EPA 625/1-74-006
PROCESS DESIGN MANUAL
FOR
SLUDGE TREATMENT AND DISPOSAL
U.S. ENVIRONMENTAL PROTECTION AGENCY
Technology Transfer
October 1974
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ACKNOWLEDGEMENTS
This design manual was prepared for the office of Technology Transfer of the U.S.
Environmental Protection Agency. Coordination and preparation of the manual was
carried out by the firm of Black, Crow and Eidsness, Gainesville, Florida, under the
direction of John R. Harrison and Dr. James B. Goodson with major contributions to the
text by Gordon Gulp of Clean Water Consultants-Culp/Wesner/Culp and Dr. James E.
Smith, Jr. of the U.S. EPA National Environmental Reasearch Center, Cincinnati, Ohio.
EPA coordination and review was carried out by Jon C. Dyer, Office of Technology
Transfer, Washington, D.C.
NOTICE
The mention of trade names of commercial products in this publication is for illustration
purposes and does not constitute endorsement or recommendation for use by the U.S.
Environmental Protection Agency.
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ABSTRACT
The purpose of this manual is to present a contemporary review of sludge processing
technology and the specific procedures to be considered, modified, and applied to meet
unique conditions.
The manual emphasizes the operational considerations and interrelationships of the
various sludge treatment processes to be considered before selecting the optimum design.
The manual also presents case histories of existing wastewater treatment plants to
illustrate the various unit processes and results.
in
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TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ii
ABSTRACT iii
LIST OF FIGURES xii
LIST OF TABLES xxi
FOREWORD xxix
Chapter
1 INTRODUCTION 1 - 1
2 METHODOLOGY AND NOMENCLATURE 2 - 1
2.1 Introduction 2-1
2.2 Methodology 2 -1
2.2.1 Working Environment of the Design Engineer 2-1
2.2.2 Essential Considerations for a Successful Plant 2-2
2.2.3 The Total System Approach to Design 2 - 2
2.2.4 The Design Team Concept 2 - 6
2.3 Sludge Processing and Disposal Nomenclature 2 - 6
2.3.1 General Considerations 2-6
2.3.2 Sludge Treatment and Disposal-Unit Processes 2 - 8
2.3.3 Sequence and Functions of the Unit Processes 2-12
2.4 References 2-12
3 OCCURRENCE OF SLUDGES AND PHYSICAL AND
CHEMICAL PROPERTIES RELATING TO PROCESSABILITY 3 -1
3.1 Occurrence of Sludges—Conventional Biological Treatment 3-1
3.2 Occurrence of Sludges—Combined Biological and Chemical
Treatment 3 - 2
3.3 Physical and Chemical Properties Relating to Processability 3 - 6
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TABLE OF CONTENTS-Cowtinned
Chapter Page
3 3.3.1 Factors Affecting Processing 3-7
3.3.2 Particle Size and Configuration 3-7
3.3.3 Surface Charge and Degree of Hydration 3-10
3.3.4 Compressibility and Water Retention 3-12
3.4 Plant Experiences with Various Processes and Types of
Sludge 3-15
3.4.1 Raw Primary Sludge 3-15
3.4.2 Effect of Anaerobic Digestion on Primary Sludge
Dewatering 3-18
3.4.3 Activated Sludge from Conventional Air Systems 3-18
3.4.4 Rationale of Design for Some Existing Activated
Sludge Plants 3 - 22
3.4.5 Improvements in Mixed Sludge Processing 3-22
3.4.6 Processing of Mixed Primary and Oxygen Activated
Sludges 3 - 26
3.4.7 Cake Release in Dewatering 3-32
3.4.8 Phosphorus Removal Process Sludges 3-33
3.5 Additional Reading 3-34
3.6 References 3-35
4 SLUDGE THICKENING (BLENDING) 4 - 1
4.1 Functions, Methods, and Occurrences 4-1
4.2 The Gravity Thickener 4 - 6
4.2.1 Performance Experiences 4-8
4.2.2 Theory of Gravity Thickening and Design Procedures 4-9
4.2.3 Gravity Thickening of Oxygen Activated Sludges 4-11
4.2.4 Capital, Operation, and Maintenance Costs for
Gravity Thickening 4-12
4.3 Air Flotation Thickening 4-12
4.3.1 Occurrence, Methods, and Process Theory 4-14
4.3.2 Operational Results 4-21
4.3.3 Advantages and Disadvantages of DAF Thickeners 4-21
4.3.4 Components of a Typical Flotation Unit System 4-23
4.3.5 Design and Performance of DAF Thickeners 4-23
4.3.6 Costs 4-27
VI
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TABLE OF CQWTENTS-Continued
Chapter Page
4 4.3.7 Integration of DAF Thickening into the Conventional
Activated Sludge Plant 4 - 28
4.3.8 Effect of Oxygen Activated Sludge 4-28
4.4 Centrifugal Thickening 4 - 30
4.4.1 Solid Bowl Conveyor Type Centrifuge—Sludge
Thickening 4 - 30
4.4.2 Disc-Noz/le Centrifuge 4-30
4,4.3 Basket (Imperforate Bowl-Knife Discharge) Centrifuge 4 - 34
4.4.4 Performance Data 4 - 34
4.5 Sludge Blending 4 - 34
4.6 References 4 - 37
5 SLUDGE STABILIZATION (REDUCTION) 5 - 1
5.1 Functions, Methods, and Occurrences 5-1
5.2 Anaerobic Digestion 5 - 1
5.2.1 Types of Anaerobic Digestion Systems 5-2
5.2.2 Design Criteria 5 - 4
5.2.3 Process Control Considerations 5-10
5.2.4 Process Performance Data 5-15
5.2.5 Upgrading Procedures 5-19
5.2.6 Typical Costs 5-20
5.3 Aerobic Digestion 5 - 20
5.3.1 Process Design 5-23
5.3.2 Process Performance Data 5-24
5.3.3 Oxygen Aerobic Digestion 5-28
5.3.4 Aerobic Digestion Costs 5-29
5.4 Chlorine Oxidation 5-29
5.5 Lime Treatment 5 - 29
5.6 Heat Treatment for Stabilization 5 - 33
5.7 Composting 5 - 34
5.7.1 Process Description 5 - 34
5.7.2 Process Performance and Costs 5-35
5.8 Additional Reading 5 - 35
5.9 References 5-38
VII
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TABLE OF CONTENTS-Continned
Chapter Page
6 SLUDGE CONDITIONING 6 - 1
6.1 Functions, Methods, and Occurrences 6-1
6.2 Considerations in Selecting a Conditioning Method 6-2
6.3 Process Chemistry—Conditioning 6-6
6.3.1 Chemical Conditioning and the Use of Poly electrolytes 6 - 6
6.3.2 Use of Inorganic Chemicals 6-13
6.3.3 Elutriation 6-13
6.3.4 Heat Treatment 6 - 14
6.4 Physical Factors in Conditioning Processes 6-15
6.4.1 Effect of Processing Prior to Conditioning 6-17
6.4.2 Conditioner Application 6-17
6.5 Conditioning for Gravity Thickening 6-19
6.6 Conditioning for Flotation Thickening 6-19
6.7 Conditioning for Dewatering 6-23
6.7.1 Rotary Vacuum Filtration 6-23
6.7.2 Centrifuges 6-23
6.7.3 Drying Beds 6-25
6.7.4 Filter Presses 6-25
6.8 Selection of Conditioning Chemicals 6-28
6.9 References 6 - 29
7 SLUDGE DEWATERING 7 - 1
7.1 Methods and Functions 7-1
7.2 Rotary Vacuum Filtration 7 - 2
7.2.1 Mechanics of Rotary Vacuum Filtration 7-2
7.2.2 Process Objectives 7-17
7.2.3 Types of Rotary Vacuum Filters 7-18
7.2.4 Machine Variables 7-29
7.2.5 Rotary Vacuum Filter Costs 7-31
7.2.6 Typical Rotary Vacuum Filter Results 7-32
7.2.7 Summation 7 - 33
7.3 Centrifugal Dewatering 7-33
Vlll
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TABLE OF CONTENTS-Conffnued
Chapter Page
7 7.3.1 Theory of Centrifugal Dewatering 7 - 34
7.3.2 Types of Centrifuges 7-41
7.3.3 Sludge Fractionation (Classification) by Centrifuge 7 - 46
7.3.4 System Requirements 7 - 49
7.3.5 Results of Centrifugal Dewatering 7-49
7.3.6 Summation 7-58
7.4 Drying Beds 7-58
7.4.1 Factors Affecting Design 7-58
7.4.2 Design Criteria for Sandbeds 7 - 58
7.4.3 Results of Sandbed Drying 7-60
7.4.4 Other Types of Drying Beds 7 - 60
7.5 Drying Lagoons 7-63
7.5.1 Factors Affecting Design 7-63
7.5.2 Design Criteria for Drying Lagoons 7-63
7.5.3 Results of Lagoon Drying 7 - 64
7.6 Pressure Filtration 7-64
7.6.1 Concept 7-64
7.6.2 System Requirements 7-68
7.6.3 Results of Pressure Filtration 7-70
7.6.4 Summation 7 - 75
7.7 Other Systems 7 - 75
7.7.1 Moving Screen Concentrator 7-76
7.7.2 Belt Pressure Filters 7-79
7.7.3 Capillary Dewatering Systems 7 - 84
7.7.4 Rotating Gravity Concentration 7 - 91
7.8 References 7 - 94
8 SLUDGE REDUCTION 8 -1
8.1 Methods, Functions, and Occurrences 8-1
8.2 Incineration 8-2
8.2.1 Composition of Sludge Feed 8-2
8.2.2 The Incineration Process 8 - 5
8.2.3 Analysis of Incineration Processes 8 - 6
8.2.4 Multiple Hearth Incineration 8-11
8.2.5 Fluidized Bed Incineration 8-17
IX
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TABLE OF CONTENTS-Co«ri««erf
Chapter Page
8 8.2.6 Flash Drying 8-22
8.2.7 Wet Air Oxidation 8-26
8.2.8 Pyrolysis 8-32
8.2.9 Other Types of Incinerators 8-33
8.3 Lime Recalcining 8-38
8.4 Air Pollution Considerations 8 - 43
8.5 References 8 - 55
9 FINAL DISPOSAL PROCESSES 9 -1
9.1 Methods, Functions, and Occurrences 9 -1
9.2 Selection of Method of Final Disposal 9 - 2
9.3 Sanitary Landfill 9 - 2
9.3.1 Design Criteria 9-3
9.3.2 Costs of Sanitary Landfill 9-3
9.4 Use of Sludge on Agricultural Land 9-5
9.4.1 Soil Considerations 9-5
9.4.2 Sludge as a Fertilizer and Soil Conditioner 9 - 5
9.4.3 Physical Process Considerations 9-10
9.4.4 Crop Considerations 9-13
9.4.5 Costs of Cropland Sludge Spreading 9-14
9.5 Land Reclamation 9-14
9.6 Land Disposal Case Studies 9-15
9.6.1 St. Marys, Pennsylvania 9-15
9.6.2 Fergus Falls, Minnesota 9-15
9.6.3 Xenia, Ohio 9-17
9.6.4 Denver, Colorado 9-18
9.6.5 Chicago, Illinois 9-19
9.7 References 9 - 20
10 CASE HISTORIES-USE OF CHEMICALS IN EXCESS
ACTIVATED SLUDGE PROCESSING 10-1
10.1 General Considerations 10-1
10.2 Washington, D.C.-Blue Plains Plant 10-1
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TABLE OF CONTENTS-Co/iftwt/ed
Chapter Page
10 10.3 St. Helens-United Kingdom 10-5
10.4 Metropolitan Toronto Main Plant 10-12
10.5 Richmond, California 10-17
10.6 Fairfax County, Virginia-Westgate Plant 10-21
10.7 Metropolitan Denver Sewage Disposal District No. 1 10-28
10.7.1 General Considerations 10-28
10.7.2 Sludge Processing System 10-28
10.7.3 Plant Loadings Experienced 10-28
10.7.4 Sludge Processing Results-1967 to 1970 10 - 32
10.7.5 Modified Denver System and Results 10-32
10.8 References 10-32
11 CASE HISTORIES OF SLUDGE TREATMENT BY HIGH
TEMPERATURE AND PRESSURE 11-1
11.1 Heat Treatment 11-1
11.2 Process Considerations 11-1
11.3 Coors-Golden, Colorado 11-5
11.4 Colorado Springs, Colorado 11-5
11.5 Borough of Pudsey—United Kingdom 11-7
11.6 Kalamazoo, Michigan 11-9
11.7 Ft. Lauderdale, Florida-Plant A 11-12
11.8 References 11-14
XI
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LIST OF FIGURES
Figure Page
2-1 Unit processes—sludge processing and disposal 2-4
2-2 Evaluation of system alternatives 2-7
2-3 Enumeration of sludge treatment processes and their functions 2-13
3-1 Particle size—common materials 3-8
3-2 Fluid flow through an incompressible sludge cake 3-13
3-3 Fluid flow through a compressible sludge cake 3-14
3-4 Raw primary filter cake 3-16
3-5 Removal of raw primary sludge cake from a vacuum filter 3-17
3-6 Effect of time on microorganism mass, COD, and percent
particle dispersion 3-20
3-7 Effect of time on biopolymer accumulation and dewaterability 3-21
3-8 Activated sludge plant where EAS is recirculated to primary
clarifiers 3-23
3-9 Activated sludge plant where EAS is mixed with primary sludge
prior to thickening and digestion 3-24
3-10 Improved sludge processing scheme for an activated sludge
plant 3-25
3-11 Use of aerobic digestion to reduce activated sludge solids in
mixed sludge processing 3-27
3-12 Improved use of anaerobic and aerobic digestion in mixed
sludge processing 3-28
3-13 Settling characteristics for air and oxygen biomass 3-29
3-14 Oxygen activated sludge system at Westgate treatment plant 3-31
xn
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LIST OF FIGURES-Continued
Figure. Page
4-1 Effect of increasing sludge solids on the final sludge volume 4-2
4-2 Effect of thickening on required digestion capacity 4-3
4-3 Effect of feed solids on performance of a rotary vacuum filter 4-4
4-4 Gravity thickener 4-7
4-5 Characteristic settling curve for slurry with hindered settling
characteristics 4-10
4-6 Costs of gravity thickening 4-13
4-7 Dissolved air flotation unit 4-16
4-8 Dissolved air flotation system 4-17
4-9 Influence of pressure of saturation on rise rate 4-19
4-10 Influence of air-to-solids ratio on float solids content 4-20
4-11 Typical DAF thickener system 4 - 24
4-12 Schematic flow diagram of a conventional activated sludge plant
incorporating a DAF thickener 4 - 29
4-13 Thickening of activated sludge by disc-nozzle centrifuge 4-32
4-14 Effect of activated sludge settleability on capture and
thickening 4-33
4-15 Eimco sludge storage tank blender mixer 4-36
5-1 Standard rate and high rate digestion 5-3
5-2 Two^tage anaerobic digestion 5-5
5-3 Anaerobic contact digestion 5-6
5-4 Influence of temperature on digestion time 5-8
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LIST OF FIGURES-Cori/inuerf
Figure • Page
5-5 Plot of volatile solids loading vs. SRT for various feed solids 5-9
5-6 Relationship between pH and bicarbonate concentration 5-12
5-7 Unit anaerobic digestion costs 5-21
5-8 Schematic of aerobic digestion system 5-22
5-9 Typical circular aerobic digester 5-25
5-10 Aerobic digestion capital cost 5-30
5-11 Composting costs 5-37
6-1 Conceptual flow sheet wastewater plant with heat treatment 6-4
6-2 Factors influencing the stability of a colloidal suspension 6 - 9
6-3 Mechanism of polymer flocculation 6-10
6-4 Structure of two polyelectrolytes's monomcric units 6-11
6-5 Typical configuration of a polyelectrolyte in solution 6-12
6-6 Schematic diagram of plant for processing heat treatment
liquor 6-16
6-7 Rotary drum conditioner 6-18
6-8 Baffled trough unit 6 - 20
6-9 Tllickening performance us affected by mass loading at constant
chemical dosage 6-21
6-10 Flow diagram of a flotation unit 6 - 22
6-11 Concurrent flow solid-bowl centrifuge 6 - 26
6-12 Sandbcd dewatcring 6-27
xiv
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LIST OF FIGURES-Continued
Figure Page
7-1 Cutaway view of a rotary drum vacuum filter 7-3
7-2 Operating zones of a vacuum filter 7-5
7-3 Corrected filter yield vs. specific resistance 7-8
7-4 Laboratory vacuum filter apparatus 7-9
7-5 Typical Buchner funnel test plot 7-11
7-6 Instantaneous filtrate flow rate 7-12
7-7 Cake processing phases rotary vacuum filter 7-13
7-8 Media size and conditioning effects on filtrate flow rate 7-16
7-9 Yield as a function of feed solids 7-19
7-10 Vacuum filtration operational labor costs as function of yield 7-20
7-11 Cake solids as a function of feed solids for different sludges 7-21
7-12 Cross section of a coil filter 7 - 23
7-13 Cutaway view of coil springs 7-24
7-14 Cross section of a belt filter 7 - 25
7-15 Cake release of a belt filter 7 - 26
7-16 Rotary vacuum filter system 7 - 28
7-17 Continuous countercurrent solid bowl conveyor discharge
centrifuge 7 - 35
7-18 Effect of bowl angle and centrifugal force on sludge solids in
drainage zone 7-38
7-19 Cross section of concurrent flow solid — bowl centrifuge 7-42
xv
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LIST OF ¥lG\3RES-Con tinned
Figure Page
7-20 Schematic diagram of a basket centrifuge 7-44
7-21 Disc type centrifuge 7-45
7-22 Reaction of particles within centrifuges 7-47
ii it
7-23 Typical dewatering performance curves for a 36 X 96 Bird
horizontal scroll centrifuge fed unconditioned primary digested
sludge 7 - 48
7-24 Summary of constituent recoveries during wet classification of
lime sludges resulting from raw wastewater coagulation 7-50
7-25 Centrifuge dewatering system 7-51
7-26 Typical flocculant piping diagram 7 - 52
7-27 Effect of polyelectrolyte dosage and pool depth on percent
solids recovery at various feed rates 7-54
7-28 Cake dryness as a function of solids recovery 7-55
7-29 Cross section of a wedgewire drying bed 7-62
7-30 Side view of a filter press 7-65
7-31 Cutaway view of a filter press 7-66
7-32 Filter press system 7 - 69
7-33 Required ash to sludge ratio as function of feed solids 7-73
7-34 Average chemical costs for pressure filtration at Cedar Rapids 7-74
7-35 Moving screen concentrator system 7-77
7-36 Moving belt concentrator yield vs. cake solids 7-78
7-37 Schematic construction of the belt filter press 7 - 80
xvi
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LIST OF FlGURES-Continued
Figure Page
7-38 Belt filter press 7 - 82
7-39 Belt filter press system 7-83
7-40 Passavant belt filter press 7 - 85
7-41 Squeegee capillary sludge dewatering unit 7 - 86
7-42 Capillary dewatering zone 7-87
7-43 Belt dewatering zone 7 - 88
7-44 Final compression zone 7-89
7-45 Rotating gravity concentrator 7-92
7-46 Schematic of MRP section 7 - 93
8-1 Sludge incineration 8-3
8-2 The effects of sludge moisture and volatile solids content on gas
consumption 8-7
8-3 Equalibrium curves relating combustion temperatures to cake
concentration 8 - 8
8-4 Impact of excess air on the cost of natural gas in sludge
incineration 8 - 9
8-5 Material balance for fluidized bed sewage sludge incineration 8-10
8-6 Cross section of a typical multiple hearth incinerator 8-12
8-7 Multiple hearth process zones 8-14
8-8 Multiple hearth incineration costs 8-16
8-9 Cross section of a fluid bed reactor 8-18
xvu
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LIST OF FIGURES-Continued
Figure Page
8-10 Fluidized bed system with air preheater 8-20
8-11 Flash dryer system 8 - 23
8-12 Sludge drying system using the jet mill principle 8 - 25
8-13 Wet air oxidation system 8-27
8-14 Skid-mounted cyclonic reactor system 8-34
8-15 Cyclone furnace 8-36
8-16 Infrared incineration system 8-37
8-17 The lime recalcining system at south Lake Tahoe 8-40
8-18 Fluidized bed system for lime recalcining 8-42
8-19 Particulate emmissions from sludge incinerators at wastewater
treatment plants 8-51
9-1 Capital and O/M costs for sanitary landfills 9-4
9-2 Relative transportation cost for liquid organic sludges 9-11
9-3 Typical spray sprinkler 9-12
9-4 Tank trunk spreading sludge in cold weather 9-16
9-5 Close-up view of sludge deflection plate 9-16
10-1 District of Columbia, plant flow diagram 10-2
10-2 District of Columbia's elutriation and filtration system 10-3
10-3 Vacuum filter operation at District of Columbia 10-6
10-4 New Blue Plains sludge processing system 10-7
xvm
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LIST OF FIGUKES-Continued
Figure Page
10-5 Solids handling at the Parr Works, St. Helens 10-8
10-6 Variation of filter yield and percent solids with time at St.
Helens 10-9
10-7 Quantity of EAS and solids concentration in the EAS as a
function of time 10-10
10-8 Metro Toronto's plant flow diagram 10-13
10-9 Variation of percent solids in elutriated sludge at Metro
Toronto for period, 1967-1970 10-14
10-10 Metro Toronto's raw sludge solids concentration from 1967 to
1971 10-15
10-11 A view of filters at Metro Toronto , 10-18
10-12 Richmond, California's plant flow diagram 10-19
10-13 Belt filters at Richmond, California 10 - 22
10-14 Original process flow diagram for Westgate plant 10-23
10-15 Westga te sedimen ta tion tank 10 - 24
10-16 Current Westgate plant flow diagram 10-26
10-17 Fairfax County's Westgate plant 10-27
10-18 Metro Denver system's flow diagram 10-29
10-19 Changes in Metro Denver's annual plant flows from 1967 to
1975 10-31
10-20 Unit costs of Metro wastewater treatment 10-33
10-21 Sludge processing costs vs. EAS/total sludge produced 10-34
XIX
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LIST OF FIGURES-Conftnwed
Figure Page
10-22 Modified Metro Denver system 10-35
10-23 Unit costs of Metro wastewater treatment from 1970 to 1972 10 - 36
11-1 Porteous process 11-2
11-2 Zimpro LPO system 11-3
11-3 Farrer process system 11-4
11-4 Flow diagram of Colorado Springs with heat treatment 11-6
11-5 Pudsey sludge system 11-8
11-6 Kalamazoo, Michigan, sludge disposal facilities 11-10
11-7 Ft. Lauderdale sludge handling system 11-13
xx
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LIST OF TABLES
Table Page
2-1 AUTHOTHERMIC COMBUSTION 2 - 3
2-2 PLANT PROCESS SELECTION CRITERIA 2 - 3
3-1 TYPICAL SLUDGE VOLUMES PRODUCED 3 -1
3-2 TYPICAL SLUDGE MASSES 3 -1
3-3 TYPICAL WATER CONTENT OF SLUDGES 3 - 2
3-4 SAMPLE CALCULATION FOR ESTIMATING SLUDGE
MASS . 3-3
3-5 SAMPLE CALCULATION OF SLUDGE QUANTITY FROM
LIME TREATMENT OF WASTEWATER 3 - 4
3-6 ADDITIONAL SLUDGE TO BE HANDLED WITH CHEMICAL
TREATMENT SYSTEMS: PRIMARY TREATMENT FOR
REMOVAL OF PHOSPHORUS 3 - 5
3-7 ADDITIONAL SLUDGE TO BE HANDLED WITH CHEMICAL
TREATMENT SYSTEMS: PHOSPHORUS REMOVAL BY
MINERAL ADDITION TO AERATOR 3 - 5
3-8 ADDITIONAL SLUDGE TO BE HANDLED WITH CHEMICAL
TREATMENT SYSTEMS: PHOSPHORUS REMOVAL BY
MINERAL ADDITION TO SECONDARY EFFLUENT 3 - 6
3-9 SLUDGE DEWATERING AS A FUNCTION OF PARTICLE
SIZE 3 - 9
3-10 SPECIFIC RESISTANCE OF VARIOUS TYPE SLUDGES 3-10
3-11 VARIATION OF SVI WITH AERATION TIME 3-11
3-12 EFFECT OF STORAGE TIME ON SLUDGE DEWATERABILITY 3-11
xxi
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LIST OF JABLES-Continued
Table Page
3-13 AQUEOUS FLUID DISTRIBUTION IN DIGESTED SLUDGE 3-12
3-14 TYPICAL PERFORMANCE DATA FOR THE VACUUM
FILTRATION OF RAW PRIMARY SLUDGE 3-15
3-15 TYPICAL PERFORMANCE DATA FOR VACUUM
FILTRATION OF DIGESTED PRIMARY SLUDGE 3-18
3-16 GRAVITY THICKENING DATA FOR AIR AND OXYGEN
ACTIVATED SLUDGES 3 - 26
3-17 FLOTATION THICKENING DATA FOR AIR AND
OXYGEN ACTIVATED SLUDGES 3 - 30
3-18 CENTRIFUGATION DATA FOR OXYGEN AND
CONVENTIONAL AERATION SLUDGES 3 - 30
3-19 PERFORMANCE DATA FOR DRUM FILTERS AND
BELT FILTERS AT WASHINGTON, D.C. 3 - 32
3-20 DEWATERING DATA ON NORTH TORONTO FERRIC
CHLORIDE-ACTIVATED SLUDGE 3-33
3-21 DEWATERING DATA ON NEWMARKET MIXED
ORGANIC/LIME SLUDGE 3 - 33
3-22 SLUDGE PROCESSING DATA FOR PHOSPHORUS
REMOVAL ALTERNATIVES AT THE LITTLE RIVER
TREATMENT PLANT 3 - 34
4-1 OCCURRENCE OF THICKENING IN WASTEWATER
TREATMENT PROCESSES 4 - 1
4-2 OCCURRENCE OF THICKENING METHODS IN SLUDGE
TREATMENT 4 - 5
4-3 GRAVITY THICKENER SURFACE LOADINGS AND
OPERATIONAL RESULTS 4 - 8
xxu
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LIST OF TAELES-Continued
Table Page
4-4 GRAVITY THICKENING DATA FOR EXCESS OXYGEN
ACTIVATED SLUDGE 4-12
4-5 OPERATING DATA FOR PLANT SCALE DAF UNITS 4 - 22
4-6 DAF THICKENING COSTS FOR VARIOUS PLANT SIZES 4 - 27
4-7 COMPARATIVE DATA ON TWO ALTERNATIVE SLUDGE
THICKENING PROCESSES 4 - 34
4-8 CENTRIFUGAL THICKENING PERFORMANCE DATA 4 - 35
5-1 ANAEROBIC DIGESTION-BIOCHEMISTRY 5 - 2
5-2 PHYSICAL AND CHEMICAL FACTORS 5 - 7
5-3 TYPICAL DESIGN CRITERIA FOR STANDARD RATE
AND HIGH RATE DIGESTERS 5-10
5-4 SUBSTANCES AND CONCENTRATIONS CAUSING
TOXICITY IN WASTEWATER SLUDGE DIGESTION 5-14
5-5 DESIGN DATA FOR CHICAGO DIGESTERS 5-16
5-6 SUMMARY OF SOUTHWEST TREATMENT PLANT
DIGESTER OPERATION 5-16
5-7 CHARACTERISTICS OF SLUDGE GAS 5-17
5-8 SUPERNATANT CHARACTERISTICS FROM
ANAEROBIC DIGESTERS 5-18
5-9 BACTERIAL SURVIVAL IN DIGESTION 5-19
5-10 AEROBIC DIGESTION DESIGN PARAMETERS 5-26
5-11 SUMMARY OF AEROBIC DEGESTION OPERATION 5 - 27
5-12 CHARACTERISTICS OF AEROBIC DIGESTION SUPERNATANT 5 - 27
xxm
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LIST OF JABLES-Continued
Table Page
5-13 RESULTS OF HIGH-PURITY OXYGEN AEROBIC DIGESTERS
SPEEDWAY, INDIANA 5 - 28
5-14 BACTERIOLOGICAL STUDIES OF SLUDGE PRODUCED IN
PLANT-SCALE TESTS AT LEBANON 5-31
5-15 EFFECT OF LIME ON FILTERABILITY OF ALUMINUM
AND IRON PRIMARY SLUDGES AT LEBANON 5-31
5-16 AVERAGE COST OF LIME ADDITION-PLANT-SCALE
TESTS AT LEBANON 5 - 32
5-17 LIME DOSE REQUIRED TO KEEP SLUDGE ph > 11.0 FOR
AT LEAST 14 DAYS 5 - 32
5-18 EFFECT OF TIME AND TEMPERATURE ON THE SURVIVAL
OF TYPICAL PATHOGENS FOUND IN SLUDGE 5 - 33
5-19 HYGIENIC QUALITY OF COMPOST 5 - 36
6-1 CONDITIONING METHODS AND PURPOSES 6 - 1
6-2 PLANT DESIGN CRITERIA 6 - 3
6-3 ADDITIONAL DESIGN CRITERIA 6 - 5
6-4 COMPARATIVE SOLIDS BALANCES VARIOUS SLUDGE
PROCESSING CONDITIONS 6 - 7
6-5 RECIRCULATED SOLIDS LOADINGS DURING DEWATERING 6 - 8
6-6 EFFECT OF ASH ADDITION ON VACUUM FILTRATION
AT INDIANAPOLIS 6-14
6-7 ESTIMATED CHEMICAL CONDITIONING DOSAGE FOR
VACUUM FILTRATION 6 - 24
6-8 CONDITIONING CHEMICAL MATERIALS 6 - 28
XXIV
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LIST OF TABLES-Continued
Table Page
7-1 THE RELATIONSHIP OF DEWATERING TO OTHER SLUDGE
TREATMENT PROCESSES FOR TYPICAL MUNICIPAL SLUDGES 7 - 2
7-2 CAKE RELEASE MEASURES USED ON BELT TYPE FILTERS
AT VARIOUS PLANT LOCATIONS 7 - 27
7-3 EVALUATION OF ALTERNATE FILTER MEDIA AT THE
CHICAGO SANITARY DISTRICT 7 - 30
7-4 TYPICAL ROTARY VACUUM FILTER RESULTS FOR
SLUDGE CONDITIONED WITH INORGANIC CHEMICALS 7 - 32
7-5 TYPICAL ROTARY VACUUM FILTER RESULTS FOR
POLYELECTROLYTE CONDITIONED SLUDGES 7 - 33
7-6 TYPICAL SOLID BOWL CENTRIFUGE PERFORMANCE 7-53
7-7 DEWATERING OF OXYGEN ACTIVATED SLUDGES IN
SOLID BOWL AND BASKET CENTRIFUGES 7 - 57
7-8 CRITERIA FOR THE DESIGN OF SANDBEDS 7 - 59
7-9 TYPICAL PERFORMANCE DATA 7-61
7-10 PRESSURE FILTRATION CONSIDERATIONS 7-67
7-11 TYPICAL FILTER PRESS PRODUCTION DATA 7-71
7-12 EUROPEAN INSTALLATIONS OF THE BELT FILTER
PRESSES 7-81
7-13 SUMMARY OF PILOT PLANT CAPILLARY DEWATERING
SYSTEM PERFORMANCE 7 - 90
7-14 CAPABILITIES OF THE MRP AND DCG UNITS 7-94
8-1 REDUCTION PROCESSES 8-1
8-2 EFFECTS OF PRIOR PROCESSES ON FUEL VALUE 8 - 4
XXV
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LIST OF TABLES-Co« tinned
Table Page
8-3 REPRESENTATIVE HEATING VALUES OF SOME SLUDGE
MATERIALS 8 - 4
8-4 SLUDGE INCINERATOR FACILITY At - SUMMARY OF
RESULTS 8 - 45
8-5 SLUDGE INCINERATOR FACILITY A2 - SUMMARY OF
RESULTS 8 - 46
8-6 SLUDGE INCINERATOR FACILITY B - SUMMARY OF
RESULTS 8 - 47
8-7 SLUDGE INCINERATOR FACILITY C - SUMMARY OF
RESULTS 8 - 48
8-8 SLUDGE INCINERATOR FACILITY D - SUMMARY OF
RESULTS 8 - 49
8-9 SLUDGE INCINERATOR FACILITY E - SUMMARY OF
RESULTS 8 - 50
9-1 FINAL DISPOSAL METHODS 9 -1
9-2 SLUDGE UTILIZATION METHODS 9 - 2
9-3 PRIMARY NUTRIENT CONTENT OF LIQUID DIGESTED
SLUDGE 9 - 5
9-4 HEAVY METAL CONTENTS IN SLUDGE 9 - 7
9-5 APPLICATION RATES TO CROPLAND 9-13
9-6 COSTS FOR LAND SPREADING DIGESTED SLUDGE 9-14
9-7 APPLICATION RATES AT ST. MARYS 9-15
9-8 OPERATING DATA FOR XENIA, OHIO LAND SPREADING
PROCEDURES 9-18
XXVI
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LIST OF JAWLES-Continued
Table Page
10-1 DISTRICT OF COLUMBIA'S SLUDGE REMOVAL PRACTICES
AND COSTS 10-4
10-2 VARIATION OF PERCENT SOLIDS IN ELUTRIATED
SLUDGE AT ST. HELENS 10-11
10-3 EFFECT OF AERATION ON EXCESS ACTIVATED SLUDGE
PRODUCTION 10-11
10-4 METRO TORONTO'S SLUDGE REMOVAL NEEDS 10-12
10-5 ELUTRIATION/FILTRATION RESULTS FOR
OCTOBER/NOVEMBER AT METRO TORONTO 10-16
10-6 ELUTRIATION AND FILTRATION RESULTS DURING
1971 AT METRO TORONTO 10-17
10-7 VACUUM FILTRATION RESULTS FOR RICHMOND,
CALIFORNIA 10-20
10-8 RICHMOND, CALIFORNIA-ELUTRIATION AND .
FILTRATION OPERATIONS' DATA 10 - 20
10-9 WESTGATE PLANT PERFORMANCE 10-21
10-10 WESTGATE OXYGEN PROCESS RESULTS 10 - 25
10-11 RESULTS FOR THICKENING AND VACUUM FILTRATION
OF WESTGATE PROCESS SLUDGE 10-28
10-12 METRO DENVER PLANT CAPACITIES 10 - 30
11-1 COLORADO SPRINGS SLUDGE PROCESSING COST 11-7
11 -2 KALAMAZOO SLUDGE PROCESSING COSTS 11-11
11-3 TOTAL SOLIDS-SLUDGE AND CENTRATE 11-12
XXVll
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FOREWORD
The formation of the United States Environmental Protection Agency marked a new en
of environmental awareness in America. This agency's goals are national in scope and
encompass broad responsibility in the area of air and water pollution, solid wastes,
pesticides, and radiation. A vital part of EPA's national water pollution control effort is
the constant development and dissemination of new technology for wastewater
treatment.
It is now clear that only the most effective design and operation of wastewater treatment
facilities, using the latest available techniques, will be adequate to meet the future water
quality objectives and to ensure protection of the nation's waters. It is essential that this
new technology be incorporated into the contemporary design of waste treatment
facilities to achieve maximum benefit of our pollution control 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 and design of present and
future wastewater treatment 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 that adequately describe and interpret 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 through the EPA Office of Technology Transfer to
describe recent technological advances and new information. Future editions will be
issued as warranted by advancing state-of-the-art to include new data as it becomes
available, and to revise design criteria as additional full-scale operational information is
generated.
xxix
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CHAPTER 1
INTRODUCTION
The purpose of this manual is to present a contemporary review of sludge processing
technology as well as procedures to be considered, modified, and applied, as appropriate,
to meet unique conditions of specific design situations. Current regulations require the
design, construction, and cost-effective operation of municipal wastewater treatment
plants capable of removing 85 percent or more influent BOD. They further require
concurrent isolation and regulated disposal of the resultant sludges. As a result, the task
faced by the environmental engineer has become much more complex.
Capital, operating, and maintenance costs of facilities required to provide the higher levels
of treatment are significantly greater than those encountered with more elementary levels
of treatment. Thus, an increased economic incentive for optimal design exists. In addition
to increased quantities of secondary biological sludges, the engineer is now confronted
with advanced waste treatment sludges as well as new and more complex industrial
wastes. A lack of regulations on sludge disposal procedures has caused the engineering
profession to concentrate their design efforts on the liquid treatment portion of the
plant. Agencies have not been accustomed to funding concurrent pilot work on sludge
processing and in the design of primary plants this was not a serious problem. However, in
secondary plants, the various liquid treatment and sludge treatment unit processes are so
highly connected and interrelated that both sections must be studied and considered as a
unified system. Unit process costs and effectiveness should be quoted in the context of a
given system. The fact that the influent waste streams at various cities and localities
deviate significantly makes any attempt at standardized design impractical.
The design engineer should have the capability for innovative design. The following items
are essential to an optimum innovative design:
1. Development and maintenance of a thorough knowledge of the various sludge
treatment unit processes.
2. Continual study of plant operational results to provide feedback for cost saving
modifications and future design.
3. Adequate pilot plant study of alternate prescreened treatment plant systems as
required by the particular circumstances.
4. Use of the systems analysis method for comparing alternate complete systems
(both liquid treatment and sludge processing) for treatment plants.
1 -1
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Case studies and generalized experiences will be discussed in detail to emphasize the
numerous sludge treatment and disposal alternatives. Data will be presented to assist the
design engineer in selecting the optimum unit processes for inclusion in a particular
conceptual design.
The results of inclusion of excess activated sludge and other sludges into processing
systems originally designed for primary sludge alone, together with the remedial
procedures employed, are also discussed in detail.
1 -2
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CHAPTER 2
METHODOLOGY AND NOMENCLATURE
2.1 Introduction
Suspended solids are usually present in the influents to a municipal wastewater treatment
plant at levels of 100 to 300 mg/1. Additional suspended solids are generated during the
various wastewater treatment processes. Some result from biological processes and others
are generated by chemical precipitation.
Sludge is a broad term used to describe the various aqueous suspensions of solids
encountered during treatment. The nature and concentration of the solids control the
processing characteristics of the sludge. Grit, screenings, and scum are not normally
considered as sludge and, therefore, are not discussed in this manual.
2.2 Methodology
It is important to consider the changing climate or working environment faced by the
engineer involved in municipal wastewater plant design. In the past, primary emphasis has
been focused on liquid phase wastewater treatment, both in plant design and in research
and development work.
Experience indicates that a lack of intense attention to the problems of sludge processing
has been prevalent in the engineering profession. As will be noted in chapters 10 and 11,
some of the activated sludge plants that came on stream in the United States over the past
5 to 15 years have been plagued with the failure of sludge processing systems to perform
as designed, either on a functional or a cost basis, or both.
2.2.1 Working Environment of the Design Engineer
The major cause of the problem just described has been the general climate and specific
working conditions regarding design of sludge processing systems facing the engineer.
Funds have not been provided in the past for adequate laboratory, pilot plant, and, most
important, plant scale process engineering work. This has limited the role of the design
engineer in developing innovative techniques. In some cases, responsibility for the design
of liquid treatment facilities at a given plant was assigned to one engineering firm while
another designed the sludge disposal facilities. This arrangement is not conducive to
effective design of an interrelated system.
Problems have plagued the effective design of sludge processing systems despite prior
comprehensive and meaningful sludge handling work. Such deficiencies occurred because
2- 1
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design was accomplished in a climate that tolerated such actions as periodic wasting of
excess solids in plant effluent and conversion of suspended solids into dissolved BOD.
With the advent of the Water Quality Act Amendments of 1972, effluent quality has
become a prime objective in the design of a wastewater treatment facility. It is now
essential to evaluate the effect of all unit processes, such as sludge treatment, on effluent
quality.
Current perspectives on the objectives of wastewater treatment plant design include the
following facts:
• Both solid and liquid fractions must be satisfactorily processed.
• Capital, operating, and maintenance costs should be optimum for the
particular situation.
• Effluent standards are going to be enforced.
2.2.2 Essential Considerations for a Successful Plant
First, optimum conceptual and detailed designs must be prepared. But, since some new
effluent standards require new processes, existing textbooks may no longer be adequate.
Up-to-date know-how in process engineering and evaluations of plant operating results
must be utilized. Second, the plant must be constructed as designed. Third, the plant
must be properly operated and maintained after construction. Fourth, continuing plant
service and development work should be carried out jointly by the engineer and owner.
Plant scale work is the most vital source of information for future process design and
modification for improved performance.
2.2.3 The Total System Approach to Design
The main point of perspective in the development of an optimum conceptual design for a
wastewater treatment plant is that each unit process must be evaluated as a part of the
total system. The most frequently encountered problem in treatment plant design is the
tendency to optimize a given subsystem such as sludge dewatering without considering
the side effects of this optimization on the overall plant operation and treatment costs. A
good example of this is dewatering by vacuum filtration. Many technical articles present
an operational analysis including only such factors as dewatering operating costs,
production rate, and cake moisture content. To really evaluate a given dewatering process
analysis should include, among other things:
• A complete material balance over the dewatering systems.
• The effect of all recycle streams on the operation and cost of other
subsystems.
2-2
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• The ratio of the quantity of volatile solids to the amount of moisture in the
filter cake for gauging calorific value.
To illustrate this point, note in Table 2-1 that the percent dry solids level at which
autogenous incineration occurs is a function of the calorific value of dry solids in filter
cake, which in turn varies with the chemical composition of the solids. The requisite dry
solids level for self-sustaining combustion varies from 18.5 percent to 41.8 percent
depending on these factors. This is in contrast to the usual claim found in technical
articles stating that a 30 to 40 percent dry solids cake is sufficient for autogenous
combustion.
TABLE 2-1
AUTHOTHERMIC COMBUSTION [1]
Sludge Parameter
Gross Calorific Value
Percent Combustible Matter in Solids
Percent Solids
Case 1
17,400
60
41.8
Case 2
29,100
75
18.5
Figure 2-1 depicts the diverse array of unit processes and possible sludge treatment
schemes for use in modern wastewater plants. This figure further illustrates the marked
complexity of these systems.
The total system approach to treatment process selection is based on criteria summarized
in Table 2-2 followed by detailed study of the several most likely total plant process
systems using the Quantitative Flow Diagram (QFD) method as outlined in Reference [3].
TABLE 2-2
PLANT PROCESS SELECTION CRITERIA
Factors Considerations
Wastewater Influent Flow, Characteristics
Liquid Effluent Standards, Disposal/Utilization
Sludge Treatment Standards, Disposal/Utilization
Constraints Existing Facilities
Local Environmental—Site Conditions
Economics—Capital and O&M Costs
Operation (Reliability)
Management
2-3
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iSLUDGEJYPEl THICKENING ISTAB^LIZATION|cONDITIONING|DEWATERINGlHEAT DRYINGJ REDUCTION I _FINAL_ I
I I Blending | Reduction | Stabilization | | | Stabilization | Disposal |
to
PRIMARY
COMPOSTING
FILTER PRESS
DRYING BEOS
CENTRIFUGE
FLASH DRYER
ROTARY
VACUUM FILTER \ JMULTIPLE HEARTH\ INCINERATION
CROPLAND
LAND RECLAIM
POWER
GENERATION
ELUTRIATION I \HORIZONTAL FILTERJ
TRAY DRYER I j WET AIR OXIDATION 11 SANITARY LANDFILLI
CYLINDRICAL
SCREEN
SPRAY DRYER
PYROLYSIS
, OCEAN DISPOSAL
FIGURE 2-1. Unit processes—sludge processing and disposal [2].
-------
The principles involved in the process selection method are further illustrated by Dague et
al. [4].
Basic Considerations
In evaluating treatment process alternatives, certain factors should always be considered.
Major factors are the wastewater to be treated, effluent requirements to be met, and
constraints within which the system must function. Each of the factors listed in Table 2-2
must be considered in relation to all other factors. It is not proper, for example, to
consider only the wastewater flow and strength in evaluating a given process. Effluent
requirements and other constraints may make the process less attractive.
Once it has been determined that a given system is capable of meeting treatment
requirements, within existing geographic and facilities constraints, evaluation is then
limited to the three remaining factors in Table 2-2: (1) economic, (2) operational, and
(3) management considerations. As with the other elements, these latter factors must be
considered in concert. The sludge treatment system with the least dollar cost may not be
the best alternative for long-range treatment objectives and this must also be considered.
There are elements upon which a dollar value cannot be placed with any degree of
reliability. This is especially true in view of rapidly changing environmental requirements.
A wastewater system that meets all requirements today may not in the future, should
standards change.
Two elements that do not readily appear as dollar costs, although they may contribute to
somewhat inflated values, are operational and management problems. The dollar costs to
operate and manage one system may be no greater than for another; however, some
systems are inherently more reliable than others.
Process Alternatives
It is common practice to classify wastewater treatment processes according to stages of
treatment. Methods of handling wastewater are classified as preliminary, primary,
secondary, and tertiary treatment. Sludge handling processes can be classified as shown in
Figure 2-1. Within each of these categories there are numerous process alternatives.
The challenge in process selection is the evaluation of the numerous system alternatives.
A first step in the evaluation is to eliminate inappropriate methods. This first cut is based
on considerations listed in Table 2-2. If standards call for a secondary level of wastewater
treatment, plus disinfection, then the engineer may elect to eliminate all tertiary
treatment methods other than disinfection. Similarly, in sludge processing, when the
plant is quite large, the engineer would likely eliminate sand drying beds from detailed
economic analysis. If the plant is small, he would probably eliminate incineration as a
method of sludge treatment. In short, there are certain alternatives that may be expelled
on the basis of engineering judgment. In making the preliminary cut of treatment
2-5
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processes, care must be taken to avoid arbitrary decisions. If the engineer excludes certain
treatment methods from further consideration, he should do so only on the basis of
sound reasoning, experience, and judgment.
Figure 2-2 illustrates the number of system alternatives that can become involved in
analyzing a treatment system. The illustration involves four stages of liquid treatment and
two stages of sludge treatment. The result is 432 system alternatives. It is highly
impractical for the engineer to prepare a preliminary design and economic analysis for
each of these alternatives without the use of a computer. The problem then is one of
computer program development. The computer should only be used to make the
calculations, not the analysis. It should be used to select the best systems, all within the
limits of sensitivity, for the overall analysis. The engineer can then evaluate the more
indeterminate aspects of each system and, finally, select the best alternative.
2.2.4 The Design Team Concept
Increased complexity of wastewater treatment plants has led some of those firms
specializing in this area to adopt the multidisciplinary approach to plant design. The
rationale for this approach is explained in detail by Voysey [5]. It is important from time
to time to take a broad look at the science of wastewater treatment, from aspects other
than those that are purely technical. It is a science involving many disciplines and the
science will develop best if each of those concerned, including operators, consider
themselves as members of design teams. This will undoubtedly result in more efficient
wastewater treatment. Further design procedures, based on the personal contact between
the designer and the operator, will afford improved designs and more efficient operation.
2.3 Sludge Processing and Disposal Nomenclature
To promote understanding and overcome problems of semantics, it is appropriate to
review the meaning and significance of certain terms used to describe various types of
sludges, processes, and equipment used for their treatment and disposal. The most
pertinent publication on wastewater treatment nomenclature is the Glossary—Water and
Wastewater Control Engineering, prepared by the Joint Editorial Board representing the
APHA, ASCE, AWWA, and WPCF. This manual will adhere to that work as closely as
possible.
2.3.1 General Considerations
The Quantitative Flow Diagram (QFD) is an important step in the design of a sludge
system. This term refers to an engineering process flow sheet which describes the total
plant processing system in a quantitative fashion. Such diagrams are essential to analysis
of various alternate total plant processing systems. Their use in the subsequent
preparation of capacity and cost summations for different combinations of unit processes
permits comparison of the cost-effectiveness of such systems.
2-6
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TREATMENT STAGES
A
x /
/i \\ ^r^\
I \v^
EFFLUENT
N = NUMBER OF ALTERNATE SYSTEMS
=2.3.4.3.2.3
= 432
SLUDGE
FIGURE 2-2. Evaluation of system alternatives.
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A material balance is also an essential part of any design. For a detailed explanation the
reader is referred to the text by Hougen et al. [6]. A material balance for a process is an
exact accounting of all materials which enter, leave, accumulate, or are depleted in the
course of a given time of operation. It is, therefore, an expression of the Law of
Conservation of Mass. The preparation of an adequate QFD or the execution of process
control or development is impossible without consideration and use of material balances.
The QFD is essentially a series of interrelated material balances.
Other terms which are relevant to sludge processing are defined as follows:
• Primary sludge is the sludge obtained from a primary settling tank. This
definition was adequate when all treatment was strictly primary treatment.
With the advent of secondary treatment and the recirculation of excess
activated sludge and possibly other sludge laden streams to the primary
sedimentation basins, the term primary sludge is subject to significant
misinterpretation and must be used with care. As originally defined, and in
its true perspective, primary sludge is that portion of the raw wastewater
solids contained in the raw plant influent which is directly captured and
removed in the primary sedimentation process.
• Biomass is a synonym for biological solids.
• Primary sedimentation is usually the first major process in wastewater
treatment works. It is not considered a sludge process. However, in some
cases the primary basins are used to capture and thicken sludge.
• Final sedimentation is used to some degree in thickening of sludge.
• Autogenous incineration refers to the combustion characteristics of a sludge
having a composition (physical and chemical) such that no auxiliary fuel is
required in incineration (except start-up and shutdown).
2.3.2 Sludge Treatment and Disposal—Unit Processes
The following categorization of processes used in treatment and disposal of sludges is set
forth:
• Thickening (Blending)
• Stabilization (Reduction)
• Conditioning (Stabilization)
• Dewatering
2-8
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• Heat drying
• Reduction (Stabilization)
• Final disposal
In classifying and describing sludge processing methods, the potential of a process to
accomplish more than one task must be taken into account. Accordingly, the
nomenclature attempts to recognize that four of the major categories (Thickening,
Conditioning, Dewatering, and Reduction) have primary as well as secondary objectives.
Sludge Thickening (Blending)
The term thickening, herein, will be used to describe an increase in solids concentration,
whether it occurs as the objective of a separate process, or as a secondary effect of a
process provided essentially for a different purpose. Thickening Methods (Blending) are
as follows:
• Gravity
• Flotation
• Centrifugation
Recognition of the need to uniformly blend or combine the two principal types of
wastewater sludges (primary and excess activated), and to keep them combined in plants
where joint processing is practiced, is not as widespread as it should be. Normally, sludge
blending can best be accomplished in a separate sludge thickening process.
Sludge Stabilization (Reduction)
Sludge stabilization processes are aimed at converting raw (untreated) sludges into a less
offensive form with regard to odor, putrescibility rate, and pathogenic organism content.
Major types of processes are:
• Anaerobic digestion
• Aerobic digestion
• Lime treatment
• Chlorine oxidation
2-9
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• Heat treatment
• Composting
Some discussion of each term follows:
Anaerobic and aerobic digestion involve the biological stabilization of sludge through
partial conversion of putrescible matter into liquid, dissolved solids, and gaseous
by-products, with some destruction of pathogens. These processes also reduce the amount
of dry sludge solids. Consequently, these processes result in stabilization and in solids
reduction or conversion.
Lime treatment and chlorine oxidation both control odor and reduce pathogens without
significantly reducing sludge solids.
Heat treatment kills pathogenic organisms. In addition, putrescible organic matter is
substantially dissolved and appears in the cooking liquor from subsequent decantation or
dewatering.
Composting is an aerobic process involving the biological stabilization of sludge. It
provides organic solids, pathogen, and odor reduction.
Sludge Conditioning
Sludge conditioning is pretreatment of a sludge to facilitate removal of water in a
thickening or dewatering process. Methods are as follows:
• Chemical (Inorganic and Organic)
• Elutriation
• Heat treatment
Chemical methods involve the use of inorganic or organic flocculants to promote
formation of a porous, free-draining cake structure. In this way, the flocculants improve
sludge dewaterability, alter sludge blanket properties, and improve solids capture. In
dewatering, flocculants increase the degree of solids capture both by destabilization and
agglomeration of fine particles and facilitate cake formation. The resultant cake becomes
the true filter media. In thickening processes, the flocculants promote more rapid phase
separation, higher solids contents, and a greater degree of capture.
Elutriation is the process of washing the alkalinity out of anaerobically digested sludge to
decrease the demand for acidic chemical conditioners and to improve settling and
dewatering characteristics. When used with primary sludge, the process is cost-effective
2-10
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and does not create undesirable effects. When elutriation is used in a plant which
combines primary and excess activated sludge prior to digestion, the mixed sludge
fractionates during the elutriation process, producing a highly polluted elutriate. The
process has been criticized because this elutriate was bypassed into the plant effluent at
some plants. However, use of flocculants in elutriation can eliminate the problem of the
polluted elutriate.
Heat treatment, herein, refers to the pressure cooking of sludges in such a manner that
little sludge oxidation occurs. The Porteous, Farrer, Zurn, and some Zimpro systems fall
into this category. Thus, heat treatment is distinct from wet air oxidation which generally
involves higher temperatures and pressures, with air injection to promote a major degree
of sludge oxidation.
Dewatering Methods
Any process which removes sufficient water from sludge so that its physical form is
changed from essentially that of a fluid to that of a damp solid, is a dewatering process.
Methods used in dewatering are best described by the equipment employed and some
major types are listed below and are discussed in detail in Chapter 7:
• Rotary vacuum filters
• Centrifuges
• Drying beds
• Filter presses
• Horizontal belt filters
• Rotating cylindrical devices
• Lagoons
Heat Drying of Sludge
Sludge drying processes involve the application of heat to evaporate sufficient moisture
and render the sludge dry to the touch and relatively free flowing. It is normal practice to
conserve energy by dewatering the sludge prior to heat drying. Principal types of dryers
are:
• Multiple hearth
• Flash dryers
2- 11
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• Tray dryers
• Spray dryers
Sludge Reduction
Sludge reduction, as defined here, pertains to processes which primarily yield a major
reduction in the volatile sludge solids. Principal methods of sludge reduction are:
• Incineration
• Wet air oxidation
• Pyrolysis
Final Disposal Methods
Final or ultimate disposal refers to the disposition of sludge in liquid, cake, dried, or ash
form, as a residue to the environment. Principal methods are:
• Cropland application
• Land reclamation
• Power generation (with solid waste)
• Sanitary landfill
• Ocean disposal
The first three methods are also utilization procedures. In instances where sanitary
landfills are used for purposes of topographic modification, this also could be construed
as utilization.
2.3.3 Sequence and Functions of the Unit Processes
Figure 2-3 summarizes the purposes and sequence of unit processes of wastewater sludge
treatment.
2.4 References
1. Gale, R. S., "Recent Research on Sludge Dewatering." F-iltr. Separ. (Sep.-Oct.
1971), pp. 531-538.
2-12
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UNIT PROCESSES
FUNCTIONS
Thickening
(Blending)
Stabilization
(Reduction)
Conditioning
(Stabilization)
Water Removal
Volume Reduction
Post Process Efficiencies
Blending
Pathogen Destruction
Volume and Weight Reduction
Odor Control
Putrescibility Control
Gas Production
Improve Dewatering or Thickening Rate
Improve Solids Capture
Improve Compactability
Stabilization
Dewatering
Heat Drying
Reduction
(Stabilization)
Water Removal
Volume and Weight Reduction
Change to Damp Cake
Reduces Fuel Requirements for Incineration/
Drying
Water Removal
Sterilization
Utilization
Destruction of Solids
Water Removal
Conversion
Sterilization
Final Disposal
Utilization (Cropland)
Utilization (Energy)
Utilization (Land Reclamation)
Disposal (Landfill)
Disposal (Ocean)
FIGURE 2-3. Enumeration of sludge treatment processes and their functions.
2- 13
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2. Stanley Consultants, "Sludge Handling and Disposal, Phase I-State of the
Art." Report to Metropolitan Sewer Board of the Twin Cities Area, Nov. 15,
1972.
3. Camp, Dresser & McKee, Inc., "Municipal Wastewater Treatment Plant Sludge
and Liquid Sidestreams." Preliminary Technical Bulletin, for the EPA, Contract
68-01-0324, 1973.
4. Dague, R. R., Walker, J. T., and Moritz, P. J., "Evaluation of Treatment Process
Alternatives: Two Case Studies." Presented at the 17th Great Plains
Wastewater Design Conference, Omaha, Nebraska, Mar. 27, 1973.
5. Voysey, J. A., "Overall Design Procedures, with Particular Reference to
Contact Between Designer and Operator." Water Pollut. Contr. (1973), pp.
231-234.
6. Hougen, O. A., Watson, K. M., and Ragatz, R. A.,Chemical Process Principles
(2nd ed., pt. 1). John Wiley & Sons: New York (1954).
2-14
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CHAPTER 3
OCCURRENCE OF SLUDGES AND PHYSICAL AND CHEMICAL PROPERTIES
RELATING TO PROCESSABILITY
3.1 Occurrence of Sludges—Conventional Biological Treatment
The various wastewater treatment processes produce different amounts and types of
sludges. Table 3-1 presents data on typical volumes of sludges produced in several
conventional treatment processes.
TABLE 3-1
TYPICAL SLUDGE VOLUMES PRODUCED
Gallons of Sludge Produced/Million Gallons Wastewater Treated
Wastewater
Treatment Process
Primary sedimentation
Trickling filter
Activated sludge
Keefer [ 1 ]
2,950
745
19,400
Fair&Imhoff[2]
3,530
530
14,600
Babbitt [3]
2,440
750
18,700
McCabe and
Eckenfelder[4]
3,000
700
19,400
This table illustrates the striking increase in the volume of sludges to be processed when a
plant is upgraded to activated sludge treatment.
Table 3-2 illustrates typical masses or weights of sludges produced by various
conventional methods of treatment.
TABLE 3-2
TYPICAL SLUDGE MASSES [5]
Wastewater
Treatment Process
Percent
Suspended Solids
Removed by
Process
Pounds of Solids
Generated by Process
per Million Gallons
Treated
Specific Gravity of
Suspended Solids
Primary sedimentation
Trickling filter
Primary plus activated
sludge
60
30
92
1,020
510
1,563
1.33
1.52
1.33
3-1
-------
Comparison of Tables 3-1 and 3-2 shows that of the three most common sludge types,
excess activated sludge is by far the most voluminous. The large volume is due primarily
to its low solids concentration. This point is further illustrated in Table 3-3.
TABLE 3-3
TYPICAL WATER CONTENT OF SLUDGES [6]
Percent Moisture
Wastewater of Sludge Ib Water/lb
Treatment Process Generated Sludge Solids
Primary sedimentation 95 19
Trickling filter
Humus-low rate 93 13.3
Humus-high rate 97 32.3
Activated sludge 99 99
While the data in Tables 3-1, 3-2, and 3-3 are typical, consideration must still be given to
the analysis of a particular wastewater and process efficiency in order to accurately
estimate sludge quantities.
3.2 Occurrence of Sludges—Combined Biological and Chemical Treatment
Addition of metallic salts at various points within conventional biological treatment
systems, for upgrading levels of treatment or phosphorus removal, results in the
production of additional quantities of sludges. Increased amounts of sludge result
primarily from precipitation of insoluble metallic compounds, largely phosphates. Table
3-4 delineates an estimation procedure for calculating the increase in sludge mass when
iron or aluminum are added at various points in the wastewater treatment sequence.
3-2
-------
TABLE 3-4
SAMPLE CALCULATION FOR ESTIMATING SLUDGE MASS (Ib/M.G.) [7]
Conventional
Feto Feto Alto Al to TF
Primary Aerator Aerator Clarifier
Primary
SS removal
Sludge solids
Fe solids
Al solids
Total
Activated Sludge
Secondary solids
Fe solids
Al solids
Total (System)
Trickling Filter
Secondary solids
Al solids
Total (System)
Cation/P Dose
(mol/mol)
50% 75% 50% 50% 50%
1,250 1,875 1,250 1,250 1,250
0 605
0
1,250 2,480 1,250 1,250 1,250
715 536
1,965 3,016
656
1,906
Basis for Sludge Mass Calculation
804
541
2,595
804
425
2,479
745
483
2,478
Ib Chemical Sludge/lb Cation
Ib/lb Al Ib/lb Fe
1.5
1.75
3.9
3.8
2.4
2.3
Assumptions
Cation/P Dose = 1.5 mol/mol to aerator
Cation/P Dose = 1.75 mol/mol to primary or before trickling filter clarifier
Influent Wastewater
BOD = 230mg/l
SS = 300mg/l
P = lOmg/1
3-3
-------
The quantity of sludge resulting from the use of lime in either primary or tertiary
treatment can be estimated from wastewater analyses as shown in Table 3-5.
TABLE 3-5
SAMPLE CALCULATION OF SLUDGE QUANTITY FROM
LIME TREATMENT OF WASTEWATER [8]
Lime Added Primary Treatment
Data Required:
On influent and effluent: alkalinity, pH, calcium hardness, phosphorus.
Change in Ionic Content
(Influent - Effluent)
HCO3, as CaCO3
CO2, as CaCO3
Mg, as CaCO3
mg/1
223
14
66
Sludge Produced
hydroxyapatite
CaC03
Mg (OH)2
mg/1
27
460
38
Total Calculated Sludge 525
Meas./Calculated
Material Balance on Ca
Ca(OH)2 dose = 390 mg/1
Input-Output = 2.9 mg/1
1.25
Following is a recent review of thirteen actual case studies giving the sludge production
data on primary, secondary, and tertiary phosphorus removal systems [7].
3-4
-------
lable 3-6 illustrates the additional sludge to be processed when chemicals are added to
the primary tanks for phosphorus removal.
TABLE 3-6
ADDITIONAL SLUDGE TO BE HANDLED WITH CHEMICAL TREATMENT
SYSTEMS. PRIMARY TREATMENT FOR REMOVAL OF PHOSPHORUS
Sludge Production
Parameter
Level of chemical
addition (mg/1)
Percent sludge
solids
Ib/M.G.
gal/M.G.
Low
Conventional Lime Addition to
Primary Primary Influent
Mean
Range
Mean
Range
Mean
Range
0
5.25
5.0-5.5
788
600-950
4,465
3,600-5,000
350-500
11.1
3.0-19.5
5,630
2,500-8,000
8,924
4,663-18,000
High
Lime Addition to
Primary Influent
800-1,600
4.4
2.1-5.5
9,567
4,700-15,000
28,254
16,787-38,000
Aluminum
Addition to
Primary Influent
13-22.7
1.2
0.4-2.0
1,323
1,200-1,545
23,000
10,000-36,000
Iron Addition to
Primary Influent
25.8
2.25
1.04.5
2,775
1,4004,500
21,922
9,000-38,000
Table 3-7 shows similar data for chemical additions to the secondary system for
phosphorus removal.
TABLE 3-7
ADDITIONAL SLUDGE TO BE HANDLED WITH CHEMICAL TREATMENT
SYSTEMS: PHOSPHORUS REMOVAL BY MINERAL ADDITION TO AERATOR
Sludge Production
Parameter
Level of chemical
addition (mg/1)
Percent sludge
solids
Ib/M.G.
gal/M.G.
Mean
Range
Mean
Range
Mean
Range
Al+++ Addition
Conventional
Secondary
0
0.91
0.58-1.4
672
384-820
9,100
7,250-12,300
to Aerator
With Al+++
Addition
9.4-23
1.12
0.75-2.0
1,180
744-1,462
13,477
7,360-20,000
Fe+++ Addition
Conventional
Secondary
0
1.2
1.0-1.4
1,059
918-1,200
10,650
10,300-11,000
to Aerator
With Fe+++
Addition
10-30
1.3
1.0-2.2
1,705
1,100-2,035
18,650
6,000-24,000
3-5
-------
Data in Table 3-8 indicate the additional sludge resulting from tertiary phosphorus
removal with chemicals.
TABLE 3-8
ADDITIONAL SLUDGE TO BE HANDLED WITH CHEMICAL TREATMENT
SYSTEMS: PHOSPHORUS REMOVAL BY MINERAL
ADDITION TO SECONDARY EFFLUENT
Sludge
Production
Parameters
Level of chemical
addition (mg/1)
Percent sludge
solids
Ib/M.G.
gal/M.G.
.
Mean
Range
Mean
Range
Mean
Range
Lime
Addition
268-450
1.1
0.6-1.72
4,650
3,100-6,800
53,400
50,000-63,000
Aluminum Iron
Addition Addition
16 10-30
2.0 0.29
2,000 507
175-781
12,000 22,066
6,000-36,000
In summation:
• Lime addition in the primary causes the greatest increase in sludge mass production.
• The minimum increase in sludge mass comes from use of alum in the aeration basins.
• Sludge mass and volume depend critically on wastewater characteristics and clarifier
performance.
• Selection of a treatment chemical and point of application should take into account
the relative sludge processability as well as added sludge mass.
3.3 Physical and Chemical Properties Relating to Processability
This section relates the measurable physical and chemical properties of types of sludges to
their processing characteristics. Further information on sludge conditioning can be found
in Chapter 6.
3-6
-------
3.3.1 Factors Affecting Processing
The sludge characteristics affecting processability are:
• Particle density
• Particle size distribution
• Surface charge
• Degree of hydration
• Compressibility (blanket and cakes)
Particle density is most important in sludge thickening since it affects the subsidence rate.
Particle size distribution affects both thickening and dewatering. As the average particle
size decreases, the surface/volume ratio increases exponentially. Increased surface area
means greater hydration, higher chemical demand, and increased resistance to dewatering.
Particle size distribution also affects compressibility.
The two principal factors promoting stable dispersions of solids in liquids are surface
charge and hydration. Surface charge must be neutralized or circumvented to promote
flocculation. Regardless of surface charge, other surface effects can cause solid particles
to adsorb film layers of water (hydration) and this also can adversely affect moisture
removal processes. Wastewater sludges all contain dispersed solids with some degree of
surface charge and hydration. It is an unfortunate fact that solids contained in essentially
all municipal wastewater sludges yield compressible and hydrophilic sludge cakes unless
the sludge prior to dewatering has been coagulated or flocculated. Cake compressibility
and migration of unstructured fines inhibits water removal in thickening and dewatering.
3.3.2 Particle Size and Configuration
Figure 3-1 shows the relative particle size of common materials. Fines and colloidal
particles are present in raw wastewater and practically all escape capture in primary
basins. The activated sludge process, in addition to removal of dissolved BOD, functions
to capture, remove, and partially metabolize these materials. Unfortunately, some of the
various wastewater processes, both liquid and solid, tend to create additional fines. The
other major processes for removing fines are coagulation and flocculation. In those rare
instances where some form of treatment is not required to promote thickening and
dewatering, relatively unhydrated materials abound in the sludge solids. Table 3-9 depicts
the relative difficulty of removing water from an unflocculated primary digested sludge
containing various particle size fractions.
3-7
-------
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TABLE 3-9
SLUDGE DEWATERING AS A FUNCTION
OF PARTICLE SIZE [9]
Mean Diameter Specific Resistance Coefficient Percent of
(microns) (sec2 /g) Compressibility Total Particles
Original, unfractionated 10.4 X 109 0.66
sample
Above 100
5-100
1-5
Below 1
2.3 X 109
4.6 X 109
13.8 X 109
-
0.73
0.70
0.42
-
10.2
75.5
8.5
5.9
As can be seen, the specific resistance to filtration of the unfractionated sludge is
dominated by the specific resistance of material under 5 microns in size, even though this
material constitutes only about 14 percent by weight of the total solids. Specific
resistance is in effect, a measure of the relative dewaterability of a sludge. It has been
defined as the pressure required to produce a unit rate of flow through a cake having a
unit weight of dry solids per unit area when the viscosity of the liquid is unity. Specific
values are determined from laboratory filtration experiments. The seeming paradox on
the coefficient of compressibility is tempered by the facts that:
• The significant compressibility is that of the unfractionated cake due to interactions
of the fractions.
• All of the fractions are sufficiently compressible to prevent unflocculated
dewatering.
• Greater compressiblity of coarser particles may result from some of these coarser
particles being agglomerates of smaller particles. In any event, the much lower
specific resistance of the coarser particles completely overshadows the
compressibility difference. Apparently, the water retention characteristics of the
fines must be the dominant factor.
Table 3-10 depicts typical specific resistance values for different types of sludges, both
coagulated and uncoagulated. Since the maximum specific resistance for feasible
mechanical dewatering is normally quoted at 1.0 X 108 (sec2/g), none of these sludges
would be readily dewaterable. Table 3-10 shows that specific resistance values can vary
significantly. The actual value for a particular sludge will be a function of both its origin
and nature. Broad experience indicates that properly conditioned raw primary sludge is
almost always the most readily dewaterable, followed by well-conditioned, digested, and
activated sludges in increasing order of difficulty.
3-9
-------
TABLE 3-10
SPECIFIC RESISTANCE OF VARIOUS TYPE SLUDGES [9]
Specific Resistance
Type Sludge (sec2 /g)
Raw 10-30 X 109
Raw (coagulated) 3-10 X 107
Digested 3-30 X 109
Digested (coagulated) 2-20 X 10 7
Activated 4-12 X 109
3.3.3 Surface Charge and Degree of Hydration
Both thickening and dewatering require aggregation or packing of individual particles into
larger clumps. Destabilization or flocculation occurs only after the dispersed particles'
normal net negative charge is essentially neutralized and some degree of surface water
desorption and particulate bridging has been brought about.
The desirable changes are brought about in three ways:
• Natural flocculation in and by activated sludge organisms through the mechanism of
excretion of natural polymeric materials.
• Use of synthetic organic polymer flocculants.
• Use of inorganic metal salts as primary coagulants.
The amount of polymeric flocculants excreted by microorganisms in the activated sludge
process is directly related to the length of aeration. In turn, the settleability of activated
sludge solids is a function of the natural polymer present and this is illustrated in Table
3-11.
3-10
-------
TABLE 3-11
VARIATION OF SVI WITH AERATION TIME [10]
Process Variation
Sludge
Volume Index
Aeration Period
(hr)
High rate activated sludge
Conventional activated sludge
Extended aeration
130-480
80-130
50-80
3-6
6-7.5
16-24
Table 3-11 shows that increased aeration times yield a more readily settleable excess
activated sludge, as measured by the sludge volume index (SVI). These results should not
be used in a dogmatic fashion. Recent evidence shows an optimum aeration period
usually exists for a given wastewater treatment system and prolonged aeration and
overproduction of naturally generated polymer can be detrimental. Pilot scale work is still
the best determinant.
Additional insight into the effect of processing variations is illustrated by the data in
Table 3-12.
TABLE 3-12
EFFECT OF STORAGE TIME ON SLUDGE DEWATERABILITY [11]
Storage Time
(days)
Temperature
Dewatering
(gpm)
Chemical Dosage
(mg/1)
8
None
40
90-100
22
66
150
100
These tests show that storage of a digested mixed sludge can have a drastic effect on
dewaterability. These results were not significantly affected by temperature.
3-11
-------
Estimated distribution of aqueous fluid in a digested mixed sludge is shown in Table 3-13.
TABLE 3-13
AQUEOUS FLUID DISTRIBUTION IN DIGESTED SLUDGE [ 12]
Percent Water
Percent Solids
Between cell water
Adhesion and capillary water
Adsorption and intracellular fluid
70
22
5
15
40
Generally, thickening removes most of the first type of water shown in Table 3-13.
Dewatering can remove most of the adhering and capillary water yielding a solids content
of 20 to 40 percent. Further dehydration requires drying or combustion.
3.3.4 Compressibility and Water Retention
Filtration is the operation of separating a heterogeneous mixture of fluid and solid
particles by means of a filter medium which permits the passage of the fluid but retains
most of the particles. Personnel engaged in work on wastewater sludge dewatering should
be familiar with the facts that cake filtration is the prevalent separation mechanism and
wastewater sludge cakes are practically always compressible and hydrophilic unless the
sludge has been previously conditioned. Cake formation rate, porosity, structure, and
water release characteristics are the dominant factors in cake filtration, not the open
space or tightness of the media. Figures 3-2 and 3-3 show the effect of compressibility on
the pressure drop through incompressible and compressible filter cakes.
In the idealized incompressible cake (see Figure 3-2) the voidage is constant throughout;
even though a pressure gradient exists, the particles do not deform or decrease porosity.
There is no dramatic increase in resistance to filtration as a function of time. In the case
of the compressible cake (see Figure 3-3) the particle deformation will decrease voidage
and thus increase resistance to filtration. Degree and rate of deformation, hence resistance
to filtration, will be more pronounced at the bottom of the cake. Compressibility and
water retention characteristics of wastewater sludge cakes are materially decreased by the
various conditioning methods to facilitate dewatering. This is because conditioning
procedures eliminate or agglomerate fine particles to minimize their decreasing cake
voidage and drainability.
3- 12
-------
FLUID FLOW
POSITION IN CAKE
CAKE <
I PRESSURE IN
I LIQUID PHASE
Pe
FIGURE 3-2. Fluid flow through an incompressible sludge cake [ 13].
-------
FLUID FLOW
POSITION IN CAKE
COMPRESSED
CAKE
PRESSURE IN
LIQUID PHASE
PC
FIGURE 3-3. Fluid flow through a compressible sludge cake [ 13].
-------
3.4 Plant Experiences with Various Processes and Types of Sludge
This section relates experience on specific types of sludges and processability. In the case
of sludge dewatering, additional material may be found in Chapter 7.
3.4.1 Raw Primary Sludge
The nature of primary sludge resulting from the initial clarification step varies to some
degree with the makeup of the collection system, ambient temperature, and relative
amounts and types of industrial wastes included. However, in nearly all cases, primary
sludge is fairly coarse and fibrous in nature, compacts well in thickening processes,
dewaters easily, and facilitates autogenous incineration when that method of solids
reduction is employed. Figure 3-4 is a close-up photo of primary filter cake.
Thickening of raw primary sludges, preferably carried out immediately in a gravity
thickener, is usually an efficient and simple process. Significant savings usually result in
subsequent processes such as digestion and dewatering.
Figure 3-5 illustrates the appearance of raw primary sludge cake during dewatering on a
vacuum filter belt. Note the excellent release from the filter belt. The heavy, thick, and
relatively dry filter cake affords a significant degree of ease in the operation.
As indicated in Table 3-14, dewatering of thickened raw primary sludge by vacuum
filtration is an efficient unit operation. It gives high yields at low costs and excellent
solids capture prevents recirculation load problems.
TABLE 3-14
TYPICAL PERFORMANCE DATA FOR THE
VACUUM FILTRATION OF RAW PRIMARY SLUDGE
Chemical
Conditioner Percent Sludge Cost Yield Cake Solids
Used Solids ($/ton)* (Ib/ft2/hr) Solid (%) Capture (%)
Cationic
polymer 1 0
1.67 10 32 90-95
Based on 1971 cost figures.
3-15
-------
FIGURE 3-4. Raw primary filter cake.
-------
10
I
*-»
-J
FIGURE 3-5. Removal of raw primary sludge cake from a vacuum filter.
-------
While primary sludges do give somewhat compressible filter cakes, they are easy to
condition with polyelectrolytes because of sufficient gross solids (~ 30 percent > 30
mesh). These solids permit rapid formation of filter cake, an adequate structural matrix
to provide excellent solids capture, and a rapid dewatering. Rapid cake forming tendency
of conditioned raw primary sludge also affords use of relatively coarse filter media with a
high percentage of solids capture.
3.4.2 Effect of Anaerobic Digestion on Primary Sludge Dewatering
Handling characteristics of primary sludges are not greatly changed by either standard or
high-rate digestion processes. The sludge still settles, compacts, and dewaters easily.
However, experience in many plants does indicate that digestion yields a sludge which is
slightly more difficult to dewater than raw primary sludge. Typical operating data are
presented in Table 3-15. When compared to data in Table 3-14, costs for filtering digested
sludge are higher and yields poorer than for raw primary sludge. Operating characteristics
during dewatering are about the same with relatively trouble-free operation being normal.
TABLE 3-15
TYPICAL PERFORMANCE DATA FOR
VACUUM FILTRATION OF DIGESTED PRIMARY SLUDGE
Percent Sludge Conditioner Yield Cake Solids
Solids Cost($/ton)* (Ib/ft2/hr) Solids (%) ' Capture (%)
12.7 2.64 7.4 28 90+
Based on 1971 cost figures.
3.4.3 Activated Sludge from Conventional Air Systems
Conventional air systems yield activated sludges that are inherently more variable in
nature than primary sludge. The principal source of variation is the configuration and
mode of operation of the particular system. However, it is possible to generalize, within
limits, on the nature and handling characteristics of biomass from essentially domestic
sanitary waste.
3- 18
-------
Activated sludge is finer in particle size than primary sludge. It is normally comprised of
60 percent to 90 percent or more cellular organic material and contains a very large
amount of water. Elemental particles of activated sludge are usually aggregated to some
extent through bioflocculation. The settling process for activated sludge is hindered
because of interparticle interferences and fluid forces resulting in occurrence of zone
settling. Further, zone settling rate and compaction tendency are sensitive to the degree
of flocculation of the activated sludge. Accordingly, the degree of flocculation of
activated sludge achieved, in either an aeration basin via bioflocculation or by the use of
flocculants, will affect the settling characteristics as well as the degree of solids capture.
Tenney, Echelberger, Coffey, and McAloon [14] illustrated the effect of biokinetic
parameters on settling and dewatering properties of resultant activated sludge. Figure 3-6
illustrates the marked effect of the microogranism growth phase on the percent dispersion
or conversely, the degree of bioflocculation occurring.
Concurrent data on the amount of natural biopolymer produced by the microorganisms
and consequent effects on dewaterability of the activated sludge are shown in Figure 3-7.
These figures show that processing characteristics of excess activated sludge and the
amount which has to be processed are functions of the particular treatment plant's
operation. While Tenney et al. [14] indicate that prolonged aeration and the associated
accumulation of natural biopolymer result in more efficient drainability, the work by
Randall et al. [15] shows that an optimum aeration time and biopolymer accumulation
occur in about 120 hours of aeration. Further aeration worsens drainability. Though
foregoing examples show the beneficial effect of a long aeration period, plants do not
normally operate activated sludge systems in such a manner that significant aerobic
digestion and improved processing characteristics result. Additional readings on the
subject of biopolymer effects on activated sludges are listed at the end of this chapter.
Biomass tends to float rather than sink after being subjected to shear in various handling
processes such as pumping. The degree of bioflocculation acheived can also help
considerably in overcoming effects of shear during handling. In some cases,
well-flocculated aggregates of activated sludge have been identified even after anaerobic
digestion and elutriation.
Experiences have shown that gravity thickening of activated sludge is not a practical
operation, particularly if the sludge has been exposed to shear and is not well
bioflocculated. Biomass, however, is very amenable to flotation thickening and
production of a 4 to 6 percent solids sludge is routine. It should also be noted that the
rheological properties of thickened activated sludge are much different from those of
primary sludge.
3-19
-------
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-------
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3.4.4 Rationale of Design for Some Existing Activated Sludge Plants
Previous observations on nature and handling characteristics of primary sludges and
excess activated sludges are important to the design and successful operation of plants
combining the two distinctly different sludges early in the solids handling process. In
many cases, plants have been designed and constructed to recirculate excess activated
sludge to the head of the plant and mix it with primary solids in the primary clarifier. The
mixed sludge is then processed in a manner analogous to methods previously described
for straight primary sludges. Figure 3-8 illustrates such a design.
Since settling and compaction properties of the excess activated sludge as previously
described prevail and dominate throughout the system, processing problems occur. Solids
capture in primary basins almost always degrades due to recirculation of activated sludge.
This results in a greater load on the secondary system and production of more excess
sludge. The combined sludges after anaerobic digestion do not settle well, and this results
in a high solids supernatant and a serious solids recirculation problem. When subjected to
elutriation, the combined sludge fractionates with resultant poor solids capture and an
additional solids recirculation problem. Further, the combined sludges do not compact
well without elutriation and this means the dewatering operation must handle a low solids
concentration sludge at high costs and low yields.
In other cases plants have been designed so that the excess activated sludge is mixed with
primary sludge just prior to gravity thickening, digestion, elutriation, and dewatering.
Such a plant is illustrated in Figure 3-9. Though the inclusion of an early thickening step
aids primary digester operation, subsequent solids processing steps are still plagued with
the same problems previously described. Difficulties experienced in plants which operate
in this manner are described more fully in the case studies presented in Chapter 10. In
these case studies, remedial measures were taken to make the sludge systems function.
Different conceptual designs could give lower costs.
3.4.5 Improvements in Mixed Sludge Processing
The design of air activated sludge systems to provide some degree of endogenous
respiration and improved sludge processability was discussed previously. It is generally
beneficial to keep air activated sludge out of the primary sludge processing stream until
just prior to the dewatering step. Assuming normal primary sludge quality and yield, and
excess air activated sludge in normal ratio and quality, the system depicted in Figure 3-10
would appear to be advantageous when raw dewatered sludge is acceptable for final
disposal.
This design has the advantage of a separate thickener for each of the sludges which after
blending provides the filter with a high solids concentration sludge. Efficient dewatering
is promoted with minimization of the deleterious effect of the heavy spiralling
3-22
-------
INFLUENT
tsi
GRIT
REMOVAL
i
1
PRIMARY
CLARIFIERS
r
i
AERATION
BASINS
FINAL
CLARIFIERS
1
I
: 4
; L
t '
ANAEROBIC
DIGESTION
ELUTRIATION
I I
VACUUM
FILTERS
TO FURTHER
PROCESSING
WASTE WATER
SLUDGE
PROCESS LIQUIDS
FIGURE 3-8. Activated sludge plant where EAS is recirculated to primary clarifiers.
-------
INFLUENT
OJ
*.
PUMPING
GRIT
REMOVAL
U
II
1
PRIMARY
CLARIFICATION
i
i
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SLUDGE
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f
i
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TO FURTHER
PROCESSING
WASTE WATER
SLUDGE
PROCESS LIQUIDS
FIGURE 3-9. Activated sludge plant where EAS is mixed with primary sludge prior to thickening
and digestion.
-------
INFLUENT
GRIT
REMOVAL
PRIMARY
BASINS
THICKENER
K)
AERATION
BASINS
SLUDGE
CONDITIONER
EFFLUENT
FINAL
CLARIFIERS
SLUDGE
BLEND
TANK
FLOTATION
THICKENER
25-30%
FILTER CAKE
FIGURE 3-10. Improved sludge processing scheme for an activated sludge plant.
-------
recirculation loads associated with the two previously mentioned design schemes. The
provision of separate flotation thickening of excess air system activated sludge is
necessary to adequately thicken this material on a reliable basis.
In the event that higher than normal ratios of air system activated sludges must be
processed, inclusion of aerobic digestion should be considered for reducing the organic
solids. Post aerobic digestion thickening is usually required and accomplished in a second
stage. Such a system is illustrated in Figure 3-11.
Use of aerobic digestion to reduce the amount of excess activated sludge to be further
processed, while maintaining or improving its inherent processability, has been
demonstrated at several plants. Metro Denver [16] is a particularly graphic example.
Where there are existing anaerobic digestion facilities, or all sludge streams must be
stabilized prior to dewatering for final disposal, the process illustrated in Figure 3-12
might well be optimum.
In this system activated sludge is excluded from the anaerobic digester train. Blending of
the two stabilized sludges just prior to dewatering eliminates problems associated with
attempts to process mixed sludges.
3.4.6 Processing of Mixed Primary and Oxygen Activated Sludges
The oxygen activated sludge process is a staged co-current gas-liquid flow system with an
improved capability for complete and simple contacting of pure oxygen with biomass. It
is more favorable than conventional air processes for endogenous respiration and hence
well bioflocculated sludge. Settling rate data for high purity oxygen and air biomasses are
shown in Figure 3-13.
It would be expected that improved sludge characteristics afforded by pure oxygen would
carry over into subsequent thickening and dewatering operations. Experience shows that
this has occurred. Gravity thickening data follows in Table 3-16.
TABLE 3-16
GRAVITY THICKENING DATA FOR AIR AND
OXYGEN ACTIVATED SLUDGES [18]
Feed
Type
Oxygen EAS
AirEAS
Oxygen mixed
Air mixed
Sludge
Percent Solids
1.7
0.9
2.3
1.1
Solids Loading
(Ib/ft2/day)
10
20
--
20
Underflow Cone.
Percent Solids
4.8
1.4-2.8
5.6
3.3(4.4)fc
Location
Louisville5
Chicago
Middlesex
Chicago
"Pilot plant data.
Data in parentheses obtained with use of Picket Stirrer.
3-26
-------
INFLUENT
OJ
PRIMARY
CLARIFICATION
r
i
AERATION
BASINS
1 1
1 U ,
1
*
GRAVITY
THICKENER
-*•
BLEND
CONDITION
*-
1
1
t
DEWATER
FINAL
CLARIFIERS
1
I *
1
1
•
^m
AEROBIC
DIGESTION
(2 STAGE)
TO FURTHER PROCESSING
FIGURE 3-11. Use of aerobic digestion to reduce activated sludge solids in mixed sludge processing.
-------
00
PRIMARY
CLARIFICATION
1
1
1
*
i
i *
I
1
AERATION
BASINS
U —
— — _
i
ANAEROBIC
DIGESTION
(2 STAGE)
-*•
BLEND
CONDITION
i
+ "
r
DEWATER
FINAL
CLARIFIERS
1
^-
I *
k |
t
•
M
AEROBIC
DIGESTION
(2 STAGE)
TO FURTHER PROCESSING
FIGURE 3-12. Improved use of anaerobic and aerobic digestion in mixed sludge processing.
-------
lO.O—i
UJ
o
z
Z 1.0—
UJ
to
_J
<
t
Z
0.1
OXYGEN
BIOMASS
AIR
BIOMASS
1,000
i I I i i 111 i ii
10,000
CONCENTRATION (mg/1)
""I
100,000
FIGURE 3-13.
Settling characteristics for air and oxygen biomass
(initial settling rate vs. concentration) [ 17].
3 -29
-------
While this data is admittedly on different sludges, the following data in Table 3-17 on air
flotation thickening also indicates the improved processing properties for oxygen
activated sludge.
TABLE 3-17
FLOTATION THICKENING DATA FOR AIR AND
OXYGEN ACTIVATED SLUDGES [18]
Feed Sludge
Type
Oxygen activated
Air activated
Percent Solids
1.7
0.9
Polymer
(Ib/ton)
2.9
9.0
Loading
(Ib/ft2/hr)
6.4-10.2
2.0-4.0
Thickened
Solids (%)
6.6
4.5
A typical comparison of dewatering information is shown in Table 3-18. This pilot data
also indicates improved processability of oxygen activated sludge.
TABLE 3-18
CENTRIFUGATION DATA FOR OXYGEN AND
CONVENTIONAL AERATION SLUDGES [18]
Type Sludge
Oxygen EAS
AirEAS
Feed
Percent Solids Rate (gpm)
2.5 95
1.0 60
Polymer
(Ib/ton)
3
12.5
Solids
Capture (%)
92
82
Cake
Solids (%)
9
8.5
In the case of the Fairfax County, Virginia, Westgate Plant, the improved aspects of
oxygen activated sludge have enabled use of the process flow diagram shown in Figure
3-14. This type system has shown significant economies. The Westgate Plant history is
more fully described in Chapter 10.
3-30
-------
INFLUENT
PRIMARY
CLARIFICATION
1
I--
1
1
J_
AERATION
BASINS
FINAL
CLARIFIERS
1
t
u>
BLEND
THICKENER
CONDITION
STABILIZER
DEWATER
•*
TO FURTHER
PROCESSING
FIGURE 3-14. Oxygen activated sludge system at Westgate treatment plant.
-------
3.4.7 Cake Release in Dewatering
A very important sludge processing characteristic which is sometimes overlooked, is the
relative ease of release of filter cake from filter media. Failure to adequately consider this
property has resulted in considerable difficulty and excess costs at several plants. While it
is sometimes possible to alter this property of sludge by process changes, an important
consideration is selection of the type of filter media.
Of the three types of rotary vacuum filters available, only the belt filter causes a severe
problem with cake release. In contrast to the drum filter which features a scraper blade
discharge from a blow-back section, and the coil filter which uses tines for release, the
belt filter has essentially no release mechanism. Among plants which have experienced
this problem are: Indianapolis; Baltimore; Richmond, California; Toronto; Washington,
D.C.; and Columbus, Ohio. A direct comparison has occurred at Washington, D.C. Table
3-19 shows production rates and chemical costs associated with use of drum filters versus
belt filters at Washington, D.C.
TABLE 3-19
PERFORMANCE DATA FOR DRUM FILTERS AND
BELT FILTERS AT WASHINGTON, D.C.
Type Filter
Drum
Belt
Drum
Belt
Yield
(Ib/ft2/hr)
3.5
3.5
4.0
4.0
Flocculation Cost
($/ton)*
5.65
8.80
3.95
7.75
'Based on 1971 cost figures.
As can be seen, the belt filters consumed substantially more ferric chloride to promote
release than the drum filters with the identical sludge.
3-32
-------
3.4.8 Phosphorus Removal Process Sludges
As noted earlier, application of metallic salts at various points in biological treatment
plants for phosphorus removal creates new and additional sludges. Thorough plant scale
data on the processing characteristics of such sludges are not widespread. Plant scale work
at the North Toronto, Ontario plant using ferric chloride has produced some results of
note. The ferric chloride was applied at the end of the aeration basins.
TABLE 3-20
<*
DEWATERING DATA ON NORTH TORONTO
FERRIC CHLORIDE-ACTIVATED SLUDGE [19]
Chemical Addition
Metal Salt
Dose (mg/1)
Mixed Sludge
Percent Solids
Cond. (Ib/ton)
Ferric
Chloride Lime
Filter Yield
(Ib/ft2/hr)
Percent Cake
Solids
Ferric Chloride
25-35
104
200
3.3
21
As seen in Table 3-20 a reasonable processing rate, cake solids concentration, and
chemical conditioning were achieved.
At the Newmarket, Ontario activated sludge plant, lime was used in the primaries with
the sludge generation and processing results shown in Table 3-21. Indicated results with
the mixture of limed primary and activated sludges at Newmarket were excellent. Good
solids capture and concentration were realized at low cost. The amount of sludge mass for
disposal tripled.
TABLE 3-21
DEWATERING DATA ON NEWMARKET MIXED
ORGANIC/LIME SLUDGE [20]
Chemical Addition
Metal Salt Dose (mg/1)
None
Lime 200
Mixed Sludge
Percent Solids
3.5
10
Solids
Produced Polymer
(tons/M.G.) OWton)
0.85
2.45 < 1
Centrifugation
Percent Cake
Solids
--
31
Solids
Capture (%)
~
97
3-33
-------
A direct comparison of sludge handling for the case of lime addition to the primary
process and the case of alum addition to the aeration process was made at the Windsor
Little River conventional activated sludge plant and is summarized in Table 3-22. First,
note that the untreated system chemical conditioning costs were abnormally high at
$16/ton, even though a good feed sludge solids concentration to the dewatering step was
realized. Lime usage gave 50 percent additional sludge, but dewatering costs per ton were
lowered. Alum use caused a lower filter yield and higher dewatering costs.
TABLE 3-22
SLUDGE PROCESSING DATA FOR PHOSPHORUS REMOVAL
ALTERNATIVES AT THE LITTLE RIVER TREATMENT PLANT [21 ]
Chemical Addition Mixed Sludge Solids Filter Yield Conditioning
Metal Salt Dose (mg/1) Percent Solids (tons/M.G.) (Ib/ft2/hr) Cost ($/ton)*
None — 6.2
Lime 125 11.6
Alum 150 5.7
0.8 5.2 16
1.2 7.2 11
1.2 4.6 18
Based on 1972 cost figures.
3.5 Additional Reading
Campbell, L. A., "The Initiation of Bioflocculation." Water Pollut. Contr. (Aug.
1972), p. 14.
U.S. Patent No. 3,763,039 October 2, 1973, "Method of Treating Sewage Using
High Polymer Ratio Flocculation Agent Biologically Produced in Situ." Inventor
George E. Wilson, Sterling, Illinois, assigning Houdaille Industries Incorporated,
Buffalo, New York.
Wallen, L. L., Davis, E. N., "Biopolymers of Activated Sludge." Environ. Sci.
Technol.,6 (2), 161 (Feb. 1972).
3-34
-------
3.6 References
1. Keefer, C. E., Sewage Treatment Works. McGraw-Hill Book Company,
Inc.: New York (1940).
2. Fair, G. M. and Imhoff, K., Sewage Treatment (2nd ed.). John Wiley & Sons,
Inc.: New York (1965).
3. Babbitt, H. E., Sewerage and Sewage Treatment (7th ed.). John Wiley & Sons,
Inc.: New York (1953).
4. McCabe, J. and Eckenfelder, W. W., Advances in Biological Waste Treatment.
Pergamon Press: New York (1963).
5. Stanley, W. E., personal communication, Washington University, St. Louis,
Missouri (1967).
6. Smith, J. E., Jr., "Ultimate Disposal of Sludges." Technical Seminar/Workshop
on Advanced Waste Treatment, Chapel Hill, N. C., Feb. 9-10, 1971.
7. Adrian, D. D. and Smith, J. E., Jr. "Dewatering Physical-Chemical Sludges,"
Applications of New Concepts of Physical-Chemical Wastewater Treatment.
Pergamon Press, Inc.: New York (Sept. 18-22, 1972), pp. 273-289.
8. Farrell, J. B., EPA Technology Transfer Design Seminar, Anaheim, California,
Nov. 1972.
9. Coackley, P. and Wilson, F., Proceedings of the Filtration Society. Filtr. Separ.
(Jan.-Feb. 1971), p. 61.
10. Hervol, H. J. and Pyle, R. H., Water Wastes Eng. (Nov. 1973), p. Fl.
11. Vermehren, P., I. Kruger A/S, Copenhagen, "Chemical Removal of Nutrient
Salts from Plant Effluent." Sixth Nordic Symposium on Water Research,
Copenhagen, Denmark.
./.
12. Bjorkman, A., "Heat Processing of Sewage Sludge.", Congress of the IRGRp
Basle, Jun. 2-5, 1969.
13. Gale, R. S. and Eng, C., "Filtration Theory with Special Reference to Sewage
Sludges." Water Pollut. Contr. (1967), p. 622.
14. Tenney, M. W., Echelberger, W. F., Coffey, J. J., and McAloon, T. J.,
"Chemical Conditioning of Biological Sludges for Vacuum Filtration. /. Water
Pollut. Contr. Fed., 42 (2), Part 2, R1-R20 (Feb. 1971).
3-35
-------
15. Randall, C. W., Turpin, J. K., King, P. H., "Activated Sludge Dewatering
Factors Effecting Drainability." /. Water Pollut. Contr. Fed., 43 (1), 102 (Jan.
1971).
16. Cohen, D. B. and Puntenny, J. L. "Metro Denver's Experience with Large Scale
Aerobic Digestion of Waste Activated Sludge." Presented at WPCF Annual
Meeting, Cleveland, Ohio, Oct. 4, 1973.
17. U.S. Environmental Protection Agency, Technology Transfer Seminar
Publication, "Oxygen Activated Sludge Wastewater Treatment Systems, Design
Criteria and Operating Experience" (Aug. 1973).
18. EPA—TTP Design Seminar for Sludge Processing and Disposal, Atlanta,
Georgia, Dec. 11-12, 1973.
19. Baldock, E. H., "Metropolitan Toronto's Experience in Phosphate Removal."
Western Canada Water and Sewage Conference, Winnipeg, Manitoba, Sep. 21,
1973.
20. Smith, A. G., "Centrifuge Dewatering of Lime Treated Sewage Sludge."
Research Paper No. 2030, Ministry of the Environment, 135 St. Clair Avenue,
W., Toronto 7 Ontario.
21. Van Fleet, G. L., Barr, J. R., and Harris, A. J., Ontario Ministry of the
Environment, "Treatment and Disposal of Chemical Phosphate Sludges in
Ontario." WPCF Meeting, Atlanta, Georgia, Oct. 1972.
3-36
-------
CHAPTER 4
SLUDGE THICKENING (BLENDING)
4.1 Functions, Methods, and Occurrences
Thickening increases the solids content of a sludge stream by partial removal of the liquid
portion. During sludge handling this process frequently occurs more than once. The purpose
is to reduce the sludge volume to be stabilized, dewatered, or hauled away as a liquid to final
disposal. Figure 4-1 shows the effect of thickening on sludge volume.
Cost benefits of reduced sludge volume on subsequent treatment processes, such as
anaerobic digestion, are illustrated in Figure 4-2. The results in Figure 4-2 were derived from
a two-stage trickling filter plant at Beaumont, Texas. Effect of increased sludge solids
content on dewatering is further documented in Chapter 7.
Figure 4-3 shows the importance of having a high solids sludge going to a mechanical
dewatering device. These results of McCarty's [ 1 ] are from a large number of plants
processing different sludges. As noted earlier, thickening occurs as an extra benefit in unit
processes provided essentially for another purpose. Individual unit processes that afford
sludge thickening are summarized in Table 4-1.
TABLE 4-1
OCCURRENCE OF THICKENING IN WASTEWATER
TREATMENT PROCESSES
Unit Process
Principal Functions
Primary Sedimentation
Elutriation Basin
Secondary Sedimentation
Gravity Thickener
Dissolved Air Flotation Thickener
Centrifuge Thickener
Clarification-liquid; sometimes blend and/or thicken sludge
Wash and thicken sludge
Clarification-liquid; partially thicken sludge
Thicken and blend sludge
Thicken sludge
Thicken sludge
Frequently, inadequate attention has been given to sludge processing considerations in the
design of sedimentation units. Dick and Ewing [3], Dick[4,5] and Mancini [6] have
graphically discussed the prevalent failure to consider both the clarification and thickening
aspects in design of sedimentation units.
The relative usage of various methods of sludge thickening in sludge treatment are delineated
in Table 4-2.
4- 1
-------
PERCENT SOLIDS IN
RAWSLUDGE
2 4 6 8 10
SOLIDS IN CONCENTRATED SLUDGE, (%)
FIGURE 4-1. Effect of increasing sludge solids on the final sludge volume [ 1 ],
4-2
-------
600,000
CAPACITY,(CU
Oi
o
o
o
o
o
K
o
o
o
o
o
UJ
CO
UJ
o
Q 300,000
UJ
og
3
O
UJ
* 200,000
\ CONVENTIONAL DESIGN
V ^ WITHOUT THICKENERS
— PL"
\
SAVING OF $175,000 IN CONSTRUCTION COST
V (COST OF THICKENERS INCLUDED)
N
\
^-^
DESIGN FOR
x^ >/ BEAUMONT
^K^
45678
PERCENTAGE SOLIDS IN SLUDGE TO DIGESTER
FIGURE 4-2. Effect of thickening on required digestion capacity [2]
4-3
-------
90
80
70
60
LU
^
<
O
5
50
I
I
I
24 6 8 10 12
SLUDGE SOLIDS CONCENTRATION, (%)
14
FIGURE 4-3. Effect of feed solids on performance of a rotary vacuum filter [ 1 ],
4-4
-------
TABLE 4-2
OCCURRENCE OF THICKENING METHODS IN SLUDGE TREATMENT
Method
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
(Elutriation)
Dissolved Air
Flotation
Dissolved Air
Flotation
Solid Bowl Conveyor
Type Centrifuge
Disc Type Centrifuge
Type Sludge
Raw Primary
Digested Primary
Raw Primary
andEAS
Raw Primary
andEAS
AirEAS
Digested Primary
and EAS Mixture
Raw Primary and
EAS
AirEAS
AirEAS
AirEAS
Other Pertinent
Considerations
Separate Air EAS
Thickening
Separate EAS
Thickening
Recirculation of Air
EAS to Primaries
Direct Mixed Sludge
Thickening
Separate Primary
Sludge Thickening
Secondary Digesters
or Elutriation
Direct Mixed Sludge
Thickening
Separate Gravity
Thickening of Primary
Separate Gravity
Thickening of Primary
Separate Gravity
Thickening of Primary
Frequency of Usage
and Relative Success
Increasing— excellent
results
Infrequent now; but
feasible
Decreasing—
usually poor
Some new installations-
marginal results
Essentially never used-
poor results
Many plants built—
requires flocculants
Rarely used
Increasing— good results
Some limited use— solid
capture problem
Some limited use— data
now being accumulated
Gravity thickening of raw or digested primary sludge is almost always an efficient and
economical process. Anaerobically digested primary sludge is normally thickened by
gravity in the secondary digester. As discussed in detail in Chapter 10, the use of primary
basins to capture and thicken both wastewater influent and recirculated excess activated
sludge solids has been found to be poor practice. The EAS solids do not resettle well in
the primary, and this results in the production of more EAS due to an increased load on
the aeration system. Further, relatively poor thickening results when the primary basins
happen to capture some of the EAS solids. Because the gravity thickeners process
mixtures of raw primary and air system excess activated sludges inefficiently and only
provide marginal solids capture, they are infrequently used at this time. The relative
degree of difficulty encountered has been correlated to the particular type of activated
4-5
-------
sludge system used and the settling properties of the excess activated sludge. Oxygen
activated sludge, however, has been shown in limited plant scale testing to be more
amenable to mixed sludge gravity thickening. The efficiency of gravity thickening of
mixed liquor in secondary sedimentation basins, as shown by current operating data is
poor. Therefore a further need for thickening of EAS exists. This is true for EAS from air
and pure oxygen systems. The city of Grand Rapids, Michigan performed a plant study
which included an unsuccessful attempt to use gravity thickeners for straight activated
sludge [7].
Elutriation had been regarded generally as a means for reducing the demand for acidic
chemical conditioner by washing out the fine solids and alkalinity. It is now normally
used also to thicken combined primary and secondary sludges after anaerobic digestion.
Such thickening is accomplished with lower volumes of wash water and polyelectrolyte
addition. In fact, elutriation is now well recognized as a postdigestion gravity thickening
process.
Use of dissolved air flotation for thickening of EAS has increased because it gives reliable
and effective results. Centrifugal thickening of EAS has not yet been widely applied.
Recent improvements, however, in the design of solid bowl conveyor units and disc units
may alter this situation.
4.2 The Gravity Thickejner
A conventional gravity thickener is shown in Figure 4-4. The thickener normally consists
of two truss-type steel scraper arms mounted on a hollow pipe shaft keyed to a motorized
hoist mechanism. A truss-type bridge is fastened to the tank walls, or, in some cases, to
steel or concrete columns, spans the tank, and supports the entire mechanism. The
thickener resembles a conventional circular clarifier with the exception of having a greater
bottom slope. Sludge enters at the middle of the thickener and the solids settle into a
sludge blanket at the bottom. The concentrated sludge is very gently agitated by the
moving rake which dislodges gas bubbles, prevents bridging of the sludge solids, and keeps
the sludge moving toward the center well from which it is removed. Supernatant liquor
passes over an effluent weir around the circumference of the thickener. McCarty [ 1 ]
found that in the operation of gravity thickeners it is desirable to keep a sufficiently high
flow of fresh liquid entering the concentrator to prevent septic conditions and resulting
odors from developing. This can be done by specifying an overflow rate of between 600
and 800 gpd/sq ft. To achieve hydraulic loadings in this range, secondary effluent is
normally blended with the sludge feed to the thickener. As mentioned previously,
chlorine also can be used for septicity prevention. Another design factor is the sludge
volume ratio (SVR) which was defined by Torpey [9] to be the volume of the sludge
blanket divided by the daily volume of sludge pumped from the thickener. This
relationship has the units of days and is used as a relative measure of the average retention
time of solids in the thickener. A long SVR is desirable for maximum concentration but
4-6
-------
Courtesy Link Belt
RAISED POSITION
OF TRUSS ARM
SCRAPER BLADES
UNDERFLOW
ELEVATION
• FIGURE 4-4. Gravity thickener [8].
-------
may lead to excessive biological decomposition. Values for SVR are normally maintained
between 0.5 and 2 days, with the lower values being used during warmer weather. Solids
loadings based on lb/hr/ft2 vary with the type of sludge; Most continuous thickeners
today are circular and designed with a side water depth of approximately 10 feet. While
sludge blanket depth is an important parameter, it has been reported that underflow
solids concentrations are independent of sludge blanket depths greater than 3 feet.
Increased sludge detention time in the thickener will result in increased underflow solids
concentration. A detention period of 24 hours has been suggested as the time required to
achieve maximum compaction. Sludge blanket depth and detention time are closely
interrelated. The sludge blanket depth may be varied with fluctuation in solids
production to achieve good compaction. During peak conditions the detention time may
have to be shortened to keep the sludge blanket depth sufficiently below the overflow
weirs to prevent excessive solids carry-over.
4.2.1 Performance Experiences
Typical solids loadings as well as thickener output concentration for various sludge types
were summarized by Newton [ 10] and are shown in Table 4-3.
TABLE 4-3
GRAVITY THICKENER SURFACE
LOADINGS AND OPERATIONAL RESULTS [10]
Type of Sludge
Solids-
Surface Loading
(lb/day/ft2)
Thickened
Sludge Solids
Concentration (%)
Separate sludges
Primary
Modified activated
Activated
Trickling filter
Combined sludges
Primary and modified activated
Primary and activated
Primary and trickling filter
20-30
15-25
5-6
8-10
20-25
6-10
10-12
8-10
7-8.5
2.5-3
7-9
8-12
5-8
7-9
4-8
-------
This data should be viewed carefully since:
• No data on quality of thickener overflows is presented and both loading and the
degree of thickening accomplished would influence this.
• Achievement of only 2 to 3 percent solids in the gravity thickening of activated
sludge further shows the inadequacy of this method for such sludges.
• Data on combined sludges is reasonable, except that few plants attain a 5 to 8
percent thickened sludge solids concentration with a mixture of primary and
activated sludges.
4.2.2 Theory of Gravity Thickening and Design Procedures
Mancini [6], and later Dick and Ewing[3], Dick and Young [11], Dick [4,5], and Edde,
Eckenfelder, and Wesley [12] called attention to the inadequacy of the Coe and Clevenger
[13], Kynch [ 14], and Talmage and Fitch [15] procedures when significant amounts of
activated sludge are involved in gravity thickening operations. It is frequently necessary,
in industrial and domestic waste treatment, to provide clarification and thickening
facilities for flocculent slurries which have hindered settling characteristics. These slurries
form a distinct solids-liquid interface when allowed to settle on a batch basis. A plot of
the elevation of the solids-liquid interface as a function of time provides a characteristic
hindered settling curve as shown in Figure 4-5.
A method of designing continuous flow clarification and thickening facilities for
suspensions which have hindered settling characteristics has been proposed by Kynch
[14] and modified by Talmage and Fitch [15]. A batch settling test is performed and a
settling curve is analyzed to determine the surface area required for clarification and the
surface area required to thicken the slurry to a desired underflow solids concentration.
Details of the classical method of design of gravity thickeners based on the analysis of
batch settling tests are fully described in the various references previously cited. It is
sufficient here to give results and conclusions by Mancini [6] and Dick et al. [3,4,5,11]
from studies on various types of sludges. The conventional thickening concepts as
outlined have been generally accepted, without qualification, as being applicable to solids
concentration problems in sanitary engineering.
Kynch [14], however, presumed all particles were of the same size and shape, and that
they were uniformly distributed in any horizontal plane. He did not discuss flocculent
particles or the applicability of the theory to compressible materials.
In contrast to the suspension considered by Kynch [14], activated sludge is comprised of
nonrigid, flocculent particles. The floe combine into aggregate particles whose size is not
necessarily uniform nor spherical, and is subject to change as dictated by the local shear
rate. Furthermore, and possibly of most importance, it seems likely that a network of
4-9
-------
^r 1.25
z
LU
u
1.0
0.75
^ 0.5
g
z>
a
0.25
o
_i
O
ZONE
-4-
INITIAL SETTLING
COMPRESSION POINT
SETTLING CURVE
10 20 30 40 50
SETTLING TIME (MIN)
FIGURE 4-5. Characteristic settling curve for slurry with hindered settling characteristics.
4-10
-------
aggregate particles could form a structured suspension exhibiting a yield strength. The
existence of such a structure would permit interparticle forces. Yet Kynch's basic
assumption is equivalent to an assumption that the only forces on particles are of
hydraulic origin.
Mancini [6] studied the gravity settling properties of several activated sludges in detail. His
major conclusions were:
• The Kynch [14] theory is valid for ideal suspensions. An example is the behavior of
sand suspensions.
• The settling behavior of activated sludge cannot be predicted by the Kynch theory.
Hence, the prevailing theories of thickening are not strictly applicable to the design
of final settling tanks and gravity thickeners for activated sludge.
• In addition to being dependent upon concentration, the rate of subsidence of
activated sludge is dependent upon sludge depth and the mixing of underlying
layers. This is true at concentrations less than that of the compression point.
• Analysis of the relationship between the initial depth of an activated sludge
suspension and its initial settling velocity provides quantitative measures of the
extent of the deviation from ideal behavior (the retardation factor) and the ultimate
settling velocity of the suspension.
• Within the concentration ranges investigated, the retardation factor of activated
sludges varied exponentially with sludge concentration.
• The retardation factor can be related to the general nature of activated sludge. At
comparable concentrations, low retardation factors are associated with sludges of
good settleability, while bulking sludges have high retardation factors.
• Settling tests for determining the solids handling capacity of activated sludge must
be conducted with depths and mixing conditions comparable to plant operating
conditions.
Plant experiences generally agree with the conclusions presented. Definitive work on
analysis of design procedures for gravity thickening has been continued by Dick [5]. These
procedures will also still be useful in considering gravity thickening of mixtures of
primary sludge and excess oxygen activated sludge.
4.2.3 Gravity Thickening of Oxygen Activated Sludges
Successful gravity thickening of a mixture of raw primary and excess oxygen activated
sludge is practiced at the Westgate treatment plant in Fairfax County, Virginia. This
4-11
-------
plant's operating experiences are discussed in detail in Chapter 10. Pilot data on separate
gravity thickening of oxygen activated sludge shows some promise as illustrated in Table
4-4.
TABLE 4-4
GRAVITY THICKENING DATA FOR
EXCESS OXYGEN ACTIVATED SLUDGE
Location
Batavia
Tonawanda
New Orleans
Louisville
Philadelphia
Middlesex
Feed Solids
Concentration Solids Loading
(%) (Ib/ft2/day)
1.7-
3.0-
2.5-
1.8-
1.5-
1.5-
3,
3,
3.
3.
2.
2,
.0 24
.5 34
.0 15
.0 12-23
.0 11-15
.0 7-9
Thickened Solids
Concentration
(%)
4.5
7.0
6
4.5
4.0
4.0
-6.0
-8.0
.0
-5.0
-5.0
-4.5
The high thickened sludge solids content and the solids loading rates achieved, indicate
that gravity thickening with this type sludge may well be applicable. Further plant scale
data if possible and delineation of overflow analyses should precede routine use of this
method.
4.2.4 Capital, Operation, and Maintenance Costs for Gravity Thickening
Gravity thickening costs were reported by Stanley Consultants. Inc. [16], in November,
1972, and these are presented in Figure 4-6.
4.3 Air Flotation Thickening
In general, air flotation thickening can be employed whenever particles tend to float
rather than sink. These procedures are also applied if the materials have a long subsidence
period and resist compaction for thickening by gravity.
4- 12
-------
COSTS ($/DRY TON)
a
8
o
o
en
r^-
en
O
cr
o°
rt
OQ
O) Ul .^ 00 KJ .-> H
-i c o o o 3
millIt
?.. D-toN
ss-
lo
ffp
3S
o o
§
ft
o'
a
O
o
S
to
CONSTRUCTION COST (MILLIONS OF DOLLARS)
-------
4.3.1 Occurrence, Methods, and Process Theory
Flotation thickening units as reviewed by Gulp [ 17] are becoming increasingly popular for
handling excess activated sludges since they have the advantage over gravity thickening
tanks of offering higher solids concentrations and a lower initial equipment cost.
Flotation, which uses rising gas bubbles to increase the buoyance of solid particles, is
accomplished in four ways:
1. Dispersed air flotation occurs when bubbles are generated by introducing air
through a revolving impeller or porous media.
2. Dissolved air-pressure flotation occurs when air is put into solution under
elevated pressures and later released at atmospheric pressure.
3. Dissolved air-vacuum flotation occurs when a vacuum is applied to wastewater
aerated at atmospheric pressure.
4. Biological flotation occurs when the gases formed by natural biological activity
are used to float solids.
Dissolved air-pressure flotation will be discussed in detail in this section. Of the preceding
techniques, it enjoys by far the widest usage for thickening sludge in the United States.
The objective of flotation-thickening is to cause the solids to separate from the water in
an upward direction by attaching minute air bubbles to particles of suspended solids. The
solid particles with attached bubbles have a specific gravity lower than water and tend to
float. The bubbles formed must have a small diameter and this is accomplished by
releasing air from a solution that has been pressurized at 40 to 80 psi. Since the solubility
of air increases with pressure, substantial quanitities of air can be dissolved. In modern
flotation practice, two general approaches to pressurization are:
• Air charging and pressurization of dilution water (frequently recycled clarified
effluent) with subsequent addition to the feed sludge.
• Air charging and pressurization of the combined dilution liquid and feed sludge.
Release of the pressurized flow into a chamber at near atmospheric pressures decreases
solubility of air, and the excess comes out of solution to form minute air bubbles (average
diameter 80 microns). These attach themselves to and are enmeshed in particles of
flocculated sludge.
There are several manufacturers of dissolved air flotation (DAF) systems used for
wastewater sludge thickening. Three widely used systems in the United States are those
4-14
-------
manufactured by (1) Komline-Sanderson, (2) Rexnord, and (3) Envirotech. Figure 4-7
shows a typical unit and a typical system installation is illustrated in Figure 4-8.
Figure 4-8 is based on the Komline-Sanderson system and serves to illustrate the basic
process considerations. In their system a portion of the flotation unit's effluent, or similar
plant process stream, is pumped to a retention tank at 60 to 70 psig. Air is fed into the
pump discharge line at a controlled rate and mixed by the action of an eductor driven by
the reaeration pump. The flow through the recycle system is metered and controlled by a
valve located immediately before the mixing of the recycle stream with the sludge feed.
Effluent recycle ratios can range from 30 to 150 percent of the influent flow. The recycle
flow and sludge feed are mixed in a chamber at the unit inlet. If flotation aids are
employed, introduction is normally in this mixing chamber. The sludge particles are
floated to the sludge blanket and the clarified effluent is discharged under a baffle and
over an adjustable weir which controls the depth of penetration of the skimming blades.
The thickened sludge is removed by a variable speed skimming mechanism. In practice,
bottom sludge collectors are also furnished for removal of any settled sludge or grit that
may accumulate.
Sludge thickening occurs in the sludge blanket, which is normally 8 inches to 24 inches
thick. The buoyant sludge and air bubbles force the surface of the blanket above the
water level, inducing drainage of water from the sludge particles. Detention time in the
flotation zone is not critical, providing the particle rise rate is sufficient and that
horizontal velocity in the unit does not produce scouring of the sludge blanket.
Design and mode of operation of the pressurization tanks differ among the
Komline-Sanderson, Rexnord, and Envirotech systems. The Envirotech and Rexnord
systems inject the liquid into the air to get the desired air dissolution while
Komline-Sanderson injects the air into the liquid. Both approaches are satisfactory and
the operating pressure of the pressurization tank is a more important variable.
The primary variables for flotation thickening are:
• Pressure
• Recycle Ratio
• Feed Solids Concentration
• Detention Period
• Air-to-Solids Ratio
• Type and Quality of Sludge
4- 15
-------
ADJUSTABLE FLOAT SKIMMER
o\
CHAIN TENSIONER
\
FLOAT
STORAGE .
SUMP
j— a_l _
i>
y
T
*
N INFLUENT
*"
L
>
^
\ *
(\
fr-
v i rn-i_> ^^
X (£)
\1 ^" *
tl
Courtesy Envirotech \
ocrk\A//^/^r» C/-OA
-J^Vj>x-
^LUDGE
DISCHARGE
BACK PRESSURE VALVE
FIGURE 4-7. Dissolved air flotation unit.
-------
Courtesy Komline-Sanderson
UNIT EFFLUENT
AUX. RECYCLE CONNECTION
(PRIMARY TANK OR
PLANT EFFLUENT)
AIR FEED
FLOTATION UNIT
RECIRCULATION PUMP
REAERATION PUMP
THICKENED SLUDGE
*~ DISCHARGE
^ FEED
•^-SLUDGE
RECYCLE
FLOW
-RETENTION TANK
(AIR DISSOLUTION)
FIGURE 4-8. Dissolved air flotation system.
-------
• Solids and Hydraulic Loading Rates
• Use of Chemical Aids
Air pressure used in flotation is important because it determines air saturation or size of
the air bubbles formed. It influences the degree of solids concentration and the subnatant
(separated water) quality. In general, either increased pressure or air flow produces
greater float (solids) concentrations and a lower effluent suspended solids concentration.
There is an upper limit, however, too much air breaks up fragile floe. The
Komline-Sanderson and Envirotech systems operate at 60 to 70 psig while the Rexnord
operates at 40 to 50 psig.
Operating pressure of the pressurization tank also has an effect on the size of the bubbles
released in the thickener. If the bubbles are too large, there will not be enough surface
area for attachment to the sludge to be floated. Generally, bubble sizes less than 100
microns in diameter, and preferably around 50 microns, are believed most desirable for
effective flotation of sludge. Although bubbles below 100 microns can be seen with the
naked eye, the only practical way to establish the size is by rise rate experiments. Such
experiments have been published [ 14] and the results indicate that a faster rise rate occurs
when air is released from a 40 psi pressure than from an 80 psi pressure, as shown in
Figure 4-9. This indicates that larger bubbles are formed when air is released from lower
pressures.
Recycle ratio and feed solids concentration are interrelated. Additional recycle of
clarified effluent does two things:
1. It allows a larger quantity of air to be dissolved because there is more liquid.
2. It dilutes the feed sludge.
Dilution reduces the effect of particle interference on the rate of separation. At the
Chicago Sanitary District, 40 percent recycle proved to be optimum. Concentration of
sludge increases and the effluent suspended solids decrease as the sludge blanket
detention period increases. In plant tests, solids concentration increased rapidly with time
up to 3 hours. Beyond 3 hours, however, no additional thickening was observed.
The air-to-solids ratio is also an important parameter because it influences the sludge rise
rate. Figure 4-10 presents data collected for several activated sludges resulting from the
treatment of various type wastes.
The air-to-solids ratio needed for a particular application is a function primarily of the
sludge's characteristics such as SVI. The most common ratio used for design of an excess
activated sludge thickener is 0.02. Figure 4-10 indicates that for a sludge which will easily
flocculate and settle (SVI below 100), an air-to-solids ratio of 0.03 should be sufficient to
achieve a thickened solids concentration of 4 percent.
4-18
-------
1,000
800
600
< 400
Q
<
ee.
200
100
80
60
20 40 60 80 100 120
PRESSURE OF SATURATION,(psig)
140
FIGURE 4-9. Influence of pressure of saturation on rise rate [ 14]
4-19
-------
0.07
0.06
0.05
W0.04
Q
_l
O
CO
E 0.03
<
0.02
0.01
SEWAGE SLUDGE .
SVI 400 1°
CHEMICAL
WASTEWATER
SLUDGE
PULP AND PAPER
WASTEWATER
SLUDGE
O
HYPOTHETICAL
EXTRAPOLATION
SEWAGE SLUDGE
SVI 85
I
234
SOLIDS IN FLOAT, (%)
FIGURE 4-10. Influence of air-to-solids ratio on float solids content [14]
4-20
-------
4.3.2 Operational Results
Like sedimentation, the type and quality of sludge to be floated affects unit performance.
Flotation thickening is most applicable to activated sludges, as stated before, but higher
float concentrations can be achieved with combined primary and activated sludge. A high
SVI represents a bulky sludge and results in poor thickener performance.
Loading rate affects performance and Table 4-5 summarizes loadings employed and
results experienced at several operating installations. Solids loading is the design
parameter which determines the surface area of the unit. Generally, the higher the solids
loading, the lower the solids in the thickened sludge.
Many different chemicals have been used in wastewater air flotation systems. Flocculating
chemicals agglomerate solids into stable floe that promote an increase in the terminal
velocity and facilitate capture of gas bubbles. Overall effect is to increase allowable solids
loadings, percentage of floated solids, and improve the clarity of the subnatant. Cationic
polyelectrolytes have been the most successful chemicals used in wastewater sludge
thickening. Chicago, for example, found a polymer dosage of 12 Ib/ton very helpful in
the flotation thickening of an activated sludge with poor settling characteristics.
Thickening of a chemically conditioned sludge with a SVI of 104 to 107 was comparable
to that obtained with untreated activated sludge having a SVI of 84. Even higher loading
rates (0.72 lb/hr/ft2 vs. 0.58 lb/hr/ft2) were achieved with the conditioned sludge.
Experience has shown that the introduction of the polymer solution into the line just as
the bubbles are being formed and mixed with the sludge produces the best results. The
decision to use or not use polymers or other chemicals must be based on an optimization
of capital costs versus the added operating costs from chemical usage.
The pronounced effect of polymer usage in flotation thickening of activated sludge is
demonstrated in Table 4-5. In three of the four locations not using flotation aids, float
cake solids were lower than a generally preferred minimum of 4 percent and the system
loadings were also lower. Nearly all installations now use flotation aids.
4.3.3 Advantages and Disadvantages of DAF Thickeners
Commonly cited considerations in selecting DAF thickening over that of gravity
thickening for processing activated sludge are:
• Reliability
• A thicker product
• A higher solids loading
• Lower capital cost
4-21
-------
TABLE 4-5
OPERATING DATA FOR PLANT SCALE DAF UNITS [ 17]
Location
Bernards ville.N.J.
Bernards ville, N.J.
Abington, Pa.
Hatboro, Pa.
Morristown, N.J.
Omaha, Nebr.
Omaha, Nebr.
Belleville, 111.
Indianapolis, Ind.
-P-
to Warren, Mich.
to
Frankenmuth, Mich.
Oakmont, Pa.
Columbus, Ohio
Levittown,Pa.
Nassau Co. ,N.Y.
Bay Park S.T.P.
Nassau Co. ,N.Y.
Bay Park S.T.P.
Nashville, Tenn.
Feed
MX/7
R.S.*
R.S.
R.S.
R.S.
R.S.
M.L.
R.S.
R.S.
R.S.
M.L.
M.L.
R.S.
R.S.
R.S.
R.S.
R.S.
R.S.
Influent
ssmg/1
3,600
17,000
5,000
7,300
6,800
19,660
7,910
18,372
2,960
6,000
9,000
6,250
6,800
5,700
8,100
7,600
15,400
Subnatant
ssmg/1
200
196
188
300
200
118
50
233
144
350
80
80
40
31
36
460
44
% Removal
ss
94.5
98.8
96.2
96.0
97.0
99.8
99.4
98.7
95.0
95.0
99.1
98.7
99.5
99.4
99.6
94.0
99.6
Float
% Solids
3.8
4.3
2.8
6.0
4.0
3.5
5.9
8.8
6.8
5.7
5.0
7.8
6-9
6-8
8.0
5.0
5.5
4.4
3.3
12.4
Loading
Ib/hr/ft2
2.16
4.25
3.0
2.95
1.70
7.66
3.1
3.83
2.1
5.2
6.5
3.0
3.3
2.9
4.9
1.3
5.1
Flow
gpm/ft2
1.2
0.5
1.2
0.8
0.5
0.8
0.8
0.4
1.47
1.75
1.3
1.0
1.0
1.0
1.2
0.33
0.66
Remarks
Standard0
Standard
Flotation Aidd
After 1 2 hours holding
Flotation Aid
Standard
Flotation Aid
After 24 hours holding
Flotation Aid
Flotation Aid
Flotation Aid
After 12 hours holding
Flotation Aid
Flotation Aid
Flotation Aid
Flotation Aid
Flotation Aid
Flotation Aid
Standard
Flotation Aid
aM.L. - Mixed liquor from aeration tanks.
6R.S. - Return sludge.
cStandard - Indicates no flotation aid and no holding before sampling.
Flotation Aid - Indicates use of coagulant-flotation aid.
-------
- A better solids capture (~ 97 percent)
• Maintenance of sludge in an aerobic condition
• Higher operating cost (power and flotation aids)
Since DAF effluent samples often have an 8 to 10 mg/1 dissolved oxygen (DO)
concentration, odor and phosphorus release problems seldom develop. Samples analyzed
for total phosphate have given average subnatant values of 14 mg/1 with average feed
values of 570 mg/1. Total phosphate in the float in these samples averaged 1,400 mg/1.
4.3.4 Components of a Typical Flotation Unit System
Figure 4-11 is a schematic flow diagram for a typical DAF thickener. The normal system
consists of the following items:
• Sludge Feed Pump (or Flow Control) and Meter
• Polymer Mix and Feed Equipment
• Flotation Thickener
• Electrical Control Panel
• Air Compressor and Receiver
• Thickened Sludge Pump
4.3.5 Design and Performance of DAF Thickeners
It is recommended that wherever possible, laboratory and pilot scale tests be carried out
on the sludge which is being considered for flotation thickening. Equipment and chemical
suppliers routinely supply methods and assistance in this area. Wood and Dick [18]
present an excellent description of test procedures. In the absence of sludge for test
purposes, procedures recommended by Jones [19] should prove satisfactory for
conventional sludges. It is necessary to know the total pounds of waste sludge, the design
loading of the unit, and the operating cycle in hours per week to size flotation units.
Performance is dependent on solids loading, concentration, hydraulic loading, and
removal efficiency, and a discussion of each follows:
• Solids Loading—A design solids loading of 2 lb/hr/ft2 is frequently selected. This
rate is achieved with the use of flotation aids and with or without the use of
auxiliary recycle, depending upon the operating situation. Although the guaranteed
4-23
-------
AUXILIARY
RECYCLE
CONNECTION
DRAIN
V
\
AIR
FEE
LINE
•UNIT EFFLUENT
FLOTATION UNIT
POLYMER FEED
PUMP
THICKENED
SLUDGE
DISCHARGE
CHEMICAL
MIX TANK
CHEMICAL
MIX TANK
SLUDGE FEED LINE
WITH METERING AND
FLOW CONTROL DEVICE
FIGURE 4-11. Typical DAF thickener system.
4-24
-------
loading with polymers is 2.0 lb/hr/ft2, operating data indicate that in most cases 3.0
lb/hr/ft2 can be expected. (The flotation thickener has a built-in capacity for 4.0 to
5.0 lb/hr/ft2, which allows for a large safety factor and flexibility in operation.)
There are times when flotation can be accomplished without flotation aids, but
generally auxiliary recycle is then employed. Loadings are generally less than 50
percent of those attainable with aids, and solids removals are questionable. Existing
data show 50 to 80 percent solids recovery at various plants and times when
flotation aids were not employed.
Concentration—A 4 percent minimum float solids concentration by weight is
normally specified for design purposes. However, a 5 to 6 percent float solids
concentration can be expected. Flotation without chemical aids generally results in a
solids concentration that is about 1 percentage point less than with flotation aids.
Further concentration is possible in a holding tank. While float density depends
upon the nature of the sludge and the amount of air entrained, tests have shown that
a weight of about 6 Ib per gal can be considered for sizing of handling and storage
facilities. After a sludge is held for several hours, the air is dissipated, and the sludge
possesses a more typical density, in addition to being further concentrated.
Hydraulic Loading—The maximum hydraulic loading or overflow rate is set at 0.80
gpm/ft2. The minimum solids concentration compatible with this hydraulic loading
and a solids loading of 2 lb/hr/ft2 is 5,000 mg/1. Lesser solids levels or higher
hydraulic loadings result in lower efficiencies and/or float solids concentrations.
• Removal Efficiency—Tests have shown that at least a 95 percent removal of
suspended solids can be expected with the flotation unit when flotation aids are
employed.
The appropriate size of a flotation unit for an existing plant can be calculated from the
solids loading, the sludge settling characteristics, and the solids concentration of the
sludge. If the thickening system is to be sized for a new plant, the following should be
considered:
• The population equivalent.
• A design loading of 0.20 Ib/capita/day of dry solids to the treatment plant.
• The quantity of wastewater solids which reach the activated sludge process; that is,
the percent of the solids that are not removed in primary treatment. Based on a
conventional activated sludge plant with primary settling, a design loading of 0.20
Ib/capita/day suspended solids, and 30 percent BOD removal in the primary,
approximately 0.085 Ib/capita/day of excess activated sludge solids will be
produced. To be conservative, 0.10 Ib/capita/day EAS production can be assumed.
4-25
-------
For a modified activated sludge system (with no primary settling) 0.17 Ib/capita/day EAS
solids production may be used unless a high BOD is expected. If unusually high BOD
loadings are anticipated, additional calculations should be performed to estimate the
quantity of sludge. For aerobically digested sludges, it may be assumed that 50 percent of
the volatile solids are destroyed and this causes the previously estimated sludge quantities
to be reduced by 35 percent. Where operating records are available in existing plants, the
amount of excess sludge should be calculated from this information.
• Use a solids loading of 2.0 lb/hr/ft2 for the flotation unit.
• Assume the number of hours per week which the unit will be operated. Although a
flotation unit does not require continuous operator attention, periodic attention to
the system should be scheduled. Generally, for plants less than 2 mgd, a 40 hr/wk
schedule is adequate. For plants 2 to 5 mgd, 80 hr/wk (2 shifts, 5 days) may be
realistic. For plants ranging from 5 to 20 mgd, 100 hr/wk (Monday a.m.-Friday
p.m.) may be used. In plants larger than 20 mgd, it is expected that operators will be
on duty 24 hr/day, 7 days per week. Of course, any information from an expected
operating schedule must be considered.
• Check the maximum hydraulic loading of 0.80 gpm/ft2.
Design Example
Population equivalent is 140,000 persons. Estimated plant size would then be 14 mgd.
Since the plant is conventional activated sludge and has primary settling tanks that
remove 50 percent of the influent solids, only 50 percent of the solids will reach the
activated sludge process. The EAS to be handled is approximately 0.10 Ib/capita/day or:
140,000 X 0.10 X 7 = 98,000 Ib/wk
For a loading of 2 lb/hr/ft2 and a 100 hr/wk operating schedule the size unit required is:
98,000 Ib/wk
2 lb/hr/ft2 X 100 hr/wk
= 490ft2
Use either two 250 ft2 units or one 500 ft2 unit. It is generally preferable to apply two
smaller prefabricated units rather than one large unit for obvious reasons of plant
flexibility.
4-26
-------
4.3.6 Costs
Stanley Consultants [16] have determined the DAF process cost given in Table 4-6.
TABLE 4-6
DAF THICKENING COSTS FOR VARIOUS PLANT SIZES
Plant Size
(mgd)
1
10
100
O&M
9.00
1.20
0.50
Cost ($/ton Dry Solids)*
Amortization
17.00
2.80
1.50
Total
26.00
4.00
2.00
Based on 1972 cost figures.
The costs are based on:
• Amortization at 7 percent for 20 years
• Labor rate of $6.25/hr
• Power cost of $0.01/kwh
• No chemicals
• Influent sludge with 0.5 percent solids concentration and a solids float of 3.5
percent
• Surface loading rate of 14.4 lb/day/ft2
• Source: Envirotech and Stanley Consultants
Use of flotation aids would add another $2 to $7 per ton. In reviewing the above costs,
due credit should be taken for the substantial benefits afforded by a consistent, effective,
and highly operable method of sludge thickening, and consequent effects on the overall
sludge treatment system operability and costs.
4-27
-------
4.3.7 Integration of DAF Thickening into the Conventional Activated Sludge Plant
For a completely successful flotation system, as presented by Jones [19], close attention
must be given to incorporation of the system into the activated sludge plant. Figure 4-12
indicates a suggested schematic flow diagram for incorporating a flotation thickener into
the conventional activated sludge plant. It can be seen that the flotation thickener does
not take the place of the final clarifier, but handles the excess activated sludge only.
Flotation unit's feed is taken from the final settling tanks, while the flotation unit's
subnatant is returned to the aeration tanks. Thickened sludge or float is delivered to a
holding tank. Sludge holdup in the tank often provides further concentration through
extended detention. Resulting supernatant can be returned to the flotation unit. The
holding tank is not an additional expense to the DAF process, since it is necessary with
any thickening device where sludge is to be delivered to a vacuum filter or a digester. The
holding tank is equipped for mixing the gravity thickened primary and DAF thickened
excess activated sludges. Such mixing appreciably improves dewatering of fresh solids on
a vacuum filter. Air at a rate of 0.5 cfm per 100 gallons of storage is an effective mixer
and prevents septicity. Decanting lines at various levels are advisable. Tank bottoms
should be sloped at least 1.5:1.0. Tank volume generally affords 24 to 48 hours holding
capacity.
Although flotation effluent is recycled during normal operation, auxiliary recirculation is
provided from the primary tank effluent. Recirculation of primary tank effluent rather
than the plant effluent minimizes the hydraulic loading and recirculated waters within the
plant. However, if primary effluent containing more than 200 mg/1 suspended solids or
unusual amounts of stringy material is anticipated, plant effluent should be employed. It
is important that feed to the flotation unit is fully controlled. In some cases where flight
scrapers are used in the final settling tanks, wide variations in feed concentrations could
result. The feed takeoff line to the flotation unit should be located to minimize
fluctuations. Sludge wasting from a sludge reaeration tank is desirable. A feed pump with
an off-on characteristic is definitely undesirable. A flow indicator and flow control devise
must be located at the flotation unit control station. Depending upon the physical layout
of the wastewater treatment plant, certain feed pumps and effluent pumps may be
necessary for the flotation unit. It is generally considered acceptable that one pump be
furnished to supply a controlled feed rate to the flotation unit. Unit effluent can be
returned to the aeration tanks or plant influent wet well by gravity. Float from the unit
can generally be delivered by gravity, to the holding tank, from which it is pumped either
to a filter or a digester.
4.3.8 Effect of Oxygen Activated Sludge
There is some evidence that activated sludges from pure oxygen systems are more
amenable to flotation thickening than activated sludges from conventional air systems.
Pilot tests at Louisville, Kentucky, indicate that with a small polymer dose of about 3
Ib/ton of dry solids, an influent solids concentration of 1.7 to 2 percent was increased to
4-28
-------
•PLANT INFLUENT
PLANT EFFLUENT-
PRIMARY
SETTLING
TANK
I
I
„ I __
I
T
AERATION
TANKS
RETURN SLUDGE-y
AUXILIARY
RECYCLE -
L*-UNIT EFFLUENT
(AND DRAIN)
I L.
HR FLOTATION
THICKENER
FINAL
SETTLING
TANK
WASTE SLUDGE
UNIT FEED
l_ ^ J
PRIMARY SLUDGE-
TO DEWATERING FACILITIES
THICKENED SLUDGE
OR FLOAT
HOLDING TANK WITH MIXING
FIGURE 4-12. Schematic flow diagram of a conventional activated sludge plant incorporating
a DAF thickener.
-------
between 6 and 7 percent solids at loading rates of 6 to 10 lb/hr/ft2 and 200 percent
recycle. The subnatant SS concentration varied from 80 to 570 mg/1 and this represented
a 97 to 99.7 percent solids recovery. Similar results have been observed on a plant scale at
the Westgate plant in Fairfax County, Virginia.
The improved thickening and dewatering characteristics of oxygen activated sludge have
been previously noted, and so the above findings are not surprising. However, as pointed
out in Chapter 10, the Westgate plant has opted for gravity thickening of combined
sludges instead of DAF thickening because their oxygen activated sludge responds to this
type treatment. This illustrates the point that where the sludges respond properly to
gravity thickening, that method is preferred. Unfortunately, few air system activated
sludges respond well to gravity thickening.
4.4 Centrifugal Thickening
There has been limited use of centrifuges for thickening of EAS. The centrifugal
thickening process can have substantial maintenance and power costs. It has been used
only where space limitations or sludge characteristics make other methods unsuitable
[20]. Further, if a particular sludge can be effectively thickened by gravity or by flotation
thickening without chemicals, centrifuge thickening is not economically feasible [16]. The
use of centrifuges for dewatering is discussed in Chapter 6. The solid bowl conveyor
disc-nozzle and basket centrifuges have been evaluated for sludge thickening.
4.4.1 Solid Bowl Conveyor Type Centrifuge-Sludge Thickening
Ettelt and Kennedy [21] evaluated flotation along with disc and solid bowl centrifugal
equipment on excess activated sludge at the Chicago Sanitary District's southwest plant
and opted for flotation thickening. This was in spite of the fact that the solid bowl
centrifuge, when processing activated sludge alone could thicken the sludge to from 6.6
to 7.5 percent. The principal difficulty with attempts to use the solid bowl conveyor type
unit is reflected in work by Ooten and Miele [22]. In order to achieve a 90 percent solids
capture and a 4 percent solids thickened activated sludge, the sludge had to be adequately
conditioned with chemicals. The chemicals added an additional processing cost of
approximately $20/ton. The solid bowl unit imposes the drastic effect of high shear on
the fragile activated sludge and chemicals are required to prevent floe breakup. Further,
the conveyability requirements of this type unit result in a high chemical demand.
4.4.2 Disc-Nozzle Centrifuge
The disc-type unit has concentrated activated sludge to about 7 percent when operated at
6,000 rpm. Operational problems, however, made its use impractical. These problems
4-30
-------
were caused by clogging of the sludge discharge nozzles, which required repeated
maintenance. Rotary screens were found effective in removing a large amount of the
oversized solids, but this resulted in low throughput. Despite these difficulties some field
testing of modified equipment has prompted the selection of disc-nozzle centrifuges by
three treatment plants. They are the Village Creek plant at Ft. Worth, Texas; the plant at
Kelowna, British Columbia and Idaho Falls, Idaho. Construction has been completed and
operation commenced at the Ft. Worth plant. Data from operation of their disc-nozzle
centrifuge indicate that:
• Activated sludge is being concentrated from 4 to 5-1/2 percent solids.
• Both rotary screens and cyclones were installed to help alleviate plugging problems.
• Rotary type screens have recently been installed and their use, together with
hot-water flushing will hopefully result in 2 to 3 weeks operation between cleanings.
A disc .centrifuge has been successfully field tested for thickening waste activated sludge
at an eastern Pennsylvania community. Using a 30-inch centrifuge with a 150-hp motor
and 300 gpm feed rate, the disc centrifuge produced a 5 percent underflow with 90
percent solids recovery. Since the plant did not have primary treatment, it was necessary
to install a screening device ahead of the centrifuge. The screening effectiveness was
demonstrated in that the nozzles of the centrifuge did not plug. The combination of
effective screening and patented recirculating system (allowing a larger nozzle size) was
instrumental to the good performance. Normally, however, disc centrifuges are not
recommended where activated sludge treatment has not been preceded by primary
treatment.
Field test results have also been reported by Vaughn and Reitwiesner [23]. The system
studied is illustrated in Figure 4-13. While it has been suggested that cyclones and screens
only be used when primary clarifiers are not used recent field results indicate the
continuous need for them. The effect of activated sludge SVI on solids capture at various
thickened sludge solids levels is shown in Figure 4-14. As can be seen the thickened sludge
solids content of 4 percent requires a SVI of about 100 to insure 90 percent solids
capture. A small amount of flocculant may be required in some instances to assist both
solids recovery and thickened sludge solids concentration.
An interesting comparison between gravity thickening of mixed sludge and a combination
involving gravity thickening of primary sludge with centrifugal thickening of activated
sludge is shown in Table 4-7.
The data again show the previously noted need for separate EAS thickening via an
efficient process. However, the comparison would have been better if it had included
flotation thickening.
4-31
-------
AERATOR
FINAL
CLARIFIER
. D
OVERFLOW
RETURN ACTIVATED
SLUDGE
I HYDROCLONE
OPTIONAL WHEN
PRIMARY CLARIFIER
USED
CLASSIFIER
ROTARY
STRAINER
DISC-NOZZLE
CENTRIFUGE
UNDERFLOW
FIGURE 4-13. Thickening of activated sludge by disc-nozzle centrifuge [23].
4-32
-------
BOD Loading - 0.2 to 0.5 @ 20° C
Based on Disc Nozzle Centrifuge Operation
General Relationship — do not use for sizing
2.0 3.0 4.0 5.0
THICKENED SLUDGE SOLIDS, (%)
FIGURE 4-14. Effect of activated sludge settleability on capture and thickening [23],
4-33
-------
TABLE 4-7
COMPARATIVE DATA ON TWO ALTERNATIVE SLUDGE
THICKENING PROCESSES [23]
Alternative No. 1
Alternative No. 2
Parameter
Unit Area, ft2 /ton/day
Capital Cost, $/ton/day
Power cost ($/ton)
Solids concentration (%)
Solids recovery (%)
Flexibility
Gravity Primary
and Secondary
250
6,300
0
5
95
None
Gravity
Primary
104
3,150
0
12
95
-
Disc-Nozzle
Centrifuge Secondary
6
5,900
1.70
5
90
-
Weighted
Total
65
4,260
0.67
9.3
90+
Excellent
Note: ft2 X 0.0929 = m2; ton (short) X 0.9078 = ton (metric)
4.4.3 Basket (Imperforate Bowl-Knife Discharge) Centrifuge
This unit has been extensively field tested at small plants for the thickening of excess
activated sludge to a 9 to 10 percent solids level. This level of solids facilitates land
disposal. The basket centrifuge's application is intermediate between normal thickening
and what can be construed as dewatering. Since there are few actual plant installations,
there is little availabh data.
4.4.4 Performance Data
Typical performance data for the disc, basket, and solid bowl centrifuges when they are
employed in the thickening of EAS, are presented in Table 4-8. Note that chemical
addition is not always required.
4.5 Sludge Blending
With the increased use of separate thickening processes for primary and activated sludges,
more consideration of this subject is in order. Thickening makes both primary and excess
4-34
-------
TABLE 4-8
CENTRIFUGAL THICKENING PERFORMANCE DATA [20]
Type of Sludge
EAS
EAS
EAS (after Roughing
Filter)
EAS (after Roughing
Filter)
EAS
EAS
EAS
EAS
Centrifuge
Type
Disc
Disc
Disc
Disc
Basket
Solid-Bowl
Solid-Bowl
Solid-Bowl
Capacity
(gpm)
150
400
50- 80
60-270
33-70
10-12
75-100
110-160
Feed Solids
(%)
0.75-1.0
-
0.7
0.7
0.7
1.5
0.44-0.78
0.5 -0.7
Underflow
Solids
(%)
5-5.5
4.0
5-7
6.1
9-10
9-13
5-7
5-8
Solids
Recovery
(%)
90+
80
93-87
97-80
90-70
90
90-80
65
85
90
95
Polymer
Requirement
(Ib/ton)
None
None
None
None
None
--
None
None
<5
5-10
10-15
activated sludge more difficult to blend. This results in the need for mechanical mixing in
sludge holding tanks. This problem has been recognized by some suppliers and
engineering firms and is well known to plant personnel. An example of the type unit
which can alleviate problems in blending thickened sludges is shown in Figure 4-15.
Contact with suppliers of mixing equipment should be made when considering sludge
blending facilities. It is also probable that in-line mixers are of interest and should be
considered. These units are designed to promote blending at minimum shear. Bubbling air
through various sludge mixtures (a previous practice) for the purpose of blending has
been found to be ineffective with thickened sludges. Use of some air injection as well as
minimum holding time should be considered to prevent septicity.
4-35
-------
BAFFLES IN
CIRCULAR TANKS
SUPERNATANT
DECANT PIPES
OPTIONAL ADJUSTABLE
SUPERNATANT DECANT
SLUDGE
IN
MANWAY
ENTRANCE
Courtesy Envirotech
DIAGONAL SUPPORT
BLENDED
SLUDGE OUT
FIGURE 4-15. Eimco sludge storage tank blender mixer.
4-36
-------
4.6 References
1. McCarty, P. L., "Sludge Concentration—Needs, Accomplishments and Future
Goals."/. WaterPollut. Contr. Fed., 38 (4), 493-507 (1966).
2. Dust, J., "Sludge Thickening Proves Economical in Beaumont, Texas." Civil
Eng., 247, 68-72 (1956).
3. Dick, R. I. and Ewing, B. B., "Evaluation of Activated Sludge Thickening
Theories." /. Sanit. Eng. Div. Proceedings of the A.S.C.E. (Aug. 1967), pp.
9-29.
4. Dick, R. I., "Role of Activated Sludge Final Settling Tanks."/. Sanit. Eng. Div.
Proceedings of the A.S.C.E. (Apr. 1970), pp. 423-436.
5. Dick, R. I., "Gravity Thickening of Sewage Sludges." Water Pollut. Contr.
(1972), pp. 368-380.
6. Mancini, J. L., "Gravity Clarifier and Thickener Design." Proceedings of the
17th Industrial Wastes Conference at Purdue University (1962), pp. 267-277.
7. Voshel, D., "Sludge Handling at Grand Rapids, Michigan, Wastewater
Treatment Plant." /. Water Pollut. Contr. Fed., 38 (9), 1506-1517 (1966).
8. Link Belt "Thickeners," Book 2959.
9. Torpey, W. N., "Concentration of Combined Primary and Activated Sludges in
Separate Thickening Tanks." Proceedings of the A.S.C.E., 70, 1275 (1944).
10. Newton, D., "Thickening by Gravity and Mechanical Means," in Sludge
Concentration, Filtration and Incineration. University of Michigan, School of
Public Health, Cont. Ed. Ser., 113, 4 (1964).
11. Dick, R. I. and Young, K. W., "Analysis of Thickening Performance of Final
Settling Tanks." Purdue Industrial Waste Conference, May 2-4, 1972.
12. Edde, H. J. and Eckenfelder, W. W., Jr., "Theoretical Concept of Gravity
Sludge Thickening: Scaling-Up Laboratory Units to Prototype Design." /.
Water Pollut. Contr. Fed., 40(8), 1486-1498 (1968).
13. Coe, H. S. and Clevenger, G. H., Trans. Amer. Inst. Mining Eng., 55, 356
(1916).
14. Kynch, G. J., "A Theory of Sedimentation." Trans. Faraday Soc., 48, 161
(1952).
4-37
-------
15. Talmage, W. P. and Fitch, E. B., "Determining Thickener Unit Areas." Ind.
Eng. Chem., 47,38(1955).
16. Stanley Consultants, "Sludge Handling and Disposal, Phase I—State of the
Art." Report to Metropolitan Sewer Board of the Twin Cities Area, Nov. 15,
1972.
17. Gulp, G. L., "Sludge Thickening." Presented at the EPA-TTP Design
Seminar—Sludge Processing and Disposal, Kansas City, Missouri, Jan. 15-17,
1974.
18. Wood, R. F. and Dick, R. I., "Factors Influencing Batch Flotation Tests." /.
Water Pollut. Contr. Fed., 45 (2), 304-315 (1973).
19. Jones, W. H., "Dissolved Air Flotation of Wastewater Sludges." Presented at
Nebraska WPC Association Meeting, Great Plains Design Conference, Omaha,
Nebraska, Mar. 26, 1968.
20. Process Design Manual for Upgrading Wastewater Treatment Plants, Office of
Technology Transfer, EPA, Washington, D.C.
21. Ettelt, G. A. and Kennedy, T. J., "Research and Operational Experience in
Sludge Dewateringat Chicago."/. Water Pollut. Contr. Fed., 38, 248 (1966).
22. Ooten, R. J. and Miele, R. P., "Centrifuging of Waste Activated and Digested
Sludges." Presented at 44th Annual WPCF Meeting, San Francisco, California,
Oct. 1971.
23. Vaughn, D. R. and Reitwiesner, G. A., "Disk-Nozzle Centrifuges for Sludge
Thickening."/. Water Pollut. Contr. Fed., 44(9), 1789-1797 (1972).
4-38
-------
CHAPTER 5
SLUDGE STABILIZATION (REDUCTION)
5.1 Functions, Methods, and Occurrences
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 this objective can also result in other basic changes in the sludge. The
selection of a certain method hinges primarily on the final disposal procedure planned for
the sludge. 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 reduction is included in the description of these
processes.
Both anaerobic and aerobic digestion are currently increasing in popularity. The former is
receiving revived attention because of the potential benefits of methane production, the
energy shortage, increasing realization that many of the previous problems experienced
were due to other wastewater process considerations, and the emphasis on final disposal
on land. 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. Composting is being practiced in several United States
cities and is being actively investigated for others. A major impetus for processes such as
anaerobic and aerobic digestion, lime treatment, and composting 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. Heat treatment has been
installed in several new United States plants to improve sludge conditioning and
dewatering economics.
5.2 Anaerobic Digestion
Excellent descriptions of this oxygen devoid process appear in the literature [ 1,2]. Table
5-1 illustrates the biochemical reactions occurring in anaerobic digestion.
Digestion is a complex biochemical process in which several groups of anaerobic and
facultative organisms simultaneously assimilate and break down organic matter. For
purposes of simplification, it is a two-phase process and can be described as follows:
1. In the first phase facultative, acid-forming organisms convert the complex
organic substrate to volatile organic acids. Acetic, propionic, butyric, and other
organic acids are formed. In this phase little change occurs in the total amount
of organic material in the system, although some lowering of pH results.
Alkaline buffering materials are also produced.
5- 1
-------
TABLE 5-1
ANAEROBIC DIGESTION-BIOCHEMISTRY [1]
Micro- Micro- Other
+ organisms Kj Nonreactive + Reactive + organisms K2 + end
Raw Sludge "A" -> Products Products "B" -*CH4 + CO2 Products
Complex Principally
substrate acid formers
Carbohydrates,
Fats, and
Proteins
CO2, H2O
Stable and
intermediate
degradation
products
Cells
Organic acids Methane
fermenters
Cellular and
other inter-
mediate
degradation
products
H2O, H2S
Cells and
stable degrada-
tion products
2. The second phase involves conversion of the volatile organic acids to primarily
methane and carbon dioxide.
This anaerobic process is essentially controlled by the methane-producing bacteria. These
bacteria grow at a relatively low rate and have generation times which range from slightly
less than 2 days to about 22 days. Methane formers are very sensitive to pH, substrate
composition, and temperature. If the pH drops below 6.0, methane formation ceases, and
there is no decrease in organic content of the sludge. The methane bacteria are highly
active in the mesophilic and thermophilic ranges. The mesophilic range is between 80°
F and 110° F, while the thermophilic range is between 113° F and 149° F. Essentially all
digesters in the United States operate within the mesophilic temperature range. Garber
[3] has recently experimented with thermophilic temperatures at the city of Los
Angeles Hyperion Plant. The main advantage was found to be that of improved
dewatering. The cost of heating to this temperature was justified by the increased
efficiency.
5.2.1 Types of Anaerobic Digestion Systems
The standard rate and high rate systems are the two main digestion processes employed.
Schematics of the processes as well as their operating criteria are given in Figure 5-1.
In practice, four types of systems have evolved from the two basic digestion modes. They
are:
1. Standard Rate Digestion—One Stage
2. High Rate Digestion—One Stage
5-2
-------
GAS OUTLET
SLUDGE INLET ^
SCUM LAYER
•////////////////// /•////////////////
SUPERNATANT
ACTIVELY
DIGESTING SLUDGE
SCUM REMOVAL
SUPERNATANT
REMOVAL
1
SLUDGE OUTLET
(A)
STANDARD RATE DIGESTION
1. UNHEATED
2. DETENTION TIME 30-60 DAYS
3. LOADING 0.03 - 0.10 Ib. VSS/cu. ft./day
4 INTERMITTENT FEEDING AND WITHDRAWAL
5. STRATIFICATION
^SLUDGE
INLET
SLUDGE OUTLET
(B)
HIGH RATE DIGESTION
1. HEATED TO 85° - 95° F
2. DETENTION TIME 15 DAYS OR LESS
3. LOADING 0.10-0.50 Ib. VSS/cu. ft./day
4. CONTINUOUS OR INTERMITTENT FEEDING
AND WITHDRAWAL
5. HOMOGENEITY
FIGURE 5-1. Standard rate and high rate digestion.
5-3
-------
3. Two-Stage Digestion
4. Anaerobic Contact Process
In the standard rate, one-stage digestion process as shown in Figure 5-1 (A), fresh sludge is
usually added to the system two or three times daily. As decomposition proceeds, three
distinct zones develop. A scum layer is formed at the top of the digester, and beneath it
are supernatant and sludge zones. The sludge zone has an actively decomposing upper
layer and a relatively stabilized bottom layer. The stabilized sludge accumulates at the
base of the digester. Supernatant is usually returned to the influent of the treatment plant
and this practice can create problems and reduce overall treatment plant efficiency even
in smaller plants.
The high rate, one-stage system (Figure 5-l(B)) requires a separate postdigestion
thickening process if dewatering is practiced. This type system is increasingly being used
in plants featuring anaerobic digestion because of the beneficial aspects of mixing,
improved process control, and lack of in-tank settling problems. Two-stage digestion is
shown in Figure 5-2.
The two-stage process can operate at various loading rates and therefore is not always
clearly defined as being either standard or high rate. It evolved as an attempt to provide
additional gas production as well as a separate settling and thickening process in the
secondary digester. The process is successful when primary sludge or combinations of
primary sludge and limited amounts of secondary sludges constitute the system's feed.
With the advent of wastewater treatment systems that are more efficient than simple
sedimentation, large quantities of activated and sometimes advanced waste treatment
(AWT) sludges are produced at the plants. This additional sludge, when placed in a
two-stage anaerobic digestion process, can cause high operating costs and poor plant
efficiencies. The basic cause of the problem is that the additional solids do not readily
settle after digestion. Typical resultant sludge processing problems are described in
Chapter 10.
Schroepfer and Ziemke [4] and McCarty [5] have fully discussed the anaerobic contact
process and it is shown in Figure 5-3. In the process operation, sludge from a high rate
digester is settled in a second-stage digester. The second-stage digester operates as a
settling basin to permit removal of microorganisms from the effluent. The organisms, as
in the activated sludge process, are returned to the digester and seed the raw waste. This
process has an increased rate of waste decomposition, when it is compared with high rate
digestion.
5.2.2 Design Criteria
Anaerobic digestion is influenced by both physical and chemical factors, some of which
are listed in Table 5-2.
5-4
-------
GAS
RELEASE
SLUDGE
INLET I
ZONE OF
MIXING
^ --
-------
GAS
RELEASE
SLUDGE
INLET
ZONE OF
ACTIVELY
DIGESTING
SLUDGE
SLUDGE RETURN
GAS
RELEASE
MIXED
LIQUOR
SUPERNATANT
REMOVAL
SUPERNATANT
DIGESTED SLUDGE
SLUDGE
DRAWOFF
FIGURE 5-3. Anaerobic contact digestion [2].
-------
TABLE 5-2
PHYSICAL AND CHEMICAL FACTORS
Physical Factors Chemical Factors
Detention Times pH
Temperature Alakalinity
Solids Concentration Volatile Acid Content
Degree of Mixing Nutrients
Solids Loading and Distribution Toxic Materials
The most important operational factors controlling the design of an anaerobic digestion
system are the combined effects of temperature mixing and the biological solids retention
time (SRT). The rate of bacterial growth and, therefore, the rate of stabilization increases
and decreases with temperature within certain limits [6]. Figure 5-4 shows the effects of
temperature on digestion time.
Time values were omitted since the digestion period is also affected by several other
factors such as pH, bacterial population, mixing, rate of feeding, and sludge
characteristics. Field investigations of high rate digestion in a controlled temperature
environment have indicated that high efficiencies are possible at liquid retention times as
low as 2 days, so long as the SRT is equal to or greater than some critical time. This
critical solids retention time (SRTc) is the time period below which digestion falls as a
result of washout of the slow-growing methane formers. The regeneration rate for the
slowest methane formers is about 10 days (SRTmin) at 95° F [6]. When the digestion
time is decreased below SRTmin, decomposition of volatile organics is slowed until
complete failure occurs at an SRTc of about 3 to 4 days.
Factors influencing the SRT are the volatile solids loading on the digester, the volatile
percentage in the total suspended solids, and the suspended solids concentration in the
raw sludge. A series of curves showing the relationship among solids loading, solids
retention time, and sludge solids is presented in Figure 5-5. The volatile solids loading to
the digester should always be adjusted, based on the volatile solids concentration in the
sludge, so that a detention time above the SRTmin is maintained. For a volatile solids
loading of 0.1 Ib/cu ft/day (see Figure 5-5), SRTc of 10 days would be met by all sludges
containing volatile solids concentrations greater than 1.5 percent.
5-7
-------
MESOPHILIC
RANGE
THERMOPHILIC
RANGE
80 100 120
TEMPERATURE, (°F)
140
FIGURE 5-4. Influence of temperature on digestion time [7]
5-8
-------
>
<
Q
Z)
O
m
Q
<
O
CO
Q
_l
O
to
LU
<
O
NOTE: (1) Inhibition occurs within shaded
(2) SRT Minimum is 10 days.
(3) Data from reference (8).
0.1
10 20 30 40 50 60
SOLIDS RETENTION TIME, (DAYS)
FIGURE 5-5. Plot of volatile solids loading vs. SRT for various feed solids.
5-9
-------
Typical design criteria for anaerobic digesters are given in Table 5-3.
TABLE 5-3
TYPICAL DESIGN CRITERIA FOR STANDARD RATE
AND HIGH RATE DIGESTERS [9]
Parameter
Solids Retention Time (SRT), days
Solids Loading, Ib VSS/cu ft/day
Volume Criteria, cu ft/ capita
Primary Sludge
Primary Sludge + Trickling
Filter Sludge
Primary Sludge + Waste
Activated Sludge
Low Rate
30 to 60
0.04 to 0. 1
2 to 3
4 to 5
4 to 6
High Rate
10 to 20
0.15 to 0.40
1-1/3 to 2
2-2/3 to 3-1/3
2-2/3 to 4
Combined Primary + Waste Biological
Sludge Feed Concentration,
percent solids (dry basis)
Digester Underflow Concentration,
percent solids (dry basis)
2 to 4
4 to 6
4 to 6
4 to 6
As noted, the high rate process requires considerably less detention time, and volume, and
operates successfully with a higher solids loading when compared to the conventional
process. This is attributed to the greater use of the digestion tank for biological activity
and improved mixing.
5.2.3 Process Control Considerations
Biochemical reaction conditions require close control for successful digestion and
excellent descriptions of the problems and measures required to correct them can be
found in the literature [10].
5-10
-------
pH
Close pH control is necessary because methane bacteria are extremely sensitive to slight
changes in pH. While the pH is usually allowed to vary from 6.6 to 7.4, it is generally wise
to maintain the pH close to 7.0. In an anaerobic digester, a great quantity of carbon
dioxide is produced during methane fermentation. The pH is, however, normally
maintained by a bicarbonate buffer system. Figure 5-6 prepared by McCarty [11]
illustrates the relationship among pH, the bicarbonate alkalinity of the digester liquor,
and the fraction of CO2 in the digester gas.
Because pH control is so important in digester operation, the dynamic nature of buffer
destruction and formation in the digester should be understood. Therefore, this process is
reviewed in the following equations for simple carbohydrates such as glucose.
acid formers
C6H12O6 >3CH3COOH
3 CH3COOH + 3 NH4HCO3 > 3 CH3COONH4 + 3 H2O + 3 CO2
methane bacteria
3 CH3COONH4 + 3 H2O > 3 CH4 + 3 NH4HCO3
The first equation represents the breakdown of glucose to acetic acid by acid-forming
bacteria. The acid is then neutralized, as shown in the second equation, by the
bicarbonate buffer. If sufficient buffer is not present, the pH will drop, and the
conversion of acetate to methane, as shown in the third equation, would be inhibited.
The buffer consumed in the second reaction is reformed in the third reaction. In a
properly operating digester a dynamic equilibrium is maintained between buffer
formation and destruction, however, when an upset occurs, it is usually the methane
bacteria which are adversely affected rather than the acid formers. Therefore, in an upset,
net buffer consumption takes place, and the process is in danger of pH failure. When this
happens, an external source of alkalinity must be supplied to maintain pH in the proper
range.
Figure 5-6 indicates that the bicarbonate alkalinity should be maintained at a minimum
level of 1,000 mg/1 as CaCO3 to ensure adequate pH control. To determine the
bicarbonate alkalinity, both the volatile acid concentration and the total alkalinity must
be measured. The bicarbonate alkalinity is then calculated as shown:
Bicarbonate Alkalinity = (Total Alkalinity - 0.8 Volatile Acids')
5- 11
-------
05
<
CD
cc
LLI
CD
Q
50
40
30
20
CM
o
o
10
LIMITS OF N
NORMAL DIGESTION s
250 500 1000 2500 5000 10,000 25,000
HC03 CONCENTRATION, (MG/L AS CaCO3)
FIGURE 5-6. Relationship between pH and bicarbonate concentration [11
5-12
-------
The 0.8 factor in the above equation is required to convert the volatile acid units from
mg/1 as acetic acid to mg/1 as CaCO3, the equivalent alkalinity unit. The volatile acid to
total alkalinity ratio should be maintained below 0.5 for good digester operation.
Temperature
The temperature response of methane bacteria is similar to that of other bacterial groups.
Digestion of wastewater sludge is almost always conducted in the mesophilic range and
the optimum temperature in this range is 95° F. More important than maintenance of a
particular temperature is maintenance of the chosen temperature for operation at a
constant value. A temperature change of 2 or 3 degrees can be sufficient to disturb the
dynamic balance between the acid and methane formers. Such a disturbance will lead to
an upset because the acid formers are able to respond more rapidly to changes in
temperature than are the methane bacteria.
Nutrients
Little knowledge is available on the nutritional requirements of methane bacteria and this
has been a stumbling block in the application of anaerobic treatment to industrial
wastewaters. Speece and McCarty [12] have reported the most definitive work on the
macronutrient and micronutrient requirements of these organisms. They indicate that
domestic wastewater appears to contain all of the nutrients required. Thus, difficulty
should only occur in digestion when a large fraction of the sludge is of industrial origin.
Potentially Toxic Materials
A review of this subject has been provided by Kugelman and Chin [13]. They indicate
that toxicity, in general, can be due to an excessive quantity of any material, even a
substance normally considered a nutrient. The concentration at which a substance starts
to exert a toxic effect is difficult to define because it can be modified by antagonism,
synergism, and acclimation. In addition, the organic loading and biological solids
retention time can cause a stress on the process and this stress can affect toxicity. The
substances which can produce toxicity when present in municipal sludge in an excessive
concentration, include heavy metals, sulfides, surface active agents, light metals, and
certain organics. All of these can gain entrance to wastewater sludge from industrial
sources. In addition, light metal cations will enter sludge if an alkaline material is added
to control the pH. Several papers [13,14,15] review the best engineering data available on
toxicity. While reference should be made to these papers for complete information,
general information on some substances is given in Table 5-4.
5- 13
-------
TABLE 5-4
SUBSTANCES AND CONCENTRATIONS CAUSING TOXICITY
IN WASTEWATER SLUDGE DIGESTION [10]
Concentration
Substance (mg/1)
Sulfides 200
Soluble Heavy Metals >1
Sodium 5,000- 8,000
Potassium 4,000 - 10,000
Calcium 2,000 - 6,000
Magnesium 1,200- 3,500
Ammonium 1,700- 4,000
Free Ammonia 150
It must be emphasized that the values in this table are only guides. If toxicity is
suspected, a thorough determination of the chemical constituents in the sludge should be
made before definite conclusions are drawn. Potential solutions to toxicity problems,
other than elimination of the chemical from the wastewater, should be evaluated in
laboratory or pilot digesters.
Process Kinetics
Lawrence and McCarty [16] have reviewed the kinetics of anaerobic digestion and show
that the overall process kinetics are controlled by methane bacteria. In addition, they
found that the removal efficiency could be characterized by the equation:
5-14
-------
Kg(l +Kd-SRT)
S0-
SRT- Km-(l +Kd ' SRT)
E =
where
E = Substrate removal efficiency expressed in decimal form
So = Influent substrate concentration
Kg, Kd, and Km = Kinetic constants
SRT = Biological solids retention time
The engineer and/or plant operator primarily controls the SRT. Thus, this becomes the
fundamental design and control parameter. SRT is analogous to the sludge age parameter
used in activated sludge system design. For a digestion system without sludge recycle, the
SRT is numerically equal to the hydraulic retention time (HRT). This analysis points up a
fallacy in current digestion criteria. Digesters are designed at present on the basis of either
the volume per individual served, the weight of volatile solids per unit volume of digester
per unit time, or HRT. Of these, the only valid criterion is HRT.
Values for the kinetic constants discussed above were determined experimentally by
Lawrence and McCarty [17]. These values indicate that at 95° F the absolute minimum
SRT for anaerobic digestion is three to four days. This value agrees well with the
minimum HRT determined by Torpey [18] in field studies. For design purposes, a longer
HRT should be utilized to provide a safety factor against upsets and to allow for
fluctuations in sludge volume. In addition, the rate limiting step in some situations is
solubilization of grease and/or protein, which requires HRT values longer than four days.
Suggested retention times for high rate digesters were given in Table 5-3.
5.2.4 Process Performance Data
Anaerobic digestion treats sludge by converting approximately 50 percent of the organic
solids to liquid and gaseous forms. Discussion of expected digester performance can best
be illustrated by an example. The Chicago Sanitary District [19] digesters were designed
as shown in Table 5-5. Note that the system design is that of high rate and must handle a
high percentage of EAS. The summary of a year's operating results for the Southwest
Treatment Plant's digesters is shown in Table 5-6. The data are for the period from July,
1964, to July, 1965, and were obtained at an average loading of 0.081 Ib/day/cu ft of
5- 15
-------
TABLE 5-5
DESIGN DATA FOR CHICAGO DIGESTERS
Parameter Value
Solids to digesters, tons/day 100
Sludge to digesters, % solids 3.3
Sludge to digesters, % volatile 67
Digester displacement, days 14
Volatile solids digested, % 40 to 45
Gas per pound volatile solids digested, cu ft 16 to 18
Heat per cubic foot of gas, BTU 600 to 650
Temperature of digesters, ° F 90 to 95
Sludge to digesters:
Activated sludge solids, % 80 to 100
Primary sludge solids, % 20 to 0
TABLE 5-6
SUMMARY OF SOUTHWEST TREATMENT PLANT DIGESTER OPERATION
Gas Produced
Alkalinity Volatile Solids cu ft/lb
Total Volatile pH (mg/1 asCaCO3) Total Volatile Reduction % Volatile Destroyed
Feed 3.1 65.3 6.5 671 82.9 54.4
Draw-
off 2.4 54.7 7.1 2,162 63.4 35.0 35.7 20.0
volatile solids, a detention time of 16 days, and temperature of 93° F. As can be seen
from the table, excellent volatile solids reduction was obtained with good g#s production
in a relatively short detention time. Not unexpectedly, the solids concentration
decreased, the pH increased slightly, and the alkalinity increased by a factor of three.
5-16
-------
Gas Production
The current energy crisis has created additional interest in the utilization of sludge gas as
an energy source. Many plants for years have used digester gases to heat facilities and
drive generators. In general, treatment of 1 mgd of municipal wastewater will provide 1
ton of mixed primary and activated sludge solids which translates to 0.2 to 0.3 Ib
solids/capita/day. An unheated digester will typically produce 0.32 to 0.56 cu ft of
gas/capita while a heated digester will produce from 0.56 to 0.74 cu ft of gas/capita. This
is equivalent to a maximum gas production of approximately 11 to 12 cu ft of gas/lb of
total solids digested. The heat value of sludge gas is approximately 566 BTU/cu ft. Should
electrical generation be considered, approximately 3.5 cu ft of gas is required to produce
1 kilowatt hour (kwh) of electricity [20,21]. Table 5-7 shows the characteristics of
sludge gas from several digester installations. As can be seen, the methane content of the
gas varies with the feed sludge. Normally, some type of off-gas treatment is necessary to
increase the heat content.
TABLE 5-7
CHARACTERISTICS OF SLUDGE GAS [20]
Constituent
Values for Various Plants
CH4
C02
H2
N2
H2S
Ho
dv
42.
47.
1.
8.
-
5
7
7
1
BTU/ft3 459
(air=l) 1-
04
61.
32.
3.
2.
0
8
3
9
667
0.
87
62.0
38.0
trace
trace
0.15
660
0.92
'ercent by Volume
67.0
30.0
-
3.0
- 0
624
0.86
70.0
30.0
-
-
.01-0.02
728
0.85
73.
17.
2.
6.
0.
7
7
1
5
06
791
0.
74
75.0
22.0
0.2
2.7
0.1
716
0.78
73-75
21-24
1-2
1-2
1-1.5
739-750
0.70-0.80
Supernatant Quality
Typical digester supernatant characteristics appear in Table 5-8. A range of values is given
for each parameter since a particular supernatant's quality is dependent upon whether the
digester has one or two stages, whether it is mixed, and how well the solids separate from
the liquor.
5- 17
-------
TABLE 5-8
SUPERNATANT CHARACTERISTICS FROM ANAEROBIC DIGESTERS[22]
Suspended solids
BOD5
COD
Ammonia as NH3
Total phosphorus as P
Primary Plants
(mg/1)
200-1,000
500-3,000
1,000-5,000
300- 400
50- 200
Trickling Filters*
(mg/1)
500- 5,000
500- 5,000
2,000-10,000
400- 600
100- 300
Activated
Sludge Plants*
(mg/1)
5,000-15,000
1,000-10,000
3,000-30,000
500- 1,000
300- 1,000
Includes primary sludge.
As can be seen, supernatant from primary sludge digestion requires only minimal concern,
when compared to the poor quality supernatant from anaerobically digested mixtures of
primary and activated sludges. Methods are available for reducing the amount of activated
sludge in the feed to the digester, and this is a way of materially improving the
supernatant quality. Methods for treating digester supernatants have been thoroughly
discussed in the literature [23]. However, elimination, rather than treatment, of highly
polluted digester supernatants is normally sound engineering.
Bacteria Inactivation
Anaerobic digestion reduces bacterial populations as shown in Table 5-9. Although
conditions in digesters are unfavorable for the multiplication of most pathogenic
organisms, they are not lethal, and the principle bactericidal effect appears to be related
to a natural die-off with time.
Impact of Alum and Iron Phosphorus Sludge on Anaerobic Digestion
Chapter 3 indicated that the addition of aluminum sulfate or ferric chloride for improved
suspended solids removal and/or phosphorus removal to the primary, biological, or
tertiary portion of the treatment plant can substantially increase both the mass and
volume of sludge to be treated. Many studies have indicated that neither ferric chloride
nor alum phosphorus sludges inhibit anaerobic digestion [24,25,26,27,28]. However,
digester performance may be altered due to an increased stress from a higher organic
loading and/or lower HRT or feed sludge alkalinity.
5- 18
-------
TABLE 5-9
BACTERIAL SURVIVAL IN DIGESTION [2]
Digestion Period Removal
Bacteria (days) (%)
Remarks
Endamoeba
hystolytica 1 2
Salmonella
typhosa 20
Tubercle
bacilli 35
Escherichia
coli 49
<1 00 Greatly reduced populations at 68° F
92 85% reduction in 6 days detention
85 Digestion cannot be relied upon for complete
destruction
<100 Greatly reduced populations at 99° F, about
the same reduction in 14 days at 72° F
At Chapel Hill, North Carolina, it was reported [29] that digester alkalinity was reduced
in the primary digester from 2,500 mg/1 to 1,500 mg/1 as CaCO3. This occurred after
addition of alum to one of two parallel trains, resulting in a need to add lime on one
occasion. Further, the secondary digester underflow concentration decreased from a
normal range of 6 to 7 percent to 3.8 percent with a coincident increase in the
supernatant SS concentration from 1,000 mg/1 to 10,000 mg/1. In spite of these
difficulties, the digestion process itself produced a normal reduction in volatile solids
throughout the entire alum treatment study.
5.2.5 Upgrading Procedures
The reader is referred to the EPA Technology Transfer Process Design Manual for
"Upgrading of Wastewater Treatment Facilities," for a thorough presentation of typical
operating results and upgrading procedures [ 10].
The methods listed here have been successful in improving the efficiency of operating
digesters.
• Prethickening of feed sludge.
• Complete mixing of digester.
5- 19
-------
• Adequate control of operating variables.
• Separate processing of excess air system activated sludge by means other
than anaerobic digestion where feasible.
• Where anaerobic digestion of mixtures of primary and excess air system
activated sludge is essential, utilize other process modifications to minimize
the relative amount of EAS in digester feed and to resolve the solid-liquid
separation problem which can result in the second-stage digester.
5.2.6 Typical Costs
Unit costs are presented in Figure 5-7. Use of this cost data should include allowance for
the effect of supernatant recycle plus effect of digestion on sludge dewaterability.
5.3 Aerobic Digestion
Aerobic digestion describes the separate aeration of waste primary sludge, waste
biological sludge, or a combination of waste primary and biological sludges in an open
tank. It is usually used to stablize excess activated sludges or the excess sludges from
small plants which do not have separate primary clarification. Figure 5-8 shows a
schematic diagram of an aerobic digestion system. The process involves the direct
oxidation of any biodegradable matter by the biologically active mass of organisms and
oxidation of microbial cellular material. These two steps are illustrated by the following
reactions:
Bacteria
organic matter + O2 > cellular matter + CO2 + H2 O
Volatile Solid
cellular matter + O2 > digested sludge + CO2 + H2 O
The second reaction called endogenous respiration is normally the predominant one
occurring in aerobic digestion. Stabilization is not complete until there has been an
extended period of primarily endogenous respiration (15 to 20 days). Major objectives of
aerobic digestion include odor reduction, reduction of biodegradable solids, and improved
sludge dewaterability. Process advantages often cited for this process over other
stabilization techniques are that it is:
5-20
-------
O
90
W
t/i
3
O
°t
H'
o
o
on
-* z
O
••3-SN^
gSSo
s. o o's'
0) (D
Is
(00
S3
og
S 3J
O
O
CD
X
O
rt— 00
to D- 10
»i "
COSTS ($71,000 CU. FT./DAY)
o ->
CONSTRUCTION COST (MILLIONS OF DOLLARS)
-------
PRIMARY SLUDGE
EXCESS
ACTIVATED OR
TRICKLING FILTER
SLUDGE
K)
NJ
/ *^. •. • 21 ^ ' V-~»-s. "ff." \
•V~V" \ *\ /• ••«•?/ I
s\- \ v /. /• -»y
SETTLED SLUDGE RETURNED TO AERODIGESTER
CLEAR
OXIDIZED
OVERFLOW
TO PLANT
FIGURE 5-8. Schematic of aerobic digestion system.
-------
• Relatively simple to operate.
• Requires a small capital expenditure compared to anaerobic digestion.
• Does not generate significant odors.
• Reduces the number of pathogenic organisms to a low level.
• Reduces the quantity of grease or hexane solubles.
• Produces a supernatant, if clarified, that is low in BOD, solids, and total P.
• Reduces sludge's respiration rate.
The process is not without disadvantages and these may include high operating cost and
unclear design parameters at present. Aerobically stabilized sludge generally has poor
dewatering characteristics on vacuum filters [30]. Ordinarily, this sludge is dewatered on
sandbeds or applied in liquid form to cropland.
5.3.1 Process Design
The important process parameters are:
• Air or oxygen requirements
• Time of aeration
• Sludge age
• Temperature
• Biodegradable volatile solids
• Processing characteristics of digested sludge
• Supernatant quality
Current practice is to provide approximately 15 days of detention time for the
stabilization of excess biological sludges, additional time is required when primary sludge
is included. Loadings will normally vary from 0.1 to 0.2 Ib VSS/ft3/day. A 40 to 50
percent reduction in volatile suspended solids content is normally obtained. It is possible
for the supernatant to contain as little as 10 to 30 mg/1 BOD, 10 mg/1 ammonia nitrogen,
and from 50 to 100 mg/1 nitrate nitrogen [31]. Oxygen requirements, exclusive of
nitrification can vary from a minimum of 3 to 30 mg/hr/gm VSS under aeration or
higher. When nitrification occurs, both pH and alkalinity are reduced.
5-23
-------
Current practice varies according to vvhether or not a separate sedimentation tank is
employed. Smaller plants employ a one-tank, batch-type system, wherein the sludge is
supplied with air and completely mixed for a protracted period of time followed by
quiescent settling and decantation. Aerobic digesters are designed similarly to rectangular
aeration tanks and use conventional aeration systems or they frequently employ circular
tanks and use an eductor tube for deep-tank aeration. Figure 5-9 shows a typical circular
aerobic'digester.
Plants processing significant amounts of sludge frequently incorporate a separate
sedimentation basin which facilitates simultaneous decantation and thickening. Aerobic
digestion in the past has largely been practiced at small plants on either contact
stabilization sludge or on mixtures of primary and activated sludge. Limited plant scale
data on the aerobic digestion process is available. Accordingly, wherever possible, pilot
work should be conducted prior to design. It should include settling, thickening, and
dewatering tests on the aerobically digested sludge alone and in combination with other
sludges to be processed. Table 5-10 presents aerobic digestion design parameters with
associated remarks [ 10].
5.3.2 Process Performance Data
Ahlberg and Boyko [33] investigated seven activated sludge facilities treating from 0.03
to 1.40 mgd of municipal wastewater. Some of these facilities had single-stage and others
had two-stage digester systems. Table 5-11 summarizes the operation of the facilities
investigated. Note the high sludge age and low volatile solids loading of the systems.
These sludges should be very well stabilized. In general the sludges fed to the digesters
had a pH of 6.7, a COD of 20,000 mg/1, and a total solids concentration of 20,000 mg/1
of which 57 percent was volatile. Following digestion, the pH was still around 6.8, the
volatile solids were reduced by approximately 50 percent, and the sludge could, on an
average, be thickened to about 3 percent solids. Characteristics of the supernatant from
settling of the digested sludge are given in Table 5-12. The range of values shown are
caused by differences in the solids-liquid separation processes, digester loadings, and
nature of the sludges [33].
The Denver, Colorado, District Plant treats 100 mgd of combined domestic and industrial
waste by the conventional activated sludge process. Under an Environmental Protection
Agency grant, Denver has been investigating aerobic digestion of dilute excess activated
sludge on a plant scale [34]. From August 1, 1972, to June 30, 1973, Denver studied,
under steady state conditions, the effects of loading rates (0.025 to 0.2 Ib VSS/ft3 /day)
and the effects of naturally varying temperatures (59 to 84° F) on the efficiency of
plant-scale, air-aerobic digestion of excess activated sludge (< 1.0 percent suspended
solids concentration). The two most important variables were discovered to be
temperature and sludge age. Optimum performance occurred at a loading rate of 0.08 Ib
VSS/ft3 /day and a temperature of 75° F, which gave a 42 percent VSS destruction. These
5-24
-------
AIR PLUG VALVE
SUPERNATANT
DRAW OFF
X
CONTROL BAFFLE
WASTE -+-
SLUDGE
DRAW-OFF
DECANT
CHAMBER
4" RETURN SLUDGE
AIR LI FT PUMP
EDUCTOR TUBE
FIGURE 5-9. Typical circular aerobic digester [32],
5-25
-------
TABLE 5-10
AEROBIC DIGESTION DESIGN PARAMETERS
Parameter
Value
Remarks
Solids Retention Time, days
Solids Retention Time, days
Volume Allowance, cu ft/capita
VSS Loading, pcf/day
Air Requirements
Diffuser System, cfm/1,000 cu ft
10-15*
15-20*
3-4
0.024-0.14
20-35*
Diffuser System, cfm/1,000 cu ft >60
Mechanical System, hp/1,000 cu ft 1.0-1.25
Minimum DO, mg/1
Temperature, ° C
VSS Reduction, percent
Tank Design
Power Requirement, BHP/10,000
Population Equivalent
1.0-2.0
35-50
8-10
Depending on temperature, type of sludge, etc.
Depending on temperature, type of sludge, etc.
Enough to keep the solids in suspension and maintain
a DO between 1-2 mg/1.
This level is governed by mixing requirements. Most
mechanical aerators in aerobic digesters require
bottom mixers for solids concentration greater than
8,000 mg/1, especially if deep tanks (>12 feet) are
used.
If sludge temperatures are lower than 15° C, additional
detention time should be provided so that digestion
will occur at the lower biological reaction rates.
Aerobic digestion tanks are open and generally require
no special heat transfer equipment or insulation. For
small treatment systems (0.1 mgd), the tank design
should be flexible enough so that the digester tank
can also act as a sludge thickening unit. If thickening
is to be utilized in the aeration tank, sock type diffusers
should be used to minimize clogging.
Excess activated sludge alone.
Primary and excess activated sludge, or primary sludge alone.
5-26
-------
TABLE 5-11
SUMMARY OF AEROBIC DIGESTION OPERATION
Parameter Range
HRT(days) 14-360
Sludge Age (days) 29-320
Actual Loading (Ib VS/cu ft/day) 0.0035-0.027
Air Supply (cfm/1,000 cu ft) 8.4-46
TABLE 5-12
CHARACTERISTICS OF AEROBIC DIGESTION SUPERNATANT
Overall
Parameter Average Range
pH 7.0 5.9-7.7
mg/1
BODS 500 9-1,700
Filtered BOD5 51 4-183
COD 2,600 288-8,140
SS 3,400 46-11,500
KjeldahlN 170 10-400
TotalP 98 19-241
Soluble P 26 2.5-64.0
5-27
-------
conditions also coincided with an oxygen requirement of 1.2 to 1.6 Ib O2/lb VSS, a
dissolved oxygen level of 1 to 2 mg/1 in the mixed liquor, a hydraulic detention time of
about 5 days, and an aeration rate of 30 cfm/1,000 ft3. The Denver project is discussed in
more detail as a case history in Chapter 10.
5.3.3 Oxygen Aerobic Digestion
Pure oxygen, rather than air, may be used in aerobic digestion to stabilize thicker sludges
in which the high oxygen uptake rates cannot be satisfied with air aeration. A
three-month plant scale study of aerobic digestion using pure oxygen in a closed system
was conducted at Speedway, Indiana [35]. Oxygenation took place in a covered 31,000
cu ft, four-stage reactor followed by a clarifier for decanting the sludge. The study was
divided into two phases, the first treating only excess activated sludge and the second
treating mixed primary and excess activated sludge. Much of the heat generated by
biological oxidation was retained within the closed system. This resulted in a significant
increase in sludge temperature and a corresponding increase in the rate of VSS
destruction. The results of this study are shown in Table 5-13. A positive DO level of 2
mg/1 was maintained.
TABLE 5-13
RESULTS OF HIGH-PURITY OXYGEN AEROBIC DIGESTERS
SPEEDWAY, INDIANA
Parameter
Feed Sludge Type
Feed TSS, percent
VSS/TSS
Biodegradable VSS/TSS
Feed Temperature, °C
Average Ambient Air Temperature, °C
Sludge Temperature-Stage 4, °C
HRT, days
Volatile Solids Loading Rate, Ib/ft3/day
VSS Reduction, percent
Biodegradable VSS Reduction, percent
Phase 1
EASa
2.14
0.17
0.45
19.5
7.5
33.0
16.3
0.064
44
94.6
Phase 2
57% EAS + 43% PSb
3.06
0.66
0.48
16.0
-2.2
31.0
11.6
0.109
43
86.6
^Excess activated sludge.
Primary sludge.
5-28
-------
5.3.4 Aerobic Digestion Costs
Ritter [36], in 1970, estimated the average power costs of aerobic digestion for three
small communities in Pennsylvania, at about $2.18/yr/lb BOD received per day, or
$0.37/yr/capita served. Capital cost for aerobic digestion is estimated in Figure 5-10.
Evaluation of aerobic digestion versus anaerobic digestion should take into account the
effect of unit process selection on overall system cost as well as the operating and capital
costs of the two-unit processes.
5.4 Chlorine Oxidation
The Purifax process oxidizes sludge with heavy doses of chlorine (about 2,000 mg/1).
Following treatment, the sludge dewaters well on sandbeds and it is stable. Purifaxed
sludges may require chemical conditioning prior to dewatering on vacuum filters, since the
sludge after treatment has a low pH (about 2). Supernatants and filtrates from the process
contain high concentrations of chloramines. A first estimate of cost would be
approximately $5/ton of dry sludge solids for the purchase of chlorine only. Other
operating costs and capital costs would increase this figure.
5.5 Lime Treatment
The addition of lime, in sufficient quantities to maintain a high pH between 11.0 and
11.5, stablizes sludge and destroys pathogenic bacteria. Lime stablized sludges dewater
well on sandbeds without odor problems. Sludge filterability can be improved with the
use of lime; however, caution is required when sludge cake disposal to land is practiced.
Disposal in thick layers could create a situation where the pH could fall to near 7 prior to
the sludge drying out, causing regrowth of organisms and resulting noxious conditions.
Essentially, no organic destruction occurs with lime treatment. The key factor in assuring
a proper stabilization process is the maintenance of a pH of around 11.0.
Farrell et al. [37] recently investigated the lime stabilization of sludge at the 1.15 mgd
Lebanon, Ohio, wastewater treatment plant. While this plant had an anaerobic digester, it
needed a simple, reliable, and inexpensive sludge treatment process to handle the
excessive solids produced by the upgrading of the facility for phosphorus removal. Both
iron and aluminum additions to the primary portion of the plant were employed. The
procedure followed in this attempt to achieve lime stabilization included the addition of
sufficient lime [Ca(OH)2 ] to elevate the sludge to a pH of 11.5, where it remained for 30
minutes. The dosage essentially maintained the pH above 11.0 for 24 hours. The mixing
of the lime slurry and the sludge was accomplished using air mixing.
5-29
-------
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NOTES:
1. Minneapolis. Mar., 1972. ENR Construction Cost Index of 1827.
2. Amortization at 7% for 20 years.
3. I nfluent sludge of 38% primary and 62% waste activated sludge with a
solids content of 3.5%.
4. 20 day volumetric displacement time.
b. Souce: EPA Cost and Manpower Report and Stanley Consultants.
FIGURE 5-10. Aerobic digestion capital cost [2]
5-30
-------
The effect of lime treatment on typical pathogenic bacteria present in sludge is shown in
Table 5-14. As can be seen, salmonella and pseudomonas were totally eliminated and the
total aerobic count was reduced by between 88 and greater than 99 percent. In later
testing, the fecal coliform and fecal streptococci were shown to be destroyed by greater
than 99 percent.
TABLE 5-14
BACTERIOLOGICAL STUDIES OF SLUDGE PRODUCED
IN PLANT-SCALE TESTS AT LEBANON
Bacterial Count (organisms/1 of sludge)
Sludge
Alum-primary
Limed alum-primary
Ferric-primary
Limed ferric-primary
Salmonella
Species
110
None detected
>24,000
None detected
Pseudomonas
aeruginosa
1,300
None detected
610
None detected
Total Aerobic
Count X 10'8
41
5.0
190
0.29
The effect of lime stabilization on the vacuum filterability of the sludge was studied in
the laboratory with a filter leaf apparatus. The results are shown in Table 5-15. Note the
filter yield in every case was increased by a factor of about two, while cake moisture was
essentially unchanged.
TABLE 5-15
EFFECT OF LIME ON FILTERABILITY OF ALUMINUM
AND IRON PRIMARY SLUDGES AT LEBANON
Sludge Property
Filter leaf test yield
(lb/hr/ft2)
Cake moisture
(Ib water/lb dry solids )
Lime Addition
Before
After
Before
After
i i i
Al Dose
(mg/1)
0.98 0.94 0.95
1.97 2.10 2.58
4.31 4.35 4.37
3.87 3.92 3.83
Fe+++ Dose
(mg/1)
1.06 1.57
1.57 2.40
4.10 3.75
4.28 3.75
5-31
-------
In every instance, the lime conditioned sludge exhibited no obnoxious odors and because
high pH conditions were present, the ammonia nitrogen concentration was reduced by
approximately 50 percent as a result of air stripping. The average cost for lime addition is
shown in Table 5-16. This information is based on a hydrated lime cost of $20/ton.
TABLE 5-16
AVERAGE COST OF LIME ADDITION-PLANT-SCALE TESTS AT LEBANON
Treatment of Resultant Sludge Ca(OH)2 Added
Raw
Chemical
A1+++
pe+++
Wastewater
Dose (mg/1)
31.8
22.7
13.6
31.0
15.5
Solids
(g/1)
24.0
28.5
20.6
20.2
17.9
(g/1)
6.0
6.3
4.4
2.2
2.2
to Sludge
(Ib/ton)
500
440
420
220
240
Lime
Cost
($/ton)
5.0
4.4
4.2
2.2
2.4
More recent information has been obtained through an EPA—Battelle study [38] on the
evaluation of lime stabilization. Results show that the pH must be maintained between
12.2 and 12.4 to insure that the sludge is stabilized and the pH should remain above 11.0
for better than 2 weeks. Paulsrud and Eikum [39] agree with these findings and
determined the lime doses required to keep the sludge at a pH greater than 11.0 for 14
days. This information is shown in Table 5-17. Battelle has estimated the costs for lime
addition to a pH of 12.2 to 12.4 including all operation and maintenance costs to range
from $8/ton for primary sludges to approximately $15/ton for biological sludges.
TABLE 5-17
LIME DOSE REQUIRED TO KEEP SLUDGE
pH >11.0 FOR AT LEAST 14 DAYS
Type
Dose (Ib Ca(OH)2 /ton sludge solids)
Primary sludge
Septic tank sludge
Biological sludge
Al sludge (secondary precipitation)
Al sludge (secondary precipitation)
+ Primary sludge (SSyy : SSp^jn
Fe sludge (secondary precipitation)
=1:1)
200- 300
200- 600
600-1,000
800-1,200
500- 800
700-1,200
5-32
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5.6 Heat Treatment for Stabilization
Heat treatment is a well known method of destroying pathogenic organisms and has been
applied successfully for disinfecting sludge. Two methods that have been applied to
sludge treatment are pasteurization and low pressure oxidation.
Pasteurization implies heating to a specific temperature for a time period that will destory
undesirable organisms in sludge. In West Germany and Switzerland, pasteurization is
required when sludge is spread on pastures during summer growth periods. Table 5-18
shows the effect of various pasteurization temperatures and times on typical pathogenic
organisms found in sludge [40]. Stern [41] recently concluded that pasteurization at 70°
C for 30 to 60 minutes is effective for destroying pathogens in digested sludge. About 75°
C for one hour is effective in reducing coliform indicators below 1,000 counts per 100 ml
as well as destroying pathogens. Stern's estimated cost for pasteurization ranged from
$5.00 to $22.00 per ton of dry sludge solids and was dependent upon the size plant, the
fuel source, and whether heat recuperation was employed.
TABLE 5-18
EFFECT OF TIME AND TEMPERATURE ON THE SURVIVAL OF
TYPICAL PATHOGENS FOUND IN SLUDGE*
Temperature ° C
Organism 50
Cysts of Entamoeba histolytica 5
Eggs of Ascaris lumbricoides 60
Brucella abortus
Corynebacterium diphtheria
Salmonella typhosa
Escherichia coli
Micrococcus pyrogene var. aursus
Mycobacterium tuberculosis var. promixis
Viruses
55 60 65
7
60 3
45
30
60
70
4
4
5
20
20
25
Pathogens completely eliminated at indicated time and temperature.
5-33
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Under normal conditions of low pressure oxidation (LPO) employment, which is to
condition the sludge for improved dewaterability, all pathogenic organisms are destroyed
due to the high temperatures achieved and the retention time. Typically, the sludge
temperature is elevated to between 350° and 400° F, pressure is raised to 180 to 210 psi,
and the retention time is between 20 and 30 minutes. Chapters 6 and 11 discuss the LPO
systems in more detail.
5.7 Composting
Composting of sludge, either separately or with municipal solid wastes has not been
widely applied in North America. Of the 18 plants constructed in the United States
between 1951 and 1969, few are currently operated and many of these are operated only
intermittently. The primary problem has been lack of a market for the stable product and
a market is required to produce revenue from the product's sale. This revenue offsets the
cost of the process and can make it economical. Many of the processes involve
composting in windrows with mechanical turning to provide oxygen for the
microorganisms to carry out the stabilization process.
5.7.1 Process Description
Though there are over 30 composting systems identified by inventor or proprietary name,
in general, the methods can be broadly classified by the digestion procedure employed.
Digestion is accomplished by the windrow with intermittent mixing procedure or by
aeration in a mechanical device.
Sequential steps usually involved in composting are:
• Preparation—Sludge that is composted without the inclusion of solid waste fractions,
must be blended with some bulking material if windrows are to be used. This
bulking material can be soil, sawdust, wood chips, etc. If a mechanical aeration
system is used; bulking agent requirements are less severe. For good digestion of the
waste, a moisture content between 45 and 65 percent by wet weight is desirable. A
potential advantage exists when combined sludge—solid waste composting is
practiced, because the digested sludge can provide nutrients and requisite moisture
to the solid waste fraction. Normal sludge to refuse ratios of roughly 0.50 to 1.00 by
weight are employed.
• Digestion—The digestion period is characterized by rapid decomposition. Air is
supplied by periodic turnings in windrow-type operations. While in mechanical
systems, forced draft or agitation in long screw conveyors is utilized. The reaction is
exothermic and the wastes reach temperatures of 140° F to 160° F or higher.
Pathogen kill and the inactivation of objectionable materials are possible at these
temperatures. Period of digestion is normally about six weeks for windrows and
5-34
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several days in mechanical aeration systems. Although the ratio of carbon to
nitrogen (C/N) can be used as a rough estimate of completion, careful testing is
required to determine operable conditions. A C/N ratio of 20 has been suggested as a
theoretical upper limit. This ratio prevents robbing of nitrogen from soil treated
with compost by further microbial activity.
• Curing—This is characterized by a slowing of the decomposition rate. The
temperature drops back to normal and the process is brought to completion. This
period takes about two more windrow weeks or one to two weeks for mechanical
systems.
• Finishing-lf municipal solid waste fractions containing nondigestible debris have
been included, some sort of screening or other removal procedure may be necessary.
5.7.2 Process Performance and Costs
Time and temperature are two major design considerations since they effect the hygienic
quality of the composting product as shown in Table 5-19 [42]. From this table it can be
seen that the water content of the material entering the system ranges between 40 and 60
percent. Typically, a mechanical system can achieve a temperature in excess of 60° C and
frequently in excess of 65° C. These materials are then pathogen-free in as little time as
one day and free of all spore formers in approximately one week.
Volume and weight reductions hinge on the type of mixture fed to the composting
process, so generalizations are not of great value. It has however been observed that from
20 to 50 percent less space is required in a landfill when the waste material has been
composted. Composting costs vary widely and ranges of $2 to $20 per ton have been
reported. Some recent capital and operating costs are shown in Figure 5-11. Revenue
from the sale of the composted product is usually $1.50 to $3.50 per ton of material fed.
5.8 Additional Reading
Dague, R. R., Mckinney, R. E., and Pfeffer, J. L., "Solids Retention in Anaerobic
Waste Treatment Systems." /. Water Pollut. Contr. Fed., 42 (2), Part 2 (1970).
Fourie, J. M., "Composting of Municipal Solid Refuse." Water Pollut. Contr. (1973),
pp. 205-208.
Haug, L. A., "Sludge Disposal May Pay for Itself." Water Wastes Eng. (Apr. 1973),
p. 72.
Kampelmacher, E. H. and vanNoorde, J. L. M., "Reduction of Bacteria in Sludge
Treatment."/. Water Pollut. Contr. Fed., 44 (2), 309-313 (1972).
5-35
-------
TABLE 5-19
HYGIENIC QUALITY OF COMPOST
Tieatment Method
Material
Water
Content (%)
Maximum
Temp.
Achieved (° C)
Hygienic
Evaluation
Remarks
Contour spreading sludge + solid
waste 55
-Contour Composting -
46
Not pathogen-free
after 5 months
Windrow spreading sludge
60
52
Not pathogen-free
after 6 months
Windrow spreading solid waste
40-60
Pathogen-free
after 3 weeks
Windrow spreading sludge + solid
waste
40-60
>55
Pathogen-free
after 3 weeks
Rotating drum
(Dano Process) solid waste
-Mechanical Composting-
45-55
>60
Pathogen-free
after 6-7 days
• Spore-free after 1
week of windrow
composting
Rotating drum sludge -t- solid
waste
approx
50 >60
Pathogen-free
after 6-7 days
• Spore-free after 1
week of windrow
composting
Rotating tower
(Multibacto
process)
Rotating tower
solid waste
sludge + solid
waste
40-50 >65
45-55 >65
Pathogen-free
after 1 day
Pathogen-free
after 1 day
• -Spore-free after 1
week of windrow
composting
Spore-free after 1
week of windrow
composting
5-36
-------
GROSS COST/TON
(SCALE RIGHT)
NET COST/TON
(SCALE RIGHT)
INITIAL CAPITAL COST
1000 DOLLARS/INSTALLED TON
(SCALE LEFT)
150 200 250 300
PLANT CAPACITY (TON/DAY)
NOTES:
1. Plant capacity is normally one or two shifts per day to achieve plant capacity.
2. Gross cost trend is the owning and operating facilities without any credits.
3. Net cost trend is for owning and operating facilities considering sales of compost
and salvaged materials.
4. All costs consider compost digested sludge with refuse.
5. Source: Composting of Municipal Solid Wastes in the United States, US
Environmental Protection Agency (1971).
FIGURE 5-11. Composting costs [2 ]
5-37
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Koch, S. G., "Anaerobic to Aerobic Digestion: Deeds and Data." J. Water Pollut.
Contr. Fed., pp. 9-10.
Oldshue, J. Y., "Mixing in Anaerobic Digesters—Tonawanda, New York." American
City (Feb. 1974), p. 80.
Pretorius, W. A., "Principles of Anaerobic Digestion." Water Pollut. Contr. (1973),
pp. 202-204.
Reynolds, T. D., "Aerobic Digestion of Waste Activated Sludge." Water Sewage
Works (Feb. 1967), pp. 37-42.
Singley, M. E. and Bridgeton, N. J., "Sludge Composting Project: A City-Farm
Relationship." Compost Sci. (Sep.-Oct. 1973), pp. 18-21.
Stanbridge, H. H., "The Consolidation and Digestion of Activated Sludge." /. Proc.
Inst., Sew. Purif. (1966), pp. 492-496.
Walker, J. M. and Willson, G. B., "Composting Sewage Sludge: Why?" Compost
Sci., 14(4), 10-12(1973).
Washington, R. R. and Symons, J. M., "Volatile Sludge Accumulations in Activated
Sludge Systems."/. Water Pollut. Contr. Fed., 34, 767 (1962).
5.9 References
1. "Anaerobic Sludge Digestion." /. Water Pollut. Contr. Fed., MOP, No. 16
(1968).
2. Stanley Consultants, Inc., "Sludge Handling and Disposal—Phase I—State of the
Art." A Report to the Metropolitan Sewer Board of the Twin Cities Area, Nov.
15, 1972.
3. Garber, Bill, City of Los Angeles, California, and Smith, J. E., Jr., National
Environmental Research Center, EPA, Cincinnati, personal communication,
1974.
4. Schroepfer, G. J. and Ziemke, N. R., "Development of the Anaerobic Contact
Process—I Pilot Plant Investigations and Economics." Sewage and Ind. Wastes,
31 (2), 164-190.
5-38
-------
5. McCarty, P. L., "Kinetics of Waste Assimilation in Anaerobic Treatment," in
"Developments in Industrial Microbiology." American Inst. Biol. Sciences, 7
(1966).
6. Ritter, E. L., "Design and Operating Experiences Using Diffused Aeration of
Sludge Digestion."/. WaterPollut. Contr. Fed., 42 (10), 1782 (1970).
7. Clark, J. W. and Viessman, W., Jr., Water Supply and Pollut. Contr., Inter
Textbook Co.: Scranton, Pennsylvania (Mar. 1966).
8. Dague, R. R., "Is the Digester Obsolete?" Presented at the 14th Annual Great
Plains Wastewater Design Conference, Omaha, Nebraska, 1970.
9. Burd, R. S., "A Study of Sludge Handling and Disposal." Federal Water
Pollution Control Administration, Publication WP-20-4 (May 1968).
10. Technology Transfer Process Design Manual for "Upgrading Existing
Wastewater Treatment Plants" (1 st revision) (1974).
11. McCarty, P. L., "Anaerobic Waste Treatment Fundamentals." Pub. Works, 95,
107-112(1964).
12. Speece, R. L. and McCarty, P. L., "Nutrient Requirements and Biological
Solids Accumulation in Anaerobic Digestion." Proceedings of the International
Conference on Water Pollution Resources, Pergamon Press, 1962.
13. Kugelman, I. J. and Chin, K. K., "Toxicity Synergism and Antagonism in
Anaerobic Waste Treatment Processes." Presented before Division of Air, Water
and Waste Chemistry, A.C.S., Houston, Texas, Feb. 1970.
14. Kugelman, I. J. and McCarty, P. L., "Cation Toxicity and Stimulation in
Anaerobic Waster Treatment." /. Water Pollut. Contr. Fed., 37 (1), 97-115
(1965).
15. Lawrence, A. W., Kugelman, I. J., and McCarty, P. L., "Ion Effects in
Anaerobic Digestion." Technical Report No. 33, Department of Civil
Engineering, Stanford University (Mar. 1964).
16. Lawrence, A. W. and McCarty, P. L., "Unified Basis for Biological Treatment
Design and Operation." /. Sanit. Eng. Div., A.S.C.E., 96 (SA3), 757-778
(1970).
17. Lawrence, A. W. and McCarty, P. L., "Kinetics of Methane Fermentation in
Anaerobic Treatment." /. Water Pollut. Contr. Fed., 41 (2), Part 2, R1-R17
(1969).
5-39
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18. Torpey, W. N., "Loading to Failure of a Pilot High-Rate Digester." Sewage and
Ind. Wastes, 27 (2), 121-133 (1955).
19. Lynam, B., McDonnel, G., and Krup, M., "Start-up and Operations of Two
New High-Rate Digestion Systems." /. Water Pollut. Contr. Fed., 40 (5), 518
(1967).
20. Herpers, H. and Herpers, E., "Importance, Production and Utilization of
Sewage Gas." KWG-Kohlenwosse-Stoffgase 72, 18, (1966).
21. "Methane Digesters for Fuel Gas and Fertilizer." New Alchemy Institute,
Newsletter No. 3 (Spring 1973).
22. Kappe, S. E., "Digester Supernatant: Problems, Characteristics, and
Treatment." Sewage Ind. Wastes, 30, 937 (1958).
23. Maliva, J. F., Jr. and DiFilippo, J., "Treatment of Supernatants and Liquids
Associated with Sludge Treatment." Water Sewage Works (1971), R-30.
24. Dow Chemical Company, "Application of Chemical Precipitation Phosphorus
Removal at the Cleveland Westerly Wastewater Treatment Plants." Report
prepared for the City of Cleveland, Ohio, by the Dow Chemical Company (Apr.
1970).
25. Derrington, R. E., Stevens, D., and Laughlin, J. E., "Enhancing Trickling Filter
Performance by Chemical Precipitations."
26. Long, D. A., Nesbitt, J. B., and Kountz, R. R., "Soluble Phosphorus Removal
in the Activated Sludge Process." Report prepared for the Water Quality
Office, U.S. EPA, Project No. 17010 EIP (Aug. 1971).
27. Thompson, J. C., "Removal of Phosphorus at a Primary Wastewater Treatment
Plant." Paper presented at spring meeting, New England Water Pollution
Control Association, Stratton, Vermont (Jun. 7, 1972).
28. Johnson, E. L., Beeghly, J. H., and Wukasch, R. F., "Phosphorus Removal at
Benton Harbor—St. Joseph, Michigan." Report prepared for Benton Harbor, St.
Joseph Joint Board of Commissioners, Michigan. (1968).
29. Brown, F. C., Little, L. W., Francisco, D. E., and Lamb, J. C., "Methods for
Improvement of Trickling Filter Plant Performance," Part II, Alum Treatment
Studies. U. S. Environmental Protection Agency, Contract No., 14-12-505,
University of North Carolina, Chapel Hill, North Carolina (1974).
5-40
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30. Cameron, J. W., "Aerobic Digestion of Activated Sludge to Reduce Sludge
Handling Costs." Presented at 45th Annual Conference Water Pollution Control
Federation, Atlanta, Georgia, Oct. 1972.
31. Smith, J. E., Jr. to Dean, R. B., EPA Memo, "Aerobic Stabilization of
Activated Sludge" (Jan. 4, 1970).
32. Hervol, H. J. and Pyle, R. H., "Aeration Got You Down?" Part II. Water Wastes
Eng. (Jan. 1974), pp. 15-20.
33. Ahlberg, N. R. and Boyko, B. I., "Evaluation and Design of Aerobic Digesters."
J. Water Pollut. Contr. Fed., 44, 634 (1972).
34. Cohen, D. B. and Puntenny, J. L., "Metro Denver's Experience with Large
Scale Aerobic Digestion of Waste Activated Sludge." Presented at 47th Annual
Conference Water Pollution Control Federation, Cleveland, Ohio, Oct. 1973.
35. Smith, J. E., Jr., Young, K. W., and Dean, R. B., "Biological Oxidation and
Disinfection of Sludges," prepublication copy (1973).
36. Ritter, E. L., "Design and Operating Experiences Using Diffused Aeration of
Sludge Digestion."/. Water Pollut. Contr. Fed., 42 (10), 1782 (1970).
37. Farrell, J. B., Smith, J. E., Jr., Hathaway, S. W., and Dean, R. B., "Lime
Stabilization of Primary Sludges."/. Water Pollut. Contr. Fed., 46, 113 (1974).
38. Courts, C. A. and Schuckrow, A. J., "Design, Development, and Evaluation of a
Lime Stabilization System to Prepare Municipal Sewage Sludge for Land
Disposal." Report for EPA, Contract No. 68-03-0203, Pacific Northwest Lab.,
Battelle Inst. (1974).
39. Paulsrud, B. and Eikum, A. S., personal communication, Norwegian Institute
for Water Research, Apr. 1974.
40. Roediger, H., "Pasteurization of Digested Sludge." Stadtehygiene (1958).
41. Stern, G., personal communication, NERC, EPA, Cincinnati, Ohio, May 1974.
42. Knoll, K. H. and Strauch, D., "Hygienisch Bakteriologischer Guteindex Der
Kompostierungsverfahren." Human-un Veterinarhygiene Handbuch Mull und
Abfallbeseitigung, 5165 Kumpf - Maas - Strub (1969).
5-41
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-------
CHAPTER 6
SLUDGE CONDITIONING
6.1 Functions, Methods, and Occurrences
Conditioning is herein defined as the pretreatment of sludge to facilitate water removal by a
thickening or dewatering process. Table 6-1 shows the usual conditioning methods, the unit
processes they are employed with, and the purposes they serve.
TABLE 6-1
CONDITIONING METHODS AND PURPOSES
Conditioning Method Unit Process
Function
Polymer Addition
Polymer Addition
Elutriation
Heat Treatment
Ash Addition
Thickening Improve loading rate, degree of concentration,
and solids capture.
Dewatering Improve production rate, cake solids content,
and solids capture.
Inorganic Chemical Dewatering
Addition
Improve production rate, cake solids content,
and solids capture.
Dewatering Decrease acidic chemical conditioner demand
and increase degree of concentration.
Dewatering Eliminate or decrease chemical use, improve
production rate, cake solids content, and
stabilization. Some conversion may also occur.
Dewatering Provide improved cake release from belt type
vacuum filters and facilitate filter pressing. It
can also result in higher filter yields and
reduced chemical requirements.
It should be noted that the first three methods involve addition of chemicals to coagulate
and/or flocculate the sludge to accomplish the functions listed. The methods do not
6-1
-------
otherwise materially change the nature of the sludges. The inorganic chemicals, however,
can change the solution pH, increase the inorganic fraction of sludge, and also affect
stabilization. Ferric chloride addition with or without lime has been the principal method
for conditioning sludges prior to dewatering. In the past ten to fifteen years, however,
polymer conditioning before dewatering has become widespread. The metal salts are
generally only used now where polymers have not yet been demonstrated as economically
effective or on sludges where polymers will not work. Polymers are also widely used for
flotation and centrifugal thickening and they are occasionally used in gravity thickening.
Elutriation was originally developed to decrease the alkalinity of anaerobically digested
sludges which reduced the demand for acidic metal salts. It is not now as widely practiced
as in previous years, although elutriation basins are still employed in many plants for
postanaerobic digestion thickening. Elutriation, depending upon the particular system's
design and operation and the type of sludge being treated, can result in fractionation of
the sludge by particle size and density. This fractionation can result in a serious
recirculation side-stream effect.
Heat treatment facilitates dewatering. At the same time it solubilizes a portion of the
treated sludge. Depending on the type of sludge treated and the kind of system employed
for treatment, this solubilization of sludge may create a cooking liquor recirculation
stream which requires a separate treatment system. This is particularly true of process
sludges which contain a high percentage of activated sludge. Naturally, treatment of this
recirculation stream will generate additional sludge to be handled. Heat treatment also
kills pathogenic organisms. Heat treatment was originally practiced in Great Britain in the
1940s as a means of processing difficult sludges. As of 1971 about 11 percent of the
British population were served by plants employing heat treatment. Heat treatment
systems are currently being designed into a number of U.S. plants [1].
Ash addition to sludge for improving the dewatering operation is occasionally employed.
Sludge incinerator ash is added to the sludge at Indianapolis and Cedar Rapids to improve
cake release and reduce the chemical requirements for good filter productivity and cake
solids concentration.
6.2 Considerations in Selecting a Conditioning Method
In the past few years, the impact of sludge processing systems on total wastewater plant
capital and operating costs has been considered more thoroughly. Sludge thickening and
dewatering can materially affect the preceding and succeeding unit processes. The
conditioning method chosen normally has a significant effect on the efficiency of the
thickening and/or dewatering operation. It should also be realized that the method of
conditioning can have a pronounced effect on the liquid treatment portion of the plant.
This can best be illustrated by a hypothetical example. Consider heat treatment, although
6-2
-------
the same kind of example could be applied to other conditioning processes, for the
conditioning of combined primary and excess activated sludge from a conventional
activated sludge plant as shown in Figure 6-1.
The proposed plant will treat 100 mgd of raw municipal wastewater containing 187,000
Ib of BODS and 98,000 Ib of suspended solids. The primary portion of the plant is
expected to remove 50 percent of the suspended solids and 25 percent of the BODS. The
primary sludge will have a solids concentration of 3 percent which can be raised to 6
percent by gravity thickening. The activated sludge portion of the plant will operate with
a MLSS of 3,000 mg/1 and a detention time of 6.5 hr. The secondary portion is expected
to remove 81 percent of the BODS remaining after primary clarification. It was estimated
that 1.065 Ib of EAS would result from removal of 1.0 Ib of BOD5 in the aeration system
and these solids can be air flotation thickened to 4 percent. These data are summarized in
Table 6-2.
TABLE 6-2
PLANT DESIGN CRITERIA
Primary Treatment Sludges
Influent suspended solids, Ib/day 98,000
Primary sludge solids to gravity thickener, Ib/day 49,000
Primary sludge solids concentration, percent 3.0
Thickened sludge solids to hold tank, Ib/day 47,000
Thickened sludge solids concentration, percent 6.0
Aeration—Final Clarifier System
MLSS mg/1 3>000
Influent BODS, Ib/day 140,000
BOD5 removed, Ib/day 113,700
Detention time, hr 55
Flotation Thickener
Influent excess activated sludge solids, Ib/day 121,000
Thickened sludge solids to hold tank, Ib/day 109,000
Thickened sludge solids concentration, percent 4.0
6-3
-------
PLANT
INFLUENT
PRIMARY
CLARIFIER
1
1
t r
GRAVITY
THICKENER
1
MMMHM
•^ •••
t i
m ^B
CENTRATE
TO
INCINERATOR
r
CENTRIFUGE
t
1
1
>--»-
DECAI
AERATION
SYSTEM
-^-
FINAL
CLARIFIER
I
_ Jfr
1
SLUDGE
HOLD TANK
t
t
FLOTATION
THICKENER
^TATE Y.
~l
DECANT
TANK
t
* -
HEAT
TREATMENT
LIQUID
EFFLUENT
FIGURE 6-1. Conceptual flow sheet wastewater plant with heat treatment.
-------
In the proposed design, primary sludge would be thickened by gravity, and the EAS
would be air flotation thickened. After combining the two thickened sludges in a holding
tank, they would be heat treated, decanted, centrifuged, and incinerated. Assumed solids
capture for the various unit processes are primary clarification, 50 percent; gravity
thickener, 96 percent; DAF, 90 percent; heat treatment decant tank, 95 percent; and the
dewatering centrifuge, 90 percent. During heat treatment, 20 percent of the solids are
destroyed by oxidation. The quantities of solids entering and leaving the heat treatment,
thickening, and dewatering unit processes together with a summary of the solids loss
during processing are given in Table 6-3. This table clearly shows the effect of solids
recirculation on the aeration portion of the plant. In this instance recirculation from the
various solids processing systems has increased the load on the aerator by 65 percent.
TABLE 6-3
ADDITIONAL DESIGN CRITERIA
Combined Sludge Processing
Heat Treatment
Influent sludge solids, Ib/day
Influent sludge solids concentration, percent
Sludge solids oxidized, Ib/day
Thickening (Decant Tank)
Influent sludge solids, Ib/day
Influent sludge concentration, percent
Dewatering (Centrifuge)
Influent sludge solids, Ib/day
Influent sludge solids, percent
Dewatered sludge solids, Ib/day
Dewatered sludge solids concentration, percent
Summary of Sludge Solids to Aeration System
156,000
4.45
31,000
125,000
3.6
119,000
8.0
107,000
30
Recirculated from sludge processing
Gravity thickener overflow
Flotation thickener underflow
Heat treatment decantate
Heat treatment centrate
Subtotal
Primary clarifier effluent
Total
% solids lost
4
10
5
10
50
Ib/day
2,000
12,000
6,000
12,000
32,000
49,000
81,000
6-5
-------
Casual examination indicates that the illustrated concept of Figure 6-1 and the preceding
design criteria could be effective with allowances for additional solids handling. Closer
study reveals that consideration of dissolved solids loadings generated in heat treatment
and recirculated was neglected. An unknown is the degree of sludge solubilization during
heat treatment. Material balances presented in Table 6-4 were prepared to explore this
unknown. Case I in this figure represents the concept described above. Various other
degrees of solubilization are assumed in Cases II and III for comparison. Table 6-5
presents a summary of solids loading recirculated from heat treatment, as derived from
Table 6-4.
The concept, described above, provided for processing of 156,000 Ib/day of sludge solids.
It allowed for recirculation of 18,000 Ib/day of suspended solids from heat treatment.
However, it now appears that the solids recirculated from this source could be
considerable and include a significant amount of solubilized solids. Imposition of the
additional recirculated solids requires consideration in the conceptual design.
Organic, suspended, and dissolved solids can generate excess activated sludge when placed
under aeration. Substantial loadings of recirculated solids would, therefore, be expected
to have an effect on the treatment plant's efficiency. A plant confronted with this type of
situation may have to expand the liquid treatment portion to accommodate the
recirculation load.
The above example illustrates the importance of sludge conditioning process evaluation
and selection in overall plant design. While this particular example illustrated a system
involving heat treatment, later case studies will discuss other conditioning systems. The
crux of the problem is to minimize recirculation effects in an economic fashion.
6.3 Process Chemistry—Conditioning
Each of the various sludge conditioning methods functions in a different way and causes
diverse chemical and physical effects. Conditioning by either organic or inorganic
chemical addition, elutriation, and heat treatment are discussed.
6.3.1 Chemical Conditioning and the Use of Polyelectrolytes
The solid particles present in sludge usually necessitate conditioning because they are fine
in particle size, hydrated, and carry an electrostatic charge. These particle properties
inhibit thickening and dewatering and are caused by the chemical composition and
surface structure of the particles. Figure 6-2 depicts a typical liquid-solid interface
relationship, and it helps to explain the difficulty so often encountered when releasing
water from sludge. Sludge in effect is a stable colloidal suspension and it is the function
of a conditioner to destabilize the suspension. The particles can lose, gain, or share
electrons by forming covalent, ionic, hydrogen, dipolar, or induced dipolar bonds.
6-6
-------
TABLE 6-4
COMPARATIVE SOLIDS BALANCES
VARIOUS SLUDGE PROCESSING CONDITIONS
Assumed conditions
Influent sludge dissolved solids
Heat treatment process
Suspended solids oxidized
Suspended solids solubilized
Thickening (decantation) process
Suspended solids capture
Thickened sludge solids content
Dewatering (centrifugal) process
Suspended solids capture
Cake solids content
Influent sludge suspended solids
(Solids concentration, 4.45%)
Solubilized solids
Suspended solids
Total solids
Solubilized solids
Suspended solids
Total solids
Solubilized solids
Suspended solids
Total solids
Negligible
20
None
95
Heat Treatment
Oxidized
None
31,000 125,000
125,000
Thickening
4- I
Decantate '
None None
6,000 119,000
6,000 119,000
Dewatering
Centrate
None None
12,000 107,000
12,000 107,000
Case
II
Negligible
20
20
95
Heat Treatment
III
Negligible
None
40
95
90
30
156,000
90
30
1h/f)av
156,000
90
30
156,000
I
Oxidized
31,000
31,000
94,000
125,000
I
Thickening
Decantate
20,400 10,600
4,700 89,300
Dewatering
Centrate
8,800
8,900
1,800
80,400
Heat Treatment
62,000
94,000
156,000
Thickening
Decantate
37,900 24,100
4,700 89,300
25,100 99,100 42,600 113,400
Dewatering
Centrate
20,500 3,600
8,900 80,400
17,700 82,200 29,400 84,000
Note: Processing by heat treatment, thickening, and dewatering as shown in Figure 6-1.
6-7
-------
TABLE 6-5
RECIRCULATED SOLIDS LOADINGS DURING DEWATERING
Type of Solids
Solubilized
Suspended
Total
Total Solids concentration*
I
None
18,000
18,000
0.58
Case
II
29,200
13,600
42,800
1.34
III
58,400
13,600
72,000
2.23
In combined centrate and decantate.
Practically all dispersed particles in wastewater carry a net negative charge. Depending on
the net charge of the suspension it may be flocculated by adsorption of polymers
(polyelectrolytes) with either positive (cationic) or negative (anionic) charges. Nonionic
(net zero charge) polymers also may function as flocculants. Anionic polymers adsorb via
isolated cationic sites on the particles. Nonionic polymers can adsorb through hydrogen
bonding, but they also normally assume some degree of negative Zeta potential in
dispersed form.
In general, the polyelectrolytes flocculate by neutralizing the surface charges on the
dispersed particles by causing the desorption of bound water and through bridging. The
latter is simultaneous attachment of the polymer to two or more solid particles. Figure
6-3 shows the variety of configurations that are thought to be involved in the flocculation
mechanism. The desorption of bound water, neutralization of surface charges, and
aggregation which are brought about by flocculants result in the formation of structured
free-draining cakes during dewatering operations. In gravity thickening the aggregates
settle more rapidly to a higher solids content, while in flotation thickening aggregation of
particles and greater compaction occur with the use of polyelectrolytes. The polymeric
flocculants most useful in conditioning are largely linear, high molecular weight materials
which carry a large charge density in an aqueous dispersion form. Figure 6-4 shows the
chemical composition of the repeating monomeric units for a typical polyanionic and
polycationic conditioner. The molecular weight of useful materials is of the order of
200,000 to 10 million. Figure 6-5 shows the typical configuration of a polyelectrolyte in
solution. This simplified figure does not show the tremendous length of the polymer's
molecule chain.
6-8
-------
LYOPHOBIC
CHARGE
PARTICULAR
SURFACE
ATTRACTIVE
DISCHARGE
.LYOPHILIC
WATER -f
PRECIPITATED
PARTICLE
FIGURE 6-2. Factors influencing the stability of a colloidal suspension [2].
-------
2ND
ORDER
- ^~
FAST
COLLOID POLYMER
1ST ORDER SLOW
VAN DER WAAL
FLOCCULATION
INTERPARTICLE
BRIDGING
NEUTRALIZATION
INTRAPARTICULAR NEUTRALIZATION
INTER. AND
INTRAPARTICLE
NEUTRALIZATION
FIGURE 6-3. Mechanism of polymer flocculation [2].
-------
r CH3 "i
i j
H 1
— O — C
H i
c=o
1
o
_ No _J
A
SODIUM
OLYMETHACRYLATE
r~ CH^ "~i
I 3
H 1
— 0— C
H 1
c=o
1
o
1
1
HCH
1
HCH
1
H-C — N — CH.
j H j
H3C-C-0
1
I— o — 1
OMAEM
FIGURE 6-4. Structure of two polyelectrolytes's monomeric units [3].
6-11
-------
0
©
/ ©
0
©
©
©
©
©
©
©
©
FIGURE 6-5. Typical configuration of a polyc'cctrolytc in solution 13]
6- 12
-------
Since an extensive variety of widely different types of polymers are currently in use, it is
impossible to generalize about the applicability of polymers to particular sludges.
Laboratory or pilot scale evaluations are essentially always required. The WPCF Manual
of Practice on Sludge Dewatering [4] has very aptly noted that seldom do any two
polymers give the same results, some polymers work best in conjunction with other
chemicals, some polymers may coagulate well but still not improve sludge dewaterability.
Further, the polymer that best improves a sludge's settleability may not be the same one
that improves its dewaterability.
6.3.2 Use of Inorganic Chemicals
Polyvalent metal ions (ferric, ferrous, and aluminum) hydrolyze in water to produce
polynuclear complexes. The important metal salts used in sludge conditioning are ferric
chloride and ferrous sulfate. They function primarily as coagulants. The hydrolyzed salts
possess a significant charge and some polymeric properties as well. Accordingly, they
provide charge neutralization and enmeshing capabilities toward dispersed material.
Hydrated lime is almost always used in conjunction with metal salts. Though lime does
have some slight dehydration effect on colloids, its use in conditioning is also for pH
control, odor reduction, disinfection, and filter aid effect.
Both power plant fly ash and sludge incinerator ash have been used successfully in the
conditioning of sludge. The properties of ash which enable it to improve dewatering of
sludge include partial solubilization of its metallic constituents, its sorptive capabilities,
and its irregular particle sizes [5]. The city of Indianapolis, Indiana, recently started using
sludge incinerator ash to condition a mixture of primary and activated sludges prior to
dewatering. The dramatic effect of ash addition on the average performance of the plant's
rotary vacuum belt filters is shown in Table 6-6. Indianapolis has succeeded in increasing
its filter productivity by as much as 500 percent and decreasing its cake moisture by as
much as 22 percent. The filtrate quality has also been improved and from a cost
viewpoint the cationic polymer requirement has been reduced by approximately 55
percent. These data were obtained in late 1972 and at that time the plant was handling
approximately 418,000 Ib of dry sludge per day and the ash to dry sludge solids ratio
varied from 0.25 to 0.50. The ash handling facilities required almost zero investment and
no additional operating cost. The relative location of the ash slurry line to the gravity
sludge thickeners provided for easy ash addition requiring only the installation of a tap,
short feed line, and a pump [6].
6.3.3 Elutriation
Elutriation is essentially a washing process once widely used for conditioning
anaerobically digested sludges prior to further conditioning with a metal salt. The process
involves countercurrent or cocurrent extraction of the soluble alkaline carbonates and
phosphates as well as fine sludge particles from the sludge by dilution with treatment
6- 13
-------
TABLE 6-6
EFFECT OF ASH ADDITION ON VACUUM FILTRATION AT INDIANAPOLIS
Parameters
Filter Yield (lbD.S./ft2/hr)
Cake Moisture (%)
Filtrate Quality
Filter Capacity
Cationic Polymer Required (Ib/ton)
Release of Cake from Media
Supplemental Fuel Requirement
Before Ash Addition
1.0-2.0
85
Poor
Insufficient
15
Poor
Little or None
After Ash Addition
5.52- 13.0
66.5
Excellent
More than Adequate
6.8
Excellent
Little or None
plant effluent and resettling. The principal purposes of the process are to reduce chemical
requirements and produce a more readily dewaterable sludge. With the advent of higher
levels of secondary treatment and consequent activated sludges, the sludge going to
elutriation contains a large amount of fine particles. Therefore, the process, unless
flocculants are used, will produce a very dirty elutriate and a heavy recirculation load.
Plant studies will be reviewed in Chapter 10 which indicate a need for a postdigestive
thickening process like elutriation in plants processing combined primary and secondary
sludges.
6.3.4 Heat Treatment
In heat treatment, temperatures of from 300 to 500° F and pressures of 150 to 400 psig
are attained for protracted periods. Significant changes in the nature and composition of
wastewater sludges result. The effect of heat treatment has been ideally likened to
syneresis, or the breakdown of a gel into water and residual solids. Wastewater sludges are
essentially cellular material. These cells contain intracellular gel and extracellular zoogleal
slime with equal amounts of carbohydrate and protein. Heat treatment breaks open the
cells and releases mainly proteinaceous protoplasm. It also breaks down the protein and
zoogleal slime, producing a dark brown liquor consisting of soluble polypeptides,
ammonia nitrogen, volatile acids, and carbohydrates. The solid material left behind is
mineral matter and cell wall debris.
Dewaterability is improved by the solubilizing and hydrolyzing of the smaller and more
highly hydrated sludge particles which then end up in the cooking liquor. While analysis
of this liquor from domestic wastewater sludges indicates the breakdown products are
mostly organic acids, sugars, polysaccharides, amino acids, ammonia, etc., the exact
composition of the liquor is not well defined [7]. Corrie and Wycombe [8] have found the
6- 14
-------
liquor to be highly polluted and contain a high proportion of nonbiodegradable matter.
This matter is largely humic acids, which can give rise to unpleasant odors and taste if
present in water which has been chlorinated prior to use for domestic supply. If industrial
wastes of various types are included in the wastewater to be treated, the actual chemical
composition of the liquor resulting from heat treatment of the sludge should be
determined by a detailed chemical analysis. The Thames Conservancy District of England
requires when heat treatment of sludge is employed in a plant that the liquor be treated
separately. The treatment process chosen must include adsorption on activated carbon of
the COD. The reasons for these requirements are that too little is as yet known about the
exact composition of soluble organic matter in the liquor to permit its recycle into the
Thames River since it is used as a downstream water supply.
Figure 6-6 gives the conceptual design recommended by the District for handling heat
treatment liquor based on their pilot plant efforts.
A review of reported analyses [8,9] of liquor from the heat treatment of sludge gives the
range of values shown:
BOD5 = 5,000 to 15,000 mg/1
COD = 10,000 to 30,000 mg/1
Ammonia = 500 to 700 mg/1
Phosphorus as P = 150 to 200 mg/1
About 20 to 30 percent of the COD is not biodegradable in a 30-day period. The volume
of cooking liquor from an activated sludge plant with heat treatment amounts to 0.75 to
1.0 percent of the wastewater flow. Based on BOD5 and solids loadings, the liquor can
represent 30 to 50 percent of the loading to the aeration system. The pH of cooking
liquors is normally in the range of 4 to 5, which necessitates chemical neutralization
and/or corrosion resistant equipment. Work by Erickson and Knopp [10] presents data
indicating few problems with treatment of cooking liquors from Zimpro systems.
6.4 Physical Factors in Conditioning Processes
Plant experiences have shown that the conditioning requirements and hence the
performance achieved in thickening and dewatering processes are affected by the manner
in which sludge is treated.
6-15
-------
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6.4.1 Effect of Processing Prior to Conditioning
Any operation which tends to irreversibly disaggregate, hydrate, or subject sludge to high
shear conditions is deleterious to solid-liquid separation procedures. One striking example
of this tendency is the effect noted when pumping sludges through pipelines from
satellite plants to a central dewatering location. It has been demonstrated that sludge is
easily and economically dewaterable prior to piping, while the sludge exiting from the
pipeline is not easily dewaterable.
Other treatment plant operations, including aeration and thickening, also subject the
sludge particles to shear forces. Although these operations normally have minimal shear
effects, there is some adverse speculation about the use of surface aerators and their
effect on floe characteristics. Unfortunately the results of no large scale comparisons
between surface and diffused aeration and their effects on floe processability have as yet
been reported.
6.4.2 Conditioner Application
The optimum sequence of addition is best determined by trial and error when two or
more conditioners are used. Normally ferric chloride is added first when used with lime.
Anionic polymer is added first when a combination of anionic and cationic polymer is
needed. In cases where experimentation is not possible, it is best to provide flexibility in
design of the system. In practically all cases, the cardinal rule for design of equipment for
mixing conditioning chemicals with sludge is to provide just enough mixing to disperse
the conditioner throughout the sludge. This is necessary to minimize floe shearing. Visual
inspection capability should be provided for the sludge-flocculant mixture from the point
of their contact on into the dewatering or thickening unit. A further design consideration
is the provision of individual conditioning units for each dewatering unit. 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 and in locating the conditioning unit relative to each
dewatering device.
One of the most widely used types of conditioning units is the rotating horizontal drum
with internal baffles, which resemble a cement mixer. Another is the baffled open trough
with paddle mixers. Vertical cylindrical tanks with propeller mixers were used at one
time, but these have not been popular in recent years. The rotating horizontal drum type
of conditioner is depicted in Figure 6-7. While this type unit normally functions well, care
should be exercised in minimizing the vertical height differential between the point of
discharge from the drum and the vat level. This is particularly important with thin,
shear-sensitive sludges. It has been helpful in some cases to baffle the point of discharge
from the chute into the vat for the prevention of localized wash-off of filter cake.
6- 17
-------
CHEMICAL FEED
CONNECTION
ROTARY
CONDITIONING
TANK
CONDITIONING
TANK SUPPORT
SLUDGE INLET LINE
CONDITIONING TANK
FEED CHUTE
FILTER VAT
(REAR SIDE)
FIGURE 6-7. Rotary drum conditioner.
Courtesy Komline-Sanderson
6-18
-------
A sectional view of the baffled trough type conditioning unit appears in Figure 6-8. This
type unit has two important features. It permits continual visual inspection of the
conditioning operation and the shear imparted to the conditioned sludge as it flows into
the vat can be minimized.
6.5 Conditioning for Gravity Thickening
Primary sludges and mixtures of primary and trickling filter sludges normally respond to
gravity thickening without conditioning. However, mixtures of primary sludge and EAS
can present a problem for gravity thickening. Flocculants are required to ensure good
solids capture and loading rate, and even then as was discussed in Chapter 4, a high
underflow solids concentration is difficult to obtain. It is generally preferable to
separately DAF thicken excess activated sludge. When flocculants are used to condition
sludge for gravity thickening, the flocculant solution should be added to either the sludge
or the dilution water on its way into the thickener. Recent experience indicates that
excess oxygen activated sludge may be amenable to gravity thickening [11]. Data on an
actual plant operation is presented in Chapter 10.
6.6 Conditioning for Flotation Thickening
Chemicals can assist flotation by increasing the solids loading rate, float cake solids
concentration, and solids capture. The first two parameters are interrelated, and this
dependency as well as the effect of polymer addition is illustrated by Figure 6-9 for an
activated sludge.
Since a minimum practical cake solids concentration in the float is usually 4 percent, the
operable conditions for this particular activated sludge and system are in the range of 4 to
6 Ib of polymer per ton of dry solids and mass loadings of 10 to 30 Ib/day/sq ft. While
such loading rates compare favorably with those of a gravity thickener, flotation units
normally achieve loading rates of 48 to 96 Ib/day/sq ft with 1 to 5 Ib/ton of a chemical
and resultant cake solids concentrations of 4 to 6 percent solids.
Figure 6-10 shows a typical layout for addition of chemical flotation aids. The chemical is
usually added at the mixing chamber where the pressurized recycle flow is mixed with the
sludge stream. The design of the inlet mixing chamber and point of polymer application
are very important. Injection of the flotation aid solution into the recycle line just as the
bubbles are being formed and mixed with the sludge produces the best results [12]. This
assures excellent mixing, minimum sludge particle shear, and positive air bubble
adsorption.
6- 19
-------
BAFFLES
INLET
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FIGURE 6-8. Baffled trough unit.
Courtesy Rexnord
6-20
-------
10
20 30
NET MASS LOADING
(LB /SQ FT/DAY)
40
FIGURE 6-9. Thickening performance as affected by mass loading at
Constant chemical dosage. Courtesy Rexnord
6-21
-------
to
UNIT
EFFLUENT
SLUDGE REMOVAL MECHANISM
RECYCLE
FLOW
••••"• «. • .'.'.'o. • ••• --.L-' • !^-v'T7 *-V1/'?.*•'.
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BOTTOM SLUDGE COLLECTOR
SLUDGE
DISCHARGE
RECYCLE
FLOW
UNIT
SLUDGE FEED
FIGURE 6-10. Flow diagram of a flotation unit.
Courtesy Komline-Sanderson
-------
6.7 Conditioning for Dewatering
An important operating variable in dewatering is the conditioning chemical and dosage.
Optimum performance of the dewatering equipment hinges on a determination of the
most economical and effective conditioning method. Conditioning for improved
dewatering by vacuum filtration, centrifugation, drying beds, and pressure filtration is
discussed.
6.7.1 Rotary Vacuum Filtration
Items of primary concern in conditioning sludge for feed to a rotary vacuum filter
include:
• Cake pickup on the drum
• Production rate
• Cake solids content
• Cake discharge from filter media
• Filtrate quality (solids capture)
The determination of optimum conditioning procedures hinges on trial and error plant
testing. However, plant scale testing is often preceded by laboratory and pilot scale
testing to narrow down the selection of conditioning systems and levels of operation. The
two most widely employed laboratory tests are the Buchner funnel and filter leaf
[4,13]. Pilot plant testing is generally conducted with a small model (3 ft diameter with a
1 ft wide face) of a plant scale rotary vacuum filter. Most manufacturers will rent pilot
equipment for test purposes. Extended testing of a limited number of operating modes
can then be conducted on a plant scale.
Table 6-7 shows estimated chemical requirements for optimum vacuum filtration of a
variety of sludges. The estimates are based on experiences at different treatment plants
throughout the United States and are listed here only for guidance [14]. All of the various
types of conditioning procedures are used in conjunction with rotary vacuum filters.
6.7.2 Centrifuges
Polymeric flocculants are normally used for sludge conditioning in solid bowl centrifuges,
which are the prevalent type used. The metal salts are generally not used because of
corrosion problems. Principal performance parameters for gauging effectiveness of
conditioners are production rate, cake solids content, and centrate solids content. A
previous deterrent to widespread use of continuous horizontal solid bowl centrifuges was
6-23
-------
TABLE 6-7
ESTIMATED CHEMICAL CONDITIONING DOSAGE FOR VACUUM FILTRATION
Type of Sludge
Primary Sludge
Limed Primary (2 12 Ib CaO/ton)
Digested Primary Sludge
Digested/Elutriated Primary
Raw (Primary + EAS)
Limed (Primary + EAS)
Digested (Primary + EAS)
Digested/Elutriated (Primary + EAS)
CaO Dose
(Ib/ton)
176
0
240
0
200
0
372
0
FeCl3 Dose
(Ib/ton)
42
42
76
68
52
40
110
125
CaO + FeCl3
($/ton)
5.01
2.81
7.86
4.56
5.98
2.68
12.02
8.38
Polymer Dose
(Ib/ton)
5
5
20
9
18
5
36
24
Polymer
($/ton)
1.65
1.65
6.60
2.97
5.94
1.65
11.88
7.92
Note: CaO cost = $0.0125/lb
FeCl3 cost = $0.067/lb
Polymer cost = $0.33/lb
-------
the problem of providing adequate solids capture with reasonable production rates ana
flocculant dosages. The crux of that problem was attainment and maintenance of floe
stability in the face of high shear conditions. This problem has been minimized by direct
injection of the flocculant into the centrifuge to avoid exposing flocculated material to
high shear. This design change reduced flocculant consumption and made the process
more efficient and competitive. Figure 6-11 shows a cross section of a typical centrifuge
with the polymer injection arrangement. The point labelled floe nozzle is the application
point for the polymer flocculant. Recently proper flocculation has permitted the
centrifuge to perform well at low rotational speeds. This reduced conditioning chemical
requirements and maintenance due to wear. Optimum conditioning can best be
determined in pilot tests.
6.7.3 Drying Beds
Use of conditioning procedures with sludge drying beds is not widespread; however,
elutriation and polymers are employed in isolated cases. Mogelnicki [14] details
experiences with elutriation and polymer conditioning.
Conditioning agents are used with sludge drying beds at the Chicago Southwest Plant to
reduce drying time and maximize bed production during fair weather. Approximately 0.5
Ib of cationic polymer per ton of dry solids is effective in decreasing the drying time from
13 to 5 days. Reported cost of the polymer is about $0.50/dry ton of sludge.
Figure 6-12 presents data from a series of tests with a digested sludge having a solids
concentration of 4 percent. This sludge was conditioned with a cationic polyelectrolyte.
The effect of various levels of polymer addition on drainage time and the time elapsed
before the cake can be readily lifted from the bed can be easily seen. The use of sludge
conditioning in this case produced significant increases in both drainage rate and ultimate
cake solids content [14].
6.7.4 Filter Presses
Filter aids such as ash and inorganic conditioners are used in dewatering operations with
filter presses. Presses depend on the exertion of massive pressures (< 200 psi) to squeeze
water out of sludge. Consequently, conditioning problems are more difficult than with
other methods and laboratory and/or pilot plant evaluation is needed. These high
pressures tend to destroy the flocculation achieved with normal conditioning.
Accordingly, relatively large doses of lime or recycled ash (1.0 to 1.5 parts ash/part dry
solids), with or without metal salts, are used. The Sheffield, United Kingdom [15] plant
uses 27.5 percent lime and 13 percent Fe2 03 on a sludge solids basis.
6-25
-------
DRAIN
PORT
SOLIDS DISCHARGE
PORTS AND PLOWS
^OVERLOAD SHEAR
DEVICE
PILLOW BLOCK AND
MAIN BEARING
SKIMMER
V CONTROL
to
ON
FLOC
INLET
EFFLUENT
DISCHARGE
CONVEYOR
GEAR DRIVE
FIGURE 6-1 1. Concurrent flow solid-bowl centrifime.
-------
to
LJJ
I
u
I
\-
CL
UU
Q
uj 4
O
2H
7% SOLIDS
56% SOLIDS
6 8 10
TIME, (DAYS)
12
14 16 18
FIGURE 6-12. Sandbed dewatering [14].
-------
6.8 Selection of Conditioning Chemicals
It is important for the design engineer, in evaluating alternate types of conditioning, to
become more familiar with the available chemicals. Table 6-8 lists several available
chemicals and indicates a broad price range.
TABLE 6-8
CONDITIONING CHEMICAL MATERIALS
Type
FeCl3
Cationic Polymer
Cationic Polymer
Anionic Polymer
Form
Liquid
Dry Powder
Liquid
Dry Powder
Price (S/lb)*
0.06
0.50-1.50
0.05-0.50
0.60-1.30
Suppliers
Two major, Several minor
About 1 0
About 10
About 1 5
Based on 1973 costs figures.
The significant points to consider in selecting these conditioning chemicals are:
• Ferric chloride is a commodity chemical, made either from waste pickle liquor or
more usually from scrap metal and chlorine. With proper handling procedures and
equipment, it is not difficult to employ in the normal 40 percent solution form.
Since it is a low priced commodity chemical, its analysis and quality can be easily
controlled with relatively simple analytical control procedures.
• The price structure of ferric chloride is relatively stable.
• Cationic polymeric flocculants, which are specialty chemicals, are available in a wide
variety. The chemical composition, functional effectiveness, and cost-effectiveness
of the various products differ greatly. Accordingly, the products of several different
companies should be evaluated to optimize efficiency.
• Because of the wide variety and divergent prices of the various polymers, one should
always compare them in terms of cost per ton of sludge solids conditioned rather
than in pounds of polymer required per ton.
6-28
-------
Since polymers are specialty chemicals, their production and composition are the
subject of continuing research and development by the suppliers. This has usually
resulted in continuing improvements in functional effectiveness and cost.
6.9 References
1. Pickford, J. (ed.), proceedings on "Sludge Treatment and Disposal." Presented
at 4th Public Health Engineering Conference, Loughborough University of
Technology, United Kingdom, Jan. 1971.
2. Priesing, C. A., "A Theory of Coagulation Useful for Design." Ind. Eng. Chem.,
54(8), 391 (Aug. 1962).
3. Ruehrwein, R. A. and Ward, "Mechanism of Clay Aggregation by
Polyelectrolytes." Soil Science, 73 485 (Jan.-Jun. 1952).
4. "Sludge Dewatering." Water Pollut. Contr. Fed. Manual of Practice No. 20
(1969).
5. Smith, J. E., Jr., Hathaway, S. W., Farrell, J. B., and Dean, R. B., "Sludge
Conditioning with Incinerator Ash." Presented at the 27th Annual Purdue
Industrial Waste Conference, May 2-4, 1972.
6. Doyle, Carlos, personal communication, Indianapolis Sanitary District, Jan.
1973.
7. Brooks, R. B., "Heat Treatment of Sewage Sludge." Water Pollut. Contr.
(1970), pp. 221-231.
8. Corrie, K. D. and Wycombe, R. D. C., "Use of Activated Carbon in the
Treatment of Heat Treatment Plant Liquor." Water Pollut. Contr. (1972) pp.
629-635.
9. Fischer, W. J. and Swan wick, J. D., "High Temperature Treatment of Sewage
Sludges." Water Pollut. Contr. (1971), pp. 355-373.
10. Erickson, A. H. and Knopp, P. V., "Biological Treatment of Thermally
Conditioned Sludge Liquors, Advances in Water Pollution Research, Pergamon
Press, 1972.
11. Robson, C. M., Block, C. S., Nickerson, G. L., and Klinger, R. C., "Operational
Experience of a Commercial Oxygen Activated Sludge Plant." Presented at
45th Water Pollution Control Federation Meeting, Atlanta, Georgia, Oct. 1972.
6-29
-------
12. Jones, Warren H., "Dissolved Air Flotation Thickening of Wastewater Sludges."
Presented at Nebraska Water Pollution Control Federation, Omaha, Nebraska,
Mar. 26, 1968.
13. Schepman, B. A. and Cornell, C. F., "Fundamental Operating Variables in
Sewage Sludge Filtration." Sewage and Ind. Wastes Journ., 28, 1443 (1956).
14. Mogelnicki, S. J., personal communication, Dow Chemical, Midland, Michigan,
May 1974.
15. Swanwick, K. H., "Control of Filter Pressing at Sheffield." Water Pollut. Contr.
(1973), pp. 78-86.
6-30
-------
CHAPTER?
SLUDGE DEWATERING
7.1 Methods and Functions
The methods used to remove sufficient water from liquid sludges so as to change the physical
form to that of a damp solid are best described in terms of the particular type of dewatering
device used. The commonly used devices include:
• Rotary vacuum filters
• Centrifuges
• Drying beds
• Lagoons
• Filter presses
• Horizontal belt filters
The relationship of the various dewatering methods to those processes which immediately
precede and follow them are summarized in Table 7-1.
An ideal dewatering operation would capture practically all the solids in the dewatered
cake at minimum cost. The resultant cake would have the physical handling
characteristics and moisture content optimal for subsequent processing. Process
reliability, ease of operation, and compatibility with the plant environment would also be
optimum.
The technology and design of all available dewatering methods is constantly under
development, particularly in the past five years. Each type, therefore, should be given
careful consideration. The applicability of a given method should be determined on a
case-by-case basis with the specifics of any given situation being carefully evaluated,
preferably in pilot tests.
7-1
-------
TABLE 7-1
THE RELATIONSHIP OF DEWATERING TO OTHER SLUDGE
TREATMENT PROCESSES FOR TYPICAL MUNICIPAL SLUDGES
Method
Rotary Vacuum Filter
Centrifuge
(Solid Bowl)
Centrifuge
(Basket)
Drying Beds
Lagoons
Filter Presses
Horizontal Belt
Filters
Pretreatment Normally Provided
Thickening
Yes
Yes
Variable
Variable
No
Yes
Yes
Conditioning
Yes
Yes
Variable
Not Usually
No
Yes
Yes
Normal Use of Dewatered Cake
Landfill
Yes
Yes
No
Yes
Yes
Yes
Yes
Land
Spread
Yes
Yes
Yes
Yes
Yes
Variable
Yes
Heat
Drying
Yes
Yes
No
No
No
Not
Usually
Yes
Incineration
Yes
Yes
No
No
No
Yes
Yes
7.2 Rotary Vacuum Filtration
7.2.1 Mechanics of Rotary Vacuum Filtration
Comprehension of the theoretical aspects of rotary vacuum filtration of wastewater
sludges plus practical application of the theory through the medium of lab, pilot, and
full-scale plant test procedures is essential in evaluating systems. The clearest and most
up-to-date expositions on the mechanisms of rotary vacuum filter dewatering have been
made by Bennett and Rein [1 ], Bennett, Rein, and Linstedt [2], and Gale [3,4,5].
Theory
Figure 7-1 shows a sectional view of a rotary filter which consists of a cylindrical drum
rotating partially submerged in a vat or pan of conditioned sludge. The drum is divided
radially into a number of sections, which are connected through internal piping to ports
in the valve body (plate) at the hub. This plate rotates in contact with a fixed valve plate
with similar parts, which are connected to a vacuum supply, a compressed air supply, and
7-2
-------
AIR AND FILTRATE
CLOTH CAULKING
STRIPS
DRUM
AUTOMATIC VALVE
FILTRATE PIPING
CAKE SCRAPER
-------
an atmospheric vent. As the drum rotates each section is thus connected to the
appropriate service. Figure 7-2 illustrates the various operating zones encountered during
a complete revolution of the drum. In the pickup or form section, vacuum is applied to
draw liquid through the filter covering (media) and form a cake of partially dewatered
sludge. As the drum rotates the cake emerges from the liquid sludge pool, while suction is
still maintained to promote further dewatering. A lower level of vacuum often exists in
the cake drying zone.
Continuous filtration is a cyclic process and operation encompasses various rate
functions. Furthermore, while one of the rate functions normally may be controlling, all
interact and, therefore, none can be ignored. Two distinct rate phenomena are
encountered in continuous operation of vacuum filters for sludge dewatering and warrant
special attention [6]. They are filter sludge cake formation rate and dewatering of filter
cake to obtain the desired final moisture content.
The hydraulics of filtrate flow were developed by Ruth, Motillon, and Montonna [7] and
Carmen [8] using Darcy's law and the Carmen-Kozeny equation. This approach was
adapted to wastewater sludge filtration by Coakley and Jones [9], Jones [10], and
Hatscheck [11], Halff [12], and Grace [13]. The theory is based on several assumptions.
These include the laminar flow condition, a constant volume of solids deposition with
each increment of filtrate, and a constant increase in filtrate flow resistance for each
volume of cake solids deposited. With these conditions, an average specific resistance for
each unit thickness of cake deposited can be assumed. Using the average specific
resistance concept, the following equation has been developed [6]. This average specific
resistance (r) is the resistance of a unit weight of cake per unit area at a given pressure and
is expressed in sec2 /g.
where
r = Average specific cake resistance (usually constant
for one slurry), sec2 /g
P = Pressure drop through filter medium and sludge cake,
cm of water
A = Area of filtering surface, cm2
b = Slope of the t/V vs. V plot in sec/ml2
7-4
-------
PICK-UP
OR FORM
FIGURE 7-2. Operating zones of a vacuum filter.
7-5
-------
M = Viscosity of filtrate in poise
w = Weight of dry sludge cake solids per unit volume
of filtrate, gm/cc
A value is obtained for "b" by conducting a simple Buchner funnel test and this is
discussed in a later section. Some derivations include the resistance of the filter medium
in series with the cake resistance. Normally, the filter media resistance is negligible, and
the term can be dropped [6]. The average specific resistance (r) is assumed to be constant
for any one slurry and operating condition. It can be altered by the application of
conditioning techniques and it is a function of the vacuum level applied. Most municipal
sludge solids deform at high vacuum levels and fill the pore openings, which increases the
resistance per unit of vacuum. This can be expressed as:
r = r'Ps
where
r ' = Constant representing the specific resistance of a
noncompressible cake
s = Cake compressibility exponent
The cake compressibility coefficient (s) varies from zero for a rigid incompressible cake to
greater than one for highly compressible cakes. For domestic wastewater sludges, this
value ranges from 0.4 to 0.85 [6] .
The following equation relates the specific resistance to the yield of a rotary vacuum
filter.
2P W Ff
x ~~ ~
where
Y = Yield of filter in mass of dry suspended solids produced
in unit time per unit of total area of filter medium
P = Pressure difference across filter cake during cake formation
7-6
-------
W = Mass of dry suspended solids per unit volume of
liquid in sludge
Ff = Fraction of total filter area used for cake formation
H = Viscosity of filtrate
Tp = Apparent specific resistance of cake measured at P
0R = Time for one revolution of the filter
Fc = Cake correction factor; the ratio of the mass of
liquid in unit mass of sludge, to the mass of filtrate
obtained when unit mass of sludge is filtered
This equation allows prediction of expected data with changes in pressure, feed
concentration, viscosity, and the cake formation time [3]. For example, if the length of
the cake formation time is quadrupled, the filtration rate is cut in half. An increase in the
temperature of the slurry, in general, decreases the viscosity and increases the cake
formation rate. As the filter feed solids concentration is increased, the solids rate also
increases. As the feed solids concentration increases, many filter cakes exhibit a more
permeable bridging and a subsequent reduction in the cake resistance. It has been noted
that the influence of feed solids concentration on the r value cannot be predicted from
theory, and thus parameters of feed solids concentration also must be employed in the
correlation methods. Physically, another factor comes into consideration, that is, because
a filter is a machine using energy to separate solids from liquids, the less liquid there is to
remove, the higher will be the rate of dry solids production. Empirically it has been
shown that the dry solids production rate is for all practical purposes directly
proportional to the feed solids concentration. Data for several experiments at a cycle time
of 120 seconds are plotted in Figure 7-3 on logarithmic scales. In this figure, the
measured yields corrected for sludge solids content are plotted against specific resistance.
The data are seen to closely lie on a straight line of slope—0.5, as theory would predict.
Test Procedures for Sizing Vacuum Filters
The two test procedures used for determining the filterability of sludges are the Buchner
funnel method and the filter leaf technique. The Buchner funnel method enables a
determination of the relative effects of various chemical conditioners and the calculation
of the specific resistance of the sludge, but it is seldom used for the calculation of
required filter area. The filter leaf test is used to determine the required filter area.
One laboratory system using a machined aluminum Buchner funnel apparatus is shown in
Figure 7-4. Typically, two pieces of No. 4 Whatman filter paper are fixed in the Buchner
7-7
-------
+ 20*
-205
I L
. I
02 03 O405 1 2 345
SPECIFIC RESISTANCE,[Jp (109s EC2/g )J
10
FIGURE 7-3. Corrected filter yield vs. specific resistance [4].
7-8
-------
6 did. ALUMINUM TUBE
PERFERATED
SUPPORT
PLATE
TO VACUUM
GASKET
LEVELING SCREW
VACUUM TIGHT VALVE
U' 'U
s -1^-:
\~~~_2
v-
TO MONOMETER
CALIBRATED
PLEXIGLASS
TUBE WITH
DRAIN
FIGURE 7-4. Laboratory vacuum filter apparatus.
7-9
-------
funnel and the sludge to be tested is introduced in a single large batch. The depth of
sludge used is about \1A cm which is approximately equal to the amount of sludge filtered
in a single pass of a rotating filter. The valve on the vacuum line is opened to initiate
filtration. The accumulative volume of filtrate is recorded at appropriate time intervals. A
plot of t/V as a function of V (as shown in Figure 7-5) permits the calculation of the
specific resistance. Typical values of specific resistance vary from 5 X 107 to 70 X 10?
sec2 /g for conditioned sludges.
A drawback of this type of testing is that the time and volume are taken on an
accumulative basis, which tends to underemphasize certain important portions of the
curve. Crook and Jones [14] have shown that the upper end of the curve includes a
plateau prior to the drying phase. This can be observed when the test is carried out in
such a way that the instantaneous flow rate, dV/dt or 1/Qj, its inverse, is measured
instead of the accumulated average flow rate. A plot of the inverse of instantaneous flow
rate as a function of the volume of sludge applied is shown in Figure 7-6. When
instantaneous flow rates are used, the initial portion of the curve is also affected, in the
manner shown. Figure 7-7 shows the correlation between the filtrate flow plot and the
normal operating cycle of a rotating vacuum filter. Four different processing phases exist.
In Phase I, solids capture increases from near zero percent, just as the media contacts the
sludge slurry, to near 100 percent capture at the end of the phase. Phase II is the
continuation of cake formation under the conditions of nearly complete solids capture.
Phase III occurs immediately after the sludge coated media leaves the vat and is
characterized by water exiting from the larger capillary pore openings. In Phase IV the
cake is further dewatered by air drying. Phase II is the only portion of the curve that
follows theory.
The Buchner funnel test enables a prediction of the effects of various conditioning
chemicals. It does not permit a precise estimate of filter size and operating characteristics.
Differences between the Buchner funnel test and an operating filter include: the sludge is
top fed to the Buchner funnel filtering medium and the test filter medium is much
tighter. Therefore, it is not possible to accurately predict the solids concentration of the
filtrate, nor cake release characteristics.
The filter leaf test, however, permits an accurate prediction of the operation of a
full-scale filter. The filter leaf test employs the use of a test leaf over which is fitted a
filtering medium identical to that which will be used on the full-scale filter. The
procedure for conducting filter leaf tests described by Eckenfelder and O'Connor [15] is
typically to:
1. Condition approximately 2 liters of sludge for filtration. The sludge should be
thickened to a minimum concentration of 2 percent or to that anticipated for
the full-scale application.
2. Apply desired vacuum to filter leaf and immerse in sample IVz min (maintain
sample mixed). The test leaf normally is inserted upside down in a
7-10
-------
UJ
h-
Ot.
o
UJ
VOLUME OF FILTRATE
FIGURE 7-5. Typical Buchner funnel test plot.
7- 11
-------
o
c*
S
Volume Filtrate or Sludge Added
FIGURE 7-6. Instantaneous filtrate flow rate.
7-12
-------
VACUUM
CAKE
COMB
CONVEYOR BELT
FILTER MEDIA
CHEMICAL
CONCRETE
SLUDGE
FIGURE 7-7. Cake processing phases rotary vacuum filter.
-------
representative slurry to simulate the cake formation zone of the drum filter.
This portion of the cycle is cake formation.
3. Bring leaf to vertical position and dry under vacuum for 3 min (or other
predetermined time). This is the cake draining and drying part of the cycle.
4. Blow off cake for 1JA min (this gives a total drum cycle of 6 min). To discharge
the cake, the leaf is disconnected and air applied (pressure not exceeding 2 psi).
5. Dry and weigh cake to determine percentage moisture. The filter rate (Y) in
lb/ft2 / hr is computed:
dry weight sludge, (gm) X cycles/hr
453.6 X test leaf area (ft2)
The test can easily be modified for other cycle times and discharge mechanisms. Filter
leafs are readily available from filter manufacturers and include instructions. It may be
necessary to adjust the above result by a factor to compensate for partial medium
blinding over a long period of operation and scale up. Although the filter leaf test is a
simple one, there are some precautions which should be observed to insure accurate
results:
• Representative sludge samples must be used.
• Several (5 to 10) tests should be run to monitor filter medium blinding.
• The test sample must be agitated to insure that it is homogeneous.
• The test filter vacuum must be regulated so that it does not vary during the
test and so that it is the same as proposed for use in full-scale operation.
Normally, the moisture content of the filter leaf test cakes is plotted as a function of a
correlating factor on rectangular coordinates. The equation for the correlating factor is:
sq ft W
where
cfm/sq ft = Air flow through the cake per unit area of filtering surface
t(j = Dry time, usually expressed in minutes
7-14
-------
P = The vacuum differential, psig
W' = Weight of the dry,cake solids in Ib per sq ft for a given
cake thickness
A decreasing moisture correlation indicates that, as the air rate through the cake per unit
of filtering area is increased, or as the vacuum differential or length of the drying time is
increased, the moisture content decreases. Conversely, if the cake thickness and
subsequently the cake weight (W') is increased, the moisture content increases. Knowing
the percentage of available drying time of the filter cycle and using the design
information (the proper cake thickness for a given type of filter, the vacuum level, and air
rate through the cake), it is possible to predict for each cycle time the discharged filter
cake moisture contents expected from the full-scale filter.
Discussion
Vacuum filtration of wastewater sludge is governed by the media's opening and the size
distribution of solid particles in the sludge. Raw primary sludges mainly contain particles
smaller than 100 mesh (0.15 mm). Filter cake formation is accomplished first by a
blinding of the media with the larger particles and this is followed by a packing of the
pores near the filter media with the fine particles [14]. An effect of elutriation is to
remove small particles that pack the pore openings. Chemical conditioning changes the
size of sludge particles and eliminates the large number of very small particles. As noted
in Chapter 6, polyelectrolytes and inorganic chemicals act differently. Both agglomerate
the fine particles, reduce the resistance, and clarify the filtrate. However, polyelectrolytes
agglomerate the fine particles and attach them to the larger ones. Iron and lime tend to
precipitate a coating on the fine particles, making them larger so that they do not pack
into the smaller pore openings. Heat treatment solubilizes many of the fine particles and
others are removed in the decant step. Two ways to increase water removal rates in
vacuum filtration are by using a coarse filter media and allowing some fine solids to pass
the media at the beginning of the filter cycle, and chemical conditioning. These
techniques are illustrated in Figure 7-8 along with the combination of media size and
conditioning considerations.
Where chemical conditioning of sludge is employed, coarse filter media is often used to
take advantage of the combined effects. Five to 10 percent solids recycle to the treatment
plant from the filtrate is common and this seems to be the most economical method to
accomplish vacuum filtration where chemical conditioning is used. When coarse filter
media is used, the machine piping maximum discharge rate controls the flow at the
beginning of the cycle. The machine variables, such as submergence and drum speed, are
not very sensitive for coarse media filters. In general, the cake will form until the
complete capture phase occurs and then the buildup will be very slow. Increasing the
formation time beyond that point does not appreciably change the cake thickness.
7-15
-------
Of.
O
o
o
MACHINE PIPING LIMITATION
CONDITIONING EFFECT
COMBINED CONDITIONING
AND MEDIA EFFECTS
VOLUME OF SLUDGE APPLIED
FIGURE 7-8. Media size and conditioning effects on filtrate flow rate.
7-16
-------
7.2.2 Process Objectives
As was noted previously, the operator of a vacuum filter strives for maximum solids
capture, filter cake yield, and filter cake solids content. He also wants to minimize his
costs. The relative importance of these objectives varies with the method of filter cake
disposal, and it is not usually possible to accomplish all of them. This necessitates striking
a reasonable balance.
Solids Capture
The amount of solids which can be recycled to a wastewater treatment process and not
affect its efficiency sometimes needs to be determined experimentally. However, in
biological plants, experience indicates that greater than 90 percent total solids capture
in the dewatering stage is usually required.
Solids capture is affected by:
• Relative proportion of suspended and dissolved solids in the sludge.
• Sludge characteristics, conditioning, and media.
• Filter drum washing.
The rotary vacuum and horizontal belt filters are capable of producing the highest filtrate
quality and hence solids capture of the various continuous dewatering alternatives. Solids
capture by vacuum filters may range from 85 to 99.5 percent depending on the type of
filter media, chemical conditioning, and solids concentration in the applied sludge. A
material balance over the projected dewatering procedure is an essential feature of
effective design.
Cake Yield
Units of expression are pounds of dry total sludge solids discharged from the media per
hour, per square foot of filter area. It is important to note the inclusion of the term
sludge solids in the definition of filter yield. When large percentages of lime, ferric
chloride, or ash are used for conditioning and largely end up in the cake, correction of
cake yields and solids contents must be made to maintain the validity of these basic
terms. Sufficient filter area must be provided so that the sludge solids removal rate
necessary to prevent excessive solids accumulation in the plant can be maintained. Since
the dewatering step is one of the two directly measurable and legitimate points for
removal of solids from the total plant process, maintenance of continuous and adequate
solids removal is absolutely essential to efficient system operation. The filter area
provided for in design should be for the peak sludge removal rate required plus a 5 to 15
7-17
-------
percent area allowance for maintenance downtime. Cake yield is affected by essentially
the same parameters of operation as is solids capture. Rotary vacuum filter cake yields
may vary from 2 to 15 lb/hr/ft2, but a yield of less than 3.5 is normally an indication of
some problem in sludge process design or operation. Raw primary yields of 7 to 15
lb/hr/ft2, digested primary yields of 4 to 7 lb/hr/ft2, and mixed digested yields of 3.5 to
5 lb/hr/ft2 are typical. The effect of feed solids content on yield is shown in Figure 7-9.
The dependence of filter cake yield on feed solids content graphically illustrates the
benefit of thickening sludges prior to dewatering by vacuum filtration. A relationship
between labor costs and filter yield has been developed by Bennett [2] for an assumed
situation and is shown in Figure 7-10. At yields below 4 lb/hr/ft2, labor costs increase
rapidly.
Cake Solids Content
Cake solids content is affected by the sludge type, sludge solids concentration, mode of
conditioning, and machine operation. The interdependence of cake solids level, feed
solids content, and type of sludge is illustrated in Figure 7-11. As with filter yield,
thickening of the feed sludge develops a drier filter cake. The data from McCarty [16] as
plotted in Figure 7-11 is a compilation of information from various plants and is an
average for the different types of sludge. The other curves are for the four types of sludge
processed at the Metro Denver plant. It should be noted that the sludges encountered at
Denver are unusually difficult to process. The Denver case history will be discussed later
in Chapter 10.
Cake solids concentration is also a very important consideration when incineration or
trucking of cake to land disposal are contemplated. As will be noted in Chapter 10, cake
solids concentration is only one of the significant parameters in gauging the effectiveness
of various preincineration sludge processing systems.
7.2.3 Types of Rotary Vacuum Filters
The three principal types of rotary vacuum filters are shown.
Type
Drum
Coil
Belt
Covering Used
Cloth
Stainless steel spring
Cloth, infrequently metal
Discharge Mechanism
Blowback section and doctor blade
Coil layer separation and tines
Small diameter roll, or flapper, or doctor blade
7-18
-------
12
11
10
9
CN"~
£«
at
3 7-
Q 5
_l
UJ
>• 4
3
2
1
D DIGESTED
X PRIMARY
OBLENDED
A ACTIVATED
345678
FEED SOLIDS (%)
10 11
FIGURE 7-9. Yield as a function of feed solids [2],
7-19
-------
10
9
8
ID
o
x
£ 4
>-
3
J_L
i i i
I
5 10 15
LABOR COST ($/TON DRY SOLIDS)
FIGURE 7-10. Vacuum filtration operational labor costs as function of yield [2].
7-20
-------
35-
30-
9 20-
O
uj 15-
U 10-
5-
McCARTY
x
PRIMARY
ACTIVATED
11gm/LCaO, 3.7 gm/L FeCI3
n i i r i
34567
FEED SOLIDS (%)
i
8
T
10
FIGURE 7-11. Cake solids as a function of feed solids for different sludges [2]
7-21
-------
The filters differ primarily in the type covering used and the cake discharge mechanism
employed. The drum filter (see Figure 7-1) also differs from the other two in that the
cloth covering does not leave the drum but is washed in place, when necessary. The design
of the drum filter provides considerable latitude in the amount of cycle time devoted to
cake formation, washing, and dewatering; while it minimizes inactive time. The drum
filter was the original type unit employed in municipal wastewater plants. Problems with
frequent washings of the drum cloth when large doses of lime were being used for
conditioning have been essentially eliminated with improved filter media and the use of
polymers.
A variation of the conventional drum filter is the top feed drum filter. In this case, sludge
is fed to the vacuum filter through a hopper located above the filter. The city of
Milwaukee, Wisconsin, is currently planning plant scale evaluation of this type of vacuum
filter [17]. The potential advantages are that gravity aids in cake formation; capital costs
may be lower since the feed hopper is smaller and no sludge agitator and related drive
equipment are required; and blinding may be reduced because the gravity formation of
the cake allows larger particles to collect first on the cloth providing a straining layer to
capture the smaller particles.
The coil type vacuum filter is shown in Figure 7-12 and uses two layers of stainless steel
coils arranged in corduroy fashion around the drum. After a dewatering cycle, the two
layers of springs leave the drum and are separated from each other so that the cake is
lifted off the lower layer of springs and discharged from the upper layer. Cake release is
essentially never a problem. The coils are then washed and reapplied to the drum. Figure
7-13 shows a cutaway view of the coil springs. The coil filter has been and is widely used
for all types of sludge. However, sludge with particles that are both extremely fine and
resistant to flocculation dewater poorly on coil filters.
The belt type filter is shown in Figure 7-14. Media on the belt filter leaves the drum
surface at the end of the drying zone and passes over a small diameter discharge roll to
facilitate cake discharge. Washing of the media next occurs before it returns to the drum
and to the vat for another cycle. This type filter normally has a small diameter curved bar
between the point where the belt leaves the drum and the discharge roll. This bar
primarily aids in maintaining belt dimensional stability. In practice it is frequently used to
insure adequate cake discharge. Figure 7-15 shows the demooning bar set at a maximum
angle of projection to help break the cake free from the belt. Remedial measures are
frequently required to obtain operable cake releases from belt filters. This is particularly
true at plants which have greasy or sticky sludges due to high activated sludge content. A
summation of locations and type of remedial action required to obtain cake release from
belt type filters appears in Table 7-2.
Rotary vacuum filters are normally supplied with essential auxiliary equipment. A
complete system is shown in Figure 7-16. Principal auxiliary equipment includes a
7-22
-------
COIL SPRING
FILTER MEDIA
WASH WATER
SPRAY PIPING
DRUM
1
to
CAKE
VACUUM AND
FILTRATE OUTLETS
AGITATOR DRIVE
AGITATOR
VAT DRAIN
FIGURE 7-12. Cross section of a coil filter.
-------
o
KJ
FIGURE 7-13. Cutaway view of coil springs.
-------
DRUM
TAKEUP ROLL
CLOTH BELT
DISCHARGE ROLL
DISCHARGE ZONE
WASH ROLL
WASH TROUGH
FIGURE 7-14. Cross section of a belt filter.
7-25
-------
to
FIGURE 7-15. Cake release of a belt filter.
-------
TABLE 7-2
CAKE RELEASE MEASURES USED ON BELT TYPE FILTERS
AT VARIOUS PLANT LOCATIONS
Location
Conditioning System
Remedial Steps Required
to Obtain Discharge
Washington, D.C.
Baltimore, Md.
Indianapolis, In.
FeCl3 and Polymer
Polymer
Ash and Polymer
Toronto, Ontario Polymer or FeCl3 /Lime
Richmond, Ca. FeCl3 and Lime
Colorado Spgs., Co. Heat Treatment
Use of excess amount of ferric
chloride 3 Ib/ton
Plastic doctor blade added
Inclusion of incinerator ash with
sludge
None successful; belt filters idle
Chemical dose increased
Occasional operator assistance
vacuum pump, filtrate receiver and pump, sludge conditioning apparatus, and a sludge
pumping system. Usually one vacuum pump is provided for each vacuum filter although
some larger plants use fewer pumps connected to a common header. In early days,
reciprocating type dry vacuum pumps were generally specified but wet type vacuum
pumps are now almost universally used. The wet type pumps are more easily maintained
and provide sufficient vacuum. Wet type pumps utilize seal water and it is essential that a
satisfactory water be used. If the water is hard and unstable, it may be necessary to
prevent carbonate buildup on the seals through the use of a sequestering agent. Each
vacuum filter must be supplied with a vacuum receiver interposed between the filter valve
and the vacuum pump. The receiver is usually designed to give a maximum air velocity of
2.5 to 5 ft/min and a minimum air detention time of 2 to 3 minutes [6]. The principal
purpose of the receiver is to separate the air from the liquid. Each receiver can be
equipped with a vacuum limiting device to admit air flow if the design vacuum is
exceeded, a condition which would cause the pump to overload. The receiver also acts as
a reservoir for the filtrate pump suction. Filtrate pumps must be sized to carry away the
water separated in the vacuum receiver. These are specially designed pumps to operate at
very low net positive suction heads and are designed for at least 20- to 22-inch Hg vacuum
at the inlet. The discharge head depends on the local conditions.
7-27
-------
to
oo
AIR TO ATMOSPHERE
SlUDGE INLET
VACUUM PUMP
FIGURE 7-16. Rotary vacuum filter system.
-------
Centrifugal style filtrate pumps are common but can become air bound unless they have a
balance or equalizing line connecting from a high point of the receiver to the eye of the
pump. Noncloggjng centrifugal style pumps are used with coil filters or with coarse metal
filter media. They permit a somewhat higher solids concentration in the filtrate.
Self-priming centrifugal pumps are used most frequently, since they are relatively
maintenance free. Self-priming, nonclogging centrifugal pumps are also available. Check
valves on the discharge side of the pumps are usually provided to minimize air leakage
through the filtrate pump and receiver back to the vacuum pump. Filtrate pumps should
be sized to accommodate the entire range of filtrate flow rates likely to be encountered.
The fact that the rate of filtrate flow is a function of the mode of conditioning must be
recognized in filtrate pump sizing. Polyelectrolytes allow the sludge to drain much more
rapidly than do inorganic conditioners. If the filtrate pumps are not sized accordingly,
full advantage of the more rapid drainage cannot be realized.
7.2.4 Machine Variables
The operation of a rotary vacuum filter is sensitive to the type of sludge and conditioning
procedures. Plant experimentation is the best way to test a potentially improved mode of
operation. One major machine variable is the media used and a great many types of filter
media are available for the belt and drum filters. Blinding characteristics and chemical
conditioning play an important role in media selection. Filter leaf tests should be
conducted with the various media as an aid in selecting the optimum one for a specific
sludge. The ideal media has the following characteristics [ 18]:
• It is able to perform the desired liquid/solid separation and give a filtrate of
acceptable clarity.
• The filter cake discharges readily from it.
• It is strong enough mechanically to give a long life.
• It is chemically resistant to the materials being handled.
• Its resistance to flow is not too great.
• It does not rapidly blind.
Obviously, some reasonable compromise must be reached between these objectives since
all of them cannot be optimized simultaneously. The Chicago Sanitary District has the
largest vacuum filter installation in the U.S. and has evaluated many different types of
filter media, reported by Shedden [19]. Table 7-3 summarizes their conclusions and
7-29
-------
TABLE 7-3
EVALUATION OF ALTERNATE FILTER MEDIA AT THE CHICAGO SANITARY DISTRICT [19]
Material
Evaluation
Criteria
Installation
Initial Cake
Pickup
Cake
Production
Elongation
Tendency
Response to
Water Wash
Response to
Detergent Wash
Response to
Acid Wash
Resistance
to Scraper
Abrasion
Useful Life
(approx. hrs)
Economic
Rating
Wool Wool Wool
12 oz 13oz 14-15 oz
Difficult Difficult Easy
Poor Fair Very
Good
Fair Fair Very
Good
Excessive Excessive High
Good Good Good
Good Good Good
Good Good Good
Poor Fair Good
1900 2400 2700
- - 4
Wool
16 oz
Difficult
Fair
Fail
High
Good
Good
Good
Good
1600
-
Wool
Treated
14-15 oz
Easy
Very
Good
Very
Good
Some
Varied
Very
Good
Very
Good
Very
Good
Good
3300
3
Vinyon
Very
Difficult
Fair
Fair
Excessive
Fair
Poor
Poor
Poor
1000
-
Nylon
100%
Very
Difficult
Fair
Fair
High
Fair
Fair
Very
Poor
Good
800 to
7600
-
Nylon
90%
Saran
10%
Very
Difficult
Fair
Fail
High
Fair
Fair
Poor
Good
3000
-
Nylon
25%
Wool
75%
Easy
Good
Fair
Some
Good
Good
Poor
Good
3300
-
Dyne!
50%
Wool
50%
Easy
Good
Good
Some
Good
Good
Good
Good
3700
-
Orion
100%
Difficult
Fair
Good
Some
Good
Fair
Poor
Fair
2000
-
Dynel
100%
Difficult
Poor
Good
Some
Good
Good
Good
Fair
Over
4000
2
Dacton
100%
Easy
Very
Good
Very
Good
Very
Little
Very
Good
Very
Good
Very
Good
Very
Good
15,000
1
-------
indicates that Dacron, a polyester, was the most suitable to their use. Other treatment
plants have found polypropylenes to be satisfactory. Polyethylenes tend to stretch when
wet and require constant operator vigilance of belt tension. Minneapolis-St. Paul has
reported a life of 12,400 hours for a Saran medium, according to Simpson and Sutton
[20]. Monofilament fabrics are the most resistant to blinding and have been used almost
exclusively in recent installations of drum or belt filters.
Up to a point, filter yield increases as the vacuum is increased. Because of the
compressible nature of wastewater sludges, there is some question whether operating
vacuums greater than 15 inches of mercury are justifiable. Only slight increases in yield
are normally experienced beyond this level [21 ]. The cost of a greater filter area must be
balanced against the higher power costs for higher vacuums. An increase from 15 to 20
inches of vacuum is reported by Schepman and Cornell [22] to have provided about 10
percent greater yield in three full-scale installations.
Increasing the drum submergence rate increases the form cycle time and usually results in
an increased yield, and thicker but wetter cake. In general, the maximum submergence
used on a sludge filter is 25 percent, although higher submergences are possible. The
submergence is usually kept between 15 and 25 percent which gives a long drying time
and keeps the cake moisture content at a minimum.
Slowing the drum speed increases the filter cycle time and produces a drier cake, but the
filter's productivity is decreased.
Proper agitation of the sludge during and after chemical conditioning is important. The
evaluation of this parameter requires variable speed mixing equipment for both the
chemical conditioning tanks and the vacuum filter pan. After chemical conditioning, the
sludge must be handled as gently as is practical. Only enough agitation should be applied
in the filter pan to prevent solids classification and keep the solids in suspension. Because
sludge viscosities vary, optimum control requires variable speed pan agitation equipment.
7.2.5 Rotary Vacuum Filter Costs
Capital costs may range from $100 to $300 per square foot depending on unit size, type
media, and auxiliary equipment [23]. Operation and maintenance costs vary widely
according to plant size, pretreatment procedures, and product quality requirements, but
$5 to $20 per ton is representative. An approximate breakdown in elements of vacuum
filter O/M costs is provided by Simpson and Sutton [24].
Percent
Labor and direct supervision 39
Chemicals and supplies 37
7-31
-------
Electric power
Maintenance
Percent
8
16
7.2.6 Typical Rotary Vacuum Filter Results
Table 7-4 presents representative data for sludges conditioned with ferric chloride and
lime.
TABLE 7-4
TYPICAL ROTARY VACUUM FILTER RESULTS
FOR SLUDGE CONDITIONED WITH INORGANIC CHEMICALS
Chemical Dose (Ib/ton)
Type Sludge
Raw Primary
Anaerobically Digested Primary
Primary + Humus
Primary + Air Activated
Primary + Oxygen Activated
Digested Primary and Air Activated
Ferric Chloride
1-2
1-3
1-2
2-4
2-3
4-6
Lime
6-8
6-10
6-8
7-10
6-8
6-19
Yield
(lb/hr/ft2)
6-8
5-8
4-6
4-5
5-6
4-5
Cake
Solids (%)
25-38
25-32
20-30
16-25
20-28
14-22
The data in this table are never a substitute for actual lab or pilot tests' results for a
particular sludge. Typical data for sludges conditioned with polyelectrolytes are shown in
Table 7-5.
The price of polymer per pound can vary considerably. Therefore, these data are
presented only to illustrate rough ranges, and a determination of accurate and meaningful
unit process costs must be considered as an integral part of a particular system.
7-32
-------
TABLE 7-5
TYPICAL ROTARY VACUUM FILTER RESULTS
FOR POLYELECTROLYTE CONDITIONED SLUDGES
Type Sludge
Chemical Cost
($/ton)
Yield
(lb/hr/ft2)
Cake
Solids (%)
Raw Primary
Anaerobically Digested Primary
Primary + Humus
Primary + Air Activated
Primary + Oxygen Activated
Anaerobically Digested Primary
and Air Activated
1-2
2-5
3-6
5-12
5-10
6-15
8-10
7-8
4-6
4-5
4-6
3.5-6
25-38
25-32
20-30
16-25
20-28
14-22
7.2.7 Summation
Rotary vacuum filtration can be and is, in most cases, an effective and efficient
dewatering method. It has been misapplied in some cases in the past. Improper selection
of media, failure to thicken the feed sludge, the cake release problem on belt filters, and
lack of proper sludge conditioning have generally been the causes of failures. In some
cases, as will be discussed in Chapter 10, plant systems have not been designed with the
proper sequence of unit processes. This makes efficient dewatering very difficult.
7.3 Centrifugal Dewatering
Centrifuges of various types have been employed for solid-liquid separation processes in
agriculture and industry for at least 50 years. For almost 25 years the continuous solid
bowl conveyor type centrifuge has been used for dewatering municipal sludges. Objectives
of centrifugal sludge dewatering are the same as for rotary vacuum filtration.
7-33
-------
7.3.1 Theory of Centrifugal Dewatering
Solids Removal and Conveyance
The centrifuge uses centrifugal force to speed up the sedimentation rate of sludge solid
particles. Figure 7-17 shows a continuous solid bowl dewatering centrifuge. The two
principal elements of this centrifuge are the rotating bowl which is the settling vessel and
the conveyor discharge of settled solids. The bowl has adjustable overflow weirs at its
larger end for discharge of clarified effluent (centrate) and solids discharge ports on the
opposite end for discharging dewatered sludge cakes. As the bowl rotates, centrifugal
force causes the slurry to form an annular pool, the depth of which is determined by the
adjustment of the effluent weirs. A portion of the bowl is of reduced diameter so that it
is not submerged in the pool and thus forms a drainage deck for dewatering the solids as
they are conveyed across it. Feed enters through a stationary supply pipe and passes
through the conveyor hub into the bowl itself. As the solids settle out in the bowl, they
are picked up by the conveyor scroll and continuously carried along to the solids outlets.
Clear effluent at the same time continuously overflows the effluent weirs. Flocculants are
normally injected into the pool.
It is extremely important to note that there are two operating zones in the horizontal
bowl conveyor centrifuge; the submerged pool and the drainage deck. Early theoretical
consideration of centrifugal dewatering mechanisms focused primarily on the relationship
between the centrifuge and a hypothetical sedimentation basin as affected by the
employment of very high "G" forces. The Sigma formula from Perry's Chemical
Engineers Handbook [25] is normally employed to describe the operation of a
continuous, horizontal, helix-type centrifuge and is shown here. This formula shows that
the rate of liquid clarification varies with the surface area of the liquid and the level of
centrifugal force.
Trbw2
S = (3r22+ri2)
2g
where
2 = Sigma centrifuge capacity factor, ft2 (Theoretical
area of gravity settling tank of equivalent sedimentation
characteristics to centrifuges)
b = Length of cylindrical bowl, ft
w = Rate of rotation, radians/second
7-34
-------
DIFFERENTIAL SPEED
GEAR BOX
/ROTATING
CONVEYOR
COVER
MAIN DRIVE SHEAVE
u
\
~7r*—FEED PIPES
U-" (SLUDGE AND
CHEMICAL)
BEARING
BASE NOT SHOWN
CENTRATE
DISCHARGE
SLUDGE CAKE
DISCHARGE
FIGURE 7-17. Continuous countercurrent solid bowl conveyor discharge centrifuge.
-------
r2 = Radius of inner bowl wall, ft
T! = Radius of retained liquid surface, ft
g = Gravitational constant, ft/sec2
Sigma and other theoretical relationships based on easily measured machine dimensions
are useful tools when employed by the centrifuge designer for estimating scale-up
relationships in geometrically similar machines. Note that a factor of two, missing from
the formula in the referenced handbook, has been included here. Unfortunately, the
widespread use of the Sigma formula in the literature and its recent publication in Perry's
Chemical Engineers Handbook have lead to some centrifuge specifications based only on
square feet of Sigma. White pointed out that this can be a serious error, because it
theoretically suggests that ample clarifying ability is the only requirement for scale-up
and desired performance. No consideration has been given of the solids conveying aspects
[26]. The centrifugal force can adversely affect what theory would indicate. This is
primarily because a solid bowl conveyor centrifuge not only has to clarify a slurry and
settle particles, but it must also accomplish the secondary function of conveying the
solids. Thus, while increasing the centrifugal force and lowering the depth of the liquid in
the bowl will theoretically increase its clarification ability, it may actually harm
clarification in the centrifuge as discussed by White [27]. The design engineer must fully
take into account the existence and impact of the drainage deck or beach zone since
significant drainage or dewatering of sludge solids occurs here.
Factors Affecting Centrifugal Dewatering
Sludge characteristics which affect centrifuge performance are essentially the same as
those listed for rotary vacuum filters. In general, those sludges which separate most
readily and concentrate to greatest thickness by plain sedimentation are those which
dewater most efficiently in centrifuges.
Machine variables of importance are:
• Bowl design
Length/Diameter Ratio
Bowl Angle
Flow Pattern
• Bowl Speed
7-36
-------
• Pool volume
• Conveyor design
• Relative conveyor speed
• Sludge feed rate
The settling time and surface area can be increased for a given diameter bowl by
increasing the length/diameter ratio. Although the detention time is increased by an
increase in bowl diameter, lower centrifugal forces result because of mechanical
limitations. Length/diameter ratios of 2.5 to 3.5 are customarily employed.
The designer can increase the length of the clarifying zone of the bowl by making the
discharge angle of the screw conveyor steeper. Centrifugal forces can also be increased.
The effect of these two variables on the settled sludge on the conveyor's incline section is
illustrated in Figure 7-18.
The slippage force (g) = sin1* while the centrifugal force (G) = 1.42 X 1(T5 (rpm)2.
Albertson and Guidi noted that as either the angle of the beach increases or the
centrifugal force level increases, the forces driving the settled sludge back down into the
pool are increased proportionately [28]. Thus, although the settling rate increases in
proportion to the centrifugal force level, the forces rejecting the material into the bowl of
the centrifuge are also proportionately increasing and preventing its discharge. Eventual
overflow of these solids with the effluent could result.
The flow pattern in the machine may be based on a countercurrent flow of liquids and
solids (as shown in Figure 7-17) or a concurrent flow as is discussed later in this chapter.
The primary operating variables are bowl speed and-pool volume. While increasing the
bowl speed increases the centrifugal forces and favors increased clarification, the settled
solids become more difficult to discharge. Excessive bowl speed tends to lock the bowl
and conveyor together and increases abrasion.
Pool depth affects both clarification and cake dryness. Lowering the pool exposes more
drainage deck area, increases the dewatering time, and produces a drier cake. Within
limits, increasing pool depth increases clarification by increasing detention time.
However, just as in plain sedimentation, too great a depth prevents a particle from
reaching the sediment zone prior to being discharged in the effluent. At too shallow a
depth, the moving conveyor tends to redisperse settled solids.
Conveyor speeds normally are designed or adjusted to a minimum turbulence inside the
pool while still providing sufficient conveying capacity. Low speeds also reduce the rate
of wear on the conveyor blades when poorly degritted sludges are handled. Increasing the
conveyor speed sometimes produces drier solids because the fines are washed from the
cake.
7-37
-------
U)
00
FIGURE 7-18.
Effect of bowl angle and centrifugal force on sludge solids in drainage
zone.
-------
The sludge feed rate is clearly one of the more important variables. It affects both clarity
and sludge cake dryness. The handling of a larger volume of sludge per unit of time in a
given bowl means less retention time and a decrease in solids recovery. It also usually
results in drier solids in the cake because of the higher loss of fines to the centrate.
Test Procedures
Successful application of continuous, solid bowl, conveyor centrifuges requires a
consideration of numerous factors. Proper scale-up is the major factor, and to obtain
predictable results, values must be available for the following variables [27]:
• Wet cake discharge rate.
• Solids dewatering time under centrifugal force.
• Conveying torque for cake solids.
• Liquid clarifying ability.
• Resistance to abrasion from slurry solids.
• Stability of centrifuge feed.
• Physical nature of solids being handled.
• Permissible chemical flocculant dosages.
Tests for scale-up can be accomplished with either laboratory or pilot tests. Pilot tests
should be conducted with a continuous, small pilot centrifuge geometrically similar to
that proposed for full-scale use. Although laboratory tests may be more convenient, they
provide little meaningful scale-up data. An evaluation of the relative effects of various
flocculants and the potential tradeoffs between high centrifugal force and little flocculant
addition and lower force with more flocculants is possible in the laboratory. The only
satisfactory method of accurately predicting the performance of a full-scale unit is the
operation of a pilot unit on the sludge involved. These pilot units are readily available
from manufacturers, who have developed scale-up factors for their pilot equipment.
These factors have proven to provide accurate predictions of full-scale performance.
These scale-up procedures are considered proprietary and are not generally available.
Vesilind [29] presents one approach to scale-up procedures. Since either the hydraulic or
the solids capacity may be limiting, scale-up is made for both and the full-scale unit
selection is then based on whichever is the governing factor. The scale-up factors are
developed in the following equations, where machine 2 is the full-scale unit and machine
1 is the pilot unit. The hydraulic capacity can be estimated as:
7-39
-------
in which
_
2 "
(QL
—
\ — -
Vcj2
The solids handling capacity can be estimated as:
fQsl
(Qs)2 = — s 03)2
IP J 1
in which
j3 = TrAcoSDNP
The nomenclature used in the preceding equations is:
QL = Liquid flow rate
V = Volume in centrifuge occupied by slurry
g = Gravitational constant
r2 = Radius of bowl wall from center line
TJ = Radius of slurry surface from center line
Qs = Solids throughout
|3 = Differential speed between bowl and conveyor
S = Scroll blades separation
D = Cylinder diameter
7-40
-------
N = Number of leads on scroll
P = Pool depth
7.3.2 Types of Centrifuges
Countercurrent
The solid bowl countercurrent centrifuge as discussed earlier is the most widely used type
for dewatering of wastewater sludge in the United States. The centrifuge assembly, as has
been shown in Figure 7-17, consists of a rotating unit comprising a bowl and conveyor
joined through a planetary gear system, designed to rotate the bowl and the conveyor at
slightly different speeds. The solid cylindrical-conical bowl, or shell, is supported between
two sets of bearings and includes a conical section at one end. This section forms the
dewatering beach over which the helical conveyor screw pushes the sludge solids to outlet
ports and then to a sludge cake discharge hopper. The opposite end of the bowl is fitted
with an adjustable outlet weir plate to regulate the level of the sludge pool in the bowl.
This plate also discharges the centrate through outlet ports either by gravity or by a
centrate pump attached to the shaft at one end of the bowl. Sludge slurry enters the
rotating bowl through a stationary feed pipe extending into the hollow shaft of the
rotating bowl. The sludge feed enters a baffled, abrasion-protected chamber for
acceleration before discharge through the feed ports of the rotating conveyor hub into
the sludge pool in the rotating bowl. The sludge pool takes the form of a concentric
annular ring of liquid sludge on the inner wall of the bowl. Separate motor sheaves or a
variable speed drive can be used for adjusting the bowl speed for optimum performance.
Bowls and conveyors can be constructed from a large variety of metals and alloys to suit
special applications. For dewatering of wastewater sludges, mild steel or stainless steel
normally has been used. Because of the abrasive nature of many sludges, hardfacing
materials are applied to the leading edges and tips of the conveyor blades, the discharge
ports, and other wearing surfaces. Such wearing surfaces may be replaced by welding
when required.
Continuous Concurrent Flow Solid Bowl Conveyor Type Centrifuge
Figure 7-19 shows a cross section of a continuous concurrent flow solid bowl conveyor
type centrifuge. Incoming sludge is carried by the feed pipe to the end of the bowl
opposite the discharge. As a result, settled solids are not disturbed by incoming feed.
Solids and liquids pass through the bowl in a smooth parallel flow pattern. Turbulence is
substantially reduced. Solids are conveyed over the entire length of the bowl before
discharge to provide better compaction, a drier cake, and reduce flocculant demands.
7-41
-------
DRAIN
PORT
POOL INTERNAL
LEVEL EFFLUENT
SKIMMER
SOLIDS DISCHARGE
PORTS AND PLOWS
PILLOW BLOCK AND
MAIN BEARING
CONVEYOR
GEAR DRIVE
SKIMMER
f CONTROL
DRIVEN FEED CONVEYOR SPLASH
SHEAVE PIPE COMPARTMENT
TRUNNION
SEALS
SOLIDS
DISCHARGE
BASE
FLOC
INLET
EFFLUENT
DISCHARGE
FIGURE 7-19. C'ross section of concurrent flow solid - bowl centrifuge.
-------
Basket Centrifuge
The basket centrifuge or imperforate bowl-knife discharge unit, as shown in Figure 7-20,
has recently been introduced primarily for use as a partial dewatering device at small
plants. Parkhurst et al. [30] have used the basket centrifuge as a clarifying device for
solid bowl centrate at the Los Angeles County Sanitary District.
Flow enters the machine at the bottom and is directed toward the outer wall of the
basket. Cake continually builds up within the basket until the centrate, which overflows a
weir at the top of the unit, begins to increase in solids. At that point, feed to the unit is
shut off, the machine decelerates, and a skimmer enters the bowl to remove the liquid
layer remaining in the unit. A knife is then moved into the bowl to cut out the cake
which falls out the open bottom of the machine. The unit is a batch device with alternate
charging of feed sludge and discharging of dewatered cake. Because of the cycle time
involved this unit has a lower capacity than continuous devices. It does, however, have
the capability of higher solids recovery without chemical addition because there is a
minimum of disturbance of the depositing solids.
Disc Centrifuge
The disc centrifuge has long been used in the chemical process industry for handling large
flows with relatively low concentrations of fine particles. The incoming stream is
distributed between a multitude of narrow channels formed by stacked conical discs.
Figure 7-21 shows a cross section of a disc centrifuge. Suspended particles have only a
short distance to settle, so that small and low density particles are readily collected and
discharge continuously through fairly small orifices in the bowl wall. Sludge
concentrations of 5- to 20-fold are accomplished. The clarification capability and
throughput range are high, but sludge concentration is limited by the necessity of
discharging through orifices of 0.050 inches to 0.100 inches in diameter, which imposes
an upper limit on the size of particle that can be handled by the disc centrifuge. Feed
must be degritted and adequately screened. Insufficient attention to these factors in the
past has led to the erroneous conclusion that disc centrifuges are not applicable to some
industrial and municipal sludges due to plugging. Plugging is a real limitation, however,
and must be considered in the design of a system employing disc centrifuges. Even if the
sludge is screened adequately, severe nozzle clogging can occur if the feed to the
centrifuge is stopped, interrupted, or reduced below some minimum value. The nozzles
immediately clog due to the collapse of the solids built up in the bowl as described by
Woodruff [31]. They are particularly of value in classifying the sludges resulting from
lime coagulation of secondary effluent. For handling organic sludges, the thickening
ability of the disc centrifuges is good but their dewatering ability leaves much to be
desired.
7-43
-------
FEED
POLYMER-,
SKIMMINGS
CAKE
CAKE
FIGURE 7-20. Schematic diagram of a basket centrifuge.
7-44
-------
SLUDGE
DISCHARGE
FIGURE 7-21. Disc type centrifuge.
7-45
-------
Lower Speed Continuous Solid Bowl Conveyor Type Centrifuges
These centrifuges have been developed primarily in Europe to achieve high solids capture
and minimize the recirculation of solids to the treatment plant without the use of high
polymer dosages. The sludge is introduced into the centrifuge with the lowest possible
acceleration and turbulence. The machine is operated at about 1,500 rpm depending on
the diameter of the centrifuge. This low rpm gives a low noise level and a minimum of
wear and tear on the rotating parts. Low conveyor differential speeds are also used.
Machines of this type are now offered by I. Kruger and Co. of Denmark and Bird
Machine. Among the reported advantages of these machines are lower capital costs, lower
power requirements, lower noise level, and reduced maintenance when compared to
higher speed centrifuges. The use of large pool volumes, reduced internal turbulence and
low centrifugal forces (500 to 800) combine to reduce shearing forces on the floe and
improve the conveying characteristics.
7.3.3 Sludge Fractionation (Classification) by Centrifuge
Excess activated sludges and hydrated fines tend to be lighter than primary sludge or well
flocculated secondary sludges. The behavior of heavy and light sludge particles in
centrifuges can be visualized from Figure 7-22. It is seen that light sludge particles require
a much longer retention time. This characteristic has been used to fractionate sludge into
a cake fraction containing essentially all the primary sludge particles and a centrate with
the lighter, excess activated or fine hydrated particles. This phenomena was used as a
means of venting the more difficult to process solids into the plant effluent where
effluent standards permitted this practice. It is also used as a means of separating various
types of sludges to permit disposal of the one stream and further processing of the other.
The former use is exemplified by past practices at some coastal plants which utilize ocean
disposal of effluent at points beyond the continental shelf. The latter use is exemplified
by the centrifugal classification at South Lake Tahoe [32] and Contra Costa, California
[33] plants for fractionation of lime sludges into a calcium carbonate-rich fraction and an
inert laden fraction.
In addition to the primary effect of residence time, the setting of pool depth materially
affects the degree of fractionation achieved. In tests at Los Angeles [30] on a digested
primary sludge containing some sludge from satellite secondary plants, it was noted that
the lower the solids capture, the drier the cake due to the venting of the fine hydrated
particles in the centrate. The low level of solids recovery (less than or equal to 50 percent
achieved with no chemical conditioning) is clearly inapplicable in most plants as indicated
by the data in Figure 7-23. However, the work at Los Angeles was carried out to select
the optimum system to cope with new effluent standards which could not be met by
previous method and equipment. By using polymers (8 to 10 Ib/ton) and a 3.4-inch pool
depth, solids recovery of as high as 95 percent were achieved under the same operating
conditions shown in Figure 7-23.
7-46
-------
HEAVY SLUDGE PARTICLES
FORCE
i
SEC.
LIGHT SLUDGE PARTICLES
FORCE
SEC.
FIGURE 7-22. Reaction of particles within centrifuges.
7-47
-------
50
-J
oo
vp"
vfis
-C45
o
40
5 35
£ U 30
c/>
U) Q
9 3
X O 25
O
< 20
15
I I I I
FEED RATE: 250 gpm (15.8 I/sec)
BOWL SPEED: 1300 rpm (900 g)
DIFFERENTIAL SPEED: 15.3 rpm
I I
1.0
I
I
1
I
I
2.0 3.0 4.0
POOL DEPTH, (inches)
5.0
6.0
FIGURE 7-23. Typical dewatering performance curves for a36"X 96 "Bird horizontal
scroll centrifuge fed unconditioned primary digested sludge [30].
-------
At the South Tahoe [32] and Contra Costa [33] plants, solid bowl centrifuges are used
on sludges resulting from lime coagulation of wastewaters. Phosphates and other inerts
are removed with the centrate after first stage centrifugation while calcium carbonate is
retained in the dewatered cake, which is later recalcined and reused. The first stage
centrate is then dewatered in a second solid bowl centrifuge and the resultant cake is
incinerated. By controlling the pool depth in the first stage centrifuge, it is possible to
control the solids capture. If the first stage machine is operated to maximize solids
capture (90 to 95 percent), then the cake contains nearly all of the inert materials as well
as the calcium carbonate desired for the recalcining process. Figure 7-24 illustrates the
classification effects on the percent recovery of various constituents of lime sludges
resulting from coagulation of raw wastewaters at Contra Costa. On the tertiary lime
sludges at South Tahoe, it was found that by operating the first centrifuge at 75 percent
capture, the calcium oxide content of the cake was improved by 15 percent over that
obtained at 95 percent. Such operation resulted in 87 percent of the usuable lime going
to the recalcining furnace and the purging of 80+ percent of phosphorus and 35+ percent
of magnesium from the recalcining system. Similar results were achieved at Contra Costa
where 90 percent of the calcium carbonate fed to the first stage centrifuge was recovered
while 50 to 75 percent of the other constituents were rejected in the centrate. The first
stage cake had a solids content of 50 percent or greater.
7.3.4 System Requirements
Most centrifuge installations include the items shown in Figure 7-25. Sludge pumps are
usually of the progressing cavity type, since constant feed is essential. Not shown, but
frequently required where gritty sludge is encountered, is a cyclone separator for
auxiliary degritting to prevent excessive wear. Details of a typical flocculant system are
shown in Figure 7-26. Although not shown in this figure, it is preferable to have separate
tanks for mixing of the flocculant and storage of the feed pump supply. This avoids
feeding incompletely mixed flocculent solution. For solid bowl machines, the flocculant
is usually added directly to the interior of the centrifuge to avoid shearing the floe. As
discussed in detail in Chapter 6, the most effective flocculant and its dosage for any given
sludge can only be determined by experimentation. Dilution of the flocculant ahead of
the centrifuge to a strength of 0.1 percent or less has often been found to give maximum
effectiveness. A dispersing eductor is normally used for this purpose.
7.3.5 Results of Centrifugal Dewatering
Data is readily available on the use of solid bowl centrifuges since they are widely used.
As noted earlier, it is difficult to predict dewatering results. However, Table 7-6 presents
typical data which can be expected with the solid bowl centrifugation of various organic
sludges.
Figure 7-27 graphically depicts the effects of the major variables on solid bowl
performance in dewatering an anaerobically digested mixture of primary and secondary
7-49
-------
20 40 60 80
RECOVERY OF TOTAL SOLIDS, (%)
100
FIGURE 7-24.
Summary of constituent recoveries during wet classification of lime
sludges resulting from raw wastewater coagulation [33].
7-50
-------
CHEMICALS FOR CONDITIONING
CENTRIFUGE
T
CAKE
OR
SOLIDS
CENTRATE
SAMPLE
TAP
SHUTDOWN
FLUSH
H2O
X
SAMPLE
TAP
>
z
SLUDGE
SLUDGE
PUMP
FIGURE 7-25. Centrifuge dewatering system.
7-51
-------
TO CENTRIFUGE
DISPERSING -, Y
EDUCTOR / T
c±n
ROTOMETER
FLOC MIXING
EDUCTOR FUNNEL
FLOCCUL^NT
FEED PUMP
./
FLOCCULANT
A MIXING TANK
FRESH WATER
OR PLANT EFFLUENT
FIGURE 7-26. Typical flocculant piping diagram.
7-52
-------
TABLE 7-6
TYPICAL SOLID BOWL CENTRIFUGE PERFORMANCE [6]
Wastewater Sludge Type
Raw or digested primary
Raw or digested primary, plus trickling filter humus
Raw or digested primary, plus activated sludge
Sludge Cake Characteristics
Solids
(%)
28-35
20-30
15-30
Solids
Recovery
(%)
70-90
(50-70)*
80-95
60-75
80-95
50-65
Chemical
Addition
no
yes
no
yes
no
New data indicate performance is in this range.
sludge. In this particular case the maximum percent solids recovery was well below 50
percent when the feed rate was 10 gpm and no flocculant was employed. However, the
deeper pond depth with a feed rate of 10 gpm and chemical addition raised the solids
recovery to 90 percent. The chemical cost for this improvement was about $5.50 per ton.
Figure 7-28 shows the effect of centrifuge operation at various capture levels on cake
solids concentration for the same case. At about 90 percent solids recovery, the cake
solids varied from 17 to 18 percent for the particular sludge and polyelectrolyte used.
Data by Albertson and Sherwood [34] on digested primary sludges indicate that 96
percent solids recovery and a cake solids concentration of 28 percent are achievable,
while at 82 percent solids recovery, a 36 percent cake solids concentration can be
achieved. Townsend [35] concluded in his work that raw primary sludges dewater to 30
to 40 percent solids with the assistance of 1.5 to 2.5 Ib/ton polymer, while digested
primary dewaters to 20 to 30 percent solids with 3 to 6 Ib/ton of polymer. Heat treated
sludges have been dewatered to 35 to 45 percent solids and no polymer was required for
85 percent capture. Recoveries of 92 to 99 percent of the solids from heat treated
primary sludges have been reported by Albertson [36] with polymer costs of $1.85 to
$5.35 per ton of dry solids. Dewatering of heat treated mixtures of activated and raw
primary sludge has produced cake solids concentrations of 40 percent with a 95 percent
solids recovery. No chemicals were required. However, the use of $4.35 per ton of
polymers in this latter case enabled a 50 percent increase in centrifuge capacity while
producing cake solids of 28 percent.
7-53
-------
SHALLOW POND DEPTH
100 -
50 -
CL
Q£
O
CURVE 1: NO POLYELECTROLYTE.
CURVE 2: 7.7 LBS/T DOSAGE.
CURVE 3: 10.2 LBS/T DOSAGE.
CURVE 4: 12.8 LBS/T DOSAGE.
1
99.99
99 90 50 10
% RECOVERY
05
100,-
50-
<
O
ioh
CURVE V. NO POLYELECTROLYTE.
CURVE 2: 2-3 LBS/T DOSAGE.
CURVE 3: 3-4 LBS/T DOSAGE.
CURVE 4: 5-6 LBS/T DOSAGE.
1
99.99
J I
99 90 50 10
% RECOVERY
0.5
DEEPER POND DEPTH
FIGURE 7-27. Effect of polyelectrolyte dosage and pool depth on percent solids
recovery at various feed rates.
Courtesy Sharpless Division ofPennwalt
7-54
-------
CO
LLJ
24
23
21
U
CO 19
Q
liJ 18
16
MECHANICAL CONDITIONS:
2100 xG
NO. 4 POND
10RPMCONV. DIFF.
LEGEND
x x NO POLYELECTROLYTE
• • POLYELECTROLYTE ADDITION
99.99 99 90 50 10
% RECOVERY
0.5
Courtesy Sharpless Division ofPennwalt
FIGURE 7-28. Cake dryness as a function of solids recovery.
7-55
-------
Table 7-7 presents data by Young, Matsch, and Wilcox [37] on the dewatering of air and
oxygen EAS by solid bowl and basket centrifuges. These sludges are from systems
treating raw wastewater or primary effluents. With both the scroll type and the basket
type centrifuges, a slightly higher cake concentration may be achieved and less chemical
conditioning may be required with oxygen excess activated sludge. Table 7-7 also
illustrates that basket centrifuges are generally capable of providing drier activated sludge
cakes than are the solid bowl machines because there is minimum disturbance of the
depositing solids. Kyte [38] found that the basket centrifuge could provide a cake of 13
to 14.6 percent solids from a feed of 1 percent solids and capture 94.5 to 97.4 percent of
the solids without polymer addition.
The basket centrifuges are capable of achieving a higher degree of dewatering than are
disc centrifuges. A basket centrifuge will typically dewater an EAS from an initial solids
concentration of 0.5 to 1.5 percent to a final solids concentration of 10 to 12 percent.
With no chemical addition, a solids capture of 90 percent is possible. For the same sludge,
a disc type centrifuge could only achieve a final solids concentration of 6 percent. While
it is sometimes preferable to obtain a higher activated sludge concentration than the 5 to
6 percent produced by disc centrifuges, there are situations where this level of
concentration is desirable. Excess activated sludge concentration of less than 1 percent
unnecessarily reduces the volumetric capacity of an anaerobic digester by a factor of 6 or
more. Similarly, wet oxidation or heat treatment processes, where expensive equipment is
involved, can be more economically sized when fed at concentrations of at least 5
percent. The disc centrifuge has also been used by Woodruff [31] to thicken activated
sludge prior to dewatering in a different centrifuge. By removing many of the fine
particles in the disc machine, the solid bowl centrifuge can produce a 22 percent cake. A
concentration of 13 to 19 percent was the best that could be achieved without prior
thickening by the disc machine.
Another approach to series centrifugation for organic sludge dewatering was studied in
Los Angeles [30] where objectionable amounts of floating and settleable material were
present in the centrate from existing solid bowl machines. This material was discharged
with the primary effluent to the ocean. The solid bowl machines operated without
polymer on digested primary sludge and produced a cake of about 35 percent solids with
35 to 40 percent solids recovery. The solids recovery could be increased to 95 percent by
the use of about 10 Ib/ton of cationic polymer, but the cake solids concentration was
reduced to 18 to 20 percent. However, variations in digested sludge characteristics
resulted in erratic recoveries at even this polymer dosage. An alternate mode of operation
studied was to pass the centrate from the solid bowl unit through a basket centrifuge and
not use polymers. Solids recovery did not exceed 80 percent, which was inadequate.
However, with a polymer dosage of as low as 2 Ib/ton, 96 percent solids recovery was
possible. The combined solid bowl and basket centrifuge cakes had a 25 percent solids
concentration. In other tests by Keith and Little [39] it was found that the series
approach with aerobically digested sludge and without polymer addition could achieve
the same process performance as a single stage solid bowl machine operating with
polymer doses of 7 to 13 Ib/ton.
7-56
-------
TABLE 7-7
DEWATERING OF OXYGEN ACTIVATED SLUDGES IN SOLID BOWL AND BASKET CENTRIFUGES [37]
Feed Solids
Concentration
(% by wt)
Centrifuge Type
Solid Bowl
t^ Raw wastewater
Primary effluent
Basket
Raw wastewater
Primary effluent
Oxygen Conventional
2-3
2-3
2-3
2-3
1.5-2.5
0.7-1.3
1.5-2.5
0.7-1.3
Feed
Oxygen
50-60
90-100
35-40
3545
Rate (gpm)
Conventional
45-55
55-65
20-35
3545
Solids
Capture (%)
Oxygen
85-90
90-95
92-96
92-97
Conventional
80-85
80-85
90-95
90-95
Cake
Concentration
(% solids by wt.)
Oxygen
10-13
8-10
9-12
10-14
Conventional
9-11
8-9
9-11
9-11
Polymer Addition
(Ib/ton dry
solids)
Oxygen Conventional
3
3
0
0
6-10
6-10
0
0
-------
7.3.6 Summation
The successful adaption of centrifugal devices to the dewatering of sludges that contain a
significant quantity of activated sludge is becoming common. Design improvements are
increasingly aimed at obtaining a minimum of 90 percent solids capture with little
chemical conditioning. The advent of the concurrent flow and the lower speed machines
should materially aid the successful adaption of solid bowl centrifuges. Plant designs
should be based upon scale-up of pilot tests whenever possible, and several manufacturers
have pilot-scale centrifuges available for evaluation. The manufacturers will provide
assistance in scaling up the pilot test data. In addition to dewatering functions, solid bowl,
basket, or disc centrifuges can also provide classification of organic or chemical sludges.
7.4 Drying Beds
7.4.1 Factors Affecting Design
The most widely used dewatering method in the United States is drying of the sludge on
open or covered sandbeds. Over 6,000 wastewater treatment plants use this method
according to Burd [40]. Although they are especially popular in small plants, drying beds
are also used by 38 percent of the cities serving populations of over 100,000.
Furthermore, sandbed drying is the most common technique employed in Europe.
Sandbeds possess the advantage of needing little operator attention and skill. Air drying is
normally restricted to well digested sludge, because raw sludge is odorous, attracts insects,
and does not dry satisfactorily when applied at reasonable depths. Oil and grease
discharged with raw sludge clog sandbed pores and thereby seriously retard drainage. The
design and use of drying beds are affected by many parameters. They include weather
conditions, sludge characteristics, land values and proximity of residences, and use of
sludge conditioning aids. Climatic conditions are most important. Factors such as the
amount and rate of precipitation, percentage of sunshine, air temperature, relative
humidity, and wind velocity determine the effectiveness of air drying.
The nature and moisture content of the sludge discharged to drying beds influences the
drying process. Sludges containing grit dry rapidly; those containing grease more slowly;
aged sludge dries slower than new sludge; primary sludge dries faster than secondary
sludge; and digested sludge dries faster than fresh sludge and cracks earlier. It is important
that wastewater sludge be well digested for optimum drying. In well digested sludge,
entrained gases tend to float the sludge solids and leave a layer of relatively clear liquid,
which can readily drain through the sand.
7.4.2 Design Criteria for Sandbeds
Design standards vary from region to region in the United States. This variance is partly
due to climatic differences. However, Table 7-8 presents typical criteria for the design of
open sand drying beds.
7-58
-------
TABLE 7-8
CRITERIA FOR THE DESIGN OF SANDBEDS [15]
Type of Digested Sludge
Primary
Primary and standard trickling filter
Primary and activated
Chemically precipitated
Area
(sq ft/capita)
1.0
1.6
3.0
2.0
Sludge Loading
Dry Solids
(Ib/sq.ft/yr)
27.5
22.0
15.0
22.0
Sandbeds can be enclosured by glass. Glass enclosures protect the drying sludge from rain,
control odors and insects, reduce the drying periods during cold weather, and can
improve the appearance of a waste treatment plant. Experience has shown that only 67 to
75 percent of area required for an open bed is needed for an enclosed bed. Good
ventilation is important to control humidity and optimize the evaporation rate. As
expected, evaporation occurs rapidly in warm, dry weather. Adaptation of mechanical
sludge removal equipment to enclosured beds is more difficult than to open drying beds.
Drying beds usually consist of 4 to 9 inches of sand which is placed over 8 to 18 inches of
graded gravel or stone. The sand typically has an effective size of 0.3 to 1.2 mm and a
uniformity coefficient of less than 5.0. Gravel is normally graded from 1/8 to 1.0 inches.
Drying beds have underdrains that are spaced from 8 to 20 feet apart. Underdrain piping
is often vitrified clay laid with open joints, has a minimum diameter of 4 inches, and has a
minimum slope of about 1 percent. Collected filtrate is usually returned to the treatment
plant.
The Ten State Standards [41 ] make the following design recommendations.
• The top 3 inches of gravel consist of 1/8- to 1/4-inch gravel
• The gravel extend at least 6 inches above the top of the underdrains
• The gravel layer should be 12 inches deep
• The sand layer should be at least 6 to 9 inches deep
7-59
-------
• Underdrains should be no more than 20 feet apart
• Bed walls should be watertight and extend 15 to 18 inches above and at least
6 inches below the surface
• Outer walls should be curved to prevent soil from washing onto the beds
• At least 2 beds should be provided
• Pairs of concrete truck tracks at 20-foot centers should be provided for all
beds
• The influent pipe should terminate at least 12 inches above the surface with
concrete splash plates provided at discharge points
Mechanical lifting of sludge has been practiced for many years at some large treatment
plants, but now it is receiving more attention as the need to minimize problems with
labor costs. Mechanical devices can remove sludges of 20 to 30 percent solids while cakes
of 30 to 40 percent are generally required for hand removal.
7.4.3 Results of Sandbed Drying
Typical performance data show rapid dewatering during the first one to two days while
water removal by drainage predominates. This is followed by two to five weeks of slow
dewatering principally by evaporation. In good weather, a cake of 45 percent solids may
be achieved in six weeks with a well digested sludge. As with any dewatering technique,
the performance of a sandbed may be improved by proper chemical conditioning and the
dewatering time may be reduced by 50 percent or more [40]. Liftable sludges have been
achieved in less than one day with proper chemical conditioning. Solids contents of 85 to
90 percent have been achieved on sandbeds.
7.4.4 Other Types of Drying Beds
In recent years, there has been increased interest in the use of paved drying beds with
limited drainage systems for dewatering wastewater sludges. The original reasons for
proposing paved bed design was to permit the use of mechanical equipment for cleaning
the beds and thereby reduce the cost of labor and sand replacement. The feasibility of
this approach has been demonstrated by field experience reported by South [42]. This
experience indicates that modification of drying beds to provide only limited drainage
facilities does not necessarily impare bed performance. In fact, the use of paved drying
beds with limited drainage resulted in shorter drying times as well as more economical
operation when compared with conventional sandbeds. The shorter time occurred
because the use of mechanical equipment for cleaning permits the removal of sludge with
a higher moisture content than in the case of hand cleaning. Paved beds have worked
7-60
-------
successfully with anaerobically digested sludges. Randall [43] and Randall and Koch
[44] determined that the use of paved beds of center drain design for dewatering
aerobically digested activated sludge is not as desirable as conventional sandbeds. Lateral
drainage of activated sludge on a paved bed is very poor and does not contribute
significantly to the dewatering process. The supernatant overlying the sludge cake may
drain but the reduced drainage area of the paved bed compared to the sandbed greatly
hampers the rate of drainage for aerobic sludges.
Wedge wire drying beds have been used sucessfully in England as reported by Burd [40]
and Gauntlett and Packham [45] and a cross-sectional view of one is shown in Figure
7-29 by Crockford and Sparham [46]. This approach prevents the rising of water by
capillary action through the media and the construction lends itself well to mechanical
cleaning. The first United States installations have been made at Rollinsford, New
Hampshire, and in Florida. The procedure used for dewatering sludge begins with the
placement of water or effluent to a depth of up to 1 inch over the wedge wire. This water
serves as a cushion permitting the added sludge to float without causing upward or
downward pressure across the wedge wire surface and prevents compression or other
disturbance of colloidal particles. The water is then run off by opening the control valve
and controlling the rate so as to prevent a turbid effluent from the bed. After the free
water has been drained, the bed is allowed to dry by drainage and evaporation until the
sludge can be removed. It is possible, in small plants, to place the entire dewatering bed in
a tillable unit from which sludge may be removed merely by tilting the entire unit
mechanically. Some typical performance data from the application of chemically
conditioned sludges to wedge wire units are shown in Table 7-9.
TABLE 7-9
TYPICAL PERFORMANCE DATA [46]
Feed Solids
Cake Solids
Sludge Type
Time Solids
Interval Capture (%)
Primary
Trickling Filter Humus
Digested Primary + EAS
Fresh EAS
Fresh EAS
Thickened EAS
8.5
2.9
3.0
0.7
1.1
2.5
25
8.8
10.0
6.2
9.9
8.1
14 days
20 hours
12 days
1 2 hours
8 days
41 hours
99
85
86
94
87
100
7-61
-------
o\
CONTROLLED DIFFERENTIAL HEAD IN VENT
BY RESTRICTING RATE OF DRAINAGE
VE
•MM
ENT
1
_ PARTITIO
rrr
WEDGEWIRESEPTUM
7
L
OUTLET VALVE TO CONTROL
RATE OF DRAINAGE
FIGURE 7-29. Cross section of a wedgewire drying bed [46].
-------
The Rollingford, New Hampshire, plant reports that it dewaters excess activated sludge
conditioned with polymers from two percent solids to a liftable condition in four hours.
7.5 Drying Lagoons
7.5.1 Factors Affecting Design
Lagoon drying is a low cost, simple system for sludge dewatering that has been commonly
used in the United States. Drying lagoons are similar to sandbeds in that the sludge is
periodically removed and the lagoon refilled. Lagoons have seldom been used where the
sludge is never removed, because such systems are limited in application to areas where
large quantities of cheap land are available. Sludge is stabilized to reduce odor problems
prior to dewatering in a drying lagoon. Odor problems with lagoons can be greater than
with sandbeds, because sludge in a lagoon retains more water for a longer period than
does sludge on a conventional sand drying bed.
Other factors affecting design include consideration of groundwater protection and access
control. Major design factors include climate, subsoil permeability, lagoon depth, loading
rates, and sludge characteristics. The design should provide for uniform distribution of
the sludge and for decanting of supernatant to speed the drying process.
7.5.2 Design Criteria for Drying Lagoons
Criteria cited in the Ten State Standards [41 ] are given below:
• The soil must be reasonably porous, and the bottom of the lagoon must be
at least 18 inches above the maximum groundwater table.
• Surrounding areas must be graded to prevent surface water from entering the
lagoon.
• The lagoon depth should not be more than 24 inches.
• At least two lagoons should be provided.
Underdrains have often been proposed where permeable soils are unavailable. However,
evaporation is the main mechanism for water removal from a lagoon.
Solids loading rates suggested for drying lagoons are 2.2 to 2.4 Ib/yr/cu ft of lagoon
capacity [41]. Other designers have made recommendations ranging from 1 sq ft/capita
for primary digested sludges in an arid climate to as high as 3 to 4 sq ft/capita for
activated sludge plants where the annual rainfall is 36 inches. A dike height of about 2
7-63
-------
feet with the depth of sludge after decanting of 15 inches has been used [6]. Sludge
depths of 2.5 to 4 feet may be used in warmer climates where longer drying periods are
possible [40]. Dikes should be of a shape and size to permit maintenance, mowing, and
trucks and front-end loaders to enter the lagoons for sludge removal.
7.5.3 Results of Lagoon Drying
Sludge will generally not dewater in any reasonable period of time to the point that it can
be lifted by a fork except in an extremely hot, arid climate. If sludge is placed in depths
of 15 inches or less, it may be removed with a front-end loader in 3 to 5 months. When
sludge is to be used for soil conditioning, it may be desirable to stockpile it for added
drying before use. One proposed approach [40] utilizes a 3-year cycle in which the
lagoon is loaded for 1 year, dries for 18 months, is cleaned, and allowed to rest for 6
months. Definitive data on lagoon drying are scarce. Sludge may be dewatered from 5
percent solids to 40 to 45 percent solids in 2 to 3 years using sludge depths of 2 to 4 feet
[40]. In England, dewatering beyond 30 percent solids in lagoons is rare.
7.6 Pressure Filtration
7.6.1 Concept
The plate and frame filter press is a batch device, which has been used in industry and in
European wastewater plants for many years to process difficult to dewater sludges. The
press consists of vertical plates which are held rigidly in a frame and which are pressed
together between a fixed and moving end as illustrated in Figure 7-30. On the face of
each individual plate is mounted a filter cloth as shown in Figure 7-31 [47]. The sludge is
fed into the press and passes through feed holes in the trays along the length of the press.
The water passes through the cloth, while the solids are retained and form a cake on the
surface of the cloth as explained by Brossman and Jensen [48]. Sludge feeding is stopped
when the cavities or chambers between the trays are completely filled. Drainage ports are
provided at the bottom of each press chamber. The filtrate is collected in these, taken to
the end of the press, and discharged to a common drain. At the commencement of a
processing cycle, the drainage from a large press can be in the order of 2,000 to 3,000
gallons per hour. This rate falls rapidly to about 500 gallons per hour as the cake begins
formation and when the cake completely fills the chamber, the rate is virtually nothing.
The dewatering step is completed when the filtrate is near zero. At this point the pump
feeding sludge to the press is stopped and any back pressure in the piping is released
through a bypass valve. The electrical closing gear is then operated to open the press. The
individual plates are next moved in turn over the gap between the plates and the moving
end. This allows the filter cakes to fall out. The plate moving step can be either manual or
automatic. When all the plates have been moved and the cakes released, the complete
pack of plates is then pushed back by the moving end and closed by the electrical closing
gear. The valve to the press is then opened, the sludge feed pump started, and the next
dewatering cycle commences.
7-64
-------
FIXED END
TRAVELLING END
ELECTRIC
CLOSING GEAR
/'
m
0
i-j
p
Q
Q
M
Q
Q
Q
p
Q
_j
Q
d
i
n
OPERATINC
\
_\ ll
JP
FIGURE 7-30. Side view of a filter press [47 ].
-------
FILTER CLOTHS
FIXED END
SLUDGE IN
FILTRATE DRAIN HOLES
FIGURE 7-31. Cutaway view of a filter press [47 ],
7-66
-------
Filter presses are normally installed well above floor level, so that the cakes can drop onto
conveyors or trailers positioned underneath the press. The dry cake release may be
assisted by introducing compressed air behind the filter cloth on both sides of the plate.
This causes the cloth to flex and dislodge the cake. Although such a technique removes
most of the cake, observations of operating presses indicate some manual attention may
still be required to remove all of the cake. The pressures which may be applied to a sludge
for removal of water by the filter presses now available range from 5,000 to 20,000 times
the force of gravity. In comparison, a solid bowl centrifuge provides forces of 3,500 g and
a vacuum filter, 1,000 g. As a result of these greater pressures, filter presses may offer
several advantages as explained by Thomas [49]. Advantages and disadvantages of
pressure filtration are listed in Table 7-10.
. TABLE 7-10
PRESSURE FILTRATION CONSIDERATIONS
Advantages Disadvantages
Higher cake solids concentrations (30 to 50 percent) Batch operation
Improved filtrate clarity High labor costs
Improved solids capture Filter cloth life limitations
Reduced chemical consumption Operator incompatibility
Cake delumping
The reasons for past limited acceptance in the United States municipal field have been
associated by Forster [50] with filter media, capacity, and discharge. At one time, the
only filter medium available was a type of canvas duck. Such cloth would blind very
rapidly with slimy sludges and had to be washed frequently. This manual operation added
appreciable cost to the operation. The maximum size of filter plates available were 36 " X
36 ", requiring an unusually long filter. Iivmost cases, multiple installations were required
to provide sufficient capacity for processing the quantities of sludge produced in a
wastewater treatment plant. The opening and closing as well as the discharge of the
accumulated filter cake was manual and required at least two men per filter press. This
labor cost, in addition to frequent filter cloth washing, made the use of the machine
unattractive. After World War II, filter press manufacturers reviewed the existing designs
7-67
-------
with a view to the elimination of these shortcomings. Manual closing and opening of filter
presses is now accomplished by hydraulic cylinders and pumping systems. Mechanized
plate shifting devices capable of moving a filter plate every three to four seconds has
replaced manual plate shifting and has made it possible for one operator to care for
several presses. The filter plate itself has received critical review. Leakage during operation
was objectionable and the relatively thick cotton duck filter medium, which doubled as a
gasket, was unsatisfactory. Present day filter plates are furnished with proper gaskets
installed in machined grooves and permit leak-proof operation of such filters.
An increase in the acceptance of pressure filters is due to the introduction of suitable
monofilament filter media. These materials, unlike multifilament filter cloth, do not
readily blind in service. The latter is woven from a yarn and will slightly increase in
diameter during service, because ultrafine solids are forced into its voids. The resultant
decrease in the size of the cloth openings reduces the flow of filtrate through the media.
No amount of washing will restore multifilament cloth to its original quality and
porosity. The use of monofilament media has virtually eliminated this problem and
greatly extended the filter cloth life. Many systems utilize an efficient precoat system for
the filter. Such a system allows for the deposition of a protective layer of porous material
on the filter media, prior to the start of the dewatering step. This prevents premature
blinding. In sludge dewatering, ash from an incinerator, fly ash from a fossil fuel power
plant, cement kiln dust, as well as buffing dust from a tannery, have been used
successfully as precoat materials. In addition to protecting the filter cloth from blinding,
the precoat acts as a parting plane and assures complete cake release during discharge.
However, it is necessary to wash even the monofilament media periodically. This task was
made easy and efficient by the development of mechanized plate washing devices.
Earlier filter presses had relatively small plate sizes (about 26 sq ft). Filter-plate size was
limited because the plate had to be shifted manually for cake discharge. However, with
the advent of completely mechanized plate shifting, a much larger plate can be built and
filtration capacity increased. This makes it possible to economically apply pressure filters
in large sludge dewatering projects. While pressure filters with a total effective filtration
area of 2,500 sq ft were once considered large, today units with an effective filtration
area of 4,500 sq ft are not uncommon.
Until recently, pressure filters, with few exceptions, operated at a maximum pressure
differential of 100 psi. Extensive studies during the early 1960s showed that pressure
differentials of up to 225 psi produced filter cake solids concentration well in excess of
50 percent. Some commercially available systems now operate near these pressures.
7.6.2 System Requirements
Sludge is normally conditioned by the addition of inorganic chemicals and, sometimes,
ash before pressure filtration. A dewatered cake shredder or delumper is needed when the
cake will be further processed by a process like incineration [51]. Figure 7-32 shows the
components of a typical system.
7-68
-------
•»—I
10
4
Os
\O
1 Sludge in 4
2 Mechanical Screen 5
3 Sludge Storage Tank
Chemical Storage Tank 6
Chemical Measurement 7
and Dilution Tank 8
Chemical Pumps 9
Conditioning Tank 10
Sludge Pumps 11
Filter Presses
Cakes Out
Filtrate Drain
FIGURE 7-32. Filter press system [47].
-------
7.6.3 Results of Pressure Filtration
Experience in the United States with pressure filtration of municipal sludges has been
limited. Table 7-11 summarizes experiences on the wide variety of sludges treated by this
technique in Europe. It is apparent that a 50 percent cake solids concentration can
readily be produced with any sludge. The three variables of importance shown here are
the quantity and type of chemical conditioner needed and cycle time. Of course ash is
free, and the quantities of inorganic chemicals required seem to vary greatly from sludge
to sludge. Two of the most significant United States filter press installations have recently
been put into operation. One dewaters alum sludges from the treatment of water in
Atlanta, Georgia [52,53], while the other dewaters a municipal wastewater sludge at
Cedar Rapids, Iowa [54]. Other major installations are now in the design stage. They
include facilities for dewatering tertiary lime and primary-secondary organic sludges at
the Tahoe-Truckee Sanitation Agency's 6 mgd plant in California and for dewatering
organic sludges and the rejects from a tertiary lime classifying centrifuge for a 60 mgd
Washington Suburban Sanitary Commission facility in Montgomery County, Maryland.
Extensive pilot dewatering tests were conducted at Atlanta, Georgia, with alum sludges.
The data from these tests served as the design basis for the full-scale filter press
installation. This facility can process 55,000 pounds/day dry solids. Filtration of the alum
sludge without conditioning agents was impractical. Fly ash was an excellent conditioner.
At a fly ash to alum ratio of 1:1 a cake of 60 percent solids was achieved in a 165 minute
filtration cycle time and a pressure of 225 psi. Fly ash was also used as a precoat. Lime also
provided excellent conditioning results. Quicklime addition in the amount of 12 percent
by weight of the dry sludge solids produced a cake with 39.5 percent solids in a cycle
time of 165 minutes. When hydrated lime added in the range of 10 to 15 percent of the
sludge solids, filter cakes of 26 to 31 percent resulted in 75 to 105 minutes. The use of
anionic polymers in amounts up to an equivalent cost of $3/ton of dry solids did not
produce good cakes. Pulverized clay was also tested and found to be ineffective because it
formed a thin, blinding film on the filter medium.
The initial operating results from the full-scale installation at Atlanta are consistent with
the above pilot results. Cakes of 40 to 50 percent solids have been produced using a 10
percent lime dosage and precoating. The filters are 44-chamber machines with a capacity
of 110 cu ft. They operate at a pressure of 225 psig and have a total area of 1,813 sq ft.
The design cycle is 90 minutes which provides a gross loading of 316 Ib/sq ft/cycle at an
anticipated solids of 5 percent. A diatomaceous precoat of 100 Ib/cycle is used. Typical
filtrate suspended solids of less than 10 mg/1 are reported. The March, 1969, bid price for
the filter press plant was $2,807,560.
Cedar Rapids, Iowa, carried out an extensive pilot study on dewatering of an
anaerobically digested mixed sludge from a 28.6 mgd two-stage trickling filter plant. Data
from this study were used in the design of a sludge processing system which included
conditioning, pressure filtration, and incineration. This system is now installed and uses a
7-70
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TABLE 7-11
TYPICAL FILTER PRESS PRODUCTION DATA [50]
Sludge Type
Raw Primary
Raw Primary
with less than
50% EAS
Raw Primary
with more than
50% EAS
Digested and
Digested with
less than 50%
EAS
Digested with
more than
50% EAS
EAS
Suspended Solids
(%)
5-10
3-6
1-4
6-10
2-6
Up to 5
Conditioning of
Dry Solids (%)
Ash
FeCl3
Lime
Ash
FeCl3
Lime
Ash
FeCl3
Lime
Ash
FeCl3
Lime
Ash
FeCl3
Lime
Ash
FeCl3
Lime
100
5
10
150
5
10
200
6
12
100
5
10
200
7.5
15
250
7.5
15
Cake Solids
(%)
50
45
50
45
50
45
50
45
50
45
50
45
Time Cycle
(hr)
1.5
2.0
2.0
2.5
2.0
2.5
1.5
2.0
1.5
2.5
2.0
2.5
combination of recycled ash and chemical conditioning for dewatering of 56,000 Ib of
digested sludge per day [54]. The two pressure filters at Cedar Rapids are Beloit
Passavant units and each has 83 chambers. The chambers have a volume of approximately
2.5 cu ft each. The filters each have 3,400 sq ft of filter area and are equipped with an
7-71
-------
expanded frame. This frame can hold as many as 100 chambers, which allows for further
increases in sludge volume. The filter plates are carbon steel construction and epoxy
coated, while the media is a monofilament polypropylene with a stainless steel mesh
backup screen and a carbon steel drainage screen. The filter operates at approximately
225 psig pressure, and the plates, during operation, are held closed by hydraulic cylinders
operating with an oil pressure at about 5,000 psig. The pressure filter plates are gasketed
with asbestos Teflon gasket material, and the filter media is caulked in place with cotton
caulking cord. Data reported on the Cedar Rapids mixed sludge indicate that it is very
difficult to dewater because of a high and variable industrial loading on the plant. The
ash/sludge solids ratio required for effective dewatering at Cedar Rapids is given in Figure
7-33. Thus, in order to function at an available and sustainable feed sludge solids level of
about 4 to 5.25 percent, from 1.70 to 2.25 pounds of ash per pound of organic sludge
was required. Further, the chemical requirements also varied with the sludge feed solid
content, and this is as shown in Figure 7-34. It is obvious from both Figures 7-33 and
7-34 that the most economical operation occurs with a concentrated feed sludge. When
the solids concentration decreases below about 5 percent, chemical requirements rapidly
escalate.
The pressure filter at Cedar Rapids has proven to be a very efficient solids liquid
separator. During the months of February to August, 1972, the filtrate averaged 74 mg/1
of suspended solids, while the sludge feed solids averaged 4.6 percent, and the
percent removal of suspended solids was essentially 100 percent. The filtrate has had an
average COD of 857 mg/1, a BOD of 504 mg/1, and a total phosphorus concentration of
less than 20 mg/1. Sludge cake discharges from the filter and is about 58 inches in
diameter, 1% inches thick, and weighs about 200 pounds. Each pressure filter produces
83 cakes per cycle, and these are discharged to a storage bunker. Shear cables are
positioned across the top of the bunker to break the large cakes into smaller pieces. Cake
is conveyed through a series of drag conveyors and elevators to the incinerator. At normal
operation cake moisture content is in the range of 36 to 38 percent, and the cake's
appearance is dense, dry, and textured.
Although the Cedar Rapids plant normally dewaters digested sludge, full-scale tests have
been run on raw primary sludge. The same chemical dosages were required as for digested
sludge with an ash to sludge solids ratio of 1 to 1. Filter cakes of 54 to 58 percent solids
were achieved with filtrate solids being comparable to those achieved with digested
sludge. However, the filtrate COD averaged 7,000 mg/1 and the BOD averaged 5,700 mg/1.
These values are much higher than those experienced with the dewatering of digested
sludge.
The costs at Cedar Rapids are related to the concentration of feed solids. The average
total capital and operating costs ranged from $26.83 per ton at 4.5 percent solids to
$18.20 per ton at 6.5 percent solids. A breakdown of costs at 5.5 percent solids is as
follows:
7-72
-------
,T
o
= 4.0
O
UJ
UJ
§ 3.0
«
5 «
Z
UJ
£ 1.0
UJ
0.
0
Q
\ DIGESTED SLUDGE
' \
s,
^0
°-o
—
1 1 1 1 1 1 1 1
23456789 10
ASH / SLUDGE RATIO
FIGURE 7-33. Required ash to sludge ratio as function of feed solids [54],
7-73
-------
-------
Average Dewatering Cost
Factor $/ton Dewatered
Operating
Capital
Chemical
Total 21.69
Incineration has normally been achieved without the use of supplemental fuel. The filter
has operated for 150 to 200 hours between media washings. Washing is achieved with a
high pressure (750 psi) nozzle and a cleaning solution containing commercial grade
detergent. Some operating difficulties have been experienced at Cedar Rapids, and
pressure filter plate warpage has been a major problem. Warpage occurring in the plate
diaphragm transfers bending to the plate frame, which in turn accelerates plate gasket
deterioration due to a warped gasket seating plane. Plate alignment also affects gasket life,
wherein poor alignment, or repositioning of the plates each cycle, causes a reshaping of
the plate gasket leading to premature failure.
7.6.4 Summation
Recent improvements in the degree of automation, filter media, and unit capacities have
led to renewed interest in pressure filtration for application to municipal sludges. Two
major systems are in operation and several others are in the design stage. The ability to
produce a very dry cake and clear filtrate are major points in favor of pressure filtration.
The cake removal cycle still requires manual assistance.
7.7 Other Systems
Several types of dewatering devices which do not fall into the categories previously
discussed are available and in use. These devices include:
• Moving screen concentrators
• Belt pressure filters
• Capillary dewatering systems
• Rotating gravity concentrators
7-75
-------
7.7.1 Moving Screen Concentrator
This device is marketed by Smith and Loveless Co. [55]. Thickened and polymer treated
sludge is distributed on a two-stage variable speed moving screen. Gravity dewatering
occurs on a first moving screen, and the partially dewatered sludge then moves onto a
second moving screen. It is passed through multiple compression stages for final
dewatering. The compression dewatering is accomplished by passing the solids through
sets of compression rollers with each successive set applying a higher pressure to the
solids. Figure 7-35 shows the moving screen concentrator. The concentrator system
depends on thickening of sludges by gravity using polymer addition, when straight
activated sludge is being processed. The conditioner may not be required in the
thickening step for other sludges.
The primary stage of the two-stage dewatering unit consists of an endless horizontal belt
filter screen driven by a variable speed gear motor. This filter screen is made from a
relatively open mesh, monofilament polyester material. The primary screen travels around
two end rolls, one of which serves as the drive roll. A third roll is located midway
between the other two rolls at a slightly elevated position. This flexing roll serves a dual
purpose. It flexes the sludge cake to allow additional drainage of free water and it forms a
conditioned sludge in the first section of the primary stage. The area where the sludge is
introducted onto the first screen is designed to create a pool. In the pool, adequate head
of one to two inches of water exists so that a thick cake is formed. Most free water drains
through the screen in this area. As the screen travels on over the flexing roll, additional
drainage of free water occurs. A drainage pan beneath the screen collects all the filtrate
water and empties it into a central waste line. Finally, the concentrated sludge passes
down a transition chute to the second dewatering stage. The primary screen is washed on
its return trip to the sludge introduction area, so that a clean surface is always exposed to
the incoming conditioned sludge. The open mesh filter screen facilitates complete
washing.
The secondary stage of the dewatering unit consists of another horizontal, endless filter
screen driven by a second variable speed gear motor. This stage is completely independent
of the primary unit. The secondary filter screen is identical to that of the primary unit.
This screen travels around two end rolls and between three sets of compression rollers.
Each set of compression rollers can be adjusted to apply the desired pressure to the sludge
being dewatered. The amount of pressure applied to the sludge depends upon the internal
strength of the sludge and is generally highest for primary sludges and lowest for digested
biological sludges. Also, the drier the sludge, the greater its strength and, for this reason,
more pressure is applied with each successive set of pressure rollers. The sludge loses more
free water with each compression, until it passes out from the last set of compression
rolls, where relatively little free water remains in the sludge cake. This sludge cake then
passes out of the secondary unit, down the discharge chute and into a dump truck or
other receptacle. The results obtained using a 40-inch wide screen at several different
locations and with various sludge types are summarized in Figure 7-36. Required polymer
7-76
-------
FIGURE 7-35. Moving screen concentrator system.
-------
18
-. 16
to
Q
3 14
O
UJ
u
12
o
§ 10
8
RAW
ANAEROBICALLY
DIGESTED
ACTIVATED
I I
I
I
I
100 200 300 400 500
YIELD ( LBS/HOUR )
600
FIGURE 7-36. Moving belt concentrator yield vs. cake solids.
7-78
-------
doses were found to range from 5 to 15 pounds per ton of dry solids depending on the
type and initial solids concentrations of the sludge being dewatered. Typical results for
dewatering of activated sludge follow:
Type
EAS
Feed Conditions
% Solids Ib/hr
0.7 298
Conditioner
$/ton*
6.60
Results
% Cake
Solids
8.8
% Solids
Capture
90
Based on 1970 costs data.
Total electrical costs for this unit were $0.16 per hour, and operator requirements were
estimated at one hour per day [56], This concentration of activated sludge could be
achieved at an equivalent solids loading of about 19 psf/hour on a 40-inch wide belt [55].
An 80-inch model is also available, and it can process from 400 to 800 Ib/hour of excess
activated sludge and 800 to 1,600 Ib/hour of primary sludge. Final cake solids of 20 to 30
percent have resulted with primary sludges. The capital cost of the 40-inch model is
reported to be about $30,000 and it provides a capacity of 200 to 400 Ib/hour of
activated sludge and 400 to 800 Ib/hour of primary sludge. The unit is normally supplied
as a package which includes conditioning and thickening equipment. The chemical mixing
and feed system is designed for easy operation and control. There are several installations
of this system in operation or being planned. Its relatively low capital and power costs
and adaptability to small plants are in its favor.
7.7.2 Belt Pressure Filters
Belt pressure filters are now being marketed by Passavant and Ralph B. Carter Co. The
Carter Company introduced a new belt filter press to the U. S. in 1971. This system as
shown schematically in Figure 7-37 was originally developed in Europe where it is widely
used. An endless filter belt (a) runs over a drive and guide roller at each end (b,c) like a
conveyor belt. The upper side of the filter belt is supported by several rollers (d). Above
the filter bed a press belt (e) runs in the same direction and at the same speed, whose
drive roller (f) is coupled with the drive roller (b) of the filter belt. The press belt can be
pressed on the filter belt by means of a pressure roller system whose rollers (g) can be
individually adjusted horizontally and vertically. The sludge which is to be dewatered is
fed on the upper face of the filter belt and is continuously dewatered between the filter
and press belts. After having passed the pressure zone, further dewatering in a reasonable
time cannot be achieved by only applying static pressures. However, a superimposition of
shear forces can effect this further dewatering. The supporting rollers of the filter belt
and the pressure rollers of the pressure belt are adjusted in such a way that the belts and
the sludge between them describe an S-shaped curve. Thus, there is a parallel
7-79
-------
DRAINING ZONE PRESS ZONE SHEAR ZONE
FIGURE 7-37. Schematic construction of the belt filter press.
7-80
-------
displacement of the belts relative to each other due to the difference in radius. After
further dewatering in the shear zone, the sludge is removed by a scraper (i).
As can be seen, the belt filter press has three processing zones along the length of the
unit. They are the initial draining zone, which is analogous to the action of a drying bed;
the press zone, which involves application of pressure, and a shear zone in which shear is
applied to the partially dewatered cake. The unit has recently been modified to a
two-stage unit as depicted in the photograph in Figure 7-38 where the initial draining
zone is on the top level followed by an additional section wherein pressing and shearing
occur. A significant feature of the belt filter press is that it employs a coarse, mesh,
relatively open weave, metal medium fabric. This is feasible because of the rapid and
complete cake formation obtainable when proper flocculation is achieved.
The belt filter press, as well as the other systems described in this section, attempt to
overcome the sludge pick up problem occasionally experienced with rotary vacuum
filters. The belt filter press supplied by Carter is supplied in system form as shown in
Figure 7-39. This system includes auxiliaries such as polymer solution preparation
equipment, and automatic process controls. The Carter belt press has the capability of
dewatering a digested combined primary and secondary sludge with an initial solids of 5.7
percent to a final cake solids concentration of 19 percent. It can do this at a rate of 6.7
Ib/sq ft/hr and a chemical conditioning cost of $4.10/ton. Table 7-12 summarizes
European installation as of 1971. This unit is currently being installed in several United
States locations.
TABLE 7-12
EUROPEAN INSTALLATIONS OF THE BELT FILTER PRESSES
Type Sludge
Primary/Biological
Primary/Biological/Chemical
Biological/Chemical
Industrial
Number of Plants
38
4
12
13
Population Equivalent
710,000
132,000
155,000
The Passavant belt filter press is somewhat similar to the Carter belt filter press, but
includes the added feature of vacuum boxes in the free drainage zone. About 6 inches of
7-81
-------
oo
to
FIGURE 7-38. Belt filter press.
-------
00
FIGURE 7-39. Belt filter press system.
-------
vacuum (Hg) are applied to obtain higher cake solids. Figure 7-40 shows the initial test
unit at Birmingham, Alabama. Passavant reports cake solids concentrations averaging 25
to 30 percent from the dewatering of mixed sludges.
7.7.3 Capillary Dewatering Systems
The Squeegee or capillary suction device is a unit offered by the Infilco Division of
Degremont and is a new type of horizontal belt filter [57]. Figure 7-41 shows a drawing
of a unit. The Squeegee incorporates a self-contained chemical feed system consisting of a
variable rate diaphragm metering pump, mixer, and storage tank containing chemical
coagulant. The sludge is metered by a variable rate sludge pump and flows to the unit
prior to entrance into the influent box. The sludge is then distributed over the screen
longitudinally through a series of openings which create a uniform level. This portion of
the operation releases free water and increases the solios concentration level by
approximately 25 percent. The free water released escapes through the screen and is
collected in a trough for discharge. After the initial free water release, the screen carrier
comes in contact with the capillary belt. The unusual feature of the Squeegee device is
this capillary dewatering zone wherein the motive force for dewatering comes from the
capillary action of the capillary belt. This belt is separated from the sludge by the carrier
screen. The capillary dewatering zone is shown in Figure 7-42. Along the longitudinal
path of the belt are a series of stations which extract the liquid from the saturated belt.
One station is shown in Figure 7-43. The number and location of these stations can be
varied on-site to optimize results depending on sludge characteristics. In these stations,
the belt is momentarily separated from the screen and sludge, while the filtrate is removed
from the belt. After the optimum amount of liquid has been extracted by the belt, the
screen carrier and belt are again finally separated. The carrier and sludge continue along
the longitudinal plane where a final compression zone extracts additional liquid for a final
dehydration as shown in Figure 7-44. The sludge cake is then removed by a doctor blade
in contact with the final compression roller and the cake drops onto a discharge chute.
Before the carrier completes the cycle it passes through a high pressure washing station
and returns for the next cycle. A belt washing station is also provided, which is controlled
by a timer and solenoid valve that washes the belt at predetermined intervals.
Considerable data have been obtained on a pilot plant scale at the Long Road treatment
plant near Pittsburg, Pennsylvania. A summary of this data appears in Table 7-13. Feed
capacities from 2 to 4.5 lb/hr/ft2 were achieved with the cake solids at discharge ranging
from 15 to 18 percent for activated sludge. Machine capacities more than twice these
values may be possible. It was found that the device can be operated without coagulant
addition at a penalty to solids capture. With chemical addition of the polyelectrolyte at
10 Ib/ton or about $4.00/ton a cake solids capture of 95 percent can be obtained when the
machine is operated at 2.0 lb/hr/ft2. Operating at higher machine capacities, i.e., 4.0
lb/hr/ft2 and higher, it appears that the ferric chloride conditioner yields more economical
system operation. Overall machine operation was found to depend on chemical addition,
7-84
-------
oo
C/l
FIGURE 7-40. Passavant belt filter press.
-------
sO
00
-------
-------
CARRIER SCREEN
CAPILLARY BELT
(SATURATED]
oo
00
BELT
DEWATERING
ROLLS
SLUDGE
FIGURE 7-43. Belt dewatering zone.
-------
SLUDGE
00
CAPILLARY
BELT
FINAL
COMPRESSION ROLL
SLUDGE CAKE
TO DISCHARGE
DOCTOR
BLADE
SLUDGE CAKE
CARRIER
SCREEN
FIGURE 7-44. Final compression zone.
-------
sludge solids loading, and screen mesh size. Total cost estimates for capillary dewatering
range from $19.36 to $39.67 per ton of dry sludge solids processed in plants of the two
to fourmgd size [58].
TABLE 7-13
SUMMARY OF PILOT PLANT
CAPILLARY DEWATERING SYSTEM PERFORMANCE
Sludge Type
Machine
Capacity
(lb/hr/ft2)
Coagulant
Cost
($/ton)*
Final Cake
Solids
(%)
Solids
Recovery
(%)
Cationic Poly electrolyte
EAS
EAS
EAS
EAS
2.0
2.0
3.0
4.3
0
4.00
Ferric Chloride
0
6.40
16-19
17-19
16
15
60-75
95
50-65
91
Cationic Polyelectrolyte
EAS
Digested
Mixed Primary
4.5
5.2
5.4
4.00
Ferric Chloride
10.00
10.00
14-15
16-18
14-15
80
88
85
Based on 1973 cost data.
Currently there is one plant scale unit in operation; and this is at Coral Springs, Florida,
where an aerobically digested sludge is being processed. Reported results are similar to
those obtained with the pilot unit an excess activated sludge. Another unit is scheduled to
be installed at St. Charles, Illinois, in early 1975.
7-90
-------
7.7.4 Rotating Gravity Concentration
This concept is employed in a unit marketed by the Permutit Co. under the trade name of
"DCG sludge dewatering unit" and it is shown in Figure 7-45. At one time, Nichols
Engineering offered a similar unit under the name "Roto-Plug" but the patent rights have
since been sold to Permutit. The unit consists of two independent cells. The cells are
formed by a fine mesh nylon filter cloth which travels continuously over front and rear
guide wheels. The filter cloth is rotated by a drive roll and sprocket assembly, which also
serves as the separator between the two cells. Dewatering occurs in the first cell, cake
formation takes place in the second'cell. Sludge is introduced in the dewatering cell,
where initial liquid solids separation takes place. In this section liquid drains through the
nylon filter cloth at a relatively high rate. The partially dewatered solids are then carried
over the drive roll separator into the second cell or cake formation zone. Here they are
continuously rolled and form into a cake of relatively low moisture content. The weight
of this sludge cake presses additional water from the partially dewatered sludge. When the
cake of dewatered solids grows to a certain size, excess quantities are discharged over the
rim of the second cell to a conveyor belt which moves the material to a disposal point.
Operation is continuous. Dewatering is accomplished entirely by gravity and without the
application of either pressure or vacuum. Blinding tendencies are minimized, even on
difficult-to-dewater sludges, because of the low differential pressure (1 inch or 2 inches)
applied, and the tendency of sludge to adhere to itself and separate cleanly from the
cloth.
The DCG unit is a concentrating device and, when more complete dewatering is required,
a multiroll press is provided as illustrated in Figure 7-46. The MRP dewatering press
consists of dual endless belts. Sludge cake, concentrated by the DCG, is fed by rotating
blades to the space between the belts and graduated pressure is applied by the rollers to
squeeze additional moisture through the cloth into the grooved support belt and thence
to the drip pan. The dewatered cake is carried by the bottom cloth to the discharge point.
The general capabilities of the DCG and the MRP units are shown in Table 7-14 [59].
The dewatering belts in the MRP unit are reported to have a life of about 6 months. The
first United States installation was made in 1962 at Caldwell, New Jersey and by 1969,
17 installations had been made [60]. Most of the installations were on aerobically
digested sludges. Production of a 25 percent cake from a 6 percent feed of raw primary
sludge without conditioning has been reported in one case [61]. The initial plant at
Caldwell, New Jersey reports polymer costs of $8 to $ 10 per ton when operating on an
anaerobically digested primary plus trickling filter humus sludge of 4 to 5 percent solids
concentration and producing 15 percent cake solids [62,59].
7-91
-------
DRIVE ROLL AND
SPROCKET ASSEMBLY
CONVEYOR
DEWATERING
CELL
SLUDGE
"INLET
FIGURE 7-45. Rotating gravity concentrator.
-------
CAKE DISCHARGE
SLUDGE INLET
EFFLUENT
FIGURE 7-46. Schematic of MRP section.
7-93
-------
TABLE 7-14
CAPABILITIES OF THE MRP AND DCG UNITS
Type Sludge
Raw Primary
Anaerobic Digested
Aerobic Digested
Excess Activated
Oxygen Activated
Influent
3
4
1.5-3.0
1.9-3.0
3
% Dry Solids
DCG
12-17
12-15
10-12
9.4-13
10
MRP
20-23
18-20
16-20
18-23
16
7.8 References
1. Bennett, E. R. and Rein, D. A., "Vacuum Filtration—Media and Conditioning
Effects," Dept. of Civil and Environmental Engineering, University of
Colorado, Boulder, Colorado.
2. Bennett, E. R., Rein, D. A., and Linstedt, K. D., "Economic Aspects of Sludge
Dewatering and Disposal."/. Environ. Eng. Div., 99, 55 (1973).
3. Gale, R. S., "The Calculation of Theoretical Yields of Rotary Vacuum Filters."
Water Pollut. Contr. (1971), p. 114.
4. Gale, R. S., "Studies on the Vacuum Filtration of Sewage Sludges." Water
Pollut. Contr. (1970), p. 514.
5. Gale, R. S., "Filtration Theory with Special Reference to Sewage Sludges."
Water Pollut. Contr. (1967), p. 622.
6. "Sludge Dewatering." WPCF Manual of Practice No. 20 (1969).
7. Ruth, B. F., Motillon, G. H., and Montonna, R. H., "Studies in Filtration." Ind.
Eng. Chem., 25, 16(1933).
7-94
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8. Carman, P. C., "A Study of the Mechanism of Filtration." Trans. Inst. Chem.
Engrs., 16, 168(1938).
9. Coakley, P. and Jones, B. R. S., "Vacuum Sludge Filtration." Sewage Ind.
Wastes, 28,963(1956).
10. Jones, B. R. S., "Vacuum Sludge Filtration." Sewage Ind. Wastes, 28, 1103
(1956).
11. Hatscheck, E., "The Mechanism of Filtration."/. Soc. Chem. Ind., 27, 538
(1908).
12. Halff, A. H., "An Investigation of the Rotary Vacuum Filter Cycle as Applied
to Sewage Sludges." Sewage Ind. Wastes, 24, 962 (1952).
13. Grace, H. P., "Resistance and Compressibility of Filter Cakes." Chem. Engr.
Prog., 49,303,367(1953).
14. Crook, M. D. and Jones, W. D., "Assessing the Filterability Characteristics of
Industrial Sludges Using an Expanded Form of Carman Equation." Brit. Chem.
Eng., 13(1),94(1968).
15. Eckenfelder, W. W., Jr. and O'Connor, D. J., Biological Waste Treatment.
Pergamon Press: New York (1961).
16. McCarty, P. L., "Sludge Concentration-Needs, Accomplishments, and Future
Goals."/. Water Pollut. Contr. Fed., 38 (4), 493-507 (1966).
17. Leary, R. D., "Top Feed Filtration of Activated Sludge-A Comparison with
Conventional Feed." Presented at the 45th WPCF Conference, Atlanta,
Georgia, 1972.
18. Purchas, D. B., "Filtration in the Chemical and Process Industries-1."
Filtration (1964), p. 256.
19. Shedden, W. C., "The Selection and Care of Sewage Sludge Filter Cloths."
Water Sewage Works, 111, 169, 211 (1964).
20. Simpson, G. D. and Sutton, S. H., "Performance of Vacuum Filters." Presented
at Inservice Training Course on Sludge Concentration, Filtration and
Incineration, University of Michigan, Continued Education Series 113 (1963).
21. Trubnick, E. H. and Mueller, P. K., "Sludge Dewatering Practice." Sewage Ind.
Wastes, 30, 1364(1958).
7-95
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22. Schepman, B. A. and Cornell, C. F., "Fundamental Operating Variables in
Sewage Sludge Filtration." Sewage Ind. Wastes, 28, 1443 (1956).
23. "Estimating Costs and Manpower Requirements for Conventional Wastewater
Treatment Facilities." EPA Report, Contract 14-12-462 (Oct. 1971).
24. Simpson, G. D. and Sutton, S. H., "Performance in Vacuum Filters in
Sludge: Concentration, Filtration and Incineration." University of Michigan,
School of Public Health (1964).
25. Perry, R. H., Chemical Engineers Handbook (4th ed.). McGraw-Hill Inc.: New
York (1963), p. 92.
26. White, W. F., "Fifteen Years of Experience Dewatering Municipal Wastes with
Continuous Centrifuges." Presented at AICHE Meeting, New York, N.Y., Nov.
1972.
27. White, F. F., "A Centrifugal for Industrial Wastes." Chem. Eng. Prog., 65 (6),
74(1969).
28. Albertson, O. and Guidi, E., "Centrifugation of Waste Sludges." /. Water
Pollut. Contr. Fed., 41 (4) 607 (1969).
29. Vesilind, P. A., "Scale-Up of Solid Bowl Centrifuge Performance."/. Environ.
Eng. Div., 100 (EE2), 479 (1974).
30. Parkhurst, J. D., Rodrigue, R. F., Miele, R. P., and Hayaski, S. T., "Summary
Report: Pilot Plant Studies on Dewatering Primary Digested Sludges." EPA
Report 670/2-73-043 (Aug. 1973).
31. Woodruff, P. H., Mimes, T. R., and Walters, J. H., "Dewatering Activated
Sludges by Two Stage Centrifugation." Water Sewage Works, 114, 429 (1967).
32. "Advanced Wastewater Treatment as Practiced at South Tahoe." EPA Report
17010 ELQ 08/71 (1971).
33. "Sludge Processing for Combined Physical-Chemical-Biological Sludges." EPA
Report EPA-R2-73-250 (Jul. 1973).
34. Albertson, O. E. and Sherwood, R. J., "Centrifuge for Dewatering Sludges."
Water Wastes Eng., 5 (4), 56 (1968).
35. Townsend, J. R., "What the Wastewater Plant Engineer Should Know About
Centrifuges." Water Wastes Eng., 66 (11), 42 (1969) and 6(12), 35 (1969).
7-96
-------
36. Albertson, O. E., "Dewatering of Heat Treated Sludges." Presented at the 42nd
WPCF Conference, Dallas, Texas, Oct. 1969.
37. Young, K. W., Matsch, L. C., and Wilcox, E. A., "Sludge Considerations of
Oxygen Activated Sludge." Presented at the University of Texas Water
Resources Symposia, Nov. 1972.
38. Kyte, K. B., "Sewage and Waste Disposal Processes." Technical Buretin
721-KBK-5, published by Pennwalt-Sharples (Mar. 1, 1971).
39. Keith, F. W. and Little, T. H., "Centrifuges in Water and Waste Treatment."
Chem. Eng. Prog., 65 (11), 77 (1969).
40. Burd, R. S., "A Study of Sludge Handling and Disposal." Federal Water
Pollution Control Administration Publication WP-20-4 (1968).
41. "Recommended Standards for Sewage Works." A Report of Committee of the
Great Lakes—Upper Mississippi River Board of State Sanitary Engineers (1968
Edition), Health Education Service, Albany, New York.
42. South, W. T., "Asphalt Paved Beds in Salt Lake City." Water Sewage Works,
105,347(1958).
43. Randall, C. W., "Are Paved Drying Beds Effective for Dewatering Digested
Sludge?" Water Sewage Works, 116, 373 (1969).
44. Randall, C. W. and Koch, C. T., "Dewatering Characteristics of Aerobically
Digested Sludge."/. WaterPollut. Contr. Fed., 4, R215 (1969).
45. Gauntlett, R. B. and Packham, R. F. "The Dewatering of a Clarification Sludge
on Drying Beds" (Abstract). Pub. Works, 102 (12), 90 (1971).
46. Crockford, J. B. and Sparham, V. R., "Developments to Upgrade Settlement
Tank Performance, Screening, and Sludge Dewatering Associated with
Industrial Wastewater Treatment." Presented at the 27th Purdue Industrial
Waste Conference, May 1972.
47. "An Introduction to Filter Presses for Effluent and Sewage Treatment."
Nichols Engineering and Research Corp. (1971).
48. Brossman, D. E. and Jensen, J. R., "The Filter Press." Ind. Water Eng., 8 (5),
18(1971).
49. Thomas, C. M. "The Use of Filter Presses for Dewatering of Sewage and Waste
Treatment Sludges." Presented at the 42nd WPCF Conference, Dallas, Texas,
Oct. 1969.
7-97
-------
50. Forster, H. W., "Sludge Dewatering by Pressure Filtration." Presented at the
American Institute of Chemical Engineers Meeting, New York, Nov. 27, 1972.
51. Tench, H. B., Phillips, L. F., and Swanwich, K. H., "The Sheffield Sludge
Incineration Plant." Water Pollut. Contr. (1972), pp. 176-185.
52. "Test Results on Basin Sediment from the Chattahoochee Water Treatment
Plant, City of Atlanta, Georgia." Beloit Passavant Corp. Report.
53. Weir, P., "Research Activities by Water Utilities-Atlanta Water Department."
J.AWWA, 64,634(1972).
54. Gerlich, J. W. and Rockwell, M. D., "Pressure Filtration of Waste Water Sludge
with Ash Filter Aid." EPA-R2-73-231 (Jun. 1973).
55. "Smith and Loveless Sludge Concentrator." Bulletin No. 7010C. Published by
Smith and Loveless Company.
56. Goodman, B. L. and Higgins, R. B., "A New Device for Wastewater Treatment
Sludge Concentration." Water Wastes Eng., 1 (8), 30 (1970).
57. "Squeegee Capillary Sludge Dewatering Unit." Infilco Bulletin 81-840 (1973).
58. Lippert, T. E. and Skriloa, M. C., "Evaluation and Demonstration of the
Capillary Suction Sludge Dewatering Device." EPA-670/2-74-017 (Mar. 1974).
59. Kneale, J. S., "Sludge Dewaterer Solves Space Problems." Pub. Works, 98 (3),
123(1967).
60. Permutit DCG Technical Data (Sep. 1, 1969).
61. Chewning, A. J., "Sludge Dewatering Problem Solved." Water Sewage Works,
109,242(1962).
62. Valente, G. A., "A New Idea in Sludge Dewatering." The American City, 80
(7), 95 (1965).
7-98
-------
CHAPTER 8
SLUDGE REDUCTION
8.1 Methods, Functions, and Occurrences
Sludge reduction processes are generally thermal ones and provide a major reduction in
the sludge solids. Although heat drying is not truly a reduction process, it occupies the
same relative position in the sequence of sludge processing as major reduction processes.
It is included in this chapter for purposes of simplification. Established and experimental
sludge reduction processes are listed and categorized in Table 8-1.
TABLE 8-1
REDUCTION PROCESSES
Reduction Process
Pretreatment Required
Additional
Processing Requirements
Established Processes
Incineration
Wet Air Oxidation
Heat Drying
Experimental Processes
Pyrolysis
Incineration/Power
or steam generation
Thickening and Dewatering
Thickening
Thickening and Dewatering
Thickening
Thickening and Dewatering
Landfill ash
Treat cooking liquor,
Landfill ash
Use dried sludge as soil
conditioner
Utilize by-products of gas,
carbon, steam. Dispose of
residue
Landfill ash
8- 1
-------
Where sufficient land was available for proper disposal of liquid, or more frequently,
dewatered sludge cake, past practice at wastewater treatment plants has been to omit
incineration or other reduction processes due to the cost. With decreasing land availability
and the possibility of more stringent standards for land disposal, reduction processes have
been receiving revived attention. This is also true of municipal solid waste disposal
practices. The concurrent increase in the price of energy has sparked wide interest in the
combined incineration of solid waste residues and wastewater sludges in reduction
systems incorporating energy recovery as a prime feature.
8.2 Incineration
The basic elements of sludge incineration are shown schematically in Figure 8-1. An
incinerator is usually part of a sludge treatment system which includes sludge thickening,
a macerating or disentegrating system, a dewatering device (such as a vacuum filter,
centrifuge, or filter press), an incinerator feed system, air pollution control devices, ash
handling facilities, and the related automatic controls. Important considerations in
evaluating incineration methods include the composition of the sludge feed and the
amount of auxiliary fuel required. Air pollution constraints and resultant equipment and
treatment requirements as well as ash disposal are also important.
8.2.1 Composition of Sludge Feed
A primary consideration in the cost-effectiveness of sludge incineration is the effect of
sludge feed composition on auxiliary fuel requirements. Other variables of importance are
the type of incinerator employed, excess air requirements, operating temperatures
necessary for odor control and other air pollution constraints. Heat yield from a given
sludge is a function of the relative amounts and elemental composition of the contained
combustible elements.
Wastewater sludges have been incorrectly classified as low grade fuels. While the heating
value of the dried sludge solids may be substantial, sludges cannot be considered as fuel
because of the high moisture content of the raw sludge. Sludges contain significant
amounts of the three principal combustible elements: carbon, hydrogen, and sulfur. It is
possible, using the Dulong formula [ 1 ], to estimate calorific value of sludges from
knowledge of the amounts of these three elements in a given sludge. However, experience
has shown that results so calculated are very often inaccurate, so that the only reliable
method of determining heat value is to carry out calorimeter tests. Inert and moisture
contents of the feed sludge also affect the heat requirements for complete combustion. In
the past, the feed sludge's dry solids content has been overemphasized, while insufficient
regard has been shown for the effect of the sludge's chemical composition and inert
content on auxiliary fuel requirements. The anaerobic digestion and heat treatment
processes reduce the volatile content and increase the inert noncombustible content with
the resultant lower fuel value for a sludge. Physical chemical sludges also have low heat
i-2
-------
AIR
oo
AUXILIARY
FUEL
SLUDGE
FEED
COMBUSTIBLE
ELEMENTS
INERTS
MOISTURE
*
t
INCINERATOR
STACK GASES
MOISTURE
EXCESS AIR
PARTICULATES
OTHER PRODUCTS OF
COMBUSTION
ASH
FIGURE 8-1. Sludge incineration.
-------
values due to inert content. Table 8-2 gives the heat value of various sludge types. Table
8-3 gives some representative heating values of various sludge constituents. Pretreatment
methods such as chemical conditioning and dewatering do result in substantial reduction
of incineration fuel requirements, but frequently they do so by creating increased energy
demands on other unit processes.
TABLE 8-2
EFFECTS OF PRIOR PROCESSES ON FUEL VALUE [ 1 ]
Calorific Value
Type Sludge (BTU/lb of dry solids)
Raw Primary 9,500
Anaerobically Digested Primary 5,500
Raw (Chem. Precip.) Primary 7,010
TABLE 8-3
REPRESENTATIVE HEATING VALUES OF SOME SLUDGE MATERIALS [1]
Material
Grease and Scum
Raw Wastewater Solids
Fine Screenings
Ground Garbage
Digested Sludge
Chemical Precipitated Solids
Grit
Combustibles
(%)
88
74
86
85
60
57
33
Heating Value
(BTU/lb of dry solids)
16,700
10,300
9,000
8,200
5,300
7,500
4,000
8-4
-------
8.2.2 The Incineration Process
Incineration is a two-step process involving drying and combustion. In addition to fuel
and air; time, temperature, and turbulence are necessary for a complete reaction. The
drying step should not be confused with preliminary dewatering; this dewatering is
usually by mechanical means and precedes the incineration process in most systems. A
sludge with a moisture content of about 75 percent is delivered to the incinerators. Since
a typical sludge might contain 3 pounds of water for each pound of dry solids and have a
volatility of 75 percent, the heat required to evaporate the water nearly balances the
available heat from combustion of the dry solids.
Drying and combustion may be done in separate units or successively in the same unit.
Manufacturers have developed diversified types of equipment. The two major incineration
systems employed in the United States are the multiple hearth furnace and the fluidized
bed incinerator. The drying and combustion process consists of the following phases: (a)
raising the temperature of the feed sludge to 212° F, (b) evaporating water from the
sludge, (c) increasing the water vapor and air temperature of the gas, and (d) increasing
the temperature of the dried sludge volatiles to the ignition point. Practical operation of
an incinerator requires that air in excess of theoretical requirements be supplied for
complete combustion of the fuel. The introduction of excess air has the effect of
reducing the burning temperature and increasing the heat losses from the furnace.
Heat is emitted by the burning of sludge in a furnace. Some of this heat is absorbed by
the furnace and lost by radiation. A large portion of the emitted heat is lost with the
stack gases, while a small portion is lost with the ash. The difference between the heat
generated and the heat lost is available for heating the incoming sludge and air.
Self-sustained combustion is often possible with dewatered raw sludges once the burning
of auxiliary fuel raises incinerator temperatures to the ignition point.
The combustion process requires, in addition to fuel and air the proper amount of time,
temperature, and turbulence for a complete reaction [2]. The reciprocal relationship
between time and temperature which is noted in many chemical processes is also of
importance in combustion. A surprisingly large part of the heat produced in the
combustion reaction (about 50 percent) must be delivered to the entering fuel and air to
keep the reaction going. This is less surprising when it is realized that the fuel and air
must be heated to a relatively high ignition temperature to maintain the process. An
important feature of burner and furnace design is the optimum combining of the required
ignition energy (heat) and the fuel and air. In the design of burners for gaseous or liquid
fuels, turbulence plays an important part in the achievement of ignition temperature. In
the design of equipment currently used for incineration, turbulence is a less important
factor and heating of the sludge to ignition temperature is usually achieved by radiation
from the hot refractory surfaces. There are a number of variables which influence the
amount of fuel required and the resulting cost for sludge incineration. Principal variables
are the moisture and volatile solids content of the sludge, and their effect on the sludge's
8-5
-------
heat content is shown in Figure 8-2. The importance of obtaining the maximum solids
concentration is illustrated by Figure 8-3. For example, at 25 percent total solids there is
only enough heat available in this particular sludge to raise the combustion products and
moisture to 900° F and this temperature is far below the accepted 1350 to 1400° F
necessary for deodorizing the stack gases of a conventional combustion unit.
Excess air added to the combustion chamber increases the opportunity of contact
between fuel and oxygen and this is essential for combustion to proceed. To insure
complete thermal oxidation, it has been necessary to maintain 50 to 100 percent excess
air over the stoichiometric amount of air required in the combustion zone. This excess air
is undesirable because it quenches the reaction temperature by pirating 12 to 24 percent
of the input BTU's for heating of the excess air. If excess air is not supplied for this
reason, it is difficult to maintain the minimum deodorizing temperature. Therefore, a
closely controlled minimum excess air flow is desirable for maximum thermal economy.
The amount of excess air required varies with the type of incineration equipment, the
nature of the sludge to be incinerated, and the disposition of the stack gases. The impact
of use of excess air on the cost of fuel in sludge incineration is shown in Figure 8-4. When
the amount of excess air is inadequate, only partial combustion of carbon occurs, and
carbon monoxide, soot, and odorous hydrocarbons are formed. Further, the heat
recovered from the partial burning of the carbon is substantially reduced, since the heat
value of carbon monoxide is only 4,400 BTU/lb.
Preheating of air is an important step in improving the thermal economy. Air preheat
affords an increase in capacity for a given size reactor since the combustion gas volume is
used more effectively and auxiliary fuel requirements are reduced. However, it should be
noted that the preheat exchanger has a significant capital cost and should only be
recommended after a complete economic evaluation of the process.
8.2.3 Analysis of Incineration Processes
An effective incineration system analysis should include material and heat balances. Cost
estimates should consider the potential incineration system's effects on other unit
processes. A typical material balance around a fluid bed incinerator is shown in Figure
8-5. Since reasonably accurate determination of the sludge input analysis is essential, the
need for sludge process evaluation work and sampling is apparent. Long-term wastewater
treatment objectives are essential for the proper sizing of an incinerator [5].
Owen [ 1 ] has presented a detailed example of a heat balance for a multiple hearth
furnace incinerating a fresh primary sludge with a volatile content of 70 percent from the
Ashland, Ohio,wastewater treatment plant. The heat balance comprised the following
quantities:
-6
-------
0 1,600
01
LU
1,400
UJ
o
1,200
1,000
ID
O
Q-
V)
O
O
CO
O
<
cc.
ID
800
600
400
200
o
Sludge heat content - 10,000 Btu/lb
volatile solids
do*
75 76 77 78 79 80 81 82 83
MOISTURE CONTENT OF FEED (%)
FIGURE 8-2. The effects of sludge moisture and volatile solids content on gas consumption.
8-7
-------
HEAT RECOVERY
WITH PREHEAT OF
COMBUSTION AIR
800 900 1000 1100 1200 1300
TEMPERATURE (°F)
1400
1500
1600
FIGURE 8-3. Equalibrium curves relating combustion temperatures to cake concentration [3],
8-8
-------
4 —
° £ 3,
Z *
«=- u_
J m 2^
01 o
1-
<
cc
$3.70/TON
SLUDGE @ 30% TS, 70% VOL & 10,000 BTU/LB
WITH GAS EXIT TEMPERATURE @ 1500°F
S0.92/TON
20
40
60
80
100
% EXCESS AIR FOR SLUDGE
EXCESS AIR FOR NATURAL GAS @ 20% (CONSTANT)
FIGURE 8-4. Impact of excess air on the cost of natural gas in sludge incineration [3].
8-9
-------
AUXILIARY FUEL
(NO. 2 OIL)
0.4246 LB.
0.3705 LB. C
0.0533 LB. H
0.0008 LB. S
AIR 23.75 LBS.
1
5.4625 LBS. 02L
[18.2875 LBS. N2J
^
LUDGE ONE DRY POUND
0.4363 LB. C ]
0.0637 LB. H
0.0024 LB. S
0.1 400 LB. ASH
0.3335 LB. O2
0.0241 LB. N
H2O 2.984 LBS. —
IN WET SLUDGE
WHICH CONTAINS
ONE DRY POUND
SLUDGE
^MO^.^^.
^^^^^^
»•
FLUIDIZED
BED
INCINER-
ATOR
(SANDS)
EXCESS AIR
20.7262 LBS.
f 2.7140 LBS. O2]
(J8.3116LBS. N2J
- ASH 0.14 LB.
•• H2O 4.027 LB.
C02 2.955 LB.
SO2 0.0064 LB.
• SAND GRANULES?
kNOx?
FIGURE 8-5. Material balance for fluidized bed sewage sludge incineration [4].
8- 10
-------
• Heat absorbed by
— Latent heat in free moisture and moisture of combustion
— Sensible heat in gases of combustion and excess air
- Ash
— Radiation
— Shaft cooling air
• Heat evolved from
— Combustibles in
• Solids
• Fuel
In a heat balance, the total heat absorbed must equal the total heat evolved.
8.2.4 Multiple Hearth Incineration
The multiple hearth furnace is the most widely used wastewater sludge incinerator in the
United States today, because it is simple, durable, and has the flexibility of burning a
wide variety of materials even with fluctuations in the feed rate. There were about 120 of
these units installed for wastewater sludge combustion, as of 1970 [3]. A typical multiple
hearth furnace is shown in Figure 8-6 and consists of a circular steel shell surrounding a
number of solid refractory hearths and a central rotating shaft to which rabble arms are
attached. Since the operating capacity of these furnaces is related to the total area of the
enclosed hearths, they are designed with diameters ranging from 54 inches to 21 ft 6
inches and from four to eleven hearths. Capacities of multiple hearth furnaces vary from
200 to 8,000 Ib/hr of dry sludge with operating temperatures of 1,700° F. The dewatered
sludge enters at the top through a flapgate and proceeds downward through the furnace
from hearth to hearth through the rotary action of the rabble arms.
The hearths are constructed of high heat duty fire brick and special fire brick shapes. The
upper or No. 1 hearth in furnaces having an even number of hearths has a central opening
or port through which sludge passes to the second hearth. The upper hearth in this case is
termed an in-feed hearth; and the second, or next lower, an out-feed hearth. The top, or
No. 1 hearth of furnaces having an odd number of hearths, is an out-feed hearth; and the
next lower, or No. 2, is an in-feed hearth. Out-feed hearths have ports or drop-holes
around the periphery of the hearth through which the sludge passes to the next lower
8- 11
-------
FLUE GASES OUT
DRYING ZONE
COMBUSTION ZONE
COOLING ZONE
COOLING AIR DISCHARGE
SLUDGE INLET
RABBLE ARM
AT EACH HEARTH
COMBUSTION
AIR RETURN
ASH DISCHARGE
COOLING AIR FAN
FIGURE 8-6. Cross section of a typical multiple hearth incinerator.
8- 12
-------
in-feed hearth. The central circular opening of the in-feed hearths are constructed to leave
a large clear opening between the edge of the hearth and the wall of the shaft. The ports
of the out-feed hearth are constructed to provide openings all around the hearth and
permit a well distributed supply of sludge to drop to the next lower hearth. These
openings tend to regulate gas velocities also.
Two doors are generally provided at each hearth. They are fitted to cast iron frames and
have machined faces to provide reasonably tight closures. An observation port with
closure is provided in each door. Since the furnace may operate at temperatures up to
2,000° F, the central shaft and rabble arms are effectively cooled by air supplied in
regulated quantity and pressure from a blower which discharges air into a housing at the
bottom of the shaft. The central shaft is an iron column cast in sections. The sections
enclosed by the furnace have a tubular inner column called the cold air tube. The annular
space between the inner tube and the outer wall of the shaft exposed to furnace heat,
serves as a passageway for hot air and is referred to as the hot air compartment. The shaft
is motor driven through means permitting adjustment of speeds of from about one-half to
one and one-half revolutions per minute. Two or more rabble arms are connected to
machined arm sockets in the shaft at each hearth. Each rabble arm is constructed with a
central tube for the purpose of conducting air from the cold air tube of the central shaft
to the extreme end of the rabble arm, thence back through an outer air space in the arm
to the shaft and through passages opening into the hot air compartment of the central
shaft. The air may be discharged to atmosphere or returned to the bottom hearth of the
furnace as preheated air, for combustion purposes.
The rabble arms provide mixing action as well as rotary and downward movement of the
sludge. The flow of combustion air is countercurrent to that of the sludge. Gas or oil
burners are provided on some of the hearths for furnishing heat for start-up or
supplemental use as required. As shown in Figure 8-7, three phases of the incineration
process occur at different levels in the furnace. In its travel across the hearths, sludge is
constantly turned and broken into smaller particles by the rotating rabble arms. Thus a
maximum sludge surface is exposed to the hot furnace gases which induces rapid and
complete drying as well as burning of sludge. The rabble arms also form spiral ridges of
sludge on each hearth, and the surface area of these ridges varies with the angle of repose
of the sludge. This angle varies with the moisture of the material. In any case, the surface
area of sludge exposed to the hot gases is considerably greater than the hearth area. Thus,
an effective area of as much as 130 percent of the hearth area is produced. While the
rabble arms provide significant solids-gas contact time on the hearths, the overall contact
time is actually still greater, due to the fall of the sludge from in-hearth and out-hearth
ports through the countercurrent flow of hot gases.
When burning sludge or any similar fuel, a measure of the work done by the heat energy
in evaporating moisture is given by the drop in temperature of the hot gases as they pass
between the combustion zone and the gas outlet of the furnace. The temperature of the
combustion gases in a boiler furnace may average 2,000° F, but by giving up heat to the
8- 13
-------
SLUDGE TEMPERATURE
-x;: 800F+ ::'•-.
FIGURE 8-7. Multiple hearth proeess zones.
8-14
-------
boiler, may leave the furnace at 500° F. Similarly, a multiple hearth sludge furnace may
generate gas temperatures exceeding 1,500° F in the combustion zone. These gases sweep
over the wet, cold sludge in the drying zone and perform useful work by giving up a
considerable portion of their heat for evaporation of moisture. In this heat exchange, the
gas temperature may drop as low as 500° F at the gas outlet. But while this exchange of
heat evaporates an important percentage of sludge moisture, it does not raise the sludge
temperature higher than about 160° F because the evaporation of water cools the mass it
leaves. Since no significant quantity of odoriferous matter is distilled, exhaust gases need
not be raised in temperature in an afterburner to destroy odors. Distillation of
odoriferous material from sludge containing 75 percent moisture does not occur until 80
to 90 percent of the water has been driven off and, by this time, the sludge is down far
enough in the incinerator to encounter gases hot enough to burn the odoriferous
materials. Generally speaking, when fuel is required to maintain combustion in a multiple
hearth furnace, a gas outlet temperature above 900° F indicates more fuel than required
for incineration is being burned.
Some incinerator installations do provide high temperature treatment of the stack gases.
The method is used is to convey the gases to a chamber where the temperature is raised
by burning auxiliary fuel in direct contact with the gases before venting to the
atmosphere. The cost of fuel for such a unit is substantial and can increase the total cost
of sludge incineration by 10 percent.
The multiple hearth system can be provided with an instrumentation system which will
convey the critical operating data to a central control panel. Temperature data can be
recorded for each hearth, cooling, and exhaust, and scrubber inlet gas. The temperature
can be controlled on each hearth to within ± 40° F. Malfunctions such as burner
shutdown, furnace overtemperature, draft loss and feed belt shutdown can be monitored.
In the event of power or fuel failure, the furnace should be shut down automatically and
the shaft cooling air fan automatically transferred to a standby power source to avoid
melting the shaft which drives the rabble arms. Problems encountered with multiple
hearth furnaces have included: (a) failure of rabble arms and teeth, (b) failure of hearths,
and (c) failure of refractories. Improvements in materials used in constructing the rabble
arms and teeth have reduced the first problem by providing a greater margin between the
yield point of the materials used and the normal operating temperatures in the furnace.
Many refractory problems result from the need for careful heating and cooling of the
furnaces during start-up and shutdown. From 24 to 30 hours are required to bring the
furnace up to temperature or to cool it. This is an operational disadvantage and a
procedure, which if not carefully followed, can contribute to refractory problems.
However, there are several installations which do operate intermittently without
significant refractory problems. Some difficulty has also been encountered with the
ultraviolet scanners which are used to monitor loss of flame on a hearth since needless
shutdowns of the furnace can result from a scanner malfunction.
As with all unit process costs, the cost of multiple hearth incineration varies with plant
size and this is shown in Figure 8-8. A detailed computer program for estimating costs of
- 15
-------
COSTS ($/DRY TON)
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O
c
50
W
oo
00
oo
CTs
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O
3
o
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asmr
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- 2. c
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o
o
o
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o
00
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CONSTRUCTION COST (MILLIONS OF DOLLARS)
-------
multiple hearth incineration has been developed. This program takes into account a large
number of variables and is based on field data from nine operating municipal sludge
incineration plants [7]. Some general cost curves for various plants have also been
developed for multiple hearth furnaces [8]. The reported costs of multiple hearth sludge
incineration vary substantially. The factors which cause these variations are the variability
from one locale to another in the moisture content and nature of the sludge, size of the
plant, chemicals used in dewatering, skill of operating and maintenance, labor and power,
and fuel costs. For example, the Cleveland Southerly plant reports [9] power costs of
$7.80 per ton of dry solids for vacuum filtration and incineration of 73,000 tons per year
(about one-third of the total appears to be related to incineration). Rochester, New York,
reports [10] an operating cost of $24.55 per ton for incineration of 390 tons of dry
solids per year with the cost expected to drop to $15.40 per ton when the load increases
to 2,180 tons per year. The South Lake Tahoe multiple hearth incineration system, which
has a rated capacity of about 3,900 tons per year, reports an operating cost of $12.71 per
ton [ 11 ].
8.2.5 Fluidized Bed Incineration
The first fluidized bed wastewater sludge incinerator was installed in 1962, and there are
now several units operating in the United States. They range in size from 220 to 5,000
Ib/hr dry solids. Ducar and Levin [12] have given a detailed description of several
installations. A typical section of a fluid bed reactor used for combustion of wastewater
sludges is shown in Figure 8-9. The fluidized bed incinerator is a vertical cylindrical vessel
with a grid in the lower section to support a sandbed. Dewatered sludge is injected above
the grid and combustion air flows upward at a pressure of 3.5 to 5.0 psig and fluidizes the
mixture of hot sand and sludge. Supplemental fuel can be supplied by burners above or
below the grid. In essence, the reactor is a single chamber unit where both moisture
evaporation and combustion occur at 1,400 to 1,500° F in either the dense or dilute
phases of the sandbed. All the combustion gases pass through the 1,500° F combustion
zone with residence times of several seconds. Ash is carried out the top with combustion
exhaust and is removed by air pollution control devices.
Several factors are involved in determining air flow. Fluidizing and combustion air must
be sufficient to expand the bed and provide proper density to prevent sludge flotation.
Excess air blows sand and products of incomplete combustion into the flue gases and
depletes stored heat energy. Minimum oxygen requirements must be met to assure
complete oxidation of all volatile solids in the sludge cake. Minimum temperatures must
be maintained to assure complete deodorization, while maximum temperature limits are
required to protect the refractory, heat exchanger, and flue piping. The quantities of
excess air are maintained at 20 to 25 percent to minimize its effect on fuel costs as was
illustrated by Figure 8-4. The capability of the fluidized bed furnace to operate at lower
excess air than typically experienced in a multiple hearth furnace is one factor accounting
for the greater heat efficiency of the fluidized bed system. The intense and violent mixing
of the solids and gases within the fluidized bed results in uniform conditions of
8- 17
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SIGHT GLASS
EXHAUST*
SAND FEED
PRESSURE
TAP fr
PREHEAT BURNER
ACCESS
DOORS
THERMOCOUPLE
-j SLUDGE INLET
FLUIDIZING
AIR INLET
FIGURE 8-9. Cross section of a fluid bed reactor.
8- 18
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temperature, composition, and particle size distribution throughout the bed. Heat
transfer between the gases and the solids is extremely rapid because of the large surface
area available.
As shown in Figure 8-10, an air preheater can be used in conjunction with a fluidized bed.
Burd [13] reported that preheating the air from 70° to 1,000° F allowed a reduction in
fuel costs from $9 to $3.50 per ton. Since air preheater cost can represent 15 percent of
the fluidized bed furnace cost, a careful economic analysis is needed to determine its
feasibility for a given situation.
The heat required for raising the sludge to the kindling point must come from the
combustion zone. While standard combustion units rely on the heat transfer from the hot
gases which contain only 16 BTU/cu ft, the expanded bed of the fluid-bed reactor has
16,000 BTU/cu ft. Because of the enormous reservoir of heat in the bed and a rapid
distribution of fuel and sludge throughout the bed, optimum contact between fuel and
oxygen and rapid transfer of heat is insured. The sandbed retains the organic particles
until they are reduced to mineral ash, and the violent motion of the bed comminutes the
ash material and prevents the buildup of clinkers. The resulting fine ash is constantly
stripped from the bed by the upflowing gases. The heat reservoir provided by the sandbed
also enables reduced start-up times when the unit is shut down for relatively short periods
(overnight). As an example, a unit can be operated 4 to 8 hours a day with little reheating
when restarting, because the sandbed serves as a heat reservoir.
Exhaust gases are usually scrubbed with treatment plant effluent and ash solids are
separated from the liquid in a hydrocyclone, with the liquid stream returned to the head
of the plant. An oxygen analyzer in the stack controls the air flow into the reactor and
the auxiliary fuel feed rate is controlled by a temperature recorder. Shutdown controls
for emergency situations should be provided.
The city of Lynnwood, Washington, installed the first commercial fluid bed system, and
raw primary sludge is combusted following gravity thickening and centrifuge dewatering.
Lynnwood's reactor is designed to receive 200 pounds per hour of dry solids with a
moisture content of 65 percent and a volatility of 75 percent. The reactor has been
operated with 20 percent excess air or about 360 scfm at a sludge feed rate of about 210
Ib/hr dried solids. No. 2 oil is used for daily reheating and as auxiliary fuel, because the
reactor has not been operated continuously. The reheat time and fuel required for
reheating are a function of the duration of shutdown. The reheat time, Tuesday through
Friday, is about 20 minutes, while for Monday it is about 1 hour [ 14].
The East Cliff Sanitary District, California, plant burns from 250 to 500 Ib/hr of raw
primary sludge in a residential area, and little auxiliary fuel is required. Auxiliary fuel is
only required for start-up on Monday morning, because the bed temperature at shutdown
exceeded 1,400° F and the overnight loss of temperature was less than 150° F. Reheating
can be accomplished at a rate of 350 to 400° F per hour using 0.24 gpm of No. 2 oil. It
has been found that the depth of the sandbed is not critical, and makeup sand is added
once every one to two months. Maximum pressure within the unit is about 3.5 psig [ 14].
8- 19
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Hot gas in
1500°F.
REACTOR
AIR
PREHEATER
Gas out
To scrubber
FIGURE 8-10. Fluidized bed system with air preheater.
8-20
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Alford [15] reported on the fluidized bed system at North Little Rock, Arkansas, and
illustrated well the considerations typically involved in the operation of such a system. A
centrifuged and degritted primary sludge with a solids concentration of 30 to 35 percent
and a volatility of 78 percent was fed to a fluidized bed equipped with a preheater at a
rate of 1,000 Ib/hr of dry solids. The low-pressure high-temperature reactor has a
diameter of 11.5 ft and an overall height of 28 ft. The bed was charged with 5,000 Ib of
high silica sand with an effective size of 0.210 mm and uniformity coefficient of 1.80.
About 100 Ib/month of sand is lost from the reactor by abrasion and disintegration. The
hot sandbed is fluidized with outside air circulated through a single pass concentric core
heat exchanger. This unit utilizes stack gas energy to heat the incoming air to about 950°
F during normal operation. Shutdown of the unit is accomplished by first stopping the
sludge feed system. Combustion air introduction is continued until the exhaust stack
gases show about 15 percent oxygen, indicating near complete combustion of sludge in
the bed. The fluidizing air feed is then shut off to retain stored heat energy. The
temperature die-off curve normally is exponential and averages a rate of 8° F/hr from
shutdown to restart. In the event the temperature in the reactor is still above 1,100° F by
the time additional sludge is accumulated in the thickener, no auxiliary fuel is required
for restart. Normally, the combustion process runs 15 to 18 hr/wk depending on the
amount of sludge received.
Some extensive maintenance problems have occurred with preheaters used in fluidized
bed systems, and scaling of the sand media has also been a problem. Most operating
problems uncovered in a survey of many existing installations, however, were caused by
jamming feed systems [12]. Screw feeds and screw pump feeds were both subject to
jamming because of either overdrying of the sludge feed at the incinerator or because of
silt carried into the feed system with the sludge. Another frequent problem has been the
burnout of spray nozzles or thermocouples in the bed.
There is very little published data on the operation and maintenance (O/M) costs of fluid
bed incineration. Albertson [16] has reported costs for fluidized systems operating
mainly with primary sludge. The principal deterrent to development of field O/M costs
has been the fact that most previous installations have been at smaller plants where record
keeping has not been sufficient. One study collected capital cost data for several size
systems [ 12]. In terms of February, 1968, dollars, the average capital cost of a 500 Ib/hr
dry solids system including the fluidized bed unit and centrifuge but excluding the
preheater, buildings, installation, or engineering costs was $195,000. A 5,000 Ib/hr
system, on the same basis had a cost of $706,000 while a 250 Ib/hr system had a cost of
$122,000. One comparison [17] of the costs of multiple hearth and fluidized bed
systems provides some insight into the inherent differences between the two systems.
Capital costs for the two competing incineration systems are often essentially equivalent
as demonstrated by results of bidding [ 18].
8-21
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8.2.6 Flash Drying
Flash drying is the instantaneous removal of moisture from solids by introducing them
into a hot gas stream. This process was first applied to the drying of wastewater sludge at
the Chicago Sanitary District in 1932. The pictorial flow diagram of the C-E Raymond
Flash Drying and Incineration System is shown in Figure 8-11 [19]. Originally, units
were designed to dry sludge for fertilizer and burn only the excess.
The flash drying system shown in Figure 8-11 is based on four distinct cycles which can
be combined in different arrangements. The first cycle is the flash drying cycle, consisting
of the hot gas duct, cage mill, mixer, uptake duct, cyclone, air lock, dry divider, and
vapor fan. The wet filter cake is blended with some previously dried sludge in a mixer to
improve pneumatic conveyance. The blended sludge and the hot gases from the furnace at
1,300° F are mixed ahead of the cage mill and flashing of the water vapor begins. The
cage mill mechanically agitates the mixture of sludge and gas and the drying is virtually
complete by the time the sludge leaves the cage mill. The sludge, at this stage, is dry at a
moisture content of 8 to 10 percent and the dry sludge is separated from the spent drying
gases in a cyclone. The dried sludge can be sent either to fertilizer storage or to the
furnace for incineration.
The second cycle is the incineration cycle. Combustion of fuel is essential to provide heat
for drying the sludge and the fuel may be gas, oil, coal, or wastewater sludge. Primary
combustion air, provided by the combustion air fan, is preheated and introduced at a high
velocity to promote complete sludge combustion. The sludge ash accumulates in the
furnace bottom and is removed periodically by a hydraulic sluicing system to an ash
lagoon or other disposal area.
The third cycle is the effluent gas cycle or induced draft cycle consisting of the
deodorizing and combustion air preheaters, dust collector, induced draft fan, and stack.
Heat recovery is practiced to improve economy. The effluent gases then pass through a
dust collector (dry centrifuge or wet scrubber) and the induced fan discharges the
effluent gases through a stack into the atmosphere. The fourth cycle may be the
fertilizer-handling cycle.
Perhaps the most notable current United States usage of this process is that by the city of
Houston, Texas [19] primarily for drying sludge for use as a fertilizer. The total
production capacity using the CE Raymond process was scheduled to reach 150 tons per
day in 1973. In the present facilities at the Northside plant in Houston, excess activated
sludge is dewatered on eight rotary vacuum filters and transported by belt conveyors to
the flash drying system. It includes mixing vacuum filtered sludge with previously dried
materials in a double paddle mixer before the combined solids are introduced into a cage
mill into which hot gas from a furnace is injected. The dried product is discharged to a
cyclone for separation of the solids from the entraining gases. The solids are fed partly to
the product conveyor and partly to the double paddle mixer to help dry the incoming
8-22
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.RELIEF VENT
HOT GAS DUCT
REFRACTORY
HOT GAS TO DRYING SYSTEM
DRYING SYSTEM
SLUDGE
COMBUSTION AIR
DEODORIZED GAS
FIGURE 8-11. Flash dryer system [19].
8-23
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sludge. The gases from the cyclone are recycled to the furnace where they pass through
the deodorizer preheater section and are subjected to a temperature of 1,200° F for
complete deodorization. The deodorized gases give up a portion of their latent heat to
incoming vapor laden gases, and subsequently give up more heat to combustion air.
The temperature at the entrance to the cage mill where the hot gases and sludge to be
dried are mixed is in the neighborhood of 1,200° F for evaporation of the moisture. At
the Cyclone, the temperature drops to around 220°F. In the subsequent heat exchange
operation, the temperature of the effluent gases is reduced to about 500° F.
The process is automated and panel boards are provided in air conditioned cubicles which
indicate and record variables such as air flow, temperatures at critical points, and
amperage on fan motors. Horn alarms indicate unsuitable temperature conditions. The
dried product is conveyed to a storage area for shipment. The organic constituents of the
finished fertilizer product are about the same as in the mixed liquor and the principal
variation is in the moisture content. The sludge on the filters averages about 3.4 percent
solids and ranges from 2.8 to 4.0 percent. The dry product after complete processing has
a moisture content of around 5.5 percent. From analysis at the time of sales of fertilizer
in January, 1972, the moisture content was 5.0 percent; ash 34.76 percent; nitrogen 5.34
percent; and available phosphoric acid, 3.93 percent. The ash content fluctuates; the
lowest on record is 26.4 percent and highest, 44.3. The variation apparently corresponds
with drought and rainfall periods. The only artificial chemical conditioning used at the
present time is to add ferric chloride to the sludge prior to filtration. In the usual
operation, this amounts to about 75 Ib per ton of dry solids or about 3.8 percent.
Throughout the experience with this operation, the city's marketing arrangements have
been scheduled on the basis of competitive bidding. The successful bidder is committed
to placing orders with the city for its entire production. At present, the contract period is
five years, which is renewable. The revenue to the city in the first six months of 1972
averaged $21 per ton f.o.b. Houston.
The material is now shipped in bulk by railroad car lots or sometimes by barge. It is
bagged for resale at the point of arrival. The present contractor has been handling it for
about 10 years, disposing of about 80 percent of the production in the citrus groves of
Florida. There has never been a time when it was not possible to dispose of the entire
sludge production by sales.
Another approach to drying of wastewater sludges for use as fertilizers is currently being
evaluated at the Blue Plains plant in Washington, D.C. [20]. A schematic of the system is
shown in Figure 8-12. Drying is achieved in a jet mill in this case. The mill has no moving
parts and offers the ability to dry and classify solids simultaneously.
As noted earlier, the use of the flash drying systems of the type shown in Figure 8-11 for
incineration alone has not proven attractive. The Metropolitan Denver Sewage Disposal
8-24
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WET SLUDGE
FROMWASTEWATER
PLANT
AIR & WATER
VAPOR
oo
N)
WET
SLUDGE
STORAGE
AIR INLET
PRODUCT
COLLECTOR
POLLUTION
CONTROL
FERTILIZER
FINISHING
PRODUCT
IN BAGS
FIGURE 8-12. Sludge drying system using the jet mill principle [20].
-------
District No. 1 plant (approximately 100 mgd in capacity) abandoned a system of this
type due to air pollution and problems of continuing explosions in the units. As an
incineration unit, the flash drying system has the disadvantages of complexity, potential
for explosions, and potential for air pollution by fine particles. An advantage is the
flexibility it offers for drying a portion of the sludge for fertilizer.
Flash drying is relatively expensive because of fuel costs (contrasted to incineration—no
heating value is realized from the sludge) and because pretreatment needs for production
of sludge, which must have some reasonable nutrient balance, are also expensive. It has
been reported that the fuel consumption for production of dried sludge is 8,000 BTU/lb.
Chicago, Illinois, reported a net cost of $45/dry ton for flash drying and abandoned the
process because of cost and air pollution considerations. However, if recent increases in
the values of fertilizers persist, the costs of sludge drying will be more largely recoverable
from sale of the dried product.
8.2.7 Wet Air Oxidation
The wet air oxidation process is based on the principle that any substance capable of
burning can be oxidized in the presence of liquid water at temperatures between 250° F
and 700° F. The process can operate on difficult to dewater waste liquors and sludges
where the solids are but a few percent of the water streams. In general, given the proper
temperature, pressure, reaction time, and sufficient compressed air or oxygen, any degree
of oxidation desired can be accomplished. By operating at lower temperatures and
pressures, the same approach may be used for sludge conditioning as covered in Chapter
6.
The wet air oxidation process has been commercialized and patented as the Zimpro
process [21]. This process has also been known as wet incineration, wet combustion, and
wet oxidation processes. Wet air oxidation does not require preliminary dewatering or
drying as required by conventional combustion processes. Water can be present up to 99
percent in this process whereas in conventional combustion it must be reduced to much
lower levels to make incineration practical.
Another significant difference is the flameless oxidation of the organics at low
temperatures of 300° F to 400° F when compared to 1,500° F to 2,700° F in the
conventional combustion processes. Air pollution is minimized because the oxidation
takes place in water at low temperatures and no flyash, dust, sulfur dioxide, or nitrogen
oxides are formed.
The general flow diagram of the Zimpro continuous wet air oxidation system is shown in
Figure 8-13. In the continuous process, the sludge is passed through a grinder which
reduces any particles greater than YA inch to about YA inch in size. Sludge and air are then
pumped into the system and the mixture is passed through heat exchangers and brought
8-26
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SLUDGE
TANK
STORAGE
SLUDGE
AIR COMPRESSOR
BIOTREATMENT
(OPTIONAL)
SOLIDS
SEPARATION
STERILE
LIQUID
(SETTLING
FILTRATION OR
CENTRIFUGATION)
*\ REACTOR
STEAM
GENERATOR
(OPTIONAL)
POWER
RECOVERY
(OPTIONAL)
CATALYTIC
GAS COLORLESS
PURIFIER EXHAUST
GAS
SEPARATOR
STERILE
INOFFENSIVE
SOLIDS
OXIDIZED SLUDGE
GASES
FIGURE 8-13. Wet air oxidation system.
8-27
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to the initiating reaction temperature. As oxidation takes place in the reactor, the
temperature increases. The oxidized products leaving the reactor are cooled in the heat
exchangers against the entering cold sludge and air. The gases are separated from the
liquid carrying the residual oxidized solids and released through a pressure control valve
to a catalytic oxidation unit for odor control. Where economic conditions make it
attractive, the gases may be expanded in power recovery equipment before being
discharged. The oxidized liquid and remaining suspended solids are released through a
level control value and the solids may be separated by settling and drainage in lagoons or
beds, or other methods such as vacuum filtration or centrifugation.
For start-up, heat is obtained from an outside source, usually a small steam generator.
With high degree oxidations and high fuel value sludges, no external heat is needed once
the process is started. Whenever the process is not thermally self-sustaining, steam may be
injected continuously to sustain the reaction temperature.
Four important parameters control the performance of wet oxidation units: temperature,
air supply, pressure, and feed solids concentration. The degree and rate of sludge solids
oxidation are significantly influenced by the reactor temperature. A much higher degree
of oxidation and shorter reaction times are possible with increased temperatures. As is the
case in conventional incinerators, an external supply of oxygen (air) is required to attain
nearly complete oxidation. The air requirement for the wet oxidation process is
determined by the heat value of the sludge being oxidized, and by the degree of oxidation
accomplished. Thermal efficiency and process economy are a function of air input, so it is
important that the optimum amount be determined. Because the input air becomes
saturated with steam from contact with reactor water, it is important to control the air
also to prevent excessive evaporation of the water. For primary wastewater sludges with a
BTU value of 7,800 BTU/lb, an air utilization of 5.75 Ib/lb is typical. For an activated
sludge with a heat value of 6,540 BTU/lb, an air utilization of 5.14 Ib/lb would result.
Sufficient pressure must be provided to prevent water vaporization because operating
temperatures are well above 212° F (typically above 500° F). Operating pressures have
varied from 150 to 3,000 psi depending upon the degree of oxidation desired. Pressures
are typically 1,000 to 1,750 psi in sludge handling installations.
The feed solids concentration has a significant effect on operating costs. It was found that
if the solids concentration could be increased from 3 percent to 6 percent, the operating
costs at the Chicago Sanitary District facility would decrease from S38/ton to about
$23/ton [22]. The solids concentration is an important factor in keeping the oxidation
self-sustaining.
A notable installation in the United States was the Chicago Sanitary District installation
at its West-Southwest plant which was operated from 1962 to 1972 but has now been
replaced by a land disposal system. This activated sludge plant treats over a billion gallons
per day of wastewater and generates 800 tons of waste sludge every day. The 1962
8-28
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installation was the first Zimpro plant erected in the U.S. for sludge treatment and
disposal. Prior to installation of the wet oxidation system, of the 800 tons per day of
sludge, up to 450 TPD was dewatered, heat dried, and sold as soil conditioner. Another
150 TPD of primary sludge was digested in Imhoff tanks and dried on sandbeds. The
remainder went to lagoons. A pilot plant of the wet air oxidation process having a
capacity of 2 TPD of sludge solids was set up and operated for about one year during
1957 to 1958. This work was successful and in 1959 a contract was signed calling for the
erection of four units each having a capacity of 50 TPD of sludge solids at 3 percent
solids concentration.
These units were placed in operation in 1962. It was soon evident that the plant had been
very conservatively designed in terms of reactor capacity and amount of air supplied, and
that the sludge processing capacity could far exceed the design value if thicker sludge
could be obtained, or if the sludge pumping and heat exchange capacity could be
increased. Because there was no means at hand for economically obtaining very much
greater than 3 percent solids, it was decided to increase the sludge pumping and heat
exchange capacity by approximately 40 percent. This expansion of the plant capacity was
carried out while the plant was in operation by modifying one unit at a time. The
nominal capacity of each of the modified units was 75 TPD with sludge at 3.4 percent
solids, having 35 g/1 COD. Usually the sludge concentration did not reach 3.4 percent and
solids processing capacity was, therefore, proportionately less. The units accomplished
approximately 70 percent COD reduction at the design capacity. At reduced solids
throughput rates, COD reductions exceeding 80 percent were obtained.
The wet air oxidation plant routinely treated a mixture of raw primary and activated
sludge solids in about equal weight proportions. The sludge was thickened by gravity
settling to approximately 3 percent solids and then pumped through grinders to storage
tanks in the oxidation facility. The sludge was ground to pass through 9/16 inch
openings. Approximately 0.3 g/1 of ammonia was added to the sludge on its way to the
storage tanks to adjust the pH to approximately 7.0, which minimized scaling in heat
exchangers. Sludge was taken from the storage tanks by centrifugal pumps that increased
the sludge pressure to about 50 psig. These fed the positive displacement high pressure
pumps which raised the sludge pressure to about 1,800 psig. Compressed air was then
introduced into the sludge at the same pressure and the mixture was passed through heat
exchangers. The sludge-air mixture was brought to approximately 400 to 420° F and
introduced into the reactor where the oxidation occurred. The oxidized material,
consisting of water, ash, steam and noncondensible gas, left the reactor at 500 to 520° F
and passed back through the heat exchangers into the separator. In the separator the
water carrying the ash was separated from the stream and noncondensible gases. The gas
and steam passed either to the turbine for power generation or directly to a water
scrubber from which they were exhausted to the atmosphere. The oxidized sludge was
discharged through a coil in the storage tank for further cooling and eventually was
discharged to a lagoon, where the ash was settled and the supernatant was returned to the
treatment plant. Tests indicated that vacuum filtration at 5.6 psf/hr would produce a
8-29
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cake with 40.5 percent solids and with filtrate solids of 0.16 to 0.6 percent with no
chemical conditioning. Centrifuging also appeared promising. Solids captures of 60 to 70
percent could be obtained, starting with a 10.8 percent solids feed, by centrifuging. The
dewatered ash contained 52 to 56 percent solids. Solids captures greater than 99 percent
were possible with the use of reasonable doses of an anionic polyelectrolyte; however, the
cake solids concentration dropped to 30 to 40 percent [23].
Each oxidation unit was normally on line for a period of 30 to 60 days. When heat
exchange efficiency was noticeably reduced, the unit was shut down, the heat exchangers
isolated from the rest of the system and washed by circulating acid or caustic through
them. This removed any accumulated scale and the unit was returned to service in 24 to
48 hours. Air compressors and sludge pumps also required periodic maintenance.
Reactors were emptied on an irregular basis, but no more than once yearly cleanings
appeared to be necessary.
Following the expansion of the facility in 1967, Chicago processed an average of about
248 tons/day over the period 1967 to 1972 at an average cost of about $35/ton.
However, late in 1972, the Chicago Sanitary District shut down all of the units in favor of
land disposal of the organic sludges. The Chicago units have not been operated since late
1972. The compressors are operated weekly and the other mechanical components
maintained to keep the wet air oxidation units available as backup to the land disposal
systems. The Sanitary District reports an improved secondary effluent quality at the
West-Southwest plant since the wet oxidation recycle liquors have been removed from the
system.
Wet oxidation of raw primary sludge has been practiced at Rye, New York [24]. The
Blind Brook treatment plant at Rye achieved a 90 percent reduction of insoluble organic
matter by operating at a temperature of 237.8° C and a pressure of 750 psi. The wet
oxidation facility has been operated intermittently on a 7 days on, 7 days off, schedule.
Auxiliary fuel has been used only when starting the unit. The oxidized sludge (ash) had
an organic content of 18.6 percent during the first year of operation (1964). After
cooling and solid-liquid separation, the BOD of the supernatant effluent averaged 8,400
mg/1. Separated ash disposal has been a problem at times due to odors from the ash
drying beds. In late 1966 [25] the operators reported few maintenance problems. Some
occasional problems had occurred from blockage of heat exchange tubes and failure of a
reactor baffle.
The operating costs for the plant at Rye, New York, have been reported as $26.80 per
ton made up of the items below:
Power = $13.60/ton($0.023/KWH)
Chemicals = 3.60/ton
8-30
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Water = 3.60/ton
Labor = 6.00/ton
McKinley [26] described the Wheeling, West Virginia, wet oxidation capital and
operating costs as:
1. Installed capital cost = $284,000 for a 5.6 ton/day facility.
2. Operating cost = $ 19.97/ton on which was broken down as follows:
Power = $6.1 I/ton
Chemicals = 4.13/ton
Fuel = 1.65/ton
Maintenance = 1.17/ton
Labor = 6.91/ton
An insoluble organic destruction of 90 percent was achieved, starting with a raw primary
sludge feed of 7.35 percent solids.
The above cost estimates do not include the cost of handling the recycled liquors or the
oxidized solids. At Chicago [23], the estimated cost of separating the oxidized solids
was:
Operation Estimated Cost/ton
Sedimentation Thickening $0.27
Vacuum Filtration 0.30
Centrifugation 1.60
An operational disadvantage of the wet air oxidation technique is the need to recycle wet
air oxidation liquors, high in organic content and in phosphorus and nitrogen, back
through the wastewater treatment process. These liquors represent a considerable organic
8-31
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load on the treatment system and the costs of handling these liquors is not usually
included in reported costs. The BOD contained in these liquors may be 40 to 50 percent
of the BOD of the unprocessed sludge and is typically high in ammonia and volatile acids
resulting from the oxidation of the nitrogen and carbon constituents of the sludge
[27,28]. The BOD of recycled liquors is typically 4,000 to 5,000 mg/1. They also contain
400 to 1,000 mg/1 NH3-N and 7,000 to 10,000 mg/1 COD. The pH is acid, in the 5 to 6
range usually.
Studies [29,30] of aerobic, biological treatment indicate that the liquors are amenable to
biological treatment with BOD removals of 90 percent or greater achieved at loading rates
of 4 Ib BOD/lb of mixed liquor volatile solids. It was estimated that if the return liquors
receive separate aerobic, biological treatment prior to recycle to the main biological plant
that the plant effluent BOD will not be increased and the effluent COD will increase by 8
percent. Direct recirculation of untreated liquors was estimated to cause a 10 percent
increase in effluent BOD and 20 percent increase in effluent COD—assuming the capacity
is available in the main plant to handle the recycled load [30].
The COD noted above to be refractory to the biological processes has also been found
difficult to remove by advanced waste treatment processes. A pilot study at Colorado
Springs, Colorado, found that some of this recycled COD would also pass through
coagulation and activated carbon adsorption processes downstream of the secondary
process.
The treatment of recycle liquors prior to return to the main plant is prudent to minimize
variations in the applied load to the main plant. Recognition of the magnitude of the
recycled load and incorporation of the capacity to handle it is imperative to successful
plant operation.
Among other design considerations are the facts that odor problems can develop from the
off-gases and from lagooning of the ash containing effluent. Air pollution caused by the
stack gases can be controlled by catalytic burning at high temperatures. Odors from
lagooning or sand drying bed operations might best be solved by dewatering the ash in a
system that includes gravity separation thickening followed by dewatering on vacuum
filters or in centrifuges.
Frequent shutdowns and maintenance problems have been reported from several
installations. The high 'pressure-high temperature system also introduces some safety
hazards. Safety considerations must play an important role in plant design.
8.2.8 Pyrolysis
Like incineration, pyrolysis is a controlled combustion process. Unlike the term
incineration, however, pyrolysis does not imply that a waste is being burned. The
pyrolysis process has been used for years by industry—for example, production of
-32
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charcoal and methanol from wood, and coal gasification. The process requires raising the
fuel to a temperature at which the volatile matter will distill, leaving carbon and inert
material behind. The carbon and volatiles do not burn in the process because the heating
occurs in an atmosphere deficient in oxygen. Volatile matter may be burned off as waste
in a secondary chamber to which air is added, or the off-gas may be cooled and
condensed to recover oils and tars or cleaned and used as fuel. Like incineration, pyrolysis
reduces the sludge volumes and sterilizes the end product. Unlike incineration, it offers
the potential advantages of eliminating air pollution and producing useful by-products.
Air pollution can be controlled because heating takes place in a closed system that allows
the collection of gases for beneficial use as a fuel or controlled burning. Those systems
ready for marketing are offered on a proprietary basis in which design, construction, and
usually operation are offered on a turnkey basis. Cost data from full-scale operating
systems are not yet available, and there is no basis on which to assess the reliability of
these new plants. Available information [31,32], however, indicates that the developers
of the new systems are willing to proceed at costs which are initially competitive with
conventional incineration.
8.2.9 Other Types of Incinerators
Cyclonic
Cyclonic reactors are designed for sludge disposal in the smaller wastewater treatment
plants. The principle of the cyclonic reactors is that high velocity air, preheated with
combustion gases from a burner is introduced tangentially into a cylindrical combustion
chamber. Concentrated sludge solids are sprayed radially towards the intensely heated
walls of the combustion chamber. This feed is immediately caught up in the rapid
cyclonic flow of hot gases and combustion takes place rapidly so that no material adheres
to the walls. The ash residue is carried off in the cyclonic flow and passes out of the
reactor.
Typically, degritted, thickened primary plus activated sludge is pumped from a thickener
to a centrifuge. The centrifuge dewaters the cake, which drops into a hopper and is
subsequently pumped into the cyclonic reactor with a small amount of compressed air.
One feature that makes this sludge combustion system attractive for some small
communities is that the unit is often sold premounted on skids as a package system.
Figure 8-14 shows a view of one type reactor with the sludge hopper mounted on the skid
as it would be shipped to the job. The system dewatering components may also be skid
mounted and the skids interconnected in the field.
The system lends itself to compact layouts. The dewatering and incinerating portions of
the system for a plant serving 5,000 people occupies a space 18 ft X 18 ft. The unit also
requires relatively short start-up times and may be brought to operating temperature
(above 1,400° F) in less than one hour.
8-33
-------
CYCLONIC REACTOR
00
u>
SLUDGE HOPPER
BLOWER
FIGURE 8-14. Skid-mounted cyclonic reactor system [33].
-------
These reactors process combined primary plus secondary sludge at nominal rates up to
100 to 130 pounds of dry solids per hour or 500 to 650 pounds of wet sludge per hour.
The detention time for the sludge within the reactor is less than 10 seconds. The
temperature is kept above 1,400° F so that the organic matter is burned above the odor
producing level. The first installation is at the Richardson Bay Wastewater Treatment
Plant at Tiburon, California.
Figure 8-15 shows another system using a cyclonic reactor applicable to larger plants is
also now available [34]. The unit is designed as a vertical cylinder with a rotating, solid
hearth on which the material for combustion lies. Unlike conventional forms of
incinerators, no combustion air is allowed to pass upwards through the material. All of
the air for combustion is injected into the furnace through tangential high velocity inlets
which impart a cyclonic, swirling pattern to the movement of the gases which pass over
the top of the burning material assuring an adequate contact of oxygen. The products of
combustion enter a central vortex before leaving the furnace through a top conical or
domed outlet. All of the solids enter the furnace at a minimum furnace temperature of
600 to 800° C with the initial temperature achieved by preheating with auxiliary fuel.
Dewatered solids are stored and are fed to the furnace by a transfer screw conveyor which
deposits the solids on the edge of the rotating hearth by means of a second water cooled
conveyor. Once on the hearth, the material travels in a predetermined depth and width
around the outer annulus as the hearth rotates, being subsequently ploughed over towards
the center of the hearth after one revolution by a fixed plow. Subsequent revolutions of
the hearth continue to move the material across towards the center in a series of
concentric annular paths until finally the incombustible material is discharged through an
ash chute in the center. This cyclone furnace is a British development and is available in
capacities to 6 tons/hr. There are no current applications in the U.S. at this time.
Electric
An all electric furnace using an infrared heat source is currently under development by
Shirco Co., Dallas, Texas, with the first full-scale sludge incineration units scheduled for
Richardson, Texas (500 Ib/hr) and Greenville, Texas (900 Ib/hr). Recent developments in
infrared lamps, coupled with the advent of silicon controlled rectifiers, semiconductor
controls and ceramic reflector materials, have provided an economical means for applying
and controlling radiant energy.
By properly applying this technology, it may be possible to approach operating costs of
natural gas with an all electric infrared incinerator utilizing smaller gas scrubber systems,
less combustion air, and lower capital investments than other incinerators presently on
the market. The shortage of natural gas and oil supplies is a factor enhancing the appeal
of this approach. Also, areas remote from natural gas or other petroleum fuel sources may
find electric infrared incineration an attractive alternate for ultimate disposal of sludge.
Figure 8-16 is a schematic of the overall system. The dewatered sludge is conveyed to the
belt conveyor which discharges the sludge into the machine onto a conveyor belt. The
8-35
-------
o
•3
•?-•
U
en
oo
00
I
-------
EXHAUST
STACK
COMBUSTION
AND
COOLING AIR
VIEWPORT
OXYGEN
ANALYZER
00
ASH
POWER D|SCHARGE
TEMPERATURE
SENSORS
POWER
SUPPLY
FIGURE 8-16. Infrared incineration system.
-------
high temperature belt conveyor carries the sludge through a drying zone and then into a
combustion zone. In the combustion zone, mounted just above the belt, is a battery of
infrared lamps which initiates and maintains the combustion. The belt then discharges the
ash into a hopper at the end of the machine. The lamps and end seals are cooled by
drawing outside air through the cooling air ducts. This preheated air is then used as
combustion air. The combustion air is then exhausted through a wet gas scrubber or
necessary air pollution equipment.
Because the heat is transferred by radiation rather than conduction or convection, the air
is not heated reducing combustion air requirements. The potential advantages of this
system appear to be:
• Low capital cost.
• Low operation and maintenance costs.
• The incinerator can be brought from ambient temperature to 1,600° F to
1,800° F within one hour.
• There is no explosion danger.
Although there is no doubt that natural gas or oil provides a cheaper source of heat than
electricity, the other savings associated with the infrared system may offset the higher
unit fuel costs. The infrared approach also scales down much better than other systems
and may be particularly attractive for small plants. This system also shows potential for
the regeneration of activated carbon. A 50 Ib/hr unit is in operation for carbon
regeneration in an industrial application in Baton Rouge, Louisiana.
8.3 Lime Recalcining
Lime is often used as a coagulant either as a tertiary step or ahead of the primary clarifier
in either a biological or a physical-chemical plant for removal of phosphorus from
wastewaters. There is, of course, considerable experience around the world with the
successful recalcining and reuse of lime used in water treatment plants and these
techniques may also be used to recalcine and reuse lime in wastewater applications.
The process of recalcining consists of heating the dewatered calcium-containing sludge to
about 1,850° F which drives off water and carbon dioxide leaving only the calcium oxide
(or quicklime). When dealing with wastewaters, the lime sludge contains inert materials
which must be wasted from the system or an infinite buildup in the quantity of sludge to
be handled will occur. When coagulating secondary effluent, these inert materials are
made up largely of magnesium hydroxide and hydroxyapatite. However, when
coagulating raw wastewater, the lime sludges will contain the many inert solids found in
8-38
-------
raw wastewater and the magnitude of the inert problem increases significantly. As
discussed in Chapter 7, centrifugation may be used to classify the inert and largely
remove them from the lime sludge.
The only significant experience in the U.S. with recalcining lime sludges from the tertiary
coagulation of municipal wastewaters is that gained from the South Lake Tahoe plant
[11,35]. At the design flow of 7.5 mgd through this water reclamation plant,
approximately 17 tons (dry CaO basis) per day of lime mud would have to be dewatered
and disposed of. Since about 93 percent by weight of this lime mud is in the form of
calcium carbonate, disposal costs would include not only dewatering and disposing of
about 34 tons of water and solids but also the loss of recoverable calcium oxide. By
recovering the lime through recalculation, the total blowdown of waste solids is reduced
to about 1.5 tons of dry solids. The cost of recalcined lime is slightly more than that of
new lime at this specific 7.5 mgd plant; however, the reuse of lime reduces by a factor of
20 the amount of water and sludge to be disposed of and, therefore, effects a substantial
overall cost savings.
The South Lake Tahoe system is shown schematically in Figure 8-17. Lime mud is
pumped from the chemical clarifier and recarbonation reaction basin to a gravity
thickener. Thickened lime mud is then pumped to 24" X 60" solid bowl concurrent flow
centrifuges operated in series for classification (see Chapter 7).
The cake from the first centrifuge is carried by a belt conveyor to a 14.3 foot diameter,
six hearth furnace in which calcium oxide and carbon dioxide are produced. The
recalcined lime is conveyed out of the furnace by gravity through a crusher to a thermal
disc cooler where lime temperatures are lowered from 700° F to 100° to 150° F, and
then into a rotary air lock. The recalcined lime is pneumatically conveyed from the rotary
air lock to a 35-ton capacity recalcined lime storage bin for eventual reuse. Stack gases,
rich in carbon dioxide, are scrubbed in a multiple tray scrubber before being exhausted to
the atmosphere. A portion of the gases are recycled to the recarbonation system to adjust
the pH of the lime coagulated wastewater to about 7.
Since April, 1968, the Lake Tahoe plant has successfully recalcined lime sludge from the
lime chemical treatment process. Over this period makeup lime has accounted for 28
percent of the calcium oxide used. Average monthly CaO values in the recalcined lime
have ranged between 51.0 percent and 74.7 percent with the average over the entire
period being 66.0 percent. It has been found that about 3.7 percent by weight of the
usuable calcium entering the furnace is lost as fly ash and captured by the wet scrubber.
At Tahoe, varying the temperatures between 1,600° F and 1,900° F has a major effect on
recalcined lime activity. Within this temperature range there is no indication that the lime
is being overturned. Recalcining lime at 1,900° F as opposed to 1,800° F produced a 5
percent increase in available calcium oxide, but very little improvement in an already
acceptable slaking rate. At 1,600° F the flour-like recalcined lime showed pronounced
8-39
-------
CENTRATE
TO PRIMARY
CLARIFIER
INFLUENT
CHANNE
. TO CO2
COMPRESSORS
MAIN
STACK
SCRUBBER
BYPASS -
DEWATEREDlr^ .
LIMESLUDGEX/
STORAGE BIN T
TRUCK
LOADING
NO 3
CENTRIFUGE
FOR LIME OR
SEWAGE
SLUDGE
REVERSIBLE
SCREW
.CONVEYOR
*
TO
SEWAGE
SLUDGE
FURNACE
t
THERMAL
DISC.
COOLER
iQ^^LOCK
w
HI T
ii.
— J C/3 O
^ UJ ^
tt -ICQ
V
LIME
FEEDEF
LIME
SLAKEF
if
FEEDER
SLAKER
PNEUMATIC
UNLOADING
AND
CONVEYING
EQUIPMENT
COMBUSTION
AIR BLOWER
RECALCINED
LIME TO
SPLITTER
BOX
FRESH
LIME TO
SPLITTER
BOX
RECALCINED
LIME BLOWER
FIGURE 8-17. The lime recalcining system at south Lake Tahoe.
8-40
-------
tendencies to agglomerage into soft, easily crushed particles of % to % inch diameter.
Many of the particles contained centers of unburned organic sludge.
The optimum furnace conditions in terms of recalcined lime activity appear to be about
1,900° F on the fourth and fifth hearths at 1.5 to 2.0 rpm rabble rate.
The large quantities of inert materials found in the sludges resulting from lime
coagulation or raw wastewater make lime recovery and reuse more difficult. One plant is
now under construction at Contra Costa, California, where a system for such lime
recovery and reuse has been developed which uses a combination of wet and dry
classification techniques [36,37]. At Contra Costa, a mixture of lime (to pH 11) and
ferric chloride (about 14 mg/1) are added to the raw wastewater as coagulants. Following
primary clarification, the wastewater receives biological treatment, including biological
nitrogen removal.
The lime sludges are thickened and then passed through series centrifugation for
classification in a manner similar to that described for the Tahoe project. Pilot data
indicate that 90 percent calcium capture is possible in the first stage centrifuge while
rejecting 40 to 85 percent of the inert materials. Following recalcination in a multiple
hearth furnace, the ash discharge from the furnace will be passed through a dry
classification device for further purging of inert materials. The dry classification device
makes a separation with air based on particle size while the preceding centrifugal wet
classification is based on particle weight. Based upon pilot tests, the use of these two
classification techniques in series offers a means of providing adequate purging of inerts
to permit lime reuse when coagulating raw wastewater.
Although the Lake Tahoe and Contra Costa waste treatment plants are both utilizing the
multiple hearth furnaces, the fluidized bed furnaces discussed. earlier may be used and
have been used successfully for recalcining in water treatment plants for many years. The
fluidized bed system for lime recalcining is shown in Figure 8-18 [38,39]. Lime mud
filter cake is fed into a paddle mixer along with dry recycled fines and quench water. The
mixture then goes to a cage mill disintegrator where precooled calciner stack gas at
1,000° F dries and disintegrates the moist solids. The resultant fine, carbonate is
conveyed by the exhaust gas to a cyclone separator. Large fraction discharge from the
cyclone is split, a portion being recycled to the mixer and a portion to the calciner feed
bin.
Calcination takes place in a two-compartment fluidized bed furnace. The upper fluid bed
of the reactor is used for low temperature calcination (1,500° to 1,600° F) of calcium
carbonate and the pelletization of calcium oxide. The lower fluid bed cools the calcined
product. In both fluid beds the solid particles are supported on a rising column of air so
that the solids behave in much the same fashion as a liquid. Fresh solids added to the bed
are quickly and uniformly distributed. The beds are held in a constant state of agitation
and suspension so that heat transfer is instantaneous and uniform.
8-41
-------
oo
i
-P-
to
FLUOSOLIDS
CALCINER
CENTRALIZED
CONTROL
PANEL
FLUIDIZING
AIR BLOWER
""""1 r>r\^^ II K li
k pt' A0j7 iPi V^
PELLETIZED
LIME
FIGURE 8-18. Fluidized bed system for lime recalcining.
-------
Lime produced by this system is in the form of pelletized particles, 6 to 20 mesh in size.
These uniform spheres are soft-burned, dust-free, and highly reactive. The chief advantage
offered over the multiple hearth approach is the pelletized product rather than the lime
dust obtained with the multiple hearth.
8.4 Air Pollution Considerations
Incineration offers the opportunity to reduce sludge to a sterile landfill and remove
offensive odors, but it also has the potential to be a significant contributor to the air
pollution problem in an urban community. The quantity and size of particulate emission
leaving the furnace of an incinerator varies widely, depending on such factors as the
sludge being fired, operating procedures, and completeness of combustion. Incomplete
combustion can form objectionable intermediate products, such as hydrocarbons and
carbon monoxide.
National air pollution standards for discharges from municipal sludge incinerators have
been promulgated which limit emissions of particulates (including visible emissions) from
incinerators used to burn wastewater sludge as follows [40]:
1. No more than 0.65 g/kg dry sludge input (1.30 Ib/ton dry sludge input).
2. Less than 20 percent opacity.
Visible emissions caused solely by the presence of uncombined water are not subject to
the opacity standard.
Available data indicate that on the average, uncontrolled multiple hearth incinerator gases
contain about 0.6 grain of particulate per standard cubic foot of dry gas [3].
Uncontrolled fluid bed reactor gases contain about 1.0 grain of particulate per standard
cubic foot [ 13]. For average municipal wastewater sludge, this corresponds to about 33
pounds of particulates per ton of sludge burned in a multiple hearth, and about 45
pounds of particulates per ton of sludge burned in a fluid bed incinerator. Particulate
collection efficiencies of 96 to 97 percent will be required to meet the standard, based on
the above uncontrolled emission rate.
Sludge incinerators differ from most other types of incinerators in that the sludge does
not supply enough heat to sustain combustion. Furthermore, there is less emphasis on
retaining ash in the incinerator and much of it is discharged in stack gases. Particulate
emissions to the atmosphere are almost entirely a function of the scrubber efficiency and
are only minimally affected by incinerator conditions. Sludge incinerators in the United
States are equipped with scrubbers of varying efficiency. These range from simple
bubble-through type units to impingent type scrubbers with pressure drops up to 20
inches of water.
8-43
-------
Existing state or local regulations in the United States tend to regulate sludge incinerator
emissions through incinerator codes or process weight regulations [41]. Many state and
local standards are corrected to a reference base of 12 percent carbon dioxide or 6
percent oxygen. Corrections to CO2 or O2 baselines are not directly related to the sludge
incineration rate due to the high percentage of auxiliary fuel required. In some
regulations, the CO2 from fuel burning is subtracted from the total when determining
compliance.
In developing the above standards, tests were conducted on the gaseous discharges from
several sludge incinerators. Stack tests were conducted by EPA at five locations, including
three multiple hearth incinerators and two fluid bed reactors as shown below [42]:
A. Fluidized bed reactor, 1,100 Ib/hr dry solids design capacity, operated at 100
percent capacity during test, equipped with a 20 inch of water pressure drop
venturi scrubber operated at 18 inches water pressure drop. Tested by EPA and
by a state agency, latter using Code Method 8 (see Table 8-4 and Table 8-5).
B. Multiple hearth (six hearths) Herreshoff incinerator, 750 Ib/hr dry solids design
capacity, operated at 64 percent capacity during test, equipped with a 6.0 inch
of water pressure drop single crossflow perforated-plate impinjet scrubber (see
Table 8-6).
C. Multiple hearth (six hearths) Herreshoff incinerator, 900 Ib/hr dry solids design
capacity, operated at 35 percent capacity during test, equipped with a 6.0 inch
water pressure drop single crossflow perforated-plate impinjet scrubber (see
Table 8-7).
D. Fluidized bed reactor, 500 Ib/hr dry solids design capacity, operated at 95
percent capacity during test, equipped with a 4.0 inch water pressure drop
single crossflow perforated-plate impinjet scrubber (see Table 8-8).
E. Multiple hearth Herreshoff incinerator, 2,500 Ib/hr dry solids design capacity,
operated at about 50 percent capacity during tests, equipped with a 2.5 inch
water pressure drop cyclonic inertial jet scrubber (see Table 8-9).
The results of these tests arc shown in Tables 8-4 to 8-9. Figure 8-19 summarizes the
results of the participate measurements. The results from the unit using a venturi scrubber
operating at 18-inch water pressure drop were used as the basis for the standard. The
other systems using other types of scrubbers operating at lower pressure drops failed to
meet the promulgated standard of 1.3 Ib/ton dry sludge input. The study of these
facilities indicated no relationship between the mass emissions and the percent of rated
capacity at which the incinerator was operating, but a strong relationship between
pressure drop across the scrubber and mass emissions was found [40]. All of the systems
easily met the opacity standard. Observations at 15 other facilities indicated they all met
a 10 percent opacity. The estimated costs of the scrubbing systems used as standard
8-44
-------
TABLE 8-4
SLUDGE INCINERATOR FACILITY Aj -SUMMARY OF RESULTS [42]
Run number
Date
Test time, minutes
Furnace feed rate,
ton/hr dry solids
Stack effluent
Flow rate, dscfm
Flow rate, dscf/ton feed
Temperature, ° F
Water vapor, vol. %
CO2 , vol. % dry
O2 , vol. % dry
CO, vol. % dry
SO2 emissions, ppm
NOX emissions, ppm
HC1 emissions, ppm
Visible emissions,
% opacity
Particulate emissions
Probe and filter catch
gr/dscf
gr/acf
Ib/hr
Ib/ton of feed
Total catch
gr/dscf
gr/acf
Ib/hr
Ib/ton of feed
1
1-11-72
108
0.550
2880
314,000
59
1.93
12.8
4.8
0.0
<0.3
4.2
<3.8
<10
0.024
0.023
0.583
1.06
0.032
0.031
0.779
1.42
2
1-12-72
108
0.560
2550
273,000
59
1.92
12.6
4.7
0.0
<0.3
5.7
<2.9
<10
0.005
0.005
0.116
0.207
0.007
0.007
0.160
0.286
3
1-12-72
108
0.560
2660
285,000
59'
2.23
11.5
6.4
0.0
<0.3
6.4
<4.1
<10
0.004
0.004
0.099
0.177
0.010
0.010
0.227
0.405
Average
108
0.557
2700
291,000
59
2.03
12.3
5.3
0.0
<0.3
5.4
<3.6
<10
0.011
0.011
0.266
0.481
0.0163
0.016
0.389
0.704
8-45
-------
TABLE 8-5
SLUDGE INCINERATOR FACILITY A2 -SUMMARY OF RESULTS [42]
Run number
Date
Test time, minutes
Furnace feed rate,
ton/hr dry solids
Stack effluent
Flow rate, dscfrn
Flow rate, dscf/ton feed
Temperature, ° F
Water vapor, vol. %
CO2 , vol. % dry
(less aux. fuel)
SO2 emissions^
Visible emissions
Ringelmann No.
Particulate emissions
Total catch
gr/dscf
(cor. to 12%CO2)
gr/acf
Ib/hr
Ib/ton of feed
1
5-3-71
60
0.325
3480
642,500
80
3.4
4.0
-
<1
0.020
0.019
0.596
1.84
2
5-4-71
60
0.325
3600
664,600
80
3.4
5.1
-
<1
0.031
0.029
0.956
2.94
3
5-4-71
60
0.325
3320
612,900
78
3.4
4.0
-
<1
0.048
0.047
1.365
4.20
Average
60
0.325
3470
640,600
79
3.4
4.4
-
<1
0.033
0.032
0.972
2.99
Note: Tested by local agency using Code Method 1. Probe and filter catch not analyzed
separately.
aNo SO2 detected.
Opacity was not recorded.
8-46
-------
TABLE 8-6
SLUDGE INCINERATOR FACILITY B-SUMMARY OF RESULTS [42]
Run number
Date
Test time, minutes
Furnace feed rate,
tons/hr dry solids
Stack effluent
Flow rate, dscfm
Flow rate, dscf/ton feed
Temperature, °F
Water vapor, vol. %
CO2 , vol. % dry
O2 , vol. % dry
CO, vol. % dry
SO2 emissions, ppm
NOX emissions, ppm
HC1 emissions, ppm
Visible emissions,
% opacity
Particulate emissions
Probe and filter catch
gr/dscf
gr/acf
Ib/hr
Ib/ton of feed
Total catch
gr/dscf
gr/acf
Ib/hr
Ib/ton of feed
1
10-13-71
120
0.237
3300
835,000
198
3.64
3.8
17.3
0.0
2.29 to 2.57
-
-
<10
0.0245
0.0187
0.690
2.91
0.0374
0.0289
1.06
4.47
2
10-14-71
120
0.236
2950
750,000
196
4.02
4.7
1.40
0.0
2.75
-
-
<10
0.0196
0.0155
0.495
2.10
0.0374
0.0287
0.945
4.00
3
10-14-71
120
0.249
2120
5 1 1 ,000
199
3.65
2.7
15.8
0.0
-
44.2 to 24.3
14.3
0.624 to 1.33
0.621
<10
0.0173
0.0132
0.315
1.26
0.0457
0.0348
0.832
3.34
Average
120
0.241
2790
699,00
198
3.77
3.7
15.7
0.0
2.53
27.6
0.858
<10
0.0205
0.0158
0.500
2.09
0.0402
0.0308
0.946
3.94
8-47
-------
TABLE 8-7
SLUDGE INCINERATOR FACILITY C-SUMMARY OF RESULTS [42]
Run number
Date
Test time, minutes
Furnace feed rate,
tons/hr dry solids
Stack effluent
Flow rate, dscfm
Flow rate, dscf/ton feed
Temperature, °F
Water vapor, vol. %
CO2 , vol. % dry
O2 , vol. % dry
CO, vol. % dry
SO2 emissions, ppm
NOX emissions, ppm
HC1 emissions, ppm
Visible emissions,
% opacity
Particulate emissions
Probe and filter catch
gr/dscf
gr/acf
Ib/hr
Ib/ton of feed
Total catch
gr/dscf
gr/acf
Ib/hr
Ib/ton of feed
1
7-15-71
80
0.111
1230
665,000
80
3.23
10.0
7.7
0.0
15.9 to 11.9
402 to 140
3.50 to 2.62
<10
0.0127
0.00985
0.127
1.14
0.0195
0.0150
0.206
1.86
2
7-15-71
80
0.149
1490
600,000
80
3.00
10.1
7.3
0.0
14.5 to 14.6
90.8 to 74.3
2.33 to 2.62
<10
0.0620
0.0477
0.620
4.16
0.0696
0.0535
0.889
5.97
3
7-16-71
80
0.146
1400
575,000
77
2.95
10.2
7.4
0.0
14.6 to 13.3
14.5 to 142
50.6 to 61. 8
2.52 to 2.62
<10
0.0196
0.0152
0.196
1.34
0.0260
0.0201
0.312
2.14
Average
80
0.135
1373
613,000
79
3.06
10.1
7.5
0.0
14.2
163
2.72
<10
0.0314
0.0242
0.314
2.21
0.0384
0.0295
0.469
3.23
8-48
-------
TABLE 8-8
SLUDGE INCINERATOR FACILITY D-SUMMARY OF RESULTS [42]
Run number
Date
Test time, minutes
Furnace feed rate,
tons/hr dry solids
Stack effluent
Flow rate, dscfm
Flow rate, dscf/ton feed
Temperature, °F
Water vapor, vol. %
CO2 , vol. % dry
O2 , vol. % dry
CO, vol. % dry
SO2 emissions, ppm
NOX emissions, ppm
HC1 emissions, ppm
Visible emission,
% opacity
Particulate emissions
Probe and filter catch
gr/dscf
gr/acf
Ib/hr
Ib/ton of feed
Total catch
gr/dscf
gr/acf
Ib/hr
Ib/ton of feed
1
7-21-71
120
0.255
1190
280,000
99
3.92
8.8
6.3
0.0
8.29 to 11.2
154 to 168
0.780 to 260
<10
0.0551
0.0468
0.562
2.20
0.0665
0.0565
0.678
2.66
2
7-21-71
96
0.237
1170
296,000
99
4.90
9.9
7.4
0.0
14.8 to 14.8
41. 2 to 42.9
4.1 6 to 1.56
<10
0.0766
0.0650
0.768
3.24
0.0859
0.0729
0.861
3.63
3
7-22-71
96
0.202
1240
368,000
95
3.48
9.1
8.2
0.0
14.2 to 15.4
17.8
187 to 170
161
2.35 to 2.09
<10
0.0545
0.0467
0.579
2.87
0.0653
0.0559
0.694
3.43
Average
104
0.231
1200
315,000
98
3.83
9.3
7.3
0.0
13.8
132
2.26
<10
0.0621
0.0528
0.636
2.77
0.0726
0.0618
0.744
3.24
8-49
-------
TABLE 8-9
SLUDGE INCINERATOR FACILITY E-SUMMARY OF RESULTS [42]
Run number
Date
Test time, minutes
Furnace feed rate,
tons/hr dry solids
Stack effluent
Flow rate, dscfm
Flow rate, dscf/ton feed
Temperature, °F
Water vapor, vol. %
CO2 , vol. % dry
O2 , vol. % dry
CO, vol. % dry
SO2 emissions, ppm
NOX emissions, ppm
HC1 emissions, ppm
Visible emissions,
% opacity
Particulate emissions
Probe and filter catch
gr/dscf
gr/acf
Ib/hr
Ib/ton of feed
Total catch
gr/dscf
gr/acf
Ib/hr
Ib/ton of feed
1
8-5-71
96
0.689
9840
-
135
16.3
4.2
14.9
0.0
2.01
62.8 to 46.0
11.9
<10
0.0260
0.0196
2.19
3.18
0.0335
0.0252
2.83
4.11
2
8-5-71
96
0.855
8510
_
145
18.6
4.3
14.9
0.0
2.07
83.5 to 75.8
6.83
<10
0.0136
0.0099
0.99
1.16
0.0221
0.0159
1.61
1.88
3
8-5-71
96
0.290
10,290
-
145
14.8
2.2
16.9
0.0
2.12
44.3 to 54.7
10.9
<10
0.0134
0.0101
1.18
4.07
0.0170
0.0128
1.50
5.17
Average
96
0.611
9547
-
142
16.6
3.6
15.6
0.0
2.07
61.2
9.88
<10
0.0177
0.0132
1.45 '
2.80
0.0242
0.180
1.98
3.72
8-50
-------
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FIGURE 8-19. Particulate emissions from sludge incinerators at
wastewater treatment plants [40].
8-51
-------
practice in the U.S. are typically about 4 percent of the total incineration facility for a
plant serving 100,000 people. The scrubber required to achieve the proposed particulate
standards would increase the cost an estimated 0.4 percent. Annual operating costs were
estimated to be increased by 0.9 percent [41].
Wastewater sludges contain metals which could be toxic if discharged into the
atmosphere. Unfortunately, there are very few data on the metals being discharged to the
atmosphere from municipal sludge incineration. The forms in which metals are found in
sludge will influence their behavior on incineration [41]. For example, if cadmium is
present in the sludge in solution as cadmium chloride, it could volatilize upon
incineration. If it is present as a precipitated hydroxide, it would probably decompose to
the oxide, but would not volatilize at the temperatures of incineration. However, it is
believed that most of the toxic metals, with the exception of mercury, will not
disproportionately appear in stack gases because of volatilization, but will be converted to
oxides and appear in the particulates removed by scrubbers or electrostatic precipitators
and in the ash.
Gaseous pollutants which could be released by sludge incineration are hydrogen chloride,
sulfur dioxide, oxides of nitrogen, and carbon monoxide. Data are presented in Tables 8-4
to 8-9 on the quantities of these materials found in stack gases. Carbon monoxide is no
threat if the incinerator is properly designed and operated. Hydrogen chloride, which
would be generated by decomposition of certain plastics, is not a significant problem at
concentrations currently observed. Consideration of the possibility of SO2 and NOX
pollution is aided by examination of the sulfur and nitrogen content of sludges. Sulfur
content is relatively low in most sludges. In addition, much of this sulfur is in the form of
sulfate, which originated in the wastewater. Sulfur dioxide is not expected to be a serious
problem. Sludge typically has a high nitrogen content from proteinaceous compounds
and ammonium ion. Limited data are available for predicting whether a high proportion
of these materials will be converted to oxides of nitrogen on combustion. From the data
available, the concentration of oxides of nitrogen from sludge incineration should be less
than 100 ppm from a properly operated incinerator and were observed to be less than 10
ppm from Facility A (Table 8-4). Considering this low concentration, the production of
oxides of nitrogen will probably not limit the use of incineration for disposing of sludge
in most cases.
Some data are available on the discharge of manganese and nickel which indicate the
following emission rates from incinerator facilities with emission controls:
Emission Factor Range
Metal Millipounds/ton (dry solids)
Mn 0.5 to 1.6
Ni 0.2 to 8.2
-52
-------
In both cases, the higher limit is represented by a single measurement.
Mercury is an example of a substance which presents special problems during
incineration. High temperatures during incineration decompose mercury compounds to
volatile mercuric oxide or metallic mercury. Fortunately, the quantity of mercury
involved is small. Based on an estimate that approximately 4,000 tons per day of sludge
are incinerated, and an average Hg concentration in sludge of 0.01 mg/g, 80 pounds of
mercury would be expelled into the atmosphere over the United States. This amount
compares to the estimated 3,000 tons per day of mercury which is discharged into the
atmosphere from the burning of coal over the earth [41 ]. Limited test data [43] indicate
that perhaps only 4 to 35 percent of the mercury entering an incinerator with emission
controls will be emitted to the atmosphere (excluding particulate forms).
In addition to the major air pollutants resulting from the burning of sludge, toxic
substances can arise due to the content of pesticides or other organic compounds in the
sludge. Unfortunately, very limited data are available on the concentrations of these
materials in municipal sludges or their fate in an incinerator. Data reported by EPA [41 ],
in a random selection of sludges, showed the following levels of materials present in the
raw sludges:
Compound Range (ppm)
Aldrin 16 (in one sludge only)
Dieldrin 0.08 to 2.0
Chlordane 3.0 to 32
DDD not detected to 0.5
DDT not detected to 1.1
PCB's not detected to 105
Pesticide and PCS determinations were made on sludges collected during the incinerator
tests at three of the five plants listed in Tables 8-4 to 8-9 [41 ]. PCB's were found in all of
these sludges, but concentrations were low (1.2 to 2.5 ppm). Pesticides and PCB's were
found only in the sludge. They were not found in the ash from either type incinerator,
nor in the inlet or outlet scrubber water. Ash can be analyzed for these materials to the
same degree of sensitivity as the sludge. A level of 0.1 microgram/g (ppm) could have
easily been detected. It is quite certain that these materials are not being carried out in
the ash.
8-53
-------
The mass flow rate of water to the scrubber is about 400 times the dry solids flow rate to
the incinerator. Consequently, the concentration at which these materials can be detected
in water must be sufficiently low to be sure that they are not escaping in the scrubber
water. Fortunately, analytical techniques are such that these materials can be detected in
water down to 0.1 nanogram/g (ppb). Thus, it is reasonable to believe that they are not in
the scrubber water.
Since the PCB's do not appear in ash or scrubber water, they are either destroyed by
incineration or remain as vapors in the water-scrubbed (and cooled) gas stream. All of
these materials have some solubility in water and it is likely that no trace would be
present in the scrubber water. Consequently, their escape as vapors from the incinerators
seems unlikely. However, one should also examine the available data on the
decomposition of PCB's and pesticides in other situations.
Rapid thermal degradation of most pesticides has been shown to begin at approximately
500° C with near total destruction at 900° C (1,652° F) [44,45]. If these materials
volatilize before burning, the use of afterburners on incinerators would be needed to
provide complete destruction. One manufacturer of pesticides achieves near total
destruction of pesticides in its multiple hearth carbon regenerator by providing an
afterburner using a 0.23 to 0.80 second retention time at 1,600 to 1,800° F.
The PCB's are even more thermally stable than most pesticides, as one would suspect. An
incinerator at St. Louis, Missouri, achieves total destruction of concentrated PCB's at
2,400° F with a retention time of 2.5 seconds. Experiments have shown, however, that
99 percent destruction is possible at 1,600 to 1,800° F in 2.0 seconds.
A privately funded study [46] found that PCB was completely destroyed in a multiple
hearth furnace burning organic sludges when the exhaust gas temperature was 1,100° F
and that 94 percent destruction occurred at normal exhaust temperatures.
The EPA Sewage Sludge Incineration Task Force [41] concluded that it has been
adequately demonstrated that existing well-designed and operated municipal wastewater
sludge incinerators are capable of meeting the most stringent particulate emission control
regulation existing in any state or local control agency. This observation coupled with the
fact that the newly promulgated federal standards are based on demonstrated
performance of an operating facility indicates that use of proper emission controls and
proper operation of the incineration system will enable a facility to meet all existing air
pollution regulations. Although only the venturi scrubber (Tables 8-4 to 8-9) met the
promulgated standard in the EPA tests. EPA [40] has stated that:
Impingement scrubbers tested by EPA did not meet the standard but, in our best
judgment, would do so if used in conjunction with an oxygen meter that
automatically regulates fuel burning rate. In our best judgment, electrostatic
precipitators could also provide more than adequate control. There are no EPA test
data on either of these control systems because during the test program there were
no existing plants using them.
8-54
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8.5 References
1. Owen, M. B., "Sludge Incineration." /. Sanit. Eng. Div., proceedings of the
A.S.C.E. (Feb. 1957), paper 1172.
2. Russell, R. A., "Theory of Combustion of Sludge." Training Course on Sludge
Concentration, Filtration, and Incineration, University of Michigan (Jan. 30 to
Feb. 1, 1963).
3. Balakrishman, S., Williamson, D. E., and Okey, R. W., "State of the Art Review
on Sludge Incineration Practice." Federal Water Quality Administration Report
17070 olV 04/70 (1970).
4. Liao, P. B. and Pilat, M. J., "Air Pollutant Emissions from Fluidized Bed
Sewage Sludge Incinerators." Water Sewage Works (Feb. 1972), pp. 68-74.
5. Reeve, D. A. D. and Harkness, N., "Some Aspects of Sludge Incineration."
Water Pollut. Contr. (1972), p. 618.
6. Stanley Consultants, Inc., "Sludge Handling and Disposal, Phase I, State of the
Art, Report to Metropolitan Sewer Board of the Twin Cities Area" (Nov. 15,
1972).
7. Unterberg, W., Sherwood, R. J. and Schnerder, G. R., "Computerized Design
and Cost Estimation for Multiple-Hearth Sludge Incinerators." EPA Report
17070 EBP 07/71 (1971).
8. "Estimating Costs and Manpower Requirements for Conventional Wastewater
Treatment Facilities." EPA Report prepared by Black and Veatch Engineers,
Contract 14-12-462(1971).
9. Guccione, E., "Incineration Slashes Costs of Sewage Disposal." Chem. Eng.
(Apr. 11, 1966).
10. "Low Cost Sewage Sludge Incineration System for Village of East Rochester,
New York." Bartlett-Snow-Pacific Brochure.
11. Gulp, R. L. and Gulp, G. L., Advanced Wastewater Treatment. Van
Nostrand-Reinhold: New York (1971).
12. Ducar, G. J. and Levin, P., "Mathematical Model of Sewage Sludge Fluidized
Bed Incinerator Capacities and Costs." Federal Water Quality Control
Administration Report No. TWRC-10 (1969).
13. Burd, R. S., "A Study of Sludge Handling and Disposal." Federal Water
Pollution Control Administration Publication WP-20-4 (May 1968).
8-55
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14. Bulletin No. 6051, Dorr-Oliver, Inc.
15. Alford, J. M., "Sludge Disposal Experience at North Little Rock, Arkansas."/.
Water Pollut. Contr. Fed. (1969), p. 175.
16. Albertson, O. E., "Low Cost Combustion of Sewage Sludges." Dorr-Oliver,
Inc., Technical Preprint No. 600-P.
17. Manchester, A. H., "Comparision Between Fluid Bed Incineration and Multiple
Hearth Incineration," private communication.
18. Copeland, G. G. and Lutes, I. G., "Fluidized Bed Combustion of Sewage
Sludge." Eng. Digest (Apr. 1973).
19. Bryan, A. C. and Garrett, M. T., Jr., "What Do You Do with Sludge? Houston
Has an Answer." Pub. Works (Dec. 1972).
20. "The Organo System." Organic Recycling Inc. (1973).
21. "Zimpro Wet Air Oxidation Units." Zimpro brochure (1968).
22. "Zimpro Wet Air Oxidation in Chicago." Zimpro catalog (1970).
23. Walters, W. R. and Ettelt, G., "Dewatering of the Ash By-Product from the Wet
Oxidation Process." Presented at the 1965 Purdue Industrial Waste Conference
(1965).
24. Harding, J. C. and Griffin, G. E., "Sludge Disposal by Wet Air Oxidation at a
Five MGD Plant."/. Water Pollut. Contr. Fed. (1965), p. 1134.
25. Swanwick, J. P., "Recent Developments in Sludge Technology in the U.S.A."
Water Pollut. Contr. (1968), p. 374.
26. McKinley, J. B., "Wet Air Oxidation Process." Water Wastes Eng. (1965),
p. 97.
27. Malina, J. F., Jr. and DiFilippo, J., "Treatment of Supernatants and Liquids
Associated with Sludge Treatment." Water Sewage Works (1971), p. R30.
28. Teletzke, G. H. et ah, "Components of Sludge and Its Wet Air Oxidation
Products."/. Water Pollut. Contr. Fed. (1967), p. 994.
29. Hurwitz, E., Telethzke, G. H., and Gitchel, W. B., Zimpro Inc., "Wet Air
Oxidation of Sewage Sludge." Water Sewage Works, 298 (1965).
8-56
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30. Erickson, A. H. and Knopp, P. V., "Biological Treatment of Thermally
Conditional Sludge Liquors." Presented at the 5th International Water
Pollution Research Conference, 1970.
31. Fife, J. A., "Solid Waste Disposal: Incineration or Pyrolysis." Environ. Sci.
Technol. (1973), p. 308.
32. "Pyrolysis and Salvage Get Demonstration Tests." The American City (Nov.
1972), p. 44.
33. The Dorr-Oliver Type CR FS Disposal System." Bulletin No. 6052, Dorr-Oliver,
Incorporated (1967).
34. Stribling, J. B., "Sludge Incineration by Cyclone Furnace." Effluent Water
Treatment Journal (Aug. 1972).
35. "Advanced Waste Water Treatment as Practiced at South Tahoe." EPA Report
17010 ELQ 08/71 (1971).
36. Horstkotte, G. A. et al., "Full-Scale Testing of a Water Reclamation System."
/. WaterPollut. Contr. Fed. (1974), p. 181.
37. Parker, D. S., Zadick, F. J. and Train, K. E., "Sludge Processing for Combined
Physical-Chemical-Biological Sludges." EPA Report R2-73-250 (Jul. 1973).
38. "Fluo Solids Lime Mud Reburning System." Dorr-Oliver Bulletin 7550-F.
39. "Dorr-Oliver Recausticizing and F/S Calcining Systems." Dorr-Oliver Bullentin
REC-2.
40. "Background Information for New Source Performance Standards" (Vol. 3).
EPA Report 450/2-74-003, APTD-1352C (Feb. 1974).
41. Final Report, "Sewage Sludge Incineration Task Force." EPA (Feb. 1970).
42. "Background Information for Proposed New Source Performance Standards"
(Vol. 2, Appendix). EPA Report APTD-13526 (Jun. 1973).
43. Roy, S. L., personal communication, Apr. 30, 1974, EPA, Research Triangle
Park, North Carolina.
44. "Basic Research on Equipment and Methods for Decontamination and Disposal
of Pesticides and Pesticide Containers." Annual Report, USDA grant number,
12-14-100-9182 (34), Mississippi State University (Jun. 1968 to Jun. 1969).
-57
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45. "Organic Pesticides and Pesticide Containers — A Study of Their
Decontamination and Combustion." Final Report, Bureau of Solid Waste
Management Contract Number CPA-69-140, Foster D. Snell, Inc. (1970).
46. "Sludge Disposal: A Burning Question." The Flowsheet, published by
Envirotech, No. 7 (1973).
8-58
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CHAPTER 9
FINAL DISPOSAL PROCESSES
9.1 Methods, Functions, and Occurrences
The need for proper treatment of all residual sludge streams is well recognized and should
be provided for in plant design. Lakes and streams are no longer acceptable for sludge
disposal. The methods of final disposal can be broadly categorized as disposal or
utilization procedures. Disposal procedures include landfills and ocean dumping as
described in Table 9-1.
TABLE 9-1
FINAL DISPOSAL METHODS
Principal
Disposal Procedure Sludge Form Main Constraints
Sanitary Landfill Dewatered cake Gas leachate, and runoff
or ash control; land availability
(Stabilized)
Ocean Dumping Liquid Oceanic and shoreline
(Thickened) pollution
The landfill is one of the major methods for final disposal of sludge and incinerator ash.
The amount of sludge that is dumped in the ocean is decreasing because of regulations.
Utilization procedures are receiving increasing attention and are described in Table 9-2.
Cropland application and land reclamation are the major sludge utilization methods. The
EPA municipal inventory shows that landfill and land spreading of sludge are used in 50
percent of the U.S. installations. Proceedings of recent symposia [1,2] on this topic are
available and contain much useful information. Other methods of synthesizing or
retrieving useful products by treating sludge with chemicals and/or heat are being studied.
9-1
-------
TABLE 9-2
SLUDGE UTILIZATION METHODS
Utilization Procedure Sludge Form Main Constraints
Cropland Application Liquid, cake Application rate,
dried, or compost unsatisfactory sludge
Land Reclamation Liquid or Application rate,
Dewatered unsatisfactory sludge,
availability of land
9.2 Selection of Method of Final Disposal
In developing an optimum conceptual design for a wastewater treatment plant, it is
increasingly apparent that determination of the method of final sludge disposal is a major
consideration. The method of final disposal determines the acceptable form of the sludge
residue and thus influences the choice of both sludge and liquid unit processes to be
employed.
In selecting a final disposal or utilization process, the method chosen should be in
accordance with local, state, interstate, and federal requirements. While no sludge
residues, grit, ash, or other solids should be discharged into the plant effluent or receiving
waters, the procedure chosen should also not result in any significant degradation of
surface or groundwater; air or land surfaces.
Since present indications are that ocean disposal could be banned, designs incorporating
sludge disposal to the ocean are questionable. It is desirable that methods chosen do not
cause a health hazard or a nuisance condition, therefore it is essential that sludges be
stabilized prior to spreading on land. The acceptable form of sludge for disposal or
utilization is partially determined by the sludge treatment method. The sludge may be in
the form of a liquid, dewatered cake, incinerator ash, compost product, or dried powder.
9.3 Sanitary Landfill
Stabilized sludge containing no free water can be satisfactorily disposed in a sanitary
landfill either alone or in a mixture with municipal solid waste. A sanitary landfill must
be managed so that wastes are systematically deposited and covered with earth to control
environmetal impacts within defined limits. This distinguishes a sanitary landfill from an
uncontrolled dumping operation. Sludges and solid wastes have in the past been disposed
9-2
-------
of in dumps not meeting proper landfill specifications. The placement of incinerator ash
or stablized sludge cake in a sanitary landfill can be an acceptable procedure when
adequate land is available and site location and operational precautions prevent the
creation of nuisance conditions or health hazards. Prior to placing sludge in a landfill it
should be sufficiently dewatered to minimize the quantity of free water present. Leachate
and runoff from a sanitary landfill should be minimized and when necessary collected and
suitably treated to prevent pollution of ground and surface waters. Therefore, sound
engineering judgment dictates that sanitary landfills not be located in an existing flood
plain.
9.3.1 Design Criteria
Wilcomb and Hichman [3] suggest that the site of a sanitary landfill be easily accessible
and safeguarded against uncontrolled gas movement from the decomposition of organic
matter. During the predesign survey, the site's geology, hydrology, and soil conditions
should be considered relative to the need for adequate protection of groundwater,
conformation of area land use planning, and provision of an adequate quantity of earth
cover.
The landfill itself should have limited access and provision for uniform spreading of
wastes in layers not over 2 feet thick, followed by compaction. The compacted wastes
should have a minimum of six inches of suitable compacted earth cover at the end of each
working day. When each portion of the landfill is completed, a uniform layer of earth
cover compacted to a minimum depth of 2 feet should be placed over it and suitable
grasses planted to prevent erosion.
Adequate monitoring of any land application or landfill site is essential. This plan must be
specifically designed for applicable local conditions and should include monitoring
groundwater observation wells, surface water, sludge and soils for heavy metals, persistent
organics, pathogens, and nitrates. Human food chain products grown in sludge aided soils
should also be monitored for heavy metals, persistent organics, and pathogens.
9.3.2 Costs of Sanitary Landfill
Even with increases in costs resulting from more stringent environmental impact control
procedures, landfilling as practiced is generally less expensive than other final disposal or
utilization procedures. An important consideration can be the dewatering requirement.
Further, land costs can be a major factor. Figure 9-1 [4] presents a range of capital and
operation and maintenance costs for landfills, excluding land costs. Quantities are
expressed in tons of wet sludge cake per day. Other investigators have reported landfilling
costs of from $ 1 to $4 per ton of dry solids [5].
9-3
-------
COSTS ($/WET TON)
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9.4 Use of Sludge on Agricultural Land
The application of sludge to farmland is a very popular utilization method because it can
be both economical and simple. Limitations include heavy metals, occasional public
resistance, and the unavailability of suitable land. The popularity of sludge spreading for
crop production and soil improvement has increased significantly over the last 10 years.
9.4.1 Soil Considerations
Soil is composed of mineral matter, organic matter, microorganisms, solutions, and air.
The soil's assimilative capacity hinges on its ability to filter, buffer, and absorb a sludge's
constituents. It chemically and biologically transforms materials and supports plants
which use the applied nutrients. Desirable soil properties for sludge assimilation are:
• Depth
• High infiltration and percolation capacity.
• Fine enough texture to have high water and nutrient holding capacity.
• Good drainability and aeration.
• Neutral or alkaline pH.
9.4.2 Sludge as a Fertilizer and Soil Conditioner
Municipal sludge contains all of the essential plant nutrients. Almost half of the nitrogen
and potassium in digested sludge is in the liquid phase, so drying or dewatering can
decrease these nutrients significantly. The ratio of potassium to nitrogen and phosphorus
in sludge is low in relation to crop needs. Therefore, the use of digested sludge at a rate
required to just supply the nitrogen needs of a crop will usually not supply enough
potassium. Table 9-3 presents the nutrient content for a liquid digested mixture of
primary and activated sludges.
TABLE 9-3
PRIMARY NUTRIENT CONTENT OF LIQUID DIGESTED SLUDGE
(25 Percent of Dry Weight)
Nitrogen Phosphate (P2 Os) Potash (K2 O)
Sludge Type
Liquid Digested 3.5-6.4 1.8-8.7 0.24-0.84
9-5
-------
Dotson [6] indicated a 2-inch application of Chicago's liquid digested sludge to cropland
would supply about 200 to 350 Ib of ammonia nitrogen, about the same amount of
organic nitrogen, 250 to 400 Ib of phosphorus (approximately 80 percent organic), and
about 60 Ib of potassium per acre. A good corn crop can utilize 150 to 250 Ib or more of
nitrogen per acre, and some grasses can use more, but increased leakage of nitrogen may
occur at these higher loadings. Crop removal and volatilization determine how much of
the soluble soil nitrogen will be leached from the soil. The principal sludge components
that determine satisfactory application rates of sludge to cropland are nitrogen, trace
elements, and pathogens.
Nitrogen is typically the first sludge component to limit the rate of sludge application to
land, since adding excess nitrogen to soil involves the very real risk of polluting the
groundwater with nitrates.
High nitrate concentrations are toxic to human and livestock. Public Health Service and
the World Health Organization drinking water standard for nitrate nitrogen is 10 mg/1. If
nitrate nitrogen is applied in amounts greater than can be removed by harvested plant
uptake, the excess nitrates can potentially contaminate ground and/or surface waters by
leaching or runoff, respectively. Furthermore, excess nitrogen can also cause a high
nitrate fodder yield in certain crops. Methemoglobinemia in infants can occur from
consuming green forage silage which contains in excess of 3.0 percent nitrate (dry basis).
The pathways of nitrogen in soils and the hazards they present to groundwater are
difficult to predict. The processes that affect the form of nitrogen in soils (mineralization,
nitrification, denitrification, immobilization, fixation, adsorption, volatilization, cation
exchange, convection, dispersion, and plant uptake) may take place concurrently, and the
rates at which they progress are determined largely by soil type and climate.
Mineralization (the conversion of organic nitrogen to ammonia) proceeds at variable rates
depending on climate and soil conditions and the nature of the organic matter, and
nitrification (the oxidation of ammonia nitrogen to nitrate nitrogen) is relatively fast in
aerobic soils with favorable temperature. On the other hand, denitrification
(transformation of nitrate nitrogen to nitrogen gas) takes place where free oxygen is
absent or deficient and where other conditions, including a supply of carbon, are
favorable for biological activity.
Microbes utilize part of the available nitrogen in soils to synthesize new cells. Ammonia
ions may be fixed by organic matter and silicate clays and protected from biological
attack (fixation). Volatilization of ammonia may be substantial from soils with high pH.
Plant uptake varies greatly, of course, and the amount of nitrogen removed in runoff
varies with precipitation patterns and farming practices.
A hypothetical model indicates that only a little more than one-half inch of digested
sludge per year could be applied without contributing nitrogen to leachates [7].
9-6
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However, at the Agricultural Research Center in Beltsville, Maryland, adding up to 160
tons of sludge solids per acre to soils with clay subsoils did not cause nitrates to leach
into groundwater at a depth of 9 feet. Ammonia compounds in excess are toxic to some
seeds. Two weeks cessation of sludge application before seeding has been adequate to
avoid this problem.
It is difficult to formulate guidelines as to the rate that sludge can be applied to cropland
without nitrate pollution of groundwater. Soil type, geology, climate, crops, and farm
management are important factors in determining the fate of nitrogen added in sludge.
Careful calculations should be made for any proposed system based upon applicable local
conditions.
Metal content of sludge varies widely. The metals of most concern are zinc, copper,
nickel, cadmium, and lead. Zinc and copper are micronutrients that may enhance crop
quality when sludge is spread thinly. The heavy metal content of sludge can vary widely
from treatment plant to treatment plant as illustrated in Table 9-4.
TABLE 9-4
HEAVY METAL CONTENTS IN SLUDGE [6]
(mg/1, Dry Basis)
Location
Dayton, Ohio
Monterey, California
Tahoe, California
Millcreek, Cincinnati, Ohio
Zinc
8,390
3,400
1,700
9,000
Copper
6,020
720
1,150
4,200
Nickel
<200
220
<400
600
Cadmium
830
<220
40
<40
It has been said that the heavy metal content is a function of industrial wastes and that
sludges from treatment plants not serving large amounts of industry should not have this
problem. Unfortunately, analysis of sludges from such cities as Washington, D.C., does
not support this supposition. Some sludges of apparently predominantly domestic origin
contain significant heavy metal concentrations. Prior to land spreading of sludges, analysis
of the influent to the treatment plant should be made. If high heavy metal concentrations
are found, sound engineering procedures to remove these metals at the source should be
initiated.
9-7
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Allowable levels of trace elements in soils are difficult to establish because of the many
complex soil and crop factors. Plants vary widely by species, varieties, and strains in
tolerance to trace elements. Interdependent soil properties greatly influence the
availability of trace elements to plants. Thus, spreading wastewater sludge on cropland,
like other disposal methods, involves an element of risk.
Usually trace element toxicities to plants are more prevalent and acute where sludges are
applied to acid soils. Also plant species exhibit rather marked differences in tolerance to
levels of trace elements in sludge-amended soils. The amount of sludge which can be
safely applied to soil, will depend upon the composition of the sludge, the kind of soil to
which it is applied, and the species of plant grown on the soil. Detailed discussions of
specific trace elements and their effects are available in the literature [8,1,2].
Pathogen control is of importance because of the possible exposure directly to sludge in
the handling and application steps and in the food chain. Although anaerobic digestion
reduces the pathogen content of sludge, a significant number of pathogens may survive
the process. Pathogens in sludge can be destroyed through:
• Storing for long periods.
• Pasteurizing at 70° C for 30 minutes.
• Adding lime to raise pH to 12.4.
• Using chlorine to stabilize and disinfect sludge.
• Using other chemicals.
Stabilization techniques are covered in Chapter 5. Long storage of wastewater sludge has
been suggested as one of the simplest methods of reducing pathogenic organisms. Storing
sludge for 30 days has reduced fecal coliforms by 99.9 percent, although some parasites
probably persist much longer when sludge is stored in lagoons. Most large municipalities
that dispose of sludge by spreading on land store it in lagoons-this provides the
flexibility needed at times when sludge cannot be spread. Access to storage lagoons should
be restricted. Sludge spread on pastures during the grazing season in Germany and
Switzerland is pasteurized. Maintaining a temperature of 70° C for 25 to 30 minutes kills
pathogens, viruses, cysts, worm eggs, and oocytes. Direct steam injection avoids fouling
and sealing of heat exchangers. Heat recovery is uneconomical for small plants, but larger
plants may use heat exchange to minimize energy requirements.
Aerosols which could contain pathogenic microorganisms may be present in the air over a
landspreading site. Where spray irrigation is used to distribute the sludge, the potential for
aerosols is increased.
9-8
-------
Three methods have been proposed to be used (either concurrently or separately) to
minimize aerosols: (1) low pressure spray, (2) downward directed spray, and (3) large
droplet spray. The potential for erosion should be checked before using these methods.
Application methods other than spraying minimize aerosols. At many reclamation sites,
however, spraying is the only practical method. Where spraying is used, spraying methods
should be selected which will minimize aerosol formation and wide buffer zones should
be used. Mosquito vectors can be controlled by eliminating ponding on the site. This
starts with properly grading the site and is made effective by maintenance to correct
minor ponding locations and prevent others from forming.
Fruits and vegetables grown on wastewater irrigated or sludge-amended soils can be
surface-contaminated with pathogenic microorganisms. The viability of these pathogens is
extremely variable and may be from a few hours to several months. Also, among the
factors which influence the survival of pathogens in the soil and on vegetation are:
• Type of organism.
• Temperature—lower temperature increases viability.
• Moisture—longevity is greater in moist soils than in dry soils.
• Type of soil—neutral, high moisture holding soils favor survival.
• Organic matter—the type and amount of organic matter present may serve as
a food or energy source to sustain the microorganisms.
The fact that some pathogens can survive the sludge digestion process, and remain viable
in the soil for periods up to several months has prompted the general recommendation
that liquid sludges should not be applied to root crops or crops intended for human
consumption in the raw form.
Pastureland and farmland used to grow forage crops are frequently used as land disposal
sites, a practice which appears to present little problem from the standpoint of disease
transmission via livestock grazing on such fields.
The potential for contamination of groundwater sources by pathogenic microorganisms is
dependent on the ability of the pathogens to survive and move through the soil system. A
number of factors including the types of soil involved, the number of microorganisms
applied to the soil surface, and the many different kinds of microorganisms contained in
both the soil system and the applied wastes all combine to complicate an assessment of
pathogen movement in soils. Fine clay soils are more effective than sandy soils for
removal of pathogens. A soil system is generalyy efficient in removing pathogens unless
movement of sludge or effluent through faults occurs. It has been reported that bacteria
do not travel horizontally more than 100 feet through granular soils, that complete
9-9
-------
removal of E. coli was noted in about 16 feet of dune sands, and viruses were removed in
a 2-foot bed of clean sand at moderate rates of application over a 7-month period.
Generally, groundwater contamination by pathogens may not be as serious a potential
hazard as surface water contamination via prolonged surface erosion and/or either direct
runoff from the sludge spreading operation or surface water runoff induced by snowmelt
or rainfall. With careful site management practices, the problem of surface runoff can be
minimized. Certainly, each site must be carefully evaluated.
9.4.3 Physical Process Considerations
In developing a system for the application of sludge to cropland, the mode of
transportation, application procedure, and rate of application must be considered.
Transportation may be accomplished by tank truck, barge, rail, or pipeline. Sludge
characteristics, elevation differences, distance, sludge volume, and land availability are
important factors in selecting a method of transporting sludge from the treatment plant
to the utilization site. Tank trucks afford flexibility in the selection of utilization sites,
and they are widely used to haul and apply sludge. The ton to mile cost is relatively high,
so small communities with available land near the treatment plant are most apt to find
the use of the tank trucks feasible. Pipelines usually entail relatively high capital and low
operating costs, so assurance of the availability of land for a long period of time is an
important consideration. Cost analysis should be used in selecting the mode of
transportation. Figure 9-2 [7] shows relative transportation costs for liquid organic
sludges.
Retention time at the treatment plant or storage facilities near the land application site
can provide for periods, when sludge spreading is not feasible. Storage also diminishes the
pathogen population and further stabilizes the sludge.
Ridge and furrow irrigation methods are used for applying sludge to the land surface.
These methods are better adapted to level land and cold climates. They also minimize
potential air-borne virus and aesthetic problems. Spray sprinkler irrigation systems
however, are more flexible, require less soil preparation, and can be used with a wider
variety of crops than the ridge and furrow methods. Figure 9-3 shows such a system. In
addition to surface sludge application several systems have been developed for
incorporating sludge of from 1 to 85 percent solids into the soil. Incorporating sludge in
the soil is a safeguard against odors, aesthetic problems, contamination of surface waters
from erosion, or other problems that can result from irrigation techniques.
Application rates depend on sludge composition, soil characteristics, climate, vegetation,
and cropping practices. Annual application rates have varied from 0.5 to more than 100
tons per acre. Applying sludge at a rate to support the nitrogen needs of a crop, usually
about 5 to 10 tons of digested sludge solids in the liquid form, avoids problems associated
with overloading the soil. A rough guide for selecting the application rate for an
acceptable sludge is given in Table 9-5.
9-10
-------
600
400
I
a:
200
§2 100
o
Q.
CO
z
<
cc.
60
40
20
20
1^1
I I I I I
TANK TRUCK-
RAILROAD TANK CAR
PIPELINE
1 I I
I I I
I
40 60 100 . 200
DISTANCE TO DISPOSAL POINT,(MILES )
400
FIGURE 9-2. Relative transportation cost for liquid organic sludges [7],
9-11
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to
FIGURE 9-3. Typical spray sprinkler.
-------
TABLE 9-5
APPLICATION RATES TO CROPLAND
Application Rate
Relative Soil Conditions (tons dry si. solids/acre/year)
Slight Limitations 10-20
Moderate Limitations <10
Specific points which must be continuously considered and/or monitored during
utilization are:
• The trace element composition of sludge, soil, and crops.
• The nitrogen content of sludge, soil, and crops, and potential nitrate
pollution of aquifers.
• The use of only disinfected sludge on low-growing fruits or vegetables to be
consumed raw.
• The hydraulic overloading of soil.
• The ultimate use of land.
• The practices to control runoff and erosion.
9.4.4 Crop Considerations
Crops vary widely in their reaction to sludge enriched soils. The particular crop species
may be adversely affected by trace elements in the sludge. Additionally, the crop may
take up and concentrate certain of these trace elements, thereby, inhibiting future use of
the harvested materials (particularly in the human food chain). The reaction of a specific
crop to sludge application is extremely site dependent. Factors such as soil type, pH,
moisture content, climate, and crop species are important. For advice concerning crops
which can be satisfactorily grown in sludge enriched soils, the local representatives of the
U.S. Department of Agriculture should be consulted.
To date, the practice of sludge spreading in forests has been limited. However, forests
offer opportunities for beneficial use of sludge to improve soil fertility and increase tree
growth. With most tree species nutrient uptake is small compared to that of cultivated
9-13
-------
crops, however in the case of some species, intensive culture operations for the
production of wood fibre is possible. High application rates might require nitrogen
removal to prevent nitrate pollution of groundwater.
9.4.5 Costs of Cropland Sludge Spreading
Table 9-6 presents actual costs data for land spreading of digested sludges. As can be
noted, the economics of this final utilization procedure can vary widely.
TABLE 9-6
COSTS FOR LAND SPREADING DIGESTED SLUDGE
Location
Chicago, Illinois
San Diego, California
Piqua, Ohio
St. Marys, Pennsylvania
Approximate
Plant Size
(mgd)
1,300
90
3.8
1.3
Cost
($/Ton)fl
60.54 = Current
35. 24 = Ultimate6
10.57
17.50-30.00
19.92
Reference
[9]
[10]
[10]
[11]
^Excludes digestion and costs are given per ton of digested solids at 1972 prices.
Ultimate costs include pipeline to be constructed. A principle variable is transportation
cost.
9.5 Land Reclamation
A large part of the previous material on cropland utilization also applies to deposition on
land for reclamation purposes. High application rates are commonly used to reclaim
strip-mine spoils or other low-quality land and may lead to water contamination if
drainage and runoff controls are not installed. Leachates that are unsuitable for
groundwater recharge may be intercepted by tile drains and treated before being released
to the environment. Surface runoff can also be impounded and treated. The great amount
of sludge sometimes used to reclaim land may cause accumulations of trace elements in
excess of normal concentrations in soils, however, soil management can maintain high
organic matter content. A nearly neutral pH minimizes toxicity to plants.
9-14
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9.6 Land Disposal Case Studies
As the pace of activity in this class of methods increases, more actual use data become
available. The periodical, Compost Science is a particular focal point for such
information.
9.6.1 St. Marys, Pennsylvania
The St. Marys, Pennsylvania treatment plant is a 1.3 mgd activated sludge facility treating
principally domestic wastewater. Anaerobically digested sludge with a 5 percent solids
concentration was applied at the rate of 900,000 gallons/year to primarily cropland for
the past 13 years. Loading of a 1,500-gallon capacity tank truck involved a 10-minute
fill-up period and about 5 minutes for spray application. The average round trip was 5 to
8 miles. Sludge was spread on hay, pasture, oats stubble, corn stubble, poor lawn, brush,
orchards, and athletic fields. Figure 9-4 and 9-5 show a typical tank truck operation and
equipment. The average application rates for two of the fields is shown in Table 9-7.
TABLE 9-7
APPLICATION RATES AT ST. MARYS
Field
Hay
Pasture
% Solids
4.25
3.7
gal/acre/yr
19,600
17,450
Tons Dry Solids
acre/year
3.54
2.60
No complaints arose in St. Marys, and farmers welcomed the sludge. Cost of disposal
averaged $19.92/ton [7].
9.6.2 Fergus Falls, Minnesota
The Fergus Falls wastewater treatment plant processes 1.6 mgd (76 percent) domestic
wastewater and 0.5 mgd (24 percent) industrial wastewater. Sludge is produced by
sedimentation and high rate trickling filters and is anaerobically digested. At one time,
sludges were dewatered on drying beds, but these were removed in 1959. Since 1959, the
digested sludge has been spread on farm fields in the summer and a city owned golf
course during the winter. The secondary digester acts as a holding tank for the sludge.
Sludge is hauled and applied with a 2,200-gallon capacity tank truck, and normally six
9-15
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FIGURE 9-4. Tank trunk spreading sludge in cold weather.
FIGURE 9-5. Close-up view of sludge deflection plate.
9-16
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loads per day are hauled by one full-time operator. The solids content of the sludge is
normally about 3.5 percent. In the summer, the sludge is applied to about 45 acres;
twenty acres of pasture and 25 acres that are planted in corn the following year. The
application is made on alternative years on the pasture land and the cornfield. The
hauling distance is about 7 miles to the golf course and about 6 miles to the farm [8].
9.6.3 Xenia, Ohio
Landspreading of liquid sludges has been utilized by Xenia's two activated sludge
wastewater treatment plants for about 13 years. Both anaerobically and aerobically
digested sludges are being disposed of by land spreading [8]. Typically, the sludges
average about 2.3 to 3.5 percent solids at 69 to 74 percent volatile solids and have an
average pH of 7 to 7.2. The influent to these plants is essentially all domestic wastewater.
Although a vacuum filter has been installed at the Glady Run plant and provisions have
been made for a vacuum filter at the new Ford Road plant, land spreading continues to
be the method of sludge disposal.
Presently, the sludge is spread on farmland used to grow field corn, wheat, and forage
crops, and on pastureland used for cattle grazing. During the growing season, the liquid
sludge is hauled extensively to pasturelands with the cattle remaining in the fields during
the spreading operation. At the present time, 10 farms are being utilized to receive the
digested sludge. These farms were obtained primarily through advertisements in the local
newspaper.
One full-time driver is employed to haul the sludges from both of the city's plants using
an over-the-road tractor-trailer unit with a tank capacity of 3,000 gallons. This is a
cumbersome unit for use in the fields and frequently gets stuck necessitating towing with
city-owned equipment. Maintenance costs and driver qualifications are higher for this
type of unit than for operation of a normal tank truck.
The land spreading operation is conducted throughout the year. However, during adverse
weather conditions, the sludges are disposed of with no further attention on old sand
drying beds at an abandoned wastewater treatment plant. An estimated 1 million gallons
of sludge were disposed of in this fashion during the wet weather of 1972. The environs
along the route to the farms are essentially rural. However, when hauling to the
abandoned treatment plant, the route is through the main business district and residential
areas. Two years of operational data are summarized in Table 9-8.
9-17
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TABLE 9-8
OPERATING DATA FOR XENIA, OHIO
LAND SPREADING PROCEDURES
1971
1972
Gallons
of
Solids Sludge
(%) (X 10 3
3.5 2,295
3.5 4,150
Diy Weight
) (tons)
335
606
Labor
Costs
($)
7,696
6,698
Fuel
Costs
($)
236
442
Maint.
Costs
($)
271
1,304
Depreci-
ation
($)
2,400
2,400
Total
Cost
($)
10,603
10,843
Cost per
ton
($)
31.65
17.89
Cost per
1,000 gal
Sludge
($)
4.62
2.61
9.6.4 Denver, Colorado
The Metropolitan Denver Sewage Disposal District No. 1 is recycling to the soil
wastewater sludge at the Lowry Bombing Range. This sludge recycling program involves
the chemical treatment and vacuum filtration of the sludge at the district wastewater
treatment plant in Commerce City, pumping of the resultant 16 percent solids sludge
cake into 42 cubic yard dump trailers, hauling of the sludge cake to the Lowry Bombing
Range, transferring of the sludge cake through a sludge transfer station to 30 cubic yard
truck spreaders, spreading of the sludge cake from the truck spreaders at a rate not to
exceed 25 dry tons per acre, and tilling into the soil of the sludge cake after drying.
The sludge at the Metropolitan Denver Sewage Disposal District No. 1 -treatment plant
consists of a mixture of raw primary sludge, anaerobically digested primary sludge from
the Denver Northside treatment plant, and aerobically digested excess activated sludge.
The three types of sludge are mixed in a holding tank and transferred as a mixture to the
sludge process building for chemical conditioning and dewatering.
To insure against water pollution problems from the sludge application, four catch basins
have been constructed near the lower end of drainage gulleys and channels at the Lowry
Bombing Range. Water samples are taken from the catch basins at regular intervals when
runoff water is present, and tested in the laboratory of the Metropolitan Denver Sewage
Disposal District No. 1. Up to the present time no significant amounts of nitrates, nitrites,
fecal coliform, and other possible problem elements have been discovered.
As part of the current expansion of the Metro Denver system, a pipeline to convey sludge
to the land disposal site will be built. Also, it is planned to change the method of sludge
application to a subsurface injection system which has been developed at Colorado State
University. The cost of this method of sludge disposal has been $45 to $55/ton at Denver
[12].
9-18
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9.6.5 Chicago, Illinois
In 1966 the Metropolitan Sanitary District (MSD) of greater Chicago started a program to
return all of the digested wastewater sludge to the soil as a liquid fertilizer and soil
conditioner.
The District's three main wastewater treatment plants have a designed capacity of 1.3 bgd
and produce about 1,000 tons of digested sludge solids per day. Four systems of handling
sludge were in use: activated sludge was heat dried and sold for fertilizer; digested sludge
was stored in lagoons; wet air oxidation was used to reduce the organic solids to ash
before the sludge was stored in lagoons; Imhoff tank sludge was dewatered on drying beds
before being hauled to a dump. None of these sludge handling and disposal methods were
satisfactory as a permanent method.
The land reclamation project plan was developed to achieve economical disposal and
utilization of the sludge. The plan provided for irrigation of low quality farmland with
digested liquid sludge. About 30,000 acres located about 50 miles from Chicago were to
be purchased. Pipeline transportation and lagoon storage at the terminal were part of the
recommended systems.
Local opposition prevented acquistion of the desired land, and available land needing
improvement was finally located in Fulton County, Illinois, nearly 200 miles from
Chicago where 10,000 acres of the strip-mined land were purchased. Seventy-one percent
of the land was formerly strip-mined. Under the initial temporary plan for Fulton
County, 7,500 wet tons/day (50 percent of the MSD's daily sludge production) sludge is
being barged down the Illinois River to a dock from which it is pumped through 11 miles
of pipeline to storage lagoons. About 10 percent of the sludge being barged to Fulton
County is pumped from the storage lagoons through irrigation pipe and sprayed on
terraced spoil banks during 8 months of the year. Runoff from the sludge-amended land
is contained in reservoirs until the water quality meets stream standards before it is
discharged.
By July, 1974, 2,000 acres of the 10,000 owned were planned to be ready for sludge
application — 1,100 acres of which were formerly strip-mined land and 900 acres row
cropland [13]. The cost to prepare the former strip-mined land has been $2,000 to
$3,000 per acre. Corn is now being grown on 900 acres. About 750 acres of the site have
been established as a recreational area.
Sludge is sprayed by a carriage mounted spray gun which sprays the sludge in a 300-foot
arc. Crops grown include corn, soybeans, and winter wheat. Application rates to the
strip-mined soil are scheduled to decrease from an initial 75 dry tons/acre/year during the
first year to 20 dry tons/acre/year over a 5-year period.
The cost of barge transportation is $36.81 per ton of sludge solids, and the total cost of
disposal is $71.54 per ton of digested solids [ 14]. Long-range land reclamation plans for
9- 19
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Fulton County include construction of a pipeline from Chicago. The total cost of sludge
disposal is expected to $35.24 per ton of solids [14]. Of the above totals, sludge
digestion comprises $9.22/ton.
Although the Chicago experience indicates the potential for converting strip-mined land
to productive agricultural land, the project is continuing to meet with some citizen
opposition [13]. About 25 percent of the registered voters in Fulton County have signed
a petition asking if agencies outside the county should be stopped from spreading sludge
on lands there. As a result, an advisory referendum on the issues has been scheduled for
November, 1974. Chief citizen concerns have been odors and the potential threats of
heavy metal buildup and pollution.
9.7 References
1. Sopper, W. E. and Kardos, L. T. (eds.), "Recycling Treated Municipal
Wastewater and Sludge through Forest and Cropland." The Pennsylvania State
University Press (1973).
2. "Proceedings of the Joint Conference on Recycling Municipal Sludges and
Effluents on Land." Champaign, Illinois, July 9-13, 1973.
3. Wilcomb, M. J. and Hickman, H. L., Jr., "Sanitary Landfill Design,
Construction, and Evaluation." EPA Solid Waste Management Office,
Publication SW-88ts (1971).
4. Stanley Engineers, "Sludge Handling and Disposal, Phase I, State of the Art."
Report to Metro Sewer Board of Twin Cities Area, Nov. 15, 1972.
5. Burd, R. S., "A Study of Sludge Handling and Disposal." Report for FWPCA,
Dept. of the Int., by the Dow Chemical Co., Publ. WP-20-4 (May 1968).
6. Dotson, G. K., "Constraints to Spreading Sewage Sludge on Cropland."
EPA-NERC Cincinnati, AWT (May 31, 1973).
7. Ewing, B. B. and Dick, R. I., "Disposal of Sludge on Land." Water Quality
Improvement by Physical and Chemical Processes, University of Texas
Press: Austin.
8. Carroll, T. E., Maase, D. L., Genco, J. M., and Ifeadi, C., "Review of
Landspreading of Liquid Municipal Sewage Sludge." Battelle-Columbus
Laboratories (publication pending, 1974).
9. Dalton, F. E. and Murphy, R. R., "Reclamation and Recycle ."WaterPoUut.
Contr., 45 (7), 1489-1507 (1973).
9-20
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10. Dotson, G. K., Dean, R. B., and Stern, G., "Cost of Dewatering and Disposing
of Sludge on the Land." Chemical Engineering Progress Symposium Series,
129, AICHE, "Water-1972 " (1973), pp. 217-226.
11. Wolfel, R. M., "Liquid Digested Sludge to Land Surfaces—Experiences at St.
Marys and Other Municipalities in Pennsylvania." Presented at the 39th Annual
Conference, Water Pollution Control Association of Pennsylvania, Aug. 11,
1967.
12. Annual Report for Metro Denver (1972).
13. "Chicago Reclaiming Strip Mines with Sludge." Civil Eng. (Jun. 1974), p. 43.
14. Kudrna, F. L., "The Prairie Plan." Landspreading Municipal Effluent and
Sludges in Florida, 1973 Workshop.
9-21
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CHAPTER 10
CASE HISTORIES-USE OF CHEMICALS
IN EXCESS ACTIVATED SLUDGE PROCESSING
10.1 General Considerations
A reliable source of information for plant design is an existing plant's operating data.
Accordingly this chapter reviews performance data which was compiled over several years
from a number of plants.
10.2 Washington, D.C.-Blue Plains Plant
Dahl, Zelinski, and Taylor [ 1 ] have excellently described many years of process
optimization work for the 253 mgd Blue Plains plant. The D.C. plant is presently a
modified, high-rate, activated sludge plant and has the processes and mode of operation
shown schematically in Figure 10-1. Prior to the 1959 addition of the modified activated
sludge system, the plant had all the facilities shown in Figure 10-1 except for the aeration
basins and final clarifiers. The rationale for the plant expansion included the supposition
that the same solids handling processes could be used for treating the mixed primary and
activated sludge as had been used for the primary sludge alone. The expansion did
include, however, a doubling of the elutriation tank capacity as well as some expansion in
total solids handling capabilities. Key assumptions were that a mixture of raw primary
sludge and excess high rate activated sludge could effectively be gravity thickened and
that elutriation of the anaerobically digested mixed sludge could work well. Hindsight
showed gravity thickening to be partially satisfactory, while elutriation was not.
A short time after starting operation of the modified secondary plant; the high solids
content of the thickener overflow and more particularly, the elutriate, caused recycle of a
large load of fine solids. The primary basins and aeration systems could not accommodate
these fine solids and hydrated activated sludge particles. They built-up in the system and
caused very poor capture in the primary sedimentation basin and gradually saturated the
entire plant causing septicity and flotation of sludge blankets. Upsets in the various solids
liquid separation procedures eventually caused essentially inoperable conditions. A
temporary solution was obtained by venting the elutriate to the plant effluent, since the
elutriate represented the largest recycle stream in the plant. This, however, was not a
satisfactory long-range solution, because it lowered plant efficiency. In effect, the
elutriate contributed 15 to 30 tons per day of BOD to the Potomac River. Early in 1965,
a high molecular weight cationic polyelectrolyte was applied to the elutriation basin's
feed in the manner shown in Figure 10-2. This process modification dramatically cleaned
up the elutriate and offered a way to solve the elutriate problem. However, the use of
chemicals in elutriation was only a trial process change and diversion of effort to other
10- 1
-------
INFLUENT
GRIT
REMOVAL
EFFLUENT
PRIMARY
.BASINS.
1 »--
1 *
I*"
AERATION
BASINS
FINAL
CLARIFIERS
THICKENER
OVERFLOW^
•«—^-iTHICKENER I
ELUTRIATE
HIGH RATE
DIGESTION
WASH WATER
ELUTRIATION ~-
B.O.D. REMOVAL 70—90%
S.S. REMOVAL 70—90%
15/25%
FILTER
CAKE
FIGURE 10-1. District of Columbia, plant flow diagram.
-------
WASH WATER
DIGESTED SLUDGE
ELUTRIATE RECYCLED
OR TO RIVER
2 STAGE
ELUTRIATION
FILTER
CAKE
X = CATIONIC POLYELECTROLYTE APPLICATION POINT
FIGURE 10-2. District of Columbia's elutriation and filtration system.
-------
plant processing studies considered potentially more efficacious delayed full realization of
the benefit of application of chemicals to elutriation. During the period 1968 to 1969,
the focus of plant studies was again on the elutriation and filtration systems. By the
application of a cationic polyelectrolyte to the elutriation system and through intensive
study of the elutriation and filtration processes, it was found possible to achieve 90
percent solids capture through the elutriation and filtration systems. The achievement of
90 percent capture produced an elutriate that could be recirculated to the head of the
primary basins and not cause any upset in plant operations. Although the use of a
cationic polymer to flocculate the digested mixture of primary and activated sludges
affected good solids capture, settling rates, and compaction; considerable benefit was also
obtained in operational control. This was brought about by the operating personnel who
devised improved methods for monitoring the performance of the elutriation basins and
the vacuum filters.
Shortly after the modified activated sludge process began operation and during the
venting of elutriate, it was only necessary to remove about 45 tons per day of sludge
solids at the filters. Ferric chloride was used for sludge conditioning and filter production
rates were about at the design figure of 3 lb/hr/ft2. Chemical costs on occasion, with
ferric chloride, were as high as $13.50 per ton. Immediately after starting chemical
treatment of the elutriation basins to effectively capture 90 percent of the solids, the
amount of solids which had to be removed by the filters increased dramatically. This was
because the plant had effectively become saturated with fine solids, resulting in a
temporarily higher than normal sludge removal rate at the filters and subsequently higher
chemical and operating costs than normal. After prolonged use of the polymer in
elutriation, a new plant equilibrium was established and the removal rate and conditioner
demand decreased. Table 10-1 shows the effect of chemical addition to the elutriation
process on solids capture and chemical requirements for good vacuum filtration of sludge.
TABLE 10-1
DISTRICT OF COLUMBIA'S
SLUDGE REMOVAL PRACTICES AND COSTS
Tons/Day Chemical Cost ($/ton)
Removed Elutriation Filtration
Elutriate to River 45 - 13.50
Post Elutriate Recycle Period 80 4.68 7.42
(Polymer in Elutriation)
After Prolonged Polymer Use 70 Total = 9.75
in Elutriation
10-4
-------
Figure 10-3 shows the excellent dewatering operation at the Washington, D.C. plant. The
filter yield obtained with the new mode of sludge processing at D.C. is over 4 lb/hr/ft2,
which is 33 percent above the design rate. Previous mention was made of a poor quality
thickener overflow stream. The use of flocculation was limitedly investigated on a
laboratory scale, and it was determined by a cost/benefit analysis that the use of
flocculants in the thickener was not advisable under prevailing operating conditions.
During the current plant expansion, separate conditioning boxes and feed systems were
provided for each filter. This was not originally the case but is essential for smooth
operation of the vacuum filters. Experience gained at many plants has indicated that
without separate conditioning and feed systems, all filters do not receive optimumly
conditioned sludge.
An interesting facet of the Blue Plains plant is the fact that flash dryer type incinerators
were installed at the time of expansion to secondary treatment. These incinerators have,
however, remained inoperative since installation and filter cake has been disposed of on
land. Land disposal in this instance was much less expensive than incineration and ash
disposal. Presently the Blue Plains plant is again being expanded in capacity and upgraded
to meet Potomac Estuary Standards for phosphorus and nitrogen removal. Since
additional sludge will be created by the new processes, present disposal methods will
require reevaluation. Figure 10-4 shows the essential features of a proposed sludge
processing system. Since incineration may be practiced, digestion has been eliminated.
Separate gravity thickening of primary sludge and flotation thickening of excess activated
sludge have been provided, and the thickened sludges would be combined and dewatered
just prior to incineration.
10.3 St. Helens-United Kingdom
The solids handling system at the St. Helens plant is shown in Figure 10-5. Excess
activated sludge is recirculated to the primary basin where it is mixed with primary
sludge. The combined primary and activated sludges are then anaerobically digested,
elutriated and vacuum filtered. During the period 1965 to 1968, the amount of excess
activated sludge to be handled by the plant increased significantly. This increase in the
quantity of EAS was attributed to an increasing degree of treatment, low retention time
in the aeration system, and recycling of a high solids elutriate stream. Since the sludge
going to the vacuum filter contained a very large amount of EAS, the filter yield
deteriorated greatly [2,3]. The four-year trend in the solids content of the elutriated
digested sludge fed to the filters is shown in Table 10-2.
Figure 10-6 shows that the variation in filter yield coincided with the variation in the
solids content of the elutriated sludge feed to the filters. Not unexpectedly, as the sludge
solids increased, so did the filter yield. Figure 10-7 further shows that the solids content
of the elutriated sludge fed to the filters varied with quantity of excess activated sludge
produced. As the amount of excess activated sludge increased, the degree of solids
concentration achieved in elutriation decreased.
10-5
-------
FIGURE 10-3. Vacuum filter operation at District of Columbia.
-------
PRIMARY
BASINS
THICKENER
EFFLUENT
AERATION
BASINS
SLUDGE
BLEND
TANK
SLUDGE
CONDITIONER
FINAL
CLARIFIERS
FLOTATION
THICKENER
25—30%
FILTER
CAKE
FIGURE 10-4. New Blue Plains sludge processing system.
-------
GRIT
REMOVAL
f '
i
1
L.
PRIMARY
CLARIFIERS
t
i
AERATION
BASINS
FINAL
CLARIFIERS
i
t
oo
ANAEROBIC
DIGESTION
Fl 1 ITPtATIOKI
CLU 1 KIM i I wIN
VACUUM
FILTERS
WASTE WATER
SLUDGE
PROCESS LIQUIDS
FIGURE 10-5. Solids handling at the Parr Works, St. Helens.
-------
6-01
FILTER YIELD (LB/HR/FT2)
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-------
TABLE 10-2
VARIATION OF PERCENT SOLIDS IN
ELUTRIATED SLUDGE AT ST. HELENS
1965 1966 1967 1968
7.16 5.68 4.08 2.08
The low aeration detention time at St. Helens is partially responsible for a greater
production of excess activated sludge. Table 10-3 compares the St. Helens plant with the
Hogsmill Valley plant [4]. The latter plant has a considerably longer detention time and
permits some endogenous respiration to occur and partially destroy the EAS solids.
Composition of the influent wastes, particularly their inert content can also affect these
figures.
TABLE 10-3
EFFECT OF AERATION ON
EXCESS ACTIVATED SLUDGE PRODUCTION
Aeration Retention EAS Ib/lb BOD
Plant Time (hr) Removed
St. Helens 4.68 1.44
Hogsmill Valley 9.9 0.74
Some success in improving filter yield has been obtained by using intermittent storage of
sludge in a lagoon to facilitate smooth operation of the elutriation system. In addition to
use of the lagoon to obtain a thicker sludge, procedural changes and improvements in
sludge conditioning equipment and dosage control have resulted in a cleaner elutriate and
a somewhat higher filter yield. Attempts to demonstrate effectiveness of polymer
flocculation in the elutriation system at St. Helens have been unsuccessful to date [2,3].
10- 11
-------
10.4 Metropolitan Toronto Main Plant
The Metro Toronto main plant is a step aeration activated sludge plant featuring
two-stage anaerobic digestion of the mixed primary and activated sludge, elutriation,
vacuum filtration with coil filters, and multiple hearth incineration. The plant's flow
diagram is shown in Figure 10-8. Excess activated sludge is recirculated to the head of the
plant. Considerable recirculation options exist for excess activated sludge, digester
supernatant, elutriate, and filtrate, however, which are not accurately reflected in the
figure.
Both the hydraulic and organic loading on the plant increased in the period 1967 to
1970, and the plant was expanded in the interim from 1969 to 1971 by installing
additional aeration basins and final clarifiers [5]. As might have been expected, the
additional pollutant load on the plant and the expansion of the activated sludge process
increased the quantity of sludge to be processed. Since there was now a much larger
quantity of EAS to be handled, as was the situation at Washington, D.C. and St. Helens, a
gradual decrease occurred in the solids concentration of the elutriated digested mixed
sludge fed to the filters. This is shown in Figure 10-9. The situation did not become
critical until 1970, when the average monthly solids concentration for the elutriated
sludge was consistently below 4 percent. The filters were then unable to remove the
sludge solids from the system by their usual mode of operation. During August 1970, the
solids content of the elutriated sludge being fed to the filters even dropped to less than 3
percent and was nonuniform. The nonuniformity and low solids caused low filter yields
and wet filter cake. Figure 10-10 clearly shows the long-term trend for the solids
concentration of the raw sludge, which was fed to the digesters. The solids concentration
fell from a level of 6 percent to 4 percent and below during 1970. This trend is also a
reflection of the increasing amounts of excess activated sludge being handled at this time.
To maintain reasonable plant operation, solids processing data indicated the four
Komline-Sanderson coil filters should process at least 3.7 Ib D.S./hr/ft2. Further, in 1971
the filters would need to process at least 4.4 Ib D.S./hr/ft2. This information is shown in
Table 10-4.
TABLE 10-4
METRO TORONTO'S SLUDGE REMOVAL NEEDS
Dry tons/mo
Filter Productivity (lb/hr/ft2)
Operable
1970
2,000
3.0
Preferred
1970
2,500
3.7
Required
1971
3,000
4.4
10- 12
-------
ri./M^ i
INFLUENT
f
I
1
L
GRIT
REMOVAL
1
PRIMARY
CLARIFICATION
I
1
\
1
1
ACTIVATED
SLUDGE
•w
FINAL
CLARIFIERS
1
jr
TLAINI
EFFLUENT
*
PRIMARY
DIGESTION
•• ^^
u
A '
1 SUPERNATANT .ELUTRIATE
SECONDARY
DIGESTION
- >
2 STAGE
ELUTRIATION
•^
^FILTRATE
VACUUM
FILTRATION
TO
INCINERAT<
WASTE WATER
SLUDGES
PROCESS LIQUIDS
FIGURE 10-8. Metro Toronto's plant flow diagram.
-------
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-------
At this juncture plant personnel decided to make a cost benefit determination for the use
of small quantities of polymer in the elutriation tanks. During October and November
1970, between one and two pounds of polymer were added to the elutriation basins per
ton of dry solids feed. Results are shown in Table 10-5. Concurrently, small and
decreasing amounts of ferric chloride were used with the polymer at the vacuum filters.
Immediate and dramatic improvements were noted in the concentration and uniformity
of the elutriated sludge solids concentration, which resulted in smooth filter operation
and yields in excess of 4.3 lb/hr/ft2. Total solids capture in the elutriation and filtration
processes was controlled at the 90 percent level.
TABLE 10-5
ELUTRIATION/FILTRATION RESULTS FOR
OCTOBER/NOVEMBER AT METRO TORONTO
1970 Period
October
November
Polymer
Elut.
1.26
1.75
Used (Ib/ton)
Filt.
7.77
8.20
Sludge Solids
(%)
3.6
4.1
Yield
(lb/hr/ft2)
4.7
4.3
Cake Solids
(%)
16
16
Since the polymer contract year ended during January 1971, a number of polymers and
various modes of elutriation system operation were evaluated. Some results are shown in
Table 10-6. It was possible to vary the solids concentration of the elutriated sludge by
varying the loading, wash water ratio, and polymer dosage to the elutriation basins. No
ferric chloride was necessary to prepare sludge for filtration. As the results in Table 10-6
show, filter yields of 4.7 to 5.8 lb/hr/ft2 were obtained at different loadings, wash water
ratios, and polymer dosages. In general the uniformity and ease of operation of the
filtration process were dramatically improved with the new mode of operation. Figure
10-11 shows the excellent cake discharge obtained and the thickness of the cake.
In effect, the sludge processing improvements just described cleared up one of the two
major recirculation streams inherent in the Toronto design. It did not, however, eliminate
the large recirculation of solids and soluble BOD from the two-stage digestion system.
This recirculation load has a deleterious effect on the amount of excess activated sludge
produced and, hence, the relative quality of mixed sludge fed to the elutriation, filtration,
and incineration systems. It is possible that separate flotation thickening and aerobic
10- 16
-------
TABLE 10-6
ELUTRIATION AND FILTRATION RESULTS
DURING 1971 AT METRO TORONTO
Vacuum Filter Performance
Elut. Flow Polymer Elutriate Solids Yield
(mgd) (Ib/ton) S.S. (mg/1) (%) (lb/hr/ft2)
Wash
Water
1.0
3.5
Digested
Sludge Elut. Filt. 1st 2nd
0.6 1.94 10.96 120 18 6.1 4.7
1.4 0.62 9.34 6,250 208 3.5 5.8
Concentration
(%)
16.0
15.4
digestion of the activated sludge, followed by mixing of aerobically digested activated
sludge with either raw or anaerobically digested primary sludge would materially improve
plant costs. This proposed process improvement would provide a readily controllable
method of thickening the excess activated sludge and reduce the amount of excess
activated sludge to be processed. It also would keep the activated sludge out of the
anaerobic digestion system which operates more efficiently with raw primary sludge and
produces a better quality supernatant. Other benefits of the process improvement would
be a reduced chemical demand for the sludge feed to the vacuum filters and an improved
cake solids concentration in the feed to the incinerators.
10.5 Richmond, California
From 1967 to 1969 the city of Richmond, California, modified its wastewater treatment
plant to provide secondary treatment by the activated sludge process. Figure 10-12 shows
the plant flow diagram. The Richmond plant has a maximum hydraulic capacity of 40
mgd with an average daily flow of 9 mgd for the period of 1969 to 1970. Flotation
thickeners were included in the plant's design to thicken EAS. Digester supernatant and
filtrate are recycled to the elutriation system. The mixed digested elutriated sludge is
vacuum filtered and either incinerated or landfilled [6]. It was speculated during design
that the inclusion of flotation thickening of the activated sludge was a positive step, but
that mixing of the primary and secondary sludges early in the solids handling process
might give problems in subsequent solid liquid separation processes such as elutriation
and dewatering. Shortly after the activated sludge system was put into operation, the
same problems that occurred at other plants occurred at Richmond. Solids accumulated
10- 17
-------
o
I
00
FIGURE 10-11. A view of filters at Metro Toronto.
-------
PLANT
INFLUENT
GRIT
REMOVAL
f
1
PRIMARY
CLARIFIERS
1
1
f
1
1 _
AERATION
BASINS
FINAL
CLARIFIERS
PLANT
EFFLUENT
Jf
DIGESTION
I
1
SECONDARY
- ~ ^DIGESTION
'\
\(
ELUTRIATION
-»
VACUUM
FILTERS
WASTE WATER
u i
SLUDGE
PROCESS LIQUIDS
FIGURE 10-12. Richmond, California's plant flow diagram.
-------
in the plant because of poor capture and compaction in elutriation, and low vacuum filter
yields and high operating costs were experienced. After much process improvement work,
plant personnel adopted regular use of a cationic high molecular weight polymer in the
elutriation basins to alleviate their solids Iiandling problems. The cost and efficiency of
the dewatering operation was dramatically improved by the use of the polymer in
elutriation and this is shown by the data in Table 10-7. Additional data on the filtration
and elutriation operations are presented in Table 10-8. Excellent capture and compaction
were achieved in elutriation. About 70 percent of the conditioning chemical cost for
sludge processing was due to the ferric chloride and lime dosages required for good
vacuum filtration.
TABLE 10-7
VACUUM FILTRATION RESULTS FOR RICHMOND, CALIFORNIA
Yield (lb/hr/ft2)
Conditioner Cost ($/ton)
Cake Solids (%)
Primary
Sludge
7-9
3.80-4.00
29-31
Mixed
No
Polymer
1-2
25-30
16-18
Sludges
Polymer in
Elutriation
5-7
11-14
20-22
TABLE 10-8
RICHMOND, CALIFORNIA-ELUTRIATION AND
FILTRATION OPERATIONS' DATA
Process
Elutriation
Filtration
Digest Sludge
Solids Cone.
(%)
3.85
FeCl3 Dosage
($/ton)
3.00
Elutriate Sludge
Solids Cone.
(%)
7.8
Lime Dosage
($/ton)
4.85
Quantity of
Polymer Added
(Ib/ton)
2.12
Filter Cake Solids
Cone. (%)
20.8
Elutriate
Solids Cone
(mg/1)
450
-
10-20
-------
Still further economies might be attained if the dewatering equipment at Richmond had
an adequate sludge cake release mechanism. The belt filters now used have essentially no
release mechanism, and this necessitates overconditioning in terms of ferric chloride and
lime dosage to obtain cake discharge. This situation is clearly shown in Figure 10-13,
where the drying cracks in the cake on the belt are evident. The cracked cake appears a
few inches above the point where the drum comes out of the vat, and this is an abnormal
condition. Since these conditions were reported, Richmond personnel have partially
alleviated the release problem by use of a polypropylene type filter cloth. They have also
eliminated the use of lime.
10.6 Fairfax County, Virginia-Westgate Plant
Robson et al. [7] have detailed the results of conversion of an overloaded intermediate
treatment level plant into an oxygen activated sludge plant which removes 90 percent of
the BODS in the incoming wastewater. Considerable data on the processing of oxygen
activated sludges have been generated at this plant. As depicted in Figure 10-14, the
wastewater was originally treated at the Westgate plant by comminution; sedimentation,
aeration of the mixed liquor, and secondary clarification in one long baffled tank; and
chlorination. The waste sludge was digested, vacuum filtered, and landfilled.
It is not usual practice to have the three unit processes of sedimentation, aeration, and
clarification in one tank. A longitudinal sectional view of this tank appears in Figure
10-15. This plant was originally designed to provide an intermediate level of treatment for
a flow of 8.0 mgd. By 1970 the plant was significantly overloaded, and for a period of
time chemical precipitation was practiced to upgrade plant effluent quality. The
performance of the Westgate plant over approximately a 20-year period is summarized in
Table 10-9.
TABLE 10-9
WESTGATE PLANT PERFORMANCE
Period
1954
1970
1971
1971-72
Design Flow (mgd)
8
12
12
12
% Removal of BODS
50+
35-40
75+
80-90
Plant Process
Original
Original
Chemical Ppt.
Oxygen
Activated Sludge
10-21
-------
FIGURE 10-13. Belt filters at Richmond, California.
10-22
-------
K>
OJ
PLANT
INFLUENT
COMMINUTION
LANDFILL
PRIMARY SEDIMENTATION
AERATION
CLARIFICATION
—»CHLORINATION
VACUUM
ILTRATION
PLANT
EFFLUENT
FIGURE 10-14. Original process flow diagram for Westgate plant.
-------
to
COMMINUTION
PRIMARY
CLARIFICATION AERATION
SECONDARY CLARIFICATION
SUMP
-AIR DIFFUSERS
/
SCRAPERS
FIGURE 10-15. Westgate sedimentation tank.
-------
The interim chemical treatment during 1971 was successful in upgrading the plant at a
relatively high treatment cost. Although present plans are to phase out the Westgate plant
within a few years, an oxygen injection system and flotation units were installed during
1971 to provide better wastewater treatment and adequate sludge processing facilities.
The current Westgate plant flow diagram is depicted in Figure 10-16. Because of the
temporary nature of the plant, no oxygen generation equipment was included. Rather,
the plant depends on bulk oxygen supplied by a tank truck. The new wastewater
treatment process has been highly successful. BOD and suspended solids removals have
exceeded goals. Stable operation of the plant has become routine, and the oxygen cost
has been a little lower than predicted. A summary of the actual results achieved appears
in Table 10-10.
TABLE 10-10
WESTGATE OXYGEN PROCESS RESULTS
%Removal of
BOD,
TSS
SVI
EAS Production
IbVSS
Ib BOD Removed
Zone
Settling Velocity
(ft/hr)
93+
90+
35-56
0.33
6.0
In addition to good wastewater treatment, it can be seen that the mixed liquor had good
settling properties as illustrated by the SVI level and zone settling velocity. The amount
of excess activated sludge produced was low at 0.33 Ib/lb of BOD removed. Although the
dissolved air flotation units functioned, it was learned during plant tests that the oxygen
activated sludge settled well. This permitted bypassing the flotation units and merely
gravity thickening a combination of the primary and excess activated sludges. Results at
Westgate for gravity thickening of the combined sludges and vacuum filtration of the
thickened sludge are shown in Table 10-11.
A small dose of flocculant is seen to be sufficient for production of a clear thickener
supernatant and thickened sludge solids concentration of 6 to 8 percent. The relatively
high solids concentration of the thickened sludge going to the vacuum filters results in
good production rates and a resultant cake solids concentration of 22 to 28 percent. Lime
and ferric chloride were used for chemical conditioning, and costs were only about
$5/ton dry solids. Sludge cake is hauled to a landfill for disposal. A key point in the
success of this gravity thickening process for the mixed sludge was the installation of
mixers on the sludge decant tank to provide adequate blending. Figure 10-17 shows the
proximity of the plant to residential areas, which shows the need for adequate odor
control.
10-25
-------
PLANT
INFLUENT
COMMINUTION
PRIMARY SEDIMENTATION
DUAL OXYGEN
ACTIVATED SLUDGE
BASINS
to
ON
SECONDARY
CLARIFIERS
(2)
Li
FILTER
CAKE
VACUUM
FILTERS
SLUDGE
DECANT
CHLORINATION
PLANT
EFFLUENT
D.A.F.
UNITS
FIGURE 10-16. Current Westgate plant flow diagram.
-------
-------
TABLE 10-11
RESULTS FOR THICKENING AND VACUUM FILTRATION
OF WESTGATE PROCESS SLUDGE
Method
Thickening
Polymer
Dosage
(Ib/ton)
Thickened
Sludge Solids
Concentration
(%)
Vacuum Filter Performance
Cake Solids
Yield Concentration
(lb/hr/ft2) (%)
Gravity 3 6-8 4.0-5.0 22-28
10.7 Metropolitan Denver Sewage Disposal District No. 1
10.7.1 General Considerations
The Metro Denver plant, which commenced operation in 1966, treats wastes from the
suburban Denver area as well as both settled wastewater and anaerobically digested sludge
from the city of Denver's Northside plant [8]. The plant's flow diagram is shown in
Figure 10-18. The Metro plant employs primary clarification, diffused air activated
sludge, and final clarification. Notable factors in plant operation are the flows from the
Denver Northside plant of settled wastewater into the aeration system and digested sludge
into the sludge processing system. Nominal designed plant capacities are shown in Table
10-12. It should be particularly noted that the ratio of the raw and digested primary
sludges to the excess activated sludge was forecast to be 0.99.
10.7.2 Sludge Processing System
The original plant design envisioned sludge processing to include dissolved air flotation
thickening of the EAS; followed by dewatering of a mixture of the DAF thickened EAS,
digested primary sludge, and raw primary sludge via rotary vacuum filters; and flash
drying of the sludge cake prior to its use as a soil conditioner.
10.7.3 Plant Loadings Experienced
The original design had contemplated a slow rise in the process flows needing primary and
secondary treatment. However, the flow magnitudes grew very rapidly, and secondary
capacity was reached in 1970. Primary capacity was reached in 1972. The growth of the
10-28
-------
o
to
SETTLED SEWAGE
FROM DENVER NORTHSIDE PLANT
METRO
RAW FLOW
DIGESTED
PRIMARY
CLARIFIERS
SLUDGE
FROM DENVER
NORTHSIDE PLANT
SLUDGE
HOLD
TANKS
i
RE
i
C
1
rCLE
t
AERATION
SYSTEM
i
DAF
THICKENERS
REO
i
rCLE
FINAL
CLARIFIERS
ROTARY
VACUUM
FILTERS
j
EFFLUENT
INCINERATOR
-<
LAND DISPOSAL
FIGURE 10-18. Metro Denver system's flow diagram.
-------
TABLE 10-12
METRO DENVER PLANT CAPACITIES
(Estimated 1970 Population of District: 870,000)
Processes
Primary Treatment
Secondary Treatment
Average Flow Maximum Flow
(mgd) (mgd)
28 50
98 234
BOD
(Ib/day)
166,350
Sludge Processing
Raw Primary Digested Primary
(Ib/day) (lb/day)
Secondary
(lb/day)
Metro Plant
Denver Northside
37,400
92,700
131,000
quantity of wastewater requiring primary treatment and the quantity of primary effluent
requiring secondary treatment over about a nine-year period is shown in Figure 10-19.
A development which had a pronounced effect on the sludge processing operation was
that only about one-third of the anticipated Denver Northside digested primary sludge
solids resulted. This is reflected in the figures below which project 1975 loadings from the
plant's operating history.
Total dry
sludge solids
(Ib/yr), 1975
Denver
Northside Digested
31,000
Metro
Primary
33,000
Metro
Excess Activated
132,000
Total
196,000
10-30
-------
ANNUAL FLOW TREATMENT
52,105
DESIGN CAPACITY;
"
1967 1968 1969 1970 1971
PRIMARY FLOW TREATMENT
1972 1973 1974 1975
SECONDARY FLOW TREATMENT
FIGURE 10-19. Changes in Metro Denver's annual plant flows from 1967 to 1975 [9].
-------
These figures show that the ratio of raw and digested primary to excess activated sludge
will be 0.485 in 1975 instead of a design consideration of a ratio of 0.99.
10.7.4 Sludge Processing Results-1967 to 1970
Considerable difficulty was encountered in sludge processing. Consistency of the sludge
going to the vacuum filters was hampered by difficulty in blending the sludges in the hold
tanks [9]. The higher than expected EAS to raw and digested primary sludge ratio
caused severe problems with the dewatering and drying or incineration operations. Design
capacities were generally not achieved.
Unit process costs for Metro Denver are shown in Figure 10-20 and were higher than
anticipated. The cost picture worsened as the quantity of EAS increased. Also the
chemical conditioning costs doubled after sludge was subjected to pipeline transport from
one plant to another.
Compilation of data over several years resulted in the correlation shown in Figure 10-21.
The high costs and difficulty encountered in attempting to process the ever increasing
amounts of sludge produced led the Metro Denver staff to test and use alternate
processing systems. To alleviate the sludge disposal problem, land disposal of dewatered
filter cake was practiced in place of drying or incineration.
10.7.5 Modified Denver System and Results
Various operational refinements of a general nature contributed to improvements in
operability and costs. In 1970 8.0 mgd of aeration capacity was adapted for aerobic
digestion of the excess activated sludge. This modification proved to be a major
improvement by diminishing the amount of EAS to be processed. Figure 10-22 shows the
flow diagram of the modified Metro Denver system. The reduction in sludge processing
costs from $60/ton to $40/ton for 1971 to 1972 was directly attributed to the partial
destruction of the organic portion of the activated sludge by aerobic digestion [9]. The
total Metro Denver treatment cost delineation is shown in Figure 10-23. From 1970 to
1973, the aerobic digestion process has saved Metro Denver in excess of $500,000.
10.8 References
1. Dahl, B. W., Zelinski, J. W., and Taylor, O. W., "Polymer Aids Dewatering and
Eliminates Solids Loss in Elutriation." Presented at the 43rd Annual WPCF
Conference, Boston, Massachusetts, Oct. 6, 1970.
2. Ashman, P. S., "Operating Experiences of Vacuum Filtration at St. Helens,"
Water Pollut. Contr. (1969), pp. 20-39.
10-32
-------
TOTALS
1968 1969
1970
$118.67**
1.33
48.70;
JJ+6.70J
255.40i
[~~| TRANSMISSION
Ijmi PRIMARY
UUll TREATMENT
mm SECONDARY
MM TREATMENT
I SLUDGE
PROCESSING
MANAGEMENT
AND SUPPORT
DEBT SERVICE
BOTHER FUNDS
, IMPLIES DEFICIT
-' FINANCING
*(1969) **(1970)
119.70 126.42
-6.69 -7.75
113.01 118.67
E.A.S.
LEGEND
PRIMARY RAW
PRIMARY
ANAEROB-
DIGEST
FIGURE 10-20. Unit costs of Metro wastewater treatment [9].
10-33
-------
100
90-
80 -J
~ 70-
IU , _
O 60~
co
50-
40-
30-
20-
10-
10 20 30 40 50 60 70
SLUDGE PROCESSING COST ($/MG)
FIGURE 10-21. Sludge processing costs vs. EAS/total sludge produced
(1968-1972) [9].
10-34
-------
o
I
OJ
SETTLED SEWAGE
DENVER NORTHSIDE PLANT
METRO
RAW FLOW
PRIMARY
CLARIFIERS
DIGESTED SLUDGE
DENVER
NORTHSIDE PLANT
SLUDGE
HOLD
TANKS
AERATION
SYSTEM
RECYCLE
LJ
i
AEROBIC
DIGESTION
RECYCLE
DAF
THICKENERS
FINAL
CLARIFIERS
EFFLUENT
RECYCLE
INCINERATOR
ROTARY
VACUUM
FILTERS
LAND DISPOSAL
FIGURE 10-22. Modified Metro Denver system.
-------
TOTALS
1970 1971 1972
$18.67** $124.74 $127.65
co
Z
o
o
z
o
O
a
TRANSMISSION
I PRIMARY
TREATMENT
SECONDARY
TREATMENT
I SLUDGE
I PROCESSING
I MANAGEMENT
I AND SUPPORT
I DEBT SERVICE
i OTHER FUNDS
, . IMPLIES DEFICIT
- FINANCING
*(1969) **(1970)
119.70 126.42
-6.69 -7.75
113.01 118.69
E.A.S.
LEGEND
PRIMARY RAW
PRIMARY
ANAEROBIC
DIGESTED
FIGURE 10-23. Unit costs of Metro wastewater treatment from 1970
to 1972 [9].
10-36
-------
3. Ashman, P. S. and Roberts, P. F., "Operating Experiences with Vacuum
Filtration at St. Helens: A Solution to the Problem," Water Pollut. Contr.
(1970), pp. 638-648.
4. Stanbridge, H. H., "Operation and Performance of the Hogsmill Valley Sewage
Treatment Works of the Greater London Council, 1958-1966." Water Pollut.
Contr., 67(1), 21 (1968).
5. Private communications with David A. Clough, Director of Metro Water
Pollution Control; Earl Baldock, Assistant Director of Water Pollution Control;
Wadid Salib, Plant Engineer, Main Plant, 1971.
6. Private communications with E. L. MacDonald, Jr., Superintendent and William
Kennedy, Plant Supervisor, City of Richmand, California, 1971.
7. Robson, C. M., Block, C. S., Nickerson, G. L., and Klinger, R. C., "Operational
Experience of a Commercial Oxygen Activated Sludge Plant." Presented at
WPCF, Atlanta, Georgia, Oct. 1972.
8. Annual Report for Metro Denver (1972).
9. Cohen, D. B. and Puntenney, J. L., "Metro Denver Experience with Large Scale
Aerobic Digestion of Waste Activated Sludge." Presented at the 46th Annual
Conference of the WPCF, Cleveland, Ohio, Oct. 4, 1973.
10-37
-------
CHAPTER 11
CASE HISTORIES OF SLUDGE TREATMENT
BY HIGH TEMPERATURE AND PRESSURE
11.1 Heat Treatment
Heat treatment processes have been used for several years in Europe and the United
States. The primary objective of heat treatment is to improve a sludge's dewaterability,
while a secondary objective is to stabilize the sludge. The available proprietary systems
include the Porteous, Zimpro LPO, and Fairer processes.
Porteous Process
The schematic diagram for a typical Porteous process is given in Figure 11-1. Heat is
applied for elevating the sludge temperature from 350° to 400° F, and the pressure is
raised to 150 to 300 psig. Steam is generally injected into the sludge, and this is followed
by a sludge/water/sludge heat exchange system as shown in the diagram. Air injection is
not normally practiced. Basic components of this system include sludge storage, grinding,
a preheater, high pressure and temperature reactor, decanter thickener, auxiliary liquid
treatment, off gas deodorizer, and a steam boiler. Colorado Springs, Colorado, is the only
U.S. installation of this process that has been operating for any significant length of time.
Two other locations commenced operation recently, and several more are planned. The
process has evolved from a batch system to a continuous system.
Zimpro LPO Process
Figure 11-2 shows a schematic for atypical Zimpro LPO system. The principal
differences between the Zimpro LPO process and the Porteous process are that air is
added to improve heat exchange characteristics and fuel consumption, and a sludge to
sludge heat exchanger is employed. About 26 United States installations of this system
are now in existence, and most of these have been constructed within the past several
years. Several more Zimpro LPO installations are under construction or planned.
Farrer Heat Treatment System
A schematic of the Farrer system is given in Figure 11-3. The Farrer system is similar to
the continuous Porteous type process. At this time, there are few systems in operation in
the United States.
11.2 Process Considerations
It is only within the past several years that significant United States operating and cost
data on heat treatment processes have become available. Further, results from the United
11 -1
-------
*L 01
ARRIVAL OF SLUDGE FOR
TREATMENT
I "\ RESIDUAL
I ./ LIQUORS
MULTIPLE DRIED
HEARTH SLUDGE
INCINERATION
KEY
— Cold raw sludge
• — ——— Hot raw sludge
• —-—-s— Hot treated sludge
Cold treated sludge
— Thickened sludge
• -- Steom
1. Raw sludge storage
2. Sludge disintegration
3. Ram pump
4. Heat exchanger
5. Reaction vessel
6. Automatic discharge valve
7. Decanter
8. Pump
9. Vacuum filter
10. Boiler for process steam
FIGURE 11-1. Porteous process.
11-2
-------
Sludge
GRINDER
V
GROUND
SLUDGE
HOLDING
TANK
HEAT
EXCHANGER
PUMP POSITIVE
DISPLACEMENT
SLUDGE PUMP
^
AIR COMPRESSOR
REACTOR
Exhaust Gas
TO INCINERATOR
OXIDIZED
SLUDGE
TANK
PRESSURE
CONTROL
VALVE
VAPOR
COMBUSTION
UNIT
Treated
Boiler
Water
FILTER
PUMP
BOILER
FIGURE 11-2. Zimpro LPO system.
-------
REACTOR
SECOND HEAT
EXCHANGER
PRE-H EATER
d
THICKENER
CONTROL
PANEL
BOILER
CIRCULATING
PUMP
LEVELING
VESSEL
AIR
COMPRESSOR
-K*H£d '
AUTOMATIC
VALVES (ONE
BACK-UP)
DECANTING
AND STORAGE
TANK
CENTRIFUGE
f
TO INCINERATION SOIL LANDFILL
CONDITIONING
GRINDER PUMP
FIGURE 11-3. Farrer process system.
-------
Kingdom are now in the technical journals. Difficulties with plants in the United
Kingdom are generally attributed to the problems of maintaining such items of
equipment as high pressure pumps, compressors, and high temperature and pressure
reaction systems. Another difficulty has been in providing adequate treatment for the
heat treated sludge cooking liquor. Plant difficulties in the United Kingdom were in some
cases attributed to the installation of systems at older plants. A new plant with
interrelated liquid and sludge heat treatment facilities might not have had troubles.
However, some of the plants that have ceased operation were specifically designed with
new liquid treatment facilities which could accommodate the heat treatment system
recirculation loads. The principal cause of process cost and effluent quality problems
appears to be a much higher degree of sludge solubilization with heat treatment than was
predicted. Available information indicates high costs of operation, maintenance, and
effluent quality problems are associated with heat treatment systems. Several U.S. plants
have ceased operating heat treatment systems due to these problems, including
Coors-Golden, Colorado; Santee, California; and Chattanooga, Tennessee. Several
additional installations of heat treatment are in the planning, design, and construction
phase.
11.3 Coors-Golden, Colorado
The Coors-Golden plant is an activated sludge system with a capacity of 5 mgd. The plant
treated a combination of domestic and brewery wastes and installed a Porteous type heat
treatment system in 1970. Coors Brewery utilizes its industrial engineering and treatment
plant personnel to improve process selection and efficiencies. The heat treatment system
was discontinued after about one year of operation due to the very high cooking liquor
recirculation load as well as corrosion and other operating and cost problems. The
cooking liquor from the sludge heat treatment system sometimes had a solids content as
high as 20,000 mg/1. Even after heat conditioning the sludge fed to the belt type vacuum
filters still required as much as 3.8 percent ferric chloride conditioning for good
dewaterability. A burnt coffee odor persisted around the entire plant during heat
treatment.
11.4 Colorado Springs, Colorado
Until the fall of 1973, Colorado Springs had a trickling filter plant, which removed 66
percent of the influsnt BOD. Plant flows were averaging 21 to 25 mgd with the actual
design capacity being 18 mgd. The influent wastewater contained some industrial waste
and had a BOD of about 300 mg/1 and a suspended solids concentration of about 345
mg/1. A Porteous unit was installed during 1968 to 1969, and it was designed to treat
2,000 Ib/hr of sludge at an operating temperature of 370° F and a pressure of 250 psi.
Figure 11-4 gives the flow diagram of the process.
11 -5
-------
PRIMARY
CLARIFIER
HIGH RATE
TRICKLING
FILTER
SECONDARY
CLARIFIER
RECYCLE
CTs
1
2 STAGE
ANAEROBIC
DIGESTION
J
PORTEOUS
PROCESS
DECANT
TANKS
VACUUM
FILTERS
TO LAND
DISPOSAL
FIGURE 11-4. Flow diagram of Colorado Springs with heat treatment.
-------
Sherwood and Phillips [1] reported vacuum filtration rates of 12 lb/hr/ft2 and
concentrations of 37 percent after heat treatment. No chemical conditioning was
required. While prior to installation of heat treatment, chemical costs were $18 to $20
per ton for ferric chloride and lime. Filtrate and decant streams were handled with no
additional aeration equipment required. No material balance data were given. Periodic
visits to the plant at the times for which the cost data were reported revealed that the
operating personnel were having problems with the recycle load from heat treatment and
were receiving odor complaints. The recycle load was greater than expected even though
this was a primary and trickling filter process plant sludge rather than an activated sludge.
Plant process work was carried out to reduce the recycle load and this included lime
precipitation of the liquor. Kochera [2] published data on an additional year of
operation. He stated that the chemical conditioning costs prior to heat treatment had
averaged from $20 to $40 per ton, while the operating costs for the Porteous system were
averaging $15 a ton (fuel, power, labor, and water). It was not clear whether the
maintenance costs were included in these figures, and apparently no allowance was made
for any costs in treating the recirculation load. Recent data [3] indicate that the average
Porteous system, vacuum filtration, and land disposal costs for 1972 were $30/ton of dry
sludge solids. The Porteous system alone was approximately $22 per ton and did not
include recirculation liquor treatment costs. Vacuum filtration costs, therefore, were
about $5/ton, and about $2.50 to $3.00/ton were required for land disposal. New
activated sludge and Porteous systems were started up in late 1973 at Colorado Springs.
Little data are available on their operation. Table 11-1 summarizes the operating costs for
the Porteous system and vacuum filter at Colorado Springs.
TABLE 11-1
COLORADO SPRINGS SLUDGE PROCESSING COST
Cost Cited $/ton Reference Source
Operating— Porteous/V.F.
Operating— Porteous/V.F.
Operating/Maintenance— Porteous/V.F. /Land
2
15
30
[1]
[2]
[3]
11.5 Borough of Pudsey—United Kingdom
The Pudsey plant installed a Fairer process in the 1969 to 1970 period for treatment of a
sludge with about 82 percent trickling filter humus and 18 percent activated sludge [4].
The sludge handling system at Pudsey is shown in Figure 11-5. The heat treatment
11 -7
-------
SLUDGE
SLUDGE
STORAGE
HEAT
TREATMENT
DECANT
TANK
I
oo
PRESS
FILTERS
t
SECONDARY
MEDIA
FILTER
CAKE
TO FARMS
1
PLASTIC
BIO. TREAT.
I
HUMUS
SLUDGE
FIGURE 11-5. Pudsey sludge system [4].
-------
process included provision of a decant tank, press filters to dewater the heat treated
sludge, and disposal of the dewatered sludge cake to land. A separate treatment system
was used for the cooking liquor and involved a plastic media trickling filter and a
secondary media filter. Successful operation of this system at the Pudsey plant was not
possible in IVz years, and severe operating and maintenance problems were encountered.
Sufficient experience was gained to make it possible to estimate costs. The estimated cost
for heat treatment and dewatering was $37.20/ton of dry solids, of which $22.32 was for
operation and maintenance and $14.88 was for capital. The cost of treating the recycled
liquors from heat treatment by plastic trickling filter and obtaining a 50 percent BOD
reduction was estimated to be about $5/ton. Thus the total cost of heat treatment
exclusive of filter cake disposal was $42.20/ton of dry solids.
In general, Great Britain has found heat treatment extremely effective in improving
filtration characteristics. However, it recognizes that approximately half of the solids go
into solution and need to be treated. If treatment is accomplished by a biological method,
more sludge is created [ 5 ].
11.6 Kalamazoo, Michigan
A Zimpro LPO unit is in operation at Kalamazoo, Michigan. The wastes entering the plant
are a combination of domestic, paper mill, and pharmaceutical wastes. The influent to the
Kalamazoo plant, however, is essentially made up of 18 mgd of municipal waste and 12
mgd of paper mill wastes. The Kalamazoo plant was, until 1966, a primary treatment
plant with anaerobic digestion and drying bed treatment of sludge. In 1967, some
secondary capacity was provided and the sludge was lagoon treated. When the lagoons
became a problem, mechanical sludge dewatering was introduced. The 34 mgd Kalamazoo
treatment plant currently has a high rate activated sludge process. This high rate activated
sludge system was designed for shock loading and treatment of up to 100,000 Ib/day of
BOD.
The initial testing program at Kalamazoo which led to the selection of the Zimpro LPO
unit was performed with both lagoon sludge and sludge to be expected in the future. The
Zimpro system was installed for sludge processing in combination with multiple hearth
incineration [6]. Figure 11-6 shows a schematic of the Kalamazoo sludge handling
system. The heat treatment system consists of three Zimpro units designed to process a
total of 97.5 tons/day of sludge. This system could readily handle the sludge from
approximately a 45 mgd facility. The units were designed to run at a temperature of
360° F and a pressure of 315 to 325 psig. Retention time in the heat treatment system is
about one hour, and demineralized water at a pH of 9.0 is required. A heat recovery
system was included in the design but has been inoperable due to the dirty off gases from
the incinerator. With an operable method for heat recovery, up to 800,000 cu ft of gas
per month could be saved. However, as noted in the section on incineration, when
particularly dry filter cake is fed to a multiple hearth incinerator, poor quality off gases
11 -9
-------
Exhaust
'FAN
f ^
STEAM
GENERATOR
WASTE
HEAT
BOILER
"";,•»••
•«^
^3*.
1
1
1
MH
SCRUBBER
VAPOR
COMBUSTION
UNIT
OXIDIZED
SLUDGE
THICKENER
FILTER
CAKE
DISPOSAL
ASH
STORAGE
BIN
VACUUM
FILTER
MULTIPLE
HEARTH
INCINERATOR
FIGURE 11-6. Kalamazoo, Michigan, sludge disposal facilities [7].
-------
result. Operation of the Kalamazoo plant's sludge thickening and dewatering processes
has been quite good. The sludge thickener provided a sludge with a solids concentration
of 9.7 percent. The vacuum filter had a production rate of 4.9 Ib/hr/sq ft and yielded a
cake with a solids concentration of 45 percent [6]. Good incinerator operation resulted.
No data were available on the decantate from the oxidized sludge thickener. The most
recent cost data (1972) on sludge processing at Kalamazoo are presented in Table 11-2.
TABLE 11-2
KALAMAZOO SLUDGE PROCESSING COSTS
Sludge Process
Thickening
Heat Treatment
Dewatering
Incineration
Amortization
Cost $/ton
2.20
7.52
5.43
6.48
10.00
31.63
This information gives a cost for thickening, heat treatment, dewatering, incineration, and
amortization of $31.63/ton. This cost does not include an estimate of the capital,
operating, and maintenance costs associated with treatment of the recycle streams from
heat treatment. In December of 1973, Kalamazoo plant personnel reported that the
recirculation load from heat treatment increases the BOD loading on the secondary
treatment system by 35 to 40 percent. Despite the fact that the aeration system at
Kalamazoo is designed for a high shock loading, this recirculation load has resulted in
some problems. Low plant BOD and suspended solids removals have occurred. BOD
removal efficiency at Kalamazoo has averaged 75 to 80 percent and the effluent contains
60 to 130 mg/1 of BOD. Suspended solids removal has averaged 70 to 80 percent and the
effluent contains 50 to 88 mg/1 suspended solids.
11-11
-------
11.7 Ft. Lauderdale, Florida-Plant A
Plant A in Ft. Lauderdale is a new 8.5 mgd activated sludge plant. A schematic of the
plant's sludge handling facility is shown in Figure 11-7. As can be seen, primary and
excess activated sludge is gravity thickened and then pumped to heat treatment. Heat
treated sludge goes first to a hold tank and then to a solid bowl scroll-type centrifuge for
dewatering. The aeration system at Ft. Lauderdale was overdesigned by 44 percent to
accommodate a predicted 9,000 Ib/day BOD load in the primary effluent as well as a
4,000 Ib/day BOD load anticipated from cooking liquor recirculation. This plant started
up in December, 1971, and a series of heat exchangers failed primarily because of
corrosion problems. An oversize exchanger was temporarily installed, and this provided
approximately one year of operation. This operation was with less than adequate facilities
and under conditions which resulted in excessive fuel consumption. During this period of
operation the sludge thickening and dewatering processes worked poorly. The heat
treated sludge thickened to only 2 to 3 percent, and the solids capture during dewatering
was less than 50 percent. A buildup of fine solids occurred throughout the system. A new
titanium heat exchanger was started up in January of 1973. As of July 19, 1973, the
same poor sludge thickening and dewatering results were being experienced. Fine solids
carry-over into the final clarifiers and odor problems were still encountered. The principal
problem has been maintaining an adequate sludge solids removal rate through the heat
treatment and dewatering systems. Remedial measures tried have included use of the fill
and draw method of thickener operation, use of flocculants in the thickening step, and
the addition of chlorine in the thickener to improve overflow clarity. Table 11-3
illustrates solids capture being experienced at Ft. Lauderdale during July, 1973.
TABLE 11-3
TOTAL SOLIDS-SLUDGE AND CENTRATE
Date
1973
JulyS
July 7
July 1 1
July 16
Thickened to
Heat Treatment
3.1
3.0
2.4
1.8
To
Centrifuge
% -
fO'
2.6
2.4
2.4
1.8
Centra te
1.2
1.0
1.2
1.3
Cake
42.2
40.6
33.2
28.8
11 -12
-------
PRIMARY
SYSTEM
CENTRATE
RECYCLE
GRAVITY
THICKENER
DEWATER
T
LAND
DISPOSAL
EXCESS
ACTIVATED
SLUDGE
HEAT
TREATMENT
FIGURE 11-7. Ft. Lauderdale sludge handling system.
11-13
-------
Less than 50 percent total solids capture in the overall heat treatment dewatering steps
still prevailed.
Old Plant A was located on the same site and employed conventional treatment methods.
Treatment costs amounted to $ 186.47 per million gallons in the last year of its operation
(1971). Costs are not reported for 1972 because of abnormalities due to equipment,
operating, and maintenance problems in New Plant A. The cost in 1973, pertaining to the
new plant, is somewhat high ($224.84 per million gallons). It is still difficult at the time
to evaluate effects of abnormal and other inflationary conditions on costs.
11.8 References
1. Sherwood, R. and Phillips, James, "Heat Treatment Process Improves
Economics of Sludge Handling and Disposal." Water Wastes Eng. 42
(1970).
2. Kochera, B., "Operation of a Thermal Treatment System for Sludge." WPCF
Meeting, Atlanta, Georgia, 1972.
3. Personal communication with plant manager, Colorado Springs, Colorado,
1973.
4. Hirst, G., Mulhall, K. G., and Hemming, M. L., "The Sludge Heat Treatment
Plant at Pudsey." Northeastern Branch of the Institute of Water Pollution
Control (Mar. 25, 1971).
5. Pickford, J. (ed.), "Sludge Treatment and Disposal." Fourth Public Health
Engineering Conference Proceedings, Department of Civil Engineering,
Loughborough University, Jan. 1971.
6. Swets, D. H., Pratt, L, and Metcalf, C., "Combined Industrial-Municipal
Thermal Sludge Conditioning and Multiple Hearth Incineration." WPCF Annual
Meeting, Atlanta, Georgia, 1972.
7. Swets, D. H., "Trials, Tribulations, and Now Triumph." Pub. Works (1971).
11 - 14
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