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 EvaporationCarver
            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 95F (35C)	    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 DigesterSacramento
        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 DigesterSacramento
        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 FSLsSacramento 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 TF)	   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


                              3-2

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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_Pagt_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  =  -  (86ooo 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!

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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  sludgesfor  example, trickling  filter
        sludgehave 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

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                                      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
concentrationsfor example, 0.5 mg/1in 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 68F  (4to 20C)(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  18F
(10C)  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 72F
        (15 to 22C) 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 59F  (10  to  15C), the same  kd  value  as  for 59 to
        75F (15 to 22C)  should  be  used, but  the Y value should
        be increased by 26  percent.   This  is based on experiments
        that compared systems at 52F  (11C)  and  70F (21C).  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 38F
        [11C] and 0.38 at 56F [21C])  (45).

       If  wastewater temperatures are  below  50F  (10C),
        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 72F  (22C) , values
        of  the  process coefficients  from the  range  59  to 72F
        (15  to  22C) 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  36F  (10C),
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 P25
   (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

        1

        0

        5

        4

        3

        2

         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 low5  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 .
     *  SllJ^=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

-------
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 103C. Four tests. Volatile contents 34 to 55 percent.

 Same samples as previous column, ashed at 550C 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 cooTng"~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 36F  (10C), the scum was a  congealed,
clotty  mass.   At 54F  (20C),  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  176F  (80F)   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

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Piping  should be heated to  a  minimum of 60F  (15C).   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 180F  (80C) 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

-------
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  ProcessOxygen 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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     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

-------
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

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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 loadthis 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 bridgetype 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

-------
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).

Pike_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_PiJLn

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

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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:



    3foot"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

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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

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      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

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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 140F (60C) is about one half less than  the saturation
of air in water at 66F (18.8C)  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.

ont0l_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 operationthat  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
LL
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  * irrir 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_Cn_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

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    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 86F (30C).   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 cleanedtwo 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 year3.0  man-
        hours .
                              5-57

-------
       High pressure  oil  filter should  be  changed  every 1,000
        operating hours0.5  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 hours40 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  140F, 50  to 60C) 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  1F  (0.6C)  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
32F to as high as  140F  (0 to  60C).   Most commonly, high-rate
digesters  are operated between  86 and  100F  (30 and  38C).
The organisms that grow  in this temperature range  are  called
mesophilic.    Another  group of  microorganisms,  the thermophilic
bacteria,  grow at  temperatures between  122 and  140F  (50 and
60C).   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 95F  [35C]),  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  95F  (35C)   (69), which  corresponds with
Torpey's  pilot-scale  study  (70),  in  which  anaerobic  sludge
digesters  operating  at 99F  (37C)  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  68F (20C)  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  95F  (35C),  7.0 days  at  77F (25C),  and 10.1
days at 50F (10C) .


        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 95F
              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

-------
  


[JIN
3"
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      3  -
      2 -
      1 -
      0

     20
      15
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  LLJ
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      10
                                   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  95F  (35C).    (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 94F
(34C),  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 95F  (35C)  will  increase the
proportion of volatile solids  destroyed during digestion.   This
response  to  temperature  change  is  not instantaneous  but  would
                              6-26

-------
    ro
    60
    40
    30
    20
    10
Q  40
ai
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0  10
   60
    50
   40
    30
    20
    IP
                                                PRIMARY SLUDGE ONLY
                              J	L
                                               * PILOT PLANT REF. (82]
                                               * PILOT PLANT REF, (S3)
                                J	_L
                                              ACTIVATED SLUDGE ONLY
 .
*   *
                                  *  PILOT PLANT REF, (13)
                                    PILOT PLANT REF. (14)
                                    PILOT PLAMT HEF, (85)
                     I     I
                                   J	I
                                      PRIMARY AND
                                      ACTIVATED SLUDGE
                                               * FULL SCALE REF. S24}
                                               * PILOT PLANT ftEF, (10)
                                                FULL SCALE REF. (27>
                     j	I
                               I
                                    I
                                         I
                                              J	L
                                                        J_
      0   200   400   600  800        1200  1400  1600  1900  2000  2200

                    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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100
90
80

70


60

50
40

30

20

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  (347F [175C])  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
(35C)  and 130F  (54C).
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 95F
                         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-
     z
     2
     I-
     u
          15  -
          10 -
     u
     LU
     Q_
     00
           5 -
                                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  ionspredominantly   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
                                                    96F (35C)
                              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 95F  (35C) (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

-------
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

-------
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 1F  (0.6C)  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 130F (49 to  55C)  (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  155F (68C).  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.
              20GF
                  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  400F
(200C).

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 140F  (52 to  60C),  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 95F (35C)  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

-------
        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 95F.


           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
                        (25F)  =  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  95F   (35C)  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  95F  (35C)   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 ->-A4\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 =  55F
- /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 = 40F

       Minimum air temperature = 10F
Qm =
          heat loss through  walls + bottom + top
        =  (0.06 Btu/sf/F/hr) ( [2    ft]55 ft/4[27.4  f t] ) (95F-40F )
        +  (0.11)(  [55 ft]2/4] (95F-40F)
        +  (0.16)(  [55 ft]2/4] (95F-10F)
        =  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 95F 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
                                   50F
        180

        187
                   Studies
                   'between
                   50- 86F
                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 140F  (60C) (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
68F  (20C)  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 113F  (45C),
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
                                    PlOT
                                                           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  113F  (45C)
(214).
     B.O
     7.0
     6,0
  I
  a.
     5.0
     4.0
     3.0
                                    LIQUID TEMP 40? (5C
             LIQUID TEMP 67F {2QC}
     ,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 50F  (10C).

       Estimated maximum  liquid  temperature (summer)  in  digester
        is 77F  (25C).
                                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
40F
60F
80F
40F
60F
80F
               2.0  pounds of oxygen per pound of volatile
                 solids destroyed when liquid temperature
                 113F or less
               1.45 pounds of oxygen per pound of volatile
                 solids destroyed when liquid temperature
                 greater than 113F
               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 50F  (10C)  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  77F  (25C):
25C 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,=6SF!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

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                                                    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

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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

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                                    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

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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.
70F
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 176F  (80C).    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 highon 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

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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 200F  (93C).
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 wastesfor   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

-------
6.6  References

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  2.   Agardy,  F.J., R.D.  Cole,  and E.A. Pearson.   "Kinetic  and
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  3.   Lawrence,  A.W.,  and P.L. McCarty.   "Kinetics  of  Methane
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  4.   Andrews, J.F.  "Dynamic Modeling of the Anaerobic Digestion
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  6.   Collins, A.S.  and  B.E.  Gilliland.   "Control  of Anaerobic
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  7.   Toerien, D.F.   "Anaerobic Digestion  -  The  Microbiology of
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  9.   Pretorius,   W.A.    "Anaerobic  Digestion  -  Kinetics  of
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 11.   USEPA.   Sludge Handling  and  Disposal Practices at Selected
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                              6-138

-------
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23.   Woods,  C.E.  and V.F.  Malina.   "Stage Digestion  of  Waste-
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                             6-140

-------
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158.   Dick, R.I.  and B.B. Ewing.   "The Rheology  of  Activated
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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

-------
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
      Studies."   Journal Water Pollu_tJ^on Control Federation.
      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  85F  (30C)  (10).  Ward and  Ashley  reported  four log
inactivation of poliovirus  in  four days  at 82F  (28C)  (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 67F  (20C)  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  121F (50C)  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  94F  (35C)  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 113F  (45C) Salmonella and
Pseudomonas  were reduced to below detectable limits in 24 hours;
at 140F(TOC), 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 (121F,  [50C])  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 39F  (4C), Stern
and Farrell  reported  major  reductions  in  fecal  coliform, total
coliform,  and Salmonella bacteria (11).   In similar tests  at 68F
(20C), the  same  bacteria  could  not  be measured after 24 weeks.
Viruses were  reduced  by 67  percent  at  39F  (4C)  and  to below
detectable limits at 68F (20C)  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

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    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
158F (70C)  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  (195F  [91C],
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
    \          64F
     V
      -\^=--
                              113F
                                       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  100F  (18  to 38C) 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  157F  (70C)  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
113F (45C)  at  1.45 pounds per  square inch  (10 kN/m2)  and  then
to 98F (35C) 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 processwhere
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  157F  (70C) 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 157F  (70C), 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 68F  to  131F  (20  to
55C);  the boiler  supplies  steam to  raise the temperature  to
157F (70C).  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/lbF;

    W  = wet sludge weight, lb;

    t  = time for heating;

    e  = boiler conversion efficiency.

If h is one Btu per lb F (864 J/kgC); e = 80 percent;    T = 63F
(35C);   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
                                               15TF
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 131F (56C1
                                              u  -r


                                                                FUEL
                                                 125 psi
                                               (175C
                                               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  400F  (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  930F  (500C) 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,  157F  (70C) 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

-------
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

-------
                          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

-------
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

-------
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

-------
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

-------
        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

-------
        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

-------
               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

-------
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

-------
        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

-------
        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

-------







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^**
^ ^ **
?*"


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

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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

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        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)  truck1
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  fans0.04
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 hydra