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
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XXXVI
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TABLE OF CONTENTS (continued
2.3.3 Time Span of Decisions .,.,,.»,,,,,.,,.. 2- 7
2.3.4 Uncertainties .................. .... 2- 8
2.3.5 The Design Team 2- 8
2.3.6 Public Involvement 2- 9
2.3.7 Social and Political Factors Affecting
Waste Export . .... 2- 10
2.4 References .... 2- 11
CHAPTER 3. DESIGN APPROACH 3- 1
3.1 Introduction .......................... .» 3- 1
3.2 Systems Approach ....................I.............. 3- 1
3.3 The Logic of Process Selection . . 3- 2
3.3.1 Identification of Relevant Criteria 3- 2
3.3.2 Identification of System Options 3- 4
3.3.3 System Selection Procedure 3- 6
3.3.3.1 Base and Secondary Alternatives ........ 3- 6
3.3.3.2 Choosing a Base Alternative:
First Cut 3- 7
3.3.3.3 Choosing a Base Alternative:
Second Cut 3- 10
3.3.3.4 Third Cut 3- 11
3.3.3.5 Subsequent Cuts 3- 12
3.3.4 Parallel Elements 3- 12
3.3.5 Process Selection at Eugene, Oregon .......... 3- 13
3.4 The Quantitative Flow Diagram 3- 18
3.4.1 Example: QFD for a Chemically Assisted
Primary Treatment Plant 3- 18
3.4.2 Example: QFD for Secondary Plant
With Filtration 3- 24
3.5 Sizing of Equipment ................................ 3- 27
3.6 Contingency Planning .................................. 3- 29
3.6.1 Example of Contingency Planning
for Breakdowns ,,..,......,...,,....,..,,..... 3- 29
3.7 Other General Design Considerations ................ 3- 34
3.7.1 Site Variations 3- 34
3.7.2 Energy Conservation .......................... 3- 35
3.7.3 Cost-Effective Analyses ...................... 3- 36
3.7.4 Checklists ........ 3- 38
3.8 References 3- 39
CHAPTER 4. WASTEWATER SOLIDS PRODUCTION AND
CHARACTERIZATION ... 4- 1
4.1 Introduction .....................I*................ 4- 1
4.2 Primary Sludge 4- 1
4.2.1 Primary Sludge Production .................... 4- 1
4.2.1.1 Basic Procedures for Estimating
Primary Sludge Production »»,» 4- 1
4.2.1.2 Industrial Waste Effect 4- 2
4.2.1.3 Ground Garbage Effect 4- 3
4.2.1.4 Other Sludges and Sidestreams 4- 3
VI
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TABLE OF CONTENTS (continued)
Page
4.2.1.5 Chemical Precipitation and
Coagulation ., ...,....,.,..,,..... 4- 3
4.2.1.6 Peak Loads 4- 3
4.2.2 Concentration Properties ..................... 4- 6
4.2.3 Composition and Characteristics .............. 4- 7
4.3 Biological Sludges ................................. 4- 9
4.3.1 General Characteristics ...................... 4- 9
4.3.2 Activated Sludge .... 4- 9
4.3.2.1 Processes Included ..........I.......... 4- 9
4.3.2.2 Computing Activated Sludge
Production - Dry Weight Basis .......... 4- 9
4.3.2.3 Example: Determination of
Biological Sludge Production 4- 19
4.3.2.4 Interaction of Yield Calculations and
the Quantitative Flow Diagram (QFD) .... 4-24
4.3.2.5 Concentration of Waste-Activated
Sludge 4- 25
4.3.2.6 Other Properties of Activated Sludge ... 4-27
4.3.3 Trickling Filters 4- 29
4.3.3.1 Computing Trickling Filter Sludge
Production - Dry Weight Basis .......... 4- 29
4.3.3.2 Concentration of Trickling Filter
Sludge 4....*..**..*...... 4- 33
4.3.3.3 Properties - Trickling Filter Sludge ... 4-34
4.3.4 Sludge from Rotating Biological Reactors , .... 4-34
4.3.5 Coupled Attached-Suspended Growth Sludges .... 4-35
4.3.6 Denitrification Sludge 4- 36
4.4 Chemical Sludges ................................... 4- 36
4.4.1 Introduction 4- 36
4.4.2 Computing Chemical Sludge
Production - Dry Weight Basis ................ 4- 37
4.4.3 Properties of Chemical Sludges 4- 38
4.4.4 Handling Chemical Sludges .................... 4- 38
4.4.4.1 Stabilization 4- 39
4.4.4.2 Chemical and By-Product Recovery 4-39
4.5 Elemental Analysis of Various Sludges 4- 39
4.5.1 Controlling Trace Elements 4- 39
4.5.2 Site-Specific Analysis ....................... 4- 41
4.5.3 Cadmium 4- 42
4.5.4 Increased Concentration During Processing .... 4-43
4.6 Trace Organic Compounds in Sludge .................. 4- 44
4.7 Miscellaneous Wastewater Solids 4- 45
4.7.1 Screenings 4- 46
4.7.1.1 Quantity of Coarse Screenings 4- 46
4.7.1.2 Quantity of Fine Screenings ............ 4- 48
4.7.1.3 Properties of Screenings ............... 4- 48
4.7.1.4 Handling Screenings 4- 48
4.7.1.5 Screenings From Miscellaneous
Locations 4- 49
4.7.2 Grit . 4- 50
VII
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TABLE OF CONTENTS (continued)
4.7.2.1 Quantity of Grit
4.7.2.2 Properties of Grit ...
4.7.2.3 Handling Grit
4.7.3 Scum
4.7.3.1 Quantities of Scum ..I..................
4.7.3.2 Properties of Scum ,
4.7.3.3 Handling Scum ..........................
4.7.4 Sept age
4.7.4.1 Quantities of Septage
4.7.4.2 Properties of Septage
4.7.4.3 Treating Septage in Wastewater
Treatment Plants ..,.,....,.,.. 4- 60
4.7.5 Backwash 4- 61
4.7.6 Solids From Treatment of Combined
Sewer Overflows 4- 62
4.8 References ..........,,.,..,.,.......*.,,,,,,,...... 4- 63
CHAPTER 5 . THICKENING . „ . 5- 1
5.1 Introduction ,, 5- 1
5.1.1 Definition 5- 1
5.1.2 Purpose ..,,...,,,,..,.............,.,........ 5- 1
5.1.3 Process Evaluation 5- 1
5.1.4 Types and Occurrence of Thickening
Processes . , 5- 2
5.2 Sedimentation Basins ......... 5- 2
5.2.1 Primary Sedimentation ........................ 5- 2
5.2.2 Secondary Sedimentation 5- 3
5.3 Gravity Thickeners • 5- 3
5.3.1 Introduction •.**..................* 5- 3
5.3.2 Theory . 5- 3
5.3.3 System Design Considerations 5- 5
5.3.3.1 Minimum Surface Area Requirements ...... 5- 6
5.3.3.2 Hydraulic Loading 5- 8
5.3.3.3 Drive Torque Requirements 5- 8
5.3.3.4 Total Tank Depth 5- 9
5.3.3.5 Floor Slope 5- 10
5.3.3.6 Other Considerations .,.,,.., 5- 11
5.3.4 Design Example 5- 12
5.3.5 Cost 5- 15
5.3.5.1 Capital Cost 5- 15
5.3.5.2 Operating and Maintenance Cost ......... 5- 15
5.4 Flotation Thickening 5- 16
5.4.1 Dissolved Air Flotation (DAF) ., 5 -18
5.4.1.1 Theory ».,.,.,. 5- 19
5.4.1.2 System Design Considerations ........... 5- 19
5.4.2 Design Example 5- 33
5.4.3 Cost 5- 35
5.4.3.1 Capital Cost 5- 35
5.4.3.2 Operating and Maintenance Costs .» 5-36
5.5 Centrifugal Thickening 5- 36
VI11
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TABLE OF CONTENTS (continued)
5.5.1 Introduction 5- 36
5.5.2 Theory , 5- 38
5.5.3 System Design Considerations 5- 39
5.5.3.1 Disc Nozzles 5- 39
5.5.3.2 Imperforate Basket 5- 45
5.5.3.3 Solid Bowl Decanter 5- 49
5.5.4 Case History ... 5- 53
5.5.5 Cost ....... .. 5- 55
5.5.5.1 Capital Cost 5- 55
5.5.5.2 Operating and Maintenance Cost ......... 5- 56
5.6 Miscellaneous Thickening Methods ................... 5- 59
5.6.1 Elutriation Basin 5- 59
5.6.2 Secondary Anaerobic Digesters 5- 60
5.6.3 Facultative Sludge Lagoons ................... 5- 60
5.6.4 Ultrafiltration ., ,. 5 -60
5.7 References ... ..,..,.,.,.,...,.,.,.,.,,,.,., 5 -60
CHAPTER 6. STABILIZATION 6- 1
6.1 Introduction ....................................... 6- 1
6.2 Anaerobic Digestion ................................ 6- 2
6.2.1 Process Description .,.,.,..,,.. 6- 2
6.2.1.1 History and Current Status 6- 2
6.2.1.2 Applicability 6- 3
6.2.1.3 Advantages and Disadvantages ........... 6- 4
6.2.1.4 Microbiology 6- 5
6.2.2 Process Variations ........................... 6- 7
6.2.2.1 Low-Rate Digestion 6- 7
6.2.2.2 High-Rate Digestion 6- 7
6.2.2.3 Anaerobic Contact Process ., 6- 15
6.2.2.4 Phase Separation 6- 16
6.2.3 Sizing of Anaerobic Digesters 6- 18
6.2.3.1 Loading Criteria ....,.»...».,...,.,... 6- 18
6.2.3.2 Solids Retention Time 6- 18
6.2.3.3 Recommended Sizing Procedure ........... 6- 20
6.2.4 Process Performance 6- 23
6.2.4.1 Solids Reduction 6- 26
6.2.4.2 Gas Production 6- 29
6.2.4.3 Supernatant Quality 6- 31
6.2.5 Operational Considerations 6- 34
6.2.5.1 pH 6- 34
6.2.5.2 Toxicity 6- 36
6.2.6 System Component Design 6- 42
6.2.6.1 Tank Design 6- 42
6.2.6.2 Heating 6- 46
6.2.6.3 Mixing 6- 52
6.2.6.4 Covers 6- 62
6.2.6.5 Piping 6- 66
6.2.6.6 Cleaning 6- 67
6.2.7 Energy Usage 6- 72
6.2.8 Costs 6- 74
IX
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TABLE OF CONTENTS (continued)
6.2.9 Design Example ............................... 6- 74
6.2.9.1 Design Loadings ..,..»» ,....,,» 6- 74
6.2.9.2 System Description ..................... 6- 75
6.2.9.3 Component Sizing , 6- 77
6.3 Aerobic Digestion .........,.,..,,.,....«,........». 6- 82
6.3.1 Process Description .......................... 6- 82
6.3.1.1 History ................... 6- 82
6.3.1.2 Current Status 6- 82
6.3.1.3 Applicability 6- 82
6.3.1.4 Advantages and Disadvantages ........... 6- 82
6.3.1.5 Microbiology ......».,,.,,.,,.,.,.. 6- 83
6.3.2 Process Variations .,,..... 6- 84
6.3.2.1 Conventional Semi-Batch Operation ...... 6-84
6.3.2.2 Conventional Continuous Operation ...... 6-84
6.3.2.3 Auto-Heated Mode of Operation ..,.,..,.. 6- 85
6.3.3 Design Considerations 6- 86
6.3.3.1 Temperature 6- 86
6.3.3.2 Solids Reduction 6- 86
6.3.3.3 Oxygen Requirements .................... 6- 88
6.3.3.4 Mixing 6- 89
6.3.3.5 pH Reduction 6- 90
6.3.3.6 Dewatering .......... 6- 91
6.3.4 Process Performance 6- 92
6.3.4.1 Total Volatile Solids Reduction 6- 92
6.3.4.2 Supernatant Quality 6- 93
6.3.5 Design Example 6- 93
6.3.6 Cost 6- 99
6.3.6.1 Capital Cost 6- 99
6.3.6.2 Operation and Maintenance Cost ......... 6- 99
6.4 Lime Stabilization 6-100
6.4.1 Process Description 6-101
6.4.1.1 History 6-101
6.4.1.2 Current Status 6-102
6.4.1.3 Applicability 6-102
6.4.1.4 Theory of the Process .» 6-103
6.4.2 Design Criteria ..."..... 6-103
6.4.2.1 pH and Contact Time 6-104
6.4.2.2 Lime Dosage 6-104
6.4.3 Process Performance .......................... 6-107
6.4.3.1 Odor Control 6-108
6.4.3.2 Pathogen Reduction 6-109
6.4.3.3 Dewatering and Settling
Characteristics 6-110
6.4.3.4 Chemical Characteristics ............... 6-110
6.4.4 Process Design ......... 6-112
6.4.4.1 Design of Lime Handling Facilities 6-112
6.4.4.2 Mixing Tank Design , 6-118
6.4.5 Costs and Energy Usage 6-121
6.4.5.1 Capital and Operating Costs 6-121
6.4.5.2 Energy Usage 6-122
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TABLE OF CONTENTS (continued)
Page
6.4.6 Design Example ............................... 6-124
6.4.6.1 Design Loading 6-124
6.4.6.2 System Description......,.,,,,...,..»,, 6-124
6.4.6.3 Component Sizing ..........,,, 6-126
6.5 Chlorine Stabilization 6-127
6.5.1 Process Description .,,,,,.,,....«..,,,»....,, 6-128
6.5.2 Uses, Advantages, and Disadvantages .......... 6-131
6.5.3 Chlorine Requirements 6-132
6.5.4 Characteristics of Chlorine-Stabilized
Materials 6-133
6.5.4.1 Stabilized Slude » 6-133
6.5.4.2 Supernatant/FiItrate/Subnatant
Quality 6-134
6.5.5 Costs .....,..,..,.,.. 6-134
6.5.5.1 Operating Costs ,.•,.... 6-135
6.5.5.2 Capital Costs 6-136
6.6 References ...... .........,..,....,,,..... 6-138
CHAPTER?. DISINFECTION 7- 1
7.1 Introduction ,.,.,.....,,.........«..*.............. 7- 1
7 . 2 Pathogenic Organisms 7- 1
7.2.1 Pathogen Sources 7- 2
7.2.2 Pathogen Characteristics 7- 2
7.2.2.1 Viruses 7- 2
7.2.2.2 Bacteria 7- 3
7.2.2.3 Parasites 7- 4
7.2.2.4 Fungi 7- 6
7.2.3 Pathogen Occurrence in the United States ..... 7- 6
7.3 Pathogen Survival During Sludge Stabilization
Processes .......,..,.,.,,,,,..,.,.,, 7- 7
7.3.1 Pathogen Reduction During Digestion .......... 7- 7
7.3.1.1 Viruses 7- 7
7.3.1.2 Bacteria 7- 8
7.3.1.3 Parasites .... ,v 7- 9
7.3.2 Long Term Storage ..»,..., 7- 10
7.3.3 Chemical Disinfection 7- 10
7.3.3.1 Lime 7- 10
7.3.3.2 Chlorine 7- 10
7.3.3.3 Other Chemicals 7- 11
7.4 Pathogen Survival in the Soil .........,.,..,.,...., 7- 11
7.4.1 Viruses ,. 7- 11
7.4.2 Bacteria 7- 11
7.4.3 Parasites 7- 12
7.5 Potential Human Exposure to Pathogens 7- 12
7.6 Heat Disinfection Processes 7- 13
7.6.1 Sludge Pasteurization ..,.,,,..............,*. 7- 14
7.6.1.1 Process Description ,.....,,.., 7- 15
7.6.1.2 Current Status ......................... 7- 16
7.6.1.3 Design Criteria ........................ 7- 16
XI
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TABLE OF CONTENTS (continued)
Page
7.6.1.4 Instrumentation and Operational
Considerations ,, 7- 17
7.6.1.5 Energy Impacts ......................... 7- 17
7.6.1.6 Cost Information 7- 17
7.6.1.7 Design Example 7- 20
7.6.2 Other Heat Processes 7- 24
7.6.2.1 Heat-Conditioning » 7- 25
7.6.2.2 Heat-Drying .............. i 7- 25
7.6.2.3 High Temperature Processes 7- 25
7.6.2.4 Composting 7- 25
7.7 Pathogen Reduction With High-Energy Radiation ...... 7-26
7.7.1 Reduction of Pathogens in Sludge With
Electron Irradiation 7- 26
7.7.1.1 Process Descritpion 7- 27
7.7.1.2 Status 7- 28
7.7.1.3 Design Considerations 7- 28
7.7.1.4 Instrumentation and Operational
Considerations 7- 30
7.7.1.5 Energy Impacts 7- 30
7.7.1.6 Performance Data . 7- 30
7.7.1.7 Production Production and Properties ... 7-31
7.7.1.8 Cost Information ...,........*.,...,.... 7- 31
7.7.2 Disinfection With Gammer Irradiation 7- 32
7.7.2.1 Process Description 7- 33
7.7.2.2 Current Status - Liquid Sludge ......... 7- 33
7.7.2.3 Current Status - Dried or Composted
Sludge 7- 34
7.7.2.4 Design Criteria 7- 35
7.7.2.5 Instrumentation and Operational
Considerations ................*..-...*.. 7- 35
7.7.2.6 Energy Impacts .• 7- 36
7.7.2.7 Performance Data 7- 37
7.7.2.8 Cost Information 7- 38
7.8 References 7- 44
CHAPTER 8. CONDITIONING 8- 1
8.1 Introduction ......,..» 8- 1
8.2 Selecting a Conditioning Process ................... 8- 1
8.3 Factors Affecting Wastewater Solids Conditioning ... 8- 1
8.3.1 General Wastewater Solids Properties ......... 8- 1
8.3.1.1 Particle Size and Distribution ......... 8- 3
8.3.1.2 Surface Charge and Degree of
Hydration 8- 4
8.3.1.3 Particle Interaction 8- 4
8.3.2 Physical Factors 8- 4
8.3.2.1 Effect of Processing Prior to
Conditioning ........................... 8- 5
8.3.2.2 Conditioner Application ................ 8- 5
8.4 Inorganic Chemical Conditioning 8- 6
8.4.1 Introduction 8- 6
Xll
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TABLE OF CONTENTS (continued)
8.4.2 Dosage Requirements 8- 6
8.4.3 Availability .............. 8- 7
8.4.4 Storage, Preparation, and Application
Equipment ........I..,,..,...,......,..,.,.... 8- 8
8.4.5 Design Example ...,.,,,......... ,.,..*. 8- 8
8.4.6 Cost 8- 9
8.4.6.1 Capital Cost 8- 9
8.4.6.2 Operation and Maintenance Cost 8- 10
8.5 Chemical Conditioning With Polyelectrolytes ,,,»,... 8-14
8.5.1 Introduction 8- 14
8.5.2 Background on Polyelectrolytes ............... 8- 14
8.5.2.1 Composition and Physical Form .......... 8- 14
8.5.2.2 Structure in Solution , 8- 17
8.5.2.3 How Polyelectrolyte Conditioning
Works 8- 17
8.5.3 Conditioning for Thickening 8- 18
8.5.3.1 Gravity Thickening ..................... 8- 18
8.5.3.2 Dissolved Air Flotation Thickening ,.,,. 8-18
8.5.3.3 Centrifugal Thickening 8- 20
8.5.4 Conditioning for Dewatering 8- 20
8.5.4.1 Drying Beds 8- 21
8.5.4.2 Vacuum Filters 8- 21
8.5.4.3 Recessed Plate Pressure Filters 8- 22
8.5.4.4 Belt Filter Presses 8- 23
8.5.4.5 Centrifuges ., 8- 24
8.5.5 Storage, Preparation, and Application
Equipment 8- 25
8.5.6 Case History 8- 25
8.5.7 Cost 8- 27
8.5.7.1 Capital Cost 8- 27
8.5.7.2 Operation and Maintenance Cost ......... 8- 29
8.6 Non-Chemical Additions ... 8- 29
8.7 Thermal Conditioning ............................... 8- 31
8.7.1 Advantages and Disadvantages 8- 33
8.7.2 Process Sidestreams 8- 34
8.7.2.1 Gaseous Sidestreams ., 8- 34
8.7.2.2 Liquid Sidestreams 8- 35
8.7.3 Operations and Cost ... 8- 36
8.7.3.1 General Considerations ................. 8- 36
8.7.3.2 USEPA Survey Results 8- 38
8.8 Elutriation 8- 39
8.9 Freeze-Thaw 8- 40
8.9.1 Indirect Mechanical Freezing .....,,,,...,.... 8- 40
8.9.2 Direct Mechanical Freezing 8- 41
8.9.3 Natural Freezing ............. 8- 41
8.10 Mechanical Screening and Grinding 8- 41
8.11 Miscellaneous Processes 8- 42
8.11.1 Bacteria 8- 42
8.11.2 Electricity 8- 42
8.11.3 Solvent Extraction 8- 43
XI 11
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TABLE OF CONTENTS (continued)
Pag_e
8.11.4 Ultrasonic ». 8- 43
8.12 References ....... .............. ...... 8- 43
CHAPTERS. DEWATERING ..•........,., 9- 1
9.1 Introduction ,,..,..........,....»,,,..,.......,.,,. 9- i
9.1.1 Process Evaluation ., 9- 1
9.1.2 Methods of Dewatering . .... 9- 3
9.2 Natural Sludge Dewatering Systems ,,,.,.,,»,,» 9- 3
9.2.1 Drying Beds , 9- 3
9.2.1.1 Basic Components and Operation .,..,.,». 9- 4
9.2.1.2 Types of Drying Beds 9- 5
9.2.1.3 Process Design Criteria ................ 9- 9
9.2.1.4 Costs ....... 9- 12
9.2.2 Drying Lagoons .......... 9- 14
9.2.2.1 Basic Concept 9- 15
9.2.2.2 Design Criteria .................... 9- 15
9.2.2.3 Costs 9- 16
9.3 Centrifugal Dewatering Systems ..................... 9- 17
9.3.1 Introduction 9- 17
9.3.2 Imperforate Basket 9- 18
9.3.2.1 Principles of Operation 9- 18
9.3.2.2 Application 9- 19
9.3.2.3 Performance 9- 19
9.3.2.4 Case History 9- 19
9.3.3 Solid Bowl Decanters 9- 23
9.3.3.1 Application 9- 23
9.3.3.2 Performance 9- 24
9.3.3.3 Other Considerations 9- 24
9.4 Filtration Dewatering Systems 9- 25
9.4.1 Introduction 9- 25
9.4.2 Basic Theory 9- 26
9.4.3 Filter Aids 9- 26
9.4.4 Vacuum Filters 9- 27
9.4.4.1 Principles of Operation 9- 28
9.4.4.2 Application . 9- 32
9.4.4.3 Performance 9- 33
9.4.4.4 Other Considerations ................... 9- 33
9.4.4.5 Case History 9- 39
9.4.4.6 Costs 9- 41
9.4.5 Belt Filter Press 9- 43
9.4.5.1 Principles of Operation 9- 45
9.4.5.2 Application 9- 46
9.4.5.3 Performance 9- 46
9.4.5.4 Other Considerations 9- 47
9.4.5.5 Design Example 9- 49
9.4.5.6 Costs 9- 51
9.4.6 Recessed Plate Pressure Filters .............. 9- 52
9.4.6.1 Principles of Operation 9- 52
9.4.6.2 Application 9- 55
9.4.6.3 Performance ............................ 9- 56
xiv
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TABLE OF CONTENTS (continued)
9.4.6.4 Other Considerations 9- 56
9.4.6.5 Case History ., 9- 59
9.4.6.6 Cost .. .,.,..,.....«,,....» 9- 60
9.4.7 Screw and Roll Press ,,.,,,*..., ,»,,,,... 9- 63
9.4.7.1 Screw Press 9- 63
9.4.7.2 Twin-Roll Press 9- 66
9.4.8 Dual Cell Gravity (DCG) Filter ..,,, 9- 67
9.4.9 Tube Filters 9- 68
9.4.9.1 Pressure Type 9- 68
9.4.9.2 Gravity Type » 9- 68
9.5 Other Dewatering Systems 9- 69
9.5.1 Cyclones 9- 69
9.5.2 Screens 9- 70
9.5.3 Electro-Osmosis 9- 70
9.6 References 9- 70
CHAPTER 10. HEAT DRYING 10- 1
10.1 Introduction 10- 1
10.2 Heat-Drying Principles .,,.,........., 10- 1
10.2.1 Drying Periods 10- 1
10.2.2 Humidity and Mass Transfer 10- 2
10.2.3 Temperature and Heat Transfer 10- 3
10.3 Energy Impacts * . 10- 5
10.3.1 Design Example 10- 6
10.3.2 Energy Cost of Heat-Dried Sludges Used
for Fertilizers .,.,., 10- 11
10.4 Environmental Impacts 10- 12
10.4.1 Air Pollution ...» 10- 12
10.4.2 Safety 10- 13
10.4.3 Sidestream Production 10- 13
10.5 General Design Criteria 10- 13
10.5.1 Drying Capacity . 10- 13
10.5.2 Storage Requirements 10- 14
10.5.3 Heat Source 10- 14
10.5.4 Air Flow 10- 14
10.5.5 Equipment Maintenance ....................... 10- 15
10.5.6 Special Considerations ...................... 10- 15
10.6 Conventional Heat Dryers , 10- 15
10.6.1 Flash-Drying 10- 16
10.6.1.1 Process Description ................... 10- 16
10.6.1.2 Case Study: Houston, Texas 10- 18
10.6.2 Rotary Dryers 10- 19
10.6.2.1 Direct Rotary Dryers .....' 10- 19
10.6.2.2 Indirect Drying .. 10- 22
10.6.2.3 Direct-Indirect Rotary Dryers 10- 24
10.6.3 Incinerators 10- 25
10.6.4 Toroidal Dryer 10- 25
10.6.4.1 Process Description ................... 10- 25
10.6.4.2 Current Status 10- 27
10.6.5 Spray-Drying 10- 27
xv
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TABLE OF CONTENTS (continued)
10.6.5.1 Process Description ........... 10- 27
10.6.5.2 Current Status ., 10- 27
10.7 Other Heat-Drying Systems 10- 28
10.7.1 Solvent Extraction--BEST Process ...» ., 10- 28
10.7.1.1 Process Description 10- 28
10.7.1.2 Current Status ...» 10- 29
10.7.1.3 Operating Experience .................. 10- 30
10.7.2 Multiple-Effect Evaporation—Carver
Greenfield Process 10- 30
10.7.2.1 Process Description ............ ....... 10- 31
10.7.2.2 Current Status ,, ...» 10- 31
10.8 References 10- 32
CHAPTER 11. HIGH TEMPERATURE PROCESSES 11- 1
11.1 Introduction ,,..»...*...,......,,....,.....»,»,,.. 11- 1
11.2 Principles of High Temperature Operations ......... 11- 2
11.2.1 Combustion Factors .......................... 11- 3
11.2.1.1 Sludge Fuel Values »»..... 11- 3
11.2.1.2 Oxygen Requirements for Complete
Combustion 11- 6
11.2.1.3 Factors Affecting the Heat Balance .... 11- 7
11.2.2 Incineration Design Example 11- 10
11.2.2.1 Problem Statement 11- 10
11.2.2.2 Approximate Calculation Method 11- 13
11.2.2.3 Theoretical Calculation Method 11- 20
11.2.2.4 Comparison of Approximate and
Theoretical Calculation Methods ....,,, 11- 24
11.2.3 Pyrolysis and Starved-Air Combustion
Calculations ...» 11- 25
11.2.4 Heat and Material Balances . 11- 28
11.3 Incineration 11- 29
11.3.1 Multiple-Hearth Furnace 11- 31
11.3.2 Fluid Bed Furnace 11- 42
11.3.3 Electric Furnace 11- 49
11.3.4 Single Hearth Cyclonic Furnace ...* 11- 55
11.3.5 Design Example: New Sludge Incineration
Process .,,.,.,,..,.,.,... 11- 59
11.3.5.1 Approach 11- 61
11.3.5.2 Preliminary Design 11- 62
11.4 Starved-Air Combustion 11- 65
11.4.1 Development and Application 11- 68
11.4.2 Advantages and Disadvantages of SAC 11- 71
11.4.3 Conversion of Existing Multiple-Hearth
Incineration Units to SAC ................... 11- 75
11.4.4 Design Example: Retrofit of an Existing
Multiple-Hearth Sludge Incinerator to a
Starved-Air Combustion Reactor 11- 76
11.4.4.1 Approach 11- 77
11.4.4.2 Preliminary Design 11- 78
xvi
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TABLE OF CONTENTS (continued)
11.4 Starved-Air Combustion ...» n_ 55
11.4.1 Development and Application » 11- 68
11.4.2 Advantages and Disadvantages of SAC ......... 11- 71
11.4.3 Conversion of Existing Multiple-Hearth
Incineration Units to SAC n- 75
11.4.4 Design Example: Retrofit of an Existing
Multiple-Hearth Sludge Incinerator to a
Starved-Air Combustion Reactor 11- 76
11.4.4.1 Approach 11- 77
11.4.4.2 Preliminary Design 11- 78
11.5 Co-Combustion of Sludge and Other Material ........ 11- 81
11.5.1 Co-Combustion with Coal and Other
Residuals .. ...» 11- 81
11.5.2 Co-Combustion with Mixed Municipal Refuse
(MMR) 11- 83
11.5.2.1 Refuse Combustion Technology .......... 11- 84
11.5.2.2 Sludge Combustion Technology 11- 87
11.5.3 Institutional Constraints ...,....,,,,,.,.,,. 11- 92
11.5.4 Conclusions about Co-Combustion 11- 94
11.6 Related Combustion Processes Used in Wastewater
Treatment , 11- 94
11.6.1 Screenings, Grit, and Scum Reduction ........ 11- 94
11.6.2 Lime Recalcination ...,..,...,.,,.,.,........ 11- 96
11.6.3 Activated Carbon Regeneration 11- 98
11.6.3.1 Granular Carbon Systems (GAC) 11- 99
11.6.3.2 Powdered Activated Carbon (PAC) ....... 11-100
11.6.3.3 Jet Propulsion Laboratory Activated-
Carbon Treatment System (JPL-ACTS) .... 11-100
11.7 Other High Temperature Processes 11-102
11.7.1 High Pressure/High Temperature Wet Air
Oxidation 11-102
11.7.2 REACT-O-THERMtm 11-109
11.7.3 Modular Starved-Air Incinerators 11-110
11.7.4 Pyro-Soltm Process 11-110
11.7.5 Bailie Process 11-113
11.7.6 Wright-Malta Process 11-113
11.7.7 Molten Salt Pyrolysis 11-115
11.8 Air Pollution Considerations 11-115
11.8.1 National Ambient Air Quality Standards
(NAAQS)-State Implementation Plans (SIP) .... 11-116
11.8.2 National Emission Standards for Hazardous
Air Pollutants (NESHAPS) ..................... 11-177
11.8.3 Standards of Performance for New
Stationary Sources (NSPS) 11-118
11.8.4 New Source Review Standards (NSR) 11-119
11.8.5 Prevention of Significant Deterioration
(PSD) 11-119
11.8.6 The Permit Process 11-120
11.8.7 Air Emissions Test Procedures 11-120
11.8.8 Design Example 11-120
xvi i
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TABLE OF CONTENTS (continued)
Page
11.8.8.1 Identify Applicable State and Local
11.8.8.2 Establish Air Pollution Abatement
11.9 Residue Disposal ..»,......,,*....»»»,,,
CHAPTER 1 2 . COMPOSTING .... ,
12.2.1 Moisture ....,..,
12.2.3 pH .,
12.2.4 Nutrient Concentration
12.3.1.2 Public Health and Environmental
12.3.2 Aerated Static Pile Process
12.3.2.1 Individual Aerated Piles ..............
12.3.2.2 Extended Aerated Piles
12.3.2.3 Current Status
12.3.2.5 Bulking Agent
12.3.2.7 Public Health and Environmental
12.3.3 Case Studies (Unconfined Systems) ...........
12.3.3.1 Joint Water Pollution Control Plant,
Carson, California ...................
12.3.3.2 Beltsville, Maryland
12.3.3.5 Cost Analysis
12.4 Confined Composting System .......................
12.4.2 Metro-Waste Aerobic Thermophilic
Bio-Reactor , ,
12.4.3 Dano Bio-Stabilizer Plant
12.4.4 BAV Bio-Reactor
CHAPTER 13. MI SCELLANEOUS PROCESSES
. 11-121
, 11-123
. 11-132
. 11-136
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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
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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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TABLE OF CONTENTS (continued)
17
17
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)
18
18
18
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
Pa
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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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
LIST OF FIGURES (continued)
Njjmbejr Page
6- 3 Single-stage, High-Rate Anaerobic Digestion
System 6- 9
6- 4 Flow Diagram for the Torpey Process ............. 6- 10
6- 5 Two-Stage, High-Rate Anaerobic Digester
System . 6- 12
6- 6 Carbon and Nitrogen Balance for a Two-Stage,
High-Rate Digestion System 6- 14
6- 7 Effect of Recycling Digester Supernatant on the
Suspended Solids Flow Through an Activated
Sludge Plant 6- 15
6- 8 Anaerobic Contact Process ....................... 6- 15
6- 9 Two-Phase Anaerobic Digestion Process ........... 6- 16
6-10 Effect of SRT on the Relative Breakdown of
Degradable Waste Components and Methane
Production ...,..,,,,..,,,.......,..,.,.,........ 6- 21
6-11 Effect of Temperature and SRT on the Pattern
of Methane Production and Volatile Solids
Breakdown 6- 22
6-12 Effect of Solids Retention Time and Temperature
on Volatile Solids Reduction in a Laboratory-
Scale Anaerobic Digester ,,.,..., 6- 27
6-13 Volatile Solids Reduction vs Temperature x SRT
for Three Types of Feed Sludges ................. 6- 28
6-14 Effect of Temperature on Gas Production ......... 6- 30
6-15 Relationship Between pH and Bicarbonate
Concentration Near 95°F (35°C) 6- 35
6-16 Cylindrical Anaerobic Digestion Tanks ........... 6- 43
6-17 Rectangular Anaerobic Digestion Tank ............ 6- 44
6-18 Egg-Shaped Anaerobic Digestion Tank at Terminal
Island Treatment Plant, Los Angeles ,,,,,...,.... 6- 45
6-19 Schematic of the Heat Reservoir System for a
Jacketed Pipe or Spiral Heat Exchanger .......... 6- 48
6-20 Spiral Heat Exchanger Operating Off Secondary
Heat Loop at Sunnyvale, California 6- 49
6-21 Effect of Solids Concentration on the Raw Sludge
Heating Requirement ,.,,, 6- 51
6-22 Circulation Patterns Produced by Draft Tube and
Free Gas Lift Mixers 6- 56
6-23 Draft Tube and Free Gas Lift Pumping Rate 6-57
6-24 Comparison of Lance and Draft Tube Mixing in
Clean Water . 6- 58
6-25 Effect of Temperature on the Viscosity of
Water 6- 60
6-26 Effect of Solids Concentration and Volatile
Content on the Viscosity of Digesting Sludge .... 6-61
6-27 Types of Digester Covers ...*.*.......*»*........ 6- 64
6-28 Overall View of Four Digesters With Downes
Floating Covers at Sunnyvale, California ........ 6- 65
6-29 Typical Digester Supernatant Collection
System 6- 68
XXXVI11
-------
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
-------
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
-------
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
-------
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
-------
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
-------
LIST OF FIGURES (continued)
Number Page
11-21 Cross Section of an Electric Infrared Furnace ... 11- 55
11-22 Flowsheet for Sludge Incineration in an
Electric Infrared Furnace ...... . . 11- 56
11-23 Cross Section of a Cyclonic Furnace .. ... 11- 58
11-24 Flowsheet for Sludge Incineration in a
Cyclonic Furnace . 11- 59
11-25 Design Example: Heat and Material Balance
in a Fluid Bed Furnace ....,,,,, 11- 64
11-26 Comparison of Excess Air Requirements:
Incineration vs. Starved-Air Combustion ......... 11- 66
11-27 Flowsheet for Starved-Air Combustion in a
Multiple-Hearth Furnace 11- 69
11-28 Design Example: Starved-Air Combustion
in a Multiple-Hearth Furnace 11- 82
11-29 Typical Grate-Fired Waterwalled Combustion
Unit 11- 84
11-30 Vertical Shaft Reactors 11- 87
11-31 Autogenous Combustion Requirements for
Co-Disposal , 11- 88
11-32 Flowsheet for Co-Combustion Full Scale Test
at the Central Contra Costa Sanitary District,
California . 11- 90
11-33 Flowsheet for Co-Combustion at the Western
Lake Superior Sanitary District, Duluth,
Minnesota 11- 93
11-34 Cross Section of the Watergate Furnace for
Scum Incineration 11- 97
11-35 JPL Activated Carbon Treatment System 11-101
11-36 Volatile Solids and COD Content of Heat
Treated Sludge 11-105
11-37 Flowsheet for High Pressure/High Temperature
Wet Air Oxidation . . . 11-106
11-38 Wet Air Oxidation - Electrical Energy
Requirements ........... ......,»...*.».. 11-108
11-39 React-0-Thermtm on Sludge/Liquid Waste
Destruction 11-109
11-40 Modular Controlled-Air Incinerator
Configurations .,....,........,............*..... 11-111
11-41 Pyro-Sol Limited Pyrolysis System 11-112
11-42 Bailie Process Flowsheet 11-113
11-43 Wright-Malta Process Flowshee 11-114
11-44 Air Emissions 11-115
11-45 San Francisco Bay Area Quality Management
District: Auxiliary Fuel and Oxygen
Correction ... * 11-126
CHAPTER 12
12- 1 Effect of Solids Content on the Ratio of Wood
Chips to Sludge by Volume 12- 4
xliv
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LIST OF FIGURES (continued)
Number
Page
14- 2 Comparison of Behaviors of Wastewater Sludge and
Water Flowing in Circular Pipelines 14- 6
14- 3 Friction Factor for Sludge, Analyzed as a Bingham
Plastic ••,,..*,.......,,.,,,......,,.....»...„,, 14- 7
14- 4 Friction Factors for Example Problem ............ 14- 12
14- 5 Pressure Drops for Example Problem 14- 13
14- 6 Viscometer Test of Sewage Sludge 14- 16
14- 7 Centrifugal Pump ..,,,.........,..,..,.,... 14- 18
14- 8 Torque Flow Pump , 14- 19
14-12 Progressive Cavity Pump ......................... 14- 23
14-13 Diaphragm Pump .................................. 14- 25
14-14 Rotary Pump 14- 27
14-15 Ejector Pump .... 14- 28
14-16 Belt Conveyor 14- 38
14-17 Inclined Belt Conveyor Features ................. 14- 41
14-18 Flexible Flat Belt Conveyor 14- 42
14-19 Screw Conveyor 14- 42
14-20 Tabular Conveyor 14- 43
14-21 Bucket Elevator 14- 44
14-22 Pneumatic Ejector ............................... 14- 45
14-23 Pneumatic Conveyor 14- 45
CHAPTER 15
15- 1 Solids Balance and Flow Diagram-Design Example
Single-Phase Concentration and Displacement
Storage 15- 13
15- 2 Effect of Various Operating Strategies on
Dewatering Unit Feed Rates ...................... 15- 17
15- 3 Proposed Design for Blending Digester—Sacramento
Regional Wastewater Treatment Plant 15- 21
15- 4 26,000 Gallon Sludge Equalization Tank (Typical
of Two) Aliso Solids Stabilization Facility ..... 15- 22
15- 5 Schematic Representation of a Facultative Sludge
Lagoon (FSL) 15- 25
15- 6 Typical Brush-Type Surface Mixer, Sacramento,
California 15- 27
15- 7 Typical FSL Layout 15- 29
15- 8 Typical FSL Cross Section 15- 29
15- 9 Layout for 124 Acres of FSLs--Sacramento Regional
Wastewater Treatment Plant 15- 30
15-10 Sacramento Central Wastewater Treatment Plant
Surface Layer Monitoring Data for FSLs 5 to 8 ... 15 -34
15-11 Sacramento Central Wastewater Treatment Plant
1977 Fecal Coliform Populations for Various
Locations in the Solids Treatment-Disposal
Process 15- 38
15-12 Typical Wind Machines and Barriers Sacramento,
California 15- 40
xlvi
-------
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
-------
LIST OF FIGURES (continued)
Number Paqe
14- 2 Comparison of Behaviors of Wastewater Sludge and
Water Flowing in Circular Pipelines ............. 14- 6
14- 3 Friction Factor for Sludge, Analyzed as a Bingham
Plastic .....,,,..,,.,.» 14- 7
14- 4 Friction Factors for Example Problem 14- 12
14- 5 Pressure Drops for Example Problem 14- 13
14- 6 Viscometer Test of Sewage Sludge ................ 14- 16
14- 7 Centrifugal Pump ................................ 14- 18
14- 8 Torque Flow Pump 14- 19
14-12 Progressive Cavity Pump . ,. 14- 23
14-13 Diaphragm Pump , 14- 25
14-14 Rotary Pump »*.»,.,.. 14- 27
14-15 Ejector Pump ,..,,.......... 14- 28
14-16 Belt Conveyor ., 14- 38
14-17 Inclined Belt Conveyor Features 14- 41
14-18 Flexible Flat Belt Conveyor 14- 42
14-19 Screw Conveyor .......... 14- 42
14-20 Tabular Conveyor 14- 43
14-21 Bucket Elevator 14- 44
14-22 Pneumatic Ejector .... 14- 45
14-23 Pneumatic Conveyor 14- 45
CHAPTER 15
15- 1 Solids Balance and Flow Diagram-Design Example
Single-Phase Concentration and Displacement
Storage 15- 13
15- 2 Effect of Various Operating Strategies on
Dewatering Unit Feed Rates 15- 17
15- 3 Proposed Design for Blending Digester—Sacramento
Regional Wastewater Treatment Plant 15- 21
15- 4 26,000 Gallon Sludge Equalization Tank (Typical
of Two) Aliso Solids Stabilization Facility ..... 15- 22
15- 5 Schematic Representation of a Facultative Sludge
Lagoon (FSL) 15- 25
15- 6 Typical Brush-Type Surface Mixer, Sacramento,
California 15- 27
15- 7 Typical FSL Layout 15- 29
15- 8 Typical FSL Cross Section , 15- 29
15- 9 Layout for 124 Acres of FSLs—Sacramento Regional
Wastewater Treatment Plant 15- 30
15-10 Sacramento Central Wastewater Treatment Plant
Surface Layer Monitoring Data for FSLs 5 to 8 ... 15 -34
15-11 Sacramento Central Wastewater Treatment Plant
1977 Fecal Coliform Populations for Various
Locations in the Solids Treatment-Disposal
Process 15- 38
15-12 Typical Wind Machines and Barriers Sacramento,
California ,.,..... 15- 40
xlvi
-------
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
-------
LIST OF FIGURES (continued)
Numb_er_ Page
18- 2 Schematic of Combined Boiler/Condenser System
for Hot Water Production 18- 12
18- 3 Process Schematic for Example of Energy Recovery
From Digester Gas 18- 16
18- 4 Energy Flowsheet for Example of Energy Recovery
From Digester Gas ............................... 18- 18
18- 5 Mean Molal Heat Capacities of Gases at Constant
Pressure (Mean Values From 77° to T°F) 18- 23
18- 6 Flowsheet for Example of Energy Recovery From
Incinerator Flue Gas 18- 27
18- 7 Steam Conditions for Example of Recovery of
Energy From Incinerator of Flue Gas ............. 18- 28
18- 8 Energy Flowsheet for Example of Energy Recovery
From Incinerator Flue Gas ....................... 18- 32
CHAPTER 19
19- 1 Wide Trenching Operation, North Shore
Sanitary District ............................... 19- 4
19- 2 Dewatered Sludge Landspreading, Metropolitan
Denver Sewage Disposal District No. 1,
Denver, Colorado .,«.*..*««*..<...,,,,,,,.,,,,.,, 19- 36
19- 3 Flow Diagram Sludge Management System, Colorado
Springs, Colorado 19- 41
19- 4 Overall Sludge Disposal Site Layout, Colorado
Springs, Colorado 19- 42
19- 5 Sludge Application Rate-DLD System Colorado
Springs, Colorado 19- 45
19- 6 Estimated Net DLD Area Requirements Sludge
Applied at 5 Percent Solids Concentration,
Colorado Springs, Colorado 19- 46
19- 7 Estimated Net DLD Area Requirements at Various
Sludge Concentrations, Colorado Springs,
Colorado ... 19- 47
19- 8 Sludge Application Rates by Subsurface Injection,
Colorado Springs, Colorado 19- 48
19- 9 Prototype Dredging Operation, Sacramento Regional
County Sanitation District 19- 50
19-10 Prototype Subsurface Injection Operations
Sacramento Regional County Sanitation
District ,,,, 19- 52
19-11 Flow Diagram - Projected 1992 Normal Solids
Treatment and Disposal Operation, Sacramento
Regional Wastewater Treatment Plant 19- 53
xlvi ii
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ACKNOWLEDGEMENTS
This design manual was prepared as part of the Technology
Transfer Series of the Center for Environmental Research
Information, U.S. Environmental Protection Agency, Cincinnati,
Ohio. Development, coordination and preparation were carried
out by Brown and Caldwell, Consulting Engineers, Walnut Creek,
California, with the assistance of Environmental Technology
Consultants, Inc., of Springfield, Virginia. Technical review
and coordination were provided by the Office of Water Program
Operations, USEPA, Washington, D.C. Additional technical
review and contributions were provided by Regions V and IX of
the USEPA, by the Metropolitan Sanitary District of Greater
Chicago, and by the Technical Practice Committee of the Water
Pollution Control Federation. USEPA project officers on this
manual were Dr. Joseph B. Farrell, Municipal Environmental
Research Center, and Dr. James E. Smith, Jr., Center for
Environmental Research Information, Cincinnati, Ohio.
xlix
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EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapter 1. Purpose and Scope
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
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
-------
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
-------
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
-------
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.
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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.
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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
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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.
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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.
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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
-------
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
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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
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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
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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
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dedicated land disposal. Before considering these, however, one
must determine what combinations of solids treatment processes
make sense for the site in question. . . .
TABLE 3-1
EXAMPLE OF INITIAL SCREENING MATRIX FOR
BASE SLUDGE DISPOSAL OPTIONS
Relevant criteria
'" ' ' • - Acceptable
Utilization/disposal Environmental Site for base
options Reliability impacts availability Cost alternative
Bag-market as
fertilizer 0 X X X 0
Agricultural land
(private) O X X X O
Agricultural land
(public) X X XXX
Forested land (pri-
vate) 0 X 0 0 O
Forested land (public) X X O 0 O
Give to citizens
(horticulture) 0 X X X O
Combine with commer-
cial topsoil O X X X O
Dedicated land dis-
posal X X XXX
Landfill X X XXX
0 = unacceptable.
X = acceptable.
Determine Practical Base Treatment Systems
Table 3-2 illustrates process compatibility matrix for treatment
alternatives. Incompatible processes and processes which are not
applicable in given locations are eliminated. The combination of
drying beds and mechanical dewatering, for example, is considered
incompatible because both dewatering and drying take place on the
drying bed; mechanical dewatering is not needed. On the other
hand, the combination of incineration and mechanical dewatering
of unstabilized sludge is generally compatible, but for the
hypothetical case investigated is ruled out because of air
pollution considerations. After first-cut analysis, seven base
treatment options are considered feasible and are further
evaluated.
Dgj^£giJJlg_P£agt_i_cal_gase_ Treatment/Disposal Combinations
Practical base treatment and disposal combinations are
then combined in a matrix, which is subjected to further
3-8
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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
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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
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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
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• 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
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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
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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
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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
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Lane County Solid Waste Division to MWMC, and (2) the
actual net volume of landfill required for sludge
disposal, allowing for sludge consolidation.
• For dedicated land disposal, seasonal water tables and
detailed groundwater migration patterns, as well as
private well locations and depths were determined.
• Estimates were made of comparative nitrate loadings which
would eventually reach the Willamette River from treated
landfill leachate discharge; from groundwater migration
from dedicated land disposal; and from filtrates from
mechanical sludge dewatering (which is subsequently
discharged with the effluent).
• Transportation modes were analyzed in detail and costed
for various sludge solids concentrations and transport
routes and distances.
These detailed analyses still left a number of viable base
alternatives. At this point, other less tangible factors
were considered. These were (1) that the chosen base
alternative(s) be compatible with desired secondary alternatives,
and (2) that flexibility and reliability be provided through the
use of parallel systems. After intensive screening, it was
decided that two base alternatives would be used: spreading of
liquid sludge on dedicated land and open-air drying followed
by landfill disposal. Both alternatives included force main
transport of digested sludge from the regional treatment plant to
a remote sludge management site, where the sludge was to be
stored in facultative sludge lagoons. Liquid sludge would be
spread on dedicated land at the sludge management site. Dried
sludge would be trucked to landfill. Operations associated with
disposal (spreading, drying, and landfilling) would be carried
out during dry weather. These systems provide the desired
flexibility and reliability and are compatible with preferred
secondary alternatives.
Several variations of sludge utilization on land were adopted
as secondary alternatives, since there was a strong feeling
that sludge should be used beneficially. The alternatives of
particular interest to the Eugene-Springfield area included
agricultural use on private farm land, use for ornamental
horticulture, in nurseries and public parks, and use in a mixture
with commercial topsoils in. landscaping. Sludge would be
provided to these outlets as the market demands. Variable demand
is particularly important in Oregon's Willamette Valley, where
prolonged winter rainfall and summer harvesting schedules control
the timing of agricultural sludge use.
The flowsheet for the Eugene system is shown on Figure 3-6.
3-16
-------
PRIMARY
SLUDGE
WASTE -
ACTIVATED
SLUDGE
GRAVITY
THICKEN
FLOTATION
THICKENER
ANAEROBIC
DIGESTERS
LONG
DISTANCE
PIPELINE
FACULTATIVE
STORAGE
LAGOONS*
AT SLUDGE MANAGEMENT SITE
ON PRIVATE FARMLAND
CITIZEN PICK-UP AT SLUDGE
MANAGEMENT SITE
DEDICATED
LAND
DISPOSAL*
DRYING
BEDS*
TRUCK
TRUCK
LANDFILL
AGRICULTURE
FIGURE 3-6
FLOWSHEET FOR THE EUGENE-SPRINGFIELD
SLUDGE MANAGEMENT SYSTEM
The ability to use base facilities and equipment for desired
secondary alternatives was a major consideration in selecting the
the force main, sludge lagoons, and
be used for dedicated land disposal
also required for agricultural use.
sludge from the sludge management site
however, be an additional expense for
base system. In Eugene,
application equipment to
of the liquid sludge are
Trucks to transport liquid
to agricultural sites will,
the secondary alternatives.
It is hoped that eventually all sludge can be utilized on
land. As indicated, however, in Table 3-5, full agricultural
utilization of sludge is estimated to be more costly than either
of the pure disposal options. This is because more equipment is
needed to transport sludge to and spread it on the agricultral
sites than is needed for the pure disposal options. Thus, as of
1979, any system which even partially incorporates agricultural
utilization will be more costly than pure disposal options. This
could change if the farmers can be persuaded to pay for the
sludge.
TABLE 3-5
ESTIMATED COSTS OF ALTERNATIVES FOR EUGENE-SPRINGFIELD
Sludge form
Alternative
Total annual cost,
million dollars
Liquid
Dried
Dedicated land disposal only
Agricultural utilization only
Landfill only
Agricultural utilization only
1.03
1.53
1.14
1. 32
3-17
-------
At the time this manual was written (1979), MWMC was involved in
public hearings aimed at selecting a suitable sludge management
site.
3.4 The Quantitative Flow Diagram
Overall system performance is the sum of the combined
performances of the system's linked processes. This is nowhere
more clearly expressed than on a Quantitative Flow Diagram (QFD).
The QFD is used to estimate loadings to the various wastewater
treatment, solids treatment, and solids disposal processes. The
QFD is the starting point for understanding process interactions
and is nothing more than a materials balance. Although balances
can be struck for components like nitrogen, phosphorus and
chemical oxygen demand, the most useful balances are those
for suspended solids. The QFDs to be presented here are for
suspended solids. Each flowsheet has its own unique set of
balance equations. In the following pages, mass balances for a
specific, rather simple flowsheet are derived, thus illustrating
the technique. The mass balance equations are then summarized
in tabular form. Mass balance equations for a more complex and
more common flowsheet are later presented, without derivation.
Two worked QFDs are presented as examples. The intent is to
demonstrate the usefulness of the method.
3.4.1 Example: QFD for a Chemically Assisted
Primary Treatment Plant
The flowsheet for a chemically assisted primary wastewater
treatment plant with anaerobic digestion and mechanical
dewatering of the sludge is shown on Figure 3-7. In this
example chemicals are added to enhance the sedimentation process.
Sidestreams from the digester and dewatering units are recycled
to the primary sedimentation basin. The calculation is carried
out in a step-by-step procedure:
1. Draw the flowsheet (as on Figure 3-7).
2. Identify all streams. For example, stream A contains raw
sewage solids plus chemical solids generated by dosing
the sewage with chemicals. Let the mass flow rate of
solids in Stream A be equal to A Ib per day.
3. For each processing unit, identify the relationship of
entering and leaving streamsto one another in terms of
mass. For example, for the primary sedimentation tank,
let the ratio of solids in the tank underflow (E) to
entering solids (A •+ M) be equal to XE. XE is actually
3-18
-------
an indicator of solids separation efficiency. The
general form in which such relationships are
expressed is:
mass of solids in stream 6
mass of~~solids entering the unit
P J
For example, Xp = =— - — 5-, Xj = p- . The processing unit's
performance is specified when a value is assigned to XQ.
DEGRITTED SEWAGE
SOLIDS
SOLIDS
GENERATED
BY CHEMICAL
ADDITION
M
PRIMARY
SEDIMENTATION
B
SUPERNATANT
FILTRATE
DIGESTION
1
DEWATERING
EFFLUENT
SOLIDS
DESTROYED
•^ (CONVERTED
TO GAS AND
WATER)
CONDITIONING
'CHEMICALS
TO ULTIMATE
DISPOSAL
FIGURE 3-7
BLANK QFD FOR CHEMICALLY-ASSISTED
PRIMARY PLANT
3-19
-------
Combine the mass balance relationships^ so as to reduce
them to one equation describing a specific stream in
terms of given or known quantities. In the calculation
to be presented, expressions will be manipulated until E,
the primary solids underflow rate, can be expressed in
terms of A, XE , X j , X^ / Xp, and X$ , quantities which the
designer would know or assume from plant influent surveys,
knowledge of water chemistry and an understanding of the
general solids separation/destruction efficiencies of the
processing involved. The calculation is carried out as
follows:
a. Define M by solids balances on streams around the
primary sedimentation tank:
(3-D
Therefore,
M = -- A (3-2)
XE
b. Define M by balances on recycle streams:
M = N + P (3-3)
N = XNE (3-4)
P = XP(S + K) (3-5)
S = XSK (3-6)
Therefore,
P = XP(1 + XS)K (3-7)
K + J + N = E (3-8)
Therefore,
K=E-J-N=E- XjE - XNE = E(l - Xj - XN ) (3-9)
and
P = XPE(1 - Xj - XN)(1 + XS) (3-10)
Therefore,
M = E[XN + XP(1 - Xj - XN)(1 + XS)] (3-110
3-20
-------
c. Equate equations (3-2) and (3-11) to eliminate M:
- - A = E[XN + Xp(l - Xj - XN)(1 + Xs)j
_
- XN - xp(i- Xj - xN)(i + xs)
E is now expressed in terms of assumed or known
influent solids loadings and solids separation/
destruction efficiencies.
Once the equation for E is derived, equations for other streams
follow rapidly; in fact, most have already been derived. These
are summarized in Table 3-6.
TABLE 3-6
MASS BALANCE EQUATIONS FOR FLOWSHEET OF FIGURE 3-7
A
E =
* - XN - XP (1-XJ-V(1 + V
M = ^ - A
E
B - d-X) (A + M)
J - XTE
J
N = XNE
K - E(1-X -X )
S = XSK
p = xp (1 + XS)K
L - K (1 + Xg)(l-Xp)
3-21
-------
DEGRITTED SEWAGE
SOLIDS
299,000
409,000
110,000
SOLIDS
GENERATED
BY CHEMICAL
ADDITION
M
56,030
0
56,030
PRIMARY
SEDIMENTATION
XE = 0.90
E
418,527
DIGESTION
Xj = 0.25
XN = 0.0
I K
313,895
i
DEWATERING
XP = 0.15
X = 0.19
317,505
T
TO ULTIMATE
DISPOSAL
46,503
104,632
-^ EFFLUENT
59,640
SOLIDS
DESTROYED
(CONVERTED
TO GAS AND
WATER)
CONDITIONING
CHEMICALS
ALL QUANTITIES ARE
EXPRESSED IN POUNDS
PER DAY
1 Ib/day = 0.454 kg/day
FIGURE 3-8
QFD FOR CHEMICALLY-ASSISTED
PRIMARY PLANT
3-22
-------
Figure 3-8 is a worked example in which all solids flow rates
are calculated. For this example the following information was
provided:
a. Based on estimates from facility planning studies,
average influent suspended solids loading is
299,000 pounds per day (136 t/day). Alum is added to
the degritted raw sewage to increase capture. The
chemical solids generated as the result of alum
addition is estimated at 110,000 pounds per day
(50 t/day). The latter figure is derived from pilot
work at Seattle, Washington, where the ratio of new
solids generated/solids in untreated raw sewage was
0.37/1 when alum ( A12 ( 804 ) 3 • 1 4H20 ) additions of
110 to 125 mg/1 were added to raw wastewater (1).
Therefore, A = 299,000 (1 + 0.37) = 409,000 pounds
per day (185 t/day).
b. Primary sedimentation solids capture is 90 percent of
the sum of sewage solids, chemical solids and recycle
solids which enter the basin. Note that solids
capture as usually computed (sewage solids basis
only) is only 84.4 percent, i.e.,
, -, _ effluent suspended solids. -,„,,
influent sewage solids
10° = 84-4 percent
c. Twenty-five percent of the suspended solids fed to
the digestion system are destroyed, i.e., converted
to gas or water (Xj = 0.25). The number assumed is
somewhat less than the usual value used (0.30-0.40),
since the biodegradable fraction of digester feed in
this instance is low because of the large proportion
of chemical solids present.
d. Digesters are not supernated (X^ = 0.0).
e. Solids capture in the dewatering units is 85 percent
(XP = 0.15) .
f. Conditioning chemicals are 19 percent by weight
of digested sludge fed to the dewatering units
(XS = 0.19).
When all loadings are expressed quantitatively and superimposed on
the flowsheet, the designer can begin to develop a feel for the
process. The effects of recycle loading and individual process
efficiencies on overall process performance can be assessed by
manipulation of the variables. Calculations can be done very
3-23
-------
rapidly when the mass balance equations (presented in Table 3-6)
are set up for solution on a computer or a programmable
calculator.
The investigator must exercise judgment in estimating the various
process efficiencies ( X@ ) . For example, one should assume
reduced efficiencies for primary sedimentation if recyle streams
contribute large quantities of solids to the sedimentation tank,
since recycled solids tend to be less easily removed than fresh
solids from the sewer system. Their mere presence in the recycle
stream is an indication of the difficulties in separating them.
3.4.2 Example: QFD for Secondary Plant with Filtration
The example just worked was relatively simple. Figure 3-9 shows
a jno_r_e_ comp 1 e x sy s tern— second ary ____ ae r obi c _ biologies It re a t me n t
followed by filtration. Mass balance equations for this system
are summarized in Table 3-7. For this flowsheet the following
information must be specified.
a. Influent solids (A).
b. Effluent solids (Q), that is, overall suspended solids
removal must be specified.
c. XE , XQ , X j , XN , XR, and Xg are straightforward
assumptions about the degree of solids removal,
addition or destruction.
d. XD, which describes the net solids destruction
reduction or the net solids synthesis in the
biological system, must be estimated from yield
data (see Section 4.3.2.4). A positive XD signifies
net solids destruction. A negative XD signifies net
solids growth. In this example 8 percent of the
solids entering the biological process are assumed
destroyed, i.e., converted to gas or liquified.
Note that alternative processing schemes can be evaluated simply
by manipulating appropriate variables. For example:
a. Filtration can be eliminated by setting XR to zero.
b. Thickening can be eliminated by setting XG to zero.
c. Digestion can be eliminated by setting Xj to zero.
d. Dewatering can be eliminated by setting Xp to zero.
e. A system without primary sedimentation can be
simulated by setting XE equal to approximately
zero, e.g. , 1 x 10~8. x^ cannot be set equal
to exactly zero, since division by Xg produces
indeterminate solutions when computing E.
3-24
-------
DEGRITTED SEWAGE
SOLIDS
A
299,000
f M
77,794 "
4 N
r 15,415
P
22,012
i
_ TREATMENT 0
CHEM1CALS D SOLIDS DESTROYED
10,831 OR SYNTHESIZED
— 40,367 ^ |R
PRIMARY
SEDIMENTATION
XE = 0.65
1
251,438
' 1
DIGESTION
XN = 0.05
Xj = 0.35
\
DEWA1
V
Xp
K
184,978
p
"ERING
0.10
L
198,111
SECONDARY
8 » -cn^MT^TinN c » FILTRATION Q fc
135,390*" ^D"VlCArJKATION 57,667*" XR = 0.70 17,300*
**"-• XD = 0.08
G F
10,034 66,892
1 '
THICKENING
X_ = 0.15
b
H
56,858
j SOLIDS DESTROYED
1U/,9U4 GAS AND WATER)
S CONDITIONING
35,146 "CHEMICALS
AM HI 1AM
EFFLUENT
TO ULTIMATE
DISPOSAL
EXPRESSED IN POUNDS
PER DAY
1 Ib/day = 0.454 kg/day
FIGURE 3-9
QFD FOR SECONDARY PLANT WITH FILTRATION
A set of different mass balance equations must be derived
if flow paths between processing units are altered. For example
the equations of Table 3-7 do not describe operations in
which the dilute stream from thickener (stream G) is returned
to the biological system instead of the primary sedimentation
tank.
3-25
-------
TABLE 3-7
MASS BALANCE EQUATIONS FOR FLOWSHEET OF FIGURE 3-9
E =
A - (r^)(Y - V
? - a - 3 (Y)
XE
Where a = Xp (1 - Xj - XN)(1 +.Xg) + XN
(1 - X_). (1 - X_)
D _ ^ . _^_
P - v ~—
XE
Y - XG + a (1 - XQ)
B = (1 - XE)E
XE
1 - XR
D = XDB
F - 3 E - T-2-
G = XQF
H = (i - XG)F
J = XT (E + H)
J
K = (1 - Xj - XN) (E|-+" H)
L = K (1 + Xg) (1 - Xp)
M = ^ - G - A
E
N = X.T (E + H)
P = Xp (1 +•Xg)K
XR
R =l^Q
1 XR
S = XSK
3-26
-------
3.5 Sizing of Equipment
The QFD described in the previous section can be an important aid
to a designer in predicting long-term (i.e., average) solids
loadings on sludge treatment components. This allows the
designer to establish such factors as operating costs and
quantities of sludge for ultimate disposal. However, it does not
establish the solids loading which each equipment item must be
capable of processing. A particular component should be sized to
handle the most rigorous loading conditions it is expected to
encounter. This loading is usually not determined by applying
steady-state models (e.g., QFD calculations) to peak plant loads.
Because of storage and plant scheduling considerations, the rate
of solids reaching any particular piece of equipment does not
usually rise and fall in direct proportion to the rate of solids
arriving at the plant headworks. Consider a system similar in
configuration to that shown on Figure 3-9. If maximum solids
loads at the headworks (Stream A) are twice the average value,
it does not necessarily follow that at that instant maximum
dewatering loads (Stream K) are twice the average dewatering
load.
To pursue this further, consider the design of a centrifuge
intended to dewater anaerobically digested primary and secondary
sludge at a small treatment plant. The flow scheme is similar to
that shown on Figure 3-9. The plant is staffed on only one shift
per day, seven days per week. The digesters are complete-mix
units equipped with floating covers. Because of the floating
covers, digester volume can vary. Secondary sludge is wasted
from the activated sludge systems to a dissolved air flotation
thickener prior to digestion whenever operators are available to
operate the thickener.
As indicated, the average loadings to the centrifuge can be
defined by the QFD, but computation of the necessary centrifuge
capacity requires analysis of both the load dampening effect of
the storage in the digesters and the plant operating schedule.
During periods of peak plant solids loadings, loads to the
dewatering units may be attenuated by storing portions of the
peak loadings within the digester. This can be done by either
mechanism 1 or mechanism 2 below, acting either singly or in
concert.
1. Digester volume is increased by allowing the digester
floating cover to rise.
2. Solids are allowed to concentrate and thus accumulate
within the digester (See Chapter 15, Section 15.2.2.2 for
example of storage by mechanism 2).
The effect of both mechanisms 1 and 2 is storage within the
digester of part of the load which would otherwise go to the
centrifuge. Thus peak dewatering loads will not be 2.5 times
the average when peak solids mass withdrawn from primary and
3-27
-------
secondary sedimentation tanks are 2.5 times the average, but
something less, for example, only 1.4 times the average value.
The degree of load dampening is a direct function of the size and
operating configuration of the digester.
Since the centrifuge will only operate when attended, the
"design" loading must account for this factor. The centrifuge
must be either capable of processing, during one shift, all the
sludge which must be extracted from the digester during the peak
day (for example, 1.4 times average quantity) or the operators
must dewater sludge for longer than one shift per day. A
judgment would be needed at this point whether to pay for
increased equipment capacity or operator overtime to handle the
peak loads. With no operator overtime, the "design" centrifuge
capacity would have to be 1.4 x 24/8 =4.2 times the average
daily digested sludge production to account for both the effect
of sludge peaking, storage volume and only one operations shift
per day.
Note that the dissolved air flotation thickener would need to be
designed for 24/8 x 2.5 = 7.5 times the average daily rate of
waste activated sludge production if it is assumed no upstream
storage is available for dampening thickener loadings, the
thickener itself has no storage capacity, and the thickener is
only operated one shift per day.
The foregoing example shows the influence of solids peaking,
storage volume and operating strategy on the selection of
design loadings for a particular sludge handling process.
Several other factors are important in selecting the capacity a
unit must have, including:
• Uncertainties. When systems are designed without the
benefits of pilot or full-scale testing, actual sludge
quantities and characteristics as well as efficiencies of
the sludge handling system components may not be known
with certainty. The degree and potential significance
of the uncertainties must be considered when developing
design criteria. This usually has the effect of
introducing a safety factor into the design so that
reliable performance can be obtained no matter what
conditions are encountered in the full-scale application.
The magnitude of the safety factor must be determined by
the designer, based on his judgement and experience.
• Equipment reliability. Greater capacity or parallel
units must be specified if there is reason to believe
that downtime for any particular units will be high.
• Sensitivity of downstream components. If losses in
efficiency of a particular sludge handling component
at peak loading conditions would cause problems for
downstream processes, this upstream process should
3-28
-------
be designed conservatively. Conversely, if reduced
efficiency could be tolerated, design need not be so
conservative.
3.6 Contingency Planning
As indicated previously, flexibility to cope with unforeseen
problems is highly desirable in any wastewater solids management
system. Such problems and possible solutions include:
• Equipment breakdowns. Downtime may be minimized by
having maintenance people on call, by advance purchase of
key spare parts, by providing parallel processing units
and by making use of storage.
• Solids disposal problems. These may include closures of
landfills, unwillingness of current users to further
utilize sludge, failure of a process to provide a sludge
suitable for utilization, strikes by sludge transporters,
and inability to dispose of sludge due to inclement
weather. Disposal problems can be reduced by providing
long-term storage and/or more than one disposal
alternative.
• Sludge production greater than expected. In some
instances this may be dealt with by operating for more
hours per week than normal or by using chemicals to
modify sludge characteristics, thus increasing solids
processing capacity.
Because of these factors, it is desirable to have more than one
process for sludge treatment and disposal. Often it is possible
to add considerable flexibility with modest investment. Backup
or alternative wastewater solids treatment processes often have
higher operating costs per ton of sludge processed than the
primary processes. This is acceptable if the alternative process
is not frequently needed and can be provided at minimum capital
cost.
3.6.1 Example of Contingency Planning for Breakdowns
Assume the plant is a 10 MGD
sludge thickening, anaerobic
dewatering as shown on Figure
include:
activated sludge facility with
digestion, and digested sludge
3-10. Pertinent design details
!• The waste activated sludge (WAS) thickener can be
operated with or without polymers. If polymers are used,
a more concentrated sludge can be produced. WAS can be
diverted to the headworks if the WAS thickener is removed
from service.
3-29
-------
WASTE ACTIVATED SLUDGE WHEN THICKENER IS INOPERATIVE
r
PRELIMINARY ^
PRIMARY . .. ACTIVATED _ DISINFECTION
p TREATMENT *^^ SEDIMENTATION " "" SLUDGE ' " DISCHARGE
GRIT, ETC.
SIDESTREAM
800 Ib/day
1
SIDESTREAM
1,000 Ib/day
1
THICKENER FEED
9,000 Ib/day
1.0% SOLIDS
r 108,000 gpd
I SLUDGE
THICKENER
PRIMARY 1 1 N PI F R F 1 OW
PRIMARY SLUDGE ONLY
10,000 Ib/day
"5.0% SOLIDS
24,000 gpd
PRIMARY SLUDGE + W.A.S.
18,000 Ib/day
2.5% SOLIDS
86,000 gpd
*
f I I
DIGESTER DIGESTER
1 2
THICKENED W.A.Sr(8,000 Ib/day)
NO POLYMER
3.5% SOLIDS
27,000 gpd
WITH POLYMER
4.5% SOLIDS
2 1,000 gpd
""^ DIGESTED SLUDGE (11,000 Ib/day)
SLUDGE
STOCKPILE
1 1
DEWATERING DEWATERING
UNIT UNIT
1 2
* *
DEWATERED CAK
33.9 yd3 @ 17
^ _ OR
""* 26.2 yd 3@ 22
1 i
D
E (10.200 Ib/dav)
% SOLIDS
% SOLIDS
TO LANDFILL
VIA 16 yd3 TRUCK
1 Ib/day = 0.454 Kg /day
1 gpd = 0.00378 m3/day
,3
FIGURE 3-10
CONTINGENCY PLANNING EXAMPLE
3-30
-------
2. Two complete-mix digesters with floating covers are
provided. Each digester has a net volume of
610,000 gallons (2,310 m3 ) at minimum cover height.
Net volume at maximum cover height is 740,000 gallons
(2,803 m3), thus total digester storage volume is
2 (740,000-610,000) = 260,000 gallons (984 m3). The
digesters are not supernated.
3. Two dewatering units are provided. Each unit, when fed
at 90 gpm (40.8 m3/hr) can produce a 22 percent solids
cake. When the dewatering units are fed at 110 gpm
(49.9 m3/hr) a 17 percent solids cake is produced. The
units are fed at 90 gpm (40.8 m3/hr) unless conditions
dictate otherwise. The bulk density of each cake is
65.5 pounds per cubic foot (1,050 kg/m3).
4. The cake is trucked to ultimate disposal. Each truck
holds 16 cubic yards (12 m3) of cake.
5. A dewatered sludge storage area of capacity 750 cubic
yards (574 m3) is available.
6. Weekends are 2.7 days long (from 5 p.m. Friday to 8 a.m.
Monday).
CjiS_e__A. All units available:
1. Digester detention time = ( ^QOQ^^OOof gpd = 24 days'
2. Dewatering operation:
a. Weekly sludge feed = 7 (24,000 + 27,000 gpd)
= 357,000 gallons (1,350 m3).
b. Hourly throughput = 2 x (90 gpm) (60 min/hr)
= 10,800 gal per hr (40.8 m3/hr).
. , t 357,000 gal
c. Operation is carried out over 10,800 gai/hr
= 33 hours per week.
d. 26.2 cubic yards (20.0 m3) of 22 percent solids
sludge cake is produced each day.
3. If dewatering is not operated over the weekend, then
51,000 gpd (2.7 days) = 138,000 gal (522 m3) of digested
sludge must be stored in the digesters during this
period. Available storage which can be obtained
by letting the floating cover rise is 260,000 gallons
(983 m3 ) . Therefore digester storage capacity is
adequate for weekend storage, including long (3.7 day)
weekends.
3-31
-------
4. Truckloads required to haul dewatered cake = 26>2 yd3/day
, , , , n , , ,,, 16 yd3/truck
=1.6 truckloads per day (11 per week).
In summary, the dewatering operation can be carried out in
a normal 5-day, 8-hour-per-day week. Time is available for
start-up and shutdown and for providing good supervision.
Digester detention time is more than adequate for good digestion.
Case B. Thickener is out of service. All other units are
available. Waste activated sludge is diverted to the plant
headworks and is subsequently removed in the primary
sedimentation tank.
1. Digester detention time = - (86°ooo° pd^ = 14 days;
short, but tolerable.
2. Dewatering operation:
a. Weekly sludge feed = 7 (86,000 gpd) = 602,000 gal
(2280 m3).
b. Hourly throughput. At 90 gallons per minute,
throughput is 10,800 gallons per hr (40.8 m3/hr). At
110 gallons per minute, throughput is 13,200 gallons
per hr (49.9 m3/hr).
c. Operating hours required. At 90 gallons per minute
„« „ T /, , • „ ^ • u 602,000 gal
(40.8 mj/hr), required operating hours = IQ 800 qph
= 56 hours per week. This requires substantial
overtime or a second shift. At 110 gallons per
minute (49.9 m3/hr), required operating hours =
gau = 46 hours per week. This reduces the
on u
13,200 gph
amount of overtime required.
d. If the dewatering units operate at 90 gallons per
minute (40.8 m3/hr), 26.2 cubic yards per day
(20.0 m3/day) of 22 percent cake is produced.
Operation at 110 gallons per minute (49.9 m3/hr)
produces 33.9 cubic yards per day (25.9 m3/day ) of
a 17 percent solids sludge cake.
3. If dewatering units are not rurt on weekends, 86,000 gal/
day x 2.7 days = 232,000 gallons (878 m3 ) must be stored
in the digesters. Digester storage capacity is adequate
for normal weekends, but not long weekends.
4.
For 22 percent cake, 11 truckloads per week are required.
For 17 percent cake, 15 truckloads per week are required.
3-32
-------
In summary, loss of the thickener reduces digester detention
time, increases required dewatering unit operating time and the
amount of trucking required for disposal of cake. The operation
can be managed, but with more difficulty. This example also
illustrates the value of the thickener.
Case C. One digester is out of service. All other units are
operating:
1. Digester detention time = 24,000°;°2?, 0^ gpd = 12 d^s«
This is only marginally adequate. By using polymers in
the thickener, assume waste activated sludge thickness
is increased from 3.5 to 4.5 percent. Detention time is
increased to 24,000°|Q21,000 gpd = 14 da^s' sti11 short'
but an improvement.
2. Dewatering operation. This is not greatly affected by
loss of the digester. It can still be operated with a
single shift and a 22 percent cake can can be produced.
3. Weekend storage. Without polymer addition to
the thickener, required storage volume is 2.7 days
x 51,000 gpd = 138,000 gallons (522 m3) . One digester
(130,000 gallons or 492 m3 ) has inadequate storage
and a dewatering machine must be run part of the
weekend. If polymer is used, required storage = 2.7
x 45,000 = 122,000 gallons (462 m3) . One digester is
marginally adequate for storage.
4. Eleven (11) truckloads per week are required to transport
the sludge cake.
In summary loss of a digester can be compensated for by using
polymer in the thickener.
Case D. One dewatering machine is out of service. All other
units are available.
1. Digestion is not affected.
2. Dewatering operation. Try the following alternatives:
a. Feed rate 90 gallons per minute (40.8 m3/hr).
51,000 gpd _ Q 4
Required operating time = 90 gpm (60 min/hrr ~ 9>4
hours per day, every day, excluding start-up and
shutdown time.
b. Feed rate is 110 gallons per minute. Required
operating time = gpm° (SO^in/hr) = 7'8 hours/day,
every day, excluding start-up and shutdown time.
3-33
-------
c. Try adding polymers to thickener and maintaining a
110 gallons per minute feed rate to the dewatering
units. Required operating time = ,,A 45/OQ0 9Pd
110 gpm (60 min/hr)
= 6.8 hours per day, every day, excluding start-up
and shutdown times.
3. Weekend digester storage is not an issue as dewatering
units must be run seven days a week.
4. Eleven (11) truckloads are required to transport
22 percent cake, 15 truckloads are required for
17 percent cake.
In summary, loss of one dewatering unit will require operation of
the remaining unit for seven days a week. overtime costs will be
high.
Case E. Truck strike lasting a month. Assuming 22 percent cake,
sludge, accumulates at about 25 cubic yards (19 m3) a day. The
sludge storage area stockpile must, therefore, be able to store
about 25 (30) = 750 cubic yards (570 m3 ) of sludge to avoid major
problems due to the strike. Odors from the stockpile could be
a problem.
Conclusion: The system as designed should be able to handle
contingencies.
3.7 Other General Design Considerations
3.7.1 Site Variations
Characteristics such as size and location of the plant and
solids disposal sites strongly influence the nature and cost of
treatment and disposal systems.
• Disposal may often be accomplished on land, thus
eliminating expensive dewatering, provided adequately
sized sites are within reasonable distances from the
treatment plant. However, dewatering is usually required
if the amount of land available for sludge disposal is
limited or if the sludge must be trucked long distances
for disposal. Sufficient land also permits long-term
storage in faculative lagoons, which can also provide
some inexpensive disinfection.
• Zoning regulations are different for different sites.
• Locations near waterways and railroads provide
opportunities for barge and rail transportation of
sludges and supplies.
3-34
-------
• Structures are less costly if foundation conditions are
good. Quite often, however, wastewater treatment plants
are located in valley bottoms, tidelands, or reclaimed
landfills where expensive foundations are required.
• Costs for labor, electricity, freight on chemicals, and
trucking can vary markedly from one region to another.
Because of these variations, the best alternative for one site is
often not the best at another site. Also, reported capital and
operating costs from one site must be carefully adjusted before
being used at another site.
3.7.2 Energy Conservation
As fossil fuel supplies become more scarce and more expensive,
energy conservation becomes increasingly important. The designer
should employ energy-efficient processes and recover energy from
sludges and sludge by-products, where practical.
The following points should be considered in the design of
energy-utilization processes:
• Energy from high temperature sources is generally more
useful than energy derived from low temperature sources,
since it can be put to a wider variety of uses.
• The evaporation of water in dryers and furnaces, consumes
large amounts of energy. Such processes should therefore
be provided with a we 11-dewatered sludge. Inert
materials such as, chemicals or ash used to condition
sludge for dewatering are, however, also energy consumers.
• Energy required for digestion and thermal conditioning is
minimized where thickening is used to reduce the water
content of process feed sludges.
• Trucking energy can be reduced if haul distances are
short and the sludge is well-dewatered.
• Energy is required for the manufacture and transportation
of chemicals. Therefore, chemicals should be added in
minimum amounts that are consistent with good operation.
Whenever possible chemicals should be employed which
require the least energy to produce and transport.
• Costs saved by reducing peak energy demands can be
subtantial. In some instances, a treatment plant's
electrical bills are largely determined by peak energy
loadings, as opposed to total energy consumed. The
designer should actively seek solutions to reduce
peak energy demand. Energy recovered from sludge and
sludge-derived fuel can be used for this purpose.
3-35
-------
• 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
-------
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
-------
3.7.4 Checklists
The following checklists provide information a designer must
have to design wastewater solids treatment and disposal systems.
Three checklists are provided.
1. A Solids Properties Checklist appears in Table 3-8. This
checklist summarizes required information concerning raw
solids entering the solids treatment system and solids
produced in the various processes and operations.
2. A Process Design Checklist appears on Table 3-9. This
checklist describes information necessary to select and
design sludge treatment and disposal processes.
3. A Public Health and Environmental Impact Checklist
appears in Table 3-10. This checklist summarizes key
interactions that must be resolved between proposed
process and the surrounding environment.
TABLE 3-10
PUBLIC HEALTH AND ENVIRONMENTAL
IMPACT CHECKLIST
1. Control of vectors (bacteria, parasites, virus, flies, rats)
2. Odor
3. Air pollution
4. Groundwater contamination
5. Surface water contamination (by run-off)
6. Soils contamination
7. Land use
8. Social-economic
9. Utilization (sludge or byproducts used beneficially)
10. Occupational safety
11. Risk of accidents involving the public
12. Control of potentially hazardous substances
13. Effects on biota including transfer and accumulation of pollutants
in the food chain
14. Use of material resources
3-3!
-------
Designers should refer frequently to these
that all relevant topics are given proper
planning stages and system design. An
checklists dealing with wastewater solids
been prepared for EPA (4). The checklists
as aids for the review of facility plans
designs and specifications and the writi
maintenance manuals.
checklists to assure
consideration during
extensive series of
management has also
are intended to serve
, for preparation of
ng of operations and
3.8 References
1. Brown and Caldwell.
IV, Chemical Treatment.
Metropolitan Seattle.
1978.
West Point Pilot Plant Study: Volume
Prepared for the Municipality of
Seattle, Washington 98101. December
2. USEPA. Energy Conservation in Municipal Wastewater Treatment.
Office of Water Program Operations. Washington, D.C., 20460.
EPA 4-30/9-77-011. March 1978.
3. Federal Register. "Cost-Effectiveness Analyses." 40 CFR 35-
Appendix A. September 1975.
4. USEPA. Evaluation of Sludge Management Systems: Evaluation
Checklist and Supporting Commentary. (in draft). Office of
Water Program Operations. Washington, D.C. 20460. August 1,
1978.
5.
Grant, E
Economy.
1964
L. and Ireson, W.G. Principles of Engineering
Fourth edition. New York; Ronald Publishing Co.
Peters, M.S. and Timmerhaus, K.D. Plant Design and Economics
for Chemical Engineers. New York. McGraw-Hill Book Co.
1962.
3-39
-------
EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapter 4. Wastewater Solids Production
and Solids
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 4
WASTEWATER SOLIDS PRODUCTION
AND CHARACTERIZATION
4.1 Introduction
This chapter principally discusses the quantities and properties
of sludges produced by primary biological and chemical wastewater
treatment processes. Screenings, grit, scum, septage, and
other miscellaneous wastewater solids, including the sludge
produced in the treatment of combined sewer overflows, are
discussed briefly.
4.2 Primary Sludge
Most wastewater treatment plants use primary sedimentation to
remove readily settleable solids from raw wastewater. In a
typical plant with primary sedimentation and a conventional
activated sludge process for secondary treatment, the dry weight
of primary sludge solids is roughly 50 percent of that for the
total sludge solids. For several reasons, primary sludge is
usually easier to manage than biological and chemical sludges.
First, primary sludge is readily thickened by gravity, either
within a primary sedimentation tank or within a separate gravity
thickener. In comparison with biological and many chemical
sludges, primary sludge with low conditioning requirements can be
mechanically dewatered rapidly. Further, the dewatering device
will produce a drier cake and give better solids capture than it
would for most biological and chemical sludges.
4.2.1 Primary Sludge Production
4.2.1.1 Basic Procedures for Estimating
Primary Sludge Production
Primary sludge production is typically within the range of 800 to
2,500 pounds per million gallons (100 to 300 mg/1) of wastewater.
A basic approach to estimating primary sludge production for a
particular plant is by computing the quantity of total suspended
solids (TSS) entering the primary sedimentation tank and assuming
an efficiency of removal. When site-specific data are not
available for influent TSS, estimates of 0.15 to 0.24 pound per
capita per day (0.07 to 0.11 kg/capita/day) are commonly used
(1). Removal efficiency of TSS in the primary sedimentation tank
4-1
-------
is usually in the 50 to 65 percent range (2). An efficiency of
60 percent is frequently used for estimating purposes, subject to
the following conditions:
• That the sludge is produced in treatment of a domestic
wastewater without major industrial loads.
• That the sludge contains no chemical coagulants or
flocculents.
• That no other sludges—for example, trickling filter
sludge—have been added to the influent wastewater.
• That the sludge contains no major sidestreams from sludge
processing.
As an example, if a designer estimates the TSS entering
the primary clarifier as 0.20 pound per capita per day
(0.09 kg/capita/day, and the removal efficiency of the clarifier
as 60 percent, the estimated primary sludge production is
0.12 pound per capita per day (0.054 kg/capita/day).
If relevant data are available on influent wastewater suspended
solids concentrations, such data should, of course, be used for
design purposes. Estimates of TSS removal efficiency in primary
sedimentation tanks may be refined by use of operating records
from in-service tanks or by laboratory testing. The "Standard
Methods" dry weight test for settleable matter estimates under
ideal conditions the amount of sludge produced in an ideal
sedimentation tank (3). Sludge production will be slightly lower
in actual sedimentation tanks.
4.2.1.2 Industrial Waste Effect
Suspended solids removal efficiency in primary sedimentation
depends to a large extent on the nature of the solids. It
is difficult to generalize about the effect that industrial
suspended solids can have on removal efficiency, but an example
illustrates that the effect can sometimes be dramatic. At
North Kansas City, Missouri, a municipal plant serves residential
customers and numerous major industries, including food
processing, paint manufacturing, soft-drink bottling, paper
manufacturing, and grain storage and milling. Raw wastewater
entering the plant had a 15-day average suspended solids
concentration of 1,140 mg/1 that was attributable to the
industries. Primary sedimentation removed 90 percent of these
solids. The quantity of primary sludge was, therefore, about
8,000 pounds per million gallons (1,000 mg/1) of wastewater
treated. This value is several times the normal one for
domestic wastewater. On two of the 15 days, removal exceeded
14,000 pounds per million gallons (1,700 mg/1) (4).
4-2
-------
4.2.1.3 Ground Garbage Effect
Home garbage grinders can significantly increase the suspended
solids load on a wastewater treatment plant. These solids are
largely settleable. Estimates of the increased primary sludge
resulting from the use of garbage grinders range from 25 percent
to over 50 percent (1,5,6).
4.2.1.4 Other Sludges and Sidestreams
Operating experience shows clearly that the amount of sludge
withdrawn from the primary sedimentation tank is greatly
increased when sludge treatment process sidestreams such as
digester supernatant, elutriate, and filtrates or centrates
and other sludges like waste-activated are recycled to the
primary sedimentation tank. Quantifying the solids entering and
leaving the primary clarifier by all streams is an important tool
for estimating primary sludge production when recycled sludges
and sludge process sidestreams contribute large quantities of
solids.
4.2.1.5 Chemical Precipitation and Coagulation
When chemicals are added to the raw wastewater for removal
of phosphorus or coagulation of nonsettleable solids, large
quantities of chemical precipitates are formed. The quantity of
chemical solids produced in chemical treatment of wastewater
depends upon the type and amount of chemical(s) added,
chemical constituents in the wastewater, and performance of the
coagulation and clarification processes. It is difficult to
predict accurately the quantity of chemical solids that will
be produced. Classical jar tests are favored as a means for
estimating chemical sludge quantities. The quantities of
suspended solids and chemical solids removed in a hypothetical
primary sedimentation tank that is processing wastewater which
has been treated by lime, aluminum sulfate or ferric chloride
addition are estimated in Table 4-1.
4.2.1.6 Peak Loads
Peak rates of primary sludge production can be several times the
average. Peak solids production levels also vary from one plant
to another. Four studies of primary sludge production rates are
summarized and presented here.
At Ames, Iowa, (9) the wastewater is basically of domestic
origin. A university contributes about 30 percent of the
volumetric and mass loads. Storm .runoff is collected and kept
separate from the domestic wastewater. For 21 years of record,
the suspended solids loads in the peak month of each year were
divided by the yearly average. The average of these ratios
4-3
-------
was 1.37. The average for comparison of peak days and peak
months over ten years of record was 1.59. Thus, in a typical
year, the maximum daily flow would be about 1.37 x 1.59, or
2.2 times the average. The maximum day's sludge production
was, therefore, expected to follow a similar pattern and was
estimated to be 2.2 times the average value.
TABLE 4-1
PREDICTED QUANTITIES OF SUSPENDED SOLIDS AND CHEMICAL
SOLIDS REMOVED IN A HYPOTHETICAL PRIMARY SEDIMENTATION TANK (7,8)
Chemical addition3
Sludge type
Suspended solids, Ib/mg
Chemical solids, Ib/mg
Total sludge production, Ib/mg
(kg/cu m)
No chemical
addition
1,041
-
1,041
(0. 13)
Lime
1, 562
2,082
3,644
(0.44)
Alumd
1,562
362
1,924
(0.23)
Iron6
1, 562
462
2,024
(0.24)
Assumes 10 mg/1 influent phosphorus concentration (as P) with
80 percent removed by chemical precipitation.
Assumes 50 percent removal of 250 mg/1 influent TSS in primary
sedimentation.
°125 mg/1 Ca(OH)2 added to raise pH to 9.5.
d!54 mg/1 A12(SO4)3 • 14 H20 added.
S84 mg/1 FeCl3 added.
Note: Assumes no recycle streams (for example, recycle of waste-activated
sludge to primary sedimentation, digester supernatant, etc.).
Secondary solids production would be cut from 833 Ib/mg without
chemical addition to 312 Ib/mg with chemical addition in this
hypothetical plant.
A study conducted in 1936 used data from Chicago, Cleveland,
Columbus, Syracuse, Rochester, and several other large American
cities (10) to show a typical relationship between peak raw
sewage solids loads entering a plant and duration of time that
these peaks persist. This relationship is shown graphically
on Figure 4-1. The curve is appropriate for large cities with a
number of combined sewers on flat.grades. The peaks occur at
least partly because solids deposited in the sewers at low flows
are flushed out by storm flows.
Data were collected over a five-year period from the West Point
plant at Seattle, Washington and used in a 1977 study (11). Peak
primary sludge loads of four- to ten-day durations were compared
with average loads. The duration of four days was selected
because it appeared to be highly significant to digester
operations at this plant, and because loads tended to drop after
about four days of heavy loading. The highest four-day primary
4-4
-------
sludge production was more than four times the normal production
from the plant's service area. Main contributors to the peak
load were:
• Solids deposits in the sewers. These deposits were
resuspended during high flows and carried to the
treatment plant. The computer-operated storage system,
which minimizes combined sewer overflows, apparently
contributed to solids deposition/reentrainment.
Storm inflow. Measurements of TSS
fluctuate widely but often show over
solids. A large portion of the West
contains combined sewers.
in storm drainage
200 mg/1 suspended
Point service area
Sludge conditioning and dewatering. Problems in these
processes have caused the sidestreams to contain more
solids than usual.
Q
<
O
500
400
Q
LLJ
cc
LLJ
LU
O
DC
LU
0.
300
200
100
10
15
20
25
30
DURATION OF PEAK LOAD, days
FIGURE 4-1
TYPICAL RELATIONSHIP BETWEEN PEAK SOLIDS LOADING
AND DURATION OF PEAK FOR SOME LARGE AMERICAN CITIES (10)
The fourth study, done in 1974, discussed two plants in
St. Louis, Missouri (12). The graphs shown on Figure 4-2
illustrate the variation in daily waste primary sludge production
as a fraction of the average waste primary sludge production with
duration of that production rate for the eight months that data
4-5
-------
were taken. Both of
loads, and both serve
sewers.
these plants have significant industrial
large areas of combined storm and sanitary
5.0
4.0
3.0
5 2.0
1.0
BISSELL POINT
MAXIMUM
MINIMUM
5.0 i-
4.0 -
3.0 -
2.0 -
1.0
LEMAY
0246
CONSECUTIVE DAYS
C - C RESULTS FROM
THE LEAST EXTREME
OF THE EIGHT MONTHS
0246
CONSECUTIVE DAYS
KEY: A - A AVERAGE OF RESULTS
OF EIGHT MONTHS
B - B MOST EXTREME
RESULTS RECORDED
IN ALL OF THE
EIGHT MONTHS
FIGURE 4-2
PEAK SLUDGE LOADS, ST. LOUIS STUDY (12)
4.2.2 Concentration Properties
Most primary sludges can be concentrated readily within the
primary sedimentation tanks. Several authors claim that a five
to six percent solids concentration is attainable when sludge is
pumped from well-designed primary sedimentation tanks (2,10,13*
14). However, values both higher and lower than the five to
4-6
-------
six percent range are common. Conditions that influence primary
sludge concentration include:
• If wastewater is not degritted before it enters the
sedimentation tanks, the grit may be removed by passing
the raw primary sludge through cyclonic separators.
However, these separators do not function properly with
sludge concentrations above one percent (15).
• If the sludge contains large amounts of fine nonvolatile
solids, such as silt, from storm inflow, a concentration
of well over six percent may sometimes be attained
(11,16).
• Industrial loads may strongly affect primary sludge
concentration. For example, at a plant receiving soil
discharged from a tomato canning operation, a primary
sludge with a 17 percent solids concentration, of which
40 percent is volatile, was recorded. Normal primary
sludge at this plant had a solids concentration of
from five to six percent solids (60 to 70 percent
volatile) (17).
• Primary sludge may float when buoyed up by gas bubbles
generated under anaerobic conditions. Conditions
favoring gas formation include: warm temperatures;
solids deposits within sewers; strong septic wastes;
long detention times for wastewater solids in the
sedimentation tanks; lack of adequate prechlorination;
and recirculating sludge liquors (18) . To prevent the
septic conditions that favor gas formation, it may be
necessary to strictly limit the storage time of sludge in
the sedimentation tanks. This is done by increasing the
frequency and rate of primary sludge pumping (19).
• If biological sludges are mixed with the wastewater, a
lower primary sludge concentration will generally result.
4.2.3 Composition and Characteristics
Table 4-2 lists a number of primary sludge characteristics.
In many cases, ranges and/or "typical" values are given. In the
absence of recirculating sludge process sidestreams, the percent
of volatile solids in the primary sludge should approximate the
percent volatile suspended solids in the influent wastewater.
A volatile solids content below about 70 percent usually
indicates the presence of storm water inflow, sludge processing
sidestreams, a large amount of grit, sludge from a water
filtration plant that was discharged to the sanitary sewer, low
volatile solids from industrial waste, or wastewater solids that
have a long detention time in the sewers.
4-7
-------
TABLE 4-2
PRIMARY SLUDGE CHARACTERISTICS
Characteristic
PH
Volatile acids, mg/1 as ace-
tic acid
Heating value, Btu/lb (kJ/kg)
Specific gravity of individ-
ual solid particles
Bulk specific gravity (wet)
BOD5/VSS ratio
COD/VSS ratio
Organic N/VSS ratio
Volatile content, percent by
weight of dry solids
Cellulose, percent by weight
of dry solids
Hemicellulose, percent by
weight of dry solids
Lignin, percent by weight of
dry solids
Grease and fat, percent by
weight of dry solids
Protein, percent by weight
of dry solids
Nitrogen, percent by weight
of dry solids
Phosphorus, percent by weight
of dry solids
Potash, percent by weight of
Range of values
5-8
200 - 2,000
6,800 - 10,000
Typical
value
Comments
Reference
0.5 - 1.1
1.2 - 1.6
0.05 - 0.06
64 - 93
60 - 80
- 15
6-30
7-35
20 - 30
22 - 28
1.5 - 4
0.8 - 2.£
10,285
7,600
1.02
1.07
65
40
40
10
3.8
1.6
Depends upon volatile content,
and sludge composition, re-
ported values are on a dry
weight basis.
Sludge 74 percent volatile.
Sludge 65 percent volatile.
Increases with increased grit,
silt, etc.
Increases with sludge thickness
and with specific gravity of
solids.
Strong sewage from a system of
combined storm and sanitary
sewers.
Value obtained with no sludge re-
cycle, good degritting; 42
samples, standard deviation 5.
Low value caused by severe storm
inflow.
Low value caused by industrial
waste.
Ether soluble
Ether extract
Expressed as N
Expressed as P205- Divide
values as P2C>5 by 2.29 to
obtain values as P.
Expressed as K20. Divide
values as K2
-------
the fragmented screenings appear in the primary sludge. Smaller
plastic and rubber items that pass through screens also appear in
the primary sludge.
Primary sludge typically contains over 100 different anaerobic
and facultative species of bacteria (24). Sulfate-reducing
and oxidizing bacteria, worm and fly eggs, and pathogenic
microorganisms are typically present.
4.3 Biological Sludges
4.3.1 General Characteristics
Biological sludges are produced by treatment processes such as
activated sludge, trickling filters, and rotating biological
contactors. Quantities and characteristics of biological sludges
vary with the metabolic and growth rates of the various micro-
organisms present in the sludge. The quantity and quality of
sludge produced by the biological process is intermediate between
that produced in no-primary systems and that produced in
full-primary systems in cases when fine screens or primary
sedimentation tanks with high overflow rates are used.
Biological sludge containing debris such as grit, plastics,
paper, and fibers will be produced at plants lacking primary
treatment. Plants with primary sedimentation normally produce
a fairly pure biological sludge. The concentrations and,
therefore, the volumes of waste biological sludge are greatly
affected by the method of operation of the clarifiers.
Biological sludges are generally more difficult to thicken and
dewater than primary sludge and most chemical sludges.
4.3.2 Activated Sludge
4.3.2.1 Processes Included
Activated sludge has numerous variations: extended aeration;
oxidation ditch; pure oxygen, mechanical aeration, diffused
aeration; plug flow; contact stabilization, complete mix, step
feed, nitrifying activated sludge; etc (2). This manual does not
discuss lagoons in which algal growth is important or lagoons
that tend to accumulate wastewater solids or biological solids.
These methods, however, can be used for predicting activated
sludge production in highly loaded aerated lagoons where the
bacteria are maintained in solution.
4.3.2.2 Computing Activated Sludge Production -
Dry Weight Basis
The quantity of waste-activated sludge (WAS) is affected by two
parameters: the dry weight of the sludge and the concentration
of the sludge. This section describes how the dry weight of
activated sludge production may be predicted.
4-9
-------
Basic J?r_ed ictive Equations
The most important variables in predicting waste-activated sludge
production are the amounts of organics removed in the process,
the mass of microorganisms in the system, the biologically inert
suspended solids in the influent to the biological process, and
the loss of suspended solids to the effluent.
These variables can be assembled into two simple and useful
equations:
Px = (Y)(sr) - (kd)(M) (4-1)
WAST = Px + INV - ET (4-2)
where:
Px = net growth of biological solids (expressed as volatile
suspended solids [VSS]), Ib/day or kg/day;
Y = gross yield coefficient, Ib/lb or kg/kg;
sr = substrate (for example, 6005) removed, Ib/day or
kg/day;
kd = decay coefficient, day~l;
M = system inventory of microbial solids (VSS) micro-
organisms, Ib or kg;
WAST = waste-activated sludge production, Ib/day or kg/day;
NV = non-volatile suspended solids fed to the process,
Ib/day or kg/day;
ET = effluent suspended solids, Ib/day or kg/day.
These equations, as stated or with slight variations, have been
widely used. Equation 4-1 dates back to 1951 (25). However,
different terms and symbols have been used by various authors in
expressing Equations 4-1 and 4-2. Table 4-3 summarizes some of
the terminology that has evolved. The technical literature
reflects some inconsistency in terminology with the term "M."
Test results reported by various authors and presented in
Table 4-3 were derived on the basis of "M" defined as mixed
liquor VSS only.
To use Equation 4-1, it is necessary to obtain values of Y and
kd. While Table 4-4 summarizes several reported values for these
parameters, it is best to determine Y and k^ on an individual
waste stream whenever possible.
4-10
-------
TABLE 4-3
ALTERNATE NAMES AND SYMBOLS FOR EQUATION (4-1)
As used in this chapter
Other symbols for Other common names for
, , „ _. • . similar quantities similar quantities
Symbol Name Dimensions M ^
Px Biological solids Mass AX , dX/dt, A, S, Accumulation, net growth,
production Time dM/dt, Rg excess microorganisms
production
Y Gross yield Mass a, Ks, c Yield coefficient, synthesis
coefficient3 Mass coefficient, growth-yield
coefficient
s Substrate removal Mass dF/dt, S, B, Fi, R Food, utilization, load
r Time
kd Decay constant 1 b, K^, Ke Endogenous respiration.
Time maintenance energy,
auto-oxidation
M Microbial solids Mass S, X, Xv Microbial mass, solids under
inventory aeration, solids inventory,
mixed liquor solids
SThe letter Y has also been used for the net yield coefficient Px/sr. The net yield
coefficient is quite different from the gross yield coefficient.
To use Equation 4-2, it is necessary to estimate
non-volatile influent solids, and Eip, effluent suspended solids.
The following are generally included within the term
• Non-volatile solids in influent sewage, including recycle
sludge liquors.
• Chemical precipitates--for example, aluminum phosphates--
when alum is added to the activated sludge process.
• Stormwater solids that are not removed in previous
processes (36).
• Normal non-volatile content of the activated sludge.
In the absence of sludge liquors, chemical precipitates,
and stormwater, activated sludge will be about 80 percent
volatile (less in extended aeration) at most municipal
treatment plants.
To compute ET, a small value such as 10 mg/1 TSS should be
used.
The following sections discuss several factors that can influence
the production of waste-activated sludge. Section 4.3.2.3 is a
detailed example of how sludge quantities should be computed.
4-11
-------
TABLE 4-4
VALUES OF YIELD AND DECAY COEFFICIENTS FOR
COMPUTING WASTE-ACTIVATED SLUDGE
Gross yield
Reference coefficient3
25 0.5
26 0.70
26, 27 0.67
28, 29 0.73
30 0.94
31 0.73
32 0.5
12 0.74
30 1.57
33 1.825
34 0.65
34 0.70
34 0.54
35 1.1
Decay
. . D
coefficient
0.055
0.04
0.06
0.075
0.14
0.06
Not calculated
(negligible)
0.04
0.07
0.20
0.043
0.048
0.014
0.09
Type of
wastewater
Primary effluent
Primary effluent
Primary effluent
Primary effluent
Primary effluent {wastewater
includes dewatering
liquors)
Primary effluent
Primary effluent (military
base)
Primary effluent (much in-
dustry)
Raw degritted including de-
watering liquors
Raw degritted
Raw degritted
Raw degritted
Raw degritted
Raw
Scale of
plant
Bench
Pilot-
Pull
Pilot
Pilot
Pilot
Pilot
Pilot
Pilot
Bench
Bench
Bench
Full
Full
Aeration
Air
Oxygen
Air
Air
Air
Oxygen
Air
Oxygen
Air
Air
Air
Air
Air
Air
Temperature,
Or-
19
Not
18
10
15
18
0
17
15
4
20
20
Not
Not
^
- 22
stated
- 27
- 16
- 20
- 22
- 7
- 25
- 20
- 20
- 21
- 21
stated
stated
Sludqe age.
days
2.8 - 22
1-4
1,2 - 8
1-12
0.5 - 8
2.5 - 17
Long
2.1 - 5
0.6 - 3
1-3
d
11 and up
d
Long
d
Long
1.1 -2.4
BOD removal
1 1 1- '
Influent
Influent minus
effluent
Influent minus
effluent
Influent minus
effluent
Influent minus
soluble ef-
fluent
Influent minus
effluent
Influent
Influent minus
soluble ef-
fluent
Influent minus
soluble ef-
fluent
Soluble in-
fluent minus
soluble ef-
fluent
Influent minus
effluent
Influent minus
effluent
Influent minus
effluent
Influent minus
effluent
3Gross yield coefficient Y, Ib (kg) VSS/lb (kg) BOD5.
Decay coefficient k , days
CMean cell residence time or sludge age 9^, measured as mass of mixed liquor
VSS divided by biological solids production P^. Note that coefficients may be
somewhat different if total system inventory of VSS (mixed liquor VSS plus VSS
in clarifiers) is used rather than just mixed liquor VSS.
Extended aeration.
Note: All values in this table are for an equation of the type px = Ysr - kdM (equation 4-1).
Effect of_Sludg_e Age and F/M Ratio
Equation (4-1) can be rearranged to show the effect of the
sludge age (9m).
(sr)
r ( 4 -
(4
(kd)(em)
where Qm = •=?- = sludge' age, days.
4-12
-------
Similarly, Equation 4-1 can be rearranged to show the effect of
the food-to-microorganism ratio (F/M):
= (Y)(sr) -
where:
C2 = coefficient to match units of sr and "F" in F/M; if sr
is 8005 removed (influent minus effluent), then C2 is
BOD5 removal efficiency, about 0.9;
F/M = food-to-microorganism ratio;
BODs applied daily
VSS (mass) in system
As 6m increases and F/M decreases, the biological solids
production Px decreases. Sludge handling is expensive, and costs
can be reduced by using high values of Qm or low values of F/M.
However, there are offsetting cost factors, such as increases in
the aeration tank volume needed, oxygen requirements for the
aerobic biological system, etc. Also, as seasons change, so
may the optimum Om and F/M for. maximum wastewater treatment
efficiency. Therefore, it is desirable to be able to operate
across a range of conditions. Obviously, trial-and-error
calculations are required to determine the least costly system.
Effect of Nitrif i cat ion
Nitrification is the bio-oxidation of ammonia nitrogen and
organic nitrogen to the nitrite and nitrate forms. Compared with
processes that are designed for carbonaceous (8005, COD) oxida-
tion only, stable nitrification processes operate at long sludge
ages (Sm) and low food-to-microorganism ratios (F/M). Also,
nitrification processes are often preceded by other processes
that remove much of the 8005 and SS. As a result, activated
sludge in a nitrification mode generally produces less waste-
activated sludge than conventional activated sludge processes.
However, there is an additional component to nitrification
sludge, the net yield of nitrifying bacteria, YN. This may be
estimated at 0.15 pounds SS per pound of total Kjeldahl nitrogen
(organic plus ammonia) removed (37). YN varies with temperature,
pH, dissolved oxygen, and cell residence time. However, detailed
measurements of Y^ are not ordinarily required for sludge
facility design because the yield of nitrifying bacteria is
small. For example, if YN is 0.15 and if the nitrifying process
removes an ammonia nitrogen concentration of 20 mg/1 and an
organic nitrogen concentration of 10 mg/1 then nitrification
would add 0.15 x (20+10) = 4.5 milligrams of nitrifying bacteria
per liter of wastewater (38 pounds per million gallons). These
4-13
-------
quantities are small compared to other sludges. In single-stage
nitrification processes, the sludge production figures must also
include the solids produced from the carbonaceous oxidation,
computed at the 0^ and F/M of the nitrifying system.
Effect of Feed Composition
The type of wastewater that is fed to the activated sludge
process has a major influence on the gross yield (Y) and
decay (k^) coefficients. Many industrial wastes contain
large amounts of soluble BOD5 but small amounts of suspended
or colloidal solids. These wastes normally have lower Y
coefficients than are obtained with domestic primary effluent.
On the other hand, wastes with large amounts of solids, relative
to BOD^, either have higher Y coefficients or require adjustments
to reflect the influent inert solids. Even among soluble wastes,
different compositions will cause different yields.
Effect ofDissolved Oxygen Concentration
Various dissolved oxygen (DO) levels have been maintained in
investigations of activated sludge processes. Very low DO
concentrations—for example, 0.5 mg/1—in conventional activated
sludge systems do appear to cause increased solids production,
even when other factors are held constant (38). However,
there is vigorous disagreement concerning solids production at
higher DO levels. Some investigators state that use of pure
oxygen instead of air reduces sludge production. This is
attributed to the high DO levels attained through the use of pure
oxygen (39,40). Other investigators in recent well-controlled
investigations have concluded that if at least 2.0 mg/1 DO is
maintained in air-activated sludge systems, then air and oxygen
systems produce the same yield at equivalent conditions (such as
food-to-microorganism ratio) (41, 42).
Efj_ect_of Temperature
The coefficients Y (gross yield) and k^ (decay) are related to
biological activity and, therefore, may vary due to temperature
of the wastewater. This variation has not been well documented
in pilot studies and process investigations. One study obtained
no significant difference due to temperature over the range
39° to 68°F (4°to 20°C)(33). However, others have observed
significant differences within the same temperature range.
Sometimes a simple exponential ("Arrhenius") equation is used for
temperature corrections to Y and k^. For instance, it has been
stated that chemical and biochemical rates double with an 18°F
(10°C) rise in temperature. Exponential equations have been
found to be accurate for pure cultures of bacteria, but are quite
inaccurate when applied to Y and k^ for the mixed cultures
found in real activated sludges (43, 44).
4-14
-------
For the design engineer, the following guidelines are recommended
until such time as process investigations and research efforts in
this area provide more consistent and reliable information:
• Wastewater temperatures in the range of from 59° to 72°F
(15° to 22°C) may be considered to be a base case. Most
of the available data are from this range. Within this
range, there is no need to make temperature corrections.
Any variations in process coefficients across this
temperature range are likely to be small in comparison to
uncertainties caused by other factors.
• If wastewater temperatures are in the range of from 50°
to 59°F (10° to 15°C), the same kd value as for 59° to
75°F (15° to 22°C) should be used, but the Y value should
be increased by 26 percent. This is based on experiments
that compared systems at 52°F (11°C) and 70°F (21°C). In
these tests, kd was the same, but Y was 26 percent
higher. (On a COD basis, Y was found to be 0.48 at 38°F
[11°C] and 0.38 at 56°F [21°C]) (45).
• If wastewater temperatures are below 50°F (10°C),
increased sludge production should be expected (46), but
the amount of increase cannot be accurately predicted
from available data. Under such conditions, there is a
need for pilot-scale process investigations.
• If wastewater temperatures are above 72°F (22°C) , values
of the process coefficients from the range 59° to 72°F
(15° to 22°C) may be used for design. The resulting
design may be somewhat conservative.
Effect of^Feed^Pattern
Various feed patterns for the activated sludge process include
contact stabilization, step feeding, conventional plug-flow, and
complete-mix. For design purposes, it appears to be best to
ignore the feed pattern when estimating solids production.
Computing Peak Rate of Waste-Activatec3
Sludge Production
Peak solids production occurs because of unfavorable combinations
of the elements in Equations 4-1, 4-3, and 4-4, presented
previously:
PX = (Y)(sr) - (kd)(M) (4-1)
PX = ____r__. (4-3)
4-15
-------
(kd)(sr)
PX = (Y)
-------
To accomplish the desired inventory reduction, solids handling
facilities must have the capacity to accept the wasted solids.
For wastewater treatment plants without major known BOD5 and SS
loading variations, allowance should be made in designing solids
processing facilities for the wasting of an additional two percent
of M per day and lasting up to two weeks. Such plants include
those serving stable domestic populations. Industrial loads
would be either small or unusually stable.
For plants with major seasonal variations in loads, allowance
should be made for wasting an additional five percent of M per
day and lasting for up to two weeks. Such plants serve resort
areas, college towns, etc. A similar allowance should be made
for plants that practice nitrification during only part of
the year. Lastly, for plants with major weekday-to-weekend
variations of over 2 to 1 in 8005 load, and medium or high
food-to-microorganism ratios of over 0.3 during the high loads,
allowance should be made for a one-day sludge wasting of up to
25 percent of M. The plant should also be able to handle wasting
of five percent of M per day and lasting for two weeks. Plants
in this category serve major industrial systems, large office
complexes, schools, and ski areas.
Since inventory reductions are not generally practiced during
peak loading periods, these above-discussed capacity allowances
should be added to average solids production. The maximum rate
of waste-activated sludge production is determined by whichever
is greater: production during peak loading or the sum of average
production plus inventory reduction allowances.
Occasionally, sludge is wasted in a pattern so that M increases
at some times and decreases at others. An example of such a
pattern is the withdrawal of WAS only during the daytime. The
Tapia, California, Water Reclamation Plant uses this pattern to
obtain good process control (51). Use of such patterns will, of
course, increase the maximum rate at which WAS must be removed.
Measurements of Sludge Yield Coeffijciejrts
Pilot studies and full-scale operating records can provide
better data for establishing sludge production design criteria
than any general compilation of data from other locations.
Measurements of the sludge yield coefficients are of two basic
types. First, both the gross yield Y and the decay k^ may be
determined. Second, observed net yields alone may be used.
Equations 4-1, 4-3, and 4-4 are used when the food-to-
microorganism ratio F/M and the sludge age, 6m, may be expected
to vary in the prototype plant. To use these equations, it is
necessary to determine the two sludge yield coefficients, Y
4-17
-------
and k^j. To establish these two
must be measured under at least
coefficients,
two different
solids production
conditions of F/M
and Q
ITT
Equation 4-1 can be rearranged slightly to Equation 4-5:
M
(4-5)
where :
sr/M =
= net growth rate = l/6m days"
lb(kg) BOD5 removed per day
Ib (kg) VSS
This equation provides a basic straight-line relationship between
PX/M and sr/M. For each condition of operation, PX/M and sr/M
are calculated and plotted, and a straight line is drawn through
the points. The slope of the line is the yield coefficient (Y),
and the intercept represents the decay coefficient (kd) . (See
Figure 4-3.)
1.00
.80
.60
BOD BASIS —
£*- = 0.67 (2L) - 0.06
M M
Y = 0.67, kd = 0.06
on .40
.20
-.20
COD BASIS
£*. = 0.34 (-!!) - 0.06
M M
Y = 0.34, kd = 0.06
.50
1.00
1.50
2.00
2.50
sr _ LB SUBSTRATE REMOVED/day
M ~
LB MLVSS
FIGURE 4-3
NET GROWTH RATE CURVES (27)
4-18
-------
If the design conditions of sr/M or 6m are known and if solids
production can be measured under these conditions, then it is not
necessary to determine both Y and k^ . Instead, a simple observed
net yield may be calculated. Equations 4-1 and 4-3 are easily
rearranged to show:
Yobs = = * - kd/(sr/M) = T___ (4-6)
where :
= ne*: yield coefficient,
_ _ lb(kg) VSS produced __
lb(kg) substrate (for example, 8005) removed
Net yield coefficients are often reported in the literature.
They are directly applicable only under the conditions of sr/M
and 6m that occurred during the experiments; they are meaningless
unless sr/M or 6m are measured also. For gathering data from
pilot plants or existing plants for use in establishing sludge
yield coefficients, several precautions should be exercised.
Either automatic dissolved oxygen (DO) control should be used in
the test or ample air or oxygen should be provided to ensure that
the mixed liquor DO concentration is over 2 mg/1 at all times.
Data from widely differing temperatures should not be plotted on
the same graph to determine Y and k^. Instead, data from each
temperature range should be used to determine Y and k^ for that
range. Each condition of sr/M or Qm should be maintained long
enough to obtain stable operation. To assure system stability,
a period of time equal to three times the sludge age should
elapse between tests. The designer should use the term I^v in
Equation 4-2 to correct the effect of sidestreams. The percent
volatile content of the solids produced should be recorded. This
will be useful in computing the total solids in the sludge.
4.3.2.3 Example: Determination of Biological
Sludge Production
This example illustrates the use of yield factors and decay
factors. Figure 4-4 shows a flow diagram for a hypothetical
plant. The problem is to prepare an initial estimate of the
loading to the waste-activated sludge thickener. Table 4-5
contains information required for this calculation, including
average and maximum day loadings and activated sludge operating
characteristics. It is assumed that the thickener in this
example will have to handle the maximum-day waste-activated
sludge production. Peak loadings of shorter duration than the
maximum day production will be handled by storing the added
suspended solids in the aeration basins. For the purposes of
this example, the sludge treatment processes such as digestion,
4-19
-------
dewatering, disinfection, thermal conditioning, and chemical
conditioning have not been identified. Depending upon the
selection and design of the sludge treatment processes, the
recycle loads from such processes could have a significant effect
upon the quantities of waste-activated sludge and primary
sludge that must be processed. When they are known, the
degradable organics (BOD) and non-volatile fractions of the
recycle streams should be added to the substrate removal (sr)
and non-volatile suspended solids (I^y) factors. Subsequent
calculations in Equations 4-1 and 4-2 are for the purposes of
obtaining a sludge mass balance, which includes the effect of
recycle streams.
COMMERCIAL WASTE
^ PRELIMINARY
TREATMENT
!
GRIT
RECYCLE
PRIMARY
TATION
PRIMARY SLUDGE
\ 1
DISINFECTION
AERATION fc FINAL AND DISCHARGE
^ TANKS CLARIFIERS
RECYCLE
1
RETURN ACTIVATED SLUDGE
WASTE -ACTIVATED SLUDGE -
TO BE CALCULATED
1
THICKENED SLUDGE
%
b
_i
u_
cc
UJ
Q
z
=>
oc
LU
U_
CC
5
(J
r
HICKENER
TREATMENT
SLUDGE FOR REUSE
OR DISPOSAL.
FIGURE 4-4
SCHEMATIC FOR SLUDGE QUANTITY EXAMPLE
Step 1. Determine 8005 load to the activated sludge process
Average day 6005 load:
5 .0 MGD x
Ib/MG
1 rag/1
/± x (1 - 0.35) = 5,150 Ib/day
4-20
-------
Maximum day 6005 load (similar calculation):
9.5 MGD x 8'i4m1/;(MG x 160 mg/1 x (1 - 0.25) = 9,510 Ib/day
TABLE 4-5
DESIGN DATA FOR SLUDGE PRODUCTION EXAMPLE
Description
Value
Description
Influent flow, mgd (m^/day)
Average day 5.0 (18,900)
Maximum day 9.5 (36,000)
Influent 8005, mg/1
Average day 190
Maximum day 160
Influent suspended solids,
mg/1
Average day 240
Maximum day 190
8005 removal in primary
sedimentation, percent
Average day 35
Maximum day 25
Suspended solids removal in
primary sedimentation
Average day 65
Maximum day 50
Ib (kg) BOD5 applied daily
Ib(kg)mixed liquor VSS
Data from other plants must be used.
1 mgd = 3,785 m /day
Note: Maximum day influent BOD,- and suspended solids concentrations
reflect a dilution from average day data due to the higher
flow.
Value
Sludge thickener capture
efficiency
Average, percent 95
Maximum day, percent 85
Food-to-microorganism
ratio3
Average 0.3
Maximum 0.5
Temperature of wastewater
Average, degrees F
(degrees C) 65 (18)
Minimum, degrees F
(degrees C) 50 (10)
Dissolved oxygen in aera-
tion tanks
Average, mg/1 2 . 5
Minimum, mg/1 2.0
Control: automatic
Effluent limitations, 30-
day average
BOD5, mg/1 30
Suspended solids, mg/1 30
Usable test data for ,
solids production None
Step 2. Determine M, the mass of microorganisms
8005 applied/day
Average: F/M =
VSS in system
= 0.3
M =
= 17,170 pounds VSS
4-21
-------
Maximum day: F/M = 0.5
M = 9/.5c° = 19,020 pounds VSS
U • D
Step 3. Determine Y, the gross yield coefficient, and k^ ,
the decay coefficient. No test data are available for this
waste, so estimates must be made from tests on other wastes.
For average conditions, use Los Angeles data from Table 4-4 (27):
Y = 0.67 pound (kg) VSS formed per pound (kg) 6005 removed;
kd = 0.06 day -1.
For maximum conditions, use minimum temperature of 36°F (10°C),
which produces the maximum Y value. Use the correction from
Section 4.3.2.2, which increases Y by 26 percent.
Ymax = 0-67 x 1.26 = 0.84; do not adjust k^
Step 4. Determine sr (substrate removal) in units to match Y.
Average daily substrate removal:
BOD5 applied 5,150 Ib/day
Effluent BOD5 (assume 10 mg/1* - 420 Ib/day
BOD5 in effluent) 4,730 Ib BOD5 removed/day
Maximum daily substrate removal:
BOD5 applied 9,510 Ib/day
Effluent BOD5 (assume 10 mg/1* - 790 Ib/day
BOD5 in effluent) 8,720 lb/BOD5 removed/day
Step 5. Determine Px, the biological solids production. Use
Equation 4-1 from 4.3.2.2:
PX = (Y)(sr) - (kd)(M)
Average :
Ib VSS produced , 7^n jj BOD5 removed
n ,_
U'b/ Ib BOD5 removed ' day
- (0.06 day-1) (17,170 Ib VSS) = 2,140 Ib VSS produced/day
*Allow 10 mg/1 for effluent BOD5, even though the plant is
permitted to discharge 30 mg/1. Activated sludge plants can
often attain 10 mg/1 effluent BOD5. Sludge capacity should be
provided for the sludge produced under such conditions.
4-22
-------
Maximum day, similar calculation:
(0 .84) (8,720) - (0 .06) (19,020) = 6,184 Ib VSS produced/day
Step 6. Compute INv- (non-volatile suspended solids fed to the
activated sludge process).
Average daily input of non-volatile suspended solids:
5.0 MGD x 8 'i^1/!^ x 24° m<3/1 x (1 ~ 0.65)(0.25*)
= 880 Ib/day
Maximum daily input of non-volatile suspended solids:
9.5 MGD x B'i4m1/{MG x 190 mg/1 x (1 - 0.50)(0.25*)
= 1,800 Ib/day
Step 7. Compute Erj, (effluent suspended solids).
Average :
5.0 MGD x -'i" * 10 mg/1 = 420 Ib/day
Maximum day:
9.5 MGD x — x 10 mg/1 = 790 Ib/day
Step 8. Compute waste-activated sludge (WAST) production
From Equation (4-2);
WAST = Px + INV ~ ET
WAST = 2,140 + 880 - 420 = 2,600 Ib TSS/day
(1,180 kg/day)
4-23
-------
Maximum day:
WAST 6,184 + 1,880 - 790 = 7,274 Ib TSS/day
(3,302 kg/day)
Step 9. Compute inventory reduction allowance.
Inventory reduction allowance = ( 0.02) (17,170) = 343 Ib/day
(156 kg/day)
In the present case, the inventory reduction allowance can be
small. Allow two percent of M per day. The 343 Ib/day computed
here is much smaller than the difference between the average and
maximum waste-activated sludge production (Step 8); therefore, if
capacity is provided for maximum solids production, then there
will be ample capacity for inventory reduction. It is not
necessary to reduce inventory during peak loads.
4.3.2.4 Interaction of Yield Calculations and
the Quantitative Flow Diagram (QFD)
The example just presented demonstrates a technique for
calculating solids production on a once-through basis; that is,
any solids associated with recycle streams were not considered in
the calculation. The QFD considers the effects of recycle
streams. Before the QFD can be constructed for biological
treatment processes, an estimate of net solids destruction or
synthesis must first be made. The relationship between solids
entering and leaving the biological unit is established via the
parameter XD, which is defined as net solids destruction per
unit of solids entering the biological unit. The data and
calculations from the previous design example allow an initial
estimate of XD to be made.
For the average flow:
1. Solids leaving the biological unit = Px + INy = 2,140
+ 880 = 3,020 pounds per day '
2. Solids entering the biological unit are equal to solids
in the primary effluent, which can be calculated from the
data on Table 4-4. Primary effluent solids = (1 - 0.65)
(240) (8.34) (5.0) = 3,503 pounds per day.
3. Net solids destruction = solids in - solids out = 3,503
- 3,020 = 483 pounds per day (219 kg/day).
483
3,503
XD = ^m = 0.138
4-24
-------
For maximum day flows:
1. Solids leaving the biological unit = 6,184 + 1,880
= 8,064 pounds per day (3,661 kg/day).
2. Solids entering the biological unit = (1 - 0.50)(190)
(8.34)(9.5) = 7,527 pounds per day (3,147 kg/day).
3. Net solids destruction = 8,064 - 7,527 = 537 pounds per
day (244 kg/day).
4. XD = - 1*1 = 0.07
umax 7,527
Once XD is known, the QFD calculation can be undertaken. After
the QFD calculation is completed, the designer may wish to make
new estimates of Px and I^y based on information derived from
the QFD calculation. For example, if the QFD calculation shows
that recycle loads are substantial, then the designer may wish to
modify estimates of sr and I^v anc^ calculate new values of Px
and INV, as indicated in Section 3.4.
4.3.2.5 Concentration of Waste-Activated Sludge
The volume of sludge produced by the process is directly
proportional to the dry weight and inversely proportional to the
thickness or solids concentration in the waste sludge stream.
Values for waste-activated sludge concentration can vary, in
practice, across a range from 1,000 to 30,000 mg/1 SS (0.1 to
3 percent SS) .
An important variable that can affect waste-activated sludge
concentration is the method of sludge wasting. A number of
different methods are illustrated in Figure 4-5. Sludge solids
may be wasted from the clarifier underflow. It has been argued
that wasting solids from the mixed liquor should improve control
of the process (2,35). In this case, waste sludge is removed
from the activated sludge process at the same concentration as
the mixed liquor suspended solids, about 0.1 to 0.4 percent.
This low concentration can be a disadvantage because a large
volume of mixed liquor must be removed to obtain a given wastage
on a dry weight basis. The most common arrangement involves
sludge wasting from the clarifier underflow, because the
concentration of sludge there is higher than in the mixed liquor.
Subsequent discussions in this section are based on sludge
wasting from the clarifier underflow.
Estimating Waste-Activated Sludge Concentration
The two primary factors that affect waste-activated sludge
concentration are the settleability of the sludge and the solids
loading rate to the sedimentation tank. These two factors have
4-25
-------
(a). WASTING FROM CLARIFIER UNDERFLOW
(b). WASTING FROM REAERATION TANK
FEED
Z
a:
uu
Ai RATION
TANK
FEED
WASTE SLUDGE
WASTE SLUDGE
(c). WASTING BY BATCH SETTLING
DURING FEED:
FEED
NO EFFLUENT
NO SLUDGE REMOVAL
NO FEED
DURING WITHDRAWAL:
TANK NOT
AERATED,
OPERATED
AS BATCH
CLARIFIER
*
WASTE SLUDGE
PROCESS EFFLUENT^
(d). WASTING FROM MIXED LIQUOR
FEED ^
RETURN ACTIVATED
I SLUDGE 1
1 T '
AERATION TANK
r i
1 REAERATION TANK |
MIXED LIQUOR _£, .„,,_,,_ J\. PROCESS EFFLUENT __
] (IF USED) j ^
I 1
1
EC
LU
LL
E
5
o
!
O
LL
oc
LU
Q
Z
D
WASTE SLUDGE
FIGURE 4-5
SLUDGE WASTING METHODS
4-26
-------
been considered in detail in the development of solids flux
procedures for predicting the clarifier underflow concentration
of activated sludge (52).
Factors Af_f_ec ting__Unde_rf_low_ C o n c en t r ation
Various factors that affect sludge settleability and the
clarifier sludge loading rate include:
• Biological characteristics of the sludge. These
characteristics may be partially controlled by mainte-
nance of a particular mean sludge age or F/M. High
concentrations of filamentous organisms can sometimes
occur in activated sludge. Reduction of these organisms
through sludge age or F/M control helps to produce more
concentrated clarifier underflow.
• Temperature. As wastewater temperatures are reduced, the
maximum attainable clarifier underflow sludge concentra-
tion (cu) is also reduced as a result of increased
water density. Also, temperature can affect the setting
properties of the sludge.
• Solids flux. The solids flux is the solids load from the
mixed liquor divided by the clarifier area (for example,
pounds per day per square foot). Higher rates of solids
flux require that clarifiers be operated at lower solids
concentration.
• Limits of sludge collection equipment. Because of the
pseudo-plastic and viscous nature of waste-activated
sludge, some of the available sludge collectors and pumps
are not capable of smooth, reliable operation when cu
exceeds about 5,000 mg/1.
• Heavy suspended solids in the sludge. If raw wastewater,
instead of primary sedimentation tank effluent, is fed to
the activated sludge process, higher cu values usually
result. Chemicals added to the wastewater for phosphorus
and suspended solids removal may similarly affect cu.
However, such additional solids will also increase the
solids load to the clarifiers.
4.3.2.6 Other Properties of Activated Sludge
Table 4-6 contains several reported measurements of the
composition and properties of activated sludge solids. Comparing
Table 4-6 with that of Table 4-2 for primary sludge, activated
sludge contains higher amounts of nitrogen, phosphorus, and
protein; the grease, fats, and cellulose amounts, and specific
gravity are lower.
4-27
-------
TABLE 4-6
ACTIVATED SLUDGE CHARACTERISTICS
pH
Heating value, Btu/lb (kJ/kg)
Range of
6.5 - 8
6,
(15,
5.5
540
200)
Can be less in high puritv oxygen
systems or if anaerobic decom-
position begins.
Baltimore , Maryland
Increases with percent volatile
content
53, 54
55
56
Specific gravity of individ-
ual solid particles
Bulk specific gravity
Color
COD/VSS ratio
Carbon/nitrogen ratio
Organic carbon, percent by
weight of dry solids
Nitrogen, percent by weight
of dry solids (expressed
as N)
17 - 41
23 - 44
4.7 - 6.7
2.4 - 5.0
Phosphorus, percent by weight 3.0 - 3.7
"of dry solids as P2°5
(divide by 2.29 to obtain 2.8 - 11
phosphorus as P)
Potassium, percent by weight 0.5 - 0.7
of dry solids as I^O
(divide by 1.20 to obtain
potassium as K)
Volatile solids, percent by 61 - 75
weight of dry solids (per-
cent ash is 100 minus 62 - 75
percent volatile) 59 - 70
Volatile solids (continued)
Grease and fat, percent by 5-12
weight of dry solids
Cellulose, percent by weight
of dry solids
Protein, percent by weight 32 - 41
of dry solids
1.08
1.0 + 7 x 10"
2. 17
12.9
6.6
14 .6
5.7
3.5
5.6
6.0
7.0
4.0
0.56
0.41
63
76
88
x C C is suspended solids concentra-
tion, in mg/1.
Some grayish sludge has been
noted. Activated sludge becomes
black upon anaerobic decomposi-
tion.
Baltimore, Maryland
Jasper, Indiana
Richmond, Indiana
Southwest plant, Chicago, Illinois
Milwaukee, Wisconsin (heat dried)
Zurich, Switzerland
Four plants
Zurich, Switzerland
Chicago, Illinois
Four plants
Milwaukee, Wisconsin
Zurich, Switzerland
Chicago, Illinois
Pour plants
Milwaukee, Wisconsin
Zurich, Switzerland
Chicago, Illinois
Milwaukee, Wisconsin
Zurich, Switzerland
Four plants
Renton, Washington (Seattle Metro),
1976 average
San Ramon, California (Valley Com-
munity Services District), 1975
average
Central plant, Sacramento County,
California, July 1977 - June
1978 average
Ether extract
Includes lignin
57
58
55
55
55
55
55
28
55
28
59
55
59
28
59
55
59
28
59
59
28
58
60
55
60
61
Several types of microorganisms are present in large numbers
in activated sludge. Floe-forming (zoogleal) bacteria include
species of Zoogloea, Pseudomonas, Arthrobacter, and Alcaligenes.
4-28
-------
Activated sludge also contains filamentous microorganisms such as
Sphaerotilus, Thiothrix, Bacillus, and Beggiatoa (62). Various
protozoa are present, including ciliates and flagellates.
4.3.3 Trickling Filters
Trickling filters are widely used in municipal wastewater
treatment. This section covers trickling filters that are used
with clarifiers. When a clarifier is not used, the trickling
filter effluent is usually fed to an activated sludge process.
Refer to Section 4.3.5 for such combinations.
4.3.3.1 Computing Trickling Filter Sludge
Production - Dry Weight Basis
Trickling filter microorganisms are biochemically similar to
microorganisms that predominate in activated sludge systems.
Consequently, solids production from trickling filters and from
activated sludge systems is roughly similar when compared on
the basis of pounds of solids produced per pound of substrate
removed. There are differences between the two systems, however,
with respect to solids production prediction methodology and the
pattern of sludge wasting. Attempts have been made to develop
solids production models consistent with biological theory
(47,63,64). However, presently (1979), empirical methods are
usually used for design purposes. Table 4-7 presents sludge
yields observed at several treatment plants and from one
long-term pilot study. These data are primarily based on heavily
loaded filters.
Equations that relate the production of suspended material in a
trickling filter can be developed in a form similar to that used
in predicting activated sludge production. The main difference
lies in the term used to define the quantity of microorganisms
in the system. In long-term studies of trickling filter
performance, Merrill (64) assumed that the total mass of micro-
organisms present in the system was proportional to the media
surface area. The resulting equation for volatile solids
production was:
Px = Y1(sr)-Ka(Am) (4-7)
where:
Px = net growth of biological solids (VSS), pounds per day or
kg per day;
Y1 = gross yield coefficient, pound per pound or kg/kg;
k<3 = decay coefficient, day"-'-;
4-29
-------
Sv =
substrate (for example, BOD5) removed, pounds per day
or kg/day = BOD5 in minus soluble effluent BOD5;
= total media surface area in reactor, square feet or
sq m.
TABLE 4-7
TRICKLING FILTER SOLIDS PRODUCTION
Unit solids production3
Plant
Stockton, California
Average of 13 months
Highest month
Lowest month
Sacramento, California
9 rioncanning months
Average
Highest month
3 canning months
Average
Dallas, Texas
Dallas, Texas
Livermore, California
San Pablo, California
Seattle , Washington-^
Total
BOD5b
basis
0.74
1.01
(5/76)
0.49
(1/77)
-
-
-
0.42
0.65.
i.io1
-
IT-ES IT-ES
BOD5o COD
basis basis
0.67 0.43
0.92 0.60
(5/76, (7/76)
7/76)
0.48 0.30
(1/77) (1/77)
-
-
-
-
_
_
0.8-0.9
SS
basis
1.00
1.17
(6/76,
1/77)
0.61
(3/76)
1.01
1.09
1.20
_
-
1.39
1.39
1.0
vss
basis
0.94
1.08
(10/76)
0.60
(3/77)
1.00
1.09
1.24
_
-
1.51
-
Solids percent
volatile
77
86
(8/76, 11/76)
64
(3/76, 6/76)
78
B3
76
_
-
84
'-
BOD5
Ioad9 Media Reference
Plastic, 27 ft2/ft3 65
27
73
(8/76)
15
16/76)
Plastic 66
_
-
-
Rock 67
Rock 67
57 Rock 2 to 4 in. 68
199 Plastic, 29 ft2/ft 37
30-250 Plastic, various 64
Solids production includes both waste sludge (clarifier underflow) and clarifier effluent solids.
Pounds volatile suspended solids (VSS) per pound 3005 removed {same as kg/kg). BOD- removal based
on total (suspended plus dissolved) measurements.
Pounds VSS per pound 6005 removed. BOD5 removal based on influent total minus effluent soluble (IT-ES)
measurements.
Pounds VSS per pound chemical oxygen demand (COD) removed. COD removal based on influent total
minus effluent soluble measurements.
Pounds total suspended solids (SS) produced per pounds SS applied.
Pounds VSS produced per pound VSS applied.
Pounds total 6005 applied per day per 1,000 cubic feet of media.
Stockton and Sacramento plants have heavy industrial loads about August to October from fruit and
vegetable canneries.
Roughing filter. For BODc, basis, BOD^ removal was computed by 6005,^ minus (0.5 times unsettled
BOD5(OUt). 1971 average data.
Pilot studies. SS basis was found to describe data well over a wide range of loadings. Wastewater
included some industrial load and recycle liquors from dewatering digested sludqe.
The production of trickling filter sludge requiring subsequent
sludge handling may be expressed:
WTFS = Px + INV - ET
(4-8)
where:
WTFS
waste trickling filter sludge production, pounds per
day or kg/day;
4-30
-------
INV = non-volatile suspended solids fed to the process,
pounds per day or kg/day;
ET = effluent suspended solids, pounds per day or kg/day.
The coefficients Y' and k^ for Equation 4-7 are obtained for
a particular system by computing the slope and intercept of a
P s
line of best fit through plotted data points for j-*- vs •=-£. VSS
"m "m
production data for three different trickling filter media designs
are given on Figure 4-6.
Nitrification in trickling filters causes a synthesis of
nitrifying bacteria. As in activated sludge, however, the
quantity is small. A value of 25 pounds per million gallons
(3 mg/1) has been suggested for design purposes (67). This
quantity must be added to the other solids produced by the
trickling filter.
It is known that temperature and loading rate affect sludge
production: "The quantity of excess sludge produced in a
low-rate trickling filter is much lower than that reported for
high-rate filters or for the activated sludge process. The lower
rate of solids accumulation may be attributable to the grazing
activities of protozoa. The activity of the protozoa is reduced
considerably at low temperatures (47)." However, there are few
data to quantify these variations.
Peak sludge loads are produced by trickling' filters. These may
be due to variations in influent load, rapid climatic changes,
and/or biochemical factors that cause unusually large amounts of
biomass to peel off from the media. The term "sloughing" is used
by some authorities to include steady state as well as peak
solids discharges. Others restrict the term "sloughing" to
unusually large discharges. In any case, peak solids loads must
be considered. Table 4-8 shows some variations due to both
unusual biomass discharges and to variations in influent load.
Table 4-9, on the other hand, shows the biomass discharge alone.
Each of the three events in Table 4-9 "occurred during periods of
light organic loadings (30 to 50 pounds BOD5 per 1,000 cubic
feet per day [0.49 to 0.81 kg/m3/day] ) which had been preceded
by periods in which exceptionally heavy organic loadings
(215 to 235 pounds BOD5 per 1,000 cubic feet per day [3.48 to
3.81 kg/m3/day]) had been applied on a sustained basis (4-14
days)" (64). Table 4-9 shows that effluent solids were much
greater than influent solids. This is quite different from
average conditions, under which effluent solids were about equal
to the influent solids.
In low-rate filters especially, there are seasonal variations in
solids production. "Slime tends to accumulate in the trickling
filter during winter operation and the filter tends to unload
the slime in the spring when the activity of the microorganisms
is once again increased" (47).
4-31
-------
Q
HI
w ->.
Q £
2 sr
o --.
O %
o >
LU
O
4
3
2
1
0
5
Q **
O o>
cc ^
.§ 4
tO T
>
3
2
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 low—5 to 10 percent of observed solids loads
4-33
-------
to activated sludge sedimentation tanks. Trickling filter sludge
also has better thickening properties than activated sludge.
Consequently, trickling filter sludge can be withdrawn at a much
higher concentration than waste-activated sludge. Concentration
data are summarized in Table 4-10.
TABLE 4-10
CONCENTRATION OF TRICKLING FILTER SLUDGE
WITHDRAWN FROM FINAL CLARIFIERS
Type of sludge
Trickling filter,
alone
Trickling filter, com-
bined with raw primary
Percent dry
solids
Comments
Reference
5-10
7
7
3
3-4
4-7
3-6
Depends on solids residence time
in trickling filter
Low-rate trickling filter
High-rate trickling filter
69
13
70
70
71
2
2,69
The solids flux method for predicting sludge concentration may be
used with trickling filter sludge (52). This method requires
measurement of initial solids settling velocity versus solids
concentration. Such relationships have been reported for at
least one trickling filter process (64).
4.3.3.3 Properties - Trickling Filter Sludge
Table 4-11 contains a few analyses of trickling filter sludge
properties. The microbial population that inhabits a trickling
filter is complex and includes many species of algae,
fungi, protozoa, worms, snails, and insects. Filter
their larvae are often present in large numbers around
filters.
bacteria,
flies and
trickling
4.3.4 Sludge from Rotating Biological Reactors
Rotating biological reactors (RBRs) are used for the same basic
purposes as activated sludge and trickling filters: to remove
BODc; and suspended solids and, where necessary, to nitrify.
The RBR process uses a tank in which wastewater, typically
primary effluent, contacts plastic media in the shape of large
discs. Bacteria grow on the discs. The discs rotate slowly on
horizontal shafts; the bacteria are alternately submerged in the
wastewater and exposed to air. Excess bacteria slough from the
discs into the wastewater. After contacting the bacteria, the
wastewater flows to a sedimentation tank, where the excess
bacteria and other wastewater solids are removed. These removed
4-34
-------
solids are RBR sludge. RBR sludge is roughly similar in quantity
by dry weight, nutrient content, and other characteristics, to
trickling filter sludge.
TABLE 4-11
TRICKLING FILTER SLUDGE COMPOSITION
Property
Volatile content, percent of
total solids
Nitrogen, percent of total
solids
Phosphorus as P2C>5, percent
of total solids
Fats, percent of total solids
Grease, percent of total
solids
Specific gravity of individ-
ual solid particles
Bulk specific gravity (wet)
Color
Value
64 - 86
1.5 - 5
2.9
2.0
2.8
1.2
6
0.03
1.52
1.33
1.02
1.025
Grayish brown
Black
Comments
Reference
See Table 4-7
Depends on length of storage
of sludge in filter.
Ether soluble.
Test slime grown in primary
effluent.
69
71
13
71
13
13
72
73
2
13
2
13
64
A small body of published data is available on RBR sludge
production rate from full-scale municipal installations. At
Peewaukee, Wisconsin, total suspended solids production has been
reported to be 0.62 to 0.82 pounds of total suspended solids
per pound BOD5 (0.62 to 0.82 kg TSS/kg) removed. The final
sedimentation tank removed 70 to 83 percent of these solids as
sludge. The biological sludge alone had a concentration of 1.5
to 5.0 percent solids. Other investigations of municipal and
industrial waste applications have concluded that sludge produc-
tion for the RBR process amounts to 0.4 to 0.5 pound of total
suspended solids per pound of BOD5 (0.4 to 0.5 kg TSS/kg BOD5)
removed (74,75,76).
4.3.5 Coupled Attached-Suspended Growth Sludges
There are several installations of coupled attached and suspended
growth processes in the United States. These dual processes
are usually installed where nitrification is required or where
strong wastes must be treated. The attached growth reactor is
a trickling filter or a rotating biological reactor. Its role
is to reduce the load on the suspended growth process. The
suspended growth process uses an aeration tank and a final
clarifier. Flow recirculation is usually practiced around
the attached growth reactor. Several reports describe these
4-35
-------
processes and note that the sludge is similar to activated
sludge, both in quantity and in characteristics (5,67,68,77,78).
The sludge characterized in Table 4-12 contains some particles
of dense solids from the attached growth reactor. These
particles may improve the thickening characteristics of the
sludge (78).
TABLE 4-12
SLUDGE FROM COMBINED ATTACHED-SUSPENDED GROWTH PROCESSES
Primary sludge mixed
with biological sludge
Solids production ™ ~~" * ™ " """"
Ib TSS produced/ Percent Percent Percent
Process Location Ib BODg removed volatile solids volatile
Roughing filter plus Livermore, California (68) 0.98 Not stated 3.3 84
nitrifying activated
sludge
Roughing filter plus San Pablo, California (37) 1.47 78.2 Not stated Not stated
nitrifying activated
sludge
4.3.6 Denitrification Sludge
Denitrification is a biological process for the removal of
nitrate from wastewater. An electron donor, carbon in primary
effluent or methanol, is added to the nitrate-bearing wastewater.
Denitrifying bacteria extract energy for growth from the reaction
of nitrate with the electron donor:
Nitrate + Electron donor (reduced state)
Nitrogen gas + Oxidized electron donor + Energy
Denitrification has been extensively studied, and a few
denitrification processes have been built into municipal plants.
Denitrifying bacteria can grow either in a suspended growth
system similar to activated sludge or in an attached growth
system similar to a trickling filter. Sludge production for
ordinary nitrified domestic waste is roughly 300 pounds per
million gallons (30 mg/1) of wastewater treated (37).
4.4 Chemical Sludges
4.4.1 Introduction
Chemicals are widely used in wastewater treatment to precipitate
and remove phosphorus, and in some cases, to improve suspended
solids removal. At all such facilities, chemical sludges are
formed. A few plants apply chemicals to secondary effluent and
4-36
-------
use tertiary clarifiers to remove the chemical precipitates. An
example of this arrangement is the plant at South Lake Tahoe,
California. However, it is more common to add the chemicals to
the raw wastewater or to a biological process. Thus, chemical
precipitates are usually mixed with either primary sludge solids
or biological sludge solids.
The discussion below is brief because the subject of chemical
sludges and their characteristics is discussed in detail
elsewhere (79-82). A 1979 publication provides considerable
background information on theoretical rates of chemical sludge
production, as well as actual operating data from wastewater
treatment plants employing chemicals for removal of phosphorus
(7). Also, production of chemical sludges in primary sedimenta-
tion is discussed in Section 4.2.2.5.
4.4.2 Computing Chemical Sludge Production -
Dry Weight Basis
Chemicals can greatly increase sludge production. The amount of
increase depends on the chemicals used and the addition rates.
There is no simple relationship between the mass of the chemical
added and the mass of sludge produced. It is beyond the scope of
this manual to describe in detail the chemistry associated with
the chemicals used in treating wastewater, and the various
solids-producing reactions that can occur. However, several
types of precipitates that are produced and must be considered
in measuring the total sludge production are listed below:
• Phosphate precipitates. Examples are A1PC>4 or
Al (H2P04~) (OH) 2 with aluminum salts, FePC>4 with iron
salts, and Ca3(P04)2 with lime (79,82,83).
• Carbonate precipitates. This is significant with lime,
which forms calcium carbonate, CaCC>3. If two-stage
recarbonation is used, a recarbonation sludge of nearly
pure CaCC>3 is formed (84).
• Hydroxide precipitates. With iron and aluminum salts,
excess salt forms a hydroxide, Fe(OH)3 or Al(OH)3.
With lime, magnesium hydroxide, Mg(OH)2f may form; the
magnesium comes from the influent wastewater, from the
lime, or from magnesium salts.
* Inert solids from the chemicals. This item is most
significant with lime. If a quicklime is 92 percent CaO,
the remaining eight percent may be mostly inert solids
that appear in the sludge.. Many chemicals supplied in
dry form may contain significant amounts of inert solids.
• Polymer solids. Polymers may be used as primary
coagulants and" to improve the performance of other
coagulants. The polymers themselves contribute little
4-37
-------
to total mass, but they can greatly improve clarifier
efficiency with a concomitant increase in sludge
production.
• Suspended solids from the wastewater. Addition of any
chemical to a wastewater treatment process affects
process efficiency. The change in sludge production must
be considered.
Quantities of the various precipitates in chemical sludges are
determined by such conditions as pH, mixing, reaction time, water
composition, and opportunity for flocculation.
Chemical sludge production, like the production of other sludges,
varies from day to day. The variation depends strongly on
chemical dosage and on wastewater flows. If the chemical dosage
is about constant in terms of milligrams per liter of wastewater,
chemical solids production will still vary, since flows fluctuate
from day to day. Changes in wastewater chemistry may also
affect the production of chemical sludge. For example,
stormwater inflow typically has a lower alkalinity than ordinary
wastewater. During storms, the production of chemical sludge
will be different from production in dry weather.
4.4.3 Properties of Chemical Sludges
Chemical sludge properties are affected mainly by the precipi-
tated compounds and by the other wastewater solids. For example,
a lime primary sludge will probably dewater better than a lime
sludge containing substantial amounts of waste-activated sludge
solids (80). Generally speaking, lime addition results in a
sludge that thickens and dewaters better than the same sludge
without chemicals. When iron or aluminum salts are added to raw
wastewater, the primary sludge does not thicken or dewater as
well as non-chemical sludge. Iron sludges dewater slightly more
easily than aluminum sludges (79). When aluminum salts are added
to activated sludge, the sludge may thicken much better than
non-chemical activated sludge (85,86). Anionic polymers can often
improve the thickening and dewatering properties of chemical
sludges.
For efficient chemical usage, feed rates must be adjusted to
match changes in wastewater flow and composition.
4.4.4 Handling Chemical Sludges
Most of the common sludge treatment processes can be used with
chemical sludges: thickening, stabilization by digestion,
incineration, etc. This section summarizes information on
stabilization and also on recovery of chemicals and by-products.
4-38
-------
4.4.4.1 Stabilization
Lime sludges may be stabilized by a small additional dose of
lime. Lime stabilization may also be used for aluminum and iron
sludges. The lime improves dewatering of these sludges by acting
as a conditioning agent. Chapter 6 discusses lime stabilization
of chemical sludges. Dewatered lime-stabilized sludges can
usually be buried in sanitary landfills.
Digestion of mixed biological-chemical sludges is generally
feasible. Pure chemical sludge will not digest. Studies done
in 1974 and 1978, however, note significant reductions in
digestibility as chemicals were added to sludge; the studies
investigated the addition of aluminum, iron, and polymer (87,88).
4.4.4.2 Chemical and By-product Recovery
Where lime use results in calcium carbonate formation, it may
be feasible to recover lime by recalcination. Tertiary lime
treatment, as practiced at the South Lake Tahoe, California,
plant is well suited to lime recovery; a recalcination process
has been operated there for several years. Where lime is added
to raw wastewater, lime recovery is more difficult but still
possible. Lime recovery does not reclaim all of the calcium,
as some is always lost with the phosphate, silica, and other
materials that must be removed from the system. Lime recovery
reduces but does not eliminate the amount of residue for
disposal. Feasibility of lime recovery depends on plant size,
amount of calcium carbonate formed, cost of new lime, and cost of
sludge disposal (81,82).
4.5 Elemental Analysis of Various Sludges
As a rule, almost anything can be found in sludge. This section
describes trace elements in all types of sludge. Data on
concentrations of the 74 elements found in wastewater sludge are
included in References 89-95.
4.5.1 Controlling Trace Elements
It is a basic principle of chemistry that elements are not
created or destroyed but chemically recombined. Therefore, the
mass of each element entering a treatment plant fixes the mass
that either accumulates within the plant or leaves it. The
mass leaving the plant does so in gaseous emissions, effluent,
a special concentrated stream, or sludge. Extracting toxic
elements from sludge appears to be impractical; source control is
the most practical way to reduce toxicants.
4-39
-------
Trace elements are present in industrial process waste,
industrial waste spills, domestic water supply, feces and urine,
and detergents. Additional trace elements are derived from:
• Chemicals in photographic solutions, paints, hobby
plating supplies, dyes, and pesticides used in households
and commercial enterprises.
• Storm inflow (this is particularly true for lead from
gasoline anti-knock compounds).
• Corrosion of water piping, which contributes zinc,
cadmium, copper, and lead (96).
• Chemicals used in wastewater treatment, sludge
conditioning, etc. Table 4-13 shows an analysis of
ferric chloride, which is an industrial by-product
(pickle liquor) of wastewater solids treatment.
TABLE 4-13
METALS IN FERRIC CHLORIDE SOLUTIONS (97)
Constituent Concentration, mg/1
Cadmium 2-3.5
Chromium 10-70
Copper 44 - 14,200
Iron 146,000 - 188,000
Nickel . 92 - 6,200
Lead ; 6 - 90
Silver 2
Zinc 400 - 2,150
aThree different liquid sources were
analyzed (43 percent FeCl3).
The quantity of toxic pollutants may be significantly reduced
by source control. At Los Angeles County, metal finishing
industries were a major source of cadmium, chromium, copper,
lead, nickel, and zinc. A source control program was developed
in cooperation with the local Metal Finisher's Association. This
program was quite successful, as shown in Table 4-14, by the
general downward trend in wastewater concentrations over time.
4-40
-------
TABLE 4-14
PROGRESS IN SOURCE CONTROL OF TOXIC POLLUTANTS (98)
Concentration in mg/1 in influent wastewater
Wastewater
pollutant
Cadmium
Chromium
Copper
Lead
Nickel
Zinc
January-June
1975
July-December
1975
January-June
1976
July-September
1976
October-December
1976
0.037
0.70
0.45
0.40
0.31
1.55
0.031
0.73
0.45
0.31
0.33
1.48
January
1977
0.029
0.78
0.45
0. 34
0. 35
1.37
0.033
0.61
0.33
0.28
0.34
1.41
0.027
0.47
0. 34
0.32
0.27
1.29
0.019
0.43
0.30
0. 34
0.21
1.17
Data for Joint Water Pollution Control Plant, Los Angeles County,
California; weekly composite samples. (13).
Occasionally, elements can be converted from a highly toxic form
to a less toxic form in wastewater treatment. Chromium is a good
example of this. In its hexavalent form, it is highly toxic, but
may be converted to the less toxic trivalent form in secondary
treatment.
4.5.2 Site-Specific Analysis
The elemental compositions of various sludges differ from one
another. If sludges are to be reused, they should be analyzed
for a number of elements. The importance of site-spec ific
analysis of sludges varies with the size of the project,
regulatory requirements, industrial activity, and the type of
reuse desired. A sampling program should recognize that:
• One plant's sludge may have 100 times or more of a
certain element than another plant's.
• There may be major variations between samples at the same
plant. A single grab sample may produce misleading
results. Careful attention to sampling and statistical
procedures will tend to reduce the uncertainty. A
detailed report on such procedures is available (99).
• Estimates of trace element sludge contamination based
on wastewater analysis are usually less useful than
estimates based on sludge testing. However, if an
element can be measured in the influent wastewater and if
flow rates are known, then a mass load (Ib or kg per day)
may be computed. For purposes of estimating sludge
contamination, it is reasonable to assume that large
trace amounts of cadmium, copper, and zinc appear in the
sludge. Analyses of sludge and supernatant samples from
a facultative sludge lagoon have shown that there is a
tendency for nickel and lead to be gradually released
from the sludge to the liquid phase (97).
• Sludge samples should be analyzed for percent solids
and percent volatile as well as for trace elements.
4-41
-------
4.5.3 Cadmium
Because it is often found in amounts that limit sludge reuse as a
soil conditioner, cadmium is a critical element. If sludge
containing cadmium is applied to agricultural cropland, some
cadmium may enter the food chain. It has been argued, with
much controversy, that the normal human dietary intake of cadmium
is already high in comparison to human tolerance limits and
that sources of additional cadmium should be strictly limited
(100,101). Table 4-15 summarizes reports on cadmium in sludge.
Chapter 18 includes a discussion of the control of sludge
application rates for the purpose of limiting cadmium levels in
soil and crops. Additional information on this subject is
provided in reference 90.
TABLE 4-15
CADMIUM IN SLUDGE
Concentration, mg/dry kg
Type of sludge
Digested
Heat dried
Anaerobic
"Other"
Not stated
Incinerator ash
Digested
Digested waste-acti-
vated
Dewatered digested
primary
Digested
Raw
Digested
Raw primary
Mesophilic digested
Thermophilic digested
Waste- activated
Anaerobically digested
chemical and waste-
activated (3.9 per-
cent average solids)
Anaerobically digested
chemical and waste-
activated (3.2 per-
cent)
Anaerobically digested
chemical and waste-
activated (4.2 per-
cent)
Raw primary
Raw primary
Raw primary
Raw primary and bio-
filter
Raw primary and bio-
filter
Raw primary and bio-
filter
Location
12 U.S. cities
4 U.S. cities
Various U.S.
Various U.S.
42 cities in England,
Wales
Palo Alto, California
Chicago (Calumet)
Chicago (West-Southwest)
Seattle (West Point)
Cincinnati (Millcreek)
Several U.S. cities
About 25 U.S. cities c
Los Angeles (Hyperion)
Los Angeles (Hyperion)0,
Los Angeles (Hyperion)
Los Angeles (Hyperion)
Chatham, Ontario0
Simcoe, Ontario
Tillsonburg, Ontario
Sacramento, California
(Northeast)
Sacramento (Rancho
Cordova)
Sacramento (Natomas)
Sacramento (Highland
Estates)
Sacramento (County Sani-
tation District 6)
Sacramento (Meadowview)
Standard
Mean deviation
89
150
106
70
-
84
-
340
48
A
130
30
75
39
140
120
110
2.6
78
9
2.8
3.0
3.5
4.1
3.6
3.1
72
200
-
-
-
-
-
-
-
K
1.51°
15
104
-
-
-
-
1.4
5
1
1.1
1.4
1.1
1.3
3.3
1.0
Median
65
67
16
14
<200
-
-
-
-
-
20
31
-
-
-
-
1.8
72
9'
, 2.6
2.6
3.6
3.8
2.5
2.6
Range
6.8
15
3
4
<200
(7
68
10
9
0
66
7
1.4
1.2
2.2
2.8
1.0
2.3
- 200
- 440
- 3,410
- 520
- 1,500
>200)
- 99
- 35
-
-
-
-
- 550
-
-
-
-
- 10
- 110
- 12
- 4.2
- 4.5
- 5.1
- 5.9
- 9.1
- 4.4
Number of
samples
12
4
98
57
42
2
-
43
100
approximate
25
20
80
-
-
"
-
225
198
40
. 5d
,
5d
,
5
5
5d
J
5d
Reference
89
89
90
90
91
92
93
102
94
95
95
95
103
103
103
103
99
99
97
97
97
97
97
97
Geometric mean.
b
Spread factor for use with geometric mean.
cConcentrations reported on wet weight basis and converted
to dry weight basis.
Weekly composites of daily samples.
4-42
-------
TABLE 4-15
CADMIUM IN SLUDGE (CONTINUED)
Concentration, mg/dry kg
Type of sludge
Raw primary and bio-
filter
Waste activated
Raw primary and waste-
activated
Raw primary
Anaerobically digested
ferric chloride
Anaerobically digested
chemical (mostly alum)
Anaerobically digested
lime
Anaerobically digested
ferric chloride
Geometric mean.
bc
Location
Sacramento (City Main)
Sacramento (Arderi)
Sacramento (Rio Linda)
Sacramento (County
Central)
North Toronto , Ontario
Point Edward, Ontario
Newmarket, Ontario
Sarnia, Ontario
.. .
Mean
10.5
5.4
9.7
29
29
8.5
7.5
76
Standard
deviation Median Range
2.0 11 7.6 - 13
2.6 6.7 2.3 - 7.7
2.9 9.1 6.2 - 14
28 12 8.3 - 72
9 - -
1.9
4.2
21
Number of
samples
5d •
5d
5
. 5d
1
f 60
61
59
40
Reference
97
97
97
97
104
104
104
104
Concentrations reported on wet weight basis and converted
to dry weight basis.
Weekly composites of daily samples.
4.5.4 Increased Concentration During Processing
Toxic elements often are non-volatile solids that remain in
sludge after volatile solids have been removed. Removal of
volatile solids such as organic matter increases the concentra-
tion of non-volatile components, expressed on a dry weight basis.
Table 4-16 shows this effect for four metals at one plant. This
increased concentration may be important if sludge reuse is
desired and if regulations limit reuse for sludge that contains
contaminants that exceed certain concentrations.
TABLE 4-16
INCREASED METALS CONCENTRATION DURING PROCESSING
Concentration, mg/kg dry weight
Element
Chromium
Copper
Nickel
Zinc
Number of samples
Note: 1977 data, Sacramento County Central treatment plant, California. Anaerobic
digesters also receive thickened waste-activated sludge (metals content not
measured).
4-43
Raw primary sludge
(79
percent volatile)
110
200
46
620
(5)
Anaerobically digested
sludge
(68 percent volatile)
160
340
63
930
(2)
Lagooned sludge
(56 percent volatile)
220
450
65
1, 400
(30)
-------
4.6 Trace Organic Compounds in Sludge
Several of the trace organic compounds found in sludge, for
example, polychlorinated biphenyls (PCBs), are toxic, slow to
decompose and widely distributed in the environment. Table 4-17
quantifies the amount of Aroclor 1254, a common PCB, found in
sludge. Three other PCBs, Aroclors 1242, 1248, and 1260, have
also been found in sludge (105,107,108). In 1970, the production
of PCBs for several end uses was halted in the United States
and was completely phased out in 1977. As of 1979, imports of
PCBs are prohibited except for a few special purposes. It is
anticipated that these measures will help to reduce PCB levels in
sludge. However, products containing PCBs are still in use, and
these chemicals are widely distributed, so that several years may
elapse before PCBs become undetectable in sludge.
TABLE 4-17
AROCLOR (PCB) 1254 MEASUREMENTS IN SLUDGE
Average
concentration of
samples with
compound detected
Location
Hamilton, Ontario
Kitchener , Ontario
Newmarket, Ontario
North Toronto, Ontario
Wet
basis,
ug/1
81
110
74
120
Dry
basis, Number of Samples with Year of sample
mg/kg samples compound detected collection
- - 1976
- - - 1976
- - 1976
- - - 1976
Reference
105
Sludge type
Undigested
Undigested (with Al)
Undigested (with Ca)
Undigested (with Fe)
Raw primary Sacramento, CA (North- 50
east)
Sacramento, (Natomas) 60
Sacramento (County 80
Central)
Ra« pr imary and biofilter Sacramento, {City Main) 30
Sacramento (County Sani- 50
tation District 6)
Sacramento (Headowview) 50
Sacramento (Rio Linda) 90
Raw primary and waste
activated
Lagooned digested primary Sacramento (County
and waste activated Central)
Digested
Heat dried
10 U.S. cities
4 U.S. cities
1.5
1.8
3.8
2.0
2.4
3.5
3.9
9.3
10
4
1977
1977
1977
1977
1977
1977
1977
1971-1972
1971-1972
Weekly composite of daily samples.
Because of their fat-soluble nature, PCBs tend to concentrate
in skimmings and scum at wastewater treatment plants. The
conventional procedure of introducing skimmings into the
digester can cause higher concentrations of PCBs in the final
sludge. Alternative disposal procedures for skimmings, such as
incineration, can reduce this problem.
Table 4-18 presents data on three chlorinated hydrocarbon
pesticides found in sludge from several treatment plants.
4-44
-------
TABLE U-18
CHLORINATED HYDROCARBON PESTICIDES IN SLUDGE (97, 106)
Compound
Hexachlorobenzpnp
Hexachlorobenzene
Lindane
Technical -qradp rhlordane
Sludge type
Waste- activated
Raw primary
Waste- activated
Raw primary
Raw primary
Raw primary
Raw primary
Lagooned anaerobically di-
Plant
Arden
County Central
Arden
Northeast
Northeast
Natomas
County Central
County Central
samples with compound
detected, mg/dry. kg
0.8
0.4
1.0
0.6
2.6
2.3
2.8
4.2
Total
samples
5a
5a
5a
5a
5a
a
5
30
Samples with
compound detected
1
2
1
1
1
2
5
3C
gested primary and waste-
activated
Waste-activated
Raw primary and waste-
activated
Raw primary and biofilter
Raw primary and biofilter
Arden
Rio Linda
Meadowview
City Main
4.4
5.5
• 0.6
19
All plants in Sacramento County, California.
Weekly composites of daily samples.
4.7 Miscellaneous Wastewater Solids
In addition to the primary, biological, and chemical sludges
discussed in previous sections, there are several other
wastewater solids that must be properly handled to achieve good
effluent, general environmental protection, and reasonable
treatment plant operations. These solids include screenings,
grit, scum, septage, and filter backwash.
When mixed with primary or secondary sludges, screenings, scum,
grit, and septage can interfere with the processing and reuse of
the sludge. Before mixing these wastewater solids with primary
and secondary sludges, design engineers should consider the
following:
• Screenings and scum detract from the final appearance,
and marketability, and utilization of sludges. They can
also clog piping, pumps, and mixers, and occupy valuable
space in digesters and other tankage.
• Scum presents a special problem when mixed with other
solids and subjected to gravity thickening, decanting, or
centrifugation. Under these conditions, scum tends to
concentrate in the sidestream and to be recycled to the
wastewater processes. Eventually some of this recycled
scum is discharged to the effluent.
• Grit can block pipelines, occupy valuable space in
digesters and other tankage, and cause excessive wear to
solids piping and processing equipment.
4-45
-------
4.7.1 Screenings
Screenings are materials that can be removed from wastewater by
screens or racks with openings of 0.01 inch (0.25 mm) or larger.
Coarse screens or racks have openings larger.than 0.25 inch
(6 mm), whereas fine screens have openings from 0.01 to 0.25 inch
(0.25 to 6 mm). If openings are larger than 1.5 inches (38 mm),
the screens are often called trash racks.
Racks and screens are usually installed to treat the wastewater
as it enters the treatment plant. Racks and coarse screens
prevent debris from interfering with other plant equipment. Fine
screens remove a significant fraction of the influent suspended
solids and 8005, thus reducing the load on subsequent treatment
processes. In this regard, fine screens may act like primary
sedimentation tanks, although they do not ordinarily remove as
much of the solids as do sedimentation tanks. Fine screens are
usually protected by upstream coarse screens or racks.
4.7.1.1 Quantity of Coarse Screenings
Coarse screenings are basically debris. Items typically
collected on coarse screens include rags, pieces of string,
pieces of lumber, rocks, tree roots, leaves, branches, diapers,
and plastics.
The quantity of coarse screenings is highly variable, but most
plants report 0.5 to 5 cubic feet per million gallons (4 ml/m3
to 40 ml/m3) on average flows. Table 4-19 shows the quantities
of screenings reported for a number of communities. The quantity
of screenings depends on:
• Screen opening size. Generally, greater quantities are
collected with smaller screen openings. This was seen
most clearly at Grand Island, Nebraska, where a change
from 0.5-inch to 1.25-inch (13 to 32 mm) openings
caused screenings production to drop from about 7 to
about 3 cubic feet per day (0.2 to 0.08 m3/day) (114) .
Tests at Chicago, Illinois, and Adelaide, Australia,
showed this tendency also (13).
• Shape of openings. For example, bar racks may have
openings 0.75 inch (19 mm) wide and over 2 feet (over
600 mm) long. Such a rack will pass twigs, ballpoint
pens, and other debris, that would be captured on a mesh-
type screen with square openings of 0.75 inch (19 mm).
• Type of sewer system. Combined storm and sanitary
sewers produce more screenings than separate sanitary
sewers. This effect is especially pronounced where much
or all of the combined wastewater is treated during and
after storms, rather than being bypassed.
4-46
-------
TABLE 4-19
SCREENING EXPERIENCE (109, 110)
Rack or screen
opening, in.
3-3/8
3
3
3
1-3/8
1-1/2
1-1/2
1-1/2
1-1/2
1-1/2
1-1/2
1-1/4
1-1/4
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
7/8
7/8
3/4
3/4
3/4
3/4
3/4
3/4
3/4
3/4
1/2
1/2
1 in. = 2.54 cm.
1 mgd = 3,785 m3/day
1 cu ft/mil gal = 7 .
City
Norwalk, Connecticut
New Haven, Connecticut
East Hartford, Connecticut
San Jose, California
New York, New York, Jamaica
Philadelphia, Pennsylvania, North
Oklahoma City, Oklahoma, Southside
Cranston, Rhode Island
Taunton, Massachusetts
Meadville, Pennsylvania
Grove City, Pennsylvania
Uniontown, Pennsylvania
Fargo, North Dakota
New York, Wards Island
New York, Owls Head
Minneapolis-St . Paul, Minnesota
New York, Hunts Point
East Bay, Oakland, California
New York, Coney Island
New York, 26th Ward
New York, Tallmans Island
Bridgeport, Connecticut, West Side
New York, Rockaway
Waterbury, Connecticut
Bridgeport, Connecticut, East Side
Duluth, Minnesota
Austin, Minnesota
Fond du Lac, Wisconsin
Findlay, Ohio
Massillon, Ohio
York, Nebraska
Marion, Ohio
Gainesville, Florida
Marshalltown, Iowa
East Lansing, Michigan
Birmingham, Michigan
Boston, Massachusetts, Nut Island
Richmond, Indiana
Detroit, Michigan
New York, Bowery Bay
Hartford, Connecticut
Portsmouth, Virginia
Sheboygan, Wisconsin
Aurora, Illinois
Topeka , Kansas
Oshkosh, Wisconsin
Green Bay, Wisconsin
Manteca, California
f
48 m3/! x 106 m3.
Flow,
mgd
11.75
8
4.0
-
65
48.2
25.0
8.32
3.5
2.5
0.8
3.0
2.7
180
160
134
120
98
70
60
40
17
15
15
14
12
9
7.2
7
5.2
5
5.0
5
4.0
3.8
1.5
125
6.2
450
40
39.0
9.7
8.0
8.0
7. 5
6.0
10.0
1.5
Screenings,
cu ft/mil gal
0. 17
1.0
1. 33
0.25
0.6
2.20
2.1
0.65
1.0
0.6
0. 1
0. 9
4.55
1.0
0.6
0. 9
0.7
1.6
1.4
1.1
0.7
0.93
1.0
2.35
2.04
0.56
1.1
5
0. 39
1.5
1. 5
2.5
3.5
0.25
0.4
1. 2
1.2
1. 2
0.47
1.1
1.6
0.82
0.25
1.42
1.30
1.7
1.2
5.2
4-47
-------
• Operating practices. Where manual cleaning is used,
operatorssometimes pass some screenings through or
around the screens. Where automatic equipment is used,
the operating pattern can greatly affect removals (112).
• Length of sewer system. The volume of screenings removed
may double with a short, as opposed to lengthy, inter-
ceptor system. This condition may be explained by the
fact that solids are more subject to disintegration with
a lengthy collection system (5). Wastewater pumping will
also tend to disintegrate large solids.
Screenings loads may increase dramatically during peak flows. It
is estimated that, at the East Bay Municipal Utility District
plant in Oakland, California, the screenings load was about
10 times the average during peak flows. For the most part, this
plant services separate sanitary sewers, but the screenings load
is concentrated (110).
4.7.1.2 Quantity of Fine Screenings
Fine screens are usually used as an alternative to conventional
primary sedimentation to remove suspended solids. Screens
with 0.09 to 0.25 inch (2 to 6 mm) openings remove about 5 to,
10 percent of suspended solids from typical municipal raw
wastewater. If 0.03 to 0.06 inch (0.8 to 1.5 mm) openings are
used instead, about 25 to 35 percent of suspended solids may be
removed (5). Higher removals increase the dry weight and the
moisture content of the screenings. For example, consider fine
screens that remove 25 percent of suspended solids from an
influent concentration of 300 mg/1. In this case, screenings are
630 dry pounds per million gallons (75 mg/1) of wastewater. If
the screenings are ten percent solids and weigh 60 wet pounds per
cubic foot (961 kg/m3), then the volume is 105 cubic feet per
million gallons (14.04 m3/lx!06 m3). This is over 25 times more
than a typical value for coarse screenings of 4 cubic feet per
million gallons (0.53 m3/lx!06 m3).
4.7.1.3 Properties of Screenings
If screenings have not been incinerated, they may contain
pathogenic microorganisms. They are also odorous and tend to
attract rodents and insects. Screenings have been analyzed
for solids content, volatile content, fuel value, and bulk
wet weight. Some of the reported values are summarized in
Table 4-20.
4.7.1.4 Handling Screenings
Screenings may be ground and handled with other sludges; direct
landfilled; and incinerated, with the ash disposed in landfill.
Table 4-21 summarizes the advantages and disadvantages of various
methods.
4-4!
-------
TABLE 4-20
ANALYSES OF SCREENINGS
Solids content,
percent dry
solids
Volatile content,
percent
Fuel value,
Btu/lb dry
solids
Bulk wet weight,
Ib/cu ft
Comments
References
20
10 - 20
B - 23
6.1
17
5,4003
80 - 90
68 - 94
96
86
7,820
Coarse screenings. Fine
screenings may have
lower solids content.
Common values
Various plants, fine
screens, 0.03 to 0.12
inch openings
Thickened ground screen-
ings from 0.75-inch
racks; after grinding,
screenings were thick-
ened on a static screen
with 0.06-inch openings.
Dewatered ground screen-
ings from 0.75-inch
racks; after grinding,
screenings were de-
watered on a rotating
drum screen with 0.03-
inch openings.
Fine screenings
13
113
113
Computed.
1 Btu/lb dry solids = 2,32 kJ/ dry solids.
1 Ib/cu ft = 16.03 kg/m3.
1 in. = 2.54 cm.
Some fecal solids accompany the larger materials such as rags and
twigs. For this reason, as well as to save labor time and cost,
it is desirable to mechanize screenings handling. Also, where
coarse screenings are landfilled or incinerated, it is desirable
to use the largest rack opening that will adequately protect
downstream processes. This will minimize the quantity of
screenings that must be handled separately.
Screenings may be transported pneumatically (116), in sluiceways,
on conveyors, and in cans, dumpsters, or covered trucks.
Screenings-water mixtures that are ground may be pumped. For
thickening and dewatering, fine static screens, drum screens,
centrifuges (113), and drum or screw presses may be used.
Chemical conditioning is not required.
4.7.1.5 Screenings from Miscellaneous Locations
Screens are occasionally used on streams other than influent
wastewater. For instance, when it is fed to a trickling filter,
primary effluent may be screened to prevent clogging of orifices
in the distributor on the trickling filter (109). At one heavily
loaded plant where regular influent screening equipment was
partially bypassed, screens installed in aeration basin effluent
channels, chlorine contact tank outlets, and other locations
prevented coarse, floating objects from being discharged with
4-49
-------
the effluent (117). Another example occurred at a plant where
digested sludge was discharged to the ocean. Fine screens were
used to prevent floatable materials from being discharged (118).
Other examples of the use of screens on streams other than
influent wastewater are the screening of overflow water from grit
separators and the screening of feed sludge to disc-nozzle
centrifuges to prevent clogging (113,119).
TABLE 4-21
METHODS OF HANDLING SCREENINGS
Method
Advantages
Disadvantages
1. Comminution within main wastewater stream
handle comminuted screenings with
other wastewater solids, e.g., primary
sludge
2. Removal from main stream, grinding or
maceration, and return to main stream
3. Removal3 from main stream, draining or
dewatering, landfill
Removal3 from main stream, dewatering,
incineration, landfill of ash.
Anaerobic digestion of fine screenings
alone (not mixed with other
solids)
Anaerobic digestion of screenings
together with scum but separate
from other sludges
Highly mech.ykized, low operating labor re-
quiremen,*.
Minimizes number of unit operations
Usually free of nuisance from flies and
odors
Widely used, familiar to plant operators
Similar to Method 1, except more complex
mechanical ly
Keeps screenings out of other sludges;
avoids disadvantages of Methods 1 and 2.
Can be fairly well mechanized.
Keeps screenings out of other sludges;
avoids disadvantages of Methods 1 and 2.
Ash is very small in volume and easy to
transport and dispose of.
If incineration is used for other sludges
and/or grit, then screenings can be
added at modest cost*
Pathogen kill
Sludge contains screenings, which may inter-
fer with public acceptance for reuse of
sludge as a soil amendment.
Sludge probably needs further maceration or
screening if it is to be pumped or
thickened in a disc centrifuge.
If sludge is to be digested, digesters must
be cleaned more often. Plastics and
synthetic fabrics do not decompose in
digesters. Aggravates digester scum pro-
blems . Ground screenings tend to
agglomerate in digesters.
Not appropriate if suspended solids removal
is required (fine screens).
Not appropriate for very large screenings
loads, especially if high grit loads are
also present (large plants, combined
sewers)
Similar to Method 1, except Method 2 can be
designed for very large flows and screen-
ings loads. Method 2 is more expensive
than Method 1 for small screenings loads.
Transport of screenings may be difficult.
Unless carefully designed and operated,
causes fly and odor nuisances and health
hazards.
Regulations for landfill disposal may strongly
affect operations.
High cost if an incinerator is required for
screenings alone.
Unless incinerator is properly designed and
operated, air pollution (odor and partic-
ulates) will be serious.
Not well adapted to wide fluctuations in
screenings quantities, unless screenings
are only a small part of the total in-
cinerator load.
Digestion was tested at large scale at
Milwaukee, Wisconsin, but found to be
impractical.. (115)
Tested at Malabar plant, Sydney, Australia,
but found to be inoperable. Material
handling was the chief difficulty.
Mechanical removal is usually practiced at large plants. Manual removal is frequently used at small plants. The advantages
of manual removal are simplicity and low capital cost; the disadvantages are high operating labor requirements and fly and odor
problems. A common arrangement at small plants is to install a single comminutor with a manually cleaned bar rack as a standby unit.
4.7.2 Grit
Grit is composed of heavy, coarse solids associated with raw
wastewater. It may be removed from wastewater before primary
sedimentation or other major processes. Alternatively, it may be
4-50
-------
removed from primary sludge after the primary sludge is removed
from the wastewater. Typical ingredients of grit are gravel,
sand, cinders, nails, grains of corn, coffee grounds, seeds, and
bottle caps.
4.7.2.1 Quantity of Grit
The amount of grit that is removed varies tremendously from one
plant to another. Table 4-22 shows grit quantities measured at
several plants. Additional values have been published elsewhere
(5,109). The quantity of grit depends on:
• Type of collection system. If a system is combined, then
street sanding, catch basin maintenance, and amount of
combined sewer overflow become important.
• Degree of sewer system corrosion. Grit may include
products of hydrogen sulfide corrosion derived from the
pipes ( 122) .
• Scouring velocities in the sewers. If scouring
velocities are not regularly maintained, grit will build
up in the sewers. During peak flows, the grit may be
resuspended, and the treatment plant may receive heavy
loads during peak flows.
* Presence of open joints and cracks_in _the__sew_e_r system .
These permit soil around the "pipe's to~e~nter "the sewers .
This effect also depends upon soil characteristics and
groundwater levels.
• Structural failure of sewers. Such failures can deliver
enormous amounts of grit to the wastewater system.
* Quantities of industrial wastes .
* S££££llJ^=iWJ^ grinders are used.
• Efficiency of grit removal at the treatment plant ( 5 ) .
• Amount of septage.
• Occurrence of construction in the service area or at the
treatment plant.
It is not possible to develop a formula which allows for all
these factors. Cautious use of available information is,
therefore, recommended. It is important to recognize that
extreme variations occur in grit volume and quantity. A generous
safety factor should be used in calculations involving the
storage, handling, or disposal of grit (5). In a new system
where there are separate sanitary sewers and favorable conditions
4-51
-------
such as adequate scouring velocities, an allowance of 15 cubic
feet per million gallons (2 m3/lxl06 m3 ) should suffice for
maximum flows. On the average, the quantity of grit in waste-
water will usually be less than 4 cubic feet per million gallons
(0.53 m3/lxl()6 m3) for separate sewer systems, (5) but
higher values have been observed (see Table 4-22).
TABLE 4-22
GRIT QUANTITIES
Plant
Quanti ty,
cu ft/mil gal
Comments
Santa Rosa, California (College
Avenue)
San Jose, California
Manteca, California
Santa Rosa, California (Laguna)
Seattle, Washington (West Point)
Dublin-San Ramon, California
Los Angeles, California
(Hyperion)
Livermore, California
Gary, Indiana
Renton, Washington
O.HH
0.3
1.4
2.5
5.2
3.2
9.5
5.0
2.1
10.7
2.6
11.2
7
2
1.0
0.3
2.4
8.6
89
1.7
4. 1
7.0
Average. Separate sewers.
Minimum month
Maximum month
Separate sewers. Older removal
.systems removed less yrit (0.3
and 1.4 cu ft/mil gal)
Average. Separate sewers.
Minimum month
Maximum month
Average. Separate sewers.
Minimum month
Maximum month
Average. Combined storm and sani-
tary sewers.
Maximum day
Average. Separate sewers.
1973 average. Separate sewers.
Average over 24 months. Separate
sewers.
Lowest month
Highest month
Annual average. Combined sewers.
Highest value on test runs.
Average over 19 months before im-
provements to grit removal equip-
ment. Separate sewers.
Average over 12 months after im-
provements.
Maximum month, following improve-
ments .
References
110
110
110
110
110
120
99
68
110
121
119
1 cu ft/mil gal = 7.48 m /.I x 10 m
4.7.2.2 Properties of Grit
Grit has been analyzed for moisture, volatiles content, specific
gravity, putrescibility, (123) particle size, and heating value.
All of these depend on the kind of sewer system and the method
of grit removal and washing.
4-52
-------
The moisture content of grit is reported as ranging from 13 to
65 percent, and the volatiles content from 1 to 56 percent (109).
Specific gravity of grit particles varies; values from 1.3 to
2.7 have been reported (109). The range for volatile solids was
8 to 46 percent (123). Particle size for grit removed from five
plants is shown in Table 4-23, along with an analysis of digester
bottom deposits. .
TABLE 4-23
SIEVE ANALYSIS OF GRIT
Percentage retained
Sieve size
4
e
10
12
20
28b
40
50
60
65b
80
100
150b
200
Sieve opening,
4.76
2.38
2.08
1.41
0.84
0.6C
0.42
0.30
0.25
0.21
0.18
0.149
0.105
0.074
Green Bay,
Wisconsin
3.7
9.1
19.8
29.6
51.7
78.2
96.1
(109)
Kenosha,
Wisconsin
Tampa,
Florida
St. Paul,
Minnesota
1-7
5-20
99.5
(109)
Renton,
Washington
2.5 - 13.5
19.5 - 34.5
50 - 74.5
71 - 88.5
90.5 - 94
97.5
99.5
(119)
Renton,
Washington
0 - 0.5
2-11
10 - 41
27 - 62
60 - 76.5
95 - 98
(119)
Digester deposits,
Los Angeles,
California
7.3
28.3
77.6
84.9
(118)
U.S. series, except as noted.
Tyler series sieve.
Dried at 103°C. Four tests. Volatile contents 34 to 55 percent.
Same samples as previous column, ashed at 550°C and resieved.
Grit quality can be varied to some extent. If a "clean" grit
with very low putrescibility is desired, it may be obtained by
grit washing and operational adjustments to the grit removal
system. However, such operations may make it impossible to
remove fine sand (of less than 0.08 inch [0.2 mm]). For example,
if a separate grit washer is used, fine sand may be recycled' in
the wash water. If it is essential, fine sand can be removed
with high efficiency. However, the sand will be accompanied by
large amounts of putrescible solids. A compromise between
cleanliness of grit and high removals of fine particles is
necessary (124). If good washing equipment is used, operators
can often remove significant quantities of fine materials without
sacrificing cleanliness. Grit should be regarded as containing
pathogens unless it has been incinerated.
4.7.2.3 Handling Grit
The first step in grit handling is
from the main stream of wastewater.
the separation of the grit
Grit may be removed from
4-53
-------
-------
4.7.3 Scum
Scum is the material that floats on wastewater, except where
flotation is involved. In a flotation unit, scum is incorporated
into the float. Scum may be removed from many treatment
units including preaeration tanks, skimming tanks, primary
and secondary sedimentation tanks, chlorine contact tanks,
gravity thickeners, and digesters. The term "skimmings" refers
specifically to scum that has been removed.
4.7.3.1 Quantities of Scum
Quantities of scum are generally small compared to those of such
wastewater solids as primary sludge and waste-activated sludge.
Table 4-24 lists some properties and quantities of scum. The
data in this table are based on scum from primary sedimentation
tanks. Scum is often removed from secondary clarifiers and
chlorine contact tanks, but there is almost no available data on
the quantities removed.
Although there is some data on the quantity of grease removed
during wastewater treatment, grease loads are not indicators of
scum quantities. As shown in Tables 4-2 and 4-3, the grease
content of primary sludge can exceed 25 percent of the total
solids. In biological sludges, it can be over ten percent.
Since the quantities of these sludges are usually large compared
to the amount of scum, it can be assumed that most of the grease
is in the sludge, not in the scum. Typically, the grease content
of raw domestic wastewater is 100 mg/1 (2), but the largest
amount of scum indicated in Table 4-24 is 17 mg/1, and in many
instances, the amounts are lower. At one plant, it was estimated
that only five percent of grease was removed in the 'scum. The
remainder was in the primary sludge (131).
Scum production is influenced by:
• Wastewater temperature,dissolved sol^s^^jand pH.
• Design and operation of grease traps at c ojm m ejr c i a 1
kitchens, gas stations^ and industries.
• Amount and character of^septage that Jj=jmi_xed with the
wastewater.
• Habits of residential population and small bus^ine^^ses.
(Spent motor^oir and coo¥Tng"~Tat are likely to be"r^moved
as scum if they reach the sewers.)
• Preaeration and prechlorination.
4-55
-------
• Efficiency of upstream processes in removing colloidal
grease. This Is true for chlorine contact tank scum,'
since chlorine breaks emulsions, allowing grease
particles to coalesce and float. Chlorine dose and
mixing may also affect contact tank scum.
• Scum that is returned from sludge handling. Anaerobic
digesters usually have a scum layer. Recycled digester
supernatant may carry portions of this scum back to the
influent wastewater. Similarly, scum may be returned in
sidestreams from gravity thickening and centrifugation.
• Scum removal equipment effectiveness. Some arrangements
produce better removal efficiencies than others. Also,
some arrangements produce a scum with a high solids
content and, therefore, a small volume.
• Tendency of sludge solids to float _in _sed_ime_ntation tanks
due to formation of gas bubbles.
• Process unit from which scum is removed. If primary
sedimentation is used, most of the scum is usually
removed there. Amounts of scum from secondary clarifiers
and chlorine contact tanks are normally small in
comparison.
e Actinomycete growths in activated sludge (50). These
growths may cause large amounts of solidsto float in the
clarifers.
At existing treatment plants, it is often possible to estimate
scum quantities from such data as scum pump operating hours or
the frequency with which scum pits must be emptied. Design
calculations should always allow for large variations in quantity
of scum.
4.7.3.2 Properties of Scum
Table 4-24 contains information on the solids content, volatile
content, fuel value, and grease content of scum. Scum usually
has a specific gravity of about 0.95 (110).
Varying quantities of vegetable and mineral oils, grease, hair,
rubber goods, animal fats, waxes, free fatty acids, calcium and
magnesium soaps, seeds, skins, bits of cellulosic material such
as wood, paper or cotton, cigarette tips, plastic and pieces
of garbage may comprise scum (110). When gases are entrained in
particles of primary and secondary sludge, these particles become
components of scum (126). At one plant, a variation in scum
consistency was noted. At 36°F (10°C), the scum was a congealed,
clotty mass. At 54°F (20°C), it flowed freely, in a manner
similar to that of four percent combined thickened sludges (126).
Scum should not be stored for more than a few days because the
grease will begin to decompose, with a resulting odor production.
4-56
-------
TABLE 1-2H
SCUM PRODUCTION AND PROPERTIES
Quantity
(volume),
Quantity (dry weight)
gal/mil gal Ib/roil gal
Treatment plant of wastewater of wastewater wastewater percent of total solid:
Dublin-San Ramon, Calif-
ornia
Lower Allen Township, nea
Harrisburg, Pa.
Northwest Bergen County,
Oakland, New Jersey
Wichita, Kansas
Minneapolis-St. Paul,
Minnesota
East Bay, Oakland, Calif-
ornia
West Point, Seattle,
Washington
Not stated
San Mateo, California
Salisbury, Maryland
Three New York City plants
Jamaica, New York City
County Sanitation Districts,
Los Angeles County, CA
Albany, Georgia
Volatile Fuel value,
mg/1 of Solids, solids, percent Btu/lb dry
solid3
250
14
. 110
• 60
29
50
64
43
51
2,9
2.3
Two samples. About 58 percent of nonvolatile solids was calcium carbonate.
91 percent of total solids were oil and grease. Scum from primary sedimentation, measured after decanting
in a heated unit.
CSludge was tending to float in the sedimentation tanks. Amount shown is estimate of pumpage. Skimming
system was unable to keep up with scum production under these poor conditions.
1 gal/mil gal =- m3/! x 106 m3
1 lb/rail gal - 0.12 kg/1 x 103 m
1 Btu/ lb dry solids =2.32 kJAg F •
From primary sedimentation, 120
domestic waste
6,900a From low lime primary sedi- 125
3.100 mentation (pH 9.4 to 9.S),
From gravity thickener 126
Grease is 30 percent of
skimmings after decant-
ing.
Grease balls from preaer- 127
ation tanks.
13,000 From pri.mary sedimentation 128
- Averaqe, July "isto^ - Jusw 129
1970b
- Maximum month 129
Minimum month 129
14,000 1965 - 1966 data 130
As pumped from primary 131
sedimentation tanks
< As above, after decanting; 13].
6.4 percent grease
- From sedimentation tanks 131
under poor conditions0
16,750 -- 114
110
From primary clarifiers. 133
Heavy grease load from
industry
From primary clarifiers; 133
about 80 percent of
- From secondary clarifiers; 133
(no primary)
— Primary sedimentation i 34
Heavy industrial load.
Ordinarily, scum should be seen as containing pathogens.
However, some scum handling processes may disinfect. If scum
has been heated to 176°F (80°F) for decanting, incinerated, or
treated with a dose of caustic soda sufficient to produce a pH of
12, few pathogens are likely to remain.
4.7.3.3 Handling Scum
Table 4-25 lists the advantages and disadvantages of various
approaches to scum disposal. Progressive cavity-type pumps have
been found suitable for pumping scum, although they are unable to
handle large grease balls (125) unless some sort of rack or
disintegrator is provided. Pneumatic ejectors are suitable if
grease does not interfere with the .controls. Piping should be
glass-lined and kept reasonably warm to minimize blockages.
4-57
-------
TABLE 4-25
METHODS OF HANDLING SCUM
Advantages
Disadvantages
Mix with other sludges, digest
aerobically.
Mix with other sludges, digest
anaerobically.
Landfill separately
Partial decomposition occurs (134), so some
of the scum does not require further
handling.
Avoids complexity of separate handling.
Widely used
Similar to Method 1, above.
Low capital cost
May cause grease balls to form, which must be
manually removed and disposed of, and which
may increase odors.
May cause petroleum contamination of sludge,
which will interfere with reuse.
Degrades appearance of sludge if to be re-
used.
May cause scum buildup due to return of scum-
containing liquors from sludge handling to
influent wastewater.
If digester is not strongly mixed, greatly
increases cleaning requirements. (119) Di-
gester cleaning is expensive and
odorous; also material still requires dis-
posal .
Even if digester is well mixed, a scum blan-
ket will form to some extent; therefore,
digester must be physically larger (136)
Degrades appearance of sludge if to be re-
used; may cause petroleum contamination.
May cause scum buildup, like Method 1.
Requires good decanting to avoid pumping
excess water to the digester.
May have very high operating cost.
Possible odors during storage.
Requires good decanting to minimize volume
and fluidity of scum.
4. Burn in open lagoon
Very low cost.
Severe air pollution (128); illegal under
present laws.
5. Incinerate in separate "Watergrate"
furnace (Nichols)a
6. Incinerate in separate single purpose
multiple-hearth furnace3
7. Incinerate in multiple-hearth furnace
with other wastewater solids3
Very small amount of ash in slurry.
Very small amount of ash
Low incrernenta1 cost
Fuel value of scum can be used to offset
fuel requirements of other solids
High capital cost, especially for small plants
Despite low emissions, may not be acceptable
to air pollution regulators.
Problems with feed systems.
High capital cost (130)
High maintenance cost
Despite low emissions, may not be acceptable
to air pollution regulators
Requires good decanting
Requires good decanting
Requires very careful feed to the furnace,-
otherwise causes high maintenance and
severe smoke problems. These problems can
be avoided. (137)
Incinerate in fluidized bed furnace
with other wastewater solidsa
Similar to Method 7.
Unless well decanted, can tax furnace
capacity. (126)
If scum is mixed with sludge before injec-
tion into furnace, unstable operation is
likely. (126)
9. Reuse for cattle feed
Provides reuse, not disposal (however, do
not expect revenue
Low capital cost
Toxic organic materials (e.g., DDT) tend to
concentrate in grease
Erratic market demand for waste grease (133),
it may be impossible to find anyone
that wants it.
Treatment for reuse must begin within a few
days; otherwise grease begins to decompose.
Requires good decanting because of long dis-
tance transportation.
Subject to interference from actinomycete
growths in activated sludge, which increase
the amount of solids that are not grease
but are in the scum.
10. Reuse for low grade soap manufacture
Same as Method 9.
Similar to Method 9, but less serious.
Caustic soda could be added at the treat-
ment plant, preventing decomposition and
probably making the material more usable
to grease reclaimers, but raising operating
costs.
11. Return to influent wastewater
Almost zero direct cost
Highly suitable for scum from chlorine con-
tact tanks, secondary clarifiers, etc.
when scum is removed from primary sedi-
mentation tanks
Slight increase in hydraulic load on the
treatment plant
Inapplicable to the main source of scum
(primary sedimentation tanks if used,
secondary clarifiers if primary tanks are
not used).
3For further information on scum incineration, see Chapter 11, High Temperature Processes.
4-58
-------
Piping should be heated to a minimum of 60°F (15°C). Higher
temperatures are preferred, especially if pipe sizes of less
than four-inch diameter (100 mm) are used or if pipe lengths are
substantial. Flushing connections and cleanouts should be
liberally provided. When scum is to be incinerated, a small
amount of fuel oil should be added as a convenient means of
ensuring that the scum can be pumped (137). An in-line grinder
should be provided if decanting or incinerating is to take place
(125,137).
Decanting (simple thickening by flotation) is occasionally used
to increase the solids content of the scum. Decanting requires
some care in design, in order to reduce the effects of unpleasant
odor and high grease and solids content in the decanted water.
At least two manufacturers market a heated decanting unit.
Heating scum to about 180°F (80°C) greatly improves the
separation of solids from water. Thus, the decanted water will
have a lower solids and grease content, whereas the thickened
scum will contain less moisture.
4.7.4 Septage
Domestic septic tank wastes (septage) may be defined as a
partially digested mixture of liquid and solid material that
originates as waterborne domestic wastes. Septage accumulates in
a septic tank or cesspool over a period of several months or
years. Normally, household wastes derive from the toilet, bath
or shower, sink, garbage disposal, dishwasher, and washing
machine. Septage may also include the pumpings from the
septic tanks of schools, motels, restaurants, and similar
establishments. Septage is frequently discharged into municipal
wastewater systems. With careful design and operation, municipal
systems can handle septage adequately (138-140).
4.7.4.1 Quantities of Septage
For Connecticut, Kolega and others (138) estimated residential
septage at 66 gallons per capita per year (250 1/capita/yr).
Some tanks were pumped only after many years of service; others
were pumped more than three times a year. Frequent pumping was
associated with seasonally high groundwater levels. Based on
the detailed observations of three tanks, Brandes recommended
designing for a septage volume of 53 gallons per capita per
year (200 1/capita/yr) (141). Others have recommended 50 to
360 gallons per capita per year (189 to 146 I/capita/year).
4.7.4.2 Properties of Septage
Table 4-26 contains a wide range of data on various constituents
of septage. Septage may foam and generally has a highly
offensive odor (140). Settling properties are highly variable.
Some samples settle readily to about 20 to 50 percent of
their original volume, whereas others show little settling.
4-59
-------
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
-------
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
-------
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
-------
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
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5. Babbitt, H.E. and E.R. Baumann. Sewe
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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
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February 1979.
8. Knight, C.H., R.G. Mondox, and B. Hambley. "Thickening
and Dewatering Sludges Produced in Phosphate Removal."
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11. Anderson, C.N. "Peak Sludge Loads at a Municipal Treatment
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12. USEPA. Cost-Effective Design of Wastewater Treatment
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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
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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.
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18. Metropolitan Engineers. West Point Waste Activated Sludge
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1976.
4-64
-------
27. USEPA. Characterization of the Activated Sludge Process
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45268. EPA-R2-73/224, April 1973.
28. Wuhrmann, K. "High-Rate Activated Sludge Aeration and Its
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29. Eckenfelder, W.W. Water Quality Engineering for Practicing
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4-65
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39. Chapman, T.D., L.C. Matsch and E.H. Zander. "Effect of
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46. USEPA. Extended Aeration Sewage Treatment in Cold Climates.
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4-67
-------
65. USEPA. Converting Rock Trickling Filters to Plastic Media;
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70. Smith, J.E., Jr. Ultimate Disposal of Sludges. Technical
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4-68
-------
78. Reimer, R.E., E.E. Hursley, and R.F. Wukasch "Pilot Plant
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Technology Transfer, Cincinnati^Ohio45268.EPA-625/1-
76-001. April 1976.
80. Brown and Caldwell. West Point Pilot Plant Study,
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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
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85. Baillod, C.R., G.M. Cressey, and R.T. Baupre'. "Influence
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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
-------
101. Stern, L. "The Great Cadium Controversy." Sludge Magazine.
Vol. 1, (2), March/April 1978. "
102. Peterson, J.R., C. Lue-Hing, and D.R. Zenz. "Chemical
and Biological Quality of Municipal Sludge" Recycling
Treated Municipal Wastewater and Sludge Through Forest
and Cropland^W. V . Sopper iTndTTr^ Kardos Editors.
Pennsylvania State University Press. p. 26, (1973).
103. Ohara, G.T., S.K. Raksit, and D.R. Olson. "Sludge
Dewatering Studies at Hyperion Treatment Plant." Journal
Water Pollution Control Federation. Vol. 50, p~912,
(1978) ."' ~~~ ' —— -
104. Chawla, V.K., J.P. Stephenson, and D. Liu. "Biological
Characteristics of Digested Chemical Sewage Sludges."
Proceedings, Sludge Handling and Disposal Seminar, Toronto^
Ontario, 1974.EnvironmentalProtection Service(Canada)7
Conference Proceedings No. 2.
105. Lawrence, J. , and H.M. Tosine. "Polychlorinated Biphenyl
Concentrations in Sewage and Sludges of Some Waste
Treatment Plants in Southern Ontario." Buileti n o f
Environmental Contamination and Toxicology. Vol18,(1) ,
(1977)."
106. Sacramento Area Consultants. Sewage Sludge Management
P r o g r a m Final Rep o r t — V q 1 _. 2, SSB_ Operation and'
P e r f o rma nee.
107. Dube, D.F.f G.D. Veith, and G.F. Lee. "Polychlorinated
Biphenyls in Treatment Plant Effluents." Journal Water
P o 11 u t i o n C o rvtr ql_ _Federati on. Vol. 46, p. 966, (1974).
108. Michigan Water Resources Commission. "Monitoring for
Polychlorinated Biphenyls in the Aquatic Environment."
1973.
109. American Society of Civil Engineers and Water Pollution
Control Federation. Manual of Practice 8 Sewage Treajtmejrt
Plant Design. 1959 Edition, (1972 printing).
110. Sacramento Area Consultants. Study of Wastewater Solids
Processing and Disposal. Report to Sacramento Regional
County Sanitation District. Sacramento, California 95814,
June 1975.
111. Merz, R.C. "Operation of Screens, Grit Chambers and
Sedimentation Tanks." Sewage Works Journal. Vol. 17,
(4) , July 1945.
112. Fisichelli, A.P. "Raw Sludge Pumping--Problems and
Interdisciplinary Solutions." Journal Water Pollution
Con t ro1 Federation. Vol. 42, (11), (1970).
4-71
-------
113. Volpe, G.J. "Static Screen and Rotostrainer Screenings
Dewatering Test." Printed as Appendix B of Study to
Wastewater Sol ids Processing and Disposal. Report
Fo Sacrame nto Reg ional County Sanitation Di s tr ict.
Sacramento, California, 95814, June 1975.
114. Carnes, B.A., and J.M. Eller. "Characterization of
Wastewater Solids." Journal Water Pollutio n Control
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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
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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
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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
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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,
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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
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Ohio 45268. EPA-600/2-77-198, September 1977.
4-73
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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
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EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapters. Thickening
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 5
THICKENING
5.1 Introduction
The purpose of this chapter is to provide the reader with
rational design and operating information on which to base
decisions about cost-effective thickening processes. Thickening
is only one part of the wastewater solids treatment and disposal
system and must be integrated into the overall treatment process,
so that performance for both liquid and solids treatment is
optimized and total cost is minimized (1-3).
5.1.1 Definition
Thickening is defined in this chapter as removal of water from
sludge to achieve a volume reduction. The resulting material is
still fluid.
5.1.2 Purpose
Sludges are thickened primarily to decrease the capital and
operating costs of subsequent sludge processing steps by
substantially reducing the volume. Thickening from one to
two percent solids concentration, for example, halves the sludge
volume. Further concentration to five percent solids reduces the
volume to one-fifth its original volume.
Depending on the process selected, thickening may also
provide the following benefits: sludge blending, sludge flow
equalization, sludge storage, grit removal, gas stripping, and
clarification.
5.1.3 Process Evaluation
Although it is good design practice to pilot thickening equipment
before designing a facility, pilot testing does not guarantee
a successful full-scale system. Designers must be cognizant of
the difficulties involved in scale-up and the changing character
of wastewater sludge and allow for them in design.
The main design variables of any thickening process are:
• Solids concentration and volumetric flow rate of the
feed stream;
5-1
-------
• Chemical demand and cost if chemicals are employed;
• Suspended and dissolved solids concentrations and
volumetric flow rate of the clarified stream;
• Solids concentration and volumetric flow rate of the
thickened sludge.
Specific design criteria for selection of a thickening process
can also be dependent on the chosen downstream process train.
Another important consideration is the operation and maintenance
(0/M) cost and the variables affecting it. In the past, 0/M
costs have not been given enough attention. This should change
as USEPA begins to implement its new Operations Check List (4)
in all phases of the Construction Grants Program.
Finally, thickening reliability is important for successful plant
operation. A reliable thickening system is needed to maintain
the desired concentration and relatively uninterrupted removal of
sludge from a continuously operated treatment plant. Sludges are
being generated constantly, and if they are allowed to accumulate
for a long time, the performance of the entire plant will be
degraded.
5.1.4 Types and Occurrence of Thickening Processes
Thickening is accomplished in sedimenation basins; gravity,
flotation and centrifugal thickeners; and in miscellaneous
facilities such as secondary anaerobic digesters, elutriation
basins, and sludge lagoons.
5.2 Sedimentation Basins
5.2.1 Primary Sedimentation
A primary clarifier can be used as a thickener under certain
conditions. Primary sludge thickens well, provided the sludge is
reasonably fresh, solids of biological origin (for example,
waste-activated sludge) are kept to a minimum, and the wastewater
is reasonably cool. If sludge of five to six percent solids
content is to be recovered from a primary sedimentation system,
it is essential that the sludge transport facilities be designed
to move those solids. This will require short suction piping,
adequate net positive suction heads on the primary sludge pump,
suction-sight glass inspection piping, and a positive means of
ascertaining the quantity pumped and the concentration of the
slurry (5).
5-2
-------
5.2.2 Secondary Sedimentation
Thickening in secondary or intermediate clarifiers has not
been successful in the past because biological sludges are
difficult to thicken by gravity. Thickening has been improved by
using side water depths of from 14 to 16 feet (4 to 5 m), suction
sludge withdrawal mechanisms rather than plow mechanisms, and
gentle floor slopes, for example, 1:12. Although thickening
within a sedimentation basin can be beneficial under certain
conditions, separate thickening is usually recommended.
5.3 Gravity Thickeners
5.3.1 Introduction
Separate, continuously operating gravity thickening for municipal
wastewater sludges was conceptualized in the early 1950's (6) .
Until that time, thickening had been carried out within the
primary clarifier. Operating problems such as floating sludge,
odors, dilute sludge, and poor primary effluent led to the
development of the separate thickening tank. Gravity thickeners
became the most commonly used sludge concentrating device; now,
however, their use is being challenged by other thickening
processes.
Table 5-1 lists advantages and disadvantages of gravity
thickeners compared to other thickeners.
TABLE 5-1
ADVANTAGES AND DISADVANTAGES OF GRAVITY THICKENERS
Advantages Disadvantages
Provides greatest sludge storage Requires largest land area
capabilities
Requires the least operational skill Contributes to the production of odors
Provides lowest operation (especially For some sludges,
power) and maintenance cost - solid/liquid separation can be erratic
- can produce the thinnest least
concentrated sludge
5.3.2 Theory
Since the early work of Coe and Clevenger (7), understanding of
gravity thickening has slowly improved (8-11). The key to
understanding the continuous gravity thickening process is
recognition of the behavior of materials during thickening.
5-3
-------
Coarse minerals thicken as particulate (nonf1occulent)
suspensions. Municipal wastewater sludges, however, are usually
flocculent suspensions that behave differently (12).
Detailed, comprehensive analysis of current gravity thickening
theory for municipal wastewater sludges is beyond the scope
of this manual; those desiring such detail should consult
Design and Operational Criteria for Thickening of Biological
Sludges
follows
(13)
(12) .
A short descriptive summary of current theory
OVERFLOW
INFLOW
ZONE OF CLEAR LIQUID
SEDIMENTATION ZONE
..^jj
THICKENING ZONE T
UNDERFLOW
ZONE OF CLEAR LIQUID
SEDIMENTATION ZONE
h
THICKENING ZONE
Cj
Cb
SOLIDS CONCENTRATION
IN THICKENER
Cj - INFLOW SOLIDS CONCENTRATION
Cb - LOWEST CONCENTRATION AT WHICH FLOCCULANT SUSPENSION IS IN
THE FORM OF POROUS MEDIUM
Cu - UNDERFLOW CONCENTRATION FROM GRAVITY THICKENER
FIGURE 5-1
TYPICAL CONCENTRATION PROFILE OF MUNICIPAL
WASTEWATER SLUDGE IN A CONTINUOUSLY OPERATING
GRAVITY THICKENER
Figure 5-1 shows a typical solids concentration profile for
municipal wastewater sludges within a continuously operating
gravity thickener. Sludge moving into the thickener partially
disperses in water in the sedimentation zone and partially flows
as a density current to the bottom of the sedimentation zone.
The solid phase of the sludge, both dispersed and in the density
current, creates floes that settle on top of the thickening
zone. Floes in the thickening zone lose their individual
character. They have mutual contacts and thus become a part of
5-4
-------
the matrix of solids compressed by the pressure of the overlying
solids. The displaced water flows upward through channels in
the solids matrix.
Generally, in decision making about thickener size, the settling
process in the sedimentation zone as well as the consolidation
process in the thickening zone should be evaluated; whichever
process (sedimentation or thickening) requires greater surface
area dictates the size of the thickener. For municipal
wastewater sludges, the thickening zone area required is almost
always greater than that for the sedimentation zone.
5.3.3 System Design Considerations
Circular concrete tanks are the most common configuration for
continuously operating gravity thickeners, though circular steel
tanks and rectangular concrete tanks have also been used.
Figure 5-2 shows a typical gravity thickener installation;
Figure 5-3 is a cross-sectional view of a typical circular
gravity thickener (14).
FIGURE 5-2
TYPICAL GRAVITY THICKENER INSTALLATION
5-5
-------
HANDRAILING
1"GROUT
INFLUENT
PIPE
1
1
BRIDGE
1
/ J/
/ r-P/
BAFFLE
SUPPORTS
/
TURNTABLE
L
EFFLUENT
WEIR
MAX. WATER SURFACE
1'3"MIN.
I
EFFLUENT
LAUNDER
1V4" BLADE /
CLEARANCE SLUf"'*
PIPE!
ADJUSTABLE
"SCRAPER SQUEEGEES
BLADES
1 ft = 0.305 m
1 in = 2.54 cm
HOPPER
SCRAPERS
FIGURE 5-3
CROSS SECTIONAL VIEW OF A TYPICAL CIRCULAR
GRAVITY THICKENER
At minimum, the following should be evaluated for every gravity
thickener: minimum surface area requirements, hydraulic loading,
drive torque requirements, and total tank depth. Floor slope and
several other considerations will also influence the final design
of the gravity thickener.
5.3.3.1 Minimum Surface Area Requirements
If sludge from the particular facility is available for testing,
the required surface area can be found by using a settling
column, developing a settling flux curve, and calculating the
critical flux (mass loading, Ibs/sq ft/hr) for that particular
sludge (4, 13, 15). In most cases, however, the sludge to be
thickened is not available, and the designer must resort to other
methods.
Table 5-2 provides criteria for calculating required surface area
when test data are not available and pilot plant work is not
reasonable. The designer must specify the sludge type (for
mixtures, the approximate proportions should be known), the range
of solids concentrations that are expected in the thickener
inflow, and the underflow concentration required for downstream
processing. Part A of the design example (Section 5.3.4)
illustrates the use of Table 5-2 in sizing gravity thickeners.
5-6
-------
TABLE 5-2
TYPICAL GRAVITY THICKENER SURFACE AREA DESIGN CRITERIA2
concentration ,
Type of sludge percent solids
Separate sludges:
Primary (PRI)
Trickling filter (TF)
Rotating biological
contactor (RBC)
Waste activated sludge
(WAS)
WAS - air
WAS - oxygen
WAS - (extended
aeration)
Anaerobically digested
sludge from primary
digester
Thermally conditioned
sludge :
PRI only
PRI + WAS
WAS only
Tertiary sludge:
High lime
Low 1 ime
Alum
Iron
Other sludges :
PRI 4 WAS
PRI 4 TF
PRI 4- RBC
PRI + 'iron
PRI 4- low lime
PRI + high lime
PRI + (WAS + iron)
PRI -f (WAS + alum)
(PRI + iron) 4 TF
(PRI + iron) + WAS
WAS 4 TF
Anaerobically digested
PRI + WAS
Anaerobically digested
PRI 4 (WAS 4 iron)
2
1
1
0.5
0.5
0.2
3
3
0.5
3
3
0.5
0.5
2.5
2
2
0.2
0.4
0.5
- 7
- 4
- 3.5
- 1.5
- 1.5
- 1.0
8
- 6
- 6
- 1.5
- 4.5
- 4.5
-
- 1.5
- 1.5
- 4.0
- 6
- 6
2
5
7.5
1.5
- 0.4
- 0.6
1.8
- 2.5
4
4
concentration,
percent solids
S
3
2
2
2
2
12
8
6
12
10
3
4
4
5
5
4.5
6.5
2
- 10
- 6
- 5
- 3
- 3
- 3
12
- 15
- 15
- 10
- 15
- 12
-
- 4
- 6
- 7
- 9
- 8
4
7
12
3
- 6.5
-8.5
3.6
- 4
8
6
Mass loading,
Ib/sq ft/hrb
0.
0.
0.
0.
0.
0.
1.
1.
0.
1.
0.
0.
0.
0.
0.
0.
0.
0.
0.
8
3
3
1
1
2
6
2
9
0
4
1
2
3
5
4
5
6
1
- 1.
- 0.
- 0.
- 0.
- 0.
- 0.
1.0
- 2.
- 1.
- 1.
- 2.
- 1.
-
- 0.
- 0.
- 0.
- 0.
- 0.
0.25
0.8
1.0
0.25
- 0.
- 0.
0.25
- 0.
0.6
0.6
2
4
4
3
3
3
1
8
2
5
25
4
6
7
8
7
7
8
3
Reference
16
16
16
16 •*>:•
17
16
18
19
19
19
18, 20
18, 20
-
20
20
16
16
16
18
18
18
18
•20 '•••• '•':.
20
18
16
18
18
Data on supernatant characteristics is covered later in this section.
Typically, this term is given in Ib/sg ft/day. Since wasting to the
thickener is not always continuous over 24 hours, it is a more
realistic approach to use Ib/sq ft/hr.
1 Ib/sq ft/hr =4.9 kg/m2/hr
5-7
-------
5.3.3.2 Hydraulic Loading
Hydraulic loading is important for two reasons. First, it
is related to mass loading. The quantity of solids entering
the thickener is equal to the product of the flow rate and
solids concentration. Since there are definite upper limits for
mass loading, there will therefore be some upper limit for
hydraulic loading. Secondly, high hydraulic loading causes
excessive carryover of solids in the thickener effluent.
Typical maximum hydraulic loading rates of 25 to 33 gallons per
square foot per hour (1,200 to 1,600 l/m^/hr) have been used in
the past but mainly for primary sludges. For sludges such as
waste-activated or similar types, much lower hydraulic loading
rates, 4 to 8 gallons per square foot per hour (200 to 400 l/m^/
hr) are more applicable (16). Table 5-3 gives some typical
operating results. Note that the hydraulic loading rate in
gallons per square foot per hour can be converted to an average
upward tank velocity in feet per hour by dividing by 7.48.
TABLE 5-3
REPORTED OPERATING RESULTS AT VARIOUS OVERFLOW RATES FOR
GRAVITY THICKENERS (20,21)
Location
Port Huron, MI
Sheboygan , WI
Grand Rapids, MI
Lakewood, OH
Influent
solids
Thickened
solids
concentration , Hydraulic
Sludge percent loading ,
type*3 solids gal/sq ft/hr
P4WAS
P+TF
P+ (TF+A1)
WAS
P+(WAS4A1)
0 6
0
0
0
3
5
2
3
8
18.
19.
4.
25.
6
0
1 •
8
Mass
loading ,
Ib/sq ft/hr
0
0
0
0
0
.34
.46
.73
.42
.6
concentration
percent
solids
4
8
7
5
5
.7
.6
.8
.6
.6
Overflow
suspended
solids ,
mg/1
2,500
400
2,000
140
1,400
Values shown are average values only. For example, at Port Huron, MI the hydraulic
loading varies between 7 to 9 gal/sq ft/hr (300-400 l/m2/hr) , the thickened solids in
the underflow between 4.0 and 6.0 percent solids; and the suspended solids in the
overflow, from 100 to 10,000 mg/1.
P = Primary sludge
TF = Trickling filter sludge
WAS = Waste-activated sludge 1 gal/sq ft/hr =40.8 1/m /hr
Al = Alum sludge 1 Ib/sq ft/hr =4.9 kg/m2/hr
Using the typical maximum hydraulic loading rates mentioned
above, maximum velocities for primary sludges are 3.3 to 4.4 feet
per hour (1.0 to 1.3m/hr) and for waste-activated sludge are
0.5 to 1.1 feet/hour (0.2 to 0.3 m/hr).
Several researchers have related overflow rates to odor control,
but odor is due to excessive retention of solids and can be
better controlled by removing the thickened sludge from the
thickener at an increased frequency.
5.3.3.3 Drive Torque Requirements
Sludge on the floor of a circular thickener resists
of the solids rake and thus produces torque.
the movement
Calculation
5-8
-------
of torque for a circular drive unit is based on the simple
cantilevered beam equation represented by Equation 5-1:
T = WR2 (5-1)
where:
T = torque, ft/lb
W = uniform load—this is sludge specific,
Ib/ft (see Table 5-4)
R = tank radius, ft
TABLE 5-4
TYPICAL UNIFORM LOAD (W) VALUES
Truss arm W,
Sludge type lb/fta
Primary only (little grit) : 30
Primary only (with grit) 40
Primary + lime 40 to 60
Waste-activated sludge (WAS)
Air 20
Oxygen 20
Trickling filter 20
Thermal conditioned 80
Primary + WAS 20 to 30
Primary + trickling filter 20 to 30
Rake arms typically have a tip speed
between 10 to 20 ft/min (3 to 6 m/min).
1 Ib/ft = 1.49 kg/m
Note that there are several levels of torque which must be
specified for a circular gravity thickener (22). Table 5-5 lists
and defines the various torque conditions.
5.3.3.4 Total Tank Depth
The total vertical depth of a gravity thickener is based on three
considerations: tank free board, settling zone (zone of clear
5-9
-------
liquid and sedimentation zone), and compression and storage zone
(thickening zone).
TABLE 5-5
DEFINITION OF TORQUES APPLICABLE TO CIRCULAR GRAVITY THICKENERS (22)
Running torque - this is the torque value calculated from equation 5-1
Alarm torque - torque setting, normally 120 percent of running, which tells the operator
that there is something wrong
Shut-off torque - torque setting, normally 140 percent of running, which would shut off
the mechanism . . .
Peak torque - torque value, determined by the supplier of the drive unit. This torque
is provided only for an instant and is normally 200 percent of the running torque
Free
Tank free board is the vertical distance between tank liquid
surface and top of vertical tank wall. It is a function of tank
diameter, type of bridge structure--half or full bridge—type of
influent piping arrangement, and whether or not skimming is
provided. It will usually be at least 2 to 3 feet (.6 to .9 m)
although free-board distances up to 7 to 10 feet (2 to 3 m) have
been used by some designers.
Settling Zone
This zone encompasses the theoretical zone of clear liquid and
sedimentation zone as shown on Figure 5-1. Typically 4 to
6 feet (1.2 to 1.8 m) is necessary, with the greater depth being
for typically difficult sludges, such as waste-activated or
nitrified sludge.
Compression and Storage Zone
Sufficient tank volume must be provided so that the solids will
be retained for the period of time required to thicken the slurry
to the required concentration. In addition, sufficient storage
is necessary to compensate for fluctuations in solids loading
rate.
Another consideration is that gas may be produced because of
anaerobic conditions or denitrif ication. Development of these
conditions depends on the type of sludge, liquid temperature,
and the length of time sludge is kept in the thickener. Plant
operating experience has indicated that the total volume in
this zone should not exceed 24 hours of maximum sludge wasting.
5.3.3.5 Floor Slope
The floor slopes of thickeners are normally greater than 2 inches
of vertical distance per foot of tank radius (17/cm/m). This is
steeper than the floor slopes for standard clarifiers. The
5-10
-------
steeper slope maximizes the depth of solids over the sludge
hopper, allowing the thickest sludge to be removed. The steeper
slope also reduces sludge raking problems by allowing gravity to
do a greater part of the work in moving the settled solids to the
center of the thickener.
5.3.3.6 Other Considerations
L j. ft ing De v i ce s
Optimum functioning of a thickener mechanism can be inhibited
by heavy accumulation of solids due to power outages or
inconsistent accumulations of heavy or viscous sludges.
Thickeners can be provided with either a manual or an automatic
lifting device that will raise the mechanism above these
accumulations. This device has not been considered necessary in
the majority of municipal wastewater treatment plants except
in applications involving very dense sludges (for example,
thermally-conditioned sludge or primary plus lime sludge).
Skimmers
Several years ago, it was rare for skimmers to be installed
on gravity thickeners. Today it is common practice to specify
skimming and baffling for new plants. The reason for the change
is the increased processing of biological sludges and the
inherent floating scum layer associated with those sludges.
Polymer Addition
Addition of polymer to gravity thickener feed has been practiced
at several plants (23,24). Results indicate that the addition
of polymers improves solids capture but has little or no effect
on increasing solids underflow concentration. (See Chapter 8 for
further discussion).
Thjjckejier Sjjpejcnatant
Thickener supernatant or overflow is normally returned to either
the primary or secondary treatment process. As indicated in
Table 5-3, the strength of the overflow, as measured by total
solids, can vary significantly. The liquid treatment system
must be sized to handle the strongest recycled load. (See
Chapter 16 for further discussion).
Pi£ke_ts_
Stirring with pickets in gravity thickeners is thought to
help consolidate sludge in the thickening zone (25). However,
the support rake mechanism usually can provide sufficient sludge
mixing to make special pickets unnecessary.
5-11
-------
jjeed Pump and Piping
The following guidelines are applicable for feed pump and piping:
• Use positive displacement feed pumps with variable
speed drives for variable head conditions and positive
feed control.
• Provide as nearly continuous pumpage as possible.
• Design piping for operational flexibility.
Th i eke ne r Und er flow; JPump __and_Pi£JLn£
For variable head conditions and typical abrasiveness of many
sludges, a positive displacement pump with variable speed drive
should be used and its operations should be controlled by some
type of solids sensor, for example, either by a sludge blanket
level indicator or solids concentration indicator. Pumps should
be located directly adjacent to the thickener for shortest
possible suction line. A positive or pressure head should be
provided on the suction side of the pump. A minimum of 10 feet
(3 m) should be provided for primary sludges and a minimum of
6 feet (2 m) for all other sludges. It is critical to provide
adequate clean-outs and flushing connections on both the pressure
and suction sides of the pump. Clean-outs should be brought to
an elevation greater than that of the water surface so that the
line may be rodded without emptying the thickener.
5.3.4 Design Example
A designer has calculated that it is necessary to thicken
a maximum of 2,700 pounds (1,225 kg) per day of waste sludge,
(dry weight). The sludge consists of 1,080 pounds (490 kg) of
primary at 4.0 percent solids and 1,620 pounds (735 kg) of
activated at 0.8 percent solids. Wasting from the primary
clarifier will be initiated by a time clock and terminated
by a sludge density meter when the sludge concentration drops
below a given value. Waste-activated sludge will be pumped from
the final clarifier 24 hours per day at 17 gallons per minute
(64 1/min).
Thickener Surface Area
Since this is a new facility and pilot testing is not possible,
the designer must utilize Table 5-2.
There are two possible thickening alternatives. The first
alternative is thickening of straight waste-activated sludge
with a maximum influent solids concentration of 0.8 percent
5-12
-------
solids. At maximum conditions, the designer has selected a mass
loading of 0.2 pounds per square foot per hour (1.47 kg/m2/hr)
and will design for a 2.0 percent solids in the underflow.
i'620 lb/day = 337.5 sq ft (31.4
(0.2 Ib/sq ft/day) (24 hrs/day)
The second alternative is thickening a combination of waste-
activated sludge and primary sludge. The density meter on
the primary clarifier will be set to allow the sludge pump to
continue as long as the solids concentration is greater than or
equal to 4.0 percent solids. The primary sludge pump will be
equipped with a variable speed controller and has a maximum rated
pumping capacity of 10 gallons per minute (38 1/min).
On a mass loading basis, the designer's past experience indicates
that surface area required for the combination of primary and
waste-activated sludge is less than that required for waste-
activated alone. However, to assure system reliability,
sufficient surface area should be provided to thicken only
waste-activated. With the addition of primary sludge, the
expected underflow solids concentration is 4.0 percent.
Hydraulic Loading
The maximum possible hydraulic flow to the gravity thickener
would be 17 gallons per minute (1.0 I/sec) of waste-activated
and 10 gallons per minute (0.63 I/sec) of primary sludge.
The designer is cognizant of the solids recycle problem from
the thickener overflow and has selected a value of 6 gallons
per square feet per hour (250 l/m2/hr) as the maximum overflow
rate.
((17 + 10) gal/min) x (60 min/hr))
6 gal/sq ft/hr
The area required for hydraulic loading is less than that
required for mass loading.
Since continuous operation of the sludge handling system is
essential, two gravity thickeners, each capable of handling the
sludge flow, will be provided. The minimum required area
is 337.5 square feet (31.4 m2), which is equivalent to a
20.7-foot (6.2 m) diameter unit. In this size range, equipment
manufacturers have standardized on 1-foot (0.3 m) increments;
therefore, a 21-foot (6.3 m) diameter, 346-square-foot (32.2 m2)
unit will be specified.
5-13
-------
Torque Requirements
The 30 pounds per foot (45 kg/m) value will be used for the truss
arm loading (Table 5-4). From Equation 5-1, the running torque
required is:
3°foot"ds x (10-5 feefc)2 = 3'307 ft-lb <465 m~k9)
The designer will specify a minimum running torque capacity
of 3,307 foot pounds (465 m-kg ) . The other torques (alarm,
shut-off, and peak) would be specified as in Table 5-5.
Depth
Because both the full and the half bridge systems work equally
well and the full bridge is less expensive to install, the
designer will use a full bridge thickener mechanism that
will rest atop the gravity thickener and will have a skimming
mechanism attached.
In order to accommodate the skimming arm beneath the bridge and
allow room to perform maintenance work, the designer has selected
24 inches (0.61 m) for the freeboard in the thickener.
From past experience, the designer has selected a typical depth
of 5 feet (1.54 m) for the settling zone.
To calculate the depth of the thickening zone, it is assumed that
the average solids concentration in the zone would be 1.4 percent
solids and that one-day storage would be utilized.
The following assumptions were made in order to arrive at this
percentage :
• Only waste-activated sludge would be thickened.
• The top of the thickening zone would hold 0.8 percent
solids .
• The bottom of the thickening zone would hold 2.0 percent
solids .
• The average concentration would be equal to 0.8 plus
2.0 quantity divided by 2.
1,620 Ib of waste-activated sludge _ _., f. ,, ,,
(0.014){8.34) (7.48 gal/cu ft)(346 sq ft) ^'Jb rt u
5-14
-------
The total vertical side-wall depth of the gravity thickener is
the sum of the free board, settling zone, and required thickening
zone. In this case, it would be 12.36 feet (3.77 m) . At this
time, no allowance has been made for the depth of the cone height
of the thickener which would reduce slightly (21 inches [.27 m]
the vertical side wall depth of the thickening zone when
subtracted from the thickening zone depth.
5.3.5 Cost
5.3.5.1 Capital Cost
Several recent publications have developed capital cost curves
for gravity thickeners (26-28). Probably the most factual
is the reference based on actual USEPA bid documents for the
years 1973-1977 (27).
According to a USEPA Municipal Wastewater Treatment Plant
Construction Cost Index - 2nd quarter 1977 (27), although the
data were scattered, a regression analysis indicated that the
capital cost could be approximated by Equation 5-2.
C = 3.28 x 104Q1-10 (5-2)
where:
C = capital cost of process in dollars
Q = plant design flow in million gallons of
wastewater flow per day
The associated costs include those for excavation, process
piping, equipment, concrete, and steel. In addition, such costs
as those for administrating and engineering are equal to 0.2264
times Equation 5-2 (27).
5.3.5.2 Operating and Maintenance Cost
Staffing
Figure 5-4 indicates annual man-hour requirements for operation
and maintenance. As an example, for a gravity thickener surface
area of 1,000 square feet (93 m^), a designer would include
350 man-hours of operation and maintenance in the cost analysis.
Power
Figure 5-5 shows annual power consumption for a continuously
operating gravity thickener as a function of gravity thickener
surface area. As an example, for a gravity thickener surface
5-15
-------
area of 1,000 square feet (93 m2) , a designer would include a
yearly power usage of 4,500 kwhr (16.2 GJ) in the cost analysis.
Figure 5-5 does not include accessories such as pumps or polymer
feed systems.
DC
O
LJ_
to
DC
ID
O
I
OC
O
2
2
<
100
4 56789 1,000 2 3 456789 10.000 2
THICKENER AREA, sq ft (1 sq ft = 0.093 m2)
4 56789
FIGURE 5-4
ANNUAL O&M MAN-HOUR REQUIREMENTS - GRAVITY
THICKENERS
Maintenance Material Costs
Figure .5-6 shows a curve developed for estimating circular
gravity thickener maintenance material costs as a function of
thickener surface area. As an example, for a gravity
1,000 square feet (93 m2), a designer
materials cost of $375. Since this
1975 cost, it must be adjusted to the
gravity
thickener surface
would estimate a
number is based on a June
current design period.
area of
yearly
5.4 Flotation Thickening
Flotation is a process for separating solid particles from a
liquid phase. Flotation of solids is usually created by the
introduction of air into the system. Fine bubbles either adhere
to, or are absorbed by, the solids, which are then lifted
to the surface. Particles with a greater density than that of
the liquids can be separated by flotation (24,29).
5-16
-------
In one flotation method, dissolved air flotation, small gas
bubbles (50-100 ym) are generated as a result of the precipita-
tion of a gas from a solution supersaturated with that gas.
Supersaturation occurs when air is dispersed through the sludge
in a closed, high pressure tank. When the sludge is removed from
the tank and exposed to atmospheric pressure, the previously
dissolved air leaves solution in the form of fine bubbles.
1,000
100
3 4 56789 1,000 2 3 456789 10,000 2 3 466788 100,000
THICKENER AREA, sq ft (1 sq ft = 0.093 m2)
FIGURE 5-5
ANNUAL POWER CONSUMPTION - CONTINUOUS OPERATING
GRAVITY THICKENERS
In a second method, dispersed air flotation, relatively large gas
bubbles (500-1000 urn) are generated when gas is introduced
through a revolving impeller or through porous media (30,31).
In biological flotation, the gases formed
activity are used to float solids (32-34).
by natural biological
In vacuum flotation, Supersaturation occurs when the sludge
is subjected initially at atmospheric pressure, to a vacuum
of approximately 9 inches ( 2'30 mm) of mercury in a closed
tank (35,36).
Although all four methods have been used in wastewater sludge
treatment systems, the dissolved air flotation process has
been the dominant method used in the United States.
5-17
-------
100 2 3456789 1,000 2 3 456789 10,000 2 3 456789
THICKENER AREA, sq ft (1 sq ft = 0.093 m2)
FIGURE 5-6
ESTIMATED JUNE 1975 MAINTENANCE MATERIAL COST FOR
CIRCULAR GRAVITY THICKENERS
TABLE 5-6
TYPES OF MUNICIPAL WASTEWATER SLUDGES BEING THICKENED
BY DAF THICKENERS
Primary only
Waste activated sludge (WAS)
WAS (oxygen) only
Trickling filter only
Primary plus WAS (air)
Primary plus trickling filter
- air only Aerobically digested WAS
Aerobically digested primary plus WAS (air)
Alum and ferrous sludge from phosphorus
removal
5.4.1 Dissolved Air Flotation (DAF)
Since the 1957 installation of the first municipal DAF thickener
in the Bay Park Sewage treatment plant, Nassau County, New York,
about 300 U.S. municipal installations (over 700 units) have been
installed. Although the principal use of the DAF thickener has
been to thicken waste-activated sludge, about 20 percent of the
installations handle other sludge types (37). Table 5-6 lists
5-18
-------
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
-------
Full, Partiaj^a^nd^Recycle Pressurization
There are three ways in which a DAF system can be operated.
The first method is called "full or total pressurization."
With this design, the entire sludge flow is pumped through the
pressure retention tank, where the sludge is saturated with
air and then passed through a pressure reduction valve before
entering the flotation chamber. A distribution device is used to
dissipate inlet energy and thus to prevent turbulence and limit
short circuiting. The primary advantage of pressurizing the
total flow is that it minimizes the size of the flotation
chamber, a significant part of the capital cost. However, the
advantage of a smaller chamber may be partially offset by the
cost of a higher head feed pump, larger pressure vessel, and more
expensive operation. Operational problems may result from floe
shearing and clogging when sludge is passed through the pressure
regulating valve.
The second method of operation is called "partial pressuriza-
tion." With this design only part of the sludge flow is pumped
through the pressure retention tank. After pressurization the
unpressurized and pressurized streams are combined and mixed
before they enter the flotation chamber. In this arrangement
the pressurizing pump and pressure vessel are smaller and the
process is not as susceptible to flow variations as is total
pressurization; this is the case when the necessary pump controls
are included in the design. The size of the flotation chamber
would be the same as that for a total pressurization system.
The third method is called "recycle pressurization." Here, a
portion of the clarified liquor (subnatant) or an alternate
source containing relatively little suspended matter is
pressurized. Once saturated with air, it is combined and mixed
with the unthickened sludge before it is released into the
flotation chamber.
The major advantage of this system over the total and partial
pressurization system is that it minimizes high shear conditions,
an important parameter when dealing with flocculent-type
sludges. Another advantage arises when wastewater sludge
streams containing stringy materials are thickened. The recycle
pressurization system eliminates clogging problems with the
pressurization pump, retention tank, and pressure release valve.
For the above reasons, recycle pressurization systems are the
most commonly used units in the United States. Figure 5-7 shows
a typical rectangular steel tank installation.
In this system, the pressure retention tank may be either
unpacked or packed (meaning that the tank is filled with a
packing material to create turbulence). The use of either is
dependent principally on the source of the pressurized recycle
f low.
The pressurized recyle flow can be obtained either from the
subnatant stream or, typically, from the secondary effluent. The
advantages of using secondary effluent are that it results in a
5-20
-------
much cleaner stream (low suspended solids and low grease content)
and allows the use of a packed pressure retention tank. A packed
tank is smaller than a packless tank, has lower associated
capital cost, and provides for a more efficient saturation of- tne
liquid stream. In this case, less air is required to achieve tne
same level of liquid saturation as a packless tank and power
requirements are lower. Packed tanks may, however, eventuallY
require cleaning, and the use of secondary plant effluent will
significantly increase the flow through the secondary treatment
system, thereby increasing pumping costs and possibly affecting
the performance of the secondary clarifier.
FIGURE 5-7
TYPICAL RECTANGULAR, STEEL TANK, RECYCLE PRESSURIZATION
DISSOLVED AIR FLOTATION THICKENER
Rectangular or .C^ir_c_ul_ar
The use of rectangular DAF thickeners has a number of
over circular units in float removal. First, skimmers can ea
be closely spaced; secondly, they can be designed to skim
entire surface. Because of the side-walls, float does not ea;
ily
the
ily
5-21
-------
move around the end of the skimmers. Bottom sludge flights are
usually driven by a separate unit and, hence, can be operated
independently of the skimmer flights. Water level in the tank
can be changed readily by adjusting the end weir. This permits
changing the depth of water and flight submergence to accommodate
changes in float weight and displacement, which affect the
ability to remove this material from the unit.
The main advantage of circular units is their lower cost in
terms of both structural concrete and mechanical equipment. For
example, two 60-foot (18 m) diameter circular units are the
equivalent of three 20-foot by 90-foot (6 m by 27 m) rectangular
units. The rectangular units require approximately 11 percent
more structural concrete, as well as more drives and controls
which increase maintenance requirements.
Concrete or Steel
Steel tanks come completely assembled and only require a concrete
foundation pad and piping and wiring hookups. Although equipment
purchase price is much higher for steel tanks, considerable field
labor and expensive equipment installation are eliminated.
Structural and shipping problems limit steel DAF units to the
smaller sizes (450 square feet [40.5 m2] or less for rectangular
units and 100 square feet [9 m2] for circular units).
For a large installation requiring multiple tanks or large
tanks, concrete tanks are more economical.
Pilot- or Bench-Scale Testing
If sludge is available, the designer should, as a minimum,
perform bench-scale testing (38,39). If money is available,
consideration should be given to renting a pilot DAF thickener
and conducting a four- to six-week test program to evaluate
the effects of such parameters as recycle ratio, air-to-solids
ratio, solids and hydraulic loading, and polymer type and dosage.
If sludge is not available, then a detailed review must be made
of experience at installations where a similiar type of sludge is
being thickened by DAF thickeners.
The first step in designing a DAF thickener is to evaluate the
characteristics of the feed stream. The designer must evaluate
the type of sludge(s) to be thickened and the approximate
quantities of each under various plant loadings and modes of
operation. If waste-activated sludge is to be thickened, the
expected range of sludge ages must be determined, since sludge
age can significantly affect DAF thickening performance (40).
Information is needed about the source of waste sludge and the
range of solids concentrations that can be expected. Also, there
should be an evaluation of any characteristic of the feed stream
that may affect air solubility--for example, concentration of
dissolved salts, and range of liquid temperatures.
5-22
-------
Surface Area
To calculate the effective surface area of a DAF thickener, a
designer must know the net solids load, solids surface loading
rate, and hydraulic surface loading rate.
Net Solids Load
Since a DAF thickener is not entirely efficient, more sludge must
be pumped into the thickener than the actual amount removed. The
actual amount removed is the net solids load. From a design
standpoint, the net load is the amount of solids that must be
removed from the liquid processing train each day. This value
divided by the appropriate solids loading rate gives the required
effective surface area.
The gross solids load is calculated by dividing the net load
by the expected solids capture efficiency of the system. The
gross solids load is important in sizing system hydraulic piping.
The allowable solids loading rate is related to the minimum
solids flux that will occur within the range of sludge
concentrations found in the thickener (41). This flux is a
function of the type of sludge processed, the float concentration
desired, and polymer used. Pounds of dry solids per square foot
per day or pounds of dry solids per square foot per hour are tne
units used to express this rate.
The effect of sludge type on the solids loading rate is shown
in Table 5-8. The loading rates indicated will normally result
in a minimum of four percent solids concentration in the float.
Actual operating data are listed in Table 5-9.
TABLE 5-8
TYPICAL DAF THICKENER SOLIDS LOADING RATES NECESSARY TO PRODUCE
A MINIMUM H PERCENT SOLIDS CONCENTRATION
Solids loading rate, Ib/sq ft/hr
Type of sludge No chemical addition Optimum chemical addition
Primary only 0.83 - 1.25 up to 2.5
Waste activated sludge (WAS)
Air 0.42 up to 2.0
Oxygen 0.6-.0.8 up to 2.2
Trickling filter 0.6-0.8 up to 2.0
Primary + WAS (air) 0.6 - 1.25 up to 2.0
Primary + trickling filter 0.83-1.25 up to 2. 5
1 Ib/sq ft/hr =4.9 kg/m2/hr
5-23
-------
TABLE 5-9
FIELD OPERATION RESULTS FROM RECTANGULAR DAF THICKENERS
Installation
Eugene , OR
Springdale, AR
Athol, MA
Westgate Fairfax, VA
Warren, MI
Frankenmuth, MI
Cinnaminso, NJ
San Jose, CA
Boise, ID
Levittown, PA
Xenia, OH
Indianapolis, IN
Columbus, OH
(Jackson Pike)
Wayne County, MI
Dalton, GA
Middletown, NJ
Sludge
typea
P+TF
P+TF
A.
Ab
Ac
AC
AC
A
P+Ad
P+AS
A
A
A
A
P+A
A
P+A
A
A
P+A
A
Solids
leading
rate,
Ib/sq f t/hr
1.25
2.5
3.2
7.0
0.58
2.0
1.9
1.6
1.0
1.17
1.13
0.54
1.00
0.83
0.75
2.0
Feed
solids
concentration ,
mg/1
5,000
20,000
8,000
14,000
11,000
5,000
8,000
5,000
23,000
17,000
4,600
5,000
5,000
8,000
6,400
4,000
10,000
6,000
4,500
12,900
10,000
Polymer
dosage ,
Ib per dry
ton solids
0
7
2
1-4
40
0
26
5
0
0
0
3
6
0
0
30
30
0
0
0
5-6
Float
concentration ,
percent
solids
4.5-5.0
6.5
4.0
7.3
5.0
3.0
3.5-5.5
4.0
7.1
5.3
4.0
3.8
4.0
6.5
8.6
2.5-3.0
3.5-4.2
3.2
4.6
6.1
4.0
Subnatant
suspended
solids,
ng/1
500
200
50
20
200
750
90
250
500
500
100
100-1,000
800
500
Ref
43
43
43
43
16
14
16
14
14
14
14
14
14
14
16
16
16
14
14
14
a P = Primary sludge
A = Waste-activated a]-",*^.,^*
TF = Trickling filter sludge
Oxygen plant
°Considerable brewery waste
Non-canning season
eCanning season
1 Ib/sq ft/hr =4.9 kg/m2/hr
1 Ib/ton =0.5 kg/t
In general, increasing the solids loading rate decreases the
float concentration. Figure 5-8 illustrates this phenomenon
without polymer addition, and Figure 5-9 with polymer addition.
The addition of
loading rate.
polyelectrolyte.will usually increase the solids
Hydraulic Loading
The hydraulic loading rate for a DAF thickener is normally
expressed as gallons per minute per square foot. When like
units are cancelled, the hydraulic loading rate becomes a
velocity equivalent to the average downward velocity of water as
it flows through the thickening tank. The maximum hydraulic rate
must always be less than the minimum rise rate of the sludge/air
particles to ensure that all the particles will reach the sludge
float before the particle reaches the effluent end of the tank.
5-24
-------
1°
1
a?
z~
g
<
o
§ 3
o
FLOAT
CONCENTRATION
SUBNATANT
SUSPENDED
SOLIDS
800
700
600
500
400
300
200
100
in
Q
o
LU
O
z
LU
o_
CD
00
01234567
SOLIDS LOADING, Ib/sq ft/hr (1 Ib/sq ft/hr = 4.9 kg/m2/hr
FIGURE 5-8
FLOAT CONCENTRATION AND SUBNATANT SUSPENDED
SOLIDS VERSUS SOLIDS LOADING OF A WASTE
ACTIVATED SLUDGE - WITHOUT POLYMERS (16)
Reported values for hydraulic loading rates range from 0.79 to
4.0 gallons per minute per square foot (0.54-2 to. 7 1/min/m2)
(32,42-46). This wide range probably indicates a lack of
understanding of the term. In some cases, the hydraulic loading
refers simply to the influent sludge flow, while in others, the
recycle flow is included. In most sources, no definition of
the term was given. Table 5-10 indicates the hydraulic loading
rates found in the literature.
Since the total flow through the thickener affects the particles,
the hydraulic loading rate should be based on the total flow
(influent plus recycle). Extensive research on waste-activated
sludge (48) has resulted in the conclusion that a peak rate of
2.5 gallons per minute per square foot (1.7 I/sec/ m^ ) should
be employed. This value is based on use of polymers. When
polymers are not used, this value is expected to be lower, but no
design criterion has been suggested at this time. Figure 5-10
shows the effects of polymer and hydraulic loading rate on DAF
thickener subnatant chemical oxygen demand (COD) (48).
Air- to- Sol ids-
Another design parameter to be considered in DAF thickening is
that of the air- to-solids (A/S) ratio. Theoretically, the
quantity of air required to achieve satisfactory flotation is
5-25
-------
directly proportional to the quantity of solids entering the
thickener (defined as gross solids load in the previous section).
For domestic wastewater sludges, reported ratios range from
0.01 to 0.4, with most systems operating at a value under 0.1.
I 6
8
a?
Z 5
LU
u
§ 3
o
FLOAT
CONCENTRATION
SUBNATANT
SUSPENDED
SOLIDS
800
700
600
500
400
300
200
100
CO
Q
_J
O
co
Q
LU
Q
I
CO
h-
CO
CO
1 23456
SOLIDS LOADING, Ib/sq ft/hr (1 Ib/sq ft/hr = 4.9 kg/m2/hr)
FIGURE 5-9
FLOAT CONCENTRATION AND SUBNATANT SUSPENDED
SOLIDS VERSUS SOLIDS LOADING OF A WASTE
ACTIVATED SLUDGE - WITH POLYMERS (16)
The appropriate A/S ratio for a particular application is a
function of the characteristics of the sludge, principally,
the sludge volume index (40), the pressurization systems air
dissolving efficiency, and the distribution of the gas-liquid
mixture into the thickening tank. Figures 5-11 and 5-12 show the
effects of A/S of float concentration and subnatant suspended
solids, with and without polymer addition.
Polymer Usage
Polymers have a
a designer must
performance with
marked effect on DAF thickener performance, and
therefore be careful to differentiate between
and without polymer use.
Polyelectrolytes may improve flotation by substantially
increasing the size of the particles present in the waste. The
particles in a given waste may not be amenable to the flotation
process because their small size will not allow proper air bubble
5-26
-------
attachment. Doubling the diameter or size of the particle can
result in a fourfold increase in the rise rate provided the
previous A/S ratio is maintained. The surface properties of the
solids may have to be altered before effective flotation can
occur. Sludge particles can be surrounded by electrically
charged layers that disperse these particles in the liquid phase.
Polyelectrolytes can neutralize the charge, causing the particles
to coagulate so that air bubbles can attach to them for effective
flotation. Thus, with use of polymers, the following operating
advantages may occur: the size of the DAF thickener may be
reduced; solids capture may be improved, thus reducing the
amount of solids recycled back to the liquid handling system; an
existing, overloaded facility in which polymers are not being
utilized may be upgraded. They also act a surfactant, thus
allowing better attachment of air bubbles.
TABLE 5-10
REPORTED DAF THICKENER HYDRAULIC LOADING RATES3
Hydraulic loading rate (gpm/sq ft)
Influent only Influent plus recycle Reference
1.5-2.5 44
2.5 45
1.0-4.0 46
0.79 47
1.25-1.5 48
0.9 3.0 49
aAll values reported are associated with polymer usage. Values
for systems not using polymer could not be found in the literature,
1 gpm/sq ft = 40.8 1/min/m
The major disadvantage of polymers is cost (polymer cost,
operation and maintenance of polymer feed equipment) when
calculated over the useful lifetime of the plant. In addition,
the actual amount required is very difficult to determine until
flotation studies can be run on the actual installation. If
polymers are to be used, it is best to design conservatively,
so that the possibility of the exceptionally high polymer
demand needed to keep marginal operation at capacity is avoided.
Table 5-9 lists current operating results of plants with and
without polymer addition.
Pressurization System
The air dissolution equipment, which consists of the pressuriza-
.tion pump, air dissolution tank, and other .mechanical equipment,
5-27
-------
is the heart of a DAF thickener system.
tion system, the designer must decide
and a quantity of pressurized flow and
affecting the performance of the system.
In sizing a pressuriza-
on an operating pressure
must be aware of factors
300 i-
200 —
Q
O
CJ
DO
CO
100
O
O
LEGEND
O WITHOUT POLYMER
D WITH POLYMER
HYDRAULIC LOADING (INFLUENT + RECYCLE) RATE (gpm/sq ft)
(1 gpm/sq ft = 40.8 l/min/m2)
FIGURE 5-10
EFFECT OF HYDRAULIC LOADING ON PERFORMANCE IN
THICKENING WASTE ACTIVATED SLUDGE (48)
Operating Pressure
Most commercial available pressurization systems operate at 40 to
80 psig (276 to 522 kN/m2). For a given A/S ratio, the air
5-28
-------
required to float the sludge can be obtained by increasing
the operating pressure of the system to dissolve more air, or
holding a lower operating pressure and increasing the volume of
pressurized flow.
g
i-
DC
I-
01
U
Z
o
CJ
o
7
6
5
4
^
2
1
^
_
-
-
_
o
o
0
0 °
Q _^r O
<^!^0
a ,4 &
W v FLOAT
* °1 , — rONCFNTRATION
^
-
I
r-^-T
bP
JO SUBNATANT
SUSPENDED
SOLIDS
-
.
bUU
700
600 E
1/5"
a
500 8
Q
Llj
Q
400 g
a.
D
in
Z
H
200 z
m
D
100
Q - _1 .K-H-X. ™*L . j_ .. . 1. _H L . _ .... _ _L . 1 .1 ..1.1 [ Q
0 .02 .04 .06 .08 .10 .12 .14 .16 .18 .20 .22 .24 .26 .28
AIR SOLIDS RATIO
FIGURE 5-11
FLOAT CONCENTRATION AND SUBNATANT SUSPENDED SOLIDS
VERSUS AIR-SOLIDS RATIO WITH POLYMER FOR A WASTE
ACTIVATED SLUDGE (16)
In one study (40), it was shown that the higher the operating
pressure of a flotation thickener system, the lower the rise rate
of the sludge. The reason for a higher rise rate at 40 psig
(276 kN/m2) than at 60 or 80 psig (414 or 552 kN/m2) is that
the optimum bubble size is predominant at this lower operating
pressure. This study concludes that attempting to raise the A/S
ratio by increasing the operating pressure is detrimental to the
thickening process. These results are important in that it will
be in the user's best interest to operate at the lowest pressure
possible. The requirement for higher head pumps, larger air
compressors, and higher pressure rated retention tanks raises the
initial cost of the process as well as operating costs.
Quantity of Pressurized Flow
For a DAF thickener to work effectively, the proper amount of
air must be present for each pound of solids to be handled
(A/S ratio). The design pressurized flow should be based on
the maximum gross solids load that the DAF thickener is designed
5-29
-------
to receive. For multiple units, each basin should have its own
independent pressurization system. This is especially important
to remember if the thickening system is designed to operate over
a wide range of influent solids concentrations and flows.
7 I-
6
<
a:
o 4
z
o
o
FLOAT
CONCENTRATION
SUBNATANT
SUSPENDED
SOLIDS
900
800
700 =g,
E
co~
600 §
o
CO
Q
500 yj
400
300
200
100
m
.02
.04
.06
.08
.10
.12 .14 .16 .18
AIR SOLIDS RATIO
.22
.24 .26
.28
FIGURE 5-12
FLOAT CONCENTRATION AND SUBNATANT SUSPENDED SOLIDS
VERSUS AIR-SOLIDS RATIO WITHOUT POLYMER FOR A
WASTE ACTIVATED SLUDGE
Factors Affecting Performance
The designer should be aware of two physical factors, air
saturation and turbulence, which can affect the performance
of the pressurizing system.
Air Saturation. The basic mechanism that makes flotation
possible is the increase in the amount of gas dissolved when
pressure is increased. The relationship between pressure and
quantity dissolved is shown in Henry's Law, which states that if
no reaction prevails between the gas and liquid phases, the
solubility of the gas is directly proportional to the absolute
pressure of the gas at equilibrium with the liquid at constant
temperature.
5-30
-------
In practice, the actual amount of air dissolved for a given air
input depends on the efficiency of the pressurization device,
liquid temperature and concentration of solutes in the liquid
stream being pressurized.
Normally a pressure retention tank is used to optimize the
air-water interface for efficient air transfer in the shortest
detention time. Depending on tank design (packed tank, packless
tank, tanks with mechanical mixers, etc.), efficiencies can range
from as low as 50 percent to over 90 percent. It is current
design practice in the United States to specify a minimum of
85 to 90 percent efficiency.
The equilibrium concentration of a gas in a liquid is inversely
related to the temperature of the liquid phase. The temperature
effect is substantial. For example, the saturation of air in
water at 140°F (60°C) is about one half less than the saturation
of air in water at 66°F (18.8°C) at one atmosphere.
The presence of salts such as chloride will normally decrease the
air solubility at a given temperature and pressure. The effect
of salt concentration on air dissolving efficiency is best
evaluated by conducting bench-scale treatability tests or a pilot
unit test program.
Turbulence. The proper amount of turbulence must be present
~at the po~int of pressure reduction to cause bubble formation.
Without the necessary turbulence, the rate at which air bubbles
form is slow and may occur too late in the process. Excessive
turbulence can result in increased bubble agglomeration and floe
shear. Under this condition, the majority of bubbles formed will
be considerably larger than the 50 to 100 ym needed for effective
flotation.
Number^of Units to_b_£JJsed
The number of DAF thickeners to be provided at a facility depends
on the following factors:
• The availability and configuration of available land.
• The operating cycle that will be used, for example, seven
days per week, 24 hours per day; five days per week;
eight hours per day; etc.
• Seasonal variability; for example, the operation of a
food processor six months of the year, the waste flow
from which will go to the municipal facility.
• The variance in average-to-peak hourly solids load that
can be expected on a day-to-day basis.
5-31
-------
Adequate capacity to thicken peak hourly waste sludge production
is necessary. In addition, provision must be made to handle
the sludge flow if a unit must be taken out of service. (See
discussion in Chapter 2).
0_therConsiderations
In addition to the system design considerations previously
discussed, the designer must also give consideration to feed
sludge line sizing, thickened sludge removal, bottom draw-off
piping, subnatant piping, pressurized flow piping, and controls.
Each of these items is briefly discussed below.
Feed _JSjLu d g e L i n e
Feed sludge flow rate must be controlled to stay within allowable
limits. This requires a flow meter that accurately measures
a high solids stream and piping large enough to handle maximum
flow.
Thickened Sludge Removal
The surface skimmer brings the thickened sludge over the
dewatering beach and deposits it in a sludge hopper. The
thickened sludge must then be pumped to the next phase of the
solids handling system. In pump selection, it is important to
remember that air has been entrained in this sludge by the
flotation thickening process. Pumps that can air lock should
not be used; positive displacement pumps are common in this
application.
For pipe sizing and final pump selection, consider that the
thickened sludge can reach concentrations in the range of ten
percent. (See Chapter 14 for further discussion).
Bottom Sludge ..Draw_0ff_
In a rectangular DAF tank, the bottom collector moves the settled
solids to the influent end of the basin. Here it is deposited
into either multiple hoppers or a cross-screw conveyor that
delivers it to a hopper. The bottom collector in a circular
DAF tank delivers the settled solids directly to a hopper in
the center of the tank. Once the solids are in the hopper, they
must be removed from the tank. Depending on where this flow
goes, it can be handled by either gravity or pumps.
One major consideration that applies to either removal system,
but particularly to gravity removal, is the static head
available. Since the draw-off point is at the bottom of the
flotation basin, the entire depth of the liquid in the basin must
be considered as available static head. Although fine control is
not required, this head must be dissipated in order to restrict
the flow. A positive displacement pump with variable speed drive
will assure control of bottom sludge withdrawal.
5-32
-------
This draw-off is at the lowest point in the basin and therefore
could also be used as a basin drain. If a tee and drain valve is
installed on this line at the outside of the tank wall, draining
can take place. The line from the drain valve can go to the
plant's drain system.
Subnatant Line
Pipe sizing should be such that it can handle the maximum total
flow (influent plus recycle) without any appreciable head loss.
Pressurized Flow Piping
Because of the high pressure requirements of this flow, the
pressurization liquor is usually delivered to the pressure tank
by a high-speed, closed impeller centrifugal pump. Piping must
be sized to handle the maximum liquid throughput rate of the
pressure tank selected.
£ont£0l_s_
The controls for a DAF thickener are dependent upon the system,
the degree of automation required, and the equipment manufac-
turer's design. They usually include, at a minimum, a pressure
controller for the pressure vessel and flow meters for the feed
and thickened sludge flows.
5.4.2 Design Example
A designer has calculated that it will be necessary to thicken a
maximum of 2,700 pounds (1,225 kg) per day of waste sludge at
0.5 to 0.8 percent solids from a contact stabilization plant
employing no primary clarification. The facility will have a
sludge handling system consisting of a DAF thickener for the
waste activated sludge, mechanical dewatering by belt press and
composting. The treatment plant will be manned eight hours per
day, seven days per week but dewatering operations will only
take place six hours per day, five days per week. Thickening
operation would take place 7.5 hours per day, five days per week.
Waste sludge flow from the final clarifier would be continuous
during the thickening operation—that is, 7.5 hours per day, five
days per week.
The designer has decided to provide polymer feed equipment for
the DAF thickener to be used in emergency situations only.
Polymers are not used in normal operation.
The designer has also decided to use a packed pressurization
tank, which requires a relatively clean source of pressurized
flow. Secondary effluent will be utilized.
Effective Surfac_e_Are_a
The maximum daily waste sludge production expected was given as
2,700 pounds (1,225 kg) of waste-activated sludge with a solids
concentration of 5,000 to 8,000 mg/1.
5-33
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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 *£ £i««rrir i
FEED
EFFLUENT
DISCHARGE
CONCENTRATING
CHAMBER
SLUDGE
DISCHARGE
FEED
EFFLUENT
DISCHARGE
ROTOR
BOWL
ROTOR
NOZZLES
SLUDGE
DISCHARGE
RECYCLE FLOW
FIGURE 5-17
SCHEMATIC OF A DISC NOZZLE CENTRIFUGE
5-41
-------
The centrifugal action causes the solids to concentrate as they
settle outward. At the outer rim of the rotor bowl, the high
energy imparted to the fluid forces the concentrated material
through the rotor nozzles. One part of this concentrated
sludge is drawn off as the thickened product and another is
recycled back to the base of the rotor and pumped back into the
concentrating chamber; there, it is subjected to additional
centrifugal force and is further concentrated before it is
once again discharged through the nozzles. This recirculation
is advantageous because it increases the overall underflow
concentration; minimizes particle accumulation inside the rotor
by flushing action; allows the use of larger nozzles, thus
decreasing the potential for nozzle plugging; and helps to
achieve a stable separation equilibrium that lends itself to
precise adjustment and control.
Disc nozzle centrifuges can be applied only to sludges consisting
of smaller particles (less than 400 m [54]) and void of
fibrous material. In early installations, severe operating
and maintenance problems occurred from pluggage (24,49,55,56).
For wastewater treatment, then, only those systems that provide
primary treatment and separate the primary sludge from the
waste-activated sludge can be equipped with a disc nozzle
centrifuge and only activated sludge can be thickened in this
way. Even for those systems that keep the necessary separation,
designers have frequently forgotten the amount of fibrous
material that can be recycled back into the aeration system
from a dirty anaerobic digester supernatant stream. This also
eventually causes severe pluggage.
Pretreatment
To further reduce operation and maintenance requirements, current
design recommendations provide for pretreatment of the disc
nozzle feed stream. Figure 5-18 shows a disc nozzle pretreatment
system.
Raw WAS is pumped to a strainer in order to remove large solids
and fibrous material. Strainers should be made of stainless
steel, should be self-cleaning, and should be easily accessible.
Approximately one percent of the inlet flow will be rejected.
The reject stream should go to the primary sludge handling
system.
After screening, the flow goes to a degritter; however, even
after aerated grit removal and primary treatment, some grit
gets into the aeration basin. Under the velocities generated in
a disc nozzle, this grit becomes abrasive and causes nozzle
deterioration. The degritter does not eliminate the problem
completely but it does increase the running time between nozzle
replacements. Approximately 10 percent of the degritter inlet
flow is rejected, and this rejected stream is usually combined
with the screen flow.
5-42
-------
STRAINER
RAW WASTE
NOZZLE
SEPARATOR
REJECT FLOW GOES
BACK TO PRIMARY
SLUDGE HANDLING SYSTEM
p) RECIRCULATION
-V-> PUMP
THICKENED
SLUDGE
FIGURE 5-18
TYPICAL DISC NOZZLE PRETREATMENT SYSTEM
Performance
Table 5-12 lists typical performance that can be expected of
disc nozzle centrifuges. In addition to the standard process
variables, the disc nozzle machine variables considered are bowl
diameter, bowl speed, operation of recycle, disc spacing, and
nozzle configuration. Possibly the most important consideration,
however, is the nature of the sludge. As with other centrifuge
applications, an increasing sludge volume index (SVI) influences
machine performance. Figure
capture and thickening (57).
5-19 shows the effect of SVI's on
TABLE 5-12
TYPICAL PERFORMANCE OF DISC NOZZLE CENTRIFUGE
Ref
5
5
5
5
24
60
Capacity,
gallons
per
minute
150
400
50-80
60-270
66
200
Feed
solids ,
percent
solids
0. 75-1.0
-?
0.7
0.7
1. 5
0.75
Underflow
solids ,
percent
solids
5-5.5
4.0
5-7
6.1
6.5-7.5
5.0
Solids
recovery,
percent
90 +
80
93-87
97-80
87-97
90
Polymer,
pounds per
dry ton
of solids
None
None
None
None
None
None
1 gpm = 3.78 1/min
1 Ib/ton = 0.5Aa/t
5-43
-------
DC
LLJ
>
o
u
LU
QC
100
90
80
70
60
50
40
30
2V/*
2.Q
3.0
4.0
5.0
6.0
THICKENED SLUDGE SOLIDS, %
FIGURE 5-19
EFFECT OF ACTIVATED SLUDGE SETTLEABILITY ON
CAPTURE AND THICKENING (57)
In general, it can be said of disc nozzle performance that the
concentration of the thickened sludge tends to increase with
increasing solids concentration in the inlet. Depending on inlet
solids concentration, thickened sludge will be five to ten times
more concentrated than the feed. The capability to concentrate
will decrease as the inlet solids become more concentrated.
Solids capture of 90 percent or better for the material fed into
the disc nozzle (after screening and grit removal) should be
obtainable without the use of polymers.
Other Considerations
As noted in the discussion of pretreatment requirements,
approximately 11 percent of the flow to the disc nozzle system is
rejected. The reject stream contains two to three percent solids
and is usually pumped to the primary sludge handling system.
The centrate stream is normally returned to the aeration tank.
This line should be designed to handle the entire flow being
pumped to the pretreatment system.
5-44
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Typically, equipment suppliers furnish disc nozzle systems
complete, including all necessary pumps. The system must be
assembled in the field.
5.5.3.2 Imperforate Basket
Imperforate basket centrifuges were first used in the U.S. in
1920, and to date, approximately 100 municipal installations
(over 300 machines) have been installed (37). About one half are
used for thickening. In fact, the largest centrifuge facility
in the world, the Joint Water Pollution Plant of the County
Sanitation Districts of Los Angeles County, California, utilizes
48 imperforate basket centrifuges. Table 5-13 lists the
advantacjos and disadvantages of an imperforate 'OH sky I: centrifuge
compared to other thickening systems.
TABLE 5-13
ADVANTAGES AND DISADVANTAGES OF IMPERFORATE BASKET CENTRIFUGE
Advantages Disadvantages
Facility can be designed so that same Unit is not continuous feed and discharged
machine can be used both for thickening Requires special structural support
and dewatering
... . , . Has the highest ratio of capital cost to
Is very flexible in meeting process .,
requirements
Is not affected by grit
Of all the centrifuges, has the lowest
operation and maintenance requirements
Compared to gravity and DAF thickener
installations, is clean looking and has
little to no odor problems
Is an excellent thickener for hard-to-handle
sludges
f Operation
Figure 5-20 is a schematic of a top feed imperforate basket
centrifuge illustrating general location of sludge inlet, polymer
feed, and centrate piping and location of cake discharge.
The following describes one complete batch operating cycle of a
basket centrifuge. When the "cycle start" button is pushed, the
centrifuge begins to accelerate. After approximately 30 seconds,
the feed pump is started through a timer relay. Depending on
the feed pump rate, it will take one to three minutes for the
bowl to reach operating speed. Sludge enters the unit through a
stationary feed pipe mounted through the curb cap. This pipe
extends to the bottom portion of the basket and ends at an angle
just above the floor in order to impart a tangential velocity to
the input stream. The duration of the feed time is controlled by
5-45
-------
either a pre-set timer or a centrate monitor that shuts the feed
pump off when a certain level of suspended solids appears in the
centrate. The centrate is normally returned to the inlet of the
secondary treatment system.
FEED
POLYMER
SKIMMINGS
KNIFE
CAKE
CAKE
FIGURE 5-20
GENERAL SCHEMATIC OF IMPERFORATE BASKET CENTRIFUGE
Deterioration in the centrate indicates that the centrifuge bowl
is filled with solids, and separation can no longer take place.
At this point, the sludge feed pump is turned off.
Turning off or diverting the feed pump decelerates the centrifuge.
When the centrifuge has decelerated to 70 rpvn's, a plow (located
by the center spindle shaft) is activated and starts to travel
horizontally into the bowl where the solids have accumulated.
5-46
-------
When the plow blade reaches the bowl wall, a dwell timer is
activated to keep the plow in the same position for approximately
5 to 15 seconds until all the solids have been discharged. When
the plow retracts, a cycle has been completed and the machine
will automatically begin to accelerate, starting a new cycle.
Application
Basket centrifuge is a good application for small plants (under
1 to 2 MGD [44 to 88 I/sec]) pumping capacity. The appropriate
plant would provide neither primary clarification nor grit
removal (that is, extended aeration, aerated lagoons, contact
stabilization), but would require:
• Thickening before aerobic or anaerobic digestion,
• Solids content of less than ten percent to minimize cost
of hauling liquid sludge for land disposal, and
• A machine that can thicken sludge part of the time and
dewater sludge part of the time.
Performance
Table 5-14 lists typical basket centrifuge performance on several
types of sludges. Figure 5-21 shows the relative influence of
one process variable as a function of feed solids content,
holding all other process variables constant.
TABLE 5-14
TYPICAL THICKENING RESULTS USING IMPERFORATE
BASKET CENTRIFUGE
Sludge type
Feed solids
concentration,
percent solids
Raw waste-activated
sludge '. ,
Aerobically
digested sludge
Raw trickling filter sludge
(rock & plastic media)
Anaerobically digested
sludge, primary and rock
trickling filter sludge
(70:30)
0.5-1.5
1-3
2-3
2-3
Average
cake solids
concentration,
percent solids
8-10
8-10
8-9
9-11
8-10
7-9
Polymer
required,
pounds dry
per ton dry
feed solids
0
1.0-3.0
0
1.0-3.0
0
1.5-3.0
0
1.5-3.0
Recovery
based on
centrate ,
percent
85-90
90-95
80-90
90-95
90-95
95-97
95-97
94-97
1 Ib/ton =0.5 kg/t
5-47
-------
FEED, % TOTAL SOLIDS
POSSIBLE
FEED, % TOTAL SOLIDS
c
1
a
o>
O)
FEED, % TOTAL SOLIDS
FEED, % TOTAL SOLIDS
o
LU
O
a.
FEED, % TOTAL SOLIDS
FEED, % TOTAL SOLIDS
FIGURE 5-21
RELATIVE INFLUENCE OF ONE PROCESS VARIABLE AS A
FUNCTION OF FEED SOLIDS CONTENT FOR IMPERFORATE
BASKET CENTRIFUGE HOLDING ALL OTHER PROCESS
VARIABLES CONSTANT
5-4!
-------
Othg_r_C£n_si_derations
In discussions of hydraulic flow rate, a distinction must be
made between instantaneous feed rate and average feed rate.
Instantaneous feed rate is the actual hydraulic pump rateto
the basket. The average feed rate includes the period of time
during a cycle when sludge is not being pumped to the basket
(acceleration, deceleration, discharge). Therefore, dividing
total gallons pumped per cycle by total cycle time gives the
average feed rate.
Basket centrifuge performance is affected by the solids feed rate
to the machine. As the solids concentration changes, the flow
rate must be adjusted. Every effort should be made to minimize
floe shear. For this reason, positive displacement cavity feed
pumps with 4 to 1 speed variation are recommended.
Cake solids concentration can only be discussed as average solids
concentration. The solids concentration in a basket centrifuge
is maximum at the bowl wall and decreases toward the center. The
solids concentration discharged will be the average for the
mixture.
The centrate stream should be returned to the secondary system.
5.5.3.3 Solid Bowl Decanter
The first solid bowl decanter centrifuge in the U.S. to operate
successfully on municipal wastewater sludge was installed in the
mid-1930's (58). Since then there have been approximately
150 installations (over 400 machines) (37). Few of these units
were used for thickening because the rotating scroll created
disturbances in the thickening sludge, and the gravity force that
had to be overcome in climbing the beach made it more difficult
for the liquid thickened sludge to be discharged.
Technological advances have made solid bowl decanters for
thickening waste-activated sludge available. Table 5-15 lists
the current advantages and disadvantages of solid bowl decanter
centrifuges in waste-activated sludge thickening.
Principles of Operation
Figure 5-22 is a schematic of a solid bowl decanter centrifuge.
The sludge stream enters the bowl through a feed pipe mounted at
one end of the centrifuge.
As soon as the sludge particles are exposed to the gravitational
field, they start to settle out on the inner surface of the
rotating bowl. The lighter liquid, or centrate, pools above the
sludge layer and flows towards the centrate outlet ports located
at the large end of the machine.
5-49
-------
TABLE 5-15
ADVANTAGES AND DISADVANTAGES OF SOLID BOWL DECANTER CENTRIFUGES
Advantages
Disadvantages
Yields high throughput in a small area
Is easy to install
Is quiet
Causes no odor problems
Has low capital cost for installation
Is a clean looking installation
Has ability to constantly achieve four to
six percent solids in the thickened sludge
Is potentially a high maintenance item
May require polymers in order to operate
successfully
Requires grit removal in feed stream
Requires skilled maintenance personnel
FEED
\
.
.';••.'; DEWATERED
' "•'.'. SOLIDS
FIGURE 5-22
SCHEMATIC OF TYPICAL SOLID BOWL DECANTER
CENTRIFUGE
The settled sludge on the inner surface of the rotating bowl is
transported by the rotating conveyor towards the conical section
(small end) of the bowl. In a decanter designed for dewatering,
the sludge, having reached the conical section, is normally
conveyed up an incline to the sludge outlet. Waste-activated
5-50
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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
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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 86°F (30°C). The plant had been unable to gravity thicken
waste-activated sludge over a maximum of 2.5 percent. In
addition, sludge blanket turnovers and other process upsets
proved troublesome. Use of polymers, dilution water, and mixing
with primary sludge did not resolve the problems associated with
gravity thickening waste-activated sludge.
After some pilot testing, two disc-nozzle centrifuges were
installed to concentrate waste-activated sludge prior to
anaer~oblcditjestiorr,and ~an equipment testing" prograirr was~
undertaken on other centrifuges. An expansion from 45 to 96 MGD
(2 to 4 mVs) was anticipated without an increase in the plant's
existing digester capacity. This meant that sludge would have to
be concentrated to at least five percent total solids.
Over a three-year period, the existing disc nozzle centri-
fuge system was redesigned and optimized and other centrifuges
(imperforate basket and solid bowl decanter) were tested.
The test program at Village Creek graphically illustrated that
the thickening characteristics of waste-activated sludges vary
markedly depending on the design and operating criteria of the
activated sludge process and on the storage conditions of the
sludges. These variations can be reduced considerably by the use
of polyelectrolyte conditioners. The effect of polyelectrolytes
on unit process costs varies; the advisability of using them must
be determined for each individual case.
Dis^c Nozzle
Testing was conducted on a 24-inch (61 cm) diameter unit,
operating at 4,290 rpm and having a 0.07-inch (1.7 mm) nozzle
opening. The optimum design for obtaining a five percent
sludge and 90 percent recovery was at 200 gpm (12.62 I/sec) and
750 pounds per hour (340 kg/hr) of solids.
In operation, the nozzles on a disc-nozzle machine will plug up
in minutes if prescreening is not provided. For activated sludge
the screen must be chosen with care. Vibrating screens can
become coated with grease and fiber. They may coat over even
when provided with spray nozzles, or they may tear from abrasion.
A rotating drum wedge wire screen with either 0.010-inch
(0.25 mm) or 0.020-inch (0.51 mm) openings offered the best
results. The rejects from this screen were about 5 to 15 percent
of the feed solids. These rejects consisted of approximately
60 percent hexane extractables and 30 percent fiber.
5-53
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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
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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
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The following provides a rough guide for operating and
maintenance requirements for a disc nozzle centrifuge:
• It is best to run a disc nozzle 24 hours per day to
prevent shutdowns from materials that will dry out
between stacked plates when machine is not operating.
• At least once every eight hours, each machine should be
inspected for general machine operation, product, and
amperage draw--l/2 man-hour per unit.
• At least once a week, each machine should be shut down
to be given a thorough flushing, and nozzles should be
removed and cleaned—two man-hours per unit.
• If grease is present in the system, the machine should
be flushed with hot water at least once every other
day--three man-hours per unit.
• Depending on sludge characteristics, the length of time
before a machine has to be completely disassembled and
cleaned is quite variable. A complete cleaning will
take approximately 16 man-hours.
• Even with good pretreatment, nozzles, holders and recycle
tubes will have to be replaced.
• Other parts that will need replacing are drive belts
and pumps.
Lmper_£cirate Basket Centrifug£
For a well designed system, operation and maintenance for one
48-inch by 30-inch (122 cm x 76 cm) basket using a hydraulic
drive can be approximated as follows:
• Normal start-up and shutdown - 0.5 man-hour.
• Observation time per eight-hour shift--!.0 man-hour.
• Basket oil change (1 quart SAE 10-40 motor oil [0.95 1]
10-40 motor oil) is required every 200 operating hours--
0.5 man-hour.
• General machine lubrication is needed every 200 operating
hours--0.5 man-hour.
• Air compressor should be serviced every 1,000 operating
hours--!.0 man-hour.
• Hydraulic oil change (65 gallons [246 1]) is required
every 3,500 operating hours or once per year—3.0 man-
hours .
5-57
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• High pressure oil filter should be changed every 1,000
operating hours—0.5 man-hour.
• If the machine is to be shut down for more than 24 hours,
the basket should be cleaned with water (tap water
pressure). This can be provided as an automatic or a
manual operation--0.5 man-hour for manual operation.
• Basket bearings should be replaced every 100,000
operating hours—40 man-hours.
Standard materials repair cost per 1,000
operating hours is $300 to $350 (June 1979).
machine
• Specific power draw for this size basket centrifuge
ranges from 1.1 to 1.3 horsepower per gallon per minute
(13 to 15 kW/l/sec) flow rate.
SoJL_ij_Bowl DecajT,ter Centr i fuge
Figure 5-25 indicates annual man-hour requirements for operation
and maintenance. Included in the curve are labor requirements
directly related to the centrifuge, sludge conditioning, and
other associated equipment.
oc
O
LL
o
X
I
^
cc
O
1,000
2 345678910 2 3 456789100 2
AVERAGE FLOW, gpm (1 gpm = 40.8 l/min)
FIGURE 5-25
ANNUAL O&M REQUIREMENTS - SOLID BOWL DECANTER
CENTRIFUGE (28)
3 456789
5-58
-------
Power
Power is dependent on machine design, but it should range from
0.28 to 0.37 horsepower per gallon per minute flow rate (3.3 to
4.4 kW/l/sec) .
Maintenance Material Costs
Figure 5-26 shows a curve developed for estimating solid bowl
decanter centrifuge maintenance material cost.
o
•a
C/3
O
O
100,000
9
8
7
6
5
3
2
10,000
7
6
5
4
1,000
I
I
I
3 456789
1 2 34567 89 10 2 34567 89100 2
AVERAGE FLOW, gpm (1 gpm = 40.8 l/min)
FIGURE 5-26
ESTIMATED JUNE 1975 MAINTENANCE MATERIAL COST
FOR SOLID BOWL DECANTER CENTRIFUGE
5.6 Miscellaneous Thickening Methods
5.6.1 Elutriation Basin
Elutriation is a satisfactory process for washing and thickening
digested primary sludges. Elutriation is also effective for
mixtures of primary and biological sludges as long as a small
5-59
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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
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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
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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
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49. Kraus, L.S. and J.R. Longley. "Concentrating Activated
Sludge with a Continuous Feed Centrifuge." jSewage Works
Journal. Vol. 11, p. 9, (1939).
50. Keith, F.W. and T.H. Little. "Centrifuges in Water and
Waste Treatment." Chemical Engineeri.nq[_Pr_o_gres_s_. Vol. 65,
(November 1969).
51. Svarovsky, L. "Separation by Centrifugal Sedimentation."
Solid-Liquid Separation, Butterworths, Inc., Ladislav
Svarovsky, editor, 1977. p. 125.
52. Vesilind, P.A. Treatment and Disposal_of_W_aj3te water Sludge.
Ann Arbor Science Publishers, 1974.
53. Ambler, C.M. "The Evaluation of Centrifuge Performance."
Chemical Engi n e e_r i n g_ Pr ogre s s . Vol. 48 #3 (1952).
54. Landis, D.M. "Centrifuge Applications." Filtration
Engendering. p. 27, (September 1969).
55. Bradney, L. and R.E. Bragstad. "Concentration of Activated
Sludge by Centrifuge." S_gw_a_g_e gJ!J_^IjJJJ5_!-.JLJ:Al __Waste *
Vol. 27, p. 404, (1955).
56. Reefer, C.E. and H. Krotz . "Experiments on Dewatering
Sewage Sludge with a Centrifuge." j3ewag_eWorks Journal,
Vol. 1, p. 120, (1929) .
57. Vaughn, D.R. and G.A. Reitwiesner. "Disc-Nozzle Centrifuges
for Sludge Thickening." Journal Water Pollution Control
Federation. Vol. 44, p. 1789, (1972).
58. Albertson, O.E. and E.E. Guidi, Jr. "Centrifugation of
Waste Sludges." Journal WaterPollution ControlFederation.
Vol. 41, p. 607, (1969).
59. McKnight, M.D., "Centrifugal Thickening of Excess Activated
Sludge." Unpublished article on experience at Village Creek
Plant, Ft. Worth, Texas. 1978.
60. Earnshaw, G. "A Storage System That Works." Sludge
Magazine. March-April, p. 29/ (1979).
61. NCASI. A Pilot Plant Study of Mechanical Dewatering Devices
Operated on Waste Activated Sludge. Prepared for National
Council of the Paper Industry for Air and Stream Improvement
Technical Bulletin 288, November 1976.
5-64
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EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapters. Stabilization
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 6
STABILIZATION
6.1 Introduction
The principal purposes of stabilization are to make the treated
sludge less odorous and putrescible and to reduce the pathogenic
organism content. Some procedures used to accomplish these
objectives can also result in other basic changes in the sludge.
The selection of a certain method hinges primarily on the final
disposal procedure planned. If the sludge is to be dewatered and
incinerated, frequently no stabilization procedure is employed.
Most stabilization methods, particularly anaerobic and aerobic
digestion, result in a substantial decrease in the amount of
suspended sludge solids. Hence, the corollary function of
conversion is included in the description of these processes.
This chapter provides detailed discussion of four processes
that have the primary function of sludge stabilization. These
processes are anaerobic digestion, aerobic digestion, lime
stabilization, and chlorine oxidation. Both anaerobic and
aerobic digestion are currently increasing in popularity. The
former is receiving revived attention from some cities and new
attention from others for several reasons. The production of
methane in anaerobic digestion is attractive in view of energy
shortages, as is the suitability of anaerobically digested
sludges to disposal on land. Also, it is being recognized
that problems experienced previously with anaerobic digestion
were actually due to other wastewater process considerations.
Interest in aerobic digestion of excess activated sludge is
growing because it has the potential for providing a good
quality liquid process stream and can produce exothermic reaction
conditions. A major impetus for processes such as anaerobic and
aerobic digestion and lime treatment is the growing emphasis on
utilization of sludge rather than mere disposal. Chlorine
oxidation is of limited use for special situations or where
septic tank wastes are involved.
Several other sludge treatment processes provide varying degrees
of stabilization, although this is not their principal function.
Composting is practiced in several United States cities and
is being actively investigated for others. This process is
considered important enough, with the emphasis on recycling of
sludge to the land, that it alone is discussed in Chapter 12.
Heat treatment, discussed in Chapter 8, has been installed in
several new United States plants to improve sludge conditioning
and dewatering economics. Some processes used to disinfect
6-1
-------
sludge, such as heat drying and pasteurization, also provide
limited stabilization. These processes are discussed in
Chapters 7 and 10.
Selection of the optimum stabilization method for any treatment
and disposal system requires an in-depth understanding of
the method and its limitations. The designer must recognize
these limitations and accommodate them in the design of the
subsequent processing and disposal steps.
6.2 Anaerobic Digestion
6.2.1 Process Description
Anaerobic digestion is the biological degradation of complex
organic substances in the absence of free oxygen. During these
reactions, energy is released and much of the organic matter is
converted to methane, carbon dioxide, and water. Since little
carbon and energy remain available to sustain further biological
activity, the remaining solids are rendered stable.
6.2.1.1 History and Current Status
Anaerobic digestion is among the oldest forms of biological
wastewater treatment. It was first used a century ago to reduce
both the quantity and odor of sewage sludges. Originally,
anaerobic digestion was1 carried out in the same tank as
sedimentation, but the two-story tanks developed in England by
Travis and in Germany by Imhoff began a trend toward separating
the two processes. Separate sludge digestion tanks came into use
in the first decades of this century. At first, these were
little more than simple holding tanks, but they provided the
opportunity to control environmental conditions during anaerobic
digestion and, thereby, improve process performance. With the
development of digester heating and, subsequently, mixing,
anaerobic digestion became the most common method of stabilizing
sludge.
As both industrial waste loads and the general degree of
wastewater treatment increased, the sludges generated by treat-
ment plants became more varied and complex. Digester systems
failed because their design and operation were empirically
developed under simpler conditions. As a result, anaerobic
sludge digestion fell into disfavor. However, interest in
anaerobic digestion of dilute wastes stimulated a new wave of
research into the process. The resulting development of steady
state models in the 1960s (1,2,3), dynamic models in the 1970s
(4,5,6), and increasing research into the basic biochemical
processes (7,8,9,10) led to significant improvements in both
reliability and performance of anaerobic digesters.
6-2
-------
Currently, sludge stabilization by anaerobic digestion is used
extensively. A 1977 survey (11) of 98 municipal wastewater
treatment plants in the United States found that 73 used
anaerobic digestion to stabilize and reduce the volume of sludge.
Because of emphasis on energy conservation and recovery and
environmental pressure to use wasteewater sludges on land, it is
expected that anaerobic digestion will continue to play a major
role in municipal sludge processing.
6.2.1.2 Applicability
A wide variety of sludges from municipal wastewater treatment
plants can be stabilized through anaerobic digestion. Table 6-1
lists some types of sludge that have been anaerobically digested
in full-scale, high rate digesters.
TABLE 6-1
TYPE AND REFERENCE OF FULL-SCALE STUDIES ON
HIGH RATE ANAEROBIC DIGESTION OF MUNICIPAL
WASTEWATER SLUDGE (13,14,15,16-34)
Reference
Mesophilic Thermophilic
Sludge type diqestion digestion
Primary and lime 16, 17 -
Primary and ferric chloride 18
Primary and alum 19
Primary and trickling filter 20, 21
Primary, trickling filter, and alum 22 -
Primary and waste activated 23, 24, 25, 26 25, 27, 23, 29
Primary, waste activated, and lime 30, 31
Primary, waste activated, and alum 30, 32, 33 -
Primary, waste activated, and ferric
chloride 30
Primary, waste activated, and sodium
aluminate 32, 33
Waste activated only (pilot plant
only) 13, 14, 15,.34 13, 14, 15
Solids-liquid separation of digested primary sludge is downgraded
by even small additions of biological sludge, particularly
activated sludge. Although mixtures of primary and biological
sludge will break down readily under anaerobic conditions, the
net rate of the reaction is slowed slightly (12). Experience
with full-scale anaerobic digestion of straight activated sludge
is limited, although laboratory (13,14) and pilot-scale studies
(15) demonstrate that separate digestion of activated sludge is
feasible.
Chemical sludges have been successfully digested anaerobically,
although in several cases, volatile solids reduction and gas
6-3
-------
production were low, compared with conventional sewage sludges
(35,36). Decreased performance appears to result from reduced
biodegradability, rather than from toxic inhibition of the
anaerobic microorganisms (35).
Anaerobic digestion is a feasible stabilizing method for
wastewater sludges that have low concentrations of toxins
and a volatile solids content above 50 percent. The obligate
anaerobic microorganisms are sensitive and do not thrive under
fluctuating operating conditions. Consequently, the process must
be carefully considered for use at treatment plants where wide
variations in sludge quantity and quality are common.
6.2.1.3 Advantages and Disadvantages
Anaerobic digestion offers several advantages over other methods
of sludge stabilization; specifically, the process:
• Produces methane, a usable source of energy. In most
cases, the process is a net energy producer, since
the energy content of digester gas exceeds the energy
demand for mixing and heating. Surplus methane
is frequently used for heating buildings, running
engines, or generating electricity (37,38,39). (Refer to
Chapter 18.)
• Reduces total sludge mass through the conversion of
organic matter to primarily methane, carbon dioxide, and
water. Commonly, 25 to 45 percent of the raw sludge
solids are destroyed during anaerobic digestion. This
can substantially reduce the cost of sludge disposal.
• Yields a solids residue suitable for use as a soil
conditioner. Anaerobically digested sludge contains
n"utr~ients and organic matter that can improve the
fertility and texture of soils. Odor levels are greatly
reduced by anaerobic digestion.
• Inactivates pathogens. Disease-producing organisms in
sludge die off during the relatively long detention times
used in anaerobic digestion. The high temperatures used
in thermophilic digestion (122 to 140°F, 50 to 60°C) have
an additional bactericidal effect. Pathogen reduction
during anaerobic digestion is discussed in Chapter 7.
Principal disadvantages of anaerobic sludge digestion are that
it:
Has a high capital cost. Very large, closed digestion
tanks are required, which must be fitted with systems for
feedina. heatinq, and mixinq the sludqe.
Udiirio a i. d L. c ^ i-i J- L. <^ *_i , wii-L^ii iiitaou. *-"^ a- j. »- v- «— ^*
feeding, heating, and mixing the sludge.
6-4
-------
Is susceptible to upsets. Microorganisms involved in
anaerobic decomposition are sensitive to small changes in
their environment. Monitoring of performance and close
process control are required to prevent upsets.
Produces a poor quality sidestream. Supernatants from
anaerobic digesters often have a high oxygen demand and
high concentrations of nitrogen and suspended solids.
Recycling of digester supernatant to the plant influent
may upset the liquid process stream or produce a build-up
of fine particles in the treatment plant. In plants that
are required to remove nitrogen from the wastewater, the
soluble nitrogen in the supernatant can cause problems
and/or increased costs of treatment.
Keeps me thane-producing bacteria growth at a slow rate.
Large reactors are required to hold the sludge for 15 to
30 days to stabilize the organic solids effectively.
This slow growth rate also limits the speed with which
the process can adjust to changes in waste loads,
temperature, and other environmental conditions (40).
6.2.1.4 Microbiology
Anaerobic digestion involves several successive fermentations
carried out by a mixed culture of microorganisms (7,10). This
web of interactions compromises two general degradation phases:
acid formation and methane production. Figure 6-1 shows, in
simplified form, the reactions involved in anaerobic digestion.
r
\
COMPLEX
SUBSTRATE
CARBOHYDRATES,
FATS AND
PROTEINS
ACID
FORMATION
A
r
MICRO-
ORGANISMS STABLE AND 1
*" , DEGRADATIO
METHANE
PRODUCTION
A
A
MICRO-
NTERMEDIATE ORGANISMS
N PRODUCTS
PRINCIPALLY ORGANIC ACIDS, CO,, METHANE
ACID FORMERS HjO, AND CELLS ^ BACTERIA
^
PH + CO + OTHER END
UH4 + UU2 + PRODUCTS
H20, H2S
CELLS AND STABLE
DEGRADATION
PRODUCTS
FIGURE 6-1
SUMMARY OF THE ANAEROBIC DIGESTION PROCESS
In the first phase of digestion, facultative bacteria convert
complex organic substrates to short-chain organic acids--
primarily acetic, propionic, and lactic acids. These volatile
6-5
-------
organic acids tend to reduce the pH, although alkaline buffering
materials are also produced. Organic matter is converted into a
form suitable for breakdown by the second group of bacteria.
In the second phase, strictly anaerobic bacteria (called
methanogens), convert the volatile acids to methane (CH4), carbon
dioxide (€02), and other trace gases. There are several groups
of methanogenic bacteria, each with specific substrate require-
ments, that work in concert to reduce complex wastes such as
sewage sludge. Tracer studies indicate that there are two major
pathways of methane formation:
• The cleavage of acetic acid to form methane and carbon
dioxide.
CH3COOH *-CH4 + C02
The reduction of carbon dioxide, by use of hydrogen
gas or formate produced by other bacteria, to form
methane.
2H20
When an anaerobic digester is working properly, the two phases of
degradation are in dynamic equilibrium; that is, the volatile
organic acids are converted to methane at the same rate that they
are formed from the more complex organic molecules. As a result,
volatile acid levels are low in a working digester. However,
methane formers are inherently slow-growing, with doubling times
measured in days. In addition, methanogenic bacteria can be
adversely affected by even small fluctuations in pH, substrate
concentrations, and temperature. In contrast, the acid formers
can function over a wide range of environmental conditions and
have doubling times normally measured in hours. As a result,
when an anaerobic digester is stressed by shock loads, tempera-
ture fluctuations, or an inhibitory material, methane bacteria
activity begins to lag behind that of the acid formers. When
this happens, organic acids cannot be converted to methane as
rapidly as they form. Once the balance is upset, intermediate
organic acids accumulate and the pH drops. As a result, the
methanogens are further inhibited, and the process eventually
fails unless corrective action is taken.
The anaerobic process is essentially controlled by the methane
bacteria because of their slow growth rate and sensitivity to
environmental change. Therefore, all successful designs must be
based around the special limiting characteristics of these
microorganisms.
6-6
-------
6.2.2 Process Variations
Experimentation over the years has yielded four basic variations
in anaerobic sludge digestion: low-rate digestion, high-rate
digestion, anaerobic contact, and phase separation.
High-rate digestion is obviously an improvement over low-rate
digestion, and its features have been incorporated into standard
practice. The anaerobic contact process and phase separation,
while offering some specific benefits, have not been used for
sludge digestion in full-scale facilities.
6.2.2.1 Low-Rate Digestion
The simplest and oldest type of anaerobic sludge stabilization
process is low-rate digestion. The basic features of this
process layout are shown on Figure 6-2. Essentially, a low-rate
digester is a large storage tank. With the possible exception
of heating, no attempt is made to accelerate the process by
controlling the environment. Raw sludge is fed into the tank
intermittently. Bubbles of sludge gas are generated soon after
sludge is fed to the digester, and their rise to the surface
provides the only mixing. As a result, the contents of the tank
stratify, forming three distinct zones: a floating layer of
scum, a middle level of supernatant, and a lower zone of sludge.
Essentially, all decomposition is restricted to the lower zone.
Stabilized sludge, which accumulates and thickens at the bottom
of the tank, is periodically drawn off from the center of the
floor. Supernatant is removed from the side of the tank and
recycled back to the treatment plant. Sludge gas collects above
the liquid surface and is drawn off through the cover.
6.2.2.2 High-Rate Digestion
In the 1950s, research was directed toward improving anaerobic
digestion. Various studies (24,41,42,43,44) documented the value
of heating, auxiliary mixing, thickening the raw sludge, and
uniform feeding. These four features, the essential elements of
high-rate digestion, act together to create a steady and uniform
environment, the best conditions for the biological process. The
net result is that volume requirements are reduced and process
stability is enhanced. Figure 6-3 shows the basic layout of this
process .
The contents of a high-rate digester are heated and consistently
maintained to within 1°F (0.6°C) of design temperature.
Heating is beneficial because the rate of microbial growth and,
therefore, the rate of digestion, increases with temperature.
Anaerobic organisms, particularly methanogens, are easily
inhibited by even small changes in temperature. Therefore, close
6-7
-------
control of the temperature in a digester helps maintain the
microbial balance and improves the balance of the digestion
process.
DIGESTER GAS
GAS
RAW SLUDGE
SCUM
\\\x\\\\\\\\\\\\\
SUPERNATANT
\\\\\\\\\\\\\\\\\\\.\\\
ACTIVELY
DIGESTING SLUDGE
\\\\\\\\\\\\\\\\v
STABILIZED
SLUDGE
SUPERNATANT
DIGESTED SLUDGE
UNHEATED
UNMIXED
INTERMITTENT FEEDING AND WITHDRAWAL
DETENTION TIME: 30-60 DAYS
LOADING RATE: 0.03-0.10 Ib VSS/cu ft/day
(0.4-1.6 kg VSS/m3/day)
FIGURE 6-2
LOW-RATE ANAEROBIC DIGESTION SYSTEM
Methane production has been reported at temperatures ranging from
32°F to as high as 140°F (0 to 60°C). Most commonly, high-rate
digesters are operated between 86 and 100°F (30 and 38°C).
The organisms that grow in this temperature range are called
mesophilic. Another group of microorganisms, the thermophilic
bacteria, grow at temperatures between 122 and 140°F (50 and
60°C). Thermophilic anaerobic digestion has been studied
since the 1930s, both at laboratory scale (13,45,46) and plant
scale (27,28,29). This research was recently reviewed by
Buhr and Andrews (47). In general, the advantages claimed for
thermophilic over mesophilic digestion are: faster reaction
rates that permit lower detention times, improved dewatering of
the digested sludge, and increased destruction of pathogens.
Disadvantages of thermophilic digestion include their higher
energy requirements for heating; lower quality supernatant,
containing larger quantities of dissolved materials (29);
6-8
-------
and poorer process stability. Thermophi1ic organisms are
particularly sensitive to temperature fluctuation. More detailed
information on the effects of temperature on digestion is
included in Section 6.2.4. Design of digester heating systems is
discussed in Section 6.2.6.2.
DIGESTER GAS
DIGESTED SLUDGE
HEATED TO CONSTANT TEMPERATURE
MIXED
CONTINUOUS FEEDING AND WITHDRAWAL
DETENTION TIME: 10-15 DAY MINIMUM
LOADING RATE: 0.10-0.50 Ib VSS/cu ft/day
(1.6-8.0 kg VSS/m3/day)
FIGURE 6-3
SINGLE-STAGE, HIGH-RATE ANAEROBIC
DIGESTION SYSTEM
Auxiliary Mixing
Sludge in high-rate digesters is mixed continuously to create
a homogeneous environment throughout the reactor. When
stratification is prevented, the entire digester is available
for active decomposition, thereby increasing the effective
detention time. Furthermore, mixing quickly brings the
raw sludge into contact with the microorganisms and evenly
distributes metabolic waste products and toxic substances.
Methods of mixing and mixing system designs are described in
Sect ion 6.2.6.3.
6-9
-------
Pre-thickening
The benefits of thickening raw sludge before digestion were
first demonstrated by Torpey in the early 1950s (24). By
gravity thickening a combination of primary and excess secondary
sludge before digestion, he was able to achieve stabilization
equivalent to digestion without thickening in one quarter of the
digester volume. In addition, liquid that had previously been
removed as digester supernatant was instead removed in the
preceding thickener. Since thickener supernatant is of far
better quality than digester supernatant, it had significantly
less adverse impact when returned to the wastewater treatment
stream. &lso, heating requirements were considerably reduced by
pre-thickening, since smaller volumes of raw sludge entered the
digesters. • ,,/
Later full-scale studies by Torpey and Melbinger (48) showed that
thickening of digester feed sludge could be improved by recycling
a portion of the digested sludge back to the gravity thickener.
This variation of high-rate digestion, often called the Torpey
process, is shown schematically on Figure 6-4. The results of
Melbinger's studies are summarized in Table 6-2.
initial effect of recirculation was to improve
further benefits were obtained. Improved thickening
sludge increased the detention time (solids retention
time) in the digesters and, thereby, enhanced solids reduction
during digestion. The result was that the volume of sludge for
final disposal was reduced by 43 percent. These results were
obtained with the same overall plant treatment efficiencies and
wastewater aeration requirements as had been achieved prior to
the recycling of digested sludge.
Torpey and
While the
thickening,
of the feed
PRIMARY
RECIRCULATING
SLUDGE
MODIFIED
AERATION
SLUDGE
i
DIGESTED SLUDGE
/^~~\ S~\
! I GRAVITY \ THICKENED /ANAEROBIC\ DIGESTED
h-HICKENEfd MIXED SLUDGE 1 DIGESTER I SLUDGE
^ TO WASTEWATER
1
TO
DISPOSAL
SUPERNATANT TREATMENT STREAM
FIGURE 6-4
FLOW DIAGRAM FOR THE TORPEY PROCESS
There is, however, a point beyond which further thickening
of feed sludge has a detrimental effect on digestion. Two
problems can result from over-concentration of feed sludge.
6-10
-------
Good mixing becomes difficult to maintain. The solids
concentration in the digester affects the viscosity,
which, in turn, affects mixing. Sawyer and Grumbling
(49) experienced difficulty in mixing when the solids
content in the digester exceeded six percent. Because
of the reduction of volatile solids occurring during
digestion, the solids concentration within the digester
is less than the feed solids concentration. Therefore,
feed solids concentrations may reach eight to nine
percent before mixing is impaired.
Chemical concentrations can reach levels that can inhibit
microbial activity. A highly thickened feed sludge
means that the contents of the digester will be very
concentrated. Compounds entering the digester, such as
salts and heavy metals, and end products of digestion,
such as volatile acids and ammonium salts, may reach
concentrations toxic to the bacteria in the digester
(50). For example, in one case, digester failure
followed a three-month period during which feed solids
concentrations ranged from 8.2 to 9.0 percent (51). It
is believed that this caused ammonium alkaline products
to reach toxic concentrations.
TABLE 6-2
RESULTS OF RECIRCULATINC DIGESTED SLUDGE TO
THE THICKENER AT BOWERY BAY PLANT, NEW YORK (48)
Without With
recirculation3 recirculation
Raw sludge
Dry weight, Ib/day 108,000 101,500
Digester feed (includes recircula-
tion)
Dry weight, Ib/day 108,000 144,300
Solids concentration, percent 8.2 9.9
Digested sludge to disposal
Dry weight, Ib/day 60,000 47,500
Solids concentration, percent 4.6 6.1
Volume, cu ft/day 20,700 12,300
aAverages for operation in 1961. Average treatment plant flow = 105 MGD.
bAverages for 15 months of operation with 33, 50, or 67 percent recirculation
of digested sludge. Average treatment flow = 101 MGD.
1 Ib/day = 0.454 kg/day
1 cu ft/day = 0.0283 rtH/day
Uniform Feeding
Feed is introduced into a high-rate digester at frequent
intervals to help maintain constant conditions in the reactor.
6-11
-------
In the past, many digesters were fed only once a day or even less
frequently. These slug loadings placed an unnecessary stress on
the biological system and destabilized the process. Although
continuous feeding is ideal, it is acceptable to charge a
digester intermittently, as long as this is done frequently (for
example, every two hours). Methods of automating digester
feeding are described in Section 6.2.6.5.
Two- S_ t a g e _ D i c[e s t_i q n
Frequently, a high-rate digester is coupled in series with
a second digestion tank (Figure 6-5). Traditionally, this
secondary digester is similar in design to the primary digester,
except that it is neither heated nor mixed. Its main function is
to allow gravity concentration of digested sludge solids and
decanting of supernatant liquor. This reduces the volume of the
sludge requiring further processing and disposal. Very little
solids reduction and gas production takes place in the second
stage (23).
DIGESTER
RAW
HEAT
SLUDGE
EXCHANGER
ACTIVE
ZONE
MIXING
TRANSFER
SUPERNATANT
l\\\\\\\\\\\\\\\\\\\\
DIGESTED
SLUDGE
SUPERNATANT
DIGESTED
SLUDGE
PRIMARY DIGESTER
SECONDARY DIGESTER
FIGURE 6-5
TWO-STAGE, HIGH-RATE ANAEROBIC DIGESTER SYSTEM
Unfortunately, many secondary digesters have performed poorly
as thickeners, producing dilute sludge and a high strength
supernatant. The basic cause of the problem is that, in most
cases, anaerobically digested sludges do not settle readily.
Basically, two factors contribute to this phenomenon (52).
6-12
-------
1. Flotation of solids. The contents of the primary
digestion tank may become supersaturated with digester
gas. When this sludge is transferred into the secondary
digestion tank, the gas will come out of solution,
forming small bubbles. These bubbles attach to sludge
particles and provide a buoyant force that hinders
settling.
2. High proportion of fine-sized particles. Fine-sized
solids are produced during digestion by both mixing (53)
and the natural breakdown of particle size through
biological decomposition (54). These fines settle poorly
and enter the supernatant. The problem is compounded
when secondary and tertiary sludges are fed into the
digesters. The solids in these sludges have quite often
been flocculated and, thus, are more easily broken up
during digestion than primary sludge solids.
The return to the head of a plant of poor quality supernatant
from two-stage digestion often has an adverse impact on the
performance of other treatment processes. Supernatant commonly
contains larger quantities of dissolved and suspended materials.
(See Section 6.2.4.3 for a more detailed description of
supernatant quality). For example, Figure 6-6 shows that at one
secondary treatment plant, most of the carbon and nitrogen
leaving the secondary digester was found in the supernatant and,
consequently, was returned to the liquid process stream. The
impact of high recycle loads on treatment at one midwestern.
plant is shown on Figure 6-7. When digester supernatant was
recycled, solids built up in the plant, and the total amount of
suspended solids in the final effluent increased by 22 percent.
Suggestions for improving liquid-solids separation in secondary
digesters have included vacuum degassing (56), elutriation (57),
and enlarging the secondary digester. However, in many cases,
particularly when biological sludges are digested, it is better
to eliminate the secondary digester altogether (52). Digested
sludge is then taken directly to either a facultative sludge
lagoon (see Chapter 15) or mechanical dewatering equipment
(see Chapter 9). Since solids capture is better in the units,
their sidestreams are of relatively high quality compared with
supernatant from secondary digesters.
A secondary digester may successfully serve the following
functions:
e ^Thickening digested primary sludge.
• Providing standby digester capacity. If the secondary
digesterIsequippedwithadequateheating, mixing, and
intake piping.
• Storing digested sludge. A secondary digester fitted
with a floating cover can provide storage for sludge.
6-13
-------
Assuring against short-circuiting of raw sludges through
digestion. This may be important for odor control if
digested sludge is transferred to open basins or lagoons
(see Chapter 15). It also provides a margin of safety
for pathogen reduction.
120
_ 100
D5
80
•a
c
o
.c
60
_• 40
20
12
10
01
c
to
en
O
-C
FEED 1ST 2ND
STAGE STAGE
CARBON
20,000 = 9.1kg x 103
LEGEND
GAS
FEED SLUDGE
TRANSFER SLUDGE
FEED 1ST 2ND
STAGE STAGE
NITROGEN
2000lb = .91kg x 103
4000lb = 1.82
GOOOIb = 2.72
SOOOIb = 3.63
10,000lb = 4.54
SUPERNATANT
STABILIZED SLUDGE
UNACCOUNTABLE
FIGURE 6-6
CARBON AND NITROGEN BALANCE FOR A TWO-STAGE,
HIGH-RATE DIGESTION SYSTEM (23)
6-14
-------
RAW SEWAGE
16,035 /
(10,520)
1 15,969
(36,801)
PRIMARY
CLARIFIER
;
PRIMARY
SUPERNATANT
L
AERATION
TANK
9,501
(15.306)
SECONDARY FINAL EFFLUENT
CLARIFIER 2;836
._ . 13 4R71
RETURN SLUDGE
SLUDGE
13,249
(19,626)
0
(30,172)
i
WASTE SLUDGE
r
ANAEROBIC-
DIGESTER
9,593
(14,645)
DIGESTED SLUDGE
DATA IN PARENTHESES WERE OBTAINED WHEN UNTREATED
SUPERNATANT WAS RETURNED TO THE HEAD OF THE PLANT.
(AVERAGE OF THREE GRAB SAMPLES). DATA NOT IN
PARENTHESES WERE OBTAINED WHEN NO SUPERNATANT
WAS RECYCLED. (AVERAGE OF THIRTEEN GRAB SAMPLES).
SOLIDS FLOWS DO NOT BALANCE BECAUSE OF GRAB
SAMPLING. ALL VALUES EXPRESSED AS Ib SS/day (1 Ib day =
0.454 kg/day).
FIGURE 6-7
EFFECT OF RECYCLING DIGESTER SUPERNATANT
ON THE SUSPENDED SOLIDS FLOW THROUGH AN
ACTIVATED SLUDGE PLANT (55)
6.2.2.3 Anaerobic Contact Process
The anaerobic contact process is the anaerobic equivalent of
the activated sludge process. As shown on Figure 6-8, the
unique feature of this variation is that a portion of the
active biomass leaving the digester is concentrated and then
mixed with the raw sludge feed. This recycling allows for
adequate cell retention to meet kinetic requirements while
operating at a significantly reduced hydraulic detention time.
POSITIVE 1 CLAfllFIED
LIQUID-SOLIDS 1 , . . .
SEPARATION LIQUID
FIGURE 6-8
ANAEROBIC CONTACT PROCESS
Positive solids-liquid separation is essential to the operation
of the anaerobic contact process. To gain any of the benefits
from recycling, the return stream must be more concentrated than
6-15
-------
the contents of the digester. The difficulties in thickening
anaerobically digested sludge have been discussed above. Vacuum
degasifiers have been used in anaerobic contact systems to reduce
the buoyancy effect of entrapped gas, thereby improving cell
settling (56).
The anaerobic contact process has found application in the
treatment of high strength industrial wastes (56,58,59), and it
has been operated successfully at a laboratory scale to stabilize
primary sludge (60). Nevertheless, this system configuration is
rarely considered in municipal anaerobic sludge digestion because
of the difficulty in achieving the necessary concentration within
the return stream.
6.2.2.4 Phase Separation
As discussed in Section 6.2.1.4, anaerobic digestion involves
two general phases: acid formation and methane production. In
the three preceding anaerobic digestion processes, both phases
take place in a single -reactor. The potential benefits of
dividing these two phases into separate tanks were discussed
as early as 1958 (61).
Subsequent research (62,63) has shown that two-phase digestion is
feasible for the treatment of sewage sludges. Figure 6-9 shows a
schematic of this multi-stage system as conceived by Ghosh, and
othe'rs (63).
DIGESTER
GAS
HEAT
RAW [ — ^ — )
SLUDGE ,
EXCHANGER
BlOf
c
MIXING
vlASS RECYC
f" POSH
| SEPAR
1
LE -, .
HEAT
EXCHANGER
, 4
1 MIXING
^ --- '
POSITIVE CLARIFIED
LIQUID-SOLIDS ! *-
SEPARATION I
LIQUID
BIOMASS RECYCLE
DIGESTED
SLUDGE
ACID DIGESTER
METHANE DIGESTER
FIGURE 6-9
TWO-PHASE ANAEROBIC DIGESTION PROCESS
An effective means of separating the two phases is essential to
the operation of anaerobic digestion in this mode. Possible
separation techniques include dialysis (62), addition of chemical
inhibitors, adjustment of the redox potential (64), and kinetic
control by regulating the detention time and recycle ratio for
each reactor (63). The latter approach is the most practical
and has been developed into a patented process (U.S. Patent
4,022,665) .
6-16
-------
Operating data for a bench-scale system, summarized in Table 6-3,
show the differences between the reactors in a two-phase system.
The acid digester has a very short detention time (0.47 to
1.20 days), low pH (5.66 to 5.86), and produces negligible
amounts of methane. Conditions in the methane digester are
similar to those found in a conventional high-rate digester,
which is operated to maintain the optimum environment for the
methanogenic bacteria. The detention time listed in Table 6-3
for the methane digester (6.46 days) is significantly lower
than the detention time in a conventional high-rate digester.
However, this is probably because the two-phase system was
operated in a bench-scale system rather than in a full-scale
system where conditions are not ideal. The main advantage of a
two-phase system is that it allows the creation of an optimum
environment for the acid fermenters. As of 1979, a two-phase
system has never been operated at a plant scale.
TABLE 6-3
OPERATING AND PERFORMANCE CHARACTERISTICS FOR
THE BENCH-SCALE, TWO-PHASE ANAEROBIC DIGESTION
OF WASTE ACTIVATED SLUDGE (63)
Parameter
Acid
digester
Methane
digester
Temperature, °C
Detention time, day
Loading,
Ib VS/day/cu ft
pH
Ammonia nitrogen, mg/1
Averaqe alkalinity,
mg/1 CaCC>3
Gas composition, mole percent
CH4
C02
N2
Gas yield, standard cu ft/lb
VS reduced
Methane yield, standard
cu ft/lb VS reduced
VS reduction, percent
Effluent volatile acid,
mg/1 HAc
37
0.47-1.20
1.54-2.67
5.66-5.86
490-600
790
37
6.46
0.18
7.12
766
4,127
19-44
73-33
8-23
0.2-0.9
0.1-0.3
8.5-31.1
3,717
69.7
29.0
1.3
17.7
11.9
29. 3
134
Combined
two-phase
system
37
6.86-7.66
0.20
7.12
766
4,127
65.9
32.3
1.8
15.7
10.7
40.2
134
1 Ib/day/cu ft = 16.0 kg/day/nT
1 cu ft/lb = .0623 m3/kg
6-17
-------
6.2.3 Sizing of Anaerobic Digesters
Determination of digestion tank volume is a critical step
in the design of an anaerobic digestion system. First, and most
important, digester volume must be sufficient to prevent the
process from failing under a11 expected conditions. Process
failure is defined as the accumulation of volatile acids
(volatile acids/alkalinity ratio greater than 0.5) and the
cessation of methane production. Once a digester turns sour,
it usually takes at least a month to return it to service.
Meanwhile, raw sludge must be diverted to the remaining
digesters, which may become overloaded in turn. Furthermore,
sludge from a sour digester has a strong, noxious odor, and
therefore, its storage and disposal are a great nuisance.
Digester capacity must also be large enough to ensure that raw
sludge is adequately stabilized. "Sufficient stabilization" must
be defined on a case-by-case basis, depending on the processing
and disposal after digestion. In the past, digested sludge
quality has been acceptable as long as the digester remained in
a balanced condition and produced methane. However, higher
levels of stabilization may be required after the 1970s because
wastewater sludges increasingly are being applied to land and
coming into closer contact with the public.
6.2.3.1 Loading Criteria
Traditionally, volume requirements for anaerobic digestion have
been determined from empirical loading criteria. The oldest
and simplest of these criteria is per capita volume allowance.
Table 6-4 lists typical design values. This crude loading factor
should be used only for initial sizing estimates, since it
implicitly assumes a value for such important parameters as per
capita waste load, solids removal efficiency in treatment, and
digestibility of the sludge. These parameters vary widely from
one area to the next and cannot accurately be lumped into one
parameter.
A more direct loading criterion is the volatile solids loading
rate, which specifies a certain reactor volume requirement for
each unit of volatile dry solids in the sludge feed per unit of
time. This criterion has been commonly used to size anaerobic
digesters. However, as early as 1948, Rankin recognized that
process performance is not always correlated with the volatile
solids loading rate. The problem stems from the fact that this
parameter is not directly tied to the fundamental component in
anaerobic digestion, the microorganisms actually performing the
stabilization.
6.2.3.2 Solids Retention Time
The most important consideration in sizing an anaerobic
digester is that the bacteria must be given sufficient time to
6-18
-------
reproduce so that they can (1) replace cells lost with the
withdrawn sludge, and (2) adjust their population size to follow
fluctuations in organic loading.
In a completely mixed anaerobic digester, cells are evenly
distributed throughout the tank. As a result, a portion of
the bacterial population is removed with each withdrawal of
digested sludge. To maintain the system in steady state,
the rate of cell growth must at least match the rate at which
cells are removed. Otherwise, the population of bacteria in the
digester declines and the process eventually fails.
TABLE 6-4
TYPICAL DESIGN CRITERIA FOR SIZING MESOPHILIC
ANAEROBIC SLUDGE DIGESTERS (65,66)
Low-rate High-rate
Parameter digestion digestion
Volume criteria,
cu ft/capita
Primary sludge 2-3 1.3
Primary sludge +
Trickling filter humus 4-5 2,7-3.3
Primary sludge +
Activated sludge 4-6 2,7 - 4
c 0.04-0.1 0.15 - 0.40
Ib VSS/day/cu ft
Solids retention time, days 30-60 10 - 20
1 cu ft/capita - ,028 m /capita
1 Ib/day/cu ft = 16.0 kg/day/m3
To ensure that the process will not fail, then, it is critical to
know the growth rate of the bacteria in the digester. It is not
practical to measure directly the rate at which the anaerobic
bacteria multiply. However, as these bacteria grow and
reproduce, they metabolize the waste and produce end products.
As a result, the bacterial growth rate can be determined by
monitoring the rate at which substrate is reduced and end
products are produced. Studies of these rates of change began in
the late 1950s and have led to an understanding of digester
process kinetics (9,10,67).
The key design parameter for anaerobic biological treatment
is the biological solids retention time (SRT), which is the
6-19
-------
average time a unit of microbial mass is retained in the system
(68). SRT can be operationally defined as the total solids
mass in the treatment system divided by the quantity of solids
withdrawn daily. In anaerobic digesters without recycle, the SRT
is equivalent to the hydraulic detention time. Recycling of a
concentrated stream back to the head of the system, which is the
unique feature of the anaerobic contact process, increases the
SRT relative to the hydraulic detention time.
Figure 6-10 illustrates the relationship between SRT and
the performance of a lab-scale anaerobic digester fed with
raw primary sludge. Specifically, the figure shows how the
production of methane, as well as the reduction of degradable
proteins, carbohydrates, lipids, chemical oxygen demand, and
volatile solids, are related to the SRT. As the SRT is reduced,
the concentration of each component in the effluent gradually
increases until the SRT reaches a value beyond which the
concentration rapidly increases. This breakpoint indicates
the SRT at which washout of microorganisms begins--that is, the
point where the rate at which the organisms leave the system
exceeds their rate of reproduction. Figure 6-10 shows that the
lipid-metabolizing bacteria have the slowest growth rate and,
therefore, are the first to washout. As the SRT is shortened
beyond the first breakpoint (occurring at an SRT between eight
to ten days at 95°F [35°C]), more types of bacteria are washed
out and performance is increasingly inhibited. The SRT can
be lowered to a critical point (SRTC) beyond which the
process will fail completely. Calculations based on process
kinetics predict an SRTC of 4.2 days for the digestion of
wastewater sludge at 95°F (35°C) (69), which corresponds with
Torpey's pilot-scale study (70), in which anaerobic sludge
digesters operating at 99°F (37°C) failed at an SRT of 2.6 days.
Performance began deteriorating sharply as the SRT was reduced
below five days.
Temperature has an important effect on bacterial growth rates
and, accordingly, changes the relationship between SRT and
digester performance. The effect of temperature on methane
production and volatile solids reduction is shown on Figure 6-11.
The significance of this relationship is that stabilization is
slowed at lower temperatures, with 68°F (20°C) appearing to be
the minimum temperature at which sludge stabilization can be
accomplished within a practical solids retention time (69). The
critical minimum solids retention time (SRTC) is also affected
by temperature. O'Rourke (69) found that the SRTC for the
digestion of a primary sewage sludge in a bench-scale digester
was 4.2 days at 95°F (35°C), 7.0 days at 77°F (25°C), and 10.1
days at 50°F (10°C) .
6.2.3.3 Recommended Sizing Procedure
The size of an anaerobic digester should be adequate to ensure
that the solids retention time in the system never falls below a
certain critical value. This design solids retention time
6-20
-------
Q
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o
01
o
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O
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2
O
H
a:
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LJJ
O
o
o
H
LU
3
BENCH-SCALE DIGESTION OF
PRIMARY SLUDGE AT 95°F
10 20 30 40
SOLIDS RETENTION TIME iSRT}, days
FIGURE 6-10
EFFECT OF SRT ON THE RELATIVE BREAKDOWN
OF DECRADABLE WASTE COMPONENTS AND
METHANE PRODUCTION (69)
6-21
-------
•
[JIN
3"
Q II
Q,o
UJ
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LU
4 -
3 -
2 -
1 -
0
20
15
cc 11.8
LLJ
o
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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 95°F (35°C). (Values for systems
operated at other temperatures are shown in Table 6-5.) This
sizing criterion is reasonable, since it corresponds with the
replication time of the slowest growing bacteria. However, this
criterion must be met under all expected conditions, including:
Peak hydraulic loading. This value should be estimated
by combining poor thickener performance with the maximum
plant loading expected during seven continuous days
during the design period.
Maximum grit and scum accumulations. Considerable
amounts of grit and scum may accumulate before a digester
is cleaned. This reduces the active volume of the tank.
Liquid level below highest level. Several feet of liquid
level variability (two to three, usually) must be
retained to allow for differences in the rate of feeding
and withdrawal and to provide reasonable operational
flexibility.
These conditions may very well occur simultaneously and,
therefore, the designer should compound them when applying
the ten-day SRT^ sizing criterion. In the past, "liberal"
detention time criteria have been applied at the average
conditions. However, problems arise during critical periods,
not when conditions are average. For this reason, the most
rational approach to sizing a full-scale facility is to apply
experimentally based design criteria (increased by a reasonable
margin of safety) to the actual set of expected peak conditions.
(An example is included in Section 6.2.9.3).
6.2.4 Process Performance
The primary result of anaerobic sludge digestion is the reduction
of both volatile solids and pathogenic organisms. Volatile
solids are degraded into smaller molecules, and eventually a
6-23
-------
large portion are converted into gas, primarily methane (CH4)
and carbon dioxide (CC>2) . Pathogens are reduced through
natural die-off because the anaerobic environment is unsuitable
for their survival. (Refer to Chapter 7). Many other chemical
and physical changes occur during anaerobic sludge digestion,
some of which are described later in this section.
TABLE 6-5
SOLIDS RETENTION TIME DESIGN CRITERIA FOR
HIGH RATE DIGESTION (71)
Solids retention time, days
Operating
temperature ,
OF
65
75
85
95
105
Minimum
(SRTC)
11
8
6
4
4
Suggested
for design
(SRTd)
28
20
14
10
10
It is not possible to predict precisely the nature and extent of
all changes occurring during anaerobic digestion. Wastewater
sludges have a complex, variable character and there are many
reactions that occur during digestion within the mixed culture of
anaerobic microorganisms. This section describes general trends
of digester performance and identifies the major influences on
anaerobic digestion.
To provide an overview of anaerobic digester performance,
operating data for a full-scale digestion facility are shown in
Tables 6-6 and 6-7. These data are for a two-stage, high-rate
digester system in which only the primary digester was heated and
mixed (23). The second tank provided a quiescent zone for the
gravity separation of digested solids from supernatant liquor.
Operating temperature in the first stage was maintained at 94°F
(34°C), and detention time in each tank was 39 days. Feed
sludge consisted of approximately equal amounts of primary
sludge and waste-activated sludge.
Essentially, all stabilization occurred in the primary digester.
In this first stage, 57 percent of the volatile solids were
converted to liquid or gas. Only 2.8 percent of the volatile
solids in the raw sludge were reduced in the secondary digester.
A similar pattern of performance is shown in Table 6-7 for
carbohydrate, lipid, and protein reduction. While data indicate
6-24
-------
that fixed solids also decreased during digestion, this is a
little understood phenomenon, and research on the subject is
continuing (76).
TABLE 6-6
AVERAGE PHYSICAL AND CHEMICAL CHARACTERISTICS OF SLUDGES
FROM TWO-STAGE DIGESTER SYSTEM (23)
Concentration, mg/1
Component
PH
Alkalinity
Volatile acids
Total solids
Fixed solids
Carbohydrates
Lipids
Carbon
Proteins, as gelatin
Ammonia nitrogen, as NH,
Organic nitrogen, as NH^
Total nitrogen, as NH-}
Feed
sludge
1,
35,
9,
9,
8,
15,
18,
1,
1,
5.7
758
285
600
000
680
310
450
280
213
346
559
Transfer
sludge
2,
18,
6,
1,
2,
6,
11,
1,
7.7
318
172
200
600
550
075
950
200
546
879
425
Supernatant
2,
12,
3,
1,
1,
4,
6,
1,
7.8
630
211
100
310
020
321
440
580
618
564
182
Stabilized
sludge
2,
32,
12,
3,
3,
10,
17,
1,
2,
7.8
760
185
800
300
100
490
910
200
691
455
146
Except pH.
TABLE 6-7
MATERIALS ENTERING AND LEAVING TWO-STAGE DIGESTER SYSTEM3 (23)
Volatile solids
Fixed solids
Carbohydrates (as
glucose)
Lipids
Feed
sludge
79.9
26.9
28.9
24.8
Quantity, tons
Transfer
sludge
Supernatant
Stabilized
sludge
34.1
19.4
4.55
6.09
Carbon
Ammonia nitrogen
Organic nitrogen
Proteins (as gelatin)
Total nitrogen (as NH3>
46.2
0.64
4.02
54.6
4.66
20.4
1.61
2.58
32.9
4.20
23.4
8.8
2.71
2.40
8.5
5.1
1.28
1.44
Gas
1st stage 2nd stage
11.8
1.64
1.50
17. 1
3.25
4. 5
0.28
0. 60
7. 1
0.89
22.1
-
-
-
0.47
2.7
0.04
Period of analysis = 33 days.
1 ton = .907 t
6-25
-------
Reduction of solids during digestion has the effect of producing
a more dilute sludge. For example, in this case, the raw
sludge fed to the system had a total solids concentration of
3.56 percent, yet the solids concentration was reduced to
1.86 percent in the first stage of digestion. Although gravity
concentration did occur in the second-stage tank, the largest
portion of the digested solids was contained in the supernatant.
At this plant, the supernatant was recycled to the primary
clarifiers and then the solids it contained either returned to
the primary digester or left the plant in the final effluent.
The preceding example illustrates the general performance of
anaerobic digesters. In the remainder of this section, three
topics are discussed in more detail: solids reduction, gas
production and supernatant quality.
6.2.4.1 Solids Reduction
Solids reduction is one of the main objectives of anaerobic
digestion. It not only makes the sludge less putrescible but
also reduces the amount of solids for ultimate disposal.
It is usually assumed that this reduction takes place only in the
volatile portion of the sludge solids. Therefore, a common
measure of digester performance is the percent of the volatile
solids destroyed. Volatile solids reduction in anaerobic
digesters usually ranges between 35 to 60 percent. The degree of
volatile solids reduction achieved in any particular application
depends on both the character of the sludge and the operating
parameters of the digestion system.
The character of the sludge determines the upper limit for
volatile solids reduction. Not all of the volatile solids
can be converted by the anaerobic bacteria. Limited research
(77 to 80) suggests that only 60 to 80 percent of the volatile
solids in municipal wastewater sludge is readily biodegradable.
The remaining fraction consists chiefly of inert organics such as
lignins and tannins. These complex organic molecules may even-
tually be degraded when held for several months in a facultative
sludge lagoon, but can be considered indigestible within the
contact times normally associated with anaerobic digestion.
The most important operating parameters affecting volatile solids
reduction are solids retention time and digestion- temperature.
As shown on Figure 6-12, volatile solids reduction climbs rapidly
to 50 to 60 percent as the SRT is increased. Beyond this point,
further reduction is minimal even with substantial increases in
the SRT. Similar curves have been produced by other researchers
(43,60,81). The shape of the response curve and the point at
which it levels out are influenced strongly by the temperature of
the digester. Figure 6-12 shows that at any given SRT, raising
the operating temperature to 95°F (35°C) will increase the
proportion of volatile solids destroyed during digestion. This
response to temperature change is not instantaneous but would
6-26
-------
ro
60
40
30
20
10
Q 40
ai
EC
I 30
8
3. 20
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
as
Z
o
1-
o
^
UJ
cc
V)
a
j
^
£/3
LLJ
_l
H
<
J
O
>
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 (347°F [175°C]) for a half hour prior to
anaerobic digestion increased volatile solids reduction and
resultant gas production. Dewaterability of the digested
sludge was also improved by thermal pretreatment.
6-27
-------
volatile solids destroyed
shows how specific gas
Conversion of volatile
(35°C) and 130°F (54°C).
no effect on specific
exceeded. Lengthening
(0.75 to 1.1 mVkg) •
Figure 6-14
production is affected by temperature.
solids is most efficient at about 95°F
Detention time, or SRT, has essentially
gas production so long as the SRT is
the SRT, however, increases the total
quantity of gas produced because volatile solids reduction is
increased. As discussed earlier, the mix of organic compounds in
the feed sludge strongly influences specific gas production
values.
20 i-
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 ions—predominantly ferrous, ferric, aluminum, and
calcium. The fate of phosphorus during the anaerobic digestion
of phosphorus-laden biological and chemical sludges has been the
subject of several studies (55,101-104). The results of these
studies are not entirely consistent. In some cases (99,101),
bound phosphorus was resolubilized during anaerobic digestion
6-32
-------
and released to the digester supernatant. The return of this
phosphorus-laden supernatant to the liquid treatment stream can
substantially reduce the net phosphorus removal efficiency of the
plant (101) and/or increase chemical demand. However, in most
studies (55,102-104), release of soluble phosphorus into digester
supernatant was minimal.
TABLE 6-10
SUPERNATANT, CHARACTERISTICS OF HIGH-RATE,
TWO-STATE, MESOPHILIC, ANAEROBIC DIGESTION
AT VARIOUS PLANTS (90,93,94,95,96)
Concentration3, mg/1
Parameter
Reference
Total solids
Total volatile solids
Primary
(95)
9,400
4,900
Primary and trickling
sludge filter sludge Primary and activated sludge
(90)b (94)
4,545
2,930
(951 (901° (94)
1,475
814
(94) (95) (96) (90)b
2,160 -
983 -
Suspended solids
Average
Maximum
Minimum
Volatile suspended solids
Average • -
Maximum
Minimum
BOD
Average
Maximum
Minimum
COD
TOC
Total (P04)-P
MH3-N
Organic N
PH 8.0
Volatile acids
Alkalinity (as CaCC>3) 2,555
Phenols
Average . - 0.23 - - 0.23 - 0.35
Maximum - 0.80 - - 0.50 - 1.00
Minimum - 0.06 - - 0.06 - 0.08
4,
17,
2,
10,
1,
277
300
660
645
850
420
713
880
200
-
-
-
-
-
-
-
-
2,205
1,660
-
4,565
1, 242
143
853
291
7.3
264
3,780
1,518
-
-
2,230
-
85
-
678
7.2
-
-
7,772
32,400
100
4,403
17,750
60
1,238
6,000
135
-
-
-
-
-
-
-
-
383
299
-
1, 384
443
63
253
53
7 .0
322
1,349
143
118
-
1,310
320
87
559
91
7.8
250
1,434
740 1,075
750
515
1,230
-
100
480
360 560
7.0 7.3
-
-
4,408
14,650
100
3,176
10,650
75
667
2,700
100
-
.
-
-
-
-
-
aUnless noted, all values are average for the sampling period studied.
bvalues indicated are a composite from seven treatment plants.
cValues indicated are a composite from six treatment plants.
Phenols
Phenols have been found in digester supernatants in concentra-
tions sufficient to inhibit biological activity (56). Typical
phenol concentrations are included in Table 6-10. The source
of phenols is not usually industrial waste discharges but
putrefaction of proteins, which begins in the human body and
continues in the sewage system. Phenols are very toxic and are
used commercially as an antiseptic. In dilute concentrations,
phenols do not necessarily kill bacteria but slow their growth
6-33
-------
and inhibit their normal metabolic activity. As a result, the
phenols contained in digester supernatant, combined with phenolic
compounds already in the sewage, may be an important cause of
sludge bulking (90). In addition, the recycling of phenols in
supernatant may contribute to odor problems.
Ammonia
As shown in Table 6-10, high levels of ammonia are often found in
digester supernatant. In plants that are nitrifying, the super-
natant return will provide a large portion of the ammonia feed to
the wastewater process. The conversion of this ammonia to
nitrate will therefore result in increased costs to provide the
required oxygen for treatment. In plants that must achieve a
nitrogen limitation in their effluent, the recycle ammonia
loadings must be carefully evaluated as to their overall effect
in meeting the standards.
6.2.5 Operational Considerations
6.2.5.1 pH
As was noted in Section 6.2.1, anaerobic digestion is a two-step
process consisting of "acid-forming" and "methane-forming" steps.
During the first step, the production of volatile acids tends
to reduce the pH. The reduction is normally countered by
destruction of volatile acids by methanogenic bacteria and the
subsequent production of bicarbonate.
Close pH control is necessary because methane-producing bacteria
are extremely sensitive to slight changes in pH. Early research
(105-107) showed that the optimum pH for methane-producing
bacteria is in the range of 6.4-7.5 and that these bacteria
are very sensitive to pH change. A 1970 study (108) seems to
indicate that the pH tolerance of methane-producing bacteria
is greater than previously thought. The bacteria are not
necessarily killed by high and low pH levels; their growth is
merely stopped. Because of the importance of these findings to
system control, more research is needed to verify these results.
Several different acid-base chemical equilibria are related to
pH. In the anaerobic digestion process, the pH range of interest
is 6.0 to 8.0, which makes the carbon dioxide-bicarbonate
relationship the most important. As Figure 6-15 indicates,
system pH is controlled by the CC>2 concentration of the gas
phase and the bicarbonate alkalinity of the liquid phase. A
digester with a given gas-phase CC-2 concentration and liquid-
phase bicarbonate alkalinity can exist at only one pH. If
bicarbonate alkalinity is added to the digester and the
proportion of CC<2 in the gas phase remains the same, digester
6-34
-------
pH must increase. For any fixed gas-phase CC>2 composition, the
amount of sodium bicarbonate required to achieve the desired pH
change is given by the following equation:
D = 0.60 (BA at initial pH - BA at final pH)
(6-1)
where:
D = sodium bicarbonate dose, mg/1
BA = digester bicarbonate alkalinity as mg/1 CaCC>3
The pH increase is less important, however, than the effect
on system buffering capacity, (that is, the system's ability
to resist pH changes). If bicarbonate alkalinity is added,
buffering capacity is increased, system pH is stabilized, and
the system becomes less susceptible to upset. The effect of
buffering capacity on anaerobic digester operations is discussed
elsewhere (110,111).
DU
40
o
oc 30
LU
20
CM
o
o
10
to s
OPERATING
'TEMPERATURE
96°F (35°C)
LIMITS OF
NORMAL
ANAEROBIC
TREATMENT
I
260 500 1000 2500 5000 10,000
BICARBONATE ALKALINITY AS CaC03, mg/1
FIGURE 6-15
RELATIONSHIP BETWEEN pH AND BICARBONATE
CONCENTRATION NEAR 95°F (35°C) (109)
25,000
6-35
-------
Bicarbonate alkalinity can be calculated from total alkalinity by
the following equation:
BA = TA - 0.71 (VA) (6-2)
where:
BA = bicarbonate alkalinity as mg/1 CaCC>3
TA = total alkalinity as mg/1 CaCC>3 determined by titration
to pH 4.0
VA = volatile acids measured as mg/1 acetic acid
0.71 is obtained by the multiplication of two factors, (0.83 and
0.85). 0.83 converts volatile acids as acetic acid to volatile
acid alkalinity as CaCC>3. 0.85 is used because in a titration
to pH 4.0, about 85 percent of the acetate has been converted to
the acid form.
It has been suggested (110) that the only sensible way to
increase digester pH and buffering capacity is by the addition of
sodium bicarbonate. Other materials, such as caustic soda, soda
ash, and lime, cannot increase bicarbonate alkalinity without
reacting with soluble carbon dioxide, which causes a partial
vacuum within the system. Above pH 6.3, lime may react with
bicarbonate to form insoluble calcium carbonate, promoting scale
formation or encrustation. Ammonia gas (NH3) could be used
without causing vacuum problems, but control of pH with sodium
bicarbonate is preferred because it provides good buffering
capacity without raising the pH as much as NH3 would. Both
sodium and ammonia can inhibit anaerobic bacteria; care must be
taken during pH control to avoid reaching toxic concentrations of
these chemicals.
6.2.5.2 Toxicity
Much of the published data on toxicity in anaerobic digestion
systems are erroneous and misleading because of inadequate
experimental techniques and a general lack of understanding
(112). Therefore, before any discussion of toxicity can take
place, a review of several fundamentals is needed.
First, for any material to be biologically toxic, it must be in
solution. If a substance is not in solution, it cannot pass
through the cell wall and therefore cannot affect the organism.
Second, toxicity is a relative term. There are many organic
and inorganic materials which, if soluble, can be either
6-36
-------
stimulatory or toxic. A good example is the effect, shown in
Table 6-11, of ammonia nitrogen on anaerobic digestion.
TABLE 6-11
EFFECT OF AMMONIA NITROGEN ON ANAEROBIC DIGESTION (113,114)
Ammonia
concentration, as N,
mg/1 Effect
50 - 200 Beneficial
200 - 1,000 No adverse effects
1,500 - 3,000 Inhibitory at pH
over 7.4 - 7.6
Above 3,000 Toxic
Acclimatization is the third consideration. When the levels
of potentially toxic materials are slowly increased within
the environment, many organisms can rearrange their metabolic
resources and overcome the metabolic block produced by the toxic
material. Under shock load conditions, there is not sufficient
time for this rearrangement to take place and the digestion
process fails.
Finally, there is the possibility of antagonism and synergism.
Antagonism is defined as a reduction of the toxic effect of one
substance by the presence of another. Synergism is defined as an
increase in the toxic effect of one substance by the presence of
another. These are important relationships in cation toxicity.
Though there are many potentially toxic materials, this section
concerns itself only with the following:
Volatile acids
Heavy metals
Light metal cations
Oxygen
Sulfides
Ammonia
Vo1a t i1e Acids
Until the 1960s, it was commonly believed that volatile acid
concentrations over 2,000 mg/1 were toxic to anaerobic digestion.
There was also considerable controversy about whether or not
alkaline substances should be added to maintain adequate buffer
capacity.
6-37
-------
In the early 1960s, McCarty and his coworkers published results
from carefully controlled studies (113,115,116). Their results
showed :
• That volatile acids, at least up to 6,000-8,000 mg/1,
were not toxic to methanogenic bacteria as long as there
was adequate buffer capacity to maintain the system pH in
the range of 6.6-7.4.
• That pH control by the addition of an alkaline material
was a valid procedure as long as the cation associated
with the alkaline material did not cause toxicity.
It was found that alkaline sodium, potassium, or ammonium
compounds were detrimental but that alkaline magnesium or
calcium compounds were not.
Heavy Metals
Heavy metal toxicity has frequently been cited as the cause
of anaerobic digestion failures. Even though trace amounts
of most heavy metals are necessary for maximum biological
development (117), the concentrations in raw wastewater sludges
could be problematic.
Heavy metals tend to attach themselves to sludge particles
(118,119). Heavy metals which cannot be detected in the influent
wastewater can be concentrated to measurable levels in the
sludge. Table 6-12 gives the range of influent concentrations of
some heavy metals. The range is quite wide, with the higher
values normally attributed to a local industrial polluter.
TABLE 6-12
INFLUENT CONCENTRATIONS AND EXPECTED REMOVALS
OF SOME HEAVY METALS IN WASTEWATER TREATMENT SYSTEMS (120,121)
Removal efficiency, percent
Heavy metal
Cadium
Chromium
+ 3
+ 6
Copper
Mercury
Nickel
Lead
Zinc
Arsenic
Iron
Manganese
Silver
Cobalt
Barium
Selenium
Influent concentration,
mg/1
< .008
< .020
< .020
< .020
< .0001
< .1
< .05
< .02
.002
< .1
.02
< .05
- 1.142
-5.8
- 5.8
- 9.6
- .068
- 880
- 12.2
- 18.00
- .0034
- 13
- .95
- .6
Secondary
treatment
20 -
40 -
0 -
0 -
20 -
15 -
50 -
35 -
28 -
72
25
-
45
80
10
70
75
40
90
80
73
Alum treatment
60
90
-
90
65
35
85
85
-
-
—
-
Below detection
*
-
-
47
79
-
"
6-38
-------
Table 6-12 gives the typical range of removal that can be
expected from standard secondary treatment. Published data seem
to indicate that the percent removal, without chemical addition,
is a function of influent concentration: the higher the influent
concentration, the higher the percent removal.
The last column of Table 6-12 shows removals of heavy metals
achieved with additions of alum. In treatment systems that
add chemical coagulants for phosphate removal, a significant
amount of influent heavy metals will also be removed (122).
Soluble and total heavy metal concentrations are often greatly
different because anions such as carbonate and sulfide can remove
heavy metals from solution by precipitation and sequestering.
Consequently, it is not possible to define precise total toxic
concentrations for any heavy metal (123). Total individual metal
concentrations that have caused severe inhibition of anaerobic
digestion are shown in Table 6-13. However, only the dissolved
fraction of these metals caused the inhibition. Table 6-14 shows
the total and soluble concentrations of heavy metals in anaerobic
digesters. Inhibition of anaerobic digestion occurs at soluble
concentrations of approximately 3 mg/1 for Cr, 2 mg/1 for Ni,
1 mg/1 for Zn, and 0.5 mg/1 for Cu (129).
TABLE 6-13
TOTAL CONCENTRATION OF INDIVIDUAL METALS REQUIRED
TO SEVERELY INHIBIT ANAEROBIC DIGESTION (123,124)
Metal
Concentration in digester contents
Metal as percent
of dry solids
Millimoles metal per
kilogram dry solids
Soluable metal,
mg/1
Copper
Cadmium
Zinc
Iron
Chromium
+ 6
+ 3
Nickel
0
1
0
9
2
2
93
08
97
56
20
60
-
1,
150
100
150
710
420
500
-
0,
1.
3.
2.
.5
-
.0
-
.0
-
.0
Except for chromium, heavy metal toxicity in anaerobic digesters
can be prevented or eliminated by precipitation with sulfides
(124-127). Hexavalent chromium is usually reduced to trivalent
chromium, which, under normal anaerobic digester pH conditions,
is relatively insoluble and not very toxic (128).
Sulfide precipitation is used because heavy metal sulfides
are extremely insoluble (129). If sufficient sulfide is not
available from natural sources, it must be added in the form of
sulfate, which is then reduced to sulfide under anaerobic
conditions.
6-39
-------
TABLE 6-14
TOTAL AND SOLUBLE HEAVY METAL CONTENT
OF DIGESTERS (124)
Metal
Total concentration,
mg/1
Soluble concentration,
mg/1
Chromium +6
Copper
Nickel
Zinc
88
27
2
11
- 386
- 196
- 97
- 390
0.03
0.1
0
0.1
- 3.0
- 1.0
- 5
- 0.7
One potential drawback of using the sulfide saturation method is
the possible production of hydrogen sulfide gas or sulfuric acid
from excess dissolved sulfide in the digester. Because of this,
it is recommended that ferrous sulfate be used as a source of
sulfide (112). Sulfides will be produced from the biological
breakdown of sulfate, and the excess will be held out of solution
by the iron in the sulfide form. However, if heavy metals
enter the digester, they will draw the sulfide preferentially
from the iron because iron sulfide is the most soluble heavy
metal sulfide. Excess sulfide additions can be monitored by
either analysing digester gas for sulfide or by the use of a
silver-silver electrode located within the digester (126,130).
Light Metal Cations
The importance of the light metal cations (sodium, magnesium,
potassium, calcium) in anaerobic digestion was shown in the mid
1960s (112,131,132). Domestic wastewater sludges have low
concentrations of light metal cations. However, significant
contributions, enough to cause toxicity, can come from industrial
operations and the addition of alkaline material for pH control.
Not only can each of these cations be either stimulatory or
toxic, depending on concentration (Table 6-15), but certain
combinations of them will form either an antagonistic or a
synergistic relationship (Table 6-16). Inhibition caused by an
excess of a certain cation can be counteracted by the addition of
one or more of the antagonist cations listed in Table 6-16.
TABLE 6-15
STIMULATING AND INHIBITORY CONCENTRATIONS
OF LIGHT METAL CATIONS (133)
Concentration, mg/1
Cation
Calcium
Magnesium
Potassium
Sodium
Stimulatory
100 - 200
75 - 150
200 - 400
100 - 200
Moderately
inhibitory
2,500 - 4,500
1,000 - 1,500
2,500 - 4, 500
3, 500 - 5, 500
Strongly
inhibitory
8,000
3,000
12,000
8, 000
6-40
-------
TABLE 6-16
SYNERGISTIC AND ANTAGONISTIC CATION COMBINATIONS (112, 132)
Toxic
cations
Ammonium
Calcium
Magnesium
Potassium
Sodium
Synergistic
cations
Calcium, magnesium, potassium
Ammonium, magnesium
Ammonium, calcium
Ammonium, calcium, magnesium
Antagonistic
cations
Sodium
Potassium, sodium
Potassium, sodium
Ammonium, calcium, magnesium,
sodium
Potassium
Oxygen
Many engineers have expressed concern over the possibility
of oxygen toxicity caused by using dissolved air flotation
thickeners for sludge thickening. Fields and Agardy (134)
injected oxygen into a bench-scale digester at the rate of
0.1 ml 02 per liter per hour (equivalent to one volume of air
per 2,100 volumes of digester contents per hour). Total gas
production fell 36.5 percent after 19 hours and ceased completely
after 69 hours. However, this rate of oxygen injection is
significantly higher than would be produced by a dissolved air
flotation thickening system. Consequently, no problems are
expected under normal circumstances.
Soluble sulfide concentrations over 200 mg/1 are toxic to
anaerobic digestion systems (125,135). The soluble sulfide
concentration within the digester is a function of the incoming
source of sulfur, the pH, the rate of gas production, and the
amount of heavy metals available to act as precipitants . High
levels of soluble sulfide can be reduced by the addition of iron
salts (136) to the liquid, or scrubbing of the recirculated gas.
Ammonia
Ammonia, produced during the anaerobic degradation of proteins
and urea, may reach toxic levels in highly concentrated
sludges (113,114,133). Two forms of ammonia are found in
anaerobic digestion: ammonium ion (NH4+) and dissolved ammonia
gas (NH3 ) . Both forms can inhibit anaerobic digestion, although
ammonia gas has a toxic effect at a much lower concentration than
ammonium ion.
The two forms of ammonia are in equilibrium and the relative
concentration of each depends on pH , as indicated by. the
following equilibrium equation:
6-41
-------
At low pH levels, the equilibrium shifts to the left and ammonium
ion toxicity is more likely to be a problem. At higher pH
levels, the equilibrium shifts to the right so that inhibition is
related to the ammonia gas concentration.
Ammonia toxicity is evaluated by analyzing the total ammonia-
nitrogen concentrations. If the total ammonia-nitrogen
concentration is from 1,500 to 3,000 mg/1 and the pH is above
7.4-7.6, inhibition may result from ammonia gas. This can be
controlled by the addition of enough HC1 to maintain the pH
between 7.0 and 7.2. If total ammonia-nitrogen levels are over
3,000 mg/1, then the NH4 + ion will become toxic no matter
what the pH level. The only solution is to dilute the incoming
waste sludge.
6.2.6 System Component Design
6.2.6.1 Tank Design
Anaerobic digestion tanks are either cylindrical, rectangular, or
egg-shaped. A simplified sketch of each tank design type is
shown on Figures 6-16, 6-17, and 6-18.
The most common tank design is a low, vertical cylinder ranging
in diameter from 20 to 125 feet (6 to 38 m) , with a side water
depth between 20 to 40 feet (6 to 12 m). Gas-lift mixing is most
effective when the ratio of tank radius to water depth is between
0.7 and 2.0 (137). The tanks are usually made of concrete, with
either internal reinforcing or post-tensioning rods or straps.
The latter design is the least expensive of the two for tanks
with diameters greater than 65 feet. Some steel tank digesters
have been constructed to diameters of 70 feet.
The floor of a cylindrical digester is usually conical, with a
minimum slope of 1:6. Sludge is withdrawn from the low point in
the center of the tank. Digestion tanks with "waffle bottoms"
have been put into operation at the East Bay Municipal Utility
District Plant in Oakland, California (138,139). Digesters with
similar bottoms have been designed for Tacoma, Washington, and
Portland, Oregon.
The principal objective of the waffle floor design is to minimize
grit accumulation and, to practically eliminate the need for
cleaning. As shown on Figure 6-16, the tank floor is subdivided
into pie-shaped hoppers, each sloping toward a separate drawoff
port along the outside edge of tank. Subdivision of the bottom
area and use of multiple drawoff ports allow steeper floor slopes
and reduce the distance that settled solids must travel. As a
result, less grit is likely to accumulate. Construction costs
6-42
-------
are higher for this type of bottom because it requires more
complex excavation, form work, and piping than a conventional
bottom. It has been estimated that the incremental construction
cost for waffle bottoms on the 90-foot (27 m) diameter digesters
in Oakland was estimated to be $120,000 per tank (1978 dollars)
(139). However, it is expected that savings will be realized
during operation because cleaning requirements will be greatly
reduced .
WITHDRAWAL
PIPE
BOTTOM PLAN
SECTION
WITHDRAWAL-1
PORTS
CONICAL BOTTOM TYPE
BOTTOM
PLAN
SECTION
WA F FjyysOTTOMj[YPE
FIGURE 6-16
CYLINDRICAL ANAEROBIC DIGESTION TANKS
The primary advantages of rectangular digestion tanks are
simplified construction and efficient use of a limited plant
site. However, it is more difficult to keep the contents of
6-43
-------
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 ±1°F (0.6°C) of design temperatures improves
process stability by preventing thermal shock.
Heating equipment must be capable of delivering enough heat
to raise the temperature of incoming sludge to operating levels
and to offset losses of heat through the walls, floor, and
cover of the digester. Methods used to transfer heat to sludge
include:
• Heat exchanger coils placed inside the tank
• Steam injection directly into the sludge
• External heat exchanger through which sludge is
circulated
• Direct flame heating in which hot combustion gases
are passed through the sludge (141)
External heat exchangers are the most commonly used heating
method. Internal heat exchanger coils were used in early
digesters; however, they are difficult to inspect and clean. This
is a serious disadvantage because the coils become encrusted,
reducing the rate of heat transfer. To minimize caking of sludge
on the coils, water circulating through the coils is kept between
120 to 130°F (49 to 55°C) (84). Typical values of heat-transfer
coefficients for hot-water coils are listed in Table 6-17.
Steam injection heating requires very little equipment but
dilutes the digesting sludge and requires 100 percent boiler
makeup water. The cost of this water may be considerable,
particularly if hardness must be removed before addition to
the boiler.
Three types of external heat exchangers are .commonly used for
sludge heating: water bath, jacketed pipe, and spiral. In the
water bath exchanger, boiler tubes and sludge piping are located
in a common water-filled container. Gravity circulation of hot
water across the sludge pipes is augmented with a pump, to
6-46
-------
increase heat transfer. The heat exchanger and boiler are
combined in a single unit, a feature which can increase the
explosion hazard in the digester area. In a jacketed pipe
exchanger, hot water is pumped counter-current to the sludge
flow, through a concentric pipe surrounding the sludge pipe. The
spiral exchanger is also a counter-flow design; however, the
sludge and water passageways are cast in a spiral. One side of
the heat exchanger is liquid, providing ready access to the
interior of the sludge passageway for cleaning. Heat transfer
coefficients for design of external heat exchangers range between
150 to 275 Btu/hr/sq ft/degree F (740 to 1,350 kg-cal/hr/m2/°C)
depending on heat exchanger construction and fluid turbulence.
To minimize clogging with rags and debris, sludge passageways in
a heat exchanger should be as large as possible. The interior of
these passageways should be easily accessible to allow the
operator to quickly locate and clear a blockage.
TABLE 6-17
HEAT TRANSFER COEFFICIENTS FOR HOT WATER
COILS IN ANAEROBIC DIGESTERS (84)
Transfer
Material surrounding coefficient (u) ,
hot water coils Btu/hr/sq ft/°F
Thin supernatant 60 - 80
Thin sludge 30
Thick sludge 8-15
1 Btu/hr/sq ft/°F - 4.9 kg-cal/hr/mV C.
A piping arrangement used to control hot water supply to a
jacketed pipe or spiral heat exchanger is shown on Figure 6-19.
Hot water is pumped through the heat exchanger and circulated
through the secondary heat loop. When the temperature of the
sludge leaving the heat exchanger falls below the set point,
some hot water from the primary heat loop is introduced through a
modulating valve into the secondary heat loop, displacing an
equal volume of cooler water back into the primary heat loop.
Balancing valves are required to assure that the secondary loop
will not be bypassed altogether and to allow adjustment of
circulation pump capacity. Supply water temperature is kept
below 155°F (68°C). Although higher temperatures will increase
the rate of heat transfer, caking of sludge will occur when
the flow of sludge is stopped. This system allows the heat
source to be -remote from the heat exchanger. This assures
maximum safety and supports the recovery of waste heat (see
Chapter 18). Figure 6-20 shows a spiral heat exchanger operating
off a secondary heat loop.
6-47
-------
Each digester should have a separate heat exchanger and in
larger plants, addition of a single heat exchanger for warming
raw sludge should be considered. Cold raw sludge should never be
added directly to the digester. The thermal shock will be
detrimental to the anaerobic bacteria, and isolated pockets of
cold sludge may form. Raw sludge should be preheated or mixed
with large quantities of warm circulating sludge before being fed
to the digester.
20G°F
PRIMARY LOOP
CIRCULATION PUMP
PRIMARY
HEAT
LOOP
MODULATING VALVE
r
SECONDARY LOOP
CIRCULATION PUMP
SLUDGE OUTLET
HEAT
EXCHANGER
SLUDGE INLET
FIGURE 6-19
SCHEMATIC OF THE HEAT
RESERVOIR SYSTEM FOR A
JACKETED PIPE OR SPIRAL
HEAT EXCHANGER
The hot water or steam used to heat digesters is most commonly
generated in a boiler fueled by sludge gas. Up to 80 percent of
the heat value of sludge gas can be recovered in a boiler.
Provisions for burning an alternate fuel source (natural gas,
propane, or fuel oil) must be included to maintain heating during
periods of low digester gas production or high heating demand.
6-48
-------
Natural gas is the most compatible alternate fuel because it has
a low heat content and, consequently, can be blended and burned
in a boiler with minimal equipment adjustment.
FIGURE 6-20
SPIRAL HEAT EXCHANGER OPERATING OFF
SECONDARY HEAT LOOP AT SUNNYVALE, CALIFORNIA
Often, waste heat from sludge gas-powered engines used to
generate electricity or directly drive equipment is sufficient to
meet digester heating requirements. Typically, 18 to 20 percent
of the low heating value of engine fuel can be recovered from
the engine cooling system (38). Engines can be cooled by
either a forced draft system in which water is pumped through the
engine or a natural draft system (termed ebullient cooling) in
which water is vaporized and circulates without pumping. The
latter method yields a higher temperature (and thus more useful)
source of heat and also increases engine life. A combination
exhaust silencer and heat-recovery unit can be used to extract
from the exhaust an additional ten to thirteen percent of the low
6-49
-------
heating value of the engine fuel (38). To prevent formation of
corrosive acids, exhaust gases should not be cooled below 400°F
(200°C).
Solar energy has been successfully used to heat anaerobic
digesters (142), freeing sludge gas for higher grade uses.
Heat is transferred to the raw sludge feed by passing the sludge
piping through a tank of solar-heated water. The optimal size
solar-heating system can supply 82 to 97 percent of the total
annual heat load from solar energy, depending on geographical
location (142). The economic attractiveness of using solar
heating for digesters, however, is strongly dependent on the
economic value of the sludge gas saved.
A unique method of generating heat is to precede anaerobic
digestion with pure oxygen aerobic digestion (143). Biologically
generated heat released in the Vaerobic reactor is sufficient to
warm the sludge to as high as 125 to 140°F (52 to 60°C), as long
as the solids concentration of the feed to the digestion system
is kept above about 3.5 percent and the tank is well insulated.
Heat balance calculations indicate that these temperatures are
only attainable when pure oxygen is used because the low gas flow
does not cool the reactor (144). The warm sludge is transferred
to the anaerobic digester, where the bulk of stabilization
occurs. In pilot tests (143), the contents of the anaerobic
digester were maintained at 95°F (35°C) without the addition of
supplemental energy, other than the power required to generate
the pure oxygen. Temperature is controlled by changing the flow
of oxygen to the aerobic digester. As yet there have been no
full-scale installations of this method of heating digesters.
Heat Required for Raw Sludge. It is necessary to raise the
temperature of the incoming sludge stream. The amount of
heat required is:
Qs =• ai_o_suae -
-------
significance of this graph is that a seemingly small change
in feed sludge concentration can have a substantial effect
on the raw sludge heating requirement.
60 r~
£• GO
^
en
So
.
Ei S.
a
* -
40
30
Pif 20
T i-
10
1
24 i 8 10
TOTAL SOLIDS CONCENTRATION, %
FIGURE 6-21
EFFECT OF SOLIDS CONCENTRATION ON THE
RAW SLUDGE HEATING REQUIREMENT
Heat Required to Make up for Heat Losses. The amount of heat
lost to the air and soil surrounding a digester depends on the
tank shape, construction materials, and the difference between
internal and external temperatures. The general expression for
heat flow through compound structures is:
Q = (U)(A)(T2 - T3)
(6-4)
where:
Q
A
= heat-loss rate, Btu/hr
= area of material normal to direction of heat flow,
sq ft
T2 = temperature within the digestion tank, °F
•6-51
-------
T3 = temperature outside the digestion tank, °F
U = heat transfer coefficient, Btu/hr/sq ft/°F, which
is directly affected by the film coefficient for
interior surface of tank, and the film coefficient for
exterior surface of tank, and inversely affected by the
thickness of individual wall material, and the thermal
conductivity of individual wall material.
Several other factors may affect the heat transfer coefficient U;
however, they may be considered negligible for the purposes of
digester design. Further discussion of heat transfer principles,
along with lists of values for film coefficients and thermal
conductivities, is available (84,145,146). Various values of U
for different digester covers, wall construction, and floor
conditions are given in Table 6-18.
TABLE 6-18
HEAT TRANSFER COEFFICIENTS FOR VARIOUS
ANAEROBIC DIGESTION TANK MATERIALS (147)
Heat transfer coefficient (u),
Material Btu/hr/sq ft/°F
Fixed steel cover (1/4 in. plate) 0.91
Fixed concrete cover (9 in. thick) 0.58
Floating cover (Dowries-type with wood
composition roof) 0.33
Concrete wall (12 in. thick) exposed to air 0.86
Concrete wall (12 in. thick), 1 in. air
space and 4 in. brick 0.27
Concrete wall or floor (12 in. thick)
exposed to wet earth (10 ft thick) 0.11
Concrete wall or floor (12 in. thick
exposed to dry earth (10 ft thick) 0.06
1 Btu/hr/sq ft/°F =4.9 kg-cal/hr/m2/°C.
1 in. = 2.54 cm
1 ft = 0.304 m
Heat losses can be reduced by insulating the cover and the
exposed walls of the digester. Common insulating materials
are glass wool, insulation board, urethane foam, lightweight
insulating concrete and dead air space. A facing is placed
over the insulation for protection and to improve aesthetics.
Common facing materials are brick, metal siding, stucco, precast
concrete panels, and sprayed-on mastic.
6.2.6.3 Mixing
Digester mixing is considered to have the following beneficial
effects:
6-52
-------
o Maintaining intimate contact between the active biomass
and the feed sludge.
• Creating physical, chemical, and biological uniformity
throughout the digester.
• Rapidly dispersing metabolic end products produced
during digestion and any toxic materials entering
the system, thereby minimizing their inhibiting effect on
microbial activity.
• Preventing formation of a surface scum layer and the
deposition of suspended matter on the bottom of the
tank. Scum and grit accumulations adversely affect
digester performance by consuming active volume in
the tank.
While the benefits of digester mixing are widely accepted,
controversy and confusion arise in attempting to answer such
questions as how much mixing is adequate, and what the most
effective and efficient method is for mixing digesting sludge.
Although general theory of slurry mixing is well developed
(148,149), little research has been focused on mixing of sludge.
Studies of mixing in full-scale digesters have been made of both
dye (150) and radioactive (105,151) tracers. These and other
studies have shown that the contents of the digester are not
completely mixed and that the degree of mixing attained is
closely related to the total power actually delivered to the
contents of the tank, irrespective of the actual mixing method
used.
A certain amount of natural mixing occurs in an anaerobic
digester, caused by both the rise of sludge gas bubbles and the
thermal convection currents created by the addition of heated
sludges. The effect of natural mixing is significant (150,152),
particularly in digesters fed continuously and at high loading
rates. However, natural mixing does not maximize the benefits of
mixing and is insufficient to ensure stable performance of the
digestion process. Therefore, mixers are an essential component
in a high-rate digestion system. Methods used for mixing include
external pumped circulation, internal mechanical mixing, and
internal gas mixing. A review of digester mixing methods is
available (57).
Ext erna 1 _Pu_mped_ CjLrcu 1 a t ion
Pumped circulation, while relatively simple, is limited in a
physical sense because large flow rates are necessary for high-
rate digester mixing. However, this method can effect
substantial mixing, provided that sufficient energy (0.2 to 0.3
hp per thousand cu ft of reactor (5 to 8 W/m3) is dissipated in
the tank (75). Greater pump power will be required if piping
6-53
-------
losses are significant. Pumped circulation is used most
advantageously in combination with other mixing systems. Besides
augmenting agitation, circulation allows external exchangers to
be used for heating the digester and uniform blending of raw
sludge with heated circulating sludge prior to the raw sludge's
entering the digester.
A pumped circulation mixing system was recently installed in an
80-foot (24 m) diameter, fixed cover anaerobic digester at the
Las Vegas Street Plant in Colorado Springs. Sludge is withdrawn
from the top-center of the tank and pumped with a 16-inch
(41 cm) horizontal, solids handling centrifugal pump to two
discharge nozzles. These nozzles are located at the base of
the sidewall, on opposite sides of the tank, and direct the
sludge flow tangentially, inducing an upward spiral motion in the
tank. Return flow from the pump can be directed to a single
scum-breaker nozzle mounted near the liquid surface. The pump
capacity is rated at 6,800 gallons per minute (429 1/s) at
21 feet total dynamic head (6.4 m) and is sufficient to pump
the entire digester contents in 3.5 hours. The new mixing system
has successfully eliminated temperature stratification and
scum buildup. Another type of pumped circulation system using
sequential pumping through multiple pipes strapped to the floor
of the digester is described in Reference 153.
Internal MechanicalMixing
Mixing by means of propellers, flat-bladed turbines, or similar
devices is widely practiced in the process industries. Its
usefulness, when applied to wastewater sludge digesters, is
limited by the nature of non-homogeneous wastewater sludge. The
large amounts of raggy and relatively inert, nonfluid material in
wastewater sludge results in fouling of the propellers and
subsequent failure of the mechanisms. The practice of grinding
screenings within the wastewater flow will accelerate ragging.
Mechanical mixers can be installed through the cover or
walls of the tank. In one design, a propeller drives sludge
through a draft tube to promote vertical mixing. Wall
installations restrict maintenance and repair to the time
when the digester has been emptied (usually every three to
five years in well maintained plants). Strong mechanical
mixing can be effected with about 0.25 hp/thousand cubic
feet of reactor (6.6 W/m3) (75).
Internal Gas Mixing
Several variations of gas mixing have been used for digesters,
including:
The injection of a large sludge gas bubble at the bottom
of a 12-inch (30 cm) diameter tube to create piston
pumping action and periodic surface agitation.
6-54
-------
• The injection of sludge gas sequentially through a series
of lances suspended from the digester cover to as great a
depth as possible, depending on cover travel.
• The free or unconfined release of gas from a ring of
spargers mounted on the floor of the digester.
The confined release of gas within a draft tube
•rne conrinea release o
positioned inside the tank.
The first method generally has a low power requirement, and
consequently, produces only a low level of mixing. As a result,
the major benefit derived from its use is in scum control. Lance
free gas lift, and draft tube gas mixing, however, can be
scaled to induce strong mixing of the digester contents. The
circulation patterns produced by these two mixing methods differ.
As shown on Figure 6-22 in the free gas lift system, the gas
bubble velocity at the bottom of the tank is zero, accelerating
to a maximum as the bubble reaches the liquid surface. Since the
pumping action of the gas is directly related to the velocity of
the bubble, there is no pumping from the bottom of the tank with
a free gas lift system. In contrast, a draft tube acts as a gas
lift pump which, by the law of continuity, causes the flow .of
sludge entering the bottom of the draft tube to be the same as
that exiting at the top. Thus, the pumping rate is largely
independent of height, as shown on Figure 6-23. The significance
of this difference is that draft tube mixers induce bottom
currents to prevent or at least reduce accumulations of settle-
able material. Velocityj profiles shown on Figure 6-24 (see
page 6-58) indicate that 'lance type mixers induce comparable
bottom velocities. Another difference among internal gas mixing
systems is that the gas injection devices in a free gas lift
system are fixed on the bottom of the digester and thus cannot be
removed for cleaning without draining the tank. To reduce
clogging problems, provisions should be made for flushing the gas
lines and diffusers with high pressure water. With the lance and
draft tube systems, the gas diffusers are inserted from the roof
and, therefore, can be withdrawn for cleaning without removing
the contents of the tank. A drawback of these systems, though,
is that the draft tube and gas lines suspended inside the tank
may foul with rags and debris contained in the digesting sludge.
Basis for Sizing Gas Lift Mixers. Three basic criteria have been
used to determine the size of gas lift mixing systems:
• Unit power (power per unit volume)
• Velocity gradient (G value)
• Unit gas flow (gas flow per unit volume)
Each of these criteria is interrelated so that one can be
calculated from the other once a few assumptions are made about
gas discharge pressure and sludge viscosity. The size of new
6-55
-------
il"
POINT OF
GAS INJECTION
DRAFT TUBE MIXER
FREE GAS LI FT _M IXE R
FIGURE 6-22
POINTS OF
GAS INJECTION
CIRCULATION PATTERNS PRODUCED BY DRAFT
TUBE AND FREE GAS LIFT MIXERS
6-56
-------
mixing systems has tended to increase in recent years as the
importance of strong mixing in anaerobic digesters has become
more widely recognized. However, oversizing of mixing systems
not only results in excess equipment and operating costs but also
may aggravate foaming problems.
O
m
o
m
h-
X
g
LU
I
LIQUID SURFACE
FREE GAS
LIFT
\
DRAFT TUBE
».*,***•**!
±
PUMPING RATE
FIGURE 6-23
DRAFT TUBE AND FREE GAS LIFT PUMPING RATE
Unit Power. The use of the unit power criterion stems from the
observation that the relative effectiveness of mixing is closely
related to the total power expended (137,152). Generally, strong
mixing can be achieved if 0.2 to 0.3 hp is used to mix each
thousand cubic feet (5 to 8 W/m^) of digester volume. The unit
power criterion is expressed in terms of the motor horsepower
used to drive the compressor. Less power is actually delivered
to the liquid because of losses in the mixing system (for
example, friction losses, compressor inefficiency).
Velocity Gradient. Camp and Stein (154) have suggested use of
the root-mean-square velocity gradient (G) as a measure of
mixing intensity expressed mathematically:
w
(6-5)
where:
f t / S & C! n
G = root-mean-square velocity gradient, —= sec"1
6-57
-------
W = power dissipated per unit volume
ft-lbforce/sec
cu ft
= lbf/sq ft/sec
w.f
(6-6.
where:
E =
V =
M =
rate of work on energy transfer (power), ft-lbf/sec, and
volume of reactor, cu ft
absolute viscosity of the liquid, lbf-sec/sq ft
The velocity gradient is a more refined design criterion for
mixing than the unit power criterion in that it takes into
account the power actually transferred to the liquid (E),
and the viscosity of the liquid ( ). Determination of these
w 3 1 n e> c fnr <"i^S lift- mivinrr in rlinAel-pva is
1 **l ^ *» ** tb* *_-*!•** *™ ^ *~ ™ »* * .r1™*«^». ™ M « ~I™-^**™M«*^. _ **r
Q
£
HI
a
0.5
1.0
10
0,6 1.0
VELOCITY, fps (1fps = 0.30m/sec)
10
30 ft FROM
AIR SOURCE
o.s
IjO
MIXER TYPE
DEPTH OF AIR RELEASE, ft (m)
17.0 (5.1)
9.5 (2.9)
12.5 (3.8)
DEPTH OF TANK = 20 ft (6.1m)
AIR FLOW RATE = 300 scfm (8.5 m3/min)
FIGURE 6-21
COMPARISON OF LANCE AND DRAFT TUBE
MIXING IN CLEAN WATER (147)
6-58
-------
When gas is discharged into a digester, liquid flow results
from the transfer of energy from the gas to the liquid as
the gas isothermally expands and rises to the surface. If
the liquid vapor pressure and the kinetic energy of the gas
are ignored, the power transferred from the gas to the liquid may
be expressed as (155):
E = 2.40 P1(Q)ln (6-7
where :
E = rate of work or energy transfer (power), ft-lbf/sec
Q = gas flow, cfm
P^ = absolute pressure at the liquid surface, psi
?2 = absolute pressure at the depth of gas injection, psi
Therefore, given a gas flow through a mixer system and the
depth of the diffuser, Equation 6-7 can be used to calculate
the power transferred to the digester liquid (E). The power
dissipated per unit volume (W) can then be calculated by
dividing the rate of energy transfer (E) by the volume of the
digester (V) .
There is little information on the rheology (flow properties)
of unstabilized wastewater sludges although some data does exist
on the rheology of anaerobically digested sludge (156,157). This
is partly because it is extremely difficult to do such studies
correctly (158). In general, digesting sludge seems to be a
pseudoplast ic material exhibiting only slight thixotropic
properties (156). Pseudoplastic liquids become less viscous at
higher shearing rates. Thixotropic liquids become less viscous
with time at a constant sharing rate. Chapter 14 has additional
information on sludge rheology.
Three parameters--temperature, solids concentration, and volatile
content appear to affect sludge viscosity. As the temperature of
sludge is increased, its viscosity is reduced. The relationship
between temperature and viscosity for water is presented on
Figure 6-25. (A similar relationship between temperature and
sludge viscosity exists, although this has not been documented.)
The viscosity of sludge increases exponentially as the solids
concentration increases (159), as shown on Figure 6-26. This
graph also shows that viscosity increases with the volatile
content of the sludge; however, the effect is only noticeable
when the solids content of the digesting sludge is greater than
three percent. The entrapment of gas bubbles in digesting sludge
may also affect viscosity, although th.e magnitude of this effect
6-59
-------
has not been measured. In general, then, it is not possible
to pinpoint the viscosity of digesting sludge although major
influences can be identified.
4.0 i-
3,0
t o
O x
« c
> g-
Ly "tl
t- I
3 U
CQ
2,0 -
1.0 -
50
70
90 110
TEMPER ATURE,°F
130
150
(1 centipoise = 2.08 x 10'5 Ib-sec/ft 2 )
FIGURE 6-25
EFFECT OF TEMPERATURE ON THE VISCOSITY OF WATER
The appropriate "G value" to use for design is difficult to
determine. In general, the "G value" should be between 50 and
80 sec"1. Walker (75) recommends a "G value" of 85 sec"1 for
substantial auxiliary mixing. A design value at the high end
of the range should be selected for a large digester with only a
single mixer, or in a case where grit or scum problems appear
likely. A lower "G value" is appropriate in cases where several
mixers are distributed through the tank or where sufficient
detention time has been provided to allow a slower rate of
6-60
-------
ffl
.2
o
9-
V
g
u
%
a,
o
ra
O
a
LU
cc
a.
a.
2000
1750
1500
1250
1QQQ
ISO
500
250
0
MEASUREMENTS MADE
WITH BROOKF1ELD
LVF VISCQMETER-
SPINDLE 2a
8
TOTAL SOLIDS, %
THE BROOKFIELD VISCOMETER OPERATES IN A VERY
LOW SHEAR STRESS RANGE, SO APPARENT VISCOSITIES
ARE VERY HIGH. THIS DATA SHOULD NOT BE USED TO
CALCULATE SLUDGE FLOW IN PIPES IN THE LAMINAR
FLOW REGIME.
FIGURE 6-26
EFFECT OF SOLIDS CONCENTRATION AND VOLATILE
CONTENT ON THE VISCOSITY OF DIGESTING SLUDGE (156)
6-61
-------
digestion. The use of a two-speed compressor provides the
capability to match mixing intensity with variations in operating
conditions.
An example of gas mixer sizing is found in Section 6.2.9.3.
Unit Gas Flow. As described in the preceding paragraphs, gas
flow through a mixing system can be related to the mixing energy
delivered to the liquid. Therefore, a simple way to size a
gas lift mixer is to specify a unit gas flow. For a draft-tube
system, 5 to 7 scf m/thousand cubic foot of digester (5 to
7 m-^/min/km3) at about 6 psig (41.4 kN/m^) is sufficient to
produce strong mixing. Less gas is required for a free-lift
system, 4.5 to 5 cfm per thousand cubic feet (4.5 to 5 m^/min/
km3) of reactor; however, the pressure must be higher since the
gas is discharged at the bottom of the tank (75). 1.5 to
2.0 cfm per foot (0.14 to 0.19 m3/min/m) of diameter (0.14 to
0.19 m^/min/m) has also been recommended for free gas lift
mixers (137 ) .
The unit gas flow can be related to the velocity gradient by
combining equations 6-5, 6-6 and 6-7 and solving for ^, the
unit gas flow:
(6-8
The values in Table 6-19 were calculated from this equation.
6.2.6.4 Covers
Anaerobic sludge digestion tanks are covered to contain odors,
maintain operating temperature, keep out oxygen, and collect
digester gas. Digester covers can be classified as either
fixed or floating. Cross sections of both types are shown on
Figure 6-27. Floating covers are more expensive but allow
independent additions and withdrawals of sludge, reduce gas
hazards, and can be designed to control formation of a scum-mat.
Fixed digester covers are fabricated from steel, reinforced
concrete and, since the mid-1970s, corrosion-proof fiberglass
reinforced polyester (FRP). In most cases, fixed covers are
dome-shaped, although conical and flat concrete covers have been
built. Concrete roofs are susceptible to cracking caused by
rapid temperature changes. Consequently, gas leakage has been a
frequent problem with reinforced concrete covers (75).
Generally, fixed-cover digesters are operated so as to maintain a
constant water surface level in the tank. Rapid withdrawals of
digested sludge (without compensating additions of raw sludge)
6-62
-------
can draw air into the tank, producing an explosive mixture of
sludge gas and oxygen. The explosive range of sludge gas in air
is 5 to 20 percent by volume (52). In addition, there have
been cases in which the liquid level under the fixed cover
has been allowed to increase sufficiently to damage the cover
structurally. Usually, this involves a tightly clogged overflow
system and a forgotten feed valve.
TABLE 6-19
RELATIONSHIP BETWEEN THE VELOCITY GRADIENT
AND UNIT CAS FLOW
G Q/v
Velocity gradient, Unit gas flow ,
1 cfm/1,000 cu ft
40 2.1
50 . 3.3
60 4.4
70 6.4
Calculated assuming depth of gas release
is 13 ft and that absolute viscosity of
sludge is the same as for water at 95°F.
1 cfm/1,000 cu ft = 1 m3/min/l,000 m3
Traditionally, floating covers have followed one of two designs:
the pontoon or Wiggins type and the Downes type (Figure 6-27).
Both types of covers float directly on the liquid and commonly
have a maximum vertical travel of 6 to 8 feet (2 to 3 m). These
cover designs differ primarily in the method used to maintain
buoyancy, which, in turn, determines the degree of submergence.
In the Wiggins design, the bottom of the cover slopes steeply
along the outer edge. This outer portion of the cover forms
an annular pontoon or float that results in a large liquid
displacement for a small degree of cover-plate submergence.
Therefore, Wiggins covers have only a portion of the annular area
submerged, with the largest portion of the cover exposed to the
gas above the liquid surface. However, for the Downes design, as
shown on Figure 6-27, the bottom of the cover slopes gradually
throughout the entire radius, thereby providing only a small
liquid displacement for a greater degree of ceiling plate
submergence. Typically, the outer one-third of the radius of the
Downes cover is in contact with the liquid. However, it is
desirable to increase the degree of submergence by adding ballast
to the cover, thus keeping the liquid level a few inches within
6-63
-------
the central gas dome. This keeps floating matter submerged and
subject to mixing action, reduces the area exposed to corrosive
sludge gas, and adds to cover stability. The fundamental
principle used to calculate ballast requirements is that at
equilibrium, a floating cover displaces a volume of liquid equal
in weight to the total weight of the cover. Ballast can be added
as concrete blocks or as a layer of concrete spread across the
upper surface of the cover.
FIXED COVERS
WIGGINS TYPE
DOWNES TYPE
FLOATING COVERS
GAS HOLDER
FIGURE 6-27
TYPES OF DIGESTER COVERS
A variation of the floating cover is the floating gas holder,
shown on Figure 6-27. Basically, a gas holder is a floating
cover with an extended skirt (up to 10 feet [3 m] high) to allow
storage of gas during periods when gas production exceeds demand.
However, storage pressure in a gas holder is low--a maximum of
15 inches water column (3.7 kN/m2). Therefore, this type of
cover will store up to three to six hours of gas production,
based on about six feet (2m) of net travel. Greater storage is
achieved by compressing the gas for high pressure storage .in
spheres or horizontal cylinders, or by providing a separate low
pressure displacement storage tank.
6-64
-------
Gas-holding covers are less stable than conventional floating
covers because they are supported entirely by a cushion of
compressible gas rather than incompressible liquid and because
they expose a large side area to lateral wind loads. To prevent
tipping or binding, ballast at the bottom of the extended skirt
and spiral guides must be provided.
Typical appurtenances for a digester cover include sampling
ports; manholes for access, ventilation, and debris removal
during cleaning; a liquid overflow system; and a vacuum-pressure
relief system equipped with a flame trap. The permissible
range of gas pressure under a digester cover is typically 0 to
15 inches of water (0 to 3.7 kN/m2). Figure 6-28 provides an
overview of four floating covered digesters with appurtenant
equipment.
FIGURE 6-28
OVERALL VIEW OF FOUR DIGESTERS WITH DOWNES
FLOATING COVERS AT SUNNYVALE, CALIFORNIA
6-65
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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
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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
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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
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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
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MIXING*
TOTAL GAS PRODUCTION a
7.3
• SURPLUS GAS
ANAEROBIC
DIGESTER
""-•^. ^-"-""^
1
CIRCULATING* \
SLUDGE HEATING . t /"
K 1,75 f
V** ns X- -*— [BOI
J
^~^~^^^ c
ALL VALUES \H UNITS OF
10B BTU/TON DRV SOLIDS
r
ij^y— ^
F ' " ' \
RAW SLUOQEb ^
V
J
.75
'
HEATING HEAT
LOSSES
FED TO THE DIGESTER
(1 Btu/lh = 0,56
RAW
SLUDGE
Raw sludge volatile solids contents, percent 75
Volatile solids reduction during digester, percent 50
Specific gas production, cu ft/lb VS reduced 15
Heat value of gas, Btu/cu ft 650
2,000 Ib/ton (.75) (.50) (15
cu ft1
Ib '
(650
) = 7.3 x 10 Btu/ton
Feed solids concentration, percent
Specific heat of sludge, Btu/lb/°F
Rise in temperature, °F
(1
(25°F) = 1.3 x 106 Btu/ton
c Makeup Heat Requirement = .
Raw Sludge Heat Requirement
Net boiler and heating system efficiency, percent 70
F'eed solids concentration, percent 4
Detention time, days 20
Mixing requirement, bhp/1,000 cu ft 0.25
2.000 Ib/ton
'(.04) 62.4 Ib/cu ft
,,,„
(2°
,,,.
(24
hr
,,. _,-
(0'25
bhp
1,000 cu ft
FIGURE 6-32
,^ _ . _
(2'547
Btu
= 2-4
Btu/ton
ENERGY FLOW THROUGH AN ANAEROBIC
SLUDGE DIGESTION SYSTEM
6-73
-------
in a waste-gas burner. The energy flow through a more complex
gas utilization system, in which gas is used to fuel an engine-
generator, is described in Chapter 18.
The energy flow diagram shown on Figure 6-32 conveys very
effectively the relative magnitude and direction of energy
exchanges in an anaerobic digestion system. This type of diagram
is helpful in the design of a gas utilization system. However,
more detail must be added and the full range of expected
conditions must be evaluated, rather than just the average
conditions depicted for this case.
More complete discussions of digester gas utilization systems can
be found in Chapter 18 and elsewhere (38,39).
6.2.8 Costs
Cost curves have been compiled that plot construction costs for
anaerobic digestion systems versus either digester volume (166),
sludge solids loading (167,168,169), or total treatment plant
flow (170,171,172). However, these curves differ significantly,
even when converted to a common cost index and plotted in terms
of a single sizing parameter (Figure 6-33). Cost curves
are generally constructed to allow comparison of equivalent
alternatives and consequently do not always describe actual
costs.
Estimated annual costs for operation and maintenance are shown
in Figure 6-34. No credit has been given in this graph for the
value of surplus sludge gas. In most cases, use of this gas
requires, construction of additional facilities for conditioning,
compressing, and burning the gas. The cost for construction
and operation of these systems (38) must be included in
calculations of the net value of surplus sludge gas.
6.2.9 Design Example
This section illustrates the basic layout and sizing of the
major components in an anaerobic sludge digestion system. For
this example, it is assumed that the treatment plant provides
activated sludge secondary treatment to a typical municipal
wastewater. A mixture of primary sludge and thickened
waste-activated sludge is to be anaerobically digested, held in a
facultative sludge lagoon, and ultimately spread as a stabilized
liquid onto land.
6.2.9.1 Design Loadings
Sludge production estimates for two flow conditions, average
and peak day, are listed in Table 6-20 (see page 6-79). The
peak loading is listed because several components must be sized
6-74
-------
to meet this critical condition. Refer to Chapter 4 for a
discussion of the procedures to determine sludge production
values. Sludge solids concentrations and the resulting sludge
volumes are also included in Table 6-20.
cc
z
UJ
I
-s
a
I
* .
H
O
U
GC
te
I
10.0 I—
9
8
7
8
S
4
1.0
9
8
7
6
5
4 -
3 -
_ REFERENCE 16?
REFERENCE 171
'CONSTRUCTION COST ONLY,
DOES NOT INCLUDE ENGINEERING
OR CONTINGENCIES.
I
J__L_J
10
34B678S1CMJ 2 346678 91,000 2
DIGESTER TANK VOLUME, 1,000 cu ft (1 cu ft = .028m3|
4 5
FIGURE 6-33
CONSTRUCTION COSTS FOR ANAEROBIC
DIGESTION SYSTEMS (111-168,171)
6.2.9.2 System Description
The conceptual design for a high-rate anaerobic digestion
system is presented on Figure 6-35 (see page 6-80). At the
heart of the system are two cylindrical single-stage, high-rate
digestion tanks operated in parallel. The contents of both
digesters are heated to 95°F (35°C) and vigorously mixed with
draft-tube gas mixers. Floating covers are used on both tanks to
keep floating material soft and submerged, and to allow in-line
storage of sludge in the digestion tanks.
6-75
-------
er
O
03
5
j
<
§
e
4!
_
o
z
.001
.OOC1
oc
111
o
CL
E
5
_
•
c
o
O
O
12
<
•*-" .00001
0.1
1.0 10
AVERAGE PLANT FLOW, MGD flMGD = 3,785m3/day)
FIGURE 6-34
OPERATING, MAINTENANCE, AND ENERGY COSTS
FOR ANAEROBIC SLUDGE DIGESTION SYSTEMS (171)
Raw primary and secondary sludges are first combined and then
heated to 95°F (35°C) in a jacketed pipe heat exchanger.
The rate of the raw sludge flow is measured with a magnetic
flowmeter. The signal from this meter is integrated to indicate
the hydraulic loading to digestion. This information is also
used to indicate equal volumes of raw sludge for even distribu-
tion to each digester. The controls are set so that each
digester is fed approximately ten times each day. Raw sludge
is mixed with circulating sludge and added to the digester
through the gas dome in the center of the cover. The operating
temperature in the digester is maintained by circulating a
6-76
-------
small volume of sludge through an external spiral heat exchanger.
Digested sludge is withdrawn daily from the bottom of the
tank and transferred by gravity to facultative sludge lagoons.
For monitoring purposes, a flowmeter is included in the digested
sludge withdrawal line. This provides a means for evenly
distributing the sludge to several lagoons. Both tanks
are operated as completely mixed primary digesters without
supernatant removal.
6.2.9.3 Component Sizing
Digestion T a n k s
Sizing criter a:
• >_IQ days solids retention time during the most critical
expected condition to prevent process failure (See
Section 6.2.3.3).
• 2.50 percent volatile solids reduction at average
conditions to minimize odors from the facultative sludge
lagoons.
Tank volume:
Raw sludge flow at peak conditions (Qp)
— Assume peak day conditions (this is conservatively large
but provides a margin of safety) .
Qp = 6,010 + 3,430 = 9,440 cu ft per day (267 m3/day)
Active volume (Va)
Va = f____ (1Q das) = 4? >2^ cu ft
Correction for volume displaced by grit and scum accumula
tions and floating cover level.
Assume:
4-ft grit deposit
2-ft scum blanket
2- ft cover below maximum
8-ft total displaced height
6-77
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Therefore, if original sidewater depth of the tank is 30 feet,
O Q __ p
active volume is only —^Q— = °-73 of the total tanks volume.
Tank volume (Vt)
v = 47,200 cu ft / 1 \
fc tank \'73/
= 64,700 cu ft per tank
Say 65,000 cu ft per tank = (1,800 m3/tank)
Solids retention time at average conditions (SRTa)
q _ 65,000 cu ft per tank (2 tanks)
a 3,200 cu ft per day + 2,000 cu ft per day
= 25.0 days, based on total volume, 50 percent
volatile solids reduction can be expected with
this solids retention time (see Section 6.2.4.1).
Tank dimensions:
Diameter (D)
Assuming initially, a 30-foot sidewater height and
neglecting the volume in the bottom cone:
^ J4(65,000 cu ft) co c ... ,,, . .
D =\— J3Q £t) L = 52.5 ft = (16.0 m)
Sidewater height (h)
Since floating covers come in 5-foot diameter increments,
enlarge diameter and adjust sidewater height:
h = 4(65,000 cu ft) =
(55 ft)2
Note: This adjustment increases displacement volume
effect and reduces active volume to —•=-=—j— or 0.71.
This is ignored in this example because of previous
conservative assumptions.
6-78
-------
TABLE 6-20
DESIGN LOADING ASSUMPTIONS
Flow condition
Peak
Parameter Average day
Sludge production, Ib dry
solids/day
Primary sludge 10,000 ' 15,000
Waste activated
sludge 5,000 7,500
Solids concentration,
percent
Primary sludge 5.0 4.0
Waste activated
sludge 4.0 3.5
Sludge volume , cu ft/day
Primary sludge 3,200 6,010
Waste activated sludge 2,000 3,430
Sludge volume =
sludge production
(solids concentration) (density of sludge)
e.g., 10,000 Ib/day _ onri ... ,,
' t n c \ i r ->—-A—4\r~r^—jnrr - 3,200 cu ft/day
(.05) (62.4 Ib/cu ft) J
I Ib/day = .454 kg/day
1 cu ft/day = .0283 m3
ft/day
Heat_Exchangers - (See Section 6.2.6.2)
Raw sludge heat exchanger capacity (Qs)
Assume:
• Peak day sludge loading
• Minimum temperature of raw sludge = 55°F
- /9,440 cu ftV (62.4 lb\ / 1 day\ / Btu \ (95oF_55o
- ~5lTTE~ V24 hrs V- Ib - °F (^^
= 982,000 Btu/hr = (247,000 kg-cal/hr;
6-79
-------
Makeup heat exchanger capacity (Qm)
Assume:
• Tank completely buried but above water table, U = 0.06
• Bottom exposed to wet soil, U = 0.11
• Cover insulated, U = 0.16
• Minimum soil temperature = 40°F
• Minimum air temperature = 10°F
Qm =
heat loss through walls + bottom + top
= (0.06 Btu/sf/°F/hr) ( [2 ft]55 ft/4[27.4 f t] ) (95°F-40°F )
+ (0.11)( [55 ft]2/4] (95°F-40°F)
+ (0.16)( [55 ft]2/4] (95°F-10°F)
= 76,029 Btu/hr = (19.2 kg-kcal/hr)
The above calculated values are used for sizing equipment.
Average heat requirements would be substantially less.
PRIMARY
SLUDGE
WASTE
ACTIVATED
SLUDGI
RAW SLUDGE
HEAT EXCHANGER
RAW SLUDGE FLOW METER
FEED CONTROL VALVE
MAKE-UP HEAT EXCHANGER
CIRCULATING
SLUDGE PUMP
DRAFT TUBE
GAS MIXER
DIGESTED SLUDGE
FLOW M6TER
DIGESTED SLUDGE
CTO FACULTATIVE
SLUDGE LAGOONSI
FIGURE 6-35
CONCEPTUAL DESIGN OF AN ANAEROBIC SLUDGE DIGESTION SYSTEM
6-80
-------
Mj-xjing (See Section 6.2.6.3)
Sizing criterion:
Assumptions:
• Velocity gradient (G) = 60 sec"1
• Plant located at sea level PI = 14.7 psi
• Gas released 13 ft below the water surface ?2 = 14.7
+ 0.434 (13) = 20.3 psi
• Viscosity of the digesting sludge is the same as for
water at 95°F or 1.5 x 10""^ lbf-sec/sq ft
Rate of energy transfer (E)
Combining Equations 6-5 and 6-6 and solving for E:
E = V M G2
= 65,000 cu ft/tank (1.5 x 10~5 lbf - sec/sq ft)(60 sec"1)2
= 3,510 ft lbf/sec/tank = (4.8 kW/tank).
This is the power delivered to the digester contents. Motor
horsepower for the compressor will be substantially higher.
Gas Flow (Q) solving Equation 6-7 for Q.
2.4 (Pl) |ln |^J
3,510 ft-lb/sec/tank
6_8)
2.
= 308 cfm/tank (0.145 mj/sec/tank)
6-81
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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
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Do not generate nuisance odors (199,200).
Will produce a supernatant low in BOD5, suspended
solids, and ammonia nitrogen (199,200).
Reduce the quantity of grease or hexane solubles in the
sludge mass.
Reduce the number of pathogens to a low level under
normal design. Under auto-heated design, many systems
provide 100 percent pathogen destruction (187).
TABLE 6-21
SELECTED AEROBIC DIGESTION STUDIES ON VARIOUS
MUNICIPAL WASTEWATER SLUDGES
Sludge tyoe
Primary sludge
Primary sludge plus
waste-activated
lime
iron
alum
waste-activated + iron
.trickling filter
waste paper
Contact stabilization sludge
Contact stabilization sludge plus
iron
a 1 urn
Waste-activated sludge
Trickling filter sludge
)indicates full-scale study results.
Studies
under
50°F
180
187
Studies
'between
50°- 86°F
Studies
over
192
181, (182 )
187
188
189
189,(190)
190
131
191
192, 197, (199)
(190)
(190), (194)
195
181, 196
133, 184
(185) (186) (187)
As with any process, there are also certain disadvantages.
In aerobic digestion processes, the disadvantages are:
Usually produce a digested sludge
mechanical dewatering characteristics.
with very poor
Have high power
small plants.
costs to supply oxygen, even for very
Are significantly influenced in
temperature, location, and type of tank
performance by
material.
6.3.1.5 Microbiology
Aerobic digestion of municipal wastewater sludges is based on
the principle that, when there is inadequate external substrate
available, microorganisms metabolize their own celluar mass. In
6-83
-------
actual operation, aerobic digestion involves the direct oxidation
of any biodegradable matter and the oxidation of microbial
cellular material by organisms. These two steps are illustrated
by the following reactions:
Organic
matter
02
Bacteria
Cellular
material
C02 + H20
(6-9)
Cellular
material
o2
Digested
sludge
C02 + H20
(6-10)
The process described by Equation 6-10 is referred to as
"endogenous respiration"; this is normally the predominant
reaction in aerobic digestion.
6.3.2 Process Variations
6.3.2.1 Conventional Semi-Batch Operation
Originally, aerobic digestion was designed as a semi-batch
process, and this concept is still functional at many facilities.
Solids are pumped directly from the clarifiers into the aerobic
digester. The time required for filling the digester depends on
available tank volume, volume of waste sludge, precipitation, and
evaporation. During the filling operation, sludge undergoing
digestion is continually aerated. When the tank is full,
aeration continues for two to three weeks to assure that the
solids are thoroughly stabilized. Aeration is then discontinued
and the stabilized solids settled. Clarified liquid is decanted,
and the thickened solids are removed at a concentration of
between two and four percent. When a sufficient amount of
stabilized sludge and/or supernatant have been removed, the cycle
is repeated. Between cycles, it is customary to leave some
stabilized sludge in the aerator to provide the necessary
microbial population for degrading the wastewater solids. The
aeration device need not operate for several days, provided no
raw sludge is added.
Many engineers have tried to make the semi-batch process more
continuous by installing stilling wells to act as clarifiers in
part of the digester. This has not proven effective (200-202).
6.3.2.2 Conventional Continuous Operation
The conventional continuous aerobic digestion process closely
resembles the activated sludge process as shown on (Figure 6-36).
As in the semi-batch process, solids are pumped directly from
6-84
-------
UNSTABILIZED
SOLIDS
AEROBIC
DIGESTER
CLAR1FIER
THICKENER
SUPERNATANT
STABILIZED SOLIDS
FIGURE 6-36
PROCESS FLOW DIAGRAM FOR A CONVENTIONAL
CONTINUOUSLY OPERATED AEROBIC DIGESTER
clarifiers into the aerobic digester. The aerator operates
at a fixed level, with the overflow going to a solids-liquid
separator. Thickened and stabilized solids are either recycled
back to the digestion tank or removed for further processing.
6.3.2.3 Auto-Heated Mode of Operation
A new concept that is receiving considerable attention in the
United States is the auto-heated thermophilic aerobic digestion
process (187,203). In this process, sludge from the clarifiers
is usually thickened to provide a digester feed solids concentra-
tion of greater than four percent. The heat liberated in the
biological degradation of the organic solids is sufficient
to raise the liquid temperature in the digester to as high
as 140°F (60°C) (187). Advantages claimed for this mode of
operation are higher rates of organic solids destruction, hence
smaller volume requirements; production of a pasteurized sludge;
destruction of all weed seeds; 30 to 40 percent less oxygen
requirement than for the mesophilic process, since few, if any/
nitrifying bacteria exist in this temperature range; and improved
solids-liquid separation through decreased liquid viscosity
(187,203,204).
Disadvantages cited for this process are that it must incorporate
a thickening operation, that mixing requirements are higher
because of the higher solids content, and that non-oxygen aerated
systems require extremely efficient aeration and insulated tanks.
6-85
-------
6.3.3 Design Considerations
6.3.3.1 Temperature
Since the majority of aerobic digesters are open tanks, digester
liquid temperatures are dependent on weather conditions and
can fluctuate extensively. As with all biological systems, lower
temperatures retard the process while higher temperatures
speed it up. Table 6-21 lists studies on aerobic digestion
of municipal sludges as a function of liquid temperature.
When considering temperature effects in system design, one
should design a system to minimize heat losses by using concrete
instead of steel tanks, placing the tanks below rather than above
grade, and using sub-surface instead of surface aeration. Design
should allow for the necessary degree of sludge stabilization at
the lowest expected liquid operating temperature, and should meet
maximum oxygen requirements at the maximum expected liquid
operating temperature.
6.3.3.2 Solids Reduction
A major objective of aerobic digestion is to reduce the mass of
solids for disposal. This reduction is assumed to take place
only with the biodegradable content of the sludge, though some
studies (205,206) have shown that there may be destruction of the
non-organics as well. In this discussion, solids reduction will
pertain only to the biodegradable content of the sludge.
The change in biodegradable volatile solids can be represented by
a first order biochemical reaction:
where:
rlM •
-fir = rate of change of biodegradable volatile solids
per unit of time - (Amass/time)
K^ = reaction rate constant - (time ~1)
M = concentration of biodegradable volatile solids
remaining at time t in the aerobic digester -
(mass/volume).
The time t in Equation 6-11 is actually the sludge age or solids
residence time in the aerobic digester. Depending on how the
aerobic digester is being operated, time t can be equal to or
6-86
-------
considerably greater than the theoretical hydraulic residence
time. The reaction rate term K^ is a function of sludge type,
temperature, and solids concentration. It is a pseudoconstant,
since the term's value is the average result of many influences.
Figure 6-37 shows a plot of various reported K^ values as a
function of the digestion temperature. The data shown are for
several different types of waste sludge, which partially explains
the scatter. Furthermore, there has been no adjustment in the
value of K,3 for sludge age. At this time, not enough data are
available to allow segregation of K^ by sludge type; therefore,
the line drawn through the data points represents an overall
average K^ value. Little research has been conducted on the
effect of solids concentration on reaction rate K^. The results
of one study with waste-activated sludge at a temperature of
68°F (20°C) are shown on Figure 6-38, which indicates that Kd
decreases with increasing solids concentration.
!B
•a
•6
^
LJJ
5
BE
z
o
Ul
cc
,40 i-
,35 —
,30 —
.25 -
.20 -
.15
,10 —
.05
X
0 -
D -
A -
-PILOT PLANT RiF
- PI LOT PLANT REF
- FULL SCALE HEF
- PILOT PLANT REF
- PILOT PLANT REF
- Pi LOT PLANT REF
PI LOT PLANT REF
PILOT PLANT REF
(2071
(208)
(185!
(208)
(209)
(196)
{210}
10 20 30 40 50
TEMPERATURE OF LIQUID IN AEROBIC DIGESTER, °C
FIGURE 6-37
REACTION RATE K
-------
.7
* .6
LU
<
QC
Z
g
o
<
HI
DC
1__ _L _ L _. { 1 1.1 I i I
6000 10,000 14,000 18,000 22,000
TOTAL SUSPENDED SOLIDS CONCENTRATION IN AEROBIC DIGESTER , mg/1
FIGURE 6-38
EFFECT OF SOLIDS CONCENTRATION ON
REACTION RATE Kd( 194)
6.3.3.3 Oxygen Requirements
Activated sludge biomass is most often represented by the
empirical equation C5H7NO2. Under the prolonged periods of
aeration typical of the aerobic digestion process, Equation 6-10
can be written as follows:
7O2
5CO2 + 3H20 + H+
(6-12)
Hypothetically, this equation indicates that 1.98 pounds
(0.898 kg) of oxygen are required to oxidize one pound (0.45 kg)
of cell mass. From pilot and full-scale studies, however, the
pounds of oxygen required to degrade a pound of volatile
solids were found to be 1.74 to 2.07 (0.789 to 0.939 kg). For
mesophilic systems, a design value of 2.0 is recommended. For
auto-thermal systems, which have temperatures above 113°F (45°C),
nitrification does not occur and a value of 1.45 is recommended
(187,203,204).
6-88
-------
The actual specific oxygen utilization rate, pounds oxygen per
1,000 pounds volatile solids per hour, is a function of total
sludge age and liquid temperature (192,199,205). In one study,
Ahlberg and Boyko (199) visited several operating installations
and developed the relationship shown on Figure 6-39. Specific
oxygen utilization is seen to decrease with increase in sludge
age and decrease in digestion temperature.
uj £ 8l°
t > 6,0
D ™
> C
-------
the process for a particular tank geometry. Figure 6-40 shows
the chart developed by Envirex Incorporated for low speed
mechanical aerators in noncircular basins. The use of this chart
is explained in the design example in Section 6.3.5.
SURFACE AREA
A^fjqus* £Mt|
(1 h "O.D8Q mj|
BOO
MO
Pl¥OT
PtDuctP SHAFT
i
FIGURE 6-40
DESIGN CHART FOR LOW SPEED MECHANICAL AERATORS IN NON-CIRCULAR
AERATION BASINS TO CALCULATE ENERGY REQUIREMENTS FOR
MEETING OXYGEN REQUIREMENTS
6.3.3.5 pH Reduction
The effect of increasing detention time on pH of sludge in the
aerobic digester during mesophilic temperature range operation is
shown on Figure 6-41.
The drop in pH and alkalinity is caused by acid formation that
occurs during nitrification. Although at one,time the low pH was
considered inhibitory to the process, it has been shown that the
6-90
-------
system will acclimate and perform just as well at the lower pH
values (186,192,213). It should be noted that if nitrification
does not take place, pH will drop little if at all. This could
happen at low liquid temperatures and short sludge ages or in
thermophilic operation (203). Nitrifying bacteria are sensitive
to heat and do not survive in temperatures over 113°F (45°C)
(214).
B.O
7.0
6,0
I
a.
5.0
4.0
3.0
LIQUID TEMP 40°? (5°C»
LIQUID TEMP 67°F {2Q°C}
,10 30 50 70
SLUDGE AGE IN AEROBIC DIGESTERS - DAYS
FIGURE 6-41
EFFECT OF SLUDGE AGE ON pH DURING AEROBIC DIGESTION
6.3.3.6 Dewatering
Although there are published reports of excellent operating
systems (193) much of the literature on full-scale operations
has indicated that mechanical dewatering of aerobically digested
sludge is very difficult (182,189,215). Furthermore, in most
recent investigations, it is agreed that the dewatering
properties of aerobically digested sludge deteriorate with
increasing sludge age (181,11,189,216). Unless pilot plant data
indicate otherwise, it is recommended that conservative criteria
be used for designing mechanical sludge dewatering facilities for
aerobically digested sludge. As an example, a designer would
probably consider designing a rotary vacuum filter for a
6-91
-------
production rate of 1.5 pounds of dry solids per square foot per
hour (7.4 kg/m^/hr), a cake solids concentration of 16 percent,
with a FeCl3 dose of 140 pounds (63.5 kg), and a lime dose
(CaO) of 240 pounds (109 kg). This assumes an aerobic solids
concentration of 2.5 percent solids. For more detailed
information on results of various types of dewatering systems,
see Chapter 9.
6.3.4 Process Performance
6.3.4.1 Total Volatile Solids Reduction
Solids destruction has been shown to be primarily a direct
function of both basin liquid temperature and the length of
time during which the sludge was in the digester. Figure 6-42
is a plot of volatile solids reduction versus the parameter
degree-days. Data were taken from both pilot and full-scale
studies on several types of municipal wastewater sludges.
Figure 6-42 indicates that, for these sludges, volatile solids
reductions of 40 to 50 percent are obtainable under normal
aeration conditions.
ui
z
o
D
O
LU
cc
w
D
-1
O
w
UJ
O
H
UJ
O
IT
UJ
O-
50
40 -
30 -
20 -
10
X
•
D
A
+
A
o
*
— PILOT
- FULL
- PILOT
- FULL
— PILOT
- PILOT
— PILOT
- FULL
PLANT
SCALE
SCALE
SCALE
PLANT
PLANT
PLANT
SCALE
REF
REF
REF
REF
REF
REF
REF
REF
(18B)
(1941
(1781
(185)
(208)
(21 It
(192)
(196!
0 200 400 600 800 1000 1200 1400 1600 1800 20OO
TEMPERATURE °C x SLUDGE AGE, days
FIGURE 6-42
VOLATILE SOLIDS REDUCTION AS A FUNCTION OF DIGESTER
LIQUID TEMPERATURE AND DIGESTER SLUDGE AGE
6-92
-------
6.3.4.2 Supernatant Quality
The supernatant from aerobic digesters is normally returned
to the head end of the treatment plant. Table 6-22 gives
supernatant characteristics from several full-scale facilities
operating in the mesophilic temperature range. Table 6-23
summarizes the current design criteria for aerobic digesters.
TABLE 6-22
CHARACTERISTICS OF MESOPHILIC
AEROBIC DIGESTER SUPERNATANT
Reference 196
Reference 199'
Reference 213
Turbidity - JTU
NO -N - mg/1
TKN - mg/1
COD - mg/1
PO.-P - mg/1
Filtered P - mg/1
BODc - mg/1
Filtered BODj - mg/1
Suspended solids - mg/1
Alkalinity - mq/1 CaCC>3
SO - mg/1
Silica - ing/1
PH
120
40
115
700
70
-
50
-
300
-
-
-
6.8
_
_
2.9-1, 350
24-25,500
2.1-930
0.4-120
5-6, 350
3-280
9-41,800
-
-
-
5.7-8.0
_
30
_
-
35
-
2-5
_
6.8
150
70
26
6.8
Average of 7 months of data.
DRange taken from 7 operating facilities.
"Average values.
6.3.5 Design Example
Given
Using the information provided in Chapter 4, a design engineer
has determined that the following quantities of sludge will be
produced at a 0.5-MGD (22 1/s) contact stabilization plant:
Total daily solids generation
Amount due to chemical sludge
Amount that will be volatile
Amount that will be non-volatile
1,262 pounds (572 kg)
0
985 pounds (447 kg)
277 pounds (125 kg)
In addition, the designer has the following information:
• Estimated minimum liquid temperature (winter) in digester
is 50°F (10°C).
• Estimated maximum liquid temperature (summer) in digester
is 77°F (25°C).
6-93
-------
System must achieve greater than 40 percent volatile
solids reduction during the winter.
A minimum of two continuously operated tanks are required
(see Figure 6-36). (This is a state requirement for
plants under 1' MGD [44 1/s]).
Expected waste sludge solids concentration to the aerobic
digester is 8,000 mg/1.
Expected thickened solids concentration for the
stabilized sludge is .three percent ,(30,000 mg/1), based
on designer's experience.
TABLE 6-23
SUMMARY OF CURRENT AEROBIC DIGESTER DESIGN CRITERIA
Days
Solids residence time required to achieve
40 percent volatile solids reduction
55 percent volatile solids reduction
Oxygen requirements
Oxygen residual
Expected maximum solids concentration
achievable with decanting
Mixing horsepower
108
31
18
386
109
64
Liquid
temperature
40°F
60°F
80°F
40°F
60°F
80°F
2.0 pounds of oxygen per pound of volatile
solids destroyed when liquid temperature
113°F or less
1.45 pounds of oxygen per pound of volatile
solids destroyed when liquid temperature
greater than 113°F
1.0 mg/1 of oxygen at worst design
conditions
2.5 to 3.5 percent solids when dealing with
a degritted sludge or one in which no
chemicals have been added
Function of tank geometry and type of
aeration equioment utilized. Should
consult equipment manufacturer.
Historical values have ranged from 0.5
to 4.0 horseoower per 1,000 cubic feet
of tank volume
1 Ib = 0.454 kg ,
1 hp/1,000 cu ft - 26.6 kw/1,000 rri
Sludge Age Required
Figure 6-42 (presented previously) offers a quick method for
calculating the number of degree days required to achieve the
40 percent volatile solids reduction required. The result is
475 degree-days. At a basin temperature of 50°F (10°C) then:
475 degree-days
10 degrees
= 47.5 d ay s
6-94
-------
Therefore, the volume of the aerobic digester must be adequate
to provide 47.5 days sludge age to meet minimum volatile solids
reduction during the winter.
During the summer, the basin temperature will be 77°F (25°C):
25°C x 47.5 day sludge age = 1,175 degree-days.
From Figure 6-42, at 1,175 degree-days, there would be 49 percent
volatile solids reduction.
Volatile Solids Reduction
For winter conditions, there would be a 40 percent volatile
solids (VS) reduction. The actual pounds of solids reduced
are:
985 Ib VS x Q>4 = 394 Ib VS reduced {179 kg/day)
For summer conditions, there would be a 49 percent volatile
solids reduction. The actual pounds of solids reduced are:
985 Ib VS x o 49 = 403 Ib VS reduced (219 kg/day)
day ' day
Oxygen Requirements
Since nitrification is expected, provisions must be made to
supply 2.0 pounds of oxygen per pound of volatile solids
destroyed (2 kg 02/kg volatile solids destroyed).
Winter conditions: 394 Ib VS dest x 2.0 Ibs 02 = 788 Ibs 02 (358 kg/day)
day Ib VS dest. day
Summer conditions: 483 1*> VS dest 2.0 Ibs 02 = 966 Ibs O2 /
day Ibs VS dest. day v •*/ -r'
During summer conditions, a minimum of 1.0 mg/1 oxygen residual
must be provided.
CalculatingJTank Volume
Sludge age in an aerobic digester can be defined as follows:
total Ib SS aerobic digester ^
Sludge age - total lb ss iost per day from aerobic digester
where SS = suspended solids.
6-95
-------
The suspended solids concentration in the digester will range
from the value of the influent suspended solids concentration or
8,000 mg/1 to the maximum value of the thickened and stabilized
solids concentration of 30,000 mg/1. On the average, the
suspended solids concentration within the digester is equal to
70 percent of the thickened solids concentration, or 21,000 mg/1.
An average poundage of suspended solids in the supernatant
can be approximated by the following equation.
(SS concentration in supernatant)(1-f)(8.34)(influent flow)
where f is the fraction of influent flow into the aerobic
digester that is retained, and 1-f is the fraction that leaves
as supernatant. The term f can be approximated by the following
equation.
f _ influent SS concentration fraction of solids
thickened SS concentration x not destroyed
For winter conditions, the fraction of solids not destroyed is:
1,262 Ib total solids - 394 Ib of solids reduced = 0 59
1,262 Ib total solids
Then, the term f for this example, is:
8'000 x 0.69 = 0.18
30,000 mg/1
Therefore, 18 percent of the influent flow into the aerobic
digester will be retained, and 82 percent will leave as
supernatant.
For a properly designed sol id s- 1 iqu id separator (under
200 gallons per day per sq ft [8.16 m3/day/m2] overflow rate),
the suspended solids concentration would be approximately
300 mg/1.
The influent flow can be found by dividing the influent solids
load (1,262 pounds per day [572 kg/day] by the influent
solids concentration [8,000 mg/1]). The result is 18,914 gallons
per day (71.5 m3/day).
The pounds of suspended solids intentionally wasted per day from
the aerobic digestion system can now be approximated from the
following expression.
(SS concentration in thickened sludge )( f )( 8 . 34) ( influent flow).
All the terms in the above equation have been previously defined.
6-96
-------
It is now possible to solve for the required tank volume for any
given sludge age. In this example, winter conditions govern,
and it was previously calculated that a 47.5-day minimum was
required. From the values previously discussed:
47 5 davs = (21,000 mg/1)(8.34)(tank volume-million gallons)
y ((300 mg/1)(1-0.18)+(30,000)(0.18))(8.34)(0.018915 mil gal)'
Tank volume = 0.233 million gallons (881 m3 )
Theoretical hydraulic detention time:
233,000 gallons ,„' ,
3 = 12.3 days
18,915 gallons per day
This is the minimum volume, to which must be added capacity for
weekend storage and precipitation requirements. For this design,
two tanks will be provided, each to have a volume capacity of
233,000 gallons (881 m3)(100 percent stand-by capacity as per
state requirements).
The actual dimensions of the tanks depend on the aeration
equipment utilized and are discussed in the following section.
Power Requirements
The designer has decided to use low-speed mechanical aerators
for mixing and oxygen transfer in the aerobic digester.
Previous calculations have indicated that the maximum oxygen
requirement was 966 pounds oxygen per day (438 kg/day). After
making corrections for plant elevation, alpha and beta factors,
water temperature, and minimum residual requirements, the
engineer calculated an overall mass transfer coefficient Kj^a of
3.53 hr"1. From this value, in conjunction with Figure 6-7,
power requirements will be calculated as follows.
Initially, a depth of 12 feet (3.65 m) is selected. Since
each tank is to be 233,000 gallons (881 m3) , the surface area
with a 12-foot (3.65 m) liquid depth would be 2,596 sq ft
(241 m2). A pivot point P is located by placing a straight-
edge across scales D and KLa of Figure 6-40. Then a line is
drawn through pivot point p connecting scale As, tank surface
area, to the required reducer shaft horsepower scale. The
required shaft horsepower for one tank would be 19 horsepower
(14.1 kW). Assuming a motor reducer efficiency of 92 percent,
total motor horsepower would equal 19 4 0.92, or 20.6 horse-
power (15.4 kW). The aerator manufacturer recommends that a
minimum 10 horsepower unit (7.5 kW) will be required to mix the
12-foot (3.65 m) liquid depth. Each 10 horsepower unit (7.5 kW)
6-97
-------
could mix an area 40 feet by 40 feet (12.1 m by 12.1 m) . After
making some calculations, the designer decides to use two
10-horsepower (7.5 kW) units in each tank, each tank being
36 feet (10.9 m) wide by 72 feet (24.5 m) long and having a total
tank depth of 14 feet (4.2 m) allowing 2 feet (0.61 m) of
free board. Figure 6-43 shows a view of the plan.
SUMMER CONDITIONS; 483 it» vs REDUCED/DAY - see itw o,Mav
WINTER CONDITIONS: 394 Ibn VS REDUCED/DAY - 788 Its 0,/day
EACH TANK; 72 ft LONG BY 36 fi WIDE x 13 It LIQUID DEPTH PLUS
2ft OF FREEBOARD
AEROBIC DIGESTER #1
18,915 §pd
8,000 ma/I §s
•**• •
TANK VOLUME = 233,000
AEROBIC DIGESTER #2
TANK VOLUME <= 233,000 gal
- 10 HP LOW SPEED MECHANICAL
AERATOR
is-
1 gpd = O.QO378 m3/day
1 cu ft = 0,0283 m3
I ft - 0.304 m
1 Ib = 0,454 kg
RECYCLE 30,000 mg/l as
iACK TO SECONDARY
TREATMENT
' 15,51 0 gpd
300 mg/l ss
WASTE STABILIZED
SLUDGE
30,000 mg/l »
FIGURE 6-«
SUMMARY OF RESULTS FOR AEROBIC DIGESTION DESIGN EXAMPLE
Clarifier Surface Area
Surface area was based on an overflow rate of 200 gallons per
square foot per day (8.16 m3/day/m2) . At an influent flow of
18,915 gallons per day (71.5 m3/day), the required surface area
is 95 square feet (8.8 m2). The designer selected a 12-foot
(3.7 m) diameter clarifier.
Supernatant Flow
It was previously calculated that 82 percent of the
to the aerobic digester would leave as supernatant,.
influent
Based on
an influent of 18,915
supernatant flow will be
plus any precipitation.
gallons per day (71.5
15,510 gallons per day
nH/day), the
(58.6 m3/day),
6-98
-------
6."3.6 Cost
6.3.6.1 Capital Cost
A regression analysis of construction bids from 1973-1977
indicated that, on the basis of USEPA Municipal Wastewater
Treatment Plant Construction Cost Index - 2nd quarter 1977, the
capital cost could be approximated by Equation 6-13 (198).
C = 1.47 x 105 Q1-14 (6-13)
where:
C = capital cost of process in dollars
Q = plant design flow in million gallons of wastewater
flow per day
The associated costs included those for excavation, process
piping, equipment, concrete, and steel. In addition, such
costs as those for administrating and engineering are equal to
0.2264 times Equation 6-13 (198).
6.3.6.2 Operation and Maintenance Cost
Although there are many items that contribute to operation
and maintenance cost, in most aerobic digestion systems, the two
most prevalent are staffing requirements and power usage.
SJbaffjLng Requirements
Table 6-24 lists labor requirements for both operation and
maintenance. The labor indicated includes: checking mechanical
equipment, taking dissolved oxygen and solids analyses, and
general maintenance around the clarifier.
Power Requirement^
In 1979, the cost of power for operating aeration equipment has
become a significant factor. It is possible to minimize power
consumption through two developments in environmental science.
• Make sure that the tank geometry- and aeration equipment
are compatible (212). The difference between optimized
and unoptimized design can mean as much as a 50 percent
difference in power consumption.
• Pace devices to control oxygen (power) input (218).
Because of temperature effects, oxygen requirements
for any given aerobic digestion system can vary as
6-99
-------
much as 20 to 30 percent between summer and winter.
One must design to meet the worst conditions (summer),
for without some type of oxygen controller, considerable
power is wasted during other times of the year.
TABLE 6-24
AEROBIC DIGESTION LABOR REQUIREMENTS (217)
Plant design flow,
MGD
Labor,
man hours per year
Operation
Maintenance
0.5
1
2
5
10
25
100
160
260
500
800
1,500
20
30
50
100
160
300
Total
120
190
310
600
960
1,800
1 MGD = 3,786 ni /day
Other Requirements
Besides manpower and power cost, the designer must consider
lubrication requirements. If mechanical aerators are being
used, each unit needs to have an oil change once, and preferably
twice, a year. Depending on horsepower size, this could be 5 to
40 gallons per unit per change (19-152 I/unit/change). Further,
the designer must make sure an adequate inventory of spare parts
are available.
6.4 Lime Stabilization
Lime stabilization is a very simple process. Its principal
advantages over other stabilization processes are low cost and
simplicity of operation. Evaluation of studies where lime
stabilization was accomplished at pH ranges of 10-11, has shown
that odors return during storage due to pH decay. To eliminate
this problem and reduce pathogen levels, addition of sufficient
quantities of lime to raise and maintain the sludge pH to 12.0
for two hours is required. The lime-stabilized sludge readily
dewaters with mechanical equipment and is generally suitable for
application onto agricultural land or disposal in a sanitary
landfill.
No direct reduction of organic matter occurs in lime treatment.
This has two important impacts. First, lime addition does not
make sludges chemically stable; if the pH drops below 11.0,
biological decomposition will resume, producing noxious odors.
Second, the quantity of sludge for disposal is not
is by biological stabilization methods. On the
reduced, as it
contrary, the
6-100
-------
mass of dry sludge solids is increased by the lime added and by
the chemical precipitates that derive from this addition. Thus,
because of the increased volumes, the costs for transport and
ultimate disposal are often greater for lime-stabilized sludges
than for sludge stabilized by other methods.
6.4.1 Process Description
6.4.1.1 History
Lime has been traditionally used to reduce odor nuisances from
open pit privies and the graves of domestic animals. Lime has
been used commonly in wastewater sludge treatment to raise the
pH in stressed anaerobic digesters and to condition sludge prior
to vacuum filtration. The original objective of lime condi-
tioning was to improve sludge dewaterability but, in time, it was
observed that odors and pathogen levels were also reduced. In
1954, T.R. Komline filed a patent (No. 2,852,584) for a method of
processing raw sludge in which heavy dosages of hydrated lime
(6 to 12 percent of total dry solids) were added specifically
to cancel or inhibit odors. However, only recently has
lime addition been considered a major sludge stabilization
alternative.
Many studies describe the effectiveness of lime in reducing
microbiological hazards in water and wastewater, but the
bactericidal value of adding lime to sludge has been noted only
recently (219-222). A report of operations at the Allentown,
Pennsylvania wastewater treatment plant states that lime
conditioning an anaerobically digested sludge to a pH of 10.2 to
11, and then vacuum filtering and storing the cake, destroyed all
odors and pathogenic enteric bacteria (233). Kampelmacher and
Jansen reported similar experiences (224). Evans noted that lime
addition to sludge released ammonia and destroyed coliform
bacteria and that the sludge cake was a good source of nitrogen
and lime to the land (225).
Lime stabilization of raw sludges has been conducted in the
laboratory and in full-scale plants. Farrell and others (226)
reported that lime stabilization of a primary sludge reduced
bacterial hazard to a negligible value, improved vacuum filter
performance, and provided a satisfactory means of stabilizing
sludge prior to ultimate disposal. Paulsrud and Eikum (227)
determined the lime dosage required to prevent odors occurring
during storage of sewage sludges. Primary biological sludges,
septic tank sludges, and different chemical sludges were used in
the study. An important finding was that lime dosages greater
than those sufficient to initially raise the pH of the sludges
were required to prevent pH decay and the return of odors during
storage. Laboratory and pilot scale work by Counts and Shuckrow
(228) on lime stabilization showed significant reductions in
pathogen populations and obnoxious odors when the sludge pH was
6-101
-------
greater than 12. Counts conducted growth studies on greenhouse
and outdoor plots which indicated that the disposal of lime-
stabilized domestic sludge on cropland would have no detrimental
effect on plant growth and soil characteristics. Disposal
of the lime-stabilized domestic sludge at loading rates up to
100 tons dry solids per acre (224 t/ha) on green-house plots and
40 tons dry solids per acre (90 t/ha) on outdoor plots had no
detrimentatal effect on plant growth.-and soil characteristics.
A full-scale lime stabilization facility was built as part
of a 1-MGD (43.8 1/s ) wastewater treatment plant in Lebanon,
Ohio. Operation began in 1976. 'A case study of lime treatment
and land application of sludge from this plant, along with a
general economic comparison of lime stabilization with anaerobic
digestion, is available (229).
6.4.1.2 Current Status
As of May 1978, lime treatment is being used to stabilize the
sludge from at least 27 municipal wastewater treatment plants in
Connecticut. Average wastewater flows treated at these plants
vary from 0.1 to 31 MGD (4.4 to 1358 1/s). In most of the
plants, incinerators have been either wholly or partially
abandoned. While few chemical or bacterial data are available,'
qualitative observations indicate that treatment is satisfactory.
Most of the communities have indicated that they will continue
with lime stabilization.
Landfill burial is the most common means of disposal for
lime-stabilized sludge. However, lime-treated sludge from eight
of the plants in Connecticut is applied onto land. At Enfield,
Connecticut, dewatered sludge is stockpiled in large mounds. The
sludge is spread onto cornfields when application is compatible
with crop cycles and weather conditions. Few nuisances are
caused by the practice. Odors have not been a problem, even when
piles have been opened for spreading of the sludge. .. In
Willimantic, Connecticut, lime-stabilized sludge is mixed with
leaves and grasses. After stockpiling, a portion of mixture
is screened and distributed to local nurseries. The remainder is
used as final cover for landfill.
6.4.1.3 Applicability
Lime stabilization can be an effective alternative when there is
a need to provide:
o Backup for existing stabilization facilities. A lime
"stabilization system can be started (or stopped), quickly.
Therefore, it can be used to supplement existing sludge
processing facilities when sludge quantities exceed
design levels, or to replace incineration during fuel
6-102
-------
shortages. Full sludge flows can be lime-treated when
existing facilities are out of service for cleaning or
repair.
• Interim sludge handling. Lime stabilization systems have
a comparatively low capital cost and, therefore, may be
cost effective if there are plans to abandon the plant or
process within a few years.
• Expansion of existing facilities or_c_ons^tructioa of new
facilities to improve odor and pa'thog^n~cc)ntrol. Lime
stabilization is particularly applicable in small plants
or when the plant will be loaded only seasonally.
In all cases, a suitable site for disposal or use of stabilized
sludge is required.
6.4.1.4 Theory of the Process
Lime addition to sludge reduces odors and pathogen levels by
creating a high pH environment hostile to biological activity.
Gases containing nitrogen and sulfur that are evolved during
anaerobic decomposition of organic matter are the principal
source of odors in sludge (228). When lime is added, the
microorganisms involved in this decomposition are strongly
inhibited or destroyed in the highly alkaline environment.
Similarly, pathogens are inactivated or destroyed by lime
addition.
High lime dosing of sludge also affects the chemical and physical
characteristics of sludge. Although the complex chemical
reactions between lime and sludge are not well understood, it
is likely that mild reactions, such as the splitting of complex
molecules by hydrolysis, saponification, and acid neutralization,
occur in the high pH environments created in lime stabilization
(228). These reactions reduce the fertilizer value of the
stabilized sludge, improve its dewaterability, and change the
character of liquid sidestreams. The nature of these chemical
changes is described in Section 6.4.3.4.
6.4.2 Design Criteria
Three fundamental design parameters must be considered in the
design of a lime stabilization system: pH, contact time,
and lime dosage. At this early stage in the development of the
process, the selection of the levels of these parameters has been
largely empirical. The results of earlier studies now can be
used as a starting point, but because of the complexity of
chemical interactions that apparently occur in lime treatment of
sludge, bench-scale and pilot studies are recommended as part of
designing a large-scale system, particularly if substantial
departures from these conditions are contemplated.
6-103
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6.4.2.1 pH and Contact Time
The primary objective of lime stabilization is to inhibit
bacterial decomposition and inactivate pathogenic organisms. The
extensive use of lime is medicinal; the masking of noxious
odors from decaying substances permits uncritical acceptance of
its use for sludge treatment. Nevertheless, evidence is needed
of its value and of the necessary dose levels and contact times
for effective treatment.
The effective factor in lime treatment is evidently the pH
level and not just the dose of lime. As with most disinfection
processes, the time of exposure (the extensive factor) is equally
as important as the pH (the intensive factor). Investigations by
Farrell and others (226), Counts and Shuckrow (228), and Noland
and others (229), have established time, pH, and processing
conditions for producing satisfactory lime stabilization.
Process performance is discussed in a subsequent section
(Section 6.4.3).
The design objective is to maintain pH above 12 for about two
hours to ensure pathogen destruction, and to provide enough
residual alkalinity so that the pH does not drop below 11 for
several days, allowing sufficient time for disposal or use
without the possibility of renewed putrefaction. The recommended
design criteria for accomplishing these objectives are:
• Treat sludge in the liquid state.
• Bring the sludge to pH 12.5 by lime addition and maintain
pH above 12.5 for 30 minutes (which keeps pH >12 for two
hours).
Farrell and others (226) attempted to determine whether the
additions of lime that would occur in conditioning of sludge for
dewatering would produce adequate stabilization. They mixed
liquid sludge with lime for two minutes, and then dewatered the
sludge on a Buchner funnel. Their results indicated inadequate
bacteriological destruction. Later results by Strauch and
others (230) in Denmark and unpublished results at Downington,
Pennsylvania (See Section 6.4.4.1) indicate that special reaction
conditions or intense mixing of sludge cake with lime can produce
satisfactory results.
6.4.2.2 Lime Dosage
The amount of lime required to stabilize sludge is determined by
the type of sludge, its chemical composition, and the solids
concentration. Table 6-25 summarizes the results of plant-scale
tests at Lebanon, Ohio, and shows that lime additions ranging
from 6 to 51 percent of the total dry solids in the sludge were
required to raise the pH to the levels indicated in the table.
These lime dosages were sufficient to keep the sludge pH above
6-104
-------
12.0 for 30 minutes. Primary sludges required the lowest
dosages, while the highest average dosages were required to raise
the pH level of waste-activated sludges. The results of studies
conducted by Paulsrud and Eikum (227) agree generally with the
Lebanon tests and are displayed in Table 6-26. Iron and alum
sludges required the highest dosages. Farrell, and others (226)
also found that alum additionally increased the lime requirement
and suggested that part of the lime added to alum sludge may be
bound as a calcium-aluminum compound.
TABLE 6-25
LIME REQUIREMENT TO ATTAIN pH 12 FOR 30 MINUTES
AT LEBANON, OHIO (228)
Solids
concentration,
percent
Sludge type
Primary sludge
Waste activated sludge
Anaerobically digested mixed
sludge
Septage
Range
3-6
1-1.5
6-7
1-4.5
Average
4.
1.
5.
2.
3
3
5
7
0
0
0
0
Lime
Ib
dosage,
Ca(OH) 2
pH, average
Ib dry solids
Range
.06-0.
.21-0.
.14-0.
.09-0.
Average
17
43
25
51
0.
0.
0.
0.
12
30
19
20
Initial
6
7
7
7
.7
.1
. 2
.3
Final
12.7
12.6
12.4
12.7
Includes some portion of waste activated-sludge.
1 Ib/lb = 1 kg/kg.
TABLE 6-26
LIME DOSES REQUIRED TO KEEP pH ABOVE 11.0
AT LEAST 14 DAYS (226)
Type of sludge
Lime dose,
Ib Ca(OH}2/lb
suspended solids
Primary sludge
Activated sludge
Septage
Alum-sludge
'
jlus pri-
mary sludge1
Iron-sludge^1
0.10
0 ,30
0.10
0.40
- 0.15
- 0.50
- 0.30
- 0.60
0.25 - 0,40
0.35 - 0.60
aPrecipitation of primary treated effluent,
Equal proportions by weight of each type
of sludge.
1
b
b = 1 kg/kg.
6-105
-------
Figure 6-44 displays the general relationship between lime dosage
and pH for a typical municipal sludge at several solids concen-
trations. Table 6-26 calculated from data on Figure 6-44, shows
that the lime dose per unit mass of sludge solids required to
attain a particular pH level is relatively constant. That is,
lime requirements are more closely related to the total mass of
sludge solids, rather than the sludge volume. Consequently,
reduction in volume by thickening may have little or no effect
on the amount of lime required, because the mass of sludge solids
is not changed.
12
11
10
9
8
o 1.0% SOLIDS
A 2.0% SOLIDS
o 3.0% SOLIDS
« 3.5% SOLIDS
• 4.4% SOLIDS
2,000
4,000
6,000
8,000
10,000
Ca(OHl2 DOSE, rng/l
FIGURE 6-44
LIME DOSES REQUIRED TO RAISE pH OF A MIXTURE OF PRIMARY SLUDGE
AND TRICKLING FILTER HUMUS AT DIFFERENT SOLIDS CONCENTRATIONS (228)
Lime additions must be sufficient to ensure that the pH of sludge
does not drop below the desired level after prolonged storage.
If insufficient lime is added, the pH will decay as the treated
sludge ages (227-229). This phenomenon is displayed on
Figure 6-45. Notice that higher lime dosages not only raise the
initial pH but, more importantly, prevent, or at least delay, the
drop in pH levels. Consequently, in practice, lime doses must be
greater than that sufficient to raise the pH to the desired
value. In most cases, significant pH decay will not occur if
enough lime is added to raise the sludge pH to 12.5 and maintain
that value for at least 30 minutes (229).
6-106
-------
02BlbLlME DOSAGE Ca{QH)2/!bSS
12
10
SS=5.07%,VSS/S£=74.6%
TEMP,=6S°F!20SCS
1lfa/lb= 1 kg/kg
12 16
DAYS OF STORAGE
FIGURE 6-45
20
24
CHANCE IN pH DURING STORAGE OF PRIMARY SLUDGE
USING DIFFERENT LIME DOSAGES
Several mechanisms of pH decay have been proposed -tnd some have
been documented (227,228). The initial pH drop results from the
uptake of atmospheric CC>2 and slow reactions of hydroxyl ions
with sludge solids. The rate of pH reduction is accelerated once
the pH reaches a point at which bacterial action can resume
production of organic acids through anaerobic microbial
degradation.
The foregoing discussion makes it clear that a dose level cannot
be defined without reference to the specific sludge. Actual
dose levels will have to be determined in bench-scale tests.
Approximate levels can be selected from the information above in
order to establish size of equipment and to estimate costs.
6.4.3 Process Peformance
Lime stabilization reduces odors and odor production potential in
sludge, reduces pathogen levels, and alters dewatering, settling,
and chemical characteristics of the sludge. The nature and
extent of the effects -produced are described in the following
paragraphs.
6-107
-------
6.4.3.1 Odor Control
Lime treatment deodorizes sludge by creating a high pH
environment in the sludge, thus eliminating or suppressing the
growth of microorganisms that produce malodorous gases. In one
laboratory study (228), the threshhold odor number of raw mixed
primary and trickling filter sludges was 8,000, while that of
lime-treated sludges ranged between 800 and 1300. The threshold
odor number defines the greatest dilution of the sample with
odor-free water to yield the least definitely perceptible odor
(231). Sufficient lime must be added to retard pH decay because
odor generation will generally resume once the pH of the sludge
falls below pH 11.0 (220,228).
Hydrogen sulfide (H2S), a malodorous gas present in dissolved
form in sludge, is a major cause of sludge odors. Figure 6-46
shows that, as the pH of sludge is raised, the fraction of total
sulfide in the H2S form decreases from about 50 percent at
pH 7 to essentially zero at pH 9. Consequently, above this pH,
there is no longer any H2S odor.
HI
O
LL.
*J
3
LL
O
I-
z
CE
UJ
a.
100
80
60
40
20
0
MOLAR SOLUTION
10
6789
pH
FIGURE 6-4S
EFFECT OF pH ON HYDROGEN SULFIDE-SULFIDE EQUILIBRIUM
During full-scale operations at the Lebanon plant (229),
intense when septic raw sludge was first pumped to
stabilization mixing tank. Odor
diffused air was applied for mixing.
sludge odor was masked by the odor of
from the sludge by the air bubbled
odor was
the lime
intensity increased when
When lime was added, the
ammonia, which was stripped
through the mixture. The
6-108
-------
ammonia odor was most intense with anaerobically digested sludge
and was strong enough to cause nasal irritation. As mixing
proceeded, the treated sludge acquired a musty, mucus-like odor.
6.4.3.2 Pathogen Reduction
Significant pathogen reductions can be achieved in sludges that
have been lime-treated to pH 12.0 (228,229). Table 6-27 lists
bacteria levels measured during the full-scale studies at
Lebanon and shows that lime stabilization of raw sludges reduced
total coliform, fecal coliform, and fecal streptococci concentra-
tions by more than 99.9 percent. The numbers of Salmonella and
Pseud omonas aerug inosa were reduced below the level of
detection. Table 6-27 also shows that pathogen concentrations in
lime-stabilized sludges ranged from 10 to 1,000 times less than
those in anaerobically digested sludge from the same plant.
TABLE 6-27
BACTERIA IN RAW, ANAEROBICALLY DIGESTED, AND
LIME STABILIZED SLUDGES AT LEBANON, OHIO (228)
Bacterial density, number/100 ml
Sludge type
Total
coliform9
Raw
Primary
Waste-activated
Septage
Anaerobically digested
Mixed primary and
waste-activated
Lime stabilized
Primary
Waste-activated
Septage
Anaerobically
digested
2.9 x 10
8.3 x 10
2.9 x 10C
8
2.8 x 10
1.2 x 10
2.2 x 10
2.1 x 10
18
7
Fecal
coliform
8.3 x 10s
2.7 x 10J
1.5 x 10'
1.5 x 10
5.9 x 10-
1.6 x 10
265
18
Fecal
streptococci
3.9 x 10!
1.0 x ' a.
6.7
10-
2.7 x 10-
1.6 x lOJ
6.8 x 10:
665
1.6 x 10-
Salmonella
Ps.
aeruginasa
62
6
6
<3
<3
<3
<3
195 .
5.5 x 10-
754
42
<3
13
<3
<3
aMillipore filter technigue used for waste-activated sludge and
septage. MPN technique used for other sludges.
To pH equal to or greater than 12.0.
CDetection limit = 3.
Information on virus destruction in sludge by lime stabilization
is scant. There are numerous investigations on removal of
viruses from wastewater by lime flocculation but little on
destruction of viruses by elevated pH. A study by Berg (233)
measured the structure of a polio virus in water by pH adjustment
alone, and indicate very rapid destruction above pH 11. Similar
effects would be expected for other animal viruses.
6-109
-------
Qualitative observation under a microscope has shown substantial
survival of higher organisms, such as hookworms, amoebic cysts,
and Ascaris ova, after contact times of 24 hours at high
pH (226). It is not known whether long-term contact would
eventually destroy these organisms. A more complete discussion
of sludge disinfection is contained in Chapter 7.
6.4.3.3 Dewatering and Settling Characteristics
Lime has been used extensively as a conditioning agent to improve
the dewaterability of sludge. Trubnick and Mueller (234)
presented detailed procedures to be followed in conditioning
sludge for filtration, using lime with and without ferric
chloride. Sontheimer (235) described the improvements in sludge
filterability produced by lime addition. A more detailed
discussion of lime conditioning is contained in Chapter 8.
The addition of lime has been shown to improve the filterability
of alum and iron primary sludges (226). Specific resistance was
reduced by a factor of approximately four, and filter yield was
increased by a factor of two when lime conditioning was used.
Counts and Shuckrow (228) studied the effect of lime treatment on
the filterability of primary sludge and trickling filter sludge
but could not detect any consistent trend.
The impact of lime stabilization on sand bed drying of sludge
has been examined by several researchers (226,228,229). Lime
additions to raw sludge increased the rate of drying at least
initially and, in one study, produced a drier final cake.
However, lime-treated primary sludge did not dry as fast as
either lime-treated or untreated anaerobically digested sludge
(229).
The settling of lime-stabilized primary and mixed sludges was
enhanced in one study (228), indicating that gravity thickening
after lime treatment may be used to reduce the volume of sludge
to be dewatered.
6.4.3.4 Chemical Characteristics
Lime stabilization causes chemical changes in the sludge. The
nature of these changes is illustrated in Tables 6-28 and 6-29,
which compile chemical data from two studies. The general effect
of lime addition is a reduction in component concentration. This
is caused by both dilution with lime slurry and loss of some
volatile sludge components to the atmosphere.
Lime-stabilized sludges have lower concentrations of soluble
phosphate, ammonia nitrogen, and total Kjeldahl nitrogen than
anaerobically digested sludge from the same plant, as shown in
Table 6-28. These lower nutrient levels reduce the agricultural
value of the sludge but, assuming nitrogen limits the rate- at
6-110
-------
reaction
form calcium-phosphate
phosphate in the super-
believed to be largely
which sludge can be applied, would allow more sludge to be
applied per acre of land. A reduction in the soluble (filter-
able) phosphate concentration is caused by the reaction between
lime and dissolved orthophosphate to
precipitate. For this reason, residual
natant/filtrate after lime treatment is
organic in nature (228). Nitrogen levels can be reduced during
lime stabilization if gaseous ammonia is stripped during air
mixing of the treated sludge. As the pH of the sludge increases
from near neutral to 12, the predominant form of ammonia shifts
from the ammonium ion (NH4+) to dissolved ammonia gas (NH3).
Some of this gas is carried off by the air bubbled through the
sludge for mixing.
TABLE 6-28
CHEMICAL COMPOSITION OF SLUDGES AT LEBANON, OHIO, BEFORE AND
AFTER LIME STABILIZATION (228)
s, mg/1
Sludge type
Primary
Before lime addition
After lime addition
Waste activated
Before lime addition
After lime addition
Anaerobically digested
mixed sludge
Before lime addition
After lime addition
Septage
Before lime addition
After lime addition
Alkalinity
1,885
4,313
1,265
5,0'JO
Total.
COD
54,146
41,180
12,310
14,700
(.6,372
IB,670
1,897
3,475
Soluble
COD
3,046
J,55b
1,043
1,618
1,011
1,809
1,223
1,537
Total
phosphate
Total
Soluble Kjeldahl
phosphate nitrogen
Ammonia
nitrogen
Total
suspended
solids
Volatile
suspended
solids
35U
283
218
263
580
381
172
134
69
36
85
25
15
2.9
25
i.4
1,656
1,374
711
1,034
2,731
1.78U
223
145
51
64
70S
494
VI
110
48,700
38,370
12,350
10,700
61,140
66,350
21,120
23,1'K
36,100
23,480
10,000
7,136
33,316
26,375
12,600
11,390
A direct result of adding lime to sludge is that the total
alkalinity will rise to a high value. This can affect the
suitability of the treated sludge for land application. The
input can be positive or negative, depending on soil conditions
at the application site. Data in Table 6-28 indicates the
magnitude of change in alkalinity.
Biochemical oxygen demand, chemical oxygen demand, and total
organic carbon concentrations increase in the liquid fraction of
wastewater sludges when lime is added (228,229). Organic matter
is dissolved in the high pH environment. Possible reactions
involved include saponification of fats arid oils, hydrolysis and
dissolution of proteins, and decomposition of proteins to form
methanol (228).
Lime stabilization usually does not produce the substantial
reductions in volatile matter associated with anaerobic and
aerobic sludge digestion. However, volatile solids concentra-
tions decreased by 10 to 35 percent after lime additions in the
6-111
-------
plant-scale studies at Lebanon (229), as shown in Table 6-28.
Reductions in total solids concentration after lime stabilization
were measured by Counts and Shunckrow (228). These reductions,
displayed in Table 6-29, are greater than can be accounted for
simply by dilution with lime slurry. It may be simply that the
lime interfered with the volatile solids analysis. However,
reactions between lime and nitrogenous organic matter may cause a
loss of sludge solids. Hydrolysis of proteins and destruction of
amino acids are known to occur by reaction with strong bases.
Volatile substances such as ammonia, water, and low molecular
weight amines or other volatile organics may possibly be formed
and lost to the atmosphere.
6.4.4 Process Design
A lime stabilization operation is divided into two operations:
lime handling and sludge mixing. Lime handling comprises
facilities for receiving storing, transporting, feeding, and
"slurrying" of the lime. The sludge mixing operation consists of
a holding tank provided with mixing. A discussion of design
considerations for these two operations follows.
6.4.4.1 Design of Lime Handling Facilities
Lime, in its various forms, is the principal and lowest cost
alkali used in industry and wastewater treatment. As a result, a
substantial body of knowledge has evolved concerning the most
efficient handling of lime. Only the basic elements of lime
system design are described in this manual. Detailed information
is contained in several references that focus on the selection,
handling, and use of lime (236-239).
Lime Character is ti_cs_
Lime is a general term applied to several chemical compounds that
share the common characteristic of being highly alkaline. The
two forms commercially available are quicklime (CaO) and hydrated
lime (Ca(OH)2). The characteristics of these two chemicals
are summarized in Table 6-30. Lime is a caustic material and can
cause severe injury to tissue, particularly to eyes. Equipment
must be designed with safe handling in mind; eyewash fountains
and safety showers should be provided, and operating procedures
should mandate use of proper handling procedures and protective
clothing.
Quicklime is derived from limestone by a high temperature
calcination process. It consists primarily of the oxides of
calcium and magnesium. The grade of quicklime most commonly used
in wastewater treatment contains 85 to 90 percent CaO.
6-112
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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.
70°F
5.6 lb/1
000 gal at
,000 gal at
1 Ib - 0.454 kg
100 Btu/lb = 55 kg-cal/kg
1 Ib/cu ft = 16 kg/m3
1 lb/1,000 gal = 0.120 g/1
If slaking is done by the lime manufacturer, hydrated lime is
delivered to the wastewater treatment plant. The manufacturer
adds only enough water for hydration, producing a dry Ca(OH)2
powder. At the waste treatment plant, the powder is then slur-
ried with more water prior to mixing with sludge. Alternatively,
slaking may be carried out at the wastewater treatment plant; the
delivered product is, therefore, quicklime. In this case, the
lime is slaked, then diluted (if necessary) prior to process
application.
Direct addition of dry quicklime to sludge and without the use
of a separate slaker, is practiced in Denmark in at least
ten Swedish treatment plants. Potential advantages are the
elimination of slaking equipment and the generation of heat,
which can improve pathogen reduction and speed dewatering
through evaporation. In one case (230), direct additions of
dry quicklime were made to raise sludge pH above 13.0 and bring
the temperature to 176°F (80°C). Salmonella and intestinal
parasites were killed within two hours. Heat generated by
slaking of quicklime does not raise temperature significantly
unless the sludge is dewatered and the lime dose is high—on the
order of 400 to 800 Ib per ton dry solids (200-400 kg/t).
The decision whether to purchase quicklime or hydrated lime
in a particular situation is influenced by a number of factors
such as size of treatment facility, material cost, and storage
6-114
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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
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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
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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 200°F (93°C).
The resulting thin paste should be held for 30 minutes after
mixing to complete hydration. Manually operated batch slaking is
a potentially hazardous operation and should be avoided if
possible. Uneven distribution of water can produce explosive
boiling and splattering of lime slurry. Use of protective
equipment should be mandatory. For small plants, the potential
gain in using the lower-priced quicklime is smaller, because lime
consumption is smaller. Use of slaked lime is safer, simpler,
and requires less labor.
Continuous slaking is accomplished in automated machines that
also dilute and degrit the lime slurry. Several types of
continuous slakers are available. They vary mainly in the
proportion of lime to water mixed initially. A volumetric or
gravimetric dry chemical feeder is used to measure quicklime as
it is moved from bulk storage to the slaker. Since quicklime is
available in a wide range of particle sizes, it is important to
match the dry feeder with the type of quicklime to be used in the
particular application.
6.4.4.2 Mixing Tank Design
A tank must be provided for mixing raw sludge with lime slurry
and then holding the mixture for a minimum contact time. Many of
the currently operating lime stabilization facilities do not have
tanks with sufficient capacity to hold sludge for more than a
few minutes. Although these operations generally have been
successful, the acceptability of very short detention times has
not been conclusively demonstrated. Because of the uncertainty
surrounding this practice, it is recommended that all lime
stabilization facilities include a tank large enough to hold the
lime sludge mixture for 30 minutes. The pH of the reacted
mixture should exceed 12.5 during this period.
6-118
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The following paragraphs discuss two aspects of mixing tank
design - tank sizing and mixing. To determine tank size, a
designer must first select a flow mode. The following section
on tank sizing describes flow modes. The subsequent section on
tank mixing covers the general types of mixers and suggests
criteria for sizing mixing systems.
Tank Sizing Considerations
Mixing tanks can be operated as either a batch process or
continuous flow process. In the batch mode, the tank is filled
with sludge, and then sufficient lime is added to maintain the pH
of the sludge-lime mixture above 12.5 for the next 30 minutes.
After this minimum contact time, the stabilized sludge can be
transferred to dewatering facilities or to either tank trucks or
a pipeline for land application. Once the holding tank is
emptied, the cycle begins again.
In the continuous flow mode, the pH and volume of sludge in the
holding tank are held constant. Entering raw sludge displaces an
equal volume of treated sludge. Lime is added continuously, in
proportion to the flow of incoming raw sludge, and thus, the
holding time would vary. The lime dose must be sufficient to
keep the contents of the tank at a pH of 12.5. Often the daily
cycle of sludge production does not match the pattern of sludge
disposal. In this case, a system could be operated on a semi-
continuous basis, where the quantity of sludge in the tank
fluctuates through the day. Here the treatment tank would be
used as a buffer between sludge production and disposal.
It is most common to operate lime stabilization systems in the
batch flow mode. Batch operations are very simple and are well
suited for small-scale, manually operated systems. When adequate
capacity is provided, the mixing tanks can also be used to
gravity thicken the lime-treated sludge before disposal. In very
small treatment plants, tank capacity should be adequate to
treat the maximum-day sludge production in one batch. This is
because small plants are generally operated only during the day,
and it is usually desirable to stabilize the entire day's sludge
in one batch. Larger plants are more likely to be manned round-
the-clock. Because sludge can be processed over the whole day,
stabilization tanks can be relatively smaller.
Continuous-flow stabilization systems require automated control
of lime feeding and therefore are usually not cost-effective for
small-scale operations. The primary advantage of continuous-flow
systems over batch systems is that a smaller tank size may be
possible. Capacity does not have to be provided for storage of
sludge between batches. Instead, the mixing tank must only be
large enough to ensure that all sludge particles are held
at high pH for a contact time sufficient toadestroy odor and
disease-producing organisms. ^
6-119
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The most important design parameter for a continuous flow,
well-mixed reactor is the nominal detention time (defined as tank
volume divided by volumetric input flow rate). Unlike a batch
tank, where contact time of all particles is the same, some
particles in a well-mixed, continuously fed tank escape after
relatively short contact. Thus, 30 minutes of pH at 12.5 in a
batch mixer might not be the same as 30 minutes residence time
in a well-mixed, continuously fed reactor.
In making a recommendation for detention time, the nature of the
treatment that occurs must be considered. Unlike some
treatments, such as irradiation, the treatment does not stop
after the treated sludge leaves the vessel. If pH is 12.5 as
the sludge leaves the mixing tank, it remains at this pH after
leaving. Consequently, a 30-minute detention time in a
continuously fed, well-mixed reactor is adequate, provided the pH
is measured in an exit line. If pH of the limed sludge appears
to fall too rapidly upon standing, it is a simple matter to move
the pH sensor and to control lime feed rate to a position further
downstream.
Thickening of raw sludge before lime addition will reduce the
mixing tank capacity requirement in direct proportion to the
reduction in sludge volume. However, the lime requirement will
be reduced only slightly by prethickening, since most of the 1ime
demand is associated with the solids (227), and total solids mass
is not changed by thickening.
Tank Mixing
Lime/sludge mixtures can be mixed with either diffused air or
mechanical mixers. The agitation should be great enough to keep
sludge solids suspended and to distribute the lime slurry evenly
and rapidly. Both diffused air and mechanical systems can
provide adequate mixing, although the former has been more
commonly used in pilot studies and full scale operations. In
addition to their mixing function, sparger air systems supply
oxygen and, thereby, can be used for sludge aeration before the
sludge is dosed with lime. If storage of unlimed sludge is
contemplated, the designer should check that the air requirement
for mixing is sufficient to meet the oxygen demand of the sludge.
Oxygen requirements are discussed in the section on aerobic
digestion.
There are disadvantages to both types of mixing systems.
Mechanical mixers are subject to fouling with rags, string, and
other debris in the sludge. Although air spargers may clog,
fouling problems are greatly reduced by mixing with air. Ammonia
will be stripped from the sludge when mixing is done with
diffused air, producing odors and reducing the fertilizer value
of the treated sludge. However, if nitrogen levels limit land
application rates, this stripping of ammonia will reduce land
requirements for disposal. A further, although probably minor,
6-120
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problem with air mixing is that CC>2 is absorbed by the sludge/
lime mixture, tending to raise the quantity of lime >required to
reach the desired pH. The selection of the method of mixing
should be based on the factors described above, coupled with an
economic evaluation.
With air mixing, coarse bubble diffusers should be used, mounted
along one of the tank walls to induce a spiral-roll 'mixing
pattern. An air supply of 20 to 30 scfm per 1,000 cubic feet
(20-30 m3/min /I,000 m3) is required for adequate mixing (241).
If the mixing tank is enclosed, ventilation should be sufficient
to remove odorous gases stripped from the sludge during mixing.
In many cases, these gases should be treated in an odor control
unit before being discharged into the atmosphere.
Mechanical mixer specifications for various tank sizes are
presented in Table 6-31. Sizing is based on two criteria:
maintaining the bulk fluid velocity (defined as the turbine
agitator pumping capacity divided by the cross sectional area of
the mixing vessel) above 26 feet per minute (8.5 m/min), and
using an impeller Reynolds number greater than 1,000. The
tank/mixer combinations in Table 6-31 are adequate for mixing
sludges with up to 10 percent dry solids and viscosity of
1,000 cp. Impellers on mechanical mixers should be designed to
minimize fouling with debris in the sludge.
6.4.5 Costs and Energy Usage
Engineering decisions are commonly based on a comparison of costs
for feasible solutions. Energy considerations are now also
becoming important in the decision-making process. This section
discusses costs and energy usage for lime stabilization systems.
6.4.5.1 Capital and Operating Costs
Cost estimates for the construction and operation of three
different size lime stabilization systems are summarized in
Table 6-32. A comparison of these costs shows that there is
a large economy of scale, especially for the capital costs.
Operation and maintenance expenses, particularly those for lime,
are more closely related to the quantity of sludge treated.
Comparisons of the cost of lime treatment with other
stabilization methods must take into account that the addition
of lime increases the quantity of solids to be handled after
stabilization. In contrast, s.ludge solids actually decrease
during anaerobic and aerobic digestion. This difference between
stabilization methods can have an important effect on costs
for final disposal of sludge. The magnitude of this cost
differential is site-specific and depends on such factors as the
method of disposal and the distance to the disposal site.
6-121
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TABLE 6-31
MECHANICAL MIXER SPECIFICATIONS FOR SLUDGE SLURRIES (228)
Tank size,
gal
5,000
15,000
30,000
75,000
100,000
ft
9.5
13.7
17.2
23.4
25.7
ter, Motor size,
hp
7 .5
5
3
20
15
10
7.5
40
30
25
20
100
75
60
50
125
100
75
Shaft speed,
rpm
125
84
56
100
63
45
37
84
68
56
37
100
68
56
45
84
68
45
Turbine
diameter, ft
2.7
3.2
3. 6
3.7
4.4
5.3
5.6
4.8
5.1
5.5
6.8
5.2
6.2
6.6
7.3
6.0
6. 5
7.8
Assumptions :
Bulk fluid velocity >26 ft/min. (8.5 m/rain.).
Impeller Reynolds number >1,000.
Mixing tank configuration.
Liquid depth equals tank diameter.
Baffles with a width of 1/12 the tank diameter,
placed at 90 degrees spacing.
Mixing theory and equations after References 155 and 242.
1 gal = 3.785 1
I ft = 0.305 m
1 hp = 0.746 kW
6.4.5.2 Energy Usage
Energy is required during both the construction and operation
of a lime stabilization system. During operation of a lime
stabilization facility, the principal direct use of energy is
electricity for mixing the lime/sludge mixture. A rough estimate
of the annual energy requirement for mixing with diffused air is
290 kWh per year per cfm of blower capacity (based on continuous
duty). This estimate was made assuming a six psig (0.4 kg/m^ )
pressure boost, standard inlet conditions, and an overall
compressor/motor efficiency of 60 percent. One horsepower of
mechanical mixing requires about 6,500 kWhr of electricity per
year. These mixing energy demands can be expressed in terms of a
primary fuel requirement (that is, fuel oil, coal, natural gas)
by applying a conversion factor of 10,700 Btu (2,700 kg-cal) per
kWh of electricity. This factor assumes a fuel conversion
efficiency of 35 percent at the power plant and a transmission
efficiency of 91 percent.
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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.
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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
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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
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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
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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.
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6.5.2 Uses, Advantages, and Disadvantages
Chlorine oxidation has been used to treat raw and digested
primary sludge, raw and digested secondary sludges, septage,
-digester supernatants, and sidestreams from dewatering processes.
The chlorine stabilization process has several attractive
features. It can be operated intermittently, so long as
sufficient storage volume is available prior to and following the
unit. Unlike biological sludge processing systems, the process
can be started up, run for a few hours, and turned off. A
constant supply of process feed is not required. As a result,
operating costs are directly dependent upon production rates, and
costs attributable to overcapacity are eliminated.
Chlorine oxidation is a chemical process and is thus opera-
tionally insensitive to factors such as toxic materials in the
sludge, which adversely affect biological stabilization systems.
It can also process feed streams of widely varying character,
such as digested sludge and digester supernatant, within a short
period of time. This flexibility is not characteristic of
anaerobic or aerobic digestion processes.
Disadvantages of the chlorine stabilization process center
on chemical, operational, and environmental factors. From a
chemical standpoint, the low pH of chlorine-stabilized sludge
may require the sludge to be partially neutralized prior to
mechanical dewatering or before being applied to acid soils.
Costs of neutralization are in addition to chlorine costs. These
are discussed in Section 6.5.5.1. As mentioned earlier, chlorine
stabilization does not reduce sludge mass nor produce methane gas
as a by-product for energy generation. The process consumes
relatively large amounts of chlorine. Special safety and
handling precautions must be used when handling this gas. If
high alkalinity wastes—for example, digested sludge, digester
supernatants--are processed, CC>2 generated during chlorination
may promote cavitation in downstream pumps.
There is concern that chlorine oxidation of sludges, septage,
and sidestreams from sludge' treatment processes could result
in increased levels of toxic chlorinated organics in the
treated materials (245). Data available are inconclusive.
Investigations are underway that will help clarify this
issue. In the meantime, measures should be taken to mitigate
environmental concerns when the chlorine oxidation processes is
used. These are:
Provisions should be made to deal with the filtrate,
centrate, or decant from the process, including return
to the wastewater treatment plant, unless this practice
leads to wastewater treatment plant upset or to
violations of effluent standards; or to treat by
activated carbon absorption or other means.
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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.
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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.
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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
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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
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for one or more pathogenic parasites. A. lumbricoides, which
produces a resistant ova, was found in over two percent of the
samples (6,7).
The frequent occurrence of enteric pathogens in the United States
population indicates that pathogens should be expected in all
wastewaters and sludges.
7.3 Pathogen Survival During Sludge Stabilization Processes
Sludge stabilization processes are ideally intended to
reduce putrescibility, decrease mass, and improve treatment
characteristics such as dewaterability. Many stabilization
processes also accomplish substantial reductions in pathogen
concentrations.
7.3.1 Pathogen Reduction During Digestion
Sludge digestion is one of the major methods for sludge
stabilization in the United States. Well-operated digesters can
substantially reduce virus and bacteria levels but are less
effective against parasitic cysts.
7.3.1.1 Viruses
Viruses are removed most readily in wastewater treatment
processes when attached to larger particles such as chemical or
biological floes. Sagik has reported primary treatment virus
removal from three percent to extensive (8). Metcalf has
measured primary treatment removals of 60 to 95 percent with a
one-hour detention time (9). Sagik and Moore have reported 70 to
99 percent removals with activated sludge (8,10).
Virus concentration ranges for raw and anaerobically digested
sludges are given in Table 7-5. The large difference between the
high and low value for the number of viruses in untreated
sludge results from several factors, including variation in virus
occurrence in the human population, differing treatment plant
removal efficiencies, and disparity in viral preconcentration and
assay techniques. Anaerobic digestion has been shown to reduce
the concentration of detectable viruses by one to several orders
of magnitude. Moore and others reported a reduction by four
orders of magnitude for poliovirus by anaerobic digestion for
30 days at 85°F (30°C) (10). Ward and Ashley reported four log
inactivation of poliovirus in four days at 82°F (28°C) (17).
Ward also found that naturally occurring ammonia (NH3) was a
viricidal agent for poliovirus, Coxsackie, and ECHO (18).
However, it was less effective against reoviruses. Digester
detention time, operating temperature, and method of operation
7-7
-------
are apparently the most important factors affecting virus
removal. Stern and Farrell report almost 50 percent virus
inactivation with sludge storage at 67°F (20°C) for two weeks
under laboratory conditions (11). Reduction continued with
longer storage. Increased operating temperature also improves
reduction. .-....•:
TABLE 7-5
PATHOGEN OCCURRENCE IN
LIQUID WASTEWATER SLUDGES
Concentration, number/100 ml
Pathogen
Virus
Bacteria
Bacteria
Bacteria
Bacteria
Bacteria
Bacteria
Parasites
Parasites
Name or species
Various
Clostridia sp
Fecal coliform
Salmonella sp .
Streptococcus faecalis
Total coliforms
Mycobacterium tubercu-
losis
Ascaris lumbricoides
Helminth eggs
Unstabilized raw
sludge3
2.5 x 103
6
8
3
5
200
200
- 7 x 104
x 106
1Q93
x 10J
x 107
x 109
10?
- 1,000
- 700
Digested sludge3'
100
2
3 x 104
BDL
4 x 104
6 x 104
0
30
- 103
x 107
- 6 x 106
- 62
- 2 x 10°
- 7 x 107
106
- 1,000
- 70
Reference
9, 10,
12
13,
11
11
11
14
15
16
11
12
Type of sludge usually unspecified.
3Anaerobic digestion; temperature and detention times
varied.
"BDL is below detection limits, <3/100 ml.
Thermophilic anaerobic digestion of sludge at a temperature
of 121°F (50°C) with a 20-day retention time at the City of
Los Angeles Hyperion Treatment Plant showed a two log greater
virus reduction than for comparable mesophilic digestion at a
temperature of 94°F (35°C) and the same time period (19). Half
the thermophilic samples, however, still showed measurable
viruses, which was unexpected; this may be due to the way that
digesters are operated. Plant-scale digesters are usually
operated on a fill-and-draw basis. If the digesters are mixed
continuously, the daily fraction of sludge which is removed to
make room for the addition of raw sludge will contain sludge that
has been in the process for only a short time. Considering this
fact, the appearance of viable pathogens in digested sludge is
not surprising.
7.3.1.2 Bacteria
Most bacteria in wastewater are readily sampled and measured.
Commonly found concentrations and types of bacteria are shown in
Table 7-5. The sensitivity of assay techniques for different
7-8
-------
bacterial species do vary, from 3 MPN per 100 ml for Salmonella
to 1,000 MPN per 100 ml for total coliform, fecal coliform,
and fecal streptococcus. In general, anaerobic digestion
reduces bacterial counts by one to four logs. Work conducted
at Hyperion, in parallel with the virus studies discussed
previously, showed thermophilic anaerobic digestion of sludge
decreased bacterial counts by two to three logs over mesophilic
digestion (19). Increasing both the temperature and the
detention time increases bacterial inactivation. Fill-and-draw
operation, however, prevents digestion from removing as large a
fraction of the bacteria as it might in another operating mode.
Farrell and Stern reported the following bacterial concentrations
in an aerobically digested waste-activated sludge (13):
fecal coliform 7 x 107 MPN per 100 ml
Salmonella 1.5 x 104 MPN per 100 ml
The Salmonella values are higher than the upper end of the
typical range of values given for anaerobically digested sludge
in Table 7-5.
For thermophilic oxygen-aerobic digestion, Ornevich and Smith
reported that increasing temperature decreased the time required
for bacteria inactivation (20). At 113°F (45°C) Salmonella and
Pseudomonas were reduced to below detectable limits in 24 hours;
at 140°F(TO°C), the time was reduced to 30 minutes.
7.3.1.3 Parasites
There is a wide variation in the apparent level of parasite
infestation from region to region in the United States (6,7,21).
Protozoa cysts should not survive anaerobic digestion, but
helminth ova definitely do and should be expected in digested
wastewater sludge unless testing proves the contrary.
The data for parasite occurrence and persistence during
wastewater treatment are much more limited than those for
bacteria. Cysts of the protozoa Entamoeba histolytica, have been
reported at about four per liter in untreated wastewater (16).
Protozoan cysts have a low specific gravity and are not likely
to be removed to any great degree in primary sedimentation.
Secondary treatment by the activated sludge process is reported
to incompletely remove all cysts. Trickling filters can
remove up to 75 percent of cysts (8). E. histolytica are easily
inactivated by well-operated mesophilic sludge digestion.
Data for helminths are also sparse; limited data for sludges,
reported in Table 7-5, indicate that digestion can cause some ova
reduction. Stern and Farrell reported that Ascaris ova survived
thermophilic (121°F, [50°C]) digestion at the Hyperion Treatment
Plant (11).
7-9
-------
7.3.2 Long Term Storage
Pathogen reduction has been recognized for years as a side
benefit of sludge storage in lagoons. Hinesley and others have
reported 99.9 percent reduction in fecal coliform density after
30-days storage (22). For an anaerobically digested sludge
stored in anaerobic conditions for 24 weeks at 39°F (4°C), Stern
and Farrell reported major reductions in fecal coliform, total
coliform, and Salmonella bacteria (11). In similar tests at 68°F
(20°C), the same bacteria could not be measured after 24 weeks.
Viruses were reduced by 67 percent at 39°F (4°C) and to below
detectable limits at 68°F (20°C) in the same time period. Recent
work by Storm and others showed fecal coliform reductions of one
to three orders of magnitude during long-term storage of an
anaerobically digested mixture of primary and waste-activated
sludge in facultative lagoons (23).
7.3.3 Chemical Disinfection
A number of chemicals used for wastewater sludge stabilization,
including lime and chlorine, also reduce the number of pathogenic
organisms in sludge.
7.3.3.1 Lime
Lime treatment of wastewater sludge is discussed in detail in
Chapter 6. Plant-scale liming of wastewater sludge was evaluated
at Lebanon, Ohio (24). Two chemical-primary sludges, one with
alum and one with ferric chloride, were limed to pH 11.5 and
placed on drying beds. After one month, galmonella sp^ and
Pseudomonas aeruginosa were undetectable. Bench testing was also
conducted on ferric chloride-treated wastewater raw sludges that
were limed to pH 10.5, 11.5 and 12.5; these sludges were sampled
after 0.5 hours and 24 hours and bacterial tests performed (24).
Pathogenic bacteria reduction improved with time and was
substantially better at pH values of 11.5 and 12.5. Qualitative
checks for higher life forms such as Ascaris ova indicated that
they survived 24 hours at a pH greater than 11.0. Virus studies
on limed sludges have not been reported, but a pH in excess of
11.5 should inactivate known viruses (11).
7.3.3.2 Chlorine
Chlorine is a strong oxidizing chemical used for disinfecting
drinking water and wastewater effluents. It is effective for
bacteria and virus inactivation if applied in sufficient quantity
to develop a free chlorine residual in the solution - be ing
treated. Chlorine is less effective in disinfecting solutions
with a high suspended solids concentration. Cysts and ova of
parasites are very resistant to chlorine. The use of chlorine
for wastewater sludge treatment is presented in Chapter 6. Few
7-10
-------
data are available on the potential of chlorine for reducing the
number of pathogenic organisms in sludge. Some samples of sludge
treated with large doses of chlorine in South Miami, Florida,
and Hartland, Wisconsin, showed large reductions in bacteria
and coliphages (25). Chlorine doses of 1,000 mg/1 applied to
waste-activated sludge (WAS) with a 0.5 percent solids concentra-
tion reduced total bacteria counts by four to seven logs and
coliform bacteria and coliphage to below detection limits.
Primary sludge with a 0.5 to 0.85 percent solids concentration
was treated with 1,000 mg/1 chlorine, and total and fecal
coliform counts were reduced below detectable limits.
7.3.3.3 Other Chemicals
Other strong oxidizing chemicals such as ozone are sometimes used
for drinking water and wastewater disinfection. While they may
prove useful for sludge disinfection, they are as yet untried.
7.4 Pathogen Survival in the Soil
An objective of reducing the number of pathogens in wastewater
treatment plant sludge is to produce a product that may be
beneficially utilized. As such, the behavior of sludge pathogens
in the soil is important. Sludge is returned to the soil by
spray irrigation, surface flooding, wet or dry surface spreading,
or subsurface injection. These techniques expose the sludge to
the sun, air, water, and soil in different ways that may strongly
affect pathogen survival.
7.4.1 Viruses
Data for the survival of viruses, bacteria, and parasites in soil
are summarized in Table 7-6. Factors that have been found to
affect survival include soil temperature, pH, clay concentration,
cation exchange capacity, specific surface area, and organic
content. Virus adsorption to soil particles is the chief
mechanism for their retention when applied to the land. Virus
adsorption in soil is reversible. Viruses survive best at
slightly alkaline pH's. Cooler temperatures prolong virus
infectiveness, as does a moisture content between 15 and
25 percent (8).
7.4.2 Bacteria
Maximum recorded bacterial survival times, vary with species, from
a little over one month to almost a year, as shown in Table 7-6.
The important variables in bacteria survival are moisture
content, moisture holding capacity, temperature, pH, sunlight,
organic matter, and competition or predation (26). Moisture
content is most important, since desiccation often leads to
7-11
-------
cellular death. Lower temperatures prolong survival, and a lower
pH increases the rate of inactivation. The presence of organics
may promote survival or even regrowth.
TABLE 7-6
PATHOGEN SURVIVAL IN SOILS
Pathogen
type
Name or species
Length of survival,
days
Reference
Virus
Virus
Virus
Virus
Virus
Bacteria
Bacteria
Bacteria
Bacteria
Bacteria
Bacteria
Bacteria
Bacteria
Parasite
Parasite
Parasite
Poliovirus
Poliovirus 1
ECHO 7
ECHO 9
Coxsackie B3
Clostridium sp
Leptospira sp
Mycobacterium tuberculosis
Salmonella sp.
Salmonella typhi
Shigella sp.
Streotococcus faecalis
Total colilorm
Entamoeba histolytica
Ascaris lumbricoides
Hookworm larvae
Up
Up
Up
Up
Up
Up
Up
to
to
to
to
to
to
to
More
Up
Up
Up
Up
Up
Up
Up
Up
to
to
to
to
to
to
to
to
84
170
170
170
170
210
43
than 180
570
120
210 ,
210
210
8
2,550
42
8
10
10
10
10
15
27
27
28
27
15
15
27
27
27
Burge reported that sludge applied by subsurface injection tends
to maintain its identity in clumps (29). Since bacteria and
viruses in sludge are associated with the solids, they may be
protected from natural predation and other environmental factors
in the sludge. Burge also stated that ammonia in sludge may be
bactericidal.
If sludge is applied by a surface method and allowed to dry
before incorporation into the soil, considerable bacterial
reduction can be achieved. This potential advantage of surface
applications must be weighed against the associated odor risk and
the cost of subsurface injection.
7.4.3 Parasites
Protozoa cysts are reported to be destroyed in eight days after
land application. Helminth ova, however, are very durable and
may survive up to seven years. Hookworm larvae may be viable for
over a month.
7.5 Potential Human Exposure to Pathogens
Man may be exposed to pathogens in wastewater sludge in a variety
of ways and at greatly varying concentrations. Figure 7-1 lays
out in simplified form some of the potential pathways. There is
7-12
-------
no firm scientific evidence to document a single confirmed case
where human disease is directly linked to exposure to pathogens
from wastewater sludge. Viable pathogens have, however, been
isolated from intermediate points in the sludge management
system, such as from surface runoff from sludge treated fields.
These factors should be considered in the selection and design of
a process for reducing the number of pathogenic organisms.
HUMAN
OB
ANIMAL
SOURCES
SLUDGE
TREATMENT
PROCESSES
(NGESTJON, DIRECT CONTACT,INHALATION
DIRECT CONTACT/INHALATION
FIGURE 7-1
POTENTIAL PATHOGEN PATHWAYS TO MAN
7.6 Heat Disinfection Processes
The number of pathogenic organisms in wastewater sludge can be
effectively reduced by applying heat to untreated or digested
sludges. Heat may be used solely for pathogen reduction as
in pasteurization or as one step in a processes designed to
stabilize sludge, improve treatability or reduce mass. The focus
of this section will be on sludge pasteurization. Other heating
processes, such as thermal processing and incineration, are
developed in Chapters 8 and 11 and will only be reviewed briefly
here.
7-13
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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
158°F (70°C) for 30 minutes inactivates parasite ova and cysts
and reduces population of measurable pathogenic viruses and
bacteria below detectable levels. For bacteria, Ward and Brandon
found that fecal streptococci were most heat-resistant, followed
by coliforms and then Salmonella (32). Nicholson indicates that
a higher temperature for a shorter time period (195°F [91°C],
10 minutes) also destroys all pathogens (33).
TABLE 7-7
TIME AND TEMPERATURE TOLERANCE FOR
PATHOGENS IN SLUDGE (30, 31, 32)
Exposure time for
organism
inactivation, min
Species
Temperature, C
50 55 60 65 70
Viruses 25
Mycobacterium tubercu-
losis 20
Micrococcus pygogenes 20
Escherichi coll 60 5
Salmonella typhi 30 4
Fecal streptococci 60
Fecal coliforms 60
Corynebacteriuin dipth-
eriae 45 , 4
Brucella abortus 60 3
Cysts of EntamoeJDa his-
tolytica 5
Eggs of Ascaris lumbri-
coides 60 7
Aspergillus flavus
conidia 60
°F = 1.8 °C + 32
7-14
-------
7.6.1.1 Process Description
The critical requirement for pasteurization is that all sludge be
held above a predetermined temperature for a minimum time period.
Heat transfer can be accomplished by steam injection or with
external or internal heat exchangers. Steam injection is
preferred because heat transfer through the sludge slurry is slow
and undependable. Incomplete mixing will either increase heating
time, reduce process effectiveness, or both. Overheating or
extra detention are not desirable, however, because trace metal
mobilization may be increased, odor problems will be exacerbated,
and unneeded energy will be expended. Batch processing is
preferable to avoid reinoculations if short circuiting occurs.
PREHEATER 1.45 psi.
CONCENTRATED
DIGESTED SLUDGE
\ 64°F
V
-\^=--
113°F
1,45 psi
BLOW-OFF
TANKS
TO THE VACUUM PUMPS
TO THE VACUUM PUMPS
,RECUPERATQR|. «,—,_.
CONDENSER
COOLING WATER
INLET_
OUTLET
PASTEURIZED
SLUDGE
TO TANKER
STORAGE BASIN
PUMPS
PUMP
STORAGE BASIN PUMP
SLUDGE
HEATING STEAM
VAPORS
VACUUM {AIR)
WATER
TO CONVERT DEGREES FAHRENHEIT TO DEGREES CENTIGRADE,
SUBTRACT 32 AND THEN DIVIDE BY 1.8.
TO CONVERT LB PER SQUARE INCH TO kN/m2,
MULTIPLY BY 6.9.
FIGURE 7-2
FLOW SCHEME FOR SLUDGE PASTEURIZATION WITH
SINGLE-STAGE HEAT RECUPERATION (11)
The flow scheme for a typical European sludge pasteurization
system with a one-stage heat recuperation system is shown on
Figure 7-2. Principal system components include a steam boiler,
a preheater, a sludge heater, a high-temperature holding tank,
blowoff tanks, and storage basins for the untreated and treated
sludge. Sludge for pasteurization enters the preheater where the
7-15
-------
temperature is raised from 64 to 100°F (18 to 38°C) by vapors
from the blow-off tank; 30 to 40 percent of the total required
heat is thus provided by recovery. Next, direct steam injection
raises the temperature to 157°F (70°C) in the pasteurizer where
the sludge resides for at least 30 minutes. Finally the sludge
is transferred to the blow-off tanks, where it is cooled first to
113°F (45°C) at 1.45 pounds per square inch (10 kN/m2) and then
to 98°F (35°C) at 0.73 pounds per square inch (5 kN/m2) (31).
For sludge flows of 0.05 to 0.07 MGD (2 to 3 1/s), a single-stage
heat recuperation system is considered economical. In the
0.11 to 0.13 MGD (4.8 to 5.7 1/s) flow range a two-stage heat
recuperator is considered economical. For flows over 0.26 MGD
(11 1/s), a three-stage heat recuperation is considered
economically attractive.
7.6.1.2 Current Status
There is only one operating municipal sludge pasteurization
facility in the United States today, a heat conditioning system
converted for pasteurization. Pasteurization is often used
in Europe and is required in Germany and Switzerland before
application of sludge, to pasture lands during the spring-summer
growing season. • Based on European experience, heat pasteuriza-
tion is a proven technology, requiring skills such as boiler
operation and understanding of high temperature and pressure
processes. Pasteurization can be applied to either untreated
or digested sludge with minimal pretreatment. Digester gas,
available in many plants, is an ideal fuel and is usually
produced in sufficient quantities to disinfect locally produced
sludge. Potential disadvantages include odor problems and
the need for storage facilities following the process—where
bacterial pathogens may regrow if sludge is reinoculated.
7.6.1-.3 Design Criteria
A pasteurization system should be designed to provide a uniform
minimum temperature of 157°F (70°C) for at least 30 minutes.
Batch processing is necessary. to prevent short circuiting and
recontamination, especially by bacteria. In-line mixing of steam
and sludge should be considered as a possible aid to increase
heat transfer efficiency and assure uniform heating. In-line
mixing will also eliminate the need to mix the sludge while it is
held at the pasteurization temperature. The system should be
sized to handle peak flows or sludge storage should be used
to reduce peak flows. Sizing of storage capacity and the
pasteurization system will depend on the type of sludge treated,
the average sludge flow, and the end use of the sludge. If
digested sludge is to be pasteurized, the digesters may have
sufficient volume to hold sludge during minor mechanical
breakdowns or when inclement weather prevents an end use such as
land application-. If sludge is to be stored after treatment and
7-16
-------
prior to pasteurization, a minimum storage volume should be two
days average flow. Storage facilities must be equipped for odor
control or with aeration capacity to prevent septic conditions.
Storage capacity for pasteurized sludge should be adequate to
hold at least four days' amount of processed sludge at average
flow. Odor control must be provided, and pilot-scale testing may
be needed to determine the best odor control process design.
Sludge thickening prior to pasteurization may be cost-effective
for increasing overall energy efficiency, but the value of
thickening should be determined on a case-by-case basis.
Piping, pumps, valves, heat exchangers, flow meters, and other
mechanical equipment should, at a minimum, be comparable to those
for thermophilic digesters. The tanks for holding sludge during
pasteurization should be corrosion-resistant.
7.6.1.4 Instrumentation and Operational Considerations
Temperature monitoring at several points in each pasteurization
system is a minimum requirement. Flow metering devices,
boiler controls, emergency pressure relief valves, and level
sensors in tanks should also be considered (see Chapter 17,
Instrumentation).
Heat pasteurization has flexibility to respond to variable
solids concentrations and flow rates, provided there is enough
basic system capacity. Expansion of facilities with parallel
modules should work well; multiple modules also improve system
reliability.
7.6.1.5 Energy Impacts
Pasteurization requires both electricity for pumping and fuel for
heating the sludge. Energy requirements for pasteurization
processes, with and without heat recovery, have been estimated
for secondary activated sludge plants where either raw or
digested sludge is pasteurized (34). A combination of primary
and waste-activated sludge with 4,800 gallons of untreated sludge
per 1,000,000 gallons (4.8 1/m3 ) raw sewage or with 3,100 gallons
of digested sludge per 1,000,000 gallons (3.1 l/m-*) raw sewage,
with a solids content of five percent and a specific heat of one
Btu per °F (1900 J/°C) were assumed. The process allowed for
10 percent heat loss and a 100 to 125 pounds per square inch
(690 to 860 kN/m2) boiler with an 80 percent efficiency. Steam
injection heats the sludge to 157°F (70°C), where it is held for
45 minutes with steam reinjection to maintain the temperature.
The energy requirements for processes with a range of wastewater
flows are summarized on Figure 7-3.
7.6.1.6 Cost Information
The only sludge pasteurization process operating in the United
States was not initially designed for pasteurization. Thus no
actual cost data are available. Costs have been estimated for
7-17
-------
the processes discussed under "Energy Impacts" (34). It was
assumed that the processes would have parallel pasteurization
reactors and four-day storage volume for the pasteurized sludge.
The use (volume of throughput per given size) for the processes
increases with increasing system size.
1,000
9
8
7
6
5
4
100
"3
o
i_
-5*
as
8
G
D
u_
3 —
i TOO
S 3
8
FUEL
WITH HEAT RECOVERY
to
£ H
cc
b
UJ
_J
10
1
PLANT CAPACITY, MGD of wastewater (1 MGD = 0,044 m3/i)
FIGURE 7-3
ENERGY REQUIREMENTS FOR SLUDGE PASTEURIZATION SYSTEMS (34)
Cost estimates were made in June 1977 for construction materials,
labor, equipment, normal excavation, contractor overhead and
profit, operating and maintenance labor, materials and supplies,
and energy. Summary graphs for these estimates are given on
Figures 7-4 through 7-7.
7-1!
-------
_ra
"o
O
u
cc
UNTREATED SLUDGE
5 10 20 30 40 50 . 60 70 SO 90100
PLANT CAPACITY, MGD of wisiewater {1 MOD = 0,044 m3/s)
FIGURE 7-4
CONSTRUCTION COSTS FOR SLUDGE PASTEURIZATION
SYSTEMS WITHOUT HEAT RECOVERY (34)
These graphs were used to estimate unit pasteurization costs
for a 50-MGD (2.2-m^/s) secondary wastewater treatment plant.
Additional assumptions made were that yard piping for the system
would cost 15 percent of the total construction cost, electricity
would cost three cents per kilowatt hour, fuel would cost
$3.00 per million Btu's ($2.84/GJ), labor would cost $10.00 per
hour, and capital was amortized over 20 years at seven percent.
The resulting pasteurization cost was $15.00 per ton ($16.50/t)
of dry solids with heat recovery. A similar calculation was made
for a 10-MGD (0.44-m-Vs) secondary plant with no heat recovery,
a cost of $33.00 per ton ($36.40/t) of dry solids was estimated.
7-19
-------
UNTREATED SLUDGE
10 20 30 40 60 60 70 80 90100
PLANT CAPACITY, MGD of wastfwatef (1 MGO = 0.044 m3/s|
FIGURE 7-5
CONSTRUCTION COSTS FOR SLUDGE PASTEURIZATION
SYSTEMS WITH HEAT RECOVERY (34)
7.6.1.7 Design Example
To establish the equipment requirements and layout for a typical
pasteurization system, digested combined primary and waste-
activated sludge from a 50-MGD (2.2 m^/s) activated sludge plant
are to be pasteurized prior to reuse by direct injection. If the
sludge is produced at a rate of 2,000 pounds of solids per
million gallons (0.24 kg/m^), and 40 percent of the solids are
7-20
-------
destroyed during
2.4 percent solids. The sludge
per million gallons (4.8 1/m3).
the flow rate is 0 . 3-MGD " "
facility
rate is 0.42
(18.9 1/s).
digestion, the resulting digested
flow rate is about 4,800 gallons
For the 50-MGD (2.2 m3/s) plant,
(13.0 1/s). If the pasteurization
is run 24 hours per day, five
MGD (18.9 1/s) or about
sludge has
days per week, the flow
300 gallons per minute
12 -
WITH HEAT
RECOVERY
WITHOUT HEAT
RECOVERY
10 20 30 • 40 50 60 70 SO 90100
PLANT CAPACITY, MGD of wastewatw ?1 MGD = Q.Q44 m3/^
FIGURE 7-6
LABOR REQUIREMENTS FOR SLUDGE
PASTEURIZATION SYSTEMS (34)
7-21
-------
16 i-
UNTREATED SLUDGE
10 , 20 30 40 SO 60 70 80 90100
PLANT CAPACITY, MGD of wastewater (1 MGD = 0,044 m3/sS
FIGURE 7-7
MAINTENANCE MATERIAL COSTS FOR SLUDGE
PASTEURIZATION SYSTEMS (34)
To select the reactor size, assume that there are two parallel
units and each can be charged, held, and emptied, in 1.5 hours.
Determining the volume per reactor:
V =
NH
where:
S =
C ••
N '
total sludge volume per week, gallons;
cycle time, hours;
number of reactors/cycle;
7-22
-------
H = total operating hours.
For this example,
v- (2.1 x 106 gallons) (1.5 hr/cycle) ., , __ .. ,.n _, ,%
V (2 reactors/cycle)(120 hr) = 13'125 ^allons <49'7 m3>
Assume a 13,500 gallon (51 m3 ) storage tank will be used to
store this sludge. Set prepasteurization storage at 2.5 times
the average daily flow, or at one million gallons (3780 m3 ) .
Set post pasteurization storage at four times the average daily
flow or 1.7 million gallons (6350 m3) . Three heat exchangers
in series heat the digested sludge from 68°F to 131°F (20° to
55°C); the boiler supplies steam to raise the temperature to
157°F (70°C). The heat exchangers can be either sludge to sludge
or sludge to water to sludge. Sludge-to-sludge exchangers should
be carefully specified as they have a history of fouling.
The sludge pumps should be sized and piped either to fill or
empty a 13,500 gallon tank (51 m3) in 30 minutes, equivalent to
450 gallons per minute (28 1/s). At least three pumps are
needed; providing one pump on standby.
The required boiler capacity is calculated with the equation:
„ _ AT h W
where:
E = energy required in Btu per hour
AT = the temperature difference between sludge from the heat
exchanger and sludge in the reactor;
h = heat capacity of the sludge, Btu/lb°F;
W = wet sludge weight, lb;
t = time for heating;
e = boiler conversion efficiency.
If h is one Btu per lb °F (864 J/kg°C); e = 80 percent; T = 63°F
(35°C); W = 112,600 lb (51,200 kg); and t = 0.5 hr; then,
E = /;:o = 17,700,000 Btu/hr (3.9 GJ/hr)
( U . b ) ( I) . o )
7-23
-------
An additional allowance of ten percent should be added to
maintain the reactor temperature for 30 minutes, giving a total
of 19.5 million Btu/hr (4.3 GJ/hr) or about 600 horsepower.
Figure 7-8 provides a schematic layout for the major process
components.
PASTEURIZATION
REACTORS
UNTREATED
OR
DIGESTED
15T°F
SLUDGEo
FROM
STORAGE
FEED
PUMPS
HE U HI ZED
.4
0 u
a <;
5"
^/X^X
S HEAT
' EXCHANGER
^
1
f
t
PASTi
iZf
SLUt
PUM
:UR-
D
)GE
PS
PAStEuftiZEO
SLUDGE
STORAGE
PREHEATED
SLUDGE 131°F (56°C1
u -r
FUEL
125 psi
(175°C
860 kN/nf*
WATER
SLUDGE
FOR
UTILIZATION
FIGURE 7-8
SYSTEM COMPONENT LAYOUT FOR SLUDGE
PASTEURIZATION WITH HEAT RECOVERY
7.6.2 Other Heat Processes
The reduction of pathogenic organisms in sludge may be an added
benefit of other sludge treatment processes. In this chapter
heat processes are subdivided into heat-conditioning, heat-
drying, high temperature combustion, and composting.
7-24
-------
7.6.2.1 Heat-Conditioning
Heat-conditioning includes processes where wet wastewater sludge
is pressurized with or without oxygen and the temperature is
raised to 350° to 400°F (177° to 240 °C) and held for 15 to
40 minutes. These processes destroy all pathogens in sludge, and
are discussed in detail in Chapter 8.
7.6.2.2 Heat-Drying
Heat-drying is generally done with a flash drier or a rotary
kiln. Limited data from analyses on Milwaukee, Wisconsin's dried
sludge, Milorganite, produced with a direct-indirect rotary
counterflow kiln type dryer, indicates it is bacteriologically
sterile (13). Data on samples of flash-dried sludge taken
in Houston, Chicago, Baltimore, and Galveston, showed no coliform
bacteria in the Houston sludge and no greater than 17 MPN/gm
dry sludge in the other sludges. Total non-confirming lactose
fermenters (spore formers) ranged from 14 MPN to 240,000 MPN per
gm (35). No tests were made for viruses or parasites; other
pathogens may also survive if some bacteria do.
Data for the Carver-Greenfield process gathered during testing by
LA/OMA showed a seven order of magnitude reduction for total and
fecal coliform, to a detectable level of less than one organism
per gram (36). Fecal streptococci were reduced six orders of
magnitude to two MPN per gram and Salmonella from 50,000 MPN per
gram to less than 0.2 MPN per gram. Ascaris ova were reduced to
less than 0.2 ova per gram.
7.6.2.3 High Temperature Processes
High temperature processes include incineration, pyrolysis, or a
combination thereof (starved-air combustion). These processes
raise the sludge temperature above 930°F (500°C) destroying
the physical structure of all sludge pathogens and effectively
sterilizing the sludge. The product of a high temperature
process is sterile unless shortcircuiting occurs within the
process.
7.6.2.4 Composting
Composting is considered here as a heat process because a major
aim of sludge composting operations is to produce a pathogen-free
compost by achieving and holding a thermophilic temperature.
Available data indicate that a well-run composting process
greatly reduces the numbers of primary pathogens (37-40).
However, windrow or aerated pile operations have not achieved a
sufficiently uniform internal temperature to inactivate all
pathogens. Adverse environmental conditions, particularly heavy
rains, can significantly lower composting temperatures. An
7-25
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additional problem with composting is the potential regrowth of
bacteria. This is particularly true with windrows where mixing
moves material from the outside of the mound to the center (40).
However, storage of compost for several months following windrow
or pile composting helps to further reduce pathogen levels.
Secondary pathogens, particularly heat-resistant fungi such
as Aspergillus, have been found to propagate rapidly during the
composting of wastewater sludges. Aspergillus apparently will
die out during storage of several months or more (22).
Enclosed mechanical composting systems may achieve sufficient
temperature, 157°F (70°C) or greater, for an adequate time; more
research can verify the efficiency of mechanical systems for
pathogen reduction.
7.7 Pathogen Reduction With High-Energy Radiation
The use of high-energy radiation for wastewater sludge
disinfection has been considered for over 25 years. Two energy
sources, beta and gamma rays, offer the best potential system
performance. Beta rays are high-energy electrons, generated with
an accelerator for use in disinfection, while gamma rays are
high-energy photons emitted from atomic nuclei. Both types of
rays induce secondary ionizations in sludge as they penetrate.
Secondary ionizations directly inactivate pathogens and produce
oxidizing and reducing compounds that in turn attack pathogens.
7.7.1 Reduction of Pathogens in Sludge With
Electron Irradiation
High-energy electrons, projected through wastewater sludge by
an appropriate generator, are being pilot tested as a means for
inactivating or destroying pathogens in sludge at the Deer Island
Wastewater Treatment Plant in Boston, Massachusetts (41). The
electrons produce both biological and chemical effects as they
scatter off material in the sludge. Direct ionization by the
electrons may damage molecules of the pathogen, particularly the
DNA in bacteria cell nuclei and the DNA or RNA of the viruses.
The electrons also cause indirect action by producing e^q
(hydrated electrons) and H and OH free radicals that react with
oxygen and other molecules to produce ozone and hydroperoxides.
These compounds then attack organics in the sludge--including
pathogens--promoting oxidation, reduction, dissociation, and
other forms of degradation.
The pathogen-reducing power of the electron beam (e-beam) depends
on the number and the energy of electrons impacting the sludge.
E-beam dose rates are measured in rads; one rad is equal to the
absorption of 4.3 x 10~6 Btu per pound (100 ergs/gin) of material.
Since the radiation distributes energy throughout the volume
of material regardless of the material penetrated, the degree
7-26
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of disinfection with an irradiation system is essentially
independent of the sludge solids concentration within the maximum
effective penetration depth of the radiation. The penetrating
power of electrons is limited, with a maximum range of 0.2 inches
(0.5 cm) in water or sludge slurries, when the electrons have
been accelerated by a potential of one million volts (MeV).
For e-beam disinfection to be effective, some minimum dosage
must be achieved for all sludge being treated. This effect
is attained by dosing above the average dosage desired for
disinfection. One method used to ensure adequate disinfection
is to limit the thickness of the sludge layer radiated so that
ionization intensity of electrons exiting the treated sludge is
about 50 percent of the maximum initial intensity. For the
0.85 MeV electrons used in the existing facility, this constraint
limits sludge layer thickness to about 0.08 inches (0.2 cm).
Accelerated electrons can induce radioactivity in substances
which they impact. However, the electron energy levels for
sludge irradiation, up to about 2 MeV, are well below the 10 MeV
needed to induce significant radioactivity with electrons.
7.7.1.1 Process Description
Disinfection with an e-beam has been proposed for use on both
untreated and digested sludges. The major system components of
the Deer Island facility shown on Figure 7-9 include the sludge
screener, sludge grinder, sludge feed pump, sludge spreader,
electron beam power supply, electron accelerator, electron beam
scanner, and sludge removal pump. A concrete vault houses the
electron beam, providing shielding for the workers from stray
irradiation, especially x-rays. X-rays are produced by the
interaction of the electrons with the nucleus of atoms in
the mechanical equipment and in the sludge. The pumps must be
progressive cavity or similar types to assure smooth sludge
feed. Screening and grinding of sludge prior to irradiation is
necessary to assure that a uniform layer of sludge is passed
under the e-beam.
At Deer Island, sludge from the feed pump discharges into the
constant head tank (see Figure 7-10), which is equipped with an
underflow discharge weir. Sludge is discharged under the weir
in a thin stream and then flows down an inclined ramp. At the
bottom of the ramp, it moves by free-fall into the receiving
tank.
The electrons are first accelerated. They leave the accelerator
in a continuous beam that is scanned back and forth at 400 times
per second across the sludge as it falls free in a thin film from
the end of the inclined ramp. The dosage is varied by adjusting
the height of the underflow weir and hence the sludge flow rate.
7-27
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HIGH VOLTAGE CABLE
ELECTRON
ACCELERATOR
FEED
SLUDGE
SLUDGE SLUDGE
SCREEN GRINDER
ELECTRON
BEAM
SCANNER
SLUDGE
FEED
PUMP
CONCRETE
SHIELDING
T
SLUDGE
SPREADER
SLUDGE
REMOVAL
PUMP
FIGURE 7-9
EQUIPMENT LAYOUT FOR ELECTRON BEAM FACILITY (41)
7.7.1.2 Status
E-beam sludge irradiation must be considered a developing
technology. The Deer Island irradiation facility, as of
August 1979, is the only e-beam facility now operated in the
United States for sludge disinfection. This pilot project is
designed to treat 0.1 MGD (4 1/s) sludge at up to eight percent
solids with a dosage of 400,000 rads. According to Shah, the
facility has been operated about 700 hours since it was brought
on line in 1976, with the longest continuous on-line time being
eight hours (42).
7.7.1.3 Design Considerations
Design criteria for an e-beam sludge facility are difficult to
establish because operational data are available from only one
pilot facility. However, the work at Deer Island provides good
baseline information. A minimum level of electron irradiation
should be 400,000 rads, which can best be supplied with a one to
two MeV electron accelerator. This energy level provides good
7-28
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penetration for 0.2-inch (0.5-cm). thick sludge layers, making the
achievement of a uniform sludge 'layer-'-less important than with
lower energy electrons. However, screening and grinding of
sludge before disinfection are still necessary to ensure uniform
spreading by this feed mechanism. The high-energy electrons,
combined with a short spacing of about 2.75-inches (7 cm) between
the scanner window and the sludge film, ensure efficient energy
transfer in the system.
INPUT
(UNTREATED Oft
DIGESTED SLUDGE)
ELECTRON
BEAM
CONSTANT
HEAD
TANK
UNDERFLOW
WEIR
INCLINED
FEED RAMP
ELECTRON BEAM
SCANNER
HIGH ENERGY
DISINFECTION
ZONE
SLUDGE
RECEIVING
TANK
.OUTPUT
(DISINFECTED
SLUDGES
FIGURE 7-10
ELECTRON BEAM SCANNER AND SLUDGE SPREADER
Only digested sludge has been irradiated at Deer Island.
Nonstabilized sludge disinfection by e-beam irradiation still
requires pilot-scale testing before any design is considered.
7-29
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Owing to the limited penetrating power of high energy electrons,
this method of treatment is probably only feasible for liquid
sludge. Piping pumps, valves, and flow meters should be
specified as equal to those used for anaerobic sludge digestion
systems.
systems.
7.7.1.4 Instrumentation and Operational
Considerations
Instrumentation needs for an e-beam facility should include
flow measurement of and temperature probes in the sludge
streams entering and leaving the irradiator. Alarms as well as
monitoring should be used to indicate variation in sludge flow
and high or low radiation doses.
Sludge disinfection by e-beam irradiation has large inherent
flexibility. The radiation source (the e-beam) can be switched
on and off as easily as an electric motor. The unit can be run
as needed, up to its maximum throughput capacity. Electron
accelerators have a proven record for reliability over at least
20 years in industrial applications and should prove dependable
in wastewater treatment applications. According to Haas, the
reliability of the electron beam generator and associated
electronics presently used for medical and industrial applica-
tions is comparable to that for the microwave radar systems at
major airports (43). Accelerators for sludge disinfection would
use the same basic components and would have similar reliability.
Other system components--pumps, screens, and grinders--are all in
common use in waste treatment plants. Cooling air for the
scanner must be provided at several hundred cfm (about 10 m^/s).
This constant introduction of cooling air leads to the generation
of ozone in the shielding vault around the accelerator. If the
ozone were vented into the plant or into the atmosphere, some air
pollution would result. At Deer.Island, this problem is avoided
by venting the cooling air through the sludge, where the ozone is
consumed by chemical reduction. These reactions provide a small
amount of additional disinfection and COD reduction.
7.7.1.5 Energy Impacts
Energy use for e-beam facilities has been estimated for the
equipment used at Deer Island. A facility with a 50-kW (50-kJ/s)
beam would require about 100 kW (lOOkJ/s) of total electrical
power including 25 kW (25 kj/s) for screening, grinding, and
pumping, 10 kW for (10 kJ/s) window cooling, and 12 kW (12 kJ/s)
for electrical conversion losses. Energy requirements for
0.1 MGD (4 1/m3) are 6 kWhr per ton (24 MJ/t) of wet sludge
at five percent solids or 120 kWhr per dry ton (480 MJ/t) (41).
7.7.1.6 Performance Data
Data for e-beam disinfection of both untreated and digested
sludges are available as a result of laboratory testing done
prior to the operation of the Deer Island facility. For
7-30
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untreated primary sludge, a dose of 400 kilorads (krads) with
3 MeV electrons reduced total bacteria count by five logs, total
coliform by more than six logs, below detectable limits, and
total Salmonella by over four logs, also below detectable
limits. Fecal streptococci were only reduced by two logs with
data indicating that some fecal streptococci are sensitive to
radiation while others are resistant.
For samples of anaerobically digested sludge irradiated at Deer
Island with 0.85 MeV electrons, total bacteria were reduced by
four logs at a dose of 280 krads, total coliform by five to six
logs at a dose of 150 to 200 krads; a dose of 400 krads reduced
fecal streptococci by 3.6 logs.
Virus inactivation has also been measured. A dose of 400 krads
will apparently reduce the total virus measured as plaque
forming units (PFU) by one to two logs. Laboratory batch
irradiation of five enteric viruses showed about two logs
reduction at a dose of 400 krads; Coxsackie virus were most
resistant while Adeno virus were least resistant. These results
correlate directly with virus size. Larger viruses are larger
targets and hence more susceptible to electron "hits" (41).
Data for parasite reduction are scarce but 400 krads will
apparently destroy all Ascaris ova (41). Comparing these perfor-
mance data with information from Table 7-5 on the quantity of
pathogens in sludge indicate that a dose of 400 krads may be
adequate to disinfect anaerobically digested sludge, but raw
sludge or aerobic sludge may require higher doses.
1.1.I.I Product Production and Properties
Odor problems are dramatically lower for irradiated sludge as
compared with pasteurized sludge (41). Irradiation of digested
sludge with an e-beam may also improve sludge dewaterability
and destroy some synthetic organic chemicals, as well as reduce
pathogen levels. Irradiation has reduced specific resistance of
sludge by up to 50 percent at a dose of 400 krads (41). Since
specific resistance is normally measured on a log scale, a
50 percent reduction may indicate minimal improvement in sludge
dewaterability.
7.7.1.8 Cost Information
The only cost estimates available on e-beam sludge treatment
process result from work done at Deer Island. The hypothetical
facility used for the cost estimate had the following
characteristics:
• Electron beam power of 75 kW (75 kJ/s).
• Accelerator voltage of 1.5 MeV.
7-31
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© Disinfection dose of 400 krad.
• Yearly throughput of 50 million gallons (190,000 m3) with
process operating 300 days per year. This throughput is
equivalent to the raw sludge from a 25-MGD (I.l-ra3/s)
activated sludge plant or the digested sludge from a
35-MGD (1.5-mVs) activated sludge plant.
The total capital cost was $600,000. The cost included the
following: accelerator component with scanner--$350,000;
automatic controls--$30,000; sludge handling equipment—$100,000;
and building construction and facility installation--$120,000.
Annual costs were as follows: capital (20 years at 10 percent)
$30,000; depreciation--$30,000 ; operation and ma intenance--
$40,000; electric power at three cents per kWhr (.83 cents per
mJ) $28,000; and water—$2,000. This cost estimate was carried
out in Boston in late 1977. At that time the ENR construction
cost index was about 2,650. The net cost was $2.53 per
1,000 gallons ($0.67/m3) of liquid sludge treated.
The energy requirements (fuel and electricity) for an irradiation
system are estimated to be 90 to 98 percent less than those for
heat pasteurization.
7.7.2 Disinfection With Gamma Irradiation
Gamma irradiation produces effects similar to those from an
electron beam. However, gamma rays differ from electrons in two
major ways. First, they are very penetrating; a layer of water
25 inches (64 cm) thick is required to stop 90 percent of the
rays from a cobalt-60 (Co-60) source; in comparison, a 1-MeV
electron can only penetrate about 0.4 inches (1 cm) of water.
Second, gamma rays result from decay of a radioactive isotope.
Decay from a source is continuous and uncontrolled; it cannot be
turned off and on. The energy level (or levels) of the typical
gamma ray from a given radioactive isotope are also relatively
constant. Once an isotope is chosen for use as a source, the
applied energy can only be varied with exposure time.
Two isotopes, Cs-137 and Co-60, have been considered as "fuel"
sources for sludge irradiators. Cs-137 has a half life of
30 years and emits a 0.660 MeV gamma ray. In the late 1970's,
it was available in the United States as a by-product from the
processing of nuclear weapons wastes. If the United States
establishes a nuclear reactor spent-fuel rod reprocessing
program, it would also be available at a rate of about 2 pounds
per ton (1 kg/t) of fuel. Co-60 has a half life of five years
and emits two gamma rays with an average energy of 1.2 MeV. It
is made by bombarding normal cobalt metal, which is stable cobalt
isotope 59, with neutrons.
7-32
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7.7.2.1 Process Description
Two general types of gamma systems have been proposed for
wastewater sludge disinfection. The first is a batch-type system
for liquid sludge, where the sludge is circulated in a closed
vessel surrounding the gamma ray source. Dosage is regulated by
detention and source strength. The second system is for dried or
composted sludge. A special hopper conveyor is used to carry the
material for irradiation to the gamma ray source. Conveyor speed
is used to control the dosage.
7.7.2.2 Current Status - Liquid Sludge
The only gamma ray system in active operation is a liquid sludge
facility at Geiselbullach (near Munich) in West Germany.
Sludge has been treated in a demonstration-scale facility
since 1973. The design capacity is 0.04 MGD (2.0 1/s) but the
initial Co-60 charge only provided radiation to treat 0.008 MGD
(0.3 1/s). The basic flow scheme is shown on Figure 7-11.
Digested sludge is pumped or otherwise moved into the vault with
the Co source and circulated until the desired dosage is reached.
The chamber is then completely emptied and recharged.
Wizigmann and Wuersching (45) reported on the efficiency of the
Geiselbullach facility when the applied dose was 260 krads in
210 minutes. Bacterial tests were made on samples of processed
sludge and showed a two-log reduction in total bacterial count,
an Enterococcus reduction of two logs, and an Enterobacteriaceae
reduction of four to five logs. Two of 40 samples were positive
for Salmonella. Bacterial regrowth was measured in sludge-drying
beds where the sludge was placed after irradiation.
Plastic encapsulated bacteria samples were also irradiated in
the system to a dosage of 260 krads. Two of nine E. coli strains
were radiation-resistant and reduced five to six logs; three
strains were totally inactivated, and four strains were
reduced six to eight logs. Tests on ten strains of Salmonella in
170 samples showed four to seven log reduction, with 85 percent
of the samples over five logs and 61 percent over six logs.
Klebsiella were reduced six to eight logs. Gram-negative
species were more sensitive to gamma radiation than gram-positive
ones, and spores were more resistant than vegetative forms. A
comparison of the disinfection results of the real sludge samples
and the plastic encapsulated cultures indicates that circulation
in the sludge system apparently did not result in a very uniform
dose exposure.
Parasite ova (Ascaris suum) circulated through the system in
plastic capsules failed to develop during three weeks of
incubation. This observation period was not adequate, however,
to assure that long-term recovery would not take place.
7-33
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SLUDGE
INLET
VENT
GROUND
LEVEL
COBALT
RODS
CONCRETE
SHIELDING
SLUDGE
OUTLET
FIGURE 7-11
SCHEMATIC REPRESENTATION OF COBALT-60
IRRADIATION FACILITY AT GE1SELBULLACH, WEST GERMANY (44)
According to the latest available reports, land spreading of the
sludges treated at GeiseIbullach has been well received by
local farmers and the general public. No radiation hazards
have resulted and the treated sludges satisfy disinfection
requirements. The competing system in Germany, heat pasteuriza-
tion, requires more energy and produces an odorous product that
is more difficult to handle.
7.7.2.3 Current Status - Dried or Composted Sludge
A dry sludge irradiation system using a gamma source is being
developed by Sandia Laboratories in Albuquerque, New Mexico. The
eight-ton-per-day (7.2 t/day) demonstration facility, containing
about one million Curie of Cs-137, underwent final testing and
start-up in June 1979. The facility will be used to irradiate
bagged composted sludge for agricultural experiments and bagged
dried raw primary sludge for testing as a cattle-feed supplement.
7-34
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Owing to the high cost of Co-60, the overall viability of any
sludge irradiation facility in the United States depends on
Cs-137 supplies. Cs-137 will be available in quantity only if
the political and technical difficulties associated with power
plant fuel rod reprocessing can be resolved. About 200 mega-
curies of Cs-137 could be available from processing wastes
from weapons manufacture and could be used for further testing.
7.7.2.4 Design Criteria
The design criteria for gamma irradiation facilities depend on
the type of wastewater sludge treated. Current literature
discussions suggest a dose of 400 krads but this level does not
ensure complete virus removal (41). The dose level should
probably be varied in relation' to other treatments the sludge
receives. A composted, bagged product with an 80 percent solids
content needs a lower dose than a mixture of raw primary and
waste-activated sludge because the dried product already has a
reduced pathogen' level owing to the drying process. Data from
the demonstration facility at Sandia Laboratory for design of a
dry facility should be available by late 1979. For a liquid
sludge facility, data on dose-response and pathogen levels
(Table 7-5 and Section 7.7.2.2) can be combined with information
from Geiselbullach to set the required radiation doses. The
storage capacity for both untreated and irradiated sludge should
be equal to that for a pasteurization facility of similar size
(see Section 7.6.1.7).
When a dry system radiation source is not in use, it should be
shielded in a steel-lined concrete vault. 'The vault should be
designed to be flooded with water during loading and unloading
of the radiation source, to shield workers from radiation.
Provision must be made for pool water treatment in the event that
the radiation source leaks. Cooling air is circulated around the
source both during system operation and down times. This air
must be filtered to prevent a radioactive air release. Since the
dried sludge is a flammable material, there must be smoke and/or
heat detection and a fire suppression system. For a liquid
storage system the treatment vessel serves as a radiation source
storage vault.
7.7.2.5 Instrumentation and Operational
Considerations
Instrumentation should include radiation detectors and flow
metering for the wet sludge system. When either facility is
operating, arrangements must be made for periodic radiation
safety inspection. The disinfection effectiveness should also
be tested by periodic sampling of the sludge before and after
disinfection.
7-35
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7.7.2.6 Energy Impacts
In May 1977, Ahlstrora and McGuire (46) projected annual energy
requirements for both wet and dry gamma irradiation facilities,
using a dose rate of 1,000 krads. Their results are summarized
on Figures 7-12 and 7-13. For a 0.1-MGD (4-1/s) facility
treating sludge with five percent solids, 300 days per year, the
unit energy use is about 5.2 kWhr per 1,000 gallons (5 MJ/m3) or
25 kWhr per ton (100 MJ/ton) dry solids. For a plant treating
35 tons per day (32 t/day) at 60 percent solids, 300 days per
year (equivalent to the solids from the previous example), the
energy use is 5.6 kWhr per ton dry (22 MJ/t) solids, almost
80 percent less than the facility treating five percent solids.
These energy uses should be compared to 120 kWhr per dry ton
(450 MJ/t) for an e-beam system.
1,080
fl
8
1
100
I
;
te
.D-"
_L
10
e. 7 a
8100
6739
1,000
SLUDGE TREATMENT CAPACITY, 0,001 MGO (4,4 x ItT5 m3/s)
FIGURE 7-12
GAMMA RADIATION TREATMENT OF LIQUID SLUDGE
POWER REQUIREMENTS (46)
7-36
-------
100
9
•£• ' 8
^ , 7
I
4 -
a
LU
cc
01
LU
I
Q,
Z
Q
10
2 3 4 5 S 7 8 9
10 100 200
SLUDGE TREATMENT CAPACITY, dry tons/day (0,907 tonne/day)
FIGURE 7-13
RADIATION TREATMENT OF DEWATERED
SLUDGE - POWER REQUIREMENTS (46)
It is important to note that the liquid system would require a
much larger Cs-137 charge since it would be treating almost
12 times the volume of material at the same dose level. However,
the rod configuration for a dry facility would be much less
efficient in terms of radiation transfer than a liquid one.
7.7.2.7 Performance Data
In June 1979 no performance data for the Sandia facility were yet
available. Data for the Geiselbullach facility are summarized in
Section 7.7.2.2.
7-37
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7.7.2.8 Cost Information
Cost estimates for both liquid and dry facilities were developed
together with the energy data of Section 7.7.2.6. The liquid
facility included the following components:
• Insulated concrete building with 25-foot (7.6-m) ceiling.
• Equalization sludge storage tank.
• Emergency water dump tank (for source shielding water).
• Irradiating capsules (radiation source).
• Steel-lined source handling pool.
• Deionizer.
• Data aquisition and control system.
• Oxygen injection facility.
• Pumps, piping, and flow meters.
• Radiation alarm.
• Fire suppression system.
A capital cost graph for the wet facility is given on
Figure 7-14; the estimates were made in May 1977. Graphs for
labor hours per year and operations and maintenance materials and
supplies are given on Figure 7-15 and 7-16, respectively. The
additional operating cost is $2.00 per 1000 gallons ($0.53/m3)
for the Cs-137 (the irradiator).
The dry system uses a bucket conveyor to move the sludge past the
radiation source (see Figure 7-17). This dry system would
include the following:
• Loading and unloading conveyors.
• Concrete shielding.
• Source-handling pool.
• Holder for the Cs-137 capsules.
• Holder moving mechanism.
• Steel building.
• Pumps.
• Ventilators.
7-38
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Filters.
Hoists.
Radiation alarm system.
Pool water testing tank.
Fire suppression system.
10,000
8
6
7
6
5
4
o
1,000
fl
i
?
%
5
100
10
_L__J_J
I
5676
100
6 6739
1,000
SLUDGE TREATMENT CAPACITY, 0,001 MGD 14.4 x 10"? m3/i|
FIGURE 7-1U
GAMMA RADIATION TREATMENT OF LIQUID SLUDGE -
CAPITAL COSTS (46)
The capital costs for the dry system are summarized on
Figure 7-18; these costs were also calculated in May 1977.
Figure 7-19 and 7-20 present labor hours, materials ,and
operations and maintenance supplies, respectively. The Cs-137
source is estimated to cost $1.55 per ton ($1.70/t) for a 10-ton-
per-day (9.1-t/d) capacity facility and $1.22 per ton ($1.35/t)
for facilities of 50 ton per day (45 t/d) and larger.
7-39
-------
10,000
a
8
7
6
ID
flC
O
(0
z
z
1,000
'O"
J_
J I
_L
-LJ.
10
3 45878 S 2 3 4667SS
100 1,000
SLUDGE TREATMENT CAPACITY, 0.001 MGO (4,4 x 1C"5 m3/sj
FIGURE 7-15
GAMMA RADIATION TREATMENT OF LIQUID
SLUDGE LABOR REQUIREMENTS (46)
If labor plus overhead is $20.00 per hour, power is three cents
per kWhr, ($0.33/GJ) and capital is amortized over 20 years at
8 percent, the cost for a 0.1-MGD (4-1/s) liquid system is
$38.50 per ton ($42.40/t) dry solids. A dry system costs
$24.00 per ton ($26.50/t) dry solids. Both these costs are
considerably higher than those for e-beam irradiation and similar
to those for heat pasteurization. -
7-40
-------
12
£
i
100
9
B
7
6
5
8
2
10
9
8
7
8
2 —
10
_L
4 S 6 7 8 9
100
2 346
SLUDGE TREATMENT CAPACITY, 0.001 MGD (4.4 * 1(T5 m3/s)
FIGURE 7-1S
GAMMA RADIATION TREATMENT OF LIQUID SLUDGE
MAINTENANCE MATERIAL SUPPLIES COSTS (46)
7 s
iroeo
FIGURE 7-17
GAMMA RADIATION TREATMENT FACILITY FOR HANDLING
25 TONS PER DAY OR MORE OF DEWATERED SLUDGE
7-41
-------
10,000
i
8
7
s
5
j!
O
<
_l
<
7
6
5
4
100
O'
I
I
2 3 456780
10 100 200
SLUDGE TREATMENT CAPACITY, ton/day (0.907 tonne/day)
FIGURE 7-18
GAMMA RADIATION TREATMENT OF DEWATERED SLUDGE
CAPITAL COST (46)
7-42
-------
fc*
J5
"5
-o
£
te
o
o
_l
t
_
o
9
8
7
6
5
4
3
2
1,OOO
9
8
7
:<
4
3
2
100
— .
—
—
„,,
—
—
_,
-•*
O^^
^**
^ ^ **
?*"
„
—
i i i i i i i i I
2 3 4S6789
10 100 200
SLUDGE TREATMENT CAPACITY, ton/day (0.907 tonne/day)
FIGURE 7-19
GAMMA RADIATION TREATMENT OF DEWATERED SLUDGE -
LABOR REQUIREMENTS (46)
7-43
-------
1,000
9
i
7
6
O
o
100
_ -°-
2 3 45fl7S9
10 100 200
SLUDGE TREATMENT CAPACITY, ton/day (0.907 tonne/day)
FIGURE 7-20
GAMMA RADIATION TREATMENT OF DEWATERED
MAINTENANCE MATERIALS AND SUPPLIES COST (46)
7.8 References
1.
2.
3.
Branden, J
Proceed ings
R. "Parasites in Soil/Sludge Systems."
of Fifth National Conference on Acceptable
Sludge Disposal Techniques, Orlando, Florida, January 31 to
February 2, 1978.
Maryland 20852, p.
Information
130.
Transfer, Inc. Rockville,
Oliver, W. M. "The Life and Times of Aspergillus Fumigatus,"
Compost Science/Land Utilization. Vol. 20, No. 2, March/
April 1979.
U.S. Public Health Service. Enteric and Neurotropic Viral
Diseases Surveillance, 1971-1975. Center for Disease
Control,Atlanta,Georgia 30333. Issued January 1977.
7-44
-------
4. U.S. Public Health Service. "Shigella Surveillance, Annual
Summary 1976." Center for Disease Control, Atlanta, Georgia
30333. Issued October 1977.
5. U.S. Public Health Service. Salmonella Surveillance,
Annual Summary 1977. Center for Disease Control, Atlanta,
Georgia 30333. Issued March 1979.
6. U.S. Public Health Service. Intestinal Parasite
Surveillance, Annual Summary 1976. Center for Disease
Control, Atlanta, Georgia 30333. Issued August 1977.
7. U.S. Public Health Service. " Intestinal Parasite
Surveillance, Annual Summary 1977 ." Center for Disease
Control, Atlanta, Georgia 30333, Issued September 1978.
8. Sagik, B.P. "Survival of Pathogens in Soils." Proceedings
of Williamsburg Conference on Management of Wastewater
R_e SJL d_u a. l._s _, Williamsburg, Virginia, November 13-14, 1975 .
OTsT Science Foundation^^i"hTngTon DTc~. 2"OlT50lRANN-AEN
74-08082, p. 30.
9. Metcalf, T.G. "Role of Viruses in Management of Environ-
mental Risks." Proceedings of Williamsburg Conference on
^anagement of Wastewater Residuals, Williamsburg, Virginia,
N'ovember" 1975. U.S. National Science Foundation, Washington
D.C. 20550.RANN-AEN 74-08082 p. 53.
10. Moore, B.F., B.P. Sagik, and C.A. Sorber. "An Assessment of
Potential Health Risks Associated with Land Disposal of
Residual Sludges." Proceedings of Third National Conference
on Sludge Management, Disposal and Utilization, Miami Beach,
Florida. December 14-16, 1976. Information Transfer, Trie.
Rockville, Maryland 20852. p. 108.
11. Stern, G., and J.B. Farrell, "Sludge Disinfection
Techniques." Proceedings of National Conference on Compost-
ing of Municipal Residues and Sludges. Washington, D.C.
August 1977 . Information Transfer, Inc., Rockville,
Maryland 20852. p. 142.
12. Fenger, B., 0. Krogh, K. Krongaard, and E. Lund. "A
Chemical, Bacteriological, and Virological Study of Two
Small Biological Treatment Plant. " Fifth Meeting of the
North Wesrt_ European Microbiological Group. Bergen, Norway
T97TT"
13. Farrell, J.B., and G. Stern. "Methods for Reducing the
Infection Hazard of Wastewater Sludge." Radiation for a
Clean Environment, Symposium Proceeding. International
Atomic Energy Agency, Vienna. 1975.
14. Lund, E. "Public Health Aspects of Wastewater Treatment."
Radiation for a Clean Environment, Symposium Proceeding.
International Atomic Energy Agency.Vienna. 1975.
7-45
-------
15. Hyde, H.C. "Utilization of Wastewater Sludge for Agricul-
tural Soil Enrichment." Journal Water Pollution Control
Federation. Vol. 48, p. 77. 1976 . "
16. Hays, B.D. "Potential for Parasitic Disease Transmission
With Land Application of Sewage Plant Effluents and
Sludges." Water Research. Vol. 11, p. 583. Pergamon
Press. 1977.
17. Ward, R.L. and C.S. Ashley. "Inactivation of Poliovirus in
Digested Sludge." Applied and Environmental Microbiology.
Vol. 31, p. 921. 1976. " - —-
18. Ward, R.L. "Inactivation of Enteric Viruses in Wastewater
Sludge." Proceedings of Third National Conference on
S_ludge Management, Disposal, and Utilizatioru^Miami Beach,
Florida. December 14-16, 1976. InformatTon~~TrransfeFT" Inc.
Rockville, Maryland,20852. p. 138.
19. Ohara, G.T. and J.E. Colbaugh. "A Summary of Observations
in Thermophilic Digester Operations." Proceedings of the
191_5 National Conference on Municipal Sludge Management
and Disposal, Anaheim, California, August 18-20, 1975.
Information Transfer, Inc., Rockville, Maryland 20852.
p. 218.
20. Ornevich, R.F. and J.E. Smith, Jr. "Pathogen Reduction in
the Thermophilic Aerobic Digestion Process." Proceedings
of the 48th Wat_er Pollution Control Federation ^Conference,
Miami Beach, Florida. October 1975.
21. Theis, J.H., V. Bolton, and D.R. Storm. "Helminth Ova in
Soil and Sludge from Twelve U.S. Urban Areas." Journal
Water Pollati_on Control Federation. Vol. 50, p. 2485
1978.
22. USEPA. Agricultural Benefits and Environmental Changes
Resulting from the Use of Digested Sludges on Field Crops.
emInterim Report on a Solid Waste Demonstration Project.
Office of Research and Development, Cincinnati, Ohio 45268.
Report SW-30d. 1971.
23. Sacramento Area Consultants. Sewage Sludge Management
Program Final Report, Volume 6 Miscellaneous Use Determina-
tions . Sacramento Regional County Sanitation District,
SacFamento, California 95814. September 1979.
24. Farrell, J.B., J.E. Smith, S.W. Hathaway, and R.B. Dean.
"Lime Stabilization of Primary Sludge." Journal Water
Pollution Control Federajti_g_n. Vol. 46, p. 113. 1974.
25. Data from B.E.F. Unit of General Signal, West Warwick, Rhode
Island 02893. Personal Communication from D.L. Moffat.
January 2, 1979.
7-46
-------
26. Gerba, C.P., C. Wallis, and J.L. Melnick. "Fate of
Wastewater Bacteria and Viruses in Soil." Journal
Irrigation and Drainage Division. ASCE. p. 152. September
1975. ___
21. Parsons, D., C. Brownlee, D. Welter, A. Mauer, E. Haughton,
L. Kornder, and M. Selzak. Health Aspects of Sewage
Effluent Irrigation. Pollution Control Branch, British
Columbia Water Resource Service, Department of Lands,
Forests and Water Resources. Victoria, British Columbia.
1975.
28. Hess, E., and C. Breer. "Epidemiology of Salmonella and the
Fertilizing of Grassland with Sewage Sludge." Zentrabblatt
Bakterioliogie Parasitenkunde. Infektious Krankheitenand
Hygeine, Abeilung I. Orig. B-161 54. 1975.
29. Burge, W.D. "Bacteria and Viruses in Soil/Sludge Systems."
Proceedings of Fifth National Conference on Acceptable
Sludge Disposal Techniques, Orlando, Florida. January 31 to
February 2, 1978. Information Transfer, Inc. Rockville,
Maryland 20852. p. 125.
30. Roediger, H. "The Techniques of Sewage Sludge Pasteuriza-
tion; Actual Results Obtained in Existing Plarxts;
Economy." International Research Group on Refuse Disposal,
Informational Bulletin Number 21-3JL. p. 325. August 1974
to December 1976.
31. Stern, G. "Pasteurization of Liquid Digested Sludge."
Proceedings of National Conference on Municipal Sludge
Mangement, Pittsburgh. June 1974. Information Transfer
Inc., Rockville, Maryland 20852. p. 163.
32. Ward, R.L. and J.R. Brandon. "Effect on Heat on Pathogenic
Organisms Found In Wastewater Sludge. Proceedings of
National Conference on Composting of Muncipal Residue and
Sludges, Washington, D.C. August 23-25, 1977.Information
Transfer Inc., Rockville, Maryland 20852.p. 122.
33. Data from Zimpro Corporation, Rothchild, Wisconsin. Personal
communication from J.R. Nicholson. July 1979.
34. Wesner, G.M. "Sludge Pasteurization System Costs."
Prepared for Battelle Northwest, Richland, Washington 99352.
June 1977.
35. Connell, C.H. and M.T. Garrett, Jr. "Disinfection Effective-
ness of Heat Drying at Sludge." Journal Water Poljhjtion
Control Federation. Vol. 35, (10). 1963.
36. Regional Wastewater Solids Management Program. "Carver-
Greenfield Process Evaluation." Los Angeles/Orange County
Metropolitan Area (LA/OMA Project). Whittier, California
90607 December 1978.
7-47
-------
37. Burge, W.D., P.B. Marsh, and P.O. Millner. "Occurrence of
Pathogens and Microbial Allergens in the Sewage Sludge
Composting Environment." Proceedings of National Composting
Conference on Municipal Residue and Sludges, Washington D.C.
August 23-25, 1977. Information Transfer, Inc., Rockville,
Maryland 20852. p. 128.
38. Kawata, K., W.N. Cramer, and W.D. Burge. "Composting
Destroys Pathogens in Sewage Sludge." Water and Sewage
Works. Vol. 124, p. 76. 1977.
39. County Sanitation Districts of Los Angeles County.
"Pathogen Inactivation During Sludge Composting."
Unpublished Report to USEPA. Whittier, California 90607.
September 1977.
40. Cooper, R.C. and C. G. Colueke. "Survival of Enteric
Bacteria and Viruses in Compost and Its Leachate." Compost
Science/Land Utilization. March/April 1979. ~~
41. Massachusetts Institute of Technology. High Energy Electron-
Irradiation of Wastewater Liquid Residuals. Report to U.S.
National Science Foundation, Washington D.C., 20550.
December 31, 1977.
42. Massachusetts Institute of Technology. Boston, Massachusetts
02139. Personal communication from D.N. Shah. May 1979.
43. Siemens Medical Laboratory Inc. Walnut Creek, California
94596. Personal communicaiton from Werner Haas. January
1979.
44. Farrell, J. B. "High Energy Radiation in Sludge Treatment--
Status and Prospects." Proceedings of the National
Conference on Municipal Sludge Management and Disposal,
Anaheim, August 18-20, 1975. Information Transfer Inc.,
Rockville, Maryland 20852.
45. Wizigmann, I. and F. Wuersching. "Experience With a Pilot
Plant for the Irradiation of Sewage Sludge: Bacteriological
and Parasitological Studies After Irradiation." Radiation
for a Clean Environment, Symposium Proceedings. Inter-
national Atomic Energy Agency. Vienna. 1975.
46. Ahlstrom, S.B. and H.E. McGuire. An Economic Comparison of
Sludge Irradiation and Alternative Methods of Municipal
Sludge Treatment. Battelle Northwest Laboratories.
Rlchland, Washington 99352. PNL-2432/UC-23. November 1977.
7-48
-------
EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapters. Conditioning
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 8
CONDITIONING
8.1 Introduction
Conditioning involves the biological, chemical, and/or physical
treatment of a sludge stream to enhance water removal. In
addition, some conditioning processes also disinfect wastewater
solids, affect wastewater solids odors, alter the wastewater
solids physically, provide limited solids destruction or
addition, and improve solids recovery.
8.2 Selecting a Conditioning Process
Conditioning always has an effect on the efficiency of the
thickening or dewatering process that follows (1-3). Any
evaluation of the conditioning process must therefore take into
consideration capital, operating and maintenance costs for
the entire system and the impact of sidestreams on other plant
processes, the plant effluent, and resultant air quality.
Figure 8-1 shows how the evaluation would look in a quantified
flow diagram.
This type of analysis is necessary because conditioning processes
differ and, therefore, produce differing consequences for the
total system. For instance, Table 8-1 compares the effects
expected with no conditioning as opposed to those expected with
polyelectrolyte conditioning or thermal conditioning prior to
gravity thickening.
8.3 Factors Affecting Wastewater Solids Conditioning
8.3.1 General Wastewater Solids Properties
Wastewater solids are composed of screenings, grit, scum and
wastewater sludges. Wastewater sludges consist of primary,
secondary, and/or chemical solids with various organic and
inorganic particles of mixed sizes; the sludges each have various
internal water contents, degrees of hydration, and surface
chemistry. Sludge characteristics that affect thickening or
dewatering and for which conditioning is employed are particle
size and distribution, surface charge and degree of hydration,
and particle interaction.
8-1
-------
GASEOUS EFFLUENT
PLOW RAT!
AMMONIA
VOLATILE ORGANIC SUBSTANCES
SOLIDS FEED
FLOW RATE
SUSPENDED SOLIDS
FILTRATE OR
CONCENTRATE STREAM
FLOW HATE
iOD
t t t
SUSPENDED SOLIDS
REFRACTORY GHGAN1CS
THICKENED OR DEWATERED
SOLIDS
FLOW RAT!
SOLIDS OQNTiMT
FIGURE 8-1
BASIC PARAMETERS FOR EVALUATION OF A
SLUDGE CONDITIONING SYSTEM
TABLE 8-1
EFFECTS OF EITHER POLYELECTROLYTE CONDITIONING OR THERMAL CONDITIONING
VERSUS NO CONDITIONING ON A MIXTURE OF PRIMARY AND WASTE-ACTIVATED
SLUDGE PRIOR TO GRAVITY THICKENINGS
Polyelectrolyte
conditioning
Thermal conditioning
Conditioning mechanism
Effect on allowable solids
and hydraulic loading rates
Effects of supernatant stream
Effects on underflow
concentration
Effects on manpower
Flocculation
Will increase
Will improve
suspended
solids capture
May increase
Little to none
Alters surface properties and
ruptures biomass cells, releases
chemical - water bonds -
hydrolysis
Will significantly increase
Will cause significant increase in
color, suspended solids, soluble
BOD_, COD and NH,-N. Improve
suspended solids capture
Will significantly increase
Requires higher skilled operators
and strong preventive maintenance
program
It is assumed that the processes involved will work well.
8-2
-------
8.3.1.1 Particle Size and Distribution
Particle size is considered to be the single most important
factor influencing sludge dewaterability (4-7). As the average
particle size decreases, primarily from mixing or shear, the
surface/volume ratio increases exponentially (8). Increased
surface area means greater hydration, higher chemical demand, and
increased resistance to dewatering. Figure 8-2 shows relative
particle sizes of common sludge materials.
01 0.01 0.1 1.0 10 102 103 104
MICRON 0,001 in. mm. cm.
10
ANGSTROM
UNITS
£
15
1 O
ITl
1
||
I
I -
8
1 £
11
1
COLLOIDS
FINE
MEDIUM
COARSE
LARGE
CLAY
SILT
FINE
SAND
COARSE
SAND
GRAVEL
FIGURE 8-2
PARTICLE SIZE DISTRIBUTION OF COMMON MATERIALS
Raw municipal wastewaters contain significant quantities of
colloids and fines, which, because of their size (1 to 10
microns), will almost all escape capture in primary clarifiers
if coagulation and flocculation are not employed. Secondary
biological processes, in addition to removing dissolved BOD,
also partially remove these colloids and fines from wastewater.
Because of this, biological sludges, especially waste-activated
sludges are difficult to thicken or dewater and also have a high
demand for conditioning chemicals.
A primary objective of conaitioi
by combining the small particles into larger
cond itioning
is to increase particle size
aggregates.
8-3
-------
8.3.1.2 Surface Charge and Degree of Hydration
For the most part, sludge particles repel, rather than attract
one another. This repulsion, or stability, may be due to
hydration or electrical effects. With hydration, a layer or
layers of water bind to the particle surface, providing a buffer,
which prevents close particle approach. In addition, sewage
solids are negatively charged and thus tend to be mutually
repulsive. Conditioning is used to overcome the effects of
hydration and electrostatic repulsion.
Conditioning is a two-step process consisting of destabilization
and flocculation. In destabilization, the surface characteris-
tics of the particles are altered so that they will adhere to one
another. This desirable change is brought about through the use
of natural polymeric material excreted by the activated sludge
organism, synthetic organic polymer, or inorganic metal salts.
Flocculation is the process of providing contact opportunities,
by means of mild agitation, so the destabilized particles may
come together.
Destabilization either with synthetic organic polyelectrolyte or
with inorganic metal salt is readily available to the plant
operator, but it represents an increase in operating cost.
The degree to which natural flocculation is available is
difficult to predict since it is dependent on the type of
activated sludge or the attached-growth biological process that
has been designed into the plant.
8.3.1.3 Particle Interaction
Municipal wastewater sludges contain large numbers of colloidal
and agglomerated particles, which have large specific surface
areas. Initially these particles behave in a discrete manner
with little interaction. As the concentration of sludge is
increased by the separational process, interaction increases. As
shown on Figure 8-3 this flocculant behavior results in three
distinct zones for a gravity thickener.
Conditioning can increase the rate of settling in the sedimenta-
tion zone, and compression thickening in the thickening zone; it
can also improve the quality of the overflow. These improvements
result from the ability of the conditioner to neutralize or
overcome the surface charge, which in turn allows the particles
to adhere to one another, thus preserving the dimensional
integrity of the sludge matrix in the thickening zone.
8.3.2 Physical Factors
The amount of conditioning required for sludges is dependent on
the processing conditions to which the sludge has been subjected
and on the mechanics of the conditioning process available.
5-4
-------
OVERFLOW
INFLOW
ZONE OF CLEAR LIQUfD
INFLOW SOLIDS CONCENTRATION
• LOWEST CONCENTRATION AT WHICH FLOCCUtANT SUSPENSION IB IN
THE FOBM OF POftOUS MEDIUM
. IJNOERPL0W CGNClNTftATIDN FROM GRAVITY
FIGURE 8-3
TYPICAL CONCENTRATION PROFILE OF MUNICIPAL
WASTEWATER SLUDGE IN A CONTINUOUSLY OPERATING
GRAVITY THICKENER (12)
8.3.2.1 Effect of Processing Prior to Conditioning
Both the degree of hydration and fines content of a sludge stream
can be materially increased by exposure to shear, heat, or
storage. For example, pipeline transport of sludge to central
processing facilities, weekend storage of sludge prior to
mechanical dewatering, and storage of sludges for long periods
of time have been shown to increase the demand for conditioning
chemicals in all types of dewatering and should be accounted for
in the design of the dewatering facility (10-13).
8.3.2.2 Conditioner Application
The optimum sequence for adding conditioner is best determined
by trial and error, when two or more conditioners are used.
With ferric chloride and lime, the ferric chloride is normally
added first. In addition, it has been shown that deterioration
of the floe after conditioning (due to both time and high shear
mixing) can be a major determinant of chemical requirement (13).
When a combination of anionic and cationic polymer is needed,
anionic polymer is added first.
-------
In order to minimize floe shearing, mixing should provide just
enough energy to disperse the conditioner throughout the sludge.
In dewatering applications, consideration should be given to
providing individual conditioning for each dewatering unit, since
it is not always economical to provide one common conditioning
unit for several dewatering units. Problems can arise in
balancing the flow rates of the various streams when starting
up or shutting down individual units. The location of the
conditioning unit relative to each dewatering device requires
optimization.
Many types of conditioning units are available. Recent USEPA
publications (14,15) describe the more common designs, design
layouts, and operating problems. Additional information can be
obtained from thickening and dewatering equipment suppliers.
8.4 Inorganic Chemical Conditioning
8.4.1 Introduction
Inorganic chemical conditioning is associated principally with
mechanical sludge dewatering, and vacuum filtration is the
most common application. The chemicals normally used in the
conditioning of municipal wastewater sludges are lime and ferric
chloride, although ferrous sulfate has also been used.
Ferric chloride is added first. It hydroylzes in water, forming
positively charged soluble iron complexes which neutralize the
negatively charge sludge solids, thus causing them to aggregate.
Ferric chloride also reacts with the bicarbonate alkalinity in
the sludge to form hydroxides that act as flocculants. The
following equation shows the reaction of ferric chloride with
bicarbonate alkalinity:
2FeCl3 + 3Ca(HCC>3)2 - *• 2Fe(OH)3 + 3CaCl2 + 6CC>2
Hydrated lime is usually used in conjunction with ferric iron
salts. Although lime has some slight dehydration effects on
colloids, it is chosen for conditioning principally because it
provides pH control, odor reduction and disinfection. CaCC>3 ,
formed by the reaction of lime and bicarbonate, provides a
granular structure which increases sludge porosity and reduces
sludge compressibility.
8.4.2 Dosage Requirements
Iron salts are usually added at a dosage rate of 40 to 125 pounds
per ton (20 to 63 kg/t) of dry solids in the sludge feed,
whether or not lime is used. Lime dosage usually varies
8-6
-------
from 150 to 550 pounds per ton (75 to 277 kg/t) of dry sludge
solids fed. Table 8-2 lists typical ferric chloride and lime
dosages for various sludges.
TABLE 8-2
TYPICAL CONDITIONING DOSAGES OF FERRIC CHLORIDE
(FeCl ) AND LIMt (CaO) FOR MUNICIPAL WASTEWATER
SLUDGES3 (16)
Sludge type
Raw primary
Raw waste-activated sludge (WAS)-air
Raw (primary + trickling filter)
Raw (primary + WAS)
Raw (primary + WAS + septic)
Raw (primary + WAS + lime)
Elutriated anaerobically digested
primary
primary + WAS (air)
Thermal conditioned sludges
Anaerobically digested sludges
primary
primary + trickling filter
primary + WAS (air)
Vacuum filter
Recessed plate
pressure filters
FeCl-
CaO
FeCl.
CaO
40-80
120-200
40-80
50-120
50-80
30-50
50-80
60-120
none
60-100
80-120
60-120
160-200
0-320
180-240
180-320
240-300
none
0-100
0-150
none
200-260
250-350
300-420
80-120 220-280
140-200 400-500
none none
80-200 220-600
All values shown are for pounds of either FeCl or CaO per ton of dry solids pumped
to the dewatering unit.
1 Ib/ton =0.5 kg/t
Inorganic chemical conditioning increases sludge mass. A
designer should expect one pound of additional sludge for every
pound of lime and ferric chloride added (13). This increases
the amount of sludge for disposal and lowers the fuel value
for incineration. Nevertheless, the presence of lime can be
beneficial because of its sludge stabilization effects. The use
of polyvalent metal salts and lime offers advantages over other
methods, because the combination can better condition sludge
which has extreme variations in quality.
8.4.3 Availability
Ferric chloride, the most widely used polyvalent metal salt
conditioner, is available in dry or liquid form, with the liquid
form being the most common. In the past, most ferric chloride
has been made from scrap metal and chlorine, but during the
past decade, much larger quantities have been made available
through conversion of waste acids from large industrial pigment
producers. It is supplied as either a 30 or 40 percent by weight
solution.
8-7
-------
Liquid ferrous sulfate, a by-product of certain industrial
processes, is not generally available in large quantities. If
availability is not at issue and testing proves it capable of
conditioning the sludge, liquid ferrous sulfate can be used like
ferric chloride.
Lime is purchased in dry form. It is readily available and
comes in many forms. Pebble quicklime (CaO) and hydrated lime
(Ca(OH)2) are most often used for sludge conditioning.
8.4.4 Storage, Preparation, and Application Equipment
There have been numerous problems such as lime scaling and FeCl3
corrosion with in-plant storage, preparation, and application of
both lime and ferric chloride. Two excellent references deal
with lime problems and how to solve them (17,18). Information on
ferric chloride can be found in USEPA's Process Design Manual for
Suspended Solids Removal (15).
8.4.5 Design Example
A designer has calculated that the rotary drum, cloth belt,
vacuum filter that will be utilized at the plant, must be capable
of dewatering a maximum of 600 pounds (272 kg) per hour of
sludge. The sludge will be a mixture of 40 percent primary and
60 percent waste-activated sludge, which will be anaerobically
digested. The vacuum filter will operate seven hours per day,
five days per week.
To design for a margin of safety in the chemical feed equipment,
the designer has used the higher values shown in Table 8-2.
Chemical feeders should be capable of adding 120 pounds per ton
(60 kg/t) of FeCl3 and 420 pounds per ton (210 kg/t) of CaO.
Maximum daily amount of sludge to be dewatered is:
600 Ib sludge x 7_hr = 4^QO lb sludge per day (1^905 kg/day)
hr day
Maximum amount of FeCl3 required per day is
4-200
The FeCl3 is available at a 40 percent solution (4.72 pounds
per gallon (0.567 kg/1) of solution).
-------
252 Ib FeCl3 ± gallon of product
~day4.72 Ib FeClo =.53.4 gallons of solution per day
day 4.72 Ib FeClo
(202 I/day)
Maximum amount of CaO required per day is:
CaO per day ,400 Kg/day)
The pebble quicklime is available at 90 percent CaO:
882 Ib CaO Ib pebble quicklime _on lu . , . . _ . .
- - - - x - c — 0.9 Ib CaO - = 980 Ib pebble quicklime per day
(45 kg/day)
The amount of extra sludge produced due to chemical addition is
estimated at one pound (0.45 kg) for every pound of FeCl3 and
pebble quicklime added. Therefore, total maximum daily dry
solids to be disposed of are:
4,200 Ib sludge + 252 Ib FeCl3 + 980 Ib quicklime
which are equal to 5,432 pounds (2,464 kg) of solids. This is
the equivalent of 27,160 pounds (12,320 kg) of wet sludge at a
minimum of 20 percent solids.
8.4.6 Cost
8.4.6.1 Capital Cost
Figure 8-4 shows the relationship between construction costs
of ferric chloride storage and feed facilities and installed
capacity. For example, if a designer needed to feed 100 pounds
(45.4 kg) per hour of ferric chloride the estimated cost would be
$330,000. Since cost are given in June 1975 dollars, the cost
must be adjusted to the proper time period. Costs for Figure 8-4
are estimated on the basis of liquid ferric chloride use.
Chemical feed equipment was sized for a peak feed rate of twice
the average. At least 15 days of storage was provided at the
average feed rate. Piping and buildings provided to house the
feeding equipment are included.
8-9
-------
in
r-.
o>
0)
c
3
J5
"5
co~
CO
o
o
O
O
D
DC
CO
2
O
O
1,000,000
9
8
7
6
5
100,000
9
8
7
6
5
4
3
2 -
10,000
UJLLLL
10
3 4 5 6 7 89100
3 456789 1,000
L_U_U_I_U
1 3 456789 10,000
INSTALLED CAPACITY, pounds Ferric Chloride Fed/Hour (1 Ib = 0.454 kg)
FIGURE 8-4
CAPITAL COST OF FERRIC CHLORIDE STORAGE AND
FEEDING FACILITIES (22)
Figure 8-5 gives construction costs of lime storage and feeding
facilities as a function of installed capacity. Cost estimates
shown on Figure 8-5 are based on the use of hydrated lime in
small plants (50 pounds per hour [22.1 kg/hr] or less) and
pebble quicklime in larger plants. Allowances for peak rates of
twice the average are built into the lime feed rates. At least
15 days of storage is provided for at the average rate. Storage
time varies from installation to installation because it is
dependent upon the relative distance to and reliability of the
chemical supply. Piping and buildings to house the feeding
equipment are included in the estimates. Estimated costs of
steel bins with dust collector vents and filling accessories are
also included.
8.4.6.2 Operation and Maintenance Cost
Figure 8-6 indicates the relationship between man-hours spent
annually for operation and maintenance and pounds of FeCl3
fed per hour. The labor includes unloading the ferric chloride
and the operation and maintenance of the chemical feed equipment.
Unloading requirements are as follows: for a 4,000-gallon
(15.1 m3) truck—1
72 per truck — 9
,5 man-hours;
man-hours.
for 50-gallon (0.19 m-
These requirements
) barrels,
are shown
8-10
-------
08
o
cc
o
Li-
en
cc
D
O
I
D
Z
3456 7891,000
3456 78910,000
INSTALLED CAPACITY, pounds Lime/Hour (as CaO) (1 Ib = 0.454 kg)
FIGURE 8-5
CAPITAL COST OF LIME STORAGE AND FEEDING FACILITIES (22)
100
10 2 3 456789100 2 3456789 1,000 2 3456 78910,000
Pounds Ferric Cloride Fed/Hour (1 Ib = 0.454 kg)
FIGURE 8-6
FERRIC CHLORIDE STORAGE AND FEEDING OPERATING
AND MAINTENANCE WORK-HOUR REQUIREMENTS (22)
8-11
-------
as man-hours per pound of chemicals fed to the process. Metering
pump operations and maintenance is estimated at five minutes per
pump per shift.
o
IT
UJ
LLI
y
DC
u
UJ
1,000
345 0, 7 8 8 10
3 4 56789 100
3 4 66789 1,000
3 4 6 6 7 8 9 10.0OO
FEEDING RATE, Ib (1 Ib = 0.454 kg)/hr
FIGURE 8-7
ELECTRICAL ENERGY REQUIREMENTS FOR A FERRIC
CHLORIDE CHEMICAL FEED SYSTEM (23)
00
o
OC
o
LL
CO
QC
=>
O
I
<
z
10,000
1,000
T
100
2 34 567891,000 2 34 5678910,000 2 34 56789100,000
Pounds Lime Fed/Hour (I Ib = 0.454 kg)
FIGURE 8-8
LIME STORAGE AND FEEDING OPERATION AND
MAINTENANCE WORK-HOUR REQUIREMENTS (22)
I-12
-------
Figure 8-7 indicates annual electric power requirements for a
ferric chloride chemical feed system.
Annual maintenance material costs are typically 3 to 5 percent of
the total chemical feed system equipment cost.
Figure 8-8 indicates man-hours for operation and maintenance as a
function of pounds of lime fed per hour. The curve consists of
lime unloading requirements and labor related to operation and
maintenance of the slaking and feeding equipment. These require-
ments are summarized as follows: slaker--one hour per eight-hour
shift per slaker in use; feeder--ten minutes per hour per feeder;
slurry pot-feed line (for slaked lime)--four hours per week.
PUMPED FEED OF
SLAKED LIME
GRAVITY FEED OF
QUICKLIME
GRAVITY FEED
OF SLAKED LIME
PUMPED FEED OF
QUICKLIME
1,000
100
3 4 567891,000 2 34 5678910,000 2
Pounds Lime Fed/Hour (1 Ib = 0.454 kg)
FIGURE 8-9
ELECTRICAL ENERGY REQUIREMENTS FOR
A LIME FEED SYSTEM (22)
3 4567 89100,000
8-13
-------
curves
Figure
a 1 ime
used in the
1,000 pounds (454 kg)
activators--2.7 to 0
collection fans—0.04
slurry feed pumps--2.2
8-9 shows annual electric power requirements for
feed system. The major components and the values
all expressed kilowatts per hour per
of lime fed are: slakers--1.6 to 0.8; bin
36; grit conveyors--0.45 to 0.06; dust
to 0.02; slurry mixers--0.027 to 0.020;
to 1.4.
Annual maintenance material costs are typically
1.5 percent of the total lime feed system equipment cost.
0.5 to
8.5 Chemical Conditioning With Polyelectrolytes
8.5.1 Introduction
During the past decade, important advances have been made in the
manufacture of polyelectrolytes for use in wastewater sludge
treatment. Polyelectrolytes are now widely used in sludge
conditioning and as indicated in Table 8-3, a large variety are
available. It is important to understand that these materials
differ greatly in chemical composition, functional effectiveness,
and cost-effectiveness.
TABLE 8-3
SUPPLIERS OF POLYELECTROLYTES
Company
American Cyanamid
Allied Colloids
Betz
Calgon
Number
of grades
and tynes
40
34
7
18
Company
Dow
Drew
Hercules
Nalco
Rohm & Hass
Number
of grades
and types
33
8
29
43
4
Selection of the correct polyelectrolyte requires that the
designer work with polyelectrolyte suppliers, equipment
suppliers, and plant operating personnel. Evaluations should be
made on site and with the sludges to be conditioned. Since new
types and grades of polymers are continually being introduced,
the evaluation process is an ongoing one.
8.5.2 Background on Polyelectrolytes
8.5.2.1 Composition and Physical Form
Polyelectrolytes are long chain, water soluble, specialty
chemicals. They can be either completely synthesized from
individual monomers, or they can be made by the chemical
1-14
-------
addition of functional monomers, or groups, to naturally
occurring polymers. A monomer is the subunit from which polymers
are made through various types of polymerization reactions.
The backbone monomer most widely used in synthetic organic
polyelectrolytes is acrylamide. As of 1979 the completely
synthesized polymers are most widely used. Polyaerylamide,
created when the monomers combine to form a long, thread-like
molecule with a molecular weight in the millions, is shown on
Figure 8-10. In the form shown polyacrylamide is essentially
non-ionic. That is to say it carries no net electrical charge in
aqueous solutions. However, under certain conditions and
with some solids, the polyacrylamide can be sufficiently
surface-active to perform as a flocculant.
— .L Pll
pt-i _„ -„.
Vrf PI — imm™
1
C = 0
,
,lr-T-_T..1 pu pi_i _,,
1
C = 0
f
\
OLJ r>Lj
•
C = 0
*
NH2 NH2 NH2
FIGURE 8-10
POLYACRYLAMIDE MOLECULE - BACKBONE OF THE
SYNTHETIC ORGANIC POLYELECTROLYTES
Anionic-type polyacrylamide flocculants carry a negative
electrical charge in aqueous solutions and are made by either
hydrolyzing the amide group (NH2) or combining the acrylamide
polymer with an anionic monomer. Cationic polyaerylamides
carry a positive electrical charge in aqueous solutions and
can be prepared by chemical modification of essentially
non-ionic-polyacrylamide or by combining the cationic monomer
with acrylamide. When cationic monomers are copolymerized with
acrylamide in varying proportions, a family of cationic
polyelectrolytes with varying degrees of charge is produced.
These polyelectrolytes are the most widely used polymers for
sludge conditioning, since most sludge solids carry a negative
charge. The characteristics of the sludge to be processed and
the type of thickening or dewatering device used will determine
which of the cationic polye lectroly tes will work best and
still be cost-effective. For example, an increasing degree
of charge is required when sludge particles become finer,
when hydration increases, and when relative surface charge
increases.
8-15
-------
Cationic polyelectrolytes are available as dry powders or
liquids. The liquids come as water solutions or emulsions. The
shelf life of the dry powders is usually several years, whereas
most of the liquids have shelf lives of two to six months and
must be protected from wide ambient temperature variations in
storage. Representative dry cationic polyelectrolytes are
described in Table 8-4. This table does not list the myriad of
available types but does show some of the differences in the
materials. The original dry materials introduced in the 1960s
were of relatively low cationic functionality or positive
charge and high molecular weight. They were produced for the
conditioning of primary sludges or easy-to-condition mixed
sludges. The incentive to produce polymers of higher positive
charge resulted largely from efforts to cope with mixed sludges
containing large quantities of biomass.
TABLE 8-4
REPRESENTATIVE DRY POWDER CATIONIC POLYELECTROLYTES
Relative cationic Molecular Approximate dosage,
Type density weight Ib/ton dry solids
Polyacrylamide copolymer Low Very high 0.5 - 10
Polyacrylamide copolymer Medium High 2-10
Polyacrylamide copolymer High Medium high 2-10
Polyamine homopolymer Complete High 2-10
Relatively low molecular weight liquid cationics with a 30 to
50 percent solids content were also available in the 1960s.
They were, however, largely displaced by the higher cationic
functionality, high molecular weight and newer, 'less costly
liquid cationics. The various liquid cationics, in either
dissolved or emulsion form, are described in representative
fashion only, in Table 8-5. These liquid cationics eliminate the
dustiness inherent in some dry powders but also require much more
storage space. The selection of a dry, liquid, or emulsion form
material usually depends on a comparison of cost-effectiveness,
ease of handling, and storage requirements.
TABLE 8-5
REPRESENTATIVE LIQUID CATIONIC POLYELECTROLYTES
Type - Molecular weight Percent solic3s_
Mannich product Low 20
Tertiary polyamine Low 30
Quaternary polyamine Very low 50
Cationic homopolymer Low to medium 16 - 20
Emulsion copolymer Low to medium 25 - 35
5-16
-------
8.5.2.2 Structure in Solution
Organic polyelectrolytes dissolve in water to form solutions of
varying viscosity. The resulting viscosity depends on their
molecular weight and degree of ionic charge. At infinite
dilution, the molecule assumes the form of an extended rod
because of the repulsive effect of the adjacent-charged sites
along the length of the polymer chain. At normal concentrations
the long thread-like charged cationic polyelectrolyte assumes the
shape of a random coil, as shown on Figure 8-11. This simplified
drawing, however, neither shows the tremendous length of the
polymeric molecular chain nor does it illustrate the very large
number of active polymer chains that are available in a polymer
solution. It has been estimated that a dosage of 0.2 mg/1 of
polyelectrolyte having a molecular weight of 100,000 would
provide 120 trillion active chains per liter of water treated.
0
0
©
©
FIGURE 8-11
TYPICAL CONFIGURATION OF A CATIONIC
POLYELECTROLYTE IN SOLUTION
8.5.2.3 How Polyelectrolyte Conditioning Works
Thickening and dewatering are inhibited by the sludge particles,
chemical characteristics, and physical configurations. Poly-
electrolytes in solution act by adhering to the sludge particle
surfaces thus causing:
• Desorption of bound surface water.
8-17
-------
• Charge neutralization.
• Agglomerization of small particulates by bridging between
particles.
The result is the formation of a permeable sludge cake matrix
which is able to release water. Figure 8-12 illustrates the
polyelectrolyte-solid attachment mechanisn. The first two
reactions noted on Figure 8-12 are the desirable ones and
represent what occurs in normal practice. The other four
reactions represent what can occur from over-dosage or too much
shear of flocculated sludge. The problems reflected in reactions
three through six rarely occur with a well-designed process.
8.5.3 Conditioning for Thickening
The various methods for thickening sludge are discussed in detail
in Chapter 5.
8.5.3.1 Gravity Thickening
Normally, the addition of polyelectrolyte is not considered in
the original design because of operating cost, but it has been
used to upgrade existing facilities (21,22). Experience to date
has indicated that the addition of polyelectrolyte to a gravity
thickener will:
• Give a higher solids capture than a unit not receiving
polymer addition.
• Allow a solids loading rate two to four times greater
than a unit not receiving polymer addition.
• Maintain the same underflow solids concentration as a
unit not receiving polymer addition.
When polyelectrolyte is used to condition sludge for gravity
thickening, it should be added into the sludge feed line. The
point of addition should provide good mixing and not cause
excessive shear before the conditioned sludge discharges into the
sludge feed well.
8.5.3.2 Dissolved Air Flotation Thickening
The effects of polyelectrolyte addition on solids capture, float
concentration, solids loading rate, and hydraulic loading rate
are covered in detail in Chapter 5.
8-18
-------
REACTION 1
INITIAL ADSORPTION AT THE OPTIMUM POLYMER DOSAGE
POLYMER
o
PARTICLE
DESTABILIZED PARTICLE
REACTION 2
FLOG FORMATION
FLOCCULATION
DESTABILIZED PARTICLES
(PERIKINETIC OR
ORTHOKINETIC)
FLOG PARTICLE
REACTION 3
^s SECONDARY ADSORPTION OF POLYMER
xj^/ NO CONTACT WITH VACANT SITES
^—
-------
8.5.3.3 Centrifugal Thickening
Centrifugal thickening includes thickening by disc nozzle,
imperforate basket, and solid bowl decanter centrifuges.
The disc nozzle unit does not utilize poly electrolyte sludge
conditioning, as it depends solely on centrifugal force
(G = 3,000 to 5,000) to achieve solids-liquid separation. The
imperforate basket centrifuge may or may not use polyelectrolyte
addition. If polymer is added, it is in the range of one to
three pounds of dry polymer per ton of feed solids (0.5 to
1.5 kg/t). This addition allows higher hydraulic feed rates and
sometimes gives better solids recovery. It does not change the
thickened solids concentration.
Solid bowl decanter centrifuges normally require as much as
20 pounds of dry polymer per ton of feed solids (10 kg/t) for
thickening of a sludge, especially a waste-activated sludge. A
new solid bowl unit has been developed for both thickening
waste-activated sludge and obtaining an 85 to 95 percent solids
capture with only 0 to 6 pounds of dry polymer per ton of feed
solids (0 to 3 kg/t).
When polyelectrolyte conditioning is used with centrifugal
thickening of sludge, several points of addition should be
provided. The optimum point of addition is influenced by
differences in polymer charge densities, required polymer
sludge reaction times, and sludge characteristics. . Recommended
points of addition are:
• Directly before the inlet side of the sludge feed pump.
• Immediately downstream of the sludge feed pump.
• To the centrifuges' sludge feed line and just before its
connection to the centrifuge.
8.5.4 Conditioning for Dewatering
The various dewatering methods are discussed in detail in
Chapter 9. Polyelectrolytes were originally used to condition
primary sludges and easy-to-dewater mixtures of primary and
secondary sludges for dewatering by rotary vacuum filters
or solid bowl decanter centrifuges. Improvement in the
effectiveness of polyelectrolytes has led to their increasing use
with all types of dewatering processes. Reasons for selecting
polyelectrolytes over inorganic chemical conditioners are:
• Little additional sludge mass is produced. Inorganic
chemical conditioners typically increase sludge mass by
15 to 30 percent.
8-20
-------
• If dewatered sludge is to be used as a fuel for
incineration, polyelectrolytes do not lower the fuel
value.
• They allow for cleaner material-handling operations.
• They reduce operation and maintenance problems.
8.5.4.1 Drying Beds
Polyelectrolyte conditioning is not widely practiced. Indica-
tions are, however, that adding 0.5 to 2.0 pounds of dry polymer
per ton of dry solids (0.25 to 1 kg/t) can increase dewatering
rates by two to four times (23,24).
8.5.4.2 Vacuum Filters
The majority of municipal vacuum filtration processes in the
United States still dewater sludge conditioned with ferric
chloride and lime. Several facilities have, however, begun
using polyelectrolytes for conditioning and have realized cost
savings (4) due to less equipment maintenance, fewer materials
handling problems, and reduction of cost in downstream sludge
processing operations (1,2,4). Table 8-6 shows addition levels
of dry polyelectrolyte used in conditioning different types
of sludge for vacuum filtration. When using polyelectrolyte
conditioning prior to vacuum filtration, the designer should be
aware that sludge formation properties can be quite different
from those of inorganic chemical conditioners. More operator
attention may be required to obtain good cake release from the
cloth. Cake dryness will probably be 10 to 15 percent lower and
the volatile content of the dry cake will be significantly
higher than if the sludge had been conditioned with ferric
chloride and lime.
TABLE 8-6
TYPICAL POLYELECTROLYTE ADDITIONS
FOR VARIOUS SLUDGES3
Sludge type
Raw primary
Waste-activated
Anaerobically digested primary
Primary plus trickling filter
Primary plus air waste-activated
Primary plus oxygen waste-activated
Anaerobically digested (primary plus air
waste-activated)
Pounds of dry polymer added per
ton of dry sol_ids_
0
1
2
.5
8
.5
.5
4
4
5
- 1.0
- 15
- 4
- 5
- 10
- 8
- 12
Data supplied by equipment manufacturers.
1 Ib/ton =0.5 kg/t
8-21
-------
8.5.4.3 Recessed Plate Pressure Filters
No published information could be found on operating experience
in the United States with polyelectrolyte conditioning of
municipal wastewater sludge prior to pressure filtration.
Several English studies indicated that polyelectrolyte
conditioning can be effectively used with pressure filtration
if done with care. Dosage must be optimized and carefully
controlled for optimum cake solids concentration, solids capture,
and ability to release the cake (25). A comprehensive study
on filter press operating experience in the North American pulp
and paper industry was recently published and gives some insight
to the use of polymers for conditioning (26). Excerpts from
the study are given below.
"Many existing pulp and paper industry installations have been
conducting polyelectrolyte evaluations on their own with, what
initially appeared to be, very encouraging results. The polymers
that have met with greatest success are those which form what can
be best described as strong 'pin-floe.1 An array of low molecular
weight cationic polymers have been cited as providing acceptable
press performance. The reasons for adopting polymer as a
conditioning agent have included (a) reduced conditioning
costs; (b) reduced quantities of solids for handling due to the
avoidance of large amounts of inorganics; and, (c) elimination of
those problems in final disposal operations that have been
associated with inorganic conditioning agents. Projected polymer
requirements vary from 3 to 30 pounds of polymer per ton of
sludge solids. "
Several mills have identified special considerations associated
with polyelectrolyte conditioning. In one instance, the polymer
conditioned cakes are discharging less readily than those with
inorganic conditioning. However, several other mills report no
noticeable difference in discharge characteristics. It is
generally observed that both cake consistencies and densities are
lower when using polymer conditioning. However, in several
instances, the difference is felt to be associated with the
bulk of the inorganic conditioning added as dry solids before
pressing."
"The handling of polymer-conditioned sludge prior to pressing
has been identified as important. Complete initial mixing of
the sludge and polymer is crucial and subsequent handling should
involve a minimum of shear. It has been proposed that mixing
be accomplished by injecting the polymer into the suction side
of a positive displacement pump or the discharge side of a
centrifugal pump. Mills have indicated the existence of an
optimum flocculation time between conditioning and pressing. One
mill reports that at the discharge of the press feed pump, the
floe is sufficiently sheared to render it very difficult to
dewater but that in the remaining 30 feet of pipe to the press,
virtually complete reflocculation occurs. At the other extreme,
several instances of intermittent sludge septicity demonstrated
8-22
-------
that extended sludge storage can be detrimental." Caution should
be exercised in extrapolating paper mill data to municipal
sludge.
8.5.4.4 Belt Filter Presses
Operating experience indicates that all belt presses require
the use of polyelectrolyte conditioning to make them work.
Compared to other mechanical dewatering processes, belt presses
seem to have the greatest need for optimizing the polymer dosage
as a function of the incoming sludge's characteristics (27).
Underconditioning results in inadequate dewatering in the initial
drainage section(s), causing either extrusion of inadequately
drained solids from the press section(s), or in extreme
instances, an uncontrolled overflow of sludge from the drainage
section(s). Underconditioned biological solids can also blind or
clog the fine mesh filter media. Overconditioning can also be a
problem. Too much polyelectrolyte can cause cake doctoring or
removal difficulties and aggravate media-blinding problems. The
type of polymer also influences the tendency of a media to blind.
In addition, overflocculated sludge may drain so rapidly that
the solids are not distributed across the media.
Table 8-7 lists typical levels of dry polyelectrolyte addition' to
condition sludges for dewatering on belt presses. The big
spread in polymer addition requirements is attributable to the
percentage of biological solids present in the total waste sludge
stream. Figure 8-13 is the result of one study and indicates
that as the percent of biological solids increases so do the
polymer requirements (27).
TABLE 8-7
TYPICAL LEVELS OF DRY POLYELECTROLYTE ADDITION
FOR BELT FILTER PRESSES3
Pounds of dry polymer added per
Sludge type ton of dry solids
Raw primary 4-8
Primary plus trickling filter 3-10
Primary plus waste-activated (air) 4-10
Waste-activated (air) 8-12
Waste-activated (oxygen) 8-12
Aerobically digested (primary plus waste-
activated {air}) 4-10
Anaerobically digested primary 2-6
Anaerobically digested (primary plus waste-
activated {air}) 3-9
Data supplied by equipment manufacturers.
1 Ib/ton = 0.5 kg/t
i-23
-------
60
~ 50
H
z
o
^ 40
Z
o
CO
IT
O
Q
m
w™
fe
o
o
d
UJ
>
O
30
= ONE OPERATING FACILITY
20
10
20
100
40 60
PERCENT BIOLOGICAL SOLIDS
FIGURE 8-13
EFFECT OF BIOLOGICAL SOLIDS ON POLYMER
REQUIREMENTS IN BELT PRESS DEWATERING (32)
8.5.4.5 Centrifuges
As was noted in the detailed discussion in Chapter 9, two types
of centrifuges can be used for dewatering: imperforate baskets
8-24
-------
and solid bowl decanters. Although many imperforate basket
centrifuges do not use polyelectrolytes for sludge conditioning
prior to dewatering, the addition of 1 to 3 pounds of dry polymer
per ton of dry feed solids (0.5 to 1.5 kg/t) can greatly reduce
overall operating cost. The reason for this reduction is that
basket centrifuges are used for dilute, difficult-to-dewater
sludges such as aerobically digested, extended aeration, and
nitrification sludges. Since the cost of polymer is offset by
the reduction in operating time, a decision is normally made in
favor of adding polymer.
Solid bowl decanter centrifuges usually require polyelectrolytes
to obtain good performance on municipal wastewater sludges.
Table 8-8 lists typical levels of dry polyelectrolyte addition to
various sludges for conditioning prior to dewatering by solid
bowl decanter centrifugation.
TABLE 8-8
TYPICAL LEVELS OF DRY POLYELECTROLYTE ADDITION FOR
SOLID BOWL DECANTER CENTRIFUGES
CONDITIONING VARIOUS SLUDGES3
Sludge type
Raw primary
Raw primary plus WAS (air)
Thermal conditioned (primary plus WAS
{air})
Thermal conditioned (primary plus
trickling filter)
Anaerobically digested
Primary
Primary plus WAS (air)
Pounds of dry polymer added per
ton of dry solids
2-5
4-10
6-10
7-10
Data supplied by equipment manufacturers.
1 Ib/ton =0.5 kg/t
8.5.5 Storage, Preparation, and Application Equipment
Storage, preparation, and application equipment for both dry and
liquid polymers are discussed in great detail in two current
USEPA publications (15,28).
8.5.6 Case History
The following summarizes the conversion of the sludge
conditioning process for the vacuum filters at the Bissell Point,
St. Louis treatment plant from an inorganic chemical to an
organic chemical process (29). The Bissell Point plant dewaters
and incinerates 35,000 dry tons (31,745 dry) of raw primary
5-25
-------
sludge per year. Conditioning of the sludge before vacuum
filtration was with ferric chloride and lime until July 1976.
Table 8-9 summarizes the solids handling systems performance
from 1972-1976.
TABLE 8-9
PERFORMANCE OF SOLIDS HANDLING SYSTEM AT
BISSELL POINT, ST. LOUIS STP 1972-1976 (29)
Cost,
_ Item Usage dollars/dry ton '
Lime dosage, Ib/dry ton , 352 6.90
Ferric chloride dosage, Ib/dry ton 64 5.09
Auxiliary fuel (natural gas), therms/
dry tonb 62 12.75
Total annual cost - 34.74
Yield (average), Ib/sq ft/hr 7.1
Solids content, percent 30
Volatile solids fraction, percent 42
All costs are adjusted to a July 1978 value.
All tons (tonnes) are net dry tons (tonnes). This is
defined as the dry tons (tonnes) of filter cake produced
less the dry quantities of chemicals required to produce
the cake.
1 Ib = 0.454 kg
1 therm = 0.116 GJ
1 Ib/sq ft/hr =4.9 kg/m /hr
1 ton = 0.907 t
Since plant startup in 1970, numerous problems have developed
from the use of ferric chloride and lime. The major problems
were:
• Lime coating of filter cloths and grid work.
• Scale buildup in filtrate and plant drainage lines.
• Constant cleanup of lime spills.
In July 1976, after six months of planning and experimentation,
the conditioning process was converted from ferric chloride
and lime to a dual polymer process utilizing either anionic or
cationic polymers. Several equipment modifications and operator
training programs had to be undertaken in order to make the
system work properly.
Grease Separation. The mixing of primary tank skimmings
with the raw sludge caused blinding of the filter cloth.
The large volume of skimmings also influenced the solids
concentration. The skimmings did not upset the ferric
8-26
-------
chloride- and lime-conditioned sludge filters as much as they
did the polymer-conditioned sludge filters. The solution
employed was to separate the skimmings and sludge and treat
each separately. Skimmings were dewatered by a modified grit
dewatering screw and then fed directly into the incinerator.
Cloth-Washing Equipment. For polyeleetrolytes to be
effective, it is mandatory that the filter cloth be cleaned
continuously. The original filter spray water system
included one spray nozzle strainer. When this strainer
had to be cleaned, the unit had to be stopped. To correct
the problem, the one strainer was replaced with a duplex-type
strainer which allowed switching of the strainers with
no change in the filter operation.
Miscellaneous Filter Improvements. Several modifications
were necessary to improve cake removal from the media. The
doctor blades were modified to fit together and against the
cloth media. Operating with polymers was found best at low
vat levels. To avoid loss of vacuum when running at low
levels, bridge blocks in the vacuum valve were installed to
modify the pickup zone.
Operator Education. It was necessary to convince the plant
operators that po~lymer usage would be beneficial to them.
An extensive educating process was conducted for several
months informing the operators of the benefits they would
obtain using polyelectrolytes.
The conversion was considered very successful. Table 8-10
summarizes performance information for the solids handling
processes after implementation of the polyeleetrolytes
conditioning process for 1977-1978. Comparison of the
performance data in Tables 8-9 and 8-10 shows that the use of
organic polymers in place of inorganic conditioners reduced
auxiliary fuel requirements by 26 percent and conditioner cost
by 53 percent. Overall annual cost per dry ton of solids was
reduced by 56 percent.
.5.7 Cost
8.5.7.1 Capital Cost
Figure 8-14 gives construction costs for polymer storage and
feed facilities as a function of installed capacity. Cost
estimates were based on the use of dry polymer. Chemical feed
equipment was chosen specifically for a 0.25 percent stock
solution. Piping and buildings to house the feeding equipment
and store the bags were included. For example, for an installed
capacity of 10 pounds (4.5 kg) of dry polymer per hour, the
approximate June 1975 cost was $110,000. The cost would need to
be adjusted to the current design period.
8-27
-------
TABLE 8-10
PERFORMANCE OF SOLIDS HANDLING SYSTEM AT
BISSELL POINT, ST. LOUIS STP 1977-1978 (29)
Item
Anionic dosage, Ib/dry ton ,
Cationic dosage, Ib/dry ton
Auxiliary, fuel (natural gas), therms/
dry ton
Total annual cost
Yield (.average) , Ib/sq ft/hr
Solids content, percent
Volatile solids fraction, percent
Usage_
0 .34
65
46
7. 8
28
56
Cost, ,
dollars/dry ton '
0.42
5.25
9.52
15.19
All costs are adjusted to a July 1978 value.
All tons (tonnes) are net dry tons (tonnes). This is defined
as the dry tons (tonnes) of filter cake produced less the dry
quantities of chemicals required to produce the cake.
1 Ib = 0.454 kg
1 therm = 0.116 GJ
1 Ib/sq ft/hr = 4.9 kg/in /hr
1 ton = 0. 907 t
UJ
D
Q
a
g
u
it
v>
Q
100,000
a
7
6
5
4
lO.QOU
5 t 7 S » 1 2 345S7B91Q 3
INSTALLED CAPACITY, Ib Polymer/hr (1 Ib = 0.454 kg)
FIGURE 8-m
3 4 § « 7 B
RELATIVE INFLUENCE OF POLYMER ADDITION ON
IMPERFORATE BASKET CENTRIFUGE PROCESS VARIABLES (22)
-------
8.5.7.2 Operation and Maintenance Cost
Figure 8-15 gives man-hours for operation and maintenance of
a dry polymer feed system as a function of pounds of chemicals
fed per hour. Unloading requirements are 16 minutes for 10-
to 50-pound (4.5 to 22.6 kg) bags. Mixing labor was estimated at
ten man-hours per 1,000 pounds (453.5 kg) of polymer under a
wastewater flow of 10 MGD (26.2 m3/s ) and three hours per
1,000 pounds (453.5 kg) of polymer for wastewater flows over
10 MGD (26.2 m^/s). Operation and maintenance requirements
were taken as 385 man-hours per year per feeder.
O
LL
O
I
z
z
100
2 34 BB7891 234 S 678910 234 56188100
POLYMER FED, Ib/hr (I Ib - 0,454 kg)
FIGURE 8-15
POLYMER STORAGE AND FEEDING OPERATION AND
MAINTENANCE WORK-HOUR REQUIREMENTS (22)
Figure 8-16 gives annual electrical power requirements for
a polymer feed system. The graph was based on the use of
plunger metering pumps and 6.4 hp hour (4.7 kWhr) for mixing of
100 pounds (45.4 kg) of polymer.
Annual maintenance material costs are typically 0.5
1.5 percent of the total polymer feed system equipment cost.
to
8.6 Non-Chemical Additions
Power plant or sludge incinerator ash has been used successfully
to improve mechanical dewatering performance on full-scale
vacuum filters and filter presses (30). The properties of ash
8-29
-------
that improve dewatering of sludge include the solubilization of
its metallic constituents, its sorptive capabilities, and its
irregular particle size (31). The advantages and disadvantages
of adding ash for sludge dewatering are given in Table 8-11.
Major advantages are lower chemical requirements and improved
cake release. Major disadvantages are the addition of a sizable
quantity of inerts to the sludge cake and additional material
handling. For installations where landfilling of sludge follows
mechanical dewatering by vacuum filters or filter presses, the
use of ash to improve the total solids content of the cake should
be evaluated. If incineration is to follow the dewatering step,
other additives such as pulverized coal or waste pulp should
receive preferential considerations (32-34). In the design of
incineration facilities, one of the
or eliminate auxiliary fuel demand.
the driest solids cake possible
by enhancing the fuel value of the
of ash to the sludge assists the dewatering device in producing a
dry cake, but it does nothing for the fuel value of the cake.
Ash has no heating value and, in fact, requires additional heat
input to raise its temperature.
main objectives is to reduce
This can be done by feeding
to the incinerator and/or
sludge solids. The addition
I
z~
o
2 <">
O ii
o ^
ce •E
o
a,
D
Z
<
1,000
3456 739100
0 234 567891 1 34 S678910 1
POLYMER FED Ib/hr (1 Ib = 0.454 kg)
FIGURE 8-16
ELECTRICAL ENERGY REQUIREMENTS FOR A
POLYMER FEED SYSTEM (22)
A pilot-scale vacuum filtration study has found pulverized coal
to be an excellent sludge conditioner for improved dewatering
(32). The coal contributed the same benefits as ash and
increased the Btu content of the sludge solids. Economic
8-30
-------
analysis showed it to be cost-effective when compared to the
addition of other supplemental fuels such as natural gas or
#2 fuel oil. A full-scale solids handling study at St. Paul,
Minnesota, demonstrated that an existing seven-hearth wastewater
sludge cake incinerator could be fed coal or wood chips with the
sludge cake to reduce consumption of natural gas or fuel oil
(35). The process was found to be economically justifiable and
practical only when a large quantity of natural gas or fuel oil
is required for sludge cake incineration.
TABLE 8-11
ADVANTAGES AND DISADVANTAGES OF ASH ADDITION
TO SLUDGE FOR CONDITIONING
Advantages Disadvantages
Substantial increase in total cake Ash handling generates considerable dust
solids
Significant improvement in filtrate Ash fines build-up
quality
Excellent cake discharge Possible equipment abrasion problems
Elimination or significant reduction Increase in materials handling problems
in use of other conditioning agents For those installations with incineration,
the addition of ash lowers the percent
volatile solids in the feed. Fuel usage
can therefore increase.
The use of waste paper as a conditioner for sludge has also
been studied in the laboratory and on a plant scale (33,34).
Some paper-conditioned sludge was dewatered on full-scale vacuum
filters (34). Results were excellent, indicating that the use of
waste paper and polymer were significantly more economical than
ferric chloride and lime.
8.7 Thermal Conditioning
This process involves .heating of wastewater sludge to
temperatures of 350° to 400°F (177° to 240°C) in a reaction
vessel under pressures of 250 to 400 psig (1,723 to 2,758 kn/m^)
for periods of 15 to 40 minutes. One modification of the
process involves the addition of a small amount of air.
Figures 8-17 and 8-18 show a general thermal conditioning
flow scheme for plants without and with the addition of air,
respectively.
Thermal conditioning of sludge was first studied by William K.
Porteous in England in the mid-1930s (36). Thermal conditioning
in the United States was first studied in the mid-1960s,
and the first facility having no air addition was installed
at Colorado Springs, Colorado, in 1969 (37-39). The
first plant with air addition was installed at Levittown,
8-31
-------
Pennsylvania, in 1967 (40). Since then, over one hundred thermal
sludge conditioning installations have been built in the United
States.
RAW SLUDGE
SLUDGE -WATER
SLUDGE HEAT
EXCHANGER
POSITIVE
DISPLACEMENT
PUMP
DECANT
LIOUOR
CAKE
FIGURE 8-17
GENERAL THERMAL SLUDGE CONDITIONING FLOW SCHEME
FOR A NON-OXIDATIVE SYSTEM
8-32
-------
RAW SLUDGE
COMPRESSED AIR
POSITIVE
DISPLACEMENT
PUMP
i IXH
SLUDGE -
SLUDGE HEAT
EXCHANGER
STEAM
BOILER _ TREATED
DECANT
LIQUOR
CAKE
FIGURE 8-18
GENERAL THERMAL SLUDGE CONDITIONING FLOW SCHEME
FOR AN OXI DATIVE SYSTEM
8.7.1 Advantages and Disadvantages
Thermal conditioning of wastewater sludges has the following
advantages:
• Except for straight waste-activated sludge, the process
will produce a sludge with excellent dewatering
characteristics. Cake solids concentrations of
30 to 50 percent are obtained with mechanical dewatering
equipment.
!-33
-------
• Processed sludge does not normally require chemical
conditioning to dewater well on mechanical equipment.
• Process sterilizes the sludge, rendering it free of
pathogenic organisms.
• If done prior to incineration, the process will provide a
sludge with a heat value of 12,000 to 13,000 Btu per
pound of volatile solids (28 to 30 kJ/g).
• Process is suitable for many types of sludges that cannot
be stabilized biologically because the presence of toxic
materials.
• Process is insensitive to changes in sludge composition.
• No length or elaborate start-up procedures are required.
The disadvantages of thermal conditioning include:
• The process has high capital cost due to the use of
corrosion-resistant materials such as stainless steel in
the heat exchangers. Other support equipment is required
for odor collection and control and high pressure fluid
transport.
• Process requires supervision, skilled operators, and a
strong preventative maintenance program.
• Process produces an odorous gas stream that must be
collected and treated before release.
• Process produces sidestreams with high concentrations of
organics, ammonia nitrogen, and color.
• Scale formation in heat exchangers, pipes, and reactor
requires acid washing.
8.7.2 Process Sidestreams
Thermal sludge conditioning produces both gaseous and liquid
sidestreams that must be considered in design.
8.7.2.1 Gaseous Sidestreams
A thermal sludge conditioning process produces odorous materials
in:
• Vapors from treated sludge in the decant or thickener
tanks.
8-34
-------
• Vacuum filter pump exhaust and vacuum filter hood
exhaust. , • . • .
• Air exhausted from the operations and hopper areas
of any enclosed mechanical dewatering system.
These odors must be treated by processing all exhaust air in
some type of odor control system. Methods of odor control
include combustion, adsorption, scrubbing, masking, dilution, and
surface evaporation (41).
8.7.2.2 Liquid Sidestreams
Thermal sludge conditioning sidestreams originate from the
conditioned sludge when it is decented, thickened or lagooned,
or when it is mechanically dewatered. The composition of
thermally conditioned sludge liquor is difficult to assess.
In one study of thermal conditioning with no air addition,
several types of sludges were treated and it was noted that in
general (42):
• The concentration of the individual components in a
heat-treatment sidestream increased in proportion to the
feed-solids concentration.
• The COD of heat-treatment liquor was proportional to
the dissolved solids for all sludges under all process
conditions.
• The organic N content of heat-treatment liquor was
proportional to the dissolved solids, there being
one relationship for activated sludge and others for
trickling filter, primary plus activated, and digested
sludges.
• The breakdown of organic N to ammonia in activated sludge
heat treatment liquor was a time-temperature phenomenon.
In general, therefore, the composition of the liquor is a
function of the type of sludge, feed volatile solids content,
reaction time, and temperature. Without a pilot scale
investigation of process feasibilityV "it is difficult to specify
design data. Table 8-12 gives ranges for various constituents
that have been reported for both the process with air addition
and the process without air addition that conditioned sludges
having 3 to 6 percent feed solids concentrations (41-50).
Table 8-13 summarizes data from the literature on filtrate
or centrate composition. Except for suspended solids, the
parameters of filtrate are similar if not equal 'to the decant
tank supernatant.
8-35
-------
TABLE 8-12
GENERAL CHARACTERISTICS OF SEPARATED LIQUOR FROM
THERMAL CONDITIONED SLUDGE3
Parameter
Suspended solids, mg/1
Dissolved solids, mg/1
COD, mg/1
BOD5, mg/1
Phosphorus0, mg/1
Total N, mg/1
Organic N, mg/1
Ammonia N, rag/1
pH
Color
Metals
Oxidative
100-20,000 .
10,000-30,000
5,000-15,000
150-200
650-1,000
400-1,700
5.0-6.5
1,000-6,000 units
~d
Non-ox idative
300-12 ,000
. 1,700-12,000
2,500-22,000
1,600-12,000
70-100 ,
700-1,700
100-1,000
30-700
5.0-6.4
2,000-8,000
_e
Mixture of 50 Dercent primary and 50 percent waste-activated at a feed solids
concentration between 3 to 6 percent.
Less than 20 percent of the COD is non-biodegradable.
"Depends on P of influent sludge.
See Reference 43.
See Reference 44.
Many methods have been used to treat the liquid sidestreams, and
they are discussed in Chapter 16.
8.7.3 Operations and Cost
Analysis of the cost of installing and operating a thermal
conditioning process should be comprehensive, as it impacts other
parts of the liquid and sludge handling system. The discussion
in this section is general; for those interested in more detail,
two recent reports are available (41,53).
8.7.3.1 General Considerations
Thermal sludge conditioning has been operating in the United
States for about ten years. During that time, over a hundred
facilities have been built and much has been learned from
past mistakes. Following are current design guidelines that
must be considered in the cost determinations for a basic thermal
sludge-conditioning system:
• If there is a chance of high chloride content (greater
than 400 mg/1) in the sewage or sludge metal with
corrosion-resistant properties greater than stainless
steel must be used in the hot heat exchanger (nearest
reactor).
$-36
-------
• All potential sources of odor (decant tank, dewatering
area, vacuum filter exhaust must be enclosed. In
addition, an air collection and treatment system must be
provided.
• Strength of the recycle streams depends on many
variables. The worst possible conditions should be
used as the design basis for the recycle liquor system.
• Good grit removal from the sludge is essential to prevent
abrasion of metal piping. The provision of grit removal
at the plant influent does not imply that grit will be
absent in the sludge stream. Large quantities of material
can blow into clarifiers and aeration tanks; therefore,
separate grit removal before the thermal-conditioning
system should be considered.
• Only the most rugged types of sludge handling pumps
should be used.
• Present-day energy economics dictate careful review of
heat recovery systems.
8.7.3.2 USEPA Survey Results
In~May and June of 1979, USEPA Technology Transfer, Cincinnati,
Ohio, conducted a survey of operation and maintenance problems at
76 thermal conditioning process facilities. Table 8-14 lists
suppliers, number of plants involved in survey, and sum of
operating experience.
Nearly all the plants contacted indicated high costs of operation
and maintenance. The high operating costs resulted mainly from
the cost of fuel for steam generation, the addition of chemicals
for boiler water treatment, and in some cases (Lexington,
Kentucky; Haverhill, Massachusetts; Poughkeepsie, New York),
the addition of chemicals to improve dewatering. Plants that
utilize waste heat from sludge cake incineration are able to cut
considerably both fuel usage and the volume of sludge (as ash)
that must be hauled. .
Maintenance costs involve replacing various parts on a somewhat
regular basis, washing the heat exchanger and reactor with acid
to remove scaling, and the costs of the manpower needed to
perform these tasks. Plants that have operating experience
express requirements for highly trained personnel, regular
preventive maintenance, and a good surveillance program. These
practices can substantially reduce maintenance costs due to
excessive shutdown time or replacement of major components that
do not normally wear out.
8-37
-------
TABLE 8-13
FILTRATE AND/OR CENTRATE CHARACTERISTICS FROM
DEWATERINC THERMAL CONDITIONED SLUDGE
Sludge type
Raw primary plus trickling
filter sludge (heavy
industrial load)
Anaerobically digested (primary
plus waste-activated) plus.
raw primary
Raw primary plus waste-
activated
Raw primary plus waste-
activated (high tannery load)
Anaerobically digested (primary
plus waste-activated) plus
raw sludge
Anaerobically digested primary
plus oxygen waste-activated
Raw primary plus trickling
filter sludge
Raw primary plus waste-
activated
Raw primary plus waste- , ~ /
activated
Raw primary plus waste-
activated
Raw primary plus waste-
activated
Raw primary plus waste-
Average values.
Dewatering process
Recessed plate pressure
filter
Rotary vacuum filter cloth
media
Rotary vacuum filter cloth
media
Rotary vacuum filter cloth
media
Sand dry beds
Recessed diaphragm plate
and frame pressure filter
Rotary vacuum filter cloth
media
Centrifuge
Centrifuge
Rotary vacuum filter cloth
media
Rotary vacuum filter cloth
media
Coil vacuum filters
Characteristics
Feed solids, percent = 9.0
Filtrate3
Total solids, mg/1 = 8,000
SS, mg/1 =150
BOD5, mg/1 = 6,500
COD, mg/1 = 12,000
Total N, mg/1 = 1,075
pH, units = 6.4
Feed solids, percent =10-15
Filtrate SSa, mg/1 = 5,000
BOD5a, mg/1 = 10,000
Feed solids, percent =6-10
Filtrate SS, mg/1 1,000
Feed solids, percent =8-13
Filtrate SS, mg/1 2,000
BOD5, mg/1 = 7,900 - 9,600
Soluble BOD5 of drainage does not ex-
ceed 6,000 mg/1
Reference
49
51
Feed solids, percent 14
Filtrate SS, mg/1 1,400
Feed solidsa, percent = 18
Filtrate SS, mg/1 9,000
BOD5, mg/1 6,800
Feed solids, percent = 6-7
Filtrate SS = 3,000 mg/1
Feed solids, oercent =6-7
Filtrate SS = 6,000-9,000 mg/1
Soluble BOD5, mg/1 = 4,200
BOD5, rag/1 = 7,300 - 9,100
Feed solids, percent 10 - 20
Filtrate, percent = 2 - 2.5
Soluble BOD5, mg/1 = 6,000 - 7,000
Feed solids, percent 13
Filtrate, percent solids =6-7
The buildup of scale in the heat exchanger, reactor, or pipes
occurs in most plants that have hard water or industrial wastes
in the influent. Regular washing with acid is practiced in all
plants with this problem. The length of operating time between
washes varies from as much as 1,500 hours to as little as
200 hours. Many plants acid-wash on a regular basis, about every
month, not only to remove scale, but to prevent its initial
buildup.
Many operators, of the non-air thermal conditioned systems
indicated that an important factor in a good maintenance program
is the upkeep of a parts inventory. This eliminates the chance
of the system being shut down over an extended period while parts
are ordered.
8-38
-------
TABLE 8-U
USEPA JULY 1979 SURVEY OF EXISTING MUNICIPAL WASTEWATER
THERMAL CONDITIONING
Total installations
Number of installations
contacted in survey
Operating more than 120 hr/
week
Operating less than 120 hr/
week
Not operating
Period of operation
Less than 1 year
Between 1 to 2 years
Between 3 to 5 years
Over 5 years
Zimpro
83
57
27
20
10
11
15
11
20
Envirotech Nichols
30 '• 6
19 0
7
6
6
1
6
9
3
Zurn
1
0
Formally called the Porteous process. Porteous process was
licensed by Envirotech in the mid-1960's.
Formally known as the Dorr Oliver Farrer System. Purchased
by Nichols in the early 1970's.
The concensus of the operators is that after the "bugs" are
worked out of the system, after the personnel have been
familiarized and trained, and after a routine maintenance program
is established, the process performs satisfactorily.
8.8 Elutriation
Elutriation is the term commonly used to refer to the washing of
anaerobically digested sludge before vacuum filtration. Washing
causes a dilution of the bicarbonate alkalinity in the sludge and
therefore reduces the demand for acidic metal salt by as much as
50 percent (54).
The process itself was patented by Center in 1941 (55). Although
it typically employs one or two tanks, any number of tanks
can be used. Two to six volumes of washwater, typically plant
effluent, flow countercurrent to one volume of anaerobically
digested sludge. Elutriation tanks are designed to act
as gravity thickeners, with a mass solids loading of 8 to
10 pounds per square foot per day (39 to 48.8 kg/m2/day).
At this time the process is not used as extensively as it had
been because, in addition to reducing alkalinity, it also washed
out 10 to 45 percent of the solids from the incoming sludge
stream (56-60). Elutriate was recycled back to the main plant
and eventually degraded the plant effluent (57,58,60).
8-39
-------
Full-scale research (60-62) has shown that the solids problem
can be solved, and 90 to 92 percent capture achieved, with
the use of polymers. Recommended current elutriation design
considerations are listed below:
• Tanks should be loaded at hydraulic loadings (total
of both sludge and washwater flow) of 200 to 300 gallons
per day per square foot (69 to 104 1/day/m2) and solids
loading of 8 to 15 pounds per day per square foot (39 to
• Tanks should have the best possible inlet structure to
minimize inlet momentum.
• Baffling should be used to prevent tank currents.
• Tanks should be provided with scum collection.
• Polymer addition should be provided.
8.9 Freeze-Thaw
In 1929, Babbit and Schlenz demonstrated the benefit of freezing
wastewater sludge (63). They noted that, after sludge was frozen
on a sand drying bed during the winter and thawed in the spring,
its drainage qualities were improved and it dried to a higher
solids content.
Research has since been conducted in three areas of freeze
conditioning: indirect and direct mechanical systems and natural
freezing.
8.9.1 Indirect Mechanical Freezing
Until recently, all mechanical freeze-conditioning research
has been oriented toward indirect freezing methods. Indirect
freezing involves the separation of the refrigerant and the
sludge by some type of partition. The studies (64-66) on
wastewater sludges indicate that freezing:
• Causes cellular dehydration and thus allows better
flocculation.
• Destroys the slirainess of biological sludges.
• Improves dewatering characteristics as measured by
sandbed and vacuum filter dewatering rates.
• Must occur slowly to be effective.
8-40
-------
Although freeze conditioning has been shown to be beneficial, it
is expensive to implement. This is because the system cannot
utilize the heat generated by the fusion of the frozen sludge to
cool the refrigerant.
8.9.2 Direct Mechanical Freezing
To overcome the above-mentioned problem, pilot work has been
conducted on direct freezing (67). In direct freezing, the
liquefied refrigerant is vaporized and dispersed through the
sludge slurry at a controlled rate. In Table 8-15, slurry
freezing (direct mechanical method) is compared to solid freezing
(indirect freezing) and several other treatment processes.
TABLE 8-15
COMPARISON OF SEWAGE SLUDGE HANDLING AND CONDITIONING PROCESSES (67)
Process
Reduction
in sludge
COD percent
Sludge
solubilization
Supernatant and
filtrate quality
Slurry freezing
Solid freezing
Anaerobic digestion
Aerobic digestion
Chemical addition
35
50-70
60-70
30-70
20-40
Low
High
High
Low
Low
_pH_
7-8
7-7.5
6-7
4-7
6-6.5
Quality
Good
Poor
Poor
Good
Moderate
Cost/ton
dry solid
6-20
5-35
15-20
15-30
10-25
1 ton = 0.907 t
8.9.3 Natural Freezing
In this method, the freezing is done by the environment.
At least one facility (68) is operating in Canada, and extensive
full-scale research is being conducted in facility design in
order to improve this method of conditioning (69).
8.10 Mechanical Screening and Grinding
In some applications, screening or grinding can be considered
part of the sludge conditioning process. A good example of
screening for conditioning is in the application of a disc nozzle
centrifuge. A stainless steel, self-cleaning screen is required
to remove large solids and fibrous material that would clog the
disc nozzle machine.
Grinding of primary sludge is an important step for some sludge
handling processes. It has also been indicated that grinding of
a thick (over 8 percent solids) sludge stream reduces viscosity,
thus making the slurry easier to pump. One outstanding example
of this is in the municipal system at Glen Cove, New York.
1-41
-------
8.11 Miscellaneous Processes
In addition to the more commonly known conditioning methods
previously discussed, research has also been conducted on more
novel methods, such as bacteria, electricity, solvent extraction,
and ultrasonic.
8.11.1 Bacteria
Autotrophic sulfur bacteria may provide cond itioning" irf added
to digested sludge prior to dewatering (54). Under aerobic
conditions, sulfur-oxidizing bacteria stimulate the production of
sulfuric acid, which, in turn, lowers the pH of the sludge
and enhances the dewatering process as measured by the specific
resistance test. In another study (70), it was shown that
filtration rates of waste-activated sludge could be increased
under anaerobic conditions with the use of the enzyme lysozyme.
8.11.2 Electricity
In extensive laboratory and pilot plant work studies, graphite
anodes and iron cathodes have been used to conditon sludge
(71-76).
These studies indicate that:
• At pH values lower than 4.0 electrical current can
condition sludge for filtration without the use of
chemicals.
• The quantity of water removed during dewatering
(vacuum filtration) was proportional to the amount of
electricity used. Thinner sludges required less current.
• Sludges electrically conditioned seemed to produce
drier cakes than chemically conditioned sludges.
The disadvantages are that:
• Anodes had to be replaced frequently because a dried
crust continually formed on them.
• The system uses a great deal of electricity; optimum
current density was approximately 0.3 amp per sq ft
(3.3 amp/m2) of anode surface, with a potential drop of
4 volts between the electrodes.
• No full-scale facilities have ever -been tested to
evaluate operating problems.
8-42
-------
8.11.3 Solvent Extraction
In 1957, research was conducted at Rockford, Illinois, with
carbon tetrachlorethylene as the solvent, with distillation
end products being dried oils, fats, and greases (77). It was
not considered to be very economical at that time.
Although solvent extraction is becoming popular in industry
(78), only recently has there been promotional activity in
the municipal field (79). To date, no municipal installations
are using the process.
8.11.4 Ultrasonic
Conditioning of sewage sludges by ultra- or supersonic vibration
has been explored (54). Ultrasonic vibrations degasify sludge,
which is beneficial, but the vibrations also tend to destroy
sludge floes, resulting in fine solids that are difficult to
dewater.
3.12 References
1. Schillinger, G.R. "Conversion of Sludge Conditioning
Chemicals." Water Pollution Con'trg_jL_Fe_djera.t.ion Deeds and
Data. Vol. 16. April 1979.
2. Nelson, J.K. and A.H. Tavery. "Chemical Conditioning
Alternatives and Operational Control for Vacuum Filtra-
tion. " Journal Water Pol 1 u t i o n Con t_r_o_l_ F e d e r a t i o n .
Vol. 50, p. 507 (1978).
3. Carry, C.W., R.P. Miele, and J.F. Stahl. "Sludge
Dewatering." Proceedings of the National Conference on
Municipal Sludge Management. I nf orma t ion Transfer Inc.
Rockville, MD (1974).
4. Bargman, R.D., W.F. Garber, and J. Nagano. "Sludge
Filtration and Use of Synthetic Organic Coagulants at
Hyperion." Sewage and Industrial Waste. Vol. 30, p. 1079
1958.
5. Coackley, P. and R. Allos. "The Drying Characteristics
of Some Sewage Sludges." Institute of Sewage Purificat.ion
Journal Proceedings. Pt. 6, p. 557. 1962.
6. Lapple, C.E. "Particle-Size Analysis and Analyzers."
Chemical Engineering. p. 149. May 20, 1968.
7. Karr, P.R. and T.M. Keinath. "Influence of Particle
Size on Sludge Dewaterability." Presented at 49th Annual
Conference Water Pollution Control Federation. Minneapolis,
Minnesota. 10/3-8/76.
8-43
-------
8. Heukelekian, H. and E. Weisberg. "Sewage Colloids." Water
and Sewage Works. Vol. 105, p. 428. October 1958. . .71
9. Kos, P. Continuous Gravity Thickening of Sludges. Dorr
Oliver Technical Reprint 705. 1978.
10. Tenney, M.W., W.F. Echelberger, Jr., J.J. Coffey, and
T.J. McAloon. "Chemical Conditioning of Biological Sludges
for Vacuum Filtration." Journal Water Pollu t^on_ _Cqntrg_l
Fede_ration. Vol. 42, p. Rl . 1970.
11. Hagstrom, L.G. and N . A . Mignone. "Whatto Consider
in Basket Centrifuge Design." Water_ anc3_Was_te_Eng_ineer ing .
p. 58. March 1978.
12. Zacharias, D.R. and K.A. Pietila. "Full-Scale Study of
Sludge Processing and Land Disposal Utilizing Centrif ugation
For Dewatering." Presented at the 50th Annual Meeting of
the Central States Water Pollution Control Federation,
Milwaukee, Wisconsin. May 18-20, 1977.
13. USEPA. Evaluation of Dewatering Devices for Producing High
Sludge Solids Cake. Office of Research and Development.
Cincinnati, Ohio 45268. Contract 68-03-2455. 1979.
14. USEPA. Performance Evaluation and Troubleshooting at
Municipal Wastewater Treatment Facilities. Office of Water
Program Operations. Washington D.C. 20460. USEPA 430/9-
78-001. January 1978.
15. USEPA. Process Design Manual for Suspended Solids Removal.
Technology Transfer, Cincinnati, Ohio 45268. USEPA 625/1-
75-003a. January 1975.
16. USEPA. Energy Conservation in Municipal Wastewater
Treatment . Office of Water Programs. Washington D.C
17
1 8 .
Washington D.C. Second edition. May 1971
19. Culp/Wesner/Culp . Cost and Performance Handbook Sludge
Handling Processes. Prepared for Wastewater Treatment and
Reuse Seminar, South Lake Tahoe. 10/26-27/75.
20. Ruehrwein, R.A. and T. Ward. "Mechanism of Clay Aggregation
by Polyelect roly tes . " Soil Science. Vole 73. p. 485.
January-June 1952.
8-44
20460. USEPA 68-03-2186, March 1977.
USEPA. Lime Use In Wastewater Treatment:
Data. Office of Research and Development,
45268. EPA 600/ 2-75-038. October 1975.
National Lime Association - Handling
Storage. Published by the National L
Design and Cost
Cincinnati, Ohio
Application and
ime Association,
-------
21. Jordan, V.J. and C.H. Scherer. "Gravity Thickening
Techniques at a Water Reclamation Plant." Journal Water
Pollution Control Federation. Vol. 42, p. 180. 1970. ~~
22. Ettelt, G.A. and T. Kennedy. "Research and Operational
Experience in Sludge Dewatering at Chicago." Journal Water
Po 11 ution Cpntrpj._Federation. Vol. 38, p. 248. 1966. ~
23. Beardsley, J.A. "Sludge Drying Beds Are Practical." Water
and Sewage Works. Part 1, p. 82. July 1976. Part 2, p. 42
August 1976.
24. Jennett, J.C. and I.W. Santry. "Characteristics of Sludge
Drying." Journal of the Sanitary^Jr^gJ.jT£e^J.j^g_DJLvision ASCE.
SA 5, p. 849. 1969.
25. Harrison, J.R. "Developments in Dewatering Wastewater
Sludges." Vol. 1 Sludge Treatment and Disposal. USEPA-ERIC
Technology Transfer, Cincinnati, Ohio 45268. October 1978.
26. NCASI. Full-Scale Operational Experience with Filter
Presses for Sludge Dewatering in the North American Pulp and
Paper Industry. Prepared for National Council of the Paper
Industry for Air and Stream Improvement. Technical Bulletin
299. October 1977.
27. NCASI. A Review of the Operational Experience With Belt
Filter Presses for Sludge Dewatering in the North American
Pulp and Paper Industry. Prepared for National Council of
the Paper Industry for Air and Stream Improvement.
Technical Bulletin 315. October 1978.
28. USEPA. Sludge Handling and Conditioning. Office of Water
Program Operations. Washington D.C. 20460. USEPA 430/9-
78-002. February 1978.
29. Schillinger, G.R. "Conversion of Sludge-Conditioning
Chemicals." Journal Water Pollution Co_n_tjrojl Federation
Deeds andData^Vol. 16.April 1979.
30. USEPA. Pressure Filtration of Wastewater Sludge with Ash
Filter Aid. Office of Research and Development, Cincinnati,
Ohio 45268. EPA-R2-73-231. 1973.
31. Smith, J.E., Jr., S.W. Hathaway, J.B. Farrell, and
R.B. Dean. "Sludge Conditioning with Incinerator Ash."
Presented 2J7jth_JPurdue Industrial Waste Conference. May
1972.'
32. Hathaway, S.W. and R.A. Olexsey. "Improving Sludge Inciner-
ation and Vacuum Filtration with Pulverized Coal." Journal
Water PollutionControlfederation. Vol. 49, pp. 2419-2430.
1977.
8-45
-------
33. Cargen, C.A. and J.F. Malina. "Effect of Waste Paper
Additions on Sludge Filtration Characteristics." Center
34.
for Water Research #24. University of Texas, Austin.
Campbell, H.W., R.W. Kuzyk, and G.R. Robertson.
Use of Pulped Newsprint As A Conditioning Aid
Vacuum Filtration of A Municipal Sludge." Progress i
1968.
"The
in the
n Water
Technology . Vol. 10, pp. 79-88. 1978.
USEPA. Draft Copy Coincineration of Sewage Sludge with Coal
or Wood Chips. Office of Research and Development,
35.
Cincinnati, Ohio 45268. EPA grant
36. Porteous, I.K. "Mechanical Treatment of Sewage Sludge by
the Steam Injection Method." Municipal Engineer. Vol. 16.
December 1966.
37. Harrison, J. and H.R. Bungay. "Heat Syneresis of Sewage
Sludges." Water and Sewage Works. Vol. 115, #5, Part I,
p. 217 and #6, Part II, p. 268. 1968.
38. Teletzke, G.H. "Low Pressure Wet Air Oxidation of
Sewage Sludge." Proceedings 20th Purdue Industrial Waste
Conference . p. 40. May 1965.
39. Sherwood, R. and J. Phillips. "Heat Treatment Process
Improves Economics of Sludge Handling and Disposal." Water
and Wastes Engineering . p. 42. November 1970.
40. Blattler, P.X. "Wet Air Oxidation at Levittown." Water and
Sewage Works. Vol. 117, p. 32. 1970.
41. USEPA. Effects of Thermal Treatment of Sludge On Munic-
ipal Wastewater Treatment Costs'^ Municipal Environmental
Research Laboratory, Cincinnati, Ohio 45268. USEPA 600/2-
78-073, June 1978.
42. Brooks, R.B. "Heat Treatment of Sewage Sludges." Journal
of the Instituteof Water Pgllu t j._g_n_^£on^ro 1 . Vol. 69,
p. 221 (1970) .
43. Sommers, L.E. and E.H. Curtin. "Wet Air Oxidation: Effect
on Sludge Composition." Journal Wate_r_Pol_lution Control
Federa t ion . Vol. 49, p. 2219 (1977).
44. Everett, J.G. "The Effect of Heat Treatment on the
Solubilization of Heavy Metals, Metals and Organic Matter
from Digested Sludge." Journal of the Institute of Water
Pollution Control. Vol. 73, p. 207 (1974).
45. Brooks, R.B. "Heat Treatment of Sewage Sludge." Third
National Chemical Engineering Conference . Mildura Victoria,
Australia. August 1975.
8-46
-------
46. Sarfert, F. "Composition of the Filtrate From Thermally
Conditioned Sludges." Water Research. Vol. 6, p. 521
(1972). ' ~~~"
47. Corrie, K.D. "Use of Activated Carbon in the Treatment of
Heat Treatment Plant Liquor." Journal of the Institute of
Water Pollution Control. Vol. 71, p. 629 (1972).
48. Whitehead, C.R. and E.J. Smith. "Sludge Heat Treatment:
Operation and Management." Journal of the Institute of
Water Pollution Control. Vol. 71, p. 31 (1976).~~
49. Hirst, G., K.G. Mulhall, and M.L. Hemming. "The Sludge Heat
Treatment and Pressing Plant at Pudsey: Design and Initial
Operating Experiences." Journal of the Institute of Water
Pollution Control. Vol. 71, p. 455 (1972) . __-___—_
50. Erickson, A.H. and P.V. Knopp. "Biological Treatment
of Thermally-Conditioned Sludge Liquors." Advances in Water
Pollution Research. Vol. II edited by S.M. Jenkins.
Published by Pergamon Press, London, 1972.
51. Jones, E.E. "Finding A Better Way To Dispose of Sludge."
Publ ic Works Magaz j._ne . March 1975.
52. SCS Engineers. Review of Techniques for Treatment and
Disposal of Phosphorus-Laden Chemical Sludges.USEPA-MERL
Contract 68-03-2432 to be published in the summer of 1979.
53. Thermal Conditioning Cost Effect ivene^^_Repg^rt. Zimpro,
Inc., November 1978. " ~" " "~™~
54. Burd, R.S. A Study of Sludge Handling and Disposal. U.S.
Department of Interior WP-20-4, May 1968^~~~~~
55. Center, A.L. U.S. Patent 2,259,688, October 21, 1941.
56. Garrity, L.V. "Sludge Disposal Practices at Detroit-
Discussion." Sewage Works Journal. Vol. 18, p. 215 (1946).
57. Sparr, A.E. "Elutriation Experience At the Bay Park Sewage
Treatment Plant." Sewage and Industrial Wastes. Vol. 26,
p. 1443 (1954).
58. Taylor, D. "Sludge Conditioning and Filtration at
Cincinnati's Little Miami Sewage Works." Sewage and
Industrial Waste. Vol. 29, p. 1333 (1957).
59. Chasick, A.H. and R.T. Dewling. "Interstage Elutriation
of Digested Sludge." Journal Wate r _JP_g_l _Tut_io_n _^iPJ} JLE2A_ F^ .9*e r ~
at ion. Vol. 34, p. 390 (1962).
60. Dahl, B.W., J.W. Zelinski, and O.W. Taylor. "Polymer Aids
in Dewatering Elutriation." Journal Water ^Pollution Control^
Federation. Vol. 44, p. 201 (1972).
8-47
-------
61. Goodman, B.L. "Chemical Conditioning of Sludges: Six Case
Histories." Water and Wastes Engineering. Vol. 3, p. 62
(1966).
62. Goodman, B.L. and C.P. Witcher. "Polymer-Aided Sludge
Elutriation and Filtration." .Journal Wa ter TPollu t ion
Control Federation. Vol. 37, p. 1643 (1965).~~~~
63. Babbitt, H.E. and H.E. Schlenz. The Effect of Freeze Drying
on Sludges. Illinois Engineering Experiment Station,
Bulletin No. 198, p. 48, 1929.
64. Sewerage Commission City of Milwaukee. Evaluation of
Conditioning and Dewatering Sewage Sludge by Freezing.
Water Pollution Control Research Series 11010 EVE 01/71.
65. Clements, G.S., R.J. Stephenson, and C.J. Regan. "Sludge
Dewatering by Freezing with Added Chemicals." Journal and
Proceedings Institute of _ S_e_w_ag_ e P u r i f i c a t i on Journal.
Part 4, p. 318 (1950) .
66. Cheng, C., D.M. Updegroff, and L.W. Ross. "Sludge
Dewatering by High Rate Freezing at Small Temperature
Differences." Environmental Science and Technology.
Vol. 4, p. 1145 (1970)."
67. Randall, C.W. "Butane Is Nearly 'Ideal' For Direct
Slurry Freezing." Water and Wastes Engineering. p. 43,
March 1978.
68. Penman, A. and D.W. Vanes. "Winnipeg Freezes Sludge,
Slashes Disposal Cost 10 Fold." Civil Engineering-ASCE.
Vol. 43, p. 65 (1973).
69. Rush, R.J. and A.R. Stickney. Natural Freeze-Thaw Sewage
Sludge Conditioning and Dewatering. Canada Environmental
Protection Service Report EPS 4-WP-79-1, January 1979.
70. Envirogenics Co. Biological Methods of Sludge Dewatering.
FWQA-W-72-05838. NTIS PB 207-480. FWQA-14-12-427, p. 147,
August 1971.
71. Slagle, E.A. and L.M. Roberts. "Treatment of Sewage and
Sewage Sludge by Electrodialysis. " Sewage Works JjQijrjna^.
Vol. 14, p. 1021 (1942).
72. Beaudoin, R.E. "Reduction o'f Moisture in Activated Sludge
Filter Cake by Electro-Osmosis." S_ewage Works Journal.
Vol. 15, p. 1153 (1943).
73. Hicks, R. "Disposal of Sewage Sludge." The Surveyor.
pp. 105, 303. April 19, 1946.
8-48
-------
74. Cooling, L.F. "Dewatering of Sewage Sludge by Electro-
Osmosis." H^i^£_£Il5_^Iliia^X_J^Il£jjl£eiiB3,' Vol. 3, p. 246
(1952).
75. Spohr, G. "Electrical Stimulation of Bacteria." Water
Works _and__ _Wa_s _t_es_ _E ng_ i_n e e r jLng Ap r i 1 1 9 6 4 .
76. Spohr, G. "Electrical Stimulation of Bacteria." U.S.
patent 3,166,501.
77. Stallery, R.H. and E.H. Eauth. "Treatment of Sewage
Sludge by the McDonald Process." Pjublic Works. p. Ill,
March 1957.
78. Hanson, C. "Solvent Extraction - An Economically Competi-
tive Process." £^l^]ILi£2^__^£i££££ill3.' ?• ^/ May 1979.
79. Olson, R.L. , R.K. Ames, H.H. Peters, E.A. Gustan, and
G.W. Bannon. "Sludge Dewatering With Solvent Extraction."
Proceedings of the National Conference Management and
Disposal of Residues From the Treatment of Industrial
Wastewaters . Washington, D.C. p. 175, February 3-5, 1975.
8-49
-------
EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapters. Dewatering
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 9
DEWATERING
9.1 Introduction
Dewatering is the removal of water from wastewater treatment
plant solids to achieve a volume reduction greater than that
achieved by thickening. Dewatering is done primarily to decrease
the capital and operating costs of the subsequent direct sludge
disposal or conversion and disposal process. Dewatering sludge
from a 5 to a 20 percent solids concentration reduces volume by
three-fourths and results in a non-fluid material. Dewatering is
only one component of the wastewater solids treatment process and
must be integrated into the overall wastewater treatment system
so that performance of both the liquid and solids treatment
schemes is optimized and total costs are minimized (1-3).
9.1.1 Process Evaluation
Several pilot-scale studies have been published that compare the
performance of various dewatering devices or techniques on
different sludge types (4-9). Table 9-1 summarizes equipment and
sludge types evaluated. One conclusion that can be drawn from
these studies is that selecting sludge dewatering processes is
still very much an art rather than a science. Bench or pilot
scale testing is always recommended before final design. This,
however, does not always guarantee successful operation of the
full-scale system. As will be shown, there are many problems
involved in the scale-up of dewatering equipment, and this,
combined with the changing character of municipal wastewater
sludges, can cause significant problems. Designers must be
cognizant of these problems and allow for them in the design of
full-scale installations.
The main variables in any dewatering process are:
• Solids concentration and volumetric flow rate of the
feed stream.
• Chemical demand and cost.
• Suspended and dissolved solids concentrations and
volumetric flow rate of the sidestream.
9-1
-------
Solids concentration
dewatered sludge.
and volumetric flow rate of the
TABLE 9-1
PILOT-SCALE SLUDGE DEWATERING STUDIES
Reference
Sludge type
Type of equipment
Mesophilic, anaerobically
digested primary sludge
from a publicly owned
treatment work (POTW)
Mesophilic and thermophilic
anaerobically digested
sludge (3/4 by weight
primary plus 1/4 by weight
waste-activated sludge)
from a POTW
Waste-activated sludge from
a pulp and paper activated
sludge plant
7,8 Raw primary sludge (1/3 by
weight) plus waste-
activated sludge (2/3 by
weight from a POTW)
9 Mesophilic, anaerobically
digested primary sludge
(1/3 by weight) plus
waste-activated sludge
(2/3 by weight) from a
POTW
Combination of a horizontal, solid bowl,
decanter centrifuge and a imperforate
basket centrifuge
Rotary drum, cloth-belt, vacuum filter
Rotary drum, coil-belt, vacuum filter
Recessed plate pressure filters
Horizontal, solid bowl, decanter centrifuge
Imperforate basket centrifuge
Rotary drum, cloth-belt, vacuum filter
Recessed plate pressure filters
Drying beds
Horizontal, solid bowl decanter centrifuge
Rotary drum, precoat vacuum filter
Recessed plate pressure filters
Belt filter press
Capillary suction
Ultraflitration
Dual cell gravity filter with multiple roll
Rotary drum, cloth-belt vacuum filter
Recessed plate pressure filters
Diaphragm recessed plate pressure filters
Belt filter press
Horizontal, solid bowl, decanter centrifuge
Rotary drum, cloth-belt, vacuum filter
Belt filter press
Specific design criteria for selection of a dewatering process
can also be dependent upon subsequent processing steps. Both the
sludge composting and the incineration process require sludge
with a relatively low solids concentration.
Another important consideration is the operation and maintenance
(0/M) cost and the variables affecting it. In the past,
O/M costs have been given little attention. This should change
as USEPA implements its new Operations Check List (10) in all
phases of the Construction Grants Program.
Finally, dewatering device reliability is important for
successful plant operation. A reliable dewatering system is
needed to maintain relatively uninterrupted removal of wastewater
solids from a continuously operated wastewater treatment process.
9-2
-------
Sludges are generated constantly, and if they are allowed to
accumulate for a long time, the performance of the entire
wastewater treatment plant will be impaired.
9.1.2 Methods of Dewatering
While numerous techniques fulfill the basic functional
definition of dewatering, they do so to widely varying degrees.
It is important to note these circumstances when
different devices. For example, drying beds
only to dewater a sludge, but also to dry
concentration of greater than 50 to 60 percent.
circumstances and particular device involved,
from a mechanical device may vary from a wet,
form, to a harder and more friable form.
comparing
can be used not
it to a solids
Depending on the
dewatered sludge
almost flowable
9.2 Natural Sludge Dewatering Systems
When land is available, sludge dewatering by nature can be
extremely attractive from both a capital and an operating cost
viewpoint. Considering escalating electrical power costs,
this method is even more attractive. Two types of systems can be
categorized as natural: drying beds and drying lagoons.
9.2.1
Drying beds
dewatering
two-thirds
Drying Beds
the
are the most widely used method of municipal sludge
in the United States (11). At the present time,
of all United States wastewater treatment plants
utilize drying beds and one-half of all the United States
municipal sludge is dewatered by this method. Although the use
of drying beds might be expected in smaller plants and in the
warmer sunny regions, they are also used in several large
facilities in northern climates (12). Table 9-2 lists advantages
and disadvantages of the drying bed method.
TABLE 9-2
ADVANTAGES AND DISADVANTAGES OF
USING SLUDGE DRYING BEDS
Advantages
Disadvantages
When land is readily available, this is
normally the lowest capital cost
Small amount of operator attention and
skill is required
Low energy consumption
Less sensitive to sludge variability
Low to no chemical consumption
Higher dry cake solids contents than fully
mechanical methods
Lack of a rational engineering design
approach allowing sound engineering
economic analysis
Requires more land than fully mechanical
methods
Requires a stabilized sludge
Must be designed with careful concern for
climatic effects
May be more visible to the general public
Removal usually labor intensive
9-3
-------
Research into the dewatering of sludge by drying beds has been
conducted since the early 1900s, when it was noted that digested
sludge dewatered more rapidly than raw sludge (13). Design data,
however, are still very empirical, and only recently has an
effort been made to develop a rational engineering design
approach (14-16). An excellent review of past work, detailed
theoretical analysis, and current understanding of the sludge
drying process is given by Adrian (14). Sludge dewatering on a
drying bed is a multi-phase process and is shown pictorially on
Figure 9-1.
RAIN IF BED \$ UNCOVERED
I
EVAPORATION DUE TO RADIATION AND CONVECTION
t t f t t t I
SLUDGE
POROUS MEDIUM - SLUDGE SUPPORT STRUCTION
* \
DRAINAGE OF WATER THROUGH POROUS MEDIUM
FIGURE 9-1
SCHEMATIC OF SLUDGE DEWATERING IN A
DRYING BED SYSTEM
9.2.1.1 Basic Components and Operation
Drying beds generally consist of a one- to three-foot (0.3-1.Om)
high retaining wall enclosing a porous drainage media. This
drainage media may be made up of various sandwiched layers
of sand and gravel, combinations of sand and gravel with cement
strips, slotted metal media, or a permanent porous media.
Appurtenant equipment includes: sludge feed pipelines and flow
meters; possible chemical application tanks, pipelines, and
metering pumps; filtrate drainage and recirculat ion lines;
possible mechanical sludge removal equipment; and a possible
cover or enclosure.
Operational procedures common to all types of drying beds involve;
• Pump 8 to 12 inches (20 to 30 cm) of stabilized liquid
sludge onto the drying bed surface.
9-4
-------
• Add chemical conditioners continuously, if conditioners
are used, by injection into the sludge as it is pumped
onto the bed.
• Permit, when the bed is filled to the desired level, the
sludge to dry to the desired final solids concentration.
This concentration can vary from between 18 to
60 percent, depending on the type of sludge, processing
rate needed, degree of dryness required for lifting, etc.
• Remove the dewatered sludge either mechanically or
manually.
• Repeat the cycle.
9.2.1.2 Types of Drying Beds
Drying beds may be classified as either conventional, paved,
wedgewire, or vacuum-assisted.
Conventional Sand Drying Beds
Sand drying beds are the oldest, most commonly used type of
drying bed. Many design variations are possible including
the layout of drainage piping, thickness and type of gravel and
sand layers, and construction materials.
Current United States practice (17-19) is to make drying beds
rectangular with dimensions of 15 to 60 feet (4.5 to 18 m) wide
by 50 to 150 feet (15 to 47 m) long with vertical side walls.
Usually 4 to 9 inches (10 to 23 cm) of sand is placed over 8 to
18 inches (20-46 cm) of graded gravel or stone. The sand is
usually 0.012 to 0.05 inches (0.3 to 1.2 mm) in effective
diameter and has a uniformity coefficient less than 5.0. Gravel
is normally graded from 1/8 to 1.0 inches (0.3 to 2.5 cm), in
effective diameter. Underdrain piping has normally been of
vitrified clay, but plastic pipe is also becoming acceptable.
The pipes should be no less than 4 inches (10 cm), should be
spaced 8 to 20 feet (2.4 to 6 m) apart, and have a minimum slope
of one percent.
Figure 9-2 shows a typical sand drying bed construction. Sand
drying beds can be built with or without provision for mechanical
sludge removal, and with or without a roof.
Paved Drying Beds
Paved drying beds have had limited use since 1954 (20). The beds
are normally rectangular in shape and are 20 to 50 feet (6 to
15 m) wide by 70 to 150 feet (21 to 46 m) long with vertical
side walls. Current practice is to use either a concrete or
asphalt lining. Normally, the lining rests on an 8- to 12-inch
(20- to 30-cm) built-up sand or gravel base. The lining should
have a minimum 1.5 percent slope to the drainage area. A minimum
four-inch (10-cm) diameter pipe would convey drainage away. An
9-5
-------
unpaved area, 2 to 3 feet (0.6 to 1 m) wide is placed along
either side or down the middle for drainage. Paved drying beds
can be built with or without a roof.
GATE
COLLECTION ^
SYSTEM -^ >J;
DRAINAGE
—£
>
t *
:^£ v.':':/" s A NO ;.. V-'-VyVV.;.;
iUZ,—.^
*_P_ **'.!••*• „*_*_»***_•••* __.«_» Aj
D'«.,
••::
. _:*x
>..-
"f
FIGURE 9-2
TYPICAL SAND DRYING BED CONSTRUCTION (18)
For a given amount of sludge, paved drying beds require more area
than sand beds. Their main advantages are that front-end loaders
can be used for sludge removal and reduced bed maintenance (21).
Figure 9-3 shows typical paved drying bed construction.
MIKJIMIJM 1_K%
SLOPE
- ASPHALT OR
CONCRETE LINING
f - -, - - ^ >
SAND
,7^'irV-,*,
SAND
\ \ \
\ \ \ \ \
DRAINAGE
FIGURE 9-3
TYPICAL PAVED DRYING BED CONSTRUCTION
9-6
-------
Wedge-Wire Drying Beds
Wedge-wire drying bed systems have been successfully used in
England for over 20 years to dewater both municipal (22) and
industrial (23,24) wastewater sludges. Used in the United States
since the early 1970s, there are presently 18 wedge-wire
installations. Ten of these installations are for municipal
wastewater sludge.
In a wedge-wire drying bed, sludge slurry is introduced onto a
horizontal, relatively open-drainage media in a way that yields a
clean filtrate and provides a reasonable drainage rate (25).
Table 9-3 lists reported advantages for this type of drying bed.
TABLE 9-3
ADVANTAGES OF A WEDGE-WIRE DRYING BED (26)
No clogging of the media
Constant and rapid drainage
Higher throughput rate than sand beds
Easy bed maintenance
Difficult-to-dewater sludges, for example,
aerobically digested can be dried
Compared to sand beds dewatered sludge is
easier to remove
Figure 9-4 shows a typical cross section of a wedge-wire bed.
The bed consists of a shallow rectangular watertight basin fitted
with a false floor of wedgewater panels. These panels have
slotted openings of 0.01 inches (0.25 mm). This false
made watertight with caulking where the
An outlet valve to control the rate
underneath the false floor.
panels abut
of dra inag e
floor is
the walls.
is located
CONTROLLED DIFFERENTIAL HEAD IN VENT
BY RESTRICTING RATE OF DRAINAGE
VENT
t
PARTITION TO FORM VENT
±r
WEDGEWIRE SEPTUM
OUTLET VALVE TO CONTROL TO CONTROL
RATE OF DRAINAGE —
FIGURE 9-U
CROSS SECTION OF A WEDGE-WIRE DRYING BED
9-7
-------
The procedure used for dewatering sludge begins with the
movement of water or plant effluent into the wedgewater unit
until a depth of approximately one inch (2.5 cm) over the
wedge-wire septum is attained. This water serves as a cushion
that permits the added sludge to float without causing upward
or downward pressure across the wedge-wire surface. The
water further prevents compression or other disturbance of the
colloidal particles. After the bed is filled with sludge, the
initially separate water layer and the drainage water are
allowed to percolate away at a controlled rate, through the
outlet valve. After the free water has been drained, the sludge
further concentrates by drainage and evaporation until there is a
requirement for sludge removal.
Vacuum-Assisted Drying Beds
The only operating vacuum-assisted drying beds at this time
are two 20 feet (6 m) by 40 feet (12 m) units built in 1976
at Sunrise City, Florida. They dewater a two percent solids
concentration, aerobically digested sludge from a contact
stabilization wastewater treatment plant (27).
The principal components of the Sunrise facility are:
• A bottom ground slab consisting of reinforced concrete.
• A layer of stabilized aggregate several inches thick
which provides support for the rigid multi-media filter
top. This space is also the vacuum chamber and is
connected to a vacuum pump.
• A rigid multi-media filter top is placed on the aggregate
support. Sludge is then applied to the surface of
this media.
The operating sequence is as follows:
• Sludge is introduced onto the filter surface by gravity
flow at a rate of 150 gallons per minute (9.4 1/s) and to
a depth of 12 to 18 inches (30 to 46 cm).
• Filtrate drains through the multi-media filter and into
the space containing the aggregate and then to a sump,
from which it is pumped back to the plant by a self-
actuated submersible pump.
• As soon as the entire surface of the multi-media filter
is covered with sludge, the vacuum system is started
and vacuum is maintained at 1 to 10 inches mercury
(3 to 34 kN/m2).
Under favorable weather conditions, this system dewaters the
dilute aerobically digested sludge to a 12 percent solids
concentration in 24 hours without polymer addition, and to
the same level in eight hours if polymer is added. This
9-8
-------
particular sludge of 12 percent solids concentration is capable
of being lifted from the bed by a fork or mechanical equipment.
The sludge will further dewater to about 20 percent solids
concentration in 48 hours.
9.2.1.3 Process Design Criteria
Covered Beds
Whenever there is the possibility of long periods of rain, snow,
or cold weather; potential odor or insect problems; or a problem
with esthetics; consideration should be given to employing covers
for the drying beds. When properly ventilated, so that air can
flow over the surface of the bed, covered sand beds can be
employed and require 25 to 33 percent less area than open sand
beds (17,26). Although covers can be provided for paved,
wedge-wire, and vacuum beds, no documentation could be found on
how covers affect or improve bed loading rates.
Sludge Conditioning
Sludge conditioning can dramatically improve drying bed
throughput (28) and should be considered as part of the design.
See Chapter 8 for further discussion on conditioning.
Sludge Removal
The majority of United States facilities employ manual labor to
remove dried sludge from drying beds. With this type of removal,
a 30 to 40 percent solids conoentrat ion is required. With
mechanical sludge removal systems (21,29,30), solids concentra-
tion between 20 and 30 percent can be handled (31). Depending
on the bed size, a tiltable unit similar to the lift and dump
mechanism of a dump truck is available for the wedge-wire drying
bed.
S ides t reams
The only sidestream from a drying bed operation is under
drainage liquor. While little is known about the characteristics
of this sidestream, Table 9-4 shows the results from one pilot
study. This flow is not normally treated separately, but is
typically returned to the plant headworks.
TABLE 9-H
CHARACTERIZATION OF SAND BED DRAINAGE (32)
Sludge type - Anaerobically digested mixture of primary and trickling filter sludge
Bed media - 6 inches of sand
Color - clear, dark amber
COD - 300-400 mg/1
BOD,- - 6-66 mg/1
- 1,900-2,360 mg/1 (over 90 percent nitrogenous)
1 inch = 2.54 cm
9-9
-------
TABLE 9-5A
SUMMARY OF RECOGNIZED PUBLISHED SAND BED SIZING CRITERIA
FOR ANAEROBICALLY DIGESTED, NON- CONDITIONED SLUDGE
Uncovered beds,
Area, Loading, Covered beds area,
Initial sludge source sq ft/capita Ib solids/sq ft/yr sq ft/capita3
Primary
Reference 33 1.0 27.5
Reference 34 1.0 - 1.5 0.75 - 1.0
Reference 36
N45° N latitude 1.25 0.93
Between 40-45° N 1.0 0.75
S40° N latitude 0.75 0.56
Primary plus chemicals
Reference 33 2.0 22
Reference 34 2.0 - 2.25 1.0 - 1.25
Reference 36
N45° N latitude 2.5 1.87
Between 40-45° N 2.0 1.50
S40° N latitude 1.5 1.12
Primary plus low rate
trickling filter
Reference 33 1.6 22
Reference 34 1.25 - 1.75 1.0 - 1.25
Reference 36
N45° N latitude 1.87 1.56
Between 40-45°N 1.50 1.25
S40° N latitude 1.12 0.93
Primary plus waste-
activated sludge
Reference 33 3.0 15
Reference 34 1.75 - 2.5 1.25 - 1.5
Reference 36
N45° N latitude 2.18 1.68
Between 40-45°N 1.75 1.35
S40° N latitude 1.31 1.01
aOnly area loading rates available for covered beds.
1 Ib/sq ft/yr =4,9 kg/m /yr
1 sq ft = 0.093 m2
BedSizing Criteria
Despite the number of drying beds in use today, the lack of
published bed sizing criteria have limited applicability. The
majority of published and professionally utilized design data
(33-36) are based on operations during the 1940s and 1950s.
Tables 9-5A and 9-5B summarize the data for sand drying beds. At
that time, sludges applied to sand beds were anaerobically
digested. They originated predominantly in primary, primary
plus low rate trickling filter, or primary plus conventional
9-10
-------
waste-activated sludge wastewater treatment processes. Many of
the sludges presently generated do not readily fall within these
categories. ( -
TABLE 9-5B
SUMMARY OF RECOGNIZED PUBLISHED STATE BED SIZING
CRITERIA FOR SAND BEDS BY USEPA REGIONS3 SQUARE FEET/CAPITA
EPA Region _I II III IV V° Yi___ _VII VIII IX Xf
uacduc uc ucuc uc u c ucuc uc
Anaerobically
digested
Primary only 1.5 1.0 1.5 0.75 0.5-1.0 1.0 1.0 1.5 1.0
Prirrary + low rate
trickling filter 1.75 1.25 1.5 0.75 0.75-1.2 0.5-1.0 0.25 1.5 1.0 1.0 1.0 1.5-2.0 1.0-1.25
Primary + sand
filter 1.0 1.0 0.5
Primary + high rate
trickling filter 1.0 1.0 1.251.25 1.0 2.0 1.25
Priirarv + waste
activated sludge 2.5 1.5 2.01.0 1.5-2.5 1.0-1.51.0 1.351.35 1.0 1.5-2.51.0-1.5
Prijnary +
chemical 2.0 1.0 1.0-1.33 1.0 1.5 1.3 3.0 2.0
Imhoff 1.5 0.75 0.66-1.0 1.0
Imhoff + low rate
trickling filter 1.0-1.2 1.0
aTaken from individual State design criteria that do not use 10 State Standards.
The states encompassed in USEPA Regions III and V do not have published reguirements at this time.
GState of Idaho: Values shown are for rainfall of 30-45 inches (76-114 cm); for rainfall between
10-30 inches (25-76 cm), reduce these values by 25 percent; for rainfall of less than 10 inches
(25 cm), reduce these values by 50 percent.
U = uncovered sand beds
C = covered sand beds
1 sq ft/capita = 0.093 m /capita
Also, most data are given in square feet of bed surface
area required for dewatering on a per capita basis. This
criterion is only valid for the characteristics of a particular
wastewater and has no rational design basis. A better criterion
for sizing sand drying beds is the pounds of solids per square
foot of bed surface area per year. The best criteria would
take into consideration climatic conditions (such as temperature,
wind velocity and precipitation), sludge characteristics,
(grit, grease, fiber, and biological content), and solids
concentration.
No generalized bed sizing criteria could be found for paved
beds. Also very little information is available from full-scale
facilities on bed sizing criteria for wedge-wire units. In one
United States, wedge-wire facility, 150 gallons per day (568 1/d)
of excess biological sludge at a two percent solids concentration
is conditioned with a polyelectrolyte and dewatered to a liftable
eight percent solids concentration in two to three hours (27).
Table 9-6 contains data on the performance of wedge-wire systems
with several different sludges.
9-11
-------
TABLE 9-6
WEDGE-WIRE SYSTEM PERFORMANCE DATA (25)
Sludge type
Primary
Trickling filter humus
Digested primary +
waste activated sludge
(WAS)
Fresh WAS
Fresh WAS
Thickened WAS
Feed solids,
percent
8.5
2.9
3.0
0.7
1.1
2.5
Sludge solids
concentration,
percent
25.0
8.8
10.0
6.2
9.9
8.1
Dewatering
time
14 days
20 hours
12 days
12 hours
8 days
41 hours
Solids capture,
percent
99
85
86
94
87
100
All sludges were chemically conditioned.
9.2.1.4 Costs
Capital _C_ost_s
Several recent publications have developed capital cost curves
for open sand beds (37-39). Probably the most accurate is the
reference based on actual USEPA bid documents for the years
1973-1977 (38).
Although the data were scattered, a regression analysis
indicated, that, on the basis of a USEPA Municipal Wastewater
Treatment Plant Construction Cost Index for the 2nd quarter 1977
(38), the capital cost could be approximated by Equation 9-1.
C = 9.89 x 104 Ql-35
(9-1)
where :
C = capital cost of process in dollars.
Q = plant design flow in million gallons of wastewater flow
per day.
The associated costs include excavation, process piping,
equipment, concrete, and steel. In addition, such costs as
those for administrating and engineering are equal to 0.2264
times Equation 9-1 (38).
Operating and Maintenance
Table 9-7 indicates open sand bed labor requirements for both
operation and maintenance. The labor indicated includes:
removal of dried sludge from the beds, sand maintenance, and
weeding as necessary.
9-12
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TABLE 9-7
SLUDGE DRYING BEDS, LABOR REQUIREMENTS (18)
Labor, hours per year
>tal bed area,
sq fta
1,000
5,000
10,000
50,000
100,000
Operation
300
400
500
1,500
2,900
Maintenance
100
180
220
710
1,500
Total
400
580
720
2,210
4,400
Assumes dry solids loading rate of 20 Ib/sq ft/yr of bed area.
1 sq ft = 0.093 m2 0
1 Ib/sq ft/yr = 4.9 kg/mVyr
3
2
10,000
B
e
7
6
in
O
u
z
z
4 —
3 "^
2 —
1,000
9
8
7
S
4
3 -
2 —
234 6678910,000 2 34 56789100,000 2
DRYING BED AREA, sq ft (1 sq ft = 0,093 m2)
FIGURE 9-5
ESTIMATED JUNE 1975 MAINTENANCE MATERIAL COST
FOR OPEN SAND DRYING BEDS (39)
3 4 i 6 789
9-13
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Figure 9-5 shows a curve developed for estimating open sand
bed maintenance material cost as a function of sand bed surface
area. As an example, for a sand bed surface area of 10,000
square feet (930 m2) , a designer would estimate a yearly
materials cost of $400. Since this number is based on a June
1975 cost, it must be adjusted to the current design period.
9.2.2 Drying Lagoons
Sludge drying lagoons are another method (12) of sludge
dewatering when sufficient, economical land is available.
Sludge drying lagoons are similar to drying beds. However, the
sludge is placed at depths three to four times greater than it
would be in a drying bed. Generally, sludge is allowed to
dewater and dry to some predetermined solids concentration before
removal and this might require one to three years. The cycle is
then repeated. Sludge should be stabilized prior to addition to
the lagoon to minimize odor problems. Large areas of lagoons can
produce nuisance odors as they go through a series of wet and dry
conditions. See Chapter 15 for further discussion. Table 9-8
lists present advantages and disadvantages for sludge drying
lagoons.
TABLE 9-8
ADVANTAGES AND DISADVANTAGES OF USING SLUDGE DRYING LAGOONS
Advantages
Disadvantages
Lagoons are low energy consumers
Lagoons consume no chemicals
Lagoons are not sensitive to sludge
variability
The lagoons can serve as a buffer in the
sludge handling flow stream. Shock
loadings due to treatment plant upsets
can be discharged to the lagoons with
minimal impact
Organic matter is further stabilized
Of all the dewatering systems available,
lagoons require the least amount of
operation attention and skill
If land is available, lagoons have a very
low capital cost
Lagoons may be a source of periodic odor
problems, and these odors may be difficult
to control
There is a potential for pollution of
groundwater or nearby surface water
Lagoons can create vector problems (for
example, flies and mosquitos)
Lagoons are more visible to the general
public
Lagoons are more land-intensive than fully
mechanical methods
Rational engineering design data are
lacking to allow sound engineering
economic analysis
Very little research has been conducted concerning sludge
drying lagoons. Dewatering occurs in three ways: drainage,
evaporation, and transpiration. Research seems to indicate
that dewatering by drainage is independent of lagoon depth.
Dewatering by drainage alone cannot produce a sludge sufficiently
dry for easy removal (40 , 41). These studies further indicate that
evaporation is the most important dewatering factor.
9-14
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•9.2.2.1 Basic Concept
Sludge drying lagoons consist of retaining walls which are
normally earthen dikes 2 to 4 feet (0.7 to 1.4 m) high. The
earthen dikes normally enclose a rectangular space with a
permeable surface. Appurtenant equipment includes: sludge feed
lines and metering pumps, supernatant decant lines, and some type
of mechanical sludge removal equipment. The removal equipment
can include a bulldozer, drag line or front-end loader. In areas
where permeable soils are unavailable, underdrains and associated
piping may be required.
Operating procedures
involve:
common to all types of drying lagoons
Pumping liquid sludge, over a period of several months or
more, into the lagoon. The pumped sludge is normally
stabilized prior to application. The sludge is usually
applied until a lagoon depth of 24 to 48 inches (0.7 to
1.4m) is achieved.
Decanting supernatant, either continuously or intermit-
tently, from the lagoon surface and returning it to the
wastewater treatment plant.
Filling the lagoon to a desired sludge depth and then
permitting it to dewater. Depending on the climate
and the depth of applied sludge, the time involved for
dewatering to a final solids content of between 20 to
40 percent solids may be 3 to 12 months.
Removing the dewatered sludge with some type of
mechanical removal equipment.
Resting (adding no new sludge) to the lagoon for three to
six months.
Repeating the cycle.
9.2.2.2 Design Criteria
Proper design of sludge drying lagoons requires a consideration
of the following factors: climate, subsoil permeability, sludge
characteristics, lagoon depth, and area management practices. A
detailed discussion of these factors follows.
Climate
After dewatering by drainage and supernating, drying in a sludge
lagoon depends primarily on evaporation. Proper size of a
lagoon, therefore, requires climatic information concerning:
• Precipitation rate (annual and seasonal distribution).
9-15
-------
• Evaporation rate (annual average, range, and seasonal
fluctuations).
• Temperature extremes.
Subsoil Permeability
The subsoil should have a moderate permeability of 1.6 x 10~4 to
5.5 x 10~4 inches per second (4.2 x 10~4 to 1.4 x 10~3 cm/s),
and the bottom of the lagoon should be a minimum of 18 inches
(46 cm) above the maximum groundwater table, unless otherwise
directed by local authorities.
SJLudge Characteristics
The type of sludge to be placed in the lagoon can significantly
affect the amount and type of odor and vector problems that can
be produced. It is recommended that only those sludges which
have been anaerobically digested be used in drying lagoons.
Lagoon.Depth arid Area
The actual depth and area requirements for sludge drying lagoons
depend on several factors such as precipitation, evaporation,
type of sludge, volume and solids concentration. Solids loading
criteria have been given as 2.2 to 2.4 pounds of solids per year
per cubic foot (36 to 39 kg/m3) of capacity (46). A minimum
of two separate lagoons are provided to ensure availability
of storage space during cleaning, maintenance, or emergency
conditions.
General Guidance
Lagoons may be of any shape, but a rectangular shape facilitates
rapid sludge removal. Lagoon dikes should have a slope of 1:3,
vertical to horizontal, and should be of a shape and size to
facilitate maintenance, mowing, passage of maintenance vehicles
atop the dike, and access for the entry of trucks and front-end
loaders into the lagoon. Surrounding areas should be graded to
prevent surface water from entering the lagoon. Return must
exist for removing the surface liquid and piping to the treatment
plant. Provisions must also be made for limiting public access
to the sludge lagoons. Chapter 15 provides a description of a
successful sludge drying lagoon operation for the Metropolitan
Sanitary District of Greater Chicago.
9.2.2.3 Costs
Current published information on capital cost of constructing
sludge lagoons is almost nonexistent. Some information is
available from a recent USEPA publication (38), and from
Chapter 15. Table 9-9 indicates labor requirements for sludge
9-16
-------
drying lagoons. The requirements include: i application of sludge
to the lagoon; periodic removal of supernatant; periodic removal
of solids; and minor maintenance requirements, such as dike
repair and weed control. No information could be found on
maintenance material costs.
TABLE 9-9
SLUDGE DRYING LAGOONS, LABOR REQUIREMENTS (18)
Labor, hours per year
Dry solids applied,
tons/year Operation Maintenance Total
100 30 55 85
1,000 55 90 145
10,000 120 . , 300 420
50,000 450 1,500 1,950
1 ton = 0.9 t
9.3 Centrifugal Dewatering Systems
9.3.1 Introduction
Centrifuges were first employed in the United States for
dewatering municipal wastewater treatment plant sludges during
the year 1920, in Milwaukee, Wisconsin, and during 1921 in
Baltimore, Maryland (42). Early centrifuges were not designed to
process extremely variable slurries such as those of municipal
wastewater treatment plants. In addition, most wastewater
treatment facilities provided little, if any, preventive
maintenance. Consequently, early installations developed numerous
operational and maintenance problems, and this led to an
anti-centrifuge reaction among environmental engineers.
By the late 1960s, equipment manufacturers were designing
and building new machines specifically for wastewater sludge
applications, and the use of centrifuges for municipal sludge
dewatering increased. In the past ten years, continuous
improvements in design and materials have led to better machines.
The machines now available (1979) require less power and
attention and produce less noise.
Two categories of centrifuges are used for municipal wastewater
sludge dewatering: imperforate basket and scroll-type decanter.
A detailed discussion of each follows. The basic theory of
thickening and process costs are presented in Chapter 5.
9-17
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9.3.2 Imperforate Basket
Basket centrifuges for dewatering municipal wastewater treatment
plant sludges were first used in the United States in 1920 (42).
Since the mid 1960s approximately 300 machines were installed in
100 municipal treatment plant applications (43). About one-half
of the installed machines are used for dewatering; the other half
and used for thickening. The largest centrifuge facility in
the world is located at the County Sanitation Districts of
Los Angeles County Carson Plant in California, and uses 48 basket
centrifuges (44). Table 9-10 lists the advantages and disadvant-
ages of a basket centrifuge compared with other dewatering
systems.
TABLE 9-10
ADVANTAGES AND DISADVANTAGES OF BASKET CENTRIFUGES
Advantages Disadvantages
Same machine can be used for both dewatering Requires special structural support
and thickening ,. .. . , ,
Except for vacuum filter, consumes more
It may not require chemical conditioning direct horsepower per unit of product
Centrifuges have clean appearance, little- processe
to-no odor problems, and fast start-up Skimming stream could produce significant
and shut-down capabilities recycle load
Basket centrifuge is very flexible in Limited size capacity
meeting process requirements . , , n , , ,
For easily dewatered sludges, has the
It is not affected by grit highest capital cost versus capacity
It is an excellent dewatering machine for
hard-to-handle sludge For most sludges, gives the lowest cake
,. , , . . i . . , .. solids concentration
It has low total operation and maintenance
costs.
Does not require continuous operator
attention
9.3.2.1 Principles of Operation
The operation of an imperforate basket centrifuge is described
in Chapter 5. There is, however, one additional operation to be
added to that discussion.
After the centrifuge bowl is filled with solids, the unit starts
to decelerate. In the thickening mode, deceleration was to a
speed of 70 rpm or lower before commencement of plowing. In the
dewatering mode, another step called "skimming" takes place
before the initiation of plowing. Skimming is the removal of
soft sludge from the inner wall of sludge within the basket
centrifuge. The skimmer moves from its position in the center of
the basket towards the bowl wall. The amount of horizontal
travel is set at the time of installation, and start-up depends
9-18
-------
on sludge type and downstream processing requirements. The
skimming volume is normally 5 to 15 percent of the bowl volume
per cycle. After the skimmer retracts, the centrifuge further
decelerates to the 70 rpm level for plowing. Skimming streams are
typically 6 to 18 gallons (22 to 66 1) per cycle with a solids
content of almost zero to eight percent. Treatment of this stream
is typically by ,returning it either to the primary or secondary
wastewater treatment system, or to some other pre-sludge handling
step such as a thickener.
9.3.2.2 Application
A basket centrifuge is well suited for small plants that do not
provide either primary clarification or grit removal (for
example, wastewater plants that use extended aeration, aerated
lagoons, and contact stabilization). These small plants require
a piece of equipment that can, at different times, dewater or
thicken conventional as well as biological sludges with a long
sludge age. Also low overall operation and maintenance, and
low operating costs, are associated with basket centrifuges.
9.3.2.3 Performance
Table 9-11 lists typical performance data for a basket centrifuge
in a number of different applications. These data are expected
values and are based on the performance of several different
installations. Table 9-12 lists the average results from two
specific operating facilities.
9.3.2.4 Case History
In 1973, a dewatering study was made in Burlington, Wisconsin, on
the wastewater treatment facility located there (46). The plant
treats a combination of domestic-industrial wastewater flow of
1.5 MGD (66 1/s) during dry weather and 2.0 MGD (88 1/s) during
wet weather. The treatment plant has no primary clarification and
uses the contact stabilization process with aerobic digestion.
Approximately 150,000 gallons (568 m^) per week of aerobically
digested sludge with a 1.4 percent solids concentration requires
disposal.
As the plant is located on a low flood plain, it was originally
necessary to truck the dilute sludge to the lagoon. In 1972,
the Wisconsin Department of Natural Resource's ordered Burlington
to discontinue use of the lagoons. Since the only options
available were landfilling or cropland application, dewatering
was required. In 1973, an engineering evaluation was performed
to select the optimum dewatering unit. The equipment evaluated
included: an imperforate basket centrifuge, a recessed plate
filter, a horizontal belt filter press, and a rotary drum vacuum
9-19
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TABLE 9-11
TYPICAL PERFORMANCE DATA FOR AN IMPERFORATE BASKET CENTRIFUGE
Sludge type
Raw primary
Raw trickling filter
(rock or plastic media)
Raw waste activated
Raw primary plus rock
trickling filter (70-30)
Raw primary plus waste
activated (50-50)
Raw primary plus rotating
biological contactor (60-40) „
Anaerobically digested
primary plus waste
activated (50-50)
Aerobically digested
Combined sewer overflow
treatment sludge
Centrate from decanter
dewatering lime sludge
Polymer
Average required. Recovery
Feed solids cake solids pounds dry based on
concentration, concentration, per ton dry centrate,'
percent solids percent solids feed solids percent
4-5
2-3
0.5-1.5
2-3
2-3
2-3
1-2
1-3
Extremely
1-2
25-30
9-10
10-12
3-10
12-14
9-11
7-9
12-14
20-24
17-20
12-14
10-12
8-10
8-11
12-14
variable -
10-13
2-3
0
1.5-3.0
0
1.0-3.0
0
1.5-3.0
1-3
0
4-6
0
1.5-3.0
4-6
0
1-3
see study by
0
95-97
90-95
95-97
85-90
90-95
95-97
94-97
93-95
85-90
98 +
75-80
85-90
93-95
80-95
90-95
EPA (4'5)
95-98
Skimming losses, if any, have not been used in calculating recovery.
1 Ib/ton =0.50 kg/t
TABLE 9-12
SPECIFIC OPERATING RESULTS FOR IMPERFORATE BASKET
County sanitation district
of Los Angeles, CA (44)
Burlington, WI (4j5J
Type of sludge
Instantaneous flow rate, gpm
Feed solids concentration,
mg/1
Polymer requirement
Cake solids content, percent
Centrate, mg/1
Skimmed volume, percent of
total basket volume
Centrate from solid bowl
decanter dewatering
anaerobically digested
primary sludge
e, gpm 50
ion,
29,000
4a
ercent 20
1,500
t of
Not given
Aerobically digested,
activated sludge from a
plant without primary
clarification
23
14,000
0
6-8
100
50
88
14,000
30b
13-15
100
14
Dry polymer at 4 Ib/ton (2.0 kg/t) of dry solids.
Combination anionic-cationic system. Thirty dollars/ton
($33/t) of dry solids.
1 gpm = 0.063 1/s
9-20
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filter. The recessed plate pressure filter option was ruled out
as too expensive for Burlington's small plant. The horizontal
belt filter press produced a low cake solids concentration and
required high levels of polymer addition at a cost of $40 per ton
($44.44/t). The vacuum filter was not selected because of high
capital cost. The imperforate basket was selected as the most
cost-effective unit. Figure 9-6 shows a flow scheme of the
Burlington wastewater treatment plant as it was operating in
1977.
The original design, as a result of the engineering evaluation,
called for one basket centrifuge to operate 40 hours per week.
This centrifuge was to dewater 96,000 gallons (370 up) per week
of sludge at a 1.8 percent solids concentration to a nine to ten
percent solids concentration without the use of polymers. This
was all based on several days of pilot plant work conducted
several months before equipment selection was made. At the
time of centrifuge start-up, the actual sludge volume to be
dewatered was 150,000 gallons (568 m^) per week at 1.4 percent
solids concentration. The column labeled "Without Polymer" in
Table 9-13 shows performance results under this condition.
Because of the 50 percent greater sludge volume and poorer
operating results than had been indicated by pilot testing, the
basket centrifuge had to operate 24 hours per day, seven days per
week. This type of operation was prohibitive for a plant the
size of the Burlington facility.
wFLUENT
SKlMMIhGS HAULED
AS LIQUID TO LAND
APPLICATION
CENTHATE TO HJAp QF PIAMT
FIGURE 9-6
1977 FLOW DIAGRAM OF BURLINGTON, WISCONSIN
WASTEWATER TREATMENT PLANT
9-21
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TABLE 9-13
OPERATING RESULTS FOR BASKET CENTRIFUGE
DEWATERING OF AEROBICALLY DIGESTED
SLUDGE AT BURLINGTON, WISCONSIN
Without With
polymer polymer
Gal/week of sludge to
dewater 150,000 150,000
Lb/week of sludge to
dewater 17,500 17,500
Instantaneous feed rate,
gpm 23 88
Feed solids concentration,
mg/1 14,000 14,000
Hr/week operation required 168 44
Labor and trucking cost
(dollars)/week at 45
percent of the time 378 99
Electricity utilized/week,
kWhr 4,888 1,584
Electricity cost at
$0.03/kWhr 146.63 47.52
Chemical cost, dollars/ton 0 30
Cake solids, percent 6-8 13-15
Skimming volume of basket,
percent of total 50 14
Cost/ton, dollars 59.96 46.74
Material was untruckable.
Material was truckable.
1 gpm = 3.78 1/min
1 gal = 3.78 1
1 Ib = 0.454 kg
1 ton = 0.907 t
1 kWhr= 3.6 MJ
The plant superintendent instituted a polymer testing program
and evaluated several hundred polyelectrolytes. The final
selection resulted in the addition of an anionic polymer to the
sludge feed line at a point several feet upstream of the sludge
entry to the basket and then the addition of a cationic polymer
at the bowl. The results of using polyelectrolytes are given
9-22
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in the column labeled "With Polymer" in Table 9-13. The results
show that operating costs were $13.22 per ton ($14.69/t) cheaper
with polymer addition than without. :The savings occurred in
reduced labor and power requirements.
9.3.3 Solid Bowl Decanters
Decanter centrifuges for dewatering municipal wastewater
treatment plant sludges were first used in the United States in
the mid 1930s. Since then, approximately 500 machines have
been placed in 175 municipal installations (43) . Most of these
installations were for dewatering applications. Table 9-14 lists
the advantages and disadvantages of a solid bowl decanter
centrifuge compared with other dewatering processes.
TABLE 9-14
ADVANTAGES AND DISADVANTAGES OF SOLID BOWL DECANTER CENTRIFUGES
Advantages Disadvantages
Centrifuges have clean appearance, little- Scroll wear potentially a high maintenance
to-no-odor problems, and fast start-up • item
and shut-down capabilities Requires grit removal or possibly a grinder
It is easy to install in the feed stream
Provides high throughput in a small surface Requires skilled maintenance personnel
area
Gives for many sludges a cake as dry as any
other mechanical dewatering process
except for pressure filtration systems
Has one of the lowest total capital cost
versus capacity ratios
Does not require continuous operator
attention
9.3.3.1 Application
Early applications of solid bowl centrifuges were for dewatering
coarse easily dewaterable municipal wastewater treatment plant
sludges. These included raw primary, anaerobically digested
primary, and lime sludges, to name a few. The application
of centrifuges to dewatering mixtures of sludges containing
greater than 50 percent by weight of waste-activated sludges was
limited because of very poor centrate quality. Advancements in
design, especially in the entrance configuration, had reduced
floe shear. The development of new polyelectrolytes has also
contributed to greatly improving centrate quality. These
developments have made the solid bowl decanter centrifuge
applicable to a much wider range of sludge types. Further
available capacities range from 6 gallons per minute (22 to
38 1/min) to over 400 gallons per minute (1,514 1/min). The
decanter can successfully operate with a highly variable feed.
9-23
-------
9.3.3.2 Performance
Table 9-15 lists operating results that can be expected when
dewatering the sludges indicated with a solid bowl decanter. The
data in this table can be used for conducting engineering
evaluations when actual test results are not available.
TABLE 9-15
TYPICAL PERFORMANCE DATA FOR A SOLID BOWL DECANTER CENTRIFUGE
Sludge type
Feed solids
concentration,
percent solids
5-8
2-5
9-12
2-5
0.5-3
1-3
9-14
13-15
7-10
10-12
4-5
2-4
4-7
1.5-2.5
Average
cake solids
concentration,
percent solids
25-36
28-36
28-35
30-35
25-30
28-35
8-12
8-10
35-40
29-35
35-40
30-35
30-50
18-25
15-18
17-21
18-23
14-16
Polymer
required,
pounds dry
per ton dry
feed solids
1-5
0
6-10
0
1-3
6-10
10-15
3-6
0
1-4
0
2-4
0
3-7
7-10
4-8
2-5
12-15
Recovery
based on
centrate,
percent
90-95 .
70-90
98 +
65-80
82-92
95 +
85-90
90-95
75-85
90-95
60-70
98 +
90-95
90-95
90-95
90-95
85-90
85-90
Extremely variable - see study by USEPA 45
Raw primary
Anaerobically digested
primary
Anaerobically diaested
primary irradiated at
400 kilorads
Waste-activated
Aerobically digested
waste-activated
Thermally conditioned
primary + waste-activated
primary + trickling filter
High lime
Raw primary + waste-activated
Anaerobically digested
(primary + waste-activated)
Anaerobically digested
(primary + waste-activated)
+ trickling filter)
Combined sewer overflow
treatment sludge
1 Ib/ton =0.50 kg/t
9.3.3.3 Other Considerations
Solid bowl decanter centrifuges are available in either
countercurrent or concurrent flow design and either "high speed"
or "low speed" design. In the countercurrent design, the sludge
feed enters through the small diameter end of the bowl, and
solids are conveyed towards the same end. In the concurrent flow
design, the sludge feed enters through the large diameter end of
the bowl and solids are conveyed towards the opposite end.
Concurrent flow units have only been in use for about ten years.
The reasons for conveying solids away from the sludge inlet are
to reduce inlet turbulence conditions and therefore reduce floe
shear and to provide a longer residence time for the solids.
Though there are reports from Europe (47) indicating advantages
of concurrent designs over countercurrent designs, United States
experience is limited. One extensive comparative study (48)
9-24
-------
showed the countercurrent design to perform best on aerobically
digested waste-activated sludge and the concurrent one to perform
best on raw waste-activated sludge.
There is considerable controversy over the benefits associated
with "high speed" or "low speed" solid bowl decanter centrifuges.
One aspect of this controversy is the definition of "high speed"
and "low speed." In a publication by one of the major suppliers
of "low speed" machines (49), "low speed" was generally defined
as a bowl speed of 1,400 rpm or less.
Manufacturers indicate that "low speed" decanter centrifuges
consume less energy; require less polymer addition to the sludge;
have a lower noise level; and require less maintenance than a
comparable "high speed" machine to satisfy the same requirements.
This combination should therefore give "low speed" machines a
significant economical advantage on a total cost per unit weight
of solids dewatered. European work seems to substantiate this
(29), but this has not been the case in the United States. In
very extensive side-by-side studies conducted at the Dallas-Fort
Worth, Texas (50), Chicago-Calumet, Illinois, (9), Chicago-West-
^Southwest, Illinois (50), Milwaukee, Wisconsin (48), and
'Columbus, Ohio-Southerly wastewater treatment plants (50), "low
speed" machines were not overall clearly advantageous compared to
the high speed ones. In fact, in most cases, they were more
expensive on a total cost basis than the "high speed" machines.
Additional information on solid bowl decanter centrifuges can be
found in Chapter 5.
9.4 Filtration Dewatering Systems
9.4.1 Introduction
Filtration can be defined as the removal of solids from a liquid
stream by passing the stream through a porous medium which
retains the solids. Figure 9-7 shows a flow diagram of a
filtration system.
SUSPENSION
REMOVED SOLIDS L
FILRATION J^-^*^*'
HARDWARE
POROUS
MiOIA
\
t
PRIVING FORCE
(PRESSURE DROP I
I
FILTRATE
FIGURE 9-7
FLOW DIAGRAM OF A FILTRATION SYSTEM (51)
9-25
-------
As indicated on Figure 9-7, a pressure drop is required in order
for liquid to flow through the porous medium. This pressure drop
can be achieved in four ways: by creating a vacuum on one side
of the porous medium, by raising the pressure above atmoshperic
pressure on one side of the medium, by creating a centrifugal
force on an area of the porous medium, and by designing to make
use of gravitational force on the medium.
Sludge filtration-dewatering processes use one or more of these
driving forces and fall under the general filtration category of
surface filters. "Surface filters are the general type of
filtration in which solids are deposited in the form of a cake on
the upstream side of a relatively thin filter medium" (54).
9.4.2 Basic Theory
All filtration theory stems from Darcy's original work in the
mid-1850s (52). Darcy found that the flow rate Q of a filtrate
of viscosity p. through a bed of thickness L and face area A was
related to the driving pressure AP. This relationship is shown
in Equation 9-2.
Q = KAAp (9-2
where K is a constant referred to as the permeability of the bed.
Many times, Equation 9-2 is written:
O - AAp
Q " MR
where R is called the medium resistance and is equal to L/K, the
medium thickness divided by the bed permeability.
Extensive research has been, and continues to be, conducted
in defining the factors involved and level of influence in
dewatering both compressible or incompressible sludges. A
comprehensive discussion on filtration has recently been
published (51). This discussion, through examples, shows
the effects of constant pressure filtration; constant rate
filtration; constant rate-constant pressure filtration;
and variable pressure and variable rate filtration on both
compressible and non-compressible sludges.
9.4.3 Filter Aids
Filter aid is material such as diatomite, perlite, cellulose, or
carbon (50) that serves to improve, or increase the filtration
rate by physical means only. Filter aids are not added directly
to the sludge body, as a conditioning agent is, but they are
added in fixed amounts to the porous medium of the particular
dewatering equipment. The amount of filter aid added is
independent of sludge solids concentration. The filter aid
literally becomes the "filtering surface" that achieves the
9-26
-------
liquid/solids separation, and the equipment functions as a filter
holder. In order to perform its function satisfactorily, the
filter aid's particles should be inert, insoluble, incompress-
ible, and irregularly shaped, porous, and small (53).
Filter aids normally assist in dewatering difficult-to-handle
industrial sludges by either vacuum filtration or pressure
filtration (54). In the past ten years, research has been
performed on the use of filter aids for improved dewatering of
municipal wastewater treatment plant sludges (55). Table 9-16
lists results obtained from several test studies in which either
a rotary drum vacuum filter or a recessed plate pressure filter
were used.
TABLE 9-16
PRECOAT3 PROCESS PERFORMANCE ON
FINE PARTICULATE SLUDGES
Sludge properties Performance
Mixture alum and
KASb - RVPFC
WAS - RVPF ,
conditioned HAS-FP
WAS - RVPF
conditioned WAS-FP
WAS - RVPF
conditioned WAS-RVPF
Alum RVPF
Alum RVPF
Diatomite.
Waste-activated sludge.
cRotary vacuum precoat filter.
Filter press.
Fly ash conditioning and precoat.
1 Ib/sq ft/hr =4.9 kg/m2/hr
1 ton = 0.907 t
1 Ib = 0.454 g
1 Ib/ton =0.5 kg/t
Feed solids Particle
concentration , size,
percent micron
0.5 4
5.0 2
2.2 10
11. 4e
1.0 - 2.0
1.0 - 2.0
1.5
1.5
0.4 - 0.8
8.0 15
Diatomite
Specific 7 Solids Cake used,
resistance x 10 , loading, solids, Ib/ton
sec^/gm Ib/sq ft/hr percent dry solids
354 0.28 26
1.00 23
3.2 2.20 25 - 30
D.30 40 - 45
40 - 790 0.55-2.09 26-33
2 - 317 0.23 - 1.44 26 - 40
53 0.88 29
16.8 2.51 25
0.3 25 - 30
118 1.37 25
820
280
160
140
200
280
120
800
120
Solids
capture ,
percent
99
99.
99.
98.
99,
98,
99.
99.
99.
99
.9 +
.9 +
.9 +
.5
.9 +
.0
.9 +
.9 +
.9 +
.9 +
9.4.4 Vacuum Filters
In vacuum filtration, atmospheric pressure, due to a vacuum
applied downstream of the media, is the driving force on the
liquid phase that moves it through the porous media.
Vacuum filters were patented in England in 1872 by William and
James Hart. The first United States application of a vacuum
filter in dewatering municipal wastewater treatment plant
sludge was in the mid-1920s (56). Until the 1960s, the drum or
9-27
-------
scraper-type rotary vacuum filter was predominant. Since then,
the belt-type filter with natural or synthetic fiber cloth, woven
stainless steel mesh, or coil springs media has become dominant.
Recently, dewatering of municipal sludges by a top feed vacuum
filter has been studied on a pilot scale (57). Results indicated
that yiel.ds could be improved by 15 to 20 percent. The full
scale operation is expected to begin in the summer of 1979.
Table 9-17 lists the advantages and disadvantages of vacuum
filtration when it is compared to other dewatering processes.
TABLE 9-17
ADVANTAGES AND DISADVANTAGES OF USING
ROTARY DRUM VACUUM FILTERS
Advantages Disadvantages
Does not require skilled personnel Consumes the largest amount of energy per
Has low maintenance requirements for unit of sludge dewatered in most
. . . . applications
continuous operating equipment
Provides a filtrate with a low suspended Requires continuous operator attention
solids concentration Auxiliary equipment (vacuum pumps) are very
loud
9.4.4.1 Principles of Operation
Figure 9-8 shows the cutaway view of a drum or scraper-type,
rotary vacuum filter. The unit consists mainly of a horizontal
cylindrical drum that rotates, partially submerged, in a vat of
conditioned sludge. The drum surface is divided into sections
around its circumference. Each section is sealed from its
adjacent section and the ends of the drum. A separate drain line
connects each section to a rotary valve at the axis of the drum.
The valve has "blocks" that divide it into zones corresponding to
the parts of the filtering cycle. These zones are for cake
forming, cake drying, and cake discharging. A vacuum is applied
to certain zones of the valve and subsequently to each of the
drum sections through the drainlines as they pass through the
different zones in the valve.
Figure 9-9 illustrates the various operating zones encountered
during a complete revolution of the drum.
About 10 to 40 percent of the drum surface is submerged in a vat
containing the sludge slurry. This portion of the drum is
referred to as the cake forming zone. Vacuum applied to a
submerged drum section causes filtrate to pass through the media
and cake to be formed on the media. As the drum rotates, each
section is successively carried through the cake forming zone to
the cake drying or dewatering zone. This zone is also under
vacuum and begins where and when a drum section carries formed
9-28
-------
CLOTH CAULKJNG
STRIPS
AUTOMATIC VALVE
AIR AND
FILTRATE
LINE
DRUM
FILTRATE PIPING
CAKE SCRAPER
SLURRY AGITATOR
VAT
AIR BLOW-BACK LINE
SLURRY FEED
FIGURE 9-8
CUTAWAY VIEW OF A DRUM OR SCRAPER-TYPE
ROTARY VACUUM FILTER
PICK-UP OR FORM
ZONE
FIGURE 9-9
OPERATING ZONES OF A ROTARY VACUUM FILTER
9-29
-------
cake out of the sludge vat. The cake drying zone represents
from 40 to 60 percent of the drum surface and terminates at the
point where vacuum is shut off to each successive section. At
this point, the sludge cake and drum section enter the cake
discharge zone. In this final zone, cake is removed from the
media. Belt-type rotary vacuum filters differ from the drum or
scraper-type units, because the drum covering or media-belt
leaves the drum. There are basically two coverings used with
belt-type units: coil springs or fiber cloth.
WASH WATER
SPRAY PIPING
INTERNAL
PIPING
VACUUM
GAUGES
DRUM
CAKE
DISCHARGE
COIL SPRING
FILTER MEDIA
VACUUM AND
FILTRATE
OUTLETS
AGITATOR
DRIVE
VAT DRAIN
FIGURE g-10
CROSS SECTIONAL VIEW OF A COIL SPRING - BELT TYPE -
ROTARY VACUUM FILTER
Figure 9-10 shows a cross sectional view of a coil filter
spring-belt type rotary vacuum. This filter uses two layers of
stainless steel coils arranged around the drum. After the
dewatering cycle, the two layers of springs leave the drum and
are separated from each other. In this way, the cake is lifted
off the lower layer of springs and can be discharged from the
upper layer. Cake release is essentially never a problem. After
cake discharge, the coils are washed and returned to the drum.
9-30
-------
The coil filter has been and is widely used for all types of
sludge. However, sludges with particles that are both extremely
fine and resistant to flocculation dewater poorly on coil
filters. Figure 9-11 shows a typical installation.
FIGURE 9-11
TYPICAL COIL SPRING - BELT TYPE -
ROTARY VACUUM FILTER INSTALLATION
Figure 9-12 shows a schematic cross section of a fiber
cloth-belt, rotary vacuum filter. Media on this type unit leaves
the drum surface at the end of the drying zone and passes over a
small-diameter discharge roll to facilitate cake discharge.
Washing of the media occurs after discharge and before it returns
to the drum for another cycle. This type of filter normally has
a small-diameter curved bar between the point where the belt
leaves the drum and the discharge roll. This bar aids in
maintaining belt dimensional stability. In practice, it is
frequently used to ensure adequate cake discharge. Remedial
measures, such as addition of scraper blades, use of excess
chemical conditioner, or addition of fly ash, are sometimes
required to obtain cake release from the cloth media. This is
particularly true at wastewater treatment plants which produce
sludges that are greasy, sticky, and/or contain a large quantity
of activated sludge. Figure 9-13 shows a typical installation.
9-31
-------
FIGURE 9-12
CROSS SECTIONAL VIEW OF A FIBER CLOTH - BELT
TYPE - ROTARY VACUUM FILTER
9.4.4.2 Application
Vacuum filters have probably been used to dewater more types
of municipal wastewater treatment plant sludges than any
other mechanical dewatering equipment. Since the mid-1920s,
more than 1,700 vacuum filters have been installed in over
800 United States municipalities (43). The era of vacuum
filtration may be declining. Improvements in other dewatering
devices, as well as the development of new dewatering devices,
have permitted municipalities to dewater their sludge as well
as they could with vacuum filters but at lower operation and
maintenance costs.
9-32
-------
FIGURE 9-13
TYPICAL FIBER CLOTH - BELT TYPE
ROTARY VACUUM FILTER
9.4.4.3 Performance
As with all types of mechanical dewatering equipment, optimum
performance is dependent upon the type of sludge and its solids
concentration, type and quality of conditioning, and how the
Selection of vacuum level, degree of drum
media, and cycle time are all critical to
Tables 9-18 and 9-19 contain expected
cloth and coil media rotary vacuum filters
indicated. Tables 9-20 and 9-21 contain
specific operating data for several wastewater treatment plants
using cloth media and coil media.
filter is operated.
submergence, type of
optimum performance.
performance data for
for the sludge types
9.4.4.4 Other Considerations
Auxiliary Equipment
Rotary vacuum filters are normally supplied with auxiliary
equipment including vacuum pump, filtrate receiver and pump, and
sludge conditioning apparatus. Figure 9-14 shows a typical
9-33
-------
complete rotary vacuum filter process. Usually, one vacuum pump
is provided for each vacuum filter, although some larger plants
use less than one pump per filter and the pumps connect to a
common header. Until the 1960s, reciprocating type dry vacuum
pumps were generally specified, but since the early 1970s wet
type vacuum pumps are universally used. The wet type pumps are
more easily maintained and provide sufficient vacuum. Wet type
pumps utilize seal water, and it is essential that a satisfactory
water be used. If the water is hard and unstable, it may be
necessary to prevent carbonate buildup on the seals through the
use of a sequestering agent. The vacuum pump requirements are
normally 1.5 to 2.0 adiabatic cubic feet per minute of air per
square foot of drum surface area at 20 inches of mercury vacuum
(1.5 m3/min/m2 at 69 kN/m2) . This is true unless the expected
yield is greater than 40 to 50 pounds per square foot per hour
(20 to 25 kg/m2/hr) and extensive sludge cake cracking is
expected. In the latter case, an air flow 2.5 times higher
should be used.
TABLE 9-18
TYPICAL DEWATERING PERFORMANCE DATA FOR
ROTARY VACUUM FILTERS - CLOTH MEDIA
Type of sludge
Raw primary (P)
Waste-activated sludge
(WAS)
P plus WAS
P plus trickling filter
(TF)
Anaerobically digested
P
P plus WAS
P plus TF
Aerobically digested no
primary clarification
Elutriated anaerobic
digested
P
P plus WAS
Thermally conditioned
P plus WAS
Feed solids
concentration,
percent
Chemical dosage,
4.5 - 9.0
2.5 - 4.5
3-7
4-8
4-8
3-7
5-10
2.5 - 6
5-10
4.5-8
6-15
lb/ton dry
FeCl3
40-80
120-200
50-80
solids
CaO
160-200
240-360
180-240
Yield,
Ib dry solids/
sq ft/hr
3.5 - 8.0
1.0 - 3.0
2.5 - 6.0
Cake,
percent
solids
27-35
13-20
18-25
40-80
60-140
50-80
60-120
180-240
60-100 200-260
80-120 300-400
80-120 250-350
150-240
0-100
0-150
3-7
3-7
2-5
3.5-8
1.5 - 4.0
4-8
3-6
4-8
23-30
25-32
18-25
20-27
16-23
27-35
18-25
35-45
aAll values shown are for pure FeCl3 and CaO. They must be adjusted for anything
else.
Filter yields depend to some extent on feed solids concentrations. Increasing
the concentration normally gives a higher yield.
1 lb/ton =0.5 kg/t
I Ib/sq ft/hr =4.9 kg/m /hr
9-34
-------
TABLE 9-19
TYPICAL DEWATERING PERFORMANCE DATA FOR
ROTARY VACUUM FILTERS - COIL MEDIA
Type of sludge
Raw primary (P)
Trickling filter (TF)
P plus waste-activated
sludge (WAS)
Anaerobically digested
Chemical dosage,"
Feed solids
concentration,
percent
8
) 4
- 10
- 6
Ib/ton dry solids
FeCl3
40-80
40-60
CaO
, .160-240
100-140
Yield,
Ib dry solids/
sq ft/hr
6 .5
6
- 8.0
- 8
Cake,
percent
solids
28-32
20-28
3-5
20-60
180-220
2.5 - 4.0
23-27
P plus FT i
P plus WAS
Elutriated anaerobically
digested primary
5 -
4 -
8 -
8
6
10
50-80
50-80
20-50
240-320
200-300
30-120
4
3.5
4
- 6
- 4.5
- 8
27-33
20-25
28-32
All values shown are for pure FeCl3 and CaO. This must be
adjusted for anything else.
Filter yields depend to some extent on feed solids concentration.
Increasing the solids concentration normally gives a higher yield.
1 Ib/ton =0.5 kg/t
2
I Ib/sq ft/hr =4.9 kg/m*/hr
TABLE 9-20
SPECIFIC OPERATING RESULTS OF
ROTARY VACUUM FILTERS - CLOTH MEDIA
Location
willoughby Eastlake, OH
Tamaqua, PA
Grand Rapids, MI
Fort Atkinson, WI
Frankenmuth, MI
Oconomowoc , WI
Genessee City, MI
Feed solids
concentration ,
Sludge type percent
P plus (WAS) plus septic 4-6
Anaerobically digested 6
(P plus WAS)
Thermally conditioned 10 - 15
(P plus WAS)
WAS 3-4
WAS 3 . 7
Anaerobically digested 2.3
(P plus WAS)
P plus WAS 8
Cake, Yield,
Conditioner used, percent Ib dry solids/ Filtrate,
percent by weight^5 solids sq ft/hr mg/1
FeCl3-
Lime -
FeCl3-
Lime -
None
FeCl3-
Lime -
FeCl3-
Lime -
Fed -
Lime -
FeCl3
Lime -
3 20 2.8 - 4.8
14
3 18 3 SS 20 - 30
26
50 6 SS 5,000
BOD 10,000
6 19 3.0 - 3.5
16
8 15 3.2
14
6 18 2.-S - 3.0 SS 500 - 1,100
20 BOD5 10
27 5.6
16
WAS = waste-activated sludge
P = primary sludge
Numbers shown are based on pure Fed, and pure CaO.
1 Ib/sq ft/hr - 4.9 kg/m2/hr
9-35
-------
Location
Blytheville, AR
York, PA
Wyomissing Valley, PA
Bayonne, NJ
Woodbridge, NJ
Shadyside, OH
Arlington, TX
TABLE 9-21
SPECIFIC OPERATING RESULTS OF ROTARY
VACUUM FILTERS - COIL MEDIA
_Sludge type
TF
Anaerobically digested
(P plus WAS)
Anaerobically digested TF
Anaerobically digested P
Anaerobically digested
(P plus WAS)
Conditioner used.
percent by weight
FeCl3
CaO
Fed,
CaO
TF FeCl3
CaO
P FeCl,
CaO
FeCl-3
CaO
FeCl3
CaO
FeCl3
CaO
- 36
- 94
- 80
- 250
- 62
- 272
- 28
- 62
- 40
- 240
- 64
- 310
- 64
- 174
Cake, Yield,
percent Ib dry solids/
solids sq ft/hr
33.1 10.4
21.1 4.7
18.2 6.0
30.9 7.8
29.7 8.0
29 4.2
25.2 8.8
WAS = waste-activated sludge; P = primary sludge. No data available for feed solids and filtrate
concentrations.
Numbers shown are based on pure Fed3 and pure CaO.
1 Ib/sq ft/hr =4.9 kg/m2/hr
FERfllC CHLfiaiOE
AIR TO
ATMQEFHEflE
SILENCER
WATSR
FILTRATE
PUMP
\
VACUUM
PUMP
FIGURE 9-14
ROTARY VACUUM FILTER SYSTEM
9-36
-------
Each vacuum filter must be supplied with a vacuum receiver
located between the filter valve and the vacuum pump. The
principal purpose of the receiver is to separate the air from the
liquid. Each receiver can be equipped with a vacuum-limiting
device to admit air flow if the design vacuum is exceeded (a
condition that could cause the vacuum pump to overload). The
receiver also functions as a reservoir for the filtrate pump
suction. The filtrate pump must be sized to carry away the water
separated in the vacuum receiver, and it is normally sized to
provide a capacity two to four times the design sludge feed rate
to the filter.
The filtrate pump should be able to pump against a minimum
total dynamic head of between 40 and 50 feet (12 to 15 m),
which includes a minimum suction head of 25-feet (7.5 m).
Centrifugal-type pumps are commonly used but can become air bound
unless they have a balance or equalizing line connecting the high
point of the receiver to the pump. Typically, nonclogging
centrifugal style pumps are used with coil filters because they
permit a somewhat higher solids concentration in the filtrate.
Self-priming centrifugal pumps are used most frequently, since
they are relatively maintenance free. Check valves on the
discharge side of the pumps are usually provided to minimize air
leakage through the filtrate pump and receiver to the vacuum
pump.
Sludge conditioning tanks are discussed in Chapter 8.
FijLter Media
A major process variable is the filter media. The ideal media
performs the desired liquid/solid separation and gives a filtrate
of acceptable clarity (58). Further, the filter cake discharges
readily from it, and it is mechanically strong enough to give
a long life. The media must be chemically resistant to the
materials being handled and provide minimal resistance to
filtrate flow. A further characteristic to be minimized is
"blinding" or clogging. All the characteristics mentioned
above need to be evaluated during the selection procedure. One
must, therefore, through experience, or bench or pilot-scale
rotary vacuum filter testing, select the best media in terms of
porosity, type of weave, material of construction, etc. for a
particular sludge. This selection is normally made at the time
of equipment start-up by the equipment supplier (15,59). The
trend over the past few years is to select a monofilament fabric,
as they seem the most resistant to blinding.
So 1 i d s Fe d Co n t e n t
The higher the feed suspended solids concentration of the sludge,
the greater will be the production rate of the rotary vacuum
filter (Figure 9-15) and the cake suspended solids concentration
(Figure 9-16). Generally, municipal wastewater treatment plant
9-37
-------
sludges are not concentrated beyond about 10 percent solids,
since above this concentration, the sludge becomes difficult to
pump, mix with chemicals, and to distribute after conditioning to
the filter. In addition, to increased production rates, higher
sludge feed concentrations result in lower chemical dosage rates
and lower cake moistures. Both of these consequences affect the
cost of sludge dewatering and ultimate disposal.
12
11
10
9
8
7
6
5
4
3
2
1
Q
_i
UJ
a
o
D DIGESTED
» PRIMARY
o BLENDED
A ACTIVATED
J
1 2 3 4 5 .6 7 8 . 9 10 11
FEED SOLIDS {%)
FIGURE 9-15
ROTARY VACUUM FILTER PRODUCTIVITY AS A
FUNCTION OF FEED SLUDGE SUSPENDED
SOLIDS CONCENTRATION (60)
The lowest feed sludge suspended solids concentration for
successful vacuum filtration is generally considered to be
3.0 percent. Below this concentration it becomes difficult to
produce sludge filter cakes thick enough or dry enough for
adequate discharge. For this reason, it is extremely important
that the design and operation of the preceding sludge processes
take into consideration the need for an optimal solids
concentration when dewatering on vacuum filters.
9-38
-------
3B
30
2B
OJ
Q 20
_j
O
Hi 15
1.0
MCCARTY
PRIMARY
ACTIVATED
11 gm/L CaO, 3,7 gm/L FeCI3
1 23456789 10
FEED SOLIDS (%)
FIGURE 9-16
SLUDGE CAKE TOTAL SOLIDS CONCENTRATION AS A FUNCTION OF
THE FEED SLUDGE SUSPENDED SOLIDS CONCENTRATION (60)
9.4.4.5 Case History
This study is summarized from a USEPA-sponsored investigation
(61). Figure 9-17 shows the 1977 flow diagram for the 13-MGD
(34 m-^/sec) Lakewood, Ohio, wastewater treatment plant. The
sludge being handled at this plant has changed several times
since the facility was built in 1938. At that time, the
plant was designed for primary treatment, with sludge being
anaerobica1ly digested and dewatered on sand drying beds.
Secondary treatment was added in 1966. Gravity thickeners, two
new anaerobic digesters, two vacuum filters, and a flash dryer
were installed to handle additional sludge. In 1974 and 1975,
the plant was further upgraded. Alum (aluminum sulfate) was
added to the aeration basin effluent channel for phosphorus
removal, and the sludge handling system (filters and dryer)
operating schedule was extended to two shifts. Finally, in 1977,
the plant was returned to single shift sludge handling, and
excess liquid sludge was hauled to land disposal.
The Lakewood plant has two polyethylene cloth belt rotary vacuum
filters. Only one can be operated at a time because of the
9-39
-------
limited capacity of the
effective area of 376
at a drum speed of one revolution per eight
submergence between 30 to 36 inches (0.76 to
flash ; dryer. _^Each filter has an
and operates best
minutes and a drum
0.91 m). A filter
square feet (35 m
is operated five days per week in either one or two 6.5-hour
shifts per day. Conditioning chemical dosages are approximately
275 pounds of dry lime (pebble lime - 72 percent CaO) per ton of
dry feed solids (137 kg/t) and 30 pounds of FeCl3 (liquid at
40 percent FeCl3) per ton of dry feed solids (15 kg/t).
COMMUNI TOflS
FIGURE 9-17
LAKEWOOD, OHIO WASTEWATER TREATMENT PLANT FLOW DIAGRAM
Prior to 1975, before alum was added for phosphorus removal
(63 mg/1 alum added), the average total solids concentration of
the digested sludge (vacuum filter solids feed) was 4.45 percent.
On the average, the sludge was dewatered to 23.8 percent solids.
After alum addition, the feed sludge solids concentration
increased to 6.5 percent, but the dewatered cake percent dropped
to 21.4.
Table 9-22 indicates operational costs for 6.5-hour and
13-hour-per-day operations based on before and after alum
addition for phosphorus removal. Because of the increase in the
number of tons from 650 dry tons per year (590 t/yr) in 1974 to
1,820 dry tons per year (1,651 t/yr) in 1976, the treatment cost
per ton of dry total solids was not much greater than it was in
1974.
9-40
-------
TABLE 9-22
OPERATIONAL COST OF LAKEWOOD, OHIO VACUUM FILTER OPERATIONS
Single shift
operation - 1974
dollars per ton
dry solids
Double shift
operation - 1976
dollars per ton
dry solids
Ferric chloride and lime
Electricity
Maintenance supplies
Maintenance and repair labor
Operational labor
Overhead
Total
8.90
1.98
1.11
3.65
3.46
2.25
21.35
8.90
1.29
1.10
3.60
6.25
3. 11
24.25
1 ton = 0.907 t
9.4.4.6 Costs
Figure 9-18 gives the 1975 capital cost as a function of filter
area for rotary vacuum filters. As an example, a 400-square-foot
(37.2 m2) area filter would cost 400,000 dollars. Since this
number is based on a June 1975 cost, it must be adjusted to the
current design period. Costs include those for filter, auxiliary
equipment, piping, and building.
The labor requirements indicated in Figure 9-19 are given as a
function of average area in use and include: start-up time and
clean-up after the- filter run, operation of filter, and operation
of sludge pumping and conditioning facilities prior to treatment.
As an example, a vacuum filter having 400 square feet (37.2 m2)
of filter area would require 550 man-hours of operation and
maintenance per year and would be included in the cost analysis.
Figure 9-20 gives power consumption as a function of filter
area. As an example, a vacuum filtration area of 400 square
feet (37.2 m2) would require 330,000 kilowatt-hours per
year (1,200 GJ/yr ) of electrical energy. If power costs are
0.05 dollars per kilowatt-hour (0.014 dollars/MJ), the cost would
be 33,000 dollars annually. Operating parameters used were based
on two adiabatic cubic feet of air per minute per square foot
(10 1/s/m2), 20 inches of vacuum (68 KN/m2), and a total dynamic
head of 50 ft (15 m) for the filtrate pump. Power required
includes that for drum drive, discharge roller, and vat agitator,
but does not include other accessory items, such as sludge feed
pump or chemical feed system.
Figure 9-21 shows a curve developed for estimating rotary drum
vacuum filter maintenance material cost as a function of filter
area. As an example, for a filtration area of 400 square feet
9-41
-------
t/J
EC
V)
3
o
tc.
o
u_
in
IT
O
I
y
(T
i
Z>
z
-z.
<
100,000
2 34 56788100 234 S8789lrtXX» 234 56789
SINGLE VACUUM FILTER AREA, sq ft (1 sq ft - 0,093 rn2)
FIGURE 9-18
ESTIMATED JUNE 1975 CAPITAL COST FOR ROTARY DRUM
VACUUM FILTERS (39)
2 -
1,000
2 3 4B67831QQ 2 3 4^67891,000 2 3 456789
, AVERAGE AREA IN USE, sq ft {1 sq ft = 0,093 m2}
FIGURE 9-19
ANNUAL OSM MAN-HOUR REQUIREMENTS - ROTARY DRUM
VACUUM FILTERS (39)
9-42
-------
(37.2 m^) , a designer would estimate a yearly materials cost of
4,000 dollars. Since this number is based on a June 1975 cost,
it must be adjusted to the current design period.
to
£
S
SI
X
O
ut
DC
>-
O
c
UJ
uu
-i
o
fie
o
01
10,000
10 234 56 789100 2 34 567Bi1,QQQ 234 56 78910,000
VACUUM FJLTRATION AREA, iq ft (1 iq ft - 0.093 m2}
FIGURE 9-20
POWER CONSUMED BY ROTARY DRUM VACUUM
FILTRATION PROCESS (39)
9.4.5 Belt Filter Press
Belt filter presses employ single or double moving belts to
dewater sludges continuously.
The early belt presses used in the United States were those
developed by Klein and by Smith and Loveless in the 1960s
(62,63). Belt filter presses are currently very popular not only
9-43
-------
in the United States (64) but in other parts of the world as well
(65). At least 20 equipment suppliers can furnish some type of
belt press. This popularity has led to many units being sold,
with very little operational experience to support the claimed
advantages. One detailed report that evaluated belt press
operating experience found that there were many operational and
maintenance problems that still needed to be solved (66). As
was pointed out by Austin (65), significant developmental work
is still being conducted. Table 9-23 lists advantages and
disadvantages of belt filter presses.
o
u
Z
2 34B678S1QG 2 34 667891,000 2 3 4567B9
AVERAGE AREA IN USE, sq ft (1 sq ft = Q.Q93 m2)
FIGURE 9-21
ESTIMATED JUNE 1975 ANNUAL MAINTENANCE MATERIAL
COST - ROTARY DRUM VACUUM FILTER (39)
9-44
-------
TABLE 9-23
ADVANTAGES AND DISADVANTAGES OF BELT FILTER PRESSES
Advantages Disadvantages
High pressure machines are capable of Very sensitive to incoming feed
producing very dry cake characteristics
Low power requirements Machines hydraulically limited
in throughput
Short media life as compared with other
devices using cloth media
9.4.5.1 Principles of Operation
Any belt filtration process includes three basic operational
stages: chemical conditioning of the feed slurry, gravity
drainage to a nonfluid consistency, and compaction of the
predewatered sludge (6).
Figure 9-22 depicts a simple belt press and shows the location
of the three stages. Although present-day belt presses are
more complex, they follow the same principles indicated in
Figure 9-22.
Good chemical conditioning is the key to successful and
consistent performance of the belt filter press, as it is
for other dewatering processes. This is fully discussed in
Chapter 8.
After conditioning, the readily drainable water is separated from
the slurry by discharge of the conditioned material onto the
moving belt in the gravity drainage section. Typically, one or
two minutes are required for drainage. Following drainage, the
sludge will have been reduced in volume by about 50 percent and
will have a solids concentration of 6 to 10 percent. "The
formulation of an even surface cake at this point is essential to
the successful operation of subsequent stages of the dewatering
cycle. The even surface prevents uneven belt tension and
distortion while the relative rigidity of the mass of sludge
allows further manipulation and gives maximum speed through the
machine" (65 ) .
The third stage of the belt press begins as soon as the sludge
is subjected to an increase in pressure, due to either the
compression of the sludge between the carrying belt and cover
belt or the application of a vacuum on the carrying belt.
Pressure can be widely varied by design, as shown by the
Pressure can be widely v
alternatives on Figure 9-23.
9-45
-------
CHEMICAL
CONDITIONAL
STAGE
POLYILECTROUTE
SOLUTION
GRAVITY
'DRAINAGE
STAGE
COMPRESSION
. DE WATER ING
STAGE
SLUQGi
CONDITIONED
sty ocs
WASH WATER
FIGURE 9-22
THE THREE BASIC STAGES OF A BELT PRESS
During pressure application, the sludge cake, squeezed between
the two belts, is subjected to flexing in opposite directions as
it passes over the various rollers. This action causes increased
water release and allows greater compaction of the sludge.
Figure 9-24 shows a typical belt press installation.
9.4.5.2 Application
Belt filter presses are being installed in many United States
municipalities to dewater many types of sludge. At this time,
there is not enough operational data available to indicate any
sludges to which a belt filter press could not be applied.
9.4.5.3 Performance
It is difficult to generalize about the operating performance
of belt presses because results depend on many factors: method
of conditioning, maximum pressure, number of rollers, etc.
Table 9-24 was developed from minimum and maximum values given in
all published data.
Published material on operating belt press installations is
very limited. Medford, New Jersey (67) reported on a belt
press dewatering aerobically digested sludge from a contact
stabilization system. Feed sludge of a 3 to 4 percent solids
concentration was dewatered to a cake of 17 to 19 percent solids
(67). Polymer was added for conditioning at 7 to 10 pounds of
dry polymer per ton of dry feed solids (3.5 to 5 kg/t). The
solids concentration in the combination washwater and filtrate
was 100 mg/1 for an overall solids capture of 99 percent.
9-46
-------
GRAVITY
DRAINAGE
[o;o;o;o;o;o;oj
******
COMPRESSION
DEWATERING
LOW PRESSURE
SECTION
J^oWalo
p;o;oio!b~**
* * * t
HIGH PRESSURE
SECTION
V * \ f f
$Lf\fi*
///y K\
/ / 1 * „
/ ^ t
III!
t Mtt tttt
4 * *
VACUUM ASSISTED
.'»> ,...:-, r -*»
P LLJ LLJ LfJ Q
If**!
FIGURE 9-23
ALTERNATIVE DESIGNS FOR OBTAINING WATER
RELEASES WITH BELT FILTER PRESSES (66)
9.4.5.4 Other Considerations
Failure of the chemical conditioning process to adjust to
changing sludge characteristics can cause operational
problems (66). If it is underconditioned, sludge does not
9-47
-------
FIGURE 9-24
TYPICAL BELT FILTER PRESS INSTALLATION
TABLE 9-24
TYPICAL DEWATERINC PERFORMANCE OF BELT FILTER PRESSES
Type of sludge
Raw primary (P)
Waste activated sludge (WAS)
P + WAS
P + trickling filter (TF)
Anaerobically digested
P
WAS
P + WAS
Aerobically digested
P + WAS
Thermal conditioned
P + WAS
Feed
solids ,
percent
3-10
1-3
0.5-1.5
3-6
3-6
4-10
3-4
3-9
1-3
6-8
4-8
Cake ,
percent
solids
28-44
16-32
12-28
20-35
20-40
26-36
18-22
18-44
12-18
20-30
38-50
Polymer,
pounds dry
per ton
dry solids
2-9
2-4
4-12
2-10
3-10
2-6
4-8
3-9
4-8
2-5
0
1 Ib/ton =0.5 kg/t
9-48
-------
drain well in the gravity drainage section, and the result
is either extrusion of inadequately drained solids from the
compression section, or uncontrolled overflow of sludge from the
drainage section. Both underconditioned and overconditioned
sludges can blind the filter media. In addition, overconditioned
sludge drains so rapidly that solids cannot distribute across the
media. Inclusion of a sludge blending tank step before the belt
press reduces this problem. See Chapter 15 for a discussion of
blending tanks.
The combined filtrate and belt washwater flow is normally about
one and one-half times the incoming flow. Some belt presses
recirculate washwater from the filtrate collection system, but
normally, secondary effluent or potable water is used. This
total flow contains between 100 and 1,000 mg/1 of suspended
solids and is typically returned either to the primary or
secondary treatment system.
Belt presses have numerous moving parts, and spare parts
should be kept available to prevent prolonged unit down-time.
Belts, bearings, and rollers deteriorate quickly, especially
in municipal wastewater treatment plants where preventive
maintenance is not normally practiced.
9.4.5.5 Design Example
The designer for an existing wastewater treatment plant has
calculated that the plant needs to dewater 5,000 dry pounds of
sludge (2,268 kg) per day, five days per week. The sludge
to be dewatered is a mixture of one part primary and two parts
waste-activated, stabilized by a two-stage, high-rate, anaerobic
digestion process. Total feed solids concentration to the belt
filter press was 2.8 percent. Pilot plant testing with a
one-meter-wide belt filter press produced the following results.
• Total solids in the dewatered sludge ranged from 23 to
30 percent, averaging 25 percent.
• Optimum polymer dosage was 6 to 8 pounds of dry polymer
per ton (3 to 4 kg/t) of dry feed solids, or 80 to
100 pounds of liquid polymer per ton (40 to 50 kg/t) of
dry feed solids.
• At the optimum polymer dosage, the total solids in
the filtrate plus washwater flow was 2,000 mg/1. The
suspended solids averaged 900 mg/1.
• Optimum hydraulic feed rate at 2.8 percent solids for a
one-meter-wide belt was 47 gallons per minute (3 1/s).
• Washwater requirements were 25 gallons per minute
(1.6 1/s).
9-49
-------
On the basis of pilot plant data, the engineer decided that one
1-meter-wide belt filter press could dewater the 5,000 pounds
(2,268 kg) of sludge in 7.6 hours. Since it was important that
the wastewater treatment plant always be able to dewater sludge,
two 1-meter-wide belt filter presses would be purchased.
The current cost of dry polymer in 50 pound (22.7 kg) bags was
$1.85 per pound ($0.84/kg); for liquid polymer in 55 gallon,
650 pound (208 1-295 kg) drums, the cost was $0.13 per pound.
Daily cost for dry polymer at 8 pounds per ton (4 kg/t) would be:
5,000 Ib solids 8 Ib poly $1.85 ,,__ nn
-" da7 x 2,000 Ib solids X Ib poly = $37'00 per
Daily cost for liquid polymer at 100 pounds per ton (50 kg/t)
would be:
5,000 Ib solids 100 Ib poly $0.13 _ $32 5Q ,
day x 2,000 Ib solids x Ib poly " $^-50 Per day
Because sludge characteristics can change with time, a dual
polymer system capable of utilizing either liquid or dry polymer
will be installed. Since liquid polymer is currently less
expensive, it will be used initially.
To allow subsequent computation of solids capture, the filtrate
flow is calculated, using a suspended solids balance and a flow
balance. The specific gravity of the feed, dewatered cake and
filtrate are assumed to be 1.02, 1.07 and 1.01, respectively.
The suspended solids balance is:
47 gal feed 8.34 x 1.02 Ib feed 0.028 Ib solids
min gal feed Ib feed
Q gal filtrate 8.34 x 1.01 Ib filtrate 900 Ib solids
min gal filtrate 106 Ib filtrate
M gal cake 8.34 x 1.07 Ib sludge 0.25 Ib solids
min gal cake Ib cake
The flow balance is:
47 gal feed 25 gal washwater
min ™ """"TnTrr
Q gal filtrate + M gal cake
min min
9-50
-------
The suspended solids and flow balances are solved simultaneously.
The flow of filtrate (Q) is 67.2 gallons per minute (254 1/m).
Solids capture
Solids in feed - solids in filtrate
Solids in feed
x 100
900
47 (8.34 x 1.02) (0.028) - 67.2 (8.34 x 1.01) TTg-
= _ _ ..- .-.- - . . ^--' v inn
47 (8.34 x 1.02) (0.028)
= 95 percent
All filtrate is returned to the secondary treatment process.
9.4.5.6 Costs
Current published information on capital cost of belt filter
presses is almost nonexistent. Some information is available
from a recent USEPA publication (68). According to this
publication, construction costs for a belt filter press,
sludge feed pump, polymer pump, and control panel to dewater
1,000 pounds (454 kg) of sludge per hour was $97,000. To dewater
2,500 pounds (1,134 kg) per hour, the cost would be $120,000.
Table 9-25 lists labor requirements for the operation and
maintenance of belt filter presses. The labor indicated includes
periodic operational adjustments and minor routine maintenance.
No information is available on maintenance material cost.
TABLE 9-25
LABOR REQUIREMENTS FOR BELT FILTER PRESSES (19)
Number Labor, hours per year
of
units Operation Maintenance Total
1 265 100 365
2 530 200 730
3 795 300 1,095
4 1,060 400 1,460
5 1,325 500 1,825
9-51
-------
9.4.6 Recessed Plate Pressure Filters
Pressure filtration for sludge dewatering evolved from the
similar practice in sugar manufacturing of forcing juices through
cloth. The first United States municipal sludge dewatering
installations, which were also the first large-scale mechanical
dewatering applications in this country, were located in
Worcester, Massachusetts, and Providence, Rhode Island, in the
early 1920s (56). Fixed- and variable-volume recessed plate
pressure filters are discussed in this section.
Fluid pressure generated by pumping slurry into the unit
provides the driving force for recessed plate pressure filters.
Performance reliability is increased by modern design concepts,
such as use of new construction materials to resist attack by
acids and alkalis; mechanization of the operating sequence to
reduce manpower requirements; and the use of membrane diaphragms
for variable volume filtration (69). Table 9-26 lists the
advantages and disadvantages of pressure filters compared with
other dewatering methods.
TABLE 9-26
ADVANTAGES AND DISADVANTAGES OF RECESSED PLATE
PRESSURE FILTERS
Advantages
Disadvantages
Highest cake solids concentration
Batch operation
High labor cost
High capital cost
Special support structure requirements
Large area requirement
9.4.6.1 Principles of Operation
Fixed-volume, recessed plate pressure filters, illustrated
on Figure 9-25, are constructed from a series of recesssed
plates. As shown on Figure 9-26, volume is provided by the
depressions on the sides of the plates.
The surfaces of both sides of the filter plate are designed so
that the filtrate drains from the filter cloth and from each
plate.
A filter cloth is mounted over the two surfaces of each filter
plate. Conditioned sludge is pumped into the pressure filter and
passes through feed holes in the filter plates along the length
of the filter and into the recessed chambers. As the sludge cake
forms and builds up in the chamber, the pressure gradually
increases to a point at which further sludge injection would
9-52
-------
FIXED OR
FEED HEAD
PLATES
MOVEABLE
HEAD
•idh
aodaj
: *
dcij
o
ri
i
: \
ho \
CLOSING
HEAD
HYDRAULIC
CLOSURE
FIGURE 9-25
SCHEMATIC SIDE VIEW OF A RECESSED PLATE PRISSURI FILTER
CAKE
SLURRY
INLET
FILTRATE OUTLETS
FIGURE 9-26
CROSS SECTION OF A FIXED-VOLUME
RECESSED PLATE FILTER ASSEMBLY
9-53
-------
be counter-productive. Pressure filters
100 pounds per square inch (690 kN/m2)
square inch (1,550 to 1,730 kN/m2).
operate at a pressure of
or 225 to 250 pounds per
A typical pressure filtration cycle begins with the closing of
the press to the position shown on Figure 9-25. Sludge is fed
for a 20- to 30-minute period until the press is effectively full
of cake. The pressure at this point is generally the designed
maximum and is maintained for a one- to four-hour period, during
which more filtrate is removed and the desired cake solids
content is achieved. The filter is then mechanically opened, and
the dewatered cake dropped from the chambers onto a conveyor belt
for removal. Cake breakers are usually required to break up the
rigid cake into conveyable form. Figure 9-27 shows a typical
pressure filter installation.
FIGURE 9-27
TYPICAL RECESSED PLATE PRESSURE
FILTER INSTALLATION AT WASSAU, WISCONSIN
Construction of a variable-volume recessed plate pressure filter
is similar to the fixed-volume filters, except that a diaphragm
is placed behind the media as shown on Figure 9-28. A dewatering
cycle begins as conditioned sludge is fed into each chamber from
a slurry inlet pipe located in the top or bottom of each plate.
Generally, about 10 to 20 minutes are required to fill the press
9-54
-------
and reach an end point determined by either instantaneous feed
rate, filtrate rate, or time. When the end point is reached,
the sludge feed pump is automatically turned off. Water or air,
under high pressure, is then pumped into the space between the
diaphragm and plate body squeezing the already
partially dewatered cake. Typically, 15 to 30
constant pressure are required to dewater the cake to
solids content. At the end of the cycle, the water
to a reservoir, plates are automatically opened, and
is discharged.
formed and
minutes of
the desired
is returned
sludge cake
SLURRY,
INLET
(TOP OR
BOTTOM)
CAKE
FILTRATE
OUTLET
(TOP OR
BOTTOM)
CLOTH
SOFT RUBBER
MEMBRANE
HIGH PRESSURE
WATER
FILTRATE
CAKE UNDER
COMPRESSION
•MOULDED
RUBBER BODY
SHAPE OF FILTER CHAMBER
DURING FILTRATION
SHAPE OF FILTER CHAMBER
DURING CAKE COMPRESSION
BY DIAPHRAGM
FIGURE 9-28
CROSS SECTION OF A VARIABLE VOLUME
RECESSED PLATE FILTER ASSEMBLY
9.4.6.2 Application
Pressure filtration is an advantageous choice for sludges of poor
dewaterability, such as waste-activated sludges, or for cases in
which it is desirable to dewater a sludge to a solids content
9-55
-------
higher than 30 percent. If sludge characteristics are expected
to change drastically over a normal operating period, or if less
chemical conditioning is desired, the variable-volume units would
probably be selected rather than the fixed-volume units.
TABLE 9-27
EXPECTED DEWATERINC PERFORMANCE FOR A TYPICAL FIXED
VOLUME RECESSED PLATE PRESSURE FILTER
Conditioning
Type of sludge
Raw primary (P)
Raw P with less than
50 percent waste
activated sludge (WAS)
Raw P with more than
50 percent WAS
Anaerobically digested
mixture of P and WAS
Less than 50 percent WAS
More than 50 percent WAS
WAS
Feed
solids,
percent
5-10
3-6
1-4
6-10
2-6
1-5
Ibs/ton
FeCl3a
100
100
120
100
150
150
dry
CaOa
200
200
240
200
300
300
dosage ,
solids
Ash
2,000
3,000
4,000
2,000
4,000
5,000
Cake with
conditioning
material ,
percent
solids
45
50
45
50
45
50
45
50
45
50
45
50
Cake without
conditioning
material,
percent
solids
39
25
39
20
38
17
39
25
37
17
37
14
Cycle
time,
hours
2.0
1.5
2.5
2.0
2.5
2.0
2.0
1.5
2.5
1.5
2.5
2.0
All values shown are for pure FeCl, and CaO. Must be adjusted for anything else.
1 Ib/ton =0.5 kg/t
1 Ib/sq ft/hr =4.9 kg/nT/hr
9.4.6.3 Performance
As of 1979, very few fixed-volume recessed plate pressure
filters are operating in the United States, and there are no
variable-volume installations operating. Table 9-27 contains
expected performance data for typical fixed-volume units, and
Table 9-28 lists actual data from operating installations.
Table 9-29 lists a performance from a large variable-volume pilot
unit (62.4 square feet [5.8 m2] of filtering area).
9.4.6.4 Other Considerations
Sludge Conditioning Process
Most systems are designed so that ferric chloride and lime
are added in batches to sludge contained in an agitated tank,
and the conditioned sludge is pumped from the tank into the
pressure filter as required. However, experience indicates
9-56
-------
TABLE 9-28
SPECIFIC OPERATING RESULTS OF FIXED VOLUME
RECESSED PLATE PRESSURE FILTERS
Percent solids
Location
Kenosha, WI
Wausau, WI
Cedar Rapids, IA
Brookfield, WI
Sludge type
Anaerobically di-
gested mixture
(P plus WAS)
Water plant plus
thermal conditioned
mixture of anaer-
obically digested
(P plus WAS)
Anaerobically di-
gested mixture
(P plus TF)
WAS plus raw P
Feed
solids ,
percent
3.5 - 5
Conditioner ,
Ib/ton dry
solidsb
FeCl3 - 54
Lime -340
Cake with
conditioning
material
41.5
Cake without
conditioning
material
35
Year and
total cost/
dollars/ton
dry solids
1975 - 61
Reference
70
2 -
34 - 45
35 - 45 Not given 71
li- 3.5 - 7
re
jested 4
P
Fly ash at 6Q
about
2,500
FeCl3 - 143 43
Ash - 1,200
Lime - 346
27 1972 - 30
25 Not given
72
61
P = primary sludge; WAS = waste-activated sludge; TF = trickling filter sludge.
All values shown for FeCl^ and CaO are for pure chemicals. Must be adjusted for
anything else.
1 Ib/ton =0.5 kg/t
1 ton = 0.907 t
TABLE 9-29
TYPICAL DEWATERINC PERFORMANCE OF A VARIABLE VOLUME
RECESSED PLATE PRESSURE FILTER
Site
1
2
3
4
5
6
7
8
9
Type of sludge
Anaerobically digested
60 P: 40 WAS
60 P: 40 WAS
40 P: 60 WAS
40 P: 60 WAS
50 P: 50 WAS
60 P: 40 WAS
Raw WAS
Raw (60 plus 40 WAS)
Thermal conditioned
50 P: 50 WAS
Feed
solids,
percent
3.8
3.2
3.8
2.5
6.4
3.6
4. 3
4.0
14.0
Chemical
dosage,3
Ib/ton dry
solids
FeCl3
120
180
120
180
80
160
180
100
0
CaO
320
580
340
500
220
320
460
300
0
Percent solids
Yield,
Ib/sq ft/hr
1.
0.
0.
0.
2.
0.
0.
0.
2.
0
7
6
6
0 .
8
6
9
5
Cake with
chemicals
37
36
40
42
45
50
34
40
60
Cake without
chemicals
30
25
32
30
39
40
25
33
60
All values shown are for pure FeCl3 and CaO. Must be adjusted for
any-thing else.
P = primary sludge; WAS = waste-activated sludge.
1 Ib/ton =0.5 kg/t
1 Ib/sq ft/hr =4.9 kg/m /hr
9-57
-------
that the prolonged agitation and tank storage time associated
with batch conditioning can result in a feed of varying and
deteriorating dewaterability. For this reason, conditioning
processes are now frequently designed to provide "in-line"
conditioning. This can be accomplished by either the continuous
pumping of sludge into a small tank and addition of chemicals,
or directly injecting conditioning chemicals into the sludge on
its way into the filter. In-line conditioning diminishes
the deleterious effects of storage and prolonged agitation.
Figure 9-29 shows a schematic for in-line conditioning.
IPOLYELECTRQUTE
MIXING TANK
ALUMINUM
CHLQHOHYDRATE
SILO
LEVEL SWITCHES CQNTRQUNG SLUDGE FEED
AND DILUTE CHEMICAL FEED PUMPS
FILTRATE TO
HEAD OF
WORKS
SLUDGE HOLDING
TANK
FIGURE 9-29
SCHEMATIC OF AN IN-LINE CONDITIONING SYSTEM FOR
RECESSED PLATE PRESSURE FILTER (73)
Feed Pump System
One major problem with pressure filters has been the need to
design a system that will pump from 30 to 2,000 gallons per
minute (1.9 to 126 1/s) of a viscous, abrasive slurry at pres-
sures of 40 to 225 pounds per square inch (276 to 1,551 kN/m^).
Ideally, the feed system should inject conditioned sludge into
the chamber as rapidly as possible but slowly enough to permit
sufficiently prompt formation of a uniform and thick enough cake
to prevent any incursion of sludge particles into the filter
cloth. Imbalance of the sludge feed and cake formation rates can
result in nonuniform, high resistance cake, or in cloth blinding
and/or initial poor filtrate quality. If a nonuniform cake is
formed or excessive fines migrate, then a long filter cycle or an
inordinate amount of cloth plugging will result.
9-58
-------
The filter feed method used for some pressure filters involves a
combination of pumps and pressure vessels. These combinations
are used to obtain a high initial feed rate of approximately
2,000 gallons per minute (126 1/s) via the pressure vessel,
followed by the use of reciprocating ram high pressure pumps to
pump at a pressure of 225 pounds per sqare inch (1,551 kN/m2)
at feed rates of 100 to 200 gallons per minute (6.3 to 12.6 1/s).
In some cases, a combination of progressive cavity pumps and
pressure vessels is used for the lower pressure, high-rate
chamber filling phase.
Cloth Washing and Cleaning
Because recessed plate pressure filters operate at high pressures
and because many units use lime for conditioning, the designer
must assume that cloths will require routine washing with high
pressure water, as well as periodic washing with acid. Practices
vary according to the particular sludge and proprietary process.
Designers should ask for recommendations from equipment suppliers
on frequency of washing.
Dewatered Cake Breakers
Design of suitable breakers is a function of the structural
properties of the dewatered cake. Pressure filter cake is
usually friable enough that use of breaker wires, bars, or
cables beneath the filter will be sufficient. If, however,
polyelectrolyte conditioning is contemplated, consideration
should be given to the resulting changes in cake structure.
9.4.6.5 Case History
This information is summarized from a recent sludge handling
investigation by USEPA (61). The 1978 flow diagram for the
5-MGD (13-m^/sec) Brookfield, Wisconsin, wastewater treatment
plant is shown on Figure 9-30. In January 1974, Brookfield
commenced treatment by the contact stabilization activated sludge
process. Addition of ferrous sulfate from pickle liquor for
phosphorus removal in the aeration tank was initiated in June
1976. The plant has one fixed-volume, recessed plate pressure
filter with a design capacity of 530 pounds dry solids per hour
(241 kg/hr).
Performance
The pressure filter is generally operated four days per week,
16 hours per day, 45 weeks per year. The other 7 weeks per year,
the sludge is applied to land. Figure 9-31 summarizes operating
performance before (letter B) and after (letter A) the addition
of ferrous sulfate. Figure 9-31 also presents a mass flow
diagram of an operating recessed plate pressure filter.
9-59
-------
RETURN ACTIVATED SLUDGE
PRIMARY SLUDGE
SCRUBBER
WATER
DIGESTER SUPERNATANT
SECONDARY
CWASTE ACTIVATED!
SLUDGE
SLUDGi
CONDITIONING
TANK
ALTERNATE
DISPOSAL Of
LIQUID SLUDGE
BY TANK TRUCK
LANDFILL
FIGURE 9-30
BROQKFIELD, WISCONSIN WASTEWATER TREATMENT
PLANT FLOW DIAGRAM
The 1976 operating and maintenance costs for the pressure filter
are combined with the incinerator operational cost in Table 9-30.
With the initiation of chemical addition for phosphorus removal,
the cost of treating and disposing of a ton of dry solids
decreased by approximately $1.33, as shown in Table 9-30. This
reduction was due to decreases in the amounts of chemical condi-
tioners and electricity required by the plate pressure filter.
These decreases were, however, partially offset by an increase in
the amount of auxiliary fuel used by the incinerator. This was
the result of decreased incinerator volatile solids feed rates.
9.4.6.6 Cost
Figure 9-32 gives fixed-volume, recessed plate pressure filter
capital cost as a function of press volume. Costs include
those for filter auxiliary equipment, piping, and building.
As an example, a pressure filter having 100 cubic feet (2.8 nH)
capacity would cost about $700,000. Since this number is based
on June 1975 cost, it must be adjusted to the current design
year.
9-60
-------
FILTER CAKE
SLUDGE TO
PRESSURE FILTER
QA = 328,000 gal/mo
TSA = 131,000 Ib/mo
%TSA = 4.77
QB •= 395,000 gal/mo
TSB = 116,000 Ib/mo
%TSB = 3.54
CONDITIONING ADMIX
ASH FECL3 LIME
79,000 Ib/mo 1,810 gal/mo 32,400 gal/mo
0.60 Ib ASH/ 8,840 Ib/mo 22,600 Ib/mo
Ib DRY SOLIDS 135 Ib FECLj/ 346 Ib LIME/
I TON DRY SOLIDS TON DRY SOLIDS
\ 1 i
98,000 Ib/mo 1,770 gal/mo 28,800 gal/mo
0.85 Ib ASH/ 8,280 Ib/mo 20,100 Ib/mo
Ib DRY SOLIDS 152 Ib FECL3/ 345 Ib LIME/
TON DRY SOLIDS TON DRY SOLIDS
Q= FLOW
TS = TOTAL SOLIDS
VS = VOLATILE SOLIDS
FS = FIXED (NONVOLATILE) SOLIDS —
%TS
%VS
%FS
= PERCENT DRY TS BY WEIGHT ""
= PERCENT DRY VS BY WEIGHT
= PERCENT DRY FS BY WEIGHT
TOTAL:
SLUDGE PLUS
ADMIX TO FILTER
WET CAKEA = 506,000 Ib/mo
TSA = 219,000 Ib/mo
VSA = 71,000 (32.6% OF Ib TS]
FSA = 148,000
%TSA > 43.4
%VSA= 14.1
%FSA = 91%
WET CAKEB * 421,000 Ib/mo
TSB = 182,000 Ib/mo
VSB = 61,000 (33.6% of OF Ib TS)
FSB = 121,000
%TSB - 43.2
%VSB = 14.5
%FSB = 28.7
QA = 362,000 gal/mo PRESSURE
TSA - 240,000 Ib/mo _.J.'t_3
%TSA = 7.95 AFTER
90 runs/mo
1.73 hrs/run
1 55 hrs/mo
QB = 426.000 gal/mo BEFORE
TSB = 243,000 Ib/mo ~}g run s/n7o^
%TSB = 6'85 2.83 hrs/run
232 hrs/mo
FILTRATE
QA = 328,000 gal/mo
TSA = 21,000 Ib/mo
QB = 397,000 gal/mo
TSB - 62.0OO Ib/mo
1 Ib = 0.454 kg
1 gallon = 3.78 I
FIGURE 9-31
PERFORMANCE DATA FOR A PRESSURE FILTER
BROOKFIELD, WISCONSIN
TABLE 9-30
PRESSURE FILTRATION AND INCINERATION OPERATIONAL COST
1976 Dollar cost
per ton dry solids
Item
FeCl3
Lime
Natural gas0
Electricity
Labor
Total'
Unit cost,
1976 dollars
Before
After
0.0305
0.001786
0.04
6.00
Includes incinerator warm-up.
1 ton = 0.907 t
9
10
ri
10
20
$62
.61
.52
.73
.40
.00
.26
8.69
10.55
12. 29
9.60
20.00
$61.13
9-61
-------
-g
U)
U>
u
E
CCS
Z
o
10,000,000
9
a
7
6
5
1,000,000
§
8
?
6
i
4
I
I
3 4 56789 TOO 2 3 4 S 6 7891,000 2
SINGLE PRESS VOLUME, cu ft fl cu ft = 0.02B m3)
3 4 5 6 7B9
FIGURE 9-32
ESTIMATED JUNE 1975 COSTS FOR FIXED VOLUME
RECESSED PLATE PRESSURE FILTERS (39)
Figure 9-33 indicates fixed-volume, recessed plate pressure
filter labor requirements. Labor requirements are based on
continuous, seven-day-per-week operation with two-hour cycles and
include operation and maintenance for both press and related
auxiliaries (chemical feed system and pumps). As an example,
a pressure filter having 100 cubic feet (2.8 m^) of capacity
would require 8,000 man-hours of operation and maintenance per
year and would be included in the cost analysis.
Figure 9-34 gives power consumption as a function of feed solids
concentration and operating volume. The graph is based on a
filter that operates continuously, seven days per week, and
has a 2-hour cycle time. Power consumption includes that for
the feed pump, open and close mechanisms, and moveable head
mechanism.
Figure 9-35 presents a graph developed for estimating annual
material and maintenance costs for a fixed-volume, recessed plate
9-62
-------
pressure filter. The graph is based on unit operation of seven
days per week with a two-hour cycle time.
DC
O
LL
Vt
(E
O
X
O
z
2 -
2 3456789100 2 34 567891,000 2 3 456789
AVERAGE FILTER PRESS VOLUME m USE, cu ft (1 cu ft = 0,028 m3}
FIGURE 9-33
ANNUAL O£M MAN-HOUR REQUIREMENTS - FIXED VOLUME
RECESSED PLATE PRESSURE FILTIR (39)
9.4.7 Screw and Roll Press
9.4.7.1 Screw Press
This dewatering device employs a screw surrounded by a perforated
steel (screen) cylinder. Sludge is pumped inside the screen and
is deposited against the screen wall by the rotating screw. The
cake that forms acts as a continuous filter. The screw moves the
progressively dewatered sludge against a containment at the
outlet and further dewaters the sludge by
action against the restriction. Figure
layout from one screw press manufacturer.
municipal wastewater treatment plants
operation, large-scale studies have been
lists typical results.
pressure of the screw
9-36 shows a typical
Although no full-scale
are known to be in
conducted. Table 9-31
9-63
-------
«
II
^.
i
5
1
Q
t
I
«
UJ
1
z
z
te
8
_j
D
Z
z
100,00?
§
8
7
6
S
4
3
2 3 45678B100 2 3 4 5 6 7 89 1,000 2 3 4 5 6 7BS
AVERAGE FILTER PflESS VOLUME IN USE, cu ft (1 cu ft = 0.028 m3)
FIGURE 9-34
FIXED VOLUME RECESSED PLATE PRESSURE
FILTER POWER CONSUMPTION (39)
I
2 34 56789100 2 34 667891,000 1 34 B67S9
AVERAGE FILTER PRESS VOLUME IN USE, cu ft (1 cu ft = 0.028 m3)
FIGURE 9-35
ESTIMATED JUNE 1975 ANNUAL MAINTENANCE MATERIAL
COST-FIXED VOLUME, RECESSED PLATE PRESSURE FILTER (39)
9-64
-------
SLUDGE FEED
WASH WATER
/MER '
CTOR
5SEL
f.
r
i
i
i
i
i
1 i
1
1
1
1
C
(.
I
(OPTIONAL)
»_•— "->!
N
:
!>
-— %_
i
'SLUDGE
CAKE
*^~^~- SCREW
FILTRATED
9 1
1
FILTRATE x-M I
PUMP (^J ,
L
FIGURE 9-36
SYSTEM SCHEMATIC FOR ONE TYPE OF SCREW PRESS SYSTEM
Location
TABLE 9-31
PERFORMANCE RESULTS FROM A SCREW PRESS
Sludge type
Stratford, CT Primary only
Feed
solids,
percent
3-5
Polymer,
Ib dry/ton
dry solids
0
Cake
solids,
percent
25-31
Filtrate,
percent
solids
0.9-1.4
Reference
74
Norwich, CT
Primary plus waste-
• activated
50:50 mixture
67:33 mixture
Anaerobically digested
mixture 60 percent
primary plus 40 percent
waste-activated
3-3.3
2.7-4.0
5.5-9.8
3.9-5.6
13-17 0.7-2.0
20-27 0.7-2.0
18.6-22.6
0.2-1.0
75
1 Ib/ton =0.5 kg/t
9-65
-------
9.4.7.2 Twin-Roll Press
Figure 9-37 shows a cross section of a twin-roll, vari-nip press.
Developed in 1970 by modifying a fixed nip twin-roll press, the
vari-nip press was installed in 17 plants by 1976. One of these
plants is municipal (76).
HOOD
CAKE DOCTOR
AND SEAL
SHREDDER
CONVEYOR
MOVEABLE
ROLL
PRESSATE
CHANNELS
PRE-THtCKENlNG
MODE
ROLL CLEANING
SHOWERS
PRESS
ROLLS
FIXED
ROLL
VARIABLE SPEED
VAT AGITATOR
VAT
SLUDGE FEED
FIGURE 9-37
CROSS SECTION VIEW OF A TWIN-ROLL VARI-NIP PRESS
The unit consists of a pair of perforated rolls, one roll fixed
and the other moveable, so that the nip (or space) between the
rolls can be varied. The horizontal rolls are mounted in a
sealed vat. Sludge is pumped into the vat under a slight
pressure of two to four pounds per square inch (14 to 28 kN/m^).
This low vat pressure moves the sludge into the nip, where it
is further dewatered by a nip pressure load of 200 to 400 pounds
per lineal inch (36 to 72 kg/lineal cm) of roll length. Filtrate
passes from the sludge through the perforated rolls and
discharges by gravity. The compressed cake is then doctored off
the rolls and discharged into a shredder and conveyor.
The "Pig's Eye Plant" at St. Paul, Minnesota has evaluated
the dewatering of mixtures of primary and waste-activated
sludge (76). Results showed that on raw primary sludge, a cake
9-66
-------
of 35 percent was obtainable after sludge conditioning with
approximately seven pounds of dry polymer per ton (3.5 kg/t) of
dry feed solids. When biological sludge was added, performance
decreased and polymer requirements increased. At a mixture
of 50:50, cake solids dropped to 28 percent, while
requirements increased to 17 pounds of dry polymer
(8.5 kg/t) of dry feed solids. The conclusion was that
an excellent dewatering unit for primary sludge.
polymer
per ton
this was
9.4.8 Dual Cell Gravity (DCG) Filter
The DCG unit consists of two independent cells formed by a
nylon filter cloth. The cloth travels continuously over guide
wheels and is rotated by a drive roll and sprocket assembly. A
cross section of a typical DCG unit is shown in Figure 9-38.
Dewatering occurs in the first cell, and cake formation, in the
second cell.
GUIDE
WMEEL
DRIVE ROLL AND
SPROCKET ASSEMBLY
NYLON
FILTER CLOTH
GUIDE WHEEL
CAKE FORMING
CELL
DEWATERING
CELL
SLUDGE
fNLET
FIGURE 9-38
CROSS SECTION VIEW OF A DUAL CELL GRAVITY FILTER
Sludge is introduced in the dewatering cell, where initial
liquid/solids separation takes place. The dewatering solids are
then carried over the drive roll separator into the second cell.
Here, they are continuously rolled and formed into a cake of
relatively low moisture content. The weight of this sludge
cake presses additional water from the partially dewatered
sludge carried over from the dewatering cell. When the cake of
dewatered solids grows to a certain size, excess quantities are
discharged over the rim of the second cell to a conveyor belt
that moves the material out of the machine.
9-67
-------
Table 9-32 summarizes the operating results from Mentor, Ohio,
which has three units to dewater an aerobically digested mixture
of primary, waste-activated sludge and a mixture of primary,
waste-activated, and alum sludge generated from phosphorus
removal.
TABLE 9-32
SUMMARY OF PERFORMANCE RESULTS FOR A DUAL CELL
GRAVITY FILTER - MENTOR, OHIO (61)
Primary
olus waste
activated sludge
Primary plus
waste activated
plus alum sludge
Feed - percent total solids
Cake - percent total solids
Polymer usage
Cationic - liquid Ibs per ton solid
Anionic - dry Ibs per ton solids
Filtrate characteristics
2.1-2.7
8.8-9.2
143
0.4
2.5-3.1
8.2-9. 1
136
0.04
Not given
1 Ib/ton =0.5 kq/t
9.4.9 Tube Filters
Tube filters can be either of the pressure type or of the
gravity type.
9.4.9.1 Pressure Type
Commonly known as tube filter presses, pressure type tube filters
have been used in industry (77). However, there are no municipal
installations. Typically, this type of device consists of an
outer cylinder, an internal rubber bladder, and an internal
perforated cylinder which is covered with a filter media. The
whole assembly is mounted vertically.
Slurry is pumped into the annular space between the bladder
and media-covered wall. When this area is full, the bladder
is filled with liquid, and the slurry is compressed against
the filter media. Filtrate flows through the media and is
discharged. When the desired cake solids concentration has been
obtained, liquid pressure is released and the cake is discharged
with a blast of air.
9.4.9.2 Gravity Type
In this application, sludge is mixed with polymer and then held
in suspended porous bags. The weight of the sludge forces water
out of the bag sides and bottom. Sludge is retained for a
maximum of 24 hours, depending upon the desired dryness, and is
then released through a bottom opening.
9-68
-------
Following is a description of the 0.5-MGD (21.9 1/s) dewatering
facility at Half Moon Bay, California.
This facility consists of four bags, each 3 feet (0.9 m) in
diameter and 9 feet (2.7 m) long with a ring at the top to
support the polyester media bag and a ring at the bottom, which
is engaged circumferentially by a motor-driven chain. The chain
twists the ring about 360 degrees, thereby closing off the bottom
so that the bag can be filled. Suspended down the center of the
bag is a polyester tube about 6 inches (15 cm) in diameter with
the end extending about 12 inches (0.3 m) beyond the bottom of
the closed ends. All four bags are mounted outdoors on a steel
framework over a concrete pad containing the drainline and
chemical conditioning system. The sludge fills the annular core,
and the filtrate seeps through the outer polyester media surface
and the inner core tube.
The batch operation practiced at Half Moon Bay is on a 24-hour
cycle consisting of a four-hour fill period (waste-activated
sludge from a complete mix aeration plant) and a 20-hour drain.
With a 1.5 percent solids feed, a 16 percent solids cake has been
obtained.
9.5 Other Dewatering Systems
Several other types of dewatering devices are available that do
not readily fall into any of the previously discussed units.
These include cyclones, screens, and electro-osmosis.
9.5.1 Cyclones
In the municipal wastewater field, cyclones or hydrocyclones
(name given to cyclones specifically designed for liquids) have
been used for cleaning and dewatering grit from grit chambers,
primary clarifiers, and anaerobic digesters since the early
1950s. Since then, over 1,400 units have been installed (43).
When a liquid stream enters a cyclone, the particles are
separated by centrifugal acceleration. Unlike centrifuges,
cyclones have no moving parts. The liquid motion inside the unit
causes the necessary acceleration. The theory of cyclones is
thoroughly covered in a recent discussion by Svarovsky (78).
By itself, a cyclone does not dewater. The underflow from the
cyclone discharges into a type of dewatering device. This device
may be as simple as a steel bin with drainage holes, or as
complex as a rotating screen screw or rake classifier. These
dewatering devices will produce a grit with a moisture content
ranging from 20 to 35 percent.
The degritted liquid stream (overflow) from a cyclone degritting
raw sludge normally goes to a gravity thickener. When the
cyclone is degritting the flow from grit chambers, the overflow
9-69
-------
is usually recycled to the grit chamber. Some designers have
found it necessary to screen this overflow to keep debris from
overwhelming the system. The drainage from the dewatering device
is collected and typically returned to the head of the treatment
plant.
9.5.2 Screens
"Screening is the process of separating grains, fragments or
lumps of a variety of sizes into groups, each of which contains
only particles in the size range between definite maximum and
minimum size limits" (79). In addition to being used in
dewatering (26), screens have also been used for primary
treatment (80), thickening (81,82), and conditioning (see
Chapter 8).
The primary use of screens in dewatering would be with bar
screenings or the underflow from grit cyclones. In one extensive
study (83), the following results were found:
• Ground bar screenings could be dewatered to six percent
solids with a static type screen.
• Ground bar screening could be dewatered to sixteen
percent solids with a revolving drum screen.
• Underflow from a grit cyclone could be dewatered to
25 percent solids with either the static or revolving
screen.
The popularity of screens is slowly increasing in the United
States because in certain applications they offer advantages in
both capital cost and operating cost.
9.5.3 Electro-Osmosis
The use of electro-osmosis for dewatering municipal wastewater
sludge has been studied on a pilot-plant scale (84). The system
consists of a vertical-mounted, endless moving belt which is
drawn over vertical plate-mounted, stainless steel cathodes,
submerged in a tank of waste sludge. Results indicated that
cakes of over 20 percent solids could be obtained from an
anerobically digested sludge having 2.6 percent feed solids.
9.6 References
1. Craig, E.W., D.D. Meredith,"and A.C. Middleton. "Algorithm
for Optimal Activated Sludge Design."
Environmental Engineering Division, ASCE.
p. 1101. 1978.
9-70
-------
Dick, R.I. and D.L. Simmons. "Optimal Integration of
Process for Sludge Management." .Proceedings 3rd National
Conference on Sludge Management Disposal and Utilization,
Miami Beach, FlT^ 12/14-16/76 , sponsored by ERDA, USEPA,
NSF and ITI, p. 20, Information Transfer Inc., Rockville,
Maryland 20852.
USEPA. Cost of Landspread ing and Hauling Sludge from
Municipal Wastewater Treatment Plants. Office o~f Sol id
~~ ~~
WasTte . Wa¥hTngTol^~lxrTbTiro~ E~PA~530/SW-619 . October
1977.
4. Carry, C.W., R.P. Miele, and J.F. Stahl. "Sludge
Dewatering." Proceedings of the National Conference on
Municipal Sludge Management. Pittsburgh , PA, 6/11-13/74 .
Sponsored by Allegheny County, PA, p. 67, Information
Transfer Inc., Rockville, Maryland 20852.
5. Ohara, G.T., S.K. Raksit, and D.R. Olson. "Sludge
Dewatering Studies at Hyperion Treatment Plant." Journal
Water Pollution Control Federation. Vol. 50, p. 912 ( 1978) .
6. USEPA. j?ilot Investigation of Secondary Sludge Dewatering
Alternatives^ Industrial En vi ronmen tal Re se arch Lab ,
Cincinnati, Ohio 45268. NTIS PB-280-982, February 1978.
7. USEPA. Evaluation of Dewatering Devices for Producing High
Solids Sludge Cake. Office of Research and Development.
Cincinnati, Ohio 45268. EPA 600/2-79-123. February 1979.
8. Cassel, A.F. and B.P. Johnson. "Evaluation of Dewatering
Units to Produce High Sludge Solids Cake." Presented
at the 51st Annual Conference Water Pollution Control
Federation. Anaheim, California. October 2, 1978.
9. Zenz, D.R., B. Sawyer, R. Watkins, C. Lue-Hing, and
G. Richardson. "Evaluation of Unit Processes for Dewatering
of Anaerobically Digested Sludge at Metro Chicago's Calumet
Sewage Treatment Plant." Presented at the 49th Annual
Conference Water Pollution Control Federation. Minneapolis,
Minnesota. October 1976.
10. USEPA. Operations Check Lists. Office of Water Program
Operations. Washington, DC 20460. MCD 48B. February 10,
1977.
11. USEPA. Cost Estimates for Construction of Publicly Owned
Wastewater Treatment Facilities - Summaries of Technical
Data. Office of Water Program Operations . Washington, DC
20460. MCD 48B. February 10, 1977.
9-71
-------
12. USEPA. Sludge Handling and Disposal Practices at Selected
Municipal Wastewater Treatment Plants. Office of Water
Program Operations. Washington, DC 20460. MCD 36. April
1977.
13. Spillner, F. "The Drying of Sludge." J3gw_a_g e Sludge.
London, England, 1912.
14. USEPA. Sludge Dewatering and Drying on Sand Beds. Office
of Research and Development, Cincinnati, Ohio 45268. EPA
600/2-78-141, August 1978.
15. Eckenfelder, W.W. and D.L. Ford. Water Pollution Control.
Pemberton Press, Austin, TX and New York, NY 1970.
16. Walski, T.M. "Mathematical Model Simplifies Design of
Sludge Drying Beds." Water and Sewage __Wg_rk_s_. p. 64.
April 1976.
17. Water Pollution Control Federation. MOP 8 Wastewater
Treatment Plant Design. Water Pollution Control Federation.
1977.
18. USEPA. Performance Evaluation and Troubleshooting at
Municipal Wastewater Treatment Facilities. Office of Water
Program Operations. Washington, DC 20460. EPA 430/
9-78-002. February 1978.
19. USEPA. Sludge Handling and Conditioning. Office of Water
Program Operations, Washington, DC 20460. EPA 430/ 9-78-
112. February 1978.
20. South, W.T. "Asphalt Paved Sludge Beds." Water and Sewage
Works. Vol. 106, p. R396. 1959.
21. Lynd, E.R. "Asphalt-Paved Sludge Drying Beds." Sewage and
IjTdjJS trjial^Was tes. Vol. 28, p. 697. 1956.
22. Lewing, V.H. "Survey of Some Methods of Sludge Dewatering."
The Surveyor. Vol. 121, #3680, p. 1521. 1962.
23. Swanwick, J.D. and Baskerville, R.C. "Dewatering and
Industrial Sludges on Drying Beds." Chemistry and Industry.
p. 338, February 20, 1965.
24. Stokes, F.E. and J.M. Harwood. "Aluminum Chlorohydrate in
Sludge Treatment." Effluent and Water Treatment Journal.
Vol. 4, p. 329. 1964.
25. Crockford, J.B. and V.R. Sparham. "Developments to Upgrade
Settlement Tank Performance, Screening, and Sludge
Dewatering Associated with Industrial Wastewater Treatment."
Proceedings of 27th Purdue Industrial Waste Conference,
Purdue University, Lafayette, Indiana 47907. 1972.
9-72
-------
26. U.S. Department of Interior. A Study of Sludge Handling and
Disposal. Federal Water Pollution Control Administration,
Office of Research and Development No. WP-20-4. May 1968.
27. USEPA. "Developments in Dewatering Wastewater Sludges."
Technology Transfer Seminar on Sludge Treatment and
Disposal. Vol. 1. Technology Transfer. Cincinnati,
Ohio 45268. October 1978.
28. Beardsley, J.A. "Sludge Drying Beds Are Practical." Water
and Sewage Works. Part 1, p. 82, July; Part 2, p.42.
August (1976).
29. Thompson, L.H. "Mechanized Sludge Drying Beds." The
Engineer. July 1966.
30. Kershaw, M.A. "Development in Sludge Treatment and Disposal
at the Maple Lodge Works, England." Journal Water Poj-jjjJbj.C)n
Cgrvy^j1_Federati_on_. Vol. 37, p. 674. 1965.
31. Water Pollution Control Federation. MOP 20 Sludge
Dewatering. Water Pollution Control Federation. 1969.
32. Jeffrey, E.A. and P.F. Morgan. "Oxygen Demand of Digested
Sludge Liquor." Sewage and Industrial Wastes. Vol. 31,
p. 20. 1959.
33. Imhoff, K. and G.M. Fair. Sewage Treatment. John Wiley &
Sons, New York, New York. 1956.
34. Water Pollution Control Federation. MOP 8 Sewage Treatment
Plant Design. Water Pollution Control Federation. 1959.
35. Haseltine, T.R. "Measurement of Sludge Drying Bed
Performance." Sewage and Industrial Wastes. Vol. 23,
p. 1065. 1951.
36. Recommended Standards for Sewage Works. Great Lakes/Upper
Mississippi River Board of State Sanitary Engineers, 1971.
37. USEPA. Areawide Assessment Procedures Manual - Volume III.
Municipal Environmental Research Laboratory. Cincinnati,
Ohio 45268. EPA 600/9-76-014. July 1976.
38. USEPA. Construction Costs for Municipal Wastewater
Treatment Plants. Office of Water Program Operations.
Washington, DC 20460. MCD 37. January 1978.
39. Culp/Wesner/Culp. Cost and Performance Handbook Sludge
Handling Processes. Prepared for Wastewater Treatment and
Reuse Seminar, South Lake Tahoe, California. October 1977.
9-73
-------
40. Jeffrey, E.A. "Laboratory Study of Dewatering Rates
for Digested Sludge in Lagoons." Proceedings of 14th Purdue
Industrial Waste Conference^ Purdue University, Lafayette,
Indiana 47907. 1959.
41. Jeffrey, E.A. "Dewatering Rates for Digested Sludge in
Lagoons." Journal Water _Polluj^io_n_^Cgntrol Federaticm.
Vol. 32, p. 1153. 1960.
42. Reefer, C.E. and H. Krotz. "Experiments on Dewatering
Sewage Sludge With a Centrifuge." Sewage Works Journal,
Vol. 1, p. 120. 1929. """" "~
43. Taken from equipment manufacturers installation lists.
44. Hansen, B.E., D.L. Smith, and W.F. Garrison. "Start-up
Problems of Sludge Dewatering Facility." Presented at the
51st Annual Conference Water Pollut i onL_Cgn t .EX)jL_Fe_de. r;a t i.on .
Anaheim, California. October 1978. _____
45. USEPA. Handling and Disposal of Sludges From Combined Sewer
Overflow Treatment Phase III - Treatability Studies.
Environmental Protection Technology Series, Office of
Research and Development, Cincinnati, Ohio 45268. EPA-600/
2-77-053C, December 1977.
46. Zacharias, D.R. and K.A. Pietila. "Full-Scale Study of
Sludge Processing and Land Disposal Utilizing Centrifugation
For Dewatering." Presented at the 50th Annual Meeting of
the Central States Water Pollution Control Federation,
Milwaukee, Winconsin. May 18-20, 1977.
47. Albertson, O.E. and E.E. Guidi, Jr. "Centrifugation of
Waste Sludges." Journal Water Pollution Control Federation.
Vol. 41, p. 607. 1969.
48. Camp, Dresser & McKee, Inc. Centrifugal Dewatering of Waste
Activated Sludge. Report on testing and equipment proposals
for Jones Island Wastewater Treatment Plant, Milwaukee,
Winconsin. October 1977.
49. Guidi, E.J. "Growth and Benefits of Low Speed Centrifuga-
tion." Water and Sewage Works. June 1977.
50. Personal communication with Mr. F.W. Keith, Jr., Director
of Environmental Technology, Sharpies-Stokes, Warminster,
Pennsylvania. April 1979.
51. Svarovsky, L. "Filtration Fundamentals." Solid-Liquid
Separation, Butterworths, Inc., Ladislav Svarovsky, editor,
1977.
9-74
-------
52. Darcy, H.P.G. "Les Fontaines Publiques de la Ville de Dijon
(The Public Wells of the City of Dijon)." V. Dalmont Paris,
1856. English translation by J.J. Fried. Water Resources
Bulletin American Wate^^Res^ou^rcqsAjssqciationTVo 1. 1, p. 4 .
1965.
53. Masters, A.L. "Filter Aids." Solid-Liquid Separation,
Butterworths, Inc. Ladislav Svarovsky, editor, 1977.
54. Basso, A.J. "Getting the Most Out of Filter Aids."
Chemical Engineering. p. 185. September 12, 1977.
55. NCASI. A Pilot Plant Study of Mechanical Dewatering
Devices Operated on Waste Activated Sludge. Prepared for
National Council of the Paper Industry for Air and Stream
Improvement. TechnicalBulletin 288, November 1976.
56. Flynn, E.O. "The Mechanical Dewatering of Sewage Sludge on
Vacuum Filters." Sewage Works Journal. Vol. 5, p. 957
1933.
57. Leary, R.D., L.A. Ernest, G.R. Douglas, A. Geinopolos and
D.G. Mason. "Top-feed Vacuum Filtration of Activated
Sludge." Journal Water Pollution Control Federation.
Vol. 46. p. 1761. 1974.
58. Purchas, D.B. "Filtration in the Chemical and Process
Industries - 1," Filtration. p. 256. 1964.
59. Vesilind, P.A. Treatment and Disposal of Wastewater
Sludges. Ann Arbor Science. Ann Arbor, Michigan 48106.
1974.
60. Bennett, E.R., D.A. Rein, and K.D. Linstedt. "Economic
Aspects of Sludge Dewatering and Disposal." Journal of the
Environmental Engineering Division ASCE. Vol. 99, p. 55.
1973.
61. USEPA. Review of Techniques for Treatment and Disposal of
Phosphorus-Laden Chemical Sludges. Office of Research and
Development. Cincinnati, Ohio 45268. EPA 600/2-79-083.
February 1979.
62. Goodman, B.L. and R.B. Higgins. "A New Device for
Wastewater Treatment Sludge Concentration." Water and
Wastes Engineering. August 1970.
63. Goodman, B.L. and R.B. Higgins. "Concentration of Sludges
by Gravity and Pressure." Proceedings of 25th Purdue Indus-
trial Waste Conference. Purdue University, West Lafayette,
Indiana 47907. 1970.
64. Dembitz, A.E. "Belt Filter Press: A New Solution
to Dewatering?" Water and Wasjtes Engineering. p. 36.
February 1978. ~
9-75
-------
65. Austin, .E.P. "The Filter Belt Press - Application and
Design." Filtration and Separation. p. 320. July/August
1978.
66. NCASI. A Review of the Operational Experience with Belt
Filter Presses for Sludge Dewatering in 'the North American
Pulp and Paper Industry. Prepared for National Council
of the Paper Industry for Air and Stream Improvement.
Te£hnJ£a^Bulletin 315. October 1978. ~
67. Eichman, B.W. "Dewatering Machine Solves Sludge Drying
Problems." Water and Sewage^Jgp_rks . p. 99 (October 1977).
68. USEPA. Innovative and Alternative Technology Assessment
Manual - Draft. Office of Water Programs. Washington, DC
20460. MCD 53. 1979.
69. Wake man, R.J. "Pressure Filtration." Sol id-Liquid
Separation. Butterworths, Inc., Ladislav Svarovsky, editor,
1977.
70. Nelson, O.F. "Operational Experience with Filter Pressing."
Water Pollution Controjl^^ede^ration - Deeds and Data. March
1978.
71. Bizjak, G.J. and A.E. Becker, Jr. "Wausau Solves Dual
Problem by Using Filter Press." W_a t e r__ a n d Wastes
Engineering . p. 28. February 1978.
72. USEPA. Pressure Filtration of Wastewater Sludge With Ash
Filter Aid. Office of Research and Development, Cincinnati,
Ohio 45268. EPA-R2-73-231 . 1973.
73. Farnham Pollution Control Works, Thames Water Authority,
England. 1977.
74. Bechir, M.H. and W.A. Herbert. "Sludge Processing Using
Som-A-Press. " Presented at the New England Water Pollution
Control Association. October 1976.
75. Taylor, J.A. Evaluation of Somat Som-A-System Dewatering
Method for the Norwich Water Pollution Control Plant,
Norwich, Connecticut. Somat Corporation. Pomeroy,
Pennsylvania. July 1978.
76. Bergstedt, D.C. and G.J. Swanson. "Evaluation of A
Twin-Roll Continuous Press For Municipal Sludge Dewatering."
Presented at the 49th Annual Conference Water P
Control Federation, Minneapolis, MN . October 1976.
77. Gwilliam, R.D. "The E.C.C. Tube Filter Press." Filtration
and Separation. March/April 1971.
78. Svarovsky, L. "Hydrocyclones . " Solid-Liquid Separation,
Butterworths, Inc. Ladislav Svarovsky, editor. 1977.
9-76
-------
79. Osborne, D.G. "Screening." Solid-Liquid Separation,
Butterworths, Inc. Ladislav Svarovsky, editor. 1977.
80. Phillips, T.G. "Screening ... A Novel Approach to Primary
Treatment." Presented at Ontario Pollution Control
Association annual meeting, Toronto, Canada. April 1977.
81. Fernbach, E. and G. Tchobanoglous. "Centrifugal Screen
Concentration for Activated Sludge Process." Water and
Sewage Works. Part I. January; Part II. February 1975.
82. Syal, R.K. "Compare Sludge Handling Alternatives." Water
and Wastes Engineering. Vol. 16, p. 60. March 1979.
83. Brown and Caldwell Consulting Engineers. Study of
Wastewater Solids Processing and Disposal. Prepared for the
Sacramento Regional County Sanitation District. June 1975.
84. Dewatering Sewage Sludge by Electro Osmosis. Part I -
Basic Studies, Part II - Scale Up Data. Prepared by the
Electricity Council Research Centre, Capenhurst, England.
NTIS PB-276 and NTIS PB-276 412, 1975 and 1976.
9-77
-------
EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapter 10. Heat-Drying
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 10
HEAT-DRYING
Heat-drying is the process of evaporating water from sludge by
thermal means. Ambient air-drying of sludges is discussed in
Chapter 9, and composting, in Chapter 12.
10.1 Introduction
In the United States, dry waste-activated sludges and those from
Imhoff tanks have been heat dried to produce a soil conditioner
and nutrient source since the early 1920s. Historically, the use
of heat-drying has been justified based on the expectation that
sales of the dried material would substantially offset process
costs. However, demand for the product has generally been low in
the fertilizer market. Milwaukee, Wisconsin; Houston, Texas;
Chicago, Illinois; and Largo, Florida, are notable exceptions
where marketing has been successful. Because revenues have
generally been low and because heat-drying is expensive, net
costs have often been high, and the process has not found wide
application. The use of heat-drying must be evaluated in
the context of overall sludge management at a given facility.
10.2 Heat-Drying Principles
Sludge is heat dried at temperatures too low to destroy organic
matter. Water vapor is carried away by a moist gas (usually
air). The designer establishes the actual conditions of
drying — for example, temperature, humidity, detention time,
velocity, and direction of flow of the gas stream across the
drying surface.
10.2.1 Drying Periods
The following are the three well-defined stages in heat-drying:
1. Initial Drying. During this stage, the sludge
"temperature and the drying rates are increased to the
steady state conditions of the second stage. Stage one
is usually short; little drying occurs during this time.
2. Steady State Drying. The time that the sludge is in this
stage is generally the longest of all the stages. The
surfaces of the sludge particles are completely saturated
with water. Surface water is replaced with water from
10-1
-------
the interior of the solid as fast as it is evaporated.
Drying proceeds as if the water were evaporated from a
pool of liquid. The solid itself does not significantly
influence the drying rate. For this drying period, the
temperature at the sludge/gas interface is ordinarily
kept at the wet-bulb temperature of the gas. As long as
unbound surface moisture is present, the solid is
heated only to the wet-bulb temperature of the gas;
solids may therefore be dried with fairly hot gases
and not themselves attain elevated temperatures. For
example, the wet-bulb temperature is 133°F (56°C) for a
gas stream that has an absolute humidity of 0.01 pounds
water per pound dry air and a temperature of 600°F
(316°C).
3. Final Drying. The final stage occurs when sufficient
water has evaporated that the solid surface is only
partially saturated. Surface water is evaporated
more rapidly than it can be replaced by water from the
interior of the solid. As a consequence, overall drying
rates are markedly lower in stage 3 than in stage 2.
During this period, the temperature of the solid/gas
interface increases because latent heat cannot be
transferred from the sludge to the gas phase as rapidly
as sensible heat is received from the heating medium.
Sludge moisture content is normally expressed in percent
moisture, percent solids, or pounds water per pound dry sludge.
The minimum sludge moisture content practically attainable with
heat drying depends upon the design and operation of the dryer,
moisture content of the sludge feed, and the chemical composition
of the sludge. For ordinary domestic wastewater sludges, sludge
moisture contents as low as five percent may be achieved.
Chemical bonding of water within the sludge, which can occur
through chemical addition for sludge conditioning, or chemicals
present in industrial sludges can increase the amount of water
retained in the dried products beyond the five percent moisture
level.
10.2.2 Humidity and Mass Transfer
Humidity is a measure of the moisture content of the gas phase
at a given temperature and is important to consider when
determining drying rates. Absolute humidity is a measure of the
weight of water per unit weight of dry gas (for example, pounds
water per pound dry air).
In heat-drying of sludge, water is transferred to the gas
phase. The driving force for transfer is the difference
between absolute humidity at the wetted solid/gas interface
10-2
-------
and the absolute humidity in the gas phase. The transfer
rate — that is, the drying rate—can be described by the following
equation:
W = KyA (Ys - Ya) (10-1)
where:
W = rate of drying, pounds water per hour (kg/hr);
Ky = mass transfer coefficient of the gas phase, pounds water
per hour per square foot per unit of humidity difference
(kg/hr/m2/unit of humidity difference);
A = area of wetted surface exposed to drying medium,
square feet (m2);
Ys = humidity at the sludge/gas interface temperature, pounds
water per pounds dry gas (kg/kg);
Ya = humidity of the gas phase, pounds water per pounds
dry gas (kg/kg).
10.2.3 Temperature and Heat Transfer
In heat-drying, the temperature difference between the heating
medium and the sludge/gas interface provides the driving force
for heat transfer.
Dryers are commonly classified on the basis of the predominant
method of transferring heat to the wet solids being dried. (1).
These methods include:
Convection (direct drying) . Heat transfer is accomplished
by direct contact between the wet sludge and hot gases. The
sensible heat of the inlet gas provides the latent heat required
for evaporating the water. The vaporized liquid is carried off
by the hot gases. Direct dryers are the most common type used in
heat-drying of sludge. Flash dryers, direct rotary dryers, and
fluid bed dryers employ this method. Convective heat transfer is
described by Equation 10-2.
tJconv = hcA (tg - ts) (10-2)
where:
qconv = convective heat transfer, Btu per hour (kJ/hr);
hc = convective heat transfer coefficient, Btu per hour
per square foot per °F (kJ/hr/m2/°C);
10-3
-------
A
= area of wetted surface exposed to gas, square feet
= gas temperature, °.F (°C);
= temperature at sludge/gas interface, °F (°C).
Conduction (indirect drying). Heat transfer is accomplished by
contact of the wet solids with hot surfaces (for example, a
retaining wall separates the wet solid and the heating medium).
The vaporized liquid is removed independently of the heating
film dryer .employs this principle. Conductive
by Equation 10-3.
wall
vaporized
medium. The thin
heat transfer is described
(10-3)
where:
^cond = conductive heat transfer, Btu per hour (kJ/hr);
ncond
m
= conductive heat transfer coefficient, Btu per hour
per°F (kJ/hr/°C);
= area of heat transfer surface, square feet (m^);
= temperature of drying medium--for example, steam,
O Ui / O (~* \ .
V ^ / / '
ts = temperature of sludge at drying surface, °F (°C).
The conductive heat transfer coefficient (hconc3) is a composite
term that includes the effects of the heat transfer surface and
sludge-side and medium-side films. Descriptions of methods for
computing hcon(~| are available in textbooks and from dryer
manufacturers (1-4).
Radiation (infrared or radiant heat-drying). Heat transfer is
accomplishedbyradiant energy supplied by electric resistance
elements, by gas-heated incandescent refractories that also
provide the advantage of convective heating, or by infrared
lamps. The Shirco Company furnace and multiple-hearth furnaces
are examples of drying equipment that use radiant heat.
Radiation heat transfer is described by Equation 10-4.
qrad = e s A a (t
.- t
(10-4)
where:
qrad = radiation heat transfer, Btu/per hour (kJ/hr);
es = emissivity of the drying surface, dimensionless;
10-4
-------
A = sludge surface area exposed to radiant source,
square feet (m^);
a = Stefan - Boltzman constant, 1.73 x 10~9 Btu/per hour
per square foot per °R (4.88 x 10~8 k cal/m2/hr/°k);
tr = absolute temperature of the radiant source, °R;
ts = absolute temperature of the sludge drying surface,
°R;
This discussion of heat drying is necessarily brief; the reader
is referred elsewhere for more information (1-5). Equations for
mass and heat transfer rates and for associated drying times for
specific dryer types are discussed in detail in these references.
It is often difficult to determine appropriate values of mass and
heat transfer coefficients to be used in these equations. Thus,
results predicted by the equations and results obtained in
practice may be divergent, perhaps critically so. Most usable
design information is obtained by testing with actual process
feeds under conditions closely simulating prototype operations.
Many dryer manufacturers provide such testing services.
10.3 Energy Impacts.
Thermal evaporation of water from sludge requires considerable
energy. The amount of fuel required to dry sludge depends
upon the amount of water evaporated. It is imperative that a
dewatering step precede heat-drying so that overall energy
requirements can be minimized. Figure 10-1 shows a relationship
between the solids content of the sludge and the energy required
to produce a product containing ten percent moisture. The energy
estimates for heat-drying of sludge must be considered rough
approximations, since values can vary considerably depending
upon the type of dryer, whether or not energy recovery is a part
of the process, the flow sheet, and the characteristics of the
sludge.
The heat required to evaporate water from the wet sludge
is composed of:
• Heat to raise the sludge solids and associated residual
water to the temperature of the sludge product as it
leaves the dryer.
• Heat to raise the water temperature to the point where it
can evaporate and then to vaporize the water (latent
heat).
• Heat to raise the temperature of the exhaust gas,
including water vapor, to the exhaust temperature.
• Heat to offset heat losses.
10-5
-------
The above-mentioned heat must be supplied by the heating medium,
for example, hot air or steam.
80
c
0
4aJ
•H1
CD
tO
O
%
Q
LU
yj
UJ J*~l
x "5
+j
LU CQ
< c
X
O
CC
Q.
O.
60
40
20
0
ASSUMPTIONS;
-10 PERCENT MOISTURE IN DRIED SLUDGE
-2000 Btu ARE REQUIRED TO EVAPORATE
ONE POUND OF WATER
10 15 20 25 30 35 40 45
PERCENT SOLIDS IN DRYER FEED
50
FIGURE 10-1
ESTIMATE OF ENERGY REQUIRED TO DRY
WASTEWATER SLUDGE AS A FUNCTION OF
DRYER FEED SOLIDS CONTENT
10.3.1 Design Example
Ten thousand pounds per hour (4,540 kg/hr) of a dewatered sludge
containing 20 percent solids is to be dried by direct contact
with hot air. The sludge temperature is 60°F (17°C). The
temperature of the air prior to heating is 70°F (22°C) and its
absolute humidity is 0.008 pounds water per pound of dry air.
The temperature of the dried sludge is 140°F (60°C). The dried
sludge is 91 percent solids and 9 percent water. The dryer
exhaust gas temperature is 240°F (116°C), and it contains
0.12 pounds of water per pound of dry air. Radiant heat
losses from the dryer structure are 1,000,000 Btu per hour
(1,054,000 kj/hr). A preheater is used to heat the air prior to
10-6
-------
its entering the dryer. Figure 10-2 is a schematic diagram for
this example. The required air flow (G), the required air
inlet temperature to the dryer (t2), and the dryer evaporative
efficiency must be calculated.
WET SLUDGE
LOADING = 1(5.000 Ib/hr
SOLIDS CONTENT = 20%
TEMP = 60°F
lNLET_AiB
VOLUME - (COMPUTE)
Y = MOISTURE CONTENT
- 0:008 Ib water/
Ib dry air
TEMP - 7Q°F
—*-
(T)
r
DRYER INLET AIR
TEMP •= t* (COMPUTE t
(1)
SLUDGE
DRYER
X
/"%
X
HEAT SUPPLIED TO PROCESS
HA - (COMPUTE)
I
t Ifa/hr = 0,454 k^'hr
= 1.054 kJ/hr
DRIED SLUDGE
SOLIDS CONTENT - 91%
TtHP'-
EXHAUST GAS
Y = MOISTURE CONTEMT
- 0.12 Ib wiltsf
Ib dry air
TEMP = 240°F
RADIATION I OSS
HR - IxlO6 I
FIGURE 10-2
SCHEMATIC FOR SLUDGE DRYING EXAMPLE
The following heat capacity information is known or assumed:
Heat Capacity,
Substance
Dry air
Dry solids
Water
Water vapor
Step 1 - Determine
Btu/lb/°F
0.24
0.25
1.0
0.45
the required air flow,
(G)
Calculate a
moisture balance of substances entering~and leaving the dryer.
1. Moisture in:
.... . , , /, n ,,,.„ Ib sludge\ / n 0 Ib water \ Q ,,,,.-, -,,
a. Moisture in sludge = 110,000 ^—^-110.8 ij-, QiU(jqe I = 8'000 lb
^,« ^- U«, ,^/TC4-/Ui^\' ' * '
per hour (3.6 t/hr).
10-7
-------
b. Moisture in inlet air = (G lb d^ airWo.008 J-b water \ =
\ hr / V Ib dry air /
Ib per hour.
2. Moisture out:
a. Moisture in sludge
= (lO,000 lb^ludgeW lb dry flidsV 9 lb water \ 200 lb
\ ' hr /y lb sludge /y 91 lb dry solidsy
per hour (91 kg/hr).
b. Moisture in air = (G lb d^ air)(o.l2 lb/ater }
y hr /y lb dry airy
3. Equate moisture in and moisture out 8,000 + 0.008 G = 200 + 0.12 G.
4. Solve for inlet air flow (G):
G = 69,600 pounds per hour (31.6 t/hr).
Step 2 - Determine the required air inlet temperature (t2).
C a 1 c u 1 a te~"a" "Ii'e'a^t baTan~ce for the d ry e r . A~~s "ub~s~t~an c e T s he a t
content with respect to a given base temperature can be
calculated by assuming the heat required to bring the substances
from the base temperature to the temperature being considered.
For this example, a base temperature of 32°F (0°C) is arbitrarily
selected, and heat content (also known as enthalpy) is calculated
with respect to it. At steady state, heat in must equal heat
out. Consider the heat content of streams entering and leaving
the dryer:
1. Heat into the dryer is the sum of
a. Heat content of sludge (H4)
(1) Heat content of dry solids
000 l^M o.20 o.25
. (10,
= 14,000 Btu per hour (14.8 GJ/hr).
(2) Heat content of water
= / lb sludge] [Q 0 lb water \ / Btu ]
I ' hr y I lb sludgey y lb°/F /
= 224,000 Btu per hour (233 GJ/hr).
(3) Summing,
H4 = 14,000 + 224,000 = 238,000 Btu per hour
(251 GJ/hr).
10-8
-------
b. Heat content of air entering the dryer (H2)
(1) Heat content of dry air
= ^69,600 j^Yo.24
= 16,700 (t2-32) Btu/hr.
(2) Determine the heat' content of the moisture
associated with the air. This includes heat
required to raise the moisture temperature from
32°F (0°C) to the dewpoint, vaporize the moisture,
and finally increase the vapor temperature to
t2. From psychrometric charts (1), the dewpoint
(the temperature at which the air in question is
saturated) of air containing 0.008 pounds of water
per pound of dry air is 50°F (10°C). From steam
tables (6), the latent heat of vaporization at
50°F (10°C) is 1,065 Btu per pound (2.5 GJ/kg).
Heat content of moisture associated with air
/ 'an cnn Ik dry air\ /_ nrio Ib water
= 69,600 - - 0.008
lb dry air
i sn -
lb/°FPu
°
1,065 ~+(0.45
Btu
= 603,000 + 250.7 (t2-50)
Btu per hour.
(3) Summing, H2 = 16,714 (t2 - 32) + 603,000 + 250.7 (t2 - 50)
= 16,960 t2 + 55,600 Btu per hour.
2. Heat out of the dryer is the sum of:
a. Heat content of the "dried" sludge
( 1 ) Heat content of the dry solids
- fin nnn Ik sludge \ /_ . lb solids \ /. -, Btu W
- 10,000 — — ^ - J ^0.20 lb sludge; ^°'25 0
= 54,000 Btu per hour (57 GJ/hr).
(2) Heat content of residual water
/,. nnn lb sludge \L on lb solids \ / 9 lb water V. . Btu
^10,000 - HF^J^20 lb sludge] (si lb solidsA1'0
x (140-32°F) = 21,400 Btu per hour (23 GJ/hr).
( 3 ) Summing ,
H3 = 54,000 + 21,400 = 75,400 Btu per hour (80 GJ/hr).
10-9
-------
b. Heat content of the exhausted air
(1) Heat content of the dry air
lb dr airVo.24 Btu\/240-32°F
lb/op
per hour (3.7 TJ/hr).
_ ,_. nnn OJ_
= 3,474,000 Btu
(2) Determine the heat content of the moisture
associated with the exhausted air. From
psychrome tr ic charts (1), the dewpoint of air
containing 0.12 pounds water per pound of
dry air is 135°F (58°C). The latent heat of
vaporization at 135°F (58°C) is 1017 Btu per
pound (2.4 GJ/kg) .
Heat content of moisture associated with
exhausted air
— I fiQ fion
+ 1017 +l(
lb dry air\ /-
hr J\ '
J 45 Btu ^ f- 40
J'45 lb/°F/^4U
, n lb water \
lb d
-135 "FJ
ry airy
= 9,7
j( 0 Btu \
^'U lb/°FJ
50,000 Btu
( ^
1 ^S ^?°P
V
per hour
(10.3 TJ/hr).
(3) Summing,
H5 = 3,474,000 + 9,750,000 = 13,224,000 Btu per hour
(13.9 TJ/hr).
c. Radiant heat loss, Hr = 1,000,000 Btu per hour
(1.05 TJ/hr).
3. Calculate an overall heat balance around the dryer. At
steady state, heat into the dryer equals heat out, that
is H4 + H2 = H3 + H5 + Hr. Therefore, 238,000
+ 16,960 t2 + 55,600 = 75,400 + 13,224,000 + 1,000,000.
4. Solve for dryer inlet air temperature (t2)
t2 = 826°F (441°C).
Step 3 - Determine the evaporative efficiency. In this example,
evaporative efficiency is defined as the heat supplied to
evaporate one pound of water, in comparison to the theoretical
heat of vaporization:
1. Determine heat supplied to the process (;HA). By an
overall heat balance around the process (including
the air preheater), HA = H3 + H5 + HR - H4- HI.
10-10
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a. From previous calculations, H3 + H5 + HR = "heat out"
= 75,400 + 13,224,000 + 1,000,000 = 14,299,000 Btu
per hour (15.0 TJ/hr).
b. From previous calculations, H4 = 238,000 Btu per
hour (251 GJ/hr).
c. Determine H]_, the heat content of the inlet air
(1) Heat content of dry air
= (69,600 £p)(0.24 jg^pH 70-32 °F) = 635,000 Btu
per hour (669 GJ/hr).
(2) Heat content of moisture associated with dry
inlet air
\
0.008
hr II--— ib dry air
1.0
Btu
+ 1065 +(0.45 1^/oJ|70-50°F) = 608,401 Btu per hour
Ib/ FJ \ j
(641 GJ/hr).
(3) Summing,
HI = 635,000 + 608,000 = 1,243,000 Btu per hour
(1.3 TJ/hr).
d. HA = 14,290,000-238,000 - 1,243,000 = 12,809,000 Btu
per hour (13.5 TJ/hr).
2. Heat supplied to evaporate 1 pound of water.
= 12 809 000 Btu of water
7,800 Ib water r . r r
(1.8 GJ/kg).
3. Heat of vaporization of water at the inlet sludge
temperature = 1060 Btu per pound (2.5 GJ/kg):
Evaporative efficiency = -, ' ft A0 (100) = 64 percent.
-L f O T •£
10.3.2 Energy. Cost of Heat-Dried Sludges Used for
Fertilizers
A simple analysis shows that heat-dried sludge is not competitive
with commercial fertilizers when the two are compared on the
basis of energy required per unit of nutrient produced. From
10-11
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Figure 10-1, the energy required to flash-dry a well-dewatered
sewage sludge (40 percent solids concentration) is approximately
5.6 x 106 Btu per ton (7.3 x 106 kJ/t) of dry solids. Assuming
that the solids are four percent nitrogen by weight and that half
of the nitrogen is in plant-available form, the energy required
to produce 1.0 ton, (0.9 t) of plant-available nitrogen is
5.5 x 1Q6 Btu 100 ton dry solids 2 ton N _ 6
ton dry solids x 4 ton N x ton available N -bu x ±u
(295 x 106 kJ).
The energy required to produce and distribute one ton of
commercial nitrogen is estimated to be 49 x 10^ Btu per ton
of nitrogen (57 x lO^.kJ/t) (7). Assuming all nitrogen in
commercial fertilizers is plant-available and that 94 percent
of the energy consumed is for production and six percent for
distribution of raw materials and finished product, then
approximately 46 x 10^ (49 kJ) is required to produce one ton
(0.9 t) of nitrogen on a commercial basis (7). This is approx-
imately 16 percent of the energy required to produce one ton of
available nitrogen from flash-dried sludge.
By similar calculations, it can be shown that one ton of
phosphorus from flash-dried sludge requires about 15 to 20 times
as much energy to produce as one ton of phosphorus from
commercial fertilizers.
10.4 Environmental Impacts
Heat-drying of sludge produces a material that usually contains
10 percent or less moisture, a moist gas stream that is ejected
to the atmosphere, and in some cases, a liquid sidestream. The
impacts of all of these products must be considered in the
design of the heat-drying facilities. Some data on pathogenic
organism survival through heat-drying processes are presented in
Chapter 7. Heat-dried sludge should not be allowed to become
rewetted, since moisture creates an environment favorable for
regrowth of organisms. Once sludge is rewetted, anaerobic
decomposition can begin with the concomitant generation of
noxious odors. This is particularly a problem for sludges that
have not been previously stabilized.
Potential users of dried sludge prefer a granular or pelletized
product. A product which is dusty, odorous, or contaminated
with materials such as plastics, strings, Or cigarette butts is
difficult to sell or give away.
10.4.1 Air Pollution
The gas stream exhausted from the dryers may be the source of
odors and visible emissions. These appear to be most significant
in high-gas velocity processes where the product is subject to
10-12
-------
abrasion and dusting occurs. The most effective control measure
for these problems is afterburning. However, afterburning
requires supplementary fuel and may be prohibitively expensive
for many installations. Cyclones, wet scrubbers, electrostatic
precipitators, and baghouses have been used with varying degrees
of success.
Wet-scrubbing, electrostatic precipitators, and baghouses were
tested for the control of odors and visible emissions from a
Toroidal dryer located at the Blue Plains plant in Washington,
D.C. The electrostatic precipitator and wet scrubber were
unable to reduce emissions sufficiently to satisfy Washington's
stringent air pollution requirements. Baghouses were effective
when operating, but they persistently caught fire as a result of
ignited grease deposits and thus were not reliable.
10.4.2 Safety
Drying systems are exposed to heavy dusting and have had problems
with fires. The combination of combustible particles, warm
temperatures, sufficient oxygen, and high-gas velocities make
these systems susceptible to fires.
10.4.3 Sidestream Production
Liquid sidestreams are produced by certain ancilliary equipment
in heat-drying (for example, wet scrubbers). These sidestreams
frequently can be recycled to the headworks of the treatment
plant but may require separate treatment.
10.5 General Design Criteria
There are several common features of heat-drying processes for
which general design criteria can be developed.
10.5.1 Drying Capacity
The number and size of the dryers depend on the type of drying
operation contemplated. If the dryers are operated continuously,
extra dryer capacity is needed so that all sludge produced can be
dried while maintenance and repairs are being performed. In
cases where non-continuous operation (for example, 40 hours per
week) is envisioned or where only one dryer is installed, the
dryer(s) must have sufficient evaporative capacity to handle all
the sludge, including that generated when the dryers are not
on line. In the latter case, wet sludge storage requirements may
be significant.
10-13
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10.5.2 Storage Requirements
The design engineer should consider storage requirements for both
the wet sludge feed and the dried product. Sufficient wet
sludge storage should be provided to allow orderly shutdown of
continuously operated drying processes (approximately three day's
production at a peak rate). Storage for the dried product
depends on the final disposal arrangement. Sales of the product
are likely to be seasonal, and considerable storage may be
necessary unless bulk buyers provide off-site storage. If
the dried product is burned as a fuel or undergoes further
processing, storage requirements are indicated by subsequent
steps in the sludge-processing system. Dust can become a problem
if the dried product is stored in bulk and is not pelletized. In
some cases, the material should be appropriately contained.
10.5.3 Heat Source
The large amounts of energy required for heat-drying dictate that
close attention be given to the source used to heat the drying
medium. Natural gas and fuel oil are most frequently used but
are becoming more expensive, and shortages have occurred in the
past few years. Energy recovery within the heat-drying system
itself provides one way of reducing energy usage; for example,
heat exchangers can be used to recover heat from the exhaust
gases. Recovery of heat from a power source within the plant is
another method; for example, Milwaukee recovers waste heat from
gas turbine exhausts. The dried sludge itself has a fuel value
and may be used as a heat source for the drying medium.
10.5.4 Air Flow
Air flow is an important consideration in the design of direct
dryers. Air flow may be cocurrent, countercurrent, or crossflow.
In direct drying, cocurrent flow offers the advantage of higher
thermal efficiency due to rapid cooling of the heating medium
near the feed end with concomitant reduced heat losses through
the dryer structure. In addition, the dried sludge is not
subjected to high-gas temperatures near the discharge end, as
it would be in counterflow operation. This is advantageous
because it minimizes distillation of odorous materials and
increases thermal efficiency somewhat by reducing heat lost with
the dried sludge.
The rates of air flow are a function of the dryer design.
However, turbulent conditions must be maintained to ensure
intimate contact between the warm air and wet sludge. Dusting
problems may limit air flow rate.
10-14
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10.5.5 Equipment Maintenance
A major maintenance problem in some dryers is erosion of
conveying equipment and drying shells by the abrasive dried
sludge. This is particularly a problem for dryers processing WAS
from activated sludge plants which have only coarse screening for
grit removal. The use of ferric chloride as a dewatering aid
may also create corrosive conditions that exacerbate the problem.
Worn conveying equipment can lead to dusting problems. Abrasive
sludge may result in replacement of rotary dryer drum shells
every few years.
10.5.6 Special Considerations
Special equipment may be needed when dried material is produced.
For example, the value of dried sludge may be increased by
nutrient supplements such as nitrogen, phosphorus, or potassium.
Also, the dried product may require finishing before sales; for
example, pelletizing or bagging operations may be needed.
In the United States, the Organiform process, developed by
Orgonics, Inc., has been used to increase the nitrogen content of
the dried sludge. This process, based on urea-formaldehyde
technology, was used in an existing heat-drying operation at
Winston-Salem, North Carolina, from 1973 to 1975, and the
prototype system is still used at a leather tanning facility in
Slatersville, Rhode Island (8). The heat-drying operation at
Winston-Salem was abandoned, however, because railroad siding
and terminal facilities for bulk storage and shipment could not
be funded. The Basel County Thermal Sludge Drying Plant in
Switzerland has provisions for adding nitrogen, phosphorus, and
potassium to the dried sludge for improvement of its fertilizing
properties.
10.6 Conventional Heat Dryers
Conventional heat-drying is usually preceded by mechanical
dewatering and may be followed by air pollution control devices
and systems which alter the form of the dried material.
Mechanical dewatering is discussed in detail in Chapter 9. It is
an important pretreatment step since it reduces the volume of
water that must be removed in the dryer. In the dryer, water
that has not been mechanically separated is evaporated without
decomposing the organic matter in the sludge solids. This means
that the solids temperature must be kept between 140 and 200°F
(60° and 93°C). A large portion of the dried sludge is often
blended with the sludge feed to the dryer, making the drying
operation more efficient by reducing agglomeration (large balls
of sludge), thus exposing a greater solids surface area to the
drying medium. Dried sludge and exhaust gases are separated in
the dryer itself and/or in a cyclone. The gas stream can go to a
10-15
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pollution control system for removal of odors and particulates.
The dried sludge is then sent to a finishing step such as
palletizing or bagging, or it is stored in bulk for marketing or
use in the next portion of the sludge management scheme.
10.6.1 Flash-Drying
Flash-drying is the rapid removal of moisture by spraying or
injecting the solids into a hot gas stream. This process was
first applied in 1932 to the drying of wastewater sludge at the
Chicago Sanitary District.
10.6.1.1 Process Description
The Combustion Engineering-Raymond Flash Drying and Incineration
Process shown on Figure 10-3, is typical of flash-drying units
used in the United States.
The flash-drying process is based on three distinct components
that can be combined in different arrangements. In the first
component, the wet sludge cake is blended with previously dried
sludge in a mixer to improve pneumatic conveyance. The blended
sludge and the hot gases from the furnace at 1,300°F (704°C) are
mixed ahead of the cage mill, and flashing of the water vapor
begins. Gas velocities on the order of 65 to 100 feet per second
(20 to 30 m/sec) are used. The cage mill mechanically agitates
the sludge-gas mixture, and drying is virtually complete by the
time the sludge leaves the cage mill. The mean residence time is
a matter of seconds. The sludge, at this stage, has a moisture
content of only 8 to 10 percent and is considered dry. The dried
sludge is then separated from the spent drying gases in a
cyclone. Temperature of the dried sludge is about 160°F (71°C),
and the exhaust gas temperature is about 220°F to 300°F (104° to
149°C). The dried sludge can be sent either to storage or to the
furnace for incineration.
The second component is the incineration process. Gas, oil,
coal, or partially dried sludge is burned in the furnace to
provide heat needed to dry the sludge. Combustion air, provided
by the combustion air fan, is preheated and injected into the
furnace at high velocity to promote complete fuel combustion.
Any ash that accumulates in the furnace bottom is periodically
removed.
The third component is the effluent gas treatment facility
or induced draft facility. This consists of the deodorizing
preheater, the combustion air heater, the induced draft fan, and
a gas scrubber. Odors are destroyed when the temperature of the
gas from the cyclone is elevated in the deodorizing preheater.
Part of the heat absorbed is recovered in the combustion air
preheater. The gas then passes through a dust collector
(generally a scrubber) and is discharged to the atmosphere.
10-16
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EXHAUST
GAS
CYCLONE
VAPOR FAN
AUTOMATIC
DAMPERS
-*— INDUCED
DRAFT FAN
EXPANSION
JOINT
EXPANSION
JOINT
EXPANSION
JOINT
DOUBLE
FLAP VALVE
MANUAL
DRY
DIVIDER
COMBUSTION
AIR PREHEATER
PRY PRODUCT
CONVEYOR
WET SLUDGE
CONVEYOR
DEODORIZING
PREHEATER
DISCHARGE SPOUT
AUTOMATIC
DAMPERS
COMBUSTION AIR FAN
REMOTE
MANUAL
DAMPERS
CAGE MILL
H OT GAS OU CT *-—™
FIGURE 10-3
FLASH DRYER SYSTEM (COURTESY OF C.E. RAYMOND)
10-17
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10.6.1.2 Case Study: Houston, Texas
The flash-drying operations at Houston, Texas, 'illustrate the
operating experience and performance of the C-E Raymond Flash
Drying process. There are four flash dryers at the 45-MGD
(1.97-m3/s) Sims Bayou plant and five flash dryers at the 75-MGD
(3.29-m3/s) Northside plant, with two additional units under
construction. The liquid process stream consists of bar
screening and activated sludge. Sludge treatment consists of
degritting, vacuum filtration with ferric chloride addition, and
flash-drying.
After gravity thickening, the sludge solids concentration is
about two percent at the Sims Bayou plant and about three percent
at the Northside plant. The cake from the vacuum filters is
about 15 percent solids. The ferric chloride additions amount
to about 75 pounds per ton (37 kg/t) of dry solids, or about
3.8 percent.
Dewatered sludge is transported to the dryers by belt conveyors.
Each flash dryer, with cage mill and 14-foot (4.3 m) diameter
cyclone, is rated at 12,000 pounds of water per hour (5,448 kg/
hr) but is operated at 9,000 to 10,000 pounds of water per hour
(4,086 to 4,540 kg/hr). Heat exchangers are provided for
high temperature deodorization and for preheating the combustion
air. The cage mill inlet temperature is 900°F to 1,150°F
(482°C to 621°C) , and the temperature at the cyclone is about
220°F (104°C). The deodorization temperature is controlled
around 1,200°F (649°C), and the stack gas temperature is 500°F to
600°F (260°C to 316°C) after heat recovery. The fuel used is
natural gas, and the heat input is about 22 million Btu per hour
(23.2 million kJ/hr) or 2,200 to 2,400 Btu per pound (5,100 kJ/kg
to 5,600 kJ/kg) water evaporated.
Moisture content of the dried product is about 5.5 percent.
About nine times as much solids on a dry weight basis are
recycled to the predryer double paddle mixer as are removed as
product. The product is conveyed to a storage area or directly
to railroad cars for shipment.
The process is automated and panel boards are provided that
indicate and record variables such as air flow, temperatures at
critical points, and amperage on fan motors. The controls are
enclosed in air-conditioned cubicles. Horn alarms indicate
unsuitable temperature conditions.
The controls for the ferric chloride feeding have proven to be
inadequate and have led to operational problems.
Dust is also a major problem at the Sims Bayou plant. The dried
sludge dust is extremely abrasive, causing wear on all mechanical
equipment. Wet sludge has also overflowed the top of the
conveyors at times, creating housecleaning problems.
10-18
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No specific cost data are available for the Houston facilities.
The dried product, Hou-actinite, is sold through a broker by
yearly contract.
10.6.2 Rotary Dryers
Rotary dryers use a sloped rotating cylinder to move the material
being dried from one end to the other by gravity. Direct,
indirect, and direct-indirect rotary dryers have been used to dry
sludge.
10.6.2.1 Direct Rotary Dryers
Direct rotary dryers have been used in the United States and in
Europe for drying sludge. These include installations at Largo,
Florida, and Stamford, Connecticut (in conjunction with a refuse
incinerator) and in Basel, Switzerland. Manufacturers include
the Heil Company, Combustion Engineering, Bartlett-Snow, and
Euranica, Inc.
Process Description
The features of a typical direct rotary drying system are
illustrated on Figure 10-4. Mechanically dewatered sludge is
added to a mixer and blended with previously dried sludge to
provide a low moisture dryer-feed. Hot gas at temperatures of
1,200°F (649°C) is added to the dryer, usually in a cocurrent
flow pattern. After the sludge has been held in the dryer for
20 to 60 minutes, the dried sludge is discharged at a temperature
of 180°F to 200°F (82 to 93°C). Exhaust gases are conveyed to a
cyclone where entrained solids are separated from the gases. The
spent gases exit at about 300°F (149°C). A portion of the dried
product is recycled, and the balance goes to a finishing step, to
further processing, or to disposal. Gaseous discharge from the
cyclone goes to an air pollution control system for deodorization
and particulate removal as necessary. Figure 10-4 shows several
alternatives for handling the exhaust gas. A long residence time
in the dryer may minimize deodorization requirements.
Design Considerations
The rotary drum usually consists of a cylindrical steel shell
that revolves at 5 to 8 rpm. One end of the dryer is slightly
higher than the other, and the wet sludge is fed into the higher
end. Flights projecting from the inside wall of the shell
continually raise the material and shower it through the dryer
gas, moving the material toward the outlet.
Gas flow through the drum may be either cocurrent or counter-
current to the sludge flow. Gas velocities must be limited to 4
to 12 feet per second (1.2 m/sec to 3.7 m/sec) to prevent dust
from being entrained with the exhaust gas.
10-19
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a.. DIRECT DISCHARGE
TO ATMOSPHERE
*• ATMOSPHERE
Alfi
BURNER '
IBCKPF
1
SCRUBBER
• ATMOSPHERE
FEED SLUPOE
v
V
ALTERNATIVES AVAILABLE FOR EXHAUST GAS DEO DO R IZ AT I ON
AND ("ARTICULATE REMOVAL
FIGURE 10-4
SCHEMATIC FOR A ROTARY DRYER
Case Study :_ _L_argo, Floric[a
The Largo, Florida, Wastewater Treatment Plant has a rated
capacity of 9 MGD (0.39 m^/s) with average summer flow of 6 MGD
(0.26 ITH/S) and winter flows greater than 9 MGD (0.39 m3/s). The
liquid process stream consists of coarse screening, grit removal,
contact stabilization activated sludge, chlorination, and
dual media filtration. Waste-activated sludge is aerobically
digested, batch gravity decanted, and thickened. Since 1976, the
thickened sludge has been dewatered by belt filter presses and
heat-dried in a rotary dryer. This system was supplied by
Ecological Services Products, Inc. (ESP).
Approximately 1.6 dry tons (1.45 t) of digested sludge is
produced daily and is processed at a rate of 2.2 tons (2.0 t) per
day for a five-day week. Typical thickened aerobic sludge is
1 to 1.1 percent solids. The belt filter presses produce a
sludge cake that is typically 10 to 12 percent solids. Polymer
is used to condition the sludge prior to filtration.
10-20
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The rotary dryer, manufactured by the Heil Company, has an
evaporative capacity of approximately 5,400 pounds water per hour
(2,450 kg/hr). The Heil dryer employs a 3-in-l drum design.
Sludge moves forward through the center cylinder, then back
through the intermediate cylinder, and forward again through the
outer cylinder toward a fan located at the discharge of the
machine. The three cylinders are concentric and are mechanically
interlocked so that they rotate at the same speed. Internal-
external flights on each cylinder repeatedly raise the sludge to
the top of the drum. This design is claimed to provide better
heat utilization by minimizing radiation losses, but maintenance
on the drums is more complex than with a single shell.
The facilities were designed assuming 1,000 pounds per hour
(454 kg/hr) of dry solids throughput, based on feeding a sludge
cake of about 20 percent solids. The dryer is water-limited
because the cake produced by the belt presses is only 10 to
12 percent solids. Actual throughput is about 600 pounds per
hour (272 kg/hr) of dry solids.
Heated air is provided by a natural gas burning furnace. Typical
dryer inlet air temperature is about 800°F (427°C), and the
outlet temperature is about 180°F (82°C). The average gas
temperature in the dryer is estimated to be about 250°F (121°C).
Off-gases from the cyclone separator are typically 120°F (49°C).
The dried product, Lar Grow, is a relatively fine pellet
produced naturally by the rotating drum. Product bulk density is
45 to 55 pounds per cubic feet (720 to 880 kg/m3). The bagged
product moisture content is about five percent. The product
is screened before bagging to remove cigarette filters and other
nondegradable materials such as plastics. In 1978, a garden
products wholesaler contracted to purchase the sludge produced
for one year (approximately 570 dry tons,[517 t]) at $54 per ton
($59/t). Because the wholesaler's markets are seasonal, the
bagged product is stored on-site for a portion of the year.
The Ecological Services Products, Inc. (ESP) sludge drying plant
was installed in 1975-76 at a contract price of $850,000 (cost of
the building not included). The approximate capital cost for the
facility can be broken down as follows: 41 percent for sludge
and polymer pumping system, belt filter presses, and polymer
preparation and feed system; 32 percent for the dryer, ductwork,
fan, cyclones, and scrubber system; and 27 percent for mechanical
conveyors, recycle bin, production storage bin and bagging
facility. According to ESP personnel, the 1978 cost for a
similar plant would be between $1.2 to $1.3 million, including
installation and startup.
Typical operating and maintenance costs for dewatering, drying,
and bagging during 1977 are shown in Table 10-1.
10-21
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TABLE 10-1
ESTIMATED 1977 COSTS FOR DEWATERINC,
DRYING AND BAGGING AT LARGO, FLORIDA (7)
Item
Polymer
Gas
Labor
Power
Annual cost,
dollars
13,000
26 ,000
21,000
11,000
Cost/ton,
dollars
23
45
36
20
Total 77,000 134
These costs are based on unit costs at Largo of $2.60 per
pound ($5.72/kg) of polymer, $1.62 per 1,000 cubic feet
($57.20/1000 m3) of natural gas, 3.4 cents per kWhr of electri-
city, and $0.24 per bag. Hence, approximately 9.9 pounds
(4.5 kg) of polymer, 27,800 cubic feet (790 m3) of natural gas,
590 kWhr of electricity, and 42 bags are used per dry ton of
product.
Although a specific deodorization system has not been included,
odor problems have been minimal. There are occasional odor
problems when sludge that is too wet enters the dryer. There
have been some problems with wear in the conveying facilities due
to the dried sludge material being more abrasive than originally
estimated. The pug mill blades and screw conveyor to the dryer
have been replaced. Replacement parts have been specified to
include heat treatment of the screw conveyor and the addition of
cellite or carborundum plates on the wearing surfaces. The
system supplier, ESP, has indicated that these changes will be
considered for future equipment. There have been few other
operating and maintenance problems.
10.6.2.2 Indirect Drying
Indirect rotary dryers have not been used in the United States
for drying sludge. Vertical thin film dryers are used at the
Dieppe, France, coincineration facility (9,10). The two LUWA
Double-Wall Dryers installed at Dieppe operate on 140 psi
(966 kN/m2) steam at a temperature of about 355°F (180°C). The
evaporaters are vertical, with top inlet and bottom outlet.
Steam generated from refuse incineration is forced into the dryer
and heats a "jacket" surrounding the incoming dewatered sludge.
The sludge is spread over the inner cylindrical surface of the
dryer by a rotor carrying self-adjusting vanes, at a top speed
of about 25 feet per second (7.6 m/sec). The water vapor travels
upward, counter to the sludge flow, and is blown into the
incinerator, where it is deodorized. The dried sludge falls onto
a conveyor belt and is incinerated with the refuse.
10-22
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Another type of indirect sludge dryer is the jacketed and/or
hollow-flight dryer and conveyor. A schematic of a jacketed
hollow-flight dryer is presented on Figure 10-5. These units can
perform the dual function of heat transfer and solids conveying
in one piece of equipment--generally a horizontal, semi-circular
trough with a jacket or coil to provide heat (10) . This
equipment has one or more agitation devices (for example, screw,
flight, disc, paddle) rotating on the axis through the center of
the trough. A significant degree of agitation is necessary to
maintain reasonable heat transfer. Simple screw conveyors
are notably poor in this regard, because increasing the speed
reduces the residence time in the dryer by moving the sludge
rapidly through the system. Heat transfer coefficients for this
type of equipment range from 15 to 75 Btu per hour per square
foot per °F (18.6 to 93 cal/sq cm/°C), depending on moisture
content and degree of agitation.
-IN! FT
BREAKER
BARS
JACKETED
VESSEL
AGITATOR
ROTARY
JOINT
DISCHARGE
FIGURE 10-5
JACKETED HOLLOW-FLIGHT DRYER
(COURTESY BETHLEHEM CORPORATION)
The agitators, paddles, or flights should also be designed to
minimize build-up on the walls of the dryer and on the agitator
itself. Generally, baffles or ploughs should be provided between
10-23
-------
the flights to improve mixing and to break up any lumps that
form. The rotating flights are often fitted with small paddles
or similar projections to improve agitation and reduce fouling of
the shell surface.
Significant increases in heat transfer can also be obtained if
the rotor is hollow and fitted for steam heating. A hollow
heated rotor often provides one to two times the heat transfer
area available in the shell.
10.6.2.3 Direct-Indirect Rotary Dryers
The direct-indirect rotary dryer is similar to indirect dryers
employing hot air or gases as the heating medium. In direct-
indirect drying, however, the heating medium is recirculated to
flow in direct contact with the drying sludge in addition to
heating the metal drying surfaces.
Case Study: Milwaukee, Wisconsin
The drying operation at Milwaukee's 200-MGD (8.76-m3/s) Jones
Island Plant employs ten direct-indirect rotary, counterflow,
kiln-type dryers for treating waste-activated sludge. The plant
is designed for continuous operation. To achieve this, nine
dryers must always be in operation. The drying system produced
over 74,000 tons (67,300 t) of dried product in 1976. Thickened
waste-activated sludge is conditioned with ferric chloride and
filtered on vacuum filters. Wet filter cake (approximately
14 percent solids) is mixed with an approximate equal weight of
previously dried .material in a screw conveyor and fed to the
direct-indirect dryers. The ten custom-built dryers are each
8 feet (2.4 m) in diameter and 57 feet (17.4 m) long. Each dryer
can evaporate approximately 10,000 pounds (4,540 kg) water per
hour (at 90 percent capacity) with an inlet air temperature of
1,200°F (649°C). The rotating drum, with lifting angles, picks
up the wet mixture that is dropped subsequently to the bottom as
a shower of particles. The sludge is continuously lifted and
dropped through the hot gases, progressing as a moving curtain
through the length of the dryer during the 45-minute drying
cycle. The granular dried sludge (Milorganite) has been sold as
a fertilizer since 1925. Rejected dust and fine particles are
pelletized, and the pellets are reground to produce granular
saleable material.
The dryer air inlet temperature is controlled at 1,200°F (649°C).
The exhausted gas leaves the dryer at 250°F (121°C) and is passed
through cyclone separators to remove fine particles. Each dryer
has its own furnace. Originally, coal was used as a fuel, then
coke oven gas (after furance modification), and then natural gas
with standby fuel oil. In the mid-1970s, gas turbines were
installed, and the gas from these turbines, at a temperature of
approximately 900°F (482°C), is now fed to the modified furnaces
and two waste heat recovery boilers. The gas burners are used to
10-24
-------
provide the additional heat necessary to maintain the dryer inlet
temperature at 1,200°F (649°C). The recovered turbine exhaust
heat supplies 70 percent of the heat required for the sludge
drying operation.
The dried sludge product is abrasive, and the wet sludge is
corrosive because of the ferric chloride used. Internals of the
drum must be replaced about every three years. The present
dryers are over 20 years old, and plans are being made to add
three direct, cocurrent rotary dryers and to rehabilitate the
existing dryers.
10.6.3 Incinerators
In sludge incineration, the temperature of the sludge is raised
to 212°F (100°C), and the water is evaporated from the sludge
before it is ignited; that is, the sludge is dried prior to
ignition. Several options are available with incinerators. If
heat inputs are reduced, the incinerator can be used as a dryer
alone. Alternatively, a portion of the dried sludge can be
removed at an intermediate point in the incinerator, with the
remainder proceeding onward to be burned. Finally, all sludge
may be incinerated.
Modifications may be required if these units are to be used for
drying alone; for example, modifications to a multiple-hearth
furnace would include fuel burners at the top and bottom hearths
plus down-draft of the gases. If the sludge is to be disposed
of, incineration provides greater volume reduction than drying
alone.
Incineration is discussed in Chapter 11. Processes include
multiple-hearth, fluid-bed, and electric furnaces.
10.6.4 Toroidal Dryer
The Toroidal (doughnut-shaped) dryer is a relatively new dryer
that is employed in the UOP, Inc. ORGANO-SYSTEMR for sludge
processing. The dryer works on a jet mill principle and contains
no moving parts. Transport of solid material within the drying
zone is accomplished entirely by high-velocity air movement.
10.6.4.1 Process Description
A simplified process flow diagram of the UOP ORGANO-SYSTEMR is
shown on Figure 10-6. The system is composed of wet sludge
storage, mechanical dewatering, sludge drying, air pollution
control, final product finishing, and storage.
The mechanical dewatering step is designed to deliver the
dewatered sludge to the dryer at about 35 percent to 40 percent
solids. The dewatered sludge is mixed with previously dried
sludge to reduce the moisture concentration of the dryer feed.
10-25
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INCOMING
MECHANICAL SECONDARY
DE WATERING STORAGE
AIR
AIR EMISSIONS
CONTROL
PRODUCT TO
CUSTOMER
TOROIDAL DRYER
PRODUCT FINISHING
FIGURE 10-6
TOROIDAL DRYING SYSTEM
Heated process air is distributed through three manifold jets to
the lower segment of the toroidal drying zone chamber. The air
from one of the three jets is directed in such a way as to
impinge upon the incoming wet feed material and propel this
material into the drying zone, where particle size reduction and
drying begins. Additional jets in the drying zone convey the
material into the toroid for additional drying, grinding, and
classifying.
Process air and solids within the toroid move at a velocity
of approximately 100 feet per second (30 meters per second).
The high-velocity gas stream reduces the size of lumps or
agglomerated feed material by impingement against the interior
walls of the drying chamber and by collision with other
particles. Wetter and heavier particles travel a path along the
internal periphery of the dryer, whereas drier and lighter
particles are swept out with the gas stream and are removed from
the drying zone. Heavy, wet particles stay in the dryer until
they are broken up and dried.
The inlet termperature is usually controlled within the range of
500°F to 1,400°F (260°C to 760°C). There is a sharp drop in the
gas temperature within the dryer when the hot inlet gas stream
meets the incoming wet sludge. The dryer exhaust temperature is
usually controlled at a specific setpoint within the range of
190°F to 300°F (90°C to 150°C). The product temperature normally
does not exceed 150°F (66°C).
10-26
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The dried sludge particles exiting the toroid are sent to a
cyclone where they are separated from the gas stream. A portion
of the dried sludge is back-mixed with the wet feed, and the
remainder is transferred to the product finishing section.
There, the dried product may be extruded (at a temperature of
140°F [60°C]), cut into pellets, and bagged, if desired.
Otherwise, the product is routed to subsequent sludge processes
including codisposal/energy recovery or land application. Gases
from the cyclone are treated by processes that may include
wet scrubbers, electrostatic precipitators, and baghouses.
Deodorizing chemicals may be required.
10.6.4.2 Current Status
The toroidal dryer has been demonstrated on a full-scale basis.
A 240-tons-water-per-day evaporative capacity ORGANO-SYSTEMR
was operated by UOP Organic Recycling at the Blue Plains
wastewater treatment plant in Washington, DC, for over three
years. Raw sludge, digested primary sludge, and waste-activated
sludge, as well as mixtures of these sludges, were processed.
This system is no longer in operation. A 24-tons-water-per-day
evaporative capacity unit is installed at UOP's West Chester,
Pennsyvania, research and development facility.
10.6.5 Spray-Drying
Spray-drying systems are similar to flash-drying systems in that
almost instantaneous drying occurs in both.
10.6.5.1 Process Description
Spray-drying involves three fundamental steps: liquid atomiza-
tion, gas/droplet mixing, and drying from liquid droplets (1).
Atomizers are usually high-pressure nozzles, or high-speed
centrifugal dishes or bowls. The atomized droplets are usually
sprayed downward .into a vertical tower through which hot gases
pass downward. Drying is complete within a few seconds; the
product is removed from the bottom, and the gas stream is
exhausted through a cyclonic dust separator.
Abrasive materials can cause problems with the atomizing devices.
Centrifugal bowls or discs apparently require less maintenance
because they are less likely to become plugged.
10.6.5.2 Current Status
A Nichols Spray Dryer was installed and operated at the
wastewater treatment plant at Ansonia, Connecticut, to dry
sludge. Dewatered sludge was sprayed into the top of a cone-like
apparatus containing rotating "wheels." The heating medium was
hot flue gases (1,300°F [705°C]) from the stack of a municipal
10-27
-------
refuse incinerator. Operation of the incinerator has been
limited to about five hours per day because of state air
pollution control requirements; the drying time was likewise
limited. A burnable, dried product with greater than 90 percent
solids has been produced with this system. The dried sludge has
been given away as a soil conditioner rather than burned in the
refuse incinerator.
10.7 Other Heat-Drying Systems
Two are currently available that differ somewhat from conven-
tional heat-drying systems. They are the Basic Extractive Sludge
Treatment (BEST) process, which employs solvent extraction,
and the Carver-Greenfield process, which uses multiple-effect
evaporation. Both of these systems employ an externally supplied
liquid to assist in the removal of water from wet sludge.
10.7.1 Solvent Extraction:—BEST Process
The BEST process is based on the use of an organic solvent
to reduce the amount of water that must be evaporated in a
conventional drying step. The process was developed by and
is available from Resource Conservation Company of Renton,
Washington.
10.7.1.1 Process Description
The BEST process, shown schematically on Figure 10-7, uses an
aliphatic amine solvent (triethylamine or TEA) to separate sludge
solids and water. The key to this process is the temperature-
sensitive miscibility properties of TEA. Below 65°F (18°C), TEA
and water solutions of any concentration are completely miscible
and form a single-phase, homogeneous solution. Above this
temperature, the mixture separates into two distinct layers, the
top layer being nearly all TEA and the bottom layer nearly all
water.
As shown in the diagram, incoming sludge is mixed with chilled,
recycled solvent. The cooled mixture is then fed into a
conventional dewatering unit, such as a vacuum filter, press,
or centrifuge. After dewatering, the wet cake is fed to a
continuous dryer operated between 250°F and 290°F (120°C and
140°C). The liquid in the wet cake contains a high percentage
of TEA. The latent heat of TEA is approximately 133 Btu per
pound (309 kJ/kg) compared to approximately 1,000 Btu per pound
(2320 kJ/kg) of water. Because of this, the drying process is
faster and uses less direct energy for drying than if the liquid
were only water. Vapors coming from the dryer are condensed
(condenser not shown) and combined with the liquid left from the
dewatering step. This solvent/water mixture is then heated and
collected in a decanter, where the components separate into two
distinct layers.
10-28
-------
1
*
-*•
so
LVENT r
DEC
140°F V
* \
k
r~\
HEAT
EXCHANGER
STILL
Y
MIX
JUNCTION
20°F| OIL
BEPF
50" F
HEAT
EXCHANGER
SLUDGE
HEAT
EXCHANGER
LIQUID/SOLID
SEPARATOR
140" F
WATER
STILL
DRYER
T
DRY PRODUCT
SOLIDS WATER
FIGURE 10-7
SCHEMATIC OF B.E.S.T. PROCESS
The solvent is drained off the top of the decanter and recycled
(after chilling) to mix with new incoming sludge. Meanwhile, the
water is decanted to a distillation column to be steam-stripped
of residual solvent, which also is recycled. Oils and fats
extracted from the sludge by the solvent are recovered in the
solvent still. The product water is returned to the headworks of
the treatment plant.
Resource Conservation Company claims that the system is entirely
closed, except for a small gas vent, and creates no environmental
problems. Air pollution and odor control equipment, if
specified, would be required to handle only a relatively small
volume of exhaust gas.
10.7.1.2 Current Status
A full-scale BEST system has yet to be operated. A 1-gallon-per-
minute (4 1/m) demonstration test unit known as "mini-BEST"
was evaluated by Metropolitan Engineers in 1975 as part of
Municipality of Metropolitan Seattle's research program.
Combinations of settled primary and thickened waste-activated
sludges were treated in the pilot facility. The study team
concluded that the BEST process was not cost-effective for
10-29
-------
Seattle Metro (12). The process was also compared by the LA/OMA
project with several other candidate sludge disposal systems and
found to be one of the more expensive alternatives for the
Los Angeles area (13).
10.7.1.3 Operating Experience
Operating experience is limited to laboratory and pilot plant
tests. Dried solids (about 5 percent water) and product water-
are disinfected as a result of the high temperature (250°F
[121°C] ) in the dryer and the high pH of the solvent solution.
Sodium hydroxide (NaOH) is added to maintain an alkaline
condition, since TEA precipitates an acidic environment. NaOH
also conditions the sludge to improve dewatering and the dryer
performance. The dried product is easy to handle and transport;
however, pelletizing may be necessary to prevent dusting and to
enhance product marketability.
Primary sludge from Seattle Metro's West Point plant, containing
3.4 percent solids and pretreated with 2 to 5 g NaOH/1 (100 to
300 pounds per ton dry solids), was blended with TEA and
centrifuged. A cake of approximately 30 percent solids was
produced. A solvent-to-sludge ratio of 6:1 was maintained. The
liquid fraction contained 60 percent solvent and 40 percent
water, which reduced the energy required to evaporate the liquid,
compared to drying of 30 percent cake with a 100 percent water
fraction. The dried product averaged 86 percent solids with
1.6 percent solvent by weight. Product water, following
decanting and solvent extraction in the water still, averaged
280 mg/1 suspended solids, contained less than 0.01 percent
solvent, and had a pH of 10.6.
This high-technology process is quite complex and may require a
competent chemical engineer to ensure efficient operation (12).
There are a relatively large number of components in the system
and, hence, maintenance costs may be high. Unpleasant odors
(ammonia-like) existed in the exhaust gas during the Seattle
study. A deodorization system may be required (12). Full-scale
data on chemical and energy requirements, as well as operating
reliability, are not currently available on the BEST system.
10.7.2 Multiple-Effect Evaporation--Carver Greenfield
Process
Multiple-effect evaporation is another technique that can be used
to remove water from sludge. The Carver-Greenfield process,
offered jointly by Foster Wheeler Energy Corporation and
Dehydro-Tech Corporation, uses this technology.
The basis of economy for multiple-effect evaporation is steam
reuse. Steam generated in the first evaporator (by evaporation
of water from sludge) is used as the heating fluid in the second
evaporator. The method is feasible if the second evaporator is
operated at a lower pressure than the first.
10-30
-------
10.7.2.1 Process Description
The Carver-Greenfield process uses a multiple-effect evaporation
process to extract water from sludge. The major steps in the
process are oil mixing, multiple-effect evaporation, oil-solid
separation, and condensate-oil separation.
The applied sludge is mixed with a petroleum hydrocarbon oil
(Number 2 fuel oil and Isoparl, an Exxon product, have been
used). The use of oil maintains fluidity in all evaporator
effects and minimizes scale formation and corrosion of heat
exchange surfaces. The sludge-oil slurry is pumped through
a grinder to the multiple-effect evaporator. The grinder
reduces the size of slurry solids to prevent obstructions in the
evaporator tubes, to optimize evaporation, and to simplify
control.
Falling-film evaporation is used; that is, the water to be
evaporated is removed as the slurry rolls down the evaporator
tubes in film flow. Steam and vapor flow is countercurrent
to the slurry flow. Vapors flow from high temperature (high
pressure) to low temperature (low pressure), while the slurry
flows from low temperature (low pressure) to high temperature
(high pressure). Steam is applied, at pressures as low as
50 psig (345 kN/m^), to the shell side of the first effect
(last stage) and its condensate returned to the boiler. The
water vapors removed from the tube side in that stage provide the
steam for the next (second) effect shell side. The water vapors
condensed in the second effect are drained to the hot well. The
steam energy, thus, is used many times. In each subsequent
effect, the vapor temperature is lower. The vapor from the last
effect (first stage) is condensed in a surface condenser and
drained to the hot well.
Oil remaining after evaporation of water is separated from the
solids by centrifuging. Oil is reused in the process, and
the dried sludge product is subjected to further processing or
disposal. The condensate from the evaporation system results
in a sidestream containing ammonia and dissolved organics,
but few inorganics. This sidestream may require subsequent
treatment. Gaseous emissions from the system must be sent to a
boiler or incinerator for odor destruction.
10.7.2.2 Current Status
According to the manufacturer, over 65 Carver-Greenfield
installations are in operation worldwide. Many of these systems
have operated at industrial facilities in the United States,
including a four-effect system at the Adolph Coors Brewery in
Golden, Colorado. This system's water evaporative capacity
is 60,000 pounds per hour (27,240 kg/hr) which allows it to
process approximately 180,000 gallons per day (682 m3/day) of
a 4 percent waste-activated sludge feed (8,10). Two systems
10-31
-------
are also operating at sewage treatment plants in Japan. The
first, installed at Fukuchiyama, is a three-effect unit which
processes combined primary and secondary sludge at rates up to
43,000 gallons per day (170 m-^/d) of 4.5 percent feed material.
The second, installed at Hiroshima, is a four-effect unit,
which can process up to 264,000 gallons per day (998 m3/d) of a
2 percent feed solids. The product at both facilities is used as
boiler fuel.
A 200-pound-per-hour (91 kg/hr) evaporative capacity single-
effect pilot unit was evaluated at the Hyperion plant in
Los Angeles by LA/OMA (14). LA/OMA engineers concluded that
the Carver-Greenfield system appeared to be a viable sludge
drying process that offered considerable energy efficiency when
compared to conventional direct and indirect contact dryers.
However, it was recommended that a large-scale facility should
be built and operated to conclusively demonstrate process
reliability and economics.
Energy requirements for a four-effect Carver-Greenfield system
with hydroextraction were projected to be about 0.44 pounds of
steam per pound of water evaporated. This value was based on
data supplied by the manufacturer, data determined for the
Coors facility, and supported by theoretical analysis of the
system. The energy requirement, including steam production, was
estimated at about 675 Btu per pound (1,568 kJ/kg) of water
evaporated. This compares favorably with the 1,200 to 2,000 Btu
per pound (2,790 to 4,650 kJ/kg) water required in most
conventional heat dryers.
10.8 References
1. Perry, R.H. and C.H. Chilton, editors. Chemical Engineers'
Handbook, Fifth Edition. New York. McGraw-Hill, 1973.
2. McCabe, W.L. and Julian C. Smith. Unit Operations of
Chemical Engineering, Third Edition. New York. McGraw-Hill,
1976.
3. Faust, A.S., Wenzel, L.A., Clump, C.W., Maus, L. and L.B.
Anderson. Principles of Unit Operations. Corrected Second
Printing. New York. John Wiley & Sons, Inc. July 1962.
4. Treybal, R.E. Mass-Transfer Operations. New York. McGraw-
Hill Book Company, Inc.. 1955.
5. Rich, L.G. Unit Operations of Sanitary Engineering. Photo-
Offset. Linvil G. Rich. Clemson, South Carolina. 1971.
6. Combustion Engineering, Incorporated. Steam Tables.
Available from Combustion Engineering, Inc., Windsor,
Connecticut 06095.
10-32
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7. USEPA. Current and Potential Utilization of Nutrients in
Municipal Wastewater and Sludge. First Draft. Office of
WaterProgram Operations. Washington, D.C. 20460.
July, 1978.
8. Yamamota, J.H., Schnelle, J.F., Jr., and J.M. O'Donnell.
"High-Nitrogen Synthetic Fertilizer Produced from Organic
Wastes." Public __Works_. January, 1975.
9. Krzeminski, J. "Sludge Drying Processes: More Flexibility—
Higher Costs." Sludge Magazine. p 32. May-June, 1978.
10.
USEPA. A Review of Techniques for Incineration of Sewage
Sludge with
Development.
December, 1976
Solid Wastes. Office
Cincinnati, Ohio 45268.
of Research and
EPA 600/2-76-288.
11. Regional Wastewater Solids Management Program., Los Angeles
Orange County Metropolitan Area (LA/OMA Project). Sludge
Processing and Disposal. A State of the Art Review.
Whittier, California. April, 1977.
12. Metropolitan Engineers. BEST Process Feasibility Study.
Prepared for Municipality of Metropolitan Seattle. October,
1975.
13. Davis. W. and R.T. Haug. "Los Angeles Faces Several Sludge
Management Problems." Water and Wastes Engineering. April,
1978.
14. Regional Wastewater Solids Management Program, Los Angeles/
Orange County Metroipo1itan Area (LA/OMA Project,
Carver-Greenfield Process Evaluation. Whittier, California.
December,1978.
10-33
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EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapter 11. High Temperature Processes
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 11
HIGH TEMPERATURE PROCESSES
11.1 Introduction
High temperature processes have been used for combustion of
municipal wastewater solids since the early 1900s. Popularity of
these processes has fluctuated greatly since their adaptation
from the industrial combustion field. In the past, combustion of
wastewater solids was both practical and inexpensive. Solids
were easily dewatered and the fuel required for combustion was
cheap and plentiful. In addition, air emission standards were
virtually non-existent.
In today's environment, wastewater solids are more complex and
include sludges from secondary and advanced waste treatment (AWT)
processes. These sludges are more difficult to dewater and
thereby increase fuel requirements for combustion. Due to
environmental concerns with air quality and the energy crisis,
the use of high temperature processes for combustion of municipal
solids is being scrutinized.
However, recent developments in more efficient solids dewatering
processes and advances in combustion technology have renewed an
interest in the use of high temperature processes for specific
applications. High temperature processes should be considered
where available land is scarce, stringent requirements for
land disposal exist, destruction of toxic materials is required,
or the potential exists for recovery of energy, either with
wastewater solids alone or combined with municipal refuse.
High temperature processes have several potential advantag
over other methods (1):
es
• Maximum volume reduction. Reduces volume and weight of
wet sludge cake by approximately 95 percent, thereby
reducing disposal requirements.
• Detoxification. Destroys or reduces toxics that may
otherwise create adverse environmental impacts (2).
• Energy recovery. Potentially recovers energy through
the combustion of waste products, thereby reducing the
.overall expenditure of energy.
11-1
-------
Disadvantages of high temperature processes include (1):
• Cost. Both capital and operation and maintenance costs,
including costs for supplemental fuel, are generally
higher than for other disposal alternatives.
• Operating problems. High temperature operations create
high maintenance requirements and can reduce equipment
reliability.
• Staffings. Highly skilled and experienced operators are
required for high temperature processes. Municipal
salaries and operator status may have to be raised in
many locations to attract the proper personnel.
• Environmental impacts. Discharges to atmosphere
(particulates and other toxic or noxious emissions),
surface waters (scrubbing water), and land (furnace
residues) may require extensive treatment to assure
protection of the environment (3).
This chapter describes both proven high temperature processes
and those having high probability of success, as indicated
by current research. Multiple-hearth and fluid bed furnaces,
the most commonly used sludge combustion equipment in the
United States, Europe, and Great Britain, are discussed, as
well as newer furnace types such as the electric furnace,
the single hearth cyclonic furnace, and modular combustion
units. New thermal processes for wastewater solids reduction
are also described. These processes include starved-air
combustion and co-combustion of sludges and other residues.
Also presented in the chapter are examples that illustrate
the methodology used in selecting and designing processes and
equipment.
11.2 Principles of High Temperature Operations
Combustion is the rapid exothermic oxidation of combustible
elements in fuel. Incineration is complete combustion.
Classical pyrolysis is the destructive distillation, reduction,
or thermal cracking and condensation of organic matter under heat
and/or pressure in the absence of oxygen. Partial pyrolysis,
or starved-air combustion, is incomplete combustion and occurs
when insufficient oxygen is provided to satisfy the combustion
requirements. The basic elements of each process are shown
on Figure 11-1. Combustion of wastewater solids, a two-step
process, involves drying followed by burning.
11-2
-------
SLUDGE
FEED
COMBUSTIBLE
ELEMENTS, INERTS,
MOISTURE
AIR
(OXYGENI —
EXCESS
AIR SUPPLEMENTAL
-1 i r— FUEL (IF REQUIRED)
FURNACE
MOISTURE, EXCESS AIR,
PARTICULATES,
NOX, SOX, HC, C02,
OTHER PRODUCTS OF
COMPLETE COMBUSTION
STACK GASES
{NOT
COMBUSTIBLE)
ASH (RELATIVELY INERT)
(A) INCINERATION (COMPLETE COMBUSTION)
HEAT
SLUDGE
FEED
COMBUSTIBLE
ELEMENTS, INERTS,
MOISTURE
FURNACE
MOISTURE, PARTICULATES,
NOX, SOX, HC, CO, C02,
CONDENSATES (TAR & OIL),
OTHER HIGHER
HYDROCARBONS
COMBUSTIBLE
OFF-GASES
(UP TO 600
Btu/cf)
RESIDUE (COMBUSTIBLE CHARACTERISTICS)
(B) PYROLYSIS (NO OXYGEN)
AIR
(OXYGEN)
i
SLUDGE
FEED
COMBUSTIBLE
MOISTURE
SUPPLEMENTAL
FUEL (IF REQUIRE
1
MOISTURE,
PARTICULATES,
NOX, SOX,
CONDENSATES (TAR & OIL),
OTHER PRODUCTS OF
INCOMPLETE COMBUSTION
f
COMBUSTIBLE
OFF-GASES
(UP TO 400
Btu/cf
RESIDUE (CAN BE UP TO 30% COMBUSTIBLE)
(Cl STARVED-AIR COMBUSTION (OXYGEN DEFICIENT)
FIGURE 11-1
BASIC ELEMENTS OF HIGH TEMPERATURE PROCESSES
11.2.1 Combustion Factors
11.2.1.1 Sludge Fuel Values
A value commonly used in sludge incineration calculations is
10,000 Btu per pound of combustibles (see Table 11-1). It is
important to clearly understand the meaning of combustibles. For
combustion processes, solid fuels are analyzed for volatile
solids and total combustibles. The difference between the two
measurements is the fixed carbon. Volatile solids is determined
by heating the fuel in the absence of air. Total combustibles
is determined by ignition at 1,336°F (725°C). By definition,
11-3
-------
the difference in weight loss is the fixed -carbon. - In the
volatile solids determination used in sanitary engineering (see
Standard Methods, Reference 5), sludge is heated in the presence
of air at 1,021°F (550°C). This measurement is higher than the
volatile solids measurement for fuels and includes the fixed
carbon. Numerically, it is nearly the same as the combustibles
measurement. In the following, if volatile solids is used in the
sense of the fuels engineer, it will be followed paranthetically
by the designation "fuels usage." If the term "volatile solids"
or "volatiles" is used without designation, it will indicate
sanitary engineering usage and will be used synonymously with
"combustibles."
TABLE 11-1
CHEMICAL REACTIONS OCCURRING DURING COMBUSTION
High heat value
„_» _
c +
c +
CO +
H2 +
CH,
2H,S
C +
Reaction
o2 — -~
1/2 02 __
1/2 02 _^
1/2 02
+ 202 _
+ 302 _
H20 (gas) — — -
Sludge combustibles —
of reaction3'13
CO,
CO
CO,
H20
C02 + 2H2°
2SO2 + 2H2O
CO + H
— CO2 + H20
-14,
-4',
-4,
-61,
-23,
-7,
+4,
100
000
400
100
900
100
700
Btu/lb
Btu/lb
Btu/lb
Btu/lb
Btu/lb
Btu/lb
Btu/lb
-10,000 Btu/lb
combustibles
of C
of C
of CO
of H2
of CH.
of H2S
of C
of
Reference
4
4
4
4
4
4
Calculated
Estimated
aNegative sign convention indicates an exothermic reaction.
High heat value assumes the latent heat of water generated
is available for use: conversely, low heat values assumes
the latent heat of water is not available hence no water
is condensed.
1 Btu/lb = 2,324 J/kg
The amount of heat released from a given sludge is a function of
the amounts and types of combustible elements present. The
primary combustible elements in sludge and in most available
supplemental fuels are fixed carbon, hydrogen, and sulfur.
Because free sulfur is rarely present in sewage sludge to any
significant extent and because sulfur is being limited in fuels,
the contributions of sulfur to the combustion reaction can
be neglected in calculations without compromising accuracy.
Similarly, the oxidation of metals contributes little to the heat
balance and can be ignored.
11-4
-------
Solids with a high fraction of combustible material; for example,
grease and scum, have high fuel values (see Table 11-2). Those
which contain a large fraction of inert materials; for example,
grit or chemical precipitates, have low fuel values. Chemical
precipitates may also exert appreciable heat demands when
undergoing high temperature decomposition. This further reduces
their effective fuel value.
TABLE 11-2
REPRESENTATIVE HEATING VALUES OF SOME SLUDGES (6)
Material
Combustibles,
percent
High heating value,
Btu/lb of dry solids
Grease and scum
Raw wastewater solids
Fine screenings
Ground garbage
Digested sludge
Chemical precip
Grit
;olids
.tated solids
74
86
85
60
57
33
16,700
10,300
9,000
8,200
5,300
7,500
4,000
1 Btu/lb = 2,324 MJ/kg
The following are experimental methods from which sludge heating
value may be estimated or computed:
• Ultimate analysis—an analysis to determine the amounts
of basic feed constituents. These constituents are
moisture, oxygen, carbon, hydrogen, sulfur, nitrogen, and
ash. In addition, it is typical to determine chloride
and other elements that may contribute to air emissions
or ash disposal problems. Once the ultimate analysis has
been completed, Dulong's formula (Equation 11-1) can be
used to estimate the heating value of the sludge.
Dulong's formula is:
°2
Btu/lb = 14,544 C + 62,208 (H2 - -) + 4,050 S (11-1)
where C, H2, 02, and S represent the weight fraction
of each element determined by ultimate analysis. This
formula does not take into account endothermic chemical
reactions that occur with chemically conditioned or
physical-chemical sludges.
The ultimate analysis is used principally for developing
the material balance, from which a heat balance can be
made.
11-5
-------
* Proximate analysis—a relatively low-cost analysis in
in which moisture content, volatile combustible matter,
fixed carbon, and ash are determined. The fuel value of
the sludge is calculated as the weighted average of the
fuel values of its individual components.
• Calorimetry—this is a direct method in which heating
value is determined experimentally with a bomb calori-
meter. Approximately 1 gram of material is burned in a
sealed, submerged container. The heat of combustion is
determined by noting the temperature rise of the water
bath. Several samples must be taken and then composited
to obtain a representative 1 gram sample. Several tests
should be run, and the results must be interpreted by an
experienced analyst. New bomb calorimeters can use
samples up to 25 grams and this type of unit should be
used where possible.
The above tests give approximate fuel values for sludges and
allow the designer to proceed with calculations which simulate
operations of an incinerator. If a unique sludge will be
processed, or unusual operating conditions will be used, pilot
testing is advised. Many manufacturers have test furnaces
especially suited for pilot testing.
11.2.1.2 Oxygen Requirements for Complete Combustion
Air is the normal source of oxygen for combustion, although
pure oxygen feed systems are sometimes used. Theoretical air
and oxygen requirements for the combustion reactions are shown
in Table 11-1. For rigorous analyses, the constants given in
Table 11-3 should be used. For general applications in which
fuel oil, methane, and/or sludge are used, a rule of thumb is
that it requires 7.5 pounds (3.4 kg) of air to release 10,000 Btu
(10.55 MJ) from sludge or supplemental fuel (7).
In practice, incinerator operations require air in excess of
theoretical requirements for complete combustion. Excess air
added to the combustion chamber increases the opportunity
for contact between the fuel and oxygen. To ensure complete
combustion, it is necessary to maintain 50 to 150 percent excess
air over the stoichiometric amount required in the combustion
zone. When the amount of excess air is inadequate, only partial
combustion of carbon occurs, and carbon monoxide, soot, and
odorous hydrocarbons are produced.
The excess air required for complete combustion adversely affects
the cost of operation, because additional heat is needed to
raise the excess air temperature to that of the exhaust gases.
Supplemental fuel may be needed to furnish this additional heat.
Thermal economy therefore demands that excess air be held to the
minimum value required to effect complete combustion. The amount
11-6
-------
of excess air required varies with the type of incineration
equipment, the nature of the sludges to be incinerated, and the
disposition of the stack .gases. , The impact of excess air use on
the cost of fuel in sludge incineration is shown on Figure 11-2.
TABLE 11-3
THEORETICAL AIR AND OXYGEN REQUIREMENTS
FOR COMPLETE COMBUSTION (H)
Ib/lb of substance
Substance Air Oxygen
Carbon
Carbon monoxide
Hydrogen
Sulfur
Hydrogen sulfide
Methane
Ethane
Ammonia
11
2
34
4
6
17
16
6
.53
.47
.34
.29
.10
.27
.12
.10
2
0
7
1
1
3
3
1
.66
.57
.94
.00
.41
.99
.73
.41
1 Ib/lb = 1 kg/kg.
11.2.1.3 Factors Affecting the Heat Balance
The heat released by burning the wastewater solids must be
sufficient to raise the temperatures of all entering substances
from ambient levels to those of the exhaust and solid residue
streams. Also, any radiant heat loss from the combustion struc-
ture must be included. If the heat is sufficient, the process is
termed autogenous. If it is not sufficient, supplemental fuel
must be burned to make up for the heat deficit.
A number of variables influence the amount of supplemental fuel
required. As shown on Figure 11-2, the amount of excess air
required to produce complete combustion has an important effect.
Water associated with the sludge also exerts significant demands.
For example, it takes almost 2,000 Btu per pound (4.64 MJ/kg) to
vaporize water and raise the temperature of the water vapor to
exhaust temperatures. When allowances are made for radiation
losses and for heating of gas streams and sludge feed solids, it
is found that approximately 3,500 Btu (3.69 MJ) are required for
every pound (0.45 kg) of water evaporated in a multiple-hearth
furnace (8).
The following example illustrates how the feed solids
concentration required for autogenous combustion is determined.
11-7
-------
io r
D I
LU
E
o
LU
K
ASSUMPTIONS
FEED: 30% SOLIDS
COMBUSTIBLES: 70% OF THE DRY SOLIDS
COMBUSTIBLE HEAT VALVE; 10,OOO Btu/lb
SUPPLEMENTAL FUEL; NATURAL GAS
i-o
DIFFERENCE DUE TO
SUPPLEMENTAL
EXCESS AIH FOR
FUEL
_L
GD TO 100
EXCESS AIR, portent
120
140
Tea
FIGURE 11-2
EFFECT OF EXCESS AIR AND EXCESS TEMPERATURE ON
SUPPLEMENTAL FUEL REQUIREMENTS
Example
A designer uses a proximate analysis to derive the following
values for a given sludge: volatile solids content(fuels
usage--66 percent, fixed carbon content—11 percent, and inert
content—23 percent. The sludge is to be dewatered and burned in
a multiple-hearth incinerator. The solids concentration required
for autogenous combustion in a multiple-hearth incinerator can be
determined.
The sludge heating value can be estimated by multiplying
the approximate fuel value of sludge--! 0,00 0 Btu per pound
(23.2 MJ/kg) by the combustible fraction in the sludge. In this
11-8
-------
example, the combustible fraction is the sum of the volatile
solids (fuels usage) and fixed carbon, or 77 percent. Therefore,
sludge heating value is:
10,000 Btu/lb x 0.77 = 7,700 Btu per pound (17.89 MJ/kg)
The minimum percent sludge solids required to maintain autogenous
combustion can be determined by equating the heat released by
combustion to the heat required by the water. Therefore:
(P)(Q) = (100 - P)(W)
where:
P = Minimum percent dry solids in sludge required for
autogenous combustion
Q = Fuel value of sludge, Btu per pound of dry solids
W = Heat required to evaporate one pound of water in a
multiple-hearth furnace, Btu
The above equation is solved for P:
P = JL. (ioo) (11-2)
Q+W
For this example:
p = i 7nn'!°7 Rnn (10°) = 31'3 Percent
7,700+3,500
If the solids could be dewatered to 31.3 percent, they would be
combusted autogenously. However, feed solids concentrations of
this magnitude are seldom achieved without chemical conditioning.
Allowances for the effect of chemical conditioning should
therefore be made. Assume conditioning requirements are
25 percent lime and 3 percent ferric chloride by weight of dry
solids fed. Therefore, for every 100 pounds (45.,4 kg) of sludge
dewatered, 28 pounds (12.7 kg) of chemicals are added. Assuming
there is no heating value in the lime and ferric chloride, the
100
combustible fraction of the feed solids is reduced to y^} x °-77
= 60 percent and the sludge heating value is 6,000 Btu per pound
(13.9 MJ/kg). Using Equation 11-2, the dewatered sludge must be
36.8 percent solids to be autogenous.
11-9
-------
Figure 11-3 shows a family of curves that can be used to
calculate the minimum percent solids required at various dry
solids heating values. This method of estimating takes into
account the effect of moisture content, inerts, and combustibles
on the combustion process and can be used for basic sizing prior
to detailed analysis.
For example, in the above analysis, a sludge heating value of
6,000 Btu per pound of solids (13.9 MJ/kg) was calculated. From
Figure 11-3, the 6,000 Btu per pound (13.9 MJ/kg) curve crosses
the break-even point at approximately 36 percent dry solids.
The importance of dewatering the sludge is illustrated on
Figure 11-4. The amount of supplemental fuel required is plotted
as a function of feed moisture content and combustible solids
concentration.
The amount of supplemental fuel can be reduced if heat can
be recovered from the process exhaust gases and reused. As
an example, heat may be transferred from the furnace flue
gas to incoming combustion air by means of heat exchangers
(recuperators). Although energy recovery can significantly
improve thermal efficiency, heat recovery equipment can be
expensive and can only be recommended after complete economic
evaluation.
11.2.2 Incineration Design Example
To evaluate combustion processes, a designer must determine if
the sludge will burn autogenously. He must also assess the
effects of different excess air rates, the effects of different
types and quantities of supplemental fuel, and combustion air
requirements.
Approximate and theoretical methods for calculating combustion
requirements are presented in the following examples A summary
is then provided that compares the results of each method.
Either method provides the information necessary for preliminary
evaluation and conceptual design of a sludge incinerator.
When an ultimate analysis of the sludge is available or a good
estimate of sludge constituents can be made, a theoretical
analysis is preferred.
11.2.2.1 Problem Statement
The dewatered sludge production rate expected for a wastewater
treatment plant is 14,000 pounds (6,350 kg) per hour at
20 percent solids. The dewatered material is a mixture of
undigested primary and waste-activated sludges, with a volatile
(combustible) content of 77 percent. The sludge temperature
is 60°F (16°C). To limit hydrocarbon emissions, an afterburner
is used to heat furnace exhaust gases to 1,400°F (760°C).
The design is based on 100 percent excess air (two times the
theoretical requirement). If supplemental fuel is required,
11-10
-------
20 i-
10
20
30
30 40
DRV SOLIDS m SLUDGE, %
FIGURE 11-3
EFFECT OF DRY SOLIDS HEATING VALUE AND SLUDGE MOISTURE
ON CAPABILITY FOR AUTOGENOUS COMBUSTION
TO
11-11
-------
30 29 28
27
SOLIDS CONTENT OF FEED SLUDGE, %
26 25 24 23 22 21
r
70
20
T~
1g 18 17
71
MOISTURE CONTENT OF FEED SLUDGE,
14 75 76 77 78 79
81 82 S3
60 61
63 64 65 66 67 68 69
SOLIDS CONTENT OF FEED SLUDGE, %
Z&OG
ASSUMPTIONS
SLUDGE HEAT VALUE - 10,000 Btu/lb COMBUSTIBLE
FURNACE EXCESS AIR - 75%
AUXILIARY FUEL EXCESS AIR - 10%
EXHAUST GAS TEMPERATURE = 1200°F
37 36 35 34 33 32 31 30
MOISTURE CONTENT OF FEED SLUDGE, %
73
FIGURE 11-4
EFFECT OF SLUDGE MOISTURE CONTENT AND COMBUSTIBLE SOLIDS
CONTENT ON SUPPLEMENTAL FUEL CONSUMPTION
11-12
-------
No. 2 fuel oil will be used. Twenty-five percent excess air will
be used for combustion of the fuel oil. The air temperature is
60°F (16°C); the absolute humidity of the air is 0.013 pounds of
water per pound of dry air. Heat capacities of dry air, water
vapor, dry sludge solids, and water are 0.256, 0.5, 0.25 and
1.0 Btu per pound per °F, respectively, (1.07, 2.1, 1.0, and
4.2 kJ/kg/°C. The latent heat of water is 970.3 Btu per pound
(2,253 kJ/kg).
11.2.2.2 Approximate Calculation Method
Assuming 10,000 Btu per pound (23.2 MJ/kg) of sludge, the heat
content of the sludge is:
10,000 y^- x 0.77 = 7,700 Btu per pound (17.9 MJ/kg)
From Figure 11-3, a value of approximately 32 percent solids in
the dewatered sludge is required for autogenous combustion.
Therefore, supplemental fuel is required and its quantity must be
determined. The demand for supplemental fuel equals the heat
required minus the heat value of the sludge.
Step 1. Sludge Heating Value
The heating value of the sludge
14,000 Ib sludge 0.2 Ib solids 0.77 Ib VS 10,000 Btu
hr x Ib sludge Ib solids Ib VS
= 21.56 x 106 Btu per hour (22.75 KJ/hr)
Step 2. Combustion Air Requirements
Therefore, combustion air requirements
- 21.56 x 1Q6 Btu 7.5 Ib dry air 2fexcess air factor)
_ x 10/000 |tu .x ^(excess air tactor)
= 32,340 pounds dry air per hour (14.68 t/hr)
Step 3. Heat Required to Raise Ambient Air Temperature
The basic formula for determining the heat required is:
Q = Mass x heat capacity x temperature change (11-3;
11-13
-------
Heat required to raise dry air from 60°F (15.6°C) to 1,400°F
(760°C)
32,340 lb dry air x O. x (lf4000p . 6()0p)
= 11.09 x 106 Btu per hour (11.70 GJ/hr)
Heat required to raise the temperature of water vapor in air
from 60°F (15.6°C) to 1,400°F (760°C)
= 32,340 lb dry air 0.013 lb water 0.5 Btu _
hr x lb air x lb-°F x (1'400 F 60 F)
= 0.28 x 106 Btu per hour (0.30 GJ/hr)
Step 4. Heat Required to Raise Solids Temperature
Heat required to raise the temperature of the volatile
(combustible) material from 60°F (15.6°C) to 1,400°F (760°C)
14,000 lb sludge 0.2 lb solids 0.77 lb VS 0.25 Btu
hr x lb sludge x lb solid x lb-°F
x (1,4QO°F - 60°F) = 0.72 x 106 Btu per hour (0.76 GJ/hr)
Heat required to raise the temperature of inerts (ash) from
60°F (15.6°C) to the ash discharge temperature of 200°F
(93.3°C)
_ 14,000 lb sludge 0.2 lb solids (1-0.77) lb inerts) 0.25 Btu
hr x lb sludge x lb solids lb-°F
x (200°F - 60°F) = 0.02 x 10^ Btu per hour (0.02 GJ/hr)
Step 5. Heat Required to Raise Temperature of Water
Associated with the Feed Sludge
This calculation does not include water formed during the
combustion reaction.
11-14
-------
Heat required to raise the water temperature from 60°F
(15.6°C) to 212°F (100°C)
= 14,000 lb sludge 0.8 lb water 10 Btu x 212oF _ 6QO
hr lb sludge lb-°F '
= 1.70 x 106 Btu per hour (1.79 GJ/hr)
Heat required to evaporate water
14,000 lb sludge 0.8 lb water 970.3 Btu
hr x lb sludge x lb
» 10.87 x 106 Btu per hr (11.46 GJ/hr)
Heat required to raise the temperature of water vapor to
1,400°F (760°C)
14,000 lb sludge 0.8 lb water 0.5 Btu
hr x lb sludge x lb-°F
= 6.65 x 106 Btu per hour (7.02 GJ/hr)
Step 6. Heat Required to Raise Temperature of
Water_Formed During ,the__Cp_mbustion Reaction
Assume water formed during the combustion reaction to
be 0.5 pound per 10,000 Btu (21.5 g/MJ) of sludge and
supplemental fuel burned (9). The heat value of the
sludge burned and supplemental fuel are equal to the heat
demands. Therefore, water formed during combustion must be
calculated on the basis of heat demands. Heat demands may be
approximated by summing the calculations thus far:
Heat requ_ir_ed for Btu/hr x 106
Air
Dry air 11.09
Water vapor in air 0.28
Sludge
Volatile solids 0.72
Inerts 0.02
11-15
-------
Heat required for Btu/hr x106
Sludge (continued)
Free water
Water 1.70
Evaporation 10.87
Water vapor 6.65
Total 31.33 (33.05 GJ/hr)
Water formed due to the combustion reaction
- 0-5 Ib 31.33 x 106 Btu = .
— ^ x /\/\/\ T-. i A i JL f ~J\J f t-JUUl H-4O k-"Cl_ 11VJ\J,1_ I / _L _L JVM/ ill. I
10,000 Btu hr » t~ r~ \ ^/ /
The heat of combustion given is the "high heat of combus-
tion," which assumes all water formed is condensed. Heat
must be provided to evaporate this water and bring it up to
exhaust temperature.
Heat required to evaporate the water
- 1.567 Ib water 970.3 Btu _ 6 Q ,
— . A i , •" X • ~/^> A J.U OL.U L/tTl. llwUl. I X . \J\J ww / 111. /
nr ID
Heat required to raise the temperature of water vapor to
1,400°F (760°C)
- 1.567 Ib water 0.5 Btu M 4nnop _ oioopx
j— x lb_0p x (1,4UU i m t)
= 0.93 x 106 Btu per hour (0.98 GJ/hr)
Step 7. Heat Required to Compensate for Radiation Losses
Assume a radiation loss of 5 percent of the total heat
demand. Total heat demand is
Heat required for Btu/hr x 106
Total from Step 6 31.33
Water formed during combustion
reaction
Evaporation 1.52
Water Vapor 0.93 ;
Total 33.78 • (3,5.;64 GJ/hr)
11-16
-------
Heat to compensate .for radiation losses
= 33'78 ^r10 BtU x 0.05 = 1.69 x 106 Btu per hour (1.78 GJ/hr)
Step 8. Determine Supplemental Fuel Required
Total heat requirements (from Step 7)
= 33.78 x 106 Btu/hr + 1.69 x 10^ Btu/hr
=35.47 x 106 Btu per hour (37.42 GJ/hr)
Total supplemental heat demand
= Heat demand minus heating value of sludge
= (35.47 x 106 - 21.56 x 10^) Btu/hr
= 13.91 x 106 Btu per hour (14.68 GJ/hr)
Therefore, supplemental fuel (No. 2 fuel oil) must be
supplied to provide 13.91 x 106 Btu per hour (14.68 GJ/hr)
of heat.
Supplemental fuel also requires air for combustion, and this
air exerts a heat demand. The air required for supplemental
fuel is 1.25 times the theoretical value needed for
supplemental fuel.
Air required for supplemental fuel
- 13.91 x 1Q6 Btu 7.5 Ib dry air , „ ,excess air factor)
_ x 10fOOQ Btu x l.2b (excess air tactor)
= 13,000 pounds dry air per hour (5,920 kg/hr)
The 13,041 pounds (5,920 kg/hr) dry air (plus any water
formed by its reaction with the supplemental fuel) must also
be raised to 1,400°F (760°C). By calculations similar to
those presented in Steps 1 through 8, it can be shown that
heat required to do this (and to account for additional
11-17
-------
radiation losses) is 20.24 x 106 Btu per hour (21.35 GJ/hr).
Since only 13.91 x 106 Btu per hour (14.68 GJ/hr) was
released by burning supplemental fuel, there is a heat
deficit of 20.24 x 106 - 13.91 x 106 = 6.33 x 105 Btu per
hour (6.67 GJ/hr). Thus, the effect of adding supplemental
fuel was to reduce but not eliminate the initial deficit of
13.91 x 106 Btu per hour (14.68 GJ/hr).
To make up for this deficit,
equivalent to 6.33 x 10^ Btu per
If 25 percent excess air is used
per hour (2,694 kg/hr) of excess
more supplemental fuel,
hour (6.68 GJ/hr) is added.
for this fuel, 5,934 pounds
air will be required. The
heat released is again insufficient to raise the air plus
water vapor formed to 1,400°F (760°C) and to make up for
additional radiation losses. The deficit for this iteration
is 2.88 x 106 Btu per hour (3.04 GJ/hr).
The calculation can be carried forward for several more
steps. Table 11-4 shows that progressively smaller addi-
tions of supplemental fuel and air are required for each
iteration and that the amount of air and fuel needed for
each iteration is a fixed fraction (0.45) of the fuel
and air needed for the previous iteration. In general,
if fuel required for each iteration is r percent of that
required for the previous iteration, then total fuel required
= (initial deficit)(l + r + r2 + r3 + ... + rn). The
term in the second bracket is an infinite geometric series
equal to rn. The series converges to
value of r is less than one (10).
if the absolute
TABLE 11-4
APPROXIMATE COMBUSTION CALCULATION -
SUPPLEMENTAL FUEL REQUIREMENTS
Heat input
—-"' • ™
Unit
Sludge
Supplemental fuel
Supplemental fuel
Supplemental fuel
Supplemental fuel
Heat value,
106 Btu/hr
21. 56
13.91
6.33
2.88
1.31
Heat value.
Unit 106 Btu/hr
Slndqe and
excess i
Supplemen a
excess i
Supplemen a
excess i
Supplemen a
excess ai
35.47
fuel and 20.24
fuel and 9.21
fuel and 4.19
fuel and 1.91
Supplemental
fuel requirements,
10& Btu/hr
13.91
6.33
2.88
1.31
0.60
.46
.45
.45
.46
Combustion
air requirements,
Ib/hr
32,340
13,041
5,934
2,700
1,228
.46
.46
aRatio of supplemental fuel to that in the previous iteration.
Ratio of air to air in the previous iteration.
CRatio in this case is not applicable since sludge is included (100 percent
1 x 10 Btu/hr = 1,055 MJ/hr
1 Ib/hr = 0.45 kg/hr
11-18
-------
The total supplemental fuel requirements can be derived from
Equation 11-4.
Total supplemental fuel = Initial deficit x ^— (11-4)
Total supplemental fuel
= 13.91 x 106
hr 1-0.45
= 25.32 x 106 Btu per hour (26.6 GJ/hr)
Step 9. Total Air Requirements
The air requirements for the supplemental fuel alone can be
found from Equation 11-5, an analog to Equation 11-4.
Total supplemental air requirements
= excess air for initial supplemental fuel addition x T^— (11-5)
Total supplemental air rquirements
= 13,041 Ib air 1
hr x 1-0.45
= 23,735 pounds dry air per hour (10,766 kg dry air/hr)
Total dry air requirements
= air for sludge plus air for supplemental fuel
(32,340 -I- 23,735) Ib dry air/hr
= 56,075 pounds dry air per hour (25,458 kg/hr)
Assuming an air density of 0.0749 pounds per cubic feet
(1.2 kg/m3):
11-19
-------
Air flow rate
56,075 Ib/hr hr -n /no u- ^ x. • *. /r n •*/ x
= —n n7AQ— x £n mi r. = 12/478 cubic feet per minute (5.9 nH/sec)
0.0749 60 min
Assume that No. 2 oil has heating value of 141,000 Btu per
gallon
Supplemental fuel rate
25.32 x 106 Btu/hr hr
141,000 Btu/gallon x 60 min
= 3.0 gallons per minute (0.18 1/s)
11.2.2.3 Theoretical Calculation Method
The method presented herein is based on the actual combustion
reactions and the method of approach used in steam generation
calculations (9). Table 11-5 is to be used for steam generation
calculations. A blank form is provided at the end of Chapter 11
for the reader's own use in making the calculations.
Step 1. (Line b) . Determine the fuel analysis and include
on the right hand side of the table (ultimate analysis).
Step 2. (Lines 1 through 12). Determine the pounds of
component, moles of component, theoretical oxygen requirement
and moles of material contributed to the flue gas by the
fuel, based on 100 pounds of fuel feed. Assume complete
combustion and no loss of combustibles to the ash.
Step 3. (Lines 13, 14, and 15). Assume the amount of excess
02 to be used (100 percent) and calculate the moles of excess
02 required.
Step 4. (Line 16) . Calculate the amount of N2 added from
the air from the total 02 (theoretical plus excess).
Step 5. (Lines 17, 18, 19, and 21). Calculate the amount of
dry air, water in the air, the amount of wet air from the
total dry air (02 + N2).
Step 6. (Lines 20 and s). Calculate the moles of all
components in the flue gas and the moles of wet and dry flue
gas.
11-20
-------
TABLE 11-5
COMBUSTION CALCULATION - MOLAL BASIS
Flu*
Malr= pnr Full Unrt (AF)
Fu* Anil, » ffcwi (AFX % by W « VM
CtoCOi
CtaCO
oo io cos-
TrtM * (T A) issuing ar b^QRSAT HI %
Ur*s I, g,
WL IIMI ynft -
Wol,
D«i,ilti..:.TriJ«»K!FiKlrv -
Fuel hen! value. Btu/lti IH1
Catnbuitibti in refute. % "C"
Carbon unbumiiii,"ib/lin Ifc liud
• % ash in fuel x
Os *nd Air, Mslti for Tow Air - If i "it
{see lined at rlgM)
0) (tilTO) re»1 - Os, llnf 12
Oi (ucera) - --ifla X 0|. tin* I?
0] (toW)wppli«i - lines 13 + H
Ns SUpplltd - j.?$ X Oi, Urn IS
Air (dry) supplied - 0;
t«it timp »f flu* pt*. !.-
Dry bulb {ambient) temp, f i
Rei huntid,
6*, &sre»™trk: preMur*. in, Hf
Stt pr«i Htp »< imB tw|Wi "
A*, pf«s«, HjO HI air. iiwi (» x q), ta. MI
^ __ .
Air (twt) nipplM -fcleil? -IB
Not« - fur air it 30 T and MO* rMttlv* lummlKj, 1 » IWB7 is gtt«» cacrf a= itsndird
D*t«r™n«Hwi of Fhw 6w wid &-ibu.hble L«l»l In BH [Hf Fwl Urrt e»fb9ninrwfus» - line kx 14.100 i««i I
Du« Is tiBbumed CO In f\tm ill - nw!« C to CO x « x 9,755 •*•• Z6 -I- 291- JRH- rtlttMkn ttt
100 x in* i far striH
• 3,4 „,,„,, x lOuv
fuol »irt - lin* 33 - Ii04 31
t Ply* s« tmlyt 1* by ORSAT. If CO I* prmtmjt In nun B»«», * cirbon
b»l«r>tt It UMd IP d«t*«-n«lri«dlili-lbiJtl«i of C, thux:
All C Ml y •mlytlt^U,
MelMCInr*iy**'llrMh^i- Molw C In CO. » (n»t« C In fuel -iMl«C
InrrtlM) x ICO, byCMSAT^I (CO,. CO) by OMHT.
M«l« In C In CO - mJM C In fu.l - mala C In rrfu» - mh. C In CO],
CtMtRAJ. HOTCS:
*
* M*AtH, u UHd In
tt By Dueeno famub (tl-H «• by «lflctawiry,
ttt R*3l*0on **w«wd to t* t flx*d pwc«r* =f Iliw U. iwtul
Copyrbjhl 1471 by th. BtbCdCk Hid WlkOX CoBKMny, Mlrw
1 It>-(,l9kf
I In. - 1,U«
i it/cu n • it
mid* |» tnli ofeta to "Ik™ Iv
of yu wl»
, T»W«w«r h«
11-21
-------
Step 7. (Lines 22 through 26)
content of
used. The
Reference 4
the gas. A base
values for mean specific
Determine the sensible heat
temperature of 60°F (15°C) is
heat can be found in
Note: mean molar specific heat = mean specific heat x
molecular weight.
Step 8. (Line 27) . Determine the latent heat of water in
the flue gas.
Step 9. (Line 28). Sum all heat in flue gas.
Step 10. (Lines 29, 30, and 31). Calculate heat losses due
to carbon in refuse (residue), unburned CO in the flue gas,
and radiation (assumed to be 5 percent). Sum all heat
losses.
Step 11. (Line 32). Determine heat value of the sludge per
100 pounds, wet basis.
Step 12. (Line 33). Determine if the sludge is autogenous
or requires supplemental fuel by subtracting line 32 from
line 31. A zero or positive number indicates that the sludge
is autogenous, supplemental fuel is not required, and the
computation is complete. A n<
Disputation is complete. A negative number shows that
supplemental fuel is necessary. The method used to determine
the amount of fuel required is shown in steps 13 through 15.
Step 13. If Step 12 indicates that supplemental fuel is
required, proceed through another theoretical calculation
method table for the supplemental fuel in the same manner as
Steps 1 through 12 (lines 1 through 33). This determines the
amount of excess heat in the fuel after the combustion
reaction. Table 11-6 illustrates the supplemental fuel
calculation for this example.
Step 14. Determine the amount
100 Ib (45 kg) of wet sludge.
of supplemental fuel per
Ib supplemental fuel required
100 Ib of sludge, wet basis
heat required from fuel (line 33, Table 11-5)
available heat from fuel (line 33, Table 11-6;
91,139 Btu/100 Ib sludge
1,165,443 Btu/100 Ib fuel
= 7.82 Ib fuel/100 Ib sludge, wet basis
11-22
-------
TABLE 11-6
COMBUSTION CALCULATION - MOLAL BASIS
Tabte «-• Comburton Cakutotloni-Molal Bwrit
32
Fu«, Q.,, »nd Air [W Unit of Fuel
fyst
CISCO:
CtoCO
C unturned,
fmk
Hj
s
COi
Fwl
Unit,
Oiui.
11.7
1.6
IJ
1,1
1
*
tJ*
Lit
M1
I
I
I
1
Htqd
TJ1
I
I
1.11
Ul
-1.11
D
O
o
o
Moles per Furl Unit (*F)
SO;
IJI
It.ll
O-i tnii Aw, Moles for Total Air -
£JM line -J al
02 i f in, IIL
MIDCITT HIPMIIY
100 '"' *8|M ^^^ lutl1
00
Fun An»t, « Fired (ftf), % 6y W at Vrt
MOO
OQ2
CO
arr (TJL) asa^ntti or by fflRSAT III %
br*s f, 5, h Far Sas
Wt (ml -Jral - I {mates E
Mai. ••« of fud - lira f , 'l
s Fuels
cd x ma. wi) Ib
Fuel 1«t »Blg«, Sty/113 21,441
Crb-:r un Burned, ib/tbo It hd
. % ish in fuel x rm—5T-TF::
E«lt tsmp at nys us, %
ent) temp, f i
!^i —
3^4 6y tt
tt
1
Hll
: II f
Rfrl hjrrid
8* Sarginrtnc SJfSSSMfS, In. Mg |§j
S*t PTOSS. N^O 3t arnb t«mpf in. Hg
A*. fftBi, Htc in v, mm {a x q). w. HI
Tet»i
Metes
Wrt
IIJ1
Clry f IPB Oas
tIJl
*Npt» - for aira« 80 f and HXni relative humidilv -7^- (1037 ii OHSI U9«l « BtBndartl,
O*»«™lMli«i af FliH fits J(id Ccmlmrtibl* Unm in Mu pv Fu*l IMI (AF>
Flye |^as constituents COi+SO;
Off, main, !i Hj f i (fgr Tj - llJl-U
In dry Me mi - mofcs MCU, »«ii M x Wep x^f" - fi'l IMfnim
tn N20 in air - mdas HjO, Sn« IB x Mc» X Hi — f"j)
in wni iwtrt. H?0 In n &-*"]
(n btent heit, HjO in fu«» - mot*s, llnss {! + 1ft) x 1 (WO * IS
T«ftl In w»t fly* g»6
Cue ta carton in titui* - line k X 14,1W
CUB tn .jnburnafi CO in HUE gas - tnntas C ta CO .:•: 12 .•: 9,755
9s
"i.j"
u
*7J.7H
71.731
111,171
I«B« ftu* BIS ^8l«6 + unb*ra
- lines 2B I- 2i f 30 4- f*SHU« ttt
100 X line i )w solid and liquid ruet
M4 y ||ne , ,x 1
T«ai ««Hi !»n pff fud unit - line 32 - line 31
OQ
Tntii
II Jit
»1 1,171
UI.711
I
i
M*M«
t Flue gtt *n*ly*» t)j OHSAT , If CO it pr*i*rt In flui D^HI, i carbtxi
balanci I* uud ID d*t*rmlo» dlttrlbMlten of C. ltn«:
All C In fwl * C in Tlus gif ««fUbJwiti - C in r**utt Mahn C in
full = X C by anilylil -i-t2,
Mal« C In rj - [mol»t C in to* - nwln C
tn r«fti»«) x I C»i by (MSAT^ t tco j + CO) br OWSAT,
lo c In CO = i»ol«i C In fuel - raolkm C m rofu» - mai« C in CO].
tt By D«*w«j f=™tiU (11-1) or far l
ttt RMtlitikin itiuflMd to b* • 1*«d iwriunt of llm tt, nwmtly ; •> $ p*r«*.
IU*j»r *> uud In Mi
ttM*, It Ow r**k»u* (lift)
from tlvt prsc»M .
1 ID • «.«• 19
1 In. -3. Ma
1 Btu/lb -
1 Ib/cu ft . !•
Copyright 1J75 fay ttiB B^KDck end Wlleanc CdO^Mny. Mlnar clkiiijw* nw* b**n
Tnilfl t« t*il« t»b(« to allnw fur Mia of un Kltfl HMtgi lludp*. T*Di* mff tM
t wltinxit pernlulon. Hlnovir. cr«ll 1 In Bibcock «1
11-23
-------
Step 15. Calculate the total fuel demand for 14,000 pounds
per hour of wet sludge (6,356 kg/hr):
Total fuel
7.82 Ib fuel
x 14,000 Ib sludge/hr
100 Ib sludge
= 1,095 pound fuel per hour (497 kg/hr)
From line i, Table 11-6, Btu value
= 1,095 Ib fuel/hr x 20,440 Btu/lb
= 22.38 x 106 Btu per hour (23.61 GJ/hr)
Step J.6. Calculate the total combustion air requirements:
From Table 11-5, line 17 combustion air required for sludge =
8.47 moles/100 Ib sludge.
From Table 11-6, line 17 combustion air required for
supplemental fuel = 61.83 moles/100 Ib fuel.
Total dry air
'8.47 moles air
^100 Ib sludge / \
\ / \~
29 Ib air
^61.83 moles air\/n nQt, Ib fuel\
\ 100 Ib fuel y^1'050 hr )
Ib mole air
= 54,040 pounds per hour (24,534 kg/hr)
11.2.2.4 Comparison of Approximate and
Theoretical Calculation Methods
Table 11-7 shows that the approximate method requires slightly
more fuel and air than the theoretical method, but the values are
close. This comparison shows that the approximate method is
suitable for preliminary evaluations. More detailed information
and combustion theory can be found in the literature (1,4,6,7,9,
and 11-16).
11-24
-------
TABLE 11-7
COMPARISON BETWEEN AN APPROXIMATE AND A THEORETICAL
CALCULATION OF FURNACE COMBUSTION
Approximate method (AM) Theoretical method (TM)
Calculation Difference
reference AM-TM . nn
Value (TM) TM * 1UU
1,661 Btu3 Table 11-4 -7.28
Ib asfed line i
91,139 Btub Table 11-4 9.01
Item
Sludge heating value
Furnace heat deficit
Supplemental fuel heating
value
Supplemental fuel required
Total combustion air
Value
10,000 Btua
Ib VS
13.91 x 106 Btu
~hr~
141,000 Btuc
gal
25. 32 x 106 Btu
hr
56,075 Ib
Calculation
reference
(AM)
Assumed
Step 8
Step 8
Step 8
Step 9
100 Ib wet sludge line 33
20,440 Btu Table 11-5 -4.19
Ib line i
22.38 x 106 Btu Step 15 13.14
hr
54,040 Ib Step 16 3.77
required h~r" Hr"
10,000 Btu/lb VS at 77 percent VS = 7,700 Btu/lb dry solids,
1,661 Btu/lb as fed v 20 percent solids = 8,305 Btu/lb dry solids.
b91,139 Btu/100 Ib wet x 14,000 Ib wet/hr = 12.76 x 106 Btu/hr.
C141,000 Btu/gal ; 7.2 Ib/gal = 19,583 Btu/lb.
1 Btu/lb = 2,324 J/kg
1 Btu/hr = 1,055 J/hr ,
1 Btu/gal = 279 kj/m3
1 Ib/hr = 0.45/hr
11.2.3 Pyrolysis and Starved-Air Combustion
Calculations
Pyrolysis and starved-air combustion have received considerable
attention recently. The yield and composition of the gas and
residue depend upon several variables. The actual interrelation-
ships are so complex that final product characteristics must be
determined empirically.
Currently, data are insufficient to provide information for
designing pyrolysis equipment. Several large pyrolysis projects
have been proposed, and some are in start-up or early operation.
However, most work to 1979 has been at laboratory scale. At this
writing, there are no full-scale pyrolysis projects proposed or
under development that use sludge alone; all are for solid waste
or specific industrial wastes.
Starved-air combustion, a partial pyrolysis process, has had
a number of successful tests, such as those conducted at
the Central Contra Costa Sanitary District (17,18), and the
Interstate Sanitation Commission (19), and several modular
combustion units have used municipal solid waste, sewage sludge,
and/or agricultural wastes. Starved-air combustion has also had
some failures such as at the Baltimore plant, which used only
solid waste. The furnace at Baltimore is now being modified for
further testing and use. Multiple-hearth furnaces have been
11-25
-------
tested for both sludge and co-disposal starved-air combustion.
This work on starved-air combustion by multiple-hearth furnaces
has been sufficient to allow development of empirical design
criteria (17-20).
Some engineers and manufacturers use a hearth loading rate
of 10 to 14 total pounds per square foot per hour (48.8 to
68.3 kg/m2/hr) over the whole effective hearth area while
assuming that up to 15 percent of the input energy remains in
the ash as a char. Other engineers and manufacturers use the
following design criteria which assumes a lower hearth loading
rate and an additional hearth area to gasify the fixed carbon.
This design results in a very low combustible content in the
ash (20):
• 15 percent of the combustible matter becomes fixed
carbon.
• Fixed carbon is gasified at a rate of 0.5 to 0.8 pounds
per square foot per hour (2.4 to 3.9 kg/m2/hr).
• Wet sludge feed rate (hearth loading rate) varies between
8 and 12 total pounds.per square foot per hour (39.0 and
58.6 kg/m2/hr).
Assuming afterburning, 85 percent of the total feed energy
remains in the afterburner gases.
ExjampJLe
Estimate the required hearth area of a multiple-hearth furnace to
burn the sludge generated from a 20 MGD (0.88 m3/s) wastewater
treatment plant by starved-air combustion and the heat content of
the hot gas from the afterburner. Assume the furnace feed is
40,000 pounds per day dry solids (18,140 kg/day). Assume that
the furnace feed is 40 percent solids and that the solids are
65 percent combustibles. Afterburning to 1400°F (760°C) is
required.
Wet sludge feed rate
40,000 Ib dry solids 1 Ib sludge 1 day
day X 0.4 Ib dry solids X 24 hr
= 4,167 pounds wet sludge per hour (1,890 kg/hr)
Fixed carbon rate
40,000 Ib dry solids 0.65 combustible solids
day Ib dry solids
11-26
-------
0.15 Ib fixed carbon 1 day
Ib combustible solids 24 hr
= 163 Ib fixed carbon per hour (73.9 kg/hr)
Estimate hearth area and multiple-hearth furnace size. Hearth
area is considered as the sum of the area required to convert the
wet sludge to the fixed carbon stage and the area needed to burn
out the fixed carbon.
Hearth area
wet sludge feed - fixed carbon feed fixed carbon feed
allowable hearth loading rate gasification rate
4,167 Ib/hr - 163 Ib/hr 163 Ib/hr
10 Ib/sq ft/hr 0.5 Ib/sq ft/hr
726 square feet (67.44 m2)
After discussions with the furnace manufacturers, a 14-foot
3-inch (4.34 m) diameter, 8 hearth unit with an effective hearth
area of 760 square feet (70.6 m2) is selected.
Estimate the heat content of hot gases leaving the afterburner:
Heat content
40,000 Ib dry solids 0.65 Ib combustible solids
day x Ib dry solids
10,000 Btu 1 day n ftc-
x Ib combustible solids x 24 hr x u*tti
= 9.2 x 106 Btu per hour (9.72 GJ/hr)
Portions of this heat can be recovered and used benefically, for
example, to generate steam or hot water (see Chapter 18).
BSP Division of Envirotech Corporation, and Nichols Engineering
and Research (now part of Wheelabrator) have developed a large
data base for evaluation of starved-air combustion operations.
11-27
-------
Even with the amount of work that has been completed to date,
however, calculations for starved-air combustion are still
empirical. Because starved-air combustion is extremely complex
and not completely understood, it is desirable to pilot any
starved-air combustion process and, where possible, test at
full-scale. There are several excellent texts and articles on
combustion, but none deal to any great degree with oxygen-
deficient combustion. Starved-air combustion is discussed in a
number of publications (17-30).
11.2.4 Heat and Material Balances
Analysis of high temperature processes must include heat and
material balances. Once provided, equipment can be sized and
operating costs estimated. Throughout the remainder of this
chapter, heat and material balances are displayed for several
alternative combustion processes, all being fed the same
hypothetical sludge. A flowsheet for a hypothetical wastewater
treatment plant is depicted on Figure 11-5. Design data for 5,
15, and 50 MGD (0.22, 0.66, and 2.19 m3/sec) wastewater treat-
ment plants using this configuration are shown in Table 11-8.
The "A" and "B" alternatives vary only in the percent solids feed
(20 percent and 40 percent, respectively) and the addition of
conditioning chemicals to obtain a dewatered cake of 40 percent
solids. Use of conditioning chemicals reduces the percent
combustibles of the "B" alternatives.
In Section 11.3, detailed heat and material balance tables are
presented for each furnace type. The tables also display the
amount of fuel and power each type of furnace requires, for each
different treatment plant alternative. Balances given are for
yearly average conditions. Operational costs can be estimated
from the requirements for supplemental fuel and connected
horsepower. General sizes and types of support facilities, such
as ash handling equipment, water supply for the air pollution
control equipment, and operating fuel requirements can also be
estimated on the basis of the data shown in the heat and material
balance tables.
In any steady-state balance, all inputs must equal all outputs.
The following is a representative example of a heat and material
balance for the Alternative IA in Section 11.3.1.
Alternative IA—Heat Balance
Inputs
Combustibles in sludge
Supplemental fuel
Total
106 Btu/hr
13.91
2.64
16.55 (17.46 GJ/hr)
11-28
-------
OutjDirbs
Furnace exhaust
Ash
Radiation
Shaft cooling air (unrecovered
portion)
Total
106 Btu/hr
15.96
0.04
0.32
0.22
16.54 (17.45 GJ/hr)
Values are essentially equal; the balance checks. Note that
shaft cooling air is an internal loop in the system. Since it is
neither an input or output, only the unrecovered portion need be
considered in the heat balance.
jAj-j-M a t e r i a lBalance
Inputs
Dry solids in the sludge
Water in the sludge
Supplemental fuel
Combustion air
Total
Ib/hr
1,806
7,224
143
22,060
31,233 (14,180 kg/hr;
Outputs
Ash
Furnace exhaust
Total
Ib/hr
415
30^817
31,232 (14,179 kg/hr;
Again, values are essentially equal; the balance checks.
Reference 23 contains valuable information on heat and material
balances.
11.3 Incineration
Incineration is a two-step oxidation process involving first
drying and then combustion. Drying and combustion may be
accomplished in separate units or successively in the same
unit, depending upon temperature constraints and control
parameters. The drying step should not be confused with
preliminary dewatering, which is usually done mechanically prior
to incineration. In all furnaces, the drying and combustion
processes follow the same phases: raising the temperature of the
feed sludge to 212°F (100°C), evaporating water from the sludge,
increasing the temperature of the water vapor and air, and
11-29
-------
increasing the temperature of the dried sludge volatiles to
the ignition point. Although presented in simplified form,
incineration is a complex process involving thermal and chemical
reactions which occur at varying times, temperatures, and
locations in the furnace.
BAR GRIT PRIMAH'Y
SCREENS REMOVAL SEDIMENTATION
AERATION
(CAK1QNACEQLS SECONDARY CHL08INE
OXIDATlONf SEplMiNTATION CONTACT
-*- SCUM TO LANDFILL
ASH TO LANDFILL
FIGURE 11-5
HYPOTHETICAL WASTEWATER TREATMENT PLANT FLOWSHEET
Manufacturers have developed a variety of equipment, each of
which has advantages and disadvantages (19, 31-34). There are
two major wastewater sludge incinerator equipment types used in
the United States: the multiple-hearth and the fluid bed. The
electric furnace, which is relatively new, has been used and, as
11-30
-------
of 1979, is planned for use in several wastewater treatment
plants. A fourth type is the single hearth cyclonic furnace.
This furnace has been used in Great Britain, but its only
application in the United States has been in industrial service.
These four systems are described in detail in this section. Heat
and material balances are included for each type, assuming each
is used in the hypothetical wastewater treatment plants described
in Figure 11-5 and Table 11-8.
TABLE 11-8
HYPOTHETICAL WASTEWATER TREATMENT PLANT DESIGN DATA
Alternative
Sewage flow, MGD
Sludge solids,
Ib/day dry basis
of dry solids
Ib/hr, dry basis
Conditioning chemicals,
Ib/hr, dry basis
Total feed to furnace,
Ib/hr, dry basis
Furnace loading rate
Ib/hr, wet basis
Volatile content of fur-
nace feed, percent of
total solids 77 65 77 65 77
I (5-MGD
:a A
5
10,320
lercent
77
hr/week 40
urnace,
; 1,806
.cals,
ib 0
lace,
; 1,806 .
furnace
• weight 20
ite,
; 9,030
flow)
B
5
10, 320
77
40
1,806
325
2,131
40
5,327
II (15-MGD
A
15
31,000
77
80
2,713
0
2,713
20
13,565
flow)
B
15
31,000
77
80
2,713
488
3, 201
40
8,003
III (50-MGD
A
50
flow)
B
50
103,000 103,000
77
168
4,292
0
4,292
20
21,460
77
168
4, 292
772
5,064
40
12,660
a
The A alternatives have a 20 percent solid feed sludge while
the B alternatives have a 40 percent solids feed sludge including
conditioning chemicals.
b!5 percent lime (CaO) and 3 percent ferric chloride (FeCl^), dry
weight basis for the 40 percent cake only.
1 MGD = 0.04 m3/s
1 Ib/day = 0.45 kg/day
1 Ib/hr = 0.45 kg/hr
11.3.1 Multiple-Hearth Furnace
The multiple-hearth furnace (MHF) is the most widely used sludge
incinerator in the United States. As of 1977, approximately
340 units had been installed for wastewater sludge combustion
(35). The MHF is durable, relatively simple to operate, and can
handle wide fluctuations in feed quality and loading rates. The
11-31
-------
MHF is designed for continuous operation. Start-up fuel require-
ments and the extended time needed to bring the hearths and
internal equipment up to temperature from a completely cold
condition normally preclude intermittent operations. The MHF is
a vertically oriented, cylindrically shaped, refractory-lined
steel shell containing a series of horizontal refractory hearths,
one above the other. MHFs are available with diameters ranging
from 4 feet-6 inches to 29 feet (1.4 to 8.8 m) and can have from
4 to 14 hearths. A cross section of a typical MHF is shown on
Figure 11-6. A central shaft extends from the bottom of the
furnace to the top and supports rabble arms above each hearth.
There are either two or four rabble arms per hearth. Each arm
contains several rabble teeth, or plows, which rake the sludge
across the hearth in a spiral pattern. Sludge is fed at the
periphery of the top hearth (see Figure 11-6) and is rabbled
toward the center, where it drops to the hearth below. On the
second hearth, the sludge is rabbled outward to holes at the
periphery of the bed. Here the sludge drops to the next hearth.
The alternating drop hole locations on each hearth and the
counter-current flow of rising exhaust gases and descending
sludge provide contact between the hot combustion gases and the
sludge feed. Good contact ensures complete combustion. The drop
holes on the "out" hearths distribute the sludge evenly around
the periphery of the hearth beneath. The drop holes also
regulate gas velocities.
Sludge is constantly turned and broken into smaller particles by
the rotating rabble arms. Thus, a large sludge surface is
exposed to the hot furnace gases. This procedure induces rapid
and complete drying, as well as burning. The rabble arms also
form spiral ridges of sludge on each hearth. The surface area of
these ridges varies with the angle of repose of the sludge, and
the angle varies with the moisture content of the material.
Because of the ridges, the actual surface area of sludge exposed
to the hot gases is considerably greater than the hearth area.
An effective area of up to 130 percent of the hearth area is
available. Two access doors are generally provided at each
hearth. They have fitted cast-iron frames and machined faces
to provide reasonably tight closure. An observation port is
provided in each door.
Figure 11-7 shows an interior cut-away view of the MHF. The
central shaft of the furnace is a hollow iron column cast
in sections; shaft speeds are adjustable from about 1/2 to
1-1/2 revolutions per minute. The hollow rabble arms are
connected to machined arm sockets in the shaft. The shaft and
rabble arms are air-cooled and normally are insulated. A cold
air tube runs up the center of the shaft. Air lances extend from
the cold air tube out to the ends of each rabble arm. Ambient
air of regulated pressure and volume is forced through the cold
air tube and lances by means of a blower. The cold air exits
from the tips of the lances, flowing backward through the space
between the lances and the rabble arm walls to the annular space
in the central shaft known as the hot air compartment. This flow
11-32
-------
COOLING AIR
DISCHARGE
SLUDGE CAKE,
SCREENINGS,
AND GRIT-
OUT_HEA_RTK
AUXILIARY
AIR PORTS
RABBLE ARM
2 OR 4 PER
HEARTH
GAS FLOW
CLINKER
BREAKER
BURNERS
SUPPLEMENTAL
FUEL
COMBUSTION AIR
SHAFT COOLING
AIR RETURN
SOLIDS FLOW
DROP HOLES
ASH
DISCHARGE
FIGURE 11-6
CROSS SECTION OF A MULTIPLE-HEARTH FURNACE
11-33
-------
HOT AIR
COMPARTMENT-
COLD AIR
TUBE
IR LANCE
RABBLE ARM TEETH
CENTER SHAFT
GEAR DRIVE
SHAFT COOLING
AIR FAN
STEEL SHELL
AIR HOUSING
COURTESY ••? DIVISION Of ENVlflQNTECH CORPORATION
FIGURE 11-7
SHAFT COOLING AIR ARRANGEMENT IN A MULTIPLE-HEARTH FURNACE
11-34
-------
of air cools the arms. The air is conducted through the hot air
compartment, cooling the shaft. The air is either discharged to
the atmosphere via the exhaust gas stack or returned to the
bottom hearth of the furnace as preheated air for combustion.
Cooling air vented to the atmosphere represents a heat loss of
roughly the same magnitude as the radiation loss from the furnace
structure.
The MHF can be divided into four zones, as shown on Figure 11-8.
The first zone, which consists of the upper hearths, is the
drying zone. Most of the water is evaporated in the drying zone.
The second zone, generally consisting of the central hearths, is
the combustion zone. In this zone, the majority of combustibles
are burned and temperatures reach 1,400°F to 1,700°F (760°C to
927°C). The third zone is the fixed carbon burning zone, where
the remaining carbon is oxidized to carbon dioxide. The fourth
zone includes the lowest hearths and is the cooling zone. In
this zone, ash is cooled by the incoming combustion air. The
sequence of these zones is always the same, but the number of
hearths in each zone is dependent on the quality of the feed, the
design of the furnace, and the operational conditions.
NORMAL
SLUDGE/ASH
TEMPERATURES
NORMAL
AIR
TEMPERATURES
DRYING ZONE
\\V\\
v 1400° to
1700°F
COMBUSTION ZONE
FIXED CARBON
BURNING ZONE
100° to
400°F
\\\\\
ASH COOLING
ZONE
SLUDGE
FLOW
AIR
FLOW
FIGURE 11-8
PROCESS ZONES IN A MULTIPLE-HEARTH FURNACE
11-35
-------
When the heating value of the sludge is insufficient to sustain
autogenous combustion, the additional heat required is supplied
by adding supplemental fuel to burners located at various points
in the furnace. Burners may operate either continuously or
intermittently and on all or selected hearths.
A measure of the quantity of water evaporated from the sludge
during burning is the drop in temperature of the hot gases as
they pass between the combustion zone and the gas outlet.
In a MHF, gas temperatures in the combustion zone may exceed
1,700°F (927°C). These gases sweep over the cold, wet sludge fed
to the drying zone, giving up considerable portions of their heat
in evaporating the water. While the temperature of the solids is
only marginally increased in the drying zone, the gas temperature
is drastically reduced, typically to the range of 600 to 900°F
(316 to 482°C). Exhaust gas temperatures should be maintained at
less than 900°F (482°C) by controlling air flow to prevent
distillation of odorous greases and tars from the drying solids.
If temperatures are so controlled, it may be possible to operate
MHFs without devices such as afterburners, which are used to
reduce odors and concentrations of unburned hydrocarbons.
However, afterburning MHF, exhaust gases will probably be needed
in areas with very stringent carbonyl and unburned hydrocarbon
emission limitations. In afterburning, furnace exhaust gases
are conveyed to a chamber where their temperature is raised by
direct contact with ignited supplemental fuel; the offending
pollutants are oxidized to CC>2 and water. Afterburning,
however, requires supplemental fuel, which raises operating
costs significantly. In this respect, the MHF is at a disadvan-
tage relative to FBF and single hearth cyclonic furnaces, which
do not require afterburning. The reason may be seen when the
air-sludge contact patterns in these furnaces are contrasted
against the pattern in the MHF. In the MHF, warm air and
unburned solids are contacted at the top of the furnace. Any
compounds distilled from the solids are immediately vented from
the furnace at temperatures too low to effect their destruction.
In contrast, temperatures in FBF and single hearth cyclonic
furnaces are high (1,200 to 1,600°F [649 to 760°C]) and nearly
uniform throughout the furnace. Sludge and air are injected into
the lower portion of the furnace, and any objectionable compounds
distilled from the solids must traverse the entire length of
the hot furnace before being vented. In the FBF and single
hearth cyclonic furnaces, therefore, the volume of the furnace
above the sludge injection zone is in effect an afterburner,
supplying ample contact time and temperature for the destruction
of pollutants. A flowsheet for the MHF process is shown on
Figure 11-9.
The MHF can be provided with instrumentation to convey critical
operating data to a central control panel. Temperature data can
be monitored for each hearth and for other points in the exhaust
gas system, such as the furnace exhaust, heat recovery device
11-36
-------
outlet, and scrubber exhaust. The temperature can be controlled
on each hearth to within + 40°F (22°C). Instrumentation such
as CC>2 or 02 meters can be used to control the flow of excess
air, thereby conserving fuel and reducing the overall operating
cost. Malfunctions such as burner shutdown, furnace over-
temperature, draft loss, and feed shutdown can be monitored.
In the event of power or fuel failure, the furnace should
be shut down automatically and the shaft cooling air fan
automatically transferred to a standby power source. This
procedure will provide continued cooling and prevent serious
deformations of the shaft and the rabble arms due to high
temperature. Further details on instrumentation are provided in
Chapter 17.
COOLING AIR
FIGURE 11-9
FLOWSHEET FOR SLUDGE INCINERATION IN A
MULTIPLE-HEARTH FURNACE
11-37
-------
Problems encountered with multiple-hearth furnaces have included
(a) failure of rabble arms and teeth, (b) failure of hearths, and
(c) failure of refractories. Improvements in materials used in
constructing the rabble arms and teeth have reduced the first
problem, increasing their ability to withstand high temperatures.
Many refractory problems result because furnaces are not
carefully heated and cooled during start-up and shutdown.
Twenty-four hours or more are required to bring the furnace up to
temperature or to cool it. This is an operational disadvantage
since start-up fuel costs can be significant. However, there are
several installations that do operate intermittently without
significant refractory problems. The normal procedures at these
installations is to fire supplemental fuel to maintain the
temperature of the furnaces during the hours when they are
not in use, thus reducing long reheat times. This procedure,
known as "hot standby" is not generally economical. MHFs should
not be operated at temperatures above 1,800°F (982°C) due to the
metals exposed to the temperature. Thus with high energy fuels
(for example, sewage scum), there may be problems with high
temperatures in the combustion zones.
Heat and material balances for the hypothetical treatment plant
alternatives listed in Table.11-8 are presented in Table 11-9 and
should be used with the flowsheet presented in Figure 11-9.
Figure 11-9 is the flowsheet for a typical multiple-hearth
furnace. Figures 11-10 through 11-15 are generalized curves for
capital and operating and maintenance costs for multiple-hearth
furnaces. Table 11-10 gives typical hearth loading rates for
multiple-hearth furnaces.
As expected, there are important differences between Alternatives
"A" (20 percent solids feed) and "B" (40 percent solids feed)
in terms of equipment size, capital costs, and operation and
maintenance costs. This illustrates the value of preparing
comparative cost tables for all options. Specific discussions
of the MHF can be found in the literature (6,15,16,31, and
37-52).
The recycle concept is relatively new in MHF applications (53).
This concept (54) is a modification of the multiple-hearth
designed "....to control sludge combustion to burn where it is
designed to burn, rather than to let it burn where it wants to
go" (55). Recycle includes three control loops: an exit gas
loop, a drying rate control loop, and a furnace combustion loop
(see Figure 11-16). The exit gas loop allows hot gases to
be exhausted from either or both the top-drying hearth and
the combustion zone. For wet sludge, most or all of the air
would be exhausted from the drying hearth, ensuring minimal fuel
consumption (conventional MHF). For hot or dry sludges, most of
the air would be drawn from the combustion zone so as to prevent
uncontrolled burning on the upper hearths.
The drying rate control loop takes the air exhausted from the
drying hearth and heats this air with exhaust gases from the
combustion zone via an air heater (recuperator). The heated
11-38
-------
exhaust from the drying zone is returned as preheated combustion
air to the furnace. This reduces the overall excess air require-
ments. The gas from the combustion zone exits from the first
recuperator and enters a second, which serves as a preheater for
makeup combustion air. Additional heat can be withdrawn from the
combustion zone gas it passes through a scrubber and is vented by
means of a heat recovery boiler.
TABLE 11-9
HEAT AND MATERIAL BALANCE FOR SLUDGE INCINERATION
IN A MULTIPLE-HEARTH FURNACE3
Alternatives
Stream
Furnace design
Diameter, ft- in.
Number of hearths
Hearth loading rate, Ib
wet solids/sq ft/hr
Sludge feed
Lb dry solids/hr
Heat value, 106 Btu/hr
Volatile content, percent
dry solids
Supplemental fuel
No. 2 fuel oil, Ib/hr
Heat value, 106 Btu/hr
Combustion air
Mass at 60° F, Ib/hr
Shaft cooling air
Mass, Ib/hr
Shaft cooling air return
Mass at 325°F, Ib/hr
Heat value, 10° Btu/hr
Shaft cooling air not
recovered
Heat loss, 10 Btu/hr
Ash
Mass at 500°F, Ib/hr "
Heat value, 106 Btu/hr
Radiation
Heat loss, 10 Btu/hr
Furnace exhaust
Mass, Ib/hr
Heat value, 10 Btu/hr
Boiler exhaust
Heat value at 500°F,
106 Btu/hr
Recoverable heat
70 percent efficiency,
106 Btu/hr
Precooler and Venturi water-
feed
Flow at 70°F, gpm
IA
•5' MGD
20 percent
solids
18-9
7
7.3
1,806
13.91
77
143
2.64
., . 22,060
19,273
16,560
1.26
0.22
415
0.04
0.32
30,817d
15.96
13.26
1.89
90
IB
5 MGD
40 percent
solids
14-3
6
9.3
b
2,131D
13.91
65
0
0
27,531
9,178
0
0
0.71
740
0.07
0.21
32,123e
12.94
9.6.4
2. 31
86
IIA
15 MGD
20 percent
solids
'22-3
7
.7.4
.2,173
20.89
77
205
3.79
32,959
24,321
20,880
1.59
0.28
624
0.06
0.41
46,102d
23.93
19.73
2.94
135
IIB
15 MGD
40 percent
solids
16-9
6
9.5
K
3,201°
20.89
65
, 0
0
41,544
13,766
0
0
1.06
1,110
0.10
0.26
48,434e
.19.48
12.28
5.04
130
IIIA
50 MGD
20 percent
solids
22-3
10
8.4
4,292
33.06
77
312
5.77
51,945
34,416
29,520
2.25
0.40
987
0.09
0.53
72,735d
37.81
31.11
4.69
215
IIIB
50 MGD
40 percent
solids
18-9
7
10.3
K
5,064S
33.06
65
0
0
66,740
19,273
0
0
1.48
1,757
0.15
0.33
77,643e
31.11
19.61
8.05
' 209
11-39
-------
TABLE 11-9
HEAT AND MATERIAL BALANCE FOR SLUDGE INCINERATION
IN A MULTIPLE-HEARTH FURNACE3 (CONTINUED)
Alternatives
Stream
IA
5 MGD
20 percent
solids
IB
5 MGD
40 percent
solids
IIA
15 MGD
20 percent
solids
IIB
15 MGD
40 percent
solids
I IIA
50 MGD
20 percent
solids
IIIB
50 MGD
40 percent
solids
Scrubber water feed
Flow at 70°F, gpm 182 174 273 260 429 41S
Scrubber drain
Flow, gpm 296 264 428 398 676 638
Temperature, °F 98 98 98 98 98 98
Gas exhaust
Mass, Ib/hr 26,667 38,938 44,278 58,646 61,116 91,393
Temperature, °| 142 170 139 168 138 166
Heat value, 10 Btu/hr 9.44 6.00 14.01 6.80 22.09 10.82
Connected power
Horsepower 238 93 305 178 305 238
Installed cost, thousand
dollars 2,000 1,600 2,200 2,000 2,400 2,000
Footnotes for Table 11-8.
All data supplied by the manufacturer.
Solids for B alternatives (40 percent solids feed), larger than A
alternatives (20 percent solids feed), due to conditioning chemicals.
See Table 11-7.
Afterburner not included.
At 800 °F.
6At 1,000 °F.
Costs as of early 1978.
1 Ib/sq ft/hr =4.9 kg/m2/hr
1 Ib/hr = 0.45 kg/hr
1 x 106 Btu/hr = 1,055 MJ/hr
1 gpm =0.06 1/s
1 ft = 0.31 m
1 in. = 0.02 m
1 MGD =0.04 m3/s
The furnace combustion control process allows the furnace
to operate with sludges with a very high volatile content (for
example, large amounts of scum) or those requiring supplemental
fuel. This loop integrates the functions of the exit gas loop
and the drying rate loop, providing for automatic control of the
process without regard to feed quality.
The manufacturer of the furnace which uses the recycle concept
claims that strict limits on gaseous emissions can be met without
use of an afterburner. The air that is exhausted has not con-
tacted wet sludge (the sludge in the drying zone) and thus has
not distilled off odors or excess hydrocarbons from the sludge .
Figure 11-16 is a flowsheet for a 50 MGD (2.2 m3/s) plant.
11-40
-------
5.0 i—
5 4.0
o
n
k_
.c
*-•
EC
^ 3.0
a
*-<
m
o
= 2,0
c
LL
DC
O
lit
cr:
_t
ASSUMPTIONS
EFFECTIVE HEARTH AREA
SQJFT
LESS THAN 400
4O&BQO
BOO-MOO
1,400-2,000
GREATER THAN 2,000
HEATOP TIME TO
REACH 1,400* F
HR
18
27
36
H
108
FREQUENCY OF STAflTt* (5 A F UNCTION OF INDIVIDUAL UNIT
FUEL li NATURAL GAS OR FUEL OIL
I
§00 1,000 1,500 2,000
EFFECTIVE HEARTH AREA, iq ft (1 sq ft - 0,093 m^j
FIGURE 11-10
MULTIPLE-HEARTH FURNACE START-UP FUEL REQUIREMENTS (36)
2,500
Disadvantages of the recycle concept include those inherent
in the MHF construction, as well as problems associated with
ducting hot gases and with recuperators. Additional instruments
and equipment add to operating and maintenance costs. These
costs may be offset by a reduction in supplemental fuel demand.
One municipal sludge installation similar to that depicted on
Figure 11-16 is under construction in San Mateo, California. The
recycle concept has been used in the MHFs for many years to
produce bone char (a "hot" feed material) in the sugar industry.
11-41
-------
-5
o
g
t-
D
EE
C/3
I
100
1
7
6
E
4
10
B
6
7
6
S
1,0
S
S
7
€
5
0,1
PLANT CAPACITY, MGD (1 MGD = 0.04 m3/sj
1 10
100
ASSUMPTIONS;
LOADING RATE = 6 Ibfa ft/hr
SLUDGE: PRIMARY + WAS SLUDGE
AT 16% SOLIDS
COSTS: JUNE 1978
100 234 E 6 7891,000 234 5678910,000 234 56786100,000
WET SLUDGE FEED, Ib/hr (1 Ib/hr = 0.46 k§/hrj
FIGURE 11-11
MULTIPLE-HEARTH FURNACE CONSTRUCTION COST (36)
11.3.2 Fluid Bed Furnace
The first fluid bed wastewater sludge furnace was installed in
1962. There are approximately 60 operating units in the United
States (35) and many more in Europe. The fluid bed furnace
(FBF) is a vertically oriented, cylindrically shaped, refractory-
lined steel shell that contains a sand bed and fluidizing air
diffusers. The FBF is normally available in sizes from 9 to
25 feet (2.7 to 7.6 m) in diameter. However, there is one
11-42
-------
industrial unit with a diameter of 53 feet*( 16.2 m) . A cross
section of the fluid bed furnace is shown on Figure 11-17. The
sand bed is approximately 2.5 feet (0.8 m) thick and sits on a
refractory-lined grid. This grid contains tuyeres through which
air is injected into the furnace at a pressure of 3 to 5 psig
(21 to 34 kN/m2 gage) to fluidize the bed. The bed expands to
approximately 100 percent of its at rest volume. Temperature of
the bed is controlled between 1,400°F and 1,500°F (760°C and
816°C) by auxiliary burners located either above or below the
sand bed. In some installations, a water spray or heat removal
system above the bed controls the furnace temperature. In
essence, the reactor is a single chamber unit in which both
drying and combustion occur in either the dense or dilute phases
in the sand bed. All of the combustion gases pass through the
combustion zone with residence times of several seconds at
1,400°F to 1,500°F (760°C to 816°C). Ash is carried out the top
of the furnace and is removed by air pollution control devices,
usually venturi scrubbers. Sand carried out with the ash
must be replaced. Sand losses are approximately 5 percent of the
bed volume for every 300 hours of operation. Feed to the furnace
is introduced either above or directly into the bed.
Air flow in the furnace is determined by several factors.
Fluidizing and combustion air must be sufficient to expand the
bed to a proper density yet low enough to prevent the sludge from
rising to and floating on top of the bed. Too much air blows
sand and products of incomplete combustion into the off-gases.
This depletes stored heat energy and increases fuel consumption
unnecessarily. Minimum oxygen requirements must be met to assure
complete oxidation of all volatile solids in the sludge cake.
Temperatures must be sufficiently high to assure complete
deodorizing but low enough to protect the refractory, heat
exchanger, and flue gas ducting. The quantities of excess air
are maintained at 20 to 45 percent to minimize effects on fuel
costs (see Figure 11-3). The fluid bed furnace operates at lower
excess air rates than typically experienced in MHF operations.
This accounts for the greater heat efficiency of the fluid bed
system at similar exit temperatures . The intense and violent
mixing of the solids and gases within the fluid bed results in
uniform conditions of temperature, composition, and particle size
distribution throughout the bed. Heat transfer between the gases
and the solids is extremely rapid because of the large surface
area available. .
There are two basic process configurations for the FBF. In
the first process, the fluidizing air passes through a heat
exchanger, or recuperator, prior to injection into the combustion
chamber. This arrangement is known as a hot windbox design. In
the second process, the fluidizing air is injected directly into
the furnace. This arrangement is known as a cold windbox design.
The first arrangement increases the thermal efficiency of the
process by using the higher temperature of the exhaust gases to
preheat the incoming combustion air. .
11-43
-------
cc
a
CO
s
m
Q
100,000
0
8
6
e
4
PLANT CAPACITY, MGO ( 1 MGO * ttQ4 m3/s}
1 10
100
10,000
§
1
7
6
e
1,000
B
8
7
6
5
4
100
ASSUMPTIONS
LOADING RAT« • e Ih/K) ft/hr
SLUDC6: PRIMARY + WAS SLUDGE AT 18% SOLIDS
J_
100 234 iB7*t 1,000 234 §678910,000 2 3 456789100,000
WET SLUDGE FEED, Ib/hr (1 Ib/hr - 0.45 kg/hrl
FIGURE 11-12
MULTIPLE-HEARTH FURNACE OPERATING AND MAINTENANCE
LABOR REQUIREMENTS (36)
Preheating the incoming combustion air from 70°F to 1,000°F (21°C
to 538°C) can yield a reduction in fuel costs of approximately
61 percent per unit wet sludge (39). Air preheating costs
can represent 15 percent of the fluid bed furnace cost;
therefore, a careful economic analysis is needed to determine
cost-effectiveness for a given situation.
11-44
-------
1,000,000
CURVE NO. SLUGSE TYPE
i
o_
1
w
"a
cp
1
d
ul
£
5
o
LLJ
Li.
PRIM, +• FERRIC CHLORIDE {F(CI_)
PRIM, + LOW LIME
PRIM, + WASTE ACTIVATED SLUDGE (W.A.8.!
PRIM, + |WAS + F*C13!
(PRIM, + FKS_) * WAS
DIGESTED PRIMARY
ASSUMPTIONS;
HEAT VALUE OF VOLATILE SOLIDS 10,000 BtuflU
LOADING BATES, fe/iq
CURVE NO, RATE
14
2.4A7J 6,B
3 18
6 M
SEE TABLE 11*10 FOR FEED SLUDGE DATA
COMBUSTION TEMPERATURE I/WO* F
DOWNTIME IS A FUNCTION OF
INDIVIDUAL SYSTEM
40% EXCESS AW, NO PR EM EATER
5TABT-UI1 FUEL NOT INCLUDED; 73,000 9wtw ft
FOP) STARTUP
FUEL IS NATURAL GAS on PUEL O»L
too
10 33 456769100 2 3 4667691,000 2 3 4 6 6 78910,000 a 3 466768100,000
DRY SLUDGE FEED, Ib/hr {1 Ib/hf - 0,45 kg^hr|
FIGURE 11-13
MULTIPLE-HEARTH FURNACE FUEL REQUIREMENTS (36)
Violent mixing in the fluidized bed assures rapid and uniform
distribution of fuel and air, and consequently, good heat
transfer and combustion. The bed itself provides substantial
heat storage capacity. This helps to reduce short-term
temperature fluctuations that may result' from varying feed
heating values. This heat storage capacity also enables quicker
start-up, if the shutdown period has been short (for example,
overnight). Organic particles remain in the sandbed until
they are reduced to mineral ash. The violent motion of the bed
comminutes the ash material, preventing the buildup of clinkers.
The resulting fine ash is constantly stripped from the bed by the
upflowing gases.
11-45
-------
I
Dt
a:
UJ
o
Q.
-I
<
u
oc
o
UJ
_i
UJ
ASSUMPTIONS:
SOLIDS CONCENTHATIOK, *
14-17
tfrSS
23-30
31
100,000
100
1 34 667891,000 234 §878910,000 234 66789100,000
EFFECTIVE HEARTH AREA, iq ft (1 iq ft = ^093 m2)
FIGURE 11-14
MULTIPLE-HEARTH FURNACE ELECTRICAL POWER REQUIREMENTS (36)
An oxygen analyzer in the stack controls air flow into the
reactor. This type of control has limited application, since air
flow ranges have upper and lower rates required for proper bed
fluidization. The rate of use of auxiliary fuel is controlled
by furnace exhaust gas temperature. Shutdown controls must
be provided for emergency situations. Further details on
instrumentation are provided in Chapter 17.
Heat and material balances for the hypothetical treatment
plant alternatives (Table 11-8) are presented in Table 11-11.
Figure 11-18 is the flowsheet for a typical FBF system.
Figures 11-19 and 11-20 are generalized curves depicting fuel and
power required for FBF systems.
11-46
-------
i
1,000,000
9
S
7
6
6
4
3
100,000
e
s
6
5
LU
o
10,000
9
8
7
i
5
4
1,000
PLANT CAPACITY, MGD (1 MGD - 0,04
I 10
I
ASSUMPTIONS;
LOADING RATE = 6 Ib/sq ftAr
SLUDGE: PRIMARY + WAS SLUDGE
AT 16% SOLIDS
COSTS: JUNE 1f78
100
234 667691.000 1 3 4 5678910,000 234 BB 780100.000
WET SLUDGE FEED, !b/hr (1 tt»/br = 0.4S kf/hr)
FIGURE 11-15
MULTIPLE-HEARTH FURNACE MAINTENANCE MATERIAL COSTS (36)
The FBF is relatively simple to operate, has a minimum of
mechanical components, and typically has a slightly lower capital
cost than the MHF. Normal operation of the FBF produces exhaust
temperature in excess of 1,400°F (760°C). Because the exhaust
gases are exposed to this temperature for several seconds,
carbonyl and unburned hydrocarbon emissions are minimal, and
strict hydrocarbon emission regulations are met without the use
of an afterburner. However, it is important that operating
conditions be optimum to assure this emission level at all times.
11-47
-------
TABLE 11-10
TYPICAL HEARTH LOADING RATES FOR A MULTIPLE-HEARTH FURNACE3
Type of sludge
Percent
solids
Primary 30
Primary plus ferric
chloride (FeCl3) 16
Primary plus low lime 35
Primary plus waste-
activated sludge (WAS) 16
Primary plus (WAS plus
FeCl3) 20
Percent
combustibles
Chemical ,
concentration,
mg/1
60
47
45
69
54
N/AU
20
298
N/A
20
plus WAS
WAS
WAS plus Fed 3
Anaerobically digested
primary
16
16
16
30
53
80
50
43
20
N/A
20
N/A
Typical wet
sludge loading
rate,c
Ib/sq ft/hr
7.0 - 12.0
6.0 - IX). 0
8.0 - 12.0
6.0 - 10.0
6.5 - 11.0
6.0 - 10.0
6.0 - 10.0
6.0 - 10.0
7.0 - 12.0
Data supplied by the manufacturer.
Assumes no dewatering chemicals.
Low number is applicable to small plants, high
number is applicable to large plants.
N/A - not applicable.
1 Ib/sq ft/hr =4.9 kg/m2/hr
Problems with the FBF have occurred primarily with feed
equipment and temperature controls. When sludge is injected
directly into the bed, screw feeders may jam if the sludge
has been overdried or if it solidifies at the point of
injection. When spray nozzles have been used, thermocouples
have occasionally burned out. These problems have generally
been solved by the use of different construction materials.
There have been some problems with preheaters and with sand
scaling on the venturi scrubber. In some installations, there
have been serious erosion problems in the scrubber due to
the excessive carryover of bed material and the resulting
sandblasting effect. The fluid bed furnace can be operated at
2,200°F (1,204°C) with appropriate design modifications and is
suitable for high energy sludges. Combustion at temperatures
over 2,000°F (1,093°C) can create many side effects such as
ash fusion, high temperature corrosion, scaling, and clinker
formation. Since a minimal amount of air is always required
for bed fluidizing, energy savings from turndown (feed reduction)
are minor. More detailed information can be found in the
literature (39,40,41,43,48,49,50, and 56-63).
11-48
-------
SLUOCi FEED
H,*UO as/to
6 2§» SOUDS"
< 10,000 BtuJIb
VOLATlLES,
30% A5IH)
GAS EXH4UST
SUPPLEMENTAL
fuEL
FIGURE 11-16
HEAT BALANCE FOR THE RECYCLE CONCEPT IN A
MULTIPLE-HEARTH FURNACE (55)
11.3.3 Electric Furnace
The first electric furnace was installed in Richardson, Texas, in
1975. The electric, or infrared, furnace (EF) is a horizontally
oriented, rectangular, steel shell containing a moving horizontal
woven-wire belt. The unit is lined with ceramic-fiber blanket
insulation. Electric furnaces are available in a range of sizes
from 4 feet (1.2 m) wide by 20 feet (6.1 m) long to 9.5 feet
(2.9 m) wide by 96 feet (29.3 m) long. Larger sizes are
currently being developed. A typical cross section is shown on
Figure 11-21.
Sludge is fed into the EF through a feed hopper that discharges
onto the woven-wire belt. Shortly after the sludge is deposited
an internal roller to a
cm), across the width of
on several new installa-
sludge layer to afford
moves under the infrared
on the belt, it is leveled by means of
layer approximately one inch thick (2.5
the belt. A rabbling device is provided
tions to break up the surface of the
better combustion. This layer of sludge
heating elements, which provide supplemental energy for the
incineration process, if required. Ash is discharged from the
end of the belt to the ash handling system. Combustion air flow
is countercurrent to the sludge flow,, with most of the combustion
air being introduced into the ash discharge end of the unit.
Excess air rates for the EF vary from 20 to 70 percent. The EF
is divided into a feed zone, a drying and combustion zone,
and an ash discharge zone. The feed and discharge zones are each
8 feet (2.4 m) long. The length of the drying and combustion
zone varies with the design.
11-49
-------
*- EXHAUST AND ASH
THERMOCOUPLE
C
SLUDGE
INLET
FLUIDIZING
AIR INLET
FLU I CM ZED A
SAND BED •
REFRACTER
ARCH
PRESSURE TAP
ASiGHT
9 GLASS
BURNER
TUYERES
FUEL
GUN
PRESSURE TAP
STARTUP
-i PREHEAT
HBURNER
_T FOR HOT
WINDBOX
FIGURE 11-17
CROSS SECTION OF A FLUID BED FURNACE
11-50
-------
TABLE 11-11
HEAT AND MATERIAL BALANCE FOR SLUDGE
INCINERATION IN A FLUID BED FURNACE3
Alternatives
IA
5 MGD
20 percent
Stream
Furnace design
Inside diameter, ft
Loading rate, Ib wet
solids/sq ft/hr
Sludge feed
Lb dry solids/hr
Heat value, 106 Btu/hr
Volatile solids, percent
dry solids
Supplemental fuel
Mass, Ib/hr
Heat value, 10 Btu/hr
Combustion air
Mass, Ib/hr
Heat value, 10 Btu/hr
Ash
Mass, Ib dry solids/hr
Heat value, 10^ Btu/hr
Water flow, gpm
Radiation
Heat loss, 10 Btu/hr
Recoverable heat
70 percent efficiency,
106 Btu/hr
Recuperator
Venturi water
Recycle water, gpm
Makeup water at 70°F, gpm
Scrubber water feed
Flow at 70°F, gpm
Scrubber drain
Flow at 130°F, gpm
Gas exhaust
Volume, cfm
Temperature, F
Connected power
Horsepower
Installed cost, thousand
dollars
solids
14
56.9
1,806
13.91
77
151
2.80
19,353
4.4
416
0.12
20
0.42
3.5d
Yes
83
10
365
391
5,042
120
218
1,100
IB
5 MGD
40 percent
solids
12
47.0
H
2,131
13.91
65
0
0
16,250
0
746
0.14
32
0.29
6.26
No
68
12
345
359
3,972
120
162
1,000
IIA
15 MGD
20 percent 40
solids
18
53.3
2,713
20.89
77
224
4.14
28,976
6.7
623
0.18
30
0.63
5.3d
Yes
124
15
548
582
7,524
120
320
1,400
IIB
15 MGD
percent
solids
14
47.0
K
3.201
20.89
65
0
0
23,576
0
1,117
. 0.26
43
0.44
9.46
No
102
19
565
600
5,949
120
234
1,100
IIIA
50 MGD
20 percent
solids
22
56.5
4,293
33.06
77
353
6.52
45,978
10.6
959
0.29
40
1.00
8.4d
Yes
197
24
863
924
12,007
120
425
1,600
IIIB
50 MGD
40 percent
solids
18
45.0
K
5,064b
33.06
65
0
0
38,620
0
1,772
0.42
70
0.71
12. 7e
No
161
30
824
900
9,459
120
350
1,500
All data provided by Dorr-Oliver, Inc.
Solids for B alternatives (40
percent solids feed) ,
alternatives (20 percent solids feed) ,
See Table 11-7.
Afterburner not required.
dAt 1,400°F.
6At 1,650°F.
Costs as of early 1978.
1 ft = 0.31 m
1 Ib/sq ft/hr =4.9 kg/m /hr
1 Ib/hr =0.45 kg/hr
1 gpm =
1 cfm =
1 MGD =
larger than A
due to conditioning chemicals.
0.06 1/s
4.72 x 10-4 m
0.04 m /s
3/s
1 x 10° Btu/hr = 1,055 MJ/hr
11-51
-------
FURNACE EXHAUST
GAS EXHAUST
FIGURE 11-18
FLOWSHEET FOR SLUDGE INCINERATION IN A FLUID BED FURNACE
A flowsheet for the typical electric furnace is shown on
Figure 11-22. Heat and material balances for the hypothetical
treatment plant alternatives (Table 11-8) are presented in
Table 11-12. In addition to the alternative cases I, II, and
III, balances for a 1 MGD (0.04 m^/s) treatment plant have
been included. The EF is suited to small wastewater treatment
plants.
The effective belt loading rate of a large EF is slightly greater
than the hearth loading rate of a multiple-hearth furnace. The
supplemental energy requirements of the EF are lower than the
requirements of the MHF, FBF, or the cyclonic furnace. Because
electricity is used to provide the supplemental energy, no fuel
is burned, and consequently, no excess air for this purpose is
required. However, when the generation efficiency of electricity
is included, the supplemental energy requirements are similar for
rather than fossil fuel, is the
Electricity is generally a more
the fossil fuel used by the other
the energy cost differential, the
all furnaces. Electricity,
energy source for the EF.
expensive energy source than
unit types. Depending upon
11-52
-------
advantage of low excess air may be reduced. when autogenous
sludge is available, the only difference between the EF and other
processes with low excess air rates would be the motive power.
1,000,000
in
IA
O
—-,
3
4^
CD
3
4-"
m
.2
E
Q*
LU
x
a
LU
(E
LU
LL
S-LUDGE TYPE
PRIMARY
PRIM. + FERRIC CHLORIDE (F»CU
PRIM, + LOW LIME
PRIM, * WASTE ACTIVATED SLU&st
PRIM. + {WAS +
(PRIM. * F«CI3J + WAS
DIGESTED PRIMARY
ASSUMPTIONS:
LOADING HATES PER TABLE 11-10
INCOMING SLUDGE TEMPERATURE IS 57 F
COMBUSTION TEMPERATURE ; 1,400* F
FOB COOL-OOWN EQUALS
STARTW TIME
FREQUENCY FOR STARTUPS IS A FUNCTION
OF INDIVIDUAL SYSTEMS
EXCESS AIR IS 106%
FUEL IS NATURAL GAS OR FUEL OIL
MO STARTUP FUiL IS INCLUOEO
(SEE FIGURE 11-10)
100
10 2 3 4 56789100 2 34 667881,000 3 34 6678910,000 214 5671»TOO.OOC
DRY SLUDGE fEf D, pounds per hour (1 Ib/hr = 0.46 kg/hrj
FIGURE 11-19
FLUID BED FURNACE FUEL REQUIREMENTS (36)
Low capital cost combined with modular construction makes the EF
attractive, especially for small treatment systems. Because of
the use of ceramic-fiber blanket insulation instead of solid
refractories, the electric furnace may be shut down and heated up
without the refractory problems that can occur in the other
furnaces. This makes the EF suitable for intermittent operation.
However, each restart requires supplemental energy (electricity),
11-53
-------
since there is no heat sink similar to the sandbed in the FBF.
Currently, no EF units are installed with a capacity of over
1,200 pounds per hour.
1,000,01X1,000
I
« ;
" 3
tl
k^
-? 100,000,000
i !
_ 6
— 4
I
•c *
LU
E
D
a
Ul
IE
o
a.
O
cc
o
LU
_i
UJ
10.000,000
i
100,000
ASSUMPTIONS:
FULL TIME
OPERATION
i i i M mi
10 234 587*9100 3 | < »67t»1,OQO 2 3 41078810,0002 3 4 6 S 788100,000
BED AREA, *q ft {1 sq ft- 0.093 m2)
FIGURE 11-20
FLUID BED FURNACE ELECTRICAL POWER REQUIREMENTS (36)
The EF appears to be a feasible alternative for both small and
large systems due to its inherent simplicity and low cost.
However, the EF requires considerably more floor space than
furnaces which are vertically oriented. Another concern is the
replacement of various components such as the woven-wire belt
(3 to 5-year life) and the infrared heaters (3-year life).
These items represent a sizable portion of the capital cost.
Replacement costs must be considered in any overall evaluation.
Connected power, whether for heating or motive power, may create
11-54
-------
a large electric demand charge in some areas. This may be the
case whether the energy is used or not. Also, time-of-day
charges could be significant. One concern is the high voltage,
240 to 480 V, required for the furnace infrared heaters. This
may create safety problems in small plants, where workers are
unaccustomed to high voltage equipment.
1 flOLLIR
r LiVELER
RADIANT
INFRARED
HEAfING
ELEMENTS !T¥?i
WOVEN WIDE
CONTINUOUS BELT
COMBUSTION
FIGURE 11-21
CROSS SECTION OF AN ELECTRIC INFRARED FURNACE
Because the gas flow in an EF runs countercurrent to the sludge
flow, the furnace will probably require an afterburner to comply
with strict carbonyl and hydrocarbon emission regulations. This
would increase the supplemental energy requirement, the amount of
equipment, and the capital and operating costs to levels greater
than those shown in Table 11-12. Allowing for the low excess air
requirements and the countercurrent flow pattern, air emission
control equipment would generally be smaller than control
equipment on MHF or FBF units of similar feed capacity.
11.3.4 Single Hearth Cyclonic Furnace
Cyclonic furnaces were developed by the British (64), and several
units are operating in Great Britain. However, as of 1979, there
are no units processing wastewater sludge in the United States.
The cyclonic furnace is sometimes called a single-rotary hearth
furnace. It is a vertical, cylindrical, refractory-lined, steel
shell, normally provided with a domed cover. There is one
rotating hearth and a fixed plow that moves the combustible
material from the outer edge of the hearth to the center. The
furnaces are currently available with hearths to 30 feet (9.1 m)
in diameter, but larger sizes can be built. The sludge is
fed by a screw feeder and deposited near the periphery of the
rotating hearth. A sectional view of the furnace is given on
Figure 11-23.
11-55
-------
6AS EXHAUST
SLUDGE SUPPLEMENTAL COOLING
FEfO ENERGY AIR
J L_l
ELECTHIC FURftACE
1 T
RADIATION ASH
AIR
FIGURE 11-22
FLOWSHEET FOR SLUDGE INCINERATION IN AN ELECTRIC INFRARED FURNACE
The cyclonic furnace design differs from the multiple-hearth
and fluid bed designs in that it does not allow the combustion
air to pass upward through the feed material. Combustion air
and supplemental fuel, if required, are injected tangentially
into the combustion chamber above the rotating hearth. This
creates a swirling (cyclonic) action that mixes the gases and
allows adequate contact between the oxygen and the furnace feed.
The gases from the combustion process spiral upward to the
outlet. The furnace exhaust temperature is approximately 1,500°F
(816°C). Heat could be recovered from the exhaust with a heat
recovery boiler followed by a recuperator. The ash is moved to
the middle of the hearth, where it drops through to a quench tank
for final disposal. The rotating hearth is sealed at the edges
by a water bath.
11-56
-------
TABLE 11-12
HEAT AND MATERIAL BALANCE FOR SLUDGE INCINERATION
IN AN ELECTRIC INFRARED FURNACE3
Alternatives
IA IB IIA IIB IIIA
5 MGD 5 MGD 15 MGD 15 MGD 50 MGD
20 percent 40 percent 20 percent 40 percent 20 percent
Furnace design
Number of units 21213
Overall width, ft 8.5 8.5 9.5 9.5 9.5
Overall length, ft 72 72 88 88 96
Belt area/furnace,
sq ft 382.6 3B2.6 560.5 560.5 616.8
solids/sq ft/hrb 11.8 13.9 12.1 14.3 11.6
Sludge feed
Heat value, 10 Btu/
hr 13.91 13.91 20.89 20.89 33.06
percent dry solids 77 65 77 65 77
Water, Ib/hr 7,224 3,200 10,582 4,800 17,172
Heat value, 10
Btu/hr 0.28 0. 12 0.41 0.18 0.65
Supplemental power
kW 280.8 0 402.5 0 643.8
Heat value, 10 Btu/ 2.98& 4.2?S 6.826
hr 00
Combustion air
Heat value, 106 Btu/
hr . 0.26 0. 36 0.38 0.54 0.61
Ash
Mass at 500°F, Ib/hr 415 747 624 1,120 987
hr 0.10 0.18 0,16 0. 28 0.24
Radiation
Heat loss, 10 Btu/
hr .36 .18 .47 .24 .77
Mass, Ib/hr 26,351f 29,3729 39,616f 44, 064 9 62,628f
Heat value, 10 Btu/
Boiler exhaust
106 Btu/hr 13.00 8.53 19.49 12.79 31.33
Recoverable heat
70 percent efficiency,
106 Btu/hr 1.37 3.85 2.05 5.81 2.25
Flow, at 70°F, gpm " 397 201' 584 314 1,049
Scrubber drain
Flow, gpm 390 196 606 306 1,081
Tempera ture,°F 120 120 120 120 120
Gas exhaust
Mass, Ib/hr 29,538 35,811 39,616 53,838 54,744
Temperature, °F 120 120 120 120 120
Heat value, 106 Btu/
' hr ' 1.98 2.77 . 2.96 4.18 4.71
Total connected power
Horsepower 22 25 30 40 50
Total installed cost j
thousand dollars 1,000 700 1,300 900 1,500
All data supplied by Shirco, Inc.
b
CSolids for B alternatives (40 percent solids feed), larger than A
See Table 11-7.
Afterburner not included.
Autogenous with combustion air preheated to 500 °F. kw =* 10,600
Btu/hr to allow for generation efficiency.
fAt 750 °F.
9At 1,200 °F.
Does not include supplemental power requirements for infrared heaters.
jCosts as of early 1978.
IIIB
50 MGD 1 MGD
40 percent 40 percent
2 1
8.5 6
88 32
479.5 94.5
13.2 11.3
33.06 2.79
65 65
7,596 641
0.29 0.02
0 0
0 0
0.85 0.07
1,772 149
0.44 0.04
.43 .07
69,732g 5,8809
20.23 1.71
9.18 0.69
498 201
485 196
120 150
85,186 7,183
120 120
6.57 0.55
60 7
1,200 300
1 ft * 0.31 in
1 sq ft = 0.093 m
1 Ib/sq ft/hr =4.9 kg/m2/hr
1 Ib/hr -0.45 kg/hr
1 x 106 Btu/hr - 1,055 MJAr
1 gpm = 0.06 1/s
1 MGD = 0.04 mVs
11-57
-------
EXHAUST
COMBUSTION AIR
CYCLONIC ACTION
ROTATING HEARTH
FfXED PLOW
TANGENTIAL
AIR PORTS
SLUDGE
INLET
BURNER ITVP)
ASH DISCHARGE IN
CENTER OF FURNACE
FIGURE tl-23
CROSS SECTION OF A CYCLONIC FURNACE
A general flowsheet for the furnace is given on Figure 11-24.
Heat and material balances for the hypothetical treatment plant
alternative (Table 11-8) are presented in Table 11-13.
The rotary hearth furnace has a relatively low capital cost
and is mechanically simple, since it has only one rotating
hearth. However, the feed mechanism is similar to that of the
fluid bed furnace and has the same plugging problem. Because
exhaust temperatures are high, afterburners or supplemental
heaters are generally not required to achieve compliance with
strict carbonyl or hydrocarbon air emission regulations. As with
the FBF, good operating conditions must be maintained if low
gaseous emission limitations are to be met. The rotary hearth
furnace requ:'res 30 to 80 percent excess air.
11-58
-------
6AS EXHAUST
FIGURE 11-24
FLOWSHEET FOR SLUDGE INCINERATION IN A CYCLONIC FURNACE
11.3.5 Design Example:
Process
New Sludge Incineration
To minimize increasing disposal costs, a municipal wastewater
treatment plant with an average daily flow of 5 MGD (0.22 m^/s)
must modify its present solids handling and disposal system. The
plant uses a conventional activated sludge process with anaerobic
digestion of combined primary sludge, waste-activated sludge, and
scum. Table 11-14 shows the basic plant data. The digested
sludge is vacuum filtered and is hauled to the local landfill.
This landfill is scheduled to close. The new landfill site has
capacity and is located several miles from the
Projected disposal costs for the new site are
treatment plant site has very little unoccupied
surrounding the plant has been heavily developed
and rendering operations. These industries
amounts of animal greases and oils to the
Naturally, they are concerned about industrial
somewhat limited
treatment plant.
very high. The
space. The area
by meat packing
discharge large
treatment plant.
sewer service charges resulting from any action by the plant.
11-59
-------
TABLE 11-13
HEAT AND MATERIAL BALANCE FOR SLUDGE
INCINERATION IN A CYCLONIC FURNACE3
Alternatives
IA
5 MGD
percent
solids
19.50
30.4
1,806
14.27b
77
132
2.48
19,665
1,100
2,280
415
0.19
0.90
NO
Yes
30,692
1,420
19.90
960
15.66
0
12
292
319
120
23,468
120
1.79
175
1,300
IB
5 MGD
40 percent
solids
13.75
30.9
1,806
14.27
77b
0
0
19,665
60
0
415
0.19
0.60
Vfes
No
23,765
1,411
13.48
500
6.87
4.63
5
197
207
110
21,209
110
1.62
125
1,000
IIA
15 MGD
20 percent
solids
24.00
30.1
2,713
21.43b
77
184
3.46
29,519
1,100
3,178
624
0.29
1.17
No
Yes
45,817
1,420
29.75
960
23.43
0
19
437
477
120
34,969
120
2.67
260
1,600
IIB
15 MGD
40 percent
solids
17.00
30.1
2,712b
21.43b
77b
0
0
29,519
60
0
624
0.29
0.80
Yes
No
35,675
1,421
20.34
500
10.32
7.01
7
296
311
110
31,838
110
2.43
190
1,100
I IIA
50 MGD
20 percent
solids
30.25
29.9
4,292
33.91b
77
546
10.28
46,694
1,100
9,430
987
0.46
2.00
No
Yes
77,143
1,420
50.10
960
39.45
0
30
699
763
120
62,225
120
4.75
460
N/A
IIIB
50 MGD
40 percent
solids
21.50
29.7
4,292b
33.91b
77b
0
0
46,694
60
0
987
0.46
1.00
Yes
No
57,424
1,420
32.38
500
16.51
11.11
15
507
535
110
49,002
110
3.74
290
1,500
Sludge feed
Lb dry solids/hr
Heat value, 106 Btu/hr
Volatile solids, percent
dry solids
Supplemental fuel
Mass, Ib/hr
Heat value, 10 Btu/hr
Primary air
Mass, Ib/hr
Temperature, F
Burner air
Mass at 60%, Ib/hr
Ash
Mass at 260°F, Ib/hr
Heat value, 106 Btu/hr
Radiation
Heat loss, 10 Btu/hr
Waste heat boiler
Recuperator
Furnace exhaust
Mass, Ib/hr
Temperature, r
Heat value, 10 Btu/hr
Boiler/recuperator exhaust
Temperature, °F,
Heat value, 10 Btu/hr
Recoverable heat - boiler
70 percent efficiency,
106 Btu/hr
Precooler water feed
Flow at 60°F, gpm
Scrubber water feed
Flow at 60°F, gpm
Scrubber drain
Flow, gpm
Temperature, F
Gas exhaust
Mass, Ib/hr
Temperature,°F
Heat value, 10 Btu/hr
Connected power
Horsepower
Installed cost , thousand
dollars
All data provided by AFB Engineers/Contractors sole U.S. distributors of the Lucas Cyclonic Furnace.
Data used by manufacturer is slightly different from that developed in Table 11-7.
Afterburners not required.
Not available.
CCosts as of early 1978.
1 ft = 0.31 m
1 Ib/sq ft/hr =4.9 kg/m /hr
1 Ib/hr =0.45 kg/hr
1 x 106 Btu/hr = 1.055 MJ/hr
1 gpm = 0.06 I/;
1 MGD = 0.04 nr3,
/s
11-60
-------
Most of the industrial wastes discharged to the plant are removed
in the primary tanks, and the result is a combined scum and
sludge with an extremely high heating value.
TABLE 11-14
DESIGN EXAMPLE: WASTEWATER TREATMENT PLANT OPERATING DATA
Parameter Value
Plant flow, MGD 5
Sludge to disposal, Ib/day dry
basis 10,320
Solids heat value, Btu/lb dry
basis 11,000
Volatile solids to digester,
percent of dry solids 77
Sludge solids content, percent''
solids by weight ^*" 20
Vacuum filter operation, hr/
week 40
1 MGD = 0.04 m /s
1 Ib/day = 0.45 kg/day
1 Btu/lb = 2,324 MJ/kg
11.3.5.1 Approach
A consultant was hired to evaluate several disposal methods,
including land disposal, composting, heat treatment, combustion,
and continuation of landfill disposal. Combustion was identified
as the most cost-effective solution. The high energy content of
the sludge and the limited available land for sludge disposal
influenced this decision. The digestion step was eliminated from
the design so that the full heat value of the sludge could be
used in combustion. It was expected that this would obviate the
need for any supplemental fuel. The existing digesters would be
converted to sludge thickening/storage units, and the existing
vacuum filters would provide an incinerator feed solids content
of approximately 20 percent.
At present, the vacuum filter operates 6 to 8 hours a day,
5 days per week. Because of the limited plant area, no space
is available for filter cake holding facilities. Therefore,
the furnace will be designed to operate in conjunction with
the vacuum filters. A review of the various furnace systems
indicated that because of the high heating value of the
11-61
-------
sludge, the intermittent operation requirements, and the space
limitations, a fluid bed system would be the most cost- and
energy-effective solution...
11.3.5.2 Preliminary Design
Fluid bed furnace manufacturers were provided the data in
Table 11-15 for analysis and development of heat and material
balances. Table 11-16 and Figure 11-25 show all sizing criteria,
as well as the requirements for peripheral equipment. On the
basis of this and additional data, a 15-foot (4.6 m) diameter
fluid bed furnace was specified. A recuperator to recover the
heat in the exhaust gas and return it to the furnace (hot wind
box design) was included.
TABLE 11-15
DESIGN EXAMPLE: SLUDGE FURNACE DESIGN CRITERIA
Parameter Value
Sludge feed
Solids content, percent by
weight 20
Volatile solids content,
percent of dry solids 77
Heat value, Btu/lb of dry
solids 11,000
Furnace operation, hr/week 40
Average solids loading rate,
Ib/hr, dry basis 1,810
1 Btu/lb = 2,324 MJ/kg
1 Ib/hr = 0.45 kg/hr
Detailed design of the complete system actually begins with
the data provided by the furnace manufacturer. More than
one manufacturer should be consulted for design data. Air
emissions must be estimated and these estimates submitted
to local, state, and federal authorities in order to obtain a
permit to construct. Because of the small orifices in the
venturi scrubber, potable makeup water at 5 gpm (0.3 1/s) is
required. The impingement scrubber water flow of 397 gpm
(24 1/s), 0.6 MGD (0.03 m^/s), will be secondary effluent.
Note that the scrubber water flow is 12 percent of the average
plant flow and approximately 25 percent of the plant's minimum
flow. Because this return flow is expected to be of low BOD
and of high SS, it will be returned to a point upstream of
11-62
-------
TABLE 11-16
DESIGN EXAMPLE: HEAT AND MATERIAL BALANCE
FOR A FLUID BED FURNACE3
Stream, unit
Connected power, hp
Startup fuel requirements
Weekday operation, 16-hr
shutdown, 106 Btu/hr
Monday morning operation,
64-hr shutdown, 106
Btu/hr
Value
Furnace design
Inside diameter, ft 15.0
Loading rate, Ib wet solids/
sq ft/hr 51.2
Sludge feed
Lb dry solids/hr 1,810
Heat value, 106 Btu/hr 19.91
Volatile solids, percent of
dry solids 77
Supplemental fuel 0
Combustion air
Mass, Ib/hr 22,950
Heat value, 10 Btu/hr 5.40
Ash
Mass, Ib dry solids/hr 416
Heat value, 10b Btu/hr 0.12
Water flow, gpm 20
Radiation
Heat loss, 10 Btu/hr 1.27
Furnace exhaust
Temperature, °F 1,400
Recoverable heat ,
70 percent efficiency, 10
Btu/hr 4 . 2
Recuperator Yes
Venturi water
Recycle water, gpm 94
Makeup water at 70 °F, gpm 5
Scrubber water feed
Flow at 70 °F, gpm 397
Scrubber water drain
Flow at 130 °F, gpm 410
Gas exhaust
Volume, cfm at 120°F 6,162
240
0.42
Data supplied by Dorr-Oliver, Inc.
At 1,400 °F.
Fuel required:
1 hr on Saturday.
1 hr on Sunday.
1/2 hr on Monday morning.
1 ft = 0.30 m ,
1 Ib/sq ft/hr =4.89 kg/m /hr
1 Ib/hr = 0.45 kg/hr
1 x 106 Btu/hr = 1,055 MJ/hr
1 cfm = 4.7 x ID"4 m3/s
1 gpro = 0.06 1/s
11-63
-------
the aeration tank (see Chapter 16). The temperature of the
sidestream, 130°F (54°C), was not considered to have an adverse
effect on the secondary process.
~f~, o» i on c
'GAS* EXHAUST
IMDUCED DRAFT FAN
SUPPLEMENTAL FUEL s
19.91 k 10EBlu/*i[
CONNECTED POWEH
t x 10°Bai/»= 106b MJ.tir
1 apro = o.oe i/s
1 Ihflif = QM Kj/hr
1 cfm - 4.? k 1CT4 m3A
FIGURE 11-25
DESIGN EXAMPLE: HEAT AND MATERIAL BALANCE IN
A FLUID BED FURNACE
Contracts for disposing of the wet ash must be established.
Methods for transporting the ash slurry, conveying it to trucks
at the plant site, and discharging it from the trucks at the
disposal site must be investigated and designed.
Options for using available excess heat should also be examined.
As shown in Table 11-16, 4.2 x 106 Btu per hour (4.4 GJ/hr)
are available for use. However, heat is available only
intermittently and not necessarily at the time it is most needed.
Another approach is to transfer the heat to hot water tanks and
use the heated water for space heating. Alternatively, the heat
can be utilized in an absorption refrigeration unit to produce
chilled water. This water can be stored and used to satisfy
subsequent space cooling demands.
11-64
-------
Other design considerations to be investigated include but are
not limited to:
• Ash dewatering methods
• Ash hauling by owner or by separate contractor
• Type of auxiliary equipment such as sludge conveyors,
fans, and feed equipment
• Heat recovery methods
• Electrical distribution
• Control philosophy
• Sophistication of instrumentation and control
• Supplemental fuel availability and storage (for start-up
and problem periods)
• Area clearance, including access platforms
• Furnace housing requirements
• Structural requirements, for example, seismic and wind
factors
• Noise levels and other safety requirements
• Heating, ventilating, and cooling the area near the
furnace
• Spare parts
• Level and quality of staffing
These points relate only to the installed system. An important
consideration is the interfacing of the existing plant with
the construction of the furnace. All systems and details
relating to the furnace should be discussed with the furnace
manufacturer and, in some cases, made the responsibility of
the manufacturer. For example, the combustion air fans, the
recuperator (if used), the heat recovery boiler (if used),
scrubbers, and some or all of the controls should be part of
the total contract.
11.4 Starved-Air Combustion
Starved-air combustion (SAC) has been demonstrated to be an
effective method for burning sludge in a furnace (17-20,22,24,
26,29,30). Strict air quality standards can be met with SAC, and
large amounts of supplemental fuel are not required.
11-65
-------
The key to SAC is the use of less than theoretical quantities
of air in the furnace--30 to 90 percent of stoichiome trie
requirements. This makes SAC more fuel-efficient than incinera-
tion in an MHF. This is shown on Figure 11-26. When a SAC-MHF
is combined with an afterburner, an overall excess air rate of
25 to 50 percent can be maintained, as compared to an excess air
rate of 75 to 200 percent for multiple-hearth incinerator with an
afterburner.
EXHAUST
EXHAUST
AFTERBURNER
(1,200° F)
SUPPLEMENTAL
FUEL
10 TO 25% EXCESS
AIR FOR FUEL
100% THEORETICAL
AFTERBURNER
{1,200° Ft .
AIR FOR FUEL
50 TO 160%
EXCESS AiR
100%
THEORETICAL AIR
0% EXCESS AIR
.JOTOJOK
THEORETICAL AIR
25 TO 50%
EXCESS AIR
10 TO 70%
THEORETICAL AIR
75% TO 200% OVER All
EXCESS AIR RATE
26% TO 50% OVERALL
excess AIR RATE
INCINERATION
STARVED-AIR COMBUSTION
ASSUMPTION: AUTOGENOUS SLUDGE FEED
FIGURE 11-26
COMPARISON OF EXCESS AIR REQUIREMENTS: INCINERATION IN A
MULTIPLE-HEARTH FURNACE VS. STARVED-AIR COMBUSTION
SAC is, in effect, incomplete combustion. The reaction products
are combustible gases, tars, and oils, and a solid char that can
have an appreciable heating value. The relative proportion
of each varies with the amount of heat applied and the feed
moisture. Generally, higher reaction temperatures yield simpler
products and greater quantities of low heating value gas. This
is at the expense of combustible solid products (25).
11-66
-------
The low heating value gases may be burned, and the heat generated
can be recovered and used beneficially. Alternatively, the gas
may be cooled and stored for subsequent off-site use. The most
effective utilization appears to be the burning of the total
gas stream, with subsequent recovery of portions of the heat
generated. Off-site use appears to be impractical because:
• The gas fuel value is low. Thus, delivery of any
significant quantity of energy requires the transport of
very large volumes of gas.
• Cooling of the gas for off-site use would result in
permanent loss of much of its heat content.
• .The condensates (tars, oils) produced when the gas is
cooled are high strength and corrosive. Containing
the condensates and disposing of them are significant
problems.
• The condensates themselves have significant heat
values. The heating values of the gas is diminshed when
condensates are removed.
In full-scale test work (17), the SAC combustible exhaust gas
was found to have a heating value of 90 Btu/standard dry cubic
foot (3.4 MJ/m^). The gas contained hydrogen, carbon monoxide,
carbon dioxide, methane, ethylene, butane, nitrogen, oxygen,
water, and some higher hydrocarbons.
SAC ash may contain combustible material; the amount depends upon
furnace operation. SAC reduces sludge to an ash containing from
3 to 30 percent combustibles, including up to 20 percent fixed
(elemental) carbon. More combustibles can be released to the
gas stream by adding more air, oxygen, or steam to the lower
part of the furnace. This has the advantage of transferring part
of the heat in the residue to the gas stream. However, the
transfer leaves the residue depleted in heat value. In some
circumstances, it may be better not to burn out the residue
completely. Conceivably, char could be used as an adsorbent or
as a filter aid for sludge conditioning prior to dewatering.
The operating temperature of the furnace can be controlled
within a wide range. The lower temperature limit is the point
when the rate of decomposition of high molecular weight organic
compounds becomes too low, about 1,300°F (704°C). The upper
temperature limit is defined by the point at which there is ash
melting or damage to refractories, about 1,800°F (982°C) (22).
One temperature consideration is that vaporization of heavy
metals must be minimized, since it is difficult to remove heavy
metals from the gas stream with conventional scrubbing equipment.
It is therefore preferable to burn the sludge at as low a
temperature as possible. Full-scale test work (17) and other
published data (18,19, and 20) indicate that 1,500°F (816°C)
appears to be a reasonable operating temperature for minimum
heavy metal vaporization. :
11-67
-------
Fluid bed, electric, and cyclonic furnaces could also be operated
in a SAC mode. To date, none has been operated in this manner
with a sludge feed. Operation in a SAC mode is particularly well
suited to the MHF. There appears to be little incentive to
operate the FBF in this mode because (1) excess air rates for SAC
and the FBF are about the same, and (2) an afterburner would be
required for a converted MHF whereas afterburning is not needed
where the FBF is used in the incineration mode. Several types of
furnaces, including an FBF (21), have been operated in the
starved-air combustion mode on wood wastes to produce charcoal.
11.4.1 Development and Application
Starved-air combustion of sludge, and/or refuse-derived fuel, was
successfully demonstrated in a full-scale test at the Central
Contra Costa Sanitary District's wastewater treatment plant in
Concord, California (17). The use of refuse-derived fuel is
discussed in Section 11-5. The furnace and an afterburner were
operated at 1,400°F (760°C) without supplemental fuel addition.
The feed was primary and trickling filter sludge from a mostly
domestic wastewater. The combined sludge had a heating value
of 9,000 Btu per pound (20.9 MJ/kg) of combustible solids, a
combustible solids content of 75 percent, and a feed sludge
solids concentration of 24 percent (17). The Concord SAC reactor
was a converted six-hearth, 16-foot 9-inch (5.1 m) diameter
MHF. Dewatered sludge was burned by using approximately
50 percent of the theoretical air requirement, and an exhaust
gas was produced with a heating value of 90 Btu per standard
dry cubic foot (3,353 MJ/m^). All of the exhaust gas was burned
in an afterburner at 1,400°F (760°C). The resulting SAC ash
contained 30 percent combustibles, of which 20 percent were
fixed carbon. Other important results and conclusions of this
two-month SAC test program were:
• Starved-air combustion was easier to control than
incineration (the furnace was also run in an incineration
mode).
• Hearth temperature could be used to control the furnace,
with air addition as the manipulated variable.
• Air addition to the furnace should be automatically
controlled.
• Particulate production per pound of solids fed was about
50 percent lower than conventional incineration.
• The completeness of the reaction depends upon the amount
of air fed, not on temperature.
• The most corrosion resistant alloys for high temperature
conditions were Type HK stainless steel and Inconel 690.
For low temperature conditions the most corrosion
resistant alloys were Hastelloy C-176 and Inconel 625.
11-68
-------
GAS EXHAUST
SHAFT COOLING
AIR RETURNED
SLUOGI
TO FURRACE
FEED
C7
AFTERBURNER
CDMBUSTJON
AlP
SHAFT COOLING AIR NOT RETURNED
FURNACE
EXHAUST
AFTtfiiUftNER
EXHAUST
-SHAFT COO LINO Ain
RETURNED TO AFTIRiURNifl
AFTERBURNER
PRECOOLER-
AND VENTURI
BOILER IX.HAUST /
INOUCiD
DRAFT FAN
MULTIPLE
HEAfltH
STARVED
AIR REACTOR
COMBUSTJQNAIR
SHAFT
VENTURI WATER
CONNECTED
COOLING AIR
FIGURE 11-27
FLOWSHEET FOR STARVED-AIR COMBUSTION IN A
MULTIPLE-HEARTH FURNACE
A flowsheet for an MHF operated as a SAC reactor is provided in
Figure 11-27. Comparison with Figure 11-9 shows the difference
to be the addition of an afterburner. Heat and material
balances for the hypothetical treatment plant alternatives
(Table 11-8) are presented in Table 11-17. Table 11-18 takes
selected data from the heat and material balances previously
presented to permit direct comparison of SAC with incineration
options. Direct comparisons are made for an autogenous sludge,
and feed rates to all systems are identical except for that to
the cyclonic furnace. SAC appears to have an advantage overall
11-69
-------
TABLE 11-17
HEAT AND MATERIAL BALANCE FOR STARVED-AIR COMBUSTION
OF SLUDGE IN A MULTIPLE-HEARTH FURNACEa
Alternative (all 40 percent solids)
Stream
Furnace design
Diameter, ft-in.
Number of hearths
Hearth loading rate, Ib wet
solids/sq ft/hr
Sludge feed
Lb dry solids/hr
Heat value, 106 Btu/hr
Volatile solids, percent dry
solids
Supplemental fuel
Combustion air
Mass, Ib/hr
Temperature, °F
Shaft cooling air
Mass, Ib/hr
Shaft cooling air return
Mass at 350 °F, Ib/hr
Shaft cooling air to stack
Mass at 325 °F, Ib/hr
Shaft cooling air to afterburner
Mass at 350 °F, Ib/hr
Ash
Mass, Ib/hr
Temperature, °F
Heat value, 106 Btu/hr
Radiation .
Heat loss, 10 Btu/hr
Furnace exhaust
Mass at 800 °F, Ib/hr
Heat value, 10° Btu/hr
Afterburner combustion air
Mass at 60 °F, Ib/hr
Afterburner exhaust •
'• Mass, Ib/hr
Temperature, °F
Heat value, 1.0 6 Btu/hr
IB
5 MGD
12-9
6
12.1
2,131
7.35
65
0
h
0D
0
9,178
6,480
0
2,698
787
500
0.23
0.44
11,010
6.82
4,382C
17,368
1,495
12.76
IIB
15 MGD
14-3
7
12.0
3,201
10.73
65
0
h
,780C
60
10,095
8,640
0
1,455
1,181
500
0.34
0.62
16,250
10.16
8,805C
26,537
1,495
19.18
IIIB
50 MGD
16-9
8
11.4
5,064
16.90
65
0
h
1,500
60
15,602
13,380
0
2,222
1,869
500
0.54
0.94
25,658
16.05
14,098C
42, 041
1,495
30.04
Boiler exhaust ,
Heat value at 500 F, 10 Btu/hr
Recoverable heat
70 percent efficiency, 10
Btu/hr
6.76
4.2
9.18
7.0
13.04
11.9
All data supplied by the manufacturer.
In addition to shaft cooling air returned to furnace.
cln addition to shaft cooling air returned to afterburner.,-
Costs as of early 1978.
1 ft = 0.30 m
1 in. = 0.02 m ,
1 Ib/sq ft/hr =4.9 kg/m /hr
1 Ib/hr = 0.45 kg/hr
1 x 106 Btu/hr = 1,055 MJ/hr
1 gpm = 0.06 1/s
1 MGD = 0.044 m3/s
11-70
-------
TABLE 11-17
HEAT AND MATERIAL BALANCE FOR STARVED-AIR COMBUSTION
OF SLUDGE IN A MULTIPLE-HEARTH FURNACE3 (Continued)
Alternative (all 40 percent solids)
IB IIB IIIB
Stream . 5 MGD 15 MGD 50 MGD
Precooler and Venturi water feed
Flow at 70 °F, gpm 51 77 121
Scrubber water feed
Flow at 70 °F, gpm 102 . 153 . 243
Scrubber drain
Flow, gpm 160 240 380
Temperature, F 98 98 98
Gas exhaust
Mass, Ib/hr 14,280 21,480 34,080
Temperature, °F 120 120 120
Heat value, 106 Btu/hr 4.62 5.96 7.94
Connected power
Horsepower 78 123 218
Installed cost, thousand dollars'3 1,400 1,600 2,300
aAll data supplied by.the manufacturer.
In addition to shaft cooling air returned to furnace.
CIn addition to shaft cooling air returned to afterburner.
dCosts as of early 1978.
1 ft = 0.30 m 1 * 106 Btu/hr = 1,055 MJ/hr
1 in. = 0.02 m -, 1 9P™ = Q-06 1/s
1 Ib/sq ft/hr =4.9 kg/m /hr 1 MGD = 0.044 m3/s
1 Ib/hr = 0.45 kg/hr
but the FBF in terms of air required, as indicated by lesser
exhaust flow rates. SAC has less connected horsepower than
the other options, arid except for the FBF, higher exhaust
temperatures and thus, greater potential for energy recovery.
Additional details of the test work and SAC application can
be found in the literature (8,17-30,65,66,67). Additional
information can also be gained by working with the furnace
manufacturers.
11.4.2 Advantages and Disadvantages of SAC
Test work, much of which is still underway, shows that SAC in an
MHF using sludge alone has many advantages over incineration or
other combustion processes.
11-71
-------
TABLE 11-18
HEAT AND MATERIAL BALANCE COMPARISON OF STARVED-AIR
COMBUSTION AND INCINERATION
Multiple-hearth Fluid bejl
Item :
Alternative IA
1 Sludge feed, Ib dry
solids/hr
Supplemental fuel,
106 Btu/hr
Furnace exhaust
Mass, Ib/hr
Temperature, °F
Recoverable heat
70 percent effi-
ciency, 10° Btu/
hr
Connected power
Horsepower
Alternative IB
Sludge feed, Ib dry
solids/hr
Supplemental fuel,
106 Btu/hr
Furnace exhaust
Mass, Ib/hr
Temperature, °F
Recoverable heat
70 percent effi-
ciency, 1Q6 Btu/
hr
Connected power
Horsepower
Alternative IIB
Sludge feed, Ib dry
solids/hr
Supplemental fuel,
106 Btu/hr
Furnace exhaust
Mass, Ib/hr
Temperature, F
Recoverable heat
70 percent effi-
ciency, 10^ Btu/
hr
Connected power
Horsepower
Alternative IIIB
Sludge feed, Ib dry
solids/hr
Supplemental fuel.
106 Btu/hr
Furnace exhaust
Mass, Ib/hr
Temperature, °F
Recoverable heat
70 percent effi-
ciency, 'lo6 Btu/
hr
Connected power
Horsepower
aSee Table 11-9.
bSee Table 11-11.
cSee Table 11-12.
dSee Table 11-13.
incinerator0
1,806
2.64
30,817
800
1.89
238
2,131
0
32,123
1,000
_
2.31
93
3,201
0
48,434
1,000
5.04
178
5,064
0
77,643
1,000
8.05
238
furnace0
1,806
2.80
19,353
1,400
3.50
218
2,131
0
16,250
1,650
6.2
162
3,201
0
23,576
1,650
9.40
234
5,064
0
38,620
1,650
12.7
350
Electric
furnace0
1,806
2.98f
26,351
750
1.37
22h
2,131
0
29,372
1,200
3.85
25h
3,201
0
44,064
1,200
5.81
40h
5,064
0
69,732
1,200
9.18
U
60"
Cyclonic
furnace
1,806
2.48
30,692
1,420
2.97g
175
1 , 806 1
0
23,765
1,411
4.63
125
2,712
0
35,675
1,421
7.01
190
4,292
0
57,424
1,420
11.11
290
Starved-air
combustion -
multiple hearth6
_
-
-
-
-
-
2,131
0
17,638
1,495
4.20
78
3,201
0
26,537
1,495
7.00
123
5,064
Q
42,041
1,495
11.90
218
See Table 11-17.
Infrared heaters (kw = 10,600 Btu to allow for generating efficiency).
^Recuperator only.
Does not include power requirements for infrared heaters.
Based data used by manufacturer is different from that for other furnaces.
1 Ib/hr =0.45 kg/hr
1 x 10° Btu/hr = 1,055 MJ/hr
11-72
-------
The SAC process provides a greater solids throughput because of
the higher allowable hearth loading rates. (This assumes that
a portion of the combustibles remain in the ash.) Operation
of a multiple-hearth furnace with SAC permits hearth loading
rates 30 to 50 percent higher than an optimum incineration
mode. This can be explained in terms of heat release and gas
velocity, although other factors also affect loading rate. In
incineration, the heat liberated in the furnace by combustion of
the feed solids must be limited to prevent high-temperature
damage to the furnace refractories. Under SAC, heat liberation
is minimized in the furnace by air control with combustibles
passing out in a gas form to an auxiliary combustion chamber, or
afterburner. The afterburner which has no moving mechanical
parts, can be designed for the high temperatures. Thus, with
the two-stage combustion process which occurs under SAC, high
furnace temperature is not a limiting condition. Gas velocity is
another factor which affects hearth loading rate. An excessive
gas velocity entrains large quantities of solids particles in
the furnace, leading to gas cleanup difficulties. With SAC,
considerably less air is used in the furnace than with incinera-
tion, and this can be traded-off against the increased volume of
combustible gases created by higher hearth loading rates.
Therefore, for a fixed maximum gas velocity, a greater hearth
loading rate can be applied with SAC than with incineration.
A second advantage offered by SAC is reduced fuel usage when
afterburning is required. Even when it is possible to dewater
the sludge feed to an autogenous state, (eliminating the need
for supplemental fuel to the furnace), a considerable quantity
of fossil fuel is still required for the afterburner in an
incineration mode. Essentially no fuel is required for the
afterburner in an SAC mode.
SAC offers more stable operation and ease of control, with
minimal furnace response to feed changes. With incineration,
increasing or decreasing the feed rate results in a corresponding
rise or fall in hearth temperature since the solids combustion
rate increases or decreases. With SAC, the extent of the
heat-generating combustion reactions are limited by the available
oxygen supply. Fluctuations in feed rate will not change the
temperature level because it does not change the amount of
combustion occurring, provided air rate does not change.
A fourth advantage of SAC over incineration is that it produces
fewer air emissions. SAC's lower furnace gas velocity, for the
same solids loading rate, results in less particle entrainment
and reduced particulate emissions. During full-scale tests at
Concord, particulate production with SAC was about 50 percent
less than incineration under equal solids feed conditions (17,
18). Furthermore, particulates leaving the furnace were larger
than those from incineration. These particulates are more easily
removed by cyclones or other simple gas cleanup equipment. At
Concord, nitrogen oxide and sulfur oxide emissions also appeared
11-73
-------
to be lower with SAC. It is probable that when organic nitrogen
is oxidized, one of the reaction products is nitrogen oxides.
The reaction is illustrated for NO:
Organic-N + 02 »- NO + H20
Thus, incineration of sludge, which contains a fairly large
organic nitrogen fraction, produces nitrogen oxides. When
organic nitrogen is subjected to SAC, however, little organic-N
is converted to nitrogen oxide because little oxygen is
available. The organic nitrogen is instead converted to ammonia.
The ammonia, when oxidized (for example, in the afterburner) is
converted to nitrogen gas and water.
4NH3 + 302 »~2N2 + 6H20
As long as afterburner temperatures are maintained below
approximately 1,600°F (871°C), conversion of N2 to nitrogen
oxide is also minimized. Thus, the key to lower nitrogen oxide
production with SAC appears to be its ability to direct organic
nitrogen destruction toward ammonia formation, rather than to
oxide formation.
Data supporting the observation of low sulfur oxide emissions in
the Concord tests are limited. Measurements indicate that much
of the sulfur in the feed solids ends up in the ash (17). With
incineration, most of the feed sulfur is delivered to the stack.
Other advantages of SAC include the fact that essentially
all equipment needed is currently available and has a long
performance history, and that most existing MHFs can be easily
retrofitted to operate in a SAC mode.
Disadvantages of SAC should also be considered in design. The
afterburner requirement can limit use of SAC in existing
installations for several reasons. The afterburner is normally
a large chamber, and space may not be available. Floor loadings
of existing buildings can easily be exceeded by a large
refractory-lined device. Supplemental fuel and air must be
supplied to the afterburner, requiring additional space for
piping and equipment.
A second disadvantage is that SAC requires more instrumentation
than does incineration. Proper control is essential for good SAC
operation; therefore, temperature controllers must be included on
each hearth to control air feed rate. Draft and other common
incinerator instrumentation must also be provided and maintained.
11-74
-------
If for some reason the furnace exhaust gases have to bypass the
afterburner, they .may create emission violations. Furnace
exhaust gases are high in pollutants, such as hydrocarbons and
other noxious products of incomplete combustion. They could
flare in the atmosphere, causing stack damage. Also, these gases
are corrosive. All construction materials in the gas stream must
be properly selected, as described previously. Corrosion results
found at the full-scale test in Concord, California, are found in
the literature (17).
Additionally, combustibles in the SAC ash may create ultimate
disposal problems. For example, in a landfill, they may not be
as inert as incinerator ash.
11.4.3 Conversion of Existing Multiple-Hearth
Incineration Units to SAC
One of the greatest advantages of SAC is that most existing units
can be converted to operate as SAC reactors. -This retrofitting
involves relatively few changes. The costs and benefits are
site-specific. One definite incentive for conversion is that
the existing unit may be able to handle increased sludge
loads without the addition of more incinerators. This incentive
is demonstrated in a design example presented later in this
section. Assuming an increase in solids loading of approximately
30 percent, the basic changes necessary are:
CHANGE
Add an afterburner (if existing
system has an afterburner, its
size may have to be increased).
If furnace is large enough, top
hearth may be used as an after-
burner; however, refractories
must be examined.
Add combustion air flow
control and temperature
controllers.
Possibly replace combustion
air fan.
Modify induced draft fan--may
need change only in speed or
damper position.
REASON
Required to burn combustible
fuel gas prior to exhaust.
Requ i red
process.
to control SAC
May be required to control
reduced air flow rate.
Required because total unit,
including afterburner, uses
approximately 50 percent
excess air, while an
MHF incinerator uses,
including afterburner, 75 to
200 percent excess air. See
Figure 11-26.
11-75
-------
CHANGE
REASON
Review and modify venturi
and wet scrubber.
Add additional emission control
equipment.
Review furnace system and
and replace remote
instrumentation.
Generally "tighten up1
system.
furnace
Required to maintain high
performance with lower air
flows. Also may need
precooler section if boiler
is not in process train.
Required depending upon
local air emission control
regulations regarding
applicability of new source
performance standards.
Modification of process may
change applicable standards.
Good practice for any major
process revision.
SAC process depends on good
air control. Peak and poke
holes must be modified along
with other openings into
the furnace to reduce
uncontrolled air leakage
into the furnace.
With these modifications and any others found necessary during
the review of the site-specific system, the retrofitted MHF
system will be suitable for SAC operation.
11.4.4 Design Example: Retrofit of an Existing
Multiple-Hearth Sludge Incinerator to a
Starved-Air Combustion Reactor
A 20-MGD (0.88-m3/s) domestic wastewater treatment plant in the
Midwest has been incinerating primary and waste-activated sludge
in two multiple-hearth furnaces. All sludge is thickened prior
to dewatering on vacuum filters that produce a 25 percent solids
feed cake. Polymers are used in the vacuum filters. The ash
from the furnaces is sluiced to ash holding ponds, and the
supernatant is recovered and returned to the plant influent
sewer. Stabilized ash is removed from the ponds at least once a
year and hauled to the local landfill.
One furnace is normally required for sludge reduction; however,
the original design provided 100 percent redundancy. The plant
is currently overloaded and both multiple-hearth furnaces are
used simultaneously about three months of the year.
11-76
-------
Substantial growth in wastewater flows are anticipated in the
next four years. Planning for an 10-MGD (0.44-m3/s) expansion
is currently underway. The design will handle projected flows
through 1988. In addition, new air emission regulations were
recently promulgated limiting hydrocarbon, carbonyl, and carbon
monoxide emissions to about half of the current incinerator
emissions. The city has been given notice to correct this
situation or be subject to fines levied by the local air quality
management district (AQMD). A time extension to review and
correct this problem has been granted to the city. Data for the
existing plant are shown in Table 11-19
TABLE 11-19
DESIGN EXAMPLE: WASTEWATER TREATMENT PLANT OPERATING DATA
Parameter Value
Plant operating conditions
Design flow, MGD 20
Total solids, Ib/day dry
basis 40,800
Volatile solids, percent of
dry solids 75
Furnace operating conditions
Operating hours/week 168
Loading rate, Ib/hr dry basis 1,700
Solids content of feed, per-
cent dry weight • . 25
Loading rate, Ib/hr wet basis 6,800
1 MGD = 0.044 m3/s
1 Ib/day = 0.45 kg/kg
1 Ib/hr = 0.45 kg/hr
11.4.4.1 Approach
The city retained a consultant to prepare a facilities plan/
project report to obtain Construction Grants funding for a plant
expansion to 30-MGD (1.32-m3/s). Because of the urgency of the
air emissions problem, the city authorized the hiring of air
pollution experts to assist the consultant in developing an
interim plan consistent with the goals of the expansion.
Following several detailed design estimates, afterburning at
1,200°F (649°C) for one-half second was determined to be the
most cost-effective solution. This approach was also felt to
guarantee a continuous and dependable operation while satisfying
all regulations.
11-77
-------
Since afterburning was proposed, it was also decided to study
SAC. SAC could possibly increase existing furnace capacity and/
or reduce the equipment to be added. Prior to review of SAC, it
was determined that, in the incineration mode, each furnace would
require an afterburner, and that a new furnace and afterburner
would be required for the plant expansion to 30-MGD (1.32-m3/s).
11.4.4.2 Preliminary Design
Two experienced multiple-hearth furnace manufacturers were
provided the data in Table 11-19. Detailed heat and material
balances were developed for incineration and starved-air
combustion—both followed by external afterburning. The schemes
used the existing vacuum filters to provide a feed cake of
25 percent solids. Also, each manufacturer was to analyze
two additional cases that entailed use of improved dewatering
equipment to produce a feed cake solids content of 35 percent.
Both of these cases used SAC, but one had an external afterburner
and the second used the top hearth of the present furnace as the
afterburner. The manufacturers were asked to use the existing
furnace to determine the capacity of each of the four systems.
The cases considered were as follows:
Case I Add an external afterburner and heat recovery boiler to
each furnace. One additional furnace is required to
satisfy future loading.
Case II Convert existing furnaces to SAC. Add an external
afterburner and heat recovery boiler to each furnace.
One additional furnace is required to satisfy future
loading.
Case III Convert existing furnaces to SAC. Add an external
afterburner and heat recovery boiler to each furnace.
Sludge feed rate calculated using allowable rates for
SAC with improved dewatering equipment (35 percent
solids). Note that afterburner temperature is 1,430°F
(777°C). No additional furnaces required for future
loading.
Case IV Convert existing furnaces to SAC and use top hearth
as an afterburner. Add a heat recovery boiler to
each furnace. Sludge feed rate calculated from
allowable rates for SAC with improved dewatering
equipment (35 percent solids) and desired afterburner
temperature of 1,200°F (649°C). No additional furnaces
are required for future loading.
Table 11-20 shows a summary of the manufacturer's calculations
for the four cases and the existing condition. An interesting
comparison can be made between Cases I and II. Both cases
use an afterburner and recover heat but Case II utilizes
SAC. Heat recovery gains by using SAC are impressive. The
city would save 1.33 x 10^ Btu per hour (1.40 GJ/hr) by using
SAC, which would produce an annual fuel savings of slightly
11-78
-------
TABLE 11-20
DESIGN EXAMPLE: HEAT AND MATERIAL BALANCES
FOR MULTIPLE-HEARTH FURNACES
Type of operation
Furnace design
Number of furnaces
Diameter, ft-in.
Number of hearths '
Hearth loading rate,
Ib wet solids/sq
f t/hr
Afterburner
Sludge feed
Lb dry solids/hr
Heat value, 106 Btu/
Volatile solids, per-
cent dry solids
Feed solids, percent
Afterburner supple-
mental fuel
Mass, Ib/hr
Heat value, 10
Btu/hr
Furnace combustion air
Mass at 60 °F, Ib/hr
Shaft cooling air
Mass, Ib/hr
Shaft cooling air return
to furnace
Mass at 350 °F,
Ib/hr
Shaft cooling air to
stack
Mass at
Ib/hr
Heat va
Btu/hr
afterburner
Mass at
Btu/hr
Ash
Mass, Ib/hr
Heat val
Btu/hr
Radiation
Heat loss, 10
N.A. - Not applicable.
Existing
condition
Incinerator
2
16-9
7
7.0
..None
1,700
12.75
75
25
N.A.
N.A.
17,833
15,939
Case I
Modified
incinerator
3C
16-9
7
7.0
External
•1,700
12.75
75
25
189
3.77
17,833
15,939
Case II
SACb
3C
16-9
7
7.0
External
1,700
12.75
75
25
128
2.44
9,822
15,939
Case III
SACb
2C
16-9
7
10.2
External
3,473
26.05
75
35
0
0
12,507
15,939
Case IV
SACb
2C
16-9
7d
10.2
Internal
(top hearth)
2,957
22.18
75
35
0
0
9,867
15,939
13,548
13,548
9,840
12,480
9,867
350 UF,
2,391
.ue, 10
0.35
ig air to
:r
350 °F,
N.A.
ir , , 425
:, 106
0.04
106 Btu/hr 0.29
2,391
0.35
0
425
0.04
0.29
5,919
1.09
180
478e
0.14
0.29
0
0
3,660
975e
0.28
0.29
0
0
6,072
866e
0.25
0.29
All data supplied by the manufacturer.
SAC - Starved-air combustion.
^Number of furnaces required in 1988 (30 MGD), for
increased sludge quantities with one furnace on
standby.
Note, top hearth is afterburner, therefore, not
included in hearth loading calculations.
"Includes combustible heat content.
Existing system does not include boiler.
1 ft = 0.30 m
1 in. = 0.02 m
1 Ib/sq ft/hr =4.9 kg/m2/hr
1 Ib/hr =0.45 kg/hr
1 x 106 Btu/hr = 1,055 MJ/hr
1 gpm = 0.06 1/s
1 MGD = 0.044 m3/s
11-79
-------
TABLE 11-20
DESIGN EXAMPLE: HEAT AND MATERIAL BALANCES FOR
MULTIPLE-HEARTH FURNACES (Continued)
Type of operation
Furnace exhaust
Mass at 800 °F, Ib/hr
Heat value, 106 Btu/
hr
Afterburner combustion
air
Mass at 60°F, Ib/hr
Afterburner exhaust
Mass, Ib/hr
Temperature, °F
Heat value, 106 Btu/
hr
Boiler exhaust
Heat value at 400 F,
106 Btu/hr
Recoverable heat
70 percent efficiency,
106 Btu/hr
Precooler and Venturi
water feed
Flow at 70 °F, gpm
Scrubber water feed
Flow at 70° F, gpm
Scrubber drain
Flow, gpm
Gas exhaust
Mass, Ib/hr
Temperature, F
Heat value, 106 Btu/hr
Existing
condition
Incinerator
24,209
12.07
N.A.
N.A.
N.A.
N.A.
N.A.
2.24"
51
243
306
21,368
120
6.35
Case I
Modified
incinerator
Case II Case III
24 ,209
12.07
2,555
26,764
1,200
15.84
9.51
4.43
51
243
306
20,759
120
7.53
SAC
4,296
19,366
1,200
14 .05
3.69'
45
282
338
15,480
120
4.11
SAC
16,145 21,454
11.23 14.87
11,534
32,987
1,430
24.24
13.92
7.22
65
456
536
26,220
120
8.52
Case IV
SAC
17,448
10.55
10,989
28,437
1,200
21.64
12.89
6.13
65
456
532
27,837
120
2.64
N.A. - Not applicable.
All data supplied by the manufacturer.
3SAC - Starved-air combustion.
'Number of furnaces required in 1988 (30 MGD), for
increased sludge quantities with one furnace on
standby.
Note, top hearth is afterburner, therefore, not
included in hearth loading calculations.
2Includes combustible heat content.
Existing system does not include boiler.
1 ft = 0.30 m
1 in. = 0.02 m
1 lb/sq'ft/hr =4.9 kg/m2/hr
1 Ib/hr = "0.45 kg/hr
1
1
106 Btu/hr = 1,055 MJ/hr
gpm
0..06 1/s
:3
1 MGD = 0.044 m /s
over $30,000 at $2.70 per 106 Btu ($2.60 per GJ). However,
it appears that this savings would not justify the conversion
when compared with the large capital expenditure.
11-80
-------
Cases III and IV both use SAC, but start with a cake that has
been dewatered to 35 percent solids. Recoverable heat quantities
are far higher than for Cases I and II. Case IV, which does not
use an external afterburner has a capital cost advantage over
Case III. Also, the system could easily handle the expected
sludge loads through the design year. The fuel savings would be
almost $90,000 per year, and the energy generated would be
sufficient to save another $250,000 per year (at 10,600 Btu/kWhr
[11.18 MJ/kWhr] and $0.05/kWhr). These savings alone would
justify capital expenditures of over $3,600,000 (20 years at
7 percent per year). In addition, there would be a capital
savings because a third furnace would not be required.
After receiving detailed cost estimates, the city authorized the
Case IV design. The flowsheet is given on Figure 11-28.
11.5 Co-Combustion of Sludge and Other Material
The net fuel value of sludge depends on the fraction of its total
combustible solids, the fuel value of those combustible solids
(generally about 10,000 Btu per pound [23.24 MJ/kg]), and the
amount of water present. Wastewater treatment plant sludge
generally has a high water content and, in some cases, fairly
high levels of inert materials. As a result, its net fuel value
is often low. Autogenous combustion of wastewater treatment
plant sludge is generally only possible when the sludge solids
content is 30 to 35 percent or greater. These solids contents
are often difficult to achieve by conventional dewatering
techniques; consequently, supplemental fuel is required for
the combustion operation. If sludge is combined with other
combustible materials in a co-combustion scheme, a furnace feed
can be created that has both a low water concentration and a heat
value high enough to sustain autogenous combustion and may be
cost-effective.
11.5.1 Co-Combustion with Coal and Other Residuals
Many materials can be combined with sewage sludge to create a
furnace feed with a higher heat value than sludge. Some of
these materials are coal; municipal solid waste; wood wastes;
sawdust; textile wastes; and agricultural wastes, such as corn
stalks, rice husks, and bagasse (68, 69). Virtually any material
that can be burned can be combined with sludge in a co-combustion
process. An advantage of co-combustion is that a municipal
or industrial waste material can often be disposed of while
providing an autogenous sludge feed, thereby solving two disposal
problems.
11-81
-------
3,64 a 10° Btv/hr
GAS IXHAUST
27,837 Ifefhr * 12BPF
9,867 Ib/hf
G1
23,lS*MJ*Bttt/hr
SLUPGi FiiD
2.967 Ih/hr DRY
M% SOLIDS FEED
a
16,438 to/ht
ASH 88B !Mw
1Btu/hr= 10S5 J/hr
llb/hrTOKShg/
Igpm - 0,08 I/I
AIR
FIGURE 11-28
DESIGN EXAMPLE: STARVED-AIR COMBUSTION IN A
MULTIPLE-HEARTH FURNACE
Recent studies have shown that the addition of pulverized coal to
liquid sludge prior to dewatering can markedly increase the cake
solids content (70-73). A drier filter cake is produced; thus,
the net heat value of the sludge-coal mix is much greater than if
the coal were added to the filter cake following filtration.
Also, the sludge-coal mix is homogenous, which leads to better
combustion. It may be possible to reduce the amounts of
inorganic filter aids (lime, ferric chloride) required and
produce an autogenous feed. This approach may become appropriate
11-82
-------
in many plants that are close to coal mines or coal-fired power
stations. Coal, however, is not a waste material, and its use to
improve filtration and increase the fuel value of the furnace
feed is not as desirable as using a combustible waste for these
purposes. Two plants, one in Rochester, New York, and an other
in Vancouver, Washington, are experimenting with sawdust as a
filter aid prior to combustion. The results to date have been
very good, but detailed data are not available. Minneapolis has
tried using woodchips as a supplemental fuel (70).
11.5.2 Co-Combustion with Mixed Municipal Refuse (MMR)
Currently there are more than twenty sludge and mixed municipal
refuse (MMR) co-combustion systems, including incineration,
pyrolysis, and starved-air combustion, that are being operated,
tested, or demonstrated in full-scale plants (74-77). The
systems described in this section have been operated at full-
scale and have been developed sufficiently to be implemented
whenever they prove cost-effective.
There are two basic approaches to co-combustion of sludge
with MMR: (a) use of refuse combustion technology by adding
dewatered or dried sludge to an MMR combustion unit, and (b) use
of sludge combustion technology by adding raw or processed MMR
as a supplemental fuel to the sludge furnace. Table 11-21
illustrates the commonly used approaches to co-combustion.
TABLE 11-21
CONVENTIONAL APPROACHES TO CO-COMBUSTION OF WASTEWATER
SLUDGE AND MIXED MUNICIPAL REFUSE
Mixed municipal refuse technology
Grate-fired (refractory or waterwalled)
Sludge dried via flue gases
Sludge dried via steam from furnace
Sludge added directly to furnace
Vertical packed bed reactors (sludge added
to bed)
Air (Andco-Torrax)
Oxygen (PUROX™, a Union Carbide System)
Sludge technology
Multiple-hearth
• Incineration
Starved-air combustion
Fluid bed . ,
11-83
-------
GAB
EXHAUST
STACK
FIGURE 11-29
SP""
TYPICAL GRATE-FIRED WATERWALLED COMBUSTION UNIT
11.5.2.1 Refuse Combustion Technology
Historically, grate-fired refractory and waterwalled combustion
units have been used to burn raw mixed municipal refuse.
Figure 11-29 illustrates this approach. This practice is common
throughout Europe, where there are several hundred installations.
When sludge disposal became a problem, the first approach was to
burn the sludge with the refuse. The quantity of sludge was
normally small compared to the refuse. This was attempted in
several locations, but efforts were generally unsuccessful, with
failures due to the following problems:
• Uniform mixing of sludge and refuse was difficult to
accomplish on a large scale. Poorly mixed feeds produced
alternate "hot" and "cold" feeds, resulting in erratic
furnace operation.
• Biodegradation of materials in the refuse/sludge holding
bins caused unacceptable odors. Detention times in these
bins are often several days long, which is sufficient for
biological action to be established.
11-84
-------
• High moisture content of the sludge and inadequate
furnace detention times sometimes caused non-autogenous
combustion and wet residues.
However, as previously stated, several systems currently in
operation have been designed specifically to incinerate MMR with
sewage sludge (78). A number of these are described below.
Sludge Drying via Steam Generated by Furnace
Several grate-fired, waterwalled combustion units in Europe are
presently incinerating refuse and sewage sludge. At Dieppe,
France, 54 tons (49 t) of MMR and 21 tons (19 t) of dried sludge
are incinerated daily (79). Digested sludge with a solids
content of four percent is pumped from the wastewater treatment
plant and dried with 350°F (177°C) process steam in two thin-film
evaporators to a solids content of 55 percent. The vapors
generated are returned to the furnace. The dried sludge is
conveyed to the charging chutes of the furnace and is mixed
with the solid waste from the receiving pit. A small plant at
Brive, France (80), is similar to that at Dieppe, except that it
uses raw sludge.
Sludge Drying via Fornace Flue _Gas_e_s
A waterwalled combustion unit at the Krefeld plant near
Dusseldorf, Germany, processes 600 tons (544 t) of MMR and
45 tons (41 t) of dry wastewater solids daily (75-77,81 ). The
facility generates electricity for the wastewater treatment plant
and incineration facility and exports hot water for use in the
community. Raw sludge, with a solids content of 5 percent,
is pumped from the wastewater treatment plant to the disposal
facility. The sludge is centrifuged to a solids content of
25 percent and then flash-dried in a vertical-shaft flash-drying
chamber with 1,500°F (816°C) flue gases at from the refuse
combustion unit. The powdered sludge is then injected into
the furnace immediately above the top of the flame (suspension
firing). The facility has been in operation for four years.
Two plants in the United States use flue gases generated in
grate-fired, refractory-walled combustion units to dry waste-
water solids prior to combustion with MMR (74). In Ansonia,
Connecticut, sludge with four percent solids is dried in
a disk-type, co-current spray dryer with 1,200°? (649°C)
incinerator flue gases. Dried sludge and vapors are injected
into the incinerator for suspension burning. The plant capacity
is 200 tons (181 t) per day of solid waste. Presently, the
sludge is not being incinerated but used as a soil conditioner.
Holyoke, Massachusetts, uses a similar incinerator and averages
250 tons (227 t) per week of refuse and 19 tons (17 t) per week
of dry sludge throughput. However, the sludge is dewatered to 28
percent solids prior to drying in a rotary unit using hot flue
gases. Dried sludge and vapors are added to the furnace above
the combustion zone.
11-85
-------
/Sludge Added JDirectly to Furnace
Recently at Norwalk, Connecticut, a process was tested in which
a stoker-fired incinerator was used to co-combust sludge and
refuse (82,83). In this project, sludge with a solids content
of five percent was sprayed onto the front wall of the charging
chute to form a thin sludge layer on top of the refuse. The
sludge layer dries and burns during the 30-minute residence time
in the combustion unit. This process has been incorporated into
the design of a plant at Glen Cove, New York, that will burn a
mixture of 25 tons (23 t) per day of sludge (20 percent solids
content) and 175 tons (159 t) per day of mixed municipal refuse.
The plant is designed to produce 2.2 megawatts of power,
sufficient to meet the demands of the wastewater treatment plant
and the incineration facility. Construction of the Glen Cove
facility is scheduled to be completed in 1982.
Vertical Packed Bed
There are two vertical packed bed, solid waste, starved-air
combustion systems currently available in the United States:
Andco-Torrax and PUROXtm (see Figure 11-30).
The Andco-Torrax system (84, 85) is a vertical shaft, slagging
type furnace in which unprocessed municipal solid waste is
charged into the unit from the top. The refuse is burned at
the bottom of the ram at 3,000°F (1,649°C) by the addition of
small quantities of air heated by countercurrent heat exchange
with the afterburner exhaust. The combustible off-gases are
afterburned at 2,000°F (1,093°C) and processed by electrostatic
precipitators. Wet sludge has been added to an existing 75 ton
(68 t) per day system, but detailed test data are not presently
available.
The PUROX system, a trademark of Union Carbide, is a vertical
furnace for combustion of a processed refuse (86, 87). Proces-
sing includes shredding and ferrous metal separation. The PUROX
system uses pure oxygen rather than air. The refuse is burned at
3,000°F (1,649°C), and a fuel gas is produced that has a heat
value of 385 Btu per standard dry cubic foot 14.3 MJ/m3 dry).
The molten slag produced at the high combustion temperature is
primarily inert silica. j A processed refuse and sludge mixture
was successfully run through the test unit for two months at
South Charleston, West Virginia (87). Average wet test feed
rates were 90 tons (82 t) per day. Test data indicated that the
refuse-to-sludge ratio was 4.26:1. Lower ratios were not tested
because the availability of sludge was limited. The pure oxygen
feed rate was approximately 0.2 tons of oxygen per ton (0.2 t
C>2/t) of feed. Fuel gas production and quality, and slag
production and quality from mixed refuse-sludge feeds, did not
differ radically from that of pure refuse combustion in the PUROX
reactor. Heavy metals in the sludge were trapped in the slag and
were not discharged with the exhaust gases.
11-86
-------
Off GAS
SHREDDED
MIXED
MUNICIPAL
REFINE
4
MIXED
MUNICIPAL
REFUSE
REFUSE .?
PLUG 'i
DRYING /
ZONE '
COMBUSTION
AIR
PRIMARY
COMBUSTION
AND
ZONE
PVROLYSIS I
I DROPOFF
AND
QUENCH
PUR0X REACTOR
(CWRTESY OF UNION CARBIDE
AN0CQ-TQRRAX REACTOR
JCOUHTESV Of ANDCO 1HCQ HPORATED}
FIGURE 11-30
VERTICAL SHAFT REACTORS
11.5.2.2 Sludge Combustion Technology
The most widely used sludge combustion methods are the multiple-
hearth and fluid bed furnace. Both types of units have
successfully burned refuse. Although the electric furnace
and the cyclonic furnace appear to be capable of refuse and
sludge combustion, no full-scale work has been done to date.
Figure 11-31 presents requirements for sustaining autogenous
combustion when sludge is mixed with refuse.
Multiple-Hearth Incineration
Several plants in Great Britain and Europe have been practicing
co-incineration in multiple-hearth furnaces for several years.
However, serious problems such as severe erosion of the hearths,
poor temperature control, refractory failures, and air pollution,
11-87
-------
50 50
40 60
30 70
LU
O
Q
-i
20 80 -
10 90
0100
ASSUMPTIONS:
MULTIPLE HEARTH FURNACE
AFTERBURNER AT 1400°F
HEAT REQUlRED/lb WATER = 3500 Btu/lb
SLUDGE COMBUSTIBLES = 10,000 Btu/lb
MWIR DRY SOLIDS - 6500 Btu/lb
MMR MOISTURE CONTENT = 25%
v>
£
g
<
oc
m
ui
LL
IU
a:
3 y
z
3
Q
LU
X
6
7
8
9
10
LU
o
Q
50
100
0
10
60
20 30 40 50
SLUDGE SOLIDS CONTENT, %
FIGURE 11-31
AUTOGENOUS COMBUSTION REQUIREMENTS FOR CO-DISPOSAL
11-88
-------
have been experienced (88). All of these problems appear to be
a direct result of poor solid waste processing prior to addition
into the furnace. Poor pre-processing causes extreme variations
in feed heat value, which in turn causes wide and uncontrollable
temperature fluctuations in the furnace. These result in
refractory failures and air emission problems.
These problems, were resolved in test work conducted at the
Central Contra Costa Sanitary District wastewater treatment
plant at Concord, California (17). All refuse was shredded,
air-classified, and screened prior to use. This provided a
feed which was relatively free of metals and had a reasonably
consistent heating value, as well as a consistent particle size.
When the furnace was operated in an incineration mode, none
of the problems encountered in Great Britain or Europe were
experienced, but temperature control was still difficult. This
was corrected by operating the furnace in a SAC mode. This work
and the European experience indicates that for MHF furnaces
pre-processing of municipal refuse is required and SAC of refuse
and sludge is preferred over MHF incineration.
MuJJ^iple-Hearth ,=iS_taryed-Air Combustion
Co-combustion of sludge with processed municipal solid waste was
first successfully performed by SAC in a multiple-hearth furnace
during a small-scale test in November 1974 at Burlingame,
California (89). A full-scale prototype test was later
implemented at Concord, California (17). A flow diagram of the
test system is given on Figure 11-32. The test SAC-MHF burned a
combination of wastewater sludge and refuse-derived fuel (RDF) in
several ratios varying from 100 percent sludge to 100 percent
RDF.
Municipal refuse was shredded, a ir-class i f iedi, and screened
to produce a refuse-derived fuel. The RDF had a heating
value of 7,500 Btu per pound (17.4 MJ/kg) of dry solids and a
moisture content of 25 percent. A combined feed rate of up to
10,000 pounds per hour (4,540 kg/hr) was applied to the 6-hearth,
16-foot 9-inch (5.1-m) diameter SAC-MHF. Because of the addition
of RDF, the heat value of the feed was greatly increased as
compared to sludge alone. This produced a fuel gas heat value
averaging 136 Btu per standard dry cubic foot (5.07 MJ/m^ dry)
and afterburner temperatures up to 2,500°F (1,371°C). Stable
furnace control was achieved by regulating the addition of air to
maintain hearth .temperature.
Results of the test indicate that to maximize energy conversion,
RDF should be fed to a mid-furnace hearth, and sludge to the top
or second hearth. In other words, the point of sludge addition
remains as in conventional systems, and the RDF is treated
like any other fuel and added to the combustion zone. The ash
handling system must be capable of handling small amounts of
metal. Test;'results indicate that autogenous combustion of
a 16 percent solids sludge cake can be accomplished with an
RDF-to-sludge wet ratio of 1:2.
11-89
-------
REFUSE
DERIVED
FU6L
ASH
HEAVY MATERIAL
TO LAHDFILL
FIGURE 11-32
FLOWSHEET FOR CO-COMBUSTION FULL SCALE TEST AT THE
CENTRAL CONTRA COSTA SANITARY DISTRICT, CALIFORNIA
This type of system is being reviewed for several plants, with
implementation expected for the Central Contra Costa Sanitary
District and the City of Memphis, Tennessee.
The flow sheet for a multiple-hearth furnace used for combustion
of sludge and solid waste is similar to Figure 11-27, except that
a refuse-derived fuel is added to the middle hearth(s). Heat
and material balances for the hypothetical treatment plant
alternatives (Table 11-8) are presented in Table 11-22. The
effect of a 20 percent sludge solids feed versus a 40 percent
sludge solids feed is again exhibited. An important item in
this table is the recoverable heat, which is four times greater
than that for other sludge-only combustion processes (see
Table 11-18). This shows the effect of the addition of refuse-
derived fuel. Also, in Case IIB, note the effect of excess air
on the afterburner temperature. With 40 percent excess air, a
temperature of 2,450°F (1,343°C) would be expected (consistent
with Cases IB and IIB); however, a temperature of 1,800°F (982°C)
occurs with an excess air rate of 150 percent.
Specific information specifically concerning co-combustion by SAC
in a MHF can be found in the literature (8,17,18,35,69,74-77,81,
89-96).
11-90
-------
TABLE 11-22
HEAT AND MATERIAL BALANCE FOR CO-COMBUSTION BY STARVED-AIR
COMBUSTION IN A MULTIPLE-HEARTH FURNACE3
Alternative
Stream
Furnace design
Diameter, ft-in.
Number of hearths
Hearth loading rate, Ib
wet solids/sq ft/hr
Sludge feed . . ' '
Lb dry solids/hr
Percent of total furnace
feed
Volatile content, percent
RDF feed
Lb dry solids/hr
Percent of total furnace
feed
Volatile content, percent?
Percent moisture
Combined feed rate
Total Ib wet solids/hr
Heat value, 106 Btu/hr
RDF to sludge ratio, wet
basis
Furnace combustion air.
Ib/hr
d
Excess air rate, percent
Ash
Mass, Ib/hr
Heat value, 10 Btu/hr
Afterburner combustion air
Mass, Ib/hr
Afterburner exhaust
Mass, Ib/hr
Heat value, 10 Btu/hr
Temperature , °F
Radiation
Heat loss, 10 Btu/hr
Recoverable heat
70 percent efficiency.
106 Btu/hr •
Connected power
Horsepower
Installed cost, thousand
dollars6
IA
5 MGD
20 percent
solids
22-3
6
11.4
1,806
50
77
7,224
50
84
20
18,060
20.28
1:1
12,753
' 40
1,749
,0.50
34,123
63,186
63.27
2,290
1.62
23
555
. 2,800
IB
5 MGD
40 percent
solids
16-9
7
10.8
2,131
50
65
4,267
• 50
'. ' ' 84
20
10,664
11.12
1:1
7,320
'. ' 40 '
1,589
.0.46
. 25,867
42,260
42.80
2,457
"i.12'.
20
'
343
5,200
IIA.,
15 MGD
20 percent
solids
25-9
6
11.8
2,713
50
77
10,850
50
84
20
27,126
30.71
1:1
': •' ,
19,316
40
2,627
0.76
' r
51,049
94,861
95.27
2,294
" 2.33
42
725
3,000 •
IIB
15 MGD
40 percent
solids
: 18-9
8
11.3
3,201
50
65
6,400
50
84
20
16,000
16.38
1:1
10,782
40
2,384
0.69
39,065
63,461
64.29
2,458
1.61
26
418
2,400
IIIA
50 MGD
20 percent
solids
25-9
9
12.5
4,292
50
77
17,172
50
84
20
42,930
47.29
1:1
29,747
40
4,158
1.20
81,838
150,355
150.9
2,294
3.57
70
725
3 , 500
IIIB
50 MGD
40 percent
solids
22-3
8
12.1
5,064
50
65
10,128
50
84
20
25,320
25.53
1:1
16,806
150
3,772
1.09
112,888
151,240
101.8
1,800
2.45
42
555
3,000
All data supplied by the Eimco BSP Division of Envirotech-Corporation.
b
Solids for B alternatives (40 percent solids feed), larger than A alternatives
(20 percent solids feed). •
Sludge volatiles heat value 10,000 Btu/lb: RDF volatiles heat value 8,500 Btu/lb.
d
For total system - furnace And afterburner. ,-.'"" , . .
Costs as of early 1978. • • „,?
1 MOD = 0.04 m /s
1 ft = 0.3 m
1 in. = 0.02 m
1 Ib/sq ft/hr =4.9 kg/m /hr
1 Ib/hr =0.45 kg/hr
1 x 106 Btu/hr = 1,055 MJ/hr
-------
EVluid Bed
Municipal solid waste and wastewater sludge have been co-
incinerated in a fluid bed furnace in Franklin, Ohio, since 1971
(97). In the solid waste separation process, a wet pulper
removes ferrous metal and heavy solids from 150 tons (136 t)
per day of shredded refuse. Fiber is recovered from the pulper
effluent by selective screening and elutriation. All unrecovered
residuals from the fiber-recovery step are conveyed to a barrel
thickener. Sludge from a 2.5-MGD (O.ll-m^/s) secondary wastewater
treatment plant is added to the thickened residuals, and the
combined stream is dewatered in a cone press to a solids content
of 45 percent before injection into the furnace. The furnace
feed is blown into the bed about one foot over the tuyeres.
Because heavy inert materials accumulate within the bed, there is
buildup in bed volume, and a small amount of bed material must be
removed periodically from the furnace. The preparation steps
reduce the amount of noncombustible material in the furnace feed
to between three and six percent and the feed size to 1/2 inch
(1.27 cm) or less (97).
In a conventional dry shredding and separation operation, the
feed stock would not be as uniform as it is at the Franklin
facility. If the feed to the fluid bed furnace is not uniform in
both size and density, heavy material tends to sift downward
through the bed. This material must be removed quickly or it
could upset the air flow through the bed. Systems have been
developed to remove settled, noncombustible material continuously
from the bed.
An FBF system using sludge cake and RDF produced by a dry
processing approach was constructed in Duluth, Minnesota, and the
shakedown operations began in 1979. A process flowsheet of the
system is presented on Figure 11-33.
11.5.3 Institutional Constraints
Co-combustion of sewage sludge with municipal solid waste is a
viable and socially beneficial approach to solids disposal
problems. Not only are both wastes disposed of in an environ-
mentally acceptable manner, but benefits can be accrued by
utilizing the waste heat or combustible exhaust gases for
energy conservation. Cost-effectiveness, however, is very
site-specific, and in general, co-combustion systems are
not economically feasible without federal and state funding
(81,98,99). This is due to the relative costs of disposal and
relative quantities of the feed material involved. For example,
solid waste quantities, dry basis, are approximately ten times
that of sludge quantities and can be disposed of at one tenth
the cost of sludge. Therefore, sludge disposal costs have a
significant impact on solid waste operations, yet solid waste is
too costly on a unit energy basis to supplement fossil fuels.
To assist in funding, the federal government has adopted
11-92
-------
guidelines for allocating costs for co-combustion systems (100).
As solid waste disposal and fossil fuel costs increase and
funding becomes more available, co-disposal economics will
become favorable in more applications. Although the technical
feasibility of co-combustion of sludge and solid waste has been
demonstrated, there remain a number of institutional constraints
that may have to be resolved prior to implementation of a large
scale co-combustion project. Because full-scale operations are
limited and the technology is growing, risk analyses should be
conducted. These analyses would provide authorities with a basis
for making a decision and with an understanding of the impacts
of that decision (101).
TO If (HJWWiHf
PROCCfS
»V. i.tH I ILUuTt EMIMION
ftSJGK/IWCF
- is*
OgHATEIHP tLUPKel
-tr "":;" :
FLUID BED
FURNACE
HIAT IMXJT ILLMXK UN -II)' BbAi
HEAT WPVr REFUIE T1.32 • 10 « Bm.^,
ASH
OPERATIC*! OF I
STEAM POWERED!
EQUIfMENT J
m-a.Mm
1r/l«ct • 2.3 lid. DRY p'ti|3
1 IU,'a4ii - P.45
llb/h •
1056 «Jli|»
FIGURE 11-33
FLOWSHEET FOR CO-COMBUSTION AT THE WESTERN LAKE SUPERIOR
SANITARY DISTRICT, DULUTH, MINNESOTA
In many localities, wastewater treatment and solid waste disposal
are controlled by different governmental agencies. Many
communities have contracts with private firms for refuse handling
and disposal that release ownership of the refuse to the
contractor. In such cases, the municipality is not able to do as
it wishes with the refuse. Some contracts are long term, lasting
15 to 20 years. Although there have been legal opinions that
these agreements can be modified for the benefit of the public,
these opinions have not been tested in court. In recent waste
11-93
-------
disposal contract negotiations, local governments have attempted
to retain ownership of the refuse, with the private firm acting
strictly as a collector and hauler. Retaining ownership of the
waste material would simplify resource recovery operations.
Consolidation of the governmental agencies responsible for
solid and liquid waste disposal would also simplify disposal
operations as they relate to co-disposal. With more emphasis
on co-combustion techniques by both federal and state agencies,
serious institutional problems may be resolved by governmental
interaction with local agencies.
11.5.4 Conclusions about Co-Combustion
Of all areas of technological growth in combustion, co-combustion
may have the greatest potential (81,102, 103) for use. Co-
combustion is a relatively new venture, and its use must
be thoroughly researched and tested, and project economics
evaluated. Many solid waste projects have failed for economic
reasons. Additionally, institutional requirements must be
satisfied before the project can reach fruition.
11.6 Related Combustion Processes Used in Wastewater
Treatment
Several high temperature processes are used in wastewater
treatment plants for purposes other than wastewater sludge
reduction. These processes include reduction of other wastewater
solids such as screenings, grit, and scum, and also the regenera-
tion of chemicals such as lime and carbon. High temperature
equipment configurations are basically the same as those
discussed in Sections 11.3 and 11.4, but some new types of
furnaces are introduced in the sections that follow.
11.6.1 Screenings, Grit, and Scum Reduction
Besides sludge, other solids produced in a wastewater treatment
plant (screenings, grit, and scum) can be processed in high
temperature systems. Some of the unique operating problems
presented by these materials are described below:
• Screenings tend to clog feed mechanisms and should be
shredded before being fed to the incinerator. Bulky and
non-combustible materials should be removed and disposed
of in a landfill.
• Grit is often odorous, extremely abrasive, normally
contains fairly large quantities of organics, and is
relatively dry, thus making it autogenously combustible
in many cases. Because of the odors, high temperature
disposal tends to be the desirable stabilization method.
11-94
-------
• Scum and grease are very difficult to handle because of
their adhesive properties; however, they have a very high
heating value up to 16,700 Btu per pound (37.8 MJ/kg) of
dry solids (Table 11-2) (104). Air flow must be adequate
to assure that the scum is totally burned; if it is not,
the furnace will smoke. To provide thorough mixing and
thus proper burning, the scum and air should be injected
into the furnace at the same point. Scum has been fed
through atomizers, but this feed system was not totally
effective because the resulting vapors and smoke have
been difficult to control (105).
A separate furnace may be difficult to justify for any one of the
above materials because their quantities, as compared with
sludge, are small. In some cases, the material can be blended
with feeds and disposed of in existing sludge furnaces. Burning
of the residues will not usually cause capacity problems.
Although scum can provide considerable heating value, it can also
create significant problems with smoking and hot spots. The
latter may damage refractory material. Screenings and grit can
also create hot spots, but they generally cause considerably
fewer problems than scum.
Complete mixing of feeds can eliminate hot spots due to
nonhomogeneity, but mixing is often difficult to achieve. The
location of the mixing step is also a serious concern. When
combined with sludge before dewatering, screenings, grit, or scum
can cause dewatering equipment problems. These can include
excessive wear, filter blocking, and poor dewatered cake release.
On the other hand, it is difficult and costly to produce a
homogeneous mixture when materials are combined after dewatering.
Since the materials are removed separately and require different
dewatering techniques, they may in many instances be disposed
of more appropriately by means other than high temperature
processing. Several plants have provided digestion for sludge,
and incineration for screenings, grit, and scum, with sludge gas
used as the fuel for the furnace. Other plants have provided
separate furnaces for scum reduction. In one plant, a separate
furnace was provided for screenings only.
Furnaces for screenings, grit, and scum in small plants (less
than 10 MGD [0.44 m^/s]), tend to be single-chamber batch
operations with little or no air emission control devices.
However, high excess air rates and large quantities of fuel are
used to make the burning relatively clean and odor-free. Such
an operation is costly. For large plants, the furnaces described
in Sections 11.3 and 11.4 are used. However, while several
multiple-hearth furnaces are used successfully for scum (106),
a starved-air combustion operation is desirable to control the
combustion process and prevent serious temperature excursions,
localized hot spots, and smoking--all typical problems when scum
is burned.
11-95
-------
To address the problem of scum burning, Nichols Engineering and
Research Corp. has developed a furnace specially suited for high
energy liquids that are lighter than water, such as grease,
waste oils, and scum. Their WATERGRATEtm furnace is shown
on Figure 11-34. It is a two-chamber, refractory-lined furnace
that uses water as the feed grate. As the material is burned,
the ash sinks and is removed. The lower chamber is a reducing
furnace (starved air), and the resulting combustible gases are
burned in the upper chamber, which functions as an afterburner,
thereby permitting better control of the process. More than
ten units have been installed and are operating. Some have
experienced severe problems with scum transport and feed systems
external to the furnace.
Other small, modular furnaces (see Section 11.7) have consider-
able potential for screening, grit, and scum reduction, provided
that pollution control devices are adequate to meet strict air
emission codes. USEPA and the State of California have been
conducting several tests on modular furnaces to determine
expected air emission levels (107).
11.6.2 Lime Recalcination
Lime is often used to remove phosphorus, suspended solids, and
trace metals from wastewater. It is generally added prior to
primary clarification (108,109) or following a biological process
(108,110). Often, energy and economic analyses indicate lime
recovery and reuse to be viable, since net lime requirements
are lower and the mass of solids for disposal is less when
recovery is practiced. There is considerable experience with the
recalcining and reuse of lime from water treatment plants. These
techniques, with suitable modifications, are also used to recover
lime in wastewater applications.
In the liquid process, the bulk of the lime reacts to form
calcium carbonate (CaC03). The resultant slurry, commonly
called lime sludge, can be thermally processed for recovery of
calcium oxide (quicklime or CaO), while simultaneously oxidizing
any entrapped organic solids. The recalcining reaction is:
CaC03 + heat -^ CaO + C02 11-17
The economics of lime recalcination as a chemical recovery
process depend upon a number of variables: efficiency of
rejection of inert material, moisture content of feed material,
thermal efficiency of the drying and recalcining system, capture
of CaO as a usable product, and reactivity (capture of CaO) in
the product (111) .
11-96
-------
THERMOCOUPLE
ACCESS DOOR
COMBUSTION AIR
INLETS (4)
THERMOCOUPLE
COMBUSTION TEMPERATURE
BETWEEN 1600 AND16QQ°F
COMBUSTION TEMPERATURE
iETWEEN 1400 AND 1600°F
CASTABLE
REFRACTORY
INSULATION
CIRCULAR STEEL
SHELL
REFRACTORY
BAFFLE
PACKAGE
AUXILIARY
FUEL SYSTEM
IGNITION AIR
INLET
MECHANICAL CRUST
BREAKER (RAKE)
FEED INLET
MAKE-UP WATER
COURTESY NICHOLS ENGINEERING AND RESEARCH CORPORATION
FIGURE 11-34
CROSS SECTION OF THE WATERCRATEtm FURNACE FOR SCUM INCINERATION
11-97
-------
Economies are realized when lime is recovered and reused, since
net lime requirements and the amount of material to be disposed
of are drastically reduced. However, lime recovery is expensive
and always energy-intensive because recalcining is endothermic.
Generally, wastewater lime sludges are low in organic material
(volatiles) that can contribute to the heat value of the sludge,
so supplemental fuel requirements to calcine the wet sludge cake
are substantial. The major operating cost of recalcination is
supplemental fuel. Fuel cost can be minimized by control of
excess air at a rate no greater than that required to assure
complete combustion and completion of the chemical reaction.
Fuel costs may also be lowered by reducing the water content of
the feed. An overall economic balance must be made to determine
if the fuel savings exceed the added cost of dewatering.
Complete recovery of spent lime cannot be expected for several
reasons. Lime sludge contains inert materials that must be
wasted from the system or the quantity of sludge to be handled
will build-up infinitely. Magnesium hydroxide and calcium
phosphate are precipitated along with CaCC>3 and should be
removed prior to recalcination to reduce recycle of inerts.
Complete rejection of Mg(OH)2 and other inerts, such as silica,
can never be achieved. However, wet and dry classification steps
can limit recycle of inerts, thus providing a relatively clean
product. These classification steps necessarily reject some
CaCC>3 and CaO, so that the recovery of available lime is limited
to 60 to 77 percent (108,112).
Three high temperature systems have been used for lime
recalcination: the multiple-hearth furnace, the fluid bed
furnace (pellet bed and sand bed), and the rotary kiln calciner.
It has also been claimed that the electric furnace has the
capability to recalcine, but no installations exist. The
multiple-hearth furnace is most frequently used in wastewater
treatment plant sludge recalcining, while the fluid bed furnace
is typically used in water treatment plants. Both the rotary
kiln and the fluid bed are widely used on industrial sludge,
primarily by the pulp and paper industry. As with other high
temperature processes, opportunities for energy conservation
and heat recovery are available. A detailed discussion of
lime recalcination is beyond the scope of this chapter. More
information is available in the literature (108-119).
11.6.3 Activated Carbon Regeneration
The use of activated carbon for removal of organic contaminants
from water and wastewater is an established practice. In most
applications, regeneration and reuse of spent carbon are required
for overall cost-effectiveness. Most carbon absorption processes
use granular carbon in packed columns. There is a growing
interest in the addition of powdered carbon to unit processes
such as activated sludge systems. Table 11-23 summarizes the
methods available for carbon regeneration (reactivation).
11-98
-------
TABLE 11-23
CARBON REGENERATION METHODS (120)
Granular Powdered
Thermal
Multiple -hearth
Fluid bed
Transport reactor
Rotary kiln
Indirect heated
vertical moving bed
Radiant heated belt
reactor
Chemical
Wet air oxidation
Chemical oxidation
Solvent extraction
X
L
, NA
X
X
X
NA
X
X
X
X
X
L
NA
X
X
NA
NA
Acid or base extrac-
tion . X NA
Biological regenera-
tion L L
X = has been done on pilot or full-scale.
L = limited success
NA = not attempted.
Typical granular and powdered carbon systems are briefly
summarized below. Also, the JPL process for carbon reactivation
in a wastewater treatment plant is discussed.
11,6.3.1 Granular Carbon Systems (GAG)
Regeneration of granulated carbon (121,122) is usually conducted
in a multiple-hearth furnace in five steps: dewatering the
slurry to about 50 percent solids, drying the carbon, pyrolyzing
the absorbed organics, oxidizing the pyrolysis residue (carbon
reactivation), and quenching the reactivated carbon in water and
washing it to remove fines.
In a multiple-hearth furnace, about 30 minutes are required
for regeneration, with dwell times of 15 minutes for drying,
5 minutes for pyrolysis, and 10 minutes for reactivation.
Loading rates for multiple-hearth furnaces must be adjusted
to provide about 1 square foot (0.09 m2) of hearth area per
40 pounds (18 kg) of spent carbon per day. The off-gases from
11-.99
-------
a carbon regeneration furnace are relatively high in carbon
particles and unburned organics. Afterburning and wet scrubbing
are suggested. The injection of steam at 1 pound per pound of
carbon (1 kg/kg) reduces the apparent density of the carbon and
increases the iodine number. Heat required for the process,
including steam but excluding any afterburner fuel requirements,
is approximately 4,250 Btu per pound (9.88 MJ/kg) of carbon
regenerated. Further details on the MHF regeneration process can
be found in the literature (123).
The electric furnace is also becoming an alternative for granular
carbon regeneration, with several units either under construction
or in the planning stages. A test unit is being installed in
Pomona, California, to develop detailed long-term data.
11.6.3.2 Powdered Activated Carbon (PAC)
During the regeneration of powdered carbon, organics must
be removed from the micropores, and since PAC is generally
associated with excess waste biomass, these solids must be
incinerated simultaneously (120,124). Also, PAC is much smaller
in particle size than GAC and must be handled with care during
combustion to prevent excessive losses and excessive loadings of
particulates on emission control systems.
Multiple-hearth systems have been used successfully to regenerate
PAC (123,125). MHF-regenerated carbon appears to be of virgin
quality and has been reused in a 40-MGD (l.VS-m^/s) plant.
Available data on a 50-gpm (30-1/s) pilot plant indicate similar
results with fluid bed technology (126).
Use of the transport reactor has been demonstrated on a
10-ton-per-day (9-t/day), full-scale facility with a recovery of
80 to 90 percent of the spent carbon. This reactor is a fast
co-current thermal plug flow system (127,128). The unit is
operated for regeneration of spent carbon from corn syrup
manufacturing.
11.6.3.3 Jet Propulsion Laboratory Activated-Carbon
Treatment System (JPL-ACTS)
Extensive laboratory and pilot testing by the Jet Propulsion
Laboratory in Pasadena, California, has led to the development of
an activated carbon treatment system for wastewater (129,130).
The system is based on starved-air combustion of sludge. All PAC
used in this process is produced by the SAC of sewage sludge and
lignite coal. The system was tested for the Orange County
Sanitary District in a 1-MGD (0.04 m3/s ) pilot plant at
Huntington Beach, California.
The flowsheet for the Orange County plant is shown on
Figure 11-35. Sludge from the primary sedimentation tank is
dewatered in a filter press to 35 percent solids. The sludge
cake is flash-dried to 90 percent solids before passing into the
11-100
-------
rotary kiln. Activated carbon and ash are generated by starved-
air combustion of the carbon-sludge solids. Activated carbon-ash
mixture is fed back to the secondary clarifier to complete the
carbon cycle. A portion of the carbon-ash is purged from the
kiln to prevent build-up of inert materials. The energy value of
the purged carbon can be recovered in a separate furnace.
CARBON + SEWAGE SOLIDS
RAW
SEWAGE
( ~ 5% SOLIDSf
(DEGRITTED)
PRIMARY
CLARtFIER
GRAVY FILTER
(MIXED MEDIA)
EFFLUENT
SEWAGE
SOLIDS -i-
CARBON
FINES
ACTIVAT1D CARBON
(10% SLURRY)
CARBON •*• SEWAGE SO LIDS
!~,5%SQL!DSt
FilTER PRESS
| DEBATE RING)
35-40%
SOLIDS
FLASH
DRYER
90%
SO Li OS
THERMAL
REGENERATION
UNIT
*
I
LIGNITE COAL
(MAKEUP ENERGY
+ CARBON)
ASH
PURGE
FIGURE 11-35
JPL ACTIVATED CARBON TREATMENT SYSTEM (129)
Various practical problems (primarily corrosion at high
temperatures) associated with the kiln and flash-dryer have
caused the developers to substitute a multiple-hearth furnace for
these two system elements. No actual test work with the MHF has
been done to date.
Activated carbon makeup requirements are dependent on adsorption
characteristics. Under some circumstances, the carbonized sludge
can satisfy the makeup requirements. Otherwise, activated carbon
makeup is necessary. Lignite coal is a source of low ash carbon
with an adsorptive capacity comparable to commercial activated
carbons. Lignite coal also provides, at low cost, the necessary
makeup energy to the system.
Preliminary economic studies by the developers indicate that the
JPL-ACTS process for wastewater treatment is competitive with
activated sludge for plant flows exceeding 175 MGD (7.67 m3/s).
11-101
-------
11.7 Other High Temperature Processes
There are a number of high temperature conversion processes that
differ substantially .from those previously discussed. Some are
presently being used for combustion or co-combustion of
wastewater sludge, and others are claimed suitable for sludge
processing. These processes include:
• High pressure/high temperature wet air oxidation
• REACTO-THERMtm (Met-Pro Corporation, Systems Division)
• Modular controlled-air incinerators for co-disposal
(Consumat, Kelly, and others)
Also, numerous thermal processes are being developed, mainly
of the pyrolysis or starved-air combustion type; which are
applicable to wastewater sludge or mixtures of sludge and solid
waste (Table 11-24). These processes are potentially important
because they produce a high heating value fuel gas that may be
directly usable in existing furnaces and burners. Of the true
pyrolytic processes (thermal decomposition in the absence of
air), only the Pyro-Sol process appears to be sufficiently
developed to be considered here for co-disposal and perhaps
sludge disposal. Some of the processes shown in Table 11-24
have been discussed previously (PUROX and Andco-Torrax). Other
developing processes with potential for sludge burning include
the Bailie process, the Wright-Malta process, and Molten Salt
pyrolysis.
11.7.1 High Pressure/High Temperature Wet Air Oxidation
Any burnable substance may be oxidized in the presence of water
at a sufficiently high temperature (flameless combustion).
Therefore, this process can be an alternative to incineration
while providing a similar ash residue (134).
The high pressure/high temperature wet air oxidation process
(HPO) is similar to thermal conditioning, except that higher
temperatures and pressures and much more air are used to effect
complete oxidation. Figure 11-36 is a composite representation
of results of wet oxidation for a typical sewage sludge, showing
volatile solids content or COD content in the solid phase and
the total sludge as a function of total oxidation in both phases.
The vertical distance between the two curves is the content
in the liquid phase. Up to about 50 percent total oxidation,
reduction in the volatile solids or COD in the liquid phase are
minimal; above 50 percent, the volatile solids and COD of both
phases are reduced to low values. At 80 percent total oxidation,
about 5 percent of the original total volatile solids in the
sludge is in the solid phase and 15 percent is in the liquid
phase.
11-102
-------
TABLE 11-24
BASIC TYPES OF PYROLYS1S, THERMAL GASIFICATION, AND
LIQUEFACTION REACTORS - NEW, DEMONSTRATED, OR UNDER
DEVELOPMENT (131,132,133)
Main products
Solids flow and
bed conditions
Examples of processes,
developers, R&D programs
Vertical-flow reactors
Moving packed bed Forest Fuels Mfg., Inc. (Antrim,
(gravity solids flow; N.H.)
also called fixed bed) Battelle Northwest (Richland, WA)
American Thermogen (location un-
known)
Andco/Torrax Process (Buffalo, NY)
H.F. Funk Process^ (Murray Hill,
NJ)
Tech-Air Crop/Georgia Inst. Tech.
(Atlanta, GA)
Union Carbide Purox Process
(Tonawanda, NY)
Motala Pyrogas (Sweden)
Urban Research & Development
(E. Granby, CT)
Wilwardco, Inc. (San Jose, CA)
U. of California (Davis,-CA)
Foster Wheeler Power Products
(London, England)
Destrugas Process (Denmark)
Koppelman Process (Encino, CA)
Moving stirred bed BSP/Envirotech (Belmont, CA)
(gravity solids flow) Nichols Research & Engr. (Belle
Mead, NJ)
Garrett Energy Research & Engr.
(Claremont, CA)
Hercules/Black, Crow & Eidsness
(Gainesville, FL)
Moving entrained bed
(may include
mechanical bed trans-
port)
Fluidized reactors
Horizontal and inclined
flow reactors
Tumbling solids bed
Occidental Petroleum Co./Garrett
Flash Pyrolysis Process (La
Verne, CA) •
Copeland Systems Inc. (Oak Brook,
IL)
Coors Brewing Co./U. Of Missouri
(Rolla, MO)
Energy 'Resources Co. (ERCO)
(Cambridge, MA)
Hercules/Black Grow & Eidsness
(Gainesville, FL)
Bailie Process/Wheelabrator
Incin. Inc. (Pittsburgh, PA)
A.D. Little Inc./Combustion
Equipment Assoc. (Cambridge,
MA/New York, NY)
Devco Management Inc. (New York,
•NY)
Monsanto Landgard/City of
Baltimore, MD Watson Energy
Systems (Los Angeles, CA)
Feedstock
Fuels or char
materials
Steam
FAR
Refuse
Refuse
Refuse
Refuse
FAR
Refuse, FAR
Refuse
Refuse
FAR, sludge
FAR
Refuse, tires
Refuse
FAR
Sludge, refuse
Sludge, wood
Manure
Refuse
X
-
_
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
-
-
-
_
-
-
-
-
-
-
X
X
-
-
Refuse
Sludges
Refuse, FAR
Refuse, FAR
Refuse
Refuse
Refuse
Refuse
Refuse
11-103
-------
TABLE 11-24
BASIC TYPES OF PYROLYSIS, THERMAL GASIFICATION, AND
LIQUEFACTION REACTORS - NEW, DEMONSTRATED, OR UNDER
DEVELOPMENT (131,132,133) (Continued)
Main products
Solids flow and
bed conditions
Horizontal and inclined
flow reactors (con-
tinued)
Agitated solids bed
Examples of processes,
developers, R&D programs
Ecology Recycling Unlimited, Inc.
(Santa Fe Springs, CA)
Pyrolenergy System/Arcalon
(Amsterdam)
Pan American Resources, Inc.
(West Covina, CA)
Kobe Steel (Japan)
JPL/Orange County, CA (Fountain
Valley, CA)
Rust Engineering (Birmingham, AL)
Tosco Corp/Goodyear Tire and
Rubber (Los Angeles, CA/Akron,
OH)
Deco Energy Co. (Irvine, CA)
Enterprise Co. (Santa Ana, CA)
Kemp Reduction Corp. (Santa
Feedstock
Refuse
Refuse, FAR
Refuse, FAR
Tires
Sludge
Refuse, sludge
Tires
Tires
Refuse
Refuse, FAR
Fuels or char
materials
X
X
X
X
X
X
X
X
X
X
Steam
-
-
-
-
-
-
-
_
-
-
Static solids bed
Molten metal or salt
beds
Floating solids bed
(horiztonal flow)
Mixed molten-salt
bed (various
possible flow
schemes)
Barbara, CA)
PyroSol (Redwood City, CA)
Thermex, Inc. (Hayward, CA)
Michigan Tech. U. (Houghton, MI)
(Puretech Pyrolysis System)
Battelle Northwest (Richland, WA)
Anti-Pollution Systems, Inc.
(Pleasantville, NJ)
Multiple-reactor systems
Combined entrained- U. of California (Berkeley, CA)
bed/static-bed
reactor system
Combined moving Battelle Columbus Laboratories
packed-bed/entrained- (Columbus, OH)
bed reactor
Combined mechanically
conveyed static-
solids-bed/moving
packed-bed reactor
Mansfield Carbon Products, Inc.
(Gallatin, TN)
Fluff from
scrapped autos
Refuse, FAR
Refuse
Refuse, sludge
Pulping liquor
Paper, biomass
Refuse
Forestry and/or agricultural residues.
Pressure above atmospheric.
11-104
-------
100
10 20 30 40 50 60 70 80 90 100
OXIDATION- %
COURTESY ZIMPRO INC.
FIGURE 11-36
VOLATILE SOLIDS AND COD CONTENT OF HEAT TREATED SLUDGE
The degree to which organic materials are oxidized is a
function of temperature, reaction time, and quantity of air
(or oxygen) supplied. The process may be applied to dilute
suspensions of sludge requiring only thickening. However, the
solids content should be four to six percent to minimize reactor
volume requirements and to maintain a thermally self-sustaining
reaction. Solids concentrations greater than about 10 percent
create problems with mixing and consequent mass transfer
of the oxygen. There is insufficient data to indicate any
advantage from use of pure oxygen rather than air as the oxidant
source.
11-105
-------
The high pressure/high temperature wet air oxidation process is
shown schematically in Figure 11-37. Thickened sludge, at about
six percent solids, passes through a grinder to reduce the size
of all feed solids to less than 1/4 inch (0.64 cm).
SLUDGE
GROUMD
SLUOGi
HEAT
EXCHANGER
GRINDER
AIR
siui-
TA
c
A tit
SLUDGE
j I |_ \
SLUDGE HIGH
FEtO
PUMP PRESSURE
SLUDGE PUMP
(POSITIVE
OlSPLACEMEMTJ
Alft
D
of—1
3| t
-I 1
" I
s
ot
X
REACTOR
STEAM
SLUDGE
INJECTION
AIR
AIR COMPRESSOR
STERILE
NGN -PUTS ESC IB Li
SOLIDS
ALTERNATE METHODS
OF DEWATERING
FILTER PRESS
VACUUM FILTER
CENTRIFUGE
BOILER
(START-UP
STEAM*
DRAINAGE iEDS
LAGOONS
LiViL
CONTROL
VALVE
OXIDIZED
SLUDGE
SLURRY
SUPERNATANT
(1) WET SCRUBBING, CARBON
AiSOHPTION, OR AFTERBURNING
COURTESY ZIMPRQ INC.
FIGURE 11-37
FLOWSHEET FOR HIGH PRESSURE/HIGH TEMPERATURE WET AIR OXIDATION
The slurry is then pressurized. The air quantity supplied is the
stoichiometric amount required for complete oxidation of the
combustible sludge solids (about 7.5 Ib per 10,000 Btu) (2 g/J).
The pressure applied must be sufficient to prevent the water from
vaporizing at the temperature selected for the reaction.
The sludge-air mixture is then passed through a heat exchanger,
where it is heated to close to the desired reaction temperature
by the reactor effluent stream and introduced into the reactor
for oxidation. Temperatures and pressures up to 500°F (260°C)
and 1,000 to 1,800 psig (6,895 to 12,411 kN/m2) are used with
detention times of 40 to 60 minutes. The oxidized slurry is then
cooled in the heat exchanger, gases are removed in a vapor-liquid
11-106
-------
separator, and the gases are reduced to atmospheric pressure
through a pressure control valve. The gases are processed to
eliminate odors. They consist mainly of oxygen, nitrogen, carbon
dioxide, and water vapor. Nitrogen oxides are formed from the
organic nitrogen present in the feed, but no nitrogen is fixed
from the air. Elemental sulfur, hydrogen sulfide, and organic
sulfur compounds are oxidized to sulfate (S04). Gas clean-up
methods have included wet scrubbing, activated carbon absorption,
afterburning with fossil fuel, and catalytic oxidation. With
the last two methods, energy recovery is possible through use of
heat recovery boilers, gas-liquid heat exchangers, and similar
methods (135-137).
Slurry from the gas-liquid separator is removed through a
liquid-level control valve and dewatered for final disposal. At
high degrees of oxidation, the residual solids resemble ash from
thermal incineration and are easily dewatered to a high solids
content by conventional means (settling, centrifugation, or
vacuum filtration).
The liquid phase is recycled to the treatment plant or given
separate treatment for reduction of the residual soluble
organics. Treatment and effects of this liquid stream are
discussed in Chapter 16.
High pressure/high temperature wet air oxidation processes
generate excess heat when they operate with a high heating
value sludge and an adequate solids content (approximately
six percent). Still, a source of high pressure steam (separate
boiler or an existing plant system) must be provided for
start-up.
There are over ten HPO systems in operation on sewage sludge in
the United States. The most notable of these are Rockland County
and Rensselaer, New York, and Akron, Ohio. These units operate
at approximately 500°F (260°C) at pressures of 1,000 to 2,000 psi
(6,895 to 13,790 kN/m2). The capacities of the units as well
as the sludge oxidized are very different in each of these plants
Rockland County processes 12.4 tons per day (11.3 t/day) of a
mixed digested primary plus waste-activated sludge. The Akron
facility (Botzum Plant) oxidizes 50 tons per day (45 t/day) of
waste-activated sludge. The Rensselaer facility oxidizes a more
conventional mixture of primary plus waste-activated sludge.
Shutdowns with HPO systems are associated with the high pressures
involved, heat exchanger scaling and corrosion, and required
supernatant liquid treatment. The HPO process may provide a good
system for oxidation of toxic and hazardous waste materials, and
research in this area is under way (138).
Lack of extensive operating data prevents reliable estimation of
the cost of HPO as a means to sludge disposal. It appears that
if equipment maintenance and replacement costs are reasonable,
the costs would be competitive with thermal processing. The only
11-107
-------
additional element of cost is treatment of the recycle stream.
Electrical energy requirements are shown in Figure 11-38.
Additional information can be found in the literature (134-139).
E
ID
fj
It
^
^_
£
g
•o
o
o
UJ
a
LU
CE
|£
LU
§
a.
y
DC
Q
111
_l
ut
PRIMARY +
WASTE
ACTIVATED
SLUDGE
WASTE
ACTIVATED
SLUDGE
ASSUMPTIONS:
SLUDGE FEED
PRIMARY + WAS = 4,6% SQUOS,
«9* VQLATILES
WAS -"3.5* SOUDS, §0% VOLATILES
VOLATILiS - 10,000 Btu/lb
REACTOR PRESSURE
PRIMARY + WAS « 1,700 pi%
1,800
CONTINUOUS OPERATION
INCLUDES:
PRiSSURlZATtON PUMPS
SLUDGE GRINDERS
DECANT TANK
BOILER FEED PUMPS
AIR COMPRESSORS
TYPE OF ENERGY REQU RED; ELECTRICAL
NOTE; FUEL IS REQUIRED
ONLY AT START-UP
1,0
3 4 G §7 6910
3456 7891QO
3456 7891,000
TREATMENT CAPACITY, gpm (1 gpm = 0.06 l/s)
FIGURE 11-38
WET AIR OXIDATION - ELECTRICAL ENERGY REQUIREMENTS (36)
Another HPO unit presently being tested for feasibility with a
feed of sewage sludge is the Vertical Tube Reactor (VTR). This
is a deep-well type of process in which the required pressure is
obtained simply by the depth of the well. The Municipal Environ-
mental Research Laboratory of USEPA is conducting test work with
a VTR system in Colorado. Data should be available in 1980.
11-108
-------
11.7.2 REACT-0-THERMtm
This is a three-stage combustion device with SAC in the first two
stages followed by complete combustion in the third stage. This
proprietary system developed by Met-Pro Corporation, Systems
Division, is unique in that auxiliary fuel and air are burned in
the primary combustion chamber (first stage) and the resulting
gases pass into the rotary chamber, where the sludge is burned.
The interior design of the rotary kiln second stage recovers the
heat generated in the first stage and transmits this heat through
a stainless steel helix and chains to the sludge. The residue,
which contains some combustibles, is deposited into a fixed,
cylindrical ash chamber, where it is removed by an auger. The
gases from the rotary chamber flow into the secondary combustion
chamber (third stage), and air and fuel are added as required to
complete the combustion of the gases and destroy odors prior to
discharge to atmosphere. The unit is available as a complete,
skid-mounted package (see Figure 11-39). The unit is primarily
designed for low-volume applications (50 to 300 gallons per hour
of wet sludge [0.05 to 0.30 1/s]). Two units are presently
operating on a physical-chemical sewage sludge in Prudhoe Bay,
Alaska.
SECONDARY
COMBUSTION
CHAMBER
PfliMAHY
COMBUSTION CHAMBER
ASH
CONVEYER
HiAT TH&NSfiR
MEDIA
ROTARY CHA«6£R
DRIVE
COURTESV METJPRQ COITORATIOM, SYSTEMS DIVISION
FIGURE 11-39
_tm
REACT-O-THERM SLUDGE/LIQUID WASTE DESTRUCTION
11-109
-------
Detailed heat and material balances are available from the
manufacturer for specific applications. Emission test data from
the manufacturer indicate that the unit, operated at rated
conditions, can meet USEPA's New Source Performance Standards.
However, the New Source Review Rule may be applicable in
some areas and Best Available Control Technology (BACT) may be
required.
11.7.3 Modular Starved-Air Incinerators
Modular controlled-air incinerators are static, and contain
two-chambers. The first chamber is operated by starved-air
combustion, and the gaseous products of combustion are passed
to the second chamber where combustion is completed and odors
are destroyed (see Figure 11-40) (107). A number of these
incinerators have been installed for municipal and industrial
solid waste. There are also units under study for co-disposal of
municipal refuse and sewage sludge (141). There are no known
installations (or test data) for sludge alone. However, the
unit appears to be suitable for sludge reduction. The units
are available in modules from 60 pounds per hour (27 kg/hr) to
250 tons per day (227 t/d). Equipment manufacturers (Consuroat,
Kelley, and others) state that USEPA New Source Performance
Standards can be met without additional air pollution control
equipment; however, the New Source Review Rule may be applicable
in some areas and BACT may be required. A test program being
funded jointly by the EPA and the State of California is
currently underway at Little Rock, Arkansas, to obtain definitive
air emission data on municipal solid waste incineration. Further
information on controlled-air incinerators is included in the
literature (107,140-145).
11.7.4 Pyro-Soltm Process
The Pyro-Sol process is a pyrolysis project presently operating
on solid waste. In the Pyro-Sol process, waste is fed to a
pyrolysis unit which, in the absence of oxygen and in the
presence of heat, causes chemical decomposition of the waste.
Products of the process are a gas and char/ash residue. A 50 to'
75-ton per day (45 to 68 t/d) (MMR), full-scale plant is in
operation in Redwood City, California. A flowsheet of that
system is presented in Figure 11-41.
The process is autogenous, but heat-up and standby energy is
provided by natural gas. A portion of the produced gas is burned
in eight radiant heat tubes to provide heat for the endothermic
pyrolysis process. The solids are fed by an airlock and moved
through the furnace by means of a vibrating conveyor.
The resulting gas (largely hydrogen [H2J and carbon monoxide
[CO]) exits from the pyrolyzer at approximately 1,100°F (543°C)
and less than 0.5 inches of water column (125 N/m2) and enters
11-110
-------
SEE NOTE
BELOW
AFTERBURNER
SECONDARY
AIR SUPPLY
COMBUSTION AIR SUPPLIED
AT HIGH VELOCITIES
CONTROL
Csl Fim Mijor Cwiflgutatkkn
(b) Second M*|&r
-SEE NOTE
BELOW
SELF-SUSTAINING
DIRECT-FLAME AFTERBURNER
FORCED AIR
Third Moiii- Cuil1l«unilijn
NOTE: STACK TO ENERGY RECOVERY EQUIPMENT
AND/OR EMiSSfON CONTROL DEVICE
OF NECESSARY)
FIGURE 11-40
MODULAR CONTROLLED-A1R INCINERATOR CONFIGURATIONS (140)
11-111
-------
a dry cyclone where the particulate matter larger than 10 microns
is removed. The hot gas is pulled through a wet scrubber/
quencher where the remaining particulates are removed. The
small amount of water that circulates to- the scrubber/quencher,
receives primary and secondary treatment, including filtration,
before disposal to the plant sewer or to an on-site treatment
plant. The scrubbed gas has a heating value of 400-500 Btu/cu ft
(14.9 to 18.6 MJ/m3).
MAKE-UP WATER
TRFATMFMT
ENCLOSURE
Tr TFFIf B MklU; ICVrirvSlE SCSUaBE3
roN,'FY-!=l I BLLWLJ
VIE RSI ING BtD
COURTtET FYBO SQL l«CORPOm4TEC-
FIGURE 11-41
PYRO-SOL LIMITED PYROLYSIS SYSTEM
The gas is transferred to a surge tank and fed from there to a
steam boiler. The steam can be used as process steam or to drive
a turbo-generator.
Pyro-Sol, with feeds of up to 50 percent moisture, can achieve
a net energy production of 60 percent of the input heat value
in the fuel gas. Due to the high recovery of input heat value
with relatively wet cakes, as compared with normal solid waste,
this process should be amenable to co-disposal and possible
sludge combustion.
11-112
-------
11.7.5 Bailie Process
The Bailie Process integrates a combustion fluid bed furnace
with a pyrolysis fluid bed reactor (146-147). The process, shown
on Figure 11-42, involves feeding solid waste into the pyrolysis
fluid bed reactor. The endothermic pyrolysis reaction is
maintained in the 1,300 to 1,500°F (704-816°C) range by recycling
hot fluidized sand from the combustion reactor. The fuel for the
combustion reaction is contained in the same recycle from the
pyrolytic reactor and from char collected in the combustion and
pyrolysis gas cyclones. Some of the pyrolysis gas is returned to
the pyrolytic reactor to control reaction kinetics. Both excess
pyrolysis gas and char may be recovered.
COMBUSTION
PRODUCTS
TO STACK
OQMSUSTION
FLUID
1ED
REACTOR
A|R BLOWER
PYflOLVSlS
GAS
PRODUCT
PVflOLVSlS GAS
RECYCLE BLOWER
FIGURE 1.1-42
BAILIE PROCESS FLOWSHEET (146)
The Bailie Process is a potentially important method of sewage
sludge pyrolysis. Less auxiliary fuel is needed for incineration
of the sludge, and a number of energy recovery options are
available. Heat from the off gases can be recovered and a
combustible fuel gas is generated.
The Bailie Process is patented and has been piloted. No
full-scale test has been conducted, but the manufacturer states
that one is planned in the near future.
11.7.6 Wright-Malta Process
The Wright-Malta Corporation (W-M) is developing a pressurizied
rotary kiln gasifier-gas turbine system for generating electric
11-113
-------
power from municipal solid waste and wastewater sludge (148,149).
Figure 11-43 shows the process in terms of energy flows. The
pressurized gasifier produces a hot, low heat value fuel gas that
is combusted and fed directly to the gas turbine. The turbine
drives an electrical generator and the associated air compressor.
The hot exhaust is used to preheat the sludge and to raise steam
temperature in a heat recovery boiler. The steam is superheated
and passed back to the kiln, where it cools and condenses,
supplying heat for the gasification process.
3310
COMBUSTION
CHAMiER
10 MW
GENfiRATOR
NOTE: ALL UNITS IN 10* Biu/day |106B MJ/d»y)
UNLESS NdfiP
FIGURE 11-43
WRIGHT-MALTA PROCESS FLOWSHEET (150)
Wastewater sludge contributes about four percent of the organic
fuel to the system. At the pressures involved, the water
evaporated from the sludge provides motive force for the turbine
in addition to the products of combustion from the fuel gas
produced. The cycle is comparable to the combined cycle system
used in electrical power generation, where hot gas turbine
exhaust flows to a boiler to produce steam. The turbine exhaust
11-114
-------
in the W-M process generates steam in the kiln. This steam,
along with the burned fuel gas, drives the turbine. The
resulting fuel efficiency .is close to the combined cycle
efficiency. This process appears ideal for very moist fuels, and
the high moisture content of the sludge is beneficial. The
Wright-Malta process has been operated in a batch mode on a bench
scale. Further progress depends on develpment of a rotary kiln
that can be operated at high pressures and temperatures.
11.7.7 Molten Salt Pyrolysis
Bench-scale studies were conducted by Battelle-Pacific Northwest
Laboratories on the pyrolysis of refuse in molten sodium
carbonate (150). The products of reaction were studied for
different conditions with steam, air, and oxygen as the gasifica-
tion agents. While the processing of municipal refuse in
the molten salt (sodium carbonate) reactor was found to be
technically feasible, the lack of a cost-effective method of ash
removal and the problems of refractory degradation have hindered
further development. This type of process is not new. However,
no information is available as to the applicability of the
process to sludge disposal.
11.8 Air Pollution Considerations
In any combustion process, air emissions are a major concern and
may be the most difficult and costly environmental consideration
to satisfy. On the federal level, the USEPA has established
standards of performance for municipal incinerators (solid waste)
and wastewater sludge incinerators. In co-combustion schemes
involving municipal solid waste and wastewater sludge, both
standards will probably apply, with allowable emissions being
prorated according to the fractions of energy in the solid waste
and in the sludge. In September, 1978, the USEPA published
proposed emission standards for new, modified, or reconstructed
electric utility steam generating units that burn fossil fuel or
a combination of fossil fuels and other fuels such as solid
wastes. These guidelines offer some indication of air pollution
requirements in co-combustion schemes.
Generally, these guidelines indicate that new sludge furnaces
will have to comply with the following standards:
• National Ambient Air Quality Standards (State Implementa-
tion Plans).
• National Emission Standards for Hazardous Air Pollutants,
subparts A and E.
• Standards of Performance for New Stationary Sources,
parts A, 0, and probably E, if co-combustion is proposed.
• New Source Review Rule.
11-115
-------
• Regulations Pertaining to Prevention of Significant
Deterioration of Air Quality.
In all cases, the minimum standards are set by the USEPA.
However, state and local jurisdictions may promugate stricter
standards.
A basic problem in evaluating any emission is predicting the
effect on the overall air basin. Projecting emissions and
estimating resulting air quality is, at best, an imperfect
science. Air basins in which critical air quality levels are
consistently exceeded have been studied in depth and have been
the object of mathematical modeling. The results of these
efforts have been mixed.
11.8.1 National Ambient Air Quality Standards (NAAQS)-
State Implementation Plans (SIP)
Federal air quality regulations are derived from the Clean Air
Act Amendments of 1970, the Energy Supply and Environmental
Coordination Act of 1974, and most recently, the Clean Air Act
Amendments of 1977 (151). The NAAQS established threshold levels
of air pollutants below which no adverse effects would occur.
These levels were designed to provide an adequate margin of
safety so as to protect the public health.
Air pollutants are classified into two groups: primary pollutants
and secondary pollutants. Primary pollutants are those emitted
directly from sources, while secondary pollutants are formed by
chemical and photochemical reactions of primary pollutants with
the atmosphere, as shown on Figure 11-44. Primary pollutants
include carbon monoxide (CO), hydrocarbons (organic gases),
oxides of nitrogen (NOX), sulfur dioxide (SC^), total suspended
particulates (TSP) and lead (Pb). Photochemical oxidants and
nitrogen dioxide (NC>2) are the principal secondary pollutants.
These form a visible brown-yellow haze. The quantity of
secondary pollutants is dependent on the availability of sunlight
as much as on the availability of primary pollutants. Health
effects of contaminants are summarized in Table 11-25.
The 1970 Amendments to the Clean Air Act required each state to
develop its own State Implementation Plans (SIP) to meet the
federal standards by 1975 or 1977, the date dependent on the
severity of the state air quality problems. The 1977 Amendments
extended the attainment deadlines and detail some appropriate
control measures. For those areas which have not yet attained
NAAQS, states must have approved implementation plan revisions by
July 1, 1979, which provide for attainment by December 31, 1982.
If a state demonstrates that such attainment is not possible, it
must submit a second plan revision by December 31, 1982, which
provides for attainment by December 31, 1987. For areas already
meeting NAAQS standards, implementation plans must include a
program to prevent significant deterioration of air quality.
11-116
-------
The USEPA guidelines require the SIPs to provide for emission
controls, transportation controls, source monitoring, ambient
air quality monitoring, and procedure for review and approval
of new sources of air pollution prior to construction. The
USEPA has the authority to approve or disapprove these plans
and to promulgate an acceptable plan if the submitted plan is
disapproved. The USEPA, state air resources boards and local air
quality management districts also have the authority to restrict
issuance of permits for construction of stationary sources if
emissions from that source would cause a violation of any air
quality standards. This is accomplished by an emission offset
policy. In both nonattainment and nondegradation areas, major
stationary sources may be constructed only by permit and must at
least meet applicable new source performance standards.
REGULATED VIA STATE
IMPLEMENTATION PLANS
(LIM«T POLLUTANTS TO
PROTECT PUBLIC HEALTH!
PRIMARY
POLLUTANTS
SUNLIGHT
SECONDARY
POLLUTANTS
11.8.2
NATIONAL AMBIENT AfR QUALITY
STANDARDS
{ CLEAN AiR AMENDMENTS OF 1977 I
FIGURE 11-44
AIR EMISSIONS
National Emission Standards for Hazardous
Air Pollutants (NESHAPS)
Subpart A of NESHAPS (40 CFR 61) comprises general provisions
covering definitions, applications, reporting, and waivers.
Subpart E deals with mercury emissions and applies to all opera-
tions that burn or dry wastewater sludge. The NESHAPS standard
(Federal Register, Vol. 40, No. 199, Tuesday, October 14, 1975)
is currently seven pounds of mercury (3.2 kg) per 24-hour period
for any source.
11-117
-------
TABLE 11-25
HEALTH EFFECTS OF AIR POLLUTANTS (152)
Pollutant lev
Air quality
level
Significant
harm
TSP
(24-hour),
(24-hour), (8-hour),
Ug/m3 mg/m3
2,100
1,600
(1-hour),
Ug/m3
N02
(1-hour)
ug/m3
Health
effect
descriptor
1,200
1,000
3,750
3,000
2,260
1,130
very
unhealthful
Unhealthful
General health effects
Cautionary statements
NAAQS
Premature death of ill
and elderly. Healthy
people will experience -
adverse symptoms that
affect their normal
activity.
Premature onset of cer-
tain diseases in addition
to significant aggrava-
tion of symptoms and
decreased exercise toler-
ance 'in healthy persons.
Significant aggravation
of symptoms and decreased
exercise tolerance in
persons with heart or
lung disease, with wide-
spread symptoms in the
healthy population.
Mild aggravation of
symptoms in susceptible
persons, with irritation
symptoms in the healthy
population.
All persons should remain
indoors, keeping windows
and doors closed. All
persons should minimize
physical exertion and avoid
traffic.
Elderly and persons with
existing diseases should
stay indoors and avoid
physical exertion. General
population should avoid out-
door activity.
Elderly and persons with
existing heart or lung
disease should stay indoors
and reduce physical activity.
Persons with existing heart
or respiratory ailments
should reduce physical
exertion and outdoor activity.
50 percent
of NAAQS
No index values reported at concentration levels below those specified by "Alert Level" criteria.
b .- •
Annual primary NAAQS.
C400 Mg/m was used instead of the 03 Alert Level of 200 Ug/m .
11.8.3 Standards of Performance for New Stationary
Sources (NSPS)
Subpart A of NSPS ( 40
covering definitions,
monitoring requirements.
that burn municipal
particulates discharged
ton (0.65 kg/t) of dry
shall not have more
co-combustion, Subpart E
charging rate greater
CFR 60) involves general provisions
performance tests, authority, and
Subpart 0 is applicable to incinerators
wastewater sludge and requires that
cannot be in excess of 1.30 pounds per
sludge feed and that the gas discharged
than 20 percent opacity (154). For
is applicable to all incinerators with a
than 50 tons per day (45 t/d) with
municipal refuse comprising 50 percent or more of the charge.
Subpart E requires that particulates discharged be no greater
than 0.08 grains per standard dry cubic foot (0.18 g/m3 dry)
corrected to 12 percent carbon dioxide.
-------
11.8.4 New Source Review Standards (NSR)
This regulation, ,40 CFR 51.18, requires a preconstruction review
of all new or modified stationary sources to determine if the
source will meet all applicable emission requirements of the
State Implementation Plans and the USEPA's Emission Offset Policy
(44 CFR 3274, January 16, 1979).
The reviewing authority is usually a state agency that can apply
stricter emission standards than the USEPA regulations. The
state also sets emission offset required for stationery sources
affected by the NSR. Federal law requires emissions offsets in
areas where NAAQS are violated for a particular pollutant if:
1. The new source could, after installation of a pollutant
control device, emit > 50 tons per year (45 t/yr) of the
offending pollutant; or
2. Could emit >100 tons per year (91 t/yr) of the offending
pollutant were there no pollution control device or were
the existing device to fail.
State and local authorities may mandate a stricter criterion. In
addition, the lowest achievable emission rate is required for any
regulated source that mandates Best Available Control Technology
(BACT).
The present definition of the term "potential emissions" is
uncontrolled emissions or, those anticipated if the emission
control device is bypassed or nonfunctional. This use of
potential emissions in the regulations has a serious effect on
which sources come under the perview of this regulation. For
example, if only one ton per year (0.9 t/yr) of actual emissions
were expected and the control device was 98 percent efficient,
the "potential emissions" would be 50 tons per year (45 t/yr).
The definition of "potential emissions" is the subject of pending
court action, and this action is expected to be settled in late
1979.
11.8.5 Prevention of Significant Deterioration (PSD)
Regulation 40 CFR 52.21 limits increases in particulate and
sulfur dioxide concentrations to specified increments above base
levels measured in attainment areas. Data on total emissions
for the entire air basin are required in order to evaluate
incremental increases in specific emissions due to operation of
any new or modified furnaces. If the potential emission rate of
a regulated pollutant(s) exceeds 250 tons per year (227 t/yr) and
the allowable emission rate exceeds 50 tons per year (45 t/yr),
then this regulation must be used and public notice is required.
11-119
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11.8.6 The Permit Process
Permits for construction and/or operation of processes that
discharge gases to the atmosphere are the primary means for
control of air emissions by a state and, in some cases, the local
jurisdictions. Regulations applicable to a specific plant site
must be thoroughly reviewed to determine if permits are required
for the proposed project. Generally, sludge incineration and
most other combustion operations require permits. The form and
stages of permit requirements will vary considerably between
state and local agencies and must be explored at that level.
Federal permits for PSD regulations may be required. In the
San Francisco Bay Area of California, for example, two stages of
permits are required (154). These are:
• Permit to construct—to be applied for and granted before
construction of a facility may proceed.
• Permit to operate--to be issued after construction and
generally after point sources have passed stack emission
tests.
11.8.7 Air Emissions Test Procedures
The criteria pollutants as defined in the Clean Air Act of 1977,
are particulate matter, SC>2, NOX, CO, hydrocarbons, and ozone.
The USEPA has promulgated stack emission sampling and test
procedures for these pollutants. However, state and local
agency procedures may differ somewhat from those of USEPA and
from each other. For example, some agencies define particulates
as filterable particulate matter while others count the total
catch (including condensible pollutants). For this reason, a
measurement made under one jurisdiction may not be directly
applicable to another.
11.8.8 Design Example
There are many regional and local variations in the rules,
test procedures, and methodologies used to attain the NAAQS.
Therefore, firm guidelines for procedures cannot be provided to
encompass all areas of the nation. Designers must determine
federal, state, and local requirements at an early project stage
and meet with USEPA Regional officers and as well as state and
local officials to negotiate changes or additions to the present
regulations based on the project design, construction, and
initial operation. This is just the start; contact must be
continued with the USEPA Regional Offices, and state and local
air quality management districts throughout the project. Also,
the Federal Register and national and statewide newsletters
should be monitored because they provide a good source for
proposed changes in requirements.
11-120
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The following design example provides a framework for project
analysis. It is based upon experience with the San Francisco Bay
Area Air Quality Management District (BAAQMD) which governs a
nonattainment area. The BAAQMD generally promulgates rules more
restrictive than federal requirements. This particular area was
selected for the example since the local authority (BAAQMD) has
developed a complex set of regulations that many areas may be
using as guidelines.
The first step is to identify the applicable emission regulations
(154-156) and then establish the requirements for emission
control devices. These requirements are reviewed with several
manufacturers to determine feasibility and cost before the device
is incorporated into the design. The last step is the startup
and testing of the control device and the receipt of a permit
to operate. To maintain the operating permit, good plant
monitoring, operations, and maintenance procedures are required.
H.8.8,.1 Identify Applicable State
and Local Regulations
New Source Review (NSR)
Combustion processes are subject to the NSR rule adopted by the
California Air Resources Board (CARB) for application by the
BAAQMD. NSR is required by the USEPA in the Bay Area and in
other regions where clean air standards are violated. NSR
governs the issuance of permits to construct new or modified
stationary sources of air pollution.
The requirements apply only to facilities that would emit large
amounts of pollutants. These requirements are that:
• The facilities must employ "best available (emission)
control technology" (BACT), Section 1308(a)(154).
• The applicant must meet current air quality regulations
regarding all sources of emission that it owns or
operates in the Bay Area. Section 1307.1 (154).
• The applicant must offset proposed emission increases in
NOX, CO, and HC with more than equivalent restrictions
at other sources in the region. Section 1309(a) (154).
The NSR rule is probably the most difficult environmental
regulation facing the designer. The NSR rule requires that
new stationary sources which emit pollutants above a certain
criterion level be approved if they use BACT. The criterion
levels are: 150 pound per day (68, kg/d) each for NOX, SOX, HC,
and TSP; and 1,500 pound per day (681 kg/d) for CO. Below these
levels, a permit may be granted without regard to NAAQS, and BACT
need not be applied. A permit can be issued where BACT is used
and the criterion is not met; however, the NSR rule allows no
exemption from BACT.
11-121
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Another requirement is that existing facilities owned or operated
by the applicant must meet all air pollution regulations. Any
wastewater treatment plant, or other facility operating under
common ownership, must be upgraded to meet existing regulations
before a new source can be added. Recent rulings make exemptions
to this doubtful.
The third requirement of the NSR rule applies to stationary
sources that will emit more than 250 pounds per day (114 kg/d)
of NOX, SOX, HC, or TSP and more than 2,500 pounds per day
(1135 kg/d) of CO. This requirement is intended to prevent the
plant from contributing to violations or increased violations of
the clean air standards. Since some standards in the Bay Area
are already being violated, no sources with controlled emissions
above this level can be built unless project proponents reduce
emissions from another source, thus offsetting the air quality
effects of the project. In other words, if BACT is employed, and
if the emission level is above 250 pound per day (114 kg/d), the
project cannot be built unless offsets are applied. The project
proponents can offset the project's emissions by modifying other
facilities to reduce emissions or by shutting down polluting
facilities.
In the past, the BAAQMD has required that the offset facilities
be in the vicinity of the proposed project so that the portion of
the air basin surrounding the project receives the benefit of
the offset. The rule also requires that the amount of emission
reduction be slightly higher than the amount of emission increase
anticipated from the project. The current offset amount is
1.2 times the emission. For example, an industry can purchase
a paint shop presently discharging 500 pounds per day (227 kg/d)
of hydrocarbons, close the shop, and credit the industry
with: 500 T 1.2 = 417 pounds per day (184 kg/d).
The feasibility of offsets depends on the availability of
suitable existing polluting plants, the cost of purchase or
modification, and the public acceptability of the offset.
If suitable plants are found, purchase of additional control
devices to reduce emissions will probably be more politically
acceptable than purchasing a privately owned facility and
closing it down. The cost of any of these alternatives would be
extremely high.
The alternative route for a large-scale plant would be to obtain
an exemption from the offset portion of the new source review
rule. The rule provides exemptions for a new stationary source
that "represents a significant advance in the development of a
technology that appears to offer extraordinary environmental or
public health benefits or other benefits of overriding importance
to the public health or welfare." An exemption granted by the
BAAQMD would require concurrence of CARB and the USEPA. While an
exemption may be provided, the likelihood that one would be given
at the present time is slight. Facilities that potentially
represent an advance in technology are normally reviewed at the
USEPA headquarters in Washington, D.C., rather than locally.
11-122
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The BAAQMD is seriously considering adoption of an NSR rule that
would apply the offset requirement only to CO and HC , but not
to NOX . This is important to any combustion process proposed,
because NOX control is unproven and very costly. BACT would
continue to apply as previously stated.
Prevention of Significant Deterioration (PSD)
The USEPA prevention of significant deterioration rule is
designed to prevent increases in air pollutant concentrations
that are below the national health standards in a particular air
basin. This is in contrast to the New Source Review Rule
designed to prevent increases in levels of air pollutants that
already exceed standards. In the San Francisco Bay Area, levels
for two pollutants, particulates and sulfur dioxide, are below or
better than standards. If BACT is applied, as required by NSR,
and if controlled emissions of SC>2 or TSP do not exceed the
50-ton per year (45-t/yr) criterion level, PSD will add no
additional constraints.
( NSPS )
Sludge incinerators will be subject to BAAQMD NSPS regulations.
These limits are 1.30 pound per ton (0.65-kg/t) of dry solids,
with gas discharge of not more than 20 percent opacity.
Limitation on Pollutant Concentrations
The BAAQMD requires, as do many other jurisdictions, that the
concentration of major pollutants in the gas stream (NOX, SOX,
HC , TSP, and CO) be limited to some maximum value. The
limits established for the San Francisco Bay Area are shown in
Table 11-26. If supplemental fuel is used in an incinerator, a
correction is required to remove the product of combustion of the
fuel from the calculation. Note that the concentrations shown in
Table 11-26 are based upon concentrations per standard dry cubic
foot (m^ dry) corrected to a standard of six percent oxygen.
This correction is applied in the design portion of this example,
11.8.6.3. Regulatory agencies vary in their treatment of these
corrections, but generally, all require the gas volumes to be
corrected to some standard concentration of C02 (usually
12 percent) or ©2 (usually six or nine percent). Some require a
supplemental fuel correction, which can have a significant effect
on the allowable emissions.
11.8.8.2 Establish Air Pollution Abatement Procedures
Requirements
The designer of an incineration facility must develop the
following information about the flue gas characteristics before
control devices can be designed: total flue gas flow rate, flue
gas temperature, particle size distribution, chemical composition
of emissions, corrosiveness of gas over the operating range, and
moisture content.
11-123
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TABLE 11-26
SAN FRANCISCO BAY AREA - MAXIMUM ALLOWABLE
POLLUTANT CONCENTRATIONS (155)
Component3 Concentration13
Particulates 0.05°
SOX 300d
NOX 175d'e
HCf 25d
BAAQMD Regulation 2.
All concentrations per dry standard cubic
foot corrected to 6 percent 02.
°Grains/sdcf (2.3 std g/m3).
d
ppm.
e
Fuel oil fired - there is no BAAQMD
standard for solid fuel.
Nonmethane hydrocarbons.
Until recently, municipal sewage sludge furnaces have been
subject only to particulate emission controls. Therefore,
limited basic data are available on emission rates of SOX and
NOX from sewage sludge furnaces. Table 11-27 presents the
available data on uncontrolled emissions from multiple-hearth
furnaces.
The following calculations and discussions are based on
Alternative IIIA (50-MGD [2.2-m3/s] plant flow with a
sludge solids concentration of 20 percent), as developed in
Section 11.2.4 (Table 11-9). The incinerator considered is
the multiple-hearth furnace (MHF) operated in the incineration
mode. Auxiliary fuel is assumed to be natural gas. Where
local regulations apply, the BAAQMD rules are used (see
Section 11.8.8.1 and Table 11-26 and Figure 11-45). Figure 11-45
is excerpted from the BAAQMD rules. Installations under other
jurisdictions will presumably have different regulations:
Step 1 - Calculate Uncontrolled EmjLs_sions of
Criteria Pollutants
a. Quantity of dry sludge solids = 51.5 ton/day (46.7 t/d) .
11-124
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b. From Table 11-27, daily emissions are:
Particulates: 51.5 tons dry solids 33 pound
day ton dry solids
= 1,700 pound/day (771.8 kg/d)
so • 51.5 tons dry solids 1 pound
2' day ton dry solids
=51.5 pound/day (23.4 kg/d)
n 51.5 tons dry solids 5 pound
*: day x ton dry solids
= 257.5 pound/day (116.9 kg/d)
51.5 tons dry solids 1 pound
day x ton dry solids
=51.5 pound/day (23.4 kg/d)
TABLE 11-27
UNCONTROLLED EMISSION RATES FROM MULTIPLE-
HEARTH FURNACES (157)
Emission factor,
Ib/ton dry sludge
Pollutant solids
Particulates
sox
NOX
Hydrocarbons
CO
33
1
5
1
0
.0
.0
.0
.0
.0
1 Ib/ton = 0.50 kg/tonne
Ste'p_2_j^j:aj1c:ul:ate Deg^ee o£ Control Required to Meet NSPS
NSPS deals only with particulate emissions (other pollutants are
covered by NSR).
a. NSPS = 1.3 Ib particulates/ton (0.65 kg/t)
„-,-, ui u • T *. 1-3 pound 51.5 ton solids
b. Allowable particulates : ^^ x — —
= 67 pound/day (30.4 kg/d)
11-125
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c. Required particulate removal efficiency:
, 67 pound day
~
1,700 pound day
,„„ .... ..
X 10° ?ercent = 96-! Percent
DIVISION 8 —CALCULATION METHODS AND GENERAL
SAMPLING PROCEDURES
CHAPTER 1—CALCULATIONS
$8100 Calculation ol
plished by the calcula:
method* which yield ei
eifitally prescribed in i
ihods prescribed in ihii Chapter I. or by
(a) Sloklnometric Combu»rion of Auxiliary Fuel
6.000 12.000 6,000 12,000
CH. + 20, CO, + 2H,0
12.000 >und«<] cubic (CMof oxygen required
{ 8! 10 Correction (or the use ol auxiliary fuel shall be as specified in
(Bill, and cor ret lion to a basis of 6^ oxygen by dry volume that! be u
specified in j BII2. For the purposes of \\ Bill and 8112 ihe term "meaj-
ured volume" ihall mean the emined. or meiered volume to be corrected,
ncpreued in tundird cubk feet.
J6ll! AUXILIARY FUEL CORRECTION. This calculation is in-
exitied if me luxiliary fuel had not been introduced, and result* obtained
by (hit procedure shall be deemed to represent such correction. The
method coniisu of four siepv
(») Calculate the amount of oxygen required (or iioichiometric combus-
tion of the •luilUrr fuel, at the rate of combustion occurring during the
period of test.
6,000 iund>rd tubii ftecCOj. 12,000 iund>rd cubic f«i H2O
(c} -100,000 + 12,000 = 412,000
(d) 412,000- 18.000 = 394.000 .undird cubic fe«
TABULATION OF VOLUME CH\NGE (SCF)
Componr,.,
CO;,
CO
o,
N,
H,O
Total
M«tu,rtl
40.1)00
8.000
21,600
281.200
49,200
400.000
Con re i, on
- 6.000
+ 12.000
- 12,000
- 6.000
F,nil
34.000
B.OOO
33.600
281.200
37,200
394,000
rected (or •uulury fud u
UUry fuel loial 400.000 itindird cubic feet during a if it period, a
atmospheric oxyRcn content) jub-
obuimil in step (b) .
(d) Diviik i hi- rr*nli «l stq> (.) l» ll.H'iii. (This is n.2095 - 0.06.)
(e) Multiply the dry volume obtained in Mcp (a) by the quotient ob-
tained in \if[) (d) IK |five 'hf (oifcned dry volume on a 6r/r oxygen basis
(0 Divide the weight <>f jir
urne obtained in sn-p (c) to j
ni. by the torrefied vol-
Component
CO,
CO
o,
Nj
H,O
Total
<"„ iVol., *«>
8-64
2-0)
8.53
71 36
9.44
10000
<•; i\ol dr%)
953
2.24
9.42
78.81
0.00
100.00
SCF
S4.000
R.OOO
SJ.600
281,200
37,200
394.000
Also
(b) Calculate the composit
Chiomeiric combuation in ox
(c) Add. to (he measured
step (»)
(d) Subtract, from the re
on and quantity of the product! of such ttoi-
gen.
vo ume, ihe amount o oxygen a cu at in
ult of step (c) . the volume of combiutioo
{ 8112 OXYGEN COR
reel the measured tone
lion fnr the use of iiutilu
measured volume for pu
Rl-CTK
if fuel is
rptiics o
)N Th.j calculauon n
this section 8112. The
mended to cor- 35b,800
to that which ^e) 02(W5
(n (7 9 ibi
: the weight of air contaminant is 7.9 pounds
(a) 394.000 - 37,200 = 356.800 SCF. dry volume
(b) .IS.fiOfl
B3HJ- =0.0942. volume fract.on of ox;
oxygen
(e) (0.782){35(i.ll<«) = 275.WM) SDCf. at fi^{ oxygen, the torretti
(f) (7 9 II)) ("OOOgr/lb) = ().20|(r/SD(;F. the corrected lonccmraiin:
775.H(M) slx;i
Where a concentration subject to trm
volume, the c
the ratio of th
(e) above
n subject to trm correction ii based on a meat
n shall conim of multiplying the concentratio
red volume to the corrected volume obtained in
FIGURE 11-45
SAN FRANCISCO BAY AREA AIR QUALITY MANAGEMENT DISTRICT:
AUXILIARY FUEL AND OXYGEN CORRECTION (155)
Step 3 - Select The Control Device for Satisfying NSPS
The actual selection of an emission control device is beyond the
scope of this manual. Equipment selection can be quite involved
and complex. Several excellent publications are listed in the
references to provide a detailed understanding of emission
control equipment (158-161). A number of publications are
available in the literature for further detail on theory,
specific furnaces, and combustion (1,4,8,9,11,12,14,17,23,38,46,
58,61,74,98,105,141,158-181). Additional sources for detailed
information include furnace manufacturers, emission control
device manufacturers, operating installations, and air quality
control consultants.
In this design example, a venturi followed by a
wet scrubber is selected. BAAQMD considers this
BACT.
tray-type
equipment
11-126
-------
Step 4 - Check Conformance with the New jaource Review
Rule (NSR)
a. The BAAQMD requires that all pollutants be below
150 pound per day (68.1 kg/d), except CO which is
1,500 pound per day (681 kg/d) (unless BACT is applied).
As per Step 1, S02, HC, and CO meet this requirement,
even as uncontrolled emissions and need not be considered
further under NSR.
Particulates and NOX require BACT. Since the venturi and
wet scrubber combination is considered BACT for partic-
ulates, the particulates criterion is satisfied.
The venturi-scrubber combination will also reduce NOX to
a certain extent. The N0-N02 distribution in flue gas
for sewage sludge incinerators is not well known. For
general combustion, N02 content represents 10 to
20 percent of the NOX and can be effectively removed
by the wet scrubber. Assuming that a ten percent N02
component of NOX is removed by scrubbing, the NOX
emission rate drops to 232 pounds per day (105 kg/d).
At present, very few control processes are effective
in reducing NOX emissions. However, major research
efforts are being made to solve the problem. The process
with the best potential has been developed and tested in
Japan only. It is a patented catalytic ammonia injection
process which reduces NOX by 90 percent. Current
research in the United States has been conducted only on
a small scale. Therefore, in effect, there are no fully
developed, available NOX control devices. Until full-
scale systems for NOX control are tested, an exemption
or variance will probably be granted.
Another potential way to reduce NOX is via combustion-
controlled processes such as SAC, reduction of excess
air, and staged combustion. Firm data are not available
with sewage sludge feed. Presently it is not known if
the 150 pounds per day (68.1 kg/day) criterion for NOX can
be met by combustion control.
b. Check to see if emissions exceed the 250 pounds per day
(114 kg/d) level at which offsets must be obtained. In
this example, the levels are below 250 pounds per day
(114 kg/d); thus offsets are not required.
Step 5 - Check that Concentrations of Criteria Pollutants
Pg__Not__Exceed_Regulatory Standards
The objective is to calculate pollutant concentrations at some
standard condition, so that the particulate emission can be
compared with emissions from other sources on an equivalent
basis. This correction is made by first calculating pollutant
11-127
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gas flow (under standard conditions), then calculating total
standard exhaust gas flow and correcting the latter for auxiliary
fuel. Finally, the pollutant concentrations are calculated
to a six percent oxygen basis. A detailed calculation for
hydrocarbons (HC) is presented below. It is assumed that HC
are not removed in the wet scrubbing system.
a. Calculate the volumetric HC flow at standard temperature
and pressure (STP). The volumetric flow rate of HC is
calculated as:
51.5 pound HC pound mole HC
day x 28 pound HC
359 cu ft day
x pound mole at STP x 1440 min."
= 0.46 standard cfm (2.17 x 10~4 std m3/s)
It is assumed HC are ethylene with a molecular weight
of 28.
b. Calculate exhaust gas flow at STP. The data in
Table 11-28 are available. Off gas temperature
and pressure are 800°F (427°C) and one atmosphere
respectively. The pollutant (NOX, S02, HC, CO,
particulate) masses are small compared to the masses of
the constituents in Table 11-28 and thus were ignored in
calculating exhaust gas volume.
Total volumetric flow rate of the exhaust stream, wet
basis; reduced to standard conditions:
.. ,._ _ 60°F + 460°F
= 44'403 scfm x 800°F + 460°F
= 18,325 scfm (8.65 std m3/s)
Note: standard conditions are taken to be 60°F (16°C) and
one atmosphere.
c. Correct for auxiliary fuel (See Figure 11-45 and
Table 11-29). The intent of this calculation is to
correct the measured exhaust gas volume to the volume
that would have existed had auxiliary fuel not been
introduced. Assume here that 100 scfm (4.72 x 10~2 std
m3/s) of natural gas was used. The combustion of
100 scfm (4.72 x 10~2 std m3/s) of natural gas is
depicted by the following equation:
CH4 + 202 ——> C02 + 2H20
100 scfm 200 scfm 100 scfm 200 scfm
11-128
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The auxiliary fuel correction procedure is:
1. Calculate the amount of oxygen, 200 scfm (0.09 std
m3/s) for s to ichiome tr ic combustion of auxiliary
fuel.
2. Calculate the quantity of combustion products
300 scfm (0.14 std m3/s) .
3. Add the oxygen calculated, 200 scfm (0.09 std m3s)
to the measured gas volume 18,325 scfm (8.65 std
m3/s), then subtract the volume of combustion
products calculated in Step 5c2, 300 scfm (0.14 std
m3/s). The sum, 18,225 scfm (8.60 std m3/s), is
gas volume corrected for auxiliary fuel (see
Table 11-29) .
Correct for oxygen (see Figure 11-45). The intent of
this calculation is to correct the measured concentration
of contaminant to that which would exist were the same
quantity of contaminant contained in a dry volume,
corrected to six percent oxygen. All calculations are
based on the final flow rate at STP per Table 11-28. The
procedure is as follows:
1. Subtract the volume of water vapor, 7,056 scfm
(3.33 std m3/s) from the final volume, 18,225 scfm
(8.60 std m3/s), to give the dry volume, 11,169 scfm
(5.27 std m3/s) .
2. Calculate the oxygen content as a decimal fraction of
the dry volume:
Scfm = 0.1035 02
11,169 scfm
3. Subtract the decimal fraction calculated in Step 5d2
from the 0.2095 (average atmospheric oxygen content):
0.2095 - 0.1035 = 0.1060.
4. Divide the result of Step 5d3 by 0.1495 (this is
0.2095 - 0.06) :
°-1060 - 0.709
0.1495
5. Multiply the dry volume obtained in 5dl, 11,169 scfm
(5.27 std m3/s), by the quotient obtained in Step
5d4, 0.7090, to get the corrected dry volume on
a six percent oxygen basis:
0.7090 x 11,169 scfm = 7,919 scfm (3.74 std m3/s)
11-129
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6. Divide the volumetric HC flow of Step 5a, 0.46 scfm
(2.17 x 10~4 std m^/s) by the corrected dry volume
on a six percent basis to obtain concentration on a
six percent basis:
0.46 scfm
7,919 scfm
106 = 58
TABLE 11-28
DESIGN EXAMPLE: EXHAUST CAS DATA FROM A
MULTIPLE-HEARTH FURNACE
Constituent
C02
N2
°2
Water vapor
Total
Ib/hr
7
39
4
20
72
,749
,623
,813
,551
,736
Percent of
total gas
volume
6
49
5
39
100
.1
.1
.2
.6
.0
Actual
• CFM
2,
21,
2,
' 17,
44,
712
793
315
583
403
1 Ib/hr =0.45 kg/hr
1 cfm = 0.028 m3/min
TABLE 11-29
DESIGN EXAMPLE: AUXILIARY FUEL CORRECTION FOR A
MULTIPLE-HEARTH FURNACE3
Component
Percent of total
gas volume
C00
2
NT
2
02
Water vapor
Total
6
49
5
39
100
.1
.1
. 2
.6
.0
CFM
at
STP
1,
8,
7,
18,
119
994
956
256
325
Final CFM at
Correction
-100
+200
-200.,
-100 : ,
ST:
i,
8,
i,
7,
18,
pb
019
994
156
056
225
See Step 5(c) .
bSTP = Standard temperature and pressure = 60°F (15.6°C) at one atmosphere.
1 cfm = 0.028 m3/min
11-130
-------
Step 6 - Compare Calculated Pollutant Concentrations
Against Emission Standards (Table 11-30)
The emissions standard is 25 ppm. The HC limit is exceeded and
afterburning will be required.
From similar calculations, the concentrations in Table 11-30 are
obtained (corrected to six percent oxygen and auxiliary fuel,
prior to afterburning).
None of the other pollutants (particulates, NOX, SOX) are in
violation of concentration standards.
TABLE 11-30
DESIGN EXAMPLE: MULTIPLE-HEARTH FURNACE POLLUTANT
CONCENTRATIONS AFTER SCRUBBING3
Pollutant Concentration Standard
Particulates,
grains/sdcf
HC , ppm
NOX , ppm
S0xe, ppm
0.04
5&c
147
17
0.05
25
175
300
aCorrected for auxiliary fuel and to 6
percent oxygen.
As ethylene.
CDoes not include afterburning.
dAs N02.
SAs S02.
1 grain/sdcf =2.3 std g/m
Step 7 - Summary
A venturi, wet tray-type scrubber and afterburning will satisfy
all emission requirements except NSR requirements for NOX. An
exemption is expected for NOX, since technology for NOX removal
is not sufficiently developed for field applications. Note that
not all jurisdictions require auxiliary fuel and oxygen correc-
tions. As shown, the corrections can have profound impacts. The
type of control scheme required may hinge upon the regulatory
agency's decision as to whether such corrections are necessary.
The procedures used in Step 5 were taken directly from Regulation
2 of the BAAQMD Regulations (155).
11-131
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11.9 Residue Disposal
The residues remaining after sludge combustion (ash, particulates
from dry scrubbing, etc.) must be disposed of. Due to the drain
of natural resources, the constructive utilization of residues,
particularly ash, is undergoing considerable research. Because
the ash concentrates the setteable material in wastewater, there
is an interest in recovering valuable scarce metals such as gold.
In Palo Alto, California, a firm is working on methods to recover
such metals from the ash (182). In this case, recovery may be
cost-effective, since the treatment plant receives the wastewater
from many electronics firms and the scarce metal content is high.
In general, however, there is no economical process to use ash;
consequently, it is typically disposed of to a landfill.
Residues (ash) from the combustion of municipal wastewater solids
generally contain high concentrations of trace metals. Leachate
from sites where incinerator ash is landfilled must be controlled
to prevent metal contamination of groundwater. Many states
are beginning to classify disposal sites according to their
relationship to nearby groundwater and the material to be
landfilled. Tables 11-31 and 11-32 describe methods used by the
State of California for classifying waste materials and disposal
sites. Typically, wastewater sludge furnace ash requires a
"protected" Class II-l site and municipal refuse incinerator
ash requires a hazardous fill site. These are described on
Table 11-32. Outside these broad classifications, the ash
will require sampling and analysis, including detailed review by
state and local health agencies. A serious problem, however, is
that no standard analyses or procedures are presently available
that allow a particular ash to be classified (leachability of
certain contaminants at various pH' s and over different times).
This type of analysis is expensive, and the results are difficult
to interpret. No data base is available to compare the labora-
tory results with actual field conditions. Work is being done in
this area and hopefully proper procedures and guidelines will be
developed.
Detailed design and operating data are beyond the scope of this
manual. More detailed discussions on landfilling ash and
sludge landfilling procedures can be found in the literature
(184,185,186) .
11-132
-------
TABLE 11-31
DESCRIPTION OF SOLID AND LIQUID WASTE CLASSIFICATIONS (183)
Group 1
Group 2
Group 3
Consist of or contain toxic
substances and substances
which could significantly
impair the quality of usable
waters.
Examples include:
• Saline fluids from water
or waste treatment pro-
cesses
• Community incinerator
ashes
• Toxic chemical toilet
waste
Industrial brines
Toxic and hazardous
fluids
Pesticides or chemical
fertilizers or their
discarded containers
Other toxic wastes
Consist of or contain
chemically or biologically
decomposable material which
does not include toxic sub-
stances nor those capable of
significantly impairing the
quality of usable waters.
Examples include:
• Garbage
• Rubbish
• Construction debris
such as paper, card-
board, rubber, etc.
• Refuse such as yard
clippings, litter,
glass, etc.
• Dead animals
• Abandoned vehicles
• Sewage treatment resi-
due such as solids from
screenings and grit
chambers, dewatered
sludge, and septic tank
pumpings
• Infectious materials
from hospitals or
laboratories
Consist entirely of nonwater
soluble, nondecomposable
inert solids.
Examples include:
• Construction and
demolition debris,
asphalt paving, inert
plastics, etc.
• Vehicle tires
• Industrial wastes such
as clay products, glass,
slags, tailings, etc
11-133
-------
TABLE 11-32
CLASSIFICATION OF WASTE DISPOSAL SITES (1&3)
Class I
Class I disposal sites are those at which
complete protection is provided for all
time for the quality of ground and surface
waters from all wastes deposited therein and
against hazard to public health and wildlife
resources. The following criteria must be
met to qualify a site as Class I:
(a) Geological conditions are naturally
capable of preventing vertical
hydraulic continuity between liquids
and gases emanating from the waste in
the site and usable surface or ground-
waters.
(b) Geological conditions are naturally
capable of preventing lateral hydraulic
continuity between liquids and gases
emanating from wastes in the site and
usable surface or groundwaters, or the
disposal area has been modified to
achieve such capability.
(c) Underlying geological formations which
contain rock fractures or fissures of
questionable permeability must be
permanently sealed to provide a com-
petent barrier to the movement of
liquids or gases from the disposal site
to usable waters.
(d) Inundation of disposal areas shall not
occur until the site is closed in
accordance with requirements of the
regional board.
(e) Disposal areas shall not be subject to
washout.
(f) Leachate and subsurface flow into the
disposal area shall be contained within
the site unless other disposition is
made in accordance with requirements of
the regional board.
(g) Sites shall not be located over zones
of active faulting or where other
forms of geological change would impair
the competence of natural features or
artifical barriers which prevent con-
tinuity with usable waters.
(h) Sites made suitable for us» by man-made
physical barriers shall not be located
where improper operation or maintenance
of such structures could permit the
waste, leachate, or gases to contact
usable ground or surface water.
(i) Sites which comply with a, b, c, e, f,
g, and h, but would be subject to
inundation by a tide or a flood of
greater than 100-year frequency may be
considered by the regional board as a
limited Class I disposal site.
Class II disposal sites are those at which Class III disposal sites are those at which
protection is provided to water quality from protection is provided to water quality from
Group 2 and Group 3 wastes. The types of
physical features and the extent of pro-
tection of groundwater quality divides
Class II sites into the two following
categories:
Class II-l sites are those overlying usable
groundwater and geologic conditions are
either naturally capable of preventing
lateral and vertical hydraulic continuity
between liquids and gases emanating from the
waste in the site and usable surface or
groundwaters, or the disposal area has been
modified to achieve such capability.
Class II-2 sites are those having vertical
and lateral hydraulic continuity with usable
groundwater but for which geological and
hydraulic features such as soil type, arti-
ficial barriers, depth to groundwater, and
other factors will assure protection of the
quality of usable groundwater underneath or
adjacent to the site.
The following criteria must be met to qualify
a site as Class II:
(a) Disposal areas shall be protected by
natural or artificial features so as
to assure protection from any washout
and from inundation which could occur
as a result of tides or floods having
a predicted frequency of once in 100
years.
(b) Surface drainage from tributary areas
shall not contact Group 2 waters in the
site during disposal operations and for
the active life of the site.
(c) Gases and leachate emanating from waste
in the site shall not unreasonably
affect groundwater during the active
life of the site.
(d) Subsurface flow into the site and the
depth at which water soluble materials
are placed shall be controlled during
construction and operation of the site
to minimize leachate production and
assure that the Group 2 waste material
will be above the highest anticipated
elevation of the capillary fringe of
the groundwater. Discharge from the
site shall be subject to waste dis-
charge requirements.
Group 3 wastes by location, construction, and
operation which prevent erosion of deposited
material.
11-134
-------
Calcu»4tkin*-n«Ql«»
ind
0«t«
13
1*
15
16
17
IS
19
20
(21
Fysl, Os, a«f Air pw Unit af Fuel
Ftwl
Gertstltueftt
CtoCO
CO to CO:
Cynbvrned,
line K
0; (deduct)
Nj
COs
H:O
*sh
Sum
Psr
Fuel
Unit,
Ib
100.0
Mul.
m
Dhri.
sor
Motes
Fuel
rtt-
ptiw
Mote
Theo
Rtq.J
0
o
o
0
Os- ind Air, Moles for Tatal Air -
(§se line dai righl)
O; (th«o) rend - Q?l line 12
Oi (eicsw) T'Aim°° x Oa, line 12
Oj (total) supplied - lines 13 I- 14
NE 4u|5ptteJ - 3.76 x. Oj, iivc IS
Air (sry; supplied ~ Qs + Nj
—*
H3Q in air
Air (wetj supplied - lines 1? i IB
Flue g«5 comrtitutnts - lines 1 to J 8, iotal
•Ncrri: —for air ?it 30 F anrtKW* relative humidity i
.
Moles per Fuel Unit fAf,
Fuel
Sourc*
OOs
-I-
SO,
.
HOP males, gaitcuj fi»(i
Fual Anal, as Flrwl (AF), % By Wl a1 Vpt
Cfo Oa CO Nj %t
Total an (TJ&.) assBTOd « &/ORSAT %
yn« f, i, h For Qiseais Fuels
Wl ftjel yftit - J (moles *ai^i x md, Wt) Ib
Mol" wt'irf'fuei"-line r : 103
! heat value, 8tu/lb
Xzfl unburned. lb/100 Ib f««l
% ash In fya y
100 -
Exit t«ip of Hue gaj, S,
Dry-bulb {ambient J temp, f i
''
Rei humid. (p5^Hrgrnttr*C chart) .
S*, barameirir: prc$$yr^, m Ng ;
Sat. preai. HjO at amS temp, in. HE
A*, press. HjO In sir, lines {o x' q), In.
Total ' Wet Flye Qas
.Moles_[
Drj Flue 4ii
• ft.03? is oftsn used as standard.
CO; + SOi
Flue G*s «nd
Fly^ £35 ^^nstJtys^ts
Me>, mean, iz to t'i ftof ft -
lo dry flue gas « males each, line 20 ,x Men X (^ — f 3)
In HjO in air malfls HjO, lint 18 x Me? x C?j— d)
I n s*ns heal, HiQ In fuel - moles, II n es (5110) x *te? M (b — r
In. ta[tnt heat MK) in fvsl - mt«BS. lines (5 + 10) x. ]D*0 x IS
Total in wet flue gis
EKje to CafbDn in refuse ™ line 1( x I^.IDO
fc« to ynbumsd CO in flue gas - nwtss C to CO x 12 x 9.75S
Total riue gas laaaes • unburttied cambuaUMe - llnaa 2S129 +• 3d t ndiotlon ttt
Heat value of fuel unit -
Tsui tiota l*n! k* fwl unit » Itn* 32 - I In* 31
in Btu per Fuel Unit g formula C It -t) ar by cjlorlnMlry,
ttfPtudlition «((umwi ID t» « flxecl ptrcenl of line li, rvjrnnally 1 to 5 percent,
Copyright 197S by th« Babcock and Wllcox Company. Minor chang«> have b««n
made to this table to allow for ease of use with sewage iludge. Table may be
uied without permission. However, credit to Babcock and Wllcox Company
should be given.
CEMERAL W7TIS;
* Sue taul for uie trf ubl« -
* IWut*, >• UMd in UMl
uble, li the r«kiu* (iih)
from [h«prot«j.
1 in. =
1 Btu/lb « J,
1 Ib/cu ft » 1*
11-135
-------
11.10 References
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11. Baumeister, T., and L.S. Marks. Standard Handbook for
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11-136
-------
17. Brown and Caldwell. Solid Waste Resource Recovery Full
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30. vonDreusche, C., and J.S. Negra. "Pyrolyses Design
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11-138
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41. Ferrel, J.F. "Sludge Incineration." Pollution Engineerinq.
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11-139
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11-140
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11-141
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TTTT." " " " " ~~ ! ' ~™ ~
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11-142
-------
93. Frazer, J.M. "Simultaneous Incineration of Refuse
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97. USEPA. A Technical, Environmental, and Economic
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1975.
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1978.
99. Jones, J.L. "The Costs for Processing of Municipal Refuse
and Sludge." Proceedings of the ConferenceonAcceptable
Sludge Disposal Techniques. 1978.
100. USEPA. Cost Allocations for Multiple Purpose Projects.
Office of Water Programs Operations. Washington, DC 20460.
Construction Grants Program Requirements, Memorandum No.
PRM 77-4. 1976.
101. USEPA. Risks and Contracts. Office of Solid Waste
Management. Washington, DC 20460. Resource Recovery Plant
Implementation: Guides for Municipal Officials, SW-157.7.
1976.
102. Krzeminski, J. "Codisposal of Sludge with Refuse." Sludge
Magazine. March-April 1978.
103. Smith, E.M., A.R. Daly. "The Past, Present, and Future
of Burning Municipal Sewage Sludg-e Along with Mixed
Municipal Refuse." Proceedings of the National Conference
on_Municipal Sludge Management and Disposal. 1975.
104. Petura, R.C., C.R. Brunner, and R.F. Bonner. "Utilization
of Sewage Skimmings as Fuel to Generate Process Steam."
Proceedings of the National Waste Processing Conference_-
ASME. 1978.
11-143
-------
105. Guillory, J.L. "Particulate Emissions Resulting from
Combustion of Municipal Sewage Skimmings." Proceeds of
the National Meeting of the American Institute of Chemical
Engineers. 1977. , "" — —
106. Ross, E.E. "Scum Incineration Experiences." Journal Water
Pollution Control Federal. Vol. 42. May 1970"! _—
107. USEPA. Evaluation of Small Modular Incinerators in
Municipal Plants. Office of Solid Waste Management.
Washington, DC 20460. Contract 68-01-3171. 1976.
108. USEPA. Lime Use in Wastewater; Design and Cost Data.
Municipal Environmental Research .Laboratory. Cincinnati
45268. EPA 600/2-75-038. October 1975.
109. USEPA. Sludge Processing for Combined Physical-Chemical-
Biological Sludges. EPA-R2-73-250. July 1973.
110. USEPA. Advanced Wastewater Treatment as Practiced at
South_Tahoe. Project 17010 ELQ. August 1971.
111. Evans, R.R. Sludge Disposal and Chemi_caJ^_R_e_c_o_y_e.j:y.
Dorr-Oliver Technical Publication. 1974.
112. Brown and Caldwell. Lime Sludge Recycling Study. Central
Contra Costa Sanitary District, California. June 1974.
113. Cardinal, P.J., and R.J. Sherwood. Plural Purpose Sludge
Incinerating and Treating, Appajratuj>_arid Method . U.S.
Patent No. 3,623,975.
114. USEPA. Solids Handling_and Reuse of Lime Sludge. Project
11010 FYM. 1974.
115. Hotz, H.J., P. Hinkley, and A. Er dm'a n. "Fluidized
Solids Lime Mud Recovery System at S. D. Warren Co."
Technical Association of the Pulp and_Pa^per Industry.
Vol. 47. November 1964.
116. Gulp, R.C., and G.L. Gulp. Advanced Wastewater^ Treatment.
Van Nostrand-Reinhold. New York. 1971.
117. Moran, J.S., and C.J. Wall. "Operating Parameters of
Fluidized Bed Lime Mud Reburning Systems." Technical
Association of the Pulp and Paper Industry. 49:174. March
1966.
118. Kroneberger, G.F. "Lime Recalcination in the U.S. Sugar
Industry." S u g a_r T e c h n o 1 o g yRei v i_ewj3. 4:1. December 1976.
119. Wall, C.J. Fluosolids Reburning of Lime Sludges. Dorr-
Oliver Technical Publication. 1974.
11-144
-------
120. Stadnik, J.G., and B.P. Hynn. "State of the Art: Powdered
Activated Carbon Addition to Activated Sludge." Proceed-
ings of the Annual Meeting of the American I"gjtjtuJ:e__g_f
Chemical Engineers. 1977.
121. USEPA. Process Design Manual for Carbon Absorption.
Technology Transfer. Cincinnati 45268. October 1973.
122. Kittredge, D. "The Economics of Carbon Regeneration--State
of the Art." Proceedings of the New England[Water Works
Association Conferen^e_. September 1978.
123. vonDreusche, C. "Process Aspects of Regeneration in a
Multiple Hearth Furnace." Presented at the National
Meeting of the American Institute of Chemical Engineers.
1974.
124. Shell, G.L., and D.E. Burns. "Powdered Activated Carbon
Application, Regeneration and Reuse in Wastewater
Treatment." Proceedings of the International Water
Pollution Control Association Conference. Tel Aviv,
Israel. 1972.
125. Rudolfs, W., and E.H. Trubnick. "Activated Carbon in
Sewage Treatment." Sewage Works Jqurnal. 7:5. September
1935.
126. Wallace, W.N., and D.E. Burns. "Factors Affecting Powdered
Carbon Treatment of a Municipal Wastewater." Journal Water
P o 11 u t JJD ri ^ControlFederation. Vol. 48. March 1976.
127. Koches, C.R., and S.B. Smith. "Reactivate Powdered
Carbon." Chemical Engineering. May 1, 1972.
128. Westvaco Corporation. U.S. Patent 3,647,716.
129. Lewis, R.E., J.J. Kalvinskas, and W. Howard. JPL Activated
Carbon Treatment System (ACTS) for Sewage. NTIS: N76-
20697. February 1976.
130. Sigler, J.E. "Activated Carbon and Fuel from Sewage
Solids." ACS Symposium Series No. 76, Solid Waste and
Res idue : Conversion by Advanced Thermal P r oc_e si_se_s_.
American Chemical Society. 1978.
131. Jones, J.L. "Converting Solid Wastes and Residues to Fuel."
Chemical Engineering. Vol. 85. January 2, 1978.
132. Jones, J.L., R.C. Phillips, S. Takaoka, and P.M. Lewis.
"Pyrolysis, Thermal Gasification, and Liquifaction of Solid
Waste and Residues—Worldwide Status of Processes (as of
Fall 1977)." Proceedings of the National Waste Processing
Conference, ASMS. May 1978.
11-145
-------
133. Klass, D.L. "Energy from Biomass and Wastes: 1978
Update." Proceedings of Symposium-Energy from Biomass
Wa. ste s . Institute of Gas Technology. 1978.
134. Wilhelmi, A.R., and P.V. Knopp. "Wet Oxidation as an
Alternative to Incineration." Presented at the Annual
Meeting of the American Institute of Chemical Engineers.
1978.
135. Flynn, B.L. "Energy Aspects of CPI Wastewater Treatment
by Wet Air Oxidation." Zimpro Technical Publication.
136. Pradt, L.A. "Wet Oxidation of Coal for Energy." Presented
at the K en.tucky Coal Conference. May 1979.
137. Pradt, L.A. "Wet Oxidation Boiler-Incinerator."
Proceedings of the National Waste Processing Conference-
Energy Conservation Through Waste Utilization - ASME.
1978.
138. USEPA. Destroying Chemical Wastes in Commercial Scale
Incinerators. Facility Report No. 4 - Zimpro, Inc. Office
of Solid Waste. Washington, DC 20460. NTIS: PB-267 987.
December 1976.
139. Pradt, L.A. "Developments in Wet Air Oxidation." Chemical
Engineering Progress. 68:12. 1972 (Updated 1976).
140. Hathaway, S.A. Design Features of Pa c k a g e In c in ejc a to r
Systems. U.S. Army Construction Engineering Research
Laboratory. NTIS: AD/A-040 743. May 1977.
141. Niessen, W.R., A.A. Kalotkin, F.C. Sapienza, and P. Nese.
"Air Pollution from Refuse - Sludge Coincineration in
Modular Combustion Units." Presented at the Mid-Atlantic
States Section of the Air Pollution Control Association.
Newark. April 1979.
142. Hofmann, R.E. "Controlled-Air Incineration - Key to
Practical Production of Energy from Wastes." Publj1cmWgrk_s
Ma_g_a_z_irie_. April 1976.
143. Martin, T.L. "A Total Package Concept for Solid Waste
Management." Public Works Magazine. April 1975.
144. "City Finds Disposal Solution." So1id Wastes Managemenit.
March 1978.
145. Titlow, E. "Refuse Incineration - Waste to Energy Systems
for the Smaller Community." Presented at the Governmental
Refuse Collection and Disposal Association Meeting. San
Leandro,California.March 1978.
146. Bailie, R.C. Production of High Energy Fue1 G a.s^
Municipal Wastes. U.S. Patent No. 3,853,498.
11-146
-------
Association. Vol. 27. October 1977.
USEPA. Environmental News. August 23, 1976.
USEPA. Inspection Manual for Enforcement of New
Performance Standards: Sewage Sludge Incinerators.
of Enforcement. Washington, DC 20460. Stationary
Source
Office
Source
147. Fluidized Bed Gasification Project, Department of Chemical
Engineering, West Virginia University. "Solid Waste; A
New Natural Resource." Morgantown, WV. May 1971.
148. Coffman, J.A., and R.H. Hooverman. "Power from Wastes
via Steam Gasification." ACS Symposium Series No. 76,
Solid Waste and Residue; Conversion by Advanced Thermal
Processes. American Chemical Society. 1978.
149. Hooverman, R.H., and J.A. Coffman. "Rotary Kiln
Gasification of Solid and Liquid Wastes." Presented at the
Annual Meeting of the American Institute of Chemical
Engineers. 1977.
150. USEPA. Feasibility Study of Use of Molten Salt Technology
for Pyrolysis of Solid Waste. Office of and Development.
Cincinnati 45268. EPA-670/2-75-014. January 1975.
151. Easton, E.B., and F.J. O'Donnell. "The Clean Air Act
Amendments of 1977, Refining the National Air Pollution
Control Strategy." Journal of the Air Pollution Control
152.
153.
Enforcement Series, EPA 340/1-75-004. February 1975.
154. San Francisco Bay Area Air Pollution Control District.
New Source Review Rules. Sections 1304 through 1311.2.
December 20, 1977.
155. San Francisco Bay Area Air Pollution Control District.
Regulation 2. Adopted May 4, 1960, and amended thereafter.
156. San Francisco Bay Area Air Pollution Control District.
Regulation 8. December 1976.
157. USEPA. Supplement for Compilation of Air Pollutant
Emission Factors. Research Triangle Park, North Carolina
27711. AP-42. 1975.
158. Bethea, R.M. Air Pollution Control Technology; An
Engineering Analysis Point of View. Van Nostrand-Reinhold
__1978^
159. USEPA. Air Pollution Engineering Manual. Office of Air
and Water Programs. Research Triangle Park, North Carolina
27711. AP-40. 2nd Ed. May 1973.
160. USEPA. Industrial Guide for Air Pollution Control.
Environmental Research Information Center. Cincinnati
45268. EPA-625/6-78-004. June 1978.
11-147
-------
161
162
163
164
165,
166,
167
169.
170.
171.
172.
173.
174.
175.
USEPA. Scrubber Handbook.
Carolina 27711
NTIS:
Research Triangle Park, North
PB 213-016. 1972.
Calvert, S. "How to Choose a Particulate Scrubber."
Chemical Engineering. 84:18. August 29, 1977.
Calvert, S. "Upgrading Existing Particulate Scrubbers."
Chemical Engineering. 84:23. October 24, 1977.
Farrell,
Pollution
Report.
J.B., H.O. Wall, and B.A. Kerdolff. "Air
from Sewage Sludge Incinerators: A Progress
Presented at the U.S./Japan Conference"^
Cincinnati. October 30, 1978.
Gilbert, W. "Troubleshooting Wet Scrubbers." Chemical
Engineering. 84:23. October 24, 1977.
Classman, I. C^mb^u_sj^iori. Academic Press. 1977.
Jacknow, J. "Environmental Aspects of Municipal Sludge
Incineration." Presented at the Fifth Conference on
Acceptable Sludge Disposal Techniques. Orlando Florida.
January IT to February T~,1978. I~n formation Transfer,
Inc., Rockville, Maryland 20852.
168. Jackn ow
Environmental Impacts from S 1 u d g e
Incineration-Present State of the Art.
Furnace Technology Committee. 1976 .
WWEMA Sludge
Kirchner, R.W. "Corrosion of Pollution Control Equipment."
Chemical Engineering Progress. 71:3. March 1975.
Marchello, J.M., and J.J. Kelly. Gas Cleaning for Air
Quality Corvtrol. Marcel Dekker, Inc. 1975.
Parker, Albert.
McGraw-Hill Inc. 197~8~7
Industrial Air Pollution Handbook.
Semrau, K.T
Scrubbers."
1977.
"Practical Process
Chemical Engineering.
Design of Particulate
84:20. September 26,
Shen, T.T.
Incineration."
Division-ASCE.
"Air Pollutants from Sewage Sludge
Journal of the Environmental
105:1. February 1979.
Stern, A. Air Pollution. Vol. Ill, 3rd Ed. Academic
Press. 1977.
Sugiki, A. "Survey of Economical and Technical Performance
for Emission Control Equipment Installed with Sludge
Incinerators . " Presented at the Fifth U.S./Japan
Conference on Sewage Treatment Technology. 1977.
11-148
-------
. D.G. Jones, L.P. Papay, S. Calvert, and S. Yung.
"Factors Influencing Plume Opacity." E nv ironmenta1 Sci e n ce
ology. Vol. 10. 1976. ~~~~——
176. Weir, A.,
"Factors ^.^^.^
and Technology
177. USEPA. Afterburner Systems Study. Office of Air Programs.
Research Triangle Park, North Carolina 27711. EPA-R2-72-
062. 1972.
178. USEPA. Air Pollution Aspects of Sludge Incineration.
Technology Transfer. Cincinnati, Ohio 45268. EPA 625/
4-75-009. June 1975.
179. USEPA. Air Pollution: Control Techniques for Hydrocarbon
and Organic Solvent Emission from Stationary Sources.
Office of Air and Waste Management. Research Triangle
Park, North Carolina 27711. NTIS: PB-240 577. October
1973.
180. USEPA. Air Pollution, Control Techniques for Particulate
Air Pollutants. Office of Air and Waste Management.
Research Triangle Park, North Carolina 27711. NTIS:
PB-240-573. 1973.
181. USEPA. Capital and Operating Costs of Selected Air
Pollution Control Systems. Office of Air and Waste
Management and Office of Air Quality Planning and
Standards. Research Triangle Park, North Carolina 27711.
EPA-450/3-76-014. 1976.
182. Gabler, R.C., and D.L. Neyland. "Incinerated Municipal
Sewage Sludge as a Secondary Source for Metals and
Phosphorus." Proceedings of the National Conference on
Sludge Management, Disposal and Utilization. Information
Transfer, Inc., Rockville, Maryland 20852. 1977.
183. California Administrative Code: Title 23; Chapter 3,
State Water Resources Control Board: Subchapter 15, Waste,
Disposal to Land.
184. USEPA. The Sanitary Landfilling of Sludge and/or Ash.
Presented at the USEPA Technology Transfer Sludge Treatment
and Disposal Seminar. Boston, Massachusetts. September
1977.
185. USEPA. Process Design Manual, Municipal Sludge Landfills.
Technology Transfer. Cincinnati, Ohio 45268. EPA-625/1-
78-010, SW-705. October 1978.
186. Reinhardt, J.J., and D.F. Kolberg. Pulp and Paper Mill
Sludge Disposal Practices in Wisconsin. April 5, 1978.
11-149
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EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapter 12. Composting
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 12
COMPOSTING
12.1 Introduction
Although sludges have been composted as a minor constituent of
refuse in many countries since the early 1900s, only since the
early seventies has major attention been directed to composting
of municipal wastewater sludges in the United States.
A major study of the composting of wastewater sludges was
conducted at Salt Lake City from 1967 to 1969 (1). This work
was followed in 1972 by research at pilot-scale wastewater
sludge composting facilities at the USDA Agricultural Research
Center at Beltsville, Maryland (2-4) and full-scale operations at
County Sanitation Districts of Los Angeles County plant at
Carson, California. Based on the operating experiences and
developments at these plants, new projects were undertaken at
Bangor, Maine (5); Durham, New Hampshire (6); and Windsor,
Ontario (7). A number of other plants are in various phases of
planning or development.
Sludge composting is the aerobic thermophilic decomposition of
organic constituents to a relatively stable humus-like material
(8). Environmental factors influence the activities of the
bacteria, fungi, and actinomycetes in this oxidation decomposi-
tion process and affects the speed and course of composting
cycles. The volatility and type of material, moisture content,
oxygen concentration, carbon/nitrogen ratio, temperature, and
pH are key determinants in the process (9). Sludge is not
rendered totally inert by composting. The composting process is
considered complete when the product can be stored without giving
rise to nuisances such as odors, and when pathogenic organisms
have been reduced to a level such that the material can be
handled with minimum risk.
Compost produced from municipal wastewater sludges can provide a
portion of the nutrient requirements for growth of crops. The
organic matter in compost is particularly beneficial as a soil
conditioner, because it has been stabilized, decomposes slowly,
and remains effective for a longer time than the organic matter
in uncomposted wastes. Composted sludge can improve the quality
of soils containing excessive amounts of sand or clay as well as
already more balanced soils. Improved physical properties
include:'
• Increased water content for sandy soils
• Increased water retention for sandy soils
12-1
-------
Enhanced aggregation
Increased aeration for clay soils
Increased permeability for clay soils
Increased water infiltration for clay soils
Greater root depth
Increased microbial population
-Decreased surface crusting (10)
The persistence of organic chemicals, pathogenic organisms, or
heavy metals in some composted sludges may restrict the use of
the material for application to crops for human consumption
(8,11). The composting process results in a significant nitrogen
reduction within the wastewater sludge and, therefore, a reduced
amount of nitrogen available to the soil and plants.
Processes for composting wastewater sludge differ from those for
composting refuse. There are several principal advantages of
sludge composting as compared to refuse composting. Sludge
composting does not require the complex materials management and
separation techniques necessary for most refuse composting
operations. Municipal wastewater sludge is more uniform in
composition causing less operating difficulties. The final
composted mixture utilizing sludge is more suitable for marketing
because it generally does not contain the plastics, metal, and
glass commonly found in refuse compost. Sludge composting is
often viewed as an alternative disposal method and does not
have to be evaluated on profit-making potential as some refuse
composting operations have been.
Classical and new solid waste composting techniques have been
modified for sludge composting. These can be classified as:
• Unconfined processes
• Windrow
• Aerated static pile
Individual pile
Extended pile
• Confined processes
Unconfined processes are not enclosed, although a roof may be
provided to protect the compost from precipitation. Unconfined
processes make use of portable mechanical equipment such as
front-end loaders or mixers for compost mixing and turning.
Confined systems utilize a stationary-enclosed container or
reactor for composting.
12.2 The Composting Process
Although each composting technique is unique, the fundamental
process is similar. The basic process steps are as follows:
• If required, bulking agents for porosity and moisture
control (for example, recycled compost, wood chips, etc.)
or feed amendments for a source of limiting nutrients
12-2
-------
such as carbon (for example, sawdust, rice hulls, etc.)
are added to the dewatered sludge to provide a mixture
suitable for composting. ,The mixture must be porous,
structurally stable, and capable of self-sustaining the
decomposition reaction.
• A temperature in the range of 130° to 150°F (55° to 65°C)
is attained to ensure destruction of pathogenic organisms
and provide the driving force for evaporation, which
reduces the moisture content.
• The compost is stored for extended periods after the
primary composting operation to further stabilize the
mixture at lower temperatures.
• Additional air drying (for example, windrowing) may be
required if the cured compost is too wet for further
processing.
• When bulking agents are reused, a separation operation is
required.
Composting represents1- the combined activity of a succession of
mixed populations of bacteria, actinomycetes, and other fungi
associated with a ^diverse succession of environments. Moisture,
temperature, pH, nutrient concentration, and availability and
concentration of oxygen supply are principal factors which affect
the biology of composting (12).
12.2.1 Moisture
Decomposition of organic matter is dependent upon moisture. The
lowest moisture content at which bacterial activity takes place
is from 12 to 15 percent; however, less than 40 percent moisture
may limit the rate of decomposition. The optimum moisture
content is in the range of 50 to 60 percent. If the mixture is
over 60 percent water, the proper structural integrity will not
be obtained.
Dewatered municipal sludges are usually too wet to satisfy
optimum composting conditions. The moisture content can be
reduced by blending the sludge with a dry bulking material or a
recycled product, and dewatering the sludge to as great an extent
as economically possible. The best approach for a particular
site can be determined from a mass balance of the particular
composting facility and by a site-specific economic analysis
based on the mass balance results. Figure 12-1 illustrates the
effect of the solids content of dewatered sludge on the required
mixing ratio of wood chips to sludge by volume for one compost
operation. The amount of wood chips needed for a 40 percent
filter cake would be about one-fifth the amount required for a
20 percent solids cake. In addition to savings on wood chips,
there would be a substantial reduction in material management
costs and site sizes (13).
12-3
-------
LU
O
LU
O
Q
D
_i
e/i
yj
O
x
O
O
O
O
cc
O
z
X
5
NOTE: THIS CURVi IS SITE-SPECIFIC FOR
ONE COMPOST OPERATION, THIS
CURVE WILL SHIFT DEPENDING ON
THE RELATIVE VOLATILITY AND
SOLIDS CONTENT OF THE WOOD
CHIPS AND SLUDGE.
10 20 30
PERCENT SOLIDS IN SLUDGE
40
50
FIGURE 12-1
EFFECT OF SOLIDS CONTENT ON THE RATIO OF WOOD
CHIPS TO SLUDGE BY VOLUME (14)
12.2.2 Temperature
For most efficient operation, composting processes depend on
temperatures of from 130° to 150°F (55° to 65°C) but not above
176°F (80°C). High temperatures are also required for the
pathogens in the sludge. Moisture content,
and shape of pile, atmospheric conditions,
the temperature distribution in a compost
temperature elevation will be less for a
inactivation of human
aeration rates, size
and nutrients affect
pile. For example,
given quantity of heat released if excessive moisture is present,
12-4
-------
as heat will be carried off by evaporation. On the other hand,
low moisture content will decrease the rate of microbial activity
and thus reduce the rate of heat evolution.
12.2.3 pH ;
The optimum pH range for growth of most bacteria is between 6 and
7.5 and between 5.5 and 8.0 for f ungi .•( 14) . The pH varies
throughout the pile, and throughout the composting operation, but
it is essentially self regulating. A high initial pH resulting
from the use of lime for dewatering will solubilize nitrogen in
the compost and contribute to the loss of nitrogen by ammonia
volatilization. It is difficult to alter the pH in the pile for
optimum biological growth, and this: has not been found to be an
effective operation control.
12.2.4 Nutrient Concentration
*'
Both carbon and nitrogen are required as energy sources for
organism growth. Thirty parts by weight of carbon (C) are
used by microorganisms for each part of nitrogen (N); a C/N ratio
of 30 is, therefore, most desirable for efficient composting,
and C/N ratios between 25 and 35 provide the best conditions.
The carbon considered in this ratio is biodegradable carbon.
Lower C/N ratios increase the loss of nitrogen by volatilization
as ammonia, ..and higher values lead to progressively longer
composting times as nitrogen becomes growth-rate limiting (12).
No other macro-nutrients or trace nutrients have been found to be
rate limiting in composting municipal wastewater sludge.
12.2.5 Oxygen Supply
Optimum oxygen concentrations in a composting mass are between
5 and 15 percent by volume (15). Increasing the oxygen
concentration beyond 15 percent by air addition will result in a
temperature decrease because of the greater air flow. Although
oxygen concentrations as low as :0.5 percent have''teen observed
inside windrows without anaerobic -symptoms^ at least 5 percent
oxygen is generally required for aerobic conditions (12).
12.2.6 Design Criteria and Procedures
The basic criteria for successful composting are that the
material to be composted be porous and 'structurally stable and
contain sufficient degradable material so that the degradation
reaction is self-sustaining (that is, heat released by oxidation
of volatile material is sufficient ' to ' raise the mixture to
reaction temperature and to bring it to required dryness). In
this section, a procedure to meet these" criteria of porosity,
structural stability, 'and sufficient biodegradability will
12-5
-------
be discussed. An equally important design consideration is
flexibility. A compost operation must be able to operate
continuously even with changes in sludge solids content and
volume. Changes in bulking agent supply and equipment failure
must also be anticipated, and the design must be flexible to deal
with these changes.
To obtain minimal assurance that the composting activity is
proceeding properly, the temperature and oxygen content within
the pile are constantly monitored. Equipment required to conduct
this monitoring includes a portable, 0 to 25 percent, dry-gas
oxygen analyzer which is used to measure the oxygen content; a
probe-thermistor-type temperature indicator, with at least a
6-foot probe and scale reading from 32° to 212°F (0° to 100°C)
is also needed. Additionally, monitoring of heavy metals, patho-
gens, and environmental parameters such as air and water quality
ensures a safe and acceptable compost and composting operation.
A comprehensive monitoring program is outlined in Table 12-1.
TABLE 12-1
SUGGESTED MONITORING PROGRAM FOR A
MUNICIPAL WASTEWATER SLUDGE
COMPOSTING FACILITY (17)
Activity/time Component Analysis Frequency
Before composting Sludqc and bulking Heavy metals and PCB's Monthly
material
During composting Aerated pile or windrows Acceptable time, temperature, Temperature and oxygen con-
dissolved oxygen relation- tent measurements taken at
ships, that is, 131°F (55°C) least 6 days during first
and 5 to 15 percent oxygen 2 weeks. (Additional
content for 3 to 5 days. measurements sometimes
required to get true
average!.
After composting Compost (prior to Certain selected indicator Monthly or bimonthly depending
marketing) heavy metals and pathogens. on use of compost.
Site monitoring during Personnel Physical examination prior Annually-
entire ope ration to employment and periodi-
cally thereafter.
Protective equipment and Continuously
clothing as needed.
Odors Odor strength Continuously, but especially
during wet periods with
•temperature inversions and
little to no wind.
Odor filter pile effective- Continuously
ness .
Log of odor complaints. Continuously
Dust Assessment of particulate Continuously but especially
concentrations. during dry period under
windy conditions
Leachate and runoff BOD and suspended solids. Monthly, downwind at locations
critical to public hea1th
concerns.
Airborne spores
Mi crometeoro logical
Numbers generated and
transported .
Temperature at 5 ft (1.5m)
and 25 ft (7.6 ml
Wind speed
Wind direction
Monthly
Continuously
Continuously
Cont inuously
12-6
-------
Pour locations for temperature and oxygen measurements at both
ends of each pile are shown on Figure 12-2,
_L
y- B
JL
2
BASE "B"
FIGURE 12-2
LOCATIONS FOR TEMPERATURE AND OXYGEN
MONITORING AT ONE END OF A WINDROW OR
INDIVIDUAL AERATED PILE
Haug and Haug (17) have shown the compost reaction is self-
sustaining when the ratio W is <_ 10. This ratio is defined
as:
_ mass of water in the initial compost mixture
mass of organics degraded under compostingconditions
In windrow and mechanical composting, porosity and structural
stability are provided when the sludge is mixed with recycled
compost product or bulking agent to obtain a solids concentration
of approximately 40 to 60 percent. With aerated pile composting,
a bulking agent such as wood chips is used to provide porosity
and structural stability. When the composting process is
complete, the bulking agents are generally screened out of the
compost and recycled back to the mix point for reuse. The fine
portion of the bulking agent is usually retained with the compost
product because it passes through the screen with the finished
compost. Fresh bulking agent must be added at the mix point to
compensate for this material loss.
Mixture degradability can be adjusted by the addition of
materials that contain high concentrations of degradable organic
material. These materials are usually dry and reduce the ratio W
by increasing the volatile fraction and decreasing the moisture
fraction of the mixture.
12-7
-------
Figure 12-3 shows a generalized mass balance diagram for the
compost process. The recycle stream could consist of finished
compost only (typical for windrow and mechanical methods),
bulking agent only (typical for aerated pile methods) or a
combination of bulking agent and finished compost. Amendment may
also be added with bulking agent. The exact quantities of the
various streams are dependent on the mass balance equations (12-1
and 12-2) derived from Figure 12-3 and the type of composting
process utilized.
A set of equations can be developed from an analysis of the
mass balance diagram. Two general equations have been arranged
that apply to all composting methods. Equation 12-1 is used
to determine the recycled compost or wood chip quantity and
Equation 12-2 is used to determine the ratio W (17):
XC(SM-SC) + XA(SM-SA) + xB(sM-sB)
XR = - (SR-SM) -- - - <12-1)
= + XB(1-SB) + XR(1-SR)
XCSCVCkc + XASAVAkA + XBSBVBkB + XRSRVRkR (12-2)
Compost Processes With No External Bulking Agent
To design a compost facility employing no external bulking agent,
the parameters Xc,So v"okC' SR' vR'kR' and SM must be determined
analytically, assumed, or calculated. The wet weight of recycled
compost (XR) is calculated, assuming no amendment or external
bulking agent addition (XA=XB=0), to provide a desired solids
content of the mixture (S^) in the 0.40 to 0.50 range:
XC(SM-SC)
(12-3)
e q
(SR-SM)
Once XR is determined for these conditions, the ratio W is
calculated :
XC(1-SC) + XR(1-SR)
~
If the ratio W is less than ten, the compost mixture has
sufficient energy available for temperature elevation and water
evaporation. The ratio number of ten is not absolute because
climatic conditions affect the thermodynamic energy requirements.
In a hot, arid climate, W may be higher because evaporation of
water from the compost mass is increased by a high humidity
12-8
-------
AMMENDMENT
EXTERNAL !
8ULKJNG AGENT
Note: RECYCLE is defined as finished compost for the windrow and
mechanical systems and as recycled wood chips for the
aerated pile system.
The exact value for these parameters must be determined
from samples of the sludge, external bulking agent,
amendment, and estimated for the recycle values unless
otherwise known.
Process Variables and Range of Average Values (in Parenthesis)
X = Total wet weight of sludge
cake produced/day.
X = Total wet weight of
amendment/day.
X = Total wet weight of
recycle/day.
X = Total wet weight of external
bulking agent/day.
X = Total wet weight of mixture/
M day.
S = Fractional solids content of
L sludge cake (0.20 to 0.55).
S = Fractional solids content of
amendment (0.50 to 0.95).
S = Fractional solids content of
recycle (0.60 to 0.75).
S = Fractional solids content of
external bulking agent (0.50
to 0.85).
S = Fractional solids content of
mixture (0.40 to 0.50).
V = Volatile solids content of
sludge cake, fraction of
dry solids (0.40 to 0.60) -
Digested; (0.60 to 0.80) -
Raw.
V = Volatile solids content of
amendment, fraction of dry
solids (0.80 to 0.95).
V = Volatile solids content of
recycle, fraction of dry
solids (0.00 to 0.90).
V = Volatile solids content of
external bulking agent,
fraction of dry solids
(0.55 to 0.90).
VM = Volatile solids content of
mixture, fraction of dry
solids (0.40 to 0.80).
k_ = Fraction of sludge cake
volatile solids degradable
under composting conditions
(0.33 to 0.56) .
kft = Fraction of amendment
volatile solids degradable
under composting conditions
(0.40 to 0.60).
k = Fraction of recycle volatile
solids degradable under
composting conditions (0.00
to 0.20) .
k = Fraction of external bulking
agent volatile solids de-
gradable under composting
conditions (0.00 to 0.40).
k = Fraction of mixture volatile
solids degradable under com-
posting conditions (0.20 to
0.60) .
FIGURE 12-3
SLUDGE COMPOSTING MASS BALANCE DIAGRAM
12-9
-------
driving force and higher initial pile temperatures. In a cold
climate, more biological energy is required to heat the pile to
normal operating temperatures and thus W may have to be as low as
seven to ten ( 17 ) .
The ratio W can be reduced by adding amendment. The parameters
SA'^A' an<3 kA are known. The amendment dry weight is assumed,
and a new recycle compost mass (XR) is calculated:
xc (SM-SC) + XA (SM-SA)
The ratio W is also recalculated:
xc d-sc) + XR (i-sR) + XA
XRSRVRkR 4- XASAVAkA
If W is still not below ten, the quantity of amendment is
increased and XR and W are recalculated until the W requirement
is satisfied.
If these guidelines are followed, a mixture with sufficient
energy to compost will be produced. The actual values for the
process parameters are site-specific and the most economical
design is dependent on accurate information about the composting
characteristics that affect the mass and thermodynamic balance.
Compost Processes _Using__^ternal__B^]LkJLng__Ag_e_n_t
Design criteria for processes using external bulking agent are
similar to those just described except that the recycle rate is
calculated in a different manner. In the former process, the
ratio of total bulking agent to sludge is specified without
regard to the mixture's moisture content, since it is not as
important as the structural integrity of the pile. The recycle
rate, XR, and makeup supply are calculated using Equations 12-7
and 12-8.
XR = (l-f2) fiXG (12-7)
XB = fi XC - XR (12-8)
where f j_ is defined as the ratio of external bulking agent
(recycle and makeup) to sludge
XR + XB
fi = --
12-10
-------
and £2 represents the fraction of total external bulking agent
lost from the process by volatilization or because it remains
with the finished compost.
f XB
f2 =
XB + XR
The values for f^ and £2 must be assumed based on operating
experience at an existing facility. The range of values for f±
are 0.75 to 1.25, and for f2 are 0.20 to 0.40. Once these
values are chosen, the amount of recycled bulking agent (XR)
and new external bulking agent ( Xg) can be calculated using
Equations 12-7 and 12-8.
The value of the ratio W is then calculated using Equation 12-2,
indicating no amendment is used (X^ =0). If W is less than or
equal to ten, then the mixture has sufficient energy to compost.
If W is greater than ten, two options for reducing the ratio are
possible. More external bulking agent can be used (that is, f]_
is increased). If the bulking agent is more volatile than the
sludge, W should be reduced. The recycle and makeup quantities
of bulking agent must be recalculated and W determined again. If
the bulking agent is of low volatile fraction, this approach will
not work because W will be reduced only slightly. In this case,
amendment must be added.
For any amount of amendment addition, the ratio W can again
be calculated using Equation 12-2. Increasing the amount of
amendment until W is below ten will result in the proper compost
energy balance.
The operation at Bangor, Maine, successfully composts sludge by
the aerated pile method in winter months. No amendment is used,
and the ratio of external bulking agent (bark) to sludge by
volume is 2.5:1. The value for W ranges from seven to ten
at this operation (17).
The best means to determining the quantities of external bulking
agent and amendment used will be a careful economic analysis of
the process and accurate estimation of the process variables.
Table 12-2 lists some of the density ranges for various compost
materials as experienced at various compost facilities.
12.3 Unconfined Composting Systems
In the United States, the windrow and aerated static pile
processes have been used almost exclusively for composting
dewatered municipal wastewater sludges. The basic steps to be
followed in these two processes are similar, but the processing
technology for the composting stage differs appreciably. In
12-11
-------
the windrow method, oxygen is drawn into the pile by natural
convection and turning, whereas in the static pile method,
aeration is induced by forced air circulation.
TABLE 12-2
DENSITIES OF VARIOUS COMPOST
BULKING AGENTS (13)
Density,
Material Ib/cu yd
Digested sludge . 1,500 to 1,700
Raw sludge 1,300 to 1,700
New wood chips 445 to 560 . .
Recycled wood chips 590 to 620
Finished compost 930 to 1,040
1 Ib/cu yd = 0.595 kg/m3
12.3.1 Windrow Process
The windrow process is normally conducted in uncovered areas and
relies on natural ventilation with frequent mechanical mixing
of the piles to maintain aerobic conditions. In areas of
significant rainfall, it may be desirable for operational
reasons to provide a roofed structure to cover the windrows for
composting sludge. The largest operating windrow process in the
United States is located at the Joint Water Pollution Control
Plant of the County Sanitation Districts of Los Angeles County in
Carson, California.
In the windrow composting process, the mixture to be composted is
stacked in long parallel rows or windrows. The cross section of
the windrows may be trapezoidal or triangular, depending largely
on the characteristics of the mobile equipment used for mixing
and turning the piles. The width of a typical windrow is 15 feet
(4.5 m) and the height is 3 to 7 feet (1 to 2 m).
Based on processing 20 percent solids sludge, land requirements
for the windrow process are greater than for the aerated pile
process. Colacicco estimates an extra 25 percent land usage for
the windrow process based on windrows 5 feet (1.5 m) high and
7 feet (2 m) wide with a two-week composting period (18). Even
more land would be necessary for the longer composting time
experienced in the Los Angeles operations.
The mixing of a bulking agent with the wet sludge cake has
enabled the windrow process to be used for composting digested
dewatered sludge. Bulking agents may include the recycled
composted sludge itself or external agents such as wood chips,
12-12
-------
sawdust, straw, rice hulls, or licorice root. The quantity of
bulking agent is adjusted to obtain a mixture solids content of
40 to 50 percent. The use of a bulking agent also increases the
structural integrity of the mixture and thus, its ability to
maintain a properly shaped windrow. Porosity of the mixed
material is greatly improved, which in turn improves the aeration
characteristics. External bulking agents can also provide a
source of carbon for the composting process. The carbon to
nitrogen (C/N) ratio of digested activated sludge is in the range
of 9 to 15:1. If wood chips are used as the bulking agent, the
C/N ratio will be raised to approximately 20 to 30:1 in the
composting mixture.
Convective air movement within windrows is essential for
providing oxygen for the microorganisms. The aerobic reaction
provides heat for warming the windrows. This causes the air
to rise, producing a natural chimney effect. The rate of air
exchange can be regulated by controlling the porosity and
size of the windrow (2). The turning of the windrow also
introduces oxygen to the microorganisms. This method of aeration
can be expensive if used excessively to obtain high oxygen
concentrations and may reduce the temperature within the windrow.
As a result of the biological decay process, temperatures in the
central portion of the windrow reach as high as 150°F (65°C).
Operating temperatures of about 140°F (60°C) may be maintained in
the central portion of the windrow for as long as ten days.
Temperatures in the outer layers are considerably cooler and may
approach atmospheric conditions. During wet periods and winter
conditions, maximum temperatures may only be 130° to 140°F (55°
to 60°C). A high temperature maintained throughout the pile for
a sufficient period of time is important to the control of
pathogens (see Chapter 7). A satisfactory degree of stabiliza-
tion is indicated by a decline in temperature, usually to about
113° to 122°F (45° to 50°C). These variations in temperature are
illustrated in Figure 12-4.
Large-scale, 270 dry tons per day (243 t/day) processing of
digested primary sludge (23 percent solids) using the windrow
process, with recycled composted sludge as the bulking agent, has
proven a viable method of sludge stabilization by the Los Angeles
County Sanitation Districts. Successful operation of the windrow
process using bulking agents such as wood chips and sawdust with
digested primary and secondary sludge has also been achieved at
Beltsville. This process has not proven suitable for composting
unstabilized primary or secondary sludges. At Beltsville during
early tests with windrows, undigested primary and waste-activated
sludges were found to produce offensive odors (3). Also,
composting of digested sludge did not kill all seeds, and these
were present in the final product.
The Los Angeles .County Sanitation Districts are currently
composting digested, centrifuged primary sludge (23 percent
solids) in windrows mixed with recycled composted sludge
12-13
-------
(60 percent solids) in a 1:2.2 ratio (dry weight). A compost
mixing machine is used to turn the mixture. Recycled compost is
added to the sludge before the windrow is constructed. Each
windrow must be turned two or three times a day for the first
five days to mix the material completely, minimize odors, and
ensure sufficient oxygen transfer. The sludge is then turned
once a day for about 30 days, depending on weather conditions.
Figure 12-5 shows a windrow being turned at Los Angeles.
o
o
EC
D
<
tr
LU
O.
S
LU
71.1
60.0
48.9
37.9
26.7
15,5
4,5
-7.8
I
I
o TEMPERATURE AT 18-INCH
DEPTH IN WINDROWS
D AMBIENT ATMOSPHERIC
TEMPERATURES
D
160
140
120
LL
O
100 Ly
QC
80
60
40
20
0
EC
LU
CL
^
LU
0 20 40 60 80 100 120 140 160 180 200 220
1 in = 2.54 cm
DAYS
FIGURE 12-4
TEMPERATURE PROFILE OF A TYPICAL COMPOST
WINDROW (12)
Large, portable, heavy materials handling equipment is required
for the windrow system. The Los Angeles operation requires four
windrow mixing-turning machines capable of turning 3,400 tons
per hour (3,084 t/hr) of a density of 1,890 pounds per cubic
yard (1,120 kg/m^). This is equivalent to a volume capacity
of 3,600 cubic yards per hour (2,752 m3/hr). Three machines
operate continuously for two shifts a day. A fourth machine
is required to provide backup whenever any of the others is
being repaired. In case of rain all four machines must operate
continuously.
12-14
-------
FIGURE 12-5
TURNING A WINDROW AT LOS ANGELES COMPOST SITE
Sawdust, shredded paper, and wood chips were the external bulking
agents used in the Beltsville windrow tests. Only shredded
paper was found to be unsatisfactory (2). The windrow area at
Beltsville was paved with 18 inches (0.46 m) of crushed stone
to support heavy equipment and the windrow composter. The area
was later paved with asphalt and then with concrete to assure
positive leachate collection and to eliminate rock pickup from
the collection equipment and damage to the screening equipment.
To start the windrow, a layer of wood chips 15 inches (0.38 m)
deep and 15 feet (4.5 m) wide was placed on the paved area.
Sludge (20 to 25 percent solids) was distributed to the chips
at a 1:3 volume ratio. The compost machine then mixed the
sludge and chips. After several turnings, the two materials
were thoroughly mixed. The windrow was turned five times a
week, flattened after two weeks to a 12-inch (0.30 m) layer and
harrowed for further drying, generally to greater than 65 percent
solids. The material was then removed from the windrow area and
stockpiled for an additional 30 days for curing purposes. Curing
was required to improve compost quality and to further control
pathogens. After curing, the composted mixture was distributed
to local government agencies as screened or unscreened material.
Wood chips separated during the screening operation were recycled
and reused as bulking agent. The use of a bulking agent may
substantially increase the cost of the composting process unless
the bulking agent is itself a waste material (7). At Beltsville,
12-15
-------
a fresh supply of wood chips was required to make up for the
estimated 25 to 30 percent lost in the composting process. Some
of the bulking agent was consumed in the biological oxidation
processes during composting, and a large portion was lost in the
screening process.
12.3.1.1 Energy Requirements
Thermodynamic considerations in the composting of sludge are
discussed in a recent article by Haug & Haug (17). As indicated
previously, the reaction is self-sustaining when the ratio W is
less than ten. Over 80 percent of the heat released by the
biological reaction is used to evaporate moisture associated with
the sludge.
In the windrow process, the only external energy requirements
are gasoline for transportation, diesel fuel for operation of
composting machines, and electricity for leachate treatment and
site services, including lighting. In the Beltsville windrow
tests, which used wood chips as a bulking agent, the following
energy consumption figures have been estimated (18).
Operating Requirements
per dry ton per day (0.9 t/day) for a
10 to 50 dry ton per day (9 to 45 t/day) operation
Labor 1.8 to 3.0 hours
Gasoline 1.1 gallons (4.5 1)
Diesel Fuel 3.3 to 4.0 gallons (13.5 to 16.5 1)
Electricity 3.0 to 8.0 kWhr (12 to 32 MJ)
Where finished compost is used as the bulking agent, and
increased windrow turning frequency is practiced, a higher diesel
fuel consumption should be expected.
12.3.1.2 Public Health and Environmental Impacts
Numerous studies have indicated that a community's wastewater
contains organisms which reflect the local prevalent endemic
diseases (19). The pathogens borne by wastewater are not
entirely inactivated during conventional sludge digestion and
drying techniques and may persist in the soil for extended
periods of time. Figure 12-6 shows this time-temperature-
destruction relationship of pathogens for windrows (20,21).
Intensive studies conducted by the Los Angeles County Sanitation
Districts indicate that total coliform and Salmonella concentra-
tions are rapidly reduced in the first ten days of composting in
the interior of windrows. For interior samples, final compost
coliform concentrations of less than one per gram have been
12-16
-------
attained, but higher values for exterior samples have been
measured consistently. Very low levels of virus, parasitic ova,
and Salmonella have been assayed in the majority of final compost
samples.
8.0
a,
i$~
g
UJ
o
z
o
o
cc
UU
b
6.0 -
4,0 ~
2.0 -
SALMONELLA
-2,0
I
3 10
1
20
1
30
1
40 5(
70
60
50
o
o
-------
Recycling large quantities of finished compost as bulking agent
provides good odor control for digested sludges, as long as
process upsets are kept under control. Interruption of regular
turning of the sludge may cause odor problems, since compost
windrows quickly become anaerobic under these circumstances.
Unpleasant odors may also be generated during periods of high
rainfall, as well as by poor mixture control and inefficient
mixing. In dry and windy areas, wetting of the compost windrows
should be practiced to prevent excessive dust generation.
A drainage and collection system is required for stormwater
runoff from the site because the contaminated water requires
treatment. The runoff may be returned to the wastewater
treatment plant. At Beltsville, a wooded area adjacent to the
site was spray irrigated (2).
Workers at a compost site should avoid inhaling dust. Respira-
tory protection, such as breathing masks, should be worn in
dusty areas, and the area should be sprinkled with water during
dry periods. Although recent experiments have shown high
concentrations of the fungus Aspergillus fumigatus, a secondary
pathogen, to be airborne at sludge composting sites, preliminary
data indicate that these higher spore levels are generally
restricted to the immediate composting area and should not pose a
significant health threat to surrounding residential, commercial,
or industrial areas (22). However, individuals with a history of
lung ailments should not work in composting operations. Research
is continuing on potential health effects of exposure to the
fungus A. fumigatus (23 to 27). For additional discussion, see
Chapter"?.
12.3.1.3 Design Example
This design example illustrates the procedure for a 10 MGD
(0.45 m^/s) municipal wastewater secondary treatment plant.
The dewatered, digested primary and secondary sludge (20 percent
solids) is generated at the rate of one dry ton per million
gallons (.00024 t/m3). The compost facility will handle ten
dry tons per day (9 t/day) at 20 percent solids, seven days per
week. The values for the process design variables are similar to
those reported for Beltsville. The availability and cost of
amendments and suitable land for the operation will strongly
influence the economic analysis of the project. This design
example, however, does not consider these site-specific economic
parameters.
The design of this windrow composting facility is based on the
following assumptions:
• The water content and total weight of the compost mixture
will be reduced by approximately 40 to 50 percent
and volatile solids content will be reduced by about
20 to 40 percent. The density will decrease by 15 to
25 percent because of evaporation.
12-18
-------
• The values for the process variables defined previously
are assumed to be as follows:
Sc = 0.20
VC = 0.50
kc = 0.45
SR = 0.70
VR = 0.35
kR = 0.15
SA = 0.90
VA = 0.90
kA = 0.50
SM = 0.40
VM = 0.50
• If the mixture has a high ratio of water to degradable
organics by weight (W ratio greater than ten), amendment
will be added to reduce W.
The amount of finished compost to be recycled can be calculated
using Equation 12-3.
XR =
Xc (SM - S
(SR - sMT
50 (0.04 - 0.20)
(0.70 - 0.40)
= 33.3 tons per day (30.3 t/day)
This indicates that if a mixture moisture content of 40 percent
is to be obtained, 0.67 tons (.67 t/t) of finished compost must
be added to each ton (0.9 tonne) of sludge cake to be composted.
The ratio W is checked using Equation 12-4 in order to determine
whether to compost.
W =
XC(1-SC) + XR(1-SR)
Xcscvckc + XRSRVRkR
50(1-0.20) + 33.3(1-0.70)
50(0.20)(0.50)(0.45) + 33.3 (0.70)(0.35)(0.15)
= 14.4
The calculated value for W is too high, indicating that amendment
addition is required. Increasing the recycle rate to create a
mixture of 50 percent solids (XR = 50 tons per day [45 t/day])
would only lower W to 13.5, because the proportion of degradable
organics does not increase significantly in the mixture.
Assuming that 1.0 ton (0.9 t) amendment per ten tons (9 t) of
sludge cake are added to the mixture, the recycle rate can be
calculated using Equation 12-5:
12-19
-------
XR =
xA(sM-sA)
(SR-SM)
50 (0.40 - 0.20) + 5 (0.40 - 0.90)
(0.70 - 0.40)
= 25.0 tons per day (22.7 t/day)
The amount of recycled compost has dropped from 0.67 tons per ton
(0.61 t/t) to 0.5 tons per ton (0.5 t/t) of sludge cake. The
ratio W is calculated using Equation 12-6:
W =
XC
XR (1
XRSRVRkR
XA (1
XASAVAkA
50(1-0.20) + 25(1-0.70) + 5(1-0.90)
50(0.20)(0.50)(0.45) + 25(0.70)(0.35)(0.15) + 5(0.90)(0.90)(0.50)
= 9.2
This mixture of sludge cake, recycled compost, and amendment
is self-sustaining and will degrade properly. Figure 12-7
process and shows the materials balance.
illustrates this
A 7-foot (2 m) high, 65-foot (20 m) long, windrow with a base
of 15 feet (4.6 m) is constructed each day. Longer windrows
can be made if the windrow is extended each day with the mixture
to be composted. The final volume of composting at the end of
six weeks of turning is approximately 65 percent of the original
volume. In continuous operation there would be about 11
windrows, 250-feet (76 m) long.
Each windrow must be turned at least two times per day for the
first five days to mix the materials completely, to minimize
odors, and to insure sufficient oxygen transfer. After the
initial five-day period, the windrows must be turned frequently
enough to maintain the proper oxygen level and temperature in the
composting material. This is dependent on weather conditions.
Other site operations must include a mixing area, maintenance and
operations building, a curing area to stockpile the finished
compost, and enough land area for handling all other site
operations and for future expansion.
Equipment required for the operation includes a windrow turning
machine; a front-end loader for site preparation, dismantling
12-20
-------
piles and loading transfer trucks; and transfer trucks to haul
the sludge and amendment to the compost facility and to haul the
finished compost away.
DIGESTED
DEW ATERED
SLUDGE
MIXING
OFF-fiASES
WINDROW
COMPOSTING
42 DAYS
RETENTION
DRYJNG
(IF REQUIRED)
6 DAYS
RETENTION
6
AMENDMENT
COMPOST
CURING AND
STORAGE
60 DAYS
CAPACITY
7 DAY PER WEEK OPERATION
LOCATION
1
2
3
4
5
6
7
WET
TONS
50
5
80
41
39
25
14
PERCENT
SOLIDS
20
90
40
70
70
70
DRY
TONS
10.0
4.5
32.0
5.0
27.0
17.5
9.5
DENSITY
(Ib/cu yd)
1,600
1,000
1,300
1,000
1,000
1,000
VOLUME
(cu yd)
63
10
123
78
50
28
PERCENT
VOLATILE
SOLIDS
50
90
50
35
35
35
1 ton = 0.907 tonne
1 Ib/cu yd = 0.6 kg/m3
1 cu yd = 0.76 m3
FIGURE 12-7
PROCESS FLOW DIAGRAM - WINDROW COMPOSTING
SLUDGE - 10 MGD ACTIVATED SLUDGE PLANT
12-21
-------
Optimum windrow compost design will do the following:
9 Minimize hauling and handling cost.
• Maximize use of existing equipment in the compost
operation.
• Minimize the use of amendment which adds to the cost and
is not recoverable.
• Maximize the solids content of the dewatered digested
sludge cake to minimize the amount of recycled compost
used for moisture control and also reduce the amount of
amendment required. The cost of dewatering should not
exceed the savings at the compost facility.
12.3.2 Aerated Static Pile Process
An aerated static pile system was developed in order to eliminate
many of the land requirements and other problems associated with
the windrow composting process and to allow composting of raw
sludge. This system consists of the following steps: mixing of
sludge with a bulking agent; construction of the composting pile;
composting; screening of the composted mixture; curing; and
storage. A diagram of an aerated pile for composting sludge is
shown in Figure 12-8.
AIR
SCREENED Oft-
UNSCREENED
COMPOST
SLUDGE AND
BULKING
AGENT
PERFORATED
PIPE
DRAIN FOR
CONOEN5ATES
— EXHAUST FAN
FILTER PILE
FIGURE 12-8
CONFIGURATION OF INDIVIDUAL AERATED PILES
12-22
-------
The forced air method provides for more flexible operation and
more precise control of oxygen and temperature conditions in
the pile than would be obtained with a windrow system. Since
composting times tend to be slightly shorter and anaerobic
conditions can be more readily prevented, the risk of odors is
reduced.
Two distinct aerated static pile methods have been developed,
the individual aerated pile and the extended aerated pile.
12.3.2.1 Individual Aerated Piles
An individual aerated pile may be constructed in a manner similar
to the Beltsville method, in which loop of perforated plastic
pipe, 4 to 6 inches (10 to 15 cm) in diameter is placed on the
composting pad, oriented longitudinally, and centered under the
ridge of the pile under construction. In order to avoid short
circuiting of air, the perforated pipe terminated at least 8 to
10 feet (2 to 3 m) inside the ends of the pile. A non-perforated
pipe that extends beyond the pile base is used to connect the
loop of perforated pipe to the blower. (See Figure 12-9).
AIR
SCREENED OR
UNSCH
COVER
BULKING ACjENT
AND SLUDGE ' /
BULKING
AGENT BASE
NON PiHFOflATiD FIFE
L- F(LTiR PILE
SCREENED
COMPOST
FIGURE 12-9
AERATION PIPE SET-UP FOR INDIVIDUAL AERATED PILE
A 6- to 8-inch (15 to 20 cm) layer of bulking agent is placed
over both the pipes and the area to be covered by the pile.
This base facilitates the movement and even the distribution of
air during composting and absorbs excessive moisture that may
otherwise condense and drain from the pile (19).
12-23
-------
At Beltsville a mixer or front-end loader is used to mix one
volume of sludge cake containing 22 percent solids and two
volumes of bulking agent. The resulting mixture contains
40 percent solids and is placed loosely upon the prepared base by
the front-end loader to form a pile with a triangular cross
section 15 feet (4.6 m) wide by 7.5 feet (2.3 m) high.
The pile is then completely covered with a 12-inch (0.3 m)
layer of cured, screened compost or an 18-inch (0.4 m) layer of
unscreened compost. This outer blanket of compost provides
insulation and prevents escape of odors during composting.
Unstabilized sludge can generate odors during dumping and initial
pile construction. Conditioning with lime during dewatering will
minimize this, however. The non-perforated pipe is connected to
a 1/3 horsepower (0.25 kW), 335 cubic feet per minute (158 1/s)
blower that is controlled by a timer (28). Aerobic composting
conditions are maintained if air is intermittently drawn through
the pile. The timing sequence for the blower is 5 minutes on and
15 minutes off for a 56-foot (17 m) long pile containing up to
80 wet tons (73 t) of sludge. If the aeration rate is too high
or the blower remains on too long, the pile will cool, and the
thermophilic process will be inhibited (12).
The effluent air from the compost pile is conducted into a small,
cone-shaped filter pile of cured, screened compost approximately
4 feet (1.2 m) high and 8 feet (2.5 m) in diameter where
malodorous gases are absorbed. The odor retention capacity of
these piles is inhibited if their moisture content is greater
than 50 percent. The odor filter pile should contain one cubic
yard (0.76 m^) of screened compost for each four dry tons (3.6 t)
of sludge in the compost pile. Filter piles are sometimes
constructed with a 4-inch (10 cm) base layer of wood chips to
prevent high back pressures on the blower.
Land area requirements are estimated at one acre per 3 to 5 dry
tons (1.0 ha/6.7 to 11.2 t) of sludge treated. The lower figure
includes space for runoff collection, administration, parking,
and general storage. The actual composting area (mixing area,
aerated piles, screening area, drying area, and storage area)
is estimated to be one acre per 5 dry tons (1.0 ha/11.2 t) of
sludge (19) .
12.3.2.2 Extended Aerated Piles
To make more effective use of available space, another static
pile configuration called the extended aerated pile has been
developed. An initial pile is constructed with a triangular
cross section utilizing one day's sludge production. Only
one side and the ends of this pile are blanketed with cured,
screened compost. The remaining side is dusted with only about
an inch (0.5 cm) of compost for overnight odor control. The
next day, additional aeration pipe is placed on the pad parallel
to the dusted side of the initial pile. The pile bed is extended
12-24
-------
by covering the additional pipe with more bulking agent and
sludge-bulking agent mixture so as to form a continuous or
extended pile. This process is repeated daily for 28 days.
The first section is removed after 21 days. After seven sections
are removed in sequence, there is sufficient space for operating
the equipment so that, a new extended pile can be started.
Figure 12-10 shows such a system. The area requirement of an
extended pile system is about 50 percent less than that for
individual piles. The amount of recycled bulking agent required
for covering the pile and bulking agent used in the construction
of the base is also reduced by about 50 percent. At Beltsville,
research into extending aerated piles in both the vertical and
horizontal directions is ongoing.
EXJUPC.25T
HtHPVJn
MERE
" MSXTUfll ~U
FIGURE 12-10
CONFIGURATION OF EXTENDED AERATED PILE
12.3.2.3 Current Status
The aerated pile system has proven effective on a full-scale
basis at Beltsville, Maryland; Bangor, Maine; Durham,
New Hampshire; Detroit, Michigan; and Windsor, Ontario. After
start-up, mean temperatures in aerated piles are 176°F (70°C);
and after stable conditions are achieved, minimum temperatures
are usually 130°F (55°C). When the piles are constructed
properly, neither excessive rainfall nor low ambient temperature
adversely affect the composting process (28).
Currently most of the interest in composting of wastewater
sludges is centered on this technique. The applicability of this
system for the treatment of undigested sludges provides it with a
significant advantage over the windrow method. Other advantages
are superior odor control, greater inactivation of pathogenic
organisms, and use of less site area. The aerated pile technique
exposes all sludge to more uniform temperature. Capital costs
are also lower for the aerated pile system, but operating costs
tend to be higher because of the cost of the bulking agent.
Comparisons of capital and operating costs using wood chips as
bulking agent in aerated piles, as well as in windrows, are made
by Colacicco (18). In experiments at Los Angeles County, it
has been found necessary to follow this technique by windrow
12-25
-------
composting for 2 to 3 days to dry off the moisture. At other
locations, the air flow is reversed without disruption of the
pile as another means to reducing moisture content.
12.3.2.4 Oxygen Supply
Centrifugal fans efficiently provide the necessary pressure to
move air through the compost and odor filter piles. Variation in
the blower pressure is a necessity for optimum conditions and a
site-specific operating parameter. The oxygen concentration in
the pile should be maintained between 5 and 15 percent; this can
be achieved with an aeration rate of about 500 cubic feet per
hour per ton (15.6 m^/hr/t) dry sludge. If the pile cools at
this air rate, the air flow must be reduced. Aeration cycles
of 20 to 30 minutes with the fan operating 1/10 to 1/2 of the
cycle have proven satisfactory (19). While the fan is not
operating, the natural convective chimney effect, typical of
windrows, takes place. In the absence of forced aeration, this
effect causes warming of the outer edges, destroying pathogens
more effectively.
Moist air drawn through the pile condenses in the slightly
cooler sections. When enough condensate accumulates, it
will drain from the pile and leach material from the sludge.
Condensed moisture which collects in the aeration pipes is
removed by a water trap. This material must be collected and
treated along with the contaminated rainfall runoff from the
site, because it can become a source of odors if allowed to
accumulate in puddles around the piles. Data is not available on
combined leachate and condensate water characteristics; the
quantity may, however, vary from 6 to 20 gallons per day (22 to
75 I/day) per pile containing 50 cubic yards (38 m3) of sludge
during dry weather (29). (Refer to Chapter 16 for further
information. )
12.3.2.5 Bulking Agent
While bulking agents are in the aerated pile composting system,
they serve primarily to maintain the structural integrity and
porosity of the pile. The quantity of external bulking agent
required is determined by the need for structural support and
porosity. The requirements for moisture control are not as
critical as adequate porosity; thus, sludge moisture can vary
considerably as long as sufficient bulking agent is added to
assure adequate porosity. The design factors discussed for
windrows do not apply here (17).
Wood chips and other bulking agents also increase the volatile
solids content of the composting mixture; volatility of new and
recycled wood chips has been reported as 90 and 86 percent,
respectively (18). The actual contribution of the wood chips to
the compost mixture is limited because their composting rate is
slower.
12-26
-------
hen wood chips are mixed with unstabilized sludge an average
volatility of about 75 percent results; this is well in excess of
the 40 to 50 percent volatility achieved in the mixture of
digested sludge and recycled compost. Volatility content is
therefore not a limiting factor in aerated pile composting of
unstabilized sludge, as it can be in the digested sludge windrow
system.
12.3.2.6 Energy Requirements
Energy costs for aerated pile composting are a small portion of
the overall operating costs. The bulk of the overall energy
requirement of the process is provided by the volatile solids in
the composting mixture. A range of resources for labor, external
bulking agent, gasoline for small vehicles, diesel fuel for the
front-end loaders, and electricity usage for leachate treatment
is listed below (18).
Operating Requirements
per dry ton per day (0.9 t/day) for a
10 to 50 dry ton per day (9 to 45 t/day;
operation (20 percent sludge)
Labor
Wood Chips
Gasoline
Diesel Fuel
Electricity
1.5 to 2.8 hours
2 to 8 cubic yards (2.1 m3)
1.1 gallon (4.1 1)
2.7 to 3.5 gallon (10.2 to 13.2 1)
7.5 to 17.5 kWhr (29.7 to 69.3 MJ)
12.3.2.7 Public Health and Environmental Impacts
Extensive studies have been made on the destruction of pathogens
in aerated piles (30). Although Salmonella, fecal coliforms,
and total coliforms initially increased in numbers, they were
reduced to essentially undetectable levels by the tenth day of
composting. Studies using "F" bacteriophage and virus as an
indicator showed that the virus was essentially destroyed by
the thirteenth day. However, survival of the virus did occur
for some time in the blanket-compost interface where lower
temperatures prevailed. Storage in a curing pile for 30 days
will complete the destruction of viruses or reduce the numbers
to an extremely low level (19). Studies have shown that the
composting process in an aerated pile is essentially unaffected
by low ambient temperatures or rainfall, which makes this system
particularly well suited to operation under difficult climatic
conditions (31). Figure 12-11 shows the time-temperature-
destruction relationship of pathogens for aerated piles (20).
Odor control is the primary environmental consideration in the
operation of an aerated pile composting system. Good odor
control results from prompt mixing of sludge and bulking agent
12-27
-------
and formation of the aerated pile. In addition, lumps of
material or puddles of liquid must not be allowed to remain in
the mixing area. No thin spots or holes should be present in
the compost blanket. There should be leakproof transport of
aeration air between blower and odor filter pile. Moisture
content within odor filter piles (Figure 12-12) should be kept
below 50 percent. Condensate, leachates, and runoff from the
piles must be collected and treated as quickly as possible. The
compost should be adequately cured before it is removed from the
area, and any unstabilized material should be recycled back into
the composting process for further treatment.
E
E
J3>
LU
o
4
cc
to
0
TOTAL COL1 FORMS
80
60
40
u
o
UJ
-------
FIGURE 12-12
ODOR FILTER PILES AT BELTSVILLE
12.3.2.8 Design Example
This design example is based on a Beltsville-type sludge
composting system utilizing existing technology and available
design criteria. The example p r^o vided is specific to a
10 million gallon per day
secondary treatment plant.
(0.45 m-^/s) municipal wastewater
The weight and volume of sludge and -bulking agent at various
points in the process must be known so that the volumetric flow
capacity of a composting facility can be determined. The basic
design decisions include the bulking agent to sludge ratio and
the ratio of new to recycled bulking agent.
The materials balance in this example is based on the following
assumptions:
« Sludge to be composted is 50 wet tons per day (45 t/d)
of undigested sludge, seven days per week, with no
digestion.
• Wood chips are added to the wet sludge at the rate of
2 cubic yards of chips per cubic yard (2.0/m3) of wet
sludge.
• Three-fourths of the chips are recovered by screening and
reuse.
12-29
-------
• The water content and total weight of the compost
mixture is reduced by approximately 30 to 40 percent
and volatile solids content is reduced by about 10 to
15 percent. The density decreases 15 to 20 percent
because of evaporation.
• The extended aerated pile system will be used.
Information on the bulk density of sludge is surprisingly scarce.
Tests conducted at Beltsville for an engineering study of a
large-scale composting facility provide some basic data on
the bulk density of sludge and wood chip bulking agents. The
following bulk densities are used in this design example (20):
Bulk Density
Pounds per
Constituent cubic yard
Dewatered Sludge 1,600 960
(20% solids)
New Wood Chips 500 300
Recycled Wood Chips 600 360
Screened Compost 865 519
Unscreened Compost 1,000 600
It is also assumed that the process variables have the following
values :
Sc=0.20 SB =0.70 SR=0.70
VC=0.75 VB=0.90 VR=0.80
kc = 0.45 kB = 0.10 kR = 0.10
Sludge composting will operate 5 days per week, 8 hours per day
using the extended aerated static pile method. The volume to be
composted per work day is as follows:
50 wet tons 7 week-days = k d (63 t/work d }
week day 5 work-days ^ J J
It is assumed that the dewatered sludge arrives on-site 5 days
per week from the dewatering operation which runs only 5 days per
week. Equalization storage to cover weekend operation of the
plant is provided for sludge in the liquid state upstream from
the dewatering process.
12-30
-------
The amount of recycled and new wood chips can be calculated using
Equations 12-7 and 12-8 and assuming f^=0.75 and f2=0.25;
XR = (1-0.25)(0.75)70 = 39.4 tons per day (35.7 t/day).
XB = (0.75)70 - 39.4 = 13.1 tons per day (11.9 t/day).
The ratio W can be calculated using Equation 12-2:
70(1-0.2) + 39.4(1-0.7) + 13.1(1-0.7)
70(0.2)(0.75)(0.45) + 39.4(0.7)(0.9)(0.1) + 13.1(0.7)(0.8)(0.1)
= 9.0
Since W is less than 10, no amendment addition is required.
The daily volume of the compost material is calculated using the
assumed values previously stated:
Mass Volume
Constituent tons/day cubic yards/day
Dewatered sludge 70 87.5
New wood chips 13.1 52.4
Recycled wood chips 39.4 131.3
Total 122.5 271.2
(111.1 t/day) (206.8 mVday)
The pile will be 8 feet (2.4 m) high and 50 feet (15 m) long.
Each day, the pile will be extended 18.5 feet (5.6 m) . The
amount of new wood chips required to construct a one-foot (0.3 m)
thick pad for the compost is as follows:
(50 ft)(18 5)(1 ft) = 34i3 cubic ds m3)/day
27 cu ft/cu yd J J
Unscreened compost is required each day to cover the pile.
This layer will be 18 inches (0.46 m) thick:
,39
12-31
-------
Figure 12-13 is the process flow diagram for the extended aerated
pile compost facility and summarizes the design materials
balance.
NiW WOOD
CHIPS
WOOD CHIP
PAO
5 DAY PER WEEK OPERATION
PERCENT
WST PERCENT DRY DENSITY VOLUME VOLATILE
LOCATION TONS SOLIDS TONS (Ib/cu yd) (cu yd) SOLIDS
70
13.1
39.4
122.5
8.6
58.7
18.6
90
32
20
70
70
41
70
65
65
60
14
9.2
27.6
50.8
6
10.3
12
58.5
18.9
1,600
500
600
900
500
725
725
975
87.5
52.4
131.3
271.2
34.3
51.4
248.3
64.6
75
90
30
80
90
65
65
45
1 too - 0.907 toon*
1 Ib/cu yd - 0.6 KB/HI*
1 cu yd - 0.76 m5
FIGURE 12-13
PROCESS FLOW DIAGRAM FOR THE EXTENDED PILE
COMPOST SLUDGE FACILITY - 10 MGD (0.44m3/s)
ACTIVATED SLUDGE PLANT
Approximately 250 feet
perforated aeration pipe,
three 4-inch (10-cm) tee
with weather protection
(76 m) of 4-inch (10-cm) diameter
50 feet (15 m) of non-perforated pipe,
connectors, and one blower/timer unit
and condensate collection system are.
required for each daily pile. Only one blower rated at 335 cubic
12-32
-------
feet per minute (158 1/s) will be used to draw air into the pile.
In general, the blower should be rated at a minimum of 150 cubic
feet per hour per wet ton (1.3 1/s/t) of sludge in the daily
pile. Non-perforated pipe should be used to connect the aeration
pipe loop to the blower. The exhausted air will be filtered in a
pile of screened compost. The filter pile will contain at least
one cubic yard of material per 30 wet tons (1 m3/35.5 t) of
sludge in the daily pile or 4 cubic yards (3 m3) for this
design. Figure 12-14 illustrates this design example. The
minimal area requirements for various composting site components
is as follows:
MINIMAL COMPOSTING AREA REQUIREMENTS
50 wet tons per day (45 t/day)
10 dry tons per day (9 t/day)
Area Required
Square
feet
5,000
Function
Truck unloading and mixing
Composting
(28 days)(50)(18.5)(1.15 excess) 30,000
Unscreened compost 10,000
Drying and screening 20,000
Compost curing and storage
(60 day)(200 cu yd/day)(27 wet tons)
(10 ft deep) + excess 33,000
New wood chip storage
(60 day)(87 cu yd/day)(27 wet tons)
(12 ft deep) + excess 15,000
Subtotal 113,000
Maintenance building, operations
building and laboratory, Lunch
room and locker room 4,000
Employee and visitor parking 5,000
Miscellaneous storage 1,000
Subtotal 10,000
Total 123,000
Square
meters
465
2,792
931
1,862
3,071
1,396
10,517
NOTE: 123,000 square feet (11,447 m2) = 3 acres (1.14
Land Utilization = 6.6 dry tons per acre (14.8 t/ha).
ha)
12-33
-------
- PAD AND PIPE BEFORE ADDITION
, Of 11*000 CHIPS AND COMPOST
Pt-PfGPATED-
*tHA
PIPt
1 in - 2.54 cm
FIGURE 12-T4
DESIGN EXAMPLE EXTENDED AERATED
PILE CONSTRUCTION
The overall space required is about 3 acres (1.2 ha) which is
0.15 acres per ton per day (0.07 ha/t/day) of dry sludge solids
composted. Reducing the bulking agent would decrease the area
required.
Although porosity is the key factor for the aerated pile, control
of moisture is important for a successful sludge composting
system. The sludge should be dewatered or mixed with sufficient
bulking agent to obtain enough porosity in the composting piles
for optimum composting conditions. For optimum composting the
composted mixture should have a solids content of not less than
40 percent or more than 50 percent. Figure 12-15 shows a compost
pile as it is being taken down.
Approximately 8.5 cubic feet per minute (4 1/s/t) of air per ton
of dry sludge solids in the pile is required. At Beltsville,
this was delivered by a centrifugal fan operating at 5 inches
differential water pressure (1.25 kN/m2) (18). The Bangor,
Maine system uses a 1/3 horsepower (0.25 Kw) blower rated at
335 cubic feet per minute (158 1/s) at 5 inches water pressure
(1.25 kN/m2) for each pile consisting of 50 cubic yards
(38 m3) sludge and 150 cubic yards (114 m3) bulking agent (7).
The blowers are operated intermittently to maintain the oxygen
level in the 5 to 15 percent range and to obtain as uniform a
temperature as possible.
12-34
-------
FIGURE 12-15
COMPOST PILES BEING TAKEN DOWN
For large composting systems, a permanent central blower system
may be considered. A header pipe could be utilized to provide
the necessary suction for each pile. Only one or two large
blowers located in a covered area would be required. Although
capital cost would be high because of the needed piping and
control devices, the operation and maintenance costs of many
individual blowers would be eliminated. On the other hand, a
central blower system is not especially flexible. Since it is
important to maintain the proper aeration rates in each pile, an
air flow metering device will be required for each pile. A
decision for or against a permanent system would be based on
economic analysis and the need
changing composting conditions.
for system flexibility to handle
The composting area should be paved. Probably the most efficient
design in a permanent facility involves the use of fixed aeration
and drainage systems. The aeration piping and drainage system
could be placed in trenches in the composting pad and the blowers
placed in permanent protected structures and equipped with water
traps and controls. The disadvantages of this type of combined
system are the high initial cost and the reduced flexibility of
operation. Possible elimination of the one-foot (0.3 m) wood
chip pad and the disposable plastic pipe processed through the
screens is a potential advantage of fixed trenches for the
12-35
-------
aeration pipes. Special precautions would be necessary to keep
the centralized aeration piping and pile drainage trenches from
clogging and to provide for condensate water drainage.
Odor filter piles should be replaced periodically. The filter
piles are replaced every other month at Bangor; during cool
weather the system has operated without significant odor problems
and with no filter piles. At Beltsville, the odor filter pile is
replaced each time the compost pile is dismantled.
After the piles are formed, they should be covered with a layer
of compost or wood chips for insulation and to prevent the dust
which is caused by excessive drying of the outer pile edges from
blowing.
Most composting facilities use a base layer of bulking agent or
unscreened compost to cover the aeration piping. However, the
piles are now constructed at Bangor with no special base layer;
the sludge-bulking agent mixture is placed directly on the
aeration piping.
Rotary or vibrating screens are commonly used to separate wood
chips for reuse. Compost containing wood chips with a moisture
content of greater than 40 to 50 percent can be difficult to
screen; the operation is therefore not conducted on rainy days.
Allowance should be made for drying the compost if the solids
content is less than 50 percent, and the screens should be
sized to handle a large volume of compost during fair weather.
Figure 12-16 is a photograph of the finished screened compost.
12.3.3 Case Studies (Unconfined Systems)
The four case studies chosen involve Los Angeles County
Sanitation District, California; Beltsville, Maryland; Bangor,
Maine; and Durham, New Hampshire. The Los Angeles system handles
80 to 120 dry tons per day (73 to 109 t/day). Beltsville
composts approximately 14 dry tons per day (12.6 t/day), Bangor
about 2 dry tons per day (1.8 t/day), and Durham around 3 dry
tons per day (2.7 t/day).
12.3.3.1 Joint Water Pollution Control Plant,
Carson, California
A large-scale windrow composting system was established in 1974
at the Joint Water Pollution Control Plant. This operation
is currently composting 400 to 600 wet tons per day (364 to
545 t/day) of anaerobically-digested, polymer-conditioned,
centrifugally dewatered, primary sludge with a 25 percent solids
content (19). The sludge is transported to the nearby compost
site in fifteen ton (13.5 t) sludge hauling trucks equipped with
end discharge and conveyor bottom trailers, provided to make
windrow construction relatively easy. Approximately 15 cubic
12-36
-------
yards (11 m3) of finished compost are added to the truck along
with the dewatered sludge. The wet and the dry materials are
initially mixed in the truck. Complete mixing is subsequently
provided by a compost turning machine in the windrow. Given the
current consistency of the sludge and the type of equipment used,
the windrows that can be constructed are about 3 feet (0.9 m)
high and 10 feet (3.0 m) wide. Typically, each windrow is about
500 feet (451 m) long and is constructed with eight to ten
truckloads of material. The windrows are placed on sixteen to
eighteen foot (14.6 to 16.5 m) centers, leaving a clear aisle for
the wheels of the turning machine.
FIGURE 12-16
FINISHED SCREENED COMPOST
When a windrow is first placed, it is turned twice to mix the
wet cake with the dry compost. Thereafter, each windrow is
turned once per day to maintain sufficient voids for the natural
passage of air and to promote drying. The process can produce
objectionable odors, particularly in the early part of the cycle
and is likely to generate dust under moderately windy conditions.
In addition to being equipped with conveyor bottom trailers, the
composting trucks have also been modified with extended sidewalls
12-37
-------
to increase their capacity, and sealed bottoms, so that they may
be used to haul wet cake on public roads. At a production rate
of 500 tons per day (450 t/day) of wet cake, about 25 hours of
truck time are required each day to construct windrows. Four
turning machines, each with a rated capacity of 3,400 tons per
hour (3,084 t/hour), are available for the operation. They are
relatively high maintenance items, and generally, only two or
three are operated. With the current sludge production and a
composting time of three weeks, about ten hours of machine time
are required to turn all the windrows each day (32).
In addition, two loaders are used for loading dry sludge into the
trucks; one crawler tractor is used for pushing windrows into
stockpiles; one grader is used for road maintenance and cleaning
between the windrows; and a water truck is used to control dust
on the plant roads.
The composting operation takes place over a 10-hour day, 7 days
per week and employs approximately twenty operators and
mechanics, excluding the sludge haulers.
Kellogg Supply Company currently uses earth movers to transfer
composted, dried sludge to a neighboring site. Kellogg has been
highly successful in distributing and selling the compost as an
organic soil conditioner.
The composting operation of the County Sanitation Districts
of Los Angeles County provides a good demonstration of the
feasibility of sewage sludge composting on a large scale.
Figure 12-17 illustrates the process flow for this operation.
12.3.3.2 Beltsville, Maryland
Many methods and concepts have been developed at Beltsville
through the integration of experimental research and practical
operation. The first method attempted at Beltsville was the
windrow process. The windrows performed well when digested
sludge was used, but odors developed when unstabi1ized,
dewatered/combined primary and secondary sludge was composted by
this method. The individual aerated pile method was developed
by the Beltsville researchers to eliminate the odor problems
associated with the windrow process.
The research programs demonstrated that either digested or
undigested sludge can be composted in the aerated pile.
Destruction of pathogens was much greater with aerated pile
composting than with windrow composting (33,34). The extended
aerated pile method was also developed at Beltsville to minimize
composting area requirements.
The extended aerated static pile process is currently used in
continuous, 5 day per week operation to compost 60 to 120 wet
tons per day (54 to 109 t/day) of dewatered, unstabilized
12-38
-------
sludge (approximately 20 to 22 percent solids). The sludge is
conditioned with lime and ferric chloride, dewatered and loaded
into tractor-trailer dump trucks at the treatment plant during
3 t) and has a
of sludge to be
sludge to the
is delivered at
the night. Each truck holds 20 wet tons (1!
watertight rear door. Depending on the quantity
composted, three to six trucks transport the
compost site in the morning. All of the sludge
once, which facilitates pile construction.
DRY
SLUDGE
LOADING
STATION
LANDFILL
STORAGE
AREA
KELLOGG
SUPPLY
COMPANY
OFF-GASES
LOCATION
1
2
3
4
5
6
WET TONS
PER DAY
1,600
2,960
1,140
1,820
1,360
460
PERCENT
SOLIDS
23
40
—
60
60
60
DRY TONS
PER DAY
368
1,184
92
1,092
816
276
DENSITY
(Ib/cu yd)
1,890
1,510
_
1,215
1,215
1,215
VOLUME
(cu yd)
1,690
3,930
—
3,000
2,240
757
PERCENT
VOLATILE
SOLIDS
55
50
—
40
40
40
1 ton = 0.907 tonne
1 Ib/cu yd = 0.6 Kg/m3
1 cu yd = 0.76 m3
FIGURE 12-17
COMPOSTING/DRYING SYSTEM - COUNTY SANITARY
DISTRICTS - LOS ANGELES (18)
The extended pile is constructed on a concrete pad approximately
100 feet (30 m) wide and about 400 feet (122 m) long. The sludge
is dumped onto the wood chips and mixed by a front-end loader, in
a 2.5:1 chip to sludge volumetric ratio.
12-39
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The composting area for each daily mixture is prepared by laying
out aeration piping on the concrete composting pad and covering
it with a 12-inch (0.3 m) layer of wood chips using a front-end
loader. The compost mixture is then placed on the wood chip base
using a front-end loader. The mixture is piled to a height of
8 feet (2.5 m), and the top and ends are then capped with an
18-inch (0.5 m) layer of unscreened finished compost or a 12-inch
(0.3 m) layer of screened, finished compost. At the end of each
day's operation, the side of the pile (which will have new
material added to it the next day) is covered with a thin layer
of compost. A pile containing 60 wet tons (54 t) of sludge and
wood chips is approximately 8 feet (2.5 m) high, 12 feet (3.6 m)
wide and 75 feet (23 m) long.
To insure proper aeration, a 1/3 horsepower (0.25 kW) blower,
rated at 335 cubic feet per minute (158 1/s) at 5 inches
differential water pressure (1.2 kN/m^) is connected to the
piping. The exhausted air is filtered through a 5 cubic yard
(3.8 m^) filter pile of screened compost for deodorization.
The blower's is operation is controlled by a timer. Currently,
blowers are operated for 8 minutes out of every 20 minutes.
At Beltsville, one blower is used to aerate 120 wet tons
(109 wet t) of primary undigested sludge mixed with wood chips.
One blower has proven sufficient for two piles when the sludge is
brought to the site at a rate of 60 wet tons per day (54 t/day).
Thus, only approximately 10 blowers and odor filter piles are
required (excluding spare equipment) to operate a 21-day extended
aerated pile facility.
Composted material is removed from the piles after 21 days.
The compost pile is dismantled by a front-end loader and moved to
the curing stockpile. The compost stays in the curing pile for
at least 30 days and is not mixed before screening and off-site
use. A mobile rotary drum screen separates the cured material,
which must be at least 60 percent solids to screen properly.
Finished, cured compost that is too wet to screen is placed in
windrows and turned as frequently as possible for 2 to 3 days
until it is sufficiently dry.
Leachate, condensate, and stormwater runoff are collected in a
holding pond at the far end of the compost facility. When the
level of the pond rises to a maximum allowable height, the water
is pumped to a forested site and sprayed on the forest floor.
Test wells at the compost site and at the land application site
have indicated no groundwater contamination with the use of this
system.
Additional research is being conducted at Beltsville on a
modification of the aerated, extended pile process, called the
extended high pile method. Since land area requirements can be
reduced by increasing pile height, one pile in the shape of a
pyramid has been constructed to a height of 18 feet (5.5 m).
Aeration pipes are installed at three elevations in the pile—at
the base, at 6 feet (2 m) , and at 12 feet (4 m) above the base.
12-40
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Those at the base and at 12 feet (4 m) levels operate at negative
pressure while the pipe at the 6-foot (2 ra) level operates at
positive pressure. Tests are presently underway to determine the
maximum height at which piles can be built and effectively
aerated with pipes placed only at the base (35).
The Beltsville staff consists of eight full-time people, two
administrative personnel and six operators, excluding the sludge
transfer truck drivers. This number is more than actually needed
for normal operations. The additional personnel are used for
special operations and to support the research demonstration
program. Each member of the operation staff is qualified on
each piece of equipment and the staff is able to perform all
preventive maintenance and much of the repair work. A list of
equipment is shown on Table 12-3. All equipment has enclosed
operator cabs so that dust and moisture do not interfere with the
equipment operators.
TABLE 12-3
BELTSVILLE EQUIPMENT (15)
3 Terex rubber-tired front loaders, 4.5 cubic yards
5 Dumr> trucks, 20 ton
1 Rotary drum screen with power unit
1 Sweco screen
1 Fixed Toledo truck scale
1 Mobile office
1 Storage building
1 Covered building - compost test, concrete floor with
aeration pipe in floor
1 Portable oxygen analyzer and temperature indicator and
probe
1 cu yd = 0.76 m
1 ton = 0.907 t
Some of the finished compost is used by the USDA at its
agricultural research center for other test programs. Most of
the compost is provided free of charge to local public works
departments who pick up the material at the Beltsville site.
The approximate material quantities used in the Beltsville
operation are based on the following: annual undigested sludge
cake (with a solids concentration of approximately 23 percent)
input of 15,000 wet tons (13,605't); ratio of 2.5:1 wood chip
bulking agent to sludge cake by volume; and 5/8-inch (1.5 cm)
screening of all compost for wood chip recovery and recycle of
75 to 80 percent; the wood chip loss/attrition rate at Beltsville
is currently about 41 percent (36). In this example, the
materials loss through composting and curing is estimated.
12-41
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The building used for test purposes at Beltsville has a concrete
floor with aeration piping built in. Channels 6 inches by
6 inches (15 cm x 15 cm) are recessed in the floor and the
aeration piping is placed into them. The channels run the width
of the building and are spaced 6 feet (2 m) apart along the
length of the building. One end of the piping is connected
to a header system and the other is closed off. One large
blower draws air through the header system. Limited tests have
suggested this arrangement will be proven successful, and a
refined version of this system is being constructed at Durham,
New Hampshire. The Beltsville structure is approximately 80 feet
(24 m) wide and 240 feet (73 m) long. Composting is conducted in
an 80 by 200 foot (24 by 61 m) area and the remainder is an
enclosed and heated equipment storage and maintenance area.
The estimated and projected costs for this extended aerated
pile operation are listed in Table 12-4. The cost for early
composting operations included extensive testing, monitoring, and
optimization. The cost per ton for this operation would be
reduced if a facility were designed to operate continuously and
to use the process as it has been optimized at Beltsville.
12.3.3.3 Bangor, Maine
In August 1975, composting operations began in Bangor to dispose
of the sludge generated by the wastewater treatment plant. An
average wastewater flow of 7 MGD (307 1/s) receives only primary
treatment. The plant produces 2,500 wet tons per year (2,268
t/year) of lime conditioned vacuum-filtered sludge cake with an
average solids content of 20 percent (5). The composting site
selected by the City of Bangor is located about 3 miles (4.8 km)
from the wastewater treatment plant.
Initially, the sludge was dumped onto a bed of bulking agent
(wood bark) in the mixing area, mixed with a front-end loader,
and formed into a compost pile. Currently, no base material is
used; the sludge bulking agent mixture is placed directly on the
pad and aeration pipes. Generally, one composting pile is
constructed per week and typically consists of 40 to 60 cubic
yards (30 to 46 m^) of undigested primary and secondary sludge
cake which is mixed in 1:2.5 ratio with about 120 to 180 cubic
yards (91 to 137 m^) of bulking agent. Bark with a less than
50 percent moisture content is used as the bulking agent.
The total area required for composting 3,000 cubic yards per year
(2,280 m-Vyear) of dewatered sludge at 20 percent solids is
about 1.7 acres (0.7 ha). Precipitation, runoff, and condensate
from the composting operation are channeled into a drainage ditch
leading to the sanitary sewer line (Figure 12-18).
The base for the compost pile is prepared using 7-foot (2 m)
lengths of perforated schedule 40 steel pipe, joined together by
short pieces of plastic pipe. The city found that the short
12-42
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lengths of steel pipe can be removed from the pile without
significant damage and reused many times. Longer pipes were used
previously but were easily bent when pulled from the pile.
TABLE 12-4
BELTSVILLE ACTUAL AND PROJECTED
OPERATING COSTS - 1977 DOLLARS (15)
Estimated October 1977 to September 1978
costs, dollars
Total, excluding off-site
Dry tons sludge/yr (23
percent solids)
Annual cost, dollars/dry
ton sludge solids
Actual
1976
On-site operations
Telephone and travel
Utilities
Fuel and oil
Sludge hauling
Labor including fringes
Miscellaneous contract
services
Wood chips
Supplies and materials
Equipment insurance
3,971
426
13,036
120,000
152,919
112,942C
73,145
32,176
3,955
512,570
3,450
149
15,000 wet
tons/yr
1, 300
2,211
10,500
132,000
125,750
27,540
144,000
22,250
4, 000
469,551
3,450
136
18,200 wet
tons/yra'b
1,300
2,211
10,500
80,000
27,540
144,000
22,250
4,000
291,801
4, 200
69
45,500 wet
tons/yra'b
1,300
3,000
25,000
125,750
37,000
350,000
35,000
4,000
581,050
10,500
55
Excluding requirements of research work.
Assume 50 percent of compost marketed unscreened and 70
percent recovery of bulking agent after screening finished
compost.
CIncludes screening performed by outside contract, screening
now performed on site.
When this analysis was conducted a wood chip attrition rate of
20 percent was used. 1979 analysis indicates that an actual value
of 41 percent should be used for wood chip attrition. (36)
1 ton = 0.907 t
The city has used unscreened compost as the bulking agent in a
number of piles. This has dramatically reduced requirements for
new bulking material, and the city plans further tests.
The compost piles are constructed as high as the front-end
loader can reach and capped with 1 to 2 feet (0.3 to 0.6 m) of
unscreened compost. The finished pile is 10 to 12 feet (3 to
4 m) high. Each pair of compost piles is provided with one
mechanical blower. Blowers are operated by timers.
12-43
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DRAINAGE DITCH
FENCE
COMPOST PILES
UNSCREENED COMPOST STORAGE
SCREENED COMPOST STORAGE SCREEN
BARK STORAGE
©UTILITY POLE
^- BLOWER HOUSING
WATER TRAP
MANHOLE TO SANITARY SEWER
FIGURE 12-18
COMPOSTING SITE LAYOUT - BANGOR, MAINE (5)
During cold weather, all available heat must be conserved to
bring the piles up to temperature. Recycled unscreened compost
provides a warm bulking agent. The interiors of the wood
bark storage piles are also sources of warm materials for
mixing. Generally, if the compost pile mixture can initially be
maintained at 39°F (4°C), the interior of the pile will warm up
to normal composting temperatures much more readily than if the
mixture falls below 39°F (4°C). Warm exhaust air recycled from
an older composting pile into the new pile also helps for the
first few days, but recycling should be discontinued after this
period because it causes high moisture levels in the new pile.
Increasing the unscreened compost blanket from 1 to 2 feet
(0.3 to 0.6 m) during the winter also helped to retain more heat
within the composting pile. The city purchased an air heater to
provide initial heat to the piles.
The piles are composted for at least 21 days. Temperature and
oxygen levels are monitored every two to five days during the
compost cycle. Blower operating cycle is adjusted according to
the performance of the pile. The aeration pipes, blowers, and
moisture traps are checked for freezing during cold weather.
At the end of the composting cycle, the pile is dismantled, and
another pile is usually constructed. The material removed from
the pile is either used as the bulking agent for the new pile or
transferred to curing.
Unstabilized sludge is not stored at the compost site.
Generally, operations are scheduled so that sludge is dewatered
and a compost pile is constructed once a week. The exact day of
pile construction is varied depending on weather conditions. The
12-44
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city has been able to compost nearly all of the sludge produced,
because it is prepared to construct the compost pile during good
weather.
A Lindig rotary drum screen is used to separate compost prior to
distribution. The drum is presently fitted with a one-inch
(2.5 cm) mesh screen. City personnel are planning to construct
a 5/8-inch (1.6 cm) screen assembly so that either
can be produced. Tests performed at Bangor
screen is capable of handling about 20 to 25
(15 to 19 mVhr)
put in the screen with a front loader. One
laborer are required during screening operations
size
indicate
cubic yards
material
that the
per hour
of feed under the best conditions. Compost is
loader operator and a
Currently, operations at Bangor are performed by treatment
plant personnel under the direction of the treatment plant
superintendent. There are no full-time composting personnel
because of the cyclical nature of the operations. Approximately
11 man-hours per week are required for a truck driver to deliver
and unload sludge at the site. Sampling and monitoring for
temperature and oxygen content require 10 man-hours per week.
Pathogen and heavy metals monitoring is performed under contract
with the University of Maine. Supervision and administration
require about 15 man-hours per week. Annual equipment and labor
requirements are shown in Table 12-5. The equipment used for
composting operations is shown in Table 12-6. This equipment is
provided by the city motor pool and is available for composting
when needed.
TABLE 12-5
ESTIMATED ANNUAL LABOR AND EQUIPMENT
REQUIREMENTS, BANGOR, MAINE (5)
Operation
Labor,
hours
Equipment,
hours
Composting
labor
front loader
Sludge hauling
labor
truck
Monitoring
labor
pickup
Administration
labor
Screening (8,000 cubic yards)
labor
screen
front loader
Maintenance
labor
572
468
520
780
1,040
100
468
468
520
520
520
1 cu yd = 0.76 m~
12-45
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TABLE 12-6
BANGOR EQUIPMENT (5)
1 Case W24B rubber-tired front loader, 4 cubic yard
1 Rubber-tired front loader, 1.5 cubic yard
1 Truck, sludge hauling
1 Mobile screen, Lindig
Small tools, as required
Miscellaneous vehicles as needed from motor uool
1 cu.yd = 0.76 m3
Approximate materials quantities for 1976 are shown in
Table 12-7. This is based on an annual sludge input of
3,000 cubic yards (2,280 m3) and a mixture of three parts bulking
agent to one part sludge.
TABLE 12-7
BANCOR MATERIALS REQUIREMENTS FOR
2,170 WET TON (1,968t) ANNUAL SLUDGE INPUT (5)
Limed raw primary sludge, wet
tons 2,170
Solids, percent 23
Cubic yards, cu yd 3,000
Density, Ib/cu yd 1,450
Dry tons 500
Static pile construction
Sludge, cu yd 3,000
Bulking agent, cu yd 9,000
Pile cover, cu yd 1,560
1 ton = 0.907 t 3
lcuyd=0.76m
1 Ib/cu yd - 0.6 kg/m
12.3.3.4 Durham, New Hampshire
Durham, New Hampshire, provides primary treatment to
approximately 1 MGD (44 1/s) of wastewater. About 15 wet tons
(13.6 t) of unstabilzed, dewatered, primary sludge (20 percent
solids) is produced each week. The treatment plant is being
upgraded to secondary treatment capability, and this is expected
to double the quantity of sludge generated.
12-46
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In an effort to cope with current sludge production and to solve
the problem of future sludge disposal, Durham investigated
several sludge utilization and disposal alternatives. Land-
filling had to be terminated because the landfill was reaching
its maximum capacity and no other suitable land was available
within the town limits. It was considered too costly and
time consuming to obtain additional land in an adjacent town.
Incineration was considered but rejected because previous
experience with burning solid waste had been unsatisfactory.
A permanent composting facility was chosen (after an extensive
pilot-scale investigation) as the sludge disposal alternative.
It was determined that this facility would best meet the needs of
the community for the following reasons (31):
• Estimated cost of the compost facility was 658,000
dollars, of which Durham, by virtue of state and Federal
funding, would pay approximately 33,000 dollars.
• The compost facility would be an integral part of the
wastewater treatment plant, and plant personnel could
operate the facility.
• Sale of finished compost would help defray the operating
costs.
• Composting returns a viable product to the land at a cost
competitive with landfilling and incineration.
The new composting facility incorporates many innovations
that reduce operation and maintenance problems. It should be
recognized that since there are many innovations in this design
that they are not a proven technology. The composting and
all other outdoor operations will take place on a concrete pad
which is easier to clean than a gravel base, prevents rocks from
mixing with the compost, and is a better year-round working
surface. The pad is sloped to allow runoff collection from the
compost piles. The runoff is recycled to the treatment plant to
provide protection for the surrounding land and streams. The pad
is 250 x 152 feet (76 x 46 m) , and is spacious enough for the
screening operation.
The aeration pipes are placed in triangular troughs 6 inches
(15 cm) deep which are recessed below the pad surface and covered
with an aluminum grating, flush with the pad. Once the aeration
pipe is in place, wood chips are used to fill up the remaining
space in the trough under the grates. It is anticipated that
chips directly under the grating will be changed occasionally,
but the pipe will be used for an extended period of time. The
sludge-wood chip mixture will then be placed directly on the
concrete pad over the grates without any wood chip base.
Figure 12-19 shows a cross section of an aeration trough with the
aeration pipe.
12-47
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A 4-foot retaining wall will be built along the edge of the
composting area of the pad. This wall will be constructed to
protect the blowers which will be located on the side away from
the composting operation and to provide a positive backstop for
front loader operations.
ALUMINUM
GRATE
3 TO 8 ft
(0.9 TO 2,4mJ
CONCRETE PAD
WOOD CHIPS
FIGURE 12-19
AERATION PIPE
CROSS SECTION OF AERATION PIPE TRENCH
DURHAM COMPOST PAD DESIGN
The sludge processing building of the new secondary treatment
plant will be placed adjacent to the composting pad. Primary
and waste-activated sludges will be mixed together prior to
coil vacuum filter dewatering to provide for more consistent
operation. The mixing tanks for both primary and secondary
sludge will be located in this building along with the condi-
tioning chemicals, chemical feed equipment and coil filters. To
provide flexibility in operation, the new plant will have a
one-week liquid storage capacity for both activated sludge
and primary sludge.
After the sludge is dewatered, a pug mill will mix it with wood
chips fed from a hopper. A conveyor belt will transport the
compost mixture from the building for pick-up by the loader
and placement on the pad. The mixing operation is conducted
inside the building to protect the operation from the weather.
Coil filter personnel will operate the mixing process, thus
minimizing personnel requirements.
Screening will be executed using a Lindig Rotary Screener
with a material throughput capacity of 280 to 400 cubic yards
per day (213 to 304 m3/day). Screening capacity exceeds
production requirements, so that the screen needs to be run only
part of the time. This frees the screen and loader operators
to undertake other tasks.
Storage bins for the composted material and chips will be
placed directly against the composting pad such that the top of
the bins are even with the pad. There will be four bins with a
12-48
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capacity of 1,200 yards (912 m3) each. Three of the bins will
be used for storing compost and one bin for the storage of wood
chips. As the composted material is screened on the composting
pad, the compost will drop into the bins for storage and curing.
A conveyor will collect the wood chips and return them to the
fourth bin for storage. The screen can be shifted to link the
compost pile being dismantled with a storage bin (31).
The storage bins will be used for curing the compost and will
have sufficient capacity for storage of all production during
winter months when no distribution is planned. The bins will
be unloaded after the sludge is cured for about four weeks. This
two-yard (1.4 m3) front-end loader will also build and dismantle
the piles, feed the screen, load the chip hopper and trucks with
the finished product, keep the pad free of ice and snow, and
provide a backup for the mixing operation. A wood chipper and
a seven-yard (5 m3) dump truck for hauling purposes are other
equipment to be used.
12.3.3.5 Cost Analysis
Comparing the cost of composting at different facilities is
extremely difficult because local factors such as the weather,
labor, and equipment are highly variable. Operations in warm,
dry climates will require less bulking agent and probably
be more successful with the screening process than operations in
cold winter areas. Labor and bulking agent costs are a large
portion of composting expenditures and vary widely according to
geographic area.
A generalized annual operating cost analysis has been performed
for an extended aerated pile system for an operation processing
ten dry tons per day (9 t/day) of sludge based on the operations
at the Beltsville facility (18, 34). This analysis is presented
in Table 12-8. A 10 dry ton per day (9 t/day) compost site
should handle the sludge generated by a secondary treatment plant
from a community of 100,000 people. The site is assumed to be
operating eight hours per day, seven days per week.
In 1976, when the original analysis was done, the operating cost
was calculated to be 40 dollars per dry ton ($44.44/t). Although
all prices have increased since then, the one item which is
significantly more expensive is wood chips. In the analysis in
Table 12-8, wood chip attrition had been estimated at 20 percent.
Analysis done in 1979 indicates that 41 percent is the actual
value. Wood chip costs have increased from the $3.50 per cubic
yard ($4.61/m3), the value indicated in Table 12-8, to a 1979
value of approximately $7.00 per cubic yard ($9.21/m3). In
addition cost for transporting sludge to the compost side must be
included.
An analysis of the capital cost is not presented, because capital
costs are site specific. The development cost of the site cannot
12-49
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be generalized, and the type of composting systems used, aerated
pile or windrow, will largely influence the capital cost. The
replacement cost of the equipment can be a large portion of the
capital cost. The largest capital cost is usually the compost
pad. The capital cost for all equipment and structures at Durham
is estimated to be about $600,000. Durham's annualized cost is
anticipated to be $80 per dry ton ($88/t) for capital and $60 per
dry ton ($66/t) for operating. This makes the total annual cost
for sludge composting at Durham $140.00 per dry ton ($154/t).
This facility is highly mechanized and may represent one of the
most capital-intensive composting operations. The operation at
Bangor, however, utilizes a portion of an abondoned taxi way and
uses the individual pile composting method. The capital costs
for this facility are estimated at about $10 to $15 per dry ton
($11 to $16/t).
Except for wood chips and labor, the best approach for estimating
annual operating costs for design purposes is to determine
the costs from a similar compost facility operating in the
same geographic area. Wood chip and labor costs must be
determined for the specific site. Capital costs are best
developed and annualized for the specific site chosen for the
facility.
TABLE 12-8
FACILITY PROCESSING 10 DRY TONS (9 t) OF
SLUDGE PER DAY a (1976 DOLLARS) (19,31)
Dollars/yr
Operations ^
Wood chips at $3.50/cu yd
Plastic pipe
Gasoline
Diesel
Electricity
Equipment maintenance
Equipment insurance
Pad, road maintenance
Water/sewer
Labor
Miscellaneous supplies
35,000
12,200
2,300
5,300
1,500
8,400
1,400
1,200
500
77,500
4,400
Total
Dollars/dry ton
9.60
3.34
0.63
1.45
0 .41
149,700
30
30
0.33
0. 14
21.23
1.20
41.01
Percent of
operating cost
23
8
1
4
1
6
6
0.
0.
52
3
100
Based on the Beltsville operation and assumed to operate
eight hours per day, seven days per week.
bln 1979 wood chips cost $6.50/cu yd at Detroit and ?7.92/cu yd
at Blue Plains. In addition the wood chip attrition rate went
from an assumed 20 percent to a confirmed 41 percent. (36)
1 ton = 0.907 t ,
1 cu yd = 0.76 m
12-50
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12.4 Confined Composting System
Mechanical composting is accomplished inside an enclosed
container or basin. Mechanical systems are designed to minimize
odors and process time by controlling environmental conditions
such as air flow, temperature, and oxygen concentration.
12.4.1 Description of Process
The primary differences among mechanical composting systems are
in the methods of process control. Some provide aeration by
tumbling or dropping the material from one floor to the next.
Others use devices which stir the composting mass. Tumbling the
compost in a rotating cylinder is another approach. In addition,
an endless belt is used to combine forced bottom aeration and
stirring. Water is added to the composting mass at critical
times to increase biological activity in some mechanical systems.
Also, some mechanical composters can introduce heat to the
composting mass to keep the composting reaction continuing at the
optimum rate during cool weather.
The brief detention times which equipment manufacturers specify
for mechanical composters do not allow adequate stabilization of
the sludge. If shorter detention times are provided, a two- to
three-month maturation period will be necessary to reduce the
remaining volatile matter. Thus, the amount of time and total
area required for mechanical processes approaches that for
unconfined processes. Mechanical processes are more capital-
intensive than unconfined processes. Currently only a few
mechanical composting processes are operating in the United
States and these are generally used to compost a mixture of
refuse and wastewater sludge. A schematic of a typical confined
composting process is shown on Figure 12-20.
12.4.2 Metro-Waste Aerobic Thermophilic
Bio-Reactor
The Metro-Waste process utilizes a compost chamber and an endless
belt to achieve adequate aeration. The endless belt lifts the
composting material to a height of three feet (0.9 m), and drops
it behind as it moves from one end of the bin to the other. A
large fan introduces air into the mixture through a perforated
floor in the compost chamber. A partial diagram of this system
is shown on Figure 12-21.
This process, including environmentally controlled buildings, is
available in module units of 10 dry tons per day (9 t/day) with
retention capacities of 7 to 21 days (37).
12.4.3 Dano Bio-Stabilizer Plant
Figure 12-22 shows a typical layout of a Dano Bio-Stabilizer
plant. The process makes use of a large, slowly rotating
12-51
-------
drum, the interior of which
Material is injected into
slowly for one to three days,
Aeration is acccomplished by
is equipped with
one end of the
and ejected from
tumbling action.
into the
oxygen.
vanes or baffles.
machine, rotated
the opposite end.
Air is injected
interior of the drum to insure a constant supply of
WASTE WATER
SLUDGE
MIXING
BULKING
AGENT
HEAT
{IF REQUIRED)
MECHANICAL
COMPOSTER
{REACTOR)
AIR
SCREENING
CURING
FINISHED
PRODUCT
FIGURE 12-20
TYPICAL PROCESS FLOW
SCHEMATIC CONFINED COMPOSTING SYSTEM
12-52
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The "maturation" or "curing" period for a Dano Bio Stabilizer
can be reduced to one month if the material is turned
occasionally (9). The Dano process is generally designed for
refuse composting with sludge addition.
TRIPPER
AG1 LOADER
T\ PERFORATED
FLOOR
PLENUM
AERATION
FIGURE 12-21
PARTIAL DIAGRAM
METRO - WASTE SYSTEM -
RESOURCE CONVERSION SYSTEMS, INC.
12.4.4 BAV Bio-Reactor
The BAV system composts municipal wastewater sludge in an upright
cylindrical reactor. Sludge is mixed with finished compost, or
other bulking agent such as sawdust, and the mixture is fed
through the top of the cylindrical reactor. The composted
mixture is drawn off the bottom of the reactor as new material
enters from the top. The detention time in the reactor is
between ten and fourteen days. Air is fed evenly throughout the
reactor and the oxygen concentration is monitored by an electric
measuring and regulating system. Municipal solid waste can also
be composted with the sludge, but then the compost would require
further processing to remove nonmagnetic metals and pieces of
wood, glass, plastic, and other non-organic materials before it
is ready for landscaping use. Figure 12-23 illustrates the BAV
process.
12.5 European Composting Experience
Of the seven European countries recently surveyed for wastewater
sludge composting practice, West Germany is the center of
activity, with more than 30 operating plants (38). Sweden
follows with 20, which are either in operation or in planning and
12-53
-------
design stages; Switzerland has nine; France has five; the United
Kingdom has one; Italy and the Netherlands have none. These
systems are located where wastewater sludge is the predominant
waste component usually mixed with municipal solid waste. The
number of sludge-only composting systems are few.
CONVEYOR FOR
REJECTS
EXHAUST AIR TO
SOIL FILTER
CONVEYOR FOR
PULVERIZED
\ MATEftfALS
X %
\
\ \
\
ADDITION OF SEWAGE
SLUDGE OR WATER
INDUSTRIAL AND
SULKY WASTES
— -^ HAUL AWAY
HAUL AWAY
HAUL AWAY
INCINERATOR
FIGURE 12-22
TYPICAL LAYOUT OF A DANO BIO-STABILIZER PLANT
The feasibility of composting wastewater sludge mixed with
a bulking agent is established in Europe, but the future of
general composting technology in Europe appears to depend on the
market economics and continued public acceptance, rather than on
technological improvements.
The predominant experience in Europe has been with enclosed
mechanical systems. This is primarily a result of attempts
to minimize compost facility area requirements. Table 12-9 lists
the various operating European wastewater sludge composting
processes.
Although numerous attempts were made from 1930 until the present,
wastewater sludge is no longer composted in windrows without
additives (sawdust, straw, bark) in Germany. Dewatered sludge,
12-54
-------
without additives, has a low porosity which impedes natural
aeration. Strong, objectionable odors developed, and caused all
attempts to be abandoned. The following illustrates recent
composting experiences, in Germany, the United Kingdom, Sweden,
and Switzerland (38).
West Germany
In the last three to five years in the Federal Republic of
Germany, about 30 plants for the composting of wastewater
sludge only have been built or are under construction. When
all of these plants are in service, they together will be
capable of managing the sludge from an equivalent population
of 800,000. The fact that these 30 confined facilities
can only together service the sludge from a population of
800,000, contrasts sharply with the fact that unconfined
processes, such as the windrow operation in Los Angeles, or
aerated static pile process in Washington, singularly each
process equal to or greater than 800,000 population. In
most cases, composting of wastewater sludge occurs with
the help of bulking materials such as sawdust or straw.
Currently, a research program is being conducted by the
German Umweltbundesamt to determine whether these processes
do indeed produce a pasteurized and pathogen-parasite-free
product. Preliminary results of this research program, as
yet unpublished, show that in some of these processes,
pasteurization is incomplete and indeed some final composted
products do contain both human and plant pathogens.
United Kingdom
As of 1978, only one operating plant located at Wanlip, near
Leicester, is composting wastewater sludge in the United
Kingdom. Although 10 to 15 years ago, municipal solid waste
(MSW) composting in Dano rotating drums was common, most of
the plants using these have shut down. In 1974, the Wanlip
plant reopened, and it now processes 1,100 tons (1,000 t) of
MSW mixed with 551 tons (500 tonnes) of digested wastewater
sludge (five percent solids) each week. The product,
packaged under the brand-name, "Lescost," is marketed with
some success throughout Great Britain.
Sweden
In 1975, the Swedish parliament passed a resolution which
emphasizes recycling through better solid waste management.
With this resolution, 20 Swedish communities or regions are
planning, or are in the process of constructing, composting
plants. At present, less than one percent of the total
MSW and wastewater sludge produced is recycled by a
composting technique. According to recent estimates by the
Swedish National Protection Board, in the next two years
approximately seven percent of the total MSW and wastewater
sludge produced will be recycled by composting methods.
12-55
-------
Switzerland
Currently, there are nine composting plants in Switzerland,
the newest of which went into operation in 1975 in Biel.
All but one, in Uzwel, mix sewage sludge with MSW. In most
cases, incineration and composting equipment are located side
by side. The composting operation is used to dispose of
sewage sludge. The incinerator burns most of the municipal
waste and the rejects from the composting installation.
Nearly all the plants use the Dano system for composting.
The auxiliary mechanical machinery, such as hammermi11s,
conveyors and screens, is usually produced by Buehler.
The construction of composting plants has almost ceased in
European countries other than Sweden. Apparently most
operating plants have difficulties in marketing the compost
at a satisfactory price. It may well be, however, that
careful operation of the plant and better marketing could
improve sales of the compost. It appears very unlikely that
a number of combined MSW/wastewater sludge composting plants
will be built in the near future. One of the reasons is that
the rejects of composting must be burnt (landfilling is, for
reasons of space, not feasible in most relatively small
European countries); therefore, an incinerator is necessary
in any case. Building a larger incinerator instead of a
combined system seems, in many cases, the simpler solution.
MIXTURE TO BE COWPOSTEO
1 .1 i, |. i_fi_*i£-,/i / f< £_
I ' T IT yvyjZj/ ycT
' /
. /t A I
-SCREW-TYPE
CONVEYER
1 AIR
COMPOST DISCHARGE
FIGURE 12-23
BAV BIOREACTOR
12-56
-------
TABLE 12-9
EUROPEAN WASTEWATER SLUDGE COMPOSTING PROCESSES (38)
Number of
Category Process operating plants
Within vessel BAV 19
Carel Fouche Languenin 1
Roediger/Fermenttechnik 1
Schnorr Valve Cell 2
Societe General
D1assainissement et de
Distribution (SGDA) '1
Triga 2
Weiss 3
Windrow BIO-Manure 1
Hazemag . -
PLM
Rotating drum Buehler 9
Dano 9
HKS 2
Pressed brick Brikollare 2
Fermentation cells Prat 1
12.6 References
1. Satriana, M.J. Large Scale Comp o s; t i r\g . Noyes Data
Corporation, Park Ridge, NJ.1974.
2. Willson, G.B. and J.M. Walker. "Composting Sewage Sludge,
How?" Compost Science Journal of Waste Recycling. p. 30.
September-October (1973).
3. Epstein, E. and G.B. Willson. "Composting Raw Sludge."
p r o c. 1975 National Conference on Municipal Sludge
Management and DisposjQ.Information Transfer Inc. p~.2457
August 1974.
4. Epstein, E.,G.B. Willson, W.D. Burge, B.C. Mullen, and
N.K. Enkiri. "A Forced Aeration System for Composting
Wastewater Sludge." Journal Water Pollution ContjrgJL
Federation. p. 688, Vol. 48, No. 4.April 1976.
5. USEPA. "Composting Sewage Sludge by High-Rate Suction
Aeration Techniques." Office of Solid Waste. Washington,
DC 10460. Interim report SW-614d. 1977.
6. Wolf, R. "Mechanized Sludge Composting at Durham, New
Hampshire." Compost Science Journal of Waste Recycling,
p. 25. November-December 1977.
7. Heaman, J. "Windrow Composting - A Commercial Possibility
for Sewage Sludge Disposal." Water andPollution Control.
p. 14. January 1975.
12-57
-------
Ehreth, D.J. and J.M. Walker. "The Role of Composting
and Other Beneficial Use Options in Municipal Sludge
Management." Proc. National Conference on Composting
9.
10.
11.
12.
13.
of Mun ic
Transfer,
Golueke,
Rodale Pr
Epstein,
Municipal
of Munic
Transfer,
J e lenek ,
Perspect
National
Sludges .
August 19
Poincelot
National
Sludges .
ipal Residues and Sludges. p. 6. Information
Inc., Rockville, MD. August 1977.
C.G. Biological Reclamation of Solid Wastes.
ess, Emmaus, PA. 1977.
E. and J.F. Parr. "Utilization of Composted
Wastes." Proc. National Conference on Composting
ipal Residues and Sludges. p. 49. Information
Inc., Rockville, MD. August 1977.
C.F., F.B. Read, and G.L. Braude. "Health
ive, Use of Municipal Sludge on Land." Proc.
Conference on Composting of Municipal Residues and
p. 27. Information Transfer, Inc., Rockville, MD.
77.
, R.P. "The Biochemistry of Composting." Proc.
Conference on Composting of Municipal Residues and
p. 33. Information Transfer, Inc., Rockville, MD.
August 1977.
Willson, G.B. "Equipment for Composting Sewage Sludge in
Windrows and in Piles". Proc. National Conference on
Composting Municipal Residues and Sludges. p. 56. Informa-
14.
tion Transfer, Inc., Rockville, MD. August 1977.
Golueke, C.G. Composting - A Study of the Process and Its
Principles. Rodale Press, Emmaus. PA. 1972.
15. Wesner, G.M. "Sewage Sludge Composting." Technology
Transfer Seminar Publication on Sludge Treatment and
Disposal. Cincinnati, OH 45628. September 1978.
16. Parr, J.F., G.B. Willson, R.L. Chaney, L.J. Sikora and
C.F. Tester. "Effect of Certain Chemical and Physical
Factors on the Composting Process and Product Quality."
Proceedings of Design of Municipal Sludge Compost
Facilities^p~. 130. Chicago, IL~. Information Transfer,
Inc., Rockville, MD. August 1978.
17. Haug, R.T., and L.A. Haug. "Sludge Composting: A
Discussion of Engineering Principles," Parts 1 & 2. Compost
Science/Land Utilization Journal of Waste Recycling.
November-December (1977)and January-February.1978.
18. Colacicco, D. "A Cost Comparison with the Aerated Pile and
Windrow Methods." Proc. National Conference on Composting
Municipal Residues and Sludges. p. 154. Information
Transfer,Inc., Rockville, MD.August 1977.
12-58
-------
19. Shuval, H.I. "Nightsoil Composting State of the Art
and Research - Pilot Study Needs." Research Working
Paper Series, P.U. report RES12, International Bank for
Reconstruction and Development, Washington, DC. November
1977.
20. Smith, D. and M.W. Selna. Pathogen Inactivation During
Sludge Composting. Internal Reports, County Sanitation
Districts of Los Angeles. September 1976, February 1977.
21. Burge, W.D. "Occurrence,of Pathogens and Microbial
Allergens in the Sewage Composting Environment." Proc.
National Conference on Composting of Municipal Residues and
STu'dg~es~. Information Transfer, Inc. , Rockville, MD. August
1977.
22. Olver, W.M. Jr., "The Life and Times of As pe rg il1 us
fumigatus." . Compost Science/Land Utilization. March-April
1979.
23. Burge, W.D., P.B. March, and P.O. Millner. "Occurrence of
Pathogens and Microbial Allergens in the Sewage Sludge
Composting Environment." Proc. 1977 National Conference on
Composting of Municipal Residues and Sludges. Information
Transfer, Inc., Rockville, MD. 1978.
24. Slueski, S. "Building Public Support for a Compost Plant."
Compost S^c^enc^e/Land Util i zation. Vol. 19, p. 10. 1978.
25. Solomon, W.R., H.P. Burge, and J.R. Boise. "Airborne
Aspergillus fumigatus Levels Outside and Within a Large
(JliHical Center." Journal Allergy Clinical Immunology.
Vol. 62, p. 56. 1978.
26. Schwartz, H.J., K.M. Citron, E.H. Chester, J. Kaimal,
P. Barlow, G.L. Baum, and M.R. Schuyler. "A Comparison of
the Prevalence of Sens itization to Aspergillus Antigens
Among Asthmatics in Cleveland and London."Journal Allergy
Clinical Immunology. Vol. 62, p. 9. 1978.
27. Slavin, R.G. "What Does A Fungus Among Us Really Mean?"
Journal_Allergy Clinical Immunology. Vol. 62, p. 7. 1978.
28. Epstein, E. "Composting Sewage Sludge at Beltsville,
Maryland". Proc. of Land Application of Residual Materials
Engineering, Foundation Conference. Publishing ASCE. New
York, NY. October 1976.
29. USEPA. Sludge Handling and Conditioning. Office of Water
Program Operations. Washington, DC 10460. EPA 430/9-78-
002. February 1978.
30. Kalinske, A.A. "Study of Sludge Disposal Alternatives for
the New York-New Jersey Metropolitan Area." Paper presented
at 48th Water Pollution Control Federation Conference, Miami
Beach, Florida. October 1975.
12-59
-------
31. Crombie, G. "Mechanized Forced Aeration Composting for
Durham, New Hampshire". Town of Durham. 1978.
32. Horvath, R.W. "Operating and Design Criteria for Windrow
Composting of Sludge." Proc. National Conference on Design
of Municipal Sludge Compost Facilities.Information
Transfer, Inc., Rockville, MD. August 1978.
33. Camp, Dresser and McKee, Inc. Alternative Sludge Disposal
Systems for the District of Columbia Water Pollution PTant
at Blue Plains, District of Columbia. Unpublished report
prepared for the Department of Environmental Services
District of Columbia, December 1975.
34. Colacicco, D., E. Epstein, G.B. Willson, J.F. Parr, and
L.A. Christensen. "Cost of Sludge Composting". USDA,
Agricultural Research Service, ARS-NE-79. Washington, DC.
February 1977.
35. Wilson, G.B., J.F. Parr, and D.C. Basey. "Criteria for
Effective Composting of Sewage Sludge in Aerated Piles and
for Maximum Efficiency of Site Utilization." Design of
Municipal Sludge Compost Facilities Conference. Information
Transfer,Inc.,Rockville, MD.August 1978.
36. Sikora, L. "Materials Balance in the Beltsville Aerated
Pile Method of Sewage Sludge Composting." Proc. National
Conference and Exhibition on Municipal and Industrial Sludge
Management. Information Transfer, Inc., Rockville, MD.
November 1979.
37. Resource Conversion Systems, Inc., Company Process Brochure,
Houston, Texas. December 1977.
38. USEPA. Evaluation of "Within Vessel" Sewage Sludge
Composting Systems in Europe. Draft Report. Municipal
Environmental Research Laboratory. Cincinnati, Ohio 45268.
Contract 68-03-2662. 1978.
12-60
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EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapter 13. Miscellaneous Processes
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
structure of the solids; a monolithic solid is much less subject
to leaching than is a granular solid. However, monolithic solids
may deteriorate if exposed to wet-dry or freeze-thaw cycles (11).
Leaching tests to estimate long-term weathering resistance
of the fixed solids are still being formulated (12). It should
be emphasized that the information presented in this paragraph
was derived from experience with sludges of an industrial origin.
Experience with municipal sludges may be similar to that with
some industrial ones.
TABLE 13-1
PARTIAL LIST OF FIXATION PROCESSES
Additive
quantity,
Vendor Process Additives percent References
Dravo Corporation Synearth3 Calcilox 7^ 3, 4
Thiosorbic lime
IU Conversion Systems, Inc. Poz-O-Teca Lime 4 3, 4, 5
Poz-0-Soila
Chemfix, Inc. Chemfix3 Portland cement 7, 3, 4
Sodium silicate 2
TRW, Systems Group0 - 1, 2 - polybutadiene 3 - 4d 6
(nonproprietary) Flyash-limestone Fly ash - 4
Limestone
Registered trademarks. Process is in full-scale use on hazardous
industrial sludge or flue gas desulfurization sludge or both.
Additive as percent by weight of dry sludge solids for flue gas
desulfurization sludge and fly ash at coal-burning power plants.
cBench scale tests.
Additive as a percent of dry sludge solids.
The cost of utilizing the chemical fixation process is affected
by the degree of dewatering required, the type of fixation
chemical(s) employed, and the method of mixing the chemical(s)
and sludge. In addition chemical fixation processes are
generally proprietary and require royalty payments. Therefore,
schemes including chemical fixation are generally more expensive
than conventional systems for processing municipal sludges.
Consequently, applications of the chemical fixation process to
municipal sludges will probably remain uncommon except when such
sludges contain significant concentrations of heavy metals or
other toxicants. Variables affecting the cost of fixation
include:
• Availability of fly ash. Some processes use fly ash
to reduce the need for other chemicals.
• Sludge dewaterability. Fixation costs increase with the
amount of water present.
13-2
-------
• Volume and mass of sludge to be treated.
• Physical properties required for the fixed sludge. A
granular product tends to cost less than a monolithic
product, for example.
• The degree to which the fixed product must resist
leaching.
• Reactivity of the sludge with the fixation chemicals.
• Unit prices of treatment chemicals. In some cases, this
factor is complicated by the fact that the chemicals are
proprietary.
13.3 Encapsulation Process
Encapsulation is the encasing of sludge in an impervious, durable
material. Encapsulation processes are expensive to employ but
are a useful treatment alternative when the sludge contains
significant concentrations of leachable toxic materials. As with
fixation processes, there is little reported experience for the
system with municipal sludges. The information presented here
has been obtained from experience with industrially derived
sludges. Two examples of encapsulation processes are discussed
below.
13.3.1 Polyethylene Process
Encapsulation of a sludge with polyethylene has been investigated
in the laboratory (6,13,14). This process involves putting a
block, a 55-gallon (208 1) drum, or other container of sludge
that has been treated by the chemical fixation process into a
bed of polyethylene powder. The polyethylene is then heated to
350°F (180°C) so that it melts and fuses into a 1/4 inch (6 mm)
thick seamless layer. The approximate amount of polyethylene
required is 4 percent by weight of the sludge to be encapsulated^
Polyethylene is tough and may be severely deformed without
rupture. Leaching tests of several materials treated by the
polyethylene process showed virtually no release of the chemical
constituent.
The extremely high system temperatures cause water to evaporate
at pressures up to about 130 psig (900 kN/m2). Therefore,
one of the following three stringent conditions must be met:
• The process must be carried out under pressure.
• The sludge must be sealed in vessels that are able to
withstand an internal working pressure of 130 psig
(900 kN/m2) before the sludge is delivered to the
encapsulation process.
13-3
-------
• The sludge must be in a thoroughly dry form such as
either sludge incinerator ash or heat-dried sludge.
13.3.2 Asphalt Process
Asphalt may be used to encapsulate wastes. In this process,
the waste is mixed with asphalt at 300°F (150°C) in such a way
that each individual particle is coated with asphalt. Moisture
is removed as steam. The coated particles are then placed in
55-gallon (208 1) drums or other containers where they cool
and form a solid, nonporous mass. The encapsulated product is
highly resistant to leaching, mechanical damage, and bacterial
attack. About one pound of asphalt is required for each pound of
dry solids (15).
Asphalt encapsulation has been used in Europe on medium-level
radioactive wastes since 1965. There is little United States
operating experience, but European experience makes it possible
to estimate costs for wastewater sludge applications. An
installation with a capacity of five hundred 55-gallon (208 1)
drums per year could handle about 84 tons (76 t) of dry sludge
solids per year. Capital and operating costs are estimated at
$1.45 million and $62,000 per year, respectively, at 1977 U.S.
price levels. Amortizing capital over twenty years at 7 percent,
the total cost is about $2,400 per ton dry solids processed
($2,600/t). This cost includes encapsulation machinery and
associated building space, drums, drum storage, asphalt, steam,
cooling water, and operating labor. It does not include
engineering (except for engineering performed by the equipment
supplier), sludge dewatering which precedes the encapsulation
process, transportation and disposal of the finished product,
treatment of contaminated steam that might be produced, or
maintenance. Possibly, cost savings can be obtained from
economies of scale and less rigorous conditions than those at
nuclear power plants.
13.4 Earthworm Conversion Process
A novel municipal wastewater sludge treatment process uses
earthworms (Oligochaete annelids) . This system is often called
"earthworm conversion," vermicomposting, or annelidic consumption
(16). Vermicomposting is different from the conventional
composting of wastewater treatment plant sludge. In the
earthworm conversion process, the worms are provided an optimum
environment to consume or metabolize the sludge and produce feces
or castings. These castings may be used as a soil conditioner.
13.4.1 Process Arrangement
Earthworm conversion is basically a simple process, and a
schematic diagram of it is shown on Figure 13-1. The process
requires worm beds and a supply of worms. Generally, digested
13-4
-------
and dewatered sludge is put into the beds, although experiments
are underway, where raw liquid sludge is placed in beds. If
anaerobic digestion is used prior to earthworm conversion,
additional pretreatment may be needed. A bulking agent such as
wood chips may be useful in some cases for keeping the bed porous
and aerobic, especially if moisture is high. Sludge is, however,
generally applied without any bulking agent. A worm bed may
take the form of a simple tray. Windrows similar to those for
composting may also be used. After the worms have consumed the
sludge, they must be separated from the castings. This may be
done with an earthworm harvester, a drum screen that rotates on
a nearly horizontal axis. Castings fall through the screen
openings while worms tumble through the length of the drum.
Table 13-2 contains some critical operational parameters for the
earthworm conversion process.
MAKE-UP
IABTHWORMS
AIR06ICALLV
DIGESTED SLUDGE _
DEWATEfllfiS
BULKING A6EHT
IF REQUIRED
1
Mixtn
BULKING AGENT
ANAERQUCALLY
DIGESTED
SLUDGE __
fIF REQ
&EVM ATE RING
PBETREAHMENT
|AEBATION,ETCj
uineOi
~l
MIXER
ftSC
WORM
BIDS
hAHIH'lYlV'PJ
HABVESTEfi
YCLED EARTHWORMS,
a
3
E
Q
UNEATEN SLUW5E PARTICLES.
AND |IF USED) BULKING AGENT
CASTINGS
FOR LAND
UTILISATION
AS SOIL
AMENDMENT
I!
isi ^
BULKING AGENT
FOR RECYCLE
j SEPARATIQW Of I
J EARTHWOflWS FROM I
*] tULKING AGENT )
IIP RESlUIfliOl 1
SURPLUS EAHTHWOBMS
FOR SALB
FIGURE 13-1
DIAGRAM OF AN EARTHWORM CONVERSION PROCESS
The main product of the earthworm conversion process is the
worm's castings. In some process arrangements there may be a net
earthworm production. The excess earthworms may then be sold for
13-5
-------
fish bait or animal protein supplement. Earthworm marketing is a
complex problem. For municipal sludge applications, surplus
earthworms may be considered a by-product; the principal product
is the castings, which can be a resource.
TABLE 13-2
PARAMETERS FOR EARTHWORM CONVERSION
Parameter Values
Detention time of sludge in worm beds 2 days (19)
32 days (18)
Worm reproductive cycle 1 to 2 months
Rate of worm feeding (15°C) 0.17 to 1.7 grams dry sludge per gram dry
worm weight per day (17)
Optimum temperature 15°C to 20°C (17)
Dry matter content of worms 20 to 25 percent (Eise_rria. foetida) (20)
Minimum solids content of the worm bed 20 percent solids
mixture
Species of worm being tested: Eisenia foetida (redworm, hybrid redworm,
tiger worm, dung worm) (17), Lumbricus rubellus (red manure worm, red
wiggler worm) (18), and Lumbricus terrestris (nightcrawler). (17).
Actual minimum solids content depends on such factors as porosity, type of
sludge, ability to keep aerobic. Experiments are being conducted to better
define these parameters.
1°F = 32 + 1.8°C
13.4.2 Advantages of the Earthworm Conversion Process
When dry, earthworm castings are essentially odorless; when damp,
they have a mild odor like a good quality topsoil. Also, the
castings have a favorable appearance. When sifted and dry, they
are granular, about 0.02 to 0.1 inches (0.5 to 3 mm) in maximum
dimension (with some fines); color is brownish gray. In a study
where municipal sludge was applied to a wheat crop, it was found
that when earthworms were added to the sludge, the germination
rate of the wheat was improved (21). The odor, appearance, and
soil supplementation advantages of the earthworm conversion
process may help in the acceptance of sludge by farmers and
householders.
Earthworm conversion affects several other sludge char-
acteristics. The oxygen uptake rate increases (17); the
acid-extractable fraction of various nutrients increases (21).
The volatile content of the solids drops slightly and humic acid
concentrations fluctuate (17). While these effects may be
beneficial, there are no data to show how the results affect
design or operation of earthworm conversion installations.
13-6
-------
The earthworm conversion process would appear to be low in cost,
although this cannot be said with certainty, since no cost data
are available for full-scale operations on sludge. The process
does not require chemicals, high temperatures, or large amounts
of electricity. Only a small amount of low-speed mechanical
equipment is needed. Significant expenditures may be required to
offset the potential operating difficulties discussed below.
13.4.3 Possible Operating Difficulties
A number of potential operating difficulties and their solutions
are listed in Table 13-3. None of these difficulties are
insurmountable. Probably it is most difficult to economically
pretreat anaerobically digested sludge so that it is nontoxic to
the worms.
13.4.4 Limitations
Limitations are:
• Earthworm conversion decreases the total nitrogen
values in the sludge because ammonia nitrogen will be
lost to the atmosphere.
• Published information to date (1979) is almost
nonexistent on full-scale municipal wastewater treatment
plant sludge operations. Consequently, costs are
unpredictable.
• TWO common ions in municipal wastewater sludge, ammonium
and copper, may be toxic to worms. Studies have found
that these ions were lethal at additions equivalent
to 180 mg NH4-N and 2,500 mg Cu per kilogram of wet
substrate (26,27). Safe limits for these elements are
not known.
• Cadmium accumulates in the worm Eisenia foetida. Zinc
apparently does not accumulate in Eisenia foetida but
does accumulate in other species (27,28). If the worms
are to be used as animal feed, the system must be
operated such that cadmium and zinc concentrations in
the worms do not exceed recommended levels for animal
consumption.
« Space requirements may rule out earthworm conversion
at some treatment plants.
• The earthworm business has been afflicted with unsound
investments and excessive claims. For example, it has
been claimed that earthworm processing is able to
reduce concentrations of heavy metals (29). Any such
13-7
-------
reduction could only be caused by simple dilution
with uncontaminated waste or by concentration of the
contaminants in the earthworms.
TABLE 13-3
POSSIBLE OPERATING DIFFICULTIES IN EARTHWORM CONVERSION
Possible difficulty
Comments
Worm drowning
Predation by birds and animals
Worm loss due to migration from the
process
Toxicity of sludge to worms
Toxicity or unpalatable nature of
dewatering chemicals
Worm shortage in the process, so that
worm additions are required
Shortage of worms for initial inventory
or restart
Temperature extremes
Shortage of enzymes
Worms must be protected from flooding.
Not a problem at San Jose - Santa Clara,
California experiments (22).
Caused by flooding, toxic sludge, unpalat-
able sludge, adjoining areas attractive
to worms, lack of artificial lighting on
rainy nights.
Significant for anaerobically digested
sludge. However, toxicity is eliminated
by exposing the sludge to air for two
months (17) or wetting sun-dried sludge
daily for 14 days (21). Stabilization by
lime or chlorine is not recommended for
sludge that will be fed to earthworms.
Toxicants such as copper salts might also
cause problems. Aerobic digestion is best
suited for sludge to be converted by
earthworms.
Avoided at Hagerstown, Md., by use of food-
grade polymer (19). Drying beds may be
used; drying beds do not usually require
chemicals.
Worms reproduce via egg capsules. These
capsules may be lost from the process in
the castings. Also, toxic conditions,
drowning, and other problems will cause
worm populations to drop. At Hagerstown,
Md. , a worm raising operation has beer.
proposed to supply the necessary make-up
worms to the sludge conversion process (19)
To begin operation, a large worm inventory
may be needed, so large that local worm
suppliers may be unable to fill it.
Gradual start-up is therefore desirable,
especially for large plants. Also, earth-
worm exchanges may become available
natiB«wide so that sludge operations can
draw on larger numbers of earthworm
suppliers.
Worm feed most rapidly at 15 to 20 degrees C;
about 5 degrees C, feeding is quite slow
(17). Freezing will kill worms. High
temperatures can also cause problems. It
may be necessary to stockpile sludge dur-
ing the winter or provide a heated
building for the conversion process.
Not a problem, despite claims by marketers
of enzyme preparations that these prepara-
tions are valuable to the process (23).
13-8
-------
TABLE 13-3
POSSIBLE OPERATING DIFFICULTIES IN EARTHWORM CONVERSION (CONTINUED)
Possible difficulty
Comments
Exposure to light
Dehydration
Salinity in castings
Contamination of castings by heavy metals,
motor oil, rags, and similar materials
Odors
Worms avoid bright light. Some sort of
cover or shade should be provided so that
worms will convert the top layer of the
sludge .
There is a minimum moisture content for the
worm bed (23).
Under some conditions, castings may have
sufficient dissolved salts to inhibit
plant growth. This problem may be elim-
inated by leaching or by mixing the
castings with other materials with lower
dissolved salts (24, 25).
Source control may be used, where feasible,
as for other processes aimed at reuse of
sludge as a soil conditioner. See
Chapter 2 for regulations on sludge pro-
ducts .
The most likely source is raw or
aerobically digested sludge, which has
been stockpiled to await earthworm con-
version .
3C = 0.555 (°F -32) .
• If a particular sludge is suitable for earthworm
conversion, that sludge should also be suitable for
reuse as a soil conditioner without being processed by
earthworms. However, earthworm conversion reduces odor,
improves texture, and may increase germination rate.
These limitations may be significant but not overwhelming. There
is considerable research and development underway. It appears
that earthworm conversion may have a role in municipal wastewater
treatment plant sludge processing.
13.5 References
1. R.K. Salas. "Disposal of Liquid Wastes by Chemical
Fixation/Stabilization - The Chemfix (R) Process."
Toxic and Hazardous Waste Disposal, Volume 1. R.B. Pojasek,
ed. Ann Arbor Science, 1979.
J.T. Schofield. "Sealosafe (SM)." Toxic and Hazardous
Waste Disposal, Volume 1.
Science ,
1979.
R.B. Pojasek, ed. Ann Arbor
3. Francis O'Donnell. "Scrubber Sludge: Nightmare for
Utilities." Sludge Magazine. Vol. 1 no. 2, p. 26. March-
April, 1978 .
13-9
-------
4. J.W. Barrier, H.L. Fawcett, and L.J. Henson. "Economic
Assessment of FGD Sludge Disposal Alternatives." Journal
Environmental Engineering Division_ ASCE. Vol. 104 ~~p~.9^T,
Oct., 1978 .
5. Hugh Mullen, Louis Ruggiano, and S.I. Taub. "Concerting
Scrubber Sludge and Flyash into Landfill Material."
Pollution Engineering. Vol. 10, no. 5, p. 71, May, 1978 .
6. USEPA. Development of a Polymeric Cementing and Encapsul-
ating Process for Managing Hazardous Wastes. Office of
Research and Development, Cincinnati, Ohio 45268. EPA-
600/2-77-045. August 1977.
7. Raymond Swan. "Indianapolis Project: From Lagoons to
Landspreading in Three Not-so-Easy Lessons." Sludge
Magaz ine. Vol. 1, no. 3, p. 16. May-June, 1978.
8. USEPA. Field Evaluation of Chemically Stabilized Sludges.
Land Disposal of Hazardous Wastes. Proceedings of the
Fourth Annual Research Symposium. San Antonio, Texas.
March 6-8, 1978. Office of Research and Development,
Cincinnati, Ohio 45268. EPA-600/9-78-016. 1978.
9. USEPA. Laboratory Assessment of Fixation and Encapsulation
Processes for Arsenic-Laden Wastes. Land Disposal of
Haz ardous W astes, Proceedings of the Fourth Annual
Research Symposium. San Antonio, Texas. March 6-8, 1978.
EPA-600/9-78-016.
10. USEPA. Pollutant Potential of Raw and Chemically Fixed
Hazardous iTTdiTstrial Wastes and Flue Gas Desulfurization
sTucfges. Interim report. Office of Research and Develop-
ment, Cincinnati, Ohio 45268. EPA-600/2-76-182. July 1976.
11. R. E. Landreth and J.L. Mahloch. "Chemical Fixation of
Wastes." Industrial Water Engineering. Vol. 14, no. 4,
p. 16. July-August 1977.
12. Robert Pojasek. "Stabilization, Solidification of Hazardous
Wastes." Environmental Science and Technology. Vol. 12,
p. 382. April 1978.
13. USEPA. Encapsulation Techniques for Control of Hazardous
Materials. Land Disposal of Hazardous Wastes, Proceedings
ofFourth Annual Research Symposium. San Antonio, Texas,
March 6-8, 1978. EPA-600/9-78-016. 1978.
14. H.R. Lubowitz and C.C. Wiles, "Encapsulation Technique
for Control of Hazardous Wastes." Toxic and Hazardous
Waste Disposal, Volume 1. R.B. Pojasek, ed. Ann Arbor
Science, Ann Arbor, Michigan 48106. 1979.
13-10
-------
15. R.D. Doyle, "Use of an Extruder/Evaporator to Stabilize
and Solidify Hazardous Wastes." Toxic and Hazardous Waste
Disposal, Vo1 ume 1. Ann Arbor Science, 1979. R.B. Pojasek,
ed. p. 65.
16. Frank Carmody, "Practical Problems in Application of
Earthworms to Waste Conversion Processes." Utilization of
Soil Organisms in Sludge Management , proceedings o~f
conference, Syracuse, New York: 6/25-17/78. National
Technical Information Service PB-286932. ed. R. Hartenstein.
17. M.J. Mitchell, R.M. Mulligan, Roy Hartenstein, and
E.F. Neuhauser. "Conversion of Sludges into "Topsoils1 by
Earthworms." C ompost Scie n ce. Vol. 18, p. 28. July-August,
1977 . :
18. David Newman. "Earthworm and Electrons: Technology's Outer
Limits." Sludge Magazine, Vol. 1, no. 1, p. 30 • January-
February 1978 .
19. Cathy Dombrowski, "Postscript: Earthworms." Sludge
Ma^gazine. Vol. 1, no. 5, p. 10 September-October, 1978 .
20. J.R. Sabine. "The Nutritive Valve of Earthworm Meal."
Utilization of Soil Organisms in Sludge Management,
proceedings of conference, Syracuse, New York: 6/15-17/78.
National Technical Information Service PB-286932. ed.
R. Hartenstein.
21. M.3. Kirkham. "Availability to Wheat of Elements in
Sludge-Treated Soil with Earthworms." Utilization of Soil
Organisms in Sludge Management, proceedings of conference,
SyracuslT;New York: 6/15-17-78. National Technical
Information Service PB-286932. ed. R. Hartenstein.
22. J.E. Collier. "Use of Earthworms in Sludge Lagoons."
U t i1i z ation of Soil Organisms in Sludge Management,
proceedings of conference.Syracuse^New York:6/15-17-78.
National Technical Information Service PB-286932. ed.
R. Hartenstein.
23. Linda Theoret, Roy Hartenstein, and M.J. Mitchell. "A Study
on the Interactions of Enzymes with Manures and Sludges."
Cgmpost_Sc ience. Vol. 19, p. 29. January-February, 1978 .
24. Soil and Plant Laboratory, Inc. Soil Fertility Analysis -
Earthworm Castings. Report on sludge-derived castings from
San Jose - Santa Clara, Calif,, experiments. May 17, 1977.
25. N. Stark, P. Pawlowski, and S. Bodmer. "Quality of
Earthworm Castings and the Use of Compost on Arid Soils."
Utilization of Soil Organisms in Sludge Management,
proceedings of conference. Syracuse,NewYork:6/15/78.
National Technical Information Service PB-286932. ed.
R. Hartenstein. p. 87.
13-11
-------
26. E.F. Neuhauser, "The Utilization of Earthworms in Solid
Waste Management", Utilization of Soil Organisms in Sludge
Management, proceedings of conference. Syracuse, New York:
6/15-17/78 . National Technical Information Service
PB-286932. ed. R. Hartenstein. p. 138. (Value converted
from ammonium acetate basis to ammonia nitrogen basis.)
27. R. Hartenstein et al., "Heavy Metals, Sludges, and
the Earthworm Eisenia foetida." Journal of Environmental
Qual i ty . In review, 1971TI
28. R.I. Van Hook, "Cadmium, Lead, and Zinc Distributions
Between Earthworms and Soils: Potentials for Biological
Accumulation." Bulletin of Environmental Contami^nat^qn.^ajrvd
Toxicology. Vol. 12, p. 509, 1974 .
29. AnProS, An Ecologically, Environmentally, & Economically
Sound Ap~proach to Sewage Sludge Management:. GTA, Inc.,
Wilmington, Delaware,1978, pamphlet.
13-12
-------
EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapter 14. Transportation
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 14
TRANSPORTATION
The fundamental objective of all wastewater treatment operations
is to remove undesirable constituents present in wastewater
and consolidate these materials for further processing and
disposal. Solids removed by wastewater treatment processes
include screenings and grit, naturally floating materials called
scumf and the remainder of the removed solids called sludge.
This chapter discusses the transportation of solids, or
the movement of sludge, scum, or other miscellaneous solids
from point to point for treatment, storage, or disposal.
Transportation includes movement of solids by pumping, conveyors,
or hauling equipment.
14.1 Pumping and Pipelines
Unless a sludge has been dewatered, it can be transported
most efficiently and economically by pumping through pipelines.
Sludge is subject to the same physical laws as other fluids.
Simply stated, work put into a fluid by a pump alters velocity,
elevation, and pressure, and overcomes friction loss. The unique
flow characteristics of sludge create special problems and
constraints. Nevertheless, sludge has been successfully pumped
through short pipelines at up to 20 percent solids by weight, as
well as in pipelines of over 10 miles (16 km) long at up to
8 percent solids concentrations.
Most of the following information is related to sludge, although
screenings, grit, and scum may also be transported by pipeline.
Mention is made of these miscellaneous solids when special
considerations are involved.
14.1.1 Simplified Head Loss Calculations
Head losses must be estimated for sludge pumping; they are not
available in standard tables. Head requirements for elevation
change and velocity are the same as for water. However,
friction losses may be much higher than friction losses in water
pipelines. Relatively simple procedures are often used in design
work; such a procedure is described below. The accuracy of these
procedures is often adequate, especially at solids contents
below 3 percent by weight. However, as the pipe length, percent
total solids, and percent volatile solids increase, these simple
14-1
-------
procedures may give imprecise or misleading results. A more
elaborate method for situations demanding greater accuracy is
given in Section 14.1.2.
In water piping, flow is almost always turbulent. Formulas
for friction loss with water, such as Hazen-Wi 11 iams and
Darcy-Weisbach, are based on turbulent flow. Sludge also may
flow turbulently, in which case the friction loss may be roughly
that of water. Sludge, however, is unlike water in that laminar
flow also is common. When laminar flow occurs, the friction loss
may be much greater than for water. Furthermore, laminar flow
laws for ordinary "Newtonian" fluids, such as water, cannot be
used for laminar flow of sludge because sludge is not a Newtonian
fluid; it follows different flow laws.
Figure 14-1 may be used to provide rough estimates of friction
loss under laminar flow conditions. This figure should be used
when:
• Velocities are at least 2.5 feet per second (0.8 m/s) .
At lower velocities, the difference between sludge and
water may greatly increase.
• Velocities do not exceed 8 feet per second (2.4 m/s).
Higher velocities are not commonly used because of high
friction loss and abrasion problems.
• Thixotropic behavior is not considered. Friction losses
may be much higher in suction piping. Also, when
starting a pipeline that has been shut down for over a
day, unusually high pressures may be needed.
• The pipe is not seriously obstructed by grease or other
materials.
As an example, consider a pipe carrying unstabilized primary
sludge. The pipe is 500 feet (152 m) long and 6 inches (150 mm)
in diameter; flow rate is 300 gallons per minute (19 1/s) .
Assume that the sludge solids concentration may be up to
7 percent solids on occasion. Using the Hazen-Williams formula
with a "C" of 100, a friction loss of 6.5 feet (2.0 m) would
apply. If laminar sludge flow occurs, Figure 14-1 gives a
multiplication factor of 5.8, so a friction loss of 38 feet
(12 m) might occur. The friction loss could easily vary from
6.5 to 38 feet (2.0 to 12 m) in actual operation due to changes
in sludge properties and factors not considered on Figure 14-1.
Grit slurries are usually dilute; also, grit particles do not
stick to each other. Therefore, ordinary friction formulas for
water are usually adequate. A velocity of about 5 feet per
second (1.5 m/s) is typically used. Low velocities may cause
deposition of grit within the pipe; high velocities may cause
erosion.
14-2
-------
14
12
x
g 10
o
8
O
u 6
G,
5 4
DIGESTED
SLUDGE
UNTREATED PRIMARY AND'
CONCENTRATED SLUDGES
01234 56789 10
SLUDGE CONCENTRATION, % solids by weight
NOTE: MULTIPLY LOSS WITH CLEAN WATER BY K TO
ESTIMATE FRICTION LOSS UNDER LAMINAR
CONDITIONS (SEE TEXT).
FIGURE 14-1
APPROXIMATE FRICTION HEAD-LOSS FOR LAMINAR
FLOW OF SLUDGE
14.1.2 Application of Rheology to
Sludge Pumping Problems
Water, oil, and most other common fluids are "Newtonian." This
means that the pressure drop is directly proportional to the
velocity and viscosity under laminar flow conditions. As the
velocity increases past a critical value, the flow becomes
turbulent. The transition from laminar to turbulent flow depends
on the Reynolds number, which is inversely proportional to
the fluid's viscosity. The viscosity is a constant for the fluid
at any given temperature. Formulas for Newtonian fluids are
available in fluid mechanics textbooks.
Wastewater sludge, however, is a non-Newtonian fluid. The
pressure drop under laminar conditions is not simply proportional
14-3
-------
to flow, so the viscosity is not a constant. Special procedures
may be used, however, to determine head loss under laminar flow
conditions, and the velocity at which turbulent flow begins.
These procedures use at least two constants to describe the fluid
instead of a single constant (the viscosity) which is used for
Newtonian fluids.
The behavior of wastewater sludge is compared with the behavior
of water on Figure 14-2. This figure is based on steady state
behavior, after thixptropic breakdown. (Thixotropic breakdown
will be discussed in a subsequent paragraph.) The following
features are notable concerning the behavior of wastewater
sludge :
• Essentially no flow occurs unless the pressure is high
enough to exceed a yield stress TO.
• Turbulent flow may occur, but a much higher velocity is
needed for sludge than for water.
• In fully developed turbulent flow, the pressure drop is
roughly that of water.
• For the laminar plastic flow region, sludge approximately
obeys the laws of a "Bingham plastic." A Bingham plastic
is described by two constants, which are the yield stress
T0 and the coefficient of rigidity, rj .
• It is also possible to consider sludge to be a
"pseudoplastic" material. In that case, two other
constants are used, and the formulas are different. The
following discussion uses the Bingham plastic approach.
14.1.2.1 Solution of Pressure Drop Equation
If the two constants ro and ?? can be determined, it is quite
easy to determine pressure drop over the entire range of
velocities with the aid of Figure 14-3 and ordinary equations for
water. To use this figure, calculate the two dimensionless
numbers (Reynolds and Hedstrom) by reading the graph. The
only real difficulty is in obtaining the two constants; see
Section 14.1.2.4.
The two dimensionless numbers are a Reynolds number, given by:
Re = .P (14_1}
14-4
-------
where:
Re = Reynolds number, dimensionless
f = density of sludge, Ib (mass)/ft3f (g/cm3)
V = average velocity, ft (cm/s)
D = diameter of pipe, ft (cm)
n = coefficient of rigidity, Ib (mass)/ft-sec, poise (same
as dyne-s/cm2 and g/cm-s);
and the Hedstrom number, given by:
DT0 gcp
He = ° (14-2)
where:
He = Hedstrom number, dimensionless
ro = yield stress, lb(foree)/ft2
gc = units conversion factor:
32.2 lb(mass)-ft/lb(force)-sec2 for English units
1.0 for metric units
2fPLV2 . , . -..
— (14-3)
where:
Ap = pressure drop due to friction, 1b(force)/ft2,
(dyne/cm2)
f = Fanning friction factor from Figure 14-3, dimension-
less
L = length of pipeline, ft (cm)
There are a few subtleties in the correct use of these equations.
First, the Reynolds number in Equation 14-1 is not the same as
a Reynolds number based on viscosity. In plastic flow, an
effective viscosity may be defined, but it is variable and it can
be much greater than the coefficient of rigidity. Consequently,
the two Reynolds numbers can differ by factors of more than ten
under some conditions. Second, many textbooks use a somewhat
14-5
-------
different definition of f, which is four times the value as
used in Equation 14-3 and Figure 14-3.' Third, care is required
with units. For English units, it is not possible to use
pounds (mass) in density at the same time as pounds (force) in
stress without introducing the conversion factor (gc) into
Equations 14-2 and 14-3. Alternatively, the "slug," English mass
unit could be used.
r
BINGHAM PLASTIC, e.* SLUDGE AFTSR
THIXQTRQPIC BREAKDOWN
NEWTONIAN FLUID, e.g. WATER
o
u
5
mtM
DZ
Is
i
Ul Z -a
(c o ~,
uj Q —
X tr. H
EH a. 3
CURVATURE IN
THIS REGION
DUE TO PLUG
FLOW IN THE
MIDDLE Of
THE PIPE
SLOPE s; COEFFICIENT
OF RIGIDITY
T
X
BAfl Of SHEA?!
PHQPQRTIO-NAL TO VELOCITY IN PIPELINE
UNITS: SECONDS''
FIGURE 14-2
COMPARISON OF BEHAVIORS OF WASTEWATER SLUDGE
AND WATER FLOWING IN CIRCULAR PIPELINES
These equations apply to the entire range from virtually zero
velocity to the fully turbulent range, except that Figure 14-3
does not allow for pipe roughness. To allow for pipe roughness,
ordinary water formulas may be used. If, for example, the
Hazen-Williams formula gives a higher pressure drop than
Equation 14-3, then pipe roughness is dominant, the flow is
14-6
-------
fully turbulent, and the pressure drop will be given by the
ordinary water formula to a sufficiently good approximation for
engineering purposes (3).
He, HEDSTROM NUMBER
cc
O
cj
<
u.
Z
g
u
CE
U
0.001
0.01
Re, REYNOLDS NUMBER (DV^/,,)
FIGURE U-3
FRICTION FACTOR FOR SLUDGE, ANALYZED AS A
BINGHAM PLASTIC
Figure 14-3 also shows whether flow is laminar or tubulent.
The friction factor f is located by the intersection of the
Reynolds and Hedstrom numbers (Re and He). If this point is
above the dashed line on Figure 14-3, or if the Reynolds number
Re is less than 2,000, the flow is laminar; otherwise it is
turbulent. For example, at Re = 10^, a Hedstrom number of
1Q4 gives turbulent flow, while a Hedstrom number of 106 gives
laminar flow.
Interpolation on logarithmic graphs such as Figure 14-3 is
somewhat difficult. This is particularly "true for the Hedstrom
number curves on Figure 14-3. If the logarithm (base 10) of He
is calculated, interpolation between lines will be linear.
Alternatively, if flow is laminar, the Buckingham equation (3,4)
may be used. Figure 14-3 incorporates the Buckingham equation in
the laminar region. The Bingham pressure loss equation is an
approximate solution of the Buckingham equation (5,3).
14-7
-------
14.1.2.2 Design Example
The designer wishes to transport anaerobically digested sludge
6 miles from one plant to another plant where there are
dewatering facilities. If transported at 5 percent solids, the
sludge quantity is 100,000 gallons per day (378 m3/day). The
sludge may be diluted or thickened, if desired, to improve
economics. All of the sludge must be pumped in a 4-hour period
each day to accommodate dewatering schedules at the receiving
plant.
It is assumed that the sludge can be considered as a Bingham
plastic using the following data from Canton, Ohio (2):
Ca_se
1
2
3
Solids
concentration,
percent
7.12
5.34
3.56
Yield stress
dyne/cm^
100
30.5
5.8
O'
Coefficient
of rigidity
y , g/cm-s
0.40
0.24
0.13
For a comparison, water has a yield stress of zero and a
coefficient of rigidity of about 0.01 g/cm-s. The pipe is
assumed to be unlined steel pipe, schedule 40; nominal pipe sizes
of 4 to 10 inches (100 to 250 mm) in diameter will be considered.
The calculation is illustrated in detail for the 8-inch (200-mm)
pipe and 7.12 percent sludge. First, the flow rate is needed.
If the sludge were at 5 percent solids, 100,000 gallons (378 m3)
of sludge would be transported daily. Since the sludge is at
7.12 percent, the volume is:
100,000 x ?512 = 70,224 gallons (266
and the flow rate is:
70,224 gallons/day
4 hours flow/day x 60 min/hr
= 292.6 gpm (18.46 1/s)
Calculations of Reynolds and Hedstrom numbers will be carried out
in the centimeter-gram-second (cgs) system because T Q and 77 are
given in cgs units. The flow rate in cgs units is:
292.6 gpm x 3.785 1/gal x 1,000 cm3/! =
60 sec/min
Cm
3/sec
14-8
-------
The internal diameter of an 8-inch (200 mm) Schedule 40 pipe is
7.981 inches (20.27 cm) and the cross sectional area is
322.7 cm2. The velocity V is the flow rate divided by the
area:
., 18,460 cm^/sec ' _ „ ,
V = '- £5 = 57.2 cm/sec
322.7 cm2
The Reynolds number is obtained from Equation 14-1:
VD 1.0 x 57.2 x 20.27 nooo .,. ,
Re = P— = n An = 2898 (dimensionless )
i) U • ft U
The Hedstrom number is obtained from Equation 14-2:
He . 5?!32fiSc = (20.27)2 x 100 x 10 x 1.0 = 25 (dimensionless)
T,2 (0.40)2
Refering to Figure 14-3, f is about 0.08. The flow is laminar,
not turbulent.
The length L is needed in cgs units:
L = 6 miles x 5,280 ft/mile x 30.48 cm/ft = 965,600 cm
Now Equation 14-3 is used to calculate pressure drop due to
friction:
Ap = 2£ P Lv2 = 2 x °-08 *l-°* 9f -60Q x (57'2)2 = 24,940,000 dyne/cm2
Convert this value to pounds per square inch:
24,940,000 dyne/cm2 =
24.94 x 106 dyne x 2.248 x HT* Pounds (force)
dyne
cm2 x 0.1550 in.2/cm2
= 362 psi (2.49 MN/m2)
14-9
-------
This value may be compared to the value for water for the same
conditions, calculated from the Hazen-Williams equation:
V = 1.318 C R0.63 S0.54 (14-4)
where:
V = average velocity, ft/sec,
C = friction coefficient,
R = hydraulic radius = -j of diameter, ft,
S = hydraulic gradient, ft/ft.
This equation may be rearranged and solved on a calculator,
or tables or nomographs may be used. In the present case,
V = 57.2 cm/sec = 1.88 ft/sec and R = 0.166 ft. With a C of
100, S is 0.00310, indicating a pressure drop of 98.2 ft or
42 psi. The drop with this sludge is 362 psi or about 9 times
higher than the drop for water.
For the various cases, calculations are summarized in Table 14-1.
Friction factor plots from Figure 14-3 are shown on Figure 14-4.
A precaution that is useful for detection of computational error
is to check to see whether the pressure drop across the pipe
calculated by the above procedure produces a sufficient shear
stress at the pipe wall to exceed the yield stress of the sludge.
If the yield stress is not exceeded, the sludge will not flow.
The pressure drop needed is calculated by setting the calculated
shear stress at the wall equal to yield stress:
n, c
= To9c (14-5
4L
where :
A po = pressure drop needed to exceed yield stress.
Results of the calculation are shown for Case I and Case 2 in
Table 14-2. Equation 14-5 is also useful as a screening
test. If TO, D, and L are known, it is possible to quickly
calculate the minimum pressure drop that could occur, regardless
of velocity or flow rate. If Apo is excessive, the diameter
D should be increased. Impractical pipe sizes could be
quickly eliminated as requiring too high a pressure drop for
consideration.
14-10
-------
Values from Table 14-1 and 14-2 are plotted on Figure 14-5.
Selection of the optimum pipe diameter and solids content
requires an economic analysis. However, it is evident that at
the more reasonable pressure drops (below 200 psi or 1400 kN/m2),
the 7.12 percent solids has a much higher pressure drop at a
given pipe diameter even though the volumetric flow rate is much
lower than for the other two cases. At 8 inches (200 mm), the
pressure drops are about the same for the 5.34 percent and the
3.56 percent sludges. However, as noted in Table 14-1, the flow
is not in the turbulent regime for the 5.34 percent sludge. This
is a disadvantage because small changes in the rheological
constants To and 77 could cause changes in f. The 3.56 percent
solids content is probably a better selection based on the
likelihood of more stable operation. At 10 inches (250 mm), the
value of f is considerably higher for the 5.34 percent sludge
than for the 3.56 percent sludge. The choices between 8-inch
and 10-inch (200 and 250 mm) diameter and 3.56 percent and
5.34 percent sludge would have to be made on the basis of minimum
overall cost. The 5.34 percent sludge will be more expensive
to transport, but this cost increase may be offset by more
economical dewatering at the plant receiving the sludge.
TABLE 14-1
SUMMARIZED CALCULATIONS FOR NON-NEWTONIAN
FLOW EXAMPLE PROBLEM
Pressure drop.
Diameter
Case in
1 4.
5.
6.
7.
10.
2 4.
5.
6.
7 .
10.
3 4.
5.
6.
7.
10.
cm
03
05
06
98
02
03
05
06
98
02
03
05
06
98
2
10
12
15
20
26
10
12
15
20
25
10
12
15
20
25
.2
.8
.4
. 3
.4
.2
.8
.4
. 3
.4
.2
.8
.4
. 3
.4
Average
velocity,
cm/sec
225
143
99.1
57.2
36.3
300
190
132
76.3
48.4
450
286
198
114
72.6
Reynolds
number ,
Re
5
4
3
2
2
12
10
8
6
5
35
28
23
17
14
,750
,580
,820
,900
,310
,780
,150
,480
,440
,130
,400
,200
,500
,800
,200
Hedstrom
number ,
He
65
103
148
257
405
55
87
126
218
343
36
56
82
141
222
,000
,000
,000
,000
,000
,300
,000
,000
,000
,000
,000
,000
,000
,000
,000
Fanning
friction
factor ,
f
•010b
•019b
•°3b
•08b
.20°
.0083
.0085
.0090
.019°
.035b
.0066
.0070
.0072
.0075
.0080
psi
sludge
1,380
775
534
362
290
2,038
673
285
152
90C
3,650°
("*
1,250=
513 =
135C
f
46C
water3
1,190
394
162
42
14
2,020
667
275
72
24
4,280
1,423
582
152
50
aCalculated from Hazen-Williams equation With a friction coefficient (C) of 100.
Flow is not in the turbulent region.
GNote that pressure drop for sludge, by equation 14-3, is less than the pressure
drop for water if C = 100. The pressure drops would be about the same if C=110.
2 2
1 psi = 6.9 kN/m = 69,000 dyne/cm
14-11
-------
Note that the pressure drop for Cases 1 and 2 is greater in all
cases than the minimum drop Apo (see Figure 14-5).
FIGURE 14-4
FRICTION FACTORS FOR EXAMPLE PROBLEM
1,0 11
US, HtDSTHOM HUM BE R
o.ooi
He, REYNOLDS NUMBER
TABLE 14-2
PRESSURE REQUIRED TO EXCEED YIELD
STRESS - EXAMPLE PROBLEM
Diameter,
in.
4.03
5.05
6.06
7.98
10.02
Pressure drop Apo, psi
Case 1
Case 2
= 100 dyne/cm^
To = 30.5 dyne/cm
548
437
363
276
220
167
133
111
84
67
Pressure drop to cause the shear, stress at pipe wall to
exceed the yield stress To- Higher pressures may be
needed to start the pipeline due to thixotropic effects
not considered in Figure 14-3.
1 in. = 2.54 cm _ 2
1 psi = 6.9 kN/m = 69,000 dyne/cm
14.1.2.3 Thixotropy and Other Time-Dependent
Effects
Besides possibly being dependent on the shearing rate, the
flow resistance of liquids can depend on the length of time of
shearing or on some function of both the time and intensity of
shearing. The most commonly encountered time-dependent change in
viscosity is a drop which occurs with time of shearing, followed
by a gradual recovery when shearing is stopped. This behavior is
called thixotropy. A familiar example is an ice cream milkshake,
14-12
-------
which "sets up" in its container and will only flow out when the
container is rapped or jarred several times. The structure
rebuilds when the rapping is stopped. Paints typically not only
are Bingham plastics but are thixotropic as well. They will flow
for a short time after being "worked" by the paint brush so brush
lines tend to disappear. Their "plastic" characteristics rebuild
quickly after shearing stops so the paint does not flow downwards
on vertical surfaces.
2,000 r-
D_
o
cc
o
UJ
a:
w)
w
UJ
DC
1,000
6QQ
400
200
100
40
1 inch = 2,54 cm
1 psi = i.9 kN/m2
10
PJPE DtABJETER,
FIGURE 14-5
PRESSURE DROPS FOR EXAMPLE PROBLEM
14-13
-------
Wastewater sludge is also thixotropic. The effect is
increasingly important as the percent solids and percent
volatile solids increase. Thixotropy has three major effects:
9 It complicates the measurement of constants such as the
yield stress rQ.
• It makes pump suction conditions very important. In one
case, a centrifugal pump produced ample pressure to move
the sludge through a hose. The pump was suspended in a
lagoon but the sludge would not flow into the pump
suction. It was found that mixers next to the pump
caused thixotropic breakdown sufficient for satisfactory
pumping (5,6).
• It raises the pressure needed to start a pipeline that
has been shut down. At one installation, this effect was
found to be significant for shutdowns exceeding one day.
An operating procedure is used to prevent this problem;
that is, if shutdowns over 8 hours are expected, the line
is purged of sludge (5,6).
Permanent degradation of yield stress can occur with time of
shearing. Intense shearing produces this result in high
polymers. This phenomena can be expected in wastewater sludges,
when shear levels are sufficiently high to physically disrupt a
portion of the particles making up the sludge. If this occurs,
it may be difficult to later thicken or dewater the sludge.
Sometimes the reduction in viscosity that occurs with time
of shearing is actually the effect of a temperature increase
produced by the energy delivered to the liquid. The general
effect of an increase in temperature with both Newtonian and
non-Newtonian liquids is a reduction in viscosity. However, for
sludge, the main effect of temperature is that low temperatures
may cause the grease fraction of the sludge to harden. Other
temperature effects appear to be unimportant, at least up to
160°F (70°C) (5,7).
There is another unusual effect that occurs in wastewater
sludge pipelines: slippage and seepage (6). Essentially, the
sludge is riding on a thin film of water next to the wall of the
pipe. This effect is noticeable at very low velocities when
starting a sludge pipeline; it partially offsets the thixotropic
effect. Seepage and slippage are hard to calculate but are
useful when starting pipelines flows (6).
14.1.2.4 Obtaining the Coefficients
Figure 14-3 cannot be used unless the yield stress o and the
coefficient of rigidity can be obtained. There is a reasonable
amount of data on anaerobically digested sludges (3,5,7,8) but
very little data on sludge that has not been digested.
14-14
-------
Several types of instruments are available for viscosity
measurements. However, only two of these types are suitable
for sludge: test pipes and rotational viscometers. Some
instruments, such as capillary viscometers, are unable to handle
the relatively large particles in sludge; other instruments,
such as ball-drop viscometers, are not suited to strongly
non-Newtonian fluids such as sludge.
Flow curves from test pipes are directly scalable to full-scale
pipes provided flow is laminar. However, the onset of turbulence
in a large pipe cannot be predicted directly from small pipe
tests. It is necessary to use the yield stress and coefficient
of rigidity, compute Reynolds and Hedstrom numbers, and use
Figure 14-3 to predict the onset of turbulence. The flow curves
obtained with test pipes do not provide fundamental rheological
data, because at a given flow rate, shear stress and rate of
shear vary across the radius of the pipe. By using the
Rabinowitsch equation, the flow curve can be transformed into
a rheologically correct shear stress versus rate of shear
curve (9). An offsetting disadvantage of test pipes is that a
high degree of experimental skill is required to get reliable
data. Also these installations are relatively expensive and
cumbersome and require large sample volumes.
For sludge, the best instrument appears to be a rotational
viscometer. In this type of machine, the test liquid is placed
between two concentric cylinders, one of which rotates. The
torque on a cylinder is measured as a function of rotational
speed. Such machines can produce approximately uniform shear
rates at given shear stresses, provided the space between the bob
(inner cylinder) and cup (outer cylinder) is small compared to
the bob radius. Viscometers in which the bob rotates and the
twisting force on the cup is measured are relatively easy to
design mechanically but turbulence occurs at low shear rates for
low viscosity materials. Turbulence onset does not occur until
much higher shear rates for viscometers in which the cup rotates
and the twisting force on the bob is measured. In both types of
viscometers, end effects become substantial if the bob and cup
are not long relative to the clearance.
There are a number of viscometers which feature rotational
movement, but either do not have constant clearances between an
inner and an outer cylinder, or do not control or measure
shearing rate or shear stress. These devices are of little value
for obtaining consistency curves for non-Newtonian liquids.
The nearly uniform shear rate achievable in rotational
viscometers allows direct measurement of the fundamental shear
stress-rate of shear curve, which is a major advantage when it
comes to application to complex flow relationships. Rotational
viscometers are simple to operate. Their primary disadvantage is
that close clearances between outer and inner cylinders are
needed to give uniform shear rates across the gap between
cylinders. Obviously too small a clearance will give erroneous
results for sewage sludges. Gap size should not be reduced below
14-15
-------
1.0 mm (0.025 inch). Sludge must be screened to remove large
particles. This creates no substantial error because a few
large particles do not strongly affect the coefficients.
A representative test curve adapted from Rimkus and Heil (5) is
shown on Figure 14-6. In this test, the viscometer speed was
gradually increased from zero to 100 rpm and then decreased.
Torque was measured and converted to shear stress, providing
"consistency curves." The upper curve (increasing speed)
shows thixotropy; the lower curve (decreasing speed) shows
behavior of the fluidized sample. The lower curve is appropriate
for pipeline design because the sludge is fluidized by passing
through a pump. In this case, the shear stress projected to zero
rpm (232 dynes/cm^) is the yield stress To; the coefficient of
rigidity rj is the slope of the straight part of the lower curve.
Even when fluidized, sludge is not exactly a Bingham plastic, as
shown by curvature in the lower curve at low rpm. This departure
from Bingham plastic conditions can be used to refine the
pressure drop calculations. The viscometer for this test was a
Haake Model RV-3 Rotoviso with sensor head MV-1.
10
20
RPM OF VISCOMETER HEAD
30 40 50 60 70
BO
90 100
,u
iA
m
c
"
Hi
cc
te
£E
<
yj
X
V)
700
600
500
400
300
200
100
/^
/ v'fr
/ V*
\v
\\
\*
\
\
- T0 = 232 dyne/cm2
235
SEC.-1 ,
100 RPM-1-
J_
SAMPLE; LAGOONED ANAEROBICALLY
DIGESTED SLUDGE
13% SOLIDS, 40% VOLATILE
I I i I I.. ..
0 20 40 60 SO 100 120 140 160 • 180 200 220
SHEAR RATE, sec.-1
FIGURE 14-6
VISCOMETER TEST OF SEWAGE SLUDGE (5)
14-16
-------
14.1.2.5 Additional Information
Sludge has been successfully and reliably pumped in the
laminar flow range. Some of the installations describedin
Section 14.1.6, Long Distance Pumping, operate in this range.
That section also contains several design recommendations.
Several researchers have investigated sludge pumping, rheology,
and related subjects (10 through 24).
14.1.3 Types of Sludge Pumps
Sewage sludges can range in consistency from a watery scum to
a thick paste-like slurry. A different type of pump may be
required for each type of sludge. Pumps which are currently
utilized for sludge transport include centrifugal, torque flow,
plunger, piston, piston/hydraulic diaphragm, progressive cavity,
rotary, diaphragm, ejector and air lift pumps. Water eductor
pumps are sometimes used to pump grit from aerated grit removal
tanks.
14.1.3.1 Centrifugal Pumps
A centrifugal pump (Figure 14-7) consists of a set of rotating
vanes in a housing or casing. The vanes may be either open or
enclosed. The vanes impart energy to a fluid through centrifugal
force. The non-clog centrifugal pump for sewage or sludges, in
comparison to a centrifugal pump designed to handle clean water,
has fewer but larger and less obstructed vane passageways in the
impeller; has greater clearances between impeller and casing;
and has sturdier bearings, shafts, and seals. Such non-clog
centrifugal pumps may be used to circulate digester contents and
transfer sludges with lower solids concentrations, such as waste
activated sludge. The.larger passageways and greater clearances
result in increased reliability at a cost of lower efficiency.
The basic problem with using any form of centrifugal pump
on sludges is choosing the correct size. At any given speed,
centrifugal pumps operate well only if pumping head is within a
relatively narrow range; the variable nature of sludge, however,
causes pumping heads to vary. The selected pumps must be large
enough to pass solids without clogging of the impellers and yet
small enough to avoid the problem of diluting the sludge by
drawing in large quantities of overlying sewage. Throttling
the discharge to reduce the capacity of a centrifugal pump is
impractical both because of energy inefficiency and because
frequent clogging of the throttling valve will occur. It is
recommended that centrifugal pumps requiring capacity adjustment
be equipped with variable-speed drives. Fixed capacity in
multiple pump applications is achieved by equipping each pump
with a discharge flow meter and using the flow meter signal
in conjunction with the variable speed drive to control the speed
14-17
-------
on pv
of the pump. Seals last longer if back suction "pumps are used,
Utilizing the back of the impeller for suction removes areas of
high pressure inside the pump casing from the location of the
seal and prolongs seal life.
DISCHARGE
BEARINGS
X
SHAFT
SEAL
NON-CLOG IMPELLER
SUCTION
CASING
FIGURE 14-7
CENTRIFUGAL PUMP
Propeller or mixed flow centrifugal pumps are sometimes used
for low head applications because of higher efficiencies, a
typical application is return activated sludge pumping. When
being considered for this type of application, such pumps must be
of sufficient size (usually at least 12 inch [300 mm] in suction
diameter) to provide internal clearances capable of passing the
type of debris normally found within the activated sludge system.
Such pumps should not be used in activated sludge systems which
are not preceded with primary sedimentation facilities.
14.1.3.2 Torque Flow Pumps
A torque flow pump (Figure 14-8), also known as a recessed
impeller or vortex pump, is a centrifugal pump in which the
impeller is open faced and recessed well back into the pump
casing. The size of particles that can be "handled by this
type of pump is limited only by the diameter of the suction or
discharge openings. The rotating impeller imparts a spiralling
motion to the fluid passing through the pump. Most of the
fluid does not actually pass through the vanes of the impeller,
thereby minimizing abrasive contact with it and reducing the
chance of clogging. Because there are no close tolerances
14-18
-------
between the impeller and casing, the chances for abrasive
wear within the pump are further reduced. The price paid for
increased pump longevity and reliability is that the pumps are
relatively inefficient compared with other non-clog centrifugals;
45 versus 65 percent efficiency is typical. Torque flow pumps
for sludge service should always have nickel or chrome abrasion
resistant volute and impellers. The pumps must be sized
accurately so that excessive recirculation does not occur at any
condition at operating head. Capacity adjustment and control is
achieved in the same manner as for other centrifugal pumps.
DISCHARGE
j i
OPEN
IMPELLER
SUCTION
FIGURE 14-8
TORQUE FLOW PUMP
14.1.3.3 Plunger Pumps
Plunger pumps (Figure 14-9) consist of pistons driven by an
exposed drive crank. The eccentricity of the drive crank is
adjustable, offering a variable stroke length and hence a
variable positive displacement pumping action. The check valves,
ball or flap, are usually paired in tandem before and after
the pump. Plunger pumps have constant capacity regardless
of large variations in pumping head, and can handle sludges
up to 15 percent solids if designed specifically for such
service. Plunger pumps are cost-effective where the installation
requirements do not exceed 500 gpm (32 1/s), a 200 feet (61 m)
14-19
-------
discharge head, or 15 percent sludge solids. Plunger pumps
require daily routine servicing by the operator, but overhaul
maintenance effort and cost are low.
DESURGING
CHAMBER
PACKING
DESURGING
CHAMBER
DISCHARGE
SUCTION
FIGURE 14-9
PLUNGER PUMP
The plunger pump's internal mechanism is visible. The pump's
connecting rod attaches to the piston inside its hollow interior
and this "bowl" is filled with oil for lubrication of the journal
bearing. Either the piston exterior or the cylinder interior
houses the packing, which must be kept moist at all times. Water
for this purpose is usually supplied from an annular pool located
above the packing; the pool receives a constant trickle of clean
water. If the packing fails, sludge may be sprayed over the
surrounding area.
Plunger pumps may operate with up to 10 feet (3m) of suction
lift; however, suction lifts may reduce the solids concentration
that can be pumped. The use of the pump with the suction
pressure higher than the discharge is not practical because flow
will be forced past the check valves. The use of special
intake and discharge air chambers will reduce noise and
vibration. These chambers also smooth out pulsations of
intermittent flow. Pulsation dampening air chambers, if used,
should be glass lined to avoid destruction by hydrogen sulfide
corrosion. If the pump is operated when the discharge pipeline
14-20
-------
is obstructed, serious damage may occur to the pump,
pipeline; this problem can be avoided by a simple
arrangement.
motor, or
shear pin
14.1.3.4 Piston Pumps
Piston pumps are similar in action to the plunger pumps, but
consist of a guide piston and a fluid power piston. (See
Figure 14-10). Piston pumps are capable of generating high
pressures at low flows. These pumps are more expensive than
other types of positive displacement sludge pumps and are
usually used in special applications such as feed pumps for heat
treatment systems. As for other types of positive displacement
pumps, shear pins or other devices must be used to prevent damage
due to obstructed pipelines.
DISCHARGE
t
J~L
DIAPHRAGM
(TYP)
HYDRAULIC
SYSTEM
SUCTION
POWER
PISTON
GUIDE
PISTON
FIGURE 14-10
PISTON PUMP
14-21
-------
A variation of the piston pump has been developed for use where
reliability and close control are needed. The pump utilizes a
fluid power piston driving an intermediate hydraulic fluid
(clean water), which in turn pumps the sludge in a diaphragm
chamber (Figure 14-11). The speed of the hydraulic fluid drive
piston can be controlled to provide pump discharge conditions
ranging from constant flow rate to constant pressure. This pump
is used primarily as a feed pump for filter presses. This
special pump has the greatest initial cost of any piston pump,
but the cost is usually offset by low maintenance and high
reliability.
FLUID
POWER
PISTON
y
SUCTION
SUCTION
FIGURE 14-11
COMBINATION PISTON/HYDRAULIC DIAPHRAGM PUMP
14.1.3.5 Progressive Cavity Pumps
The progressive cavity pump (Figure 14-12) has been used
successfully on almost all types of sludge. This pump comprises
a single-threaded rotor that operates with an interference
clearance in a double-threaded helix stator
A volume or "cavity" moves "progressively"
discharge when the rotor is rotating, hence the
cavity." The progressive cavity pump may
discharge heads of 450 feet (137 m) on sludge.
available to 1,200 gpm (75 1/s). Some progressive cavity
pumps will pass solids up to 1.125 inches (2.9 cm) in diameter.
made of rubber.
from suction to
name "progressive
be operated at
Capacities are
14-22
-------
Rags or stringy material should be ground up before entering this
pump. The rotor is inherently self-locking in the stator
housing when not in operation, and will act as a check valve
for the sludge pumping line. An auxiliary motor brake may be
specified to enhance this operational feature.
UNIVERSAL
JOINT
SUCTION
DISCHARGE
DRIVE
CAVITIES
FIGURE 14-12
PROGRESSIVE CAVITY PUMP
The total head produced by the progressive cavity pump is divided
equally between the number of cavities created by the threaded
rotor and helix stator. The differential pressure between
cavities directly relates to the wear of the rotor and stator
because of the slight "blow by" caused by this pressure
difference. Because wear on the rotor and stator is high, the
maintenance cost for this type of pump is the highest of any
sludge pump. Maintenance costs are reduced by specifying the
pump for one class higher pressure service (one extra stage)
than would be used for clean fluids. This creates many extra
cavities, reduces the differential pressure between cavities, and
consequently reduces rotor and stator wear. Also, speeds should
not exceed 325 rpm in sludge service, and grit concentrations
should be minimized.
Since the rotor shaft has an eccentric motion, universal joints
are required between the motor shaft and the rotor. The
design of the universal joint varies greatly among different
manufacturers. Continuous duty, trouble-free operation of these
universal joints is best achieved by using the best quality (and
usually most expensive) universal gear joint design. Discharge
pressure safety shutdown devices are required on the pump
14-23
-------
discharge to prevent rupture of blocked discharge lines. No-flow
safety shutdown devices are often used to prevent the rotor and
stator from becoming fused due to dry operation. As previously
mentioned, these pumps are expensive to maintain. However, flow
rates are easily controlled, pulsation is minimal, and operation
is clean. Therefore, progressive cavity pumps are widely used
for pumping sludge.
14.1.3.6 Diaphragm Pumps
Diaphragm pumps (Figure 14-13) utilize a flexible membrane that
is pushed or pulled to contract or enlarge an enclosed cavity.
Flow is directed through this cavity by check valves, which may
be either ball or flap type. The capacity of a diaphragm pump is
altered by changing either the length of the diaphragm stroke
or the number of strokes per minute. Pump capacity can be
increased and flow pulsations smoothed out by providing two
pump chambers and utilizing both strokes of the diaphragm for
pumping. Diaphragm pumps are relatively low head and low
capacity units; the largest available air-operated diaphragm pump
delivers 220 gpm (14 1/s) against 50 feet (15 m) of head. The
distinct advantage of the diaphragm pumps is their simplicity.
Their needs for operator attention and maintenance are minimal.
There are no seals, shafts, rotors, stators, or packing in
contact with the fluid; also, diaphragm pumps can run in a dry
condition indefinitely.
Flexure of the diaphragm may be accomplished mechanically (push
rod or spring) or hydraulically (air or water). Diaphragm life
is more a function of the discharge head and the total number of
flexures than the abrasiveness or viscosity of the pumped fluid.
Power to drive air driven diaphragm pumps is typically double
that required to operate a mechanically driven pump of similar
capacity. However, hydraulically operated (air or water)
diaphragms generally outwear mechanically driven diaphragms by a
considerable amount. Hydraulically driven diaphragm pumps
are suitable for operation in hazardous explosion-prone areas;
also a pressure release means in the hydraulic system provides
protection against obstructed pipelines. Typical repairs to a
diaphragm pump usually cost less than $75 (1978 basis) for parts
and require approximately two hours of labor. In some locations,
high humidity intake air will cause icing problems to develop at
the air release valve and muffler on an air driven diaphragm
pump. A compressed air dryer should be used in -the air supply
system when such a condition exists.
The overall construction of some diaphragm pumps, the common
"trash pump," is such that abrasion may cause the lightweight
casings to fail before the diaphragms, since the pumps are not
designed for continuous service. For wastewater treatment
applications the mechanical diaphragm "walking beam" pumps
are more appropriate. These pumps are dependable, have quick
14-24
-------
cleanout ball or flap check valves and are presently used to
handle scum and sludge at numerous small plants throughout the
country.
DIAPHRAGM
CHECK
VALVE
FIGURE 14-13
DIAPHRAGM PUMP
One air-driven diaphragm pump is sold in a package expressly
intended for pumping sludge from primary sedimentation tanks
and gravity thickeners. The basic pump package consists of a
single-chambered, spring return diaphragm pump, an air pressure
regulator, a solenoid valve, a gage, a muffler, and an electronic
transistorized timer. This unit pumps a single 3.8 gallon
(14.4 1) stroke after an interval of time. The interval is
readily adjusted to match the pumping rate to the rate of
formation of the sludge blanket in the sedimentation tank or
thickener. The large single stroke capacity of this pump has
several maintenance advantages. Not only is total flexure count
reduced, but ball valve flushing is improved, so large particles
cause less difficulty. The maximum recommended solids size is
7/8 inch (2.2 cm). Pump stroke speed is constant regardless of
the selected pump flow so that minimum scouring velocities are
always maintained in the discharge piping during the pumping
surge.
14-25
-------
The traditional sequence of intermittent pumping for primary
sedimentation tanks has been to thicken for an interval without
pumping and then draw the sludge blanket down. A relatively
long interval is required by pump motors, since frequent motor
starts can cause over heating. Theoretically if the sludge
concentration is 10 percent on the bottom and decreases to
8 percent at the top of the pumped sludge zone, then the pumped
average is 9 percent. However, by using air drive, a diaphragm
pump can operate with starts every few seconds instead of every
several minutes or longer. The manufacturer claims its system
will draw single intermittent pulses from the 10 percent bottom
layer since the sludge blanket depth is maintained at a virtually
constant height. Downstream sludge treatment processes can have
greater solids capacity because more concentrated sludges can be
obtained.
The City of San Francisco ran independent pump evaluation tests
in 1975 (25). They concluded that proper use of air-driven
diaphragm pumps will increase the sedimentation tanks' ability
to concentrate sludges. The sludge collection system in the
sedimentation tanks and the sludge pumping equipment had to be
controlled together to give optimum thickening. Savings in
operations and maintenance as well as improved thickening were
accomplished by lowering the overall average rate of sludge
withdrawal and making the sludge collectors work continuously at
a reduced rate instead of intermittently. When considering such
a pump installation, the capacity requirement is based on the
maximum rate at which the sludge blanket forms in the tank and
not the capacity required to maintain minimum pipe velocities.
14.1.3.7 Rotary Pumps
Rotary pumps (Figure 14-14) are positive displacement pumps in
which two rotating synchronous lobes essentially push the fluid
through the pump. Because rotary pump lobe configurations can be
designed for a specific application, rotary pumps are suitable
for jobs ranging from air compressor duty to sewage sludge
pumping. Rotational speed and shearing stresses are low. Sewage
pumping lobes are noncontact and clearances are factory changed
according to the abrasive content of the slurry. It is not
recommended that the pumps be considered self-priming or suction
lift pumps although they are advertised as such. Experience at
one plant indicates that the pump operates best with a bottom
suction and top -discharge. Only very limited operational data
are available for rotary pumps used on sludge. Two manufacturers
now advertise hard metal two-lobed pumps for sludge usage. Lobe
replacement for these pumps appears to be less costly than
rotor and stator replacement on progressive cavity pumps. One
manufacturer is offering hard rubber three-lobed rotary pumps,
which are used successfully for sludge pumping in Europe. Test
units of this pump are presently being evaluated in the United
States. To date these tests have been unsuccessful due to
14-26
-------
the failure of the lobe liners,
positive displacement pumps, must be
obstructions.
Rotary pumps, like other
protected against pipeline
DISCHARGE
SUCTION
FIGURE 14-14
ROTARY PUMP
14.1.3.8 Ejector Pumps
Sewage ejectors use a charging pot which is intermittently
discharged by a compressed air supply (See Figure 14-15).
Ejectors are most applicable for incoming average flow rates
less than 150 gpm (9 1/s). These pumps require a positive
suction and usually discharge to a vented holding tank or basin.
Scum and sludge can incapacitate the standard mechanical or
electronic probe-type level sensors offered by most manufacturers
to sequence pot discharge; custom instrumentation may be
necessary. Large flushing and cleanout connections should be
provided. If ejectors are to be used to discharge sludge to an
anaerobic digester where the air could produce an explosive
mixture, special precautions should be taken to see that the
units cannot bleed excessive quantities of air into the digester.
Ejector pumps have been used in some installations to pump
thickened waste-activated sludge produced by the dissolved air
flotation process.
14.1.3.9 Gas Lift Pumps
Gas lift pumps use low pressure gas released within a confined
riser pipe with an open top and bottom. The released gas bubbles
rise, dragging the liquid up and out of the riser pipe. Air is
commonly used, in which case the pump is called an air lift pump.
14-27
-------
Air lift pumps are used for return activated sludge and similar
applications; gas lift pumps using digester gas are used to
circulate the contents of anaerobic digesters. The main
advantage of these relatively inefficient pumps is the complete
absence of moving parts. Gas lift sludge pumps are usually
limited to lifts of less than 10 feet. The capacity of a lift
pump can be varied by changing its bouyant gas supply. Reliable
gas lift pumping requires the gas supply to be completely
independent of outside flow or pressure variables. Gas lift
pumps with an external gas supply and circumferential diffuser
can pass solids of a size equivalent to the internal diameter of
the confining riser pipe without clogging. When the gas is
supplied by a separate inserted pipe, the obstruction created
negates this non-clog feature. Gas lift pumps, because of
their low lifting capability, are very sensitive to suction and
discharge head variations, and to variations in the depth of
bouyant gas release. Special discharge heads are usually
required to enhance the complete separation of diffused air once
the discharge elevation has been reached.
DISCHARGE
AIR CHARGE
CONNECTION
SUCTION
ISOLATION
VALVE
CHECK
VALVE
CHECK
VALVE
FIGURE 14-15
EJECTOR PUMP
14.1.3.10 Water Eductors
Water eductors use the suction force (vacuum) created when a high
pressure water stream is passed through a streamlined confining
tube (venturi). Like the air lift pump, water eductors have no
14-28
-------
moving parts. When water is required to transport a solid
material, the water eductor becomes a very convenient pump.
Most water eductors with reasonable water demands cannot pump
solids of golf ball size. They have, however, been successfully
used to remove grit from aerated grit removal tanks and discharge
the grit into dewatering classifiers.
14.1.4 Application of Sludge Pumps
The previous section describes the types of pumps available for
sludge pumping. This section describes appropriate applications
for these pumps and identifies some limitations and constraints.
This section covers screenings, grit, and scum as well as sludge.
Suction conditions require special attention when pumping sludge.
When pumping water or other Newtonian fluids, calculations of net
positive suction head (NPSH) can be used to determine permissible
suction piping arrangements. However, sludge is a non-Newtonian
fluid, especially at high solids concentrations. This behavior
may drastically reduce the available NPSH. Consequently, long
suction pipelines should be avoided and the sludge pump should be
several feet below the liquid level in the tank from which the
sludge is to be pumped. If these conditions are not met, a pump
will not be able to handle sludge at high concentrations.
Special precautions are usually required to reliably pump
screenings and grit. Screenings should be ground up and pumped
by pumps with the ability to pass large material. Torque flow
pumps are ideal for this application. Grit pumping requires
special abrasion and non-clogging considerations. Both
screenings and grit pumps should be easy to disassemble with
quick access to the volute and impeller.
Table 14-3 presents an application matrix that identifies the
various types of sludges or solids normally encountered in
wastewater applications, and provides a guide for the suitability
of each type of pump in that service.
14.1.5 Pipe, Fittings, and Valves
Materials for wastewater solids pipelines include steel; cast and
ductile iron; pretensioned concrete cylinder pipe; thermoplastic;
fiberglass reinforced plastic; and other materials. Steel and
iron are most common. With steel or iron, external corrosion may
occur in unprotected buried lines; corrosion may be adequately
controlled under most conditions by coatings and, where needed,
cathodic protection. Inside' the pipe, a lining of cement,
plastic, or glass may be used to protect the pipe from internal
corrosion and abrasion. With raw sludges and scum, linings
have an additional function: they provide a smooth surface
that greatly retards accumulations of grease on the pipe wall
(26, 27). With anaerobically digested sludge, linings may be
14-29
-------
useful to prevent crystals of struvite from growing on the pipe
wall. (Refer to the anaerobic digestion portion of Chapter 6 for
control of struvite). Smooth linings are especially valuable in
pump suction piping and in key portions of piping (header pipes
and the like) where maintenance shutdowns would cause process
difficulties.
TABLE U-3
APPLICATIONS FOR SLUDGE PUMPS
Misce3laneous solids
Primary sludge
Secondary sludge
Centrifugal 0
Torque flow 5
Plunger 0
000
4 J 5
044
Settled
3
4
4
Thickened
2
3
4
Trickling
filter
4
4
4
Thickened
sludge
Activated — ——-•••- •
4 Oa 3
4 Oa 4
1 1C 4
Lagooned
Digested sludge, sludge,
percent percent
Mixed Thickened Wet Dry
- ; low efficiency
Daily attention
required
Progressive
cavity
Piston/hydraulic
diaphragm
Diaphragm
Rotary
Pneumatic ejector
Air lift
Water eductor
Float may cause air binding.
Varying quality and head conditions requires positive flow control.
Restricted to low flows.
Maximum li percent solid .
High discharge pressure nly.
nding.
Should be preceded by gr
Large bore pumps may be
Batch Pneumatic Ejector
DShort distance only.
sed with m-line grinding.
High
Low lift
Low lift
Key:
0 - Unsuitable
1 - Use only under special circumstance.
2 - Use with caution
3 - Suitable with limitations
4 - suitable
5 - Best type to use
Fittings and appurtenances must be compatible with sludge and
pipe. Long sweep elbows are preferred over short radius elbows.
Grit piping may be provided with elbows and tees made of special
erosion resistant materials.
Valves of the nonlubricated eccentric plug type have proven
reliable in sludge pipeline service. Care must be taken if a
cleaning tool is to pass through the valves. Grit pipelines are
usually equipped with tapered lubricated plug valves.
Wastewater solids piping should be designed for reasonably
convenient maintenance. Even under good conditions, pipe may
occasionally have erosive wear, grease deposits, or other
difficulties. Pipe in tunnels or galleries is more accessible
than buried pipe. An adequate number of flanged joints,
mechanical couplings, and take-down fittings should be provided.
It is recommended that 4 to 6 inches (10 to 15 cm) be considered
the minimum diameter for wastewater solids pipelines to minimize
14-30
-------
grease clogging or particle blockage and facilitate maintenance.
Blind flanges and cleanouts should be provided for ease of line
maintenance. Gas formation by wastewater solids left for long
periods in confined pipe or equipment can create explosive
pressures; therefore, provision should be made for flushing and
draining all pipes, pumps, and equipment. The pressure rating of
wastewater solids pipelines should be adequate for unusual as
well as routine operating pressures. Unusual pressures
will occasionally occur due to high solids concentrations,
pipe obstructions, gas formation, water hammer, and cleaning
operations.
Temperature changes may cause stress in the pipe. Temperatures
are changed by heated material as it enters cold pipe; flushing;
and the use of hot fluids during cleaning to remove grease. Pipe
should be designed to accommodate such stresses.
14.1.6 Long Distance Pumping
Sludge may be pumped for miles. A pipeline is frequently
less expensive than the alternatives of trucks, rail cars, or
barging (see Section 14.1.3 and reference 28), especially if, by
pipelining, mechanical dewatering can be avoided. Pipelines may
have less environmental impact along their routes than trucks.
14.1.6.1 Experience
Tables 14-4 and 14-5 describe some typical pipelines for
unstabilized and digested sludges. There is considerable
additional U.S. experience; see Tables 14-6 and 14-7. An
examination of these tables shows that:
• Centrifugal pumps are widely used, even on unstabilized
sludge.
• Operating pressures are usually below 125 psig (860 kN/m2
gage).
® Velocities are usually below 3.5 ft/sec (1.1 m/s).
* If the volatile solids content of the sludge is low,
the sludge can be pumped at a high total solids
concentration. This is well illustrated by the lagoon
sludge pipelines, which have operated at up to 18 percent
solids; lagooned sludge has a very low volatile content.
In some cases, sludge thickening at the receiving location was
adversely affected by the shearing or the septicity that occurred
in the pipelines. Special flushing practices after pipeline use
or use of a pipe cleaning device were not used in several cases.
Need for these techniques seem to depend on the nature of the
sludge being pumped, although experience is not conclusive on
this point.
14-31
-------
TABLE 14-4
TYPICAL LONG PIPELINES CARRYING
UNSTABILIZED SLUDGE
Length, mi
Diameter, in.
Pipe material
Percent solids
Flow rate, gpm
Velocity, ft/sec
Total pressure, psl g
Pump type
Operating schedule
Use of cleaning tool
Septicity of sludge
Commen s
Cleveland, OH
13.2
12
Cast iron, un lined
activated13
3- 3.5C
350C
1.0°
150 - 175°
Centrifugal, three in
series
Continuous
Every 4-6 weeks
-
D.,fficulty with
at receiving plant
Indianapolis , IN
7.5
Twin 14
Ductile iron
activated
0.75 - 1.75
1,000 minimum
2 minimum
90 normal
Centrifugal
Continuous
None
YPS
sludge that has
been pumped from
Southport
Jacksonville , FL
District II to
7
8
-
activated
3
500 normal
3
90 normal
Centrifugal , two in
series
30 - 60 minutes every
two hours
Possible, not needed
Yes
Heat treatment de
ering ess >
Kansas City, MO
West Side to Big
6.6
12
Ducti le iron
rimary
0.4 - 1.0
1,000
2.8
65
Centrifugal
Continuous
Weekly
Some; chlorine used
receiving plant
Philadelphia, PA
Southeast to Southwest
5
8a
Ductile iron
scum
2.5 - 5
500
3
90 normal
Centrifugal
Continuous
Every 1 to 2 weeks
Not much odor
Two ductile iron lines will replace a single line. The old lines is subject to external corrosion and
will be abandoned over most of its length. The new lines have polyethylene wrap and cathodic protection.
Pickle 1iquor is added to primary treatment for phosphorus removal. Skimmings are handled separately.
Data from Reference 10. Later, sludge thickness was decreased to 1-2 percent solids to reduce operating
pressures and line breaks.
There is a heavy grease buildup in the pipe, especially in winter.
1 mi = 1.6 km
1 in. = 25.4 mm
1 gpm = 0.063 1/s
1 ft/sec = 0. 30 m/s
1 psig = 6.9 kN/m2 qage
14.1.6.2 Design Guidance
Proper pre-planning of a pipeline installation is of great
importance. For example, a pump breakdown or a plugged pipeline
has a great impact on plant operation, and its likelihood can be
greatly minimized by good initial design and equipment selection.
If digestion is to be part of the system, the digesters may
be located either before or after the long sludge pipeline.
However, sludge is much easier to pump after it has been
digested. In addition, raw sludges may cause problems related to
thickening, odors, and corrosion at the receiving point, since
septic conditions may develop in the pipeline. If raw sludge is
to be pumped long distances, the least environmental impact will
result if the pipeline contents are discharged directly into
anaerobic digesters.
14-32
-------
TABLE 14-5
TYPICAL LONG PIPELINES CARRYING
DIGESTED SLUDGE
Length, mi
Diameter, in.
Material
Sludge type
Percent solids
Percent volatile
Flow rate, gpm
Velocity, ft/sec
Pumps
Operating schedule
- e o ^ go
Chicago , IL .
agoon no.
1.7
16
Steel
Lagooned
13 average
15 maximum
40
1 , 300
2.1
Centrifugal with
mixers
Intermittent
None
Denver, CO
Northside to Metro
'- 2
Twin 8.
Cast iron
Anaerobically-digested
primary
4-7
49
700
2
40 — 60
Centrifugal
1-2 hr/day, not
flushed
None
Fort Wayne,
3
12, some 10
Unlined cast iron
Digested
5 maximum
35 - '40
600
1.6
20 - 30
Centrifugal
3 hr/day , can flush
but not needed
None
Rahway Valley Sanitary
Authority , NJ
3 •
8
_
Anaerobically di-
gested primary and
3-4
-
500
3
80
Two-stage centrifugal ,
formerly recipro-
cating
4 hr/day, not flushed
h
Not needed
San Diego, CA
Point Lcma
7.5
8
Fiber reinforced plastic
Anaerobically digested
primary
Up to 7.56
57
550 - 60C
3.5
155
Torque flow
5 times/week, flushed
before and after use
None
Temporary pipeline to clean Lawndale lagoon no. 28 (5,6). No longer in service.
Also, a 25-mi pipeline has been designed but not yet constructed, as of early 1979.
Fiber reinforced plastic replaced a lined and coated steel pipe that corroded.
Anaerobically digested primary and waste-activated sludges with phosphorus-precipitating chemicals.
Dilution water is needed sometimes to get the sludge started. Once it is moving, the dilution water
may be shut off, depending on pressure.
Non-clog centrifugal pumps are suitable for ordinary digested sludge. A nickel-alloy torque flow pump
is being added for digester cleaning and septic tank waste.
Three pumps in series, two of which have variable speed drives.
In the past, a novel ice bag tool was used (26).
1 mi = 1.6 km
1 in. = 25.4 mm
1 gpm = 0.063 1/s
1 ft/sec = 0. 30 m/s
1 psig =6.9 kN/m gage
Sludge that has been piped for a long distance may experience
floe breakdown. If this occurs, thickening and dewatering may be
impaired. Chemical conditioning may require a higher chemical
dose; thermal conditioning may produce a sludge with poorer
dewatering properties.
The following special design features should be considered for
long distance pipelines:
1. Provide two pipes unless a single pipe can be shut down
for several days without causing problems in wastewater
treatment system. ;
2. Consider external corrosion and pipe loads just as
for any other utility pipeline, for example, water
or natural gas. External corrosion has been a problem on
some long sludge pipelines. Electrical return currents,
14-33
-------
TABLE 14-6
LONG PIPELINES FOR UNSTABILJZED SLUDGE
ADDITIONAL LOCATIONS
City
Austin, TX
Houston, TX
Jersey City, NJ
Knoxville, TN
Linden-Roselle, NJ
Miami, FL
San Francisco, CA
There are additional
Two 16-in. pipes over
CTwo pipes.
1 mi = 1.6 km
1 in. - 25.4 mm
Length,
Treatment plants mi
Walnut Creek v "•"
Southwest
Simms Bayou to Northside 6.8
Eastside to Westside 2.5
Loves Creek to Third 3.2
Creok system
Linden-Roselle Sewerage 1
Authority
.
North Point to Southeast 6
__ .
system
pipelines in Houston {26} .
most of the route .
Diameter of
pipe , in. Sludge type
12 Primary, waste-activated
"a- Waste-activated
zi ' Primary
6 Primary, trickling
filter
24 Primary
b . .
10 Primary with ferric
chloride
C -
Percent
solids Pump
' 1-1.2 Positive di
. 0.5-1 Centrifugal
4 Plunger, 3
(maximum)
1-3 Centrifugal
2-4 Centrifugal
i
1 Centrifugal
variable
type
splacement
, 2 in series
speed
, 2 -speed
speed
TABLE 14-7
LONG PIPELINES FOR DIGESTED SLUDGE
ADDITIONAL LOCATIONS
Length,
Location mi
Austin, TX - Govalle plant 7
Boston, MA - Nut Island 4.5
plant
Chicago, IL - West-Southwest 5.5
plant to Lawndale lagoons
Chicago, IL - 1970 rail 3.5
loading3
Chicago, IL - barge loading 1 . 0
Chicago, IL - Calumet 1
lagoons
East Rockaway, NY - Bay 1. 5
Park plant
Evansville, .IN 3.5
Fulton County, IL 10.8
Los Angeles, CA - Hyperion 7
Morgantown , WV ""4.5
Philadelphia, PA - Southwest 1
Wantagh, NY - Cedar Creek 11
Temporary pipeline, now out of service.
1 mi = 1.6 km
1 in. = 25.4 mm
Diameter of
pipe, in..
10
12
16
12
16
18
16
8
20
20
2"
-
10
Type of digestion
Aerobic
Anaerobic
-Anaerobic
Anaerobic, lagoon
Anaerobic , lagoon
Anaerobic, lagoon
,); Anaerobic
Anaerobic
Anaerobic, lagoon
Anaerobic, aerobic, diluted
~"1 " Anaerobic
Anaerobic, laqoon
Percent solids Pump type
0.8 , Positive displacement
„ . 3 Centrifugal, reciprocating
3.5-4.5 ' Centrifugal
4-15
9.2 average
,8 - 18 Centrifugal with mixers
12 ' - Centrifugal
3 . 7 Variable speed
1-9 Torque flow, plunger
4-8 Centrifugal
0.9 -
"Reciprocal ing
10-12 normal
15 average
2 . 5 Centrifugal , 3 stage
14-34
-------
acid soils, saline groundwater, and other factors may
cause serious difficulty unless special corrosion control
measures .are'used. -Advice of specialists on the need for
cathodic protection is advised. , :
3. Provide for adding controlled amounts of water to dilute
the sludge or flush the line. Primary effluent may be
used in raw sludge pipelines; disinfected final effluent
may be preferred for digested sludge pipelines. The
water connection should have a flow rate indicator. The
flushing water should flow at about 3 fps (0.9 m/s) .
4. Provide for inserting and removing a cleaning tool
("pig," "go-devil") which can be sent through the line if
needed (10, 28a, 28b). Such cleaning may be frequently
required if unstabilized sludge is pumped, even if scum
is handled separately. If tool cleaning is to be used,
some additional recommendations apply:
a. Valves must provide an unobstructed waterway to pass
the tool.
b. Flushing water pressure should be sufficient to push
the tool through the full length of pipeline.
c. Pipe bend fittings should be 45-degree or, if
possible, 22-1/2-degree. Some cleaning tools will
pass 90-degree bends, but such bends are likely to be
trouble spots. Length/radius of bends should be
checked with the tool supplier.
d. A recording or totalizing flowmeter should be
provided. ' '" (See Chapter 17, Instrumentation.) If the
tool gets stuck in the line, the flow record can be
used to compute the number of gallons pumped since
the tool was inserted. Thus, the tool can be located
and retrieved.
5. The pipeline route should be selected for ease of
maintenance.
6. At high points, air or gas relief valves should be
provided. With care, automatic relief valves can be
made reliable on digested sludge lines; however, in
unstabilized sludge lines, grease and debris generally
cause automatic valves to be unreliable. Simple manual
blowoff valves are generally better for unstabilized
sludge. Air and gases_ from sludge pipelines may be
odorous. In confined spaces, the air or gas may also be
toxic, flammable, explosive, and corrosive.
7. If sludge is to be pumped at more than about 3 percent
solids, the pumps and pipeline should be designed for
high and variable friction head losses. Sludge may flow
14-35
-------
•more like a Bingham plastic than an ordinary Newtonian
fluid. A multiplication factor, such as those on
Figure 14-1, should not be used. A more accurate design
method, such as the one in Section 14.1.2, should be
used.
8. If centrifugal pumps are used, flow rates will be
somewhat unpredictable because of the varying flow
resistance properties of the sludge. Storage provisions
should be made for these variations. Pumps should be
capable of operating at shutoff head with very low flow
during pipeline startup.
9. Positive displacement pumps may experience difficulty
when starting a long sludge pipeline. The thixotropic
nature of sludge may cause very high resistance to flow
during start-up. Consequently excessive pressures may be
generated by positive displacement pumps. To avoid this
problem, variable speed drives should be provided and the
pumps should be started at low speeds. An air chamber
(see Section 14.1.3.3) may be installed on the discharge
side of the pumps; the chamber will assist in start-up,
as well as dampen pulsations. With digested sludge, a
relief valve piped back to the digesters may be used near
the pumps.
10. For very long lines, a booster pumping station may be
required. If positive displacement booster pumps
are used, a holding tank should be provided. It is
practically impossible to match booster pumping rates to
the sludge flow reaching the booster station unless
centrifugal pumps are used.
11. Waterhammer is best controlled by limiting velocity.
Unless a special evaluation is made, velocities should
not exceed about 3 fps (0.9 m/s). Even lower velocities
may be required in some cases.
14.1.7 In-Line Grinding
In-line grinders are used to reduce the size of sludge solids to
prevent problems with the operation of downstream processes.
Grinders require high maintenance; therefore they should not be
installed unless shown to be absolutely necessary. For locations
where a grinder may be installed in the future, removable spool
pieces should be inserted into the pipeline to facilitate the
later installation of a grinder. Grinders may be applicable
to streams carrying debris, rags or stringy materials, but
are usually not needed for streams carrying only secondary
(biological) sludge. Grinders have often been installed
preceding equipment with ball or flapper check valves. However,
utilizing dual check valving, proper stroke seating can be
14-36
-------
obtained and the grinders can often be eliminated. Grinders
remain a necessity upstream from small diameter, high pressure
positive displacement pumps.
Sophisticated, slow speed, hydraulic or electric grinders that
can sense blockages and clear themselves by reverse operation are
now available. Special combination centrifugal pump-grinders
are available for use as digester circulation pumps, and are
effective in preventing rag .balls. .Experience indicates such
pumps require as much maintenance as grinders.
14.2 Dewatered Wastewater Solids Conveyance
Dewatered or dried sludges, screenings, ash, and grit can
be conveyed by most forms of industrial materials handling
equipment, including belt, tubular, and screw conveyors; slides
and inclines;, elevators; and pneumatic systems. Each may be used
to advantage in certain applications. Because the consistency
of wastewater solids is highly variable, and because the solids
are often difficult to move and may tend to flow, the design
of this equipment must consider the most severe conditions
that may be expected.
14.2.1 Manual Transport of Screenings and Grit
A common method of handling screenings or grit is simply to place
a mobile container (29) beneath the discharge point and to
periodically empty the mobile container into a larger container
to be hauled away to a landfill. The mobile container may
have wheels for ease of movement or it may be maneuvered by an
overhead crane. The principal disadvantage of this approach is
the amount of manual labor required. However, for small or
isolated operations this may be the most appropriate method.
14.2.2 Belt Conveyors
Troughed belt conveyors are simple and reliable (Figure 14-16).
They may be equipped with load-cell weigh-bridge sections for
totalization of conveyed solids weight. (See Chapter 17,
Instrumentation). Totalization is useful when an accurate solids
balance must be calculated for a dewatering facility or treatment
plant. Sludge concentrated enough to maintain a semi-solid
shape (15 percent) can be conveyed at about 18 degrees maximum
inclination on troughed belt conveyors. Sludges with a higher
solids content can be moved up steeper slopes. Where wash sprays
are utilized, splash pans should be provided on the underside of
belts to direct the used washwater to a proper disposal point.
Such splash protection will assist in keeping the area dry
and preventing head and tail pulley slippage. Head and tail
pulley lagging -(grooving), crowning, and other auxiliary ways of
maintaining ;belt guidance should be thoroughly reviewed with
conveyor manufacturers before specifying a troughed belt
14-37
-------
installation. Most troughed belt installations for sludge
currently utilize steel idlers and pulleys with lubricated
anti-friction bearings. The fisheries industry, which also uses
conveyors in constantly wet applications, is successfully using
lubricated thermoplastic (TFE, Delrin) idler bearings with
Schedule 80 PVC pipe rollers; these provide longer service life
than is achieved with all steel construction.
HEAD
PULLEY
FEED CHUTE/
LOADING SKIRTS
TAIL
PULLEY
TROUGHED BELT
CARRYING IDLERS
TAIL TAKEUP
FRAME
DRIVE
UNIT
DISCHARGE
CHUTE
CROSS SECTION
TROUGHED
BELT IDLERS
SUPPORT
BENTS
CROSS SECTION
FLAT BELT IDLERS
FIGURE 14-16
BELT CONVEYOR
In sludge applications, belt failures usually occur first at the
zipper-like mechanical belt seams. Endless belts with field
vulcanized seams may be specified to eliminate this mode of
failure. Belt material must be resistant to dilute sulfuric
acid, formed by the reaction of hydrogen sulfide and moisture.
Material selection must also consider oil, grease and a multitude
of other elements found in sludge.
Belt conveyors have been successfully used to transport coarse
solids removed from mechanically cleaned bar racks, and can be
used to transport grit. Special consideration should be given to
the type of belt design, construction materials, bearings,
type of drive and controls. Since screenings are heavily laden
be designed to contain and direct
disposal. A means of changing belt
so that a range of loads can be
with water, the belt must
draining water to a point of
speeds should be provided
accommodated.
The handbook on belt conveyors for bulk materials by the Conveyor
Equipment Manufacturers Association (30) is a good reference for
general design of belt conveyors. However, there is little
14-38
-------
specific information available relating to the special problems
associated with the cohesive, non-uniform properties of dewatered
sludge. Experience at existing facilities using this type of
conveying equipment and transporting sludge with similar
characteristics provides the most useful design information.
The experience of the County Sanitation Districts of Los Angeles
County in the first three years of operation of a two-stage
digested sludge dewatering station provides useful guidance
for conveying centrifuge-dewatered digested sludge (31), The
facility includes solid bowl centrifuges as a first stage, after
which the centrate is screened and then dewatered using basket
centrifuges. The system uses belt conveyors to transport
dewatered sludge between production, storage, and truck loading.
The system has 44 belt conveyors totaling approximately one-half
mile in length. Troughed conveyor belts carry both first stage
centrifuge cake at 32 percent solids and second stage centrifuge
cake at 17 percent solids. Dewatered sludge is usually stored in
the twelve storage bins at 22 to 24 percent solids and then
transported to trucks by additional belt conveyors.
Helpful guidelines resulting from start-up of this facility
include the following:
1. Reduction of splashing at transfer points: The dump
point should be enclosed and the drop distance minimized.
Skirtboards (stationary sidewalls at edges of belts)
should be used at critical areas and covered if
necessary. Rubber gaskets from hoppers to skirtboards and
on the bottom of skirtboards may be required to reduce
splashing or spillage. Where long drops cannot be
avoided transfer chutes should have interior impact
baffles to dissipate the momentum of falling sludge.
2. Removal of sludge from returning belts: Counter-weighted
rubber-bladed scrapers at head pulleys are not effective
in scraping sludge off return belts and are a maintenance
problem. The use of adjustable tension finger-type
scrapers is recommended. To avoid problems with idler
roller vibration and irregularities, and to ensure
continuous contact, scrapers should be installed beyond
the idler on the flattened portion of the belt.
3. Assuring minimum pulley slippage: Appurtenances that
contact the dirty side of the belt should be avoided.
Figure 14-17 illustrates both the undesirable and the
recommended design features of inclined belt conveyors.
Snubber pulleys and trippers (devices that remove the
moving material from the belt) cannot be successfully
used for sludges. Gravity counterweight take-ups should
be avoided, and screw take-ups should be used instead.
Where long lifts are required, multiple short belts
should be used instead of one long belt to avoid the need
for gravity take-ups.
14-39
-------
4. Importance of housekeeping facilities: Notwithstanding
the care taken to avoid spillage or splashing, sludge
handling facilities are dirty, and must be designed to
facilitate cleanup. Non-skid cover plates, rather than
grating, should be used for all access areas except those
immediately over storage hoppers. Convenient hose
stations should be located to serve all areas. Floors
and slabs should be provided with exaggerated drainage
slopes (up to one inch per foot [8 cm/m]) and should
drain to liberally distributed drain sumps. Special care
should be used at all transfer points, take-up pulleys,
and dump points to minimize sludge spillage or splashing,
or to provide surroundings that are easily cleaned.
Flexible conveyors are now available in styles with integral
pockets, sidewalls and cleats that allow steep, high capacity
operations on almost all materials (Figure 14-18). The belts may
change inclination at several points in their run. They are best
cleaned by a combination brush and spray cleaner. Except for
belt pockets, sidewalls, and cleats, their mechanical components
are similar to those on troughed belts; maintenance costs for
mechanical drives and rollers are also similar.
There are patented flexible conveyors that can not only change
inclination but also change direction or even spiral vertically
upwards. One unit may replace several straight line belts.
These units are not actually belts but segmental chain and
sprocket-driven mechanisms with interlocked, pleated rubber
trough sections. Drive mechanism wear and corrosion is high in
comparison with flat belt conveyors. These conveyors are not
recommended where there is sufficient room to allow installation
of multiple conventional troughed or pocketed conveyors.
14.2.3 Screw Conveyors
Screw conveyors (Figure 14-19) are silent, reliable, and
economical (32). They are used for horizontal movement of grit
or sludge, or may be used to convey dewatered sludge up inclines.
(The degree of incline depends upon sludge moisture content
and consistency). Conservative sizing, abrasion resistant
construction materials, and integral wash down systems within
enclosed housings are recommended for solids handling facilities.
All enclosed housings should have numerous quick opening access
plates for maintenance and observation. Screw conveyors for
dewatered sludge should not have internal intermediate bearings
because sludge can pile up on the bearing and restrict or prevent
flow. For this reason, screw conveyor lengths should be limited
to 20 feet. Screw conveyors with reversible direction, or with
several slide gate controlled discharge openings in the bottom of
the conveyor housing, allow the point of conveyor discharge to be
changed as appropriate, providing flexibility of operation.
14-40
-------
"TRIPPER WITH
SHUTTLE BELT FOR
TRANSFER TO MULTIPLE
BINS
._- SNUBBER
O-* PULLEY
•GRAVITY COUNTERWEIGHT TAKEUP
FOR CONVEYOR BELT TENSION
UNDESIRABLE LAYOUT
'NOT RECOMMENDED FOR USE WITH SLUDGE
TRANSFER OF CONVEYOR MATERIALS
HEAD
PULLEY
MOVEABLE RUBBER
BLADED PLOWS
HEAD
PULLEY
TAIL
PULLEY
DUMP INTO
TANK AT HEAD
OF CONVEYOR
SCREW TAKEUP
FOR CONVEYOR
BELT TENSION
RECOMMENDED LAYOUT
FIGURE 14-17
INCLINED BELT CONVEYOR FEATURES (31)
14-41
-------
FLEX IBLY CLEATED AND
' SIDE WALLED
FLAT BELT CONVEYOR
FIGURE 14-18
FLEXIBLE FLAT BELT CONVEYOR
INLET
DISCHARGE
FIGURE 14-19
SCREW CONVEYOR
Screw conveyors have been successfully used for transporting grit
but their application to screenings is questionable because rags
may become entangled on the conveyor shaft. Oversized objects,
such as sticks, can jam the screw or fall out of the conveyor,
creating housekeeping problems. To reduce wear, ;_open or ribbon
type screw conveyors are sometimes used for grit.
14-42
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14.2.4 Positive Displacement Type Conveyors
Positive displacement type conveyors include tubular conveyors
and bucket elevators. Tubular conveyors (Figure 14-20) are
tubular conduits through which circular flights are pulled by
chains. They may be used for the horizontal transportation of
dry solids such as incinerator ash or semi-dry grit. They are
several times as expensive as flat belts per linear foot, but
require much less room, are fully enclosed and air tight, and can
be routed anywhere a conduit will fit. Maintenance is high.
Most plants utilizing these conveyors routinely replace the chain
elements at least once per month.
FIGURE 14-20
TUBULAR CONVEYOR
Bucket elevators (Figure 14-21) incorporate chain and sprocket
driven buckets in a manner similar to the tubular conveyors
except that the chain flights are not in continual contact
with the product. As a result, mechanical longevity is greatly
increased. They are usually restricted to vertical lifts with
limited horizontal displacement.
14.2.5 Pneumatic Conveyors
Pneumatic conveyors are usually not appropriate for dewatered
sludge, but can effectively handle screenings, grit, and dry
finely divided materials such as incinerator ash. Screenings and
1.4-43
-------
grit can be easily transported, even over long distances, through
the use of a batch pneumatic ejector system (Figure 14-22).
Such pneumatic ejector systems have provided good service for
distances up to one-half mile and up to 100 feet of lift. The
transport system between the points of loading and discharge is a
totally enclosed pipe, which is clean and odor-free and can be
easily routed along available passages. The entire system
utilizes a minimum of moving parts. Consideration must be given
to the use of abrasion resistant materials, especially at pipe
bends, and an air pressure system consistent with the distance
and lift to be traversed.
MATERIAL
OUT
MATERIAL
FIGURE 1U-21
BUCKET ELEVATOR
Continuous pneumatic conveying systems (Figure 14-23), either
pressure or vacuum type, are widely used where dry, particulate
materials are to be transported. Their use in sludge transport
is limited to materials such as incinerator ash. Where long
distances or complex routings are involved pneumatic conveyor
systems are especially well suited to ash transport.
Ash is an extremely abrasive material and rotary valves and elbow
segments in particular must be carefully specified to provide
maximum abrasion resistance. The blowers may require noise
shielding.
14.2.6 Chutes and Inclined Planes
Chutes and inclined planes for sludge, screenings, ash, and
grit should be tested for minimum inclination on the specific
14-44
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transported product whenever possible. In general, inclinations
for dewatered sludge should be greater than 60 degrees from
the horizontal. For dry bulk materials, such as ash, the
inclinations should at least be greater than the material's
natural angle of repose.
DISCHARGE
HOPPER
SCREENINGS
OR GRIT
INLET
CONTROLS
GATE
AIR ^^>
COMPRESSOR
RECEIVER J
u
FlGURt Tl-22
PNEUMATIC EJECTOR
MATERIAL
AND AIR
AIR OUT
MATERIAL
OUT
FIGURE 14-23
PNEUMATIC CONVEYOR
14-45
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14.2.7 Odors
Open sludge conveyance can be a source of odors. All solids
transporting facilities should be well ventilated and, if
necessary, provided with odor control for the vented air. Even
with stabilized sludges, if large holding or ' equalization tanks
are required for the pumping system, floating covers or special
odor control facilities for venting tank air should be provided
when the detention time is greater than several hours. See
Chapter 15 for more detailed information on sludge storage.
14.3 Long Distance Wastewater Solids Hauling
It is often necessary to transport the wastewater solids for long
distances, that is, beyond the boundaries of the wastewater
treatment plant site. This may be done by pumping if the
material is sludge or scum (covered in Section 14.1.6) or by
other methods, which shall be termed long distance hauling. For
this chapter, long distance hauling is limited to trucking, rail
transport, and barging.
Ettlich (28), in developing cost formulas for transport of
wastewater sludge, makes the following general observations about
the comparative economics of the long distance sludge hauling
methods:
1. Transportation of dewatered sludge
« Total annual cost for railroad is less than truck for
all annu'al sludge volumes (7,500 to 750,000 cu yd
[5730 to 573,450 m3] and distances (20 to 320 miles
[32 to 515 km]) studied with and without terminal
facilities for loading and unloading sludge to the
transport vehicle.
• Railroad facilities are more capital intensive than
truck facilities.
• Transport equipment can be leased for both truck and
railroad transport.
2. Transportation of liquid sludge
« Truck is the least expensive mode for one way
distances of 20 miles (30 km) or less and sludge
volumes less than 10 to 15 million gallons (38,000 to
57,000 m3) per year.
• Pipeline is the least expensive mode for all cases
when the annual sludge volume is greater than
approximately 30 to 70 million gallons (110,000 to
260,000 m3)•
14-46
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« Pipeline is not economically attractive for annual
sludge volumes of 10 million gallons (38,000 m3) or
less because of the high capital investment.
• Pipeline is capital intensive and the terminal points
are not easily changed. Pipeline is ideal for large
volumes of sludge transported between two fixed
points.
• Rail and barge are comparable over the 7 to
700 million gallons (30,000 to 2,600,000 m3) volume
range for long haul distances.
• Barge is more economical than rail for short to
medium distances for annual sludge volumes greater
than 30 million gallons (110,000 m3).
While much information is available on costs of transporting
sludge in specific situations (33, 34, 35, 36) there is a wide
disparity in reported costs since there are so many variables in
each situation. Consequently it is much more accurate to utilize
an approach such as Ettlich's, than to rely upon cost estimates
from other treatment plants where conditions may be quite
different (28).
14.3.1 Truck Transportation
For most small plants and some large -plants, the use of trucks
is the best approach. Trucking provides a viable option for
transport of both liquid and dewatered sludge. Trucking provides
flexibility not found in other modes of transport since terminal
points and route can be changed readily at low cost (35) .
Provided trucks are leased rather than purchased, a truck hauling
option is not capital intensive and allows more flexibility than
pumping or other transport modes. This flexibility is valuable
since locations of reuse or disposal may change.
14.3.1.1 Types of Trucks
Sludge hauling trucks are similar to standard highway trucks
because both types of trucks must use public roads and comply
with their overall vehicle width, height and gross weight
restrictions. Most of the variability can be seen in sludge
containment body configuration. For the majority of cases, which
involve comparatively short distances with one-way travel times
less than one hour, ease and speed of loading and unloading
are of paramount importance. The larger trucks are the most
economical except for one-way haul distances less than ten miles
and annual sludge volumes less than 3,000 cubic yards for
dewatered sludge and for less than one million gallons per
14-47
-------
year for liquid sludge. Generally, diesel engines are used in
the larger trucks and are the economical choice for small trucks
that are operated at high annual mileage (35).
Where it is determined that economic, environmental, and
institutional considerations allow direct land application
of liquid digested sludge, special tank trucks are available
equipped with specially designed spreaders, auger beaters, and/or
special application apparatus. Some manufacturers equip their
trucks with subsoil injectors for sub-surface treatment. Use of
such dual purpose trucks allows transport and ultimate disposal
without an intermediate storage/pumping step. Specialized
tanks or trucking equipment can be custom built for specific
applications. One company produces flexible tanks designed to
fit on a flatbed truck (37).
Spillage or leakage from sludge hauling operations are
unacceptable because of aesthetic and health considerations.
This has meant a shift away from belly-dump vehicles, even for a
very well dewatered sludge cake. There is increased concern for
covering the top of the sludge to minimize both odor release
during transit and the chance of spillage due to sudden stops or
accidents. Consequently, tank^-type bodies are gradually becoming
the most common, even for mechanically dewatered sludges. These
vehicles require unusually large hatch openings for loading
purposes, and well designed water-tight hatches or tailgates
for unloading. Tanks for liquid sludge transport are of more
standard design, but the provision of internal baffles to
minimize load shifting is recommended for highway transport.
14.3.1.2 Owned Equipment vs. Contract Hauling
The foregoing concerns apply equally whether or not the
wastewater treatment management agency contracts out its sludge
hauling or uses its own vehicles. The choice between utilizing
agency personnel or contracting for private companies to drive
sludge trucks is often decided not on the basis of cost, but on
the size of the plant. Smaller plants favor the use of both
their own vehicles and staff.
The choice of contract hauling can be limited to the provision of
tractor units and driver services, with the trailers owned by the
agency. This has two major benefits. First, treatment plant
staff, assigned to sludge handling and/or dewatering operations
are working in the immediate vicinity of the trailers, and can
therefore re-spot the trailers under a conveyor belt at the best
times. Second, with most contracts awarded for only one to three
year terras, the contractor would otherwise need to figure in his
bid price a very rapid amortization of custom trailers, which may
be of no further use to him if he is not re-awarded the contract
at a later date, even though they may have a useful life far in
excess of the contract period. Since it is economically sensible
to operate with more trailers than tractor units, trailer cost
depreciation can be a significant overall cost factor.
14-48
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•14.3.1.3 Haul Scheduling
A common problem, usually not recognized, is the need to properly
schedule trucking operations. In general, the total cost
of truck transport will be decreased (per unit of material
hauled) if the daily period of truck operation is increased,
because capital intensive equipment is better utilized. However,
restrictions may be placed on any significant truck operations,
such as requiring specific routes or limiting operations to
daylight hours (35). Such haul scheduling may require the
provision of some form of temporary sludge storage at the
plant. See Chapter 15 for sludge storage information. Whenever
intermittent operations are possible, however, mechanically
dewatered sludge is usually loaded directly from a conveyor
belt. Using trucks or trailer bodies as temporary storage may
not be the most economical method when drivers' work hours,
overtime pay, and the cost of re-spotting trailers under a belt
are considered.
In designing sludge handling facilities, it is desirable to
provide several dump points with the capability to quickly shift
from one to another. If trailers are used, the ability to fill
several units before the tractor unit returns adds flexibility to
scheduling and reduces storage requirements. If the receiving
vessel for dewatered sludge is not self-powered (such as a
trailer), consideration should be given to movable dump conveyors
to allow the load to be distributed uniformly within the vessel.
Dewatered sludge will mound when loaded from a single point.
This may prevent effective utilization of the transport vessel.
14.3.1.4 Trucking Costs
When considering sludge trucking, it is worthwhile to remember
that pumping equipment can handle digested sludge at least up to
20 percent solids concentration, and to note that the layout
and design of loading and unloading facilities can contribute
markedly to cost savings. A more detailed breakdown of relative
costs associated with truck transportation is available (28).
14.3.2 Rail Transport
Rail transport is suitable for transporting sludges of any
solids concentration. It is, however, not a common method of
transporting sludge in the United States.
14.3.2.1 Advantages and Disadvantages of
Rail Transport
Rail transport has a lower energy cost per unit volume of sludge
than pipelining and truck hauling, and once found to be feasible
has a right-of-way already established, which is not usually the
14-49
-------
case with a pipeline. Rail transport can suffer from many of the
same problems as pipelines, such as large unrecoverable capital
expenditures and fixed terminal points. In addition, it has some
of the same problems associated with trucking, such as an ongoing
administrative burden, vulnerability to labor disputes and
strikes, risk of spills, and because of the labor requirements,
an operational cost that will rise continually. However, special
circumstances may favor rail hauling. For example, if sludge is
to be used to rehabilitate strip-mined lands, a rail line may
have been built for hauling out the coal. That line would still
be available for the transport of sludge.
14.3.2.2 Routes
The construction of a new railroad line may not be cost-effective
or even possible for the sole purpose of transporting wastewater
sludge. New construction is normally limited to a short spur
from a mainline railroad or the provision and/or expansion
of small switching yards on a large treatment plant site in
conjunction with chemical delivery facilities. Any attempt at
longer new lines is impractical. This limits the overall route
selection, generally between the treatment plant site and
the final sludge disposal point, to railroad lines already in
existence. In turn, this will limit either the selection of
rail for sludge transport or severely limit the choice of or
subsequent change in disposal site location.
14.3.2.3 Haul Contracts
Railroad cars must be hauled by a railroad company, except
possibly for switching. Therefore a contract must be obtained
with the railroad. Since this contract hauling is a major cost
element, and since the railroad often cannot provide rapid and
realistic cost estimates, some time and consideration will be
required.
Railroads are a regulated utility; this complicates the rate
quotation process. Rates are of two general types: a "class
rate" and a "special commodity rate." The class rates are
readily obtained, but are usually prohibitively expensive for
sludge. To obtain a special commodity rate, the following
procedure is necessary:
1. An application is submitted to the railroad, including a
complete description of what is to be shipped; how it is
to be shipped (type of material, liquid or solid);
precisely where it is to be shipped; the frequency of
shipping (how much per day, per week); the approximate
loading and unloading time; what other types of materials
are similar in form, concentration, and makeup to
the material being shipped (for example, Code 5630,
North Coast Freight Bureau, "tankage"—a commodity used
14-50
-------
in production of fertilizers); and a statement of the
price the shipper would be '.willing to pay in cents per
100 pounds net weight (45.4 kg).
2. The local railroad—-the .. carrier—reviews the load,
distance, terrain, switching requirements, and
competition and calculates a rate.
3. The rate is published by the local freight bureau (for
example, for Seattle, Washington, the North Coast Freight
Bureau) for a notice period of 30 days for review by
other, possibly competing, carriers, and by one of the
five regional freight bureaus: Western, Southwest,
Central, Southern, or Northeastern. The regional freight
bureaus are conglomerations of the local ones and they
regulate and control prices between bureau jurisdictions.
4. Comments and appeals of rates can be made to the
Interstate Commerce Commission (ICC). An appeal of a
proposed rate will cause that rate to be suspended for a
seven-month period for the case to be heard by the
suspension board of ICC and for the carrier to justify
that rate. Historically, appeals have caused proposed
rates to be eliminated from the carriers' tariffs. This
effectively eliminates the option of rail transport of
sludge for this locality.
Generally speaking, railroads are interested in providing sludge
transportation. However, many railroads are unfamiliar with
sludge hauling; similarly, many environmental engineers are
unfamiliar with railroad procedures (38).
14.3.2.4 Railcar Supply
There are three methods of ensuring railcar equipment adequacy:
by leasing, by outright purchase, or through placement of the
required number of cars in "assigned service" by the carrier
under the terms of the haul contract. Generally, an assigned
service option is only available for a solid (dry) or semi-solid
(mechanically dewatered) sludge which can be transported in
hopper cars. A liquid sludge must be carried in tank cars which
are not normally available "free" from the railroad. As a
generalization, the amortization of the purchase of either
type of car (at approximately $90,000 to $120,000 new) will be
at considerably higher cost than the rental or lease fee.
Consequently, it is expected that the assigned service option
would be selected for hopper cars, and a lease arrangement
negotiated with a private tank car rental company for tank cars.
Railroad hopper car use is subject to minimum shipment fees per
car and certain demurrage criteria. For example, a single hopper
car minimum shipment is 180,000 pounds (82,000 kg) and demurrage
criteria are that the car must be loaded within 48 hours and
14-51
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unloaded within 24 hours. Reference time is 7 a.m. If a car is
delivered between midnight and 8 a.m., the time begins -at 7 a.m.
the same day. If a car is delivered between 8 a.m. and midnight,
the time begins at 7 a.m. the following day. Typical hopper car
capacities are 2,600, 3,215, and 4,000 cubic feet (75, 91, and^
113 m3 ) , with the smallest size being typically the most readily
available . .
Tank cars are normally rented by the month from private tank car
rental companies with a minimum five-year commitment. A large
non-insulated coiled car (coiled to prevent freezing during the
winter months) will rent for approximately $450 per month
(1978 prices). Tank car capacities are typically 10,000 to
20,000 gallons (37,850 to 75,700 1). The selection of rail
transport, with its high transit times, for more putrescible
sludges without special gas venting and control equipment, should
be avoided. Typical minimum tank and hopper car requirements are
shown in Table 14-8.
TABLE 14-8
TYPICAL MINIMUM TANK CAR REQUIREMENTS (28)
Car loads'
Approximate secondary
treatment plant size,
MGD
10
50
100
Annual sludge One-way
volume, MG distance, mi
7.5 20
40
80
160
320
15 20
40
80
160
320
75 20
40
80
160
320
150 20
40
80
160
320
Per
year
375
375
375
375
375
750
750
750
750
750
3,750
3,750
3,750
3,750
3,750
7,500
7,500
7,500
7,500
7,500
Per
day
1
1
1
1
1
2
2
2
2
2
10
10
10
10
10
21
21
21
21
21
Cars j
required
5
5
7
8
9
9
9
13
15
17
47
47
68
78
89
97
97
139
160
181
aCar size 20,000 gal (76 m3).
Estimate assumes that ample storage is available so
that extra cars are not required for peak sludge
production or scheduling problems.
1 MGD = 0.044 m3/s
1 MG = 3,785 m3
1 mi = 1.6 km
1 gal = 3.8 1
14-52
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The exact calculation of car requirements is very site- and
area-specific and should be checked directly for any given
situation. It should be recognized that the speed of railroad
transport will depend in part on the track conditions and on the
railroad's normal traffic schedule; the track conditions may also
limit the loads carried per car, and hence the size and number of
cars required. As a guide only, typical transit times are shown
in Table 14-9.
TABLE 14-9
TYPICAL TRANSIT TIMES FOR
RAILROAD TRANSPORTATION
One-way distance, Round-trip transit time,3
miles days
20 4
40 4
80 6
160 7
320 8
aFor estimating rail car demand, an allowance of 25
to 50 percent should be added to accommodate
scheduling and car holdup problems. Also, the
transit time does not include time for loading
and unloading, which must be estimated separately.
14.3.2.5 Ancillary Facilities
Railroad transport of sludge requires loading storage and
equipment (tanks, pumps, and piping for liquid sludge and hoppers
and conveyors for dewatered sludge), railroad sidings, and
unloading equipment. Unloading is ordinarily accomplished by
gravity. Car maintenance and storage will be undertaken by
the owner of the cars—not normally the wastewater treatment
authority—but car cleaning and washdown facilities may be
required.
14.3.2.6 Manpower and Energy Requirements
The wastewater authority will have labor requirements for loading
and unloading railroad cars and for associated maintenance;
14-53
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estimates are given in Table 14-10. Data on energy demands
associated with railroad transport are not readily available,
but energy demands are relatively low compared with other
transportation modes. The fuel consumed in transporting the
sludge should nevertheless be estimated for inclusion in the
sludge management program's energy effectiveness analysis.
TABLE 14-10
MANPOWER REQUIREMENTS FOR RAILROAD TRANSPORT (28)
Liquid sludge Dewatered sludge
Annual volume,
mil gal
7 . 5
15
150
750
Labor, manhours/yr
Operation
4, 124
4,124
10,500
28,500
Maintenance
130
260
500
1,200
Annual volume,
thousand cu yd
7.5
15
150
750
Labor, manhours/yr
Operation
1,650
3, 300
4, 125
10, 000
Maintenance
130
260
500
1,200
1 cu yd = 0.76 m
1 mil gal = 3,785 mj
14.3.3 Barge Transportation
Barge transport for the ocean dumping of sludge has been
practiced for many decades around the world. Recent decisions to
limit ocean dumping, combined with rapidly escalating costs for
dewatering or drying sludges, have led to more consideration
of barge transport of liquid sludges between the wastewater
treatment plant or plants and land disposal sites many miles
distant. Barge transportation of sludges is generally only
feasible for liquid sludges (to the solids concentration limit at
which it may be pumped) and over longer distances, generally over
30 miles. Additional information is available (28,36,39).
14.3.3.1 Routes and Transit Times
It is evident that the key feature in consideration of barge
transportation is the proximity to a suitable waterway. However,
in planning a barge transport system, the transit time also plays
a critical role. The traffic on the waterway; physical features
such as drawbridges, locks, and height limitations,and natural
characteristics such as currents, tides, and even wave heights
will all affect the transit time. Local operators familiar
with the waterway should be contacted for information and a
conservative safety factor should be applied. Loading and
unloading times then must be added to estimate the overall
turnaround time — the key feature when contracting for towing
service. Towing speeds and cost estimates are given in
Table 14-11.
14-54
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TABLE U-11
TUG COSTS FOR VARIOUS BARGE CAPACITIES3
Average velocity, knotsb
Barge Capacity, Tug costs,C
barrels Loaded Unloaded dollars/hour
25,000 6 7 120
50,000 7 8 150
100,000 8 10 195
Source: Foss Tug, Seattle, Washington, a division
of Dillingham Corporation, various personal inter-
views with Metropolitan Engineers/Brown and
Caldwell staff members, 1975 through 1976.
Velocities in open water. Waterway restrictions
reduce average speeds.
Costs are for late 1975 and early 1976. Inflation
has been at about 15 percent per annum compounded
since 1976.
1 barrel = 159 1
1 knot = 0.51 m/s = 1.85 km/hr
14.3.3.2 Haul or System Contracting
Only for very large plants should ownership of the motive power
unit(s) (tug or powered barge) be considered. Self-propelled
barges are no longer generally considered cost-effective when
initiating a new system, although the specifics of any particular
case could modify this conclusion. This means the choice for
most wastewater treatment authorities narrows down to contracting
for either complete barge transport services or for tug service
alone. Full service contracts may prove the best for small
operations with intermittent transport requirements. Moderate to
large plants will generally favor contract towing only, with the
barge(s) owned by the authority (although Chicago's barging
system is a full service contract). Contractual agreements
should clearly define in detail all services to be provided and
include a barging schedule. In certain cases it may be possible
for two or more wastewater treatment authorities to join in a
common contractual agreement whereby sludge from two or more
plants is picked up in tandem by the one haul contractor.
14-55
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14.3.3.3 Barge Selection and Acquisition
Both the useful life and salvage value of barges tend to be high.
This will often lead to a decision to purchase rather than
lease equipment. Size and number of barges will depend on
plant size and the specific sludge processing system.
Some data on typical barge sizes and costs are given in
Table 14-12. Physical dimensions of barges are not standardized,
since they are usually custom built within certain dimensions set
by some waterway constriction, such as lockage limitations. Lead
times on construction are about two years. Barge proportions are
commonly length to breadth 4 or 5 to 1, and breadth to depth
3 or 4 to 1. For inland waterways, about two feet (0.6 m) of
freeboard under the maximum loaded condition is usually adequate.
Barges are very common in the 20,000 to 25,000 barrel (3,200 to
4,000 m3) capacity range. Construction costs in 1976 were about
$6 per cubic foot ($212/m3) for a 25,000 barrel (4,000 m3) barge,
with only a slight reduction in unit costs as size increases, to
about $5.50 per cubic foot ($194/m3) at the 100,000 barrel
(16,000 m3) size. Greater flexibility in operations will usually
dictate the choice of smaller barges, unless distances are about
200 miles (330 km) or more and number of waterway restrictions
low. Then the increased speed offered by a larger tug/barge
combination will substantially cut transit time and thus reduce
towing fees.
TABLE 14-12
.a
TYPICAL BARGE SIZES AND COSTS'
Cost,C
Capacity,
barrels
Length
240
240
_
_
286
320
Dimensions,
Breadth
52
60'
_
_
62
70
ft
Depth
15
13.5
_
_
18
20
Draft
_
13.5
-
-
-
16
18
thousand
Newd'e
_
1,100
-
-
-
1,750
2,300
dollars
Usedf
225
-
-
650
625
-
~
14,000
20,000
23,000
27,000
33,000
35,000
50,000
aExamples are for barges custom built for liquid sludges but do not include
pumps necessary for unloading.
One barrel equals 42 gallons (159 1).
°Costs are for 1976. Inflation in new and used barges has been about 15
percent per annum compounded 1976 through 1979.
dSource: L. R. Gloston and Associates, Naval Architects, Seattle, Washington.
Construction costs were approximately 50 cents/lb of steel in the barge ($1.10/kg) in
1976 and are about 80 cents/lb ($1.80/kg) in 1979.
Source: William Drury Company, Seattle, Washington, communication to
Metropolitan Engineers/Brown and Caldwell, September 30, 1976.
1 barrel = 0.16 m3
1 ft = 0.30 m
1 cent/lb = $0.022/kg
14-56
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14.3.3.4 Ancillary Facilities
A critical factor in determining the feasibility of barging
sludge lies in the cost of facilities for loading and offloading,
and receiving the sludge. If the treatment plant is not close to
the waterway, it may be desirable to locate a sludge storage
tank or lagooon near the barge loading dock. For a tank, design
would need to be similar to an unheated digester because of
continued anaerobic decomposition. Lagoons should be operated as
facultative sludge lagoons. In either case, costs of the tank or
lagoon should be included in the barge system costs.
In most cases, it is desirable to load and meter the flow from a
fixed pumping station located on a fixed wharf. Offloading is
often accomplished by a pump mounted on the barge itself.
The disposal site should be located near a dock capable of
mooring a suitably sized barge. Floating docks are usually more
expensive in both marine and freshwater environments than fixed
wharfs, due to the complexity of anchoring devices capable of
sustaining the loads imposed by a large barge. In certain
instances, however, a floating dock may be more acceptable from
an environmental standpoint.
Unloading to a land pipeline typically takes about 6 hours. If a
tug must remain with the barge during the unloading period, rapid
unloading becomes economically important.
14.3.3.5 Spill Prevention and Cleanup
One important element of a barge transportation system is a
well developed spill prevention and cleanup program. Spills
resulting from accidents during transport can result in serious
water pollution and associated health problems. Sludge spills
should be contained immediately and transferred to storage tanks
or another barge as quickly as possible to reduce risks. The
risk of spills during loading and unloading can be minimized by
careful attention to design and operator training.
14.4 References
1. Metcalf & Eddy, Inc. Wastewater Engineering; Treatment^
Reuse. McGraw-Hill. 1979 (second edition).
2. Hanks, R.W. and B.H. Dadia. "Theoretical Analysis of the
Turbulent Flow of Non-Newtonian Slurries in Pipes."
American Institute of Chemicjil^Enginee rs Journal . Vol. 17,
p. 554. May 1971.
3. Caldwell, D.H. and H.E. Babbitt. "The Flow of Muds,
Sludges, and Suspensions in Circular Pipe." Transactions of
American Institute of Chemical Engineers^. Vol. 37, p. 237.
April 25, 1941.
14-57
-------
4. Buckingham, E. "On Plastic Flow Through Capillary Tubes."
Proceedings of the American Society of _Testing and
Material's^Vol. 21, p. 1154.1921.~~
5. Rimkus, R.R. and R.W. Heil. "The Rheology of Plastic Sewage
-Sludge." Proceedings of the Second National Conference on
Complete Water Reuse. Chicago, Illinois: 5/4-8/75.
American Institute of Chemical Engineers. L.K. Cecil, ed.
p. 722.
6. Rimkus, R.R. and R.W. Heil. "Breaking the Viscosity
Barrier." Proceedings of the Second National Conference on
Complete Water Reuse. Chicago, Illinois; 5/4-8/75.
American Institute of Chemical Engineers. L.K. Cecil, ed.
p. 716.
7. Kenny, J.P. Bulk Transport of Waste Slurries to Inland and
Ocean Disposal Sites. Volume III. Bechtel Corporation.
1969. Published by National Technical Information Service
as PB 189759/BE.
8. Babbitt, H.E. and D.H. Caldwell. "Laminar Flow of Sludges
in Pipes with Particular Reference to Sewage Sludge."
University of Illinois Engineering Experiment Station,
Bulletin Series, No. 319.1939.
9. Rabinowitsch, B. Z. Physical Chemistry. Vol. 145A, p. 1.
1929.
10. Wolfs, J.R. "Factors Affecting Sludge Force Mains." Sewage
and Industrial Wastes. Vol. 22, p. 1. January 1950.
11. Holland, F.A. Fluid Flow for ChemicalEngineers. Chemical
Publishing Company. 1973.
12. Bourke, J.D. "Sludge Handling Characteristics in Piped
Systems." Proceedings of the Northern Regional Conference
of the California Water Pollution Control Association.
Monterey, California: 10/19-20/73.
13. Babbitt, H.E. and D.H. Caldwell. "Turbulent Flow of Sludges
in Pipes." University of Illinois Engineering Experiment
Station, Bulletin Series, No. 323. 1940.
14. Hedstrom, B.O.A. "Flow of Plastic Materials in Pipes."
Industrial Engineering Chemistry. Vol. 44, p. 651. 1952.
15. Behn, V.C. and R.M. Shane. "Capillary vs. Pipeline in
Determining Sludge Flow Behavior." Wa_te r & Sewage Works.
Vol. 110, p. 272. July 1963.
16. Alves, G.E., D.F. Boucher, and R.L. Pigford. "Pipeline
Design for Non-Newton ion Solutions and Suspensions."
Chemical Engineering Progress. Vol. 48, p. 385. 1952.
14-58
-------
17. Hanks, R.W. "The Laminar-Turbulent Transition for Fluids
With a Yield Stress." American Institute of Chemical
Engineers Journal. Vol. 9, No. 3, p. 306. 1964. "~
18. Hanks, R.W. and D.R. Pratt. "On the Flow of Bingham Plastic
Slurries on Pipes and Between Parallel Plates." Society of
Petroleum Engineers Journal. p. 342. December 1967.
19. Kenny, J.P., E.J. Wasp, and T.L. Thompson. "A Design iModel
for Pipeline Flow of Solid Wastes." Water-1970. Chemical
Engineering Progress Symposium Series. American Institute
of Chemical Engineers. Vol. 67, no. 107, p. 364. 1971.
20. Dick, R.I. and B.B. Ewing. "The Rheology of Activated
Sludge." Journal Water Pollution Control Federation.
Vol. 39, p. 543. 1967.
21. Bingham, B.C. Fluidity and Plasticity. McGraw-Hill. 1922.
22. Brisbin, S.G. "Flow of Concentrated Raw Sewage Sludges in
Pipes." Journal of the Sanitary Engineering Division ASCE.
Vol. 83, no. SA3, p. 1274. June 1957.
23. Chou, T.L. "Flow of Concentrated Raw Sewage Sludges in
Pipes." Journal of the Sanitary Engineering Division ASCE.
Vol. 84, no. SAl, p. 1557. February 1958.
24. Vesilind, P.A. "Treatment and Disposal of Wastewater
Sludges." Ann Arbor Science. Chapter 4. 1979 (Second
Edition).
25. City of San Francisco. "Primary Sludge Pump Evaluation."
Prepared by the City's Division of Sanitary Engineering.
October 1975.
26. Sparr, A.E. "Pumping Sludge Long Distances." Journal Water
Pollution Control Federation. Vol. 43, p. 1702. August
1971.
27. Williams, M.L. "A Guide to the Specification of Glass Lined
Pipe." Water & Sewage Works. Vol 124, no. 10, p. 76.
October 1977.
28. USEPA. Transport of Sewage Sludge. U.S. Environmental
Protection Agency report EPA-600/2-77-216. December 1977.
28a. Weller, L.W. "Pipeline Transport and Incineration." Water
Works and Wastes Engineering. Kansas City, Missouri,
installation. September 1965.
28b. Wirts, J.J. "Tips and Quips—Contribution from Cleveland."
Sewage Works J qu rna_l. Vol. 20, No. 3, p. 571. May 1948.
29. Tchobanoglous, G.> H. Theisen, and R. Eliassen. S o_li_d
Wastes. McGraw-Hill. Chapter 5. 1977.
30. Conveyor Equipment Manufacturers Association. Belt
Conveyors for Bulk^Materials. Cahners Publishing Company.
1966.
14-59
-------
31. Hansen, B.E., D.L. Smith, and W.E. Garrison. "Start-up
Problems of Sludge Dewatering Facility." Proceedings of the
51st Annual Water Pollution Control Federation Conference.
October 1978. Anaheim, California.
32. Conveyor Equipment Manufacturers Association. S_c_re_w
Conveyers. Book No. 350. 1971.
33. Dallon, F.E. and R.R. Murphy. "Land Disposal IV:
Reclamation and Recycle." Journal Water Pollution Control
Federation. Vol. 45, no. 7, p. 1489 (July 1973).
34. USEPA. Cost of Landspreading and Hauling Sludge from
Municipal Wastewater Treatment Plants. U.S. Environmental
Protection Agency, Office of Solid Wastes, Cincinnati,
Ohio 45268. EPA/530/SW-619, Oct. 1977.
35. Ettlich, William F. "Economics of Transport Methods of
Sludge." Proceedings of the Third National Conference on
Sludge Management; Disposal and Utilization. Miami Beach,
Florida. December 14-16, 1976. Information Transfer Inc.,
p. 7.
36. Guarino, C.F., M.D. Nelson, S.A. Townsend, T.E. Wilson, and
E.F. Ballotti. "Land and Sea Solids Management Alternatives
in Philadelphia." Journal Water Pollution Control
Federation. Vol. 47, noT 11, p. 2551. November 1975.
37. Billings, C.H., S.H. Conner, J.R. Kircher, and G.M. Scales.
1979 Public Works Manual. Public Works Journal Corp. 1979.
; p. D-49.
38. Heller, N. "Working With the Railroad." Proceedings of the
Third National Conference on Sludge Management: Disposal
~an d~~uFiTTz a t i o n^ Miami Beach, Florida. December 14-16,
1976. Information Transfer Inc. p. 50.
39. USEPA. "Evaluation of Sludge Management Systems: Evalua-
tion Checklist and Supporting Commentary." Technical
bulletin prepared by Culp/Wesner/Culp. April 1979 draft.
To be published.
14-60
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EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapter 15. Storage
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 15
STORAGE
15.1 Introduction
Storage is an integral part of every wastewater solids treatment
and disposal system, since it is necessary to the assurance
that the system will be used to full capacity. Recent emphasis
on the control of wastewater solids treatment and disposal
mandates that effective storage be provided. Storage that is
compatible with the objectives of a system must be incorporated
into its design to enhance both the system's reliability and its
efficiency.
15.1.1 Need for Storage
Storage allows different processes to operate on schedules which
best fit overall system objectives and precludes the need to
force all processes to operate on the same schedule. For
example, solids are generated from the wastewater treatment
system 24 hours per day, but it may be most convenient to operate
the solids processing system only on the day shift. Solids must
therefore be stored during off-hours. Storage must also be
provided between adjacent treatment or disposal processes which
operate at different rates--for example, between centrifuges
(which discharge solids at 100 tons per hour [91 t/hr]) and
incinerators (which must be fed at 50 tons per hour [45 t/hr]).
In addition, it must be provided upstream from virtually any land
disposal system, since sludge can usually be applied to land only
part of the year, whereas the waste treatment plant generates
solids all year around.
15.1.2 Risks and Benefits of Solids Storage
Within Wastewater Treatment System
Stored solids can be washed from the wastewater treatment
system, thereby degrading effluent quality. They may also
become septic, with the same effect. As a general rule, solids
should not be stored in wastewater treatment systems unless
storage provides benefits that clearly outweigh the risks
involved. For many small plants, if sludge processing units
are operated only on the day shift, the benefits do outweigh
the risks. These plants frequently store solids within the
wastewater treatment process for periods as long as 24 hours.
Large plants, which typically process sludge around-the-clock,
15-1
-------
make less frequent use of storage within the wastewater treatment
system. The main exception to this rule is the storage of solids
within wastewater stabilization ponds, where solids and dead
algae settle to the bottom of the ponds and anaerobi cal ly
decompose. These solids are seldom removed and often accumulate
for many years with no deleterious effect.
15.1.3 Storage Within Wastewater Sludge
Treatment Processes
Solids can be stored within sludge treatment processes with
fewer adverse effects than if they were stored within the waste-
water treatment system. Whereas the processes of disinfection,
conditioning, mechanical dewatering, high-temperature conversion,
and heat-drying do not provide storage, those of gravity
thickening, anaerobic and aerobic digestion, air drying, and
composting do. Used judiciously, these processes can store
enough solids to enable necessary adjustments to be made in rates
of flow between processes. One or two of these processes can
provide cost-effective storage for periods exceeding one month.
However, because of process limitations, some cannot provide
storage for minimum periods of three to four days even though
they can store for periods of three to four weeks and longer.
15.1.4 Effects of Storage on Wastewater Solids
If wastewater solids are to be stored for any extended period
of time, they must be stable. Stable liquid sludge with less
than ten percent solids can be stored in facultative sludge
lagoons, anaerobic sludge lagoons, or aerated basins. When
it is air dried to greater than 30 to 40 percent solids, stable
sludge can be stored safely and without odors in relatively
small, confined structures or in unconfined stockpiles. It is
impractical to store unstabilized dewatered or partially dried
sludge (sludge containing more than 10 percent and less than
30 percent solids) for much longer than three to four days
because septic conditions and problems associated with septicity
(odors, poor solids transport properties) can develop.
Wastewater solids are usually stored in concentrated form.
If these solids are biodegradable, indigenous oxygen supplies
can rapidly be depleted and anaerobic decomposition begins.
Anaerobic decomposition is often, but not always ,accompanied
by the production of undesirable odors. However, anaerobic
decomposition will not occur if:
• Biodegradable materials present are insufficient to
support biological activity. For example, screenings
and grit are relatively non-odorous, provided they have
been processed and transported hydraulically prior to
final dewaterings. The washing action which occurs
15-2
-------
during these operations reduces the concentration of
putrescible organic material. Conversely, if processed
and transported mechanically (that is, without washing),
they may be the source of strong odors when subsequently
stored.
• Oxidizing conditions can be maintained. Agents such as
oxygen, chlorine, and hydrogen peroxide can be used
to this end if the sludge is in liquid form. Forced
aeration or physical manipulation can be used to maintain
the aerobic condition if solids are dewatered and
managed, as is done in composting.
• Moisture is reduced to discourage biological activity.
For example, air dried stabilized sludge with a solids
content greater than 40 to 50 percent and unstabilized
heat-dried sludges can be stored indefinitely without
nuisance, provided rewetting does not occur.
• pH is adjusted to values above approximately 12 and
below approximately 4 by adding chemicals like lime or
chlorine. Note that pH extremes must be maintained.
These treatments do not destroy putrescible materials,
and the biocidal effects caused by extreme pH are lost as
the pH drifts toward neutral values as the result of
interaction with atmospheric carbon dioxide.
The fact that anaerobic digesters and facultative sludge
lagoons have operated without nuisance odors clearly indicates
that storage can be accomplished under anaerobic conditions
without adverse effects. Work on facultative sludge lagoons in
Sacramento documents these conclusions (1).
Nuisance odors will not develop in anaerobic storage when
sufficient methane bacteria are present. If the methane bacteria
are destroyed, however, serious odor problems may result. As an
example, consider anaerobically digested sludge which is placed
on a drying bed or in a drying lagoon. The top layer of sludge
is dewatered, and methane bacteria die as the sludge aerates and
dries. Odor levels are extremely low, since the sludge is too
dry to support anaerobic biological activity. Should the surface
of the sludge be re-wetted (for example, by rainfall or surface
flooding), however, anaerobic activity would resume, the organic
acid concentration would rapidly increase, and odors would
increase to nuisance levels. Odor problems experienced with
approximately 580 acres (235 ha) of drying lagoons at San Jose,
California, immediately following a rainstorm, is an example of
this type of problem (2).
Not all the effects of solids storage are negative. Storage
of anaerobically digested sludge in the liquid state can be
beneficial for its ultimate disposal. If such sludge is stored
15-3
-------
for several years without being contaminated by freshly digested
sludge, its organics content (40 to 50 percent) and its content
of pathogenic bacteria, viruses, and parasites will be greatly
reduced (1,3).
15.1.5 Types of Storage
Wastewater solids may be stored in facilities within the
treatment system, within the sludge treatment and disposal
system, and within tanks, lagoons, bins, or stockpiles provided
primarily for storage. This latter group is divided into two
divisions, those provided for either liquid or dewatered sludge.
The use of wastewater and sludge treatment facilities for solids
storage must not adversely affect their treatment capability. If
this potential exists, then facilities dedicated primarily to
storage must be provided.
Three methods of storage are described as follows:
• Single-Phase Concentration. Solids accumulate in a
completely-mixedvesselas a result of increasing
concentration. The solids concentration is uniform
throughout and vessel volume is constant. For example,
solids buildup within the aeration reactor of an
activated sludge system if solids are not wasted.
• Two-Phase Concentration. Storage is within the solids
layer of a liquid/solids separation device. Volume of
the solids layer increases; however, total system volume
remains constant. For example, solids are accumulated
in a gravity thickener by terminating sludge withdrawal
from the thickener and allowing the sludge blanket to
build up.
• Displacement. Solids are stored as a result of changing
total system volume. For example, solids can accumulate
within digesters with floating covers by displacement
storage, since the covers can rise to accommodate greater
volumes of sludge.
Storage may be accomplished by two or three methods operating
in concert. For example, solids can accumulate in a floating
cover equipped secondary digester by simultaneous two-phase
concentration and displacement.
Storage may be further categorized as follows by detention time:
• Equalization Storage Solids detention time should not
——————exceed three to four days.
• Short-Te rm_S to rage Solids detention time should not
~"exceed three to four weeks.
15-4
-------
Long-Term Storage
Solids detention
than one month.
time is greater
Table 15-1 lists wastewater solids
method, and detention time category.
storage by type, facility,
TABLE 15-1
WASTEWATER SOLIDS STORAGE APPLICABILITY
Detention tim
Type
Equalizing
(3 to 4
days)
Short term
(3-4 weeks)
Long term
(Greater
than 1 month)
Comments
Storage within waste-
water treatment
processes
Grit removal
Primary sedi-
mentation
Aeration reactors
Two-phase concentra-
tion
Two-phase concentra-
tion
Single-phase concen-
tration
Use of wastewater treatment processes for
storage must not adversely affect treat-
ment efficiency.
Storage time depends on sewer system grit
loading to plant.
Storage for over
Secondary sedi- Two-phase concentra-
mentation tion
Imhoff tanks Two-phase concentra-
tion
Community septic Two-phase concentra-
tanks tion
Wastewater sta- Single and two-phase
bilization ponds concentration
Storage within sludge
treatment processes
Gravity thickeners Two-phase concentra-
tion
Anaerobic digesters Single and two-phase
concentration and
displacement
Temperature sensitive.
24 hours.
Storage within extended aeration systems,
for example, oxidation ditches , can
exceed 3 weeks if accomplished in con-
junction with secondary sedimentation
concentration.
Highly temperature sensitive. Storage for
over 8 hours requires chemicals.
Lightly loaded systems can store for over
6 months. Most systems will require
solids removal every 4 to 6 weeks.
Sludge from many septic tanks is removed
only once in several years.
Aerated ponds operate like aeration
reactors. Other ponds use two-phase
concentration and can store solids for
many years.
Use of sludge treatment processes for
storage must not adversely affect sludge
treatment efficiency.
Temperature sensitive . Usually not used
with WAS. Storage for over 24 hours re-
quires chemicals.
Floating covers allow for displacement
storage. Two-phase concentration stor-
age impracticable if WAS present.
Single-phase concentration storage pos-
sible if digesters operated in conjunc-
tion with primary sedimentation
concentration changes.
15.2 Wastewater Treatment Storage
Influent variability and fixed effluent requirements make
operational flexibility a necessity for every wastewater
treatment plant. One of the most cost-effective means of
providing flexibility for small plants is to assure that
treatment processes contain storage within themselves.
15.2.1 Storage Within Wastewater Treatment Processes
Listed in Table 15-1 are several wastewater treatment processes
that can provide solids storage. The following sections describe
ways in which this storage can be used effectively.
15-5
-------
TABLE 15-1
WASTEWATER SOLIDS STORAGE APPLICABILITY (Continued)
Detention time
Type
Equalizing
(3 to 4
days)
Short term
(3-4 weeks)
Long term
{Greater
than 1 month)
Storage within sludge
treatment processes
(continued)
Aerobic digesters
Composting
Drying beds
Facilities provided
primarily for stor-
age of liquid sludge
Single and two-phase
concentration and
displacement
Two-phase concentra-
tion and displace-
ment
Two-phase concentra-
tion and displace-
ment
Holding tanks
Facultative sludge
lagoons
Single and two-phase
concentration and
displacement
Two-phase concentra-
tion
Anaerobic liquid
sludge lagoons
Two-phase concentra-
tion
Decanting can be limiting. Short-term
storage possible if digesters operated
in conjunction with sedimentation con-
centration. Displacement storage
requires aeration systems which will
operate with variable level.
Evaporation with process accomplishes two-
phase concentration. Processed solids
not removable for 3 to 4 weeks.
Initial settling accomplishes two-phase
concentration. Processed solids not
normally removable for 3 to 4 weeks.
Storage limited to equalizing by high
costs of detention and continuous
mixing.
Time required for initial settling limits
storage to short or long term.
Mechanics of sludge removal makes short-
term storage very expensive. Odor free
operation requires anaerobically di-
gested solids. Organic loadings must be
restricted and surface agitation pro-
vided. Odor mitigation required when
surface area exceeds 30 to 40 acres.
Time required for initial settling limits
storage to short or long term.
Mechanics of sludge removal makes short-
term storage very expensive. Odor
minimization requires anaerobic digested
solids. Usually operated without organic
loading restriction. No surface agita-
tion provided, potential odor risk
high, although no quantifying data
available.
15.2.1.1 Grit Removal
Grit removal basins and channels may be used to store unusually
heavy grit loadings which, when combined sewer systems are
involved, generally arrive at the treatment plant after a dry
spell and during the first flush of a storm. Storage must be
provided to contain all of the grit which could accumulate during
the storm. The required storage volume is a function of grit
loading and the rate at which the grit can be transferred out of
the basin or channel. Where grit is transferred manually (for
example, in small plants with duplicate channels), the designer
may wish to provide storage sufficient to hold grit during
periods when the plant may be unattended (long weekends). Grit
production figures are shown in Chapter 4. .. ,•
15-6
-------
TABLE 15-1
WASTEWATER SOLIDS STORAGE APPLICABILITY (Continued)
Detention time
Type
Equalizing
(3 to 4
days)
Short term
{3-4 weeks)
Long term
(Greater
than 1 month)
Facilities provided
primarily for stor-
age of liquid sludge
(continued)
Aerated storage
Facilities provided
primarily for stor-
age of dewatered
sludge
Sludge drying
lagoons
Confined hoppers
or bins
Unconfined stock-
piles
Single and two-phase
concentration and
displacement
Two-phase concentra-
tion and displace-
ment
Displacement
Displacement
High energy demand usually restricts
detention time. Same limits as
aerobic digesters.
Initial settling accomplishes two-phase
concentration. Process solids not
normally removable for one to two
months. Odor minimization requires
anaerobically digested solids. Can be
odorous if aerobically stabilized sur-
face layers begin to decompose
anaerobically when rewetted.
Moist (15 to 30 percent solids) dewatered
sludge can present major material manage-
ment and odor production problems if
storage time exceeds 3 to 4 days.
Structures usually too expensive for
long-term storage. Short-term storage
can be successful with dry (greater than
30 to 40 percent solids) stabilized
sludges.
Requires stabilized dry (greater than 30
to 40 percent solids) sludge. Stock-
piles are usually covered in very wet
climates. Natural freeze drying is
possible.
Special techniques or equipment may be needed to transfer heavy
grit accumulations. If grit is transferred mechanically (by
flight, bucket, and screw conveyors), the equipment must be able
to start while the entire basin or channel is filled with grit.
If grit is transferred hydraulically, air agitation should be
used to loosen up the accumulated solids during the removal
operation. Hydraulic removal can be accomplished by eductors,
air-lift pumps, or special centrifugal pumps. When special
centrifugal (torque-flow or vortex) pumps are used, the grit
should be loosened up in the immediate vicinity of the pump
suction by a high-velocity water jet. More design information on
grit removal facilities is available (4,5).
15.2.1.2 Primary Sedimentation
If storage is provided in primary sedimentation, solids
processing systems can operate at rates independent of
the rate at which solids are removed from the wastewater.
This is especially useful for small plants which are not
manned continuously and for any size plant that experiences
large diurnal or seasonal fluctuations in settleable solids.
15-7
-------
Concentration of sludge removed from the primary sedimentation
tank may be controlled to some degree if the depth of the sludge
layer in the sludge removal hoppers is controlled. Hopper sides
should be sloped at least 60 degrees off the horizontal so
that solids can flow by gravity to the pump suction. Primary
sedimentation tank storage capacity should be sufficient to allow
suitably sized primary sludge pumps to remove the peak sludge
loadings. Otherwise the solids may interfere with the gathering
function of the longitudinal sludge collectors in rectangular
tanks or the main collector in circular clarifiers.
Efficient use of primary sedimentation storage requires the use
of a control timer, density, and blanket level instrumentation.
Ideally, all three devices can control primary sludge pump
operations. Blanket level sets the time when the pump starts;
control timers set the cyclical period when the pumps can
share the discharge piping (if necessary) and the minimum pump
operating period if the density of the pumped sludge is below
the required concentration; and density sets the time when the
pump shuts down. Chapter 17 provides more information on this
instrumentation.
More design information on primary sedimentation tank design
is available (4,5).
Design Example
The designer of a 7 . 5-MGD (0.33-m3/s) average design flow
wastewater treatment plant wishes to determine the available
sludge storage volume in three rectangular primary sedimentation
tanks, the tanks are designed to treat a peak wet weather
flow of 20 MGD (0.88 m3/s). Available storage will determine
the maximum time allowed between sludge pumping cycles and the
maximum capacity of the sludge pumps.
Tank design is based on conservative experience involving
overflow rates and mean velocities at average design flows. Each
tank is 110 feet (33.5 m) long and 19 feet (5.8 m) wide, with an
average sidewater depth of ten feet (3.05 m). Longitudinal
collectors operating continuously bring the settled sludge to the
head end of the tank, where it is conveyed to the sludge removal
hopper by cross-collectors. The sludge is then pumped from the
removal hopper on a timed cycle with density and blanket level
instrumentation. Cross collection channels and sludge removal
hoppers have been laid out to aid in the concentration, storage,
and removal of the collected sludge by providing steep side
slopes, ample depths, and short suction pipelines. Combined
storage volume of the cross collector channel and removal hopper
of the selected tank design is approximately 350 cubic feet
(9.9 m3) for each tank.
It is assumed that peak and wet weather flows will be of at
least eight hours duration and will have an average suspended
solids content of 200 mg/1. Primary sedimentation tank removal
15-8
-------
efficiency is assumed to be only 50 percent at peak wet weather
flow, down from 60 percent at average design flow, because of
higher overflow rate and higher mean velocity. Using these
assumptions, the solids collected in each primary sedimentation
tank during the storm can be calculated as follows:
J20_MGD) (200 rog/1) (0.50) (8.33 Ib/gal)
— - (3 tanks) (24 hr/day) - = 231 lb/hr(105 kg/hr)
Primary sludge solids concentration and wet bulk specific
gravity are assumed to be six percent and 1.07, respectively.
Using these assumptions, the volume produced in each tank can
be calculated as follows:
= 58 ft3/hr (1.6 m3/hr)
(0.06) (1.07) (62.4 lbs/ft3)
By dividing this production into the storage volume available,
the designer finds the maximum period of time between pump
cycles to be slightly greater than six hours.
The primary sludge piping to the digester is arranged so that
only one primary sludge pump can operate at a time. To assure
sufficient pumping capacity to handle the peak wet weather
sludge, it is necessary that each pump operate only one-third of
the time. Each pump,, therefore, must have the capacity to remove
all of the sludge stored during the six-hour cycle in two hours.
This capacity is calculated as follows:
,n nooK/ n- /. . = 21.6 gpm (1.36 1/s)
(0.06) (8.92 Ib/gal) (60 min/hr) ^
As an additional safety factor, to assure maximum reliability
and operational flexibility, this pumping rate is doubled and
rounded off to 50 gallons per minute (3.2 1/s). The pump
selected (a diaphragm pump, see Chapter 14) can be adjusted down
to 25 gpm (1.6 1/s) if higher flow rates are found to pull liquid
instead of concentrating solids.
15.2.1.3 Aeration Reactors and Secondary
Sedimentation
Solids are stored in aeration reactors and secondary sedimen-
tation tanks whenever there is an increase in the solids
concentration of the mixed liquor. This solids increase requires
the two processes to be operated as one, with the sedimentation
tank providing the two-phase concentration necessary to fully
15-9
-------
utilize the single-phase concentration storage capabilities of
the reactors. Reactors should be designed with the flexibility
to operate either in the plug flow, step feed, reaeration or
contact stabilization modes or any combination of these. Given a
fixed reactor size, maximum solids storage capability is provided
when the process operates in a combination of the reaeration and
contact stabilization modes. Often the ability to switch between
complete plug flow and partial reaeration modes allows the solids
to be removed from the hydraulic flow stream and prevents their
loss when that stream receives a shock loading. Operation in the
step feed mode also minimizes the solid loading rates to the
secondary sedimentation tanks. This solids storage flexibility
should be provided regardless of whether the source of aeration
comes from dissolved air or pure oxygen. Plug flow nitrifying
aeration systems, which are often required to retain solids
for two to three weeks, operate at maximum efficiency when
the hydraulic and organic loadings have the least diurnal
fluctuation. This uniformity is often achieved in smaller plants
through upstream flow equalization. Oxidation ditches are a
simple type of aeration reactor found in many small treatment
plants. More design information on aeration reactors and flow
equalization is available (4-8).
Secondary sedimentation tank two-phase concentration storage is
vital to the successful operation of an aeration system. Design
of secondary sed invent at ion facilities usually involves the
use of the solids flux theory, which is discussed briefly in
Chapter 5 and in detail in references 9 and 10. To take maximum
advantage of the concentration capabilities, secondary sedimenta-
tion tanks are usually from 150 to 200 percent deeper than
primary sedimentation tanks (14 to 20 feet [4.3 to 6.1 m]-)«
Blanket level instrumentation is commonly used to keep track
of sludge storage levels within the secondary sedimentation
tanks. Instrumentation for this determination is discussed in
Chapter 17. More design information on secondary sedimentation
tanks is available (4,5,7).
15.2.1.4 Imhoff and Community Septic Tanks
Both the Imhoff and the community septic tank were in use long
before most of the sludge treatment processes discussed in this
manual. For this reason, it is not surprising that their design
includes significant sludge storage capabilities. Imhoff tanks
are still in use in many of the older treatment plants, and
therefore, still provide those plants with extensive solids
storage capacity in what are essentially unheated low rate
anaerobic digesters (see Chapter 6). The storage capacity of
Imhoff and septic tanks is part of the empirical design criteria
for these facilities. While their future use may be limited
because of today's secondary treatment mandate, both processes
offer low cost primary treatment for upgrading small community
wastewater stabilization pond facilities. In Newman, California,
existing community septic tanks are being upgraded to provide
15-10
-------
primary treatment for a 0.76-MGD (33.3-1/s) complete treatment
plant with pond stabilization for secondary treatment and
overland flow for tertiary treatment (11). More information on
Imhoff and community septic tank design is available (4,12,13).
15.2.1.5 Wastewater Stabilization Ponds
Wastewater stabilization ponds are cost-effective because
of their ability to store solids. Pure aerobic wastewater
stabilization ponds provide only single-phase concentration type
storage, whereas the more commonly used anaerobic and facultative
ponds, can provide for long-term, two-phase concentration type
storage of removed settleable and created biological solids.
When debris is thoroughly removed from their influent, secondary
facultative ponds can store most of the wastewater solids from a
large secondary treatment plant for many years. In Sunnyvale,
California, secondary treatment facultative stabilization ponds
covering 425 acres (172 ha) have been receiving the majority of
sewage solids from a 15-MGD (657-1/s) plant for the past ten
years with no ill effects. Sunnyvale's tertiary treatment
facilities for algae and nitrogen removal return all solids
removed by dissolved air flotation and gravity filtration to the
ponds (13). Primary sludge is removed from the plant before the
primary effluent is discharged into the pond and anaerobically
stabilized in complete-mix digesters. Supernatant from these
digesters is discharged daily into the plant's influent. Most of
the solids eventually find their way to the facultative pond.
Bottom sludge is withdrawn every week or ten days from the
complete-mix digesters and discharged to anaerobic sludge
lagoons. The primary sedimentation effluent, and the uncaptured
and unrecycled contents of the supernatant merge with the
anaerobic bottom layers in the secondary treatment facultative
stabilization ponds.
Primary wastewater (usually anaerobic stabilization) ponds that
receive raw sewage must be drained and cleaned approximately
every five to ten years, depending on loadings. Secondary
wastewater (usually facultative stabilization) ponds that are
sufficiently deep (6 to 8 feet [1.8 - 2.4 m] ) and that receive
only those solids generated by biological activity probably
never require cleaning. More design information on wastewater
stabilization ponds is available (14).
15.2.2 Storage Within Wastewater Sludge
Treatment Processes
Table 15-1 lists wastewater sludge treatment processes that
provide some degree of solids storage. The following paragraphs
discuss how much of this storage capability can be used and how
its use can be made as effective as possible.
15-11
-------
15.2,2.1 Gravity Thickeners
Gravity thickeners separate liquid from primary and fixed-growth
biological secondary solids. In this sense, they function
like primary and secondary sedimentation facilities. Cool
temperatures and chemicals which retard septicity enable
gravity thickeners to store sludge for .several days. Equipment
precautions recommended for primary sedimentation facilities
apply to gravity thickeners. Using the same type of calculation
indicated in the primary sedimentation design examples, storage
capacity can be increased by providing extra depth. For more
design information on gravity thickeners see Chapter 5.
15.2.2.2 Anaerobic Digesters
Anaerobic digesters provide all three types of storage. Those
with floating covers have the flexibility to store about 20 to
25 percent of the digester's volume. The cover movement is used
to provide displacement storage. Fixed cover digesters must be
protected from excessive vacuum or pressure conditions whenever
an attempt is made to achieve displacement storage.
Secondary digesters can be used for two-phase concentration
storage by means of liquid-solids separation as long as they
are not treating stabilized biological suspended growth
(waste-activated) secondary sludge. Biological fixed growth
secondary sludge normally does not use secondary digester,
two-phase concentration storage. More and more treatment plants
are finding that the stabilization of waste-activated sludge has
a major impact on digester operation. Without waste-activated
sludge, the liquid-solids separation process in secondary
digesters can concentrate and store solids for considerable
periods of time. These time periods usually equal the time
required to fill the secondary digester at design flow rates and,
depending on the quality of acceptable supernatant, can often be
extended.
All digesters can be used to provide equalization storage.
Digesters may be used to equalize peak loadings and thereby make
downstream dewatering more cost-effective as the following
example illustrates.
Design Example
This example illustrates how the digester storage can be used
to "damp-out" solids surges and thus prevent overloading of
downstream dewatering units.
Consider a primary wastewater treatment plant with the flow
scheme and average loads depicted on Figure 15-1. Average
loading to the dewatering units is 103,313 pounds per day
(46,904 kg/d). Dewatering unit capacity is 200,000 pounds
15-12
-------
FEED
270,100 = RAW INFLUENT SOLIDS, ibs/day
GRIT CHAMBER
31,500
RECIRCULATED
SOLIDS
259r262
PRIMARY
SEDIMENTATION
65% CAPTURE
90,742
168,520
SUPERNATANT
(ZERO)
DIGESTERS
(39% SOLIDS
DESTRUCTION)
65,723
20,663
102,797
««
516
103,313
FILTER
(80% CAPTURE)
FILTRATE I f
T <
I
CENTRIFUGES
(80% CAPTURE)
I f
GRIT
PRIMARY
EFFLUENT
SOLIDS
CONVERTED
TO GAS AND
WATER
POLYMER
CENTRATE I
82.650 ^ SOLIDS TO
^ WASTE
Ibs/day - 0.454 kg/day
FIGURE 15-1
SOLIDS BALANCE AND FLOW DIAGRAM-DESIGN EXAMPLE
SINGLE-PHASE CONCENTRATION AND DISPLACEMENT STORAGE
15-13
-------
per day (90,800 kg/d); under average loading conditions, the
dewatering units are clearly not stressed. The treatment plant,
however, receives flow from a combined sanitary/storm sewer
network. During storms, hydraulic loadings increase dramatically
as a result of infiltration and inflow to the sewer system.
Plant solids loadings also increase sharply as the result of
solids being carried into the sewer by run-off and the scouring
of previously accumulated materials from the sewer system.
From historical records, the peak 5-day solids loading (average
load for^ the most heavily loaded five consecutive days) is
433,000 pounds per day (196,582 kg/d). This is 2.57 times
greater than the average digester load. If the storage available
upstream of the dewatering units is not utilized, dewatering
unit loading would also be 2.57 times the average value or
265,000 pounds per day (120,310 kg/d). The dewatering units
would therefore be overloaded. Overload can be prevented,
however, if digester storage is properly utilized. Solids can be
stored within the digester so that, during peak loading periods,
dewatering capacity is not exceeded. The accumulated solids can
be released after the storm has passed and the dewatering units
are no longer stressed.
the digesters by either of two
or in concert.
Solids may be stored in the
mechanisms, acting either singly
• The digester working volume is increased by allowing the
floating covers to rise (displacement storage).
• The digester feed is thickened to a greater degree than
previously. As a result, the solids concentration of the
digested material increases (single-phase concentration).
The following analysis examines how one of several possible
operating strategies can be implemented. It is assumed that
the system is at average conditions (see Figure 15-1) when a
large storm occurs and for the next five days average digester
loadings increase to 433,000 pounds per day (196,582 kg/d). At
the onset of the storm, the plant operator decides to ease a
potentially serious dewatering overload situation by (1) allowing
the floating covers to rise at the rate of one foot per day
(0.305 m/d) and (2) by thickening the raw sludge withdrawn from
the primary sedimentation basin from the normal five percent
concentration to seven percent concentration. The additional
thickening is accomplished by allowing sludge to accumulate
to greater depths in the primary sedimentation tanks cross-
collection trough and sludge hoppers. The intent of these two
operations is to maintain digested solids mass flow rates below
200,000 pounds per day (90,800 kd/d) to prevent dewatering unit
overload.
15-14
-------
The effects of these operations can be estimated from an
unsteady state analysis of digester operations. The basic
predictive equation is derived by an unsteady state mass balance:
1. Solids in - solids out - solids destroyed = solids
accumulated.
a. Solids in = QCi
b. Solids out = (Q-k)C
c. Solids destroyed = QCi X
d. Solids accumulated = — -.. •
dt
Where:
Q = digester feed rate, volume per time;
Ci = digester feed solids concentration, mass per volume;
C = digester sludge solids concentration, mass per
volume;
k = rate of liquid accumulation in the digesters due to
rise of floating covers, volume per time;
X = fraction of digester feed destroyed by digestion,
dimens ionless;
V = digester liquid volume;
t = time.
2. Summing the terms:
QCi - (Q-k)C - QCiX =
3. The right-hand side of the above equation can be further
developed:
- V * C - V + CK
4. Simplifying:
QCi (1-X) - QC = V |
15-15
-------
5. Make the simplifying assumptions that digester feed flow,
feed concentration, and liquid volume are constant for the
period t. The above equation is integrated and solved
for C.
C = Ci(l-X) - [Ci(l-X)-C0] exp - £
15-1
Equation 15-1 predicts digested sludge solids concen-
tration at any time beyond initiation of the operating
strategy. Co is defined as digested sludge concentration
at the time the operating strategy is put into operation.
Note that the product of digested sludge concentration
(C) and digester effluent liquid flow (Q-k) is the load
which the dewatering units must process.
TABLE 15-2
CALCULATIONS FOR DIGESTER EFFLUENT MASS
FLOW RATE FROM EQUATION 15-1
Digester
volume
Time after
start of
Operating strategy days
A. Floating cover rise = 0
1 ft/day; digester feed 0*
thickened to 7 percent 1
2
3
4
5
remains at 5 percent 1
4
5
C. Floating covers are not Q~
allowed to rise; di- 0
gester feed thickened 1
to 7 percent 2
3
4
5
1 Ib/day = 0.454 kg/d
1 gpd = 0.00378 m3/d
1 gal = 0.00378 m3
Dige
Ib/day percent
168
433
433
43:
433
433
433
168
433
433
433
433
168
433
433
433
433
433
433
,520 5
,000 7
,000 7
,000 7
,000 7
,000 7
,000 7
,520 5
,000 5
,000 5
,000 5
,000 5
5
,520
,000
,000
,000
,000
,000
,000
feed
gpd
396,051
72 ,875
72 ,875
72 ,S75
72 ,875
72 ,875
72 ,875
396.051
1,017,626
1,017,626
1,017,626
1,017,626
1,017,626
396,051
26,875
26,875
26,875
26,875
26,875
26,875
due to rise
covers , grid
0
,j
176 2 ].
176
176
176
176
176
176,
176
176
2 >
2 3
2 3
2 •
A
0
24 .
24 •
24 ^
>:•
i:
C=
£
I,:
0
il
Digester
5.97 x
5.97 x
6.05 X
6.23 x
6.41 x
6.58 x
6.76 X
5.97 x
5.97 x
6.05 X
6.53 x
6.76 x
5.97 x
5.97 x
5.97 x
5.97 x
5.97 x
5.97 x
5.97 x
'«?
iob
10"
10°
10o
10o
10°
wl
106
$
10
in6
10b
10*
10G
10
10
wf:
wl
iob
Fractional
0. 39
0.20
0.20
0.20
0.20
0.20
0.20
0. 39
0.20
0.20
0.20
0.20
0.20
0. 39
0.20
0.20
0.20
0.20
0.20
0.20
Digester
effluent
gpd
396,051
550,632
550,632
550,632
590,632
590,632
550,632
396,051
841, 383
841,383
841, 383
841,383
396,051
726,875
726,875
726,875
726,875
726,875
726,875
Digested
sludge solids
percent
3.05
3.05
3.38
3.58
i.78
3.96
4.11
3.05
3. as
3.20
3.49
3.56
3.05
3.05
3.34
3.6O
3.83
4.03
4.21
Dewatering
unit
Ib/day
102,797
142,919
156,429
167,772
177,373
185,561
192,594
102, 97
218, 85
228, 03
249, 40
254, 89
102, 97
188, 64
206, 45
222, 55
236, 28
249, 78
260, 88
Calculations related to the operating strategy just described are
summarized in Table 15-2 part A. The digested solids mass flow
rates are calculated just before the storm (t = 0~), immediately
after the storm begins (t = 0+) and for each of the next
five consecutive days. It is assumed digester loading increases
in one step from 168,520 pounds per day (76,508 kg/d) to
433,000 pounds per day (196,582 kg/d) at t = 0. Digested sludge
liquid volume at the beginning of the storm is 5.97 x 10^ gallons
(22,600 m3). Each 1 foot (0.305 m) of cover rise increases tank
volume by 176,243 gallons (667 m3) . Solids destruction within
the digesters is assumed to be 39 percent ( X = 0.39) during
15-16
-------
average conditions, dropping to 20 percent ( X = 0.20) during
the storm due to decreased digester retention time. The calcula-
tion shows that dewatering capacity (200,000 pounds per day
[90,800 kg/d] ) is not exceeded during the storm, thus the
operating strategy has been successful. Calculations for two
other strategies which were not successful are also included.
The results are shown graphically on Figure 15-2.
300,000 i-
BJ
~a
a
•3 200,000
>
TJ
S
LLI"
Q
LU
100,000 -
Z
ec
LJJ
H
3
LLI
Q
STRATEGY 8.
FEED SOLIDS CONCENTRATION
5%; ALLOW COVERS TO RISE 1 ft/day
STRATEGY C.
FEED SOLIDS CONCENTRATION
7%; COVERS ARE STATIONARY
CAPACITY OF
DEWATERING UNITS
STRATEGY A.
FEED SOLIDS CONCENTRATION
7%; ALLOW COVERS TO RISE 1 ft/dav
1 ft/day = 0.305 iti/day
0 \ 1 3
TIME AFTER START OF STORM, days
FIGURE 15-2
EFFECT OF VARIOUS OPERATING STRATEGIES
ON DEWATERING UNIT FEED RATES
15-17
-------
15.2.2.3 Aerobic Digesters
To use an aerobic digester for two-phase concentration type
storage, the normally highly agitated contents must be made
quiescent and the solids made to settle from the liquor before
the whole mass becomes anaerobic and starts to decompose and
create nuisance odors. Chemical treatment can facilitate solids
settling. Without successful decanting, only single-phase
concentration type storage and displacement type storage can be
used by aerobic digesters. When displacement type storage is
used with a fixed surface area, the liquid surface must rise or
fall. Under such conditions, the aeration and mixing source must
automatically adjust to such changes. Floating mechanical units
and fixed-bottom mounted diffusers are both adaptable to these
requirements; fixed mechanical aerators are not. Long-term
storage in aerobic digesters will have a relatively low capital
cost and a very high operating (energy) cost. Often, evaporation
can account for significant concentration of the stored solids.
As long as the solids remain aerobic throughout the digester,
the odor impact of such storage is very minimal. For more
information on aerobic digesters, see Chapter 6.
15.2.2.4 Composting
Composting is one of the two wastewater solids processes with
storage capabilities that are effective for long-term storage.
Once the wastewater solids have been stabilized by composting,
the curing step can be extended as long as storage is required.
This curing step usually involves nothing more than the placing
of the composted material in unconfined stockpiles exposed to the
atmosphere. As long as there are no site restrictions, this
method of storage can be very economical, for it is actually just
another use of time needed for curing and removing the material
to its point of final disposal. For more design information on
composting, see Chapter 12.
15.2.2.5 Drying Beds
Drying beds are used extensively by many smaller plants in
conjunction with anaerobic and aerobic digestion. They are
operated on a fill and draw basis and are often used to provide
two-phase concentration and displacement type storage between
production and disposal. To assure adequate storage capability,
the designer should allow for up to 50 percent excess drying bed
area. More design information on sludge drying beds can be found
in Chapter 9.
15.3 Dedicated Storage Facilities
When solids storage within wastewater treatment processes and
sludge solids treatment processes cannot provide the operational
flexibility necessary to maintain cost-effective solids treatment
15-18
-------
and disposal, these within-process storage capabilities must
be augmented with special dedicated storage facilities. These
dedicated storage facilities can provide storage for sludge in
either the liquid or dewatered state, and may, depending on
design considerations and upstream treatment, be utilized for any
of the three detention times listed in Table 15-1.
15.3.1 Facilities Provided Primarily for
Storage of Liquid Sludge
Usually, dedicated liquid storage facilities consist of one
of the three types listed in Table 15-1. Although listed as
primarily for storage of liquid sludge, any of these facilities
that are used for anything other than equalizing storage (3 to
4 days) will also provide some degree of solids treatment.
Holding tanks, without air agitation, and facultative sludge
lagoons usually continue anaerobic stabilization. Holding
tanks, with air agitation, and aeration basins continue aerobic
stabilization. As these are side benefits to the main design
functions of these facilities, they have been ignored for the
purpose of these classifications. However, if the storage is for
a long term, then the additional treatment afforded certainly
must be taken into account in setting final disposal criteria.
15.3.1.1 Holding Tanks
Holding tanks are commonly provided as an integral part of most
conditioning processes and many stabilization processes. Holding
tanks may be used for blending different materials as well as for
equalizing storage, thereby assuring that the downstream sludge
treatment process is uniformly loaded, both in quality and
quantity. Holding tanks also often provide the decanting
facilities for sludge treatment processes, such as thermal
conditioning, which create products that support two-phase
concentration.
Holding tanks that are to be used for blending must be maintained
in a homogeneous condition. Such tanks can thus provide only
single-phase concentration type storage or displacement type
storage. Usually such tanks are relatively small, with detention
times measured in hours instead of days. Most of the storage,
therefore, is provided by volume adjustments. Holding tanks that
involve blending and provide equalizing storage are usually
limited to a batch, or a near-batch, type of operation or
continuous level adjustments. Tank contents can be mixed by
mechanical impellers, hydraulic recirculation, or gas agitation.
Each method's applicability may be restricted by the type of
material requiring the blending. For example, mechanical
impellers are not applicable when unground sludge is being
stored. The use of gas agitation and recirculation mixing is
normally only limited by the volume which must be blended.
15-19
-------
If the holding tank is located downstream from a sludge treatment
process, special precautions may be required. For example, if
downstream from anaerobic digestion and planned for more than a
few hours of storage, the blending tank should be designed with a
cover and be equipped to collect and remove combustible digester
gas. If downstream from chlorine stabilization, it should be
designed to function in a very low pH (acid) environment.
Whatever its function, however, the holding tank must be designed
to eliminate the production of malodorous gaseous discharges.
This elimination is made especially difficult when the holding
tank must provide equalizing storage and operate on a batch
basis. Unless the solids supplied to the holding tank are
completely stabilized (a condition seldom encountered with
wastewater sludge), the tank's use for extended periods of
storage will result in the creation of nuisance odors.
Even short periods of storage of unstabilized primary and
secondary sludges in a holding tank can produce nuisance odors if
no form of temporary inhibiting treatment has been applied.
Decant tanks following thermal conditioning often create major
odor problems. There are many ways of dealing with the odorous
gases created by these holding tanks—for example, by passing the
gases back through the aeration system, activated carbon filters,
chemical scrubbers, and incinerators. The best design solution,
however, is to minimize their creation.
Design Examples
The Sacramento California Regional Wastewater Treatment Plant,
now under construction, is to be provided with a holding and/or
blending tank that will be capable of receiving the daily
anaerobically digested sludge discharged from nine complete-mix
digesters (15). This digested sludge discharge will vary from
0.56 to 0.94 MGD per day (24.5 to 41.2 1/s ) over the next
20 years. The blending tank will be 110 feet (33.5 ra) in
diameter and will have a 38.5-foot (11.7 m) sidewater depth. It
will be provided with a Downes type floating cover that will have
a vertical movement of at least 14 feet (4.3 m) . This floating
cover movement will allow the blending tank to mix the entire
daily discharge from all the nine digesters prior to discharging
its daily accumulation to downstream facultative sludge lagoons.
This blending tank will provide a complete separation between the
operational control of the complete-mix tank and the controlled
feeding of the 20 lagoons. Total solids retention time of the
blended sludges will be at least three days, and approximately
one-third of the liquid contents of the blending tank will be
displaced each day. Except for the provision for the extra
floating cover travel and the use of bottorn mounted gas
diffusers, the blending tank will have the same design as the
four complete-mix tanks now under construction. This method of
blending and containment will minimize the release of odorous
gases and maintain a safe control on the production and use of
the digester gas during the blending operation. Figure 15-3
shows a sectional sketch of this proposed blending digester. In
15-20
-------
Aliso, California, two 26,000-gallon blending tanks are being
proposed to blend and equalize unstabilized sludge flows from
several sources at the Aliso Regional Solids Stabilization
Facility (16). These tanks are being provided with hydraulic
mixing and fixed covers. The gas cap above the varying liquid
level within the fixed covered tanks will be maintained at a
constant pressure by an intertie with the low pressure digester
gas system. This intertie will eliminate the need for special
odor control equipment, minimize the danger from the possible
production of an explosive gas-air mixture, and negate the need
for some highly complicated pressure control system to protect
against a rapid drawdown that might pull a vacuum or air into the
blending tank. Figure 15-4 shows a sectional elevation of this
raw sludge blending tank.
WATEB SURFACE IW.SVl
WO"
GAS DOME
TYPE
FLO AT INC COVER
6" PMSSU PS-VACUUM
FtEUif ANO FLAME
THAT
14™ AND W" 0(A
GAS COLLECTION ANP
SLUDGE SUWLY
r PIPELINES
SLOPf TO GAS DOME
HOSES FQfi
COVER TflAVEL
,_J1 ,„„.
'7—^
j| l_igjgT !'°" ABOVE ipTTQM QF GA$ DOME
STABIOTY_CONCR:ETE^ BALLA5TJjlMG_
__^ 1W DIA
•COVER SUPPORT CORBEL |TYP OF "t*\
\
PROVIDE «S*
SPIRAL
GUIDES
1 ft = 0,305 m
1 in - 2.54 cm
8" OIA Cl HC SLUDGE
- SUCTION AND
f SUPPLY PIPING
(TYPICAL OF 4J
L—
GAS DIFFUSES
ASSEMBLY
(TYP Of
DlflESIER
PVMP STATION
FIGURE 15-3
PROPOSED DESIGN FOR BLENDING DIGESTER—SACRAMENTO
REGIONAL WASTEWATER TREATMENT PLANT
General Comm^enj:s_
While very little specific design guidance is provided in
the literature for sludge holding tanks, the major issue that
must be dealt with is the same as for most sludge treatment
processes—material management. Wastewater sludge can contain
almost anything. If a holding tank design is to incorporate
mechanical mixing, which can be incapacitated by stringy
15-21
-------
material, the designer must make sure that material is either
removed or cut up before reaching the blending tank. Likewise,
hydraulic mixing pumps must be of the non-clog type or the
material reduced in particle size by grinding so that it can pass
through the minimum clearances of the type of pump used.
ULTRASONIC LEVEL
TRANSMITTER
6" DIA DIGESTED
SLUDGE
24" OfA M,H.
6" PRESSURE VACUUM
RELIEF AND FLAME TRAP
6" FLAME TRAP
6" DIA
LOW PRESSURE
SLUDGE GAS
CONNECTION
TO DIGESTERS
MAX W.L. E LEV 214.3
42 DIA
ACCESS M.H.
ELEV 209,5
x GROUND E- =v 207,5
EQUIPMENT PIT
EQUALIZING
CIRCULATING
PUMP
6" DIA CIRC
PUMP
DISCHARGE
MIN W.L, ELEV 196-0
DISCHARGE NOZZLE
TO ASSURE MIXING
t ELEV 195.5
6" DIA
RAW
SLUDGE
SUPPLY
DIG ESTER SUPPLY
PUMP (T'YP OF 5
FOR TWO TANKS}
S6" DIACiRC PUMP
SUCTION
sftSiPatfSas?
1
iNV ELEV 191.5
i" DIA DIGESTER
SUPPLY SUCTION
1 ft = 0.305 m
1 in •» 2.56 ui*
6" DIA SUCTION
TO OTHER
*•
DIGESTER
SUPPLY
PUWIPS
AND DRAIN
SUMP
TABLE 15-4
26,000 GALLON SLUDGE EQUALIZATION TANK (TYPICAL OF TWO)
ALISO SOLIDS STABILIZATION FACILITY
15-22
-------
The other major design problem involves the control of odors that
are so often an integral part of any type of sludge holding tank.
The Sacramento and Aliso holding tank design examples indicate
two very successful means of . dealing with such odors (that is,
containing and incorporating them with the low pressure
digester gas system). In many locations stabilized material is
held within the holding tank only a few hours. Under these
circumstances, their design depends on minimum odor generation, a
reasonable assumption given the short retention period. Often
decant tanks and conditioning blending tanks cannot depend on
either of these methods of odor control. The designer should be
very aware that when such a situation exists it will be expected
that odors will be confined and treated to the point where their
discharge ceases to create a nuisance. Odor control is a very
complicated subject. Designers are referred to a Manual of
Practice soon to be released by a Joint Committee of the ASCE and
Water Pollution Control Federation.
15.3.1.2 Facultative Sludge Lagoons
Introduction
Sludge lagoons have been used for years to store wastewater
solids. Unfortunately, most of this use has been with complete
disregard to the aesthetic impact on the surrounding environment.
Such misuse has become so widespread that just the use of
the term "sludge lagoon" is often enough to eliminate their
consideration in present-day alternatives analyses.
Recent studies in Sacramento, California, based on the successful
operation of facultative sludge lagoons in Auckland, New Zealand,
indicate that sludge lagoons can be designed to be environmen-
tally acceptable and still remain extremely cost-effective
(17,18). The facility studied in Sacramento provides storage for
at least five years of sludge production. The sludge stored in
the facultative sludge lagoon continues to stabilize without
creating an odor level unacceptable to the surrounding neighbor-
hood. Table 15-3 lists the advantages and limitations of using
facultative sludge lagoons for long-term storage.
Facultative sludge lagoons (FSLs) are designed to maintain
an aerobic surface layer free of scum or membrane-type film
build-up. The aerobic layer is maintained by keeping the annual
organic loading to the lagoon at or below a critical area loading
rate and by using surface mixers to provide agitation and mixing
of the aerobic surface layer. The aerobic surface layer of the
FSLs is usually from one to three feet (0.30 to 0.91 m) in depth
and supports a very dense population of between 50 x 10-^ and
6 x 10" organisms/ml of algae (usually Chorella ) . Dissolved
oxygen is supplied to this layer by algal photosynthesis, by
direct surface transfer from the atmosphere, and by the surface
mixers. The oxygen is used by the bacteria in the aerobic
15-23
-------
degradation of colloidal and soluble organic matter in the
digested sludge liquor, while the digested sludge solids settle
the bottom of the basins and continue their anaerobic
Sludge liquor or supernatant is periodically
to
decomposition.
returned to the plant's liquid process stream.
TABLE 15-3
ADVANTAGES AND LIMITATIONS OF USING FACULTATIVE SLUDGE
LAGOONS FOR LONG-TERM STORAGE
Advantages
Limitations
Provides long-term storage with
acceptable environmental impacts
(odor and groundwater contamination
risks are minimized).
Continues anaerobic stabilization, with up
to 45 percent VS reduction in first year.
Decanting ability assures minimum solids
recycle with supernatant (usually less
than 500 mg/1) and maximum concentration
for storage and efficient harvesting
(>6 percent solids) starting with digested
sludge of <2 percent solids.
Long-term liquid storage is one of few
natural (no external energy input) means
of reducing pathogen content of sludges.
Energy and operational effort requirements
are very minimum.
Once established, buffering capacity is
almost impossible to upset.
Allows for all tributary digesters to
operate as primary complete-mix units
(one blending unit may be required for
large installations).
Provides environmentally acceptable place
for disposal of digester contents during
periodic cleaning operations.
Sludge harvesting is completely independent
from sludge production.
Can only be used following anaerobic
stabilization. If acid phase of
digestion takes place in lagoons they
will stink.
Large acreages require special odor
mitigation measures.
Requires large areas of land, for
example, 15 to 20 gross acres (6 to
8 ha) for 10 MGD, (438 1/s) 200
gross acres (80 ha) for 136 MGD
(6,000 1) carbonaceous activated
sludge plants.
Must be protected from flooding.
Supernatant will contain 300-600 mg/1
of TKN, mostly ammonia.
Magnesium ammonia phosphate (struvite)
deposition requires special supernat-
ant design.
The nutrient and carbon dioxide released in both the aerobic and
anaerobic degradation of the remaining organic matter within
the digested sludge are, in turn, used by the algae in the
cyclic-symbiotic relationship. This vigorous relationship
maintains the pH of the FSL surface layer between 7.5 and 8.5,
which effectively minimizes any hydrogen sulfide (r^S) release
and is believed to be one of the major keys to the successful
operation of this sludge storage process.
Facultative sludge lagoons must operate in conjunction with
anaerobic digesters. They cannot function properly (without
major environmental impacts) when supplied with either
15-24
-------
unstabilized or aerobically digested sludge. If the acid phase
of anaerobic stabilization becomes predominant, the lagoons will
stink. Figure 15-5 provides a schematic representation of the
reactions in a typical FSL.
in
§1
ui
LU
H
in
D
1
o
II
UJ Q
^ a.
<
V.
FIGURE 15-5
SCHEMATIC REPRESENTATION OF A FACULTATIVE
SLUDGE LAGOON (FSL)
Current Status
Facultative sludge lagoons were installed initially in 1960
in the Auckland, New Zealand, Manukau sewage treatment plant
to provide for the storage and disposal of that plant's
anaerobically digested primary sludge. Although lagoons were
installed at Dublin-San Ramon, California in 1965, Medford,
Oregon in 1971, and other sites in the United States since 1960
in an attempt to duplicate the successful Auckland installation,
it was not until 1974 that the area loading became the critical
criterion for their success. Studies at Sacramento since 1974,
with approximately 40 acres (16.2 ha) of FSLs, have determined
that the standard annual loading rate can be doubled during the
15-25
-------
warm, long, sunny days of July, August, and September. Reduced
algae activity during the colder winter months indicates that the
standard loading rate should not be exceeded.
Since 1974, additional FSLs have been placed in service at
Corvallis, Oregon - 4.5 acres (1.82 ha) and Salinas, California -
6.0 acres (2.43 ha). Other FSLs are being built or are under
design for Eugene-Springfield, Oregon - 25 acres, (10.1 ha);
Red Bluff, California - 0.93 acres (0.38 ha); Sacramento,
California - 84 acres (34 ha); Flagstaff, Arizona 7.3 acres
(2.95 ha); and Colorado Springs, Colorado - 60 acres (24.3 ha).
Successful operation was experienced this past winter under
freezing conditions at Corvallis, Oregon. Experience to date
indicates the design criteria established at Sacramento are
applicable under other climatic conditions.
Design Criteria
Design considerations for the FSLs include the area loading
rate, surface agitation requirements, dimensional and layout
limitations, and physical factors. All have been developed
during the studies conducted over the past five years at the
Sacramento lagoons.
Area Loading Rate. To maintain an aerobic top layer, the
annual organic loading rate to that FSL must be at or below
20 pounds of volatile solids (VS) per 1,000 square feet per day
(1.0 t VS/ha-d). Lagoons have been found to be capable of
receiving the equivalent of the daily organic loading rate every
second, third, or fourth day without experiencing any upset.
That is, lagoons have assimilated up to four times normal daily
loadings as long as they have had three days of rest between
loadings. Loadings as high as 40 pounds VS per 1,000 square feet
per day (1.0 t VS/ha-d) have been successfully assimilated for
several months during the warm summer and fall. Experiments on
small basins loaded to failure indicate that peak loadings up to
90 pounds VS per 1,000 square feet per day (4.4 t VS/ha-d) can be
tolerated during the summer and fall as long as they do not occur
for more than one week.
Surface Agitation Requirements. Experiments on FSLs that were
continuously loaded at the standard rate indicate FSLs cannot
function in an environmentally acceptable manner without daily
operation of surface agitation equipment. Observations indicate
the brush-type mixer is required to breakup the surface film that
forms during the feeding of the lagoon. If this film is not
dissipated, a major source of oxygen transfer to the surface
layer is eliminated. FSLs with surface areas of from 4 to
7 acres (1.6 to 2.8 ha) require the operation of two surface
mixers from 6 to 12 hours per day to successfully maintain scum-
free surface conditions. All of the successful installations
to date have used brush-type floating surface mixers to achieve
the necessary surface agitation. Figure 15-6 shows a typical
brush-type surface mixer. Recent experiments indicate that
15-26
-------
two brush-type mixers with 8-foot-long (2.4-rn) rotors turning at
approximately 70 rpm and driven by 15 horsepower (11.2 kW) motors
are required for a 4 to 7 acre (1.6 to 2.8 ha) lagoon. The
mixers need to operate 12 hours per day. Lagoons of much less
(1.62 ha) should be able to achieve the same results
with 6-foot (1.8-m) long rotors and 5-horsepower
Operation time is expected to be about the same
per day. FSLs of larger than 7 acres (2.8 ha)
to be cost-effective because of the need to
of service during sludge removal operations.
than 4 acres
with two mixers
(3.7 kW) motors.
number of hours
have not been found
take the lagoons out
FIGURE 15-6
TYPICAL BRUSH-TYPE SURFACE MIXER,
SACRAMENTO, CALIFORNIA
Brush type mixers have been used to limit the agitation to the
surface layer of the FSLs. So far this has been an acceptable
application; however, there is some question as to their
applicability for very cold climates. Several submerged pump-
type floating aerators have been reviewed, and they could be
15-27
-------
adapted to provide the necessary surface agitation if the
brush-type could not function under severe freezing conditions.
Two mixers are used per FSL to assure maximum scum break-up in
those areas of the lagoon where the prevailing wind deposits the
daily loading of scum. The agitation and mixing action of the
two mixers located at opposite ends or sides of the lagoon also
act to maintain equal distribution of the anaerobic solids.
Dimensional and Layout Limitations. FSL size is usually
determined by the number of lagoons required to assure adequate
surface area, while sludge is removed from a lagoon. If the
removed sludge is to be reused, several spare lagoons are
required to keep full lagoons out of service for the 2- to 3-year
pathogen die-off period (3). The maximum area for a single
lagoon area is somewhat arbitrary but is based on the most
practical size for loading, surface agitation, mixing, and
removal requirements. Large, 4 to 7 acre (1.6-2.8 ha) individual
lagoons would be applicable only to plants with over 70 acres
(28 ha) of FSLs. FSLs as small as 150 feet (45.7 m) on a side
have been operated successfully.
Lagoon depth was established by the practical limitation of
commercially available dredges with a proven capability of
removing wastewater solids from beneath liquid surfaces.
Equipment that meets this requirement is available to extract
sludge from FSLs up to 11-1/2 and 15 feet (3.5 and 4.7 m) of
depth. For plants <10 MGD (440 1/s) , the 11-1/2 foot (3.5 m)
depth dredge should be adequate. For plants >10 MGD (440 1/s)
the 15-foot (4.7-m) depth should be used to provide additional
storage flexibility. If surface agitation must be maintained
by submerged pump type aerators, it may be necessary to employ
the deepest lagoon possible to assure adequate separation between
the aerobic zone and the anaerobic settling zone of the FSL.
Contractors can supply dredge equipment for a lagoon, either with
or without the manpower to operate it.
FSLs are usually best designed to have a long and a short
dimension with the shortest dimension oriented parallel to the
direction of the maximum prevailing wind. The longer side is
made conducive to efficient dredge operation, while the short
side's parallel orientation to the prevailing wind direction
helps to minimize wave erosion on the surrounding levees.
Figure 15-7 is a typical FSL layout, while Figure 15-8 is a
typical FSL cross section.
When the area of FSLs exceeds 40 acres (16.2 ha), the potential
cumulative effect of large odor emission areas to the vicinity
must be considered. Figure 15-9 shows the layout for the
124 acres (50.2 ha) of Sacramento FSLs that were sited on the
basis of the least odor risk to surrounding areas.
Work at Sacramento has also determined that batteries of FSLs
totalling 50 to 60 acres (20 to 24 ha) are about the maximum size
for most effectively reducing the transport of odors.
15-28
-------
PREVAILING WIND DIRECTION
! '' 1
\ I
/OVERFLOW r
I * AUTOMAT 1C /
/ / PHNTRDI VAl UP -/
/ - J IT
j- -y |
t
- SLUDGE REMOVAL
-------
LAYOUT FOR 124 ACRES OF FSLs—SACRAMENTO
REGIONAL WASTEWATER TREATMENT PLANT
15-30
-------
located upstream from the prevailing winds to minimize scum
build-up in its vicinity. FSL supernatant will precipitate
magnesium ammonia phosphate (struvite) on any rough surface
that is not completely submerged. It has also been found to
precipitate inside cavitating pumps. This crystalline material
can completely clog cast iron fittings and pump valves when the
surface goes through a fill-and-draw cycle or when its operation
results in the presence of diffused air. The most practical
approach to successful elimination of this problem has been to
use PVC piping throughout the FSL supernatant process and to
design the process for gravity return to the plant influent, with
a minimum of critical depth conditions. If pumping is required,
submerged slow-speed non-clog centrifugal pumps with low suction
and discharge velocities (to minimize cavitation) will be the
most trouble-free. All equipment that cannot be PVC or other
smooth non-metallic material should be coated with a smooth,
impervious surface.
Two digested sludge feed lines are provided, each with its
own automatic valve, to assure adequate distribution of solids
over the whole volume of the FSL. Surface mixers are downstream
of the prevailing winds. The harvested sludge dredge hookup is
centrally located. Lagoon dike slopes are conservative--three
horizontal to one vertical—with adequate rip-rap provided in the
working zone of the surface level. Sufficient freeboard is
provided to protect against any conceivable overtopping of the
dikes. Digested sludge feed pipelines are located directly
below the bottom of the lagoons, with the inlet surrounded by a
protective concrete surface. All piping within the basin is
out of the way of future dredging operations.
Many of the physical considerations for the basins have
been required by the State Dam Safety Agency. Larger FSLs
most probably will come under some regulatory agency whose
responsibility involves seeing that earthen structures used to
confine large quantities of liquid a significant height above
the existing ground surface are safe. It is wise to check early
to ascertain what, where, and how these agencies will be involved
in FSL design.
Operational Cqnisidera,t_ions_
Operational considerations can be divided into three categories:
the loading or placement of sludge into the FSLs; their routine
operation; and the removal of their solids. Considerations
listed below were developed during the five years of study on the
Sacramento lagoons.
§^££^.^H£_aJl^_t2a.^AD.S• FSLs should be initially filled with
eTFriTervtfi I d~eaTry, "that effluent should then have about three to
six weeks for development of an aerobic surface layer prior to
the introduction of digested sludge. All FSLs should be
loaded daily, with the loading distributed equally between FSLs.
Loadings should be held below 20 pounds VS per 1,000 square feet
15-31
-------
per day (1.0 t VS/ha-d) on an average annual basis. As indicated
earlier, considerable flexibility does exist. Loads can vary
from day to day, and batch or intermittent loading of once every
four days or less is acceptable. Shock loadings, such as with
digester cleanings, should be distributed to all operating FSLs
in proportion to the quantity of sludge inventory they possess.
FSLs should be loaded during periods of favorable atmospheric
conditions, particularly just above ground surface, to maximize
odor dispersion. The fixed and volatile sludge solids loadings
to the FSLs and their volatile contents should be monitored
quarterly.
Daily Routine. Surface mixers should operate for a period
of between 6 and 12 hours. Operation should not coincide with
FSL loading and should always be during the hours of minimum
human exposure (usually midnight to 5 a.m.) and during periods of
favorable atmospheric conditions. FSL supernatant return to the
wastewater treatment process should be regulated to minimize
shock loadings of high ammonia. Supernatant return flows should
be monitored so that their potential impact on the liquid
treatment process can be discerned. The sludge blanket in a
lagoon should not be allowed to rise higher than two feet below
the operating water surface.
SJ1u_d_g_e__R_emo_v_a_!. FSLs that are to be emptied of accumulated
solids should be removed from routine operation at least 30 days
prior to the removal of any solids. Pathogen safe reuse requires
removal from operations for two to three years (3). Sludge
removal should be limited to those FSLs that are concentrating
the sludge solids to six to eight percent. During FSL sludge
removal operations, the water surface level should not be allowed
to drop more than 12 to 18 inches (30 to 46 cm) below its normal
operating level.
Energy Impacts
Energy requirements of FSLs are relatively small because FSLs use
solar energy. The sun supplies the needed energy for the algal
photosynthesis. In turn, the algal cells supply the dissolved
oxygen to support the aerobic bacterial action in the surface
layer. The only outside power used in normal FSL operation is for
surface agitation, supernatant pumping and treatment, and the
supply and removal of the sludge. For the 124-acre (50.2 ha)
Sacramento installation, it was recently calculated that these
energy requirements could equal 31,700' x 10^ Btu per year
(33,400 GJ/yr) when the FSLs became fully loaded in 1990 (19).
As loading is based on area, the energy impact of FSLs will be
255 x 10^ Btu/yr/acre (670 GJ/yr/ha). With maximum odor source
control and transport reduction measures, this energy use will
increase to 294 x 10^ Btu per year per acre (765 GJ/yr/ha).
As no chemicals or major structures are involved, all FSL energy
impacts are direct. There are no secondary impacts.
15-32
-------
Actual Performance Data
The following figures and tables report the actual performance of
the eight FSLs in operation at the Sacramento Central Wastewater
Treatment Plant. Although the plant is designed as a 24-MGD
(I.l-m3/s) carbonaceous activated sludge secondary wastewater
treatment plant -with anaerobic digestion for solids
stabilization, it treats the total solids from three upstream
secondary treatment plants, the total annual flow of which is
considerably greater than its own. Solids from those upstream
plants are transported to the Central plant by its tributary
sewer collection system. The Central plant also receives a
substantial solids loading (up to 35 percent daily surcharge)
from seasonal canning operations. Table 15-4 indicates the FSL
loadings for the four years from 1975 through 1978.
TABLE 15-4
SACRAMENTO CENTRAL WASTEWATER TREATMENT
PLANT VOLATILE REDUCTIONS, DIGESTED
SLUDGE QUANTITIES AND FSL AREA LOADINGS
Year
Digester volatile
reduction, percent
Digested solids to FSLs FSL loading
Annual average
.total solids, Percent Percent
103 lb/daya volatile solids
Annual average
Ib volatile solids,
103 sq ft/daya
1975
1976
1977
1978
52
50
51
45
44 .1
35.9
44.0
52.7
63
67
68
66
1.7
1.6
1.6
1.6
22.5
15.9
17.1
20.7
Dry weight.
Source: Treatment plant records.
Ib = 0.4536 kg.
sq ft = 0.0929 sq m.
Figure 15-10 summarizes typical surface layer data for four of
the FSLs for July 1977 through June 1978. Unfortunately, some
turbidity and algae count data are missing, but the seasonal
trend is quite apparent. Table 15-5 summarizes the FSL's design
data and provides the necessary background to understand the FSL
solids inventory in Table 15-6. Data from Table 15-6 was used to
calculate a volatile solids reduction of 42 percent. Solids
profiles are taken quarterly in all FSLs.
Recycled FSL supernatant quality for 1978 is given in Table 15-7,
and complete mineral, 'heavy metals, and chlorinated hydrocarbon
data for digested, FSL, and harvested solids for 1977 is provided
in Table 15-8. While the specific conductance in the supernatant
remains high (2,500 to 4,300 mhos/cm), the supernatant contains
very little of the heavy metals. Rainfall increases the quantity
15-33
-------
of supernatant and decreases its strength. Winter-specific
conductivity always dropped in Sacramento following significant
rainfall. The only solution to this problem would seem to be to
reduce the heavy metals concentrations in the unstabilized
sludge.
^f:
r
j A 3 0 N 0 J * tt * M ;
FIGURE 15-10
SACRAMENTO CENTRAL WASTEWATER TREATMENT
PLANT SURFACE LAYER MONITORING DATA
FOR FSLs 5 TO 8
15-34
-------
Public Health and Environmental Impact
FSLs have been found to have the following insignificant
environmental impagts at Sacramento during five years of study:
• No vector impacts
• No groundwater impacts
• Controlled pathogen impacts
• Acceptable odor impacts
TABLE 15-5
SACRAMENTO CENTRAL WASTEWATER TREATMENT
PLANT FSL DESIGN DATA
Depth from
Date placed water surface
FSL in operation to bottom, ft
7/73
8/73
9/74
11/74
8/76
8/76
11/75
11/75
Area at water
bh from
surface
ttom, ft
11
11
14
14
15
15
15
15
'otal
surface
1,000 ft2
(acres)
164.
(3.
164.
(3.
244.
(5.
229.
(5.
204 .
(4.
204.
(4.
270.
(6.
270.
(6.
1,749.
(40.
0
8)
0
8)
2
6)
0
3}
2
7)
2
7)
0
2)
0
2)
6
1)
Volume below
sludge blanket,
1,000
1,
1,
2,
1,
1,
1,
2,
2,
15,
cu
030.
030.
137.
983.
851.
850.
689.
689.
259.
ft
4
4
0
0
0
0
0
0
8
Loading capacity
of basin,3
1,000 Ib
3.
3.
4 .
4.
4.
4.
5.
5.
31.
VS/day
28
28
88
58
08
08
40
40
80
aCapacity of lagoon based on a design volatile solid (VS) loading
of 20 lb/1,000 ft2 of water surface area per day.
1 ft = .3048 m .
1 ft2 = .0929 m .
1 Ib = 0.4536 kg.
1 cu ft = 28.32 1.
TABLE 15-6
SACRAMENTO CENTRAL WASTEWATER TREATMENT
PLANT FSL SLUDGE INVENTORY, DRY TONS
Parameter
Digested sludge
VS TS
VS TS
3,925 2,6^0 4,580 2,995 5,398 3,416 3,801 3,5^ 2,222 1.4C.1 2,211 1,454 3,4Sf> 2,31? 3,275 2,177 30,898 20,106
Stored sludge 1,973 B&O 1,009 l,62l) 2,y50 1,721 3,845 I ,W2 1 ,45'J Hl<> 1, 173 719 3,792 2,214 3,208 1,076 21,399 11,727
Quantities account for sludyc that has been (1) added to the SSB.s, (2) applied to land
(1,256 dry tons in 1974, l,f>B8 in 1975, 976 in 197C, and 1,930 in 1977) and (3) transferred
between basins since beginning of operations.
b
Quantities calculated based on data obtained from sludge samples collected July 12, 1978.
15-35
-------
TABLE 15-7
SACRAMENTO CENTRAL WASTEWATER TREATMENT
PLANT RECYCLED FSL SUPERNATANT QUALITY
10/5/78 10/6/78 10/7/78 10/11/78 10/30/78 12/20/78 Average
BOD
TP04
Sulf ides
COD
TKN
pH
SS
NH3-N
140
51
0
-
-
-
-
-
140
50
0
-
-
-
-
-
140
66
0
-
-
-
-
-
96
' 120
0
910
220
7.7
470
-
200
.'• ! no
0
960
360
7.7
420
300
110 143
80, 79
0
874 935
394 290
7.8 7.7
728 445
335 300
In mg/1 except for pH.
Vector Impacts. Rodents and flies have apparently not bred
around the FSLs for the last five years. Scum control is
obviously the key to elimination of this problem.
Groundwater Impacts. Groundwater contamination is nonexistent.
Monitoring wells surrounding the 40 acres (16.2 ha) of existing
FSLs in Sacramento have been sampled monthly and have never shown
any indication of groundwater contamination traceable to the
lagoons. Tests show that sludge which settles to the bottom
quickly and effectively seals off the lagoon contents from the
surrounding soils. Undisturbed soil samples taken directly from
the bottom of a lagoon with a limited history (one to two
years) and a lagoon with a long history (four to five years)
confirm that the FSL contents have a limited penetration into the
surrounding soils. These studies indicate that the sealing of
FSLs is a combination of soil pore plugging by suspended and
colloidal materials within the sludge and the formation of
mucus-like materials that create an impermeable membrane between
the stored sludge and the underlying soil. Sandy soils take
longer to seal than silty clay soils, but both achieve complete
sealing in two to three months.
The two- to six-inch (5.08 to 15.24 cm) engineered fill seal
provided over the natural bottom and side slopes of the typical
FSL cross-section on Figure 15-8 assures that none of the FSL
start-up sewage or diluted sludge content escapes during the
natural sealing process.
Pathogen Impacts. It has been recognized for many years that
long-term liquid storage signif i.cantly reduces the pathogenic
microorganism content in sludge (3). Studies at Sacramento
confirm this for the most common bacteria. Figure 15-11
indicates that the fecal coliform population decreases as the
sludge passes through the sludge management system. Studies of
parasitic protozoans and their cysts, helminths and their eggs
(ova), and virus were inconclusive either because insufficient
15-36
-------
numbers were found or the techniques required for reasonable
reproducibility were unavailable to the project. The system of
disposal selected, that of dedicated land disposal, made further
investigatory work unnecessary.
TABLE 15-8
SACRAMENTO CENTRAL WASTEWATER TREATMENT PLANT
COMPARISON OF DIGESTED FSL AND REMOVED
SLUDGE ANALYTICAL DATA
Stored sludge
Constituent
Alkalinity
Chloride0
Ammonia0
Soluble phosphorus (P)c
Sulfate
Percent dry weight
Total phosphorus (P)
Total nitrogen (N)
pprc^dry weight
Calcium
Magnesium
Potassium
Sodium
Arsenic
Beryl li-um
Cadmium
Chromium
Copper
Lead
Mercury
Molybdenum
Nickel
Selenium
Silver
Zinc
PCB 1242
PCB 1254
Tech chlordane ,
Other pesticidals
Units as noted
Cd/Zn ratio, percent
Total solids, percent
Volatile solids, per-
cent of total
pH c
Specific conductance ,
umhos/cm
1.8
8.7
21,000
5,800
5,500
9,200
47
<2.2
12
165
340
185
3.7
<22
63
1.6
28
930
1.3
1.7
68
7.5
4,742
2.0
5.1
27,000
8,200
3,200
3,100
75
24
218
410
134
5.3
<13.4
58
1.7
26
1,700
<2.8
5.5
3.8
0.30
1.4
7.0
55
7.3
5,109
1.9
5.2
25,000
7,900
3,900
3,450
72
<1.1
26
245
398
123
5.1
<16
72
1.4
26
1,500
<3.1
5.3
4.0
0.27
1.7
6.3
55
7.3
1.7
5.2
21,000
7,900
3,800
3, 500
89
19
224
385
96
70
1.6
26
1, 300
<2.9
4.0
3.6
0.25
1.5
6.1
53
7.3
5,743
28,000
6,300
2,900
3,300
101
16
243
721
134
5.2
<12.5
115
1.4
23
1,325
<2.6
4.8
4.0
0.22
1.0
7.6
52
7.3
4,914
1.4
5.4
28,000
5,500
2,600
4,100
22
14
173
400
116
5.0
<13.7
46
4.1
34
1,207
<2.3
4.7
3.9
0.25
1:1
4.7
60
7.2
4,434
FSL 6
1,687
166
452
50
77
1.6
6.2
24,000
5,300
3,000
5,600
28
13
220
477
183
5.8
<15.4
48
3.2
38
1,400
<2.6
3.8
4.2
0.25
1.3
3.4
62
7.4
4,093
FSL 7
2,239
171
613
51
68
1.6
5.8
26,000
6,300
3,100
4,600
82
21
278
456
153
5.8
<12.2
60
2.6
35
1,400
<3.0
6.6
5.9
0.27
1.5
4.8
61
7.3
5,061
FSL 8
2,175
186
600
49
49
1.4
5.1
21,000
3,500
3,200
4,200
62
17
188
353
121
4.2
<11 . 8
53
1.4
27
1,090
<3.0
3.3
3.8
0.23
1.5
5.7
52
7.3
4,760
Removed
sludge3
2,069
171
573
45
151
1.9
5.9
24,000
8,600
4,500
5,400
15.4
19
181
384
159
5.6
< 13
77
5.6
28
1,200
< 2 . 1
4.6
5.0
<0.7
1.5
4.1
54
7.4
4,731
aValues are averages from samples collected during 1977.
As CaCO-, determined by potentiometrie titration of supernatant.
As CaCO-, determined by potentiometrie titration of supernatant.
CDetermined on supernatant; other determinations run on solution resulting from acid digestion of whole sample.
Other pesticidals include residues such as DDT, DDE, dieldrin, etc.
Analysis not performed.
Odor Impacts. Odor impacts change in direct proportion to
theFSL1ssurface area. In most small plants (those requiring
<40 acres [16.2 ha] of FSLs), controlling the loading rate, using
adequate surface agitation, providing sufficient buffering area
and carefully selecting the best time periods for feeding and
surface agitation operation are sufficient to achieve acceptable
levels of odor risk. Table 15-9 shows the annual odor risk
analysis developed for the existing 40 acres (16.2 ha) of FSLs at
the Sacramento site before the installation of the barriers and
wind machines (1). No high technology mitigation has been
15-37
-------
required to maintain this acceptable risk level. For larger
areas of FSLs, additional odor control measures would probably be
required. These might include the installation of a blender
digester to keep raw sludge from short circuiting to the FSLs,
vacuum vaporization to remove entrained odors from the digested
sludge prior to its discharge into the FSLs, separation of
batteries of FSLs, construction of special 12-foot (3.7 m) high
barriers around the FSLs, to ensure maximum odor dispersion
at low wind speeds, and the use of wind machines to aid odor
dispersion when the atmosphere is calm. Figure 15-12 shows
typical wind machines and barriers at the Sacramento FSLs.
o
i
I
c
d
LU
OJ
Q
w
(3
BE
o
IQ10 i
108
10*
107
106
10B
104
103
10*
10°
Iff1
102
-
—
_,
—
_
-
-
-
^
—
JAN APR JUL OCT
JAM APR JUL OCT
JAN APB JUL OCT
1
MAY JUL StP
JUN AUG OCT
-
-
-
-
-
-
-
_
_,
-
jyt oct
RAW
SLUDGE
DIGESTED
SLUDGE
FSL
STORED
SLUDGE
FSL
REMOVED
SLUDGE
TREATED
SOILS
FIGURE 15-11
SACRAMENTO CENTRAL WASTEWATER TREATMENT
PLANT 1977 FECAL COLIFORM POPULATIONS FOR
VARIOUS LOCATIONS IN THE SOLIDS TREATMENT-
DISPOSAL PROCESS
15-38
-------
TABLE 15-9
SACRAMENTO CENTRAL WASTEWATER TREATMENT PLANT ODOR RISK
FOR 40 ACRES OF FSLa, ANNUAL EVENTS (DAYS)
Downwind odor Direction towards which wind is blowing
concentration, "
C N NE E SE S SW W NW Total
*
5d
iod
2.8
0.3
0.08
• 2.1
0.3
0.06
3.2
0.5 .
0.10
7.3
1.2
0.20
11.5
1.6
0.3
6.7
0.8
0.2
4.1
0.4
0.1
3.1
0.3
0.1
38.9
5.4
1.1
Includes source control mitigation - controlled organic surface loading rate,
adequate surface mixers, and controlled feeding and mixer operating times -
and odor transport mitigation - 2,000 to 5,000 feet of buffer.
2 ou/cf barely detectable ambient odor criteria.
5 ou/cf threshold complaint conditions.
10 ou/cf consistent complaint conditions.
1 AC = .4047 ha.
foot = 0.3048 m.
1 cf = 0.02832 m3.
The odors from 40 acres (16.2 ha) of FSLs at Sacramento have
proven to be completely acceptable. An analysis of the expected
annual odor risks for the 124 acres (50.2 ha) of FSLs to be
constructed for the new regional treatment plant (see
Figure 15-9) is shown in Table 15-10 (1). This analysis shows
that with the installation of complete control measures, the
incidence of threshold complaint odor levels at the plant
boundary (2,000 to 5,000 feet [610 to 1,520 m] downwind) will be
less than once every two years, regardless of wind direction, and
once every seven years for the worst specific wind direction.
This level of odor risk was found to be acceptable in the public
environmental impact hearings.
Cost Information
The major elements involved in determining FSL costs are land and
earth moving. Both are usually quite site specific. Normally,
land costs vary less predictably than construction costs.
A typical FSL storage facility for a 10-MGD (438-1/s) secondary
carbonaceous activated sludge treatment plant with primary
sedimentation, anaerobic digestion, and norma^l strength domestic
and industrial sewage will cost about $1.5 million to construct
and $25,000 per year to operate. Construction costs are based on
a 3500 Engineering News Record Construction Cost Index and do
not include the cost of land. Operation costs are based on 1978
wage rates and do not include dredge operators or any other
removal costs. ;s . • .
15-39
-------
. • V,:*&v#. '.-:•-.• ^
' f 3ti"'J-'"'>"'':"»^'"->^-'; -' ;- •>"..
'" ' ••
FIGURE 15-12
TYPICAL WIND MACHINES AND BARRIERS
SACRAMENTO, CALIFORNIA
Construction costs include the installation of three complete
four-acre (1.62 ha) FSLs. This is assumed to be the capacity
needed to meet the annual digested sludge loading rate criterion
of 20 pounds VS per 1,000 square feet per day (1.0 t VS/ha-d).
It is based on a conservative unstabilized sludge production
rate and a nominal 50 percent volatile solids reduction in the
anaerobic digesters. The three lagoons will provide capacity for
daily loading, digester cleaning, and maintenance and storage for
intermittent removal to dedicated land disposal. FSLs are
assumed to be 15 feet (4.6 m) in depth and have 3:1 dike side
slopes. If they are required, purchase of the dredge and booster
pump would add another $150,000 to $180,000 to the construction
costs.
Odor control costs, including blending digester, vacuum
vaporizer, barriers, and wind machine could increase the
construction costs another $250,000 and the operation costs
15-40
-------
another $25,000 per year. As indicated by the odor impact
assessment, sufficient area to ensure maintenance of loading
criteria, together with surface agitators and proper buffer,
would make it possible to avoid the cost of the aforementioned
more extensive odor mitigation facilities.
TABLE 15-10
SACRAMENTO REGIONAL WASTEWATER TREATMENT PLANT ULTIMATE
ODOR RISK FOR 124 ACRES OF FSLa, ANNUAL EVENTS (DAYS)
Downwind odor Direction towardg_which wind is blowing
concentration, ' : """
C N NE E SE S SW W NW Total
2b
^
10d
0
0
0
.44
.08
.02
0.15
0.02
<0.01
0.18
0.03
<0.01
0.41
0.06
0.01
0
0
0
.85
.13
.02
0.31
0.04
0.01
0.22
0.03
0.00
0.33
0.05
0.01
2.9
0.44
<0.09
Includes source control mitigation - controlled organic surface loading rate,
adequate surface mixers , blending digester, vacuum vaporization and controlled
feeding and mixer operation times, and odor transport mitigation - 2,000 to
5,000 feet of buffer and, separation of groups of FSLs, barriers and wind
machines.
2 ou/cf barely detectable ambient odor criteria.
Q
5 ou/cf threshold complaint conditions.
10 ou/cf consistent complaint conditions.
1 AC = .40407 ha.
foot = 0.3048 m.
1 cf = 0.02832 m3.
Construction costs for the 124 acres (50.2 ha) of FSLs with
complete odor mitigation facilities for the Sacramento Regional
Wastewater Treatment Plant are estimated to be $28.7 million.
This includes almost $3.3 million for the existing 40 acres
(16.2 ha) of FSLs with barrier wall and wind machines. This
acreage will store the solids from a 136-MGD (5,960-1/s) secon-
dary carbonaceous activated sludge treatment plant. Operation
costs are estimated to be $650,000 per year.
15.3.1.3 Anaerobic Liquid Sludge Lagoons
Many such lagoons are being operated throughout the United
States. One system that has collected some meaningful data is
the 220.2 acres (89.1 ha) in operation at the Metropolitan
Sanitary District of Greater Chicago (MSDGC) Prairie Plan land
reclamation project in Fulton County, Illinois. In a personal
communication R.R. Rimkus, Chief of Maintenance and Operations
MSDGC provided the layout shown on Figure 15-13 of the four
lagoons at this site. He reports that Lagoons 1 and 2 have been
in service for eight and seven years, respectively, and Lagoons
3a and 3b for six years. Lagoons 1 and 2 have an average depth
of 35 feet (10.7 m), plus or minus one foot (0.3 m), while
Lagoons 3a and 3b are about 18 feet (5.5 m) deep. Lagoons 3a and
3b are utilized more for supernatant treatment and storage.
15-41
-------
3 a TRANSFER
HOLDING BASINS
1 acre = 0,405 ha
FIGURE 15-13
ANAEROBIC LIQUID SLUDGE LAGOONS, PRAIRIE PLAN LAND
RECLAMATION PROJECT, THE METROPOLITAN SANITARY
DISTRICT OF GREATER CHICAGO
15-42
-------
Rimkus further indicates barged anaerobica1ly digested
waste-activated sludge from Chicago is discharged into Fulton
County Lagoons 1 and 2 throughout the year, when river shipment
conditions permit, at a frequency of about 20 days per month.
Solids loading varies between 65,000 to 95,000 dry tons
(59,000 to 86,200 t) per year. Based on the total loading
received by Lagoons 1 and 2 and the volatile solids content of
the digested sludge equaling 57 percent, the organic loading rate
to the Fulton County Lagoons varies between 36 and 50 pounds VS
per 1000 square feet per day (1.7 to 2.4 t/ha-d). This is
considerably above the 20 pounds VS per 1000 square feet per day
(1.0 t/ha-d) established at Sacramento to maintain facultative
conditions within the lagoons. If the area of all four lagoons
is considered, this organic loading rate drops to 21 to 29 pounds
VS per 1,000 square feet per day (1.0 to 1.4 t/ha-d), which is
close to the facultative sludge lagoon concept.
Rimkus reports that the solids concentration of sludge pumped
from the barge to the lagoons varies from four to six percent by
weight. Further, the sludge pumped from lagoons to fields in
1978 varied from 3.57 to 5.93 percent by weight. The average
annual quantity of removed sludge is 60,000 dry tons (54,400 t).
Mean value for volatile solids content of 1978 removed sludge
was 47.5 percent. If the barged sludge volatile content is
57 percent, then the lagoons are reducing the volatile solids
by 17 percent. Data for sludge removed in 1978 are given in
Table 15-11. Sludge removal is usually accomplished in about
115 days, between May 1 and November 15.
According to Rimkus, Fulton County supernatant is disposed of on
1,320 acres (534.2 ha) of alfalfa-brome hay fields. Average
annual quantity to dispose equals 700,000 wet tons (634,900 t)
with an average ammonia content of 109.9 mg/1 and an average TKN
content of 156.4 mg/1. Table 15-12 provides other data on lagoon
supernatant. Dissolved oxygen (D.O.) measurements taken in
the summer and fall of 1977 in Lagoons 3a and 3b indicate the
surface D.O. ranged between 0.9 and 8.5 mg/1,while the bottom
D.O. ranged between 0.4 and 2.6 mg/1. The lowest lagoon
temperature during this period was 40.6°F (15.5°C). The lagoon
surface is frozen between 45 and 60 days per year, with scum
build-up experienced only during periods of new sludge input. No
surface agitation equipment is used on any of the lagoons. The
nearest residence to the lagoon is approximately 6,000 feet
(1,800 m) from the perimeter of the installation. No information
is available regarding odors or odor complaints.
15.3.1.4 Aerated Storage Basins
To use aerated storage basins successfully for wastewater solids,
a design must meet the following criteria:
• Basin contents must be sufficiently mixed to assure
uniformity of solids concentration and complete
dissemination of oxygen.
15-43
-------
• Sufficient oxygen must be
conditions throughout the
solids concentration.
available to maintain aerobic
basin at maximum attainable
• Liquid level variation must be sufficient to
accommodate maximum storage needs under anticipated
rainfall.
TABLE 15-11
1978 REMOVED SLUDGE-PRAIRIE PLAN LAND
RECLAMATION PROJECT, THE METROPOLITAN
SANITARY DISTRICT OF GREATER CHICAGO3
Constituent
pH, units
EC, urahos/cm
Total phosphorus
Kjeldahl nitrogen - N
Nitrogen as ammonia - N-NHj
Alkalinity as CaC03
Cloride - Cl
Iron - Fe
Zinc - Zn
Copper - Cu
Nickel - Ni
Magnesium - Mn
Potassium - K
Sodium - Na
Manganese, Mg
Calcium - Ca
Lead - Pb
Chromium - Cr
Cadmium - Cd
Aluminum - Al
Mercury - Hg
Total solids, percent
Total volatile solids, percent
Minimum,
mg/lb
7.2
2,500
900
1,276
772
1,640
228
1,000
87
44.8
9
8.5
80
30
80
710
25.9
90. 6
7.5
340
0.132
3.57
43.5
Maximum,
mg/lb
7.9
6,800
2,960
2,905
1,338
5,750
752
2,900
231
124
28
28.3
200
120
810
1, 800
54.5
513
20.2
900
1.920
5.93
50.0
Mean,
mg/lb
_
4, 675
1,416
2,329
1,046
3, 630
388
1,938
171
81.6
18
18. 0
166
88
450
1,185
42.1
175
13.2
679
0.417
4.75
47.5
Mean content
Ib/dry ton
_
-
59.6
98. 1
44. 0
153
16. 3
81.6
7. 2
3.44
0.76
0. 758
6.99
3. 7
18. 9
49. 9
1.77
7.37
0. 556
28. 6
0. 018
2, 000
950
aLiquid fertilizer applied to fields from May 23, 1978 to November 18, 1978.
Results are based on 24 weekly composite samples. Data supplied by Metropolitan
Sanitary District of Greater Chicago.
mg/1 unless otherwise noted.
1 Ib = 0.4536 kg
1 ton = .907 t
Mixing Requirements
Equipment required for aerated storage basins is similar to that
for aerobic digestion (see Chapter 6). Unfortunately for the
designer, mixing capability for various types of static or
mechanical aeration devices varies greatly. Fixed or floating
15-44
-------
turbine or propeller-type aerators are often affected by very
limited side boundaries, while brush-type aerators and aspirating
pumps often have almost unlimited side boundaries but rather
restricted vertical mixing capabilities. Submerged static
aeration devices are excellent for vertical mixing but are always
limited by very confined side boundaries. The designer should
rely on a performance-type specification to achieve desired
results. The equipment supplier should be given information
about the configuration of the basin, its liquid level operating
range, the maximum solids concentration expected, and the level
of dissolved oxygen to be maintained. The designer is expected
to have established the most cost-effective basin configuration
based on loading, site-specific conditions and available aeration
equipment requirements. A maximum horsepower limit should be
established, and the specifications should include a bonus to be
added to the bid price and a penalty to be subtracted from the
bid price based on the energy costs involved when the equipment
meets the required performance. A guarantee should be used to
assure that the final installation will meet the performance
requirement.
TABLE 15-12
1973/1974 SUPERNATANT-PRAIRIE PLAN RECLAMATION
PROJECT, THE METROPOLITAN SANITARY
DISTRICT OF GREATER CHICAGO*
Mean value,
Constituent mg/1
BOD -
BOD -
COD -
COD -
TSS
total
soluble
total
soluble
170
62
951
695
276
Range ,
mg/1
28 -
20 -
325 -
328 -
52 -
466
114
2,120
1,026
1,041
Data supplied by The Metropolitan Sanitary
District of Greater Chicago.
Oxygen Requirements
Oxygen requirements to maintain aerobic conditions within an
aerobic storage basin will be considerably less than that
required for aerobic digesters if the material being stored has
been stabilized prior to its introduction to the basin. Minimum
15-45
-------
measurable dissolved oxygen levels of about 0.5 mg/1 are quite
adequate to maintain a basin free from anaerobic activity, as
long as it is provided with adequate mixing. If the basin
influent is not sufficiently stabilized to minimize oxygen
requirements, then the aerobic storage basin must be designed for
oxygen requirements similar to aerobic digesters (see Chapter 6).
Oxygen transfer capabilities are similar to mixing capabilities
for the various types of applicable equipment. The design should
therefore include oxygen transfer requirements as part of the
performance requirement indicated in the preceding section on
mixing specifications.
Level Variability
Often, aerated storage basins cannot be decanted, because
solids settle when the aerator is turned off, and anaerobic
decomposition may also occur, resulting in odor production.
Attempts at in-basin decanting without aeration and mixer
shutdown will usually result in the recycling of the concentrated
solids back to the liquid process. Separate continuous decanting
is usually possible either by sedimentation or dissolved
air flotation. Evaporation will also quite often result in
significant liquid removal. Aerobic storage basins that do
not have separate decanting facilities must be operated on
single-phase concentration or displacement storage concepts.
The single-phase concentration concept will function as described
for aerobic digesters. The displacement concept, however, will
require liquid level variability and make aerated storage basin
equipment installation quite complicated. Under such conditions,
this equipment must be capable of maintaining adequate mixing
and oxygen transfer over the complete range of liquid level
variation. This requirement may cause this equipment to have
varying mixing and aeration capabilities, depending on the basin
depth. Variable speed drives, multi-speed drives, or variation
in the quantity of diffused air should be investigated. At no
time should the equipment be operated under conditions that will
waste energy. Mixing and aeration design requirements and layout
details can be found in Chapter 6.
15.3.2 Facilities Provided Primarily for Storage of
Dewatered Sludge
Dedicated dewatered sludge storage of wastewater solids can
include the storage of easily managed dry solids (>60 percent
solids) or hard to manage wet solids (15 to 60 percent solids).
Dry solids are usually the product of heat-drying, high
temperature conversion, or air-drying processes and can be stored
by standard dry material storage techniques. Descriptions of
these techniques are readily available in materials processing
textbooks, and, if desired, more detailed data is available
(20,21). The storage of wet solids is another matter, however.
The successful application of common storage techniques to this
15-46
-------
normally unstable organic material is practically impossible.
The most commonly accepted methods of providing dedicated storage
for wet organic material involves the use of drying sludge
lagoons, placing the material in some type of confined structure
or placing it in unconfined stockpiles. All three methods can
involve special'design considerations.
15 . 3 . 2 .1 •• Drying Sludge Lagoons
Drying sludge lagoons are probably the most universally practiced
method of storing of wet organic sludge. Actually, the material
arrives at the lagoons in a liquid form, but as described under
Chicago's actual performance data, most of the storage capability
is derived while the material is in a partially dewatered state.
Unfortunately, many existing applications of this method
of storage are being operated with sludge that has not been
anaerobically stabilized prior to its discharge to the lagoons.
In some cases, drying sludge lagoons are used after aerobic
digestion, and in other cases they have been used as digesters
with no upstream stabilization. In these instances, odors that
are quite unacceptable to the surrounding community are produced.
When such lagoons are considered a means of ultimate disposal,
they are called "permanent lagoons." Because permanent sludge
lagoons have sometimes been the source of strong odors, they are
often rejected as a means to store sludge, either in the liquid
or semisolid state (22). A detailed discussion of design
criteria for drying sludge lagoons can be found in Chapter 9.
Performan c e__Dat. a
Several reasonably successful drying sludge lagoon operations do
exist. An investigation of their actual performance, however,
indicates that these lagoons are acceptable because they receive
adequately stabilized anaerobically digested sludge and do not
normally generate the odors associated with the acid phase of
anaerobic stabilization.
San Jose, California. The San Jose/Santa Clara Water Pollution
Control Plant in San Jose, California, is a secondary treatment
plant that operates on the Kraus modification of the activated
sludge process during its seasonal canning loading period.
The plant stores its anaerobically digested primary and
waste-activated sludge in 73 sludge lagoons on 580 acres (235 ha)
of land immediately adjacent to the plant (2). In 1978 the plant
operated both anaerobic liquid sludge lagoons and drying sludge
lagoons with 35 either filled or more than half filled with
liquid sludge and 32 containing 2 feet (0.60 m) or less of dried
sludge. Three lagoons have never been used,'and three have been
dredged and are now empty. The drying sludge lagoons were filled
in layers of approximately one foot (0.3 m), and each layer was
allowed to dry by evaporation prior to the addition of the next
15-47
-------
layer. The drying lagoon operation took place from 1974 until
1976, when operational limitations and odor production resulted
in the return to anaerobic liquid sludge lagoon storage. Liquid
sludge lagoon storage had been practiced prior to 1974.
As a result of existing operations, the present storage capacity
of the lagoons will last until 1986. Because the plant does not
have existing dewatering facilities, it will not be able to
dispose of over 900,000 gallons per day (3,400 1/d) of liquid
sludge without providing additional sludge treatment facilities
by 1986. Studies are now under way evaluating alternative
dewatering and drying processes and facilities for the disposal
and use of dewatered and dried sludge.
Residents living in areas near the sludge lagoons have become
increasingly concerned about odors produced by the lagoons.
During 1976, several complaints were registered with the Air
Pollution Control Board. The area most affected is a residential
community just southeast of the plant. Correlation of complaints
with atmospheric conditions indicates that the greatest odor risk
occurs with a northwest wind and when dry weather is followed by
heavy rain. This points to the danger of rewetting the dried
surface layers and anerobically stabilized material and confirms
that this can create strong odors.
Chic a go, Illinois
The Metropolitan Sanitary District of Greater Chicago (MSDGC)
operates 30 drying sludge lagoons, each with an average storage
capacity of 200,000 cubic yards (153,000 m3) and a storage depth
of 16 feet (4.9 m) (23). Figure 15-14 provides a plan view of a
typical lagoon. Anaerobically digested sludge is pumped to the
MSDGC lagoons at a solids content of about 4 percent. Volatile
content of this material is approximately 57 percent. Sludge is
usually applied to each available lagoon in 6-inch (152-mm)
layers in rotation. Rotations are repeated.
Supernatant appears on the lagoon surface approximately five to
seven days after each fresh sludge application. It is then
drained from the surface and returned to the West-Southwest
Sewage Treatment Works by removing one or more stop logs from the
draw-off box. Once the supernatant is decanted, the eight to
ten percent solids sludge is further concentrated by evaporaion.
Evaporation tapers off, however, as an aerobic sludge crust
develops. Supply sludge concentration (4 percent solids) is
beneficial, as it covers the entire lagoon surface with only a
slight gradient from the point of application. Any higher
concentration would inhibit this coverage and reduce the
evaporative surface area per unit volume. Lagoons that have been
filled to capacity by this method have an average solids content
of 18 to 22 percent by weight. Volatile solids content of this
material is in the range of 35 to 40 percent, indicating that
the lagoons are producing about a 34 percent volatile solids
reduction.
15-48
-------
DRAW - OFF BOX & TRUSS
CRESCENT SCRAPER
AND CARRIER
SLACKLINE CRANE
SLUDGE INFLUENT
TAIL ANCHORAGE
(BULLDOZER!
DRAGLINE (LOADING
PARTIAL OEWATERED SLUDGE)
FIVE AXLE DUMP TRUCK
LAGOON PERIMETER
ADJACENT LAGQQMS
FIGURE 15-14
PLAN VIEW OF DRYING SLUDGE LAGOON NEAR
WEST-SOUTHWEST SEWAGE TREATMENT WORKS, CHICAGO
Once the drying sludge lagoons are filled, they are taken out of
service and preconditioned to provide an improved drainage
gradient. For this purpose, the sludge is excavated from the
area adjacent to the draw-off box and the slope within the lagoon
is allowed to stabilize to the point at which the area remains
free of solids. Excavation is by pump with nearby
additional water, if necessary, to assure sludge
Figure 15-15 illustrates a cross section of this
preconditioning is complete. When the sludge has
the lagoon is left dormant through the following
reasonably
mixers and
f lu id i ty .
area after
s tabilized,
winter and
drained by
early spring.
gravity to the
Trapped
draw-off
water and
structure.
rainfall runoff are
Once relatively dry weather returns, a slackline cable system is
utilized with a dragline crane to further condition the sludge.
The slackline system, which is shown on Figure 15-16, is used to
improve the lagoon surface drainage and to scrape as much of the
dried crust as possible to the side of the lagoon. This system
provides the following four operational benefits:
• Drier sludge is scraped to the side, where it can be
reached by portable dragline or clamshell and loaded onto
dump trucks.
• Piling sludge along sides improves lagoon drainage
pattern and profile.
• Removal of crust exposes wetter sludge to atmosphere for
optimum evaporation.
15-49
-------
• Some of dried crust mixes with wetter material during
removal and increases the wet sludge solids content.
MONORAIL BEAM
WALKWAY
STEEL TRUSS
MONORAIL HOIST
SEWER
Vi%£S?FZ£^*^^
DRAW-OFF
BOX
LAGOON BOTTOM
FIGURE 15-15
CROSS SECTION OF DRAW-OFF BOX AREA DRYING
SLUDGE LAGOON NEAR WEST-SOUTHWEST SEWAGE
TREATMENT WORKS, CHICAGO
Figure 15-16 shows the location of the equipment during lagoon
partial dewatering and removal operation.
CRANE
CRESCENT
SCRAPER
TAIL
ANCHORAGE
(BULLDOZER}
LAGOON BOTTOM
FIGURE 15-16
CROSS SECTION OF DRYING SLUDGE LAGOON WITH
SLACKLINE CABLE NEAR WEST-SOUTHWEST
TREATMENT WORKS, CHICAGO
15-50
-------
Once the sludge crust is scraped to the side of the lagoon,
it is removed by portable dragline or clamshell, loaded onto
watertight five-axle dump trucks, and delivered to the general
public for reuse. This lagoon sludge, at its time of delivery,
usually has an average solids content of 30 to 35 percent by
weight. Tree nurseries, sod farms, landfills, and stripped land
are among the major users of this material. In 1977, the MSDGC
disposed of 69,362 dry tons (62,925 t) of drying lagoon sludge at
an average cost of $16.75 per dry ton ($18.47/t). In 1978,
production was expected to exceed 100,000 dry tons (90,700 t) at
a cost of $17.76 per dry ton ($19.58/t). Preconditioning costs
are approximately $3.00 per dry ton ($3.31/t), which makes the
cost for the whole operation about $21.00 per dry ton ($23.15/t).
Preconditioning is accomplished by MSDGC manpower and equipment,
and the services of the slackline, dragline, and trucks are
contracted out. The overall operation requires little capital
investment, minimal lead time, and limited effort. Natural
processes are optimized and odors minimized. The level of odor
involved has not been qualified.
15.3.2.2 Confined Hoppers or Bins
A designer is often tempted to take advantage of the volumetric
reduction in material provided by the dewatering process and lay
out his sludge disposal system based on short and long-term
storage (3 weeks to >6 months) of the dewatered product. If the
product is too wet (<30 percent solids), several problems
may arise with this type of storage. These problems include
continuing decomposition, liquefaction, and concentration and
consolidation. Although each may have its own result, all three
problems are interrelated and combine to limit the use of this
type of storage to equalization storage and then only if special
attention is given to controlling the difficulties. A brief
description of some of these difficulties is given in the
following paragraphs.
Continuing Decomposition
Unless it is stabilized to non-reactive levels (<50 percent
by weight), the biodegradable volatile organic material of
wastewater solids will continue to decompose if the moisture
content is too high (solids content <30 percent). This
decomposition will reduce organic material and generate gaseous
byproducts. Depending on the stage and sometimes the type of
stabilization employed prior to dewatering, the method of
conditioning for dewatering, and the dewatering method itself,
gaseous byproducts may or may not be odorous. For example, a
biodegradable volatile content of <50 percent would result in
strong odors; aerobically stabilized dewatered sludge would be
more subject to strong odors than anaerobically stabilized
dewatered sludge; polymer-conditioned dewatered sludge would be
more subject-to strong odors than lime and ferric conditioned
dewatered sludge; and centrifuged dewatered sludge would be more
subject to strong odors than vacuum filtered dewatered sludge.
15-51
-------
Enclosed structures are often used in this type of storage to
assure odor-free operation. Such structures may be extremely
hazardous if the designer fails to recognize the potentially
explosive nature of some of these gaseous byproducts and assure
that they are never mixed with air within the combustible
range. If such protection involves the replacement of the
displaced volume, it may become the limiting feature of the
storage structure's ability to manage the sludge.
One solution to this problem is to treat the volume above the
solids as part of the digester gas storage system. However, this
is only practical if the overall solids treatment system uses
anaerobic digestion for stabilization and the gas collection
system has sufficient capacity to fill the void created by
storage discharge within the required period of time. Major
problems of such a system are the sealing of sludge supply and
discharge and the assurance of accessibility for maintenance.
To eliminate the discharge and supply problems and assure
convenient access to the storage loading equipment, the enclosed
area of the storage structures should be sufficiently ventilated.
The area must be ventilated with about 20 to 30 air changes per
hour. Air movement should be felt by the operators who work in
the area. To assure ventilation of all areas, regardless of any
continuously or intermittently operating openings, both supply
and exhaust air should be managed by powered fans. All exhaust
air should pass through an odor removal system. The quantity
of exhaust ventilation air should be slightly greater than the
quantity of supply ventilation air to assure a negative pressure
within the area and minimize leakage that might bypass the odor
removal system. The atmosphere of enclosed areas should be
monitored with hydrocarbon detectors (see Chapter 17) to provide
ample warning if the gas release begins to develop dangerous
mixtures of methane and air.
Liquefaction
When the reduction of putrescible organic material is carried out
within a confined structure used for short or long-range storage
(three to four weeks to more than one month), the liquefaction of
dewatered solids occurs. Liquefaction is negligible when the
storage is limited to equalization (three to four days). The
designer must be aware of the effects of this liquefaction
and realize that as the liquid or moisture content of the
sludge increases, the difficulties of transport also increase.
An example of this liquefaction, in which no evaporation or
additional moisture is assumed to be added during storage, can be
seen in the following calculation:
Typical Liquefaction Calculation
Dewatered digested sludge (polymer conditioner used)
Solids to be stored, dry wt, tons 1,000 (907 t)
15-52
-------
Total solids (TS) content, percent 20
Volatile solids, percent 65
Assumed reduction of VS during
6 months storage, percent 20
Water content of dewatered sludge, tons 5,000 (4,535 t)
VS,. dry tons at start of storage 650 (590 t)
VS, dry tons at end of storage 520 (472 t)
Fixed solids, dry tons (unchanged) 350 (317 t)
TS, dry tons at end of storage 870 (789 t)
Total solids content at end of storage,
percent 14.8
The example indicates how a reasonably dry, dewatered digested
sludge (20 percent solids) can be liquefied to a fairly wet,
digested sludge (14.8 percent solids) if the putrescible organic
material continues to be reduced. The speed of this reduction is
greatly affected by temperature and organic content in the
dewatered sludge. Thus, liquefaction will be a greater problem
in warm climates or during the hot summer seasons. If lime and
ferric chemicals are used to condition the digested sludge for
dewatering, liquefaction will be greatly reduced, both because
of the lower overall organic content of the material and the
inhibiting effects of the chemicals on the bacterial reduction of
the putrescible organic matter.
Concentration and Consolidation
The material handling properties of the dewatered sludge entering
the storage facilities often do not resemble those of the
material discharged from the same facility. The method of
controlling the discharge must be flexible enough to adapt to
these changes in properties at any time. A live bottom discharge
for variable positive control and back-up isolating valves for
positive shut-off if the live bottom equipment fails or the
material starts to run like water are mandatory when the volume
of storage greatly exceeds the volumetric capacity of the
transport system receiving the discharge. As long as the
storage structure's volume does not exceed the capacity of the
transport system receiving the discharge, and that transport
system is of the bulk handling type (for example, truck, rail
car, or barge) the discharge control can be a simple open-close
valve. Water collecting, tracked, hopper valves with remote
motor or air cylinder operation can be used for this control.
Facilities whose storage volume exceeds the discharge transport
system capacity or whose transport system is of the continuous
rate type (for example, conveyor belts, screw conveyors, and
15-53
-------
pipelines) must be provided with a discharge system capable of
infinite variability under all degrees of moisture content or
concentration. Such systems must be provided with remote
controls that are capable of detecting overloads prior to
their overwhelming the transport system. The controls must be
capable of automatically closing the discharge control system's
back-up, open-close isolating valve. , Sonic level detectors and
capacitance probes can be used for this function. Chapter 17
provides additional information on this type of level detection
instrumentation.
The use of polymers to condition the sludge prior to dewatering
can have a major effect on its ability to be stored conveniently
in the dewatered state. Hansen reports that high polymer
doses used experimentally (testing a belt filter press) at the
Los Angeles County plant created a dewatered sludge that was
quite viscous. This material tended to act like glue and was
extremely difficult to remove from conveyors especially at
transfer points and the head point above the hoppers. The
material could be stored, but required a positive type of
unloading system at the storage discharge to assure that the
lumps were pushed onto the discharge conveyor.
When exceptionally dry dewatered sludge (greater than 30 percent
solids) is stored, bridging can be a very difficult problem.
None of the facilities investigated had successfully solved this
problem. It is suggested that any large system which anticipates
storing dewatered sludge much dryer than 30 percent solids set up
a test facility to develop a reliable system for overcoming this
difficulty.
Performanc e Data
Probably one of the most successful confined bin dewatered sludge
storage facilities is located at the County Sanitation Districts
of Los Angeles County Joint Water Pollution Control Plant in
Carson, California. The Joint Water Pollution Control Plant
(JWPCP) provides advanced primary wastewater treatment for about
350 MGD (15.3 m3/s) of wastewater. The JWPCP also receives the
sludge from five tertiary treatment plants that employ activated
sludge followed by multimedia filtration and have a combined
capacity of 120 MGD (5.2 m3/s), Sludge from all six plants
is treated at the JWPCP using the anaerobic stabilization
(digestion), dewatering (centrifugation), and composting
(windrow) processes.
In June 1979, Mischeri reported the centrifuges were producing
about 400 to 600 wet tons (360 to 540 t) of dewatered digested
sludge each day with a 25 percent average solids content. Twelve
storage bins, each capable of holding 550 wet tons (500 t) of
dewatered sludge, are provided to equalize 24-hour-per-day
centrifuge production with 10-hour-per-day windrow construction.
The storage bins also provide the five-day storage needed to
assure continuous dewatering when both the composting and backup
15-54
-------
sanitary fill disposal options are unavailable due to excessive
rainfall. The facilities have been in service about three
years, and according to Hansen, the maximum period of disposal
unavailability has not exceeded two days to date, although there
have been times when all twelve of the bins have been filled with
dewatered sludge. An isometric sketch of the JWPCP storage and
truck loading station is shown on Figure 15-17.
END OF CONVEYOR
ANfy TAKFUVS OVFB
frW SiOHAUt BINS
SCKtW
cOhvEVOrt
S4J=lGE
BINS
;• RUSTIC SOOA
A ', SCflUBBtHS
ASSEMBLY
CONTROL
SYSTEM
!TVP Oi- 12:
*z—^f ~£—«2C
FROM DEWAT6R
-------
content is greater than 18 percent and the sludge is not left in
storage more than a few days. Bubbles, which can be observed in
the standing water on top of the stored sludge, attest to the
fact that decomposition is continuing in the bins.
Each storage bin is fabricated of steel, is 30 feet in diameter,
and tapers at the bottom to a five-foot-square discharge. The
taper is at 30 degrees off the vertical. Hansen indicates this
taper seems to eliminate bridging, except during the storage
of extremely dry (greater than 30 percent solids) sludge.
The five-foot-square (1.5-m square) discharge is equipped with
five 12-inch (30.5 cm) diameter continuous screw conveyors
(live-bottom system) that can be operated in any combination or
number to positively control the stored sludge discharge to the
discharge conveyor belt. Normal operation requires only the
three middle screw conveyors to be in service. A cylinder-type
plug valve with five ten-inch (25.4-cm) long by eight-inch
(20.3-cm) wide openings has been provided to assure positive
isolation between the live-bottom system and the discharge
conveyor. The plug valve is fabricated of 0.406-inch (1.03 cm)
steel wall, 12-inch (30.5 cm) O.D. steel pipe, approximately
five feet (1.5 m) long and is actuated by a pneumatic cylinder
that positively rotates the valve 90 degrees from a full open to
a tight shut-off position. An isolating bull gate that can be
hydraulically forced between the bottom of the storage bin and
the top of the live-bottom assembly is also provided. It can be
used to cut off sludge discharge should the live-bottom assembly
fail with a load in the hopper. It has been suggested that a
hydraulically operated gate valve or knife-gate valve could also
be used to provide this isolation. An isometric view of this
discharge control system is shown on Figure 15-18.
Hansen reports the storage facilities were built in 1973 at a
cost of $3 million. Sludge variability during start-up created
several problems that have now been successfully solved (24).
Solutions included: simplifying the supply to the storage
tanks by equipping each with a plow and moving the end of the
supply belts over the end hoppers; providing the live-bottom
discharge system with a positive discharge isolation valve; and
increasing the ventilation level in the supply and storage areas
to achieve the "breeze" atmosphere necessary to satisfy operator
safety concerns.
15.3.2.3 Unconfined Stockpiles
Unconfined stockpiles are a major method of providing long-term
storage for dewatered sludge. This method is used primarily for
the storage of air-dried, anaerobic or aerobic stabilized sludge
at thousands of small plants across the country. Probably the
largest storage and weathering installation is operated by the
Metropolitan Sanitary District of Greater Chicago (MSDGC) at
their West-Southwest Sewage Treatment Works (WSW-STW). All of
the air-dried Imhoff sludge at WSW-STW is stored and aged up to
15-56
-------
several years
land and then
"Nu-Earth" (23). The air-dried material weathers to
50 percent moisture after one to two years of aging.
on between 50 and 100 acres (20 and 40 ha) of
made available for delivery to the public as
less than
BULL GATE
HYDRAULIC
OPERATOR
80" DiA DISCHARGE
FflQM 550 Wit ION STOftAGfc
BIN
LtVf BOTTOM
DRIVE ASSEMBLY
1?" DIA
5=0" LONG
live BOTTOM
DISCHARGE
CONTROL
CONrnoi
VALVE
PNEUMATIC
OPFBflTQR
MAINTENANCE
TRACK
EMERGENCY
LIVE BOTTOM
ISOLATING BULL GATE
BULL GATE
OPERATING TRACK
LIVi BOTTOM OVERLOAD
COUPLING-DISENGAGED
TD CONTROL NUMBER OF
SCREW CONVENORS IN OPERATION
1 f1 - 0.105 m
1 in m 2.54 on
1 ton - 0.80? I
'CONVEYOR LOAD
MONITORING
PHOBFS
FIGURE 15-18
STORAGE BIN DISCHARGE CONTROL SYSTEM, JOINT
WATER POLLUTION CONTROL PLANT, LOS ANGELES
COUNTY, CALIFORNIA
Unconfined stockpiles of mechanically dewatered stabilized
sludge, which has less than 25 percent solids, usually are
destroyed (loose all semblance of stability) when exposed to
extensive rainfall. While it is possible to maintain such a
stockpile for equalizing or short-term storage, especially in
very dry climates like the southwest, long-term storage is
usually quite impossible. Stabilized sludges with a high chemical
content (greater than 40 percent lime plus some ferric) or a very
low organic content (less than 50 percent volatile solids)
sometimes prove to be exceptions. Highly stabilized lagooned
15-57
-------
sludges can also be one of these exceptions. Such open
stockpiles usually quickly absorb atmospheric moisture and
rapidly deteriorate in climates with intense or frequent
rainfall.
Covered stockpiles are often used in those areas where rainfall
is intense or frequent to assure the dewatered sludge integrity
during periods of equalizing storage. Such stockpiling is
usually limited because of the expense of developing covered
areas of sufficient size to provide adequate storage area and
equipment accessibility. The North Shore Sanitary District
(NSSD) (25), north of Chicago, Illinois, disposes of their
anaerobically stabilized (digested) dewatered sludge in deep
trenches on a 300 acre (121 ha) site. During 10 to 20 days
per year, the NSSD disposal operation is abandoned due to wet
conditions, and the dewatered sludge is stored in a covered and
enclosed building for disposal within a few days. The building
is enclosed to maintain odor control. The District also
frequently liberally sprinkles the dewatered sludge with lime
during transport and storage to maintain odor control.
Unfortunately, no quantitative work has been published regarding
the odor risk of stockpiling dewatered sludge. Drying lagoons,
like those operated at San Jose, California, do create malodorous
conditions in surrounding urban areas during or immediately
after being wetted by rainfall. Work in Sacramento, California,
however, indicates that odors are generated cumulatively in
direct relationship to the area covered by the odor producing
sludge (1). Good housekeeping around such stockpiles is
mandatory to assure proper rodent control.
15.4 References
1. Sacramento Area Consultants. Sewage Sludge Management
Program Final Report, Volume 4, SSBs and Odors, 1978.
Sacramento Regional County Sanitation District. Sacramento,
California 95814. September 1979.
2. San Francisco Bay Region Wastewater Solids Study,
San Francisco Bay Region Sludge Management Plan. Volume V,
San Jose/Santa Clara Project and Environmental Impact
Report. p. 2-4. Oakland, California 94620. December 1978.
3. USEPA. Communication to J.B. Farrell, Ultimate Disposal
Section to Office of Solids Waste. Best Management
Technology Definitions for (a) Sludge Stabilization and
(b) Additional Pathogen Reducing Processes. MERLE.
Cincinnati, Ohio 45268. November 1978.
4. Metcalf and Eddy, Inc. Wastewater Engineering: Treatment
Disposal, Reuse - Second Edition. McGraw-Hill Book Company.
p. 322 and 353. 1979.
15-58
-------
5. Water Pollution Control Federation. Manual of Practice
No. 8, Wastewate r P1 an _t_D e s i g n . WPCA Washington, D.C.
p. 57. 1977.
6. USEPA. MERL Publication Series. ^Evaluation of Flow
Equalization in Municipal Wastewater Treatment. Cincinnati^
Ohio, 45268. EPA-600/2-79-096. May 1979.
7. USEPA. Technology Transfer Upgrading Existing Wastewater
Treatment Plants. USEPA Cincinnati, Ohio 45268. October
1974.
8. Berk, W.L. The Design, Construction and Operation of the
Oxidation Ditch. RAD-211, Lakeside Equipment Corporation,
1022 E. Devon Avenue, Bartlett, Illinois 60103.
9. Dick, R.I., E.L. Thakston, and W.W. Eckenfelder, Jr., Ed.
Water Quality Engineering New Concepts and Developments.
Jenkins Publishing Co., Austin and New York. 1972.
10. Keinath, T.M, M.D. Ryckman, C.H. Dana, Jr., D.A. Hofer.
Design and Operational Criteria for Thickening of Biological
Sludges, Parts I, II, III, IV. Water Resources Research
Institute, Clemson University. September 1976.
11. Tucker, D.L., N.D. Vivado. "Design of an Overland Flow
System at Newman, California." Proceeds of. the 51st Annual
Water Pollution Control Federation Conference. Anaheim,
California. October 1978.
12. Liptak, B.C., Ed. Environmental Engineers Handbook,
Volume I Water Pollution. Chilton Book Comany. p. 807.
Radnor, Pennsylvania. 1974.
13. State of California Water Resources Control Board. Final
Report-Phase I, Rural Wastewater Disposal Al ternatjLyjss.
Sacramento, California, p. 12. September 1977.
14. USEPA. Upgrading Lagoons. Technology Transfer Seminar
Publication. Cincinnati, Ohio 45268. EPA-626/4-73-0016.
Revised June 1977.
15. Sacramento Area Consultants. Sewage Sludge Management
Program Final Report, Volume 7, Environmental Impact Report
and Advanced Site Planning. Sacramento Regional County
Sanitation District. Sacramento, California 95814.
September 1979.
16. Brown and Caldwell. Joint Regional Wastewater and Solids
Treatment Facility Project Design Report. Moulton-Miguel
Water District-Aliso Water Management Agency. March 1978.
17. Sacramento Area Consultants. Sewage Sludge Management
Program Final Report, Volume 2, SSB Operation and
Performance. Sacramento Regional County Sanitation
District. Sacramento, California 95814. September 1979.
15-59 *
-------
18. Sacramento Area Consultants. Study of Wastewater Solids
Processing and Disposal, Appendix C.
County Sanitation
June 1975.
District,
Sacramento Regional
Sacramento, California 95814.
19
Sacramento
Technology
Area Consultants. Innovative and Alternative
Documentation Sacramento Regional Wastewater
20,
Treatment Plant - Solids Project.
County Sanitation
April 1979.
Sacramento Regional
District. Sacramento, California 95814.
Hawk, B.C.
Pitsburg,
1971.
, Ed. Bulk Materials Handling.
School of Engineering. Pittsburg,
University of
Pennsylvania.
21. National Lime Association. Lime Handling Application and
Storage in Treatment Processes, Bulletin 213
D.C. Second Edition. 1971.
Washington,
22.
Ve s ilind,
Sludges.
Michigan.
P.A,
Treatment and Disposal of Wastewater
Ann-Arbor
1974.
Science Publishers, Inc. Ann Arbor,
23.
24.
25.
Rimkus, R.R., J.M. Ryan, R.W. Dring. "A New Approach to
Dewatering and Disposal of Lagooned Digested Sludge."
Proceeds of the Annual Convention, ASCE, Chicago, IljLino^s.
October 1978.
B.E. Hansen, D.L. Smith, W.E. Garrison. " Start-Up
Problems of Sludge Dewatering Facility." Proceeds of the
51st Annual Water Pollution Control Federation Conference,
A n a h e i m, C a 1 if or_n_ia.. October 1978.
Lukasik, G.D., J. W. Cormack. Development and Operation of
a Sanitary Landfill for Sludge Disposal - North Shore
ganitary District. North Shore Sanitary District. 1976.
15-60
-------
EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chater 16. Sidestreams from Solids Treatment
Processes
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 16
SIDESTREAMS FROM SOLIDS
TREATMENT PROCESSES
Sidestreams are a major reason why solids treatment and disposal
facilities often become trouble spots at wastewater treatment
plants. Failure to account for these sludge processing liquors
in the wastewater treatment design can result in overloading of
the treatment facility. It has been conventional practice to
return sludge sidestreams to the treatment plant at a convenient
point, usually at the headworks, with no pretreatment and with
little concern for its pollutant loadings. These sidestreams can
increase the organic loading by 5 to 50 percent, depending on the
type and number of solids treatment processes used.
The major objectives of this chapter are to describe the
sidestreams produced by sludge treatment processes, factors that
affect sidestream quality, and options available to designers in
managing the sidestreams. Information on the pollutant loads of
the sidestream produced by a particular process is presented in
the chapter dealing with that process.
16.1 Sidestream Production
Sidestreams are produced when wastewater solids are concentrated,
and when water, usually plant effluent, is used to remove odors
or particulate matter from flue gases, or to wash and transport
debris from structures and equipment. Some sidestreams require
special attention because of their impact on a wastewater
treatment plant's efficiency.
Usually several sidestreams are produced at a particular plant.
Figure 16-1 is a flow diagram showing eight wastewater solids
sidestreams: (1) screenings centrate, (2) grit separator
overflow, (3) gravity thickener supernatant, (4) dissolved air
flotation subnatant, (5) decantate following heat treatment,
(6) vacuum filter filtrate and washwater, (7) scrubber water from
furnace flue gas cleanup, and (8) overflow from biological odor
removal system.
This chapter devotes special attention to the most pronounced
examples of the problem--anaerobic digester supernatant and
thermal conditioning liquor. For additional information on
production and treatment of wastewater solids sidestreams,
several publications are available. Municipal Wastewater
Treatment Plant Sludge and Liquid Sidestreams deals with side-
streams from severalsolids handlingandtreatment processes (1).
16-1
-------
Effects of Thermal Treatment of Sludge on Municipal Wastewater
Treatment Costs describestheincreased wastewatertreatment
capacity required by use of thermal conditioning (2).
WA5TEIVATER
WAST LWATEH SOLIDS
i —-. NON-CHLOBJNATFO EF
miTfi'l GASLLhJEDISCHAIGEE
•0— SlOESTBtAKlS
r
i
n
SCREENINGS
REMOVAL
GRIT
MtMOV,lL
HLI
'*
i PRIMARY
SEniMEWATIC
AERATION
i
©
i
c
1
._
'}
1
' '& v-^
!!' i
' «' t
i ' - :i u
SCREENINGS OVEEFLCW _,Ci"M Il_j Gil AMITY
DEYVATcRE-t SCREEN "* "TPrtAT^fl " ' THICItENEa
" "-" I j_
1
•
' 0
ainLoscfti
I ODOR --®-*-|
! KtMO'-'AL
h«
""V-
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'*"•""'_
V
h 0 _
?Or
V'y
• 1~©
'• DECANTING
® I
*
1'HlCKC
"t
J
" 1
f FU,
KHUBEiNG
" ~"| ~
1 FINAL
INC;NE!I4TOR i~ n •-.—*•
i DISPOSAL
FIGURE 16-1
EXAMPLE OF SIDESTREAM PRODUCTION
16.2 Sidestream Quality and Potential Problems
The interrelationship between a wastewater treatment plant's
effluent quality requirements and the processes used for solids
treatment and disposal must be carefully scrutinized during
planning and design to avoid problems caused by sidestreams.
Generally, more sophisticated wastewater treatment plants produce
greater quantities of more difficult-to-manage biological and
chemical sludges, When processed, these sludges may indirectly
cause the production of sidestreams containing large quantities
of soluble and colloidal materials including nutrients.
Sidestream quality from a specific process is strongly affected
by upstream solids handling processes. Vacuum filter filtrate
and washwater quality, for example, are determined by the
upstream conditioning or stabilization process.
16-2
-------
Sidestreams should be returned to points in the wastewater
treatment process which will -result in treatment of the side-
stream and prevent nuisances and operational problems. The
return points shown on Figure 16-1 comply with this requirement.
Runoff from sludge composting areas and leachate from sludge
landfilling areas may pose a unique problem, since it may be
difficult and costly to return them to the treatment plant if
the landfill or composting site is remote from the treatment
facility. Chian and DeWalle extensively investigated the
composition and treatment of sanitary landfill leachate,
including anaerobic biological filtration, chemical precipita-
tion, chemical oxidation, and activated carbon treatment (3).
Data are also available on groundwater monitoring near sewage
sludge or combined solid waste sewage sludge landfills (4,5,6).
In addition, USEPA's Process Design Manual, Municipal Sludge
Landf ills, discusses methods of handling leachate (7). In dry
climates, leachate can often be recycled to the landfill site.
At Beltsville, Maryland, runoff from a composting site is stored
in a pond and periodically sprayed on a forest floor. Monitoring
wells have been installed, and no groundwater contamination has
been detected. At Durham, New Hampshire, and Bangor, Maine,
runoff is recycled back to the treatment works without pretreat-
ment. At Sacramento, California, runoff has been returned from
a dedicated land disposal site to the plant headworks and has
been monitored for several parameters (8) . It was found that
runoff is polluted with constituents, particularly the first
runoff following the spreading of sludge. The concentrations of
insoluble constituents such as heavy metals, however, were
1 ow.
16.3 General Approaches to Sidestream Problems
Several general approaches to preventing or solving problems that
may result from sidestreams can be identified:
• Modification of solids treatment and disposal systems
to eliminate particular sidestreams.
• Modification of previous solids processing steps to
improve sidestream quality from a particular solids
treatment process.
• Changing the timing, return rate, or return point for
reintroducing sidestreams into the wastewater treatment
process.
• Modification of wastewater treatment facilities to
accommodate sidestream loadings.
• Provision of separate sidestream treatment prior to
return.
Potential applications for each of these are described.
16-3
-------
16.3.1 Elimination of Sidestream
Although not generally practical, specific situations arise in
which it is possible to modify the solids treatment and disposal
system and eliminate a troublesome sidestream. A particular case
involves anaerobic digester supernatant, which has often been
identified as a source of problems when a mixture of primary and
waste-activated sludges is digested. Mignone has pointed out
that where mechanical dewatering follows anaerobic digestion, it
would be beneficial to eliminate the secondary (unmixed) digester
by converting it to a primary mode (9,10,11). There would be no
variable supernatant stream, only a predictable filtrate or
centrate stream of low solids content which would be amenable to
biological treatment.
16.3.2 Modification of Upstream
Solids Processing Steps
Thickening of sludge prior to anaerobic digestion by the use of
gravity, flotation, or centrifugal thickeners can improve the
quality and reduce the quantity of digester supernatant (12).
Residence time in the digesters is increased and/or smaller
digesters can be constructed. Liquor that would otherwise be
produced by the secondary digester as supernatant is produced
instead in the thickening step. Its quality will be better, and
it will have a lesser impact when returned to the wastewater
treatment facility.
Other digester operating parameters such as organic loading and
temperature also affect supernatant quality. An increase in
organic loading will generally result in poorer supernatant
quality (13). Thermophilic digestion produces poorer supernatant
quality than mesophilic digestion.
Substitution of an equivalent solids treatment process for
another may also reduce sidestream problems. For example,
substitution of chemical conditioning for elutriation or heat
treatment can reduce the level of contaminants in sidestreams
from subsequent dewatering steps.
The high colloidal content of elutriate has been successfully
reduced in several instances by addition of chemicals,
particularly polymer, to the elutriation process. In 1973 the
sludge treatment system at the District of Columbia's Blue Plains
plant (a 253-MGD [11.l-m3/sec] facility) consisted of gravity
thickening, single-stage anaerobic digestion, elutriation of
digested sludge, chemical sludge conditioning, and vacuum filtra-
tion. Large quantities of fines and activated sludge solids
were recycled with the elutriate, and the primary clarifiers and
aeration process could not accommodate them. Solids accumulated
in the plant; upsets occurred in both the wastewater and sludge
treatment systems; and it became necessary to temporarily
discharge elutriate .directly to the plant effluent. Eventually,
addition of polyelectrolyte to the elutriation process, coupled
16-4
-------
with intensive effort on the part of plant staff to improve
elutriation and vacuum filtration performance, resulted in a
90 percent solids capture through the two processes.
The Metropolitan Toronto main plant and the Richmond, California,
facility experienced the same results as the Blue Plains plant.
An example of successful use of polymer to improve elutriation is
shown in Table 16-1 (14).
TABLE 16-1
EFFECT OF POLYMER ON ELUTRIATION (14)
Parameter
Before
polymer
use
After
polymer
use
Elutriate suspended
solids,, mg/1 3,385 365
Solids capture, percent 65.1 95.3
Underflow solids con-
centration, percent 3.5 4.3
16.3.3
Sidestreams
facilities
sidestreams
rather than
overloads.
fluctuations
Change in Timing, Return Rate,
or Return Point
are normally returned to the wastewater treatment
at the plant headworks. In general, return of
to plant headworks should be at a low, steady rate
in slugs, since these are likely to cause upsets and
In instances where there are high diurnal load
and the plant is approaching capacity, consideration
should be given to returning sidestreams during off-peak hours,
thus equalizing wastewater loadings. Adverse effects on primary
treatment facilities, such as septicity, odors, and floating
sludge can be avoided by returning sidestreams to the biological
treatment process influent. Alternatively, mixing supernatant
with waste-activated sludge before returning it to the headworks
may also aid in reducing odors because of the adsorptive nature
of the activated sludge particles.
16.3.4 Modification of Wastewater
Treatment Facilities
Liquid treatment facilities should be designed with the capacity
to treat recycled sidestreams whenever the sidestream will
contain significant concentrations of pollutants or have a large
hydraulic impact. Table 16-2 shows an example of the effect of
supernatant return on suspended solids and phosphorus loadings at
16-5
-------
an activated sludge plant using two-stage anaerobic digestion
(15). Table 16-3 shows estimated increases in 6005 treatment
capacity required by sidestreams from several sludge treatment
processes (16) .
TABLE 16-2
EFFECT OF SUPERNATANT RETURN (15)
Suspended solids, Ib/day
Phosphorus, Ib/day
Point of measurement
Raw wastewater
To primary clarifiers
To secondary clarifiers
Final effluent
Primary sludge
Waste activated sludge
With
supernatant
return3
10,520
36,801
15,306
3,467
19,626
14,645
Without
supernatant
return
16,035
15,969
9,501
2,836
13,249
9, 593
With
supernatant
return3
756
1,304
991
435
299
453
Without
supernatant
return
857
914
803
500
156
287
Returned ahead of primary clarifiers.
The Central Contra Costa Sanitary District Water Reclamation
Plant, an advanced waste treatment facility, removes
nutrients through chemical-primary treatment and nitrification-
denitrification. Recycled sidestreams were taken into account in
plant design by allowing for additional loads of 12 percent for
8005 and 21 percent for suspended solids. Recycled streams
include gravity thickener overflow, centrate from a two-stage
dewatering centrifuge, and drainwater from a wet scrubber.
Sidestreams may contain compounds that are difficult to remove
in wastewater treatment facilities. For example, the nonbio-
degradable COD in heat treatment liquor will pass through normal
secondary treatment unchanged. Digester and sludge lagoon
supernatant may contain high concentrations of nutrients. In
some instances separate treatment may be appropriate. The
Metropolitan Sanitary District of Greater Chicago has conducted
several investigations involving nitrification and nitrogen
removal from sludge lagoon supernatant,,using both attached
growth and suspended growth biological processes (17,18,19).
In evaluating solids treatment and disposal processes, both the
direct costs of the solids treatment and disposal systems and the
indirect costs associated with return of sidestreams to the
wastewater treatment facilities should be included in the cost-
effectiveness analysis. The cost of handling the increased
sidestream flows may or may not be negligible, but capital and
operating expenses will surely increase as a result of the 8005
and suspended solids load of the returned stream. Major
16-6
-------
components of such indirect costs include increased aeration tank
size and blower capacity (for diffused air-activated sludge
systems), increased sludge treatment capacity, increased power
requirements for blowers, and increased labor for operating and
maintaining more heavily loaded secondary treatment facilities.
Additional costs will also be incurred if odor control facilities
are required.
TABLE 16-3
ESTIMATED INCREASE IN WASTEWATER STREAM
BIOLOGICAL TREATMENT CAPACITY REQUIRED TO
HANDLE SIDESTREAMS FROM VARIOUS SOLIDS
TREATMENT PROCESSES (16)
Required capacity
Treatment process increase, percent
Liquid sludge to land 0
Raw sludge to drying beds 7
Chemical conditioning and 6-11
filter pressing
Rotoplug dewaterer 10 - 30
Digestion and drying beds 0.6
Digestion, chemical con- 5
ditioning, and filter
pressing
Digestion, chemical con- 4
ditioning, and vacuum
filtration
Heat treatment of raw 30
sludge
Heat treatment of di- 7
gested sludge
Indirect solids treatment costs for handling sidestreams will
vary significantly. The indirect costs associated with heat
treatment have been estimated as 20 percent of the direct thermal
treatment costs. A report has been prepared describing the
effects of sludge heat treatment on overall wastewater treatment
costs (2).
16.3.5 Separate Treatment of Sidestreams
Most sidestreams from properly operating solids treatment and
disposal systems can be recycled to the wastewater treatment
facilities without significant problems. In many cases two-stage,
anaerobic digester supernatant return to the wastewater treatment
16-7
-------
^facilities causes operating difficulties. Heat treatment is less
widely used, but it results in conversion of some of the COD to
the soluble form. Furthermore, a portion of the COD can be
nonbiodegradable .
16.3.5.1 Anaerobic Digester Supernatant
In most cases, 6005 and suspended solids are of concern, although
under certain circumstances, nitrogen and phosphorus removal may
also be desirable. Anaerobic digester supernatant characteris-
tics are summarized in Chapter 6, and typical values are given as
a part of the example on Figure 16-2. Table 16-4 lists possible
treatment processes for each major constituent (20). Chemical
treatment of digester supernatant has been studied for many years
(21,22,23). Rudolfs and Gehm studied coagulation using ferric
chloride, lime, caustic soda, sulfuric acid, chlorine, bentonite
clay, and zeolite (21). It was found that a lime/ferric chloride
combination gave the best results and 150 mg/1 ferric chloride
and 1,200 mg/1 lime reduced turbidity from 420 to 110 units.
The carbon dioxide in digester supernatant will react with the
lime to form calcium carbonate precipitate. Lime requirements
and the quantity of lime sludge produced can be reduced
significantly by first air stripping carbon dioxide from the
supernatant. This may also release odors, and for this reason,
its use should be approached with caution. Because lime raises
the pH of the supernatant and under high pH conditions the
ammonia molecule tends to be in the nondissociated form, ammonia
stripping can be affected after coagulation. The relatively high
temperature of digester supernatant also aids ammonia stripping
for the same reason.
Figure 16-2 shows a possible treatment scheme for digester
supernatant based principally on chemical coagulation (20). Also
shown are probable removals and common influent and expected
effluent concentrations. Straight aeration of digester super-
natant at plant scale has also been attempted (12,24,25) .
Even where the supernatant after aeration was not settled prior
to return and no discernible improvement in quality resulted, it
was found that wastewater treatment operation improved, probably
as a result of better settling in the primary clarifiers.
Biological filters, either aerobic or anaerobic, appear to be
feasible methods of biologically treating digester supernatant.
The Greater London Council studied aerobic biofilter treatment of
supernatant liquor using coke as the filter medium (26). At a
1:1 dilution with clarified plant effluent, 85 to 90 percent
removal and 60 percent ammonia removal were obtained.
Howe suggested storage of digester supernatant in lagoons
for long periods to reduce contaminant levels (22) . In one
experiment, a detention time of 60 days reduced 6005, suspended
solids, color, and ammonia by about 85 percent; hydrogen sulfide
16-8
-------
was reduced by approximately 95 percent. Facultative sludge
lagoons designed for long-term storage have been found to reduce
levels of all contaminants except ammonia;(see Chapter 15).
SUPERNATANT
BQDg = 7,&QQ
SS^ 5,000
ORG.N = 400
P * ISO
NMg» goo
C02 - 1,000
C02 STRIPPING
REDUCTIONS
95-98 PERCENT CO2
LIME/
FERRIC CHLORIDE
COAGULATION
PLUS SETTLING
1
70-85 PERCENT BQD5
80-90 PERCENT SS
65-70 PERCENT ORG.N
85-95 PERCENT P
AMMONIA
STRJPPING
85-90 PERCENT NH3
j • TREATED
i SUPERNATANT
BODg * 1,750 mg/1
SS= 750
ORG.N =' 150
P - 15
NH3 = 75
CO2 = 50'
FIGURE 16-2
POSSIBLE TREATMENT SCHEME
:FOR ANAEROBIC DIGESTER SUPERNATANT (20)
16-9
-------
TABLE 16-4
POSSIBLE DIGESTER SUPERNATANT
TREATMENT PROCESSES (20)
Constituent
Suspended
solids
BOD:
Processes
Coagulation, settling,
microstraining
Removal with suspended
solids, stripping of
volatile acids, bio-
logical .treatment, ad-
sorption on activated
carbon
Phosphorus
Nitrogen
CO,
Removal with suspended
solids, chemical pre-
cipitation, ion exchange
Removal with suspended
solids (limited),
ammonia stripping, ion
exchange
Lime addition, air strip-
ping
The chlorine stabilization process (see Chapter 6) has also been
used to treat digester supernatant before it is returned to the
treatment plant (Table 16-5). Low chlorine doses (100 to
300 mg/1) have little effect on 8005 and COD levels, but
according to the manufacturer, they may be used to reduce odors
and improve treatability of the supernatant. Very high dosages
(1,500 to 2,000 mg/1) are required to appreciably reduce the
levels of oxygen demanding materials in the supernatant liquor.
16.3.5.2 Thermal Conditioning Liquor
Heat treated sludge liquor, which is received as decantate
and filtrate or centrate, contains high levels of soluble
pollutants and a significant fraction of nonbiodegradable COD.
The color level of the liquor may dlso be high, affecting
the color of the final effluent (27). Furthermore, chlorination
of effluent containing recycled heat treatment liquor may cause
taste and odor problems if the receiving stream is used for
drinking water supply (28).
Loll has cited average BOD5 loading increases of 7 to 15 percent
and COD increases of 10 to 20 percent at wastewater facilities
recycling untreated liquor (29). Recycle of heat treatment
16-10
-------
liquor at Colorado Springs, Colorado, caused the 6005 loading
to be increased by 20 percent and the suspended solids load by
30 percent (27).
TABLE 16-5
CHLORINE TREATMENT OF DIGESTER SUPERNATANT
Value3
Supernatant treated at indicated
chlorine dose, mg/1
Untreated
Parameter supernatant 500 1,500 1,800 1,900 2,000
Suspended solids,
percent 1.9 1.7 1.8 1.7 1.7 2.0
Chlorine residual, mg/1 0 0 0 10 80 190
pH 6.8 5.8 5.5 4.8 4.4 2.7
Specific conductance,
micromhos
Alkalinity, mg/1
BOD5, mg/1
COD, mg/1
Total nitrogen, mg/1
Total phosphate
phosphorus, mg/1 510 430 440 400 380 260
1,950
1,100
2,600
43,900
2, 100
2,750
170
2,600
43,100
2,200
2,380
83
2,600
40,800
1,900
2,500
60
2,200
32,000
1,600
2,600
32
2,000
31,200
1,400
4,500
0
1,500
20,200
1, 100
SBased on results obtained with Purifax laboratory unit.
Trickling filters, the activated sludge process, anaerobic
biological filtration, and aerobic digestion have been used to
treat the liquor. To reduce the nonbiodegradable COD, activated
carbon has been used. Ozonation or chlorination can also be used
to reduce COD levels.
Loll has described experiments using autothermal therraophilic
aerobic digestion of heat treatment liquors (29). Because
the reactions are exothermic, the process is thermally
self-supporting.
Presented on Figure 16-3 are the results of batch aerobic
digestion tests. Note that the temperature rose during the
period of most rapid degradation. The results of continuous flow
tests are presented in Table 16-6 at residence times of five
and ten days. The COD reduction is significantly less than the
BOD5 reduction, reflecting the nonbiodegradable character of a
portion of the waste.
Erickson and Knopp used the activated sludge process for heat
treatment liquor (30). They reported a COD reduction of
83 percent and a BOD5 reduction of 98 percent with an aeration
time of 41 hours. Results are shown in Table 16-7, (page 16-14).
16-11
-------
15
o COD
X BOD5
D VOLATILE ACIDS
TOC
TEMPERATURE
60
50
40
8
0
30
o
*.
LJJ
ac
DC
LU
0.
s
LU
I-
20
10
5 10
TEST LENGTH, days
FIGURE 16-3
AEROBIC DIGESTION OF
HEAT TREATMENT, BATCH TESTS (29)
Anaerobic biological filtration of heat treatment liquor has been
tested for use at the City of Los Angeles Hyperion treatment
plant (31). The waste-activated sludge treatment scheme is shown
on Figure 16-4. The anaerobic filter, originally developed by
Young and McCarty is similar to the conventional aerobic trick-
ling filter in that organisms are attached to the media surface
and a short hydraulic detention time results (32). Advantages
16-12
-------
are that the production of methane can result in energy recovery
and that no power is required for oxygen addition. Care must
be taken, however, to avoid any plugging from periodic high
suspended solids loadings. Results of a two-month test are shown
in Table 16-8 (31). At a hydraulic detention time of two days,
8005 and COD removals averaged 85 and 76 percent, respectively.
This study concluded that detention time could be reduced to
about 0.5 to 1.0 days without significant deterioration in
performance. Other pilot scale tests on anaerobic filtration of
heat treatment liquor have been conducted. One study reported
COD removals of approximately 65 percent at detention times of
3.5 days and organic loadings of 125 Ib COD per 1,000 cubic feet
per day (2.0 kg/m3/day)(33) .
TABLE 16-6
AEROBIC DIGESTION OF HEAT TREATMENT LIQUOR (29)
Residence time,
days
Parameter 5 10
Temperature, °c 38 34
COD
Influent, mg/1 13,500 12,400
Effluent, mg/1 4,100 3,800
Reduction, percent 66 71
BOD5
Influent, mg/1 6,900 6,100
Effluent, mg/1 510 250
Reduction, percent 94 96
Figure 16-5 illustrates the AS pilot treatment scheme used in
a pilot study in Great Britain (28). The purpose of the study
was to reduce the quantity of refractory organics entering
the Thames River from treatment plants conditioning sludge
with heat treatment. The study was prompted by the fact that the
Thames is used for water supply, and possible taste and odor
problems would result from chlorinating the water; in addition,
there was uncertainity about the exact composition and effects of
the organics in the liquor. The process can reduce COD from
20,000 mg/1 to about 100 mg/1, or by approximately 99.5 percent.
The chlorine oxidation process can also be used for treating
liquor from thermal sludge conditioning. 6005 and COD levels
are reduced by approximately 25 to 35 percent. The odor is
changed from noxious to chlorinous or medicinal. The color is
16-13
-------
changed from dark brown to yellow or tan which may allow the
liquor to go undetected when diluted in the liquid stream.
Results of a pilot test on Zimpro process liquor are shown in
Table 16-9. A flow diagram indicating sampling point locations
is shown on Figure 16-6.
TABLE 16-7
ACTIVATED SLUDGE TREATMENT OF THERMAL
CONDITIONING LIQUOR (30)
Aeration time,
hours
Parameter
21.8
40 .9
Temperature,
°C
COD
Influent, mg/1
Effluent, mg/1
Reduction, percent
BOD5
Influent, mg/1
Effluent, mg/1
Reduction, percent
33.4
10,600
4,300
59
4,700
400
91
31.7
11,900
2,000
83
5,900
110
98
METHANE
CAR BOM DIQXiDE
WASTE
ACTIVATED
CONCENTRATION
» 1%
i r
UNDERFLOW
TO TREATMENT
PLANT
\THICKENI NG/
HEAT r. _A /
IEATMEMT , 5,^ \ ~~ '/
Y
m
1 1
DEWATERING f-
1
i * p
! *
i
HEAT
TREATMENT
LIQUOR
EFFLUENT
u^r RECYCLED TO
TREATMENT
PLANT
ANAEROBIC
FILTRATION
CAK.E
FIGURE 16-4
FLOW DIAGRAM, ANAEROBIC FILTRATION OF
HEAT TREATMENT LIQUOR (31)
16-14
-------
TABLE 16-8
AEROBIC BIOLOGICAL FILTRATION OF THERMAL
CONDITION LIQUOR (31)
Parameter
Value
Hydraulic detention time, days
Temperature, °C
COD
Influent, mg/1
Effluent, mg/1
Reduction, percent
BOD5
Influent, mg/1
Effluent, mg/1
Reduction, percent
Suspended solids
Influent, mg/1
Effluent, mg/1
Total solids
Influent, mg/1
Effluent, mg/1
Volatile acids
Influent, mg/1
Effluent, mg/1
Alkalinity, as CaCO3
Influent, mg/la
Effluent, mg/1
PH
Influent3
Effluent
2.0
32
9,500
2,300
76
3,000
450
85
110
100
8,800
4,900
520
300
2,200
3,500
7.1'
7.1
Decant liquor.
pH following thermal conditioning was
approximately 5.5; 1,600 mg/1 alkalinity
added to influent for pH adjustment.
16-15
-------
HEAT TREATMENT LIQUOR
COD 20,000 mg/1 APPROXIMATELY
I
ROUGHING
FILTER
(COD 3,000 mg/|)
1
—1
ACTIVATED
CARBON
COLUMN
l_
"I
ACT IV
CARE
con
AERATION
TANKS
49 MRS.
DETENTION
I
ACTIVATED
CARBON
COLUMN
I
I
*** EFFLUENT TO SEWAGE
TREATMENT WORKS
(COD 10Qmg/i}
(COD 900 mg/|)
FIGURE 16-5
SCHEMATIC DIAGRAM OF PLANT FOR PROCESSING
HEAT TREATMENT LIQUOR (2)
TABLE 16-9
CHLORINE OXIDATION TREATMENT
OF THERMAL CONDITIONING LIQUOR
Parameter
COD, mg/1
Suspended solids, mg/1
Total solids, mg/1
Total volatile solids, percent
Ammonia, mg/1
Chlorine dose, mg/1
Chlorine residual after three
hours, mg/1
pH
40
19
24
t
1
,664
,300
,500
63 .1
225
0
0
5.1
Value
2
31,280
15,400
16,800
65.5
209
1, 000
0
3.7
a , b
3
3,910
172
5,700
66.4
209
1,000
0
3 .5
4
70, 380
51,600
52,000
56.1
269
1,000
0
3.9
For location of sampling point, see Figure 16-6.
""Data taken at Canton Water Pollution Control Center,
May 10 and 11, 1977.
16-16
-------
FROM
Z1MPRO
PROCESS
DECANTING
1
PURIFAX
TREATMENT
rT1
DECANTING
1
TO
DEWATERING
NOTE: CIRCLED NUMBERS DESIGNATE SAMPLING
POINTS; SEE TABLE 16-9 FOR QUALITY DATA.
FIGURE 16-6
CHLORINE TREATMENT OF HEAT TREATMENT LIQUOR
16.4 References
1. Municipal Wastewater Treatment Plant Sludge and Liquid
2.
3.
Sidestreams. USEPA Report No. EPA
Water Program Operations. p. 119.
430/9-76-007.
June 1976.
Office of
Ewing , L . J ,
of Thermal
, Jr., H.H. Almgren,
Treatment of Sludge
and R.L. Gulp. Effects
on Municipal Wastewater
7.
Treatment Costs
102
June 1978
USEPA Report No. EPA-600/2-78-073 .
Chian, E.S.K., and F.B. DeWalle. "Sanitary Landfill
Leachates and Their Treatment." Proceedings ASCE, Journal
of the Environmental Engineering Division. Vol. 102, p. 411.
1976.
H .T
Phung, R.P. Stearns, and J.J. Walsh.
Disposal of Municipal Wastewater Treatment
Lof y , R . J .
Subsurface _
Sludge, Environmental Assessment. USEPA Office of
Waste, prepublication issue, Contract No. 68-01-4166.
Solid
1978.
Sikora, L.J., C.M. Murray, N.H. Frankos, and J.M. Walker.
"Water Quality at a Sludge Entrenchment Site." Groundwater.
Vol. 16. 1978.
Walker, J.M. , L
Kaminski. USEPA.
by Small Communities
Ely, P
Sewage
Hundenmann, N. Frankos, and A.
Sludge Entrenchment System for Use
EPA-600/2-78-018. February 1978
USEPA, Technology Transfer
Municipal Sludge Landfills. p.
Process Design Manual
195. October 1978.
16-17
-------
8. Sacramento Area Consultants. Sewage Sludge Management
Program, Final Report, Volume 5, Dedicated Land Disposal
Study. Sacramento Regional County Sanitation District.
Sacramento, California 95814. September 1979.
9. Mignone, N.A. "Digester Supernatant Does Not Have To Be a
Problem." Water& Sewage Works. p. 57. December 1976.
10. Mignone, N.A. "Survey of Anaerobic Digestion Supernatant
Treatment Alternatives." Water & __Sew_age Works. p. 42.
January 1977.
11. Mignone, N.A. "Elimination of Anaerobic Digester
Supernatant." Water & Sew_age_Works;. p. 48. February 1977.
12. Kappe, S.E. "Digester Supernatant: Problems, Character-
istics, and Treatment." Sewage and I ndustrial Wastes.
Vol. 30, p. 937. 1958.
13. Mueller, L., E. Hindin, J.V. Lundsford, and G.H. Dunstan.
"Some Characteristics of Anaerobic Sludge Digestion - I,
Effect of Loading." Sewage and Industrial Wastes. Vol. 31,
p. 669. 1959.
14. Burd, R.S. "Use of New Polyelectrolytes in Sewage Sludge
Conditioning." Proceedings of the 2nd yande_rbilt_ Sanitary
Engineering Conference. May 1963.
15. Geinopolos, A., and F.I. Vilen. "Process Evaluation -
Phosphorus Removal." Journal Water Pollution Control
Federation. Vol. 43, pp. 1975-1990. 1971.
16. Clough, G.F.G. "The Effect of Sludge Treatment Processes
on the Design and Operation of Sewage Treatment Plants."
Water Pollution Control. Vol. 76., p. 452. 1977.
17. Lue-Hing, C., A.W. Obayashi, D.R. Zenz, B. Washington, and
B.M. Sawyer. "Nitrification of a High Ammonia Content
Sludge Supernatant by Use of Rotating Discs." Presented
at the 29th Annual Purdue Industrial Waste Conference.
West Lafayette, Indiana. p. 245. May 1974.
18. Lue-Hing, C., A.W. Obayashi, D.R. Zenz, B. Washington, and
B.M. Sawyer. "Biological Nitrification of a High Ammonia
Content Sludge Supernatant. Under Ambient Winter and Summer
Conditions by Use of Rotating Discs." Presented at the
47th Annual New X°rk Water Pollut_ion__Control Conference.
January 1975.
19. Prakasam, T.B.S., W.E. Robinson, and C. Lue-Hing. "Nitrogen
Removal From Digested Sludge Supernatant Liquor Using
Attached and Suspended Growth Systems." Presented at
the 32nd Annual Purdue Industrial Waste Confere nee.
West Lafayette, Indiana. p. 745. May 1977.
16-18
-------
20. Malina, J.F., and J. DiFilippo. "Treatment of Supernatant
and Liquids Associated with Sludge Treatment." Water &
Sewage Works, Reference Number, p. R-30. 1971. ~"
21. Rudolfs, W., and L.S. Fontenelli. "Supernatant Liquor
Treatment with Chemicals." Sewage Works Journal. Vol. 17,
p. 538. 1945 .
22. Howe, R.H. "What To Do with Supernatant." Wa __s t e s
Vol. 30, p. 12. 1959 . —
23. Reefer, C.E., and H. Kratz, Jr. "Treatment of Supernatant
Sludge Liquor By Coagulation and Sedimentation." Sewage
Works Journal. Vol. 12, p. 738. 1940 . "
24. Erickson, C.V. "Treatment and Disposal of Digestion Tank
Supernatant Liquor." Sewage Works Journal. Vol. 17,
p. 889. 1945 .
25. "The PFT Supernatant Liquor Treater." Sewage Works Journal.
Vol. 15, p. 1018. 1943 .(Author anonymous).
26. Brown, B.R., L.B. Wood, and H.J. Finch. "Experiments on
the Dewatering of Digested and Activated Sludge." Wa_tejr
Pollution Control . Vol. 71, p. 61. 1972.
27. Boyce, J.D. and D.D. Gruenwald. "Recycle of Liquor from
Heat Treatment of Sludge." Journal Water Pollution Control
Fe_der_ati£n. Vol. 47, pp. 2482-2489, 1975 .
28. Corrie, K.D. "Use of Activated Carbon in the Treatment of
Heat Treatment Plant Liquor." Water Pollution Contro^L.
Vol. 71, p. 629 . 1972 .
29. Loll, U. "Treatment of Thermally Conditioned Sludge
Liquors." Water Research. Vol. 11, pp. 869-872. 1977 .
30. Erickson, A.H. and P.V. Knopp. "Biological Treatment of
Thermally Conditioned Sludge Liquors." Proceedings of
the 5th International Water Pollution Control Research
Conference, San Francisco. Vol. II, p. 30. 1970 .
31. Haug, R.T., S.K. Raksit, and G.G. Wong. "Anaerobic Filter
Treats Waste Activated Sludge." Water & Sewage Works.
p. 40. February 1977.
32. Young, J.C. and P.L. McCarty. "The Anaerobic Filter
for Waste Treatment." Journal Water Pollution Control
Federation. Vol. 41, Research Supplement, p. R160. 1969 .
33. USEPA Pilot Scale Anaerobic Filter Treatment of High
Strength Heat Treatment Liquors; . MERL, Cincinnati,
Ohio 45268. Draft, Undated Contract No. 68-03-2484.
16-19
-------
EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapter 17. Instrumentation
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 17
INSTRUMENTATION
17.1 Introduction
Wastewater solids treatment and disposal systems are generally
under-instrumented in comparison to other treatment systems,
such as those in water supply or chemical processing plants (1).
While the economics and operating efficiencies of various
measuring devices, control equipment, and operator interface
displays should be carefully evaluated by the treatment system
designer, increased use of instrumentation is recommended. This
chapter examines instruments suitable for sludge treatment and
disposal facilities.
17.1.1 Purposes of Instrumentation
Most of the measuring devices described in this chapter are
"on-line" equipment designed for essentially unattended
operation. However, some critical data can be obtained only by
the use of portable test or laboratory equipment that requires
manual operation or attention. On-line instrumentation serves
the following purposes in a wastewater solids treatment system:
Reduces labor
Reduces chemical consumption
Reduces energy consumption
Improves treatment process efficiency and reliability
Provides information for planning
Verifies compliance with discharge requirements
Assures personnel safety
17.1.2 Instrumentation Justification
and Design Considerations
Some uses of instrumentation—for example, to reduce labor,
chemical consumption, or energy consumption--will be justified
primarily from an economics viewpoint. Economics may, however,
be a secondary consideration in decisions to install instrumenta-
tion for any of the other purposes listed above. For instance,
instrumentation for providing planning information and/or for
verifying compliance with discharge requirements may be justified
on non-economic grounds. The information provided may be
essential for planning new facilities and/or improving existing
facilities. Such information may also be required for monitoring
17-1
-------
treatment results for reports to various .government agencies.
Economic considerations will also be secondary for those systems
requiring continuous monitoring to protect operating personnel.
Economic analyses of instrumentation, when required, must
include both capital and operation and maintenance (0/M) costs.
0/M costs can be high, especially in sludge management, where the
materials being measured are usually debris-laden and sometimes
corrosive. A 1976 USEPA study found that many wastewater
treatment instruments are not properly operated or maintained
and quickly fall into disuse (1). This is particularly true
in small plants where the maintenance staff usually does not
include full-time instrumentation specialists, and where contract
instrumentation specialists are unavailable. The designer
must consider whether proper operation and maintenance will be
available before incorporating instrumentation into a plant's
design. In larger plants (20 to 30 MGD [0.88 to 1.3 m3/s]),
0/M staffs should include full-time instrumentation specialists.
Aside from the cost evaluations and 0/M requirements discussed
above, several factors will influence the selection of
instruments for a specific application. These include:
• Characteristics of the process fluid, particularly the
water, grease, grit, and gas content and the degree of
variability in the influent material from day to day.
o Configuration of process piping, channels, or vessels.
• Requirements relating to instrument measurement range and
accuracy.
• Utility availability (instrument air, purge water,
electricity, etc.).
The instrumentation information presented in Tables 17-1 through
17-12 is applicable to a wide variety of sludge treatment and
disposal processes. The tables list the process and process
variables, the measurements, and the suggested instruments
for individual process steps in treatment and disposal. The
specific instruments listed in these tables should be considered
as candidates, not as specific recommendations.
More detailed information about the various instruments is
available including illustrations, descriptions, and lists of
manufacturers (2). Note, however, that although many specific
instruments are used in both sludge processing and in conven-
tional industrial processes, some manufacturers are not active in
the wastewater field. The suitability of their instruments for
sludge applications has not been established.
17-2
-------
TABLE 17-1
THICKENING
Process arid process
variables
Gravity Thickener
Feed sludge
Dilution water
Tank sludge depth
Supernatant
Collection equipment
Thickened sludge
Measurements
Flow
F.I ow
Blanket level
Polymer or chemicals
Flotation Thickener
Feed sludge
Thickened float or
sludge
Subnatant
polymer or chemicals
(See Table 17-12,
Torque or power
draw
Flow
Pressure
Density
Level
Flow
Weight
Flow
Pipe empty
Flow
Suggested instruments
Venturi with diaphragm sensors
Magnetic
Doppler
We ir
Pump displacement
Venturi
Magnetic
Ultrasonic
Propeller
Orifice
Optical
Ultrasonic
Sidestreams)
Shearpin
Ammeter
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Diaphragm
Nuclear
Optical
Ultrasonic
Tape and float
Capacitance
Ultrasonic
Magnetic
Rotameter
Pump displacement
Static
Venturi with diaphragm sensors
Magnetic
Doppler
Capacitance
Nuclear
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
(See Table 17-12, Sidestreams)
Level Capacitance
Ultrasonic
Tape and float
Flow Magnetic
Rotameter
Pump'' displacement
Weight
Static
17-3
-------
TABLE 17-1
THICKENING (Continued)
Process and process
variables Measurements Suggested instruments
Flotation Thickener (continued)
Dissolution system (assuming Flow Venturi
subnatant recycle Magnetic
or full make-up) Ultrasonic
Propeller
Orifice
Pressure Bourdon
Diaphragm
Air supply Flow Rotameter
Pitot tube
Pressure Bourdon
Diaphragm
Centrifuge
Feed sludge Flow Magnetic
Pump displacement
Pipe empty Capacitance
Nuclear
Centrate (See Table 17-12, Sidestreams)
Thickened sludge Level Ultrasonic
Flow Pump displacement
Pressure Bourdon with cylindrical seal
Density Nuclear
Optical
Ultrasonic
Centrifuge operation Vibration Accelerometer
Displacement probes
Torque or power Ammeter
draw
Polymers or chemicals Level Capacitance
Ultrasonic
Tape and float
Flow Magnetic
Rotameter
Propeller
Pump displacement
Weight Static
17-4
-------
TABLE 17-2
'STABILIZATION
Process and process
variables
Measurements
Suggested instruments
Anaerobic Digesters
Feed sludge
Digester liquid surface
Floating cover
Fixed cover
Gas holding cover
Digester contents
Circulating sludge
Digested sludge
Supernatant
Digester gas
Hot water heating system
Atmospheric monitoring
Flow
Pressure
Density
Level
Level
Level
Temperature
pH and ORP
Pressure
Temperature
pH and ORP
Flow
Pressure
Density
pH and ORP
(See Table 17-12,
Flow
Pressure
Compos ition
Heat value
Pressure
Temperature
Hydrocarbons
Odors
Venturi wjth diaphragm sensors
Magnetic
Doppler
Bourdon with cylindrical seal
Nuclear
• • Optical
Ultrasonic
Tape (attach to cover)
Bubbler with nitrogen purge
Diaphragm
Capacitance
Ultrasonic
Diaphragm (differential pressure)
RTD
Portable selective-ion
Bourdon with cylindrical seal
RTD (pad type)
Selective-ion (pipeline mtg) "
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type) '
Nuclear
Optical
Ultrasonic
Portable selective-ion
Sidestreams)
Orifice
Turbine
Vortex
Diaphragm
Chromatograph
Calorimeter
Bourdon
RTD
Catalytic
Portable olefactometer
17-5
-------
TABLE 17-2
STABILIZATION (Continued)
Process and process
variables
Measurements
Suggested instruments
Aerobic Digesters
Feed sludge
Digester liquid surface
Digester contents
Sedimentation tank
Supernatant
Recycled sludge
Digested sludge
Lime Treatment
Feed Sludge
Flow
Pressure
Density
Level
Temperature
Suspended solids
Dissolved oxygen
pH or ORP
Blanket level
(See Table 17-12, Sidestreams)
Flow
Density
Flow
Pressure
Temperature
Density
pH and ORP
Flow
Treated sludge
Pressure
Density
pH and ORP
Flow
Pressure
Temperature
Density
pH and ORP
Venturi with diaphragm sensors
Magnetic
Doppler
Bourdon with cylindrical seal
Nuclear
Optical
Bubbler
Diaphragm
Capacitance
Ultrasonic
RTD
Optical
Polarographic
Galvanic
Thallium
Portable selective-ion
Optical
Ultrasonic
)
Venturi with diaphragm sensors
Magnetic
Doppler
Nuclear
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Ultrasonic
Selective-ion (pipeline mtg)
Magnetic
Doppler
Venturi with diaphragm seal
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Portable selective-ion
Magnetic
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Ultrasonic
Selective-ion (pipeline mtg)
17-6
-------
TABLE 17-2
STABILIZATION (Continued)
Process and process
variables
Measurements
Suggested instruments
Lime Treatment (continued)
Chemicals
Chlorine Treatment
Feed sludge
Treated sludge
Chemicals
Level
Flow
Weight
Flow
Pressure
Density
Flow
Pressure
Temperature
Density
PH
. Flow
Pressure
Weight
Ultrasonic
Magnetic
Pump displacement
Static
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Magnetic
Doppler
Bourdon with cylindrical seal
RTD
Nuclear
Optical
Selective-ion (pipeline mtg)
Rotaraeter
Orifice
Bourdon with diaphragm seal
Static
17-7
-------
TABLE 17-3
DISINFECTION
Process and process
variables
Measurements
Suggested instruments
Pasteurization
Feed sludge
Pasteurization system
Pasteurized sludge
Steam supply
Electron _lrr_adiation
Feed Sludge
Irradiation system E-beam
monitoring
Irradiated sludge
Level
Flow
Pressure
Density
Pressure
Temperature
Time
Level
Flow
Pressure
Flow
Pressure
Temperature
Level
Flow
Pressure
Temperature
Density
Power draw
Flow
Pressure
Temperature
Bubbler
Diaphragm
Capacitance
Ultrasonic
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Bourdon with flush diaphragm
seal
RTD (pad type)
Digital
Synchronous motor
Ultrasonic
Pump displacement
Bourdon with cylindrical seal
Nozzle
Orifice
Bourdon with steam service
siphon
RTD
Bubbler
Diaphragm
Capacitance
Ultrasonic
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Ultrasonic
Ammeter
Venturi with diaphragm seal
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
17-8
-------
TABLE 17-3
DISINFECTION (Continued)
Process and process
variables
Measurements
Suggested instruments
Electron Irradiaticm
(continued)
Cooling air
Gamma Irradiation
Feed sludge
Irradiation system
Radiation
Irradiated sludge
Flow
Flow loss
Level
Flow
Pressure
Density
Dosage
Safety
Flow
Pressure
Temperature
Radiation
Pitot tube
Vane
Differential pressure
Thermal
Bubbler
Diaphragm
Capacitance
Ultrasonic
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Geiger counter
Geiger counter
Dosimeter
Badge
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Transport displacement
Bourdon with cylindrical seal
RTD (pad type)
Geiger counter
17-9
-------
TABLE 17-4
CONDITIONING
Process and process
variables
Measurements
Suggested instruments
Inorganic Chemical Conditioning
Feed sludge Flow
Chemicals
(aluminum sulfate,
aluminum chloride,
lime ferric chloride,
ferrous sulfate)
Organic Chemical Conditioning
Feed sludge
Polymers
Non-Chemical Additions
Feed sludge
Miscellaneous materials
(ash, pulverized coal,
sawdust, wastepaper
Pressure
Density
Level
Flow
Pressure
Weight
Flow
Pressure
Density
Level
Flow
Pressure
Weight
Flow
Pressure
Density
Level
Weight
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Ultrasonic
Tape and float
Magnetic
Doppler
Pump displacement
Bourdon with chemical seal
Static
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Capacitance
Ultrasonic
Tape and float
Magnetic
Rotameter
Pump displacement
Bourdon with cylindrical seal
Static
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Capacitance
Ultrasonic
Static
Mass flow
17-10
-------
TABLE 17-4
CONDITIONING (Continued)
Process and process
variables
Measurements
Suggested instruments
Thermal Conditioning
Feed sludge
Conditioning
Solids separation
Flow
Pressure
Temperature
Density
Pipe empty
Pressure
Temperature
Level
Blanket level
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Sonic
Capacitance
Nuclear
Bourdon with cylindrical
RTD (pad type)
Thermocouple
Ultrasonic
Optical
Ultrasonic
seal
Atmospheric monitoring
Decant liquor
Conditioned sludge
Steam supply
Air supply
Elutriation
Feed sludge
Odors
(See Table 17-12,
Flow
Pressure
Temperature
Flow
Pressure
Temperature
Flow
Pressure
Flow
Pressure
Density
Portable olefacttometer
Panel
Sidestreams)
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
'RTD (pad type)
Nozzle
Orifice
Bourdon with steam siphon
RTD
Venturi
Rotometer
Orifice
Bellows
Diaphragm
Venturi with diaphragm sensors
Magnetic
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
17-11
-------
TABLE 17-4
CONDITIONING (Continued)
Process and process
variables
Measurements
Suggested instruments
Elutriation (continued)
Solids separation
Elutriate
Conditioned sludge
Wash water
Level
Blanket level
Bubbler
Diaphragm
Ultrasonic
Optical
Ultrasonic
(See Table 17-12, Sidestreams)
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Pressure Bourdon with cylindrical seal
Flow Venturi
Magnetic
Rotameter
Propeller
17-12
-------
TABLE 17-5
DEWATERINC
Process and process
variables
Measurements
Suggested instruments
Drying beds
Feed sludge
Bed contents
Dewatered sludge
Drainage and surface
runoff
Weather
Atmospheric monitoring
Drying Lagoons
Feed sludge
Flow
Pressure
Density
Moisture content
Flow (volume)
Weight
Moisture content
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Portable ohmmeter
Lab test
Transport displacement
Static
Portable ohmmeter
Lab test
(See Table 17-12, Sidestreams)
Anamometer
Wind speed (15 ft (4.6 m))
above ground
Wind direction (15 ft
(4.6 m)) above
ground
Temperature, dry bulb
(5 and 25 ft (1.5 and
and 7.6 m)) above
ground
Relative humidity
Rainfall
Solar radiation
Odors
Flow
Pressure
Density
Wind vane
RTD with solar shield
Thermistor with solar shield
RTD with lithium chloride
cloth (wet bulb tempera-
ture)
Tipping bucket
Thermopile
Portable olefactometer
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
17-13
-------
Process and process
variables
TABLE 17-5
DEWATERING (Continued)
Measurements
Suggested instruments
Drying Lagoons (Continued)
Lagoon contents
Harvested sludge
Supernatant and
surface runoff
Weather
Atmospheric
monitoring
Centrifugal Dewatering
Feed sludge
Centrate
Centrifuge operation
Dewatered sludge
Polymers or chemicals
Moisture content
Flow (volume)
We ight
(See Table 17-12,
Portable ohmmeter
Lab test
Transport displacement
Static
Wind speed (15 ft
(4.6 m)) above ground
Wind direction (15 ft
(4.6 m above ground
Temperature (5 and 25
ft (1.5 and 7.6 m))
ground
Relative humidity
Sidestreams)
Anemometer
Rainfall
Solar radiation
Odors
Flow
Pressure
Density
Pipe empty
Wind vane
ft RTD with solar shield
above Thermistor with solar shield
RTD with lithium chloride
cloth (wet bulb temperature)
Tipping bucket
Thermopile
Portable olefactometer
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Capacitance
Nuclear
(See Table 17-12, Sidestreams]
Torque of power draw
Vibration
Flow (volume)
Weight
Moisture content
Level
Flow
Pressure
We ight
Ammeter
Accelerometer
Displacement probes
Pump displacement
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
Capacitance
Ultrasonic
Tape and float
Magnetic
Rotameter
Propeller
Pump displacement
Bourdon with chemical seal
Static
17-14
-------
TABLE 17-5
DEWATERING (Continued)
Process and process
variables
Measurements
Suggested instruments
Filtration Dewatering
Feed sludge
Vacuum filter
Operation
Filtrate
Spent wastewater
and rejected
feed sludge
Washwater
Belt filter presses
Operation
Filtrate
Spent wastewater
and rejected
feed sludge
Washwater
Recessed plate filter
presses
Operation
Filtrate
Spent washwater
and reject
feed sludge
Flow
Pressure
Density
Pipe empty
Level
Pressure
Speed
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Capacitance
Nuclear
Capacitance
Ultrasonic
Bourdon with chemical seal
Reluctance pick-up
(See Table 17-12, Sidestreams)
(See Table 17-12, Sidestreams)
Flow
Pressure
Pressure
Speed
Venturi
Rotameter
Propeller
Orifice
Bourdon
Bourdon or bellows with
chemical seal
Diaphragm
Reluctance
(See Table 17-12, Sidestreams)
(See Table 17-12, Sidestreams)
Flow
Pressure
Pressure
Venturi
Rotameter
Propeller
Orifice
Bourdon
Bourdon with cylindrical seal
(See Table 17-12, Sidestreams)
(See Table 17-12, Sidestreams)
17-15
-------
17-5
DEWATERINQ (Continued)
Process and process
variables
Filtration Dewatering
(Continued)
Dewatered sludge
Polymers or chemicals
Cyclonic Separation
Feed wastewater solids
Overflow
Underflow
Screening
Feed wastewater
Measurements
Flow
Height
Moisture content
Level
Flow
Pressure
Weight
Flow
Pressure
Density
Suggested instruments
Pump displacement
Transport displacement
Static
Mass flow
Portable ohmeter
Lab test
Capacitance
Ultrasonic
Tape and float
Magnetic
Rotameter
Propeller
Pump displacement
Bourdon with chemical seal
Static
Magnetic
Doppler •-
Bourdon with cylindrical seal
Nuclear (sludge system only)
Ultrasonic
(See Table 17-12, Sidestreams)
Flow (volume) Transport displacement
Feed wastewater solids
Automatic bar screens
Hydraulic sieve bends
Moving screens
Screened liquid
Level
Flow
Level
Flow
Pipe empty
Torque or power draw
Level
Level
Speed
(See Table 17-12, Sidestreams)
Bubbler
Diaphragm
Venturi
Magnetic
Doppler
Weirs and flumes
Bubbler
Diaphragm
Ultrasonic
Venturi with diaphragm sensors
Magnetic ....'.
Doppler
Pump displacement
Capacitance
Nuclear
Shear pin
Ammeter
Bubbler
Diaphragm
Bubbler
Diaphragm
Bubbler
Diaphragm
17-16
-------
TABLE 17-6
HEAT DRYING
Process and process variables
Measurements
Flash drying
Feed sludge
Drying operation
Dried sludge
Hot air furnace
Burner operation
Fuel
Combustion air
Heated air
Fan monitoring
Scrubber water
Direct rotary dryer
Feed sludge
Drying operation
Dried sludge
Flow, volume
Weight
Moisture content
Pipe empty
Temperature
Flow, volume
Temperature
Weight
Moisture content
Flame monitoring
Flow
Flow
Pressure
Temperature
Temperature
Flow loss
Suggested instruments
Pump displacement
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
Capacitance
Nuclear
RTD (pad type)
Transport displacement
RTD (pad type)
Static
Mass flow
Portable ohmmeter
Lab test
Ultravilot scanner
Pitot tube
Orifice
Positive displacement
Pitot tube
Orifice plate
Diaphragm
Bellows
RTD
Thermocouple
Vane
Differential pressure
Thermal
Vibration
Flow, volume
Weight
Accelerometer
(See Table 17-12, Sidestreams)
Transport displacement
Moisture content
Temperature
Speed
Torque or power draw
Flow, volume
Temperature
Weight
Moisture content
Static
Mass flow
Portable ohmmeter
Lab test
RTD (pad type)
Reluctance
Shearpin
Ammeter
Transport displacement
RTD (pad type)
Static
Mass flow
Portable ohmmeter
Lab test
17-17
-------
TABLE 17-6
HEAT DRYING (Continued)
Process and process variables
Direct rotary dryer (continued)
Hot air furnace
Burner operation
Fuel
Combustion air
Heated air
Fan monitoring
Scrubber water
Indirect and direct-indirect
rotary dryers
Feed sludge
Drying operation
Dried sludge
Hot air furnace
Burner operation
Fuel
Combustion air
Heated air
Fan monitoring
Scrubber water
Measurements
Flame monitoring
Flow
Flow
Pressure
Temperature
Temperature
Flow loss
Vibration
(See Table 17-12,
Flow, volume
Weight
Moisture content
Tempera ture
Speed
Torque or power draw
Flow, volume
Temperature
Weight
Moisture content
Flame monitoring
Flow
Flow
Pressure
Temperature
Temperature
Flow loss
Suggested instruments
Vibration
Ultravilot scanner
Pitot tube
Orifice
Vortex
Positive displacement
Pitot tube
Orifice
Diaphragm
RTD
Thermocouple
Vane
Differential pressure
Thermal
Accelerometer
Sidestreams)
Pump displacement
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
RTD (pad type)
Reluctance
Shearpin
Ammeter
Transport displacement
RTD (pad type)
Static
Mass flow
Portable ohmmeter
Ultraviolet scanner
Pitot tube
Orifice
Vortex
Positive displacement
Pitot tube
Orifice
Bellows
Diaphragm
RTD
Thermocouple
Vane
Differential pressure
Thermal
Accelerometer
(See Table 17-12, Sidestreams)
See Table 17-12, Sidestreams.
17-18
-------
TABLE 17-6
HEAT DRYING (Continued)
Process and process variables
Incinerators
Torodial_dry_ers_
Liquid or dewatered solids
storage
Dewatering
Feed sludge
Drying operation
Dried sludge
Measurements
Suggested instruments
Hot air furnace
Burner operation
Fuel
Combustion air
Heated air
Fan monitoring
Scrubber water
Spray drying
Feed sludge
Drying operation
Dried sludge
Hot air supply
(See Table 17-7, High Temperature Processes)
(See Table 17-11, Storage)
(See Table
Flow, volume
Weight
Moisture content
Temperature
Flow, volume
Temperature
Weight
Moisture content
Flame monitoring
Flow
Flow
Pressure
Temperature
Temperature
Vibration
Flow loss
17-5, Dewatering)
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
RTD (pad type)
Transport displacement
RTD (pad type)
Static
Mass flow
Portable ohmmeter
Lab test
Ultraviolet scanner
Pitot tube
Orifice
Vortex
Positive displacement
Pitot tube
Orifice
Bellows
Diaphragm
RTD
Thermocouple
Accelerometer
Vane
Differential pressure
Thermal
(See Table 17-12, Sidestreams)
Flow
Pressure
Density
Temperature
Flow, volume
Temperature
Weight
Moisture content
Temperature
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
RTD (pad type)
Transport displacement
RTD (pad type)
Static
Mass flow
Portable ohmmeter
Thermocouple
See Table 17-12, Sidestreams.
"'see Table 17-7, High Temperature Processes.
'See Table 17-11, Storage
See Table 17-5, Dewatering.
17-19
-------
TABLE 17-6
HEAT DRYING (Continued)
Process and process variables
Measurements
Suggested instruments
extraction
Feed sludge
Cooled sludge
Extraction system
Dried sludge
Product water
Hot air
Chilled water
Multiple-effect evaporator
Feed sludge
Fluidizing system
Fluidizing tank
Flow
Pressure
Temperature
Density
Temperature
Pressure
Temperature
Flow, volume
Temperature
Weight
Moisture content
Flow
Pressure
Temperature
Suspended solids
Chemical oxygen demand
Temperature
Flow
Pressure
Temperature
Flow
Pressure
Temperature
Density
Level
Temperature
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Ultrasonic
RTD (pad type)
Bourdon with chemical seal
RTD
Transport displacement
RTD (pad type)
Static
Mass flow
Portable ohmmeter
Lab test
Venturi
Magnetic
Pump displacement
Bourdon with chemical seal
RTD
Optical
TOC analyzer
RTD
Thermocouple
Rotameter
Propeller
Orifice
Bourdon
RTD
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Ultrasonic
Ultrasonic
RTD (pad type)
17-20
-------
TABLE 17-6
HEAT DRYING (Continued)
Process and process variables
Multiple-effect evaporator
(continued) , T~
Fluidizing system (continued)
Fluidizing pump
Measurements
Suggested instruments
Feed tank
Feed pump
Evaporation system
Dried sludge
Condensate
Recycled oil
Steam supply
Pressure
Temperature
Level
Temperature
Flow
Pressure
Temperature
Pressure
Temperature
Flow, volume
Temperature
Weight
Moisture content
Flow
Pressure
Temperature
Level
Flow
Pressure
Temperature
Flow
Pressure
Temperature
Bourdon with cylindrical seal
RTD (pad type)
Ultrasonic
RTD (pad type)
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Bourdon with chemical seal
RTD (pad type)
Transport displacement
RTD (pad type)
Static
Mass flow
Portable ohmmeter
Lab test
Rotameter
Orifice
Bourdon with chemical seal
RTD (pad type)
Ultrasonic
Orifice
Pump displacement
Bourdon with diaphragm seal
RTD
Nozzle
Orifice
Bourdon with steam siphon
RTD
17-21
-------
TABLE 17-7
HIGH TEMPERATURE PROCESS
Process and process
variables
Measurements
Suggested instruments
Incineration
Feed sludge
Furnace operation
Multiple-hearth
Fluid-bed
Electric
Single-hearth cyclonic
Ash
Combustion air
Recycled flue gas
Afterburner
Multiple-hearth furnace
Electric furnace
Exhaust (stack gas)
Flow (volume)
Temperature
Weight
Moisture content
Temperature
Speed
Torque of power
draw
Flame monitoring
Pressure
Temperature
Flame monitoring
Temperature
Speed
Power draw
Temperature
Speed
Torque or power
draw
Flame monitoring
Flow (volume)
Temperature
Weight
Flow loss
Pressure
Temperature
Temperature
Temperature
Flame monitoring
Temperature
Power draw
Pressure
Temperature
Oxygen content
Opacity
Other measurements
as required by
local air quality
management dis-
tricts
Pump displacement
Transport displacement
RTD
Static
Mass flow
Portable ohmmeter
Lab test
Thermocouple
Reluctance
Shear pin
Ammeter
Ultraviolet scanner
Bourdon with diaphragm seal
Thermocouple
Ultraviolet scanner
Thermocouple
Reluctance
Ammeter
Thermocouple
Reluctance
Shear pin
Ammeter
Ultraviolet scanner
Transport displacement
Thermocouple
Static
Mass flow
Vane
Differential pressure
Thermal
Diaphragm
Bellows
Thermocouple
Thermocouple
Thermocouple
Ultraviolet scanner
Thermocouple
Ammeter
Bellows
Diaphragm
Thermocouple
Paramagnetic
Catalytic
Ceramic
Optical
As required
17-22
-------
TABLE 17-7
HIGH TEMPERATURE PROCESS (Continued)
Process and process
variables
Measurements
Suggested instruments
Incineration (continued)
Heat recovery system
Flue gas
Boiler
Steam produced
Scrubbing water
Fuel
Electric furnace
Other furnaces
Temperature
Level
Pressure
Temperature
Flow
Thermocouple
Float (cage mounted)
Bourdon
Thermocouple
Nozzle
Orifice
Pressure Bourdon with steam siphon
Temperature Thermocouple
(See Table 17-12, Sidestreams)
Starved Air Combustion
Feed sludge
Furnace operation
Ash
Combustion air
Afterburner
Power draw
Level
Flow
Pressure
Flow (volume)
Temperature
Weight
Moisture content
Pressure
Temperature
Flow (volume)
Temperature
Weight
Flow loss
Pressure
Temperature
Temperature
Flame monitoring
Ammeter
Diaphragm
Tape and float
Positive displacement
Orifice
Bourdon
Bellows
Diaphragm
Pump displacement
Transport displacement
RTD
Static
Mass flow
Portable ohmmeter
Lab test
Bellows
Diaphragm
Thermocouple
Transport displacement
Thermocouple
Static
Mass flow
Vane
Differential pressure
Thermal
Bellows
Diaphragm
RTD
Thermocouple
Ultraviolet scanner
17-23
-------
TABLE 17-7
HIGH TEMPERATURE PROCESS (Continued)
Process and process
variables
Starved Air__Cgmbus_tion (continued)
Exhaust (stack gas)
Measurements
Suggested instruments
Heat recovery system
Flue gas
Boiler
Steam produced
Scrubbing water
Fuel
Flue gas for after-
burner
Supplemental fuel
Pressure
Temperature
Oxygen content
Opacity
Other measurements
as rquired by
local air quality
management dis-
tricts
Temperature
Level
Pressure
Temperature
Flow
Pressure
Temperature
Bellows
Diaphragm
Thermocouple
Paramagnetic
Catalytic
Ceramic
Optical
As required
Thermocouple
Float (cage mounted)
Bourdon
Thermocouple
Nozzle
Orifice
Bourdon
RTD
(See Table 17-12, Sidestreams)
Watergate Furnace
Feed scum
Furnace operation
Exhaust (stack gas)
Pressure
Level
Flow
Pressure
Flow
Pressure
Density
Level
Temperature
Flame monitoring
Pressure
Temperature
Bellows
Diaphragm
Diaphragm
Tape and float
Orifice
Positive displacemeent
Bourdon
Bellows
Diaphragm
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Diaphragm
Ultrasonic
Thermocouple
Ultraviolet scanner
Bellows
Diaphragm
Thermocouple
17-24
-------
TABLE 17-7
HIGH TEMPERATURE PROCESS (Continued)
Process and process
variables
Measurements
Suggested instruments
Watergate Furnace (Continued)
Exhaust (stack gas)
(continued)
Oxygen content
Paramagnetic
Catalytic
Ceramic
Optical
As required
Scrubbing water
Fuel
Opacity
Other measurements
as required by
local air quality
management dis-
tricts
(See Table 17-12, Sidestreams)
Level Diaphragm
Tape and float
Flow Orifice
Positive displacement
pressure Bellows
Diaphragm
Co-Combustion with Hunicipal Refuse
Feed sludge
Liquid state Flow
Dewatered state
Municipal refuse
Furnace operation
Grate fired
Multiple-hearth
Pressure
Density
Flow (volume)
Weight
Moisture content
Flow (volume)
Weight
Moisture content
Temperature
Flame monitoring
Temperature
Speed
Torque or power
draw
Flame monitoring
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
Thermocouple
Ultraviolet scanner
Thermocouple
Reluctance
Shear pin
Ammeter
Ultraviolet scanner
17-25
-------
TABLE 17-7
HIGH TEMPERATURE PROCESS (Continued)
Process and process
variables
Measurements
Suggested instruments
Co-Combustion with Municipal Refuse
(continued) ™ ™"
Furnace operation (continued)
Fluid-bed
Ash
Combustion air
Recycled flue gas
Afterburner
Multiple hearth
Exhaust (stack gas)
Heat recovery system
Flue gas
Boiler
Steam produced
Scrubber water
Pressure
Temperature
Flame monitoring
Flow (volume)
Temperature
Weight
Flow loss
Pressure
Temperature
Temperature
Temperature
Flame monitoring
Pressure
Temperature
Oxygen content
Opacity
Other measurements
as required by
local air quality
management dis-
tricts
Temperature
Level
Pressure
Temperature
Flow
Pressure
(See Table 17-12,
Bellows
Diaphragm
Thermocouple
Ultraviolet scanner
Transport displacement
Thermocouple
Static
Mass flow
Vane
Differential pressure
Thermal
Diaphragm
Bellows
RTD
Thermocouple
Thermocouple
Ultraviolet scanner
Bellows
Diaphragm
Thermocouple
Paramagnetic
Catalytic
Ceramic
Optical
As required
Thermocouple
Float (cage mounted)
Bellows
Diaphragm
Thermocouple
Nozzle
Orifice
Bourdon with steam siphon
Sidestreams)
17-26
-------
TABLE 17-8
COMPOSTING
Process and process
variables
Measurements
Suggested instruments
Unconfined
Windrow
Feed sludge
Composting
Composted sludge
Amendment or bulking
agent
Leachate and surface
runoff
Level
Flow (volume)
Weight
Moisture content
Temperature
Moisture content
Aerobic condition
Flow (volume)
Weight
Moisture content
Flow (volume)
Weight
Moisture content
Capacitance
Ultrasonic
Transport displacement
Static
Portable ohmmeter
Portable thermometer
Portable ohmmeter
Lab test
Portable galvanic cell
Portable polarographic cell
Truck displacement
Static
Portable ohmmeter
Lab test
Transport displacement
Static
Portable ohmmeter
Lab test
(See Table 17-12, Sidestreams)
Weather
Atmosperic monitoring
Aerated pile
Feed sludge
Wind speed (15 ft
(4.6 m ) ) above
ground
Wind direction
(15 ft (4.6 m))
above ground
Temperature (5 and
25 ft (1.5 and
7.6 m)) above
ground
Relative humidity
Solar radiation
Odors
Level
Flow (volume)
Weight
Moisture content
Anemometer
Wind vane
RTD
Thermistor
with solar shield
RTD with lithium chloride cloth
(wet bulb temperature)
Thermophile
Portable olefactometer
Capacitance
Ultrasonic
Transport displacement
Portable ohmmeter
Lab test
17-27
-------
TABLE 17-8
COMPOSTING (Continued)
Process and process
variables
Measurements
Suggested instruments
Unconfined (continued)
Aerated pile
Feed sludge
Composting
Composted sludge
Aeration air
Amendment or bulking
agent
Leachate and surface
runoff
Weather
Level
Flow (volume)
Weight
Moisture content
Temperature
Moisture content
Aerobic condition
Flow (volume)
Weight
Moisture content
Flow
Flow (volume)
Weight
Moisture content
Capacitance
Ultrasonic
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
RTD
Portable thermometer
Portable ohmmeter
Lab test
Portable galvanic cell
Portable polarographic cell
Transport displacement
Static
Portable ohmmeter
Lab test
Venturi
Pitot tube
Orifice
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
(See Table 17-12, Sidestreams)
Wind speed (15 ft Anemometer
Atmospheric monitoring
(4.6 m)) above
ground
Wind direction
(15 ft (4.6 m))
above ground
Temperature (5 and
25 ft (1.5 and
7.6 m)) above
ground
Relative humidity
Rainfall
Solar radiation
Odors
Wind vane
RTD
)
Thermistor)
with solar shield
RTD with lithium chloride cloth
(wet bulb temperature)
Tipping bucket
Thermopile
Portable olefactometer
17-28
-------
TABLE 17-8
COMPOSTING (Continued)
Process and process
variables
Measurements
Suggested instruments
Confined Systems
Feed sludge
Composting
Composed sludge
Amendment or bulking
agent
Atmospheric monitoring
Level
Flow (volume)
Weight
Moisture content
Temperature
Moisture content
Aerobic condition
Level
Flow (volume)
Weight
Moisture content
Level
Flow (volume)
Weight
Moisture content
Odors
Capacitance
Ultrasonic
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
RTD
Portable ohmmeter
Portable galvanic cell
Portable polarographic cell
Capacitance
Ultrasonic
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
Capacitance
Ultrasonic
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
Portable olefactometer
17-29
-------
TABLE 17-9
MISCELLANEOUS CONVERSION PROCESSES
Process and process
variables
Fixation
Feed sludge
Fixed sludge
Encapsulation
Feed sludge
Polyethlene system
Asphalt system
Encapsulated sludge
Earthworm Conversion
Feed sludge
Castings (egesta)
Measurements
Flow (volume)
Moisture content
Moisture content
Flow (volume)
Moisture content
Pressure
Temperature
Temperature
Flow (volume)
Flow (volume)
Temperature
Moisture content
Flow (volume)
Moisture content
Suggested instruments
Transport displacement
Portable ohmmeter
Lab test
Portable ohmmeter
Lab test
Transport displacement
Portable ohmmeter
Lab test
Bellows with diaphragm seal
RTD
Thermocouple
RTD
Thermocouple
Transport displacement
Transport displacement
Portable thermometer
Portable ohmmeter
Lab test
Transport displaceeraent
Portable ohmmeter
Lab test
17-30
-------
TABLE 17-10
TRANSPORTATION
Process and process
variables
Measurements
Suggested instruments
Pumping
Centrifugal and torque
flow pumps
Variable speed drive
Pumped sludge
Positive displacement
pumps
Variable speed drive
Pumped sludge
Pipelines
Corrosion, electrolytic
Pig location
Conveying
Continuous belt
Positive displacement
Pneumatic ejection
Open screw
Trucking
Barging
Railroad Cars
Speed
Vibration
Flow
Pressure
Empty pipe
Speed
Flow
Pressure
Empty pipe
Power draw
Flow
Underspeed
Level (volume
overload)
Weight
Underspeed
Flow (volume)
Level (volume
overload)
Underspeed
Flow (volume)
Weight
Level
Flow (volume]
Weight
Tachometer generator
Reluctance
Accelerometer
Venturi with diaphragm sensors
Magnetic
Doppler
Bourdon with cylindrical seal
Capacitance
Nuclear
Reluctance
Reluctance (revolution counter)
Bourdon with cylindrical seal
Capacitance
Nuclear
Ammeter
Venturi with diaphragm sensors
Magnetic
Pump displacement
Reluctance
Capacitance
Ultrasonic
Mass flow
Reluctance
Transport displacement
Capacitance
Ultrasonic
Reluctance
Transport displacement
Vehicle detection
Static
Bubbler
Diaphragm
Ultrasonic
Transport displacement
Vehicle detection
Static
17-31
-------
TABLE 17-11
STORAGE
Process and process
variables
Measurements
Suggested instruments
Wastewater Treatment
Sedimentation facilities
Aeration reactors
Imhoff and septic tanks
Oxidation ditches
Stabilization ponds
Density
Suspended solids
Blanket level
Suspended solids
Blanket level
Density
Suspended solids
Suspended solids
Nuclear
Optical
Optical
Optical
Ultrasonic
Optical
Optical
Ultrasonic
Nuclear
Optical
Optical
Optical
Wastewater Solids Treatment (See Individual Process Tables)
Liquid Storage
Holding Tanks
Feed sludge
Tank liquid surface
Fixed cover
Floating cover
Tank contents
Circulating sludge
Discharged sludge
Flow
Pressure
Temperature
Density
Level
Level
Temperature
pll
Pressure
Temperature
pH
Flow
Pressure
Temperature
Density
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Ultrasonic
Bubbler
Diaphragm
Capacitance
Ultrasonic
Tape (attach to cover)
RTD
Portable selective-ion
Bourdon with cylindrical seal
RTD (pad type)
Selective-ion (pipeline mtg)
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Ultrasonic
17-32
-------
TABLE 17-11
STORAGE (Continued)
Process and process
variables
Measurements
Suggested instruments
Confined Hoppers or Bins
Feed sludge
Hopper or bin contents
Discharged sludge
Atmosperic monitoring
Unconfined Stockpiles
Feed sludge
Stockpiled sludge
Harvested sludge
Weather
Atmospheric monitoring
Flow (volume)
We ight
Moisture content
Level
Weight
Level (volume over
load)
Flow (volume)
Weight
Moisture content
Hydrocarbons
Odors
Flow (volume)
Weight
Moisture content
Moisture content
Flow (volume)
Weight
Moisture content
Wind speed (15 ft
(4.6 m) above
ground
Wind direction
(15 ft (4.6 m))
above ground
Temperature (5 and
25 ft (1.5 and
7.6 m)) above
ground
Relative humidity
Rainfall
Solar radiation
Odors
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
Capacitance
Ultrasonic
Static
Capacitance
Ultrasonic
Transport displacement
Static
Mass flow
Portable ohmmeter
Lab test
Catalytic
Portable olefactometer
Transport displacement
Static
Portable ohmmeter
Lab test
Portable ohmmeter
Lab test
Transport displacement
Static
Portable ohmmeter
Lab test
Anemometer
Wind vane
RTD )
Thermistor)
with solar shield
RTD with lithium chloride
cloth (wet bulb temperature)
Tipping bucket
Thermopile
Portable olefactometer
17-33
-------
TABLE 17-11
STORAGE (Continued)
Process and process
variables
Measurements
Suggested instruments
Facultative Sludge Lagoons
Feed sludge
Lagoon contents
Harvested sludge
Supernatant
Weather
Atmospheric monitoring
Anaerobic Sludge Lagoons
Feed sludge
Flow
Pressure
Density
pli
Conductivity
Blanket level
Dissolved oxygen
Flow
Pressure
Density
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Portable selective-ion
Portable conductivity probe
Portable optical
Portable ultrasonic
Portable galvanic
Portable polarographic
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
(See Table 17-12,
Wind speed (15 ft
(4.6 m)) above
ground
Wind direction
(15 ft (4.6 m))
above ground
Temperature (5 and
25 ft (1.5 and
7.6 m)) above
ground
Relative humidity
Rainfall
Solar radiation
Odor
Flow
Pressure
Density
Sidestreams)
Anemometer
Wind vane
RTD
Thermistor
with solar shield
RTO with lithium chloride cloth
(wet bulb temperature)
Tipping bucket
Thermopile
Portable olefactometer
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Lagoon contents
Sludge blanket
Portable optical
Portable ultrasonic
17-34
-------
TABLE 17-11
STORAGE (Continued)
Process and process
variables
Measurements
Suggested instruments
Anaerobic Sludge Lagoons
(Continued")
Harvested sludge
supernatant
Weather
Atmospheric monitoring
Aerat e d_Basin
Feed sludge
Basin contents
Supernatant
Solid State Storage
Drying sludge lagoons
Flow
Pressure
Venturi with diaphragm sensors
Magnetic
Doppler
Bourdon with cylindrical seal
(See Table 17-12, Sidestreams)
Wind speed (15 ft
(4.6 m)) above
ground
Wind direction
(15 ft (4.6 m))
above ground
Temperature (5 and
25 ft (1.5 and
7.6 m)) above
ground
Relative humidity
Rainfall
Solar radiation
Odors
Flow
Pressure
Dissolved oxygen
Flow
Pressure
Density
Anemometer
Wind vane
RTD )
Thermistor)
with solar shield
RTD with lithium chloride cloth
(wet bulb temperature)
Tipping bucket
Thermopile
Portable olefactometer
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Portable galvanic
Portable polarographic
Venturi with diaphragm sensors
Magnetic
Doppler
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
(See Table 17-12, Sidestreams)
(See Table 17-5, Dewatering)
17-35
-------
TABLE 17-12
SIDESTREAMS
Process and process
variables
Measurements
Suggested instruments
Thickening
Gravity supernatant
Flotation subnatant
Centrifuge centrate
Stabilization
Anaerobic digestion
supernatant
Aerobic digestion
supernatant
Flow
Pressure
Suspended solids
Flow
Pressure
Suspended solids
Flow
Pressure
Suspended solids
Level
Flow
Pressure
Density
Sludge blanket
Suspended solids
PH
Chemical oxygen
demand
Ammonia
Level
Flow
Pressure
Venturi with diaphragm sensors
Magnetic
Doppler
Weirs and flumes
Pump displacement
Bourdon with diaphragm sensors
Optical
Venturi with diaphragm sensors
Magnetic
Doppler
Weirs and flumes
Pump displacement
Bourdon with diaphragm sensors
Optical
Venturi with diaphragm sensors
Magnetic
Doppler
Weirs and flumes
Pump displacement
Bourdon with diaphragm sensors
Optical
Bubbler
Diaphragm
Tape and float
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Sonic
Optical
Selective-ion (pipeline mtg)
TOC Analyzer
Selective-ion analyzer
Bubbler
Diaphragm
Float and tape
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
17-36
-------
TABLE 17-12
SIDESTREAMS (Continued)
Process and process
variables
Measurements
Suggested instruments
Stabilization (continued)
Aerobic digestion
supernatant (continued)
Atmospheric monitoring odors
Conditioning
Thermal liquor
(decant and filtrate)
Elutrlation elutriate
Flow
Pressure
Temperature
Density
Suspended solids
pH
Ammonia
Level
Flow
Pressure
Temperature
Dens ity
Suspended solids
pH
Chemical oxygen
demand
Ammonia
Level
Flow
pressure
Dens ity
Suspended solids
pH
Ammonia
Blanket level
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Ultrasonic
Optical
Selective-ion (pipeline mtg)
Selective-ion analyzer
Portable olefactometer
Bubbler
Diaphragm
Float and tape
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
RTD (pad type)
Nuclear
Optical
Ultrasonic
Optical
Selective-ion (pipeline mtg)
TOC analyzer
Selective-ion analyzer
Bubbler
Diaphragm
Float and tape
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Optical
Selective-ion (pipeline mtg)
Selective-ion analyzer
Optical
Ultrasonic
17-37
-------
TABLE 17-12
SIDESTREAMS (Continued)
Process and process
variables
Measurements
Suggested instruments
Cond_ition_in£ (continued)
Atmospheric monitoring
Dewatering
Drying bed drainage
and surface runoff
Drying lagoons super-
natant and surface
runoff
Centrifuge centrate
Vacuum, belt press, recessed
plate and frame and screw
and roll press filters
Filtrate
Odors
Level
Flow
Pressure
Suspended solids
PH
Ammonia
Level
Flow
Pressure
Suspended solids
pH
Level
Flow
Pressure
Density
Suspended solids
pH
Ammo n i a
Blanket level
Level
Flow
Portable defactpmeter
Panel
Bubbler
Diaphragm
Float and tape
Venturi with diaphragm sensors
Magnetic
Doppler
Weirs and flumes
Pump displacement
Bourdon with cylindrical seal
Portable optical
Portable selective-ion
Lab test
Bubbler
Diaphragm
Ultrasonic
Magnetic
Doppler
Weirs and flumes
Pump displacement
Bourdon with cylindrical seal
Portable optical
Portable selective-ion
Bubbler
Diaphragm
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Bourdon with cylindrical seal
Nuclear
Optical
Ultrasonic
Optical
Selective-ion (pipeline mtg)
Selective-ion analyzer
Optical
Ultrasonic
Bubbler
Diaphragm
Float and tape
Venturi with diaphragm sensors
Magnetic
Doppler
Weirs and flumes
Pump displacement
17-38
-------
TABLE 17-12
SIDESTREAMS (Continued)
Process and process
variables
Measurements
Suggested instruments
Dewatering (Continued)
Filtrate (continued)
Spent washwater and
rejected feed sludge
Cyclonic separation
Overflow
ScreenLing_
Screening liquid
High Temperature Processes
and Heat Drying
Scrubber water
supply
Discharge
Pressure
Suspended solids
pH
Ammo n i a
Level
Flow
Dens ity
Suspended solids
pH
Ammonia
Level
Flow
Level
Flow
Density
Level
Flow
Pressure
Temperature
Flow
Temperature
Suspended solids
Bourdon with diaphragm seal
Optical
Selective-ion (pipeline mtg)
Selective-ion analyzer
Bubbler
Diaphragm
Venturi with diaphragm sensors
Magnetic
Doppler
Pump displacement
Nuclear
Optical
Ultrasonic
Optical
Selective-ion (pipeline mtg)
Selective-ion analyzer
Bubbler
Diaphragm
Magnetic
Doppler
Weirs and flumes
Bubbler
Diaphragm
Venturi
Magnetic
Doppler
Weirs and flumes
Nuclear
Sonic
Bubbler
Diaphragm
Float and tape
Rotameter
Propeller
Orifice
Bourdon
RTD
Venturi
Magnetic
Ultrasonic
Orifice
RTD
Optical
17-39
-------
TABLE 17-12
SIDESTREAMS (Continued)
Process and process
variables
Measurements
Suggested instruments
Composting Leachate and
Surface Runoff
Storage
Facultative sludge
lagoon supernatant
Anaerobic sludge
lagoon supernatant
Landfilling Leachate and
Surface Runoff
Level
Flow
Pressure
Suspended solids
PH
Chemical oxygen
demand
Level
Flow
Pressure
Suspended solids
pH
Ammonia
Level
Flow
Pressure
Suspended solids
Ammo n i a
Level
Flow
Pressure
Suspended solids
pH
Chemical oxygen
demand
Ammonia
Bubbler
Diaphragm
Float and tape
Venturi with diaphragm sensors
Magnetic
Doppler
We irs
Bourdon with cylindrical seal
Optical
Selective-ion (pipeline mtg)
Total organic carbon
analyzer
Bubbler
Diaphragm
Ultrasonic
Magnetic
Doppler
Weirs and flumes
Bourdon with cylindrical seal
Portable optical
Portable selective-ion
Lab test
Bubbler
Diaphragm
Ultrasonic
Magnetic
Doppler
Weirs and flumes
Bourdon with cylindrical seal
Portable optical
Portable selective-ion
Lab test
Bubbler
Diaphragm
Ultrasonic
Float and tape
Venturi with diaphragm sensors
Magnetic
Doppler
Weirs and flumes
Pump displacement
Bourdon with cylindrical seal
Optical
Selective-ion (pipeline mtg)
Total organic carbon
Analyzer
Selective-ion analyzer
17-40
-------
17.2 Measurements
This section briefly describes each of the instrumentation
devices listed in Tables 17-1 through 17-12.
17.2.1 Level Measurements
Level measurements are required for both displacement volume and
open channel flow instrumentation. Some instruments have almost
unlimited applicability, while others are restricted because of
material interferences. Sometimes these interferences can be
minimized or eliminated with special purging; however, the
designer must provide the support systems required if such
instruments are to be reliable.
17.2.1.1 Bubblers
The pneumatic bubbler remains the most universally applicable
liquid level measuring device in wastewater treatment facilities.
In its simplest form, a bubbler consists of a dip tube through
which a constant small flow of purge gas, usually air, is
discharged. The gas flow prevents the liquid from rising in the
dip tube; therefore, the pressure required to maintain the gas
flow is directly proportional to the depth of a liquid above the
dip tube outlet. This pressure can be measured by virtually any
pressure measurement device, some of which are described in
Section 17.2.3. The bubbler can be used with almost any liquid,
but clogging may be a problem when solids are present. Clogging
can be controlled by frequent purging with high pressure air.
Where the use of an air purge is undesirable, such as in
anaerobic digesters, nitrogen or natural gas can be used for
purging the bubbler dip tube. Figure 17-1 shows a typical
bubbler schematic with air purge capabilities.
17.2.1.2 Diaphragms
Bubbler dip tube clogging problems can be overcome by use of
diaphragm level element. A diaphragm is usually 3 to 4 inches
(7.6 to 10. cm) in diameter and serves as one wall of what is,
essentially, a box. Inside the box, a pneumatic, hydraulic,
or electric mechanism transmits any pressure exerted on the
diaphragm. The entire box is submerged in a vessel, or the
diaphragm may be inserted in the wall of the vessel by means
of a standard pipe flange. In either case, the pressure exerted
on the diaphragm is directly proportional to the depth of
liquid above the diaphragm. The type of diaphragm shown on
Figure 17-2 is air-purged and produces a back pressure similar to
a bubbler. Thus, both the air supply and pressure measuring
devices are similar to those used in bubbler systems. The
air-purged diaphragm can, therefore, be used as a replacement for
existing bubblers. The second type of diaphragm uses a filled
17-41
-------
COMPRESSED
AIR SUPPLY
3Qpsig
MINIMUM
3/B" PVC JACKETED - -.
COPPER TUBING
ON PAN EL (TYP)
DRAIN PLUG —
{REQUIRED WHEN
AIR SUPPLY FROM
ABOVE!
FILTER REGULATOR
ASSEMBLY W/GAUGE
BULKHEAD
- ^FITTINGS
DIFFERENTIAL PRESSURE •
REGULATOR W/NEEDLE
VALVE
ROTAMETER W/CHECK•
VALVE, 0-2 SCFH
Inchtt x 0.39 - em
p»i| x 0.14i - kN/m*
SCREW ADAPTER
ItCAPTO PERMIT
RODDING
3/4" SCHED 80
PVC PIPE
UJ
NOTCH
1/2" x 1/3"
FIGURE 17-1
TYPICAL BUBBLER SCHEMATIC WITH
AIR PURGE CAPABILITIES
17-42
-------
COMPRESSED
AIR SUPPLY
30 psis
MINIMUM
3/8" PVC JACKETED
COPPER TUBING
ON PANEL (TYP)
DRAIN PLUG ——-
(REQUIRED WHEN
AIR SUPPLY FROM
ABOVE)
1/2" PIPE
[ 1/2" BALL
'VALVE
PRESSURE,
ELEMENT I ~\J
FT
01
T .:
FILTER REGULATOR-
ASSEMBLY W/GAUGE
DIFFERENTIAL PRESSURE
REGULATOR W/NEEDLE
VALVE
RQTAMETER
VALVE, 0-2 SCFH
1" NPTPIPE
CONNECTION
/BULKHEAD
±^f FITTINGS
• 1/4- PVC TUBES
inches v. 0,39
(%tig x 0,145 =
on
TEFLON COATED
FIBERGLASS DIAPHRAGM
INSTRUMENT
CONNECTION
1"SCHED 80
PVC PIPE
(SUPPORT ONLY}
DIAPHRAGM
ELEMENT
tSEE DETAIL)
FIGURE 17-2
TYPICAL BUBBLER SCHEMATIC WITH
DIAPHRAGM ELEMENT
17-43
-------
capillary tube between the diaphragm and a conventional pressure
transmitter, thus eliminating the need for an air supply. When
a filled capillary tube is employed, the volumetric displacement
of the diaphragm is critical, and pressure indicators or
transmitters should have as low a displacement as possible so
that diaphragms with low movement can be used. Capillary filling
fluid should have a low thermal expansion coefficient to limit
errors resulting from temperature changes. Diaphragms are
flush-mounted and have no crevices to accumulate solids. The
almost insignificant movement required for accurate measurement
is maintained even when the diaphragm is coated with grease.
17.2.1.3 Capacitance Transmitters
In recent years, several electronic level measuring devices
have appeared. Capacitance and ultrasonic instruments are
particularly applicable for sludge measurements. The capacitance
liquid level transmitter consists of a steel rod or cable
(probe), usually teflon-coated, which is installed in the tank.
The probe forms one plate of a capacitor, and the liquid, which
must be conductive and grounded, forms the second. The probe
insulation forms a dielectric between these two plates. The
electrical capacitance between the probe and the liquid is
proportional to the axial length of probe immersed in liquid. An
electrical instrument measures this capacitance and provides a
signal proportional to level. This signal can be used to provide
either on-off or continuous level measurement. Gross changes in
fluid conductance can affect calibration of capacitance probes.
This is not ordinarily a problem in sludge handling facilities.
One disadvantage with capacitance instruments is that, even with
teflon-coated probes, greasy material can adhere and cause
errors. Improved electronics has reduced this problem on some
units. Very successful level measurements have been made for all
types of sludge, including the fluid level inside fixed cover
digesters. Capacitance instruments are also used to measure the
level of dry solids in bins or silos. In this application, a
bare rod or cable (probe) is used, and the side of the tank
serves as the ground plate. Where tanks are non-metallic, a
metallic ground plate can be installed on the side of the tank.
The material in the tank then serves as the dielectric and must
have a stable dielectric constant significantly different from
air. Since many solids contain large and varying amounts of air,
the use of capacitance probes for solids level measurement is
frequently unsuccessful.
17.2.1.4 Ultrasonic Transmitters
Ultrasonic level instruments operate with the level transmitter
completely out of contact with the process material. This is a
very appealing advantage for sludge treatment and disposal
processes. A transducer suspended above the material emits an
ultrasonic pulse toward the liquid. The pulse bounces off
17-44
-------
the material surface and the reflected pulse returns to the
transducer. The time that elapses between the transmitted pulse
and the received pulse is related to the distance between the
transducer and the reflecting surface by the speed of sound in
air. Unfortunately, the speed of sound in air is affected by
both temperature and humidity, and the reflected signal is also
scattered. These conditions substantially weaken the received
pulse. All these problems can be overcome with more complex
electronics and larger transducers, but experience to date (1979)
with these units has been very poor. Their use cannot be
recommended until their serviceability has been proved under
treatment plant conditions.
17.2.1.5 Tape-Supported Floats
Tape-supported floats are suitable for level measurements of many
liquids. Floats are suspended from a tape that winds on a drum.
The drum is provided with either a counterweight or a "constant
tension" spring to remove all the slack from the tape. The
position of the drum is a very accurate measure of float position
and, hence, liquid level. Standard units provide an accuracy of
0.01 feet (3.09 mm) for local readout and can be equipped with
electric signal transmission for remote readout. In the case of
floating cover digesters, the cover becomes the float, and the
same drum assembly provides measurement of level in the digester.
Tape-supported floats are often located within concentric wells
to isolate them from the turbulence of the liquid surface being
measured. To assure maximum reliability, these wells are usually
purged with water at rates sufficient to keep a continuous
flow from the well, even during periods of maximum rising levels.
Such installation is usually not practical when the material
being monitored contains significant amounts of debris and
grease.
17.2.2 Flow Measurements
Flow is an important measurement for sludge treatment and
disposal operations. Accuracy has been an ongoing problem with
all types of flowmeters. Venturi-type flow tubes, orifice
plates, and weirs are regarded as standard flow measuring devices
providing proper approach conditions—the length of straight pipe
upstream and downstream from the device--are maintained. This by
itself is a strong argument in favor of their use. In many
situations, proper approach conditions cannot be obtained or a
wider range of operation is needed. Some in-plant method should
be provided to "prove" the accuracy of non-standard meters.
A liquid flowmeter can frequently be calibrated by discharging
a flow into a tank of known dimensions and measuring the change
in level. In other cases, meters may be compared with other
meters of proven accuracy. For comparison, the meter under
test must be left in the actual plant piping, or a test stand
with an identical piping configuration must be provided. Flow
17-45
-------
measurements of wastewater sludge are difficult to take. The
designer must select the instrument with care, recognizing that
reliability may be a far more important criterion than accuracy.
17.2.2.1 Venturi Tubes
Venturi-type flow tubes have been successfully used for all
sludges, including primary sludge. Venturi tubes are classical
differential pressure producers that function according to
Bernoulli's relationships. A Venturi tube has a restricted
throat. A pressure drop is produced as the fluid accelerates
through the throat. The pressure drop is proportional to the
square of the liquid velocity and is measured by differential
pressure instruments, as described in Section 17.2.3. Modern
flow tubes operate on the same principles; they are improvements
on early Venturi devices. However, they are less expensive and
produce less residual head loss. When used for sludge flow
measurement, the pressure taps must be protected from plugging.
This can be done by water purge or by use of diaphragms similar
to those described earlier and installed in the tube wall. The
disadvantage of a Venturi tube is the narrow usable flow ranges
available. Anything over 3 to 1 is usually accomplished at
reduced accuracy. If air-purging is used, the Venturi tube
static lines require careful sloping to eliminate errors caused
by trapped bubbles. Water-purged systems require a source water
free of both soluble and insoluble solids to avoid clogging of
flow control needle valves. Consideration should be given to the
potential impact of purging water on downstream sludge processes.
17.2.2.2 Nozzles
Flow nozzles are similar in operation to Venturi tubes but
are substantially less expensive. Residual head loss is much
greater than for Venturi tubes and approaches that of an orifice
plate installation. Flow nozzles do not wear out as quickly as
orifice plates and can handle fluids containing limited solids.
The most common application of the flow nozzle is for steam flow
measurement.
17.2.2.3 Magnetic Meters
The magnetic flowmeter functions according to Faraday's law
which, in simple terms, states that when an electrical conductor
(in this case water) passes through a magnetic field, an
electrical voltage is developed at right angles to the direction
of the field and to the direction of the movement. If the
magnetic field is constant, the voltage is proportional to the
conductor's velocity. Hence, a magnetic flowmeter is simply a
tube with magnetic coils that uses electronics to measure the
voltage produced. In the past, a number of poor applications has
17-46
-------
put magnetic flowmeters in disfavor. When they are properly
applied with modern electronics, magnetic flowmeters are now as
reliable as any other flow measuring devices.
Flow velocity of primary sludge through a magnetic flow tube
should be in the range of 5 to 25 feet per second (1.5 to
7.6 m/s), providing a usable range of 5:1. The lower limit is
established by the minimum scouring action required to keep
electrodes free of grease. The upper limit is necessary to
limit erosion of the tube's plastic liner. Flow velocity for
secondary sludges may be extended down to 3 feet per second
(0.9 m/s) because less grease is present. For intermittent flow,
velocities may be extended up to 30 feet per second (9 m/s)
because less grit is present. Combining these conditions
provides a usable range for secondary sludges of 10:1. Magnetic
flowmeter manufacturers generally recommend certain accessories,
such as electrode cleaning devices, when metering sludge.
Purchase specifications should clearly state the application and
require provision of all recommended accessories. Properly
applied and installed, modern magnetic flowmeters are giving
excellent service in many installations.
17.2.2.4 Ultrasonic Meters
Ultrasonic meters are a fairly new development, and no two meters
work exactly the same. There are two basic types. The first and
most common one, which is listed as the ultrasonic device in this
chapter, consists of a pair of transducers mounted on opposite
sides of the pipe and displaced so that one transducer is one
pipe diameter downstream from the other. The first transducer
emits an ultrasonic pulse, and the time it takes this pulse to
reach the second transducer is measured. The system is then
reversed. The second transducer emits the pulse, and the time
until the first transducer receives this pulse is measured. The
travel time is known as propagation time. In one case, flow
velocity decreases propagation time, and in the reverse case,
increases the propagation time. The difference in time between
the two measurements is directly proportional to flow velocity.
The ultrasonic flow measuring system is relatively insensitive to
factors that normally affect the speed of sound (for example,
temperature). This is because the effects are cancelled as
the signals reverse. However, some difficulties have been
experienced with this technique. Most sludge is sonically
opaque. The signal cannot travel between the transducers, even
with a high power. Therefore, at this time, this type of meter
is not considered reliable.
17.2.2.5 Doppler Meters
The second type of ultrasonic flowmeter depends on the Doppler
effect. A continuous ultrasonic signal is emitted into the
pipe by the transducer. This signal is reflected by solids
17-47
-------
or bubbles in the liquid stream and is returned to a second
transducer at a frequency different from that transmitted.
This difference is related to the velocity of the material that
caused the reflection. Presently, difficulties prevent the
practical application of this technique. The frequency change is
affected by the velocity of sound, which in turn is affected by
temperature in the fluid. Furthermore, in sludge applications,
the particles or bubbles causing reflections will very probably
be located close to the pipe walls, and their velocity may not be
representative of average fluid velocity. Hence, the accuracy of
this type of meter is questionable. Actual field experience with
this type of meter is not extensive.
17.2.2.6 Rotameters
Rotameters are commonly used for both gas and liquid flow
measurements of clear homogeneous fluids. Their use in sludge
management is primarily for chemical feed systems, air flows, and
purge systems. A rotameter consists of a "float" in a conically
shaped tube. The "float" does not actually float, since it must
sink into the fluid being measured. The size of the rotameter
orifice increases as the "float" rises in the tube; therefore,
the upward force on the "float" for any fluid velocity decreases
as the float rises. The equilibrium point between "float" weight
and upward force due to flow velocity is an indication of flow.
Rotameters are constructed of a wide variety of materials,
including metals and plastics. They can be constructed to
measure almost any type of fluid. Rotameters are available up to
3-inch (8 cm) pipe size. Larger pipes are accommodated by
installing an orifice plate parallel to the rotameter so that a
known fraction of the flow passes through the rotameter. This is
called a "by-pass" rotameter. If the float is made of magnetic
material or contains an iron core, a magnet mounted on the
outside of the tube can be made to follow it. This magnet can be
attached to a transmitting mechanism to provide remote readout.
17.2.2.7 Propeller Meters
Relatively clean, non-corrosive fluids flowing through large
pipes (2 inches [5 cm] or larger) can be readily measured with
propeller meters. Propeller meters can provide local readout or
can be equipped with transmitting mechanisms for remote readout
or recording. They are not applicable for sludge flows, but can
provide reliable, cost-effective service for support systems.
17.2.2.8 Pitot Tubes
Pitot tubes very economically measure flow in pipes of almost
any size. The pitot tube produces a differential pressure
proportional to the square of the fluid velocity, which may be
17-48
-------
measured by differential pressure transmitters described in
Section 17.2.3. One commercial unit provides four ports spaced
across the diameter of the pipe and averages the impact pressure
of each to provide compensation for irregular flow profiles.
The pitot tube produces a very small pressure differential for
liquid flow velocities typically used in treatment plants and,
therefore, is not particularly suitable for liquid service. It
is frequently suitable for gas flows where wide flow ranges are
not required. The small tube entrances make it completely
unsuitable for use with sludge flows.
17.2.2.9 Weirs and Flumes
Weirs and flumes provide a simple, very accurate method of
measuring liquid flows in open channels. They are not applicable
to pressure systems. Any of the level measuring instruments
described in Section 17.2.1 provide a means of measuring the
liquid level behind the weir or at the critical point in a flume.
Weirs are not suitable for flows with large amounts of settleable
solids or debris. This material will collect behind the weir and
change weir measuring characteristics.
17.2.2.10 Orifice Plates
Gas flow measurement is commonly required where anaerobic or
aerobic digesters are used. Gas produced by anaerobic digesters
is dirty and corrosive. Permissible head losses in anaerobic and
aerobic digesters are often very low and the range of operating
requirements extreme. An orifice plate can be used in this
service. Orifice plates are similar in theory to Venturi tubes;
that is, pressure drop through the device is proportional to the
square of the liquid velocity. Orifice plates, however, lack a
smooth recovery cone and, consequently, have a much greater
residual head loss. The advantage of the orifice plate, other
than lower cost, is the ease with which it can be changed. The
optimum size of orifice plate can be readily installed for any
flow. Quick change fittings permit changing of orifice plates
without disturbance of a piping run.
17.2.2.11 Turbine Meters
Turbine meters, which provide good service in gas flow
applications, consist of flow directing channels, a suitable
turbine blade, gearing, shafting, and a readout device. In the
simplest form, the output shaft directly drives the readout
register. Where remote readout is desired, the shaft rotation
actuates an electrical switch. Each switch closure represents a
discrete quantity of gas. The meter must be specially designed
for dirty and corrosive gas. Moderate maintenance is required to
keep the meter clean. The turbine meter's ability to operate
over wide ranges makes it attractive for the measurement of
anaerobic digester gas.
17-49
-------
17.2.2.12 Vortex Meters
The Vortex shedding flowmeter is a comparatively new meter that
is also applicable to anaerobic digester gas flow measurement.
The meter consists of an obstruction placed in the pipeline with
sensors that detect the vortices caused by the obstruction. The
flow is proportional to the number of vortices produced. These
meters are suitable for Reynolds Numbers above 5,000 and readily
provide a usable operating range of 100:1.
17.2.2.13 .Positive Displacement
Meters
Orifice plates, turbine meters, and Vortex meters have all
provided adequate instrumentation fo.r anaerobic digester gas
flow. However, these instruments cannot provide the absolute
accuracy of positive displacement meters at the very low flows
encountered during digester operations. Positive displacement
meters can be of the rotating cavity (lobe) or the diaphragm
type. Positive displacement meters are probably the oldest meter
used for digester gas measurements. In recent years, they have
been almost completely replaced by the in-line meters described
in the previous paragraphs. Positive displacement meters are
frequently used for clean oil or clean gas flow measurements and
are inherently useful over an extremely wide operating range.
The meter's cavities, exposed bearings, and/or close clearances
make them unsuited for dirty gas service.
17.2.2.14 Pump and Transport
Displacement Systems
Sludge transport systems should not be overlooked as flow
measurement devices. Progressive cavity and other positive
displacement pumping equipment can be equipped with speed
monitors or cycle counters that provide a fairly accurate
flow indication. None of the problems usually associated
with flowmeters operating on sludge are encountered. Where
materials are trucked, the number of truck loads will provide a
rough measure of quantities. If the trucks are also weighed-in
and -out, accurate measurements can be obtained.
17.2.3 Pressure Measurement
Pressure measurement is basic to many level and flow measuring
systems, as well as to the measurement of individual process
pressures. As a result, pressure elements are without a doubt
the most highly developed instruments used in industry.
17-50
-------
17.2.3.1 Bourdons or Bellows
Pressure Elements
The bourdon tube is the most commonly used pressure element for
pressure ranges of 15 pounds per square inch (103 kN/m^)
or greater. The bourdon tube is essentially a piece of
tubing closed at one end and bent in an arc. When pressure is
applied to the tube, it tends to straighten. The movement
produced at the free or closed end is amplified by mechanical
linkage to operate a pointer or transmitter mechanism. Bellows
are frequently used when lower pressures must be measured or
greater movement is required for direct actuation of control
mechanisms. Bourdon tubes are rarely used in modern industrial
process pressure transmitters. Bellows elements are frequently
used in process pressure transmitters for pressure ranges from
10 inches water pressure (2.4 kN/m^) to as high as 600 pounds per
square inch (4.14 MN/m^). Bellows elements are also readily
adaptable to differential pressure measurements and absolute
pressure measurements.
Chemical Seals
Both bourdon tubes and bellows are unsuitable for direct
measurement of fluids containing solids. Collecting solids
within the pressure element is the problem. Corrosive fluids
also must be kept out of the pressure element. A "chemical seal"
is used for these applications. The most common chemical seal
consists of a small metal or elastomer diaphragm, one side of
which is exposed to the process fluid. Sometimes this exposed
side is purged with water or mounted flush with the fluid
containment vessel. The other side of the seal is close-coupled
or connected by a capillary tube to the measuring element and
filled with a suitable fluid such as silicon oil. For very
dirty, grease-laden process fluids such as wastewater sludge,
an in-line tubular or cylindrical chemical seal, as shown on
Figure 17-3, must be used to assure operational reliability.
This seal is constructed as an elastomer tube of the same size
as the process pipe line and mounted within a flanged steel
pipe spool. The space between the elastomer and steel spool
is sealed, filled with a suitable fluid (anti-freeze when
necessary), and connected directly to the pressure element.
Pressure' elements with electrical contacts and cylindrical
chemical seals should be used immediately downstream from all
positive displacement pumps transporting wastewater sludge. This
will provide a reliable system for emergency shutdown whenever
the pump "discharge pressure becomes excessive.
Chemical seals used to isolate corrosive fluids from pressure
elements are available in a great variety of materials. Care
must be exercised in the application of any chemical seal to
ensure that it has sufficient displacement to operate the
measuring element. Use of chemical seals for ranges of less
than 50 pounds per square inch (345 JcN/m^) can be expected to
introduce significant errors in the measurement.
17-51
-------
FIGURE 17-3
CYLINDRICAL CHEMICAL SEAL FOR
SLUDGE PRESSURE MEASUREMENT
17.2.3.2 Diaphragms
Pressu ^e^E^l^em en_ t
Where a direct mechanical readout is not required from a pressure
element, the diaphragm pressure transmitter is suitable for
any application where bourdon tubes or bellows would be used.
The force-balance is the oldest type of diaphragm pressure
transmitter and continues to be widely applied in industry and
wastewater treatment. Newer types such as the strain gauge,
reluctance, and capacitance, are functionally similar and are
becoming more common. Diaphragm pressure transmitters are
available to measure gauge pressure, differential pressure, or
absolute pressue, with ranges as low as 1-inch water column to
10,000 pounds per square inch (250 N/m2 to 80 MN/m2) or higher.
Chemical Seals
Chemical seals are not generally required with diaphragm pressure
transmitters for solids bearing fluids because the measuring
element itself is an essentially flat diaphragm. Chemical seals
are still frequently used for corrosion protection and, in high
temperature applications, to separate transmitter electronics
from temperatures above permissible levels. Chemical seals are
also used with differential pressure configurations to permit
flush-mounting of the diaphragms to the process at two physically
separated locations.
17.2.4 Temperature Measurements
Stabilization, disinfection, conditioning, composting, and
heat processes in sludge treatment all may require temperature
information to assure successful operation. Temperature
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instrumentation is relatively simple; however, successful
application requires locating probes to obtain representative
readings without obstructing sludge flow. The designer must be
aware of these application restrictions and locate and specify
instruments correctly.
17.2.4.1 Resistance Temperature
Detectors (RTDs)
Resistance temperature detectors (RTDs) are applied at
temperatures up to about 1,000°F (540°C). This is well within
the range of most sludge temperatures. RDTs work on the basis of
the fact that the electrical resistance of metals changes with
temperature. Electronic amplifiers measure this resistance
change and provide an output proportional to temperature.
Thermistors are sometimes used for special temperature
measurement applications. A thermistor is a temperature-
sensitive semi-conductor. Like RTDs, the thermistor's
resistance changes with temperature, but the change is extremely
non-linear. The advantage of using a thermistor is that a
large change in resistance can be obtained over a very narrow
temperature range.
17.2.4.2 Thermocouples
For processes with temperatures in excess of approximately
1,000°F (540°C) (for example, incineration), RTDs are not
suitable and thermocouples must be used. Thermocouples consist
of two junctions of dissimilar metals. One junction, the
measuring junction, is placed in or on the material to be
measured. The second, or reference, junction is located in a
constant temperature zone, or the measuring instrument may
include an artificial reference junction. The Peltier effect
states that at any junction of dissimilar metals, an electric
motive force (voltage) will be produced. Thus, two voltages,
(one at each junction) are produced in a series circuit. The
measuring instrument detects the difference between these
two voltages and produces an output proportional to process
temperature. Thermocouples produce very small voltages at low
temperatures. More importantly, the difference in voltage
produced by the reference and measuring junction is very small.
For this reason, thermocouples are not generally used to measure
small variations in temperature. Thermocouples are generally
less expensive than RTDs but require greater attention to
installation procedures to reduce electrical interference.
Wiring for thermocouples must be especially matched to the
thermocouple junction material.
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17.2.5 Weight Measurements
Two types of weight measurements are of interest in sludge
handling facilities. The first is the common static measurement.
The second is the weight per unit of time, which is actually a
mass flow measurement.
17.2.5.1 Static
Mechanical scales are frequently used foi: static weight measure-
ments. Such scales consist of a platform or vessel and a system
of pivots and levers to provide a usable readout. Mechanical
scales are constructed to measure anything from 0.40 ounces (11 g)
in the laboratory to 100,000 pounds (45.4 Mg) or more to weigh a
railroad car. Many modern scales use load cells under the
platform to eliminate the complex lever and pivot system. Load
cells are placed under one or more of the platform support
points, and the output of the cells is summed to obtain the total
weight. In some cases, the number of load cells can be lower
than the number of support points because load symmetry allows
multiplying the output of the installed cells by a factor to
account for the missing cells. Load cells may be either the
hydraulic or strain gauge type. Hydraulic load cells resemble
a piston that converts force to a hydraulic pressure. This
pressure is readily monitored by pressure instruments, as
previously described. Strain gauge load cells consist of a
calibrated structural member to which a resistance wire element
is attached. When the structural member is strained by an
applied force, the resistance wire element's dimensions change.
Hence, its electrical resistance changes. Suitable electronic
circuitry converts these small resistance changes to an
electrical output proportional to the force applied to the cell.
17.2.5.2 Mass Flow
Mass flow measurements involve a fixed transport system such as a
belt conveyor. Mass flow measurement on a belt conveyor is made
by supporting one or two conveyor belt idler rollers on a scale,
measuring the conveyor speed as described in Section 17.2.8, and
multiplying weight and speed together to obtain mass flow.
Modern belt scales using load cells and two idlers are very
accurate and are easily maintained. Nuclear belt scales can
provide this function without contacting either the belt or
the material being weighed. This may be an advantage in some
installations. Nuclear scales are almost identical in operation
to nuclear density meters. The only difference is that a nuclear
scale is calibrated to monitor total mass in its path rather than
the change in mass caused by suspended solids in a liquid. This
is a less difficult application, and premium radiation monitors
are not required, but nuclear source decay still causes the
calibration to drift. The radioactive source is a controlled
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substance subject to United States Nuclear Regulatory Commission
restrictions and regulations. It requires special training,
safety precautions, and testing. This adds to operation and
maintenance costs.
All conveyor mass flow scales measure the total mass of material
on the platforms or belt. No differentiation is made between
solids and water; therefore, the reading is most accurate if the
moisture content is constant or can be measured.
17.2.6 Density and Suspended
Solids Measurements
Sludge density and suspended solids are the same measurement from
an instrumentation point of view. However, they are quite
different from the operation standpoint. Sludge density is a
common term used to describe the concentration of solids in
sludge mixtures in which solids are the favored component.
Sludge density is usually expressed in percent solids by
weight. Suspended solids is a common term used to describe the
concentration of solids in water in which the liquid element is
the favored component--for example, the solids present in the
plant influent or the solids left in the supernatant after
gravity thickening. Suspended solids are usually expressed
in weight of solids per unit volume of water. There is no
instrument available that directly measures either sludge density
or suspended solids. Instruments that are used actually measure
nuclear radiation absorption, light transmission or reflection
(optical), or sonic attenuation characteristics of the mixture.
These measurements are then empirically correlated to sludge
density or suspended solids concentration. In most cases, this
correlation does not remain constant, and periodic recalibration
is necessary. The frequency of this recalibration is dependent
on the characteristics of the liquid being monitored. In no case
do these instruments provide adequate accuracy for reporting
purposes, although they can be used for control. Laboratory
analysis is usually required to obtain the accuracy necessary to
develop QFD (see Chapter 3) diagrams. Nuclear and opacity
density measurements can be used in conjunction with control
systems to allocate sludge automatically to various process
facilities on a mass flow basis. Sonic density measurements are
usually only applicable to the on and off control of sludge
pumping equipment.
17.2.6.1 Density
Nuclear
Nuclear density gauges usually work well on primary and mixed
primary and secondary sludges in the higher concentration range.
They usually have limited applicability to secondary sludge
alone. The nuclear density gauge consists of a small radioactive
source, usually cesium-137, and a detector placed on opposite
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sides of the pipe. Gamma radiation is emitted and absorbed
by the material in the pipe in direct proportion to its density.
However, the difference in radiation absorption between plain
water and water containing the suspended solids concentration
must be significant for nuclear meters to function well. These
meters are generally effective where suspended solids concentra-
tions are in the range of 1 to 10 percent. The radioactive
source itself decays, and the high gain amplifiers suffer from
gain changes resulting from component aging. Both factors cause
the instrument calibration to change rapidly, and frequent
adjustment is required to maintain accuracy. When nuclear
density gauges are to be used to measure sludge solids concentra-
tion, they must be specified with special premium low-drift
amplifiers. The source is a controlled substance subject to
United States Nuclear Regulatory Commission restrictions and
regulations. When properly installed and maintained, nuclear
density gauges have functioned quite successfully with wastewater
sludge .
Optical
Optical type meters are usually used to measure sludge density
concentrations of less than 3 percent. These instruments use
either light transmittance or a combination light transmittance/
scatter measurement and are suitable for concentrations from
0.2 percent to 10 percent solids. Units that employ a mechanical
wiper to keep the optics clean have been very successful.
Caution must be exercised in the application of these units to
primary sludge, which may contain grit that damages optical
surfaces and wipers.
Ultrasonic
The sonic density gauge is a relatively new product proposed for
measuring the density of primary sludge. The sonic density gauge
consists of two ultrasonic transducers mounted on opposite
sides of a pipe section. Ultrasonic signals emitted from one
transducer pass through the material in the pipe to the second
transducer. Suspended solids in the signal path attenuate this
signal. The signal received decreases in strength with an
increase in suspended solids. The relationship between the
strength of the signal received and the suspended solids
concentration is non-linear but sufficiently predictable to be
used for control of sludge pumps. Sonic density meters have been
used successfully and are much less expensive than nuclear
density meters.
17.2.6.2 Suspended Solids Measurements
The optical instrument described for providing density
measurements is also suitable for suspended solids measure-
ments. Instruments are available with a range from 0-30 to
0-30,000 mg/1. The mechanical wiper optical unit is generally
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the most suitable for this application. Surface scatter types
with no optics in contact with the process fluid are also
usable. Care should be taken to exclude larger solids, such as
particles of floating debris, that are frequently present in the
liquid being monitored. More information on suspended solids
instrumentation is available (3).
17.2.7 Time Measurements
Digital and synchronous motor batch (reset) timers are available
for control of sludge management and support services. Digital
timers provide one second resolution up to about 2 3/4 hours.
Synchronous motor timers provide 0.1 minute resolution up to
16 hours and 0.1 hour resolution up to 1,000 hours. Both types
use power line frequency as a time reference. They are designed
to reset at zero at the completion of a cycle. If time functions
cannot be interrupted by power failure or the wastewater plant
generates all its own electrical power, the designer must take
special precautions to see that all control timers function as
required on the emergency standby or continuous plant electric
power frequency. Digital timers can be obtained with an internal
quartz crystal to provide their frequency reference. They can
therefore operate independently of power line frequency.
17.2.8 Speed Measurements
Speed is readily measured either by a tachometer-generator
coupled to equipment or by a reluctance pick-up. Tachometer-
generators are generally more expensive and require higher
maintenance than reluctance pick-ups. This is because
tachometers have their own bearings, brushes, and usually a
timing belt coupling. A reluctance pick-up installation consists
of a split gear bolted around a shaft on the equipment. The
pick-up is then mounted in close proximity to the gear teeth.
Suitable electronics amplify the pulses that come from the
pick-up each time a gear tooth passes and converts these pulses
to a voltage or current output proportional to speed.
Electronic trip units can be used with either tachometer-
generators or reluctance pick-ups to permit these devices to be
used as underspeed switches. Mechanical underspeed switches
are also available. Tachometer-generators and mechanical units
are not reliable for operating speeds that are normally below
50 revolutions per minute. Reluctance pick-'-'p systems can
provide reliable operation at virtually any speed.
17.2.9 Moisture Content Measurements
Measurement of the moisture content of dewatered sludge is
necessary if the output of weighing equipment is to be directly
interpreted as weight of dry solids.
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There is no proven on-line instrumentation for measuring
moisture in sludge. Consideration of available options leads
to essentially two possibilities--a manual resistance probe
and laboratory tests. A manual resistance probe must be
considered a very approximate instrument since it is not actually
measuring moisture, and resistance measurements (moisture
content) will vary significantly with the contact pressure
between the monitored material and the probe. However, the
portable resistance probe can provide the accuracy needed for
immediate process control measurements; for example, compost
piles or windrows. The laboratory test is the only moisture
measurement method, however, that can provide the repeatable
accuracy demanded for QFD calculations (see Chapter 3). Special
infrared drying equipment with integral weighing instrumentation
is available to make such laboratory testing both convenient and
efficient.
7.2.10 Dissolved Oxygen Measurements
Three types of dissolved oxygen probes are commonly used in
wastewater treatment plants for measuring the dissolved oxygen
level in liquid streams containing high levels of suspended
solids. These include the galvanic cell type, the polarographic
cell type, and the thallium cell type.
Each of these cell types has its own proponents. The galvanic
cell is probably the most commonly used in existing wastewater
treatment plants. Both the galvanic cell and the polarographic
cell use a membrane (usually teflon) through which oxygen can
migrate into an electrolyte in which the electrodes are immersed.
Membrane cleaning and electrolyte replenishment require a
significant maintenance effort with these cells. The thallium
cells dispense with the membrane and immerse the electrodes
directly in the fluid to be analyzed. None of these cells is
applicable for measuring dissolved oxygen in liquids having
solids contents much higher than 2 percent.
17.2.11 pH Measurements
Modern selective-ion pH sensors with "non-flowing" reference
electrodes are suitable for measuring the pH of sludge. The
non-flowing reference electrode replaced the liquid reference
junction in which the electolyte (generally potassium chloride)
flowed continually from a reservoir into the process stream.
These systems sometimes plugged, causing erroneous readouts.
Non-flowing reference electrodes use a semi-solid electrolyte
that does not require frequent replenishment or reservoir
pressurization to maintain flow. Electrodes should be installed
in lines where sludge flows pass the sensor, maintaining a fresh
sample at the measuring point; for example, circulation lines.
Electrode assemblies should be designed to hold electrodes
essentially flush with the pipe wall. The electrodes should be
easily removable for cleaning or replacement.
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17.2.12 Chemical Oxygen Demand Measurements
Often liquid sidestreams from sludge treatment processes carry
significant levels of organics back into the liquid processing
system. The chemical oxygen demand measurements can be useful
in determining the strength of these sidestream organic loadings
and, therefore, provide input on their effect on the liquid
treatment processes downstream from their point of recycle.
Automated wet chemistry analyzers are capable of making a
standard chemical oxygen demand (COD) analysis, but these units
have not given satisfactory service under wastewater treatment
plant conditions. The total organic carbon (TOC) analyzer is
more suitable operationally, providing suitable correlation
can be established between TOC analyzer measurements and COD
laboratory data. There are several units on the market, and each
operates somewhat differently. Operation of one TOC unit is as
follows: The sample is treated with HC1 to remove inorganic
carbon as C02» It is then oxidized in a thermal reactor and
the resulting C02 measured by an infrared analyzer.
TOC analyzers operate with moderate-sized samples and can handle
suspended solids. However, they are high maintenance devices
requiring daily servicing.
17.2.13. Ammonia Measurements
A selective-ion electrode is available for measuring ammonia.
Ions other than ammonia frequently interfere with accurate
measurement and elimination of interferences requires treatment
of the sample before the measurement is made. Package analyzers
are available to prepare the sample and make the measurement.
Since custom sample preparation is frequently required, a sample
should be submitted to the analyzer manufacturer prior to
purchasing this type of equipment.
17.2.14 Gas Measurement and Analysis
17.2.14.1 Composition Analyzer
The composition of digester gas is a useful parameter for
monitoring the health of the anaerobic digestion process (see
Chapter 6). The chemical process industries make extensive use
of on-line gas chromatographs for measuring gas composition. The
heart of the chromatograph is the "column." The column is a
length of tubing filled with an absorbent material. As a gas
sample passes through this column, different gas components are
first absorbed, then released back into the gas stream. The
rate of absorption/release is different for each component and,
as a consequence, each component emerges from the column at a
different time. The components are thus separated from one
another. A detector at this exit measures the eluting gas,
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and its output is plotted as a function of time. The resulting
plot consists of a series of peaks and valleys, with each peak
representing the detector's response to one of the gases being
measured. Each peak can be associated with a specific component,
since the time (relative to sample injection) at which the
component peak will emerge from the column is known. The area
under each of the peaks is proportional to the gas concentration.
Even though this unit is called "on-line" it is a batch
instrument which, at best, might make four measurements per
hour. The on-line mass spectrometer is also capable of these
measurements but is even more expensive than the chromatograph.
Digester gas samples for analysis must be stripped of hydrogen
sulfide and filtered to remove solids before passing through
analysis equipment. Sample lines must be heat-traced to avoid
moisture condensation. With adequate sample preparation, gas
analysis instruments should function without undue maintenance;
however, at present, no data is available on a successful
wastewater treatment plant installation of any of these
instruments.
17.2.14.2 Calorimeter
A suitable instrument for measuring the heat value of digester
gas is a calorimeter, which essentially burns a gas sample. The
instrument must be located in an area free of drafts, which can
affect its accuracy or even extinguish the flame. Instrument
response is slow. This should be of no consequence during
monitoring applications, however, since digester gas composition
will normally change slowly. Care must be exercised, however, if
the instrument is to be used to control mixing of digester gas
with other gases to maintain a constant heat value or if the
instrument is to be used with a multiple sampling scheme for
monitoring several digesters. Calorimeters have been used
successfully in full-scale operations at wastewater treatment
plants.
17.2,15 Stack Gas Measurements
and Analysis
On-line analysis of boiler or furnace stack gas composition is
used frequently and has proven successful. It is directly
applicable to wastewater solids systems incorporating heat drying
and high temperature processes. These measurements are used
for combustion control and are usually mandatory if air pollution
is to be minimized. Obtaining a representative sample and
conditioning it for the analyzer are the biggest problems
in application of these instruments. There are a number of
different parameters that may require measurement to meet air
pollution control requirements, but oxygen is the parameter
normally used to control the air-fuel ratio. Two types of stack
gas oxygen analyzers are commonly used. The older unit is the
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paramagnetic type and the more modern is the catalytic type.
A new system uses a ceramic element for which the manufacturer
claims satisfactory operation on dirty flue gases without
clean-up. Precautions are required where sulfides are present.
Oxygen analysis equipment is normally included as part of the
combustion control system of a furnace.
17.2.16 Odor Measurements
Odor measurements are required during sludge management to
assure that the treatment and disposal processes selected meet
regulatory agency requirements (no nuisance). There are no
instruments for on-line measurement of odors. A device called a
"direct reading olfactometer" (DRO) provides a means to make a
semi-objective manual measurement of odors directly in the
location affected. Figure 17-4 shows a close-up of the DRO
assembly and a DRO in use with subject and controller. The DRO
is essentially a breathing mask with carbon filter, rotameters,
and valves to permit mixing known ratios of filtered and
unfiltered ambient air. The subject conditions his nose by
breathing 100 percent filtered air and then the operator adds
increasing amounts of unfiltered air until the subject indicates
he detects an odor. Repeated measurements with the same subject
permit detection of changes in odor conditions or odor levels at
different locations, within an accuracy of about plus or minus
25 to 50 percent. However, no absolute measurement exists.
Standard test procedures call for the use of odor panels (usually
six people) who rate odor levels from bagged samples taken at the
location affected. The panel usually works in a filtered air
environment, where absence of extraneous odors can be guaranteed.
17.2.17 Aerobic Condition Measurements
Aerated pile composting operations require the measurement of
oxygen concentration in the pile. The portable oxygen indicator
frequently used for personnel safety monitoring is applicable to
this service. These instruments operate on the same principle as
the catalytic or polarographic cell dissolved oxygen analyzers
described in Section 17.2.10 but are designed to be portable,
with a hand pump for drawing a gas sample.
17.2.18 Blanket Level Measurements
Measurements of sludge blanket level in sedimentation tanks and
gravity thickeners can be accomplished with optical (turbidity)
type instruments and with ultrasonic ' instruments, as described
in Section 17.2.6. The success of this measurement is dependent
on the characteristics of the sludge blanket. A well defined
blanket interface provides a readily detectable change in
suspended solids concentration. Where the blanket is poorly
defined, this measurement is not satisfactory.
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t'Jt
ASSEMBLY DETAILS OF DRO SHOWING
SUBJECT'S MASK AND CONTROL'S DILUTION METERS
SUBJECT AND CONTROL MEASURING FOR ODORS IN FIELD
BATTERY PACK USED FOR PORTABILITY
FIGURE 17-4
DIRECT READING OLFACTOMETER (DRO) (4)
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Air-lifts with intakes at multiple elevations have been provided
in a number of plants for drawing a sludge sample. The
individual air lifts for a tank can be manifolded together
and the flow passed through a turbidimeter to provide remote
monitoring. The individual air lifts are actuated in sequence,
and a sludge profile is obtained. A. turbidimeter of the falling
stream type is recommended for this service. It should be
installed at the air-lift location.
17.2.19 Hydrocarbons and Flammable
Gas Detectors
Methane is the flammable gas most likely to occur in sludge
management. Catalytic detectors, available from a number of
manufacturers, are sensitive to any flammable gas and ordinarily
may be installed in the space to be monitored, thus eliminating
sampling systems. The detector consists of a heated catalytic
element exposed to the ambient air and a similar reference
element isolated from ambient air. If flammable gas is present,
the exposed element temperature will rise above the reference
probe as the gas is oxidized. This temperature difference
results in a change in electrical resistance, which is measured
by the detector's electronics. These units should be calibrated
periodically with a standard reference gas. Catalytic probe life
is definitely limited, and periodic replacement is required.
Under very severe conditions, the probe may lose sensitivity
in less than a year. When these conditions occur, a sampling
system to clean up the sample and remove the moisture should be
considered.
17.2.20 Radiation Monitoring
If gamma radiation is used in sufficient quantities to effect
treatment, safety monitoring will be required to protect
personnel. Note that nuclear density and weight equipment
uses such small gamma sources that no significant hazard exists,
and personnel safety monitoring is not required. Personnel
safety monitoring requires monitoring of the radiation levels in
the exposed spaces to detect abnormal leakage from the process
and individual monitoring to detect exposure of that individual
to radiation. Space monitoring is accomplished by suitable
geiger counters. Personnel who are not normally exposed to
radiation can be adequately monitored by badges containing
photographic emulsion. Personnel who may absorb radiation
during job performance will have to carry instrumentation
capable of accurately accumulating the amount of radiation
absorbed to control dosage to acceptable limits. Specialists in
nuclear monitoring must be consulted if this type of process is
contemplated.
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17.2.21 Machinery Protection
Wastewater solids treatment and disposal machinery requires
protection similar to that required by the machinery in other
wastewater treatment systems. However, some solids system
protective instrumentation is unique. This section deals with
this unique protective instrumentation.
17.2.21.1 Empty Pipe Detectors
Empty pipe detectors were developed to provide protection for
sludge pumps that might be operated with no fluid in the suction
pipe. This protection is particularly applicable to positive
displacement or progressive cavity pumps, which can suffer
extensive stator damage if operated without fluid. Capacitance
elements fabricated as a wafer to fit between pipe flanges are
most commonly used. The theory of operation is identical to
the capacitance level elements described earlier. Nuclear
level switches can also be used for this application but require
more mounting space. The nuclear device clamps onto the outside
of the pipe and operates much like the nuclear density meter
described earlier. This is a very simple application of the
nuclear unit. The unit used has a much lower cost than nuclear
units required for density measurement. The nuclear device may
be easier to install in existing plants since existing piping
would not have to be disturbed as long as sufficient space is
available.
17.2.21.2 Vibration - Acceleration and
Displacement Systems
Vibration detectors are provided for most machinery that operates
at high rotational speeds--for example, centrifuges. Vibration
detectors are usually capable of giving advance warning of
incipient machine failure. This allows for orderly shutdown,
thereby minimizing damage to both process and machinery. The
cost of the protection afforded is usually justified only for
larger pieces of equipment. Two types of detectors are generally
applicable: acceleration and displacement. Accelerometers are
less expensive than displacement systems and provide moderate
protection to lower value machinery, such as thickening or
dewatering centrifuges. Displacement probes are mounted rigidly
to a bearing pedestal or similar stationary object and provide a
very accurate measure of actual shaft movement in the journals.
A large number of displacement probes are required to provide
full protection. Their installation and alignment is rather
complex when compared to the accelerometer, which is simply
attached to the machine housing. As a result, displacement
installations must be carefully engineered and are relatively
expensive. Displacement systems are generally used on large,
high-speed machinery, such as centrifugal blowers in sizes of
500 horsepower or greater.
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17.2.21.3 Flow Loss Monitors
Gas or air flow can be effectively monitored for loss of flow by
vane switches, differential pressure switches, and thermal flow
switches. Vane switches are the simplest, but require fairly
high velocities for reliable operation. A differential pressure
rise from the suction to the discharge side of a fan or blower,
or a differential pressure loss from the suction to the discharge
side of a filter or other piping element provides a simple
monitor of flow that is adequate for most purposes. Where
precision operation is required, particularly at low velocities,
a thermal flow switch is most suitable. These devices consist of
a heated element that is convection-cooled by air flow. The
change in heat loss of the element provides reliable detection of
gas or air flow.
Vane switches, differential pressure switches, and thermal flow
switches are also applicable to liquid flows. However, the vane
switch is unsuitable for solids-bearing fluids, such as sludge.
One thermal flow switch is constructed as a smooth rod. If
installed at an angle with the pipe radius or into an elbow, this
unit is applicable to solids bearing fluids. Differential
pressure devices must be provided with chemical seals if they are
to be successfully applied to solids-bearing fluids.
17.2.21.4 Overload Devices
All electric motor drives at wastewater treatment plants are
provided with thermal overloads. However, these units are not
fast enough to protect the driven machinery from damage due
to mechanical blockage. Collector drives, in particular,
are virtually always provided with some type of instantaneous
protection from excessive torque. One of the most common
applications involves the circular collector of secondary
sedimentation tanks and circular gravity thickeners. The
simplest overload device for such equipment is the shear pin.
The shear pin has the disadvantage of working only once. When it
has provided protection for one overload, it must be replaced
with an identical pin. As a result, several mechanical resetable
overload devices have been used. The one most commonly used
today is an instantaneous over-current relay or ammeter with
high alarm contacts installed in the motor circuit. These
units are simple, very reliable, and also provide a continuous
indication of load. This is useful for detecting any abnormal
load build-ups.
17.2.21.5 Flame Safeguard Equipment
Wastewater solids systems that use boilers or furnaces to
maintain anaerobic digestion, heat drying, or high temperature
processes require flame safeguard instrumentation. Flame
safeguard equipment shuts off the oil and gas burners in case of
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ignition loss. Ultraviolet light detectors provide virtually
instantaneous protection since ultraviolet energy is present
only in the actual flame. This equipment is normally provided
as part of the burner control package. This package also
includes sequencing systems as necessary to ensure the purging of
explosive gases from the fire boxes before burner relighting
following a flame-out.
17.3 Sampling Systems
Sampling systems include sample transport and sample condi-
tioning. Where practical, measuring elements should be installed
directly into process vessels. In some cases, however, immersing
a measuring element directly into the process is not possible or
desirable. Some analyzers simply are not adaptable to direct
on-line measurements. In other cases, the cost of an analyzer is
so great that it must be time-shared between several sample
streams.
Anytime a sample must be transported a significant distance, care
must be taken to ensure that the sample delivered to the analyzer
is fresh and that critical characteristics do not change during
the transport time. Pumps and piping materials must not be
corroded by the sample nor should they in any way affect the
sample composition. Fluid velocities in transport lines must be
kept high enough to prevent solids settlement and to limit
transit time. Flow to the analyzer should be continuous to
maintain clean lines and deliver a fresh sample immediately to
the analyzer, where sample switching is practiced. Where pumps
are required, they must be suitable for continuous operation
without excessive maintenance. Where switching systems are used
to direct multiple samples into an analyzer, three-way diverter
valves are required for each sample stream, with one port to
the drain and the other to the sampler.
Solenoid-controlled, pneumatically operated ball valves are
recommended for sample switching. These units are capable of
handling many operations without excessive maintenance and
can provide slow operation of the ball valve and, therefore,
smooth switching of the sample stream. Electric motor-operated
ball valves can be used but life expectancy of ball valves in
repetitive operations is short. Rapid direct switching with
large solenoid valves causes significant pressure stress on
sample valve piping. If solenoid valves are used to switch
samples directly, some system must be provided to absorb
water hammer. Large, three-way solenoid valves with suitable
characteristics are not readily available; therefore, two two-way
valves, one normally open and one normally closed, are usually
required to obtain the three-way switching function.
Some type of program timer is required to control sample valves
and synchronize readout devices with L'ne samples. The time
program must also consider the settling time of an analyzer
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switching samples. The settling time includes both
transport d e 1 a " s required to ^ e t new samples through pi^in0
between sv/itching valves and time for the analyzer itself to
reach new readings. In many cases, an analysis is used only for
recording or indication (that is, not for control purposes), and
it is acceptable to provide a single output instrument with some
means of identifying the sample source currently being measured.
In systems where the output of the analyzer is used for control,
some means must be provided of holding the last value of the
parameter during periods that the analyzer is working on other
samples. It is also essential to review control system dynamics
in an intermittently sampled data environment. In general,
completely different control strategies are required for
intermittently sampled data systems than for continuous data
systems.
Gas sample lines should be heat-traced to avoid condensation
within the lines. In cold climates, exposed liquid samples will
also require heat tracing to avoid freezing.
Sample preparation is critical to satisfactory operation of
analysis equipment. The degree of grinding and/or filtering
required depends on the nature of the analysis and the equipment.
In general, the aid of the analyzer manufacturer should be
enlisted in working out a suitable system. More information on
sample transport is available (5).
17.4 Operator Interface
17.4.1 Location
Modern electronic instruments that provide information to
operators (for example, indicators and recorders) are designed
for installation in clean, air-conditioned control rooms.
Field locations are usually not suitable for these instruments
unless additional protection is provided. Hydrogen sulfide is
present in many process areas, and if it is allowed to contact
instruments that are not designed for this atmosphere, failures
may result from corrosion. Some process areas are classified as
hazardous, so that electrical equipment must be explosion-proof.
Explosion-proof electronic operator interface instruments are not
available. To be usable in a hazardous area, non-explosion-proof
instruments must be enclosed in a suitable box. This makes them
virtually inaccessible and, therefore, difficult to use and
maintain. Where instruments must be located in a contaminated
or hazardous process area, pneuma.tic instruments, which are
inherently explosion-proof and are fairly resistant to dirt and
corrosion, should be considered. Where pneumatic instruments
are not practical, air purging of cabinets or special filters
may provide adequate protection to electronic instruments.
A suitable remote control room is the most desirable solution.
17-67
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17.4.2 Indicator Boards
Sludge handling systems are frequently designed with considerable
operating flexibility, with large numbers of valves and many
possible flow configurations. As a minimum, some means is
required to tell the operator what the present flow configuration
is. A chalk board can be used for this purpose, but it does not
readily provide a graphic picture of the piping configuration.
Therefore, in more complicated plants, some type of graphic
indicator board is desirable to prevent errors. In the simplest
form of indicator board, a graphic panel is produced with
manually moveable flags or indicating lights with which the
operators indicate current valve positions and pump operation.
Such a system can give an excellent picture of the present
operating configuration, but is dependent on the operators to set
the flags correctly. The use of limit switches on valves and
indicating lamps is more reliable and also provides the operator
with a ready means to check the validity of the valve settings
and pump selection. Figures 17-5 and 17-6 show two examples of
graphic panels with indicating lights for showing valve or gate
positions.
17.5 References
1. U S E P A. Instrumentation and Automation Experiences in
Wastewater Treatment Facilities. MERL. Cincinnati, Ohio
45268. EPA-600/2-76-198. October 1976.
2. Liptak, B.C., editor. Instrument Engineers Handbook.
Chilton Book Company. Radnor^Pennsylvania." 1969. '
3. US EPA. Advanced Automatic Control Strategies for the
Activated Sludge Treatment Process. ERIC. Cincinnati, Ohio
45268. EPA-67(I/2-75::OT
-------
.
.
~ ' Vr-
FIGURE 17-5
AERATION CONTROL GRAPHIC PANEL AND CONSOLE
LIGHTS SET MANUALLY ON GRAPHIC PANEL
17-69
-------
FIGURE 17-6
INCINERATOR-DIGESTER CONTROL GRAPHIC PANEL
LIGHTS CONTROLLED BY REMOTE VALVE LIMIT SWITCHES
17-70
-------
EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapter 18. Utilization
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 18
UTILIZATION
18.1 Introduction
Utilization refers to the beneficial use of sludge or sludge
by-products. Sludge disposal options that do not involve
beneficial use (for example, when disposal is the only goal) are
discussed in Chapter 19.
Sludge may be used as a:
• Soil amendment. Sludge contains both crop nutrients and
organic matter. Sludge can be used as a fertilizer
and in the reclamation of disturbed lands, such as
construction sites, strip-mined lands, gravel pits, and
clear-cut forests. It may be used to stabilize bank
spoils and moving sand dunes.
• Source of heat and work. Energy may be recovered from
the gas produced during anaerobic stabilization, or
partial or full pyrolysis of sludges, and from the direct
burning of sludges. This energy may be converted to
heat or work and put to a variety of in-plant uses, or
exported for uses outside the plant.
• Source of other useful products. Other useful products
include waste treatment chemicals, landfill toppings,
industrial raw materials, animal feed, and materials of
construction.
The thrust of recent legislation has been to encourage beneficial
reuse. The Federal Water Pollution Control Act of 1972
(PL 92-500) stated that "The Administrator shall encourage waste
treatment management which results in the construction of revenue
producing facilities for . . . the recycling of potential sewage
pollutants through the production of agriculture, silviculture,
or agricultural products, or any combination thereof." The Clean
Water Act (CWA) of 1977 (PL 95-217) offered further incentives
for projects that involved innovative and alternative technology
(for example, sludge utilization, energy recovery). In
addition, the CWA requires the establishment of industrial
waste pretreatment programs with the objective of reducing
toxic pollutant loadings to municipal treatment facilities.
Implementation of pretreatment programs will make more municipal
solids suitable for reuse.
18-1
-------
The pretreatment program supplements programs established by the
Toxic Substance Control Act (PL 94-469) which authorized USEPA
to obtain production and test data from industry on selected
chemical substances and regulate them where they pose an
unreasonable risk to the environment. Steps towards the goal
of furthering sludge utilization were taken by the Resources
Conservation and Recovery Act (RCRA) (PL 94-580), which
authorized USEPA to develop treatment and application rate
criteria for sludge to be applied to land growing food-chain
crops, as well as to nonagricultural areas. RCRA also authorized
funds for research, demonstrations, training, and other
activities related to development of other resource recovery
schemes.
At the same time, it is recognized that there are potential
hazards associated with wastewater sludge utilization and that
utilization without careful planning, management, and operation
could present a danger to human health and to the environment.
18.2- Sludge as a Soil Amendment
Approximately 1.3 million dry tons per year (1.2 million t/yr),
or 31 percent of the treated municipal sludge generated in the
United States today, is applied to the land for productive
use. The quantities of treated sludge projected for ultimate
disposal by 1990 range from 5.6 to 7.6 million dry tons per year
(5.1 to 6.9 t/yr). The sludge quantities generated will depend
in .great part upon the extent to which municipalities adopt
land treatment of wastewater. Land treatment, which is an
alternative to conventional forms of wastewater treatment,
reduces substantially the amount of sludge produced.
18.2.1 Perspective
The impact of sewage sludge on the national commercial
fertilizer market is relatively insignificant. This is shown in
Table 18-1, where the amount of nutrients in currently utilized
and potentially usable sludges are compared against the
nutrients presently consumed in the form of commercial
chemical fertilizers. Nitrogen, phosphorus, and potassium in
currently utilized sludges are estimated to be only 0.2, 0.9, and
0.1 percent, respectively, of those nutrients consumed with
chemical fertilizers. If all United States wastewater sludges
were applied to land, these percentages would increase to 0.6,
3.2, and 0.4 percent, respectively. If the value per pound of
nutrient in the sludge was the same as that paid by farmers
for the corresponding commercial nutrient, the monetary
value of utilized nutrient sludges in 1978 was $9.5, $26.0, and
$1.7 million per year for nitrogen, phosphorus, and potassium,
respectively.
18-2
-------
TABLE 18-1
COMPARISON OF CURRENT AND POTENTIAL SLUDGE UTILIZATION
TO COMMERCIAL FERTILIZER CONSUMPTION IN THE UNITED STATES (1)
Nutrient usage, 1,000 ton/yr
Nutrient
A.
Nutrients in
currently
utilized sludges
B. Nutrients in
potentially
useable sludges
C. Nutrients
presently consumed
in commercial
fertilizers
Nitrogen as N
Phosphorus as P
Potassium as K
21.63
21.5
4.3
65. 3a
80.7
18.9
10,642
2,453
4,841
A, as
percent
of C
0.2
0.9
0.1
B, as
percent
of C
0.6
3.2
0.4
Nitrogen in sludge expressed as available N, assumed to be
50 percent of total N.
While the values of nutrients in sludge are small relative to the
current dollar values of commercial fertilizers, they are by no
means insignificant to those who would benefit monetarily. For
example, wastewater treatment plants could reduce operating costs
by sludge sales or by elimination of more expensive treatment and
disposal methods. Sludge users, for example, private citizens,
can obtain nutrients for lawns and gardens at low cost.
It is estimated that by the year 1990, annual savings in
treatment costs could be $100-$500 million if sewage sludge
utilization were increased to about 50 percent (2). This
utilization increase could result, in part, from the incentives
for innovative and alternative technologies provided by the
1977 CWA if various constraints to sludge utilization, including
regulations, are not overly stringent. If 50 percent of sewage
sludge were utilized on land, about $50 million (1978 dollars) in
nutrients and organic matter could be recovered and utilized for
growing crops and improving soil structure.
A number of locations where various sludge utilization options
are currently being employed are listed in Table 18-2. Some of
these operations have only recently started up (for example,
Madison, Wisconsin), while others have been in operation for as
long as 50 years (for example, Los Angeles County, California).
18.2.2 Principles and Design Criteria for Applying
Wastewater Sludge to Land
Certain basic elements are common to all land application
projects, no matter how or where the sludge is to be applied.
These elements include preliminary planning, site selection,
process design (which includes determination of sludge
18-3
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application rates), facilities design, and facility management
and operation. Full and complete discussions of each of
these elements are too lengthy to be included in this manual.
Therefore, this section will provide only a brief outline. For
full details, the reader should consult Reference 3. At this
writing, this is USEPA's primary reference for the utilization of
sewage sludges on land. The entire subject of sludge use on
land will be covered more extensively in a future Technology
Transfer design manual.
TABLE 18-2
EXAMPLES OF COMMUNITIES PRACTICING LAND UTILIZATION (2)
Communities
Wastewater
flow,
MGD
Sludge
utilized,
dry ton/day
Description
Landspreading of liquid sludges
Clinton, New Jersey 1
Rochester, Indiana 1
Little Falls, Minnesota 1
Peru, Indiana 2.5
Bowling Green, Ohio 3.5
Muncie, Indiana 17
Salem, Oregon 30
Madison, Wisconsin 36
Seattle, Washington 150
Chicago, Illinois 909
Composting
Durham, Mew Hampshire 0.8
Burlington, Vermont 5.8
Toms River, New Jersey 6.5
Bangor, Maine 7
Windsor, Ontario 21
Camden, New Jersey 32
Philadelphia, Pennsylvania 113
Washington, D.C. 300
Los Angeles, California 440
Drying
Little Falls, Minnesota
Largo, Florida
Marion, Indiana
Fort Worth, Texas
Houston, Texas
Toledo, Ohio
Milwaukee, Wisconsin
Denver, Colorado
Chicago, Illinois
0.5
0.8
0.6
0.8
1.7
10
8
27
28
165
0.7
2.3
7.8
2
25
12
30
55
150
1
8
9
75
73
78
132
140
909
0.4
2.5
0.2
41
18
35
190
125
131
PO, PL
MO, PL
MO, ML, PL
MO, PL
MO, PL
MO, PL
MO, PL
FO, PL
PO, PL
MO, ML
MO, GAM
MO, PL, ML
ML, PL
MO, GAM
MO
MO, GAM
MO, ML, PL
PO, GAM, S
MO, S
Drying bed, ML, PL
Heat dry, S
MO, PL
Drying bed, MO, ML
Heat dry, S
PO, PL, Filter cake
Heat drying, MO, S
MO, ML, Filter cake
Heat dry, MO, S
a PO - Privately operated (contractor)
PL - Private land
MO - Municipally operated
ML - Municipal land
FO - Farmer operated
GAM - Giveaway to municipality
S - Sale
18-4
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18.2.2.1 Preliminary Planning
Preliminary planning consists of the following steps:
• A planning team is formed of individuals who are
interested in the proposed program and whose expertise
and support are required. A major activity of the
planning team is to solicit and obtain public support
for the program, particularly the support of potential
sludge users and local government. The importance of
obtaining public support cannot be overemphasized. Many
utilization projects have failed because planners have
failed to recognize this necessity.
• Basic data is collected, including sludge quantities and
characteristics, climatic conditions and local, state,
and federal regulations.
18.2.2.2 Site Selection
Site selection consists of:
• Preliminary screening. A rough estimate of total acreage
required is obtained by dividing total sludge quantity
by an assumed application rate. Land that might be
available within about 30 miles is identified; obviously
unsuitable sites are immediately eliminated. If this
rough analysis indicates that sufficient land is
available, a more detailed study of potential sites is
initiated.
• Site identification. Potentially available sites
remaining after preliminary screening are characterized
as to topography, land use, soil characteristics,
geology, and distance from treatment plant. The
characterization at first is general, taken from
published and readily available sources of information,
such as soils surveys and topographical maps. The least
suitable sites are eliminated by an objective ranking
procedure, similar to the second-cut analysis described
in Process Selection Logic, Chapter 3. The procedure
is reiterated, with more detailed and site-specific
information in each iteration, until finally the best
site or sites are determined.
• Site acquisition. Sites are acquired either by outright
purchase or by the municipality obtaining a contract for
the right to use private land for sludge utilization.
18.2.2.3 Process Design
Process design involves selecting suitable crops and determining
appropriate sludge application rates as well as application
methods. Although basic design goals (maximization of crop yield
18-5
-------
and quality, and minimization of environmental damage) remain
constant regardless of projected land use, design procedures
differ for applications on agricultural, forested, and reclaimed
lands:
• Application on agricultural land. Sludge should be
applied to agricultural land at a rate equal to
the nitrogen uptake rate of the crop unless lesser
application rates are required because of cadmium
limitations. Annual loading rates for cadmium on
soils have been set at 1.8 pounds per acre per year
(2.0 kg/ha-yr) for food chain crops; however, this value
can be regarded as provisional and may be revised on the
basis of ongoing and future research and future federal
regulations. The basis for the nitrogen criterion is to
minimize nitrate leaching to groundwater. The annual
limit for cadmium is chosen to minimize uptake by crops
and the potential for long-term, sub-clinical adverse
effects on human health. Site lifetime limits are
established on the basis of maximum cumulative loadings
of lead, zinc, copper, nickel, and cadmium. These limits
are designed to allow growth and use of food-chain crops
at any future date.
• Application on forested land. As with agronomic crops,
the harvesting of a forest stand removes the nutrients
accumulated during growth. However, the amounts removed
in forest harvesting annually are significantly lower
than in agronomic crop harvesting. Uptake by vegetative
cover is negligible. Therefore, forest systems rely
primarily on soil processes (denitrification) to minimize
nitrate leaching into groundwater. As a result, nutrient
loadings on forested lands must generally be less than
those on agricultural sites. No annual limitations are
set for cadmium, since no food-chain crops are grown.
Lifetime metals limits used for agricultural sites are
suggested for forested land; this would minimize metal
toxicity to trees and allow growth of other crops if the
area were cleared at a future date.
• Application on reclaimed land. Sludge is usually applied
to impoverished lands at rates sufficient to satisfy the
nutrient requirements of the cover crop.
18.2.2.4 Facilities Design
Once the site has been chosen and crops and approximate sludge
application rates have been decided upon, the project can proceed
to the facility design stage. This phase of the project is
site-specific and consists of:
• Detailed site investigations. On-site soil analyses are
conducted to determine such factors as available
phosphorus and potassium, soil pH and lime requirements,
18-6
-------
cation exchange capacity, and organic matter. Such
information will allow for finalizing sludge application
rates determined in the Process Design phase. Soil
should be characterized to provide baseline data against
which subsequent analyses can be compared. This will
allow documentation of changes in the physical and
chemical properties of the soil due to sludge applcation.
* Determining pre-application treatment. This refers to
upstream sludge treatment, including thickening,
stabilization, disinfection, conditioning, dewatering,
and drying (see Chapters 5 through 10 for detailed
discussions). For new plants, the method of sludge
disposal or utilization may dictate the preapplication
proces&ing configuration. For existing plants,
pre-application treatment! influences sludge form and
composition, and thus affects application rate,
the method of spreading, and the mode of sludge
transportation.
• Determining sludge application mode. The application
mode depends upon the sludge form. Liquid sludge can be
spread by tank truck, sprayed, injected, or applied by
the ridge-and-furrow technique. Dewatered sludges are
usually applied by conventional fertilizer spreading
equipment. See Chapter 19 for a discussion of sludge
application techniques.
• Determining sludge storage requirements. Storage should
be provided when sludge cannot be spread (for example,
during inclement weather). Storage can also provide
additional stabilization and disinfection. See
Chapter 15 for information on storage.
18.2.2.5 Facility Management, Operations,
and Monitoring
Once the system has been constructed, it must be made to run
smoothly and efficiently:
• Operations must be scheduled. Spreading must be timed to
satisfy farming requirements. If the municipality
grows its own crops, tilling, planting, and harvesting
operations must also be scheduled.
* Operations must be managed to reduce off-site impacts
(odors, contamination of groundwaters, and surface
waters).
• Operations must be monitored to assure that the system is
operating as intended. Sludge must be analyzed to ensure
its acceptability to the user and to provide a record of
nutrient and metal additions to the soil. Soil, crops,
18-7
-------
groundwaters, and surface waters need to be monitored
only if sludge nutrients are applied at rates exceeding
the uptake capacity of crops or soils.:
18.3 Sludge as an Energy Source
Whether produced from direct burning of sludge or from the
combustion of sludge-derived fuels such as digester gas or
pyrolysis gas, the end product is energy. Heat can be made to
perform a variety of useful functions.
18.3.1 Perspective
The precipitous rise in energy prices during the 1970s has
generated intense interest in the conservation and recovery of
this precious commodity. For example, the United States Energy
Research and Development Administration (now the Department of
Energy) has proposed one-seventh of the United States energy
requirements be produced by bioconversion processes (for example
anaerobic digestion) by the year 2020 (4). Clearly, however,
this awesome quantity of energy will not be generated from
municipal wastewater sludge; there is simply insufficient sludge.
Very large external organic sources (for example, manure from
feed lots or municipal refuse) and external processing systems
(energy farms) will be required to effect such production. As
with utilization of sludge on land, the impact of energy recovery
from municipal sludges will be largely local, that is, it will be
felt most strongly at the treatment plant and in its immediate
vicinity. Here, the effects can be significant.
As Figure 6-32 indicates, the energy value of methane generated
from the anaerobic digestion process exceeds the energy
requirements of the digestion process. The excess can be used to
supply the energy needs of other plant processes. In some
instances, the gas generated is sufficient to supply the energy
needs of the entire wastewater treatment plant, with excess gas
available for sale. Notable examples are the British Southern
and Mogden plants and the County Sanitation Districts of
Los Angeles County Joint Disposal Plant (5). Heat recovery
is possible even if digestion is not used, for example, heat
recovery from coincineration of sludge and municipal refuse is
expected to provide all the energy needs of the Central Contra
Costa Sanitary District (CCCSD) plant in Concord, California (6).
In January 1978, the State of California Public Utilities
Commission (PUC) passed a resolution directing all state
utilities to augment cogeneration projects by setting up new rate
schedules covering interruptible electric service; by creating
new specific rates to encourage cogeneration, including revisions
to standby rates; and by developing guidelines covering the
price and conditions for the purchase of energy and capacity
from cogeneration facilities owned by others (7). The term
cogeneration in this context means the production of power by
utilization of waste heat; it also covers power produced through
18-8
-------
the burning of alternative fuels, such as municipal waste.
The resolution significantly changes the economics of power
generation at California Wastewater treatment plants and
encourages the use of in-plant energy recovery.
On June 27, 1979, the Federal Energy Regulatory Commission issued
proposed regulations providing for the qualification of small
power production and cogeneration facilities under Section 201 of
the Public Utility Regulatory Policies Act of 1978 (8). The
proposed regulations are set up to assure opportunities for
small power producers (<80 MW) to sell electricity to.electric
utilities when such electricity is generated through the use of
renewable energy sources (such as sludge) or recovered process
heat.
These regulatory actions are an indicator of future trends in the
United States as the country seeks to increase its non-fossil
fuel energy production. The designer should be aware of their
impacts on future planning for using sludge as an energy source.
The recovery of energy in the form of fuels and heat from
municipal sludges will be discussed in detail in the following
sections.
18.3.2 Recovery of Energy From Sludge
Figure 18-1 shows on one diagram processes which release energy
from sludge; devices which convert the released energy to useful.
forms; useful energy forms; and suggested applications of
recovered energy, either at the wastewater treatment plant or
off-site. Special consideration must be made when designing
processes to recover energy from wastewater sludge. Some of
these considerations are discussed below.
18.3.2.1 Treatment of Digester Gas
The treatment required depends on the digester gas1 anticipated
use. Treatment is minimal if the gas is burned in a boiler or in
a high temperature internal combustion engine. Conversely, if
it is sold for utilities as a natural gas substitute it must be
upgraded to natural gas quality. This involves treatment to
remove part iculates, H2S, CC>2, and water. As a general rule, gas
treatment should be avoided to as great a degree as possible. It
is preferable to set up recovery systems that can be operated
with untreated digester gas.
Particulates are carried over with the gas as it leaves the
digester. They may be removed in large sedimentation traps and
cyclonic separators.
H2S is most commonly removed by iron-sponge scrubbers. The
"sponge" consists of wood shavings impregnated with iron oxide.
H2S reacts with iron oxide to form nonvolatile ferric sulfide.
The sponge can be regenerated with air. Sponge capacity is
18-9
-------
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FIGURE 18-1
THE RELEASE, CONVERSION, FORMS AND USES
OF ENERGY FROM SLUDGE
18-10
-------
about 0.6 pounds of sulfur per pound of iron oxide (0.6 kg/kg).
Problems have been experienced with fouling of the iron-sponge by
oils and greases entrained in the digester gas. Iron-sponge
scrubbers are commercially available. Other H2S scrubbing
processes are less commonly used and are proprietary.
CC>2 removal processes can be divided into three broad categories;
absorption (both physical and chemical), adsorption, and
cryogenic processing. Many CC>2 removal processes also remove
H2S. The only process which has received much use in wastewater
treatment plants is absorption in water; this process has
been tested at Modesto, California, and Los Angeles County,
California. In 1976, total costs for a water scrubbing unit
of 1,000,000 cubic feet per day (28,300 m3/d) capacity were
estimated at $2.50 per million Btu ($2.37/GJ) of energy (9).
Some methane is also absorbed during the scrubbing process; costs
were based on energy leaving the scrubber as opposed to energy in
the untreated gas. Of this, $0.15 per million Btu ($0.14/GJ) was
attributed to the cost of iron-sponge H2S removal, which must
necessarily precede the water scrubber. It was estimated that
this unit would produce 2 MGD (87 1/s) of spent scrubbing water.
Costs for treating the spent scrubbing water were included in the
estimate. These units are commercially available.
Gas leaves the digestion system at approximately 95°F (35°C) and
is saturated with water vapor. During transport the gas is
cooled. Condensate formed must be removed to protect downstream
equipment. Water traps should be installed at low spots in the
gas pipe and at frequent intervals. If moisture must be reduced
substantially, adsorption drying or glycol dehydration can be
used.
18.3.2.2 Gas-Burning Equipment
Corrosion Factors
One of the major problems associated with recovering heat
from digester gas is corrosion caused by S02 and 803, the
combustion products of I^S. If the exhaust gas temperature is
allowed to drop below its dewpoint, the condensate which
forms is acidic as the result of absorbing S02 and 803. The
acidic condensate is corrosive to metallic elements of the
exhaust-carrying system. There are two alternatives to alleviate
the problem. The first is scrubbing of I^S from the gas
before combustion. The second is maintaining the exhaust gas at
temperatures considerably greater than its^ dewpoint, to prevent
condensation. This generally requires that the water temperature
of any boiler or engine using unscrubbed gas be at least 212°F
(100°C). Also, stack gas temperatures should not be allowed
to drop below 350° to 400°F (177° to 204°C). Use of unscrubbed
digester gas is preferred. Equipment fueled by unscrubbed
digester gas should not be used in intermittent service, since
condensation will occur each time the unit is shut down.
18-11
-------
Shutdowns should be minimized. Similarly, the equipment should
be designed so that even when operated at its lowest loadings,
exhaust gas temperatures are sufficiently high to prevent
condensation.
VENT
HEAT TRANSFER
TUBES
CONDENSER
FREE-LIQUID
SURFACE
RETURN
WATER
STEAM-
WATER
MIXTURE
BOILER
RETURN HOT
FROM WATER
HEAT TO
DEMAND HEAT
DEMAND
FIGURE 18-2
SCHEMATIC OF COMBINED BOILER/CONDENSER
SYSTEM FOR HOT WATER PRODUCTION
18-12
-------
Bo i 1 e r s
Scotch-type tube boilers and cast iron sect ional ized boilers
have both worked well with untreated digester gas as long as the
water or steam temperatures are maintained above 212°F (100°C).
Figure 18-2 illustrates an effective method for hot water
production using boilers. The heat source (the boiler) and heat
demands are not directly tied together, but separated by a
condenser. The condenser is mounted directly above the boiler.
The specific gravity of the steam/water mixture produced in the
boiler tubes is less than that of the water returning to the
boiler. The mixture is displaced upward into the condenser,
gives up its heat, then flows by gravity back to the boiler. A
natural circulation pattern is thus set up.
If heat supply exceeds heat demand, the excess heat is released
by venting steam from the condensers. Temperature control is
automatic, being set by the vent pressure. Advantages of this
system are simplicity, elimination of costs associated with
pumping, automatic temperature control, and independent
operation of the boiler from other heat sources and heat demands.
Independent operation is particularly important; it allows the
boiler to operate at its own best conditions, without being
affected by the operations of other components of the system.
Digester gas can be used to fuel reciprocating engines and gas
turbines. Prime movers convert part of the fuel's energy to
work, rejecting the remainder as waste heat. Thermal efficiency
can be dramatically improved if portions of the rejected heat can
be recovered and used for process or building heating. Waste
heat recovery is more efficient if prime movers are run hot,
since heat rejected at higher temperatures can be put to a
greater variety of uses than heat rejected at low temperature.
Also, exhaust systems last longer because 802-803 corrosion is
reduced .
Reciprocating Engines. Engines may be cooled using either a
forced circulation system in which water is pumped through the
engine, or a natural draft system. The equipment configuration
for natural circulation cooling is similar to that described
for boiler natural circulation systems except the engine
replaces the boiler in the flow diagram (see Figure 18-2). The
advantages of natural circulation cooling are the same as those
discussed for natural circulation boiling. Cooling system
pressures are limited to about 10 psig (69 kN/m2 ) ; if operated at
higher pressures cooling water could leak past the cylinder
liner seals and into the cylinder. The maximum cooling water
temperature is thus about 240°F (116°C), corresponding to the
temperature of saturated steam at 10 psig (69 kN/m2 ) . Engines
using natural circulation cooling are relatively small, typically
developing less than 1,500 horsepower (1,120 kW) . Flow rates
developed by natural circulation cooling may be insufficient to
18-13
-------
cool larger engines. Flow rates may be increased by installing
a booster pump in the circulating loop near the entrance to the
engine jacket. There are reciprocating engines on the market
designed to operate at temperatures in the 160°to 180°F (71° to
82°C) range. However, they are not recommended for services
with unscrubbed digester gas because of potential problems
with 302-303 corrosion. Heat recovered from the engine jacket is
typically used to sustain the digestion process and for space
heating .
Reciprocating engines commonly employed in wastewater treatment
plants fall into two categories; dual-fuel (compression ignited)
and spark ignited engines. Dual-fuel engines use a blend of
diesel fuel and digester gas; the fraction of diesel fuel can be
varied from a minimum of 4 percent all the way to 100 percent of
the mixture. Dual-fuel engines are typically used if there is
insufficient digester gas to satisfy power demands. Dual-fuel
engines have been specified for new plants where digester gas
production is expected to lag behind power demands for several
years.
Spark-ignited engines are generally used when there is sufficient
digester gas to satisfy power demands. Spark-ignited engines can
operate on several different types of fuel (for example, digester
gas and natural gas). Special carburetors are provided to blend
digester gas with an air-diluted backup fuel (for example,
natural gas) during infrequent periods when not enough digester
gas is available to satisfy power requirements. Spark-ignited
engines are less complex then dual-fuel engines, are available
in smaller sizes, and are less costly to operate since expensive
diesel fuel is not required.
Naturally aspirated feed systems are preferred to turbocharged
systems for spark-ignited engines. Turbocharged systems require
that gas be delivered at high pressure, which means the gas must
be first compressed, then delivered through a fuel metering
system with restricted openings. Gas impurities (oils,
greases, and water) are condensed when the gas is compressed and
cooled; these impurities often clog the fuel metering system.
Naturally aspirated systems operate at low pressures (<0.5 psig
[3.4 kN/m^j ) . with careful design of the gas transport systems,
compression of the feed gas is not required. Low pressure fuel
metering systems also have relatively large openings compared
to metering systems used with turbocharged units. For these
reasons, naturally-aspirated fuel systems are therefore less
susceptible to clogging than systems with turbocharged units.
Engines represent a large capital investment and should be
conservatively designed to protect that investment. For
four-stroke engines it is recommended that brake mean effective
pressure (BMEP) not exceed 80 to 85 psig (550 to 590 kN/m2) to
minimize strain on the equipment. Engine speeds in the 700 to
1,000 rpm are preferred as are average piston speeds in the range
of 1,200-1,500 feet per minute (370 to 460 m/min). Heavy-duty
industrial engines should be specified, not automotive engines.
18-14
-------
Gas Turbines. Gas turbines have had relatively limited use to
date. Where used, there have been fouling problems which are
inherent with compressing a dirty gas through fuel metering
systems with small clearances. However, new developments in
the turbine field and the fact that less NOX is produced by
turbines than by reciprocating engines has led to a second look
at turbines, particularly in nonattainment air quality areas. A
new system that uses a relatively low (4/1) pressure ratio
turbine with recuperation has the potential to solve many of the
problems which plagued earlier installations (10). The normally
low efficiency of the low pressure ratio turbine is boosted by
preheating the compressed air with heat recovered from the
exhaust gas. Ignition for this turbine can be staged to minimize
NOX generation. Emissions control is particularly important in
non-attainment areas where new stationary sources must use
Best Available Control Technology (BACT). BACT for reciprocating
engines is considered to be catalytic denitrification, while BACT
for low pressure ratio turbines can be staged ignition.
18.3.2.3 Generators
Generators may be synchronous or induction types. Synchronous
generators are by far the most common. However, in smaller sizes
(below 5 or 10 MW) induction units are generally less expensive
than synchronous units. They are also easier to maintain since
they require no governor or synchronizing equipment. Induction
generators have the disadvantage of being unable to operate
unless parallelled with synchronous generation, either utility or
in-plant. Thus an induction generator by itself cannot be used
to provide emergency power.
18.3.3 Examples of Energy Recovery
The following two examples demonstrate calculations for two of
the most commonly encountered energy recovery practices. Other
examples and case histories can be found in References 11 and 12.
18.3.3.1 Energy Recovery from Digester Gas
Gas from an anaerobic digestion system is to be utilized to help
supply plant energy needs in a 30 MGD (1.3 m^/s) activated
sludge plant. Digester gas will be used to fuel a spark-ignited
internal combustion engine equipped with natural circulation
cooling. The engine will drive an electrical generator. The
electricity generated will be us.ed to power various plant motor
drives. Heat recovered from the engine cooling jacket and from
the exhaust silencer will be used for space and process heating.
It is hoped that sufficient heat will be recovered to supply at
least digester heat requirements; any excess heat recovered will
be used for "other" process heating. It is anticipated that heat
recovered from the engine jacket (usually low temperature heat)
18-15
-------
will be used to make hot water for digester heating, while heat
recovered from the exhaust silencer (high temperature heat) will
be used to generate steam. Figure 18-3 is the system flowsheet.
CCL'J
FOR
H IG»* TEUPE RATUR F
ii-SSB* F
FIGURE 18-3
PROCESS SCHEMATIC FOR EXAMPLE OF ENERGY
RECOVERY FROM DIGESTER GAS
The following data is estimated for the sludges and digester gas:
• Digester feed = 50,000 pounds per day (22,700 kg/d), dry
weight basis. The feed solids are 75 percent volatile.
The sludge is 4 percent solids by weight.
• Fifty percent of the volatile solids (VS) are destroyed
during digestion.
• Raw sludge temperature is 60°F (16°C).
• Fifteen standard cubic feet (0.42 m^) of digester gas
are generated for every pound (0.454 kg) of VS destroyed.
• The gas composition is 66 percent CH4, 28.3 percent
CC>2, and 5.7 percent water (by volume). Other gases
(H2' H2^f N2) are present but not in sufficient
quantities to affect the heat balance.
18-16
-------
• 619 Btu (648 kJ) of heat are produced for every standard
cubic foot (28.3 liters) of digester gas combusted.
The plant has the following energy requirements, which could
be supplied in part or in whole by energy recovery from
digester gas:
• 1,000 kW of electricity.
• Energy for raw sludge and digester heating (to be
computed).
• 15 x 1Q6 Btu per day (15.8 x GJ/d) for miscellaneous
heating.
The following calculations are required:
• Determine the energy value of the digester gas.
• Determine if energy that can be recovered from the
combusted gas is sufficient to satisfy the energy
requirements listed above.
• Provide an energy flow diagram.
• Determine overall heat recovery efficiency.
To make comprehension of this example easier, the energy flow
diagram is presented first (see Figure 18-4). The calculation
is divided into four sections, as illustrated by the numbered
"boxes" on the diagram. The magnitudes of the energy streamd
shown on Figure 18-4 are developed in the following calculations:
Determine the Energy Value of the Digester Gas (Bpx_lj_
1. Digester gas flow rate
50,000 Ib solids\ /0.75 Ib VS\ /0.5 Ib VS destroyed
day / \ lb solids/ \ lb vs fed
15 scf \
ib vsdestroyed
= 281'250 scfd (8'157
2. Energy value of the gas
= (281,250 scfd) (619 Btu/scf)
= 174 x 106 Btu per day (183.5 GJ/d)
Strictly speaking, the energy value of the digester gas
should include not only the heat of combustion but the
heat contents (enthalpy) of the reactants (air, fuel gas)
18-17
-------
calculated with respect to a selected base temperature.
However, the heat contents of the reactants are very small
compared to the heat of combustion and may be neglected with
very little loss of accuracy and with a substantial reduction
in amount of calculations necessary.
ENE1GY VALUE
>i'SU'H
in. a
FIGURE 18-4
ENERGY FLOWSHEET FOR EXAMPLE OF
ENERGY RECOVERY FROM DIGESTER GAS
Make a Heaj_B a^£1.9_e__A£o_u.£id__th e_ E_n g j. n e / G e n e r a tor (Box 2 )
1. Assume 28 percent of the energy value of the fuel gas is
converted to work.
Work produced
= 0.28 (174 x 106 Btu/day)
= 48.7 x 106 Btu per day (51.3 GJ/dJ
Assume 90 percent of the work produced can be converted to
electricity .
18-18
-------
Electricity
= 0.90 (48.7 x 106 Btu/day) = 43.9 x 10^ Btu/day (46.2 G J/d )
This is equivalent to 535 kW. Since average plant electrical
demand is 1,000 kW, auxiliary power must be purchased.
2. Assume 33 percent of the energy value of the fuel gas is
recovered in the engine jacket water.
Energy recovered in the jacket water
= 0.33 (174 x 106 Btu/day)
= 57.4 x 106 Btu per day (60.5 GJ/d )
3. Assume the radiant heat loss from the engine is 4 percent of
the energy value of the fuel gas.
Radiation loss
= 0.04 (174 x 106 Btu/day) = 7.0 x 106 Btu per day (7.4 GJ/d)
4. Assume 5 percent of the energy value of the fuel gas is
transferred to lubricating oil.
Heat loss to oil
= 0.05 (174 x 106 Btu/day) = 8.7 x 10^ Btu per day (9.2 GJ/d)
5. Heat in the exhaust gas is the difference between the energy
value of the fuel gas and the heat losses determined in items
1 through 5.
Heat in the exhaust gas
= (174.0 - 48.7 - 57.4 - 7.0 - 8.7) x 10^
= 52.2 x 106 Btu per day (55.0 GJ/d)
Determine Whether Sufficient Heat can be Recovered From the
Jacket Cooling Wa t e r __tg_S at. i jjE y _ DjLg e s t er _ H eating Requirements
( Box 3 ) — — ---
1. Energy required to heat raw sludge
50,000 Ib solids/day W 1.0 Btu
O.04 Ib solids/lb sludge/\lb sludge/0?
= 42.0 x 106 Btu per day (44.3 GJ/d)
2. Determine energy required for circulating sludge heating.
The purpose of the circulating sludge heater is to make
up for any heat lost through the digester structure. Heat
18-19
-------
loss calculations similar to these shown in Chapter 6,
Section 6.2.6.2, indicates that for the digester of this
example, losses are on the order of 5.0 x 10^ Btu per day
(5.3 GJ/d).
3. Determine heat loss in the hot water circulating loop. There
is very little heat loss because this is a closed system
(see Figure 18-3). The only losses will be through the
insulation. It is roughly assumed that heat loss is 5 percent
of the heat leaving the engine jacket.
Heat loss
= 0.05 (57.4 x 106)= 2.9 x 106 Btu per day (3.0 GJ/d)
4. Total heat required for the digestion system
= (42.0 + 5.0 + 2.9) x 106 = 49.9 x 106 Btu/day (52.6 GJ/d)
5. Heat available in the cooling water minus total heat required
for the digestion system
= (57.4 - 49.9) x 106 = 7.5 x 106 Btu/day (7.9 GJ/d)
To keep the internal combution engine adequately cooled,
this heat must be rejected in some manner. The heat may be
rejected by renting steam from the condenser. In this case,
however, the designer has chosen to use the extra heat for
building heat, thereby utilizing rather,than wasting it.
Determine if Sufficient Heat can be Recovered from the Hot
C~ombu"ition~Gases Leaving the Engine to Satisfy "Other"~Trocess
Requirements (Box 4)
From previous calculations, the heat available in the hot
combustion gas is 52.2 x 106 Btu per day (55.0 GJ/d). Not
all of this heat can be recovered for use. Practical limits
exist to the degree to which the hot gas can be cooled. For
example, the hot gases must be substantially warmer than the
material being heated to carry out heat transfer in an exchanger
of reasonable size and cost. In this example, however, the lower
temperature limit is set at 350°F to preclude corrosion that
might occur by condensation of water vapor on the inside of the
exhaust stack walls. The designer must therefore determine if
sufficient heat can be obtained to satisfy "other" process uses
when the hot combustion gases are cooled to 350°F (117°C) in the
exhaust silencer. Since the heat content of the hot combustion
gases is known (52.2 x 106 Btu per day [55.0 GJ/d]), heat
available can readily be calculated once the heat content of
the gas at 350°F (117°C) has been determined. This is calculated
as follows:
1. First calculate the volume of exhaust gas. Gas production
can be predicted from stoichiometry:
18-20
-------
CH4 + 202 - »*C02 + 2H20 (18-1)
a. C02 present = CC>2 in digester gas plus C02 formed
by combustion of methane.
1. From previous calculations, digester gas production
is 281,250 standard cubic feet per day (8,157 m3/d ) .
2. Unburned digester gas contains 28.3 percent CC>2 by
volume .
CC>2 associated with digester gas
= 0.283 (281,250 scfd) = 79,593 scfd (2,252 m3/d )
3. From Equation 18-1, one cubic foot of CC>2 is formed
for every cubic foot of methane burned. Digester gas
contains 66 percent methane by volume.
CC>2 formed by combustion of methane
= 0.66 (281,250 scfd) = 185,625 scfd (5,253 m3/d )
4. Total C02 volume
= 79,593 -f 185,625 = 262,218 scfd (7,505 m3/d )
b. CH4 present: none, all converted to CC>2 .
c. 02 present: assume that air supplied exceeds theoretical
requirements by 10 percent. Oxygen associated with this
excess is not consumed. From Equation 18-1, theoretical
oxygen requirements are two cubic feet of oxygen for
every cubic foot of methane burned.
Oxygen in excess of theoretical requirements
/ 0.66 ft3 CH4 \
= (2) (0.10) — — — __-____±__~ ) (281,520 scfd)
\ ft-^ digester gas /
= 37,125 scfd (1,050 m3/d )
d. N2 present: N2 associated with the air passes through
the system unchanged in quantity.
18-21
-------
N2 flow
/ 0.66 ft3 CH4 \
= 281,250 scfd - = - — 1
yft-15 digester gas/
([1.10 x 2] ft3 02 delivered \ /Q.79 ft3 N2 \
ft3 CH4 ) \0.21 ft3 02 /
= 1,536,265 scfd (43,476 m3/d)
e. H20 present = H2O in digester gas plus that created by
combustion of methane.
1. Digester gas contains 5.7 percent ^0 by volume.
H2O in digester gas
= 0.057 (281,250 scfd) = 16,031 scfd (453 m3/d)
2. From Equation 18-1, two cubic feet of H20 are formed
for every cubic foot of methane burned.
H20 formed
/ 0.66 ft3 CH4 \ I 2 ft3 H20\
= 281,250 scfd - 5 - -- ^— |{ - =5 - —I
\ft3 digester gas/ \ ft3 CH4 /
= 371,250 scfd (10,506 m3/d )
3. Total water = 16,031 + 371,250 = 387,281 scfd
(10,960 m3/d)
f. Total gas flow = 262,218 + 37,125 + 1,536,265 + 387,281
= 2,222,889 scfd (62,907 m3/d )
2. Next calculate the heat content of the exhaust gas at 350°F
(117°C). The heat content of the exhaust gas is the sum of
the heat contents of its individual components. The heat
content of any component at 350°F is the sum of the sensible
and latent heats required to raise the component from an
arbitrarily selected base temperature to 350°F (177°C). Mean
heat capacity data for several gases is shown on Figure 18-5.
The base temperature for Figure 18-5 is 77°F (25°C). The
mean heat capacity of a gas over the range 77°F to 350°F is
the value found at 350°F.
a. Heat content of CC>2
-77°P, ,262,218 scfd,
= 1.9 x 106 Btu per day (2.0 G J/d )
18-22
-------
I 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400 4800 5200
TEMPERATURE.0*1
!°F =1,B°e 4321
FIGURE 18-5
MEAN MOLAL HEAT CAPACITIES OF GASES AT CONSTANT
PRESSURE (13) (MEAN VALUES FROM 77° to T°F)
b. Heat content of 02
7.2 Btu /Ib mole\
Ib mole/°F \^359 scfy
= 0.2 x 106 Btu per day (0.2 GJ/day)
\ ,-^ i oc ~~f*\
) (37,125 scfd)
Heat content of N2
6.8 Btu /Ib mole^
Ib mole/°F \359 scf
= 7.9 x 106 Btu/day (8.4 GJ/d )
(350°-77°F) (1,536,265 scfd)
Heat content of water. In this calculation, water is
pictured as heated in a liquid state to the dew point,
evaporated, and heated as a vapor to final temperature.
Other approaches can also be used; these are described in
thermochemistry textbooks.
18-23
-------
387,281 scfd H20 \ , nn
Water comprises 2f222/889 scfd totalj 10°
= 17.4 percent by volume of the exhaust gas. The
dew point for gas containing 17.4 percent water by
volume is 135°F (58°C).
2. Heat to raise liquid water to the dew point
387,281 scfd \ / 18 Btu
scf/lb mole/lib mole/°F
/ \ I
= 1.1 x 106 Btu per day (1.2 GJ/d)
3. Heat to vaporize water at the dew point
387,281 scfd \/18,720 Btu\
359 scf/lb mole M Ib mole )
= 20.2 x 106 Btu per day (1.19 GJ/d)
4. Heat to raise water vapor from the dew point
to 350°F
_/ 8.2 Btu \( 387,281 scfd \ . ?[-no -. ,, ,.„ .
\lb mole/°F/\359 scf/lb mole/1 '
= 1.9 x 106 Btu per day (2.0 GJ/d)
5. Total heat content of water = (1.1 + 20.2 + 1.9)
x 106 = 23.2 x 106 Btu per day (24.5 GJ/d).
e. Heat content of exhaust gas at 350°F (117°C)
= (1.9 + 0.2 + 7.9 + 23.2) x 106 = 33.2 x 106 Btu per day
(35.0 GJ/d)
3. Energy available to satisfy "other" requirements
= (52.2 - 33.2) x 106 = 19.0 x 106 Btu per day (20.0 GJ/d).
4. Determine heat loss in steam/condensate circulating loop.
There will be very little heat loss because this is a
closed system (see Figure 18-3). Assume losses are roughly
5 percent of the heat transferred from the exhaust silencer.
Heat loss
= 0.05 (19.0 x 106 Btu/day) = 0.9 x 106 Btu per day
(1.0 GJ/d)
18-24
-------
5. Heat available for "other" process demands
= (19.0 - 0.9) x 106 = 18.1 x 106 Btu per day (19.1 GJ/d)
The available heat is sufficient to satisfy the demands.
Deter mi n e EJ:_fi_c_i e n c y_ g f _t h e En er; g y_R e covery System
There are several methods for evaluating the efficiency of the
energy recovery system. One approach is to compute the useful
heat and work recovered as a percentage of the energy input.
1. Useful heat and work:
a. Electrical energy = 43.9 x 106 Btu per day (46.2 GJ/d).
b. Raw sludge heating = 42.0 x 106 Btu per day (44.2 GJ/d).
c. Circulating sludge heating = 5.0 x 10^ Btu per day
(5.3 GJ/d).
d. "Other" process heating = 15.0 x 10^ Btu per day
(15.8 GJ/d).
e. Space heating = 7.5 x 106 Btu per day (7.9 GJ/d).
2. Energy input from digester gas = 174 x 10*> Btu/day
(183.4 GJ/d).
-, „ . , ff. . /43.9 + 42.0 + 5.0 + 15.0 + 7.5\ inn
3. Computed efficiency = I 1 100
\ 174.0 /
= 65 percent
This activated sludge plant is not able to supply all its energy
needs using digester gas (insufficient electrical energy).
Generally, digester gas is sufficient to satisfy the energy
requirements of most primary treatment plants but not activated
sludge plants, since aeration blowers generally have high
electrical demands.
18.3.3.2 Recovery of Energy from Incinerator Flue Gas
A wastewater treatment plant of 125 MGD (5.48 m^/s) capacity uses
incineration to process 190,000 pounds per day (82,260 kg/d) of
combined primary and waste-activated sludges. Heat is recovered
from the flue gases as electricity and steam in a steam turbine
power cycle, using a waste heat boiler. The designer's objective
is to maximize work production (electricity and direct power).
18-25
-------
Steam is not used for space or process heating. A flow sheet of
the process is shown on Figure 18-6. The following additional
information is provided:
• The flue gas heat content is 606 x 106 Btu per day
(639 GJ/d), based on an assumed gas composition and
gas temperature, using methods described in the example
of Section 18.3.3.1. Similarly, the heat content of the
stack gas is 250 x 106 Btu per day (263 GJ/d). Heat
losses from the boiler structure are 18 x 10^ Btu per
day (19 GJ/d).
• The boiler produces superheated steam at 615 psia
(4,261 kN/m2) and 825°F (441°C), which is then fed to
a steam turbine, called the "main turbine."
• Steam is withdrawn from the turbine at three points.
First, 50,000 pounds per day (22,700 kg/d) are withdrawn
at 165 psia (1,143 kN/m2) and applied to drives for
pumps and compressors. This is called "process" steam.
Second, a quantity (to be computed) is withdrawn and used
for preheating of the boiler feedwater. This is called
"preheat" steam. The remaining steam, which is "primary"
steam, is exhausted at 1 psia (6.9 kN/m2). The
efficiency of the turbine (actual to theoretical work
output) is assumed to be 76 percent.
• Exhausted "process" steam from the pump and compressor
drives is condensed at 1 psia (6.9 kN/m2), combined
with the "primary" condensate, and sent to the feedwater
heater. "Primary" and "process" condensates are
assumed to be saturated water at the exhaust pressure
(1 psia [6.9 kN/m2]).
• "Preheat" steam is mixed with "primary" and "process"
condensates in the feedwater heater to produce a
saturated feedwater at 300°F (149°C).
• The feedwater is pressurized to 615 psia (4,261 kN/m2),
and returned to the boiler.
The following information is desired:
• Steam and condensate flow rates.
• Electric power generated.
• Pump and compressor work produced by the "process" steam.
• Energy recovery efficiency.
18-26
-------
"FEFD" STIAM 825' f. 615 psis
"PROCESS" STEAM 165
PIRFCT
POWE R
TURBINE
DRIVTIS;
FOR PUMPS
AND
COMPRESSORS
I pit - 6.93 hN/m
PUMP
FEED WATER
HEATER
FIGURE 18-6
FLOWSHEET FOR EXAMPLE OF ENERGY RECOVERY
FROM INCINERATOR FLUE GAS
Analyze the Operation jOf__the Main Turbine
Turbine operations can be analyzed using a Mollier diagram. A
Mollier diagram is a plot of enthalpy versus entropy for specific
two-phase systems which display lines for constant pressure,
temperature, percent moisture, and superheat, among others.
Figure 18-7 is a Mollier diagram for the steam-water system.
Note that the terms "enthalpy" and "heat content" are equivalent
and will be used interchangeably in the following discussion.
The "state line" concept is used for turbine analysis. The state
line describes the steam condition at every point within the
turbine. The line can be drawn once any two points describing
steam conditions in the turbine are established. For this
example, the turbine feed steam and the "primary" steam exhaust
conditions will be determined, then plotted on the Mollier
diagram of Figure 18-7.
1. The turbine feed steam condition (615 psia [4,261 kN/m2]),
825°F [441°C]) is plotted as point A on the Mollier Diagram
(see Figure 18-7). Figure 18-7 is not detailed, so that
data points and state lines can be clearly seen. More
detailed diagrams are available (14,15).
18-27
-------
3650
1600
585° F,
165 PSIS,
1320 Btu/lb
448°F,
87 psia,
1258 Btu/lb
1 piia,
1036 Btu/lb
B. ISENTROPICALLY EXPANDED
"PRIMARY" STEAM
1.1 1.2 1.3 1.4 1.5 1.6 1.7 l.B 1.4 2.0 Z,l 2.2 2.3
ENTROPY, Btu/lb/*F (1 Btu/Jb/°F - 4.18 kJ/kg^C)
MOLLIER CHART COURTESY OF BABCOCK AND WILCOX
FIGURE 18-7
STEAM CONDITIONS FOR EXAMPLE OF RECOVERY OF ENERGY
FROM INCINERATOR OF FLUE GAS
18-28
-------
Determine the "primary" steam exhaust condition. If the
turbine were 100 percent efficient, the steam would expand
isentropically, that is, the entropy of the steam at any
point in the turbine would be identical to the entropy
of the feed steam and the state line would be vertical
(dashed line in the Mollier Diagram). The "primary" exhaust
steam condition would be located at the intersection
of the vertical state line and the exhaust pressure
(1 psia [6.9 kN/m2]), at point B. Enthalpy of the steam at
point B is 915 Btu per pound (2.13 MJ/kg).
However, turbines are not 100 percent efficient since
isentropic expansion is never attained. The energy which can
be extracted from the steam in practical applications is only
a percentage of that which can be extracted by isentropic
expansion. This is expressed by Equation 18-2.
/ Hi - H2p \
\ HI - H2i ;
Turbine efficiency =( u^ _ „**; )100 (18-2)
Where:
HI = enthalpy of inlet steam, Btu/lb.
H2p = enthalpy of steam exhausted from a practical
turbine, Btu/lb.
H2i = enthalpy of steam exhausted from an ideal
turbine, Btu/lb.
The efficiency described by Equation 18-2 is the actual
work output relative to theoretical output—it is less than
100 percent because of irreversibility in the expansion of
gases in the turbine. Mechanical losses in the turbine and
generator are not included.
For the practical turbine, enthalpy of the exhausted
steam (H2p) can be computed from Equation 18-2. For the
turbine of the example (76 percent efficient).
H2p = U,420 - y^J (1,420 - 915) (18-2)
= 1,036 Btu per pound (2,405 kJ/kg)
The "primary" exhaust steam condition for the practical
turbine is located at point C, the intersection of the
exhaust pressure (1 psia [6.9 kN/m2]) and enthalpy value
1,036 Btu per pound (2,405 kJ/kg) . The state line for the
practical turbine is then drawn between points A and C.
18-29
-------
3. The "process" steam condition must lie on the state line. It
is located at the intersection of the state line and the
"process" steam operating pressure (165 psia [1,145 kN/m2] ),
at point D.
4. As with the "process" steam, the "preheat" steam condition
can be determined once its pressure is known. Pressure can
be determined by the following reasoning:
a. "Preheat" steam pressure is essentially equal to the
pressure in the feedwater heater (pressure drop through
the lines connecting the turbine and feedwater heater is
assumed negligible).
b. The feedwater heater is a direct contact device.
Sufficient "preheat" steam is mixed with "primary" and
"process" condensates to form a two-phase system at
300°F (149°C). Thus the feedwater heater system is a
saturated system.
c. The feedwater heater pressure, therefore is the pressure
of saturated steam at 300°F (149°C), which is 67 psia
(464 kN/m2).
The "preheat" steam condition is located at the intersection of
the state line and the 67 psia (464 kN/m2) constant pressure
line (point E). Enthalpy of the "preheat" steam is 1,258 Btu per
pound (2,921 kJ/kg).
Determine St e am and C QJ}djsn sat e_F1 ows
1. Circulating steam rate is computed by a heat balance around
the boiler.
a. Enthalpy of the water entering the boiler is assumed
equal to that leaving the feedwater heater; that is,
pumping affects the enthalpy value negligibly. This
is a justifiable assumption for the pumping of liquids.
From steam tables (14,15), the enthalpy of saturated
water at 300°F (149°C) is 270 Btu per pound (627 kJ/kg).
b. By previous calculations, enthalpy of superheated steam
leaving the boiler is 1,420 Btu per pound (3,297 kJ/kg).
c. From the problem statement, heat absorbed in the boiler
= 338 x 106 Btu per day (356 GJ/d).
d. Therefore steam circulating rate
338 x 106 Btu/day
(1,420 - 270) Btu/lb
= 293,900 pounds per day (133,400 kg/d).
18-30
-------
2. "Process," "primary," and "preheat" steam rates are
determined by mass and heat balances around the feedwater
heater. Let X and Y be the flow rates for "primary" and
"preheat" steam, respectively. Equation 18-3 is the mass
balance around the feedwater heater.
293,900 = X + Y + 50,000 (18-3)
Equation 18-4 is the heat balance for the feedwater heater.
293,900 (270) = 70 X + 1258 Y + 70 (50,000) (18-4)
Enthalpies of the "process" and '"primary" condensates
(70 Btu per pound or 162 kJ/kg) are for saturated water
at 1 psia (6.93 kN/m2). Solving Equations 18-3 and 18-4
simultaneously, "primary" and "preheat" steam rates are
194,626 pounds per day (88,350 kg/d) and 49,274 pounds
per day (22,370 kg/d), respectively.
At this point, construction of an energy flowsheet should
be initiated (see Figure 18-8). This allows the designer
to see all pertinent data on one sheet and gives a feeling
for the magnitude of the various energy flows.
Determine Electrical Energy Generated
Work produced is the sum of the total enthalpy changes across the
turbogenerator: :
1. Work from "process" steam
= 50,000 Ib/day (1,420,- 1,320 Btu/lb)
= 4.90 x 106 Btu per day (5.16 GJ/d)
2. Work from "preheat" steam
= 49,274 Ib/day (1,420 - 1,258 Btu/lb)
= 7.98 x 106 Btu per day (8.41 GJ/d) b
3. Work from "primary" steam
= 194,620 Ib/day (1,420 - 1,036 Btu/lb)
= 74.73 x 106 Btu per day (78.77 GJ/d)
4. Total work produced
= (4.90 + 7.98 + 74.73) x 10^
= 87 x 106 Btu per day (92.3 GJ/d)
5. Assume mechanical efficiency of the turbine/generator
combination is 95 percent. • •
18-31
-------
Net electricity produced
= 83.2 x 106 Btu per day (87.7 GJ/d)
This is equivalent to 1,015 kW of electricity
tkttl?
8i*.*b
-tiP St..*!
MAiM 7UHBIKE
TlUl BfflSXlDbafuNi^
5
STfsarrusAi 1
WEA- ^ V
r^
•'-j
^:;
F«k*s.tS^p-
'FHIMiM^Y-
S;TE*M
IW^iS !W*«
uvk t-iM-f.
SOv^i^ie1 huH^
?nocsss
TUPHfNl
DftlVEf^'
/"
FIGURE 18-8
ENERGY FLOWSHEET FOR EXAMPLE OF ENERGY
RECOVERY FROM INCINERATOR FLUE GAS
De t e rm i n e
^ t e am Cycle
Enthalpy of the "process" steam is 1,320 Btu per pound (3,065 kJ/
kg). Enthalpy of the exhausted steam can be determined using
the same technique employed for analysis of the main turbine.
Isentropic expansion of process steam (initially at point D,
Figure 18-5) to 1 psia (6.9 kN/m2 ) produces an exhaust gas
of enthalpy 950 Btu per pound (2,206 kJ/kg ) . Assume process
turbines are 50 percent efficient.
1. Enthalpy of exhausted steam
= 1,320 - yj£ (1,320 - 950)
= 1,135 Btu per pound (2,635 kJ/kg )
18-32
-------
2. Work produced
= (50,000 Ib/day) (1,320 - 1,135 Btu/lb)
= 9.2 x 106 Btu per day (9.7 GJ/d)
3. Assuming mechanical losses of 5 percent, work delivered
= (9.2 x 106 Btu per day) (0.95) = 8.8 x 106 Btu per day
(9.3 GJ/d)
This is equivalent to 107 kW.
Determine Energy Recovery Efficiency
Assume heat removed in the condensers is not used beneficially,
but discharged to the atmosphere via cooling towers.
1. Energy recovery, .based on heat transferred to steam
106 + 8.8 x 106
= A 8 3. 2 x
100 = 27.2 percent
338 x 106
2. Energy recovery, based on heat in the incinerator flue gas
.00 = 15.2 percent
= /(83.2 x 1Q6 + 8.8 x 106)\
\ 606 x 106 /
Compare the recovery of this example (15 percent) against the
recovery of energy from digester gas (65 percent), as illustrated
by the example in Section 18.3.3.1. Greater efficiency was
obtained by the internal combustion system because:
1. No heat was lost prior to the work producing step. In
contrast, fully 41 percent of the heat available in the
incinerator flue gas was rejected in the waste heat recovery
boiler before any useful work could :be extracted (see
Figure 18-8).
2. With the internal combustion system, waste heat from the
work producing step was used benefically (for digester
and space heating). In contrast, waste heat from the steam
condensers was not used benefically but rejected to the
environment. It is difficult to use this heat since it is
available at only a very low temperature (102°F [39°C]).
These two examples demonstrate the general rule that energy
recovery schemes whose sole effect is the production of work are
not likely to be efficient.
18-33
-------
It should not be inferred from the examples that energy recovery
from flue gases must necessarily be inefficient. In this
example, the objective of the designer in recoverying heat from
incinerator flue gas was to maximize work. Had he chosen to
exhaust steam from either of the turbines at higher pressures and
used it for heating purposes or had he used "process" steam
solely for heating, some work would have been sacrificed but
thermal efficiency could have been substantially improved. The
point to be made here is that the designer should examine a wide
range of options when analyzing energy recovery operations.
18.3.4 Other Factors Affecting Heat Recovery
The previous calculations point out some of the factors a
designer must consider in conducting a heat recovery analysis.
They are by no means the only factors; much more detail must be
added. For example:
• The full range of conditions expected at the plant
must be evaluated, not just average conditions. Energy
supply and energy demand schedules must be established.
Heat recovery equipment must be sized to handle peak
demands. Storage requirements for primary and backup
fuels must be determined.
• A source of backup energy must be available in the event
that plant energy recovery systems experience partial or
total failure.
• The physical and chemical nature of flue gases
generated must be considered (for example, temperature,
corrosiveness, particulate concentration, and moisture
content) .
• The equipment must be designed to withstand the
conditions to which it will be subjected. Appropriate
materials of construction must be used.
• Any solid, liquid or gaseous residual from the heat
recovery operation must be collected and disposed of in a
safe and environmentally sound manner.
• Chemical and physical treatments for makeup and
circulating water or steam must be established.
\
• Manpower to operate the heat recovery system must be
determined. Specialists may be required for certain
equipment, for example, stationary engineers for high
pressure boilers and engine specialists for internal
combustion engines.
• Control strategies must be decided upon, and instrumenta-
tion to carry them out must be provided.
18-34
-------
Economic analyses must be performed to determine
if the system can be economically justified. As
a ru 1 e-o f -1 h umb, the larger the plant, the more
sophisticated the heat recovery system which can be
justif ied.
18.4 Other Uses of Wastewater Solids and Solid By-Products
Wastewater solids may sometimes be used beneficially in ways
other than as a soil amendment or as a source of recoverable
energy. Lime and activated carbon have been recovered from
sludges for many years at plant scale. These applications are
discussed in Chapter 11. Stabilized sludge, when mixed with
soil, is used as interim or final cover over completed areas of
refuse landfills (see Chapter 19). Wastewater scum has been
collected (sometimes purchased) by renderers at several treatment
plants for use as a raw material in the manufacturing of cosmet-
ics and other products. Grit, particularly incinerated grit, may
be used as an aggregate, for example, as a road sub-base.
Other beneficial uses of wastewater solids have been considered;
some have been tested on a laboratory or plant scale. These
include:
• Recovery of ammonia from the filtrate or centrate
following anaerobic digestion and dewatering of sludge.
Ammonia is stripped from the liquor, absorbed in sulfuric
acid and crystallized as ammonium sulfate.
• Recovery of ammonia and phosphates by precipitation of
MgNH4P04 from digester supernatants. The precipitate
is used as a fertilizer.
• Addition of sludge to processes designed to compost
or anaerobically digest municipal refuse. In such
situations, sludge serves principally as a nutrient
source.
• Recycling of wastewater solids for use as a foodstuff
for livestock (cattle, sheep, goats, poultry, and fish).
Note that solids used for this purpose have generally not
originated from municipal wastewater treatment plants,
but from systems treating purely industrial or animal
wastes. However, the use of dried municipal sludge
disinfected by gamma irradiation is being investigated as
a food source for grazing animals.
• Use of wastewater solids as an organic substrate in worm
farming (see Chapter 13).
• Use of sludge as a raw material for the production of
powdered activated carbon (see JPL/ACTS process/
Chapter 11).
18-35
-------
18.5 References
1. USEPA. Current and Potential Utilization of Nutrients in
Municipal Wastewater and Sludge, Volume 2. Office of
Water Program Operations. Washington, D.C. 20640.
Contract 68-01-4820. July 21, 1978.
2. Walker, J.M. "Overview: Costs, Benefits and Problems of
Utilization of Sludges," pp. 167-174. 18th National
Conference and Exhibition on Municipal Sludge Management.
Miami Beach, Florida. Information Transfer, Inc.
Rockville, Md. 1979.
3. USEPA. "Principals and Design Criteria for Sewage Sludge
Application on Land." Sludge Treatment and Disposal,
Part 2. Technology Transfer, Cincinnati, Ohio 45268.'
EPA-625/4-78-012. October 1978.
4. Chicago Sun-Times. "U.S. Bares Solar Energy Program to
Year 2020." p. 29. August 14, 1975.
5. Ward, P.S. "Digester Gas Helps Most Energy Needs."
Journal Water Pollution_Control Fed. Vol. 46, p. 620.
1974.
6. Brown and Caldwell. Solid Waste Resource Recovery
Study. Prepared for the Central Contra Costa Sanitary
District, Walnut Creek, California. August 1974.
7. California Public Utilities Commission. Staff Report on
California Cogeneration Activities. Utilities Division.
San Francisco, California. January 17, 1978.
8. U.S. Department of Energy. Proposed Regulations Providing
for Qualification of Small Power Production and Cogen-
eration Facilities Under Section 201 of the Public
Utility Regulatory Policies Act of 1978. Federal Energy
Regulatory Commission, Washington, D.C., Rm 79-54. June
1979.
9. Sacramento Area Consultants. Sacramento Regional Waste-
water Program - Study of Methane Uses. Sacramento
Regional County Sanitation District. Sacramento,
California. June 1976.
10. Alpha National Inc. Solid Waste and Biomass Low Btu Gas
Conversion System Program. 1301 East El Segundo Blvd.,
El Segundo, California. April 1978.
11. National Bureau of Standards. Waste Heat Management
Guidebook. NBS Handbook 121. Washington, D.C. U.S.
Government Printing Office. 1976.
18-36
-------
12. USEPA. Energy Conservation in Municipal Wastewater
Treatment. USEPA Office of Water Program Operations.
Washington, D.C. 20640. EPPA 430/9-77-001/. March
1978.
13. Hougen, O.A., Watson, K.M., and R.A. Ragatz. Chemical
Process Principles. 2nd Ed. New York. John Wiley and
Sons. 1956.
14. Keenan, J.H. and F.G. Keyes. Thermodynamic Properties of
Steam. 4th Ed. New York. John Wiley and Sons. 1936.
15. Combustion Engineering, Inc. Steam Tables. Available
from the Public Relations and Advertising Department,
Windsor, Connecticut. 1967.
18-37
-------
EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Chapter 19. Disposal to Land
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
CHAPTER 19
DISPOSAL TO LAND
19.1 Introduction
Wastewater sludge may not always be used as a resource because
of land acquisition constraints or because they contain
high levels of metals and other toxic substances. In these
situations, the sludge must be further processed by other
methods. Non-utilization disposal processes are the subject of
this chapter.
As discussed in Chapter 2, ocean disposal is no longer considered
appropriate. Consequently, land disposal processes are being
optimized so that the increasing amounts of municipal wastewater
sludge produced by the adopted secondary treatment standards can
be accepted. Two principal land disposal methods, landfilling
and dedicated land disposal, differ in application rates and
methods of application. Typical landfill operations involve
dewatered-sludge subsurface application rates, often several feet
in depth. Dedicated land disposal operations, however, typically
involve repetitive liquid sludge applications, which may only
raise the land surface a few inches per year.
19.1.1 Regulatory Agency Guidance
Development of formalized methods for sludge disposal to land is
recent. Major efforts in this area have been encouraged and
funded by the USEPA since 1974. The reader is referred to
Chapter 2 for a discussion of some of the guidance and regulatory
documents which deal with the recent federal laws to control the
disposal of wastewater solids. State and local guidance has also
been provided. Extensive sludge research is being funded by
USEPA and various states.
19.2 Sludge Landfill
19.2.1 Definition
Sludge landfill can be defined as the planned burial of
wastewater solids, including processed sludge, screenings, grit,
and ash at a designated site. The solids are placed into a
prepared site or excavated trench and covered with a layer of
soil. The soil cover must be deeper than the depth of the plow
19-1
-------
zone (about 8 to 10 inches [20.3 to 25.4 cm]). For the most
part, landfilling of screenings, grit, and ash is accomplished
with methods similar to those used for sludge landfilling.
19.2.2 Sludge Landfill Methods
Sludge landfill methods can be grouped into three general
categories: sludge-only trench fill, sludge-only area fill, and
co-disposal with refuse. General site and design criteria are
discussed under these categories. A detailed discussion of
sludge landfills is presented in the USEPA Technology Transfer
Process Design Manual, Municipal Sludge Landfills (1). The
remaining parts of the landfill portion of this chapter summarize
the information presented in this design manual. Other
information on the disposal of wastewater sludge in sanitary
landfills is available (2).
19.2.2.1 Sludge-Only Trench Fill
The sludge-only trench method involves excavating trenches so
that dewatered sludge may be entirely buried below the original
ground surface. In some locations, liquid stabilized and
unstabilized sludges (Blue Plains, Washington, D.C. and Colorado
Springs, Colorado) have been buried by the trench fill method.
In this method, the sludge is deposited directly into the trench
from a haul vehicle. Normal operating procedure requires daily
coverage. Trench disposal is appropriate for unstabilized sludge,
because the immediate application of cover material reduces
associated odors. Vector control requires daily cover, except
during very cold weather.
Narrow Trenches
Trenches are defined as narrow when their widths are less than
10 feet (3 m). Disposal in narrow trenches is applicable to
sludges with a relatively low solids content of from 3 to
28 percent. The application rates range from 1,200 to
5,600 cubic yard of sludge per acre (2,270 to 10,580 m3/ha).
Excavated material can be either used immediately to cover
adjacent sludge - filled trench or stockpiled alongside and used
to cover the trench from which it was removed. The surface soil
cover thickness is about 4 feet (1.3 m). Excavation and covering
equipment operates from surface areas adjacent to the trench.
Wide Trenches
Trenches are defined as wi'de when they have widths greater
than 10 feet (3 m). Material excavated from the trenches is
stockpiled neatly and used as cover for the trench. Disposal in
wide trenches is suitable for sludges with solids contents of
20 percent or greater. The application rates range from 3,200 to
14,500 cubic yards of sludge per acre (6,050 to 27,400 m3/ha).
19-2
-------
The surface cover thickness depends on the solids concentration
of the sludge. The covered sludge will only be capable of
supporting equipment when the solids concentration of the sludge
exceeds 25 to 30 percent and the sludge has been tooped with 3 to
5 feet (1 to 2 m) of soil.
The wide trench method has two distinct advantages; it is less
land-intensive than the narrow trench method and groundwater
protection can be provided by liners. The use of liners permits
deeper excavations. The primary disadvantage of the wide trench
method is the need for sludge solid contents of greater than
20 percent. Sludge with solid contents of greater than 30 to
35 percent will not flow, and extra effort is therefore required
to spread them evenly in the trench. Figure 19-1 provides two
views of a wide trenching operation at the North Shore Sanitary
District just north of Chicago, Illinois.
19.2.2.2 Sludge-Only Area Fill
In the sludge-only area fill method, the sludge is mixed with
soil and the mixture is placed on the original ground surface.
This method requires substantial amounts of imported soil but may
be suitable in areas where groundwater is shallow (liners can be
easily installed) or bedrock prevails (that is, where excavation
is neither possible nor required). Stabilized sludge is
best suited for this method, since daily cover is not usually
provided. Adequate drainage and runoff control are necessary to
prevent contamination of nearby surface waters.
A rea^_ J?i_1 l^M o un. d
Area fill mound applications are generally suitable for
stabilized sludges with solids concentrations of 20 percent or
more. Soil is mixed with sludge to provide bulk and stability
before hauling to the filling area. At the filling area, the
mixture is placed in 6 foot (2 m) mounds and then covered with
3 to 5 feet (1 to 1.5 m) of soil. A level area is required for
disposal; however, the use of earthen containment structures
permits disposal in hilly areas.
Area Fill Layer
Area fill layer applications are suitable for stabilized sludge
with solids as low as 15 percent. Soil is mixed with sludge,
either at the filling area or at a special mixing area. The
sludge/soil mixture is spread in even layers of approximately
1 foot (0.3 m) thick, and 3 to 5 feet (1 to 1.5 m) of soil are
added for final cover.
Level ground is preferred for this type of operation, but mildly
sloping terrain can be used.
19-3
-------
The District's operation consists of opening 20 feet (6,1 m)
deep trenches on 300-acre (121.5 ha) site with large
backhoe equipment. This same equipment is used to cover
each layer of sludge with a layer of soil and cap each trench
with several feet of soil. Production in 1976 was 30 dry
tons/day (27 t/day).
Dewatered sludge is dumped from trucks directly into the
trench. Various equipment is shown in the background.
Also, the dewatered sludge storage building is shown in
the background. Sludge is stored inside the building on
weekends for transfer to trenches during daytime hours
Monday through Friday.
FIGURE 19-1
WIDE TRENCHING OPERATION, NORTH SHORE
SANITARY DISTRICT
19-4
-------
D i ke^Conta i nme n t
Dike containment applications require sludge with a solids
content of 20 percent or greater. This method is suitable for
either stabilized or unstabilized sludge. Sludge is usually not
mixed with a bulking agent. If the disposal site is level,
earthen dikes are used on all four sides of the containment area.
If the site is at the toe of the hill, only a partial diking is
required. Access is provided to the top of the dike so that
haul vehicles can dump sludge directly into the containment.
Depending on the type of equipment used, the interim cover will
vary from 1 to 3 feet (0.3 to 1.0 m) and the final cover from
3 to 5 feet (1.0 to 1.5 m) . Although diked containment is an
efficient disposal method from the standpoint of land use, it may
necessitate controls for leachate outbreaks.
19.2.2.3 Co-Disposal with Refuse
The term co-disposal is used when municipal sludge is disposed of
at a refuse landfill. There are distinct trade-offs in using
co-disposal method rather than the sludge-only methods.
Sludge can be disposed of in this manner if it is mixed with
refuse or with soil. Mixing techniques are discussed in detail
in the USEPA Office of Solid Waste Report, Disposal of Sewage
Sludge into a Sanitary Landfill (2).
Sludge/Refuse Mixture
Stabilized or unstabilized sludge with a solids content of three
percent or greater is mixed with the refuse. Normally sludge
content is approximately ten percent of the sludge/refuse
mixture. The sludge is applied on top of the refuse at the
working face of the landfill. The sludge and refuse are
thoroughly mixed before they are spread, compacted, and covered
with soil. An interim cover of approximately one foot (0.3 m)
and a final cover of two feet (0.6 m) is used. Application rates
range from 500 to 4,200 cubic yards of sludge per acre (950 to
7,900 m3/ha).
Sludge/Soil Mixture
In this operation, sludge is mixed with soil and the mixture is
used as cover for a refuse landfill. This method requires
stabilized sludge with at least a 20 percent solids content.
It promotes vegetation growth over completed landfill areas
without the use of fertilizer. However, it may cause odors,
since the sludge is not completely buried. A final soil cover
could be added if necessary to eliminate this problem.
19-5
-------
19.2.2.4 Suitability of Sludge for Landfilling
Some wastewater treatment sludges may not be suitable for
landfilling by any of the methods described above. For
sludge-only landfills, the solids concentration should be
15 percent or more. Although soil may be used as a bulking agent
to effectively increase the solids concentration to this level,
cost-effectiveness may become a problem. Solids concentrations
down to three percent are tolerated for co-disposal, but the
absorptive capacity of the refuse should not be exceeded. An
assessment of the suitability of various sludge types is given in
Table 19-1. In general, only stabilized and dewatered sludges
are recommended for landfill disposal.
19.2.3 Preliminary Planning
The purpose of the preliminary planning activity is to select a
disposal site and suitable method(s) of disposal. Preliminary
planning is followed by detailed design, initial site develop-
ment, site operation and maintenance, and final site closure.
Site selection is the major activity during the preliminary
planning phase. Since the selection of a site is not completely
independent of the selection of a method, the preliminary
planning phase should also include the determination of sludge
characteristics and the identification of alternate landfill
methods for each site. Chapter 2 of Municipal Sludge Landfill
(1) provides an excellent discussion on public participation in
this and other phases of the project.
19.2.3.1 Sludge Characterization
Sludge must be characterized as to quantity and quality.
Chapter 4 provides further discussion on sludge characterization.
Sludge_Quantity
An estimate of the average sludge quantity is necessary to
establish landfill area requirements and the probable life of
the disposal site. Data on minimum and maximum sludge quantities
are important for developing an understanding of daily operating
requirements. Maximum daily sludge quantities will govern
equipment and storage facility sizing and daily operating
schedules.
Sludge Quality
The character of the sludge to be landfilled is directly related
to the choice of a landfill method. Sludge quality and the
corresponding leachate can be roughly correlated; design of
leachate treatment facilities is more effective if sludge quality
is known.
19-6
-------
TABLE 19-1
SUITABILITY OF SLUDGES FOR LANDFILLINC
Sludge only landfilling Co-disposal landfilling
Type of sludge Suitability Reason Suitability Reason
Liquid - unstabilized
Gravity thickened primary,
WAS and primary, and WAS NS OD, OP NS OD, OP
Flotation thickened primary
and WAS, and WAS without
chemicals NS OD, OP NS OD, OP
Flotation thickened WAS with
chemicals NS OP NS OD, OP
Thermal conditioned primary
or WAS NS OD, OP MS OD, OP
Liquid - stabilized
Thickened anaerobic digested
primary and primary, and
WAS NS OP MS OP
Thickened aerobic digested
primary and primary, and
WAS NS OP MS OP
Thickened lime stabilized
primary and primary, and
WAS NS OP MS OP
Dewatered - unstabilized
Vacuum filtered, lime
conditioned primary S - S -
Dewatered - stabilized
Drying bed digested and
lime stabilized S - S -
Vacuum filtered, lime
conditioned digested S - S
Pressure filtered, lime
conditioned digested S - S -
Centrifuged, digested and
lime conditioned digested S - S
Heat dried
Heat dried digested S - S -
High temperature processed
Incinerated dewatered
primary and primary, and
WAS S S -
Wet-air oxidized primary
and primary, and WAS NS OD, OP MS OD, OP
WAS - Waste-activated sludge
NS - Not suitable
MS - Marginally suitable
S - Suitable
OD - Odor problems
OP - Operational problems
Parameters that should be analyzed are discussed briefly below.
Although all of these may not be critical to the design of a
particular disposal system, a complete analysis is necessary,
because the sludge must be adequately characterized.
• Concentration. Concentration or solids content of sludge
is related to the nature of wastewater treatment and
sludge processing steps. The type and operation of
19-7
-------
dewatering equipment may have a significant effect on the
sludge concentration. A certain degree of flexibility
should be incorporated into the design of landfills to
compensate for the variability in solids concentration of
dewatered sludge.
• Volatile content. Volatile solids are a measure of the
organic content present in the solid fraction of sludge.
This organic matter is eventually broken down into
methane gas and other digestion by-products. Typically,
volatile solids represent 60 to 80 percent of the total
solids in raw primary sludge and 30 to 60 percent in
anaerobically digested primary solids.
• Nitrogen. Nitrogen found in sludge is a potential source
of groundwater pollution. The total quantity and type of
nitrogen are of importance. Nitrate is relatively mobile
in soil and is therefore of concern.
• Inorganic ions. Inorganic ions such as heavy metals are
found in most municipal sludges. These are more readily
leached if soil and sludge are acidic. If near neutral
or alkaline conditions are maintained, the metals will
not be as readily leached from the sludge or through the
soil.
• Bacteriological quality. Sludge treatment systems reduce
the number of pathogens and the possibility of pathogenic
contamination associated with landfilling of sludges;
however, they do riot provide a sterile product.
• Toxic organic compounds. Toxic organic compounds can
present potential contamination problems. Solids
contaminated with toxic materials must be placed in
appropriately designated disposal facilities.
• pH. Acidic conditions promote leaching of heavy metals
and other compounds from the sludge.
19.2.3.2 Selection of a Landfilling Method
Relationships between the characteristics of alternative
landfill sites, the characteristics of the sludge to be
landfilled, and the landfill method need to be considered in the
preliminary planning process. These relationships are summarized
in Table 19-2.
19.2.3.3 Site Selection
Site selection is a critical process in the planning of a sludge
landfill project. It is directly related to the method of
ultimate disposal. The site finally selected must be suitable
19-8
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for the type of sludge to be disposed of and situated in a
convenient, yet unobtrusive, location. Chapter 4 of Municipal
Sludge Landfill (1) provides an in-depth approachTo site
selection.
TABLE 19-2
SLUDGE AND SITE CONDITIONS
Method
Narrow trench
Wide trench
Area fill mound
Area fill layer
Diked containment
Sludge/refuse mixture
Sludge/soil mixture
Sludge solids
content, percent
15 - 28
>20
>20
Appropriate
sludge
characteristics
Unstabilized or
stabilized
Unstabilized or
stabilized
Stabilized
Appropriate
hydrogeology
Deep ground water and
bedrock
Deep groundwater and
bedrock
Shallow groundwater
Appropriate
ground slope
<20 percent
<10 percent
Suitable for ste<
£15
£20
>2Q
Unstabilized or
stabilized
Stabilized
Unstabilized or
stabilized
Stabilized
or bedrock
Shallow groundwater
or bedrock
Shallow groundwater
or bedrock
Deep or shallow
groundwater or bed-
rock
Deep or shallow
groundwater or bed-
rock
terrain as long as
level area is pre-
pared for mounding
Suitable for medium
slopes but level
ground preferred
Suitable for steep
terrain as long as
a level area is pre-
pared inside dikes
<30 percent
<5 percent
Site Considerations
The following factors must be considered during the evaluation of
possible landf ill sites. Information on these factors should
therefore be collected and assessed in advance of the final
decision making process.
• Haul distance. The most favorable haul conditions
combine level terrain and minimum distances.
* Site life and size. The site 1ife and s ize are directly
related to the quantity and characteristics of the sludge
and the method used for landfilling. Since the entire
site cannot be used as fill area, both the gross area and
the usable or fill area must be cons idered in determining
the site size. Initially, the life of the site can be
estimated. As the landfill is used, the expected
life should be reevaluated to ensure adequate capacity
for future operations.
* Topography. In general, sludge landf illing is limited to
sites with minimum slopes of one percent and maximum
slopes of 20 percent. Flat terrain tends to result in
ponding, whereas steep slopes erode.
19-9
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Surface water. The location and extent of surface waters
in the vicinity of the landfill site can be a significant
factor in the selection process. Existing surface waters
and drainage near proposed sites should be mapped and
their present and proposed uses outlined. Leachate
control measures including collection and treatment may
be required as part of the landfill design.
Soils and geology. Soil is an important determinant in
the choice of an appropriate sludge landfilling site.
Properties such as texture, structure, permeability,
pH, and cation exchange capacity, as well as the
characteristics of soil formation, may influence the
selection of the site. The geology of possible landfill
sites should be thoroughly examined to identify any
faults, major fractures and joint sets. The possibility
of aquifer contamination through irregular formations
must be studied.
Groundwater. Data on groundwaters in the vicinity of
potential landfill sites is essential to the selection
process. Knowledge of characteristics such as the depth
to groundwater, the hydraulic gradient, the quality and
use of the groundwater, and the location of recharge
zones is essential for determining the suitability of a
potential landfill site.
Vegetation. The type and quantity of vegetation in the
area o~fproposed landfill sites should be considered
in the evaluation. Vegetation can serve as a natural
buffer, reducing visual impact, odor, and other
nuisances. At the same time, clearing a site of timber
or other heavy vegetation can add significantly to the
initial project costs.
Meteorology. Prevailing wind direction, speed,
temperature and atmospheric stability should be evaluated
to determine potential odor and dust impacts downwind of
the site.
Environmentally sens it i ve a re a s. Environmentally
sens itive areas such as the wetlands, flood plains,
permafrost areas, critical habitats of endangered
species, and recharge zones of sole source aquifers
should be avoided if at all possible when selecting
a landfill site.
Archaeological and histo r i c a 1 significance^. The
archaeological and hlsTorTcal™ s ignif icance 6T"proposed
sites should be determined early in the evaluation
process. Any significant finds at the selected site must
be accommodated prior to final approval.
19-10
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• Site access. Haul routes should be major highways, or
arterials, preferably those with a minimum of traffic
during normal transport hours. Proposed routes should be
studied to determine impacts on local use and the
potential effects of accidents. Transport through
nonresidential areas is preferable to transport through
residential areas, high-density urban areas, and areas
with congested traffic. The access roads to the site must
be adequate for the anticipated traffic loads.
• Land use. Zoning restrictions, and future development
on potential sites should be considered in the selection
process. Ideally, the sludge landfill site should be
located on land considered unsuitable for higher uses;
however, the designer should be aware that this may
be a politically sensitive issue and maximum public
participation must be assured.
• Costs. Cost-effectiveness of each potential landfill
site must be evaluated. Factors to be included in the
economic evaluation include capital costs and operating
and maintenance (O&M) costs. In the latter category,
sludge hauling may prove to be a significant component.
The trade-offs between high capital and high OS.H costs
will depend on the design life of the landfill. These
trade-offs will become evident when the total annual
(amortized capital and O&M costs) are compared. This
evaluation should be performed in accordance with the
methods outlined in the cost-effectiveness analysis
section discussed in Chapter 3.
Sj^te Selection Methodology
The selection procedure can be roughly divided into three phases:
initial inventory and assessment of sites, screening of potential
sites, and final site selection.
Initial inventory and assessment is designed to develop a list of
potential sites that can be evaluated and rapidly screened to
produce a manageable number of candidate sites. Information used
in this phase is generally available and readily accessible.
Investigation of each option becomes more detailed as the
selection procedure progresses.
I n i t i a1_Assessment of Site
Initial assessments will consist of identifying Federal, State,
and local regulatory constraints, eliminating inaccessible areas,
locating potential sites, roughly assessing the economic
feasibility of such sites, and performing preliminary site
evaluations. The less desirable sites are eliminated on the basis
of preliminary economics, regulatory, and technical information.
A public participation program is initiated (4). Attitudes of
the public should be determined early. The public may assist in
identifying candidate sites.
19-11
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Screening of Candidate Sites. Sites remaining after the initial
assessment are subjected to closer scrutiny. Information used in
evaluating each option is more detailed and somewhat more site-
specific than in the initial assessment. Remaining sites may be
rated by a scoring system including both objective and subjective
evaluations (Chapter 3). Table 3-4 serves as an example of
a rating system. Candidate systems with lowest overall ratings
are eliminated, and the higher rated systems are carried forward
for final evaluation.
Site selection findings for the remaining candidate systems
should provide input into an environmental impact report, if
required. Public attitudes toward the remaining sites should also
be determined.
Final Site Selection and Site Acquisition. Methodology for final
site selection is similar to that for the screening procedure
just discussed, in that rating systems are still used. However,
each site remaining is investigated in greater detail. Public
hearings may also be scheduled so that final inputs can be
received from local government officials and the public.
Once the best sites are determined, they must be acquired. Site
acquisition should begin immediately following acceptance of the
program by local, State, and Federal regulatory authorities.
The several acquisition procedures include: purchase option,
outright purchase, lease, condemnation and/or other court action,
and land dedication.
It will generally prove advantageous to purchase the site
rather than hold a long-term lease. The managing agency's
responsibility will normally extend well beyond the life of the
site. Certain advantages may also be gained by leasing with an
option to buy the site at the time of planning approval. A
purchase option assures the availability of land upon completion
of the facility planning process. This approach also allows time
for the previous owner to gradually phase out operations, if
desired.
19.2.4 Facility Design
19.2.4.1 Regulations and Standards
Local, State, and Federal regulations and standards must be fully
understood before the landfill is designed. Consideration must
be given to requirements governing the degree of sludge
stabilization, the loading rates, the frequency and depth of
cover, monitoring, and reporting. The design should conform to
all building codes and should include adequate buffer zones to
protect public roads, private structures, and surface waters.
19-12
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Obtaining permits for construction and operation of sludge
landfills can be a long and costly process. To minimize delays
associated with this task, permit application should be initiated
early in the design stage. A sound regulatory-consultant
relationship and a mutual understanding should be developed.
The following is a partial list of the permits which may be
required:
• NPDES permit—if landfill is in wetlands.
• Army Corps of Engineers permit — for construction of
levees, dikes, or containment structures to be placed in
the water in a wetlands area.
• Office of Endangered Species permit—if landfill is
located in critical habitat of an endangered species.
• Solid Waste Management permit.
• Special Use permit.
• Highway Department permit.
• Construction permit.
• Building permit.
• Drainage and/or Flood Plain Alteration permit.
19.2.4.2 Site Characteristics
Site characteristics should be clearly described and analyzed to
ensure the suitability of the landfill site and the method of
landfilling. Design phase work will build upon planning phase
data but will be carried to a higher level of detail and include
working drawings.
Sit.e_Plan
The site plan should contain the following minimum information:
• Boundaries of fill area and buffer zones.
• Topographic features and slopes of fill area and buffer
zones.
• Location of surface water, roads, and utilities.
• Existing and proposed structures and access roads.
• Vegetation to remain and to be removed; areas to be
revegetated.
19-13
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Soils
The soil characteristics at the landfill site should be
thoroughly catalogued and mapped. The information of most
importance to the design and operation of the landfill includes
depth, texture, structure, bulk density, porosity, permeability,
moisture, stability, and ease of excavation. Areas with rocky
soils or extensive rock outcrops should be noted. The pH and
cation exchange capacity have a direct bearing on heavy metal
transport through the soil. Translocation of metals must be
considered to ensure protection of surface and groundwater
supplies.
Grouridwcrteir
The groundwater aquifers underlying the landfill site must be
located. Depth of the aquifer under varying conditions should be
determined at several locations. Other characteristics such as
the direction and rate of flow, the hydraulic gradient, the
quality, and present and planned uses should also be established.
Location of the primary recharge zones is critical in protecting
quality.
Subsurface Geology
The geological formations underlying the landfill are important
in establishing the design parameters. Critical design
parameters include the depth, distribution, and characteristics
of subsurface soils in relation to stability and groundwater
transmissability.
Climate
Climate can influence many factors in the design of landfills.
Climatic conditions effect rate of organic decomposition, the
composition and quantity of leachate and runoff, the day-to-
day fill operations, and the dispersion of odors and dust.
Information such as seasonal temperature, precipitation,
evaporation, wind direction and speed and atmospheric stability,
can be obtained from a local weather station.
Land JJ se
The present and proposed use of the landfill site and adjacent
properties should be evaluated. If the site is already dedicated
to refuse or sludge disposal, it is unlikely that expanding it
will result in adverse impacts. However, if the site is located
in or near a populated area, extensive control measures may be
needed to eliminate concerns and minimize any public nuisance
which would detract from the va.lue of adjacent properties.
19.2.4.3 Landfill Type and Design
More than one sludge landfill method may be suitable for the
selected site, as shown in Table 19-21. If this is the case, a
method must be selected before the final design is begun.
19-14
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Maximizing utilization of the site is an important consideration
in method selection. If daily cover is to be applied, the daily
sludge generation rate will affect the net capacity of the
site. If several ^days are required to fill a trench, as the
result of low sludge generation, and cover is required each day,
then the ratio of sludge/cover will be less than for sites
managing larger sludge quantities. The net sludge capacity will
be higher at sites where trenches are filled each day.
The amount by which the net capacity of the site will be reduced
will vary with the landfill methods, the specific site, and the
daily sludge generation rate. Before a final method is selected,
estimates of net capacity and site life should be made for each.
Additional design criteria are summarized in Table 19-3 (1).
TABLE 19-3
LANDFILL DESIGN CRITERIA
Sludge
solids Trench
content, width, Bulking Bulking
Cover Sludge
thickness, ft Imported application
soil rate,
Method
Sludge only-trench fill
Wide trench
20-28°
h9d
"10
10
NO
No --
No
Interim Final
3-4
4-5
required
°
No
cu yd/acre
3,200-14,500
Equipment
'
machine
Track loader, dragline,
scraper, track
Sludge only-area fill
Area fill mound
Area fill layer
Diked containment
Codisposal with refuse
Sludge/refuse mixture
Sludge/soil mixture
>20C'd -
>15d
20-28C
>28d
v H
>3d
>20d
Volume basis unless otherwise noted.
In actual fill areas.
Yes Soil
Yes Soil
No Soil
No Soil
Yes Refuse
Yes Soil
1 ft = 0.
1 cu yd =
1 acre =
0.5-2 soil:
1 sludge
0.25-1 soil:
1 sludqe
0.25-
1 sludge
4-7 tons refuse ;
1 wet ton sludge
1 soil:
1 sludqe
305 m
0. 765 cu m
0.405 ha
3
0.5-1
0.5-1
0.5-1
Yes 3,000-14,000 Track loader, backhoe
with loader, track
dozer
Yes 2,000-9,000 Track dozer, grader,
track loader
Yes 4,800-15,000 Dragline, track dozer,
scraper
500-4,200 Draqline, track dozer
1,600 Tractor with disc,
grader, track loader
Land-based equipment.
Sludge-based equipment
But sometimes used.
19.2.4.4 Ancillary Facilities
Ancillary facilities may be needed in association with the
landfill site. These are described briefly in the following
sections.
Lea.chate^ Contro 1 s
Leachate from the landfill site must be contained and treated
to eliminate potential water pollution and/or potential public
19-15
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health problems. In many cases, leachate containment and
treatment may be required by state or local regulations.
Numerous methods are available for controlling leachate, includ-
ing drainage, natural attenuation, soil or membrane liners, or
collection and treatment. The method and the design features
chosen are specific for each project. Table 19-4 depicts
sludge-only leachate quality for one site sampled over two years.
TABLE 19-4
LEACHATE QUALITY FROM SLUDGE-ONLY LANDFILL
Constituents Values
Constituents
PH 6.7
TOC 1,00(K
COD 5,100
Ammonia nitrogen 198
Nitrate nitrogen 0.28
Chloride 6.7
Sulfate 10
0
Specific conductivity 3,600
Cadmium 0.017
Chromium 1.1
Copper 1.3
Iron 170
Mercury 0.0004
Nickel 0.31
Lead 0.60
Zinc 5.0
aData from "Site 8" monitored from July 1975
through September 1977. First received
sludge in 1973. Receives unstabilized
primary and WAS, gravity thickened and
centrif uged . Sludge is lagooned, allowed
to dry, and covered with soil. Soil
characteristics: sand and gravel, glacial
deposites .
Specific conductivity in micromhos/cm, pH
in units, all others in mg/1 .
°Ranged from 3,000 mg/1 to 1 mg/1.
Limited to early part of sampling program.
f\
Ranged from 10,000 micromhos/cm 340
micromhos/cm .
19-16
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Gas Control
Gas produced by decomposition of organic matter is potentially
dangerous. This condition is of particular concern if the
landfill is located near a populated area. Methane gas, in
particular, is highly explosive if confined in an enclosed area.
Control of the gases produced at the landfill must be provided.
Two widely accepted methods control paths of gas migration.
Permeable methods usually consist of a gravel-filled trench
around the fill area for intercepting migrating gas and venting
it to the atmosphere. Impermeable methods consist of placing a
barrier of low permeability material, such as compacted clay,
around the fill area to minimize lateral movement of gas. This
method provides for gas venting through the cover material. In
general, methane recovery is not cost-effective at sludge-only or
small co-disposal sites.
Roads
Paved access and on-site roads are necessary at the landfill
site. Temporary roads may be constructed of well compacted
natural soil or gravel. Considerations should include grades,
road surface and stability, and climate. Grades in excess
of ten percent should be avoided. Provisions should be made to
allow trucks to turn around within the site area.
Soil S
Storage area should be provided for on-site stockpiling of
transported soils where on-site soils are insufficient or their
use inappropriate. The quantity and type of soil to be stockpiled
depends on the individual demands of the landfill. Stockpiles
may also be desirable for winter operations where frozen ground
may limit excavation.
I nc 1 erne n t We athej:Areas
Special landfill areas should be placed near the entrance to
the site so that operations may be continued during inclement
weather. Paved or all-weather roads should be provided for
working these sites.
Structures
An office and employee facilities should be located at the
landfill site. For large operations, a permanent structure
should be provided. At smaller sites a trailer might suffice.
An equipment barn and shop may be desirable for some locations.
Utilities
Electrical, water, communication and sanitary services should be
provided for large landfill operations. Chemical toilets, bottled
water, and on-site electrical generation may reduce the cost of
obtaining services from utility companies. This approach may be
appropriate for remote sites.
19-17
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Fencing
The landfill site should be fenced. Access should be limited to
one or two secured entrances. The height and type of fence
should suit local conditions. A 6-foot (1.8 m) chain link fence
topped with barbed wire will restrict trespassers; a wooden fence
or hedge is effective for screening the operation from view, and
a 4-foot (1.2 m) barbed wire fence will keep cattle or sheep away
from the site area.
Lighting
Portable lighting should be provided if landfill operations are
carried out at night. Permanent lights should be installed for
all structures and heavily used access roads.
A cleaning program should be required for frequently used
equipment. A curbed wash pad and collection basin should be
provided to contain the contaminated washwater for treatment.
Monitoring We 11s
It is crucial to monitor groundwater. The number, type, and
location of monitoring wells and monitoring frequency should be
designated to meet specific conditions associated with the
landfill.
Depending on the size and location of the landfill, landscaping
may be an important design factor. The aesthetic acceptability
of the landfill is critical, especially in an urban or densely
populated area. In general, shrubbery chosen should require
little maintenance and become an effective visual barrier.
19.2.4.5 Landfill Equipment
A wide variety of equipment may be required for a sludge
landfill. The type of equipment depends on the landfill method
employed and on the quantity of sludge to be disposed of.
Equipment will be required for sludge handling, excavation,
backfilling, grading, and road construction. Table 19-5 presents
typical equipment performance characteristics for various
sludge landfilling methods.
19.2.4.6 Flexibility and Reliability
Because sludge characteristics and quantities may change, a
landfill site should be designed with maximum flexibility. Since
the life of a landfill is difficult to accurately predict,
19-18
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expansion may be needed sooner than originally planned or it
may be delayed. Any change in wastewater treatment or sludge
management processes may affect the nature and quantity of sludge
produced. Operational modifications may be needed if these
changes are drastic. The landfill design should be such that
changes can be made without major disruption to operations.
TABLE 19-5
LANDFILL EQUIPMENT PERFORMANCE CHARACTERISTICS
Equipment type
Landfill
method Submethod
Soil hauling
Mixing
Sludge hauling
Mounding
Covering
Soil hauling
Mixing
Sludge hauling
Layering
Covering
•Diked con- Soil hauling
tainment Dike construe
Covering
Codisposal Sludge/refuse Spreading
Covering
Haulii
Cover
Legend
G = Good. Fully capable of performing function listed. Equipment could be selected solely on basis of function listet
F = Fair. Marginally capable of performing function listed. Equipment should be selected on basis of full capabilitii
in other function.
Reliability is another important factor in designing a landfill
operation. • Operation should continue even in inclement weather.
Special work areas and storage facilities should be available
on site for emergency operations or unexpected equipment
failures.
19.2.4.7 Expected Performance
Although the overall performance of a sludge landfill may be
difficult to predict accurately, certain operating parameters
should be estimated. The site life depends on many factors;
an estimate .is needed for purposes of economic evaluations
and future planning. Sludge application rate and soil cover
19-19
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requirements should be estimated before scheduling initial
operations. Performance can be more closely predicted after
actual operating experience is gained.
19.2.4.8 Environmental Impacts
Specific areas of environmental impact vary among landfill
locations. Crucial impact areas include: traffic, land use,
air quality, surface and groundwater quality, public health,
aesthetics, wildlife, and habitats of endangered species.
Adverse impacts should be mitigated during the site selection
process or by specific measures in the design.
19.2.5 Operation and Maintenance
A sludge landfill should be viewed as an ongoing construction
site. Unlike conventional construction, however, the operating
parameters of a sludge landfill often change and may require
innovative alterations and contingency plans. An effective
landfill requires a detailed operational plan. Equipment
selection should be compatible with sludge characteristics,
site conditions, and landfill method.
Operational procedures can be separated into those specific to
the landfill method and those applicable to sludge landfills
in general. Method-specific procedures include: site
preparation, sludge unloading, sludge management and covering.
These procedures are discussed in detail in Municipal Sludge
Landfill (1).
General procedures include scheduling, equipment selection and
maintenance, management and reporting, safety, and environmental
controls. These items are discussed in Sanitary Landfill Design
and Operation (2). Important points are summarized below.
19.2.5.1 Operations Plan
As with any construction activity, sludge landfilling must
proceed according to detailed plans and operating schedules. The
operation plan should address all relevant method-specific or
general operating procedures for the landfill, including:
• Hours of operation.
• Measuring procedures.
• Traffic flow and unloading procedures.
• Special wastes handling.
• Cover excavation, stockpiling, and placement.
19-20
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• Maintenance procedures and schedules.
• Inclement weather operations.
• Environmental monitoring and control practices.
An operations plan is an important tool for providing continuity
of activities, monitoring and control of progress, and personnel
training.
19.2.5.2 Operating Schedule
Major features of the operating schedule include: hours of
operation, availability of qualified personnel, site preparation
schedules, and equipment maintenance schedules. The hours of
operation must be such that the site is open when sludge is to be
received. If variations in the rate of receipt are expected
during the day, it may be desirable to schedule for equipment and
personnel accordingly. The schedule may need to provide for the
application of daily soil cover.
19.2.5.3 Equipment Selection and Maintenance
Equipment selection depends largely upon the landfill method,
design dimensions, and sludge quantity. Selection must be based
upon the functions to be performed and the cost of alternate
machines. Table 19-5 summarized general selection criteria.
Table 19-6 presents examples of equipment choices for seven
landfill schemes.
TABLE 19-6
TYPICAL EQUIPMENT TYPE AND NUMBER AS A FUNCTION
OF LANDFILL METHOD AND SITE LOADING
Trench method Area fill method - Codisposal method*1
Narrow trench Wide trench Hound Laver Diked containment Sludge/refuse Sludge/soil
Trenching
machine 1 2
Backhoe with
loader 1 1 I9 1 I9 I9 I9 1
.vator 1
;k loader 1 lg 1 I9 1 1 1 1 1
1 loader ' 11 I9
ck dozer I9 1 1 23 I9 1 1 29 I9 1 1 1 1 1 29 2 1 I9 I9 1
aper I9 1 I9 I9 1 I9 lg I9 1 lg 1
gline ' 111
Total 12235 12 224 1245 5122341 233 4- - 1 121124
Additional equipment only.
Scheme 1-10 wet TPD.
C Scheme 2 - 50 wet TPD.
Scheme 3 - 100 wet TPD.
GScheme 4 - 250 wet TPD.
19-21
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Equipment maintenance can be more expensive than the amortized
annual purchase cost. A scheduled preventive maintenance program
should be followed to control maintenance costs. Operators
should perform routine daily maintenance (for example, check
fluid levels, cleaning, etc.). The operating schedule should
provide periods for thorough maintenance.
19.2.5.4 Management and Reporting
Management and reporting activities include the maintenance
of activity records, performance records, required regulatory
reports, cost records, on-site supervision and public relations
activities. Activity records include equipment and personnel
accounts, sludge and (if applicable) solid waste receipts, cover
material quantities and used site area layouts. These records
become bases for scheduling site development, gauging efficiency,
and any billing as required.
Performance records may be required as a part of the regulatory
process. Regulatory agencies may perform periodic inspections on
a scheduled or an unscheduled basis. Operating and supervisory
personnel must be aware of these requirements.
For the purposes of safety and control, the site should be
staffed with two or more persons. At smaller sites, where only
one operator is required, daily visits or phone checks should be
made.
19.2.5.5 Safety
Providing a safe working environment at the landfill site should
be a part of general O&M, and certain safety features should be
built into the design. Certain practices must be followed daily
to provide safe working conditions. The operations plan should
have a separate safety section, as well as specific safety
guidelines for each operation and feature of the landfill.
S_o_il_and Fill Stability
The stability of the soil and fill can present a critical safety
problem, particularly with the use of large equipment. Disturbed
and filled areas should be approached cautiously as should muddy
areas or areas subject to erosion.
Equipment Operation
The operation of large, earth-moving equipment presents the
potential for accidents. Only fully trained operators should
be allowed to use such equipment. Regular maintenance and safety
checks can greatly reduce the number of accidents associated with
equipment failure and operator error.
19-22
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Gas Control
Caution must be used when dealing with gas control equipment.
The O&M manual should contain a complete set of instructions on
the safe servicing of gas control and monitoring equipment, and
the operation of this equipment should be explained periodically
at operation and safety training sessions.
19.2.5.6 Environmental Controls
The protection of the environment and public health are important
aspects of the landfill operation. The operations plan should
contain guidelines for providing this protection and actual
operations should conform to the guidelines. General require-
ments are summarized in Table 19-7. Critical areas are discussed
below.
TABLE 19-7
POTENTIAL ENVIRONMENTAL PROBLEMS AND
CONTROL PRACTICES
Environmental problems
Siltation
and
Control practice Spillage erosion Mud Dust Vectors Odors Noise Aesthetics Health Safety
Safety program X
Maintain washrooms for person-
nel X
Training of new personnel X XXXXXX X XX
Use safety clamps on truck
tailgates X x
Maintain road markings and
trench barriers X X
Ma intain fencing X X
Apply insecticide X X
Maintain buffer areas and grass XXX XX X
Proper equipment maintenance X XX
Spray water/oil/liquid asphalt X X
Truck wash pad (to clean trucks) XXX
Maintain grass waterways,
diversion ditches, rip rap XX X
Fi nal grading of disturbed
areas X X
Revegetation of disturbed
areas XXX X
Chemical masking agent X
Limeonsite X XX XX
Workers supplied with
aerators XX XX
Cover sludge daily XX XXX
Water diverted away from site X X
Environment
Environmental protection is generally focused on leachate
and runoff controls for preventing surface and groundwater
contamination. Trench liners must be kept intact during and
after filling operations. Drainage systems should be checked
19-23
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to see that they are functioning as designed. If monitoring
indicates that adverse environmental impacts are occurring or
pending, immediate corrective action should be taken.
Public Health
Protection of public health should be a foremost concern in the
operation of sludge landfills. Protection of water supplies and
particularly sole source aquifers is an obvious responsibility.
In addition, control of potential disease by reduction of
vectors, the adequate venting of explosive or toxic gases, and
the restriction of access to the landfill site are the
responsibility of the operators.
Social Welfare
Minimizing the negative aesthetic impacts of a sludge landfill
can greatly increase public acceptance. Control of odors, noise,
and other nuisances is generally straight-forward and should be
accomplished as part of the daily operating routine. Efforts
should be made to reduce the undesirable social impacts of the
fill operation.
19.2.6 Site Closure
In closing a sludge landfill site, certain criteria must be met
to make the site publicly acceptable. These criteria are
established according to the type of landfill and the location,
size, and ultimate use of the site. The procedures for site
closure should be included in the operations manual and updated
or modified as the original landfill plan is altered.
19.2.6.1 Ultimate Use
The ultimate use of the site should be described and illustrated
in the O&M manual or in a separate document describing the
closure of the site. The actual work involved in completing the
site will depend on its ultimate use and on the care taken in
day-to-day fill operations.
19.2.6.2 Grading at Completion of Filling
When each section of the landfill is completed, the final cover
should be graded according to a predetermined plan. It is
imperative that no sludge become or remain exposed after the
grading has been completed.
19-24
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19.2.6.3 Final Grading
Final grading of the site is to be performed after sufficient
time has elapsed to allow for initial settlement. The final
grading plan should be designed in accordance with the intended
ultimate use of the landfill site. It is important that all
sludge be completely cohered to the specified depth with cover
material.
19.2.6.4 Landscaping
The landscaping plan should reflect the intended ultimate use of
the landfill site. Where practical, landscaping may be done on
completed sections before the entire fill project is completed.
19.2.6.5 Continued Leachate and Gas Control
Since decomposition of the organics in the sludge may continue
even after the landfill has been completed, an ongoing monitoring
and control program must be maintained. Leachate and gas must be
controlled even after the filling operations have stopped. Th-e
completion plans should clearly outline this program.
19.2.7 Landfilling of Screenings, Grit and Ash
Screenings and grit normally contain some putrescible materials
and, if landfilled, should be covered every day. Odors from
temporarily uncovered solids may be alleviated by sprinkling the
solids with lime. Special care should be exercised to assure
vector control (for example, safe poisons for rodent control,
spraying for flies, and animal-proof fencing to keep pets from
the area).
Residues (ash from the combustion of municipal wastewater solids)
generally contain high concentrations of trace metals. Leachate
from sites where incinerator ash is landfilled must be controlled
to prevent metals contamination of groundwater. In California,
for example, wastewater sludge furnace ash must be placed
in a "protected" Class II-l site. See Chapter 11 for more
information.
19.3 Dedicated Land Disposal
19.3.1 Definition
Dedicated land disposal means the application of heavy sludge
loadings to some finite land area which has limited public
access and has been set aside or dedicated for all time to the
disposal of wastewater sludge. Dedicated land disposal does not
19-25
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mean in-place utilization. Dedicated sites typically receive
liquid sludges. While application of dewatered sludges is
possible, it is not common. In addition, disposal of dewatered
sludge in landfills is generally more cost-effective.
As with any other land disposal technique, dedicated land
disposal requires the wastewater sludge be stabilized prior
to application. Once the sludge has been stabilized, however,
it can be applied to the dedicated land in either the liquid
or the dewatered state. Use of anaerobically digested sludge
minimizes odor and potential nuisances.
Many existing wastewater treatment plants practice some form of
dedicated land disposal. However, precautions necessary for
assuring that this method of disposal is not harmful to the
environment have not always been practiced.
19.3.2 Background
Dedicated land disposal was first developed as an informal
practice in response to the need to reduce high operational costs
associated with sludge disposal. The practice was applicable
particularly in cases where the plant site had adequate acreage
or where adjacent land was available and hauling costs to the
nearest landfill were high. Groundwater contamination, odor
production, and aesthetic concerns were not usually addressed in
this informal practice.
A more sophisticated approach to dedicated land disposal had to
be taken as sludge quantities increased with higher treatment
levels, and on-site sludge disposal was perceived as associated
with environmental problems. Recent research on this method of
sludge disposal has developed key environmental controls which
are covered in subsequent sections.
The use of dedicated land disposal has several major advantages.
These include flexibility in managing sludges in excess of
utilization demand; minimum land use because sludge application
rates per acre are maximized; inexpensive dewatering through
the use of solar energy instead of the relatively expensive
electrical energy required for mechanical dewatering; relatively
low capital and operating costs (6).
Dedicated land disposal is applicable as a disposal method for
liquid, dewatered, or dried sludges. To maximize the advantage
of low-cost solar drying and minimize the cost of upstream sludge
processing, disposal of liquid sludge is the most cost-effective
approach. Disposal of sludges in the liquid form requires
storage capacity. Facultative sludge lagoons (FSLs), as
discussed in Chapter 15, can provide that storage. FSLs provide
a buffer between continuous sludge production and intermittent
land disposal operations. Disposal of the thickened (solids
concentration of 6 to 8 percent) sludges from the FSLs will
commence 1 to 5 years after the first anaerobically stabilized
sludge is discharged into FSLs.
19-26
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19.3.3 Site Selection : ::
There are five major considerations in selecting an appropriate
dedicated land disposal (DLD) site. These considerations are
ownership, groundwater patterns, topography, soil types, and
availability of sufficient land. All are discussed briefly in
the following paragraphs.
19.3.3.1 Ownership by Wastewater Treatment Authority
By definition, the selected DLD site will be dedicated in
perpetuity to the sludge disposal function. Long-term buildup
of heavy metals and salts in the soil surface layers will
make the site unsuitable for future direct agricultural use.
Public access to these sites must be restricted because of
their potential pathogen contamination. These factors require
complete control and thus ownership of the site by the wastewater
treatment authority. However, merely because certain elements
accumulate to toxic concentrations on that site does not mean
that the surface soils are forever useless.
19.3.3.2 Groundwater Patterns
Groundwater movement must be considered in the selection of a
DLD site. Groundwater flow patterns must be known in order to
protect present or future domestic water supply wells. The
following three control options are possible:
1. Choice of a site with an isolated groundwater pattern.
This option requires that there be well-defined
groundwater migration to a river or the ocean; In this
case, there must be no intermediate domestic source
wells. An adequate subsurface buffer strip between the
site and the receiving waters should be provided to
permit further potential pollutant attenuation, uptake,
or dilution.
2. Choice of a site with a tight/low permeability surface
and/or subsurface soil layer which essentially prevents
DLD leachate from reaching the groundwater. In this
option, additional monitoring wells may be required
to confirm the design assumptions over the long term.
3. Construction of an artificial leachate control barrier
composed of a minimum 2-foot (0.6 m) depth clay layer
under the entire site and deep cutoff trenches at the
groundwater downstream end of the site for leachate
collection and recycling. It should be noted that when
there are low-permeable soils too close to the surface,
liquid disposal operations can be hindered. Shallow
clays can cause ponding and reduced loading rates
with these systems.
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19.3.3.3 Topography
Natural topography is an important consideration in selecting a
OLD site. Natural slopes greater than 0.5 percent will have to
be modified to prevent erosion. The lack of vegetation on
the disposal site increases the potential erosion problem and
subsequent runoff control. The use of level or nearly level land
eliminates erosion problems. Graded or terraced sites can be
used, but increased earthmoving costs are involved.
19.3.3.4 Soil Types
Most soil types can accommodate one or another form of DLD system
with proper protection of ground and surface waters as outlined
above. Preference should, however, be given to soils with a
moderate to high cation exchange capacity (CEC), typically
greater than 10 milliequivalents per 100 grams.
Desirable soil conditions include restrictive permeability,
minimal ponding, and freedom from boulders. Technical assistance
in the areas of soil science, soil agronomy, and soil engineering
is recommended, so that the impacts of specific soil types on the
project can be accurately evaluated.
19.3.3.5 Availability of Sufficient Land
The amount of land required depends upon the quantity of sludge
generated and upon the acceptable loading rates. Sufficient land
must be available to ensure the integrity of the system.
19.3.4 Storage
Storage should be considered for DLD systems under certain
climatic conditions and for increased operational efficiency
and control. As discussed earlier, FSLs are recommended
to meet these conditions and to assist in flow buffering (see
Chapter 15).
19.3.4.1 Climatic Influences
In most areas of the country, rainfall is seasonal, and in some
the ground may be frozen to a depth which makes it unworkable
during the winter. These conditions mean ^that dedicated land
disposal operations can occur only during the drier months. As a
minimum, provision for six months of sludge storage is required.
Systems designed for handling liquid sludge in Sacramento,
California (6), and in Corvallis, Oregon (7,8,9), are designed
for 18 to 60 months storage of anaerobically digested sludge in
FSLs. This allows upstream systems to operate through a winter-
summer-winter cycle and without disposal problems during a wet
spreading season.
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19.3.4.2 Operational Storage
Even where climate is not severe enough to require sludge
storage, storage may still be warranted for operational
efficiency. If storage is provided, routine equipment
maintenance can take place during normal work hours. Emergency
situations, such as those which require the retention of
unstabilized sludge for very short periods during any plant
upset, can be responded to effectively (10).
19.3.5 Operational Methods and Equipment
Dedicated land disposal has achieved recent prominence because
of its application to the problem of direct disposal of liquid
sludges. Systems that are designed to deliver and manage liquid
sludges on DLD sites are of primary interest (11).
19.3.5.1 Liquid Sludge
Application of liquid sludge is desirable because it simplifies
upstream processes. Dewatering processes are not required, and
inexpensive liquid transport and application systems can be used.
Four common surface application methods for the liquid sludge are
described in the following paragraphs. The first three are
irrigation systems and the fourth is a mobile tank application
system subsurface applications methods are described in the final
paragraph. Summaries of certain characteristics of those methods
are given in Tables 19-8 and 19-9.
Spraying
Wastewater sludge can be applied to the land using either fixed
or portable irrigation systems. These systems must either be
designed specifically to handle solids without clogging, or
liquid sludges must be screened. A 1/8-inch (0.32 cm) mesh
rotary strainer will perform satisfactorily.
It is advantageous to spray sludges because operating labor is
reduced, less land needs to be prepared, and a wide selection of
commercial equipment is available. Fixed irrigation systems can
be highly automated, whereas operator attention is required
for portable sprinkler systems. Sprinklers can operate
satisfactorily on rough, wet land unsuitable for tank trucks or
injection equipment.
Disadvantages of spraying sludges include power costs associated
with the use of high-pressure pumps, the potential for aerosol
pollution from entrained pathogens, odors, potential for ponding
of the sludge, and adverse public reaction. Preferred spray
systems direct the sludge toward the ground. Modified versions
of center pivot systems provide for low pressure at the nozzles,
minimizing odors and aerosols. Such designs minimize direct
airborne transport of sludge, control application rates and
distribution, and minimize aerosol formation and transport.
19-29
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TABLE 19-8
SURFACE APPLICATION METHODS AND EQUIPMENT FOR LIQUID SLUDGES
Method
Characteristics
Spray (sprinkler) fixed
or portable
Overland flow and
flooding
Ridge and furrows
Tank truck
Large orifice required on
nozzles; large power and low
labor requirement; wide
selection of commercial equip-
ment available; sludge must be
flushed from pipes when use
stops for longer than 2 to 3
days.
Used on sloping ground with or
without vegetation with no
runoff permitted; suitable for
emergency operation; difficult
to get uniform aerial applica-
tion; use of gated or perforated
pipe requires screening of
sludge prior to application;
sludge must be flushed from
pipes when use stops for longer
than 2 to 3 days.
Land preparation needed; lower
power requirements than spray;
limited to low solids con-
centration (less than 3 percent
works best).
Capacity 500 to 3,800 gallons;
larger volume trucks will re-
quire flotation tires; can use
with temporary irrigation setup;
with pump discharge can spray
from roadway onto field.
Topographical and seasonal
suitability
Can be used on a sloping
land; can be used year-
round if the pipes are
drained in winter; odor
and aerosol nuisances may
•occur.
Can be applied from all-
weather ridge roads.
Between 0.3 and 1.0 percent
slope depending on solids
concentration and
condition of soil. Fill-
able land not usable on
wet or frozen ground.
Tillable land; not usable
on very soft ground.
1 gal = 3.8 1
Overland Flow and Controlled Flooding
Overland flow (wild flooding) and controlled flooding (border
check flooding) are common irrigation techniques. Both of these
use gated or perforated pipe to assure aerial uniformity. DLD
experiments with these techniques on stabilized lagooned sludge
at Sacramento, California (12), indicate that neither resulted in
the satisfactory surface spreading of such sludge. Wild flooding
spread the sludge too far laterally and quite unevenly downslope.
Border check flooding took care of the lateral spreading, but
the downslope could not be adjusted to varying sludge solids
concentrations. Therefore, the sludge either collected at the
top of the sloped field (when there was too little slope for the
percent solids concentration) or at the bottom of the sloped
field (when there was too much slope for the percent solids
concentration). Both flooding techniques resulted in the
accumulation of excessive amounts of sludge on limited areas;
reapplication was thus limited and problems such as odors and
19-30
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vectors were an outcome. In both techniques, clogging problems
were experienced with standard water irrigation gated and
perforated piping. This indicated that either special distribu-
tion piping would be required for use with sludge or the sludge
would have to be screened in a manner similar to that indicated
for sprinkler application.
TABLE 19-9
SUBSURFACE APPLICATION METHODS AND EQUIPMENT
FOR LIQUID SLUDGES
Method
Flexible irrigation hose
(umbilical cord system)
with subsurface injection
or surface discharge3
Tank truck with subsurface
injection or surface
discharge
Farm tank trailer and
tractor with surface dis-
charge3
Farm tank trailer and
tractor with subsurface
injection3
Characteristics
Topographical and seasonal
suitability
Pipeline or tanker pres-
surized supply; 650 ft hose
connected to manifold dis-
charge on plow or disc
pulled by tracked vehicle;
abrasive wear can result in
short hose life; subsurface
injection by means of very
small furrow behind knife-
edge cutting disk and/or
narrow plow; surface dis-
charge into furrow
immediately ahead of plow-
application rate of 50 to
100 wet ton/acre/pass.
500 - 3,800 gallon 4-wheel
drive commercial equipment
available; subsurface
injection by means of very
small furrow behind knife-
edge cutting disk and/or
narrow plow; surface dis-
charge into furrow
immediately ahead of plow-
application rate of 50 to
100 wet ton/acre/pass.
Sludge discharged into fur-
row ahead of plow mounted
on tank trailer - applica-
tion of 170 to 225 wet
ton/acre/pass. Sludge
spread in narrow bank on
ground surface and imme-
diately plowed under -
application rate of 50 to
125 wet ton/acre/pass.
Sludge discharged into chan-
nel opened and covered by
a tillable tool mounted on
tank trailer - application
rate 25 to 50 wet ton/acre/
pass .
Tillable land; not usable on
wet or frozen ground.
Tillable land; not usable on
wet or frozen ground.
Tillable land; not usable on
wet or frozen ground.
Tillable land; not usable on
wet or frozen ground.
Vehicle reaccess to area receiving application dependant on
water content and application rate of liquid sludges.
1 gal = 3.8 1
1 ton/acre = 2.25 t/ha
19-31
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Ridge and_JFur;row
The ridge and furrow sludge application method is similar to that
used in agricultural systems. At the high application rates and
given low solids content, the ridge and furrow method offers
better control than gated or perforated pipe systems used for
overland flow or controlled flooding. Key factors in the success
of ridge and furrow application are the solids concentration of
the sludge, the furrow slope, and the condition of the soil. The
effect of the solids concentration and the furrow slope on sludge
application, determined from a study in Sacramento, California
area (34), is summarized in Table 19-10. Generally, for
a well-stabilized sludge, the furrow slope should be about
0.1 to 0.2 percent per one percent sludge solids concentration,
particularly for sludges which behave like water (less than
3 to 4 percent solids). Sludges with much greater solids
concentrations cannot be successfully surface spread by the ridge
and furrow technique. As long as the soil remains loose and
friable, satisfactory ridges and furrows can be created and
friction losses can be tolerated. Excessive reapplications of
sludges with high moisture contents can create soils which clump.
This makes ridge and furrow construction difficult and increases
friction losses to intolerable levels.
Advantages of ridge and furrow irrigation include simplicity,
flexibility, and lower energy requirements. Disadvantages
include the settling of solids at the heads of furrows, the
need for a well-prepared site with proper gradients, and the
impossibility of maintaining a friable soil. In addition,
ponding of sludge in the furrows can result in odor problems.
Often, ridge and furrow sludge irrigation also involves a
covering operation. This must be carefully considered, laid out,
and tested prior to installation so that maximum efficiency in
application and land use is assured.
Tank Truck Surface Spreading
A common method of liquid sludge surface application is direct
spreading by tank trucks, tractors, and farm tank wagons with
capacities of 500 to 3,800 gallons (2 to 14 m3 ) . Sludge is
spread from a manifold on the rear of the truck or wagon as the
vehicle is driven across the field. Application rates can be
controlled either by valving the manifold or by varying the speed
of the truck.
The principal advantages of a tank truck system are low capital
investment and ease of operation. The system is flexible in
that a variety of application sites, pastures, golf courses,
farmland, athletic fields and the like, can be served. This
permits utilization of sludge, with a dedicated land disposal
system as a reliable backup disposal mode.
19-32
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TABLE 19-10
FURROW SLOPE EVALUATION
Slope, Percent
percent solids*3 Observations0
0.1 3.1 Sludge ponded or flowed
very slowly. On slopes
this flat slight
variations in grade
causing ponding. Gen-
erally unsatisfactory.
.2 3.1 No ponding, sludge flowed
slowly. Minimum grade
for 3 percent solids.
Would be too flat for
5 percent solids.
.3 3.1 Sludge flowed evenly at a
moderate rate. Excel-
lent slope for 3 per-
cent solids.
.4 - .5 2.7 Sludge flowed evenly at a
moderate rate. If
sludge-furrow was not
covered when full all
the sludge would flow
to the low end and pond
a0.1 percent equals 0.1 ft of fall/100 ft
of run. (0.1 m/100 m)
Percent solids expressed determined in a
dry weight basis.
s-~<
All observations are based on 12 in. (.30 cm)
deep furrows. Soil in excellent friable
condition. Deeper furrows would permit the
use of flatter slopes.
Disadvantages of this system include wet-weather problems
and the high operating costs for sludge hauling. Standard,
highway-operable tank trucks are not able to enter sites when the
ground is soft. Consequently, storage or wet-weather handling
alternatives must be available. Another disadvantage is that
truck traffic damages soil structure and compresses the soil,
thus yielding higher bulk densities and reduced infiltration
capaci ties.
19-33
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To maximize disposal time during days'when the site can be used,
a highway vehicle with a 3,000 to 6,000 gallon (11.7 to 22.7 m3)
capacity tank can be 'used ;to transport the sludge to the OLD
site. Sludge is then transferred to one or more off-road
application trucks. These trucks should be equipped with high
flotation tires and four-wheel drive for working wet sites.
Subsurface Injection
Subsurface injection involves a principle of incorporation, which
involves cutting a furrow, delivering sludge into that furrow,
and covering the sludge and furrow, all in one operation.
Modifications include methods in which the sludge is injected
beneath the soil surface or incorporated by use of a disk.
Advantages of incorporation include: immediate mixture of sludge
and soil, elimination of potential odor and vector problems from
ponding, and control of surface runoff. Incorporation procedures
are also favored when sludge utilization is desired, because less
nitrogen is lost from the soil through ammonia volatilization.
The principal disadvantages of incorporation are its complex
management procedures and the fact that the equipment cannot be
effectively used on wet or frozen ground.
19.3.5.2 Dewatered Sludge
Application of dewatered sludge is similar to application
of solid or semi-solid fertilizers, lime, or animal manure.
Sludge can be spread with bulldozers, loaders, graders, or box
spreaders and then plowed or disked in. Spiked tooth harrows
used for normal farming operations may be too light to bury
sludge to the required depth. Use of heavy-duty industrial
discs or disk harrows may be required. Methods and equipment
for application of dewatered sludges are shown in Table 19-11.
Figure 19-2 shows views of Denver Metro's dewatered sludge
landspreading operations.
The principal advantage of using dewa-tered sludge is that
conventional equipment for application of fertilizer and lime
and for tillage can be used. Another advantage is that dewatered
sludge may be applied at higher rates" than liquid sludge.
Problems of flooding and ponding and subsequent site access
associated with the high hydraulic loading rates of liquid
sludge applications are avoided. The disadvantage is the higher
energy and operational costs associated with sludge dewatering
and the treatment required for resulting sidestream. The
disadvantages appear to outweigh the advantages, since dewatered
sludge is infrequently used for OLD.
19-34
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TABLE 19-11
METHODS AND EQUIPMENT FOR APPLICATION OF
DEWATERED SLUDGES
Method
Characteristics
Spreading Truck-mounted or tractor-
powered box spreader
(commercially available);
sludge spread evenly on
ground; application rate
controlled by over-the-
ground speed; can be
incorporated by disking
or plowing.
Piles or win-
drows
Reslurry and
handle as in
Tables 19-8 and
19-9.
Normally hauled by dump
truck; spreading and
leveling by bulldozer or
grader needed to give
uniform application; 4
to 6 inch layer can be
incorporated by plowing.
Suitable for long hauls by
rail transportation.
19.3.5.3 Sludge Application Rates
Sludges should be applied such that soils can dry sufficiently
between sludge applications to allow the passage of sludge
distribution vehicles. Sludge application does not create excess
leachate or runoff. Application should also be managed so that
the soil does not become anaerobic and generate odors.
Adverse moisture conditions can be avoided for the most part if
sludge application rates are not allowed to exceed the net soil
evaporation rate (that is, evaporation minus precipitation).
Using this guideline, water should be removed by evaporation as
rapidly as it is added with the sludge and the fields should dry
out prior to subsequent sludge applications. Since on the
average, all water is removed by evaporation, none should remain
to percolate or become runoff. The environmental hazard and
operating costs associated with controlling these streams
are thus minimized. Given this premise, sludge should be
applied only when the net soil evaporation rate is positive.
The Colorado Springs case example discusses this approach.
Operations will tend to be seasonal, intensive during warm, dry
conditions and slowed down during wet or cold conditions. Sludge
application must, of course, be terminated when the ground is
frozen.
19-35
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The Denver Metro uses large trucks to transport dewatered sludge cake to its rela-
tively isolated disposal site at the former Lowry Bombing Range 25 miles (40 km)
from the treatment plant. This picture shows transfer of sludge to smaller dump
trucks for spreading in the field. Sludge is spread by allowing it to drop from the
truck as it is driven through the field. At one time the District used a manure
spreader instead of dump truck for sludge spreading purposes.
After spreading, sludge is incorporated into the soil by
plowing with this 6-bottom, 2-way moldboard plow.
Annual application was about 30 dry tons per acre
(67 dry t/ha) in 1976. Nine or ten months later, the
same land received another application of sludge.
FIGURE 19-2
DEWATERED SLUDGE LANDSPREADING, METROPOLITAN
DENVER SEWAGE DISPOSAL DISTRICT NO. 1,
DENVER, COLORADO
19-36
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It should be noted that runoff and leachate controls are still
required even though the system, on the average, eliminates all
water by evaporation. Leachate and runoff must be expected,
since periods will occur when the net soil evaporation rates are
less than expected or where more sludge than permissible is
applied.
Organic rather than hydraulic limitations may govern,
particularly when dewatered sludges are applied. Odors can
develop if the soil/sludge layer does not remain aerobic.
Maintenance of the aerobic condition depends on rate of sludge
application, the sludge to soil ratio, temperature, and frequency
of soil turning or disking.
19.3.6 Environmental Controls and Monitoring
In general, environmental controls for dedicated land disposal
are not as severe as those for sludge utilization. The basic
requirement is that activities do not cause any nuisance
off-site. Control of all transport mechanisms for potential
pollutants, specifically via surface and groundwater, and through
aerosols and odor is required. If the sludge is well stabilized,
vector controls will be negligible.
19.3.6.1 Site Layout
Good site planning is the key to environmental pollution control
for OLD. Initial site selection should be based on slope, soil
type, and isolation possibilities from ground and surface water.
Subsequent detailed planning can significantly enhance final
environmental control measures.
Division of the OLD site into several fields is desirable for
operational and environmental controls. Individual fields should
be in the range of 10 to 100 acres (4 to 40 ha), and 50 acres
(20 ha) is typical. For the umbilical cord subsurface
injection method, a minimum dimension of 1,300 feet (400 m)
is desirable. This will allow a tractor dragging a 650-foot
(200 m) hose to cover a field, side-to-side, when the sludge
hydrant is located in the center of the field. Smaller sites are
more amenable to the use of tank vehicle systems.
The breakdown of the site into smaller areas will permit easier
terracing. First, fields with fairly uniform elevations must
be chosen, and slopes must then be regraded for the chosen
application method.
Beyond the site subdivision, plans for larger OLD systems should
include a layout of "nurse centers." These are take-off points
on a fixed distributional system for re-filling application
trucks in order to minimize their unproductive travel time
and undesirable extra field compaction. They also serve as
19-37
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hookups to the tractor-drawn umbilical cord system. Usually, the
nurse centers consist of a small (4- to 6-inch [10 to 15 cm] in
diameter) sludge force main riser with a quick connect coupling,
coming from a pump station or dredge operating in a sludge
storage lagoon. A small storage tank or vault (maximum volume
twice the tanker capacity) is often added at the nurse center to
simplify pumping control and to permit sludge pickup by a field
tanker by means of suction. Sludge should not be allowed to
remain unmixed in the vault for more than 30 minutes. Mixing of
the tank contents will prevent liquid-solid separation, which
could cause wide variations in solids concentrations at pickup
and, therefore, uneven solids application rates to the site.
19.3.6.2 Groundwater Controls
There are two distinct kinds of groundwater control for OLD
sites. The first involves complete collection of any and all
leachate from the site followed by either recycling back to
the treatment plant or further on-site treatment. The second
involves monitoring groundwater migration patterns from the
site and assuring that the quality of external waters are not
reduced.
In the case of the first, the site should be underlain with an
impervious soil, hardpan, or rock. Although it is possible to
prepare this barrier artificially using clay or a liner, it is
usually not economically feasible to do so. Thus, the original
site selection determines the degree of vertical containment.
Horizontal movement of groundwater is prevented by the use
of diking and cutoff trenches. Leachate is then collected
together with surface runoff.
In the case of the second, extensive surveys may be necessary to
determine natural groundwater migration patterns. The direction
of leaching must be determined. Design should be such that final
concentrations of potential pollutants either in the off-site
groundwater or in surface water do not exceed contamination
guidelines preset by the applicable regulatory authority.
19.3.6.3 Surface Water Runoff Controls
Each OLD site should be graded such that all surface runoff would
drain toward one point near the edge or toward the corner of
the field. Each site should be surrounded by a berm to keep
uncontaminated surface runoff out and to contain contaminated DLD
runoff. A center drain should either direct the contaminated
runoff back to the nearest manhole on a facultative sludge lagoon
supernatant system, or be connected to a pump which directs the
runoff back to the treatment plant or to a separate on-site
treatment system. Temporary holding of the runoff to permit
settling of settleable solids and monitoring may be desirable.
If the stored water is found to be of sufficient quality to meet
discharge standards, it can be released without any treatment.
19-38
-------
The primary mechanism for water removal at most sites is
evaporation. Runoff can be minimized by adjusting sludge
loadings so that they are less than or equal to the net soil
evaporation rate (evapotranspiration rate minus precipitation).
Runoff control can be aided by disking in the sludge soon after
application, thereby preventing downward movement of the liquid
sludge.
19.3.6.4 Air Pollution Control
Two air pollution concerns are aerosol transport and odor.
There must be adequate buffer zones around the OLD site.
Operationally, systems which minimize the length of time sludge
is directly exposed to the air are preferred. It is possible to
incorporate special design features for air pollution control,
for example, vacuum stripping of the digested sludge to remove
odors prior to land application.
19.3.6.5 Site Monitoring
Monitoring requirements for OLD are relatively straightforward.
Groundwater monitoring is essential and should be conducted
from a pattern of groundwater wells located primarily at the
downstream boundary of the site. In addition to groundwater,
collected leachate and surface water runoff streams must be
monitored to determine if and when such streams must be treated.
For air pollution control, olfactometer measurements (see
Chapter 17) could be taken regularly, particularly during calm
periods and preceding and during times of air inversions. If
odors are a major problem, operations could be stopped during
periods of calm winds and temperature inversion.
19.3.7 Costs
Extensive cost data are not available on OLD. Cost estimates
are, however, available from a new system developed at Colorado
Springs, Colorado, and a large prototype system at Sacramento,
California. These cost estimates are discussed in the case
examples to follow. These DLD costs are quite site-specific, and
extrapolations from the Colorado Springs and Sacramento cost data
should be made with caution.
19.3.8 Case Examples
The relatively recent acceptance of dedicated land disposal
makes the selection of case examples limited, particularly for
small plants. Colorado Springs, Colorado, a medium-sized system,
and Sacramento, California, a large system, are discussed in the
following sections.
19-39
-------
19.3.8.1 Colorado Springs, Colorado
The analyses for and design of a sludge management program for
Colorado Springs was based on population and average dry-weather
flow figures (see Table 19-12).
TABLE 19-12
COLORADO SPRINGS POPULATION AND WASTEWATER
FLOW PROJECTIONS
Year
1978
ir-90
2005
Ultimate
Population,
thousands
230
330
440
—
ADWF , mgd
25
36
48
60
Planning
designation
Present
Phase I
Phase II
Phase III
An interim sludge management system employs anaerobic digestion
of primary and waste-activated sludge at the wastewater treatment
plant site. 150,000 gallons (570 m3 ) of digested sludge of
2.5 percent solids concentration is produced each day. The
sludge is trucked from the treatment plant site to two 5-acre
(2.0 ha) 15-feet (4.6 m) deep temporary storage lagoons located
20 miles (32 km) away. The sludge is later removed from the
lagoons by two special four-wheel drive high flotation-tired tank
vehicles equipped with suction devices and subsurface injected on
an adjacent dedicated land disposal site. Capacities of the
subsurface injection (SSI) vehicles are 3,600 and 3,800 gallons
(13.6 and 14.4 m3).
The Colorado Springs sludge management system is being
substantially modified and upgraded (14). A schematic of the
modified system is shown on Figure 19-3, and an overall layout
of the sludge disposal site on Figure 19-4. Estimated capital
and operating costs for the various facilities are shown in
Table 19-13. . ' .
The soils at the DLD site consist of Verdos Alluvium, Piney Creek
Alluvium, and a weathered Pieere Shale having low to very low
permeabilities, in the range of 1.0 x 10~4 £o ]_. Q x io~6 crn per
second.
•?•,",;-• ' •-'•».. •, "
Monthly average temperatures range from 29°F to 71°F (-1°C to
22°C). Effective soil evaporation occurs to a depth of about
2 feet (0.6 m), and moisture profiles from SSI test sites show
a maximum downward migration of moisture to,a depth of 22 inches
(57 cm), after application of liquid sludge.
19-40
-------
COLORADO SPRINGS
WWTF
THICKENERS
! i i
DUAL SLUDGE PIPELINE
COLORADO SPRINGS WWTF
TO HANNA RANCH
20 MILES
ANAEROBIC DIGESTION
FACULTATIVE SLUDGE
BASINS (FSB)
AGRICULTURAL I
REUSE BY [
SUBSURFACE [
INJECTION f
(AG/SSt) t
SUPERNATANT
TREATMENT
AT
HANNA RANCH
DEDICATED LAND
DISPOSAL BY
SUBSURFACE
INJECTION
(DLD/SSO
i
ON-SITE REUSE
THE TERM FACULTATIVE SLUDGE BASIN (FSB) IN USED INTERCHANGEABLY
WITH FACULTATIVE SLUDGE LAGOON (FSL)
FIGURE 19-3
FLOW DIAGRAM SLUDGE MANAGEMENT SYSTEM, COLORADO
SPRINGS, COLORADO
19-41
-------
4» ^tf*.
.21 /4H i.
FIGURE 19-4
OVERALL SLUDGE DISPOSAL SITE LAYOUT
COLORADO SPRINGS, COLORADO
w " &""—"*»—l»<*l«t
-------
TABLE 19-13
COLORADO SPRINGS PROJECTED COST OF SLUDGE
MANAGEMENT SYSTEM
Phase cost, thousand dollars3
Item i ii
Capital cost
Raw sludge conveyance system 3,552 98 98
Anaerobic digesters 5,539 2,289 2,313
Facultative sludge basins 3,924 1,236 2,068
Subsurface injection system" 1,696 756 841
Supernatant lagoons 461 - 75
Supernatant treatment facility ' 1,681 217
Subtotal, capital cost 16,853 4,596 5,394
Engineering and contingencies0 5,899 1,609 1,888
Total, capital cost 22,752 6,205 7,282
Present worth^
Capital costf - 22,473d N/Ae
Operation and maintenance cost"? - 8,048 N/A
Total, present worth of project
cost - 30,521 N/A
Equivalent annual cost - 2,881 N/A
3Costs based on an ENR cost index of 2600, March 1979, Denver.
SSI system includes FSB dredge; harvested sludge distribution pumps, piping and nurse
tanks; SSI tank vehicles; and site preparation including grading, cutoff trenches and
monitoring facilities, but excluding land costs, which were approximately $1,400 per
acre in 1972.
CAllowance for engineering and administrative expense and contingencies is based on
35 percent of construction cost.
Present worth costs based on an interest rate of 7 percent and projected construction
dates of Phase I and II facilities for a 20-year planning period.
ePhase III not included—beyond 20-year planning period.
'Salvage values based on assumed life of equipment and computed on straight-line
depreciation.
gBased on uniform series present worth for fixed costs and gradient series for variable
costs.
1 acre = .91 ha
Note: The term facultative sludge basin (FSB) is used
interchangably with facultative sludge lagoon (FSL).
There were no groundwater supplies which could be endangered on
the 160-acre (65 ha) disposal site or the immediately adjacent
areas. However, to provide maximum protection of the environ-
ment, the system was designed to minimize percolate production.
The design approach was to match sludge application and net
soil evaporation rates. Net soil evaporation calculations are
presented on a month-to-month basis in Table 19-14. Note that
gross soil evaporation was estimated to be a fixed fraction
(70 percent) of the evaporation which would occur from a free
water surface (a lake).
19-43
-------
TABLE 19-14
COLORADO SPRINGS CLIMATIC CONDITIONS AFFECTING
SLUDGE DISPOSAL
Month
Lake
evaporation
a,b
January
February
March
April
May
June
July
August
September
October
November
December
Annual
Precipitation
5.15
6 .44
7.62
8.26
6.99
5.39
4 .13
43 .98
0
0
1
1
2
2
3
2
1
1
0
0
.71
.73
.56
.91
.14
.16
.00
.32
.55
.11
.95
.67
18 .81
Net lake ,
a . c . a ,d
i evaporation
-0
-0
-1
3
4
5
5
4
3
3
0
0
25
.71
.73
.56
.24
. 30
.46
.26
.67
.84
.02
.95
.67
.17
Net soil3'6
evaporation
1.
2.
3.
2.
2.
2.
1.
16.
_
_
70
37
17
78
57
22
78
_
-
59
All values shown in inches.
Developed from "Interim Study of Land - Incorporated Sewage
Sludge" at Colorado Springs, Colorado, December 1978, by
Waste and Land Systems, Inc.
Q
Includes precipitation and snow assuming 10 percent of snow depth
is equivalent to precipitation depth in inches.
Lake evaporation minus precipitation.
Q
Estimated net soil evaporation based on 70 percent of lake
evaporation less precipitation.
1 in. = 2.54 cm
Allowable sludge application rates were calculated on a monthly
basis for the months of April to October, assuming a sludge
content of 5 percent. Results of this analysis, shown on
Figure 19-5 indicated total allowable sludge application on a
6-month and 7-month operating basis to be 86.0 and 95.8 dry tons
per acre (193 to 215 t/ha), respectively. The range of required
land area for the more restrictive 6-month period is shown on
Figure 19-6. Area required for average loadings (14.3 tons per
acre per month [32.1 t/ha-mo]) is shown by the "average" curve.
Area required if the sludge could be applied for all the
months at the maximum June rate (18.6 tons per acre per month
[41.7 t/ha-mo]) is shown by the "low range" curve. Similarly,
area required if all the sludge were applied at the minimum
October rate is shown by the "high range" curve. A second
analysis shown on Figure 19-7, indicated the range of area
requirements based on variations in sludge solids content of 4 to
6 percent, using average solids loadings (14.3 tons per acre per
month or 32.1 t/ha-mo).
With respect to surface water controls, cutoff ditches will be
constructed to prevent surface runoff from the disposal site.
The injection pattern will be parallel to the contours of the
area to reduce the potential for soil erosion and surface runoff
19-44
-------
from sludge-amended soils. To the south, the entire sludge
disposal area is contained behind a retention dam designed
to prevent runoff from reaching an existing ash disposal site.
This dam, designed for a flood level equivalent to a once in a
1,000-year recurrence interval, provides containment of both
surface runoff and upstream percolate. Although the operation of
the tank vehicle SSI system was based on a well-defined DLD area
with ground slopes typically 3 to 6 percent, portions of the mesa
area which have slopes of less than 10 percent can also be used
for injection. The maneuverability and freedom of movement of
the detached vehicles allows maximum site utilization.
**
*
II
O)
i
Q
Q
|
_i
Q.
UJ
O
10
FES APR JUN AU6 OCT
MONTHS
DEC
(1 t/acre = 2.24 t/ha)
FIGURE 19-5
SLUDGE APPLICATION RATE-OLD SYSTEM,
COLORADO SPRINGS, COLORADO
19-45
-------
1ft
O
UJ
o:
Q
D
O
o
LLJ
Q
ISO i-
140
120
ICO
BO
60
40
20
NOTE: BASED ON SSI OPERATING
PERIOD MAY THROUGH
OCTOBER, ASSUMING
SLUDGE APPLIED AT
5 PERCENT SOLIDS
1970
1980
I960
2030
2010
2020
2030
YEARS
FIGURE 19-6
ESTIMATED NET OLD AREA REQUIREMENTS SLUDGE APPLIED
AT 5 PERCENT SOLIDS CONCENTRATION,
COLORADO SPRINGS, COLORADO
The operation of the DLD/SSI system commences with harvesting of
the sludge from the facultative sludge basins (FSBs) (faculative
sludge lagoons [FSLs] ) are referred to as facultative sludge
basins at Colorado Springs) (15). The sludge is transferred from
the basin to a sludge receiving/distribution station by a dredge
equipped with a diesel-driven pump. From the station, the
harvested sludge is conveyed through a distribution system
consisting of 12-inch (30 cm) diameter pipes to a series of DLD
nurse tanks. The fiberglass nurse tanks are each 7,500 gallons,
twice the volume of the SSI vehicle tank. The nurse tanks are
19-46
-------
I®
£
IP
O
o
II
LU
O
o.
tft
Q
Q
Q
LU
I-
o
o
LU
Q
160
140
120
100
80
60
40
20
0
NOTE: BASED ON SSI OPERATING
PEBIQD MAY THROUGH
OCTOBER AND SLUDGE
SOLIDS CONTENT AS SHOWN
1970 1980
1990 2000 2010
YEARS
2020
2030
FIGURE 19-7
ESTIMATED NET OLD AREA REQUIREMENTS AT VARIOUS
SLUDGE CONCENTRATIONS, COLORADO SPRINGS, COLORADO
buried below ground and protected with a concrete slab on grade.
A steel pipe fitted with a gate valve and couplings extends from
the bottom of the tank to above the ground surface to feed the
SSI vehicles. The harvested sludge distribution system is valved
to allow any combination or number of nurse tanks to be placed
into service. The network is designed to allow approximately
1,000 lineal feet (305 m) of injection area between nurse tanks
to optimize the injection operation and minimize downtimes caused
by travel with empty tanks. Depending on climatic conditions,
19-47
-------
O
in
m
CL
a.
£
u
o
a.
CL.
s
D
o
1.50
1.25
1,00
0.75
BASIS:
1. INJECTION VEHICLE MOVING AT 1.5 mph (2,4 kWhr)
2. SLUOCi SOLIDS CONTENT OF 5 PERCENT
0-50
0.25
6.0
5.0
4.0
3,0
2.0
Q
E
I
V!
o
z
O
1.0 y
0.
CL
g
.j
o
300 350 400 450 500 550 600 §50 100
SLUDGE INJECTION RATE, gpm (1 gpm = 3.78 l/min)
FIGURE 19-8
SLUDGE APPLICATION RATES BY SUBSURFACE INJECTION,
COLORADO SPRINGS, COLORADO
the sludge injection cate can be adjusted to correspond with the
soil conditions in the injection area and will vary through
the sludge application season, as shown on Figure 19-5. The
relationship between sludge injection rate and solids application
rate on the basis of both liquid sludge and dry solids is shown
on Figure 19-8. Based on the estimated turnaround time for tank
refilling and normal maintenance, a net injection time of about
3 hours per day per vehicle can be expected. One dredge can
harvest sludge from the FSLs at a rate sufficient to feed two SSI
vehicles. Equipment requirements and operating characteristics
are shown in Table 19-15.
While the DLD/SSI system for Colorado Springs is designed as a
base disposal system, it can be used as a secondary, or utiliza-
tion, option without significant additional expense. Eventual
agricultural utilization of a major portion of the sludge
production is, in fact, a defined goal of the chosen system. See
Chapter 3 for discussion of base and secondary disposal options.
19-4!
-------
TABLE 19-15
COLORADO SPRINGS DEDICATED LAND DISPOSAL/
SUBSURFACE INJECTION SYSTEM DESIGN DATA
Item Phase I Phase II Phase III
Facultative sludge basins (FSBs)
Basin dredge
Number 1 23
Maximum capacity, gpm 1,400 1,400 1,400
Solids capacity, percent
Maximum 888
Average 5 55
Pumping head, feet *; 65 65 65
Diesel engine power, hp : 175 175 175
Dedicated land disposal (OLD) ;
Harvested sludge application
Quantity, dry tons per day3 43.2, 58.4 76.7
Volume, gpda'b 203,790 274,360 360,440
Percent volatile 50 50 50
Average percent solids 5 '-5 5
Average annual application
6-month operating period, dry tons
per acre 86.0 36.0 86.0
7-month operating period, dry tons
per acre 95.8 95.8 95.8
OLD area required, acres
Maximum 85 115 150
Average ... 60 85 110
OLD distribution system
Nurse tanks
Number . 12 18 24
Capacity, each gal . 7,500 • 7,500 7,500
SSI vehicles
Number 2 45
Tank capacity, each gal 3,600 3,600 3,600
Injection rate, gpm
Maximum 700 700 700
Average . ' - '. 500 500 500
Average vehicle speed,' mph 1.5 1.5 1.5
Injection width, feet .-.. 12 12 12
Volume injected, gallons per vehicle
per day
Maximum 116,000 116,000 116,000
Average 100,000 100,000 100,000
Vehicle coverage, acres per vehicle
per day 6.5 6.5 6.5
Tillage tractors
Number ., 1 11
Assuming 120 day per year operation.
Assuming 5 percent solids.
Note: The term facultative sludge basins (FSBs) is
used interchangably with facultative sludqe
lagoons (FSLs) . . . '; ,
1 gpm = 0.06 1/s
1 ft = 0.30 m
1 hp = 746 W
1 ton/day = .91 t/day
1 gpd =3.8 I/day , ,
1 ton/acre = 2.24 t/ha • -,
1 acre = .405 ha
1 gal =3.81
1 mph = 1.61 km/hr . , , ,
1 gal/vehicle =,.3.8 I/vehicle .
1 acre/vehicle = .405 ha/vehicle
19-49
-------
19.3.8.2 Sacramento, California
Sacramento, California has been the site of much of the work
associated with the development of dedicated land disposal
technical criteria. The Regional Wastewater Treatment Plant
Sludge Management Program for the Sacramento Regional County
Sanitation District was approved by the Regional Board of
Directors in January 1979 (15) and the Environmental Impact
Report (EIR) (6) was certified at that time. The sludge planning
period for the treatment plant is divided into two phases;
Stage I includes operations to be conducted from 1980 to 1992,
and Stage II is devoted to operations for the period 1992-1999.
The sludge management program was approved after 3 to 4 years of
monitoring and detailed investigations directed primarily
at determining the engineering, economic, and environmental
aspects of storing liquid anaerobically digested sludge in solid
storage basins (SSBs) (12). Precise operational and design
criteria were developed for the Sacramento SSB/DLD system to
assure efficient operation and environmental acceptability. Most
investigative work was conducted on a large prototype SSB/DLD
subsurface injection system and therefore did not suffer the
problems normally associated with scaling up a pilot system.
LJlv * *~" ' * M *y , ' 5
FIGURE 19-9
PROTOTYPE DREDGING OPERATION, SACRAMENTO
REGIONAL COUNTY SANITATION DISTRICT
Initial work commenced in 1974. Site preparation included
installation of groundwater monitoring wells. The prototype
20-acre OLD system has been in full operation since 1976, and
data has been collected and analyzed for 1976 through 1978 and
for part of 1979. Figure 19-9 illustrates the prototype dredging
19-50
-------
operations at Sacramento, while Figure 19-10 illustrates
prototype subsurface injection operations with a close-up view of
the injector units.
The sludge applied to the Sacramento DLD site has been
anaerobically digested and then subjected to long-term storage in
the SSBs. Application rates were planned at 100 dry tons/acre
(224 dry t/ha); rates of 97 tons/acre (217 t/ha) were achieved
without problems in the 1977 tests. The application rates are
controlled by the water content of the sludge removed from the
SSBs, since DLD operates primarily on a solar evaporation basis.
New equipment installed at sludge removal operations in 1979 has
increased the solids concentration to over 6 percent, with
better than 8 percent achieved for several hours. It is expected
that when the FSLs are fully developed, an average harvested
sludge concentration of 6 percent can be sustained. The
following text discusses the final DLD subsurface injection
system for Sacramento based on experience gained over the 1976 to
1979 period.
Table 19-16 shows projected flows and loadings for the Sacramento
wastewater treatment plant for 1985. Figure 19-11 is a flow
diagram of the solids treatment and disposal system. The numbers
thereon give the solids flow in dry tons per day for operations
through 1992.
TABLE 19-16
SACRAMENTO REGIONAL WASTEWATER TREATMENT PLANT
PROJECTED 1985 WASTEWATER FLOW AND LOADINGS
Parameter Value
Seasonal3
ADWF,b MGD 136.2
BOD5, 1,000 Ib/day 243.3
Suspended solids, 1,000 Ib/day 246.3
Nonseasonal
ADWF, MGD 122.7
PWWF,C MGD 248.7
aSeasonal = canning season, mid-June to
mid-October.
ADWF = average dry-weather flow.
CPWWF = peak wet-weather flow.
1 Ib/day - 0.454 kg/day.
1 MGD = 0.044 m3/sec.
19-51
-------
View of tractor pulling sludge injector units
Close up view of prototype sludge injector units
FIGURE 19-10
PROTOTYPE SUBSURFACE INJECTION OPERATIONS
SACRAMENTO REGIONAL COUNTY SANITATION DISTmCT
19-52
-------
RECQMMiNOEQ PROJECT
GAS
APID BOTO5TPAINERS
RECYCLE
TO
PLANT IN FALL
AWDW NTER
PERIODS
^ ON I _. ^ " "'.'„'" '"1.
^^""^P*1" r s L * fc H c L* IN
^° ACT: VATE D CAR
:ABBON
EVAPORATION
ssa SUPER it AT AN r
\
114 SURFACE ACRES
SSB'sCFFER
- TERM
E AND
EVAPORATION
WINTER RUNOFF
RECYCLE TO
REGIONAL PLANT
DECOMPOSITION
HARVESTED SLUDGE
11,500 tans Ktlids .' year (98% solidi I
LAND DISPOSAL- 1 SB ACHES
HIGHLY STA61L! JED SLUOGE IS INCORKJflATED INTO
THE SURFACE SOU, lAY«S FQBQRVlNG . CONTINUED
AEROBIC DECOMPOSITION OCCURS OVEfi TIME.
SITE IS REUSED ANNUALLY.
SIX OP V WONTHS
THE TERM 'SOLID STORAGE BASINS' (SSB's) IS USED INTERCHANGEABLY
WITH FACULTATIVE SLUDGE LAGOONS (FSL'sl
(1 ton/yr = 0.91 t/yrl
(1 acre = 0.405 ha)
FIGURE 19-11
FLOW DIAGRAM - PROJECTED 1992 NORMAL SOLIDS
TREATMENT AND DISPOSAL OPERATION, SACRAMENTO
REGIONAL WASTEWATER TREATMENT PLANT
19-53
-------
The flow schematic indicates that not all the sludge will be
managed by the SSB/DLD subsurface injector system. Through
the 1980s, there will be sufficient furnace capacity in an
incinerator (designed for screenings, grit, and scum) to handle
about 30 percent of the primary sludge production. A total of
one month per year shutdown of the incinerator was assumed,
two weeks for maintenance during the time of low solids
production in spring, and two weeks miscellaneous upset.
Estimated sludge production rates at the Sacramento plant are
given in Table 19-17.
TABLE 19-17
SACRAMENTO REGIONAL WASTEWATER TREATMENT PLANT
PROJECTED DIGESTED SLUDGE PRODUCTION
Estimated
solids
parameters
Solids
concentration,
percent
Volatile
solids,
percent
Digested solids, with
incinerators operating
Average annual 2.2-2.5
Maximum seasonal 2.4-2.7
Digested solids, with
incinerators not
operating
Average annual 2.5-2.7
Maximum seasonal 2.6-3.0
Average annual SSB solids
removed^ 6
Total solids production, ton/day
Stage I
1980
1985
1992
Dry
Wet
Dry
Dry
57-59 51.2 2,278 54.2 2,366 67.0 2,818
57-62 70.0 2,715 74.4 2,810 81.5 3,340
57-60 65.0 2,500 70.0 2,600 77.4 3,070
58-63 81.2 2,860 89.1 3,000 92.7 3,550
43.0 38.6
643 40.8
679 50.7
844
Stage II
1999
Dry
75.5 3,035
94.0 3,640
85.0 3,380
107.0 3,860
57.0
951
The term solid storage basins (SSBs) is used interchangably with facultative sludge lagoons (FSLs).
Actual daily removal rates are higher, since solids are harvested for only part of the year (May-October).
1 ton/day =2.24 t/ha
The layout of the existing and future dedicated land disposal
sites in relation to the Sacramento Regional Wastewater Treatment
Plant is shown in Chapter 15, Figure 15-9.
Operation of the OLD system is practiced from May through
October. Several methods of sludge application were tested, and
for the prevailing conditions at the Sacramento site, subsurface
injection utilizing a flexible hose and injection unit mounted
behind a crawler-tractor fitted with extra-wide tracks worked
best.
Two dredges will take care of operations through 1992, (Stage I)
dredging solids at about a 6 percent solids concentration
from the lower depths of the SSBs and pumping the sludge to the
DLD site. Booster pumping is required to pump 6 percent solids
19-54
-------
material over the maximum 8,000-foot (2,440 m) distance.
Four-inch (10 cm) diameter flexible hoses connect the tractor-
injectors with hydrants located throughout the DLD sites. Four
tractor-injectors are needed to handle the two-dredge disposal
operation. In normal operation of these facilities, freshly
applied solids remain unexposed to the atmosphere. The DLD sites
are loaded at 100 dry tons to the acre (224 dry t/ha) each
season. Sludge is supplied to approximately match the net soil
evaporation rate. The soil evaporation rate in Sacramento is
about half the evaporation rate which occurs from a free water
surface (lake evaporation rate). Stage I DLD operations will
utilize 185 acres (75 ha) in five 37-acre (15 ha) sites. Regular
disking of the site is necessary to break up the soil/sludge
surface and expose more of the loaded soil to the atmosphere.
Existing subsoils are fairly impervious and are underlain by
hardpan. The local groundwater supply is not endangered. Free
groundwater was measured at depths of 13 to 46 feet (4.0 to
14.0 m), with an average depth of 31.6 feet (9.7 m) for nine
borings. The aggregate coefficient of permeability for the
composite layered interval of the surface soils is on the order
of 5 x 10~8 cm/sec. Cemented strata were encountered in the
borings at depths ranging from 5 to 10 feet (1.5 to 3.0 m), with
thicknesses of approximately 12 to 21 feet (3.7 to 6.4 m) and
permeability coefficients of 2.2 x 10~8 cm/sec to 3.7 x 10"10 cm/
sec. Effective sealing of the surface soils from vertical
leachate movement to groundwater is thus achieved (6,12).
Increases in the concentrations of nitrates and chlorides have
not been observed below the impervious strata (12).
Runoff is collected in detention basins and returned to the
regional plant influent after storm flows have subsided. Some
data on DLD runoff water quality are given in Table 19-18.
TABLE 19-18
SACRAMENTO TEST DLD RUNOFF WATER ANALYSIS
Constituent : 12/18/77 12/28/77 1/05/78 i/09/!!
Zn, mg/1 0.05 0.05 0.25, 0.12
Cu, mg/1 0.050 0.043 0.101 0.064
Cd, mg/1 0.001 0.001 0.001 0.001
Ni, mg/1 , ; ; ..0.090 . 0.090 0.16 0.078
Pb, mg/1 . 0.014 0.008 . 0.016 , 0.028
.Hg, mg/1 , ' . ' 0^0001 0.0001 , 0.0002 0.0002
TKN,a mg/1 ' 30 ,24 17 7.6
Turbidity, NTU ' ' " • 3.0 1.5 170 . .37
TSS, mg/1 •'.'•. : :. 26 - 16 442 38
EC x 103 4.0 3.6 1.9 1-1
pH 7.2 8.4 7.4 7.4
NO,, mg/1 440 310 43 31
NH^, mg/1 ••..:'• 36-' ' 5 • •• • .2 . • . • 1
aTotal Kjeldahl Nitrogen.
19-55
-------
For runoff control, typical OLD sites are sloped transversely at
a maximum of 0.5 percent and spread outward in both directions
from the centerline. A longitudinal slope of 0.1 to 0.2 percent
is also provided. Runoff drains from each OLD site to ditches on
both sides. To prevent erosion, the maximum field slope will be
held to 0.5 percent and water velocity in the ditches will be
limited to 5 feet per second (1.5 m/s). Runoff from the ditches
is collected in a fairly flat (0.1 percent transverse slope)
detention basin, one per OLD site; the basin is situated
approximately 3 feet (0.9 m) lower than the main operational part
of the DLD site. Each basin has a capacity of 12.7 acre-feet
(123 m^) and a maximum depth of 2 feet (0.6 m). The basins are
designed to contain two 4-inch (10 cm) 24-hour storms (the
25-year maximum rainfall for two 24-hour periods). The runoff is
drained from the basins via corner inlet structures fitted with
controlled release rate weirs and is transferred through a
21-inch (0.53 m) runoff return pipe to a flow metering structure.
Then the runoff is drained to an interceptor sewer connected
to the wastewater treatment plant. A flood control levee is
constructed on the lowest three sides of the SSB/DLD area such
that the entire site is protected from flooding. Provision
is also made for the collection, retention, and pumping of
uncontaminated site runoff trapped by the flood control levee.
In this regard, facilities (including a pump station) are
designed to handle the same storm conditions (two 25-year,
24-hour storms, one day apart) as are runoff facilities for
the DLD sites. A runoff of 80 percent is assumed based on a
saturated ground condition.
Final DLD sites have a gross area of about 50 acres (20 ha),
including space for drainage, road access, and injector turning.
As indicated earlier, this results in a net usable area of
37 acres (15 ha) for each of the five sites. Each site is
approximately 1,300 feet (400 in) wide, which allows a tractor
dragging a 650-foot (200 m) hose to cover the entire width of
the field. The sludge hydrant is then located in the center of
the field. Sites are approximately 1,600 feet (560 m) long,
calculated from the area required to allow two injectors to
operate continuously on the same field 6 hours per day, 5 days
per week, during the peak dry months of June, July, and August.
Peak dry month operation assumes sludge removed from the FSLs can
be applied to the same site twice a week. During May, September,
and October, it is assumed the application of sludge removed from
the SSBs will be limited to one once a week. Thus, application
rates during June, July, and August are approximately double
(10 inches per month [25 cm/month]) the rates of May, September,
and October (5 inches per month [13 cm/month]). Each DLD site is
provided with six field hydrants for injector feed connection,
located at 230-foot (70 m) centers down the middle of each site.
The field hydrants each have a foulproof pressure sensing device,
a manual isolation valve, and a swivel-elbow assembly designed
for quick coupling to a 4.5-inch (11 cm) injector feed hose.
19-56
-------
Operationally, SSB sludge removal piping is flushed with FSL
supernatant at the end of each week's run with the flushing water
returned to the FSLs. Sludge removal operations themselves are
restricted to reducing the water level in the FSLs no more than
14 inches (36 cm) below normal operational levels. The water
level is never allowed to drop low enough to expose the sludge
blanket. The blanket is maintained below its maximum elevation
which is another 10 inches (25 cm) below the absolute minimum
water level.
Key DLD equipment for Sacramento includes:
• Two SSB dredges, each generating 1,400 g p m
(7,630 m^/day) average flow at 6 percent solids
concentration.
• Two 200- to 250-horsepower (150 to 187 kW) diesel powered
floating booster pumps connected to the dredges with
variable speed pump operation to compensate for
variations in sludge solids concentrations.
• Four 60- to 80-horsepower (45 to 60 kW) crawler-tractors
with 30-inch (0.76 m) wide tracks and nine to ten rear-
mounted subsurface injector sweeps, each with 2-inch
(5 cm) diameter feed hoses. Path width is 13 to 14 feet
(4.0 to 4.2 m), speed 1.0 to 1.5 miles per hour (1.6 to
2.4 km/hr), and average capacity 700 gpm (3,800 m3/day)
each.
• One four-wheel drive, rubber-tired, 150-horsepower
(112 kW) tillage tractor with heavy disk unit which can
be raised clear of the ground.
Staffing requirements for full Phase I DLD operations are
expected to reach 11 people on a 6-month seasonal basis, May
through October, to remove the sludge from the SSBs and inject it
into the soil at the five DLD sites. Personnel needs are given
in Table 19-19. Fifteen other full-time personnel are needed for
the whole solids processing and disposal system exclusive of
anaerobic digestion, with their time only partially attributable
to DLD operations. The 15 staff are composed of one at the
screenings, grit and ash landfill, six in general operational
maintenance, six in mechanical and electrical maintenance, and
two in management and monitoring.
Ongoing requirements and possible concerns associated with
DLD operations include the need to lime the soil (at about
one ton of lime per acre per year [2.24 t/ha/yr]) to maintain a
proper pH and hence decrease mobility of metal cations. Also,
the useful life of the present type of 4-inch (10 cm) diameter
feed hose is unacceptably short. The possibility of using
different hose construction or a different brand is being
explored. Finally, after a 20-year operation, DLD soils,
19-57
-------
building up at about 0.75 inch (1.9 cm) per year, will have
increased in salinity to about 8,000 mg/1 in the saturation
extract. This concentration is not expected to affect the
bacterial decomposition of the organic matter, however.
TABLE 19-19
SACRAMENTO REGIONAL WASTEWATER TREATMENT PLANT
PROJECTED 1985 OLD STAFFING REQUIREMENTS
Number of
Description staff
One operator for each dredge/booster pump combination 2
Relief operator for dredges 1
One operator for each tractor/injector 4
Relief operators for tractor/injectors 2
Operator for tillage tractor 1
Supervisor 1
Total
11
Costs for sludge treatment and disposal at Sacramento are given
in Table 19-20. Costs do not include the main battery of
anaerobic digesters but do include the costs of a blending
digester (see Chapter 15).
19.4 References
1. U S E P A. P rocess Design Manual:^ Municipal Sludge Landfills .
Environmental Research information Center, Office of Solid
Wastes, Cincinnati, Ohio 45268. EPA-625/1-78-010, SW-705.
October 1978.
2. USEPA. Disposal of Sewage Sludge into a Sanitary Landfill.
Office ofSolidWastes,Washington, D.C. 20460. SW-71d.
1974.
3. Lukasik, G.D., and J.M. Cormack. "Development and Operation
of a Sanitary Landfill for Sludge Disposal - North Shore
Sanitary District." North Shore Sanitary District,
vtfaukegan, Illinois. 1976.
4. USEPA. Regulations on Public Participation in Programs
Under the Resource Conservation and Recovery Act, The Safe
Drinking Water Act, and The Clean Water Act~i Office of'
Waste and Hazardous Materials, Washington, D.C. 20460.
40 CFR 25, 44 CFR 10292. February 16, 1979.
19-58
-------
TABLE 19-20
SACRAMENTO REGIONAL WASTEWATER TREATMENT PLANT
PROJECTED COSTS OF SLUDGE MANAGEMENT SYSTEM
FOLLOWING ANAEROBIC DIGESTION
Item
Capital cost
Levee/drainage
Blending digester
Odor-stripping facilities
Solid storage basins (SSBs)
Existing SSB modifications
Barriers and wind machines
DLD sites
Electrical and controls
Wetlands/agricultural land
Landfill
Subtotal, construction cost
Administration, engineering
and contingencies'3
Landc
Sludge handling equipment
Total, capital cost
Operational cost, annual
Labor6
Materials and supplies
Power and fuel^
Site monitoring
Total annual operating cost
h
Annual costs
Stationary facilities
Mobile equipment*
Land]
Operational costs
Total annual costs
Phase I:
1980-1992
670
3, 910
980
7,810
480
1,020
2,480
1,640
840
280
20,110
4,840
2,690
1, 150
28,790
574
248
126
30
978
2,218
144
185
978
3,525
Costs, thousand dollars
Phase II:
1992-1999
additional costs
2,730
420
100
3,250
750
1,150
5,150
112
25
18
5
160
359
48
160
597
Total
670
3,910
980
10,540
480
1,440
2,480
1,740
840
280
23,360
5, 590
2,690
2, 300
33,940
585
273
144
35
1, 138
2, 577
192
185
1, 138
4,122*
Costs based on an ENR cost index of 3900, Sacramento, 1980.
Allowance for administrative and engineering expense, and contingencies. Includes
staging allowance for additional work in Stage I to accommodate Stage II.
CLand costs are $ 1,500/acre.
Operational costs are based on estimated 1980 prices for solids loads at the midpoint
of each staqe, i.e., 1985 for Stage I and 1996 for Stage II. .,
eTotal average annual cost per full-time individual of $28,000 in 1980, including all
fringe benefits and administrative overhead expenses. (20 1/2 person staff 1985,
24 1/2 person staff 1996).
Materials and supplies include special allowances for flexible hose for DLD operation
($25,000/yr), activated carbon for odor-stripping (11,200 Ib/yr) percent allowances for
equipment (3 percent), structures (1 percent), and earthwork (1/2 percent) construction
costs.
gElectrical power projected at 2.9 cents/kWhr and diesel fuel at 80 cents/gal in bulk
in 1980.
Amortization at 6 7/8 percent over a 25-yr life.
1Mobile facilities have various useful lives. No salvage value assumed.
^Land value assumed the same at the end of 20 years.
k ' '
Weighted average annual total program cost $3,824,000.
1 acre = 0.405 ha .. .
1 kWhr = 3.6 MJ
1 qal = 3.8 1
1 Ib = 0.453 kq
19-59
-------
5 . U S E P A. Subsurface Disposal of Municipa1 Wastewater
Treatment Sludge . Office of Soli d~~W a s t e s , Wa shington,
D.C. 20460. SW-167c. 1978.
6. Sacramento Area Consultants. Sewage Sludge Management
Program Final Report, Volume 7, Environmental Report and
Advanced Site Planning. Sacramento Regional County
Sanitation District. Sacramento, California 95814.
September 1979.
7. Brown and Caldwell. Corvallis Sludge Disposal Study. City
of Corvallis, Oregon. April 1977.
8. Brown and Caldwell. Corvallis Sludge Disposal Predesign
Report. City of Corvallis, Oregon. March 1978.
9. Brown and Caldwell. Amendment to Corvallis Wastewater
Treatment Program Environmental Assessment Dedicated Land
Disposal Project. City of Corvallis, Oregon. April 1978.
10. Uhte, W.R. "Wastewater Solids Storage JBasins: A Useful
Buffer Between Solids __S_tabilization and Final Disposal."
Presented at the 48th Annual Conference of the California
Water Pollution Control Association, Lake Tahoe, California.
April 14, 1976.
11. USEPA. "Principals and Design Criteria for Sewage Sludge
Application on Land." In Sludge Treatment and Disposal,
Part 2. Environmental Research Information Center.
Cincinnati, Ohio 45268. EPA-625/4-78-012. October 1978.
12. Sacramento Area Consultants. Sewage Sludge Management
Program Final Report, Volume 5. Dedicated Land Disposal
S_tiacry_. Sacramento Regional County Sanitation District.
S~acramento, California 95814. September 1979.
13. USEPA. Comprehensive Summary of Sludge Disposal Recycling
History. Office of Research and Development. Cincinnati,
Ohio 45268. EPA-600/2-77-054. April 1977.
14. Brown and Caldwell. Preliminary Draft: Colorado Springs
Long-Range Sludge Management Study. City of Colorado
Springs, Colorado 80947. April 1979.
15. Sacramento Area Consultants. Sewage Sludge Management
Program Final Report, Volume 1, SSMP Final Report, Work
Plans, Source Survey. Sacramento Regional County Sanitation
District. Sacramento, California 95814. September 1979.
16. Sacramento Area Consultants. Sewage Sludge Management
Program Cost Increases. Letter to Sacramento Regional
County Sanitation District. Sacramento, California 95814.
May 18, 1979.
19-60
-------
EPA 625/1-79-011
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
Appendix A. Metric Equivalents
U.S. ENVIRONMENTAL PROTECTION AGENCY
Municipal Environmental Research Laboratory
Office of Research and Development
Center for Environmental Research Information
Technology Transfer
September 1979
-------
APPENDIX A
METRIC EQUIVALENTS
METRIC CONVERSION TABLES
Recommended Units
Description
Length
Area
Volume
Mass
Time
Force
Unit
meter
kilometer
millimeter
centimeter
micrometer
square meter
square kilometer
square millimeter
hectare
cubic meter
cubic centimeter
liter
kilogram
gram
milligram
tonne
second
day
year
newton
Symbol
m
km
mm
cm
m2
km?
2
mm^
ha
3
Cm3
1
|(g
g
mg
t
s
day
yr or
8
N
Comments
Basil -SI unit
The hectare (10.000
m2) is a recognised
multiple unit and
will remain in inter
national use,
The liter is now
recognized as the
special name for
the cubic decimeter
Basic SI unit
1 tonne = 1,000 kg
Basic SI unit
Neither the day nor
the year is an SI unit
but both are impor-
tant.
The newton is that
force that produces
1 m/s2 m a mass
of 1 kg.
English
Eauivalents
39. 37 in. = 3.28 ft •
1.09yd
062 mi
0.03937 in.
03937 in
3.937 X 10'3=103A
10.744 sq ft
' 1 196 sq yd
6.384 sq mi =
247 acres
0.1 55 sq in.
0 00155 sq m.
2471 acres
35314 cu ft =
1.3079cuyd
0.061 cum.
1.057 qt = 0.264 g»l
= 0.81 X 10"* acre
ft
2.205 Ib
0.035 oz = 15.43 gr
0.01543 gr
0.984 ton (long! =
1.1023 ton (short)
0.22481 Ib (weight)
- 7.5 poundals
Description
Velocity
linear
angular
Viscosity
Pressure
Temperature
Work, energy,
quantity of heat
Power
Application of Units
Description
Precipitation,
run -off,
evaporation
River flow
Flow in pipes,
conduits, chan-
nels, over weirs,
pumping
Discharges or
abstractions,
yields
Usage of water
Density
Unit
millimeter
cubic metar
per second
cubic meter per
second
liter per second
cubic meter
per day
cubic metar
per year
liter per person
per day
kilogram per
cubic meter
Symbol
mm
m3/s
m3/s
l/s
m3/day
m3/yr
I/person
day
kg/m3
Comments
For meteorological
purposes it may be
convenient to meas
ure precipitation in
terms of mass/unit
area(kg/m3).
1 kg/sq m
Commonly called
the cumec
1 l/s = 86.4 m3/day
The density of
water understand
ard conditions is
1 000 kg/m3 or
l.OOOg/t
English
Equivalent!
35.314 cfs
15.85 flpm
I.83X 10'3 flpm
0.264gcpd
0.0624 Ib/cuft
Description
Concentration
BOD loading
Hydraulic load
per unit area,
e.g. filtration
rates
Hydraulic load
per unit volume:
a.g. biological
filters, lagoons
Air supply
Pipes
diameter
length
Optical units
Recommended Units
Unit
meter per
second
millimeter
per second
kilometers
per second
radians per
second
per second
liter per second
poise
newton per
square meter
kilonewton per
square meter
kiloyram (force)
per square
centimeter
degree Kelvin
degree Celsius
(oule
kilojoule
watt
kilowatt
joule per second
Symbol
m/s
mm/s
km/s
rad/i
3
l/s
poise
N/m?
kN/m?
kgf/cm2
K
C
J
kJ
W
kW
J/l
Comments
the cumec
The newton is not
et well known as
le unit of force
nd kgf cm2 will
early be used for
ome time. In this
ield the hydraulic
head expressed m
meters if an accept
able alternative
Basic SI iinii
The Kelvin and
Celsius degrees
are identical
The use of the
Celsius scale is
recommended as
it is the former
centigrade scale
joule = 1 N m
1 watt * 1 J/s
English
Equivalents
3.28 fpi
000328 IPS
2.230 mph
15 850 gpm
= 2.120cfm
15.85 gpm
0.0672/lb/
sec It
000014 psi
0 145 psi
14.223 ps.
5F
- - 17 77
2.778 X ID'7
kw hr =
3.725 X10'7
hphr»0.737SS.
h-lb > 9.48 X
10-' Blu
2.778 kw hr
Application of Units
Unit
milligram per
litit
kilogtam per
cubic meter
per day
cubic miter
per square meter
per day
cubic meter
per cubic meter
per day
cubic meter or
liter of free air
per second
milhmattf
mettr
lumen per
square meter
Symbol
mfl/l
kg/m3day
m3/m2 day
m3/m3diy
m3/s
l/s
mm
m
lumen/m^
Comments
If thts is con
verted to a
velocity, it
should be ex
pressed in mm/s
(1 mm/i -86.4
m3/m2day}.
tnjlitt
Equiwllfili
1 ppm
0.0624 Ib/cu-lt
d*y
3.28cutt/iqlt
0.03937 in.
39. 37 in. •
3.28 II
0 092 II
andla/iq ft
ft U.S. GOVERNMENT PRINTING OFFICE: 1981—757-064/0276
A-l
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