab test
Bellows with  diaphragm seal
RTD
Thermocouple
RTD
Thermocouple
Transport displacement
Transport displacement
Portable thermometer
Portable ohmmeter
Lab test

Transport displaceeraent
Portable ohmmeter
Lab test
                                     17-30

-------
                                     TABLE 17-10

                                 TRANSPORTATION
   Process and process
        variables
                                    Measurements
                        Suggested  instruments
Pumping

  Centrifugal and torque
    flow pumps

      Variable speed drive
      Pumped sludge
  Positive displacement
    pumps
      Variable speed drive
      Pumped sludge
  Pipelines

    Corrosion, electrolytic
    Pig location
Conveying

  Continuous belt




  Positive displacement

  Pneumatic ejection

  Open screw



Trucking



Barging



Railroad Cars
Speed

Vibration

Flow



Pressure
Empty pipe
Speed
Flow
Pressure
Empty pipe
Power draw
Flow
Underspeed
Level (volume
  overload)
Weight

Underspeed

Flow (volume)

Level (volume
  overload)
Underspeed

Flow (volume)

Weight

Level
Flow (volume]

Weight
Tachometer generator
Reluctance
Accelerometer

Venturi with diaphragm  sensors
Magnetic
Doppler
Bourdon with cylindrical  seal
Capacitance
Nuclear
Reluctance
Reluctance (revolution  counter)
Bourdon with cylindrical  seal
Capacitance
Nuclear
Ammeter
Venturi with diaphragm sensors
Magnetic
Pump displacement
Reluctance
Capacitance
Ultrasonic
Mass flow

Reluctance

Transport displacement

Capacitance
Ultrasonic
Reluctance

Transport displacement
Vehicle detection
Static

Bubbler
Diaphragm
Ultrasonic

Transport displacement
Vehicle detection
Static
                                        17-31

-------
                                     TABLE 17-11

                                       STORAGE
   Process and process
        variables
                                    Measurements
                                                          Suggested  instruments
Wastewater Treatment
  Sedimentation facilities
  Aeration reactors
  Imhoff and septic tanks
  Oxidation ditches
  Stabilization ponds
Density

Suspended solids
Blanket level

Suspended solids
Blanket level

Density

Suspended solids
Suspended solids
Nuclear
Optical
Optical
Optical
Ultrasonic

Optical

Optical
Ultrasonic
Nuclear
Optical

Optical

Optical
Wastewater Solids Treatment   (See Individual Process Tables)
Liquid Storage
  Holding Tanks
    Feed sludge
  Tank liquid surface
    Fixed cover
    Floating cover

  Tank contents


  Circulating sludge



  Discharged sludge
Flow
                                 Pressure
                                 Temperature
                                 Density
Level
Level

Temperature
pll

Pressure
Temperature
pH

Flow
                                 Pressure
                                 Temperature

                                 Density
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Ultrasonic
Bubbler
Diaphragm
Capacitance
Ultrasonic
Tape (attach to cover)

RTD
Portable selective-ion

Bourdon with cylindrical seal
RTD (pad type)
Selective-ion (pipeline mtg)

Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)

Nuclear
Optical
Ultrasonic
                                        17-32

-------
                                    TABLE  17-11

                                STORAGE  (Continued)
   Process and process
        variables
                                    Measurements
                                                         Suggested instruments
Confined Hoppers or Bins

  Feed sludge
  Hopper or bin contents
  Discharged sludge
  Atmosperic monitoring


Unconfined Stockpiles
  Feed sludge
  Stockpiled sludge


  Harvested sludge




  Weather
  Atmospheric monitoring
Flow (volume)
We ight

Moisture content

Level

Weight
Level (volume over
  load)
Flow (volume)
Weight

Moisture content
Hydrocarbons
Odors
Flow (volume)
Weight
Moisture content

Moisture content


Flow (volume)
Weight
Moisture content


Wind speed (15 ft
    (4.6 m) above
   ground

Wind direction
   (15 ft (4.6 m))
   above ground

Temperature  (5 and
   25 ft (1.5 and
   7.6 m)) above
   ground

Relative humidity

Rainfall
Solar radiation

Odors
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test

Capacitance
Ultrasonic
Static

Capacitance
Ultrasonic
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test

Catalytic
Portable olefactometer
Transport displacement
Static
Portable ohmmeter
Lab test

Portable ohmmeter
Lab test
Transport displacement
Static
Portable ohmmeter
Lab test

Anemometer
                                                       Wind  vane
                                                       RTD       )
                                                       Thermistor)
            with solar shield
                                                       RTD with  lithium  chloride
                                                         cloth  (wet  bulb temperature)

                                                       Tipping  bucket
                                                       Thermopile

                                                       Portable  olefactometer
                                        17-33

-------
                                     TABLE 17-11

                                STORAGE  (Continued)
   Process and process
        variables
                                    Measurements
                                                          Suggested  instruments
Facultative Sludge Lagoons

  Feed sludge
  Lagoon contents
  Harvested sludge
  Supernatant

  Weather
  Atmospheric monitoring

Anaerobic Sludge Lagoons

  Feed sludge
   Flow



   Pressure
   Density



   pli
   Conductivity
   Blanket  level

   Dissolved  oxygen


   Flow
                                 Pressure
                                 Density
       Venturi  with  diaphragm  sensors
       Magnetic
       Doppler
       Pump  displacement
       Bourdon  with  cylindrical seal
       Nuclear
       Optical
       Ultrasonic

       Portable selective-ion
       Portable conductivity probe
       Portable optical
       Portable ultrasonic
       Portable galvanic
       Portable polarographic

       Venturi  with  diaphragm  sensors
       Magnetic
       Doppler
       Pump  displacement
       Bourdon  with  cylindrical seal
       Nuclear
       Optical
       Ultrasonic
(See  Table  17-12,

   Wind  speed  (15  ft
     (4.6 m))  above
     ground
   Wind  direction
     (15 ft (4.6 m))
     above  ground

   Temperature (5  and
     25  ft  (1.5  and
     7.6 m)) above
     ground

   Relative humidity

   Rainfall
   Solar radiation

   Odor
   Flow
                                 Pressure
                                 Density
Sidestreams)

       Anemometer
                                                       Wind  vane
                                                       RTD
                                                       Thermistor
                    with solar  shield
                                                       RTO  with  lithium chloride cloth
                                                         (wet  bulb  temperature)
                                                       Tipping bucket
                                                       Thermopile

                                                       Portable  olefactometer
       Venturi with diaphragm sensors
       Magnetic
       Doppler
       Pump displacement
       Bourdon with cylindrical  seal
       Nuclear
       Optical
       Ultrasonic
  Lagoon contents
   Sludge blanket
       Portable optical
       Portable ultrasonic
                                        17-34

-------
                                     TABLE 17-11

                                STORAGE (Continued)
   Process and process
      variables
                                    Measurements
                                                          Suggested  instruments
Anaerobic Sludge Lagoons
  (Continued")
  Harvested sludge
  supernatant

  Weather
  Atmospheric monitoring

Aerat e d_Basin
  Feed sludge
  Basin contents
  Supernatant

Solid State Storage
  Drying sludge lagoons
   Flow
                                 Pressure
                         Venturi with diaphragm sensors
                         Magnetic
                         Doppler
                         Bourdon with cylindrical seal
(See  Table  17-12,  Sidestreams)
   Wind  speed  (15  ft
     (4.6  m))  above
     ground
   Wind  direction
     (15 ft  (4.6 m))
     above ground

   Temperature (5  and
      25 ft  (1.5 and
      7.6  m))  above
      ground

   Relative  humidity

   Rainfall
   Solar radiation

   Odors
   Flow
                                 Pressure
   Dissolved oxygen
                                 Flow
                                 Pressure
                                 Density
                                                       Anemometer
                                                       Wind  vane
                                                       RTD        )
                                                       Thermistor)
            with solar shield
RTD with lithium chloride cloth
  (wet bulb temperature)
Tipping bucket
Thermopile
Portable olefactometer
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Portable galvanic
Portable polarographic
Venturi with diaphragm sensors
Magnetic
Doppler
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
(See Table 17-12,  Sidestreams)
(See Table 17-5,  Dewatering)
                                        17-35

-------
                                   TABLE 17-12

                                  SIDESTREAMS
   Process and process
        variables
                                    Measurements
                                                         Suggested instruments
Thickening

  Gravity supernatant
  Flotation subnatant
  Centrifuge centrate
Stabilization
  Anaerobic digestion
    supernatant
  Aerobic digestion
    supernatant
Flow
Pressure
Suspended solids
Flow
Pressure
Suspended solids

Flow
                                 Pressure
                                 Suspended  solids
Level
                                 Flow
Pressure
Density
Sludge blanket

Suspended solids
PH
Chemical oxygen
  demand
Ammonia

Level
                                 Flow
                                 Pressure
Venturi with diaphragm sensors
Magnetic
Doppler
Weirs and flumes
Pump displacement

Bourdon with diaphragm sensors
Optical

Venturi with diaphragm sensors
Magnetic
Doppler
Weirs and flumes
Pump displacement

Bourdon with diaphragm sensors
Optical

Venturi with diaphragm sensors
Magnetic
Doppler
Weirs and flumes
Pump displacement
Bourdon with diaphragm sensors
Optical
Bubbler
Diaphragm
Tape and float

Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement

Bourdon with cylindrical seal
Nuclear
Optical
Sonic
Optical
Selective-ion (pipeline mtg)
TOC Analyzer

Selective-ion analyzer


Bubbler
Diaphragm
Float and tape
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement

Bourdon with cylindrical seal
                                        17-36

-------
                                     TABLE 17-12

                             SIDESTREAMS (Continued)
   Process and process
        variables
                                   Measurements
                         Suggested  instruments
Stabilization (continued)
  Aerobic digestion
    supernatant (continued)
  Atmospheric monitoring odors

Conditioning
  Thermal liquor
    (decant and filtrate)
  Elutrlation elutriate
Flow
                                 Pressure
                                 Temperature
                                 Density


                                 Suspended  solids
                                 pH
                                 Ammonia
Level
                                 Flow
Pressure
Temperature
Dens ity


Suspended solids
pH
Chemical oxygen
  demand
Ammonia
Level


Flow
                                 pressure
                                 Dens ity


                                 Suspended  solids
                                 pH
                                 Ammonia
                                 Blanket  level
                      Venturi  with diaphragm sensors
                      Magnetic
                      Doppler
                      Pump  displacement
                      Bourdon  with cylindrical seal
                      RTD  (pad  type)
                      Nuclear
                      Optical
                      Ultrasonic
                      Optical
                      Selective-ion  (pipeline mtg)
                      Selective-ion  analyzer
                      Portable  olefactometer
Bubbler
Diaphragm
Float and tape
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical  seal
RTD (pad type)
Nuclear
Optical
Ultrasonic
Optical
Selective-ion (pipeline mtg)
TOC analyzer

Selective-ion analyzer
Bubbler
Diaphragm
Float and tape
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical  seal
Nuclear
Optical
Ultrasonic
Optical
Selective-ion (pipeline mtg)
Selective-ion analyzer
Optical
Ultrasonic
                                        17-37

-------
                                     TABLE 17-12

                             SIDESTREAMS  (Continued)
   Process and process
        variables
                                    Measurements
                                                          Suggested  instruments
Cond_ition_in£ (continued)

  Atmospheric monitoring


Dewatering

  Drying bed drainage
    and surface runoff
  Drying lagoons super-
    natant and surface
    runoff
  Centrifuge centrate
  Vacuum, belt press, recessed
    plate and frame and screw
    and roll press filters

      Filtrate
Odors
Level
                                 Flow
                                 Pressure
                                 Suspended  solids
                                 PH
                                 Ammonia

                                 Level
                                 Flow
Pressure
Suspended solids
pH

Level

Flow
                                  Pressure
                                  Density


                                  Suspended solids
                                  pH
                                  Ammo n i a
                                  Blanket level
 Level
                                  Flow
                      Portable  defactpmeter
                      Panel
Bubbler
Diaphragm
Float and tape

Venturi with diaphragm sensors
Magnetic
Doppler
Weirs and flumes
Pump displacement
Bourdon with cylindrical seal
Portable optical
Portable selective-ion
Lab test

Bubbler
Diaphragm
Ultrasonic

Magnetic
Doppler
Weirs and flumes
Pump displacement
Bourdon with cylindrical seal
Portable optical
Portable selective-ion

Bubbler
Diaphragm
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Optical
Selective-ion (pipeline mtg)
Selective-ion analyzer
Optical
Ultrasonic
Bubbler
Diaphragm
Float and tape
Venturi with diaphragm sensors
Magnetic
Doppler
Weirs and flumes
Pump displacement
                                         17-38

-------
                                     TABLE 17-12

                              SIDESTREAMS  (Continued)
   Process and process
        variables
                                    Measurements
                                                          Suggested  instruments
Dewatering (Continued)
      Filtrate (continued)
  Spent washwater and
    rejected feed sludge
Cyclonic separation
  Overflow
ScreenLing_
  Screening liquid
High Temperature Processes
  and Heat Drying
  Scrubber water
    supply
  Discharge
Pressure
Suspended solids
pH
Ammo n i a

Level

Flow
Dens ity


Suspended solids
pH
Ammonia



Level

Flow
Level

Flow



Density






Level


Flow


Pressure
Temperature

Flow
                                 Temperature
                                 Suspended solids
 Bourdon with diaphragm seal
 Optical
 Selective-ion  (pipeline mtg)
 Selective-ion  analyzer

 Bubbler
 Diaphragm
 Venturi with diaphragm sensors
 Magnetic
 Doppler
 Pump  displacement
 Nuclear
 Optical
 Ultrasonic
 Optical
 Selective-ion  (pipeline mtg)
 Selective-ion  analyzer
 Bubbler
 Diaphragm
 Magnetic
 Doppler
 Weirs  and  flumes
 Bubbler
 Diaphragm
 Venturi
 Magnetic
 Doppler
 Weirs  and  flumes
 Nuclear
 Sonic
 Bubbler
 Diaphragm
 Float and tape
 Rotameter
 Propeller
 Orifice
 Bourdon
  RTD
 Venturi
 Magnetic
 Ultrasonic
 Orifice
RTD
 Optical
                                        17-39

-------
                                     TABLE 17-12

                              SIDESTREAMS  (Continued)
   Process and process
        variables
                                    Measurements
                                                          Suggested  instruments
Composting Leachate and
  Surface Runoff
Storage
  Facultative sludge
    lagoon supernatant
  Anaerobic sludge
    lagoon supernatant
Landfilling Leachate and
  Surface Runoff
                                 Level
                                 Flow
                                 Pressure
                                 Suspended  solids
                                 PH
                                 Chemical oxygen
                                   demand
Level
                                 Flow
                                 Pressure
                                 Suspended  solids
                                 pH
                                 Ammonia
                                 Level
Flow


Pressure
Suspended solids

Ammo n i a


Level
                                 Flow
                                 Pressure
                                 Suspended  solids
                                 pH
                                 Chemical oxygen
                                  demand

                                 Ammonia
                      Bubbler
                      Diaphragm
                      Float  and  tape

                      Venturi with  diaphragm sensors
                      Magnetic
                      Doppler
                      We irs

                      Bourdon with  cylindrical seal
                      Optical
                      Selective-ion (pipeline mtg)
                      Total  organic carbon
                      analyzer
                      Bubbler
                      Diaphragm
                      Ultrasonic
                      Magnetic
                      Doppler
                      Weirs  and flumes
                      Bourdon with  cylindrical seal
                      Portable optical
                      Portable selective-ion
                      Lab  test

                      Bubbler
                      Diaphragm
                      Ultrasonic

                      Magnetic
                      Doppler
                      Weirs  and flumes
                      Bourdon with  cylindrical seal
                      Portable optical
                      Portable selective-ion
                      Lab  test
                      Bubbler
                      Diaphragm
                      Ultrasonic
                      Float  and tape

                      Venturi with  diaphragm  sensors
                      Magnetic
                      Doppler
                      Weirs  and flumes
                      Pump displacement
                      Bourdon with  cylindrical  seal
                      Optical
                      Selective-ion (pipeline mtg)
                      Total  organic carbon
                      Analyzer

                      Selective-ion analyzer
                                       17-40

-------
17.2  Measurements

This  section briefly describes  each  of  the  instrumentation
devices listed  in Tables 17-1 through 17-12.


    17.2.1  Level Measurements

Level measurements  are required  for  both displacement volume and
open channel flow instrumentation.   Some instruments have almost
unlimited applicability,  while  others are restricted because of
material  interferences.   Sometimes   these  interferences  can be
minimized or eliminated  with  special  purging;  however,  the
designer  must  provide the  support systems  required  if  such
instruments are  to be  reliable.
        17.2.1.1   Bubblers

The pneumatic  bubbler  remains  the  most universally applicable
liquid level  measuring device in wastewater  treatment  facilities.
In its  simplest  form,  a  bubbler consists  of a dip tube through
which  a constant  small  flow of  purge gas,  usually  air,  is
discharged.   The  gas flow prevents  the  liquid  from rising in the
dip tube; therefore, the  pressure  required  to maintain the gas
flow is directly  proportional  to the  depth of  a liquid  above the
dip tube outlet.   This  pressure  can be  measured by virtually any
pressure  measurement device,  some  of  which are described in
Section 17.2.3.   The bubbler  can be used with  almost  any liquid,
but clogging  may  be a problem when  solids are  present.   Clogging
can be  controlled  by frequent  purging  with high pressure  air.
Where  the use  of  an  air  purge  is undesirable,  such as  in
anaerobic digesters, nitrogen  or  natural  gas can  be  used for
purging  the  bubbler  dip tube.   Figure  17-1 shows a typical
bubbler schematic with air purge capabilities.


        17.2.1.2   Diaphragms

Bubbler  dip  tube clogging problems can be overcome by use of
diaphragm level  element.   A  diaphragm  is  usually 3  to  4 inches
(7.6 to  10. cm)  in diameter  and  serves  as  one wall  of   what is,
essentially,  a box.  Inside  the box,   a pneumatic,  hydraulic,
or electric  mechanism  transmits  any  pressure  exerted on the
diaphragm.   The entire  box is  submerged   in a  vessel, or the
diaphragm may be inserted  in  the  wall of  the vessel  by means
of a standard pipe flange.   In either case,  the pressure exerted
on  the diaphragm is  directly  proportional  to the  depth  of
liquid  above the  diaphragm.   The  type of diaphragm  shown on
Figure 17-2  is air-purged and produces a back pressure similar to
a bubbler.   Thus, both  the  air supply and pressure measuring
devices  are  similar  to  those  used in bubbler  systems.   The
air-purged diaphragm  can, therefore, be  used as a  replacement for
existing bubblers.   The  second  type of diaphragm uses   a filled


                            17-41

-------
                 COMPRESSED
                 AIR SUPPLY
                    3Qpsig
                  MINIMUM
3/B" PVC JACKETED - -.
COPPER TUBING
ON PAN EL (TYP)
DRAIN PLUG —
{REQUIRED WHEN
AIR SUPPLY FROM
ABOVE!
    FILTER REGULATOR
    ASSEMBLY W/GAUGE
     BULKHEAD
- ^FITTINGS
                      DIFFERENTIAL PRESSURE •
                      REGULATOR W/NEEDLE
                      VALVE
                                ROTAMETER W/CHECK•
                                VALVE, 0-2 SCFH
Inchtt x 0.39 - em
p»i| x 0.14i - kN/m*
               SCREW ADAPTER
               ItCAPTO PERMIT
               RODDING
               3/4" SCHED 80
               PVC PIPE
                                                                      UJ
                                                                    NOTCH
                                                                   1/2" x 1/3"
                                    FIGURE 17-1

                       TYPICAL BUBBLER SCHEMATIC WITH
                             AIR PURGE CAPABILITIES
                                        17-42

-------
                 COMPRESSED
                  AIR SUPPLY
                    30 psis
                   MINIMUM
3/8" PVC JACKETED
COPPER TUBING
ON PANEL (TYP)
DRAIN PLUG ——-
(REQUIRED WHEN
AIR SUPPLY FROM
ABOVE)
                       1/2" PIPE

                        [ 1/2" BALL
                        'VALVE
                                                       PRESSURE,
                                                       ELEMENT I  ~\J

                                                                 FT
                                           01
T   .:
     FILTER REGULATOR-
     ASSEMBLY W/GAUGE
                      DIFFERENTIAL PRESSURE
                      REGULATOR W/NEEDLE
                      VALVE
                                RQTAMETER
                                VALVE, 0-2 SCFH

                       1" NPTPIPE
                       CONNECTION
                                                              /BULKHEAD
                                                         ±^f FITTINGS
                                                             • 1/4- PVC TUBES
inches v. 0,39
(%tig x 0,145 =
           on
                TEFLON COATED
                FIBERGLASS DIAPHRAGM
                                                   INSTRUMENT
                                                   CONNECTION
                                                                          1"SCHED 80
                                                                          PVC PIPE
                                                                          (SUPPORT ONLY}
                                                                             DIAPHRAGM
                                                                             ELEMENT
                                                                             tSEE DETAIL)
                                      FIGURE  17-2

                        TYPICAL BUBBLER SCHEMATIC WITH
                                 DIAPHRAGM ELEMENT
                                         17-43

-------
capillary tube between the diaphragm and a conventional pressure
transmitter,  thus eliminating the  need  for  an  air supply.   When
a filled capillary tube is employed, the volumetric displacement
of  the  diaphragm  is  critical,  and  pressure  indicators  or
transmitters  should  have  as  low  a displacement  as  possible  so
that diaphragms with  low movement  can be used.  Capillary filling
fluid should have  a  low thermal  expansion  coefficient  to  limit
errors  resulting  from temperature  changes.    Diaphragms  are
flush-mounted  and  have  no crevices  to  accumulate solids.    The
almost  insignificant movement required  for  accurate  measurement
is maintained even  when  the diaphragm is coated with grease.


        17.2.1.3  Capacitance  Transmitters

In  recent  years,  several electronic level measuring devices
have  appeared.   Capacitance and  ultrasonic  instruments  are
particularly  applicable  for sludge  measurements.  The capacitance
liquid  level  transmitter  consists  of a  steel  rod  or  cable
(probe), usually teflon-coated,  which is installed  in  the  tank.
The probe forms  one  plate of  a  capacitor,  and  the liquid,  which
must  be  conductive  and  grounded,  forms  the second.    The  probe
insulation  forms a  dielectric  between  these  two plates.   The
electrical  capacitance  between  the probe and  the liquid  is
proportional  to the  axial  length of probe immersed in liquid.   An
electrical  instrument measures  this  capacitance  and  provides  a
signal proportional  to level.  This signal can be used to provide
either on-off or continuous level measurement.   Gross changes in
fluid conductance can affect  calibration of capacitance probes.
This  is  not  ordinarily  a  problem  in sludge handling facilities.
One disadvantage with capacitance instruments is that,  even with
teflon-coated  probes,  greasy  material  can adhere and  cause
errors.   Improved  electronics has reduced   this  problem  on some
units.  Very  successful  level  measurements have been made for all
types of  sludge, including  the  fluid  level inside  fixed  cover
digesters.   Capacitance  instruments are  also used to measure the
level of dry  solids  in  bins  or  silos.   In  this application,  a
bare  rod or cable  (probe)   is  used,  and the  side of  the tank
serves  as  the ground plate.   Where tanks  are non-metallic,  a
metallic ground  plate can be  installed  on  the  side of  the  tank.
The material in  the  tank  then serves  as the dielectric and must
have  a  stable  dielectric  constant  significantly different from
air.  Since  many solids  contain  large and varying amounts of air,
the  use  of  capacitance probes  for solids   level  measurement  is
frequently  unsuccessful.


        17.2.1.4  Ultrasonic  Transmitters

Ultrasonic  level instruments  operate  with  the  level transmitter
completely  out of contact with  the  process  material.   This is  a
very  appealing  advantage  for  sludge  treatment and  disposal
processes.   A  transducer  suspended above the material  emits  an
ultrasonic  pulse  toward  the liquid.   The  pulse bounces  off
                              17-44

-------
the material surface  and the  reflected pulse  returns to  the
transducer.   The time that elapses between the transmitted  pulse
and the  received  pulse is related  to the distance  between  the
transducer and  the  reflecting  surface by the speed  of  sound  in
air.   Unfortunately,  the  speed of  sound  in  air is  affected  by
both temperature and humidity,  and  the  reflected  signal  is  also
scattered.   These  conditions  substantially weaken  the  received
pulse.   All  these problems  can be overcome  with more  complex
electronics  and  larger  transducers, but experience to date (1979)
with  these   units  has  been  very  poor.   Their  use cannot  be
recommended  until their  serviceability  has  been proved under
treatment plant  conditions.


        17.2.1.5  Tape-Supported Floats

Tape-supported floats are  suitable for level measurements of  many
liquids.   Floats are suspended from  a  tape that  winds on a  drum.
The drum  is  provided with either  a  counterweight  or a "constant
tension"  spring to  remove all the  slack from  the tape.    The
position  of  the  drum is  a  very accurate measure of float position
and, hence,   liquid level.   Standard  units provide an accuracy of
0.01 feet (3.09  mm) for  local  readout  and can  be  equipped  with
electric  signal  transmission  for remote  readout.   In the case of
floating  cover  digesters,  the  cover  becomes  the float,  and  the
same drum assembly  provides measurement of level in the digester.
Tape-supported floats are  often located  within  concentric  wells
to  isolate them from the  turbulence of  the liquid  surface  being
measured.  To assure maximum reliability,  these wells are usually
purged with  water at   rates  sufficient  to  keep  a continuous
flow from the well, even during  periods of maximum rising levels.
Such  installation  is  usually not practical  when the material
being  monitored  contains significant  amounts of  debris  and
grease.


    17.2.2  Flow Measurements

Flow  is   an   important  measurement   for  sludge  treatment  and
disposal  operations.  Accuracy  has  been  an ongoing  problem  with
all  types of flowmeters.   Venturi-type flow  tubes,  orifice
plates, and  weirs  are regarded as  standard flow measuring devices
providing proper approach  conditions—the  length of straight pipe
upstream and downstream from the device--are maintained.  This by
itself  is a  strong argument in  favor of their use.   In  many
situations,   proper  approach  conditions  cannot be obtained  or a
wider range  of operation  is needed.    Some in-plant method should
be  provided  to  "prove"  the  accuracy of non-standard  meters.
A  liquid  flowmeter can frequently be calibrated  by discharging
a  flow into  a tank of  known  dimensions  and measuring the change
in  level.   In other cases,  meters may  be  compared with other
meters  of proven  accuracy.    For comparison,  the meter  under
test must be left in  the actual  plant  piping,  or  a test  stand
with an  identical  piping  configuration  must  be  provided.   Flow


                              17-45

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measurements of  wastewater  sludge are  difficult  to take.   The
designer must select the  instrument  with  care,  recognizing that
reliability may be a far more important criterion than accuracy.


        17.2.2.1   Venturi  Tubes

Venturi-type flow tubes  have  been  successfully used  for  all
sludges, including primary sludge.   Venturi  tubes are  classical
differential pressure  producers that  function according  to
Bernoulli's relationships.   A  Venturi tube  has a restricted
throat.   A pressure  drop  is  produced as  the  fluid accelerates
through the  throat.   The  pressure  drop is  proportional  to  the
square  of  the  liquid velocity  and  is measured  by  differential
pressure instruments, as  described  in  Section  17.2.3.   Modern
flow tubes operate on the same principles;  they are improvements
on early Venturi  devices.   However,  they  are less expensive  and
produce  less residual  head  loss.   When  used  for sludge  flow
measurement, the  pressure taps  must  be  protected  from  plugging.
This can be  done  by  water  purge  or  by use  of diaphragms  similar
to those described earlier and  installed in  the  tube wall.   The
disadvantage of a Venturi  tube  is the narrow usable flow ranges
available.   Anything  over 3 to 1  is usually  accomplished  at
reduced  accuracy.   If  air-purging  is used,  the Venturi  tube
static lines require careful  sloping  to  eliminate errors caused
by trapped bubbles.   Water-purged systems require a source water
free of both soluble and insoluble  solids to  avoid clogging  of
flow control needle valves.  Consideration  should  be given to the
potential  impact  of purging water on downstream sludge processes.


        17.2.2.2   Nozzles

Flow  nozzles are similar  in operation  to Venturi  tubes  but
are substantially  less  expensive.   Residual head  loss  is  much
greater than for Venturi tubes and approaches that of  an  orifice
plate   installation.   Flow  nozzles do  not wear  out as  quickly  as
orifice plates and can  handle fluids  containing  limited  solids.
The most common application of the flow nozzle is for  steam flow
measurement.
        17.2.2.3  Magnetic  Meters

The magnetic flowmeter functions  according  to Faraday's  law
which,  in simple terms, states that when an electrical conductor
(in  this  case  water)  passes  through a  magnetic  field,  an
electrical voltage is developed at right angles to the direction
of  the  field and  to  the  direction  of the movement.    If  the
magnetic  field  is  constant,  the  voltage is proportional  to  the
conductor's velocity.   Hence,  a  magnetic flowmeter is  simply  a
tube with magnetic  coils  that  uses  electronics  to measure  the
voltage produced.   In the past, a number of poor applications has
                              17-46

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put magnetic  flowmeters in disfavor.  When they  are  properly
applied with modern electronics,  magnetic flowmeters are now as
reliable as any  other  flow measuring devices.

Flow velocity of primary sludge  through a magnetic flow tube
should  be in  the  range of  5 to  25  feet per  second  (1.5 to
7.6 m/s), providing a usable  range of  5:1.   The lower limit is
established   by the minimum  scouring action required to keep
electrodes  free of grease.   The  upper  limit  is  necessary to
limit erosion of  the  tube's   plastic  liner.   Flow velocity for
secondary sludges may  be  extended down to  3  feet  per second
(0.9 m/s) because  less grease  is present.   For  intermittent  flow,
velocities  may be extended  up to 30  feet per second (9 m/s)
because  less grit is  present.    Combining  these  conditions
provides a usable range  for  secondary  sludges of 10:1.   Magnetic
flowmeter manufacturers  generally  recommend certain  accessories,
such  as  electrode cleaning  devices,   when  metering  sludge.
Purchase specifications  should clearly  state the application and
require  provision of all  recommended  accessories.   Properly
applied  and  installed,  modern magnetic flowmeters are giving
excellent service  in many installations.


        17.2.2.4  Ultrasonic Meters

Ultrasonic meters  are  a  fairly new development,  and  no two  meters
work exactly the same.   There  are two basic types.   The  first and
most common one, which is listed as the ultrasonic  device in  this
chapter,  consists  of  a  pair  of transducers mounted  on  opposite
sides of  the  pipe and  displaced  so  that one  transducer is one
pipe diameter downstream from  the  other.   The  first transducer
emits an  ultrasonic pulse, and the  time  it takes  this pulse to
reach  the second  transducer  is  measured.  The system is then
reversed.   The  second transducer  emits  the pulse,  and the  time
until the first transducer  receives this  pulse is measured.  The
travel  time is known as propagation time.  In one case, flow
velocity  decreases propagation time,  and  in  the  reverse   case,
increases the propagation time.   The  difference in  time between
the two  measurements is directly  proportional  to flow  velocity.
The ultrasonic flow measuring  system is relatively  insensitive to
factors  that  normally  affect  the  speed  of  sound   (for  example,
temperature).   This  is because  the  effects  are  cancelled as
the  signals  reverse.   However,  some  difficulties have been
experienced with  this  technique.    Most  sludge  is sonically
opaque.   The  signal cannot travel  between  the transducers,  even
with a  high power.  Therefore, at this time,  this  type of  meter
is not considered  reliable.
        17.2.2.5   Doppler Meters

The second  type  of  ultrasonic  flowmeter  depends  on the Doppler
effect.   A continuous  ultrasonic  signal is emitted  into the
pipe  by  the transducer.   This  signal  is reflected  by solids


                             17-47

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or bubbles in  the  liquid stream and  is  returned to  a  second
transducer at  a  frequency  different  from  that transmitted.
This difference is  related to  the velocity of the material that
caused  the reflection.   Presently, difficulties prevent  the
practical application of  this technique.  The  frequency change is
affected by the velocity  of sound,  which  in turn is  affected by
temperature in  the  fluid.   Furthermore, in sludge applications,
the particles or  bubbles  causing  reflections  will very probably
be located close to  the pipe walls,  and  their  velocity may not be
representative of  average  fluid  velocity.   Hence,  the accuracy of
this type of meter is questionable.   Actual  field  experience with
this type of meter is not  extensive.
        17.2.2.6  Rotameters

Rotameters  are  commonly  used  for both  gas  and  liquid  flow
measurements of  clear  homogeneous  fluids.   Their use  in sludge
management is primarily for chemical  feed systems, air flows, and
purge systems.  A rotameter consists of a "float" in a conically
shaped tube.  The "float" does not actually float, since it must
sink  into the  fluid being measured.   The  size  of the rotameter
orifice increases as  the "float" rises in  the  tube;  therefore,
the upward  force on the  "float"  for  any fluid velocity decreases
as the float rises.   The equilibrium  point between "float" weight
and upward  force due  to flow  velocity  is  an indication  of flow.
Rotameters  are  constructed  of a  wide variety of materials,
including  metals  and  plastics.  They can  be constructed  to
measure almost any  type of fluid.  Rotameters  are  available up to
3-inch  (8  cm)  pipe  size.   Larger  pipes  are  accommodated  by
installing  an orifice  plate parallel to the rotameter  so that a
known fraction of the  flow passes through the  rotameter.   This is
called a  "by-pass"  rotameter.   If  the  float is  made of  magnetic
material  or  contains  an  iron  core,  a magnet mounted  on the
outside of the tube  can be made to  follow it.  This magnet can be
attached to  a transmitting mechanism  to provide  remote readout.


        17.2.2.7  Propeller Meters

Relatively  clean,  non-corrosive fluids  flowing through large
pipes  (2  inches [5  cm]  or larger)  can be  readily measured with
propeller meters.   Propeller meters  can provide local readout or
can be  equipped  with  transmitting  mechanisms  for remote readout
or recording.  They are  not applicable  for sludge flows, but can
provide reliable, cost-effective  service for support systems.


        17.2.2.8  Pitot Tubes

Pitot  tubes very economically measure flow in  pipes of almost
any  size.    The pitot  tube  produces   a differential  pressure
proportional to  the square of the  fluid velocity,  which may be
                              17-48

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measured  by differential pressure  transmitters  described  in
Section 17.2.3.  One  commercial  unit  provides  four ports spaced
across the diameter of the pipe and averages the impact pressure
of each  to provide compensation for irregular flow profiles.
The pitot  tube  produces  a very  small pressure  differential  for
liquid flow  velocities  typically used in  treatment  plants  and,
therefore, is not particularly suitable  for  liquid service.   It
is frequently suitable for gas flows  where wide flow ranges  are
not required.   The  small  tube entrances make  it completely
unsuitable for use  with sludge  flows.


        17.2.2.9 Weirs and Flumes

Weirs  and flumes  provide a  simple, very accurate method  of
measuring liquid flows in open  channels.   They are not applicable
to pressure systems.  Any of  the level measuring  instruments
described in Section 17.2.1  provide a  means  of measuring  the
liquid level behind the weir or at the critical point in a flume.
Weirs are not suitable for flows  with  large amounts of settleable
solids or debris.  This material  will  collect behind  the weir and
change weir measuring  characteristics.


        17.2.2.10  Orifice Plates

Gas flow  measurement is commonly required  where anaerobic  or
aerobic digesters are used.   Gas produced by anaerobic digesters
is dirty and corrosive.   Permissible head losses in anaerobic and
aerobic digesters are often very  low  and the range of operating
requirements extreme.   An orifice  plate can  be  used  in  this
service.   Orifice plates are  similar in  theory to Venturi tubes;
that is,  pressure drop through the device is proportional to the
square of  the  liquid  velocity.   Orifice  plates,  however,  lack a
smooth  recovery cone and,  consequently, have a  much  greater
residual  head  loss.   The advantage of the orifice plate,  other
than lower cost, is the  ease with which  it can  be  changed.   The
optimum size  of orifice  plate can be readily  installed  for  any
flow.   Quick change  fittings  permit  changing of  orifice plates
without disturbance of a  piping run.


        17.2.2.11  Turbine Meters

Turbine  meters,  which provide good  service  in  gas flow
applications,  consist of flow directing  channels, a suitable
turbine blade,  gearing,  shafting, and a  readout device.   In  the
simplest  form,  the  output shaft  directly  drives  the  readout
register.    Where remote  readout  is desired,  the  shaft rotation
actuates  an  electrical switch.   Each switch closure represents a
discrete  quantity of  gas.  The meter  must  be specially designed
for dirty and corrosive gas.  Moderate maintenance  is required to
keep  the  meter  clean.   The  turbine meter's  ability to  operate
over  wide ranges  makes   it attractive   for  the measurement  of
anaerobic digester  gas.


                             17-49

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        17.2.2.12   Vortex  Meters

The Vortex shedding flowmeter  is  a  comparatively  new  meter  that
is also  applicable  to anaerobic  digester  gas  flow measurement.
The meter consists of an obstruction placed in the pipeline  with
sensors that detect the vortices caused by the obstruction.   The
flow is proportional  to  the  number  of  vortices  produced.   These
meters are suitable for  Reynolds Numbers above 5,000 and  readily
provide a usable operating  range of  100:1.
        17.2.2.13  .Positive  Displacement
                   Meters

Orifice  plates,  turbine meters, and  Vortex meters have  all
provided  adequate instrumentation fo.r  anaerobic digester  gas
flow.   However,  these  instruments  cannot provide  the  absolute
accuracy of  positive  displacement  meters at the  very  low  flows
encountered  during  digester operations.    Positive  displacement
meters  can be of  the  rotating cavity  (lobe)  or the diaphragm
type.  Positive displacement meters are probably the oldest  meter
used for digester  gas  measurements.   In  recent years,  they have
been almost  completely  replaced by  the  in-line meters described
in  the  previous paragraphs.   Positive   displacement  meters  are
frequently used  for clean oil or clean gas flow measurements and
are  inherently  useful   over  an  extremely wide  operating  range.
The  meter's  cavities,  exposed  bearings,  and/or close clearances
make them unsuited  for  dirty gas service.
        17.2.2.14  Pump and  Transport
                   Displacement  Systems

Sludge  transport  systems  should  not be  overlooked  as  flow
measurement devices.   Progressive cavity  and  other  positive
displacement  pumping  equipment  can  be  equipped  with  speed
monitors  or  cycle  counters that provide  a  fairly  accurate
flow  indication.    None  of  the  problems  usually  associated
with  flowmeters operating on  sludge  are  encountered.   Where
materials are  trucked,  the  number  of truck  loads will provide a
rough measure  of quantities.   If the  trucks are also weighed-in
and -out, accurate  measurements  can be obtained.
    17.2.3  Pressure Measurement

Pressure  measurement  is  basic  to many  level  and  flow measuring
systems,  as well as  to  the measurement  of individual  process
pressures.   As  a result, pressure  elements  are  without  a doubt
the most highly developed instruments  used  in  industry.
                              17-50

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    17.2.3.1   Bourdons or Bellows

Pressure Elements

The bourdon tube is  the most  commonly used pressure element for
pressure  ranges  of 15  pounds  per  square   inch  (103  kN/m^)
or  greater.   The  bourdon  tube  is essentially  a  piece  of
tubing closed at one end and  bent  in an arc.   When pressure is
applied  to the  tube,  it  tends to  straighten.   The  movement
produced at  the  free or closed  end  is  amplified by mechanical
linkage to operate  a pointer  or  transmitter mechanism.   Bellows
are frequently used when  lower pressures must  be  measured or
greater  movement is required for  direct actuation of control
mechanisms.  Bourdon tubes are  rarely used in modern industrial
process pressure transmitters.   Bellows elements are frequently
used  in  process  pressure  transmitters  for pressure  ranges from
10 inches water pressure (2.4  kN/m^) to as  high as 600 pounds per
square  inch  (4.14 MN/m^).  Bellows elements are also readily
adaptable  to differential pressure measurements and  absolute
pressure measurements.

Chemical Seals

Both  bourdon  tubes  and   bellows   are  unsuitable  for  direct
measurement  of fluids  containing  solids.   Collecting  solids
within  the  pressure element   is  the  problem.   Corrosive  fluids
also must be  kept out of the pressure element.  A  "chemical seal"
is  used  for  these  applications.   The most common chemical seal
consists of  a small metal or elastomer  diaphragm,  one side of
which  is  exposed  to the process fluid.    Sometimes this exposed
side  is purged  with water   or  mounted  flush  with the  fluid
containment vessel.   The other side  of  the seal  is close-coupled
or  connected  by  a  capillary   tube  to the  measuring  element and
filled  with  a  suitable fluid  such  as  silicon  oil.   For very
dirty,  grease-laden  process   fluids  such  as  wastewater sludge,
an  in-line tubular  or  cylindrical chemical  seal,  as  shown on
Figure  17-3,  must  be used to assure operational reliability.
This  seal  is  constructed as  an elastomer  tube of the  same size
as  the process  pipe line  and  mounted  within a flanged  steel
pipe  spool.   The space between the elastomer and  steel  spool
is  sealed,  filled   with  a suitable fluid  (anti-freeze when
necessary),  and connected directly  to the pressure  element.
Pressure' elements  with electrical  contacts and  cylindrical
chemical  seals  should  be   used  immediately  downstream  from all
positive displacement pumps transporting wastewater sludge.  This
will  provide  a reliable system  for  emergency shutdown whenever
the pump "discharge pressure becomes excessive.

Chemical  seals  used to isolate  corrosive fluids from pressure
elements  are  available  in  a  great  variety  of materials.   Care
must  be exercised  in the  application of any chemical seal to
ensure  that  it has  sufficient  displacement  to  operate  the
measuring  element.   Use of  chemical seals for  ranges of less
than  50  pounds per   square inch  (345  JcN/m^)  can  be  expected to
introduce significant errors in  the measurement.
                             17-51

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                            FIGURE 17-3

                  CYLINDRICAL CHEMICAL SEAL FOR
                   SLUDGE PRESSURE MEASUREMENT

        17.2.3.2  Diaphragms

Pressu ^e^E^l^em en_ t

Where a direct mechanical readout is not required from a pressure
element,  the diaphragm pressure  transmitter  is  suitable  for
any  application  where  bourdon tubes  or  bellows would  be  used.
The  force-balance  is   the  oldest type  of diaphragm  pressure
transmitter and  continues  to  be  widely applied  in  industry  and
wastewater treatment.   Newer types  such  as  the strain gauge,
reluctance,  and  capacitance,  are  functionally  similar and  are
becoming  more  common.   Diaphragm  pressure  transmitters  are
available  to  measure  gauge pressure, differential  pressure,  or
absolute pressue, with  ranges  as  low as 1-inch  water  column to
10,000 pounds per square inch  (250 N/m2  to  80 MN/m2)  or higher.

Chemical Seals

Chemical seals are not  generally required with diaphragm pressure
transmitters for solids bearing  fluids because the measuring
element itself is an essentially flat diaphragm.  Chemical  seals
are  still  frequently used  for  corrosion  protection  and, in high
temperature  applications,  to  separate   transmitter  electronics
from temperatures above permissible levels.   Chemical  seals  are
also  used  with  differential  pressure configurations  to  permit
flush-mounting of the diaphragms to the process at two physically
separated locations.
    17.2.4  Temperature  Measurements

Stabilization,   disinfection,  conditioning,  composting,  and
heat processes  in  sludge treatment all  may  require  temperature
information  to  assure  successful  operation.    Temperature
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instrumentation   is  relatively  simple;  however,   successful
application  requires  locating  probes to  obtain representative
readings without  obstructing  sludge  flow.   The designer must be
aware of  these  application  restrictions  and  locate  and specify
instruments  correctly.
        17.2.4.1  Resistance Temperature
                 Detectors (RTDs)

Resistance  temperature  detectors  (RTDs)  are applied  at
temperatures  up to about  1,000°F  (540°C).   This  is well within
the range of  most sludge temperatures.  RDTs  work on the basis of
the fact  that  the  electrical  resistance  of  metals changes  with
temperature.   Electronic amplifiers  measure this  resistance
change and provide  an output proportional  to  temperature.

Thermistors  are  sometimes  used  for  special  temperature
measurement  applications.    A thermistor   is  a  temperature-
sensitive  semi-conductor.    Like  RTDs,   the   thermistor's
resistance changes  with  temperature,  but  the  change is extremely
non-linear.   The  advantage  of  using a thermistor  is  that  a
large change  in  resistance  can be obtained   over  a very narrow
temperature range.
        17.2.4.2   Thermocouples

For  processes with  temperatures  in excess  of approximately
1,000°F  (540°C)  (for  example,  incineration), RTDs  are  not
suitable and thermocouples must be  used.  Thermocouples consist
of  two junctions of  dissimilar metals.   One junction,  the
measuring  junction,  is placed  in  or  on  the  material to  be
measured.    The  second,  or reference,  junction  is  located  in  a
constant  temperature  zone,  or  the measuring  instrument  may
include an  artificial  reference  junction.   The Peltier  effect
states  that  at  any junction of  dissimilar  metals,  an  electric
motive  force  (voltage)  will  be produced.   Thus,  two voltages,
(one  at  each  junction)  are produced in  a  series circuit.   The
measuring   instrument   detects  the  difference  between  these
two  voltages  and  produces an output  proportional  to  process
temperature.  Thermocouples produce very small  voltages  at  low
temperatures.    More  importantly,  the  difference  in  voltage
produced by the reference and measuring junction is very small.
For this reason,  thermocouples  are not generally used to measure
small  variations  in  temperature.    Thermocouples  are generally
less  expensive  than  RTDs  but  require greater  attention  to
installation  procedures  to  reduce electrical interference.
Wiring  for thermocouples must  be  especially  matched  to  the
thermocouple junction material.
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    17.2.5  Weight  Measurements

Two  types of  weight measurements are  of interest  in sludge
handling facilities.   The  first is  the common static measurement.
The second is  the  weight  per  unit of time, which  is  actually  a
mass flow measurement.
        17.2.5.1  Static

Mechanical scales are frequently used foi: static weight measure-
ments.  Such scales consist of a platform or vessel  and a system
of  pivots  and  levers to  provide  a usable  readout.   Mechanical
scales are constructed to  measure anything from 0.40 ounces (11 g)
in the laboratory to 100,000 pounds (45.4 Mg)  or more  to weigh  a
railroad  car.    Many modern  scales  use load  cells  under  the
platform to eliminate the  complex  lever  and pivot  system.   Load
cells  are placed  under  one  or more  of the  platform  support
points, and the output of  the  cells is summed to obtain the total
weight.   In  some cases,   the  number  of  load cells can  be  lower
than  the  number  of support points because  load  symmetry allows
multiplying  the output of the installed  cells by a  factor  to
account  for  the missing  cells.  Load cells  may be either  the
hydraulic or strain  gauge type.   Hydraulic  load  cells resemble
a piston  that  converts   force  to  a  hydraulic  pressure.   This
pressure  is  readily monitored  by   pressure  instruments,  as
previously described.   Strain gauge load  cells  consist  of  a
calibrated structural member  to which a  resistance  wire element
is  attached.    When the  structural  member is  strained by  an
applied force,  the  resistance wire element's  dimensions change.
Hence,  its  electrical  resistance  changes.   Suitable  electronic
circuitry  converts these  small resistance  changes to  an
electrical output proportional to  the force applied  to the cell.


        17.2.5.2  Mass Flow

Mass flow measurements involve a fixed transport system such  as a
belt  conveyor.   Mass flow measurement on  a belt conveyor is  made
by  supporting one or two  conveyor  belt idler rollers on a scale,
measuring the conveyor speed  as described in Section 17.2.8, and
multiplying  weight  and  speed together to obtain  mass flow.
Modern belt scales  using  load cells and  two  idlers  are  very
accurate  and  are easily  maintained.  Nuclear belt scales can
provide  this  function  without contacting either  the  belt  or
the  material  being  weighed.   This may  be  an  advantage  in  some
installations.   Nuclear  scales are almost identical in operation
to nuclear density meters.  The only difference is that a nuclear
scale  is calibrated to monitor total mass  in its path rather  than
the  change in mass caused  by  suspended solids in a liquid.   This
is  a less  difficult  application,  and  premium radiation monitors
are  not required,  but  nuclear source  decay  still causes  the
calibration to  drift.   The radioactive  source  is  a  controlled
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substance subject to United States  Nuclear  Regulatory  Commission
restrictions  and regulations.   It requires special  training,
safety precautions, and testing.   This adds  to  operation and
maintenance costs.

All conveyor mass flow scales measure  the total  mass of material
on  the platforms  or belt.    No  differentiation is made between
solids and water; therefore,  the  reading  is  most accurate  if the
moisture  content  is  constant  or can be measured.


    17.2.6  Density  and Suspended
            Solids Measurements

Sludge density and suspended  solids are the  same measurement from
an  instrumentation  point  of view.   However,  they  are  quite
different  from  the  operation standpoint.    Sludge  density is  a
common term used to describe the concentration  of  solids in
sludge mixtures  in which  solids  are  the favored  component.
Sludge  density  is  usually  expressed  in percent   solids  by
weight.    Suspended  solids  is a common term  used to describe the
concentration  of solids in water in which  the  liquid  element is
the  favored  component--for  example,  the solids present  in the
plant  influent  or  the solids  left  in  the supernatant  after
gravity  thickening.   Suspended solids  are usually  expressed
in  weight of  solids per  unit  volume of  water.   There  is no
instrument available that directly measures  either  sludge density
or  suspended solids.  Instruments that are  used  actually measure
nuclear  radiation absorption,  light  transmission  or  reflection
(optical), or  sonic  attenuation  characteristics of the mixture.
These measurements  are then empirically correlated  to sludge
density or  suspended solids  concentration.   In  most cases, this
correlation does not remain constant,  and periodic recalibration
is  necessary.   The  frequency  of  this  recalibration is dependent
on  the characteristics  of the liquid being monitored.   In no case
do  these instruments  provide  adequate accuracy  for  reporting
purposes,  although  they can be  used for control.   Laboratory
analysis  is usually  required to obtain the  accuracy necessary to
develop  QFD  (see Chapter  3)  diagrams.   Nuclear and  opacity
density  measurements  can  be used  in conjunction with  control
systems  to allocate  sludge automatically to  various  process
facilities on a mass flow basis.   Sonic  density measurements are
usually  only applicable  to the on  and off control  of sludge
pumping equipment.


        17.2.6.1   Density

Nuclear

Nuclear  density  gauges  usually  work  well  on  primary  and mixed
primary and secondary sludges in  the higher concentration  range.
They  usually have  limited applicability  to  secondary sludge
alone.  The nuclear  density gauge consists of a  small radioactive
source,   usually  cesium-137,  and  a  detector  placed  on opposite


                             17-55

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sides  of  the pipe.   Gamma radiation  is emitted  and  absorbed
by the material  in the pipe in direct proportion  to  its density.
However,  the  difference  in  radiation  absorption between plain
water  and  water  containing  the  suspended solids  concentration
must be significant for nuclear meters to function well.   These
meters are generally effective where  suspended  solids concentra-
tions  are  in the  range  of 1 to  10  percent.  The  radioactive
source itself decays,  and  the high gain  amplifiers suffer from
gain changes resulting from component aging.  Both  factors cause
the  instrument  calibration  to  change  rapidly,  and  frequent
adjustment  is  required  to  maintain accuracy.   When  nuclear
density gauges are to  be used  to measure sludge  solids concentra-
tion,  they  must  be  specified with  special premium  low-drift
amplifiers.   The source is  a controlled substance  subject to
United  States Nuclear Regulatory Commission  restrictions and
regulations.    When properly  installed  and  maintained,   nuclear
density gauges have functioned quite successfully  with wastewater
sludge .

Optical

Optical type  meters  are  usually  used  to  measure sludge  density
concentrations of  less than  3 percent.   These  instruments use
either light transmittance  or a combination  light transmittance/
scatter measurement  and are  suitable for concentrations from
0.2 percent  to 10 percent solids.   Units that employ a mechanical
wiper  to  keep   the  optics  clean have  been very  successful.
Caution must  be  exercised  in the  application of these units to
primary sludge,  which may contain  grit that damages  optical
surfaces and wipers.

Ultrasonic

The sonic  density gauge is  a relatively  new  product  proposed for
measuring  the density  of primary sludge.   The sonic  density gauge
consists  of  two ultrasonic  transducers  mounted  on  opposite
sides  of  a  pipe  section.   Ultrasonic  signals  emitted  from one
transducer pass  through the  material  in the pipe to the  second
transducer.    Suspended solids in  the signal  path  attenuate this
signal.   The signal  received  decreases  in strength with  an
increase  in suspended solids.   The  relationship  between the
strength  of  the  signal  received  and  the  suspended  solids
concentration is  non-linear  but  sufficiently predictable to be
used for control of sludge  pumps.   Sonic density meters have been
used  successfully  and are  much  less  expensive  than  nuclear
density meters.


        17.2.6.2  Suspended  Solids  Measurements

The  optical  instrument   described  for  providing  density
measurements is  also suitable  for  suspended solids  measure-
ments.   Instruments  are  available  with a range  from  0-30 to
0-30,000 mg/1.   The  mechanical  wiper  optical unit is generally


                              17-56

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the most  suitable  for  this application.   Surface  scatter  types
with  no optics  in  contact with  the  process  fluid  are  also
usable.   Care  should be  taken  to  exclude  larger  solids,  such  as
particles of floating debris, that are frequently present in the
liquid  being  monitored.    More  information on  suspended  solids
instrumentation is available (3).
    17.2.7  Time Measurements

Digital and synchronous motor batch (reset) timers are available
for control of  sludge  management  and  support  services.   Digital
timers provide  one  second resolution  up  to about  2  3/4 hours.
Synchronous motor  timers provide 0.1  minute resolution  up to
16 hours  and  0.1  hour  resolution  up  to  1,000  hours.   Both  types
use power line  frequency as a time reference.   They are designed
to reset at zero at  the completion of  a  cycle.  If time functions
cannot be  interrupted  by  power failure or  the  wastewater  plant
generates all  its own  electrical  power,  the  designer  must take
special precautions  to see that  all  control  timers  function as
required  on  the  emergency standby or continuous  plant electric
power frequency.  Digital  timers can be  obtained with an internal
quartz crystal  to provide their  frequency  reference.   They  can
therefore operate independently  of power line  frequency.
    17.2.8  Speed Measurements

Speed  is  readily measured either  by  a  tachometer-generator
coupled  to equipment or  by a  reluctance  pick-up.   Tachometer-
generators are  generally  more  expensive  and  require  higher
maintenance   than  reluctance  pick-ups.    This  is  because
tachometers have  their own bearings,  brushes,  and usually a
timing belt coupling.  A reluctance  pick-up installation consists
of  a  split gear bolted around a shaft  on the equipment.   The
pick-up  is then mounted  in close proximity to  the  gear  teeth.
Suitable  electronics  amplify the  pulses  that  come from  the
pick-up  each  time  a  gear tooth passes and converts these  pulses
to a voltage or current output  proportional to  speed.

Electronic trip units can be  used  with  either  tachometer-
generators  or  reluctance  pick-ups to permit these devices to be
used  as  underspeed  switches.   Mechanical underspeed  switches
are also available.   Tachometer-generators and mechanical units
are not  reliable  for  operating speeds  that are  normally below
50  revolutions  per  minute.    Reluctance  pick-'-'p systems  can
provide  reliable operation at  virtually any speed.
    17.2.9  Moisture Content Measurements

Measurement of  the  moisture content of  dewatered sludge  is
necessary  if the  output  of weighing equipment is to be directly
interpreted as weight of  dry solids.


                              17-57

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There  is  no  proven  on-line  instrumentation  for  measuring
moisture in  sludge.    Consideration of  available  options  leads
to  essentially two  possibilities--a  manual resistance  probe
and  laboratory   tests.   A manual  resistance  probe must  be
considered  a very  approximate instrument since it  is  not actually
measuring  moisture,   and  resistance  measurements  (moisture
content)  will vary  significantly  with  the contact  pressure
between  the monitored  material and the  probe.  However,  the
portable resistance probe  can provide  the  accuracy needed  for
immediate  process control measurements;  for example,  compost
piles  or windrows.  The laboratory  test is the only moisture
measurement method, however,  that can provide  the repeatable
accuracy demanded  for QFD calculations  (see  Chapter  3).  Special
infrared drying equipment with integral weighing  instrumentation
is available to make such laboratory  testing  both convenient and
efficient.


    7.2.10   Dissolved Oxygen Measurements

Three  types of dissolved  oxygen  probes  are commonly  used  in
wastewater   treatment plants  for  measuring  the  dissolved oxygen
level  in  liquid  streams containing high levels of  suspended
solids.  These include  the galvanic cell  type, the polarographic
cell type,  and  the thallium cell type.

Each of these  cell  types has its own proponents.    The galvanic
cell is probably  the most  commonly  used  in  existing  wastewater
treatment  plants.   Both  the  galvanic cell and the polarographic
cell use  a membrane (usually  teflon)  through which oxygen  can
migrate into an electrolyte in which the electrodes are immersed.
Membrane  cleaning  and  electrolyte  replenishment  require  a
significant maintenance effort with  these  cells.    The thallium
cells  dispense with the membrane and  immerse  the electrodes
directly in  the fluid  to be analyzed.   None of  these cells  is
applicable  for measuring  dissolved oxygen in  liquids  having
solids  contents much higher than 2 percent.


    17.2.11  pH Measurements

Modern  selective-ion  pH sensors with  "non-flowing"  reference
electrodes  are suitable for  measuring the  pH of sludge.   The
non-flowing  reference  electrode  replaced  the  liquid  reference
junction in which  the  electolyte  (generally  potassium chloride)
flowed  continually from a reservoir  into the process  stream.
These  systems  sometimes plugged,  causing erroneous  readouts.
Non-flowing  reference  electrodes  use  a  semi-solid  electrolyte
that does   not require  frequent  replenishment  or  reservoir
pressurization  to  maintain  flow.   Electrodes  should  be installed
in lines where sludge flows pass  the  sensor,  maintaining a fresh
sample  at  the  measuring  point; for example, circulation lines.
Electrode  assemblies  should  be  designed  to  hold  electrodes
essentially flush  with  the pipe wall.   The electrodes should  be
easily  removable for cleaning or replacement.
                             17-58

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    17.2.12  Chemical  Oxygen Demand Measurements

Often  liquid  sidestreams  from  sludge treatment processes  carry
significant levels of  organics  back into the  liquid  processing
system.   The  chemical  oxygen  demand measurements  can be  useful
in determining the strength of these sidestream organic  loadings
and,  therefore,  provide  input on  their effect  on the liquid
treatment processes  downstream from  their point  of recycle.
Automated wet  chemistry  analyzers  are capable of  making  a
standard  chemical oxygen demand (COD) analysis, but  these  units
have  not  given satisfactory service under  wastewater  treatment
plant  conditions.   The total  organic  carbon  (TOC)   analyzer  is
more  suitable  operationally, providing suitable  correlation
can  be  established between TOC  analyzer measurements and  COD
laboratory data.  There  are  several units on the market,  and each
operates  somewhat differently.   Operation of one  TOC  unit  is  as
follows:   The sample is  treated  with HC1  to  remove inorganic
carbon  as C02»   It is  then oxidized in a  thermal  reactor  and
the resulting C02 measured  by an infrared analyzer.

TOC analyzers operate  with moderate-sized samples  and can  handle
suspended  solids.   However, they  are  high  maintenance devices
requiring daily servicing.


    17.2.13.  Ammonia Measurements

A  selective-ion  electrode  is  available  for measuring  ammonia.
Ions  other  than ammonia  frequently  interfere  with accurate
measurement and  elimination of  interferences  requires  treatment
of the  sample before the measurement is  made.  Package analyzers
are  available  to prepare  the  sample and make  the  measurement.
Since custom sample preparation is frequently required,  a  sample
should  be submitted  to  the  analyzer  manufacturer  prior  to
purchasing this type of  equipment.


    17.2.14  Gas Measurement and Analysis


        17.2.14.1  Composition  Analyzer

The  composition  of  digester gas   is a  useful  parameter  for
monitoring the  health of  the  anaerobic  digestion  process  (see
Chapter 6).   The chemical  process  industries make extensive  use
of on-line gas chromatographs  for measuring gas composition.  The
heart of  the chromatograph is the  "column."   The column is a
length  of tubing filled with  an  absorbent  material.   As  a  gas
sample  passes  through this  column,  different  gas  components  are
first absorbed,  then released back into  the  gas stream.   The
rate  of absorption/release  is  different  for  each  component and,
as  a consequence, each component emerges  from the   column  at  a
different time.   The components  are thus  separated from  one
another.   A detector at  this  exit measures  the eluting  gas,
                              17-59

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and its output is plotted as a function of time.   The  resulting
plot consists of  a  series  of  peaks and valleys, with  each peak
representing  the detector's response to one  of the gases being
measured.   Each  peak  can be associated with a  specific  component,
since  the  time  (relative to  sample  injection)   at  which  the
component  peak will  emerge from  the  column  is known.   The area
under each  of the  peaks is proportional to  the gas  concentration.
Even  though this  unit  is  called  "on-line" it is a  batch
instrument  which, at  best,  might  make  four  measurements per
hour.   The  on-line  mass  spectrometer  is  also capable of these
measurements  but is even more  expensive than  the  chromatograph.

Digester gas  samples for  analysis  must be stripped of hydrogen
sulfide and  filtered to  remove  solids before passing through
analysis equipment.   Sample lines  must be heat-traced to avoid
moisture condensation.    With   adequate  sample preparation,  gas
analysis instruments should function without  undue  maintenance;
however,  at  present,  no  data  is available on  a  successful
wastewater  treatment plant  installation of  any  of  these
instruments.
        17.2.14.2   Calorimeter

A suitable  instrument  for  measuring  the  heat value of  digester
gas is a calorimeter,  which essentially burns  a  gas  sample.  The
instrument must be located in an area free of drafts, which can
affect  its  accuracy  or even  extinguish  the  flame.    Instrument
response  is slow.   This  should be  of  no consequence during
monitoring applications,  however,  since digester gas composition
will normally change slowly.  Care must be exercised, however,  if
the instrument  is  to  be  used to control  mixing  of  digester gas
with  other  gases to maintain  a constant heat  value or if the
instrument  is to be used  with  a multiple sampling scheme for
monitoring  several  digesters.   Calorimeters   have  been used
successfully  in  full-scale  operations at  wastewater treatment
plants.
    17.2,15  Stack  Gas  Measurements
             and Analysis

On-line analysis of  boiler  or furnace stack gas  composition  is
used  frequently and has proven successful.   It  is  directly
applicable to wastewater  solids systems incorporating heat  drying
and high  temperature  processes.  These measurements  are used
for combustion control  and are usually mandatory if air  pollution
is  to  be  minimized.  Obtaining a  representative  sample and
conditioning it  for  the  analyzer  are the  biggest  problems
in  application of  these instruments.   There  are  a number  of
different  parameters that  may require  measurement  to  meet air
pollution control  requirements, but  oxygen  is  the parameter
normally used to control  the air-fuel ratio.   Two  types of  stack
gas oxygen  analyzers are  commonly  used.   The older unit  is the


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paramagnetic type and  the  more modern  is  the catalytic  type.
A new  system uses a ceramic  element  for which the manufacturer
claims  satisfactory operation on  dirty  flue  gases  without
clean-up.    Precautions  are  required where sulfides are present.
Oxygen  analysis  equipment  is  normally  included  as part of  the
combustion control system of a furnace.
    17.2.16  Odor Measurements

Odor  measurements  are  required  during sludge  management  to
assure  that  the treatment and  disposal  processes  selected meet
regulatory agency  requirements  (no nuisance).   There  are  no
instruments for on-line measurement of odors.  A device called a
"direct  reading  olfactometer"  (DRO)  provides a means  to  make a
semi-objective  manual measurement  of  odors  directly  in  the
location affected.   Figure 17-4 shows  a  close-up of  the  DRO
assembly and a  DRO  in  use  with  subject  and  controller.   The DRO
is essentially  a  breathing  mask with carbon filter, rotameters,
and  valves  to  permit  mixing  known ratios  of  filtered  and
unfiltered ambient  air.  The  subject  conditions his  nose  by
breathing  100  percent  filtered air  and   then  the  operator adds
increasing amounts of  unfiltered  air  until  the subject indicates
he detects an odor.  Repeated measurements with the same subject
permit detection of changes in  odor  conditions or odor levels  at
different  locations, within  an  accuracy   of  about  plus  or minus
25 to 50 percent.   However,  no  absolute  measurement  exists.
Standard test procedures  call for  the use of  odor panels  (usually
six people) who rate odor levels from bagged  samples taken at the
location affected.   The panel  usually  works  in a filtered  air
environment,  where absence  of extraneous  odors  can  be guaranteed.


    17.2.17  Aerobic Condition Measurements

Aerated  pile  composting operations  require the  measurement  of
oxygen concentration in  the pile.  The portable oxygen indicator
frequently used  for personnel safety  monitoring is applicable  to
this service.   These instruments operate  on  the same principle as
the  catalytic  or polarographic cell  dissolved oxygen analyzers
described  in  Section  17.2.10 but are designed to be portable,
with a hand pump for drawing  a gas sample.


    17.2.18  Blanket Level  Measurements

Measurements  of  sludge blanket  level in   sedimentation tanks and
gravity  thickeners  can be  accomplished  with optical  (turbidity)
type  instruments  and with ultrasonic ' instruments,  as described
in Section 17.2.6.  The  success of this  measurement is dependent
on the  characteristics of the  sludge blanket.  A well defined
blanket  interface  provides  a  readily detectable  change  in
suspended  solids concentration.  Where the blanket  is  poorly
defined, this measurement is  not satisfactory.


                             17-61

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       t'Jt
        ASSEMBLY DETAILS OF DRO SHOWING
  SUBJECT'S MASK AND CONTROL'S DILUTION METERS
SUBJECT AND CONTROL MEASURING FOR ODORS IN FIELD
       BATTERY PACK USED FOR PORTABILITY
                   FIGURE 17-4

      DIRECT READING OLFACTOMETER (DRO) (4)
                     17-62

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Air-lifts with intakes at multiple  elevations have been provided
in  a  number  of plants  for  drawing a  sludge sample.   The
individual  air lifts for a  tank  can be manifolded  together
and the  flow  passed  through  a  turbidimeter to provide  remote
monitoring.   The individual air lifts are actuated in sequence,
and a sludge profile is obtained.   A.  turbidimeter of the falling
stream  type is recommended  for  this service.   It  should be
installed at the air-lift  location.
    17.2.19  Hydrocarbons and Flammable
             Gas  Detectors

Methane  is the  flammable  gas most  likely to occur  in  sludge
management.   Catalytic detectors,  available  from  a  number of
manufacturers,  are sensitive  to any  flammable  gas  and ordinarily
may be installed  in  the space  to  be  monitored, thus eliminating
sampling  systems.   The  detector  consists  of a heated catalytic
element  exposed  to  the ambient  air and  a similar  reference
element isolated  from ambient air.   If flammable gas is present,
the exposed  element  temperature  will rise  above  the reference
probe  as the  gas is oxidized.   This  temperature  difference
results in a  change  in  electrical resistance,  which is measured
by the detector's electronics.   These units  should be calibrated
periodically  with a  standard  reference gas.   Catalytic probe life
is definitely limited, and  periodic replacement  is  required.
Under  very severe conditions,  the  probe may  lose  sensitivity
in less  than  a  year.   When  these conditions  occur, a sampling
system to  clean  up  the  sample  and remove the  moisture should be
considered.
    17.2.20  Radiation  Monitoring

If  gamma  radiation is  used  in  sufficient  quantities  to effect
treatment,   safety monitoring   will   be  required  to  protect
personnel.    Note  that  nuclear density  and weight  equipment
uses such small gamma sources that no  significant  hazard exists,
and  personnel safety  monitoring  is  not required.   Personnel
safety monitoring requires monitoring  of  the  radiation levels in
the  exposed  spaces  to  detect abnormal leakage from the process
and  individual monitoring to detect exposure  of that  individual
to  radiation.   Space  monitoring  is  accomplished  by suitable
geiger  counters.   Personnel who  are not normally exposed to
radiation  can  be  adequately  monitored  by   badges  containing
photographic emulsion.   Personnel  who  may absorb  radiation
during  job  performance  will  have  to  carry  instrumentation
capable  of  accurately  accumulating   the  amount of  radiation
absorbed to  control dosage to acceptable  limits.   Specialists in
nuclear monitoring  must  be consulted  if this  type of process is
contemplated.
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    17.2.21  Machinery Protection

Wastewater solids  treatment  and  disposal  machinery requires
protection  similar  to that  required  by the machinery in  other
wastewater treatment systems.   However,  some  solids system
protective  instrumentation  is  unique.   This section  deals  with
this unique protective instrumentation.


        17.2.21.1  Empty  Pipe Detectors

Empty  pipe detectors were  developed  to provide  protection  for
sludge pumps that might be operated with no fluid in the  suction
pipe.   This protection  is   particularly applicable  to  positive
displacement or  progressive  cavity  pumps, which  can suffer
extensive  stator damage  if  operated  without  fluid.   Capacitance
elements  fabricated  as  a wafer to fit  between  pipe  flanges  are
most  commonly used.   The theory of  operation is identical  to
the  capacitance  level  elements described earlier.   Nuclear
level  switches can also be used for this application but  require
more mounting space.  The nuclear device clamps onto the  outside
of  the pipe and operates much like  the  nuclear density meter
described earlier.   This is a very  simple  application of  the
nuclear unit.  The  unit  used has  a  much lower  cost than  nuclear
units  required for  density  measurement.   The nuclear device  may
be  easier to install  in existing plants since  existing  piping
would  not have  to  be disturbed  as long as  sufficient space  is
available.
        17.2.21.2  Vibration -  Acceleration and
                   Displacement Systems

Vibration detectors are provided for most machinery that operates
at  high  rotational  speeds--for example,  centrifuges.   Vibration
detectors are  usually  capable of giving  advance  warning  of
incipient  machine  failure.   This  allows for  orderly  shutdown,
thereby  minimizing  damage  to  both  process  and machinery.   The
cost  of  the  protection  afforded  is usually justified  only  for
larger pieces of equipment.   Two types of detectors are generally
applicable:   acceleration  and  displacement.   Accelerometers  are
less  expensive  than  displacement  systems  and  provide  moderate
protection to  lower  value machinery,  such  as  thickening  or
dewatering centrifuges.  Displacement  probes are mounted rigidly
to  a  bearing  pedestal  or similar stationary object and provide a
very  accurate measure  of  actual shaft  movement in the journals.
A  large  number  of  displacement probes  are  required  to provide
full  protection.   Their  installation  and  alignment is  rather
complex  when compared to  the accelerometer,  which is  simply
attached to  the  machine housing.   As  a  result, displacement
installations must  be  carefully  engineered  and  are  relatively
expensive.   Displacement  systems are  generally used on  large,
high-speed machinery,  such  as  centrifugal  blowers  in  sizes  of
500 horsepower or greater.


                             17-64

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        17.2.21.3   Flow Loss Monitors

Gas or air flow can be effectively monitored  for  loss of  flow by
vane switches, differential pressure switches, and  thermal  flow
switches.    Vane  switches are  the simplest,  but require  fairly
high velocities for reliable operation.   A differential  pressure
rise from the  suction to the discharge side of a fan or  blower,
or a differential  pressure loss from the suction to  the discharge
side  of  a  filter  or other piping  element provides  a simple
monitor  of flow  that is  adequate  for  most purposes.   Where
precision operation is required,  particularly at  low velocities,
a thermal flow switch  is  most suitable.  These devices consist of
a heated  element that is  convection-cooled  by  air flow.  The
change in heat loss  of the element provides reliable detection of
gas or air flow.

Vane  switches, differential pressure switches, and  thermal  flow
switches are also applicable to liquid  flows.   However,  the  vane
switch is unsuitable  for solids-bearing fluids,  such as  sludge.
One  thermal flow  switch is constructed  as a smooth  rod.  If
installed at an angle  with  the  pipe radius or into an elbow,  this
unit  is applicable  to  solids bearing  fluids.   Differential
pressure devices  must  be  provided  with chemical seals if  they are
to be successfully applied  to solids-bearing fluids.


        17.2.21.4   Overload  Devices

All  electric motor drives  at  wastewater  treatment plants are
provided with  thermal overloads.   However,  these  units  are not
fast  enough to protect  the driven machinery from damage due
to  mechanical  blockage.   Collector drives,   in particular,
are  virtually  always  provided  with  some  type of  instantaneous
protection from  excessive  torque.   One  of the  most common
applications  involves   the circular  collector  of  secondary
sedimentation tanks  and  circular  gravity  thickeners.   The
simplest overload  device for  such  equipment  is  the shear  pin.
The shear pin has the  disadvantage of working only once.   When it
has  provided  protection  for one  overload,  it must  be  replaced
with an identical  pin. As  a result,  several mechanical  resetable
overload  devices have been used.  The one  most commonly  used
today  is an instantaneous  over-current  relay  or  ammeter  with
high  alarm contacts  installed  in  the  motor circuit.   These
units  are  simple,  very  reliable,  and also provide  a  continuous
indication  of  load.   This   is  useful  for  detecting any  abnormal
load build-ups.


        17.2.21.5  Flame  Safeguard Equipment

Wastewater solids  systems  that  use  boilers  or  furnaces to
maintain anaerobic  digestion,  heat drying, or high  temperature
processes   require  flame   safeguard  instrumentation.   Flame
safeguard equipment shuts off  the  oil and  gas burners in  case of


                             17-65

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ignition loss.   Ultraviolet  light  detectors  provide virtually
instantaneous protection since  ultraviolet energy  is  present
only in  the  actual  flame.   This equipment is normally provided
as  part of  the  burner  control package.   This package  also
includes sequencing  systems as necessary to ensure the purging of
explosive  gases  from the fire  boxes before burner  relighting
following a flame-out.


17.3  Sampling Systems

Sampling  systems include  sample transport  and  sample  condi-
tioning.  Where practical, measuring elements  should be installed
directly into process vessels.  In some cases,  however, immersing
a measuring element  directly into the process  is  not possible or
desirable.    Some analyzers  simply   are  not adaptable  to direct
on-line measurements.  In other cases, the cost of an  analyzer is
so  great that it must  be  time-shared between  several  sample
streams.

Anytime a sample  must be  transported a significant distance, care
must be taken to  ensure that the  sample delivered  to the analyzer
is  fresh and  that critical  characteristics do not change during
the  transport time.   Pumps  and piping materials must  not be
corroded by the  sample  nor should they  in any  way  affect the
sample  composition.   Fluid velocities in transport lines must be
kept  high  enough  to prevent solids  settlement and  to  limit
transit time.   Flow  to  the  analyzer should be  continuous to
maintain clean lines and  deliver a  fresh  sample  immediately to
the analyzer, where   sample  switching is practiced.   Where pumps
are  required, they must  be suitable for  continuous operation
without excessive maintenance.  Where switching  systems are used
to  direct  multiple  samples  into  an  analyzer,  three-way diverter
valves  are required for  each sample stream, with  one port to
the drain and the other to the sampler.

Solenoid-controlled, pneumatically  operated ball  valves are
recommended  for  sample  switching.    These  units  are  capable of
handling many operations  without  excessive maintenance and
can  provide slow operation of  the ball  valve  and,  therefore,
smooth  switching  of the  sample  stream.   Electric motor-operated
ball  valves  can  be  used  but  life  expectancy  of ball valves in
repetitive operations   is  short.   Rapid direct  switching with
large  solenoid  valves  causes  significant pressure  stress on
sample  valve  piping.   If  solenoid valves are  used  to switch
samples directly,  some  system must  be provided  to  absorb
water  hammer.    Large, three-way solenoid valves with  suitable
characteristics are  not readily  available; therefore,  two two-way
valves,  one  normally  open  and one   normally closed, are usually
required to obtain the three-way  switching function.

Some  type  of  program  timer  is required  to control  sample valves
and  synchronize  readout devices  with  L'ne samples.   The time
program must  also  consider  the settling time  of  an  analyzer


                              17-56

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       switching  samples.   The  settling  time  includes  both
transport  d e 1 a " s required  to ^ e t  new samples  through pi^in0
between  sv/itching valves and  time  for the analyzer itself to
reach new readings.   In many cases,  an  analysis is  used  only for
recording or indication (that is,  not  for  control purposes), and
it is acceptable to provide  a single output  instrument with some
means of identifying the sample source  currently  being measured.
In systems where the output  of the analyzer  is  used  for  control,
some means must be provided of  holding the last  value of the
parameter during periods  that  the analyzer  is  working  on  other
samples.   It is also essential to review control  system  dynamics
in an  intermittently  sampled data  environment.   In general,
completely  different  control  strategies  are  required  for
intermittently sampled data systems than  for continuous data
systems.

Gas  sample lines should  be heat-traced to avoid  condensation
within the lines.  In cold climates, exposed liquid  samples will
also require heat tracing  to  avoid freezing.

Sample  preparation  is  critical  to satisfactory  operation of
analysis equipment.  The degree of grinding  and/or filtering
required depends on  the  nature of  the analysis  and the equipment.
In general, the  aid of  the analyzer  manufacturer  should be
enlisted in  working out a suitable  system.  More information on
sample transport is  available (5).


17.4  Operator  Interface
    17.4.1  Location

Modern  electronic  instruments  that provide information  to
operators  (for  example,  indicators  and recorders) are  designed
for  installation  in  clean,  air-conditioned control rooms.
Field locations  are  usually not  suitable  for  these  instruments
unless  additional protection  is provided.   Hydrogen  sulfide  is
present in  many  process  areas,  and  if it  is  allowed  to contact
instruments that are not  designed for this atmosphere,  failures
may result  from  corrosion.  Some process  areas are classified  as
hazardous,  so that electrical equipment must be explosion-proof.
Explosion-proof  electronic operator interface instruments are not
available.  To be usable  in a hazardous area, non-explosion-proof
instruments must be  enclosed in a suitable  box.  This  makes  them
virtually inaccessible  and, therefore,  difficult  to use  and
maintain.    Where instruments  must be  located  in  a  contaminated
or  hazardous process  area, pneuma.tic instruments,  which  are
inherently  explosion-proof and are fairly  resistant to  dirt  and
corrosion,  should  be   considered.   Where  pneumatic  instruments
are  not practical,  air  purging  of  cabinets or special  filters
may  provide  adequate protection to electronic instruments.
A suitable remote control room  is  the  most desirable solution.
                              17-67

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    17.4.2  Indicator  Boards

Sludge handling  systems  are frequently designed with considerable
operating  flexibility,  with  large numbers  of valves and  many
possible  flow  configurations.   As a  minimum,  some means  is
required to tell the operator  what the present  flow configuration
is.  A chalk board can be used for this purpose, but it does not
readily provide  a graphic picture of  the  piping configuration.
Therefore,  in more complicated  plants, some type of graphic
indicator board is desirable  to prevent errors.  In the simplest
form  of  indicator board,  a  graphic  panel  is  produced  with
manually  moveable flags or  indicating lights with which the
operators  indicate current  valve positions  and  pump operation.
Such  a system  can give  an  excellent  picture of the present
operating configuration,  but is dependent on  the operators to set
the flags  correctly.   The use  of limit switches  on  valves and
indicating lamps  is more reliable and also provides the operator
with  a  ready  means to check  the  validity of the valve settings
and pump  selection.   Figures  17-5  and  17-6  show two examples of
graphic panels with indicating lights  for  showing valve  or  gate
positions.
17.5  References

1.   U S E P A.  Instrumentation  and Automation  Experiences  in
    Wastewater  Treatment  Facilities.    MERL.    Cincinnati,  Ohio
    45268.  EPA-600/2-76-198.   October  1976.

2.   Liptak,  B.C.,  editor.    Instrument  Engineers  Handbook.
    Chilton Book Company.   Radnor^Pennsylvania."  1969.         '

3.   US EPA.    Advanced  Automatic  Control  Strategies  for  the
    Activated Sludge Treatment  Process.   ERIC.   Cincinnati,  Ohio
    45268.  EPA-67(I/2-75::OT
-------
                           .
                 .
                  ~  '   Vr-
                FIGURE 17-5



AERATION CONTROL GRAPHIC PANEL AND CONSOLE

   LIGHTS SET MANUALLY ON GRAPHIC PANEL
                   17-69

-------
                  FIGURE 17-6

  INCINERATOR-DIGESTER CONTROL GRAPHIC PANEL
LIGHTS CONTROLLED BY REMOTE VALVE LIMIT SWITCHES
                     17-70

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EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
               Chapter 18. Utilization
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

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                          CHAPTER 18

                          UTILIZATION
18.1   Introduction

Utilization  refers to  the  beneficial use  of  sludge or sludge
by-products.    Sludge  disposal options that  do  not  involve
beneficial use (for example, when disposal  is  the  only goal)  are
discussed in  Chapter 19.

Sludge may be used  as  a:

     •  Soil  amendment.  Sludge  contains  both  crop  nutrients  and
        organic  matter.  Sludge can  be used as  a fertilizer
        and  in  the  reclamation  of disturbed lands,  such  as
        construction  sites,  strip-mined  lands, gravel pits,  and
        clear-cut  forests.   It  may be  used  to  stabilize bank
        spoils and  moving  sand dunes.

     •  Source of heat and work.  Energy  may be  recovered from
        the  gas  produced  during anaerobic stabilization,  or
        partial  or  full  pyrolysis of sludges, and from the  direct
        burning  of  sludges.  This  energy may be  converted  to
        heat  or  work  and  put to a  variety of  in-plant  uses,  or
        exported  for uses  outside the plant.

     •  Source of other  useful products.  Other useful products
        include   waste  treatment  chemicals,  landfill  toppings,
        industrial  raw materials, animal feed, and  materials  of
        construction.

The thrust of recent legislation has been to encourage beneficial
reuse.    The Federal  Water Pollution Control  Act  of 1972
(PL 92-500) stated that "The Administrator shall  encourage waste
treatment management which results in the construction of revenue
producing facilities  for .  . .  the recycling of potential  sewage
pollutants through the  production of  agriculture,  silviculture,
or agricultural  products,  or any combination thereof."  The Clean
Water Act  (CWA)  of  1977  (PL 95-217) offered  further incentives
for projects  that involved  innovative and alternative technology
(for  example,   sludge utilization,  energy  recovery).   In
addition,  the CWA requires  the establishment  of industrial
waste  pretreatment  programs with  the  objective  of reducing
toxic  pollutant loadings  to municipal treatment facilities.
Implementation of pretreatment programs will make  more municipal
solids suitable  for  reuse.


                             18-1

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The pretreatment  program  supplements programs established  by  the
Toxic Substance Control Act  (PL  94-469)  which  authorized  USEPA
to obtain  production and test data  from industry on selected
chemical  substances  and  regulate  them where  they  pose   an
unreasonable  risk to  the  environment.   Steps towards the goal
of furthering  sludge utilization  were  taken by  the Resources
Conservation  and Recovery  Act  (RCRA) (PL  94-580),  which
authorized  USEPA to  develop  treatment and  application rate
criteria  for  sludge  to be applied to  land  growing food-chain
crops,  as  well  as  to nonagricultural areas.  RCRA also authorized
funds  for  research,  demonstrations,  training, and  other
activities  related  to development of  other resource recovery
schemes.


At the  same time, it  is recognized that there  are potential
hazards associated with wastewater sludge utilization and that
utilization without  careful  planning,  management,  and  operation
could present  a danger to human health and to the environment.
18.2-  Sludge  as  a Soil Amendment


Approximately 1.3  million  dry  tons  per year (1.2 million  t/yr),
or 31  percent of  the  treated  municipal  sludge  generated  in  the
United  States  today,  is  applied  to  the  land  for productive
use.    The quantities  of  treated sludge  projected  for  ultimate
disposal by 1990 range  from  5.6 to  7.6 million dry tons  per year
(5.1  to  6.9  t/yr).  The sludge quantities generated will  depend
in .great part  upon the  extent to which municipalities adopt
land  treatment  of wastewater.  Land treatment,  which  is an
alternative  to  conventional   forms  of  wastewater treatment,
reduces substantially the amount of  sludge produced.
    18.2.1  Perspective


The  impact  of  sewage  sludge  on  the  national  commercial
fertilizer market is relatively  insignificant.   This  is  shown  in
Table 18-1, where the amount of nutrients in currently  utilized
and  potentially usable sludges  are compared against the
nutrients  presently  consumed  in the  form  of  commercial
chemical  fertilizers.   Nitrogen,  phosphorus,  and potassium  in
currently utilized sludges are estimated to be only 0.2,  0.9, and
0.1  percent,  respectively,  of  those  nutrients  consumed with
chemical  fertilizers.  If  all  United States wastewater  sludges
were applied to  land,  these  percentages would  increase  to  0.6,
3.2, and  0.4 percent,  respectively.  If the value per  pound  of
nutrient  in  the sludge  was  the same  as  that  paid  by  farmers
for  the  corresponding  commercial  nutrient,  the monetary
value of  utilized nutrient  sludges in 1978 was  $9.5,  $26.0, and
$1.7 million per  year  for  nitrogen,  phosphorus,  and potassium,
respectively.


                             18-2

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                            TABLE 18-1

      COMPARISON OF CURRENT AND POTENTIAL SLUDGE UTILIZATION
    TO COMMERCIAL FERTILIZER CONSUMPTION IN THE UNITED STATES (1)
                     Nutrient usage, 1,000 ton/yr
   Nutrient
             A.
  Nutrients in
  currently
utilized sludges
B. Nutrients in
  potentially
useable sludges
  C. Nutrients
presently consumed
  in commercial
   fertilizers
Nitrogen as N
Phosphorus as P
Potassium as K
21.63
21.5
4.3
65. 3a
80.7
18.9
10,642
2,453
4,841
 A, as
percent
 of C

  0.2

  0.9

  0.1
 B, as
percent
 of C

  0.6

  3.2

  0.4
 Nitrogen in sludge expressed as available N, assumed to be
 50 percent of total N.
While the values of nutrients  in sludge are small relative to the
current  dollar  values of  commercial fertilizers,  they  are by no
means  insignificant  to those  who would benefit  monetarily.   For
example, wastewater treatment  plants could reduce operating costs
by sludge sales or by  elimination of more expensive treatment and
disposal methods.   Sludge  users, for  example,  private citizens,
can obtain nutrients  for  lawns and gardens at low cost.

It  is  estimated  that  by  the  year  1990,  annual  savings  in
treatment costs  could  be $100-$500  million  if sewage  sludge
utilization  were  increased  to about  50  percent  (2).    This
utilization  increase  could result,  in part,  from the  incentives
for  innovative  and  alternative technologies  provided  by  the
1977 CWA if  various constraints  to  sludge utilization, including
regulations,  are  not  overly  stringent.   If  50  percent of sewage
sludge were utilized  on  land,  about $50 million  (1978 dollars) in
nutrients and organic matter  could  be  recovered and utilized for
growing  crops and improving soil structure.

A  number of  locations where  various  sludge  utilization options
are  currently being employed  are listed  in Table 18-2.  Some of
these  operations  have only recently started  up  (for example,
Madison, Wisconsin),  while others have been  in  operation for as
long as  50 years (for example, Los Angeles County, California).
    18.2.2  Principles  and  Design Criteria for Applying
            Wastewater  Sludge to Land


Certain  basic  elements  are  common  to  all  land application
projects,  no  matter  how or  where the  sludge is  to  be applied.
These  elements  include  preliminary planning,  site  selection,
process  design  (which  includes  determination  of  sludge
                               18-3

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application  rates),   facilities  design,  and  facility  management
and   operation.    Full  and   complete   discussions  of   each   of
these elements  are  too  lengthy  to  be  included  in  this  manual.
Therefore,  this section  will  provide  only  a brief outline.    For
full  details,   the reader  should  consult  Reference  3.    At  this
writing,  this  is USEPA's primary  reference  for  the  utilization of
sewage  sludges on land.   The  entire  subject  of  sludge  use  on
land will  be  covered  more  extensively  in a  future  Technology
Transfer  design manual.
                                 TABLE 18-2

        EXAMPLES OF COMMUNITIES PRACTICING LAND UTILIZATION  (2)
        Communities
Wastewater
  flow,
   MGD
  Sludge
 utilized,
dry ton/day
Description
Landspreading of liquid sludges
  Clinton,  New Jersey               1
  Rochester, Indiana                1
  Little Falls, Minnesota            1
  Peru, Indiana                  2.5
  Bowling Green, Ohio             3.5

  Muncie, Indiana                 17
  Salem, Oregon                   30
  Madison,  Wisconsin               36
  Seattle,  Washington             150
  Chicago,  Illinois               909

Composting
  Durham, Mew Hampshire           0.8
  Burlington, Vermont             5.8
  Toms  River, New Jersey          6.5
  Bangor, Maine                    7

  Windsor,  Ontario                21
  Camden, New Jersey               32
  Philadelphia, Pennsylvania       113
  Washington, D.C.               300
  Los Angeles, California          440
Drying
  Little Falls, Minnesota
  Largo, Florida
  Marion, Indiana
  Fort Worth, Texas

  Houston,  Texas
  Toledo, Ohio
  Milwaukee, Wisconsin
  Denver, Colorado
  Chicago,  Illinois
                0.5
                0.8
                0.6
                0.8
                1.7

                 10
                  8
                 27
                 28
                165
                0.7
                2.3
                7.8
                  2

                 25
                 12
                 30
                 55
                150
1
8
9
75
73
78
132
140
909
0.4
2.5
0.2
41
18
35
190
125
131
              PO, PL
              MO, PL
              MO, ML,  PL
              MO, PL
              MO, PL

              MO, PL
              MO, PL
              FO, PL
              PO, PL
              MO, ML
              MO, GAM
              MO, PL, ML
              ML, PL
              MO, GAM

              MO
              MO, GAM
              MO, ML, PL
              PO, GAM,  S
              MO, S
                          Drying bed,  ML, PL
                          Heat dry, S
                          MO, PL
                          Drying bed,  MO, ML

                          Heat dry, S
                          PO, PL, Filter cake
                          Heat drying, MO, S
                          MO, ML, Filter cake
                          Heat dry, MO, S
a PO - Privately operated (contractor)
  PL - Private land
  MO - Municipally operated
  ML - Municipal land
  FO - Farmer operated
 GAM - Giveaway to municipality
   S - Sale
                                    18-4

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        18.2.2.1  Preliminary  Planning

Preliminary planning consists  of  the  following  steps:

     •  A  planning  team  is  formed  of  individuals who  are
        interested in  the  proposed program  and  whose expertise
        and  support are  required.    A  major  activity  of  the
        planning  team  is  to  solicit  and  obtain  public  support
        for  the  program,  particularly  the  support  of potential
        sludge  users  and  local  government.    The  importance of
        obtaining public support cannot be overemphasized.   Many
        utilization projects  have  failed  because  planners  have
        failed to recognize  this  necessity.

     •  Basic data is  collected, including sludge quantities and
        characteristics,  climatic  conditions  and  local,   state,
        and federal regulations.


        18.2.2.2  Site Selection

Site selection consists of:

     •  Preliminary screening.  A rough estimate of  total  acreage
        required  is  obtained  by dividing  total  sludge quantity
        by  an assumed  application  rate.   Land  that  might be
        available within about 30 miles  is  identified; obviously
        unsuitable  sites  are  immediately  eliminated.    If  this
        rough  analysis  indicates that sufficient land is
        available, a  more  detailed study  of potential sites is
        initiated.

     •  Site identification.  Potentially available sites
        remaining  after preliminary  screening are  characterized
        as  to topography,  land  use, soil characteristics,
        geology,  and distance  from  treatment  plant.   The
        characterization  at first  is general,  taken from
        published  and  readily  available  sources of  information,
        such  as  soils  surveys  and topographical maps.  The least
        suitable  sites  are  eliminated  by  an  objective  ranking
        procedure, similar  to  the  second-cut  analysis described
        in  Process Selection  Logic, Chapter 3.   The procedure
        is  reiterated, with  more  detailed and site-specific
        information  in each  iteration,  until  finally  the  best
        site or sites are  determined.

     •  Site acquisition.   Sites are acquired  either by outright
        purchase or by the  municipality obtaining a  contract for
        the right to use private  land for sludge utilization.

        18.2.2.3  Process  Design

Process design  involves selecting suitable crops  and determining
appropriate  sludge  application  rates as  well as  application
methods.  Although basic design goals  (maximization  of  crop yield


                              18-5

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and quality,  and minimization  of environmental  damage)  remain
constant  regardless of  projected land  use,  design procedures
differ for applications on agricultural,  forested, and reclaimed
lands:

    •  Application  on  agricultural  land.    Sludge should  be
        applied to  agricultural  land at  a  rate  equal  to
        the  nitrogen  uptake rate of the  crop  unless  lesser
        application  rates  are required because  of cadmium
        limitations.   Annual  loading  rates for  cadmium  on
        soils  have  been  set at  1.8  pounds per  acre per  year
        (2.0  kg/ha-yr)  for food chain crops;  however, this value
        can be regarded as provisional and may be revised on the
        basis of ongoing  and  future  research  and  future  federal
        regulations.   The basis for the nitrogen criterion is  to
        minimize nitrate  leaching to  groundwater.    The  annual
        limit for cadmium is  chosen  to minimize uptake by crops
        and  the  potential  for  long-term, sub-clinical  adverse
        effects  on human health.   Site lifetime  limits  are
        established  on the  basis  of  maximum  cumulative loadings
        of lead,  zinc,  copper, nickel,  and cadmium.  These limits
        are designed  to allow growth and use of food-chain crops
        at any future  date.

     •  Application on  forested  land.   As with  agronomic crops,
        the  harvesting of a forest  stand removes  the nutrients
        accumulated  during growth.  However,  the amounts removed
        in forest  harvesting annually  are   significantly  lower
        than  in agronomic crop harvesting.  Uptake by vegetative
        cover  is  negligible.   Therefore, forest systems  rely
        primarily on  soil processes (denitrification) to minimize
        nitrate leaching  into groundwater.  As a result, nutrient
        loadings on  forested  lands must  generally be  less  than
        those on agricultural sites.   No annual limitations are
        set  for  cadmium,   since no  food-chain  crops  are grown.
        Lifetime metals  limits  used  for  agricultural  sites are
        suggested for  forested  land;  this would  minimize  metal
        toxicity to trees and allow growth of other crops if the
        area  were cleared at  a future  date.

     •  Application on  reclaimed land.  Sludge  is usually applied
        to impoverished lands at rates sufficient to satisfy the
        nutrient requirements of the  cover crop.


        18.2.2.4  Facilities  Design

Once  the  site  has  been  chosen  and crops and  approximate sludge
application rates have  been  decided upon,  the project can proceed
to  the  facility design   stage.   This phase  of the project  is
site-specific and consists of:

     •  Detailed site  investigations.   On-site soil analyses are
        conducted to  determine  such  factors  as  available
        phosphorus  and  potassium,  soil pH and lime requirements,


                              18-6

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        cation  exchange capacity,  and  organic matter.   Such
        information  will allow for finalizing  sludge  application
        rates determined in  the Process  Design  phase.   Soil
        should be characterized to provide  baseline data against
        which subsequent analyses  can be  compared.   This will
        allow documentation  of  changes  in  the  physical and
        chemical properties of  the soil due to sludge applcation.

     *  Determining  pre-application  treatment.   This refers  to
        upstream sludge  treatment,  including  thickening,
        stabilization,   disinfection,  conditioning,  dewatering,
        and  drying   (see  Chapters  5  through 10  for detailed
        discussions).   For  new plants,  the method of sludge
        disposal or  utilization  may dictate  the  preapplication
        proces&ing  configuration.    For  existing  plants,
        pre-application treatment!  influences  sludge form and
        composition,   and  thus affects  application  rate,
        the  method  of  spreading,  and  the mode  of  sludge
        transportation.

     •  Determining  sludge  application  mode.    The  application
        mode depends upon the sludge form.   Liquid  sludge  can  be
        spread by tank  truck,  sprayed,  injected,   or  applied  by
        the  ridge-and-furrow  technique.   Dewatered  sludges are
        usually  applied by  conventional  fertilizer spreading
        equipment.    See Chapter 19 for  a discussion of  sludge
        application  techniques.

     •  Determining  sludge storage requirements.   Storage  should
        be provided when  sludge cannot be spread  (for  example,
        during  inclement weather).  Storage can  also provide
        additional  stabilization  and  disinfection.   See
        Chapter  15 for  information on storage.


        18.2.2.5 Facility Management,  Operations,
                 and Monitoring

Once  the  system has been  constructed,  it must be made to run
smoothly and efficiently:

     •  Operations must  be scheduled.  Spreading  must be  timed  to
        satisfy  farming  requirements.    If  the  municipality
        grows its own  crops,   tilling, planting, and harvesting
        operations must  also  be scheduled.

     *  Operations  must  be  managed to  reduce  off-site  impacts
        (odors,  contamination  of  groundwaters,   and  surface
        waters).

     •  Operations must  be monitored to assure that the system  is
        operating as intended.   Sludge must be analyzed to ensure
        its acceptability  to the user  and to  provide a record  of
        nutrient and metal additions to  the soil.   Soil,  crops,
                             18-7

-------
        groundwaters,  and  surface  waters  need  to  be  monitored
        only if sludge nutrients are  applied  at  rates  exceeding
        the uptake capacity of crops or soils.:


18.3  Sludge as an Energy  Source

Whether  produced  from direct  burning  of sludge  or  from  the
combustion of  sludge-derived  fuels such as digester gas  or
pyrolysis gas,  the end product  is  energy.   Heat can be  made  to
perform a variety  of  useful functions.


    18.3.1  Perspective

The  precipitous  rise in  energy prices during  the  1970s  has
generated  intense  interest  in the conservation  and recovery  of
this precious  commodity.   For  example,  the United States Energy
Research  and  Development  Administration  (now the  Department  of
Energy)  has proposed  one-seventh of the  United States  energy
requirements be produced by bioconversion processes (for example
anaerobic  digestion)  by  the year  2020   (4).   Clearly,  however,
this  awesome  quantity of  energy will  not  be  generated  from
municipal wastewater  sludge;  there  is  simply insufficient sludge.
Very  large  external  organic  sources  (for  example, manure  from
feed  lots  or municipal refuse)  and  external  processing  systems
(energy  farms)  will  be required to effect  such  production.   As
with utilization of sludge on land, the impact of energy recovery
from municipal sludges will be  largely local,  that is,  it will be
felt  most  strongly  at the treatment  plant  and  in its  immediate
vicinity.  Here, the  effects  can be significant.

As  Figure  6-32  indicates,  the  energy  value of methane  generated
from the  anaerobic  digestion process  exceeds the energy
requirements of the digestion process.  The  excess can be used to
supply  the  energy needs  of other  plant  processes.    In  some
instances,  the  gas generated  is sufficient to supply  the energy
needs of  the entire  wastewater  treatment plant,  with  excess gas
available  for  sale.   Notable examples are  the  British Southern
and  Mogden plants  and  the  County Sanitation  Districts  of
Los  Angeles County  Joint Disposal  Plant  (5).   Heat  recovery
is  possible even  if  digestion  is not used,  for example,  heat
recovery  from  coincineration of sludge  and municipal  refuse is
expected  to provide  all  the energy  needs  of  the Central Contra
Costa Sanitary District (CCCSD)  plant  in  Concord, California (6).

In  January  1978, the State  of  California  Public   Utilities
Commission  (PUC) passed  a resolution  directing  all  state
utilities to augment cogeneration projects  by  setting up new rate
schedules  covering  interruptible electric  service; by creating
new specific rates to encourage  cogeneration,  including revisions
to  standby  rates;  and by developing guidelines  covering the
price and conditions  for the purchase  of energy  and  capacity
from  cogeneration  facilities  owned by  others  (7).   The  term
cogeneration  in this  context means  the  production of  power by
utilization  of  waste  heat; it also covers power  produced through
                              18-8

-------
the  burning of  alternative  fuels,  such as municipal waste.
The  resolution significantly changes  the economics  of power
generation  at  California  Wastewater  treatment  plants   and
encourages the  use  of  in-plant energy recovery.

On June 27, 1979,  the  Federal Energy Regulatory Commission issued
proposed  regulations  providing  for  the  qualification  of  small
power production and cogeneration facilities under Section 201 of
the  Public Utility Regulatory  Policies  Act  of 1978 (8).   The
proposed  regulations  are set up  to assure  opportunities  for
small power  producers  (<80  MW)  to sell  electricity to.electric
utilities when such electricity is generated  through the  use of
renewable  energy  sources  (such  as sludge) or  recovered  process
heat.

These regulatory actions  are  an indicator of future trends in the
United  States  as   the  country  seeks  to  increase  its  non-fossil
fuel energy  production.   The designer should be  aware  of  their
impacts on future  planning for using sludge as an energy  source.

The  recovery of  energy  in the  form of  fuels  and  heat  from
municipal  sludges  will  be discussed  in detail  in  the  following
sections.


    18.3.2  Recovery of Energy From Sludge

Figure  18-1  shows  on  one diagram  processes which release energy
from sludge; devices which convert the released  energy  to useful.
forms;  useful  energy  forms;  and  suggested  applications  of
recovered  energy,  either at  the  wastewater  treatment  plant  or
off-site.   Special consideration must  be made when designing
processes to recover  energy from wastewater sludge.   Some of
these considerations are  discussed below.


        18.3.2.1  Treatment  of Digester Gas

The  treatment required depends on  the  digester  gas1  anticipated
use.  Treatment is minimal if  the gas  is burned in a boiler or in
a  high  temperature internal combustion engine.   Conversely,  if
it  is sold  for  utilities  as  a  natural  gas  substitute it  must be
upgraded  to natural  gas quality.  This  involves treatment to
remove part iculates, H2S,  CC>2, and water.   As a general  rule, gas
treatment should be avoided  to as great a degree as possible.  It
is  preferable  to  set   up  recovery  systems that  can  be  operated
with untreated  digester gas.

Particulates are  carried over with the  gas  as it leaves the
digester.   They may be removed  in large  sedimentation  traps and
cyclonic separators.

H2S  is  most commonly removed  by  iron-sponge  scrubbers.   The
"sponge"  consists  of wood shavings  impregnated  with  iron oxide.
H2S  reacts  with  iron  oxide to form  nonvolatile  ferric  sulfide.
The  sponge  can be regenerated with air.  Sponge capacity is


                              18-9

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                          THE  RELEASE,  CONVERSION, FORMS AND USES
                                         OF ENERGY  FROM  SLUDGE
                                                        18-10

-------
about 0.6 pounds of sulfur per pound of  iron oxide  (0.6 kg/kg).
Problems have  been  experienced with fouling of  the  iron-sponge by
oils and  greases entrained  in  the digester gas.   Iron-sponge
scrubbers  are  commercially available.    Other H2S  scrubbing
processes are  less  commonly used and are proprietary.

CC>2 removal  processes  can be divided into three  broad categories;
absorption  (both  physical  and   chemical),   adsorption,  and
cryogenic processing.    Many  CC>2  removal  processes  also remove
H2S.  The only process which  has  received much  use in wastewater
treatment  plants  is  absorption   in  water;  this  process  has
been tested at Modesto,  California,  and Los  Angeles  County,
California.   In 1976, total costs for a water scrubbing  unit
of  1,000,000  cubic feet  per day  (28,300 m3/d) capacity  were
estimated at  $2.50 per million  Btu ($2.37/GJ) of  energy  (9).
Some methane is  also absorbed during the scrubbing process; costs
were based on  energy leaving the scrubber as opposed  to energy in
the untreated  gas.  Of this, $0.15 per million  Btu  ($0.14/GJ) was
attributed to  the  cost  of  iron-sponge  H2S  removal,  which  must
necessarily  precede the water scrubber.   It was estimated  that
this unit would produce  2  MGD (87  1/s)  of spent scrubbing water.
Costs for treating  the spent scrubbing water were included in the
estimate.  These units are commercially available.

Gas leaves the digestion system  at approximately 95°F  (35°C) and
is  saturated  with water  vapor.   During transport the gas is
cooled.   Condensate formed must  be removed to  protect downstream
equipment.   Water traps should  be  installed at  low spots in the
gas pipe and at frequent intervals.   If moisture must be reduced
substantially,  adsorption drying  or glycol dehydration  can be
used.
        18.3.2.2   Gas-Burning Equipment

Corrosion Factors

One  of the  major  problems associated with  recovering  heat
from  digester gas  is  corrosion caused  by S02  and  803,  the
combustion products  of  I^S.   If the exhaust gas temperature is
allowed  to  drop below  its  dewpoint,  the  condensate  which
forms  is  acidic  as  the result  of  absorbing S02 and  803.   The
acidic  condensate  is  corrosive  to metallic  elements  of  the
exhaust-carrying  system.   There are two alternatives  to  alleviate
the  problem.   The   first  is  scrubbing  of I^S  from  the  gas
before combustion.   The second  is  maintaining the exhaust gas at
temperatures  considerably  greater  than its^ dewpoint, to prevent
condensation.   This  generally requires that the  water temperature
of any  boiler  or  engine using  unscrubbed  gas  be at least 212°F
(100°C).   Also,  stack  gas  temperatures should not be allowed
to drop below 350° to 400°F (177° to 204°C).  Use of unscrubbed
digester  gas  is  preferred.   Equipment  fueled by  unscrubbed
digester gas should   not be  used in  intermittent service,  since
condensation  will  occur  each time the  unit  is  shut  down.
                             18-11

-------
Shutdowns should  be minimized.   Similarly, the  equipment should
be designed  so that  even when  operated  at its  lowest  loadings,
exhaust gas  temperatures  are  sufficiently  high  to  prevent
condensation.
                          VENT
                                           HEAT TRANSFER
                                           TUBES
              CONDENSER
                                    FREE-LIQUID
                                    SURFACE
                                              RETURN
                                              WATER
STEAM-
WATER
MIXTURE
            BOILER
                           RETURN   HOT
                            FROM   WATER
                            HEAT     TO
                           DEMAND  HEAT
                                  DEMAND
                            FIGURE 18-2

              SCHEMATIC OF COMBINED BOILER/CONDENSER
                 SYSTEM FOR HOT WATER PRODUCTION
                               18-12

-------
Bo i 1 e r s

Scotch-type  tube boilers  and cast iron  sect ional ized boilers
have both worked well with untreated digester  gas  as  long  as  the
water or  steam  temperatures  are  maintained  above  212°F  (100°C).
Figure  18-2  illustrates  an effective method  for  hot  water
production using boilers.   The heat source (the boiler)  and heat
demands  are not  directly  tied  together,  but separated by a
condenser.  The  condenser  is  mounted  directly  above  the boiler.
The specific gravity of the  steam/water mixture produced  in  the
boiler  tubes  is less than that  of the water  returning  to  the
boiler.   The  mixture is  displaced upward  into  the  condenser,
gives up  its heat, then flows by gravity  back  to  the  boiler.  A
natural circulation pattern is thus  set up.

If heat supply  exceeds  heat  demand,  the excess heat  is  released
by  venting  steam from  the condensers.   Temperature  control  is
automatic, being set by the vent pressure.  Advantages of this
system  are  simplicity, elimination  of costs associated with
pumping,  automatic temperature  control,   and independent
operation  of  the boiler  from  other heat sources and heat  demands.
Independent  operation is  particularly  important;  it  allows  the
boiler  to operate at  its own best conditions,  without  being
affected by  the  operations  of  other  components  of  the  system.
Digester gas can  be  used to fuel reciprocating engines and  gas
turbines.   Prime movers  convert part of the  fuel's energy to
work, rejecting the remainder as waste heat.   Thermal efficiency
can be dramatically improved  if  portions of the rejected  heat  can
be  recovered and  used for process  or building heating.  Waste
heat  recovery  is more efficient if prime movers  are run hot,
since  heat rejected  at  higher temperatures  can be  put to  a
greater variety of  uses  than heat rejected at  low  temperature.
Also, exhaust  systems  last  longer because 802-803 corrosion is
reduced .

Reciprocating Engines.  Engines may  be cooled using  either  a
forced circulation system in which  water  is  pumped  through  the
engine,  or a natural  draft  system.   The  equipment  configuration
for  natural  circulation  cooling is similar  to that described
for  boiler  natural  circulation systems  except   the  engine
replaces the boiler in the flow diagram (see Figure  18-2).   The
advantages of natural circulation cooling are  the same  as those
discussed  for  natural   circulation boiling.   Cooling system
pressures  are limited  to  about  10 psig (69 kN/m2 ) ;  if  operated at
higher  pressures  cooling water  could  leak past  the cylinder
liner seals and  into the cylinder.   The maximum cooling water
temperature is  thus  about 240°F  (116°C),  corresponding  to  the
temperature of  saturated  steam  at 10 psig (69  kN/m2 ) .  Engines
using natural circulation  cooling are relatively small,  typically
developing less  than  1,500  horsepower (1,120  kW) .   Flow rates
developed  by natural circulation  cooling  may be insufficient to


                             18-13

-------
cool larger engines.   Flow  rates may be increased by installing
a booster pump  in  the  circulating  loop  near the entrance to the
engine jacket.   There  are  reciprocating engines on  the market
designed  to operate  at temperatures  in the 160°to  180°F (71° to
82°C)  range.   However,  they are not recommended for  services
with  unscrubbed  digester  gas  because of  potential  problems
with 302-303 corrosion.  Heat recovered  from the  engine  jacket is
typically used  to sustain  the  digestion process and  for space
heating .

Reciprocating  engines  commonly  employed  in  wastewater  treatment
plants fall into two categories; dual-fuel  (compression  ignited)
and  spark  ignited engines.  Dual-fuel  engines use  a  blend of
diesel fuel and digester gas; the fraction of diesel fuel can be
varied from a minimum of 4 percent all the way to 100 percent of
the  mixture.   Dual-fuel engines are  typically  used  if  there is
insufficient digester  gas  to satisfy power  demands.    Dual-fuel
engines  have  been specified  for new plants  where  digester gas
production  is expected  to  lag  behind power  demands  for several
years.

Spark-ignited  engines are generally used  when  there is sufficient
digester  gas to  satisfy power demands.   Spark-ignited engines can
operate on several different types  of  fuel (for example, digester
gas  and  natural gas).   Special  carburetors are provided to blend
digester gas with  an  air-diluted  backup fuel (for  example,
natural  gas) during  infrequent  periods  when not enough digester
gas  is  available  to satisfy  power  requirements.  Spark-ignited
engines  are  less  complex then  dual-fuel  engines,  are  available
in smaller  sizes, and  are less  costly to operate since  expensive
diesel fuel is  not required.

Naturally aspirated  feed systems are preferred  to  turbocharged
systems  for spark-ignited engines.  Turbocharged systems require
that gas be delivered at high pressure,  which means  the gas must
be  first compressed,  then  delivered through  a fuel  metering
system  with  restricted  openings.   Gas  impurities  (oils,
greases,  and water)  are  condensed when the gas is compressed and
cooled;  these  impurities often  clog  the fuel  metering system.
Naturally aspirated  systems  operate  at  low  pressures  (<0.5 psig
[3.4 kN/m^j  ) .   with  careful design of the gas transport systems,
compression of  the feed  gas is  not required.   Low pressure fuel
metering   systems  also  have  relatively   large  openings  compared
to metering systems used  with  turbocharged units.   For these
reasons,  naturally-aspirated  fuel  systems are therefore less
susceptible to  clogging than systems  with  turbocharged units.

Engines  represent  a  large capital  investment and  should  be
conservatively   designed  to  protect  that  investment.    For
four-stroke engines  it  is  recommended  that  brake mean  effective
pressure  (BMEP)  not  exceed  80 to 85 psig (550  to  590 kN/m2)  to
minimize  strain on  the equipment.   Engine  speeds in  the 700 to
1,000 rpm are  preferred as  are average piston  speeds in  the range
of 1,200-1,500  feet  per minute  (370  to  460 m/min).   Heavy-duty
industrial engines should be specified,  not  automotive engines.


                              18-14

-------
Gas Turbines.   Gas  turbines  have had  relatively  limited  use  to
date.   Where  used, there  have  been fouling  problems  which  are
inherent  with compressing a dirty gas  through fuel metering
systems with  small clearances.  However,  new developments  in
the  turbine field  and the fact that less  NOX is produced  by
turbines than by  reciprocating  engines has  led  to a second look
at turbines, particularly in nonattainment air quality areas.   A
new  system that  uses a  relatively  low (4/1)  pressure  ratio
turbine with recuperation has the potential  to solve many of the
problems which plagued earlier  installations  (10).  The normally
low  efficiency of  the low pressure  ratio turbine  is  boosted  by
preheating the  compressed  air with  heat  recovered from  the
exhaust gas.  Ignition for this  turbine can be staged to minimize
NOX  generation.   Emissions  control  is  particularly  important  in
non-attainment areas where  new stationary sources must  use
Best Available Control Technology (BACT).  BACT  for reciprocating
engines is considered  to  be  catalytic denitrification, while BACT
for low pressure  ratio turbines  can  be  staged  ignition.


        18.3.2.3   Generators

Generators  may be synchronous  or  induction  types.   Synchronous
generators are by  far  the most common.  However,  in smaller sizes
(below 5 or 10  MW)  induction  units are generally less expensive
than synchronous  units.   They are  also easier to maintain since
they require no governor  or  synchronizing equipment.   Induction
generators have  the  disadvantage  of being  unable to  operate
unless parallelled with synchronous  generation,  either utility or
in-plant.    Thus an  induction  generator by itself cannot  be used
to provide emergency power.


    18.3.3  Examples of Energy Recovery

The  following  two examples demonstrate  calculations  for  two  of
the  most commonly  encountered energy  recovery practices.   Other
examples and case  histories  can  be  found  in References 11 and 12.


        18.3.3.1   Energy  Recovery from  Digester  Gas

Gas  from an anaerobic digestion  system is to be utilized to help
supply  plant  energy  needs  in   a  30  MGD (1.3  m^/s)  activated
sludge plant.   Digester gas will be used to fuel a spark-ignited
internal  combustion  engine  equipped  with  natural circulation
cooling.   The  engine   will  drive an electrical  generator.   The
electricity generated  will be us.ed  to  power various plant motor
drives.  Heat  recovered  from  the engine  cooling jacket and from
the  exhaust silencer will be used for space and process heating.
It is  hoped that  sufficient heat will  be recovered  to supply  at
least digester heat requirements; any excess heat recovered will
be used for "other"  process heating.  It  is anticipated that heat
recovered  from the  engine  jacket (usually low temperature heat)


                              18-15

-------
will be  used  to make hot water for digester heating, while  heat
recovered from  the  exhaust  silencer  (high  temperature heat)  will
be used to generate steam.  Figure 18-3 is  the  system flowsheet.
 CCL'J
                                                          FOR
                                                       H IG»* TEUPE RATUR F
                                                       ii-SSB* F
                            FIGURE 18-3

              PROCESS SCHEMATIC FOR EXAMPLE OF ENERGY
                    RECOVERY FROM DIGESTER GAS

The following data is estimated for  the sludges  and  digester  gas:

     •  Digester feed = 50,000 pounds  per day  (22,700 kg/d), dry
        weight basis.   The  feed  solids are  75  percent  volatile.
        The sludge is 4  percent solids  by  weight.

     •  Fifty percent of  the  volatile  solids (VS)  are  destroyed
        during digestion.

     •  Raw sludge temperature is 60°F  (16°C).

     •  Fifteen standard  cubic  feet  (0.42  m^) of  digester gas
        are generated for  every pound  (0.454  kg) of  VS destroyed.

     •  The  gas  composition   is  66 percent CH4,  28.3 percent
        CC>2,  and  5.7 percent water (by  volume).   Other gases
        (H2'  H2^f  N2)  are  present  but  not  in sufficient
        quantities to affect the  heat  balance.
                              18-16

-------
    •   619  Btu (648 kJ)  of heat are produced  for  every  standard
        cubic foot  (28.3  liters) of digester gas combusted.

The plant has the following  energy requirements,  which could
be  supplied in part or  in whole by  energy  recovery  from
digester gas:

    •   1,000 kW of  electricity.

    •   Energy  for raw  sludge  and digester  heating  (to be
        computed).

    •   15  x  1Q6  Btu   per  day  (15.8 x GJ/d)  for miscellaneous
        heating.

The following calculations  are required:

    •   Determine  the  energy value of the digester gas.

    •   Determine  if  energy  that  can be  recovered  from the
        combusted gas  is sufficient  to satisfy  the  energy
        requirements listed above.

    •   Provide  an  energy  flow diagram.

    •   Determine  overall  heat recovery efficiency.

To  make  comprehension of  this  example  easier,  the  energy  flow
diagram is presented  first  (see  Figure 18-4).  The  calculation
is  divided  into four sections,  as  illustrated  by the  numbered
"boxes" on  the  diagram.    The  magnitudes  of  the energy  streamd
shown  on Figure  18-4 are  developed in the following calculations:

Determine the Energy Value  of the Digester Gas (Bpx_lj_

1.  Digester gas flow  rate


        50,000 Ib  solids\  /0.75 Ib VS\ /0.5 Ib VS destroyed
              day       /  \ lb solids/ \     lb vs fed

            15 scf     \
       ib vsdestroyed
                       = 281'250 scfd (8'157
2.  Energy value  of  the gas

    = (281,250  scfd)  (619 Btu/scf)

    = 174 x 106 Btu  per day  (183.5 GJ/d)

    Strictly  speaking, the energy value  of the  digester gas
    should  include  not  only  the  heat  of  combustion  but the
    heat  contents  (enthalpy)  of the  reactants  (air,  fuel  gas)


                             18-17

-------
    calculated with  respect  to  a  selected  base  temperature.
    However,  the  heat contents  of the reactants  are  very small
    compared  to the heat  of  combustion and may be neglected with
    very little loss of accuracy and with  a substantial reduction
    in amount of calculations necessary.
                                ENE1GY VALUE
                                  >i'SU'H
                                  in. a
                            FIGURE 18-4

                 ENERGY FLOWSHEET FOR EXAMPLE OF
                ENERGY RECOVERY FROM DIGESTER GAS
Make a Heaj_B a^£1.9_e__A£o_u.£id__th e_ E_n g j. n e / G e n e r a tor  (Box 2 )


1.  Assume  28 percent of the  energy value  of the fuel gas is
    converted to work.


    Work produced

    = 0.28 (174 x 106 Btu/day)

    = 48.7 x 106 Btu per day (51.3 GJ/dJ


    Assume 90  percent of  the  work produced  can be converted to
    electricity .
                              18-18

-------
    Electricity

    = 0.90 (48.7  x  106  Btu/day)  =  43.9 x 10^ Btu/day (46.2 G J/d )

    This is equivalent  to  535 kW.  Since average plant electrical
    demand is  1,000  kW,  auxiliary  power must be purchased.

2.   Assume  33 percent  of  the  energy value  of the fuel  gas is
    recovered  in  the engine  jacket water.

    Energy recovered in the  jacket water

    = 0.33 (174 x 106 Btu/day)

    = 57.4 x 106  Btu per day (60.5 GJ/d )

3.   Assume the radiant  heat loss from the engine is 4 percent of
    the energy value of the  fuel gas.

    Radiation  loss

    = 0.04 (174 x 106 Btu/day)  = 7.0  x 106 Btu per day (7.4 GJ/d)

4.   Assume  5  percent  of  the  energy value of  the fuel  gas is
    transferred to  lubricating  oil.

    Heat loss  to  oil

    = 0.05 (174 x 106 Btu/day)  = 8.7  x 10^ Btu per day (9.2 GJ/d)

5.   Heat in the  exhaust gas is the difference between the energy
    value of the  fuel gas  and the  heat losses determined in items
    1 through  5.

    Heat in the exhaust gas

    = (174.0 - 48.7  - 57.4  - 7.0 - 8.7) x 10^

    = 52.2 x 106  Btu per day (55.0 GJ/d)

Determine  Whether  Sufficient  Heat  can be  Recovered  From  the
Jacket Cooling  Wa t e r __tg_S at. i jjE y _ DjLg e s t er _ H eating  Requirements
( Box 3 )                               — — ---

1.  Energy required to  heat raw  sludge


        50,000 Ib solids/day  W  1.0 Btu
       O.04 Ib solids/lb  sludge/\lb  sludge/0?

    = 42.0 x 106 Btu per  day (44.3 GJ/d)


2.  Determine  energy  required  for  circulating  sludge  heating.
    The purpose of  the  circulating  sludge heater is  to  make
    up for  any  heat lost  through the digester  structure.   Heat


                              18-19

-------
    loss  calculations  similar to  these shown  in Chapter 6,
    Section 6.2.6.2,  indicates that  for the digester of this
    example,  losses are on  the  order  of 5.0 x  10^  Btu per day
    (5.3 GJ/d).

3.   Determine  heat  loss  in the hot water circulating  loop.  There
    is very little heat loss  because this is  a  closed system
    (see  Figure  18-3).  The only  losses  will be  through the
    insulation.  It  is  roughly assumed that heat  loss  is  5 percent
    of the heat  leaving  the  engine jacket.

    Heat loss

    = 0.05 (57.4  x  106)= 2.9  x 106 Btu per day  (3.0 GJ/d)

4.   Total heat required  for  the digestion system

    = (42.0 +  5.0 + 2.9) x 106 = 49.9 x 106  Btu/day  (52.6 GJ/d)

5.   Heat available  in  the cooling water minus total heat  required
    for the digestion  system

    = (57.4 -  49.9) x  106 =  7.5 x 106 Btu/day (7.9  GJ/d)

    To keep the  internal combution engine adequately cooled,
    this heat  must  be rejected in some manner.   The heat may be
    rejected by renting steam from  the  condenser.  In this case,
    however,  the designer has  chosen  to use the extra heat for
    building heat,  thereby utilizing rather,than wasting  it.

Determine if  Sufficient  Heat  can be  Recovered  from  the Hot
C~ombu"ition~Gases Leaving the  Engine to Satisfy "Other"~Trocess
Requirements (Box 4)

From  previous  calculations,  the   heat available  in  the  hot
combustion gas  is 52.2 x  106  Btu per  day (55.0  GJ/d).   Not
all of  this heat  can be recovered for use.   Practical limits
exist  to the  degree  to which  the  hot gas  can  be  cooled.  For
example,  the  hot gases must be  substantially  warmer  than the
material being heated to carry  out  heat transfer in  an  exchanger
of  reasonable  size  and cost.  In this example,  however,  the  lower
temperature  limit  is set at 350°F to preclude corrosion that
might occur by condensation of water vapor on the inside of the
exhaust  stack  walls.   The  designer must therefore  determine if
sufficient heat can be  obtained to  satisfy  "other"  process  uses
when the hot combustion gases are cooled to  350°F  (117°C)  in the
exhaust silencer.  Since the heat  content of the  hot combustion
gases  is  known  (52.2  x  106  Btu  per  day  [55.0 GJ/d]),  heat
available can readily  be calculated once  the  heat content of
the gas at 350°F  (117°C) has  been determined.  This  is calculated
as  follows:

1.   First  calculate  the  volume of  exhaust  gas.  Gas production
    can be predicted  from stoichiometry:


                             18-20

-------
    CH4 +  202 - »*C02 +  2H20                     (18-1)
a.  C02  present  = CC>2  in digester  gas plus C02  formed
    by combustion of methane.

    1.  From previous  calculations,  digester  gas  production
        is 281,250 standard cubic feet  per day (8,157  m3/d ) .

    2.  Unburned  digester  gas contains  28.3  percent  CC>2  by
        volume .


        CC>2 associated with digester gas


        = 0.283  (281,250 scfd)  = 79,593  scfd (2,252 m3/d )


    3.  From Equation  18-1, one  cubic  foot of  CC>2  is formed
        for every cubic foot of methane burned.  Digester gas
        contains 66 percent methane by  volume.


        CC>2 formed by combustion of methane

        = 0.66  (281,250 scfd)  = 185,625  scfd (5,253 m3/d )


    4.  Total C02 volume


        = 79,593 -f 185,625 = 262,218 scfd (7,505 m3/d )


b.  CH4 present: none, all converted to CC>2 .


c.  02 present:  assume that air  supplied exceeds theoretical
    requirements by 10 percent.   Oxygen  associated  with  this
    excess is not  consumed.   From Equation 18-1,  theoretical
    oxygen  requirements  are  two  cubic feet of oxygen for
    every cubic  foot of methane burned.


    Oxygen in excess of theoretical requirements


                 /      0.66 ft3 CH4     \
    = (2) (0.10)    — — — __-____±__~ ) (281,520  scfd)
                 \   ft-^  digester gas    /


    = 37,125 scfd (1,050  m3/d )
d.  N2 present:   N2 associated with  the air passes  through
    the system unchanged in quantity.
                          18-21

-------
        N2  flow
                       /    0.66  ft3  CH4  \
        = 281,250  scfd  - = - — 1
                       yft-15  digester gas/
          ([1.10  x 2] ft3  02  delivered \ /Q.79  ft3  N2  \

                  ft3  CH4            ) \0.21  ft3  02  /

        = 1,536,265  scfd (43,476  m3/d)

    e.   H20  present = H2O  in digester  gas  plus that created  by
        combustion of  methane.
        1.   Digester gas contains  5.7  percent ^0  by  volume.

            H2O in digester gas

            = 0.057 (281,250 scfd)  = 16,031 scfd  (453 m3/d)

        2.   From Equation 18-1,  two cubic  feet of  H20 are  formed
            for every cubic foot of methane burned.

            H20 formed

                           /  0.66  ft3 CH4   \ I 2  ft3  H20\
            = 281,250 scfd  - 5 - -- ^— |{ - =5 - —I
                           \ft3  digester gas/ \  ft3 CH4 /

            = 371,250 scfd (10,506  m3/d )


        3.   Total water =  16,031  + 371,250 =  387,281 scfd
            (10,960 m3/d)

    f.   Total gas flow =  262,218 +  37,125  +  1,536,265  + 387,281
        = 2,222,889 scfd (62,907 m3/d )

2.   Next calculate  the  heat content of the  exhaust  gas  at  350°F
    (117°C).  The heat  content  of  the exhaust gas is  the  sum  of
    the  heat  contents of  its individual  components.    The  heat
    content of any  component  at 350°F  is the sum  of  the sensible
    and  latent heats  required to  raise  the  component from  an
    arbitrarily selected base temperature to 350°F (177°C).   Mean
    heat capacity data for several  gases is shown  on  Figure 18-5.
    The  base  temperature for Figure  18-5  is  77°F  (25°C).   The
    mean heat capacity of  a gas over the  range 77°F  to  350°F  is
    the value found at 350°F.

    a.   Heat content of CC>2

                                     -77°P, ,262,218  scfd,
        = 1.9 x 106 Btu per day (2.0 G J/d )
                              18-22

-------
     I   400  800 1200 1600  2000 2400 2800 3200 3600  4000 4400 4800 5200

                          TEMPERATURE.0*1
                           !°F =1,B°e 4321

                         FIGURE 18-5

      MEAN MOLAL HEAT CAPACITIES OF GASES AT CONSTANT
         PRESSURE  (13)  (MEAN VALUES FROM 77° to T°F)
b.  Heat content  of  02

        7.2 Btu    /Ib mole\
      Ib mole/°F   \^359  scfy

    = 0.2 x 106 Btu  per day (0.2 GJ/day)
                                        \  ,-^  i oc  ~~f*\
                                        )  (37,125  scfd)
    Heat content  of  N2
       6.8 Btu     /Ib mole^
      Ib mole/°F   \359  scf

    = 7.9 x  106 Btu/day (8.4 GJ/d )
                             (350°-77°F)  (1,536,265  scfd)
    Heat  content  of  water.   In  this  calculation, water  is
    pictured  as heated  in a  liquid state  to the  dew  point,
    evaporated,  and heated  as a vapor  to final  temperature.
    Other approaches  can also be used;  these  are  described in
    thermochemistry textbooks.
                           18-23

-------
                               387,281  scfd H20  \  , nn
            Water comprises   2f222/889  scfd totalj  10°

            = 17.4 percent  by volume  of  the exhaust  gas.   The
            dew point  for  gas containing 17.4  percent water by
            volume is 135°F  (58°C).

        2.   Heat to raise  liquid  water  to  the dew point

                387,281 scfd  \ /  18  Btu
                  scf/lb mole/lib  mole/°F
                            / \          I

            = 1.1 x 106  Btu  per day  (1.2  GJ/d)

        3.   Heat to vaporize water at  the dew point

                387,281  scfd \/18,720  Btu\
              359 scf/lb mole  M   Ib mole  )

            = 20.2 x 106 Btu per day  (1.19 GJ/d)

        4.   Heat to raise water vapor  from the dew point
            to 350°F


            _/  8.2 Btu  \( 387,281 scfd  \ . ?[-no -. ,, ,.„  .
             \lb mole/°F/\359  scf/lb mole/1          '

            = 1.9 x 106  Btu  per day  (2.0  GJ/d)


        5.   Total  heat  content of  water =  (1.1 + 20.2  + 1.9)
            x 106 = 23.2 x 106  Btu per  day (24.5  GJ/d).

    e.   Heat content of  exhaust gas at  350°F  (117°C)

        = (1.9 + 0.2 + 7.9 + 23.2) x 106  = 33.2  x 106 Btu per day

          (35.0 GJ/d)

3.   Energy  available to  satisfy "other" requirements

    = (52.2 - 33.2) x  106 =  19.0 x 106  Btu per day (20.0 GJ/d).

4.   Determine  heat  loss  in steam/condensate  circulating  loop.
    There  will  be very  little  heat loss  because  this  is  a
    closed  system  (see  Figure  18-3).   Assume losses are roughly
    5 percent of the heat transferred  from the exhaust silencer.

    Heat loss

    = 0.05  (19.0 x 106 Btu/day)  =  0.9  x 106 Btu per day

      (1.0  GJ/d)
                             18-24

-------
5.  Heat available for "other"  process  demands

    = (19.0 - 0.9) x 106 = 18.1 x  106 Btu  per day  (19.1 GJ/d)

    The available heat is sufficient to satisfy  the demands.


Deter mi n e EJ:_fi_c_i e n c y_ g f _t h e En er; g y_R e covery  System

There are  several methods for   evaluating the  efficiency of the
energy recovery  system.   One  approach  is to  compute the useful
heat and work recovered as a percentage of the energy  input.

1.  Useful heat and work:

    a.  Electrical energy = 43.9 x 106  Btu per day  (46.2  GJ/d).

    b.  Raw sludge heating = 42.0  x 106 Btu  per  day  (44.2 GJ/d).

    c.  Circulating  sludge  heating  = 5.0  x   10^  Btu  per day
        (5.3 GJ/d).

    d.  "Other"  process  heating =  15.0 x  10^  Btu  per day
        (15.8 GJ/d).

    e.  Space heating = 7.5 x 106  Btu per  day (7.9 GJ/d).


2.  Energy  input  from  digester gas  =  174  x  10*>  Btu/day

    (183.4 GJ/d).


-,   „    .  ,  ff. .       /43.9 +  42.0  + 5.0 + 15.0 +  7.5\ inn
3.  Computed efficiency = I	1 100
                          \          174.0               /

                        = 65 percent


This activated sludge plant  is   not able to supply all  its energy
needs  using  digester  gas  (insufficient  electrical  energy).
Generally, digester  gas  is sufficient  to satisfy  the energy
requirements of  most  primary treatment plants  but not activated
sludge  plants,   since  aeration   blowers generally have  high
electrical demands.
        18.3.3.2  Recovery of  Energy  from  Incinerator Flue Gas


A wastewater treatment plant of  125 MGD  (5.48 m^/s) capacity uses
incineration to process  190,000  pounds  per day (82,260 kg/d) of
combined primary and waste-activated sludges.  Heat is recovered
from the  flue  gases  as  electricity  and  steam in a steam turbine
power cycle, using  a  waste heat  boiler.  The  designer's objective
is  to  maximize work  production  (electricity  and  direct power).
                              18-25

-------
Steam is not used for space or process heating.   A flow sheet of
the process  is  shown on Figure  18-6.   The following additional
information is provided:

     •  The  flue gas  heat content  is  606  x 106  Btu per  day
        (639  GJ/d),  based  on an  assumed gas  composition  and
        gas temperature, using methods  described  in the example
        of Section 18.3.3.1.  Similarly, the heat content of  the
        stack gas  is 250  x  106  Btu per  day  (263 GJ/d).  Heat
        losses from  the  boiler  structure are   18 x  10^  Btu  per
        day (19  GJ/d).

     •  The  boiler  produces superheated steam  at  615 psia
        (4,261 kN/m2)  and  825°F  (441°C),  which  is   then  fed  to
        a steam turbine,  called the "main  turbine."

     •  Steam  is withdrawn  from the turbine at three  points.
        First, 50,000 pounds per day (22,700 kg/d) are withdrawn
        at  165  psia  (1,143 kN/m2)  and applied  to drives  for
        pumps and compressors.   This  is called  "process" steam.
        Second,  a quantity  (to be computed) is withdrawn and used
        for preheating  of  the boiler feedwater.  This  is called
        "preheat" steam.  The  remaining  steam, which  is "primary"
        steam,   is  exhausted  at 1  psia  (6.9  kN/m2).    The
        efficiency  of  the   turbine  (actual to  theoretical  work
        output)  is  assumed  to  be  76 percent.

     •  Exhausted "process"  steam  from  the pump  and compressor
        drives  is  condensed  at  1  psia  (6.9 kN/m2),  combined
        with the "primary"  condensate,  and sent to the feedwater
        heater.    "Primary" and "process"  condensates  are
        assumed  to  be  saturated  water  at the  exhaust  pressure
        (1 psia  [6.9  kN/m2]).

     •  "Preheat" steam  is mixed  with   "primary" and  "process"
        condensates in the  feedwater  heater  to  produce  a
        saturated feedwater at 300°F  (149°C).

     •  The feedwater is pressurized to  615  psia  (4,261 kN/m2),
        and returned  to  the boiler.


The following information is  desired:

     •  Steam and condensate  flow rates.

     •  Electric  power  generated.

     •  Pump and  compressor work  produced  by the "process" steam.

     •  Energy recovery  efficiency.
                              18-26

-------
         "FEFD" STIAM 825' f. 615 psis
                              "PROCESS" STEAM 165
                                                         PIRFCT
                                                         POWE R
                                          TURBINE
                                          DRIVTIS;
                                          FOR PUMPS
                                          AND
                                          COMPRESSORS
   I pit - 6.93 hN/m
                 PUMP
                            FEED WATER
                            HEATER
                           FIGURE 18-6

            FLOWSHEET FOR EXAMPLE OF ENERGY RECOVERY
                    FROM INCINERATOR FLUE GAS

Analyze the Operation jOf__the  Main Turbine


Turbine operations  can  be analyzed using a Mollier  diagram.   A
Mollier diagram is a plot of  enthalpy  versus entropy  for specific
two-phase  systems  which display  lines  for constant  pressure,
temperature,  percent  moisture,  and superheat, among  others.
Figure  18-7 is a Mollier diagram for  the  steam-water  system.
Note that  the  terms  "enthalpy" and "heat content" are  equivalent
and will be used interchangeably  in the  following discussion.


The "state line" concept is used  for  turbine analysis.  The state
line describes the  steam condition  at  every point within the
turbine.   The  line can  be drawn once any   two points  describing
steam  conditions  in the  turbine are  established.  For  this
example, the turbine  feed  steam  and   the "primary" steam exhaust
conditions will  be determined,  then  plotted  on  the  Mollier
diagram of Figure 18-7.

1.  The turbine  feed steam  condition (615  psia  [4,261 kN/m2]),
    825°F  [441°C])  is plotted  as point  A on the Mollier Diagram
    (see Figure 18-7).   Figure 18-7 is not detailed,  so that
    data  points  and state  lines  can  be   clearly   seen.   More
    detailed diagrams are available (14,15).
                              18-27

-------
3650
1600
         585° F,
         165 PSIS,
         1320 Btu/lb
         448°F,
         87 psia,
         1258 Btu/lb
                                                        1 piia,
                                                        1036 Btu/lb
                                             B. ISENTROPICALLY EXPANDED
                                                "PRIMARY" STEAM
        1.1     1.2    1.3    1.4    1.5    1.6    1.7    l.B    1.4    2.0     Z,l    2.2    2.3
                     ENTROPY, Btu/lb/*F (1 Btu/Jb/°F - 4.18 kJ/kg^C)

                                    MOLLIER CHART COURTESY OF BABCOCK AND WILCOX

                                 FIGURE 18-7

       STEAM CONDITIONS FOR EXAMPLE OF RECOVERY OF ENERGY
                     FROM INCINERATOR OF FLUE GAS
                                   18-28

-------
Determine  the "primary" steam  exhaust condition.   If the
turbine were  100  percent  efficient,  the steam would expand
isentropically,  that is,  the  entropy of the  steam at any
point  in  the  turbine would  be identical  to the  entropy
of  the feed  steam and the  state line  would be  vertical
(dashed line in the Mollier Diagram).   The "primary" exhaust
steam condition  would be  located  at the  intersection
of  the vertical  state  line  and the exhaust pressure
(1 psia [6.9 kN/m2]), at point B.  Enthalpy  of the steam at
point B is 915 Btu  per pound (2.13 MJ/kg).

However,  turbines are  not  100 percent  efficient  since
isentropic expansion  is never attained.  The  energy which can
be extracted from the  steam in practical applications is only
a  percentage  of  that which  can  be extracted by  isentropic
expansion.  This  is expressed by Equation 18-2.
                    / Hi - H2p \
                    \ HI - H2i ;
Turbine efficiency =( u^ _ „**; )100                    (18-2)


Where:

    HI   = enthalpy of inlet steam, Btu/lb.

    H2p = enthalpy  of   steam exhausted from  a  practical
          turbine, Btu/lb.

    H2i = enthalpy  of steam  exhausted  from  an  ideal
          turbine, Btu/lb.

The  efficiency described  by Equation  18-2 is  the actual
work output relative to theoretical output—it  is less than
100  percent because  of irreversibility  in  the  expansion of
gases  in  the  turbine.   Mechanical  losses in the  turbine and
generator are  not  included.

For  the  practical  turbine,  enthalpy of  the exhausted
steam  (H2p)  can be  computed from Equation  18-2.   For the
turbine of the example  (76 percent efficient).


H2p  =  U,420  - y^J  (1,420 - 915)                     (18-2)


= 1,036 Btu per pound (2,405 kJ/kg)
The  "primary"  exhaust steam  condition for  the practical
turbine  is  located at  point C,  the  intersection  of the
exhaust  pressure  (1 psia  [6.9 kN/m2])  and  enthalpy value
1,036 Btu  per  pound (2,405  kJ/kg) .   The state line for the
practical turbine is then drawn between points A and C.
                         18-29

-------
3.  The "process"  steam condition must lie on the state  line.   It
    is located  at the intersection of the  state line and  the
    "process"  steam operating  pressure  (165  psia [1,145  kN/m2] ),
    at point  D.

4.  As with  the  "process"  steam,  the  "preheat"  steam  condition
    can be determined once its pressure is  known.   Pressure  can
    be determined  by  the following reasoning:

    a.  "Preheat"  steam  pressure  is essentially equal to  the
        pressure  in the feedwater heater  (pressure  drop  through
        the lines  connecting  the  turbine and feedwater  heater is
        assumed  negligible).

    b.  The  feedwater  heater is  a direct  contact  device.
        Sufficient "preheat"  steam  is  mixed with  "primary"  and
        "process"  condensates  to  form  a two-phase  system  at
        300°F (149°C).   Thus  the  feedwater  heater system is  a
        saturated  system.

    c.  The feedwater heater  pressure,  therefore is the  pressure
        of saturated  steam  at 300°F  (149°C),  which is 67 psia
        (464  kN/m2).

The "preheat" steam condition  is located  at  the intersection of
the state line  and the 67 psia  (464 kN/m2)  constant pressure
line (point E).  Enthalpy of the "preheat" steam is 1,258  Btu  per
pound  (2,921  kJ/kg).

Determine St e am  and C QJ}djsn sat e_F1 ows

1.  Circulating  steam rate is computed  by a  heat balance  around
    the boiler.

    a.  Enthalpy  of  the  water entering the  boiler is assumed
        equal to  that leaving the  feedwater  heater;  that  is,
        pumping affects  the  enthalpy value  negligibly.  This
        is a  justifiable  assumption  for the  pumping of  liquids.
        From  steam tables (14,15),  the enthalpy of  saturated
        water at 300°F  (149°C)  is 270 Btu per pound (627  kJ/kg).

    b.  By previous calculations,  enthalpy of  superheated  steam
        leaving  the boiler is  1,420 Btu per pound (3,297  kJ/kg).

    c.  From  the problem statement,  heat absorbed in the  boiler

        = 338  x  106 Btu per day (356 GJ/d).

    d.  Therefore  steam circulating rate

            338  x  106  Btu/day
          (1,420 - 270) Btu/lb

        = 293,900  pounds per day (133,400 kg/d).


                             18-30

-------
2.   "Process,"  "primary," and "preheat"  steam  rates  are
    determined  by mass  and  heat  balances  around  the  feedwater
    heater.  Let X  and Y be the flow rates  for  "primary" and
    "preheat"  steam, respectively.   Equation 18-3  is  the mass
    balance around the feedwater heater.

    293,900 = X  + Y  + 50,000                              (18-3)

    Equation 18-4 is the heat balance for the feedwater  heater.

    293,900 (270) =  70 X + 1258 Y + 70  (50,000)           (18-4)

    Enthalpies   of  the  "process"  and '"primary"  condensates
    (70 Btu  per pound or  162  kJ/kg)  are  for  saturated water
    at 1 psia  (6.93 kN/m2).   Solving Equations  18-3  and 18-4
    simultaneously, "primary"  and "preheat" steam rates are
    194,626  pounds  per  day (88,350   kg/d)  and  49,274 pounds
    per day (22,370  kg/d), respectively.

    At this  point,  construction  of an energy  flowsheet  should
    be initiated  (see  Figure  18-8).   This allows the  designer
    to see  all  pertinent data  on one   sheet and gives  a  feeling
    for the magnitude of the various energy flows.

Determine Electrical Energy  Generated

Work produced is the sum of  the total enthalpy changes across the
turbogenerator:                                                 :

1.   Work from "process" steam

    = 50,000 Ib/day  (1,420,- 1,320 Btu/lb)

    = 4.90  x 106 Btu per day (5.16 GJ/d)

2.   Work from "preheat" steam

    = 49,274 Ib/day  (1,420 - 1,258 Btu/lb)

    = 7.98  x 106 Btu per day (8.41 GJ/d) b

3.   Work from "primary" steam

    = 194,620  Ib/day (1,420  - 1,036 Btu/lb)

    = 74.73 x  106 Btu per day (78.77 GJ/d)

4.   Total work  produced

    = (4.90 +  7.98 + 74.73)  x 10^

    = 87  x  106  Btu per day (92.3 GJ/d)

5.   Assume  mechanical  efficiency of  the  turbine/generator
    combination  is 95 percent.                  •  •


                             18-31

-------
    Net electricity produced
    = 83.2 x 106 Btu per  day  (87.7 GJ/d)

    This is equivalent  to 1,015 kW of electricity
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                                                 TUPHfNl
                                                 DftlVEf^'
                                                    /"
                             FIGURE 18-8

              ENERGY FLOWSHEET FOR EXAMPLE OF ENERGY
                 RECOVERY FROM INCINERATOR FLUE GAS
De t e rm i n e
                                          ^ t e am Cycle
Enthalpy of  the  "process"  steam is 1,320 Btu per pound  (3,065 kJ/
kg).   Enthalpy  of the  exhausted  steam can  be determined  using
the  same technique  employed  for  analysis  of  the  main  turbine.
Isentropic  expansion of  process steam  (initially  at point D,
Figure  18-5)  to  1  psia  (6.9  kN/m2 )  produces  an  exhaust  gas
of  enthalpy 950 Btu per  pound  (2,206 kJ/kg )  .  Assume process
turbines are 50  percent  efficient.

1.  Enthalpy of  exhausted  steam

    = 1,320  -  yj£  (1,320 - 950)

    = 1,135  Btu  per pound  (2,635 kJ/kg )
                               18-32

-------
2.  Work produced
    = (50,000  Ib/day)  (1,320 - 1,135 Btu/lb)

    = 9.2 x 106  Btu per day  (9.7 GJ/d)
3.   Assuming mechanical  losses  of  5  percent, work  delivered

    = (9.2 x 106  Btu per day) (0.95) = 8.8 x 106 Btu  per day
      (9.3 GJ/d)

    This is equivalent  to 107 kW.


Determine Energy  Recovery Efficiency


Assume heat removed in the  condensers  is  not  used beneficially,
but discharged to the atmosphere via cooling towers.

1.  Energy recovery, .based on heat transferred  to steam

              106 + 8.8 x 106
= A 8 3. 2 x
                              100 = 27.2 percent
              338  x  106


2.  Energy recovery,  based on heat in the incinerator  flue gas


                               .00 = 15.2 percent
= /(83.2 x 1Q6  +  8.8 x 106)\
 \        606  x  106       /
Compare  the  recovery  of this  example  (15  percent)  against the
recovery of energy  from digester gas (65 percent),  as  illustrated
by the  example in  Section 18.3.3.1.   Greater efficiency was
obtained by the internal combustion system because:

1.  No  heat  was lost  prior to  the work  producing  step.   In
    contrast,  fully 41 percent  of the  heat available  in the
    incinerator flue gas  was rejected  in  the  waste heat recovery
    boiler  before  any  useful work  could :be  extracted  (see
    Figure 18-8).

2.  With  the  internal combustion  system, waste  heat from the
    work  producing  step  was  used benefically  (for  digester
    and space heating).   In contrast,  waste heat from the steam
    condensers  was not used  benefically  but rejected  to the
    environment.   It  is difficult  to  use this heat  since it is
    available at only  a very low temperature (102°F  [39°C]).

These  two examples  demonstrate the  general rule  that  energy
recovery schemes  whose sole  effect  is  the production of work are
not likely to be efficient.
                             18-33

-------
It should not be inferred from the examples  that  energy  recovery
from  flue gases  must  necessarily  be inefficient.    In this
example,  the objective of the designer  in recoverying heat from
incinerator  flue gas was to  maximize work.   Had  he  chosen to
exhaust steam from  either of the turbines at  higher  pressures and
used  it  for heating  purposes or  had he used "process" steam
solely for  heating, some work  would have  been  sacrificed but
thermal efficiency  could have been substantially  improved.  The
point to  be  made here  is that  the  designer should examine  a wide
range of  options when  analyzing energy recovery operations.


    18.3.4  Other Factors Affecting Heat Recovery

The  previous  calculations  point out  some  of  the  factors  a
designer  must  consider  in  conducting a heat  recovery analysis.
They are  by  no  means  the only factors;  much more detail must be
added.  For  example:

     • The   full  range  of  conditions expected  at  the plant
       must  be evaluated,  not just  average conditions.    Energy
        supply and  energy demand  schedules  must  be  established.
       Heat  recovery equipment  must be sized  to  handle peak
       demands. Storage requirements  for primary and backup
        fuels must  be  determined.

     • A source of backup  energy  must be  available  in the  event
       that  plant  energy recovery systems experience partial or
       total failure.

     • The  physical  and   chemical  nature of   flue  gases
       generated  must be considered  (for example,  temperature,
       corrosiveness,  particulate concentration,  and  moisture
       content) .

     • The  equipment   must  be  designed   to withstand  the
       conditions  to  which it  will  be subjected.   Appropriate
       materials of construction must be used.

     • Any   solid,  liquid  or gaseous  residual  from  the heat
       recovery operation must be collected  and disposed of in a
       safe  and environmentally sound manner.

     • Chemical   and physical  treatments  for  makeup  and
       circulating water or steam must be established.
                                                    \
     • Manpower to operate  the  heat recovery system  must be
       determined.   Specialists  may be required  for  certain
       equipment,  for  example,  stationary  engineers  for high
       pressure boilers and engine specialists  for  internal
       combustion  engines.

     • Control strategies must be decided upon, and  instrumenta-
       tion  to carry  them out must be provided.
                             18-34

-------
        Economic analyses  must  be performed  to determine
        if  the  system can  be  economically  justified.   As
        a  ru 1 e-o f -1 h umb,  the  larger  the  plant,  the  more
        sophisticated the heat  recovery  system which  can be
        justif ied.
18.4  Other Uses of  Wastewater Solids and Solid By-Products

Wastewater solids may  sometimes be used beneficially in ways
other  than as a soil amendment or as a source  of  recoverable
energy.   Lime  and  activated carbon have been  recovered from
sludges for many  years  at plant scale.  These applications  are
discussed  in  Chapter 11.   Stabilized sludge, when mixed with
soil,  is used as  interim  or  final  cover  over  completed  areas  of
refuse  landfills (see  Chapter  19).  Wastewater scum has been
collected (sometimes purchased)  by  renderers at several treatment
plants for use as a raw material in the manufacturing  of cosmet-
ics and other  products.  Grit, particularly incinerated grit,  may
be used as an  aggregate, for  example, as a road sub-base.

Other beneficial uses of wastewater solids  have been considered;
some  have  been tested  on a  laboratory or plant scale.  These
include:

     •  Recovery of  ammonia from  the  filtrate or  centrate
        following anaerobic digestion and dewatering  of  sludge.
        Ammonia  is stripped from the  liquor,  absorbed in sulfuric
        acid and crystallized as ammonium sulfate.

     •  Recovery of  ammonia  and phosphates  by precipitation  of
        MgNH4P04  from  digester   supernatants.     The  precipitate
        is used  as a fertilizer.

     •  Addition  of sludge  to   processes  designed  to compost
        or  anaerobically  digest  municipal  refuse.    In such
        situations,  sludge  serves  principally   as  a nutrient
        source.

     •  Recycling of  wastewater solids for use  as  a  foodstuff
        for livestock (cattle,  sheep, goats,  poultry,  and  fish).
        Note that solids used for this purpose have generally  not
        originated  from  municipal  wastewater  treatment  plants,
        but from  systems   treating  purely  industrial  or  animal
        wastes.   However,  the   use  of dried  municipal sludge
        disinfected  by gamma  irradiation is being  investigated as
        a food source for  grazing animals.

     •  Use of wastewater solids as an organic substrate in worm
        farming  (see Chapter  13).

     •  Use of  sludge  as  a  raw material  for  the production  of
        powdered activated  carbon  (see  JPL/ACTS  process/
        Chapter  11).
                             18-35

-------
18.5  References

 1.     USEPA.  Current and Potential Utilization of Nutrients  in
        Municipal Wastewater and Sludge, Volume 2.      Office   of
        Water  Program Operations.   Washington,   D.C.  20640.
        Contract 68-01-4820.  July 21, 1978.

 2.     Walker, J.M.   "Overview:  Costs,  Benefits and Problems  of
        Utilization  of  Sludges,"  pp.  167-174.   18th National
        Conference  and Exhibition on Municipal Sludge Management.
        Miami  Beach, Florida.   Information  Transfer,  Inc.
        Rockville,  Md.  1979.

 3.     USEPA.  "Principals and Design Criteria for  Sewage Sludge
        Application  on  Land."    Sludge Treatment  and Disposal,
        Part 2.  Technology Transfer,  Cincinnati,  Ohio 45268.'
        EPA-625/4-78-012.  October 1978.

 4.     Chicago Sun-Times.   "U.S.  Bares Solar Energy  Program  to
        Year 2020."  p. 29.  August 14,  1975.

 5.     Ward,  P.S.  "Digester Gas  Helps Most  Energy Needs."
        Journal Water Pollution_Control  Fed.    Vol.   46,  p.  620.
        1974.

 6.     Brown  and  Caldwell.    Solid Waste Resource Recovery
        Study.   Prepared  for  the Central Contra Costa  Sanitary
        District, Walnut Creek, California.  August  1974.

 7.     California  Public  Utilities  Commission.   Staff Report  on
        California  Cogeneration Activities.   Utilities  Division.
        San Francisco, California.  January 17, 1978.

 8.     U.S. Department of Energy. Proposed Regulations Providing
        for Qualification  of  Small  Power Production  and Cogen-
        eration Facilities  Under  Section   201 of  the  Public
        Utility Regulatory Policies Act  of 1978.   Federal Energy
        Regulatory  Commission,  Washington, D.C.,  Rm  79-54.  June
        1979.

 9.     Sacramento  Area  Consultants.   Sacramento Regional Waste-
        water  Program -  Study  of  Methane  Uses.    Sacramento
        Regional   County  Sanitation  District.   Sacramento,
        California.  June 1976.

10.     Alpha National  Inc.   Solid Waste and Biomass Low Btu Gas
        Conversion  System Program.  1301  East  El Segundo Blvd.,
        El  Segundo, California.  April 1978.

11.     National  Bureau  of Standards.   Waste  Heat Management
        Guidebook.  NBS  Handbook  121.   Washington,  D.C.   U.S.
        Government  Printing Office.  1976.


                             18-36

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12.      USEPA.   Energy  Conservation  in Municipal Wastewater
        Treatment.    USEPA  Office  of  Water  Program Operations.
        Washington,  D.C. 20640.    EPPA  430/9-77-001/.   March
        1978.

13.      Hougen,  O.A., Watson,  K.M.,  and R.A.  Ragatz.   Chemical
        Process  Principles.   2nd Ed.  New York.   John  Wiley and
        Sons.   1956.

14.      Keenan,  J.H.  and  F.G. Keyes.  Thermodynamic Properties of
        Steam.  4th  Ed.   New  York.  John Wiley and Sons.  1936.

15.      Combustion  Engineering,  Inc.   Steam Tables.   Available
        from  the  Public Relations  and  Advertising  Department,
        Windsor,  Connecticut.   1967.
                              18-37

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EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
             Chapter 19.  Disposal to Land
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                           CHAPTER 19

                        DISPOSAL TO LAND
19.1  Introduction

Wastewater sludge may  not  always be used as  a  resource because
of  land  acquisition  constraints or  because they  contain
high  levels  of  metals and other toxic substances.    In  these
situations,  the  sludge  must  be further  processed  by  other
methods.  Non-utilization disposal processes  are  the  subject  of
this chapter.

As discussed  in Chapter 2, ocean disposal is no longer considered
appropriate.    Consequently,  land  disposal  processes are  being
optimized so that the  increasing  amounts of municipal wastewater
sludge produced by the  adopted  secondary treatment standards can
be  accepted.   Two principal  land disposal  methods,  landfilling
and dedicated land disposal,  differ  in application rates  and
methods  of  application.  Typical landfill operations  involve
dewatered-sludge subsurface application rates, often several feet
in depth.  Dedicated  land  disposal operations, however, typically
involve  repetitive  liquid sludge  applications,  which  may only
raise the land surface a few inches per year.


    19.1.1  Regulatory Agency Guidance

Development of formalized methods for sludge disposal to land is
recent.   Major efforts in  this  area  have  been encouraged and
funded  by the  USEPA  since  1974.   The reader  is referred  to
Chapter 2 for a discussion of  some of the guidance and regulatory
documents which deal with the recent federal laws to control the
disposal of wastewater solids.   State and local guidance has also
been  provided.  Extensive  sludge research is being funded  by
USEPA and various states.
19.2  Sludge Landfill


    19.2.1  Definition

Sludge  landfill  can  be defined as  the planned  burial  of
wastewater solids, including processed sludge, screenings, grit,
and  ash at a  designated site.   The solids  are placed  into a
prepared  site  or excavated trench  and  covered with  a  layer  of
soil.   The  soil  cover must  be  deeper  than the depth of the plow


                              19-1

-------
zone  (about 8 to 10  inches  [20.3 to 25.4  cm]).   For the most
part,  landfilling of  screenings,  grit,  and ash  is  accomplished
with methods similar to those used for sludge landfilling.


    19.2.2  Sludge Landfill Methods

Sludge  landfill  methods  can  be  grouped  into  three general
categories:   sludge-only trench fill,  sludge-only  area fill,  and
co-disposal  with  refuse.   General  site  and design  criteria  are
discussed under  these categories.   A detailed  discussion of
sludge  landfills  is  presented  in the USEPA Technology  Transfer
Process  Design Manual,  Municipal  Sludge Landfills  (1).  The
remaining parts of the landfill portion of this chapter summarize
the  information presented  in  this  design  manual.   Other
information on the disposal of  wastewater sludge  in sanitary
landfills is available (2).
        19.2.2.1   Sludge-Only Trench Fill

The  sludge-only  trench method  involves  excavating trenches  so
that dewatered sludge  may  be  entirely  buried  below the  original
ground  surface.   In  some  locations,  liquid  stabilized  and
unstabilized sludges  (Blue Plains,  Washington,  D.C. and  Colorado
Springs, Colorado) have been  buried  by the trench  fill  method.
In this method, the sludge is deposited directly  into  the trench
from a  haul  vehicle.   Normal  operating procedure requires  daily
coverage.  Trench  disposal  is appropriate for unstabilized sludge,
because the immediate application  of cover  material   reduces
associated odors.   Vector control requires daily  cover,  except
during very cold  weather.

Narrow Trenches

Trenches are defined  as  narrow  when their widths  are less  than
10 feet (3 m).  Disposal  in  narrow trenches  is applicable  to
sludges with  a  relatively  low  solids content of  from 3  to
28  percent.   The  application  rates range  from 1,200  to
5,600  cubic yard of  sludge  per acre  (2,270  to  10,580  m3/ha).
Excavated  material  can  be  either used immediately  to  cover
adjacent sludge - filled trench or stockpiled  alongside  and used
to cover the trench from which it was removed.  The surface soil
cover thickness is about 4  feet  (1.3 m).  Excavation and  covering
equipment operates from surface  areas adjacent to  the trench.

Wide Trenches

Trenches  are  defined as  wi'de  when  they  have  widths   greater
than  10 feet  (3  m).   Material  excavated  from the trenches  is
stockpiled neatly and used as cover for the trench.  Disposal  in
wide  trenches  is suitable for  sludges with solids contents  of
20 percent or greater.  The application rates range from  3,200  to
14,500  cubic yards  of sludge per  acre  (6,050  to 27,400  m3/ha).


                             19-2

-------
The surface  cover  thickness  depends on the solids  concentration
of  the sludge.   The  covered  sludge will  only be  capable of
supporting equipment when the solids  concentration  of  the  sludge
exceeds 25 to 30 percent and  the sludge has  been tooped with  3 to
5 feet (1 to 2 m) of soil.

The wide  trench  method has  two distinct  advantages;   it is  less
land-intensive  than  the narrow  trench method  and groundwater
protection can be provided by liners.  The use  of liners permits
deeper excavations.  The primary  disadvantage of the wide  trench
method is the need  for sludge solid contents  of  greater  than
20  percent.   Sludge with  solid contents of  greater  than  30 to
35 percent will  not flow, and extra  effort is therefore  required
to  spread  them  evenly  in the trench.   Figure 19-1 provides two
views  of  a  wide  trenching  operation at the North Shore  Sanitary
District just north of Chicago,  Illinois.
        19.2.2.2  Sludge-Only Area Fill

In  the  sludge-only area  fill  method,  the  sludge  is mixed with
soil and  the  mixture is  placed  on the original ground  surface.
This method requires substantial  amounts  of  imported  soil but  may
be  suitable in  areas where groundwater is shallow  (liners  can be
easily installed)  or bedrock prevails  (that  is,  where  excavation
is  neither  possible  nor  required).   Stabilized  sludge   is
best suited  for this method,  since daily  cover is not  usually
provided.  Adequate  drainage and  runoff  control  are  necessary to
prevent contamination of  nearby surface waters.

A rea^_ J?i_1 l^M o un. d

Area  fill  mound  applications  are  generally  suitable  for
stabilized sludges with  solids  concentrations  of  20 percent  or
more.    Soil  is mixed with  sludge to provide bulk and  stability
before  hauling  to  the filling area.  At  the filling area,  the
mixture  is placed  in 6  foot  (2  m) mounds  and then  covered with
3 to 5  feet  (1  to  1.5 m) of soil.   A  level  area is  required  for
disposal;  however,  the  use of  earthen  containment structures
permits disposal in hilly areas.

Area Fill Layer

Area fill  layer applications are  suitable for stabilized  sludge
with solids  as  low  as 15 percent.  Soil  is mixed  with  sludge,
either  at the  filling  area or  at a special mixing area.   The
sludge/soil  mixture  is   spread in  even  layers   of  approximately
1 foot  (0.3  m)  thick, and 3 to  5 feet (1  to 1.5 m) of  soil  are
added for final cover.

Level ground  is preferred for  this type  of  operation,  but mildly
sloping terrain can be used.


                              19-3

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The District's operation consists of opening 20 feet (6,1 m)
deep trenches on 300-acre (121.5 ha) site with large
backhoe equipment. This same equipment is used to cover
each layer of sludge with a layer of soil and cap each trench
with several feet of soil. Production in 1976 was 30 dry
tons/day (27 t/day).
Dewatered sludge is dumped from trucks directly into the
trench. Various equipment is shown in the background.
Also, the dewatered sludge storage building is shown in
the background. Sludge is stored inside the building on
weekends for transfer to trenches during daytime hours
Monday through Friday.

                     FIGURE 19-1

      WIDE TRENCHING  OPERATION,  NORTH SHORE
                 SANITARY DISTRICT
                        19-4

-------
D i ke^Conta i nme n t


Dike  containment  applications  require  sludge with  a  solids
content of  20  percent  or greater.   This  method  is  suitable  for
either stabilized or unstabilized sludge.   Sludge is usually  not
mixed  with a  bulking  agent.  If  the disposal  site is  level,
earthen dikes are used  on all  four sides of the containment area.
If the site  is  at  the  toe of  the hill,  only  a partial  diking is
required.   Access  is  provided to the top of the  dike so that
haul  vehicles can  dump  sludge directly  into the  containment.
Depending on the  type  of equipment  used,  the  interim cover will
vary from  1  to 3  feet  (0.3  to 1.0  m) and the final cover from
3 to  5  feet (1.0  to 1.5 m) .   Although diked  containment  is an
efficient disposal method from  the standpoint of land use,  it  may
necessitate controls for  leachate outbreaks.
        19.2.2.3   Co-Disposal with Refuse


The term co-disposal  is  used when municipal sludge is disposed  of
at a  refuse  landfill.    There  are  distinct trade-offs  in  using
co-disposal method rather than  the sludge-only methods.


Sludge  can be disposed  of  in  this manner  if  it is mixed with
refuse or with soil.   Mixing techniques  are  discussed  in detail
in the  USEPA  Office of  Solid  Waste  Report,  Disposal  of  Sewage
Sludge into a  Sanitary  Landfill  (2).


Sludge/Refuse  Mixture


Stabilized or  unstabilized sludge  with a solids content of  three
percent or greater is  mixed with  the refuse.    Normally  sludge
content  is approximately  ten percent  of  the  sludge/refuse
mixture.  The  sludge is  applied  on  top of the  refuse at the
working  face  of  the  landfill.   The  sludge and  refuse are
thoroughly mixed  before  they are spread,  compacted,  and covered
with  soil.  An  interim  cover of approximately one  foot  (0.3  m)
and a final cover of  two feet (0.6 m)  is  used.   Application rates
range from 500 to 4,200  cubic  yards  of  sludge per  acre  (950  to
7,900 m3/ha).


Sludge/Soil Mixture

In this operation, sludge is mixed with  soil and  the mixture  is
used  as cover  for a  refuse landfill.   This method requires
stabilized sludge with  at  least  a 20 percent solids content.
It promotes  vegetation growth over  completed  landfill areas
without  the use of  fertilizer.  However,  it  may cause odors,
since the  sludge  is  not completely  buried.   A final  soil  cover
could be added if necessary to eliminate  this problem.


                             19-5

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        19.2.2.4  Suitability  of Sludge for Landfilling

Some  wastewater  treatment  sludges  may not  be  suitable  for
landfilling  by  any of  the  methods  described above.    For
sludge-only   landfills,  the   solids   concentration  should  be
15 percent or more. Although soil  may  be used as a bulking  agent
to effectively  increase the solids  concentration  to  this  level,
cost-effectiveness may  become  a problem.   Solids  concentrations
down  to  three percent  are  tolerated  for co-disposal, but  the
absorptive capacity  of  the  refuse  should not  be exceeded.   An
assessment of  the  suitability  of various sludge types is given in
Table 19-1.   In general,  only stabilized and  dewatered  sludges
are recommended for landfill disposal.


    19.2.3  Preliminary  Planning

The purpose of  the preliminary planning  activity  is  to select  a
disposal   site  and suitable  method(s)   of disposal.   Preliminary
planning   is  followed by detailed design, initial  site develop-
ment,  site operation and maintenance, and final site closure.

Site  selection is the  major  activity during  the preliminary
planning  phase.  Since the selection of a site is not completely
independent  of  the  selection of  a   method,   the  preliminary
planning   phase  should also  include the  determination  of  sludge
characteristics and  the  identification of  alternate landfill
methods for each  site.   Chapter  2  of  Municipal  Sludge Landfill
(1) provides an excellent discussion on  public  participation in
this and  other phases of the project.


        19.2.3.1  Sludge Characterization

Sludge  must  be  characterized   as  to  quantity  and quality.
Chapter 4 provides further discussion on sludge characterization.

Sludge_Quantity

An  estimate of  the  average  sludge quantity  is  necessary  to
establish  landfill area requirements   and the  probable life  of
the disposal  site.  Data on  minimum  and maximum sludge quantities
are important for  developing an understanding of daily operating
requirements.   Maximum  daily sludge quantities  will govern
equipment and  storage  facility sizing  and  daily  operating
schedules.

Sludge Quality

The character of  the  sludge to be landfilled is directly related
to  the  choice  of a landfill  method.   Sludge  quality and  the
corresponding  leachate can  be  roughly correlated; design of
leachate  treatment facilities  is more effective if sludge quality
is known.
                              19-6

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                                  TABLE 19-1

                  SUITABILITY OF SLUDGES FOR LANDFILLINC

                             Sludge only landfilling      Co-disposal landfilling

       Type of sludge	    Suitability	 Reason      Suitability     Reason
Liquid - unstabilized
  Gravity thickened primary,
    WAS and primary,  and WAS          NS         OD, OP          NS         OD,  OP
  Flotation thickened primary
    and WAS,  and WAS  without
    chemicals                      NS         OD, OP          NS         OD,  OP
  Flotation thickened WAS with
    chemicals                      NS           OP           NS         OD,  OP
  Thermal conditioned primary
    or WAS                         NS         OD, OP          MS         OD,  OP

Liquid - stabilized
  Thickened anaerobic digested
    primary and primary, and
    WAS                            NS           OP           MS           OP
  Thickened aerobic digested
    primary and primary, and
    WAS                            NS           OP           MS           OP
  Thickened lime stabilized
    primary and primary, and
    WAS                            NS           OP           MS           OP

Dewatered - unstabilized
  Vacuum filtered, lime
    conditioned primary              S            -             S            -

Dewatered - stabilized
  Drying bed digested and
    lime stabilized                 S            -             S            -
  Vacuum filtered, lime
    conditioned digested             S            -             S
  Pressure filtered,  lime
    conditioned digested             S            -             S            -
  Centrifuged, digested and
    lime conditioned  digested        S            -             S

Heat dried
  Heat dried digested               S            -             S            -

High temperature processed
  Incinerated dewatered
    primary and primary, and
    WAS                            S                         S            -
  Wet-air oxidized primary
    and primary, and  WAS             NS         OD, OP          MS         OD,  OP
WAS - Waste-activated sludge
NS  - Not suitable
MS  - Marginally suitable
S   - Suitable
OD  - Odor problems
OP  - Operational problems

Parameters  that should  be  analyzed  are  discussed briefly below.
Although  all of these  may not  be  critical  to the design of a
particular  disposal   system,  a   complete  analysis   is   necessary,
because  the  sludge  must be  adequately  characterized.

      •   Concentration.   Concentration  or  solids content  of sludge
          is  related  to  the  nature  of  wastewater treatment  and
          sludge  processing  steps.    The  type   and  operation  of


                                    19-7

-------
        dewatering  equipment  may  have a significant effect on the
        sludge  concentration.   A certain degree  of  flexibility
        should  be  incorporated  into  the design of  landfills  to
        compensate  for  the  variability in solids concentration of
        dewatered sludge.

     •   Volatile  content.   Volatile  solids  are a  measure  of  the
        organic  content present in the solid fraction of sludge.
        This organic  matter  is  eventually  broken  down  into
        methane  gas and other digestion by-products.   Typically,
        volatile  solids represent 60 to  80  percent  of  the total
        solids in  raw  primary  sludge and  30  to  60 percent  in
        anaerobically digested  primary solids.

     •   Nitrogen.   Nitrogen found  in sludge  is a potential source
        of  groundwater  pollution.  The total quantity and type of
        nitrogen  are  of importance.  Nitrate is relatively mobile
        in  soil and is  therefore of concern.

     •   Inorganic ions.   Inorganic  ions  such as  heavy metals  are
        found in  most municipal sludges.   These are more readily
        leached  if  soil  and  sludge  are  acidic.   If  near neutral
        or  alkaline  conditions  are maintained,  the  metals  will
        not be as readily leached from the sludge  or through  the
        soil.

     •   Bacteriological quality.   Sludge  treatment systems reduce
        the number  of pathogens  and the possibility of pathogenic
        contamination  associated  with  landfilling  of  sludges;
        however,  they do riot  provide a sterile product.

     •   Toxic organic compounds.   Toxic   organic  compounds  can
        present  potential  contamination  problems.   Solids
        contaminated with  toxic materials must be placed  in
        appropriately designated  disposal facilities.

     •   pH.  Acidic conditions promote leaching of  heavy metals
        and other compounds from the sludge.


        19.2.3.2  Selection of  a  Landfilling Method

Relationships  between  the  characteristics  of   alternative
landfill  sites,  the  characteristics of  the  sludge to  be
landfilled, and the  landfill method need to be considered in the
preliminary planning process.  These relationships are summarized
in Table 19-2.


        19.2.3.3  Site  Selection

Site selection is a  critical process in the planning of a sludge
landfill  project.   It  is directly  related  to  the method  of
ultimate disposal.    The site  finally selected  must  be suitable


                              19-8

-------
for  the  type  of sludge  to  be disposed of  and  situated  in  a
convenient,  yet  unobtrusive,   location.   Chapter  4  of  Municipal
Sludge  Landfill (1)  provides  an  in-depth  approachTo  site
selection.
                               TABLE 19-2

                       SLUDGE AND SITE CONDITIONS
      Method
Narrow trench


Wide trench

Area fill mound




Area fill layer


Diked containment




Sludge/refuse mixture


Sludge/soil mixture
Sludge solids
content, percent
15 - 28
>20
>20
Appropriate
sludge
characteristics
Unstabilized or
stabilized
Unstabilized or
stabilized
Stabilized
Appropriate
hydrogeology
Deep ground water and
bedrock
Deep groundwater and
bedrock
Shallow groundwater
Appropriate
ground slope
<20 percent
<10 percent
Suitable for ste<
£15
£20
>2Q
         Unstabilized or
           stabilized
         Stabilized
         Unstabilized or
           stabilized
         Stabilized
                       or bedrock
Shallow groundwater
 or bedrock
                      Shallow groundwater
                       or bedrock
Deep or shallow
 groundwater or bed-
 rock

Deep or shallow
 groundwater or bed-
 rock
 terrain as long as
 level area is pre-
 pared for mounding

Suitable for medium
 slopes but level
 ground preferred
Suitable for steep
 terrain as long as
 a level area is pre-
 pared inside dikes
   <30 percent
                                        <5 percent
Site  Considerations

The following  factors must be considered during the evaluation  of
possible  landf ill  sites.    Information  on  these  factors  should
therefore be  collected  and  assessed  in  advance of  the  final
decision making  process.

      •   Haul distance.  The  most  favorable  haul  conditions
         combine  level terrain and  minimum distances.

      *   Site life and size.   The  site 1ife  and s ize are  directly
         related  to the quantity and  characteristics of  the sludge
         and  the  method used for   landfilling.   Since   the entire
         site cannot be used  as fill  area, both  the gross  area and
         the usable or fill area must be cons idered in determining
         the  site  size.    Initially,   the life  of   the  site  can  be
         estimated.   As   the landfill  is  used,  the expected
         life  should  be  reevaluated  to  ensure adequate  capacity
         for future operations.

      *   Topography.  In general,  sludge  landf illing is  limited  to
         sites  with minimum slopes  of  one  percent and   maximum
         slopes  of  20  percent.    Flat terrain  tends  to result  in
         ponding,  whereas  steep slopes erode.
                                 19-9

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Surface water.   The  location and extent of surface waters
in the vicinity of the  landfill site can be a significant
factor in the  selection process.  Existing surface waters
and drainage  near  proposed sites  should  be mapped  and
their  present  and  proposed  uses outlined.    Leachate
control measures including collection  and  treatment  may
be required as  part  of  the  landfill design.

Soils  and geology.   Soil  is an  important  determinant  in
the choice  of  an  appropriate sludge  landfilling  site.
Properties  such as  texture,  structure,  permeability,
pH, and  cation exchange capacity,  as well as  the
characteristics of  soil  formation,  may  influence  the
selection of the site.   The geology of possible landfill
sites  should  be  thoroughly  examined  to identify  any
faults, major fractures and joint sets.  The possibility
of  aquifer contamination  through  irregular  formations
must be studied.

Groundwater.   Data  on  groundwaters  in the vicinity  of
potential  landfill  sites  is  essential  to  the  selection
process.  Knowledge  of characteristics such as  the depth
to groundwater, the  hydraulic gradient, the  quality  and
use of the groundwater,  and the  location of  recharge
zones   is  essential  for determining  the suitability of a
potential landfill site.

Vegetation.  The type  and  quantity  of vegetation in  the
area  o~fproposed landfill sites  should  be considered
in  the evaluation.   Vegetation  can serve as  a natural
buffer,  reducing   visual  impact,   odor,  and other
nuisances.  At  the  same  time,  clearing a  site  of timber
or  other  heavy vegetation can add  significantly  to  the
initial project costs.

Meteorology.    Prevailing  wind  direction,  speed,
temperature and atmospheric stability  should be evaluated
to determine potential odor and dust  impacts downwind of
the site.

Environmentally sens it i ve  a re a s.   Environmentally
sens itive areas such as  the wetlands,  flood  plains,
permafrost  areas,  critical habitats of  endangered
species,  and  recharge zones of  sole source  aquifers
should be avoided  if at  all possible when  selecting
a landfill site.

Archaeological and  histo r i c a 1  significance^.    The
archaeological  and  hlsTorTcal™ s ignif icance  6T"proposed
sites  should  be  determined early  in  the evaluation
process.  Any significant  finds  at  the  selected site must
be accommodated prior to  final approval.
                      19-10

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     •  Site access.   Haul  routes should  be  major  highways,  or
        arterials,  preferably  those  with  a minimum  of traffic
        during normal  transport  hours.   Proposed routes  should be
        studied to determine  impacts on  local  use and  the
        potential  effects  of  accidents.    Transport  through
        nonresidential areas is preferable  to transport through
        residential  areas,  high-density  urban areas,  and  areas
        with congested traffic.  The access  roads to the  site must
        be adequate  for  the  anticipated  traffic loads.

     •  Land use.    Zoning restrictions, and  future development
        on potential sites should be considered in the  selection
        process.   Ideally,  the sludge  landfill  site  should  be
        located on  land  considered  unsuitable for  higher  uses;
        however,  the  designer  should  be  aware  that  this  may
        be  a  politically sensitive  issue and  maximum public
        participation  must be assured.

     •  Costs.    Cost-effectiveness  of  each  potential  landfill
        site must be  evaluated.   Factors  to  be  included  in the
        economic evaluation include capital  costs  and  operating
        and maintenance  (O&M)  costs.  In  the latter  category,
        sludge hauling may prove  to  be  a significant component.
        The trade-offs  between  high  capital and high  OS.H  costs
        will depend on  the design  life of  the  landfill.  These
        trade-offs  will  become evident when the total  annual
        (amortized  capital  and  O&M costs)  are compared.   This
        evaluation  should  be  performed  in accordance  with  the
        methods  outlined in the cost-effectiveness  analysis
        section discussed  in Chapter 3.

Sj^te Selection Methodology

The selection procedure  can  be roughly divided  into three phases:
initial inventory and  assessment of sites,  screening of  potential
sites, and final site  selection.

Initial inventory and  assessment is designed to develop  a list of
potential  sites  that  can be  evaluated  and  rapidly  screened  to
produce a manageable number  of candidate sites.  Information used
in  this  phase is generally available  and readily accessible.
Investigation  of  each  option  becomes more detailed as  the
selection procedure  progresses.

I n i t i a1_Assessment of  Site

Initial assessments  will  consist  of  identifying  Federal, State,
and local regulatory constraints,  eliminating  inaccessible areas,
locating   potential  sites,  roughly   assessing   the  economic
feasibility of  such  sites,  and  performing  preliminary  site
evaluations. The less  desirable  sites  are eliminated on  the basis
of preliminary economics,  regulatory,  and  technical information.
A public  participation  program  is initiated  (4).   Attitudes  of
the public  should be  determined early.  The public may  assist in
identifying candidate  sites.


                              19-11

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Screening of Candidate Sites.   Sites  remaining  after the initial
assessment are subjected to closer  scrutiny.  Information used in
evaluating each option  is  more detailed and somewhat more site-
specific than in the  initial assessment.  Remaining sites may be
rated by a scoring  system including both objective and subjective
evaluations (Chapter 3).   Table  3-4  serves  as an example of
a rating  system.   Candidate systems  with lowest overall ratings
are eliminated,  and the higher rated systems are carried forward
for final evaluation.

Site  selection findings  for  the  remaining  candidate  systems
should  provide input into an environmental  impact report, if
required. Public attitudes  toward the remaining sites should also
be determined.


Final Site Selection and Site  Acquisition.  Methodology for final
site  selection  is   similar  to  that for  the  screening  procedure
just discussed,  in  that rating systems are still used.   However,
each  site  remaining is investigated  in greater  detail.   Public
hearings  may also be  scheduled  so  that final inputs can be
received from local government  officials and the  public.

Once the best sites are determined, they must be acquired.   Site
acquisition should  begin immediately  following acceptance of the
program by local,  State,  and Federal  regulatory  authorities.
The  several  acquisition procedures  include:    purchase  option,
outright purchase,  lease,  condemnation and/or other  court action,
and land dedication.

It  will  generally prove   advantageous  to  purchase  the  site
rather  than hold  a  long-term lease.   The  managing agency's
responsibility will normally extend well  beyond  the life of the
site.   Certain  advantages  may  also be  gained  by leasing  with an
option  to buy  the site at the  time of  planning  approval.   A
purchase option assures the availability of land upon completion
of the facility  planning process.   This  approach  also allows time
for  the previous  owner to gradually phase  out operations, if
desired.
    19.2.4  Facility Design
        19.2.4.1  Regulations  and  Standards

Local, State,  and Federal regulations  and  standards must be fully
understood before the  landfill  is designed.   Consideration must
be  given  to requirements  governing  the  degree  of sludge
stabilization,  the  loading rates, the  frequency and  depth of
cover, monitoring, and  reporting.   The design should conform to
all building  codes  and should  include adequate  buffer  zones to
protect public roads, private  structures,  and  surface waters.


                              19-12

-------
Obtaining  permits  for  construction  and  operation  of  sludge
landfills can be a  long  and  costly  process.   To minimize delays
associated with  this task,  permit application should be initiated
early  in  the  design  stage.   A  sound regulatory-consultant
relationship and a mutual understanding  should be developed.

The  following is  a partial  list  of the permits which  may  be
required:

     •  NPDES permit—if  landfill is  in wetlands.

     •  Army Corps of  Engineers  permit — for  construction  of
        levees,  dikes,  or containment structures to be placed  in
        the water in a  wetlands area.

     •  Office  of  Endangered Species  permit—if  landfill  is
        located  in  critical habitat of an endangered species.

     •  Solid Waste Management permit.

     •  Special  Use permit.

     •  Highway  Department  permit.

     •  Construction permit.

     •  Building permit.

     •  Drainage and/or Flood Plain Alteration permit.


        19.2.4.2  Site  Characteristics

Site characteristics should  be clearly described and analyzed  to
ensure  the  suitability of   the  landfill  site and  the  method  of
landfilling.  Design  phase work will build  upon  planning phase
data but will be carried to  a higher level of detail and include
working drawings.

Sit.e_Plan

The site plan should contain the  following minimum information:

     •  Boundaries  of fill  area  and  buffer zones.

     •  Topographic features  and slopes  of  fill area  and buffer
        zones.

     •  Location of surface water,  roads, and  utilities.

     •  Existing and  proposed  structures  and  access  roads.

     •  Vegetation to  remain and  to be removed;  areas  to  be
        revegetated.


                              19-13

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Soils

The  soil  characteristics at  the  landfill  site  should  be
thoroughly  catalogued and  mapped.   The information of most
importance to the  design  and operation of the landfill includes
depth, texture,  structure, bulk density, porosity, permeability,
moisture, stability, and  ease  of excavation.   Areas  with  rocky
soils or  extensive rock  outcrops should  be  noted.   The pH  and
cation exchange  capacity  have a  direct bearing  on heavy  metal
transport through  the  soil.  Translocation  of  metals must  be
considered  to  ensure  protection  of  surface and  groundwater
supplies.

Grouridwcrteir

The  groundwater  aquifers  underlying  the  landfill  site must  be
located.   Depth  of the  aquifer  under varying conditions should be
determined at several  locations.  Other characteristics  such  as
the  direction and rate  of  flow, the  hydraulic gradient,  the
quality,  and present and planned  uses  should also be  established.
Location of the primary recharge zones  is critical in  protecting
quality.

Subsurface Geology

The  geological  formations  underlying  the  landfill are important
in  establishing  the  design parameters.    Critical  design
parameters include  the  depth,  distribution,  and  characteristics
of subsurface soils in relation to  stability and  groundwater
transmissability.

Climate

Climate  can  influence  many factors  in  the design of  landfills.
Climatic  conditions  effect  rate  of  organic  decomposition,  the
composition and quantity of leachate  and  runoff,  the day-to-
day  fill operations,   and  the dispersion  of odors  and  dust.
Information  such  as  seasonal  temperature,  precipitation,
evaporation, wind  direction  and speed and atmospheric  stability,
can be obtained  from a  local weather station.

Land JJ se

The  present  and  proposed  use of  the  landfill  site  and adjacent
properties should be evaluated.   If  the  site is already dedicated
to refuse  or  sludge disposal,  it is  unlikely  that  expanding  it
will  result in adverse  impacts.  However,  if the  site  is located
in or near  a populated area,  extensive control  measures may  be
needed to eliminate concerns  and minimize  any  public  nuisance
which would detract from the va.lue of  adjacent properties.


    19.2.4.3  Landfill  Type and Design

More  than one sludge  landfill method  may  be suitable for  the
selected site, as  shown in  Table  19-21.   If  this is the case,  a
method must be selected before  the final design is begun.


                              19-14

-------
 Maximizing utilization of  the  site  is  an important consideration
 in method selection. If  daily cover is  to  be applied,  the  daily
 sludge generation  rate  will  affect  the  net  capacity  of  the
 site.   If  several ^days  are required  to fill a  trench,  as  the
 result  of low  sludge generation, and cover  is required each  day,
 then  the  ratio of  sludge/cover will  be  less  than  for sites
 managing  larger  sludge quantities.  The  net sludge capacity  will
 be higher at sites where  trenches  are  filled each  day.

 The amount by which  the net  capacity of  the site  will be  reduced
 will  vary with  the  landfill methods,  the specific  site,   and  the
 daily  sludge generation rate.  Before  a final method is selected,
 estimates of net  capacity  and  site  life  should be made for  each.

 Additional design  criteria are summarized in Table 19-3 (1).
                               TABLE 19-3

                        LANDFILL DESIGN CRITERIA
             Sludge
             solids  Trench
             content, width, Bulking Bulking
  Cover         Sludge
thickness, ft Imported application
         soil   rate,
Method
Sludge only-trench fill


Wide trench





20-28°
h9d




"10
10


NO

No --
No
Interim Final



3-4
4-5
required

°

No

cu yd/acre



3,200-14,500

Equipment

'
machine
Track loader, dragline,
scraper, track
Sludge only-area fill
Area fill mound

Area fill layer

Diked containment

Codisposal with refuse
Sludge/refuse mixture

Sludge/soil mixture

>20C'd -

>15d

20-28C
>28d
v H
>3d

>20d

Volume basis unless otherwise noted.
In actual fill areas.

Yes Soil

Yes Soil

No Soil
No Soil

Yes Refuse

Yes Soil

1 ft = 0.
1 cu yd =
1 acre =
0.5-2 soil:
1 sludge
0.25-1 soil:
1 sludqe
0.25-
1 sludge

4-7 tons refuse ;
1 wet ton sludge
1 soil:
1 sludqe
305 m
0. 765 cu m
0.405 ha
3

0.5-1




0.5-1

0.5-1



                                                  Yes  3,000-14,000  Track loader, backhoe
                                                              with loader, track
                                                              dozer

                                                  Yes  2,000-9,000  Track dozer, grader,
                                                              track loader

                                                  Yes  4,800-15,000  Dragline, track dozer,
                                                              scraper
                                                      500-4,200  Draqline, track dozer
                                                       1,600    Tractor with disc,
                                                              grader, track loader
 Land-based equipment.

 Sludge-based equipment

 But sometimes used.
          19.2.4.4  Ancillary Facilities

 Ancillary  facilities  may be  needed  in association  with  the
 landfill  site.   These  are described briefly  in the  following
 sections.

 Lea.chate^ Contro 1 s

 Leachate from  the  landfill  site  must  be contained  and  treated
 to  eliminate  potential  water  pollution  and/or potential  public
                                 19-15

-------
health  problems.    In many  cases,  leachate  containment and
treatment  may  be  required  by  state  or  local  regulations.
Numerous methods are available  for  controlling  leachate,  includ-
ing drainage,  natural  attenuation,  soil  or membrane liners, or
collection and  treatment.   The method and  the design  features
chosen  are  specific  for  each project.    Table  19-4  depicts
sludge-only leachate quality for one site  sampled  over two years.
                            TABLE 19-4

            LEACHATE QUALITY FROM SLUDGE-ONLY LANDFILL

                   Constituents             Values
               Constituents
                 PH                             6.7
                 TOC                          1,00(K
                 COD                          5,100

               Ammonia nitrogen                 198

               Nitrate nitrogen                0.28
                 Chloride                       6.7
                 Sulfate                         10
                                                  0
               Specific conductivity          3,600
                 Cadmium                      0.017
                 Chromium                       1.1
                 Copper                         1.3
                 Iron                           170

                 Mercury                     0.0004
                 Nickel                        0.31
                 Lead                          0.60
                 Zinc                           5.0
           aData from "Site 8" monitored from July 1975
            through September 1977.   First received
            sludge in 1973.  Receives unstabilized
            primary and WAS,  gravity thickened and
            centrif uged .   Sludge is  lagooned,  allowed
            to dry, and covered with soil.  Soil
            characteristics:   sand and gravel, glacial
            deposites .
            Specific conductivity in micromhos/cm, pH
            in units, all others in  mg/1 .
           °Ranged from 3,000 mg/1 to 1 mg/1.
            Limited to  early part of sampling program.
           f\
            Ranged from 10,000 micromhos/cm 340
            micromhos/cm .
                              19-16

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Gas Control

Gas produced  by  decomposition  of organic matter  is  potentially
dangerous.   This  condition  is  of particular concern if  the
landfill  is located  near a populated area.   Methane gas,  in
particular, is highly  explosive if confined in an enclosed  area.

Control of  the gases  produced  at  the  landfill  must  be provided.
Two widely accepted  methods  control paths of gas migration.
Permeable  methods  usually consist of a gravel-filled trench
around the  fill  area  for  intercepting  migrating gas  and venting
it to  the  atmosphere.   Impermeable methods consist  of placing  a
barrier of  low  permeability material,  such  as compacted  clay,
around the  fill  area  to minimize  lateral  movement of  gas.   This
method provides  for gas venting  through  the  cover material.   In
general,  methane  recovery  is  not cost-effective at sludge-only or
small co-disposal sites.

Roads

Paved  access and on-site roads  are necessary at the  landfill
site.   Temporary  roads may be  constructed of well compacted
natural soil  or  gravel.    Considerations  should include grades,
road  surface  and  stability,  and climate.   Grades  in excess
of ten percent should  be  avoided.  Provisions  should  be made to
allow trucks to turn around within the site area.

Soil S
Storage  area  should be  provided  for  on-site  stockpiling  of
transported soils where on-site soils are  insufficient  or  their
use inappropriate.  The quantity and  type of soil to be stockpiled
depends on  the  individual demands  of the  landfill.   Stockpiles
may also be  desirable for winter  operations  where frozen ground
may limit excavation.

I nc 1 erne n t We athej:Areas

Special  landfill areas should be placed  near the entrance  to
the site  so that  operations  may  be continued  during  inclement
weather.   Paved  or all-weather  roads  should  be provided for
working these sites.

Structures

An  office  and  employee  facilities should  be  located at the
landfill  site.   For large operations, a permanent structure
should be  provided.   At  smaller sites  a  trailer might  suffice.
An equipment barn and shop may  be desirable for some locations.

Utilities

Electrical, water, communication  and sanitary services  should  be
provided for large landfill operations. Chemical toilets, bottled
water, and on-site  electrical  generation may  reduce  the cost  of
obtaining services from utility companies.   This approach may  be
appropriate for remote sites.


                             19-17

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Fencing

The landfill site should be fenced.   Access  should  be  limited  to
one or two secured  entrances.   The height and  type of fence
should suit local conditions.   A 6-foot (1.8 m)  chain  link fence
topped with barbed  wire  will restrict trespassers;  a wooden fence
or hedge is effective for screening the operation  from view, and
a 4-foot (1.2  m)  barbed  wire fence will keep cattle or  sheep away
from the site  area.

Lighting

Portable lighting should be provided if  landfill  operations are
carried out at night.   Permanent  lights  should  be  installed for
all structures and  heavily  used  access roads.
A  cleaning  program should be  required  for frequently used
equipment.  A curbed wash pad  and collection basin should  be
provided to contain the  contaminated washwater for treatment.

Monitoring We 11s

It  is  crucial to monitor  groundwater.  The  number, type, and
location of  monitoring  wells and  monitoring frequency  should  be
designated   to  meet specific  conditions  associated  with the
landfill.
Depending on the size  and  location  of the landfill,  landscaping
may be  an  important  design  factor.   The aesthetic  acceptability
of the  landfill  is  critical,  especially  in an  urban  or  densely
populated  area.   In general,  shrubbery  chosen should require
little maintenance and  become an effective visual barrier.
        19.2.4.5  Landfill Equipment

A  wide  variety  of  equipment  may  be required  for  a sludge
landfill.  The  type  of  equipment  depends  on  the  landfill method
employed and  on  the quantity of  sludge  to be disposed of.
Equipment  will be  required for  sludge  handling,  excavation,
backfilling,  grading,  and road construction.  Table 19-5 presents
typical  equipment  performance  characteristics  for various
sludge landfilling methods.


        19.2.4.6  Flexibility and Reliability

Because  sludge characteristics and  quantities  may  change,  a
landfill site should  be  designed with maximum flexibility.   Since
the  life of  a  landfill  is difficult to accurately  predict,
                              19-18

-------
expansion may  be needed  sooner  than  originally  planned or  it
may  be  delayed.   Any  change  in  wastewater  treatment  or  sludge
management processes may affect the nature and quantity of  sludge
produced.   Operational  modifications  may  be  needed  if these
changes are  drastic.    The  landfill  design  should be  such  that
changes can be  made  without  major disruption to  operations.
                              TABLE 19-5

           LANDFILL EQUIPMENT PERFORMANCE CHARACTERISTICS
                                           Equipment type
      Landfill
      method  Submethod
                 Soil hauling
                 Mixing
                 Sludge hauling
                 Mounding
                 Covering

                 Soil hauling
                 Mixing
                 Sludge hauling
                 Layering
                 Covering
          •Diked con-   Soil hauling
           tainment   Dike construe
                 Covering
     Codisposal Sludge/refuse Spreading
                 Covering
                 Haulii
                 Cover
     Legend
      G = Good. Fully capable of performing function listed. Equipment could be selected solely on basis of function listet
      F = Fair. Marginally capable of performing function listed. Equipment should be selected on basis of full capabilitii
            in other function.
Reliability  is another important  factor  in  designing  a  landfill
operation. •  Operation  should  continue  even in  inclement  weather.
Special  work  areas  and  storage  facilities   should  be available
on   site  for  emergency  operations  or  unexpected   equipment
failures.
         19.2.4.7   Expected Performance

Although  the  overall performance of a sludge  landfill may be
difficult  to  predict accurately,  certain  operating  parameters
should  be estimated.  The site life depends on many  factors;
an estimate  .is  needed  for  purposes  of   economic evaluations
and future planning.  Sludge  application rate  and  soil  cover


                                 19-19

-------
requirements should  be  estimated  before scheduling initial
operations.   Performance  can be more closely  predicted after
actual operating experience  is gained.


        19.2.4.8  Environmental Impacts

Specific  areas  of  environmental  impact  vary  among  landfill
locations.    Crucial  impact areas  include:   traffic,  land use,
air quality, surface and  groundwater quality,  public health,
aesthetics,  wildlife,   and habitats  of  endangered  species.
Adverse impacts  should  be  mitigated during  the site  selection
process or by specific measures in the design.


    19.2.5   Operation and Maintenance

A  sludge  landfill should be  viewed  as an ongoing  construction
site.    Unlike  conventional  construction, however,  the  operating
parameters  of a sludge  landfill often change  and may require
innovative  alterations  and contingency  plans.   An  effective
landfill  requires  a detailed  operational  plan.   Equipment
selection  should be  compatible with  sludge  characteristics,
site conditions, and landfill method.

Operational procedures  can  be  separated  into those specific to
the landfill method and  those  applicable to  sludge  landfills
in  general.    Method-specific procedures  include:    site
preparation,  sludge unloading,  sludge  management  and  covering.
These  procedures are discussed  in detail in  Municipal  Sludge
Landfill (1).

General procedures include  scheduling,  equipment  selection and
maintenance,  management  and reporting,  safety,  and  environmental
controls.   These  items  are discussed in Sanitary  Landfill  Design
and Operation (2).  Important points are summarized  below.


        19.2.5.1  Operations Plan

As  with any  construction  activity,  sludge landfilling must
proceed according to detailed plans and operating schedules.  The
operation  plan  should  address all  relevant  method-specific or
general operating procedures for the landfill,  including:

     •  Hours of operation.

     •  Measuring procedures.

     •  Traffic  flow and  unloading procedures.

     •  Special  wastes handling.

     •  Cover excavation, stockpiling, and placement.


                             19-20

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     •  Maintenance procedures  and  schedules.

     •  Inclement weather operations.

     •  Environmental monitoring  and control practices.

An operations plan  is  an important  tool for providing  continuity
of activities,  monitoring  and control  of progress, and  personnel
training.


        19.2.5.2  Operating Schedule

Major  features  of the  operating   schedule  include:   hours  of
operation,  availability  of  qualified personnel, site  preparation
schedules,  and  equipment  maintenance  schedules.   The  hours  of
operation  must  be such that the site is open when  sludge is to be
received.  If variations  in  the rate of  receipt are expected
during the day,  it may be desirable to schedule  for equipment and
personnel  accordingly.   The schedule may need to  provide  for the
application of  daily soil cover.


        19.2.5.3  Equipment Selection and Maintenance

Equipment   selection depends   largely   upon  the  landfill  method,
design dimensions,  and sludge quantity.   Selection must be based
upon  the  functions to  be  performed  and the cost of alternate
machines.   Table  19-5  summarized  general selection criteria.
Table  19-6 presents  examples  of   equipment  choices for  seven
landfill  schemes.

                             TABLE 19-6

          TYPICAL EQUIPMENT TYPE AND  NUMBER AS A FUNCTION
                 OF LANDFILL METHOD AND SITE LOADING

            Trench method                Area fill method -              Codisposal method*1
       Narrow trench    Wide trench     Hound        Laver     Diked containment  Sludge/refuse   Sludge/soil
Trenching
 machine        1  2

Backhoe with
 loader   1  1   I9 1             I9 I9 I9 1

  .vator       1

  ;k loader           1  lg  1   I9 1  1 1  1  1

  1 loader             '            11         I9

  ck dozer    I9 1 1  23   I9  1 1  29    I9 1  1  1 1 1  29 2  1  I9 I9 1

  aper                  I9 1     I9 I9 1   I9 lg I9 1     lg 1

  gline                            '             111
  Total   12235  12  224  1245  5122341  233  4-  - 1  121124



Additional equipment only.

Scheme 1-10 wet TPD.

C Scheme 2 - 50 wet TPD.

Scheme 3 - 100 wet TPD.

GScheme 4 - 250 wet TPD.
                                19-21

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Equipment maintenance  can  be more  expensive  than the amortized
annual purchase cost.   A scheduled preventive maintenance program
should  be followed  to control  maintenance  costs.   Operators
should  perform routine daily maintenance (for  example,  check
fluid  levels,  cleaning, etc.).   The  operating  schedule should
provide periods for thorough  maintenance.


        19.2.5.4  Management  and  Reporting

Management  and  reporting activities  include the  maintenance
of  activity  records,  performance   records,  required  regulatory
reports,  cost  records,  on-site  supervision and public relations
activities.    Activity  records  include equipment  and personnel
accounts, sludge and  (if applicable) solid waste  receipts, cover
material  quantities and used site  area layouts.   These records
become bases  for scheduling site  development, gauging  efficiency,
and any billing as required.

Performance  records may  be required as a  part of the  regulatory
process.  Regulatory agencies may perform periodic inspections on
a scheduled or  an unscheduled basis.  Operating and  supervisory
personnel must be aware of  these  requirements.

For  the  purposes of  safety and  control,  the  site  should be
staffed with  two  or  more persons.   At smaller sites, where only
one operator  is  required,  daily  visits or phone checks should be
made.
        19.2.5.5  Safety

Providing a safe working environment at the landfill site  should
be a part  of  general O&M,  and certain safety features should be
built into the  design.  Certain practices must be followed daily
to provide safe working conditions.   The operations plan  should
have a  separate safety  section,  as  well  as  specific  safety
guidelines for each operation and  feature  of  the  landfill.

S_o_il_and Fill Stability

The  stability of the  soil and  fill  can present a critical  safety
problem, particularly with the use of large  equipment.  Disturbed
and  filled areas should be  approached cautiously as should muddy
areas or areas subject to erosion.

Equipment Operation

The  operation  of  large, earth-moving equipment presents the
potential  for accidents.    Only  fully  trained  operators  should
be allowed to use such equipment.   Regular maintenance  and  safety
checks can greatly reduce the number of  accidents associated with
equipment failure and operator error.


                              19-22

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Gas  Control
Caution  must  be  used  when  dealing  with  gas  control  equipment.
The  O&M  manual  should  contain a  complete set  of  instructions on
the  safe  servicing of  gas  control and  monitoring equipment,  and
the  operation of  this  equipment  should  be explained  periodically
at operation and safety training sessions.
         19.2.5.6  Environmental Controls
The protection of the environment  and public health  are  important
aspects  of  the  landfill operation.  The  operations plan  should
contain  guidelines  for  providing  this  protection  and actual
operations  should  conform  to  the guidelines.    General  require-
ments are  summarized in Table 19-7.   Critical areas  are  discussed
below.
                               TABLE 19-7
                POTENTIAL ENVIRONMENTAL PROBLEMS AND
                           CONTROL PRACTICES
                                        Environmental problems
                            Siltation
                              and
      Control practice     Spillage   erosion  Mud Dust Vectors  Odors Noise  Aesthetics  Health  Safety
 Safety program                                                             X
 Maintain washrooms for person-
  nel                                                              X
 Training of new personnel       X     XXXXXX      X     XX
 Use safety clamps on truck
  tailgates                X                                              x
 Maintain road markings and
  trench barriers            X                                              X
 Ma intain fencing                                                 X          X
 Apply insecticide                                X                      X
 Maintain buffer areas and grass         XXX         XX      X
 Proper equipment maintenance     X                             XX
 Spray water/oil/liquid asphalt                X   X
 Truck wash pad (to clean trucks)               XXX
 Maintain grass waterways,
  diversion ditches, rip rap           XX                        X
 Fi nal grading of disturbed
  areas                         X                             X
 Revegetation of disturbed
  areas                         XXX                    X
 Chemical masking agent                                  X
 Limeonsite               X                   XX                XX
 Workers supplied with
  aerators                                XX                XX
 Cover sludge daily                                XX          XXX
 Water diverted away from site           X     X
Environment
Environmental  protection  is   generally  focused  on  leachate
and  runoff controls  for  preventing  surface  and  groundwater
contamination.   Trench  liners  must be  kept  intact during  and
after  filling  operations.    Drainage  systems  should  be  checked
                                  19-23

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to see  that they are  functioning as  designed.   If  monitoring
indicates  that  adverse  environmental  impacts  are  occurring  or
pending, immediate corrective  action  should  be taken.

Public Health


Protection of public health should  be  a foremost concern in the
operation of sludge landfills.  Protection of water supplies and
particularly sole source  aquifers  is an obvious responsibility.
In addition,  control  of potential  disease  by  reduction  of
vectors, the adequate  venting of explosive or  toxic  gases, and
the  restriction  of  access  to  the  landfill site  are  the
responsibility  of the operators.

Social Welfare

Minimizing  the  negative  aesthetic impacts  of  a sludge landfill
can greatly increase public acceptance.  Control of odors, noise,
and other  nuisances  is  generally  straight-forward  and should be
accomplished as  part  of  the  daily  operating  routine.   Efforts
should  be  made  to reduce  the  undesirable  social impacts  of the
fill  operation.
    19.2.6  Site Closure


In closing  a  sludge  landfill site,  certain criteria must be met
to  make  the  site  publicly  acceptable.   These  criteria  are
established according to  the  type  of landfill and the location,
size,  and ultimate  use of the site.   The procedures  for site
closure should  be  included  in the operations manual and updated
or modified as the original landfill  plan  is altered.
        19.2.6.1  Ultimate Use

The ultimate use of the site should be described and illustrated
in  the  O&M  manual or  in a separate document describing  the
closure of the site.  The  actual work involved  in completing the
site will  depend  on its  ultimate  use  and on  the  care  taken in
day-to-day fill operations.
        19.2.6.2  Grading at Completion  of  Filling


When each  section  of  the landfill is completed, the final cover
should  be graded  according  to  a  predetermined  plan.   It is
imperative that no sludge become or  remain exposed  after the
grading has been completed.


                              19-24

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        19.2.6.3  Final Grading

Final grading  of  the  site  is to  be  performed after  sufficient
time has  elapsed to  allow for initial  settlement.   The final
grading plan should  be designed  in accordance with the  intended
ultimate  use of  the  landfill site.   It is important that all
sludge be  completely  cohered  to the  specified  depth  with cover
material.
        19.2.6.4  Landscaping

The landscaping plan should reflect the intended ultimate use of
the landfill site.   Where practical,  landscaping may be done on
completed sections before the  entire fill  project is  completed.


        19.2.6.5  Continued Leachate and Gas  Control

Since decomposition  of  the organics in the  sludge may continue
even after the  landfill has been completed, an  ongoing monitoring
and control program must be maintained.  Leachate and gas must be
controlled even after  the  filling  operations have  stopped.   Th-e
completion plans should clearly outline this  program.


    19.2.7  Landfilling of Screenings, Grit and Ash

Screenings and  grit  normally  contain  some putrescible materials
and,  if  landfilled,  should be  covered  every day.  Odors  from
temporarily uncovered solids may be alleviated by  sprinkling the
solids with  lime.    Special  care should  be  exercised  to assure
vector control  (for example,  safe  poisons  for  rodent control,
spraying   for flies,  and animal-proof  fencing  to keep  pets  from
the area).

Residues   (ash from the combustion of municipal  wastewater solids)
generally contain high  concentrations of  trace metals.  Leachate
from sites where incinerator ash is  landfilled  must be controlled
to prevent metals  contamination of  groundwater.   In California,
for  example,  wastewater  sludge  furnace  ash  must be  placed
in  a  "protected" Class  II-l  site.   See Chapter  11  for  more
information.
19.3  Dedicated Land Disposal


    19.3.1  Definition

Dedicated  land  disposal means  the application  of  heavy  sludge
loadings  to  some  finite  land  area  which  has limited public
access and  has  been set aside or  dedicated  for all time  to the
disposal of wastewater  sludge.   Dedicated  land  disposal  does not


                              19-25

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mean  in-place  utilization.    Dedicated  sites  typically  receive
liquid  sludges.   While  application of  dewatered  sludges  is
possible, it is  not  common.   In addition,  disposal of dewatered
sludge in landfills  is  generally more cost-effective.

As  with any  other  land disposal  technique,  dedicated  land
disposal requires  the wastewater  sludge be  stabilized  prior
to  application.   Once  the sludge has been  stabilized,  however,
it  can  be  applied to  the dedicated land  in either  the  liquid
or  the  dewatered state.  Use of  anaerobically  digested  sludge
minimizes odor  and potential  nuisances.

Many existing wastewater  treatment plants practice some  form  of
dedicated  land disposal.   However, precautions necessary  for
assuring that this  method   of  disposal is  not  harmful  to  the
environment have  not always  been practiced.


    19.3.2   Background

Dedicated  land  disposal was first  developed  as  an informal
practice in response to the  need to reduce high operational costs
associated  with  sludge disposal.    The  practice  was  applicable
particularly in  cases  where  the  plant  site  had adequate  acreage
or  where adjacent land was  available  and  hauling costs  to  the
nearest  landfill were high.  Groundwater  contamination,  odor
production, and aesthetic concerns were not usually addressed  in
this informal practice.

A more  sophisticated approach  to  dedicated  land  disposal  had  to
be  taken as sludge  quantities  increased  with  higher treatment
levels,   and on-site  sludge  disposal  was  perceived as  associated
with environmental  problems.  Recent  research on  this  method  of
sludge  disposal  has  developed key environmental  controls  which
are covered in  subsequent  sections.

The use  of dedicated land disposal has several major advantages.
These  include  flexibility  in managing  sludges  in  excess  of
utilization demand;   minimum  land  use because sludge  application
rates per  acre are  maximized; inexpensive  dewatering through
the use  of solar energy instead of the relatively  expensive
electrical  energy required for mechanical dewatering;  relatively
low capital and operating  costs  (6).

Dedicated  land disposal  is  applicable  as a  disposal  method  for
liquid,   dewatered, or  dried  sludges.   To maximize the advantage
of low-cost solar drying and  minimize the cost of upstream sludge
processing, disposal of liquid sludge is the most cost-effective
approach.   Disposal  of  sludges in  the  liquid  form requires
storage  capacity.     Facultative  sludge  lagoons  (FSLs),  as
discussed in Chapter 15, can provide that storage.  FSLs  provide
a  buffer between continuous  sludge  production  and intermittent
land  disposal  operations.   Disposal of  the thickened  (solids
concentration  of 6  to 8 percent)  sludges  from the  FSLs  will
commence 1  to  5  years  after  the  first  anaerobically  stabilized
sludge is discharged into  FSLs.


                              19-26

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    19.3.3  Site Selection               :      ::

There are five  major  considerations  in  selecting  an appropriate
dedicated land  disposal  (DLD)  site.   These  considerations  are
ownership,  groundwater  patterns, topography, soil types,  and
availability of  sufficient  land.   All are  discussed  briefly  in
the following paragraphs.


        19.3.3.1  Ownership  by Wastewater Treatment Authority

By  definition,  the  selected  DLD  site will be  dedicated  in
perpetuity  to  the sludge disposal function.   Long-term buildup
of  heavy metals and  salts  in the  soil  surface  layers  will
make  the site unsuitable  for future direct  agricultural  use.
Public  access  to these sites  must  be restricted because  of
their potential  pathogen contamination.   These  factors require
complete control and thus ownership of the site by the wastewater
treatment authority.   However,  merely  because  certain  elements
accumulate  to  toxic concentrations  on  that site does  not  mean
that the surface soils are  forever useless.
        19.3.3.2  Groundwater  Patterns

Groundwater movement  must be considered  in  the selection of  a
DLD site.   Groundwater flow patterns must be known  in  order  to
protect  present  or  future domestic water  supply wells.  The
following three control options  are possible:

    1.  Choice of  a  site with  an  isolated  groundwater  pattern.
        This  option  requires that  there be  well-defined
        groundwater migration to a river  or  the ocean;   In this
        case,  there must be no  intermediate  domestic  source
        wells.  An  adequate  subsurface buffer strip  between  the
        site  and  the  receiving waters  should be provided  to
        permit further  potential  pollutant  attenuation,  uptake,
        or dilution.

    2.  Choice of  a  site with  a  tight/low  permeability  surface
        and/or subsurface soil  layer which  essentially  prevents
        DLD leachate from  reaching  the  groundwater.   In  this
        option,  additional  monitoring  wells may be required
        to confirm the design  assumptions  over the long term.

    3.  Construction of  an  artificial  leachate control  barrier
        composed  of  a minimum  2-foot  (0.6  m) depth  clay layer
        under  the  entire site  and  deep  cutoff  trenches  at  the
        groundwater  downstream end of  the  site  for leachate
        collection and recycling.   It  should be noted that when
        there are  low-permeable soils  too close to the  surface,
        liquid  disposal  operations  can  be  hindered.   Shallow
        clays can cause ponding and reduced loading rates
        with these systems.


                              19-27

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        19.3.3.3  Topography

Natural topography is an  important  consideration  in  selecting  a
OLD site.  Natural slopes greater  than  0.5  percent will  have  to
be  modified  to  prevent  erosion.    The lack  of  vegetation  on
the disposal  site increases  the  potential  erosion  problem  and
subsequent runoff control.  The use of level or nearly level land
eliminates erosion problems.   Graded or  terraced sites  can  be
used,  but increased earthmoving costs are involved.


        19.3.3.4  Soil Types

Most soil types can accommodate one or another form of DLD system
with proper protection of ground and surface  waters  as outlined
above.   Preference should,  however, be given to soils  with  a
moderate  to  high cation  exchange  capacity  (CEC),  typically
greater than  10 milliequivalents per  100 grams.

Desirable soil  conditions  include restrictive  permeability,
minimal ponding, and  freedom  from boulders.  Technical assistance
in the areas  of soil  science,  soil  agronomy, and soil  engineering
is recommended, so that  the impacts of specific soil types on the
project can be  accurately  evaluated.


        19.3.3.5  Availability of Sufficient Land

The amount of  land required  depends  upon  the  quantity  of sludge
generated and upon the acceptable loading rates.  Sufficient land
must be available to  ensure the integrity of the system.


    19.3.4 Storage

Storage  should  be  considered  for  DLD systems  under  certain
climatic  conditions and  for increased operational  efficiency
and  control.  As discussed earlier,  FSLs  are recommended
to  meet  these  conditions  and to  assist in flow  buffering  (see
Chapter 15).


        19.3.4.1  Climatic Influences

In most areas  of  the country, rainfall  is  seasonal,  and  in some
the ground may be frozen to a  depth which makes  it unworkable
during the winter.   These  conditions  mean ^that  dedicated  land
disposal  operations can  occur only  during the drier months.  As  a
minimum,  provision for  six months of sludge storage is  required.
Systems  designed for  handling  liquid  sludge  in Sacramento,
California (6),  and  in  Corvallis,  Oregon  (7,8,9), are designed
for 18 to 60 months  storage  of  anaerobically  digested  sludge  in
FSLs.   This allows upstream systems to operate through  a  winter-
summer-winter cycle  and without  disposal  problems during  a wet
spreading season.
                              19-28

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        19.3.4.2  Operational  Storage

Even  where  climate  is  not  severe  enough to  require  sludge
storage,  storage  may  still  be warranted  for  operational
efficiency.    If  storage  is  provided,  routine  equipment
maintenance can take  place  during  normal  work  hours.   Emergency
situations,  such  as  those  which  require  the  retention  of
unstabilized sludge  for very  short periods  during any  plant
upset, can be responded  to  effectively  (10).


    19.3.5  Operational  Methods  and Equipment

Dedicated  land  disposal  has  achieved  recent  prominence  because
of  its  application  to the problem of  direct  disposal  of liquid
sludges.   Systems that are designed to deliver and manage liquid
sludges on DLD sites are  of  primary  interest (11).


        19.3.5.1  Liquid  Sludge

Application of  liquid sludge  is desirable because it simplifies
upstream processes.    Dewatering  processes  are  not required,  and
inexpensive liquid transport and application systems can be used.
Four common surface  application  methods for  the liquid sludge are
described  in the following  paragraphs.   The first three  are
irrigation systems  and  the  fourth is  a  mobile  tank application
system subsurface  applications methods  are described in the final
paragraph.   Summaries of  certain characteristics of those methods
are given in Tables  19-8  and 19-9.

Spraying

Wastewater sludge can be  applied to  the  land  using either fixed
or  portable  irrigation  systems.   These  systems  must  either  be
designed  specifically  to handle  solids without clogging,  or
liquid  sludges must be screened.   A 1/8-inch  (0.32 cm)  mesh
rotary strainer will perform satisfactorily.

It  is  advantageous  to spray  sludges  because operating  labor  is
reduced, less land needs to be prepared,  and a wide selection of
commercial equipment  is available.  Fixed irrigation systems can
be  highly automated, whereas  operator  attention is required
for  portable  sprinkler  systems.    Sprinklers  can  operate
satisfactorily on rough, wet  land  unsuitable  for tank trucks  or
injection equipment.

Disadvantages of  spraying sludges include power costs associated
with  the  use  of  high-pressure pumps,  the  potential  for  aerosol
pollution from entrained pathogens,  odors, potential for ponding
of  the  sludge, and  adverse public reaction.   Preferred  spray
systems direct  the  sludge  toward the  ground.   Modified versions
of center  pivot systems provide  for low pressure at the nozzles,
minimizing odors and aerosols.  Such designs minimize  direct
airborne  transport  of  sludge,  control  application rates  and
distribution,  and  minimize  aerosol formation and transport.
                              19-29

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                                TABLE 19-8

     SURFACE APPLICATION METHODS AND EQUIPMENT FOR LIQUID SLUDGES
        Method
                             Characteristics
 Spray (sprinkler) fixed
  or portable
 Overland flow and
   flooding
 Ridge and furrows
 Tank truck
Large orifice required on
  nozzles; large power and low
  labor requirement; wide
  selection of commercial equip-
  ment available; sludge must be
  flushed from pipes when use
  stops for longer than 2 to 3
  days.

Used on sloping ground with or
  without vegetation with no
  runoff permitted; suitable for
  emergency operation; difficult
  to get uniform aerial applica-
  tion; use of gated or perforated
  pipe requires screening of
  sludge prior to application;
  sludge must be flushed from
  pipes when use stops for longer
  than 2 to 3 days.

Land preparation needed; lower
  power requirements than spray;
  limited to low solids con-
  centration (less than 3 percent
  works best).
                      Capacity 500 to 3,800 gallons;
                        larger volume trucks will re-
                        quire flotation tires; can use
                        with temporary irrigation setup;
                        with pump discharge can spray
                        from roadway onto field.
Topographical and seasonal
      suitability

Can be used  on a sloping
  land; can  be used year-
  round if the pipes are
  drained in winter; odor
  and aerosol nuisances may
  •occur.
Can be applied from all-
  weather ridge roads.
Between 0.3 and 1.0 percent
  slope depending on solids
  concentration and
  condition of soil.  Fill-
  able land not usable on
  wet or frozen ground.

Tillable land; not usable
  on very soft ground.
 1 gal =  3.8 1
Overland Flow and  Controlled Flooding

Overland  flow   (wild  flooding)  and  controlled  flooding  (border
check flooding)  are  common  irrigation  techniques.  Both  of  these
use  gated  or  perforated  pipe  to  assure  aerial  uniformity.    DLD
experiments  with   these techniques on  stabilized  lagooned sludge
at Sacramento,  California  (12),  indicate  that neither  resulted in
the  satisfactory surface spreading of such sludge.  Wild flooding
spread the sludge  too far  laterally and quite unevenly  downslope.
Border  check  flooding  took  care  of  the  lateral  spreading,   but
the  downslope  could  not  be adjusted  to varying sludge solids
concentrations.     Therefore,  the  sludge  either  collected at  the
top  of the  sloped  field  (when  there  was too little slope for  the
percent  solids concentration)  or  at  the  bottom of  the sloped
field  (when there  was  too  much  slope  for  the  percent solids
concentration).    Both  flooding  techniques   resulted  in   the
accumulation of excessive  amounts  of  sludge on  limited  areas;
reapplication was   thus  limited  and problems  such as  odors   and
                                  19-30

-------
vectors were  an  outcome.    In  both  techniques,  clogging  problems
were  experienced  with   standard   water  irrigation   gated  and
perforated  piping.    This  indicated  that  either  special  distribu-
tion  piping  would be required  for  use  with  sludge  or  the  sludge
would  have   to be screened  in  a manner  similar  to  that  indicated
for sprinkler  application.
                                   TABLE 19-9

              SUBSURFACE APPLICATION METHODS AND EQUIPMENT
                              FOR LIQUID SLUDGES
            Method
  Flexible irrigation hose
    (umbilical  cord system)
    with subsurface injection
    or surface  discharge3
  Tank truck with subsurface
    injection or surface
    discharge
  Farm tank trailer and
    tractor with  surface dis-
    charge3
  Farm tank trailer and
    tractor with  subsurface
    injection3
                                  Characteristics
                            Topographical and seasonal
                                   suitability
Pipeline or tanker pres-
  surized supply; 650 ft hose
  connected to manifold dis-
  charge on plow or disc
  pulled by tracked vehicle;
  abrasive wear can result in
  short hose life; subsurface
  injection by means of very
  small furrow behind knife-
  edge cutting disk and/or
  narrow plow; surface dis-
  charge into furrow
  immediately ahead of plow-
  application rate of 50 to
  100 wet ton/acre/pass.

500 - 3,800 gallon 4-wheel
  drive commercial equipment
  available; subsurface
  injection by means of very
  small furrow behind knife-
  edge cutting disk and/or
  narrow plow; surface dis-
  charge into furrow
  immediately ahead of plow-
  application rate of 50 to
  100 wet ton/acre/pass.

Sludge discharged into fur-
  row ahead of plow mounted
  on tank trailer - applica-
  tion of 170 to 225 wet
  ton/acre/pass.  Sludge
  spread in narrow bank on
  ground surface and imme-
  diately plowed under -
  application rate of 50 to
  125 wet ton/acre/pass.

Sludge discharged into chan-
  nel opened and covered by
  a tillable tool mounted on
  tank trailer - application
  rate 25 to 50 wet ton/acre/
  pass .
Tillable land;  not usable on
  wet or frozen ground.
Tillable land;  not usable on
  wet or frozen ground.
Tillable land;  not usable on
  wet  or frozen ground.
Tillable land;  not usable on
  wet or frozen ground.
   Vehicle reaccess to area  receiving application dependant on
   water content and application rate of  liquid sludges.

   1 gal = 3.8 1
   1 ton/acre = 2.25 t/ha
                                      19-31

-------
Ridge and_JFur;row

The ridge and furrow sludge  application method is similar to that
used in agricultural systems.   At the high application rates and
given  low solids content,  the ridge and  furrow method offers
better control  than gated  or  perforated  pipe systems  used  for
overland  flow or controlled  flooding.  Key factors in the success
of ridge  and  furrow application  are  the  solids  concentration of
the sludge, the furrow  slope, and the condition of the soil.  The
effect of the solids concentration and the furrow slope on sludge
application,  determined  from  a study in  Sacramento,  California
area  (34),  is  summarized  in Table  19-10.    Generally,  for
a  well-stabilized  sludge,  the  furrow  slope should be  about
0.1 to 0.2 percent  per one percent  sludge solids concentration,
particularly for sludges which behave  like water  (less  than
3  to  4  percent solids).   Sludges with  much  greater solids
concentrations cannot be successfully surface spread by the  ridge
and furrow technique.   As  long  as the soil  remains loose  and
friable,  satisfactory ridges and  furrows can  be created  and
friction  losses  can be  tolerated.    Excessive  reapplications  of
sludges with  high moisture  contents can create soils which clump.
This makes ridge and furrow construction difficult and increases
friction losses to intolerable  levels.


Advantages of  ridge and  furrow  irrigation  include  simplicity,
flexibility, and   lower  energy  requirements.    Disadvantages
include  the  settling  of solids  at the heads  of furrows,  the
need  for a well-prepared  site with proper  gradients,  and  the
impossibility  of  maintaining a friable  soil.   In addition,
ponding of sludge in the furrows can result in odor problems.


Often,  ridge  and  furrow   sludge  irrigation  also  involves  a
covering  operation.   This must  be carefully considered, laid out,
and tested prior to installation so that  maximum  efficiency  in
application and land use is  assured.

Tank Truck Surface Spreading

A  common  method of  liquid  sludge surface  application  is direct
spreading  by  tank  trucks,  tractors,  and  farm tank  wagons  with
capacities of  500  to  3,800 gallons  (2  to 14  m3 ) .   Sludge  is
spread from a manifold  on the  rear  of  the  truck  or wagon as the
vehicle  is driven  across the  field.   Application rates can  be
controlled either by valving the manifold or by varying the  speed
of the truck.


The principal advantages of a  tank  truck  system  are  low capital
investment and  ease of operation.   The  system is flexible  in
that  a  variety  of  application sites, pastures, golf  courses,
farmland,  athletic fields  and the like,  can be served.   This
permits  utilization of sludge, with  a  dedicated  land  disposal
system as a reliable backup  disposal mode.
                              19-32

-------
                            TABLE 19-10

                     FURROW SLOPE EVALUATION
            Slope,    Percent
            percent   solids*3        Observations0
              0.1       3.1     Sludge ponded or flowed
                               very slowly.  On slopes
                               this flat  slight
                               variations in grade
                               causing ponding.  Gen-
                               erally unsatisfactory.

               .2       3.1     No ponding,  sludge flowed
                               slowly.  Minimum grade
                               for 3 percent solids.
                               Would be too flat for
                               5 percent  solids.

               .3       3.1     Sludge flowed evenly at a
                               moderate rate.  Excel-
                               lent slope for 3 per-
                               cent solids.

            .4  -  .5     2.7     Sludge flowed evenly at a
                               moderate rate.  If
                               sludge-furrow was not
                               covered when full all
                               the sludge would flow
                               to the low end and pond
          a0.1 percent equals  0.1 ft of fall/100 ft
           of run.   (0.1 m/100 m)

           Percent  solids expressed determined in a
           dry weight basis.
          s-~<
           All observations are based on  12 in.  (.30 cm)
           deep  furrows.  Soil in excellent friable
           condition.  Deeper  furrows would permit the
           use of flatter slopes.

Disadvantages of   this  system   include  wet-weather  problems
and  the  high operating costs for sludge hauling.   Standard,
highway-operable  tank trucks are  not  able  to enter  sites when  the
ground is  soft.   Consequently,  storage  or wet-weather  handling
alternatives  must  be available.    Another  disadvantage is that
truck  traffic  damages soil  structure  and  compresses  the  soil,
thus  yielding higher bulk  densities  and reduced  infiltration
capaci ties.


                              19-33

-------
To maximize disposal time during days'when the site can  be  used,
a highway vehicle with a 3,000 to 6,000 gallon (11.7  to  22.7  m3)
capacity  tank can be 'used  ;to transport  the  sludge to the  OLD
site.   Sludge  is then transferred to  one  or  more  off-road
application trucks.   These  trucks  should be  equipped with  high
flotation tires  and four-wheel drive  for working wet sites.

Subsurface Injection


Subsurface injection  involves  a  principle of incorporation,  which
involves  cutting  a furrow,  delivering sludge  into  that  furrow,
and  covering the sludge  and  furrow,  all  in  one  operation.
Modifications  include methods  in which  the  sludge  is  injected
beneath the soil  surface or  incorporated by use of a disk.


Advantages of  incorporation  include:  immediate mixture of  sludge
and soil, elimination of potential  odor and vector problems  from
ponding, and control  of  surface  runoff.  Incorporation procedures
are also favored  when sludge utilization is desired, because less
nitrogen is lost  from the soil through ammonia volatilization.


The principal disadvantages  of incorporation are its complex
management procedures and the fact that  the  equipment cannot  be
effectively used  on wet  or frozen ground.
        19.3.5.2  Dewatered Sludge


Application of  dewatered  sludge  is  similar  to  application
of  solid  or semi-solid fertilizers,  lime,  or  animal manure.
Sludge can  be  spread  with bulldozers,  loaders,  graders, or  box
spreaders  and  then  plowed or  disked  in.    Spiked  tooth  harrows
used  for  normal  farming operations  may be  too light to bury
sludge  to the  required depth.   Use of heavy-duty industrial
discs or  disk  harrows may be  required.   Methods  and  equipment
for  application of  dewatered  sludges  are shown  in  Table  19-11.
Figure  19-2 shows  views  of  Denver  Metro's  dewatered sludge
landspreading operations.


The  principal  advantage  of   using  dewa-tered  sludge  is that
conventional equipment  for application  of  fertilizer and  lime
and for tillage can  be used.  Another advantage is that dewatered
sludge  may  be applied  at higher rates" than  liquid sludge.
Problems  of flooding and ponding and subsequent site access
associated  with  the  high  hydraulic  loading rates  of liquid
sludge applications  are avoided.   The disadvantage is  the  higher
energy and  operational  costs  associated with  sludge  dewatering
and  the  treatment  required   for  resulting  sidestream.    The
disadvantages appear to outweigh  the advantages,  since dewatered
sludge is  infrequently used for OLD.
                              19-34

-------
                           TABLE 19-11

             METHODS AND EQUIPMENT FOR APPLICATION OF
                       DEWATERED SLUDGES
                Method
     Characteristics
           Spreading         Truck-mounted or tractor-
                              powered box spreader
                              (commercially available);
                              sludge spread evenly on
                              ground; application rate
                              controlled by over-the-
                              ground speed; can be
                              incorporated by disking
                              or plowing.
           Piles or win-
            drows
           Reslurry and
            handle as in
            Tables 19-8 and
            19-9.
Normally hauled by  dump
  truck; spreading  and
  leveling by bulldozer  or
  grader needed to  give
  uniform application; 4
  to 6 inch layer can be
  incorporated by plowing.

Suitable for long hauls  by
  rail transportation.
        19.3.5.3   Sludge  Application Rates

Sludges should be  applied  such that soils  can  dry  sufficiently
between  sludge  applications  to allow  the passage  of  sludge
distribution vehicles.  Sludge  application does not create excess
leachate or runoff.  Application  should  also  be  managed so that
the soil does  not become  anaerobic and generate odors.

Adverse moisture conditions can be  avoided  for  the  most part  if
sludge application rates are not  allowed  to exceed  the  net soil
evaporation rate  (that  is,  evaporation  minus  precipitation).
Using this  guideline, water  should  be  removed  by evaporation  as
rapidly as it is added  with the sludge and the fields  should dry
out  prior to  subsequent  sludge applications.   Since  on  the
average,  all water is removed by evaporation,  none should remain
to percolate  or become  runoff.  The environmental hazard  and
operating   costs  associated  with  controlling  these   streams
are  thus minimized.   Given  this  premise,  sludge  should  be
applied  only  when the net soil evaporation  rate is positive.
The  Colorado  Springs  case  example  discusses  this  approach.
Operations will tend to  be seasonal,  intensive  during warm,  dry
conditions and slowed down  during wet or cold conditions.  Sludge
application must,  of course,  be  terminated when the ground  is
frozen.
                              19-35

-------
The Denver Metro uses large trucks to transport dewatered sludge cake to its rela-
tively isolated disposal site at the former Lowry Bombing Range 25 miles (40 km)
from the treatment plant.  This picture shows transfer of sludge to smaller dump
trucks for spreading in the field. Sludge is spread by allowing it to drop from the
truck as it is driven through the field. At one time the District used a manure
spreader instead of dump truck for sludge spreading purposes.
            After spreading, sludge is incorporated into the soil by
            plowing with this 6-bottom, 2-way moldboard plow.
            Annual application was about 30 dry tons per acre
            (67 dry t/ha)  in 1976.  Nine or ten months later, the
            same land received another application of sludge.

                                FIGURE 19-2

            DEWATERED SLUDGE LANDSPREADING, METROPOLITAN
                 DENVER SEWAGE DISPOSAL DISTRICT NO. 1,
                            DENVER, COLORADO
                                   19-36

-------
It should  be  noted  that runoff and leachate controls  are  still
required even though the system, on the  average,  eliminates all
water  by  evaporation.  Leachate  and  runoff must be expected,
since periods will occur when the  net  soil evaporation  rates are
less  than expected  or where  more  sludge than  permissible  is
applied.

Organic  rather  than hydraulic  limitations  may   govern,
particularly  when dewatered sludges  are applied.  Odors can
develop  if  the  soil/sludge  layer  does  not  remain  aerobic.
Maintenance of the aerobic  condition  depends  on rate  of  sludge
application,  the  sludge to soil ratio,  temperature,  and  frequency
of soil turning or disking.


    19.3.6  Environmental Controls and  Monitoring

In general,  environmental  controls  for dedicated  land  disposal
are  not  as severe as  those  for  sludge utilization.    The  basic
requirement   is  that  activities  do  not cause  any  nuisance
off-site.   Control of  all  transport  mechanisms for potential
pollutants, specifically via  surface and groundwater,  and through
aerosols and  odor is  required.  If the  sludge  is well  stabilized,
vector controls will  be negligible.


        19.3.6.1   Site  Layout

Good site planning is  the key to environmental  pollution control
for  OLD.   Initial site selection should  be based  on  slope,  soil
type, and  isolation possibilities  from ground  and surface water.
Subsequent detailed planning  can significantly enhance  final
environmental control  measures.

Division  of  the  OLD site  into several fields   is  desirable for
operational and environmental controls.  Individual fields should
be in  the range  of  10  to 100  acres  (4 to 40  ha), and  50  acres
(20  ha)  is  typical.   For the  umbilical   cord subsurface
injection method, a  minimum  dimension of 1,300  feet  (400  m)
is desirable.  This will  allow a tractor dragging  a  650-foot
(200  m)  hose to cover a  field,  side-to-side,  when  the sludge
hydrant is located in the  center of the field.   Smaller  sites are
more amenable to  the  use of  tank vehicle systems.

The  breakdown of  the  site  into smaller areas  will permit easier
terracing.   First,  fields  with  fairly uniform  elevations  must
be chosen, and  slopes must then be  regraded  for  the chosen
application method.

Beyond  the site  subdivision, plans for larger  OLD systems should
include a  layout  of  "nurse  centers."   These are take-off points
on a fixed distributional  system  for  re-filling application
trucks  in order to  minimize  their  unproductive travel  time
and  undesirable  extra field  compaction.  They also  serve  as


                             19-37

-------
hookups to the tractor-drawn  umbilical  cord system.  Usually, the
nurse centers consist of a small  (4-  to  6-inch  [10 to 15 cm]  in
diameter) sludge force main riser with a quick connect coupling,
coming  from  a  pump station or  dredge  operating in a  sludge
storage  lagoon.   A small storage  tank or  vault  (maximum volume
twice the tanker capacity)  is often added at the  nurse center to
simplify pumping control and  to  permit sludge  pickup  by  a field
tanker  by  means of suction.  Sludge  should not  be  allowed  to
remain unmixed in the vault for more than 30 minutes.   Mixing  of
the  tank contents  will  prevent   liquid-solid  separation,  which
could cause  wide  variations   in  solids concentrations  at pickup
and, therefore,  uneven solids  application rates to  the site.


        19.3.6.2  Groundwater Controls

There  are  two  distinct kinds of groundwater  control  for OLD
sites.   The first  involves  complete  collection  of any  and all
leachate from  the  site followed by either recycling  back  to
the  treatment  plant or  further  on-site  treatment.   The second
involves monitoring  groundwater migration patterns from the
site  and  assuring  that  the  quality  of external waters  are not
reduced.

In  the  case  of  the first,  the site should  be  underlain  with  an
impervious soil, hardpan, or rock.  Although  it  is  possible  to
prepare  this  barrier  artificially using  clay or a liner,  it  is
usually  not  economically feasible  to  do  so.   Thus, the original
site  selection  determines  the  degree of  vertical containment.
Horizontal  movement  of groundwater  is  prevented  by  the  use
of  diking  and  cutoff  trenches.   Leachate is  then  collected
together with surface  runoff.

In  the case  of  the  second,  extensive surveys may  be necessary  to
determine natural groundwater migration patterns.   The direction
of  leaching must be determined.   Design should be such that final
concentrations  of  potential   pollutants  either  in  the off-site
groundwater or in  surface  water do  not  exceed  contamination
guidelines preset by the applicable regulatory authority.


        19.3.6.3  Surface Water Runoff  Controls

Each OLD site should be graded such that all  surface runoff would
drain  toward one point near the  edge or  toward  the  corner  of
the  field.   Each site  should be  surrounded by  a berm  to  keep
uncontaminated surface runoff out and  to contain contaminated DLD
runoff.   A  center  drain should   either direct  the contaminated
runoff back to the nearest manhole on  a facultative sludge lagoon
supernatant  system, or  be  connected  to a  pump  which directs the
runoff  back  to  the treatment plant or to  a  separate  on-site
treatment  system.  Temporary holding of  the  runoff  to permit
settling of  settleable  solids and  monitoring  may  be  desirable.
If  the stored water is found  to be of sufficient quality  to meet
discharge standards, it can  be released without any treatment.


                              19-38

-------
The  primary  mechanism  for  water  removal  at  most  sites  is
evaporation.    Runoff  can be  minimized  by  adjusting  sludge
loadings  so that they  are less than or  equal to the net  soil
evaporation rate (evapotranspiration  rate minus precipitation).

Runoff control  can be aided by  disking  in  the sludge soon after
application, thereby preventing  downward movement  of the liquid
sludge.
        19.3.6.4  Air Pollution Control

Two  air pollution  concerns  are  aerosol transport and  odor.
There  must  be  adequate  buffer  zones   around  the  OLD  site.
Operationally,  systems which minimize the  length of  time sludge
is directly exposed to the air are preferred.   It is possible to
incorporate special  design features  for  air  pollution  control,
for example, vacuum  stripping  of the digested sludge  to remove
odors prior to  land application.


        19.3.6.5  Site Monitoring

Monitoring requirements  for OLD  are  relatively straightforward.
Groundwater monitoring  is essential  and should  be conducted
from a  pattern of groundwater wells located primarily  at  the
downstream  boundary  of  the site.   In addition  to groundwater,
collected  leachate  and  surface  water runoff  streams  must be
monitored to determine if  and when such streams must be treated.
For  air  pollution  control,  olfactometer   measurements  (see
Chapter  17)  could be  taken  regularly, particularly  during calm
periods  and  preceding and during  times  of air  inversions.   If
odors  are  a major  problem,  operations  could  be stopped during
periods of calm winds and temperature inversion.
    19.3.7  Costs

Extensive cost  data  are not  available  on OLD.   Cost estimates
are, however, available  from  a  new system developed at Colorado
Springs,  Colorado, and  a large  prototype  system  at Sacramento,
California.   These  cost estimates are  discussed in the  case
examples to follow.   These DLD costs are quite site-specific, and
extrapolations from the  Colorado  Springs and Sacramento cost data
should be made with caution.
    19.3.8  Case Examples

The  relatively  recent acceptance  of dedicated land  disposal
makes  the  selection  of  case  examples  limited,  particularly  for
small plants.   Colorado Springs,  Colorado, a medium-sized system,
and Sacramento,  California, a large system,  are discussed in the
following sections.


                              19-39

-------
        19.3.8.1  Colorado Springs, Colorado

The  analyses  for and  design  of a sludge management program for
Colorado  Springs  was  based  on population and average  dry-weather
flow figures  (see Table 19-12).
                            TABLE 19-12

            COLORADO SPRINGS POPULATION AND WASTEWATER
                         FLOW PROJECTIONS
Year


1978

ir-90

2005

Ultimate
Population,
thousands
230
330
440
—
ADWF , mgd
25
36
48
60
Planning
designation
Present
Phase I
Phase II
Phase III
An  interim  sludge  management system employs anaerobic  digestion
of primary and waste-activated sludge at the  wastewater  treatment
plant  site.   150,000 gallons (570 m3 )  of  digested  sludge of
2.5  percent solids  concentration  is  produced each  day.  The
sludge  is  trucked  from  the  treatment plant site  to two  5-acre
(2.0 ha)  15-feet  (4.6  m) deep temporary  storage lagoons  located
20  miles (32 km) away.   The sludge is later removed from the
lagoons by two special four-wheel  drive  high  flotation-tired  tank
vehicles equipped with suction devices and subsurface injected on
an  adjacent dedicated land disposal site.   Capacities of the
subsurface  injection  (SSI)  vehicles  are 3,600  and 3,800  gallons
(13.6 and 14.4 m3).

The  Colorado  Springs  sludge  management  system  is  being
substantially modified  and upgraded  (14).   A  schematic of the
modified system  is  shown on  Figure  19-3,  and  an overall  layout
of  the  sludge  disposal site  on Figure  19-4.  Estimated  capital
and  operating  costs  for  the various  facilities are  shown in
Table 19-13. .                   '   .

The soils at the DLD  site consist  of  Verdos Alluvium, Piney Creek
Alluvium, and  a weathered Pieere  Shale having  low  to very low
permeabilities,  in the range  of 1.0  x 10~4 £o  ]_. Q x io~6  crn per
second.
                                   •?•,",;-•  '   •-'•»..   •, "
Monthly  average  temperatures  range  from  29°F  to  71°F  (-1°C to
22°C).    Effective  soil  evaporation  occurs to  a  depth of about
2  feet  (0.6 m), and moisture profiles  from  SSI  test sites  show
a maximum downward migration of moisture to,a depth of  22  inches
(57 cm), after application  of  liquid  sludge.
                              19-40

-------
                         COLORADO SPRINGS
                               WWTF
                            THICKENERS
                               ! i  i
                                    DUAL SLUDGE PIPELINE
                                    COLORADO SPRINGS WWTF
                                    TO HANNA RANCH
                                    20 MILES
                        ANAEROBIC DIGESTION
                        FACULTATIVE SLUDGE
                            BASINS (FSB)
  AGRICULTURAL  I
    REUSE BY    [
   SUBSURFACE   [
    INJECTION    f
     (AG/SSt)     t
                             SUPERNATANT
                              TREATMENT
                                 AT
                             HANNA RANCH
DEDICATED LAND
  DISPOSAL BY
  SUBSURFACE
   INJECTION
   (DLD/SSO
i
  ON-SITE REUSE
THE TERM FACULTATIVE SLUDGE BASIN (FSB) IN USED INTERCHANGEABLY
WITH FACULTATIVE SLUDGE LAGOON (FSL)

                            FIGURE 19-3

      FLOW DIAGRAM SLUDGE MANAGEMENT SYSTEM, COLORADO
                        SPRINGS, COLORADO
                               19-41

-------
                                          4»  ^tf*.
                                          .21  /4H i.
             FIGURE 19-4
OVERALL SLUDGE DISPOSAL SITE LAYOUT
    COLORADO SPRINGS, COLORADO
                 w "  &""—"*»—l»<*l«t

-------
                                  TABLE 19-13

               COLORADO SPRINGS PROJECTED COST OF SLUDGE
                             MANAGEMENT SYSTEM

                                            Phase cost, thousand dollars3
                 Item                      i            ii
    Capital cost
      Raw sludge conveyance system             3,552            98           98
      Anaerobic digesters                     5,539         2,289        2,313
      Facultative sludge basins                3,924         1,236        2,068
      Subsurface injection system"             1,696           756          841
      Supernatant lagoons                      461            -           75
      Supernatant treatment facility     '      1,681           217

         Subtotal, capital cost              16,853         4,596        5,394

      Engineering and contingencies0           5,899         1,609        1,888

         Total, capital cost                22,752         6,205        7,282

    Present worth^
      Capital costf                             -        22,473d         N/Ae
      Operation and maintenance cost"?             -         8,048          N/A

         Total, present worth of project
           cost                                -        30,521          N/A

    Equivalent annual cost                       -         2,881          N/A
3Costs based on an ENR cost index of 2600, March 1979,  Denver.
 SSI system includes FSB dredge; harvested sludge distribution  pumps, piping and nurse
 tanks; SSI tank vehicles; and site preparation including grading, cutoff trenches and
 monitoring facilities, but excluding land costs, which were approximately $1,400 per
 acre in 1972.
CAllowance for engineering and administrative expense and contingencies is based on
 35 percent of construction cost.
 Present worth costs based on an interest rate of 7 percent and projected construction
 dates of Phase I and II facilities for a 20-year planning period.
ePhase III not included—beyond 20-year planning period.
'Salvage values based on assumed life of equipment and computed on straight-line
 depreciation.
gBased on uniform series present worth for fixed costs and gradient series for variable
 costs.

 1 acre = .91 ha
Note: The term facultative sludge basin  (FSB) is used
      interchangably with facultative sludge lagoon (FSL).


There  were  no  groundwater  supplies which could  be  endangered  on
the 160-acre   (65  ha)  disposal  site or  the  immediately  adjacent
areas.     However,  to  provide  maximum  protection of  the  environ-
ment,  the system was  designed  to minimize  percolate production.
The design  approach  was  to  match  sludge   application and net
soil evaporation  rates.    Net  soil evaporation  calculations  are
presented on   a  month-to-month  basis   in Table  19-14.   Note that
gross  soil   evaporation was  estimated  to   be  a  fixed  fraction
(70 percent)   of  the  evaporation  which  would   occur from  a free
water surface  (a  lake).
                                    19-43

-------
                             TABLE 19-14

           COLORADO SPRINGS CLIMATIC CONDITIONS AFFECTING
                           SLUDGE DISPOSAL
   Month
    Lake
evaporation
                     a,b
  January
  February
  March
  April
  May
  June
  July
  August
  September
  October
  November
  December

  Annual
                           Precipitation
    5.15
    6 .44
    7.62
    8.26
    6.99
    5.39
    4 .13
                43 .98
0
0
1
1
2
2
3
2
1
1
0
0
.71
.73
.56
.91
.14
.16
.00
.32
.55
.11
.95
.67
                               18 .81
Net lake ,
a . c . a ,d
i evaporation
-0
-0
-1
3
4
5
5
4
3
3
0
0
25
.71
.73
.56
.24
. 30
.46
.26
.67
.84
.02
.95
.67
.17
Net soil3'6
evaporation



1.
2.
3.
2.
2.
2.
1.


16.

_
_
70
37
17
78
57
22
78
_
-
59
 All values shown in inches.
 Developed from "Interim Study of Land - Incorporated Sewage
 Sludge" at Colorado Springs, Colorado, December 1978, by
 Waste and Land Systems, Inc.
Q
 Includes precipitation and snow assuming 10 percent of snow depth
 is equivalent to precipitation depth in inches.
 Lake evaporation minus precipitation.
Q
 Estimated net soil evaporation based on 70 percent of lake
 evaporation less precipitation.
1 in. = 2.54 cm

Allowable sludge  application  rates were  calculated  on a  monthly
basis for  the  months  of  April to  October, assuming  a sludge
content  of 5  percent.    Results  of  this  analysis,  shown  on
Figure  19-5  indicated  total  allowable  sludge  application  on  a
6-month  and 7-month  operating basis  to be  86.0  and 95.8  dry  tons
per  acre (193  to  215 t/ha),  respectively.   The  range  of  required
land  area  for  the  more  restrictive  6-month  period  is  shown  on
Figure  19-6.   Area  required for average loadings  (14.3  tons  per
acre  per month [32.1 t/ha-mo])  is shown by  the "average"  curve.
Area required if   the   sludge  could  be  applied  for  all  the
months  at  the  maximum  June rate  (18.6 tons  per  acre per  month
[41.7  t/ha-mo])  is  shown  by  the  "low  range" curve.   Similarly,
area required  if all  the sludge were  applied  at  the minimum
October  rate  is  shown  by  the  "high  range"  curve.   A second
analysis  shown on  Figure 19-7,  indicated  the  range  of area
requirements based on variations in sludge  solids content of  4 to
6 percent,  using  average solids  loadings (14.3  tons per  acre  per
month or 32.1  t/ha-mo).

With  respect  to surface  water  controls, cutoff ditches will  be
constructed to prevent   surface  runoff  from  the  disposal  site.
The  injection  pattern  will  be  parallel to  the contours of  the
area  to reduce  the  potential  for  soil erosion and surface  runoff
                                19-44

-------
from  sludge-amended soils.   To  the  south,  the entire  sludge
disposal  area  is  contained  behind a  retention  dam  designed
to prevent  runoff from reaching  an existing  ash  disposal site.
This  dam, designed  for a flood  level  equivalent to a  once  in a
1,000-year  recurrence interval,  provides  containment of  both
surface runoff and  upstream  percolate.  Although the operation of
the tank vehicle  SSI  system  was  based  on a well-defined DLD area
with ground slopes typically 3 to 6 percent, portions of the mesa
area which  have  slopes of  less than 10  percent  can also be used
for  injection.   The  maneuverability  and freedom of  movement of
the detached vehicles allows maximum site utilization.
           **
           *
           II
           O)
           i
           Q
           Q

           |
           _i
           Q.

           UJ

           O

           10
                   FES    APR    JUN    AU6     OCT
                                MONTHS
DEC
           (1 t/acre = 2.24 t/ha)
                            FIGURE 19-5
                SLUDGE APPLICATION RATE-OLD SYSTEM,
                    COLORADO SPRINGS, COLORADO
                               19-45

-------
         1ft
         O
         UJ
         o:
         Q
         D
         O
         o
         LLJ
         Q
             ISO i-
             140
             120
             ICO
             BO
60
             40
             20
                     NOTE:  BASED ON SSI OPERATING
                          PERIOD MAY THROUGH
                          OCTOBER, ASSUMING
                          SLUDGE APPLIED AT
                          5 PERCENT SOLIDS
               1970
        1980
I960
2030
2010
2020
2030
                                  YEARS

                            FIGURE 19-6

         ESTIMATED NET OLD AREA REQUIREMENTS SLUDGE APPLIED
                AT 5 PERCENT SOLIDS CONCENTRATION,
                    COLORADO SPRINGS, COLORADO
The operation  of the DLD/SSI system  commences with harvesting of
the sludge  from the facultative  sludge  basins (FSBs)  (faculative
sludge  lagoons  [FSLs] )  are referred to as  facultative sludge
basins at Colorado  Springs)  (15).  The sludge is transferred from
the basin  to a sludge receiving/distribution station by  a dredge
equipped with a diesel-driven pump.    From the  station,  the
harvested  sludge  is  conveyed  through  a  distribution system
consisting  of  12-inch (30 cm)  diameter pipes to  a  series of DLD
nurse tanks.   The fiberglass nurse tanks  are each 7,500  gallons,
twice the  volume of  the  SSI vehicle  tank.   The  nurse tanks are
                               19-46

-------
         I®
         £
         IP
         O

         o
         II
         LU
         O
         o.
         tft
         Q
         Q
Q
LU
I-

o
o
LU
Q
             160
             140
             120
             100
             80
             60
             40
             20
              0
                        NOTE: BASED ON SSI OPERATING
                            PEBIQD MAY THROUGH
                            OCTOBER AND SLUDGE
                            SOLIDS CONTENT AS SHOWN
               1970    1980
                   1990    2000    2010


                         YEARS
2020
2030
                             FIGURE 19-7

           ESTIMATED NET OLD AREA REQUIREMENTS AT VARIOUS
        SLUDGE CONCENTRATIONS, COLORADO SPRINGS, COLORADO
buried  below  ground and  protected with a  concrete slab on  grade.
A steel  pipe  fitted with  a  gate valve and  couplings extends  from
the bottom of  the  tank to  above the  ground surface  to feed  the
SSI vehicles.   The  harvested sludge  distribution system is  valved
to allow any  combination or  number of  nurse tanks  to be  placed
into  service.   The  network  is designed  to allow  approximately
1,000  lineal  feet  (305 m)  of  injection area between nurse tanks
to optimize the injection operation  and  minimize downtimes  caused
by travel with  empty  tanks.    Depending  on  climatic conditions,
                               19-47

-------
          O
          in
          m
          CL
          a.
         £
         u
         o
         a.
         CL.
         s
         D
         o
             1.50
             1.25
             1,00
             0.75
                  BASIS:
                  1. INJECTION VEHICLE MOVING AT 1.5 mph (2,4 kWhr)
                  2. SLUOCi SOLIDS CONTENT OF 5 PERCENT
             0-50
             0.25
                                                    6.0
                                                    5.0
                                                    4.0
                                                    3,0
                                                    2.0

                                                         Q
     E
     I
     V!
     o
    z
    O
1.0  y
    0.
    CL
                                                        g
                                                        .j
                                                        o
                   300 350 400 450  500 550 600 §50 100

                 SLUDGE INJECTION RATE, gpm (1 gpm = 3.78 l/min)

                             FIGURE 19-8

          SLUDGE APPLICATION RATES BY SUBSURFACE INJECTION,
                    COLORADO SPRINGS, COLORADO
the sludge  injection cate can be  adjusted to correspond with  the
soil  conditions in  the injection  area and  will vary  through
the sludge application  season,  as shown  on  Figure  19-5.   The
relationship between  sludge  injection rate and solids  application
rate on  the basis of both  liquid sludge  and dry  solids is  shown
on Figure 19-8.   Based on the estimated turnaround time for tank
refilling and  normal maintenance,  a net  injection time of  about
3 hours  per day per vehicle can  be expected.   One  dredge  can
harvest sludge from  the  FSLs at  a rate sufficient  to feed  two  SSI
vehicles.   Equipment requirements and  operating   characteristics
are shown in Table 19-15.

While  the  DLD/SSI system for Colorado  Springs is designed as  a
base disposal  system, it can be used as a secondary,  or utiliza-
tion,   option  without significant  additional  expense.   Eventual
agricultural  utilization  of  a  major portion   of   the  sludge
production  is, in fact,  a defined goal of the chosen system.   See
Chapter 3 for discussion  of  base  and secondary disposal options.
                               19-4!

-------
                                  TABLE 19-15

           COLORADO SPRINGS  DEDICATED LAND DISPOSAL/
            SUBSURFACE  INJECTION SYSTEM DESIGN DATA

      Item                                  Phase I         Phase  II       Phase III
Facultative sludge basins  (FSBs)

  Basin dredge
    Number                                      1                 23
    Maximum capacity,  gpm                    1,400             1,400         1,400
    Solids capacity,  percent
      Maximum                                   888
      Average                                   5                 55
    Pumping head,  feet  *;                      65                65            65
    Diesel engine  power, hp    :               175               175           175

Dedicated land disposal  (OLD)                                         ;

  Harvested sludge application
    Quantity, dry  tons per day3               43.2,             58.4          76.7
    Volume, gpda'b                        203,790           274,360       360,440
    Percent volatile                            50                50            50
    Average percent solids                       5          '-5             5
    Average annual application
      6-month operating  period, dry tons
       per acre                             86.0              36.0          86.0
      7-month operating  period, dry tons
       per acre                             95.8              95.8          95.8

  OLD area required,  acres
    Maximum                                    85               115           150
    Average                 ...             60                85           110

  OLD distribution system
    Nurse tanks
      Number              .                     12                18            24
      Capacity, each  gal                .    7,500       •      7,500         7,500

  SSI vehicles
    Number                                      2                 45
    Tank capacity, each  gal                  3,600             3,600         3,600
    Injection rate, gpm
      Maximum                                 700               700           700
      Average            .     ' -         '.     500               500           500
    Average vehicle speed,' mph                 1.5               1.5           1.5
    Injection width,  feet       .-..               12                12            12
    Volume injected,  gallons per vehicle
      per day
      Maximum                             116,000           116,000       116,000
      Average                             100,000           100,000       100,000
    Vehicle coverage,  acres per vehicle
      per day                                 6.5               6.5           6.5

  Tillage tractors
    Number          .,                           1                 11
 Assuming  120 day per year operation.

 Assuming  5 percent solids.

Note:   The term facultative sludge basins  (FSBs) is
       used interchangably with facultative sludqe
       lagoons (FSLs) . .        . ';  ,

1 gpm  = 0.06 1/s
1 ft = 0.30 m
1 hp = 746 W
1 ton/day  = .91 t/day
1 gpd  =3.8 I/day           ,  ,
1 ton/acre = 2.24 t/ha    •      -,
1 acre = .405 ha
1 gal  =3.81
1 mph  = 1.61 km/hr .        ,                ,     ,
1 gal/vehicle =,.3.8 I/vehicle  .
1 acre/vehicle = .405 ha/vehicle
                                      19-49

-------
        19.3.8.2  Sacramento, California

Sacramento,  California has been  the site of much of  the  work
associated  with the  development  of dedicated  land  disposal
technical  criteria.   The Regional  Wastewater Treatment Plant
Sludge Management  Program  for  the  Sacramento Regional County
Sanitation  District  was approved  by  the Regional  Board  of
Directors  in January  1979  (15) and  the  Environmental Impact
Report (EIR)  (6) was certified at that time.  The sludge planning
period  for the treatment plant  is divided  into two phases;
Stage I  includes operations to  be  conducted  from  1980  to  1992,
and Stage II  is devoted to operations for the period 1992-1999.

The sludge management  program was approved after 3  to 4  years  of
monitoring  and  detailed  investigations directed  primarily
at  determining the engineering,  economic,  and environmental
aspects of storing  liquid anaerobically  digested sludge  in  solid
storage  basins (SSBs)  (12).   Precise  operational and design
criteria  were developed for the  Sacramento SSB/DLD system  to
assure efficient operation and environmental acceptability.   Most
investigative work  was  conducted  on a  large  prototype SSB/DLD
subsurface  injection system and  therefore did not suffer the
problems normally associated with scaling up a pilot system.
                      LJlv * *~"   ' * M *y   ,          ' 5
                           FIGURE 19-9

            PROTOTYPE DREDGING OPERATION, SACRAMENTO
                REGIONAL COUNTY SANITATION DISTRICT


Initial  work  commenced  in  1974.   Site preparation  included
installation of groundwater monitoring wells.   The  prototype
20-acre  OLD  system  has been  in  full  operation since  1976,  and
data  has  been  collected  and  analyzed  for 1976 through 1978  and
for part of 1979.   Figure  19-9 illustrates the prototype dredging
                             19-50

-------
operations   at  Sacramento,  while  Figure  19-10  illustrates
prototype subsurface  injection  operations with a close-up view of
the injector units.

The  sludge applied  to the  Sacramento  DLD  site  has been
anaerobically digested  and then subjected to long-term storage in
the SSBs.   Application rates were planned at  100  dry tons/acre
(224 dry  t/ha);  rates  of 97 tons/acre  (217  t/ha)  were achieved
without problems  in  the  1977  tests.   The  application rates  are
controlled  by the  water  content  of the  sludge  removed from  the
SSBs,  since DLD operates primarily on a solar evaporation basis.
New equipment installed at sludge removal operations in 1979  has
increased the  solids  concentration to over  6  percent, with
better than 8 percent achieved  for several hours.  It is expected
that when the FSLs  are  fully  developed,  an average  harvested
sludge  concentration  of  6 percent can  be  sustained.    The
following text discusses  the final  DLD  subsurface  injection
system for Sacramento based  on  experience gained over the 1976 to
1979 period.

Table 19-16 shows  projected  flows and loadings for the Sacramento
wastewater  treatment  plant for 1985.   Figure 19-11  is  a flow
diagram of the solids treatment and disposal system.  The numbers
thereon give  the  solids  flow in  dry  tons per day for operations
through 1992.


                           TABLE 19-16

         SACRAMENTO  REGIONAL WASTEWATER TREATMENT PLANT
            PROJECTED 1985 WASTEWATER FLOW AND LOADINGS

                         Parameter               Value
           Seasonal3
             ADWF,b MGD                          136.2
             BOD5, 1,000 Ib/day                  243.3
             Suspended solids,  1,000  Ib/day       246.3
           Nonseasonal
             ADWF, MGD                           122.7
             PWWF,C MGD                          248.7
           aSeasonal = canning season,  mid-June  to
            mid-October.

            ADWF = average dry-weather  flow.

           CPWWF = peak wet-weather flow.

           1 Ib/day - 0.454 kg/day.
           1 MGD = 0.044 m3/sec.
                              19-51

-------
        View of tractor pulling sludge injector units
     Close up view of prototype sludge injector units
                    FIGURE 19-10

  PROTOTYPE SUBSURFACE INJECTION OPERATIONS
SACRAMENTO REGIONAL COUNTY SANITATION DISTmCT
                       19-52

-------
                                  RECQMMiNOEQ  PROJECT
                                                                                   GAS
                                                  APID BOTO5TPAINERS
     RECYCLE
     TO
     PLANT IN FALL
     AWDW NTER
     PERIODS
           ^ ON  I	_.   ^ " "'.'„'" '"1.
               ^^""^P*1"  r s L * fc H c L* IN
           ^°          ACT: VATE D CAR
                                                                                    :ABBON
                                                                EVAPORATION
                         ssa SUPER it AT AN r
\
                                                        114 SURFACE ACRES
                        SSB'sCFFER
                            - TERM
                              E AND
                    EVAPORATION
     WINTER RUNOFF
     RECYCLE TO
      REGIONAL PLANT
                                                                            DECOMPOSITION
                                                       HARVESTED SLUDGE
                                                       11,500 tans Ktlids .' year (98% solidi I
                       LAND DISPOSAL- 1 SB ACHES
               HIGHLY STA61L! JED SLUOGE IS INCORKJflATED INTO
               THE SURFACE SOU, lAY«S FQBQRVlNG .  CONTINUED
               AEROBIC DECOMPOSITION OCCURS OVEfi TIME.
               SITE IS REUSED ANNUALLY.
                                                       SIX OP V WONTHS
THE TERM 'SOLID STORAGE BASINS' (SSB's) IS USED INTERCHANGEABLY
WITH FACULTATIVE SLUDGE LAGOONS (FSL'sl

(1 ton/yr = 0.91 t/yrl
(1 acre = 0.405 ha)

                                     FIGURE  19-11


             FLOW DIAGRAM  -  PROJECTED 1992 NORMAL SOLIDS
           TREATMENT AND  DISPOSAL OPERATION, SACRAMENTO
                 REGIONAL WASTEWATER TREATMENT PLANT
                                          19-53

-------
The  flow schematic  indicates  that  not  all the sludge will be
managed  by  the  SSB/DLD subsurface  injector  system.   Through
the  1980s,  there will  be sufficient  furnace  capacity  in an
incinerator  (designed  for  screenings,  grit,  and  scum)  to  handle
about  30  percent  of the primary sludge production.   A  total of
one  month per year shutdown  of the  incinerator was assumed,
two  weeks  for  maintenance during  the  time  of  low  solids
production  in  spring,  and  two  weeks  miscellaneous   upset.
Estimated  sludge  production  rates  at  the  Sacramento plant are
given  in Table  19-17.

                               TABLE 19-17

          SACRAMENTO REGIONAL WASTEWATER TREATMENT PLANT
                 PROJECTED DIGESTED SLUDGE PRODUCTION
      Estimated
       solids
      parameters
   Solids
concentration,
  percent
Volatile
solids,
percent
Digested solids, with
  incinerators operating
   Average annual         2.2-2.5
   Maximum seasonal        2.4-2.7

Digested solids, with
  incinerators not
  operating
   Average annual         2.5-2.7
   Maximum seasonal        2.6-3.0

Average annual SSB solids
  removed^                  6
                                           Total solids production, ton/day

                                              Stage I
                                      1980
                                               1985
                                                        1992
                                    Dry
                                        Wet
                                             Dry
                                                      Dry
            57-59  51.2  2,278 54.2 2,366  67.0  2,818
            57-62  70.0  2,715 74.4 2,810  81.5  3,340
            57-60  65.0  2,500 70.0 2,600  77.4  3,070
            58-63  81.2  2,860 89.1 3,000  92.7  3,550
                               43.0  38.6
                                         643  40.8
                                                  679 50.7
                                                           844
                                                                Stage II
                                                                 1999
                                                               Dry
                                  75.5  3,035
                                  94.0  3,640
                                  85.0  3,380
                                 107.0  3,860
                                                               57.0
                                                                    951
 The term solid storage basins (SSBs) is used interchangably with facultative sludge lagoons (FSLs).
 Actual daily removal rates are higher, since solids are harvested for only part of the year (May-October).
1 ton/day =2.24 t/ha


The  layout of the  existing  and  future dedicated  land  disposal
sites in relation to the  Sacramento Regional  Wastewater Treatment
Plant is shown in Chapter 15, Figure 15-9.

Operation  of   the  OLD  system  is  practiced  from  May  through
October.   Several methods  of sludge  application were tested,  and
for  the  prevailing  conditions at  the  Sacramento site,  subsurface
injection  utilizing  a  flexible  hose  and  injection  unit mounted
behind   a  crawler-tractor  fitted  with   extra-wide  tracks  worked
best.

Two  dredges will  take  care  of operations through 1992,  (Stage  I)
dredging  solids  at  about  a  6  percent   solids  concentration
from the  lower depths  of  the SSBs and  pumping  the sludge to  the
DLD  site.   Booster  pumping is  required  to pump  6 percent solids
                                 19-54

-------
material  over  the  maximum  8,000-foot  (2,440  m)  distance.
Four-inch  (10  cm)  diameter flexible  hoses  connect  the  tractor-
injectors  with  hydrants located  throughout  the DLD  sites.   Four
tractor-injectors  are  needed  to handle  the  two-dredge  disposal
operation.  In  normal operation of  these facilities,  freshly
applied solids remain unexposed  to the atmosphere.   The DLD sites
are  loaded  at  100  dry tons  to the  acre  (224 dry  t/ha)  each
season.  Sludge  is supplied  to approximately match  the  net  soil
evaporation  rate.    The soil  evaporation rate  in Sacramento  is
about  half  the  evaporation  rate which  occurs  from a  free water
surface  (lake  evaporation  rate).   Stage I  DLD operations  will
utilize 185 acres  (75 ha) in five 37-acre (15 ha)  sites.   Regular
disking  of the  site is necessary to break up the soil/sludge
surface and expose more of  the loaded  soil  to the  atmosphere.

Existing subsoils are fairly impervious and  are  underlain  by
hardpan.   The  local groundwater  supply is not  endangered.   Free
groundwater was  measured  at depths  of  13  to 46  feet  (4.0  to
14.0  m), with an  average  depth of 31.6 feet  (9.7 m)  for  nine
borings.   The  aggregate  coefficient  of  permeability  for the
composite  layered  interval  of the surface soils is  on the order
of 5  x 10~8 cm/sec.  Cemented  strata were encountered  in the
borings at depths  ranging from 5 to 10  feet (1.5  to 3.0  m),  with
thicknesses  of  approximately  12 to 21 feet (3.7  to 6.4  m)  and
permeability coefficients of 2.2  x 10~8 cm/sec  to  3.7 x 10"10 cm/
sec.   Effective  sealing  of  the surface soils  from  vertical
leachate  movement  to groundwater   is  thus  achieved  (6,12).
Increases  in the  concentrations of nitrates  and  chlorides  have
not been observed  below the impervious strata  (12).

Runoff is collected  in  detention  basins  and returned  to the
regional plant  influent after  storm  flows have subsided.   Some
data on DLD runoff water quality  are given in Table  19-18.


                            TABLE 19-18

             SACRAMENTO TEST DLD RUNOFF WATER ANALYSIS

    Constituent      : 12/18/77       12/28/77       1/05/78       i/09/!!	
   Zn, mg/1             0.05         0.05         0.25,         0.12
   Cu, mg/1             0.050        0.043        0.101        0.064
   Cd, mg/1             0.001        0.001        0.001        0.001
   Ni, mg/1     ,  ;  ;   ..0.090    .    0.090         0.16        0.078

   Pb, mg/1           .  0.014        0.008     .   0.016   ,     0.028
  .Hg, mg/1     ,  '  . '  0^0001        0.0001  ,      0.0002       0.0002
   TKN,a mg/1            '  30        ,24           17          7.6
   Turbidity, NTU ' ' "   •    3.0          1.5          170  .        .37

   TSS, mg/1  •'.'•.   :   :. 26  -         16          442          38
   EC x 103              4.0          3.6          1.9          1-1
   pH                   7.2          8.4          7.4          7.4
   NO,, mg/1              440          310           43          31
   NH^, mg/1   ••..:'•    36-'        '  5  •  ••     •  .2 . • .    •    1
aTotal Kjeldahl Nitrogen.
                              19-55

-------
For runoff control, typical OLD sites are sloped transversely  at
a maximum  of  0.5  percent and spread outward  in both  directions
from the centerline.   A longitudinal slope of  0.1 to 0.2  percent
is also provided.   Runoff drains from each OLD site to ditches  on
both sides.  To prevent erosion,  the maximum field slope  will  be
held to  0.5  percent  and water  velocity  in the  ditches  will  be
limited to 5  feet per second (1.5  m/s).   Runoff from the  ditches
is collected  in a fairly  flat  (0.1 percent  transverse slope)
detention  basin,  one  per OLD  site;  the  basin  is  situated
approximately  3  feet  (0.9 m) lower than the main operational  part
of the  DLD site.   Each  basin has  a capacity of  12.7  acre-feet
(123 m^) and  a maximum  depth of 2 feet  (0.6 m).   The  basins are
designed  to  contain  two  4-inch  (10 cm)  24-hour  storms  (the
25-year maximum  rainfall for two 24-hour periods).  The  runoff  is
drained from  the basins  via  corner  inlet structures  fitted  with
controlled release  rate  weirs  and is  transferred  through  a
21-inch (0.53  m)  runoff  return pipe  to a flow metering  structure.
Then the  runoff is drained  to  an  interceptor sewer connected
to the  wastewater treatment  plant.  A  flood control levee  is
constructed on the lowest  three  sides  of  the  SSB/DLD  area  such
that the  entire site  is  protected from  flooding.   Provision
is also made for  the  collection,   retention,  and  pumping  of
uncontaminated  site runoff  trapped  by  the  flood  control levee.
In  this regard,   facilities  (including  a  pump  station)  are
designed  to  handle  the same storm  conditions  (two  25-year,
24-hour  storms, one  day apart)  as are runoff facilities for
the DLD  sites.   A runoff  of  80 percent  is assumed based  on a
saturated ground  condition.


Final  DLD  sites have a gross area of about  50 acres (20  ha),
including space  for drainage,  road access, and injector turning.
As  indicated  earlier,  this  results in  a net  usable  area  of
37 acres  (15 ha)  for  each  of  the  five sites.   Each  site  is
approximately  1,300  feet   (400 in) wide,  which allows  a  tractor
dragging a 650-foot  (200 m) hose to cover the  entire width  of
the field.   The sludge  hydrant is then located  in the  center  of
the field.  Sites are  approximately 1,600  feet (560 m) long,
calculated from the  area  required to allow  two injectors  to
operate continuously  on  the  same  field  6 hours  per day,  5  days
per week, during  the  peak dry  months of June, July, and August.


Peak dry month operation assumes sludge removed from the FSLs can
be applied to  the  same  site twice a  week.  During May,  September,
and October,  it  is  assumed  the application of sludge removed  from
the SSBs will be limited to one once a  week.   Thus,  application
rates  during  June, July,  and August are  approximately double
(10 inches per month [25 cm/month])  the  rates  of May,  September,
and October (5  inches per month  [13  cm/month]).  Each DLD site  is
provided with six  field hydrants for  injector  feed  connection,
located at 230-foot (70 m)  centers down  the middle of  each  site.
The field hydrants  each  have  a foulproof pressure sensing  device,
a manual  isolation valve,  and a  swivel-elbow  assembly  designed
for quick coupling  to a  4.5-inch (11  cm)  injector feed hose.


                              19-56

-------
Operationally,  SSB sludge  removal piping  is  flushed with FSL
supernatant at the  end  of each week's run with the flushing  water
returned to the FSLs.   Sludge  removal  operations  themselves are
restricted to reducing the water level in the  FSLs  no  more than
14 inches  (36 cm)  below normal  operational  levels.   The  water
level is  never  allowed to  drop low enough to  expose the sludge
blanket.  The blanket  is maintained  below  its  maximum  elevation
which is  another  10  inches  (25  cm)  below the  absolute  minimum
water level.

Key DLD equipment for Sacramento includes:

     •  Two   SSB   dredges,  each  generating  1,400  g p m
        (7,630  m^/day) average  flow  at  6   percent  solids
        concentration.

     •  Two 200- to 250-horsepower  (150 to 187 kW)  diesel  powered
        floating booster pumps  connected to  the dredges with
        variable   speed pump  operation  to   compensate for
        variations  in sludge  solids concentrations.

     •  Four 60- to 80-horsepower (45 to 60 kW) crawler-tractors
        with 30-inch  (0.76  m) wide tracks and  nine  to  ten  rear-
        mounted subsurface  injector sweeps,  each with 2-inch
        (5 cm) diameter feed  hoses.   Path width is 13 to  14 feet
        (4.0 to 4.2 m), speed 1.0  to 1.5 miles  per  hour  (1.6  to
        2.4 km/hr), and average capacity 700 gpm  (3,800  m3/day)
        each.

     •  One  four-wheel  drive,  rubber-tired,  150-horsepower
        (112 kW) tillage tractor with  heavy  disk  unit  which can
        be raised clear of  the ground.

Staffing  requirements for  full  Phase I  DLD operations are
expected  to reach 11  people on a 6-month  seasonal basis, May
through October, to remove  the sludge from the SSBs and  inject  it
into the  soil at the five  DLD  sites.   Personnel needs  are  given
in Table 19-19.  Fifteen other  full-time  personnel are needed for
the  whole solids   processing and disposal  system exclusive  of
anaerobic digestion,  with their time only partially attributable
to DLD  operations.   The 15 staff are  composed  of one at the
screenings, grit  and   ash  landfill,  six in  general operational
maintenance,  six  in  mechanical and  electrical  maintenance, and
two  in management and monitoring.

Ongoing requirements  and  possible  concerns  associated  with
DLD  operations include the need to  lime  the soil  (at  about
one  ton of  lime per acre per year  [2.24  t/ha/yr]) to maintain a
proper  pH  and hence  decrease mobility of metal cations.   Also,
the  useful  life of the present type of  4-inch  (10  cm) diameter
feed hose  is  unacceptably  short.   The possibility  of  using
different hose  construction  or  a  different brand  is  being
explored.   Finally,   after a  20-year  operation,  DLD soils,


                             19-57

-------
building up  at about  0.75  inch (1.9  cm) per  year, will have
increased in  salinity  to about 8,000 mg/1  in the saturation
extract.  This concentration  is  not  expected  to affect the
bacterial decomposition  of  the  organic matter, however.


                            TABLE 19-19

         SACRAMENTO REGIONAL WASTEWATER TREATMENT PLANT
              PROJECTED 1985 OLD STAFFING REQUIREMENTS

                                                   Number of
               Description                             staff
   One operator for each dredge/booster pump combination           2

   Relief operator for dredges                               1

   One operator for each tractor/injector                       4

   Relief operators for tractor/injectors                       2

   Operator for tillage tractor                              1

   Supervisor                                            1
      Total
                                                     11
Costs for  sludge treatment  and  disposal at  Sacramento are given
in  Table  19-20.   Costs  do  not  include  the main battery of
anaerobic digesters  but do  include  the  costs  of a  blending
digester (see Chapter  15).
19.4  References

 1.   U S E P A.   P rocess  Design  Manual:^  Municipal Sludge Landfills .
     Environmental  Research  information  Center, Office  of Solid
     Wastes,  Cincinnati,  Ohio  45268.   EPA-625/1-78-010,  SW-705.
     October  1978.

 2.   USEPA.   Disposal of  Sewage  Sludge into a Sanitary Landfill.
     Office  ofSolidWastes,Washington,  D.C.  20460.   SW-71d.
     1974.

 3.   Lukasik, G.D.,  and J.M.  Cormack.   "Development and Operation
     of  a Sanitary  Landfill  for Sludge  Disposal -  North Shore
     Sanitary District."   North   Shore  Sanitary  District,
     vtfaukegan, Illinois.   1976.

 4.   USEPA.   Regulations  on Public  Participation  in  Programs
     Under the  Resource Conservation  and Recovery  Act,  The Safe
     Drinking Water Act,  and The  Clean Water Act~i  Office of'
     Waste  and  Hazardous Materials, Washington,  D.C.  20460.
     40 CFR 25, 44 CFR 10292.  February 16,  1979.


                               19-58

-------
                                   TABLE 19-20

       SACRAMENTO REGIONAL WASTEWATER TREATMENT PLANT
          PROJECTED COSTS OF SLUDGE MANAGEMENT SYSTEM
                      FOLLOWING ANAEROBIC DIGESTION
              Item
Capital  cost
  Levee/drainage
  Blending  digester
  Odor-stripping facilities
  Solid  storage basins (SSBs)
  Existing  SSB modifications

  Barriers  and wind machines
  DLD sites
  Electrical and controls
  Wetlands/agricultural land
  Landfill

  Subtotal, construction cost

  Administration, engineering
    and  contingencies'3
  Landc
  Sludge handling equipment
    Total,  capital cost

Operational cost, annual
  Labor6
  Materials and  supplies
  Power and fuel^
  Site monitoring
    Total annual  operating cost

                      h
Annual costs
  Stationary  facilities
  Mobile equipment*
  Land]
  Operational  costs

    Total annual costs
                                   Phase  I:
                                  1980-1992
                                      670
                                     3, 910
                                      980
                                     7,810
                                      480

                                     1,020
                                     2,480
                                     1,640
                                      840
                                      280

                                    20,110
                                     4,840
                                     2,690
                                     1, 150

                                    28,790
                                       574
                                       248
                                       126
                                        30

                                       978
                                     2,218
                                       144
                                       185
                                       978

                                     3,525
                                               Costs, thousand dollars

                                                      Phase  II:
                                                      1992-1999
                                                  additional costs
2,730


  420

  100



3,250


  750

1,150

5,150
  112
   25
   18
    5

  160
  359
   48
  160

  597
                                                                           Total
   670
 3,910
   980
10,540
   480

 1,440
 2,480
 1,740
   840
   280

23,360
 5, 590
 2,690
 2, 300

33,940
   585
   273
   144
    35

 1, 138
 2, 577
   192
   185
 1, 138

 4,122*
 Costs based on  an  ENR cost index of 3900,  Sacramento, 1980.

 Allowance for administrative and engineering  expense, and contingencies.   Includes
 staging allowance  for additional work in  Stage  I to accommodate Stage  II.

CLand costs are  $ 1,500/acre.
 Operational costs  are based on estimated  1980 prices for solids loads  at  the midpoint
 of each staqe,  i.e., 1985 for Stage I and  1996  for Stage II.            .,
eTotal average annual cost per full-time individual of $28,000 in 1980,  including all
 fringe benefits and administrative overhead expenses. (20 1/2 person staff  1985,
 24 1/2 person staff 1996).

 Materials and supplies include special allowances for flexible hose for DLD operation
 ($25,000/yr), activated carbon for odor-stripping (11,200 Ib/yr)  percent  allowances for
 equipment (3 percent), structures  (1 percent),  and earthwork (1/2 percent)  construction
 costs.
gElectrical power  projected at 2.9 cents/kWhr  and diesel fuel at 80 cents/gal in bulk
 in 1980.

 Amortization at 6  7/8 percent over a 25-yr life.
1Mobile facilities  have various useful lives.  No salvage value assumed.

^Land value assumed the same at the end of  20  years.
k                        '                                                '
 Weighted average  annual total program cost $3,824,000.

1 acre = 0.405 ha                                          ..  .
1 kWhr = 3.6 MJ
1 qal = 3.8 1
1 Ib = 0.453 kq
                                        19-59

-------
 5 .   U S E P A.   Subsurface Disposal  of Municipa1  Wastewater
     Treatment Sludge .   Office  of Soli d~~W a s t e s , Wa shington,
     D.C.  20460.   SW-167c.   1978.

 6.   Sacramento  Area  Consultants.   Sewage  Sludge Management
     Program  Final  Report,  Volume  7,  Environmental Report  and
     Advanced Site Planning.   Sacramento  Regional County
     Sanitation  District.   Sacramento,  California 95814.
     September 1979.

 7.   Brown  and Caldwell.  Corvallis Sludge Disposal  Study.   City
     of Corvallis,  Oregon.   April  1977.

 8.   Brown   and   Caldwell.     Corvallis  Sludge  Disposal Predesign
     Report.   City of  Corvallis, Oregon.  March  1978.

 9.   Brown  and  Caldwell.   Amendment  to  Corvallis Wastewater
     Treatment Program  Environmental  Assessment  Dedicated  Land
     Disposal Project.   City of  Corvallis, Oregon.  April 1978.

10.   Uhte,  W.R.    "Wastewater Solids  Storage JBasins:   A  Useful
     Buffer  Between  Solids __S_tabilization  and  Final Disposal."
     Presented at  the  48th Annual Conference  of  the California
     Water  Pollution Control Association, Lake  Tahoe, California.
     April  14, 1976.

11.   USEPA.     "Principals  and  Design Criteria  for Sewage Sludge
     Application  on Land."  In  Sludge  Treatment  and Disposal,
     Part  2.  Environmental  Research Information  Center.
     Cincinnati,  Ohio  45268.   EPA-625/4-78-012.  October 1978.

12.   Sacramento  Area  Consultants.   Sewage  Sludge Management
     Program  Final  Report,  Volume 5.    Dedicated Land  Disposal
     S_tiacry_.   Sacramento  Regional County  Sanitation District.
     S~acramento,  California  95814.  September 1979.

13.   USEPA.     Comprehensive Summary of  Sludge  Disposal Recycling
     History.  Office of  Research  and Development.   Cincinnati,
     Ohio  45268.   EPA-600/2-77-054.  April 1977.

14.   Brown  and Caldwell.   Preliminary  Draft:   Colorado Springs
     Long-Range Sludge  Management  Study.  City  of  Colorado
     Springs, Colorado  80947.  April 1979.

15.   Sacramento  Area  Consultants.   Sewage  Sludge Management
     Program  Final Report, Volume 1,  SSMP  Final Report,  Work
     Plans,  Source Survey.   Sacramento Regional  County Sanitation
     District. Sacramento,  California 95814.   September 1979.

16.   Sacramento  Area  Consultants.   Sewage  Sludge Management
     Program Cost Increases.   Letter  to  Sacramento  Regional
     County Sanitation  District.   Sacramento,  California  95814.
     May 18,  1979.
                              19-60

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EPA 625/1-79-011
              PROCESS DESIGN MANUAL
                        FOR
          SLUDGE TREATMENT AND DISPOSAL
            Appendix A. Metric Equivalents
      U.S. ENVIRONMENTAL PROTECTION AGENCY

      Municipal Environmental Research Laboratory
           Office of Research and Development
      Center for Environmental Research Information
                 Technology Transfer
                   September 1979

-------
                                    APPENDIX A
                              METRIC EQUIVALENTS

                             METRIC CONVERSION TABLES
Recommended Units

Description
Length






Area









Volume








Mass




Time





Force









Unit
meter

kilometer
millimeter
centimeter
micrometer

square meter

square kilometer

square millimeter
hectare




cubic meter

cubic centimeter

liter




kilogram
gram
milligram
tonne


second
day

year


newton









Symbol
m

km
mm
cm


m2

km?
2
mm^
ha




3

Cm3

1




|(g
g
mg
t


s
day

yr or
8

N









Comments
Basil -SI unit











The hectare (10.000
m2) is a recognised
multiple unit and
will remain in inter
national use,




The liter is now
recognized as the
special name for
the cubic decimeter

Basic SI unit

1 tonne = 1,000 kg


Basic SI unit
Neither the day nor
the year is an SI unit
but both are impor-
tant.

The newton is that
force that produces
1 m/s2 m a mass
of 1 kg.






English
Eauivalents
39. 37 in. = 3.28 ft •
1.09yd
062 mi
0.03937 in.
03937 in
3.937 X 10'3=103A

10.744 sq ft
' 1 196 sq yd
6.384 sq mi =
247 acres
0.1 55 sq in.
0 00155 sq m.
2471 acres




35314 cu ft =
1.3079cuyd
0.061 cum.

1.057 qt = 0.264 g»l
= 0.81 X 10"* acre
ft


2.205 Ib
0.035 oz = 15.43 gr
0.01543 gr
0.984 ton (long! =
1.1023 ton (short)







0.22481 Ib (weight)
- 7.5 poundals









Description
Velocity
linear






angular






Viscosity


Pressure









Temperature








Work, energy,
quantity of heat





Power


Application of Units

Description
Precipitation,
run -off,
evaporation






River flow


Flow in pipes,
conduits, chan-
nels, over weirs,
pumping

Discharges or
abstractions,
yields



Usage of water


Density




Unit
millimeter








cubic metar
per second

cubic meter per
second

liter per second

cubic meter
per day

cubic metar
per year

liter per person
per day

kilogram per
cubic meter



Symbol
mm








m3/s


m3/s


l/s

m3/day


m3/yr


I/person
day

kg/m3




Comments
For meteorological
purposes it may be
convenient to meas
ure precipitation in
terms of mass/unit
area(kg/m3).

1 kg/sq m

Commonly called
the cumec






1 l/s = 86.4 m3/day








The density of
water understand
ard conditions is
1 000 kg/m3 or
l.OOOg/t
English
Equivalent!









35.314 cfs





15.85 flpm

I.83X 10'3 flpm





0.264gcpd


0.0624 Ib/cuft




Description
Concentration


BOD loading



Hydraulic load
per unit area,
e.g. filtration
rates




Hydraulic load
per unit volume:
a.g. biological
filters, lagoons

Air supply



Pipes
diameter
length


Optical units
Recommended Units

Unit

meter per
second
millimeter
per second
kilometers
per second

radians per
second

per second

liter per second

poise


newton per
square meter

kilonewton per
square meter

kiloyram (force)
per square
centimeter

degree Kelvin
degree Celsius







(oule




kilojoule
watt
kilowatt
joule per second

Symbol

m/s

mm/s

km/s


rad/i

3


l/s

poise


N/m?


kN/m?


kgf/cm2



K
C







J




kJ
W
kW
J/l

Comments











the cumec






The newton is not
et well known as
le unit of force
nd kgf cm2 will
early be used for
ome time. In this
ield the hydraulic
head expressed m
meters if an accept
able alternative
Basic SI iinii
The Kelvin and
Celsius degrees
are identical
The use of the
Celsius scale is
recommended as
it is the former
centigrade scale
joule = 1 N m





1 watt * 1 J/s


English
Equivalents

3.28 fpi

000328 IPS

2.230 mph




15 850 gpm
= 2.120cfm

15.85 gpm

0.0672/lb/
sec It

000014 psi


0 145 psi


14.223 ps.



5F
- - 17 77







2.778 X ID'7
kw hr =
3.725 X10'7
hphr»0.737SS.
h-lb > 9.48 X
10-' Blu
2.778 kw hr



Application of Units

Unit
milligram per
litit

kilogtam per
cubic meter
per day

cubic miter
per square meter
per day





cubic meter
per cubic meter
per day


cubic meter or
liter of free air
per second


milhmattf
mettr


lumen per
square meter

Symbol
mfl/l


kg/m3day



m3/m2 day







m3/m3diy




m3/s

l/s


mm
m


lumen/m^

Comments







If thts is con
verted to a
velocity, it
should be ex
pressed in mm/s
(1 mm/i -86.4
m3/m2day}.
















tnjlitt
Equiwllfili
1 ppm


0.0624 Ib/cu-lt
d*y


3.28cutt/iqlt

















0.03937 in.
39. 37 in. •
3.28 II

0 092 II
andla/iq ft
ft U.S. GOVERNMENT PRINTING OFFICE: 1981—757-064/0276
                                          A-l

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