United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
EPA-600'2-79-083
August 1979
Research and Development
&EPA
Review of
Techniques for
Treatment and
Disposal of
Phosphorus-Laden
Chemical Sludges
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1 Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7 Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-79-083
August 1979
REVIEW OF TECHNIQUES FOR TREATMENT
AND DISPOSAL OF PHOSPHORUS-LADEN
CHEMICAL SLUDGES
by
Curtis J. Schmidt
LeAnne E. Hammer
Michael D. Swayne
SCS ENGINEERS
Long Beach, California 90807
Contract No. 68-03-2432
Project Officer
R. V. Villers
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. .ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental
Research Laboratory, U.S. Environmental Protection Agency, and
approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the U.S.
Environmental Protection Agency, nor does mention of trade names
or commercial products constitute endorsement or recommendation
for use.
11
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FOREWORD
The Environmental Protection Agency was created because of
increasing public and governmental concern about the dangers of
pollution to the health and welfare of the American people. Nox-
ious air, foul water, and spoiled land are tragic testimony to
the deterioration of our natural environment. The complexity of
that environment and the interplay between its components require
a concentrated and integrated attack on the problem.
Research and development constitute that necessary first
step in a solution of the problem, and involve a definition of
the problem, the measurement of its impact, and a search for
solutions. The Municipal Environmental Research Laboratory
develops new and improved technology and systems for the preven-
tion, treatment, and management of wastewater and solid and haz-
ardous waste pollutant discharges from municipal and community
sources; for the preservation and treatment of public drinking
water supplies; and to minimize the adverse economic, social,
health, and aesthetic effects of pollution. This publication is
one of the products of that research; a most vital communications
link between the researcher and the user community.
This report relates the actual operating experiences of
treatment plants which practice phosphorus removal by chemical
addition, indicating that all of the various sludge treatment
unit processes for thickening, stabilization, conditioning, dewa-
tering, and reduction are adversely affected by phosphorus
removal. The more promising methods for handling phosphorus-
laden chemical sludges were identified as pressure filtration of
iro'n sludges, flotation thickening of iron and aluminum sludges,
thermal conditioning of iron sludges, and land disposal of lime
sludges.
Francis T. Mayo
Director
Municipal Environmental Research
Laboratory - Cincinnati
111
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ABSTRACT
Removal of phosphorus from wastewater normally entails pre-
cipitation of phosphates by the addition of a chemical, generally
either lime or a salt of iron or aluminum. A consequence of phos-
phorus removal, therefore, is the production of sludge which is
laden with chemical precipitates. A survey of 174 municipal
plants using chemicals to remove phosphorus from wastewater was
conducted in order to quantify the effects of chemical addition
on the sludge handling and disposal operations at full-scale
plants.
Because of the generation of chemical sludge, phosphorus
removal adversely affects a treatment plant in two ways. First,
the volume or mass of sludge that must be handled and disposed of
is significantly increased. Second, the resulting combined chem-
ical-organic sludges thicken, dewater, and incinerate differently,
and often with more difficulty than do organic sludges alone.
Both these factors combine to compound the problem of processing
and disposing of sludge, and to increase the cost of its handling.
Of the three types of chemicals normally considered for phos-
phorus removal, iron salts generally appear to have the least
overall adverse effect upon subsequent sludge handling. Primary
addition of the phosphorus removal chemical often has advantages
over secondary addition in terms of the volume and solids concen-
tration of the combined primary/secondary sludge. At existing
plants which have gone to phosphorus removal, it has generally
not been cost-effective to add a tertiary chemical flocculation
and clarification step, unless exceptionally high effluent quality
was a goal in addition to phosphorus removal. Neither has it been
found economical at existing plants to separate chemical-1aden
sludges from other sludges for handling if the previous practice
has been to combine all the sludges (e.g., primary and secondary,
or primary, secondary, and tertiary). Several older plants which
have inadequate volume capacity for sludge handling have found
that pumping chemical-laden waste activated sludge to the primary
clarifier influent for settling with the primary sludge reduces
the volume of combined sludge to be treated. At these plants,
sludge handling considerations have either been judged to outweigh
the problem of deterioration of primary effluent quality, or an
increase in secondary clarifier efficiency has counteracted the
problem. These conclusions about existing plants should not be
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applied to the design of new plants because of the different con-
straints on costs and options.
Plant operating experiences have shown that all of the vari-
ous sludge treatment unit processes for thickening, stabilization,
conditioning, dewatering, and reduction are adversely affected by
phosphorus removal. The adverse impact is reduced when adequate
capacity is available to handle the increased sludge quantity.
Compared to other alternatives, relatively few problems have been
encountered with pressure filtration of iron sludges, flotation
thickening of iron and aluminum sludges, thermal conditioning of
iron sludges, and land disposal of lime sludges.
This report was submitted in fulfillment of Contract No. 68-
03-0268 by SCS Engineers under the sponsorship of the U.S. Envi-
ronmental Protection Agency. This report covers the period from
July 15, 1976, to September 14, 1977. Work was completed as of
October 1978.
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CONTENTS
Foreword ill
Abstract iv
Figures ix
Tables xii
Acknowledgments xvi11
1. Introduction 1
2. Project Scope and Methods 3
3. Phosphorus Removal Impacts on Sludge 6
Types of chemical sludges 6
Quantities of chemical sludges generated 12
Solids concentration and percent volatile solids
of chemical sludges generated 26
4. Prevalence of Various Treatment and Disposal
Methods for Chemical Sludges 29
Introduction 29
Sludge thickening 29
Sludge stabil ization/reduction 29
Sludge conditioning/stabilization 33
SI udge dewatering 33
Sludge heat drying 33
SI udge reduction 33
Sludge final disposal 33
5. Thickening of Chemical Sludges 34
Gravity thickening 34
Flotation thickening 40
6. Stabilization of Chemical Sludges 46
Anaerobic digestion 46
Aerobic digestion 60
Composting 67
7. Condition of Chemical Sludges 71
Chemical conditioning 71
Thermal conditioning 72
vi i
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CONTENTS (continued)
8. Dewatering of Chemical Sludges 82
Drying beds 82
Vacuum f il t rat ion 88
Dry i ng lagoons 93
Pressure filtration 98
Centrifugation 105
9. Reduction of Chemical Sludges - Incineration 112
Introduction 112
Questionnaire survey 112
Case studies 115
Literature 116
Conclusions 117
10. Disposal of Chemical Sludges - Transport and Land
Disposal 119
Introduction 119
Questionnai re Survey 119
Literature 121
Conclusions 123
11. State-of-the-Art Appraisal 124
Bibliography 133
Appendices
A. Phosphorus-Laden Sludge Management
Questionnaire Form 141
B. Outline for Collection of Field Data 149
C. Case Studies 161
Introduction to Case Studies 161
Case Study C: South Bend, Indiana 161
Case Study D: Sheboygan, Wisconsin 181
Case Study E: Coldwater, Michigan 195
Case Study F: Lakewood, Ohio 215
Case Study G: Mentor, Ohio 242
Case Study H: Brookfield, Wisconsin (Fox River) 256
Case Study I: Midland, Michigan 278
Case Study J: Port Huron, Michigan 293
Case Study K: Pontiac, Michigan 308
v i i i
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FIGURES
Number Page
1 Points of chemical addition for phosphorus
removal 7
2 Cubic meter (m ) sludge per kg phosphorus
removed (gal sludge per Ib removed) 22
3 3
3 Cubic meter (m ) sludge per mil m plant
influent 23
4 Kilogram (kg) sludge per kg phosphorus
removed 24
5 Kilogram (kg) sludge per m plant influent
(Ib/MG plant influent) 25
C-l South Bend, Indiana, wastewater treatment
plant flow diagram 166
C-2 Tertiary upflow clarifier configuration,
South Bend, Indiana 169
C-3 Flow pattern for South Bend anaerobic digester 173
C-4 Sheboygan, Wisconsin, wastewater treatment
plant flow diagram 185
C-5 Sheboygan, Wisconsin, gravity thickener
hydraulic and mass balance 186
C-6 Coldwater, Michigan, wastewater treatment
plant flow diagram 199
C-7 Hydraulic and solids mass balances for wastewater
and sludge treatment operations, Coldwater,
Michigan 202
C-8 Flocculation channel, Coldwater, Michigan 206
C-9 Lakewood, Ohio, wastewater treatment plant
flow diagram 218
IX
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FIGURES (continued)
Number Page
C-10 Lakewood, Ohio, anaerobic digester
configuration
.222
C-ll Lakewood, Ohio, hydraulic and solids mass balance
during 63 mg/£ alum addition (Jan thru Dec 74) 227
C-12 Lakewood, Ohio, hydraulic and solids mass balance
before alum addition (Nov 72 thru Oct 73) 228
C-13 1976 anaerobic digestion mass balance during
double shift vacuum filter and flash dryer
operation, Lakewood, Ohio ....234
C-14 Anaerobic digestion mass balance during liquid
sludge hauling, Lakewood, Ohio 235
C-15 Greater Mentor wastewater treatment plant
wastewater flow diagram 245
C-16 Dual cell gravity thickeners and appurtenances,
Mentor, Ohio 252
C-17 Brookfield, Wisconsin, wastewater treatment
plant flow diagram 259
C-18 Materials balance for primary wastewater
treatment operations at Brookfield, Wisconsin 262
C-19 Materials balance for secondary wastewater
treatment operations at Brookfield 263
C-20 Brookfield, Wisconsin pressure filtration and
incineration facilities 267
C-21 Pressure filter performance, Brookfield,
Wisconsin 268
C-22 Mul tiple -hearth incinerator performance,
Brookfield, Wisconsin •. 270
C-23 Midland, Michigan, wastewater treatment plant
flow diagram 281
C-24 Midland, Michigan, sludge treatment flow diagram 286
C-25 Chemical feed points to aeration tanks, Port Huron,
Michigan 295
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FIGURES (continued)
Number PA16
c-26 Port Huron, Michigan, wastewater treatment
plant flow diagram 297
C-27 Port Huron, Michigan, gravity thickener
hydraulic and mass balance 300
C-28 Wastewater treatment unit process flow
diagram, Pontiac, Michigan 315
C-29 Sludge handling unit process flow diagram,
Pontiac, Michigan 31 6
C-30 Pontiac, Michigan, hydraulic and materials
balance 318
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TABLES
Number Page
1 Results of Plant Survey 4
2 Prevalence of Phosphorus Removal Methods
(Chemicals and Points of Addition) among
Plants Responding to Questionnaire Survey 9
3 Combination of Chemical-Laden and Other Sludges for
Processing as Practiced by Plants in Questionnaire
Survey 11
4 Influence of Plant Size on Type(s) of Chemical(s)
Used for Phosphorus Removal among Plants in
Questionnaire Survey 13
5 SS and BOD Removal Efficiencies and Dry Weights
of Suspended Solids Removed at a Hypothetical
Activated Sludge Plant 15
6 Total Dry Weight of Suspended Solids and Chemical
Solids Removed during Treatment Processes at
a Hypothetical Activated Sludge Plant 18
7 Theoretical Kilograms of Solids Generated per
Kilogram of Phosphorus Removed at a Hypothetical
Activated Sludge Plant 19
8 Comparison of Theoretical Solids Generation Rates
with Results of Questionnaire Survey 26
9 Solids Characteristics of Chemical Sludges
with and without Combination with Other
Plant Sludges 28
10 Prevalence of Treatment and Disposal Processes for
Chemical Sludges among Plants Responding to
Questionnaire Survey. 30
11 Impacts of Chemical Sludge upon Gravity Thickening
Performance as Reported in Questionnaire Response. .. .35
XI 1
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TABLES (continued)
Number Pa
12 Impacts of Chemical Sludges upon Flotation
Thickener Performance as Reported in
Questionnaire Response ............................... 4
13 Impacts of Chemical Sludges upon Anaerobic
Digester Performance as Reported in
Questionnaire Response ............................... 47
14 Impacts of Chemical Sludges upon Aerobic
Digester Performance as Reported in
Questionnaire Response ............................... 61
15 Projected Costs of Various Sludge Handling
Alternatives Following Aerobic Digestion
at Portage Lake, Michigan ($) ........................ 65
16 Average Sidestream Characteristics at Grand
Haven, Michigan ...................................... 74
17 Characteristics of Plants in Study of Thermal
Conditioning of Chemical Sludges ..................... 76
18 Characteristics of Streams and Sidestreams
Associated with Sludge Treatment Operations .......... 78
19 Results of Pilot Scale Centrifugation of Midland
Thermally Conditioned Iron Sludge ..... . .............. 80
20 Comparison of Sidestreams from Plant and Pilot
Dewatering Operations ............................... : 81
21 Impacts of Chemical Sludges upon Drying Bed
Performance as Reported in Questionnaire
Response ............................................. 83
22 Polymer Application to Drying Beds ....... . .............. 87
23 Changes in Vacuum Filter Performance Reported
as a Result of Phosphorus Removal Chemical
SI udge Addition ...................................... 89
24 Impacts of Chemical Sludges upon Drying
Lagoon Performance as Reported in
Questionnaire Response ............................... 95
25 Sludge Treatment/Disposal Methods Used by
Plants Practicing Pressure Filtration ............... 100
* • •
xi 11
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TABLES (continued)
Number Page
26 Impacts of Chemical Sludges upon Pressure
Filter Performance as Reported in
Questionnaire Response '°'
27 Pilot Filter Press Test at Holland, Michigan 104
28 Centrate Characteristics from Various Scroll
Centrifugation Runs 1 08
29 Impacts of Chemical Sludges on Centrifuge
Performance as Reported in Questionnaire
Response - .....110
30 Impacts of Phosphorus-Laden Chemical Sludges on
Incinerator Performance as Reported in
Questionnaire Response 113
31 Bibliography Information Index 127
32 Key to Bibliography Information Index 131
C-l Description of Case Study Sites According to Plant
Sel ection Factors 162
C-2 Wastewater Treatment Process Design Parameters,
South Bend, Indiana 167
C-3 Summary of 1976 Wastewater Characteristics and
Treatment Performance, South Bend, Indiana 168
C-4 Sludge Treatment Process Design Parameters
South Bend, Indiana 171
C-5 Chemical Sludge Production and Gravity
Thickening, South Bend, Indiana .....174
C-6 Anaerobic Digester Gas Production, South
Bend, Indiana 176
C-7 A sample of Historical Data Indicating Average
Plant Influent Characteristics, Suspended
Solids, and BOD Removal, Sheboygan, Wisconsin 182
C-8 Vacuum Filtration and Incineration Performance
before and after Phosphorus Removal ,
Sheboygan , Wisconsin 189
xi v
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TABLES (continued)
Number
Pai
C-9 Influent and Effluent Wastewater Characteristics
and Removal Efficiencies, Coldwater, Michigan 196
C-10 General Plant Description Summary, Coldwater,
Michigan 200
C-ll Sludge Pumped from Primary Clarifiers to
Primary Digester, Coldwater, Michigan 208
C-12 Sludge Transferred from Digesters to Drying
Beds (or Lagoon), Coldwater, Michigan 211
C-13 Plant Influent Wastewater Flow Rates,
Lakewood, Ohio 216
C-14 Plant SS, BOD, and Phosphorus Concentrations
before and during Alum Addition for Phosphorus
Removal, Lakewood, Ohio • 226
C-15 Plant SS, BOD, and Phosphorus Removals before
and during Alum Addition for Phosphorus
Removal, Lakewood, Ohio 229
C-16 1976 Sludge Treatment Data during Do'uble Shift
Vacuum Filter and Flash Dryer Operation
at Lakewood, Ohio 234
C-17 Costs of Treating Alum Sludge during Single and
Double Shift Vacuum Filter and Flash Dryer
Operation, Lakewood, Ohio 238
C-18 Costs of Alum Sludge Treatment during Single
and Double Shift Vacuum Filter and Flash
Dryer Operation, Lakewood, Ohio 240
C-19 Influent and Effluent Wastewater Characteristics
and Removal Efficiencies, Mentor, Ohio 243
C-20 General Plant Description Summary, Mentor, Ohio 246
C-21 Aerobic Digester Sludge Characteristics,
Mentor, Ohio 250
C-22 Performance of Dual Cell Gravity Concentrators,
Mentor, Ohio 254
xv
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TABLES (continued)
Number £lfl£
C-23 Influent and Effluent Wastewater Characteristics
and Removal Efficiencies, Brookfield, Wisconsin.... 257
C-24 General Plant Description Summary, Brookfield,
Wisconsin 260
C-25 Changes in Primary Clarifier Performance as the
Result of Chemical Addition for Phosphorus
Removal, Brookfield, Wisconsin 264
C-26 Impacts of Chemical Addition for Phosphorus
Removal on Final Clarifiers, Brookfield,
Wisconsin 266
C-27 Pressure Filter Performance, Brookfield,
Wi scons in 273
C-28 Pressure Filter and Incinerator Operational
Cost per t (ton) Dry Solids, Brookfield,
Wisconsin 277
C-29 Phosphorus Removal Impacts on Plant BOD and
SS Removals, Midland, Michigan 282
C-30 Impacts of Phosphorus Removal: Sludge
Conditioning, Thickening, and Dewatering
Characteristics -- Variability with
Conditioning Method, Thermal Conditioner
Temperature, and the Recycling of Tertiary
Filter Backwash Water, Midland, Michigan 287
C-31 A Comparison of the Characteristics of the
Sludges Produced with Alum and Ferris
Chloride, Midland, Michigan 290
C-32 Additional Cost for High Temperature Thermal
Conditioning, Midland, Michigan 292
C-33 Direct Effects of Conditioning Method on
Performance and Operational Costs of Solids
Handling, Port Huron, Michigan 304
C-34 Wastewater Characteristics and Removal
Efficiencies, Pontiac, Michigan 310
xvi
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TABLES (continued)
Number Page
C-35 General Plant Description Summary, Pontiac,
Michigan 312
C-36 Primary Clarifier Waste Stream Characteristics
Pontiac, Michigan 317
C-37 Raw and Digested Sludge and Supernatant
Characteristics, Pontiac, Michigan 320
C-38 Vacuum Filter Performance, Pontiac, Michigan......... 322
C-39 Incinerator Performance, Pontiac, Michigan 323
xvi i
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ACKNOWLEDGMENTS
The project officer for this contract was R. V. Villiers,
Ultimate Disposal Section, Wastewater Research Division, Municipal
Environmental Research Laboratory, EPA, Cincinnati, OH. SCS
Engineers personnel were Lee Hammer, Curt Schmidt, Don Sherman,
Mike Swayne, and Mark Montgomery.
We wish to
and assistance:
thank the following people for their cooperation
Dr. J. B. Farrell
Municipal Environmental Research
Laboratory, EPA
Cincinnati, OH
Mr. G. L. Van Fleet and
Mr. P. Seto
Sanitary Engineering Branch,
Ontario Ministry of the
Environment
Toronto, Ontario
Mr. Charles J. Sluskonis
Greater Mentor Wastewater
Treatment Plant
Mentor, OH
Mr. John F. Budde
Fox River Water Pollution
Control Center
Brookfield, WI
Mr. J. Michael Jeter
Bureau of Wastewater
South Bend, IN
Mr. John Hennessey and
Mr. Jim Spangler
Pontiac Wastewater Treatment
Plant
Pontiac, MI
Mr. Larry Marshall
Lakewood Wastewater Treatment
Plant
Lakewood, OH
Mr. Willis W. Stubbe
Sheboygan Wastewater Treatment
Plant
Sheboygan, WI
Mr. David
Coldwater
Plant
Coldwater,
H. McKay
Wastewater
MI
Treatment
Mr. Paul S. Hendricks
Port Huron Wastewater
Treatment Plant
Port Huron, MI
Mr. Larry Dull and
Mr. Arthur Mass
Midland Wastewater Treatment
Plant
Midland, MI
xv m
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SECTION 1
INTRODUCTION
Starting in the late sixties, regulatory agencies began
placing limits upon the phosphorus content of treated wastewater
effluents discharged to many surface receiving waters. As a
result, over 400 municipal sewage treatment plants in the United
States and Canada are now required to implement additional chemi-
cal treatment steps to remove 80 percent or more of the phos-
phorus contained in the raw sewage. There are several chemicals
which can be added to precipitate phosphorus in the form of phos-
phates from: iron salts, aluminum salts, or lime. All such pro-
cesses generate sludge which is laden with chemical precipitates.
This report focuses upon the problems being experienced in muni-
cipal sewage sludge management as a result of the addition of
these chemical-laden sludges to the sludge treatment processes.
The municipal plants which were studied were operated to
achieve at least 80 percent removal of the phosphorus contained
in the raw sewage. At higher or lower removal rates, the impacts
on sludge which are described in the report would be correspond-
ingly greater or lesser. A few plants which were investigated
were not reaching 80 percent removal at the time of the study
because of problems in plant design, unusual waste characteris-
tics, plant hydraulic overload, or, in a few cases, inadequate
sludge handling capacity.
Included early in this report are sections which describe
the changes in sludge volume and characteristics experienced by
treatment plants as a result of adding chemicals to remove phos-
phorus. Emphasis is upon the differences in volume and charac-
teristics experienced by treatment plants as a result of adding
chemicals to remove phosphorus. Emphasis is upon the differen-
ces in volume and characteristics resulting from use of each of
the common phosphorus removal chemicals and the effects of utili-
zing alternate points of chemical addition in the sewage treat-
ment chain.
Subsequently, the report discusses the effect of the chemical
laden sludges upon typical sludge treatment unit processes (e.g.,
thickening, digestion, dewatering, incineration, etc.). Problems
experienced by various treatment plants are cited, and solutions
that were developed are described. The information presented is
1
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derived from a questionnaire survey (174 responses) of treatment
plants practicing phosphorus removal, field investigation case
studies of nine selected plants, and a review of pertinent liter-
ature. Whenever possible, treatment plant names and locations
are referenced so that readers may obtain additional information
di rectly.
Case study reports of field investigations at selected treat-
ment plants are provided in the appendices. These case studies
provide detailed information pertinent to sludge handling at a
variety of plants which are generally representative of typical
phosphorus removal technology.
It is necessary to warn the reader that sludge generation
and management are complex subjects. Every sewage treatment
plant is a unique combination of variables in raw sewage charac-
teristics, treatment unit design, operational procedures, etc.
For these reasons, it is often difficult to successfully trans-
fer technology or information on sludge volume and mass from one
plant to another. However, the experience of others can serve
as a background basis for possible solutions to, or avoidance of,
similar sludge management problems. It is our hope that this
report will be helpful in guiding sewage treatment plant design-
ers and operators toward a realization of potential sludge manage'
ment problems and solutions when phosphorus removal operations
are necessary.
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SECTION 2
PROJECT SCOPE AND METHODS
Removal of phosphorus from wastewater normally entails pre-
cipitation of phosphates by the addition of a chemical, generally
either calcium hydroxide or a salt of iron or aluminum. A con-
sequence of phosphorus removal, therefore, is the production of
a sludge which is laden with chemical precipitates. Because of
the generation of chemical precipitates, phosphorus removal
impacts adversely on a treatment plant in two ways. First, the
volume or mass of sludge that must be handled and disposed of is
significantly increased. Second, the resulting combined chemical-
organic sludges thicken and dewater differently, and often with
more difficulty than do organic sludges alone. Both these factors
combine to compound the problem of processing and disposing of
sludge, and to increase the cost of its handling.
Although increasingly more treatment plants are being required
by regulatory agencies to remove phosphorus, the information neces-
sary to competently design or modify engineering works to ade-
quately handle the phosphorus-laden chemical sludges is still
largely not available. EPA's Office of Research and Development
perceived the need for a project to assemble and evaluate the
information needed to detail the most viable methods for handling
and disposing of chemical-1aden sludges produced by phosphorus
removal, and to identify those techniques which are most cost-
effective for dealing with them. This no small task because of
the variety of types of chemical-laden sludges and the number of
methods available for treatment and disposal of them.
It was the objective of this project to conduct extensive
data gathering and in-depth evaluation of available hard engineer-
ing data, including operating experience of plants practicing
phosphorus removal. It is hoped that the information in this
report, when combined with other EPA research on the subject
will enable the future preparation of satisfactory definitive
guidelines for use by federal and state agencies and consulting
engineers for treatment and disposal of chemical sludges.
A literature search was conducted to gather all published
and unpublished literature pertinent to the management of chemi-
cal sludges. Approximately 80 documents were reviewed and found
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to contain helpful information. These are listed in the biblio-
graphy to this report; To make the bibliography more useful, it
is preceded by a matrix index which enables the user to determine
the subjects covered in each document, and conversely to identify
all the documents which include information about a subject of
interest.
An extensive questionnaire survey was made in early 1977 of
treatment plants practicing phosphorus removal, as identified by
state regulatory agencies, EPA regional offices, and the Canadian
Ministry of the Environment of Ontario. A copy of the question-
naire mailed out is shown in Appendix A to this report. Question-
naires were mailed to over 400 plants. A number of plants replied
that they will have phosphorus removal at some time in the future.
A total of 361 plants were identified as currently practicing
phosphorus removal. Table 1 shows the number of plants identi-
fied as removing phosphorus (361); those that responded to the
survey (174) are broken down by state, plus Canada.
TABLE 1. RESULTS OF PLANT SURVEY
State
(Name)
Cal if ornia
Colorado
Illinois
Indiana
Michigan
Minnesota
New York
Ohio
Pennsylvania
Wisconsin
Texas
Canada
Identified Plants
in State (No.)
2
1
22
26
91
12
8
34
13
59
1
92
Plants Responding
to Survey (No.)
1
1
5
7
59
4
4
16
9
26
1
41
TOTAL 361
Considering the length of the questionnaire, the response of
almost 50 percent is excellent. Phone call follow-ups were
required to obtain the response received, and to clarify those
questionnaire responses that contained confusing information.
The questionnaire asked for a great deal of detailed information,
and respondents supplied whatever data they had. Unfortunately,
hard engineering and cost data were usually not available to them.
Questionnaire responses are tabulated and/or summarized throughout
the report where appropriate to the subject being discussed.
Based upon the questionnaire survey, literature information,
and recommendations from knowledgeable persons, a total of nine
operating treatment plants were selected for field investigations.
An average of one person week was spent gathering all available
historical and current operating and design data at each selected
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plant. Emphasis was placed upon determining the effects of the
chemical sludge addition on sludge treatment processes, and upon
solving the problems encountered. Appendix B shows the field
investigation outline form used. Appendices C through K comprise
the case studies themselves. Information gathered during the case
studies is utilized throughout the report to enhance discussions
of various unit treatment processes.
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SECTION 3
PHOSPHORUS REMOVAL IMPACTS ON SLUDGE
TYPES OF CHEMICAL SLUDGES
In this report a chemical sludge is any sludge containing
chemical precipitates derived from phosphorus removal. The vari-
ous types of chemical sludges can be defined by the type(s) of
chemical(s) used, the type of plant, the point(s) of chemical
addition, and whether or not and how the primary, secondary, and
tertiary sludges are combined for processing.
It appears that the only chemicals used by full-scale plants
for phosphorus removal are lime or salts of iron or aluminum.
Some plants use more than one chemical. Lime usually comes to
the plant as quicklime (CaO). It is usually slaked to calcium
hydroxide lime (Ca(OH)2) before it is added to wastewater. The
slaking process requires a great deal of equipment and some addi-
tional operator attention and is partly responsible for the rela-
tively infrequent use of lime for phosphorus removal. The iron
salts that are used are ferrous sulfate (FeS04), ferrous chloride
(FeCl2)> and ferric chloride (FeCls). Both ferrous sulfate and
ferrous chloride are waste products of the pickling process in
steel industries. Waste pickle liquor containing ferrous iron
is the cheapest source of iron which can be used for phosphorus
removal in some areas of the country. Ferric iron can also
occasionally be obtained as a by-product from certain industries
although this is rare. The aluminum salts that are used for phos-
phorus removal are aluminum chloride (AlClo), aluminum sulfate or
alum ^12(504)3), and sodium aluminate (NaA102). Polymers are
often used as coagulant aids with any of the phosphorus removal
chemicals.
In most modern wastewater treatment plants, several points
are available for chemical addition to precipitate phosphorus
(see Figure 1). However, differences exist in the points of
addition which are available depending on the type of plant and
the type of chemical in use.
Primary addition of any of the phosphorus removal chemicals
can be practiced. Primary addition of ferrous iron salts is a
special case because the phosphates are not always precipitated
in the primary clarifier, as is the case with other chemicals.
-------
INFLUENT
WASTEWATER
r
PRIMARY ADDITION
i
PRIMARY
CLARIFICATION
CHEMICAL-LADEN
PRIMARY SLUDGE
CHEMICAL PRECIPITANT
SECONDARY ADDITION
i ! i
i i i
_i
TERTIARY ADDITION
BIOLOGICAL
TREATMENT
1
t .,
SECONDARY
CLARIFICATION
1
t
CHEMICAL
FLOCCULATION/
CLARIFICATION
EFFLUENT
WASTEWATER
CHEMICAL-LADEN
SECONDARY SLUDGE
CHEMICAL-LADEN
TERTIARY SLUDGE
Figure 1. Points of chemical addition for phosphorus removal.
-------
In order for ferrous iron to affect precipitation of phosphates
to a significant extent, the sewage must either be aerated or the
pH raised by adding lime or sodium hydroxide. If ferrous iron is
added to the primary stage without also adding a base to raise
the pH, almost all of the phosphorus passes through the primary
treatment without being removed. If the plant has activated
sludge treatment, the ferrous ions will be oxidized to ferric
ions in the aeration basins, and phosphates will be precipitated
and removed in the secondary clarifier.
Secondary addition of any chemical except lime can be prac-
ticed. Lime addition to the secondary treatment stage is not
practiced because the high pH accompanying lime addition would be
detrimental to the biological population of the system. Secondary
addition includes addition before, directly into, or just after
the activated sludge aeration basins or trickling filters.
Tertiary addition of chemicals in a third stage clarifier
is occasionally practiced. Either lime, aluminum salts, or ferrous
iron salts are used. Some plants are designed with third-stage
treatment primarily because it is effective in removing BOD, SS,
and metals in addition to phosphorus. Since it is expensive
to build a tertiary clarifier, this practice is uncommon when
phosphorus removal alone is the effluent quality objective.
In trickling filter plants, chemicals are seldom added just
ahead of, or directly dosed to, the filters themselves, but in
activated sludge plants, chemicals are frequently added just prior
to or directly into the aeration basins. In trickling filter
plants, the chemical would be added before the final clarifiers
to avoid staining and clogging of the filters, while in an acti-
vated sludge plant it can be advantageous to add the chemicals
to the aeration basins to allow utilization of the mixing action
which provides dispersion of the chemical in the wastewater. It
is sometimes found more effective to add the chemical 2/3 or 3/4
of the way through the aeration basin to minimize floe disruption.
Table 2 presents the results of the questionnaire survey
on the prevalence of the various phosphorus removal methods
(chemicals and points of addition) among the plants which res-
ponded. It also indicates the number of these plants which use
a polymer as a flocculation aid. The table shows that chemical
addition to the secondary treatment stage is more prevalent than
primary or tertiary addition. Sixty-two percent of the plants
in the survey practiced secondary chemical addition whereas 26
percent practiced primary addition and six percent added the
chemical in a tertiary step involving chemical flocculation and
clarification. The remaining six percent used a combination of
points of addition. The percentages are basically the same when
the plants which did not have either a primary or secondary treat-
ment step are excluded from consideration.
8
-------
vo
TABLE 2. PREVALENCE OF PHOSPHORUS REMOVAL METHODS (CHEMICALS AND POINTS
OF ADDITION) AMONG PLANTS IN QUESTIONNAIRE SURVEY
Point(s) of
Chemical Addition
Primary
Secondary
Tertiary
Primary and Secondary
Primary and Tertiary
Secondary and Tertiary
Total Number
Primary
Without
Secondary
10F3, 3A
NA
NA
NA
NA
NA
10F3, 3A
Aerated Lagoon or
Activated Sludge
Without Primary *
NA!
12F3, 2F2, 14A
1L+A
NA
NA
1F3
13F3, 2F2, 14A,
1L+A
Number of Plants
Primary Plus **
Activated Sludge
6L, 4FV3F,,, 5A,
1L+A J i
26F3, 9F2, 36A,
4L, 2F3, 1A
1F2, !L-i-F3, 1F,+A
2F3, 1A z
1L+A
NA
10L, 34F3, 13F2, 43A,
1L+F3, 2L+A, 1F2+F,,
1F2+A *
*
by Plant Type
Primary Plus
Trickling Filter
12F,, IF,, 1A
^ €~
3F3, 5A
1F3, 1A
1F3, 1L+A
NA
NA
17F3, 1F2, 7A, 1L+A
Total
6L, 26F3,4F2,9A
41F3, 11F,, 55A,
1F2+F3 i
4L, 3F3, 2A, 1L+A
3F3, 1F2, 1A.1L+F,,
1L+A, 1F2+A J
1L+A
1F3
10L, 74F3, 16FZ,
67A, 1L+F3, 4L+A
Total
Using Polymer
16F.,, 4A, 1L+A
J
20F3, 3F2, 12A
3L, 2F3
2F3, 1A, 1L+A
1L+A
1F3
3L, 41F-, 3F,, 17A,
3L+A 6 i
L = Lime;
F3 = Ferric iron salt;
A = Aluminum salt;
F- = Ferrous iron salt.
fTwo plants in this category have aerated lagoons but do not produce secondary sludge.
*Three plants in this category have aerated lagoons with sludge removal from final clarifiers; seven plants have the extended
aeration modification of the activated sludge process.
* Three plants in this category have the extended aeration modification of the activated sludge process
f NA = Not applicable.
-------
Iron salts, either ferric chloride (FeCls), ferrous chloride
2)> or ferrous sulfate (FeS04), were the most common chemi-
cals used among the plants in the survey. Fifty-two percent of
the plants were using iron. Seventy-five plants used ferric
chloride. Both primary and secondary addition of ferric chloride
were common, with secondary addition being more prevalent. Ter-
tiary ferric chloride addition was occasionally practiced. One
important factor influencing the frequency of iron usage was the
availability of waste pickle liquor from steel mills in the Great
Lakes Region. All of the 18 plants in the survey which used fer-
rous iron were obtaining it as an industrial by-product. Most of
these plants added the chemical to the secondary stage. The
plants which added it to the primary did not accompany it with
a base, and therefore the actual phosphate removal occurred mainly
in the secondary stage.
Thirty-eight percent of the plants in the survey used alumi-
num. Secondary addition was much more common than primary, pos-
sibly due to difficulty in getting adequate mixing of the alumi-
num with primary addition. Two plants practiced tertiary addition
of aluminum. Fifteen plants reported using lime -- four of them
in combination with another chemical. Ten plants added it to the
primary stage, while five added it in a tertiary step. With
respect to polymer usage, 43 percent of the plants using lime, and
19 percent of the plants using ferrous iron, employed a polymer.
Considering the two most commonly used chemicals -- ferric iron
and aluminum -- polymer was used in combination with the former
55 percent of the time, but it was used with the latter only 25
percent of the time.
The chemical-1aden primary, secondary, or tertiary sludge
resulting from lime, aluminum, or iron addition to remove phos-
phorus can be processed separately from the other sludges pro-
duced in the plant, or all of the sludges can be combined for
processing (e.g., primary and secondary, or primary, secondary,
and tertiary). Sludges can be combined in a sludge-processing
unit such as a thickener, digester, holding tank, or mixing tank.
Alternatively, primary and secondary sludges are often combined
by pumping the waste secondary sludge to the primary clarifier
influent where it is mixed with raw sewage. Solids derived from
both the secondary sludge and the raw sewage are thus removed
together in the primary clarifiers. Table 3 shows that, except
in the case of chemical-1aden tertiary sludges, it is more com-
mon to combine chemical-laden and other sludges than to treat
them separately. Tertiary sludges were treated separately at six
of the ten plants in the questionnaire survey practicing tertiary
chemical addition. At the other four plants, the sludges were
combined in or before a digester or a gravity thickener. Among
33 plants practicing primary chemical addition and having both
primary and secondary sludges, the chemical-1aden primary sludge
was combined with the secondary sludge at 31 plants. Thirty-nine
percent of these combined by pumping the waste secondary sludge
10
-------
TABLE 3. COMBINATION OF CHEMICAL-LADEN AND OTHER SLUDGES FOR
PROCESSING AS PRACTICED BY PLANTS IN QUESTIONNAIRE SURVEY
Point at Which Chemical -Laden
and Other Sludges Were Combined
Plants Without Primary or
Secondary Clarification
• No primary sludge
• No secondary sludge
Plants With Both Primary and
Secondary Clarification
• Sludges not combined
* Combined in or before
primary clarifier
• Combined in or before
thickener
• Combined in or before digester
• Combined in or before dewatering
device
Total
Number of Plants by Point(s) of Chemical
Primary
Addition
Only
*
NA
13
2
12
13
6
0
46
Secondary
Addition
Only
28
NA
10
26
14
24
6
108
Tertiary
Addition
Only
1
NA
6
0
2
2
0
11
Primary
and
Secondary
Addition
NA
NA
1
3
1
3
0
8
Addition
Primary
and
Terti ary
Addition
NA
NA
0
0
1
0
0
1
Secondary
and
Tertiary
Addition
1
NA
0
0
0
0
0
1
Total
30
13
19
41
31
35
6
175+
NA - Not applicable.
One plant with tertiary addition is counted twice because it has no primary sludge, and
it has secondary and tertiary sludges which are not combined.
-------
to the primary clarifier influent; 42 percent combined sludges in
or before a gravity thickener; and 19 percent in or before a
digester. Among 80 plants with secondary chemical addition and
both primary and secondary sludges, the sludges were combined
at 70 plants. Thirty-seven percent of these plants combined the
sludges by pumping to the primary clarifier influent; 20 percent
combined sludges in or before a gravity thickener; 34 percent in
or before a digester; and 9 percent in or before a dewatering
device.
Table 4 examines the influence of plant size on the type(s)
of chemical(s) used for phosphorus removal among plants in the
questionnaire survey. The table indicates that the majority of
plants (74 percent) treats no more than 5 mgd of wastewater.
There is no apparent correlation between plant size and chemical
used.
QUANTITIES OF CHEMICAL SLUDGES GENERATED
Introduction
The additional sludge generated may be a major concern when
implementing chemical addition for phosphorus removal. This sec-
tion will first present a discussion of stoichiometric relation-
ships to determine the theoretical quantities of chemical pre-
cipitates by addition of lime, iron, and aluminum chemicals for
phosphorus removal. Second, information taken from literature
sources on sludge generation quantities will be presented; and
finally, the sludge generation information developed from the
treatment plant survey will be presented. Literature and survey
information is derived from actual plant experience and will
enable comparison of theoretical and actual sludge generation
rates. Actual generation is expected to be greater than theore-
tical rates because of additional solids and BOD capture during
chemical treatment.
There are several methods for estimating the mass of sludge
which will be generated in a particular plant by phosphorus
removal. The best method to use at an existing installation is
a plant-scale test. When a full-scale test is infeasible at
an existing plant or when an engineer is designing a new plant,
a pilot plant test is the next best method. If this is impossi-
ble but the actual wastewater to be treated is available, sludge
mass should be estimated from jar tests with the wastewater. The
jar test results can be corroborated with estimates derived
using a calculation method (see Reference 27 for a procedure).
The calculation method is most effective when the parameter of
chemical dosage can be determined using jar tests. The jar test
relates chemical dosage to other parameters; iron or aluminum
dosage is related to phosphorus removal; lime dosage is related
to alkalinity of the water and the pH level required. If jar
tests cannot be carried out, then judgements of these parameters
must be made on the basis of past experience.
12
-------
TABLE 4. INFLUENCE OF PLANT SIZE ON TYPE(S) OF CHEMICAL(S) USED FOR
PHOSPHORUS REMOVAL AMONG PLANTS IN QUESTIONNAIRE SURVEY
CO
Chemical (s)
Used
Lime
Ferric iron
salt
Ferrous iron
salt
Aluminum salt
Lime and Ferric
Lime and Aluminum
Ferrous and Ferric
Ferrous and Aluminum
Total
Number of Plants by Size
0.05-1 .0 mgd
3
31
3
28
0
0
0
0
65
1 .0-5.0 mgd
6
24
7
21
1
4
1
0
64
5.0-10.0 mgd
0
7
1
9
0
0
0
1
18
10.0-20.0 mgd
0
7
3
7
0
0
0
0
17
over 20.0 mgd
1
5
2
2
0
0
0
0
10
Total
10
74
16
67
1
4
1
1
174
-------
Estimates of sludge production have been made on a purely
empirical basis by noting increases in production at plants which
have instituted phosphorus removal. This method is useful only
for generalizations because of differences in sewage characteris-
tics and plant design and performance. Lime, iron, or alum addi-
tion to an existing plant greatly affects the solids and BOD
removal efficiency of the plant; therefore, the increase in sludge
production caused by chemical addition depends greatly on what
the original removal efficiency was.
Theoretical Sludge Generation Quantities
Knight, Mondoux, and Hambley(40) have calculated sludge
generation rates at a hypothetical sewage treatment plant. The
hypothetical plant has the following values:
Flow - 3,785 m3/day (1.0 mgd)
BOD - 200 mg/£
SS - 250 mg/£
Total Dry Weight SS per Day - 947 kg (2086 Ib)
P - 10 mg/£
P effluent limitation - 2 mg/£
Table 5 presents, on both dry weight and percentage bases,
the wastewater solids removal assumed for this discussion for
activated sludge systems: 1) with no chemical addition, 2) with
chemical addition for phosphorus removal prior to the primary
clarifier, and 3) with tertiary chemical addition for phosphorus
removal. The daily weights of dry solids are derived simply by
multiplying percentage removals by total suspended solids weights
in the influent. Thus, the weights of chemical solids are not
included. These calculated weights do not allow for the organics
consumed by endogenous respiration or for the soluble BOD con-
verted during aeration into secondary sludge solids. For typical
municipal wastewater of the composition shown, respiration and BOD
conversion have a negligible effect on the weight values. How-
ever, with a high BOD, low suspended solids wastewater, a more
detailed calculation would be advisable (see Reference 26 for a
procedure).
Lime--
Phosphorus is precipitated by lime as hydroxyapati te, Ca,- OH
. Assuming 89 percent phosphorus removal and 10 mg/£ in
the influent, the dry weight of this precipitate from a 3,785
m3/day (1.0 mgd) plant is 163 kg/day (359 Ib/day).
14
-------
TABLE 5. SS AND BOD REMOVAL EFFICIENCIES AND DRY WEIGHTS OF SUSPENDED
SOLIDS REMOVED AT A HYPOTHETICAL ACTIVATED SLUDGE PLANT*
Primary Treatment
No Chemical Addition
Primary Chemical
Addition
Tertiary Chemical
Addition
Percent
SS
50
75
50
Removal
BOD
35
50
35
kg/day
(Ib/day)
SS Removed
473
(1041)
709
(1562)
473
(1041)
Secondary Treatment Tertiary Treatment
Percent
_^
SS
90
90
90
Removal
BOD
90
90
90
kg/day Percent Removal
(Ib/day)
SS Removed SS BOD
378
(833)
142
(312)
378 95 95
(833)
kg/day
(Ib/day)
SS Removed
—
47
(104)
Total
kg/day
(Ib/day)
SS Removed
851
(1874)
851
(1874)
898
(1978)
*We1ght of chemical solids and additional SS removed during chemical
treatment not Included.
-------
The percentage of phosphorus removed is essentially a func-
tion of pH, with substantial removal occurring as low as pH 9.0.
Better than 90 percent removal can be obtained at pH 11.0. The
pH required to obtain 80 percent removal of phosphorus is below
that at which substantial precipitation of Mg(OH)2 occurs. This
affects the thickening and dewatering characteristics of the
sludge, since magnesium hydroxide is bulky and dewaters slowly
to a very low consistency.
The lime.dosage needed to attain any desired pH level depends
on wastewater alkalinity, hardness, and the relative quantities
of calcium and magnesium ions. In practice, the bulk of the lime
added is precipitated in the chemical sludge as calcium carbonate.
If all the lime added was precipitated as CaCOs, 1 kg of Ca(OH)2
would yield 1.35 kg of CaCOs. If tne lime was used UP in ma9~
nesium precipitation, 1 kg of Ca(OH)2 would produce 1.35 kg of
CaCOs Plus °-39 kg-of Mg(OH)2 for a total of 1.74 kg. One kg of
Ca(OH)2 precipitates 1.36 kg of hydroxyapatite.
Although it is not possible, according to Knight, Mondoux,
and Hamble (40), to accurately calculate the total amount of
solids that will precipitate per pound of lime added, Parrel!
(26) has presented a procedure for estimating the required lime
dose and the quantity of sludge produced for tertiary treat-
ment of a wastewater with lime. All of these authors agree that
if the wastewater which will actually be treated is available,
the best way to determine lime dose and sludge quantity is by
performing jar tests.
o
For the example of the 3,785 m /day (1 mgd) hypothetical
plant of Knight, Mondoux, and Hambley (40), assuming that 125 mg/£
of Ca(OH)2 is required to increase the pH to 9.5, and that chemi-
cal precipitates are produced at the rate of 2.0 kg/kg of Ca(OH)2
added, then the total weight of chemical precipitates for the
plant would be 945 kg/day (2082 Ib/day). This is roughly equal
to the amount-of sludge solids produced without chemical addition.
Aluminum Salts--
Phosphorus is precipitated by aluminum ions to form alumi-
num - Al2(S04)s . H20. A 60 percent excess over the stoichio-
metric ratio of aluminum to phosphorus is necessary because alum
neutralizes alkalinity to lower pH. The excess aluminum is pre-
cipitated essentially as aluminum hydroxide (Al (OH)3).
In removing 80 percent of the phosphorus from a 3,785 m3/day
(1 mgd) plant, the alum added will produce 119 kg (262 Ib) of
A1P04 and 45 kg (100 Ib) of A1(OH)3 for a total weight of chemi-
cal solids of 164 kg/day (363 Ib/day).
Iron Salts--
Phosphorus is precipitated by ferric ions to form FeP04- The
reaction is very similar to aluminum precipitation, with a simi-
lar excess .of ferric ions needed over the stoichiometric Fe/P
16
-------
ratio. The excess ferric is precipitated as ferric hydroxide,
Fe(OH)3. Ferrous iron may also be used. If primary removal is
practiced, using ferrous iron and a base to raise the pH, phos-
phorus precipitates as Feo(P04)?. However, the stoichiometric
ratio is 1.5 for ferrous insteaa of 1.0 with ferric. If secon-
dary removal is practiced, using ferrous iron in the absence of
a base, the plant must have some modification of the activated
sludge process. In this situation, the ferrous ion, Fe(II), is
converted to the ferric state, Fe(III). Phosphorus precipitates
as FeP04 and excess iron precipitates as ferric hydroxide. In
removing 80 percent of the phosphorus from a 3,785 m3/day plant,
ferric chloride added in 60 percent excess will produce 147 kg
(324 Ib) of ferric phosphate and 63 kg (138 Ib) of ferric hydrox-
ide for a total weight of chemical solids of 210 kg (462 Ib).
Summary--
'Adding together the weights calculated for sewage and chemi-
cal solids gives the comparison shown in Table 6. The weight
of additional SS removed by chemical addition is not included.
The table shows the percentage of total plant solids produced at
each treatment stage (primary, secondary, and tertiary). For
instance, it indicates that the chemical solids produced during
lime addition make up over 50 percent of the total plant solids.
The dry weight of the total plant solids produced with lime addi-
tion is approximately twice that produced with either aluminum
or iron. This is true whether the chemical is added at the pri-
mary or tertiary stage. The weight of total plant solids produced
with either aluminum or iron is approximately equal.
The data presented in Table 6 were used to calculate the
theoretical weights of solids generated per kg of phosphorus
removed, shown in Table 7. These theoretical quantities will
be compared later in this section with values for the weights
of sludge per kg of phosphorus removed obtained from the litera-
ture and the treatment plant survey. The literature and survey
data are based on actual plant operation and include the addi-
tional weight of SS removed during chemical treatment.
Sludge Generation Quantities as Reported in the Literature
Recently there have been many studies reported in the liter-
ature dealing with the effectiveness of chemical addition for
phosphorus removal. Unfortunately, since wastewater effluent
quality was the major concern of many of these studies, quantita-
tive determinations of sludge genreation rates were lacking. There-
fore the sludge generation values presented here, because they
represent only one or two information sources, should not be con-
strued as average sludge generation values. These data should
merely be used to augment the sludge generation data from the
treatment plant survey found later in this section.
17
-------
CO
TABLE 6. TOTAL DRY WEIGHT OF SUSPENDED SOLIDS AND CHEMICAL SOLIDS REMOVED
DURING TREATMENT PROCESSES AT A HYPOTHETICAL ACTIVATED SLUDGE PLANT*
SS Removed
Duri ng
Primary
Treatment
SS Removed
During
Secondary
Treatment
SS Removed
During
Tertiary
Treatment
Chemi cal
Solids
Produced
Total
Plant
Sludge
Solids
kg/day
(lb/day)
+
r
kg/day
(lb/day)
%
kg/day
(lb/day)
%
kg/day
(lb/day)
'%
kg/day
(Ib/day)
%
No Chemical
Addition
473
(1041)
55.5
378
(833)
44.5
—
—
851
(1874)
100
Primary Chemical
Addition
Lime
709
(1562)
39.5
142
(312)
7.9
—
9~45
(2082)
52.6
1796
(3956)
100
Al urn
709
(1562)
69.8
142
(312)
14.0
—
164
(362)
16.2
1015
(2236)
100
Iron
709
(1562)
66.8
142
(312)
13.4
—
210
(462)
19.8
1015
(2336)
100
Tertiary Chemical
Addition
Lime
473
(1041)
25.6
378
(833)
20.5
47
(104)
2.6
945
(2082)
51.3
1843
(4060)
100
Alum
473
(1041)
44.5
378
(833)
35.6
47
(104)
4.4
164
(362)
15.5
1062
(2340)
100
Iron
473
(1041)
42.7
378
(833)
34.1
47
(104)
4.2
210
(462)
19.0
1108
(2440)
100
*Weight of additional SS removed during chemical treatment not included.
tPercent of total sludge produced in plant.
-------
TABLE 7. THEORETICAL KILOGRAMS OF SOLIDS GENERATED
PER KILOGRAM OF PHOSPHORUS REMOVED AT A
HYPOTHETICAL ACTIVATED SLUDGE PLANT*1"
Primary Chemical Addition
kg sludge SS/kg P removed
Lime
Aluminum
Iron
59
33
35
Tertiary Chemical Addition
Lime
Aluminum
Iron
61
35
37
* Derived from Table 6
t Weight of additional SS removed during chemical treatment
not included.
19
-------
Lime--
Primary addition of 200 mg/£ of Ca(OH)2 in Ontario, Canada,
to achieve an 80 percent reduction in total phosphorus (10.3 to
2.0 mg/l) resulted in an increase in sludge generation from 0.17
kg/m3 (141 Ib/MG) to 0.49 kg/m3 (4082 Ib/MG) (73). The latter
generation rate equals 59 kg of sludge per kg of phosphorus
removed. Another primary plant mentioned in the same report
showed a sludge increase from 0.16 kg/m3 (1316 Ib/MG) to only
0.23 kg/m3 (1933 Ib/Mg) when adding 125 mg/a hydrated lime to
achieve 80 percent phosphorus removal. A theoretical discussion
of phosphorus removal reports 400 mg/a Ca(OH)o added to reduce
effluent phosphorus from 11.5 to 0.3 mg/a will produce 0.75 kg/m3
(6290 Ib/MG) or approximately 68 kg of sludge per kg of phosphorus
removed (19).
Aluminum Salts--
Primary addition of 150 mg/a aluminum sulfate (alum) at the
Barrie, Ontario, wastewater treatment plant to achieve 88 percent
phosphorus removal resulted in sludge generation of 0.3 kg/m3
(2428 Ib/MG) or 14 kg of sludge generated per kg of phosphorus
removed. Secondary alum addition at the Barrie plant of 75 and
100 mg/a generated sludge at 0.39 and 0.31 kg/m3 (3275 and 2591
Ib/MG) respectively. At one Canadian treatment plant, the pri-
mary addition of 150 mg/a alum caused a negligible sludge increase
from 0.24 to 0.25 kg/m3 (2033 to 2049 Ib/MG); at a second Canadian
plant, primary addition of 150 mg/a alum caused a sludge quantity
increase of 55 percent from 0.16 to 0.24 kg/m3 (1316 to 2033 Ib/MG)
(73).
Iron Salts--
A Canadian activated sludge plant adding 37.5 mg/a ferric
chloride to achieve over 95 percent phosphorus removal, experi-
enced an increase in sludge production from 6,200 m3/mil m3 to
7,600'm3/mil m3 with primary addition of FeCls and 7,682 m3/mil m3
with secondary addition of FeCls (2). Primary addition of 20 mg/£
FeCls at Sarnia, Ontario, wastewater treatment plant to reduce
the effluent phosphorus concentration by 80 percent (5.71 to
1.16 mg/a) , resulted in a sludge production of 0.2 kg/m3 (1,643
Ib/MG), or 43 kg of sludge per kg of phosphorus removed (30). The
previously mentioned EPA Technology Transfer Publication predicts
that an 80 mg/£ FeCls dosage used to reduce a phosphorus concen-
tration from 11.5 to 0.3 mg/a will produce 0.32 kg/m3 (2,662 1b/
MG) or 28.5 kg of sludge per kg of phosphorus removed (19).
Sludge Generation Quantities as Reported by the Treatment
Plant Survey
Approximately 100 plants responding to the questionnaire sur-
vey provided information pertinent to sludge volumes and/or weight.
Often only fragmentary data or crude estimates were provided. In
addition, it is difficult to compare sludge generation by dif-
ferent treatment plants because such individual factors as raw
20
-------
sewage characteristics, plant design, etc., may have a profound
effect and distort the results. Nevertheless, the responses were
analyzed to look for confirmation of the theoretical and litera-
ture information presented in the previous subsections. The ques-
tionnaire data are displayed in the form of bar graphs in Figures
2 through 5. The bars indicate average values based on the
responses of several treatment plants. Each bar displays a dif-
ferent type of information. Therefore, the value for each bar is
generally the average of data from a different set of plants than
was the basis for any other bar. This explains some of the incon-
sistencies in the results.
Figure 2 summarizes the reported sludge volume generated per
unit mass of phosphorus removed. The numbers refer to sludge
volume before thickening. The following trends are apparent:
• Lime addition generates roughly twice the volume of sludge
as does alum or iron addition
t Chemical additions to the secondary treatment process
generate substantially more sludge volume than chemical
additions to the primary treatment process. However, if
the secondary sludge is recycled back to the primary tank,
the overall plant sludge volume is not appreciably more
than if the chemicals were initially added to the primary
treatment process. In other words, there appears to be
a definite disadvantage, in terms of total raw sludge
volume generated, to handling the secondary sludge sepa-
rately. Presumably, the secondary sludge thickens in
the primary tank.
Figure 3 summarizes the reported volume of sludge generated
per unit volume of plant influent. As in Figure 2, the numbers
refer to sludge volume before thickening. The trends found in
Figure 3 are similar to the Figure 2 results. In addition, suf-
ficient questionnaire data was available to compare increases in
sludge volume resulting from the chemical addition with sludge
volumes generated prior to the chemical addition. Increases in
sludge volume resulting from the addition of iron, aluminum, or
lime to the primary clarifiers were:
• 25 percent for iron salt additions
« 58 percent for aluminum salt addition
• huge for lime addition.
As previously stated, the results shown are averages of the
values reported by many plants and should not be used for pre-
dicting actual sludge volume at a specific plant since many other
plant-specific variables affect sludge volume. It should also
be noted that the huge lime sludge volumes reported are greatly
decreased by sludge thickening.
21
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-------
Figure 4 summarizes the reported mass of sludge generated
per unit mass of phosphorus removed. The results were inconclu-
sive in defining differences in sludge weight generated by the
three common types of chemicals, i.e., lime, aluminum salts, and
iron salts.
Table 7 shows the calculated weights of solids generated at
a hypothetical activated sludge plant with addition of various
chemicals. The effect of additional sludge generation due to
improved plant efficiency is not included. These sludge weights
are compared in Table 8 with the averages from Figure 4. It can
be seen that the average rates of sludge mass generation based on
the questionnaire survey are higher than the theoretical quanti-
ties calculated. This is a reasonable finding because the survey
data reflect the total increase in sludge mass due to phosphorus
removal, including the effect of improved plant efficiency.
TABLE 8. COMPARISON OF THEORETICAL SOLIDS GENERATION RATES
WITH RESULTS OF QUESTIONNAIRE SURVEY
Chemical Added
kg of dry soli
Theoretical *
Lime 60
Aluminum 34
Iron 36
ds/kg of P removed
Questionnaire"1"
70
35 - 75
40
* Derived from Table 7.
t Taken from Figure 4.
Figure 5 summarizes the reported weight of sludge generated
per m3 of sewage treated. Of particular interest on this figure
are the increases in sludge weight reported after chemical addi-
tion was implemented. The average weight of the total plant
sludge increased 54 percent with iron addition, 18 percent with
alum addition. Overall, the reported sludge weights were on the
high side of the ranges expected from the theoretical calculations
presented earlier in this section.
SOLIDS CONCENTRATION AND PERCENT VOLATILE SOLIDS OF CHEMICAL
SLUDGES GENERATED
In addition to changes in the amounts of sludges generated,
chemical addition for phosphorus removal also has.an effect on
sludge physical characteristics. The major impacts on sludge
26
-------
characteristics, no matter which chemical is added for phosphorus
removal, are on the sludge total and volatile solids concentra-
tions. Table 9, which is based on the questionnaire survey of
plants practicing phosphorus removal , shows the solids charac-
teristics of the sludges before thickening or other processing.
The sludges produced by iron primary addition can be seen to have
had the greatest average TS concentrations. Somewhat less,con-
centrated were the sludges produced by iron tertiary and alum
primary addition. The average TS concentrations of the iron ter-
tiary addition and alum primary addition sludges were similar.
The least concentrated sludges were those produced by iron and
alum addition to the secondary step. The average TS concentra-
tions of the iron secondary addition and alum secondary addition
sludges were similar. The table indicates that for all of the
iron and alum sludges, the average sludge TS concentration tended
to be greater when the chemical sludge was combined with the other
sludges produced in the plant.
The sludge VS fraction (as a percentage of the TS concentra-
tion) appears to have been highest when alum was added to the
secondary step. Alum sludges had higher VS fractions than iron
sludges; and sludges produced by secondary iron or alum addition
had higher VS fractions than those produced by primary or tertiary
addition. The table shows that iron-tertiary sludge had a very
low VS fraction; but that, when it was combined with the other
sludges produced in the plant, the total plant sludge had a normal
VS fraction.
The information in the table shows that the sludges produced
with lime had low TS concentrations. This information could be
misleading, however, unless it is remembered that the sludges
were sampled before thickening. Lime sludges are readily thick-
ened to around 10 percent TS. The VS fraction of the lime-tertiary
sludge was low, even when combined with other plant sludges.
Unfortunately, no information on the volatile contents of sludges
produced by primary addition of lime was contained in the
questionnaire.
27
-------
TABLE 9. SOLIDS CHARACTERISTICS OF CHEMICAL SLUDGES
WITH AND WITHOUT COMBINATION WITH OTHER PLANT SLUDGES
Type of chemical sludge
and whether combined with
other plant sludge(s)
Iron addition to primary step -
Primary sludge
Total plant sludge
Iron addition to secondary step -
Secondary sludge
Total plant sludge
Iron addition to tertiary step -
ro Tertiary sludge
00 Total plant sludge
Alum addition to primary step -
Primary sludge
Total plant sludge
Alum addition to secondary step -
Secondary sludge
Total plant sludge
Lime addition to primary step -
Primary sludge
Total plant sludge
Lime addition to tertiary step -
Tertiary sludge
Total plant sludge
Sludge characteristics (before thickening, digestion, etc.)
Total solids (%)
Volatile solids (% of TS)
Range Average
3.4 - 8.0 5.
2.31-10.0 5.
0.2 - 4.0 0.
0.5 - 7.75 4.
4.0 4.
4.64- 5.0 4.
3.3 - 4.35 3.
3.96- 5.0 4.
0.4 - 4.4 1.
1.0 - 7.0 3.
0.7 - 1.5 1.
0.64- 0.82 0.
2.5 - 4.0 3.
1.95 1.
26
73
93
13
0
82
95
49
41
82
1
73
3
95
Range
45-69
40-70
50-70
42-72
35
62
61-67
46-70
60-78
52-70
N/A
N/A
11-30
39
Average
57
57
62
62
35
62
65
59
67
59
N/A
N/A
21
39
Note: N/A = not available.
-------
SECTION 4
PREVALENCE OF VARIOUS TREATMENT AND
DISPOSAL METHODS FOR CHEMICAL SLUDGES
INTRODUCTION
The sludges resulting from chemical addition for phosphorus
removal at municipal wastewater treatment facilities generally
require treatment before final disposal. The many unit processes
used for handling and disposal of these chemical sludges can be
segregated into the following categories: sludge thickening; sta-
bilization/reduction; conditioning/stabilization; dewatering;
heat drying; reduction; and final disposal.
The prevalence of certain unit processes within the above-
mentioned categories, as determined from the treatment plant
questionnaire survey will be discussed on the following pages.
This narrative should be reviewed in conjunction with the infor-
mation on Table 10.
SLUDGE THICKENING
Gravity thickening was the most prevalent thickening tech-
nique for all types of chemical sludges, and was practiced by 85
percent of the plants with a thickening step. Flotation thicken-.
ing was used for thickening of waste activated sludges or occa-
sionally for combined primary sludge and waste activated sludge.
This was in accordance with the common application of air flota-
tion to the separate thickening of waste-activated sludges. The
sludges that were flotation thickened were produced with secon-
dary addition of iron or aluminum, except in one case of primary
addition of iron. Interestingly, centrifuge thickening was prac-
ticed at six plants using lime, but was not applied to the thick-
ening of iron or aluminum sludges.
SLUDGE STABILIZATION/REDUCTION
Anaerobic and aerobic digestion were the most prevalent
sludge stabilization/reduction processes. Among plants using
iron salts, anaerobic digestion was used considerably more fre-
quently than was aerobic digestion. In contrast, aerobic diges-
tion was used almost as often as was anaerobic digestion among
plants using aluminum salts. Anaerobic digestion of a lime sludge
29
-------
TABLE 10. PREVALENCE OF TREATMENT AND DISPOSAL PROCESSES FOR CHEMICAL SLUDGES
AMONG PLANTS RESPONDING TO QUESTIONNAIRE SURVEY *
SI udge
Treatment and
Disposal Unit
Processes
Major Chemical Used and Point of Addition
Prim-
ary
Iron Salt
Second-
ary
Terti-
ary
Aluminum Salt
Prim- Second- Terti-
ary ary ary
Prim-
ary
Lime
Second-
ary
Terti-
ary
Total
CO
o
Thickening
Gravity
Flotation
Centrifuge
Stabilization/
Reduction
Composting
Aerobic Dig.
Anaerobic Dig.
18
1
0
1
4
27
25
4
0
0
11
37
2
0
0
0
1
1
4
0
0
1
1
5
18
5
0
0
24
25
1
0
0
0
0
1
8
0
5
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
76
1Q
5
2
41
97
Lime
Stabilization
Conditioning/
Stabilization
Chemical
Conditioning
Elutriation
11 12 1 2 12 0 20 0 40
1200000003
Thermal
Conditioning
0 11
(continued)
-------
TABLE 10 (continued)
SI udae
Treatment and
Disposal Unit
Processes
Prim-
ary
Major
Iron Salt
Second-
ary
Chemical Used and Point of Addition
Terti-
ary
Aluminum Salt
Prim- Second- Terti-
ary ary ary
Prim-
ary
Lime
Second-
ary
Terti-
ary
Total
co
Dewatering
Pressure Filter
Air Drying Beds
Centrifuge
Vacuum Filter
Horizontal Moving
Screen Concen-
trator
Cylindrical
Rotating Gravity
Filter
Lagoon
Heat Drying
Flash Dryer
Multiple Hearth
Dryer
1
17
0
12
0
3
20
0
13
0
0
0
0
1
0
1
2
1
1
0
1
18
4
10
2
0
0
0
0
0
2
1
0
0
0
0
0
0
0
0
1
0
1
0
1
8
58
6
37
3
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
16
1
2
(continued)
-------
TABLE 10 (Continued)
on j
Sludge
Treatment and
Disposal Unit
Processes
^^W-A^^BVW^M^^^W
Prim-
ary
Iron Salt
Second-
ary
^^^^^^^^^^^^^^^^^^^^^^ta^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^«*^WMV-WII^^^^^^WW^^^^^^^^^^^^^^»«
Major Chemical Used and Point of Addi
Terti -
ary
Aluminum Salt
Prim- Second- Terti-
ary ary ary
Prim-
ary
tion
Lime
Second-
ary
^B^^^^_^^^HBVM^i^HHH_Hlk
Terti-
ary
^^^^^^^^•W^V^MM^^BB
Total
CO
Reduction
Incineration 7
Final Disposal
Agricultural 18
Fields, Lawns,
Gardens
Land Reclamation
Sanitary Landfill 16
Private- or 8
Authori ty-owned
Dump- site
5
39
1
18
10
0
1
0
1
0
0
7
1
1
2
5
30
6
13
8
Q
1
0
0
0
5
5
1
4
2
Q
Q
0
0
0
0
1
0
0
1
22
102
9
53
31
One plant may use more than one method of thickening, dewatering, etc.
-------
was occurring at one plant. Three plants reported having compost-
ing operations involving iron or aluminum sludges. Chemical sta-
bilization by lime was practiced infrequently.
SLUDGE CONDITIONING/STABILIZATION
Chemical conditioning was the most prevalent conditioning
method and was used for conditioning all types of chemical sludges
prior to dewatering. Thermal conditioning was in use at eleven
plants which had iron or aluminum sludges. Elutriation was also
practiced by three plants with iron sludges.
SLUDGE DEWATERING
Dewatering of chemical sludges on air drying beds was more
common than other dewatering methods, including vacuum filtration,
which followed it in frequency. Both pressure filters and cen-
trifuges were also receiving application at a number of plants,
pressure filters being used for iron, aluminum and lime sludges,
and centrifuges for aluminum and lime sludges only. Horizontal
and cylindrical screens were infrequently- used. Sludge lagoons
were common, but often they were not the sole dewatering method
used at a plant.
SLUDGE HEAT DRYING
Heat drying by either flash or multiple hearth dryers was
practiced in only three plants.
SLUDGE REDUCTION
Incineration with multiple hearth or fluidized bed inciner-
ators was practiced at 22 plants. Most of these plants handled
iron sludges, but both aluminum and lime sludges were also incin-
erated.
SLUDGE FINAL DISPOSAL
The two major final disposal sites for chemical sludges were
agriculture fields, lawns, or gardens; and sanitary landfills or
dumps. A sanitary landfill is an engineered operation involving
the spreading of refuse on land in thin layers which are com-
pacted and covered with earth each day. In contrast, a dump is
a state-approved site where sludge is spread or buried on land
(not to be later removed) and is not as closely controlled as a
sanitary landfill; neither does it receive refuse.
33
-------
SECTION 5
THICKENING OF CHEMICAL SLUDGES
GRAVITY THICKENING
Introduction
As previously shown in Table 10, a total of 76 (44 percent)
of the plants responding to the questionnaire survey maintained
gravity thickeners. Among the plants which reported having a
thickening step, 85 percent utilized gravity thickening. Gravity
thickening is simple and is the least expensive of the available
thickening processes used for sludge concentration prior to
digestion and/or dewatering. The process allows blending of
sludges and sludge flow rate equalization, thereby improving the
uniformity of feed to the subsequent processes.
Questionnaire Survey
Nine plants which were surveyed indicated that chemical addi-
tion was having a significant impact on their gravity thickening
process. The impacts were attributed to both increased sludge
quantities and changed sludge characteristics and ranged from
improvements in thickening to adverse impacts. The experiences
of five of these plants are summarized in Table 11. Four other
plants were case study sites and will be discussed further on.
Sludge settleabi1ity was a problem at two of the three
plants using iron; the third plant experienced improved settle-
ability during iron addition. The two plants using lime found
that the sludge thickened readily to 7 to 10 percent TS. Insuf-
ficient thickener capacity for the additional sludge was a con-
cern at one of these plants, however.
Case Studies
The case studies in the appendices contain detailed infor-
mation on the effects of chemical addition on gravity thickening.
The particular case studies which deal with this subject are
Sheboygan, Wisconsin; Port Huron, Michigan; Lakewood, Ohio; and
South Bend, Indiana. The experiences at these plants are similar
to the experiences of plants responding to the questionnaire sur-
vey. However, some unusual highlights of the case studies can
be mentioned.
34
-------
TABLE 11. IMPACTS OF CHEMICAL SLUDGE UPON GRAVITY THICKENING PERFORMANCE
AS REPORTED IN QUESTIONNAIRE RESPONSE
Location
Size
m3/day
(mgd)
Type of Raw
Sludge Treated
Impacts of Chemical Sludge
CO
Mil ford,
Michigan
Wayne Co.-
Wyandotte,
Michigan
Ford Lake
WWTP,
Ypsilanti,
Michigan
Virginia,
Minnesota
Hatfield Twp.-
Colmar,
Pennsylvania
3,000 Iron-secondary and
(0.8) • primary from a con-
ventional activated
sludge plant
291,400 Iron-primary and
(77) secondary from a
pure oxygen activat-
ed sludge plant
22,700 Iron-secondary and
(6) primary from a step
aeration activated
sludge plant
8,700 Lime-primary and
(2.3) secondary from a con-
ventional waste acti-
vated sludge plant
8,700 Lime-primary,
(2.3) aluminum-tertiary and
secondary from a
complete mix acti-
vated sludge plant
The sludge thickens better and is forming more
supernatant. There is more room for sludge in the
thickeners, which are operated as holding tanks.
The sludge is harder to thicken.
(fluffier).
It is lighter
It has been very difficult to handle the extra
sludge generated by phosphorus removal because the
thickener bulks back to the aeration tanks before
wasting is finished.
The plant converted from anaerobic digestion and
drying beds to gravity thickening, lime stabiliza-
tion and lagoons in anticipation of phosphorus re-
moval with lime. The thickener produces an 8 to
10 percent TS sludge.
The sludge thickens to 7.0 percent TS. Although
sludge settleability is fine, the additional sludge
is overloading the thickener. The thickener over-
flow contains 8,400 mg/jn, SS and 3,500 mg/Jl BOD. The
superintendent feels that the high solids content of
the overflow is not hurting the plant because the
associated aluminum mixes with the raw sewage and
lime and may improve primary clarification.
-------
At the Sheboygan, Wisconsin, case study site, primary and
secondary sludges were fed to a single thickening unit and gravity
thickened to an average TS'cOncentration of 8.6 percent before
chemical addition for phosphorus removal began. After secondary
addition of ferric chloride was incorporated into the treatment
system, the sludge was gravity thickened to 7.76 percent TS on
the average. The thickened sludge solids concentration was lower
after phosphorus removal was started despite an increase in the
average concentration of the primary sludge fed to the thickener.
Problems with floating of the sludge blanket and resulting high
solids overflow were experienced in thickener operation both
before and after phosphorus removal, but were more frequent after
phosphorus removal. The problems with high floating sludge tended
to coincide with industrial waste discharges. A means of reducing
thickener solids overflow was found which involved adding polymer
separately to the primary and secondary sludge feed lines to
achieve the same concentration of polymer in the sludge in each
line. In other words, the amount of polymer added to each line
was proportional to the sludge flow. The polymer was added
through a plastic hose to a point immediately before the discharge
end of the feed lines into the thickener center well. The usual
dosage was 3 ppm. During the first 4 months after proportional
polymer addition was started, the thickener overflow TS concen-
tration averaged only 154 mg/a, compared to previous concentra-
tions as high as 2,000 mg/a TS.
At Port Huron, Michigan, aluminum sulfate was added to the
activated sludge basins for phosphorus removal. The plant used
a Dorr-Oliver "Densludge" type SD gravity thickener. Waste acti-
vated sludge was pumped to a box ahead of the thickener where it
was combined with primary sludge and polymer. On the average,
the raw sludge thickened from 0.56 to 4.68 percent total solids,
and the overflow contained approximately 2,480 mg/£ SS. However,
due to variations in the activated sludge wasting rate and inter-
mittent incinerator operation, performance of the gravity thickener
was very inconsistent.
The thickened sludge concentration reached almost 6 percent
on a few good days; only about 4 percent on poor days. The over-
flow solids concentration varied greatly from day to day between
about 100 mg/£ and 10,000 mg/£. These variations were related to
the addition of alum to the aeration basins. Alum addition
increased the mass of activated sludge which was generated and
wasted. This waste activated sludge had poorer thickening charac-
teristics than the primary sludge, so that when activated sludge
wasting rates were high, thickening was poorer. Low thickened
sludge solids concentrations led to higher polymer requirements
for chemical conditioning.
Recently at Port Huron, modifications in thickener opera-
tion have been made with significant results. Two identical grav-
ity thickeners are used rather than just one. In the past, the
sludge blanket depth approached 2.1 m (7 ft) in the single 3.4
36
-------
m- (10 ft-) deep thickener. Polymer addition was necessary to
aid solids capture and thickening. With two thickeners, the
sludge blanket depth is maintained between 1.2 and 1.5 m (4 and
5 ft), and no polymer is used.
The savings in polymer cost amounts to $2.75/t ($2.50/ton)
of dry solids. It is also reported that a reduction in the
amount of phosphorus recycled to the head of the aeration basins
in the thickener overflow has occurred. This may mean a slight
decrease in the cost of phosphorus removal chemicals.
The Lakewood, Ohio, plant treats aluminum waste activated
and primary sludges which are combined in gravity thickeners.
The thickener overflow is returned to the head of the plant.
Lakewood is an older facility whose effluent quality is impaired
due to hydraulic overloading and inadequate solids handling capa-
city. Alum addition adversely affected gravity thickener opera-
tion as the result of the generation of additional sludge solids.
The mass of waste activated sludge pumped to the thickeners
increased by 4.3 t/mil rrr of (360 Ib/MG) of wastewater treated.
These additional solids increased the overloading of the thick-
eners, and problems with bulking and poor overflow quality became
more severe. The thickener overflow quality deteriorated to the
extent that the overflow contributed a higher loading of SS to
the primary clarifiers than did the plant influent. The plant
alleviated the problem by removing sludge from the thickeners at
a faster rate by increasing the capacity for digestion, dewater-
ing, and final disposal. Inadequate capacity in these areas had
been causing a bottleneck in the solids-handling system.
At South Bend, Indiana, lime was added to two tertiary up-
flow clarifiers for phosphorus removal. The volume of lime
sludge generated was 1,158 m3/day (306,000 gal/day). The average
sludge TS concentration was 4.0 percent. Gravity thickening
reduced the sludge volume to 454 nr/day (120,000 gal/day) and con-
centrated the sludge to 10.2 percent TS.
Literature
Accounts of gravity thickening of chemical sludges presented
in the literature are summarized in the following paragraphs.
An EPA pilot plant using lime for phosphorus removal and
lime recovery by recalcination found that the recycling of the
lime markedly improved the thickening properties of the sludge
(5). With lime recycling, the solids were reduced to 26 percent
of their original volume after one hour of settling. Without
lime recycling the same volume reduction required approximately
5 hours. Typically the gravity thickener produced sludges of 15
to 20 percent solids for either sludge type.
For a 15 percent solids thickener underflow concentration,
the improved settling characteristics increased the thickener
37
-------
2
capacity from a loading rate of 92 kg/m /day without lime recy-
cling to 1,470 mg/m^/day with recycling. Since the increase in
thickener capacity was much greater than the actual increase in
solids loading, less thickener area was required during lime
recycling.
The Holland, Michigan, wastewater treatment plant adds lime
to the primary clarifiers for phosphorus removal (43). Excess
activated sludge from the final clarifiers is pumped to the thick-
ener and combined with the primary sludge. An adequate supply of
dilution water is mixed with the activated sludge fed to the
thickener. The dilution water, clarified secondary effluent,
serves two purposes. It contains dissolved oxygen which helps
keep the thickener sludge from becoming septic. It also lowers
the solids concentration in the feed to a level where the parti-
cles are not hindered in settling. The thickener has produced
an underflow solids concentration of 7.4 to 17 percent TS.
To improve thickener operation at Holland, the following
modifications were made: The thickener was provided with vertical
pickets to help release supernatant from the sludge in compres-
sion; additional vertical mixing bars were attached to aid this
process; and there was a modification to the rakes to prevent
furrowina the sludge blanket.
The centrate from the sludge centrifuges was originally
returned to the sludge blanket in the thickener. The return of
centrate to this blanket disrupted both the thickening and clari-
fication process. The centrate lines were re-routed to the ash
thickener. The overflow from the ash thickener was returned to
the plant influent.
An EPA study by the Wastewater Research Division of the
Municipal Environmental Research Laboratory (76) examined the
thickening characteristics of an aerobically digested aluminum
sludge with and without polymer conditioning. Aerobically
digested waste activated sludge grom a 2 to 3 mgd plant was
thickened in a 2-a graduated cylinder. The results were poor:
At initial sludge TS concentrations of greater than 1 percent,
virtually no thickening occurred. With an initial sludge TS
concentration of 0.86 percent, thickening to 1.62 percent was
possible. Polymer conditioning improved thickening somewhat,
but the final sludge TS concentration was still below 3 percent.
Significant thickening occurred only at an initial TS concentra-
tion of less than 1 percent both with and without polymer con-
ditioning.
- At Portage Lake, Michigan, gravity thickening of an aerobi-
cally digested alum sludge was measured in laboratory cylinders (2)
38
-------
(See page 64 for a description of the Portage Lake plant.) Waste
activated sludge produced alternatively with and without alum
addition to the plant's aeration basins was digested in full-scale
aerobic digesters. The laboratory tests showed that the sludge
produced with alum addition thickened more rapidly and to a higher
solids concentration than the sludge produced without alum addi-
tion.
Conclusions
The impacts of chemical addition for phosphorus removal on
sludge gravity thickening are attributable to both increased
sludge quantities and changed sludge characteristics.
Lime addition for phosphorus generally improved the thicken-
ing characteristics of the sludge. Gravity thickened lime
sludges typically have an average TS concentration of 10 percent,
but the approximate range is 7 to 20 percent. In contrast, sludge
thickening characteristics may be either favorably or adversely
affected by the addition of iron or aluminum salts. At each
plant, the impact will depend on the influent wastewater charac-
teristics, the type of wastewater treatment, the relative propor-
tions of primary and secondary sludges, and the sludge pumping
procedures.
Thickener overloading is a frequent result of the increased
amounts of sludge generated with any of the phosphorus removal
chemicals. Even slight overloading causes serious problems
because of the sensitivity of gravity thickeners to the critical
control variables of loading rate and sludge blanket depth. In
most plants where problems have occurred, however, thickener
capacity was already insufficient before phosphorus removal
began.
The information which has been presented suggests several
ways of modifying plant operation to overcome some of the adverse
impacts of chemical sludges on thickening. For instance, adding
lime or a polymer to the sludge as it enters the thickener can
be helpful. At Sheboygan, Wisconsin, it was found that the suc-
cess of polymer addition depended upon finding the right point
of addition and dosage rate through a considerable amount of
experimentation.
At several of the case study sites, return sidestreams were
suspected of disrupting thickener operation. Recycle streams
such as digester supernatant and centrifuge centrate can carry
heavy loads of difficult-to-settle solids which have a negative
impact on sludge settleability. Treating these sidestreams or
rerouting them to a different point in the plant may indirectly
cause an improvement in thickener operation.
39
-------
FLOTATION THICKENING
Introduction
Flotation thickening is becoming an increasingly popular
method of sludge thickening prior to further handling. Among the
variables that affect flotation thickening performance are:
Sludge feed solids concentration
Sludge detention period
Type and quality of sludge
Solids and hydraulic loading rates
Use of chemical aids.
As a general rule, the higher the solids loading rate, the
lower the solids concentration of the thickened sludge. When
thickening a mixture of primary and waste activated sludge, a
higher solids loading rate is allowed and a thicker sludge is
produced than when thickening waste activated sludge alone. Higher
loading rates and thicker sludges also result from chemical aids
(usually cationic polyelectrolytes), which increase the capture
of solids, improving the quality of the subnatant.
Chemical addition for phosphorus removal can affect process
performance by changing the relative amounts of primary and waste
activated sludge generated in a plant, by altering feed sludge
solids concentrations, or by modifying feed sludge quality. Chem-
ical addition can also affect process economics by changing the
relative amounts of primary and waste activated sludges generated.
On an economic basis, flotation thickening is considered to be
most applicable to waste activated sludges, and is generally used
when a plant has no primary sludge or when separate gravity thick-
ening of primary sludge is provided.
Questionnaire Survey
Used by 6 percent of the plants responding to the question-
naire survey, flotation was shown to be the second most common
method of chemical sludge thickening. Unlike the more common
gravity thickening, which is applied to both trickling filter and
waste activated sludges, flotation is applied only to waste acti-
vated sludge. Nine plants which responded to the questionnaire
survey commented upon the performance of their flotation units
and the impacts of the chemical sludges. Table 12 summarizes
these comments, indicating the type of sludge treated at each
plant and the type of sludge treatment system.
All but one of the plants treated iron or aluminum waste
activated sludges. These sludges were produced by secondary
addition of the chemical and were not mixed with primary sludge
before thickening. The other plant was an exception because it
practiced primary chemical addition and it thickened a mixture
40
-------
TABLE 12. IMPACTS OF CHEMICAL SLUDGES UPON FLOTATION THICKENER PERFORMANCE
AS REPORTED .IN QUESTIONNAIRE RESPONSE
Plant Location
Type of Sludge
Treated
Impacts of Chemical
Sludge
Performance with Chemical
Sludge
Further Treatment
Warren,
Michigan
Zilwaukee,
Michigan
MeHenry,
Illinois
Alum-secondary
(aeroblcally
digested)
Iron-secondary
(aerobically
digested) from
a plant with
no primary
treatment
Iron-primary
and secondary
(aerobically
digested)
Although the feed sludge solids
concentration remained the same,
It was necessary to reduce the
hydraulic loading rate by 10
or 20 percent. No change in
polymer dosage was necessary.
The plant has always treated
iron sludge and, therefore,
comparisons cannot be made.
The SVI of the chemical sludge
is only 60 to 70 compared to a
high SVI before iron addition
The proportion of waste acti-
vated sludge 1n the thichener
feed has increased. Before Iron
addition, cationic polymer was
used as a thickening aid and
the sludge was easily concen-
trated to 7 to 8 percent TS.
With the iron sludge it has
been necessary to use both
anionic and cationic polymers
and the thickened sludge only
reaches 5 to 6 percent TS.
Sludge at 1 percent TS is
thickened to 5 percent.
The sludge from the aerobic
digester averages only 1.5
percent TS. It was thick-
ened by flotation to only
2.5 percent TS. Flotation
was discontinued because it
could not deliver sludge
fast enough for the vacuum
filters. Polymer as a
thickening aid was tried
and it lowered the SS con-
centration of the subnatant
from 50 to 12 mg/fc, but had
no other effect.
Blending with primary sludge;
Polymer conditioning;
Vacuum filtration;
Incineration
Polymer conditioning;
Vacuum filtration;
Sanitary landfill
Polymer conditioning;
Vacuum filtration;
Croplands
(continued)
-------
TABLE 12 (continued)
Plant Location
Type of Sludge
Treated
Impacts of Chemical
Sludge
Performance with Chemical
SIudge
Further Treatment
Gurnee,
Illionis
Iron-secondary No impacts were noticed.
Oak Creek,
Wisconsin
Iron-secondary No impacts were noticed.
ro
Willoughby-
Eastlake,
Ohio
Alum-secondary No impact;were noticed.
Middleburg
Hts., Ohio
Alum-secondary
(aerobically
digested) from
plant with no
primary treat-
ment
The average thickened sludge
TS concentration was 2.5
percent before alum addition
and since has been 3.3 per-
cent TS.
The feed sludge at 8500
ppm SS is thickened to 4.8
to 5.1 percent TS. The
subnatant contains 70 to
80 mg/Ji SS, Polymer is
used to aid solids capture.
Sludge at 1.0 to 1.5 per-
cent TS is thickened to
3.5 to 4.0 percent. The
subnatant SS concentration
ranges under 500 ppm.
Polymer is used when a
high loading rate is needed
to keep up with a high
activated sludge wasting
rate.
The sludge can be concen-
trated to 4 percent, or as
high as 7 percent. At the
higher concentration, the
sludge loading rate is
slower and the subnatant
contains more solids. The
subnatant SS concentration
varies from 52 to 3000
mg/n. Polymer to aid thick-
ening is not used.
The subnatant SS concen-
tration is averaging 300
mg/4 SS.
Combined with primary sludge;
Lime and FeClj conditioning;
Vacuum filtration; Sanitary
landfill
Combined with primary sludge;
Anaerobic digestion; Drying
lagoons; Croplands or sanitary
landfill
Combined with primary sludge;
Lime and Fed? conditioning;
Vacuum filtration; Incinera-
tion or dumpsite
Lime and FeClg conditioning;
Vacuum filtration; Incinera-
tion
(continued)
-------
TABLE 12 (continued)
Type of Sludge
Plant Location Treated
Impacts of Chemical
Sludae
Performance with Chemical
Sludge
Further Treatment
Kenosha,
Wisconsin
Frankenmuth,
Michigan
Iron-secondary No impacts were noticed
CO
Primary sludge
is anaerobically
digested then
sent to acti-
vated sludge
aeration basins.
The aluminum-
secondary sludge
is flotation
thickened
The plant has tried iron for
phosphorus removal but feels
that the sludge is easier to
thicken and.dewater when
sodium aluminate is used.
Sludge feed at 1 percent TS
is thickened to 4 percent
TS. No polymer is used to
aid thickening because it
seems to provide no signi-
ficant improvement.
Feed sludge at 0.6 percent
SS can be concentrated to
3.5 percent TS, or as high
as 4.0 percent. At the
higher concentration the
loading rate is slower,
however. Thickening is
easiest when the sludge
density index of waste
activated sludge is kept
above one. A small concen-
tration of polymer is used
(.011 kg/kg of solids) to
help the sludge fall off the
skimmers neatly, but it does
not increase the sludge
solids concentration. The
subnatant averages 209 mg/j.
SS and 100 mg/fc BOD.
Combined with primary sludge;
Anaerobic digestion; Lime
and FeCl3 conditioning;
Pressure filtration; Crop-
lands
Lime and FeClq conditioning;
Vacuum filtration; Sanitary
landfill
-------
of primary and secondary sludges. Aerobic digestion of the sludge
before thickening was practiced by half of the plants listed in
the table, while fewer anaerobically digested after thickening.
All nine plants had a dewatering step, most using vacuum filters,
but one using a pressure filter and another using drying lagoons.
Four of the plants reported a significant change in flota-
tion unit performance related to the chemical sludges, while four
reported no change at all. A ninth plant could not comment on
the impact of chemical addition, but described the performance of
their flotation unit with a chemical sludge. Among the four
plants that noted significant changes, one reported a positive
impact: an increase in the thickened sludge TS concentration.
The other three plants reported negative impacts: the need to
reduce the sludge hydraulic loading rate, the need to use both
anionic and cationic polymers as thickening aids, and a decrease
in the thickened sludge TS concentration.
Some of the plants reported that the addition of a polymer
to aid thickening was not necessary; others found that polymer
addition increased the allowable sludge loading rate or improved
the subnatant quality. One plant noted that thickener perform-
ance was aided by running the bottom flights of the unit contin-
ually instead of just when the bottom pumps were on. Two plants
related good performance to either a high sludge volume or sludge
density index.
Li terature
A theoretical discussion of sludge generation rates by
Knight, et al. (40), indicated that the waste activated sludge
portion of the total sludge produced in a plant is changed when
a phosphorus removal chemical is added ahead of the primary clar-
ifiers. Theoretically, in a conventional activated sludge plant,
44.5 percent of the total sludge mass is waste activated; in the
same plant with primary chemical addition, only 8 to 14 percent
of the total sludge mass is waste activated. This is because of
the increased SS and BOD removals that occur during primary
treatment when chemicals are added. According to the authors,
because only 18 to 14 percent of the sludge is waste activated,
separate flotation thickening of waste activated sludge is less
likely to be economical in a plant with primary chemical addition
than in a conventional plant or one with secondary or tertiary
addition. The advantages of flotation thickening would be dimin-
ished with such a small percentage of waste activated sludge.
j
At Portage Lake, Michigan, the flotation thickening capacity
of a,n aerobically digested alum sludge was measured in batch
laboratory-scale tests (2). (See page 64 for a discussion of
the Portage Lake plant.) Waste activated sludge produced alter-
natively with and without alum addition to the plantl:s aeration
44
-------
basins was digested in full-scale aerobic digesters. The labora-
tory-scale tests indicated no difference between the thickening
properties of the sludges produced.
Cone!us ions
In some .cases, chemical sludges may flotation thicken just
as well as, or better than, regular sludges. However, in other
cases, chemical sludges require a lower sludge loading rate or
different polymers, and lower concentrations of thickened sludge
solids are achieved. Little information is available for drawing
further conclusions about the flotation thickening properties of
various chemical sludges. It is necessary that further informa-
tion be collected, however, in order to make judgements about the
cost effectiveness of flotation thickening of chemical sludges.
In particular, information on allowable solids loading rates with
chemical sludges is needed. When the cost effectiveness of flo-
tation thickening is compared to that of gravity thickening, a
higher sludge loading rate is frequently one of the most impor-
tant factors in favor of flotation.
It is also necessary to determine allowable loading rates
for chemical sludges so that design engineers can size equipment
properly. It seems that presently, equipment is often undersized.
At some of the plants listed in Table 12, a low sludge through-
put rate was a problem. The plants were having trouble achieving
a high rate that would avoid backlogs of sludge in the clarifiers,
and that would provide thickened sludge fast enough for subsequent
dewatering operations. The plants also preferred a high rate so
that they could run their sludge handling equipment for only one
shift per day.
45
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SECTION 6
STABILIZATION OF CHEMICAL SLUDGES
ANAEROBIC DIGESTION
Introduction
As previously shown in Table 10, 97 (56 percent) of the
plants responding to the questionnaire survey reported that they
have anaerobic digestion. As with other unit processes, it is
necessary to evaluate the anaerobic digestion process as a com-
ponent of the entire sludge treatment/disposal system. For exam-
ple: In order to properly compare the costs of anaerobic diges-
tion of regular vs. chemical sludges, differences in dewatering,
disposal, and supernatant treatment costs must be analyzed for
the total sludge management system.
Questionnaire Survey
Twenty-one plants which were surveyed indicated that chemical
addition was having a significant impact on their anaerobic diges-
tion process. Table 13 summarizes the experiences of those
plants. All but one of the plants were treating iron and alum
sludges produced by primary or secondary addition of the chemical.
A single plant treated a lime sludge. At this plant there were
reportedly no adverse impacts of this type of sludge on anaerobic
digestion, but a slight increase in digester pH was noted.
As the table shows, the most common impact of chemical addi-
tion at the plants was a sudden increase in raw sludge volume and
mass. A number of plants also noted changes in the settling
characteristics of the sludge. The common results of these
impacts were:
t Increased energy requirements for sludge mixing, pumping
and heating
• Difficulty in achieving adequate digester mixing and
heating
• Increased labor requirements for sludge pumping
• Increased digester gas production
46
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TABLE 13. IMPACTS OF CHEMICAL SLUDGES UPON ANAEROBIC DIGESTER PERFORMANCE
AS REPORTED IN QUESTIONNAIRE RESPONSE
Location
Richardson, TX
Size-
m3/day
(mgd)
6,720
(1.8)
Type of
Sludge Treated
•*V^VMV^M^H^^^^^^VM*-MW»4laiM4W«M*M
Alum-secondary and
primary from a
trickling filter
plant
Ashland, WI
4,542
(1.2)
Alum-secondary and
primary from a
step-aeration ac-
tivated sludge
plant
Impacts of Chemical Sludge
The raw sludge mass increased
by about .033 kg/m3 (275 lb/
MG). The volume of digested
sludge increased by about 50
percent. In the past, when
alum was added before the pri-
mary clarifiers, the solids in
the digester units stratified
and upset the digestion pro-
cess. When the point of addi-
tion was moved to the second-
ary stage, no digestion
problems were observed.
The plant began secondary
treatment and vacuum fi.ltra-
tion at the same time that
phosphorus removal was begun.
At the time, of only primary
treatment, digester super-
natant characteristics were
good. With secondary treat-
ment and phosphorus removal,
however, solids-liquid sepa-
ration in the secondary di-
gesters no longer occurred.
Adding polymer to the
digester did not solve the
problem.
Performance with
Chemical Sludge
m*f^^f-^—~~~~t*—f, mmmmmm m !•
The digested
sludge averages
5 percent TS dis-
posal by vacuum
filtration and
incineration.
Digested sludge
vacuum filtered
and disposed of
in a landfill.
(continued)
-------
TABLE 13 (continued)
Location
Parry Sound,
Ontario
Size-
m3/day
(mgd)
3,220
(0.85)
Type of
Sludge Treated
Iron-primary from
a plant with no
secondary treat-
ment; gravity
thickening before
digestion
Impacts of Chemical Sludge
The sludge is more difficult
to digest. It is necessary
to add 113 kg/mo (250 Ib/mo)
of lime to increase the
alkalinity in the digester.
Performance with
Chemical Sludge
Digested sludge
averages 8 per-
cent TS. Sludge
dried on drying
beds and dis-
posed of in a
landfill.
Cedarburg, WI
00
Fergus,
Ontario
Three Rivers, MI
4,542 Alum-secondary
(1.2) and primary from
a conventional
activated sludge
plant; gravity
thickening be-
fore digestion
2,270 Iron-secondary
(0.6) and primary from
a conventional
activated sludge
plant
4,730 Alum-primary or
(1.25) lime primary and
secondary from a
conventional acti-
vated sludge plant
The raw sludge volume in-
creased. The pH in the
digester was raised to 8.2.
The volume of digested
sludge increased, raising
disposal costs.
Foaming in the digesters
occurred. The volume of
digested sludge was in-
creased, raising disposal
costs.
With alum, increased raw sludge
volume and mass. Poorer solids-
liquid separation in secondary
digester; therefore, less super-
natant can be removed. With
lime, the huge volume of sludge
generated plugged all the lines
and filled the digesters, so it
was discontinued.
Digested sludge
is applied to
drying beds or
lagoons, or
hauled directly
to croplands.
Digested sludge
at 5 percent TS
is applied to
croplands.
Supernatant
contains 2.5
percent TS and
1,920 mg/X, BOD.
Digested sludge
TS concentration
is 5.3 percent.
(continued)
-------
TABLE 13 (continued)
Location
Size-
m3/day
(mgd)
Type of
Sludge Treated
Impacts of Chemical Sludge
Performance with
Chemical Sludge
Watt's Creek
STP, Shirley's
Bay, Ontario
18,620 Alum-secondary and
(4.92) primary from a con-
ventional activated
sludge plant
vo
The raw sludge volume in-
creased 65 percent or more
as the percent solids de-
creased from 3.8 to 3.0
percent TS. The sludge mass
increased by about 1,770
kg/day (3,900 lb/day). The
retention time in the di-
gesters was reduced, re-
sulting in digester failure.
Supernatant TS concentration
increased. Raw sludge vola-
tile content decreased from
about 70 to 64 percent.
Digested sludge TS decreased
from 5 or 6 percent to. 3
percent with an increase in
volatile matter from 50 to
55 percent. The digester
temperature is hard to main-
tain due to insufficient
heat exchanger capacity with
the increase in sludge
volume. Additional labor
costs for raw and digested
sludge pumping are estimated
at $32,500/yr. Several experi-
mental changes in operating
procedures are planned: use of
iron for phosphorus removal
with primary addition to
Supernatant con-
tains 2.5 percent
TS.
(continued)
-------
TABLE 13 (continued)
Location
Size-
nrVday
(mgd)
Type of
Sludge Treated
Performance with
Impacts of Chemical Sludge Chemical Sludge
Watt's Creek
STP, Shirley's
Bay, Ontario
(cont'd)
Gladstone, MI
2,839
(0.75)
en
O
Silver Bay, MN
2,190
(0.58)
Alum-secondary
and primary from
a bio-surf
treatment plant
Alum-secondary
and primary
(gravity thickened
from a trickling
filter plant)
hopefully produce a more
compact sludge; settling of
supernatant in a holding
tank; liming of primary
sludge to improve settle-
ability; operation as a con-
tact stabilization plant.
Increased volume and mass of
sludge requires additional
labor and electricity for
pumping. Additional elec-
tricity is also required for
mixing and natural gas for
heating. Labor costs in-
creased by $ll,400/yr (2080
labor hours). Cost of'elec-
tricity for mixing sludge in
digester increased by
$14.60/day (400 kwh/day).
The sludge does not settle as
well in the secondary di-
gester. A heavier sludge
was obtained by returning the
secondary sludge to the pri-
mary clarifiers.
Solids-liquid separation in
the single-stage digester did
not occur after alum addition.
Both the raw and
digested sludge
average 4.0 per-
cent solids. The
raw sludge is
65 percent
volatile.
(continued)
-------
TABLE 13 (continued)
Location
Size-
m3/day
(mgd)
Type of
Sludge Treated
Impacts of Chemical Sludge
Performance with
Chemical Sludge
Ford Lake WWTP,
Ypsilanti, MI
Duffin Creek WPCP,
Whitby, Ontario
Wei land, Ontario
22,700 Iron-secondary and
(6.0) primary from a step-
aeration activated
sludge plant;
gravity thickening
before digestion
11,350 Lime-primary and
(3.0) iron-secondary from
a step-aeration
activated sludge
plant
34,800 Alum-secondary and
(9.2) primary from a step-
aeration activated
sludge plant
Increased sludge volume is
crowding the digester.
Digestion seems to be poorer
because of decreased volatile
fraction.
No adverse impacts on an-
aerobic digestion. Digester
pH has increased a small
amount. No additional costs
were incurred.
Both raw sludge volume and
mass increased. The TS con-
centration of the raw sludge
decreased from 5.89 to 4.89
percent. More energy was
required for digester heating
and sludge pumping. Sludge
settling in the secondary
digester appeared slower.
Pumping costs increased by
$6,000/yr for labor and
$12,000/yr for electricity in
1976. The cost of additional
natural gas for digester
heating was $2,000 in 1976.
The volume and mass of di-
gested sludge increased,
raising disposal costs.
Raw sludge before
thickening is 2.5
percent TS and 70
percent volatile.
Digested sludge
is 10 percent TS.
Digested sludge
TS concentration
is 5.7 percent.
The combined raw
sludge averages
4.89 percent TS
and the digested
sludge averages
10 percent TS.
Sludge disposal
is by trucking
liquid to crop-
lands.
(continued)
-------
TABLE 13 (continued)
Location
Size-
m3/day
(mgd)
Type of
Sludge Treated
Impacts of Chemical Sludge
Performance with
Chemical Sludge
Niagara Falls,
Ontario
31i600
(8.3)
tn
Weyauwega, WI
961
(0.25)
Iron-primary from
a plant with no
secondary treatment
Iron-secondary
from a conven-
tional activated
sludge plant with
no primary treat-
ment. Gravity
thickening before
digestion
Raw sludge volume increased
from 53 to 76 m3/day (14,000
to 20,000 gpd) as TS concen-
tration dropped from 9 to 7
percent. Sludge mass increased
by about 454 kg (1,000 lb)/day.
Digester gas production in-
creased. The digester gas is
utilized for digester and
building heating, so natural
gas consumption was reduced.
Additional energy was required
to pump sludge to and from the
digesters and through the heat
exchanges.
The plant operators estimate
by visual observation that the
volume of raw sludge has
doubled. Digestion is now
more difficult. There is lit-
tle or no gas production.
Dewatering of the sludge by
vacuum filter is more difficult.
More ferric chloride and lime
are used for conditioning.
Digested sludge
TS concentra-
tion averages
12 percent.
Digested sludge
pumped to lagoons
and later trucked
to croplands.
The digested
sludge is 5 to
6 percent TS.
It is chemically
conditioned,
vacuum filtered
to 18 to 20
percent TS and
trucked to
croplands.
(continued)
-------
TABLE 13 (continued)
Location
Size-
nr/day
(mgd)
Type of
Sludge Treated
Impacts of Chemical Sludge
Performance with
Chemical Sludge
Ottawa, Ontario
276,000
(73)
Ul
Belleville,
Ontario
29,900
(7-9)
Port Dal nousie
PCP, St.
Catherines,
Ontario
28,600
(7.55)
Alum-primary from
a plant with no
secondary treat-
ment
Iron-secondary or
alum-secondary and
primary from a
conventional acti-
vated sludge plant
Iron-secondary and
primary from a con-
ventional activated
sludge plant
Raw sludge mass increased
from 4,100 to 17,700 kg/day
(9,000 to 39,000 Ib/day) as
TS decreased from 4.8 to
4.35 percent. Raw sludge
volume increased by about
90 percent. Liquid-solids
separation in digester is
more difficult. Increased
digester gas production
experienced.
When iron was tried for
3 months, the digester
became upset; the pH dropped
to 4; volatile acids pro-,
duction increased; gas pro-
duction fell. Presently,
with alum there are no
problems. There was a
slight increase in sludge
production.
A 30 percent increase in
sludge volume resulted in
an increased cost for
natural gas to heat
digesters.
Digester super-
natant is 1.92
percent TS. Raw
sludge feed is
67.2 percent
volatile.
Digested sludge
is 3.5 percent
TS.
Raw alum sludge
is 3.9 percent
TS, 70 percent
volatile.
Digested alum
sludge is 4.3
percent TS, 59
percent volatile.
Supernatant
contains 1 to 2
percent TS.
Raw sludge is
4.1 percent TS,
62.6 percent
volatile.
Digested sludge
is 4.3 percent
TS.
(continued)
-------
TABLE 13 (continued)
Location
Size-
m3/day
(mgd)
Type of
Sludge Treated
Impacts of Chemical Sludge
Performance with
Chemical Sludge
Port Weller PCP,
St. Catherines,
Ontario
Sarnia, Ontario
en
South Lyon, MI
Lakeview WPCP,
Mississauga,
Ontario
33,700
(8.91)
30,300
(8.0)
2,157
(.57)
159,000
(42)
Alum-secondary and
primary from a con-
ventional activated
sludge plant
Iron-primary from
a plant with no
secondary treat-
ment
Iron-secondary and
primary from a con-
ventional activated
sludge plant
Iron-secondary and
primary from a
step aeration acti-
vated sludge plant
A 26 percent increase in
sludge volume resulted in
an increased cost for
natural gas to heat digesters.
Raw sludge volume increased
from 83 to 113 m3/day (22,000
to 30,000 gpd). Sludge TS
concentration decreased from
5.5 percent to 4.0 percent TS.
Sludge mass increased by 36
kg/day (80 Ib/day). Digester
gas production increased from
1,820 to 2,240 m3/day (65,000
to 80,000 ft3/day). No ad-
verse effects on anaerobic
digestion were observed.
It is necessary to add polymer
to the digester to aid
settling.
Digester pH and alkalinity
were lowered. It is some-
times necessary to add lime
to the digester.
Raw sludge is
59.6 percent
volatile.
Raw sludge is
7 percent TS, 62
percent volatile.
Digested sludge
is 6 percent TS.
The supernatant
TS concentration
is 1.8 percent
and the BOD con-
centration is
6,000 to 10,000
-------
t Poor solids-liquid separation and high supernatant solids
concentrations.
Less common results were:
• Poor digestion, accompanied by moderate decreases in
digester pH, decreases in volatile matter destruction, or
decreases in gas production
• Digester upsets, with symptoms such as the absence of gas
production, low pH, low volatile destruction, and increased
production of volatile acids
t Primary digester stratification
t Foaming in digesters.
Adverse impacts on anaerobic digestion were reported both by
plants using iron and plants using alum and with both primary and
secondary addition of the chemical. There was one case in which
a plant experienced problems when using iron, but observed no
significant adverse impacts after switching to alum. Another
plant found that problems disappeared when the point of addition
of alum was switched from the primary stage to the secondary stage.
Some plants achieved good thickening of sludge in digesters while
others achieved none at all.
Several plants reported average TS concentrations of their
digested sludge. These ranged from 3.5 to 12 percent, with the
average value being 6.7 percent.
The reduced volatile solids fraction of the raw sludge was
felt to be significantly affecting digestion at two plants. Other
plants did not report any problems resulting from the reduced VS
fraction. The values for average VS fraction which were reported
ranged from 59.6 to 70 percent of TS with the average value being
65 percent of TS. In many cases the VS loading was increased
when the plant treated chemical sludge, and thus resulted in
increased gas production per cubic meter of digester space.
Case Studies
The case studies in the appendices contain detailed informa-
tion on the effects of chemical addition on anaerobic digestion.
The particular case studies which deal with this subject are
Lakewood, Ohio; Coldwater, Michigan; South Bend, Indiana; and
Pontiac, Michigan. The experiences at those plants are similar
to the experiences of plants responding to the questionnaire sur-
vey. However, some unusual highlights of the case studies can be
mentioned.
55
-------
At Lakewood, Ohio, anaerobic digester supernatant quantity
was increased greatly when the digesters were overloaded with
alum sludge. The excess supernatant was generated, not because a
greater sludge volume was being fed to the digesters, but because
sludge was removed from the digesters at a slower rate due to
reduced vacuum filter and flash dryer capacity. The supernatant
became very high in total solids. Because of the recirculation
of this sidestream to the head of the plant, there were adverse
impacts on the performance of both primary and secondary treat-
ment and gravity thickener operation. By finding ways to pump
more sludge out of the digester, the volume and solids concentra-
tion of the supernatant was decreased. Longer operation of the
vacuum filter and flash dryer was one method used. Hauling liquid
sludge was found to be a more successful method. Eventually, how-
ever, it was concluded that it would be necessary to hire a con-
tractor to clean out the secondary digesters, removing the grit
and other heavy materials which were reducing the effective volume
of the digesters.
At Pontiac, Michigan, an increased volume of raw sludge was
pumped to the digesters at a lower average solids concentration
during phosphorus removal with ferric chloride and polymer. The
net effect was an increase in the mass of primary sludge pumped
to the digesters per pound of SS in the plant influent. There
was an increased volume of supernatant, but at a considerably
lower average solids concentration, indicative of better sludge
sett!eabi1ity within the digesters. The chemical sludge had no
negative effects on digestion. The plant manager felt that pri-
mary addition of the chemical had several benefits for plant oper-
ation which would not be realized with secondary addition: The
primary sludge settled well and was easily pumped. The removal
of additional solids during primary clarification because of
increased clarifier efficiency with ferric and polymer use actually
reduced the load on the aeration system. At times the additional
primary sludge seemed to be balanced by reduced volume of waste
activated sludge generated because of reduced sludge growth in
the aeration basins. The increased efficiency of the primary
clarifiers was very helpful in containing (and recycling) solids
from the digester supernatant. If these solids were not contained
they would result in the degradation of secondary treatment. The
supernatant volume was always large because of inadequate sludge
disposal resulting from down time on the vacuum filters. The two
vacuum filters were inadequate from the start because of frequent
shut-downs due to lime-scale problems. The filter media is acid
cleaned once per week, but the internal piping must still be taken
apart and cleaned on an annual basis. Polymer conditioning of the
sludge was tried but hasn't been as effective as conditioning with
1 ime.
At Coldwater, Michigan, the TS concentration of the sludge
entering the digesters has increased since the start-up of iron
56
-------
and polymer addition for phosphorus removal. Most of the addi-
tional solids produced and fed to the digesters were non-volatile.
The volatile fraction of the sludge decreased. But the mass of VS
fed to the primary digester has increased from 590 kg VS/day
(1,300 Ib VS/day) before phosphorus removal to 720 kg VS/day
(1,590 Ib VS/day) during phosphorus removal. The amount of diges-
ter gas produced increased from roughly 5,040 m3/mo (180,000 ft3/mo)
to 11,200 m3/mo (400,000 ft3/mo).
At South Bend, Indiana, combined primary and waste activated
sludge was gravity thickened and then digested in two-stage anaer-
obic digesters. Before being fed to the primary digesters, the
sludge was preheated. The plant accomplished phosphorus removal
by mixing ferric chloride and polymer with the wastewater in ter-
tiary upflow clarifiers. The resulting iron sludge was pumped
from the clarifiers to a chemical sludge gravity thickener.
Because of plant ,design, the thickened iron sludge could not be
preheated before it was fed to the digesters. It therefore was
capable of suppressing the temperature in the digester into which
it was fed. This resulted in a loss of digester gas production.
Since most of the gas production took place in the primary diges-
ters, the chemical sludge was fed into one of the secondary diges-
ters. The result was a 4 to 5°C loss in temperature in this
digester, but a decrease in total digester gas production was
avoided.
Literature
Accounts of anaerobic digestion of iron and aluminum sludges
available in the literature can be summarized as follows:
• The presence of iron and aluminum precipitates in digesting
sludge does not inhibit the action of the anaerobic bacteria
(22, 32, 44, 51, 63) .
0 The volume of gas produced per pound of VS introduced to
the digester is similar for conventional sludges and iron
or aluminum sludges. Gas composition is also similar (42,
51).
t During anaerobic digestion of ferric iron and aluminum
sludges, solubilization and release of precipitated phos-
phorus into the supernatant does not occur (22, 32, 44, 51,
63).
• A difference in the behavior of primary and waste activated
sludges containing phosphates removed by the addition of
ferrous iron occurs when they are digested. A significant
release of phosphorus can take place upon anaerobic diges-
tion of the waste activated sludges. In contrast, there
is an uptake of phosphorus when the primary sludges are
digested. This difference in behavior is not hard to
57
-------
explain. The ferrous iron added during primary treatment
remains in the ferrous form before and after the digestion
of primary sludge. However, the ferrous iron added to the
aeration tank is mostly oxidized to the ferric form, which
is then reduced back to ferrous upon anaerobic digestion,
causing a release of phosphorus.
• When digesting ferric iron or aluminum sludges, high con-
centrations of iron or aluminum ions do not occur in the
digester sludge or supernatant (44, 51).
Successful anaerobic digestion of a lime sludge was demon-
strated at the 9,092 m3/day (24 mgd) Newmarket, Ontario, activated
sludge plant (6, 72). The plant achieved 80 percent phosphorus
removal by adding 200 mg/a lime (as Ca(OH);?} to the raw wastewater
The addition of lime caused the mass of primary sludge TS to
increase by 0.32 kg/m3 (2,670 Ib/MG). The primary sludge contain-
ing relatively small quantities of waste activated sludge, was
treated by two-stage anaerobic digestion with ultimate disposal
on croplands. During lime addition, failure of the digestion pro-
cess was caused by the high pH (10.0 to 10.5) of the raw sludge.
Intermittent and frequent overdosing of lime was also responsible.
It was found that by letting the sludge sit in the primary
clarifiers longer the pH was lowered to 9.4 and the digesters
could operate satisfactorily. Modifications were made so that a
sludge blanket 1.5 to 2 ft (0.46 to 0.6 m) was maintained in the
clarifiers. After this modification, the digesters continued to
operate satisfactorily with the primary sludge being fed at 8 to
12 percent TS compared to 3 to 4 percent TS before chemical addi-
tion. The digester operated at a pH between 7.2 and 7.4, with
supernatant soluble phosphorus concentrations of 6 to 8 mg/i.
Digested sludge was hauled at 10 to 11 percent solids, compared
to 3 to 4 percent before lime addition, and volumes were somewhat
1 ess.
The experience at Barrie, Ontario, illustrated that decreas-
ing the dosage of the phosphorus removal chemical by 25 percent
could improve sludge characteristics for digestion (72). Dosing
at 200 mg/£ alum to the raw sewage to produce a 0.5 mg/jj, total
phosphorus concentration in the plant effluent resulted in a pri-
mary sludge concentration of 2.76 percent TS. This resulted in an
increased volume of sludge being transferred to the digester, a
decrease in digester temperature and gas production, and an
increase in volatile acids. At an alum dosage of 150 mg/£ an
effluent phosphorus concentration of 1.2 mg/£ was achieved, and
the TS concentration of the primary sludge rose to 4.25 percent.
This returned digester operation to normal.
Conclusions
Hydraulic or solids overloading of anaerobic digesters is
the main problem to result from chemical addition for phosphorus
58
-------
removal. This results in inadequate liquid retention time, poor
mixing, poor heating, and inadequate room for solids-liquid sep-
aration. Volatile destruction is then poor; gas production is
lowered; and supernatant quality deteriorates.
The easiest remedies for these situations lie in the design
stage. For instance, poor mixing is currently a major defect in
the design of anaerobic digesters treating both conventional and
chemical sludges. With chemical sludges, and especially lime
sludges, present mixing is even more likely to be inadequate.
In cases where present mixing is suspected to be inadequate, mix-
ing equipment should be operated full time, with every advantage
taken to any pipeworks and pumps which can be utilized to provide
additional circulation.
The situation is similar with regard to sludge heating in
digesters. Chemical sludges can aggravate existing heating prob-
lems. When there is a separate chemical sludge, it may be possi-
ble to avoid loss of digester temperature in the primary digester
by introducing the chemical sludge into the secondary digester.
This would prevent a suppression of digester gas production in the
primary digester where most of the gas is usually produced.
Another major problem resulting from chemical addition is
poor solids-liquid separation in the digester. Both iron and alu-
minum sludges can contain floes which do not settle readily.
Methods of improving the settling of these sludges include adding
polymer to the digester and sludge liming before digestion. In
some situations it may be possible to produce a sludge with better
settling characteristics by changing the type of phosphorus remo-
val chemical used, the chemical dosage, or the point of addition.
Also, in some cases, using a polymer in conjunction with the iron
or aluminum for phosphorus removal could improve solids-liquid
separation in the digester.
When poor supernatant is a problem as the result of digester
crowding, several steps can be taken to reduce the crowding. Raw
sludge prethickening, when possible, can reduce sludge volume,
saving digester space. Sometimes thickening can be achieved in
existing holding tanks or by allowing sludge to remain on clari-
fier bottoms longer. Also, in some cases, recirculating waste
biological sludge to the primary clarifiers will produce a thicker
combined sludge.
Faster removal of sludge from digesters is another way of
reducing digester crowding. Many plants may not be removing sludge
at a fast enough rate because of lack of disposal ability. A pop-
ular way to meet this need is with liquid sludge hauling.
When faced with a poor supernatant, a plant may choose to
avoid drawing off supernatant except at times when it is clear.
However, there is then less reduction occurring in the volume of
59
-------
sludge for disposal. In other situations, a plant which has
started phosphorus removal may be forced to remove more superna-
tant because of inadequate means of disposing of the additional
sludge which is fed to the digesters. Settling of supernatant
solids in a holding tank may then be possible to avoid the deter-
ioration of wastewater treatment performance.
Cleaning of digesters to remove grit is expensive, but it
is ultimately found to be necessary in many cases where greater
space and longer retention time in digesters are needed. It is
not uncommon to find that effective digester volume has been
reduced drastically by accumulated grit.
AEROBIC DIGESTION
Introduction
As previously shown in Table 10, 41 (24 percent) of the
plants responding to the questionnaire survey reported that they
had aerobic digestion. Aerobic digestion is mostly used by smal-
ler treatment plants and/or for treatment of waste secondary
sludges. Principal advantages cited for aerobic digestion over
anaerobic digestion are simpler operations, lower initial capital
cost, and better supernatant quality. The major disadvantage is
the much higher operating cost due to high energy consumption.
Two basic types of aerobic digestion systems exist. Single-
stage digestion is a batch-type operation where the sludge is sup-
plied with air and completely mixed for a period of time, followed
by quiescent sett!ing and decanting of supernatant, all in one tank.
Two-stage digestion incorporates a separate tank for settling and
supernatant decanting. Two-stage digestion is a continuous-feed
rather than a batch operation.
As with other unit processes, it is necessary to evaluate the
aerobic digestion process as a component of the entire sludge
treatment/disposal system. For example: In order to properly
compare the costs of the aerobic and anaerobic digestion systems,
differences in dewatering, disposal, and supernatant treatment
costs must be analyzed for the total sludge management system.
Questionnaire Survey
Four plants which were surveyed reported problems with aero-
bic digestion of chemical sludges. Table 14 summarizes the
experiences of those plants.
The most common impact of chemical addition was a sudden
increase in raw sludge volume and mass. This resulted in the
need for more secondary digester capacity, the need for an
increased air supply rate to achieve adequate mixing, and higher
disposal costs. Hauling the liquid digested sludge to croplands
60
-------
TABLE 14. IMPACTS OF CHEMICAL SLUDGES UPON AEROBIC DIGESTER PERFORMANCE
AS REPORTED IN QUESTIONNAIRE RESPONSE
Location
Size-
m3/day
(mgd)
Type of
Sludge Treated
Impacts of Chemical Sludge
Performance with
Chemical Sludge
Portland, MI
CTl
1,135
(0.3)
Merrickville, OH
3,030
(0.8)
Iron-primary and
iron-secondary
from an activated
sludge plant
Iron-secondary
from an extended
aeration acti-
vated sludge
plant without
primary treat-
ment
Total raw sludge volume in-
creased from 8 to 19 m3/day
(2,000 to 5,000 gal/day).
Sludge TS decreased from
5.0 percent to 2.4 percent.
Sludge ma-ss increased by
about 91 kg (200 lb)/day.
A new secondary digester was
constructed to handle the
additional volume. The
volume and mass of digested
sludge increased, raising
disposal costs.
The plant began operation with
phosphorus removal on-^stream.
Therefore, there is no com-
parison between digester
operation with and without
the iron sludge. However,
the following operational
problems resulted from design
errors: (1) the air lift
pumps could not remove the
heavy sludge from the digester
fast enough; (2) the mass of
sludge was greater than anti-
cipated, so the air supply
rate was greater than pre-
dicted.
Digested sludge
TS is 5.0 percent.
Supernatant con-
tains 0.2 percent
TS.
(continued)
-------
TABLE 14 (continued)
Location
Size-
nr/day
(mgd)
Type of
Sludge Treated
Impacts of Chemical Sludge
Performance with
Chemical Sludge
Addison, MI
1,510
(0.4)
en
ro
Columbia Boro,
PN
4,920
(1.3)
Alum-primary from
an aerated lagoon
plant
Alum-secondary
from a contact
stabilization
activated sludge
plant without
primary treat-
ment
The sludge volume fed to the
digester increased and the
sludge was heavier and more
difficult to pump. Mixing
the sludge in the digesters
was also more difficult so
the air supply rate had to
be raised to accomplish this.
Sludge volume decreased from
113 to 90 m3/day (30,000 to
24,000 gal/day)as TS in-
creased from 0.5 percent
to 1.25 percent. An addi-
tional 230 kg/day (500 lb/
day) dry TS is fed to di-
gester. Digester superna-
tant volume has decreased.
The volume and mass of di-
gested sludge has increased,
raising disposal costs.
The digester
feed is 5.7 rrr/day
(1,500 gal/day)
primary sludge at
3.3 percent TS.
The digested
sludge is 6 per-
cent TS. Super-
natant concen-
trations are
320 mg/& SS and
117 mg/& 8005.
The digested
sludge is 3 per-
cent to 4 per-
cent TS. Super-
natant return is
68 m3/day
(18,000 gal/day)
at less than
0.5 percent TS
containing 500
to 1,000
BOD5.
-------
was the most common disposal method used. The digested sludge
TS concentrations varied from 3 to 6 percent.
No problems with deterioration of supernatant quality were
reported as the result of either the additional sludge feed quan-
tities or the changed sludge characteristics. This is significant
because poor supernatant quality is often an indicator of diges-
ter overloading. Apparently, the capacity of the existing aerobic
digesters was usually adequate to handle the additional quantities
of«chemical sludges. One plant reported the need for additional
secondary digester capacity to handle the chemical sludge, but
did not require additional primary digester capacity.
Literature
Accounts of aerobic digestion of chemical sludges available
in the literature can be broken down into laboratory-scale studies
on the one hand and pilot- or full-scale studies on the other hand.
The results of laboratory scale studies can be summarized as fol-
1 ows:
t Aerobic digestion of waste activated sludge is not affected
to an appreciable degree, nor is it inhibited by the pre-
sence, of iron or aluminum precipitates (28).
t The release of soluble organic carbon and nutrients into
the liquid phase is not enhanced by the presence of iron
and aluminum precipitates (28).
t An aeration period of 10 to 15 days provides satisfactory
stabilization of iron- and alum- waste-activated sludges
at a temperature of 20°C (28).
• Dewatering characteristics of digested iron- and alum-waste
activated sludges are poor, especially when long aeration
periods are employed in batch treatment (28).
t With iron- and alum-waste-activated sludges, batch digester
operation results in a greater destruction of sludge vola-
tile solids and a lower sludge oxygen uptake rate than
semi-continuous operation. However, the latter method pro-
vides better sludge dewaterabi1ity and better supernatant
quality (28).
• Aerobic digestion can be successfully applied to lime-pri-
mary sTudges, and the kinetics of the digestion process
are not appreciably affected except at high lime dosages
(33).
• When treating lime-primary sludges, the digestion system
shows good buffering capacity, and the pH of the system
63
-------
is always maintained at 8.5 or above. The latter is con-
trary to the process characteristics with primary or waste-
activated sludge alone, which are a low buffering capacity
and a pH reaching as low as 4.0 (33).
t Aerobically digested lime-primary sludges have good set-
tling and dewatering characteristics (33).
It should be kept in mind that the above conclusions were
based on laboratory research. An experimental study of full-scale
plant operation was conducted at the Portage Lake, Michigan,
wastewater treatment plant (2). The plant influent flow of 6,400
m3/day (1.7 mgd) was split after passing through the aerated grit
chamber. The rest of the plant was divided into two identical
halves, each side consisting of a contract stabilization activated
sludge system and an aerobic sludge digester. The two 765-m3
(201,990-gal) digesters were single-stage, batch-type operation
units. During the study, one half of the plant was fed an alum
dosage of 84 mg/£ just ahead of the contact basin, while the
other half did not receive chemical feed.
Alum addition increased the aerobic digester sludge feed vol-
ume by 16 percent and the dry weight by 50 percent. It increased
the digested sludge volume by 37 percent and the dry weight by 92
percent. However, the test was carried out in the summer, and it
was projected that under average annual conditions there would be
only a 20 percent increase in digested sludge volume and a 66 per-
cent increase in dry weight. The alum-biological sludge was le'ss
amenable to aerobic digestion than the biological sludge. The
alum-biological sludge TSS reduction observed through aerobic
digestion under the summer conditions of the study was relatively
low, on the order of 12 percent. It was predicted that the aver-
age annual rate would be even less (2).
The aerobically digested sludge gravity thickening capacity,
as measured in laboratory cylinders, was higher for the alum-bio-
logical sludge than for the biological sludge. Batch laboratory-
scale flotation thickening tests indicated no difference between
the thickening properties of the two digested sludges. The alum
sludge was concentrated by flotation thickening from 0.75 to 3.5
percent SS without polymer addition at an air to solids ratio of
0.03. Under these apparently optimal conditions, the underflow
SS concentration was 88 mg/& SS. It is likely that addition of
cationic polymer could have improved the performance (2).
Laboratory vacuum filtration tests conducted by the Buchner
funnel method indicated that the alum-biological sludge filtered
slightly better than the biological sludge. Both sludges were
conditioned with ferric chloride. Vacuum filter leaf tests indi-
cated that the conditioned alum-biological sludge could be dewa-
tered from 1.6 percent SS to a cake concentration of 16 percent
SS at a filtration rate of 14.6 kg/m2/hr (3.0 Ib SS/hr/sq ft)
(2).
64
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An evaluation of the feasibility of sludge dewatering by
centrifugation was conducted using a 305-mm (12-in) diameter pilot-
scale basket centrifuge. Several runs were performed at various
feed rates and feed solids concentrations. After application of
the manufacturer's scale-up procedure, the results indicated that
a full size 1.22-m (48-in) diameter basket centrifuge could dewa-
ter the digested alum-biological sludge at 1.6 percent feed SS
to a solid cake containing 16 percent TS at a rate of 158.9 kg
dry solids/hr (350 Ib/hr) with no chemical addition. The SS con-
centration ranged from 300 mg/£ to 600 mg/£, and the solids
recovery was 96 percent (2).
Because of the good thickening characteristics of the alum-
biological sludge, it was predicted that a more concentrated sludge
could be removed from the digester by converting it to a two-stage
unit. Plans were made to implement this by installing a concrete
partition in the existing digester and an air lift pump for trans-
ferring sludge from the bottom of the first stage to the second
stage. In year-round operation, it was estimated that digested
alum-biological sludge of 1.25 percent SS would be obtained with
the one-stage and 1.7 percent SS with the two-stage process (2).
Based on the study results, the costs of various alternatives
for further sludge processing and disposal were considered. All
of the alternatives assumed ultimate disposal on land at a loca-
tion about 11 km (7 mi) from the plant. The alternatives were:
direct trucking by tanker at 1.7 percent SS; centrifugation or
vacuum filtration and hauling cake at 16 percent TS; and flotation
thickening and tanker hauling at 4 percent SS. Table 15 summa-
rizes the estimated costs of each alternative (2).
TABLE 15 PROJECTED COSTS OF VARIOUS SLUDGE HANDLING
ALTERNATIVES FOLLOWING AEROBIC DIGESTION AT PORTAGE LAKE,
MICHIGAN, $ (2)
Direct Basket Vacuum
Trucking Centrifuge Filter Flotation
Capital investment 79,000 143,000 137,000 202,000
Annual debt service 8,951 16,203 15,523 22,888
Annual operation 16,109 12,046 14,795 18,209
and maintenance
Total annual cost 25,060 28,249 30,318 41,097
65
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The estimated capital costs included all mechanical equip-
ment, building space, and vehicles required. Operation and main-
tenance costs included labor, power, fuel, chemicals, snow removal,
and disposal site maintenance. Aerobic digestion costs were not
included. Annual debt service was based on 7.5 percent interest
over a useful life of 15 years. All costs were intended to reflect
January 1975 levels. The projected costs showed direct trucking
to be the most economical alternative primarily because of the
relatively short haul distance. However, if the one-way haul dis-
tance were increased from 11 km to 16 km (10 mi), the centrifuga-
tion system would yield the minimum annual cost (2).
Conclusions
Aerobic digesters can be a cost-effective means of stabiliz-
ing chemical sludges when attempts are made by plant management
to find procedures yielding maximum efficiency. Often, however,
plant personnel prefer to operate them merely as aerated sludge
holding tanks. The research conducted at Portage Lake, Michigan,
(2) showed that the TSS reduction observed through aerobic diges-
tion of alum-waste activated sludge was relatively low compared
to that observed for waste activated sludge, but that the alum
sludge thickened to a higher solids concentration. The investi-
gators concluded that the chief utility of aerobic,digestion of
chemical-biological sludge may be sludge storage and thickening.
No other reports of poor digestibility of chemical sludges were
found in the literature.
The information which has been presented suggests the follow-
ing ways of modifying aerobic digester operation to achieve bet-
ter performance when treating chemical sludges:
• Converting a one-stage to a two-stage system. This is
reported to increase the digested sludge solids concentra-
tion and sludge dewaterabi1ity, while not adversely affect-
ing supernatant characteristics. This method may, however,
yield less volatile solids destruction and require a higher
oxygen supply rate.
• Installing a new secondary digester (or converting an
unused existing basin) to handle the additional chemical
sludge. It may not be necessary to install a new primary
digester for moderate increases in sTudge volume.
Although no reports of poor solids-liquid separation in aero-
bic digesters treating chemical sludges were found in the present
study, there are situations in which this problem will occur. In
this case, the addition of polymer may aid settling of the sludge
in the digester and reduce the volume for disposal. This method
could result in significant savings by reducing trucking costs
for hauling liquid sludge. At Escanaba, Michigan (35), solids-
liquid separation could not be achieved in aerobic digesters
66
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treating waste activated sludge. Addition of 25 to 30 mg/£ of
a cationic polymer resulted in sludge concentration of 1.8 to
2.2 percent TS.
COMPOSTING
Composting is being viewed with increasing interest as a
sludge management alternative. It is appealing because it con-
verts sludge into a product that is aesthetically acceptable,
essentially free of pathogens, and easy to handle. The compost
produced can be used to improve soil structure, increase its
retention of water, and provide nutrients for plant growth. How-
ever, the fact that the process may be relatively expensive (com-
pared to land application of liquid digested sludge, for example)
has prevented its widespread use in the past. Several investiga-
tions in the United States and Canada are presently underway to
determine if composting can be made economical for widespread use.
Since sludge characteristics are an important factor in
treatment costs, the economics of composting are likely to differ
for chemical sludges and regular sludges. In the preceding sec-
tions of this report, the characteristics of regular sludges and
chemical sludges resulting from phosphorus removal were compared.
In this section, information related to the effects of phosphorus
removal on sludge composting is presented. It is intended to help
answer questions about the ability of chemical sludges to dry as
expected to remain aerobic, and to support biological growth. It
will also look at the value of the chemical sludge compost as a
fertilizer and soil conditioner.
Questionnaire Results and Case Studies
As previously shown in Table 10, only two (1 percent) of the
plants responding to the questionnaire survey were composting
their chemical sludges. Both of these plants were case study
sites. At the Midland, Michigan, site, an informal sludge com-
posting operation was underway. The operation utilized vacuum
filtered sludge cake containing iron precipitates from the plant's
primary addition of iron for phosphorus removal. The sludge cake
may have been particularly suitable for composting because of the
plant's thermal conditioning step. Thermal conditioning enabled
production of a filter cake which was very dry (about 50 percent
dry TS); it pasteurized the sludge; and it eliminated the need
for using chemical conditioners such as iron or lime which would
further increase the chemical content of the sludge cake.
All of the plant's 4.6 m3/day (10.8 yd3/day) of filter cake
was transported approximately 6.5 mi to the city of Midland's
sanitary landfill site, but not all of it was composted. Some of
the sludge was used for land reclamation at the landfill site.
The remainder was informally made into compost by the city's
67
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Department of Forestry and used as soil conditioner in their on-
site ornamental tree nursery. Both sludge-leaf compost and sludge-
sawdust compost were made in piles, with a minimum of attention
and labor from the City Forester and a few employees. The compost
piles were simply turned over once every three days.
The compost greatly enriched and conditioned the native hard
clay soil at the site. The texture of the soil was greatly
improved by mixing in compost. Over part of the tree nursery, 15
to 24 cm (6 to 8 in) of sludge cake was disced directly into the
soil, while on other plots, up to 0.6 cm (2 ft) of sludge-leaf
compost was used. The sludge-leaf compost was not as rich a fer-
tilizer as the sludge disced in alone, but it may have improved
the texture of the soil more. Whether sludge-leaf compost or
sludge alone was disced in,placing a sludge-sawdust mixture (T/2
sludge - 1/2 sawdust) on top of the soil produced even richer
growth. Iron sludge composting at Midland, therefore, was both
successful and valuable to the city.
It has not been possible to assess the costs invol ved because
the operation was very informal and the quantity of sludge used
was small. It can be reasoned, however, that the costs would
have been raised as the result of iron addition for phosphorus
removal had not the initial impact of iron addition on filter
cake moisture content been overcome. The initial impact was an
increase in filter cake moisture content, meaning higher costs
for sludge cake hauling and perhaps a slower rate of composting.
Filter cake dryness was restored, however, by raising the temper-
ature of the thermal conditioning unit, thus avoiding these prob-
lems at Midland.
At Windsor, Ontario, a sludge disposal arrangement existed
between the Little River Pollution Control Plant and a commercial
composting operation. After the plant hauled the sludge 11 mi to
the composting site, it payed $2.20/t ($2.00/ton) to the commer-
cial operation to handle the sludge from there. The commercial
firm mixed the sludge with sawdust where it is dumped from the
truck in the field. The sludge then dried in the field without
being turned over. When dry, it was taken into a barn where it
was worked considerably and piled. The temperature inside the
piles reached 66° to 71°C (150° to 160°F). The operation was
monitored by the Ontario Ministry of the Environment to see that
it met health standards. The product was shredded and then
bagged. Further details of the private operation were proprie-
tary. The sludge-sawdust compost material was sold in bags for
$198/t ($180/ton) retail and $88/t ($80/ton) wholesale. Unbagged
compost was sold in bulk for $16/m3 ($12/yd3). The product was
marketed'as a soil additive and organic fertilizer primarily for
home gardening and commercial greenhouse applications.
At Little River, alum was added to the raw sewage for phos-
phorus removal. The undigested sludge was centrifuged before
68
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being trucked to the composting site. Alum addition resulted in
an increase in the average moisture content of the centrifuge
cake from 80 percent to 84 percent. This increase in moisture
content raised the costs of hauling the sludge cake, and probably
also slowed the rate of drying of the sludge in the compost piles.
The owner of the composting operation reported having no problems
as the result of the presence of aluminum precipitates in the
sludge. However, he felt that a lime sludge would make a better
product, which would cost perhaps $2.20/t ($2.00/ton) less to
make.
Presently, the cost to the plant to dispose of 9,604 t/yr
(10,589 tons/yr) of dry sludge TS from the centrifuges is esti-
mated at $3.73/t ($3.38/ton). This includes the cost of sludg-e
hauling and the fee paid to the composting operation to take the
sludge.
The City of Windsor was planning to start a composting oper-
ation at its other plant, the West Windsor Pollution Control
Plant. Sludge was to be composted in static piles with forced
ventilation. Wood chips were to be used as the bulking agent.
Preliminary tests indicated that there was no difficulty in com-
posting the plant's undigested iron-primary sludge. The cost of
composting 31,750 t/yr (35,000 tons/yr) of dry sludge TS was
estimated at $7.50/t ($6.80/ton) of dry solids, a very low esti-
mate compared to many others fqund in the literature for sewage
sludges. The estimate included no land capital cost because the
land was already owned by the city. Similarly, there were to be
no sludge hauling fees since the land was adjacent to the plant.
A total site preparation cost of $85,000 was estimated, with an
equipment cost of $114,000. Assuming a ten-year life span for
the equipment and site preparation, the total capital cost was
$20,000/yr, or $0.66/t ($0.60/ton).
The operating costs were estimated at $217,000/yr or $6.84/t
($6.20/ton) dry sludge. The value of the compost produced was
expected to be about $8.82/t ($8.00/ton) in bulk sales, making
the project very economical.
Conclusions
At at least two sites in the United States and Canada, a
valuable compost was produced from all or part of the chemical
sludge from a municipal wastewater plant with phosphorus removal.
Based on the experiences of these plants, it appears that the
presence of iron and aluminum precipitates in the sludge does not
inhibit the growth of the microorganisms which are necessary for
composting. The quality and value of the compst as a soil condi-
tioner and fertilizer can be very high for iron and aluminum
sludges. It is likely that the value of a lime sludge compost
would be even higher because of the beneficial effects of the
69
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lime on soil pH; and one compost manufacturer thought that lime
sludge compost would be cheaper to produce by the ton.
There may be some problems with composting of iron and alu-
minum sludges, however. The chemical sludges often cannot be
dewatered by vacuum filter or centrifuge to a moisture content
as low as that to which regular sludges can be dewatered. When
this is the case, the cost of hauling the sludge cake to the com-
posting site increases. In addition, a wetter sludge could be
expected to require more time for drying and composting to occur,
increasing land requirements and operating costs. More bulking
agent and/or more ventilation of the compost piles could also
be necessary to keep the wetter sludge aerobic.
70
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SECTION 7
CONDITIONING OF CHEMICAL SLUDGES
CHEMICAL CONDITIONING
Among the plants participating in this study, 40 (23 percent)
out of 174 practiced sludge conditioning with chemicals such as
lime, ferric chloride, and polyelectrolytes. The survey showed
chemical conditioning to be much more than other conditioning
methods for chemical sludges.
The purpose of chemical conditioning is to improve the per-
formance of subsequent dewatering processes. Maximum dewatering
performance requires the optimization of many more operating vari-
ables for chemical conditioning than for thermal conditioning.
The type(s) of chemical conditioner(s), the dosage(s), and the
method(s) of application must be chosen, with a wide range of
choices and combinations possible. The success of the condition-
ing process can only be measured by its effects on subsequent
dewatering, incineration, and disposal processes. Therefore,
chemical conditioning has been discussed in each of the sections
of this report dealing with dewatering and also in the section on
incineration. The reader is referred to those sections for
detailed information from the literature and from the field inves-
tigations and questionnaire survey which were part of this study.
In general, the most common impact of phosphorus removal on
chemical conditioning requirements for sludges has been the rais-
ing of the dosages required for successful dewatering. Increased
chemical dosages are undesirable not only because of the effect
on incinerator auxiliary fuel requirements: with inorganic and
inert chemical conditioners, increased chemical dosages provide a
drier cake on a total solids basis, but the cake is wetter on a
sludge (as opposed to total) solids basis. That is, the kg
water/kg dry sludge solids fed to the incinerator is higher.
Thus, more fuel is required for incineration.
As far as auxiliary fuel requirements, the polyelectrolyte
conditioners have an inherent advantage, and the inert materials
such as fly ash and sludge ash have inherent disadvantages. For
the same cake TS content, the polyelectrolyte-conditioned sludge
cake will require significantly less auxiliary fuel while the
ash-conditioned sludge cake significantly more auxiliary fuel
than the lime and ferric chloride-conditioned cake.
71
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With regard to chemical sludges, then, particular attention
should be paid to cases where polyelectrolyte conditioners have
been successful in producing dry sludge cakes and have kept down
incinerator fuel requirements. Also promising are cases where,
because of the presence of phosphorus removal chemicals in sludge,
fewer chemicals were required for conditioning. In scanning the
sections of this report on dewatering, the reader will note that
in few instances have these ideal situations occurred, but in
some cases such as pressure filtration of an iron sludge and cen-
trifugation of lime sludge, there were promising results.
THERMAL CONDITIONING
Introduction
Thermal conditioning is a technique which has been increas-
ingly used by consulting engineers in recent years. Two types of
thermal conditioning exist. The Zimpro process, often termed
low-pressure oxidation, differs from the Porteous or Farrer pro-
cesses in that air is injected into, rather than excluded from,
the process reactor. Thermal sludge conditioning has several
benefits. It stabilizes the sludge, enabling further handling
without pathogens, odors, or putrefaction. It changes the cellu-
lar structure of the sludge, enabling thickening to a relatively
high solids concentration and thus reducing the volume to be
vacuum filtered; and it improves the dewatering characteristics
of the sludge, increasing filter yield and cake solids concen-
tration. Its disadvantages are the capital and operating costs
of the process and production of a high COD decantate.
Questionnaire Survey
As previously shown in Table 10. 11 (6 percent) of the
plants responding to the questionnaire survey reported that they
have thermal conditioning. Of these 11 plants, 7 practiced anaer-
obic digestion before the thermal conditioning step. The
digested and thermally conditioned sludge from these plants was
either applied to drying beds, applied directly to croplands, or
vacuum filtered. At the other four plants which did not anaer-
obically digest, a gravity thickening step preceded thermal con-
ditioning. At these plants the thermally conditioned sludge was
either vacuum filtered or centrifuged.
Only three of the questionnaire respondents commented on the
performance of thermal conditioning at their plant or the impact
of the chemical sludge. At the Maumee River plant in Waterville,
Ohio, a contact stabilization activated sludge plant, a combined
alum-secondary and primary sludge was treated. The sludge was
gravity thickened, underwent Zimpro, Inc. low-pressure oxidation,
and was dewatered on a belt vacuum filter. The plant used polymer
for further conditioning of the sludge before dewatering. A filter
cake of 35 percent TS was achieved.
72
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At the Grand Haven, Michigan, plant, a 14,000 m3/day (3.7 mgd)
modified activated sludge plant, a combined iron-primary and waste
activated sludge was treated. The raw sludge had an average TS
concentration of 4.38 percent. The VS fraction was 65 percent of
TS. The sludge was gravity thickened, underwent Zimpro, Inc.
low-pressure oxidation, and was vacuum filtered and incinerated.
The thermally conditioned sludge had a TS concentration of 11.8
percent. Although the sludge was very difficult to dewater
because of a tannery waste received by the plant, thermal con-
ditioning was enabling a filter cake TS concentration of over 30
percent. The high TS concentration of the filter cake enabled
economical operation of the incinerator. A comparison of the
characteristics of Grand Haven's filtrate, thickener supernatant,
and thermal conditioning decantate is shown in Table 16.
The plant was looking for an alternative to thermal condition-
ing for use when the unit was down for repair. Chemical condition-
ing had been tried with more than 50 different polymers, but none
of them produced a filter cake of greater than 19 percent TS.
The North Olmsted, Ohio, plant, a contact stabilization acti-
vated sludge plant, treated combined aluminum-secondary and pri-
mary sludge. The sludge was gravity thickened, underwent Zimpro,
Inc. low-pressure oxidation, and was vacuum filtered. The plant
achieved a filter cake of 46 percent TS and reported that the
performance was not affected by the addition of sodium aluminate
for phosphorus removal. The thermal conditioner decantate con-
tained 3,000 mg/l SS and 2,631 mg/l BOD.
Case Studies
The case studies in the appendices contain detailed infor-
mation on the effects of chemical sludges on thermal condition-
ing. The particular case studies which deal with this subject
are similar to the experiences of other plants responding to the
questionnaire survey. However, unusual highlights of the case
studies can be mentioned.
At Port Huron, Michigan, the costs of thermal conditioning
using the Farrer system were compared with the costs of chemical
conditioning with polymer. The plant practiced centrifugation
and incineration of the conditioned sludge. When thermal condi-
tioning, the plant reduced its costs for conditioning polymer and
fuel oil (for incineration) by $12.63/t ($11.46/ton) and $37.71 t
($34.20/ton), respectively. The operational and maintenance
expenses involved in thermal conditioning included $20.94/t
($19.00/ton) for natural gas, $1.08/t ($0.98/ton) for electricity,
$1.76/t ($1.60/ton) for boiler water conditioning and cleaning
chemicals, $1.68/t ($1.52/ton) odor control, and $9.52/t
($8.64/ton) for sidestream treatment. The major operational and
maintenance concern, however, was the cost of supplies and labor
73
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TABLE 16. AVERAGE SIDESTREAM CHARACTERISTICS AT
GRAND HAVEN, MICHIGAN
Type of Sidestream Thickener Thermal Conditioner
Supernatant Decantate Filtrate
Volume (m3/day (gal/day)) 889 (235,000) 140(37,000) 31(8,230)
SS (mg/A) 425 2,295 2,358
VSS (% of SS) 70.3 60.8 62.6
COD (mg/A) N/A 20,900 18,267
BOD (mg/A) 173 7,737 6,755
-------
for equipment maintenance and repair. This cost was unknown,
because the unit was in use for a limited time only. The equip-
ment suffered from extreme corrosion and erosion of heat exchanger
and connecting piping, and its operation was eventually discontin-
ued to alleviate the need to replace all the parts with stainless
steel. Polymer conditioning was then relied upon. The case con-
cluded that if the unit could have been operated at an equipment
maintenance and repair cost of less than $15.35/t ($13.92/ton) ,
it would have been more cost-effective than polymer conditioning.
Without thermal conditioning, the sludge fed to the centri-
fuges was of a lower solids concentration, and more time was needed
to dewater it. The filter cake solids concentration was also
reduced, and the capacity of the incinerator was reduced by about
454 kg/hr (1,000 Ib/hr). Because of the high costs of operating
under these conditions, the plant was evaluating several alterna-
tive treatment systems.
At Midland, Michigan, ferric chloride addition was found to
have adversely affected the performance of the Zimpro, Inc. ther-
mal conditioner. The impaired performance was evidenced mainly
by the poorer vacuum filter yield and lower filter cake TS concen-
tration. It was found that excellent performance could be restored
by raising the temperature of the thermal conditioner from 185°C
(365°F) to 202°C (395°F).
i
The effect of ferric chloride addition on the thermal condi-
tioner decantate SS and BOD concentrations was unknown. During
ferric addition, the decantate was low in SS (478 mg/£) but high
in BOD (6,000 mg/£). The vacuum filter filtrate SS concentration
was high before, and even higher after, ferric addition began.
The BOD concentration of the filtrate was also high before ferric
addition began but was not further increased by ferric addition.
When alum addition for phosphorus removal was tried at Mid-
land there were poor conditioning and dewatering results even at
the higher thermal conditioner temperature.
Literature
A single literature account of thermal conditioning of chem-
ical sludges was found (26). Dewatering of the thermally condi-
tioned sludges by vacuum filtration, centrifugation, or on drying
beds was studied. Three plants using the Zimpro, Inc. low-pres-
sure oxidation system were involved. The characteristics of
these plants are described in Table 17. At the Midland and Lucas
County plants, the thermally conditioned sludges were vacuum fil-
tered, while at Defiance sludge was applied to drying beds. Some
of the Defiance sludge was anaerobically digested rather than
thermally conditioned before being applied to the drying beds.
Data from the plants were gathered which characterized the
75
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TABLE 17. CHARACTERISTICS OF PLANTS IN STUDY OF THERMAL
CONDITIONING OF CHEMICAL SLUDGES (26)
Plant Location
Type of Raw
Sludge Treated
(m3/day (mgd))
Reactor
Temperature
(°C (OF))
Reactor
Pressure
(kg/ cm2 (psi))
Further Treatment/
Disposal
Defiance, Ohio
0>
Midland, Michigan
Lucas County,
Ohio
Iron-primary and
secondary from an
activated sludge
plant
Iron-primary and
secondary from a
trickling filter
plant
Aluminum-primary
from a contact
stabilization
activated sludge
plant with no
primary treatment
171 (340)
16.52 (235)
193 (380)
182 (360)
31.63 (450)
21.09 (300)
Sludge drying beds;
dried sludge stock-
piling on land
adjacent to plant
Vacuum filters;
land disposal of
filter cake
Vacuum filters; land
disposal of filter
cake
-------
sidestreams associated with thermal conditioning, vacuum filtra-
tion, and anaerobic digestion, and are presented in Table 18. In
the case of the decantate from the three thermal conditioning
units, the TS concentrations averaged 1 percent, with a VS fraction
of 74 to 87 percent of TS. The decantates contained 0.3 to 0.6
percent organic carbon, almost all of which was soluble in each
case. These observations indicate that the decantate exerted a
high oxygen demand on the plant biological oxidation processes
when it was returned to the plant. Sidestreams from subsequent
dewatering processes would be expected also to have been high in
soluble organic carbon. The amount of phosphorus, and either iron
or aluminum, in the thermal conditioner decantate varied widely
between the plants.
The Midland thermally conditioned iron sludge was dewatered
in a pilot-scale, solid-bowl scroll centrifuge at two different
bowl speeds and loading rates. The results of this test are given
in Table 19. At the higher of the two bowl speeds and loading
rates tested, both the centrate TS concentration and the percen-
tage of feed solids recovered in the cake were greater than at
their lower rates.
Table 20 contains a comparison of the sidestream characteris-
tics from plant vacuum filter operations and pilot-scale centri-
fugation of the plant sludges. In the two comparisons made, the
vacuum filter filtrate was lower in organic carbon, phosphorus,
and iron or aluminum than the Scroll or basket centrifuge centrate.
Conclusions
Few problems have been experienced in thermal conditioner
operation as the result of chemical addition for phosphorus
removal. Methods of improving thermal conditioner operation when
treating chemical sludges may include:
t Providing sludge storage facilities or alternate disposal
methods such as liquid sludge hauling for periods when the
thermal conditioner is down for repair. This is particu-
larly important at plants practicing incineration because
chemical conditioning as a substitute may not provide a
filter cake of adequate dryness. Repair requirements for
thermal conditioners seem to be moderate to substantial.
• Raising the temperature of the unit. This may reduce
decantate SS concentrations and improve sludge dewaterabil-
ity.
There is some indication that vacuum filters may be more suit-
able for the dewatering of thermally conditioned chemical sludges
than centrifuges. However, this should be investigated further,
and filter presses should also be evaluated.
77
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TABLE 18. CHARACTERISTICS OF STREAMS AND SIDESTREAMS
ASSOCIATED WITH SLUDGE TREATMENT OPERATIONS (26)
Stream or Fe or
Sidestream Al
Lucas County
Raw sludge 2,200
Midland
Raw sludge 7,250
Lucas County
Conditioned
sludge 3,045
Midland
Conditioned
sludge 12,233
Defiance
Conditioned
sludge 5,570
Lucas County
Decantate 5
Midland
Decantate 70
Defiance
Decantate 122
Defiance
Digested
sludge 2,545
Defiance
Supernatant 635
Midland
Filter cake 73,000
Midland
Filtrate 5,200
P
mg/a
1,157
1,867
1,955
4,317
1,483
20
723
66
656
26
17,000
2,300
Fe or Al
to P ratio
2.0
3.9
1.5
2.8
3.3
0.2
0.1
1.8
3.9
2.4
4.2
2.3
TOC
mg/£
4,800
41,830
10,650
—
52,400
2,937
6,000
5,090
10,600
6,400
500,000
4,000
SOC TS
2,441 29,450
5,467 86,333
4,950 47,412
— 200,000
5,019 110,000
2,887 9,183
5,800 10,667
3,902 10,340
2,050 35,000
2,180 17,500
7,400 598,000
3,200 7,200
% vs
57
54
53
41
48
87
76
74
47
53
42
62
(continued)
78
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TABLE 18 (continued)
Stream or
Sidestream
Fe or
Al
P
mg/fc
Fe or AT
to P ratio
TOC
SOC
mg/t,
TS
VS
Lucas County
Filter cake 30,336 15,218 2.0
Lucas County
Filtrate 95 69 1.4
Lucas County
Filter cloth
wash 100 64 1.6
Lucas County
Digested
sludge 1,680 950 1.8
338,000 45
3,811 3,489 7,878 82
4,700 4,500 8,500 85
2,100 270 18,300 43
79
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TABLE 19. RESULTS OF PILOT SCALE CENTRIFUGATION OF
MIDLAND THERMALLY CONDITIONED IRON SLUDGE (26)
«o
O'
Bowl Speed, rpm
4,000
4,500
Loading Rate
.kg/hr (Ib/hr)
19.2 (42.3)
24.7 (54.4)
Cake Solids
% TS
33.3
49.0
Centrate Sol ids
% TS Polymer
4.5 no
9.6 no
Solids
Recovery, %
82.9
97.1
-------
eo
TABLE 20. COMPARISON OF SIDESTREAMS FROM PLANT AND
PILOT DEWATERING OPERATIONS (26)
Operation
Midland:
Vacuum filtration
filtrate
Scroll centrifugation
centrate
Lucas County:
Vacuum filtration
filtrate
Scroll centrifugation
centrate
Basket centrifugation
centrate
Fe or Al
to P ratio
520
640
95
458
260
P TOC SOC TS
mg/x, mg/& mg/& mg/&
230 4,000 3,200 7,200
170 5,600 4,400 9,600
69 3,811 3,489 7,878
354 6,560 4,180 14,500
200 9,800 2,000 8,000
% MS
62
64
82
68
84
-------
SECTION 8
DEWATERING OF CHEMICAL SLUDGES
DRYING BEDS
Introduction
A total of 58 (33 percent) of the plants responding to the
questionnaire survey utilized drying beds for sludge dewatering.
Historically, sludge drying beds have been mainly used by smaller
communities. Principal advantages of drying beds are their sim-
plicity and low maintenance costs. The chemical cost and operat-
ing complexity of mechanical dewatering equipment are additional
factors which favor drying beds. Disadvantages include their
large land requirements, inability to dewater effectively year-
round in certain areas, and potential odor problems.
Questionnaire Survey
Nine of the questionnaire respondents which have drying beds
reported problems with the handling of chemical sludges. All of
those plants used iron or aluminum salts for phosphorus removal.
Table 21 summarizes the experiences of-those plants. Plants
using lime did not report any impacts, either positive or nega-
tive, on drying bed operation other than the need for additional
bed space to handle the extra sludge generated.
The most common problems experienced in drying bed operation
were increases in sludge volume and mass, and poor sludge dewater-
ability. These problems resulted in: construction of additional
drying beds, addition of chemicals to improve dewatering; modifi-
cations in sludge application procedures, lengthened sludge drying
times, or the abandonment of sludge drying beds in favor of ano-
ther dewatering process. The point of chemical addition did not
seem to influence the performance of iron or aluminum sludges on
drying beds. Plants with chemical addition to primary or secon-
dary stages reported the same general problems.
Parry Sound, Ontario, a primary plant adding ferric chloride
for phosphorus remova-1 , reported rapid clogging of their sand
drying beds with solids. This problem was alleviated by replacing
the original bed sand with a coarser sand and by replacing the
bed sand more frequently.
83
-------
TABLE 21. IMPACTS OF CHEMICAL SLUDGES UPON DRYING BED PERFORMANCE
AS REPORTED IN QUESTIONNAIRE RESPONSE
Location
Size-
nr/day
(mgd)
Type of
Sludge Treated
Impacts of
Chemical Sludge
Performance with
Chemical Sludge
Addison, Michigan
Escanaba, Michigan
00
to
Rogers City, Michigan
1,480 Alum-primary (aerobi-
(0.39) cally digested) from
an aerated lagoon
plant.
7,040 Iron-primary (anaero-
(1.86) bically digested)
from an activated
sludge plant.
2,650 Iron-secondary (an-
(0.7) aerobically digested)
from a conventional
activated sludge
plant with no pri-
mary treatment.
The chemical sludge takes
longer to dewater on the
beds.
The sludge is not more
difficult to dewater, but
there is a larger volume.
A method was devised to
add polymer to the sludge
as it enters the drying
beds to increase the bed
turnover rate. Also,
additional cement strips
were added in the drying
beds to allow mechanical
sludge removal. This also
increases the bed turnover
rate.
(continued)
The sludge at
3.2 percent TS
is applied to
sand drying beds.
When applied 8
to 12 in deep,
the sludge is
ready for re-
moval by hand in
30 to 45 days.
It is best to fill
the beds to only
-------
TABLE 21 (continued)
Location
Size-
m3/day
(mgd)
Type of
Sludge Treated
Impacts of
Chemical Sludge
Performance with
Chemical Sludge
Rogers City, Michigan
(cont'd)
Flushing, Michigan
00
Charlevoix, Michigan
Coldwater, Michigan
4,580 Iron-secondary and
(1.21) primary (anaerobically
digested) from a con-
ventional activated
sludge plant.
1,100 Iron-secondary and
(0.29) primary (anaerobi-
cally digested) from
a complete mix acti-
vated sludge plant.
6,780 Iron-primary and
(1.79) secondary (anaero-
bically digested)
from a trickling
filter plant.
8 in for fastest
drying and easiest
removal.
The estimated
operation and
maintenance costs
for the drying
beds were about
$700 in 1976.
Before phosphorus
removal was begun, the
plant had only primary
sludge. The sludge
at that time took one
half as long to dewater
and was slightly less
odorous.
The sludge volume was three
times greater after phos-
phorus removal, causing the
plant to construct 7 new
drying beds. Cement bottoms
were put in both the old and
new beds. The sludge takes
30 to 50 percent longer to
dry.
(continued)
-------
TABLE 21 (continued)
Location
Size-
m3/day
(mgd)
Type of
Sludge Treated
Impacts of
Chemical Sludge
Performance with
Chemical Sludge
Parry Sound, Ontario
Ashland, Wisconsin
00
Three Rivers, Michigan
3,200 Iron-primary (anaero-
(0.85) bically digested)
from a plant with no
secondary treatment.
4,540 Alum-secondary and
(1.2) primary (anaerobically
digested) from a con-
ventional activated
sludge plant.
4,730 Alum-primary and
(1.25) secondary (anaero-
bically digested)
from a conventional
activated sludge
plant.
The sludge is finer and
more difficult to dewater.
To improve dewatering, a
more coarsely screened
sand was put in and the
sand is changed more often.
Sludge drying beds were
abandoned because the
gelatinous alum sludge
would not dewater. The
plant began using a vacuum
filter instead.
The sludge takes longer
to dry on the beds.
-------
The drying beds at Ashland, Wisconsin, an activated sludge
plant with secondary addition of aluminum sulfate, were abandoned
because of difficulties in dewatering the gelatinous alum sludge.
This plant switched to ferric chloride sludge conditioning and
vacuum filter dewatering.
Literature
Literature accounts of drying bed dewatering of chemical
sludges are few, although the recent rise of chemical addition
for phosphorus removal should change this pattern.
An excellent discussion of chemical sludge dewatering on
sand beds (48) relates sludge characteristics to sand bed design.
It concludes:
• The most critical sludge characteristic with regard to
the use of sand beds is sufficient sludge compressibility
to prevent sand bed penetration. Compressibility is the
variation of specific resistance with pressure. It influ-
enced the rate of water loss of sludges applied to sand
beds.
• Chemical sludge drainage rates depend upon the specific
resistance and applied solids concentration.
• Air drying of chemical sludges occurs in two distinct
phases. Initially, a slow drying rate occurs, followed
by a more rapid drying.
• The slow drying rate appears to be governed by the applied
depth and drained solids concentration. Rapid drying is
approximately equal to the rate of free surface water
evaporation.
The addition of a high molecular weight anionic polymer to
the drying beds at the Escanaba, Michigan, plant had a dramatic
effect on the sludge disposal operation (35). The plant's aero-
bically digested sludge failed to dewater and dry out in a rea-
sonable length of time. The sludge had a tendency to penetrate
and plug the sand bed. Polymer addition to the sludge as it
entered the beds resulted in faster drying time. Before this
operation, extensive hauling of liquid sludge for land disposal
was necessary. The sludge was flocculated by placing a simple
plywood mixing box (4 ft by 4 ft by 1.5 ft) under the bed dis-
charge valve. The box contained a baffle to provide mixing and
a notched overflow weir. Polymer was added to the sludge as it
entered the box by means of a portable tank and a small electric
pump mounted on a trailer. A used fuel tank (1.04 m3 or 275 gal)
with the top cut out made an excellent low cost tank. As the
polymer was added to the mixing box, the sludge rate was adjusted
until a large floe formed. The final concentration of polymer
86
-------
was dependent on the sludge TS concentration (Table 22). Several
polymers were tried, with a high molecular weight anionic proving
to be the most effective.
TABLE 22. POLYMER APPLICATION TO DRYING BEDS
Sludge
% Solids
1.8
2.0
2.2
Cat ionic
mq/ a
83
92
104
Anioni
mg/£
23
26
29
c
Drying time was evaluated by filling each of two beds with
56.8 m3 (15,000 gal) of the same sludge. One bed had polymer
added while the other did not. During the test period there was
almost constant daily sunshine and minimal rainfall. At the end
of 12 days, the bed with the polymer was dry enough to clean,
while the one without polymer required 34 days to dry. During
the summer, drying time varied depending on climatic conditions
and the mass of dry sludge applied to the bed. Average drying
time in ideal weather was 10 to 14 days with the polymer added.
Another benefit derived from the polymer addition was a
reduction in odor from the drying beds. The relatively high
volatile content of the digested sludge generated an offensive
odor when it remained in the liquid state for long periods.
Concl usions
Improved drying bed performance can be achieved if the fol-
lowing modifications are made:
t Improving the performance of upstream facilities, e.g.,
thickeners, digesters
• Adding chemicals to improve sludge dewatering character-
istics
t Optimizing sludge loading rates and bed turnover rates
• Changing the drying bed filter material
• Covering open beds where climatic conditions adversely
affect performance.
87
-------
VACUUM FILTRATION
Introduction
As shown in Table 10, vacuum filtration is utilized by
approximately 21 percent of the sewage treatment plants responding
to the project questionnaire. Vacuum filtration is an important
sludge treatment unit process for many plants generating phospho-
rus-laden chemical sludges. It has received substantial attention
in both the technical literature reviewed and the case studies
conducted. It was reported that vacuum filtration operational
characteristics and performance results were definitely affected
by the inclusion of chemical sludges with primary and/or secondary
sludges normally produced. The resulting changes were not in all
cases detrimental, however, and solutions to the problems encoun-
tered were often achieved by treatment plant personnel.
In the following discussion it is assumed that the reader is
familiar with the theory and mechanical operation of vacuum fil-
ters. The operator of a vacuum filter strives for maximum solids
capture, filter cake yield, and filter cake solids content, as
well as to minimize costs. Solids capture is usually expressed
as the percent of the dry weight of sludge TS retained on the
filter, i.e., that portion not returning to the treatment plant
in the filtrate. A solids capture of at least 90 percent is
desired.
Filter cake yield is expressed in terms of pounds of dry
total sludge TS discharged from the filter media per hour per
square foot of filter media. When chemicals are used for phos-
phate removal, they increase the percentage of non-volatile solids
in the sludge. When comparing filter cake yields, correction
should be made to account for this difference.
Cake solids content is expressed in terms of percent weight
of dry TS in the sludge cake. The cake solids content normally
increases with an increase in the TS concentration of the sludge
being fed to the filter.
Questionnaire Survey
Table 23 summarizes the comments of 11 plants utilizing
vacuum filtration which responded to the questionnaire survey.
Many plants reported that vacuum filtration problems resulted
from the inclusion of phosphorus-laden chemical sludges with the
biological sludges previously processed. Typically, the increased
sludge volume and solids mass stressed the capacity of the vacuum
filter. In one extreme case, a plant reported increasing their
vacuum filtration operation from 40 hr/wk up to 168 hr/wk. Doub-
ling of operational time, e.g., from one to two shifts, was fairly
common.
88
-------
TABLE 23. CHANGES IN VACUUM FILTER PERFORMANCE REPORTED AS A RESULT OF
PHOSPHORUS REMOVAL CHEMICAL SLUDGE ADDITION
Location
Type of Sludge
Changes in Vacuum Filter Performance Resulting from
Phosphorus Removal Chemical Sludge Addition
London,
Ontario
Lime-primary and secondary
(conditioned with Fed3)
from an activated sludge
plant
Filter cake solids increased from 16.8 percent to
19.3 percent TS. Filter yield increased by 30 percent.
Cost of chemical conditioning increased from $15 to
$16 per ton of dry solids.
00
Sheboygan,
Wisconsin
Iron-secondary and primary
from a trickling filter
plant
Filter yield decreased by 25 percent,
Hatfield
Township,
Pennsylvania
Lime-primary and alum-
secondary
Tremendous increase in sludge volume, required going
to a 24 hr/day, 7 day/wk operation on vacuum filter
as opposed to the former 40 hr/wk operation.
Willoughby-
Eastlake,
Ohio
Alum-secondary and primary
from an activated sludge
plant
The sludge filter cake averages about 20 percent
higher moisture content than formerly because of
the alum addition. Filter discharge characteristics
(solids capture) deteriorated. Experiments with
various filter cloths showed that a sateen weave
provided the best results. Lime and ferric chloride
sludge conditioners also impared filter operation.
(continued)
-------
TABLE 23 (continued)
Location
Type of Sludge
Changes in Vacuum Filter Performance Resulting from
Phosphorus Removal Chemical Sludge Addition
Warren,
Michigan
Alum-secondary and primary
from an activated sludge
plant.
Many other plant changes were incorporated con-
currently with the implementation of phosphorus re-
moval, making it impossible to identify specific
changes caused by the alum addition. However, it
appeared that the alum addition caused the sludge cake
solids content to drop from 20 percent down to 16
percent. In addition, the sludge conditioning
chemical was changed to polymer instead of lime and
ferric chloride. As the result of more moisture in
the sludge cake, subsequent incineration energy
(natural gas) costs increased.
Ypsilanti,
Michigan
Iron-secondary and primary
from an activated sludge
plant
The plant found it very difficult to handle the 50
percent increase in sludge generated by implementation
of phosphorus removal. The vacuum filter operation
time was increased by 50 percent, and solids increased
in the filtrate and throughout the plant.
Lakeville,
New York
Iron-tertiary and primary
and secondary from a
trickling filter plant
Changed from vacuum filtration of raw sludge to
vacuum filtration of anaerobically digested sludge.
More polymer was needed to treat the sludge.
Hilton,
New York
Alum-secondary and primary
from an activated sludge
plant
This plant operates both vacuum filters and centri-
fuges. They found ng changes due to alum addition for
phosphorus removal. The vacuum filter yielded a 20 per-
cent solids cake (sludge conditioned with lime and
ferric chloride). The centrifuge yielded a 19 percent
solids cake (polymer added).
(continued)
-------
TABLE 23 (continued)
Location
Type of Sludge
Changes in Vacuum Filter Performance Resulting from
Phosphorus Removal Chemical Sludge Addition
Lakewood,
Ohio
Alum-secondary and primary
from an activated sludge
plant
Frankenmuth,
Michigan
Alum-secondary and primary
sludge (anaerobically
digested) from an activated
sludge plant
The mixed T:liquor suspended solids concentration in-
creased from 2000 to 5000 mg/£, and the return sludge
solids concentration increased from 1.2 percent to
2 percent. The vacuum filter operating time has in-
creased to two shifts instead of one. The filter cake
solids concentration has decreased to 20 percent from
the former 25 percent because the alum sludge is more
difficult to dewater. Overall the cost of plant opera-
tion has increased about 40 percent due to the addi-
tion of alum for phosphorus removal. The increased
cost includes labor (3 men at $32,000/yr), lime and
ferric chloride sludge conditioning at about
$21,000/yr, furnace fuel at $83,000/yr, and in-
creased hauling costs to the landfill.
The luxury uptake of phosphorus in the activated
sludge unit due to high BOD in the influent (brewery
waste) has made the required phosphorus removal
obtainable with a minimum amount of chemical addition.
No problems have been experienced with vacuum
filtration.
Mil ford,
Michigan
Iron-secondary and primary
from an activated sludge
plant
The plant experienced greatly improved vacuum filter
operation after the implementation of phosphorus
removal. The vacuum filter yield increased from
24 kg/m2/hr (5 Ib/ft/hr) to 68 kg/m2/hr
(14 Ib/ft/hr) and cake solids increased from 16 per-
cent to 19 percent. In addition, there was a decrease
in the quantity of lime and ferric chloride needed
for sludge conditioning.
-------
Also commonly reported was the requirement to change dosages
or types of conditioning chemicals used (polymers, ferric chlo-
ride, etc.). as a result of different sludge characteristics
caused by the introduction of chemical sludges.
Literature
Most literature sources indicated that increased filter cake
yield can be expected from phosphorus-laden chemical sludges.
Typical comments include the following:
• Laboratory vacuum filtration tests conducted by the Bli'ch-
ner funnel method indicated that the alum digester sludge
filtered slightly better than the control sludge. Both
sludges were effectively conditioned by ferric chloride
at 4 percent by weight of TSS. Specific resistances were
10.9 x 105 sec^/g and 7.6 x 10)5 sec2/g for the control
and alum digester sludges, respectively, after condition-
ing. Vacuum filter leaf tests indicated that the condi-
tioned alum-biological sludge could be dewatered from 1.6
percent TSS at a filtration rate of 14.6 kg TSS/m2/hr
(3.0 Ib/ft2/hr) (2).
• A dual polyelectrolyte sludge conditioning sequence made
it possible to attain vacuum filter yields of up to 49
kg/mz/hr (10 Ibs/ft2/hr) on the phosphorus removal sludge
solids (22).
• Alum addition to wastewater for the removal of phosphorus
also produced a waste activated sludge which is easier to
dewater. Also, when this sludge is combined with raw pri-
mary sludge, the resulting mixture again shows an
increased ability to be dewatered (51).
• Vacuum filtration of iron sludge at the North Toronto
plant showed the filter yield to increase from 11.08 to
23.14 kg/m2/hr (2.27 to 4.74 Ib/ft2/hr). However, these
data are not considered typical in view of the fact that
an extremely low capture of solids was obtained. No data
are available on sludge conditioning chemical requirements
at this plant (72).
Conclusions
Some plants have experienced adverse effects on vacuum fil-
ter operation as the result of the addition of phosphorus-laden
chemical sludges, while other plants have not. Unfortunately,
there are no clear patterns apparent on which to base recommenda-
tions for "best" treatment. Treatment plants which are imple-
menting phosphorus removal should conduct bench-top (or prefer-
ably pilot) tests, to determine the combination of phosphorus
removal chemicals and sludge conditioning chemicals to obtain
92
-------
optimum vacuum filter performance. Experiments with various fil-
ter media are also suggested.
Some generalizations may be made as follows:
• Many plants reported operating their vacuum filters addi-
tional shifts to handle the additional chemical sludges
generated.
• It is advantageous to thicken and condition the sludge
prior to vacuum filtration in order to increase filter
yield and filter cake solids concentration.
• Ferric chloride and lime are reported successful in con-
ditioning alum sludges. Even with conditioning, however,
it is common for alum sludges to have a lower percentage
of filter cake solids; al urn siudges are more difficult to
dewater.
• Polymers have been reported successful in conditioning
iron sludges.
• Filtration of combined primary-secondary sludges is pre-
ferable to filtration of secondary sludges alone.
• Iron sludges are more corrosive, so system component
materials should be selected for corrosion resistance.
• Lime addition for phosphorus removal greatly increases
the amount of sludge generated, and system components
should be sized to accommodate the anticipated increase.
Lime scaling may be a problem on wetted surfaces, so
maintenance access for cleaning should be provided.
• Many dramatic improvements in sludge filterabi1ity have
been reported as resulting from the proper use of polymers
during sludge treatment steps prior to filtration.
Reported changes in filter yield and filter cake solids
showed a confused pattern, with some plants experiencing increased
yields and solids, and others showing poorer results than were
experienced with non-chemical sludges. Alum sludge particularly
was singled out as often difficult to dewater.
DRYING LAGOONS
Introduction
Drying lagoons form a simple, low-cost sludge dewatering
system which is limited in application to areas where large quan-
tities of cheap land are available. Of plants which responded
to the questionnaire survey and/or were field investigated during
93
-------
this study, drying lagoons were used by 15 out of 174 (9 percent).
Lagoons were the third most common dewatering method for chemical
sludges, following drying beds and vacuum filtration.
In evaluating the impact of phosphorus removal by chemical
addition on drying bed operation, we must consider how sludge
drying rates will be affected and how plants may respond to the
need for additional drying bed capacity for increased sludge
volume. Sludge drying rate is important because sludges that
take longer to dry have lower solids loading rates, and they
require more space/ton of dry solids. Sludges that dewater slowly
also tend to be the cause of odor problems. The need for addi-
tional drying lagoon capacity is often problematical for plants
because of the expenses involved in making more space available.
Questionnaire Survey and Case Studies
As shown in Table 24, relatively little information on
lagoon operation was obtained from the surveyed plants. Evi-
dently, little monitoring of sludge drying rates, dried sludge
TS concentrations, etc., is performed. The comments in Table 24
indicate that often lagoons were used as stand-by or supplemen-
tary operations in addition to other dewatering methods, and
since they were not relied on closely, they were not monitored.
Furthermore, at some of the plants, lagoons were treated as stor-
age areas rather than as a dewatering method. In some cases,
the plants did not expect to completely fill their lagoons for
several years, so they had not been particularly concerned over
control of dewatering rates or over sludge removal.
On the other hand, Table 24 also contains comments from
plants which had run out of lagoon space and had been forced
either to clean out the filled lagoons to make space available,
or to purchase land and construct new ones. Both of these alter-
natives were considered expensive by the plants.
Some of the plants mentioned that their need for additional
lagoon space was created by the additional sludge generated by
phosphorus removal. Other adverse impacts of phosphorus removal
which were mentioned included slower sludge drying rates, failure
of the sludge to dewater, and odor problems. In contrast, there
were other reports of chemical sludges which settled readily in
the lagoons, formed clear supernatants, and posed no odor prob-
1 ems.
Conclus ions
Drying lagoon operation is often made more expensive and
difficult by the need to handle chemical sludges because of
greater sludge volumes and slower drying rates. Plants must be
prepared to provide more space for the extra sludge volume by
constructing new lagoons or by cleaning out existing ones which
94
-------
TABLE 24. IMPACTS OF CHEMICAL SLUDGES UPON DRYING LAGOON PERFORMANCE AS
REPORTED IN QUESTIONNAIRE RESPONSE
Plant
Location
Type of Sludge Treated
Impacts of Chemical Sludge Performance with Chemical Sludge
Virginia,
Minnesota
VD
vn
Ottawa,
Ontario
(Green
Creek)
Berrien
Springs,
Michigan
Lime-primary and secondary
(gravity thickened) from
an activated sludge plant
Alum-primary (anaero-
bically digested) from a
plant with no secondary
treatment
Iron-tertiary from an
activated sludge plant
When phosphorus removal was
begun with lime, the plant's
digester and drying beds were
abandoned. It was believed
that the lime sludge could
not be fed to the digester
without causing problems. A
gravity thickener and drying
lagoons were chosen instead.
The sludge was limed at the
thickener to reduce odors,
using 200 Ib/day lime at
$0.15/lb.
Alum addition created about a
90 percent increase in sludge
volume, or approx. an addi-
tional 378 m^/day (100,000
gpd). This necessitated the
construction of 28.5 acres
extra lagoon space (for a
total of 56 acres) at a cost
of $690,000.
The chemical sludge is pumped
directly to a holding lagoon
which has a 20-yr design capa-
city. Organic sludge is
treated separately.
An 8 to 10 percent TS sludge was
generated by the thickener. But
the sludge would not dry in the
lagoons. The sludge also emitted
a very offensive odor after a
period of time even when heavily
limed. The lagoons were aban-
doned and the sludge is hauled
at 8 to 10 percent TS to a dis-
posal site.
There have been no odor
problems.
(continued)
-------
TABLE 24 (continued)
Plant
Location
Type of Sludge Treated
Impacts of Chemical Sludge Performance with Chemical Sludge
Coldwater, Iron-primary and secondary
Michigan (anaerobically digested)
from a trickling filter
plant
vo
en
North
Madison,
Ohio
Lime-tertiary from an
activated sludge plant
Sturgis, Alum-tertiary from a
Michigan trickling filter plant
The sludge drying lagoon was
cleaned out shortly after
chemical addition for phos-
phorus removal was initiated
to provide.additional sludge
drying space. This was
necessitated by a slower
sludge drying rate as well as
greater quantities of digest-
ed sludge.
The plant has had phosphorus
removal since it started
operation in November of 1974.
Therefore, it is not possible
to compare operation with and
without chemical sludge.
Sludge is sent to the lagoons
on a sporadic basis at times
when the drying beds are full
and there is a need to make
room in the digesters. No formal
records have been kept as to the
characteristics of the sludge in
the lagoon.
The lime sludge is pumped to 2
lagoons which are 61 m by 30.5 m
(200 ft by 100 ft). The sludge
settles and a clear supernatant
is decanted by a pipe at the
1.5-m (5-ft) water height.
Cleaning the lagoons is an
expensive problem. Both are full.
$7,000 was paid in 1976 to a
private contractor to clean 1/4
of a lagoon.
The chemical sludge is not com-
bined with primary and secondary
sludges. It is pumped separately
to so-called "dry lagoons."
When a lagoon is filled it is
allowed to dry and then scari-
fied. This has not been done yet
as a laqoon has not been filled.
(continued)
-------
TABLE 24 (continued)
Plant
Location
Type of SLudge Treated Impacts of Chemical Sludge Performance with Chemical Sludge
Ludington, Lime-tertiary from an
Michigan activated sludge plant
Watt's
Creek
STP,
Shirley's
Bay,
Ontario
Niagara
Falls,
Ontario
Alum-secondary and primary
(anaerobically digested)
from an activated sludge
plant
Iron-primary (anaero-
bically digested) from a
plant with no secondary
treatment
The chemical sludge is not com-
bined with primary and secondary
sludge. It is Dumped to lagpons
which are not expected to be
filled for 20 yrs.
With alum addition the raw
sludge volume has increased
65 percent or more. The
digested sludge solids concen-
tration decreased from 5.5
percent to 3 percent, meaning
an even greater increase in
digested sludge volume. In-
creased holding lagoon space
is planned to accommodate the
extra sludge. An additional
sludge hauling cost of $25,000
is anticipated for 1976.
The sludge from the digesters
has a lower TS concentration
and the volume is greater.
The lagoons must be filled to
deeper depths to accommodate
the extra volume. The sludge
takes longer to dry in the
lagoons and does not reach the
same solids concentration as
before.
-------
have been used simply as storage areas. They must also antici-
pate the possibility of slower sludge drying rates, causing an
even further demand for greater capacity. Where capacity is
unavailable, alternative means of sludge handling such as liquid
sludge hauling or mechanical dewatering will have to be consid-
ered.
PRESSURE FILTRATION
Introduction
Pressure filters have long been used in Europe to process
difficult-to-dewater sludges. During the last five years, there
has been a substantial increase in the use of pressure filters in
the U.S. Improvements in equipment and greater quantities of
difficult-to-dewater sludges, such as iron and alum sludges,
account for the increase.
For conventional (non-chemical) sludges, the process pro-
duces a drier filter cake than either vacuum filtration or cen-
trifugation. Total solids concentrations of 40 to 60 percent
are generally achieved with undigested primary and secondary
sludges, and concentrations of 40 to 45 percent TS are typical
for digested primary and secondary sludges. Sludge thickening
is usually required prior to filtration for all but primary
sludges. Proper sludge conditioning is necessary in order to
achieve the best filter results.
Evidently, the only conditioning method utilized at present
is chemical conditioning. Polymer, lime, ferric chloride, fly
ash, lime kiln flue dust, and sludge incinerator ash are the con-
ditioners used. Frequently, a combination of approximately 5
percent (of the dry weight of the sludge) FeCl3 and 10 percent
lime is employed. The use of an additional 100 to 250 percent
ash can usual ly increase filter cake dryness by about 5 percent.
For conventional sludges, the cost of dewatering by pressure
filtration is higher than the cost of vacuum filtration or cen-
trifugation. However, the drier cake produced may result in cost
savings in the downstream disposal processes which offset or
exceed the higher cost. The highest operation and maintenance
expenses for pressure filtration are typically the cost of labor
and the cost of chemical conditioners.
When dewatering chemical sludges, both the performance char-
acteristics of the process and the energy, labor, and chemical
requirements are altered. Consequently, the costs associated
with pressure filtration, and also with the downstream disposal
processes, are changed. It is desirable to determine whether
chemical sludges enhance or decrease the cost-effectiveness of
pressure filtration in relation to other dewatering methods.
98
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Questionnaire Survey
A total of eight (46 percent) of the plants responding to
the questionnaire survey reported that they have pressure filters.
Only six questionnaire respondents commented upon the performance
of pressure filtration at their plant or the impacts of the chem-
ical sludge. Table 25 indicates the sizes of the six plants and
the sludge treatment/disposal methods which were used. Thicken-
ing and digestion of the raw sludge were common. Two plants
practiced incineration of the dewatered filter cake. A variety
of sludge conditioners were used prior to dewatering. A single
plant used an organic polymer. The others, including the two
plants with incineration, used inorganic conditioners.
Table 26 presents data and comments from the questionnaires
pertaining to pressure filter performance with conventional and
chemical sludges. Four of the plants used plate and frame-type
pressure filters and two used unusual types of filter processes.
The plants, which had iron and lime sludges, reported that, when
handling the chemical sludges, they needed smaller dosages of
chemicals to condition the sludges before dewatering. These
plants had filter cake solids concentrations of 40 to 50 percent
TS.
Case Studies
The case study of Brookfield, Wisconsin, in Appendix H, stu-
dies the impacts of an iron sludge on the plant's plate and frame
pressure filter. Phosphorus removal with ferrous sulfate resulted
in an increase in the mass of sludge generated which was offset
by a decrease in the weight of chemicals needed for conditioning.
Although roughly 30 percent more sludge TS was generated during
phosphorus removal, the dry weight of the filter feed, consisting
of both sludge and conditioning chemicals, was greater by only
15 percent. The dosage of incinerator ash for conditioning was
lowered from 85 percent to 60 percent. The FeCl3 dosage also
decreased somewhat, to a dosage of 6 to 8 percent. The lime
dosage remained constant at 17 percent.
The chemical sludge did not alter the TS concentration of
filter cake, which remained at 43 percent. The VS fraction of
cake TS decreased only slightly, from 33.6 to 32.6 percent. Two
positive impacts of the chemical sludge were a decrease in the
average length of a filter run from 2.83 to 1.73 hr/run and an
increase from 75 to 90 percent in the filter cake solids recovery.
The costs of sludge treatment and disposal at Brookfield
were lower for the iron sludge. A decrease of about $1.47/t
($1.33/ton) occurred because of decreases in the amounts of chem-
ical conditioners and electricity used by the pressure filter.
99
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TABLE 25. SLUDGE TREATMENT/DISPOSAL METHODS USED BY PLANTS PRACTICING PRESSURE FILTRATION
Plant Location
Saline, MI
Westfield, NY
Kenosha, HI
Coloma, MI
o Brookfield, WI
Hatfield
Township
Col mar, PA
Size
(m3/day
(mgd))
4,807
(1.27)
5,678
(1-5)
75,700
(20)
4,542
(1.2)
9,084
(2.4)
8,706
(2.3)
Thickening
None
Gravity
thickening
Flotation
thickening
of WAS
Gravity
thickening
Gravity
thickening
Gravity
thickening
Digestion
Anaerobic
digestion
Aerobic
digestion
Anaerobic
digestion
None
Aerobic
digestion
of WAS
None
Chemical Conditioning
FeCls, Lime, Fly Ash
conditioning
Polymer conditioning
Fed 3, Lime
conditioning
Lime Kiln Flue Dust
conditioning
Fed 3, Lime,
Incinerator Ash
conditioning
Lime conditioning
Incineration
None
None
None
None*
Multiple
hearth
incineration
Multiple
hearth
incineration
Disposal
Sanitary
landfill
Sanitary
landfill
Cropland
Cropland
Sani tary
landfill
Sanitary
landfill
* When the plant began operation in 1973, multiple hearth incineration was used and chemical conditioning
was performed with lime and incinerator ash.
t In the past, FeCls was also used for chemical conditioning.
-------
TABLE 26. IMPACTS OF CHEMICAL SLUDGES UPON PRESSURE FILTER PERFORMANCE
AS REPORTED IN QUESTIONNAIRE RESPONSE
Plant
Location
Type of Sludge Treated
Impacts of Chemical Sludge Performance with Chemical Sludge
Saline,
Michigan
Westfield,
New York
Kenosha,
Wisconsin
Iron-primary and secondary
from a trickling filter
plant
Aluminum-secondary from
a two-stage long-term
activated sludge plant
with no primary treat-
ment
Iron-secondary and pri-
mary from a conventional
waste activated sludge
plant
The entire plant was expanded
and the pressure filter was
installed when phosphorus re-
moval was begun. Therefore
there is no comparison between
pressure filter operation with
and without the iron sludge
The plant incorporated phos-
phorus removal since it went
into operation. Therefore,
there is no data on the im-
pacts of the aluminum sludge.
The amount of FeCl3 required
for chemical sludge condition-
ing was reduced from 5 percent
to 3% percent. (The lime
dosage is 18 to 22 percent.)
The digested sludge at 9 percent
TS produces a filter.cake which
averages 41 percent TS. The
cake vs fraction averages 62
percent of TS.
The plant is using an unusual
type of belt filter press which
is similar to a vacuum filter
with a belt squeezing the sludge
to the cloth. The sludge is
difficult to dewater because it
is all waste activated. The
filter cake TS concentration
averages 11.5 percent. Polymer
conditioning of the aluminum
sludge was found to be more
successful than conditioning
with FeClo and lime.
The digested sludge at 4.8 per-
cent TS produces a filter cake
which averages 40 percent TS.
(continued)
-------
TABLE 26 (continued)
Plant
Location
Type of Sludge Treated
Impacts of Chemical SLudge Performance with Chemical Sludge
Coloma,
Michigan
o
ro
Brookfield
Wisconsin
Hatfield
Twp,
Col mar,
Penn-
sylvania
Lime-primary and
secondary from a
filter plant
alum-
trickling
Iron-secondary and pri-
mary from a contact
stabilization activated
sludge plant
Lime-primary secondary
and alum-tertiary from
a complete-mix acti-
vated sludge plant
The use of lime increases the
TS concentration of the thick-
ened sludge. It also decreases
the requirement for Time kiln
flue dust for chemical condi-
tioning.
See the following case study
discussion of Brookfield.
The amount of sludge generat-
ed when a plant expansion in-
cluding phosphorus removal
took place was much greater
than expected. At that time
the plant was practicing
vacuum filtration and incin-
erating a 22 percent TS filter
cake. The incinerator loading
rate was too low to keep up
with the amount of sludge gen-
erated. In response to this
problem, the vacuum filter was
The estimated average filter
cake TS concentration is 40 per-
cent. The plant used incinera-
tion when it began operation but
it was very expensive because
of the low VS content of the
sludge. Due to the large amounts
of lime, lime flue dust and in-
cineration ash used for phos-
phorus removal and sludge
conditioning the VS fraction of
filter cake TS was only 20 to
30 percent.
The plant has an unusual type of
filter press which has been
adapted from the old belt vacuum
filter. Sludge is actually
vacuum filtered and then carried
by the long vacuum filter belt
to a diaphragm-type filter press.
The system produces a 45 to 50
percent TS filter cake which has
a VS fraction ranging from 27
to 40 percent of TS. The filter
contains 299 rag/* SS, 16 mg/'A
VSS, 917 mg/fc COD, and
(continued)
-------
TABLE 26 (continued)
Plant
Location Type of Sludge Treated Impacts of Chemical Sludge Performance with Chemical Sludge
adapted to a diaphragm-type 872 mg/£ BOD.
filter press. They produced a
45 to 50 percent filter cake.
This allowed a higher incin-
erator loading rate. The addi-
tion of lime for phosphorus
removal is believed to be re-
ducing the lime requirement
for chemical conditioning.
**>
-------
Literature
An evaluation of lime sludge dewatering by centrifugation
and pressure filtration was made at the Holland, Michigan, waste-
water treatment plant (43). The design of the plant included
addition of lime to the primary step for phosphorus removal and
aluminum addition to the activated sludge treatment step for
effluent polishing. Sludge treatment was provided by primary
sludge degritting, primary and waste activated sludge gravity
thickening, centrifugation and fluidized bed incineration.
With this system, a 30 to 32 percent TS centrifuge cake was
incinerated, and the cost of fuel (natural gas) for incinerator was
found to be a major operational expense at $26.92/t of dry cake
solids (24.42/ton) in 1974. Pressure filtration was then con-
sidered as a means of reducing the fuel requirement by increasing
the sludge cake TS concentration. The resvilts of a pilot test
are shown in Table 27. A 0.093 m2 (1 ft2) pilot press with
three plates and two chambers was operated at 125 psig pressure
for this test.
TABLE 27. PILOT FILTER PRESS TEST AT HOLLAND, MICHIGAN (43)
Total
Sol ids
in Feed
% TS
11.2
Fil tration
Rate ?
(gal/hr/fl; )
(l/hr/m2)
178
(4.33)
Cake
Cone .
%
41.54
with thick-
ness 1.25"
Pressure
kg/m^
(psig)
8.8
(125)
Polymer
, kg/t ,
(Ib/ton)
1.5
(3 Ib/ton)
Hercules 859-C
Based on the pilot test data, estimates of the performance
and cost of full-scale pressure filtration were made. It was
assumed that polymer conditioning of the sludge would be used.
It was estimated that a pressure filter operating at 225 psig
would produce a cake of 45 percent TS. The filtration cycle
would be 1 hr based on a feed sludge TS concentration of 8 per-
cent. With the incinerator operating at a freeboard temperature
of 827°C (1,520°F), and with a 28 percent volatile filter cake,
the fuel cost would be $12.03/t ($11.15/ton).
Pressure filtration was compared to dewatering with a cen-
trifuge or with the existing vacuum filters at the Hatfield
Township, Pennsylvania, plant (31). This plant was described in
Tables 25 and 26. A mixture of lime-primary, waste activated
and alum-tertiary sludges was dewatered by the three different
methods under similar operating conditions. The pressure filter
104
-------
produced a drier cake with greater recovery of solids than either
the vacuum filter or tlie solid bowl centrifuge. A 41 percent TS
filter cake was achieved with sludge conditioning with 15 percent
1 ime.
Conclusions
There is insufficient data to make conclusions about the
dewatering of alum and waste activated sludges with pressure fil-
ters. But there are indications that the results are not as good
for these sludges as they are for iron or lime sludges and for
primary or combined primary and secondary sludges.
The results of the questionnaire survey, case studies, and
literature search have not included any mention of adverse
impacts of iron or lime sludges on pressure filtration. These
sludges dewater well on pressure filters, with.filter cake TS
concentrations of 40 to 50 percent and often with reduced chemi-
cal requirements for sludge conditioning. Because of the reduc-
tion in chemical costs, the cost-effectiveness of this dewatering
method alternative can be favorably affected.
CENTRIFUGATION
Introduction
Sewage sludge dewatering ~by centrifuges began to evolve in
the 1950's. Their use has been gradually increasing due to bet-
ter engineering and materials of construction. Of plants which
responded to the questionnaire survey and/or were field investi-
?ated during this study, centrifuges are used by 6 out of 174
3 percent). Several additional plants have conducted promising
pilot tests with centrifuges and are considering their future
full-scale use to help solve sludge management problems.
Proper centrifuge design is complex, and it is assumed that
the reader has a basic knowledge of centrifuge design and opera-
tional factors. Essentially, centrifuges have the same objec-
tives as vacuum filters, e.g., maximum solids capture, maximum
yield per unit of energy consumed, high solids concentration in
the sludge cake, and low total cost. In addition, since centri-
fuges are high maintenance units, it is desirable to have good
resistance to abrasion and long-living moving parts.
Most centrifuges dewatering municipal sewage sludge are of
the continuous, solid bowl, conveyor type. A relatively new,
low-speed, solid bowl centrifuge which was developed in Europe
is now available in the United States and is reported to have
some significant advantages. Basket centrifuges are also avail-
able.
Because each sludge and treatment plant situation is unique,
the information presented in the following subsection is not
105
-------
intended to provide universal answers to specific plant problems.
Rather, the information is intended to be helpful to those seek-
ing possible avenues of further investigation. The best method
of predicting the true performance of full-scale centrifuges is
experiments with pilot units on the actual sludges to be treated.
These pilot units are available from manufacturers who have devel-
oped scale-up design factors for their units.
Literature
There are hundreds of literature references pertinent to
centrifuging of sewage sludge. This study concentrated upon
those which discussed centrifuging of phosphorus-laden chemical
sludges. Generally, the literature favorably evaluates centri-
fugation of chemical sludges (particularly lime sludges), as
evidenced by the selected summaries below.
In connection with the lime treatment nutrient removal pro-
gram at the Newmarket, Ontario WPCP plant, a 2-mo raw sludge
dewatering study using a Sharpies solid bowl centrifuge was
undertaken (65). Centrifuge variables, such as sludge feed con-
centration, sludge feed rate, sludge pH, and speed differential
were examined. The project was successful and the optimum
results produced from centrifuging raw lime primary sludge were
as follows:
(a) 98 to 99 percent SS capture
(b) Polymer dosages of less than 0.5 kg/t (1 Ib/ton) of dry
solids
(c) Consistent centrate qualities of less than 700 ppm SS
(d) Sludge cake of 27 to 34 percent TS
(e) Feed to centrate, total phosphorus reduction of 95
percent
(f) Centrate total phosphorus levels in the order of 10 to
30 ppm.
Using the Sharpies P3000 Super-D-Canter centrifuge, solids
recovery and cake dryness could be attained to some degree with
unconditioned lime sludge. Augmentation with polyelectrolytes
showed increased overall solids recovery and vast improvements
in centrate quality.
The OrangeCounty Water District practiced chemical clarifi-
cation of secondary effluent with lime at a pilot wastewater
reclamation facility. They removed lime sludge from the pilot
tertiary clarifier and treated it in a small pilot thickener and
a 15-cm (6-in) solid bowl centrifuge. Sludge was removed from
106
-------
the clarifier at 1 to 2 percent TS and could be thickened in the
pilot thickener to 10 to 20 percent TS. The centrifuge data
showed that the thickened sludge could be dewatered to over 50
percent TS in a Sharpies P-600 Super-D-Canter centrifuge.
Sludge dewatering studies (2) at the Portage Lake Water and
Sewer Authority, Michigan, concluded that alum-biological sludges
were amenable to conventional sludge treatment processes. Of the
various concentrating and dewatering schemes investigated for the
alum-biological sludge, two-stage aerobic digestion followed by
basket centrifugation appeared to be the most economical. How-
ever, because of the short distance to the disposal site, direct
trucking was estimated to produce a 10 percent savings over cen-
trifugation.
In a detailed report by Martin and Nardozzi (43), perfor-
mance and cost information for lime-biological sludge treatment
at Holland, Michigan, was provided. The city of Holland used
lime precipitation of phosphorus in the primary clarifier, with
A1C13 addition to the flocculator prior to biological treatment.
The thickened, combined primary solids and waste activated sludge
was fed to solid bowl centrifuges prior to incineration. The
centrifuges produced a cake of 30 to 32 percent total dry solids,
with 97 percent solids capture.
The polymer (Hercules 869-C) dosage rate was 2.0 to 2.5 kg/t
(4 to 5 Ib/ton) of dry solids. When polymer was used for condi-
tioning, more of the finer, less dense particles were captured.
The data revealed that the highest cake solids were achieved when
no polymer was used. As the capture increased with polymer addi-
tion, the cake became wetter, because the fines and associated
water were captured. Even so, polymer addition was advisable
because a cleaner centrate ensured a lighter recirculating load
and a minimum sludge age in the plant.
The EPA, in 1975, conducted pilot scale investigations (26)
of dewatering chemical sludges with vacuum filters and centri-
fuges. The investigations were made at various midwestern plants
and were plagued by mechanical difficulties with the trailer-
housed pilot equipment. The most significant centrifuge results
were the characteristics of the centrate from various pilot-scale
centrifugation runs. These data, shown in Table 28 provide a
tool with which to study the impact the various sidestreams
could have on plant operation and performance. Note particularly
the heat-treated iron sludge from the Midland, Michigan, plant
which demonstrated a high loading rate and cake solids concentra-
tion, but also showed a high solids and organic load in the cen-
trate. This centrate returned to the plant could cause opera-
tional difficulties.
The EPA study further concluded that:
Increasing the polymer dosage decreased the cake solids.
107
-------
TABLE 28. CENTRATE CHARACTERISTICS FROM VARIOUS SCROLL CENTRIFUGATION RUNS (26)
Sludge
Soli ds
Loading Rate Cake Solids Centrate Solids Polymer Recovery
Bowl Speed, rpm Kg/hr (Ib/hr) % % %
Beaver Creek 5000
Waste Activated
Alum-Sludge
Beaver Creek 5000
Aerobic Digested
Alum-Sludge
Midland 4000
Heat Treated 4500
Iron-Sludge
0.671
1.26
1.61
1.64
2.42
3.20
4.86
7.44
19.2
24.7
(1.48)
(2.77)
(3.55)
(3.62)
(5.32)
(7.05)
(10.7)
(16.4)
(42.3)
(54.4)
4.3
5.1
5.42
5.79
10.9
6.32
6.37
15.2
33.3
49.0
0.01
0.02
0.03
0.01
0.04
0.16
0.33
0.09
4.5
9.6
yes
yes
yes
*
yes
yes
VeS
*
yes
yes
no
no
100
95.8
95.3
94.2
99.7
90.0
84.6
99.2
82,9
97.1
Added to sludge ahead of centrifuge.
-------
This was also borne out in separate Canadian studies.
t Much better results are achieved from polymer addition
at the centrifuge, rather than ahead of the centrifuge.
Higher solids recovery, better centrate quality, and
higher cake solids concentration were achieved by polymer
addition at the centrifuge.
• Increasing the feed rate resulted in decreasing the solids
recovery using aerobically digested alum-secondary sludge.
• For similar conditioning, both solids recovery and cake
solids concentration are generally less for chemical
sludges than for conventional sludges.
Questionnaire Survey and Case Studies
As shown in Table 29, there was relatively little informa-
tion obtained from the questionnaires pertinent to centrifugation
of chemical sludges. More complete information was obtained from
the Port Huron, Michigan, and South Bend, Indiana, case studies
in the appendices.
The Port Huron case study report examines some of the factors
influencing centrifuge dewatering of an alum sludge:
• Centrifuge loading rate was higher with thermal sludge
conditioning than with ^polymer conditioning.
• The dosage of polymer required for conditioning was higher
when the feed sludge TS concentration was low as the
result of high activated sludge wasting rates.
t The TS concentration of the dewatered cake was higher
with thermal conditioning than with polymers. The cake
TS concentration apparently did not vary as the result of
changes in the feed sludge TS concentration if polymer
dosages and feed rate were adjusted.
• The average centrate TS concentration was lower during
thermal conditioning while the volatile fraction of TS
in the centrate was higher.
t The TS capture could be maintained at at least 80 percent
by the operator. It was necessary for the operator to
check the centrate of each centrifuge separately, several
times each day. He then made separate adjustments of
feed rate and polymer dosage for each centrifuge.
It was not possible to judge the impacts of alum addition
for phosphorus removal on centrifuge performance at this plant
because alum had been in use since the plant was upgraded. How-
ever, it seemed likely that the quantity of waste activated
109
-------
TABLE 29. IMPACTS OF CHEMICAL SLUDGES ON CENTRIFUGE PERFORMANCE
AS REPORTED IN QUESTIONNAIRE RESPONSE
Plant Location
Type of Sludge Treated
Summary of Comments
Lower Allen
Township, Camp
Hill,
Pennsylvania
Lime-primary and secondary
(gravity-thickened) from
an activated sludge plant.
There was a great increase in sludge mass. The
centrifuge dewatering operational costs greatly
increased. The lowered volatile content in the
sludge cake increased the subsequent incineration
operational costs. Had to reroute centrate
return flow so that instead of being added to the
thickeners, it was sent to the head of the
plant because the centrate return was causing
odor and floating solids problems in the thick-
eners. They had difficulty finding polymers
that were effective in conditioning the sludge
by adding to the centrifuges. The dewatered cake
contains 30 percent TS.
Hilton, New York
Alum-secondary and primary
(gravity-thickened) from
an activated sludge plant.
Centrifuge operation was not significantly
affected by addition of the chemical sludge.
Polymer continued to be added at a rate of 6
to 7-1/2 kg/t (12 to 15 Ibs/ton), and cake solids
remained at about 19 percent.
-------
relative to primary sludge was greater because of alum addition
to the aeration basins. Greater quantities of waste activated
sludge meant higher polymer conditioning requirements at the
plant.
The South Bend case study report describes centrifugation
of a lime/iron sludge resulting from tertiary addition of both
lime and iron to secondary effluent. The sludge was gravity
thickened and fed to the centrifuges at 10.2 percent TS. The
cake produced contained 43 percent TS. There were problems with
maintenance of the centrifuges. The heavy lime sludge was respon-
sible for breaking and frequent equipment overhauls.
Conclusions
The information which was presented indicates that thickened
lime-primary, or primary plus secondary, sludges can be dewatered
to 27 to 34 percent TS in a centrifuge. Centrifuge dewatering of
thickened lime-tertiary sludges apparently produces an even drier
cake of 40 to 50 percent TS. At one plant, a combined alum-sec-
ondary and primary sludge was gravity thickened and centrifuged
to 19 percent TS. Centrifuge performance depends heavily on
choosing the right conditioning method. Polymers are usually
used. The point of polymer addition is important, some experi-
ences indicating that the polymer should be added at the centri-
fuge rather than before it. The polymer dosage is also important
and should be adjusted as feed sludge consistency and TS concen-
tration vary.
Iron and aluminum chemical sludges often require higher con-
ditioning polymer dosages and lower feed rates than conventional
sludges. Poor centrate quality and low cake TS concentrations
result otherwise. Closer operator attention may also be neces-
sary with iron and aluminum sludges because of inconsistent grav-
ity thickener performance, meaning variations in feed sludge
q u a 1 i ty.
Ill
-------
SECTION 9
REDUCTION OF CHEMICAL SLUDSES - INCINERATION
INTRODUCTION
Incineration uses thermal energy to provide a major reduc-
tion in the weight and volume of solids requiring disposal. The
major factor in evaluating incineration performance and costs is
the composition of the sludge feed. Sludge composition, i.e.,
moisture, volatile solids, and inert contents, influences feed
rate capacity and auxiliary fuel requirements, among other vari-
ables, and, consequently, sludge incineration cost effectiveness.
Due to their inert content, chemical sludges have been recognized
as poor incineration feed (43 and 45).
QUESTIONNAIRE SURVEY
As previously shown in Table 10, 22 of the plants (13 per-
cent) responding to the questionnaire survey reported that they
incinerated their chemical sludges. Six plants surveyed indicated
that chemical addition for phosphorus removal was having a signi-
ficant impact on their incineration systems. Table 30 summarizes
the experiences of these plants. All plants incinerated combined
primary and secondary sludges. Lime, ferric chloride, and alum
were used at two of the six respondents.
Problems with incineration encountered by the plants sur-
veyed generally agreed with literature reporting the difficulties
with incineration of chemical sludges. The lower TS concentra-
tion and lower volatile content of the phosphorus-laden sludges
made auto-combustion of the sludge harder to attain resulting in
soaring auxiliary fuel requirements.
Increased fuel requirements caused a few plants to abandon
their incinerators and adopt other sludge handling methods, i.e.,
dewatering and hauling. In addition, the increased inert content
of the chemical sludges put a burden on incinerator ash handling
equipment and, in certain cases, clinker buildup within the
incinerators resulted. Problems with clinkers resulted in perio-
dic downtime for incinerator maintenance. The Wyandotte, Michi-
gan, plant used primary additions of ferric chloride for phospho-
rus removal and reported clinker buildup within its multiple hearth
incinerator. The clinkers plugged the multiple he.arth dropKoles
and were identified as the source of increased rabble arm wear.
112
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TABLE 30. IMPACTS OF PHOSPHORUS-LADEN CHEMICAL SLUDGES ON INCINERATOR PERFORMANCE
AS REPORTED IN QUESTIONNAIRE RESPONSE
Location
Size-m /day
(mgd)
Type of Sludge Treated
Impacts of Chemical Sludge
Warren, Michigan
Wyandotte, Michigan
Coloma, Michigan
Camp Hill, Pennsylvania
123,000 Alum * secondary and primary
(32.5) from a conventional activated
sludge plant
291,400 Iron -primary and secondary
(77) from a pure oxygen activated
sludge plant
4,500 Lime - primary and alum sec-
(1.2) ondary from a trickling
filter plant
7,600 Lime - primary and secondary
from a complete mix acti-
vated sludge plant
Higher gas costs for incineration
resulted.
In the multiple hearth incinera-
tor there has been a major problem
with clinker buildup, causing more
time to be spent cleaning up the
drop holes in the hearth and more
wear on the rabble arms.
The addition of chemicals for
phosphorus removal has lowered
the sludge volatile content to
20 to 30 percent, causing the need
for continuous use of auxiliary
fuel (natural gas).
The multiple hearth incinerator
was designed to recalcinate and
reclaim lime; this step was eli-
minated due to economic consid-
erations. With the lower volatile
content of the sludge, the incin-
erator requires between 80 and 100
gal of auxiliary No. 2 fuel oil
per dry ton of sludge.
(continued)
-------
TABLE 30 (continued)
Location
Size-rn /day
(mgd)
Type of Sludge Treated
Impacts of Chemical Sludge
Sheboygan, Wisconsin
Richardson, Texas
54,900
(14.5)
6,700
(1.78)
Iron - secondary and primary
from a trickling filter plant
Alum -secondary and primary
from a trickling filter plant
The incinerator requires an addi-
tional 12 to 15 gal/hr of fuel
oil due to the lower volatile
content of the solids.
The plant originally used anaer-
obic digesters and drying beds
for sludge disposal. With alum
addition for phosphorus removal,
the volume of sludge to be dis-
posed of increased by 50 percent.
Dewatering equipment was installed
and an electric incinerator de-
signed to incinerate the dewatered
cake. Leaf tests were run on the
sludge, and these showed the
sludge could be dewatered to 20 to
25 percent dry TS with a 60 per-
cent volatile fraction, and the
dewatering equipment and electric
incinerator were designed on this
basis. However, in practice, the
best results from dewatering were
15 percent dry TS and a 45 to
50 percent volatile fraction.
This sludge was not judged to be
suitable for incineration, and
conditioning with lime, ferric
chloride, and different polymers
did not improve the system's
performance.
-------
CASE STUDIES
The case studies in the Appendices contain detailed informa-
tion on the effects of chemical addition on incineration. The
particular case studies dealing with this subject are Sheboygan,
Wisconsin; Port Huron, Michigan; and Brookfield, Wisconsin.
The Sheboygan, Wisconsin, wastewater treatment plant operates
a Dorr-Oliver FluoSolids incinerator. This fluidized bed incin-
erator was designed to handle a sludge feed of 25 percent TS with
a 73 to 74 percent volatile fraction. With the addition of fer-
ric chloride for phosphorus removal, the sludge feed TS concen-
tration was 22 percent with a volatile fraction of 65 percent.
These deviations from the design capacity of the incinerator unit
resulted in a decrease in the incinerator's loading capacity and
an increase in auxiliary fuel consumption. The decrease in capa-
city was judged to be approximately 121 kg (266 Ib) of dry TS/hr,
and No. 2 fuel oil consumption was judged to increase by approx-
imately 143 £/t (34 gal/ton).
There have also been increased incinerator maintenance and
repair problems at the plant due to slag formation and corrosion.
On one occasion, four tuyeres blew out because several of them
were plugged with slag. On another occasion, a pressure buildup
in the reactor signaled a problem, and a large piece of slag was
found blocking the exhaust line. After this experience, the
operator performed a visual inspection of the exhaust line from
the roof duct inspection port every 3 to 4 mo.
In addition, the plant manager suggested that the ferric
chloride used for phosphorus removal was responsible for a high
rate of metal corrosion in ducts, especially in the elbows of
the scrubber system. Gradually, these parts were being replaced
wi th stainless steel .
The Port Huron, Michigan, plant, which was handling an alum
sludge, also operated a Dorr-Oliver FluoSolids incinerator. For
a period of time, Port Huron thermally conditioned sludge with a
Farrer System thermal conditioner prior to centrifugation and
incineration. This step provided major savings on chemicals and
sludge conditioning fuel oil. It also increased the solids load-
ing to the incinerator. With thermal conditioning, the sludge
burning rate reached 1,043 kg/hr. Dorr-Oliver has estimated that
the incinerator capacity would be reduced to 590 kg/hr for non-
thermally conditioned sludge.
The cost of sludge thermal conditioning amounted to $35.00/t
($31.74/ton) excluding equipment maintenance and repair supplies.
Prior to thermal conditioning, chemical conditioning costs were
$50.30/t ($45.66/ton). It therefore appeared that operation and
maintenance of thermal conditioning at the Port Huron plant was
cost effective as long as the cost of equipment maintenance and
repair supplies was less than $15.30/t ($13.92/ton).
115
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LITERATURE
A review of the literature on incineration of chemical slud-
ges reveals a common set of problems: greater sludge volume,
lower calorific value, increased moisture content, formation of
clinkers due to iron or aluminum content, and increased inert
content. The Canadian Environmental Protection Service has sug-
gested solutions to handle these incineration problems (61).
Problem
Greater sludge volume
Lower calorific value
Increased moisture content
Formation of clinkers
Increased inert content
Solution
Increase capacity or run time
Increase auxiliary fuel
Improve dewatering or increase
auxiliary fuel
Decrease incineration temperature
below fusion point
Increase ash disposal capacity
In a wastewater treatment facility using large amounts of
lime for phosphorus removal, consideration must be given to the
possibility of recalcination of the chemical sludge. When lime
is used, calcium carbonate and calcium hydroxyapatite are the
major sludge components. The hydroxyapatite containing the
removed phosphorus is fairly stable, but the calcium carbonate
can be recalcined to recover lime. The decision to recalcinate
lime should involve an economic study considering the cost of
lime, the capital costs necessary to achieve recalcination, the
additional operating costs of recalcination, and the cost of
incinerator ash disposal. The latter item is one that very often
is not considered in such an economic analysis, but reuse of 65
to 85 percent of the lime can significantly reduce the volume of
ash generated from the incineration process. If costs associated
with this final ash disposal are significant, it can have a marked
effect upon the total economic picture.
The Holland, Michigan, wastewater treatment plant used pri-
mary addition of lime for precipitation of phosphorus and a small
amount of Al+3 for effluent polishing (43). The primary and
waste-activated sludges were combined and dewatered for incinera-
tion in a Dorr-Oliver FluoSolids reactor. In operating this fluid-
ized bed reactor, Holland found that the sand in the bed served as
a seed for chemical material to form agglomerates. The resultant
bed material tended to be spherical, decreasing significantly the
116
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angle of repose. This resulted in backsifting of the sand through
the distribution tuyeres into the windbox of the reactor. This
problem was solved by plugging alternate tuyeres to increase pres-
sure drops across the distribution plate and adding a gravel type
base (olivine) to the top of the distribution plate. Backsifting
was eliminated.
Another problem occurred at the feed tubes through which
sludge entered the fluidized bed. The combined organic-chemical
sludge baked into a solid mass when exposed to the heat of the
reactor for a sustained period of time. To counteract this, the
feed tubes to the reactor were shortened to decrease heat trans-
fer from the bed. Normally, a plug was left in. the reactor feed
tube at the conclusion of daily operations. Because a plug would
solidify under these conditions, it was necessary to replace the
material with peat moss or digested sludge from a nearby digester.
A tool was later developed to remove the baked plugs.
A study was undertaken at the Minneapolis and St. Paul, Min-
nesota, wastewater treatment plants to determine the relationship
between chemical conditioner additions and incinerator auxiliary
fuel requirements. Ferric chloride and lime addition at greater
than 5 and 15 percent of the weight of the dry solids produced a
drier feed cake, but it also increased the kg of water fed per kg
of dry TS fed into the incinerator. Any increases in the kg
water/kg dry TS fed into the incinerator also increased the aux-
iliary fuel requirements for c^ake incineration. The basic prob-
lem, then, in reducing auxiliary fuel requirements was seen to
be one of reducing the ratio of water to sludge solids in the
cake, and not one of reducing the moisture content of the cake
on a percent TS basis.
In addition, the auxiliary fuel requirements were compared
using alternative sludge conditioners. A polyelectrolyte-condi-
tioned sludge was found to require significantly less auxiliary
fuel than a 1 ime-and-ferriochloride-conditioned cake.
CONCLUSIONS
Various solutions to the inherent probl ems associated with
incineration of phosphorus-laden chemical sludges have been pre-
sented in this, discussion. Unfortunately, at some wastewater
treatment plants modification of the incineration system to
incorporate these solutions surpasses the cost effectiveness of
the system. Therefore, a detailed cost analysis of the benefits
of incineration modification versus other sludge handling methods
must be made.
Generally, there are many indications that sludge handling
systems designed to include incineration will not be cost effec-
tive for the treatment of chemical sludges unless they incorpor-
ate those sludge pretreatment technologies most capable of pro-
viding a filter cake of high volatile content and low moisture
117
-------
content. The advantages and disadvantages of these techniques
were discussed at length in previous sections with reference to
their effects on incineration.
118
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SECTION 10
DISPOSAL OF CHEMICAL SLUDGES - TRANSPORT AND LAND DISPOSAL
INTRODUCTION
Table 10, which is based on the questionnaire responses of
174 plants practicing phosphorus removal by chemical addition,
shows that virtually all of the plants practiced some type of land
disposal of sludge. Even among plants practicing incineration,
most periodically disposed of sludge cake on land or in landfills.
Except in a few cases where sludge was lagooned or stockpiled at
the plant, sludge transport to the final disposal was required.
Transport costs sometimes comprised a very significant portion of
the overall costs of land disposal. Sludge hauling by tank or
dump truck was by far the most widespread method of transport.
However, pipeline transport has successfully been used for both
lime and iron sludges at South Bend, Indiana, as described in
Case Study No. C (see Appendices).
QUESTIONNAIRE SURVEY AND CASE STUDIES
Because of the impacts of phosphorus removal on sludge pro-
duction and treatment, plants were hauling more sludge in liquid
and cake form to land disposal sites than before. The question-
naire responses verified that most of the plants which histori-
cally practiced sludge hauling had to haul greater volumes of
sludge after phosphorus removal for numerous reasons:
1) Additional raw sludge solids were produced.
2) The raw sludge TS concentration was lower.
3) Sludge thickening was adversely affected.
4) Digester solids-liquid separation was poorer.
5) The dewatered cake contained more moisture.
At Manitowoc, Wisconsin, for example, alum addition increased
sludge volume so that it was necessary to incorporate into daily
operation the use of a 9.5-m3 (2,500-gal) tank truck as well as
the regular ll.S-ne (3,000-gal) truck. The capital cost of the
truck was $43,382.00 in 1976, and the estimated operating cost
was $17,590.00 in 1977.
At the Port Weller plant in St. Catherines, Ontario, a 26
percent increase in sludge volume with alum addition caused a 23
119
-------
percent increase in the annual cost of liquid sludge hauling.
Kingston, Ontario, reported that its sludge mass doubled with
iron addition, and its liquid sludge hauling effort was 60 per-
cent greater. Several other plants, ranging in size from 1,510
to 51,100 m3/day (0.4 to 13.5 mgd), reported increases of $5,000.00
to $25,000.00 for hauling in 1976 due to iron or alum addition.
The questionnaire responses also indicated that many plants
which historically have not hauled liquid sludge or sludge cake
found it necessary to start doing so after phosphorus removal
because:
1) the chemical sludge was unsuitable for the usual treat-
ment processes, or
2) the quantity of chemical siudge exceeded the capacity of
existing treatment facilities.
Liquid fludge hauling was frequently used by plants with
inadequate dewatering and incineration capacity to dispose of
extra sludge resulting from phosphorus removal. At Hatfield
Township, Pennsylvania, for example, the pressure filter and
incinerator could not handle all of the plant's sludge even when
run for 24 hr/day, 7 days/wk, so liquid sludge was hauled perio-
dically by a contractor. Other plants may have had the ability
to mechanically dewater the extra sludge, but they preferred to
haul it as liquid in order to avoid running the equipment longer
and hiring more personnel. For instance, at Lakewood, Ohio, the
plant could not handle the additional sludge volume generated by
alum addition on its schedule of vacuum filter and flash dryer
operation for one shift each day. Rather than extending filter
and dryer operation to two shifts per day and hiring three more
employees, the plant decided to have sludge hauled from the
digesters at $132.30/t ($120.00/ton). Details on costs and the
effects on plant performance are contained in Case Study No. F
(see Appendices)>
The questionnaire responses revealed only two plants --
Virginia, Minnesota, and Colotna, Michigan -- which were forced
to haul all of their sludge to land disposal after phosphorus
removal, abandoning their existing dewatering or incineration
facilities. At the Virginia plant, anaerobic digestion and dry-
ing beds were used for sludge handling previous to phosphorus
removal. In anticipation of phosphorus removal with lime, they
were replaced with a gravity thickener and lagoons. The sludge
refused to dry in the lagoons, so they were abandoned and liquid
sludge was then hauled to a disposal sitfe. This involved hauling
sludge three times/wk as opposed to cleaning the drying beds
twice each year. The plant was planning to purchase a tandem or
triaxle tank truck for $40,000.00 to haul 189 m3 (50,000 gal) of
liquid sludge per week, and a heated enclosure for the truck and
piping changes for $40,000.00. The estimated operating costs
are $4,500.00/yr.
120
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At Coloma, pressure filtration and incineration of sludge
were practiced when the plant went into operation. Phosphorus
removal with primary addition of lime and secondary addition of
alum was begun a year later. Because of the large volumes of
lime and flue dust used for sludge conditioning, the volatile
content of the sludge was only from 20 to 30 percent, and incin-
erator auxiliary fuel consumption was high. Flower farmers in the
area were contacted, and they agreed to take all of the sludge
for its ability to improve the sandy, low pH soil on their farms.
Phosphorus removal can reduce the number of suitable land
disposal alternatives which are available to a particular plant.
For instance, sludge TS concentration of at least 20 percent is
usually needed for landfilling of sludge along with other mater-
ials such as municipal solid waste, and at least a 40 percent TS
concentration is necessary for landfilling of sludge alone. Iron
and aluminum sludges are generally more difficult to dewater to
these concentrations than conventional sludges with many of the
dewatering methods available. Some lime sludges can easily be
dewatered to 40 percent TS, but the plant must have the equipment
to handle the large quantities of sludge which are generated with
lime. Furthermore, there must be room in the landfill for the
greater quantities of sludge which usually result from phosphorus
removal. At Toledo, Ohio, for instance, it was necessary to
switch from landfilling of vacuum filter cake to cropland appli-
cation because of the additional volume of cake produced during
phosphorus removal with ferric chloride.
LITERATURE
As with conventional sewage sludges, there is concern over
the beneficial or detrimental effects of chemical sewage sludge
application to croplands. There is special concern with regard
to chemical sludges because concentrations of nutrients, metals,
and organic and microbial constituents can be higher in these
sludges than in conventional sludges. The use of lime, aluminum
salts, or iron salts for phosphorus removal precipitates most of
the metallic cations contained in the wastewater as well as the
bulk of the phosphorus. These metals and the iron, aluminum, or
calcium of the precipitating chemicals are concentrated in the
sludge. A limited amount of investigation has been conducted
into the effects on crop yield, heavy metal toxicity to crops
and animals, and groundwater quality of applying these sludges
to croplands. Of primary concern has been the state of heavy
metals in sludge and the possibility of resollubilization in the
soil. Resolubilization makes the metals available to be concen-
trated in crops and leachate, presenting the possibilities of
toxicity tO'Crops and aquatic life and accumulation in the human
food chain and drinking water supply. Since resolubilization is
most likely to occur in low pH soils, it is expected that lime
sludges will present the least problem because of their tendency
to raise the soil pH (72).
121
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Kirkham and Dotson studied the growth of barley which was
irrigated with liquid sludges produced with the addition of fer-
ric chloride and alum to raw wastewater (39). The barley was
grown in pots of loam soil for 4 mo. The chemical sludges did
not limit the growth of the barley compared to plants fertilized
with inorganic fertilizer or non-chemical sludge.
Chawla, et al . , (15) conducted 1-yr lysimeter investigations
using alum, iron, and lime sludges from various Ontario treatment
plants. They planted orchard grass in loamy sand and silt loam
amended with sludge. They found that grass yields comparable to
fertilizer treatment could be achieved at the highest application
rates (900 kg/ha nitrogen) of lime and iron sludges but that alum
sludge produced much lower yields. Cumulative effects of higher
zinc equivalent, higher aluminum contents and low potassium
amounts in alum sludges might have retarded the plant growth.
Very high concentrations of petroleum hydrocarbons in the alum
sludge possibly caused some plant toxicity resulting in lower
yields.
Increasing the rates of sludge application increased the
nitrogen content of the plants; phosphorus did not show any mean-
ingful trend; potassium contents decreased with increasing rates
of sludge addition. Nickel, copper and zinc concentrations in
plant tissues were reasonably low and similar to the control and
fertilizer treatments. One year after sludging, no definite chem-
ical changes in the soil systems were observed. Decline in pH
values under highest rates of iron and alum sludges suggested a
trend toward increased mineralization and acidification due to
a lack of calcium.
Chawla and others (14) also conducted an intesive study of
the biochemical characteristics of chemical sewage sludge from
various municipal plants in the province of Ontario. The study
pointed out that sludge from sewage treatment plants must be
characterized on an individual basis to determine if possibly
hazardous concentrations of heavy metals or other contaminants
exist. The sludges studied varied widely in their biochemical
compositions. It should be remembered that high levels of speci-
fic metals in sludge are virtually always attributable to speci-
fic industries in the area served by the plant.
Where heavy metals are concentrated to a considerable degree
in sludge, Scott and Horlings (58) suggest that they can be eas-
ily removed by extraction with dilute acid. They experimented
with anaerobically produced sludges, and found that both metals
and phosphate can be extracted from thickener underflow sludge
or dewatered filter cake. The products of this extraction can
be further treated by other conventional means to recover phos-
phate or specific metallic components.
122
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CONCLUSIONS
Hauling sludge liquid or cake to a land disposal site has
been a common solution to many of the sludge processing problems
created by phosphorus removal. This has been the case even
though phosphorus removal generally increases sludge hauling
costs because of greater sludge volumes and the lower solids con-
centrations of many chemical sludges. In some cases, phosphorus
removal has shifted the economics of sludge processing favor of
hauling rather than dewatering and/or incineration. In other
cases, sludge hauling simply provides an interim solution to
problems although it is not necessarily the most cost-effective
alternative.
Cropland application of chemical sludges has raised many
serious questions with regard to the transmittal of contaminants
to plants and animals. The use of lime, aluminum salts, or iron
salts for phosphorus removal precipitates most of the metallic
cations contained in the wastewater as well as the bulk of the
phosphorus. These constituents, as well as the iron, aluminum,
or calcium of the precipitating chemicals,are concentrated in
the sludge. Chemical sludges therefore contain nutrients and
other elements which can be beneficial to plant growth. Lime
sludges can improve low pH, low calcium, or low phosphorus soils,
Chemical sewage sludges must be characterized on an individual
basis to determine if possibly hazardous concentrations of heavy
metals or other contaminants exist.
123
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SECTION 11
STATE-OF-THE-ART APPRAISAL
It is evident that the addition of chemical sludges to the
regular sewage treatment plant primary and secondary sludges
often has a significant impact upon subsequent sludge handling.
The volume and mass of the sludge increase and the percentage of
volatile solids decreases. Except in the case of lime sludges,
thickening and dewatering efficiencies are often adversely
affected. Chemical conditioning requirements change. Where
incineration is used, an increased need for supplemental fuel is
reported. The extent and seriousness of these and other effects
vary greatly between treatment plants. Each treatment plant is
unique, and neither the problems nor their solutions are univer-
sal. However, certain generalizations can be made from the data
obtained during this investigation.
Of the three chemicals normally considered for phosphorus
removal -- lime, iron salts, or aluminum salts « iron salts
generally appear to have the least overall adverse effect upon
subsequent sludge handling. This conclusion is based primarily
upon two factors:
1. The addition of lime generates a much greater mass of
sludge than does the addition of iron or aluminum salts.
2. The chemical sludge generated by the addition of alumi-
num salts is usually more difficult to thicken and/or
dewater than the sludge generated by the addition of
iron salts.
Obviously, for many plants these two advantages are over-
ridden by other considerations or else iron salts would be rou-
tinely used by all plants. Other considerations include phospho-
rus removal efficiency, geographical variations in chemical costs
and the corrosiveness of iron salts.
The next decision to be made is where in the sewage treat-
ment chain to apply the phosphorus removal chemical. For a typ-
ical activated sludge treatment plant, there appears to be some
advantage to adding iron or aluminum salts just ahead of the aer-
ation tank or directly to the mixed liquor, where good mixing is
achieved prior to discharge to the secondary clarifier. When
124
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using a ferrous iron salt, unless it is accompanied by a base,
removal of phosphorus takes place at the secondary treatment
stage if the chemical is added before the primary clarifier.
If lime is the chemical used, it is added to the primary
treatment step or occasionally to a special tertiary treatment
process. Lime is never added to the secondary biological process.
Several older plants which have inadequate volume capacity
for sludge handling have found that pumping chemical-1aden waste
activated sludge to the primary clarifier influent for settling
with the primary sludge reduced the volume of combined sludge to
be treated. At these plants, sludge handling considerations have
either been judged to outweigh the problem of deterioration of
primary effluent quality, or an increase in secondary clarifier
efficiency has counteracted the problem.
It has become common practice to thicken sludge in a gravity
thickener prior to further sludge treatment. In virtually all
cases, chemical sludges containing iron salts thicken much better
than sludge containing aluminum salts. Best results with either
iron or aluminum sludges are obtained with the addition of a poly-
mer (dosage range 0.5 to 1.0 mg/ji). It has also been found that
the lower the dosage of iron or aluminum salt used the easier
the resulting sludge is to thicken. For this reason, and to save
chemical costs, it is recommended that chemical feed equipment
be automatically controlled to prevent overdosage of more chemi-
cal than required to achieve the phosphorus reduction needed.
If it is necessary to thicken secondary biological sludge
separately, experience indicates that air flotation thickening
is superior to gravity thickening. Again, the addition of poly-
mers substantially improves performance.
Centrifuge dewatering of primary or combined chemical sludges
is greatly enhanced by polymer addition. Sludges containing iron
salts dewater much better than sludges containing aluminum salts.
Lime sludges dewater very well.
Anaerobic and aerobic sludge digestion is reported to be
essentially uninhibited by the addition of chemical sludges,
there being no toxic effects from the presence of the chemical
precipitates or the pH of the sludge. However, there is fre-
quently the need for more volume to handle the increased sludge
mass while maintaining proper retention time. When iron and
aluminum sludges are added to anaerobic digesters, there is also
commonly an adverse effect on supernatant quality and digested
sludge solids concentration because of poor solids-liquid separa-
tion.
Vacuum filtration of chemical sludges presented problems at
a number of plants due to increased solids mass, sludge volume,
125
-------
and/or poorer sludge dewatering characteristics. Experimentation
with chemical conditioning, e.g., polymer dosages, lime addition,
etc., generally led to improved vacuum filter performance. In
addition, changes in filter media were reported helpful. The
City of Milwaukee, Wisconsin, found in pilot tests that top feed
vacuum filtration of iron sludges was more effective than conven-
tional bottom feed filters. As was the case with centrifuges,
iron sludges are generally reported easier to dewater than alumi-
num sludges.
Thermal conditioning of chemical sludges is generally
reported successful prior to vacuum filtration or centrifugation.
Sludge cakes of 35 percent TS and above are routinely achieved.
Potential negative aspects are similar to those for non-chemical
sludges: sidestreams have high dissolved organic strength, and
operation and maintenance costs are high. One plant reported
excessive corrosion and erosion of the thermal conditioning unit
components, but it is not known if the problem was aggravated by
the chemical component of the sludge.
Because of the impacts of phosphorus removal on sludge pro-
duction and treatment, plants are now hauling more sludge in liq-
uid and cake form to land disposal sites than before. Hauling
sludge as a liquid or cake rather than dewatering or incinerating
has been a common solution to many of the difficulties experienced
by plants in dewatering and incinerating chemical sludges. In
some cases, phosphorus removal has shifted the economics of sludge
processing in favor of hauling rather than dewatering and/or
incineration. Especially in the case of lime sludges, it is
being found that land application of cake is preferable to lagoon
storage or incineration. In other cases, hauling simply provides
an interim solution to problems, although it is not necessarily
the most cost effective alternative. It is frequently relied
upon as an interim solution by plants which have inadequate capa-
city to handle the additional sludge generated by phosphorus
removal with existing facilities.
In view of the large amounts of chemical sludges being
applied to land, it is important that the negative or beneficial
effects on plants and animals be considered. Chemical sludges
contain nutrients and other elements which are beneficial to
plant growth. Lime sludges can improve low pH, low calcium, or
low phosphorus soils. Chemical sewage sludges must be character-
ized on an individual basis to determine if possibly hazardous
concentrations of heavy metals or other contaminants exist.
126
-------
TABLE 31. BIBLIOGRAPHY INFORMATION MATRIX
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TABLE 31 (continued)
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TABLE 31 (continued)
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41
42
43
44
45
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TABLE 31 (continued)
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-------
TABLE 32. KEY TO BIBLIOGRAPHY INFORMATION MATRIX
I Level of investigation:
L) Laboratory
P) Pilot
(F) Full-scale demonstration
(A) Actual operation
(R) Review
II Audience:
(M) Management
(E) Engineering
(S) Scientific
III Chemical precipitant:
Aluminum salt
(A) Alumi
(L) Lime
(I) Iron salt
(P)
Polymer
IV Point of addition:
(P) Pre-primary
(S) Pre-secondary or
secondary
(A) After secondary
V Sludges processed:
(1) Raw primary + WAS
(2) Digested primary +
(3 Raw primary + TF
(4 Digested primary
(5 Raw primary
(6) Digested primary
(7) Separate chemical
(8) Raw secondary
(9) Digested secondary
VI Thickening/blending:
WAS
(1
(2
(3
(4)
Gravity thickener
Flotation thickener
Stirred thickener
Centrifuge
VII Stabilization/reduction:
1) Composting
2) Aerobic digestion
3) Anaerobic digestion
4) Wet air oxidation
5) Pyrolysis
6) Chemical stabilization
VIII Conditioning/stabilization:
(1) Chemical conditioning
(2) Elutriation
(3) Thermal conditioning
(4) Radiation treatment
(5) Freezing
IX Dewatering:
(1) Pressure filter
(2) Air drying beds
(3) Centrifuge
(4) Vacuum filter
(5) Horizontal moving
screen concentrator
(6) Belt filter press
(7) Cylindrical rotating
gravity filter
(8) Capillary suction
(9) Lagoon
(10) Moving bed sand filter
(continued)
-------
TABLE 32 (continued)
CO
IN5
X Heat drying/incineration:
1) Flash or spray dryer
2) Tray dryer
3) Rotary kiln furnace
(4) Rotary kiln dryer
(5) Multiple hearth dryer
(6) Multiple hearth
incinerator
(7) Fluidized bed inciner-
ator
XI Final disposal:
ili
1) Agricultural fields
2) Land reclamation
(3) Power generation
4) Sanitary landfill
5) Ocean disposal
Private or authority-
owned dumpsite
Commercial or non-
commerical soil condi-
tioner
Resource recovery
(7)
(8)
XII Cost coverage:
XIII Sludge characteristics:
(R) Generation rates
(C) Chemical/physical charac-
teristics
* Article covers information
category generally
C) Capital costs
0) Operating costs
-------
BIBLIOGRAPHY
1. Alvord, E. T., et al. , "Phosphorus Removal by Ferrous
Iron and Lime," 11010 EGO 01/71, U.S. Environmental
Protection Agency, January 1971.
2. Baillod, C. R. , Cressey, G. M., and Beaupre, R. T.,
"Influence of Phosphorus Removal on Solids Budget,"
JWPCF, 49 (1): 131-145, 1977.
3. Baldock, E. H., "Metropolitan Toronto's Experience in
Phosphate Removal," Presented at Western Canada Water
and Sewage Conference, September 21, 1973, Toronto.
4. Bell, G. R., Libby, D. V., and Lordi, D. T., "Phos-
phorus Removal Using Chemical Coagulation and a Con-
tinuous Countercurrent Filtration Process, 17010EDO
06/70, Federal Water Quality Administration, U.S.
Dept. of the Interior, June 1970.
5. Bennett, S. M., and Bishop, D. F., "Solids Handling and
Reuse of Lime Sludge," Contract No. 14-12-818, U.S.
Environmental Protection Agency.
6. Black, S. A., "Anaerobic Digestion of Lime Sewage Sludge,"
Research Report No. 50, Canada-Ontario Agreement on
Great Lakes Water Quality, Environment Canada, Ottawa,
1976.
7. Boyko, B. I., "Aerobic and Anaerobic Sludge Digestion,"
In: Proceedings of the Sludge Handling and Disposal
Seminar, September 18-19, 1974, Toronto, pp. 46-62.
8. Boyko, B. I., and Rupke, J. W. G., "Phosphorus Removal
within Existing Wastewater Treatment Facilities,"
Research Report No. 44, Canada-Ontario Agreement on
Great Lakes Water Quality, Environment Canada, Ottawa,
1976.
9. Brandt, H. T., and Kuhns, R. E., "Apollo County Park
Wastewater Reclamation Project, Antelope Valley,
California," EPA-600/2-76-022, U.S. Environmental Protec-
tion Agency, March 1976.
10. Burns, D. E., and Shell, G. L., "Physical Chemical
Treatment of a Municipal Wastewater Using Powdered
Carbon." EPA-R2-73-264, U.S. Environmental Protection
Agency, May 1972.
133
-------
BIBLIOGRAPHY (Continued)
11. Burns, D. E., and Shell, G. L., "Physical-Chemical
Treatment of a Municipal Wastewater Using Powdered
Carbon," EPA-R2-73-264, U.S. Environmental Protection
Agency, August 1973.
12. Buzzel, J. C., Jr., and Sawyer, C. N., "Removal of
Algal Nutrients from Raw Wastewater with Lime," JWPCF,
39 (10): R16-R24, 1961.
13. Campbell, H., and LeClair, B. P., "Effects of Control
Variables and Sludge Characteristics on the Performance
of Dewatering and Thickening Devices," In: Proceedings of
the Sludge Handling and Disposal Seminar, September
|_a
to,
18-19, 1974, Toronto, pp. 280-314.
14. Chawla, V. K. , Stephenson, J. P., and Liu, D. , "Bio-
chemical Characteristics of Digested Chemical Sewage
Sludges," In: Proceedings of the Sludge Handling and
sp
63-94
Disposal Seminar, September 18-19, 1974, Toronto, pp.
ge
74,
15. Chawla, V. K., Bryant, D. N., and Liu, D., "Disposal
of Chemical Sewage Sludges on Land and Their Effects on
Plants, Leachate and Soil Systems," In: Proceedings
of the Sludge Handling and Disposal Seminar, September
18-19, 1974, Toronto, pp. 207-233.
16. Connell, C. H., "Phosphorus Removal and Disposal from
Municipal Wastewater," 17010 DYB02/71, U.S. Environ-
mental Protection Agency, February 1971.
17. Cornwell, D. A., and Zoltek, J. Jr., "Recycling of
Alum Used for Phosphorus Removal in Domestic Waste-
water Treatment," JWPCF. 49(4): 600-612, 1977.
18. Gulp, R. L., Evans, D. R., and Wilson, J. C., "Advanced
Wastewater Treatment as Practiced at South Tahoe,"
EPA-WQO-17010-ELQ-08/71 , U.S. Environmental Protection
Agency, August 1971 .
19. Gulp, G., Suhr, L. G., and Evans, D. R., "Physical-
Chemical Wastewater Treatment Plant Design," EPA
Technology Transfer Seminar Publication, August 1973.
20. Derrington, R. E., Stevens, D. H., and Laughlin, J. E.,
"Enhancing Trickling Filter Plant Performance by
Chemical Precipitation," EPA-670/2-73-060, U.S. Environ-
mental Protection Agency, August 1973.
134
-------
BIBLIOGRAPHY (Continued)
21. Dorr-Oliver, Inc., "Phosphate Extraction Process,"
Stamford, Conn., 1968.
22. Dow Chemical Company, "Application of Chemical Preci-
pitation Phosphorus Removal at the Cleveland Westerly
Wastewater Treatment Plant," Prepared for the City of
Cleveland, Ohio, April 1970.
23. Dunseth, M. G., et al., "Ultimate Disposal of Phosphate
from Waste Water by Recovery as Fertilizer," 17070ESJ01/
70, Federal Water Quality Administration, U.S. Dept. of
the Interior, January 1970.
24. Eikum, A. S., Carlson, D. A., and Lundar, A., "Phos-
phorus Release during Storage of Aerobically Digested
Sludge," JWPCF, 47(2): 330-337, 1975.
25. Parrel! , J. B., "Design Information on Dewatering
Properties of Wastewater Sludges," In: Proceedings of
the Sludge Handling and Disposal Seminar, September
18-19, 1974, Toronto, pp. 269-279.
26. Farrell , J. B., "Interim Report of Task Force on
Phosphate Removal Sludges," EPA-670/2-75-013, U.S.
Environmental Protection Agency, 1975.
27. Fowlie, P. J. A., and Shannon, E. E., "Utilization of
Industrial Wastes and Waste By-Products for Phosphorus
Removal: An Inventory and Assessment," Research Report
No. 6, Canada-Ontario Agreement on Great Lakes Water
Quality, Environment Canada, Ottawa, June 1973.
28. Ganczarczyk, J., and Hamoda, M.F.D., "Aerobic Digestion
of Organic Sludges Containing Inorganic Phosphorus
Precipitates, Phase 1," Research Report No. 3, Canada-
Ontario Agreement on Great Lakes Water Quality, Environ-
ment Canada, 'Ottawa, June 1973.
29. Gray, I. M. , "Phosphorus Removal Study at Barrie WPCP,"
Research Branch, Ontario Ministry of the Environment,
Toronto, 1972.
30. Gray, I. M., "Phosphorus Removal Study at the Sarnia
WPCP," Research Report No. 14, Canada-Ontario Agreement
on Great Lakes Water Quality, Environment Canada, Ottawa,
1972.
135
-------
BIBLIOGRAPHY (Continued)
31. Greenland, T. W. , and Gaines, F. R. , "Hatfield Town-
ship, Pennsylvania Advanced Waste Treatment Plant,"
Tracy Engineers, Inc., Camp Hill, Pennsylvania, 1977.
32. Grigoropoulos, S. G., Vedder, R. C., and Max, D. W.,
"Fate of Aluminum-Precipitated Phosphorus in Acti-
vated Sludge and Anaerobic Digestion," JWPCF, 43(12):
2366-2382, 1971.
33. Hamoda, M. F., and Ganczarczyk, J., "Aerobic Diges-
tion of Sludges Precipitated from Wastewater by Lime
Addition," JWPCF. 49(3): 375-387, 1977.
34. Hudgins, R. R., and Silveston, P. L., "Wet Air
Oxidation of Chemical Sludges," Research Report No.
12, Canada-Ontario Agreement on Great Lakes Water
Quality, Environmental Canada, Ottawa, March 1973.
35. Jacke, R. , "Polymer Cuts Disposal Costs," Water and
Sewage Works, 123(6): 99-100, 1976.
36. Jenkins, D. , Ferguson, J. F., and Menar, A. B.,
"Chemical Processes for Phosphate Removal," Water
Research. 5: 369-389, 1971.
37. Johnson, E. L., Beeghly, J. H., and Wukasch, R. F.,
"Phosphorus Removal at Benton Harbor-St. Joseph,
Michigan," Dow Chemical Company, Midland, Michigan,
1969.
38. Keith, F. W. , Jr., "Centrifuges — Types and Applications,"
In: Proceedings of the Sludge Handling and Disposal
Seminar. September 18-19, 1974, Toronto, pp. 351-368.
39. Kirkham, M. B., and Dotson, G. K., "Growth of Barley
Irrigated with Wastewater Sludge Containing Phosphate
Precipitants," In: Proceedings of the National Confer-
ence on Municipal Sludge Management, June 11-13, 1974,
pp. 97-106.
40. Knight, C. H., Mondoux, R. G., and Hambley, B., "Thick-
ening and Dewatering Sludges Produced in Phosphate
Removal," Paper presented at Phosphorus Removal Design
Seminar, May 28-29, 1973, Toronto.
136
-------
BIBLIOGRAPHY (Continued)
41. Koers, D. A., "Aerobic Digestion of Wastewater Sludges,"
Paper presented at Technology Transfer Seminar on
Sludge Handling Disposal, February 16-19, 1977, Calgary,
Alberta.
42. Malhotra, S. K., Parrillo, T. P., and Hartenstein,
A. G., "Anaerobic Digestion of Sludges Containing Iron
Phosphates," Journal of the Sanitary Engineering Divi-
sion, ASCE, 97(SA5):629-646, October 1971.
43. Martin, L., and Nardozzi, A. D., "Operational Consid-
erations Associated with Chemical and Biological Sludge
Generated by Lime Precipitation," Paper presented at
the National WPCF Conference, October 1976, Minneapolis,
Minnesota.
44. Mignone, N. A., "Anaerobic Digester Supernatant Does
Not Have to be a Problem," Water and Sewage Works.
123(12): 57-59, 1976. ~~
45. Minton, G. R., and Carlson, D. A., "Primary Sludges
Produced by the Addition of Lime to Raw Waste Water,"
Water Research, 7: 1821-1847, 1973.
46. Motamedi, M., "Dewatering of Ferric Chloride Coagu-
lation Sludge," Water Research. 9: 861-864, 1975.
47. Mulbarger, M. C., et al., "Lime Clarification, Recovery,
Reuse, and Sludge Dewatering Characteristics," JWPCF,
41(12): 2070-2085, 1969.
48. Novak, J. T., and Montgomery, G. E., "Chemical Sludge
Dewatering on Sand Beds," Journal of the Environmental
Engineering Division, ASCE, IQI(EEl):1-14, February
TWT.
49. Opferkuch, R. E., Ctvrtnicek, T. , and Mehta, S. M.,
"Study of Utilization and Disposal of Lime Sludges
Containing Phosphates," EPA-R2-73-282, U.S. Environmental
Protection Agency, June 1973.
50. O'Shaughnessy, J. C., et al. , "Digestion and Dewatering
of Phosphorus-Enriched Sludges," JWPCF, 46(8): 1914-
1926, 1974.
137
-------
BIBLIOGRAPHY (Continued)
51. O'Shaughnessy, J. C., et al., "Soluble Phosphorus
Removal in the Activated Sludge Process. Part II:
Sludge Digestion Study," EPA-1701O-EIP-10/71 , U.S.
Environmental Protection Agency, October 1971.
52. Proceedings of the Technical Seminar on Physical-
Chemical Treatment, Ontario Ministry of Health
Laboratories, March 9, 1972.
53. Recht, H. L., and Ghassemi, M., "Kinetics and Mechan-
ism of Precipitation and Nature of the Precipitate
Obtained in Phosphate Removal from Wastewater Using
Aluminum (III) and Iron (III) Salts," 17010EKI 04/70,
Federal Water Quality Administration, U.S. Dept. of
the Interior, April 1970.
54. Roe, I. P., "Sludge Dewatering Using the Kruger
Centrifuge," In: Proceedings of the Sludge Handling
and Disposal Seminar, September 18-19, 1974, Toronto,
pp. 369-384.
55. Salib, W. A., "Sludge Handling and Disposal Practices
in Metropolitan Toronto," In: Proceedings of the
Sludge Handling and Disposal Seminar, Septemhier 18-19,
1974, Toronto, pp. 452-465.
56. Schmid, L. A., and McKinney, B. E., "Phosphate Removal
by a Lime-Biological Treatment Scheme," JWPCF, 41(7):
1259-1276, 1969.
57. Schroeder, W. H., "Principles and Practices of Sludge
Incineration," Paper presented at the Technology
Transfer Seminar on Sludge Handling Disposal, February
1977, Calgary, Alberta.
58. Scott, D. S., and Horlings,H., "Removal of Phosphates
and Metals from Sewage Sludge," Research Report No.
28, Canada-Ontario Agreement on Great Lakes Water
Quality, Environment Canada, Ottawa, 1973.
59. Scott, D. S., and Horlings, H., "Removal of Phosphates
and Metals from Sewage Sludges," In: Proceedings of
the Sludge Handling and Disposal Seminar, September
18-19, 1974, Toronto, pp. 413-443.
60. Scott, D. S., and Horlings, H., "Removal of Phosphates
and Metals from Sewage Sludges," Environmental Science
and Technology. 9(9): 849-855, 19757
138
-------
BIBLIOGRAPHY (Continued)
61. Shannon, E. E., Plummer, D., and Fowlie, P.J.A.,
"Aspects of Incinerating Chemical Sludges," In:
Proceedings of the Sludge Handling and Disposal
Seminar, September 18-19. 1974. Toronto. PP. 391-412.
62. Singer, P. C., "Anaerobic Control of Phosphate by
Ferrous Iron," JWPCF, 44(4): 663-669, 1972.
63. Smart, J., "Anaerobic Sludge Digestion Processes,"
Paper presented at Technology Transfer Seminar on
Sludge Handling Disposal, February 1977, Calgary,
Alberta.
64. Smart, J., and Boyko, B. I., "Full Scale Studies on
the Thermophilic Anaerobic Digestion Process," Research
Report No. 59, Canada-Ontario Agreement on Great Lakes
Water Quality, Environment Canada, Ottawa, 1977.
65. Smith, A. G., "Centrifuge Dewatering of Lime Treated
Sewage Sludge," Paper No. W2030, Research Branch,
Ontario Ministry of the Environment, Toronto, 1972.
66. Stickney, R., and LeClair, B. P., "The Use of Physico-
chemical Sludge Characteristics and Bench Dewatering
Tests in Predicting the Efficiency of Thickening and
Dewatering Processes," In: Proceedings of the Sludge
Handling and Disposal Seminar, September 18-19, 1974,
Toronto, pp. 315-350.
67. "Studies on Removal of Phosphates and Related Removal
of Suspended Matter and Biochemical Oxygen Demand,"
May-October 1967, Lake Odessa, Michigan, Wastewater
Section, Division of Engineering, Michigan Dept. of
Public Health.
68. Tofflemire, T. J., et al., "Tertiary Treatment for
Phosphorus Removal by Alum Addition at Richfield Springs,
New York," Research Paper No. 39, Environmental Quality
Research Unit, New York State Dept. of Environmental
Conservation, March 1976.
69. U.S. Environmental Protection Technology Transfer,
"Physical-Chemical Wastewater Treatment Plan Design,"
EPA Technology Transfer Seminar Publication, 1973.
70. University of Guelph, Ontario, "Land Disposal of Sewage
Sludge. Vol. I," Research Report No. 16, Canada-
Ontario Agreement on Great Lakes Water Quality,
Environment Canada, Ottawa, 1973.
139
-------
BIBLIOGRAPHY (Continued)
71. Van Fleet, G. L., Barr, J. R., and Harris, A. J.,
"Treatment and Disposal of Chemical Phosphate Sludge
in Ontario," JWPCF. 46(3): 582-587, 1974.
72. Van Fleet, G. L., Barr, J. R., and Harris, A. J.,
"Treatment and Disposal of Chemical Phosphate Sludge
in Ontario," Presented by J. R. Barr at the Annual
Conference of the Water Pollution Control Federation,
October 1972, Atlanta, Georgia.
73. Van Loon, J. C., "Heavy Metals in Agricultural Lands
Receiving Chemical Sewage Sludges," Research Report
No. 9, Canada-Ontario Agreement on Great Lakes Water
Quality, Environment Canada, Ottawa, March 1973.
74. Van Loon, J. C., "Heavy Metals in Agricultural Lands
Receiving Chemical Sewage Sludges. Vol. II," Research
Report No. 25, Canada-Ontario Agreement on Great Lakes
Water Quality, Environment Canada, Ottawa, 1975.
75. Van Loon, J. C., "Heavy Metals in Agricultural Lands
Receiving Chemical Sewage Sludges. Vol. Ill," Research
Report No. 30, Canada-Ontario Agreement on Great Lakes
Water Quality, Environment Canada, Ottawa.
76. Villiers, R. V., "Thickening and Dewatering Phosphorus-
Laden Chemical Sludges, Field Operation--!974,"
U.S. Environmental Protection Agency, 19.74.
77. Williams, T. C., "Phosphorus is Removed at Low Cost,"
Water and Wastes Engineering, 13(11): 52-63, 1976.
78. Wood, G. M., "Land Application of Processed Organic
Wastes," In: Proceedings of the Phosphorus Removal
Design Seminar Conference, May 28-29, 1973, Environ-
ment Canada, Toronto.
79. Zaloum, R., "Activated Sludge Characterization and
Settling," In: Proceedings of Phosphorus Removal
Design Seminar, May 28-29, 1973, Environment Canada,
Toronto.
140
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OMB #158-5-77002
February 1978
APPENDIX A
PHOSPHORUS-LADEN SLUDGE MANAGEMENT QUESTIONNAIRE
When completed, mail to:
SCS Engineers
4014 Long Beach Boulevard
Long Beach, California 90807
(213) 426-9544
I. GENERAL INFORMATION
1. Full plant name:
2. Address:
3. Phone number:
4. Name of person completing questionnaire:
5. Title:
6. Name of alternate contact for technical information:
7. Title:
8. Approximate time when (a) primary plant was built:
(b) secondary treatment was installed:
(c) phosphorus removal was begun:
9. Has the plant ever participated directly in any EPA, university, or other
study pertinent to phosphorus-laden sludge management?
Yes No
II. RAW SEWAGE INFLUENT INFORMATION (Recent monthly averages)
1. Vol ume: mgd
2. pH: BOD5: mg/t COD: mg/£ SS: mg/£
Total-P: lmg/£ VSS mg/£
3. Estimated percentage of industrial wastes %
4. Do you get sludge from the water purification plant? Yes
No Don' t know
5. Storm and sanitary sewers combined or separate (check one)
141
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III. PLANT DESIGN INFORMATION (Please attach flow diagram if available),
1. Primary treatment (circle below)
Screening
Aerated grit chamber
Non-aerated grit chamber
Others (please explain)
2.
3.
Gravity settling
Flocculation chamber
Secondary treatment (circle below)
Conventional activated sludge
Trickling filter
Final clarifier
Others (please explain)_
Modified activated sludge:
Step-aeration
Contact stabilization
Extended aeration
Complete mix
Pure oxygen
Sludge treatment/disposal -
Circle boxes on Figure 1 on the next page and connect with arrows
to show your sludge treatment/disposal chain. Indicate any
points of chemical addition as well.
IV. PHOSPHORUS REMOVAL INFORMATION (Give recent monthly averages)
1. Effluent total-P: mg/£
2. Chemicals added and chemical dosage -
a. Chemicals added to b. Percent of
remove phosphorus:
(circle below)
Lime
Ferric chloride
Ferrous sulfate
A!um(aluminum sulfate)
Sodium aluminate
Polymer
Other(name below)
chemical in
sol'ution added:*
% Ca(OH)2
% Fed 3
% FeS04
% A12(S04)3 ~
c. Average
amounts added:*
9Pd
_gpd
__SPd
*Explanation of parts b and c: We assume you add a solution containing
a certain percentage of actual chemical to the wastewater. Give that
percentage in part b. Then tell how much of that solution you add in
part c. Use other units such as gal/mil gal of sewage rather than gpd
if you wish, but please indicate units. If you wish to respond to this
section in another way, by giving the dosage of dry chemical in mg/i or
Ib/day, for example, write the appropriate information below:
142
-------
Indicate the sources of the chemicals by circling beiow and writing the name
of the chemical next to its source:
Chemical manufacturer
Industrial waste product
Drinking water purification sludge
Other (specify) ^__
Point of chemical addition:(circle below)
Before primary clarifier Primary clarifier
Activated sludge tank Trickling filter
Activated sludge effluent channel Trickling filter effluent channel or
After secondary clarifier pipe
Other (please explain) Secondary clarifier
V. SLUDGE HANDLING INFORMATION
1. Does your plant combine all of its sludges --primary, waste activated or
trickling filter, and chemical — together for dewatering and disposal, etc.?
Yes No
2. If answer to previous question #1 is no, indicate what sludges you treat or
dispose of individually: (circle below)
Primary Secondary Chemical
Combined Primary/Secondary Other (specify)
3. Chemical conditioners used for dewatering: (circle below)
Lime Ferric chloride Polymer Others (specify)
143
-------
Circle applicable boxes and connect with lines to show sludge
treatment/disposal chain.
(1)
(2)
(3)
(4)
(5)
(6)
(7)
SLUDGE TYPE THICKENING STABILIZATION CONDITIONING
BLENDING REDUCTION STABILIZATION
DEWATERING
HEAT DRYING REDUCTION FINAL
STABILIZATION DISPOSAL
PRIMARY
SECONDARY
CHEMICAL
GRAVITY
FLOTATION
CENTRIFUGE
COMPOSTING
AEROBIC
DIGESTION
ANAEROBIC
DIGESTION
LIME
TREATMENT
CHLORINE
TREAT.
-
-
CHEMICAL
ELUTRJATION
HEAT TREAT.
TRAY
DRYER
r-
L
INCINER-
ATION
WET AIR
OXIDATION
PYROLYSIS
-
CROPLAND
LAND
RECLAM
POWER
GENERATION
SANITARY
LANDFILL
OCEAN
DISPOSAL
PRIVATE OR
AUTHORITY-
- OWNED
DUMPSITE
-------
VI. SLUDGE CHARACTERISTICS (Recent monthly averages)
1. Sludge characteristics before thickening or stabilization by
digestion, heat,or chemical treatment:
(For those who answered question V-2 above, there is room for
the characteristics of more than one type of sludge.)
Type of sludqe:
Volume (gpd)
TS (%, dry wt)
VS (X of TS)
COD (mg/*)
Substitute other
units (e.g. , mg/a )
( j
( ,
( )
Sidestreams generated by sludge treatment units: (circle below)
Centrate Digester supernatant
Filtrate Thickener supernatant Other(specify)
Sidestream characteristics:
Type of sidestream:
Volume (gpd)
SS ( X, dry wt)
VSS( X- of SS)
COD(mq/i)
BODi;(mg/*)
Substitute other units
(e.g.,.mg/£, TS, etc.)
( )
( )
( )
( }
( )
Final TS content of treated liquid sludge before drying, dewatering,
or ultimate disposal: % TS, dry wt.
VII. PHOSPHORUS REMOVAL IMPACT ON PLANT OPERATION
1. Additional sludge generated - (Give typical monthly averages)
a. Total sludge volume before plant had phosphorus removal: .
b. Sludge TS before plant had phosphorus removal: X dry wt
c. Wastewater flow volume before plant had phorphorus removal: _
d. Wastewater SS before plant had phosphorus removal:_ mg/x.
JPd
or mg/n_
mgd "
u. waai.ewai.ei JJ UGIWIC pmm. >•«•- r..—~r — - . -•
e. If possible, estimate how much additional sludge is generated as a result
of phosphorus removal: #/day dry solids _
2 How have changes in sludge volume or mass and sludge characteristics affected
plant operations? For instance, does the phosphorus-laden sludge require
more time, energy, equipment, etc. to treat/dispose of than the sludge before
P-removal did? How? Is the phosphorus-laden sludge more difficult to settle,
thicken, dewater, digest, ect.? How? How did you. solve any problems^which
resulted from phosphorus removal? Please respond to these questions in as
much detail as possible. This is-the most important part of the questionnaire.
We appreciate your thoughtful response.
145
-------
Please use other side or extra pages if needed.
3. If not covered above, please discuss any adverse effects of the phosphorus-
laden sludge on anaerobic digestion?
4. Amount of extra labor, energy, equipment, and supplies needed because of
changes in sludge volume or mass and sludge characteristics (please estimate)-
a. New equipment for sludge treatment/disposal:
Equipment item
Year
Added
Capital cost
($)
Estimated operating
cost ($/yr)
Year
b. Additional labor for maintenance and operation of all sludge treatment/
disposal equipment:
Area where additional labor
was needed
Number of additional labor
hours (hr/yr)
Cost
(S/hr)
146
-------
c. Additional energy and fuel requirements for operation of all sludge
treatment/disposal equipment:
Equipment item
or area of impact
Type of
energy/fuel
Number of additional
KWH, MCF, etc./yr
Cost (S/KWH
or MCF,. etc.!
d. Additional chemical supplies for sludge treatment/disposal: (Include
chemicals for conditioning, thickening, oxidation, etc.)
Chemical
Area of use of chemical
Additional Ib of chemical,
(cu ft oxygen), etc. /day
Cost($/lb
or cu ft)
e. Additional outside costs related to sludge treatment/disposal (e.g., increased
sludge volume means must pay contractor more to haul it away):
Type of additional outside cost
Cost(5/yr)
Year
5. Effects of. P-removal- (Answer only questions which, apply to your plant and for
which you-have avail able-data or can:estimate. You may- answer part of a question),
a. Incineration or mechanical heat drying rate (#wet cake/hr*)
Increase? Before » #wet cake/hr
Decrease? After *
Same?
"~#wet cake/hr
b. Dewataring rate (Idry sludge/hr*)
Increase? Before: »
Decrease? After *
Same?
Idry sludge/hr
#dry sludge/hr
*Note parts a & b: If the size of the dewatering device, incinerator, or
mechanical heat dryer was changed between the "before" and "after" periods,
report the "yield" instead of the "rate" by dividing by the effective area or
volume of the device, and note this change.
147
-------
c.
d.
Cost of chemical
Increase?
Decrease?
Same?
conditioning ($/dry
Before =
After =
ton solids
dewatered):
$/dry ton solids
$/dry ton solids
cake TS content (% TS, dry wt):
Before = %, dry wt
%, dry wt
Dewatered
Increase?
Decrease?
Same ?
(Current dewatered cake VS content (% of TS):
After =
Thank you for your cooperation in providing us information vital to the success-
ful completion of this research. If there are any questions, please contact
Lee Hammer or Michael Swayne at SCS Engineers, phone collect: (213) 426-9544.
148
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APPENDIX B
OUTLINE FOR COLLECTION OF FIELD DATA
I. GENERAL INFORMATION
A. Full plant name; operating authority name
B. Addresses
C. Phone numbers
D. Names and titles of personnel interviewed by SCS
E. Approximate times when all additions and modifications
to plant affecting sludge were made
F. History of experimentation at plant with modes of
operation affecting sludge
1. Points of chemical addition
2. Chemicals used for P-removal
3. Chemical conditioners used
4. Equipment used
5. Machine and process operating parameters
6. Inclusion of chemical sludge with other sludges
7. Disposal method
6. Previous studies done
H. Design engineer name, address, and phone
I. Equipment manufacturers' names, addresses, and phones
J. Engineering consultant's name, address, and phone
K. Unusual wastes accepted at plant
L. Variations in flow
M. Before and after periods dates and modes of operation
N. Design flow
0. Average flow
P. Variations in wastewater treatment operations
Q. Unusual problems in plant operation affecting plant
performance
II. GEN.ERAL DESCRIPTION OF WASTEWATER TREATMENT OPERATIONS
A. Diagram of wastewater treatment facilities
1. Treatment units and arrangement
149
-------
B. Machine operating parameters
1. Grit chamber (if chemical is added here)
2. Flocculators (if chemical is added here)
3. Clarifiers
4. Activated sludge or trickling filter units
5. Channels where chemicals may be added
6. Tertiary treatment units
a. dimensions
b. type
III. PROCURING, STORING, MIXING, PUMPING, DISTRIBUTION, AND
ADDITION OF CHEMICALS FOR PHOSPHORUS REMOVAL
A. Diagram of chemical handling operations
1. Handling facilities and arrangement
B. Description of all procedures pertaining to these
operations
1. Quantities and rates
2. Types of chemicals
3. Types of equipment
4. Automated or manual handling and addition
5. Locations
6. Timing
7. Personnel
IV. DETAILED DESCRIPTION OF SLUDGE TREATMENT AND DISPOSAL OPERATION?
A. Diagram of sludge treatment/disposal facilities including
sidestream and sludge recirculation (treatment units and
arrangement)
1. Storage (including storage in clarifiers)
2. Screening, degritting
3. Pumping
4. Blending
5, Thickening
6. Conditioning
7. Stabi1izing
8. Dewatering
9. Reduction
10. Disposal
11. Sidestream treatment
B. Machine operating parameters
1 . Dimensions
2. Shape
150
-------
3. Type
4. Brand name
5. Mechanical variations
6. Continuous or batch flows accepted
7. Expected life of equipment
8. Etc.
C. Machine operating variables
1. Speed
2. Temperature
3. Pressure
4. Vacuum
5. Cycle time
6. Fuel consumption >
7. Etc.
D. Process operating variables
1. Hydraulic flow rates in and out
2. Chemical addition rates
3. Run lengths
4. Solids flow rate
5. Solids loading
6. Hydraulic loading
7. Etc.
E. Process performance variables (excluding screening, degrit-
ting, and pumping)
1. Input and output stream characteristics
a. total solids content
b. sludge solids content
c. volatile solids content
d. BOD or COD concentration
e. total phosphorus concentration
f. heavy metals concentrations
g. etc.
2. Performance characteristics
a. yield
b. volatile solids reduction
c. volume reduction
d. cake thickness
e. etc.
F. Diagrams of hydraulic and materials balances (excluding
screening, degritting, and pumping)
1. Partition of loading between outputs
151
-------
a. TS
b. sludge solids
c. VS
d. BOD or COD
e. total-P
f. heavy metals
V. OPERATIONAL PROBLEMS ATTRIBUTABLE TO THE PRODUCTION OF
CHEMICAL SLUDGE
A. Impact on wastewater treatment operations
1, Changes in process performance variables
a,
b
clarifiers
activated sludge or trickling filter units
1) input and output characteristics of
all units
a)
b)
c)
d
e
f
SS concentration
BOD or COD concentration
VSS concentration
total-P concentration
heavy metals concentration
viruses, etc.
pH, alk., sol. org. carbon
2) performance characteristics
a) settleabil ity or sludge volume
index of sludge
b) #SS produced/#BOD removed in
activated sludge (or TF)
c) #/day BOD removed in activated
sludge (or TF)
2. Changes in hydraulic and materials balances of
primary and secondary treatment units
a. wastewater flow
b. SS
c. BOD or COD
d. VSS
e. others = P, pH, alk. (all forms), hardness
(all forms)
B. Additional sludge quantities, including sidestreams
1. From clarifiers
2. From thickeners
3. From conditioners
152
-------
4. From stabilizers
5. From dewatering unit
6. From reduction unit
7. To disposal
8. To sidestreams
a. additional mass of dry sludge
1) observed mass
2) calculated mass
b. additional volume of liquid sludge
1) observed volume
Impact on sludge treatment and disposal operations
1. Impact on facilities
a. uninitiated changes in process operating
variables
1) flow rates
2) loading rates
3) retention times
4} etc.
b. impact on physical condition and functioning
of equipment, including functioning of new
equipment
1) corrosion
2) grit
3) pH
4) mixing
5 foaming
6 wear
7 mechanical failure
2. Impact on performance
a. changes in process performance variables
1) input and output characteristics
a) TS, VS, sludge solids, SS (sidestreams)
b) BOD, COD
c) etc.
2) performance characteristics
) yield
) volume reduction
153
-------
c) volatile solids reduction
d) etc.
b. changes in hydraulic and materials balances
1) partition of loading between outputs
a ) TS, VS, sludge sol ids
b ) BOD or COD
c ) etc.
Problem resolution
1. Personnel-initiated changes in machine and process
operating variables
a. machine temperature, pressure, etc.
b. chemical addition
c. fuel consumption, etc.
d. run len.gths
e. etc.
2. New construction and equipment required
a. type, space required, etc.
b. installation time, safety measures, etc.
3. New operation and maintenance activities
a. cleaning, adjustments, breakdowns, etc.
b. setting levels, monitoring, starting and
shutting off, keeping records
c. amount of labor involved
d. new personnel required
Impact on meeting external constraints restricting
sludge treatment and disposal operations
1. Environmental constraints
a. moisture content of land-filled sludge;
heavy metal content of agricultural land-
applied sludge; etc.
2. Economic constraints
a. budget limitations; specific chemical or fuel
costs, etc.
3. Land and materials availability constraints
a. land for disposal; industrial by-product
chemicals, etc.
154
-------
4. Social-acceptability constraints
a. putrefaction odor control; disposal site
location, etc.
VI. OPERATIONAL COSTS OF SLUDGE TREATMENT/DISPOSAL
A. Operations
1. Phosphorus removal operations
a. procuring chemicals
b. storing chemicals
c. mixing chemicals
d. pumping chemicals
e. distribution of chemicals
f. addition of chemicals
2. Wastewater treatment operations
a. flocculators and clarifiers
b. activated sludge or trickling filter units
c. return sludge pumping
3. Sludge treatment/disposal operations
a. storage (including sludge storage in clarifiers)
b. screening, degritting
c. pumping (including sludge and sidestream recir-
culation)
d. blending
e. thickening
f. conditioning
g. stabilizing
h. dewatering
i. reduction
j. disposal
k. sidestream treatment
B. Costs
1. Operating and maintenance costs
a. labor costs
1 } operation and supervision
2) maintenance
b. supply costs (costs at the site)
1) chemicals
155
-------
a) solids
b) liquids
c) gases
2) water
3) maintenance supplies
c. energy costs (costs at the site)
1 ) natural gas
2) electricity
3) fuel oil
d. outside servicing and contracting
service and maintenance of equipment
residual disposal
e. overhead
2. Investment costs
a. construction
1 ) site preparation
2 ) structures
3 ) buildings
b. mechanical equipment on an installed basis
c. piping on an installed basis
d. electrical on an installed basis
e. instrumentation on an installed basis
f. engineering design
g. land
Cost variables
1. Items or categories of labor, supplies, energy, and
services utilized
2. Unit costs of labor, supply, energy, and service
i terns
3. Amounts of labor, supply, energy, and service items
used
4. Components of outside servicing and contracting,
overhead, engineering design, construction, and land
costs
-------
5. Cost break-down by components of outside servicing
and contracting, overhead, engineering design,
construction, and land costs
6. Description of mechanical equipment, instrumentation,
piping, electrical, construction, and land items.
157
-------
COST MODULE
Investment Costs
1. Land: value/acre In general locality $
Treatment system acres $/acre
Auxiliary support acres $/acre
(includes utilities,
offices, roads, etc.)
Total investment (excluding land and support buildings)
and year of module installation:
$ Year
Installation time (excluding design) months,
useful life , salvage value (items)
$
Structures (steel, dikes, slabs): $
$
$
$
5. Piping and-valves (type, length,
size): $
$
$
$
6. Mechanical equipment (type, size): $
$
$
$
7. Electrical system: $
$
$
158
-------
Cost Module (Continued)
8. Instrumentation (type): $
$
$
9. Support (enclosures, access): $
$
$
10. Engineering design cost: $
11. Legal and administrative cost: $
12. How was system financed?
13. Was module individual or part of a larger- installation?
B. Operation and Maintenance Costs for 19_
1. Days per year of operation
2. Operation assumptions (flow or activity/day)
3. Labor requirements:
Category Hrs/day $/hr
a.
b.
c.
159
-------
Cost Module (Continued)
4. Energy requirements:
Electricity KWH/day , $/KWH
Fuel (unit) /day, $/unit
5. Chemical costs
Type $/unit as _
Type $/unit as _
Type $/unit as _
6. Other supplies (type)
$/unit
7. Maintenance (labor and parts) $/year
8. Overhead (administration)
Percent attributed to module $/year
9. Residual disposal
Transportation $/unit
Disposal $/unit
10. Laboratory or analysis costs (type)
$/unit
160
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APPENDIX C
CASE STUDIES
INTRODUCTION TO CASE STUDIES
Nine case study sites were selected which were rated rela-
tively high on the basis of completeness of historical operating
data, cooperation of plant personnel, a history of problem-solving
and innovation at the plant, performance of dewatering devices,
and potential for demonstrating the impacts of phosphorus removal
on sludge. The plants were also chosen to represent a variety of
possibilities with respect to the following factors:
Plant size
Plant type
Phosphorus removal chemical(s)
Point(s) of .chemical addition
Point of combination of chemical-laden and other sludges
Sludge treatment and disposal methods.
Table C-l presents a description of the nine plants with
respect to these factors.
CASE STUDY C: SOUTH BEND, INDIANA
Introduction
The South Bend Wastewater Treatment Plant adopted tertiary
treatment with lime in 1974 in anticipation of stricter Indiana
effluent regulations covering BOD, SS, and dissolved oxygen.
The existing secondary treatment plant was found incapable of
meeting the anticipated effluent requirements without consider-
able enlargement of plant capacity. An upflow clarifier system
was installed to provide the tertiary treatment because it per-
mits effective polishing of the effluent to remove BOD and SS as
well as phosphorus. If phosphorus removal alone had been the
objective, tertiary treatment with lime probably would not have
been chosen. While the tertiary facilities were under construc-
tion, a study by Tenech, a wastewater engineering consulting firm
for the city of South Bend, indicated that Time should not be
used because of the high cost and the huge amounts of sludge
which would be generated - an estimated 907 t (100 tons)/day.
161
-------
O»
ro
TABLE C-l. DESCRIPTION OF CASE STUDY SITES
ACCORDING TO PLANT SELECTION FACTORS
Plant Size
Plant Type
Phosphorus
Removal Chemi-
cal(s)
Point(s) of
Chemical
Addition
Point of
Combination
of Chemical -
Laden and
Other Sludges
Sludge Treat-
ment and Dis-
posal Methods
South Bend
42.0 mgd
Step Aera-
tion Acti-
vated Sludge
Fed,, Poly-
mer
Tertiary
Anaerobic
Digester
Gravity
Thickener,
Anaerobic
Digester,
Air Dry-
Ing on
Land .Non-
commercial
Fertilizer
Sheboygan
10.5 mgd
Trickling
Filter
Fed,, Poly-
mer
Secondary
Gravi ty
Thickener
Gravity
Thickener,
Vacuum Fil-
ter, Flu-
Hized Bed
Incinerator
Coldwater
2.4 mgd
Trickling
Filter
Fed,, Poly-
mer J
Primary
Before Pri-
mary Clari-
fier
Anaerobic
Digester,
Drying Beds,
Dump site
Midland
6.5 mgd
Trickling
Filter
Fed,, Poly-
mer
Primary
Before Pri-
mary Clari-
fier
Thermal Con-
ditioner,
Vacuum Fil-
ter, Sani-
tary Land-
fill, Com-
posting,
Non-com-
mercial Soil
Conditioner
Port Huron
11.6 mgd
Activated
Sludge
Al2(S04)3.
Polymer
Secondary
Before Gra-
vity Thick-
ener
Gravity
Thickener,
Centrifuge,
Fluidized
Bed Incin-
erator, Ash
Thickener,
Ash Vacuum
Filter
Pontiac
25.5 mgd
Activated
Sludge
Fed,, Poly-
mer
Primary
Before Pri-
mary Clarifier
Anerobic
Digester,
Vacuum Fil-
ter, Incin-
erator
Lakewood
13.4 mgd
Activated
Sludge
A12(S04)3
Secondary
Before
Gravity
Thickener
Anaerobic
Digester,
Flash Dryer,
Drying Beds.
Non-commer-
cial Soil
Conditioner,
Liquid Sludge
Hauling
Mentor
5.3 mgd
Activated
Sludge
A12(S04)3
Secondary
Sludges Not
Combined
Aerobic Di-
gester, Dual
Cell Gravity
Concentrator,
Agricultural
Land Applica-
tion
Brookfield
2.4 mgd
Contact Stabil-
ization Acti-
vated Sludge
FeS04
Secondary
Before Primary
Clarifier, In
Aerobic Digester,
or Before Chemi-
cal Conditioning
Pressure Filter,
Multiple Hearth
Incinerator
-------
When the two upflow clarifiers were put into operation, one was
started on lime and the other on ferric chloride for comparison.
Tenech's predictions were borne out during the 18-mo qualifica-
tion period before the U.S. Environmental Protection Agency
approved the plant's performance and the city of South Bend's
control. The plant immediately converted to full-scale operation
with ferric chloride. By converting to ferric the savings in
chemical costs amount to $720,000.00/yr. The following report
will discuss the dramatic differences in sludge generation rates'
and sludge processing with lime and with ferric chloride.
South Bend is a well-managed plant with relatively few oper-
ational difficulties. It receives no abnormal industrial wastes
because there is a minimal amount of industry now remaining in
the community. Studebaker Motor Company quit its South Bend
operations in the early 1960's. A brewery also closed down about
3 yr ago, reducing the plant influent BOD from approximately 150
to the present 70 to 80 mg/t. Suspended solids were similarly
reduced to about 80 mg/t and digester gas production decreased
somewhat.
There are no dramatic seasonal fluctuations in flow or waste
loading. The major problem, because of combined sewers in the
city, is tremendous hydraulic variation due to infiltration. The
impact of high storm flows is that the plant influent is very
dilute in BOD and SS, and it contains a great deal of grit and
bitumen washed off the streets. During these periods, the TS
content of the sludge pumped from the primary clarifiers is low,
but subsequent thickening gets the solids concentration back to
4 or 5 percent before the sludge is pumped to the digester. A
primary sludge degritter helps reduce the grit load on the diges-
ters. Even so, it is necessary to clean one of the digesters
each year on a rotating basis.
History
A description of plant modifications affecting sludge pro-
duction and characteristics follows:
1956 - The original primary and secondary treatment plants
were constructed to handle 90,840 m3/day (24 mgd)
average dry weather flow.
1974 - The facilities were modified and expanded to handle
181,680 m3/day (48 mgd) average dry weather flow.
The new equipment consisted of:
An automated mechanical bar screen
A primary sludge gravity thickener
A sludge degritter
Three waste-activated sludge centrifuges
One additional aeration tank
163
-------
• Two additional 6,586 m3 (1.74 mil gal) secondary
clarifiers
• A chemical sludge gravity thickener
• Three lime sludge centrifuges
• Two upflow clarifier tertiary treatment units.
April
1975 - Phosphorus removal was initiated with lime addition
to one upflow clarifer and ferric and polymer addi-
tion to the other. The chemical sludges were com-
bined, gravity thickened, centrifuged, and trucked
to a storage area.
Sept.
1975 - The use of the waste-activated sludge centrifuges was
discontinued. Waste-activated sludge was recircu-
lated to the wet well ahead of primary clarifiers.
The use of the lime sludge centrifuges was discontin-
ued. Chemical sludge was pumped to the sludge
lagoons 4 km (2.5 mi) from the plant.
Oct.
1976 - Lime addition for phosphorus removal was discontinued.
Ferric chloride and polymer were added to both of the
upflow clarifiers. The chemical sludge was gravity-
thickened and fed to the anaerobic digester.
Chemical Addition for Phosphorus Removal'
Lime Addition--
Approximately 36.3 t/day (40 tons/day) lime as CaO were
added to remove phosphorus in one of the upflow clarifiers. The
average dosage was approximately 523 mg/a.
The lime dosage was regulated by checking the pH of the
water in the upflow clarifiers. The pH was maintained between
9.6 and 10.2. The ideal pH was considered to be 9.8. Above pH
10.2, the generation of sludge was dramatically increased due to
the co-precipitation of magnesium hydroxide. Below pH 9.6, poorer
phosphorus removal was achieved.
Ferric Chloride and Polymer Addition--
Liquid ferric chloride (37 percent FeCK) containing approx-
imately 1.1 t/day (1.2 tons/day) Feds, and T7 kg/day (38 Ib/day)
polymer are added to remove phosphorus in each clarifier. The
average dosages are approximately 16 mg/£ Fe Cls and 0.2 mg/£
polymer. The chemical feed rate is selected to achieve 85 per-
cent reduction of the phosphorus concentration entering the
plant. The feed rate is controlled by manual adjustment of the
feed pump settings to compensate for changes in influent flow
rate.
164
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General Description of Wastewater Treatment Operations Affecting
SIudge
Figure C-l presents a general treatment plant flow diagram.
Design parameters for the clarifiers and aeration basins are pre-
sented in Table C-2. Table C-3 presents a summary of 1976 influ-
ent flow characteristics and SS, BOD, and total phosphorus remov-
als during primary, secondary, and tertiary treatments.
The raw wastewater is degritted by gravity in non-aerated
grit chambers. Degritting reduces the grit load on the anaerobic
digesters, increasing the volatile solids concentration of the
digester feed and preventing grit accumulation in the sides and
corners of the digesters.
Primary treatment removes only about 39 percent of the SS
and 22 percent of the BOD entering the plant. The eight rectan-
gular primary tanks have chain-type straight line collector arms
for scraping the sludge into the end hoppers. The withdrawal of
primary sludge from the hoppers is an automatic operation. Each
of the eight clarifiers is pumped in series for three minutes in
a continuous cycle.
Suspended solids and BOD removals during secondary treatment
averaged 82 and 80 percent, respectively, in 1976. According to
the plant chemist, this high degree of treatment is due to a low
food-to-microorganism ratio in the mixed liquor. The activated
sludge microorganisms are in a near-starvation phase so that they
efficiently degrade the volatile material in the wastewater.
The five octagonal or circular secondary clarifiers have
revolving nozzle-type chain belt collector arms which continuously
scrape the sludge to the center hoppers. Sludge is continuously
pumped from each hopper and either wasted to the wet well or
returned to the aeration tanks. The amount of sludge which is
returned averages 37 percent of the flow entering the plant.
Figure C-2 shows the configuration of the two 61-m (200-ft)
diameter tertiary upflow clarifiers. Flow is directed from the
secondary clarifiers via a 196-cm (77-in) line to a pair of gates
which divert the flow into two parallel clarifiers. At this
point, ferric chloride is dosed to serve as a flocculant. Flow
entry to the clarifier is obtained through a 4.9-m (16-ft) dia-
meter upward flow cone in the bottom of the center of each clar-
ifier. Here a polymer is mixed into the upward flow and agitated
by an eight-bladed propeller. The solids then gain entry into
the flocculation zone which is surrounded by a shroud which
serves as a baffle. The clarifier is designed so that the water
recirculates within the shroud approximately eight times before
it reaches the velocity at which it flows out from under it and
down through the sludge blanket. As it flows through the sludge
blanket, the flocculated solids are captured. The clarified
water moves to the surface and exists over two concentric weirs.
165
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ST. JOSEPH
RIVER
CJ1
PARK
LAND
Figure C-1 . South Bend, Indiana, wastewater treatment plant flow diagram.
-------
TABLE C-2. WASTEWATER TREATMENT PROCESS DESIGN PARAMETERS,
SOUTH BEND, INDIANA
Unit
Description
Plant Design Flow Capacity
Primary Clarifiers (8)
Aeration Tanks (4)
Secondary Clarifiers (5)
Upflow Clarifiers (2)
0.18 mil m3/day (48 mgd) Avg. dry weather
0.30 mil m3/day (80 mgd) Avg. wet weather
Shape: Rectangular
Size: 12.2 m x 36.6 m x 2.9 m SWD
(40 ft x 120 ft x 9.5 ft SWD) ,
Total Capacity: 10,214 m3 (364,800 ft3)
Detention: 82 min @ Avg. dry weather flow
Type: Step Aeration
Shape: Four Bay Rectangular Tanks
Size: Each Bay 76.3 m x 7.3 m x 3.7 m SWD
(250 ft x 24 ft x 12 ft SWD)
Total Capacity: 32,260 m3 (1,152,000 ft3)
Air Blower Capacity: 1,896 m3/min
(67,730 ft3/min)
Shapes: 3 Octagonal, 2 Circular
Sizes: 34.3 m x 3.7 m SWD (112.5 ft x
12 ft SWD) Octagonal Tanks;
44.2 m x 4.3 m SWD (145 ft x
14 ft SWD) Circular Tanks
Total Capacity: 24,640 m3 (6.51 mil gal)
Detention: 192 min @ Avg. dry weather
flow
Shape: Circular
Size: 61.0 m x 4.9 m SWD (200 ft x
16 ft SWD)
Total Surface Area: 5.840 m2 (62,840 ft2)
Overflow Rate: 31.2 m3/day/m2
(766 gal/day/ft2) @
Avg. dry weather flow
Detention: 3.76 hr @ Avg. dry weather
flow
Total Capacity: 28,460m3 (7.52 mil gal)
167
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TABLE C-3. SUMMARY OF 1976 WASTEWATER CHARACTERISTICS AND
TREATMENT PERFORMANCE, SOUTH BEND, INDIANA
Population
Connected 150,000
Equivalent BOD 156,643
Equivalent suspended solids 152,330
Wastewater Flow
mil m3/day (mgd) 0.17 (44.9)
mil m3 treated 61.9 (16,378)
m3 (gallons) per capita daily flow 1.13 (299)
Suspended Solids
Influent, mg/i 82
Primary effluent, mg/£ 49
Secondary effluent, mg/£ 9
Final effluent 8
% removal in primary 39
% removal in secondary 82
% removal in tertiary 17
% removal overall 90
BOD5
Influent, mg/l 71
Primary effluent, mg/£ 56
Secondary effluent, mg/£ 10
Final effluent, mg/£ 2
% removal in primary 22
% removal in secondary 80
% removal in tertiary 79
% removal overall 97
Total Phosphorus
Influent, mg/£ 2.29
Primary effluent, mg/£ 2.19
Secondary effluent, mg/£ 0.93
Secondary effluent, kg/day (Ib/day) 158 (349)
Final effluent, mg/£ 0.24
Final effluent, kg/day (Ib/day) 41 (90)
% removal in tertiary 74
% removal overall 88
168
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vo
ACID
LIME OR POLYMER
SECONDARY
EFFLUENT
DISCHARGE
TO CHLORINA-
TION AND
RIVER
SKIMMERS
FERRIC CHLORIDE
SLUDGE
Figure C-2. Tertiary upflow clarifier configuration, South Bend, Indiana.
-------
When lime is used for phosphorus removal, it is added
directly to the propeller mixing zone in the upflow cone of the
clarifier. No polymer is needed. In contrast, polymer is neces-
sary with ferric chloride to produce an ideally settling floe.
The 244-cm (96-in) line leading to the center well acts as a
long plug-flow reactor in which the ferric floe can develop to
the correct size before it reaches the upflow cone. The low-velo-
city, large propel 1ermixer at that point can disrupt the floe or,
with the addition of the polymer at that point, enhance the set-
tling characteristics of the floe. The polymer is also necessary
since without it temperature gradients produced by the sun passing
over the clarifier during the day produce vertical water circula-
tion and floe escapes. The polymer tends to weigh down the floe
and prevent resulting instability of the sludge blanket.
The plant experiences some problems with the upflow clari-
fiers during periods of storm flow. To compensate for flow var-
iations, the operator must regulate the speed of the variable-
speed propeller mixer in the upflow cone. Speeding up the pro-
peller tends to circulate the sludge solids in a tight circle
with the shroud and prevents the velocity vectors from going out-
ward.
Operational parameters for the upflow clarifiers during fer-
ric and polymer addition have been determined by experience. An
empirically determined relation between the i>5 concentration in
the shroud and clarifier performance forms the basis for deciding
when to pump sludge. The SS concentration is sampled once each
day and maintained between 1,500 and 2,500 ppm. At greater than
2,500 ppm, sludge is pumped out of the center donut-like collec-
tion well. At less than 1,500 ppm, pumping is stopped and sludge
allowed to accumulate. When the concentration goes as high as
4,000 ppm, the sludge blanket is too high and floe is lost over
the weirs. When the concentration falls below 1,500 ppm, the
sludge blanket is too thin and poor phosphorus removal results.
Because of the low capacity of the sludge pumps, pumping is con-
tinuous from one clarifier or the other.
Clarifier operation with lime addition differs from operatior
with ferric in that sludge is continuously pumped from each clar-
ifier. An efficient sludge pumping system is necessary for the
heavy lime sludge -- sludge lines should have few elbows. In
addition, when lime is used, reduction of the final effluent pH
below 9.0 is necessary, requiring the addition of approximately
9.5 mj (2,500 gal)/day sulfuric acid to the effluent weir. On
the other hand, because of the high pH produced with lime, no
chlorine disinfection is necessary.
Detailed Description of Sludge Treatment and Disposal Operations
Table C-4 presents design parameters for the major sludge
treatment steps at South Bend. The treatment and disposal of
theJime/iron sludge produced at the plant in the past and the
170
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TABLE C-4. SLUDGE TREATMENT PROCESS DESIGN PARAMETERS,
SOUTH BEND, INDIANA
Unit
Description
Primary Sludge Gravity Thickener (1)
Waste-Activated Sludge
Centrifuges (3)
Anaerobic Digestion Tanks (4)
Chemical Sludge Gravity
Thickener (1)
Lime Sludge Centrifuges (3)
Shape: Circular
Size: 18.3 m x 3.0 m SWD (60 ft x
10 ft SWD)
Volume: 792 m3 (28,280 ft3)
Surface Area: 263 m2 (2.828 ft2)
Solids Loading: 28 kg/m2/day
(5.7 Ib/fWday)
Type: Bird Continuous Scroll Solid
Bowl
Solids Feed Rate: 400 kg/hr (882 Ib/hr)
Hydraulic Feed Rate: 276 £/min
(73 gal/min)
Centrate BOD5: 3 to 8 mg/l
Centrate SS: 6.7 mg/£
Type: 2-Primary, 2-Secondary
Shape: 33.5 m x 7.8 m SWD + 2.7 m cone
(110 ft x 25.5 ft SWD + 9 ft
cone)
Total Capacity: 30,280 m3 (8 mil gal)
Gas Storage Capacity: 7,000 m3 @
28,124 kg/m2
(250,000 ft3 @
40 ps)
Volatile Loading Rate: 0.6 kg VS/m-Vday
(0.037 Ib
VS/ft3/day)
Shape: Circular
Size: 30.5 m x 3.0 m SWD
(100 ft x 10 ft SWD)
Surface Area: 730 m2 (7,854 ft2)
Volume: 2,200 m3 (78,540 ft3)
Solids Loading: 75.2 kg/m2/day
(15.4 Ib/ft2/day)
Type: Bird Continuous Scroll Solid
Bowl
Hydraulic Feed Rate: 125£/min
(33 gal/min)
171
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iron sludge presently produced are described below. It should
be recalled that the chemical sludges have always been gravity-
thickened separately from the plant's organic sludge. Gravity-
thickened sludge is presently anaerobically digested along with
the organic sludges normally digested. The lime/iron sludge
which was produced in the past, however, was never introduced
into the digesters, but was centrifuged and trucked to a storage
area.
A brief description of the procedures for handling the
organic as well as the chemical sludges follows so that the
digester performance can be related to sludge characteristics.
As previously mentioned, waste-activated sludge is recirculated
to the wet well ahead of the primary clarifiers. An estimated
55 percent of the solids removed from the primary clarifiers is
derived from the recirculated secondary sludge. This percentage
is rather high because of the high SS removal efficiency of the
plant's secondary relative to its primary treatment operation.
Degritting--
Sludge from the primary clarifiers is pumped at less than
one percent TS to allow effective degritting. Sludge degritting
reduces the grit load on the anaerobic digesters, thus increasing
the volatile solids concentration of the digester feed and pre-
venting grit accumulation in the sides and corners of the diges-
ters. The VS fraction of TS in the digester feed rose from
approximately 62 percent to 65.5 percent of TS because of sludge
degritting. A slight increase in digester gas production may
have resulted.
Organic Sludge Thickening--
Degritted sludge at less than one percent TS is pumped to
the gravity thickener. The gravity thickening step raises the
TS concentration of the sludge before it is pumped to the diges-
ters, so that digester space is not wasted and large amounts of
digester supernatant are not formed and pumped back to the wet
well. The average TS concentration of the thickened sludge
which was pumped to the digesters was 4.7 percent in 1976. The
VS fraction of TS averaged 67.5 percent. On the average, approx-
imately 267 m^/day (70,530 gal/day) thickened sludge were pumped
from the thickener to tlie digesters in 1976. The sludge is con-
tinuously drawn off at 378 to 568 &/min (100 to 150 gal/min).
Chemical Sludge Thickening--
The single chemical sludge gravity thickener is continuously
fed with iron sludge at approximately 0.16 m3/min (43 gal/min).
When one of the tertiary clarifiers was operated with lime,
sludge feed to the thickener was continuous at 0.89 m3/min (234
gal/min). The thickened iron sludge is removed from the hopper
172
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at the bottom of the thickener once every 3 days. Pumping lasts
for 7.8 hr at 0.38 m^/min (TOO gal/min). Formerly, the lime/
iron sludge was pumped to the three lime centrifuges every day at a
total rate of 0.48 nH/day (126 gal/min) for 16 hr/day. A com-
parison of the amounts of raw and thickened sludge produced with
iron and lime/iron is presented in Table C-5.
Table C-5 reveals that the mass of raw lime sludge produced
by a single clarifier was 38 times the mass of iron sludge pro-
duced by both clarifiers. The TS concentration of the raw iron
sludge is 1.0 percent, while that of the raw lime sludge was 4.0
percent. The combined lime/iron sludge was thickened to 10.2
percent TS. The amount of thickened iron sludge produced was
59 m3/day (15,570 gal/day) by volume and 2.36 t/day (2-6 tons/day)
by weight. In contrast, it is estimated that the amount of lime
sludge which would have been produced if lime had been used in
both tertiary clarifiers would be approximately 833 m3/day
(224,000 gal/day) by volume and 907 t/day (100 tons/day) by
weight.
Anaerobic Digestion--
There are four 33.5-m (110-ft)-diameter anaerobic digestion
tanks on-site with floating covers. The operation of the diaes-
tion tanks follows the pattern shown in Figure C-3.
THICKENED
CHEMICAL
SLUDGE
THICKENED
ORGANIC
SLUDGE
DIGESTED
SLUDGE
Figure C-3. Flow pattern for South Bend anaerobic digester.
Only the first two digestion tanks (No. 1 and No. 3) are
heated and mixed. Mixing is accomplished with the Carter gas
recirculation system. Each of the two digesters contains six
Carter Aero-Hydraulic guns. Digester gas is pumped down vertical
tubes to the bases of the guns, creating gas bubbles at the bot-
tom which lift the sludge to the top of the tank and create the
circulatory patterns in the tank. In the past, the plant had
experienced severe upsets and souring of the digesters because
of inadequate mixing. With the installation of the Carter Sys-
tem, operation has been excellent and there have been no upsets.
173
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TABLE C-5. CHEMICAL SLUDGE PRODUCTION AND GRAVITY THICKENING,
SOUTH BEND, INDIANA
WASTEWATER TREATED
0
mil m /day
(mgd)
RAW SLUDGE PRODUCTION
m /day
(gal/day)
% TS, dry weight
•2
kg TS/mil m wastewater treated
(Ib/MG)
THICKENED SLUDGE PRODUCTION
m /day
(gal /day)
% TS, dry weight
VS, % of TS
kg TS/day
(Ib/day)
kg VS/day
(Ib/day)
t TS/day
(tons/day)
Iron Sludge
0.14
(36.7)
236
(62,280)
1.0
16,900
(141)
59
(15,570)
4.0
35
2,358
(5,194)
825
(1,818)
2.36
(2.6)
Lime Sludge
0.07
(18.35)
1,129
(298,200)
4.0
649,858
(5,420)
N/A
N/A
N/A
N/A
N/A
N/A
Lime/Iron Sludgi
0.14
(36.7)
N/A
N/A
N/A
454
(120,000)
10.2
11
46,345
(102,081)
5,094
(11,220)
46.26
(51) '
Note: N/A = Not Available
174
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Digester heating is accomplished in two ways. Sludge from
the No. 1 and No. 3 digesters is pumped through a heat exchange
system. The boiler for the heat exchange system is run on methane
digester gas. There is also a sludge preheating system of the
heat-exchange type. Sludge from digester No. 1 is pumped to the
preheating unit. The partially digested sludge mixes with the
primary plus waste-activated sludge coming from the organic
sludge gravity thickener. The digesting sludge feeds the raw
sludge, and the mixture is heated. The preheater utilizes heat
from cooling water coming off the plant's air compression engines,
which are also run on methane digester gas.
Because of plant design, the thickened chemical sludge can-
not also be preheated before being pumped into the digester.
Therefore, it is at a lower temperature than the organic sludge
and is capable of suppressing the temperature in the digester
into which it is fed. This could result in a loss of digester
gas production. Since most of the gas production takes place in
the No. 1 digester, the chemical sludge is fed into the No. 3
digester. The result has been a 4°to 5° loss in temperature in
this digester, but a decrease in total gas production has been
avoided.
Intermittent pumping of organic sludge to the No. 1 digester
occurs for 12 hr/day (2 out of every 4 hr). 7 days/wk. The pri-
mary digester receives an average of 267 m3/day (70,530 gal/day).
It receives 12,550 kg TS/day (27,650 Ib TS/day) and 8,470 kg MS/
day (18,660 Ib VS/day). Pumping of iron sludge to the No. 3
digester occurs for 7.8 hr every third day. The average amount
pumped is 59 m3/day (15,570 gal/day). This contains approximately
2,358 kg (5,194 Ib) of TS and 825 kg (1,818 Ib) of VS. Some
degradation of the VS contained in the iron sludge is occurring,
as evidenced by the increase in digester gas production which has
occurred. The level of digester gas production before and during
addition of the iron sludge to the digester is shown in Table C-6.
Iron sludge was first fed to the digester on 10/14/76. During
iron sludge addition, digester gas production was higher by about
0.11 m3 gas/kg VS destroyed (1.87 ft3/lb VS). The level of
digester gas production is still low compared to the nationwide
average of 0.86 to 1.11 m3 gas/kg VS destroyed (14 to 18 ft3 gas/
Ib VS destroyed). It is possible that the waste-activated sludge
has been so severely degraded as the result of the low food to
microorganism ratio in the aeration basins that 1ittle volatile
matter remains, causing the low gas production.
Some time ago, a fluoride dye tracer study of the digester
was performed to calculate the actual liquid retention time based
on an input of 378.5 m3/day (100,000 gal/day). Instead of the
expected 20 days, the dye tracer study indicated a shorter deten-
tion time of approximately 15 days.
175
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TABLE C-6.
ANAEROBIC DIGESTER GAS PRODUCTION
SOUTH BEND, INDIANA
Month/
Year
1/75
2/75
3/75
4/75
5/75
6/75
7/75
8/75
9/75
10/75
11/75
12/75
1/76
2/76
3/76
4/76
5/76
6/76
7/76
8/76
9/76
10/76
11/76
12/76
1/77
2/77
3/77
1000 m3
gas/day
5.26
5.12
5.12
5.10
5.66
4.79
4.00
3.70
4.68
5.18
5.29
4.73
4.48
5.54
4.84
5.18
4.56
5.15
4.62
4.23
5.26
5.40
5.40
5.71
5.38
5.80
5.40
1000 ft3
gas/day
188
183
183
182
202
171
143
132
167
185
189
169
160
198
173
185
163
184
165
151
188
193
193
204
192
207
193
m gas/kg
VS destroyed
0.54
0.45
0.49
0.44
0.53
0.52
0.47
0.42
0.57
0.60
0.59
0.49
0.55
0.58
0.57
0.60
0.57
0.59
0.56
0.50
0.59
0.69
0.67
0.73
0.73
0.67
0.61
ft3 gas/lb
VS destroyed
8.7
7.3
7.9
7.1
8.6
8.5
7.7
6.9
9.3
9.8
9.6
7.9
9.0
9.4
9.2
9.8
9.3
9.5
9.1
8.2
9.6
11 .2
10.9
11.9
11.8
10.9
9.9
176
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Sludge is withdrawn from the No. 2 and 4 digestion tanks
approximately every three days as space is needed in the No. 1
and 3 tanks. Sludge is removed from the No. 2 and 4 tanks
through conventional bottom outlets. Withdrawal from the No. 1
and 3 tanks is through "spider drawoffs," three pipes near the
bottom which extend toward the center of the tank and branch at
the ends into three arms. The spider which is used to draw off
sludge is rotated each time.
The digested sludge removed from the digester averages 5.5
percent TS with a VS fraction of 47.2 percent. Iron sludge addi-
tion had no effect on the TS and VS concentrations of the digested
sludge. The average volume of sludge removed from the digester
was 118 m3/day (31,300 gal/day) before iron siudge .addition and
160 m3/day (42,200 gal/day) during iron sludge addition. So the
volume of digested sludge increased by 42 m3/day (10,900 gal/day).
During the same period, the volume of supernatant increased by
8 m3/day (2,100 gal/day).
Disposal of Digested Sludge--
Digested sludge is pumped through a 15-cm (6-in) steel pipe-
line from the digester directly to a "sludge farm" which is 4 km
(25 mi) away. The sludge is pumped with a 100-HP, 126-ft pumping
head, centrifugal pump. At the farm, the sludge can be routed
to any one of seven different fields of varying size. The diked
fields are filled with approximately 76 cm (30 in) of sludge.
Approximately 757 m3 (200,000 gal) of sludge are pumped to a
field at any one time. It takes approximately 2 to 3 mo to fill
a field. The sludge running at 5 to 6 percent TS is allowed to
dewater on the fields through percolation and evaporation for a
time before a No. 4230 John Deere tractor of special design mixes
up the sludge and exposes wet section to the air. There are only
a few operators who are capable of driving this tractor, since
the front end has a capacity to lift up, depending on the field
conditions. A weight is added to the front end for better balance.
The front wheel spread can also be varied to achieve any width
desired. The wheels on the rear are specially designed cleat-
type steel wheels capable of tracking through sludge and mud.
The 91 ac at the site were purchased around 1967. The
site became operational that same year. The fields are effec-
tively screened from the roadway by a tract of trees approximately
0.2 mi in width. The seven fields are located on the back of the
property.
Sludge is pumped to the fields year round, but normally
worked with the tractor only in the summer. Depending on weather
conditions and other variables, the sludge fields are worked with
the John Deere tractor to produce a relatively dry sludge cake.
The tractor wheels work down as far as 30 to 38 cm (12 to 15 in)
into the soil and sludge to work it into a homogenous mixture.
After 18 mo, the material is removed with a front-end pa.yloader
177
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and stockpiled elsewhere on the property. Last year, for example,
contractors removed almost all of the stockpiled sludge and used
it on various construction projects around South Bend.
Four lagoons were originally constructed on the site for
receipt of digester wash water. Treated effluent is used in
cleaning out the digesters. This wash water contains grit,
detritus and residual sludge having some BOD. Odors from the
lagoons do not seem to be a problem. The No. 4 lagoon is now
being filled in with rubbish and covered over. Some of the
dried soil-sludge mixture produced at the site will be used as
surface cover on this lagoon.
The site supposedly has a clay layer underneath it. How-
ever, in an exposed section being dug out for fill material and
for making a deeperfield for later sludge filling, there were
0.6 to 0.9 m (2 to 3 ft) of surface soils underlain by approxi-
mately 3.0 to 3.7 m (10 to 12 ft) or more of silty sand. The
spreading area is part of a large meandering ancient stream or
flood plain area.
Li me/Iron Sludge Dewatering and Disposal--
When the plant was using lime to remove phosphorus from half
of the wastewater, the thickened lime/iron sludge produced was
dewatered by centrifuges. The thickened sludge, at 11 percent
TS, was fed to each of the three centrifuges at a rate of 159
£/min (42 gal/miri). Because of the large volume of lime/iron
sludge to be dewatered, it was necessary to run all three cen-
trigues for approximately 16 hr/day, 7 days/wk. If the plant
had used lime to remove phosphorus from all of the wastewater,
the volume of lime sludge would have been almost doubled, and
this amount could not have been centrifuged.
There were several problems with operation of the centrifuges.
The first problem has been mentioned: the total centrifuge capa-
city was inadequate to handle all of the sludge. The design
engineers estimated that only 27 t/day (30 tons/day) of dry
siudge TS would be produced by the two upflow clarifiers operating
with lime, while the actual production figure for both clarifiers
is closer to 91 t (100 tons). Other operational problems resulted
because of the poor instructions given to the plant on centrifuge
operation and maintenance procedures. The heavy chemical sludge
caused breaking of shear pins, and frequent equipment overhauls
were necessary. Maintenance costs were high during the 6-1/2 mo
that the centrifuges were in operation.
The lime/iron sludge was centrifuged to a cake that was 43
percent TS, and 8 percent of the solids were volatile. The cake
was trucked out to a storage site. This disposal method was not
satisfactory and was only meant to be temporary until the plant
stopped using lime for phosphorus removal. One of the problems
178
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with this method was the spillage of lime from the truck onto
the road, discoloring the pavement.
Sludge Treatment and Disposal Costs
The best cost data available from plant record is related
to anaerobic digestion. Unfortunately, the operational costs for
chemical sludge thickening and centrifugation cannot be broken
apart from the operational costs of handling phosphorus removal
chemicals and tertiary upflow clarifier operation. Because no
special problems were involved in the chemical sludge thickener
operation, the costs can be assumed to be similar to those of a
similar organic sludge thickening operation. Of course, the
total cost to thicken per ton of dry solids is related to the
mass of sludge treated. For instance, the total cost of lime
sludge treatment at South Bend would be much less per ton of dry
solids than the total cost of treating iron sludge. This results
from the mass of lime sludge being much greater than the mass of
iron sludge. The size of the thickener, however, remains the
same in each case, and it is run for the same number of hours
each day.
The disadvantage of treating lime sludge comes in the dis-
posal stage. At South Bend, it would not have made sense to send
the lime sludge to the digesters and the sludge farm, as can be
done with the iron sludge. While there is probably adequate room
in the four digestion tanks to handle the large amount of lime
sludge, the digesters would likely serve mainly as holding tanks
for this sludge because of the inability to preheat the sludge
and the low volatile content.
Pilot plant studies have shown that the lime sludge can be
compacted in the thickener to as high as 16 percent TS,and even
at this high concentration, the sludge can be pumped fairly
easily.
It would be possible, then, to pump the lime sludge to the
sludge farm, but ultimate disposal of the lime sludge would still
be a problem because of its volume. Parkland disposal would not
use up all of the sludge. Direct trucking of thickened and per-
haps centrifuged lime sludge to agricultural land seems to be the
best disposal alternative. Dewatering the sludge to a high sol-
ids concentration would lower the trucking costs. Pilot tests
showed that the lime sludge centrifuges can operate with a feed
sludge solids concentration of up to 30 percent TS. By feeding
sludge at 16 percent TS, a fairly dry cake could be produced
which would lower trucking costs.
The following costs related to digester operation were
available from plant records.
179
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Item 1975 1976
Operational Labor $74,981.18 $71,012.53
(4 full-time operators)
Maintenance of Equipment $33,458.26 $35,235.74
and Structures
Electricity $10,462.40 Not Available
Miscellaneous $10,903.11 $4,164.04
The capital cost for the four digestion tanks was $5,059,200
in 1956. This does not include the cost of the Carter sludge
mixing system which was added later. The cost of the two sludge
heat exchangers for tanks No. 1 and 3 plus piping and valve con-
trols, and the sludge pump which pumps sludge from the No. 1
digester to the preheater, was $63,000 in 1956. The two digester
gas storage spheres cost $225,000 each.
The acquisition costs for 91 ac of farmland were $100,800
in 1969. Land is very available near the treatment plant. The
capital cost expenditures related to farm operations included
purchase of the land; site development costs, including road
improvements; installation of piping for the seven fields,
including cross-connections, valves, boxes, and miscellaneous
tubing and hoses; construction of a 4-km (2.5-mi), 15.24-cm (6-in)
steel pipeline; and purchase of one John Deere Model 4230 special
tractor, one front-end payloader, two multiple harrow plows, one
rbtotiller disc harrow, and two special pieces of drag equipment.
The operating costs of farm operation disposal have been esti-
mated at $1.60/t ($1.45/ton) of dry sludge TS.
The additional costs for sludge treatment resulting from
iron addition at South Bend were very minor, since there was
plenty of room available in the digester. The chemical sludge
increased the volume of digester feed by 22 percent and the mass
of feed TS by 19 percent. The capital and operating costs of
the chemical sludge gravity thickener, including pumping to and
from, are the major additional costs.
Summary and Conclusions
A comparison of tertiary upflow clarifier operation with
lime and with ferric chloride showed that the mass of raw lime
sludge generated per mil m3 of wastewater treated was 77 times
that of the raw iron sludge produced. The TS concentration of
the raw lime sludge was 4.0 percent. Gravity thickening of the
lime sludge mixed with a relatively insignificant amount of iron
sludge produced a TS concentration of 10.2. The sludge could be
.centrifuged to 43 percent TS. The centrifuged sludge cake was
180
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trucked to a storage area unti 1 difficulties arose regarding com-
plaints along the route travelled by the trucks. Then the thick-
ened sludge was pumped through a 4-km (2.5-mi), 15.24-cm (6-in)
pipeline to lagoons located at the plant's secluded sludge farm.
This was the mode of disposal until the plant discontinued using
lime. The plant feels that the most satisfactory method of lime
sludge disposal would be land application of thickened or centri-
fuged cake. Although there were problems with centrifugation -
high maintenance and electricity costs for centrifuge operation
and insufficient capacity provided by the design engineers - the
centrifuges operated with feed concentrations of up to 30 percent
TS and produced a correspondingly dry cake in pilot tests. These
tests also showed that the lime sludge could be compacted in the
gravity thickener to 16 percent TS and even at that high concen-
tration it was easily pumped and flowed well.
The thickened iron sludge was fed to the anaerobic digester.
By feeding it to the second of the two primary digesters which
follow in series, a suppression of total gas production was
avoided. This suppression could have occurred because of the
plant's inability to preheat the chemical sludge as it does its
organic sludge. Some temperature suppression occurred in the
second digester as the result of chemical sludge feed, but most
of the gas is produced in the primary digester,so the impact was
not great. Other than the fact that a total decrease did not
occur, the effect of the iron sludge on gas production is uncer-
tain. Initially, there appeared to be an increase in the amount
of gas produced per quantity of VS destroyed, but later this
trend disappeared.
Digested sludge was pumped out to a sludge farm where it
was dried and mixed with topsoil in fields with a special tractor
and then given away to contractors and the city of South Bend for
parks. The sludge farm appears to be an excellent concept for
areas where land is available. The concept was developed by
South Bend's Manager of Sanitary Operations who is a former far-
mer. The operating cost of the sludge farm operation is estimated
at $1.60/t ($1.45/ton) of dry sludae TS.
CASE STUDY D: SHEBOYGAN, WISCONSIN
Introduction
Sheboygan provides an example of secondary addition of ferric
chloride. It also provides an example of vacuum filtration of
undigested (raw) ferric sludge. The data from this plant can be
compared with data from plants, such as Midland, Michigan, and
Windsor, Ontario, which have practiced vacuum filtration of raw
ferric sludges produced by primary addition of the chemical.
Sheboygan operates a fluidized bed incinerator. The impact
of ferric addition on energy requirements, maintenance, and repair
181
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requirements for this type of system can be examined at the plant.
The performance can be compared with that of other plants such as
in Brookfield, Wisconsin, where a ferric sludge undergoes pressure
filtration and multiple hearth incineration.
Overall, Sheboygan is representative of the many older waste-
water treatment plants which are awaiting future expansions and
are overloaded both hydraulically and with respect to sludge hand-
ling capacity in the meantime. Because of the hydraulic loading
problems, the plant is fortunate that only 4 percent of the sewers
are storm rather than sanitary sewers. Industrial waste discharges
contribute 40 percent of the flow to the plant. Included are
industrial phenols, plastics, and leather tanning waste. Whether
because of hydraulic conditions or because of industrial wastes,
SS and BOD removal are poor. Table C-7 below presents some his-
torical data on plant influent characteristics and suspended sol-
ids and BOD removal. The plant's nominal design treatment capa-
city is 0.060 mil m^/day (16 mgd) average daily flow with 0.83 mil
m3/day (22 mgd) maximum flow. Its discharge permit allows efflu-
ent concentrations of 80 mg/£ SS and 70 mg/£ BOD. Phosphorus
removal performance is also poor due to both hydraulic conditions
and the lack of sludge handling capacity.
TABLE C-7. A SAMPLE OF HISTORICAL DATA INDICATING
AVERAGE PLANT INFLUENT CHARACTERISTICS, SUSPENDED
SOLIDS AND BOD REMOVAL, SHEBOYGAN, WISCONSIN
April-Sept April-Sept April, May,
1970 1973 July, Sept,
Oct, Nov
1976
3
Influent m /day
(mgd) 45,800 (12.1) 54,900 (14.5) 14,300 (10.9)
Influent SS (mg/£) 202 184 195
Effluent SS (mg/£) 34 53 48
Influent BOD (mg/£) 196 211 219
Effluent BOD (mg/£) 58 81 58
History
The following describes historical plant modifications
affecting sludge production and characteristics.
182
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1968 - Began sludge treatment with Dorr-Oliver vacuum filtra-
tion and Fluo Solids incineration system. Old anaerobic digesters
converted to holding tanks for excess liquid sludge. Excess sludge
applied to drying beds.
Jan. 1972 - Began phosphorus removal by chemical addition.
Feb.-Apr. 1972 - Discontinued use of incinerator heat exchan-
ger. Excess liquid sludge hauled to farmlands instead of applied
to drying beds.
Apr. 1973 - Began polymer addition to thickener.
Jun. 1975 - Thickener overflow recirculated to head of the
primary tanks instead of before the trickling filters.
Oct. 1975 - Began keeping records of phosphorus removal chem-
ical dosages and influent and effluent phosphorus concentrations.
Jan. 1977 - Began proportional addition of polymer to thick-
ener feed 1ines.
Chemical Addition for Phosphorus Removal
In order to remove phosphorus at this treatment plant, liquid
ferric chloride is added to a division well which distributes the
effluent from the trickling filters to the two final clarifiers.
Anionic polymer (Hercules 831) is then added to the center feed
wells of the final clarifiers. The average ferric chloride dosage
was 30 mg/£ dry FeCIo in 1976, with polymer dosage average 208
kg/mil m3 (1.75 Ib/MG). Because accurate records were not kept
before 1976, chemical dosages before that time are unknown.
The reduction of the total phosphorus concentration in the
wastewater averaged only about 54 percent in 1976. Average influ-
ent and effluent concentrations were approximately 7.02 ppm and
3.26 ppm total phosphorus, respectively. Phosphorus removal was
very poor on the average, partly because during this period ferric
chloride addition was stopped whenever thickener performance deter-
iorated. This occurred frequently. In 1977, when thickener per-
formance was improved and ferric chloride addition rarely had to
be stopped, average phosphorus removal increased to 66.5 percent.
Influent and effluent phosphorus concentrations averaged 10 ppm
and 3.3 ppm, respectively. The plant is required to reduce its
effluent phosphorus concentration below 4 ppm to meet its dis-
charge permit requirements. Eighty percent removal cannot be
achieved at the plant because of its design. In order to get bet-
ter removal, more contact time is needed between the addition of
ferric chloride and polymer, but this cannot be achieved with the
present plant design.
183
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General Description of Wastewater Treatment Operations Affecting
Sludge
Figure C-4 presents a general treatment plant flow diagram.
The plant has no facilities for grit removal from the raw sewage.
Primary settling occurs in two primary clarifiers having a total
capacity of 5,680 m3 (1.5 MG). Thickener overflow, vacuum filter
filtrate, and incinerator scrubber water join the raw sewage before
it enters the primary clarifiers. Occasionally, when the thickener
is severely overloaded and cannot accept it, secondary sludge is
pumped to the primary clarifiers. Primary sludge is continually
removed from the bottoms of both primary clarifiers and pumped to
the thickener by way of the primary sludge degritter.
Primary effluent is pumped over two rock-filled trickling
filters. The trickling filter effluent is split and enters two
10.7-m (35-ft) diameter final clarifiers. Secondary sludge is
removed continually from the bottoms of both clarifiers and pumped
directly to the thickener.
Detailed Description of Sludge Treatment and Disposal Operations
Degritting--
Because there is no grit removal from the raw sewage, primary
sludge degritting is essential. Grit lowers the volatile content
of the sludge fed to the incinerator and increases the amount of
fuel required to burn it. The Dorr-Oliver cyclone sludge degrit-
ter efficiently degrits primary sludge of less than one percent
solids concentration.
Thickening--
Degritted primary sludge and sludge from the final clarifiers
enter a single Dorr-Oliver gravity thickener through separate
15.2-cm (6-in) feed lines. The thickener is 12.2 m (40 ft) in
diameter, and its depth is 3.05-m (10 ft) liquid sidewal plus the
cone at the bottom. It is equipped with scraper arms having ver-
tical pickets for continuously stirring sludge in the thickener
as it is scraped over the bottom toward the center. Sludge is
sucked into a sludge pape through eight portholes in the base of
the thickener center column. It is then pumped to the vacuum
filters when they are operating. On weekends, when the filters
not operating, the operator has the option of pumping sludge to the
holding tanks for later land application or not pumping at all.
Figure C-5 presents combined hydraulic and mass balances con-
structed around the thickener. It presents information on average
sludge flow rates, total and volatile solids concentrations, and
masses obtained from monthly reports. It is based on monthly
averages of data for a 6-mo period of operation with no ferric
addition and a similar later period with ferric addition.
184
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LAKE
MICHIGAN
FLOW
METER
00
en
AljH TRUCKED
TO LANDFILL
LIQUID SLUDGE-
HAULED TO FARMLAND
Figure C-4. Sheboygan, Wisconsin, wastewater treatment plant flow diagram.
-------
AVERAGE SEWAGE FLOW: 12.1 MGD
6,690 »TS/DAY
3,980 »VS/DAY
0.7 XTS
59 XVS
122.000 GPD ™*»g<
7,220 *TS/DAY
4,150 »VS/DAY
0.2 XTS
57 X VS
440,000 GPD §LUDG|ARY
13 910 tf T^/F>A Y
8, 130 * VS/DAY
0.3 XTS
58 XVS
THI CKEN—
562.000 GPD £p FEED
| 1
FOR PERIOD OF OP
PHOSPHORUS REMOVAL •
AVERAGE SEWAGE FLOW: 14.5 MGD
14,000 »TS/DAY
9,340 »VS/DAY
1.4 XTS
64 XVS
128.000 GPD «££'
7,460 0TS/DAY
4,540 »VS/DAY
0.2 XTS
61 XVS
447 nnn rpn SECONDARY
H*t f , uuu CJHU SLUDGE
13,890 »VS/DAY
0. 5 XTS —
/63 XVS
575.000 GPD THICKEN-
N/A «TS/DAY
N/A *VS/DAY
. 04 XTS
N/A XVS
N/A GPD THICKENER
OVERFLOW
/"
^/
11,670 HITS/DAY
3,790 KVS/DAY
8.6 XTS
65 XVS
., __- rDn SLUDGE TO
16,550 GPD FILTER
I Y
f- I THICKENER I
\ A*.
V
^\
N/A »TS/DAY
N/A »VS/DAY
8.6 XTS
65 XVS
N/A CPD SLUDGE TO
N/A GPD DRYING BEDS.
ERATION BEFORE
- APRIL-SEPT. 1970
N/A (TS/DAY
N/A »VS/DAY
. 2 XTS
N/A XVS
N/A GPD THICKENER
OVERFLOW
J
/
V
CENER ]
-A
'V
24,090 KTS/DAY
7.930 *VS/DAY
7. 8 XTS
66 XVS
37,800 GPD p^L?||sTD
N/A »TS/DAV
N/A »VS/DAY
7. 8 XTS
66 XVS
N/A GPD SLUDGE
HAULED
FOR PERIOD OF OPERATION DURING
PHOSPHORUS REMOVAL - APRIL-SEPT. 1973
NOTE. RATES ARE CALCULATED ON AN AVERAGE DAILY FLOW BASIS
WHETHER FLOWS OCCUR EVERY DAY OR NOT.
Figure C-5 . Sheboygan, Wisconsin,
mass balance.
gravity thickener hydraulic and
186
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The hydraulic-mass balances show that during both periods,
thickener performance, as measured by the solids concentration of
the thickened sludge, was good. During the first period, part of
1970, with no ferric addition, a combined primary and secondary
sludge of about 0.3 percent solids was thickened on the average
to 8.6 percent TS. During the 1973 period, with ferric addition,
the sludge feed averaged 0.46 percent TS and was thickened on the
average to 7.76 percent TS. The thickened sludge solids concen-
tration was lower in 1973, despite a higher concentration in the
feed sludge and despite the addition of polymer to the sludge
feed lines. An anionic polymer was used beginning in April of
1973, to aid sludge settling and to maintain a clear overflow.
Problems with floating of the sludge blanket and resulting high
overflow solids concentration were experienced in thickener oper-
ation both prior to and after phosphorus removal. They seem
related to industrial waste discharges. Poor settleabi1ity was
more frequent during ferric addition than before. In 1970, the
average overflow solids concentration was 441 mg/£ TS, while in
1973, during ferric addition, it was 1,764 mg/£. The plant oper-
ators responded to thickener upsets by stopping ferric addition
and discontinuing sludge feed whenever thickener upsets occurred.
Secondary sludge was wasted to the primary tanks at these times
and the bottoms of these tanks were used for storage until the
thickener regained stability. As a result, the primary sludge
solids concentration and sludge mass was greater in 1973.
The total mass of primary and secondary sludge pumped to the
thickeners was higher in 1973 by about 44,400 kg/mil m^ (370 Ib
TS/MG). This amounts to an increase of approximately 2.18 t (2.4
tons) of dry sludge each day at a 49,200 mVday (13 mgd) influent
flow rate. The total volume of sludge pumped to the thickeners
was actually lower in 1973, however, because of the higher solids
concentration.
The thickener has an overflow rate of approximately 18,300
to 18,700 £/day/m2 (450 to 460 gal/day/ft^). A sludge blanket of
at least 1.22 m (4 ft) must be maintained in the thickener to avoid
pumping a thin sludge. The blanket depth is normally maintained
at close to 1.52 m (5 ft).
The anionic polymer which is used in the thickener is the
same one used for phosphorus removal (Hercules 831). Since poly-
mer addition began, in April 1973, many different points of addi-
tion and dosages have been tried. It was added, for instance, to
the secondary sludge feed line about 12.2 m (40 ft) before it
enters the thickener, providing a 2- to 3-min contact time. Addi-
tion directly to the center well through a plastic hose was also
tried both with and without a diffuser on the end of the line to
distribute the polymer. None of the methods tried produced a
stable sludge blanket. In January of 1977, a successful method
was found. The method involves adding polymer separately to the
187
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primary and secondary sludge feed lines to achieve the same con-
centration of polymer in the sludge in each line. In other words,
the amount of polymer added to each line is proportional to the
sludge flow through the line. The polymer is added to each feed
line through a plastic hose. The point of addition is immediately
before the feed lines discharge in the center feed well of the
thickener. The usual dosage is 3 ppm.
Since January of 1977, when proportional polymer addition
was started, the thickener overflow has had a very low average TS
concentration. The average for January through April was only
154 mg/£.
Sludge is pumped from the thickener to either the holding
tanks or the vacuum filters. The amount of sludge pumped to the
holding tanks has not been recorded until recently. During six
months from September 1976 through February 1977, sludge was
pumped to the holding tanks for an average of 28.4 hr/wk. It is
pumped during the 33 hr on weekends when the vacuum filters are
not operating. The average number of gallons pumped per day was
7,535 and the mass of solids per day pumped was 2,000 kg (4,400 Ib),
During the same period, the average volume pumped to the vacuum
filters was about 127,000 £/day (34,000 gal/day) and the sludge
mass filtered was 8,810 kg/day (19,400 Ib/day).
Unfortunately, the hydraulic-mass balances in Figure 2 do
not show how phosphorus removal affected the quantity of sludge
pumped to the vacuum filters, or the amount of excess pumped to
the holding tanks. The data necessary to calculate these quanti-
ties are unavailable. The increased amount of sludge which Figure
C-5 shows was pumped to the filters in 1973 and was a result of
longer operation of the filters and incinerator. This longer oper-
ation was a result of the elimination of certain problems experi-
enced when the incinerator was new. Since 1972, the incinerator
has been operated on its present schedule.
Liquid Sludge Hauling--
Liquid sludge from the holding tanks is trucked out and
applied to agricultural lands by a private contractor. The con-
tractor charges by the gallon for this service. The number of
gallons hauled is not the same as the number of gallons pumped
to the holding tanks. The tanks are not mixed, so the sludge
becomes heavy in them. It must be diluted with water during pump-
ing so that it will flow into the truck. Data on the quantities
of sludge hauled in 1970 and 1973 are unavailable. In 1976,
22,700 m3 (6 MG) were hauled, but only approximately 10,400 m3
(2.75 MG) were pumped to the holding tanks.
Dewatering--
Sludge dewatering is accomplished with two vacuum filters
which were installed in 1957 and rebuilt in 1968 with Dorr-Oliver
188
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components to be compatible with the Dorr-Oliver Fluo Solids
incineration system. The filters have a design filtering capa-
b]lity of 454 kg (1,000 Ib) TS/hr each at a feed solids concentra-
tion of 27 percent TS. The two rotary drum filters each have a
filtering area of 18.6 m2 (200 ft2), face width of 2.44 m (8 ft),
and diameter of 2.44 m (8 ft). The filter medium is polyethylene
cloth. The filter drum speed is 7 min/revolution. This is the
slowest drum speed setting possible. Sludge is conditioned prior
to filtration with Hercules 814 cationic polymer and ferric chlor-
ide. The vacuum filters are operated for 24 hr/day on Monday
through Friday, 15 hr/day on Saturday, and are shut down on Sun-
day.
Table C-8 presents a comparison of filter performance before
and after phosphorus removal. Data on incineration rate and incin-
erator fuel consumption are also presented. The data are based
on monthly reports for the 1970 "before phosphorus removal" and
1973 "after phosphorus removal" time periods previously mentioned.
In addition, data from a similar 6-mo period in 1976 is presented.
The plant manager felt that the 1976 data were more representative
of the results after phosphorus removal than the 1973 data.
TABLE C-8. VACUUM FILTRATION AND INCINERATION
PERFORMANCE BEFORE AND AFTER PHOSPHORUS REMOVAL,
SHEBOYGAN, WISCONSIN
1970 1973 1976
Filter yield, kg TS/rr,2/hr 20.3 16.1 12.8
(lbTS/ft2/hr) (4.16) (3.3) (2.62)
Filter feed solids % TS 8.6 7.76 7.0
Cake dryness % TS 25.5 24.7 21.5
Cake volatile VS (% of TS) 65.45 65.9 65.4
Filtrate solids ppra SS C58 383 442
814 Polymer, kg/t (Ib/
ton) 1.91 (3.82) 1.28 (2.55) 1.20 (2.4)
FeClj, kg/t (Ib/ton) NA NA 66.3 (132.5)
Incinerator feed
kg/hr (Ib) hr*
Fuel consumption
(gal/ton)
rate,
, 1/t
755
246
(1,662)
(59)
578
451
(1
(1
318)
08)
475
517
(1,046)
(124)
*• Dry-solids fed to incinerator based on solids in filter feed.
Amount of solids lost in filtrate unknown.
Before phosphorus removal, ferric chloride was used for con-
ditioning less than 20 percent of the time. Most of the time,
polymer was used alone. After phosphorus removal, cake dryness
189
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was usually only about 18 percent TS without ferric conditioning;
but with ferric conditioning, a 21-percent TS cake could be
achieved.
Though ferric conditioning raised cake solids content, it
did not improve filter yield. Filter yield can be raised by
increasing filter drum speed, but at the expense of cake dryness.
The volatile content of the sludge cake does not appear to
have decreased with phosphorus removal. Filtrate quality also
was not adversely affected. Polymer dosage decreased somewhat as
ferric chloride usage became regular.
Incineration--
The plant operates a Dorr-Oliver "F/S System" Fluo Solids
incinerator. It has a design capacity of 122 kg/hr (2,684 Ib/hr)
dry filter cake. The cake is expected to contain 25 percent TS
with 73 to 74 percent volatile content. The incinerator's overall
height is 12.2 m (40 ft) and the cylinder's inside diameter is
3.05 m (10 ft). The expanded sand bed is 1.83 to 2.13 m (6 to 7
ft) deep and contains 7.26 t (8 tons) of silica sand. A construc-
tion plate supports the bed and allows fluidizing air to pass
upward. It is a self-supported refractory arch with metal tuyeres
which pass the air. The windbox chamber is below this. The wind-
box size is 1.07 m by 2.74 m (3.5 ft by 9 ft). A preheat burner
mounted on the windbox preheats the fluidized bed in the reactor
prior to sludge feed. A preheat burner fuel oil pump furnishes
No. 2 fuel oil to preheat the burner. An atomizing blower supplies
pressurized air to atomize the fuel oil in the preheat burner.
Once the reactor bed temperature is preheated to 621°C (1,T50°F),
which is the auto-ignition temperature of sludge, No. 2 fuel is
injected directly into the bed by the bed guns. The preheat burner
is turned down but continues to serve as the method of heating the
fluidizing air which is discharged through the windbox into the
bed. The fluidized air comes from a centrifugal blower which dis-
charges air at 4.5 to 5.0 psig into the reactor. Before April 21,
1972, the fluidizing air was heated by a heat exchanger rather
than by the preheat burner. Problems with the heat exchanger
prompted its discard, and the preheat blower became the method of
fluidizing air heating. Without the heat exchanger, oil consump-
tion is higher by about 129 a/t (31 gal/ton). Without the heat
exchanger, the incinerator capacity is reduced by about 159 kg/hr
(350 dry Ib/hr). If the sludge feed rate is increased above its
capacity, the excess air level in the bed will be too low, and
more fluidizing air must be supplied to counteract this. The
incinerator's capacity is limited by its size in the sense that
size limits the rate at which fluidizing air can be accepted.
When fluidizing air is discharged in excess of 2,100 scfm, the
metal tuyeres through which the air must pass upward into the bed
are blown out.
190
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Exhaust gases from the incinerator enter a Venturi scrubber
system. Gases are scrubbed of ash and cooled and discharged to
the atmosphere. Ash slurry is pumped to a Dorr-Clone wet cyclone
and classifier system for dewatering. Dewatered ash is carried
in a 3.06-m3 (4-yd3) truck to a small landfill. 2.29 m3 (3 yd3)
of ash are produced each day. The scrubber water is recirculated
back to the head of the plant. A single sample of the scrubber
water showed a SS concentration of 758 ppm and a pH of 6.4.
Solids are processed in the incinerator continually, except
for 32 hr on the weekends. Before the incinerator is shut down,
the temperature is run up to 871°C (1,600°F). Over the next 32 hr
the temperature falls to about 643°C (1,190°F). Before beginning
again for the next week, preheating takes place for about 1 hr to
get the temperature to 677°C (1,250°F) when feeding can start. The
usual operating bed temperature is 721°C (1,330°F). Fluidizing air
is normally discharged at 2,000 scfm.
Phosphorus removal has adversely affected incinerator capa-
city and fuel consumption because of the increased moisture con-
tent of the sludge cake fed to the incinerator. In Table 2, the
incinerator feed rate and fuel consumption rate in 1970 and 1976
are indicative of the performance before and during phosphorus
removal. However, part of the feed rate decrease and fuel con-
sumption rate increase is due to the removal of the heat exchanger.
The effect of phosphorus removal alone is judged to be an average
decrease of approximately 121 kg (266 Ib) dry solids/hr in the feed
rate and an increase of 142 l/t (34 gal/ton) in fuel oil consumption
There have also been increased incinerator maintenance and
repair problems with phosphorus removal due to slag formation and
corrosion. On one occasion, four tuyeres blew out because several
tuyeres were plugged with slag. On another occasion, a pressure
build-up in the reactor signalled a problem, and a big piece of
slag was found clogging the exhaust line. After this experience,
the operator has done visual inspection of the exhaust line from
the roof duct inspection port every 3 to 4 mo. The plant manager
believes that ferric chloride is also responsible for a high rate
of corrosion of the metal in ducts, especially in the elbows of
the scrubber system. Gradually, these parts are being replaced
with stainless steel.
Sludge Treatment and Disposal Costs
Operational Costs--
The additional costs for sludge treatment and disposal which
resulted from phosphorus removal which have been estimated include
the cost of chemical conditioners, polymer for the thickener, and
fuel oil. The current costs of liquid sludge hauling, incinerator
ash disposal, electricity for vacuum filter motors and incinerator
motors, repair equipment for the incinerator, maintenance supplies
191
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for filtration and incineration combined, and labor for filtra-
tion and incineration combined, have been estimated.
In assessing the operational costs of sludge treatment and
disposal at Sheboygan, the following assumptions were made:
1. All costs are shown as per ton of dry solids filtered,
except for the cost of liquid sludge hauling which is
per ton of solids hauled.
2. Current costs are based on a filtration rate of 9,080 kg
TS/day or 3,310 t/yr (20,000 Ib TS/day or 3,650 ton/yr)
at a feed concentration of 7.0 percent TS; sludge hauling
rate of 728 t/yr or 0.023 mil m3/yr (803 ton/yr or 6.0
MG/yr).
3. The unit cost factors used are:
(1977 costs)
Ferric chloride $.0277/kg ($.0126/lb)
Conditioning polymer $3.74/kg ($1.70/lb)
Anionic polymer $3.13/kg ($1.42/lb)
Contractor's sludge $3.01/m3 ($0.0114/gal)
haul ing fee
Fuel oil $113.34/m3 (0.429/gal)
Electricity $0.01622/kwh
4. Labor for filter and incinerator operation consists of:
160 Operator I man-hr/wk,
20 Operator II man-hr/wk
The cost breakdown is:
Changes in costs
resulting from Current
phosphorus removal costs
Ferric chloride conditioning +$1.65/t $1.84/t
assuming 8,000 gal/yr were (+$1.50/ton) ($1.67/ton)
used before phosphorus
removal
Polymer conditioning -$2.43/t $4'.49/t
(-$2.21/ton) ($4.08/ton)
192
-------
Changes in costs
resulting from Current
phosphorus removal costs
Thickening polymer
No. 2 fuel oil
Electrical power for chemical
conditioning and vacuum filter
operation
Electrical power for incinerator
operation
Labor for filter and incinerator
1976
Liquid sludge hauling
Charge for ash landfill use
Equipment for incinerator
repair - 1976
Equipment for vacuum filter
repair - 1976
Supplies for vacuum filter and
incinerator maintenance - 1976
+$2.20/t
(+$2.00/ton)
+$16.08/t
(+$14.59/ton)
, Unknown
Unknown
Unknown
Unknown
None
Unknown
Unknown
Unknown
$2.20/t
($2.00/ton)
$58.65/t
($53.20/ton)
$2.77/t
($2.52/ton)
$3.43/t
($3.12/ton)
$16.61/t
($15.07/ton)
$29.85/t
($27.08/ton)
None
$15.10/t
($13.70/ton)
None
$4.96/t
($4.50/ton)
Since filter yield and incinerator rates were decreased with
phosphorus removal, the cost per ton of dry solids for electricity
and labor for filter and incinerator operation must have increased.
Because of more hours of operation of the filter and the incinera-
tor, the cost of maintenance and repair supplies and equipment per
ton of solids must also have increased. The amounts of these
increases are unknown because it is not known how much additional
sludge was fed to the filters as a result of phosphorus removal.
Similarly, it is not known how much additional sludge was fed to
the filters as a result of phosphorus removal. However, since the
TS concentration of the thickened sludge was lower with phosphorus
removal, the cost per ton of dry solids to haul must have increased.
Capital Costs--
The vacuum filtration and incineration system was
in 1967. The work included installation of two vacuum
instal1ed
filters
193
-------
(all new, except for the agitators) and a fluidized bed incinera-
tor; removing a wall from an existing building; and adding a small
section to the existing building to house the ash dewatering equip-
ment. The capital cost of this project, including installation,
electrical work and piping, planning and engineering fees, etc.,
was $738,935.18. The planning and engineering fees alone were
$46,708.10. It was financed by floating a bond issue for $396,518.00
and obtaining federal aid for $388,469.00.
Summary and Conclusions
The initial impact of phosphorus removal on sludge handling
appears to have been an increase in the mass of secondary sludge
generated. Concurrently, the problem of intermittent floating of
the thickener sludge blanket was worsened considerably, as evi-
denced by the large increase in the average solids concentration
of the thickener overflow. Whenever thickener upsets occurred,
phosphorus removal was temporarily halted; sludge feed to the
thickener was temporarily halted; and secondary sludge was pumped
to the primary clarifiers. Sludge was stored in the bottoms of
the primary clarifiers until the thickener stabilized. Some thick-
ening occurred in the clarifier bottoms because of this storage.
Therefore, after phosphorus removal was started, the sludge pumped
from the primary clarifiers to the thickener was heavier. The
solids concentration of the secondary sludge was unchanged. The
total volume of thickener feed sludge did not increase with phos-
phorus removal because of the sludge concentrating going on in the
primari es.
The additional mass of sludge pumped to the thickener after
phosphorus removal was started has been calculated from available
data. It is 44,000 kg/mil m3 (370 Ib TS/MG) of wastewater treated,
or 2.18 t (2.4 additional tons) TS/day assuming that 49,200 m3/day
(13 mgd) of wastewater are treated. Part of this increase in
sludge mass was due to the recirculation of more solids in the
thickener overflow. Part was due to the use of ferric chloride
and polymer for phosphorus removal. Unfortunately, available data
for the period we are concerned with do not permit calculation of
the volume and total solids mass of the thickener overflow or the
sludge pumped to the holding tanks. Therefore, we do not know what
fraction of the additional 5.75 t TS/mil m3 (2.4 tons TS/MG) which
entered the thickener went out in the overflow, to the vacuum fil-
ters, or the the holding tanks.
We do know, however, that phosphorus removal resulted in a
change in the dewatering characteristics of the thickened sludge.
Comparing data from 6-mo periods in 1970 and 1976 which were before
and after phosphorus removal began, we see that the average TS con-
centration of the thickened sludge decreased from 8.6 percent to
7.0 percent TS. Filter cake dryness decreased from 25.5 percent
to 21.5 percent TS. The volatile content of the filter cake did
not change. Filtrate SS concentration decreased. Ferric chloride
194
-------
conditioning became necessary to achieve a filter cake TS concen-
tration greater than 18 percent. Previously only about 20 percent
of the sludge had been conditioned with ferric chloride. Because
of ferric conditioning, polymer conditioner dosage decreased. As
a result, the cost for conditioning chemicals decreased by about
$0.78/t ($0.71/ton).
Because of the higher moisture content of the filter cake,
incinerator fuel oil consumption rose and the fuel oil cost rose
by $16.10/t ($14.60/ton) of dry solids. In addition, the rate at
which cake could be burned in the incinerator was lower by about
121 kg (266 Ib) TS/hr. Because the incineration rate was lower,
the vacuum filtration rate was lowered to match. Filter yield
became only 12.8 kg TS/m2/hr (2.62 Ib TS/ft2/hr). A higher filter
yield is possible, but at the expense of cake dryness. It is dif-
ficult to consider filter yield Independently of the incineration
capacity in this situation.
In response to the adverse effect of phosphorus removal on
thickener sludge blanket stability and overflow solids concentra-
tion, efforts were made to find a method of adding polymer to the
thickener which would improve the situation. A really successful
method was not found until January of 1977. Before this, in June
of 1975, the thickener overflow was rerouted to go to the primary
clarifier influent instead of the wet well ahead of the trickling
filters. Poor thickener overflow quality had an adverse effect
on plant BOD and SS removal and plant effluent quality. BOD and
SS removal are so dependent upon influent flow and strength, how-
ever, that it has not been possible to present data showing the
effect of the thickener overflow. Nevertheless, improvement in
plant BOD and SS removal can be considered to be benefits derived
from the addition of polymer to the thickener. Another, benefit,
better phosphorus removal, occurred because ferric and polymer
addition no longer had to be halted frequently for thickener
upsets. The costs of.polymer addition, on the other hand, are the
cost of the polymer itself, which is $2.21/t ($2.00/ton) of dry
solids, and the cost of disposing of the additional sludge solids
which are generated as a result of polymer usage.
CASE STUDY E: COLDWATER, MICHIGAN
Introduction
The Coldwater, Michigan treatment plant provides an example
of the primary addition of ferric chloride for phosphorus removal,
with and without the use of anionic polymer. The major components
of the Coldwater treatment plant are trickling filters, pressure
sand filters, anaerobic digesters, and sludge drying beds.
Wastewater influent to the Coldwater plant is delivered by
separate sanitary sewers, although some older portions of the
collection system do experience groundwater infiltration. Less
195
-------
than 5 percent of the wastewater flow is from industrial sources.
One of the industrial sources is a slaughterhouse, which contri-
butes 379 m3/day (0.1 mgd) with BOD concentrations between 200
and 800 mg/£. The other industrial source is a rendering plant
which contributes 1,140 m3/day (0.3 mgd) with BOD concentrations
as hiqh as 2,700 mg/£.
Three years of treatment plant operational data have been
selected for study as follows:
• 1971 - Prior to initiation of chemical addition for
phosphorus removal.
• 1973 - With chemical addition of both ferric chloride
and anionic polymer.
t 1976 - With chemical addition of ferric chloride only.
Table C-9 presents the average influent and effluent waste-
water characteristics and plant removal efficiencies for each
time period.
TABLE C-9. INFLUENT AND EFFLUENT WASTEWATER CHARACTERISTICS AND
REMOVAL EFFICIENCIES, COLDWATER, MICHIGAN
FLOW m3/d
SS (mg/£)
BOD (mg/l)
TP CmgAe)
(mgd)
- influent
effluent
% removal
infl uent
effl uent
% removal
infl uent
effl uent
% removal
1971
4,660 (1.23)
156
20
87
297
30
90
_ _ ..
_ _ _
1973
7,310 (1 .93)
114
7
93
289
21
93
6.2
1.2
80
1976
9,200 (2.43)
168
15
91
253
26
90
5-4
0.7
86
History
Modifications to the plant which have affected sludge
production and characteristics are detailed below:
1952 Original secondary wastewater treatment plant
structed. Treatment units consisted of a .grit
chamber, two primary clarifiers, two trickling
filters, one final clarifier (identical to the
196
-------
primary clarifiers), and chlorination. Sludge from
the final clarifier was returned to the head of the
primary clarifiers, settled, and subsequently
removed from the bottom of the primary clarifiers
along with the settled wastewater solids. This
sludge was pumped to the primary sludge digester
where it was heated and mixed and then transferred
to the secondary digester. Sludge in the secondary
digester was allowed to settle without heating or
mixing, while the supernatant was returned to the
flocculation channel ahead of the primary clarifiers.
Sludge from the secondary digester was eventually
transferred to one of the sludge drying beds. The
dry sludge cake was disposed of at a landfill site.
1971 Construction of tertiary sand filters began.
April 1972 Plant flow increased by 946 mg3/d (0.25 mgd) when
new trunk sewer from "State Home" connected.
July 1972 Two new final clarifiers on line and one old secondary
clarifier converted to perform primary clarification
in parallel with two existing primary clarifiers;
third trickling filter also on line at this time.
Fall 1972 Existing five sludge drying beds paved and seven new
paved sludge drying beds constructed; new above-
ground, unheated holding digester added; new gas
lifter system installed in the existing primary
di gester.
Sept. 1972 Tertiary sand filters put into operation.
Dec. 1972 Chemical addition for phosphorus removal began;
ferric chloride initially added to the aerated grit
chamber, while anionic polymer was added to the head
of the aerated flocculation at the flash mixer
channel .
1976 Anionic polymer addition suspended.
1977 Anionic polymer addition resumed using liquid
polymer instead of dry polymer.
Chemical Addition for Phosphorus Removal
Liquid ferric chloride is added to the head of the grit
chamber. Anionic polymer (Haviland Chemical, Poly-floe M-P) is
then added at the flash mixer at the head of the flocculation
channel. Approximately 9.4 min contact time at 7,570 m3/day
(2 mgd) plant flow is provided in the flocculation channel. The
chemical dosage is selected to achieve 90 percent phosphorus
197
-------
removal. The average ferric chloride dosage was 40 mg/£ dry
FeCl3 until 1977, when it was raised to 75 mg/l. The average
dosage of anionic polymer is 4 mg/£.
General Description of Wastewater Treatment Operations Affecting
Sludge
A flow diagram for the wastewater and sludge treatment
operations is presented in Figure C-6. Table C-10 gives a
summary of major equipment at the Coldwater plant utilized for
wastewater treatment and sludge handling. In addition to the
specific modifications to the wastewater treatment plant listed
in Table C-9, there are some seasonal variations in plant opera-
tions and plant influent characteristics.
Generally, two trickling filters are operated to provide
secondary treatment. The third trickling filter is not operated
unless it can be run for at least three months. It has been
frequently necessary, during the winter months, to use only one
trickling filter. This serves to increase flow rates through
the filter, thereby avoiding freeze-ups.
A variation in plant influent characteristics is due to
the dumping of the concentrated wastes pumped out of septic
tanks. These dumpings occur with highest frequency during the
summer months, causing brief but significant increases in plant
influent characteristics. Specific data describing these
influents were not available; however, plant personnel indicated
that the quantities of wastes delivered to the plant have
remained relatively constant throughout the study periods.
A flow diagram and materials balance of the Coldwater plant
are shown in Figure C-7. The data presented are based on infor-
mation contained in the monthly reports submitted by the plant
to the state of Michigan, during the years selected for study
(viz. - 1971, 1973, and 1976). From the figure, it can be seen
that wastewater solids information is presented in terms of
suspended solids (SS), while sludge and sidestream information
is given as total solids (TS). In spite of the inconsistency of
the units of expression, all available solids information has
been presented to qualitatively substantiate the observed impacts
of chemical additions for phosphorus removal on plant solids
handling operations. The remainder of this section briefly
describes each of the wastewater treatment unit operations which
have significant impacts on sludge,
Flocculation Channel--
Figure C-8 is a flow diagram of the flocculation channel.
The flocculation channel receives raw influent wastewater after
degritting, chemical addition for phosphorus removal, and
pressure sand filter backwash water return have taken place.
T98
-------
to
vo
SEPTIC TANK
WASTE
RAW
SEWAGE
WET
WELL
SCREW PUMP
PARSHALL
FLUI^E
COMMI-
NUTOR
GRIT
CHAMBER
FLOCCULATION
CHANNEL
EC
CHLORI-
NATION
TERTIARY
SAND
FILTERS
HUMUS SLUDGE
TRICKLING
FILTERS
PRIMARY
CLARIFIERS
§
d
u
DUMP SITE
DRIED
SLUDGE
DRYING
BEDS
DIGESTED
SLUDGE
SLUDGE
ANAEROBIC
DIGESTERS
<
2
LU
(/)
o:
Figure C-6. Coldwater, Michigan, wastewater treatment plant flow diagram.
-------
TABLE C-10.
GENERAL PLANT DESCRIPTION SUMMARY,
COLDWATER, MICHIGAN
Item
No. and Description
Comminutor
/
Grit Chamber
Flash Mixer
1 with 91.4 cm (36 in) impeller at 84 rpm;
motor - 5 hp, 30, 60 hz, 460 v.
27.7 m (91 ft) length x 1 .30 m (4.25 ft)
width x 1.37 m (4.5 ft) height
3 at 27.7 m (91 ft) x 4.88 m (16 ft) width
x 2.74 m (9 ft) height
3 at 34.1 £/min (40 gal/min) - one pump
use at a time; motors - 3 hp, 30, 60 hz,
460 v.
3 pairs of vertical chains with wooden
flights at 3.05 m (10 ft) intervals;
motors - 2 at 1/2 hp, 30, 60 hz, 460 v.
3 at 35.1 m (115 ft) dia. x 2.44 m (8 ft)
side wall depth (SWD) rack-filled (pit
run) field stone, 3.81 to 10.2 cm (1.5
to 4 in dia). Flow sprayed continuously
from fixed nozzles rotating at 2 to 20
rpm
Secondary Clarifiers 19.8 m (65 ft) dia. x 3.05 m (10 ft) SWD
Flocculation
Channel
Primary Clarifiers
Primary Sludge
Pumps
Primary Sludge
Col lectors
Trickling Filters
Secondary
Sludge Pumps
Secondary Sludge
Collectors
Pressure Sand
Filters
Chlorination
2 at 75 rpm; motors - 2 hp, 30, 60 hz,
460 v.
2 rotating collector arms; motors - 3/4 hp,
30, 60 hz, 460 v.
4 multiple media at 1.83 m (6 ft) dia.
x 4.57 (15 ft) length
(continued)
200
-------
TABLE C-10 (continued)
Item
No. and Description
Primary Digester
Primary Digester
Secondary Digester
Holding Digester
Sludge Drying Beds
Sludge Lagoon
958,000 L (253,000 gal) capacity with
13.7 m (45 ft) dia. x 5.49 m (18 ft) SWD;
fixed concrete dome and 3 gas lift eductor
tube encased by an internal water heating
jacket assembly; eductor tube capacity -
41,600 I (11,000 gal)/min total; water
jacket heat exchange capacity 500,000
Btu/hr; motor - 10 hp, 30, 60 hz, 460 v;
boiler converter pump - 2 hp, 30, 60 hz,
460 v.
958,000 I (253,000 gal) capacity at 13.7
(45 ft) dia. x 5.49 m (18 ft) SWD with
fixed concrete dome
142,000 I (374,000 gal) capacity at 16.8
(55 ft) dia. x 5.79 m (19 ft) SWD; with
fixed steel dome
m
m
Concrete paved;
length x 7.62 tn
(125 ft) height
12 at 32.9 m (108 ft)
(25 ft) width x 3.81 m
700,000 £ (185,000 gal) capacity at 30.5 m
(100 ft) length x 15.2 m (50 ft) width x
1.52 m (5 ft) depth
201
-------
FECL,
ANIONIC
PLANT INFLUENT
ADDI- BACKWASH POLYMER
TION WATER ADDITION
0=2.43X10
SS=3410 (69%
VSS = 2370
BOD = 5120
TP=110
VOLATILE)
m
r-
0=1.93X10
SS=1840 (76%)
VS5=1400
BOD=4650
TP=100
0=1.23X10
SS=1600 (71%)
VSS=1140
BOD=3050
TP=N/A-
0=50000
TS=810 SS=340
VSS=240
BOD=170
TP=6
Q=40000
TS=645 SS=170
VSS=130
BOD=120
TP=5
Q=N/A
TS=N/A SS=N/A
VSS=N/A
BOD=N/A
TP=N/A
i i \ >
TS=0
SECONDARY
CLARIFIER
SLUDGE
0=5500032.5%TS
TS=11500
TS=80
Q=25000S)1 . 8%TS
TS=3800
TS=N/A
0=350031.0%TS
TS=300
GRIT
REMOVAL
FLOCCULATION CHANNEL
\
GRIT
REMOVED
Q=FLOW - GPD
SS=SUSPENDED SOLIDS - 0/DAY
VSS=VOLATILE SUSPENDED SOLIDS - #/DAY
TS=TOTAL SOLIDS - #/DAY
BOD=5-DAY BOD - #/DAY
TP=TOTAL PHOSPHOROUS - #/DAY
PH=STANDARD UNITS
N/A=NOT AVAILABLE
Figure C-7. Hydraulic and solids mass balances for wastewater and
sludge treatment operations, Coldwater, Michigan.
202
-------
DIGESTER
SUPERNATANT
RETURN
0=479030. 8%TS
TS=320(48%VOLATILE)
« PH=N/A
BOD=70
PRIMARY- TREATMENT
PERCENT REMOVALS
SS=-65%
VSS=-37
BOD=+29
TP=-10
0=319032.7%TS
TS=720(45%VOLATILE)
PH=7.3
BOD=70
Q=301030.3%TS
TS=75 (31SVOLATILE )
PH=7.1
BOD=20
SS=36
VSS=41
BOD=55
TP=51
SS=58
VSS=63
BOD=43
TP=N/A
FLOCCULATION
CHANNEL
PRIMARY
CLARIFIERS
PRIMARY CLARIFIER
SLUDGE TO
PRIMARY DIGESTER
PRIMARY EFFLUENT
SS=5620(58%VOLATILE)
VSS=3240
BOD=3620
.TP=120
SS=1240(66%VOLATILE)
VSS=820
BOD=2370
TP=51
SS=630(67%VOLATILE
VSS=430
BOD=690
TP=N/A
PRIMARY EFFLUENT
TO TRICKLING FILTERS
PRIMARY CLARIFIER SLUDGE
0 0=610038.3%TS
N TS=4220(51%VOLATILE)
- PH=N/A
[2 0=521036.3%TS
ov TS=2740(8%VOLATILE)
"* PH=6.6
-« 0=387035. 6%TS
£ TS=1800(72%VOLATILE)
- PH=6.7
Figure C-7 (continued)
203
-------
PRIMARY EFFLUENT
SS=5620(58%VOLATILE)
VSS=3240
BOD=3620
TP=120
SS=1240(66%VOLATILE)
VSS=820
BOD=2370
TP=151
SECONDARY TREATMENT
PERCENT REMOVALS SECONDARY EFFLUENT
SS=88%
VSS=87
BOD=81
TP = 81
SS=640(66%VOLATILE)
VSS=430
BOD=690
TP = 23
SS=77
VSS=76
BOD=81
TP=47
•SS=290(69%VOLATILE
VSS=200
BOD=460
TP = 27
^ SS=630(67%VOLATILE)
N VSS=430
« BOD=1690
TP=N/A
SS=67
VSS=70
BOD=82
TP=N/A
SS=210(62%VOLATILE)
VSS=30
800=310
TP=N/A
PRIMARY
CLARIFIER
EFFLUENT
SLUDGE TO
PRIMARY DIGESTER
0=610038.3%TS
TS=4220(51%VOLATILE)
PH=N/A
0=521036.3%TS
TS=2740(8%VOLATILE)
PH=6.6
0=387035.6%TS
TS=1800(72%VOLATILE)
PH = 6.7
1
i TRICKLING
1 FILTERS
1
SECONDARY
CLARIFIERS
SECONDARY
SLUDGE
SLUDGE TO DRYING
BEDS (OR LAGOON)
0=1310311.4%TS
TS=1250(41%VOLATILE)
PH=N/A
0=202538.4%TS
TS=1420(50%VOLATILE)
PH=7.2
0=86036.5%TS
TS=465(54%VOLATILE)
PH=7.1
SUPERNATANT
PRIMARY
DIGESTERS
SECONDARY
AND
HOLDING
DIGESTERS
Figure C-7 (continued)
204
-------
PRESSURE SAND FILTERS
PERCENT REMOVALS
SS=53%
£ VSS=56
2 BOD=25
TP=26
TERTIARY EFFLUENT
SS=3 00 (6 556 VOLATILE)
VSS=190
800=520
TP=17
OVERALL
PLANT PERCENT
REMOVALS
SS=91%
VSS=92%
BOD=90%
TP=85X
SS=59
P VSS=65
2 BOD=26
TP=19
SS=120(58%VOLATILE)
VSS=70
BOD=340
TP=22
SS=93%
BDD=95%
BOD=93%
TP=78%
SS=N/A
h! VSS=N/A
2 BOD=N/A
TP=N/A
SS=N/A
VSS=N/A
BOD=N/A
TP=N/A
SS=87%
VSS=89%
BOD=90%
TP=N/A
PRESSURE
SAND
FILTERS
CHLORINATION
AND DISCHARGE
BACKWASH
WATER
SLUDGE
DRYING
BEDS
(OR LAGOON)
TO LANDFILL
Figure C-7 (continued)
205
-------
RAW INFLUENT
WASTEWATER
FROM GRIT
CHAMBER
PRESSURE
SANDFILTER
BACKWASH WATER
11
FERRIC CHLORIDE
ADDITION
SECONDARY CLARIFIER
(HUMUS) SLUDGE RETURN
DIGESTER
- SUPERNATANT
RETURN
ANIONIC POLYME5
ADDITION AND-*"
FLASH MIXER
=>
I
TO PRIMARY
CLARIFIERS
Figure C-8. Flocculation channel, Coldwater, Michigan
206
-------
Digester supernatant and secondary clarifier sludge enter the
flocculation channel approximately 6.10 m (20 ft) from the flash
mixer at the head of the channel. Ferric chloride solutions
are added to the gri.t chamber, which is ahead of the flash
mixer, and anionic polymer addition takes place at the flash
mixer. As previously mentioned, polymer addition began in 1972,
was discontinued in 1976, and was reinstituted in 1977.
With the advent of chemical addition for phosphorus removal,
the amount of solids entering the flocculation channel in the
pressure sand filter backwash water has increased steadily.
Solids contained in the supernatant also increased considerably
after phosphorus removal began, although somewhat less signi-
ficantly after polymer addition was suspended. The most signi-
ficant increase, however, has been in sludge solids returned to
the flocculation channel from the secondary clarifiers. The
combined effect of these increases has caused no problems in
the flocculation channel but has resulted in huge increases in
solids loadings on the primary clarifiers.
-~«x
*
Primary Clarifiers--
The primary clarifiers receive the raw influent wastewater
and the returned sidestreams from the flocculation channel.
Sludge is collected and removed from the bottom of each clarifier
3 times/day, 7 days/wk. During the intervals between sludge
collection and pumping, a sludge blanket as thick as two feet
builds up on the bottoms of the primary clarifiers. Settled
sludge is pushed to the sludge hopper in each clarifier by
wooden flights attached to two parallel endless chains. The
sludge collectors are turned on a half hour prior to sludge
pumping from the clarifier and are operated until sludge pumping
is completed.
Sludge collected in the sludge hoppers is pumped by the
two primary sludge pumps to the primary digester from one
clarifier at a time. The sludge solids concentration is
approximately 10 percent TS when pumping begins. Pumping con-
tinues until the solids concentration is reduced to roughly
3.5 percent TS.
Since the initiation of chemical addition for phosphorus
removal, the amount of solids influent to the primary clarifiers
and both the quantity and total solids concentration of the
sludge pumped from the primary clarifier to the primary digester
has increased. .Concurrently, the volatile fraction of these
solids has steadily declined. Due to variations in plant
influent suspended solids concentrations and flow rates (pre-
viously characterized in Table C-9), the impacts of phosphorus
removal on sludge generation rates cannot be adequately compared
on the basJs of pounds of primary sludge solids to the digester
per mg of plant influent. Instead, Table C-ll relates the mass
207
-------
of total solids to the digester to the mass of suspended solids
in the plant influent.
TABLE C-ll. SLUDGE PUMPED FROM PRIMARY CLARIFIERS
TO PRIMARY DIGESTER, COLDWATER, MICHIGAN
Year
1976
1973
1971
.e/day (gal/day)
23,100 (6,100)
19,700 (5,210)
14,600 (3,870)
TS
C%)
8.3
6.3
5.6
Volatile
(%)
51
58
72
kg(lb)TS/kg
.(lb), S-S
plant Influent
1.56 (1.24)
0.67 (1.48)
0.51 (1.12)
kg(lb)VS/kg(lb)
VSS
plant Influent
0.41 (0.91)
0.51 (1.13)
0.52 (1.12)
kg(lb)FS/kg(lb)
FSS
' plant Influent
0.90 (2.00)
1.18 (2.61)
0.49 (1 .09)
On this basis, it can be seen that the mass of volatile
solids to the digester per pound volatile suspended solids in
the plant influent remained roughly the same after the addition
of both ferric chloride and polymer began and decreased slightly
when only ferric chloride was added. On the other hand, the.
mass of total solids to the digester per pound suspended solids
in the plant influent showed a significant increase when both
polymer and ferric chloride were added and a moderate increase
when only ferric chloride was added. These increases are
apparently attributable to the increases in the nonvolatile
(fixed) portion of the sludge solids, due to chemical additions
for phosphorus removal .
Trickling Filters--
A notable impact of chemical addition for phosphorus removal
on the trickling filters at the Coldwater plant has been the
reddish-brown discoloration of the trickling filter structure
and media. Although BOD loadings to the trickling filters have
increased significantly, percent removals of BOD have remained
constant. Higher BOD concentrations are thus found in the
trickling filter effluent.
Secondary Clarifiers--
we.l 1
Each
arms
to a
col 1ector
moves the
Scum then
is pumped
clarifier
Trickling filter effluent flows by gravity to a division
which distributes the flow to the two secondary clarifiers.
clarifier. is equipped with a pair of rotating collector
equipped with squeegees to continuously move settled sludge
sludge hopper at the bottom of the tank. One of the
arms is equipped with a traveling scum collector which
collected grease and scum to a fixed scum box assembly.
enters a scum well along with the secondary sludge and
to the flocculation channel at the head of the primary
208
-------
Sludge which has settled to the bottoms of the secondary
clarifiers is pumped twice a day to the flocculation channel.
Sludge is pumped from one final clarifier for the same amount
of time. Both pumps are used to develop enough suction to
prevent clogging of the pumps. When sludge pumping begins, the
sludge solids concentration is approximately 5 percent TS.
Sludge pumping continues until the liquid delivered to the
flocculation channel no longer has a black color, which corres-
ponds roughly to a solids concentration of 0.1 percent TS.
Since the beginning of chemical addition for phosphorus
removal, both the secondary sludge volume and total solids con-
centration have increased, significantly. This has caused a
huge increase in the TS mass pumped to the flocculation channel.
This increase is the most notable change in plant operations
since chemical additions for phosphorus removal were started.
Pressure Sand Filters--
Secondary effluent flows to the pressure sand filters
where approximately half of the remaining suspended solids and
one fourth of the remaining BOD and phosphorus are removed.
Backwash water from the pressure sand filters is discharged to
a holding basin where some solids settling takes place. Back-
wash water in excess of basin capacity overflows the holding
basin weir and enters the flocculation channel. The quantities
and characteristics of the backwash water were shown in Figure
C-10.
Detailed Description of Sludge Treatment and Disposal Operations
Primary Digester--
Sludge enters the primary, digester 3 times/day, 7 days/wk
from the primary clarifiers. The primary digester is heated and
mixed. It is generally maintained at a temperature between 29
and 35°C (85 and 95°F). Heating and mixing is intermittent,
taking place for at least 16 hr/day, with longer periods during
the fall and winter months. Mixing of the digester takes place
by compressing the collected gas from the destruction of
volatile materials and discharging it through three gas eductor
tubes enclosed in a hot water jacket in the center of the
digester. The boiler used to heat the jacket water can be
operated on natural gas or methane digester gas.
The primary digester is maintained full at all times,
except when sludge is withdrawn and transferred to the secondary
(or, on occasion, holding) digester. During a digester sludge
transfer, between 114 and 265 m3 (30,000 and 70,000 gal) are
pumped during the day shift over a period of three or more days.
This is done in an effort to maximize operator efficiency, since
the positions of a considerable number of valves must be changed
209
-------
each time sludge is transferred to or from a different location
(primary, secondary, or holding digester, or sludge drying beds).
After a sludge withdrawal from the primary digester, sludge
transfer does not take place again until the digester has been
refilled. This is because the primary digester is the only
digester which is heated or mixed and is supposed to account for
the majority of the volatile destruction taking place in all the
digesters.
Secondary Digester--
Prior to the plant modifications in 1972, the secondary
d.igester was virtually identical to the primary diaester. At
present, tne secondary digester sludge is neither heated nor
mixed, although the side walls of the digester are surrounded
by soil. Consequently, the sludge which is transferred from
the primary to the secondary digester retains much of its heat,
allowing digestion and destruction of volatile materials to
continue while solids separation takes place. Supernatant from
this digester flows by gravity to the scum well ahead of the
primary clarifiers. Although minor adjustment of the super-
natant overflow pipe height is possible, the quality of the
supernatant returned is largely dependent on the need for
providing additional space in the secondary digester by allowing
more supernatant of less than optimum solids concentration to
be returned to the primary clarifier. This effect appears to
have been most pronounced during 1973, when an estimated 25
percent of the total solids sent to the primary digester were
returned to the flocculation channel as supernatant.
Holding Digester--
The holding digester was installed along with the other
plant modifications of 1972. This digester is situated above-
ground and is neither heated nor mixed. As a result, only a
small amount of volatile solids destruction takes place in this
digester, particularly during the winter months.
Sludge from the secondary digester (and occasionally from
the primary digester) is transferred to the holding digester for
storage until weather conditions or space allow transfer of
sludge to the sludge drying beds. Thus, digester sludge handling
operations are directed at having as little sludge as is possible
in the holding digester at the beginning of autumn to maximize
the amount of digester volume available for storage of sludge
generated during the winter months when the sludge drying beds
are essentially inoperative. Due to the format of the data sub-
mitted by the treatment plant to the state of Michigan, charac-
teristics and quantities of sludge stored in the secondary and
holding digesters are indistinguishable.
210
-------
Digester Performance--
It, is difficult to assess the impact of chemical addition
for phosphorus removal on the individual digesters due to the
limitation in available data. No measurements of the volume of
supernatant returned to the flocculation channel from the secon-
dary or holding digester are available. Therefore, it has been
assumed that the volume of supernatant returned is equal to the
difference between the volume of sludge pumped to the primary
digester minus the volume of sludge sent to the sludge drying
beds (and lagoons).
Since the start-up of chemical addition for phosphorus
removal, the percent total solids of sludge entering and leaving
the digester has increased. A steady decrease in the fraction
of volatile solids in the sludge fed to the digesters has been
observed. But the mass of VS fed to the primary digester/day
increased as follows: 590 kg VS/day (1,300 Ib VS/day) in 1971;
722 kg VS/day (1,590 Ib VS/day) in 1973; and 976 kg VS/day
(2,150 Ib VS/day) in 1976. The amount of digester gas produced
increased from roughly 5,040 m3/mo (180,000 ft3/mo) before phos-
phorus removal to more than 11,200 m3/mo (400,000 ft3/mo) after
phosphorus removal.
The sludge transferred from the secondary and holding diges-
ters to the sludge drying beds (or the lagoons), also reflects
the trend of decreasing volatile solids fraction with increasing
total solids concentration. Table C-12 indicates that the
increase in the mass of TS transferred from the digester to the
sludge drying beds, per pound of suspended solids in the plant
influent, was due to a large extent to the increase in the non-
volatile (fixed) portion of the solids. Once again, this effect
appears to have been most pronounced when both ferric chloride
and polymer were added to the flocculation channel. Furthermore,
less volatile destruction took place in the digestion process
overall during this period. This resulted in a more than doubled
mass of volatile solids in the sludge transferred to the drying
beds, per pound of volatile suspended solids in the plant influent,
as shown in Table C-12. This is in spite of the fact that, as
TABLE C-12. SLUDGE TRANSFERRED FROM DIGESTERS
TO DRYING BEDS (OR LAGOON), COLDWATER, MICHIGAN
Year
1976
1973
1971
•tyday
5,960
7,660
3,870
gal/day
1,310
2,025
860
TS
U)
11.4
8.4
6.5
Volatile
IX)
41
50
54
kg TS
kg(lb) SS
Plant Influent
0.167 (0.37)
0.349 (0.77)
n.131 (0.29)
kg VS
kg(lb) VSS
Plant Influent
0.099 (0.22)
0.231 (0.51)
0.099 (0.22)
kg FS
kg(lb) FSS
Plant Influent
0.322 (0.71)
0.730 (1.61)
0.213 (0.47)
211
-------
was shown in Table C-ll, the sludge fed to
contained nearly equal amounts of volatile
on the same basis.
Sludge Drying Beds--
the primary
sol ids when
digester
compared
Sludge is transferred to the sludge drying beds from the
holding digester (or occasionally the secondary digester) at a
rate largely dependent on prevailing weather conditions. Once
each bed is filled, sludge is not removed until it is determined
to be sufficiently dry, roughly 6.5 percent moisture by weight.
Depending on weather conditions, each bed can be filled three to
four times per year. The sludge is transferred to the beds in
the fall, generally remains through the cold winter months, and
is removed in the spring. As shown in Figure C-7, the TS con-
centration of the sludge transferred to the drying beds has
increased considerably, while the volatile fraction of the solids
has decreased since the addition of phosphorus removal chemicals.
As a consequence of these trends, combined with the effects of
paving the former drying beds, the sludge now takes 30 to
50 percent longer to reach the dryness desired for removal and
disposal.
Sludge Lagoon--
The sludge drying lagoon was excavated shortly after chemi-
cal addition for phosphorus removal was initiated to provide addi
tional sludge drying space. '^6 s was necessitated by the longer
sludge drying times, as well as the greater-than-anticipated
quantities of digested sludge generated. Sludge is sent to the
lagoon on a sporadic basis at times when the drying beds are full
and there is a need to make room in the digesters. No formal
records have been kept as to the characteristics of the sludge
in the lagoon.
Sludge Treatment and Disposal Costs
Operating and Maintenance Costs--
The annual O&M costs for the Coldwater plant are based on
the following unit costs:
Item
Fe C13
Natural Gas
Electricity
Labor (including
benefi ts)
supplies and
Unit Cost
$0.11/kg ($0.05/lb)
$0.0215/m2 ($0.002/ft3)
$0.0368/KWH
$7.75/hr
212
-------
O&M costs for the 1976-77 fiscal year were as follows:
Item $/Yr
Operator and Superintendent Labor $ 45,000
Operating Supplies 40,000
Electricity 4,160
Natural Gas 3,660
Water 7,840
Maintenance and Repairs 25^000
Total $125,660
Cost/m3 (MG) influent $0.0374/m3 ($142/MG)
The impacts of- chemical addition on plant O&M costs are dif-
ficult to assess, since major plant modifications occurred at
the time when phosphorus removal was initiated. According to
plant personnel, most sludge treatment equipment operated at the
plant requires approximately twice the maintenance when compared
with the period prior to the beginning of phosphorus removal.
Since plant flows have nearly doubled in the interim, the rela-
tive increase/MG of plant influent has been negligible.
Capital Costs--
The capital cost of the original Coldwaterplant was financed
by 30 year general obligation bonds providing $650,000 at 2.25
percent interest beginning October 1, 1951. The additions and
modifications to the plant were financed beginning October 1, 1970
as follows:
Federal Grant $301,500.00
State Grant $800,000.00
Revenue Bonds $550,000.00
Total $1,651,500.00
The revenue bonds mature in 17 years, at an initial interest
rate of 4.5 percent per annum. In 1983, the interest rate begins
to increase, reaching 5.5 percent at maturity.
Summary and Conclusions
There has been a significant increase in sludge and side-
stream solids concentrations entering and leaving every unit of
the Coldwater, Michigan, wastewater treatment plant, beginning
213
-------
with the flocculation channel through the sand drying beds. The
impacts of chemical addition on the treatment units were as
follows:
Flocculation Channel - more solids entering in the following
sidestreams:
t Secondary clarifier sludge
t Digester supernatant
• Pressure sand filter backwash water
t Ferric chloride and an ionic polymer additions.
Primary Clarifiers:
• Increased average SS loading
t Increased volumes of primary sludge pumped to pri-
mary digester at higher average TS concentration
M
t Decreased average BOD and SS removal efficiencies.
Trickling Filters:
• Increased average BOD and TS influent and effluent
concentrations
• Discoloration of filter media.
Final Clarifiers:
• Increased average SS loading
• Increased volumes of sludge pumped to flocculation
channel at higher average TS concentration.
Pressure Sand Filters:
• Increased SS loading
• More backwash water at higher average SS concentra-
ti on.
Digesters:
• Increased .volumes of sludge feed at higher average
TS concentration
• Higher supernatant average SS concentration.
Sand Drying Beds:
t Increased quantities of sludge requiring longer
d ry i n g times.
214
-------
In conclusion, a solids build-up has occurred throughout the
p.lant. The additional solids have been almost entirely nonvola-
tile. Although(0&M costs have nearly doubled since phosphorus
removal began* the plant flow doubled at the same time, resulting
in no increase in treatment costs per million gallons of plant
influent.
CASE STUDY F: LAKEWOOD, OHIO
Introduction
The Lakewood, Ohio, experience is similar to that encoun-
tered at many older facilities which have inadequate sludge
handling capacity. Soon after the plant was built in 1966, it
became apparent that available flash drying capacity was inade-
quate, and as the equipment has aged, its Capacity has further
decreased. Alum addition for phosphorus removal further
decreased the dryer's capacity by increasing the moisture con-
tent of the filter cake, and alum has also increased the amount
of sludge to be handled. A bottleneck in the solids handling
end of the plant has resulted. This had led to high solids
buildup and recirculation throughout the plant and a high SS
concentration in the plant effluent.
In spite of the problems created by inadequate sludge
handling capacity, the plant management stayed, until 1975, with
a filter and dryer operation schedule of one 8-hr shift/day
because of budget considerations. It has been possible to
observe the effects on plant performance of extending the oper-
ation of two shifts.
It has also been possible to observe the effects on plant
performance of hauling liquid sludge from the digesters. It
appears that, liquid sludge hauling as a sludge management
alternative is frequently relied upon by plants which have
additional sludge to handle because of phosphorus removal but
do not have the necessary existing sludge handling capacity.
As with other wastewater treatment plants, sludge produc-
tion at Lakewood is affected by changes in wastewater flow
volume. The plant was designed to treat 0.05 mil m3 (13 MG) of
wastewater per day. The actual average dry weather flow is only
0.045 mil m3/day" ('12 mgd). But peak flow rates occurring during
wet weather are considerably higher, since sanitary and storm
sewers in the area are 40 percent combined. Table C-13 shows the
average and peak influent flow rates for each year since 1973.
Before September of 1975, peak flows reached almost 0.076 mil m3/
day (20'mgd). Since then, they have been kept at 0.060 mil. m3/
day (16.0 mgd) by bypassing the excess to the river. Before
bypassing was used, on days of heavy flow, treatment efficiency
was reduced and effluent suspended solids concentrations reached
400 mg/£.
215
-------
TABLE C-13. PLANT INFLUENT WASTEWATER FLOW RATES, LAKEWOOD, OHIO
Year
1973
1974
1975
1976
1977 (thru
May)
Average
Total mil m^/day
0.055
0.055
0.050 •
0.051
0.042 "
Total mgd
14.6
14.6
13.3
13.4
11.2
Peak .
Total mil m3/day
0.075
0.073
0.067
0.060
0.060
Total mgd
19.8
'• 19.4
17.7
16.0
16.0
~ =»
History
The following description details historical plant changes
which have affected sludge characteristics:
1938 - Original Imho'ff plant constructed; sludge anaerobi-
cally digested and dewatered on sand drying beds.
1966 - Conventional activated sludge plant constructed;
gravity thickeners, two new anaerobic digesters,
two vacuum filters, and a flash dryer added to
handle additional sludge.
1974 - Alum addition for phosphorus removal begun.
1975 - Sludge handling system (filters and dryer) operating
schedule extended to two shifts to handle additional
sludge generated.
1977 - Returned to single shift sludge handling; hauled
excess liqu.id sludge to land disposal.
Chemical Addition for Phosphorus Removal
(48.4 percent Al2(S04)3
sludge effluent channel, an
T4 H20) is added to
Liquid alum
the mixed liquor in the activated
open concrete channel leading to the final division well ahead
of the secondary clarifiers. The alum is added through a non-
submerged pipe at a point about 2 m (5 ft) from the aeration
tank effluent weir and 20 m (.70 ft) before the final division
we! 1 .
The chemical dosage is controlled by manual adjustment of
the chemical feed pump settings. The pumps are normally reset
at midnight and at eight in the morning each day to compensate
for the smaller wastewater flow at night. The same settings
are used each day unless the plant effluent phosphorus concen-
216
-------
tration increases, in which case more chemical is added. A
total of about 4,160 m3 (1,100 gal) of alum are used each day.
Each liter liquid contains 0.650 g/nT of Al2(S04)s • 14 H20 (5.4
lb/gal). Therefore, the average wastewater concentration is
approximately 63 ppm Al2(S04)3 • 14 H20.
Presently, the plant is not achieving the required 1 mg/£
effluent total phosphorus concentration because ft cannot
handle the additional sludge which is produced when alum dosage
is increased or a polymer is used. Only 65 percent reduction of
the total phosphorus concentration of the influent is achieved
within the plant. The average final effluent concentration is
6.4 mg/£.
General Description of Wastewater Treatment Operations
Figure C-9 shows the treatment system flow diagram.
Suspended solids in the raw sewage influent plus biological and
chemical solids generated by the activated sludge and phosphorus
removal operations are collected by the primary and secondary
clarifiers and then are gravity thickened. A portion of the
solids are converted to liquid and gas by the digesters, with
the remainder being either hauled to agricultural lands, dried on
sand beds, or vacuum filtered and flash dried. The under-
drainage from the sludge drying beds and the thickener opera-
tions are returned to the front of the plant. Other sidestreams
from sludge treatment operations (.digester supernatant, vacuum
filter filtrate and flash dryer scrubber water) are collected
and returned to the system at the primary division well.
Descriptions of sludge-significant wastewater treatment
unit operations follow.
Primary Clarifiers--
^
The primary clarifiers consist of four circul ar .Eimco units,
with 19.8 m (65 ft) diameters, 308 m2 (3,320 ft2) surface areas,
and 2.9 m (9.4 ft) side water depths. Each clarifier holds
883 m3 (0.233 MG). Scum is collected in scum boxes and pumped
to the primary digesters. Sludge is mechanically scraped to a
hopper in the center of the sloping clarifier bottom by four
revolving arms. The sludge hoppers are 0.9 m (3 ft) deep,
0.9 m (3 ft) by 0.6 m (2 ft) wide at the top, and narrowing like
a square cone to a 15 cm (6 in) sludge outlet pipe at the bottom.
Although there is room in the primary clarifier bottoms,
sludge storage here is avoided. Sludge is continually scraped
from the bottoms of the clarifiers, and it continually flows by
gravity from the hoppers to the primary sludge well. The
primary clarifiers are not equipped or operated to achieve
.thickening, although less frequent removal of sludge could
probably be practiced to allow thickening of sludge to a higher
217
-------
ro
oo
COHHINUTORS
PRIMARY DIVISION
WELL
THICKENER OVERFLOW
SLUOGC I
PRIMARY
st_unfif?
WELL
SECONDARY
SLUDGE
WELL
8
o
E
SECONDARY (WASTE
ACTIVATED! SLUDGE
SUPERNATANT
ft 1 IMP
1
OVERFLOW
FILTRATE
SCRUBBER WATER
AGRICULTURAL
LANDS
SLUDGE
FLASH
DRYER
DRIED SLUDGE
BAGS OR BULK
SOIL
' CONDITIONING
Figure C-9. Lakewood, Ohio, wastewater treatment plant flow diagram.
-------
solids concentration. This is avoided, however, because the
plant manager wishes to avoid problems in thickener operation
which might be caused by a heavier sludge feed.
Secondary Clarifiers--
The secondary clarifiers are four circular Eimco units
with 23 m (75 ft) diameters, 411 m2 (4,420 ft2) surface areas.
and side water depths of 3.6 m (11.7 ft). Each has a 1,465 m3
(0.387 MG) capacity. The sludge collection mechanism is the
plow and scraper type like that used in the primary clarifiers.
The sludge hopper is 1.2 m (4 ft) deep and 0.6 m (2 ft) by 0.9 m
(3 ft) wide at the top. It then narrows to a 20 cm (.8 in)
sludge outlet pipe at the bottom.
The secondary clarifiers are heavily relied upon for sludge
storage because the plant is overloaded with secondary sludge
which has no place to go. A significant portion of this sludge
is due to alurr- addition for phosphorus removal. A sludge
blanket about 2.3 m (7.5 ft) deep has built up in the secondary
clarifiers. The detention time of sludge in the secondary
clarifier bottoms may be somewhere around 1.3 hr, even though
sludge is continually scraped to the hoppers and continually
flows by gravity to the secondary sludge well and then to the
primary sludge well. Pumping of sludge from the primary and
secondary sludge wells is also continuous.
Activated Sludge Aeration Basins--
The eight aeration basins are operated as conventional
activated sludge treatment units with somewhat tapered aeration.
Mixing and aeration both are accomplished with diffused air.
Each basin holds 1,950 m3 (0.515 MG), giving a total capacity
of 15,600 m3 (4.12 MG). Primary effluent and return activated
sludge enter and flow the lengths of four of the eight basins,
then flow down the other four basins before emptying into the
eff1uent channel .
Because the gravity sludge thickener is overloaded, 97 to
98 percent of the secondary sludge which is produced is recir-
culated to the aeration basins as return activated sludge.
This has resulted in a high mixed liquor suspended solids con-
centration .
Detailed Description of Sludge Treatment and Disposal Operations
Thickening--
The two circular Eimco gravity thickeners have 10.7 m (35
ft) diameters and 3 m (10 ft) side water depths. The thickeners
were designed to operate with an influent total solids concentra-
tion of less than 0.2 percent. Mechanisms for sludge stirring
219
-------
or polymer addition are absent. Sludge is fed at the center of
the top of each clarifier. Two collector arms on the bottom
of each thickener scrape the thickened sludge toward circular
center hoppers which are approximately l.lm (36 ft 6 in) in
diameter and 0.6 m (2 ft) deep. Sludge is pumped from the
hoppers to the digesters. Sludge from the #1 thickener is
pumped to the #1 primary digester, while that-from the #2
thickener is pumped to the #2 primary digester.
The sludge thickener is continuously fed combined primary
and secondary from the primary sludge well at a rate of 3.240
m3/min (850 gal/min). Sludge is removed from the thickener
intermittently at 0.282 m3/min (75 gal/min) for 15 min each hour.
The top of the sludge layer in each thickener is near the over-
flow weir. This overloaded condition is responsible for solids
carryover and frequent thickener bulking.
Digestion--
There are four above-ground anaerobic digestion tanks. The
primary digesters are 22.9 m (75 ft) in diameter with a 6.8 m
(22.5 ft) side water depth, while the secondary digesters are
18.3 m (60 ft) in diameter with a side water depth of 5.8 m
(19 ft). The sludge flow pattern through the digesters is
shown in Figure C-10. Sludge flows by gravity from the primaries
to the secondaries.
FROM
THICKENED
#1
SECONDARY
DIGESTER
#1
PRIMARY
DIGESTER
#2
SECONDARY
DIGESTER
EROM
THICKENER
#2
PRIMARY
DIGESTER
TO
SLUDGE
DRYING BEDS
HOLDING
TANK
TO
VACUUM
FILTERS
HAULED TO
AGRICULTURAL LANDS
Figure C-10. Lakewood, Ohio, anaerobic digester configuration
The primary digesters are mixed and heated. The mixing
mechanism is a "circulating mixer" consisting of a propeller on
a shaft inside of a draft tube. Each primary digester has
three circulating mixers, each capable of circulating at least
473 £/sec (7,500 gal/min) o.f water.
220
-------
The temperature in the primary digesters is maintained at
77.2 to 87.2°C (95 to 105°F). Heating is accomplished by recir-
culating sludge through two external heat exchangers. Sludge
recirculation through .the heat exchangers also provides limited
mixing of the contents of the primary digesters. The two hot
water boilers serving the heat exchangers can run on either
methane digester g.as or No. 2 fuel oil. Throughout most of the
year, digester gas alone is used. It is consumed in the winter
at a rate of 896 to 1,060 m3/day (32.000 to 38,000 ft3/day) per
boiler and at a rate of 360 to 560 m3/day (13,000 to 20,000 ft3/
day) in the summer. In addition, approximately 3.785 m3/yr
(1,000 gal/yr) fuel oil are needed for digester heating.
The primary digesters have uninsulated anchored steel dome
covers while the secondaries have insulated floating covers.
Methane gas is collected from all four digesters and compressed
into two storage spheres for use in digester and building
heating.
The secondary digesters are neither mixed nor heated.
Sludge in the secondaries stays warm at about 87°F due to pre-
vious heating in the primaries. Supernatant can be drawn from
the secondary digesters at three different levels, 1.8 m (6 ft),
1.5 m (5 ft), and 1.2 m (4 ft) from the top.
Each primary digester is fed from one thickener for 15 min/
hr, 24 hr/day. Therefore each primary digester is fed for a
total of 6 hr/day at 0.284 nr/rain (75 gal/min), 7 days/wk.
Sludge overflow from the primaries to the secondaries and super-
natant overflow from the secondaries is more or less continuous
by gravity. From each secondary digester, 19 m3 (5,000 gal) of
sludge are pulled to tank trucks 4 times a day, 5 days a week.
In addition, sludge flows by gravity 0.144 m3/min (38 gal/min)
from each secondary digester to the holding tank for approxi-
mately 3 hr and 15 min/day, 5 days/wk. When the filter and dryer
was.operated 2 shifts a day, as before liquid sludge hauling was
practiced, the flow rate from each digester was about 0.16 m3/
min (42 gal/min) about 7 hr/day, 5 days/wk. Finally, each sand
drying bed is fille-d twice a year with digested sludge.
The digested sludge holding tank is a simple 49,000-m3
(13,000-gal) tank. It is unmixed, and no dilution water is
added to it or supernatant removed.
The digesters have not been cleaned since they were
installed in 1966, and their effective volume is presumed to
be considerably reduced from their total volume. There is
evidence that the sides and corners of the secondary digesters
are filled in with a very heavy sludge (estimated at about 15
percent solids). •
221
-------
Dewatering-Vacuum Filters--
Two Eimco belt-type rotary drum vacuum filters are used
for dewatering digested sludge. Each filter has a drum diameter
of 3 m MO ft), a length of 3.6 m (12 ft), and filtering area
of 35 m2 (376 ft^). The filters have 24 drum compartments.
The vacuum pumps have a capacity of 15 m^/min (545 ft^/min) at
51 cm (20 in) Hg vacuum. The filter drive motors are variable-
speed, enabling a drum speed of 1.5 to 9 min per revolution.
The filter medium is polyethylene cloth. The filter tanks are
equipped with swing-type agitators capable of completing 6 to 18
cycles/min.
The design filtering performance specifies a capability of
filtering a sludge of 1.5 to 3 percent solids at a rate of 35
to 65 kg/nr/hr (1.25 to 2.25 1b/ft2/hr) when properly conditioned
with ferric chloride and lime. The maximum suspended solids
concentration of the filtrate is specified as 500 ppm.
Only one vacuum filter can be operated at a time because
of the limiting capacity of the flash dryer. One filter operates
5 days/wk and for 6.5 hr/day when operated one shift, and 13 hr/
day when operated two shifts. Conditioning chemical dosages
are roughly 137 kg dry 1ime/t of dry solids (275 Ib dry lime/
ton), and 0.283 m3 (63 gal) liquid ferric chloride (40 percent
FeCls) or 15 kg. FeClg/t of dry solids (30 Ib FeClg/tj. The
ferric chloride is diluted to 17 percent before mixing with
sludge in the flash mixer, and the lime is slaked. The lime
is purchased as pebble lime, 72 percent CaO (calcium oxide).
The filter drum speed has been set at 8 rain/revolution and the
depth of drum submergence is around 0.76 to 0.91 m (30 to 36 in).
Dewatering-Drying Beds--
The six sludge drying beds are greenhouse-enclosed sand
beds which are ventilated through open doors and windows. The
beds consist of 23 cm (9 in) of sand underlain by 8 to. 23 cm
(3 to 9 in) of gravel. The effective size of the filter sand
is 0.25 to 0.50 mm with a uniformity coefficient not greater
than 4.0. Bed filtrate collects underneath in a lengthwise
row of drain tile under the middle of each .bed. These conduits
empty into a long intersecting drain which runs to the raw
sewage lift station. The beds are 8.5 m (27 ft 8 in) wide, and
50 m (163 ft) long. The concrete bed walls rise 0.5 m (1.5 ft)
above the sand. Dried sludge is lifted from the beds manually
with a fork. The dried sludge is then made available to the
public. Interested persons can drive into the plant and pick up
the sludge, or it can be delivered to them in a 2-1/2-ton dump
truck.
Sludge is applied to the drying beds twice a year. The
first application is in the spring and the second at the end of
222
-------
the summer. All but two beds are filled at these times. The
two beds are available for use later, should the filter and
dryer system be down for repair.
Heat Drying--
The plant has a flash drying system by C-E Raymond Combus-
tion Engineers. Filter cake is transported by a belt conveyor
to the dryer. The design capacity of the dryer is 825 kg dry
solids/hr (1,820 Ib/hr). The design is based upon the following
sludge cake feed characteristics:
t Moisture content 75 percent
• Volatile matter 51.4 percent
• 3,000 kg cal/kg (5,400 Btu/lb) of dry solids.
The furnace burns either No. 2 fuel oil or methane digester gas.
Sludge drops off the conveyor into a pug mill mixer where
it is thoroughly mixed with dried sludge. The sludge then
passes through a cage mill where it is pulverized and entrained
with rising air and gases which have been heated in a furnace.
Sludge and hot gases rise 9 m (.30 ft) through a vertical duct
to a cyclone separator. The dried sewage solids are separated
from the hot gases and dust which are drawn off by an induced
draft fan. The hot gases go either back down to the pug mill
to be recycled through the system or they go to a deodorizing
preheater where some of their heat is recovered in preheating
combustion air. From the preheater, the gases go into the fur-
nace to be deodorized. Then they pass through the combustion
air preheater for further heat recovery before entering the wet
scrubber.
The wet scrubber consists of a wet centrifugal precleaner
followed by a dynamic precipitator with ,a water spray. The
dust-laden scrubber water is discharged and returned to the head
of the treatment plant.
The dried sludge passes from the cyclone separator through
an airlock, and 80 to 85 percent of it is recycled back to the
pug mill to be mixed with wet filter cake while 15 to 20 percent
is conveyed to storage.
The dryer is furnished with a pneumatic dried sludge cooling
and conveying system to receive the product from the drying
system and deliver it to the storage bin. It consists of a
cyclone collector, fan, and wet scrubber. There is also a
dried sludge bagging machine and bulk loading conveyor. Inter-
ested persons can pick up the bagged product at the plant for
use on lawns, gardens, etc. Deliveries are made by the plant
to bulk users in a truck that carries 4 tons.
223
-------
The flash dryer operates whenever sludge is being filtered.
However, the furnace, is started up an hour before filtering
begins. When sludge is being fed, the furnace is at a tempera-
ture of roughly 815°C (1,500°F), and the hot gases are at about
593°C (1,100°F) when they are mixed with the sludge feed. The
furnace is run on No. 2 fuel oil at 210,900 kg/m2 (300 psi)
pressure or occasionally on methane gas at 4,218 kg/nr (6 psi)
pressure. When drying two shifts per day, approximately 3.785
m3/day (1,000 gal/day) fuel oil are consumed; when drying one
shift per day, about 2.08 nr/day (550 gal/day) are consumed.
Disposal of Liquid Sludge--
The plant has contracted a private firm to haul liquid
digested sludge. The company specializes in hauling liquid
municipal sludge, lime water sludge, and septic tank sludge.
Hauling of Lakewood liquid sludge has been going on only since
June of 1977. A "tee" was installed on the line running from
the digesters to the holding tank, so that sludge could be
sucked directly into a tank truck with a vacuum pump. When the
sludge is heavy, a piston pump, normally used to pump to the
drying beds, is also applied. TJie tank trucks which are used
hold 17,000, 18,900, or 20,800m3 (4,500, 5,000, or 5,500 gal)
of sludge.
Sludge is hauled approximately 115 km (70 mi) to agricul-
tural lands outside of the Cleveland suburbs. From the tank
truck the sludge is forced into a special truck or "field
jimmy" which is adapted for spreading liquid sludge on fields.
Normally eight 19,000-m3 (5,000-gal) loads of digested
sludge are hauled per day, 5 days/wk.
Impact of Phosphorus Removal on Wastewater Treatment Operations
Alum is added for phosphorus removal to the channel between
the aeration basins and the final clarifiers. Final clarifi-
cation is directly affected by the alum by increasing the capture
of SS and BOD. The resulting chemical sludge indirectly affects
primary and secondary clarification and activated sludge treat-
ment by altering the composition of the sidestreams and flows
entering these processes.
Wastewater samples for SS, BOD, and phosphorus analyses
are taken at an insufficient number of points in the treatment
process to allow a complete analysis of the impact of the
chemical sludge on wastewater treatment operations. For example,
a single sampling point exists before the primary clarifiers,
ahead of the point where the sidestreams from the sludge treat-
ment processes join the raw sewage. There is no sample of the
wastewater which actually enters the primary clarifiers.
224
-------
Table C-14 presents data taken from plant monthly reports on
wastewater SS, BOD, and total phosphorus and orthophosphorus
concentrations. Routine samples for SS and BOD determination
are taken of the raw sewage at the magnetic flow meter building,
of the primary clarifier effluent, and of the final effluent.
Phosphorus determinations are made only on the raw sewage and
final effluent. The table presents the average concentrations
for 1 yr with no alum addition, and 1 yr during which 63 mg/£
was added to the sewage.
There is only a small difference between the plant influent
and primary effluent SS and BOD figures. This indicates that
the return sidestreams from the thickeners and digesters are
contributing heavy solids loadings to the primary clarifiers.
The solids accumulate in the primary clarifiers from whence they
are fed back to the thickeners and digesters, thus being trapped
essentially in a closed-loop system. Table C-14 shows that the
primary effluent SS concentration was higher when alum was
added than before, presumably due to the heavier solids loading
contributed by the sidestreams entering the primary clarifiers.
Suspended solids mass balances were constructed about the
primary clarifiers to the extent possible with available plant
data. Figure C-ll is a mass balance which is derived from a year
of performance data during which 63 mg/£ alum addition was
practiced. Figure C-12 portrays the situation for a year with
no alum addition, but data for this period is incomplete.
The information in Figure C-ll shows that the sidestreams
are contributing a loading of 6,948 kg/day (15,305 Ib/day) SS
to the primary clarifiers. This figure is greater than the
6,025 kg/day (13,272 Ib/day) contained in the plant influent.
By far the most significant sidestream in terms of solids input
is the thickener overflow, carrying 6,433 kg/day (.14,171 Ib/day)
SS. The plant superintendent has indicated that the thickener
overflow quality deteriorated when alum addition was started,
but the average increase in solids concentration is unknown.
The digester supernatant also contributes a large amount of
solids, and a comparison of Figures C-ll and C-12 shows that the
supernatant quality was poorer during alum addition.
During alum addition, higher SS and BOD loadings were
carried into the aeration basins in the primary effluent.
Table C-15 shows that the plant's final effluent concentrations
did not change significantly, however. This indicates that the
addition of alum allowed increased capture of solids and BOD in
the final clarifiers. Table C-15 shows the "removals" (or the
differences between primary and final effluent concentrations)
of SS and BOD during secondary treatment with and without alum
addition. There were on the average 0.015 kg/m3 (125 Ib/MG)
additional SS and 600 kg/mil m3 (5 Ib/MG) additional BOD removed
during alum addition. These figures do not give the whole
225
-------
TABLE C-14. PLANT SS, BOD, AND PHOSPHORUS CONCENTRATIONS BEFORE AND
DURING ALUM ADDITION FOR PHOSPHORUS REMOVAL, LAKEWOOD, OHIO
Wastewater flow
m3/day (mgd):
Plant influent SS
Primary effluent SS
Final effluent SS
(mg/£) :
Plant effluent BOD5
(mg/£):
Primary effluent 6005
(mg/£):
Final effluent BODs
(mg/£):
Plant influent total
phosphorus (mg/£):
Effluent total phosphorus
(mg/£) :
Plant influent ortho-
phosphorus:
Effluent ortho-phosphorus:
No alum addition
(Nov. 72 to Oct. 73)
55,700 (14.9)
111
94.6
32
97
84
13
11.8
9.5
6.9
8.2
63 mg/l alum addition
(Jan. 74 to Dec. 75)
54,600 (14.6)
109
105.6
28
108
84.6
13
18.29
6.4
8.6
4.15
226
-------
ro
ro
55
SS
W .
13,270
109
OAK
6 MGD
'y
/^i
uy<
SEWAGE
INFUJF.NT
VACUJM FILTER
' KGD FILTRATE
SS 1,130
SS 1.360
SCRUBBER
' MGD WATER
!Tv
p
SS
SS
I'l.
3^'l2p
28
y
O)A1
x
SECONDARY
HOD EFFLUENT
:TICN 1
i3,s6o ^y
TS 17,300 °/f
„_. WASTE
•°94 MGO ACT. SLUDGE
SCRUBBER WATER FILTRATE
NOTE: HEAVILY OUTLINED BOXES CONTAIN ESTIMATED VALUES. N/A = NOT AVAILABLE.
Figure C-ll. Lakewood, Ohio, hydraulic and solid mass balance during 63 mg/£ alum addition
(Jan through Dec 74)
-------
13.600
SS III
14,9 HGD
V
,/WY
X
RAH SEWAGE
INFLUENT
•x
Xn»v
X
M FILTER
IE
N/A
N/A
u,/
/DAY
X
•"- "SET
ro
ro
oo
H.A.
TS 93O.OOO
J--6AV
3r
H7A. DRIED SIUW£
H.A.
IS 237,700
*&
y,
N.A, FILTER CAKE
N/A
TS «.5S3
NQlESp N/A * NOT AVAILABLE,
tCAVlLV OUTLINED
BOXES CONTAIN
ESTIMATED VALUES.
1453* CHEMICALS
SCRLBOER MATER
Figure C-12.
Lakewood, Ohio, hydraulic and solid mass balance before alum addition
(Nov 72 through Oct 73)
-------
TABLE C-15. PLANT SS, BOD, AND PHOSPHORUS REMOVAL BEFORE AND DURING
ALUM ADDITION FOR PHOSPHORUS REMOVAL, LAKEWOOD, OHIO
Removal
Secondary Treatment
No Alum
kg SS/nr (Ib/MG)
kg BOD/m3 (Ib/MG)
% SS
% BOD
Removal
kg P/m3 (Ib/MG)
Total
Ortho
% P
Total
Ortho
kg SS/m3 (Ib/MG)
kg BOD/m3 (Ib/MG)
% SS
% BOD
63 mg/ZAIum
Difference
0.06 (522.1) 0.077 (647.2)
0.071 (592.1) 0.072 (597.1)
66.2 73.5
84.5 84.6
Total Plant
0.017 (125.1)
0.001 (5.0)
7.3
0.1
No Alum
0.002 (19.2)
0.0013 (10.8)
19.5
18.8
0.08 (658.9)
0.084 (700.5)
71.2
86.6
63 mg/l Alum
0.018 (15.2)
0.004 (37.1)
65.0
51.7
0.081 (675.5)
0.095 (792.3)
74.3
88.0
Difference
0.016 (133.3)
0.006 (47.9)
45.5
70.5
0.002 (16.6)
0.010 (91.8)
3.1
1.4
229
-------
picture, however, because the aeration basin influent includes
the return activated sludge in addition to the primary effluent.
The mass balance figures, C-ll and C-12, indicate the role of the
return activated sludge solids. Because of a higher TS concen-
tration in the return activated sludge, and an increase in the
mass of solids returned during alum addition, the solids loading
to the aeration basins went up and the mixed liquor suspended
solids concentration rose by 56 percent. With this information,
it is possible to calculate the SS mass removed in the final
clarifiers before and after alum addition. The MLSS concentration
is taken as the SS concentration of the final clarifier influent.
Effluent SS concentration and wastewater flow rate are also known.
The mass of SS removed can thus be calculated as 2.85 kg/m3
(23,827 Ib SS/MG) before alum addition, and 4.48 kg/m3 (37,380 Ib
SS/MG) during alum addition. The difference is 1.63 kg/m3
(13,553 Ib SS/MG). On a per-day basis, there were 86.1 kkg/day
(95 tons/day) more secondary sludge SS generated during alum
addition.
Impact of Phosphorus Removal on Sludge Treatment and Disposal
Operations
Additional Sludge Quantity Produced by Alum Addition--
Figures C-ll and C-12 summarize all available information on
volume and mass changes in sludges and sidestream quantities
after alum addition. Data contained in monthly reports indicate
that as a result of alum addition to the channel between the
aeration basins and the secondary clarifiers, the average TS
concentration of the secondary sludge rose from 1.17 to 1.73
percent TS. The volume of secondary sludge remained the same,
however. After alum addition, approximately 0.015 mil m3/day
(4.1 mgd) secondary sludge were still generated, with 0.015 mil
m3/day (4 mgd) still being returned to the aeration basins and
0.004 mil m3/day (0.1 mgd) still being wasted to the gravity-
thickeners. Because of the higher solids concentration of the
sludge, however, calculation shows that an average of 78.9 kkg/
day (87 ton/day) more secondary sludge TS were generated, or
1.5 kg/m3 (12,554 Ib/MG). This is similar to the estimate of
86.1 kkg/day (95 ton/day), or 1.62 kg/m3 (13,553 Ib/MG) addi-
tional sludge SS arrived at in the last section by a calculation
of mass balance.
Of the 78.9 kkg/day (87 ton/day) additional sludge TS,
rned to t
(2.5 ton/day)
e . gay onay aona
approximately 76.6 kkg/day (84.5 ton/day) are returned to the
aeration basins, while approximately 2.26 kkg/day
are wasted to the thickeners
Operational Problems Attributable to the Production of Chemical
SI udge--
Gravity thickeners—Al urn addition created essentially no
change in the hydraulic balance of the thickener. However, the
230
-------
solids concentrations and, therefore, the solids loads of the
sludge feed, the thickener overflow, and the thickened sludge,
were increased.
Approximately 2,270 kg/day (5,000 Ib/day) of additional
secondary sludge TS were pumped to the gravity thickeners after
alum addition. Figures C-ll and C-12 show that the mass of
gravity thickened sludge which was pumped to the digesters
increased by a similar amount. The total solids concentration
of the thickened sludge which was pumped to the digesters rose
from 4.34 to 5.6 percent.
It is likely that the average SS concentration of the
thickener overflow was higher after alum addition than before,
due to more severe thickener overloading and more frequent
thickener bulking. The thickener overflow, heavily loaded with
solids, is returned to the primary clarifiers where it contri-
butes to primary sludge. Since the primary sludge is pumped
back to the thickeners, a circular flow path exists. A heavy
solids load is recirculated along this path.
The thickeners were designed for a surface settling rate
of 33 m3/day/m2 (811 gal/day/ft2). Both before and after alum
addition, the actual rate was much higher than this, about
46.6 m3/day/m2 (1,143 gal/day/ft^). According to the plant
superintendent, the thickener was designed for a sludge feed of
less than 2,000 ppm TS. The actual feed concentration was at
least 2,400 ppm before and during alum addition. A sludge
blanket ranging from about 1.8 m (6 ft) to the entire water
depth of the thickener has built up. The supernatant SS concen-
tration ranged from 22 to 3,974 ppm in the course of 1 mo during
alum addition. Thickener bulking occurs frequently with periods
of stable operation in between. Bulking may have been somewhat
less frequent before alum addition according to the plant
superintendent.
Digesters — The volume of sludge pumped to the digesters was
essentially unchanged by alum addition. The hydraulic balance of
the digesters was altered, however, because the volume of removed
sludge decreased while the volume of supernatant increased. Pre-
sumably, the volume of sludge removed was decreased because of
reduced filter and dryer capacity.
The TS concentration of the supernatant increased to the
point that it was almost as high in solids as the digesting
sludge itself. This occurred because there was little room in
the digesters for solids-liquid separation.
The average volatile solids concentration as a percentage
of total solids of the sludge fed to the primary digesters
decreased from 63.8 before alum addition to 57.8 with alum addi-
tion. The percent volatile solids of the digested sludge
231
-------
decreased from 50.2 before al um addition to 44.8 with alum addi-
tion. The percent volatile reduction, as calculated by the for-
mul a
MO - inn n % ASH in raw x % VS in digested>
VR - 100 x (1- g vS in raw x % ASH in digested'
as 42.8 before alum addition and 40.7 percent with alum addition.
The average gas production during one year of operation before
phosphorus removal was 1,994 m3/day (71,200 ft3/day), while the
average production of methane during 1 yr with alum addition was
2,178 m3/day (77,800 ft3/day). The digester gas characteristics
were not measured before alum addition, but a single recent anal-
ysis is available showing the following composition:
Carbon dioxide (C02) 33%
Methane (CH4) 67%
Carbon monoxide 0.01%
Hydrogen sulfide (H2S) 100 ppm
Sulfur dioxide 2 ppm
Nitrogen dioxide (NOg) <0.1
A recent study analysis also showed the following composition of
the digested sludge:
Potassium (k) 0.08%
Total Kjeldahl nitrogen 646 yg/gram dry basis
Nitrate nitrogen 23.75 yg/gram dry basis
Total phosphate 1,064 yg/gram
Cadmium 0.32 ppm
Zinc 1,475 ppm
Copper 25.3 ppm
Chromium 13.5 ppm
Mercury 0.135 ppm
Lead 1.25 ppm
Vacuum filtering and flash drying — Unfortunate! v. reliable
measurements of flow rates or volumes of filter feed, filtrate,
filter cake, and dried sludge are unavailable for before 1977.
Total solids concentrations of filter feed and filter cake are
available, however. The average TS concentration of the digested
sludge (filter feed) rose from 4.45 percent when no alum was used
to 6.53 percent during alum addition. Despite this increase in
feed solids concentration, the filter cake which was produced
during alum addition was wetter than it had been before. Average
cake TS dropped from 23.8 percent to 21.4 percent.
Since measurements of the volume or mass of sludge filtered
are unreliable for years prior to 1977, the impact of alum addi-
tion on chemical conditioner dosages required for vacuum filter-
ing cannot be determined. During the first 5 mo of 1977, the
average lime dosage used was 214 kg/kkg (429 Ib/ton) of dry sol-
ids and the average ferric chloride dosage was 25.5 kg/kkg (51
Ib/ton) of dry solids.
232
-------
Average filter yield during this period was 0.94 kg (2.08
Ib sludge solids/ft2/hr). The filter yield can be assumed to
have decreased as a result of alum addition, because filter yield
at the plant cannot exceed the rate at which cake can be fed to
the dryer. The rate at which cake could be dried necessarily
decreased with alum addition, because the sludge cake was wetter.
Problem resolution--The basic problem facing the plant was
its inability to move sludge all the way through and out of the
system at a fast enough rate. Operation of the vacuum filters
and dryer for one shift each day and use of the drying beds per-
mitted the removal of only about 49.2 to 64.3 m3/day (13,000 to
17,000 gal/day) digested sludge from the digesters. By running
the filters and dryer 14 hr/day, instead of 6.5, it became pos-
sible to process 98.4 to 128.7 m3/day (26,000 to 34,000 gal/day)
digested sludge. The most obvious plant improvement which resulted
was the reduction of the volume of supernatant. Pumping less
supernatant reduced the load of solids circulating and recirculat-
ing through the primary clarifiers, thickeners, and digesters as
sludge, thickener overflow, and supernatant. An improvement in
plant effluent quality resulted. The average SS concentration of
the final effluent quality resulted. The average SS concentration
of the final effluent was only 18 mg/£ in 1976 when operating for
two shifts/day.
The year 1976 was a period of operation during which the
filter and dryer were run on two shifts. .Table C-16 presents
average values for various items recorded by the plant during
this period. Based on this information, Figure C-13 presents a
solids mass balance which was constructed around the digesters.
Comparing it with the mass balances in Figures C-ll and C-12 indi-
cates that removal of sludge solids and sludge gallons from the
digesters increased when the plant went to two shifts, while the
quantity of sludge feed stayed about the same. This led to fewer
gallons of supernatant returned to the primaries. It also resulted
in more room in the digester for solids-liquid separation. A
higher quality supernatant resulted, with only about 1.5 percent
solids rather than the previous 3.1 percent.
In conclusion, by extending the time of filter-dryer opera-
tion, the adverse impacts of alum addition on digester supernatant
were overcome. Poor supernatant quality probably had been one of
the factors responsible for thickener bulking according to the
plant superintendent, so the adverse effect of alum addition on
the thickeners was probably also overcome by running two shifts.
However, even with these improvements, the plant was still over-
loaded with sludge, as it had been even before alum addition,
and some of the adverse effects of alum addition had not been
overcome.
As a solution, the hauling of liquid digested sludge was
begun, and filter-dryer operation was returned to one shift per
day. Liquid sludge hauling began at the beginning of June of
233
-------
TABLE C-16. 1976 SLUDGE TREATMENT DATA DURING DOUBLE SHIFT
VACUUM FILTER AND FLASH DRYER OPERATION AT LAKEWOOD, OHIO
Average sewage flow m3/day (gal/day)
o
Sludge fed to digester m /dav
(gal/day)
TS digester feed (%)
•a
Digested sludge removed m /day
(gal/day)
TS digested sludge (%)
Sludge filtered m /day (gal/day)
Hours of filter operation
0.05 (13.4)
187 (49,424)
85.3
4.6
(22,530)
5.8
84.7 (22,393)
12.5 hr/day, 5 day/wk
AVE. SEWAGE FLOW = 13.4
MGD
ASSUME 25% TS DESTRUCTION
10,898
TS 58,000
Ibrs/
XOAY
MG/'
X
DIGESTED
.022530 MGO SLUDGE
NOTE: HEAVILY OUTLINED BOXES CONTAIN ESTIMATED VALUES.
Figure C-13.
1976 anaerobic digestion mass balance during double shift vacuum
filter and flash dryer operation at Lakewood, Ohio.
234
-------
1977, so limited data are available on its effects. Figure
m3 sludge/day (39,096 gal
digester in June 1977. The
C-14 shows that an average of 148
sludge/day) were removed from the u i yc-j K._• ... -».._.- -
average quantity of sludge filtered/day was 32.3 m6 (8,546 gal)
The remaining volume of digested sludge, 115.6 m3/day (30,550
gal/day), was hauled out as liquid. While the quantity of
sludge pumped into the primary digesters was not increased by
the plant operators, more sludge was removed than ever before.
In fact, the sludge removal rate exceeded the sludge feed rate.
N/A
N/A
0 .01093
MGD SUPERNATANT
i
1 ASSUME 25%
17,523
Tb7
DAY
TS 42,000
MG/,
.050
MGD
THICKENED
SLUDGE
ANAEROBIC
DIGESTERS
19,564
/
DAY
TS 60,000
MG/,
DIGESTED
.039 MGD SLUDGE
NOTES: N/A =• NOT AVAILABLE
HEAVILY-OUTLINED BOXES CONTAIN ESTIMATED VALUES.
Figure C-14.
Anaerobic digestion mass balance during
liquid sludge hauling, Lakewood, Ohio.
Remova'l of sludge from the digesters at this rate resulted
in less supernatant return and better solids-liquid separation
in the secondary digesters. The quantity of supernatant decreased
to 41.4 m3/day (10,930 gal/day), and if the TS concentration is
assumed to have remained at 1.5 percent, then only 620.6 kg TS/day
(1,367 Ib TS/day) were returned to the primaries in the superna-
tant. The impacts on plant operation included a reduction of the
mass of primary sludge. Consequently, more secondary sludge
could be wasted to the thickeners. This was done, with the result
that the MLSS concentration in the aeration basins came down to
about 3,500 to 4,000 ppm, compared to 4,500 ppm when the plant
was removing less sludge from the digesters.
It was hoped that one of the results
the digesters at a high rate would be the
of removing sludge from
dislodging of much of
235
-------
the heavy sludge that is apparently lodged in the sides and cor-
ners of the secondary digesters. To aid in getting rid of the
heavy sludge, the secondary digester contents were occasionally
recirculated by withdrawing from the bottom and pumping back into
the middle. At first, some dislodging of heavy sludge occurred
at the fast sludge removal rate. After a couple of months, how-
ever, the situation began changing drastically. Apparently, the
effective volume of the digesters was so reduced by heavy sludge,
grit and rags that there was only a very short detention time in
the secondary digesters at the high sludge removal rate. The TS
concentration of the sludge removed from the digesters decreased
from 6.0 percent TS to 3.0 to 3.5 percent TS. Solids-liquid
separation was not occurring.
Consequently, the sludge removal rate was cut in half. The
plant manager is now planning to hire a private contractor to
pump out both secondary digesters, removing the heavy material.
Sludge Treatment and Disposal Costs
In assessing the operational costs of sludge treatment and
disposal at Lakewood, the following assumptions were made:
1. All costs are shown as per ton (metric and English) of
dry TS removed from the digesters.
2. Costs are based on a digested sludge removal rate of
1,180 t/yr (1,300 ton/yr) in 1974 and 1,810 t/yr (2,000
ton/yr) in 1976.
3. The unit cost factors which were used are:
Item 1977 Costs
Electricity $0.045/kwh
Fuel oil $1.454/4 ($0.384/gal)
Ferric chloride $0.118/4 ($0.031/gal)
(40 percent FeCla)
Lime $0.015/kg ($0.033/lb)
Labor categories
(L) Laborer $5.00/hr
(R) Repairman $6.43/hr
(M) Maintenance person $5.70/hr
(0) Filter-dryer $5.43/hr
operator
The costs of handling the alum sludge were estimated for
1974, when the filter and dryer were operated during one shift
per day, and for 1976, when they were operated for two shifts per
day. Operation for two shifts was a response to the additional
sludge load imposed on the plant by alum addition. Therefore,
the additional costs of running for two shifts can be attributed
to the alum addition.
236
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The operational cost breakdown is shown in Table C-17. The
total treatment costs for each process are summarized in Table
C-18. Because of the increase in the number of tons of dry TS
removed from the digesters in 1976, the treatment cost per ton
of dry TS was not much greater than it was in 1974. The total
treatment cost was $133.58/t ($121.19/ton) in 1974 and $138.13/t
($125.28/ton) in 1976.
These costs can be compared with the cost of hauling liquid
sludge. The private contractor's fee for hauling the sludge and
applying it to agricultural fields with a "field jimmy" is $0.11/£
($0.03/gal). The cost per ton of dry TS of this method depends
upon the TS concentration of the liquid sludge. A more concen-
trated sludge improves the cost-effectiveness of the method.
Good digester performance will result in both reduced sludge vol-
ume and a more concentrated sludge, which will increase the cost-
effectiveness of liquid sludge hauling. When the digesters at
Lakewood were producing a 6.0 percent TS sludge, the cost of
hauling was $132.30/t ($120/ton). This is less than the treat-
ment using the filters and dryer.
Summary and Conclusions
The Lakewood, Ohio, plant is an older facility (built in
1966) which has treated average annual flows ranging from 42,000
to 55,000 nH/day (11.2 to 14.6 mgd) during the period examined
in this case study. The plant's effluent quality is poor due to
hydraulic overloading and inadequate solids handling capacity.
The plant treats alum-waste activated and primary sludges which
are combined in the gravity thickeners. The basic solids-handling
problem facing the plant has been its inability to remove sludge
from the digesters at a fast enough rate. The plant is equipped
with vacuum filters and a flash dryer for sludge dewatering and
drying. Over the last three years, the flash dryer has functioned
at loadings of 360 to 450 kg (800 to 1,000 Ib) TS/hr. This is
360 to 450 kg/hr (800 to 1,000 lb/hr) less than the dryer's design
capability rating. It has been possible to operate only one of
the two vacuum filters at a time because of the limiting capacity
of the flash dryer.
Alum addition for phosphorus removal placed a squeeze on the
plant's already overloaded sludge handling system. It increased
the amount of sludge generated, and at the same time decreased
the capacity of the vacuum filter and flash dryer to process the
sludge. This created a bottleneck in the solids handling end of
the plant, leading to high solids buildup and recirculation
throughout the plant.
Although no part of the plant was designed specifically for
sludge storage, the whole plant has become what could be described
as one big sludge storage tank. In addition to the solids buildup
in the thickeners and digesters, the thickener overflow and
237
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TABLE C-17. COSTS OF TREATING ALUM SLUDGE DURING SINGLE AND DOUBLE SHIFT
VACUUM FILTER AND FLASH DRYER OPERATION, LAKEWOOD, OHIO
Single Shift
Operation (1974)
Double Shift
Operation (1976)
Gravity Thickening
Electricity (kwh/yr)
($/t ($/ton))
*Maintenance supplies ($/yr)
($/t ($/ton))
Maintenance labor (hr/yr)
($t ($/toh))
Operating costs and overhead
118,000
$4.50 ($4.03)
$100
$0.09 ($0.08)
32 (M)
$0.15 ($0.14)
none
118,000
$2.92 ($2.65)
$100
$0.06 ($0.05)
32 (M)
$0.10 ($0.09)
none
Anaerobic Digestion
Electricity (kwh/yr)
($/t ($/ton))
Fuel oil (gal/yr)
($/t ($/ton))
*Maintenance supplies ($/yr)
($/t ($/ton))
Maintenance labor (hr/yr)
($/t ($/ton))
Operational labor (hr/yr)
($/t ($/ton))
Overhead ($/yr)
($/t ($/ton))
360,000
$13.74 ($12.46)
1,000
$0.32 ($0.29)
$2,630
$2.23 ($2.02)
832 (M)
$4.02 ($3.65)
1,100 (L)
$4.66 ($4.23)
$3,230
$2.73 ($2.48)
360,000
$8.93 ($8.10)
1,000
$0.21 ($0.19)
$2,630
$1.44 ($1.31)
832 (M)
$2.61 ($2.37)
1,100 (L)
$3.03 ($2.75)
$3,230
$1.78 ($1.61)
(continued)
238
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TABLE C-17 (continued)1
Single Shift
Operation (1974)
Double Shift
Operation (1976)
Chemical Conditioning and Vacuum
Filtration
Fed3 and lime
($/t ($/ton))
Electricity (kwh/yr)
($/t ($/ton))
Maintenance supplies ($/yr)
($/t ($/ton))
Maintenance and repair labor (hr/yr)
($/t ($/ton))
Operational labor (hr/yr)
($/t ($/ton))
Overhead ($/yr)
($/t ($/ton))
$9.81 ($8.90)
57,300
$2.18 ($1.98)
$1,450
$1.22 ($1.11)
792 (M,R)
$4.02 ($3.65)
830 (0)
$3.81 ($3.46)
$2,920
$2.48 ($2.25)
$9.81 ($8.90)
57,300
$1.42 ($1.29)
$2,200
$1.21 ($1.10)
1,200 (M,R)
$3.97 ($3.60)
2,490 (0)
$6.89 ($6.25)
$6,225
$3.43 ($3.11)
Flash Drying
Electricity (kwh/yr)
($/t ($/ton))
*Fuel oil ($/yr)
($/t ($/ton))
*Maintenance supplies ($/yr)
($/t ($/ton))
Maintenance labor (hr/yr)
($/t ($/ton))
tOperational labor (hr/yr)
($/t ($/ton))
Overhead ($/yr)
($/t ($/ton))
326,500
$12.46 ($11.30)
$42,900
$36.38 ($33.00)
$3,036
$2.57 ($2.33)
1,188 (M,R)
$6.04 ($5.48)
914 (L) 1,240 (0)
$9.59 ($8.70)
$5,830
$4.94 ($4.48)
609,500
$15.12 ($13.71)
$77,000
$42.45 ($38.50)
$4,600
$2.54 ($2.30)
1,800 (M,R)
$5.95 ($5.40)
1,830 (L) 3,730 (0)
$15.37 ($13.94)
$12,220
$6.74 ($6.11)
(continued)
239
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TABLE C-17 (continued)
Single Shift Double Shift
Operation (1974) Operation (1976)
Flash Drying (cont'd)
Approximate average income—
dried product
($/t ($/ton)) $5.64 ($5.12) $2.15 ($1.95)
1976 costs.
Includes the cost of bagging and trucking the dried sludge.
TABLE C-18. COSTS OF ALUM SLUDGE TREATMENT DURING SINGLE AND DOUBLE SHIFT
VACUUM FILTER AND FLASH DRYER OPERATION, LAKEWOOD, OHIO
Single Shift Double Shift
Operation (1974) Operation (1976)
Gravity thickening: $4.74 ($4.30) $3.08 ($2.79)
Anaerobic digestion: $27.70 ($25.13) $18.00 ($16.33)
Chemical conditioning
and vacuum filtration: $23.52 ($21.35) $26.73 ($24.25)
Flash drying: $77.62 ($70.41) $90.32 ($81.91)
Total: $133.58 ($121.19) $138.13 ($125.28)
240
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digester supernatant sidestreams carried heavy solids loads back
to the head of the plant. The solids accumulated in the primary
clarifiers from which they were fed back to the thickeners and
digesters, thus being trapped essentially in a closed-loop system.
Furthermore, because the thickeners were overloaded, 97 to 98 per-
cent of the secondary sludge which was produced was recirculated
through the aeration basins and back to the final clarifiers.
The solids accumulated in the secondary clarifiers, and then were
fed back to the aeration basins, forming a second closed loop.
An additional 2.3 t/day (2.5 ton/day), or 4.3 t/mil m3 (360
Ib/MG), of secondary sludge were wasted to the thickeners during
alum addition (Figures C-ll and C-12). The volume of secondary
sludge wasted did not increase,. however. The volume of thickened
sludge pumped to the digesters also did not change significantly,
but the sludge TS concentration increased from 4.3 to 5.6 per-
cent. The increase in the mass of sludge pumped to the digesters
was approximately the same as the amount of additional secondary
sludge wasted to the thickeners.
During alum addition, the sidestreams from the sludge treat-
ment operations contributed a loading of 6,948 kg SS/day (15,300
Ib SS/day) to the primary clarifiers. This figure is greater
than the 6,025 kg/day (13,270 Ib/day) which was contained in the
plant influent. By far the most significant sidestream in terms
of solids input was the thickener overflow, carrying 6,433 kg SS/
day (14,170 Ib SS/day). Alum addition increased the overloading
of the thickeners, and problems with bulking and poor overflow
quality became more severe.
The rate at which sludge could be flash dried was reduced
during alum addition because the moisture content of the filter
cake rose from 76.2 percent to 78.6 percent. Filter yield was
then cut back so that it would not exceed the drying rate. The
volume of sludge removed from the digesters each day was decreased
in response to the reduced filter and dryer capacity. This
resulted in the production of 22.7 m3/day (6,000 gal/day) more
digester suprenatant. The digesters were overloaded to the
extent that the supernatant TS concentration rose from 2.4 to 3.1
percent. The supernatant TS loading on the primary clarifiers
increased by 1,362 kg TS/day (3,000 Ib/day);
Alum addition to the activated sludge effluent channel
affected wastewater treatment operations in several ways:
t The primary clarifier influent concentrations of SS and
BOD were increased due to greater loadings from the thick-
ener overflow and digester supernatant.
t The primary clarifier effluent concentrations of SS and
BOD did not increase, however, indicating that the addi-
tional loadings were removed as sludge.
241
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• The MLSS concentration in the aeration basins rose by 56
percent due to a higher TS concentration of the return
activated sludge.
• The capture of SS and BOD during secondary clarification
was increased, preventing an increase in final effluent
SS and BOD5 concentrations.
t Phosphorus removal was increased from 20 percent to only
65 percent. Greater removal could have been achieved at
higher alum dosages or with polymer addition, but the
plant was not equipped to handle the additional sludge
which would have been generated.
In order to remove sludge from the digesters at a faster
rate and thus alleviate the solids-handling problem, the length
of filter and dryer operation was extended to 14 hr/day instead
of 6.5. A reduction in the volume of supernatant and improved
supernatant quality resulted. More recently, the plant returned
filter and dryer operation to one shift per day and began hauling
liquid sludge. Initially, during liquid sludge hauling, the rate
of sludge removal from the digesters exceeded the feed rate by
908 kg TS/day (2,000 Ib TS/day). As a result, the volume of
supernatant was further reduced and there was more room in the
secondary digesters for solids-liquid separation.
The indirect effects of longer filter-dryer operation and
liquid sludge hauling included less primary sludge production,
less frequent thickener bulking, a reduction of the MLSS concen-
tration in the aeration basins, and improved plant final effluent
q u a 1 i ty.
CASE STUDY G: MENTOR, OHIO
Introduction
The Greater Mentor Wastewater Treatment Plant provides an
example of secondary addition of alum for phosphorus removal.
The chemical sludge at the plant is handled separately from the
primary sludge. The alum-secondary sludge wasted from the secon-
dary clarifiers is sent to an aerobic digester followed by dual
cell gravity concentrators (DCG's) with final disposal on crop
lands. Wastewater influent to the plant is delivered by separate
sanitary sewers and contains approximately 12 percent industrial
waste.
History
The history of modifications to the plant which have affected
sludge, follows:
• Late 1965 - Start-up of primary plant
242
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• Early 1969 - Start-up of ferrous iron and lime addition
for phosphorus removal
t October 6, 1973 - Start-up of secondary treatment using
contact stabilization activated sludge process
• October 7, 1973 - End of ferrous iron and lime addition
• October 14, 1974 - Start-up of alum addition for phospho-
rus removal
• June 22, 1976 - Switch from contact stabilization to con-
ventional activated sludge process
t March 25, 1977 - Return to contact stabilization activated
sludge process
• July 1977 - Switch from aluminum sulfate to sodium alum-
inate addition for phosphorus removal.
Two periods were selected for comparison purposes to deter-
mine the impact of chemical additions for phosphorus removal.
The period from mid-October 1973 to mid-October 1974 was selected
as a period without phosphorus removal. The period from July 1976
to March 1977 was selected as a period during which phosphorus
removal with alum was practiced. Table C-19 presents the daily
average influent and effluent wastewater characteristics and plant
removal efficiencies during the two periods.
TABLE C-19. INFLUENT AND EFFLUENT WASTEWATER
CHARACTERISTICS AND REMOVAL EFFICIENCIES,
MENTOR, OHIO
October
to
October
'73
'74
July '
to
March
'76
'77
Plant Flow mil m3/day (mgd) 0.017 (4.60) 0.02 (5.29)
SS (rng/n) Primary Influent 186 172
Primary Effluent 113 91
Plant Effluent 36 35
% Removal - Primary 39 47
% Removal - Plant 81 80
BOD(mgA) Primary Influent 206 187
Primary Effluent 147 137
Plant Effluent 32 26
% Removal - Primary 29 27
% Removal - Plant 84 86
TP (mg/£) Primary Influent .28 25
Plant Effluent 21 14
X Removal - Plant 25 44
243
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Chemical Addition for Phosphosrus Removal
tO
Liquid aluminum sulfate (9.0 percent Al ) was added to the
effluent channel leading from the aeration basins to the final
clarifiers while the plant was operated as a conventional acti-
vated sludge system with four aeration basins. Approximately
2.6 m3 (690 gal") of liquid alum were used per day. The actual
feed rate of solution was changed twice a day to account for diur-
nal variations in plant flow. From 7 a.m. to 5 p.m. the solution
was added at a rate which achieved a dosage of 12.2 mg/£ Al+3,
with a plant flow of about 0.017 mil m3/day (4.5 mgd), At 5 p.m.
the chemical feed rate was changed to maintain the same chemical
dosage at a plant flow of about 0.024 mil m3/day (6.5 mgd).
In addition to alum, the plant has used several other chemi-
cals for phosphorus removal. Initially, ferrous iron and lime
were added ahead of the primary clarifiers during the period when
the plant only provided primary treatment. More recently, sodium
aluminate addition ahead of the aeration basins was tested.
According to plant personnel, sodium aluminate addition caused
the pH of the sludge in the aerobic digester to increase signifi-
cantly. This, in turn, decreased the effectiveness of the DCG's.
As a result, the use of sodium aluminate was abandoned.
General Description of Wastewater Treatment Operations Affecting
Siudge
A wastewater flow diagram is presented in Figure C-15. Table
C-20 is a summary of the wastewater and .sludge handling unit and
mechanical equipment. Influent wastewater flows to the pump sta-
tion, where it passes through either a comminutor or a bar screen
before dropping into the raw wastewater wet well. The wastewater
is then pumped to the aerated grit chamber where heavy, abrasive
solids are removed. The wastewater then flows to the primary
clarifiers where settleable and floating solids are removed.
The primary effluent flows by gravity to the aeration basins
where it is mixed with return sludge. From the aeration basins
the mixed liquor flows to the secondary clarifiers where the
sludge and solids in the wastewater are settled out. The secon-
dary effluent is chlorinated and discharged to Lake Erie. A por-
tion of the sludge from the secondary clarifiers is returned to
the reaeration basins and is subsequently recycled by mixing with
the aeration basin influent.
Sludges removed for sludge processing from the primary and
secondary treatment systems are handled separately. Primary
sludge is pumped to the anaerobic digesters or to the heat treat-
ment unit. Digested sludge is then chemically conditioned and
sent to the vacuum filters. Thermally conditioned sludge from
the heat treatment unit is sent without chemical conditioning to
another set of vacuum filters. Vacuum filter sludge cake is dis-
posed of on croplands.
244
-------
ISJ
*»
tn
LABORATORY ft
FILTER eUlLOINO
Figure C-15. Greater Mentor"wastewater treatment plant wastewater flow diagram.
-------
TABLE C-20.
GENERAL PLANT DESCRIPTION SUMMARY,
MENTOR, OHIO
Unit
Plant Design Capacity
Comminutor
Bar Screen
Grit Chamber
Primary Clarifiers
No. and Description
0.045 mil m2/day (12.0 mgd) .
78.4 m3 (2,800 ft3) aerated.
2 at 21 .3 m (70 ft) dia. x 2.7 m
Primary Sludge Pumps
Primary Sludge Collectors
Influent Flume to
Aeration Basins
Aeration Basins
Secondary Clarifiers
(9 ft); overflow weir - 2 at
193.5 m (635 ft) length.
2 at 7.5 hp;0.47 m3/min (125 gal/
min) capacity.
2 at 1.6 revolutions/hr, sludge
scrapers on the bottom of a rotating
arm direct the sludge on the bottom
of each clarifier to the hopper in
the center of the tank; motors -
2 at 1/2 hp.
48.5 m (159 ft) length x 1.1 m
(3.5 ft) width x 0.9 m (3 ft)
effective dia.; 3 2-in air diffuser
headers at 17-20 diffusers each
capable of discharging up to 12 cfm
at 10.6 in water.
6 at 37.5 m (123 ft) length x 7.3 m
(24 ft) width x 4.6 m (15 ft) dia.
3 multistage centrifugal compressors
(blowers) at 300 hp each capable of
supplying 5600 scfm at 7 psig; 5
4-in diffuser headers at 32 diffusers
per basin, each capable of discharg-
ing up to cfm 26.9 cm (10.6 in)
water; aeration basin #6 is used
as an aerobic digester.
3 at 28 m (92 ft) dia. x 2.4 m
(8 ft) SWD; overflow weir 3 at
88.1 in (289 ft) length.
(continued)
246
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TABLE C-20 (continued)
Unit
No. and Description
Secondary (Return)
Sludge Pumps
Secondary Sludge
Col 1ector
Anaerobic Digesters
Vacuum Filter (for
anaerobic digester
siudge)
Sludge Heat Treatment
(for raw primary
siudge)
2 at 25 hp, 7.6 m /min
gal/min); 1 at 40 hp,
(4200 gal/min).
(2,000
15.9 m3/min
3 at 1.8 revolutions/hr, sludge
scrapers on bottom of pair of ro-
tating arms with 5 6-in "suction"
pipes per arm; motors - 3 at 3/4
hp; scrum removal by travelling
weir attached to torque tube of
scraper arms.
ft3),
0.57
2 at 1719.8 m° (61,420
recirculation pumps - 2 at
m3/min (150 gal/min); heat exchanger
boiler - 1 at 500,000 BTU/hr.
1.8 to 3.6 kg (4 to 8 Ib/hr)
loading rate at 30 percent dry
sol ids.
3.8 m3/hr (1,000 gal/hr)
at 5 percent dry solids.
temperatures °C (°F).
feed rate
Sludge
Inlet
Maximum
Outlet
12.7 ( 55)
202.4 (400)
37.4 (100)
Disintegrators 2
mil m3/min (8-25
solids.
at 30,280-94,625
gal/min) of 3-10%
High pressure pumps 2 at 49,205
75,700mil m3/min (13-20 gal/min)
at 300 psig.
Head exchanger, 3.03-4.54 m /hr
(800-1200 gal/hr).
Reactor, 3.03-4.54 m3/hr (800-
1,200 gal/hr) at 275 psig and
210°C (4140F).
(continued)
247
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TABLE C-20 (continued)
Unit
No. and Description
Sludge Holding Tank
(Thickener)
Dual Cell Gravity (DCG)
Concentrator
Lime Clarification
Supernatants and
Filtrates
of
Decant and Thickening Tanks
2
Solids loading - 97.7 kg/m /day
(20 Ib/ft2/day) maximum.
Sludge pumps - 2 at 18,925-180,250
mil m3/min (5-50 gal/min).
Vacuum filter (for heat) treatment
sludge) - 302.8 kg/hr (667 Ib/hr)
loading rate at 30 percent dry
solids.
84.4 m (22,300 gal) capacity.
3 at 11.35 m3/hr (3,000 gal/hr)
total capacity (aerobically digested
sludge); main drives - 3 at 1/2 hp;
internatl conveyors - 3 at 1h hp,
110 v.; external conveyors -
2 at 2 hp, 220 v., 61 cm (24 in)
width, capacity - 2.8 m3 (100 ft3)
sludge at 1,297 kg/m3 (80 lb/ft3);
sludge pumps - 3 adjustable speed
and stroke plunger type at 2 hp,
440 v., 26,495 to 211,960 mil
m3/min (7 to 56 gal/min); sludge
pump wet wells - 1 at 33.1 m3
(8,740 gal), 1 at 41.6 m^ (10,980
gal); chemical mixing - 4 plastisol
lined steel tanks at .38 m3 (100
gal), with 1/4 hp mixers operating
at 1,725 rpm; chemical feed pumps
- 3 dual head at 1/4 hp, 75,700
mil m3/hr (20 gal/hr) head;
coagulation - 3 tanks; filter media
fine mesh, nylon cloth.
ft)
6.7 m (22 ft) length x 6.7 m (22
width x 3.7 m (12 ft) dia.; lime
storage bin - 3.7 m (12 ft) dia. x
6.5 SWD; maximum lime feed rate
.25 m3/hr (8.75 ft3/hr; lime sludge
pumps - 2 at 0.19 m3/min .(50 gal/
min).
248
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Alum-secondary sludge is pumped to a sludge holding tank and
on to the aerobic digester, or is sent directly to the aerobic
digester. Following aerobic digestion, the sludge is dewatered
by the DCG's. Dewatered sludge is disposed of at a privately
owned dumpsite. Aerobic digester and sludge holding tank decan-
tate and DC6 concentrate are returned to the head of the aeration
basins.
The plant also has a lime clarifier in which anaerobic diges-
ter supernatant, vacuum filter filtrate, DCG concentrate, and
sludge holding tank decantate can be treated to reduce phosphorus
concentrations. At present, the lime clarifier is not operated
on a continuous basis. The anaerobic digester supernatant and
vacuum filter filtrate are returned to the head of the primary
clarifiers.
Detailed Description of Mastewater Treatment Operations Affecting
SIudge
Since the alum-secondary sludge is handled separately from
primary sludge, this section will emphasize plant operations
affecting secondary sludge generation and handling.
Primary Clarifiers--
Since alum addition was initiated, the quantity and charac-
teristics of the primary clarifier effluent have changed as pre-
viously indicated in Table C-19. ~ Specifically, there has been a
noticeable improvement in primary clarifier performance in terms'
of suspended solids removals. This has led to a decrease in sol-
ids loadings on the secondary treatment system. At the same time,
concentrations of BOD's discharged from the primary clarifiers
have also decreased. Plant data does not, however, fully account
for slugs of concentrated waste from septic tank pumpouts deliv-
ered to the treatment plant by truck. During the period without
phosphorus removal, two to six 3.78-m3 (1,000-gal) loads entered
the system daily. This practice was terminated prior to alum
addition.
Aeration Basins--
During the period without phosphorus removal, the aeration
basins were operated as a contact stabilization system with two
contact basins and one reaeration basin. At that time, the con-
tact basins contained 2,400 to 2,800 mg/& MLSS, with approximately
70 percent volatile solids, and the reaeration basin was operated
at 5,000 to 7,000 mg/£ MLSS with raughly the same percent vola-
tile solids as the contact basins.
During the studied period with aluminum sulfate addition the
system was operated as a conventional activated sludge treatment
plant with four aeration basins, operated at concentrations of
249
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3,000 to 7,500 mg/£ MLSS, with 62 percent volatile solids.
Return sludge with 6,000 to 20,000 mg/i suspended solids was
recycled at a rate equal to 40 to 50 percent of plant forward
flow. This led to considerably longer sludge ages of the mixed
liquor in the aeration basins, causing the cells to enter an
endogenous growth phase. Less sludge was wasted per pound of
BOD removed in the secondary treatment system during this period
than during the period without phosphorus removal.
Secondary Clarifiers--
Sludge is removed from the bottoms of the secondary clari-
fiers on a continuous basis by the sludge draw-off pipes attached
to the arms of the sludge collector mechanisms. This sludge is
returned to the aeration basin system at a pumping rate that
varies between 25 and 50 percent of the plant forward flow. For
approximately 1 hr/8-hr shift, sludge is wasted to sludge pro-
cessing at a pumping rate approximately equal to 100 percent of
a plant forward flow. This high rate of pumping is necessary to
remove the sludge blanket which accumulates to up to 1.8 m (6 ft)
at the center of the clarifier during periods of low rate return
sludge pumping.
Aerobic Digester--
During the period prior to alum addition, one of the six
aeration basins was used as an aerobic digester. Generally, dur-
ing the period of alum addition, one aeration basin was used as
an aerobic digester, although two aeration basins were used as
aerobic digesters for a short time. Characteristics of the sludge
influent to and effluent from the aerobic digester are given in
Table C-21.
TABLE C-21. AEROBIC DIGESTER SLUDGE CHARACTERISTICS,
MENTOR, OHIO
October '73 to
October '74
July '76 to
March '77
Influent Sludge:
WAS kg (lb) dry solids
per month
Total Solids (%)
Volatile (%)
38,600 (85,000)
0.5-1.5
70
45,400 (100,000)
0.8-2.0
63
Effluent Sludge (to
DCS Concentrators):
Total Solids (%)
Volatile (%)
2.0-2.6
68
2.4-2.9
56
250
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The aerobic digesters are aerated and mixed by aeration
equipment identical to that in all other aeration basins at the
Mentor plant. Sludge in the digesters is maintained at approxi-
mately 27°C by the diffused air which is heated by the air com-
pressors. Supernatant is decanted from the top of the aerobic
digester less than once per day. Prior to drawing off superna-
tant, air to the tank is turned off and sludge wasting discontin-
ued for 2 to 6 hr . After settling has taken place, supernatant
is withdrawn slowly to prevent mixing in the tank. During super-
natant withdrawal, concentrations of dissolved oxygen in the tank
drop as low as 0.1 mg/£. When decanting is complete, air is
turned on again and aeration takes place until the dissolved oxy-
gen levels reach 4 to 5 mg/a. This takes approximately 1 day,
at which point the tank is again ready for decanting.
Normally, after 1 to 2 wk of continuously wasting sludge
into the tank and drawing off supernatant, the sludge in the tank
becomes so thick that no additional water can be decanted off.
At this time sludge must be pumped to the DCG's. To avoid this
situation, the DCG's are operated daily if at all possible.
The impacts of chemical additions for phosphorus removal on
aerobic digester operations have been an increase in the concen-
tration of total solids fed to the digester coupled with a
decrease in the fraction of those solids which are volatile.
The destruction of volatile solids,by the aerobic digester was
also observed to increase after alum addition to the secondary
treatment system was initiated.
Dual Cell Gravity Concentrators--
Three DCG's are located in the sludge handling building.
These units are used to dewater the aerobically digested sludge
prior to land disposal of wet sludge cake. Chemical storage
tanks, pumps, and piping are provided for adding polymers to the
sludge ahead of the mixing tank as previously detai1ed inTab!e C-20.
Figure C-16 presents a schematic diagram of the DCG's and
appurtenances. As shown in the diagram, sludge from the aerobic
digesters enters one of two sludge wet wells prior to pumping to
the DCG units. Conditioning chemicals may be injected into the
sludge either immediately following the sludge pumps or immedi-
ately preceding the coagulation tanks.
The chemical feed system consists of three chemical feed
pumps and four chemical storage tanks. Four tanks are furnished
in case both anionic and cationic polymers are required for coa-
gulation. Sludge enters the bottom of coagulation tanks, mixes
with coagulation chemicals, and overflows through four outlets
near the top of each tank.
251
-------
COAGULATOR TANKS
ro
01
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CONVEYOR TO
TRUCK OR
SLUDGE HOLD-
ING TANK
D.C.G.
CONCEN-
TRATOR
CHEMICAL
FEED"
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1
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p
D.C.G.
CONCEN-
TRATOR
t
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4
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CONCEN-
TRATOR
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CONCENTRA-
TOR PUMPS
OVERFLOW
AND COA-
C5ULANT.DRAIN
TO
LINE
CLARIFICATION
LINE SLUDGE
AEROBICALLY
DIGESTED
SLUDGE
Figure C-16. Dual cell gravity thickeners and appurtenances, Mentor, Ohio.
-------
Sludge slurry from the coagulation tanks enters the first
cell of each of the dual cell concentrators at four points. The
cells are formed by a moving, fine mesh nylon filter cloth. A
hump in the center of the cloth divides each unit into a thick-
ening cell and a compression cell. Coagulated solids entering
the first or thickening cell are trapped on the cloth and carried
over the hump to the compression cell while the filtrate drains
through the cloth. In the compression cell, the solids form a
plug, or cake roll, which in turn presses additional moisture out
of the sludge. Excess quantities of sludge cake are discharged
over the rim of the second cell onto one of the internal conveyor
belt system. This system is comprised of one horizontal conveyor
belt which receives the sludge from the internal conveyors and
discharges the sludge cake to the second, inclined conveyor
belt. The second conveyor belt extends outside the sludge hand-
ling building and discharges the sludge cake to a waiting dump
truck.
Normally, three DCG units are utilized 90 percent of the
time and two DCG units are used 10 percent of the time. The
units are not operated at all unless at least two units are in
working order. During the winter months, subfreezing weather
often prevents conveyor system operation, even with the actual
DCG units in good repair. Lack of trucks available for hauling
wet sludge cake can also prevent DCG operation. Specific mechan-
ical problems which occur in the DCG system were identified by
plant personnel as follows: screens and mesh tear and develop
holes; zippers break from seals or loose teeth; seals wear, break,
crack, or get caught in drive sprockets; conveyor bearings require
replacement; polymer pumps clog due to the formation of scale;
main drive unit and conveyor motors burn out.
According to plant personnel, the DCG units are operated as
often as possible. Although no operation or maintenance problems
have been directly attributable to the handling of alum sludge,
the units have been operated somewhat less frequently since the
initiation of alum addition. The difference appears negligible,
however, if the fact is considered that the units were operated
less than 45 percent of the available hours per month during
both periods.
Performance of the DCG units is shown in Table C-22. Since
the initiation of alum addition, the following operational changes
have taken place:
• The volume of sludge fed per hour of operation has
decreased slightly.
• The mass of dry solids fed to the DCG's per hour of oper-
ation has increased slightly.
• The amount of cationic polymer added per gallon of sludge
fed to the DCG's has increased slightly (although the
253
-------
TABLE C-22. PERFORMANCE OF DUAL CELL GRAVITY
CONCENTRATORS, MENTOR, OHIO
Estimated
Monthly Averages
Hours of operation
Percent TS feed sludge
(including conditioners)
q
m (gal)feed sludge/hr
of operation
kg (Ib) feed sludge TS/hr
of operation
q
kg (Ib) cationic polymer/m
(1000 gal) feed sludge
q
kg (Ib) anionic polymer/mil m
(1000 gal) feed sludge
kg (Ib) wet sludge cake/mo
Percent TS sludge cake
kg (Ib) cake TS/mo
kg (Ib) cake TS/MG
plant influent
Without
P-Removal
October '73
to October '74
330
2.1-2.7
3.7
(980)
77.2-99.9
(170-220)
1.7
(14.4)
473
(0.0394)
299,000
(658,000)
8.8-9.2
26,800
(59,000)
1.54
(12,830)
With
P-Removal
July '76
to March '77
315
2.5-3.1
3.5
(920)
86.3-109
(190-240)
1.9
(15.9)
708
(0.0059)
345,000
(760,000)
8.2-9.1
30,900
(68,000)
1.54
(12,850)
254
-------
amount added per pound of solids fed to the DCG's has
remained the same).
t The amount anionic polymer added per gallon of sludge
fed to the DCG's has decreased considerably.
• The concentration of total solids in the wet sludge cake
removed from the DCG's has decreased marginally.
• Overall, the mass of TS concentrated per million gallons
of plant influent has remained the same.
Summary and Conclusions
Since the initiation of alum addition for phosphorus removal,
no increase in the mass of secondary sludge TS removed from the
plant by DCG units has been observed. This unexpected result
can be explained by a combination of several factors. These fac-
tors, and other impacts of chemical addition for phosphorus
removal on the plant, are highlighted below.
First, there has been a general decrease in the loadings of
SS and BODs influent to the secondary treatment system. This has
been the result of both improved primary clarifier performance
and the termination of septic tank pump-out deliveries to the
plant. In fact, the actual magnitude of the decrease due to ter-
mination of septic tank waste deliveries is somewhat greater than
indicated by available plant data. As a result of plant compo-
site sampling which takes place once every 3 hr, 24 hr/day, slugs
of septic tank wastes within the primary treatment system often
go largely undetected.
Second, the aeration system was fun as a contact stabiliza-
tion process during the period without phosphorus removal. Dur-
ing the period of alum addition for phosphorus removal, the plant
was run as a conventional activated sludge plant. Operationally,
the aeration system showed significant increases in MLSS concen-
trations, decreased percentages of volatile solids, and longer
sludge ages. This had led to a reduction of the mass of sludge
wasted to the aerobic digester per pound of BOD removed.
There have been several other changes observed in plant
operations since alum addition was begun, although they are not
necessarily atrributable to the alum addition. Specifically,
the average concentration of solids in sludge entering the aero-
bic digester has increased, the volatile fraction of these solids
has been reduced, and there appears to be greater volatile
destruction taking place in the digester.
Finally, initiation of alum addition, changes in wastewater
characteristics, and variations in plant operations have combined
to produce several minor changes in DCG concentrator operators.
255
-------
The volume of sludge fed to the DCG units per hour of operation
has decreased slightly, but the mass of TS fed, has increased some-
what. The necessary dosage of cationic polymer used to condition
the sludge fed to the DCG's increased slightly, while the dosage
of anionic polymer used decreased. Overall, the TS concentration
in the sludge cake decreased marginally, and the mass of dry cake
produced per million gallons of plant influent remained approxi-
mately the same. Thus, alum addition has had only minor impacts
on plant operations.
CASE STUDY H: BROOKFIELD, WISCONSIN (FOX RIVER)
Introduction
The Brookfield plant provides an example of secondary addi-
tion of ferrous sulfate for phosphorus removal. It also provides
an example pf pressure (plate and frame) filtration and multiple
hearth incineration of an iron chemical sludge which is composed
of combined primary and waste activated sludges. Incinerator
performance and the impact of ferrous sulfate addition can be
compared with other plants (such as Sheboygan, Wisconsin) which
operate fluidized bed incinerators.
Raw wastewater influent to the Brookfield plant contains no
industrial wastes. Monthly averages of daily influent flows range
from 6,430 to 15,100 m3/day (1.7 to 4.0 mgd) with an average of
approximately 9,080 m^/day (2.4 mgd). The plant was designed to
handle an average daily flow of 18,900 m3/day (5.0 mgd) and thus
currently is operating at approximately half capacity.
Table C-23 presents plant influent and effluent wastewater
characteristics and removal efficiencies for the periods selected
for comparison before and after the initiation of chemical addi-
tion for phosphorus removal. Table C-23 indicates that the efflu-
ent suspended solids concentrations for the two periods .were quite
different. However, the influent wastewater sampling point actu-
ally includes sidestream flows returned to the head of the plant.
Thus, according to plant personnel, the raw influent wastewater
characteristics remained unchanged with the difference due to an
increase in sidestream suspended solids.
History
Construction of the Brookfield plant was begun in August
1971, with wastewater treatment beginning on January 2, 1974.
The following shows the history of modifications to the plant
affecting sludge production and characteristics.
January 1974 - Wastewater treatment operations begun using
the contact stabilization activated sludge process.
February 1976 to July 1976 - Two final clarifiers in opera-
tion (a single final clarifier was in operation during all other
periods).
256
-------
TABLE C-23. INFLUENT AND EFFLUENT WASTEWATER
CHARACTERISTICS AND REMOVAL EFFICIENCIES, BROOKFIELD, WISCONSIN
After P-removal Before P-removal
initiated initiated
Aug. and Sept. '76, Jan., Feb., May,
Jan., Feb., & May '77 Aug., & Sept. '75
Flow m3/d (mgd) 8,250 m3/d (2.18 mgd) 9,580 m3/d (2.53 mgd)
SS (mg/jt) -
influent 265* 212
effluent 18 13
% removal 93 94
BOD (mg/£) -
influent
effl uent
% removal
113*
7
94
103
20^"
81
*Includes sidestreams returned to lift station at head of plant.
''"In 1976, effluent BOD concentrations improved when polishing
lagoon volume was decreased, reducing retention times from
approximately 12 days down to 7 hours. Thus, the apparent
decrease in effluent BOD concentration after phosphorus
removal was initiated was due to the size reduction of the
polishing lagoon.
March and April 1975 - Chemical additions for phosphorus
removal were tested, using several different chemicals.
June 1976 - Chemical addition for phosphorus removal using
ferrous sulfate (pickle liquor) was initiated.
Chemical Addition for Phosphorus Removal
Pickle liquor, or liquid ferrous sulfate (12.4 percent Fe),
is added to the activated sludge mixed liquor at a point 3/4 of
the distance between the head end and the discharge end of the
contact basin. The quantity of pickle liquor to be added is
determined by testing the phosphorus concentration in the efflu-
ent from the primary clarifiers once a week. During periods of
low flow, the chemical delivery pumps are adjusted to deliver
257
-------
approximately 0.908 kg (2.0 Ib) of iron by weight per kg (Ib) of
phosphorus in the primary effluent. During periods of high flow,
the pumps are adjusted to deliver approximately 0.77 kg (1.7 Ib)
of iron per kg (Ib) of phosphorus. Approximately 246 m3 (65,000
gal) of pickle liquor are added each year, at a rate of 0.673 m3
(178 gal) of liquid or 0.70 m3 (184 Ib) Fe/day.
General Description of Wastewater and Sludge Treatment Operations
Figure C-17 presents a flow diagram of the Brookfield treat-
ment plant. Table C-24 is a summary of the specifications of the
major components of the treatment system. The following para-
graphs describe the general sequence of wastewater treatment
operations at the Brookfield plant.
Raw influent wastewater passes through a comminutor and is
discharged to the plant influent lift station wet well. At the
lift station wet well the raw wastewater mixes with sidestreams
from several plant operations, including pressure filter filtrate,
incinerator scrubber water, digester supernatant, and waste acti-
vated sludge. From the lift station, the raw wastewater and
returned sidestreams are pumped to the primary clarifiers where
primary sedimentation takes place. The primary effluent flows
by gravity to the preaeration chamber followed by the contact
basin, where chemical addition for phosphorus removal takes place.
The mixed liquor then passes to the final clarifier where the
overflow is chlorinated and discharged.
Sludge from the bottom of the final clarifier is pumped to
the reaeration basin as return activated sludge or is wasted to
one of several other locations. In general, waste activated
sludge is pumped to the aerobic digester or returned to the lift
station wet well at the head of the plant. Sludge from the aero-
bic digester is subsequently transferred to the sludge processing
holding tank. Sludge from the primary clarifiers is generally
pumped to the sludge processing holding tank or the aerobic
digester.
Sludge received by the sludge processing holding tank is fed
along with chemical conditioners to the plate and frame pressure
filter for dewatering. Dewatered sludge cake is either inciner-
ated in the multiple hearth furnace or hauled away from the plant
as wet sludge cake for disposal on crop land or at a landfill.
Incinerator ash is used as a sludge conditioner prior to pressure
filtration, or hauled to a landfill for disposal.
Detailed Description of Wastewater and Sludge Treatment Operations
Figures C-18 and C-19 present materials balances for primary
and secondary wastewater treatment operations at Brookfield.
Data presented with a subscript letter A are for the period after
phosphorus removal was initiated, while data with the subscript
letter B are for the period before phosphorus removal was initia-
ted. 258
-------
ro
ui
ID
RETURN ACTIVATED SLUDGE
PLANT
INFLUENT
LIFT
STATION
AND
COMMINUTOR
SCRUBBER
WATER
PRIMARY
CLARIFIERS
CONTACT
STABILIZATION
ACTIVATED
SLUDGE
FINAL
CLARIFIER
t
PRIMARY SLUDGE
DIGESTER SUPERNATANT
FILTRATE
MULTIPLE
HEARTH
INCINERATOR
SLUDGE
CAKE
CHLORINE
CONTACT
TANK
3
X
"71
SECONDARY
(WASTE ACTIVATED)
SLUDGE
AEROBIC
DIGESTER
PRESSURE
FILTER
SLUDGE
CONDITIONING
TANK
ASH
ASH
STORAGE
TANK
DIGESTED
SLUDGE
SLUDGE
HOLDING
TANK
ALTERNATE
DISPOSAL OF
LIQUID SLUDGE
BY TANK TRUCK
LANDFILL
Figure C-17. Brookfield, Wisconsin, wastewater treatment plant flow diagram
-------
TABLE C-24.
GENERAL PLANT DESCRIPTION SUMMARY, BROOKFIELD,
WISCONSIN
Unit
No. and Description
Comminutor
Lift Station
Primary Clarifiers
Primary Sludge
Helithickener
Primary Sludge
Pumps
Aeration Basins
Air Diffusers
Final Clarifiers
Waste Activated
Sludge Pump
Return Activated
Sludge Pumps
2 at 16.80 m (55 ft) dia. x 2.44 m (8 ft)
side wall depth (SWD); sludge collectors -
2 Walker Process Equipment type RSP with 2
collector arms; motors - 2 at 3/4 hp, 30,
60 hz, 480 v.
2 at 45.70 cm (18 in) dia. x 45.70 cm
(18 in) pitch x 5.48 m (18 ft) length;
Walker Process Equipment Helithickener Cross
Collectors, 10-15 rpm; hopper dimensions -
2 at 21.3 m (70 ft) length x 0.762 m (2.5 ft)
width x 1.680 m (5.5 ft) height; collector
arm motors - 2 at 3/4 hp, 30, 60 hz, 480 v.;
helithickener motors - 2 at 1/2 hp, 30, 60 hz,
480 z.
2 at 0.3 m3/min (80 gal/min); 5 hp, 30,
60 hz, 40 v.
3 at 28.30 m (93 ft) length x 9.140 m (30 ft)
width x 4.570 (15 ft) height; air compressors -
3 at 2,190 scfm; motors - 3 squirrel cage
induction motors at 100 hp, 30, 60 hz,
480 v-
6 Walker Process Equipment EASEOUT header
assemblies with 12 saddle-mounted MONOSPARJ
diffusers (per basin) at 8 scfm/diffuser.
2 at 22.90 m (75 ft) dia. x 3.660 m (12 ft)
SWD; sludge collectors - 2 Walker Process
Equipment type SWD equipped with surface
skimmer and 2 collector arms with 7.62 to
15.2(3 to 6 in) suction pipes; collector motors
2 at 1/2 hp, 30, 60 hz, 480 v.
1 at 0.3 m3/min (80 gal/min); 5 hp, 30,
60 hz, 480 v.
2 at 5,680i/min (1,500 gal/min); 10 hp,
30, 60 hz, 230 v.
Chlorine Contact
Tank
Lagoon
Aerobic Digester
Sludge Processing
Holding Tank
1 at 12.20 m (40 ft) dia. x 3,660 m (12 ft)
SWD
1 at 1,350 m2 (14,500 ft2) x 2.130 m (7 ft)
depth
1 at 11.0 m (36 ft) x 11.0 m (36 ft) x
7.62 m (25 ft) SWD with Walker Process Unit
Equipment Rollaer diffusers
1 at 4.270 m (14 ft) x 4.270 m (14 ft) x
2.130 m (7 ft) SWD
(continued)
260
-------
TABLE C-24 (continued)
Unit
No. and Description
Pressure Filter
Pressure Filter
Appurtenances
Incinerator
1 plate and frame type Beloit-Passavant Series
5,200 - 46 chambers at 0.042 m3 (1.5 ft3)
each with 112 m3(l21 ft2)total area; 241.0 kg
(530 lb) dry solids/hr capacity; epoxy
coated carbon steel construction filter
plates; filter media-monofilament nylon with
stainless steel 10 x 10 mesh backup wire;
galvanized carbon steel drainage member.
1 mix tank at 4,390 m2 (120 ft3) with 2
agitators at 6 rpm; contact tank at 2.44 m
(8 ft) dia, x 4.27 m (14 ft) length, 1,510)1
(4.850 gal) capacity; 3 rpm agitator powered
by 3 hp TEFC motor; equalization tank at
1.680 m (5.5 ft) dia. x 2.680 m (8.8 ft)
height, 1,610 s, (425 gal) capacity; filtrate
weir tank; precoat tank at 13.30 m (3.5 ft)
dia. x 23.6 m (8.8 ft) height; 2 sludge-
transfer pumps at 189s, (50 gal/min), 1 1/2 hp;
pumps at 189* (50 gal)/min, 1 1/2 hp; precoat
pump at 40 hp, 30, 60 hz, 460 v.
5 multiple hearths Nichols-Herreshoff sludge
incincerators at 3.96 m (13 ft) outside
depth x 5.33 m (17.5 ft) height; 242.0 kg
(510 lb) dry solids/hr capacity fed at
1,230 kg (2,700 lb)/hr with ash at 1.5:1,
moisture 50 percent, 27 percent volatile;
auxi'11 iary natural gas burners in hearths
no. 3 and 5; operating temperature 667 to
871°C (1,200 to 1,600°F); induced draft
exhaust gas fan at 661,000 cms2/sec (1,400
cfm), 5 hp, 30, 60 hz, 480 v; ash screw
conveyors.
Incinerator
Exhaust Gas
Scrubber
3-stage flooded tray type wet scrubber with
quencher, scrubber, and cyclonic impinge-
ment separator; scrubber water pump at
25 hp, 30, 60 hz, 480 v.
261
-------
RAW INFLUENT
WASTEWATER AND
RETURNED SIDE-
STREAMS FROM
WET WELL OF LIFT
STATION AT HEAD
OF PLANT
PRIMARY TREATMENT
PERCENT REMOVALS
SSA=67%
VSSA=62%
FSSA=78%
BODA=36%
QA=2.18X10
SSA=4825(72%VOLATILE)
VSSA=3450
FSSA=1375
BODA=2060
QB=2.53X10
SSB=4480(74%VOLATILE)
VSSB=3350
FSSB=1130
BODB=2180
SSB=64%
VSSB=63%
FSSB=69%
BODB=32%
PRIMARY
CLARIFIERS
SSA=1600(81%VOLATILE)
VSSA=1300
FSSA=300
BODA=1310
TPA=115
SYMBOLS & UNITS
SSB=1600(78%VOLATILE)
VSSB=1250
FSSB=350
BODB=1490
TPB=N/A
PRIMARY SLUDGE
QA=1100034.25%TSS
SSA=3225(67%VOLATILE)
VSSA=2150
FSSA=1075
BODA=750
QB=134003)2. 90%TSS
SSB=2880(73%VOLATILE)
VSSB=2100
FSSB=780
BODB=690
Q=FLOW GAL/DAY UNLESS OTHER-
WISE SPECIFIED
% RETURN Q = (RETURN FLOW 7
FORWARD FLOW) X 100%
SS=SUSPENDED SOLIDS - #/DAY
VSS=VOLATILE SUSPENDED SOLIDS -
0/DAY
FSS=FIXED (NONVOLATILE) SUSPENDED SOLIDS - #/DAY
TP=TOTAL PHOSPHOROUS - #/DAY
BOD=5 DAY BOD - #/DAY
I - ADEQUATE DATA NOT AVAILABLE FOR ACCURATE MASS BALANCE
II - A PORTION OF THE WASTE SLUDGE IS RETURNED TO WET WELL
OF LIFT STATION AT HEAD OF PLANT
N/A=NOT AVAILABLE
Figure C-18.
Materials balance for primary wastewater
treatment operations at Brookfield, Wisconsin
262
-------
SECONDARY TREATMENT
PERCENT REMOVALS
SSA=79%
VSSA=81%
FSSA=77%
BODA=91%
QA=3.27X106 TPA = 8«
SSA=67000 SSB=83% SSA = 330 ( 765SVOLAT I LE )
CONTACT BASIN VSSA=38000 VSSB=82% VSSA=250
pc;c;A = pQnnn ccc: — Q •a * ccc — -7/1
MLSSA=2440 mg/l
MLVSSA=i39o mg/l
MLFSSA=i050 mg/l
%VSSA=57
MI ^c; — — lAin \no/f
MLVSSB=2050 mg/l
MLFSSB=360 mg/l
%VSSB=74
REAERATION BASIN
MLSsA=545o mg/l
MLVSSA=3060 mg/l
MLFSSA=2390 mg/l
%VSSA=56
MLSSB=4210 mg/l
MLVSSB=3070 mg/l
MLFSSB=ii40 mg/l
%VSSB=73
TO SLUDGE
HOLDING AND/OR
SVIA=64 BODB=72%
TPB=N/A
FINAL
CLARIFIER
QB=3. 64X106
SSB=42000 V. s>
VSSB = 31000 ^X.X^
FSSB=11000 J
SVIa=95
D
RETURN SLUDGE
QA=1 . 09X1063. 55%TSS
% RETURN QA=50
c; Q — T
A~~
VSSA=I
FSSA=I
QB=1 . 1 1X1 O6 S. 42%TSS
% RETURN QB=43
SSB=39000
VSSB=29000
FSSB=10000
WASTE SLUDGE
QA=42000a.55%TSS
SSA=1910(56%VOLATILE)
VSSA=1070
FSSA=840
PROCESSING ^ QB=35000S,.42%TSS
UNITS °R slB=1190
RECYCLE TO VSSR=870 ( 73%VOLATILE )
HEAD OF PLANT II _,on
' i>i5p3~~«3^-U
BODA=130
TPA=18
TO CHLORINATION
AND DISCHARGE
^ SSB=280(79%VOLATILE)
VSSB=220
FSSB=60
BODB=420
TPB-40
Figure C-19. Materials balance for secondary wastewater
treatment operation at Brookfield, Wisconsin.
263
-------
Primary Clarifiers--
Primary sedimentation of the raw influent wastewater and
returned sidestreams takes place in the two primary clarifiers
Each clarifier is equipped with two sludge collector arms which
continually move the settled solids towards the sludge hopper
which contains a rotating "he!ithickener." Sludge in the heli-
thickener hopper and collected clarifier surface scum are com-
bined and pumped to the sludge holding and/or processing units
one to three times daily, five days/week.
Since the initiation of phosphorus removal, the suspended
solids loading on the primary clarifiers has increased by 27 per-
cent or 202 kg SS/M3 (445 Ib SS/MG) of plant influent. However,
since the average suspended solids concentration of the sludge
increased from 2.9 percent to 3.5 percent SS, there was a net
increase of 157 kg SS/M3 (345 Ib SS/MG) pumped to sludge proces-
sing. Further details of the impact on the primary clarifiers
are shown in Table C-25.
TABLE C-25. CHANGES IN PRIMARY CLARIFIER PERFORMANCE AS THE RESULT
OF CHEMICAL ADDITION FOR PHOSPHORUS REMOVAL, BROOKFIELD, WISCONSIN
Chances
Primary Clarifier
Influent Concentrations
Primary Clarifier
Effluent Concentrations
Primary Sludge
Pumped
ss
kg/mil m3
(lb/MG)
+91 ,000
(+455t)
+20,000
(+100)
(+69,000)
(+345)
VSS ,
kg/mil m3
(lb/MG)
52,000
(+260)
+20,000
(+100)
32,000
(+160)
FSS* ,
kg/mil n)3
(lb/MG)
+37,000
(+185)
No charge
+37,000
(+185)
BOD
kg/mil m3
(lb/MG)
+17,000
(+85)
+ 2,000
(+10)
+ 15,000
(+75)
Flow
n)3/nnl m'
(gal/MG)
NA#
NA
-50,000
(-250)
*FSS = non-volatile (fixed) suspended solids.
t+ indicates increase; - indicated decrease.
*NA - Not Applicable.
Aeration Basins--
Currently the activated sludge system is being operated as
a contact stabilization process, using one of the three aeration
basins as the contact basin, and one as the reaeration basin.
(The third basin is operated as an aerated sludge storage tank
apart from the activated sludge process). Prior to entering the
contact basin, primary effluent is received by a preaeration
chamber where it is mixed with flow from the reaeration basin.
264
-------
Since the initiation of chemical addition for phosphorus
removal, the average MLSS concentration in the contact basin has
increased by nearly 75 percent, with the majority of this increase
due to a 190 percent increase in the non-volatile (fixed) portion.
As a result, the VSS fraction of MLSS decreased from 74 percent
VSS prior to phosphorus removal to 57 percent VSS since phospho-
rus removal was initiated. Since there was virtually no change
in the amount of nonvolatile SS entering the contact basin from
the primary clarifier, it is assumed that the entire increase is
due to the addition of phosphorus removal chemicals and subse-
quent reactions.
In the reaeration basin, the MLSS concentration has increased
by nearly 30 percent since phosphorus removal was begun. The
entire change is due to a 110 percent increase in the concentra-
tion of fixed SS in the mixed liquor (MLFSS). Consequently, the
volatile fraction of the reaeration basin mixed liquor (MLVSS)
decreased from 73 percent to 56 percent VSS.
Final Clarifiers--
The final clarifiers receive the mixed liquor effluent from
the contact basin. Sludge is withdrawn from the bottoms of the
clarifiers on a continuous basis and pumped as' return sludge to
the reaeration basin. As necessary, sludge is wasted to one of
the sludge holding tanks, sludge processing units, or returned
to the head of the plant. Roughly 25 percent of the waste sludge
is returned directly to the head of the plant. Thus, approxi-
mately 75 percent is sent to one of the sludge holding or pro-
cessing units. (The portion sent to the aeration basin not in
use for secoridary treatment is later returned to the head of the
plant.)
Since phosphorus removal was begun, the settleabi1ity of the
secondary sludge has improved. This was evidenced by a decrease
in sludge volume index from an SVI of 99 prior to phosphorus
removal to an SVI of 64 since phosphorus removal was begun.
After phosphorus removal, the pumping of return activated
sludge from the final clarifier to the reaeration basin increased
from 43 percent to 50 percent of the plant's forward flow. The
amount of waste activated sludge pumped also increased by 5,430
m3/mil m3 (5,430 gal/MG) of plant influent, a 40 percent increase.
At the same time, the return and waste activated sludge SS con-
centrations increased from 4,200 mg/£ SS to 5,500 mg/£ SS, caus-
ing an additional 48,600 kg SS/mil m3 (405 Ib SS/MG) of plant
influent to be wasted. Table C-26 shows these impacts of chemi-
cal addition for phosphorus removal, as well as the impacts on
final clarifier effluent.
265
-------
TABLE C-26. IMPACTS OF CHEMICAL ADDITION FOR PHOSPHORUS REMOVAL ON
FINAL CLARIFIERS, BROOKFIELD, WISCONSIN
Changes
Final Clarifier
Effluent Concentrations
Waste Activated
Sludge Pumped
Returned Activated
Sludge Pumped*
Flow ,
n)3/mil m3
(gal/MG)
NAf
+5,430
+0.061 x
106
ss
kg/mil mj
Og/MG)
+ 4,800
(+40)
+48,600
(+405)
vss
kg/mil m3
(Ib/HG)
+ 3,000
(+25)
+17,400
(+145)
FSS
kg/mil mj
(lb/MG)
+ 1 ,800
(+15)
+ 31,000
(260)
BOD
-13,200
(-110)
NA
NA
*Adequate data not available for accurate mass balance.
+NA = Not Applicable
Sludge Treatment and Disposal Operations
After sludge has been removed from the primary or final clar-
ifiers, it can be stored prior to sludge processing in three
locations as follows:
t Aeration basin (not in use for secondary treatment)
• Aerobic digester
t Sludge processing holding tank.
Unfortunately, it was not possible to thoroughly assess the
impact of chemical addition for phosphorus removal on each of the
individual sludge storage units. This was due to the wide vari-
ability of sludge storage and disposition operations and the
shortness and discontinuous nature of the periods available for
study in comparison to the retention times of the individual
sludge holding units. An additional problem in evaluating impacts
has been the necessity to use suspended solids concentration for
constructing mass balances around clarifiers and the activated
sludge process, but to use total solids concentrations throughout
the discussion of sludge processing.
The remainder of this section discusses each of the sludge
storage units, sludge volume reduction units, and disposal oper-
tions was shown previously in Figure C-17. A diagram of sludge
treatment and disposal facilities is shown in Figure C-20. Flow
diagrams showing quantities, and characteristics of sludge and
conditioning chemicals fed to the pressure filter and incinerator
during the periods before and after phosphorus removal , are shown
in Figures C-21 and C-22.
Aeration Basin (not in use for secondary treatment)--
At present, only two of the three aeration basins are used
as part of the activated sludge system. Instead, the third is
used to provide additional aerated sludge storage capacity. An
266
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ro
BUCKET
GRINDER ELEVATOR
I SLUDGE CAKE_CONVEYOR _ . —
SLUDGE FROM AEROBIC DIGESTER
AND PRIMARY CLARIFIER
Figure C-20. Brookfield, Wisconsin, pressure filtration and incineration facilities.
-------
SLUDGE FROM
AEROBIC DIGESTOR
SLUDGE PROCESSING
HOLDING TANK
SLUDGE FROM
PRIMARY
CLARIFIERS
SLUDGE TO
PRESSURE FILTER^
QA=328000 GAL/MO
TSA=131000 Ib/MO
*TSA=4.77
CONDITIONING ADMIX
ASH
79000 Ib/MO
0. 60 lb ASH/
1b DRY SOLIDS
FECL3
1810 GAL/MO
8440 Ib/MO
4.27 lb FECL3/
TON DRY SOLIDS
i
QB=395000 GAL/MO
TSB=1 16000 Ib/MO
%TSB=3.54
98000 Ib/MO
0.85 lb ASH/
"lb DRY SOLIDS
1770 GAL/MO
8280 Ib/MO
4.70 lb FECL3/
TON DRY SOLIDS
P=FLOW
TS=TOTAL SOLIDS
VS=VOLATILE SOLIDS
FS=FIRED (NONVOLATILE) SOLIDS
%TS=PERCENT DRY TS BY WEIGHT
%VS=PERCENT DRY VS BY WEIGHT
%FS=PERCENT DRY FS BY WEIGHT
Figure C-21 . Pressure filter performance, Brodkfield, Wisconsin
268
-------
FILTER CAKE
WET CAKEA=506000lb/MO
TSA=219000 lb/MO
VSA=71000(32.6% OF 1 b TS )
FSA=148000
%TSA=43.4
%VSA=14.1
%FSA=91%
LIME
32400 GAL/MO
22600 lb/MO
346 lb LIME/
TON DRY SOLIDS
TOTAL:
SLUDGE PLUS
ADMIX TO FILTER
QA=362000 GAL/MO
TSA=240000 lb/MO
%TSA=7.95
28800 GAL/MO
20100 Tb/MO
345 lb LIME/
TON DRY SOLIDS
QB=426000 GAL/MO
TSB=243000 lb/MO
%TSB=6.85
WET CAKEB=421000 Ib/MO
TSB=182000 "lb/MO
VSB=61000(33.6% OF 1 b TS )
FSB=121000
%TSB=43.2
%VSB=14.5
%FSB=28.7
PRESSURE
FILTER
AFTER
90 RUNS/MO
1.73 HRS/RUN
155 HRS/MO
BEFORE
79 RUNS/MO
2.83 HRS/RUN
232 HRS/MO
FILTRATE
UJ
3
(-•
111
*
O
QA=328000 GAL/MO
TSA = 21000 lb/MO
FILTRATE-
TANK
FILTRATE
WEIR TANK
QB=397000 GAL/MO
TSB=62000 lb/MO
TO PRECOAT
TANK
Figure 21 (continued^
269
-------
RETURNED 70 WET
WELL OF LIFT
STATION AT HEAD
OF PLANT
SCRUBBER WATER
Q=0.2 MGD
SS=200 1 b/DAY
WHEN OPERATING
SLUDGE
FROM
PRESSURE
FILTER
CAKE
INCINERATOR
FEED
FEED =308000 1 b/MO
TSA=133700 lb/MO
VSA=43400 1 b/MO
FS = 89900 1b/MO
FEEDB=416000 1
TSB=179700 1 b/MO
VSB=60300 lb/MO
FSB=1 19400 1 b/MO
INCINERATOR
OPERATION
AFTER
253 HRS/MO
1220 1 b/HR
172 1 b VS/HR
BEFORE
284 HRS/MO
1460 l.tXHR
212 lb VS/HR
SLUDGE CAKE HAULED
AFTER
198000 1 b/MO
BEFORE
4800 1 b/MO
ASH FOR RECYCLE AS
CONDITIONER OR PRECOAT
AFTER
87700 lb/MO
BEFORE
97800 1 t/MO
ASH HAULED
AFTER
Q=FLOW
TS=TOTAL SOLIDS
VS=VOLATILE SOLIDS
FS=FIXED (NONVOLATILE)
FEED=WET CAKE
22001 b/MO
BEFORE
21600 Ib/MO
SOLIDS
Figure C-22.
Multiple hearth incinerator performance, Brookfield,
Wisconsin.
270
-------
estimated 60 percent of the plant's waste activated sludge and
5 percent of the primary sludge enters this unit by pumping as
necessary from the bottoms of the primary or final clarifiers.
Since the aeration basin is not equipped for the collection and
pumping of concentrated sludge, solids contained in the aeration
basin are pumped from the aeration basin without being allowed
to settle. This sludge is then sent to the wet well of the lift
station at the head of the plant. Subsequently, it is pumped to
the primary clarifiers along with the raw influent wastewater and
other returned sidestreams. There, the combined raw and returned
solids are settled and concentrated, after which they are gener-
ally sent to one of the sludge holding units other than the aera-
tion basin.
Aerobic Digester--
An estimated 80 percent of the sludge routed to the aerobic
digester is from the primary clarifiers while 20 percent is from
the final clarifiers. The aerobic digester supernatant is
decanted periodically to the wet well at the head of the plant,
after the aeration equipment has been turned off and the solids
have settled to the bottom of the digester. The sludge is pumped
from the bottom of the digester to the sludge processing holding
tank when the aeration equipment has been turned off. Since
phosphorus removal was initiated, the aerobically digested sludge
has averaged 1.1 percent TS. Before phosphorus removal began,
the sludge contained 1.5 percent TS.
Sludge Processing Holding Tank--
An estimated 85 percent of the sludge routed to this unit
is from the primary clarifiers and 15 percent from the aerobic
digester. Occasionally, supernatant is decanted from this tank
and returned to the wet well at the head of the plant. Sludge
from the sludge processing holding tank is generally fed to the
pressure filter (although liquid sludge can be pumped into a
tank truck and hauled from the plant for spreading on croplands).
Figure C-21 presents data on the average characteristics of the
sludge from the sludge processing holding tank.
Pressure Filter--
The plate and frame pressure filter is generally operated
4 days/wk, 16 hr/day, 45 wk/yr. (To facilitate data comparisons,
sludge quantities and flow rates are expressed as daily amounts
per 7-day week.) A filter run begins by pumping sludge from the
sludge processing holding tank (sludge supply tank) to the mix
tank where it is blended with admix materials into a uniform
slurry. The admix material consists of incinerator ash, ferric
chloride solution of 570 kg FeCl3/ra3 (4.75 Ib FeCls/gal) of solu-
tion, and hydrated lime diluted to an 8 percent solution.
271
-------
Next, the sludge and conditioning admix slurry are pumped
from the mix tank to the contact tank. This tank is provided to
allow sufficient residence time for conditioning reactions to
take place prior to feed to the filter. The tank is equipped
with an agitator blade which turns at 3 rpm to prevent the sus-
pension from clinging to the tank walls. An equalizing tank
receives the slurry from the contact tank and holds it until
pumping from the tank to the filter begins.
Prior to filtration, precoat consisting of filtrate and
incinerator ash fed from the precoat tank at 100 psi to cause is
a uniform precoat slurry to flow through the filter. Following
filter precoating, sludge and admix from the equalizing tank are
pumped to the filter, initially at 379 i (100 gal/min) and 225 psi
pressure to preclude the loss of precoat from the filter media.
When the tank has delivered approximately one half of the con-
ditioned sludge to the filter, an air supply valve is opened to
complete the emptying of the tank at a reduced but constant pres-
sure of 100 psi. Throughout the filter run, the filter feed pump
automatically decreases output as the pumping head increases, and
eventually stalls out against a terminal pressure of 225 psi
(which coincides with zero flow through the filter).
*
Upon completion of the filtration cycle, sludge cake is dis-
charged from the filter and dropped into a bunker where it is
stored. A portion of the filtrate from the pressure filter oper-
ation is returned to the wet well at the head of the plant while
the remainder of the filtrate is returned to the precoat tank.
Sludge cake is removed from the bunker by a drag flight conveyor,
discharged onto an inclined conveyor, and then fed to the incin-
erator. Alternatively, if may be discharged to a bypass conveyor
which discharges the sludge cake to a truck outside the building.
Data describing the operation of the pressure filter during
the period before and after phosphorus removal was initiated is
shown in Table C-27. After phosphorus removal, there was^a slight'
decrease in the volume of sludge fed from the sludge processing
tank to the chemical mix tank each month which is equal to a
decrease of 185 m3/mil m3 (185 gal/MG) of plant influent. How-
ever, the total solids in the sludge holding tank increased from
3.54 percent TS before phosphorus removal to 4.77 percent TS
after phosphorus removal. As a result the mass of total solids
fed to the chemical mix tank from the sludge processing holding
tank has increased by 30 percent, or 56,405 kg/mil m^ (470 Ibs/MG)
of plant influent.
272
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TABLE C-27. PRESSURE FILTER PERFORMANCE,
BROOKFIELD, WISCONSIN
Pressure Filter Input
m3 (gal) feed/run
m3 (gal) feed and admix
run
kg (Ib) TS feed/run
kg (Ib) TS feed and
admix run
kg (Ib) TS feed/hr
kg (Ib) TS feed and
admix/hr
After
13.7 (3,640)
15.2 (4.020)
661 .0 (1 ,455)
1 ,200 (2,670)
384.0 (845)
704.0 (1,550)
Before
18.9 (5,000)
20.4 (5,380)
667.0 (1,470)
1 ,400 (3,075)
227 (500)
477.0 (1 ,050)
Pressure Filter Output
kg (Ib) wet cake/run
kg (Ib) wet cake/hr
kg (Ib) TS in cake/run
kg (Ib) TS in cake/hr
kg (Ib) TS in filtrate/run
kg (Ib) TS in filtrate/hr
% feed and admix TS
recovered in cake
After
2,550 (5,620)
1 ,480 (3,265)
1,100 (2,430)
638 (1,405)
107 (235)
61.3 (135)
90
Before
2,420 (5,330)
824 (1 ,815)
1 ,044 (2,300)
370 (815)
356 (785)
125 (275)
75
When the sludge gets to the chemical mix tank, incinerator
ash, FeCl3, and lime are added, again changing the quantity and
solids concentration of the sludge before it gets to the filter.
The dosage of ash was lower by about 0.1140 kg ash/kg of dry
solids (0.25 Ib ash/lb of dry solids) after phosphorus removal.
The average dosage decreased from 0.386 to 0.272 kg ash/kg of dry
solids (0.85 to 0.60 Ib ash/lb of dry solids). Similarly, the
dosage of ferric chloride decreased from 2.35 kg FeCla/t (4.70
Ib FeCl3/ton) of dry solids to 2.14 kg FeCl3/t (4.27 Ib FeCl3/ton)
of dry solids after phosphorus removal. The dosage of lime
remained constant at approximately 173.0 kg lime/t (345 Ib lime/
ton) of dry solids. The net result was that the volume of sludge
plus admix delivered to the filter from the mix tank was lower by
80 m3/mil m3 (80 gal/MG) of plant influent after phosphorus
removal. And there was a total solids increase from 6.85 percent
TS to 7.95 percent TS in the sludge fed to the filter. This is
equivalent to an increase in the amount of pressure filter feed
of 55,200 kg TS/mil m3 (460 Ib TS/MG) of plant influent, or an
increase of 15 percent.
273
-------
Filter cake characteristics showed only minor variations
between the periods before and after phosphorus removal. Speci-
fically, the TS concentration of the cake increased from 43.2 to
43.4 percent. The VS fraction of cake TS decreased somewhat from
33.6 percent of TS to 32.6 percent of TS. Table C-27 compares
pressure filter performance before and after phosphorus removal.
Since the initiation of chemical addition for phosphorus
removal, pressure filter performance has improved significantly.
The sludge mass fed per hour (not including admix materials) has
increased from 227 to 384 kg TS/hr (500 to 845 Ib TS/hr), and
the mass of feed plus admix/hr increased from 477 to 704 kg/hr
(1,050 to 1,550 Ib/hr). The length of the average pressure fil-
ter run decreased by 40 percent from 2.83 hrs/run to 1.73 hrs/run.
In addition, the percentage of the solids fed to the filter which
are recovered in the cake increased from 75 percent to 90 percent.
The combined effect was a production increase of 658 kg wet cake/
hr (1,450 Ib wet cake/hr) and 268 kg TS/hr (590 Ib TS/hr).
Incinerator--
During periods of incinerator operation, the incinerator is
run 24 hrs/day, 4 days/wk with an additional half day for incin-
erator start-up, and another half day for cool-down. The cake
elevating conveyor raises pulverized filter cake to the jtqp of
the incinerator, where the cake enters the first hearth/'After
the cake has been fully incinerated and reaches the bottom hearth,
the ash is discharged into a screw conveyor which in turn dis-
charges to an ash grinder, followed by a bucket elevator which
ultimately discharges into the ash storage bin. Ash stored in
the bin is used for precoating the pressure filter or condition-
ing the sludge prior to pressure filtration. Excess ash can be
discharged from the ash bin into a truck for disposal on-site
followed by subsequent transfer to an off-site landfill.
Exhaust from the incinerator is either vented to the air
pollution control system or returned to the lower hearths through
the warm air duct for preheating. The air pollution control
equipment consists of a 3-stage, flooded-tray type wet scrubber.
The first stage consists of a hot gas quencher where water is
sprayed into the gases until they are cooled. The second stage
is the scrubber where water is sprayed to entrain particulate
matter. Finally, the gases are exhausted through the third stage
which consists of a cyclonic impingement separator followed by
the exhaust stack. Scrubber waters pass through the scrubber
once, after which they are discharged to the wet well at the
head of the plant.
274
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The operating temperatures recorded at the hearths and the
scrubber system are:
Hearth 1 - 427°C (800°F)
Hearth 2 - 867°C (900°F)
Hearth 3 - 843°C (1,550°F)
Hearth 4 - 704°C (1,300°F)
Hearth 5 - 468°C (600°F)
Cooling Air Outlet - 38°C (100°F)
Scrubber Inlet - 93°C (200°F)
Scrubber Outlet - 248°C (280°F)
Incinerator fuel consumption is approximately 18.5 to 19.6 m3/hr
(660 to 700 ft3/hr) of operation (natural gas).
Figure C-22 shows performance data for the multiple hearth
incinerator. From this data and the information presented in the
general plant description summary (Table C-24), the incinerator
can be seen to be presently operating at an average feed rate
which is approximately one-half of the design capacity. Although
not attributable to phosphorus removal, there has been a shift
in the method of final disposal used. Before phosphorus removal
an average of 1.8 t/mo (2 tons/mo) wet pressure filter cake were
hauled from the plant rather than incinerated. More recently,
nearly 90.7 t/mo (100 tons/mo) of wet cake are being hauled.
The plant manager apparently has found it cost effective to keep
the incinerator down for a week rather than operate at less than
capacity. Currently, consideration is being given to hauling
all the wet sludge cake produced as a cost effective alternative
to incineration.
Since the beginning of phosphorus removal, incinerator feed
rates have declined from 663.0 to 554.0 kg wet cake/hr (1,460 to
1,220 Ib wet cake/hr) of operation. This has reduced the feed
rate of volatile solids from 96.3 to 78.1 kg VS/hr (212 to 172
Ib VS/hr) of operation. As a result, an additional 14 m3 (500
ft3) of natural gas are required per hour of operation.
Prior to the initiation of phosphorus removal, approximately
9.98 t (11 tons) of incinerator ash were hauled from the plant
each month, while an average of only 0.907 t (1 ton) of ash was
hauled each month after phosphorus removal was initiated. The
remainder of the ash generated by the incinerator is recycled as
admix for pressure filter sludge conditioning or pressure filter
precoat. During periods of incinerator operation, approximately
757 m3/day (0.2 mgd) of scrubber water containing approximately
90.8 kg (200 Ib) SS/day are generated and returned to the head
of the plant.
275
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Sludge Treatment and Disposal Costs
Operation and Maintenance Costs--
With the initiation of chemical addition for phosphorus
removal, the cost of sludge treatment and disposal has decreased
by approximately $1.60/t ($1.33/ton) of dry solids filtered, as
shown in Table C-28. This reduction was due to decreases in the
amounts of chemical conditioners and electricity used by the
plate and frame pressure filter. These decreases were partially
offset by an increase in the amount of auxiliary fuel used by the
incinerator as the result of decreased incinerator volatile
sol ids feed rates.
The mass of solids filtered increased from 182 to 237 t/mil
m3 (0.76 to 0.99 tons/MG) of plant influent with phosphorus
removal. Theoretically, sludge treatment costs should have
increased by 28 percent going from $179,000.00 to $229,000.00
($47.30 to $60.50/MG) of plant influent. However, increased wet
sludge cake hauling (at little or no charge) and reduced periods
of incinerator operation prevented any actual increase in the
cost of sludge treatment per MG of plant influent.
•
Capital Costs--
The total plant cost in 1973 was $3.6 million. The plant
was financed by 15 year general obligation bonds at 4.37 percent
interest, maturing in 1986. The pressure filter equipment and
piping cost $385,000.00. The cost of the multiple hearth incin-
erator was $336,000.00.
Summary and Conclusions
Since the initiation of chemical addition for phosphorus
removal, the Brookfield wastewater treatment plant has shown an
increase in the average solids loadings to each wastewater treat-
ment and sludge handling unit process. The only exception to
this observation was a decrease in total amount of sludge solids
sent to the incinerator, which was due to increased wet sludge
cake hauling after pressure filtration. There has been an
increase of approximately 0.056 kg TS/m3 (470 Ib TS/MG) of plant
influent pressure filtered as the .result of phosphorus removal'.
Fortunately, due to the improved dewatering performance of the
pressure filter, presumably attributable to phosphorus removal
chemical addition, conditioning admix dosages of ash and ferric
chloride were reduced. In addition, the total solids recovery
by the pressure filter also improved significantly, improving
filtrate qua!ity.
Since this plant is operating well below capacity, the addi-
tional phosphorus laden chemical sludge at this plant has caused
no operational problems. Furthermore, since wet sludge cake can
276
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TABLE C-28. PRESSURE FILTER AND INCINERATOR OPERATIONAL
COSTS PER t (ton) DRY SOLIDS, BROOKFIELD, WISCONSIN
Item
Fed 3*
Lime
Natural1"
Gas
Electrici ty*
Labor**
(O&M)
TOTAL
Unit
Cost
$
83.20/m3
(0.315/gal)
0.067/kg
(0.0305/lb)
O.Q64/m3
(0.001786/ft3)
0.04/kwh
6.00/hr
Units
Vton)
After
0.115
(27.6)
173
(346)
(213)
6,880
240
3.33
Used per t
dry solids
Before
0.13
(30.5)
172
(345)
(204)
6,570
260
3.33
Costs per
After
9.58
(8.69)
11.63
(10.55)
13.55
(12.29)
10.58
(9.60)
22.05
(20.00)
67.39
(61 .13)
t (ton) Dry
Before
10.59
(9.61)
11 .60
(10.52)
12.93
(11 .73)
11.47
(10.40)
22.05
(20.00)
68.64
(62.26)
Sol ids
Change
-1.01
(-0.92)
+0.03
(+0.03)
+0.62
(+0.56)
-0.88
-0.80
No
Change
-1 .24
-1 .13
*During 1976, the plant installed ferric chloride bulk storage facilities reducing
ferric chloride unit costs $206/m3 ( $0.780/gal ), $1 ,190/m3 ($0.31 5/gal ). The
unit cost for bulk purchases has been used for the purposes of cost comparisons.
''"includes incinerator warm-up.
^Estimated decrease due to shorter length of filter press runs.
**Assumes no sludge cake hauled from plant.
-------
be hauled from the plant at little or no charge, increased quan-
tities of more easily dewaterable sludge have actually decreased
the cost of sludge handling per ton of dry solids.
CASE STUDY I: MIDLAND, MICHIGAN
Introduction
In 1960, when the Midland plant was designed as a two-stage,
high-rate trickling filter plant, various alternative means of
sludge handling were considered. The one chosen was vacuum fil-
tration of the undigested sludge with disposal in a sanitary land-
fill. In 1970, it became necessary to upgrade the Midland plant
to provide for phosphorus removal and a higher level of BOD and
SS removal. Facilities were added for chemical additions of
either ferric chloride or alum plus polymer, and rapid sand fil-
ters were added to filter the effluent prior to discharge. These
changes meant that it would be necessary to handle additional
sludge solids from the chemical addition and the tertiary filtra-
tion both, and this was considered in the design of the sludge
handling facilities.
Besides the increased mass of sludge to be disposed of, two
other factors were considered in the design of the new sludge
handling facilities. One was the new requirement by the Michigan
Department of Natural Resources that sludge stabilization and
dewatering to at least 50 percent dry solids be performed if the
plant was to continue sanitary landfill disposal. Previously,
raw sludge at a concentration of about 25 percent TS had been
landfilled. The other factor that was considered was cost.
Of the various alternatives considered for the solids hand-
ling facilities, the addition of thermal conditioning with con-
tinued utilization of the existing vacuum filters and sanitary
landfill disposal appeared the least costly. Other alternatives
considered included anaerobic digestion and farmland disposal of
liquid sludge or sludge cake; incineration of sludge cake; and
replacing the vacuum filters with a filter press. Thermal condi-
tioning was the only method which could provide both sludge sta-
bilization and a 50 percent TS filter cake. It thus enabled con-
tinued use of the sanitary landfill as well as the greatest reduc
tion in the volume of sludge for disposal. Thermal conditioning
also appeared to require less operational labor than any of the
other alternatives.
When the thermal conditioning option was selected in 1970,
the energy shortage was not a factor considered. At the present
time, the alternatives are being re-evaluated by Midland due to
the increased awareness of the energy shortage. This case study
will investigate the impact of ferric chloride and polymer addi-
tion ahead of the primary clarifiers on the cost-effectiveness
278
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of the present system. Other plants considering the use of ther-
mal conditioning and vacuum filtration can benefit from this dis-
cussion.
The plant was designed in 1963 to handle 0.025 mil m3/day
(6.5 mgd) municipal wastewater from a population of 35,000 and
effluent from the sanitary facilities at the Dow Chemical Co.
complex nearby. Every year since 1972, the actual average flow
has exceeded the design average. For the year July 1975 through
June 1976, the flow averaged 0.029 mil m3/day (7.63 mgd). A flow
equalization basin at the head of the plant evens out daily var-
iations in flow, thus avoiding the need for a total plant capa-
city increase.
A high degree of treatment is provided by the wastewater
system, which incorporates two-stage trickling filters, tertiary
multi-media filtration, and phosphorus removal by chemical addi-
tion. The system provides approximately 90 percent reduction of
both BOD and SS. Average influent BOD and SS concentrations are
about 106 and 151 ppm, respectively. Industrial wastes comprise
-an insignificant part of the plant influent and create no waste-
water or sludge treatment problems.
History
Historical plant modifications which have affected sludge
production and characteristics are listed below:
May 1972 - Thermal sludge conditioning replaced polymer con-
ditioning.
March 29, 1973 - Ferric chloride and polymer addition to
primary influent for phosphorus removal started.
May 1, 1973 - Discontinued thermal conditioning and returned
to polymer conditioning.
May 29, 1973 - Raised reactor temperature of thermal cpndi-
tioning unit from 185° to 202°C (365°K to 395°F).
June 1973 - Tertiary mixed media filters added; backwash
water recirculated to head of plant.
Chemical Addition for Phosphorus Removal
Liquid ferric chloride (40 percent Feds) is added to the
raw sewage between the raw sewage comminutor and the raw sewage
pumps. Flow-proportional feed pumps are set to achieve a concen-
tration of approximately 42 mg/'£ FeCl3. The turbulence created
by the pumps aids in mixing the ferric chloride. Ferric phos-
phate begins to form, and the mixture enters a grit chamber where
an anionic polymer supplied by Dow Chemical Co. is added for
279
-------
coagulation. The usual dosage of dry polymer is 0.17 mg/a. The
contact time between the ferric and polymer addition is 3 to 4
min. After polymer is added there is another 1.5 min or so con-
tact time before the flow enters the primary clarifiers. Approx-
imately 1.05 t (1.16 tons) per day dry FeCU and 4.3 kg (9.5 Ib)
per day dry polymer are used. The plant's average phosphorus
removal efficiency is approximately 80 percent, and phosphorus
is generally reduced to a final effluent concentration of less
than 1 mg/£, meeting discharge permit requirements.
General Description of Wastewater Treatment Operations Affecting
SIudge
Figure C-23 presents a general treatment plant flow diagram
for Midland. "It can be seen that s'idestreams from the sludge
treatment processes, backwash water from the tertiary filters,
and waste sludge from the intermediate and final clarifiers all
join the raw sewage before it enters the primary clarifiers.
The clarifiers themselves are six rectangular tanks, with a total
capacity of 0.0023 mil m3 (610,500 gal). Each tank is 25.9 m
(85 ft) long by 4.8 m (16 ft) wide by 3.05 m (10 ft) deep. The
bottoms of the tanks are flat. Sludge is continuously scrapped
along the bottom of each tank by a drag conveyor with seventeen
flights to a hopper at one end. Each hopper is 4.88 m (16 ft)
long, 1.22 m (4 ft) deep, 1.4 m (4 ft 6 in) wide at the top and
0.635 m (2 ft 1 in) wide at the bottom. Sludge is pumped from
each tank once every 4 hr for 8 min, at 0.03 m3/min (75 gal/min).
In the past, sludge had been pumped for 15 min instead of 8.
Decreasing the pumping time allowed a longer sludge detention
time in the clarifier bottoms, which resulted in a sludge solids
concentration of 7 percent TS instead of the former 6 percent TS.
This increase in sludge solids concentration has had no effect
on sludge handling operations except that less sludge volume is
now pumped to the holding tanks and little or no supernatant is
formed in the holding tanks and returned to the primaries.
It is clear that adding ferric chloride and polymer ahead
of the primary clarifiers has affected primary clarifier perfor-
mance and operation by increasing BOD, SS, and total phosphorus
removal efficiency, and by increasing the amount of sludge solids
generated. However, the amounts of these increases are difficult
to determine. Other changes in plant operation took place about
the same time that phosphorus removal was started. These changes
affected clarifier efficiency and sludge production, so the
effect of phosphorus removal alone is obscured. These plant
modifications were the shortening of the sludge pumping time,
affecting sludge solids concentration as already discussed, and
the recycling of tertiary filter backwash to the head of the
plant. The backwash water contains the sol ids .removed in the
tertiary filters. An unknown fraction of these solids is removed
in the primary clarifiers, thus increasing sludge production by
an unknown amount.
280
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TITTABAWASSEE
RIVER
CHLORINE
CONTACT
TANK
TERTIARY
FILTERS
FINAL
CLARIFIERS
2ND STAGE
TRICKLING
FILTERS
INTERMI-
DIATE
CLARIFIERS
1ST STAGE
TRICKLING
FILTERS
BACKWASH
WATER
SLUDGE
ro
00
RAW SEWAGE
COMMINUTOR
FLOW
SPLITTING
VENTURI
FLOW METER
GRIT
CHAMBER
FLOW
EQUALI-
ZATION
FILTRATE
LAND RECLAMATION-^-
DECANTATE
PRIMARY
CLARIFIERS
VACUUM
FILTERS
CONDI-
TIONED
SLUDGE
DECANT
TANK
21MPRO,
INC.
THERMAL
CONDI-
TIONING
RAW SLUDGE
HOLDING
TANKS
Figure C-23. Midland, Michigan, wastewater treatment plant flow diagram
-------
There was one month of operation between the commencement of
phosphorus removal and the start-up of the tertiary filters. Dur-
ing that month only 19 mg/£ Fed 3 were added instead of the pre-
sent 40 to 50 mg/£. Comparing primary clarifier efficiency dur-
ing that month with the performance before phosphorus removal, we
see increased BOD and SS removal with FeCl3 addition. This is
shown in Table C-29.
TABLE C-29. PHOSPHORUS REMOVAL IMPACTS ON PLANT
BOD AND SS REMOVALS. MIDLAND, MICHIGAN _
No Fed 3 Added 19 mg/l FeCl3
Total mil m3 (M6) wastewater treated/mo:
n q, /?dq ?n n qt.
percent removed by primary treatment - _ u>yt ^<**-*|J u-*5
BOD 26 48
SS 32 67
Percent removed by first-stage
biological treatment - BOD 34 38
SS 29 42
Percent removed by second-stage
biological treatment -
Total plant percent removal -
BOD
SS
BOD
SS
51
14
62.4
63.9
38
37
80.2
89.0
The effluent from the six primary tanks flows to two first-
stage stone-filled trickling filters, 24.4 m (80 ft) in diameter
by 1.82 m (6 ft) deep. The effluent from these is collected in
the underdrain system and flows to two intermediate clarifiers.
Each clarifier is 18.3 m (60 ft) in diameter by 3.04 m (10 ft)
deep. Trickling filter humus settles in the tanks and is contin-
uously scraped to the center hoppers. Sludge is pumped from each
hopper for 1 hr once every 8 hr. The sludge is returned to the
primary clarifier influent. The effluent is pumped to the second
stage trickling filters which are just like the first-stage fil-
ters. The final clarifiers are 21.9 m (72 ft) in diameter by
2.74 m (9 ft) deep. Sludge is removed from them as in the inter-
mediate clarifiers and returned to the primary clarifier influent,
The flow is then pumped to tertiary multi-media filters.
It appears that phosphorus removal has improved overall
plant performance with regard to BOD and SS removal. Because of
the increased removals in the primary clarifiers, the BOD and SS
loadings on the secondary part of the plant are decreased, appar-
ently enabling better trickling filter performance. This is
borne out by the data in Table C-29.
282
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Detailed Description of Sludge Treatment and Disposal Operations
Storage--
The two raw sludge holding tanks have served as thickeners
in the past, but now serve only as holding tanks since thickening
is occurring in the bottoms of the primary tanks. Each tank is
square, 4.41 m (14.5 ft) on a side, and 6.09 m (20 ft) deep.
Each has a capacity of 59.5 m3 (15,710 gal). Sludge is pumped
from these tanks to the thermal conditioner when it is in opera-
tion.
Conditioning--
Thermal conditioning is practiced with the Zimpro, Inc., low-
pressure oxidation system. The principal difference between this
system and the Porteous or Farrer processes i s the addition of air to
the reactor during the treatment operation. The system is really
a low-pressure form of wet-air oxidation. Sludge is first ground
to eliminate large particles. High pressure pumps then bring the
sludge to a system pressure of 400 psig, before it is mixed with
air from an air compressor. A heat exchanger transfers heat to
the incoming sludge-air mixture from the outgoing sludge. The
heated sludge then passes to a reactor, where steam injected from
a boiler brings it to a temperature between 177°C and 204°C (350°
and 400°F). The sludge leaves the reactor after 20 min, passes
back through the heat exchanger, and enters two sludge thickening
tanks. The combination of heat, air, and pressure in the reactor
results in a cellular breakdown of the sludge solids, allowing a
further gravity thickening to occur readily in the tanks.
Sludge is pumped to the thermal conditioner at 2.08 a/sec
(33 gal/min) when it is in operation. This is about 2.8 days/wk
on the average, for 24 hr/day. The reactor temperature is pre-
sently maintained at 202°C (395°F). Approximately 14,000 m5
(500,000 ft3) of natural gas/mo are used to fuel the boiler.
Thermal sludge conditioning has several benefits. It sta-
bilizes the sludge, enabling further handling without pathogens,
odors, or putrefaction. It changes the cellular structure of the
sludge, enabling thickening to a relatively high solids concen-
tration and thus reducing the volume to be vacuum filtered. And
it improves the dewatering characteristics of the sludge, increas
ing filter yield and cake solids concentration.
Thickening--
After thermal conditioning the sludge is thickened and
stored in two tanks which are constructed like the two raw sludge
holding tanks. Conditioned sludge is pumped into tank #1 until
it is full. The decantate is then drawn off, and the thickened
sludge is pumped over into tank #2. There is a screw mechanism
283
-------
in the center of tank #2 which is turned on whenever sludge is
pumped to the vacuum filters. It mixes the denser sludge near
the bottom of the tank with the less dense sludge near the top.
The solids retention time in the thickener varies considerably
between about 1.5 and 3 days because of intermittent vacuum fil-
ter operation. Sludge retention time and sludge blanket depth
are not critical control variables for the operation of a condi-
tioned sludge thickener as they often are for the operation of a
raw sludge gravity thickener.
Dewatering--
Dewatering is accomplished by two Eimco rotary drum vacuum
filters. Some machine operating parameters for these vacuum fil-
ters are given below:
.2
Filter area: 18.6 mfc (200 1
Filter media: polyethylene
(28 x 32 threads/in)
Vacuum: 50.8 cm (20 in) Hg
cloth, 11 x 12.6 threads/cm
The drum speed is set at
dr.um submergence is 5.08
are usually run at once.
average of 4 hr/day.
3 min per revolution, and the depth of
to 10.2 cm (2 to 4 in). Both filters
They are operated 3.7 days/wk for an
Chemical conditioning with anionic and cationic polymers
has been used in the past. Approximately 0.150 kg (0.3 lb) of
anionic polymer/t (ton) of dry solids and 2.50 to 4.00 kg (5 to
8 lb) of cationic polymer/t (ton) of dry solids were used. Ther-
mal conditioning eliminated the need for chemical conditioners.
Ultimate Disposal--
Until about 1973, vacuum filter cake was buried in trenches
at the city of Midland landfill site. A private contractor was
hired by the plant to dig the trenches and bury the sludge. The
filter cake at that time was unstabilized sludge. With thermal
conditioning it became possible to use the sludge in a variety
of other more useful ways. The sludge cake is still trucked out
to the sanitary landfill site in two 6-wheel drive liftainer
trucks belonging to the plant. Each truck carries a 4.59-m3
(6-yd3) load, and an average of 1.8 loads are hauled per day. A
round trip is approximately 13 mi long and takes about 30 min.
The sludge is stockpiled at the landfill site where the city of
Midland can utilize it. The expenses involved in further hand-
ling are borne by the city of Midland using their General Fund.
t
Some of the sludge is used for land reclamation at the land'
fill site. After a section of landfill has been filled with
refuse to the desired elevation, it is capped with clay soil.
284
-------
Sludge is disced into this clay soil covering to enrich and con-
dition it. The soil is then seeded with grass. In the future,
when landfilling has ceased, the site will be grass covered and
suitable for recreational use.
Some of the sludge is informally made into compost by the
city's Department of Forestry and used as soil conditioner in
their on-site ornamental tree nursery. Sludge or compost is
disced into the soil, greatly enriching it and enhancing tree
growth. Both sludge-leaf compost and sludge-sawdust compost are
made in piles with a minimum of attention and labor.
Impact of Phosphorus Removal on Sludge Treatment and Disposal
Operations
The sludge treatment flow diagram below (Figure C-24) shows
as completely as possible the sludge and sidestream quantities
handled at Midland. Approximately 75.7 m3 (20,000 gal) of raw
sludge containing 4,540 kg (10,000 Ib) TS (7 percent TS) are
pumped to the raw sludge holding tanks each day. Approximately
53.0 m3 (14,000 gal) of thickener decantate are removed and the
remaining 22.7 rrn (6,000 gal) of concentrated (18 percent TS)
sludge are vacuum filtered. The decantate contains only about
475 ppm SS, so we have estimated that only a few hundred pounds
TS are removed in the decantate and that the rest are filtered.
The vacuum filter filtrate is relatively high in suspended solids,
containing 8,000 to 9,000 ppm. Several hundred pounds TS may be
lost in the approximately 11.40 m3/day (3,000 gal/day) of fil-
trate. Approximately 8.26 m3 (10.8 yd3) filter cake (50 percent
TS) are formed each day. The dry weight of this filter cake is
approximately 4,090 kg (9,000 Ib).
The impacts of ferric chloride addition to the raw sewage
on sludge treatment and disposal can be judged using plant data
collected since 1970. The impacts upon sludge conditioning,
thickening, and dewatering characteristics have varied with the
conditioning method used, the thermal conditioner temperature,
and the recycling of tertiary filter backwash water. Table C-30
consolidates available data indicating these relationships. The
values presented are based on monthly averages as recorded in
plant monthly reports.
Sludge Production and Sludge Solids Concentrations--
As discussed earlier, the impact of phosphorus removal on
raw sludge quantity and solids concentration is obscured by sev-
eral factors:
1. Modifications of sludge pumping procedures which
increased the detention time of the sludge in the pri-
mary clarifiers;
285
-------
SECONDARY
SLUDGE
00
PRIMARY
CLARIFIERS
50,000 GPD RAW SLUDGE
6.9% TS
10,000 #TS/DAY
FILTRATE
9,000 PPM SS
14,000 GPD DECANTATE
478 PPM SS
6,000 GPD CONDITIONED SLUDGE
18% TS
RAW
SLUDGE
HOLDING
TANKS
THERMAL
CONDITIONER
CONDITIONED
SLUDGE
GRAVITY
THICKENERS
10.8 CU YD/DAY FILTER CAKE
50% TS
TO ULTIMATE DISPOSAL
Figure C-24. Midland, Michigan, sludge treatment flow diagram
-------
oo
TABLE C-30. IMPACTS OF PHOSPHORUS REMOVAL: SLUDGE CONDITIONING,
THICKENING, AND DEWATERING CHARACTERISTICS—VARIABILITY WITH
CONDITIONING METHOD, THERMAL CONDITIONER TEMPERATURE, AND
THE RECYCLING OF TERTIARY FILTER BACKWASH WATER, MIDLAND, MICHIGAN
Variable
-5
1000 013 raw sludge/
mil m sewage
(1000 gal/MG)
% TS raw sludge
3
kg sludge TS/mil m
(Ib TS/MG)
Raw sludge VS
(% of TS)
Polymer dosage-
anionic kg/t dry
solids (Ib/ton)
Polymer dosage-
cationic kg/t dry
solids (Ib/ton)
o
m thickener decantate
(1000 gal)
mg/£ SS thickener
decantate
mg/£ BOD thickener
• • • ^ f r*j
decantate
No Fed 3
Polymer Cond.
4.15 (4.15)
4.55
189,000
( 1,575)
72
0.15 (0.3)
2.5 to 4.0
(5 to 8)
0
0
0
No Fed 3
Therm. Cond.
185°C (3650F)
2.74 (2.74)
6.5
178,200
( 1,485)
59
0
0
N/A
N/A
N/A
19 mg/£ Fed 3
Therm. Cond.
185°C
2.92 (2.92)
5.2
151,900
( 1,266)
59
0
0
N/A
N/A
N/A
19 mg/Jl Fed 3
Polymer Cond.
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
25-60 mg/£ FeCl3
Therm. Cond.
2020C (395°F)
Tertiary Filters
1.98
6.9
1,139
(13,6619.8)
55.6
0
0
61 (16.1)
478
6,000
(continued)
-------
oo
00
TABLE C-30 (continued)
Variable
% TS sludge to filter
2
Filter yield-kg/m /hr
(16/ft2/hr)
mg/jl SS filtrate
Filtrate VSS (% of SS)
mg/£ BOD filtrate
% TS filter cake
No FeCl3
Polymer Cond.
6
23.9 (4.9)
647
73.25
2,724
25.5
No Fed3
Therm. Cond.
1850C (3650F)
13
39 (8)
4,057
61.4
7,795
48
19 mg/ji FeCl3
Therm. Cond.
185°C
9
20.2 (4.14)
5,682
52
N/A
44
19 mg/i FeCl3
Polymer Cond.
9.5
14.7 (3.0)
3,469
45.3
2,950
39.3
25-60 mg/£ FeCl3
Therm. Cond.
202°C (395°F)
Tertiary Filters
79
78 (16)
8,288
47
6,769
54
Filter cake VS
(% of TS)
3
m cake hauled/t
dry solids filtered
(yd3/ton)
72.25
57.9
56.4
4.03 (4.78) 3.00 (3.56) 3.27 (3.88)
47.8
3.46 (4.1)
46.4
2.66 (3.16)
-------
2. Availability of only one month's data with FeCl3 addi-
tion before the tertiary filters were placed in opera-
tion;
3. Tertiary filter operation, involving the recycling of
backwash water containing the solids removed in the fil-
ters .
Sludge Thickening and Conditioning--
Raw sludge thickening to approximately 7.0 percent solids
has been possible both with and without ferric addition. The
thickening can occur in either the primary clarifiers or the raw
sludge holding tanks. Thermal conditioning of the raw sludge
changes the cellular structure of the sludge particles so that
the sludge can be thickened even further prior to vacuum filtra-
tion. When there was no ferric addition for phosphorus removal,
the thermally conditioned sludge thickened readily to 13 percent
solids. With 19 mg/a FeCls addition, however, thickening to-only
9 percent TS occurred. This adverse effect of ferric addition
was overcome by raising the temperature of the thermal condi-
tioner from 185°C to 202°C (365°F to 395°F). The sludge then
thickened to 19 percent TS. The effect of ferric addition on
the conditioned sludge thickener decantate is unknown. The
decantate is currently low in SS (478 ppm), but high in BOD (6,000
ppm).
Sludge Dewatering--
Thermal conditioning has improved vacuum filter yield, rais-
ing it from 23.90 kg/m2/hr (4.9 Ib/ft2/hr) with polymer condi-
tioning to 39.10 kg/m2/hr (8 Ib/ft2/hr) with thermal conditioning.
The addition of 19 mg/Jl FeCls caused a decrease to only 20.2 kg/
m2/hr (4.14 Ib/ft2/hr) for thermally conditioned sludge and 14.7
kg/m2/hr (3.0 Ib/ft2/hr) for chemically conditioned sludge. Rais-
ing the temperature of the thermal conditioner enabled a filter
yield of 78.2 kg/m2/hr (16 Ib/ft2/hr).
The filtrate suspended solids concentration increased con-
siderably with the switch to thermal conditioning. FeCls addi-
tion apparently caused an even further increase. Tertiary filter
operation may also have caused some increase because of the recy-
cling of fine solids in the backwash water. The volatile frac-
tion of the filtrate SS concentration appears to have decreased
with the switch to thermal conditioning. This could be explained
by the solubilization of volatile suspended solids during heat
treatment and their removal in soluble form in the decantate.
Concurrently, the BOD of the filtrate increased due to this sol-
ubilization. It appears that FeCla addition further decreased
289
-------
the VSS fraction of the filtrate, and tertiary filter operation
may have had a similar effect. It does not appear that the fil-
trate BOD was affected by Feds addition or tertiary filter oper-
ation.
The filter cake total solids concentration was greatly
increased by thermal conditioning. Cake solids concentration
was adversely affected by FeCla addition, more so with polymer
conditioning than with thermal conditioning. Raising the temper-
ature of the thermal conditioner produced a dryer cake, somewhat
dryer even than was achieved without Feds addition. The VS
fraction of cake TS appears to have been decreased by thermal
conditioning, with further decreases resulting from ferric chlor-
ide addition and from tertiary filter operation.
Sludge Cake Hauling--
As filter cake solids concentration increased, the volume
of filter cake which must be hauled to the ultimate disposal site
decreases. Table C-30 shows that the volume of cake hauled per
ton of dry solids filtered was highest when polymer conditioning
was used, and lowest when the thermal conditioner was operating
at 202°C (395°F). With thermal conditioner operation at only
185°C (365°F), the adverse impact of ferric chloride addition
resulted in more sludge cake volume to be hauled, and thus in more
trips to the disposal site.
Experimental Addition of Alum, to the Raw Sewage--
A full-scale test of alum addition to the raw sewage (80 mg/l
alum as A1203) was conducted at the plant during October of 1975.
Enough alum was added to provide 80 percent phosphorus removal
within the plant. Table C-31 presents the test results.
TABLE C-31. A COMPARISON OF THE CHARACTERISTICS OF THE SLUDGES
PRODUCED WITH ALUM AND FERRIC CHLORIDE, MIDLAND, MICHIGAN
Alum FeCl3
Raw sludge, % TS 6.5 7.6
Thermally conditioned sludge,
% TS 16.0 22.5
Filter vield, Kg/m2/hr 23.5 76.7
(Ib/ft2/hr) (4.8) (15.7)
Filter cake, % TS 41 56
290
-------
Considerable problems were experienced in handling the
sludge produced with alum. Separation of solids and supernatant
in the thermally conditioned sludge thickener did not occur as
rapidly as with ferric chloride. The TS concentrations of the
thickened sludge was lower. The filter cake TS concentrations
were only 41 percent compared to 56 percent with ferric chloride.
Sludge with less than 50 percent TS is not suitable for the pre-
sent method of disposal, indicating that alum sludge would require
additional processing such as incineration. The filter rate was
considerably slower with alum, it taking 3.3 times as long to
filter a ton of alum sludge. This would mean adding one or pos-
sibly two full-time operators to the staff. Also, since the
sludge pumped from the primary tanks was thinner, it was deter-
mined that the thermal conditioner would have to operate 17 per-
cent longer to process,it.
Sludge Treatment and Disposal Costs
In response to the adverse effects of phosphorus removal on
sludge handling, the temperature of the thermal conditioner was
raised. This restored satisfactory thickening and dewatering.
Raising the thermal conditioner temperature from 185°C to 202°C
(365°F to 395°F) meant an increase in the amount of natural gas
required to fuel the boiler. The cost for natural gas increased
by approximately $4.87/t ($4.34/ton) of dry solids as outlined
in Table C-32. This is the only significant cost increase for
sludge treatment and disposal which has been identified by this
case study.
It has not been possible to establish the additional quantity
of sludge solids generated by phosphorus removal, although it is
certain that the quantity of sludge did increase. Without raising
the temperature of the thermal conditioner, the additional sludge
quantity would have been responsible for more vacuum filter run-
ning hours and more trips to the sludge cake disposal site. But
at the higher thermal conditioner temperature, these trends were
countered by a higher filter yield and cake solids content. It
is therefore likely that the additional sludge quantity was han-
dled without a significant increase in the costs of vacuum filter
operation and sludge cake hauling. The plant presently runs the
vacuum filters for about 94 hr/mo and makes about 79 trips/mo to
the sludge cake disposal site.
Summary and Conclusions
Table C-30 has summarized the discussion of the variations in
the performance of the sludge treatment processes. Thermal con-
ditioning improved vacuum filter yield and cake dryness, but raised
the BOD concentration of the thickener decantate and the filtrate
and the SS concentration of the filtrate. Ferric chloride addi-
tion adversely affected filter yield, cake dryness, and filtrate
SS concentration of the filtrate. Ferric chloride addition
291
-------
TABLE C-32. ADDITIONAL COST FOR HIGH-TEMPERATURE THERMAL CONDITIONING, MIDLAND, MICHIGAN
(Assume sludge feed at 6.5% TS; natural gas
cost of $0.089/m3 ($2.50/1000 ft3) in 1976}
Before Phosphorus Removal After Phosphorus Removal Difference
185°C (365°F) Cond. Temp. 202°C (395°F) Cond. Temp.
,1000 m3 natural gas/mil m3
sludge (1000 ft3/MG)
$/mil m sludge
($/MG)
$/t dry solids
($/ton)
4,365 (590)
$3,900 ($14.75)
$6.00 ($5.44)
7,841 (1,060)
$7,000 ($26.50)
$10.78 ($9.78)
3,477 (470)
$3,100 ($11.75)
$4.78 ($4.34)
-------
adversely affected filter yield, cake dryness, and filtrate SS
concentration. Even with the adverse impacts of ferric addition,
however, filter yield and cake dryness were better with thermal
conditioning than with polymer conditioning. Raising the temper-
ature of the thermal conditioner overcame the adverse effects of
ferric addition which were mentioned except for the increase in
filtrate SS concentration. Filtrate SS concentration remained
high, probably due to the recirculation of tertiary filter back-
wash -water rather than due to ferric addition.
The decrease in vacuum filter cake volatile solids fraction
(percent of TS) which occurred with thermal conditioning and fur-
ther occurred with ferric addition would be an adverse impact for
a plant practicing sludge cake incineration. Ferric chloride
addition appears to have lowered the volatile fraction of cake
solids from 58 down to 48 percent of TS.
A cost analysis has shown that the only significant cost
increase for sludge treatment and disposal attributable to ferric
addition was the cost of raising the thermal conditioner temper-
ature to 202°C (395°F). The additional natural gas required to
operate at the higher temperature had a cost of approximately
$4.78/t ($4.34/ton) of dry solids (Table C-32).
Some general observations about this case study can be made.
Apparently, the characteristics of the Midland sludge are such
that it has thickened and dewatered readily both before and after
phosphorus removal. It is likely that this is related to the
facts that the plant uses trickling filters, has no significant
industrial wastes, and is not overloaded. It has also been shown
that ferric addition to the raw sewage produces a better sludge
than alum addition (Table C-31).
In conclusion, thermal conditioning at Midland has been suc-
cessful, but phosphorus removal has increased the energy require-
ments of the process at a time when energy is becoming more
expensive.
CASE STUDY J: PORT HURON, MICHIGAN
Introduction
Port Huron is an example of an activated sludge plant
practicing alum addition to the secondary part of the plant.
Built from 1972 through 1974, it is representative of the
newer wastewater treatment plants. Characteristic of these
newer plants is the possession of adequate sludge handling
capacity. At older plants, in contrast, problems related to
lack of sludge handling capacity tend to dominate sludge
handling considerations. Another characteristic of many
newer plants is that phosphorus removal by chemical addition
has been incorporated into their original design rather than
293
-------
added later. These plants have not handled sludge when no
chemical sludge was being generated.
At Port Huron, sludge treatment consists of gravity
thickening, thermal conditioning, polymer conditioning,
centrifuge dewatering, and fluidized bed incineration. The
system has been operated and evaluated both with and without
inclusion of the thermal conditioning step. At plants where
incineration seems to be the proper disposal method, thermal
conditioning prior to dewatering has been cited as a way to
reduce overall fuel costs. The combination of thermal con-
ditioning and incineration has recently been chosen for the
design of several new facilities. Centrifuges for dewatering
are also finding application in these facilities. Investiga-
tion of the Port Huron facility gives us insight into the
actual performance of an entire system using these techniques
for handling phosphorus-laden chemical sludge.
Every plant has its peculiarities of design and influent
wastewater characteristics. At Port Huron, the significant
ones which affect sludge are related to the low flow rate and
SS and BOD concentrations of the influent. Designed to
provide secondary treatment for an average flow of 0.075 mil
m^/day (20 mgd), the plant treated an average flow of only
0.04 mil m3/day (11.6 mgd) in 1976. Because of this dis-
crepancy between plant design and actual flow, there is a long
aeration basin retention time and a low food to microorqanism
(F/M) ratio. The activated sludge microorganisms are in a
near-starvation phase and have a slow growth rate. This probably
means that sludge generation is somewhat lower than typical for
this type of plant.
Plant influent SS and BOD concentrations are quite low
due to the fact that 20 percent of the wastewater entering
the plant originates from industries, mainly of the metal
manufacturing, coating, and plating types. Wastewater SS
and BOD are usually about 86 and 58 ppm, respectively.
Because of the low concentrations, these constituents are more
efficiently removed by biological treatment and settling
than by primary settling. The production of secondary relative
to primary sludge is higher than would be the case if influent
SS and BOD concentrations were greater. This is undesirable
because the waste activated sludge tends to be harder to
thicken than the primary sludge.
History
Historical plant modifications affecting sludge production
and characteristics follow:
1972 - Initiated expansion of old primary plant and construc-
tion of secondary treatment facilities, phosphorus removal facili-
ties, and solids handling system.
294
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January 1975 - Commenced operation of new facilities
including phosphorus removal by chemical addition.
December 1976 - Discontinued use of
unit for sludge conditioning. Increased
conditioners used for d_ewatering.
Chemical Addition for Phosphorus Removal
heat treatment
dosage of polymer
Liquid alum (48.8 percent Al2(S04)3 • 141^0) is added
to the activated sludge mixed liquor as it flows over the
effluent weirs of the aeration basins. Figure C-25 shows how
wastewater enters and leaves the aeration tanks and the alum
(A) and polymer (P) feed points. Alum is added at six points
to the mixed liquor as it falls over the effluent weirs into
six effluent channels leading to the final clarifiers. Alum
is added at the farthest end of each weir from the final
tanks, while polymer is added at the other end, 30.5 m (100
ft) closer to the finals. The alum is diluted as it is split
into the six feed lines at a ratio of 5 to 1.
TO FINAL CLARIFIER
ft
ft
>>
Si
ft
Figure C-25.
Chemical feed points to aeration tanks
Port Huron, Michigan.
The alum dosage is adjusted according to plant phosphorus
removal efficiency. Operators notice changes in removal
efficiency and decide whether to increase or decrease the
alum dosage. When a
certain dosage is desired, a computer is
295
-------
signalled and the alum feed pump is automatically paced by
the computer according to the wastewater flow to achieve
that dosage. The average dosage has been approximately 52
ppm Al2(S04)3 • 14^0 over the first 7 mo of 1977. Anionic
polymer (Hercules 847) is used. The computer proportions
the polymer feed pumping rate to the wastewater flow to
achieve a desired dosage. The dosage is selected by plant
personnel based on visual clarity of the final clarifiers and
final effluent. The average dosage has been 0.275 ppm for
January through July of 1977.
General Description of Wastewater Treatment Operations Affecting
SIudge
Figure C-26 presents a general plant flow diagram. The
plant has two aerated grit chambers complete with mechanical
grit removal and mechanical oil and scum removal facilities.
Eight primary clarifiers having a total capacity of 4,000 m3
(138,400 ft3) are designed to handle an average flow of 0.12
mil m3/day (33 mgd) and a peak flow of 0.22 mil m3/day (58 mgd)
A flow-equalizing retention basin for primary effluent with
0.022 mil m3 (5.7 MG) capacity is used. The clarifiers are
rectangular. In each clarifier, sludge is scraped to a
hopper at one end by a chain and flight mechanism. In the
hopper, a screw mechanism scrapes sludge to a valve opening
in the center. Sludge is collected from a pair of clarifiers
at a time for 7 out of every 28 min. The flights and screws
are turned on, a valve opens from two clarifiers and sludge
is pumped from them at a combined rate of 0.984 m3/min
(260 gal/min) for the 7 min. Primary sludge is pumped at
less than one percent solids for efficient degritting. Scum
from the surfaces of the clarifiers enters a scum well which
is discharged once a day to the thickener splitter box.
Secondary biological treatment is accomplished with con-
ventional complete mix activated sludge. Three aeration
basins providing a total capacity of 0.013 mil m3 (3,350,000
gal) are mixed and aerated by diffused air. Their design
detention time is 3 hrs, but the actual detention time in
these basins averages about 6.9 hrs. The F/M ratio (figured
using MLVSS rather than MLSS concentration) is maintained
near 0.1 mg/£. The return sludge rate averages 63 percent
Of flow. MLSS and MLVSS concentrations have averaged 5,250
and 3,000 ppm, respectively. Sludge density index averages
around 1.7 percent.
Three square final clarifiers provide a total final
settling capacity of 0.011 mil m3 (2.9 MG). Sludge is sucked
up hydraulically from the bottom of each clarifier by pickup
pipes and flows by gravity to a manifold in the center of the
clarifier bottom. Sludge is pumped out to return or waste
from one clarifier at a time. The operator checks the depth
296
-------
INTER-
CEPTOR
ro
Figure C-26. Port Huron, Michigan, wastewater treatment plant flow diagram..
-------
of the sludge blanket in each clarifier once a shift with
the help of installed photoelectric cell detectors. He pumps
sludge out as necessary to prevent the blanket depth from
exceeding 0.914 m (3 ft). The average blanket depth is
probably about 0.457 m (1.5 ft). The operator decides how
much sludge to return and how much to waste on the basis of
MLVSS in the aeration basins and desired F/M ratio. Scum is
skimmed from the surfaces of the clarifiers into hoppers and
discharged to the waste sludge lines.
Detailed Description of Sludge Treatment and Disposal Operations
Degritting--
A Dorr-Oliver "Dorr-Clone" cyclone sludge degritter is
employed to remove 95 percent of 250 mesh grit from the
primary sludge. Sludge degritting, in addition to the raw
sewage grit removal, is necessary to limit wear on the sludge
centrifuges.
Thickening--
Primary and secondary sludge, and scum from the primaries,
secondaries, and aerated grit chamber flow to a Dorr-Oliver
"Densludge" Type SD gravity thickener. Only one of the three
thickeners present is used. A splitter box is present for
dividing sludge flow between the three thickeners. A low
cationic polymer is fed at the splitter box at a dosage of
3 ppm to aid solids capture and thickening. Each 16.8-m
(55-ft) diameter thickener is of 738 m3 (195,000 gal) capacity.
The liquid sidewall is 3.35 m (11 ft) plus the cone at the
bottom. It is equipped with scraper arms having vertical
pickets for continuously stirring sludge in the thickener as
it is scraped from the bottom to a hopper. Another scraper
pushes sludge to a single draw-off point in the hopper from
which sludge is pumped out to either the centrifuges or the
thermal conditioner.
Thickener dilution water can be provided by three pumps
which deliver final effluent to the splitter box. Dilution
water is not used, however. Partial dilution is effected as
a result of the low TS concentration at which primary sludge
is pumped. Further dilution would be required to raise the
thickener overflow rate into its design operating range of
16 to 32 m3/day/m2 (400 to 800 gal/day/ft?). But the thickener
is operated at a much lower overflow rate of approximately
7.13 m3/day/m2 (175 gal/day/ft2). This is because its volume
is effectively reduced by its high sludge blanket.
A mechanical skimmer collects scum from the surface of the
thickener. The scum trough is so small, however, that once a
day the thickener must be scraped manually. Scum adversely
298
-------
affects thickener operation by blocking the overflow weir and
reducing weir overflow length. The scum baffle which is
present is not deep enough to prevent this.
Figure C-27 presents information on average sl'udge flow
rates, total and volatile solids concentrations, and masses
in the form of a combined hydraulic and mass balance diagram
constructed around the thickener. The diagram is based on
monthly averages of data for January through July of 1977.
During this period, the thermal conditioner was not in
operation.
Figure C-27 shows that the average performance of the
thickener is satisfactory, with a combined primary-waste
activated sludge of about 0.56 percent TS thickening to 4.68
percent TS and the overflow containing about 2,480 mg/£ TS.
But it does not show the inconsistency which exists in results
from day-to-day due to variations in activated sludge wasting
rates and intermittent incinerator operation. Thickened
sludge concentration reaches almost 6 percent on a few good
days, while on poor days it is only about 4 percent. The
overflow solids concentration varies greatly from day-to-day
between about 100 mg/£ and 10,000 mg/£. These variations
are related to the addition of alum to the aeration basins.
Alum addition increases the mass of activated sludge which is
generated and wasted. Waste-activated sludge has poorer
thickening characteristics than primary sludge, so that when
activated sludge wasting rates are high, thickening is poorer.
This leads at times to low thickened sludge solids concen-
trations and high polymer dosage requirements for chemical
condi tion ing.
Primary sludge is continually wasted to the thickener
at 0.016 m^/sec (260 gal/min). Activated sludge is normally
wasted at 0.18 nwmin (50 gal/min), but the rate varies between
0.096 and 0.36 m3/min (25 and 100 gal/min). Wasting continues
during all or part of the day, depending on how much needs to
be wasted. Sludge is withdrawn from the thickener at an
average rate of 0.18 m3/min (52 gal/min) when the centrifuges
and incinerator are operating. In 1977, they have operated
3 out of 4 wks/mo and for 2 to 5 days/wk (average 3.35 days/
wk). A sludge blanket deeper than 1.2 (4 ft) is normally
maintained in the thickener to get good thickening action.
If the blanket is less than 0.61 m (4 ft), water can be
drawn while pumping sludge, so this is avoided. The usual
blanket depth is 1.5 to 2.1 m (5 to 7 ft). When the activated
sludge wasting rate is high and the incinerator has not been
running, a sludge blanket of up to 3.05 m (10 ft) builds- up
in the thickener, and overflow quality deteriorates. The
thickener is in general slightly overloaded, and plans have
been made to alleviate this by using one of the other two
thickeners available. There is concern, however, about
299
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AVERAGE SEWAGE FLOWi 11.9 MGD
CO
O
O
7,680
3,530
.25
48.
374,400
0TS/DAY
0VS/DAY
XTS
XVS
PRIMARY
SLUDGE
8,030
4,440
.25
55.
388,400
0TS/DAY
0VS/DAY
XTS
%VS
THICKENER
GPD OVERFLOW
12,020
7,910
3. 3
66
43, 900
#TS/OAY
#VS/DAY
XTS
XVS
GPD WAS
11,700 0TS/DAY
7,000 0VS/DAY
4.70 %TS
60
%VS
29,900 GPD
SLUDGE
REMOVED
NOTE. RATES ARE CALCULATED ON AN AVERAGE DAILY FLOW BASIS WHETHER FLOWS OCCUR
EVERY DAY OR NOT.
Figure C-27. Port Huron, Michigan, gravity thickener hydraulic and mass balance.
-------
increasing the solids detention time in the thickeners which,
it was observed at the plant, results in poorer dewatering
than can be achieved with "fresher" sludge.
Condition ing--
The two alternative methods of sludge conditioning which
have been practiced are thermal conditioning with the Farrer
system and chemical conditioning with polymers. Polymers were
used in low concentrations even with thermal conditioning.
The Farrer system has two high-pressure feed pumps
which force thickened sludge through two sludge disintegrators
and to the Farrer heat exchanger. The preheated sludge flows
to the Farrer reactor where steam, which has been purged of
air, is injected. The sludge residues in the reactor for 20
to 30 min under 300 psi pressure at a temperature of 204°C
(400°F). When sludge leaves the reactor, it flows through
the heat exchanger to a decant tank. Decant tank overflow is
returned to the head of the plant while sludge is withdrawn
by the four centrifuge feed pumps. The system was designed
to handle 20,600 kg/day (45,300 dry Ib/day) of a 4.61 percent
TS sludge feed. It was designed to produce a sludge capable
of being centrifuged to 31 percent TS.
Polymer is added to the sludge in the lines which feed
each of the four centrifuges at a point about 0.305 m (1 ft)
before the sludge enters the hub of the scroll. Hercules
849 and 874 cationic polymers (1/2:1/2) are used. The average
polymer dosage was 3.38 kg/t (6.75 Ib/ton) of dry solids with
thermal conditioning and 6.48 kg/t (12.94 Ib/ton) without
thermal conditioning.
With thermal conditioning, solids were processed (con-
ditioned, dewatered, and burned) only about 25 percent of the
time. The equipment was usually run 24 hr/day for 5 days and
then kept down for 16 days. (Without thermal conditioning,
solids were processed about 36 percent of the time.) Thermal
conditioning was discontinued after November of 1976 due to
extreme corrosion and erosion of the carbon steel making up
the heat exchanger piping and connecting piping.
Thermally conditioned sludge from the decant tank averaged
8.6 percent TS. The decant tank overflow averaged abour 3,000
ppm SS with the volatile fraction averaging 45 percent. The
pH of this sidestream varied between 6.0 and 7.2.
Dewatering--
Dewatering of thickened and chemically or thermally/
chemically conditioned sludge is accomplished by centrifuga-
tion. The plant operates four horizontal fully-continuous
301
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solid bowl Dorr-Oliver "Merco-Bowl" centrifuges. Some
machine operating parameters for these centrifuges are given
below:
Largest inside bowl diameter: 41.9 cm (16.5 in)
Inside bowl length: 121.9 cm (48 in)
Bowl shape: conical-cylindrical
Scroll configuration: dual flight
Centrate disposition: pumped to plant influent
The machines are operated at a bowl speed of 3,000 rpm,
scroll speed differential of 17 rpm and pool depth of
7.874 cm (3.1 in). When solids processing is underway, all
four centrifuges are operated. Machine operating variables
such as sludge feed rate and polymer dosage are monitored
and controlled for each centrifuge separately by the operator.
He makes adjustments which will enable at least 80 percent
solids recovery. Centrate TS concentration is determined
several times each day for each centrifuge.
Centrifuge and incinerator performance will be discussed
together and related to type of sludge conditioning.
Incineration--
The plant has a Dorr-Oliver "F/S System" fluosolids
incinerator. Centrifuge cake falls by gravity through chutes
into sludge transfer screw conveyors. They convey it to the
progressive cavity pumps which feed the reactor. The reactor
cylinder is 4.57 m (15 ft) in diameter and contains a 1.52-m
(5-ft) expanded bed of silica sand. A fluidizing air blower
discharges into the reactor through the windbox. Hot gases
and ash exit through the top of the reactor into a water seal
expansion joint followed by a Venturi-type scrubber. They
then enter a gas scrubber with a radial vane and water trays.
The gases exit from the exhaust stack through a plume suppres-
sion burner. The ash slurry flows to ash pumps which deliver
it to a 5.49-m (18-ft) diameter Dorr-Oliver ash thickener.
Overflow from this thickener is recycled to the plant influent
except for a portion which is used in the scrubber system.
The ash is pumped to a vacuum filter (Dorr-Oliver) which is
1.220 m (4 ft) in diameter, has a 0.914 m (3 ft) face width,
and is designed to handle 590 kg (1,300 lb)dry solids/hr.
The filtrate is returned to the head of the plant. The
dewatered ash is trucked 19.3 km (12 mi) to a landfill along
with grit from the aerated grit chambers and sludge degritter.
3
Fluidizing air is normally discharged at 1.65 to 1.70 m /
sec (3,500 to 3,600 scfm). The bed pressure differential is
maintained at 1.19 to 1.22 m (47 to 48 in) of water. The
normal reactor operating temperature is approximately 721°C
(1,330°F). Although solids processing is intermittent, the
302
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incinerator temperature is always maintained above 663°C
(1,225°F) over the next 30 hrs. It takes about 227 I (60 gal)
of No. 2 fuel oil to raise the temperature from 663°C to 871°C
(1,225°F to 1,600°F).
The incinerator was designed to operate in conjunction
with the thermal conditioner. Its design capacity is 1,040 kg
(2,300 Ib) of dry solids/hr for a 31 percent TS feed that is
58 percent volatile.
Performance and Operational Costs of Sludge Treatment and
Disposal
Centrifuge and incinerator performance and operational
costs have been greatly affected by the sludge conditioning
method used. Table C-33 summarizes the direct effects of
conditioning on the performance and operational costs of
solids handling. Centrifuge cake TS concentration was improved
by thermal conditioning and the pounds of solids which could
be fed to the incinerator per hour was higher. This reduced
incinerator fuel consumption. During thermal conditioning
the burning rate covered a range of 420 to 1,320 kg TS/hr
(929to 2,900 Ib TS/hr) which included the design capacity
for thermally conditioned sludge of 1,040 kg/hr (2,300 Ib/hr).
Dorr-Oliver has estimated that the capacity is reduced to
590 kg/hr (1,300 Ib/hr) for non-thermal1y conditioned sludge.
Although total cake solids is higher, percent volatile is
lower with thermal conditioning, due to the solubi1ization of
volatile solids and removal in the decantate. This is a dis-
advantage as far as fuel consumption for incineration, but the
disadvantage is outweighed by having less water in the cake
to be evaporated. The major savings on utilities with thermal
conditioning were for fuel oil and conditioning polymer.
Major expenses were for natural gas and electricity and
maintenance and repair of the thermal conditioner itself.
Actual figures on maintenance and repair were not available
based on the plant's experience because of the corrosion
problem. Before normal operation could be achieved, the
existing system would have to be rebuilt. The top five rows of
the heat exchanger would have to be replaced with Schedule
160 inner tubes and Schedule 80 outer tubes, rather than the
present Schedule 40 tubes. The total cost of labor for solids
handling was the same with and without thermal conditioning.
When thermal conditioning was discontinued, labor was shifted
from that area to the incinerator.
The indirect effects of conditioning method on per-
formance and operational costs of solids handling must also be
considered. As mentioned, thermal conditioning solubilizes
some of the volatile solids in the sludge which are removed
in the decantate. The high BOD concentration of the decantate
is a burden on the activated sludge treatment. The cost of
303
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TABLE C-33. DIRECT EFFECTS OF CONDITIONING METHOD ON
PERFORMANCE AND OPERATIONAL COSTS OF SOLIDS HANDLING, PORT HURON, MICHIGAN
Unit cost factors:
Fuel Oil
Natural Gas
Electricity
Centrifuge Polymer
Thickening Polymer
Gasoline
$ .1004/Jt
$87.07/1,000 m
$ 0.03/kwh
$ 4.40/kg
$ 2.97/kg
$ .185A
(1976 costs)
3
kg TS/hr (Ib TS/hr) to centrifuges
and incinerator
kg VS/hr (Ib VS/hr) to incinerator
Centrifuge cake % TS
Centrifuge cake % VS
Incinerator bed temp., °C (°F)
kg conditioning polymer/t (Ib/ton)
Vt ($/ton)
1,000 ft3 gas used/ton
$/t ($/ton)
3
m fuel oil used/t (gal/ton)
$/t ($/ton)
kwh electricity used/t (kwh/ton)
thermal cond.
centrifuges
incinerator
thickener
$/t ($/ton)
Thermal cond. steam boiler water
conditioning and cleaning
chemicals est. $/t ($/ton)
$ 0.38/gal
$ 2.27/ft3
$ 0.03/kwh
$ 2.00/lb
$ 1.35/lb
$ 0.70/gal
With thermal
conditioning
777 (1,712)
350 (771)
27-32.6
45.05
738 (1,360)
With chemical
conditioning
556 (1,224)
342 (753)
18.8-21.0
61.6
732 (1,350)
3.06 (6.75) 5.67 (12.5)
$14.88 ($13.50) $27.51 ($24.96)
8.37
$20.94 ($19.00)
0.309 (74.1) 0.685 (164.1)
$31.04 ($28.16) $68.75 ($62.36)
36.1 (32.7)
176 (159) - est. 176 (159) - est.
374 (339) - est. 374 (339) - est.
9.92 (9) -est. 9.92 (9) -est.
$17.88 ($16.22) $16.79 ($15.23)
$ 1.76 ($ 1.60)
(continued)
304
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TABLE C-33 (continued)
Odor control (therm, cond.) and
sand replacement (incin.),
est. $/t ($/ton)
3
Dewatered ash hauled, m /t
(yd3/ton)
est. $/t ($/ton) for gasoline
kg thickening polymer/t
(Ib/ton)
$/t ($/ton)
Cost of labor,* $/t ($/ton) - est.
1) Operational and minor equip-
ment maint.
thickener
thermal conditioner
centrifuges
incinerator
Total
2) General maintenance
thickener
thermal conditioner
centrifuges
incinerator
Total
3) Equipment repair
thickener
thermal conditioner
centrifuges
incinerator
Total
4) Ash and grit handling
Total maintenance and repair
supplies for solids handling,
$/t, ($/ton) - est.
Total supervision for solids
handling, $/t ($/ton) - est.
With thermal With chemical
conditioning conditioning
$ 3.85 ($ 3.50) $ 2.18 ($ 1.98)
0.961 (1.14) 0.961 (1.14)
$ 0.08 ($ 0.08) $ 0.08 ($ 0.08
0.926 (1.85) 0.926 (1.85)
$ 2.75 ($ 2.50) $ 2.75 ($ 2.50)
$ 5.90 ($ 5.35) $ 5.90 ($ 5.35)
$ 5.90 ($ 5.35)
$16.00 ($17.64) $17.64 ($16.00)
$16.00 ($17.64) $23.54 ($21.35)
$47.08 ($42.60) $47.08 ($42.60)
$ 3.53 ($ 3.20) $ 3.53 ($ 3.20)
$ 1.76 ($ 1.60)
$ 5.30 ($ 4.81) $ 5.30 ($ 4.81)
$ 3.53 ($ 3.20) $ 5.29 ($ 4.80)
$14.12 ($12.81) $14.12 ($12.81)
$ 0.49 ($ 0.44) $ 0.49 ($ 0.44)
$ 0.49 ($ 0.44)
$ 1.95
$ 1.47
$ 1.95 ($ 1.77
$ 0.98 ($ 0.89
$ 3.91 ($ 3.54
$ 3.91
$ 1.77)
$ 1.33)
$ 3.54)
$ 3.57 ($ 3.24) $ 3.57 ($ 3.24)
$36.38 ($33.00) $21.49 ($19.50)
$12.50 ($11.34) $12.50 ($11.34)
(continued)
305
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TABLE C-33 (continued)
Total direct costs, $/t ($/ton)
thermal conditioner
centrifuges
incinerator
thickener
With thermal
conditioning
With chemical
conditioning
$210.79 ($191.19) $220.78 ($200.25)
$ 33.62 ($ 30.50)
$ 45.04 ($ 40.86) $ 57.68 ($52.32)
$ 66.67 ($ 60.49) $112.55 ($102.09)
$ 12.96 ($ 11.76) $ 12.96 ($ 11.76)
Based on 1,090 t (1,200 tons) dry solids/yr in 1976.
and wages, including benefits.
1976 salaries
306
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increasing the power of the air blower to handle this BOD
loading has been estimated at $9.52/t ($8.64/ton). The actual
BOD concentration of the decantate is not available. Average
centrate TS concentration was lower with thermal conditioning
(966 vs 2,600 ppm) and volatile concentration was higher
(63 vs 56.3 percent of TS).
Summary and Conclusions
Alum addition to the secondary rather than the primary
part of the plant resulted in an increased mass of secondary
rather than primary sludge. It is not known whether the
solids concentration or volume of secondary sludge was
affected by alum addition as well. Experience at the plant
shows that the waste activated sludge is harder to thicken
than the primary sludge. When the solids concentration of
the thickened sludge is lower than average as a result of
high activated sludge wasting rates, the cost of chemical
conditioning prior to dewatering is increased. Solids con-
centration of the dewatered cake apparently does not vary in
direct response to the variations in feed concentration, as
long as adequate conditioning takes place and the centrifuge
feed rate is properly adjusted. At the sludge loading and
sludge removal rates in use, one 738-m3 (195,000-gal)
thickener is of insufficient capacity. This is evidenced by
the high sludge blanket which sometimes builds up and causes
deterioration of effluent quality. Rather than operate two
thickeners, which might have deleterious effects on dewatering
due to a long solids detention time, "polymer was added to
increase solids capture. It is recommended, however, that
the design criteria for sizing thickeners be studied closely
as problems in this area have been observed at several plants.
Especially at plants where alum addition to the secondary
stage is practiced, additional thickener capacity is needed.
The use of thermal conditioning resulted in savings of
$50.30/t ($45.70/ton) of dry sludge when compared with the
cost of chemical conditioning alone. The additional expenses
(excluding equipment maintenance and repair supplies)
resulting from thermal conditioning amounted to $35.00/t
($31.70/ton). Therefore, it appears that operation and
maintenance of thermal conditioning at the plant are currently
cost-effective, as long as the cost of equipment maintenance
and repair supplies is less than $15.30/t ($13.92/ton).
Without thermal conditioning, the capacity of the
incinerator is decreased from 1,040 kg/hr to 490 kg/hr
(2.300 Ib/hr to 1,300 Ib/hr). It was assumed in the design
of the incinerator that the plant would be producing 18,000
kg/day (39,700 Ib/day) thickened sludge, but it actually
produced only 5,310 kg/day (11,700 Ib/day) due to the low
wastewater flow. The incinerator capacity is therefore
307
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presently adequate at 590 kg/hr (1,300 Ib/hr), but when the
plant influent flow increases in the future, incinerator
capacity will become limiting. It was estimated that three
additional centrifuges and a 6.71-m (22-ft) diameter incinerator
reactor rather than the present 4.57-m (15-ft) reactor would
be necessary to handle 1,040 kg/hr (2,300 Ib/hr) of sludge
with chemical conditioning alone. Including this considera-
tion in the analysis, operation and maintenance of chemical
conditioning will not be cost-effective in the future unless
additional capital costs are incurred.
Recent Developments
Since the original investigation of the Port Huron
plant and the writing of this case study, modifications in
thickener operation have been made with significant results.
Two of the three identical gravity thickeners available at
the plant are now in use rather than just one. Each of the
two thickeners now receives approximately 12.6 I/sec
(200 gal/min) of dilution water plus 8.2 I/sec (130 gal/min)
of primary sludge and 1.6 £/sec (25 gal/min) of waste
activated sludge. They are operating at an overflow rate of
approximately 22 m3/day/m2 (540 gpd/ft2). The sludge blanket
depth in each thickener is maintained between 1.2 and 1.5 m
(4 and 5 ft).
In the past, where only one thickener was in use, the
thickener was overloaded with sludge, although the surface
overflow rate was low and no dilution water was used. The
sludge blanket depth in each thickener averaged 1.5 to
2.1 m (5 to 7 ft). Polymer addition was necessary to aid
solids capture and thickening. Presently, with two thickeners
in operation, solids capture and thickening are reportedly
slightly improved although no polymer is used. The savings
in polymer cost amounts to $2.75/t ($2.50/ton) of dry solids.
It is also reported that a reduction in the amount of
phosphorus recycled to the head of the aeration basins in the
thickener overflow has occurred. This may mean a slight
decrease in the cost of phosphorus removal chemicals.
CASE STUDY K: PONTIAC, MICHIGAN
Introduction
The Sewage Treatment Division of the city of Pontiac,
Michigan, operates two wastewater treatment plants a short dis-
.tance from each other. The East Boulevard plant is a conven-
tional activated sludge treatment plant with a design average
flow of 32,600 m3/day (8.6 mgd). The Auburn plant is also a
conventional activated sludge treatment plant, with rapid sand
filtration facilities for the tertiary treatment of both East
Boulevard and Auburn secondary effluents. The Auburn Plant
308
-------
design average flow is 64,000 m3/day (16.9 mgd), giving a com-
bined design average flow for the two plants of 96,500 m3/day
(25.5 mgd). A flow diagram for the treatment process is shown
in Figures C-28 and C-29. Wastewater influent to the Pontiac
plants is delivered by combined storm and sanitary sewers.
Approximately 40 percent of the influent consists of pretreated
industrial process water (plating waste) from which heavy metals
have been removed. This industrial waste is characterized by a
relatively low concentration of BOD.
Two periods of plant operation have been selected for com-
parison purposes to determine the impacts of chemical additions
for phosphorus removal. The period from January to June 1969
was selected as the period during which both primary and
secondary addition of ferric chloride took place for phosphorus
removal. Table C-34 presents the average wastewater character-
istics and plant removal efficiencies of the two Pontiac
plants combined.
History
In 1919, a secondary wastewater treatment plant, consisting
of Imhoff tanks and trickling filters, was constructed at the
East Boulevard site. In 1938, activated sludge aeration basins
and an anaerobic digester were added. A primary settling tank
and additional sludge treatment facilities were constructed in
1952. In 1962, a second plant with secondary treatment
facilities was constructed at the Auburn site. The plant also
contained anaerobic sludge digestion, vacuum filter dewatering,
and incineration facilities. The Imhoff tanks and trickling
filters at the East Boulevard plant were removed from service
in 1962. In 1973, both plants were modified. A second primary
settling tank and a flow equalizing retention basin were added
at the East Boulevard site, and additional primary and secondary
settling tanks and tertiary treatment facilities were added at
Auburn.
Historical modifications to the plants affecting sludge are
outlined in the following description:
• May 12-15, 1969 - Initial jar tests of phosphorus
removal chemicals
• Aug.-Dec., 1969 - Full-scale testing of various chemi-
cal additions for phosphorus removal
• 1970 - Plants experimented with ferric chlo-
ride at the Auburn plant, with the use
of various polymers
1971 - 1973 ~ Continuous addition of ferric chloride
at the Auburn plant, with the use of
various polymers
309
-------
TABLE C-34. WASTEWATER CHARACTERISTICS AND REMOVAL EFFICIENCIES,
PONTIAC, MICHIGAN
2
Plant Flow m /day (mgd)
SS Primary Influent (mg/&)
Primary Effluent (mg/#)
Plant Effluent (mg/ji)
Primary Removal (%)
Plant Removal (%}
VSS Primary Influent (mg/Jl)
Primary Effluent (mg/£)
Plant Effluent (mg/£)
Primary Removal (%}
Plant Removal (%)
BOD Primary Influent (mg/ji)
Primary Effluent (mg/jj)
Plant Effluent (mg/£)
Primary Removal (%)
Plant Removal (X)
TP Primary Influent (mg/jt)
Primary Effluent (mg/#)
Plant Effluent (mg/ji)
Primary Removal (%)
Plant Removal (%)
Jan. - June '69
81 ,400 (21.5)
184
182
23
1
87
100
106
13
-6
87
100
90
10
10
90
«•
-
-
-
_
Jan. - June '77
104,000 (27.6)
105
66
3
37
97
62
36
2
42
97
81
60
5
26
94
3.8
1.9
0.2
50
95
310
-------
Plant expansion begun (specific addi-
tions presented in general plant
description summary); incinerator
di smantled
t 1975 - 1976 - Various points of alum addition with
and without polymer, plus secondary
addition of ferric chloride used at
the Auburn plant
• 1977 - Primary and secondary addition of ferric
chloride at both plants; primary addi-
tion of anionic polymer at East Boule-
vard plant with some secondary addition
of cationic polymer at Auburn plant
• March 1977 - Incinerator expansion completed and
returned to service
Chemical Addition for Phosphorus Removal
The points of addition of the phosphorus removal chemicals
were shown on Figure C-28. Liquid ferric chloride solution
(approximately 41 percent FeCla) is added to the raw influent
wastewater at a dosage of 8 to 12 mg/l Fe"1"3. Anionic polymer
(Dru-Floc 2270) is mixed to achieve a 0.25 percent solution for
addition at a dosage rate of 0.13 mg/£ to the raw sewage.
Liquid cationic polymer is added at a dosage rate of 0.17 mg/£
to the aeration basin prior to secondary settling.
General Description of Wastewater Treatment Operations Affecting
SI udge--
Table C-35 presents a description of wastewater treatment
and sludge handling units at the Pontiac plants.
As Figure C-28 shows, sludge removed from the primary
settling tanks is the only sludge sent directly to s'ludge
processing by the anaerobic digester. Waste-activated sludge
and plant sidestreams--sand filter backwash water, anaerobic
digester supernatant, and vacuum filter f il trate--are all
returned to the wastewater stream ahead of the primary settling
tanks. Suspended solids are removed from the sludge and side-
streams in the primary tanks, from whence they are sent to the
sludge processing units as part of the primary sludge.
Figure C-29 shows a sludge handling unit process flow diagram
of the primary sludge removed from both the East Boulevard and
Auburn plants. This processing consists of two-stage anaerobic
digestion, followed by chemical conditioning, vacuum filtration,
and incineration. Incinerator ash is disposed of either as a
wet ash slurry in a lagoon on site, or as dry ash in a landfill.
311
-------
TABLE C-35
GENERAL PLANT DESCRIPTION SUMMARY,
PONTIAC, MICHIGAN
Aeration Tanks
(E)-4 9 54.6 m (179 ft) 00 x 6.1 m (20 ft) (w) x 3.4 m (11 ft)
(SWD) per pass, two passes per tank;(A)- 4 0 63.7 m (209 ft)
(1) x 11.1 m (36.5 ft) (w) x 3.7 m (12 ft) (SWD); 2 @ 72.5 m
(238 ft) (1) x 16.5 m (54 ft) (w) x 4.3 m (14.25 ft) (SWD).
Aeration Equipment
"? 3
(E) - diffused air blowers - 294 m /min (10,500 ft /min) , 8 surface.
aerators @ 5 hp, 1,800 rpm; (A).- diffused air blowers - 280 m3
(10,000 ft^/min); 16 submerged turbine aerators at 10 hp,
37 rpm; 10 two-speed surface aerators at 40 hp, 68/45 rpm.
Secondary Settling Tanks
(E) -4 @ 30.8 m (101 ft) (1) x 10.1 m (33 ft)
(9.5 ft) (SWD);(A)~2 @ 27.4 m (90 ft) x 2.6
2
-------
TABLE C-35 (continued)
Filter Cake Converying and Storage System
Five conveyor sections - one in front of each pair of vacuum
filters, one cross conveyor, one inclined to the top of the
incinerator, and one across the top of the incinerator to the
sludge filter cake hopper; storage hopper @ 61.m3 (8 yd3)
capacity.
Incinerator
Multiple hearth furnace capacity - 5.4 t/hr (6 ton/hr) @
25 percent TS prior to 1974, expanded to 7.3 t/hr (8 ton/hr)
at 25 percent TS in 1977; number of hearths - 7; 10 auxiliary
burners @ 2 per hearth in hearths No. 2, 3, 4, 5, and 6;
minimum operating temperatures:
Hearth No. Temperature °C.(°F.)
1 477.4 (900)
2 532.4 (1 ,000)
3 642.4 (1,200)
4 807.4 (1,500)
5 697.4 (1,300)
6 532.4 (1,000)
7 422.4 (800)
Center shaft and rabble arm drive @.,10 hp, 1.0 rpm;-induced
draft exhaust fan @ 100 hp, 358.4 nT/min (12,800 ft°/m1n);
center shaft cooling air fan @ 10 hp, 106.4 m-Vmin (3,800
ft3/min).
Incinerator Exhaust Gas Scrubber
Flooded tray-type wet scrubber with quencher, scrubber, and
Venturi; inlet gas temperature @ 422.40lC. (800 F.); exhaust
gas temperature @ 37.4*C. (100°F.); 2 scrubber water supply
pumps Ca 2.3 m3/min (610 gal/min).
Wet Ash Handling System
Ash hopper with mixer and vibrating screen; ash discharge
pump 1.5 m3/min (400 gal/min); agitator pump @ 0.23 m3/min
(60 gal/min).
(continued)
313
-------
TABLE C-35 (continued)
Dry Ash Handling System
o
Two screw conveyors and one bucket elevator @ 5.6 m /hr
(200 ft3/hr); one rotary conditioner; one ash storage bin
6 78.4 m3 (2,800 ft3).
Notes
E = East Boulevard Plant
A = Auburn Plant
DIA = Diameter
SWD = Side Water Depth
314
-------
en
I
u
m
CD
CO
•a
01
4J
fl
JJ
U
0)
4-1
en
(3
Raw Sewage
Bar Screens
S Aerated Grit Chamber
.Anionic Polymer
Primary Settling Tanks
Ferric
Chloride
Flow Equalization
Retention Basin
Aeration Tanks
Cationic Polymer
Final Settling Tanks
Return
Activated
Sludge
Rapid Sand Filters
Chlorine Control Tanks
Discharge
Figure C-2b
Wastewater treatment unit process flow diagram,
Pontiac, Michigan.
315
-------
East Blvd Plant
Primary
Sludge
Auburn Plant
Primary
i Sludge
Digestion
Supernatant
To Head of^
Plant or to
Aeration Basins
Digestion
Supernatant
Digested Sludge
To Head of Plant
or to Aeration
Basins
Conditioning
Tanks
Lime
Ferric
Chloride
Vacuum
Filters
Filtrate
to Head of
Plant
Filtercake
Incineration
Ash
Figure C-29.
Sludge handling unit
Pontiac, Michigan.
process flow diagram,
316
-------
The lagooned ash is also sent to a landfill after drying. A
materials balance of key wastewater treatment and sludge handling
unit operations is superimposed on .the plant flow diagram shown
i'n Figure C-30. A brief description of each of the wastewater
treatment unit processes which have significant impacts on
sludge, follows:
Primary Clarifiers--
The primary clarifiers receive the raw influent wastewater
and returned sidestreams, the most significant of which are
characterized in Figure C-30. At the East Boulevard plant, sludge
is removed on a continuous b'asis from the bottoms of the
primary clarifiers to the sludge hopper. At the Auburn plant,
sludge is removed during 3 out of every 4 hrs, 24 hrs/day from
the bottom of the clarifiers to the wet well adjacent to each
clarifier. Also, at the Auburn plant, sludge blankets regularly
build up to a depth of 1.5 m (5 ft), although actual depths are
not regularly monitored. Depths of the sludge blankets at the
East Boulevard plant are also considerable, although not as great
as those at the Auburn plant.
Sludge pumping from the sludge hoppers at East Boulevard and
the wet wells at Auburn takes place during approximately 1 hr out
of every 8 hr per clarifier. This sludge is then pumped to the
primary digester at the respective plant.
Since the initiation of chemical addition for phosphorus
removal, several significant changes in primary clarifier opera-
tions have been observed. Specifically, there have been signifi-
cant improvements in primary clarifier removal efficiency of
influent BOD and SS, as previously shown in Table C-34. Due to
variations in plant influent characteristics, Table C-36 is pre-
sented to further highlight the changes in primary clarifier
operations.
TABLE C-36. PRIMARY CLARIFIER WASTE STREAM
CHARACTERISTICS, PONTIAC, MICHIGAN
kg TS/kg SS (Ib TS/lb SS)
in Plant Influent
Jan. to June Jan. to June
Waste Stream
Digester supernatant
Waste activated sludge
Primary sludge (to digesters)
1969
0.55 (1.21)
0.65 (1.44)
1.03 (2.26)
1977
0.39 (0.86)
<.65 (<1.44)*
1.20 (2.65)
*Estimated due to reduced BOD loadings
on secondary system.
317
-------
CO
«-J
oo
"
'77
0-0.347X10
...._ S3-I4600
WAS VS * 93OO
^9^9
77
0* 27.6 XIO6
S3 > 24200
VSS « I42OO
BOD - 18600
TP = 870
AERATED
RAW ,__ GRIT
INFLUENT CHAMBER
69
0=2I.5XI06
SS « 33IOO
VSS * 17900
BOD • 17900
TP • N/l
1 r ™ nfH
LEGEND
Q * FLOW (6AL/OAY)
SS * SUSPENDED SOLIDS (LBS/DAY)
VSS -VOLATILE SUSPENDED
SOLIDS (L8S/DAY)
BOD»BOD5 (LBS/DAY)
TP* TOTAL PHOSPHOROUS (LBS/DAY)
TS* TOTAL SOLIDS (LBS/DAY)
VS1* VOLATILE SOLIDS (LBS/DAY)
pH= STANDARD UNITS
Q«0.50XI06
, TS* 47500
77
S3 -18300
VS3<830O
BOD* 13700
TP*440
^ PRIMARY AERATION SECONDARY
" CLARIFIERS ' BASINS " CLARIFIERS
§
69
83*32700
VSS- 19000
BOD* 16200 OAC
Tf*« tJj*A l»rlO
'77
Q- 139000
TS * 64100
VS* 34400
pH* 8.8
PRIMARY
DIGESTERS
-co
Q*IO900O
TS*74700
VS* 39800
pH * 6.7
1
SUPERNATANT )
l**^^^^^^^^^^^l>*— ^"•^^^^•'^••"^•••^•^^•^^^^•^••••'i*"*'i-***iB««"M*J"J"J"J"J>"J"J"«*JBJ**M"J"*H*J-J*M"|
'77
83 * 770
VSS « 830
BOD* 1070
TP*60
TO
, SAND _ CHLORINATION
FILTERS " AND
DISCHARGE
•
69
33=4200
VSS* 2400
BOD* 1 700
TP- N/A
1 • ™ H/«
'77
Q- 45000
TS* 29400
VS> 13500
pH* N/A TQ
SECONDARY VACUUM FILTERS
DIGESTERS , AND
K<* INCINERATOR
0*48000
TS«320OO
VS* I6OOO
pH* 7.3
77
0=92000
FS* 20800
/S»9900
>H* 6.9
'69
0=69000
TS* 37600
VS* 18400
pH* 7.2
Figure C-30. Pontlac, Michigan, hydraulic and materials balance
-------
Significant decreases are apparent in the relative amounts
of solids returned in the digester supernatant to the head of the
primary clarifiers. A decrease in the quantities of solids con-
tained in the waste activated sludge returned to the head of the
primary clarifiers is also assured, since there were significant
decreases in BOD loadings on the secondary treatment system.
Overall, Table C-37 shows that sludge removed from the primary
clarifiers contains significantly greater mass of solids, since
chemical addition for phosphorus removal began in terms of the
kilograms of TS per kilogram of SS in the plant influent.
Secondary Clarifiers--
Sludge is removed from the bottoms of the secondary clari-
fiers on a continuous basis at both the East Boulevard and Auburn
plants. The majority of sludge withdrawn from the bottoms of
these clarifiers is pumped as return activated sludge to the head
of the aeration basins. A smaller portion is pumped as waste
activated sludge to the head of the primary clarifiers to maintain
the desired average mixed liquor suspended solids concentration.
Since the initiation of chemical addition for phosphorus removal,
there has been a considerable decrease in the volume of waste
activated sludge per million gallons of plant influent.
Detailed Description of Sludge Treatment and Disposal
Operations
Digesters--
Table C-37 describes the raw and digested sludge and super-
natant characteristics befdre and after the initiation of
chemical addition for phosphorus removal. During the period with
dual point ferric addition, an increased volume of raw sludge
at a lower average solids concentration was sent to the primary
digesters. This led to a net increase in the mass of sludge
TS fed to the digester per pound of SS in the plant influent.
Most of the additional sludge TS fed to the digesters were
volatile (fixed) solids.
Digester supernatant returned from the secondary digesters
to the head of the primary clarifiers also followed the trend
of increased volume at considerably lower solids concentrations.
The net result was a significant decrease in the mass of super-
natant TS per pound SS in the plant influent. These changes
are indicative of improved digesting sludge settleability.
Lastly, the volume and solids concentrations of the
digested sludge removed from the secondary digester were observed
to have decreased. However, there were large increases in the
mass of digested sludge TS per kilogram of SS in the plant
inf1uent.
319
-------
TABLE C-37. RAW AND DIGESTED SLUDGE AND SUPERNATANT
CHARACTERISTICS, PONTIAC, MICHIGAN
Raw Sludge
m3/day (gpd)
% TS
% Volatile
pH
kg TS/kg SS influent to plant
kg VS/kg VSS influent to plant
*
kg FS/kg FSS influent to plant
Supernatant
m /day (gpd)
% TS
% Volatile
pH
kg TS/kg SS influent to plant
kg VS/kg VSS influent to plant
kg FS/kg FSS influent to plant
Digested Sludge
m /day (gpd)
% TS
% Volatile
pH
kg TS/kg SS influent to plant
kg VS/kg VSS influent to plant
kg FS/kg FSS influent to plant
*FS = Nonvolatile (fixed) solids
FSS = Nonvolatile (fixed) suspenc
Jan. -June 1969
413 (109,000)
8.2
53
6.7
2.26
2.22
2.30
261 (69,000)
6.5
49
7.2
1.14
1.03
1.19
182 (48,000)
8.7
47
7.3
0.97
0.83
1 .26
Jed solids
•Ian. -June 1977
526 (139,000)
5.5
54
6.8
2.65
2.42
2.97
348 (92,000)
2.7
48
6.9
0.86
0.70
1.09
170 (45,000)
7.8
46
N/A
1.32
0.95
1.59
320
-------
Overall, the performance of the digesters improved signifi-
cantly after the initiation of chemical addition for phosphorus
removal. Digesting sludge settleabi1ity improved, as evidenced
by major decreases in the mass of digester supernatant solids
returned to the head of the primary clarifiers. Furthermore,
the destruction of volatile solids influent to the digesters
increased from 16 percent to 33 percent when the two periods
are compared. A secondary impact of the increased volatile
destruction was an increase in digester gas production from
l,9fin no to 2,240 m3/day (70,000 up to 80,000 ft3/day).
Vacuum Filters--
Since the initiation of chemical addition for phosphorus
removal, the vacuum filters have been operated 24 hr/day for an
average of 13.8 days/mo, as opposed to 19.5 days/mo prior to
the initiation of phosphorus removal. Table C-38 summarizes
average vacuum filter performance for the two periods. During
the period prior to the initiation of phosphorus removal, the
vacuum filters were operated between 4 and 5 days/mo. During the
period with dual point addition of ferric chloride to the waste-
water, the vacuum filters were operated approximately 3 days/wk.
Since the initiation of chemical addition for phosphorus
removal, there have been only minor changes in the dosage rates
of conditioning chemicals added to the sludge prior to vacuum
filtration. However, the filter yield rate has decreased from
19.8 to 17.8 g TS/hr (4.05 to 3.65 Ib TS/ft2/hr). In addition,
the VS fraction of filter cake TS decreased from 33.5 to 30.5
percent of TS. Overall, however, there has been an increase
in the capture of sludge solids in the filter cake. This has
resulted in over a 40 percent decrease in the mass of filtrate
solids returned to the head of the primary clarifiers.
Incinerator--
The multiple hearth incinerator is operated continuously
during vacuum filter operations. Thus, the incinerator is oper-
ated 24 hr/day, 3 days/wk. Twelve additional hours are required
to bring the furnace up to combustion temperatures, and 8 hours
are required for cool-down. Table C-39 summarizes incinerator
performance data for the two periods listed. It should be noted
that the second period describes incinerator performance after
the 1974 incinerator expansion was completed.
321
-------
TABLE C-38. VACUUM FILTER PERFORMANCE, PQNTIAC, MICHIGAN
January - June 1969
CO
P
m (galj sludge fed
% TS of sludge
kg (lb) sludge solids fed
kg lime per kg sludge dry solids
kg ferric chloride/kg sludge dry solids
Filter yield (kg sludge TS/m2 filter
i u. „ .» / U u ~ £ nnxtMi*.-:,*.. / 1 U TC/.C4-'-/U.A\\
Average
Per Day
of Filter.
Operation
299 (79,000)
8.2
24,500 (54,100)
0.143
0.029
in n / jt nr \
Average
Per Day
of Plant
Operation
191 (50,500)
8.2
15,700 (34,500)
0.143
0.029
i rt n i » r\t?\
January - June 1977
Average
Per Day
of Filter
Operation
375 (99,000)
7.3
27,400 (60,300)
0.156
0.025
•IT rt /*» /• i- \
Average
Per Day
of Plant
Operation
170 (45,000)
7.3
12,300 (27,000)
0.156
0.025
"i"t r* /*"& r* r \
kg (Ib) wet filter cake
% TS of filter cake
% VS of filter cake
kg (lb) filter cake dry solids
kg (lb) filter cake VS
kg (lb) filtrate total solids
% capture of sludge and conditioner
solids
74,500 (164,000)
33.3
37.7
24,800 (54,600)
9,310 (20,500)
3,950 (8,700)
85
47,700 (105,000)
33.3
37.7
15,900 (35,000)
5,990 (13,200)
2,540 (5,600)
85
94,900 (209,000)
30.5
39..0
28,900 (63,700)
11 ,300 (24,900)
3,670 (7,200)
89
43,100 (95,000)
30.5
39.0
13,200 (29,000)
5,130 (11,300)
1 ,500 (3,300)
89
-------
TABLE C-39. INCINERATOR PERFORMANCE,
PONTIAC, MICHIGAN
Inci nerator
Hours of operation
kg (Ib) wet cake/h
kg (Ib) cake VS/hr
m3 (thousand ft3)
m3 natural gas/kg
(ft3/lb wet cake)
m3 natural gas/kg
(ft3/lb VS)
Feed
per
r of
of
natu
wet
cake
month
operation
operation
ral gas/mo
cake
VS
Jan.
June
368
3,860 (
499 (
30.5 (
0.02 (
0.17 (
1
8
1
1
0
2
to
969
,500)
,100)
,090)
.35)
.70)
April
June 1
348
3,720 (
454 (
34.2 (
0.03 (
0.22 (
to
977
8,
1,
1,
0.
3.
200)
000)
220)
43)
55)
Since the initiation of chemical addition for phosphorus
al , there has been a decrease in the mass of wet cake fe
remova
the incinerator
decrease in the
to
mass of wet cake fed
per hour of operation. There has also been a
mass of volatile solids fed to the incinerator per
hour of operation. As a result of these factors and incinerator
capacity expansion, there has been an increase in the auxiliary
fuel requirements per pound of wet cake and per pound of cake
volatile solids fed to the incinerator.
Summary and Conclusions
There have been many changes in the Pontiac plant's waste-
water and sludge handling operations as a result of chemical
additions for phosphorus removal, changes in plant influent
characteristics, and plant equipment additions and modifications.
These impacts are highlighted below, for each of the unit
operations listed:
Primary Clarifiers:
• Improved SS and BOD removal efficiencies
t Decreased solids loading on the primary clarifiers
from the return of digester supernatant, waste-
activated sludge, and vacuum filter filtrate
• Increased mass of primary sludge TS generated per
kilogram SS influent to plant; additional sludge
solids mainly nonvolatile.
323
-------
Digesters:
t Increased volume of sludge pumped to digesters
at lower average solids concentration, with net
effect being an increase in the mass of primary
sludge TS fed to the digesters per kilogram
of SS in the plant influent
• Increased volume of supernatant, but at a
considerably lower solids concentration,
indicative of better sludge settleabi1ity with-
in the digesters
• Increased volatile destruction occurring within
the digesters
• Increased digester gas production.
Vacuum Filters:
t Decreased filter yield
§ Decreased filter cake total solids and
volatile solids concentrations
• Increased capture of sludge solids in the cake
t Decreased solids in filtrate returned to head
of primary clarifiers.
Incinerator:
t Decreased feed rates in terms of kilograms wet
cake and kilograms volatile solids per hour of
operation
§ Increased fuel consumption per hour of operation,
per kilogram wet cake feed and per kilogram
volatile solids fed.
324
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
. REPORT NO.
EPA-600/2-79-083
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
5. REPORT DATE
REVIEW OF TECHNIQUES FOR TREATMENT AND DISPOSAL
OF PHOSPHORUS-LADEN CHEMICAL SLUDGES
August 1979 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Curtis J. Schmidt, LeAnne E. Hammer, and
Michael D. Swayne
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
SCS Engineers
4014 Long Beach Boulevard
Long Beach, California 90807
10. PROGRAM ELEMENT NO.
1BC821QBC611B), SOS
11. CONTRACT/GRANT NO.
68-03-2432
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory—Cin.,OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: R. V. Villiers (513) 684-7664
16. ABSTRACT
This report summarizes the effects of phosphorus removal by chemical addition on
sludge handling and disposal options at full-scale wastewater treatment plants.
American and Canadian plants which generate phosphorus-laden chemical sludges were
surveyed by questionnaire, and 174 responses were received. Investigations at
selected plants that were using a variety of phosphorus removal chemicals, points of
chemical addition, and sludge treatment/disposal methods were conducted. The plant
operating experiences have shown that all of the various sludge treatment unit
processes for thickening, stabilization, conditioning, dewatering, and reduction are
adversely affected by phosphorus removal. The adverse effects result from both
increases in sludge quantity and changes in sludge characteristics. The adverse
impacts are reduced when adequate capacity is available to handle the increased sludge
quantity. However, many plants have inadequate capacity, and therefore have been
forced to find innovative solutions to problems. This report documents such problem-
solving attempts and the results achieved. It also compares the various sludge
handling alternatives, and finds that relatively few problems have been encountered
with pressure filtration of iron sludges, flotation thickening of iron and aluminum
sludges, thermal conditioning of iron sludges, and land disposal of lime sludges. The
report contains a bibliography of literature dealing with phosphorus-laden sludges,
with an indication as to the scope of each reference.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Group
Chemical removal (sewage treatment)
Sludge digestion
Sludge drying
Sludge disposal
Phosphorus-laden
chemical sludges
Phosphorus removal
(sewage treatment)
Sludge treatment and
disposal
13B
8. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
• • • • ^*. • ft ** f* V VH V ^ M ^ '
SECURITY CLASS I.
UNCLASSIFIED
21. NO. OF PAGES
343
20. SECURITY CLASS (This page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (Rev. 4-77)
325
« U.S. GOVERNMENT PRINTING OFFICE: 1979 -657-060/5460
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