United States
Environmental Protection
Agency
EPA-625/1-82/014
Technology Transfer
Design
Manual
Dewatering Municipal
Wastewater Sludges
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EPA-625/1-82-014
PROCESS DESIGN MANUAL
FOR
DEWATERING MUNICIPAL WASTEWATER SLUDGES
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Municipal Environmental Research Laboratory
October 1982
Published By
U.S. ENVIRONMENTAL PROTECTION AGENCY
Center For Environmental Research Information
Cincinnati, Ohio 45268
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NOTICE
The mention of manufacturers' names, trade names or commercial products in
this publication is for informational or illustrative purposes and does not
constitute endorsement or recommendation for use by the U. S. Environmental
Protection Agency.
11
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ABSTRACT
This manual presents a critical review of municipal wastewater sludge
dewatering process technology. Particular emphasis is given to the development
of a procedure for the selection and design of a dewatering process.
Included in the manual are discussions of sludge characteristics, dewatering
processes, their performance capabilities and operational variables, chemical
conditioning, cost and energy considerations, and case-study information.
Dewatering processes discussed are basket centrifuge, low G and high G solid
bowl centrifuges, belt filter press, vacuum filter, fixed volume and variable
volume recessed plate filter presses, drying bed, sludge lagoon, and
gravity/low pressure devices.
111
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ACKNOWLEGEMENTS
There were four groups of participants involved in the preparation of this
manual: (1) the contractor, (2) the authors, (3) the technical directors, and
(4) the technical reviewers. The contractor for this project was SCS Engineers
of Long Beach, California. The manual was written by personnel from
Culp/Wesner/Culp of Santa Ana, California. Technical direction was provided by
U. S. Environmental Protection Agency (EPA) personnel from the Office of
Research and Development in Cincinnati, Ohio. The technical reviewers were
experts in wastewater treatment plant sludge dewatering and included univer-
sity professors, wastewater equipment manufacturers, consultants, and govern-
ment officials. Each reviewer provided an invaluable constructive critique of
the manual. The membership of each group is listed below.
MANUAL PREPARATION
Culp/Wesner/Culp, Santa Ana, CA
Authors: Robert C. Gumerman
Bruce E. Burris
In-house Review: Gordon L. Gulp
Production Staff: Linda McKinney, Joanne Vogelsang, Al Herron (illustrations)
TECHNICAL DIRECTION
Project Officer: Roland V. Villiers , MERL, EPA, Cincinnati, OH
Technical Directors: Joseph B. Farrell, MERL, EPA, Cincinnati, OH
James E. Smith, Jr., CERI, EPA, Cincinnati, OH
TECHNICAL REVIEW
1. Richard T. Moll, Sharpies-Stokes, Pennwalt Corporation, Warminster, PA
2. Walter E. Garrison, Los Angeles County Sanitation Districts, Whittier, CA
3. Thomas J. LeBrun, Los Angeles County Sanitation Districts, Carson, CA
4. Albert B. Pincince, Camp, Dresser and McKee, Boston, MA
5. John T. Novak, Virginia Polytechnic and State University, Blacksburg, VA
6. Tom Komline, Komline-Sanderson, Peapack, NJ
7. Paul R. Erickson, Rexnord Corporation, Milwaukee, WI
8. Kenneth A. Pietila, Rexnord Corporation, Milwaukee, WI
9. Stephen H. Silverman, KHD Humboldt Wedag, Atlanta, GA
10. E. D. Simmons, Passavant Corporation, Birmingham, AL
11. Denis Lussier, CERI, EPA, Cincinnati, OH
12. James F. Wheeler, OWPO, EPA, Washington, D.C.
13. Charles F. von Dreusche, Jr., Nichols Engineering & Research Corporation,
Belle Mead, NJ
14. David DiGregorio, Envirotech, Salt Lake City, UT
15. Bala Krishnan, OEET, EPA, Washington, D.C.
16. Robert Bastian, OWPO, EPA, Washington, D.C.
17. Alan F. Cassel, Water Pollution Control Division, Arlington, VA
18. Glenn Reierstad, Bird Machine Company, South Walpole, MA
iv
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CONTENTS
Chapter
ABSTRACT iii
ACKNOWLEDGEMENTS iv
CONTENTS v
LIST OF FIGURES viii
LIST OF TABLES xii
INTRODUCTION
1.1 Purpose and Scope 1
1.2 Objectives of Dewatering 2
1.3 Location of the Dewatering Process 2
1.4 Guide to Intended Use 2
1.5 References 4
SLUDGE CHARACTERISTICS AFFECTING DEWATERING
2.1 Introduction 5
2.2 Characteristics Affecting Dewatering 5
2.3 References 8
DEWATERING PROCESS DESCRIPTIONS
3.1 Introduction 9
3.2 Centrifugation 10
3.3 Belt Press Filtration 22
3.4 Vacuum Filtration 27
3.5 Pressure Filtration 30
3.6 Drying Bed 36
3.7 Sludge Lagoon 42
3.8 Gravity/Low Pressure Dewatering 43
3.9 References 47
CAPABILITIES OF DEWATERING PROCESSES
4.1 Introduction 49
4.2 Performance Capabilities of Mechanical
Dewatering Processes 50
4.3 Process Operational Variables Which Affect
Dewatering Results 56
4.4 Effect of Dewatering on Sludge Volume 60
4.5 References 64
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CONTENTS (continued)
Chapter Page
5 CHEMICALS USED IN DEWATERING
5.1 Introduction 67
5.2 Ferric Chloride 69
5.3 Lime 70
5.4 Polymers 71
5.5 Waste Pickle Liquor (Ferrous Chloride) 71
5.6 References 72
6 STRATEGY FOR DEWATERING PROCESS SELECTION
6.1 Introduction 73
6.2 Stage 1 - Initial Screening of Dewatering
Processes 76
6.3 Stage 2 - Initial Cost Evaluation 92
6.4 Stage 3 - Laboratory Testing 96
6.5 Stage 4 - Field Testing 102
6.6 Stage 5 - Final Evaluation Based on Detailed
Design Parameters 105
6.7 References 107
7 COMPARATIVE COST ANALYSES OF SLUDGE TREATMENT AND
DISPOSAL SYSTEMS
7.1 Introduction 110
7.2 Cost Comparison for One Ton Per Day Sludge
Handling Systems 111
7.3 Cost Comparison for Five Ton Per Day Sludge
Handling Systems 111
7.4 Cost Comparison for Fifty Ton Per Day Sludge
Handling Systems 114
7.5 References 121
8 ENERGY CONSIDERATIONS IN DEWATERING PROCESS SELECTION
8.1 Introduction 122
8.2 Direct Energy Requirements for Dewatering 122
8.3 Indirect Energy Requirements for Dewatering 125
8.4 Total Energy Requirements for Dewatering 127
8.5 References 128
VI
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CONTENTS (continued)
Chapter
SUMMARY OF RECENT SIDE-BY-SIDE COMPARISONS OF
DEWATERING PROCESSES AT TEN TREATMENT PLANTS
9.1 Introduction 130
9.2 County Sanitation Districts of Los Angeles
County (California) 130
9.3 County Sanitation Districts of Orange County
(California) 134
9.4 Irvine Ranch Water District (California) 136
9.5 Metropolitan Denver Sewage Disposal District
No. 1 (Colorado) 137
9.6 Metropolitan Sanitary District of Greater
Chicago (Illinois) 140
9.7 Middlesex County Sewerage Authority
(New Jersey) 142
9.8 Milwaukee Metropolitan Sewerage District
(Wisconsin) 143
9.9 Nassau County (New York) 144
9.10 San Jose-Santa Clara Water Pollution
Control Plant (California) 145
9.11 Blue Plains Wastewater Treatment Plant
(District of Columbia) 148
9.12 References 151
APPENDIX A
APPENDIX B
APPENDIX C
BIBLIOGRAPHY
MANUFACTURERS OF DEWATERING EQUIPMENT
EXAMPLE CALCULATIONS SHOWING SLUDGE VOLUMES
PRODUCED BY DIFFERENT DEWATERING TECHNIQUES
COST OF DEWATERING EQUIPMENT
154
157
160
199
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FIGURES
Number
3-1 Basket Centrifuge in Sludge Feed and Sludge
Plowing Cycles 11
3-2 Continuous Countercurrent Solid Bowl Centrifuge 14
3-3 Continuous Concurrent Solid Bowl Centrifuge 15
3-4 Nomograph and Equation Used to Calculate G-Force
for Solid Bowl Centrifuge 21
3-5 The Three Basic Stages of a Belt Filter Press 23
3-6 Operating Zones of a Rotary Vacuum Filter 27
3-7 Cross Sectional View of a Coil Spring, Belt-Type
Rotary Vacuum Filter 28
3-8 Cross Sectional View of a Cloth, Belt-Type
Rotary Vacuum Filter 30
3-9 Cross Section of a Fixed Volume Recessed Plate
Filter Press Assembly 33
3-10 Operational Cycle for a Lasta Diaphragm Filter Press 34
3-11 Operational Cycle for an Envirex-NGK Diaphragm
Filter Press 35
3-12 Cross Section of a Dual Cell Gravity Unit 45
3-13 Cross Section of a Smith & Loveless Concentrator 46
4-1 Dewatered Sludge Cake Percent Solids for Mixtures
of Digested Primary (P) and Digested Waste
Activated Sludge (WAS) 51
4-2 Dewatered Sludge Cake Percent Solids for Raw Primary
and Raw WAS 53
4-3 Dewatered Sludge Cake Percent Solids for Mixtures of
Raw Primary and Secondary Sludges 54
4-4 Dewatered Sludge Cake Percent Solids for Mixtures of
Digested Primary and Secondary Sludge and Heat
Treated Primary and Secondary Sludge 55
Vlll
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FIGURES (continued)
Number Page
4-5 Effect of Percent Sludge Solids on Sludge Volume 61
4-6 Effect of Inorganic Conditioning Chemical Dosage
on Dewatered Sludge Volume 63
6-1 General Schematic for Solids Handling Showing Most
Commonly Used Methods of Treatment and Disposal 74
6-2 Five Stages of Analysis in Selection of a
Dewatering Process 75
6-3 Process Flow Diagram and Design Criteria for a Solids
Handling System Using Anaerobic Digestion, Belt
Filter Press Dewatering, Truck Haul and Composting 93
6-4 Filter Leaf Test Apparatus 97
6-5 Time/Filtrate Volume Vs. Filtrate Volume Plot Used
in Specific Resistance Testing 98
6-6 Use of Specific Resistance to Determine Optimum
Chemical Dosage 99
6-7 Capillary Suction Time (GST) Test Set-up 100
6-8 Filterbelt Press Simulator - Effect of Pressure and
Time on Cake Solids Concentration 101
6-9 Filterbelt Press Simulator - Effect of Polymer Dosage
on Cake Solids Concentration 101
6-10 Typical Plot - Variation of Cake Solids Concentration
With Centrifugal Acceleration 103
7-1 Sludge Treatment and Disposal Systems Evaluated For
5 Ton Per Day Cost Analyses 113
7-2 Sludge Treatment and Disposal Systems Evaluated For
50 Ton Per Day Cost Analyses 119
8-1 Direct and Indirect Energy Requirements for Sludge
Dewatering Processes 126
C-l Construction Cost for Basket Centrifuges 172
IX
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FIGURES (continued)
Number Page
C-2 Basket Centrifuges - Building Energy, Process Energy
and Maintenance Material Requirements 173
C-3 Basket Centrifuges - Labor and Total Annual Operation
and Maintenance Cost 174
C-4 Construction Cost for Low G Solid Bowl Centrifuges 175
C-5 Low G Solid Bowl Centrifuges - Building Energy,
Process Energy and Maintenance Material Requirements 176
C-6 Low G Solid Bowl Centrifuges - Labor and Total Annual
Operation and Maintenance Cost 177
C-7 Construction Cost for High G Solid Bowl Centrifuges 178
C-8 High G Solid Bowl Centrifuges - Building Energy,
Process Energy and Maintenance Material Requirements 179
C-9 High G Solid Bowl Centrifuges - Labor and Total
Annual Operation and Maintenance Cost 180
C-10 Construction Cost for Belt Filter Press 181
C-ll Belt Filter Press - Building Energy, Process Energy
and Maintenance Material Requirements 182
C-12 Belt Filter Press - Labor and Total Annual Operation
and Maintenace Cost 183
C-13 Construction Cost for Vacuum Filters 184
C-14 Vacuum Filters - Building Energy, Process Energy
and Maintenance Material Requirements 185
C-15 Vacuum Filters - Labor and Total Annual Operation
and Maintenance Cost 186
C-16 Construction Cost for Recessed Plate Filter Press 187
C-17 Recessed Plate Filter Press - Building Energy, Process
Energy and Maintenance Material Requirements 188
C-18 Recessed Plate Filter Press - Labor and Total Annual
Operation and Maintenance Cost 189
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FIGURES (continued)
Number Page
C-19 Construction Cost for Diaphragm Filter Press 190
C-20 Diaphragm Filter Press - Building Energy, Process
Energy and Maintenance Material Requirements 191
C-21 Diaphragm Filter Press - Labor and Total Annual
Operation and Maintenance Cost 192
C-22 Construction Cost for Sand Drying Beds 193
C-23 Sand Drying Beds - Diesel Fuel and Maintenance
Material Requirements 194
C-24 Sand Drying Beds - Labor and Total Annual Operation
and Maintenance Cost 195
C-25 Construction Cost for Sludge Dewatering Lagoons 196
C-26 Sludge Dewatering Lagoons - Diesel Fuel and
Maintenance Material Requirements 197
C-27 Sludge Dewatering Lagoons - Labor and Total Annual
Operation and Maintenance Cost 198
XI
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TABLES
Number
2-1 Viscosity of Water as a Function of Temperature 7
3-1 Advantages and Disadvantages of Basket Centrifuges 12
3-2 Common Design Shortcomings of Basket Centrifuge
Installations 13
3-3 Advantages and Disadvantages of Solid Bowl Decanter
Centrifuges 18
3-4 Common Design Shortcomings of Solid Bowl Decanter
Centrifuge Installations 19
3-5 Advantages and Disadvantages of Belt Filter Presses 25
3-6 Common Design Shortcomings of Belt Filter Press
Installations 26
3-7 Advantages and Disadvantages of Vacuum Filtration 31
3-8 Common Design Shortcomings of Vacuum Filter
Installations 31
3-9 Advantages and Disadvantages of Filter Presses 37
3-10 Common Design Shortcomings of Filter Press
Installations 38
3-11 Advantages and Disadvantages of Sand Drying Beds 40
3-12 Common Design Shortcomings of Sand Drying Bed
Installations 40
3-13 Advantages and Disadvantages of Sludge Lagoons 44
3-14 Advantages and Disadvantages of Gravity/Low
Pressure Dewatering Devices 44
4-1 Operational Variables for Dewatering Processes 57
5-1 Chemical Conditioners Commonly Used for Different
Dewatering Processes 67
5-2 Typical Dosages of Chemical Conditioners for
Different Dewatering Processes 68
XI1
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TABLES (continued)
Number Page
5-3 Crystallization Temperatures for Ferric Chloride
Solutions 69
6-1 Compatibility of Dewatering Equipment with Plant Size 80
6-2 Dewatering Process Compatibility with Subsequent
Treatment or Ultimate Disposal Techniques 81
6-3 Typical Solids Capture of Dewatering Processes 85
6-4 Evaluation of Environmental Considerations of
Dewatering Processes 87
6-5 Capital and O&M Cost Estimates - Solids Handling
System Including Anaerobic Digestion, Belt Filter
Press Dewatering, Truck Haul, and Composting 95
7-1 Design Criteria for 910 kg/Day (1 ton/day)
Sludge Handling Cost Analyses 112
7-2 Cost Summary for 910 kg/Day (1 ton/day) Capacity
Sludge Treatment and Disposal Systems 113
7-3 Design Criteria for Sludge Handling Cost Analyses
5 and 50 Ton Per Day Systems 115
7-4 Cost Summary for 4,540 kg/day (5 ton/day) Capacity
Sludge Treatment and Disposal Systems 118
7-5 Cost Summary for 45,400 kg/day (50 ton/day) Capacity
Sludge Treatment and Disposal Systems 120
8-1 Direct Energy Requirements for Sludge Dewatering -
Case Study Results 123
8-2 General Ranges of Direct Energy Requirements for
Sludge Dewatering 124
8-3 Indirect Energy Requirements for Sludge Dewatering 127
9-1 Summary of Results from Ten Evaluations of
Mechanical Dewatering Equipment 131
9-2 Design Criteria and Cost Comparison for Dewatering
at County Sanitation Districts of Orange County
(California) 135
xiii
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TABLES (continued)
Number Page
9-3 Results from Field Testing of Belt Filter Presses
at County Sanitation Districts of Orange County
(California) 136
9-4 Results and Operating Costs from Field Testing at
Metropolitan Denver Sewage Disposal District No. 1 138
9-5 Results of Field Testing at the Metropolitan Sanitary
District of Greater Chicago Calumet Plant 140
9-6 Results of Field Testing at the Metropolitan Sanitary
District of Greater Chicago West - Southwest Plant 141
9-7 Test Results at Middlesex County Sewerage Authority 142
9-8 Design Criteria Developed From Laboratory and
Pilot-Scale Tests at San Jose - Santa Clara Water
Pollution Control Plant 147
A-l Manufacturers of Dewatering Equipment 155
xiv
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Chapter 1
INTRODUCTION
1.1 Purpose and Scope
This manual has been prepared to present up-to-date information on dewatering
processes applicable to municipal wastewater sludge, as well as to present a
strategy to be used in the selection of these processes. The manual both
complements and supplements those chapters in the EPA - Process Design Manual
for Sludge Treatment and Disposal that discuss conditioning and dewatering
(1). Significant advances have been made in dewatering technology since
preparation of the latter manual.
Information is presented on design parameters, performance capabilities,
design deficiencies, and cost and energy requirements for all dewatering
processes. The manual specifically discusses those processes where the most
extensive progress has been made including centrifugation, belt press filtra-
tion, and pressure filtration.
The manual is current as of the summer of 1982 and includes detailed
discussions of and presentation of case history information on the newer
process equipment, including solid bowl centrifuges with backdrive capability
and optimized bowl design, third generation belt filter presses, and diaphragm
filter presses. Provided also are the capabilities of these and other dewater-
ing processes by presenting data from full-scale field testing and operating
installations. Information presented is restricted to sludges produced during
primary and secondary wastewater treatment. Chemical sludges produced during
advanced wastewater treatment are not considered.
In general, the manual has been prepared for use by experienced, engineers
involved in the design and selection/specification of dewatering equipment.
Federal, state, and local decision-making officials, however, will also find
useful information here. Little background information is presented on solids
handling processes other than dewatering, although the strategy approach
presented for the selection of a dewatering process is strongly dependent on
analysis of the entire sludge handling and disposal operation. For more
information on sludge handling processes, refer to the references at the end
of each chapter and to the Bibliography.
The major types of dewatering processes discussed in this manual include:
Centrifugation
Basket Centrifuges
Solid Bowl Centrifuges - high G and low G
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Belt Press Filtration
Vacuum Filtration
Pressure Filtration
Fixed Volume
Variable Volume
Drying Beds
Sludge Lagoons
Gravity/Low Pressure Dewatering
All of these are in common usage today, although processes such as the basket
centrifuge and the vacuum filter are rarely seen in new installations. The
manual does not discuss processes which have been installed at one or two
plants or processes which do not have a proven background of performance.
1.2 Objectives of Dewatering
The general objectives of dewatering are to remove water and thereby reduce
the sludge volume, to produce a sludge which behaves as a solid and not a
liquid, and to reduce the cost of subsequent treatment and disposal processes.
No generally accepted lower limit exists for the percent solids content of a
dewatered sludge. In many cases, the lower limit is set by the requirements
for subsequent treatment and disposal. However, the lower limit is always
significantly higher than the percent solids content of a thickened sludge.
This manual considers the use of low pressure first generation type belt
presses which dewater sludge to a 10-122 solids concentration, as well as
drying beds which produce a 60-702 solids content cake.
1.3 Location of the Dewatering Process
The type and order of processes used for solids treatment, dewatering,
transport, and disposal vary widely from plant to plant. Generally however,
the dewatering process is preceded by a stabilization process, such as anaero-
bic or aerobic digestion, thickening by either gravity, centrifugation or air
flotation, and chemical or heat treatment conditioning. In some cases, raw
sludge, particularly raw primary sludge, may be dewatered directly, although
the method of ultimate disposal would have to be considered in such a
decision. After the dewatering operation, further stabilization may be
provided by composting, volume and organic reduction may be accomplished by
incineration, or the dewatered sludge may be ultimately disposed of by
transport to a landfill or a site for landspreading.
1.4 Guide To Intended Use
This manual is organized to allow users to locate particular information and
to concentrate on specific areas of interest as easily as possible. The
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following brief chapter and appendix descriptions are provided as an
introduction to the organization of the manual.
Chapter 2 - Sludge Characteristics Affecting Dewatering
Seven sludge characteristics which significantly affect dewatering
capabilities and conditioning requirements are discussed along with the
interrelationships between these characteristics.
Chapter 3 - Dewatering Process Descriptions
Descriptions are presented for dewatering processes in common usage. Included
in these descriptions are operational principles, key advantages and disadvan-
tages, and common design shortcomings.
Chapter 4 - Capabilities of Dewatering Processes
Performance capabilities of dewatering processes are discussed for a. variety
of different types of sludge and sludge mixtures. Graphic presentations are
included to illustrate the capabilities of dewatering processes. The impact of
process operational variables on dewatering results and the influence of
dewatering on sludge volume are also discussed.
Chapter 5 - Chemicals Used in Dewatering
Major conditioning chemicals used in dewatering, their applications and
typical conditioning requirements are discussed. Important considerations
which the designer should recognize in addition to performance and cost are
included.
Chapter 6 - Strategy for Dewatering Process Selection
A strategy applicable to selection of a dewatering process for new or existing
facilities is described. Five stages of analysis comprise this strategy, which
is a progressive selection procedure. Processes are given increasing scrutiny
as more detailed cost, operational, and design data are collected.
Chapter 7 - Comparative Cost Analyses of Sludge Treatment and Disposal Systems
Comparative cost analyses are presented for three sizes of sludge handling
systems: 910, 4,540 and 45,400 kg/day of dry sludge solids. Design criteria
and flow diagrams are presented for each system evaluated, and a ranking of
systems based upon total annual cost is presented.
Chapter 8 - Energy Considerations in Dewatering Process Selection
Direct energy requirements for dewatering and indirect energy requirements
associated with production of conditioning chemicals are described and quanti-
fied. Graphic and tabular comparisons are included for each dewatering
process .
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Chapter 9 - Summary of Recent Side-By-Side Comparisons of Dewatering
Processes at Ten Treatment Plants
Evaluations conducted by ten large utilities in various parts of the U.S. are
described. The utilities' findings, conclusions, and progress made to date
relative to installation of additional dewatering equipment are presented.
Appendix A - Manufacturers of Dewatering Equipment
An up-to-date listing of manufacturers of centrifuges, belt filter presses,
vacuum filters, filter presses, and drying bed systems is presented.
Appendix B - Example Calculations Showing Sludge Volumes Produced By
Different Dewatering Processes
Example calculations are presented for major dewatering processes. The
calculations are self descriptive and are the basis for several figures in
Chapter 4.
Appendix C - Cost of Dewatering Equipment
Construction and operation and maintenance cost curves are presented for nine
dewatering processes. These construction cost estimates are for installed
equipment, and include all concrete structures, housing, pipes and valves,
electrical and instrumentation equipment and installation labor. Operation and
maintenance requirements are presented individually for labor, building elec-
trical, process electrical, diesel fuel, and maintenance materials. A complete
description of the design assumptions used for the development of the cost
data is presented.
1.5 References
1. "Process Design Manual For Sludge Treatment and Disposal," USEPA - Center
for Environmental Research Information, Cincinnati, Ohio, 45268,
EPA-625/1-79/011, September 1979.
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Chapter 2
SLUDGE CHARACTERISTICS AFFECTING DEWATERING
2.1 Introduction
Many factors influence the dewaterability of a sludge. They include the source
of the sludge and prior treatment or storage which can change the sludge
characteristics prior to dewatering. A number of characteristics can be used
to define the ability of a sludge to be dewatered. Some of these characteris-
tics are readily measured with equipment available at most plants, while
others are difficult or impossible for the plant operator to measure in
day-to-day operation, and can only be measured with sophisticated analytical
techniques and equipment.
2.2 Characteristics Affecting Dewatering
2.2.1 General Considerations
In general, all characteristics relate to the difficulty of forcing sludge
solids closer together, or to the difficulty of water movement through the
voids between the sludge solids. The purpose of sludge conditioning is to
counteract adverse characteristics which decrease the rate or degree of water
removal.
The sludge characteristics which most significantly affect dewatering and
conditioning requirements are:
Particle surface charge and hydration
Particle size
Compressibility
Sludge temperature
Ratio of volatile solids to fixed solids
Sludge pH
Septicity
These characteristics and their interrelationships are discussed in the
following sections.
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2.2.2 Particle Surface Charge and Hydration
Sludge particles have a negative surface charge and repel each other as they
are forced together. This repulsive force increases exponentially as the
sludge particles are forced closer together. Additionally, sludge particles
weakly attract water molecules to their surface either by adsorption or by
capillary action between particles. Although the water is only weakly held at
the particle surface, it does interfere with dewatering.
Conditioning chemicals are used to overcome the effects of surface charge and
surface hydration. Typically used chemicals are organic polymers, lime, and
ferric chloride. Generally they act by reducing or eliminating the repulsive
force, thus permitting the particles to come together or flocculate. Water can
be more readily removed at a higher rate during the subsequent mechanical
dewatering.
2.2.3 Particle Size
Particle size is generally recognized as the most important factor influencing
dewaterability. As average particle size decreases, the surface area for a
given sludge mass increases. The effects of increasing the surface area
include:
• Greater electrical repulsion between sludge particles due to a larger
area of negatively charged surface.
• Greater fractional resistance to the movement of water.
• Greater attraction of water to the particle surface due to more
adsorption sites.
Particle size is influenced by both the sludge source and prior treatment.
Generally, primary sludge has a larger average particle size than secondary
sludge. This is because fine and colloidal solids tend to pass through the
primary clarifier. Some of these same particles are then removed in the
secondary clarifier along with the less dense, flocculated cellular material
that is created during biological treatment. Sludge treatment prior to
dewatering, particularly by aerobic or anaerobic digestion, also decreases the
average particle size. This is the principal reason that digested sludge is
more difficult to dewater than raw sludge. Other conditions which can result
in decreased particle size are mixing, storage, and sludge transport. There-
fore, to maximize the dewaterability of a sludge, use of these conditions
should be minimized.
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2.2.4 Compressibility
If sludge particles were idealized incompressible solids, the solids would not
deform, and the void area between particles would remain constant during
mechanical dewatering. In such an ideal situation, resistance to filtration
would be proportional to sludge depth, and there would be no increase in
resistance to filtration as dewatering progresses. Unfortunately, sludge
particles are compressible to a degree, which results in particle deformation
and a reduction in the void area between particles. This reduction in void
volume inhibits the movement of water through the compressed portion of the
sludge cake, and reduces the rate of dewaterability.
Proper conditioning improves dewaterability primarily by producing a
flocculant matrix of solids in relatively clear water prior to initiation of
filtration. When this matrix is deposited on a filtering medium, the bulk cake
retains a substantial porosity. However, if an excess pressure drop occurs
across the sludge floe, the conditioned sludge cake may collapse, resulting in
a decrease in filtration rate. The net result of conditioning is more rapid
removal of water, principally due to the higher rate of water removal at the
start of the filtration cycle.
2.2.5 Sludge Temperature
As sludge temperature increases, the viscosity of the water present in the
sludge mass decreases. Viscosity is particularly important in centrifuges,
since sedimentation is a key component of the centrifugation process. Accord-
ing to Stokes Law, the terminal settling velocity during centrifugal accelera-
tion varies according to an inverse linear relationship with viscosity of the
water. For example, if viscosity is decreased by 50%, the rate of centrifugal
acceleration is increased by 100%. To illustrate the relationship beween
water temperature and viscosity of water, Table 2-1 is presented-
TABLE 2-1
VISCOSITY OF WATER AS A FUNCTION OF TEMPERATURE
Temperature - °C Viscosity-Centipoises
10 1.308
15 1.140
20 1.005
25 0.894
30 0.800
35 0.723
1 centipoise = 0.001 pascal seconds
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A complete discussion of the influence of centrifugal acceleration on
centrifuge operation is included in Reference 1.
Dewatering processes which utilize filtration principles would not be expected
from theory to be affected by sludge temperature as greatly as centrifuges.
Information available from manufacturers of vacuum filters, belt filter
presses, and filter presses confirms this expectation.
2.2.6 Ratio of Volatile Solids to Fixed Solids
Sludges tend to dewater better as the percentage of fixed solids increases,
assuming all other factors are equivalent. One high G centrifuge manufacturer
utilizes the percentage of fixed solids as a key parameter in sizing of equip-
ment (2). (See Section 3.2.2.1 for a description of low G and high G centri-
fuges). According to this manufacturer, the cake from centrifugal dewatering
of an anaerobically digested mixture of primary and waste activated sludge
shows a 5% increase in its solids concentration when the percentage of
volatile solids in it decreases from 70% to 50% (2).
2.2.7 Sludge pH
Sludge pH affects the surface charge on sludge particles, as well as
influences the type of polymer to be used for conditioning. Generally, anionic
polymers are most useful when the sludge is lime conditioned and has a high
pH, while cationic polymers are most suitable at pH slightly above or below
neutral.
2.2.8 Septicity
Septic sludge is more difficult to dewater and requires higher dosages of
chemical conditioners than fresh sludge, assuming other conditions are equal.
This phenomenon has been experienced at many locations, and is most likely due
to a reduction in the size of sludge particles and to generation of gases that
remain entrained in the sludge.
2.3 References
1. Vesilind, P.A., "Treatment and Disposal of Wastewater Sludges," Revised
Edition, Ann Arbor Science Publishers, Ann Arbor, Michigan, 1980.
2. Personal communication, Richard T. Moll, Manager of Process Engineering,
Sharpies-Stokes Division, Pennwalt Corporation, Warminster, Pennsylvania,
June 1982.
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CHAPTER 3
DEWATERING PROCESS DESCRIPTIONS
3.1 Introduction
A wide variety of mechanical dewatering processes are available, in addition
to evaporation/percolation processes such as sand drying beds and sludge
lagoons. This chapter briefly discusses for each process its operational
principles, key advantages and disadvantages, and design shortcomings.
Detailed performance information for each process is presented in Chapter 4.
Chemical conditioning requirements for the different dewatering processes are
presented in Chapter 5. The processes which are described and the order in
which they are presented are as follows:
• Centrifugation
Basket centrifuge
Solid bowl centrifuge
• Belt press filtration
• Vacuum filtration
• Pressure filtration - fixed volume and variable volume
• Drying bed
Sand drying bed
Paved drying bed
Wedgewater drying bed
Vacuum-assisted drying bed
• Sludge lagoon
• Gravity/low pressure dewatering
Rotating cylindrical gravity dewatering device
Low pressure belt press
At present, belt filter presses and solid bowl centrifuges are the mechanical
devices most commonly selected for dewatering municipal wastewater sludges.
Vacuum filters, although commonly installed up to the mid-1970's, are rarely
selected today. Basket centrifuges have never been a common selection for
municipal sludge dewatering. Filter presses have seldom been selected due to
their high capital and operating costs, yet for certain cases where a very dry
cake is required, a filter press can be cost-effective. The gravity/low
pressure dewatering devices are still occasionally selected for small plants
where a lower cake solid concentration is desired or acceptable. Drying beds
and lagoons have commonly been used at small plants which have land available
and in larger plants which have both high evaporation and available land.
A list of manufacturers of currently available dewatering equipment is
contained in Appendix A. Although the list is intended to be up-to-date and
-------
complete, it is possible that some manufacturers are excluded. Due to the
dynamic nature of the equipment manufacturing business, it is probable that
some companies on the list may in the future discontinue the manufacture of
the equipment for which they are listed. References such as the Journal Water
Pollution Control Federation, Pollution Equipment News, and Water & Wastes
Digest should be consulted for additional suppliers.
3.2 Centrifugation
Centrifugal dewatering of sludge is a process which uses the force developed
by fast rotation of a cylindrical drum or bowl to separate the sludge solids
and liquid. In the basic process, when a sludge slurry is introduced to the
centrifuge, it is forced against the bowl's interior walls, forming a pool
of liquid. Density differences cause the sludge solids and the liquid to
separate into two distinct layers. The sludge solids "cake" and the liquid
"centrate" are then separately discharged from the unit. The two types of
centrifuges used for municipal sludge dewatering, basket and solid bowl, both
operate on these basic principles. They are differentiated by the method of
sludge feed, magnitude of applied centrifugal force, method of solids and
liquid discharge, cost, and performance. A third centrifuge type, the
disc-nozzle centrifuge, has been used for thickening waste activated sludge
(WAS), but does not produce a dewatered material. It will not be discussed in
this manual.
3.2.1 Basket Centrifuge
The imperforate basket centrifuge is a semi-continuous feeding and solids
discharging unit that rotates about a vertical axis. A schematic diagram of a
basket centrifuge in the sludge feed and sludge plowing cycles is shown in
Figure 3-1. Sludge is fed into the bottom of the basket and sludge solids form
a cake on the bowl walls as the unit rotates. The liquid (centrate) is
displaced over a baffle or weir at the top of the unit. Sludge feed is either
continued for a preset time or until the suspended solids in the centrate
reach a preset concentration.
After sludge feeding is stopped, the centrifuge begins to decelerate, and a
special skimmer nozzle moves into position to skim the relatively soft and low
solids concentration sludge on the inner periphery of the sludge mass. These
skimmings are typically returned to the plant headworks or the digesters.
After the skimming operation, the centrifuge slows further to about 70 rpm,
and a plowing knife moves into position to cut the sludge away from the
walls; the sludge cake then drops through the open bottom of the basket. After
plowing terminates, the centrifuge begins to accelerate and feed sludge is
again introduced. At no time does the centrifuge actually stop rotating.
The cake solids concentration produced by the basket machine is typically not
as dry as that achieved by the solid bowl centrifuge. However, the basket
centrifuge is especially suitable for dewatering biological or fine solids
10
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LIQUID SLUDGE IN
CENTRATE
OVERFLOW
PLOWING KNIFE
RETRACTED
PLOWING KNIFE
EXTENDED
BASKET
SLUDGE FEED
SLUDGE PLOWING
DEWATERED SOLCS OUT
FIGURE 3-1
BASKET CENTRIFUGE IN SLUDGE FEED AND
SLUDGE PLOWING CYCLES
sludges that are difficult to dewater, for dewatering sludges where the nature
of the solids varies widely, and for sludges containing significant grit. The
basket centrifuge is most commonly used for thickening WAS. Advantages and
disadvantages of an imperforate basket centrifuge compared to other dewatering
processes are presented in Table 3-1. Common design shortcomings experienced
in basket centrifuge installations are presented in Table 3-2.
Performance of a basket centrifuge is measured by the cake solids content, the
solids capture, the required polymer dosage, and the average feed rate or
solids throughput. Cake solids concentration must be considered as average
solids content, since the solids content is maximum at the bowl wall and
decreases toward the center. The polymer requirement for a basket centrifuge
is generally lower than that required by other mechanical dewatering equip-
ment. The average feed rate includes the period of time during a cycle when
sludge is not being pumped to the basket (acceleration, deceleration,
11
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TABLE 3-1
ADVANTAGES AND DISADVANTAGES OF BASKET CENTRIFUGES
Advantages
Same machine can be used for both
thickening and dewatering
Is very flexible in meeting process
requirements
Is not affected by grit
Little operator attention is required;
full automation is possible
Compared to belt filter press and
vacuum filter installations, is clean
looking and has little to no odor
problems
Is excellent for dewatering hard-to-
handle sludges, although sludge cake
solids are only 10-15% for digested
primary + WAS
Flexibility in producing different
cake solids concentrations because
of skimming ability
Disadvantages
Unit is not continuous feed and
discharge
Requires special structural support,
much more than a solid bowl
centrifuge
Has a high ratio of capital cost to
capacity
Discharge of wet sludge can occur if
there is a machine malfunction or if
the sludge is improperly
conditioned.
Provision should be made for noise
control.
Continuous automatic operation
requires complex controls.
discharge). Therefore, dividing total gallons pumped per cycle by total cycle
time gives the average feed rate. Solids throughput can be determined using
the average feed rate, the percent feed solids, and the solids capture.
A basket centrifuge can be a good application in small plants with capacities
in the range of 0.04 to 0.09 cu m/s (1 to 2 mgd); where thickening is
required before or after stabilization, or where dewatering to 10 to 12
percent solids is adequate. The basket centrifuge is sometimes used in larger
plants. For example, at the Los Angeles County Sanitation Districts' Joint
Water Pollution Control Plant at Carson, California, 44 basket centrifuges are
used to dewater anaerobically digested primary sludge from a 15.3 cu m/s
(350 mgd) advanced primary treatment plant. Typical results achieved are 21%
cake solids, at a polymer consumption of 1.5 g/kg (3 Ib/ton) and a solids
capture of 95 percent, from a feed solids concentration of about 3 percent.
The ability to be used either for thickening or dewatering is an advantage of
the basket centrifuge. A basket centrifuge will typically dewater a 50:50
blend of anaerobically digested primary and waste activated sludge to
12
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TABLE 3-2
COMMON DESIGN SHORTCOMINGS OF BASKET CENTRIFUGE INSTALLATIONS
Shortcomings
Resultant Problems
Solution
Rigid piping connections
to centrifuge
Inadequate structural
support
Inadequate solids capture
due to insufficient
machine capacity or no
provision for polymer feed
Electrical control panels
located in same room with
centrifuges, conveyor
belts, etc.
No provision for centrate
sampling
No flow meters on sludge
feed lines
Cracked or leaking pipes Use flexible connect-
ors
Cracks in supports
Redesign and recon-
struct
High solids content in Add more machines or
centrate properly condition
sludge
Corrosive atmosphere
deteriorates controls
Process control is
hampered
Process control is
hampered
Redesign and relocate
controls in separate
room away from
corrosive atmosphere
Install sample tap in
the centrate line
Install flow meters
10-15% solids. Detailed performance data for basket centrifuges are presented
in Chapter 4.
3.2.2 Solid Bowl Centrifuge
Solid bowl centrifuge technology has greatly advanced in the past five to six
years, as both the conveyor life and machine performance have been improved.
At many treatment plants in the U.S., older solid bowl centrifuge installa-
tions have required very high maintenance expense due to rapid wear of the
conveyor and reduced performance. Recently the use of replaceable ceramic
tiles in low-G centrifuges (less than 1,100 G's) and sintered tungsten carbide
tiles in high-G centrifuges (greater than 1,100 G's) have greatly increased
the operating life prior to overhaul. In addition, several centrifuge manufac-
turers also offer stainless steel construction, in contrast to carbon-steel
construction, and claim use of this material results in less wear and vibra-
tion caused by corrosion. Revised bowl configurations and the use of new
automatic backdrives and eddy current brakes have resulted in improved
reliability and process control, with a resultant improvement in dewatering
performance. In addition, in recent years several centrifuge manufacturers
have reduced the recommended throughput of their machines in direct response
13
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to competition from the belt filter press. This has allowed for an increase in
solids residence time in the centrifuge and subsequent improvement in cake
dryness.
As opposed to the semi-continuous feed/discharge cycles of the imperforate
basket centrifuge, the solid bowl centrifuge, also called decanter or scroll
centrifuge, is a continuously operating unit. This centrifuge, shown in
Figure 3-2, consists of a rotating horizontal cylindrical bowl containing a
screw type conveyor or scroll which rotates also, but at a slightly lower or
higher speed than the bowl. The differential speed represents the difference
in revolutions per minute (rpm) between the bowl and the conveyor. The
conveying of solids requires that the screw conveyor rotate at a different
speed than the bowl. The rotating bowl, or shell, is supported between two
sets of bearings and at one end necks down to a conical section that acts as a
dewatering beach or drainage deck for the screw type conveyor. Sludge enters
the rotating bowl through a stationary feed pipe extending into the hollow
shaft of the rotating conveyor and is distributed through ports in this hollow
shaft into a pool within the rotating bowl.
COVER
DEWATERING BEACH
DIFFERENTIAL SPEED
GEAR BOX
MAIN DRIVE
SHEAVE
CENTRATE
DISCHARGE
PORT
(ADJUSTABLE)
BEARING |
FEED PIPES
(SLUDGE AND
CONDITIONING CHEMICAL)
BASE NOT SHOWN
CENTRATE
DISCHARGE
CAKE
DISCHARGE
FIGURE 3-2
CONTINUOUS COUNTERCURRENT SOLID BOWL
CENTRIFUGE
14
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The centrifuge illustrated in Figure 3-2 operates in the countercurrent mode.
Influent sludge is added through the feed pipe; under the influence of centri-
fugal force, sludge solids settle through the liquid to the bowl wall because
their density is greater than that of the liquid. The solids are then moved
gradually by the rotating conveyor from left to right across the bowl, up the
dewatering beach to outlet ports and from there drop downward into a sludge
cake discharge hopper. As the settled sludge solids move from left to right
through the bowl toward the sludge cake outlet, progressively finer solids are
settled centrifugally to the rotating bowl wall. The water or centrate drains
from the solids on the dewatering beach and back into the pool. Centrate is
actually moved from the end of the feed pipe to the left, and is discharged
from the bowl through ports in the left end, which is the opposite end of the
centrifuge from the dewatering beach. The location of the centrate removal
ports is adjustable, and their location establishes the depth of the pool in
the bowl.
A second variation of the solid bowl centrifuge is the concurrent model shown
in Figure 3-3. In this unit, liquid sludge is introduced at the far end of the
bowl from the dewatering beach, and sludge solids and liquid flow in the same
direction. General construction is similar to the countercurrent design except
that the centrate does not flow in a different direction than the sludge
solids. Instead, the centrate is withdrawn by a skimming device or return tube
located near the junction of the bowl and the beach. Clarified centrate then
flows into channels inside the scroll hub and returns to the feed end of the
machine where it is discharged over adjustable weir plates through discharge
ports built into the bowl head.
FEED PIPES
(SLUDGE AND
CHEMICAL
CONDITIONER)
ROTATING CONVEYOR/SCROLL
ROTATING BOWL
GEAR REDUCER
DEWATERING / BACKDRIVE
CENTRATE
DISCHARGE
CENTRATE
WITHDRAWAL
SLUDGE CAKE
DISCHARGE
FIGURE 3-3
CONTINUOUS CONCURRENT SOLID BOWL CENTRIFUGE
15
-------
A relatively new development in solid bowl decanter centrifuges is the use of
a backdrive to control the speed differential between the scroll and the bowl.
The objective of the backdrive is to control the differential to give the
optimum solids residence time in the centrifuge and thereby produce the
optimum cake solids content. A backdrive of some type is considered essential
when dewatering secondary sludges because of the fine particles present. The
backdrive function can be accomplished with a hydraulic pump system, an eddy
current brake, B.C. variable speed motor, or a Reeves type variable speed
motor. The two most common backdrive systems are the hydraulic backdrive and
the eddy current brake.
Cake solids content increases of 4% or more relative to machines without a
backdrive are achievable, although it must be recognized that the effective
capacity of the machine is decreased by utilizing a backdrive to produce a
higher solids content cake. A backdrive unit will generally not reduce the
quantity of polymer required, but it will increase overall stability of
centrifuge performance when the feed solids characteristics vary.
The eddy current brake backdrive is commonly provided by one high G centrifuge
manufacturer. The eddy current brake is attached to the pinion shaft of the
gearbox and consists of a stationary field coil and a brake rotor on the
shaft. When a B.C. voltage is applied to the stationary field coil, magnetic
flux lines are created in the brake rotor. The amount of flux in the rotor is
a function of the speed differential between the rotor and the field coil as
well as the B.C. current applied to the field coil. This flux produces eddy
currents which create a resistance to turning, or a braking action. Thus,
varying the B.C. voltage applied to the stationary field coil results in a
change in the speed differential between the bowl and the scroll.
An automatically-controlled variable speed hydraulic backdrive is commonly
provided by several low G centrifuge manufacturers to control the speed
differential between the scroll and the bowl. The differential is controlled
to maintain a constant torque on the scroll shaft, with the resulting produc-
tion of a high solids content sludge cake. A hydraulic pump and a hydraulic
backdrive motor are the two principal components of the hydraulic backdrive
unit. The hydraulic backdrive is a noise producing operation, whereas the eddy
current brake is silent.
Most centrifuge installations have the centrifuge mounted a few feet above the
floor, and use a belt conveyor to move dewatered cake away. Other methods of
installing a solid bowl centrifuge are to put the centrifuge on the second
floor of a two story building and drop the dewatered cake into either trucks
or a storage hopper on the first level; to mount the centrifuge about a foot
off the floor and to drop cake into a screw conveyor built into the floor; or
to let the centrifuge cake drop into an open throated progressive cavity type
pump for transfer of the cake to a truck, incinerator, or storage.
Centrifuge performance is measured by the percent solids of the sludge cake,
the percent solids capture, the overall quality of the centrate, the solids
loading rate, and the polymer requirement. The performance of a particular
centrifuge unit will vary with the sludge feed rate and the characteristics of
the feed sludge, including percent solids, sludge temperature and ash content.
16
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Centrifuge performance is also affected by polymer selection and the dosage
utilized as well as its point of introduction. Centrifuge performance on a
particular sludge will also vary with bowl and conveyor design, bowl speed,
differential speed, and pool volume. Bowl and conveyor design are not vari-
able after installation. Although pool depth is variable on solid bowl units,
up to several hours of labor may be required to change the pool depth.
Increasing the pool depth will normally result in a wetter sludge cake but
better solids recovery, however, this is not necessarily true on newer
machines equipped with an automatic backdrive.
Bowl speed is not normally varied on most centrifuge models once a centrifuge
is installed. An increase in bowl speed normally results in a drier sludge
cake and better solids recovery, although in some cases it may result in
shearing of the sludge floe and a reduction in solids capture. With the
addition of polymer internally into the bowl of the centrifuge, a capability
available from several manufacturers, no shearing occurs since both the
polymer and the solids are up to bowl speed when the formation of the floe
occurs. Conveyor differential speed normally can be varied, yet it may require
some disassembly of the machine. On centrifuges equipped with an automatic
backdrive, the differential speed can be easily varied. Increasing the differ-
ential between the bowl speed and the scroll speed normally results in a
wetter sludge cake, poorer solids recovery, and higher machine throughput. On
the other hand, reducing the differential speed produces a dryer cake,
increases solids capture, and decreases machine throughput. Operating at too
low a differential speed can cause the pile of solids formed in front of the
scroll conveyor blade to increase in overall height such that it infringes on
the clarified liquid area. This may result in the skimming of some fine solids
from the top of the cake pile to the centrate, lowering solids capture. Too
low of a differential speed, unless adequately controlled, can also result in
plugging the centrifuge, if solids are removed at a slower rate than they are
fed to the machine.
Some of the advantages and disadvantages of a solid bowl decanter centrifuge
compared with other dewatering processes are presented in Table 3-3, and
Table 3-4 lists common design shortcomings associated with solid bowl
centrifuges.
The ability to be used either for thickening or dewatering provides
flexibility and is a major advantage for solid bowl centrifuges. For example,
a centrifuge can be used to thicken ahead of a filter press, reducing chemical
usage and increasing solids throughput. During periods of downtime of the
filter press, the solid bowl centrifuge can serve as an alternate dewatering
device. Another advantage of the solid bowl centrifuge for larger plants is
the availability of equipment with the largest sludge throughput capability
for single units of any type of dewatering equipment. The larger centrifuges
are capable of handling 19 to 44 1/s (300 to 700 gpm) per unit depending on
the sludge's characteristics. The centrifuge also has the ability to handle
higher than design loadings, such as a temporary increase in hydraulic loading
or solids concentration, and the percent solids recovery can usually be main-
tained with the addition of more polymer; while the cake solids concentration
will drop slightly, the centrifuge will stay on line.
17
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TABLE 3-3
ADVANTAGES AND DISADVANTAGES OF SOLID BOWL DECANTER CENTRIFUGES
Advantages
Clean appearance, little to no odor
problems, and fast start-up and shut-
down capabilities
Easy to install and requires a
relatively small area
Does not require continuous operator
attention
Can operate with a highly variable
feed solids concentration on many
sludge types
Can be operated either for thicken-
ing or dewatering
High rates of feed per unit, thus
reducing the number of units
required
Use of low polymer dosages when
compared to other devices, except
the basket centrifuge
Can handle higher than design
loadings with increased polymer
dosage, although cake solids content
may be reduced
Disadvantages
Scroll wear can be a high
maintenance item. Hardsurfacing and
abrasion protection materials are
extremely important in reducing wear
Prescreening or a grinder in the
feed stream is recommended
Requires skilled maintenance
personnel in large plants where
scroll maintenance is performed
Noise is very noticeable, especially
for high G centrifuges and hydraulic
backdrive units
Vibration must be accounted for in
designing electronic controls and
structural components
High power consumption for a high G
centrifuge
A condition such as poor centrate
quality can be easily overlooked
since the process is fully
contained
Requires extensive pretesting to
select correct machine settings
before placement in normal service
Solid bowl centrifuges are typically capable of dewatering a 50:50 mixture of
anaerobically digested primary and secondary sludges to a 15-21% solids
concentration. More detailed performance data are presented in Chapter 4.
3.2.2.1 Low G vs High G Solid Bowl Centrifuge Controversy
Solid bowl centrifuges are currently available as both low G and high G
machines. A low G machine operates at bowl speeds causing centrifugal forces
of 1,100 times the force of gravity or less. In the Process Design Manual for
18
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TABLE 3-4
COMMON DESIGN SHORTCOMINGS OF SOLID BOWL DECANTER CENTRIFUGE INSTALLATIONS
Shortcomings
Resultant Problems
Solution
Improper materials used
for scroll tips
Inability to remove bowl
assembly during main-
tenance
Rigid piping used to
connect feed pipe to
centrifuge
Excessive wear
Bowl is bulky and heavy
and can not be removed
without using lifting
equipment.
Cracked or leaking pipes
or pipe connections
Grit present in sludge Excessive centrifuge wear
Electronic controls,
structural components,
and fasteners not
designed for vibration
Electrical connections
become loose; structural
components and fasteners
fail
Electrical control panels
located in same room with
centrifuges, conveyor
belts, etc.
Corrosive atmosphere
deteriorates controls
Replace with harder,
more abrasion-resist-
ant tips
Install overhead
crane
Replace with flexible
connections
Install a degritting
system on the sludge
or on the wastewater
prior to sludge
removal
Isolate sensitive
electronic controls
from vibration; re-
design and construct
structural components
and fasteners to
resist vibration
Redesign and relocate
controls in separate
room away from
corrosive atmosphere
19
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Sludge Treatment and Disposal (1), the low G and high G centrifuges are
described as "low speed" and "high speed" centrifuges. "Low speed" centri-
fuges are defined as those operating at a bowl speed of 1,400 rpm or less (1).
Note that the gravity force level (G) increases with the bowl diameter, as
shown in the nomograph and equation on Figure 3-4. It can be seen that a 61 cm
(24 in) diameter centrifuge operating at 1,400 rpm would develop 668 G's,
while a 144 cm (56.5 in) diameter centrifuge also operating at 1,400 rpm would
develop centrifugal force of 1573 G's. For a small diameter centrifuge, even
the low G machines would typically be operating above 1,400 rpm in order to
achieve a higher G force. Therefore, G force is a better method of describing
solid bowl centrifuges than bowl speed alone, since G force takes into account
both bowl speed and bowl diameter. However, because of the common usage, both
"G" and "speed" will be used in this manual to describe solid bowl scroll
centrifuges.
There has been considerable controversy over the benefits of low G and high G
centrifuges. Low G decanter centrifuge manufacturers claim that their machines
consume less energy, have a lower noise level, and require less maintenance
than comparable high G machines. On the other hand, high G decanter centrifuge
manufacturers claim that their machines require less polymer and achieve a
higher throughput because of the higher G forces utilized. Resolution of
whether or not low G centrifuges have a lower total annual cost than high G
centrifuges can only be determined after side-by-side tests are conducted with
a particular sludge and the design parameters are known for each machine.
There have been few cases where simultaneous side-by-side testing with exactly
the same sludge between low G and high G solid bowl centrifuges has been
conducted. One recent side-by-side dewatering test between Sharpies' high G
centrifuge and KHD Humboldt Wedag's low G centrifuge occurred in June 1982 at
the Littleton-Englewood, Colorado wastewater treatment plant. A report
summarizing the comparison was expected to be completed by the end of 1982. An
additional side-by-side dewatering test was initiated during the summer of
1982 by the City of San Francisco.
The materials used in constructing a solid bowl centrifuge are also a source
of controversy between low G and high G centrifuge manufacturers. Abrasive
wear on scroll conveyor blades or flights has traditionally been the item of
greatest maintenance, both in terms of time and expense. Several factors tend
to influence the rate of abrasive wear including the abrasiveness of the
sludge, the centrifugal force at the bowl wall, the differential speed, and
the abrasion resistance of the material used to form scroll blade tips.
Manufacturers of low G, concurrent flow centrifuges maintain that their
machines are much less prone to scroll tip wear than high G countercurrent
flow machines, because the low G machines operate at lower centrifugal forces
and lower differential speeds. Manufacturers of high G machines maintain that
their problems with high abrasive wear rates can be overcome by the use of the
proper abrasion resistant materials. A method of measuring wear rates and
volume loss on abrasion resistant materials is the ASTM G65-80 (Procedure A)
test.
20
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DIAMETER
(INCHES)
— 7
— 8
-9
-10
— 15
— 20
I-25
— 30
— 40
-50
L-60
NOTE: 2.54 cm = 1 in.
RPM
5000 —
4000 —
3000 —
2500-^
2000 —
1500—
1000 —
900 —
800 —
700—
600—
500 —
"G" FORCE
5000—i
4000 —
3000 —
2500-^
2000 —
1500 —
70,414
1000-
900-
800-
700-
600-
500-
400 —
300-
250-
200—'
FIGURE 3-4
NOMOGRAPH AND EQUATION USED TO CALCULATE
G-FORCE FOR SOLID BOWL CENTRIFUGE
21
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Various types of hardfacing have been used to reduce wear on scroll tips.
These include many different welder applied metallic hardfacings (such as
Colmonoy #6, Eutalloy, and Stellite) as well as tungsten carbide and ceramic
tiles. Field replaceable ceramic tiles have recently been recommended by
several low G manufacturers because of their long life, relatively low
replacement cost and ease of replacement. However, they are more fragile than
metallic hardfacings, tending to chip easily. They also may occupy more space
in the bowl and not form as smooth a surface on the conveyor blades as do
metallic hard facings. Ceramic tiles can be glued on to the flights although
in some cases they are both glued and bolted to the flights. Tungsten carbide
tiles have an extremely long life hardfacing, but one study found them to be
5-10 times as expensive as ceramic tiles (3). One high-G centrifuge manufac-
turer claims that sintered tungsten carbide tiles are no more than 2 times as
expensive as ceramic tiles (4). Sintered tungsten carbide tiles are generally
welded to the flights and are usually required for only the portion of the
conveyor blade near the dewatering beach (4).
One manufacturer of low G centrifuges using ceramic tile hardsurfacing
material will routinely guarantee scroll conveyor life for 15,000-20,000 hours
between rebuilds (5). One high G centrifuge manufacturer will routinely
guarantee scroll conveyor life for 30,000 hours using highly abrasion resis-
tant sintered tungsten carbide tiles (4). Experience with low G concurrent
flow centrifuges at the Los Angeles County Sanitation Districts' Carson Plant
has indicated that conventional welder applied hardfacing has an operating
life of about 5,000 hours (3).
3.3 Belt Press Filtration
Belt filter presses employ single or double moving belts to continuously
dewater sludges through one or more stages of dewatering. In the past few
years, belt filter presses and solid bowl centrifuges have become the most
frequently selected dewatering devices. At least 14 equipment suppliers can
furnish a type of belt press, as listed in Appendix A.
All belt press filtration processes include three basic operational stages:
chemical conditioning of the feed sludge, gravity drainage to a nonfluid
consistency, shear and compression dewatering of the drained sludge.
Figure 3-5 depicts a simple belt press and shows the location of the three
stages. Although present-day presses are usually more complex, they follow the
same principle indicated in Figure 3-5. The dewatering process is made effec-
tive by the use of two endless belts of synthetic fiber. The belts pass around
a system of rollers at constant speed and perform the function of conveying,
draining, and compressing. Many belt presses also use an initial belt for
gravity drainage, in addition to the two belts in the pressure zone.
Good chemical conditioning is very important for successful and consistent
performance of the belt filter press. A flocculant (usually an organic
polymer) is added to the sludge prior to its being fed to the belt press. Free
water drains from the conditioned sludge in the gravity drainage stage of the
press.
22
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STAGE I
CHEMICAL
CONDITIONING
MIXER OR
IN-LINE
INJECTION
POLYMER
SOLUTION
STAGE 3
SHEAR &
COMPRESSION
DEWATERING
\
$, WASH SPRAY
/«>\\
FILTRATE
J
vyy¥*
/ DEWATERED
WASH WATER
FIGURE 3-5
THE THREE BASIC STAGES OF A BELT FILTER PRESS
The sludge then enters a two-belt contact zone, where a second upper belt is
gently set on the forming sludge cake. The belts with the captured cake
between them pass through rollers of generally decreasing diameter. This stage
subjects the sludge to continuously increasing pressures and shear forces.
Pressure can vary widely by design, with the sludge in most pro.,ses moving
from a low pressure section to a medium pressure section. Some presses
include a high pressure section which provides additional dewatering.
Progressively, more and more water is expelled throughout the roller section
to the end where the cake is discharged. A scraper blade is often employed for
each belt at the discharge point to remove the cake from the belts.
Two spray-wash belt cleaning stations are generally provided to keep the belts
clean. Typically, secondary effluent can be used as the water source for the
spray-wash. High pressure jets can be equipped with a self-cleaning device
used to continuously remove any solids which may tend to plug the spray
nozzles.
Belt press performance is measured by the percent solids of the sludge cake,
the percent solids capture, the solids and hydraulic loading rates, and the
required polymer dosage. Several machine variables including belt speed, belt
tension, and belt type influence belt press performance (6).
23
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Belt speed is an important operational parameter which affects cake solids,
polymer dosage, solids recovery, and hydraulic capacity. Low belt speeds
result in drier sludge cakes. At a given belt speed, increased polymer dosages
result in higher cake solids. With an adequate polymer dose, solids recoveries
are improved by lowering belt speeds. Hydraulic capacity increases at higher
belt speeds; however, the solids capture drops. Depending on desired perfor-
mance, the belt speed setting can be used to produce a variety of different
results.
Belt tension has an effect on cake solids, maximum solids loading, and solids
capture. In general, a higher belt tension produces a drier cake but causes a
lower solids capture, at a fixed flow rate and polymer dose. A possible draw-
back of using higher tension is increased belt wear. For sludges with a large
quantity of WAS, the belt tension must be reduced to contain the sludge
between the belts. The maximum tension which will not cause sludge losses from
the sides of the belts should be used. The high pressure zones on belt
presses may cause problems with some WAS blends and may be unusable or require
the lowest pressure setting possible.
Belt type is important in improving overall performance. Most belts are woven
of polyester filaments. Belts are available with weaves of different coarse-
ness and different strengths. A belt with a coarser and stronger weave may
require higher polymer dosages to obtain adequate solids capture.
Failure of the chemical conditioning process to adjust to changing sludge
characteristics can cause operational problems. If sludge is underconditioned,
improper drainage occurs in the gravity drainage section, and either extrusion
of inadequately drained solids from the compression section or uncontrolled
overflow of sludge from the drainage section may occur. Most manufacturers'
belt presses can be equipped with sensing devices which can be set to automa-
tically shut off the sludge feed flow in case of underconditioning. Both
underconditioned and overconditioned sludges can blind the filter media. In
addition, overconditioned sludge drains so rapidly that solids cannot distri-
bute across the belt. Vanes and distribution weirs included in the gravity
drainage section help alleviate the problem of distribution of overconditioned
sludge across the belt. Inclusion of a sludge blending tank before the belt
press can also reduce this problem. Scraper units and filtrate trays are sites
where solids build up. A belt press installation should be designed for daily
washdown by hosing; therefore, drainage and safe walking areas around the
press are important.
The flow rate required for belt washing is usually 50 to 100 percent of the
flow rate of sludge to the machine and the pressure is typically 690 kPa
(100 psi) or more. The combined filtrate and belt washwater flow is normally
about one and one-half times the incoming sludge flow. Some belt presses
recirculate washwater from the filtrate collection system, but normally,
secondary effluent or potable water is used. This combined flow of washwater
and filtrate contains between 500 and 2,000 mg/1 of suspended solids and is
typically returned either to the primary or secondary treatment system.
Belt presses have numerous moving parts, that include up to 25 to 30 rollers
and 50-75 bearings. Spare parts should be kept available to prevent prolonged
24
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unit down-time. Belts, bearings, and rollers can deteriorate quickly, if
maintenance is inadequate. However, most parts are small and easily acces-
sible, so that even small facilities should have little difficulty in
maintaining these replacement parts.
Table 3-5 lists some of the advantages and disadvantages of the belt filter
press compared to other dewatering processes. Common design shortcomings
associated with belt filter press installations are listed in Table 3-6. When
dewatering a 50:50 mixture of anaerobically digested primary and waste activa-
ted sludge, a belt filter press will typically produce a cake solids concen-
tration in the 18-23% range. More complete performance data are presented in
Chapter 4.
TABLE 3-5
ADVANTAGES AND DISADVANTAGES OF BELT FILTER PRESSES
Advantages Disadvantages
High pressure machines are capable
of producing drier cake than any
machine except a filter press
Low power requirements
Low noise and vibration
Operation easy to understand for
inexperienced operator because all
parts are visible and results of
operational changes are quickly
and readily apparent
Continuous operation
Media life can be extended when
applying the low belt tension
typically required for municipal
sludges
Very sensitive to incoming feed
characteristics and chemical
conditioning
Machines hydraulically limited in
throughput
Short media life as compared with
other devices using cloth media
Wash water requirement for belt
spraying can be significant
Frequent washdown of area around
press required
Require prescreeening or grinding of
sludge to remove large objects and
fibrous material
Can, like any filtration device,
emit noticeable odors if the sludge
is poorly stabilized
Require greater operator attention
than centrifuge
Condition and adjustment of scraper
blades is a critical feature that
should be checked frequently
Typically require greater polymer
dosage than a centrifuge
25
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TABLE 3-6
COMMON DESIGN SHORTCOMINGS OF BELT FILTER PRESS INSTALLATIONS
Shor tcomings
Improper tracking of
filter belt
Inadequate wash water
supply
Improper belt type
Inadequate control
of conditioning
Wash water not metered
Spray wash unit poorly
sealed
Inadequate mixing time
for polymer and feed
sludge before belt
press
No flow meters on
sludge feed lines
Resultant Problems
Belt creeps off rollers
and dewatering operation
must be stopped for repair
Sludge buildup on belts
and/or rollers
Frequent tearing or
wrinkling or inadequate
solids capture
Frequent under-
conditioning or
overconditioning of
sludge
Difficult to calculate
solids capture
Fine mist escapes from
spray wash unit
increasing moisture/
corrosion problems
Underconditioning of
sludge
Solution
Process control is
hampered
Repair or adjust
automatic tracking
device, if one exists.
If not, attempt to
add such a device
Increase spray water
pressure or install
new spray heads
Experiment with
different belt types
and install proper
belt for actual
conditions
Install a feedback
control system which
monitors sludge
solids content and
sets required polymer
addition
Install a water meter
in wash water line
Replace or modify
spray wash unit to
provide better seal
around belt
Move polymer injection
point upstream toward
feed pumps to increase
mixing time or install
polymer/sludge mixing
before belt presses
Install flow meters
26
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3.4 Vacuum Filtration
The most common means of mechanically dewatering municipal wastewater sludge
up until the mid-1970's was vacuum filtration. A vacuum filter consists
basically of a horizontal cylindrical drum which rotates partially submerged
in a vat of sludge. The filter drum is divided into multiple compartments or
sections by partitions (seal strips). Each compartment is connected to a
rotary valve by a pipe. Bridge blocks in the valve divide the drum compart-
ments into three zones which are referred to as the cake formation zone, the
cake drying zone, and the cake discharge zone. The filter drum is submerged to
about 25% of its depth (variable) in a vat of conditioned sludge, and this
submerged zone is the cake formation zone. Vacuum applied to the submerged
drum section causes filtrate to pass through the media and sludge cake to be
retained on the media. As the drum rotates, each section is successively
carried through the cake forming zone to the vacuum drying zone (See Figure
3-6). This zone begins when the filter drum emerges from the sludge vat. The
cake drying zone represents from 40 to 60 percent of the drum surface and ends
at the point where the internal vacuum is shut off. At this point, the sludge
cake and drum section enter the cake discharge zone, where sludge cake is
removed from the media.
CAKE
DISCHARGE
ZONE
CAKE FORMATION
ZONE
FIGURE 3-6
OPERATING ZONES OF A ROTARY VACUUM FILTER
27
-------
The discharge cycle varies with the type of medium used. Up until the 1960's,
the drum or scraper type rotary vacuum filter was predominant. Since then, the
belt-type rotary filter has become dominant. There are two coverings that are
most commonly used with belt-type units: coil springs and fiber cloth (woven
cloth or metal belt). Belt-type filters differ from the drum or scraper-type
units because the drum covering leaves the drum.
Figure 3-7 shows a cross sectional view of a coil spring, belt-type vacuum
filter. This filter uses two layers of stainless steel coils arranged around
the drum. After the dewatering cycle, the two layers of springs leave the drum
and are separated from each other. In this way, the cake is lifted off the
lower layer of springs and can be discharged from the upper layer. Cake
release is usually not a problem if the sludge is properly conditioned. After
cake discharge, the coils are spray washed and returned to the drum just
before the drum enters the sludge vat.
WASH WATER
SPRAY PIPING
INTERNAL PIPING
con. SPRING
FILTER MEDIA
VACUUM GAUGES
VACUUM AND
FILTRATE OUTLETS
CAKE DISCHARGE
VAT
FIGURE 3-7
CROSS SECTIONAL VIEW OF A COIL SPRING,
BELT-TYPE ROTARY VACUUM FILTER
28
-------
The coil springs, which have 7 to 14 percent open area, act to support the
initial solids deposit which in turn serves as the filtration media. Because
of the open area of the springs, it is important that the feed solids concen-
tration be high or that it contain sufficient fibrous material to control the
loss of fine solids. Sludges with particles that are both extremely fine and
resistant to flocculation dewater poorly on coil filters, and solids capture
is low. Cloth media is required when filtering unthickened sludge that is
predominantly secondary solids.
Figure 3-8 shows a schematic cross section of a fiber cloth, belt-type rotary
vacuum filter. Media on this type unit leaves the drum surface at the end of
the drying zone and passes over a small-diameter discharge roll to facilitate
cake discharge. Washing of the media occurs after discharge and before it
returns to the drum for another cycle. This type of filter normally has a
small-diameter curved bar between the point where the belt leaves the drum and
the discharge roll. This bar aids in maintaining belt dimensional stability.
In practice, it is frequently used to ensure adequate cake discharge. Remedial
measures, such as addition of scraper blades, use of excess chemical condi-
tioner, or addition of fly ash, are sometimes required to obtain cake release
from the cloth media. This is particularly true at wastewater treatment plants
which produce sludges that are greasy, sticky, and/or contain a large quantity
of waste activated sludge. In general, cloth media made from staple fiber
produces cleaner filtrate but has lower throughput than cloth media made from
monofilament fiber.
The performance of vacuum filters may be measured by several criteria includ-
ing the yield, the efficiency of solids removal, and the cake characteristics.
Yield, the most common measure of filter performance, is expressed in terms of
kg dry solids in the cake discharged from the filter per sq m of effective
filter area per hour (Ib/sq ft/hr). A typical range of vacuum filter yields
for anaerobically digested primary and waste activated sludge is about 17-29
kg/sq m/hr (3.5 - 6 Ib/sq ft/hr).
The efficiency of solids removal, or percent solids recovery, is the actual
percentage of feed solids recovered in the filter cake. Solids removals on
vacuum filters with adequate chemical conditioning range from about 85 percent
for coarse mesh media to 99 percent with close weave, long nap media. The
recycled filtrate solids impose a load on the treatment plant and should
normally be kept to a practical minimum. However, it may be necessary to
reduce the percent recovery in order to deliver more filter output and thus
keep up with sludge production. Cake solids concentration is another important
parameter used in evaluating vacuum filter performance.
Table 3-7 lists some of the advantages and disadvantages of vacuum filtration
relative to other dewatering processes, and Table 3-8 lists design short-
comings which have been noted at a number of vacuum filter installations.
Typically, a vacuum filter will produce a cake with a solids concentration of
between 15 and 20% (including conditioning chemicals) on a 50:50 blend of
anaerobically digested primary and waste activated sludge. More detailed
performance data for vacuum filters are presented in Chapter 4.
29
-------
FILTER DRUM
INTERNAL PIPING
CLOTH FILTER MEDIA
DISCHARGE ROLL
CAKE
DISCHARGE
»WASH
WATER
WASH TROUGH SPRAY
PIPING
SLUDGE LEVEL
FILTER
AGITATOR
FILTER VAT
FIGURE 3-8
CROSS SECTIONAL VIEW OF A CLOTH.
BELT-TYPE ROTARY VACUUM FILTER
In 1978, an evaluation was made of the feasibility of using a high pressure
belt press following vacuum filtration to produce a drier cake and to increase
vacuum filter throughput (7). At that time, three manufacturers were marketing
such devices. At present, however, all three manufacturers have ceased market-
ing high pressure presses for this application. The manufacturers indicate
that the principal reason for withdrawing from this application is the
difficulty of transferring the vacuum filter cake to the belt press in a
satisfactory manner (8) (9) (10).
3.5 Pressure Filtration
The two types of filter presses which are commonly available to dewater
municipal wastewater sludges are the fixed volume recessed plate filter press
and the variable volume recessed plate filter press, also referred to as the
diaphragm filler press. The recessed plate filter press is often confused with
the plate and frame filter press, which is not commonly marketed to dewater
30
-------
TABLE 3-7
ADVANTAGES AND DISADVANTAGES OF VACUUM FILTRATION
Advantages
Operation is easy to understand
because formation and discharge
of sludge cake are easily visible
Continuous operation
Will continue to operate even if the
chemical conditioning dosage is not
optimized
Coil spring media has very long life
compared to any cloth filter media
Has low maintenance requirements for
a continuously operating piece of
equipment except in certain cases
with lime conditioning
Disadvantages
Consumes a large amount of energy
per unit of sludge dewatered
Vacuum pumps are noisy
Can emit strong odors if the sludge
is poorly stabilized
Lime and ferric chloride conditioning
can cause considerable maintenance-
cleaning problems
The use of lime for conditioning can
produce strong ammonia odors with
digested sludge
Requires at least 3 percent feed
solids to achieve adequate cake
formation and discharge
TABLE 3-8
COMMON DESIGN SHORTCOMINGS OF VACUUM FILTER INSTALLATIONS
Shortcomings
Improper filter media
Improper chemical
conditioning used
Inadequate water
pressure for spray
nozzles
Re s u11 an t Prob1em
Filter blinds, provides
inadequate solids capture,
and/or poor cake release
Poor solids capture, low
solids loading rate, and
low cake solids concen-
tration
Improperly cleaned media
Solution
Replace media after
testing for optimum
media
Change to correct
chemical conditioners
Provide booster pumping
to maintain 345 kPa
(50 psig) minimum
pressure
31
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municipal wastewater sludges, although several installations do exist. The
recessed plate filter press is also referred to as a chamber filter press. In
the fixed volume recessed plate filter press, liquid sludge is pumped by high
pressure pumps into a volume between two filter cloths, held in place by a
rigid framework. As a result of the high pressure that the sludge is under, a
substantial portion of the water in the feed sludge passes through the filter
cloth and drains from the press. Sludge solids and the remaining water
eventually fill the void volume between the filter cloths, and continued
pumping of solids to the press is no longer productive. At this point, pumping
is stopped and the press is opened to release the dewatered sludge cake prior
to initiation of a new cycle. In a variable volume recessed plate or diaphragm
filter press, sludge is pumped into the press at a low pressure until the
volume of the press has been filled with a loosely compacted cake, then
sludge pumping is stopped and the diaphragm is inflated for a preset time. For
the diaphragm press, although most of the water removal occurs when sludge is
being pumped into the press, a significant quantity of water is also removed
after the diaphragm is inflated.
In the fixed volume recessed plate press, filter media is used on both sides
of the filtering volume. As shown in Figure 3-9, sludge is pumped into the
volume between the cloth media, and water is expelled through the media.
Sludge pumping is at relatively high pressures, up to 1,550 kPa (225 psi), and
the driving force for movement of water through the cloth is this high
pressure. Low pressure recessed plate presses are also available which operate
at about 690 kPa (100 psi). When little or no additional filtrate is being
produced, the pumping is stopped, the press is opened, and sludge cake falls
from the press. Periodic washing of the filter cloth is required as the high
pressure tends to cause blinding of the cloth. Since lime conditioning is
normally required, periodic acid washing is also required to remove lime
scale.
The diaphragm press is a relatively new innovation, which uses a diaphragm to
further compress the sludge solids after low pressure, about 690 kPa
(100 psi), sludge pumping into the press is ineffective in promoting further
dewatering. The diaphragm is expanded by pumping either air or water into the
diaphragm at pressures up to between 1,480 kPa (215 psi) and 1,965 kPa
(285 psi), depending upon the manufacturer. After a pre-set time has elapsed,
the diaphragm is deflated and the press opens, allowing the cake to drop out
the bottom. Periodically the filter cloth is washed, by permanent spray
nozzles. Figure 3-10 shows the basic configuration of one cell of Ingersoll
Rand's Lasta diaphragm press and the four separate stages of operation. Figure
3-11 shows the operational cycle of the Envirex-NGK diaphragm press.
The diaphragm press has several advantages over the fixed volume recessed
plate press. First, a dryer cake with a relatively uniform moisture content
is produced. This uniformity generally does not occur in the fixed volume
press, because low solids content feed sludge which produces the filtering
pressure is being continually added; thus, the inner part of the cake in each
cell is generally of low solids content. The second key advantage of the
diaphragm press is an overall shorter cycle time and therefore a higher
production thoughput. The primary reason for this shorter cycle is that the
diaphragm creates a more effective and uniform pressure on the sludge cake
32
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CAKE FORMS
IN THIS VOLUME
FILTRATE
FILTER CLOTH
SLUDGE FE
RIGID PLATE ASSEMBLY
WHICH HOLDS FILTER CLOTH
* *
FILTRATE
FILTER CLOTH
FIGURE 3-9
CROSS SECTION OF A FIXED VOLUME RECESSED PLATE
FILTER PRESS ASSEMBLY
than occurs when liquid sludge is pumped into the chamber. Two other
advantages of the diaphragm press are the lower operation and maintenance
requirements for the sludge feed pumps, and the ability to dewater a marginal-
ly conditioned sludge to a high solids content. Generally, a fixed volume
recessed plate press can not dewater a marginally conditioned sludge to a
satisfactory cake concentration. Another advantage of the diaphragm press is
that it does not require a precoat while a precoat is frequently required with
a fixed volume press.
The principal disadvantage of the diaphragm press is its higher initial cost,
which can be two to three times the cost of a fixed volume recessed plate
33
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ITytFTl
Filter cloth*
Filtering chamber •
Filtrate
2 t
6
Feed slurry
- Diaphragm
O O
STEP I -LOW PRESSURE
FILTRATION
T7&™
Diaphragm
i A High pressure
01v ««t«.
water
O O
STEP 2-COMPRESSION OF SLUDGE
BY THE DIAPHRAGM
STEP 3-CAKE DISCHARGE
Wash Water
STEP 4-FILTER CLOTH WASHING
FIGURE 3-10
OPERATIONAL CYCLE FOR A LASTA DIAPHRAGM FILTER PRESS
(Courtesy of Ingersoll-Rand)
34
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SQUEEZING WATER
PLATE WITH
DIAPHRAGM
DIAPHRAGM
PLATE
FILTER CLOTH
FEED SLUDGE
A/VT"
CLOTH
SUSPENSION
- FILTRATE
DIAPHRAGM
FILTRATE
STEP 1 - FILTRATION
STEP 2 - SQUEEZING
VIBRATING SHOE
\ w \ w
J C CAKE J \. CAKE J V
STEP 3 - CAKE DISCHARGE
WASHING
CYLINDER
WASHING
NOZZLE
WASH WATER
STEP 4 - CLOTH WASHING
FIGURE 3-11
OPERATIONAL CYCLE FOR AN ENVIREX-NGK DIAPHRAGM FILTER PRESS
(COURTESY OF REXNORDJ
35
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press with the same daily throughput. Another disadvantage is that although
the diaphragm press has a lower cycle time, the capacity of the largest
diaphragm filter press is generally less than that of the largest fixed volume
recessed plate filter press.
Filter press performance is measured by the solids content in the feed sludge,
required chemical conditioning dosages, cake solids content, total cycle time,
solids capture, and the yield, in kg/sq m/hr (Ib/sq ft/hr). These performance
parameters are all interrelated; for example, as the feed solids content
increases, the required chemical dosages and total cycle time usually
decrease, while the filter yield, or throughput, usually increases. As the
chemical conditioning dosage is increased up to the optimum level, the cake
solids content, solids capture, and yield all increase, while the cycle time
decreases. It should be noted that increasing chemical conditioning beyond the
optimum level can increase the overall volume and reduce the heat value of the
filter cake because of the addition of large quantities of inorganic
chemicals.
Control of filter presses may be manual, semi-automatic, or fully automatic.
Labor requirements for operation will vary dramatically depending on the
degree of instrumentation utilized for control. In spite of automation, opera-
tor attention is often needed during the dump cycle to insure complete separa-
tion of the solids from the media of the filter press. Process yields can
typically be increased 10 to 30 percent by carefully controlling the optimum
cycle times with a micro-controller. This is important since the capital costs
for filter presses are very high.
Table 3-9 presents the principal advantages and disadvantages of filter
presses compared to other dewatering processes. Common design shortcomings
associated with filter press installations are listed in Table 3-10, along
with solutions for these shortcomings. The fixed volume recessed plate filter
press will typically dewater a 50:50 blend of digested primary and waste
activated sludge to between 35-42% solids, while a diaphragm press will
produce a 38-47% solids cake on the same sludge. These cake solids concentra-
tions include large amounts of inorganic conditioning chemicals. Chapter 4 of
this manual presents more complete performance data for each type of press.
3.6 Drying Bed
Although the expression "drying bed" originally referred to a sand drying bed,
three other types of beds are also available: paved drying beds, wedgewater
filter beds, and vacuum assisted drying beds. Drying beds generally work best
in areas with little rainfall; however, they are extensively used in small
plants even in localities where rainfall averages up to 102 cm (40 in) per
year. It is important that sludge be well stabilized before being applied to
drying beds. If poorly stabilized or raw sludge is applied to sand beds,
dewatering will occur very slowly and substantial problems will result. Two
major problems are odor production and the occurrence of flies resulting from
further biological stabilization on the bed.
36
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TABLE 3-9
ADVANTAGES AND DISADVANTAGES OF FILTER PRESSES
Advantages
High solids content cake
Can dewater hard-to-dewater sludges,
although very high chemical
conditioning dosages or thermal
conditioning may be required
Very high solids capture
Only mechanical device capable of
producing a cake dry enough to meet
landfill requirements in some
locations
Disadvantages
Large quantities of inorganic
conditioning chemicals are commonly
used for filter presses
Polymer alone is generally not used
for conditioning due to problems
with cake release and blinding of
filter media. Experimental work on
polymer conditioning is continuing.
High capital cost especially for
diaphragm filter presses
Labor cost may be high if sludge is
poorly conditioned and if press is
not automatic
Replacement of the media is both
expensive and time consuming
Noise levels caused by feed pumps
can be very high
Requires grinder or prescreening
equipment on the feed
Acid washing requirements to remove
calcified deposits caused by lime
conditioning can be frequent and
time consuming
Batch discharge after each cycle
requires detailed consideration to
ways of receiving and storing cake,
or of converting it to a continuous
stream for delivery to an
incinerator
37
-------
TABLE 3-10
COMMON DESIGN SHORTCOMINGS OF FILTER PRESS INSTALLATIONS
Shortcomings
Improper conditioning
chemicals utilized
Insufficient filter
cloth washing
Inability to transport
dewatered cake from
dewatering building
Resultant Problems
Blinding of filter cloth
and poor cake release
Blinding of filter cloth,
poor cake release, longer
cycle time required,
wetter cake
Cake buildup and spillage
onto the floor
Solution
Improper filter cloth
media specified
Inadequate facilities
when dewatering a digested
sludge with a very fine
floe.
Poor cake discharge;
Difficult to clean
Poor cake release
Feed sludge is too dilute
for efficient filter
press operation
Sludge feed at only one
end of large filter
press
Long cycle time and
reduced capacity
Unequal sludge
distribution within
the press
Switch conditioning
chemicals or dosages
Increase frequency
of washing
Install cake breakers;
redesign angle of
screw conveyors or
belt conveyors to 15°
maximum angle.
Alternatively, use a
heavy duty flight
conveyor.
Change media
(1) Try two-stage
compression cycle with
first stage at low
pressure to build up
thickened sludge
"media" before
increasing pressure
(2) If this fails,
install precoat
storage and feed
facilities
Thicken sludge before
feeding to filter
press
Use equalizing tank or
centrifugal pump to
feed at opposite end
of press
38
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3.6.1 Sand Drying Beds
The operative dewatering principles involved in sand drying bed installations
are evaporation and percolation. Percolation may be either to the groundwater,
or to underdrain tiles located underneath the sand drying bed, and it
generally occurs quickly after sludge application. Evaporation is then respon-
sible for any further water removal. In some locations, environmental
constraints due to leaching of nitrogen compounds and other constituents have
resulted in the requirement to seal the bottom of the drying bed with an
impermeable liner. In this case, an underdrain system would be mandatory for
proper dewatering. To enhance the capabilities of sand drying beds in climates
with high precipitation rates, the use of covered drying beds has occasionally
been practiced. A key to the proper operation of covered beds is to provide
good ventilation.
Sludge conditioning is possible prior to application of sludge to the drying
beds. Such conditioning is generally not economically justified unless it is a
short term remedy until additional bed area can be constructed, or if adverse
weather has decreased the effectiveness of the beds. Long term operation with
polymer conditioning may also be practical if there is insufficient area for
drying bed expansion. Use of lime and ferric chloride for conditioning could
in certain cases result in chemical blinding of the sand layer.
In cases where underdrains are used, the gravel layer is typically 30 to 46 cm
(12 to 18 in) deep, the sand is 15 to 30 cm (6 to 12 in) deep, and the drain-
age pipes are located 3 to 6 m (10 to 20) apart. Sludge is applied in a layer
between 20 to 30 cm (8 to 12 in) across the entire bed and allowed to drain
and dry until the sludge is caked and cracked. At this point the dried sludge
is removed either manually or mechanically. Caking and cracking will generally
occur when the solids content reaches 35 to 40%, and this is the content at
which most sludge is removed. Sludge may, however, be removed at higher or
lower solids contents, depending upon the operator's ability to remove it from
the bed, and upon the disposal method for the dried sludge. Drying time
varies in a nonlinear manner with the depth of the applied sludge. For
example, a 20 cm (8 in) layer of sludge may dry in one-half the time required
for a 30 cm (12 in) layer of sludge. The optimum sludge application depth must
be determined on a sludge by sludge basis, and will be a function of the total
bed area, the number of beds, the digester capacity, the climate, and the
desired cake solids content for removal of dried sludge from the beds.
Advantages and disadvantages of sand drying beds are listed in Table 3-11, and
common design shortcomings are listed in Table 3-12.
Sludge removal from drying beds may be either manually or with a front-end
loader. Depending upon bed thickness, use of mechanical equipment can cause
problems because of its weight. Additionally, a portion of sand is lost as the
sludge is removed, and periodic sand replenishment is necessary.
39
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TABLE 3-11
ADVANTAGES AND DISADVANTAGES OF SAND DRYING BEDS
Advantages
Low capital cost—excluding land
Low operational labor/skill requirement
Low energy
Low maintenance material cost
Little or no chemicals required
High cake solids content possible
Disadvantages
Weather conditions such as rainfall
and freezing weather have an impact
on usefulness
Requires large land areas
High labor requirement for sludge
removal
May be aesthetically unpleasing,
depending on location
Potential odor problem with poorly
stabilized sludge
TABLE 3-12
COMMON DESIGN SHORTCOMINGS OF SAND DRYING BED INSTALLATIONS
Shortcomings
Inadequate Bed Area
Inadequate access for
removal of dried sludge
Inadequate drainage
system
Poor sludge distribu-
tion on the beds
Resultant Problem
Sludge must be removed
before it is dry enough;
conditioning chemicals
may be required
Solution
Construct additional
beds or use conditioning
chemicals
Dried sludge must be moved Construct roadway
considerable distance to
reach hauling truck
Longer than necessary
drying time
Inadequate use of bed
area
Improper sand gradation Slow drainage
between beds; cast con-
crete treadways in beds
for vehicle access; use
planks on bed to support
vehicles
Add additional drainage
pipes
Partition large beds
into smaller beds; level
sand in beds
Remove and replace sand
40
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3.6.2 Paved Drying Bed
To alleviate the problem of mechanical sludge removal equipment damaging the
underdrain pipes, paved drying beds were developed. In this concept, the beds
are paved with asphalt or concrete, and have approximately a 1 .5 to 2% slope
toward the center. A perforated drainage pipe is located in the center beneath
a sand drainage strip, at an elevation below the paved bed. The key advantage
of this type of bed is the ability to use mechanical equipment for sludge
removal without causing damage to underdrain pipes or loss of sand. The main
disadvantages are high capital cost and a larger land area requirement than
for sand beds.
3.6.3 Wedgewater Drying Beds
This type drying bed uses a wedgewater panel media placed in an open concrete
basin. The concrete basin may be either a new structure, or an existing sand
drying bed retrofitted by removing the sand and pouring a concrete bottom. The
Wedgewater panel media acts as a false bottom, and the volume beneath is used
for collection and removal of water which percolates through the media. Two
types of Wedgewater panel media are available. One is constructed of stainless
steel and the other is constructed of polyurethane. The stainless steel media
requires supports to be placed on the concrete floor of the basin, trtiile the
newer polyurethane media has integrally molded supports and is self-
supporting. The polyurethane media is manufactured in one square foot pieces,
each two inches high, which lock together using an integrally molded locking
arrangement. Both types of media can support a small front-end loader, when
properly installed.
Prior to introducing sludge, the valve controlling removal of drainage water
is closed, and the beds are filled with water to slightly above the media
surface. The sludge is then introduced, and the initial drainage rate from the
sludge is controlled by controlling the rate of water removed from the volume
beneath the media. Controlled drainage for a period of 15 minutes to 2 hours
is recommended by the manufacturer to maintain sludge porosity and reduce
compression of the sludge matrix. After the controlled drainage phase, the
sludge is allowed to further dewater by natural drainage for up to 24 hours.
It can then be removed.
According to the manufacturer, aerobically digested sludges can be dewatered
on a wedgewater drying bed to 8 to 12% solids within 24 hours and anaerobi-
cally digested sludges can be dewatered to 16 to 20% in 24 hours. The manufac-
turer indicates that the Wedgewater drying bed is most practical for the
smaller treatment plant which has an average daily flow of 0.13 cu m/s (3 mgd)
or less (11). As of July 1982, approximately 35 installations were operating,
with 8 new projects under construction.
41
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3.6.4 Vacuum-Assisted Drying Bed
Vacuum assisted drying beds use a porous media filter plate set above an
aggregate filled support plenum, which drains to a sump. A relatively small
vacuum pump is connected to draw vacuum from the sump. When polymer condition-
ed sludge is added to the bed surface, dewatering begins by gravity drainage.
When the maximum sludge level in the bed is reached, 30 to 46 cm (12 to 18
in), flow of conditioned sludge is stopped, and the vacuum pump operation
begins at 2.5 to 25 cm (1 to 10 in) of mercury. At the point when the cake
cracks, the vacuum pump is shut off, and the sludge can be mechanically
removed using a front-end loader.
The porous media filter plate is a specially fabricated material consisting of
a thin carborundum plate overlying a layer of sized aggregate Which is held
together with epoxy. The media filter plates are supplied in sheets, which are
caulked together after they are placed on the aggregate filled plenum. Caulk-
ing is also used around the periphery of the bed in order to provide a vacuum
seal. A typical size for one bed is 6 by 12 meters (20 by 40 ft), with a 1 hp
(0.7 kW) vacuum pump required for this size.
The manufacturer claims that a polymer conditioned anaerobically digested
sludge can be dewatered to 12-16% solids in less than 24 hours. Polymer cost
in this application would be about $8-12 per ton of dry solids. Typical design
loadings are about 10 kg/sq m (2 Ib/sq ft) per application, or about 30-57
1/sq m (8-15 gal/sq ft) per application. There are six installations at muni-
cipal wastewater treatment plants in the U.S. which have been installed since
1979, and at least three more installations are currently under construction.
Filtrate is low in suspended solids, generally less than 10 mg/1 (12).
3.7 Sludge Lagoon
Sludge lagoons are not a commonly utilized dewatering process, and little
definitive design criteria are available. Two types of sludge lagoons may be
utilized: storage lagoons and drying lagoons. The objective of storage lagoons
is to store sludge in relatively deep earthen or concrete basins for a multi-
year period, until a method of disposal is available. On the other hand,
drying lagoons are relatively shallow and are designed for in-place drying of
the sludge. In either type of lagoon, it is usually necessary to periodically
decant supernatant from the top of the lagoon and return it to the wastewater
treatment facility.
Sludge storage lagoons are between 1.5 and 4.6 m (5 and 15 ft) deep. The
duration of storage may be anywhere from 1 to 5 years, with the storage time
established by the ultimate form of disposal and variable local factors. At
some plants, storage lagoons have been used either because there was no method
available for disposal, or because the disposal methods could not accept all
of the sludge.
42
-------
Sludge drying lagoons are relatively shallow, with sludge being applied to a
depth generally between 15 and 38 cm (6 and 15 in). Water removal from lagoons
is by evaporation, and decanting is also frequently practiced. After sludge
has reached an air dried state, it is typically removed either by a front-end
loader or other mechanical equipment.
Table 3-13 lists advantages and disadvantages of sludge lagoons. Because there
are no defined guidelines for lagoon design, it is difficult to enumerate
common design shortcomings. However, areas in which the most mistakes occur in
lagoon design are: (1) too steep a bank slope, making bank maintenance
difficult; (2) inability to easily decant supernatant from the lagoon surface;
(3) an inadequate number of lagoons, even though overall volume of lagoons is
sufficient; (4) surface water is not diverted away from the lagoon; (5) no
ramps into the lagoon to allow entrance of sludge removal equipment; and (6)
insufficient concern is given to visual aesthetics and/or odor potential.
3.8 Gravity/Low Pressure Dewatering
Several manufacturers market devices which concentrate sludge by gravity
drainage or a combination of gravity drainage and low pressure pressing. For
descriptive purposes, they are referred to in this section as rotating cylin-
drical gravity dewatering devices and low pressure belt presses. The most
commonly used units are the Permutit Dual Cell Gravity Unit (DCG) sometimes in
conjunction with a Multiple Roll Press (MRP), the Ralph B. Carter Company
sludge Reactor-Thickener, and the Smith and Loveless Sludge Concentrator.
These devices are typically capable of producing a dewatered sludge with a
cake solids concentration in the range of 8 - 12%. The devices rely on large
dosages of polymer to condition the sludge. As a result, they are typically
considered for small plants where the annual cost of even large dosages of
conditioning chemicals is small.
A characteristic of gravity/low pressure dewatering devices is their
simplicity and relatively low cost compared to other dewatering devices. They
are quite useful where a large sludge volume reduction is required, as long as
the requirement for final sludge concentration does not exceed 12%. The large
volume changes which are experienced in dewatering from 3 - 4% to 10 - 12% are
illustrated in Figure 4-5. A sludge idiich is dewatered to only 8 percent
solids is often desired when the ultimate disposal method is land application
using a sludge truck designed for spreading or subsurface injection.
Table 3-14 lists advantages and disadvantages of these types of dewatering
device.
3.8.1 Rotating Cylindrical Gravity Dewatering Device
This type of equipment uses a cylindrical framework covered with a filter
media on the interior. As the device rotates, the conditioned sludge is
continuously exposed to clean filter media, which enhances gravity drainage.
These devices are sized on the basis of hydraulic loading.
43
-------
TABLE 3-13
ADVANTAGES AND DISADVANTAGES OF SLUDGE LAGOONS
Advantages
Low energy, labor, maintenance
material, and chemical requirements
Low capital cost - excluding land
Relatively insensitive to operational
upsets in the treatment system
Some organic decomposition will take
place
Disadvantages
Visually unattractive
Potential odor source
Potential problems with flies and
mosquitos
Requires more land than most other
dewatering concepts
TABLE 3-14
ADVANTAGES AND DISADVANTAGES OF GRAVITY/LOW PRESSURE
DEWATERING DEVICES
Advantages
Low energy and maintenance require-
ments
Low capital costs
Requires little operator skill
Low space requirements
Very little noise
Very useful for dewatering sludge to
8 percent solids level often required
for land application
Disadvantages
Only suitable for smaller plants
due to limited capacity per
machine
Can not produce a solids concen-
tration much above 10 - 12%
without excessive chemical use.
Require relatively large
conditioning chemical costs
44
-------
The Permutit DCG unit uses two cylindrical cells and a single piece of filter
cloth, as shown in Figure 3-12. The purpose of the first cell is dewatering,
while the second cell is used for additional dewatering and cake formation. A
variable rim depth on the discharge end of the second cell is used to control
sludge depth in this cell.
CONVttOt
FILTRATE OUCHMfil
FIGURE 3-12
CROSS SECTION OF A DUAL CELL GRAVITY UNIT
CCourtesy of the Permutit Company)
The Ralph B. Carter Company sludge Reactor—Thickener operates on the same
theory as the DCG unit, but only a single cylinder and combination screen made
of stainless steel and polyester weave are utilized. The Carter Reactor Thick-
ener system is also used on some Carter Belt Filter presses in place of a
gravity drainage zone. The manufacturers claim this increases the hydraulic
capacity of the belt press because the reactor thickener is more efficient
than gravity drainage (13). Paduska and Stroupe found this to be true based on
testing of an industrial waste activated sludge (14).
Performance data for the DCG indicate the capability of dewatering an
aerobically digested mixture of primary and waste activated sludge from 2.5%
to 9%, and an aerobically digested primary sludge from 2.5% to 8%. In general,
45
-------
both manufacturers indicate that their units are capable of dewatering most
sludges to at least 8-10% solids. In 1980, Permutit reportedly had over 20 DCG
installations on municipal applications (15). The Ralph B. Carter Company
reports that four Reactor-Thickener Units (without belt presses) for municipal
treatment plants have been installed since 1979 (13).
3.8.2 Low Pressure Belt Presses
Low pressure belt presses are the Smith and Loveless Sludge Concentrator and
Permutit MRP. The Sludge Concentrator, shown in Figure 3-13, is skid mounted,
and consists of a flashmix/flocculator, a gravity dewatering screen, and a
dewatering screen which passes under a series of rollers, with each roller
exerting higher pressure. Both belts are open mesh, and are variable speed.
Smith & Loveless reports that they had over 140 U.S. installations in 1981 and
that more than 15 Sludge Concentrators have been installed at municipal treat-
ment plants since 1980 (16). The Permutit MRP is a single pass, low pressure
spring loaded device, which presses sludge between two moving belts. This
device was developed to provide further dewatering of output from a DCG unit.
CONDITIONED
SLUDGE
POLYMER
SOLUTION
WASH SPRAY DISCHARGE
CHUTE
COMPRESSION
ROLLERS
FILTRATE
DISCHARGE
CHUTE
SLUDGE
SUPPORT PLATFORM
FILTER SCREEN
DEWATERED
SLUDGE
CAKE
FIGURE 3-13
CROSS SECTION OF A SMITH & LOVELESS CONCENTRATOR
46
-------
These devices are significantly less costly then the more complex, higher
pressure belt presses, which produce a higher solids content cake. Typical
sludge cakes produced by these low pressure presses are in the range of 8 to
12% with polymer dosages of 5 to 7.5 g/kg (10 to 15 Ib/ton) depending on the
sludge type. When the MRP is used after the DCG unit, sludge concentrations up
to 15% have been claimed by the manufacturer.
3.9 References
1. "Process Design Manual For Sludge Treatment and Disposal," USEPA - Center
for Environmental Research Information, Cincinnati, Ohio, 45268,
EPA-625/1-79/011, September 1979.
2. "Innovative and Alternative Technology Assessment Manual," USEPA - Office
of Water Program Operations, Washington, D.C., 20460, MCD-53,
EPA-430/9-78-009, February 1980.
3. Bachtel, David R., "Operation and Maintenance of a Low Speed Scroll
Centrifuge," County Sanitation District No. 2 of Los Angeles County,
California, March 1982.
4. Personal communication with Richard T. Moll, Manager of Process
Engineering, Sharpies-Stokes Division, Pennwalt Corporation, Warminster,
Pennsylvania, June 9, 1982.
5. Personal communication with Stephen H. Silverman, Sales Manager,
Centrifuge Division, KHD Humboldt Wedag, Atlanta, Georgia, June 1982,
6. Trubiano, R., Bachtel, D., LeBrun T., and Horvath, R. , "Parallel
Evaluation of Low Speed Scroll Centrifuges and Belt Filter Presses for
Dewatering Municipal Sewage Sludge," Draft EPA Report, Contract
68-03-2745, 1981. (Authors are with County Sanitation Districts of Los
Angeles County, Whittier, California.)
7. Harrison, J.R., "Review of Developments in Dewatering Wastewater
Sludges," Sludge Treatment and Disposal, Volume 1 - Sludge Treatment,
USEPA - Center for Environmental Research Information, Cincinnati, Ohio,
45268, EPA-625/4-78-012, October 1978.
8. Personal communication, Don Cline, Fulco Sales, Garden Grove, California,
Sales Representative for Envirotech, August 1981.
9. Personal communication, Don Herman, Herman-Phinney-Kodmur, Los Angeles,
California, Sales Representative for Parkson Corporation, June 1981.
10. Personal communication, Eric Hammarstron, Komline Sanderson Engineering
Corporation, Peapack, New Jersey, June 1981.
11. Personal communication, Vince Spalding, Hendrick Fluid Systems,
Carbondale, Pennsylvania, July 1982.
47
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12. Personal communication, Rob Ramsay, U. S. Environmental Products, Santa
Ana, California, April 1982.
13. Personal communication, LeRoy A. Swenson, Ralph B. Carter Company,
May 25, 1982.
14. Poduska, R.A. and Stroupe, R.C., "Belt-Filter Press Dewatering Studies,
Implementation, and Operation at the Tennessee Eastman Company Industrial
Activated Sludge Wastewater Treatment System," presented at the 35th
Annual Purdue Industrial Waste Conference, May 1980.
15. Personal communication, Bob Nagle, Permutit Company, Inc., Paramus, New
Jersey, June 1982.
16. Personal communication, Don Aholt, Smith & Loveless, Inc., Lenexa,
Kansas, May 27, 1982.
48
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CHAPTER 4
CAPABILITIES OF DEWATERING PROCESSES
4.1 Introduction
To define the capabilities of dewatering processes, a comprehensive and
critical review was made of all available experience from full-scale
operations. Engineering judgment was used in interpreting the data reviewed,
and it is possible that others would reach different conclusions from the same
information. Sources of information included the published literature,
communication with manufacturers, literature from manufacturers, wastewater
treatment plant contacts, communication with consultants, discussions with
government officials, and the authors' own files. It is realized that there
may exist information that differs from that presented here and which was not
readily available to the writers of this manual.
Data were obtained from side-by-side comparisons of different dewatering
processes as well as side-by-side comparisons of the same type of equipment
supplied by different manufacturers. Similarly, data were obtained for
equipment permanently installed at plant sites. Much of the information
gathered included the newer advances in dewatering technology: the solid bowl
centrifuge with backdrive capability, third generation belt filter press, and
the diaphragm filter press.
The principal factors which influence the capabilities of dewatering
processes, and which were considered in the writers' review, are:
• Source of Sludge - Domestic wastewater
- Domestic wastewater with a varying percentage of
industrial wastewaters
• Type of Sludge - Primary
- Biological (WAS, TF, RBC, etc)
- Combinations of primary and biological
• Sludge Solids Concentration
• Prior Handling of Sludge - Thickening
- Stabilization
- Storage
- Transport
• Process Design - Conditioning provisions
- Operational flexibility
49
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4.2 Performance Capabilities of Mechanical Dewatering Processes
Based upon an evaluation of the performance information collected, a series of
four figures was developed to illustrate the typical performance of mechanical
dewatering processes with different types of sludges. Each figure presents a
range for the sludge cake solids concentration expected from each dewatering
process. The cake solids concentration varies for several reasons. First and
most importantly, the cake solids concentration produced by any dewatering
process can be influenced by the sludge feed rate and by changing the parame-
ters that influence the process operation. Principal process operational
variables will be described in Section 4.3 of this Chapter. Naturally, both
overall economics and the degree of solids capture need to be considered in
determining the optimum operation of the dewatering process. Secondly, choice
and quantity of conditioning chemicals added can dramatically change the final
sludge cake solids concentration; again, economics are a key factor to be
considered in selecting the optimum chemical dosage. Third, no sludge consis-
tently exhibits the same dewatering characteristics, and sludges from differ-
ent plants exhibit wide variations in their ability to be dewatered. A number
of factors are responsible for such variations, including the influence of
industrial discharges on sludge composition, particularly its organic content,
and the variability of preceding processes in the sludge treatment system such
as thickening, storage or holding, transport, and stabilization operations.
Figures 4-1 to 4-4 are presented to illustrate the capabilities of mechanical
dewatering processes on different types of sludge, and each of these figures
is described in subsequent paragraphs. In utilizing the information in these
figures, the reader is cautioned that the cake solids concentrations given do
not correct for any inorganic conditioning chemicals, do not take into account
the cost of chemical conditioning, and do not take into account the percent
recovery obtained. The data are, however, based on reasonable levels of
chemical conditioning and solids recoveries for the processes considered.
Figure 4-1 provides typical ranges for dewatered sludge cake solids
concentrations produced by mechanical dewatering processes on digested primary
and waste activated sludge (WAS) combinations. It is apparent from this figure
that as the percentage of WAS increases, the achievable cake solids concen-
tration decreases, and similarly, that 100% primary sludge is much easier to
dewater than 100% WAS. Figure 4-1 also illustrates the differences in the
capabilities of various mechanical dewatering processes. The diaphragm filter
press will typically produce the driest, most highly dewatered sludge cake of
any mechanical dewatering process, while the fixed volume recessed plate or
conventional filter press (both high and low-pressure) will produce the next
highest solids content cake. Belt filter presses, solid bowl centrifuges, and
vacuum filters can all produce similarly dewatered cakes, although belt
presses are generally capable of producing the driest cake of these three
processes. Basket centrifuges generally produce a cake somewhat lower in
solids concentration than the other dewatering processes. It should be noted
that the cake solids contents for the diaphragm filter press, the conventional
filter press, and the vacuum filter will usually include large amounts of
inorganic conditioning chemicals. These additives reduce the actual sludge
solids content.
50
-------
10
DEWATERED SLUDGE CAKE, PERCENT TOTAL SOLIDS
20 30 40 50
60
100% P( 0% WAS
70% Pi 30% WAS
50% P: 50% WAS
30% Pt 70% WAS
B.
'
B. CENT.
^
B. CENT.
, CENT.
( V.F.
B. CENT.
CENT.
1 V'FN
, B.P.
CENT.
CENT. g
V.F. (
B.P.
, CENT,
V.F.
CENT. .
CENT. ,
V.F.,
B.P.
B.P^
B.P.,
F.P.
1 F'P- ,
I
F.P. (
D.F.P.
F.P.
. D.F.P.
F.P. §
D.F.P.
D.F.P.
D.F.P.(
— — 1
LEGEND
CENT. -SOLID BOWL CENTRIFUGE B. CENT. -BASKET CENTRIFUGE REFERENCES - 1,2,3,4,5,6,7,
V.F. -VACUUM FILTER F.P. -FILTER PRESS 8,9,10,11,12,
B.P. -BELT PRESS D.F.P. -DIAPHRAGM FILTER PRESS 13,14,15,16,
17,18,19,20.
FIGURE 4-1 21,22.23,24
OEWATERED SLUDGE CAKE PERCENT SOLIDS FOR MIXTURES OF DIGESTED
PRIMARY (P) AND DIGESTED WASTE ACTIVATED SLUDGE (WAS)
-------
In Figure 4-2, typical de-watered sludge cake solids concentrations are shown
for raw primary and raw WAS. From Figure 4-2, it can be seen how much more
difficult it is to dewater the raw WAS than the raw primary sludge. Also, by
comparing Figures 4-1 and 4-2, it is evident that most mechanical processes
can dewater raw sludge to between 2 to 5% higher solids concentration than
digested sludge. This difference partially occurs because anaerobic digestion
produces a larger proportion of fine-sized particles than is typically found
in raw sludge, and these smaller particles tend to hinder dewatering as
discussed in Chapter 2. Anaerobic digestion also significantly reduces the
quantity of sludge solids to be dewatered; however, the sludge solids concen-
tration is also significantly reduced, which adversely affects dewatering.
Figure 4-3 shows typical dewatered sludge cake solids concentrations for raw
primary plus raw WAS, raw trickling filter (TF) sludge, raw primary plus raw
TF sludge, and raw primary plus raw rotating biological contactor (RBC)
sludge. Data were not available for the performance of all mechanical dewater-
ing processes with all types of sludges, and therefore for some of the sludge
types only one or two dewatering processes are shown. This does not necessar-
ily mean that only the processes shown are appropriate for dewatering that
type of sludge, and the equipment manufacturer should be consulted for
specific advice on particular applications. A comparison between raw primary
plus WAS and raw primary plus TF sludge shows that TF sludge is generally
easier to dewater than WAS. Raw primary plus RBC sludge is also easier to
dewater than raw primary plus WAS, In general, these variations are the result
of the denser nature of the attached growth TF and RBC sludges and the fact
that suspended growth WAS contains more fine material.
Figure 4-4 presents typical data for the dewatering of digested TF sludge and
digested primary plus TF sludge. These data, when compared with the data for
raw primary plus TF sludge and raw TF sludge in Figure 4-3, again illustrate
that digestion increases the difficulty in dewatering. Also shown in Figure
4-4 are data for thermal conditioned primary plus WAS and primary plus TF.
Thermal conditioning will produce a sludge with excellent dewatering charac-
teristics, because cellular solids have been broken down and the intercellular
liquid contents are released. However, there are also a number of unfavorable
aspects of thermal conditioning which must be considered.
The overall conclusions which can be reached after comparing the data
presented in Figures 4-1 to 4-4 are:
• Solid bowl centrifuges and vacuum filters produce comparable cake
solids concentrations.
• A third generation belt filter press can produce a cake with up to a
several percent higher solids content than can a solid bowl centrifuge
or vacuum filter.
• Diaphragm filter presses produce sludge cakes with a 2-6% higher
solids concentration than a conventional fixed volume filter press.
52
-------
DEWATERED SLUDGE CAKE, PERCENT TOTAL SOLIDS
c
RAW PRIMARY
RAW WAS
) 1
^B.
(S 4 L
.D.C.G
0 2
,S A L,
D.C.G. ,
CENT.
.. . -. . 4
. V.F.
SENT.,
•—4
0 3
V.F.
B.CENT.
B.P.,
0 4
CENT. .
..__j
B.P.
P.P. (
D.F.P.
0 5
P.P.
t D.F.P.
0 6
LEGEND
CENT. -SOLID BOWL CENTRIFUGE D.F.P.
V.F. -VACUUM FILTER S 4 L
B.P. -BELT PRESS
B. CENT. -BASKET CENTRIFUGE D.C.G.
F.P. -FILTER PRESS
-DIAPHRAGM FILTER PRESS
-SMITH & LOVELESS SLUDGE
CONCENTRATOR
-PERMUTIT DUAL CELL
GRAVITY UNIT
FIGURE 4-2
DEWATERED SLUDGE CAKE PERCENT SOLIDS
FOR RAW PRIMARY AND RAW WAS
REFERENCES
-1,2,3,5,6.18,
22,25,26,27
-------
DEWATERED SLUDGE CAKE, PERCENT TOTAL SOLIDS
10 20 3O 40 50
60
RAW P 4 WAS
(Approx. l«l ratio)
RAW TF
RAW P 4 TF
RAW P 4 RBC
ts
D.I
B.
K-
B.CENT.
i. Lj
;.Gt|
CENT.
— I
.B.CENT.
.B.CENT.
1 1
CENT.,
V.F.,
B.P.
V.F.
CENT.
1 V'F- i
B.P.
F.P.
D.F.P.
D.F.P.
LEGEND
CENT. -SOLID BOWL CENTRIFUGE
V.F. -VACUUM FILTER
B.P. -BELT PRESS
B. CENT. -BASKET CENTRIFUGE
F.P. -FILTER PRESS
D.F.P. -DIAPHRAGM FILTER PRESS
S 4 L -SMITH 4 LOVELESS SLUDGE
CONCENTRATOR
D.C.G. -PERMUTIT DUAL CELL
GRAVITY UNIT
RBC -ROTATING BIOLOGICAL
CONTACTOR
P -PRIMARY
WAS -WASTE ACTIVATED SLUDGE
TF -TRICKLING FILTER
REFERENCES
-1,2,4,7,15,18,
22,28,29,30
FIGURE 4-3
DEWATERED SLUDGE CAKE PERCENT SOLIDS FOR MIXTURES OF
RAW PRIMARY AND SECONDARY SLUDGES
-------
DEWATERED SLUDGE CAKE, PERCENT TOTAL SOLIDS
ui
c
DIGESTED TF
DIGESTED P & TF
THERMAL COND.
P & WAS
THERMAL COND.
P & TF
) 1
0 2
. V.F.
0 3
V.F. ,
0 4
CENT.
CENT.
0 5
D.F.P. ,
V.F. (
B.P.
0 6
D.F.P.
LEGEND
CENT. -SOLID BOWL CENTRIFUGE P.P.
V.F. -VACUUM FILTER D.F.P.
B.P. -BELT PRESS TF
P -PRIMARY WAS
-FILTER PRESS
-DIAPHRAGM FILTER PRESS
-TRICKLING FILTER
-WASTE ACTIVATED SLUDGE
REFERENCES - 1,18
FIGURE 4-4
DEWATERED SLUDGE CAKE PERCENT SOLIDS FOR MIXTURES OF
DIGESTED PRIMARY AND SECONDARY SLUDGE AND HEAT TREATED
PRIMARY AND SECONDARY SLUDGE
-------
• Digested primary sludge can be dewatered to a significantly higher
solids content than digested WAS. The extent varies among the
different dewatering processes.
• Raw sludge can typically be dewatered to a solids concentration that
is 2 to 4% higher than that for the same sludge which has been
digested.
• TF and RBC sludges, either raw or digested, dewater to a higher solids
content than WAS.
• All processes exhibit a range of probable sludge cake solids
concentrations, due to varying loading or feed rates, amount and type
of conditioning utilized, equipment operational variables, and the
variability of the sludge composition from location to location.
4.3 Process Operational Variables Which Affect Dewatering Results
As the prior section illustrates, a substantial range in dewatered sludge cake
solids concentrations is evidenced for all mechanical dewatering processes.
One reason is variable sludge composition from plant to plant, while an
important second reason is the number of variables associated with operation
of the dewatering process. All dewatering processes have several operational
variables which influence process performance. The four key factors normally
used to evaluate process performance are: cake solids concentration; percent
solids capture; process throughput; and conditioning chemical requirements. It
is not possible to vary process operation to simultaneously optimize all four
process performance indicators. For example, a change in process operation to
increase cake solids concentration without changing conditioning chemical
dosage, would likely result in decreases in process throughput and solids
capture. The process operator must determine which of the four process
performance indicators are most important and change the operational variables
to achieve the desired results.
For the dewatering processes included in this manual, Table 4-1 lists key
operational variables, and these operational variables are discussed in the
following sections.
4.3.1 Basket Centrifuge
Increasing the bowl speed and the time at full speed will increase cake solids
content and usually solids capture, although increasing the time at full speed
will reduce machine throughput. An increase in the depth of skimming will
result in a drier cake, but it will return more solids back to the plant for
subsequent retreatment. Polymer dosage increase will increase cake solids
concentration and percent solids capture up to a point. An increase in sludge
feed rate will increase the throughput, but may require more polymer and
produce a lower cake solids concentration with a lower solids capture.
56
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TABLE 4-1
OPERATIONAL VARIABLES FOR DEWATERING PROCESSES
1. BASKET CENTRIFUGE
A. Bowl speed
B. Time at full speed
C. Depth of skimming
D. Sludge feed rate
E. Polymer conditioner
• Dosage utilized
• Point of addition
2. SOLID BOWL CENTRIFUGE
A. Bowl/conveyor differ-
ential speed
B. Pool depth
C. Sludge feed rate
D. Polymer conditioner
• Dosage utilized
• Point of addition
3. BELT FILTER PRESS
A. Belt speed
B. Belt tension
C. Washwater flow and pressure
D. Belt type
E. Sludge feed rate
F. Polymer conditioner
• Dosage utilized
• Point of addition;
contact time; mixing
4. VACUUM FILTER
A. Quantity of wash E^O used
B. Drum Speed
C. Vacuum level
D. Conditioning chemicals -
type & dosage
E. Drum submergence
F. Vat agitation
G. Filter media used
5. CONVENTIONAL FILTER PRESS
A. Pressure of feed sludge
B. Filtration time
C. Use of Precoat
D. Conditioning chemicals -
type & dosage
E. Cloth washing frequency
F. Filter cloth used
6. DIAPHRAGM FILTER PRESS
A. Pressure of feed sludge
B. Filtration time
C. Diaphragm pressure
D. Diaphragm squeezing time
E. Conditioning chemicals
• Type & Dosage
• Point of addition
F. Filter cloth used
G. Frequency of cloth washing
7. DRYING BEDS
A. Depth of sludge application
B. Conditioning of sludge
C. Duration of drying time
D. Method of sludge cake removal
8. SLUDGE LAGOONS
A. Frequency of sludge addition
B. Method of sludge removal
C. Method of supernating
9. GRAVITY/LOW PRESSURE DEWATERING
A. Rate of sludge feed
B. Polymer concentration
C. Belt speed
D. Force applied by rollers
E. Depth of dewatered sludge in
cylindrical devices
57
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4.3.2 Solid Bowl Centrifuge
Increasing the bowl speed will in theory increase the cake dryness, because of
higher gravitational force. However, in some cases the increased shear of the
sludge floe which occurs when the sludge is fed will tend to offset the advan-
tage of the higher bowl speed. Shearing of the sludge floe at increased G
forces is usually not a problem in a centrifuge where the polymer is added
internally and the floe is formed after both the polymer and feed have reached
the speed of the centrifuge. In a solid bowl conveyor centrifuge, the scroll
operates at a slightly slower or higher speed than the bowl. As the scroll
speed approaches the bowl speed, the resultant differential speed is reduced
and machine capacity decreases. As the bowl-conveyor differential speed
increases, solids are removed from the machine quicker, thereby increasing
machine capacity. Offsetting this, however, is the usual production of a
wetter cake, when the solids are removed faster from the machine. Use of a
backdrive to maintain either a constant torque or a constant differential
speed between the scroll and the bowl will usually result in a drier sludge
cake, but at the same time will decrease machine throughput. An increase in
the pool depth will result in increased solids capture, but generally a wetter
sludge cake is produced. Increasing the polymer dosage will generally increase
both cake dryness and solids capture, although an increase in solids capture
can cause the cake solids content to be reduced as more fine material is
captured.
4.3.3 Belt Filter Press
Machine throughput can be increased by increasing belt speed, with the usual
result being production of a lower solids content cake, because both gravity
drainage time and press time are decreased. Increased belt tension will
promote a drier cake, but solids capture will normally decrease, and belt wear
will increase. An increase in washwater flow and/or pressure can increase cake
solids concentration, if the washwater was not adequately cleaning the belt.
Also, the more porous the belt, the drier the cake and lower the solids
capture. An increase in sludge feed rate can increase machine throughput if
the belt speed is high enough to move the sludge, and if the polymer dosage is
high enough to maintain solids capture. As polymer dosage increases, both cake
solids and solids capture increase, until an upper limit is reached. Point of
polymer addition can be important to allow sufficient contact time before the
conditioned sludge is applied to the belt press.
4.3.4 Vacuum Filter
In cases where insufficient cloth washing is used, increasing the amount of
cloth wash water will increase the machine throughput and will help to some-
what increase cake dryness. A high drum speed will increase machine throughput
but may decrease solids content of the cake. A high vacuum level will increase
the cake solids content at the expense of increased energy consumption. An
58
-------
increased drum submergence will increase machine capacity but will decrease
drying time and may decrease solids content of the cake. Vat agitation is
necessary for proper cake formation, but over-agitation will result in break-
ing up the sludge floe and poor solids capture. The addition of scraper
blades, use of excess chemical conditioner, or addition of fly ash, are some-
times required to obtain cake release from cloth media vacuum filters. This
is especially true if the sludges are greasy, sticky, and/or contain a large
quantity of waste activated sludge.
4.3.5 Fixed Volume Filter Press
Use of a higher feed pressure and a longer cycle time will increase cake
solids concentration, although the latter will decrease machine throughput.
Use of a precoat will improve solids capture, reduce filtration time, and
preserve the media's efficiency. A precoat would normally be required only for
a digested sludge which has very fine floe, or to obtain an adequate cake
release. For a "sticky" sludge, use of a precoat actually saves time by
significantly reducing the cloth washing frequency. Conditioning is particu-
larly important. To achieve an adequate cake release and a reasonable filtra-
tion time, lime and ferric chloride are typically required for conditioning,
although thermal conditioning can also be used and there has been some limited
success using polymers. A correct conditioning chemical dosage will result in
a dry cake, while an incorrect dosage will decrease machine throughput due to
the use of excess chemicals, or will produce a wet sludge cake. Frequent
filter cloth washing will increase machine throughput, cake dryness, and cloth
life, while use of a suitable filter cloth will increase solids capture and
probably the machine capacity.
4.3.6 Diaphragm Filter Press
Feed sludge pressure and pumping time only have a moderate effect on the
product cake. More significant factors are the diaphragm pressure and the
diaphragm squeezing time, both of which increase cake dryness when they are
increased. Influence of other variables is similar to the fixed volume filter
press. The type of filter cloth used is generally established by wear and
abrasion resistance, ease of cake release, and quality of the filtrate (solids
capture).
4.3.7 Drying Beds
Bed capacity is maximized by using shallow sludge applications and
conditioning the sludge with polymer. Naturally, longer drying times will
produce a greater cake solids content; however, if the cake is dry enough to
be removed using mechanical equipment and if the bed capacity is required for
the application of wet sludge, or if there is the potential for substantial
rainfall, it may be necessary to remove the dried sludge prior to the
59
-------
achievement of optimum dryness. Sludge cake removal can be performed manually
in very small plants, although typically it is removed with a front-end loader
or grader.
4.3.8 Sludge Lagoons
Use of relatively infrequent sludge applications will result in better
settling, a higher cake solids concentration and fewer solids in the recycle.
If sludge is removed by a dragline and allowed to dry on the lagoon periphery,
it will have a higher solids content than if a dredge is used for solids
removal. Supernatant can be removed by fixed pipelines at several depths in
the lagoon or by lowering a submersible pump or suction line to the desired
depth in the lagoon.
4.3.9 Gravity/Low Pressure Dewatering
Both rotating cylindrical gravity dewatering devices and low pressure belt
presses will produce a higher solids content cake at lower sludge feed rates
and higher polymer dosages. These are the two most important operational
factors. Other operational factors are the depth of sludge in the rotating
cylindrical devices, and the belt speed and roller pressure in low pressure
belt presses.
4.4 Effect of Dewatering on Sludge Volume
As cake solids content increases, dramatic reductions in sludge volume occur,
as shown in Figure 4-5. However, as higher cake solids concentrations are
achieved, the percentage volume reduction is not as great. For example,
increasing dewatering from 10 to 15% solids reduces volume by 35%, vfoile
increasing dewatering from 20 to 25% solids only reduces volume by 21%. In
other words, as the final dewatered cake concentration increases from a low
level to a higher level, the incremental volume reduction becomes lower. This
relationship is an important factor.
In certain situations, the relatively inexpensive dewatering processes that
dewater from 3% solids to 8 to 12% cake solids may be economically justified
even though hauling and disposal costs may be higher. This is because a volume
reduction of 70 to 80% can be achieved with even the gravity/low pressure
devices. Also, in situations involving further dewatering beyond 20%, volume
and weight reductions may not be justified on an overall economic basis. While
no economic decisions can be made based solely on Figure 4-5, the relationship
presented is often useful in the initial screening stage of evaluating
dewatering concepts, where dewatering requirements and possible ways of accom-
plishing them are being evaluated. This is discussed in detail in Chapter 6.
60
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VOLUME -LITERS
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Another way of evaluating differing cake solids concentrations is to consider
the moisture content in terms of mass of water per mass of solids. For
example, filter press cake of 40 percent solids contains 1 1/2 Ib of water/lb
of solids, while a vacuum filter cake of 15-20 percent solids contains 4-5.7
Ib of water/lb of solids. Therefore, if the sludge is to be reduced by
incineration, 3 to 4 times as much water would have to be heated and vaporized
from the vacuum filter cake compared to the filter press cake.
The use of an inorganic chemical conditioning chemical will increase the mass
of sludge solids and may increase both the overall sludge mass and volume.
This effect is shown in Figure 4-6 for conditioning chemical usages of 50,
100, 200 and 300 g/kg (100, 200, 400, and 600 Ib/ton) of dry weight solids.
This Figure indicates that, if conditioning chemicals are added equivalent to
approximately 20% of the sludge weight with the sludge cake solids concentra-
tion fixed, the sludge volume is increased by 20%.
It is, however, possible to reduce the volume of the sludge cake produced by
using inorganic conditioning chemicals. For example, if a vacuum filter
produced a 15 percent cake on a primary sludge when conditioning with polymer,
the volume from Figure 4-6 (based on 910 kg or 2,000 Ib of solids) would be
about 5.6 cu m (200 cu ft). If an inorganic conditioning chemical dosage of
100 g/kg (200 Ib/ton) increased the cake solids concentration to 25 percent
solids, the volume would be reduced to about 3.7 cu m (130 cu ft). This is a.
volume reduction of 35 percent. These important factors must be incorporated
into the initial screening process and the initial cost evaluation, as
described in Chapter 6.
Example calculations showing the computation of sludge volumes produced by
different dewatering processes are shown in Appendix B. The cake volume
comparison in Appendix B shows that the cake volumes (smallest volume to
largest) per unit weight of solids dewatered including conditioning chemicals
are: drying bed; diaphragm filter press; fixed volume filter press; sludge
lagoons; belt press; solid bowl centrifuge; vacuum filter; basket centrifuge;
and gravity/ low pressure dewatering devices. Some caution, however, must be
applied to the use of cake volumes shown for drying beds and sludge lagoons
since a very wide range of sludge cake solids can be produced. Sludge lagoons
may produce a sludge with solids concentrations ranging from 5 to 40 percent,
while drying beds may produce sludge cakes ranging from 15 to more than 70
percent solids. Given a sufficient drying time, a well designed and operated
drying bed can produce a drier sludge (with a lower volume) than any mechani-
cal device.
From the standpoint of trucking and subsequent handling, caution must be
exercised in comparing filter press cake volumes with cake volumes of other
mechanical devices which do not produce as dry a cake. For the filter press
cake, the "bulk" volume of the sludge is the important factor, as it accounts
for air spaces between the pieces of cake. For other types of sludge cake, the
"true" volume is the important criterion, since the cake produced is generally
moist enough to readily compact. The increase in volume for the filter press
cake is not possible to quantify without actual testing.
62
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Z6--
20--
15--
(0- •
7-
6-
5--
1000
»00
eoo
700
600
500
400
300
ZOO
ISO
u
I
2--
I 100
111
|90
§ 80
m
70
60
50
I.O--
9--
8--
7-.
6--
3..
40
30
15
3--
'% ' ' ' ' 8'o '
CHEMICAL CONDITIONING
Ik/ton dw»
(I Ib/toii m 0.5
BASED ON 910 kg (2000 Ik)
OF DRY SLUDGE SOLIDS
600
400
200
100
0
25 ' ' ' ' 30 ' ' ' ' 35 ' ' ' ' 40 ' ' ' ' 45 ' ' ' ' 50
CAKE SOLIDS. PERCENT
FIGURE 4-6
EFFECT OF INORGANIC CONDITIONING CHEMICAL DOSAGE
ON DEWATERED SLUDGE VOLUME
63
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4.5 References
1. "Process Design Manual For Sludge Treatment and Disposal," USEPA - Center
for Environmental Research Information, Cincinnati, Ohio, 45268,
EPA-625/1-79/011, September 1979.
2. Harrison, J. R. , "Review of Developments in Dewatering Wastewater
Sludges," Sludge Treatment and Disposal, Volume 1 - Sludge Treatment,
USEPA - Center for Environmental Research Information, Cincinnati, Ohio,
45268, EPA-625/4-78-012, October 1978.
3. Trubiano, R., Bachtel, D., LeBrun, T., and Horvath, R., "Parallel
Evaluation of Low Speed Scroll Centrifuges and Belt Filter Presses for
Dewatering Municipal Sewage Sludge," Draft EPA Report, Contract
68-03-2745, 1981. (Authors are with County Sanitation Districts of Los
Angeles County, Whittier, California)
4. Gulp, Gordon L. and Hinrichs, Daniel J., "Municipal Wastewater Sludge
Management Alternatives," prepared for the EPA Technology Transfer
National Conference on 208 Planning and Implementation, 1977. (Authors
are with Culp/Wesner/Culp, Cameron Park, California)
5. "Mechanical Dewatering Study - Los Angeles County Sanitation Districts,"
LA/OMA Project, Regional Wastewater Solids Management Program, Los
Angeles-Orange County Metropolitan Area, September 1980. (County
Sanitation Districts of Los Angeles County, Whittier, California)
6. "Mechanical Dewatering Study - Orange County Sanitation Districts,"
LA/OMA Project, Regional Wastewater Solids Management Program, Los
Angeles-Orange County Metropolitan Area, September 1980. (County
Sanitation Districts of Orange County, Fountain Valley, California)
7. Villiers, R. V. and Farrell, Joseph B., "A Look at Newer Methods for
Dewatering Sewage Sludges," Civil Engineering - ASCE, December 1977.
8. CH2M-Hill, "Michelson Water Reclamation Plant - Engineering Report for
Dewatering Equipment Selection," Irvine Ranch Water District, Irvine
California, June 1979.
9. Tavery, M. A., "Evaluation of Sludge Dewatering Equipment at the Metro
Denver Sewage District," paper presented at the Colorado AWWA-WPCA
Technical Activities Committee, May 3, 1979. (Author is with the
Metropolitan Denver Sewage Disposal District No. 1, Denver, Colorado).
10. Parkson Corporation, "Summary of Test Results on Magnum Press at Metro
Denver S.T.P., Denver, Colorado, October 30 - November 3, 1978."
(Parkson Corporation, Fort Lauderdale, Florida)
11. Madden, J., "Tait-Andritz Sludge Dewatering Report to Metropolitan Denver
Sewage Disposal District No. 1," January 10, 1979. (Author is with
Pfister and Associates, Denver, Colorado)
64
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12. Pennwalt Sharpies, "Technical Department Report for Metro-Denver Sewage
Disposal District No. 1," January 25, 1979. (Sharpies-Stokes Division,
Pennwalt Corporation, Warminster, Pennsylvania)
13. John Carollo Engineers, "Design Memorandum No. 5 - Dewatering Methods,"
County Sanitation Districts of Orange County, Fountain Valley,
California, April 1979.
14. Consoer, Townsend & Associates Ltd., "Draft Project Report - Sludge
Processing Facilities Plan For the Cities of San Jose and Santa Clara,
California," May 1980.
15. Ettlich, William F., Hinrichs, Daniel J., and Lineck, Thomas S.,
"Operations Manual - Sludge Handling and Conditioning," USEPA - Office of
Water Program Operations, Washington, D.C., 20402, EPA-430/9-78-002,
February 1978.
16. Hansen, Blair E., Garrison, Walter E., and Smith, Donald L., "Start-up
Problems of Sludge Dewatering Facility," Journal WPCF, October 1980.
17. Zenz, D. R., et al., "Evaluation of Unit Processes for Dewatering of
Anaerobically Digested Sludge at Metro Chicago's Calumet Sewage Treatment
Plant," The Metropolitan Sanitary District of Greater Chicago, October
1976.
18. Ingersoll-Rand, Unpublished data on filter press and centrifuge test
results, 1977 - 1979. (information from Wayne B. Gendron, Ingersoll-Rand,
Nashua, New Hampshire)
19. Moser, J.H., et.al., "Milwaukee Water Pollution Abatement Program Solids
Handling Study," Milwaukee Metropolitan Sewerage District, May
1981. (Author is with Milwaukee Metropolitan Sewerage District)
20. Sawyer, Bernard; Watkins, Robert; and Lue-Hing, Cecil, "Evaluation of
Unit Processes for Mechanical Dewatering of Anaerobically Digested Sludge
at Metro Chicago's West-Southwest Sewage Treatment Plant," Paper
presented at the 31st Annual Purdue Industrial Waste Conference, May
1976. (Authors are with the Research and Development Department of The
Metropolitan Sanitary District of Greater Chicago)
21. Greenhorne & O'Mara Engineers, "Nassau County Sludge Study Composting and
Dewatering Demonstration Program—Final Report," July 1979. (Greenhorne &
O'Mara Engineers are in Riverdale, Maryland)
22. Passavant Corporation, Unpublished data on filter press and belt filter
press test results. (Information received from E. D. Simmons, Vice-
President, Technical Services, Passavant, Birmingham, Alabama, May 1982)
23. East Bay Municipal Utility District, Unpublished Data on Low Speed Solid
Bowl Centrifuge, (information received from Daryl G. Deruiter, Associate
Environmental Engineer, EBMUD, Oakland, California, June 1982)
65
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24. Sharpies, Stokes Division, Pennwalt Corporation, Unpublished Data on
High-G Solid Bowl Centrifuge Test Results. (Information received from
Richard T. Moll, Manager of Process Engineering, Sharpies-Stokes,
Warainster, Pennsylvania, June 1982)
25. Schillinger, George R. , "Conversion of Sludge - Conditioning Chemicals,"
Deeds & Data, WPCF Highlights, April 1979.
26. Cassel, Alan F. and Johnson, Berinda P., "Evaluation of Dewatering
Devices For Producing High-Solids Sludge Cake," USEPA - Municipal
Environmental Research Laboratory, Cincinnati, Ohio 45268,
EPA-600/2-79-123, August 1980.
27. Marx, C. J. and Keay, G. F. P., "Towards A Rational Sludge Disposal
Policy For Johannesburg, "Presented at the Institute of Water Pollution
Control Conference, Pretoria, South Africa, June 1980. (Author Keay is
with City Engineer's Dept., Johannesburg, South Africa)
28. Ingersoll-Rand, "Lasta Filter Press Demonstration Detroit Metropolitan
Wastewater Treatment Plant, July 16 - August 10, 1979 and August 20 -
August 30, 1979, March 1980. (information received from Wayne B. Gendron,
Ingersoll-Rand, Nashua, New Hampshire)
29. Gulp, G. L., "Handbook of Sludge Handling Processes - Cost and
Performance," Garland STPM Press, New York, 1979.
30. Kupper Associates and Metcalf & Eddy, Inc., "Pilot Plant Dewatering
Testing for the Recommended Land-Based Sludge Management Plan," Middlesex
County Sewerage Authority, New Jersey, January 1979.
66
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CHAPTER 5
CHEMICALS USED IN DEWATERING
5.1 Introduction
Inorganic chemicals, such as lime and ferric chloride, and organic polymers
are typically used to condition a sludge prior to dewatering. These chemicals
destabilize the surface charge on the sludge particles, and flocculate the
sludge particles into a matrix which is more easily dewatered than the
discrete particles, as discussed in Chapter 3. When comparing various condi-
tioning chemicals, a number of factors must be evaluated in addition to
performance and chemical costs. Among these factors are the volume/weight
changes in the sludge, the difficulty of storing and handling the chemicals,
chemical availability, and increased maintenance of the dewatering or
subsequent sludge handling equipment due to the chemical(s) utilized.
This Chapter discusses, for the major conditioning chemicals, important
considerations which the designer should recognize in addition to performance
and cost. For additional information on chemical handling and feeding,
references 1 and 2 should be consulted. Table 5-1 outlines the most common
applications of conditioning chemicals for each dewatering process, and Table
5-2 presents typical dosages. For belt filter presses, vacuum filters and
filter presses, the type of media used is also a factor which affects chemical
dosages.
TABLE 5-1
CHEMICAL CONDITIONERS COMMONLY USED FOR
DIFFERENT DEWATERING PROCESSES
PROCESS LIME*
Basket Centrifuge
Solid Bowl Centrifuge
Belt Filter Press
Vacuum Filter C
Filter Press C
Drying Beds
Sludge Lagoons
Gravity/Low Pressure Devices
FERRIC CHLORIDE*
C
C
POLYMER
C
C
C
C
P
P
None
Required
LEGEND:
C - Common Usage
P - Possible; Used in certain situations, but usage is not common
*Lime and ferric chloride are typically used together
67
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TABLE 5-2
TYPICAL DOSAGES OF CHEMICAL CONDITIONERS
FOR DIFFERENT DEWATERING PROCESSES1
Process/Chemical
Basket Centrifuge
Polymer
Solid Bowl Centrifuge
Polymer
Belt Filter Press
Polymer
Vacuum filter
Polymer^
Lime ^
Ferric Chloride-*
Filter Press
Ferric Chloride-*
Raw Primary
g/kg
(Ib/ton)
0-2
(0-4)
1 - 2.5
(2-5)
2-4
(4-8)
2-5
(4-10)
80 - 100
(160-200)
20 - 40
(40-80)
110 - 140
(220-280)
40 - 60
(80-120)
Raw
Primary & WAS
g/kg
(Ib/ton)
0.5 - 2.5
(1-5)
2-5
(4-10)
2-5
(4-10)
3-6
(6-12)
90 - 160
(180-320)
25 - 60
(50-120)
110 - 160
(220-320)
40 - 70
(80-140)
Anaerobically
Digested
Primary & WAS
I/kg
(Ib/ton)
1 - 3
(2-6)
3-5
(6-10)
4 - 7.5
(8-15)
150 - 210
(300-420)
30 - 60
(60-120)
110 - 300
(220-600)
40 - 100
(80-200)
1. These typical dosages correspond to the typical recoveries shown in
Table 6-3. Polymer requirements are for dry polymer and lime requirements
are for lime as CaO.
2. Polymer can sometimes be substituted for lime and ferric chloride in
conditioning raw sludges for vacuum filtration.
3. Lime and ferric chloride are typically used together at these dosages.
68
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5.2 Ferric Chloride
Ferric chloride addition to sludge results in the formation of positively
charged iron complexes which neutralize the negatively charged sludge
particles. Reaction also occurs between alkalinity and ferric chloride,
resulting in insoluble ferric hydroxide, which acts to flocculate the
destabilized sludge particles.
Ferric chloride may be purchased as a liquid or solid, although most utilities
purchase it in the liquid form. The liquid form is generally 20 to 45% ferric
chloride and contains 12 to 17% iron by weight. Ferric chloride solutions are
generally fed at the concentration received from the supplier, as dilution can
lead to hydrolysis reactions and the precipitation of ferric hydroxide. An
important consideration in the use of ferric chloride is its corrosive nature.
Special materials must be utilized in handling, with the recommended materials
being: epoxy, rubber, ceramic, Hypalon, PVC, vinyl, synthetic resins, and
Penton. Contact with skin and eyes must be avoided. Rubber gloves, goggles or
a face shield, and a rubber apron must be used when handling ferric chloride.
Spillage should also be prevented, as staining of concrete and other surfaces
will result. The corrosiveness and the staining capability make solution feed
and measurement somewhat more difficult than with other chemicals, but
specialized equipment constructed of acceptable materials is available.
Ferric chloride can be stored for long periods without deterioration.
Customarily, it is stored in above ground tanks constructed of resistant plas-
tic or in lined steel tanks. An important consideration is the potential for
crystallization at low temperatures, which generally leads to locating tanks
indoors, or using tank heaters and insulation. The crystallization temperature
varies with the concentration of ferric chloride in the solution, as shown in
Table 5-3.
TABLE 5-3
CRYSTALLIZATION TEMPERATURES FOR FERRIC CHLORIDE SOLUTIONS
Solution Strength Freezing Temperature of an
% FeCl3 Unagitated Solution
°F °C
20 -5 -21
25 -25 -32
30 -50 -46
35 -40 -40
40 -10 -23
45 +30 -1
69
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Interestingly, the lowest freezing points fall in the concentration range of
30 to 35% FeCl-}, and higher freezing points occur at both more dilute and
more concentrated solutions.
Ferric chloride is most commonly used in conjunction with lime in vacuum
filter and filter press installations.
5.3 Lime
Two types of dry lime are customarily used in sludge treatment: pebble lime
(CaO), also called quicklime, and hydrated lime (Ca(OH)o). Quicklime is less
expensive to purchase and is generally used in larger facilities. It does
require slaking (addition of water to produce calcium hydroxide) prior to use.
Hydrated lime is the more costly form of lime, but is commonly used in smaller
facilities due to its convenience.
Lime should have a minimum CaO content of 88 to 90% in order to be acceptable.
Dolomitic limestone containing magnesium carbonate is often unacceptable,
because it does not have this CaO content.
When lime as calcium hydroxide is added to sludge, the calcium hydroxide
reacts with calcium bicarbonate to form calcium carbonate (CaCO^), which is
insoluble. The high pH conditions are conducive to release of ammonia from
digested sludge. The use of lime as a conditioning agent for sludge can
accomplish the following:
Increase sludge porosity
Decrease sludge matrix compressibility
Dehydrate (to a degree) sludge solids
Raise pH
Help control odor formation
Provide disinfection
Flocculate fine solids
The extent that each of these is accomplished depends on the lime dose. Lime
is most frequently utilized for conditioning prior to a vacuum filter or a
filter press. Most commonly it is used in conjunction with ferric chloride.
Either form of lime can be purchased in bulk form or in bags. Typically,
quicklime is purchased in bulk and hydrated lime is purchased in bags. If
purchased in bags, a waterproof building should be used for storage, with the
maximum storage time generally restricted to less than 60 days. If bags of
quicklime are allowed to become wet, slaking will start within the bag, and
the resultant heating and swelling may cause the bags to burst. If stored in
bulk, the storage hoppers should be both water tight and air tight. Lime is
not corrosive to steel or concrete, and either can be used as a storage bin.
The bottom slope on the bin should be about 60° from horizontal, and bin
agitators may be necessary for bulk hydrated lime storage.
70
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5.4 Polymers
Polymers are popular for sludge conditioning because they are generally easy
to handle, store, and feed, and create little additional volume of sludge
solids. Polymers may be purchased in the dry form, as emulsions, or as
liquids, with the latter being the most expensive (when comparing active
ingredients) because significant quantities of water must be transported along
with the polymer. If purchased in the dry form, polymers must be thoroughly
mixed with water according to manufacturer's recommendations prior to use.
The most common form of polymer for sludge conditioning is the cat ionic
polymer. These polymers react with the negatively charged sludge particles,
destabilize them and agglomerate the particles by forming bridges among them.
Anionic and nonionic polymers are also useful in conditioning, but they are
generally used in conjunction with inorganic conditioning agents. In this
role, the polymer is responsible for agglomeration of sludge particles which
have already been destabilized by the inorganic agent.
Polymers are most frequently utilized in belt filter press, centrifuge, and
occasionally vacuum filter and drying bed applications. There continues to be
research into the use of polymers in filter press applications. Key advantages
of polymers is the low dosages required, compared to inorganic chemicals, and
the insignificant amount of dry solids added by polymer conditioning.
5.5 Waste Pickle Liquor (Ferrous Chloride)
Waste pickle liquor, a by-product of steel processing operations, is available
in some parts of the country which are near such operations. To oxidize the
ferrous iron to ferric iron, chlorine must be added to the waste pickle
liquor. The oxidized pickle liquor is then suitable as a replacement for
ferric chloride in conditioning applications.
Waste pickle liquor contains 20 to 25% ferrous chloride, and generally weighs
between 1.19 and 1.25 kg/1 (9.9 and 10.4 Ib/gal). As a result of its produc-
tion, free acid is present at a concentration of about 2% by weight.
Continuous availability of waste pickle liquor is a factor which should be
considered. Often, provisions are made for storage and feed of ferric chloride
when waste pickle liquor is unavailable.
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5.6 References
1. Heim, Nancy E., and Burris, Bruce E., "Chemical Aids Manual for
Wastewater Treatment Facilities," USEPA - Office of Water Program
Operations, Washington, D.C., 20460, MO-25, EPA-430/9-79-018, December
1979.
2. "Process Design Manual for Sludge Treatment and Disposal," USEPA - Center
for Environmental Research Information, Cincinnati, Ohio, 45268,
EPA-625/1-79/011, September 1979.
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CHAPTER 6
STRATEGY FOR DEWATERING PROCESS SELECTION
6.1 Introduction
The most important factor which must be kept in mind when either evaluating or
selecting a dewatering process is the inherent influence that both the prior
treatment processes and subsequent disposal practices have. A dewatering
process can not be evaluated independently without consideration of the other
processes involved in the overall solids handling system. Selection of a
dewatering process requires evaluation of the complete solids handling system.
This can be a complex procedure because of the vast number of combinations of
unit processes which are available for thickening, stabilization, condition-
ing, dewatering, and ultimate disposal. Figure 6-1 presents a general
schematic of a typical solids handling system and the unit processes which are
most commonly utilized to perform each of these functions.
The strategy involved in selection of a dewatering process at either new or
existing plants involves five stages of analysis, as shown in Figure 6-2. The
stages represent a screening procedure in which dewatering processes under
consideration are given increasing scrutiny as more detailed cost, operation-
al, and design data are collected. The components of each of these stages
are:
Stage 1 - Initial Screening of Dewatering Processes
A large number of factors are reviewed to determine if any processes can
be eliminated prior to the intial cost analysis. Factors to be considered
in the initial screening include: compatibility with plant size and
existing facilities, including type and quantities of sludge produced;
compatibility with the planned or existing ultimate disposal technique;
compatibility with labor availability, degree of conditioning required,
and land availability; environmental considerations; and field experience
with equipment or processes at other operating installations.
Stage 2 - Initial Cost Evaluation
Based on the best estimates of design and operational criteria for the
potentially feasible dewatering processes, an initial cost evaluation
should be conducted. In many cases, 10 to 20 complete solids handling
alternatives, which may include four or five different dewatering
processes, are evaluated in this initial stage. Generally, three to five
of the lowest cost alternatives are selected for more detailed
evaluation.
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THICKENING
PRIMARY SLUDGE
Source Thickening
in Primary Clarifier
Gravity
THICKENING
SECONDARY SLUDGE
Dissolved Air
Flotation
Centrifuge - Basket,
Solid Bowl, or
Disc Nozzle
OTARII I7ATIHM 1— *
CONDITIONING
•
DEWATERING
• Compost
• Incinerate
1
AL
Anaerobic Digestion
Aerobic Digestion
Wet Air Oxidation
Aerobic-Anaerobic
Digestion
Chlorine Oxidation
Lime Stabilization
Ferric Chloride
Lime
Lime & Ferric
Chloride
Polymer
Heat Treatment
Elutriation
Basket Centrifuge
Solid Bowl Centrifuge
Belt Filter Press
Vacuum Filter
Filter Press
Drying Beds
Sludge Lagoons
Gravity/Low Pressure
Devices
• Land Application
• Landfill
• Ocean Disposal
FIGURE 6-1
GENERAL SCHEMATIC FOR SOLIDS HANDLING SHOWING MOST COMMONLY
USED METHODS OF TREATMENT AND DISPOSAL
-------
STAGE
1
INITIAL SCREENING OF
DEWATERING CONCEPTS
STAGE
2
INITIAL COST
EVALUATION
STAGE
3
LABORATORY
TESTING
STAGE
4
FIELD LEVEL
TESTING
STAGE
5
FINAL EVALUATION
BASED ON DETAILED
DESIGN PARAMETERS
Figure 6-2. Five Stages of Analysis in Selection
of a Dewatering Process
Stage 3 - Laboratory Testing
Laboratory testing should be conducted on the dewatering processes
selected in Stage 2 to further define design criteria for the more favor-
able dewatering techniques. This laboratory testing may be conducted at
the plant or by equipment manufacturers in their laboratories.
Stage 4 - Field Testing
After Stage 3, two or three dewatering techniques may remain. The
objective of Stage 4 is to conduct on-site testing of the pilot-scale or
full-scale equipment required for each process. This testing further
defines equipment design parameters; chemical, labor and energy require-
ments; and potential O&M problem areas. Since there are a number of
manufacturers who supply equipment for the same dewatering process, it
may also be desirable to evaluate equipment from more than one equipment
manufacturer in Stage 4. The need and justification for field testing
depends in part upon the size of the treatment plant in question. At
very small plants with a capacity of less than 0.04 cu m/s (1 mgd), it
may not be cost-effective to conduct pilot-scale or full-scale testing.
Instead laboratory or bench-scale testing by the manufacturer may be
adequate.
Stage 5 - Final Evaluation Based on Detailed Design Parameters
After Stage 4 is completed, accurate scale-up and sizing of equipment is
performed by the design engineer, with the aid of the equipment manufac-
turer; and more refined estimates can be made of the capital cost, labor,
energy, chemical, and maintenance material requirements for the
dewatering process under consideration. This information can also be
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supplemented with information from other plants using the same process.
Additionally, judgements can be made by the operating utility based on
performance and operational problems experienced in Stage 4. Based upon
more accurate capital and operation and maintenance cost information, a
final cost evaluation can be made in conjunction with an evaluation of
other parameters. The net result of Stage 5 is the selection of a dewat-
ering process to be used and in many cases the preferred manufacturer.
Throughout this five stage process, decisions based on trade-offs will
continually be made. In many cases, the total annual cost of two or more
solids treatment systems are nearly identical, and the decision must be made
on some basis other than cost. Frequently such a decision is based upon
capital cost vs. O&M cost considerations, ease of equipment operation, energy
requirements, performance, or other factors. A significant point to keep in
mind is that often the decision is not clear cut.
It is important to realize that the overall complexity of analysis will vary
depending on the size of the plant and whether or not a new solids handling
system is being designed or an old one upgraded. If the solids handling system
is all new, there will probably be few constraints on processes to be evalua-
ted, conditioning method to be used, and ultimate disposal techniques to be
considered. In other situations, if the entire treatment facility is new, or
if it is being upgraded from primary to secondary, sludge of the correct
composition will not be available for conduct of field tests. Relative to
plant size, Stage 4 is generally not conducted for most small capacity plants,
those less than 0.04 cu m/s (1 mgd). For the small plant, it is usually more
economical to design facilities based on laboratory or bench-scale testing,
often performed by the manufacturer of the equipment, than it is to conduct
the field-scale testing.
The following sections discuss in detail, the five stages required in the
analysis:
6.2 Stage 1 - Initial Screening of Dewatering Processes
The purpose of the initial screening is to eliminate early in the analysis
processes which are not acceptable for any of a variety of reasons. Factors to
be considered in the initial screening include:
Compatibility with existing facilities
Compatibility with size of plant
Compatibility with ultimate disposal technique(s)
Influence of secondary treatment and prior sludge treatment
Conditioning requirements
Solids capture during dewatering
Labor requirements
Environmental considerations
Long term utility
Plant location
Experience at other operating installations
Bias by individuals or agencies
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6.2.1 Compatibility With Existing Facilities
Existing facilities which must be considered in evaluating dewatering
processes include:
• Type of dewatering equipment presently utilized, and its compatibility
with future requirements
• Existing conditioning chemical storage and feed facilities
• Existing building used for dewatering and ancillary equipment
• Existing site constraints
• Existing sludge transport facilities
Considerations which relate to existing facility requirements are now
discussed.
6.2.1.1 Existing Dewatering Equipment
Existing dewatering equipment customarily plays a major role in the selection
of additional equipment, particularly if space is available and has been
planned for expansion of the present dewatering facilities. If existing equip-
ment is providing satisfactory performance (from both a cost and operational
standpoint) for the plant staff, and if the product cake is suitable for the
ultimate disposal technique which is being used, in all likelihood the same
dewatering process would be desired in the expansion. This would be particu-
larly true if the dewatering facilities had been designed to accomodate more
equipment of the same type. In perhaps the majority of situations, existing
equipment is providing unsatisfactory performance and requires more chemicals
or energy than originally anticipated. In other cases the sludge characteris-
tics have adversely changed since design, and the original equipment can not
be operated at the original design capacity. In some cases existing equipment
can not perform as well or as efficiently as some of the newer equipment
available, or the cake produced by existing equipment is not suitable for the
future ultimate disposal technique.
In a large percentage of the cases involving expansion of dewatering
facilities, the plant staff is dissatisfied with the operation of the existing
equipment. Typical situations are: (1) vacuum filter installations where lime
coating of the filter media, filter drum, and filtrate piping presents an
expensive and continuing maintenance problem; (2) filter press installations
that often have chemical requirements substantially higher than originally
expected; (3) older existing solid bowl centrifuge installations where a great
deal of scroll maintenance is required due to abrasive wear and/or where the
operating performance is poor; and (4) drying beds or lagoons where odors, the
visual impacts, intensive labor requirements or difficulty with sludge removal
make the process an operations problem for the plant staff. These situations
and other possible situations not described here can cause headaches for the
operation and maintenance staffs and can decrease effective dewatering
capacity and increase operating costs when equipment must be taken out of
service for repairs and/or cleaning.
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Variation in sludge characteristics after design and installation of
equipment, and therefore variation in the ability of the sludge to be dewater-
ed, presents a particularly vexing problem. Invariably, this leads to higher
conditioning and energy requirements than originally projected, and in some
cases the inability to produce a dewatered cake suitable for ultimate
disposal. In some instances, equipment may need to be operated at less than
design capacity due to changed sludge characteristics. Evaluation should be
conducted to determine the likelihood and severity of changes in sludge feed
rate and characteristics. If significant variations are anticipated, equipment
which is less sensitive to such changes should be selected.
6.2.1.2 Existing Conditioning Chemical Storage and Feed Facilities
Most plant staffs have a preference for the types of chemicals which they
desire to handle, and a bias against ones which they dislike to handle.
Assuming that conditioning chemicals presently utilized are acceptable to the
plant staff, this would give an added preference to dewatering processes which
utilize the same chemicals. Another factor is the cost and availability of
storage and feed facilities. It is important to assess the unused capacity of
both storage and feed facilities. If unused capacity is available, this must
be considered in process selection, particularly in the Stage 2 cost
analysis.
6.2.1.3 Existing Building Used for Dewatering Equipment
As discussed previously, often space has been planned and constructed for the
same type of equipment as that presently used. This is an important factor to
consider.
Also to be considered is the present building's structural capacity for
modern heavyweight equipment, and whether the building has sufficient height
for the equipment being considered. Dewatering equipment like solid bowl
centrifuges, belt filter presses, and filter presses frequently discharge
dewatered solids downward. This requires elevated mounting to allow for
conveyor belts under the equipment and can be incompatible with low roof
buildings.
Building structural capacity also must be analyzed. Heavy equipment such as a
filter press may not be compatible with a building originally designed for a
centrifuge installation, even though both have bottom discharge of cake
solids. Basket centrifuges require greater structural support than solid bowl
centrifuges. In the case where there is an existing overhead crane, heavy
equipment may exceed the allowable capacity, and lighter equipment should
perhaps be considered.
An important factor which the designer must continually keep in mind, even if
a new method of dewatering is selected, is the usefulness of the existing
facilities. In many cases, the new dewatering process will only be used to
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supplement existing facilities. In other situations, because of a change in
the ultimate disposal technique or because of generally unsatisfactory opera-
tion, the new facilities will replace existing facilities. When this occurs,
rather than removing existing facilities, strong consideration must be given
to their use as standby or backup facilities to the new facilities. Often they
can be used on a short-term basis in this role even if they do not produce a
sufficiently dry cake for the disposal technique used.
Equipment previously used for dewatering is occasionally converted to sludge
thickening prior to anaerobic digestion. This may be especially advantageous
if anaerobic digester capacity is lacking and expansion of the digester
capacity is being considered. Examples include use of centrifuges to thicken
WAS prior to digestion, or use of any dewatering device to dewater a portion
of the sludge and then blend the dewatered cake with the dilute feed sludge to
produce a thickened sludge. An economic analysis should be conducted prior to
such use, as chemical and energy requirements may be significantly greater
than alternative techniques. Solids capture efficiency must also be considered
in such conversions.
6.2.1.4 Existing Site Constraints
Drying beds and sludge lagoons both require considerable land area. Expanded
use of these processes may not be practical if land is unavailable, or if
environmental constraints make continued use unacceptable. However, often the
existing beds or lagoons can be used in conjunction with a different dewater-
ing process.
6.2.1.5 Existing Sludge Transport Facilities
This consideration would probably relate only to a. decision of whether or not
to dewater. For example, if a considerable investment had previously been made
in trucks or a pipeline and pumping facilities for liquid sludge transport,
the decision may be made not to dewater. Another possibility could be to
dewater at a site remote from the treatment facility if liquid transport
facilities are available. Although these decisions are generally made on the
basis of cost, their recognition during the initial screening may save
substantial time in the decision making process.
6.2.2 Compatibility With Size of Plant
Use of uncomplicated sludge handling systems increases the chances for
successful operation in any size plant. Complex equipment is especially
unsuited for small plants for several reasons. First, the amount of operator
time available generally decreases as plant size decreases. Second, the
overall skill of both operations and maintenance personnel is not as great at
small plants. Third, less complex equipment is generally less expensive to
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purchase, and since little economy of scale occurs for small plants, this is
of particular benefit.
The choice of the dewatering process to be used is customarily left up to the
designer and owner. No specific rules exist for which processes should be used
for a particular size of installation. However, certain guidelines do exist
based on results experienced at plants across the U.S. These guidelines are
summarized in Table 6-1, which presents a matrix showing compatibility of
different dewatering techniques with various plant sizes.
Designers should only use the information presented in Table 6-1 as a guide.
Every situation must be considered independently, as location specific
considerations can have a large influence on the dewatering process. For
example, drying beds and sludge lagoons may be cost-effective at a plant
larger than 0.44 cu m/s (10 mgd) if weather is favorable and land is available
at reasonable cost. Other variations may occur, when more than one type of
dewatering is used at a plant, or where a plant is a regional solids handling
center, and solids treatment capacity is in excess of liquid treatment
capacity.
TABLE 6-1
COMPATIBILITY OF DEWATERING EQUIPMENT WITH PLANT SIZE
<0.04 cu m/s 0.04-0.44 cu m/s >0.44 cu m/s
MGD) (1-10 MGD) (>10 MGD)
Basket Centrifuge X X
Solid Bowl Centrifuge X X
Belt Filter Press X1 X X
Vacuum Filter X X
Filter Press X X
Drying Beds X X
Sludge Lagoons X X
^Only low pressure press is commonly used in this range
6.2.3 Compatibility With Ultimate Disposal Technique
This is the most important factor in the screening process. Careful attention
must be paid to the methods of ultimate disposal available, and the solids
content required for disposal by them. A potentially costly situation which
should be avoided is for the dewatering process to remove more water than
necessary for the selected or available disposal technique.
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Table 6-2 presents general guidelines for the compatibility of the principal
ultimate disposal techniques with the seven principal methods of dewatering.
Similar to the information presented in Table 6-1, the information in
Table 6-2 must be evaluated on a case-by-case basis. There are undoubtedly
exceptions to these general guidelines, and it is not intended that the
designer completely eliminate from consideration any process which does not
fit the guidelines.
TABLE 6-2
DEWATERING PROCESS COMPATIBILITY WITH SUBSEQUENT TREATMENT OR
ULTIMATE DISPOSAL TECHNIQUES
Dewatering Process
Basket Centrifuge
Solid Bowl Centrifuge
Belt Filter Press
Vacuum Filter
Filter Press
Drying Bed
Sludge Lagoon
Incineration* Composting
X
X
X
X
x-
X
X
Agricultural
Land
Application
X
X
X
X4
X*
X
X
Landfill2
X
X
X
X
X
X
X
1. Solids content required for self-sustaining combustion will vary depending
upon the percent of solids that are organic and the calorific value of the
organics.
2. Some states and municipalities have rigid requirements on the solids content
of sludges placed in landfills. Local regulations should be checked by the
designer.
3. Suitability of this method depends on organic content of sludge.
Thermodynamics of composting must be evaluated. (For more information see
references 1, 2, and 3.) Generally, sludges with 20% or greater solids content
can be composted, depending on the degree of prior stabilization and weather
conditions.
4. Soil characteristics are important. For some alkaline soils (i.e. some
calcareous soils), land application may not be desirable because of lime
in the dewatered sludge cake. For soils with a high sodium content, however,
addition of calcium can beneficially increase the calcium/sodium ratio and
result in improved tilth. There are very few soils where a problem would be
anticipated due to application of sludge cake. Advice of agriculturists is
recommended.
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It should be recognized, however, that just because a dewatering process is
compatible, it may not be the most cost-effective technique. For example, use
of a filter press is compatible with landfilling, but a belt filter press
would in most cases represent a more cost-effective method of dewatering prior
to landfilling.
Designers should remember that the objective of dewatering is only to remove
sufficient water to produce a sludge compatible with the selected disposal
technique. Removal of additional water is not cost-effective and may require
the unnecessary expenditure of energy and chemicals.
Table 6-2 indicates that only the filter press, solid bowl centrifuge, belt
filter press, and vacuum filter are compatible with incineration. It is
frequently concluded that only a filter press can produce a dewatered cake
compatible with incineration. This conclusion is based upon the criteria that
only an autogeneous or nearly autogenous sludge should be incinerated.
However, in some instances where digestion is not used or where the sludge is
thermally conditioned, nearly autogenous cakes can be produced by belt filter
presses, solid bowl centrifuges, and vacuum filters. This is particularly true
if only raw primary sludge is being dewatered.
6.2.4 Influence of Secondary Treatment and Prior Sludge Treatment
6.2.4.1 Secondary Treatment
The influence of the type of secondary treatment on the sludge produced and
its dewaterability was reviewed in Chapter 4. The most important conclusion
reached was that both trickling filter (TF) and rotating biological contactor
(RBC) sludges dewater better than waste activated sludge (WAS). This is true
whether sludges are raw or digested. The difference between the TF/RBC sludges
and WAS is due to the nature of the biological growth. The TF and RBC sludges
are from attached growth biological systems which produce dense, easily
settleable sludge solids. The WAS is from a suspended growth system, in which
the sludge is dispersed in nature, with large amounts of water contained
between dispersed sludge particles. The nature of the WAS is also strongly
dependent upon the process variation of the activated sludge process which is
utilized. High rate systems using high food to microorganism ratios (F/M >0.4)
produce quantities (by weight) of biomass which are greater than the conven-
tional activated sludge process (F/M of 0.2 - 0.4), and this biomass is
particularly difficult to dewater due to the large quantities of intercellular
water. Problems have also been experienced at plants using the extended
aeration modification of the activated sludge process (F/M < 0.15), because of
the pin-point, discrete nature of the biological floe.
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6.2.4.2 Prior Sludge Treatment
Prior sludge treatment by thickening, digestion or storage will affect the
ease of dewaterability. Thickening, generally used before digestion to reduce
digester capacity, produces beneficial results due to a higher solids content
feed to the dewatering process. Conversely, both digestion and storage are
usually detrimental to the ease of dewatering.
Vacuum filters generally have a requirement for the feed sludge total solids
concentration to be at least 3.0 percent. If the feed sludge is wore dilute
than this, it becomes difficult to form a cake on the filter that is thick
enough or dry enough for adequate discharge. For this reason the role of the
preceding sludge handling processes is an especially important one.
The major benefit associated with feeding a thickened sludge to mechanical
dewatering processes is the higher solids throughput obtainable. This can
reduce the number of machines required and the overall space requirements. All
dewatering devices are to some extent hydraulically limited because the
hydraulic capacity is exceeded before the solids capacity. The belt filter
presses offered by several manufacturers have a larger or separate gravity
drainage zone which allows them to be loaded with either a wetter feed sludge
or a higher solids loading rate. For these machines the gravity drainage zone
actually thickens the sludge to at least 6 percent solids, and it is doubtful
that prethickening would substantially reduce the number of machines required,
unless the feed solids are below about 2 percent. For a filter press, a
thicker feed sludge reduces the fill time and overall cycle time because there
is less water to force through the cake and filter media, and therefore the
total required filter area is reduced.
Another benefit of a thicker feed sludge is a reduction in the chemical
requirements for conditioning. Although there may be an increase in solids
loading rates and some reduction in chemical requirements, there generally
will not be a major increase in cake solids achievable, especially for a
centrifuge, a belt press, or a filter press. One mechanical dewatering device
which may show a significant increase in cake solids content due to higher
sludge feed concentrations is a vacuum filter.
Although it is not a common problem, it is possible to have a feed sludge for
dewatering which is too thick to be easily handled. A suggested maximum feed
solids concentration of 7 to 8% is recommended for dewatering equipment.
Solids contents higher than 8% will tend to make the sludge difficult to
transfer to the dewatering unit.
Anaerobic digestion generally degrades the dewaterability of raw sludge
because of fines produced by the process. This fine material is hard to
dewater due to its large surface area, its difficulty in being flocculated
during conditioning, and its compressibility during dewatering. This overall
difficulty often further results in poor capture of the fines during dewater-
ing, and recycle of fine material to the plant liquid handling processes. The
best solution for the problem of fine material produced during digestion is
adequate conditioning prior to dewatering. Because the mass of solids is
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reduced by anaerobic digestion, the actual number of dewatering devices
required may be less. In addition to the problem of fine material, anaerobic
digestion also creates alkalinity during the breakdown of organic matter. This
alkalinity reacts with ferric chloride, ferrous sulfate, and lime if they are
used for conditioning, with a resultant increase in the quantity of
conditioning chemical(s) required.
Storage of sludge prior to dewatering generally is detrimental to
dewaterability. Storage may be necessary, however, to equalize the rate of
sludge feed to the dewatering process.
6.2.5 Conditioning Requirements
Conditioning before dewatering can be by thermal treatment or by chemical
addition. Chemical conditioning is much more common than thermal conditioning,
but thermal treatment has the advantage of substantially improving the
dewaterability of a sludge without requiring the addition of chemicals. At
existing thermal conditioning facilities, plant operating personnel may be
dissatisified due to problems with complexity, odor generation, and the
high-strength recycle stream produced. Additionally, thermal treatment
facilities installed before 1973, when fuel and electricity prices began to
rapidly escalate, may not be justified on a cost-effective basis at current
fuel prices. These costs should be reevaluated when expansion plans are
formulated.
As discussed relative to existing facilities, preference by plant personnel
for specific chemicals, as well as the availability of storage and feed
facilities for certain chemicals, are important considerations. Other factors
which need to be considered are availability and cost of chemicals. If the
conditioning chemical(s) is readily available, on-site storage times can be
reduced and a savings will result in storage facility costs. Chemical cost is
also a factor and it will vary from location to location, depending upon its
source. The impact of these costs will become evident during the cost
comparison (Stage 2 of the evaluation procedure), but it is important that
they be recognized during the initial screening phase also.
6.2.6 Solids Capture During Dewatering
Incomplete capture of solids during dewatering, with recycle of these solids
to the plant headworks, will generally not create a problem if solids capture
exceeds 90% and plant effluent suspended solids (SS) limits are 30 mg/1. When
effluent SS limits are 20 mg/1 or 10 mg/1, often times 95% solids capture may
be necessary to allow the plant to meet effluent discharge limitations.
Solids loss during dewatering generally occurs by two mechanisms: (1) solids
passage through the filtering media, or with centrifuges, solids lost in the
centrate, and (2) incomplete separation of solids from the media and the need
to spray wash the media prior to the next application of sludge.
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The percentage of solids captured is highly variable and depends upon the type
of sludge being dewatered (particularly the percentage of waste activated
sludge), whether the sludge has been stabilized or not, the type and amount of
conditioning chemicals utilized, the type of equipment used for dewatering,
and the desired percent solids in the dewatered sludge. Table 6-3 lists
typical ranges of solids capture exhibited by dewatering processes. The solids
concentrations of liquid sidestreams from dewatering processes (such as
centrate, filtrate, and percolated liquid) are inversely proportional to
percent solids capture. For detailed information on the characteristics of
these liquid sidestreams, see Reference 3.
TABLE 6-3
TYPICAL SOLIDS CAPTURE OF DEWATERING PROCESSES
Typical Solids
Process Capture
Basket Centrifuge 80 - 98
Solid Bowl Centrifuge 90 - 98
Belt Filter Press 85 - 95
Vacuum Filter 88 - 95
Filter Press >98
Drying Beds >99
Sludge Lagoons >99
Gravity/Low Pressure Devices 88 - 95
Note: Solids captures shown are for properly operated dewatering systems
with well conditioned sludge. With improper operation, solids
capture as low as 50% has been noted for some processes.
The centrifugation process relies on centrifugal settling of solids; because
of this, centrifuges classify the solids, settling the heavier solids first.
Other dewatering processes which rely on filtration in general achieve more
even distribution of solids captured. Because of this difference in operation,
it is possible for a buildup of fines to occur in treatment plants using
centrifuges, if the centrifuge is operating improperly due to inadequate
conditioning or due to a malfunction.
6.2.7 Labor Requirements
Two labor factors are important in evaluating dewatering processes: the amount
of labor required and the skill of labor required. Both the amount and skill
of labor required typically increase as the plant size increases. At small
plants, operators generally have little time available for operation or
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maintenance of complex pieces of equipment. They also often have no desire to
attempt proper operation of a complex mechanical dewatering device.
Generally drying beds, lagoons, or low pressure belt presses are best suited
to small facilities. Little time or skill is required for drying beds or
lagoons. Low pressure belt press/gravity drainage processes require rather
large polymer dosages to achieve dewatering, but they are relatively easy to
operate and maintain.
6.2.8 Environmental Considerations
Environmental factors should be considered in the initial screening process.
The key considerations which relate specifically to the dewatering process
include:
Energy Requirements
Noise
Vibration
Odor Potential
Aesthetics (visual impact)
Groundwater Contamination
An evaluation of each of these environmental considerations is presented in
Table 6-4 for the principal dewatering processes, and they are further
discussed in the following sections.
6.2.8.1 Energy Requirements
Energy requirements for mechanical dewatering equipment are moderate to high,
with the exception of belt presses and other low pressure or gravity drainage
type devices, which have relatively low energy requirements. Generally, the
requirement for energy is proportional to the degree of dryness required in
the cake. To some extent, energy utilization can be reduced by increasing
the level of conditioning, but this would generally not be cost-effective.
Conditioning chemical dosage adjustment would usually only be made to increase
machine capacity, increase solids capture, or aid in cake removal from the
dewatering equipment. Drying beds and sludge lagoons require energy only for
pumping sludge to the beds or lagoons and for the equipment used to remove
dewatered sludge from the beds or lagoons. Energy requirements for these solar
processes are low in relation to that needed by mechanical dewatering
equipment.
Energy requirements for the dewatering equipment can not be considered without
also taking into account the energy and costs required for subsequent trans-
portation and disposal of the dewatered sludge. Often dewatering to 20 to 25%
cake solids and hauling at this solids content is more cost-effective and less
energy consuming than the alternative of dewatering to 35 to 45% solids and
incinerating. Naturally it is environmentally desirable to always utilize the
overall lowest energy consuming processes for treatment, transportation and
86
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TABLE 6-4
EVALUATION OF ENVIRONMENTAL CONSIDERATIONS OF DEWATERING PROCESSES
ENVIRONMENTAL CONCERN
00
Process
Basket Centrifuge
Solid Bowl Centrifuge
Belt Filter Press
Vacuum Filter
Filter Press
Drying Beds
Sludge Lagoons
Energy
Requirement
High
Moderate to High
Low
Moderate to High
Moderate to High
Low**
Low**
Noise
Moderate
Moderate to High
Low
Moderate
Moderate
None***
None***
Vibration
High
High
Low
Low
Low
None
None
Odor
Potential*
Low
Low
Moderate
Moderate
Moderate
High
High
Potential For
Visual Groundwater
Impact Contamination
None
None
None
None
None
High
High
None
None
None
None
None
High
High
*Rating is based on dewatering a poorly stabilized sludge. If sludge is well stabilized, there should be
no significant odor from any dewatering process.
**Energy required is electricity for sludge pumping and diesel fuel for equipment used to remove
dewatered/dried sludge.
***Noise levels for drying beds and lagoons can be high during cleaning due to heavy equipment and
"beep-type" signaling device required when operating in reverse.
-------
ultimate disposal. Often, however, the alternative with the lowest overall
cost for capital and operation and maintenance does not have the lowest energy
consumption. In such a situation, different individuals or utilities reach
different conclusions; some select the lower cost alternative while others
select the higher cost alternative which uses less energy.
A further discussion of energy requirements is presented in Chapter 8 of this
manual.
6.2.8.2 Noise
For equipment located indoors, noise is a consideration for plant operators,
and for equipment located out of doors, noise is a consideration for both
operators and neighbors to the treatment facility. The processes for which
noise is a potential problem are basket centrifuges, solid bowl centrifuges,
vacuum filters, and filter presses. High speed solid bowl centrifuges create
more noise problems than the lower speed models. Noise resulting from vacuum
filters and filter presses is primarily caused by vacuum pumps and high
pressure hydraulic pumps, respectively. Often, vacuum pumps are located by the
designer in another room away from vacuum filters, and this isolates vacuum
filter noise problems.
6.2.8.3 Vibration
Only centrifuges create major vibration problems, and this is essentially a
design consideration since with proper use of vibration isolators, the vibra-
tions caused during normal centrifuge operation can be effectively dampened.
Vibration can also be an indication of inadequate maintenance or the need for
maintenance. These are factors which should be considered relative to the
location of other equipment and operator's stations.
6.2.8.4 Odor Potential
Odor is a key concern relative to residential, commercial, and industrial
neighbors of the treatment facility. It may also be of concern, depending on
the design, for indoor installations which have poor ventilation. Another
factor to consider is that operators may find the odor to be objectionable
enough that they may prefer to stay away from the process, perhaps creating
operation and/or maintenance problems. The kinds of odor vrtiich may present
problems are hydrogen sulfide, mercaptans, indole, skatole, and ammonia.
Ammonia is often released when the sludge pH is raised by the addition of lime
for sludge conditioning.
Drying beds and sludge lagoons present the highest odor potential, but only if
sludge has not been adequately stabilized before application to the beds or
lagoons. If sludge is properly stabilized, only an earthy or musty odor will
88
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emanate from the beds or lagoons, and this odor is normally not offensive.
During normal operation raw sludge or poorly stabilized sludge should not be
placed on drying beds. On a temporary basis, raw sludge that has been raised
to pH 12 by lime addition could be placed on drying beds.
For partially digested sludge, use of centrifuges will generally not create an
odor problem, while use of vacuum filters, belt filter presses, and filter
presses may create a moderate odor problem.
For lime conditioned sludges, a localized occasional odor problem may occur
due to ammonia release. This is generally not a problem, although good venti-
lation should be provided to protect workers and to prevent corrosion of metal
surfaces and electrical equipment.
6.2.8.5 Aesthetics (Visual Impact)
Depending on the plant location, lagoons and drying beds may create aesthetic
problems. Landscaping around the plant perimeter as well as directly around
the beds or lagoons will help, as will berms. Well chosen, isolated locations
on the plant site, or perhaps locations in remote areas, may be most suitable
for the beds or lagoons.
In mild climates, centrifuge installations may be outdoors. This should
generally not create a visual problem, as centrifuges are visually compatible
with other plant equipment.
6.2.8.6 Groundwater Contamination
Unlined lagoons or drying beds may allow downward percolation of water toward
the groundwater. The quantity and quality of this percolate will depend upon
how much the sludge has been previously thickened and the character of the
soil. Clay and clay containing soils will allow passage of water at very low
rates while filtering out solids. In general sand and gravel will allow down-
ward passage at relatively high rates. Additionally, sludge lagoons often seal
themselves to a certain extent, greatly reducing percolation. Drying bed
underdrains also greatly reduce percolation, catching most of the percolating
water so that it can be returned to the plant headworks for treatment. The
quality of the percolating water will also change as it moves downward through
the soil. The reactions which occur will depend on the type of soil, how long
the soil has been used for this purpose, whether the soil is aerobic or
anaerobic, and the depth to groundwater. Seasonal variations in groundwater
levels may also create problems in certain areas and should be investigated
prior to process selection.
Sealing of drying beds or the use of underdrains should be considered if tests
indicate that groundwater contamination will be likely to occur. Drying bed
bottoms are commonly sealed with concrete or asphalt, while lagoons are sealed
most often with a compacted bentonite clay layer or an impermeable plastic
membrane.
89
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6.2.9 Long Term Utility
The appropriateness and cost-effectiveness of the dewatering process is
closely linked with long-term consistency of sludge quality and the ultimate
disposal method which is utilized. Evaluation of the long term utility of a
dewatering process should involve consideration of these factors. For example,
if the type of secondary treatment is changed from trickling filtration to
activated sludge, the resultant sludge characteristics will change and this
can affect the cost of dewatering and the overall appropriateness of the
dewatering process being used. Another example would be the expansion of sand
drying beds. Although such an expansion may be cost-effective at the present
time, if the same land is required for future expansion of the liquid handling
components of the treatment process, it may be preferable to select a
different dewatering strategy rather than changing a few years later. Another
example would be selection of a belt press, centrifuge, or vacuum filter for
dewatering prior to landfill, when the landfill has only a few years of life
remaining, and no other nearby landfill site is available; in such a case, it
may be appropriate to consider a filter press, because of the probability of a
long truck haul to a landfill or land application site. In addition, the lower
volume of a filter press cake may help extend the life of the existing
landfill. Considerations such as these are important in evaluation of the long
term utility of a dewatering process.
6.2.10 Plant Location
Factors associated with plant location and the effect on dewatering process
selection include:
• Land Availability
• Proximity To Ultimate Disposal Location
• Proximity To Developed Areas
6.2.10.1 Land Availability
Plant location can greatly influence land availability for plant expansion,
for construction of land intensive dewatering processes such as drying beds
and lagoons, and for providing a buffer zone around the plant. Land availabil-
ity is a factor in expansion of an existing plant including its dewatering
facilities. A plant in an isolated location is more likely to have room for
expansion or for drying beds or lagoons than a plant located in a heavily
populated area. Land availability can also be important to provide room for a
buffer zone around a plant, to help control odor problems with close neighbors
or to allow landscaping to visually shield the plant.
Available land does not have to be immediately adjacent to the plant in cases
where a plant is located in a heavily populated area. Although not common,
90
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dewatering facilities can be located a distance from Che main plant processes
with liquid sludge transported by pipeline or truck to the location for
dewatering.
6.2.10.2 Proximity To Ultimate Disposal Location
Dewatering options are closely tied to either an existing or a future ultimate
disposal method. If a landfill or a site for composting or land application is
located nearby, this can influence the selection of the dewatering process,
since dewatering to a very dry cake such as that produced by a filter press
may be unnecessary. However, if there is no ultimate disposal location
nearby, dewatering by filter press for long distance hauling or for incinera-
tion may be cost-effective.
6.2.10.3 Proximity to Developed Areas
The proximity of the wastewater treatment plant to residential, commercial, or
industrial development can limit the options available for dewatering. Sand
drying beds and lagoons both have a potential for major odor problems. In
addition, lagoons and drying beds can be visually unattractive and may require
either landscaping around the perimeter or construction of berms.
6.2.11 Experience at Other Operating Installations
Experience at other operating installations with similar sludges is another
important screening criterion. There are undoubtedly other wastewater treat-
ment plants somewhere in the region which have had similar dewatering
problems, and evaluation of their experiences can be invaluable. It is for
this reason that Chapter 4 with the capabilities of dewatering processes and
Chapter 9 with summaries of side-by-side comparisons were included in this
manual. For drying beds or lagoons, nearby locations with similar weather are
particularly good indicators of the type of performance which can be
expected.
6.2.12 Bias by Individuals or Agencies
The plant owner, plant operators, and/or the enforcement agency may have
preconceived biases for or against certain processes. The extent and rationale
for such biases should be investigated by the designer. A decision should be
made relative to the reasonableness of the bias and whether it should be used
as a basis for eliminating a process from further consideration.
91
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6.3 Stage 2 - Initial Cost Evaluation
The purpose of the initial cost evaluation is to develop budget level cost
estimates for sludge treatment processes and techniques which remain after the
initial screening process, and to eliminate techniques which are not remotely
cost-effective. Importantly, this cost evaluation must include not only the
dewatering process, but also the entire sludge handling system, which includes
prior treatment processes and subsequent transport and ultimate disposal of
the sludge cake. Because of the number of treatment, dewatering,
transportation, and ultimate disposal processes available, up to 10 to 20
process combinations may be evaluated in the initial cost evaluation.
This initial cost evaluation uses budget level cost curves and cost data for
development of total sludge treatment and disposal system costs. Generally,
budget level cost estimates can be expected to be within +^15% of true cost,
assuming appropriateness of the design parameters used to develop the costs. A
number of references are available which present capital and O&M cost data for
many sludge handling processes from which budget level costs can be developed
(3-11). Construction cost and operation and maintenance cost curves for nine
different dewatering processes are presented in Appendix C. The curves are
based on April 1982 cost levels.
Equipment costs can also be obtained from equipment manufacturers and
equipment suppliers. Often manufacturers do not have comprehensive data
available on O&M requirements and can offer only general guidelines. Thus O&M
requirements should be based on the above references, the Appendix C cost
data, or the experience of the designer or manufacturer.
It is important that costs and O&M requirements presented in these or other
references be updated to either the time of the subject cost analysis, or some
future time, depending upon the requirements of the project. The simplest
approach is to use a single composite index to update construction costs. The
index most commonly used for this purpose is the Engineering News Record (ENR)
Construction Cost Index (CCl). The ENR Construction Cost Index average for 20
cities was 348.64 in April 1982.
The initial cost evaluation should develop capital and O&M costs, and express
the total cost in either total annual cost or present worth cost. The relative
cost-effectiveness of alternatives is then evaluated by ranking the alterna-
tives on the basis of either the total annual cost or the present worth cost.
An example cost evaluation, similar to one which should be conducted to
develop budget level costs during the initial cost evaluation phase, has been
prepared for one solids handling alternative for a 1.1 cu m/s (25 mgd) acti-
vated sludge plant operating at design capacity. A process flow diagram and
design criteria for solids treatment and handling are shown on Figure 6-3.
For this example, a raw wastewater flow of 25 mgd has suspended solids and
BODj concentrations of 275 mg/1 and 230 rag/1, respectively. Primary clarifi-
cation removes 65% of the suspended solids and 35% of the BODj; the primary
sludge has a 5% solids content with 65% of the solids being volatile. During
92
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PROCESS FLOW DIAGRAM
PRIMARY SLUDGE
SECONDARY
SLUDGE
CENTRIFUGE
POLYMER
ANAEROBIC
DIGESTION
BELT FILTER
PRESS
TRUCK
HAUL
WINDROW
f*nUPOQTINfi
SITE
DISTRIBUTE
TO POTENTIAL
USERS
VO
DESIGN CRITERIA
Avg. Flow - 1.1 cu ml a (25 mgd)
Primary Sludge - 16,920 kg/d (37,300 Ib/d)
338 cu tn/d (89,400 gal/d) @ 52
Secondary Sludge - 11,340 kg/d (25,000 Ib/d)
1509 cu m/d (398,700 gal/d)
e 0.75%
Centrifuge Output - 174 cu m/d (46,000 gal/d)
@ 6.5%
Anaerobic Digester - 20 day detention time
Single stage complete mix
Belt Filter Press
Feed = 18,200 kg/d (40,135 Ib/d)
Feed Rate = 284 kg/hr/m (627 Ib/hr/m)
Product
-------
biological treatment, the remaining 65% of 8005 is converted to cellular
material: 0.8 pounds of cells are produced per pound of BOD5 removed. WAS
has a 0.75% solids content, and the solids are 80% volatile. It is thickened
to a 6.5% solids content using a low-speed solid bowl centrifuge. During
complete mix single-stage anaerobic digestion, 50% of the volatile solids are
destroyed, resulting in a feed to the belt press of 18,200 kg/d (40,135 Ib/d)
at a rate of 284 kg/hr/m (627 Ib/hr/m). Polymer is used to condition the
sludge at a rate of 6.5 g/kg (13 Ib/ton). The belt press cake of 20% solids is
hauled in a 22.9 cu m (30 cu yd) truck to a remote composting location.
Composting is by the windrow technique with frequent turning of the windrows,
particularly in the early stages. Composting time is 30 days followed by 60
days on-site storage prior to stabilization. No income was included for sale
of the compost product.
Installed, operating and standby equipment design capacities are shown in
Table 6-5 for sludge handling processes. To develop construction costs and
building energy requirements, installed capacity was used as the basis.
Operating capacity was used as the basis for labor, energy, maintenance
material and chemical requirements.
Capital and operation and maintenance costs for sludge handling operations in
this example are shown in Table 6-5. Based upon the capital cost of
$6,318,200, and an annual cost of $483,500/year, the total annual cost for
sludge handling is $1,287,400/year. Development of construction cost and O&M
requirements for the low-speed solid bowl centrifuge used for WAS thickening
and the belt filter press used for digested sludge dewatering are based upon
Appendix C, Figures C-4 to C-6 and C-10 to C-12, respectively. Other construc-
tion costs and O&M requirements were developed using references (3) through
(11). Unit costs used for labor, electrical, digester gas, and diesel fuel are
shown in Table 6-5.
Costs developed using this general approach will have an accuracy of ^ 15% of
the actual equipment cost, which is sufficient for this stage of the analysis.
After determining costs for each of the different alternatives, about four or
five of the lowest cost alternatives should be selected for more detailed
scrutiny, and refinement of cost data by determining equipment loadings from
laboratory and field scale testing. In many situations, more than one dewater-
ing process would be included in the remaining four to five alternatives.
Naturally such cost estimates, although accurate to +^15%, must be based on
conservative estimates of dewatering throughput rates at this stage of the
analysis, since no laboratory or field testing on the actual sludge has yet
been performed. It is possible that the facility may be overdesigned by as
much as 50 to 100 percent in this initial cost evaluation. Designs tailored
to the specific sludge require field and/or laboratory test data. This type of
information is required before final cost estimates for the dewatering
alternatives can be made.
94
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TABLE 6-5
CAPITAL AND O&M COST ESTIMATES
SOLIDS HANDLING SYSTEM INCLUDING ANAEROBIC DIGESTION,
BELT FILTER PRESS DEWATERING, TRUCK HAUL, AND COMPOSTING
EQUIPMENT DESIGN
PROCESS
Centrifuge -
Low G - gpm
Anaerobic Digestion - cu ft
VD
Ul
Polymer Feed
Belt Filter
Truck Haul
Compost Site
Equipment
- Ib/day
Press - gpm
& Required
INSTALLED
420
362,000
260
210
1 Tractor
2 Trailers
—
OPERATING STANDBY
280 140
362,000
260
140 70
1 Tractor
2 Trailers —
—
CONSTRUCTION
COST
$
705
2,062
38
760
110
838
,000
,000
,000
,000
,000
,000
LABOR ELECTRICITY NATURAL GAS DIESEL FUEL
hr/yr kwh/yr btu/yr gal/yr
2,600 770,000
4,000 1,150,000 (52,560X106)*
250 28,000
1,600 250,000
2,900 — — 19,500
2,000 — — 17,500
MAI NTENANCE
MATERIALS CHEMICALS
15
9
4
8
12
$/yr $/yr
,000
,000
500 189,800**
,000
,800
,000
TOTAL CONSTRUCTION COST
Engineering, Contingencies, Contractors Overhead
& Profit, Legal, Fiscal and Administrative, and
Interest during Construction - 402
TOTAL CAPITAL COST
4,513,000 13,350 2,198,000 (52,560X106) 37,000
49,300
1,805,200
512/HR
$ 0.05/KWH $1.30/106BTU $ 1.15/GAL
$6,318,200 $160,200/YR $ 109,900/YR ($68,300/YR) $42,600/YR
189,800
$49,300/YR $189,800/YR
AMORTIZED CAPITAL COST = $ 803,900/YR***
TOTAL OSM COST = $ 483,500/YR
TOTAL ANNUAL COST = $1 ,287 ,400/YR
*This represents captured gas which is available after digester heating.
**Polymer cost is $2.00/lb.
***A11 facilities are amortized at 10% and 20 years except for trucks and compost site equipment, which are amortized at 10% and 8 years.
NOTE1 Construction costs are based on installed capacity, while OSM and chemical requirements are based on operating capacity.
Costs for centrifuge are from Appendix C, Figure C-4 to C-6, and for the belt press, Figure C-10 to C-I2.
Metric conversions: gpm x .0631 = 1/s; cu ft x .0283 = cu m; Ib x 0.454 - kg; btu x 1.055 = kJ; gal x 3.785 = 1
-------
6.4 Stage 3 - Laboratory Testing
A number of different tests can be performed in the laboratory to determine
the dewaterability of sludge. These tests serve a number of purposes,
including:
• Development of sizing criteria for full or pilot-scale installations
• Testing the influence of conditioning techniques
• Use as an operational control technique
The more commonly utilized laboratory testing procedures are described in the
following discussion.
6.4.1 Filter Leaf Test
This test is utilized for performance evaluation, sizing, and operational
control of vacuum filters. With the filter leaf test it is possible to vary
the solids content of the feed sludge, sludge conditioning, filter media,
cycle time, percent filter submergence, and vacuum level. Its intent is to
duplicate as closely as possible the actual operation of a vacuum filter
(12)(13). The equipment required to conduct the filter leaf test is shown in
Figure 6-4.
The procedure for the filter leaf test is to place a portion of filter cloth
identical to that used or planned for use with the vacuum filter on the test
apparatus. Vacuum in the test apparatus is adjusted to be equivalent to the
actual vacuum in the cake forming stage of filter operation, and this vacuum
is maintained for a time equivalent to the time of cake formation. The filter
cloth portion of the apparatus is then withdrawn from the agitated sludge and
maintained at a vacuum equivalent to the vacuum used during the drying stage
of vacuum filter operation. The cake solids content on the filter leaf and
the suspended solids content of the filtrate can be analyzed to determine
performance results. Generally, experiments are made with a variety of
chemical conditioning agents and dosages so that an approximate optimum
conditioning dosage and dewatering rate can be established.
The principal advantage of the filter leaf test is that it uses the same
filtering media as the vacuum filter. However, in order for the results to be
accurate, the sludge must be representative, the sample must be uniformly
stirred, and vacuum and cycle times must be identical to those utilized for
the full scale vacuum filter. Extrapolation of results to those which would be
expected from a pilot scale or full-scale unit should be done with the help of
an equipment manufacturer. Otherwise, the results are only an indication of
potential performance.
96
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VACUUM REGULATING
VALVE
VACUUM GAUGE
FLEXIBLE HOSE
\
STIRRED SLUDGE
MIXTURE
VACUUM
SOURCE
FILTER CLOTH
FIGURE 6-4
FILTER LEAF TEST APPARATUS
6.4.2 Specific Resistance Testing
The Specific Resistance Test, also known as the Buchner Funnel method, is used
to determine the dewaterability of sludge (13)(14)(15). A Buchner funnel with
a paper filter is mounted on top of a graduated cylinder, and a vacuum is
applied to the graduated cylinder. As a mixed sample of sludge is added to the
Buchner funnel, the volume of filtrate is recorded at preestablished time
intervals, and a plot is made of time/filtrate volume vs filtrate volume, as
shown in Figure 6-5.
97
-------
u
UJ
CO
M>
V -ML
FIGURE 6-5
TIME/FILTRATE VOLUME VS. FILTRATE VOLUME PLOT
USED IN SPECIFIC RESISTANCE TESTING
Using the slope of this line, specific resistance (r) is calculated from the
formula:
2 PA2b
where:
uw
r = specific resistance - m/kg
P = pressure of filtration - Pa
A = area of filter - sq m
b = slope of time/volume vs volume curve - s/m"
u = viscosity of filtrate - Pa.s
w = weight of dry solids per volume of filtrate - kg/cu m
Although it is possible to utilize laboratory specific resistance data to
calculate filter size and loadings, this procedure is not recommended because
of dissimilarities between the specific resistance test and actual vacuum
filter operation. Key dissimilarities are the use of top feed into the
Buchner funnel in contrast to pickup of sludge from the feed tank in actual
practice, and use of a paper filtering medium rather than the actual filtering
medium used on the vacuum filter.
The best use of the specific resistance test is to indicate the influence of
varying dosages of conditioning chemicals on sludge dewaterability. Figure 6-6
illustrates such a plot and its usefulness in determining optimum conditioning
chemical dosage.
98
-------
o
2
I
IU
o
o'4+
'2
I '°
Ui
oc
1 .0" +
0.
CO
IOIO +
OPTIMUM CONDITIONING CHEMICAL DOSAGE
4-
0 i 2 3 4
CONDITIONING CHEMICAL DOSAGE - % BY WEIGHT
FIGURE 6-6
USE OF SPECIFIC RESISTANCE TO DETERMINE OPTIMUM
CHEMICAL DOSAGE
6.4.3 Capillary Suction Time
The Capillary Suction Time (CST) is a simple and easy laboratory test to
conduct (13)(16)(17). The test gives a quick indication of sludge dewaterabil-
ity but the results are only meaningful when they are correlated with specific
resistance or some other test of sludge dewaterability. After this correlation
has been established, operation at the desired specific resistance can be
accomplished by operating at the CST corresponding to this specific
resistance.
The concept of the CST test is to measure the time required for the liquid
portion of the sludge to travel one centimenter, or any other fixed distance,
on a sheet of blotter paper. The device used to run the CST is shown in
Figure 6-7. As illustrated, a timer is used to measure the time required for
sludge movement between the two electrodes.
As an example of how to use the CST test, a typical CST time for an
unconditioned sludge is 200 seconds. For a filter press, this sludge must be
sufficiently conditioned to obtain a CST time of 10 seconds or less to produce
a cake where positive discharge is assured (18)(19). The CST test is very
useful in screening conditioning agents and evaluating the effect of
conditioner dosage on sludge dewaterability.
99
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BLOTTER PAPER
SLUDGE
FIGURE 6-7
CAPILLARY SUCTION TIME (CST) TEST SET-UP
6.4.4 Filterbelt Press Simulator
An instrument designed to test specifically for sludge dewaterability on a
belt filter press was devised in Sweden (20) (21). In this device, sludge is
placed in a filtration cell and pressed by a stainless steel piston into a
section of filter medium. The press is equipped with a pressure recording
device and a filtrate recording device. Shearing action, similar to that which
occurs as belts pass around rollers in a full-scale belt press, is simulated
by using a piece of filter media on the end of the piston, and then rotating
the piston.
Usefulness of the data generated is principally for operational control
purposes, and insufficient information is available to determine how closely
the results correlate with full-scale performance data. There are two differ-
ent graphic techniques to analyze data. One approach shows the cake solids
content versus pressing time, and this curve can be plotted for varying
pressures, as shown in Figure 6-8. This figure is based on no gravity drainage
prior to pressing. The second approach is to plot the cake solids content
versus conditioning chemical dosage. A typical plot is shown in Figure 6-9. On
either plot, filtrate suspended solids concentration can also be plotted if
desired.
Halde feels this test is much more applicable to belt filter presses than any
other dewatering test method, such as specific resistance or the Capillary
Suction Time (20). In one series of tests, four samples of the same sludge
were conditioned with four different chemicals. Although the specific
100
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25-r
z
UJ
o
0}
CO
Ul
1C
o
60 90 120 ISO
PRESSING TIME - SECONDS
FIGURE 6-8
FILTERBELT PRESS SIMULATOR - EFFECT OF PRESSURE AND
TIME ON CAKE SOLIDS CONCENTRATION
if
i
Ul
o
a>
o
5
co
UJ
x
o
25-
20--
BASED ON A FIXED TIME AND PRESSURE
6 d
POLYMER DOSAGE - g/kg DRY SOLIDS
l'2
FIGURE 6-9
FILTERBELT PRESS SIMULATOR - EFFECT OF POLYMER
DOSAGE ON CAKE SOLIDS CONCENTRATION
101
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resistance was the same for each of the conditioned sludges, cake solids
contents according to the filterbelt press simulator ranged from 8% to 23%
(including conditioning chemical). This result appears to indicate that this
test may be more applicable to belt presses than specific resistance testing.
6.4.5 Laboratory Scale Centrifuge Testing
Laboratory centrifuge techniques have been developed. These tests are useful
for determination of the effect of centrifugal force on cake solids concentra-
tion, the influence of centrifuge retention time on cake concentration, and
the influence of conditioning chemicals on cake solids concentrations. The
first two, centrifugal force and retention time in the centrifuge are the
factors with the greatest influence on effectiveness of the centrifuge.
The most frequently used laboratory test technique, sometimes called the
bottle centrifuge method, is to spin a graduated centrifuge tube at different
G forces or for different lengths of time. At the termination of testing, the
centrate is decanted and the cake solids are measured. A typical plot of data
which could be obtained by this technique is shown in Figure 6-10. Retention
time in the centrifuge and conditioning chemical dosage will affect the shape
of the curve and the cake solids concentration achievable. This test can also
be used to evaluate the effect of various polymer dosages on sludge dewater-
ability. It does not take into account the agitation and drainage which will
occur in a horizontal solid bowl centrifuge and thus the cake solids in the
full-scale unit may be higher than that predicted by the bottle centrifuge
test. Such tests are used by manufacturers to determine quickly on unknown
applications whether or not a centrifuge is feasible. It provides an excellent
tool for judging success, but is not effective for scale-up and sizing of
equipment.
Vesilind has proposed a modification of this technique in which a strobe light
is utilized to allow continuous observation of the sludge cake/centrate
interface (13). This technique can be useful for predicting optimum detention
time in the centrifuge.
6.5 Stage 4 - Field Testing
Following the initial screening, initial cost evaluation, and laboratory
testing, often two or more dewatering alternatives have similar overall costs
and it is often necessary to field test different dewatering processes. The
need and justification for field level testing depends in part upon the size
of the wastewater treatment plant. At very small plants with a capacity less
than 0.04 cu m/s (1 mgd), it may not be cost-effective to conduct pilot-scale
or full-scale testing. Instead laboratory or bench-scale testing by the
manufacturer may be adequate.
102
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25--
20--
<
-------
Pilot-scale devices are usually the smallest production scale machines
available. Although these are production machines, they are typically referred
to as pilot-scale units, while the larger units are typically referred to as
full-scale units. For example, a pilot-scale belt filter press would normally
be only 0.5 m or 1.0 m wide, versus a 2 m wide full-scale unit. Pilot-scale
solid-bowl centrifuges may have a capacity of only 0.95 to 2.2 1/s (15 to 35
gpm), while the largest centrifuges have capacities in excess of 25 1/s
(400 gpm). Pilot-scale filter presses often have only four filtering chambers,
while the largest presses have more than 40 chambers.
Scale-up from the small pilot-scale units to the larger full-scale units is
generally predictable and can usually be estimated by the manufacturer based
upon previous field test experience. Thus, often pilot-scale field testing is
all that is required prior to the final economic evaluation and selection of a
dewatering process. However, it is not always true that scale-up from pilot-
scale to full-scale units is predictable. For example, at the Los Angeles
County Sanitation Districts, both pilot- and full-scale testing of belt filter
presses and low-speed centrifuges was conducted (22)(23). Of the two belt
presses evaluated in detail, one manufacturer's belt press had a 50 percent
lower sludge throughput capacity in the pilot-scale testing, but had a
throughput capacity that was more than 30 percent higher than the other
machine in the full-scale testing. It was also concluded that the performance
of smaller belt presses was consistently better than that of larger units on
all sludge blends tested. Some consideration should therefore be given by the
designer to possible changes in performance when scaling-up the design and
performance of pilot-scale belt filter presses for full-scale operation.
Field testing of dewatering processes can be on an intermittent or continuous
basis, depending upon the ability to provide sludge feed and to provide for
the removal of sludge cake. In addition, the chemical conditioning cost
increases in proportion to the length of testing. Ten different test programs
performed at various locations in the United States are summarized in
Chapter 9. The total time required for field testing ranged from several weeks
to over six months. Pilot- and full-scale units were not operated continuously
over the entire testing period; in most cases, the units from different
manufacturers were operated at different times and not concurrently. At least
15 separate runs covering three days or more each appears to be the minimum
needed to evaluate the many variables associated with each dewatering unit.
Field testing of several dewatering processes provides specific design
criteria which can be used to develop a more realistic cost comparison between
units. If practical, simultaneous pilot- or full-scale testing of the
different dewatering devices being evaluated is recommended to obtain directly
comparable results. In summary, field testing should include the following:
• Field test as many machines as possible.
• Use actual sludge (or sludges) to be dewatered in final plant, if
possible.
• Test various levels and types of conditioning.
• Field test machines using same sludge feed in simultaneous side-by-
side comparisons, if possible.
104
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Field test machines which are as close in size to design machines as
possible.
Have at least one engineer or senior operator be involved in field
testing program. Do not rely entirely on manufacturer's representative
for evaluation and reporting of test results.
Verify or modify design criteria established in initial cost
evaluation.
6.6 Stage 5 - Final Evaluation Based on Detailed Design Parameters
After the results from the laboratory and/or field level testing are
available, it will be possible to scale-up or size the actual equipment which
would be used in the final detailed design. A final evaluation should then be
made, with the objective being to determine the validity of the prior design
criteria, assumptions and conclusions which were made. A final cost analysis
should be performed on the lowest cost alternatives from Stage 2, using the
same approach to the cost evaluation that was taken in Stage 2. Generally, the
lowest cost alternative will be selected, assuming all other factors are
comparable.
The most probable factors which may have changed based on results of field
and/or laboratory testing are:
Cake solids concentration
Solids capture during dewatering
Conditioning requirements
Equipment throughput per unit of time
Equipment cost based on manufacturer's quotations
Operator acceptance and ease of equipment maintenance
Maintenance cost and energy consumption
Reliability of equipment
Ability to handle variations in sludge quantity and quality
6.6.1 Cake Solids Concentration
Cake solids concentration will affect the cost of subsequent transportation
and disposal. If cake solids are lower than original projections, transporta-
tion and disposal costs will increase, and in some instances, the method of
ultimate disposal may become unsuitable. Situations where this may occur
involve either incineration or composting of the dewatered cake, or landfill
operations which require a minimum solids concentration for disposal. A
related factor which may have changed from original assumptions for a filter
press is the ease of cake release. To achieve an adequate cake release may
require considerably more or less chemicals than originally planned.
105
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6.6.2 Solids Capture During Dewatering
If solids capture is lower than originally expected, this will place a higher
solids load on the liquid handling portion of the treatment plant. This
increased solids recycle could adversely affect plant performance and result
in poorer effluent quality. Low solids capture during laboratory or field
testing may indicate the need for reevaluation of the selected dewatering
process.
6.6.3 Conditioning Requirements
Conditioning chemical dosages are highly dependent on the character of the
sludge being dewatered, as discussed in Chapter 5. In the initial cost evalua-
tion, chemical dosages must be based upon "typical" dosages for similar
sludges. After laboratory and/or field testing, more definitive information
will be available on dosages, and this more accurate data should be used in
the final cost evaluation.
6.6.4 Equipment Throughput Per Unit Time
A variation in throughput from the initial assumptions will affect the number
of pieces of equipment required and therefore the capital cost, and perhaps,
the O&M costs of the dewatering process. A large variation could change the
conclusion of the cost evaluation, and this should be checked during this
phase of the project.
6.6.5 Equipment Cost Based on Manufacturer's Quotations
At this point in the selection process, many contacts will have been made with
manufacturers, and equipment from several manufacturers may have been tested.
Based upon refined design criteria, equipment manufacturer's will be able to
furnish more accurate equipment costs than the budget level estimates used in
Stage 2 - Initial Cost Evaluation.
6.6.6 Operator Acceptance and Ease of Equipment Maintenance
The acceptance by plant operators of new equipment plays a large role in how
efficiently the equipment operates and how well it is maintained. Generally,
a piece of equipment which is difficult to operate and maintain will probably
not operate at peak efficiency or be adequately maintained. In several of the
studies evaluated in Chapter 9, the final decision between two types of equip-
ment with comparable costs was made on the basis of overall operability. The
106 \
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operator's role is particularly important in selecting between belt filter
presses and centrifuges, which have comparable overall costs.
6.6.7 Maintenance Cost and Energy Consumption
Full-scale testing can be helpful in gathering additional data on maintenance
requirements and energy consumption. Variations from original estimates may
occur in belt or filter media life, frequency of replacement of minor parts,
wash water requirements for a filter press or a belt filter press, and overall
energy consumption per unit weight of dry solids.
6.6.8 Reliability of Equipment
If equipment is prone to frequent breakdowns, maintenance costs will increase
and throughput will decrease. Effort should be directed to selection of the
most reliable equipment available when all other factors are comparable.
Should equipment be selected that is prone to frequent breakdowns, the normal
tendency is for plant operators to dislike both operating and maintaining the
equipment. The end result is that the equipment operates at low efficiency,
is poorly maintained, and will probably need replacement well before its
anticipated useful life.
6.6.9 Ability to Handle Variations in Sludge Quantity and Quality
Variations in sludge quality occur frequently, and without appropriate changes
in conditioning chemical dosages, the conditioned sludge will be difficult to
dewater. Belt filter presses are particularly susceptible to having large
changes in solids recovery due to changes in sludge quality or flow. Centri-
fuges and vacuum filters can handle variations in sludge feed fairly well,
although cake solids and solids recovery may be reduced somewhat. Proper
conditioning is important to all dewatering processes, however, and it is a
necessary part of a process to be able to quickly and easily change chemical
dosages in response to changes in sludge characteristics or flow.
6.7 References
1. Haug, R. T., "Compost Engineering - Principles and Practice," Ann Arbor
Science Publishers, Inc., Ann Arbor, Michigan, 1980.
2. Wesner, G. M., "Sewage Sludge Composting," Sludge Treatment and Disposal,
Volume 2 - Sludge Disposal, USEPA - Center for Environmental Research
Information, Cincinnati, Ohio, 45268, EPA-625/4-78-012, October 1978.
107
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3. "Process Design Manual For Sludge Treatment and Disposal," USEPA - Center
for Environmental Research Information, Cincinnati, Ohio, 45268,
EPA-625/1-79/011, September 1979.
4. Gulp, G. L., "Handbook of Sludge Handling Processes - Cost and
Performance," Garland STPM Press, New York, 1979.
5. "Innovative and Alternative Technology Assessment Manual," USEPA - Office
of Water Program Operations, Washington, D.C., 20460, MCD-53,
EPA-430/9-78-009, February 1980.
6. Gulp, Gordon L. and Hinrichs, Daniel J., "Municipal Wastewater Sludge
Management Alternatives," prepared for the EPA Technology Transfer
National Conference on 208 Planning and Implementation, 1977. (Authors
are with Culp/Wesner/Culp, Cameron Park, California)
7. Benjes, H. H., Jr., Faisst, J. A., and Lineck, T. S., "Capital and O&M
Cost Estimates for Biological Wastewater Treatment Processes, "EPA
Contract No. 68-03-2556, August 1979. (Lead author is with
Culp/Wesner/Culp, Denver, Colorado)
8. "Cost and Performance Handbook - Sludge Handling Processes,"
Culp/Wesner/Culp, Wastewater Treatment and Reuse Seminar, South Lake
Tahoe, California, October, 1977. (Culp/Wesner/Culp, Cameron Park,
California)
9. Ettlich, W. E., "Transport of Sewage Sludge," USEPA - Center for
Environmental Research Information, Cincinnati, Ohio, 45268,
EPA-600/2-77-216, December 1977.
10. Gulp, G. L., et al., "Costs of Chemical Clarification of Wastewater,"
U.S. EPA Task Order Contract 68-03-2186, 1976. (Lead author is with
Culp/Wesner/Culp, Cameron Park, California)
11. Ewing, L. J., Jr., Almgren, H. H., and Gulp, R. L., "Effects of Thermal
Treatment of Sludge on Municipal Wastewater Treatment Costs," USEPA,
Center for Environmental Research Information, Cincinnati, Ohio, 45268,
EPA-600/2-78-073, 1973.
12. "Sludge Dewatering," Manual of Practice No. 20, Water Pollution Control
Federation, Washington, D. C., 1969.
13. Vesilind, P. A., "Treatment and Disposal of Wastewater Sludges," Revised
Edition, Ann Arbor Science Publishers, Ann Arbor, 1980.
14. Rich, Linvil G., "Unit Operations of Sanitary Engineering," John Wiley
and Sons, Inc., New York, 1961.
15. Coakley, P. & Jones, B. R. S. , "Vacuum Sludge Filtration, I.
Interpretation of Results by the Concept of Specific Resistance, Sewage &
Industrial Wastes, Vol. 28, p. 963, 1956.
108
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16. Baskerville, R.C., and Gale, R.S., "A Simple Automatic Instrument for
Determining the Filterability of Sewage Sludges", Water Pollution Control
(Br), Vol. 67, p.233, 1968.
17. Gale, R.S., and Baskerville, R.C., "Capillary Suction Method for
Determination of the Filtration Properties of a Solid/Liquid Suspension,"
Chemistry and Industry, 1967.
18. Pietila, Kenneth A. and Joubert, Paul J., "Examination of Process
Parameters Affecting Sludge Dewatering with a Diaphragm Filter Press,"
Journal Water Pollution Control Federation, Vol. 53, p. 1708, 1981.
19. Personal communication, Ken Pietila, Rexnord, Inc., Milwaukee, Wisconsin,
February 1982.
20. Halde, Rolf E., "Filterbelt Pressing of Sludge - A Laboratory
Simulation," Journal Water Pollution Control Federation, Vol. 52, p. 310,
February 1980.
21. Baskerville, R.C., et al., "Laboratory Techniques for Predicting and
Evaluating the Performance of a Filterbelt Press," Filtration and
Separation, Vol. 15, p. 445, 1978.
22. "Mechanical Dewatering Study - Los Angeles County Sanitation Districts,"
LA/OMA Project, Regional Wastewater Solids Management Program, Los
Angeles-Orange County Metropolitan Area, September 1980.
23. Trubiano, R., Bachtel, D., LeBrun, T., and Horvath, R. , "Parallel
Evaluation of Low Speed Scroll Centrigues and Belt Filter Presses for
Dewatering Municipal Sewage Sludge," Draft EPA Report, Contract
68-03-2745, 1981. (Authors are with County Sanitation Districts of Los
Angeles County, Whittier, California)
109
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CHAPTER 7
COMPARATIVE COST ANALYSES OF SLUDGE TREATMENT AND DISPOSAL SYSTEMS
7.1 Introduction
In this chapter, capital and operation and maintenance costs are presented for
complete sludge treatment and disposal systems utilizing many of the different
dewatering processes described in this manual. Comparative cost analyses were
made for three sizes of sludge handling systems: 910, 4,540 and 45,400 kg per
day (1, 5, 50 tons per day) of dry sludge solids, which correspond approxima-
tely to 0.04, 0.2 and 2.2 cu m/s (1, 5 and 50 mgd) capacity wastewater treat-
ment plants. These sludge quantities are for raw primary and secondary sludge
prior to anaerobic digestion.
To develop capital and operation and maintenance costs, the same approach used
in Figure 6-3 and Table 6-5 was followed. First, processes were sized using
design criteria presented in this chapter. Construction costs for dewatering
processes were obtained from the cost curves presented in Appendix C, which
include equipment cost, excavation and site work, concrete structures, instal-
lation labor, electrical and instrumentation, piping, and housing. Construc-
tion costs for other sludge handling processes were based on references
(1-6). All costs were updated to April 1982. As in Table 6-5, construction
costs were increased by 40% to account for engineering, contingencies,
contractors' overhead & profit, legal fiscal and adminstrative, and interest
during construction. Land costs were included at $4950/ha ($2000/acre).
Capital costs were amortized at 10% and 20 years, except for trucks,
composting equipment, and front-end loaders, which were amortized at 10% and 8
years.
Operation and maintenance requirements were calculated as shown in Table 6-5.
The O&M requirements were developed in terms of labor, electricity, natural
gas, diesel fuel, maintenance material and chemicals. Unit cost factors used
for each of these O&M categories are as follows:
Category Unit Cost Factor
Labor $8 or 12/hr*
Electricity $0.05/kwh
Natural Gas $1.30/106 btu**
Diesel Fuel $1.15/gal
Maintenance Materials $/yr
Chemicals $2/lb polymer
*$8/hr used for 910 kg/day (1 ton/day) systems and $12/hr used
for larger systems.
**Value of excess digester gas remaining after digester heating.
110
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Users of this manual should recognize that the cost estimates presented in
this chapter are based on a great number of assumptions relating to design and
loading criteria. Since these loading criteria most certainly will vary from
location to location, the costs developed and presented herein should be
utilized for general purposes only.
7.2 Cost Comparison for One Ton Per Day Sludge Handling Systems
Four systems are compared for treatment of 910 kg/day (1 ton/day) of a
primary/waste activated sludge (WAS) mixture. These systems are:
• belt press thickening of WAS, anaerobic digestion, lagoons
• belt press thickening of WAS, anaerobic digestion, sand drying beds
• belt press thickening of WAS, anaerobic digestion, low pressure belt
press
• belt press thickening of WAS, anaerobic digestion, and vacuum assisted
drying beds.
All systems used low pressure belt press thickening (such as Smith & Loveless
Sludge Concentrator) of WAS and on-site disposal of dried sludge. Design
criteria used for the sizing and loading of process equipment are listed in
Table 7-1.
Capital and operation and maintenance costs for these alternatives, as shown
in Table 7-2, indicate that the sludge lagoons are the lowest cost alterna-
tive, followed by sand drying beds, low pressure belt presses, and vacuum
assisted drying beds. Although the latter two alternatives are more costly
than lagoons or sand drying beds, they have potential application where land
is unavailable for conventional sand drying beds or where lagoons are
aesthetically or otherwise unacceptable.
7.3 Cost Comparison for Five Ton Per Day Sludge Handling Systems
Eight treatment and disposal systems were evaluated for 4,540 kg/day (5
ton/day) of raw primary and waste activated sludge. These sludge handling
systems, as shown in Figure 7-1, consisted of low G solid bowl centrifuge
thickening of WAS, anaerobic digestion, dewatering, 16 km (10 mile) one-way
truck haul, and landfill of dewatered sludge. With the exception of the
dewatering process, all other components of the system were the same. The
dewatering processes evaluated were:
Basket centrifuge
Low G solid bowl centrifuge
High G solid bowl centrifuge
Belt filter press
Vacuum filter
Fixed volume filter press
Sand drying beds
Sludge lagoons
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TABLE 7-1
DESIGN CRITERIA FOR 910 kg/Day (l ton/day)
SLUDGE HANDLING COST ANALYSES
A. SOLIDS PRODUCTION
Primary 0.14 kg/cu m (1150 Ib/mil gal) - 60% volatile;
5% solids
Secondary Waste 0.11 kg/cu m (950 Ib/mil gal) - 80% volatile;
Activated Sludge 0.5% solids
B. SECONDARY SLUDGE THICKENING
Low pressure belt press
5% solids output
Polymer dosage - 6 g/kg (12 Ib/ton)
C. ANAEROBIC DIGESTION
Single stage, completely mixed
15 day hydraulic detention time
Volatile solids loading - 2.6 kg V.S./cu m/dy (0.16 Ib V.S./cf/d)
50 percent reduction of volatile solids
Flare digester gas remaining after digester heating
D. CHEMICAL CONDITIONING
Process Conditioner Dosage
g/kg (Ib/ton)
Lagoons None — —
Sand drying beds None
Low pressure belt press Polymer 7.5 (15)
Vacuum assisted
drying beds Polymer 2.5 (50)
E. DEWATERING EQUIPMENT - CAKE SOLIDS AND LOADING RATE
Lagoons - Volume = 1110 cu m (39,600 cu ft)
No sludge removal
Sand drying beds - 50% cake solids
78 kg/sq m/yr (16 Ib/sq ft/yr)
Vacuum assisted drying beds - 15% cake solids
9.8 kg/sq m/d (2 Ib/sq ft/d)
One application/day
Low pressure belt press - 12% cake solids
50 kg/hr/m (75 Ib/hr/ft)
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TABLE 7-2
COST SUMMARY FOR 910 KG/DAY (1 TON/DAY) CAPACITY
SLUDGE TREATMENT AND DISPOSAL SYSTEMS
Sludge
Dewatering System
Sludge Lagoons
Sand Drying Beds
Low Pressure
Belt Press
Vacuum Assisted
Drying Bed
Capital Cost
Thousand $
84
294
154
294
O&M Cost
$1,000/YR
25.7
11.1
38.3
35.3
Total Annual
Cost
$1,OOQ/YR
35.6
45.6
56.4
69.8
Percent
Higher Than
Lowest Cost
28%
58%
96%
PRIMARY SLUDGE
p-
ANAEROBIC
DIGESTION
DEWATERING
WASTE
ACTIVATED
SLUDGE
CENTRFUGE
THICKEN
TRUCK
HAUL
LANDFILL
DISPOSAL
BASKET CENTRIFUGE
SOLID BOWL CENTRIFUGE - LOW G
SOLID BOWL CENTRIFUGE - HIGH G
BELT FILTER PRESS
VACUUM FILTER
FIXED VOLUME FILTER PRESS
BAND DRYING BEDS
SLUDGE LAGOONS
FIGURE 7-1
8LUDQE TREATMENT AND DISPOSAL SYSTEMS EVALUATED
FOR 5 TON PER DAY COST ANALYSES
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These dewatering techniques were selected on the basis of their applicability
to systems of this general size. A flow diagram for the systems evaluated is
shown in Figure 7-1 and design criteria used in the cost analyses are shown in
Table 7-3.
Capital costs, operation and maintenance costs, and total annual costs are
presented in Table 7-4. As shown in this table, the sludge treatment system
using centrifuge thickening of WAS, anaerobic digestion of primary and WAS,
dewatering by sludge lagoons and then a 16 km (10 mile) one-way truck haul to
landfill disposal has the lowest total annual cost. The next lowest cost
dewatering alternatives are high G solid bowl centrifuges, followed in order
of increasing total annual cost by low G solid bowl centrifuges, belt filter
presses, sand drying beds, basket centrifuges, and vacuum filters. The fixed
volume filter press has the highest total annual cost (59 percent higher than
sludge lagoons).
7.4 Cost Comparison For Fifty Ton Per Day Sludge Handling Systems
Nine treatment and disposal systems were evaluated for 45,400 kg/day (50
tons/day) of raw sludge. These systems were:
• Centrifuge thicken WAS, anaerobic digestion, low G solid bowl
centrifuge, truck haul, landfill
• Centrifuge thicken WAS, anaerobic digestion, high G solid bowl
centrifuge, truck haul, landfill
• Centrifuge thicken WAS, anaerobic digestion, belt filter press, truck
haul, landfill
• Centrifuge thicken WAS, anaerobic digestion, vacuum filter, truck
haul, landfill
• Centrifuge thicken WAS, anaerobic digestion, fixed volume filter
press, truck haul, landfill
• Centrifuge thicken WAS, anaerobic digestion, diaphragm filter press,
truck haul, landfill
• Centrifuge thicken WAS, fixed volume filter press, multiple hearth
incineration, truck haul, landfill
• Centrifuge thicken WAS, heat treatment, vacuum filter, incinerate,
truck haul, landfill
• Heat treatment, vacuum filter, incinerate, truck haul, landfill
For the first six of these systems, one-way haul distances of 16 km (10 miles)
and 64 km (40 miles) were also evaluated, to determine the sensitivity of
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TABLE 7-3
DESIGN CRITERIA FOR SLUDGE HANDLING COST ANALYSES
5 and 50 Ton Per Day Systems
A. SOLIDS PRODUCTION
Primary
Secondary Waste
Activated Sludge
B. SECONDARY SLUDGE THICKENING
0.14 kg/cu m (1150 Ib/mil gal) - 60% volatile;
5% solids
0.11 kg/cu m (950 Ib/mil gal) - 80% volatile;
0.5% solids
Low G Solid Bowl Centrifuge - 6% solids output
No chemicals added
C. ANAEROBIC DIGESTION
Single stage, completely mixed
15 day hydraulic detention time
Volatile solids loading - 2.6 kg V.S./cu m/dy (0.16 Ib V.S./cf/dy)
50 percent reduction of volatile solids
Sell digester gas remaining after digester heating
D. CHEMICAL CONDITIONING
Conditioner
1. Basket Centrifuge* Polymer
2. Solid Bowl Centrifuge***
Low G
High G
3. Belt Filter Press***
4. Vacuum Filter***
5. Fixed Volume
Filter Press***
6. Diaphragm Filter
Press**
7. Drying Beds*
8. Lagoons*
Polymer
Polymer
Polymer
Lime
Ferric Chloride
Lime
Ferric Chloride
Lime
Ferric Chloride
Polymer
None
Raw Sludge Digested Sludge
g/kg (Ib/ton) g/kg (Ib/ton)
3 (6)
Not Applicable
Not Applicable
4
4
(8)
(8)
(12)
::
140 (280)
40 (80)
—
180
60
180
60
180
60
(360)
(120)
(360)
(120)
(360)
(120)
2.5
(5)
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TABLE 7-3 (Continued)
E. HEAT TREATMENT (Thermal Conditioning)**
Option without WAS Thickening
Feed Solids Concentration = 1.8%
Loading Based on sludge flow of 29 1/s for 45,400 kg/day (461
gpm for 50 ton/day)
Option with WAS Thickening
Feed Solids Concentration = 5.4%
Loading Based on Sludge Flow of 9.5 1/s for 45,400 kg/day (151
gpm for 50 ton/day)
Recycled Liquor Treatment includes increased aeration capacity
Odor control using carbon adsorption
F. DEWATERING EQUIPMENT - CAKE SOLIDS AND LOADING RATES
Raw Digested Heat Treated
Sludge Sludge Sludge
Basket Centrifuge*
Cake Solids - % — 14
Loading - Based on
hydraulic loading
to the unit
Solid Bowl Centrifuge***
Cake Solids - % — 18
Loading - Based on
hydraulic loading
to the unit
Belt Filter Press***
Cake Solids - % ~ 22
Loading - 1/s/m — 3.2 —
Loading - gpm/m — 50 —
Vacuum Filter***
Cake Solids - % — 18 35
Loading - kg/sq m/hr — 20 34
Loading - Ib/sq ft/hr — 47
Fixed Volume Filter Press***
Cake Solids - Z 40 36
Cycle Time 2.5 hr 2.5 hr
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TABLE 7-3 (Continued)
Raw Digested Heat Treated
Sludge Sludge Sludge
Diaphragm Filter Press**
Cake Solids - % — 45
Loading - kg/sq m/hr — 4.9 —
Loading - Ib/sq ft/hr — 1.0
Drying Beds*
Cake Solids - % -- 50
Loadings - kg/sq m/yr — 78
Ib/sq ft/yr — 16
Lagoons*
Cake Solids - % — 302
Loading - Assumes sludge added to lagoons intermittently for 18
months, then rested for 6 months before removal
G. INCINERATION**
Multiple Hearth Furnace
Combustion is self-sustaining with 35Z feed solids
Fuel is required for startup only
24 hr/day operation, 6 start-ups/year
Loading of 44 kg/sq m/hr (9 Ib/sq ft/hr) for 45,400 kg/day
(50 ton/day) plant
H. TRUCK HAUL
One way distance = 16 km (10 Mi)
Type of Trucks:
4,540 kg/day (5 ton/day) plant - 7.6 cu m (10 cu yd) gasoline
45 400 kg/day (50 ton/day) plant - 22.9 cu m (30 cu yd) diesel
Operational Criteria
4,540 kg/day (.5 ton/day) plant - 10 hr/day maximum haul time
45,400 kg/day (50 ton/day) plant - 16 hr/day permissible haul
time
Ash density of 800 kg/cu m (50 Ib/cu ft)
I. LANDFILL DISPOSAL
$1.96 per cu m ($1.50 per cubic yard)
*5 ton/day systems only
**50 ton/day systems only
***Both 5 and 50 ton/day systems
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TABLE 7-4
COST SUMMARY FOR 4,540 KG/DAY (5 TON/DAY) CAPACITY
SLUDGE TREATMENT AND DISPOSAL SYSTEMS
Sludge Capital Cost O&M Cost Annual Cost Higher Than
Dewatering System million $ $l,000/yr $l,000/yr* Lowest Cost
Sludge Lagoons 1.87 85 313
Solid Bowl Centrifuge -
High G 2.10 121 378 17Z
Solid Bowl Centrifuge -
Low G 2.14 123 386 23%
Belt Filter Press 2.19 121 387 24%
Sand Drying Beds 2.34 115 398 27%
Basket Centrifuge 2.54 102 411 31%
Vacuum Filter 2.38 160 450 44%
Fixed Volume Filter
Press 2.74 168 498 59%
Note: Facilities include centrifuge thickening of V&S, anaerobic digestion,
dewatering, truck haul, and landfill disposal
*Capital cost converted to annual cost using a CRF of 0.11746 (10%, 20 yr)
for all facilities except trucks, for which a CRF of 0.18744 (10%, 8 yr) was
used.
overall cost to distance hauled. Flow diagrams for the systems evaluated are
shown in Figure 7-2, and design criteria used for process sizing are listed in
Table 7-3.
Capital, O&M and total annual costs for the systems evaluated are shown in
Table 7-5. For a 16 km (10 mile) haul distance, the lowest cost system is for
centrifuge thickening, anaerobic digestion, and dewatering by low G or high G
solid bowl centrifuge with cake hauling to a landfill. Costs for these two
systems are virtually identical. The system using belt press dewatering is 7%
more costly than the centrifuge dewatering systems. A fixed volume filter
press is more cost-effective than either a vacuum filter or diaphragm filter
press. As the dewatered sludge haul distance is increased from 16 km (10
miles) one-way to 64 km (40 miles), the processes which produce a drier cake
solids become somewhat more cost-effective, although the general ranking of
the alternative concepts is the same with the exception that the diaphragm
filter press becomes more cost-effective than the vacuum filter.
118
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PRIMARY SLUDGE
— •
ANAEROBIC
DIGESTION
DEWATERING
WASTE
ACTIVATED
SLUDGE
CENTRIFUGE
THICKEN
TRUCK
HAUL
LANDFILL
DISPOSAL
SOLID BOWL CENTRIFUGE - LOW G
SOLID BOWL CENTRIFUGE - HIGH G
BELT FILTER PRESS
VACUUM FILTER
FIXED VOLUME FILTER PRESS
DIAPHRAGM FILTER PRESS
PRIMARY SLUDGE
WASTE
ACTIVATED
SLUDGE
CENTRIFUGE
THICKEN
FILTER PRESS
DEWATERING
MULTIPLE
HEARTH
INCINERATION
PRIMARY SLUDGE
r 1
' CENTRFUGE L
] THICKEN i
I 1
(OPTIONAL)
VACUUM FILTER
DEWATERING
INCINERATION
TRUCK HAUL
LANDFILL DISPOSAL
FIGURE 7-2
SLUDGE TREATMENT AND DISPOSAL SYSTEMS EVALUATED
FOR 50 TON PER DAY COST ANALYSES
119
-------
TABLE 7-5
COST SUMMARY FOR 45,400 KG/DAY (50 TON/DAY)
CAPACITY SLUDGE TREATMENT AND DISPOSAL SYSTEMS
Sludge
Dewatering System
Capital Cost
million $
• Centrifuge/Anaerobic Digestion/
Dewater/Truck Haul/Landfill
16 km (10 Mile) One-Way Haul
Solid Bowl Centrifuge -
Low G 6.61
Solid Bowl Centrifuge -
High G
Belt Filter Press
Fixed Volume Filter Press
Vacuum Filter
Diaphragm Filter Press
64 km (40 Mile) One-Way Haul
Solid Bowl Centrifuge -
High G
Solid Bowl Centrifuge -
Low G
Belt Filter Press
Fixed Volume Filter Press
Diaphragm Filter Press
Vacuum Filter
6.61
O&M Cost
$l,000/yr
Total
Annual Cost
$l,000/yr*
Percent
Higher Than
Lowest Cost
648
803
1,446
6.50
7.03
8.43
7.54
10.39
671
694
726
930
706
1,456
1,540
1,733
1,840
1,943
—
7%
20%
27%
34%
1,588
6.50
7.07
8.44
10.40
7.66
807
820
817
781
1,128
1,605
1,674
1,828
2,019
2,061
1%
5%
15%
27%
30%
Centrifuge/Filter
Press/Incinerate
Heat Treatment/Vacuum
Filter/Incinerate
13.51
Centrifuge/Heat
Treatment/Vacuum Filter/
Incinerate 17.62
22.39
945
1,165
1,587
2,544
3,255
4,227
76%**
125%**
192%**
* Capital cost converted to annual cost using a CRF of 0.11746 (10%, 20 yr)
** Percent higher than cost of solid bowl centrifuge with 16 km (10 mile) haul
120
-------
The cost estimates shown in Tables 7-2, 7-4, and 7-5 are only presented to
illustrate the procedure of evaluating dewatering processes as a part of a
complete sludge treatment system. These cost estimates are based upon the cost
curves presented in Appendix C, several other references (1 to 6), and upon
the specific design criteria assumed in this analysis. The costs presented are
general in nature, and therefore, should only be used for general purposes
until detailed, site-specific cost estimates can be prepared.
7.5 References
1. Gulp, G. L., "Handbook of Sludge Handling Processes - Cost and
Performance," Garland STPM Press, New York, 1979.
2. Benjes, H. H., Jr., Faisst, J. A., and Lineck, T. S., "Capital and O&M
Cost Estimates for Biological Wastewater Treatment Processes, "EPA
Contract No. 68-03-2556, August 1979. (Lead author is with
Culp/Wesner/Culp, Denver, Colorado)
3. "Cost and Performance Handbook - Sludge Handling Processes,"
Culp/Wesner/Culp, Wastewater Treatment and Reuse Seminar, South Lake
Tahoe, California, October 1977. (Culp/Wesner/Culp, Cameron Park,
California)
4. Ettlich, W. E., "Transport of Sewage Sludge," USEPA - Center for
Environmental Research Information, Cincinnati, Ohio, 45268,
EPA-600/2-77-216, December 1977.
5. Gulp, G. L., et al., "Costs of Chemical Clarification of Wastewater,"
U.S. EPA Task Order Contract 68-03-2186, 1976. (Lead author is with
Culp/Wesner/Culp, Cameron Park, California)
6. Ewing, L. J., Jr., Almgren, H. H., and Culp, R. L., "Effects of Thermal
Treatment of Sludge on Municipal Wastewater Treatment Costs," USEPA -
Center for Environmental Research Information, Cincinnati, Ohio, 45268,
EPA-600/2-78-073, 1973.
121
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CHAPTER 8
ENERGY CONSIDERATIONS IN DEWATERING PROCESS SELECTION
8.1 Introduction
Energy required for sludge dewatering is important because energy costs can
influence overall project cost as well as process selection. This chapter
presents an analysis of both "direct" energy requirements, as well as the
"indirect" energy requirements associated with production of the conditioning
chemicals. Consideration of both the direct and indirect energy requirements
is important in conducting an energy sensitivity analysis. Such an analysis
should be conducted to determine the impact of energy cost escalations at
rates greater than or less than the average inflation rate.
As is stressed in other portions of this manual, evaluation of the dewatering
process can not be performed independently of other sludge treatment and
disposal processes. This is true from an energy standpoint also. In many
process flow concepts, dewatering energy requirements may be low, but the
overall energy requirement for solids treatment, transportation, and disposal
may be high. In other cases the converse is true.
8.2 Direct Energy Requirements for Dewatering
In order to compare direct energy requirements of the various dewatering
options available, information was gathered from full scale equipment opera-
ting at wastewater treatment plants across the country, from dewatering equip-
ment manufacturers, and from the EPA report "Energy Conservation in Municipal
Wastewater Treatment—MCD-32" (1). A summary of this information is presented
in Table 8-1. It is important to note that while all of the sludges shown in
Table 8-1 are digested mixtures of primary and waste activated sludge (WAS),
the percentage of difficult-to-dewater WAS in the mixture varies from 25 to 90
percent. Generally, as the ratio of primary to secondary sludge decreases, the
energy required for dewatering increases.
Based upon the information in Table 8-1, ranges in electricity and fuel
requirements for sludge dewatering were developed and are presented in
Table 8-2. The ranges in total energy requirements presented for each dewater-
ing process further illustrate the fact that sludges from different wastewater
treatment plants vary greatly in their dewaterability. This variability
emphasizes the need for full-scale testing of equipment comparable to the
actual equipment which would be installed in order to define the actual
throughput rates achievable. In addition to sludge quality, there are several
122
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TABLE 8-1
DIRECT ENERGY REQUIREMENTS FOR SLUDGE DEWATERING - CASE STUDY RESULTS
Energy Requirement, kwh/ton dry solids
INFORMATION SOURCES
to
Dewatering
Process
(Sludge Type)1
Drying Beds
Vacuum Filter
Basket Centrifuge
Solid Bowl Centrifuge
Low- Speed
High-Speed
Fixed Volume Filter Press
Metro
Chicago
Calumet
Plant (2)
(Approx.
40:60)
—
7002
—
53
88
25
Metro
Chicago
West-S.W.
Plant (3)
(10:90)
—
2472
—
2132
79
35 37
L.A.
County
(4)(5)
(75:25)
—
—
1153
65
20
Irvine,
Calif. (6)
(Approx .
50:50)
_.
—
—
100
58
Metro Orange Co. San Joae
Denver (7) San. Dist.(8) Calif. (9)
(50:50) (70:30) (50:50)
„
60
—
72-147 48 22-29
52-87
8-12 7 —
52 41-54
Data From
Manufacturers
(10 - 13)
(Approx .
50:50)
—
46-58
—
30,33,38
60-90
10-15
EPA
Report (1)
(65:35)
3-4
38-58
89-107
33
29-54
Diaphragm Filter Press — — — —- — —
'All sludges are digested mixtures of primary and waste activated sludge, unless noted. Ratio shown is (Primary:WAS)
^These values seem high but are the values reported in the literature
-'Digested primary sludge
Metric Conversion: 1 kwh/ton « 0.0011 kwh/kg
45-55
-------
Process
Basket Centrifuge
Solid Bowl
Centrifuge
Low-Speed
High-Speed
Belt Filter Press
Vacuum Filter
Fixed Volume
Filter Press
Diaphragm Filter
Press
Drying Beds
Sludge Lagoons
TABLE 8-2
GENERAL RANGES OF DIRECT ENERGY REQUIREMENTS FOR SLUDGE DEWATERING
Fuel
kj/kg dry
solids
23
(Btu/ton)
(20,000)
Electricity
Total
Equivalent
Electricity^
102 - 170 (88,000-146,000)
kwh/kg dry
solids
0.105-0.140
0.035-0.070
0.070-0.105
0.011-0.029
0.046-0.070
0.046-0.070
0.041-0.064
0.001-0.002
0.001-0.002
(kwh/ton)
(90-120)
(30-60)
(60-90)
(10-25)
(40-60)
(40-60)
(35-55)
(1-2)
(1-2)
kwh/kg dry
solids
0.105-0.140
0.035-0.070
0.070-0.105
0.011-0.029
0.046-0.070
0.046-0.070
0.041-0.064
0.003-0.004
0.010-0.018
(kwh/ton)
(90-120)
(30-60)
(60-90)
(10-25)
(40-60)
(40-60)
(35-55)
(3-4)
(9-16)
dewatering a digested 50:50 mixture of primary and WAS at 3 percent feed solids.
2Fuel converted to equivalent electricity using a factor of 11,080 kJ per kwh (10,500 BTU/kwh) and an
electrical generation efficiency of 32.5%.
-------
other variables which affect dewatering energy requirements: (1) solids
concentration of sludge feed; (2) conditioning method selected; (3) number of
machines - more energy is generally required to run two smaller machines than
one large machine of equivalent capacity, although this is not generally true
for solid bowl centrifuges if the same G force is used in both small and large
centrifuges; (4) solids throughput achieved; and (5) differences in machines
produced by different manufacturers.
The energy requirements for drying beds shown in Table 8-2 include an
electricity requirement for sludge pumping to the beds and a fuel requirement
for operating a front-end loader used for sludge removal. The power required
for pumping sludge to the drying beds is based on a TDH of 4.6 m (15 feet).
This value could be low in some plants where long pumping distances are
required, or high in a plant where there is gravity flow to the beds. Appro-
priate corrections need to be made for situations significantly different than
4.6 m (15 feet) of TDH. In smaller plants it is possible that manual labor is
used for sludge removal and not a front-end loader.
The direct energy requirements presented in Table 8-2 are also shown in
Figure 8-1. For the seven mechanical dewatering processes presented, the
order of direct energy used, from lowest to highest, is:
Belt filter press
Low G solid bowl centrifuge
Diaphragm filter press
Fixed volume filter press
Vacuum filter
High G solid bowl centrifuge
Basket centrifuge
8.3 Indirect Energy Requirements for Dewatering
Most of the sludge dewatering processes operate more efficiently when the
sludge is conditioned, typically with chemicals, prior to dewatering.
Secondary energy is indirect energy required to produce consumables
(chemicals) used in wastewater and sludge treatment processes. Consideration
of these secondary energy requirements is supplemental to any cost-effective-
ness analysis that may be performed in evaluating alternatives. However, the
future cost of chemicals is directly affected by increases in the cost of
energy, and this would be apparent in an energy sensitivity analysis which
included secondary energy. A dewatering alternative having a relatively high
secondary energy requirement has a greater dependence on energy than is
indicated by the direct energy alone. Indirect energy requirements for sludge
dewatering are shown in Table 8-3 and Figure 8-1. As shown, processes which
utilize polymer conditioning (centrifuges and belt filter press) have low
indirect energy requirements, while processes which utilize lime and ferric
chloride conditioning (vacuum filter and filter presses) have high indirect
energy requirements.
125
-------
OT
Q
.22
w .20
ec.
o
10 * I 8
s.
X
.16
ui
2 • •«
o
o
a:
u
o
K
U
200
- 180
160
140
to
-a
i'20
tn
.12
.10
.08
5 .04 -
§ .02
OO
80
60
40
20 -
INDIRECT ENERGY
DIRECT ENERGY
TYPICAL RANGE FOR
DIRECT ENERGY
n
^
#Z
^
^2
/> /v
^
X/ /v
// //
&. &.
BASKET LOW SPEED HIGH SPEED BELT VACUUM FIXED DIAPHRAGM DRYING
CENTRIFUGE SOLID BOWL SOLID BOWL FILTER FILTER VOLUME FILTER BEDS
CENTRIFUGE CENTRIFUGE PRESS FILTER PRESS PRESS
NOTEi Sludg* typ« It dlg««ttd primary and WAS, approximately SOtSO ratio.
FIGURE 8-1
DIRECT AND INDIRECT ENERGY REQUIREMENTS
FOR SLUDGE DEWATERING PROCESSES
-------
TABLE 8-3
INDIRECT ENERGY REQUIREMENTS FOR SLUDGE DEWATERING*(1)
Dewatering
Process
Basket Centrifuge
Solid Bowl
Centrifuge
Belt Filter
Vacuum Filter
Filter Press
Conditioning
Chemical
Polymer
Polymer
Polymer
Lime
FeCl3
Lime
FeClo
Chemical Dosage
g/kg (Ib/ton)
(6)
4
6
150
40
120
50
(8)
(12)
(300)
(80)
(240)
(100)
Indirect
Electrical Energy
kwh/kg kwh/ton
dry solids dry solids
0.0007
0.0009
0.0013
0.099
0.044
0.079
0.055
(0.6)
(0.8)
(1.2)
(90)
(40)
(72)
(50)
*Sludge type is digested primary + WAS,
Use of polymer conditioning has been tested at a number of filter press
installations, but the results have been generally unsatisfactory, due to poor
cake release, poor solids capture, and low cake solids concentrations. For
vacuum filtration, the Process Design Manual for Sludge Treatment and Disposal
reports that several facilities have realized cost savings using polymers for
conditioning (14). However, more operator attention may be required to obtain
good cake release, and the overall cake solids content may be somewhat lower
while the volatile solids content of the dry cake will be higher. For some
sludges, especially digested sludges and sludges containing large quantities
of WAS, polymer conditioning may not be feasible.
8.4 Total Energy Requirements for Dewatering
Total energy requirements for sludge dewatering, including both direct and
indirect energy, are summarized in Figure 8—1. As shown, processes which
utilize polymer conditioning have the lowest total energy requirements.
When selecting a sludge dewatering system, it is important to evaluate not
only the energy required for dewatering, but the overall energy requirements
for sludge treatment and disposal. There are cases where the selected
127
-------
dewatering process consumes more energy than other alternatives, but the total
sludge treatment and disposal energy requirements are lower. In some
instances, the most cost-effective dewatering alternative may require more
energy than other alternatives. In such cases an energy sensitivity analysis
should be made to determine the effect of escalating energy costs. For
example, if energy costs outpace inflation by 10 or 20 percent over the next
10 years, the cost-effective alternative at current energy prices may no
longer be cost-effective at future energy prices.
8.5 References
1. Wesner G. M., et al., "Energy Conservation in Municipal Wastewater
Treatment," USEPA - Office of Water Program Operations, Washington,
D. C., 20460, MCD-32, EPA-430/9/77-011, March 1978.
2. Zenz, D. R., et al., "Evaluation of Unit Processes for Dewatering of
Anaerobically Digested Sludge at Metro Chicago's Calumet Sewage Treatment
Plant," The Metropolitan Sanitary District of Greater Chicago, October
1976.
3. Sawyer, Bernard; Watkins, Robert; and Lue-Hing, Cecil, "Evaluation of
Unit Processes for Mechanical Dewatering of Anaerobically Digested Sludge
at Metro Chicago's West-Southwest Sewage Treatment Plant," Paper
presented at the 31st Annual Purdue Industrial Waste Conference, May
1976. (Authors are with the Research and Development Department of The
Metropolitan Sanitary District of Greater Chicago)
4. Trubiano, R. , Bachtel, D., LeBrun, T. , and Horvath, R. , "Parallel
Evaluation of Low Speed Scroll Centrifuges and Belt Filter Presses for
Dewatering Municipal Sewage Sludge," Draft EPA Report, Contract
68-03-2745, 1981. (Authors are with County Sanitation Districts of Los
Angeles County, Whittier, California)
5. Personal Communication, Thomas J. LeBrun, Supervisor of Research Section,
Joint Water Pollution Control Plant, County Sanitation Districts of Los
Angeles County, Carson, California, June 1982.
6. CH2M~Hill, "Michelson Water Reclamation Plant - Engineering Report for
Dewatering Equipment Selection," Irvine Ranch Water District, Irvine
California, June 1979.
7. Tavery, M. A., "Evaluation of Sludge Dewatering Equipment at the Metro
Denver Sewage District," paper presented at the Colorado AWWA-WPCA
Technical Activities Committee, May 3, 1979. (Author is with the Metro-
politan Denver Sewage Disposal District No. 1, Denver, Colorado).
8. John Carollo Engineers, "Design Memorandum No. 5 - Dewatering Methods,"
County Sanitation Districts of Orange County, Fountain Valley,
California, April 1979.
128
-------
9. Consoer, Townsend & Associates Ltd., "Draft Project Report - Sludge
Processing Facilities Plan For the Cities of San Jose and Santa Clara,
California," May 1980.
10. Ingersoil-Rand, Unpublished data on filter press and centrifuge test
results, 1977 - 1979. (information received from Wayne B. Gendron,
Ingersoll-Rand, Nashua, New Hampshire)
11. Personal communication, Brian Scholes, KHD Humboldt Wedag, Atlanta,
Georgia, April 1981.
12. Personal communication, Gordon Wilson, Ingersoll-Rand sales
representative, from Tom Ponton Industries, Santa Fe Springs, California,
June 1979.
13. Personal communication, Dick Gray, Komline Sanderson Engineering
Corporation, Peapack, New Jersey, July 1982.
14. "Process Design Manual For Sludge Treatment and Disposal,"
USEPA - Municipal Environmental Research Laboratory, Cincinnati, Ohio,
45268, EPA-625/1-79/011, September 1979.
129
-------
Chapter 9
SUMMARY OF RECENT SIDE-BY-SIDE COMPARISONS OF DEWATERING
PROCESSES AT TEN TREATMENT PLANTS
9.1 Introduction
In this chapter, the evaluation studies of dewatering alternatives conducted
by ten large utilities in various parts of the U.S. are summarized. In each of
the evaluations, pilot tests and full-scale field tests of at least two and
sometimes three or four different types of mechanical dewatering processes
were considered.
Of the ten evaluations, four recommended belt filter presses, four recommended
solid bowl centrifuges, one recommended a fixed volume recessed plate filter
press, and for one study no recommendation was made. A summary of these
studies showing the equipment evaluated and the type of dewatering equipment
recommended by the evaluation is shown in Table 9-1.
These ten studies are presented to show the manner in which different large
utilities approached the selection of dewatering equipment. Although eight of
the ten utilities selected centrifuges or belt presses, this does not neces-
sarily imply that all utilities of comparable size should select one of these
two types of dewatering devices. Each application is unique, and decisions on
dewatering equipment selection should be made using the approach recommended
in Chapter 6.
9.2 County Sanitation Districts of Los Angeles County (California)
During a two-year period from 1977 through 1979, several pilot-scale and some
full-scale mechanical dewatering equipment including belt filter presses,
centrifuges, and a diaphragm filter press were tested at the Joint Water
Pollution Control Plant in Carson. At the time of the dewatering evaluations,
no method of ultimate disposal had yet been selected.
Six different manufacturers sent pilot-scale belt filter presses to the
Sanitation Districts for testing (1):
Ashbrook-Simon-Hartley Winklepress
Parkson Magnum Press
Tait-Andritz SDM
Carter Belt Filter Press
Komiine-Sanderson Unimat
Envirotech Eimco High-Solids Press
130
-------
TABLE 9-1
SUMMARY OF RESULTS FROM TEN EVALUATIONS OF MECHANICAL
DEWATERING EQUIPMENT
Utility
County Sanitation Districts
of Los Angeles County -
References 1, 3, and 4
County Sanitation Districts
of Orange County (Ca.) -
References 6, 7, and 8
Irvine Ranch Water District
(Ca.) - Reference 9
Metropolitan Denver Sewage
Disposal District No. 1.-
References 10, 11, 12, 13,
and 14
Metropolitan Sanitary
District of Greater
Chicago - References 15,
16, and 17
Middlesex County Sewerage
Authority (N.J.) -
References 18 and 19
Milwaukee Metropolitan
Sewerage District -
References 20 and 21
Nassau County (N.Y.) -
References 22, 23, and 24
San Jose—Santa Clara Water
Pollution Control Plant -
References 25 and 26
District of Columbia
Wastewater Treatment Plant
at Blue Plains - References
27 and 28
Equipment
Evaluated*
BFP, C(LS),
C(HS), DFP
BFP, C(LS),
FP
BFP, C(LS)
BFP, C(LS),
C(HS), VF
BFP, C(LS),
C(HS), VF
BFP, DFP,
FP
BFP, C(HS),
FP
BFP, DFP,
FP
BFP, C(LS),
C(HS), DFP,
FP, VF
BFP, DFP,
FP, VF,
VF Retrofit
Recommended
Dewatering Equipment
Centrifuge—Low Speed
with hydraulic back
drive
Belt Filter Press
Belt Filter Press
Centrifuge
Centrifuge—Either low
speed or high speed
Belt Filter Press
Fixed Volume Filter
Press
Belt Filter Press
Centrifuge—Either low
speed or high speed
Preliminary evaluation-
no recommendation
*BFP = Belt Filter Press
C(HS) = High Speed Centrifuge
C(LS) = Low Speed Centrifuge
DFP = Diaphragm Filter Press
FP = Fixed Volume Filter Press
VF = Vacuum Filter
131
-------
The Eimco High-Solids Press is not actually a belt filter press but rather a
vacuum filter which advances sludge cake intermittently to a pneumatically
operated press. Use of the unit was considered infeasible because of the
requirement for large amounts of lime and ferric chloride for conditioning.
The other five belt filter presses were tested on a number of blends of
digested primary and waste activated sludge (WAS). With a blend of 30 percent
primary and 70 percent WAS, cake solids concentrations of 20 percent and
solids recoveries of 90 percent were obtained on the Winklepress and the
Magnum Press. Cake solids concentrations on the Andritz SDM were 16 to 17
percent. Polymer requirements for the belt filter presses were 3 to 6 g/kg (6
to 12 Ib/ton) dry solids. Descriptions, sketches, and differences in design
and operation for many of the different manufacturers' belt filter presses are
presented in an EPA Technology Transfer Seminar Publication (2).
Three different types of pilot-scale centrifuges were tested (1):
Sharpies PM-35000 High-Speed Solid Bowl (Scroll) Centrifuge
Kruger 250 Low-Speed Solid Bowl (Scroll) Centrifuge
Robatel Basket Centrifuge (existing)
The Robatel basket centrifuge required only 2 to 2.5 g/kg (4 to 5 Ib/ton) of
polymer but could produce a cake solids concentration of only 15 to 20 percent
on blends consisting of 30 to 50 percent WAS. Also, the basket centrifuges
used a batch operation and required much operator and maintenance attention.
Because of high cake disposal and O&M costs, the basket centrifuge was not
considered further for dewatering blends of digested primary and WAS.
The Sharpies centrifuge produced cake solids concentrations of 22 to 26
percent solids with 94 to 99 percent solids recoveries for the 100 percent
digested primary sludges. The polymer doses ranged from 2 to 3 g/kg (4 to 6
Ib/ton). For a 60 percent primary and 40 percent WAS mixture, cake solids
concentrations were 16 to 20 percent, recoveries 94 to 97 percent, and polymer
dosages 2.5 to 4.5 g/kg (5 to 9 Ib/ton).
The Kruger centrifuge had problems with plugged centrate tubes after a few
months and operation was judged to be unpredictable and unreliable. It was
estimated that on 100 percent digested primary sludge a cake solids concentra-
tion of 24 to 27 percent and 90 percent solids recovery could be achieved
using a polymer dosage of 3.5 to 4.0 g/kg (7 to 8 Ib/ton). On a mixture of 70
percent primary and 30 percent WAS, polymer requirements were 7.5 g/kg (15
Ib/ton) to achieve adequate solids racoveries.
The Ingersoll Rand Lasta automatic diaphragm filter press was evaluated in a
four chamber pilot unit. The Lasta press produced sludge cakes ranging from 40
to 53 percent solids, however, lime and ferric chloride requirements were very
high. On a 50:50 blend of digested primary and WAS, a cake solids concentra-
tion of 49 percent was produced, however 47 percent lime and 21 percent ferric
chloride were required for conditioning, resulting in a corrected cake sludge
solids of only 26 percent. Use of the Lasta diaphragm filter press was
considered impractical because of the large amounts of conditioning chemicals
required. Belt filter presses and scroll centrifuges proved to be the most
effective dewatering devices.
132
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Following the extensive pilot-scale testing study, two belt filter presses and
two low speed scroll centrifuges were selected for full-scale parallel
evaluation (3):
Ashbrook-Simon-Hartley Winklepress Model 3V - 2.2 m belt width
Parkson Magnum Press Model MP50 - 2.0 m belt width
Bird HB5900 Centrifuge - 0.9 m (36 in) bowl diameter
Kruger 280 MC Centrifuge - 0.8 m (32 in) bowl diameter
The Sanitation Districts did not evaluate full-scale high speed centrifuges
such as are manufactured by Sharpies. They chose to evaluate only low speed
centrifuges because they felt low speed centrifuges had lower energy, polymer
and maintenance time requirements for their operation (3).
The Magnum Press consistently produced about 4% drier cakes for all the sludge
blends tested than did the Winklepress. However, polymer dosages for the
Winklepress were consistently 1.5 to 2.0 g/kg (3 to 4 Ib/ton) lower than for
the Magnum Press. Solids recovery was also considerably better on the Winkle-
press. Because of superior performance of the Winklepress, it was used in
comparisons with the centrifuge. Cake solids of 23 percent were achieved on
the Winklepress for a 75/25 blend of digested primary to WAS with a polymer
dose of 4.5 g/kg (9 Ib/ton). On a 50/50 blend, the Winklepress produced a 22
percent cake with 89 percent solids recovery and a polymer dose of 7.3 g/kg
(14.6 Ib/ton).
The Bird centrifuge produced 19 percent cake solids and 95 percent recovery at
a polymer dose of about 6 g/kg (12 Ib/ton) on the 50/50 blend of digested
primary and WAS. Although the operation of both the belt filter press and the
centrifuge was judged to be unpredictable and fairly unstable on the 50/50
blend, it appeared on the average that the belt filter press could produce a
drier cake but required more polymer than the centrifuge.
The results of the test work led to the following conclusions: 1) within the
accuracy of the tests, there was no significant cost advantage for either the
belt filter presses or the low speed scroll centrifuges equipped with automa-
tically controlled hydraulic backdrives; the slightly drier cakes produced by
belt filter presses and their lower power costs were offset by their increased
polymer requirements; 2) low speed scroll centrifuge operation may be more
difficult for the novice operator to understand because the process was not as
visible as it was for the belt filter press; the centrifuges also produced
more noise and vibration; 3) belt filter presses were found to require greater
maintenance due primarily to a belt life of only three to six months; were
susceptible to acute loss of solids recovery due to changes in sludge quality
or flow; required greater operator attention and frequent washdown; emitted
noticeable odors; and required prescreening of sludge to remove large objects
and fibrous materials; and 4) low speed scroll centrifuges with hydraulic
backdrives were judged to be preferable to belt filter presses for dewatering
digested sewage sludge (3).
As a result of the side-by-side testing evaluation, the Districts advertised
for bids for low-G scroll centrifuges in 1980. In August 1980, the bid was
awarded to KHD Humboldt Wedag to provide 19 S-4-1 centrifuges to dewater an
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anaerobically digested 50:50 blend of primary and WAS. An 8.8 cu m/s (200 mgd)
pure oxygen activated sludge plant to be completed in 1983 will generate WAS
to be dewatered along with the existing primary sludge. The centrifuges, to be
installed in two separate buildings with nine centrifuges in one building and
ten in another, are expected to be on-line by late 1983 or early 1984 (4).
In October and November 1981 a Sharpies PM 35000 high G solid bowl centrifuge
and a KHD Humboldt Wedag S3-0 low G centrifuge were tested at the District 32
water reclamation plant in Valencia, California (4). The Sharpies centrifuge
was equipped with all of the Polymizer features and a manual version of the
eddy current brake, and the Humboldt centrifuge was equipped with a hydraulic
backdrive. The centrifuges were tested several weeks apart on an anaerobically
digested 50:50 blend of primary and WAS. The feed sludge quality was somewhat
different at the time the two machines were tested:
Feed Solids Volatile Solids
Sharpies 2.7% TS 68%
Humboldt 3.0% TS 66%
At the start of the tests, the Districts' criteria for acceptable performance
was a minimum cake solids of 15% and solids recovery of 95%. The Sharpies
machine never had a solids capture above 90%, and the cake solids content
ranged up to 11.5 percent for solids captures of 80 to 90% and polymer dosages
of 3.5 to 7 g/kg (7 to 14 Ib/ton). The Humboldt machine was able to produce a
cake solids concentration up to 16 percent with a 91 percent capture and a
polymer dosage of 7 g/kg (14 Ib/ton).
Based upon the test results, the Humboldt centrifuge was rated as a viable
alternative for the dewatering facility and the Sharpies centrifuge was rated
as unacceptable. It should be noted that the centrifuge tests were not simul-
taneous side-by-side tests. A representative from Sharpies described the
difference in sludge quality (as noted by volatile solids content) as the
major reason for the performance differences between the machines (5).
9.3 County Sanitation Districts of Orange County (California)
An evaluation of dewatering processes was undertaken by the Orange County
Sanitation Districts during 1979 and 1980. The dewatering equipment processed
an anaerobically digested mixture of primary and WAS consisting of about 70
percent primary and 30 percent WAS. The sludge was generated from a 6.0 cu m/s
(138 mgd) primary treatment plant and a 2.0 cu m/s (46 mgd) activated sludge
plant. At the time of the dewatering evaluation, ultimate sludge disposal
alternatives had not yet been evaluated. A desktop cost evaluation of centri-
fuges, filter presses and belt filter presses was made in a prior design
memorandum in 1979, and the results are shown in Table 9-2 (6).
Almost concurrently, pilot-scale field tests using one-meter presses were
conducted on three different belt filter presses (7), to see if the actual
belt press performance matched the design criteria in the earlier memorandum.
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TABLE 9-2
DESIGN CRITERIA AND COST COMPARISON FOR DEWATERING
AT COUNTY SANITATION DISTRICTS OF ORANGE COUNTY (CALIFORNIA)
Centrifuge
Filter Press
Belt Filter Press
Unit Used
No. Units Required
Rated Capacity
Cake Solids
Solids Capture
Chemical Use
Power Usage
Construction Costs - Dewatering
Construction Costs - Storage
Annual Operation and Mainte-
nance Costs
Dewatering Costl - $/Mg ($/ton)
Disposal Costl - $/Mg ($/ton)
Total Sludge Handling Costsl -
$/Mg ($/ton)
Bird Model
HB 64000
8
12.6 1/s
(200 gpm)
Pas savant
Model 20
4
56 m3
(2,000 cu ft)
Ashbrook
"Winklepress"
Model 3V
12
7.9 1/s
(125 gpm)
22%
6 g polymer/
kg dry solids
65 kw/unit
$5,082,000
860,000
1,057,000
43.55 (39.50)
20.77 (18.84)
64.32 (58.34)
95%
200 g lime/kg dry solids
80 g FeCl3/kg dry solids
57.3 kwh/Mg solids
$8,805,000
725,000
1,230,000
63.16 (57.29)
11.81 (10.71)
74.97 (68.00)
24%
6 g polymer/kg
dry solids
6 kw/unit
$4,572,000
820,000
1,100,000
42.76 (38.78)
18.52 (16.80)
61.28 (55.58)
•'•Present worth analysis using 6 7/8 % interest for a 10-year planning period.
-------
The results of the field tests shown in Table 9-3, compared favorably with the
previously established design criteria and belt filter presses were selected
as the recommended dewatering method (7).
TABLE 9-3
RESULTS FROM FIELD TESTING OF BELT FILTER PRESSES AT COUNTY
SANITATION DISTRICTS OF ORANGE COUNTY (CALIFORNIA)
Press
EIMCO
Tait-Andritz
Winkle
Design
Feed
Rate
1/s
3.22
28
30
3.32
Feed
Solids
2.13
2.33
2.33
2.7
Polymer
Average Dose
4.4
6.0
5.9
6.0
Average
Capture
96.6
94.7
95.6
90.0
Primary/
Avg. Cake Secondary
Solids Sludge
19.34
23.33
21.50
24.00
69/31
75/25
75/25
75/25
Four 2.2-m Winklepresses will be installed at Plant No. 1 by about December
1982 to dewater a digested blend of primary and air waste activated sludge. In
addition, ten 2.2-m Winklepresses began operation in June 1982 at Plant No. 2
dewatering digested primary sludge. Preliminary results indicate that a 30%
solids cake can be achieved at a polymer dosage of 3.5 to 4 g/kg (7 to 8
Ib/ton) and that up to a 40% cake can be obtained at 6.5 g/kg (13 Ib/ton)
polymer. By late 1982 a new pure oxygen activated sludge plant will be opera-
ting, and the belt presses will be used to dewater a blend of digested primary
and oxygen WAS. The belt presses at both plants will replace existing high G
and low G solid bowl centrifuges. It is currently planned that the belt
presses will be operated during the period from 10 pm to 12 noon four or five
days per week as required, to keep the power costs as low as possible during
the peak electrical demand period (8).
9.4 Irvine Ranch Water District (California)
During 1979 the Irvine Ranch Water District conducted a brief dewatering
equipment evaluation for the Michelson Water Reclamation Plant (9). At the
time of the evaluation, about 4,540 dry kg/d (5 tons/d) of aerobically
digested, centrifuge-dewatered sludge were being transported to a sanitary
landfill for disposal. The existing low speed Bird centrifuges were at that
time producing a 11-13% solids cake, with 95% solids capture and a polymer
cost of $44/dry Mg ($40/dry ton).
When the requirements for landfill disposal were raised to a minimum cake
solids of 15 percent, an equipment evaluation was undertaken. The criteria for
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selecting new dewatering equipment were to produce a minimum of 15 percent
cake solids at substantially lower operating costs than were possible with the
existing centrifuges.
Four types of dewatering equipment were discussed as possible options for the
Michelson Plant: existing centrifuges; belt filter presses; vacuum filters;
and filter presses. Vacuum filters were not evaluated in detail because of
their typical requirement for large quantities of lime and ferric chloride.
Filter presses were eliminated early from consideration because of their high
costs and requirements for lime and ferric chloride. This left belt filter
presses to be compared with the existing centrifuges.
A pilot test of a 1-m Winklepress on site proved that a belt press could
achieve a dewatered cake of 15 to 17 percent solids at a polymer cost of about
$33/Mg ($30/ton) and a capture of 94 to 95 percent. An economic evaluation
showed that leasing a 2-m belt press and operating an interim facility would
reduce the annual cost of operation from about $480,000 to about $280,000. In
addition, it was recommended that two belt filter presses be purchased if the
leased unit operated satisfactorily for at least two months. Satisfactory
operation of the belt filter press was obtained, and the existing centrifuge
building was modified by the addition of two 2.2-m Winklepresses.
9.5 Metropolitan Denver Sewage Disposal District No. 1 (Colorado)
The Metro Denver Sewage District currently operates a 7.5 cu m/s (172 mgd)
activated sludge plant. In 1979 the District completed an evaluation of three
types of sludge dewatering equipment: belt filter presses, centrifuges and
vacuum filters (10). The study was undertaken because the District could not
process all the anaerobically digested sludge produced with six existing coil
spring vacuum filters. At the time of the evaluation, the sludge disposal
option chosen for the treatment plant was an agricultural reuse system. An
alternate form of dewatering and disposal, however, was required on an interim
basis. The staff recommended that pilot-scale production models of centrifuges
and belt presses be brought to the District for on-site testing so the opera-
tional characteristics and costs could be evaluated and compared with the
vacuum filters.
Two manufacturers of belt presses, the Parkson Corporation and the
Tait-Andritz Company, and two firms which manufacture centrifuges, the
Pennwalt-Sharples Corporation and Bird Machine Company, were invited to
demonstrate their pilot units at the District. The above companies were
selected primarily on their ability to provide equipment for evaluation prior
to January 1979. The pilot equipment was operated by the associated companies
under the supervision of the District's Operations Control Specialist. The
average performance and costs of the belt press, centrifuge and vacuum filter
are compared in Table 9-4 (10).
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TABLE 9-4
RESULTS AND OPERATING COSTS FROM FIELD TESTING
AT METROPOLITAN DENVER SEWAGE DISPOSAL DISTRICT NO. 1
Feed Sludge,
% TS
% VS
Alkalinity, mg/1
Chemical Conditioning
System
Cake Solids, % TS
Solids Recovery, %
Chemicals, $/Mg ($/ton)
Labor, $/Mg ($/ton)
Power, $/Mg ($/ton)
Water, $/Mg ($/ton)
Haul, $/Mg ($/ton)
TOTAL OPERATING COST,
$/Mg ($/ton)
Belt
Filter Press
3.1
62
5,200
FeCl3 &
Anionic
Polymer
17.2
90-95
48.14 (43.66)
6.48 (5.88)
0.28 (0.25)
2.01 (1.82)
26.79 (24.30)
83.70 (75.91)
Centrifuge
3.0
64
4,820
Cationic
Polymer
13.0
90-95
22.19 (20.13)
4.32 (3.92)
3.09 (2.80)
0.00 ( .00)
29.91 (27.13)
Vacuum Filter
3.1
62
5,180
Cationic
Polymer
9.5
75-80
54.12 (49.09)
6.48 (5.88)
1.59 (1.44)
0.36 (0.33)
40.68 (36.90)
59.51 (53.98) 103.24 (93.64)
Based upon fairly limited data it appears that the Sharpies high G centrifuge
produced either a drier cake or used somewhat less polymer than the Bird low G
centrifuge (11). The Sharpies PM 35,000 centrifuge was able to produce cake
solids contents of 10-11 percent with 8 Ib/ton polymer, 11.5 - 12.5 percent
with 9 Ib/ton polymer and at a 50 percent lower flow rate, 14-14.5 percent
solids with 12 Ib/ton polymer. The Bird HB 2500 centrifuge was able to produce
cake solids contents of 10-11 percent with 9-12 Ib/ton polymer and, at a 50
percent lower flow rate, 12.5 - 13.5 percent with 11-12 Ib/ton polymer.
The chief difference between the belt press, the centrifuge, and the existing
vacuum filter was the chemical conditioning. The sludge fed to the centrifuge
and vacuum filter was flocculated using a cationic polymer. In the belt press,
the use of the cationic polymer produced large, fluffy floes that squeezed out
the sides of the belt in the low pressure zone and squeezed into the belt mesh
in the high pressure area. This hydrophilic characteristic of the floe contri-
buted to wet cakes and poor solids recoveries. The Parkson Corporation had
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tested successfully a ferric chloride-anionic polymer combination in their
laboratory. When this dual chemical system was applied to the pilot belt
press, there was a large improvement in cake solids (13 to 19%) and in solids
recoveries (85 to 93%). It was also significant that the sludge loadings
increased 50% after switching to the dual conditioning.
As indicated, the centrifuge had the lowest overall operating cost. The belt
press, using the dual chemical system, had a high cost due to the use of
ferric chloride, which accounted for 80% of the chemical cost. Vacuum filtra-
tion had the highest chemical cost. Based upon the field test results and
final evaluation, solid bowl centrifuges were selected as the recommended
method of dewatering. On August 18, 1981, Metro Denver received bids from two
low G manufacturers and one high G manufacturer to provide one large solid
bowl centrifuge to dewater an anaerobically digested blend (45 percent
primary: 55 percent oxygen WAS). The contract was awarded to KHD Humboldt
Wedag to provide one S-6 low G centrifuge to dewater 32 1/s (500 gpm) to 16
percent solids or to thicken 47 1/s (750 gpm) to 6 percent solids. The centri-
fuge has been installed and was operational by June 1982. Although the one
centrifuge can handle the total sludge flow, consideration is being given to
purchasing a second centrifuge for standby capacity and flexibility. Current
plans are to dispose of digested thickened sludge by land application or to
dewater sludge for possible composting when land is unavailable for applica-
tion of liquid sludge (12).
Two manufacturers of diaphragm filter presses also conducted laboratory or
bench-scale dewatering tests on the Metro Denver sludge, although at a later
date than the field-testing of vacuum filters, belt presses and centrifuges.
Ingersoll Rand conducted laboratory-scale tests on April 20-21, 1981 on a
digested sludge which contained 3.0 percent total solids, 2.7 percent suspend-
ed solids, and 27.9 percent ash. The following results were obtained (13):
Feed Cake Solids
Conditioning Chemicals Time Solids Loading Rate Solids Capture
30% Lime, 10% FeCl3 5 min 1.68 kg/sq m/hr
(0.34 Ib/sq ft/hr) 37.9% 99.5%
30% Lime, 10% FeCl3 7 min 1.85 kg/sq m/hr
(0.38 Ib/sq ft/hr) 33.4% 99.5%
30 Ib/ton polymer,* 0.77 kg/sq m/hr
Pfizer X-99 (0.16 Ib/sq ft/hr) 20.7%
*These results with polymer were considered unfavorable for pressure
filtration by Ingersoll Rand.
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Envirex also conducted bench-scale diaphragm filter press tests, with the
following results obtained (14):
Conditioning Chemicals Feed Solids Cake Solids Solids Capture
15% Lime, 5% FeCl3
30% Lime, 10% FeCl3
5.8%
3.0%
34%
32%
99.
99.
9.6 Metropolitan Sanitary District of Greater Chicago
During 1976 the District completed two evaluations of mechanical dewatering
methods, one for the 9.6 cu m/s (220 mgd) Metro Chicago Calumet Sewage Treat-
ment Plant (15) and one for the 52.6 cu m/s (1200 mgd) Metro Chicago West-
Southwest Plant (16). Following dewatering the sludge was stored and dewatered
on land to greater than 30 percent solids before distribution to the general
public. Similar dewatering equipment was field-tested at both locations:
• Carter Belt Filter Press (Pilot Scale)
• Passavant Vac-U-Press (Full Scale)
• Komline Sanderson Vacuum Filter (Existing Full Scale)
• Sharpies Centrifuge (Pilot Scale)
• Bird Centrifuge (Pilot Scale)
At the Calumet Plant the dewatering results shown in Table 9-5 were obtained
with an anaerobically digested mixture of 30 to 45 percent primary sludge, and
55 to 70 percent WAS, and a sludge feed solids concentration of about 2 to 3
percent (15).
TABLE 9-5
RESULTS OF FIELD TESTING AT THE METROPOLITAN SANITARY DISTRICT
OF GREATER CHICAGO CALUMET PLANT
Carter BFP
Passavant BFP
K.S. Vac. Fil.
(FeCl3)
K.S. Vac. Fil.
(FeCl3 + CaO)
Sharpies Cen.
Bird Cen.
*1974 Prices
CAKE SOLIDS
SOLIDS CAPTURE
22.2
19.0
16.6
19,
20,
19.9
85.7
90.0
89.0
95.0
93.2
98.8
CHEMICAL
COSTS
$/dry Mg
11.58
11.41
15.44
20.95
7.72
16.32
SOLIDS
LOADING
28.1 kg/sq m/hr
6.1 kg/sq m/hr
POWER USAGE
kwh/dry Mg
28
46
10.3 kg/sq m/hr 1544
20.8 kg/sq m/hr 772
87.2 kg/hr 97
177 kg/hr 58
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At the West-Southwest Plant the dewatering results shown in Table 9-6 were
obtained with an anaerobically digested mixture of 10 percent primary and 90
percent WAS and a feed solids content of 3.5 to 4 percent (16).
TABLE 9-6
RESULTS OF FIELD TESTING AT THE METROPOLITAN SANITARY DISTRICT
OF GREATER CHICAGO WEST - SOUTHWEST PLANT
CAKE SOLIDS
SOLIDS CAPTURE
Carter BFP
Passavant BFP
K.S. Vac. Fil.
(FeCl3)
K.S. Vac. Fil.
(FeCl3 + CaO)
Sharpies Cen.
Bird Cen.
*1974 Prices
12.1
14.2
13.1
15.5
15.3
17.1
82.0
90.0
92.0
92.5
96.4
97.6
CHEMICAL
COSTS*
$/dry Mg
17.64
13.89
13.23
13.78
13.67
13.56
SOLIDS
LOADING
20.5 kg/sq m/hr
6.8 kg/sq m/hr
POWER USAGE
kwh/dry Mg
39
41
35.2 kg/sq m/hr 453
61.1 kg/sq m/hr 272
85.4 kg/hr 87
95.3 kg/hr 235
At the West-Southwest Plant, centrifuges produced the driest cake and achieved
the highest solids capture at chemical conditioning costs approximately equal
to the other unit processes tested. Based upon the test results, centrifuges
were selected for the West-Southwest Plant.
At the Calumet Plant, centrifuges were selected because the percent solids
recovery was higher than for the belt filter presses, and the cake solids
concentration was nearly as high as that produced by the Carter belt press.
Also, the Sharpies centrifuge had the lowest chemical cost of the devices
evaluated.
Eleven Sharpies PC 81,000 high G centrifuges were installed at the
West-Southwest Plant by January 1981 to dewater 159 dry Mg/d (175 tons/d) of
digested sludge. The dewatering facility first achieved full production on a
monthly basis in August 1981. Current operation of the facility consists of
centrifuge dewatering of a portion of the digested sludge (10% primary:90%
WAS) to a nominal 15 percent solids, then blending it with the remaining
digested sludge at 4 percent solids to form a 7-8 percent sludge mixture. The
sludge mixture is barged 322 km (200 mi) for land disposal on a 6,070 ha
(15,000 ac) farm (17).
Five HS-805M high G centrifuges supplied by Ishikawajima-Harimic Heavy
Industries Co. of Tokyo, Japan (IHI) through Marubeni America Corporation were
installed at the Calumet Plant by January 1982 to dewater 91 dry Mg/d (100
tons/d) of digested sludge. The dewatering facility had not yet begun
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full-scale operation as of May 1982. Based upon test results and bid
performance specifications it is expected that a dewatered cake solids
concentration of about 20 percent will be produced on the digested sludge (30%
primary:70% WAS). It is expected that the dewatered sludge will be further
dewatered in lagoons to about 50% solids before disposal in landfills (17).
9.7 Middlesex County Sewerage Authority (New Jersey)
During 1978 the Middlesex County Sewerage Authority pilot tested four filter
presses and four belt filter presses for sludge dewatering (18). The units
tested were:
Passavant High-Pressure Fixed Volume Filter Press (Pilot Scale)
Nichols Low-Pressure Fixed Volume Filter Press (Bench Scale)
Ingersoll Rand Lasta Diaphragm Filter Press (Pilot Scale)
Rexnord Diaphragm Filter Press (Bench Scale)
Ashbrook-Simon-Hartley Belt Filter Press
Komline-Sanderson Belt Filter Press
Parkson Belt Filter Press
Tait-Andritz Belt Filter Press
During the testing period the feed sludge varied from 2.4 to 4.5 percent total
solids concentration. A mixture of raw primary and WAS on about a 50:50 ratio
was used for the testing. The test results shown in Table 9-7 were obtained
during the field scale testing.
TABLE 9-7
TEST RESULTS AT
MIDDLESEX COUNTY SEWERAGE AUTHORITY
Unit Cake Solids* Chemical Requirements
Filter Presses
Recessed Plate Presses
High-Pressure 36 18% Lime, 7% FeCl3
Low-Pressure 30 - 34 18% Lime, 6%
Diaphragm Presses 40 20% Lime, 6%
Belt Filter Presses 20 - 30 Polymer, $13-18/Mg
*Note: Cake Solids Concentrations include conditioning chemicals.
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Both belt filter presses and filter presses were considered to be capable of
producing sludge cake suitable for landfilling, composting, starved air
combustion, and co-disposal operations. A cost analysis indicated that belt
press dewatering was the most economical system from among these disposal
alternatives evaluated, and belt filter presses were selected. Although belt
filter presses were selected for dewatering, as of July 1982 there are no
plans for design and installation of belt presses. Current plans are to
continue using barges for ocean disposal of liquid sludge (19).
9.8 Milwaukee Metropolitan Sewerage District (Wisconsin)
Field testing of pilot-scale and full-scale thickening and dewatering
equipment was conducted by the Milwaukee Metropolitan Sewerage District at the
Jones Island and South Shore Wastewater Treatment Plants during the period
from June 1980 to January 1981 (20). The following dewatering units were
tested on an anaerobically digested mixture of primary and WAS:
1. Centrifuge
• Sharpies PM-35,000 Polymizer horizontal solid bowl unit
2. Belt Filter Press
• Passavant 2-m Vac-U-Press
• Komline-Sanderson 0.5-tn Kotnpress
• Ralph B. Carter 0.8-m Model 32 unit
3. Fixed Volume Filter Press
• Passsavant press (four round chambers, each with effective filtra-
tion area of 0.56 sq m [6.05 sq ft])
• Edwards and Jones press (four square chambers, each with effective
filtration area of 0.30 sq m [3.21 sq ft])
4. Diaphragm Filter Press (14)
• Envirex press (six square chambers, each with a total filtration
area of 0.97 sq m [10.4 sq ft])
Based on a feed solids concentration of 2.5 to 3.0 percent from the anaerobic
digester, both the centrifuge and belt press could produce a cake solids
concentration of 18 percent with a 95 percent solids recovery. However, the
belt press typically required 50 percent more polymer than the centrifuge - 6
versus 4 g/kg (12 versus 8 Ib/ton). The centrifuge option was shown to be 12
percent less costly than a belt press on a present worth basis.
For the filter press tests, with a feed solids concentration of 2.4 percent
solids, typically a 41 percent cake solids was achieved using a lime dose of
35% (as CaO) and a ferric chloride dose of 5.5%. Centrifuge thickening the
sludge before the filter press reduced the required chemical dosages to
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17 to 24 percent lime and to 3.2 to 4.4 percent ferric chloride and produced a
cake solids of about 38 percent. Prethickening also increased the machine
throughput anywhere from 64 to 125 percent (20). Diaphragm filter press tests
produced cake solids concentrations of 35 to 55 percent total solids with
solids recoveries greater than 99.5 percent (14).
Fixed volume filter presses were selected as the recommended dewatering
process for both the Jones Island Plant and the South Shore Plant in the
Milwaukee Solids Handling Studies as of May 1981 (20). At the Jones Island
Plant, filter cake would be landfilled in a sludge-only landfill. At the
South Shore Plant, sludge storage in a building would be required during
winter months. The ability to stack filter press cake in a ten-foot pile with
a front-end loader with minimal drainage favored the filter press cake over
the centrifuge or belt press cake. There was some concern over applying a
filter press cake containing substantial quantities of lime to the alkaline
soils available for land application. Thickening ahead of the filter press was
recommended, in part because this greatly reduced the quantity of lime
required for conditioning.
In May 1982 the District's plans were somewhat different than those described
above. For the Jones Island Plant, the current plan is to use filter presses
for dewatering primary sludge from primary clarifiers which are not yet
constructed. Waste activated sludge will be thickened by solid bowl centri-
fuges and dewatered on existing vacuum filters for continued production of
Milorganite. For the South Shore Plant, the current plan is to use existing
flotation thickening of waste activated sludge and centrifugal dewatering of a
digested blend of primary and waste activated sludge (21).
9.9 Nassau County (New York)
During 1978 and 1979, a sludge handling demonstration project was conducted as
part of the Nassau County Sludge Management Plan to evaluate dewatering and
composting of sewage sludge (22). The Cedar Creek Water Pollution Control
Plant processes 72.6 Mg (80 tons) of sludge solids per day. Dewatering
machines evaluated from the various manufacturers included:
Komline-Sanderson Belt Filter Press (Unimat) - 0.5 m
Ashbrook-Simon-Hartley Belt Filter Press (Klam press) - 0.5 m
Passavant Recessed Plate Filter Press (Pilot Scale)
Shriver Diaphragm Filter Press (Pilot Scale)
Envirex Diaphragm Filter Press (Bench Scale)
Nichols Engineering Diaphragm Filter Press (Full Scale)
Anaerobically digested sludge solids concentrations ranged from 1 to 5 percent
and averaged about 2.3 percent. Both the fixed volume recessed plate and
diaphragm filter presses were able to dewater the sludge to a solids content
of 35 percent or greater (including chemicals). Chemical requirements for all
filter presses tested were significantly higher than reported by equipment
manufacturers for similar sludge types. The fixed volume recessed plate press
required 45 to 67 percent lime and 15 to 27 percent ferric chloride. The
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Envirex diaphragm press required 27 to 53 percent lime and 8 to 14 percent
ferric chloride. The Nichols diaphragm filter press was able to produce a cake
solids content of 31 percent when conditioning with polymer at a dosage of
18.5 g/kg (37 Ib/ton). The polymer, however, was an experimental polymer not
commercially available. The Nichols Engineering report also recommended the
use of a precoat when conditioning the sludge with polymer.
The Komline-Sanderson belt filter press produced a sludge cake of about 20
percent solids, operating on a sludge solids feed of 2.5 percent and an
average polymer dosage of 11 g/kg (22 Ib/ton) dry solids, for a polymer cost
of $43.65/Mg ($39.60/ton). The Ashbrook-Simon-Hartley belt filter press was
able to produce a sludge cake of 30 percent solids, but required an average
polymer dosage of 23 g/kg (46 Ib/ton) dry solids, for a polymer cost of $90/Mg
($82/ton). Thus, by increasing polymer dosage, cake solids as high as 30
percent were achieved with a belt filter press. Advantages of the belt filter
press cake were that it had relatively few inert chemical solids and it was
easily broken up, which are desirable cake characteristics for disposal either
by composting or by incineration.
Based upon the results of the dewatering demonstration and an economic
evalution of each treatment alternative, the belt filter press system was
selected as the most cost-effective and most compatible with the disposal
options considered feasible for implementation (composting or incineration).
A dewatering building housing eight 2.5-m Belt Press Dewatering belt filter
presses has been constructed (23), yet the facility was not being operated as
of May 1982. There was tremendous public opposition to the plans for dewater-
ing, composting, and landfill disposal, due to the possibility of contamina-
ting the major water aquifer on Long Island. Because of this, the digested
sludge is currently barged at 2.5 percent solids 19 km (12 mi) off the coast
for ocean disposal. Current plans are for a continuation of this method of
sludge disposal (24).
9.10 San Jose-Santa Clara Water Pollution Control Plant (California)
In 1977, a facilities planning study for the handling and disposal of
wastewater sludge solids at the 6.26 cu m/s (143 mgd) activated sludge plant
was started (25). In a preliminary screening the following devices or methods
to achieve the required unit processes were reviewed for the purpose of
developing system alternatives.
Unit Process Method
1. Stabilization 1. Chlorine Oxidation
2. Lime Treatment
3. Heat Treatment
4. Composting Raw Sludge
5. Aerobic Digestion
6. Anaerobic Digestion
7. Aerobic-Anaerobic Digestion
145
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Unit Process
Method
2. Primary Sludge
Thickening
3. Conditioning
4. Sludge Dewatering
5. Final Disposal
1. Centrifuge
2. Gravity Thickening
3. Dissolved Air Flotation
1. Polymer
2. Elutriation
3. Heat Treatment
4. Ferric Chloride and Lime
1. Rotary Vacuum Filter
2. Centrifuge
3. Filter Press
4. Belt Filter
5. Sandbed Drying
6. Asphalt Drying
7. Drying of Lagoon Sludge
1. Compost and Market Product
2. Landfill On-Site
3. Landfill - Off-Site
Primary sludge thickening was considered because during the canning season
(July through August), large quantities of primary solids are removed.
Primary sludge thickening would reduce the volume of sludge for digestion and
was considered as an alternative to increasing digester capacity.
All stabilization alternatives were excluded except anaerobic digestion and
lime stabilization primarily because of high costs and incompatibility with
existing anaerobic digestion facilities. Elutriation for digested sludge
conditioning was eliminated because of incompatibility with existing secondary
treatment facilities.
All other methods were retained at this stage of analysis. Several pilot plant
and laboratory studies were conducted to obtain information needed for the
development and comparison of project alternatives, including anaerobic
digestion, heat treatment of digested sludge, mechanical dewatering, primary
sludge thickening, lime stabilization, and large scale solar dewatering.
The following types of mechanical dewatering devices were field tested on
bench-scale and pilot-scale units during canning and noncanning seasons on an
anaerobically digested mixture of primary and waste activated sludge:
1. Centrifuge (high and low speed)
2. Belt Filter Press
3. Filter Press (high pressure fixed volume and diaphragm)
4. Vacuum Filter
The San Jose sludge was found to be difficult to dewater on all types of
mechanical dewatering devices. Higher than expected chemical dosages and lower
146
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cake solids were experienced. Design criteria developed from these tests are
shown in Table 9-8 (25).
TABLE 9-8
DESIGN CRITERIA DEVELOPED FROM LABORATORY AND PILOT-SCALE TESTS AT
SAN JOSE - SANTA CLARA WATER POLLUTION CONTROL PLANT
1. Centrifuge (Pilot Scale Tests)
Cake Solids
Recovery
Polymer Demand
15%
90%
7 g/kg (14 Ib/ton) canning season
5 g/kg (10 Ib/ton) noncanning season
2. Belt Press (Pilot Scale Tests)
Cake Solids
Recovery
Polymer Demand
20%
90%
11.5 g/kg (23 Ib/ton) canning season
10 g/kg (20 Ib/ton) noncanning season
3. Vacuum Filter (Filter Leaf Tests)
Cake Solids
Recovery
Chemical Demand
20%
90%
10% FeCl3
30% Lime
4. Filter Press (Bench Scale and Pilot Scale Tests)
Cake Solids
Recovery
Chemical Demand
35%
99%
27% Lime, 12% FeCl^ canning season
20% Lime, 10% FeCl3 noncanning season
Based on review of available solids handling unit processes and on the results
of the pilot studies, fifteen project alternatives were developed. Alterna-
tives were compared on the basis of cost, environmental impact, land use,
energy use, reliability and flexibility. In this comparison stage, belt filter
presses were considered to have equivalent overall costs as centrifuges but
were eliminated from consideration because there was less belt filter opera-
ting experience available. Based on this comparison, five alternatives were
selected for more detailed analysis:
1) Lagoon Drying, On Site Landfill for all sludge
2) Centrifuge Dewatering, Composting for portion of sludge,
Sandbed Dewatering and On Site Landfill for remainder
147
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3) Centrifuge Dewatering to Compost for portion of sludge,
Lagoon Drying and On Site Landfill for remainder
4) Filter Press Dewatering, Composting for portion of sludge,
Sandbed Drying and On Site Landfill for remainder
5) Filter Press Dewatering, Composting for portion of sludge,
Lagoon Drying and On Site Landfill for remainder
These alternatives were more closely compared by determining factors for
comparison, assigning a relative weight to each factor and assigning a value
for each alternative. The factors used in the final evaluation were:
annual cost land use
environmental effects dewatering flexibility
dewatering experience disposal flexibility
weather dependency chemical use
market constraint resource recovery
energy
Alternative 2, consisting of anaerobic digestion for stabilization, a
combination of centrifuging, sandbed drying, and lagoon drying for dewatering,
and compost/market and landfill for disposal, had the highest total score and
was selected as the apparent best alternative system. No preference was made
for either high-speed or low-speed centrifuges.
As of May 1982, no design or construction of the recommended dewatering
facilities had begun. There were plans to construct additional anaerobic
digesters, and this would precede any construction of new dewatering facili-
ties. Current practice is to dispose of digested sludge in on-site sludge
lagoons (26).
9.11 Blue Plains Wastewater Treatment Plant (District of Columbia)
In 1976 and 1977, a study was conducted of pilot-scale dewatering devices
capable of producing high-solids sludge cakes (27). This study was funded by
EPA Region III and EPA's Municipal Environmental Research Laboratory in
Cincinnati.
The pilot-scale dewatering processes investigated were:
Vacuum filter
Vacuum filter retrofit (add-on) units - three manufacturers
Belt Press - two manufacturers
Fixed volume filter press - two manufacturers, one high pressure and one
low pressure
Diaphragm filter press - three manufacturers
148
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The ultimate plan at the time of this study was to dewater and incinerate the
sludge. While the plant had vacuum filters, incinerators had not been
obtained. Recent fuel cost increases appeared to have changed the cost-
effectiveness of incineration of vacuum filtered sludge cake, and the study
was conducted to evaluate several dewatering processes capable of producing
cakes with significantly higher solids contents than vacuum filters.
Feed solids to the units averaged 5% total solids with a range of 2.4 to 10%.
Several different ratios of raw primary sludge to raw WAS were tested, with
emphasis on a 33:67 ratio. Conditioning chemicals investigated were lime,
ferric chloride and polymer.
Conclusions of the study (27) were:
Chemical Conditioning
• The lime and ferric chloride dosages required to produce a filterable
sludge varied with the percentage of WAS. Fibrous primary sludge
filtered quite readily; WAS required greater quantities of condition-
ers and was more difficult to dewater. Generally, a 3/1 ratio of
lime-to-ferric chloride was optimum for conditioning the raw Blue
Plains sludge. Bench-scale filterability tests were found to be useful
when optimizing and controlling the lime and ferric chloride dosages.
• Polymer conditioning of the raw 33:67 mixture of primary-to-WAS sludge
was generally ineffectual. No single polymer was found which could
adjust to the daily variations in the quality of sludge received from
the primary and secondary treatment processes.
Filter Press-General
• Each of the filter presses was capable of dewatering all sludge ratios
and total feed solids in the range of 2.4 to 10% to at least a 30%
solids cake. The diaphragm press, however, was the only unit capable
of dewatering the marginally conditioned sludges to the 35% solids
required for an autocombustible cake.
• Once a minimum chemical conditioning requirement of lime and ferric
chloride for adequate dewatering was established, increases in filtra-
tion yields (up to 20%) were obtained by slight increases in chemical
dosages.
• In all the presses, suspended solids recovery in the filter cake was
greater than 99%. The quantity of suspended solids in the filtrate was
affected primarily by the type of filter cloth used and the degree of
chemical conditioning.
• The filter presses did not satisfactorily dewater polymer conditioned
sludges.
• The average specific resistance-to-filtration parameter was correlated
directly with filter press yield.
149
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Filter Press - Diaphragm Unit
• Average results for conditioning with 19.6% lime and 6.5% FeCl3, and
dewatering a 33:67 mixture of raw primary and WAS were a 38.7% solids
cake with a yield between 2.39 and 2.93 kg/sq m/hr (0.49 to 0.6 Ib/sq
ft/hr). The pumping pressure required to feed the press was always
less than 690 kPa (100 psig). The pumping cycle time averaged 17
minutes and was controlled by monitoring the total solids feed rate. A
squeezing pressure of 1,470 kPa (213 psig) was generally used. The
squeezing cycle time (18 minutes) was controlled by filtering to a
specified filtrate flow rate.
• Different filter cloths were tested on both presses. All gave
acceptable filtrate quality, but cloth life, resistance to abrasion,
etc., were not evaluated.
Filter Press - Fixed Volume Unit
• The high-pressure press (225 psig) had an averge filtration yield of
1.51 kg/sq m/hr (.31 Ib/sq ft/hr) and required 62.3% more filtration
area than the diaphragm presses to produce equivalent results. The
low-pressure press (100 psig) had an average full-scale yield of 1.07
kg/sq m/hr (.22 Ib/sq ft/hr) and needed 126.8% more filter area than
the diaphragm presses to produce equivalent results.
• Cycle time on the presses averaged 2-3 hours and was determined by
filtering to a specified filtrate flow rate.
• The cakes from the fixed volume presses always contained a dry outer
section and a wetter inner core. This resulted in a substantial
variation in the solids content across the cake.
Belt Press
• Because of the highly variable sludge at Blue Plains, no polymer was
found that could adjust to these variations and adequately condition
the sludge at all times. The operation of the belt press, therefore,
was not consistent.
• With thickened sludge feeds, the press capacity, final cake solids,
and polymer consumption were all affected by the percentage of waste-
activated sludge. The unit performed best when dewatering high
percentages of fibrous, primary sludge.
• Suspended solids recovery in the filter cake averaged only 95%. This
was felt to be insufficient due to plant discharge requirements.
Vacuum Filter Retrofit Unit
• The only vacuum filter retrofit device which showed promise was the
high-pressure section of the continuous belt press when used to
further dewater the vacuum filter cake. Cake solids of 35% were
150
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• achieved in bench-scale work; however, demonstration of the system in
a full-scale test was not successful because of problems with feeding
the vacuum filtered cake to the press.
Economic Comparison
• The belt press at $35.63/Mg ($32.39/ton) and the vacuum filter at
$43.01/Mg ($39.10/ton) provided the lowest cost for dewatering.
• Dewatering costs for each of the filter presses were nearly equal with
unit costs of approximately $60.50/Mg ($55.00/ton).
As of July 1982, the Blue Plains plant is vacuum filtering two sludge types:
anaerobically digested sludge and raw sludge, which have been conditioned with
lime and ferric chloride. Dewatered sludge is composted. Future plans are
indefinite, but should incineration become a viable alternative, it is likely
the District would elect to dewater by pressure filtration. This choice is
because of the desire to produce an autogenous sludge cake (28).
9.12 References
1. "Mechanical Dewatering Study - Los Angeles County Sanitation Districts,"
LA/OMA Project, Regional Wastewater Solids Management Program, Los
Angeles-Orange County Metropolitan Area, September 1980.
2. Harrison, J. R., "Review of Developments in Dewatering Wastewater
Sludges," Sludge Treatment and Disposal, Volume 1 - Sludge Treatment,
USEPA - Center for Environmental Research Information, Cincinnati, Ohio,
45268, EPA-625/4-78-012, October 1978.
3. Trubiano, R., Bachtel, D., LeBrun, T., and Horvath, R. , "Parallel
Evaluation of Low Speed Scroll Centrifuges and Belt Filter Presses for
Dewatering Municipal Sewage Sludge," Draft EPA Report, Contract
68-03-2745, 1981. (Authors are with County Sanitation Districts of Los
Angeles County, Whittier, California)
4. Personal communication, Thomas J. LeBrun, Supervisor of Research Section,
Joint Water Pollution Control Plant, County Sanitation Districts of Los
Angeles County, Carson, California, June 1982.
5. Personal communication, Richard T. Moll, Manager of Process Engineering,
Sharpies-Stokes Division, Pennwalt Corporation, Warminster, Pennsylvania,
June 9, 1982.
6. John Carollo Engineers, "Design Memorandum No. 5 - Dewatering Methods,"
County Sanitation Districts of Orange County, Fountain Valley,
California, April 1979.
151
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7. "Mechanical Dewatering Study - Orange County Sanitation Districts,"
LA/ DMA Project, Regional Wastewater Solids Management Program, Los
Angeles-Orange County Metropolitan Area, September 1980.
8. Personal communication, Blake P. Anderson, Chief of Operations, County
Sanitation Districts of Orange County, Fountain Valley, California, June
1982.
9. CI^M-Hill, "Michelson Water Reclamation Plant - Engineering Report for
Dewatering Equipment Selection," Irvine Ranch Water District, Irvine,
California, June 1979.
10. Tavery, M. A., "Evaluation of Sludge Dewatering Equipment at the Metro
Denver Sewage District," paper presented at the Colorado AWWA-WPCA Tech-
nical Activities Committee, May 3, 1979. (Author is with the Metropolitan
Denver Sewage Disposal District No. 1, Denver, Colorado).
11. Personal communication, Mary Ann Tavery, Metropolitan Denver Sewage
Disposal District No. 1, Denver, Colorado, March 1981.
12. Personal communication, Colin McKenna, Facilities Engineer, Metropolitan
Denver Sewage Disposal District No. 1, Denver, Colorado, June 1982.
13. Inger soil-Rand, "Laboratory Test Report - Denver Metro - Anaerobically
Digested Sludge," Ingersoll-Rand Company, Nashua, New Hampshire, April
20-21, 1981.
14. Personal communication, Kenneth A. Pietila, "Detailed Review of Draft
Process Design Manual for Dewatering Municipal Wastewater Sludge,"
February 26, 1982. (Author is with Rexnord in Milwaukee, Wisconsin)
15. Zenz, D. R., et al., "Evaluation of Unit Processes for Dewatering of
Anaerobically Digested Sludge at Metro Chicago's Calumet Sewage Treatment
Plant," The Metropolitan Sanitary District of Greater Chicago, October
1976.
16. Sawyer, Bernard; Watkins , Robert; and Lue-Hing, Cecil, "Evaluation of
Unit Processes for Mechanical Dewatering of Anaerobically Digested Sludge
at Metro Chicago's West-Southwest Sewage Treatment Plant," Paper
presented at the 31st Annual Purdue Industrial Waste Conference, May
1976. (Authors are with the Research and Development Department of The
Metropolitan Sanitary District of Greater Chicago)
17. Personal communication, David R. Zenz, Coordinator of Research,
Metropolitan Sanitary District of Greater Chicago, May- June , 1982.
18. Kupper Associates and Metcalf & Eddy, Inc., "Pilot Plant Dewatering
Testing For The Recommended Land-Based Sludge Management Plan," Middlesex
County Sewerage Authority, New Jersey, January 1979.
19. Personal communication, Allan Jacobs, Vice President, Metcalf & Eddy,
Somerville, New Jersey, July 1982.
152
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20. Moser, J.H., et.al., "Milwaukee Water Pollution Abatement Program Solids
Handling Study," Milwaukee Metropolitan Sewerage District, May 1981.
(Author is with Milwaukee Metropolitan Sewerage District)
21. Personal communication, John H. Moser, Milwaukee Metropolitan Sewerage
District, Milwaukee, Wisconsin, May 1982.
22. Greenhorne & O'Mara Engineers," Nassau County Sludge Study Composting and
Dewatering Demonstration Program—Final Report," July 1979. (Greenhorne &
O'Mara Engineers are in Riverdale, Maryland)
23. Personal communication, Ray Advendt, Greenhorne & O'Mara Engineers,
Riverdale, Maryland, May 1982.
24. Personal communication, John J. Pascucci, Department of Public Works,
Nassau County, New York, May 1982.
25. Consoer, Townsend & Associates Ltd., "Draft Project Report - Sludge
Processing Facilities Plan For the Cities of San Jose and Santa Clara,
California," May 1980.
26. Personal communication, Douglas C. Humphrey, Sanitary Engineer, San
Jose/Santa Clara Water Pollution Control Plant, San Jose, California, May
1982.
27. Cassel, Alan F. and Johnson, Berinda, P., "Evaluation of Dewatering
Devices for Producing High-Solids Sludge Cake," USEPA - Municipal
Environmental Research Laboratory, Cincinnati, Ohio, 45268,
EPA-600/2-79-123, August 1979.
28. Personal communication, Russ Thomas, Superintendent of District of
Columbia's Wastewater Treatment Plant at Blue Plains, Washington, D. C.,
July 30, 1982.
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APPENDIX A
MANUFACTURERS OF DEWATERING EQUIPMENT
During the last several years, a number of well known manufacturers have
withdrawn from the production of dewatering equipment, while others have
entered the field with new products. Table A-l presents a listing of suppliers
of different dewatering equipment which is intended to be up-to-date and
complete, although it is possible that some manufacturers are excluded. Due to
the dynamic nature of the equipment manufacturing business, it is probable in
the future that some on the list may discontinue making the equipment.
References such as the Journal Water Pollution Control Federation, Pollution
Equipment News, and Water & Wastes Digest should be consulted for additional
suppliers.
This listing is presented as an aid to individuals involved in the selection
of equipment, and does not represent an endorsement of any particular manufac-
turer or piece of equipment by either the EPA or Culp/Wesner/Culp. Suppliers
are listed alphabetically, and the order of presentation does not constitute
an order of preference.
154
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TABLE A-l
MANUFACTURERS OF DEWATERING EQUIPMENT
CENTRIFUGES
Basket (imperforate Bowl)
• Ametek
• Robitel
• Sharpies
• Western States
Solid Bowl (Decanter or Scroll)
Alfa Laval
Bird
Dorr-Oliver
Ingersoll Rand (Kruger)
KHD Humboldt Wedag
Marubeni America Corporation (IHI)
Sharpies (Polymizer)
Westfalia
BELT FILTER PRESSES
Low Pressure
• Permutit (DCG/MRP)
• Smith & Loveless (Sludge
Concentrator)
VACUUM FILTERS
High Pressure
Arus-Andritz (SDM-SM Press)
Ashbrook-Simon-Hartley (Winklepress
& Klampress)
Belt Dewatering
Clow (Hydropress)*
Envirex
Envirotech (EVT Belt Press)
Euramia
Infilco - Degremont (Flocpress)
Komline - Sanderson (Kompress)
Koppers (Enelco Von Roll Rollpress)
Parkson (Magnum Press)
Passavant (Vac-U-Press)**
Performance Systems, Inc.
Ralph B. Carter
Ametek (Industrial only)
Dorr-Oliver (industrial only)
Envirex
Envirotech
Ingersoll Rand
Komiine-Sanderson
*0nly a 0.5 meter wide press is available
**Combination press and vacuum type process, available with or without vacuum
155
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TABLE A-l
(Continued)
FILTER PRESS
Fixed Volume Type
Clow
Edwards and Jones
Envirotech (Shriver Press)
Hoesch
Koppers - Environmental
Development Corporation
Netzsch
Passavant
Performance System, Inc.
D. R. Sperry and Company
William R. Perrin Incorporated
Diaphragm Type
• Edwards and Jones
• Envirex (NGK)
• Ingersoll Rand (Lasta)
•' Johnson Progress
• Performance Systems, Inc
DRYING BED SYSTEMS
• U.S. Environmental Products (Rapid Sludge Dewatering System)
• Hendrick Fluid Systems (Wedgewater Filter Bed)
• International Sludge Reduction Company (Vacuum Drying Beds)
• Infilco-Degretnont (Vacuum Drying Beds)
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APPENDIX B
EXAMPLE CALCULATIONS SHOWING SLUDGE VOLUMES PRODUCED BY
DIFFERENT DEWATERING TECHNIQUES
This appendix presents example calculations that (1) show how to use
information in the manual; (2) illustrate how to determine sludge cake
volumes; and (3) compare the sludge cake volumes produced by different
dewatering processes. See Chapter 4, Section 4.4 of the manual for a detailed
discussion of the comparisons presented here.
Sludge type:
Dewatering
Technique
Digested (Primary + WAS), 50:50 Blend
Assume 2,000 Ib dry solids
Basket Centrifuge
Solid Bowl Centrifuge
Belt Filter Press
Vacuum Filter
Fixed Volume
Filter Press
Diaphragm Filter
Press
Drying Beds
Sludge Lagoons
Gravity/Low Pressure
Devices
Cake Solids
10-15, Use 13%
15-21, Use 18%
18-23, Use 20%
15-20, Use 18%
35-42, Use
38-47, Use 43%
15-70, Use 50%
5-40, Use 25%
8-12, Use 10%
Chemicals Required
6 Ib/ton polymer
8 Ib/ton polymer
12 Ib/ton polymer
15% Lime, 4% FeCl3
20% Lime, 10% FeCl3
20% Lime, 10% FeCl3
None
None
15 Ib/ton polymer
NOTE: The specific gravities used in the following calculations are based
upon the addition of lime and ferric chloride with the resultant
production of calcium carbonate and ferric hydroxide. The following
specific gravities were used to develop the specific gravity of the
sludge cake mixture: volatile solids, 1.0; fixed solids, 2.5; ferric
hydroxide, 3.4; and calcium carbonate, 2.8. The calculations are based
upon the assumption that reaction products are equivalent in weight to
the lime and ferric chloride added.
157
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BasketCentrifuge
Includes chemicals-
„, J „ , , , 2,006 Ib dry solids
Sludge Cake Volume = 2
(0.13) (8.34 Ib/gal) (1.05) (7.48 gal/cu ft)
\
d — *
% solids — * ^-Specific gravity of digested
sludge cake
= 236 cu ft
Solid Bowl Centrifuge
2,008 Ib
Cake Volume = r—; :—: :—; - = 167 cu ft
(0.18) (8.34) (1.07) (7.48)
Belt Filter Press
2,012 Ib
Cake Volume = r ; :—; = 149 cu ft
(0.20) (8.34) (1.08) (7.48)
Vacuum Filter
2,380 Ib
(0.18) (8.34) (1.07) (7.48)
Fixed Volume Filter Press
2,600 Ib
Cake Volume = — — —— r = 94 cu ft
(0.38) (8.34) (1.17) (7.48)
Diaphragm Filter Press
2,600 Ib
(0.43) (8.34) (1.19) (7.48)
Drying Beds
2,000 Ib
(0.50) (8.34) (1.23) (7.48)
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Sludge Lagoons
2,000 Ib
(0.25) (8.34) (1.10) (7.48)
Gravity/Low Pressure Devices
2,015 Ib
(0.10) (8.34) (1.04) (7.48)
ft
A comparison of the sludge cake volumes produced by the various dewatering
processes is tabulated below. The largest cake volume, produced by the
gravity/low pressure devices, is used as a basis for comparing the cake
volumes. For example, drying beds produce a cake volume which is only 17
percent of the volume produced by the gravity/low pressure devices.
Cake Volume Comparison
Basket Centrifuge
Solid Bowl Centrifuge
Belt Filter Press
Vacuum Filter
Fixed Volume
Filter Press
Diaphragm Filter Press
Drying Beds
Sludge Lagoons
Gravity/Low Pressure
Devices
Percentage of
Volume Gravity/Low Pressure Devices
cu ft
235 76%
167 54
149 48
198 64
94 30
81 26
52 17
117 38
311 100
159
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APPENDIX C
COST OF DEWATERING EQUIPMENT
C.I Introduction
This section presents costs for the construction and operation of nine
different dewatering processes. These processes in the order that they are
presented are:
Basket Centrifuge
Solid Bowl Centrifuge - Low G
Solid Bowl Centrifuge - High G
Belt Filter Press
Vacuum Filter
Filter Press - Fixed Volume
Filter Press - Diaphragm
Sand Drying Beds
Sludge Dewatering Lagoons
For each of these processes, curves are presented for construction cost,
process and building energy, diesel fuel, maintenance material costs, labor,
and total O&M cost.
C.I.I Construction Cost
The construction cost curves were developed from data supplied by equipment
manufacturers, from actual bid prices, as well as from unit cost take-offs
from both actual and conceptual designs. In developing the aggregate construc-
tion cost, separate cost estimates were made for eight principal components:
(1) excavation and site work; (2) manufactured equipment; (3) concrete; (4)
steel; (5) labor; (6) pipe and valves; (7) electrical equipment and process
instrumentation; and (8) housing. This approach was used to enhance the
accuracy of the cost data. Following development of the construction costs,
15% was added for contingencies which might be expected to be encountered
during construction. The construction cost for each unit process is presented
as a function of the most applicable design parameter for the process. For
example, solid bowl centrifuge and belt press costs are presented in terms of
gpm of machine capacity, vacuum filter costs are presented versus square feet
of filter surface area, and plate and frame press costs are presented versus
cubic feet of machine capacity. This approach of selecting a most applicable
design parameter was utilized in both developing and presenting costs, as it
allows the costs to be utilized with the greatest degree of flexibility.
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The construction cost curves were developed using specific conceptual designs
for equipment sizing and layout. In these conceptual designs, single units of
equipment were used up to the maximum feasible size, and in larger installa-
tions multiple pieces of equipment were used. When preliminary cost analyses
are being conducted for smaller installations, however, often multiple units
are desired for operational flexibility or standby purposes. In these cases,
it is recommended that the cost curve be entered with the desired size, then
multiply the cost by the number of units, and finally reduce this cost by a
factor of 25-35% for economy of scale.
Construction cost curves are based upon costs experienced in April 1982. It
should be recognized that the curve for construction cost is not capital cost.
The curve does not include costs for special site work, general contractor
overhead and profit, engineering, land, legal, fiscal, and administrative work
and interest during construction. These cost items are all more directly
related to the total cost of a project rather than the cost of any one of the
individual unit processes. These costs are therefore most appropriately added
following cost summation of the individual unit processes, if more than one
unit process is required. Typically, these costs add 35 to 45%, depending on
project size and complexity, to the actual construction costs which are shown
in the curves.
C.I.2 Operation and Maintenance Cost
Operation and maintenance requirements were developed from information
collected at existing wastewater treatment facilities. For newer types of
equipment for which actual full-scale operating data are limited or not avail-
able, such as the diaphragm filter press, O&M requirements which are presented
are based upon the manufacturers' estimates and the experience of the
authors.
Electrical energy requirements are presented for both building-related energy
and process energy. Building energy includes heating, cooling, lighting and
ventilation, and was based upon the required building size and an annual
requirement of 904 kwh/sq m/yr (84 kwh/sq ft/yr). This number represents an
average for 21 cities across the U.S., but it is highly variable and depends
on heating and cooling requirements. It is suggested that this nunfcer be
adjusted either upward or downward depending upon locally experienced require-
ments. Process energy requirements are for motors required to drive and other-
wise operate the dewatering mechanism and appurtenant equipment. Process
energy requirements will be constant from location to location. Electrical
energy costs are expressed in terms of kwh/yr, and, in calculating annual O&M
costs, the electrical cost component can be calculated using the local
electrical cost in $/kwh. Certain processes such as sand drying beds and
sludge dewatering lagoons require use of equipment which utilizes diesel fuel.
Curves which are presented for diesel fuel requirements are presented in terms
of gallons of fuel required per year.
Maintenance material cost includes the cost of periodic replacement of
component parts necessary to keep the process operable and functioning.
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Examples of maintenance material items which are required are valves, motors,
instrumentation, and other process items of similar nature. Maintenance
material cost shown in the curves are based upon April 1982 costs. The main-
tenance material requirements do not include the cost of chemicals required
for process operation since chemical requirements will vary widely from sludge
to sludge.
The labor requirement curve includes both operation and maintenance labor
and is presented in terms of hours per year. Labor requirements were based
upon 24 hour per day operation, including any required clean-up time.
A curve is also presented for total annual O&M costs. This curve was developed
using an electrical energy cost of $0.05/kwh, a diesel fuel cost of $0.30/1
($1.15/gal) and a labor cost of $12/hour. If significantly different labor,
electrical or diesel fuel costs are experienced, the total annual O&M cost
should be adjusted as appropriate.
C.2 Basket Centrifuge
C.2.1 Construction Cost
Basket style centrifuges, because of design and operating features, are
ideally suited to dewatering of light and hard-to-handle sludges such as waste
activated sludge. Construction costs are for single units at smaller capaci-
ties and multiple units at larger capacities. Centrifuge costs are for
automatic machines operating on a preprogrammed cycle, an approach which
requires only minimal operator attention.
In addition to the basic machines, the costs include equipment for polymer
preparation, storage, and application. If other conditioning chemicals are
used, the costs would have to be adjusted accordingly. The costs do not
include sludge and centrate pumping, sludge conveying, and sludge storage. It
was assumed that centrifuges are located in two story concrete block buildings
with bottom discharge to trucks or storage bins. Housing requirements were
developed from equipment manufacturers' recommended layouts.
Figure C-l presents construction costs for basket centrifuge installations
with total installed machine capacities between 0.15 and 30.7 1/s (3500 and
700,000 gpd).
C.2.2 Operation and Maintenance Cost
Electrical energy requirements were computed from connected and operating
horsepower information provided by equipment manufacturers. Basket centrifuge
operating horsepower, computed on the basis of a complete cycle involving
machine acceleration, sludge feeding, skimming, decelerating, and sludge
plowing, averages 40 to 60 percent of the connected horsepower. Electrical
162
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power for polymer preparation and feeding is included, but energy for sludge
pumps, centrate pumping and sludge conveying equipment is not included.
Maintenance costs were obtained from equipment manufacturers and from
operating installations and represent an industrywide average of annual
expenditures for maintenance, replacement parts, lubrication, and other
consumable items associated with basket centrifuge operation. Maintenance
material costs do not include the cost of polymers.
Labor requirements for O&M assume 24 hours per day of operation, with
occasional downtime for maintenance as required. The major portion of the
operating labor is devoted to machine start-up and adjustment, polymer prepar-
ation, and required maintenance.
Electrical requirements and maintenance material costs are shown in Figure
C-2, while labor and annual O&M costs are shown in Figure C-3. Annual O&M
costs are based upon $0.05/kwh for electricity and $12/hr for labor. Polymer
costs are not included in the annual O&M costs. It should be recognized that
operation and maintenance costs will vary widely depending on sludge dewater-
ing characteristics and specific operating conditions related to the installa-
tion, and appropriate adjustment should be made if conditions vary signifi-
cantly from those stated above.
C.3 Solid Bowl Centrifuge - Low G
C.3.1 Construction Cost
Costs for low-G solid bowl centrifuges, also commonly called low speed
decanter or low speed scroll centrifuges, are shown in Figure C-4. According
to the definition used in the cost development, low G refers to centrifuges
operating at G forces generally less than 1,100. The costs are based on
centrifuges with capacities between 0.63 and 126.4 1/s (10 and 2000 gpm). At
capacities greater than 31.6 1/s (500 gpm) multiple units are utilized.
Centrifuges were assumed to be equipped with automatically controlled back-
drive units. In addition to the cost of the centrifuge, costs are included for
polymer storage, preparation, and feed equipment. Although housing is not
necessary in moderate climates, housing costs are included. Costs do not
include sludge or centrate pumping, or conveyance of the sludge cake from the
dewatering building.
C.3.2 Operation and Maintenance
Process energy usage was computed from manufacturers' information on connected
and operating horsepower for main drive and back drive units and for polymer
preparation and feed equipment. If back drive is not utilized, power costs
would decrease by 5 to 20%, depending on the centrifuge manufacturer and the
163
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method of controlling the backdrive. The process energy does not include
energy related to feed sludge pumping and handling of dewatered sludge.
Maintenance material costs were developed from data furnished by equipment
manufacturers. These maintenance material costs are lower than experienced at
most operating installations, since the new ceramic tile conveyor tips were
assumed to be utilized in this installation.
Labor requirements for operation and maintenance were computed based on 24
hr/day of continuous operation. The major portion of the operating labor is
devoted to polymer preparation, machine start-up and adjustment, and occasion-
al maintenance involving machine and motor lubrication. Periodically, exten-
sive maintenance will be required for replacement of the ceramic tile conveyor
tips and bearing replacement, although the ceramic tiles should not require
replacement more than every 15-20,000 hours of operation.
It is important to realize that the cost curves do not include the cost for
purchase of polymer. Polymer usage is highly variable between machines
produced by different manufacturers and between different sludge types.
Polymer costs must be added separately. Figure C-5 presents process and
building electrical requirements as well as maintenance material costs. Figure
C-6 presents labor requirements and total O&M costs. Total O&M costs were
calculated using $0.05/kwh for electrical energy and $12/hr for labor.
C.4 Solid Bowl Centrifuge - High G
C.4.1 Construction Cost
High G solid bowl centrifuges operate at G forces greater than 1,100. These
high G forces are developed by high speed operation up to 3300 rpm. Machine
throughput is significantly affected by the polymer dosage, and therefore the
construction cost for a given feed rate varies with the polymer dose, as shown
in Figure C-7. In this figure, single machines were assumed to be used for
feed rates up to 31.5 1/s (500 gpm), with multiple units being used for higher
feed rates. All machines are equipped with automatically controlled eddy
current backdrive and have sintered tungsten carbide conveyor tips. Polymer
storage preparation, and feed equipment is included in the costs, but costs
for sludge feed pumping and centrate pumping are not included.
C.4.2 Operation and Maintenance Cost
Process energy was calculated from information supplied by a manufacturer of
high G centrifuges and assumes use of an eddy current backdrive. Energy
requirements could be reduced between 5 to 20% if the backdrive is not
utilized. Included in the process energy requirements are the main drive
motor, the eddy current backdrive, and equipment required for polymer prepara-
tion and feed. Energy required for feed sludge pumping and handling of the
dewatered sludge is not included.
164
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Maintenance material costs are relatively low due to the use of the long
lasting sintered tungsten carbide conveyor tips. Maintenance material require-
ments include replacement of the conveyor tips every 30,000 hours of opera-
tion, as well as replacement of other necessary components of the centrifuge
and the electrical controls.
Operation and maintenance labor requirements are based upon 24 hours per day
of continuous operation. Most operational labor is devoted to polymer prepara-
tion and machine start-up and adjustment. Occasional maintenance is required
for lubrication, with more extensive maintenance required approximately every
30,000 hours for replacement of the sintered tungsten carbide conveyor tips.
The cost curves presented do not include the cost of polymer. The polymer
dosage is highly dependent on the characteristics of the sludge being dewater-
ed, and polymer dosage will also have a great influence on the throughput of
the centrifuge, as shown in Figure C-7. Figure C-8 presents process and build-
ing electrical requirements as well as maintenance material costs. Figure C-9
presents labor requirements and total O&M costs. Total O&M costs were
calculated using $0.05/kwh for electrical energy and $12/hr for labor.
C.5 Belt Filter Press
C.5.1 Construction Cost
The new third generation belt filter presses are becoming increasingly popular
for dewatering a wide range of different types of sludges. As contrasted to
earlier generations of belt filter presses, which used short contact time and
low pressures, the newer presses rely on longer pressing times and multiple
passes over a series of rollers. Such passing over rollers creates shear
between the sludge particles, exposing new surfaces and enhancing water
removal.
Construction costs are for belt filter press dewatering systems that include
the belt press unit, wash water pump, conditioning tank, feed pump, polymer
storage tank and pump, belt conveyor, and electrical control panel. Machines
are generally sized using metric dimensions and are rated on the basis of
sludge flow in gpm/m of belt width. For mixtures of digested primary and
secondary sludges, a value of 3.2 1/s/m (50 gpm/m) of belt width is a typical
loading recommendation, and was used in the conceptual layouts used in the
cost development. Higher loadings are possible in some cases if the sludge can
be easily dewatered.
Estimated construction costs are presented in Figure C-10 as a function of
total installed machine capacity.
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C.5.2 Operation and Maintenance Cost
Process energy requirements were developed from the total connected horsepower
for the belt drive unit, belt wash water pump, conditioning tank, feed pump,
polymer pump and tanks, belt conveyor, and electrical control panel. A belt
filter loading of 3.2 1/s/m (50 gpm/m) of machine width was used in selecting
unit sizes and determining power requirements. Twenty-two hours of continuous
operation with 2 hr of downtime for routine maintenance was assumed in
calculating process energy requirements.
Labor and maintenance requirements were estimated from information provided by
equipment manufacturers, as well as information from plants operating belt
filter presses. The maintenance material requirements assume the replacement
of a set of belts every 6 months in continuous service.
Figures C-ll and C-12 present operation and maintenance requirements for the
belt filter press. As operation and maintenance costs vary widely depending on
the nature and solids concentration of the sludge being processed, and adjust-
ments to these O&M requirements may have to be made on a case-by-case basis.
Conditioning chemical costs are not included in the total annual O&M cost
curve.
C.6 Vacuum Filters
C.6.1 Construction Cost
Costs for vacuum filter installations are presented in Figure C-13. The costs
include the vacuum filter, conditioning tank, vacuum and filtrate pump
assemblies, vacuum receiver, a short belt conveyor for the dewatered sludge,
feed sludge piping, lime and ferric chloride storage and feed facilities,
electrical controls, and necessary housing for the entire assembly.
C.6.2 Operation and Maintenance Cost
Electrical energy curves are presented for both process and building energy.
Process energy is for vacuum filter drum drive, cake discharge roller, vacuum
and filtrate pumps, tank agitators, and the dewatered sludge belt conveyor.
Process energy requirements were calculated for a sludge solids loading of 8.3
kg dry solids/sq m/hr (1.7 Ib/sq ft/hr). Building sizes are based upon
conceptual layouts for various total filter areas, and energy requirements are
based upon 904 kwh/sq m of building/year (84 kwh/sq ft/yr).
Labor and maintenance material requirements are based upon operating
experience at operating dewatering facilities. Labor requirements are based
upon 24 hour per day operation, and will have to be adjusted if filters are
operated for only one or two shifts per day. Maintenance material costs are
166
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for periodic repair and replacement of equipment. Costs are not included for
purchase of the lime or ferric chloride utilized for conditioning, since
chemical requirements are highly variable from sludge to sludge, and are not
generally a function of vacuum filter surface area.
Electrical energy and maintenance material costs are shown in Figure C-14, and
labor and total O&M costs are shown in Figure C-15. Total O&M costs were
calculated using a rate of $0.05/kwh for electrical energy and a rate of
$12/hr for labor. Conditioning chemical costs are not included in the total
O&M cost.
C.7 Filter Press - Recessed Plate
C.7.1 Construction Cost
The recessed plate filter press has gained popularity for dewatering sludges
because it can produce a high solids content cake suitable for incineration
or any other subsequent process requiring a high solids content sludge. The
introduction of semi-automatic and fully automatic presses along with other
labor and maintenance saving improvements has further stimulated interest in
filter presses.
Construction costs, as shown in Figure C-16, were developed for a series of
single and multiple recessed plate filter press systems ranging in size from
0.12 to 25.4 cu m (4.3 to 896 cu ft). The largest single press utilized in
the cost estimates had a capacity of 6.3 cu m (224 cu ft). The construction
costs include the filter press, feed pumps (including one standby), a lime
storage bin and feeders, ferric chloride liquid solution storage and feeders,
a sludge conditioning and mixing tank, an acid wash system, and housing.
Housing costs are for a two story, concrete block building, with the filter
press located on the upper floor and discharging through a floor opening to a
truck located on the lower level.
C.7.2 Operation and Maintenance Cost
Operation and maintenance costs were developed for a filter loading of 80 to
90 kg dry solids/cu m/hr (5 to 5.6 Ib/cu ft/hr), a dry solids density of 1030
kg/cu m (64 Ib/cu ft), and 19 hr of operation/day. The remaining 5 hr/day
would be devoted to press preparation, sludge removal, cleanup, and press
maintenance.
Most of the process energy consumed by the filter press is related to
operation of the sludge feed pump. Energy is also consumed by the open-close
mechanism and the tray mover. Pumping power requirements were calculated for a
solids loading of 4 percent at a cycle time of 2.25 hr, with a 20 minute turn-
around time between cycles. Power required for chemical preparation, mixing,
167
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and feeding is also included in process energy. Energy requirements related to
building heating, cooling, lighting, and ventilation were based upon a usage
of 904 kwh/sq m/yr (84 kwh/sq ft/yr).
Maintenance material costs and labor requirements were estimated based on
manufacturers' experience and data from a number of operating installations.
Process and building electrical requirements and maintenance material
requirements are shown in Figure C-17, and labor and annual O&M costs are
shown in Figure C-18. Annual O&M costs do not include the cost for lime and
ferric chloride conditioning chemicals.
C 8 Filter Press - Diaphragm
C.8.1 Construction Cost
The diaphragm filter press has several operational advantages over a
conventional recessed plate type filter press. One of the more important
advantages is the production of a higher solids content cake, often up to 8%
solids higher. Other advantages include more positive cake release, a shorter
overall cycle time, lower pumping pressure for sludge fed to the press, and
the ability to successfully dewater poorly conditioned sludges. Diaphragm type
presses are generally fully automatic, including automatic cloth washing. The
product cake solids content is varied by changing the time of compression,
with compression being created by inflating the diaphragm.
Construction costs shown in Figure C-19 are for diaphragm presses with press
areas between 111 and 1398 sq m (1200 and 15,050 sq ft). The largest machine
manufactured is 557 sq m (6000 sq ft), and the larger areas shown in Figure
C-19 are for multiple presses. The construction costs shown include the
diaphragm press, feed pump, pumps for the diaphragm and cloth washing, vacuum
pumps, an air compressor and receiver, lime and ferric chloride storage and
feed facilities, and all electrical and controls necessary for complete auto-
matic operation. Housing is for a two story, concrete block building, with the
filter press discharging through an opening, in the floor to a truck on the
lower level.
C.8.2 Operation and Maintenance Cost
Operation and maintenance costs were developed for a 4% feed of anaerobically
digested sludge, chemically conditioned with 5% ferric chloride and a 20%
lime. Press loading was 4.9 kg/sq m/hr (1.0 Ib/sq ft/hr), without chemicals,
and cake discharge was taken at 35%. Press operation time was 19 hours per
day, with the remaining time dedicated to press cleanup and maintenance.
Process energy requirements are for the sludge feed pump, the air pump for
inflating the diaphragm, and a vacuum pump for removal of liquid sludge
168
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remaining in the internal piping prior to opening the press. Energy is also
required to open and close the press, for cloth washing, and for conditioning
chemical preparation and feed. Building energy requirements are based upon 904
kwh/sq m/yr (84 kwh/sq ft/yr).
Maintenance material costs consist principally, over 90%, of replacement of
diaphragms and filter cloths. Other costs are for miscellaneous equipment
parts and for miscellaneous electrical components.
Labor required is for both operation and maintenance, with the majority of the
labor devoted to operational requirements. Labor requirements are based upon
operational experience of the manufacturer.
Electrical requirements for process energy and building energy, as well as
maintenance material requirements are presented in Figure C-20. Labor and
annual O&M costs are shown in Figure C-21. Conditioning chemical costs are not
included in the annual O&M cost, since they vary widely between different
sludges. Chemical costs must be added separately to arrive at a total annual
O&M cost.
C.9 Sand Drying Beds
C.9.1 Construction Cost
Sand drying beds are an economical method of producing a dry sludge cake from
digested sludge. Sludge thickening prior to application on the drying beds is
not required, although thickening will decrease the area of beds required, and
will also decrease the time required for sludge drying. Dewatering on the
sand beds is by a combination of draining and air drying, and beds perform
best when both of these processes are optimized. Removal of dried sludge is
normally accomplished by front-end loader. Although sand drying beds offer a
low-cost approach to sludge drying, this advantage may be offset by the amount
and cost of the land area required and poor performance during cold and/or wet
periods.
Cost estimates are for uncovered and unlined sand drying beds. The estimates
include the sludge distribution piping, 23 cm (9 in) of sand media overlying
23 cm (9 in) of gravel media, 0.6 m (2 ft) high concrete dividers between
beds, and an underdrain system to remove percolating water. Land costs and
lining to prevent downward percolation are not included in the cost estimates.
If bed lining or land purchase are required, the costs would have to be
adjusted accordingly.
Construction cost estimates are presented in Figure C-22.
169
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C.9.2 Operation and Maintenance Cost
Diesel fuel requirements are for a front-end loader to remove dried sludge
from the beds and to prepare the bed for the next sludge application. A clean-
ing and preparation time of 3 hr for a 372 sq m (4,000 sq ft) bed, a diesel
fuel consumption of 15 1/hr (4 gal/hr), and 20 cleanings/bed per year were
used to calculate fuel requirements.
Maintenance material requirements are for replacement of sand lost during bed
cleaning. One-quarter inch of sand loss per cleaning was used to calculate
maintenance material costs.
Labor costs are for sludge removal, bed preparation, and changing of valves to
direct sludge flow to different drying beds. Labor costs were based upon
experience at a number of different locations.
The diesel fuel and maintenance material requirements are presented in
Figure C-23 and labor and total annual O&M cost are presented in Figure C-24.
Total annual O&M cost is based on a labor rate of $12/hr and a diesel fuel
cost of $0.30/1 ($1.15/gal).
C.10 Sludge Dewatering Lagoons
C.10.1 Construction Cost
Sludge dewatering or storage lagoons are used at many plants to receive,
store, and partially dewater waste sludge before further treatment or ultimate
disposal. Depending on the climate for solar/air drying and the ability of
water to percolate from the lagoon, sludge can thicken to a solids content of
15 to 40 percent (20 to 25 percent average) during 6 months of storage.
Generally, when sufficient land area is available, lagooning represents the
lowest cost system for sludge dewatering. Other factors must also be
considered however, particularly aesthetics.
Construction costs are for unlined lagoons with a 3 m (10 ft) sludge depth and
a 0.6 m (2 ft) freeboard depth. Dikes were assumed to have a 3 m (10 ft) crest
width and 3:1 side slopes. It was assumed that the excavation volume is equal
to the dike fill volume. Lagoons were designed with an inlet structure that
would prevent disturbance of settling material, and an outlet structure to
skim clarified water.
Construction costs are presented in Figure C-25. The costs are shown as a
function of effective volume, which is the volume of the lagoon minus free-
board volume. The costs do not include land cost or pond lining.
170
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C.10.2 Operation and Maintenance Cost
Operation and maintenance requirements are primarily associated with sludge
removal from the lagoons. Removal is generally done with a front-end loader or
with dragline dredging. Dredging is used to allow further dewatering by air
drying on the lagoon periphery. After air drying, the concentrated sludge is
removed by a front end loader. The costs and requirements presented are for a
combination of these approaches. Sludge was assumed to be removed from a
lagoon, on the average of once every 2 years, and hauled in dump trucks to
within 1 mile of the lagoons. If a further haul distance is required, the
additive cost of this hauling must be added.
Energy costs are for diesel fuel for the front end loader and the dragline, as
well as for trucks to haul sludge one mile from the lagoons. Requirements are
for removal of 20% sludge, which is generally the lowest concentration that
sludge is removed from a lagoon. Requirements are expressed in terms of the
volume of sludge removed annually.
Periodic repair and maintenance of the lagoon dikes and the roadway at the top
of the dike is required. These costs comprise the maintenance material costs.
Labor requirements consist of labor required for sludge removal from the
lagoons, loading the sludge into dump trucks, hauling the sludge 1.6 km
(1.0 mi) from the plant site, and maintenance of the roadways.
Figure C-26 presents diesel fuel and maintenance material costs, while
Figure C-27 presents labor and total annual O&M costs. Cost for total annual
O&M is based on $0.30/1 ($1.15/gal) for diesel fuel and $12/hr for labor.
171
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Figure C-8
High G solid bowl centrifuges - building energy,
process energy and maintenance material requirements
179
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Figure C-9
High G solid bowl centrifuges - labor and
total annual operation and maintenance cost
180
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Figure C-10
Construction cost for belt filter press
181
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Belt filter press - labor and total annual
operation and maintenance cost
183
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Figure C-13
Construction cost for vacuum filters
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184
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Vacuum filters - building energy, process energy
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185
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Figure C-15
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186
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Figure C-16
Construction cost for recessed plate filter press
187
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Recessed plate filter press - building energy, process energy
and maintenance material requirements
188
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234 5678910
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Figure C-18
Recessed plate filter press - labor and total annual
operation and maintenance cost
189
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Figure C-19
Construction cost for diaphragm filter press
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190
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Figure C-20
Diaphragm filter press - building energy, process
energy and maintenance material requirements
191
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Figure C-22
Construction cost for sand drying beds
100,000
193
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Figure C-24
Sand drying beds - labor and total annual
operation and maintenance cost
100,000
195
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Figure C-25
Construction cost for sludge dewatering lagoons
196
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MAINTENANCE MATERIAL -$/YR
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10,000
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Figure C-27
Sludge dewatering lagoons - labor and total annual
operation and maintenance cost
198
-------
BIBLIOGRAPHY
Adam, F. , "Dehydration of Fine Suspensions by Means of the Settling
Centrifuge," Environmental Protection Engineering (Ger.), Vol. 3, p. 7,
January/February, 1977; Chem. Abs., Vol. 89, 11610.
Adam, F., "Dewatering of Fine Suspensions by Means of Settling Centrifuge
Plant II—Dewatering Plant for Municipal and Industrial Sludges."
Environmental Protection Engineering (Ger.), Vol. 4, p. 5, January 1978;
Poll. Abs. 79-01809.
Agrawal, S., and Goyal, A., "Vacuum Filtration Calculation," Poll. Eng.,
Vol. 10, p. 60, March 1978.
Alt, C., "Pusher Centrifuges—Optimum Conditions," Filtration and
Separation, Vol. 17, p. 47, 1980.
Ames, R. K., et al., "Sludge Dewatering/Dehydration Results with
Mini-B.E.S.T.," Proc. 30th Ind. Waste Conf., Purdue Univ., Ann Arbor Sci.
Publ., Inc., Ann Arbor, Mich., 897, 1977.
"Areawide Assessment Procedures Manual - Volume III, Municipal
Environmental Research Laboratory, Cincinnati, Ohio 45268, EPA
600/9-76-014, July 1976.
Atwell, J. S., "Sludge Dewatering Techniques Must Meet Pollution Control
Requirements," Pulp & Paper, Vol. 42, p. 180, November 1978.
Austin, E. P., "The Filter Belt Press - Application and Design,"
Filtration and Separation, p. 320, July/August 1978.
Bassett, D. J., et al., "Performance Comparison of Filter Press, Belt
Press and Centrifuge Units for Dewatering of Citrus Processing/Activated
Sludge," Proc. 33rd Ind. Waste Conf., Purdue, Univ., Ext. Ser. , 1978.
Basso, A. J., "Getting the Most Out of Filter Aids," Chemical
Engineering, p. 185, September 1977.
Beardall, L. R., and Scarlett, B., "Separation and Classification on
Rotating Surfaces," Filtration & Separation, Vol. 17, p. 248, 1980.
Beardsley, J. A., "Sludge Drying Beds are Practical," Water and Sewage
Works, Part 1, p. 82, July; Part 2, p. 42. August 1976.
199
-------
Bechir, M.H., and Herbert, W.A., "Sludge Processing Using Som-A-Press,"
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* US GOVERNMENT PRINTING OFFICE 1982 -559-092 /0450
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