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 EvaporationCarver
Greenfield Process 10- 30
10.7.2.1 Process Description ............ ....... 10- 31
10.7.2.2 Current Status ,, ...» 10- 31
10.8 References 10- 32
CHAPTER 11. HIGH TEMPERATURE PROCESSES 11- 1
11.1 Introduction ,,..»...*...,......,,....,.....»,»,,.. 11- 1
11.2 Principles of High Temperature Operations ......... 11- 2
11.2.1 Combustion Factors .......................... 11- 3
11.2.1.1 Sludge Fuel Values »»..... 11- 3
11.2.1.2 Oxygen Requirements for Complete
Combustion 11- 6
11.2.1.3 Factors Affecting the Heat Balance .... 11- 7
11.2.2 Incineration Design Example 11- 10
11.2.2.1 Problem Statement 11- 10
11.2.2.2 Approximate Calculation Method 11- 13
11.2.2.3 Theoretical Calculation Method 11- 20
11.2.2.4 Comparison of Approximate and
Theoretical Calculation Methods ....,,, 11- 24
11.2.3 Pyrolysis and Starved-Air Combustion
Calculations ...» 11- 25
11.2.4 Heat and Material Balances . 11- 28
11.3 Incineration 11- 29
11.3.1 Multiple-Hearth Furnace 11- 31
11.3.2 Fluid Bed Furnace 11- 42
11.3.3 Electric Furnace 11- 49
11.3.4 Single Hearth Cyclonic Furnace ...* 11- 55
11.3.5 Design Example: New Sludge Incineration
Process .,,.,.,,..,.,.,... 11- 59
11.3.5.1 Approach 11- 61
11.3.5.2 Preliminary Design 11- 62
11.4 Starved-Air Combustion 11- 65
11.4.1 Development and Application 11- 68
11.4.2 Advantages and Disadvantages of SAC 11- 71
11.4.3 Conversion of Existing Multiple-Hearth
Incineration Units to SAC ................... 11- 75
11.4.4 Design Example: Retrofit of an Existing
Multiple-Hearth Sludge Incinerator to a
Starved-Air Combustion Reactor 11- 76
11.4.4.1 Approach 11- 77
11.4.4.2 Preliminary Design 11- 78
xvi
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TABLE OF CONTENTS (continued)
11.4 Starved-Air Combustion ...» n_ 55
11.4.1 Development and Application » 11- 68
11.4.2 Advantages and Disadvantages of SAC ......... 11- 71
11.4.3 Conversion of Existing Multiple-Hearth
Incineration Units to SAC n- 75
11.4.4 Design Example: Retrofit of an Existing
Multiple-Hearth Sludge Incinerator to a
Starved-Air Combustion Reactor 11- 76
11.4.4.1 Approach 11- 77
11.4.4.2 Preliminary Design 11- 78
11.5 Co-Combustion of Sludge and Other Material ........ 11- 81
11.5.1 Co-Combustion with Coal and Other
Residuals .. ...» 11- 81
11.5.2 Co-Combustion with Mixed Municipal Refuse
(MMR) 11- 83
11.5.2.1 Refuse Combustion Technology .......... 11- 84
11.5.2.2 Sludge Combustion Technology 11- 87
11.5.3 Institutional Constraints ...,....,,,,,.,.,,. 11- 92
11.5.4 Conclusions about Co-Combustion 11- 94
11.6 Related Combustion Processes Used in Wastewater
Treatment , 11- 94
11.6.1 Screenings, Grit, and Scum Reduction ........ 11- 94
11.6.2 Lime Recalcination ...,..,...,.,,.,.,........ 11- 96
11.6.3 Activated Carbon Regeneration 11- 98
11.6.3.1 Granular Carbon Systems (GAC) 11- 99
11.6.3.2 Powdered Activated Carbon (PAC) ....... 11-100
11.6.3.3 Jet Propulsion Laboratory Activated-
Carbon Treatment System (JPL-ACTS) .... 11-100
11.7 Other High Temperature Processes 11-102
11.7.1 High Pressure/High Temperature Wet Air
Oxidation 11-102
11.7.2 REACT-O-THERMtm 11-109
11.7.3 Modular Starved-Air Incinerators 11-110
11.7.4 Pyro-Soltm Process 11-110
11.7.5 Bailie Process 11-113
11.7.6 Wright-Malta Process 11-113
11.7.7 Molten Salt Pyrolysis 11-115
11.8 Air Pollution Considerations 11-115
11.8.1 National Ambient Air Quality Standards
(NAAQS)-State Implementation Plans (SIP) .... 11-116
11.8.2 National Emission Standards for Hazardous
Air Pollutants (NESHAPS) ..................... 11-177
11.8.3 Standards of Performance for New
Stationary Sources (NSPS) 11-118
11.8.4 New Source Review Standards (NSR) 11-119
11.8.5 Prevention of Significant Deterioration
(PSD) 11-119
11.8.6 The Permit Process 11-120
11.8.7 Air Emissions Test Procedures 11-120
11.8.8 Design Example 11-120
xvi i
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TABLE OF CONTENTS (continued)
Page
11.8.8.1 Identify Applicable State and Local
11.8.8.2 Establish Air Pollution Abatement
11.9 Residue Disposal ..»,......,,*....»»»,,,
CHAPTER 1 2 . COMPOSTING .... ,
12.2.1 Moisture ....,..,
12.2.3 pH .,
12.2.4 Nutrient Concentration
12.3.1.2 Public Health and Environmental
12.3.2 Aerated Static Pile Process
12.3.2.1 Individual Aerated Piles ..............
12.3.2.2 Extended Aerated Piles
12.3.2.3 Current Status
12.3.2.5 Bulking Agent
12.3.2.7 Public Health and Environmental
12.3.3 Case Studies (Unconfined Systems) ...........
12.3.3.1 Joint Water Pollution Control Plant,
Carson, California ...................
12.3.3.2 Beltsville, Maryland
12.3.3.5 Cost Analysis
12.4 Confined Composting System .......................
12.4.2 Metro-Waste Aerobic Thermophilic
Bio-Reactor , ,
12.4.3 Dano Bio-Stabilizer Plant
12.4.4 BAV Bio-Reactor
CHAPTER 13. MI SCELLANEOUS PROCESSES
. 11-121
, 11-123
. 11-132
. 11-136
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. 12- 51
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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
-------
LIST OF FIGURES (continued)
Number
Page
14- 2 Comparison of Behaviors of Wastewater Sludge and
Water Flowing in Circular Pipelines 14- 6
14- 3 Friction Factor for Sludge, Analyzed as a Bingham
Plastic ,,..*,.......,,.,,,......,,.....»...,, 14- 7
14- 4 Friction Factors for Example Problem ............ 14- 12
14- 5 Pressure Drops for Example Problem 14- 13
14- 6 Viscometer Test of Sewage Sludge 14- 16
14- 7 Centrifugal Pump ..,,,.........,..,..,.,... 14- 18
14- 8 Torque Flow Pump , 14- 19
14-12 Progressive Cavity Pump ......................... 14- 23
14-13 Diaphragm Pump .................................. 14- 25
14-14 Rotary Pump 14- 27
14-15 Ejector Pump .... 14- 28
14-16 Belt Conveyor 14- 38
14-17 Inclined Belt Conveyor Features ................. 14- 41
14-18 Flexible Flat Belt Conveyor 14- 42
14-19 Screw Conveyor 14- 42
14-20 Tabular Conveyor 14- 43
14-21 Bucket Elevator 14- 44
14-22 Pneumatic Ejector ............................... 14- 45
14-23 Pneumatic Conveyor 14- 45
CHAPTER 15
15- 1 Solids Balance and Flow Diagram-Design Example
Single-Phase Concentration and Displacement
Storage 15- 13
15- 2 Effect of Various Operating Strategies on
Dewatering Unit Feed Rates ...................... 15- 17
15- 3 Proposed Design for Blending DigesterSacramento
Regional Wastewater Treatment Plant 15- 21
15- 4 26,000 Gallon Sludge Equalization Tank (Typical
of Two) Aliso Solids Stabilization Facility ..... 15- 22
15- 5 Schematic Representation of a Facultative Sludge
Lagoon (FSL) 15- 25
15- 6 Typical Brush-Type Surface Mixer, Sacramento,
California 15- 27
15- 7 Typical FSL Layout 15- 29
15- 8 Typical FSL Cross Section 15- 29
15- 9 Layout for 124 Acres of FSLs--Sacramento Regional
Wastewater Treatment Plant 15- 30
15-10 Sacramento Central Wastewater Treatment Plant
Surface Layer Monitoring Data for FSLs 5 to 8 ... 15 -34
15-11 Sacramento Central Wastewater Treatment Plant
1977 Fecal Coliform Populations for Various
Locations in the Solids Treatment-Disposal
Process 15- 38
15-12 Typical Wind Machines and Barriers Sacramento,
California 15- 40
xlvi
-------
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 DigesterSacramento
Regional Wastewater Treatment Plant 15- 21
15- 4 26,000 Gallon Sludge Equalization Tank (Typical
of Two) Aliso Solids Stabilization Facility ..... 15- 22
15- 5 Schematic Representation of a Facultative Sludge
Lagoon (FSL) 15- 25
15- 6 Typical Brush-Type Surface Mixer, Sacramento,
California 15- 27
15- 7 Typical FSL Layout 15- 29
15- 8 Typical FSL Cross Section , 15- 29
15- 9 Layout for 124 Acres of FSLsSacramento Regional
Wastewater Treatment Plant 15- 30
15-10 Sacramento Central Wastewater Treatment Plant
Surface Layer Monitoring Data for FSLs 5 to 8 ... 15 -34
15-11 Sacramento Central Wastewater Treatment Plant
1977 Fecal Coliform Populations for Various
Locations in the Solids Treatment-Disposal
Process 15- 38
15-12 Typical Wind Machines and Barriers Sacramento,
California ,.,..... 15- 40
xlvi
-------
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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RIVER
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MULTIPLE EFFECT
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PROCESSES
RESOURCE RECOVER1
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DISPOSAL ON LAND
FINAL INSTITUTION
\ 1
FOREST INDUSTRY
OTHER INDUSTRY
FARMS
HOUSEHOLD
METRO
LOCAL
STATE
FEDERAL
OTHERS
FIGURE 3-2
COMPONENTS FOR SYSTEM SYNTHESIS
3.3.2 Identification of System Options
Candidate systems are synthesized from an array of components,
such as these shown on Figure 3-2. Wastewater and solids
management components are listed as a reminder that all
components of the train must be considered. Figure 3-3
illustrates how Figure 3-2 can be used to develop a specific flow
sheet. Process streams can be drawn on copies of the master
3-4
-------
drawing. Relevant information such as solids concentrations and
mass flow rates can be entered directly on the flow sheet, if
desired. The advantages of using arrays such as Figure 3-2 are
that nearly all potential options are identified and process
streams are clearly displayed.
SECONDARY TREATMENT
TERTIARY TREATMENT
EFFLUENT RECEIVER
__ .
THERMAL r~_'._ VACUUM FILTER ! I
fl \ L-J
CHEMICAL I L^j BELT FILTER g
SOLVENT EXTRACTION
MULTIPLE EFFECT
UTILIZATION
RESOURCE RECOVERY
DISPOSAL ON LAND
AGRICULTURE [
FOREST j
DEDICATED LAND DISPOSAL
PERMANENT LAGOONS
LAND RECLAMATION
FINAL INSTITUTION
r~~~~~~
FOREST INDUSTRY
OTHER INDUSTRY
FARMS
HOUSEHOLD
/METRO
i.«M
STATE
FEDERAL
OTHERS
1 -" l
CONFINED
L, .,-.. - _J
UNCONFINED
1 CHEMICAL FIXATION
ENCAPSULATION ;
1 EARTH WORM '
1 CONVERSION |
ASH ^
KEY:
WASTEWATER
^^^^ SLUDGE
SIDESTREAMS
FIGURE 3-3
FLOWSHEET DEVELOPED FROM COMPONENTS FOR
SYSTEM SYNTHESIS
3-5
-------
3.3.3 System Selection Procedure
The process selection procedure consists of (1) developing
treatment/disposal systems which are compatible with one another
and appear to satisfy local relevant criteria, and (2) choosing
the best system or systems by progressive elimination of weaker
candidates. Related to these are the concepts of base and
secondary alternatives.
3.3.3.1 Base and Secondary Alternatives
A base alternative is defined as a wastewater solids management
system which, during evaluation, appears able to provide reliable
treatment and disposal at all times under all circumstances for
sludges. It therefore meets the prime criterion of reliability.
It must also satisfy the following seven conditions:
1. It must be legally acceptable.
2. Sites for processing and disposal operations must be
readily available.
3. Environmental and health risks must be sufficiently low
to satisfy the public and all agencies having
jurisdiction.
4. It must be competitive with cost to other alternatives on
a first-round analysis.
5. The necessary equipment and material must be readily
available.
6. The contractor must be able to begin construction
immediately following design and have the system
operational almost immediately after construction.
7. Financing of the system must be straightforward and
assured.
A secondary alternative is defined as a wastewater solids
management system which does not meet the prime criterion of
reliability, that is, the system cannot accept all of the sludge
under all circumstances all of the time. This does not mean
secondary alternatives ^are without value; they may in fact be
used to great advantage in tandem with base alternatives and
may in fact accept a greater quantity of sludge than the base
alternative. As an example, a city's horticultural market may be
insufficiently developed to accept all of the city's sludge all
of the time; therefore, horticulture cannot be considered a base
disposal alternative. However, it may cost less to release the
sludge to horticulture than to dispose of it by means of city's
base disposal alternative, for example, landfilling. The
3-6
-------
city should therefore make every effort to dispose of as many
solids as possible via horticulture, the secondary alternative.
However, should the secondary alternative fail or be interrupted
for any reason, the sludge going to the secondary system must be
readily and quickly diverted back to the base alternative, which
must remain fully operational and thus immediately capable of
receiving the entire sludge flow.
3.3.3.2 Choosing a Base Alternative: First Cut
The purpose of the first cut is to rapidly and with minimum
effort produce a list of candidate base alternatives which
are technically feasible and reasonably cost-effective. The
alternatives must be environmentally acceptable and implementable
in the time frame of the project. Analyses are qualitative at
this stage. The first cut involves determination of:
1. Practical base disposal options.
2. Practical base solids treatment systems.
3. Practical treatment/disposal combinations.
Determi_nat.ion_of_ Practical Base Disposal Options
The method of solids disposal usually controls the selection of
solids treatment systems and not vice versa. Thus, the system
selection procedure normally begins when the solids disposal
option is specified.
In the first cut, feasible base disposal alternatives and
relevant criteria are set up in matrix form. An example is shown
in Table 3-1. Feasible alternatives are those which appear to be
suitable for the situation at hand. Obviously inapplicable
alternatives would not be included in this matrix. Only those
criteria which the planner see's as critical for the site at hand
should be considered in this first cut. Other, less critical
criteria can be considered in subsequent iterations, where more
in-depth investigation is needed for each of the candidate
processes.
For the hypothetical situation described in Table 3-1, nine
utilization/disposal options are considered feasible and are
set up for evaluation. The criteria most important to the site
are judged to be reliability, environmental impacts, site
availability and cost. Base disposal alternatives are judged to
be practical only if they satisfy all the relevant criteria.
In Table 3-1, utilization of sludge on private agricultural land
is an unacceptable base disposal alternative. Reasons for this
might be insufficient acreage or a lack of assurance that the
farmers would accept all of the sludge. Alternatives which would
seem to satisfy relevant criteria for base disposal alternatives
are utilization on public agricultural land, landfill, and
3-7
-------
dedicated land disposal. Before considering these, however, one
must determine what combinations of solids treatment processes
make sense for the site in question. . . .
TABLE 3-1
EXAMPLE OF INITIAL SCREENING MATRIX FOR
BASE SLUDGE DISPOSAL OPTIONS
Relevant criteria
'" ' ' - Acceptable
Utilization/disposal Environmental Site for base
options Reliability impacts availability Cost alternative
Bag-market as
fertilizer 0 X X X 0
Agricultural land
(private) O X X X O
Agricultural land
(public) X X XXX
Forested land (pri-
vate) 0 X 0 0 O
Forested land (public) X X O 0 O
Give to citizens
(horticulture) 0 X X X O
Combine with commer-
cial topsoil O X X X O
Dedicated land dis-
posal X X XXX
Landfill X X XXX
0 = unacceptable.
X = acceptable.
Determine Practical Base Treatment Systems
Table 3-2 illustrates process compatibility matrix for treatment
alternatives. Incompatible processes and processes which are not
applicable in given locations are eliminated. The combination of
drying beds and mechanical dewatering, for example, is considered
incompatible because both dewatering and drying take place on the
drying bed; mechanical dewatering is not needed. On the other
hand, the combination of incineration and mechanical dewatering
of unstabilized sludge is generally compatible, but for the
hypothetical case investigated is ruled out because of air
pollution considerations. After first-cut analysis, seven base
treatment options are considered feasible and are further
evaluated.
Dgj^£giJJlg_P£agt_i_cal_gase_ Treatment/Disposal Combinations
Practical base treatment and disposal combinations are
then combined in a matrix, which is subjected to further
3-8
-------
culling. Table 3-3 shows the matrix of base treatment/disposal
combinations made by bringing forward the base disposal and
treatment options from Tables 3-1 and 3-2. Incompatible
combinations and systems ruled out by local constraints are then
eliminated. For example, undewatered wastewater solids are
not generally disposed of in landfills. An example of local
constraints is the ruling out of applying lime stabilized sludge
on agricultural lands because of already high soil pH.
TABLE 3-2
EXAMPLE OF PROCESS COMPATIBILITY MATRIX
Digestion options
Undigested sludge options
Anaerobically or
aerobically digested
Final processing
step
No further processing
Drying beds
Heat dry
Pyrolysis
Incineration
Compost
Mechanically
dewatered
xa
0
X
0
0
X
Not
dewatered
X
X
o
0
0
o
Not stabilized
Mechanically
dewatered
ob
O
0
0
0
X
Not
dewatered
O
O
O
0
O
O
Lime
stabilized
Mechanically
dewatered
X
0
0
0
0
o
Thermally
conditioned
Mechanical ly
dewatered
0C
0
0
0
0
0
Wet air
oxidation
Mechanically
dewatered
0
0
0
O
0
0
X = generally compatible .
TABLE 3-3
EXAMPLE OF TREATMENT/DISPOSAL COMPATIBILITY MATRIX
Treatment options
Digested sludge options
Viable
disposal
Agricultural
(public)
Landfill
local
options
land
Dedicated land disposal
Mechanically
dewatered
xa
X
X
Mechanically
dewatered,
heat dry
X
X
X
Mechanically
dewatered,
compost
X
oc
o
Not
mechanically
dewatered
X
o
X
Not
mechanically
dewatered,
drying beds
X
X
X
Undigested sludge options
Mechanically
dewatered ,
composted
X
X
X
Lime
stabilized,
mechanically
dewatered
f
X
0
X - generally compatible.
b
0 - generally compatible, but ruled out by local considerations.
0 - generally not compatible.
The number of candidate base treatment/disposal systems is thus
reduced. For the hypothetical case of Table 3-3, sixteen systems
remain for further evaluation.
3-9
-------
3.3.3.3 Choosing a Base Alternative: Second Cut
The purpose of second-cut analyses is to further reduce the list
of candidate systems. Analyses are more quantitative than in the
first cut, but the level of effort used to investigate each
option is not yet intensive. Information used in the second cut
is general and readily available, for instance, equipment cost
curves which are not site-specific, areawide evaluation of soils,
geology, hydrology, topography and land use, and general energy
costs.
One approach is to set up a numerical rating system for the
remaining candidate systems, such as that shown in Table 3-4.
The list of criteria to be considered may be expanded beyond
those critical criteria used in first cut analyses to encompass
the full range of criteria listed on Figure 3-1, or any fraction
of it. This follows the principle that as the list of candidate
process narrows, each will be analyzed in greater detail.
TABLE 3-4
EXAMPLE OF NUMERICAL RATING SYSTEM FOR
ALTERNATIVES ANALYSIS
Ratings of alternatives
Categories and criteria
Effectiveness
Flexibility
- Reliability
Sidestream effects
Track record
Relative
weight3
3
S
3
2
ARb
4
3
10
5
WRC
12
15
30
10
AR
6
5
9
7
WR
18
25
27
14
AR
9
5
5
4
stive 3 Alternative 4
WR AR WR
27 5 15 ..
25 2 10 ..
15 6 18 ..
8 9 18 ..
AR
6
2
7
6
WR
18
10
21
12
Compatibility
With existing land use
plans
With areawide wastewater,
solid waste and air
pollution programs
With existing treatment
facilities
Economic impacts
- Net direct costs
- Net indirect
costs
3
4
4
1
3
5
7
8
9
20
28
8
fi
5
8
9
18- 3
20 6
32 0
9 6
3
24 ,
32
6
5
8
9
3
15
32
36
33
21
12
28
8
Environmental impacts
- Public health
Administrative burdens
Level of effort
Marketing respons-
ibilities
- Resolution of juris-
dictional disputes
Public relations
Total weighted alternative
ratingd
1,576
6
10
1,430
7
14
4
10
4
18
1,317
3Relative importance of criteria as perceived by reviewer; scale, 0 to 5; no importance rated zero, most important rated 5.
Alternative rating. Rates the alternatives according to their anticipated performance with respect to the various criteria;
scale 0 to 10; least favorable rated zero, most favorable rated 10.
CWeighted rating. Relative weight for each criteria multiplied by alternative rating.
Sum of weighted ratings for each alternative.
3-10
-------
In the second cut, subjective judgments are combined with
technical measurements. Numerical values are assigned to all
criteria for all alternative systems. The planner's perception
of the relative importance of each criterion is indicated on a
rating scale, say of 0 to 5, with highest ratings given to
criteria the planner considers to be of greatest importance,
and the lowest to those of least important. For example,
if reliability is highly valued for the site in question,
reliability may be assigned a relative weight of 5.
Next, each alternative system is rated according to its
anticipated performance with respect to the various criteria,
again by using a rating scale, say 0 to 10. An alternative which
rates favorably is given high scores; one which rates less
favorably is given lesser scores. For example, an alternative
which is not dependable may be rated at 2 with respect to
reliability.
The relative weight is then multiplied by the alternative rating
to produce a weighted rating for each criteria/alternative
combination. For the examples described in the previous two
paragraphs, the weighted rating for the alternative in question
with respect to reliability is 5 x 2 = 10.
Finally, the weighted ratings are summed for each alternative to
produce a total or overall rating. Systems with lowest overall
ratings are eliminated, with higher rated systems carried forward
for further evaluations. I.n the example shown in Table 3-4,
Alternatives 3 and 4 are eliminated and Alternatives 1, 2, and n
are carried forward.
3.3.3.4 Third Cut
The third cut uses the same methodology as the second, but
the number of alternatives remaining is more limited; typically
to a maximum of 3 to 5--and the analysis is more detailed.
Information may include:
Analyses of potential sludge disposal sites (soils,
geology, and groundwater).
Local surveys to determine marketability of sludge and
sludge by-products.
Possible effects of industrial source control/
pretreatment programs on process viability and quality of
sludge for disposal.
Data oriented literature search.
Detailed analysis of effect of candidate systems on the
environment (air, water, land).
3-11
-------
Information developed from site-specific pilot work.
Mass balances.
Energy analyses.
Detailed cost analyses.
3.3.3.5 Subsequent Cuts
Subsequent cuts are even more detailed. Analyses are repeated
until the optimum base treatment/disposal alternative is defined.
3.3.4 Parallel Elements
By means of the procedure discussed above, a base alternative is
selected. However, the optimum system may include more than just
this base alternative. A number of parallel elements may be
involved which provide flexibility, reliability, and operating
advantages. For example, the base alternative for the system
depicted on Figure 3-4 is thickening, anaerobic digestion,
storage in facultative sludge lagoons, and spreading of liquid
sludge on agricultural land. Parallel elements cosnsist of
the application of liquid sludge on forest land and drying
beds followed by distribution for horticultural purposes. If
horticultural and forest land outlets were each large enough to
accept all of the sludge under all circumstances and at all
times, three base alternatives are then available. If not, the
forest land and drying beds/horticulture applications would be
considered secondary alternatives.
BASE ALTERNATIVE
r
THICKEN
ANAEROBIC
DIGESTERS
FACULTATIVE
LAGOON
PARALLEL
SYSTEMS
SPREAD ON
AGRICULTURAL
LAND
APPLICATION
ON
FOREST
LAND
FIGURE 3-4
PARALLEL ELEMENTS
3-12
-------
The concept of providing for more than one base alternative may
at first seem contradictory but a given base alternative might
not always be reliable because unpredictable events might occur.
For example, new owners of farmland may decide they do not wish
to accept sludge, or a disaster or strike could interrupt one
method of transporting sludge to its ultimate destination. To
minimize risks, therefore municipalities may wish to provide more
than one base alternative. The selection procedure presented in
Section 3.3.3 has the advantage of clearly depicting which is the
second or even third most desirable base alternative.
Parallel base alternatives are more common in large systems,
which are generally located in urban areas where land is scarce
than in small plants, which are usually located in rural areas
where land is more plentiful and temporary storage and disposal
options therefore more numerous. Large plants may maintain two
or three base alternatives to ensure solids disposal. Since this
may increase the cost of operation, it leads to the observation
that very large systems do not necessarily benefit from economies
of scale when it comes to wastewater solids disposal.
3.3.5 Process Selection at Eugene, Oregon
Eugene, a city of 100,000 people, is located at the southern end
of the agricultural Willamette Valley in Western Oregon. The
Metropolitan Wastewater Management Commission (MWMC) was formed
in 1977 to implement the findings of a facility planning effort
which called for the construction of a regional sewage treatment
plant. The plant, to be constructed on the site of the existing
Eugene plant, will serve the whole metropolitan area. This area
is composed of the cities of Eugene, Springfield, and urbanized
portions of Lane County.
Regionalization and upgrading of the plant to meet a 10/10 summer
effluent standard for BOD5 and suspended solids prior to
discharge to the Willamette River, means that sludge quantities
are dramatically increased. The plant is to serve a population
of 277,000 by the year 2000. Design average dry weather flow is
49 MGD (2.15 m3/s) , wet weather flow is 70 MGD (3.07 m3/s), and
peak wet weather flow is 175 MGD (7.67 m3/s).
The plant will use an activated sludge process, with flexibility
for operation in plug, step, contact stabilization, or complete
mix modes. Provision is also made for the addition of mechanical
flocculators in the secondary clarifiers and tertiary filtration
if either or both prove desirable at a later date.
It was decided early that sludge thickening would be economical,
regardless of the sludge management system which would eventually
be used. Consequently, two existing thickeners, one gravity and
one flotation, will be retained for thickening primary sludge,
waste-activated sludge, or a combination of the two.
3-13
-------
A key provision in the selection of a suitable sludge management
system was that the system be fully operational by the time the
wastewater treatment system is started up. This seemingly
straightforward condition was complicated by the fact that
planning for the sludge system did not start until design of the
wastewater treatment plant was already under way. This meant
that the sludge management system would be forced to fit into
an already developed plan for the wastewater treatment facility
(which is by no means unusual).
As a first cut, sludge disposal options were immediately
developed and screened for acceptability as part of a base
alternative, using a matrix similar to that developed in
Table 3-1. Practical treatment systems were identified from a
process compatibility matrix similar to Table 3-2. Practical
disposal/processing combinations were then developed in a matrix
form (as in Table 3-3). Physically incompatible or otherwise
unsuitable combinations were eliminated in this matrix. A
flowsheet was then prepared for the remaining options, with
necessary intermediate storage and transport requirements added
in. The flowsheet of alternatives for Eugene second cut analysis
is shown on Figure 3-5.
OLD * DEDICATED LANO DISPOSAL
FIGURE 3-5
CANDIDATE BASE ALTERNATIVES FOR EUGENE-SPRINGFIELD
3-14
-------
It is worth noting that utilization on agricultural land could
not be considered as a base alternative despite the large
agricultural acreage north of Eugene and the fact that the new
regional plant is on the north side of the city. It would have
been a requisite that MWMC own sufficient farmland (2,000 to
3,000 acres) to accept all of the sludge generated. The cost
of purchasing such acreage was deemed unacceptably high;
furthermore, there was opposition to converting private land to
public land. Thus agricultural utilization was not considered
further in the search for a base alternative.
The second cut analysis was more quantitative. Information used
was general and readily available. For example, costs were taken
from current cost curves, and certain environmental impacts
were assessed from projects with similar disposal systems and
soil/groundwater conditions. With numerical data established
for each criterion, a rating table was produced similar to that
of Table 3-4. The data were developed by the project engineers,
but the ratings were analyzed extensively by the Citizens
Participation Committee (CPC) on sludge management which had been
recruited from the population at large at the very beginning of
the project. The committee was composed of various vested
interest groups, representatives of government agencies and
private unaffiliated citizens who were interested in the project.
Systems with the lowest total ratings were then eliminated.
Incineration was found to be unacceptable primarily because
it would impact the already limited dilution capacity available
during the summer in the trapped valley airshed of Eugene;
pyrolysis was eliminated primarily because of its perceived
inability to meet the construction deadline for plant start-up;
and lime stabilization with disposal to landfill was eliminated
primarily on a cost-effective basis. At the end of the second
cut analysis, all alternatives which could accommodate raw
sludges were eliminated, since, as indicated, most raw sludge
options (incineration, pyrolysis, lime stabilization) were not
viable and there was a strong desire to make use of existing
digesters. A decision was made to combine primary and secondary
sludge in order to avoid the cost and problems of constructing
and operating separate systems for each.
The same methodology used in the second cut was used in the
third; however, data used in the analysis were more site
specific, so that economic and environmental comparisons could be
better refined. As examples:
Actual routes were selected to off-site facilities;
river crossings were defined, and decisions were made on
routing pipes under bridges or jacking under freeways.
For disposal at the local sanitary landfill, estimates
were made of (1) the contribution of the sludge to
landfill leachate production and subsequent marginal
leachate treatment costs to be passed back from the
3-15
-------
Lane County Solid Waste Division to MWMC, and (2) the
actual net volume of landfill required for sludge
disposal, allowing for sludge consolidation.
For dedicated land disposal, seasonal water tables and
detailed groundwater migration patterns, as well as
private well locations and depths were determined.
Estimates were made of comparative nitrate loadings which
would eventually reach the Willamette River from treated
landfill leachate discharge; from groundwater migration
from dedicated land disposal; and from filtrates from
mechanical sludge dewatering (which is subsequently
discharged with the effluent).
Transportation modes were analyzed in detail and costed
for various sludge solids concentrations and transport
routes and distances.
These detailed analyses still left a number of viable base
alternatives. At this point, other less tangible factors
were considered. These were (1) that the chosen base
alternative(s) be compatible with desired secondary alternatives,
and (2) that flexibility and reliability be provided through the
use of parallel systems. After intensive screening, it was
decided that two base alternatives would be used: spreading of
liquid sludge on dedicated land and open-air drying followed
by landfill disposal. Both alternatives included force main
transport of digested sludge from the regional treatment plant to
a remote sludge management site, where the sludge was to be
stored in facultative sludge lagoons. Liquid sludge would be
spread on dedicated land at the sludge management site. Dried
sludge would be trucked to landfill. Operations associated with
disposal (spreading, drying, and landfilling) would be carried
out during dry weather. These systems provide the desired
flexibility and reliability and are compatible with preferred
secondary alternatives.
Several variations of sludge utilization on land were adopted
as secondary alternatives, since there was a strong feeling
that sludge should be used beneficially. The alternatives of
particular interest to the Eugene-Springfield area included
agricultural use on private farm land, use for ornamental
horticulture, in nurseries and public parks, and use in a mixture
with commercial topsoils in. landscaping. Sludge would be
provided to these outlets as the market demands. Variable demand
is particularly important in Oregon's Willamette Valley, where
prolonged winter rainfall and summer harvesting schedules control
the timing of agricultural sludge use.
The flowsheet for the Eugene system is shown on Figure 3-6.
3-16
-------
PRIMARY
SLUDGE
WASTE -
ACTIVATED
SLUDGE
GRAVITY
THICKEN
FLOTATION
THICKENER
ANAEROBIC
DIGESTERS
LONG
DISTANCE
PIPELINE
FACULTATIVE
STORAGE
LAGOONS*
AT SLUDGE MANAGEMENT SITE
ON PRIVATE FARMLAND
CITIZEN PICK-UP AT SLUDGE
MANAGEMENT SITE
DEDICATED
LAND
DISPOSAL*
DRYING
BEDS*
TRUCK
TRUCK
LANDFILL
AGRICULTURE
FIGURE 3-6
FLOWSHEET FOR THE EUGENE-SPRINGFIELD
SLUDGE MANAGEMENT SYSTEM
The ability to use base facilities and equipment for desired
secondary alternatives was a major consideration in selecting the
the force main, sludge lagoons, and
be used for dedicated land disposal
also required for agricultural use.
sludge from the sludge management site
however, be an additional expense for
base system. In Eugene,
application equipment to
of the liquid sludge are
Trucks to transport liquid
to agricultural sites will,
the secondary alternatives.
It is hoped that eventually all sludge can be utilized on
land. As indicated, however, in Table 3-5, full agricultural
utilization of sludge is estimated to be more costly than either
of the pure disposal options. This is because more equipment is
needed to transport sludge to and spread it on the agricultral
sites than is needed for the pure disposal options. Thus, as of
1979, any system which even partially incorporates agricultural
utilization will be more costly than pure disposal options. This
could change if the farmers can be persuaded to pay for the
sludge.
TABLE 3-5
ESTIMATED COSTS OF ALTERNATIVES FOR EUGENE-SPRINGFIELD
Sludge form
Alternative
Total annual cost,
million dollars
Liquid
Dried
Dedicated land disposal only
Agricultural utilization only
Landfill only
Agricultural utilization only
1.03
1.53
1.14
1. 32
3-17
-------
At the time this manual was written (1979), MWMC was involved in
public hearings aimed at selecting a suitable sludge management
site.
3.4 The Quantitative Flow Diagram
Overall system performance is the sum of the combined
performances of the system's linked processes. This is nowhere
more clearly expressed than on a Quantitative Flow Diagram (QFD).
The QFD is used to estimate loadings to the various wastewater
treatment, solids treatment, and solids disposal processes. The
QFD is the starting point for understanding process interactions
and is nothing more than a materials balance. Although balances
can be struck for components like nitrogen, phosphorus and
chemical oxygen demand, the most useful balances are those
for suspended solids. The QFDs to be presented here are for
suspended solids. Each flowsheet has its own unique set of
balance equations. In the following pages, mass balances for a
specific, rather simple flowsheet are derived, thus illustrating
the technique. The mass balance equations are then summarized
in tabular form. Mass balance equations for a more complex and
more common flowsheet are later presented, without derivation.
Two worked QFDs are presented as examples. The intent is to
demonstrate the usefulness of the method.
3.4.1 Example: QFD for a Chemically Assisted
Primary Treatment Plant
The flowsheet for a chemically assisted primary wastewater
treatment plant with anaerobic digestion and mechanical
dewatering of the sludge is shown on Figure 3-7. In this
example chemicals are added to enhance the sedimentation process.
Sidestreams from the digester and dewatering units are recycled
to the primary sedimentation basin. The calculation is carried
out in a step-by-step procedure:
1. Draw the flowsheet (as on Figure 3-7).
2. Identify all streams. For example, stream A contains raw
sewage solids plus chemical solids generated by dosing
the sewage with chemicals. Let the mass flow rate of
solids in Stream A be equal to A Ib per day.
3. For each processing unit, identify the relationship of
entering and leaving streamsto one another in terms of
mass. For example, for the primary sedimentation tank,
let the ratio of solids in the tank underflow (E) to
entering solids (A + M) be equal to XE. XE is actually
3-18
-------
an indicator of solids separation efficiency. The
general form in which such relationships are
expressed is:
mass of solids in stream 6
mass of~~solids entering the unit
P J
For example, Xp = = - 5-, Xj = p- . The processing unit's
performance is specified when a value is assigned to XQ.
DEGRITTED SEWAGE
SOLIDS
SOLIDS
GENERATED
BY CHEMICAL
ADDITION
M
PRIMARY
SEDIMENTATION
B
SUPERNATANT
FILTRATE
DIGESTION
1
DEWATERING
EFFLUENT
SOLIDS
DESTROYED
^ (CONVERTED
TO GAS AND
WATER)
CONDITIONING
'CHEMICALS
TO ULTIMATE
DISPOSAL
FIGURE 3-7
BLANK QFD FOR CHEMICALLY-ASSISTED
PRIMARY PLANT
3-19
-------
Combine the mass balance relationships^ so as to reduce
them to one equation describing a specific stream in
terms of given or known quantities. In the calculation
to be presented, expressions will be manipulated until E,
the primary solids underflow rate, can be expressed in
terms of A, XE , X j , X^ / Xp, and X$ , quantities which the
designer would know or assume from plant influent surveys,
knowledge of water chemistry and an understanding of the
general solids separation/destruction efficiencies of the
processing involved. The calculation is carried out as
follows:
a. Define M by solids balances on streams around the
primary sedimentation tank:
(3-D
Therefore,
M = -- A (3-2)
XE
b. Define M by balances on recycle streams:
M = N + P (3-3)
N = XNE (3-4)
P = XP(S + K) (3-5)
S = XSK (3-6)
Therefore,
P = XP(1 + XS)K (3-7)
K + J + N = E (3-8)
Therefore,
K=E-J-N=E- XjE - XNE = E(l - Xj - XN ) (3-9)
and
P = XPE(1 - Xj - XN)(1 + XS) (3-10)
Therefore,
M = E[XN + XP(1 - Xj - XN)(1 + XS)] (3-110
3-20
-------
c. Equate equations (3-2) and (3-11) to eliminate M:
- - A = E[XN + Xp(l - Xj - XN)(1 + Xs)j
_
- XN - xp(i- Xj - xN)(i + xs)
E is now expressed in terms of assumed or known
influent solids loadings and solids separation/
destruction efficiencies.
Once the equation for E is derived, equations for other streams
follow rapidly; in fact, most have already been derived. These
are summarized in Table 3-6.
TABLE 3-6
MASS BALANCE EQUATIONS FOR FLOWSHEET OF FIGURE 3-7
A
E =
* - XN - XP (1-XJ-V(1 + V
M = ^ - A
E
B - d-X) (A + M)
J - XTE
J
N = XNE
K - E(1-X -X )
S = XSK
p = xp (1 + XS)K
L - K (1 + Xg)(l-Xp)
3-21
-------
DEGRITTED SEWAGE
SOLIDS
299,000
409,000
110,000
SOLIDS
GENERATED
BY CHEMICAL
ADDITION
M
56,030
0
56,030
PRIMARY
SEDIMENTATION
XE = 0.90
E
418,527
DIGESTION
Xj = 0.25
XN = 0.0
I K
313,895
i
DEWATERING
XP = 0.15
X = 0.19
317,505
T
TO ULTIMATE
DISPOSAL
46,503
104,632
-^ EFFLUENT
59,640
SOLIDS
DESTROYED
(CONVERTED
TO GAS AND
WATER)
CONDITIONING
CHEMICALS
ALL QUANTITIES ARE
EXPRESSED IN POUNDS
PER DAY
1 Ib/day = 0.454 kg/day
FIGURE 3-8
QFD FOR CHEMICALLY-ASSISTED
PRIMARY PLANT
3-22
-------
Figure 3-8 is a worked example in which all solids flow rates
are calculated. For this example the following information was
provided:
a. Based on estimates from facility planning studies,
average influent suspended solids loading is
299,000 pounds per day (136 t/day). Alum is added to
the degritted raw sewage to increase capture. The
chemical solids generated as the result of alum
addition is estimated at 110,000 pounds per day
(50 t/day). The latter figure is derived from pilot
work at Seattle, Washington, where the ratio of new
solids generated/solids in untreated raw sewage was
0.37/1 when alum ( A12 ( 804 ) 3 1 4H20 ) additions of
110 to 125 mg/1 were added to raw wastewater (1).
Therefore, A = 299,000 (1 + 0.37) = 409,000 pounds
per day (185 t/day).
b. Primary sedimentation solids capture is 90 percent of
the sum of sewage solids, chemical solids and recycle
solids which enter the basin. Note that solids
capture as usually computed (sewage solids basis
only) is only 84.4 percent, i.e.,
, -, _ effluent suspended solids. -,,,
influent sewage solids
10° = 84-4 percent
c. Twenty-five percent of the suspended solids fed to
the digestion system are destroyed, i.e., converted
to gas or water (Xj = 0.25). The number assumed is
somewhat less than the usual value used (0.30-0.40),
since the biodegradable fraction of digester feed in
this instance is low because of the large proportion
of chemical solids present.
d. Digesters are not supernated (X^ = 0.0).
e. Solids capture in the dewatering units is 85 percent
(XP = 0.15) .
f. Conditioning chemicals are 19 percent by weight
of digested sludge fed to the dewatering units
(XS = 0.19).
When all loadings are expressed quantitatively and superimposed on
the flowsheet, the designer can begin to develop a feel for the
process. The effects of recycle loading and individual process
efficiencies on overall process performance can be assessed by
manipulation of the variables. Calculations can be done very
3-23
-------
rapidly when the mass balance equations (presented in Table 3-6)
are set up for solution on a computer or a programmable
calculator.
The investigator must exercise judgment in estimating the various
process efficiencies ( X@ ) . For example, one should assume
reduced efficiencies for primary sedimentation if recyle streams
contribute large quantities of solids to the sedimentation tank,
since recycled solids tend to be less easily removed than fresh
solids from the sewer system. Their mere presence in the recycle
stream is an indication of the difficulties in separating them.
3.4.2 Example: QFD for Secondary Plant with Filtration
The example just worked was relatively simple. Figure 3-9 shows
a jno_r_e_ comp 1 e x sy s tern second ary ____ ae r obi c _ biologies It re a t me n t
followed by filtration. Mass balance equations for this system
are summarized in Table 3-7. For this flowsheet the following
information must be specified.
a. Influent solids (A).
b. Effluent solids (Q), that is, overall suspended solids
removal must be specified.
c. XE , XQ , X j , XN , XR, and Xg are straightforward
assumptions about the degree of solids removal,
addition or destruction.
d. XD, which describes the net solids destruction
reduction or the net solids synthesis in the
biological system, must be estimated from yield
data (see Section 4.3.2.4). A positive XD signifies
net solids destruction. A negative XD signifies net
solids growth. In this example 8 percent of the
solids entering the biological process are assumed
destroyed, i.e., converted to gas or liquified.
Note that alternative processing schemes can be evaluated simply
by manipulating appropriate variables. For example:
a. Filtration can be eliminated by setting XR to zero.
b. Thickening can be eliminated by setting XG to zero.
c. Digestion can be eliminated by setting Xj to zero.
d. Dewatering can be eliminated by setting Xp to zero.
e. A system without primary sedimentation can be
simulated by setting XE equal to approximately
zero, e.g. , 1 x 10~8. x^ cannot be set equal
to exactly zero, since division by Xg produces
indeterminate solutions when computing E.
3-24
-------
DEGRITTED SEWAGE
SOLIDS
A
299,000
f M
77,794 "
4 N
r 15,415
P
22,012
i
_ TREATMENT 0
CHEM1CALS D SOLIDS DESTROYED
10,831 OR SYNTHESIZED
40,367 ^ |R
PRIMARY
SEDIMENTATION
XE = 0.65
1
251,438
' 1
DIGESTION
XN = 0.05
Xj = 0.35
\
DEWA1
V
Xp
K
184,978
p
"ERING
0.10
L
198,111
SECONDARY
8 » -cn^MT^TinN c » FILTRATION Q fc
135,390*" ^D"VlCArJKATION 57,667*" XR = 0.70 17,300*
**"- XD = 0.08
G F
10,034 66,892
1 '
THICKENING
X_ = 0.15
b
H
56,858
j SOLIDS DESTROYED
1U/,9U4 GAS AND WATER)
S CONDITIONING
35,146 "CHEMICALS
AM HI 1AM
EFFLUENT
TO ULTIMATE
DISPOSAL
EXPRESSED IN POUNDS
PER DAY
1 Ib/day = 0.454 kg/day
FIGURE 3-9
QFD FOR SECONDARY PLANT WITH FILTRATION
A set of different mass balance equations must be derived
if flow paths between processing units are altered. For example
the equations of Table 3-7 do not describe operations in
which the dilute stream from thickener (stream G) is returned
to the biological system instead of the primary sedimentation
tank.
3-25
-------
TABLE 3-7
MASS BALANCE EQUATIONS FOR FLOWSHEET OF FIGURE 3-9
E =
A - (r^)(Y - V
? - a - 3 (Y)
XE
Where a = Xp (1 - Xj - XN)(1 +.Xg) + XN
(1 - X_). (1 - X_)
D _ ^ . _^_
P - v ~
XE
Y - XG + a (1 - XQ)
B = (1 - XE)E
XE
1 - XR
D = XDB
F - 3 E - T-2-
G = XQF
H = (i - XG)F
J = XT (E + H)
J
K = (1 - Xj - XN) (E|-+" H)
L = K (1 + Xg) (1 - Xp)
M = ^ - G - A
E
N = X.T (E + H)
P = Xp (1 +Xg)K
XR
R =l^Q
1 XR
S = XSK
3-26
-------
3.5 Sizing of Equipment
The QFD described in the previous section can be an important aid
to a designer in predicting long-term (i.e., average) solids
loadings on sludge treatment components. This allows the
designer to establish such factors as operating costs and
quantities of sludge for ultimate disposal. However, it does not
establish the solids loading which each equipment item must be
capable of processing. A particular component should be sized to
handle the most rigorous loading conditions it is expected to
encounter. This loading is usually not determined by applying
steady-state models (e.g., QFD calculations) to peak plant loads.
Because of storage and plant scheduling considerations, the rate
of solids reaching any particular piece of equipment does not
usually rise and fall in direct proportion to the rate of solids
arriving at the plant headworks. Consider a system similar in
configuration to that shown on Figure 3-9. If maximum solids
loads at the headworks (Stream A) are twice the average value,
it does not necessarily follow that at that instant maximum
dewatering loads (Stream K) are twice the average dewatering
load.
To pursue this further, consider the design of a centrifuge
intended to dewater anaerobically digested primary and secondary
sludge at a small treatment plant. The flow scheme is similar to
that shown on Figure 3-9. The plant is staffed on only one shift
per day, seven days per week. The digesters are complete-mix
units equipped with floating covers. Because of the floating
covers, digester volume can vary. Secondary sludge is wasted
from the activated sludge systems to a dissolved air flotation
thickener prior to digestion whenever operators are available to
operate the thickener.
As indicated, the average loadings to the centrifuge can be
defined by the QFD, but computation of the necessary centrifuge
capacity requires analysis of both the load dampening effect of
the storage in the digesters and the plant operating schedule.
During periods of peak plant solids loadings, loads to the
dewatering units may be attenuated by storing portions of the
peak loadings within the digester. This can be done by either
mechanism 1 or mechanism 2 below, acting either singly or in
concert.
1. Digester volume is increased by allowing the digester
floating cover to rise.
2. Solids are allowed to concentrate and thus accumulate
within the digester (See Chapter 15, Section 15.2.2.2 for
example of storage by mechanism 2).
The effect of both mechanisms 1 and 2 is storage within the
digester of part of the load which would otherwise go to the
centrifuge. Thus peak dewatering loads will not be 2.5 times
the average when peak solids mass withdrawn from primary and
3-27
-------
secondary sedimentation tanks are 2.5 times the average, but
something less, for example, only 1.4 times the average value.
The degree of load dampening is a direct function of the size and
operating configuration of the digester.
Since the centrifuge will only operate when attended, the
"design" loading must account for this factor. The centrifuge
must be either capable of processing, during one shift, all the
sludge which must be extracted from the digester during the peak
day (for example, 1.4 times average quantity) or the operators
must dewater sludge for longer than one shift per day. A
judgment would be needed at this point whether to pay for
increased equipment capacity or operator overtime to handle the
peak loads. With no operator overtime, the "design" centrifuge
capacity would have to be 1.4 x 24/8 =4.2 times the average
daily digested sludge production to account for both the effect
of sludge peaking, storage volume and only one operations shift
per day.
Note that the dissolved air flotation thickener would need to be
designed for 24/8 x 2.5 = 7.5 times the average daily rate of
waste activated sludge production if it is assumed no upstream
storage is available for dampening thickener loadings, the
thickener itself has no storage capacity, and the thickener is
only operated one shift per day.
The foregoing example shows the influence of solids peaking,
storage volume and operating strategy on the selection of
design loadings for a particular sludge handling process.
Several other factors are important in selecting the capacity a
unit must have, including:
Uncertainties. When systems are designed without the
benefits of pilot or full-scale testing, actual sludge
quantities and characteristics as well as efficiencies of
the sludge handling system components may not be known
with certainty. The degree and potential significance
of the uncertainties must be considered when developing
design criteria. This usually has the effect of
introducing a safety factor into the design so that
reliable performance can be obtained no matter what
conditions are encountered in the full-scale application.
The magnitude of the safety factor must be determined by
the designer, based on his judgement and experience.
Equipment reliability. Greater capacity or parallel
units must be specified if there is reason to believe
that downtime for any particular units will be high.
Sensitivity of downstream components. If losses in
efficiency of a particular sludge handling component
at peak loading conditions would cause problems for
downstream processes, this upstream process should
3-28
-------
be designed conservatively. Conversely, if reduced
efficiency could be tolerated, design need not be so
conservative.
3.6 Contingency Planning
As indicated previously, flexibility to cope with unforeseen
problems is highly desirable in any wastewater solids management
system. Such problems and possible solutions include:
Equipment breakdowns. Downtime may be minimized by
having maintenance people on call, by advance purchase of
key spare parts, by providing parallel processing units
and by making use of storage.
Solids disposal problems. These may include closures of
landfills, unwillingness of current users to further
utilize sludge, failure of a process to provide a sludge
suitable for utilization, strikes by sludge transporters,
and inability to dispose of sludge due to inclement
weather. Disposal problems can be reduced by providing
long-term storage and/or more than one disposal
alternative.
Sludge production greater than expected. In some
instances this may be dealt with by operating for more
hours per week than normal or by using chemicals to
modify sludge characteristics, thus increasing solids
processing capacity.
Because of these factors, it is desirable to have more than one
process for sludge treatment and disposal. Often it is possible
to add considerable flexibility with modest investment. Backup
or alternative wastewater solids treatment processes often have
higher operating costs per ton of sludge processed than the
primary processes. This is acceptable if the alternative process
is not frequently needed and can be provided at minimum capital
cost.
3.6.1 Example of Contingency Planning for Breakdowns
Assume the plant is a 10 MGD
sludge thickening, anaerobic
dewatering as shown on Figure
include:
activated sludge facility with
digestion, and digested sludge
3-10. Pertinent design details
! The waste activated sludge (WAS) thickener can be
operated with or without polymers. If polymers are used,
a more concentrated sludge can be produced. WAS can be
diverted to the headworks if the WAS thickener is removed
from service.
3-29
-------
WASTE ACTIVATED SLUDGE WHEN THICKENER IS INOPERATIVE
r
PRELIMINARY ^
PRIMARY . .. ACTIVATED _ DISINFECTION
p TREATMENT *^^ SEDIMENTATION " "" SLUDGE ' " DISCHARGE
GRIT, ETC.
SIDESTREAM
800 Ib/day
1
SIDESTREAM
1,000 Ib/day
1
THICKENER FEED
9,000 Ib/day
1.0% SOLIDS
r 108,000 gpd
I SLUDGE
THICKENER
PRIMARY 1 1 N PI F R F 1 OW
PRIMARY SLUDGE ONLY
10,000 Ib/day
"5.0% SOLIDS
24,000 gpd
PRIMARY SLUDGE + W.A.S.
18,000 Ib/day
2.5% SOLIDS
86,000 gpd
*
f I I
DIGESTER DIGESTER
1 2
THICKENED W.A.Sr(8,000 Ib/day)
NO POLYMER
3.5% SOLIDS
27,000 gpd
WITH POLYMER
4.5% SOLIDS
2 1,000 gpd
""^ DIGESTED SLUDGE (11,000 Ib/day)
SLUDGE
STOCKPILE
1 1
DEWATERING DEWATERING
UNIT UNIT
1 2
* *
DEWATERED CAK
33.9 yd3 @ 17
^ _ OR
""* 26.2 yd 3@ 22
1 i
D
E (10.200 Ib/dav)
% SOLIDS
% SOLIDS
TO LANDFILL
VIA 16 yd3 TRUCK
1 Ib/day = 0.454 Kg /day
1 gpd = 0.00378 m3/day
,3
FIGURE 3-10
CONTINGENCY PLANNING EXAMPLE
3-30
-------
2. Two complete-mix digesters with floating covers are
provided. Each digester has a net volume of
610,000 gallons (2,310 m3 ) at minimum cover height.
Net volume at maximum cover height is 740,000 gallons
(2,803 m3), thus total digester storage volume is
2 (740,000-610,000) = 260,000 gallons (984 m3). The
digesters are not supernated.
3. Two dewatering units are provided. Each unit, when fed
at 90 gpm (40.8 m3/hr) can produce a 22 percent solids
cake. When the dewatering units are fed at 110 gpm
(49.9 m3/hr) a 17 percent solids cake is produced. The
units are fed at 90 gpm (40.8 m3/hr) unless conditions
dictate otherwise. The bulk density of each cake is
65.5 pounds per cubic foot (1,050 kg/m3).
4. The cake is trucked to ultimate disposal. Each truck
holds 16 cubic yards (12 m3) of cake.
5. A dewatered sludge storage area of capacity 750 cubic
yards (574 m3) is available.
6. Weekends are 2.7 days long (from 5 p.m. Friday to 8 a.m.
Monday).
CjiS_e__A. All units available:
1. Digester detention time = ( ^QOQ^^OOof gpd = 24 days'
2. Dewatering operation:
a. Weekly sludge feed = 7 (24,000 + 27,000 gpd)
= 357,000 gallons (1,350 m3).
b. Hourly throughput = 2 x (90 gpm) (60 min/hr)
= 10,800 gal per hr (40.8 m3/hr).
. , t 357,000 gal
c. Operation is carried out over 10,800 gai/hr
= 33 hours per week.
d. 26.2 cubic yards (20.0 m3) of 22 percent solids
sludge cake is produced each day.
3. If dewatering is not operated over the weekend, then
51,000 gpd (2.7 days) = 138,000 gal (522 m3) of digested
sludge must be stored in the digesters during this
period. Available storage which can be obtained
by letting the floating cover rise is 260,000 gallons
(983 m3 ) . Therefore digester storage capacity is
adequate for weekend storage, including long (3.7 day)
weekends.
3-31
-------
4. Truckloads required to haul dewatered cake = 26>2 yd3/day
, , , , n , , ,,, 16 yd3/truck
=1.6 truckloads per day (11 per week).
In summary, the dewatering operation can be carried out in
a normal 5-day, 8-hour-per-day week. Time is available for
start-up and shutdown and for providing good supervision.
Digester detention time is more than adequate for good digestion.
Case B. Thickener is out of service. All other units are
available. Waste activated sludge is diverted to the plant
headworks and is subsequently removed in the primary
sedimentation tank.
1. Digester detention time = - (86°ooo° pd^ = 14 days;
short, but tolerable.
2. Dewatering operation:
a. Weekly sludge feed = 7 (86,000 gpd) = 602,000 gal
(2280 m3).
b. Hourly throughput. At 90 gallons per minute,
throughput is 10,800 gallons per hr (40.8 m3/hr). At
110 gallons per minute, throughput is 13,200 gallons
per hr (49.9 m3/hr).
c. Operating hours required. At 90 gallons per minute
« T /, , ^ u 602,000 gal
(40.8 mj/hr), required operating hours = IQ 800 qph
= 56 hours per week. This requires substantial
overtime or a second shift. At 110 gallons per
minute (49.9 m3/hr), required operating hours =
gau = 46 hours per week. This reduces the
on u
13,200 gph
amount of overtime required.
d. If the dewatering units operate at 90 gallons per
minute (40.8 m3/hr), 26.2 cubic yards per day
(20.0 m3/day) of 22 percent cake is produced.
Operation at 110 gallons per minute (49.9 m3/hr)
produces 33.9 cubic yards per day (25.9 m3/day ) of
a 17 percent solids sludge cake.
3. If dewatering units are not rurt on weekends, 86,000 gal/
day x 2.7 days = 232,000 gallons (878 m3 ) must be stored
in the digesters. Digester storage capacity is adequate
for normal weekends, but not long weekends.
4.
For 22 percent cake, 11 truckloads per week are required.
For 17 percent cake, 15 truckloads per week are required.
3-32
-------
In summary, loss of the thickener reduces digester detention
time, increases required dewatering unit operating time and the
amount of trucking required for disposal of cake. The operation
can be managed, but with more difficulty. This example also
illustrates the value of the thickener.
Case C. One digester is out of service. All other units are
operating:
1. Digester detention time = 24,000°;°2?, 0^ gpd = 12 d^s«
This is only marginally adequate. By using polymers in
the thickener, assume waste activated sludge thickness
is increased from 3.5 to 4.5 percent. Detention time is
increased to 24,000°|Q21,000 gpd = 14 da^s' sti11 short'
but an improvement.
2. Dewatering operation. This is not greatly affected by
loss of the digester. It can still be operated with a
single shift and a 22 percent cake can can be produced.
3. Weekend storage. Without polymer addition to
the thickener, required storage volume is 2.7 days
x 51,000 gpd = 138,000 gallons (522 m3) . One digester
(130,000 gallons or 492 m3 ) has inadequate storage
and a dewatering machine must be run part of the
weekend. If polymer is used, required storage = 2.7
x 45,000 = 122,000 gallons (462 m3) . One digester is
marginally adequate for storage.
4. Eleven (11) truckloads per week are required to transport
the sludge cake.
In summary loss of a digester can be compensated for by using
polymer in the thickener.
Case D. One dewatering machine is out of service. All other
units are available.
1. Digestion is not affected.
2. Dewatering operation. Try the following alternatives:
a. Feed rate 90 gallons per minute (40.8 m3/hr).
51,000 gpd _ Q 4
Required operating time = 90 gpm (60 min/hrr ~ 9>4
hours per day, every day, excluding start-up and
shutdown time.
b. Feed rate is 110 gallons per minute. Required
operating time = gpm° (SO^in/hr) = 7'8 hours/day,
every day, excluding start-up and shutdown time.
3-33
-------
c. Try adding polymers to thickener and maintaining a
110 gallons per minute feed rate to the dewatering
units. Required operating time = ,,A 45/OQ0 9Pd
110 gpm (60 min/hr)
= 6.8 hours per day, every day, excluding start-up
and shutdown times.
3. Weekend digester storage is not an issue as dewatering
units must be run seven days a week.
4. Eleven (11) truckloads are required to transport
22 percent cake, 15 truckloads are required for
17 percent cake.
In summary, loss of one dewatering unit will require operation of
the remaining unit for seven days a week. overtime costs will be
high.
Case E. Truck strike lasting a month. Assuming 22 percent cake,
sludge, accumulates at about 25 cubic yards (19 m3) a day. The
sludge storage area stockpile must, therefore, be able to store
about 25 (30) = 750 cubic yards (570 m3 ) of sludge to avoid major
problems due to the strike. Odors from the stockpile could be
a problem.
Conclusion: The system as designed should be able to handle
contingencies.
3.7 Other General Design Considerations
3.7.1 Site Variations
Characteristics such as size and location of the plant and
solids disposal sites strongly influence the nature and cost of
treatment and disposal systems.
Disposal may often be accomplished on land, thus
eliminating expensive dewatering, provided adequately
sized sites are within reasonable distances from the
treatment plant. However, dewatering is usually required
if the amount of land available for sludge disposal is
limited or if the sludge must be trucked long distances
for disposal. Sufficient land also permits long-term
storage in faculative lagoons, which can also provide
some inexpensive disinfection.
Zoning regulations are different for different sites.
Locations near waterways and railroads provide
opportunities for barge and rail transportation of
sludges and supplies.
3-34
-------
Structures are less costly if foundation conditions are
good. Quite often, however, wastewater treatment plants
are located in valley bottoms, tidelands, or reclaimed
landfills where expensive foundations are required.
Costs for labor, electricity, freight on chemicals, and
trucking can vary markedly from one region to another.
Because of these variations, the best alternative for one site is
often not the best at another site. Also, reported capital and
operating costs from one site must be carefully adjusted before
being used at another site.
3.7.2 Energy Conservation
As fossil fuel supplies become more scarce and more expensive,
energy conservation becomes increasingly important. The designer
should employ energy-efficient processes and recover energy from
sludges and sludge by-products, where practical.
The following points should be considered in the design of
energy-utilization processes:
Energy from high temperature sources is generally more
useful than energy derived from low temperature sources,
since it can be put to a wider variety of uses.
The evaporation of water in dryers and furnaces, consumes
large amounts of energy. Such processes should therefore
be provided with a we 11-dewatered sludge. Inert
materials such as, chemicals or ash used to condition
sludge for dewatering are, however, also energy consumers.
Energy required for digestion and thermal conditioning is
minimized where thickening is used to reduce the water
content of process feed sludges.
Trucking energy can be reduced if haul distances are
short and the sludge is well-dewatered.
Energy is required for the manufacture and transportation
of chemicals. Therefore, chemicals should be added in
minimum amounts that are consistent with good operation.
Whenever possible chemicals should be employed which
require the least energy to produce and transport.
Costs saved by reducing peak energy demands can be
subtantial. In some instances, a treatment plant's
electrical bills are largely determined by peak energy
loadings, as opposed to total energy consumed. The
designer should actively seek solutions to reduce
peak energy demand. Energy recovered from sludge and
sludge-derived fuel can be used for this purpose.
3-35
-------
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 sludgesfor example, trickling filter
sludgehave been added to the influent wastewater.
That the sludge contains no major sidestreams from sludge
processing.
As an example, if a designer estimates the TSS entering
the primary clarifier as 0.20 pound per capita per day
(0.09 kg/capita/day, and the removal efficiency of the clarifier
as 60 percent, the estimated primary sludge production is
0.12 pound per capita per day (0.054 kg/capita/day).
If relevant data are available on influent wastewater suspended
solids concentrations, such data should, of course, be used for
design purposes. Estimates of TSS removal efficiency in primary
sedimentation tanks may be refined by use of operating records
from in-service tanks or by laboratory testing. The "Standard
Methods" dry weight test for settleable matter estimates under
ideal conditions the amount of sludge produced in an ideal
sedimentation tank (3). Sludge production will be slightly lower
in actual sedimentation tanks.
4.2.1.2 Industrial Waste Effect
Suspended solids removal efficiency in primary sedimentation
depends to a large extent on the nature of the solids. It
is difficult to generalize about the effect that industrial
suspended solids can have on removal efficiency, but an example
illustrates that the effect can sometimes be dramatic. At
North Kansas City, Missouri, a municipal plant serves residential
customers and numerous major industries, including food
processing, paint manufacturing, soft-drink bottling, paper
manufacturing, and grain storage and milling. Raw wastewater
entering the plant had a 15-day average suspended solids
concentration of 1,140 mg/1 that was attributable to the
industries. Primary sedimentation removed 90 percent of these
solids. The quantity of primary sludge was, therefore, about
8,000 pounds per million gallons (1,000 mg/1) of wastewater
treated. This value is several times the normal one for
domestic wastewater. On two of the 15 days, removal exceeded
14,000 pounds per million gallons (1,700 mg/1) (4).
4-2
-------
4.2.1.3 Ground Garbage Effect
Home garbage grinders can significantly increase the suspended
solids load on a wastewater treatment plant. These solids are
largely settleable. Estimates of the increased primary sludge
resulting from the use of garbage grinders range from 25 percent
to over 50 percent (1,5,6).
4.2.1.4 Other Sludges and Sidestreams
Operating experience shows clearly that the amount of sludge
withdrawn from the primary sedimentation tank is greatly
increased when sludge treatment process sidestreams such as
digester supernatant, elutriate, and filtrates or centrates
and other sludges like waste-activated are recycled to the
primary sedimentation tank. Quantifying the solids entering and
leaving the primary clarifier by all streams is an important tool
for estimating primary sludge production when recycled sludges
and sludge process sidestreams contribute large quantities of
solids.
4.2.1.5 Chemical Precipitation and Coagulation
When chemicals are added to the raw wastewater for removal
of phosphorus or coagulation of nonsettleable solids, large
quantities of chemical precipitates are formed. The quantity of
chemical solids produced in chemical treatment of wastewater
depends upon the type and amount of chemical(s) added,
chemical constituents in the wastewater, and performance of the
coagulation and clarification processes. It is difficult to
predict accurately the quantity of chemical solids that will
be produced. Classical jar tests are favored as a means for
estimating chemical sludge quantities. The quantities of
suspended solids and chemical solids removed in a hypothetical
primary sedimentation tank that is processing wastewater which
has been treated by lime, aluminum sulfate or ferric chloride
addition are estimated in Table 4-1.
4.2.1.6 Peak Loads
Peak rates of primary sludge production can be several times the
average. Peak solids production levels also vary from one plant
to another. Four studies of primary sludge production rates are
summarized and presented here.
At Ames, Iowa, (9) the wastewater is basically of domestic
origin. A university contributes about 30 percent of the
volumetric and mass loads. Storm .runoff is collected and kept
separate from the domestic wastewater. For 21 years of record,
the suspended solids loads in the peak month of each year were
divided by the yearly average. The average of these ratios
4-3
-------
was 1.37. The average for comparison of peak days and peak
months over ten years of record was 1.59. Thus, in a typical
year, the maximum daily flow would be about 1.37 x 1.59, or
2.2 times the average. The maximum day's sludge production
was, therefore, expected to follow a similar pattern and was
estimated to be 2.2 times the average value.
TABLE 4-1
PREDICTED QUANTITIES OF SUSPENDED SOLIDS AND CHEMICAL
SOLIDS REMOVED IN A HYPOTHETICAL PRIMARY SEDIMENTATION TANK (7,8)
Chemical addition3
Sludge type
Suspended solids, Ib/mg
Chemical solids, Ib/mg
Total sludge production, Ib/mg
(kg/cu m)
No chemical
addition
1,041
-
1,041
(0. 13)
Lime
1, 562
2,082
3,644
(0.44)
Alumd
1,562
362
1,924
(0.23)
Iron6
1, 562
462
2,024
(0.24)
Assumes 10 mg/1 influent phosphorus concentration (as P) with
80 percent removed by chemical precipitation.
Assumes 50 percent removal of 250 mg/1 influent TSS in primary
sedimentation.
°125 mg/1 Ca(OH)2 added to raise pH to 9.5.
d!54 mg/1 A12(SO4)3 14 H20 added.
S84 mg/1 FeCl3 added.
Note: Assumes no recycle streams (for example, recycle of waste-activated
sludge to primary sedimentation, digester supernatant, etc.).
Secondary solids production would be cut from 833 Ib/mg without
chemical addition to 312 Ib/mg with chemical addition in this
hypothetical plant.
A study conducted in 1936 used data from Chicago, Cleveland,
Columbus, Syracuse, Rochester, and several other large American
cities (10) to show a typical relationship between peak raw
sewage solids loads entering a plant and duration of time that
these peaks persist. This relationship is shown graphically
on Figure 4-1. The curve is appropriate for large cities with a
number of combined sewers on flat.grades. The peaks occur at
least partly because solids deposited in the sewers at low flows
are flushed out by storm flows.
Data were collected over a five-year period from the West Point
plant at Seattle, Washington and used in a 1977 study (11). Peak
primary sludge loads of four- to ten-day durations were compared
with average loads. The duration of four days was selected
because it appeared to be highly significant to digester
operations at this plant, and because loads tended to drop after
about four days of heavy loading. The highest four-day primary
4-4
-------
sludge production was more than four times the normal production
from the plant's service area. Main contributors to the peak
load were:
Solids deposits in the sewers. These deposits were
resuspended during high flows and carried to the
treatment plant. The computer-operated storage system,
which minimizes combined sewer overflows, apparently
contributed to solids deposition/reentrainment.
Storm inflow. Measurements of TSS
fluctuate widely but often show over
solids. A large portion of the West
contains combined sewers.
in storm drainage
200 mg/1 suspended
Point service area
Sludge conditioning and dewatering. Problems in these
processes have caused the sidestreams to contain more
solids than usual.
Q
<
O
500
400
Q
LLJ
cc
LLJ
LU
O
DC
LU
0.
300
200
100
10
15
20
25
30
DURATION OF PEAK LOAD, days
FIGURE 4-1
TYPICAL RELATIONSHIP BETWEEN PEAK SOLIDS LOADING
AND DURATION OF PEAK FOR SOME LARGE AMERICAN CITIES (10)
The fourth study, done in 1974, discussed two plants in
St. Louis, Missouri (12). The graphs shown on Figure 4-2
illustrate the variation in daily waste primary sludge production
as a fraction of the average waste primary sludge production with
duration of that production rate for the eight months that data
4-5
-------
were taken. Both of
loads, and both serve
sewers.
these plants have significant industrial
large areas of combined storm and sanitary
5.0
4.0
3.0
5 2.0
1.0
BISSELL POINT
MAXIMUM
MINIMUM
5.0 i-
4.0 -
3.0 -
2.0 -
1.0
LEMAY
0246
CONSECUTIVE DAYS
C - C RESULTS FROM
THE LEAST EXTREME
OF THE EIGHT MONTHS
0246
CONSECUTIVE DAYS
KEY: A - A AVERAGE OF RESULTS
OF EIGHT MONTHS
B - B MOST EXTREME
RESULTS RECORDED
IN ALL OF THE
EIGHT MONTHS
FIGURE 4-2
PEAK SLUDGE LOADS, ST. LOUIS STUDY (12)
4.2.2 Concentration Properties
Most primary sludges can be concentrated readily within the
primary sedimentation tanks. Several authors claim that a five
to six percent solids concentration is attainable when sludge is
pumped from well-designed primary sedimentation tanks (2,10,13*
14). However, values both higher and lower than the five to
4-6
-------
six percent range are common. Conditions that influence primary
sludge concentration include:
If wastewater is not degritted before it enters the
sedimentation tanks, the grit may be removed by passing
the raw primary sludge through cyclonic separators.
However, these separators do not function properly with
sludge concentrations above one percent (15).
If the sludge contains large amounts of fine nonvolatile
solids, such as silt, from storm inflow, a concentration
of well over six percent may sometimes be attained
(11,16).
Industrial loads may strongly affect primary sludge
concentration. For example, at a plant receiving soil
discharged from a tomato canning operation, a primary
sludge with a 17 percent solids concentration, of which
40 percent is volatile, was recorded. Normal primary
sludge at this plant had a solids concentration of
from five to six percent solids (60 to 70 percent
volatile) (17).
Primary sludge may float when buoyed up by gas bubbles
generated under anaerobic conditions. Conditions
favoring gas formation include: warm temperatures;
solids deposits within sewers; strong septic wastes;
long detention times for wastewater solids in the
sedimentation tanks; lack of adequate prechlorination;
and recirculating sludge liquors (18) . To prevent the
septic conditions that favor gas formation, it may be
necessary to strictly limit the storage time of sludge in
the sedimentation tanks. This is done by increasing the
frequency and rate of primary sludge pumping (19).
If biological sludges are mixed with the wastewater, a
lower primary sludge concentration will generally result.
4.2.3 Composition and Characteristics
Table 4-2 lists a number of primary sludge characteristics.
In many cases, ranges and/or "typical" values are given. In the
absence of recirculating sludge process sidestreams, the percent
of volatile solids in the primary sludge should approximate the
percent volatile suspended solids in the influent wastewater.
A volatile solids content below about 70 percent usually
indicates the presence of storm water inflow, sludge processing
sidestreams, a large amount of grit, sludge from a water
filtration plant that was discharged to the sanitary sewer, low
volatile solids from industrial waste, or wastewater solids that
have a long detention time in the sewers.
4-7
-------
TABLE 4-2
PRIMARY SLUDGE CHARACTERISTICS
Characteristic
PH
Volatile acids, mg/1 as ace-
tic acid
Heating value, Btu/lb (kJ/kg)
Specific gravity of individ-
ual solid particles
Bulk specific gravity (wet)
BOD5/VSS ratio
COD/VSS ratio
Organic N/VSS ratio
Volatile content, percent by
weight of dry solids
Cellulose, percent by weight
of dry solids
Hemicellulose, percent by
weight of dry solids
Lignin, percent by weight of
dry solids
Grease and fat, percent by
weight of dry solids
Protein, percent by weight
of dry solids
Nitrogen, percent by weight
of dry solids
Phosphorus, percent by weight
of dry solids
Potash, percent by weight of
Range of values
5-8
200 - 2,000
6,800 - 10,000
Typical
value
Comments
Reference
0.5 - 1.1
1.2 - 1.6
0.05 - 0.06
64 - 93
60 - 80
- 15
6-30
7-35
20 - 30
22 - 28
1.5 - 4
0.8 - 2.£
10,285
7,600
1.02
1.07
65
40
40
10
3.8
1.6
Depends upon volatile content,
and sludge composition, re-
ported values are on a dry
weight basis.
Sludge 74 percent volatile.
Sludge 65 percent volatile.
Increases with increased grit,
silt, etc.
Increases with sludge thickness
and with specific gravity of
solids.
Strong sewage from a system of
combined storm and sanitary
sewers.
Value obtained with no sludge re-
cycle, good degritting; 42
samples, standard deviation 5.
Low value caused by severe storm
inflow.
Low value caused by industrial
waste.
Ether soluble
Ether extract
Expressed as N
Expressed as P205- Divide
values as P2C>5 by 2.29 to
obtain values as P.
Expressed as K20. Divide
values as K2
-------
the fragmented screenings appear in the primary sludge. Smaller
plastic and rubber items that pass through screens also appear in
the primary sludge.
Primary sludge typically contains over 100 different anaerobic
and facultative species of bacteria (24). Sulfate-reducing
and oxidizing bacteria, worm and fly eggs, and pathogenic
microorganisms are typically present.
4.3 Biological Sludges
4.3.1 General Characteristics
Biological sludges are produced by treatment processes such as
activated sludge, trickling filters, and rotating biological
contactors. Quantities and characteristics of biological sludges
vary with the metabolic and growth rates of the various micro-
organisms present in the sludge. The quantity and quality of
sludge produced by the biological process is intermediate between
that produced in no-primary systems and that produced in
full-primary systems in cases when fine screens or primary
sedimentation tanks with high overflow rates are used.
Biological sludge containing debris such as grit, plastics,
paper, and fibers will be produced at plants lacking primary
treatment. Plants with primary sedimentation normally produce
a fairly pure biological sludge. The concentrations and,
therefore, the volumes of waste biological sludge are greatly
affected by the method of operation of the clarifiers.
Biological sludges are generally more difficult to thicken and
dewater than primary sludge and most chemical sludges.
4.3.2 Activated Sludge
4.3.2.1 Processes Included
Activated sludge has numerous variations: extended aeration;
oxidation ditch; pure oxygen, mechanical aeration, diffused
aeration; plug flow; contact stabilization, complete mix, step
feed, nitrifying activated sludge; etc (2). This manual does not
discuss lagoons in which algal growth is important or lagoons
that tend to accumulate wastewater solids or biological solids.
These methods, however, can be used for predicting activated
sludge production in highly loaded aerated lagoons where the
bacteria are maintained in solution.
4.3.2.2 Computing Activated Sludge Production -
Dry Weight Basis
The quantity of waste-activated sludge (WAS) is affected by two
parameters: the dry weight of the sludge and the concentration
of the sludge. This section describes how the dry weight of
activated sludge production may be predicted.
4-9
-------
Basic J?r_ed ictive Equations
The most important variables in predicting waste-activated sludge
production are the amounts of organics removed in the process,
the mass of microorganisms in the system, the biologically inert
suspended solids in the influent to the biological process, and
the loss of suspended solids to the effluent.
These variables can be assembled into two simple and useful
equations:
Px = (Y)(sr) - (kd)(M) (4-1)
WAST = Px + INV - ET (4-2)
where:
Px = net growth of biological solids (expressed as volatile
suspended solids [VSS]), Ib/day or kg/day;
Y = gross yield coefficient, Ib/lb or kg/kg;
sr = substrate (for example, 6005) removed, Ib/day or
kg/day;
kd = decay coefficient, day~l;
M = system inventory of microbial solids (VSS) micro-
organisms, Ib or kg;
WAST = waste-activated sludge production, Ib/day or kg/day;
NV = non-volatile suspended solids fed to the process,
Ib/day or kg/day;
ET = effluent suspended solids, Ib/day or kg/day.
These equations, as stated or with slight variations, have been
widely used. Equation 4-1 dates back to 1951 (25). However,
different terms and symbols have been used by various authors in
expressing Equations 4-1 and 4-2. Table 4-3 summarizes some of
the terminology that has evolved. The technical literature
reflects some inconsistency in terminology with the term "M."
Test results reported by various authors and presented in
Table 4-3 were derived on the basis of "M" defined as mixed
liquor VSS only.
To use Equation 4-1, it is necessary to obtain values of Y and
kd. While Table 4-4 summarizes several reported values for these
parameters, it is best to determine Y and k^ on an individual
waste stream whenever possible.
4-10
-------
TABLE 4-3
ALTERNATE NAMES AND SYMBOLS FOR EQUATION (4-1)
As used in this chapter
Other symbols for Other common names for
, , _. . similar quantities similar quantities
Symbol Name Dimensions M ^
Px Biological solids Mass AX , dX/dt, A, S, Accumulation, net growth,
production Time dM/dt, Rg excess microorganisms
production
Y Gross yield Mass a, Ks, c Yield coefficient, synthesis
coefficient3 Mass coefficient, growth-yield
coefficient
s Substrate removal Mass dF/dt, S, B, Fi, R Food, utilization, load
r Time
kd Decay constant 1 b, K^, Ke Endogenous respiration.
Time maintenance energy,
auto-oxidation
M Microbial solids Mass S, X, Xv Microbial mass, solids under
inventory aeration, solids inventory,
mixed liquor solids
SThe letter Y has also been used for the net yield coefficient Px/sr. The net yield
coefficient is quite different from the gross yield coefficient.
To use Equation 4-2, it is necessary to estimate
non-volatile influent solids, and Eip, effluent suspended solids.
The following are generally included within the term
Non-volatile solids in influent sewage, including recycle
sludge liquors.
Chemical precipitates--for example, aluminum phosphates--
when alum is added to the activated sludge process.
Stormwater solids that are not removed in previous
processes (36).
Normal non-volatile content of the activated sludge.
In the absence of sludge liquors, chemical precipitates,
and stormwater, activated sludge will be about 80 percent
volatile (less in extended aeration) at most municipal
treatment plants.
To compute ET, a small value such as 10 mg/1 TSS should be
used.
The following sections discuss several factors that can influence
the production of waste-activated sludge. Section 4.3.2.3 is a
detailed example of how sludge quantities should be computed.
4-11
-------
TABLE 4-4
VALUES OF YIELD AND DECAY COEFFICIENTS FOR
COMPUTING WASTE-ACTIVATED SLUDGE
Gross yield
Reference coefficient3
25 0.5
26 0.70
26, 27 0.67
28, 29 0.73
30 0.94
31 0.73
32 0.5
12 0.74
30 1.57
33 1.825
34 0.65
34 0.70
34 0.54
35 1.1
Decay
. . D
coefficient
0.055
0.04
0.06
0.075
0.14
0.06
Not calculated
(negligible)
0.04
0.07
0.20
0.043
0.048
0.014
0.09
Type of
wastewater
Primary effluent
Primary effluent
Primary effluent
Primary effluent
Primary effluent {wastewater
includes dewatering
liquors)
Primary effluent
Primary effluent (military
base)
Primary effluent (much in-
dustry)
Raw degritted including de-
watering liquors
Raw degritted
Raw degritted
Raw degritted
Raw degritted
Raw
Scale of
plant
Bench
Pilot-
Pull
Pilot
Pilot
Pilot
Pilot
Pilot
Pilot
Bench
Bench
Bench
Full
Full
Aeration
Air
Oxygen
Air
Air
Air
Oxygen
Air
Oxygen
Air
Air
Air
Air
Air
Air
Temperature,
Or-
19
Not
18
10
15
18
0
17
15
4
20
20
Not
Not
^
- 22
stated
- 27
- 16
- 20
- 22
- 7
- 25
- 20
- 20
- 21
- 21
stated
stated
Sludqe age.
days
2.8 - 22
1-4
1,2 - 8
1-12
0.5 - 8
2.5 - 17
Long
2.1 - 5
0.6 - 3
1-3
d
11 and up
d
Long
d
Long
1.1 -2.4
BOD removal
1 1 1- '
Influent
Influent minus
effluent
Influent minus
effluent
Influent minus
effluent
Influent minus
soluble ef-
fluent
Influent minus
effluent
Influent
Influent minus
soluble ef-
fluent
Influent minus
soluble ef-
fluent
Soluble in-
fluent minus
soluble ef-
fluent
Influent minus
effluent
Influent minus
effluent
Influent minus
effluent
Influent minus
effluent
3Gross yield coefficient Y, Ib (kg) VSS/lb (kg) BOD5.
Decay coefficient k , days
CMean cell residence time or sludge age 9^, measured as mass of mixed liquor
VSS divided by biological solids production P^. Note that coefficients may be
somewhat different if total system inventory of VSS (mixed liquor VSS plus VSS
in clarifiers) is used rather than just mixed liquor VSS.
Extended aeration.
Note: All values in this table are for an equation of the type px = Ysr - kdM (equation 4-1).
Effect of_Sludg_e Age and F/M Ratio
Equation (4-1) can be rearranged to show the effect of the
sludge age (9m).
(sr)
r ( 4 -
(4
(kd)(em)
where Qm = =?- = sludge' age, days.
4-12
-------
Similarly, Equation 4-1 can be rearranged to show the effect of
the food-to-microorganism ratio (F/M):
= (Y)(sr) -
where:
C2 = coefficient to match units of sr and "F" in F/M; if sr
is 8005 removed (influent minus effluent), then C2 is
BOD5 removal efficiency, about 0.9;
F/M = food-to-microorganism ratio;
BODs applied daily
VSS (mass) in system
As 6m increases and F/M decreases, the biological solids
production Px decreases. Sludge handling is expensive, and costs
can be reduced by using high values of Qm or low values of F/M.
However, there are offsetting cost factors, such as increases in
the aeration tank volume needed, oxygen requirements for the
aerobic biological system, etc. Also, as seasons change, so
may the optimum Om and F/M for. maximum wastewater treatment
efficiency. Therefore, it is desirable to be able to operate
across a range of conditions. Obviously, trial-and-error
calculations are required to determine the least costly system.
Effect of Nitrif i cat ion
Nitrification is the bio-oxidation of ammonia nitrogen and
organic nitrogen to the nitrite and nitrate forms. Compared with
processes that are designed for carbonaceous (8005, COD) oxida-
tion only, stable nitrification processes operate at long sludge
ages (Sm) and low food-to-microorganism ratios (F/M). Also,
nitrification processes are often preceded by other processes
that remove much of the 8005 and SS. As a result, activated
sludge in a nitrification mode generally produces less waste-
activated sludge than conventional activated sludge processes.
However, there is an additional component to nitrification
sludge, the net yield of nitrifying bacteria, YN. This may be
estimated at 0.15 pounds SS per pound of total Kjeldahl nitrogen
(organic plus ammonia) removed (37). YN varies with temperature,
pH, dissolved oxygen, and cell residence time. However, detailed
measurements of Y^ are not ordinarily required for sludge
facility design because the yield of nitrifying bacteria is
small. For example, if YN is 0.15 and if the nitrifying process
removes an ammonia nitrogen concentration of 20 mg/1 and an
organic nitrogen concentration of 10 mg/1 then nitrification
would add 0.15 x (20+10) = 4.5 milligrams of nitrifying bacteria
per liter of wastewater (38 pounds per million gallons). These
4-13
-------
quantities are small compared to other sludges. In single-stage
nitrification processes, the sludge production figures must also
include the solids produced from the carbonaceous oxidation,
computed at the 0^ and F/M of the nitrifying system.
Effect of Feed Composition
The type of wastewater that is fed to the activated sludge
process has a major influence on the gross yield (Y) and
decay (k^) coefficients. Many industrial wastes contain
large amounts of soluble BOD5 but small amounts of suspended
or colloidal solids. These wastes normally have lower Y
coefficients than are obtained with domestic primary effluent.
On the other hand, wastes with large amounts of solids, relative
to BOD^, either have higher Y coefficients or require adjustments
to reflect the influent inert solids. Even among soluble wastes,
different compositions will cause different yields.
Effect ofDissolved Oxygen Concentration
Various dissolved oxygen (DO) levels have been maintained in
investigations of activated sludge processes. Very low DO
concentrationsfor example, 0.5 mg/1in conventional activated
sludge systems do appear to cause increased solids production,
even when other factors are held constant (38). However,
there is vigorous disagreement concerning solids production at
higher DO levels. Some investigators state that use of pure
oxygen instead of air reduces sludge production. This is
attributed to the high DO levels attained through the use of pure
oxygen (39,40). Other investigators in recent well-controlled
investigations have concluded that if at least 2.0 mg/1 DO is
maintained in air-activated sludge systems, then air and oxygen
systems produce the same yield at equivalent conditions (such as
food-to-microorganism ratio) (41, 42).
Efj_ect_of Temperature
The coefficients Y (gross yield) and k^ (decay) are related to
biological activity and, therefore, may vary due to temperature
of the wastewater. This variation has not been well documented
in pilot studies and process investigations. One study obtained
no significant difference due to temperature over the range
39° to 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 low5 to 10 percent of observed solids loads
4-33
-------
to activated sludge sedimentation tanks. Trickling filter sludge
also has better thickening properties than activated sludge.
Consequently, trickling filter sludge can be withdrawn at a much
higher concentration than waste-activated sludge. Concentration
data are summarized in Table 4-10.
TABLE 4-10
CONCENTRATION OF TRICKLING FILTER SLUDGE
WITHDRAWN FROM FINAL CLARIFIERS
Type of sludge
Trickling filter,
alone
Trickling filter, com-
bined with raw primary
Percent dry
solids
Comments
Reference
5-10
7
7
3
3-4
4-7
3-6
Depends on solids residence time
in trickling filter
Low-rate trickling filter
High-rate trickling filter
69
13
70
70
71
2
2,69
The solids flux method for predicting sludge concentration may be
used with trickling filter sludge (52). This method requires
measurement of initial solids settling velocity versus solids
concentration. Such relationships have been reported for at
least one trickling filter process (64).
4.3.3.3 Properties - Trickling Filter Sludge
Table 4-11 contains a few analyses of trickling filter sludge
properties. The microbial population that inhabits a trickling
filter is complex and includes many species of algae,
fungi, protozoa, worms, snails, and insects. Filter
their larvae are often present in large numbers around
filters.
bacteria,
flies and
trickling
4.3.4 Sludge from Rotating Biological Reactors
Rotating biological reactors (RBRs) are used for the same basic
purposes as activated sludge and trickling filters: to remove
BODc; and suspended solids and, where necessary, to nitrify.
The RBR process uses a tank in which wastewater, typically
primary effluent, contacts plastic media in the shape of large
discs. Bacteria grow on the discs. The discs rotate slowly on
horizontal shafts; the bacteria are alternately submerged in the
wastewater and exposed to air. Excess bacteria slough from the
discs into the wastewater. After contacting the bacteria, the
wastewater flows to a sedimentation tank, where the excess
bacteria and other wastewater solids are removed. These removed
4-34
-------
solids are RBR sludge. RBR sludge is roughly similar in quantity
by dry weight, nutrient content, and other characteristics, to
trickling filter sludge.
TABLE 4-11
TRICKLING FILTER SLUDGE COMPOSITION
Property
Volatile content, percent of
total solids
Nitrogen, percent of total
solids
Phosphorus as P2C>5, percent
of total solids
Fats, percent of total solids
Grease, percent of total
solids
Specific gravity of individ-
ual solid particles
Bulk specific gravity (wet)
Color
Value
64 - 86
1.5 - 5
2.9
2.0
2.8
1.2
6
0.03
1.52
1.33
1.02
1.025
Grayish brown
Black
Comments
Reference
See Table 4-7
Depends on length of storage
of sludge in filter.
Ether soluble.
Test slime grown in primary
effluent.
69
71
13
71
13
13
72
73
2
13
2
13
64
A small body of published data is available on RBR sludge
production rate from full-scale municipal installations. At
Peewaukee, Wisconsin, total suspended solids production has been
reported to be 0.62 to 0.82 pounds of total suspended solids
per pound BOD5 (0.62 to 0.82 kg TSS/kg) removed. The final
sedimentation tank removed 70 to 83 percent of these solids as
sludge. The biological sludge alone had a concentration of 1.5
to 5.0 percent solids. Other investigations of municipal and
industrial waste applications have concluded that sludge produc-
tion for the RBR process amounts to 0.4 to 0.5 pound of total
suspended solids per pound of BOD5 (0.4 to 0.5 kg TSS/kg BOD5)
removed (74,75,76).
4.3.5 Coupled Attached-Suspended Growth Sludges
There are several installations of coupled attached and suspended
growth processes in the United States. These dual processes
are usually installed where nitrification is required or where
strong wastes must be treated. The attached growth reactor is
a trickling filter or a rotating biological reactor. Its role
is to reduce the load on the suspended growth process. The
suspended growth process uses an aeration tank and a final
clarifier. Flow recirculation is usually practiced around
the attached growth reactor. Several reports describe these
4-35
-------
processes and note that the sludge is similar to activated
sludge, both in quantity and in characteristics (5,67,68,77,78).
The sludge characterized in Table 4-12 contains some particles
of dense solids from the attached growth reactor. These
particles may improve the thickening characteristics of the
sludge (78).
TABLE 4-12
SLUDGE FROM COMBINED ATTACHED-SUSPENDED GROWTH PROCESSES
Primary sludge mixed
with biological sludge
Solids production ~~" * " """"
Ib TSS produced/ Percent Percent Percent
Process Location Ib BODg removed volatile solids volatile
Roughing filter plus Livermore, California (68) 0.98 Not stated 3.3 84
nitrifying activated
sludge
Roughing filter plus San Pablo, California (37) 1.47 78.2 Not stated Not stated
nitrifying activated
sludge
4.3.6 Denitrification Sludge
Denitrification is a biological process for the removal of
nitrate from wastewater. An electron donor, carbon in primary
effluent or methanol, is added to the nitrate-bearing wastewater.
Denitrifying bacteria extract energy for growth from the reaction
of nitrate with the electron donor:
Nitrate + Electron donor (reduced state)
Nitrogen gas + Oxidized electron donor + Energy
Denitrification has been extensively studied, and a few
denitrification processes have been built into municipal plants.
Denitrifying bacteria can grow either in a suspended growth
system similar to activated sludge or in an attached growth
system similar to a trickling filter. Sludge production for
ordinary nitrified domestic waste is roughly 300 pounds per
million gallons (30 mg/1) of wastewater treated (37).
4.4 Chemical Sludges
4.4.1 Introduction
Chemicals are widely used in wastewater treatment to precipitate
and remove phosphorus, and in some cases, to improve suspended
solids removal. At all such facilities, chemical sludges are
formed. A few plants apply chemicals to secondary effluent and
4-36
-------
use tertiary clarifiers to remove the chemical precipitates. An
example of this arrangement is the plant at South Lake Tahoe,
California. However, it is more common to add the chemicals to
the raw wastewater or to a biological process. Thus, chemical
precipitates are usually mixed with either primary sludge solids
or biological sludge solids.
The discussion below is brief because the subject of chemical
sludges and their characteristics is discussed in detail
elsewhere (79-82). A 1979 publication provides considerable
background information on theoretical rates of chemical sludge
production, as well as actual operating data from wastewater
treatment plants employing chemicals for removal of phosphorus
(7). Also, production of chemical sludges in primary sedimenta-
tion is discussed in Section 4.2.2.5.
4.4.2 Computing Chemical Sludge Production -
Dry Weight Basis
Chemicals can greatly increase sludge production. The amount of
increase depends on the chemicals used and the addition rates.
There is no simple relationship between the mass of the chemical
added and the mass of sludge produced. It is beyond the scope of
this manual to describe in detail the chemistry associated with
the chemicals used in treating wastewater, and the various
solids-producing reactions that can occur. However, several
types of precipitates that are produced and must be considered
in measuring the total sludge production are listed below:
Phosphate precipitates. Examples are A1PC>4 or
Al (H2P04~) (OH) 2 with aluminum salts, FePC>4 with iron
salts, and Ca3(P04)2 with lime (79,82,83).
Carbonate precipitates. This is significant with lime,
which forms calcium carbonate, CaCC>3. If two-stage
recarbonation is used, a recarbonation sludge of nearly
pure CaCC>3 is formed (84).
Hydroxide precipitates. With iron and aluminum salts,
excess salt forms a hydroxide, Fe(OH)3 or Al(OH)3.
With lime, magnesium hydroxide, Mg(OH)2f may form; the
magnesium comes from the influent wastewater, from the
lime, or from magnesium salts.
* Inert solids from the chemicals. This item is most
significant with lime. If a quicklime is 92 percent CaO,
the remaining eight percent may be mostly inert solids
that appear in the sludge.. Many chemicals supplied in
dry form may contain significant amounts of inert solids.
Polymer solids. Polymers may be used as primary
coagulants and" to improve the performance of other
coagulants. The polymers themselves contribute little
4-37
-------
to total mass, but they can greatly improve clarifier
efficiency with a concomitant increase in sludge
production.
Suspended solids from the wastewater. Addition of any
chemical to a wastewater treatment process affects
process efficiency. The change in sludge production must
be considered.
Quantities of the various precipitates in chemical sludges are
determined by such conditions as pH, mixing, reaction time, water
composition, and opportunity for flocculation.
Chemical sludge production, like the production of other sludges,
varies from day to day. The variation depends strongly on
chemical dosage and on wastewater flows. If the chemical dosage
is about constant in terms of milligrams per liter of wastewater,
chemical solids production will still vary, since flows fluctuate
from day to day. Changes in wastewater chemistry may also
affect the production of chemical sludge. For example,
stormwater inflow typically has a lower alkalinity than ordinary
wastewater. During storms, the production of chemical sludge
will be different from production in dry weather.
4.4.3 Properties of Chemical Sludges
Chemical sludge properties are affected mainly by the precipi-
tated compounds and by the other wastewater solids. For example,
a lime primary sludge will probably dewater better than a lime
sludge containing substantial amounts of waste-activated sludge
solids (80). Generally speaking, lime addition results in a
sludge that thickens and dewaters better than the same sludge
without chemicals. When iron or aluminum salts are added to raw
wastewater, the primary sludge does not thicken or dewater as
well as non-chemical sludge. Iron sludges dewater slightly more
easily than aluminum sludges (79). When aluminum salts are added
to activated sludge, the sludge may thicken much better than
non-chemical activated sludge (85,86). Anionic polymers can often
improve the thickening and dewatering properties of chemical
sludges.
For efficient chemical usage, feed rates must be adjusted to
match changes in wastewater flow and composition.
4.4.4 Handling Chemical Sludges
Most of the common sludge treatment processes can be used with
chemical sludges: thickening, stabilization by digestion,
incineration, etc. This section summarizes information on
stabilization and also on recovery of chemicals and by-products.
4-38
-------
4.4.4.1 Stabilization
Lime sludges may be stabilized by a small additional dose of
lime. Lime stabilization may also be used for aluminum and iron
sludges. The lime improves dewatering of these sludges by acting
as a conditioning agent. Chapter 6 discusses lime stabilization
of chemical sludges. Dewatered lime-stabilized sludges can
usually be buried in sanitary landfills.
Digestion of mixed biological-chemical sludges is generally
feasible. Pure chemical sludge will not digest. Studies done
in 1974 and 1978, however, note significant reductions in
digestibility as chemicals were added to sludge; the studies
investigated the addition of aluminum, iron, and polymer (87,88).
4.4.4.2 Chemical and By-product Recovery
Where lime use results in calcium carbonate formation, it may
be feasible to recover lime by recalcination. Tertiary lime
treatment, as practiced at the South Lake Tahoe, California,
plant is well suited to lime recovery; a recalcination process
has been operated there for several years. Where lime is added
to raw wastewater, lime recovery is more difficult but still
possible. Lime recovery does not reclaim all of the calcium,
as some is always lost with the phosphate, silica, and other
materials that must be removed from the system. Lime recovery
reduces but does not eliminate the amount of residue for
disposal. Feasibility of lime recovery depends on plant size,
amount of calcium carbonate formed, cost of new lime, and cost of
sludge disposal (81,82).
4.5 Elemental Analysis of Various Sludges
As a rule, almost anything can be found in sludge. This section
describes trace elements in all types of sludge. Data on
concentrations of the 74 elements found in wastewater sludge are
included in References 89-95.
4.5.1 Controlling Trace Elements
It is a basic principle of chemistry that elements are not
created or destroyed but chemically recombined. Therefore, the
mass of each element entering a treatment plant fixes the mass
that either accumulates within the plant or leaves it. The
mass leaving the plant does so in gaseous emissions, effluent,
a special concentrated stream, or sludge. Extracting toxic
elements from sludge appears to be impractical; source control is
the most practical way to reduce toxicants.
4-39
-------
Trace elements are present in industrial process waste,
industrial waste spills, domestic water supply, feces and urine,
and detergents. Additional trace elements are derived from:
Chemicals in photographic solutions, paints, hobby
plating supplies, dyes, and pesticides used in households
and commercial enterprises.
Storm inflow (this is particularly true for lead from
gasoline anti-knock compounds).
Corrosion of water piping, which contributes zinc,
cadmium, copper, and lead (96).
Chemicals used in wastewater treatment, sludge
conditioning, etc. Table 4-13 shows an analysis of
ferric chloride, which is an industrial by-product
(pickle liquor) of wastewater solids treatment.
TABLE 4-13
METALS IN FERRIC CHLORIDE SOLUTIONS (97)
Constituent Concentration, mg/1
Cadmium 2-3.5
Chromium 10-70
Copper 44 - 14,200
Iron 146,000 - 188,000
Nickel . 92 - 6,200
Lead ; 6 - 90
Silver 2
Zinc 400 - 2,150
aThree different liquid sources were
analyzed (43 percent FeCl3).
The quantity of toxic pollutants may be significantly reduced
by source control. At Los Angeles County, metal finishing
industries were a major source of cadmium, chromium, copper,
lead, nickel, and zinc. A source control program was developed
in cooperation with the local Metal Finisher's Association. This
program was quite successful, as shown in Table 4-14, by the
general downward trend in wastewater concentrations over time.
4-40
-------
TABLE 4-14
PROGRESS IN SOURCE CONTROL OF TOXIC POLLUTANTS (98)
Concentration in mg/1 in influent wastewater
Wastewater
pollutant
Cadmium
Chromium
Copper
Lead
Nickel
Zinc
January-June
1975
July-December
1975
January-June
1976
July-September
1976
October-December
1976
0.037
0.70
0.45
0.40
0.31
1.55
0.031
0.73
0.45
0.31
0.33
1.48
January
1977
0.029
0.78
0.45
0. 34
0. 35
1.37
0.033
0.61
0.33
0.28
0.34
1.41
0.027
0.47
0. 34
0.32
0.27
1.29
0.019
0.43
0.30
0. 34
0.21
1.17
Data for Joint Water Pollution Control Plant, Los Angeles County,
California; weekly composite samples. (13).
Occasionally, elements can be converted from a highly toxic form
to a less toxic form in wastewater treatment. Chromium is a good
example of this. In its hexavalent form, it is highly toxic, but
may be converted to the less toxic trivalent form in secondary
treatment.
4.5.2 Site-Specific Analysis
The elemental compositions of various sludges differ from one
another. If sludges are to be reused, they should be analyzed
for a number of elements. The importance of site-spec ific
analysis of sludges varies with the size of the project,
regulatory requirements, industrial activity, and the type of
reuse desired. A sampling program should recognize that:
One plant's sludge may have 100 times or more of a
certain element than another plant's.
There may be major variations between samples at the same
plant. A single grab sample may produce misleading
results. Careful attention to sampling and statistical
procedures will tend to reduce the uncertainty. A
detailed report on such procedures is available (99).
Estimates of trace element sludge contamination based
on wastewater analysis are usually less useful than
estimates based on sludge testing. However, if an
element can be measured in the influent wastewater and if
flow rates are known, then a mass load (Ib or kg per day)
may be computed. For purposes of estimating sludge
contamination, it is reasonable to assume that large
trace amounts of cadmium, copper, and zinc appear in the
sludge. Analyses of sludge and supernatant samples from
a facultative sludge lagoon have shown that there is a
tendency for nickel and lead to be gradually released
from the sludge to the liquid phase (97).
Sludge samples should be analyzed for percent solids
and percent volatile as well as for trace elements.
4-41
-------
4.5.3 Cadmium
Because it is often found in amounts that limit sludge reuse as a
soil conditioner, cadmium is a critical element. If sludge
containing cadmium is applied to agricultural cropland, some
cadmium may enter the food chain. It has been argued, with
much controversy, that the normal human dietary intake of cadmium
is already high in comparison to human tolerance limits and
that sources of additional cadmium should be strictly limited
(100,101). Table 4-15 summarizes reports on cadmium in sludge.
Chapter 18 includes a discussion of the control of sludge
application rates for the purpose of limiting cadmium levels in
soil and crops. Additional information on this subject is
provided in reference 90.
TABLE 4-15
CADMIUM IN SLUDGE
Concentration, mg/dry kg
Type of sludge
Digested
Heat dried
Anaerobic
"Other"
Not stated
Incinerator ash
Digested
Digested waste-acti-
vated
Dewatered digested
primary
Digested
Raw
Digested
Raw primary
Mesophilic digested
Thermophilic digested
Waste- activated
Anaerobically digested
chemical and waste-
activated (3.9 per-
cent average solids)
Anaerobically digested
chemical and waste-
activated (3.2 per-
cent)
Anaerobically digested
chemical and waste-
activated (4.2 per-
cent)
Raw primary
Raw primary
Raw primary
Raw primary and bio-
filter
Raw primary and bio-
filter
Raw primary and bio-
filter
Location
12 U.S. cities
4 U.S. cities
Various U.S.
Various U.S.
42 cities in England,
Wales
Palo Alto, California
Chicago (Calumet)
Chicago (West-Southwest)
Seattle (West Point)
Cincinnati (Millcreek)
Several U.S. cities
About 25 U.S. cities c
Los Angeles (Hyperion)
Los Angeles (Hyperion)0,
Los Angeles (Hyperion)
Los Angeles (Hyperion)
Chatham, Ontario0
Simcoe, Ontario
Tillsonburg, Ontario
Sacramento, California
(Northeast)
Sacramento (Rancho
Cordova)
Sacramento (Natomas)
Sacramento (Highland
Estates)
Sacramento (County Sani-
tation District 6)
Sacramento (Meadowview)
Standard
Mean deviation
89
150
106
70
-
84
-
340
48
A
130
30
75
39
140
120
110
2.6
78
9
2.8
3.0
3.5
4.1
3.6
3.1
72
200
-
-
-
-
-
-
-
K
1.51°
15
104
-
-
-
-
1.4
5
1
1.1
1.4
1.1
1.3
3.3
1.0
Median
65
67
16
14
<200
-
-
-
-
-
20
31
-
-
-
-
1.8
72
9'
, 2.6
2.6
3.6
3.8
2.5
2.6
Range
6.8
15
3
4
<200
(7
68
10
9
0
66
7
1.4
1.2
2.2
2.8
1.0
2.3
- 200
- 440
- 3,410
- 520
- 1,500
>200)
- 99
- 35
-
-
-
-
- 550
-
-
-
-
- 10
- 110
- 12
- 4.2
- 4.5
- 5.1
- 5.9
- 9.1
- 4.4
Number of
samples
12
4
98
57
42
2
-
43
100
approximate
25
20
80
-
-
"
-
225
198
40
. 5d
,
5d
,
5
5
5d
J
5d
Reference
89
89
90
90
91
92
93
102
94
95
95
95
103
103
103
103
99
99
97
97
97
97
97
97
Geometric mean.
b
Spread factor for use with geometric mean.
cConcentrations reported on wet weight basis and converted
to dry weight basis.
Weekly composites of daily samples.
4-42
-------
TABLE 4-15
CADMIUM IN SLUDGE (CONTINUED)
Concentration, mg/dry kg
Type of sludge
Raw primary and bio-
filter
Waste activated
Raw primary and waste-
activated
Raw primary
Anaerobically digested
ferric chloride
Anaerobically digested
chemical (mostly alum)
Anaerobically digested
lime
Anaerobically digested
ferric chloride
Geometric mean.
bc
Location
Sacramento (City Main)
Sacramento (Arderi)
Sacramento (Rio Linda)
Sacramento (County
Central)
North Toronto , Ontario
Point Edward, Ontario
Newmarket, Ontario
Sarnia, Ontario
.. .
Mean
10.5
5.4
9.7
29
29
8.5
7.5
76
Standard
deviation Median Range
2.0 11 7.6 - 13
2.6 6.7 2.3 - 7.7
2.9 9.1 6.2 - 14
28 12 8.3 - 72
9 - -
1.9
4.2
21
Number of
samples
5d
5d
5
. 5d
1
f 60
61
59
40
Reference
97
97
97
97
104
104
104
104
Concentrations reported on wet weight basis and converted
to dry weight basis.
Weekly composites of daily samples.
4.5.4 Increased Concentration During Processing
Toxic elements often are non-volatile solids that remain in
sludge after volatile solids have been removed. Removal of
volatile solids such as organic matter increases the concentra-
tion of non-volatile components, expressed on a dry weight basis.
Table 4-16 shows this effect for four metals at one plant. This
increased concentration may be important if sludge reuse is
desired and if regulations limit reuse for sludge that contains
contaminants that exceed certain concentrations.
TABLE 4-16
INCREASED METALS CONCENTRATION DURING PROCESSING
Concentration, mg/kg dry weight
Element
Chromium
Copper
Nickel
Zinc
Number of samples
Note: 1977 data, Sacramento County Central treatment plant, California. Anaerobic
digesters also receive thickened waste-activated sludge (metals content not
measured).
4-43
Raw primary sludge
(79
percent volatile)
110
200
46
620
(5)
Anaerobically digested
sludge
(68 percent volatile)
160
340
63
930
(2)
Lagooned sludge
(56 percent volatile)
220
450
65
1, 400
(30)
-------
4.6 Trace Organic Compounds in Sludge
Several of the trace organic compounds found in sludge, for
example, polychlorinated biphenyls (PCBs), are toxic, slow to
decompose and widely distributed in the environment. Table 4-17
quantifies the amount of Aroclor 1254, a common PCB, found in
sludge. Three other PCBs, Aroclors 1242, 1248, and 1260, have
also been found in sludge (105,107,108). In 1970, the production
of PCBs for several end uses was halted in the United States
and was completely phased out in 1977. As of 1979, imports of
PCBs are prohibited except for a few special purposes. It is
anticipated that these measures will help to reduce PCB levels in
sludge. However, products containing PCBs are still in use, and
these chemicals are widely distributed, so that several years may
elapse before PCBs become undetectable in sludge.
TABLE 4-17
AROCLOR (PCB) 1254 MEASUREMENTS IN SLUDGE
Average
concentration of
samples with
compound detected
Location
Hamilton, Ontario
Kitchener , Ontario
Newmarket, Ontario
North Toronto, Ontario
Wet
basis,
ug/1
81
110
74
120
Dry
basis, Number of Samples with Year of sample
mg/kg samples compound detected collection
- - 1976
- - - 1976
- - 1976
- - - 1976
Reference
105
Sludge type
Undigested
Undigested (with Al)
Undigested (with Ca)
Undigested (with Fe)
Raw primary Sacramento, CA (North- 50
east)
Sacramento, (Natomas) 60
Sacramento (County 80
Central)
Ra« pr imary and biofilter Sacramento, {City Main) 30
Sacramento (County Sani- 50
tation District 6)
Sacramento (Headowview) 50
Sacramento (Rio Linda) 90
Raw primary and waste
activated
Lagooned digested primary Sacramento (County
and waste activated Central)
Digested
Heat dried
10 U.S. cities
4 U.S. cities
1.5
1.8
3.8
2.0
2.4
3.5
3.9
9.3
10
4
1977
1977
1977
1977
1977
1977
1977
1971-1972
1971-1972
Weekly composite of daily samples.
Because of their fat-soluble nature, PCBs tend to concentrate
in skimmings and scum at wastewater treatment plants. The
conventional procedure of introducing skimmings into the
digester can cause higher concentrations of PCBs in the final
sludge. Alternative disposal procedures for skimmings, such as
incineration, can reduce this problem.
Table 4-18 presents data on three chlorinated hydrocarbon
pesticides found in sludge from several treatment plants.
4-44
-------
TABLE U-18
CHLORINATED HYDROCARBON PESTICIDES IN SLUDGE (97, 106)
Compound
Hexachlorobenzpnp
Hexachlorobenzene
Lindane
Technical -qradp rhlordane
Sludge type
Waste- activated
Raw primary
Waste- activated
Raw primary
Raw primary
Raw primary
Raw primary
Lagooned anaerobically di-
Plant
Arden
County Central
Arden
Northeast
Northeast
Natomas
County Central
County Central
samples with compound
detected, mg/dry. kg
0.8
0.4
1.0
0.6
2.6
2.3
2.8
4.2
Total
samples
5a
5a
5a
5a
5a
a
5
30
Samples with
compound detected
1
2
1
1
1
2
5
3C
gested primary and waste-
activated
Waste-activated
Raw primary and waste-
activated
Raw primary and biofilter
Raw primary and biofilter
Arden
Rio Linda
Meadowview
City Main
4.4
5.5
0.6
19
All plants in Sacramento County, California.
Weekly composites of daily samples.
4.7 Miscellaneous Wastewater Solids
In addition to the primary, biological, and chemical sludges
discussed in previous sections, there are several other
wastewater solids that must be properly handled to achieve good
effluent, general environmental protection, and reasonable
treatment plant operations. These solids include screenings,
grit, scum, septage, and filter backwash.
When mixed with primary or secondary sludges, screenings, scum,
grit, and septage can interfere with the processing and reuse of
the sludge. Before mixing these wastewater solids with primary
and secondary sludges, design engineers should consider the
following:
Screenings and scum detract from the final appearance,
and marketability, and utilization of sludges. They can
also clog piping, pumps, and mixers, and occupy valuable
space in digesters and other tankage.
Scum presents a special problem when mixed with other
solids and subjected to gravity thickening, decanting, or
centrifugation. Under these conditions, scum tends to
concentrate in the sidestream and to be recycled to the
wastewater processes. Eventually some of this recycled
scum is discharged to the effluent.
Grit can block pipelines, occupy valuable space in
digesters and other tankage, and cause excessive wear to
solids piping and processing equipment.
4-45
-------
4.7.1 Screenings
Screenings are materials that can be removed from wastewater by
screens or racks with openings of 0.01 inch (0.25 mm) or larger.
Coarse screens or racks have openings larger.than 0.25 inch
(6 mm), whereas fine screens have openings from 0.01 to 0.25 inch
(0.25 to 6 mm). If openings are larger than 1.5 inches (38 mm),
the screens are often called trash racks.
Racks and screens are usually installed to treat the wastewater
as it enters the treatment plant. Racks and coarse screens
prevent debris from interfering with other plant equipment. Fine
screens remove a significant fraction of the influent suspended
solids and 8005, thus reducing the load on subsequent treatment
processes. In this regard, fine screens may act like primary
sedimentation tanks, although they do not ordinarily remove as
much of the solids as do sedimentation tanks. Fine screens are
usually protected by upstream coarse screens or racks.
4.7.1.1 Quantity of Coarse Screenings
Coarse screenings are basically debris. Items typically
collected on coarse screens include rags, pieces of string,
pieces of lumber, rocks, tree roots, leaves, branches, diapers,
and plastics.
The quantity of coarse screenings is highly variable, but most
plants report 0.5 to 5 cubic feet per million gallons (4 ml/m3
to 40 ml/m3) on average flows. Table 4-19 shows the quantities
of screenings reported for a number of communities. The quantity
of screenings depends on:
Screen opening size. Generally, greater quantities are
collected with smaller screen openings. This was seen
most clearly at Grand Island, Nebraska, where a change
from 0.5-inch to 1.25-inch (13 to 32 mm) openings
caused screenings production to drop from about 7 to
about 3 cubic feet per day (0.2 to 0.08 m3/day) (114) .
Tests at Chicago, Illinois, and Adelaide, Australia,
showed this tendency also (13).
Shape of openings. For example, bar racks may have
openings 0.75 inch (19 mm) wide and over 2 feet (over
600 mm) long. Such a rack will pass twigs, ballpoint
pens, and other debris, that would be captured on a mesh-
type screen with square openings of 0.75 inch (19 mm).
Type of sewer system. Combined storm and sanitary
sewers produce more screenings than separate sanitary
sewers. This effect is especially pronounced where much
or all of the combined wastewater is treated during and
after storms, rather than being bypassed.
4-46
-------
TABLE 4-19
SCREENING EXPERIENCE (109, 110)
Rack or screen
opening, in.
3-3/8
3
3
3
1-3/8
1-1/2
1-1/2
1-1/2
1-1/2
1-1/2
1-1/2
1-1/4
1-1/4
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
7/8
7/8
3/4
3/4
3/4
3/4
3/4
3/4
3/4
3/4
1/2
1/2
1 in. = 2.54 cm.
1 mgd = 3,785 m3/day
1 cu ft/mil gal = 7 .
City
Norwalk, Connecticut
New Haven, Connecticut
East Hartford, Connecticut
San Jose, California
New York, New York, Jamaica
Philadelphia, Pennsylvania, North
Oklahoma City, Oklahoma, Southside
Cranston, Rhode Island
Taunton, Massachusetts
Meadville, Pennsylvania
Grove City, Pennsylvania
Uniontown, Pennsylvania
Fargo, North Dakota
New York, Wards Island
New York, Owls Head
Minneapolis-St . Paul, Minnesota
New York, Hunts Point
East Bay, Oakland, California
New York, Coney Island
New York, 26th Ward
New York, Tallmans Island
Bridgeport, Connecticut, West Side
New York, Rockaway
Waterbury, Connecticut
Bridgeport, Connecticut, East Side
Duluth, Minnesota
Austin, Minnesota
Fond du Lac, Wisconsin
Findlay, Ohio
Massillon, Ohio
York, Nebraska
Marion, Ohio
Gainesville, Florida
Marshalltown, Iowa
East Lansing, Michigan
Birmingham, Michigan
Boston, Massachusetts, Nut Island
Richmond, Indiana
Detroit, Michigan
New York, Bowery Bay
Hartford, Connecticut
Portsmouth, Virginia
Sheboygan, Wisconsin
Aurora, Illinois
Topeka , Kansas
Oshkosh, Wisconsin
Green Bay, Wisconsin
Manteca, California
f
48 m3/! x 106 m3.
Flow,
mgd
11.75
8
4.0
-
65
48.2
25.0
8.32
3.5
2.5
0.8
3.0
2.7
180
160
134
120
98
70
60
40
17
15
15
14
12
9
7.2
7
5.2
5
5.0
5
4.0
3.8
1.5
125
6.2
450
40
39.0
9.7
8.0
8.0
7. 5
6.0
10.0
1.5
Screenings,
cu ft/mil gal
0. 17
1.0
1. 33
0.25
0.6
2.20
2.1
0.65
1.0
0.6
0. 1
0. 9
4.55
1.0
0.6
0. 9
0.7
1.6
1.4
1.1
0.7
0.93
1.0
2.35
2.04
0.56
1.1
5
0. 39
1.5
1. 5
2.5
3.5
0.25
0.4
1. 2
1.2
1. 2
0.47
1.1
1.6
0.82
0.25
1.42
1.30
1.7
1.2
5.2
4-47
-------
Operating practices. Where manual cleaning is used,
operatorssometimes pass some screenings through or
around the screens. Where automatic equipment is used,
the operating pattern can greatly affect removals (112).
Length of sewer system. The volume of screenings removed
may double with a short, as opposed to lengthy, inter-
ceptor system. This condition may be explained by the
fact that solids are more subject to disintegration with
a lengthy collection system (5). Wastewater pumping will
also tend to disintegrate large solids.
Screenings loads may increase dramatically during peak flows. It
is estimated that, at the East Bay Municipal Utility District
plant in Oakland, California, the screenings load was about
10 times the average during peak flows. For the most part, this
plant services separate sanitary sewers, but the screenings load
is concentrated (110).
4.7.1.2 Quantity of Fine Screenings
Fine screens are usually used as an alternative to conventional
primary sedimentation to remove suspended solids. Screens
with 0.09 to 0.25 inch (2 to 6 mm) openings remove about 5 to,
10 percent of suspended solids from typical municipal raw
wastewater. If 0.03 to 0.06 inch (0.8 to 1.5 mm) openings are
used instead, about 25 to 35 percent of suspended solids may be
removed (5). Higher removals increase the dry weight and the
moisture content of the screenings. For example, consider fine
screens that remove 25 percent of suspended solids from an
influent concentration of 300 mg/1. In this case, screenings are
630 dry pounds per million gallons (75 mg/1) of wastewater. If
the screenings are ten percent solids and weigh 60 wet pounds per
cubic foot (961 kg/m3), then the volume is 105 cubic feet per
million gallons (14.04 m3/lx!06 m3). This is over 25 times more
than a typical value for coarse screenings of 4 cubic feet per
million gallons (0.53 m3/lx!06 m3).
4.7.1.3 Properties of Screenings
If screenings have not been incinerated, they may contain
pathogenic microorganisms. They are also odorous and tend to
attract rodents and insects. Screenings have been analyzed
for solids content, volatile content, fuel value, and bulk
wet weight. Some of the reported values are summarized in
Table 4-20.
4.7.1.4 Handling Screenings
Screenings may be ground and handled with other sludges; direct
landfilled; and incinerated, with the ash disposed in landfill.
Table 4-21 summarizes the advantages and disadvantages of various
methods.
4-4!
-------
TABLE 4-20
ANALYSES OF SCREENINGS
Solids content,
percent dry
solids
Volatile content,
percent
Fuel value,
Btu/lb dry
solids
Bulk wet weight,
Ib/cu ft
Comments
References
20
10 - 20
B - 23
6.1
17
5,4003
80 - 90
68 - 94
96
86
7,820
Coarse screenings. Fine
screenings may have
lower solids content.
Common values
Various plants, fine
screens, 0.03 to 0.12
inch openings
Thickened ground screen-
ings from 0.75-inch
racks; after grinding,
screenings were thick-
ened on a static screen
with 0.06-inch openings.
Dewatered ground screen-
ings from 0.75-inch
racks; after grinding,
screenings were de-
watered on a rotating
drum screen with 0.03-
inch openings.
Fine screenings
13
113
113
Computed.
1 Btu/lb dry solids = 2,32 kJ/ dry solids.
1 Ib/cu ft = 16.03 kg/m3.
1 in. = 2.54 cm.
Some fecal solids accompany the larger materials such as rags and
twigs. For this reason, as well as to save labor time and cost,
it is desirable to mechanize screenings handling. Also, where
coarse screenings are landfilled or incinerated, it is desirable
to use the largest rack opening that will adequately protect
downstream processes. This will minimize the quantity of
screenings that must be handled separately.
Screenings may be transported pneumatically (116), in sluiceways,
on conveyors, and in cans, dumpsters, or covered trucks.
Screenings-water mixtures that are ground may be pumped. For
thickening and dewatering, fine static screens, drum screens,
centrifuges (113), and drum or screw presses may be used.
Chemical conditioning is not required.
4.7.1.5 Screenings from Miscellaneous Locations
Screens are occasionally used on streams other than influent
wastewater. For instance, when it is fed to a trickling filter,
primary effluent may be screened to prevent clogging of orifices
in the distributor on the trickling filter (109). At one heavily
loaded plant where regular influent screening equipment was
partially bypassed, screens installed in aeration basin effluent
channels, chlorine contact tank outlets, and other locations
prevented coarse, floating objects from being discharged with
4-49
-------
the effluent (117). Another example occurred at a plant where
digested sludge was discharged to the ocean. Fine screens were
used to prevent floatable materials from being discharged (118).
Other examples of the use of screens on streams other than
influent wastewater are the screening of overflow water from grit
separators and the screening of feed sludge to disc-nozzle
centrifuges to prevent clogging (113,119).
TABLE 4-21
METHODS OF HANDLING SCREENINGS
Method
Advantages
Disadvantages
1. Comminution within main wastewater stream
handle comminuted screenings with
other wastewater solids, e.g., primary
sludge
2. Removal from main stream, grinding or
maceration, and return to main stream
3. Removal3 from main stream, draining or
dewatering, landfill
Removal3 from main stream, dewatering,
incineration, landfill of ash.
Anaerobic digestion of fine screenings
alone (not mixed with other
solids)
Anaerobic digestion of screenings
together with scum but separate
from other sludges
Highly mech.ykized, low operating labor re-
quiremen,*.
Minimizes number of unit operations
Usually free of nuisance from flies and
odors
Widely used, familiar to plant operators
Similar to Method 1, except more complex
mechanical ly
Keeps screenings out of other sludges;
avoids disadvantages of Methods 1 and 2.
Can be fairly well mechanized.
Keeps screenings out of other sludges;
avoids disadvantages of Methods 1 and 2.
Ash is very small in volume and easy to
transport and dispose of.
If incineration is used for other sludges
and/or grit, then screenings can be
added at modest cost*
Pathogen kill
Sludge contains screenings, which may inter-
fer with public acceptance for reuse of
sludge as a soil amendment.
Sludge probably needs further maceration or
screening if it is to be pumped or
thickened in a disc centrifuge.
If sludge is to be digested, digesters must
be cleaned more often. Plastics and
synthetic fabrics do not decompose in
digesters. Aggravates digester scum pro-
blems . Ground screenings tend to
agglomerate in digesters.
Not appropriate if suspended solids removal
is required (fine screens).
Not appropriate for very large screenings
loads, especially if high grit loads are
also present (large plants, combined
sewers)
Similar to Method 1, except Method 2 can be
designed for very large flows and screen-
ings loads. Method 2 is more expensive
than Method 1 for small screenings loads.
Transport of screenings may be difficult.
Unless carefully designed and operated,
causes fly and odor nuisances and health
hazards.
Regulations for landfill disposal may strongly
affect operations.
High cost if an incinerator is required for
screenings alone.
Unless incinerator is properly designed and
operated, air pollution (odor and partic-
ulates) will be serious.
Not well adapted to wide fluctuations in
screenings quantities, unless screenings
are only a small part of the total in-
cinerator load.
Digestion was tested at large scale at
Milwaukee, Wisconsin, but found to be
impractical.. (115)
Tested at Malabar plant, Sydney, Australia,
but found to be inoperable. Material
handling was the chief difficulty.
Mechanical removal is usually practiced at large plants. Manual removal is frequently used at small plants. The advantages
of manual removal are simplicity and low capital cost; the disadvantages are high operating labor requirements and fly and odor
problems. A common arrangement at small plants is to install a single comminutor with a manually cleaned bar rack as a standby unit.
4.7.2 Grit
Grit is composed of heavy, coarse solids associated with raw
wastewater. It may be removed from wastewater before primary
sedimentation or other major processes. Alternatively, it may be
4-50
-------
removed from primary sludge after the primary sludge is removed
from the wastewater. Typical ingredients of grit are gravel,
sand, cinders, nails, grains of corn, coffee grounds, seeds, and
bottle caps.
4.7.2.1 Quantity of Grit
The amount of grit that is removed varies tremendously from one
plant to another. Table 4-22 shows grit quantities measured at
several plants. Additional values have been published elsewhere
(5,109). The quantity of grit depends on:
Type of collection system. If a system is combined, then
street sanding, catch basin maintenance, and amount of
combined sewer overflow become important.
Degree of sewer system corrosion. Grit may include
products of hydrogen sulfide corrosion derived from the
pipes ( 122) .
Scouring velocities in the sewers. If scouring
velocities are not regularly maintained, grit will build
up in the sewers. During peak flows, the grit may be
resuspended, and the treatment plant may receive heavy
loads during peak flows.
* Presence of open joints and cracks_in _the__sew_e_r system .
These permit soil around the "pipe's to~e~nter "the sewers .
This effect also depends upon soil characteristics and
groundwater levels.
Structural failure of sewers. Such failures can deliver
enormous amounts of grit to the wastewater system.
* Quantities of industrial wastes .
* 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
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4.8 References
1. Water Pollution Control Federation. MOP 8 Wastewater
Treatment Plant Design. Water Pollution Control
Federation. 1977.
2. Metcalf and Eddy, Inc. Wastewater Engineering; Treatment
Disposal, Reuse. 2nd Edition. McGraw-Hill Book Company.
1979.
3. American Public Health Association, American Water Works
Association, Water Pollution Control Federation. Standard
Methods for the Exami n at ion__of__W a t e r and W a s t e w a t e r .
14th Edition. 1975.
4. Schmidt, O.J. "Wastewater Treatment Problems at North
Kansas City, Missouri. Journal Water Pollution Control
Federation. Vol. 50, p. 635. (1978).
5. Babbitt, H.E. and E.R. Baumann. Sewe
Treatment. Eighth Edition, Wiley, 1958.
6. Brisbin, S.G. "Flow of Concentrated Raw Sludges in Pipes."
Journal Sanitary Engineering Diyj_s_ion_,___ASCE_. Volume 83,
SA3, 1957.
7. USEPA Review of Techniques for Treatment and Disposal of
Phosphorus-Laden Chemical Sludges. Office of Research and
Development,Cincinnati,Ohio,45268. EPA-600/2-79-083,
February 1979.
8. Knight, C.H., R.G. Mondox, and B. Hambley. "Thickening
and Dewatering Sludges Produced in Phosphate Removal."
Paper presented at Phosphorous Removal Design Seminar,
May 28-29, 1973, Toronto.
9. Young, J.C., J.L. Cleasby, and E.R. Baumann. "Flow
and Load Variations in Treatment Plant Design." Journal
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10. Fischer, A.J. "The Economics of Various Methods of Sludge
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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
Facilities Based on Field Derived Parameters. Office of
Research an Development,Cincinnati, Ohio,45268. EPA-670/
2-74-062. July 1974.
4-63
-------
13. Babbitt, H.E. Sewerage and Sewage Treatment. 6th Edition
Wiley. 1947. ' -
"
14. Smith, J.E., Jr. "Ultimate Disposal of Sludges
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Hill, North Carolina. February 9-10, 1971.
15. Prazink, J.A. "Process Control in the Real World." Deeds
and_ Data (WPCF). July 1978. ~" -
16. Burd, R.S. A Study of Sludge Handling and Disposal.
Federal Water Pollution Control Administration. Report
WP-20-4. 1968.
17. Dewante and Stowell and Brown and Caldwell. 1973 Study,
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1974.
18. Metropolitan Engineers. West Point Waste Activated Sludge
Withholding Experiment . Report to Municipality of
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19. USEPA. Process Design Manual for Suspended Solids Removal
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22. Municipality of Metropolitan Seattle, Washington. Data
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23. Teletzke, G. "Wet Air Oxidation of Sewage Sludges".
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25. Heukelekian, H., H.E. Orford, and R. Manganelli. "Factors
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26. Colbaugh, J.E. and A. Liu. "Pure Oxygen and Diffused Air
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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
Relation to Stream Sanitation." -Sewage_an_d_I ndustr ial
Wastes. Vol. 26, (1), (1954). ~
29. Eckenfelder, W.W. Water Quality Engineering for Practicing
Engineers. Barnes & Noble, 1970. ~~~
30. Brown and Caldwell. West Point Pilot Plant Study, Vol. II,
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to Treat Waste From Military Field Installations': An
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Down flow. Bubble Contact Aeratioru Part II , Final ReportV
University ol Texas, Austin, ~ Texas 78712 EHE-740-01,
July 1974.
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37. USEPA. Process Design Manual for Nitrogen Control.
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38. Hopwood, A. P. and A.L. Downing. "Factors Affecting the
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4-65
-------
39. Chapman, T.D., L.C. Matsch and E.H. Zander. "Effect of
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45. Gujer, W. and D. Jenkins. "The Contact Stabilization
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46. USEPA. Extended Aeration Sewage Treatment in Cold Climates.
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52.
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4-67
-------
65. USEPA. Converting Rock Trickling Filters to Plastic Media;
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66. Derived from E. Herr. Special Solids Balance Operating
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70. Smith, J.E., Jr. Ultimate Disposal of Sludges. Technical
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73. Stanley, W.E. Personal Communication, 1967, as cited in
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76. Gillespie, W.J., D.W. Marshall, and A.M. Springer, "A
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77. Stenquist, R.J., D.S. Parker, W.E. Loftin and R.C. Brenner.
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4-68
-------
78. Reimer, R.E., E.E. Hursley, and R.F. Wukasch "Pilot Plant
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at National Conference on Environmental Engineering,
July 12-14, 1976, Seattle, Washington.
79. USEPA. Process Design Manual for Phosphorus Removal.
Technology Transfer, Cincinnati^Ohio45268.EPA-625/1-
76-001. April 1976.
80. Brown and Caldwell. West Point Pilot Plant Study,
Vol. IV, Chemical Treatment. Report for Municipality of
Metropolitan Seattle, Seattle, Washington, 98101,
December 1978.
81. Gulp, R.L. , G. Wesner, and G. Gulp. Handbook of Advanced
Wastewater Treatment, Second Edition. Van Nostrand
Reinhold, 1978.
82. USEPA Lime Use in Wastewater Treatment: Design and Cost
Data_. Office of Research and Development, Cincinnati,
Ohio 45268. EPA-600/2-75-038, October 1975.
83. Scott, D.S., and H. Harling. Removal of Phosphates and
Metals from Sewage Sludge. Environmental Protection
Service (Canada) , Research" Report No. 28, about 1975.
84. Merrill, D.T. and R.M. Jorden "Lime-induced Reactions
in Municipal Wastewaters." Journal Water Pollution Control
Federation. Vol. 49, (12), (1975).
85. Baillod, C.R., G.M. Cressey, and R.T. Baupre'. "Influence
of Phosphorus Removal on Solids Budget." Journal Water
Pollution Control Federation. Vol. 49, (1), (1977).
86. Finger. R.E. "Solids Control in Activated Slduge Plants
with Alum." Journal Water Pollut i q n _C_qn trol^ Feder a t i o n.
Vol. 45, (8), (.1973) .
87. Gossett, J.M., P.L. McCarty, J.C. Wilson, and D.S. Evans,
"Anaerobic Digestion of .Sludge from Chemical Treatment."
Journal Water Pollution Control Federation. Vol. 50,
pT 513V (1978V. " "
88. Parker, D.S., D.G. Niles and F.J. Zadick "Processing
of Combined Physical-Chemical-Biological Sludge." Journal
Water Pollution^ _C_g_ n t_r o j^_ Federation . Vol. 46 p. 2281,
(1974).
89. Furr, A.K. "Multielement and Chlorinated Hydrocarbon
Analysis of Municipal Sewage Sludges of American Cities."
E n v i r o nm e n t^a 1 S c i e_n_ce_ a nd Technology;. Vol. 10, (7),
(1976).
4-69
-------
90. Sommers, L.E. "Chemical Composition of Sewage Sludges and
Analysis of Their Potential Use as Fertilizers. "Journal
of_Ejwironmen_tal Quality. Vol. 6, (2), (1977). ~~~ ~~
91. Berrow, M.L., and J. Webber. "Trace Elements in Sewage
Sludges." Journal of Science of Food and Agriculture
Vol. 23, January 1972. "~ ~ ~~" ~ ~~~
92. Gulbrandsen, R.A., N. Rait, O.J. Krier, P.A. Baedecker,
and A. Childress. "Gold, Silver, and Other Resources
in the Ash of Incinerated Sewage Sludge at Palo Alto,
California, a Preliminary Report." U.S. Geological Survey
Circular 784, 1978. ~
93. Bernard, H. "Everything You Wanted to Know About Sludge
But Were Afraid to Ask." Proceedings of the 1975 National
Conference on Municipal Sludge Management andDisposal.
Information Transfer, Inc. 1975.
94. Metropolitan Engineers. Draft Facility Plan for Upgrading
Metro Puget Sound Plants-System Wide Volume, Part 1, Basis
of Planning. Report to Municipality of Metropolitan
Seattle, Seattle, Washington 98101. 1976.
95. USEPA "Elemental Analysis of Wastewater Sludges from
33 Wastewater Treatment Plants in the United States."
Pretreatment and Ultimate Disposal of Wastewater Solids.
Office of Research and Development^Cincinnati,Ohio 45268.
EPA-902/9-74-002, May 1974.
96. Carun, G.F., and L.J. McCabe. "Problems Associated with
Metals in Drinking Water." Journal AmericanJWater Works
Association. Vol. 67, (11), (1975).
97. Sacramento Area Consultants. Sewage Sludge Management
Program Final Report-Vol. I SSMP Final Report, Work Plans,
Source SurvVy. Sacramento Reg ional County Sani tat ion
District, Sacramento, California 95814. September 1979.
98. Eason, J.E., J.G. Kremer, and F.D. Dryden "Industrial Waste
Control in Los Angeles County." Journal Water Pollution
Control Federation. Vol. 50, p. 672, (1978).
99. Monteith, H.D., and J.P. Stephenson. Development of an
Efficient Sampling Strategy to Chara cterize Digested
Sludges"^Environmental Protection Service (Canada) ,
T'raining and Technology Transfer Division, Research Report
No. 71, 1978.
100. Epstein, E., and R.L. Chaney. "Land Disposal of Toxic
Substances and Water-Related Problems." Journal Water
Pollution Control Federation. Vol. 59, (8), (1978).
4-70
-------
101. Stern, L. "The Great Cadium Controversy." Sludge Magazine.
Vol. 1, (2), March/April 1978. "
102. Peterson, J.R., C. Lue-Hing, and D.R. Zenz. "Chemical
and Biological Quality of Municipal Sludge" Recycling
Treated Municipal Wastewater and Sludge Through Forest
and Cropland^W. V . Sopper iTndTTr^ Kardos Editors.
Pennsylvania State University Press. p. 26, (1973).
103. Ohara, G.T., S.K. Raksit, and D.R. Olson. "Sludge
Dewatering Studies at Hyperion Treatment Plant." Journal
Water Pollution Control Federation. Vol. 50, p~912,
(1978) ."' ~~~ ' -
104. Chawla, V.K., J.P. Stephenson, and D. Liu. "Biological
Characteristics of Digested Chemical Sewage Sludges."
Proceedings, Sludge Handling and Disposal Seminar, Toronto^
Ontario, 1974.EnvironmentalProtection Service(Canada)7
Conference Proceedings No. 2.
105. Lawrence, J. , and H.M. Tosine. "Polychlorinated Biphenyl
Concentrations in Sewage and Sludges of Some Waste
Treatment Plants in Southern Ontario." Buileti n o f
Environmental Contamination and Toxicology. Vol18,(1) ,
(1977)."
106. Sacramento Area Consultants. Sewage Sludge Management
P r o g r a m Final Rep o r t V q 1 _. 2, SSB_ Operation and'
P e r f o rma nee.
107. Dube, D.F.f G.D. Veith, and G.F. Lee. "Polychlorinated
Biphenyls in Treatment Plant Effluents." Journal Water
P o 11 u t i o n C o rvtr ql_ _Federati on. Vol. 46, p. 966, (1974).
108. Michigan Water Resources Commission. "Monitoring for
Polychlorinated Biphenyls in the Aquatic Environment."
1973.
109. American Society of Civil Engineers and Water Pollution
Control Federation. Manual of Practice 8 Sewage Treajtmejrt
Plant Design. 1959 Edition, (1972 printing).
110. Sacramento Area Consultants. Study of Wastewater Solids
Processing and Disposal. Report to Sacramento Regional
County Sanitation District. Sacramento, California 95814,
June 1975.
111. Merz, R.C. "Operation of Screens, Grit Chambers and
Sedimentation Tanks." Sewage Works Journal. Vol. 17,
(4) , July 1945.
112. Fisichelli, A.P. "Raw Sludge Pumping--Problems and
Interdisciplinary Solutions." Journal Water Pollution
Con t ro1 Federation. Vol. 42, (11), (1970).
4-71
-------
113. Volpe, G.J. "Static Screen and Rotostrainer Screenings
Dewatering Test." Printed as Appendix B of Study to
Wastewater Sol ids Processing and Disposal. Report
Fo Sacrame nto Reg ional County Sanitation Di s tr ict.
Sacramento, California, 95814, June 1975.
114. Carnes, B.A., and J.M. Eller. "Characterization of
Wastewater Solids." Journal Water Pollutio n Control
Federation. Vol. 44, (8), (1972).
115. Townsend, D.W. Water Works and Sewerage. 1933. As cited
by Metcalf, L. and H.P. Eddy. American Sewage Practice,
Vol. Ill, Disposal of Sewage. Third Edition, McGraw-Hill,
1935.
116. Blanchard, C.T. "Pressurized Air Simplifies Conveying
Sewage Solids." Water and Sewage Works. Vol. 123, (11),
(1976).
117. Sacramento Area Consultants. Northeast Treatment Plant
Interim Improvements. Report to Sacramento Regional County
SanitationDistrict, Sacramento, California, 95814.
September 1976.
118. Garber, W.F., and G.T. Ohara. "Operation and Maintenance
Experience in Screening Digested Sludge." Journal Water
Po11u t ion C on t r oj^ Fe^d e r a t ion. Vol. 44, (8), (1972).
119. Finger, R.E., and J. Parrick. "Optimization of Grit
Removal at a Wastewater Treatment Plant." Presented at
51st Annual Conference, Water Pollution Control Federation,
Anaheim, California, October 3, 1974.
120.
121.
Questionnaire for San Francisco Bay Region Wastewater
Solids Study, Oakland, California 94611, 1977.
Mathews, W.W. 1957 Operators' Forum, W.D. Itatfield,
Leader. Sewage and Industrial Wastes. Vol. 30, April
1958.
122. USE PA. Process Design Manual for Sulfide Control in
Sanitary Sewerage Systems. Technology Transfer,
Cincinnati, Ohio.45268.EPA 625/1-74-005. October 1974.
123. Fischer. Sewage Works Journal 1930. As cite by
Metcalf, L. and H.P. E ddy. American Sewage Practice,
Vol. Ill, Disposal of Sewage. Third Edition,McGraw-Hill,
1935.
124. Cooper, T.W. 1963 Operators' Forum, A.J. Wahl, President.
Journal Water Pollution Control Fedejrati^qn. Vol. 36, (4),
(1964).
4-72
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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
Pollution Control Federation. Vol. 42, (5), (1970).
131. Burwell, H.W. Memoranda on West Point Treatment Plant
Operations, Municipality of Metropolitan Seattle,
Washington 98101. 1976.
132. Banerji, S.K., C.M. Robson, and B.S. Hyatt, Jr. "Grease
Problems in Municipal Wastewater Treatment Systems."
Proceedings of 29th Industrial Waste Conference^_May7-9,
19J7J, Purdue University, p. 768, 1975.
133. Donaldson, W. "Utilization of Sewage Grease." Sjsjv^ajgjg
Works Journal. Vol. 16, (3), May 1944.
134. Harris, R.H. Skimmings Removal and Disposal Studies.
A report submitted to the Research and Development Section,
County Sanitation Districts of Los Angeles County, Whittier,
California 90607. September 28, 1964.
135. Brown and Caldwell Study of Operations, Phase 1 Report of
City of Albany, Georgia 31702. March 1979.
136. Metropolitan Engineers. Sludge Handling and Disposal
Interim Measures. Report toMunicipalityofMetropolitan
Seattle, Seattle, Washington 98101. November 1975.
137. Lyon, S.L. "Incineration of Raw Sludges and Greases." WPCF
Deeds and Data, April 1973.
138. USEPA. Treatment and Disposal of Wastes Pumped from Septic
Tanks. Office of Research and Development, Cincinnati,
Ohio 45268. EPA-600/2-77-198, September 1977.
4-73
-------
139. USEPA. Septage Treatment and Disposal. Prepared for 1977
Technology Transfer Seminar Program. Technology Transfer,
Cincinnati, Ohio 45268. 1977.
140. USEPA. Treatment and Disposal of Septic Tank Sludges, A
Status Report. Design Seminar Handout, Small Wastewater
Treatment Facilities. Technology Transfer, Cincinnati,
Ohio 45268. January 1978.
141. Brandes, M. "Accumulation Rate of Septic Tank Sludge
and Septage." Journal Water
^
Vol. 50, (5), (1978).
142. USEPA. Handling and Disposal of Sludges from Combined Sewer
Overflow Treatment. Office of Research and Development,
Cincinnati, Ohio 45268. EPA-600/2-77-053a, b, and c.
May 1977.
4-74
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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 loadthis is sludge specific,
Ib/ft (see Table 5-4)
R = tank radius, ft
TABLE 5-4
TYPICAL UNIFORM LOAD (W) VALUES
Truss arm W,
Sludge type lb/fta
Primary only (little grit) : 30
Primary only (with grit) 40
Primary + lime 40 to 60
Waste-activated sludge (WAS)
Air 20
Oxygen 20
Trickling filter 20
Thermal conditioned 80
Primary + WAS 20 to 30
Primary + trickling filter 20 to 30
Rake arms typically have a tip speed
between 10 to 20 ft/min (3 to 6 m/min).
1 Ib/ft = 1.49 kg/m
Note that there are several levels of torque which must be
specified for a circular gravity thickener (22). Table 5-5 lists
and defines the various torque conditions.
5.3.3.4 Total Tank Depth
The total vertical depth of a gravity thickener is based on three
considerations: tank free board, settling zone (zone of clear
5-9
-------
liquid and sedimentation zone), and compression and storage zone
(thickening zone).
TABLE 5-5
DEFINITION OF TORQUES APPLICABLE TO CIRCULAR GRAVITY THICKENERS (22)
Running torque - this is the torque value calculated from equation 5-1
Alarm torque - torque setting, normally 120 percent of running, which tells the operator
that there is something wrong
Shut-off torque - torque setting, normally 140 percent of running, which would shut off
the mechanism . . .
Peak torque - torque value, determined by the supplier of the drive unit. This torque
is provided only for an instant and is normally 200 percent of the running torque
Free
Tank free board is the vertical distance between tank liquid
surface and top of vertical tank wall. It is a function of tank
diameter, type of bridge structure--half or full bridgetype of
influent piping arrangement, and whether or not skimming is
provided. It will usually be at least 2 to 3 feet (.6 to .9 m)
although free-board distances up to 7 to 10 feet (2 to 3 m) have
been used by some designers.
Settling Zone
This zone encompasses the theoretical zone of clear liquid and
sedimentation zone as shown on Figure 5-1. Typically 4 to
6 feet (1.2 to 1.8 m) is necessary, with the greater depth being
for typically difficult sludges, such as waste-activated or
nitrified sludge.
Compression and Storage Zone
Sufficient tank volume must be provided so that the solids will
be retained for the period of time required to thicken the slurry
to the required concentration. In addition, sufficient storage
is necessary to compensate for fluctuations in solids loading
rate.
Another consideration is that gas may be produced because of
anaerobic conditions or denitrif ication. Development of these
conditions depends on the type of sludge, liquid temperature,
and the length of time sludge is kept in the thickener. Plant
operating experience has indicated that the total volume in
this zone should not exceed 24 hours of maximum sludge wasting.
5.3.3.5 Floor Slope
The floor slopes of thickeners are normally greater than 2 inches
of vertical distance per foot of tank radius (17/cm/m). This is
steeper than the floor slopes for standard clarifiers. The
5-10
-------
steeper slope maximizes the depth of solids over the sludge
hopper, allowing the thickest sludge to be removed. The steeper
slope also reduces sludge raking problems by allowing gravity to
do a greater part of the work in moving the settled solids to the
center of the thickener.
5.3.3.6 Other Considerations
L j. ft ing De v i ce s
Optimum functioning of a thickener mechanism can be inhibited
by heavy accumulation of solids due to power outages or
inconsistent accumulations of heavy or viscous sludges.
Thickeners can be provided with either a manual or an automatic
lifting device that will raise the mechanism above these
accumulations. This device has not been considered necessary in
the majority of municipal wastewater treatment plants except
in applications involving very dense sludges (for example,
thermally-conditioned sludge or primary plus lime sludge).
Skimmers
Several years ago, it was rare for skimmers to be installed
on gravity thickeners. Today it is common practice to specify
skimming and baffling for new plants. The reason for the change
is the increased processing of biological sludges and the
inherent floating scum layer associated with those sludges.
Polymer Addition
Addition of polymer to gravity thickener feed has been practiced
at several plants (23,24). Results indicate that the addition
of polymers improves solids capture but has little or no effect
on increasing solids underflow concentration. (See Chapter 8 for
further discussion).
Thjjckejier Sjjpejcnatant
Thickener supernatant or overflow is normally returned to either
the primary or secondary treatment process. As indicated in
Table 5-3, the strength of the overflow, as measured by total
solids, can vary significantly. The liquid treatment system
must be sized to handle the strongest recycled load. (See
Chapter 16 for further discussion).
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 operationthat is, 7.5 hours per day, five
days per week.
The designer has decided to provide polymer feed equipment for
the DAF thickener to be used in emergency situations only.
Polymers are not used in normal operation.
The designer has also decided to use a packed pressurization
tank, which requires a relatively clean source of pressurized
flow. Secondary effluent will be utilized.
Effective Surfac_e_Are_a
The maximum daily waste sludge production expected was given as
2,700 pounds (1,225 kg) of waste-activated sludge with a solids
concentration of 5,000 to 8,000 mg/1.
5-33
-------
The maximum net hourly load (actual amount of solids that must be
captured and removed per hour by the thickener) is:
(2,700 Ib/day) (7 days/wk) _. . ...
TT75 hrs/day)(5 day/wk operation) = 504 Ib/hr (228.6 kg/hr)
The sludge being thickened is considered to be equivalent to a
straight waste-activated sludge even though primary solids are
mixed with it. From Table 5-8, a value of 0.42 pounds per square
foot per hour (2.1 kg/m2/hr) is selected.
504 Ib/hr max. net load , ._. ... . . nn 9,
0.42 Ib/sq ft/hr loading rate = 1 '20° Sc< ft (108 m2)
Based on the solids loading rate (hydraulic loading rate needs to
be checked), the maximum effective surface area required is
1,200 square feet (108 m2).
Feed, pressurized recycle, thickened sludge, and subnatant must
be calculated to determine pump size and piping requirements.
Feed Flow Rate - Both the gross solids load and minimum solids
concentration must be known to calculate the feed flow rate.
The gross solids load is the amount of solids that must be fed to
the thickener in order for the system to capture and thicken the
required net solids load. The maximum net hourly load has
already been calculated to be 504 pounds per hour (228.6 kg/hr).
Since polymers are not to be used during normal operation, a
capture efficiency of 85 percent is used (standard for the
industry) . The maximum gross solids load is then calculated as
follows:
= 593 Ib/hr (269 kg/hr)
n
0.85 efficiency factor
The minimum solids concentration expected is 5,000 mg/1 . The
maximum feed flow rate can now be calculated as follows:
' 237 «*" <897
(34M60 min/hr)
Pressurized recycle flow rate - The design pressurized flow
should be based on the maximum gross solids load expected from
the DAF thickener. For this example, the maximum hourly gross
solids load used was 593 pounds per hour (269 kg/hr).
5-34
-------
After discussing the operating conditions with several DAP
thickener equipment suppliers, the engineer designed for a
maximum of 237 gallons per minute (14.95 I/sec).
Thickened sludge flow rate - The maximum hourly net solids load
was 504 pounds per hour (228.6 kg/hr). At the minimum four
percent solids concentration, the expected flow rate can be
calculated as follows:
504 Ib/hr r ' . .. _,. , .
T0)4)(8.34)(60 min/hr) = 25'2 9pm (1'59 1/sec>
Subnatant flow rate - This rate is equal to the maximum total
flow into the tank 237 gallons per minute (14.95 I/sec) feed
plus 237 gallons per minute (14.95 I/sec) recycle.
Hydraulic Surface Loading Rate
Based on solids loading, the minimum thickener surface area
was calculated to be 1,200 square feet (108 m2). The total
maximum flow rate (influent plus recycle) was calculated to be
474 gallons per minute (1,794 1/min). The maximum hydraulic
surface loading rate would be:
= 2'53 9Pm/S(3 ft
The 2.53 gallon per minute per square foot (105 1/min/m2) is on
the high side for a system that does not employ polymer addition.
Under maximum conditions, polymer usage would be required.
Number of Units
Only one unit will be used, with an adequate spare parts
inventory to minimize down time.
Manufacturers
Several reputable manufacturers of DAF thickeners were contacted
for their comments on the designer's calculations and proposed
application.
5.4.3 Cost
5.4.3.1 Capital Cost
Several recent publications have developed capital cost curves
for DAF thickeners (26-28). As discussed in Section 5.3.5.1,
the most factual is the reference based on actual USEPA bid
5-35
-------
documents for the years 1973-1977 (27). Although the data were
scattered, a regression analysis indicated the capital cost could
be approximated by Equation 5-3:
C = 2.99 x 104Q1-14
(5-3)
where:
C = capital cost of process in dollars;
Q = plant design flow in mil gal wastewater flow per day.
The associated costs include, those for, excavation, process
piping, equipment, concrete and steel. In addition, such cost as
those for administrating and engineering are equal to 0.2264
times Equation 5-3 (27).
5.4.3.2 Operating and Maintenance Costs
Staffing
Figure 5-13 indicates annual man-hour requirements for operations
and maintenance. As an example, for a DAF thickener surface area
of 1,000 square feet (93 m2) a designer would include 2,700 man-
hours of operation and maintenance in the cost analysis.
Power
Figure 5-14 shows annual power consumption for a continuously
operating DAF thickener as a function of DAF thickener surface
area. As an example, for a DAF thickener surface area of
1,000 square feet (93 m^), a designer would include a yearly
power usage of 720,000 kWhr (2,592 GJ) in the cost analysis.
Figure 5-14 does not include accessories such as pumps or polymer
feed systems.
Maintenance Material Cost
Figure 5-15 shows a curve developed for estimating DAF thickener
maintenance material cost as a function of DAF thickener
surface area. As an example, for DAF thickener surface area of
1,000 square feet (93 m^), a designer would estimate a yearly
materials cost of $275. Since this number is based on a June
1975 cost, it must be adjusted to the current design period.
5.5 Centrifugal Thickening
5.5.1 Introduction
The concept of using centrifuges for thickening municipal
wastewater sludges (waste-activated sludge) was first considered
in the United States in the late 1930's (49). At that time, disc
5-36
-------
nozzle centrifuges were used. Early installations used machines
developed for industrial processing. Equipment manufacturers
did not appreciate that the composition of municipal wastewater
sludges is extremely variable from plant to plant and within a
plant, and that most wastewater treatment facilities provided
little, if any, of the preventive maintenance common in
industrial applications.
oc
O
LL
CO
cc.
D
o
X
I
^
cc
o
z
z
<
10
3 456789 100 2 34567 891,000 2
THICKENER AREA, sq ft (1 sq ft = 0.093 m2)
FIGURE 5-13
ANNUAL O&M MAN-HOUR REQUIREMENTS -
DAF THICKENERS (28)
4 56789
early installations developed numerous operational
Dlems; thus, for a period of time designers
and users did not favor the centrifuge
Consequently, early installations developed nt
and maintenance problems; thus, for a period
By the late 1960's, equipment manufacturers had designed new
machines specifically for wastewater sludge applications, and
centrifuges began to be used once again. Considerable experience
resulted in improved application of centrifuges and centrifuge
5-37
-------
support systems (chemical conditioning and chemical feed systems,
pumps, and electrical controls). Today, more sophisticatd
machines are being built that require less power and attention
and produce less noise.
o
CD
co
n
i_
_c
g
t
5
D
CO
Z
O
o
cc
LU
O
a.
D
Z
Z
3 456789 100 2 3456789 1,000 2
THICKENER AREA, sq ft (1 sq ft = 0.093 m2)
3456789
FIGURE 5-14
ANNUAL POWER CONSUMPTION - CONTINUOUS OPERATING
DAF THICKENERS (28)
At present, disc nozzle, imperforate basket and scroll-type
decanter centrifuges are used in municipal wastewater sludge
thickening.
5.5.2 Theory
Centrifugation is an acceleration of sedimentation through
the use of centrifugal force. In a settling tank, solids sink to
the bottom and the liquid remains at the top. In a centrifuge,
the rotating bowl acts as a highly effective settling tank.
Space limitations within this manual make it impossible to
discuss the theory and mathematics involved in centrifugation.
Complete discussions can be found in other references (50-52).
5-38
-------
10,000
7
6
5
4
_ro
"5
to
C/3
O
u
1,000
g
8
7
6
5-
3 -
2 -
100
2 34 567891,000 234 5678910,000 2 3 456789
THICKENER AREA, sq ft (1 sq ft = 0.093 m2)
FIGURE 5-15
ESTIMATED JUNE 1975 MAINTENANCE MATERIAL COST
FOR DAF THICKENERS (28)
One aspect that should be mentioned about centrifuge theory,
because of its misapplication by the wastewater design profes-
sion, is the use of a Sigma factor to evaluate bids from
different centrifuge manufacturers. First developed in 1952 (53),
the Sigma concept is an established me thbd derve lcTb!f3tTcT
predict the sedimentation performance of centrifuges that are
geometrically and hydrodynamically similar. It cannot, however,
be used in engineering bid specifications to compare different
units when the two basic assumptions, geometric and hydrodynamic
similarity, are not valid. This is normally the case in
scroll-type decanters.
5.5.3 System Design Considerations
5.5.3.1 Disc Nozzles
Disc nozzles were first used in the United States in 1937 (49).
To date, approximately 90 machines have been installed at over
5-39
-------
50 municipalities (37). Table 5-11 lists the advantages and
disadvantages of a disc nozzle as compared to other thickening
systems. Figure 5-16 shows a typical disc nozzle centrifuge.
TABLE 5-11
ADVANTAGES AND DISADVANTAGES OF DISC NOZZLE CENTRIFUGES
Advantages
Disadvantages
Yields highly clarified centrate without
the use of chemicals
Has large liquid and solids handling
capacity in a very small snace
Produces little or no odor
Can only be used on sludges with particle
sizes of 400 vm or less
Requires extensive prescreening and grit
removal
Requires relatively high maintenance if
designed
Requires skilled maintenance personnel
FIGURE 5-16
TYPICAL DISC NOZZLE CENTRIFUGE IN THE FIELD
5-40
-------
Principles of Operation
Figure 5-17 features a cut away view of a disc nozzle centrifuge.
The feed normally enters through the top (bottom feed is also
possible) and passes down through a feedwell in the center of the
rotor. An impeller within the rotor .accelerates and distributes
the feed slurry, filling the rotor interior. The heavier solids
settle outward toward the circumference of the rotor under
increasingly greater centrifugal force. The liquid and the
lighter solids flow inward through the cone-shaped disc stack.
These lighter particles are settled out on the underside of the
discs, where they agglomerate, slide down the discs, and migrate
out to the nozzle region. The gap of 0.050 inches (1.27 mm)
between the discs means that the particles have a short distance
to travel before settling on the disc surface. The clarified
liquid passes on through the disc stack into the overflow chamber
and is then di-sefetrge.' tk- . ^Vthe *£ £i««rrir i
FEED
EFFLUENT
DISCHARGE
CONCENTRATING
CHAMBER
SLUDGE
DISCHARGE
FEED
EFFLUENT
DISCHARGE
ROTOR
BOWL
ROTOR
NOZZLES
SLUDGE
DISCHARGE
RECYCLE FLOW
FIGURE 5-17
SCHEMATIC OF A DISC NOZZLE CENTRIFUGE
5-41
-------
The centrifugal action causes the solids to concentrate as they
settle outward. At the outer rim of the rotor bowl, the high
energy imparted to the fluid forces the concentrated material
through the rotor nozzles. One part of this concentrated
sludge is drawn off as the thickened product and another is
recycled back to the base of the rotor and pumped back into the
concentrating chamber; there, it is subjected to additional
centrifugal force and is further concentrated before it is
once again discharged through the nozzles. This recirculation
is advantageous because it increases the overall underflow
concentration; minimizes particle accumulation inside the rotor
by flushing action; allows the use of larger nozzles, thus
decreasing the potential for nozzle plugging; and helps to
achieve a stable separation equilibrium that lends itself to
precise adjustment and control.
Disc nozzle centrifuges can be applied only to sludges consisting
of smaller particles (less than 400 m [54]) and void of
fibrous material. In early installations, severe operating
and maintenance problems occurred from pluggage (24,49,55,56).
For wastewater treatment, then, only those systems that provide
primary treatment and separate the primary sludge from the
waste-activated sludge can be equipped with a disc nozzle
centrifuge and only activated sludge can be thickened in this
way. Even for those systems that keep the necessary separation,
designers have frequently forgotten the amount of fibrous
material that can be recycled back into the aeration system
from a dirty anaerobic digester supernatant stream. This also
eventually causes severe pluggage.
Pretreatment
To further reduce operation and maintenance requirements, current
design recommendations provide for pretreatment of the disc
nozzle feed stream. Figure 5-18 shows a disc nozzle pretreatment
system.
Raw WAS is pumped to a strainer in order to remove large solids
and fibrous material. Strainers should be made of stainless
steel, should be self-cleaning, and should be easily accessible.
Approximately one percent of the inlet flow will be rejected.
The reject stream should go to the primary sludge handling
system.
After screening, the flow goes to a degritter; however, even
after aerated grit removal and primary treatment, some grit
gets into the aeration basin. Under the velocities generated in
a disc nozzle, this grit becomes abrasive and causes nozzle
deterioration. The degritter does not eliminate the problem
completely but it does increase the running time between nozzle
replacements. Approximately 10 percent of the degritter inlet
flow is rejected, and this rejected stream is usually combined
with the screen flow.
5-42
-------
STRAINER
RAW WASTE
NOZZLE
SEPARATOR
REJECT FLOW GOES
BACK TO PRIMARY
SLUDGE HANDLING SYSTEM
p) RECIRCULATION
-V-> PUMP
THICKENED
SLUDGE
FIGURE 5-18
TYPICAL DISC NOZZLE PRETREATMENT SYSTEM
Performance
Table 5-12 lists typical performance that can be expected of
disc nozzle centrifuges. In addition to the standard process
variables, the disc nozzle machine variables considered are bowl
diameter, bowl speed, operation of recycle, disc spacing, and
nozzle configuration. Possibly the most important consideration,
however, is the nature of the sludge. As with other centrifuge
applications, an increasing sludge volume index (SVI) influences
machine performance. Figure
capture and thickening (57).
5-19 shows the effect of SVI's on
TABLE 5-12
TYPICAL PERFORMANCE OF DISC NOZZLE CENTRIFUGE
Ref
5
5
5
5
24
60
Capacity,
gallons
per
minute
150
400
50-80
60-270
66
200
Feed
solids ,
percent
solids
0. 75-1.0
-?
0.7
0.7
1. 5
0.75
Underflow
solids ,
percent
solids
5-5.5
4.0
5-7
6.1
6.5-7.5
5.0
Solids
recovery,
percent
90 +
80
93-87
97-80
87-97
90
Polymer,
pounds per
dry ton
of solids
None
None
None
None
None
None
1 gpm = 3.78 1/min
1 Ib/ton = 0.5Aa/t
5-43
-------
DC
LLJ
>
o
u
LU
QC
100
90
80
70
60
50
40
30
2V/*
2.Q
3.0
4.0
5.0
6.0
THICKENED SLUDGE SOLIDS, %
FIGURE 5-19
EFFECT OF ACTIVATED SLUDGE SETTLEABILITY ON
CAPTURE AND THICKENING (57)
In general, it can be said of disc nozzle performance that the
concentration of the thickened sludge tends to increase with
increasing solids concentration in the inlet. Depending on inlet
solids concentration, thickened sludge will be five to ten times
more concentrated than the feed. The capability to concentrate
will decrease as the inlet solids become more concentrated.
Solids capture of 90 percent or better for the material fed into
the disc nozzle (after screening and grit removal) should be
obtainable without the use of polymers.
Other Considerations
As noted in the discussion of pretreatment requirements,
approximately 11 percent of the flow to the disc nozzle system is
rejected. The reject stream contains two to three percent solids
and is usually pumped to the primary sludge handling system.
The centrate stream is normally returned to the aeration tank.
This line should be designed to handle the entire flow being
pumped to the pretreatment system.
5-44
-------
Typically, equipment suppliers furnish disc nozzle systems
complete, including all necessary pumps. The system must be
assembled in the field.
5.5.3.2 Imperforate Basket
Imperforate basket centrifuges were first used in the U.S. in
1920, and to date, approximately 100 municipal installations
(over 300 machines) have been installed (37). About one half are
used for thickening. In fact, the largest centrifuge facility
in the world, the Joint Water Pollution Plant of the County
Sanitation Districts of Los Angeles County, California, utilizes
48 imperforate basket centrifuges. Table 5-13 lists the
advantacjos and disadvantages of an imperforate 'OH sky I: centrifuge
compared to other thickening systems.
TABLE 5-13
ADVANTAGES AND DISADVANTAGES OF IMPERFORATE BASKET CENTRIFUGE
Advantages Disadvantages
Facility can be designed so that same Unit is not continuous feed and discharged
machine can be used both for thickening Requires special structural support
and dewatering
... . , . Has the highest ratio of capital cost to
Is very flexible in meeting process .,
requirements
Is not affected by grit
Of all the centrifuges, has the lowest
operation and maintenance requirements
Compared to gravity and DAF thickener
installations, is clean looking and has
little to no odor problems
Is an excellent thickener for hard-to-handle
sludges
f Operation
Figure 5-20 is a schematic of a top feed imperforate basket
centrifuge illustrating general location of sludge inlet, polymer
feed, and centrate piping and location of cake discharge.
The following describes one complete batch operating cycle of a
basket centrifuge. When the "cycle start" button is pushed, the
centrifuge begins to accelerate. After approximately 30 seconds,
the feed pump is started through a timer relay. Depending on
the feed pump rate, it will take one to three minutes for the
bowl to reach operating speed. Sludge enters the unit through a
stationary feed pipe mounted through the curb cap. This pipe
extends to the bottom portion of the basket and ends at an angle
just above the floor in order to impart a tangential velocity to
the input stream. The duration of the feed time is controlled by
5-45
-------
either a pre-set timer or a centrate monitor that shuts the feed
pump off when a certain level of suspended solids appears in the
centrate. The centrate is normally returned to the inlet of the
secondary treatment system.
FEED
POLYMER
SKIMMINGS
KNIFE
CAKE
CAKE
FIGURE 5-20
GENERAL SCHEMATIC OF IMPERFORATE BASKET CENTRIFUGE
Deterioration in the centrate indicates that the centrifuge bowl
is filled with solids, and separation can no longer take place.
At this point, the sludge feed pump is turned off.
Turning off or diverting the feed pump decelerates the centrifuge.
When the centrifuge has decelerated to 70 rpvn's, a plow (located
by the center spindle shaft) is activated and starts to travel
horizontally into the bowl where the solids have accumulated.
5-46
-------
When the plow blade reaches the bowl wall, a dwell timer is
activated to keep the plow in the same position for approximately
5 to 15 seconds until all the solids have been discharged. When
the plow retracts, a cycle has been completed and the machine
will automatically begin to accelerate, starting a new cycle.
Application
Basket centrifuge is a good application for small plants (under
1 to 2 MGD [44 to 88 I/sec]) pumping capacity. The appropriate
plant would provide neither primary clarification nor grit
removal (that is, extended aeration, aerated lagoons, contact
stabilization), but would require:
Thickening before aerobic or anaerobic digestion,
Solids content of less than ten percent to minimize cost
of hauling liquid sludge for land disposal, and
A machine that can thicken sludge part of the time and
dewater sludge part of the time.
Performance
Table 5-14 lists typical basket centrifuge performance on several
types of sludges. Figure 5-21 shows the relative influence of
one process variable as a function of feed solids content,
holding all other process variables constant.
TABLE 5-14
TYPICAL THICKENING RESULTS USING IMPERFORATE
BASKET CENTRIFUGE
Sludge type
Feed solids
concentration,
percent solids
Raw waste-activated
sludge '. ,
Aerobically
digested sludge
Raw trickling filter sludge
(rock & plastic media)
Anaerobically digested
sludge, primary and rock
trickling filter sludge
(70:30)
0.5-1.5
1-3
2-3
2-3
Average
cake solids
concentration,
percent solids
8-10
8-10
8-9
9-11
8-10
7-9
Polymer
required,
pounds dry
per ton dry
feed solids
0
1.0-3.0
0
1.0-3.0
0
1.5-3.0
0
1.5-3.0
Recovery
based on
centrate ,
percent
85-90
90-95
80-90
90-95
90-95
95-97
95-97
94-97
1 Ib/ton =0.5 kg/t
5-47
-------
FEED, % TOTAL SOLIDS
POSSIBLE
FEED, % TOTAL SOLIDS
c
1
a
o>
O)
FEED, % TOTAL SOLIDS
FEED, % TOTAL SOLIDS
o
LU
O
a.
FEED, % TOTAL SOLIDS
FEED, % TOTAL SOLIDS
FIGURE 5-21
RELATIVE INFLUENCE OF ONE PROCESS VARIABLE AS A
FUNCTION OF FEED SOLIDS CONTENT FOR IMPERFORATE
BASKET CENTRIFUGE HOLDING ALL OTHER PROCESS
VARIABLES CONSTANT
5-4!
-------
Othg_r_C£n_si_derations
In discussions of hydraulic flow rate, a distinction must be
made between instantaneous feed rate and average feed rate.
Instantaneous feed rate is the actual hydraulic pump rateto
the basket. The average feed rate includes the period of time
during a cycle when sludge is not being pumped to the basket
(acceleration, deceleration, discharge). Therefore, dividing
total gallons pumped per cycle by total cycle time gives the
average feed rate.
Basket centrifuge performance is affected by the solids feed rate
to the machine. As the solids concentration changes, the flow
rate must be adjusted. Every effort should be made to minimize
floe shear. For this reason, positive displacement cavity feed
pumps with 4 to 1 speed variation are recommended.
Cake solids concentration can only be discussed as average solids
concentration. The solids concentration in a basket centrifuge
is maximum at the bowl wall and decreases toward the center. The
solids concentration discharged will be the average for the
mixture.
The centrate stream should be returned to the secondary system.
5.5.3.3 Solid Bowl Decanter
The first solid bowl decanter centrifuge in the U.S. to operate
successfully on municipal wastewater sludge was installed in the
mid-1930's (58). Since then there have been approximately
150 installations (over 400 machines) (37). Few of these units
were used for thickening because the rotating scroll created
disturbances in the thickening sludge, and the gravity force that
had to be overcome in climbing the beach made it more difficult
for the liquid thickened sludge to be discharged.
Technological advances have made solid bowl decanters for
thickening waste-activated sludge available. Table 5-15 lists
the current advantages and disadvantages of solid bowl decanter
centrifuges in waste-activated sludge thickening.
Principles of Operation
Figure 5-22 is a schematic of a solid bowl decanter centrifuge.
The sludge stream enters the bowl through a feed pipe mounted at
one end of the centrifuge.
As soon as the sludge particles are exposed to the gravitational
field, they start to settle out on the inner surface of the
rotating bowl. The lighter liquid, or centrate, pools above the
sludge layer and flows towards the centrate outlet ports located
at the large end of the machine.
5-49
-------
TABLE 5-15
ADVANTAGES AND DISADVANTAGES OF SOLID BOWL DECANTER CENTRIFUGES
Advantages
Disadvantages
Yields high throughput in a small area
Is easy to install
Is quiet
Causes no odor problems
Has low capital cost for installation
Is a clean looking installation
Has ability to constantly achieve four to
six percent solids in the thickened sludge
Is potentially a high maintenance item
May require polymers in order to operate
successfully
Requires grit removal in feed stream
Requires skilled maintenance personnel
FEED
\
.
.';.'; DEWATERED
' "'.'. SOLIDS
FIGURE 5-22
SCHEMATIC OF TYPICAL SOLID BOWL DECANTER
CENTRIFUGE
The settled sludge on the inner surface of the rotating bowl is
transported by the rotating conveyor towards the conical section
(small end) of the bowl. In a decanter designed for dewatering,
the sludge, having reached the conical section, is normally
conveyed up an incline to the sludge outlet. Waste-activated
5-50
-------
sludge is too "slimy" to be conveyed upward without large doses
of polyelectrolyte. In the newly designed machines, maximum
pool depths are maintained; in addition, a specially designed
baffle is located at the beginning of the conical section. This
baffle, working in conjunction with the deep liquid pool, allows
hydrostatic pressure to force the thickened sludge out of the
machine independent of the rotating conveyor. This design
eliminates the need for polymer addition to aid in conveying
thickened sludge up the incline towards the sludge discharge and
allows only the thickest cake at the bowl wall to be removed.
Figure 5-23 shows a typical installation of a centrifuge designed
for thickening.
FIGURE 5-23
SOLID BOWL DECANTER CENTRIFUGE INSTALLATION
Application
Because of the specially designed baffle, the new type of
thickening decanter centrifuge can be used to thicken only
straight waste-activated or aerobically digested waste-activated
5-51
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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
-------
The following provides a rough guide for operating and
maintenance requirements for a disc nozzle centrifuge:
It is best to run a disc nozzle 24 hours per day to
prevent shutdowns from materials that will dry out
between stacked plates when machine is not operating.
At least once every eight hours, each machine should be
inspected for general machine operation, product, and
amperage draw--l/2 man-hour per unit.
At least once a week, each machine should be shut down
to be given a thorough flushing, and nozzles should be
removed and cleanedtwo man-hours per unit.
If grease is present in the system, the machine should
be flushed with hot water at least once every other
day--three man-hours per unit.
Depending on sludge characteristics, the length of time
before a machine has to be completely disassembled and
cleaned is quite variable. A complete cleaning will
take approximately 16 man-hours.
Even with good pretreatment, nozzles, holders and recycle
tubes will have to be replaced.
Other parts that will need replacing are drive belts
and pumps.
Lmper_£cirate Basket Centrifug£
For a well designed system, operation and maintenance for one
48-inch by 30-inch (122 cm x 76 cm) basket using a hydraulic
drive can be approximated as follows:
Normal start-up and shutdown - 0.5 man-hour.
Observation time per eight-hour shift--!.0 man-hour.
Basket oil change (1 quart SAE 10-40 motor oil [0.95 1]
10-40 motor oil) is required every 200 operating hours--
0.5 man-hour.
General machine lubrication is needed every 200 operating
hours--0.5 man-hour.
Air compressor should be serviced every 1,000 operating
hours--!.0 man-hour.
Hydraulic oil change (65 gallons [246 1]) is required
every 3,500 operating hours or once per year3.0 man-
hours .
5-57
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High pressure oil filter should be changed every 1,000
operating hours0.5 man-hour.
If the machine is to be shut down for more than 24 hours,
the basket should be cleaned with water (tap water
pressure). This can be provided as an automatic or a
manual operation--0.5 man-hour for manual operation.
Basket bearings should be replaced every 100,000
operating hours40 man-hours.
Standard materials repair cost per 1,000
operating hours is $300 to $350 (June 1979).
machine
Specific power draw for this size basket centrifuge
ranges from 1.1 to 1.3 horsepower per gallon per minute
(13 to 15 kW/l/sec) flow rate.
SoJL_ij_Bowl DecajT,ter Centr i fuge
Figure 5-25 indicates annual man-hour requirements for operation
and maintenance. Included in the curve are labor requirements
directly related to the centrifuge, sludge conditioning, and
other associated equipment.
oc
O
LL
o
X
I
^
cc
O
1,000
2 345678910 2 3 456789100 2
AVERAGE FLOW, gpm (1 gpm = 40.8 l/min)
FIGURE 5-25
ANNUAL O&M REQUIREMENTS - SOLID BOWL DECANTER
CENTRIFUGE (28)
3 456789
5-58
-------
Power
Power is dependent on machine design, but it should range from
0.28 to 0.37 horsepower per gallon per minute flow rate (3.3 to
4.4 kW/l/sec) .
Maintenance Material Costs
Figure 5-26 shows a curve developed for estimating solid bowl
decanter centrifuge maintenance material cost.
o
a
C/3
O
O
100,000
9
8
7
6
5
3
2
10,000
7
6
5
4
1,000
I
I
I
3 456789
1 2 34567 89 10 2 34567 89100 2
AVERAGE FLOW, gpm (1 gpm = 40.8 l/min)
FIGURE 5-26
ESTIMATED JUNE 1975 MAINTENANCE MATERIAL COST
FOR SOLID BOWL DECANTER CENTRIFUGE
5.6 Miscellaneous Thickening Methods
5.6.1 Elutriation Basin
Elutriation is a satisfactory process for washing and thickening
digested primary sludges. Elutriation is also effective for
mixtures of primary and biological sludges as long as a small
5-59
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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
O
o
01
o
C
O
Q
«
2
O
H
a:
z
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
z
LU
4 -
3 -
2 -
1 -
0
20
15
cc 11.8
LLJ
o
CJ
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 ionspredominantly ferrous, ferric, aluminum, and
calcium. The fate of phosphorus during the anaerobic digestion
of phosphorus-laden biological and chemical sludges has been the
subject of several studies (55,101-104). The results of these
studies are not entirely consistent. In some cases (99,101),
bound phosphorus was resolubilized during anaerobic digestion
6-32
-------
and released to the digester supernatant. The return of this
phosphorus-laden supernatant to the liquid treatment stream can
substantially reduce the net phosphorus removal efficiency of the
plant (101) and/or increase chemical demand. However, in most
studies (55,102-104), release of soluble phosphorus into digester
supernatant was minimal.
TABLE 6-10
SUPERNATANT, CHARACTERISTICS OF HIGH-RATE,
TWO-STATE, MESOPHILIC, ANAEROBIC DIGESTION
AT VARIOUS PLANTS (90,93,94,95,96)
Concentration3, mg/1
Parameter
Reference
Total solids
Total volatile solids
Primary
(95)
9,400
4,900
Primary and trickling
sludge filter sludge Primary and activated sludge
(90)b (94)
4,545
2,930
(951 (901° (94)
1,475
814
(94) (95) (96) (90)b
2,160 -
983 -
Suspended solids
Average
Maximum
Minimum
Volatile suspended solids
Average -
Maximum
Minimum
BOD
Average
Maximum
Minimum
COD
TOC
Total (P04)-P
MH3-N
Organic N
PH 8.0
Volatile acids
Alkalinity (as CaCC>3) 2,555
Phenols
Average . - 0.23 - - 0.23 - 0.35
Maximum - 0.80 - - 0.50 - 1.00
Minimum - 0.06 - - 0.06 - 0.08
4,
17,
2,
10,
1,
277
300
660
645
850
420
713
880
200
-
-
-
-
-
-
-
-
2,205
1,660
-
4,565
1, 242
143
853
291
7.3
264
3,780
1,518
-
-
2,230
-
85
-
678
7.2
-
-
7,772
32,400
100
4,403
17,750
60
1,238
6,000
135
-
-
-
-
-
-
-
-
383
299
-
1, 384
443
63
253
53
7 .0
322
1,349
143
118
-
1,310
320
87
559
91
7.8
250
1,434
740 1,075
750
515
1,230
-
100
480
360 560
7.0 7.3
-
-
4,408
14,650
100
3,176
10,650
75
667
2,700
100
-
.
-
-
-
-
-
aUnless noted, all values are average for the sampling period studied.
bvalues indicated are a composite from seven treatment plants.
cValues indicated are a composite from six treatment plants.
Phenols
Phenols have been found in digester supernatants in concentra-
tions sufficient to inhibit biological activity (56). Typical
phenol concentrations are included in Table 6-10. The source
of phenols is not usually industrial waste discharges but
putrefaction of proteins, which begins in the human body and
continues in the sewage system. Phenols are very toxic and are
used commercially as an antiseptic. In dilute concentrations,
phenols do not necessarily kill bacteria but slow their growth
6-33
-------
and inhibit their normal metabolic activity. As a result, the
phenols contained in digester supernatant, combined with phenolic
compounds already in the sewage, may be an important cause of
sludge bulking (90). In addition, the recycling of phenols in
supernatant may contribute to odor problems.
Ammonia
As shown in Table 6-10, high levels of ammonia are often found in
digester supernatant. In plants that are nitrifying, the super-
natant return will provide a large portion of the ammonia feed to
the wastewater process. The conversion of this ammonia to
nitrate will therefore result in increased costs to provide the
required oxygen for treatment. In plants that must achieve a
nitrogen limitation in their effluent, the recycle ammonia
loadings must be carefully evaluated as to their overall effect
in meeting the standards.
6.2.5 Operational Considerations
6.2.5.1 pH
As was noted in Section 6.2.1, anaerobic digestion is a two-step
process consisting of "acid-forming" and "methane-forming" steps.
During the first step, the production of volatile acids tends
to reduce the pH. The reduction is normally countered by
destruction of volatile acids by methanogenic bacteria and the
subsequent production of bicarbonate.
Close pH control is necessary because methane-producing bacteria
are extremely sensitive to slight changes in pH. Early research
(105-107) showed that the optimum pH for methane-producing
bacteria is in the range of 6.4-7.5 and that these bacteria
are very sensitive to pH change. A 1970 study (108) seems to
indicate that the pH tolerance of methane-producing bacteria
is greater than previously thought. The bacteria are not
necessarily killed by high and low pH levels; their growth is
merely stopped. Because of the importance of these findings to
system control, more research is needed to verify these results.
Several different acid-base chemical equilibria are related to
pH. In the anaerobic digestion process, the pH range of interest
is 6.0 to 8.0, which makes the carbon dioxide-bicarbonate
relationship the most important. As Figure 6-15 indicates,
system pH is controlled by the CC>2 concentration of the gas
phase and the bicarbonate alkalinity of the liquid phase. A
digester with a given gas-phase CC-2 concentration and liquid-
phase bicarbonate alkalinity can exist at only one pH. If
bicarbonate alkalinity is added to the digester and the
proportion of CC<2 in the gas phase remains the same, digester
6-34
-------
pH must increase. For any fixed gas-phase CC>2 composition, the
amount of sodium bicarbonate required to achieve the desired pH
change is given by the following equation:
D = 0.60 (BA at initial pH - BA at final pH)
(6-1)
where:
D = sodium bicarbonate dose, mg/1
BA = digester bicarbonate alkalinity as mg/1 CaCC>3
The pH increase is less important, however, than the effect
on system buffering capacity, (that is, the system's ability
to resist pH changes). If bicarbonate alkalinity is added,
buffering capacity is increased, system pH is stabilized, and
the system becomes less susceptible to upset. The effect of
buffering capacity on anaerobic digester operations is discussed
elsewhere (110,111).
DU
40
o
oc 30
LU
20
CM
o
o
10
to s
OPERATING
'TEMPERATURE
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
-------
6.2.6.5 Piping
The piping system for an anaerobic digester is an important
component of the design. Many activities take place during
the operation of a digester: feeding of raw sludge, circulation
of sludge through the heat exchanger, withdrawal of digested
sludge and supernatant, and collection of sludge gas. The piping
system should be designed to allow these activities to occur
concurrently, yet independently. Flexibility should also be
built into the piping system to allow operation in a variety of
modes and to ensure that digestion can be continued in the event
of equipment breakdown or pipe clogging.
Feeding of incoming sludge into anaerobic digesters can be
automated to load the tanks frequently and uniformly. Switching
feeds between several tanks can be controlled based on either
time, hydraulic flow, or solids flow. A time-controlled feed
system uses a repeat-cycle timer to sequentially open and
close the feed valve for each digester. Switching between
digesters can occur every thirty minutes to four hours. A
flow-controlled feed system uses a flowmeter on the raw sludge
pipeline, in combination with a totalizer, to load preset volumes
to each digester. These may or may not be equal depending on the
individual characteristics of each digester. A feed control
system based on solids flow requires the measurement of both raw
sludge flow and density. Since density is correlated with the
concentration of solids in sludge, these two signals can be
combined to yield a measure of the solids mass being fed to the
digesters. Selection of flowmeters and density meters for sludge
is discussed in Chapter 17.
Raw sludge should enter the digester in the zone of intense
mixing to disperse the undigested organics quickly. Raw
sludge, before entering the digester, should be mixed with warm
circulating sludge to seed the incoming sludge and avoid thermal
shock. The introduction of cold feed sludge into regions where
there is no local mixing results in the feed sludge sinking to
the digester bottom and becoming an isolated mass.
Digested sludge is usually drawn off the bottom of the tank,
although means to withdraw sludge from at least one other
point should be provided in case the main line becomes plugged.
A supernatant collection system, when required, should have
drawoff points at three or more elevations to allow the operator
to remove the clearest supernatant. An example of a supernatant
collection system is shown on Figure 6-29. The telescopic valve
is used to adjust the water surface level in the digester. An
unvalved overflow with a vent as a jsiphon breaker is provided to
ensure that the tank cannot be overfilled.
Special consideration should be given in the design of
sludge-piping systems to prevent the deposition of grease and
clogging with debris. Sludge piping generally has a minimum
diameter of 6 inches (150 mm), except for pump discharge lines
6-66
-------
in small plants, where four-inch (100 mm) diameter pipes may
be acceptable. Where possible, considering these minimum pipe
size recommendations, velocities in sludge pipelines should
be maintained above four feet per second (1.2 m/sec) to keep
sludge solids in suspension. The hydraulics of sludge piping is
described in detail elsewhere (84,160). Glass lining of cast
iron and steel pipe will prevent the buildup of grease and is
recommended for all pipes conveying scum and raw sludge. The
grease content of sludge is typically reduced by 50 percent or
more during digestion, so that glass lining is not warranted for
pipes carrying digested or circulating sludge. Sludge piping is
generally kept as short as practicable, with a minimum number of
bends. Long radius elbows and sweep tees are preferred for
changes in direction. Provisions are commonly made for cleaning
sludge lines with steam, high pressure water, or mechanical
devices. These provisions should include blind flanges, flushing
cocks, and accommodation for thermal expansion.
A problem unique to anaerobic digestion systems is the buildup of
crystalline inorganic phosphate deposits on the interior walls of
the tank and downstream piping. This encrustation will increase
pipeline friction, displace volume in the digestion tank, and
foul downstream mechanical equipment (102). This chemical
scale has formed not only in digested sludge lines, but also on
mechanical aerators for facultative sludge lagoons and in pipes
carrying either digester supernatant or filtrate/cent rate.
Laboratory analyses have identified this material as magnesium
ammonium phosphate (MgNH4P04 6 1^0), more commonly known
as guanite or struvite. It has a specific gravity of 1.7,
decomposes when heated, and is readily soluble only in acid
solutions. Methods successfully used to prevent this buildup
include (161):
Aerobic digestion of the sludge stream with the highest
phosphate content
Dilution of digested sludge flows to prevent super-
saturation and to raise pipeline velocities
Limiting magnesium ion concentration in the stream
Substitution of PVC pipe for cast-iron pipe to reduce
interior roughness
6.2.6.6 Cleaning
Anaerobic digestion tanks can become partially filled with a
bottom layer of settled grit and a top layer of floating scum.
These accumulations reduce the volume available for active
digestion and thereby degrade the performance of the digesters.
Periodically, the digestion tank must be drained and these
6-67
-------
deposits removed. This cleaning process is usually expensive and
unpleasant. Furthermore, it can disrupt normal processing of
sludge for as long as several months. Therefore, attention
should be given during design to (1) reducing the rate at which
grit and scum can accumulate, and (2) making it easy to clean the
digester when it becomes necessary.
TOP OF DIGESTER
VENT
DIGESTER
OVERFLOW
SUPERNATANT
COLLECTION BOX
TELESCOPIC VALVE
MAX, W,a ELEVATION
GROUT
M)N. W.S. ELEVATION
SUPERNATANT
DRAWOFF PORTS
TO PLANT HEADWORKS
TO SECONDARY DIGESTER,
HEADWORKS, OR SUPERNATANT
TREATMENT
FIGURE 6-29
TYPICAL DIGESTER SUPERNATANT COLLECTION SYSTEM
6-68
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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
-------
MIXING*
TOTAL GAS PRODUCTION a
7.3
SURPLUS GAS
ANAEROBIC
DIGESTER
""-^. ^-"-""^
1
CIRCULATING* \
SLUDGE HEATING . t /"
K 1,75 f
V** ns X- -* [BOI
J
^~^~^^^ c
ALL VALUES \H UNITS OF
10B BTU/TON DRV SOLIDS
r
ij^y ^
F ' " ' \
RAW SLUOQEb ^
V
J
.75
'
HEATING HEAT
LOSSES
FED TO THE DIGESTER
(1 Btu/lh = 0,56
RAW
SLUDGE
Raw sludge volatile solids contents, percent 75
Volatile solids reduction during digester, percent 50
Specific gas production, cu ft/lb VS reduced 15
Heat value of gas, Btu/cu ft 650
2,000 Ib/ton (.75) (.50) (15
cu ft1
Ib '
(650
) = 7.3 x 10 Btu/ton
Feed solids concentration, percent
Specific heat of sludge, Btu/lb/°F
Rise in temperature, °F
(1
(25°F) = 1.3 x 106 Btu/ton
c Makeup Heat Requirement = .
Raw Sludge Heat Requirement
Net boiler and heating system efficiency, percent 70
F'eed solids concentration, percent 4
Detention time, days 20
Mixing requirement, bhp/1,000 cu ft 0.25
2.000 Ib/ton
'(.04) 62.4 Ib/cu ft
,,,
(2°
,,,.
(24
hr
,,. _,-
(0'25
bhp
1,000 cu ft
FIGURE 6-32
,^ _ . _
(2'547
Btu
= 2-4
Btu/ton
ENERGY FLOW THROUGH AN ANAEROBIC
SLUDGE DIGESTION SYSTEM
6-73
-------
in a waste-gas burner. The energy flow through a more complex
gas utilization system, in which gas is used to fuel an engine-
generator, is described in Chapter 18.
The energy flow diagram shown on Figure 6-32 conveys very
effectively the relative magnitude and direction of energy
exchanges in an anaerobic digestion system. This type of diagram
is helpful in the design of a gas utilization system. However,
more detail must be added and the full range of expected
conditions must be evaluated, rather than just the average
conditions depicted for this case.
More complete discussions of digester gas utilization systems can
be found in Chapter 18 and elsewhere (38,39).
6.2.8 Costs
Cost curves have been compiled that plot construction costs for
anaerobic digestion systems versus either digester volume (166),
sludge solids loading (167,168,169), or total treatment plant
flow (170,171,172). However, these curves differ significantly,
even when converted to a common cost index and plotted in terms
of a single sizing parameter (Figure 6-33). Cost curves
are generally constructed to allow comparison of equivalent
alternatives and consequently do not always describe actual
costs.
Estimated annual costs for operation and maintenance are shown
in Figure 6-34. No credit has been given in this graph for the
value of surplus sludge gas. In most cases, use of this gas
requires, construction of additional facilities for conditioning,
compressing, and burning the gas. The cost for construction
and operation of these systems (38) must be included in
calculations of the net value of surplus sludge gas.
6.2.9 Design Example
This section illustrates the basic layout and sizing of the
major components in an anaerobic sludge digestion system. For
this example, it is assumed that the treatment plant provides
activated sludge secondary treatment to a typical municipal
wastewater. A mixture of primary sludge and thickened
waste-activated sludge is to be anaerobically digested, held in a
facultative sludge lagoon, and ultimately spread as a stabilized
liquid onto land.
6.2.9.1 Design Loadings
Sludge production estimates for two flow conditions, average
and peak day, are listed in Table 6-20 (see page 6-79). The
peak loading is listed because several components must be sized
6-74
-------
to meet this critical condition. Refer to Chapter 4 for a
discussion of the procedures to determine sludge production
values. Sludge solids concentrations and the resulting sludge
volumes are also included in Table 6-20.
cc
z
UJ
I
-s
a
I
* .
H
O
U
GC
te
I
10.0 I
9
8
7
8
S
4
1.0
9
8
7
6
5
4 -
3 -
_ REFERENCE 16?
REFERENCE 171
'CONSTRUCTION COST ONLY,
DOES NOT INCLUDE ENGINEERING
OR CONTINGENCIES.
I
J__L_J
10
34B678S1CMJ 2 346678 91,000 2
DIGESTER TANK VOLUME, 1,000 cu ft (1 cu ft = .028m3|
4 5
FIGURE 6-33
CONSTRUCTION COSTS FOR ANAEROBIC
DIGESTION SYSTEMS (111-168,171)
6.2.9.2 System Description
The conceptual design for a high-rate anaerobic digestion
system is presented on Figure 6-35 (see page 6-80). At the
heart of the system are two cylindrical single-stage, high-rate
digestion tanks operated in parallel. The contents of both
digesters are heated to 95°F (35°C) and vigorously mixed with
draft-tube gas mixers. Floating covers are used on both tanks to
keep floating material soft and submerged, and to allow in-line
storage of sludge in the digestion tanks.
6-75
-------
er
O
03
5
j
<
§
e
4!
_
o
z
.001
.OOC1
oc
111
o
CL
E
5
_
c
o
O
O
12
<
*-" .00001
0.1
1.0 10
AVERAGE PLANT FLOW, MGD flMGD = 3,785m3/day)
FIGURE 6-34
OPERATING, MAINTENANCE, AND ENERGY COSTS
FOR ANAEROBIC SLUDGE DIGESTION SYSTEMS (171)
Raw primary and secondary sludges are first combined and then
heated to 95°F (35°C) in a jacketed pipe heat exchanger.
The rate of the raw sludge flow is measured with a magnetic
flowmeter. The signal from this meter is integrated to indicate
the hydraulic loading to digestion. This information is also
used to indicate equal volumes of raw sludge for even distribu-
tion to each digester. The controls are set so that each
digester is fed approximately ten times each day. Raw sludge
is mixed with circulating sludge and added to the digester
through the gas dome in the center of the cover. The operating
temperature in the digester is maintained by circulating a
6-76
-------
small volume of sludge through an external spiral heat exchanger.
Digested sludge is withdrawn daily from the bottom of the
tank and transferred by gravity to facultative sludge lagoons.
For monitoring purposes, a flowmeter is included in the digested
sludge withdrawal line. This provides a means for evenly
distributing the sludge to several lagoons. Both tanks
are operated as completely mixed primary digesters without
supernatant removal.
6.2.9.3 Component Sizing
Digestion T a n k s
Sizing criter a:
>_IQ days solids retention time during the most critical
expected condition to prevent process failure (See
Section 6.2.3.3).
2.50 percent volatile solids reduction at average
conditions to minimize odors from the facultative sludge
lagoons.
Tank volume:
Raw sludge flow at peak conditions (Qp)
Assume peak day conditions (this is conservatively large
but provides a margin of safety) .
Qp = 6,010 + 3,430 = 9,440 cu ft per day (267 m3/day)
Active volume (Va)
Va = f____ (1Q das) = 4? >2^ cu ft
Correction for volume displaced by grit and scum accumula
tions and floating cover level.
Assume:
4-ft grit deposit
2-ft scum blanket
2- ft cover below maximum
8-ft total displaced height
6-77
-------
Therefore, if original sidewater depth of the tank is 30 feet,
O Q __ p
active volume is only ^Q = °-73 of the total tanks volume.
Tank volume (Vt)
v = 47,200 cu ft / 1 \
fc tank \'73/
= 64,700 cu ft per tank
Say 65,000 cu ft per tank = (1,800 m3/tank)
Solids retention time at average conditions (SRTa)
q _ 65,000 cu ft per tank (2 tanks)
a 3,200 cu ft per day + 2,000 cu ft per day
= 25.0 days, based on total volume, 50 percent
volatile solids reduction can be expected with
this solids retention time (see Section 6.2.4.1).
Tank dimensions:
Diameter (D)
Assuming initially, a 30-foot sidewater height and
neglecting the volume in the bottom cone:
^ J4(65,000 cu ft) co c ... ,,, . .
D =\ J3Q £t) L = 52.5 ft = (16.0 m)
Sidewater height (h)
Since floating covers come in 5-foot diameter increments,
enlarge diameter and adjust sidewater height:
h = 4(65,000 cu ft) =
(55 ft)2
Note: This adjustment increases displacement volume
effect and reduces active volume to =-=j or 0.71.
This is ignored in this example because of previous
conservative assumptions.
6-78
-------
TABLE 6-20
DESIGN LOADING ASSUMPTIONS
Flow condition
Peak
Parameter Average day
Sludge production, Ib dry
solids/day
Primary sludge 10,000 ' 15,000
Waste activated
sludge 5,000 7,500
Solids concentration,
percent
Primary sludge 5.0 4.0
Waste activated
sludge 4.0 3.5
Sludge volume , cu ft/day
Primary sludge 3,200 6,010
Waste activated sludge 2,000 3,430
Sludge volume =
sludge production
(solids concentration) (density of sludge)
e.g., 10,000 Ib/day _ onri ... ,,
' t n c \ i r ->-A4\r~r^jnrr - 3,200 cu ft/day
(.05) (62.4 Ib/cu ft) J
I Ib/day = .454 kg/day
1 cu ft/day = .0283 m3
ft/day
Heat_Exchangers - (See Section 6.2.6.2)
Raw sludge heat exchanger capacity (Qs)
Assume:
Peak day sludge loading
Minimum temperature of raw sludge = 55°F
- /9,440 cu ftV (62.4 lb\ / 1 day\ / Btu \ (95oF_55o
- ~5lTTE~ V24 hrs V- Ib - °F (^^
= 982,000 Btu/hr = (247,000 kg-cal/hr;
6-79
-------
Makeup heat exchanger capacity (Qm)
Assume:
Tank completely buried but above water table, U = 0.06
Bottom exposed to wet soil, U = 0.11
Cover insulated, U = 0.16
Minimum soil temperature = 40°F
Minimum air temperature = 10°F
Qm =
heat loss through walls + bottom + top
= (0.06 Btu/sf/°F/hr) ( [2 ft]55 ft/4[27.4 f t] ) (95°F-40°F )
+ (0.11)( [55 ft]2/4] (95°F-40°F)
+ (0.16)( [55 ft]2/4] (95°F-10°F)
= 76,029 Btu/hr = (19.2 kg-kcal/hr)
The above calculated values are used for sizing equipment.
Average heat requirements would be substantially less.
PRIMARY
SLUDGE
WASTE
ACTIVATED
SLUDGI
RAW SLUDGE
HEAT EXCHANGER
RAW SLUDGE FLOW METER
FEED CONTROL VALVE
MAKE-UP HEAT EXCHANGER
CIRCULATING
SLUDGE PUMP
DRAFT TUBE
GAS MIXER
DIGESTED SLUDGE
FLOW M6TER
DIGESTED SLUDGE
CTO FACULTATIVE
SLUDGE LAGOONSI
FIGURE 6-35
CONCEPTUAL DESIGN OF AN ANAEROBIC SLUDGE DIGESTION SYSTEM
6-80
-------
Mj-xjing (See Section 6.2.6.3)
Sizing criterion:
Assumptions:
Velocity gradient (G) = 60 sec"1
Plant located at sea level PI = 14.7 psi
Gas released 13 ft below the water surface ?2 = 14.7
+ 0.434 (13) = 20.3 psi
Viscosity of the digesting sludge is the same as for
water at 95°F or 1.5 x 10""^ lbf-sec/sq ft
Rate of energy transfer (E)
Combining Equations 6-5 and 6-6 and solving for E:
E = V M G2
= 65,000 cu ft/tank (1.5 x 10~5 lbf - sec/sq ft)(60 sec"1)2
= 3,510 ft lbf/sec/tank = (4.8 kW/tank).
This is the power delivered to the digester contents. Motor
horsepower for the compressor will be substantially higher.
Gas Flow (Q) solving Equation 6-7 for Q.
2.4 (Pl) |ln |^J
3,510 ft-lb/sec/tank
6_8)
2.
= 308 cfm/tank (0.145 mj/sec/tank)
6-81
-------
6.3 Aerobic Digestion
Aerobic digestion is the biochemical oxidative stabilization of
wastewater sludge in open or closed tanks that are separate from
the liquid process system.
6.3.1 Process Description
6.3.1.1 History
Studies on aerobic digestion of municipal wastewater sludge
have been conducted since the early 1950's (175,176). Early
studies (177,178) indicated that aerobic digestion performed as
well as, if not better than, anaerobic digestion in reducing
volatile solids in sludge. Aerobic digestion processes were
economical to construct, had fewer operating problems than
anaerobic processes, and produced a digested sludge that drained
well. By 1963, at least one major equipment supplier (179) had
approximately 130 installations in plants with flow from 10,000
to 100,000 gallons per day (37.8 to 378 m3/day) . By the late
1960's and early 1970's, consulting engineers across the country
were specifying aerobic digestion facilities for many of the
plants they were designing.
6.3.1.2 Current Status
As of early 1979, numerous plants use aerobic digestion, and
several of them are quite large (11). Because of significant
improvements in design and control of anaerobic processes,
coupled with the significant mid-1970 jump in energy costs,
the continued use of aerobic digestion, except in the small
facility, is much in doubt.
6.3.1.3 Applicability
Although numerous lab and pilot-scale studies have been conducted
on a variety of municipal wastewater sludges, very few docu-
mented, full-scale studies have been reported in the literature.
Table 6-21 lists some of these aerobic digestion studies and
provides information on the type of sludge studied, temperature
of digestion, scale of study, and literature reference.
6.3.1.4 Advantages and Disadvantages
Various advantages have been claimed (66,197) for aerobic
digestion over other stabilization techniques, particularly
anaerobic digestion. Based on all current knowledge, the
following advantages can be cited for properly designed and
operated aerobic digestion processes:
Have capital costs generally lower than for anaerobic
systems for plants under 5 MGD (220 1/s) (170).
Are relatively easy to operate compared to anaerobic
systems.
6-82
-------
Do not generate nuisance odors (199,200).
Will produce a supernatant low in BOD5, suspended
solids, and ammonia nitrogen (199,200).
Reduce the quantity of grease or hexane solubles in the
sludge mass.
Reduce the number of pathogens to a low level under
normal design. Under auto-heated design, many systems
provide 100 percent pathogen destruction (187).
TABLE 6-21
SELECTED AEROBIC DIGESTION STUDIES ON VARIOUS
MUNICIPAL WASTEWATER SLUDGES
Sludge tyoe
Primary sludge
Primary sludge plus
waste-activated
lime
iron
alum
waste-activated + iron
.trickling filter
waste paper
Contact stabilization sludge
Contact stabilization sludge plus
iron
a 1 urn
Waste-activated sludge
Trickling filter sludge
)indicates full-scale study results.
Studies
under
50°F
180
187
Studies
'between
50°- 86°F
Studies
over
192
181, (182 )
187
188
189
189,(190)
190
131
191
192, 197, (199)
(190)
(190), (194)
195
181, 196
133, 184
(185) (186) (187)
As with any process, there are also certain disadvantages.
In aerobic digestion processes, the disadvantages are:
Usually produce a digested sludge
mechanical dewatering characteristics.
with very poor
Have high power
small plants.
costs to supply oxygen, even for very
Are significantly influenced in
temperature, location, and type of tank
performance by
material.
6.3.1.5 Microbiology
Aerobic digestion of municipal wastewater sludges is based on
the principle that, when there is inadequate external substrate
available, microorganisms metabolize their own celluar mass. In
6-83
-------
actual operation, aerobic digestion involves the direct oxidation
of any biodegradable matter and the oxidation of microbial
cellular material by organisms. These two steps are illustrated
by the following reactions:
Organic
matter
02
Bacteria
Cellular
material
C02 + H20
(6-9)
Cellular
material
o2
Digested
sludge
C02 + H20
(6-10)
The process described by Equation 6-10 is referred to as
"endogenous respiration"; this is normally the predominant
reaction in aerobic digestion.
6.3.2 Process Variations
6.3.2.1 Conventional Semi-Batch Operation
Originally, aerobic digestion was designed as a semi-batch
process, and this concept is still functional at many facilities.
Solids are pumped directly from the clarifiers into the aerobic
digester. The time required for filling the digester depends on
available tank volume, volume of waste sludge, precipitation, and
evaporation. During the filling operation, sludge undergoing
digestion is continually aerated. When the tank is full,
aeration continues for two to three weeks to assure that the
solids are thoroughly stabilized. Aeration is then discontinued
and the stabilized solids settled. Clarified liquid is decanted,
and the thickened solids are removed at a concentration of
between two and four percent. When a sufficient amount of
stabilized sludge and/or supernatant have been removed, the cycle
is repeated. Between cycles, it is customary to leave some
stabilized sludge in the aerator to provide the necessary
microbial population for degrading the wastewater solids. The
aeration device need not operate for several days, provided no
raw sludge is added.
Many engineers have tried to make the semi-batch process more
continuous by installing stilling wells to act as clarifiers in
part of the digester. This has not proven effective (200-202).
6.3.2.2 Conventional Continuous Operation
The conventional continuous aerobic digestion process closely
resembles the activated sludge process as shown on (Figure 6-36).
As in the semi-batch process, solids are pumped directly from
6-84
-------
UNSTABILIZED
SOLIDS
AEROBIC
DIGESTER
CLAR1FIER
THICKENER
SUPERNATANT
STABILIZED SOLIDS
FIGURE 6-36
PROCESS FLOW DIAGRAM FOR A CONVENTIONAL
CONTINUOUSLY OPERATED AEROBIC DIGESTER
clarifiers into the aerobic digester. The aerator operates
at a fixed level, with the overflow going to a solids-liquid
separator. Thickened and stabilized solids are either recycled
back to the digestion tank or removed for further processing.
6.3.2.3 Auto-Heated Mode of Operation
A new concept that is receiving considerable attention in the
United States is the auto-heated thermophilic aerobic digestion
process (187,203). In this process, sludge from the clarifiers
is usually thickened to provide a digester feed solids concentra-
tion of greater than four percent. The heat liberated in the
biological degradation of the organic solids is sufficient
to raise the liquid temperature in the digester to as high
as 140°F (60°C) (187). Advantages claimed for this mode of
operation are higher rates of organic solids destruction, hence
smaller volume requirements; production of a pasteurized sludge;
destruction of all weed seeds; 30 to 40 percent less oxygen
requirement than for the mesophilic process, since few, if any/
nitrifying bacteria exist in this temperature range; and improved
solids-liquid separation through decreased liquid viscosity
(187,203,204).
Disadvantages cited for this process are that it must incorporate
a thickening operation, that mixing requirements are higher
because of the higher solids content, and that non-oxygen aerated
systems require extremely efficient aeration and insulated tanks.
6-85
-------
6.3.3 Design Considerations
6.3.3.1 Temperature
Since the majority of aerobic digesters are open tanks, digester
liquid temperatures are dependent on weather conditions and
can fluctuate extensively. As with all biological systems, lower
temperatures retard the process while higher temperatures
speed it up. Table 6-21 lists studies on aerobic digestion
of municipal sludges as a function of liquid temperature.
When considering temperature effects in system design, one
should design a system to minimize heat losses by using concrete
instead of steel tanks, placing the tanks below rather than above
grade, and using sub-surface instead of surface aeration. Design
should allow for the necessary degree of sludge stabilization at
the lowest expected liquid operating temperature, and should meet
maximum oxygen requirements at the maximum expected liquid
operating temperature.
6.3.3.2 Solids Reduction
A major objective of aerobic digestion is to reduce the mass of
solids for disposal. This reduction is assumed to take place
only with the biodegradable content of the sludge, though some
studies (205,206) have shown that there may be destruction of the
non-organics as well. In this discussion, solids reduction will
pertain only to the biodegradable content of the sludge.
The change in biodegradable volatile solids can be represented by
a first order biochemical reaction:
where:
rlM
-fir = rate of change of biodegradable volatile solids
per unit of time - (Amass/time)
K^ = reaction rate constant - (time ~1)
M = concentration of biodegradable volatile solids
remaining at time t in the aerobic digester -
(mass/volume).
The time t in Equation 6-11 is actually the sludge age or solids
residence time in the aerobic digester. Depending on how the
aerobic digester is being operated, time t can be equal to or
6-86
-------
considerably greater than the theoretical hydraulic residence
time. The reaction rate term K^ is a function of sludge type,
temperature, and solids concentration. It is a pseudoconstant,
since the term's value is the average result of many influences.
Figure 6-37 shows a plot of various reported K^ values as a
function of the digestion temperature. The data shown are for
several different types of waste sludge, which partially explains
the scatter. Furthermore, there has been no adjustment in the
value of K,3 for sludge age. At this time, not enough data are
available to allow segregation of K^ by sludge type; therefore,
the line drawn through the data points represents an overall
average K^ value. Little research has been conducted on the
effect of solids concentration on reaction rate K^. The results
of one study with waste-activated sludge at a temperature of
68°F (20°C) are shown on Figure 6-38, which indicates that Kd
decreases with increasing solids concentration.
!B
a
6
^
LJJ
5
BE
z
o
Ul
cc
,40 i-
,35
,30
.25 -
.20 -
.15
,10
.05
X
0 -
D -
A -
-PILOT PLANT RiF
- PI LOT PLANT REF
- FULL SCALE HEF
- PILOT PLANT REF
- PILOT PLANT REF
- Pi LOT PLANT REF
PI LOT PLANT REF
PILOT PLANT REF
(2071
(208)
(185!
(208)
(209)
(196)
{210}
10 20 30 40 50
TEMPERATURE OF LIQUID IN AEROBIC DIGESTER, °C
FIGURE 6-37
REACTION RATE K
-------
.7
* .6
LU
<
QC
Z
g
o
<
HI
DC
1__ _L _ L _. { 1 1.1 I i I
6000 10,000 14,000 18,000 22,000
TOTAL SUSPENDED SOLIDS CONCENTRATION IN AEROBIC DIGESTER , mg/1
FIGURE 6-38
EFFECT OF SOLIDS CONCENTRATION ON
REACTION RATE Kd( 194)
6.3.3.3 Oxygen Requirements
Activated sludge biomass is most often represented by the
empirical equation C5H7NO2. Under the prolonged periods of
aeration typical of the aerobic digestion process, Equation 6-10
can be written as follows:
7O2
5CO2 + 3H20 + H+
(6-12)
Hypothetically, this equation indicates that 1.98 pounds
(0.898 kg) of oxygen are required to oxidize one pound (0.45 kg)
of cell mass. From pilot and full-scale studies, however, the
pounds of oxygen required to degrade a pound of volatile
solids were found to be 1.74 to 2.07 (0.789 to 0.939 kg). For
mesophilic systems, a design value of 2.0 is recommended. For
auto-thermal systems, which have temperatures above 113°F (45°C),
nitrification does not occur and a value of 1.45 is recommended
(187,203,204).
6-88
-------
The actual specific oxygen utilization rate, pounds oxygen per
1,000 pounds volatile solids per hour, is a function of total
sludge age and liquid temperature (192,199,205). In one study,
Ahlberg and Boyko (199) visited several operating installations
and developed the relationship shown on Figure 6-39. Specific
oxygen utilization is seen to decrease with increase in sludge
age and decrease in digestion temperature.
uj £ 8l°
t > 6,0
D
> C
-------
the process for a particular tank geometry. Figure 6-40 shows
the chart developed by Envirex Incorporated for low speed
mechanical aerators in noncircular basins. The use of this chart
is explained in the design example in Section 6.3.5.
SURFACE AREA
A^fjqus* £Mt|
(1 h "O.D8Q mj|
BOO
MO
Pl₯OT
PtDuctP SHAFT
i
FIGURE 6-40
DESIGN CHART FOR LOW SPEED MECHANICAL AERATORS IN NON-CIRCULAR
AERATION BASINS TO CALCULATE ENERGY REQUIREMENTS FOR
MEETING OXYGEN REQUIREMENTS
6.3.3.5 pH Reduction
The effect of increasing detention time on pH of sludge in the
aerobic digester during mesophilic temperature range operation is
shown on Figure 6-41.
The drop in pH and alkalinity is caused by acid formation that
occurs during nitrification. Although at one,time the low pH was
considered inhibitory to the process, it has been shown that the
6-90
-------
system will acclimate and perform just as well at the lower pH
values (186,192,213). It should be noted that if nitrification
does not take place, pH will drop little if at all. This could
happen at low liquid temperatures and short sludge ages or in
thermophilic operation (203). Nitrifying bacteria are sensitive
to heat and do not survive in temperatures over 113°F (45°C)
(214).
B.O
7.0
6,0
I
a.
5.0
4.0
3.0
LIQUID TEMP 40°? (5°C»
LIQUID TEMP 67°F {2Q°C}
,10 30 50 70
SLUDGE AGE IN AEROBIC DIGESTERS - DAYS
FIGURE 6-41
EFFECT OF SLUDGE AGE ON pH DURING AEROBIC DIGESTION
6.3.3.6 Dewatering
Although there are published reports of excellent operating
systems (193) much of the literature on full-scale operations
has indicated that mechanical dewatering of aerobically digested
sludge is very difficult (182,189,215). Furthermore, in most
recent investigations, it is agreed that the dewatering
properties of aerobically digested sludge deteriorate with
increasing sludge age (181,11,189,216). Unless pilot plant data
indicate otherwise, it is recommended that conservative criteria
be used for designing mechanical sludge dewatering facilities for
aerobically digested sludge. As an example, a designer would
probably consider designing a rotary vacuum filter for a
6-91
-------
production rate of 1.5 pounds of dry solids per square foot per
hour (7.4 kg/m^/hr), a cake solids concentration of 16 percent,
with a FeCl3 dose of 140 pounds (63.5 kg), and a lime dose
(CaO) of 240 pounds (109 kg). This assumes an aerobic solids
concentration of 2.5 percent solids. For more detailed
information on results of various types of dewatering systems,
see Chapter 9.
6.3.4 Process Performance
6.3.4.1 Total Volatile Solids Reduction
Solids destruction has been shown to be primarily a direct
function of both basin liquid temperature and the length of
time during which the sludge was in the digester. Figure 6-42
is a plot of volatile solids reduction versus the parameter
degree-days. Data were taken from both pilot and full-scale
studies on several types of municipal wastewater sludges.
Figure 6-42 indicates that, for these sludges, volatile solids
reductions of 40 to 50 percent are obtainable under normal
aeration conditions.
ui
z
o
D
O
LU
cc
w
D
-1
O
w
UJ
O
H
UJ
O
IT
UJ
O-
50
40 -
30 -
20 -
10
X
D
A
+
A
o
*
PILOT
- FULL
- PILOT
- FULL
PILOT
- PILOT
PILOT
- FULL
PLANT
SCALE
SCALE
SCALE
PLANT
PLANT
PLANT
SCALE
REF
REF
REF
REF
REF
REF
REF
REF
(18B)
(1941
(1781
(185)
(208)
(21 It
(192)
(196!
0 200 400 600 800 1000 1200 1400 1600 1800 20OO
TEMPERATURE °C x SLUDGE AGE, days
FIGURE 6-42
VOLATILE SOLIDS REDUCTION AS A FUNCTION OF DIGESTER
LIQUID TEMPERATURE AND DIGESTER SLUDGE AGE
6-92
-------
6.3.4.2 Supernatant Quality
The supernatant from aerobic digesters is normally returned
to the head end of the treatment plant. Table 6-22 gives
supernatant characteristics from several full-scale facilities
operating in the mesophilic temperature range. Table 6-23
summarizes the current design criteria for aerobic digesters.
TABLE 6-22
CHARACTERISTICS OF MESOPHILIC
AEROBIC DIGESTER SUPERNATANT
Reference 196
Reference 199'
Reference 213
Turbidity - JTU
NO -N - mg/1
TKN - mg/1
COD - mg/1
PO.-P - mg/1
Filtered P - mg/1
BODc - mg/1
Filtered BODj - mg/1
Suspended solids - mg/1
Alkalinity - mq/1 CaCC>3
SO - mg/1
Silica - ing/1
PH
120
40
115
700
70
-
50
-
300
-
-
-
6.8
_
_
2.9-1, 350
24-25,500
2.1-930
0.4-120
5-6, 350
3-280
9-41,800
-
-
-
5.7-8.0
_
30
_
-
35
-
2-5
_
6.8
150
70
26
6.8
Average of 7 months of data.
DRange taken from 7 operating facilities.
"Average values.
6.3.5 Design Example
Given
Using the information provided in Chapter 4, a design engineer
has determined that the following quantities of sludge will be
produced at a 0.5-MGD (22 1/s) contact stabilization plant:
Total daily solids generation
Amount due to chemical sludge
Amount that will be volatile
Amount that will be non-volatile
1,262 pounds (572 kg)
0
985 pounds (447 kg)
277 pounds (125 kg)
In addition, the designer has the following information:
Estimated minimum liquid temperature (winter) in digester
is 50°F (10°C).
Estimated maximum liquid temperature (summer) in digester
is 77°F (25°C).
6-93
-------
System must achieve greater than 40 percent volatile
solids reduction during the winter.
A minimum of two continuously operated tanks are required
(see Figure 6-36). (This is a state requirement for
plants under 1' MGD [44 1/s]).
Expected waste sludge solids concentration to the aerobic
digester is 8,000 mg/1.
Expected thickened solids concentration for the
stabilized sludge is .three percent ,(30,000 mg/1), based
on designer's experience.
TABLE 6-23
SUMMARY OF CURRENT AEROBIC DIGESTER DESIGN CRITERIA
Days
Solids residence time required to achieve
40 percent volatile solids reduction
55 percent volatile solids reduction
Oxygen requirements
Oxygen residual
Expected maximum solids concentration
achievable with decanting
Mixing horsepower
108
31
18
386
109
64
Liquid
temperature
40°F
60°F
80°F
40°F
60°F
80°F
2.0 pounds of oxygen per pound of volatile
solids destroyed when liquid temperature
113°F or less
1.45 pounds of oxygen per pound of volatile
solids destroyed when liquid temperature
greater than 113°F
1.0 mg/1 of oxygen at worst design
conditions
2.5 to 3.5 percent solids when dealing with
a degritted sludge or one in which no
chemicals have been added
Function of tank geometry and type of
aeration equioment utilized. Should
consult equipment manufacturer.
Historical values have ranged from 0.5
to 4.0 horseoower per 1,000 cubic feet
of tank volume
1 Ib = 0.454 kg ,
1 hp/1,000 cu ft - 26.6 kw/1,000 rri
Sludge Age Required
Figure 6-42 (presented previously) offers a quick method for
calculating the number of degree days required to achieve the
40 percent volatile solids reduction required. The result is
475 degree-days. At a basin temperature of 50°F (10°C) then:
475 degree-days
10 degrees
= 47.5 d ay s
6-94
-------
Therefore, the volume of the aerobic digester must be adequate
to provide 47.5 days sludge age to meet minimum volatile solids
reduction during the winter.
During the summer, the basin temperature will be 77°F (25°C):
25°C x 47.5 day sludge age = 1,175 degree-days.
From Figure 6-42, at 1,175 degree-days, there would be 49 percent
volatile solids reduction.
Volatile Solids Reduction
For winter conditions, there would be a 40 percent volatile
solids (VS) reduction. The actual pounds of solids reduced
are:
985 Ib VS x Q>4 = 394 Ib VS reduced {179 kg/day)
For summer conditions, there would be a 49 percent volatile
solids reduction. The actual pounds of solids reduced are:
985 Ib VS x o 49 = 403 Ib VS reduced (219 kg/day)
day ' day
Oxygen Requirements
Since nitrification is expected, provisions must be made to
supply 2.0 pounds of oxygen per pound of volatile solids
destroyed (2 kg 02/kg volatile solids destroyed).
Winter conditions: 394 Ib VS dest x 2.0 Ibs 02 = 788 Ibs 02 (358 kg/day)
day Ib VS dest. day
Summer conditions: 483 1*> VS dest 2.0 Ibs 02 = 966 Ibs O2 /
day Ibs VS dest. day v */ -r'
During summer conditions, a minimum of 1.0 mg/1 oxygen residual
must be provided.
CalculatingJTank Volume
Sludge age in an aerobic digester can be defined as follows:
total Ib SS aerobic digester ^
Sludge age - total lb ss iost per day from aerobic digester
where SS = suspended solids.
6-95
-------
The suspended solids concentration in the digester will range
from the value of the influent suspended solids concentration or
8,000 mg/1 to the maximum value of the thickened and stabilized
solids concentration of 30,000 mg/1. On the average, the
suspended solids concentration within the digester is equal to
70 percent of the thickened solids concentration, or 21,000 mg/1.
An average poundage of suspended solids in the supernatant
can be approximated by the following equation.
(SS concentration in supernatant)(1-f)(8.34)(influent flow)
where f is the fraction of influent flow into the aerobic
digester that is retained, and 1-f is the fraction that leaves
as supernatant. The term f can be approximated by the following
equation.
f _ influent SS concentration fraction of solids
thickened SS concentration x not destroyed
For winter conditions, the fraction of solids not destroyed is:
1,262 Ib total solids - 394 Ib of solids reduced = 0 59
1,262 Ib total solids
Then, the term f for this example, is:
8'000 x 0.69 = 0.18
30,000 mg/1
Therefore, 18 percent of the influent flow into the aerobic
digester will be retained, and 82 percent will leave as
supernatant.
For a properly designed sol id s- 1 iqu id separator (under
200 gallons per day per sq ft [8.16 m3/day/m2] overflow rate),
the suspended solids concentration would be approximately
300 mg/1.
The influent flow can be found by dividing the influent solids
load (1,262 pounds per day [572 kg/day] by the influent
solids concentration [8,000 mg/1]). The result is 18,914 gallons
per day (71.5 m3/day).
The pounds of suspended solids intentionally wasted per day from
the aerobic digestion system can now be approximated from the
following expression.
(SS concentration in thickened sludge )( f )( 8 . 34) ( influent flow).
All the terms in the above equation have been previously defined.
6-96
-------
It is now possible to solve for the required tank volume for any
given sludge age. In this example, winter conditions govern,
and it was previously calculated that a 47.5-day minimum was
required. From the values previously discussed:
47 5 davs = (21,000 mg/1)(8.34)(tank volume-million gallons)
y ((300 mg/1)(1-0.18)+(30,000)(0.18))(8.34)(0.018915 mil gal)'
Tank volume = 0.233 million gallons (881 m3 )
Theoretical hydraulic detention time:
233,000 gallons ,' ,
3 = 12.3 days
18,915 gallons per day
This is the minimum volume, to which must be added capacity for
weekend storage and precipitation requirements. For this design,
two tanks will be provided, each to have a volume capacity of
233,000 gallons (881 m3)(100 percent stand-by capacity as per
state requirements).
The actual dimensions of the tanks depend on the aeration
equipment utilized and are discussed in the following section.
Power Requirements
The designer has decided to use low-speed mechanical aerators
for mixing and oxygen transfer in the aerobic digester.
Previous calculations have indicated that the maximum oxygen
requirement was 966 pounds oxygen per day (438 kg/day). After
making corrections for plant elevation, alpha and beta factors,
water temperature, and minimum residual requirements, the
engineer calculated an overall mass transfer coefficient Kj^a of
3.53 hr"1. From this value, in conjunction with Figure 6-7,
power requirements will be calculated as follows.
Initially, a depth of 12 feet (3.65 m) is selected. Since
each tank is to be 233,000 gallons (881 m3) , the surface area
with a 12-foot (3.65 m) liquid depth would be 2,596 sq ft
(241 m2). A pivot point P is located by placing a straight-
edge across scales D and KLa of Figure 6-40. Then a line is
drawn through pivot point p connecting scale As, tank surface
area, to the required reducer shaft horsepower scale. The
required shaft horsepower for one tank would be 19 horsepower
(14.1 kW). Assuming a motor reducer efficiency of 92 percent,
total motor horsepower would equal 19 4 0.92, or 20.6 horse-
power (15.4 kW). The aerator manufacturer recommends that a
minimum 10 horsepower unit (7.5 kW) will be required to mix the
12-foot (3.65 m) liquid depth. Each 10 horsepower unit (7.5 kW)
6-97
-------
could mix an area 40 feet by 40 feet (12.1 m by 12.1 m) . After
making some calculations, the designer decides to use two
10-horsepower (7.5 kW) units in each tank, each tank being
36 feet (10.9 m) wide by 72 feet (24.5 m) long and having a total
tank depth of 14 feet (4.2 m) allowing 2 feet (0.61 m) of
free board. Figure 6-43 shows a view of the plan.
SUMMER CONDITIONS; 483 it» vs REDUCED/DAY - see itw o,Mav
WINTER CONDITIONS: 394 Ibn VS REDUCED/DAY - 788 Its 0,/day
EACH TANK; 72 ft LONG BY 36 fi WIDE x 13 It LIQUID DEPTH PLUS
2ft OF FREEBOARD
AEROBIC DIGESTER #1
18,915 §pd
8,000 ma/I §s
**
TANK VOLUME = 233,000
AEROBIC DIGESTER #2
TANK VOLUME <= 233,000 gal
- 10 HP LOW SPEED MECHANICAL
AERATOR
is-
1 gpd = O.QO378 m3/day
1 cu ft = 0,0283 m3
I ft - 0.304 m
1 Ib = 0,454 kg
RECYCLE 30,000 mg/l as
iACK TO SECONDARY
TREATMENT
' 15,51 0 gpd
300 mg/l ss
WASTE STABILIZED
SLUDGE
30,000 mg/l »
FIGURE 6-«
SUMMARY OF RESULTS FOR AEROBIC DIGESTION DESIGN EXAMPLE
Clarifier Surface Area
Surface area was based on an overflow rate of 200 gallons per
square foot per day (8.16 m3/day/m2) . At an influent flow of
18,915 gallons per day (71.5 m3/day), the required surface area
is 95 square feet (8.8 m2). The designer selected a 12-foot
(3.7 m) diameter clarifier.
Supernatant Flow
It was previously calculated that 82 percent of the
to the aerobic digester would leave as supernatant,.
influent
Based on
an influent of 18,915
supernatant flow will be
plus any precipitation.
gallons per day (71.5
15,510 gallons per day
nH/day), the
(58.6 m3/day),
6-98
-------
6."3.6 Cost
6.3.6.1 Capital Cost
A regression analysis of construction bids from 1973-1977
indicated that, on the basis of USEPA Municipal Wastewater
Treatment Plant Construction Cost Index - 2nd quarter 1977, the
capital cost could be approximated by Equation 6-13 (198).
C = 1.47 x 105 Q1-14 (6-13)
where:
C = capital cost of process in dollars
Q = plant design flow in million gallons of wastewater
flow per day
The associated costs included those for excavation, process
piping, equipment, concrete, and steel. In addition, such
costs as those for administrating and engineering are equal to
0.2264 times Equation 6-13 (198).
6.3.6.2 Operation and Maintenance Cost
Although there are many items that contribute to operation
and maintenance cost, in most aerobic digestion systems, the two
most prevalent are staffing requirements and power usage.
SJbaffjLng Requirements
Table 6-24 lists labor requirements for both operation and
maintenance. The labor indicated includes: checking mechanical
equipment, taking dissolved oxygen and solids analyses, and
general maintenance around the clarifier.
Power Requirement^
In 1979, the cost of power for operating aeration equipment has
become a significant factor. It is possible to minimize power
consumption through two developments in environmental science.
Make sure that the tank geometry- and aeration equipment
are compatible (212). The difference between optimized
and unoptimized design can mean as much as a 50 percent
difference in power consumption.
Pace devices to control oxygen (power) input (218).
Because of temperature effects, oxygen requirements
for any given aerobic digestion system can vary as
6-99
-------
much as 20 to 30 percent between summer and winter.
One must design to meet the worst conditions (summer),
for without some type of oxygen controller, considerable
power is wasted during other times of the year.
TABLE 6-24
AEROBIC DIGESTION LABOR REQUIREMENTS (217)
Plant design flow,
MGD
Labor,
man hours per year
Operation
Maintenance
0.5
1
2
5
10
25
100
160
260
500
800
1,500
20
30
50
100
160
300
Total
120
190
310
600
960
1,800
1 MGD = 3,786 ni /day
Other Requirements
Besides manpower and power cost, the designer must consider
lubrication requirements. If mechanical aerators are being
used, each unit needs to have an oil change once, and preferably
twice, a year. Depending on horsepower size, this could be 5 to
40 gallons per unit per change (19-152 I/unit/change). Further,
the designer must make sure an adequate inventory of spare parts
are available.
6.4 Lime Stabilization
Lime stabilization is a very simple process. Its principal
advantages over other stabilization processes are low cost and
simplicity of operation. Evaluation of studies where lime
stabilization was accomplished at pH ranges of 10-11, has shown
that odors return during storage due to pH decay. To eliminate
this problem and reduce pathogen levels, addition of sufficient
quantities of lime to raise and maintain the sludge pH to 12.0
for two hours is required. The lime-stabilized sludge readily
dewaters with mechanical equipment and is generally suitable for
application onto agricultural land or disposal in a sanitary
landfill.
No direct reduction of organic matter occurs in lime treatment.
This has two important impacts. First, lime addition does not
make sludges chemically stable; if the pH drops below 11.0,
biological decomposition will resume, producing noxious odors.
Second, the quantity of sludge for disposal is not
is by biological stabilization methods. On the
reduced, as it
contrary, the
6-100
-------
mass of dry sludge solids is increased by the lime added and by
the chemical precipitates that derive from this addition. Thus,
because of the increased volumes, the costs for transport and
ultimate disposal are often greater for lime-stabilized sludges
than for sludge stabilized by other methods.
6.4.1 Process Description
6.4.1.1 History
Lime has been traditionally used to reduce odor nuisances from
open pit privies and the graves of domestic animals. Lime has
been used commonly in wastewater sludge treatment to raise the
pH in stressed anaerobic digesters and to condition sludge prior
to vacuum filtration. The original objective of lime condi-
tioning was to improve sludge dewaterability but, in time, it was
observed that odors and pathogen levels were also reduced. In
1954, T.R. Komline filed a patent (No. 2,852,584) for a method of
processing raw sludge in which heavy dosages of hydrated lime
(6 to 12 percent of total dry solids) were added specifically
to cancel or inhibit odors. However, only recently has
lime addition been considered a major sludge stabilization
alternative.
Many studies describe the effectiveness of lime in reducing
microbiological hazards in water and wastewater, but the
bactericidal value of adding lime to sludge has been noted only
recently (219-222). A report of operations at the Allentown,
Pennsylvania wastewater treatment plant states that lime
conditioning an anaerobically digested sludge to a pH of 10.2 to
11, and then vacuum filtering and storing the cake, destroyed all
odors and pathogenic enteric bacteria (233). Kampelmacher and
Jansen reported similar experiences (224). Evans noted that lime
addition to sludge released ammonia and destroyed coliform
bacteria and that the sludge cake was a good source of nitrogen
and lime to the land (225).
Lime stabilization of raw sludges has been conducted in the
laboratory and in full-scale plants. Farrell and others (226)
reported that lime stabilization of a primary sludge reduced
bacterial hazard to a negligible value, improved vacuum filter
performance, and provided a satisfactory means of stabilizing
sludge prior to ultimate disposal. Paulsrud and Eikum (227)
determined the lime dosage required to prevent odors occurring
during storage of sewage sludges. Primary biological sludges,
septic tank sludges, and different chemical sludges were used in
the study. An important finding was that lime dosages greater
than those sufficient to initially raise the pH of the sludges
were required to prevent pH decay and the return of odors during
storage. Laboratory and pilot scale work by Counts and Shuckrow
(228) on lime stabilization showed significant reductions in
pathogen populations and obnoxious odors when the sludge pH was
6-101
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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
-------
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 highon the
order of 400 to 800 Ib per ton dry solids (200-400 kg/t).
The decision whether to purchase quicklime or hydrated lime
in a particular situation is influenced by a number of factors
such as size of treatment facility, material cost, and storage
6-114
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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.
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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.
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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
6-127
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rapid, reactor volumes are relatively small, reduced system size
and initial costs. The process results in no appreciable
destruction of volatile solids, and unlike anaerobic digestion,
yields no methane gas for energy generation and little sludge
mass reduction.
Chlorine-stabilized sludges are buff-colored, weak in odor,
sterile, and generally easy to dewater, either mechanically or on
drying beds. The stabilized sludge has been used as a soil
conditioner. However, there is concern about its use on cropland
and its disposal in landfills because of its high acidity, high
chloride content, and potential for releasing chlorinated
hydrocarbons and heavy metals. The stabilized sludges are
corrosive unless pH has been adjusted. Process equipment that
comes into contact with sludges that have not been neutralized
must be constructed of acid-resistant materials or be coated with
protective films.
6.5.1 Process Description
Chlorine treatment stabilizes sludge by both reducing the number
of organisms available to create unpleasant or malodorous condi-
tions and making organic substrates less suitable for bacterial
metabolism and growth. Some of the mechanisms responsible are
oxidation, addition of chlorine to unsaturated compounds, and
displacement of hydrogen by chlorine.
The immediate reaction from addition of gaseous chlorine to water
is shown below:
HOC1 + H+ Cl~ (6-14)
In the chlorine stabilization process, sufficient acid is
produced to reduce the pH of the sludge to a range of 2 to 3.
Dissociation of HOC1 to H* and OC1~ is suppressed by low pH
and therefore is not significant. C12 and HOC1 are highly
reactive and powerful bactericides and viricides. The chloride
ion has no disinfection capability.
The process stream immediately following the chlorine addition
is substantially a chlorine solution containing sludge. The
solution contains (in molecular form) as much as ten percent of
the total chlorine species present. The predominant species in
solution is undissociated HOC1. HOC1 and C12 react with sludge
to oxidize ammonia to chloramines and organic nitrogen to organic
chloramines. Other reduced ions, such as Fe+2 and S~2, are
oxidized at the same time. Some of the oxidized end products,
such as chloramines and organic chloramines, are germicidal and
viricidal (244).
The chlorine stabilization unit consists of a disintegrator, a
recirculation pump, two reaction tanks, a chlorine eductor, and a
pressure control pump. A chlorine evaporator and/or a
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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 wastesfor example, digested sludge, digester
supernatants--are processed, CC>2 generated during chlorination
may promote cavitation in downstream pumps.
There is concern that chlorine oxidation of sludges, septage,
and sidestreams from sludge' treatment processes could result
in increased levels of toxic chlorinated organics in the
treated materials (245). Data available are inconclusive.
Investigations are underway that will help clarify this
issue. In the meantime, measures should be taken to mitigate
environmental concerns when the chlorine oxidation processes is
used. These are:
Provisions should be made to deal with the filtrate,
centrate, or decant from the process, including return
to the wastewater treatment plant, unless this practice
leads to wastewater treatment plant upset or to
violations of effluent standards; or to treat by
activated carbon absorption or other means.
6-131
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If the treated sludge leaving the pressurized chlorinator
is discharged to a tank sparged with air, the gases from
the tank should be vented away from workers.
Treated solids should be disposed of with care. Consid-
eration should be given to:
1. Using secured landfills or landfills located at
hydrogeographically isolated sites.
2. Treating leachates from secured landfills to prevent
contamination of surface or groundwaters.
3. Directly incorporating the solids into soils at rates
sufficiently low to minimize leachate production.
Direct incorporation as opposed to surface spreading
should be used to prevent consumption of solids by
grazing animals.
4. Using erosion control measures to prevent runoff
contaminated with toxic chlorinated compounds from
entering surface waters.
5. Providing adequate monitoring of facilities to assure
detection of unexpected problems.
6.5.3 Chlorine Requirements
Chlorine demand varies with the characteristics of each waste
stream. Demand can be estimated from Table 6-33 for cases in
which a combination of sludges and/or sidestreams makes up the
process feed. The demand of a sludge produced by combining two
streams is the weighted average of the demands of the individual
streams. For example, using Table 6-33 one estimates that the
demand of a sludge composed of five volumes of 0.7 percent
waste-activated sludge and one volume of four percent primary
sludge is about (17 + 5[7])/6 = 9 pounds per thousand gallons
(1 kg/1,000 1) .
If a chlorine residual is desired to provide added protection
against septicity, then additional chlorine should be added in
an amount equal to the required residual. For instance, if
a residual of 200 mg/1 chlorine is required for the waste-
activated/primary sludge combination just discussed, then
an additional (200 mg/1)(0.00834 Ib/thousand gal) = 1.7 pounds of
mg/1
chlorine per 1,000 gallons of sludge (0.20 kg/1,000 1) should
be added, bringing the total chlorine addition to 10 to 11 pounds
per 1,000 gallons (1.2 to 1.3 kg/1,000 1) of sludge.
BIF Division of General Signal (246) states that, for solids
concentrations other than those shown, the chlorine demand
per gallon varies in proportion to the solids concentration. For
example, if the solids concentration were to double, chlorine
demand would also double.
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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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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
-------
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
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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
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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 processwhere
bacterial pathogens may regrow if sludge is reinoculated.
7.6.1-.3 Design Criteria
A pasteurization system should be designed to provide a uniform
minimum temperature of 157°F (70°C) for at least 30 minutes.
Batch processing is necessary. to prevent short circuiting and
recontamination, especially by bacteria. In-line mixing of steam
and sludge should be considered as a possible aid to increase
heat transfer efficiency and assure uniform heating. In-line
mixing will also eliminate the need to mix the sludge while it is
held at the pasteurization temperature. The system should be
sized to handle peak flows or sludge storage should be used
to reduce peak flows. Sizing of storage capacity and the
pasteurization system will depend on the type of sludge treated,
the average sludge flow, and the end use of the sludge. If
digested sludge is to be pasteurized, the digesters may have
sufficient volume to hold sludge during minor mechanical
breakdowns or when inclement weather prevents an end use such as
land application-. If sludge is to be stored after treatment and
7-16
-------
prior to pasteurization, a minimum storage volume should be two
days average flow. Storage facilities must be equipped for odor
control or with aeration capacity to prevent septic conditions.
Storage capacity for pasteurized sludge should be adequate to
hold at least four days' amount of processed sludge at average
flow. Odor control must be provided, and pilot-scale testing may
be needed to determine the best odor control process design.
Sludge thickening prior to pasteurization may be cost-effective
for increasing overall energy efficiency, but the value of
thickening should be determined on a case-by-case basis.
Piping, pumps, valves, heat exchangers, flow meters, and other
mechanical equipment should, at a minimum, be comparable to those
for thermophilic digesters. The tanks for holding sludge during
pasteurization should be corrosion-resistant.
7.6.1.4 Instrumentation and Operational Considerations
Temperature monitoring at several points in each pasteurization
system is a minimum requirement. Flow metering devices,
boiler controls, emergency pressure relief valves, and level
sensors in tanks should also be considered (see Chapter 17,
Instrumentation).
Heat pasteurization has flexibility to respond to variable
solids concentrations and flow rates, provided there is enough
basic system capacity. Expansion of facilities with parallel
modules should work well; multiple modules also improve system
reliability.
7.6.1.5 Energy Impacts
Pasteurization requires both electricity for pumping and fuel for
heating the sludge. Energy requirements for pasteurization
processes, with and without heat recovery, have been estimated
for secondary activated sludge plants where either raw or
digested sludge is pasteurized (34). A combination of primary
and waste-activated sludge with 4,800 gallons of untreated sludge
per 1,000,000 gallons (4.8 1/m3 ) raw sewage or with 3,100 gallons
of digested sludge per 1,000,000 gallons (3.1 l/m-*) raw sewage,
with a solids content of five percent and a specific heat of one
Btu per °F (1900 J/°C) were assumed. The process allowed for
10 percent heat loss and a 100 to 125 pounds per square inch
(690 to 860 kN/m2) boiler with an 80 percent efficiency. Steam
injection heats the sludge to 157°F (70°C), where it is held for
45 minutes with steam reinjection to maintain the temperature.
The energy requirements for processes with a range of wastewater
flows are summarized on Figure 7-3.
7.6.1.6 Cost Information
The only sludge pasteurization process operating in the United
States was not initially designed for pasteurization. Thus no
actual cost data are available. Costs have been estimated for
7-17
-------
the processes discussed under "Energy Impacts" (34). It was
assumed that the processes would have parallel pasteurization
reactors and four-day storage volume for the pasteurized sludge.
The use (volume of throughput per given size) for the processes
increases with increasing system size.
1,000
9
8
7
6
5
4
100
"3
o
i_
-5*
as
8
G
D
u_
3
i TOO
S 3
8
FUEL
WITH HEAT RECOVERY
to
£ H
cc
b
UJ
_J
10
1
PLANT CAPACITY, MGD of wastewater (1 MGD = 0,044 m3/i)
FIGURE 7-3
ENERGY REQUIREMENTS FOR SLUDGE PASTEURIZATION SYSTEMS (34)
Cost estimates were made in June 1977 for construction materials,
labor, equipment, normal excavation, contractor overhead and
profit, operating and maintenance labor, materials and supplies,
and energy. Summary graphs for these estimates are given on
Figures 7-4 through 7-7.
7-1!
-------
_ra
"o
O
u
cc
UNTREATED SLUDGE
5 10 20 30 40 50 . 60 70 SO 90100
PLANT CAPACITY, MGD of wisiewater {1 MOD = 0,044 m3/s)
FIGURE 7-4
CONSTRUCTION COSTS FOR SLUDGE PASTEURIZATION
SYSTEMS WITHOUT HEAT RECOVERY (34)
These graphs were used to estimate unit pasteurization costs
for a 50-MGD (2.2-m^/s) secondary wastewater treatment plant.
Additional assumptions made were that yard piping for the system
would cost 15 percent of the total construction cost, electricity
would cost three cents per kilowatt hour, fuel would cost
$3.00 per million Btu's ($2.84/GJ), labor would cost $10.00 per
hour, and capital was amortized over 20 years at seven percent.
The resulting pasteurization cost was $15.00 per ton ($16.50/t)
of dry solids with heat recovery. A similar calculation was made
for a 10-MGD (0.44-m-Vs) secondary plant with no heat recovery,
a cost of $33.00 per ton ($36.40/t) of dry solids was estimated.
7-19
-------
UNTREATED SLUDGE
10 20 30 40 60 60 70 80 90100
PLANT CAPACITY, MGD of wastfwatef (1 MGO = 0.044 m3/s|
FIGURE 7-5
CONSTRUCTION COSTS FOR SLUDGE PASTEURIZATION
SYSTEMS WITH HEAT RECOVERY (34)
7.6.1.7 Design Example
To establish the equipment requirements and layout for a typical
pasteurization system, digested combined primary and waste-
activated sludge from a 50-MGD (2.2 m^/s) activated sludge plant
are to be pasteurized prior to reuse by direct injection. If the
sludge is produced at a rate of 2,000 pounds of solids per
million gallons (0.24 kg/m^), and 40 percent of the solids are
7-20
-------
destroyed during
2.4 percent solids. The sludge
per million gallons (4.8 1/m3).
the flow rate is 0 . 3-MGD " "
facility
rate is 0.42
(18.9 1/s).
digestion, the resulting digested
flow rate is about 4,800 gallons
For the 50-MGD (2.2 m3/s) plant,
(13.0 1/s). If the pasteurization
is run 24 hours per day, five
MGD (18.9 1/s) or about
sludge has
days per week, the flow
300 gallons per minute
12 -
WITH HEAT
RECOVERY
WITHOUT HEAT
RECOVERY
10 20 30 40 50 60 70 SO 90100
PLANT CAPACITY, MGD of wastewatw ?1 MGD = Q.Q44 m3/^
FIGURE 7-6
LABOR REQUIREMENTS FOR SLUDGE
PASTEURIZATION SYSTEMS (34)
7-21
-------
16 i-
UNTREATED SLUDGE
10 , 20 30 40 SO 60 70 80 90100
PLANT CAPACITY, MGD of wastewater (1 MGD = 0,044 m3/sS
FIGURE 7-7
MAINTENANCE MATERIAL COSTS FOR SLUDGE
PASTEURIZATION SYSTEMS (34)
To select the reactor size, assume that there are two parallel
units and each can be charged, held, and emptied, in 1.5 hours.
Determining the volume per reactor:
V =
NH
where:
S =
C
N '
total sludge volume per week, gallons;
cycle time, hours;
number of reactors/cycle;
7-22
-------
H = total operating hours.
For this example,
v- (2.1 x 106 gallons) (1.5 hr/cycle) ., , __ .. ,.n _, ,%
V (2 reactors/cycle)(120 hr) = 13'125 ^allons <49'7 m3>
Assume a 13,500 gallon (51 m3 ) storage tank will be used to
store this sludge. Set prepasteurization storage at 2.5 times
the average daily flow, or at one million gallons (3780 m3 ) .
Set post pasteurization storage at four times the average daily
flow or 1.7 million gallons (6350 m3) . Three heat exchangers
in series heat the digested sludge from 68°F to 131°F (20° to
55°C); the boiler supplies steam to raise the temperature to
157°F (70°C). The heat exchangers can be either sludge to sludge
or sludge to water to sludge. Sludge-to-sludge exchangers should
be carefully specified as they have a history of fouling.
The sludge pumps should be sized and piped either to fill or
empty a 13,500 gallon tank (51 m3) in 30 minutes, equivalent to
450 gallons per minute (28 1/s). At least three pumps are
needed; providing one pump on standby.
The required boiler capacity is calculated with the equation:
_ AT h W
where:
E = energy required in Btu per hour
AT = the temperature difference between sludge from the heat
exchanger and sludge in the reactor;
h = heat capacity of the sludge, Btu/lb°F;
W = wet sludge weight, lb;
t = time for heating;
e = boiler conversion efficiency.
If h is one Btu per lb °F (864 J/kg°C); e = 80 percent; T = 63°F
(35°C); W = 112,600 lb (51,200 kg); and t = 0.5 hr; then,
E = /;:o = 17,700,000 Btu/hr (3.9 GJ/hr)
( U . b ) ( I) . o )
7-23
-------
An additional allowance of ten percent should be added to
maintain the reactor temperature for 30 minutes, giving a total
of 19.5 million Btu/hr (4.3 GJ/hr) or about 600 horsepower.
Figure 7-8 provides a schematic layout for the major process
components.
PASTEURIZATION
REACTORS
UNTREATED
OR
DIGESTED
15T°F
SLUDGEo
FROM
STORAGE
FEED
PUMPS
HE U HI ZED
.4
0 u
a <;
5"
^/X^X
S HEAT
' EXCHANGER
^
1
f
t
PASTi
iZf
SLUt
PUM
:UR-
D
)GE
PS
PAStEuftiZEO
SLUDGE
STORAGE
PREHEATED
SLUDGE 131°F (56°C1
u -r
FUEL
125 psi
(175°C
860 kN/nf*
WATER
SLUDGE
FOR
UTILIZATION
FIGURE 7-8
SYSTEM COMPONENT LAYOUT FOR SLUDGE
PASTEURIZATION WITH HEAT RECOVERY
7.6.2 Other Heat Processes
The reduction of pathogenic organisms in sludge may be an added
benefit of other sludge treatment processes. In this chapter
heat processes are subdivided into heat-conditioning, heat-
drying, high temperature combustion, and composting.
7-24
-------
7.6.2.1 Heat-Conditioning
Heat-conditioning includes processes where wet wastewater sludge
is pressurized with or without oxygen and the temperature is
raised to 350° to 400°F (177° to 240 °C) and held for 15 to
40 minutes. These processes destroy all pathogens in sludge, and
are discussed in detail in Chapter 8.
7.6.2.2 Heat-Drying
Heat-drying is generally done with a flash drier or a rotary
kiln. Limited data from analyses on Milwaukee, Wisconsin's dried
sludge, Milorganite, produced with a direct-indirect rotary
counterflow kiln type dryer, indicates it is bacteriologically
sterile (13). Data on samples of flash-dried sludge taken
in Houston, Chicago, Baltimore, and Galveston, showed no coliform
bacteria in the Houston sludge and no greater than 17 MPN/gm
dry sludge in the other sludges. Total non-confirming lactose
fermenters (spore formers) ranged from 14 MPN to 240,000 MPN per
gm (35). No tests were made for viruses or parasites; other
pathogens may also survive if some bacteria do.
Data for the Carver-Greenfield process gathered during testing by
LA/OMA showed a seven order of magnitude reduction for total and
fecal coliform, to a detectable level of less than one organism
per gram (36). Fecal streptococci were reduced six orders of
magnitude to two MPN per gram and Salmonella from 50,000 MPN per
gram to less than 0.2 MPN per gram. Ascaris ova were reduced to
less than 0.2 ova per gram.
7.6.2.3 High Temperature Processes
High temperature processes include incineration, pyrolysis, or a
combination thereof (starved-air combustion). These processes
raise the sludge temperature above 930°F (500°C) destroying
the physical structure of all sludge pathogens and effectively
sterilizing the sludge. The product of a high temperature
process is sterile unless shortcircuiting occurs within the
process.
7.6.2.4 Composting
Composting is considered here as a heat process because a major
aim of sludge composting operations is to produce a pathogen-free
compost by achieving and holding a thermophilic temperature.
Available data indicate that a well-run composting process
greatly reduces the numbers of primary pathogens (37-40).
However, windrow or aerated pile operations have not achieved a
sufficiently uniform internal temperature to inactivate all
pathogens. Adverse environmental conditions, particularly heavy
rains, can significantly lower composting temperatures. An
7-25
-------
additional problem with composting is the potential regrowth of
bacteria. This is particularly true with windrows where mixing
moves material from the outside of the mound to the center (40).
However, storage of compost for several months following windrow
or pile composting helps to further reduce pathogen levels.
Secondary pathogens, particularly heat-resistant fungi such
as Aspergillus, have been found to propagate rapidly during the
composting of wastewater sludges. Aspergillus apparently will
die out during storage of several months or more (22).
Enclosed mechanical composting systems may achieve sufficient
temperature, 157°F (70°C) or greater, for an adequate time; more
research can verify the efficiency of mechanical systems for
pathogen reduction.
7.7 Pathogen Reduction With High-Energy Radiation
The use of high-energy radiation for wastewater sludge
disinfection has been considered for over 25 years. Two energy
sources, beta and gamma rays, offer the best potential system
performance. Beta rays are high-energy electrons, generated with
an accelerator for use in disinfection, while gamma rays are
high-energy photons emitted from atomic nuclei. Both types of
rays induce secondary ionizations in sludge as they penetrate.
Secondary ionizations directly inactivate pathogens and produce
oxidizing and reducing compounds that in turn attack pathogens.
7.7.1 Reduction of Pathogens in Sludge With
Electron Irradiation
High-energy electrons, projected through wastewater sludge by
an appropriate generator, are being pilot tested as a means for
inactivating or destroying pathogens in sludge at the Deer Island
Wastewater Treatment Plant in Boston, Massachusetts (41). The
electrons produce both biological and chemical effects as they
scatter off material in the sludge. Direct ionization by the
electrons may damage molecules of the pathogen, particularly the
DNA in bacteria cell nuclei and the DNA or RNA of the viruses.
The electrons also cause indirect action by producing e^q
(hydrated electrons) and H and OH free radicals that react with
oxygen and other molecules to produce ozone and hydroperoxides.
These compounds then attack organics in the sludge--including
pathogens--promoting oxidation, reduction, dissociation, and
other forms of degradation.
The pathogen-reducing power of the electron beam (e-beam) depends
on the number and the energy of electrons impacting the sludge.
E-beam dose rates are measured in rads; one rad is equal to the
absorption of 4.3 x 10~6 Btu per pound (100 ergs/gin) of material.
Since the radiation distributes energy throughout the volume
of material regardless of the material penetrated, the degree
7-26
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of disinfection with an irradiation system is essentially
independent of the sludge solids concentration within the maximum
effective penetration depth of the radiation. The penetrating
power of electrons is limited, with a maximum range of 0.2 inches
(0.5 cm) in water or sludge slurries, when the electrons have
been accelerated by a potential of one million volts (MeV).
For e-beam disinfection to be effective, some minimum dosage
must be achieved for all sludge being treated. This effect
is attained by dosing above the average dosage desired for
disinfection. One method used to ensure adequate disinfection
is to limit the thickness of the sludge layer radiated so that
ionization intensity of electrons exiting the treated sludge is
about 50 percent of the maximum initial intensity. For the
0.85 MeV electrons used in the existing facility, this constraint
limits sludge layer thickness to about 0.08 inches (0.2 cm).
Accelerated electrons can induce radioactivity in substances
which they impact. However, the electron energy levels for
sludge irradiation, up to about 2 MeV, are well below the 10 MeV
needed to induce significant radioactivity with electrons.
7.7.1.1 Process Description
Disinfection with an e-beam has been proposed for use on both
untreated and digested sludges. The major system components of
the Deer Island facility shown on Figure 7-9 include the sludge
screener, sludge grinder, sludge feed pump, sludge spreader,
electron beam power supply, electron accelerator, electron beam
scanner, and sludge removal pump. A concrete vault houses the
electron beam, providing shielding for the workers from stray
irradiation, especially x-rays. X-rays are produced by the
interaction of the electrons with the nucleus of atoms in
the mechanical equipment and in the sludge. The pumps must be
progressive cavity or similar types to assure smooth sludge
feed. Screening and grinding of sludge prior to irradiation is
necessary to assure that a uniform layer of sludge is passed
under the e-beam.
At Deer Island, sludge from the feed pump discharges into the
constant head tank (see Figure 7-10), which is equipped with an
underflow discharge weir. Sludge is discharged under the weir
in a thin stream and then flows down an inclined ramp. At the
bottom of the ramp, it moves by free-fall into the receiving
tank.
The electrons are first accelerated. They leave the accelerator
in a continuous beam that is scanned back and forth at 400 times
per second across the sludge as it falls free in a thin film from
the end of the inclined ramp. The dosage is varied by adjusting
the height of the underflow weir and hence the sludge flow rate.
7-27
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HIGH VOLTAGE CABLE
ELECTRON
ACCELERATOR
FEED
SLUDGE
SLUDGE SLUDGE
SCREEN GRINDER
ELECTRON
BEAM
SCANNER
SLUDGE
FEED
PUMP
CONCRETE
SHIELDING
T
SLUDGE
SPREADER
SLUDGE
REMOVAL
PUMP
FIGURE 7-9
EQUIPMENT LAYOUT FOR ELECTRON BEAM FACILITY (41)
7.7.1.2 Status
E-beam sludge irradiation must be considered a developing
technology. The Deer Island irradiation facility, as of
August 1979, is the only e-beam facility now operated in the
United States for sludge disinfection. This pilot project is
designed to treat 0.1 MGD (4 1/s) sludge at up to eight percent
solids with a dosage of 400,000 rads. According to Shah, the
facility has been operated about 700 hours since it was brought
on line in 1976, with the longest continuous on-line time being
eight hours (42).
7.7.1.3 Design Considerations
Design criteria for an e-beam sludge facility are difficult to
establish because operational data are available from only one
pilot facility. However, the work at Deer Island provides good
baseline information. A minimum level of electron irradiation
should be 400,000 rads, which can best be supplied with a one to
two MeV electron accelerator. This energy level provides good
7-28
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penetration for 0.2-inch (0.5-cm). thick sludge layers, making the
achievement of a uniform sludge 'layer-'-less important than with
lower energy electrons. However, screening and grinding of
sludge before disinfection are still necessary to ensure uniform
spreading by this feed mechanism. The high-energy electrons,
combined with a short spacing of about 2.75-inches (7 cm) between
the scanner window and the sludge film, ensure efficient energy
transfer in the system.
INPUT
(UNTREATED Oft
DIGESTED SLUDGE)
ELECTRON
BEAM
CONSTANT
HEAD
TANK
UNDERFLOW
WEIR
INCLINED
FEED RAMP
ELECTRON BEAM
SCANNER
HIGH ENERGY
DISINFECTION
ZONE
SLUDGE
RECEIVING
TANK
.OUTPUT
(DISINFECTED
SLUDGES
FIGURE 7-10
ELECTRON BEAM SCANNER AND SLUDGE SPREADER
Only digested sludge has been irradiated at Deer Island.
Nonstabilized sludge disinfection by e-beam irradiation still
requires pilot-scale testing before any design is considered.
7-29
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Owing to the limited penetrating power of high energy electrons,
this method of treatment is probably only feasible for liquid
sludge. Piping pumps, valves, and flow meters should be
specified as equal to those used for anaerobic sludge digestion
systems.
systems.
7.7.1.4 Instrumentation and Operational
Considerations
Instrumentation needs for an e-beam facility should include
flow measurement of and temperature probes in the sludge
streams entering and leaving the irradiator. Alarms as well as
monitoring should be used to indicate variation in sludge flow
and high or low radiation doses.
Sludge disinfection by e-beam irradiation has large inherent
flexibility. The radiation source (the e-beam) can be switched
on and off as easily as an electric motor. The unit can be run
as needed, up to its maximum throughput capacity. Electron
accelerators have a proven record for reliability over at least
20 years in industrial applications and should prove dependable
in wastewater treatment applications. According to Haas, the
reliability of the electron beam generator and associated
electronics presently used for medical and industrial applica-
tions is comparable to that for the microwave radar systems at
major airports (43). Accelerators for sludge disinfection would
use the same basic components and would have similar reliability.
Other system components--pumps, screens, and grinders--are all in
common use in waste treatment plants. Cooling air for the
scanner must be provided at several hundred cfm (about 10 m^/s).
This constant introduction of cooling air leads to the generation
of ozone in the shielding vault around the accelerator. If the
ozone were vented into the plant or into the atmosphere, some air
pollution would result. At Deer.Island, this problem is avoided
by venting the cooling air through the sludge, where the ozone is
consumed by chemical reduction. These reactions provide a small
amount of additional disinfection and COD reduction.
7.7.1.5 Energy Impacts
Energy use for e-beam facilities has been estimated for the
equipment used at Deer Island. A facility with a 50-kW (50-kJ/s)
beam would require about 100 kW (lOOkJ/s) of total electrical
power including 25 kW (25 kj/s) for screening, grinding, and
pumping, 10 kW for (10 kJ/s) window cooling, and 12 kW (12 kJ/s)
for electrical conversion losses. Energy requirements for
0.1 MGD (4 1/m3) are 6 kWhr per ton (24 MJ/t) of wet sludge
at five percent solids or 120 kWhr per dry ton (480 MJ/t) (41).
7.7.1.6 Performance Data
Data for e-beam disinfection of both untreated and digested
sludges are available as a result of laboratory testing done
prior to the operation of the Deer Island facility. For
7-30
-------
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
-------
Owing to the high cost of Co-60, the overall viability of any
sludge irradiation facility in the United States depends on
Cs-137 supplies. Cs-137 will be available in quantity only if
the political and technical difficulties associated with power
plant fuel rod reprocessing can be resolved. About 200 mega-
curies of Cs-137 could be available from processing wastes
from weapons manufacture and could be used for further testing.
7.7.2.4 Design Criteria
The design criteria for gamma irradiation facilities depend on
the type of wastewater sludge treated. Current literature
discussions suggest a dose of 400 krads but this level does not
ensure complete virus removal (41). The dose level should
probably be varied in relation' to other treatments the sludge
receives. A composted, bagged product with an 80 percent solids
content needs a lower dose than a mixture of raw primary and
waste-activated sludge because the dried product already has a
reduced pathogen' level owing to the drying process. Data from
the demonstration facility at Sandia Laboratory for design of a
dry facility should be available by late 1979. For a liquid
sludge facility, data on dose-response and pathogen levels
(Table 7-5 and Section 7.7.2.2) can be combined with information
from Geiselbullach to set the required radiation doses. The
storage capacity for both untreated and irradiated sludge should
be equal to that for a pasteurization facility of similar size
(see Section 7.6.1.7).
When a dry system radiation source is not in use, it should be
shielded in a steel-lined concrete vault. 'The vault should be
designed to be flooded with water during loading and unloading
of the radiation source, to shield workers from radiation.
Provision must be made for pool water treatment in the event that
the radiation source leaks. Cooling air is circulated around the
source both during system operation and down times. This air
must be filtered to prevent a radioactive air release. Since the
dried sludge is a flammable material, there must be smoke and/or
heat detection and a fire suppression system. For a liquid
storage system the treatment vessel serves as a radiation source
storage vault.
7.7.2.5 Instrumentation and Operational
Considerations
Instrumentation should include radiation detectors and flow
metering for the wet sludge system. When either facility is
operating, arrangements must be made for periodic radiation
safety inspection. The disinfection effectiveness should also
be tested by periodic sampling of the sludge before and after
disinfection.
7-35
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7.7.2.6 Energy Impacts
In May 1977, Ahlstrora and McGuire (46) projected annual energy
requirements for both wet and dry gamma irradiation facilities,
using a dose rate of 1,000 krads. Their results are summarized
on Figures 7-12 and 7-13. For a 0.1-MGD (4-1/s) facility
treating sludge with five percent solids, 300 days per year, the
unit energy use is about 5.2 kWhr per 1,000 gallons (5 MJ/m3) or
25 kWhr per ton (100 MJ/ton) dry solids. For a plant treating
35 tons per day (32 t/day) at 60 percent solids, 300 days per
year (equivalent to the solids from the previous example), the
energy use is 5.6 kWhr per ton dry (22 MJ/t) solids, almost
80 percent less than the facility treating five percent solids.
These energy uses should be compared to 120 kWhr per dry ton
(450 MJ/t) for an e-beam system.
1,080
fl
8
1
100
I
;
te
.D-"
_L
10
e. 7 a
8100
6739
1,000
SLUDGE TREATMENT CAPACITY, 0,001 MGO (4,4 x ItT5 m3/s)
FIGURE 7-12
GAMMA RADIATION TREATMENT OF LIQUID SLUDGE
POWER REQUIREMENTS (46)
7-36
-------
100
9
£ ' 8
^ , 7
I
4 -
a
LU
cc
01
LU
I
Q,
Z
Q
10
2 3 4 5 S 7 8 9
10 100 200
SLUDGE TREATMENT CAPACITY, dry tons/day (0,907 tonne/day)
FIGURE 7-13
RADIATION TREATMENT OF DEWATERED
SLUDGE - POWER REQUIREMENTS (46)
It is important to note that the liquid system would require a
much larger Cs-137 charge since it would be treating almost
12 times the volume of material at the same dose level. However,
the rod configuration for a dry facility would be much less
efficient in terms of radiation transfer than a liquid one.
7.7.2.7 Performance Data
In June 1979 no performance data for the Sandia facility were yet
available. Data for the Geiselbullach facility are summarized in
Section 7.7.2.2.
7-37
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7.7.2.8 Cost Information
Cost estimates for both liquid and dry facilities were developed
together with the energy data of Section 7.7.2.6. The liquid
facility included the following components:
Insulated concrete building with 25-foot (7.6-m) ceiling.
Equalization sludge storage tank.
Emergency water dump tank (for source shielding water).
Irradiating capsules (radiation source).
Steel-lined source handling pool.
Deionizer.
Data aquisition and control system.
Oxygen injection facility.
Pumps, piping, and flow meters.
Radiation alarm.
Fire suppression system.
A capital cost graph for the wet facility is given on
Figure 7-14; the estimates were made in May 1977. Graphs for
labor hours per year and operations and maintenance materials and
supplies are given on Figure 7-15 and 7-16, respectively. The
additional operating cost is $2.00 per 1000 gallons ($0.53/m3)
for the Cs-137 (the irradiator).
The dry system uses a bucket conveyor to move the sludge past the
radiation source (see Figure 7-17). This dry system would
include the following:
Loading and unloading conveyors.
Concrete shielding.
Source-handling pool.
Holder for the Cs-137 capsules.
Holder moving mechanism.
Steel building.
Pumps.
Ventilators.
7-38
-------
Filters.
Hoists.
Radiation alarm system.
Pool water testing tank.
Fire suppression system.
10,000
8
6
7
6
5
4
o
1,000
fl
i
?
%
5
100
10
_L__J_J
I
5676
100
6 6739
1,000
SLUDGE TREATMENT CAPACITY, 0,001 MGD 14.4 x 10"? m3/i|
FIGURE 7-1U
GAMMA RADIATION TREATMENT OF LIQUID SLUDGE -
CAPITAL COSTS (46)
The capital costs for the dry system are summarized on
Figure 7-18; these costs were also calculated in May 1977.
Figure 7-19 and 7-20 present labor hours, materials ,and
operations and maintenance supplies, respectively. The Cs-137
source is estimated to cost $1.55 per ton ($1.70/t) for a 10-ton-
per-day (9.1-t/d) capacity facility and $1.22 per ton ($1.35/t)
for facilities of 50 ton per day (45 t/d) and larger.
7-39
-------
10,000
a
8
7
6
ID
flC
O
(0
z
z
1,000
'O"
J_
J I
_L
-LJ.
10
3 45878 S 2 3 4667SS
100 1,000
SLUDGE TREATMENT CAPACITY, 0.001 MGO (4,4 x 1C"5 m3/sj
FIGURE 7-15
GAMMA RADIATION TREATMENT OF LIQUID
SLUDGE LABOR REQUIREMENTS (46)
If labor plus overhead is $20.00 per hour, power is three cents
per kWhr, ($0.33/GJ) and capital is amortized over 20 years at
8 percent, the cost for a 0.1-MGD (4-1/s) liquid system is
$38.50 per ton ($42.40/t) dry solids. A dry system costs
$24.00 per ton ($26.50/t) dry solids. Both these costs are
considerably higher than those for e-beam irradiation and similar
to those for heat pasteurization. -
7-40
-------
12
£
i
100
9
B
7
6
5
8
2
10
9
8
7
8
2
10
_L
4 S 6 7 8 9
100
2 346
SLUDGE TREATMENT CAPACITY, 0.001 MGD (4.4 * 1(T5 m3/s)
FIGURE 7-1S
GAMMA RADIATION TREATMENT OF LIQUID SLUDGE
MAINTENANCE MATERIAL SUPPLIES COSTS (46)
7 s
iroeo
FIGURE 7-17
GAMMA RADIATION TREATMENT FACILITY FOR HANDLING
25 TONS PER DAY OR MORE OF DEWATERED SLUDGE
7-41
-------
10,000
i
8
7
s
5
j!
O
<
_l
<
7
6
5
4
100
O'
I
I
2 3 456780
10 100 200
SLUDGE TREATMENT CAPACITY, ton/day (0.907 tonne/day)
FIGURE 7-18
GAMMA RADIATION TREATMENT OF DEWATERED SLUDGE
CAPITAL COST (46)
7-42
-------
fc*
J5
"5
-o
£
te
o
o
_l
t
_
o
9
8
7
6
5
4
3
2
1,OOO
9
8
7
:<
4
3
2
100
.
,,
_,
-*
O^^
^**
^ ^ **
?*"
i i i i i i i i I
2 3 4S6789
10 100 200
SLUDGE TREATMENT CAPACITY, ton/day (0.907 tonne/day)
FIGURE 7-19
GAMMA RADIATION TREATMENT OF DEWATERED SLUDGE -
LABOR REQUIREMENTS (46)
7-43
-------
1,000
9
i
7
6
O
o
100
_ -°-
2 3 45fl7S9
10 100 200
SLUDGE TREATMENT CAPACITY, ton/day (0.907 tonne/day)
FIGURE 7-20
GAMMA RADIATION TREATMENT OF DEWATERED
MAINTENANCE MATERIALS AND SUPPLIES COST (46)
7.8 References
1.
2.
3.
Branden, J
Proceed ings
R. "Parasites in Soil/Sludge Systems."
of Fifth National Conference on Acceptable
Sludge Disposal Techniques, Orlando, Florida, January 31 to
February 2, 1978.
Maryland 20852, p.
Information
130.
Transfer, Inc. Rockville,
Oliver, W. M. "The Life and Times of Aspergillus Fumigatus,"
Compost Science/Land Utilization. Vol. 20, No. 2, March/
April 1979.
U.S. Public Health Service. Enteric and Neurotropic Viral
Diseases Surveillance, 1971-1975. Center for Disease
Control,Atlanta,Georgia 30333. Issued January 1977.
7-44
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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) truck1
72 per truck 9
,5 man-hours;
man-hours.
for 50-gallon (0.19 m-
These requirements
) barrels,
are shown
8-10
-------
08
o
cc
o
Li-
en
cc
D
O
I
D
Z
3456 7891,000
3456 78910,000
INSTALLED CAPACITY, pounds Lime/Hour (as CaO) (1 Ib = 0.454 kg)
FIGURE 8-5
CAPITAL COST OF LIME STORAGE AND FEEDING FACILITIES (22)
100
10 2 3 456789100 2 3456789 1,000 2 3456 78910,000
Pounds Ferric Cloride Fed/Hour (1 Ib = 0.454 kg)
FIGURE 8-6
FERRIC CHLORIDE STORAGE AND FEEDING OPERATING
AND MAINTENANCE WORK-HOUR REQUIREMENTS (22)
8-11
-------
as man-hours per pound of chemicals fed to the process. Metering
pump operations and maintenance is estimated at five minutes per
pump per shift.
o
IT
UJ
LLI
y
DC
u
UJ
1,000
345 0, 7 8 8 10
3 4 56789 100
3 4 66789 1,000
3 4 6 6 7 8 9 10.0OO
FEEDING RATE, Ib (1 Ib = 0.454 kg)/hr
FIGURE 8-7
ELECTRICAL ENERGY REQUIREMENTS FOR A FERRIC
CHLORIDE CHEMICAL FEED SYSTEM (23)
00
o
OC
o
LL
CO
QC
=>
O
I
<
z
10,000
1,000
T
100
2 34 567891,000 2 34 5678910,000 2 34 56789100,000
Pounds Lime Fed/Hour (I Ib = 0.454 kg)
FIGURE 8-8
LIME STORAGE AND FEEDING OPERATION AND
MAINTENANCE WORK-HOUR REQUIREMENTS (22)
I-12
-------
Figure 8-7 indicates annual electric power requirements for a
ferric chloride chemical feed system.
Annual maintenance material costs are typically 3 to 5 percent of
the total chemical feed system equipment cost.
Figure 8-8 indicates man-hours for operation and maintenance as a
function of pounds of lime fed per hour. The curve consists of
lime unloading requirements and labor related to operation and
maintenance of the slaking and feeding equipment. These require-
ments are summarized as follows: slaker--one hour per eight-hour
shift per slaker in use; feeder--ten minutes per hour per feeder;
slurry pot-feed line (for slaked lime)--four hours per week.
PUMPED FEED OF
SLAKED LIME
GRAVITY FEED OF
QUICKLIME
GRAVITY FEED
OF SLAKED LIME
PUMPED FEED OF
QUICKLIME
1,000
100
3 4 567891,000 2 34 5678910,000 2
Pounds Lime Fed/Hour (1 Ib = 0.454 kg)
FIGURE 8-9
ELECTRICAL ENERGY REQUIREMENTS FOR
A LIME FEED SYSTEM (22)
3 4567 89100,000
8-13
-------
curves
Figure
a 1 ime
used in the
1,000 pounds (454 kg)
activators--2.7 to 0
collection fans0.04
slurry feed pumps--2.2
8-9 shows annual electric power requirements for
feed system. The major components and the values
all expressed kilowatts per hour per
of lime fed are: slakers--1.6 to 0.8; bin
36; grit conveyors--0.45 to 0.06; dust
to 0.02; slurry mixers--0.027 to 0.020;
to 1.4.
Annual maintenance material costs are typically
1.5 percent of the total lime feed system equipment cost.
0.5 to
8.5 Chemical Conditioning With Polyelectrolytes
8.5.1 Introduction
During the past decade, important advances have been made in the
manufacture of polyelectrolytes for use in wastewater sludge
treatment. Polyelectrolytes are now widely used in sludge
conditioning and as indicated in Table 8-3, a large variety are
available. It is important to understand that these materials
differ greatly in chemical composition, functional effectiveness,
and cost-effectiveness.
TABLE 8-3
SUPPLIERS OF POLYELECTROLYTES
Company
American Cyanamid
Allied Colloids
Betz
Calgon
Number
of grades
and tynes
40
34
7
18
Company
Dow
Drew
Hercules
Nalco
Rohm & Hass
Number
of grades
and types
33
8
29
43
4
Selection of the correct polyelectrolyte requires that the
designer work with polyelectrolyte suppliers, equipment
suppliers, and plant operating personnel. Evaluations should be
made on site and with the sludges to be conditioned. Since new
types and grades of polymers are continually being introduced,
the evaluation process is an ongoing one.
8.5.2 Background on Polyelectrolytes
8.5.2.1 Composition and Physical Form
Polyelectrolytes are long chain, water soluble, specialty
chemicals. They can be either completely synthesized from
individual monomers, or they can be made by the chemical
1-14
-------
addition of functional monomers, or groups, to naturally
occurring polymers. A monomer is the subunit from which polymers
are made through various types of polymerization reactions.
The backbone monomer most widely used in synthetic organic
polyelectrolytes is acrylamide. As of 1979 the completely
synthesized polymers are most widely used. Polyaerylamide,
created when the monomers combine to form a long, thread-like
molecule with a molecular weight in the millions, is shown on
Figure 8-10. In the form shown polyacrylamide is essentially
non-ionic. That is to say it carries no net electrical charge in
aqueous solutions. However, under certain conditions and
with some solids, the polyacrylamide can be sufficiently
surface-active to perform as a flocculant.
.L Pll
pt-i _ -.
Vrf PI imm
1
C = 0
,
,lr-T-_T..1 pu pi_i _,,
1
C = 0
f
\
OLJ r>Lj
C = 0
*
NH2 NH2 NH2
FIGURE 8-10
POLYACRYLAMIDE MOLECULE - BACKBONE OF THE
SYNTHETIC ORGANIC POLYELECTROLYTES
Anionic-type polyacrylamide flocculants carry a negative
electrical charge in aqueous solutions and are made by either
hydrolyzing the amide group (NH2) or combining the acrylamide
polymer with an anionic monomer. Cationic polyaerylamides
carry a positive electrical charge in aqueous solutions and
can be prepared by chemical modification of essentially
non-ionic-polyacrylamide or by combining the cationic monomer
with acrylamide. When cationic monomers are copolymerized with
acrylamide in varying proportions, a family of cationic
polyelectrolytes with varying degrees of charge is produced.
These polyelectrolytes are the most widely used polymers for
sludge conditioning, since most sludge solids carry a negative
charge. The characteristics of the sludge to be processed and
the type of thickening or dewatering device used will determine
which of the cationic polye lectroly tes will work best and
still be cost-effective. For example, an increasing degree
of charge is required when sludge particles become finer,
when hydration increases, and when relative surface charge
increases.
8-15
-------
Cationic polyelectrolytes are available as dry powders or
liquids. The liquids come as water solutions or emulsions. The
shelf life of the dry powders is usually several years, whereas
most of the liquids have shelf lives of two to six months and
must be protected from wide ambient temperature variations in
storage. Representative dry cationic polyelectrolytes are
described in Table 8-4. This table does not list the myriad of
available types but does show some of the differences in the
materials. The original dry materials introduced in the 1960s
were of relatively low cationic functionality or positive
charge and high molecular weight. They were produced for the
conditioning of primary sludges or easy-to-condition mixed
sludges. The incentive to produce polymers of higher positive
charge resulted largely from efforts to cope with mixed sludges
containing large quantities of biomass.
TABLE 8-4
REPRESENTATIVE DRY POWDER CATIONIC POLYELECTROLYTES
Relative cationic Molecular Approximate dosage,
Type density weight Ib/ton dry solids
Polyacrylamide copolymer Low Very high 0.5 - 10
Polyacrylamide copolymer Medium High 2-10
Polyacrylamide copolymer High Medium high 2-10
Polyamine homopolymer Complete High 2-10
Relatively low molecular weight liquid cationics with a 30 to
50 percent solids content were also available in the 1960s.
They were, however, largely displaced by the higher cationic
functionality, high molecular weight and newer, 'less costly
liquid cationics. The various liquid cationics, in either
dissolved or emulsion form, are described in representative
fashion only, in Table 8-5. These liquid cationics eliminate the
dustiness inherent in some dry powders but also require much more
storage space. The selection of a dry, liquid, or emulsion form
material usually depends on a comparison of cost-effectiveness,
ease of handling, and storage requirements.
TABLE 8-5
REPRESENTATIVE LIQUID CATIONIC POLYELECTROLYTES
Type - Molecular weight Percent solic3s_
Mannich product Low 20
Tertiary polyamine Low 30
Quaternary polyamine Very low 50
Cationic homopolymer Low to medium 16 - 20
Emulsion copolymer Low to medium 25 - 35
5-16
-------
8.5.2.2 Structure in Solution
Organic polyelectrolytes dissolve in water to form solutions of
varying viscosity. The resulting viscosity depends on their
molecular weight and degree of ionic charge. At infinite
dilution, the molecule assumes the form of an extended rod
because of the repulsive effect of the adjacent-charged sites
along the length of the polymer chain. At normal concentrations
the long thread-like charged cationic polyelectrolyte assumes the
shape of a random coil, as shown on Figure 8-11. This simplified
drawing, however, neither shows the tremendous length of the
polymeric molecular chain nor does it illustrate the very large
number of active polymer chains that are available in a polymer
solution. It has been estimated that a dosage of 0.2 mg/1 of
polyelectrolyte having a molecular weight of 100,000 would
provide 120 trillion active chains per liter of water treated.
0
0
©
©
FIGURE 8-11
TYPICAL CONFIGURATION OF A CATIONIC
POLYELECTROLYTE IN SOLUTION
8.5.2.3 How Polyelectrolyte Conditioning Works
Thickening and dewatering are inhibited by the sludge particles,
chemical characteristics, and physical configurations. Poly-
electrolytes in solution act by adhering to the sludge particle
surfaces thus causing:
Desorption of bound surface water.
8-17
-------
Charge neutralization.
Agglomerization of small particulates by bridging between
particles.
The result is the formation of a permeable sludge cake matrix
which is able to release water. Figure 8-12 illustrates the
polyelectrolyte-solid attachment mechanisn. The first two
reactions noted on Figure 8-12 are the desirable ones and
represent what occurs in normal practice. The other four
reactions represent what can occur from over-dosage or too much
shear of flocculated sludge. The problems reflected in reactions
three through six rarely occur with a well-designed process.
8.5.3 Conditioning for Thickening
The various methods for thickening sludge are discussed in detail
in Chapter 5.
8.5.3.1 Gravity Thickening
Normally, the addition of polyelectrolyte is not considered in
the original design because of operating cost, but it has been
used to upgrade existing facilities (21,22). Experience to date
has indicated that the addition of polyelectrolyte to a gravity
thickener will:
Give a higher solids capture than a unit not receiving
polymer addition.
Allow a solids loading rate two to four times greater
than a unit not receiving polymer addition.
Maintain the same underflow solids concentration as a
unit not receiving polymer addition.
When polyelectrolyte is used to condition sludge for gravity
thickening, it should be added into the sludge feed line. The
point of addition should provide good mixing and not cause
excessive shear before the conditioned sludge discharges into the
sludge feed well.
8.5.3.2 Dissolved Air Flotation Thickening
The effects of polyelectrolyte addition on solids capture, float
concentration, solids loading rate, and hydra |