&EPA
United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
EPA-600/2-78-073
June 1978
Research and Development
Effects of Thermal
Treatment of Sludge
on Municipal
Wastewater
Treatment Costs
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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. Eliminalion of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are: i
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research I
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 fo 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 pojnt 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-600/2-78-073
June 1978
EFFECTS OF THERMAL TREATMENT OF SLUDGE
ON MUNICIPAL WASTEWATER TREATMENT COSTS
by
Lewis J. Ewing, Jr.,
Howard H. Almgren,
Russell L. Gulp
Culp/Wesner/Culp-Clean Water Consultants
El Dorado Hills, California 95630
Contract No. 68-03-2186
Project Officer
Francis L. Evans, III
Task Officer
R. V. Villiers
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 publica-
tion. 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 commerci&l products constitute endorsement or
recommendation for use.
IX
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our national environment.
The complexity of that environment and the interplay between its components
require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solu-
tion and it involves defining the problem, measuring its impact, and search-
ing for solutions. The Municipal Environmental Research Laboratory develops
new and improved technology and systems for the prevention, treatment, and
management of wastewater and solid and hazardous 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 presents data from which construction costs and operating
and maintenance requirements may be estimated for thermal treatment of
municipal wastewater sludge. The use of the information contained in this
report facilitates making cost analyses for alternative solutions to pro-
posed projects.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
iii
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ABSTRACT
Data for estimating average construction costs and operation and main-
tenance requirements are presented for (1) thermal treatment of municipal
wastewater sludges (2) handling, treatment, and disposal of the strong
liquor generated, and (3) controlling odors produced. Size ranges covered
are treatment plants of 1 to 100 mgd, and sludge handling facilities of 1
to 100 tons per day. Estimating .data are included for many separate pro-
cess functions associated with thermal treatment of sludge, processing of
the sidestreams, and control of odors produced. Where possible, cost com-
ponents are presented in a manner that will allow adjustment to fluctuating
costs for labor, materials, and energy.
The data presented provide means of estimating costs and operating and
maintenance requirements for a variety of facilities on an average basis,
but they do not supplant the need for detailed study of local conditions or
recognition of changing design requirements in preparing estimates for
specific applications.
This report was submitted in partial fulfillment of Contract Number
68-03-2186, under sponsorship of the U. S. Environmental Protection Agency.
The report covers the period August 1975 to July 1977, and work was
completed as of September 1977.
IV
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CONTENTS
Foreword . . „ iii
Abstract „ iv
Figures vi
Tables. • „ viii
Acknowledgments ......... ix
I. Introduction ..... 1
II. Conclusions and Recommendations 2
III. Task Objective 6
IV. Study Procedure. 8
V. Thermal Treatment Processes 10
VI. Thermal Treatment Process Sidestreams... 24
VII. Case Histories 32
Experience summary 45
Supernatant - filtrate recycle . • 47
Chemical analyses. ...... 48
VIII. Direct and Indirect Costs of Thermal Treatment of Sludge . . 49
Indirect costs for treating odorous off-gas 68
Summary of direct and indirect costs 77
Bibliography 85
Appendices
A. Japanese experience with heat treatment 90
B. List of metric conversions 103
C. Update of Case Histories 104
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FIGURES
Number Page
1 Porteous process 13
2 Wet air oxidation system 15
3 Barber-Colman Purtec process 17
4 Nichols heat treatment process 19
5 Zurn sludge heat treatment process 20
6 Schematic diagram for processing heat treatment liquor 30
7 Direct unit construction costs for thermal treatment 51
8 Direct construction costs for thermal treatment ........ 52
9 Annual direct fuel requirements for thermal treatment 54
10 Annual direct electrical energy requirements for thermal
treatment 55
11 Annual direct cost of fuel and electrical energy for
thermal treatment 57
12 Operating and maintenance requirements for thermal
treatment 58
13 Operating and maintenance labor costs for thermal treatment . . 60
14 Materials and supplies for thermal treatment 61
15 Incremental cost for construction of recycled liquor
treatment facilities 65
16 Incremental electrical energy requirement for recycled
liquor treatment 66
17 Incremental electrical energy cost for recycled liquor
treatment 67
VI
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FIGURES (continued)
Number Page
18 Incremental requirements for operation and maintenance
labor for recycled liquor treatment. . 69
19 Incremental cost of operation and maintenance labor for
recycled liquor treatment 70
20 Incremental cost of materials and supplies for recycled
liquor treatment 71
21 Construction cost for odor control systems 75
22 Operation and maintenance costs for odor control systems. ... 76
23 Direct and indirect costs for thermal treatment ........ 84
VI1
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TABLES
Number
1
2
3
4
5
6
A-l
A-2
A-3
A-4
Thermal Treatment Plants Cntacted
Installation and Operating Date for Plants Visited
33
34
Utility, Chemical, and Labor Requirements for Odor
Control Systems ....................... 73
Costs for Odor Control Systems
Example Calculation of Direct & Indirect Costs
Direct and Indirect Construction Cost for Thermal
Treatment (Solids Basis)
Direct and Indirect Operation and Maintenance Cost for
Thermal Treatment (Solids Basis)
Summary of Direct .and Indirect Cost for Thermal
Treatment (Solids Basis) .......
Design Data for Heat Treatment Facilities in
Projected Sites. ... ..... *•
Operational Data of Heat Treatment Plants (From
April, 1972 to March, 1973)
Comparison of Capital Costs Per Dry Ton of Solids
Between Heat Treating System and Digestion-
Dewatering System (1975 Dollars)
Comparison of Upkeep Costs and Depreciation Costs for
Heat Treating and Digestion-Dewatering Systems
(Dollars Per Dry Ton) . ,
74
79
80
81
83
92
95
100
1°2
viix
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ACKNOWLEDGMENTS
Gulp, Wesner, Gulp, Clean Water Consultants, are grateful to the
owners and operators of municipal thermal treatment plants, equipment
manufacturers, consulting engineers, and the U. S. Environmental Pro-
tection Agency for data and information necessary for the preparation of
this report.
IX
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SECTION I
INTRODUCTION
Thermal treatment of sludges produced in municipal wastewater treatment
is used.as a conditioning process. It involves heating sludge under pressure
for short periods of time with or without the addition of air or oxygen. The
process may serve a number of purposes by proper selection of temperature,
contact time, and amount of air added. Where destruction of pathogenic
organisms is required, the sludge may be pasteurized. To improve thicken-
ing or dewatering characteristics, the sludge may be heat treated or sub-
jected to low, intermediate, or high degrees of wet air oxidation. With
high oxidation, the amount of ash remaining is about the same as with con-
ventional incineration; but wet air oxidation does not require preliminary
dewatering or drying as do conventional combustion processes. Sludges con-
taining only 1 percent solids may be processed, whereas a minimum of about
15 percent solids is required for economical incineration.*
By coagulating the solids and breaking down the gel structure of sludge,
thermal treatment processes produce significant changes in the nature and
composition of the sludge. Water that was tightly bound to raw sludge
solids is released along with proteins, carbohydrates, and zoogleal slimes
forming a dark brown cooking liquor. The liquor may be separated from the
cooked solids by decanting, centrifuging, filtering, or draining on granular
beds. Under some conditions of heat treatment, the end products are almost
odorless; but under many other conditions, they may have a strong odor,
sometimes described as a scorched coffee odor.
Liquor from thermal treatment of sludge contains high concentrations of
materials that have been solubilized in the process, including BOD (bio-
chemical oxygen demand), COD (chemical oxygen demand), ammonia, and phos-
phorus. As much as 30 percent of the COD may be nonbiodegradable in a 30-
day period.
Although much is known about thermal treatment of wastewater sludges
and the results that can be attained, cost data are fragmentary and often
do not include information on the costs for handling, treating, and dis-
posing of the strong liquor or for controlling odors produced. The impact
of such costs on total treatment costs is relatively unknown. To ascertain
the full and true costs of thermal treatment of sludge, this report collects
and analyzes costs associated with the process from a number of operating
installations.
* The processing of a 1% sludge in an intermediate or high oxidation mode
can be done autothermally, whereas, incineration of 15% solids sludge
cake will require outside auxiliary fuel.
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SECTION II
CONCLUSIONS MFD RECOMMENDATIONS
1. Recommendations and observations based on field visits to 28 thermal
treatment systems include:
a. Pilot plant operation before design is desirable.
b. The auxiliary units for which back-up should be provided include
sludge feed pumps, grinders, air compressors, high pressure pumps,
boiler waterfeed pumps, and boilers.
c. Effluent grit removal from the sludge is essential.
d. The treated sludge storage tank should be equipped with decanting
facilities.
e. Digestion before heat treatment is not desirable.
f. Operating temperatures should be kept as low as possible consistent
with adequate sludge conditioning.
g. The lead operator for the thermal treatment system should have a
good mechanical aptitude and be able to do frequent, routine
preventive maintenance.
h. The need to remove scale from heat exchangers is the most frequent
reason for routine shutdowns of thermal treatment systems.
i. High chloride wastewaters (i.e., in coastal areas where seawater
intrusion may occur) require the use of especially corrosion
resistant materials in the thermal treatment system.
j. Odor problems are common and adequate control facilities should be
used. The methods most commonly used for control of odors are
high temperature incineration, adsorption on activated carbon,
and chemical scrubbing.
k. The BOD load imposed on the wastewater treatment plant by recycle
of the liquors from the thermal treatment system can constitute
20 percent of the total influent BOD load. The recycle liquors
usually impart a color to the final plant effluent and may signifi-
cantly degrade the effluent quality if the wastewater treatment
plant does not have adequate capacity to treat the recycle loads.
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2. The following cost information directly related to the thermal treatment
system was developed:
a. Capital costs for thermal treatment systems vary from about
$50,000/gpm of thermal treatment system capacity for a 10 gpm
system to $10,000/gpm for a 200 gpm system. The higher unit cost
for the smaller systems reduces the economic feasibility for small
plants.*
b. Typical fuel requirements are 900-1,000 Btu/gallon for thermal
systems not practicing air addition and 300-600 Btu/gallon with air
addition.
^
c. Average electrical consumption averaged 22 Kwh/10 gallons for
plants with air addition and 10 Kwh/10^ gallons without air addi-
tion.
d. Operation and maintenance labor constitutes a significant fraction
of overall costs, ranging from 6,000 man-hours/year for a 10 gpm
system to 20,000 man-hours/year for a 200 gpm system.
e. Costs for materials and supplies range from $5,000/year for a 10
gpm system to $20,000/year for a 200 gpm system.
3. The following cost information .related to indirect costs for handling
and treating the recycle liquors was developed (based upon increasing
the capacity of an existing activated sludge system to handle the re-
cycle load):
a. Increased capital costs primarily result from the need to increase
aeration tank volume and air supply capabilities.
b. Increased energy is required for added aeration needed to treat
the recycled liquor.
c. Increased labor is needed for maintaining and operating the added
aeration capacity and related settling and pumping systems.
4, Costs for treating the off-gas from the thermal, treatment system typi-
cally constitutes 5-10% of the total costs for thermal treatment.
Carbon adsorption is the most costly technique for odor control. Incin-
, eration is economically attractive in the smaller plants and chemical
scrubbing in the larger plants.
5.' Based upon unit costs of $7/hr for labor, $0.03/Kwh for electricity,
and $2.80/10 Btu and amortization of capital costs over 20 years at
7% interest, the following typical costs for heat treatment were
determined (all costs are dollars/ton of dry solids processed):
* See Appendix B for a list of metric conversions.
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O & M Cost
Direct Indirect Total
Total Cost
4.11
3.18
2.93
1.83
1.98
101.64
33.97
24.38
14.03
12.94
150.14
46.46
32.52
19.10
16.58
4.93
3.67
3.50
2.99
2.87
155.07
50.13
36.02
22.09
19.45
256.71
84.10
60.40
36.12
32.39
Sludge Construction Cost
Ton/Day Direct Indirect Total
1 97.53
5 30.79
10 21.45
50 12.20
100 10.96
Cost curves are presented which enable evaluation of costs where
other unit costs are appropriate.
The advantages of thermal treatment systems include:
a. Conditions sludges so that they dewater more readily and more
completely than when conditioned by other means. This provides a
reduction in dewatering and incineration costs and may enable
operation of a conditioriing-dewatering-incineration system without
the need for supplemental fuel.
b. Can provide pasteurization of the sludge rendering it free of
pathogenic organisms.
c. Can provide oxidation of sludges without the preliminary dewater-
ing required for incineration and with substantially reduced air
pollution potential.
d. Can provide reduction of sludges containing toxic materials which
would render biological ;stabilization processes ineffective.
The disadvantages of thermal treatment systems include:
a. Greater mechanical complexity than most systems that municipal
treatment plant operators are familiar with. Operators are often
reluctant to work on or around the high temperature-pressure
equipment.
b. Requires substantial operator attention and frequent maintenance.
Many of the systems visited reported substantial downtime due to
maintenance problems.
c. Produces odorous off-gas that must be collected and deodorized
before release.
d. Transforms insoluble organic substances in the sludge into soluble
materials which appear in a high strength cooking liquor. Recycle
of this liquor to the wastewater treatment plant can degrade
effluent quality if the plant does not have adequate capacity to
handle the recycle load. Recycled liquor may also result in re-
fractory organics appearing in the effluent causing color and
increased COD levels.
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e.
At small scale the comparatively high cost of support equipment
makes heat treatment facilities more costly to build than other
sludge treatment plants. Operations are expensive because of the
constant attention required by a skilled operator. Also, the
necessary operational skill might not be available to a small
plant. •
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SECTION III
TASK OBJECTIVE
The general objective of this study has been the procurement of organ-
ized information pertaining to the impact that thermal conditioning of sludge
has on municipal wastewater treatment plant costs. The impact of costs
associated with the handling and disposing of thermal process supernatant has
been determined as well as the capital, operating, and maintenance costs
associated with the thermal process. In some cases, where actual operating
costs could not be obtained, engineering estimates have been made.
The objective of this task has not been to make a sanitary engineering
appraisal of the advantages or disadvantages or any particular thermal treat-
ment process for conditioning sludge, but was intended to be an independent
survey of all the costs associated with the various processes commercially
available for thermally conditioning those sludges normally generated during
the treatment of municipal wastewater.
In pursuing the general task objective, there were a number of detailed
objectives to be accomplished. Capital, operating, and maintenance cost
information on the most frequently used thermal process designs in the United
States were obtained first hand by surveys, plant visits, correspondence, and
telephone contact. Since there is little information on the costs associated
with handling and disposing of liquid and gaseous side streams from thermal
conditioning processes, particular emphasis has been placed on this area.
Cost data have been based preferentially on actual plant operating exper-
ience. Published cost data have been used only when it has been verified to
represent facts accurately. In those instances where cost data were not
available from construction or operation experience, engineering estimates
have been made based on sound fundamentals. Where possible, costs have been
developed for different-sized, similar type thermal systems to provide
insight into scale-up economics. -Information has been developed on the costs
for handling the strong liquors produced by thermal treatment of sludge
including: (1) direct recycle to the main plant, and (2) pretreatment prior
to plant recycle. The costs for odor control of thermal process off-gases
have been ascertained. The cost effects of heat treatment liquor recycle on
plant effluent quality have been determined.
It was intended that plants practicing phosphorus removal by chemical
addition be studied so that the impact of phosphorus-laden chemical sludges
on thermal process costs could be analyzed and so that any impact of
phosphorus-chemical laden thermal supernatant on the treatment plant could
be evaluated. Very few plants practicing both phosphorus removal and thermal
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treatment were found and the data available from those plants were not suffi-
cient to permit detailed analyses.
The temperature, pressure, and detention time of the thermal process
affect several items of cost including: (1) operation and maintenance of the
thermal process, (2) the size of the load recycled to the main plant in the
strong liquor, and (3) the dewatering characteristics of the heat treated
sludge.
The cost of air addition in thermal treatment has been investigated. In
addition to the direct cost of compressing the air, there are other indirect
considerations such as: (1) differences in heat transfer efficiencies,
(2) materials of construction which must be used to resist increased corro-
sion tendencies in the presence of oxygen, and (3) differences in odor
production, type of sludge, and strength of strong liquor recycled.
Because virtually all of the equipment for thermal treatment of sludge
in the United States has been supplied in the past by Zimpro or BSP (Enviro-
tech), this study deals almost exclusively with the processes and equipment
used by these two manufacturers.
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SECTION IV
STUDY PROCEDURE
LITERATURE REVIEW
A search was made of published information on thermal treatment of
wastewater sludge and associated costs. Bibliographies were prepared on
thermal treatment and on odor control. They appear at the end of this
report.
CORRESPONDENCE
A list of thermal treatment installations was compiled from information
supplied by equipment manufacturers. Letters requesting operating informa-
tion were sent to all 43 cities and sewer districts which had been operating
thermal plants for one year or more. Data were obtained in response to these
requests from about 10 sources. Follow up letters to 20 of the original
correspondents who did not answer [produced only one reply. On the other
hand, good response was obtained from follow-up letters seeking missing in-
formation from those who answered 'the original request for data.
FIELD VISITS !
The best method for collecting the data needed in this task proved to be
field visits to operating plants following advance arrangements with the
city or sewering agency to do so. ' This method was by far the most produc-
tive. Although complete information on costs is just not available in some
cases, a field visit makes it possible to determine this definitely, as well
as to collect all of the data that are available and to view the installation
first hand. First hand observation of plant operations yields information
and understanding which cannot be gained in any other way.
A tabulation of the 28 plants visited and the type of thermal treatment
equipment installed at each of them is given in Section VI of this report.
Visits were also made to the offices and manufacturing plant of Zimpro
at Rothschild, Wisconsin and the offices of BSP at Menlo Park, Ccilifornia to
gather background data and to learn which of their installations might yield
the best cost records.
TELEPHONE CONTACTS
The telephone was used to make final arrangements for field visits and
to fill information gaps in written submittals or in field data. It was also
8
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used to verify questionable data.
DATA ANALYSIS
The cost data collected in this survey were in terms of various calendar
dates and from different geographical locations. By methods described in
more detail later, these costs were transformed into national average costs
as of March, 1975. This facilitates cost comparisons and makes the cost
figures more useful in development of new projects. Wherever possible, labor
costs for operation and maintenance have been expressed in terms of man-hours
per year as well as in March, 1975 dollars. Graphical representations have
been used to show dollar costs and man-hours of effort for various aspects
of construction, operation, and maintenance of thermal systems. The graphs
included in this report are shown in the List of Figures. Discussions which
appear later describe in more detail the assumptions and methods used to
develop individual graphs or curves.
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SECTION V
THERMAL TREATMENT PROCESSES
GENERAL
As mentioned in the Introduction, sludges resulting from the treatment
of municipal wastewaters may be subjected to high temperatures under pressure
for any of several purposes such as destruction of pathogenic organisms,
improvement of sludge dewatering, 6r, with air addition, partial or complete
oxidation. Thermal treatment may also be used to process wastes containing
toxic materials which prevent the use of biological methods.
Pasteurization ',
Heat treatment is a well known and effective method for destroying
pathogenic organisms, and it has been applied successfully for disinfecting
sludge. Pasteurization at 70°C (159°F) for 30 to 60 minutes will destroy
pathogens in digested sludge. Under normal conditions of temperature and
retention time used in low pressure oxidation, all pathogenic organisms are
destroyed. Time and temperature are inversely related in heat sterilization;
the higher the temperature, the shorter the required exposure time. For
example, at 120°C (249°F) in a laboratory autoclave (compressed steam) it re-
quires only 6 minutes to kill even !very resistant spores.
An advantage of a sterile, non-infectious residue is that it can be used
on land or disposed of without biological hazards to human health.
Sludge Conditioning
If the goal of thermal treatment goes beyond elimination of pathogens to
conditioning of sludge for dewatering and reducing or eliminating chemical
requirements, this can be done by increasing temperatures. To maintain
reaction temperatures higher than the minimums used in Pasteurization it is
necessary also to increase operating pressures in order to prevent flashing
of the water to steam or burning.
The EPA Technology Transfer "Process Design Manual for Sludge Treatment
and Disposal," (October 1974) includes the following discussion of sludge
conditioning by heat treatment which is appropriate for quotation here:
"In heat treatment, temperatures of from 300 to 500°F and pressures
of 150 to 400 psig are attained for protracted periods. Significant
changes in the nature and composition of wastewater sludges result. The
effect of heat treatment has been ideally likened to syneresis, or the
10
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breakdown of a gel into water and residual solids. Wastewater sludges
are essentially cellular material. These cells contain intracellular
gel and extracellular zoogleal slime with equal amounts of carbohydrate
and protein. Heat treatment breaks open the cells and releases mainly.
proteinaceous protoplasm. It also breaks down the protein and zoogleal
slime, producing a dark brown liquor consisting of soluble polypeptides,
ammonia nitrogen, volatile acids, and carbohydrates. The solid material
left behind is mineral matter and cell wall debris."
"Dewaterability is improved by the solubilizing and hydrolyzing of
the smaller and more highly hydrated sludge particles which then end up
in the cooking liquor. While analysis of this liquor from domestic
wastewater sludges indicates the breakdown products are mostly organic
acids, sugars, polysaccharides, amino acids, ammonia, etc., the exact
composition of the liquor is not well defined."
"A review of reported analyses of liquor from the heat treatment of
sludge gives the range of values shown: BOD5 = 5,000 to 15,000 mg/1,
COD = 10,000 to 30,000 mg/1, Ammonia = 500 to 700 mg/1, and Phosphorus
as P = 150 to 200 mg/1. About 20 to 30 percent of the COD is not bio-
degradable in a 30-day period. The volume of cooking liquor from an
activated sludge plant with heat treatment amounts to 0.75 to 1.0 per-
cent of the wastewater flow. Based on BOD5 and solids loadings, the
liquor can represent 30 to 50 percent of the loading to the aeration
system. The pH of cooking liquors is normally in the range of 4 to 5,
which necessitates chemical neutralization and/or corrosion resistant
equipment."
"Plant experiences have shown that the conditioning requirements
and hence the performance achieved in thickening and dewatering pro-
cesses are affected by the manner in which sludge is treated."
The same degree of improvement in filterability and settleability of
activated sludge, humus, digested sludge, and mixtures of activated and pri-
mary sludges can, within limits, be accomplished by various combinations of
time and temperature. Long-time and low temperature treatment is usually
the most economical. For a given holding time, lower temperatures are re-
quired to reduce the specific resistance for filtration of humus and digested
sludge than for activated sludge.
"Over-cooking" can lead to an increase in specific resistance due to
breakdown of fibrous material which would otherwise aid filtration. Crude
fiber degradation is nearly linear with increase in temperature and oxida-
tion. Both specific resistance and the quality of heat treatment liquor can
be accurately estimated from laboratory batch test results according to
comparisons which have been made of the performance of full-scale heat treat-
ment plants with predictions based on laboratory tests. The pH at which
sludges are heat treated has an effect on the resulting specific resistance
to filtration. Low pH values are much more effective, but corrosion problems
are increased. Cooling of heat conditioned sludges prior to atmospheric
exposure can reduce odor problems. Increases in the solids content of heat
11
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treatment process feed reduces the dewaterability of the conditioned sludge
and proportionally increases the liquor's content of dissolved COD, nitrogen,
and phosphorus. About 6,000 mg/1 pf COD is produced for every 1% feed solids
present. Increased heat treatment temperature increases soluble nitrogen and
decreases the suspended solids of recycle liquor.
A schematic diagram for a typical Porteous process is given in Figure 1.
In this process heat is applied to elevate the sludge temperature to 350° to
400°P, and the pressure is raised to 150 to 300 psig. Steam is generally
injected into the sludge, and this is followed by a sludge/water/sludge heat
exchange system as shown in the diagram. This type of arrangement is known
as liquid-coupled heat exchange. It is used because of the difficulties en-
countered with plugging when sludge was used in the annular pipe of a double-
pipe heat exchanger. Air injection is not normally practiced. Basic
components of this system include isludge storage, grinding, a preheater, high
pressure and temperature reactor, [decant thickener, auxiliary liquid treat-
ment, off gas deodorizer, and a steam boiler.
Wet Air Oxidation
The basis of the wet air oxidation process, which is also referred to as
wet incineration or wet combustion, is that any substance which can be burned
can be oxidized in the presence of oxygen and liquid water at temperatures of
250° to 1,650°F. The process can operate on difficult to dewater waste
liquors and sludges where the solids are but a few percent of the water
streams. In general with the prop;er temperature, pressure, reaction time,
and sufficient compressed air or oxygen, any degree of oxidation desired can
be accomplished. By operating at Slower temperatures and pressures, the same
approach may be used for sludge conditioning.
The wet air oxidation process has been commercialized and patented as
the Zimpro process. Wet air oxidation does not require preliminary dewater-
ing or drying as required by conventional dry combustion processes. Water
can be present up to 99 percent in this process whereas in conventional
combustion it must be reduced to much lower levels to make incineration
practical.
A significant feature of wet oxidation is the flameless oxidation of the
organics at temperatures of 300°F to 400°F for low oxidation and about 700°F
for high oxidation when compared to 1,500°F to 2,700°F in conventional com-
bustion processes. Air pollution is minimized because the oxidation takes
place in water at low temperatures and no flyash, dust, sulfur dioxide, or
nitrogen oxides are formed.
The terms used to categorize the degree of wet oxidation - low oxidation,
intermediate oxidation, and high oxidation - refer to the degree of reduction
in the COD of the sludge. Higher temperatures are required to effect higher
degrees of oxidation, and the higher temperatures, in turn, require the use
of correspondingly higher pressures in order to prevent flashing to steam or
burning.
Thermal conditioning can be accomplished without oxidation.
12
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SLUDGE (\
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DISPOSAL
COLD RAW SLUDGE
HOT RAW SLUDGE
HOT TREATED SLUDGE
COLD TREATED SLUDGE
THICKENED SLUDGE
STEAM
RECIRCULATED COOLING WATER
Figure.1. Porteous process.
13
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Low oxidation, even as low as 5%, oxidizes sulfur and odor-producing
organics (which are highly volatile).
The effects of increased holding time in the thermal reactor are to in-
crease the solubilization of COD and color and to degrade fibrous material
Which would otherwise aid vacuum filtration of the conditioned sludge.
The operating temperature and pressure ranges for the three oxidation
categories are given below:
Oxidation
Low
Intermediate
High
Reduction In
Sludge COD,
Percent
5
40
92-98
Temp. F'
350-400
450
675
Pressure, psi
300-500
750
1,650
With high oxidation the amount of sludge ash is about the same as with
incineration.
The general flow diagram of the Zimpro continuous wet air oxidation
system is shown in Figure 2. The principal differences between-the Zimpro
process and the Porteous process are that in the Zimpro process air is added
for oxidation, for improvement of heat exchange characteristics, and for
reduction of fuel requirements, and that a sludge-to-sludge heat exchanger is
employed. In the continuous process, the sludge is passed through a grinder
which reduces the size of sludge particles to about 1/4 inch. Sludge and air
are then pumped into the system and the mixture is passed through heat ex-
changers and brought to the initiating reaction temperature. As oxidation
takes place in the reactor, the temperature increases. The oxidized products
leaving the reactor are cooled in the heat exchangers against the entering
cold sludge and air. The gases are separated from the liquid carrying the
residual oxidized solids and released through a pressure control valve to a
catalytic oxidation unit for odor Icontrol. Where economic conditions make
it attractive the gases may be expanded in power recovery equipment before
being discharged. The oxidized liquid and remaining suspended solids are
released through a level control valve and the solids may be separated by
settling and drainage in decant tanks, lagoons or sand beds, or by other
methods such as vacuum filtration or centrifugation.
For start-up, Beat is obtained from an outside source, usually a small
steam generator. With high degree oxidations and high-fuel-value sludges,
no external heat is needed once the process is started. Whenever the pro-
cess is not thermally self-sustaining, steam may be injected continuously to
maintain the reaction temperature.
Four important parameters control the performance of wet oxidation
units: temperature, air supply, pressure, and feed solids concentration.
The degree and rate of sludge solids oxidation are significantly influenced
by the reactor temperature. Much higher degrees of oxidation and shorter
reaction times are possible with increased temperatures. As is the case
with conventional incinerators, an external supply of oxygen (air) is
14
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RAW
SLUDGE
SLUDGE
TANK
STORAGE
GRINDER
AIR .
AIR
COMPRESSOR
HEAT
EXCHANGER
^T „ n
i — " — vj ^
SLUDGE
PUMP ,
— •* a.
f-+Z.
f^
^*\
i
—)
REACTOR
SOLIDS
SEPARATION
SUPERNATANT
(SETTLING
FILTRATION OR
CENTRIFUGATION)
STEAM
GENERATOR
TREATED
SLUDGE
ODOR
CONTROL
SYSTEM
EXHAUST
GAS
SEPARATOR
Figure 2. Zimpro wet air oxidation system.
15
-------
required to attain nearly complete oxidation. The air requirement for the
wet oxidation process is determined by the heat value of the sludge being
oxidized, and by the degree of oxidation accomplished. Thermal efficiency
and process economy are functions |of air input, so it is important that the
optimum amount be determined. Because the input air becomes saturated with
steam from contact with the liquid in the reactor, it is important to control
the air also to prevent excessive Evaporation of the water. For primary
wastewater sludges with a BTU value of 7,800 BTU/lb, an air utilization of
5.75 Ib/lb is typical. For an activated sludge with a heat value of 6,540
BTU/lb, an air utilization of 5.14 Ib/lb is typical.
The feed solids concentration has a significant effect on operating
costs. If the solids concentratipn is increased from 3 percent to 6 percent,
•the operating costs may be reduced by as much as 40 percent.
EQUIPMENT
Equipment for thermal conditioning of sludge is manufactured and suppli-
ed in the U.S. by BSP (Porteous system), Zimpro (wet oxidation), Barber-
Colman (Purtec Wetox system), Zurri (sludge heat treat process), and Nichols
(heat treatment process).
Zimpro
In the preceding section, the Zimpro process is described and is illus-
trated by Figure 2. The majority of operating installations in this country
are of Zimpro manufacture. They are for the most part low oxidation plants,
but there are also intermediate ozidation, high oxidation, and heat treat-
ment installations of Zimpro equipment.
BSP (Porteous)
The BSP-Porteous system is described in the previous section and is
illustrated by Figure 1. There are a number of operating plants in the U.S.
utilizing BSP heat treatment equipment.
Barber-Colman
Figure 3 presents a flow sheet for the Barber-Colman Puretec Process.
The continuous process begins with the maceration of the incoming sludge
which is then pumped through a liquid phase and vapor phase tube-in-tube
heat exchanger. Thermal energy from the reactor effluent is recovered and
heats the incoming material. The (material is then introduced into the Wetox
Reactor where it is oxidized at 600 psi at 450° - 465°F temperature. Once
'the reactor is preheated the process becomes self-sustaining due to the
highly efficient destruction of organic waste liberating sufficient energy
to maintain temperature.
The organic waste introduced into first compartment is violently agi-
tated to insure rapid reaction, and cascades to subsequent compartments to
complete the oxidation process. The incoming material blends with the much
16
-------
W
03
OJ
O
s
o
3
S
td
ffl
ro
0)
17
-------
larger agitating mass already present in the reactor. In the final compart-
ment, liquid and vapor phases are separated and conducted separately to heat
exchangers for thermal energy exchange. After cooling, the vapor phase con-
densate is let down to atmospheric pressure as is the liquid phase. The
flow chart shows the use of lime as a neutralizing agent. This process re-
duces the acidic effluents to a slightly alkaline solution and precipitates
the heavy metals. The lime neutralizing agent is recovered and recycled for
re-use.
There are no operating municipal installations in the U.S.
Nichols Heat Treatment Process
This process (previously marketed as the Dorr-Oliver Farrer System) of
continuous .heat treatment is shown in Figure 4.
The heat treatment process is comprised of a two stage heat exchanger
followed by an economizer. The economizer section of the heat exchanger pre-
heats incoming sludge to approximately 300°F by controlling the outgoing
treated sludge temperature at 85°F. A booster heat exchanger raises the pre-
heated sludge to the desired final temperature of 360° - 380°F by means of
heat from the boiler. The heated jsludge then flows through a specially
designed reactor, which eliminates short circuiting and insures the desired
retention time for full sludge conditioning.
The boiler for the booster heat exchanger can be fired by any of the
conventional fuels including'digester gas. However, when used in conjunction
with incineration, a waste heat boiler can be utilized for additional overall
economy in operating costs. A central, automated control panel with full
instrumentation minimize operation attention. Following heat treatment the
sludge moves to a continuous flow decanting tank for separation.
The Nichols process is used at a plant serving York, Pa. .and five in-
stallations of the Farrer system were made in the U.S. (San Bermidino, Calif.;
Elkhart, Ind.; Port Huron, Mich.; Glouster, N.J.; Norwalk, Conn.).
Zurn '
The Zurn Heat Treat Process is shown diagrammatically in Figure 5. The
incoming sludge is ground, pumped at 250 psi through a heat exchcinger, heated
to 380°F, held in a reactor for 45 minutes, cooled in a heat exchanger, held
in a thickening tank, and dewatered on a vacuum filter. The vacuum filtrate
and the thickener tank overflow are recycled through the wastewater treatment
plant. :
There is a Zurn system installed at Mentor, Ohio which serves an area
in Lake County, Ohio.
Equipment Operating Conditions
In the Pasteurization, thermal conditioning, and wet air oxidation
sludge processing systems there are equipment service conditions much like
18
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BOILER
THICKENING
AND STORAGE
HEAT
EXCHANGER
BOOSTER
REACTOR
DEWATERING
UNIT
,
v-y
OVERFLOW RETURN TO
1
1 1
PLANT OR PRETREAT
INCINERATION
LAND SOIL
FILL CONDITIONER
Figure 4. Nichols heat treatment process.
19
-------
SLUDGE
DISINTEGRATOR
-HIGH PRESSURE
I PUMP
HEAT
EXCHANGER
REACTOR
COOLER
MAKE UP
r-WATER PUMP
FLOW CONTROL
STATION
THICKENING
TANK
FILTER
CAKE-
*•
uj
WAI
REC
PUd
V
I
•ER
CIRCULATION
/IP
j —
T-C
\
Xh
s
-sTv-
1
HIGH TEMPERATURE-
WATER BOILER
-LEVEL CONTROLLED
EXPANSION TANK
RECYCLE
SUPERNATANT
& FILTRATE
TO PLANT
-FILTRATE RECYCLED
THROUGH PLANT
Figure 5. Zurn sludge heat treatment process
20
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those in conventional wastewater treatment plants. However, much of the
equipment operates under more severe conditions which are not familiar to
most treatment plant operators and which may be uncommon to some design en-
gineers and plant supervisors.
Sludge grinders, macerators, disIntegraters, and the like operate under
the same conditions found in conventional plants. Plastic objects, gravel,
grit, and pieces of wood and metal which pass through primary screens and
other devices intended to accomplish their removal cause much difficulty in
the grinding step. Almost constant operator attention and frequent main-
tenance is required.
The temperatures and pressures used in thermal treatment are higher than
those found elsewhere in wastewater treatment plants and require special con-
sideration in plant design, in the preparation of operator training manuals
and courses of instruction, and in plant operation and maintenance.
When air is added under high pressure, the air compressor maintenance
requirements differ from those of conventional low pressure equipment. Pro-
per lubricants must be used at regular, manufacturer-prescribed time inter-
vals. Care must be taken to keep carbon or other foreign particles out of
the compressor cylinders to avoid scoring of the pistons, cylinder walls, or
shafts. Compressor piping must be designed to prevent accidental backup of
liquids into the air system and particularly into the compressors. Shaft
packing must be properly installed and maintained. Many plant operators
fear for their safety when working with or near high pressure and temperature
equipment and piping. This fear can only be overcome by furnishing full
information to the operators concerning the design and proper operation of
tiie system. High pressure pumps require special attention in operation and
maintenance.
Many of the high pressure pumps used in thermal treatment systems are
units especially designed for this service and complete data on design and
construction features as well as operation and maintenance requirements are
essential if the pumps are to be kept in continuous operation without ex-
cessive repair costs. 7
The high temperatures used create special problems of deposition, scal-
ing, and corrosion which must be taken into account in the design of the
system, the selection of materials of construction, and in operation and
maintenance. The presence of calcium, sulfates, or chlorides in the sludge
adds to the difficulties in controlling corrosion and deposition at high
temperatures. Calcium sulfate deposits can accumulate rapidly and clog
heat exchangers or reactor vessels and piping. Chlorides may be responsible
for creating special corrosion or metal stress problems. Corrosion may also
result from the presence of organic acids, oxygen, or sulfides. The use of
various stainless steels and of titanium is common to resist severe corro-
sion conditions imposed on wetted metal parts. When air is added to sludge
undergoing thermal treatment 316L SS is a minimum material of construction.
Scale and other deposits in heat exchangers or reactors are often re-
moved by washing with 5 percent nitric acid solution for from 4 to 6 hours
21
-------
up to a day. The required frequency for this acid wash may vary from a few
weeks to semi-annually depending upon the chemical composition of the sludge
being processed. In all cases special treatment or conditioning must be pro-
vided for boiler feed water.
Grit, which is present in all sludge in varying amounts, can cause mod-
erate to disastrous results in thermal treatment systems. Grit can accumu-
late at some low velocity point in the process and subsequently passes
through the following portions of the system in a batch or slug that can be
particularly damaging to grinders, pumps, or piping. Return bends in piping
are especially vulnerable to erosion from grit action. Under the worst
conditions return bends may be .eroded away completely by grit in only a few
weeks. Adequate grit removal from;the raw wastewater and from the sludge
are absolute necessities for successful operation of thermal treatment sys-
tems.
COST IMPACT OF THERMAL TREATMENT ON OTHER PLANT PROCESSES
The use of thermal treatment of sludge affects the cost of other treat-
ment plant processes, decreasing some and increasing others. The calculation
of the total cost must include direct capital, operating, and maintenance
costs for sludge handling plus or minus the indirect net cost effect of
sludge handling on other treatment plant processes.
Reductions
Improvements in the thickening and dewatering characteristics of sludge
provided by thermal treatment may reduce the size or capacity of thickening
or dewatering equipment required, with accompanying cost savings. If dewat-
ered sludge is to be hauled, then the lower sludge moisture content resulting
from thermal treatment will reduce the tonnage to be transported and the
cost for disposal. If the dewatered sludge is to be incinerated, then the
lower moisture content will reduce incinerator fuel and power consumption.
In many cases, the need for chemical additives to aid sludge dewatering is
eliminated. Thermal treatment processes also present opportunities for
energy recovery from heat values in the waste sludge for various in-plant
processes.
Increases
Thermal treatment processes transform many insoluble organic substances
In the sludge into soluble materials which appear in the cooking liquor.
The liquor overflows the decant tabk or thickener or is extracted in the
centrifuge, vacuum filter, drying bed, or other dewatering device. With a
few possible exceptions, additional costs are involved in processing the BOD
load produced in heat treatment. >
A second added cost is that for controlling odors produced in heat
treatment. Odors emanating from the treated sludge may be released to the
atmosphere from the decant tank or thickener, the vacuum pump exhaust, the
dewatering device, the strong liquor pretreatment device, or in other places.
It is necessary to collect the foul air and to deodorize it before release
22
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to the,outside air in order to avoid nuisance conditions at the plant and in
surrounding areas. Special consideration and provisions for positively con-
trolling odors are essential in every instance.
The cost impact of thermal treatment processes on other plant processes
will be discussed in greater detail later in this report, and appropriate
costs will be presented.
23
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SECTION VI
THERMAL TREATMENT PROCESS SIDESTREAMS
OFF-GAS
Sources and Character
There are five principal sources of odor in thermal sludge treatment:
(1) vapors from treated sludge storage (decant tank or thickener), (2) vac-
uum filter pump exhaust, (3) vacuum filter hood exhaust, (4) exhausted air
from working atmosphere in filter and loading hopper areas, and (5) vapors
from strong liquor pre-treatment devices. The air from sources (1), (2), and
(5) are high in hydrocarbon content, while that from sources (3) and (4) are
low in hydrocarbon, content.
The odorous gases produced arte simple, low molecular weight, volatile
organic substances, consisting of aldehydes, ketones, various sulphurous
compounds, and organic acids. Each class of compounds that is present in the
vapors contains members of its homologous series beginning with the lowest
molecular weight and continuing on to perhaps the Cj or CQ member. For ex-
ample, the aldehydes that are present begin with formaldehyde and include
acetaldehyde, propionaldehyde, etc., continuing on to caproic aldehyde. In '
the laboratory the total organic content of the exhausted process vapors can
be measured by means of the flame ionization detector. The odor level of
each uncontrolled odor source associated with thermal sludge conditioning
units is dependent to a high degree on the total hydrocarbon content.
Odor measurement and determination of odor threshold levels are best
done by a test panel using the human nose rather than by monitoring instru-
ments. If the odor threshold, as determined by the human test panel, is
reached when one volume of the odorous sample is diluted to 100 volumes with
odor free air, the odor concentration in the odorous sample is reported to be
100 odor units per cubic foot.
Collection
A foul air collection system is an essential part of any thermal treat-
ment system. Sludge decant tanks, thickeners, strong liquor pretreatment
tanks, loading hoppers, and vacuum filters should be covered or hooded so
that the odorous gases can be collected and subjected to methods for hydro-
carbon reduction before release to; the atmosphere. The vacuum filter pump
exhaust pipe should also be discharged through an odor control device.
24
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Odor Control Methods
Odor control methods include: combustion, adsorption, scrubbing, mask-
ing, dilution, and surface evaporation control. The concentrated off-gases
are best controlled by use of incineration, adsorption, scrubbing, or combin-
ations thereof. The dilute off-gases may be treated by adsorption or scrub-
bing, or, in some cases, by masking and dilution.
Water Scrubbing Plus Incineration—
For high hydrocarbon air streams, the highest degree of odor control can
be obtained by water scrubbing followed by incineration. The scrubbing por-
tion of this system consists of a packed bed unit which uses plant effluent
water at rates of 20 to 30 gpm per 1,000 cfm.
The incineration portion of this system can be either direct flame in-
cineration at 1500°F or catalytic incineration at 800°F. The oxidation
catalysts that are commonly used in catalytic incineration are supported
platinum or palladium materials. Odor levels of the exhausted air from this
system can be reduced to less than 100 O.U./SCF if the incinerator is oil
fired, and less than 25 O.U./SCF if gas fired.
The low hydrocarbon odor sources are high volume streams, and the oper-
ational cost of afterburning is prohibitive.
Water Scrubbing Plus Adsorption—
In scrubbing methods, the odorous substances are removed by solubiliza-
tion, condensation, or chemical reaction with the scrubbing medium. Scrub-
bing media that are commonly used for odor control are potassium permangan-
ate, sodium hydroxide, or sodium hypochlorite. Two to four pounds of
potassium permanganate are required per pound of hydrocarbon removed.
In the adsorption method, the odor substances are removed from the
odorous gas stream by adsorption on activated carbon or silica gel. When
the adsorption method is applied to odor control of processes, the adsorp-
tive medium, activated carbon or silica gel, must be capable of regeneration
usually by steaming.
High hydrocarbon sources can be treated in an odor control system com-
posed of a water scrubber followed by an activated carbon adsorption unit.
The water scrubber is the same as that described above. The carbon adsorp-
tion unit is a multiple bed adsorber that is sized to minimize the required
number of steam regenerations. Normally, the carbon bed would be sized so
that only one steam regeneration per day would be required. Treating a
1,000 CFM gas stream would require a dual bed carbon system containing 1,800
pounds of carbon per bed. This sizing would permit an adsorption cycle of
24 hours. After a 24 hour adsorption time, the second carbon bed would be
-• placed in the adsorption cycle and the spent bed would be steam regenerated.
The regeneration cycle requires low pressure steam at a maximum of 50 psig
for a period of one hour. The steam and desorbed organic compounds from the
bed are condensed and collected. The aqueous condensate is returned to the
head of the treatment plant and the liquid organic phase is incinerated.
This water scrubbing-carbon adsorption odor control system can produce an
25
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exhaust with an odor level ranging from 50 to 300 O.U./SCF.
A word of caution is in order when odors due to hydrogen sulfide or
mercaptans are to be adsorbed on activated carbon. These sulfur compounds
may be air oxidized to elemental sulfur which is not removable by steam
stripping and which may plug the carbon bed.
The applicability of carbon adsorption to low hydrocarbon odor sources
must be decided on a case-by-case basis.
Multiple Scrubbers—
A third option for treating high hydrocarbon sources is a multiple
scrubber system. The multiple scrubber system would contain at least two
and preferably three scrubbing stages. In all cases, the first scrubbing
stage of the multiple scrubber system should be water scrubbing using plant
effluent water at a rate of about 27 gpm per 1,000 CFM. The second and
third stages should be chemical scrubbing stages using a combination of
scrubbing'media selected from 5 percent sodium hydroxide, 3 percent sodium
hypochlorite, and 3 percent potassium permanganate. The potassium perman-
ganate solution affects the highest degree of hydrocarbon reduction and,
hence, the highest odor reduction. One of the most effective multiple
scrubber systems would consist of three stages and would utilize plant efflu-
ent water in the first stage, 5 percent sodium hydroxide in the second stage,
and 3 percent potassium permanganate in the final stage. This system would
affect a hydrocarbon reduction of 80 to 90 percent and would produce an
exhaust with an odor level of 100 to 250 O.U;/SCF.
In one plant a nitrifying trickling filter has been used as a biological
scrubber with the primary effluent serving as scrubber water.
As with carbon adsorption, the suitability of scrubbing for low hydro-
carbon sources must be decided on| an individual basis. The greatest con-
sideration in choosing between carbon adsorption and scrubbing is usually
the space requirement of the selected system. The space requirement for
the carbon adsorption system is quite large, whereas, the chemical scrubbing
system can be accomodated in lesser space.
Additives and Dilution—
When additives are used, odot control is achieved by masking or counter-
acting odorous substances. Odor counteraction implies that the intensity of
an odor is reduced as detected byj the human olfactory sense. In contrast,
odor masking implies that the odor is obscured. Actually, both terms,
masking and counteracting, are used interchangeably. Masking agents include
orange fragrance and Malabate (proprietary chemicals).
In the dilution method, the intensity of an odor is reduced by diluting
the odorous stream with odor-free; air. This dilution can be achieved by
actual addition of air to the odotous stream or by affecting dilution sub-
sequent to exhausting by utilizing tall stacks or by increasing the exhaust
gas velocity.
26
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The chemical reaction of the hydrocarbon compounds from the thermal
sludge conditioning odor sources with ozone is rather slow and often times
produces peroxides and hydroperoxides which are more offensive than the
original hydrocarbon. Hence, direct ozonation is not an effective odor con-
trol method. However, if the hydrocarbon odor sources are diluted with other
non-hydrocarbon odor sources which react with ozone, then the composite gases
might be deodorized by the ozonation method.
The ozonation method for controlling odors from thermal sludge treatment
units should be considered only when total odor control is to be provided
for the entire treatment plant. An example of this type of odor control
system is the operating system at Midland, Michigan. In this system the
gases from the sludge conditioning building, which include both high and low
hydrocarbon sources, are combined with gases from the covered trickling fil-
ters, from a flow equalizing tank, from the primary building, and from the
grit building. The gases from the sludge conditioning building comprise
one-fourth of the total 28,000 CFM air flow through the ozone contact
chamber. The composite gases in the ozone contact chamber are normally
treated with one to two ppm ozone and a detention time of 30 seconds. The
total capacity of the ozone generators is 34 pounds per day of ozone which
could produce a maximum ozone concentration of approximately seven ppm.
The use of additives, whether masking agents or odor counteractants,
cannot be considered as an ultimate solution of any odor problem. However,
this odor control method might be employed on a short term basis until a
more absolute method could be implemented.
The use of the dilution method for odor control is often suspect. All
of the varieties of this method are dependent on subsequent atmospheric
dispersion which is prone to failure during certain weather conditions.
Dilution methods are usually employed in conjunction with one or more of the
above hydrocarbon reducing methods and, hence, aid in the overall effective-
ness of each applied odor control method.
Additives and dilution may be applied in series for treatment of low
hydrocarbon sources.
Surface Evaporation Control—
Odors escaping from the surface of open tanks have been controlled by
covering the tank surface with small floating plastic balls which reduce
evaporation and thereby reduce odors.
THERMAL TREATMENT LIQUORS
Sources and Composition
The strong liquors containing the materials solubilized during heat
treatment of sludge may be separated from the solids (1) during storage in
decant tank, thickener, or lagoon, and (2) in the dewatering step using a
vacuum filter, centrifuge, sand drying bed, or other method. The quantity
of liquor is about 0.5 percent of plant inflow.
27
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The concentrations of COD, nitrogen, and phosphorus in the liquor are
proportional to the feed solids concentration. For example, about 6,000 mg/1
of COD is solubilized for every 1 percent solids in the heat treatment pro-
cess feed. Increases in the temperature used in thermal treatment increase
the amount of nitrogen solubilized during the process, and as much as 70
percent of the total nitrogen in sludge may go into solution at high temper-
attires. On the other hand, suspended solids may decrease at increased
temperatures. Effects of increased holding time in thermal reactors are to
increase the solubilization of COD and color.
Some of the substances present in thermal treatment liquor and the gen-
eral ranges of concentration are tabulated below:
Substances in
Strong Liquor
SS
COD
BOD
NH3-N
Phosphorus
Color
Concentration Range,
mg/1 (except as shown)
100
10,000
5,000
400
150
1,000
20,900
30,000
15,000
1,700
200
6,000 units
The exact composition of thermal treatment liquor varies widely depend-
ing upon temperature, reaction time, feed solids, air addition, sludge com-
position, and other factors which[are considered in more detail in the
review of data gathered from operating thermal treatment installations later
in this report.
A limited amount of available information on the fate of nutrients dur-
ing the thermal treatment shows that 60-90 percent of the phosphorus present
in the feed typically remains in treated sludge while 60 to 80 percent of the
ammonia feed remains with the liquor.
Direct Recycle
Thermal treatment liquor often is recycled through the main treatment
plant, being introduced to the raw sewage or primary effluent. This places
an additional load upon the system principally in the form of oxygen demand,
suspended solids, and color, "in most cases the color of the final effluent
is increased. This may also be true of the BOD and SS in the effluent if the
main plant is fully loaded or if pperation of the main plant is not changed
(i.e., increase air supply) to handle the increased loadings. If effluent
BOD and SS quality is maintained by adjustments in treatment, then the total
cost of treatment is increased. The effects of recycle can be mitigated to
some extent by storing the thermal treatment liquor and returning to the
treatment plant at a uniform rate or during off-peak hours. The BOD and SS
in thermal treatment liquor may amount to 10 to 50 percent of the total
plant load, as will be seen in later sections of this report.
Separate Treatment and Disposal
Another method for handling [liquors processes the sidestream separately
28
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and with no return of any liquor to the main treatment plant. Sometimes
digester supernatant and waste activated sludge are combined with the thermal
treatment liquor for separate processing; one example of this is the instal-
lation at Indio, California where aerated lagoons with long retention provide
excellent service. Lagoon effluent is blended with plant effluent for
discharge.
Separate Treatment Prior to Recycle
In order to reduce the load on the main treatment plant and maintain
final effluent quality, thermal conditioning liquor is often subjected to
separate treatment prior to its reintroduction to the main plant flow stream
by addition to raw sewage or primary effluent. Again, digester supernatant
may be combined with the liquor for pretreatment.
The Thames Conservancy District of England requires pre-treatment of
liquor and recommends a conceptual design including a roughing filter, 49
hours aeration, and 2 to 3 stages of downflow contact with granular activated
carbon (Figure 6). This pre-treatment scheme is designed to.reduce the COD
of, the liquor from 20,000 to 100 mg/1 before recycle to the treatment works.
Plain aeration, extended aeration, and activated sludge treatment have
also been used for pretreatment of thermal treatment liquors with and with-
out dilution with sewage. BOD reductions by conventional activated sludge
pre-treatment of liquor have been reported as high as 90 percent. In some
cases as much as 30 percent of the liquor COD has proven to be nonbiodegrad-
able. It may be necessary to collect and deodorize aeration basin off-gases
before release.
Thermophilic aerobic digestion is another process that has potential for
treating high-strength liquors. Because of the high endogenous decay value
reported for liquors, this process should be very efficient and should be
studied.
Another proposed method calls for pre-treatment of liquor by anaerobic
filters for 75 to 90 percent BOD reduction followed by chlorination to oxi-
dize sulfur compounds. The effluent would go the secondary treatment system.
Off-gases from the anaerobic filter would be burned. A potential advantage
of this type of process is that the amount of biological solids produced is
much less than in aerobic processes.
Because of the high nitrogen content, land disposal of heat treatment
liquors has also been proposed.
Effects of Recycle on Plant loadings and Process
There are a number of variables which influence decisions regarding the
handling and final disposal of thermal treatment liquor.
As already mentioned, thermal treatment liquor varies widely to compos-
ition and strength depending upon raw sludge character, feed solids concen-
tration, treatment temperature, contact time, amount of air added, and other
29
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HEAT TREATMENT LIQUOR
COD 20,000 APPROXIMATELY
ROUGHING
'FILTER
— 1
ACTIVATED
CARBON
COLUMN
1
ACT
CA
CO
(COD 3,000)
AERATION
TANKS
49 HRS.
DETENTION
EFFLUENT TO SEWAGE
TREATMENT WORKS
(COD 100)
(COD 900)
After Fig. 6-6, EPA Technology Transfer "Process Manual
for Sludge Treatment and Disposal" (October 1974)
Figure 6. Schematic diagram of plant for processing heat
treatment liquor;.
30
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factors. Laboratory or pilot plant tests can yield useful information along
these lines.
Untreated liquor may be recycled directly to raw wastewater or biologi-
cal process influent.
The liquor may be processed entirely independently from the main waste-
water flow. The thermal treatment liquor may be combined with digester
supernatant or waste activated sludge and the mixture processed separately.
In these cases, the effluent from the treated sidestream may be added to the
raw wastewater, the primary effluent, or the final effluent, or it may be
disposed of separately from the main plant flow.
The discharge requirements will influence many of the decisions to be
made regarding handling of thermal treatment liquor treatment and disposal.
Liquors from thermal treatment may produce increases in the main plant
effluent BOD, COD, SS, NH3, color, or phosphorus content which are sufficient
to require separate pre-treatment or improved in-plant treatment to meet
discharge requirements. The cost impact of the pre-treatment or expanded
plant facilities to meet the standards are a principal concern of this study.
Usually the primary considerations in direct recycle are the need for hand-
ling of added BOD and SS loads. The fact that these added loads can make
up anywhere from 10 to 50 percent of the total load on the plant is signifi-
cant from a cost as well as a plant performance standpoint. The costs of
separate processing for thermal treatment liquors or the incremental costs
associated with recycle represent added costs for wastewater treatment.
Means for estimating these costs will be developed later in this report
along with estimating procedures for direct costs involved in thermal treat-
ment of sludge.
31
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SECTION VII
CASE HISTORIES
GENERAL
Plant visits by CWC engineers have been a major source of data for this
study. At each of these visits an extensive interview was conducted with
management or chief operating personnel. In most cases the plant was also
inspected. A detailed report form was completed for each interview. These
forms were submitted in the monthly reports and will not be repeated in the
final report. Table 1 lists the 36 treatment plants contacted.
A total of 28 plants were visited, with an-additional seven plants
supplying data by correspondence and one plant supplying a limited amount of
data by telephone. Of the plants visited, 21 were manufactured by Zimpro
and 7 by BSP. One high oxidation and three intermediate oxidation Zimpro
plants were visited.
A brief description of special features in the installation and opera-
tions of the plants visited is provided in this chapter. These case histor-
ies are grouped according to the type of heat treatment they employ. Plants
where data were obtained by correspondence or 'telephone are not described.
Table 2 is a tabulation of installation and operating conditions for the
plants visited. An experience summary of the plant visits follows the case
histories.*
PLANTS VISITED
North Olmstead, Ohio
All
The City of North Olmstead has installed a Zimpro low oxidation sludge
conditioning process at its activated sludge plant. The plant includes
phosphorus removal, with sodium aluminate,- and tertiary micro-strainers
sludge from the plant is gravity-thickened, then heat treated, vacuum-
filtered, and given to local residents as a soil conditioner. The heat
treatment is normally operated 24|hours per day, seven days per week.
The return liquid streams from the oxidized sludge tank and vacuum
filters are given separate, extended aeration treatment in an unused aera-
tion tank. Three days aeration reduces the BOD to about 40 mg/1. This
effluent is sent to the gravity thickener along with the sludges from pri-
mary sedimentation, waste activated sludge, and the micro-strainer. The
underflow from the thickener is sent to the heat treatment and the overflow
* See Appendix C for more current summary of case histories.
32
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TABLE 1
THERMAL TREATMENT PLANTS CONTACTED
Plants Visited
Colorado Springs, Colo.
Portland, Oregon
Greshain, Oregon
Vancouver , Washington
Muskogee, Oklahoma
Denton, Texas
Indio, California
Clark County, Nevada
So. Milwaukee, Wisconsin
Rothschild, Wisconsin
Merrill, Wisconsin
Wausau, Wisconsin
Terre Haute, Indiana
North Olmsted, Ohio
Bedford Heights, Ohio
Akron, Ohio
Canton, Ohio
Lucas County, Ohio
Columbus, Ohio (Jackson Pike)
Cambridge, Md.
Lancaster, Pa. (South Plant)
Millville, New Jersey
Le vittown , Pa .
Westchester Co. , N.Y.
(Blind Brook)
Rockland Co., N.Y.
Groton, Conn.
Glover sville- Johnstown, N.Y.
Cincinnati, Ohio (Muddy Creek)
Data by Correspondence
Speedway, Indiana
Midland, Michigan
Kalamazoo, Michigan
Defiance, Ohio
May f ie Id , Ken tucky
Amsterdam, N. Y.
Grand Haven, Michigan
Checked by Telephone
Chattanooga, Tennessee
Month
Visited
June
Aug.
Aug.
Aug.
Sept.
Sept.
Oct.
Oct.
Oct.
Oct.
Oct.
Oct.
Oct.
Dec.
Dec.
Dec.
Dec.
Dec.
Dec.
Jan.
Jan.
Jan.
Jan.
Jan.
Jan.
Jan.
Jan.
Jan.
Manuf ac tur er
BSP
BSP
BSP
BSP
BSP
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
BSP
Zimpro
BSP
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Type
thermal
thermal
thermal
thermal
thermal
low
low
low
interm, batch
low
thermal
low
low
low
low
high
low
low
low
low
low
low
low
interm.
interm.
thermal
low
thermal
low
low
low
low
low
low
low
interm.
33
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TABLE 2
INSTALLATION AND OPERATING DATA FOR PLANTS VISITED
Plant and Type
tow Oxidation
Donton, Texas
India, California
Clark Co., Nevada
Rathschild, Wisconsin
Haraau, Wisconsin
Torre Haute, Indiana
North Olmsted, Ohio
Bedford Heights, Ohio
Canton, Ohio
Lucas Co., Ohio
Coluftbus, Ohio
(Jackson Pike)
Cartridge. Hd.
Lancaster, Pa.
(South Plant)
Hillville, M.J.
Levittovn, Pa.
Glove rsville-Johnstown
Now York
Thermal, Conditioning
Colorado Springs, Colo
Portland, Oregon
Gr«shan, Oregon
Vancouver, Wash.
Muxkogea, Oklahoma
Merrill, Wisconsin
Groton, Conn.
Size'1'
gpcn
33
30
117
6.7
38
SO
34
26
2x75
33
200
65
40
36
35
'100
. 83
75
2x75
33
66
50
25
32
Cincinnati (Muddy Creek)
Ohio 67
Intermediate Oxidation
So. Milwaukee, Wise.
Westehester Co. , H.Y.
(Blind Brook)
Rodeland Co., M-Y.
High Oxidation
Akron, Ohio
LECEilD
D " digested
p « primary sludge
TF • trickling filter
Batch
2 t/d
83
50
2x75
humus
Equipment
Manufacturer
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
Zimpro
BSP
BSP
BSP
BSP
BSP
BSP
Zimpro
BSP
BSP
Ziropro
Zimpro
Zimpro
Zimpro
Operating
Temperature
op
1 390
390
350
360 ,
, 375
270
390
: 370
365
380
350
; 350
330
360
350
400
380
400
375
365
400
400
' 375
I 350
j soo
450
: 400
; 550
Operating
Pressure
psig
350
350
400
300
350
300
300
350
300
370
300
350
325
340
300
210
200
210
275
170
250
300
250
275
400
750
800
1650
Type
of
Sludge
P+AS
p
P+TF
P+AS
P+AS
DP+AS
P+AS
P+AS
DP+AS
AS
P+AS
P+AS
P+AS
P+AS
P+AS
P+AS
P+AS
P+TF
P+AS
P+AS
P+AS
P+TF
P+AS
P+AS
P+AS
DP+AS
P
P+AS
AS
Initial
Operation
Date
1970
1969
1974
1969
1969
1972
1973
1970
1973
1973
1972
1975
1973
1970
1967
1972
1974
1968
1975
1973
1974
1972
1974
1974
1973
1961
-
1969
1971
Liquid
Plant
. Design
Flow
mgd
6
5
32
1.5
11.2
9
3.6
30
5
80
8.1
12
5
10
13.5
30
14 /
100
6
12
6.5
2.1
5.5
15
6
5
10
87.5
Present''
Average
Flow
mgd
7.3(75)
1.8(75)
27.0(75)
0.7(74)
6.5(75)
11.3(74)
6.0(75)
2.3(76)
21.0(75)
2.2(75)
60.0(75)
4.2(76)
9.0(76)
2.5(76)
9.0(76)
10.6(76)
16.0(74)
77.8(75)
3.8(75)
7.0(75)
6.5(75)
1.2(74)
8.0(76)
3.0(75)
2.5(76)
15.0(76)
90.0(74)
AS - waste activated sludge :
(1) In SOM cases size in gpn has been estimated from reported capacity in tons per day.
(2) Figure in parenthesis is the year for which flow was reported.
34
-------
from the thickener is sent to the head of the treatment plant. This separate
recycle treatment works well and it is planned to build a special treatment
tank for this purpose when the aeration tank needs to be returned to sewage
treatment.
Odors from the heat treatment off-gas have been a severe problem at
North Olmstead. The plant is located on low ground near the river bank with
homes on the surrounding higher ground. Odors are now satisfactorily con-
trolled by use of a city-designed and built high temperature gas after-burner
which receives all off-gas from the oxidized sludge tank and the vacuum
filters. Ozone treatment was tried, but it was not adequate. The original
Zimpro supplied catalyst burner was retained for treatment of the gas-liquid
separator off-gas with the addition of an air scrubber using effluent water.
The oxidized sludge tank and vacuum filters are fully enclosed.
Bedford Heights, Ohio
The City of Bedford Heights has a Zimpro low oxidation heat conditioning
plant at its activated sludge plant. Forty percent of the Bedford Heights
flow is industrial with a high amount of pickling liquor, which accounts for
a high phosphorus removal without special chemical treatment. Activated
sludge is wasted to primary sedimentation and the combined sludge is pro-
cessed by the Zimpro unit. The heat treatment plant has had extensive
mechanical problems which have resulted in a 50 percent down-time from 1970
to summer 1975, when the heat treatment-was taken out of service. These
mechanical problems have centered on the grinders, boiler, and heat exchanger.
The oxidized sludge tank was converted from an old digester and not equipped
with decanting facilities, so an abandoned and undersized sludge thickener
was adapted for limited decanting. The plant was operated 24 hours per day,
seven days per week.
The recycle streams were returned to the head end of the plant. They
did not cause any treatment problems, since the plant has extra, unused air
capacity, however, there was concern about effluent color during low flows.
The recycle streams caused odors from the liquid treatment tanks. Because
of the odors, pre-aeration of the recycle streams in old septic tanks was
tried. It showed a good degree of treatment. Hydrogen peroxide was also
added to the recycle stream.
Off-gas from the oxidized sludge tank caused odor problems. The ori-
ginal catalyst burner was not effective in odor treatment, nor were acid and
caustic scrubbers or ozone. If the plant is put back in service and natural
gas can be obtained, it is planned to purchase a high temperature after-
burner for off-gas treatment.
Akron, Ohio
The City of Akron has two separate processes for the treatment of sludge
from its activated sludge plant. The primary sludge is vacuum filtered raw
and incinerated. The waste activated sludge is thickened and treated in a
high oxidation Zimpro plant. The volatile solids reduction in the high
oxidation heat treatment is 86 percent and the COD reduction is 83 percent.
35
-------
The oxidized sludge is sent to two decant tanks which are converted from
trickling filters. The decant liquid is returned to the plant. There is
no problem in treating this recycle liquor because the high oxidation in the
reactor has reduced the BOD. When the decant tank is filled with sludge
which takes about one year, it is slowly drained. The resulting solids have
about 50 percent moisture and are removed with mechanical equipment and
hauled to a landfill. The decant tanks are uncovered and there is no odor
problem. The plant is equipped with a scrubber using effluent water and a
gas-fired incinerator to treat process off-gas.
The City has had a satisfactory operating record with the equipment.
Down periods are confined to routine maintenance or waiting for replacement
parts. Because they have dual units, they generally have been able to keep
one unit operating by shifting parts between units. Once the reactor is up
to temperature the reaction is thermally self-sustaining. A steam turbine
was installed for waste heat power recovery, but the system has not been
used much. The mechanical design has been modified and they hope to use it
regularly in the future.
An analysis of operating costs by the City for six months in 1974
showed the cost of primary sludge disposal by vacuum filtration and inciner-
ation to be $86 per dry ton. The cost of waste activated sludge disposal
for the same period by high oxidation heat treatment and settling was $78
per dry ton.
Canton, Ohio ;
The City of Canton has installed a Zimpro low oxidation heat condition-
ing process for treating the sludge from its activated sludge plant. Plant
operations are presently in a period of transition due to new plant construc-
tion. The new sludge handling facilities have been completed, but the new
activated sludge plant is still under construction. At present activated
Sludge is wasted to the primary tanks. About half of the combined sludge
from the primary tanks is anaerobically digested before heat conditioning
and the other half is heat conditioned raw. The conditioned sludge is
vacuum filtered and trucked to a landfill. When the liquid plant expansion
is completed, anaerobic digestion well be discontinued and the dewatered
sludge will be incinerated. The raw sewage is about one-third industrial
with a high metal content. After frequent initial mechanical problems, heat
treatment is now a reliable and satisfactory process. Canton has a two
reactor system with an extra high pressure pump, air compressor and boiler.
The major operating problem is in solids dewatering. The anaerobically
digested sludge does not filter will after heat treatment. The solution has
been to use a mixture of one-half digested and one-half undigested sludge.
Poor filtration resulted in a build up of fine solids in the aeration tanks
and plant effluent. No chemicals are used in filtration. The recycle
liquors are returned to the head of the aeration tanks. There is no problem
in treating these recycle liquors,\although extra air is required and there
is a noticeable color to the effluent. Odors are not a significant problem
at the plant. Off-gas from the decant tank are treated in a gas incinerator
supplied by Zimpro. The air-water;separator after the reactor is bypassed
because it accumulated grit.
36
-------
Lucas County, Ohio
Lucas County, Southwest of Toledo on the Maumee River, has a Zimpro low
oxidation heat conditioning process for its contact stabilization wastewater
treatment plant. There is no primary sedimentation. There is phosphorus
removal by application of liquid alum to the aeration tank outlet. A self-
cleaning rotary screen has been installed after the secondary sedimentation
tanks and before the gravity sludge thickener. Use of the screen has allowed
by-passing the sludge grinders. The sewage is domestic in origin, with no
significant industrial discharges. The heat treatment operates well. The
only significant down-time has been due to mechanical problems during start-
up. The heat treatment is operated 7 hours per day, five days per week.
The local water supply from Lake Erie is soft, and frequent acid washing has
not been necessary. The treated and dewatered sludge is stockpiled on the
plant grounds. The supernatant and filtrate are returned to the head of the
contact tank. There are no problems in treating the recycled BOD load since
the plant flow is below rated capacity. There are no odor problems and
normally no odor control equipment is operated. The plant is in a rural
location.
Columbus, Ohio (Jackson Pike Sewage Treatment Plant)
The Jackson Pike Sewage Treatment Plant of the City of Columbus uses a
low oxidation Zimpro sludge process to condition the sludge from its acti-
vated sludge plant. The single 200 gpm unit operates 24 hours per day,
seven days per week to process a major portion of the sludge. The remaining
sludge is anaerobically digested, chemically conditioned and vacuum filtered,
with a small portion of the raw sludge vacuum filtered directly. A second
200 gpm unit was bid in January, 1976 and eventually will replace the
anaerobic digestion. The present vacuum filters for dewatering the heat
treated sludge will be replaced with centrifuges. The sludge is disposed of
by landfill or incineration. It is expected that incineration will be ther-
mally self-sustaining when the second Zimpro unit is operating. Operation
of the thermal process has been irregular, with 35 percent down-time.
Grinder failures and high pressure pump problems have been frequent. The
local water supply has 8 grains of hardness, which is sufficient to cause
scaling problems in the heat exchanger. The acid washing has not been very
effective. Supernatant and filtrate are returned to the head of the
aeration tank. Odor from the heat treatment units is noticeable on the
plant grounds. A Zimpro supplied fume incinerator is used on the decant
tank off-gas. There is no odor control on the filter room. It is planned
to install ozone odor control equipment in the new installation.
The City of Columbus is also installing three 200 gpm Zimpro units in
its Southerly treatment plant. These units are to be computer controlled to
improve efficiency and balance flows but it is not expected that the com-
puter will reduce the manpower required to operate the units.
Cambridge, Maryland
The City of Cambridge has recently begun operation of its Zimpro low
oxidation heat conditioning process. The plant conditions both primary and
37
-------
waste activated sludge. There is [considerable sea water infiltration into
the older sewers laid along the shores of Chesapeake Bay. Initial operation
of the thermal process was delayed about two years because the heat exchanger
tubes were changed from stainless [steel to titanium due to concern about
corrosion from infiltrated sea water. Operation has been intermittent the
first half year due to startup problems. Cambridge sewage has large seasonal
industrial loads from processing fish and farm products which have a high
grease content. The operators have been observing a rising grease content
in the sludge and are concerned that this may be related to the conditioning
plant operations. The supernatant and filtrate from the process are returned
to the plant headworks. They do not cause any operating problems since there
is adequate excess aeration capacity available. The heat treatment is oper-
ated one eight-hour shift per day [from three to five days per week depending
upon the amount of industrial sludge. The treated and dewatered sludge is
hauled 16 miles to a landfill for ^disposal.
Local residents are vigorously protesting odors from the hecit treatment
processes. There are homes directly across the street from the treatment
plant. Off-gases from the decant tank are treated in a gas incinerator
supplied by Zimpro. The vacuum filter is in a separate room of the sludge
building and without odor control equipment. Plant personnel hope to con-
trol odors from the recycle liquor by constructing covers over the hopper
areas of the primary tanks and discharging the supernatant and filtrate at
a depth of six feet into the primary tank, instead of the present discharge
to the headworks.
Lancaster, Pennsylvania
The City of Lancaster operates a Zimpro low oxidation heat treatment
process at its Stanley D. Nissley 'Water Pollution Control Plant (South Plant)
to condition primary and waste contact stabilization sludge. The plant
operates satisfactorily 24 hours per day, five days per week. The dewatered
sludge is given to farmers to put on agricultural land. The recycled streams
are returned to the head end of the plant. There are relatively few odor
complaints and these occur only during unusually still air. Therefore,
the high temperature odor incinerator for the decant tank off-gases is not
normally operated.
Millville, New Jersey
The City of Millville has operated a Zimpro low oxidation sludge con-
ditioning process for six years. Operation has been continuous with only
minor problems during the initial startup. During the inspection the plant
was down for its first major overhaul. The plant treats primary and waste
activated sludge from a contact stabilization plant. Dewatered sludge is
stockpiled at the plant and given,away. No adjustments are made in the oper-
ations for the recycle streams and there are no problems in treating the
recycle. Supernatant is displaced from the decant tank only when the heat
treatment is running eight hours per day, five days per week. It is re-
turned to the head of the plant with the filtrate. However, the entire
plant is operated even though the[present flow is only one-half of the de-
sign capacity. There are some homes in the general plant areas, but only a
38
-------
few of these complain about odor during the summer when the air is still.
The plant is equipped with a catalytic burner for the decant tank off-gas,
but this burner is never used. There is no other odor control equipment at
the plant.
Levittown, Pennsylvania
The Levittown Plant of the Lower Bucks County Joint Municipal Authority
has employed a Zimpro low oxidation'process for its activated sludge plant
since March, 1967. This unit operates 24 hours per day, seven days per week,
with a shutdown every 21 days' for acid washing. The unit operates reliably
with one operator-mechanic on one shift and with operators on other shifts
checking the unit and cleaning the grinders. The supernatant and filtrate
recycle streams are returned to the head end of the plant. They do not cause
any operating problem, even though the plant flow is near its rated capacity.
Sludge after dewatering is hauled to a landfill. The off-gas from the heat
treatment process is bubbled into a tank containing plant effluent contin-
uously pumped from the chlorine contact basin. The overflow from the tank
returns to primary sedimentation and sludge is removed from the tank once
per year. There are no odor complaints, even from a popular restaurant
about one hundred yards from the plant.
Westchester County, New York
The Blind Brook Water Pollution Control Plant of Westchester County is
located in Rye, New York. This intermediate oxidation .Zimpro plant condi-
tions the primary sludge from a primary sedimentation plant. An 85 percent
reduction in volatile solids is obtained from the intermediate oxidation.
The boiler is only needed for startup, when the reactor is up to temperature
the oxidation is thermally self-sustaining. The unit operates 24 hours per
day, seven days per week. Acid washing is necessary every 6 to 7 days be-
cause of scale problems caused by sea water infiltration. One hundred
pounds of soda ash is added each day to the sludge storage tank before the
reactor to help control this problem. The plant was out of service for
rebuilding the heat exchanger and compressor during CWC's inspection. The
Blind Brook Plant is scheduled for expansion to an activated sludge plant.
When expanded, the Zimpro plant will be shut down and sludge will be pumped
to another treatment plant which is expected to install a Zimpro low oxida-
tion process. The heat treatment decant and filtrate are discharged to an
effluent wet well downstream of the chlorination basin. The recycle streams
are considered disinfected and, therefore, are allowed to go directly into
Long Island Sound via the plant outfall. The off-gases are bubbled into the
same effluent wet well. The plant is located near homes and a recreational
park, but there have not been odor complaints. The dewatered solids are
hauled to a landfill.
Rockland County, New York
The Rockland County activated sludge sewage plant is located near West
Nyack, New York. The intermediate oxidation Zimpro plant conditions primary
and secondary sludge. The sewage flow presently exceeds the plant rated
capacity, however, return of the supernatant and filtrate from the sludge
39
-------
conditioning to the head of the plant does not upset the plant. This is due
to a high volatile solids reduction, around 90 percent, with the intermediate
oxidation plant. The heat treatment operated fairly steadily from startup in
summer 1969 until late 1973 before;a series of lengthy down periods due to
mechanical problems and lack of paits began. The main problems have been
with the boiler and its feed pump,|the air compressor, and the heat exchang-
er. Control of odors from the oxidized sludge tank has been a difficult
problem. The district has used several gas burners, but with low temperature
or installation problems. The district was installing a new unit which will
have an exhaust temperature of 1500°F, and will be equipped with a heat ex-
changer to recover 55 percent of the heat and raise the incoming air to 780°P.
Groton, Connecticut
The Fort Hill Water Pollution Control Facility of the town of Groton
has recently installed a BSP thermal sludge conditioning process for its new
activated sludge plant. During the present very low flow conditions, the
heat treatment is used only periodically to condition both primary and secon-
dary sludge. Supernatant and concentrate are returned to the head end of
the plant without incident. The decant tank is vented through an afterburner
for off-gas treatment. All of the heat treatment and dewatering equipment is
within a sludge handling building. There have been no odor complaints,
although a strong odor is noticeable within the building while the thermal
plant is running. The dewatered sludge is trucked to a landfill, but will
be incinerated at the plant in the future when the quantities of sludge are
larger. The heat treatment piping was originally designed as cast iron, but
was changed before installation to: glass-lined ductile iron to avoid corro-
sion problems.
Gloversville and Johnstown, New York
The joint wastewater treatment plant of Gloversville and Jortstown has a
Zimpro low oxidation heat treatment process for primary and secondary sludge
conditioning. The sewage contains discharges from many industries, includ-
ing 22 canneries. The wastewater jvolume is 57 percent industrial, with a
BOD of 550 mg/1 and a suspended solids of 700 mg/1, 70 percent volatile.
The treatment plant has primary sedimentation, high rate roughing trickling
filters, followed by activated sludge. The heat treatment plant operates
24 hours per day, seven days per week, but shuts down every 10 days for acid
washing. During the first three years of operation, forced plant shutdowns
equalled 40 percent of the time. ;There have been less shutdowns during the
past year. Heat exchanger scaling and mechanical repairs caused the shut-
downs.
The supernatant from the oxidized sludge tank has been recycled to the
roughing filter, as originally designed, and also to primary sedimentation
and the aeration tanks without upsetting the plant. However, because of
odors emitted by the supernatant it is not desirable to expose it to the
atmosphere on the filters. Sometimes the oxidized sludge does not decant
well and the supernatant containsjup to one percent fine solids. Therefore,
because of odor emissions and fine solids, the supernatant at present is
discharged directly to the river without treatment. The filtrate is
40
-------
returned to primary sedimentation.
The odor problems were analyzed in a recent engineering report. This
report claims the major source of odors is from aftergrowths in the oxidized
sludge tank. The tank is essential for plant operations because oxidized
sludge must be held 24 hours to thicken it sufficiently for vacuum filtra-
tion. Tank cleanings and the introduction of biocides to the sludge have not
been successful in preventing these odors. Presently one barrel per day of
peroxide is added to the tank for odor control. Off-gases from the tank are
treated- in a two stage scrubber, the first stage uses sodium hydroxide and
the second stage uses hypochlorite.
Cincinnati, Ohio
The Muddy Creek Treatment Plant of the Metropolitan Sewer District of
Greater Cincinnati has installed a BSP thermal sludge conditioning process.
Primary and waste activated sludge is conditioned for vacuum filtration and
incineration. The recycle streams are returned to the aeration tanks. Since
the plant is operating at one-half of design capacity, there is no upset in
the aeration process. The off-gases from the decant tank are passed through
an afterburner at 1400°F. Little odor can be detected near the afterburner
stack. The heat treatment plant has operated only five months in the 2 1/2
years since its initial start-up. At the time of inspection the plant had
been down seven months waiting for a new boiler installation. The downtime
has resulted from many mechanical, piping and instrumentation problems which
has resulted in replacement of many items of original equipment.
Indio, California
The Valley Sanitary District's activated sludge plant at Indio uses a
Zimpro low oxidation plant to heat condition its primary sludge. The heat
treatment plant has operated reliably since 1969 with a minimum of downtime.
Acid washing of the heat exchanger is required only once every year or two.
The plant is operated eight hours per day, five days per week. The heat
exchanger and separator are flushed out with each shutdown. Secondary efflu-
ent is stored in a secondary lagoon and then pumped to pasture irrigation.
Waste activated sludge, Zimpro supernatant, and filtrate are discharged to
an aerated lagoon for treatment. Overflow from the aerated lagoon is
blended with secondary effluent for disposal. There is no recycle from the
Zimpro process to the activated sludge plant. Off-gas from the heat treat-
ment process are discharged at a three foot depth in the aerated lagoon.
There is no odor problem -at the plant. Dewatered sludge is taken by a pri-
vate fertilizer company and blended with manure. Analysis by the District
for fiscal year 1974-75 determined the total operations and maintenance cost
to be $37.20 per ton.
Clark County, Nevada
The trickling filter plant of the Clark County Sanitation District, near
Las Vegas, has recently installed a Zimpro low oxidation heat conditioning
process. Primary sludge and trickling filter humus will be heat treated,
vacuum filtered and incinerated. Heat treatment supernatant and
41
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vacuum-filter filtrate will be given fourteen hours aeration before recycle
to the head-end of the plant. Off-gases from the decant tank, vacuum filters
and thickeners will be burned at 1200°P.
South Milwaukee, Wisconsin
South Milwaukee has a batch operated Zimpro intermediate oxidation heat
treatment for its activated sludge plant. The plant produces primary and
secondary sludge which includes a ^phosphorus sludge produced by the addition
of sodium aluminate. The sewage flow is 3 mgd. All the sludge is anaerobi-
cally digested. The Zimpro process handles two-thirds of it with most of
the remainder hauled directly to land disposal and with a small portion going
directly to drying beds. The heat treatment plant began operation in 1961
and was modified to batch operations in 1965. Each batch is about 5,700
gallons of digested sludge at about three percent solids. The sludge is
pumped to a storage tank and from there transferred to the reactor with a low
pressure centrifugal pump. Steam is injected to the reactor until the tem-
perature reaches 400°F. Compressed air is added to the bottom of the reactor
to 500 psi. Air and steam addition continue for 16 to 19 hours, then reactor
pressure is reduced to 180 psi, the steam is then diverted to the storage
tank to preheat a new batch of sludge and the reactor contents are discharged
to a lagoon for drying. The continuous stream of off-gases from the reactor
are diffused into the primary tank effluent channel. There is no dewatering
equipment, decant liquor from the lagoon is returned to the primary tank and
the sludge from the lagoon is removed to a landfill. For 1975 South Milwau-
kee has determined that the operation and maintenance costs for its heat
treatment process were $87.30 per ton of dry solids.
Rothschild, Wisconsin
The small activated sludge plant at Rothschild is operated by Zimpro.
A low oxidation Zimpro plant built in 1969 conditions the primary and waste
activated sludge. Experimental work on heat treatment, recycle disposal,
and odor control is conducted on this 6.7 gpm plant. Dewatered sludge is
disposed of in a landfill.
Merrill, Wisconsin
Zimpro operates a thermal conditioning plant at the Merrill activated
sludge plant. This heat treatment plant is similar to a Zimpro low oxida-
tion plant except that no air is added to the reactor. The 25 gpm plant
began operation in 1974 and operates at 300 psi and 390°F. Primary and
waste activated sludge are processed. Dewatered sludge is hauled to a land-
fill. Off-gases are treated within a jet impingement scrubber and with
ozonation.
Wausau, Wisconsin
Wausau has installed a Zimpro low oxidation heat treatment at its acti-
vated sludge plant. The heat treatment began operation in 1969 and processes
primary and waste activated sludge 24 hours per day, five days per week.
The plant operated with the air compressor out of operation in the summer of
42
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.1975. Without air addition severe odor problems developed. Also air addi-
tion improves heat transfer because of turbulence. The odor control is
through an afterburner at 1200°F. Dewatering is done by a filter press
supplied by Zimpro. Five thousand four hundred pounds per day of lime-alum
water treatment plant sludge is added to the 10,800 pounds per day of sewage
treatment plant sludge. The decant tank overflow is returned to the head
end of the plant and the press filtrate is returned to the secondary part of
the plant. Filter press cake is disposed of on the land or to landfill.
Terre Haute, Indiana
Terre Haute has a Zimpro low oxidation sludge conditioning process at
its activated sludge plant. Seventy percent of the sewage flow is industrial
and commercial, however, the BOD and suspended solids are in the normal
municipal range. The 16 ton per day plant treats digested primary and waste
activated sludge. Oxidized sludge supernatant and filtrate are returned to
the primary tank and dewatered sludge is applied to the land at the plant.
Off-gases are treated in a burner which has a maximum temperature of 1000°F.
A higher temperature or a catalyst is needed to reduce the odorsj- In 1974,
110 tons of solids were processed and plant downtime was 208 days. In 1974
costs per ton of solids were determined as: Capital - $9.23, Operations -
$23.46, and Maintenance - $83.60 for a total cost per ton of $116.29. The
extensive' downtime in 1974 was due to equipment failures, scaling of piping
and heat exchangers, and delays in obtaining parts.
Denton, Texas
Denton has installed a Zimpro low oxidation heat conditioning process
at its 6 mgd activated sludge plant. The heat treatment processes primary
and waste activated sludge. After heat treatment the sludge is cooled in a
tank and discharged to sand dewatering beds. The filtrate from the beds is
returned to the head end of the plant. Gas from the gas-liquid separator
goes to a diffuser in the junction box and some gas from the holding tank
goes to a waste burner. Sludge from the beds is trucked to a ranch for
disposal. There has been a considerable amount of downtime and maintenance
on the•heat treatment process and plant records indicate that only about 10
percent of the sludge is processed through the heat treatment. Sewage flow
to the plant is now 7.3 mgd and the City is considering a plant expansion.
Indications are that the heat treatment process will continue to be used in
an expanded plant.
Muskogee, Oklahoma
The City of Muskogee has a BSP-Porteous thermal sludge conditioning pro-
cess at its two stage trickling filter plant. The 50 gpm plant, which
began operation in mid-1972, processes primary sludge and trickling filter
humus. Off-gases from the decant tank, centrifuge and screw conveyor are
exhausted through an afterburner.
Gresham, Oregon
The activated sludge plant at Gresham has installed a BSP thermal type
4:3
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heat conditioning process. Primary and waste activated sludge are heat
treated, decanted, and centrifuged. The present sewage flow is 3.8 mgd with
the plant rated at 6 mgd capacity. Initial operation of the heat treatment
was in 1973. The downtime for 1974 was estimated at 21 days. The increase
in BOD to be removed due to recycled heat treatment supernatant and centrate
is calculated at 41 percent. The increase in total suspended solids to be
given heat treatment due to the recycled supernatant and centrate is calcu-
lated at 59 percent. Off-gas from the blending tank, reactor, and decant
tank is exhausted throug~h a burner at 1200 °F.
Vancouver, Washington
At 66 gpm BSP thermal type heat conditioning plant began operation in
1974 at the Vancouver, Washington'activated sludge plant. Primary and waste
activated sludge are heat conditioned, vacuum-filtered and incinerated. The
increase in BOD to be removed duetto recycled heat treatment supernatant and
filtrate is 35 percent. There is;an eleven percent increase in total sus-
pended solids to be given heat treatment due to the recycled heat treatment
supernatant and filtrate. Decanted heat treatment liquor and filtrate are
returned to the primary tank. In 1974 2,782 dry tons of sludge were pro-
cessed. The cost for heat treatment was $37.65 per dry ton and the cost for
incineration,was $9.64 per ton.
Portland, Oregon
Portland has two BSP thermal'treatment units at its Columbia Boulevard
activated sludge plant. Primary and waste activated sludge are heat treated,
vacuum-filtered and trucked to a landfill. Estimates of the increase in BOD
to be removed due to recycled heat treatment supernatant and filtrate are
16-28 percent. The increase in total suspended solids to be given heat
treatment due to recycled supernatant and filtrate is 16-28 percent. Off-
gases from the reactor are burned at 1200°F, other off-gases are ozonated.
Heat treatment downtime is estimated at 33 percent for unit no. 1 and 30
percent for unit no. 2.
Colorado Springs, Colorado
Colorado Springs has installed two BSP-Porteous thermal sludge condi-
tioning units. Colorado Springs has two sections to its secondary plant, 14
mgd of trickling filters and 30 mgd of activated sludge. One heat treatment
unit handles primary sludge and trickling filter humus, the other unit
handles primary and waste activated sludge. The first unit began operation
in 1968 with the trickling filter plant and the second unit began operation
in 1973. Downtime on the first unit is 30 percent and on the second unit 33
percent. The increase in BOD to be removed due to recycled heat treatment
supernatant and filtrate is 21 percent. The increase in total suspended
solids to be given heat treatment due to recycled supernatant and filtrate
is 30 percent. Heat treatment affects plant costs by increasing the aera-
tion time to 8 hours instead of six and adding 12 percent to the power con-
sumption. The color of the final plant effluent is increased 60 to 100
standard color units. For the first five months of 1975 heat treatment costs
per ton of dry solids were: Operation expense - $37.28, Operation
44
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labor - $14.24, and Maintenance labor and expense - $7.26.
EXPERIENCE SUMMARY
As a result of many field inspections and analysis of the data obtained,
several aspects of thermal conditioning of sludge may be reviewed. It may
be important to record comments gained as a result of this study, realizing
that opinions may vary on these technical matters. An observation may come
from the opinions of operators at only a few locations or from a problem
unique to a location. The time and resources available for this study have
not allowed verification of all these comments. The amount of information
available to the design engineer when considering thermal conditioning of
sludge is not large, therefore, even though all comments cannot be fully
documented, they may be of value in putting the process in perspective.
Design
Pilot plant operation before design, regardless of size, appears to be
desirable because of the large number of variables. Pilot plant operations
may point up problem areas which should be given special consideration in
design and also may provide special design criteria.
The basic criterion of design is the sludge flow rate. This flow rate
determines detention time in the heat exchangers which should remain con-
stant. The weight of sludge treated in a day is dependent on the hours of
operation and influent solids concentration.
Heat treatment plants, do not seem to be economical in small sizes. At
small scale the comparatively high cost of support equipment such as boiler,
air compressor, and decant tank makes heat treatment facilities more costly
to build than other sludge treatment plants. Operations are expensive be-
cause of the constant attention required by a skilled operator. Also, the
necessary operational skill might not be available to a small plant.
Multiple support units with interconnecting piping should be provided.
The most essential units are the sludge feed pumps, grinders, high pressure
pumps, and boilers. Without these units the plant cannot be run, therefore,
there should be at least two units with one unit adequate to run the plant.
For the Zimpro equipment it is not absolutely essential to have a backup
air compressor since the plant can be run without air addition, as a thermal
plant. Multiple reactors or heat exchangers are not necessary and, in fact,
may not be desirable because the smaller reactors may be less stable ther-
mally. A service life of 10 to 20 years is projected for piping and reactors
constructed of proper materials. Zimpro's current design practice is to use
No. 360L stainless steel throughout the piping, reactor vessel, and valving.
Carbon steel does not appear to have sufficient corrosion and erosion resis-
tance. One plant has installed glass-lined ductile iron. Where chlorides
are high, Zimpro has installed titanium in its heat exchangers (chlorides
greater than 400 mg/1). High chlorides are found most often where there is
sea water intrusion. Some plants have been shut down for extended periods
due to a lack of parts for the boiler water feed pump, and it is recommended
that spare parts should be kept at the plant for this pump and all other
45
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essential mechanical items.
Grit removal must be provided for the raw sludge. The raw sludge grin-
der is a troublesome unit, requiring frequent maintenance. The grinder
should be protected as much as possible from grit, large objects, trash and
rags. The Zimpro low oxidation units are designed to operate with the addi-
tion of air. They can be used without air as thermal sludge conditioners,
however, the fuel consumption is substantially increased. The Zimpro Company
representative stated that operation without air would double the boiler
fuel consumption. The other effects of air addition were less readily ob-
served. Based on the plant visits, it appears that air addition does
beneficially affect the odor of the off-gases, the strength and odor of the
recycle stream, and the dewaterability of the sludge.
It is important to have the oxidized sludge storage tank equipped with
decanting facilities. The oxidized sludge thickens readily to mox-e than -10
percent in the storage tank. This improves the dryness of the filter cake
and decreases the size of filters required.
Influent Sludge
In the low oxidation units, a ^higher sludge concentration is more de-
sirable because less steam is needed. Many plants operate with a 3 percent
solids concentration, however, six percent is more desirable. It is better
not to digest the sludge before heat treatment as this a.ffects the; dewater-
ing of the treated sludge. The breakdown of solids materials in the digester
means a low filtering rate and a thin cake. Thermal conditioning of sludge
is intended to treat the scum as well as the settled sludge. Generally
plants experience no problems with scum treatment, however, in one plant
where the raw waste had a high grease content from food processing, the
operator was concerned with grease jbuild-up from cycling through the thermal
treatment.
Operating Conditions
The reactor temperature and pressure affect the amount of recycle BOD
and dewaterability of the oxidized .sludge. In general temperature should
be kept as low as possible, consistent with adequate conditioning of the
sludge. Higher temperatures breakdown the sludge particles, solubilizing
more BOD and decreasing the fibrous material in the sludge which is essen-
tial to high filtration rates and thick cakes. The temperature and pressure
of the reactor varies through the day especially where the plant is only
run part of each day or where the raw sludge concentration varies consider-
ably. Therefore, the temperature and pressure reported in the data tables,
indicates the average point around which there is some variation.
Thermal treatment units always have an operator in attendance when they
are running. The lead operator of the thermal conditioning crew should be
machinery oriented and able to do routine frequent preventive maintenance.
With proper preventive maintenance,; some units have run years without signi-
ficant problems. It is possible that a separate maintenance staff can care
for the equipment, however, in plants where the chief operator takes the
46
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lead in maintenance work there seems to be fewer problems.
Maintenance Problems
The need to clean the heat exchanger is the most frequent reason for
routine shutdown of thermal treatment systems. Scale builds up in the heat
exchanger tubes and is removed, in some systems, by recirculating hot nitric
acid through the tubes for about a day. The frequency of acid washing is
highly variable, some plants requiring it every week to two weeks and other
plants run 26 weeks before requiring acid wash. The frequency of acid wash-
ing seems related to the hardness of the water in the service area. Scale is
removed from the reactor less frequently and some times"this is done by hand
chipping.
A major overhaul of mechanical equipment should be expected every five
to eight years. This would include, major work on the air compressor, high
pressure pumps and boiler. More frequent overhaul of the grinder is to be
expected and the, boiler may also need more frequent attention depending upon
the feed_water chemical quality and chemical conditioning.
Odor
The existence of odor problems seems to be a highly individual matter.
Some plants operate without odor control equipment without complaint, yet
others have severe problems due to odors. Site conditions are an important
factor in the determining the extent of odor control equipment required. The
proximity of housing to the plant, local topography, and wind and atmospheric
conditions in the area are important in considering the odor potential at a
given plant. The most severe odor problems are with the off-gas from the
gas-liquid separator and the decant tank. The recycle liquor may also be
odorous. The off-gas can be successfully treated. The most frequent method
is to use high temperature incineration (1,5QO°F). -The operation of the
incinerator requires about as much fuel as the steam boilers and, therefore,
may not be possible in some gas short areas. The substitution of fuel oil
to operate the incinerator may also.be costly. The current recommended odor
treatment procedure by Zimpro is to treat off-gas in a dry packed granular
carbon tower where the carbon is regenerated with steam. The air flow
through the decant tank is about doubled over that of process air to provide
more rapid air changes in the tank. Liquid scrubbers with caustic, perman-
ganate, or chlorine have also been successful. In certain cases, off-gas is
merely bubbled through a tank or lagoon of chlorinated effluent. The results
with ozone treatment are less clear, some operators claiming success with
that type of treatment and others saying they had tried it and it did not
work. Low temperature catalyst odor treatment has not been successful. Con-
version of low temperature units to high temperature operation has generally
not been successful because of the inadequate gas capacity and overheating
of the stack.
Supernatant-Filtrate Recycle
The BOD of the recycled liquors from the thermal treatment system can
constitute a substantial overload on a wastewater treatment plant not
47
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originally designed with surplus aeration capacity. The recycle BOD load
can be about 20 percent of the influent BOD load on the aeration system. The
hydraulic recycle loading on the plant, however, is very small and generally
can be ignored. The recycle liquor can be introduced at the hea.d-end of the
plant which is most common or at ;the aeration tanks. The BOD is almost all
soluble. There are some situations, however, where solids capture on vacuum
filters is poor, and fine solids are returned to the plant. If this situa-
tion continues, there can be a buildup of solids in the plant which settle
in the final clarifiers, are returned to the heat treatment and from there
returned to the treatment plant in the filtrate. Waste biological solids
produced from the recycle soluble BOD are equal to a net yield of about 0.6
Ib waste activated sludge per Ib of recycled BOD.
Most plants visited were operating at less than design load, therefore,
they had adequate aeration capacity available to treat the recycle loads.
In most cases, the operators did not feel there was extra labor involved in
recycle treatment although they did acknowledge that extra blower power may
be required to treat the BOD.
Color in the effluent appears to be variable. Some plants are concerned
with it, most plants, however, do not consider it a problem.
Chemical Analyses
Only a minimum of chemical testing information is available from plant
records. Plants do not routinely test their heat treatment streams because
they feel data are of relatively little use to them for purposes of process
control and the tests are difficult and time consuming for the average plant
laboratory.
48
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SECTION VIII
DIRECT AND INDIRECT COSTS OF THERMAL TREATMENT OP SLUDGE
GENERAL
This section presents information, estimates, and curves for the cost of
thermal treatment plants. Subsections cover construction, operation, and
maintenance costs for the thermal treatment process itself, the handling and
treating of strong liquors from the treated sludge decanting tank and de-
watering equipment, and the treating of odorous gases.
As discussed in Section III, information related to costs of thermal
treatment was obtained from several sources including surveys of operating
plants, review of available literature and manufacturers' data, and prepara-
tion of engineering estimates. The surveys and the manufacturers' data
yielded much information on direct costs although in many instances some
conversion was required to put the data into a form usable for this study.
Very little information on the indirect costs of or requirements for treat-
ment of liquors and off-gas was forthcoming from the surveys. Engineering
estimates, information from the literature review, and information from man-
ufacturers were used almost exclusively to arrive at the indirect costs.
The costs are presented in various ways. First, costs for thermal
treatment alone or for its components are compared with the physical variable
most controlling for the particular cost. The separate listing of items
such as materials, fuel and labor, which vary with time and location, pro-
vides a base for adjustment to local conditions. Costs for the various com-
ponents were combined and expanded to plot thermal treatment costs versus
wastewater treatment plant size for a typical wastewater and plant. These
generalized cost curves provide a quick preliminary estimate of the approxi-
mate total cost of thermal treatment.
CONSTRUCTION COSTS (THERMAL TREATMENT)
In order to estimate the costs to construct thermal treatment facilities
of various capacities, construction cost data were obtained from the records
of approximately thirty thermal treatment plants, from manufacturers, and
from engineering estimates. Bidding information and records from plants
frequently did not contain breakdowns indicating what portions of the cost
were for thermal treatment and, where some breakdown was provided, it often
covered the total costs for sludge handling and included prethickening,
> storage, thermal treatment, decanting, vacuum filter or centrifuge dewater-
ing, incineration, and engineering. Information from some plants included
49
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the costs for buildings, which were| sometime quite elaborate, large, included
room for expansion, and housed dewaitering and incineration systems as well
as thermal treatment facilities. Building costs for some plants were re-
ported to be over two-thirds of the costs for thermal treatment equipment.
Because of these differences in reported costs, the costs for thermal
treatment alone were separated for comparison and plotting. The resulting
costs for thermal treatment include sludge feed pumps; grinders; heat ex-
changers; reactors; boilers; gas separators; air compressors where appli-
cable; decanting tanks; standard odor control systems; and piping, controls,
wiring and installation services usually furnished by the equipment or
system manufacturer. Not included in the basic thermal treatment costs are
buildings; footings; piping, electrical work and utilities not supplied by
the equipment manufacturer; sludge storage and thickening prior to thermal
treatment; sludge dewatering, incineration or disposal; land; and engineering
fees.
The separate costs for thermal treatment are presented in terms of
March 1975, national average costs. In escalating costs to later dates, it
should be considered that the escalation determined from the EPA-STP index
may not adequately reflect the increased costs for high temperature, equip-
ment-dominated processes such as thermal treatment. Costs are plotted as
cost per unit of thermal treatment capacity versus thermal treatment capa-
city ($ per gpm vs. gpm) in Figure 7 and as cost for thermal treatment versus
thermal treatment capacity ($ vs. gpm) in Figure 8.
A second curve is plotted on each of the above two figures to include
the costs for typical building, foundation and utility needs for the thermal
treatment systems. The building costs represent single-story, concrete or
masonry construction with built-up!roofing, insulation and heat and vent
systems, and assume that reactors and decant tanks will be located outside
of the building. The costs also include piping and wiring within the
building, foundations for internal; and external equipment, and a limited
amount of sitework. Building sizes provide for easy access to equipment and
a control room. For larger installations, where multiple units are antici-
pated, space for some standby equipment is included. Typical building sizes
range from 1500 sq ft for a 10 gpm plant to 5250 sq ft for a 200 gpm plant.
The construction cost per square foot of building was estimated to be $36.
The curves show a rapid rise in unit construction costs for plants
smaller than about twenty gallons per minute, indicating that there is a
limiting plant size below which high cost may make the process economically
infeasible. For large plants, above about 150 gallpns per minute, the
increased use of multiple treatment units and of standby units results in a
lower limit for unit cost per gpm of capacity. This lower limit appears to
be in the range of $9000 to $12,00:0 per gpm. Data for these larger plants
are sparse, however, and some plants reported lower unit costs.
At the present time, the minimum direct cost of a thermal treatment
plant is estimated to be approximately $350,000 regardless of plant size.
50
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51
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52
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FUEL AND ELECTRICAL ENERGY REQUIREMENTS (THERMAL TREATMENT)
Fuel and electrical energy use and cost data were obtained from the rec-
ords of several operating plants, from equipment manufacturers, and from
calculations. For analysis and comparison, cost data again were reduced to
unit requirements and then recomputed as costs based on common unit pricing.
Because both fuel and electrical energy usages are dependent primarily on the
volume of sludge processed, volume treated was used as the.common denominator
for both utilities. Fuel and electrical energy requirements were analyzed,
respectively, as million Btu per gallon and kwh per gallon treated. Uniform
unit prices and annual volumes were than applied to obtain annual costs.
Fuel is used chiefly as a saurce of heat to produce steam. The amount
of fuel used is influenced by the temperature to which the reactor contents
must be raised, the efficiencies of the boiler and heat exchanging systems,
insulation or heat losses from the system, and the degree of heat-producing
oxidation which takes place in the reactor. Some reduction in the unit heat
requirement for an increase in plant size was noted in reported data. This
is thought to result from more uniform and constant operation of the system,
greater heat transfer and insulation efficiencies and possibly a greater
amount of oxidation in the larger units. Plants adding air to .their heat
exchangers and reactors and experiencing some oxidation had lower fuel re-
quirements. The annual fuel requirements based on 8000 hours of operation
at capacity vs plant size are shown in Figure 9.
Typical fuel requirements averaged 900 to 1000 Btu/gallon for plants not
practicing air addition and 300 to 600 Btu/gallon, depending on the degree
of oxidation obtained, for plants practicing air addition. Curves in this
report are based on fuel requirements of 900 Btu/gallon for thermal condi-
tioning plants and 500 Btu/gallon, corresponding to about five percent
oxidation, for low oxidation plants. The above fuel requirements do not
include allowances for treatment of off-gas.
Electrical energy needs are determined by sizes and efficiencies of
driven machinery such as sludge and boiler water pumps, grinders, thickeners
and, in plants where air addition is practiced, air compressors. Electrical
energy is also needed for lighting and other building uses. Because the
majority of the plants investigated did not meter separately the energy used
in the thermal treatment process, and there was substantial variance among
the available data, supplementary estimates of electrical energy use were
made based on typical equipment sizes and efficiencies. Average unit energy
usages were found to be 22 kwh/10^ gallons for plants practicing air addition
and 10 kwh/10^ gallons for plants practicing only thermal conditioning.
Annual electrical energy usages for the two types of plants based on the
same criteria as above for fuel are shown in Figure 10. A separate curve is
included on the figure for estimating the energy requirements for building
needs.
As can be seen, the electrical energy required for air addition is
approximately equal to the remainder of the load for thermal conditioning.
The curve also shows that for small plants, the load for miscellaneous
building requirements is a substantial portion of the total plant load.
53
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1,000,000.
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THERMAL TREATMENT CAPACITY, GPM
Figtare 9. Annual direct fuel requirements for thermal treatment.
54
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treatment.
55
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Annual costs versus plant sizes for both fuel and electrical energy are
shown in Figure 11. To determine these annual costs, unit costs of $2.80 per
million Btu for fuel and $0.03 per kwh for electrical energy were applied to
the usages shown in Figure 9 and 10. These unit costs may appear high when
compared with those reported by some plants visited. However, it is doubtful
that low unit costs such as $0.01 per kwh for electrical energy and $1.00
per million Btu for natural gas some plants have reported will continue to
be available.
MANPOWER REQUIREMENTS (THERMAL TREATMENT)
Labor for operation and maintenance presents one of the highest areas
of cost in the operation of a thermal treatment plant. For plants below
about 50 gpm the cost for labor exceeds the cost of energy. At 10 gpm the
cost for labor may be in excess of two and one-half times the cost for
energy.
Labor requirements can be divided into two types: operation and main-
tenance. In this study operation comprises time spent reading and logging
data on the process, controlling and adjusting the various systems and
components, and laboratory work. The functions covered by maintenance in-
clude cleaning and repairing process components, general upkeep of the
process area, checking and repairing of controls and instrumentation, and
performing preventative maintenance. In some plants these operation and
maintenance functions may vary or may overlap.
For use in this study, labor costs and manhour requirements reported by
plant operators were reduced to manhours per year based on an assumed 8000
hours per year operation of the thermal system. The labor requirements for
maintenance and operation are plotted separately on Figure 12 as manhours
per year versus thermal treatment capacity. Data and the figures show that
for mid-sized plants, one operator per shift is employed. As the plant size
decreases below about 40 gpm the operator can perform other duties as well
as operate the thermal treatment plant. Reported data indicate that for
small-sized plants the operator may also operate dewatering facilities or
provide some of the maintenance required for thermal treatment. For larger
plants more operation is required, reaching about two operators per shift
at 200 gpm. Some plants, however, report using two or more operators at
plants much smaller than 200 gpm.
Reported data on requirements for maintenance labor vary considerably
from plant to plant and an average confirmed by calculation was used for
plotting. In general, maintenance labor is approximately one-fourth of
•the operating labor, ranging from the equivalent of one maintenance man for
one shift at a 50 gpm plant to one, and one-half men for one shift at a 200
gpm plant. The amount of maintenance required depends greatly on the design
and operation of the plant, particularly on equipment and materiatls used for
construction. It is also dependent on the skill and knowledge of the main-
tenance personnel and the design of and adherence to a preventative mainten-
ance program. Plants practicing a well designed maintenance program
appeared to have less maintenance problems and to require less overall time
for maintenance.
56
-------
1,000
100
9
8
7
6
5
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100,000
9
8
7
6
5
10,000
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OPERATION
MAINTENANCE
3 4 5 6789
10
3 4 5 6 789
100
2 3456 789
1,000
THERMAL TREATMENT CAPACITY, GPM
Figure 12. Operating and maintenance requirements for thermal
treatment. !
58
-------
Maintenance as discussed above and as shown in Figure 12 is for preven-
tative work and routine repair work. It includes daily clean-up, lubrica-
tion, replacement of seals and gaskets, acid or other normal cleaning of
heat exchanger tubing, painting and similar work done on a daily, weekly,
monthly, etc. basis. It does not include major overhal functions which
should be accomplished on a periodic basis every six or seven years.
Labor requirements for major overhaul work such as reactor cleaning;
pipe and tube replacement; pump, compressor and boiler working parts replace-
ment and other similar items are not included in Figure 12. For this type
of work, except in large plants, the skills of specially trained manufac-
turers' representatives or contracted specialists should be utilized. Costs
for this work is covered below under "Materials and Supplies".
Figure 13 is included to indicate the manpower costs for thermal treat-
ment . The graph is based on the hours reported in Figure 12. An average
unit labor cost of $7.00 per hour is used for the extension. Hourly costs
will, of course, vary with the particular circumstance of plant size and
location. Based on the illustrative labor cost, the labor costs for thermal
treatment vary from $0.08 per gallon for a 200 gpm plant to $0.17 per gallon
for a 50 gpm plant to $0.56 per gallon for a 10 gpm plant.
MATERIALS AND SUPPLIES (THERMAL TREATMENT)
Annual costs for several reporting plants were summarized and are shown
in Figure 14. Curve A shows the normal annual cost for materials and sup-
plies required to operate and maintain the thermal treatment system. These
costs are plotted against thermal treatment plant capacity and include
materials and parts such as seals, packing, coatings, lamps, bearings,
grinder blades, and other items used in scheduled and normal maintenance.
They also include operating supplies such as lubricants, cleaning chemicals,
boiler feed water, and water treating chemicals. These costs vary from
about $5000 per year for a ten gpm plant to approximately $20,000 per year
for a 200 gpm plant.
Besides the normal, periodic maintenance required for a plant and
•covered by Curve A, additional costs for major overhaul work are incurred.
This work includes such items as motor rewinding; major overhauls of pumps
and compressors; major, non-routine rehabilitation or replacement of heat
exchanger tubing piping and controls; and refitting of boilers. This type
of work is required at an average interval of about six to seven years,
depending on the conditions at a particular plant. Because labor for this
type of major work is often contracted, labor costs are treated as part of
the overhaul and are included in its cost under this section. Curve B shows
the combination of these costs with those included under Curve A to give the
total annual cost for materials and supplies. The inclusion of major over-
haul work increases the annual materials cost by about 45 percent over that
required for routine and preventative maintenance materials.
There was considerable variation among the costs for materials in
seemingly similar plants and it appeared that three factors tended to govern
the costs. The first of these was the adequacy of the preventative
59
-------
1,000,000
Si 00,000
o
o
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ca
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a.
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10 100 1,000
THERMAL TREATMENT CAPACITY, GPM
Figure 13. Operating and maintenance labor costs for thermal
treatment.
60
-------
1,000,000
^ 100,000
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10,000
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NORMAL ANNUAL COST
ANNUAL COST WITH A L
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THERMAL TREATMENT CAPACITY, GPM
2 3 456789
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Figure 14. Materials and supplies for'thermal treatment.
61
-------
maintenance program. In plants where a good program was practiced, overall
costs for parts and supplies generally were lower. Where maintenance was
neglected, more major failures were found to occur with a need for greater
expenditure for parts.
The second factor involved the design of the plant and selection of
materials of construction. If a higher grade of materials and equipment
were selected for initial construction and if the plant were designed with
ease of maintenance in mind, less maintenance and better maintenance were
found and hence less need for replacement was noted.
Finally, need for materials and supplies was affected by the quality of
the area's water supply. In areas having high amounts of hardness and high
mineral contents in their water supplies, more scaling and corrosion were
noted in equipment, particularly in heat exchangers. Scaling, along with
the increased amount of cleaning required, resulted in both an increase in
replacement parts for boilers and heat exchangers and an increased amount of
chemicals for boiler water treatment and heat exchanger cleaning.
INDIRECT COSTS FOR HANDLING AND TREATING RECYCLE LIQUORS
Costs associated with the handling and treating of the strong liquors
resulting from thermal processing of sludge often have been neglected when
comparing thermal treatment with other sludge conditioning processes. These
costs can affect substantially the: total cost of treatment. Depending on
the method chosen to handle and treat the liquors, the characteristics of
the liquors, the sewage treatment process, the discharge requirements and
other factors, the costs for processing the liquors may reach 20 percent of
the direct costs for thermal treatment.
This section presents information for preparing initial estimates for
comparison of processes. The information is presented in the form of graphs
showing the cost of constructing and operating facilities to handle process
liquors. The graphs are based on one of the many sets of conditions and
processes which might be encountered or used in design of thermal treatment
facilities. Many other sets of conditions could be encountered in actual
practice and their costs can be estimated in a manner similar to that used
herein.
The method selected considers direct recycle of liquor to the activated
sludge section of the main treatment plant. This process was selected be-
cause it is the one most often used in the plants studied. A liquor quality
of a BOD of 6500 mg/1, and a suspended solids concentration of 5000 mg/1
(75 percent volatile) is used. It was also assumed that the incoming sewage
to the plant has BOD and suspended solids concentrations of 250 mg/1 each
and is subjected to primary treatment prior to mixing with the recycled
liquor.
Other conditions are that: (1) the process, prior to the addition of
the recycled liquor, is fully loaded so that no capacity to accept the
liquor remains, (2) the sizes of ivarious equipment and structures in the
process must be increased to maintain the same mixed liquor solids
62
-------
concentration and cell residence time, (3) the combined primary and secon-
dary sludges are thickened to 4.5 percent solids prior to thermal treatment,
and (4) slight degradations in effluent color, COD and BOD are acceptable.
Design calculations were made to size plant components before and after
adding the recycled liquor. In this way, incremental costs and requirements
could be estimated by comparing before and after values.
The illustration below shows the process and the amounts of solids flow-
ing in various process streams.
0 = 1 MGD
BOD = 250MG/L
SS=250MG/L
RAW _
SEWAGE
SS =2085DAY
PRIMARY
AND
THICKENING
Q = O.3GPM
SS=I426*/DAY
SOLIDS=4O%
WASTE .
SLUDGED
BOD=114 7*/ DA'
SECONDARY
AND
THICKENING
Q = l MGD
SS= 208*/DAY
Q = 3.7 GPM
SS = 225*/DAY
BOD=29I^DAY
DEWATERING
THERMAL
TREATMENT
EFFLUENT
Q = 4.0 GPM
SOLIDS=4'/2%
SS=165I*/DAY
SOLIDS AND FLOW DIAGRAM FOR A TYPICAL
ACTIVATED SLUDGE THERMAL TREATMENT SYSTEM
Recirculation of liquor of the above stated quality results in an addi-
tional solids load on sludge processing facilities. This load is approxi-
mately 300 Ibs per day per mgd of raw influent. The increase comes
primarily from the nonvolatile and non-degradable fractions of the solids in
the return liquor and from cell production from the recycled BOD. This load
increases the amount of sludge to be processed by about 15 percent.
For the case under study, recirculation also increases the BOD loading
to secondary treatment by 20 percent.
Treatment plant capacity was selected as the variable having the great-
est influence on costs for liquor treatment. Other variables, particularly
the BOD and suspended solids concentrations in the raw sewage, also influ-
ence the cost of liquor treatment but do not vary over as great a range as
influent flow. Their use would also result in a more complex and difficult
procedure for estimating cost.
63
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Construction Costs (Liquor Treatment)
Construction cost for handling and treating liquors from thermal treat-
ment was estimated for the above use. The estimate uses the same basis as
was used earlier under direct construction costs and is taken primarily from
Black and Veatch, "Estimating Costs and Manpower Requirements for Conven-
tional Wastewater Treatment Facilities", EPA Project 17090 DAN, October 1971
(B&V). Construction costs taken from B&V are up-dated to March 1975 national
average values.
The estimate includes costs for increasing the size of an activated
sludge system using diffused-air for aeration. The only areas of significant
cost increase is in the aeration tanks and air supply system.
Because the increase in hydraulic loading is very small, there is almost
no increase in cost for clarification, piping and pumping. An allowance of
10 percent above the cost increase for aeration is provided, however, to
cover those small costs and increases for site work, yard piping, utilities
and support facilities.
Costs to provide sludge storage ahead of thermal treatment are not in-
cluded in the differential cost. , Such storage like many other items is
common to treatment plants whether or not thermal treatment is used so that
no differential would result. If the addition of storage is required, its
cost can be easily estimated from various cost references and added to the
costs developed hexein.
Figure 15 shows the incremental cost for plant enlargement versus treat-
ment plant capacity.
Energy Requirements (Liquor Treatment)
The indirect energy requirements to operate' processing facilities for
recycled liquor treatment were estimated for the' same system as used for
estimates of construction costs. Again, increments were determined based on
the differences in operating requirements with and without recycled liquor
included.
The increment in energy requirements results almost entirely from the
increase in aeration capacity needed to treat the recycled liquor. In-
creases in energy requirements for other operations such as clarifier and
thickener drives and pumping are negligible in comparison.
The requirements are shown in Figure 16 as kilowatt hours per year for
8000 hours of operation versus treatment plant capacity. The annual cost
for that energy based on a unit cost of three cents per kilowatt hour is
shown in Figure 17.
Manpower Requirements (Liquor Treatment)
The requirements for operation and maintenance labor needed for the
sewage treatment facility expanded to treat the recycle liquor are shown in
64
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Figure 15. Incremental cost for construction of recycled liquor
treatment facilities.
65
-------
10,000
9
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TREATMENT PLANT CAPACITY, MGD
Figure 16. Incremented electrical energy requirement for recycled
liquor treatment.
66
-------
ANNUAL ELECTRICAL ENERGY COST, 103 DOLLARS
— • o
•• N oi 4> 01 en eno->ia>«
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TREATMENT PLANT CAPACITY, MGD
Figure 17. Incremental electrical energy cost for recycled liquor
treatment.
67
-------
Figure 18. Again, these curves reflect the incremental between wastewater
treatment with and without recycling of the liquor. The requirements in-
clude time for operating and controlling the secondary treatment and thick-
ening processes, maintaining aeration equipment and basins, and maintaining
settling and pumping systems.
Figure 19 presents annual cobts for performing the above functions and
is based, as for direct labor costs for thermal treatment, on a unit labor
cost of $7.00 per hour.
Materials and Supplies (Liquor Treatment)
The incremental cost for supplies and materials needed for operation
and maintenance of the secondary process are shown in Figure 20. These
costs represent the difference in expected annual expenditure for a treat-
ment plant with and without recycle liquor. Costs are taken from B & V and
are updated to March 1975. They cover materials such as paint, lubricants
and replacement parts, bearings, seals, air filters, diffusers, lab supplies,
manufacturer's assistance and other items normally required in a secondary
treatment plant.
INDIRECT COSTS FOR TREATING ODOROUS OFF-GAS
The sources and characteristics of odorous gas and vapor streams eman-
ating from thermal treatment processes and the methods for their control
were discussed in Section VI, including incineration, adsorption, scrubbing,
masking, the use of chemical additives, dilution and prevention by limiting
evaporation. All of these methods have been used alone or in combination
in attempts to control odors from various locations in treatment plants.
The choice of method is dependent; on the make-up of the odor, its strength,
the environment around the plant, cost and other factors.
The methods most commonly used and most generally effective for control-
ling odors from thermal treatmentj are high temperature incineration,
adsorption on activated carbon, and chemical scrubbing. Costs for these
three methods are developed in this section.
Other methods such as oxidation with ozone, low temperature catalytic
burning, and masking in most case? have not proven effective or reliable
and are not discussed further. Inexpensive methods such as dilution or
submerged discharge into various processes may or may not be applicable for
particular situations and also are not considered.
The costs given in this section represent the costs necessary to treat
concentrated, high-hydrocarbon gab streams coming primarily from gas
separators or covered decanting tanks. Commonly, five to ten percent of the
total costs for thermal treatment are represented by the requirements for
odor control. Costs for treating| comparable gas discharges from dewatering
processes are considered common to other sludge handling systems within a
plant so are not covered separately. They can be determined, however, using
the information given below. Likewise, costs for ventilating, heating, and
air conditioning of the thermal treatment building are not included. These
68
-------
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THERMAL TREATMENT CAPACITY, GPM
Figure 18. Incremental requirements for operation and maintenance
labor for recycled liquor treatment.
69
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Figure 19. Incremental cost of[operation and maintenance labor for
recycled liquor treatment.
70
-------
100,000
a
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2
10,000
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Figure 20. Incremental cost of materials and supplies for recycled
liquor treatment.
71
-------
costs are included in the building costs given in the section on direct
costs.
The costs developed for the three methods are representative of complete
odor control systems and include costs for collection of gas; ducting; fans;
chemical feeding, mixing, and storage equipment; automatic control systems;
disposal of removed and waste materials, and discharge of treated gas as
well as for odor removal itself. Table 3 lists the requirements for the
three methods based on a typical li,000 cfm odor control system. A concen-
trated gas stream of 1,000 cfm corresponds to a thermal treatment plant
size of 200 to 250 gpm or a sewage! treatment plant size of 50 to 60 mgd.
The incineration or afterburning process considered consists of pre-
treatment by water scrubbing using' treated effluent in a packed bed and
direct flame incineration at 1,500^ with recovery of forty percent of the
input heat. The carbon adsorption process includes prescrubbing with efflu-
ent, dual-bed adsorption on activated carbon, regeneration of carbon with
low pressure steam, condensation of vapors, and incineration of the waste
organic stream. The chemical scrubbing system utilizes three stages of
scrubbing in packed beds. The first two stages use secondary effluent and
a final stage uses a buffered, potassium permanganate solution.
In general, all three systems are capable of reducing the total hydro- •
carbon content of thermal treatment off-gases by approximately 85 to 90
percent. However, because of the Difference found in sewages, sludges,
reactor conditions, economies of size, etc., not all are applicable or even
useable in every situation. Pilot tests or, at .least detailed analyses, are
recommended to determine the best Method, to set design criteria,, and to
verify anticipated results and costs. Using the typical requirements listed
in Table 3, costs are developed for each of the three control systems.
These costs for the three 1,000 cfm systems are shown in Table 4,
The listed costs assume the same basis as used elsewhere in this report.
Labor is charged at $7.00 per hour and fuel and electrical energy are
charged respectively at $2.80 per!million Btu and $0.03 per kwh. Costs for
construction assume that the odor!control system is constructed as a part
of a complete thermal treatment system and are March, 1975 national average
costs.
Figure 21 compares construction'costs and odor control facility size
for the three processes. In all but the smallest plants, chemical scrubbing
has the lowest initial costs. Carbon, because of its more complex control
system and greater amount of equipment (boiler, insulated pressure vessels,
liquid burner, etc.) is the most Mostly in all cases. Incineration has the
lowest initial cost for very smalfL plants but increases in cost more rapidly
than the other methods.
Operation and maintenance cobts for the three treatment methods are
shown versus size in Figure 22. [costs for incineration are dominated by the
high fuel requirement which is related directly to air flow. La.bor costs
• for incineration are quite low.
72
-------
TABLE 3
UTILITY, CHEMICAL, AND LABOR REQUIREMENTS
FOR ODOR CONTROL SYSTEMS
( 1)
System v '
Incineration
Carbon
Adsorption
Chemical
Scrubbing
Effluent Potable O & M
Fuel Power Water Water Labor
btu/ kwh/ Gals./ Gals./ Chemicals Hrs./
Day Day Day Day Ibs/day Day
53 x 106 176 22 x 103 - - 0.5
1.5 x 106 211 181 x 103 - 25(2) 1
211 86 x 103 390 97(3) 1
(1) Based on 1,000 cfm
(2) Buffered KMnO4
(3) Make-up Carbon
73
-------
TABLE 4
COSTS FOR ODOR CONTROL SYSTEMS
$/Day : . Total (2-
System*1* Fuel- Power Water Chemical Labor $/Day $/Year Construction
Incinera-
tion 148.40 5.30
3.50 157.20 52,400 88,100
Carbon 6.40 6.30
Adsorption
25.00 7.00 44.70 14,900 182,300
Chenti. cal
Scrubbing -
6.30 1.20 67.90 7.00 82.40 27,500 46,700
(1) Based on 1,000 cfm
(2) Based on 8,000 hrs/year operation of the thermal
treatment system
74
-------
SEWAGE TREATMENT PLANT CAPACITY, MGD
1,000
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Figure 21. Construction cost for odor control systems.
75
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PTION
G
10,000
SYJSTEM CAPAC!TY,CFM
Figure 22. Operation and maintenance costs for odor control systems.
76
-------
The predominant item for scrubbing is the cost of the oxidizing chemi-
cals and for the adsorption process is the make-up carbon. The requirements
for both these materials are directly proportional to the system capacity.
From the standpoint of total cost for odor control, incineration and
scrubbing are the least expensive methods for use in very small plants. As
size increases, however, total costs for scrubbing and adsorption become
more attractive.
It must be emphasized again, that odor control systems for use in ther-
mal treatment plants must be selected on the basis of what will adequately
treat the specific off-gas involved. Only after it is determined that more
than one process will perform adequately, can selection be made on the basis
of cost.
SUMMARY OF DIRECT AND INDIRECT COSTS
In this section, construction costs and operation and maintenance costs
for direct thermal treatment and for handling and treatment of process
liquors developed earlier are combined for a typical plant and sewage.
Whereas these costs were developed on the basis of the process variable most
controlling of the cost — gpm of thermal treatment capacity for direct
costs and mgd of treatment plant capacity for indirect costs — they are
combined on the basis of weight of sludge to be processed.
Weight is used as the basis for general presentation because it is the
unit most commonly used in the comparisons and costing of solids handling
processes. For processes such as thermal treatment in which costs are
largely related to liquid flow, certain obvious limitations and complexities
arise in using a weight basis. In this report the complexities are handled
by developing a procedure for converting and then developing costs for one '
representative case. The procedure involves first determining the charac-
teristics of the raw sewage to be treated, selecting the liquid treatment
process to be used, and making process design calculations which normally
would be made during the preliminary design of a sewage treatment plant. The
primary parameters to be determined -are the weight of sludge to be handled,
the concentration of the sludge to be treated, and the characteristics of the
recycled liquor streams. A mass balance similar to that shown earlier is
helpful in determining the effect of recycle on the total amount of sludge
to be processed. For quick estimates, the various parameters can be esti-
mated with reasonable accuracy. From these calculations the flow of sludge
can be determined.
Figures 7 through 14 can then be used to determine the direct costs and
requirements and, knowing the raw sewage flow, the costs and requirements
for treating the recycled liquor can be determined from Figures 15 through
20. Although costs for odor control are included in Figure 8, if desired,
five percent can be subtracted from the indicated cost, and odor control
costs more accurately determined from Figure 21 and added to the remainder.
An example of the calculation required to determine construction and
operation and maintenance costs for a typical thermal treatment facility
77
-------
is given in Table 5. The table lists the basic assumptions made, lists the
steps to follow, and the curves to iuse. The example uses those curves giving
operation and maintenance requirements directly in dollars per year. How-^
ever, if different unit costs for labor and energy are applicable, the basic
curves can be used in an identical [procedure to get annual requirements in
man hours, Btu's and kwh's. These requirements can then be multiplied by
the appropriate unit costs to arrive at total cost.
Total costs for a typical thermal treatment system are developed and
shown in Tables 6 and 7. The factors and parameters used to develop the
tables have been discussed above under "Indirect Costs" and are for a
primary/secondary treatment plant using the activated sludge process, thick-
ening, and vacuum filtration. Raw I sewage to the plant has BOD and suspended
solids concentration of 250 mg/1 each. Prior to thermal treatment, sludge
is thickened to 4 1/2 percent solids and after thermal treatment, sludge is
dewatered to 40 percent solids. The analysis shows that approximately 1.1
tons of solids per million gallons ^are produced from the raw sewage and
recycled liquor, and that 4 gpm pet million gallons flows to the thermal
treatment facility. The thermal plant is sized for 8,000 hours per year of
operation (a down time of approximately 9 percent). Construction costs are
amortized over twenty years at seven percent.
Table 6 shows the combined construction cost for plants ranging in size
from one to 100 tons per day. These costs are exclusive of engineering,
administrative and financing costs. Apparent from the table is that, be-
cause of the very high unit construction cost for small thermal treatment
plants, construction costs for liquor treatment is only a small percentage
of the total cost. The percentagejof construction costs related to liquor
treatment increases rapidly as the plant size increases and reached 15 per-
cent at 100 tons per day. For the given conditions, total construction cost
varies from $393,000 for a one ton per day plant to $5,003,000 for a 100
ton per day plant. These costs result in amortized construction costs of
approximately $100 per ton for a one ton/day plant to $13 per ton for a
100 ton per day plant.
The operation and maintenance costs for thermal treatment plants follow
the same pattern as construction costs, starting quite high -- $155 per ton
for a one ton per day plant, and dropping rapidly to less than $20 per ton
for a 100 ton per day plant. The ratio between direct and indirect costs
follows almost the same pattern found for construction costs.
Analysis of the various elements of operation and maintenance costs
shows that indirect costs result primarily from the energy required for
aeration. Direct costs are dominated by the high requirements for operating
labor. The cost of direct operating labor in a one ton per day plant is
approximately 60 percent of the total O & M cost. This percentage drops to
about 26 percent for a 100 ton per day plant but the figure is still quite
significant.
Since operating labor is such a major factor in small plants and con-
struction costs do not decrease in proportion to decreases in thermal system
capacity, it is apparent that a size is reached below which continuous
78
-------
TABLE 5
EXAMPLE CALCULATION OF DIRECT & INDIRECT COSTS
A.
Basis
1. Q = 10 mgd
2.
3.
4.
5.
Solids to be treated =1.1 tons/mgd
Solids concentration to thermal treatment = 4.5%
Annual hours of operation = 8,000 (Downtime = 9%)
Thermal treatment process = low oxidation
B.
D.
General
1. Weight of sludge to be treated
10 mgd x 1.1 ton/mgd = 11 tons/day
2. Size of thermal treatment unit
11 tons/day @ 4 1/2% solids = 40 gpm
40 gpm x (8760 hrs/yr - 8,000 hrs/yr)
Capital Costs
Direct Cost From Figure 8, Curve B
Indirect Cost From Figure 15
Construction Cost
Engineering
Legal and Administrative
Interest during Construction
Total Capital Cost
Operation and Maintenance Cost
Direct Operations Labor (Figure 13)
Direct Maintenance Labor (Figure 13)
Indirect Operations Labor (Figure 19)
Indirect Maintenance Labor (Figure 19)
Direct Fuel (Figure 11)
Direct Power (Figure 11)
Indirect Power (Figure 7)
Direct Materials (Figure 14, Cuve B)
Indirect Materials (Figure 20)
E.
Annual O & M Cost
Total Annual Cost
Amortized Capital Cost @ 7% - 20 .years
$1,200,000 x 0.09439 =
Annual O S M Cost
= 44 gpm
$ 890,000
120,000
$1,010,000
100,000
10,000
80,000
$1,200,000
55,900
12,300
J 1,300
900
30,000
19,000
10,400
12,300
900
$ 143,000
113,300
143,000
Total
Cost per million gal. =
Cost per ton of solids =
$256,300
365 x 10
$256,300
365 x 11
$70
$68
$ 256,300
79
-------
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81
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operation is no longer desirable and that savings would be achieved by find-
ing the optimum relationship between hours of operation, design capacity,
and sludge storage.
Table 8 summarizes the direct, indirect, and total costs for construc-
tion and for operation and maintenance for several sludge capacities.
The total costs for thermal treatment - direct and indirect - range from
$257 per ton to $32 per ton for sludge capacities of one to 100 tons per
day. It must be kept in mind, however, that the costs presented above are
for one representative but particular set of conditions. One additional
figure (Figure 23) is presented to serve as a quick visual reference to the
costs for thermal treatment. It is based on the same set of conditions used
above, and shows total construction cost, amortized construction cost, annual
O & M cost, and total annual cost ^for a range of plant sizes. Plant sizes
are shown for both solids loading ;to the thermal treatment system over a
range of one to 100 tons per day and sewage treatment plant flow over a
range one to 100 mgd.
82
-------
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THERMAL TREATMENT LOADING, TONS/DAY
Figure 23. Direct and indirect costs for thermal treatment.
84
-------
BIBLIOGRAPHY
THERMAL TREATMENT OF SLUDGE
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2, 32 (1970). '
Boyle, J. D., and Gruenwald, D. D., "Colorado Springs Activated Sludge Plant
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Conference, Oct.,1974.
Brooks, R. B., "Heat Treatment of Activated Sludge." Water Poll. Control
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(G. B.), 69, 221, (1970).
Corrie, K. D., "Use of Activated Carbon in the Treatment of Heat-Treatment
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Dean, R. B., "Ultimate Disposal of Waste Water Concentrates to the Environ-
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ditioned Sludge Liquors", Advances in Water Pollution Research, p. II
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Everett, J. G., and Nagai, I., "Effect of Concentration of Sewage Sludge
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85
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Everett, J. G., "Biodegradability of Sewage Sludge Heat Treatment Liquor."
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Harding, J. C., and Griffin, G. E., "Sludge Disposal by Wet Air Oxidation at
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Harrison, J., and Bungay, H. R., "Heat Syneresis of Sewage Sludges, Part I;
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Harrison, J., Bungay, H. R., and Lord, A. M., "Heat Syneresis of Sewage
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Harrison, J. R.', "Earn an A if You [Cope With the C's and D's of Solids."
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Hurwitz, E., Teletzke, G. H., and Gitchel, W. B., Zimpro Inc., "Wet Air
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Kalbskopf, K. H., "Thermal Conditioning Tests of Activated Sludge and
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86
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Kochera, B., "Operation of a Thermal Treatment System for Sludge." WPCF
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88
-------
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(1960).
Post, N., "Counteraction of Sewage Odors," Sewage and Industrial Wastes,
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Environmental Protection Agency Report, EPA-670/2-73-093.
Raleigh, C. W., "The Chemical Control of Wastewater Sulfides," Deeds & Data,
WPCF, p. D-l, Jan., 1975.
Sawyer, C. N. & Kahn, P. A., "Temperature Requirements For Odor Destruction
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Water and Sewage Works, 152-154 (April 1964).
Stokes, R. S., and Uhte, W., "Clean Up Down - Under Sewage Odors," American
City, p. 43, October, 1974.
Stone, R., "Sewage Treatment System Odors and Air Pollutants," JSED, ASCE,'
p. 905, August 1970.
Sutton, G. P., "Odors and Air Pollution from the Treatment of Municipal
Waste Water," Paper, APCA, 1971.
Zack, S. I., "Sludge Dewatering and Disposal," Sewage and Industrial
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Units," Sept. 1974.
89
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APPENDIX A
JAPANESE EXPERIENCE WITH HEAT TREATMENT
GENERAL
As reported in EPA 670-9-75-005 (May 1975), Proceedings of the Third US-
Japan Conference on Sewage Treatment'Technology, a study was conducted on
heat treating of sludges to improve dewaterability. The study was; undertaken
by the Japanese Government's Ministry of Construction and was under the dir-
ection of the Committee for Investigation into Sludge Handling and Disposal
of the Japan Society of Civil Engineers.
The purposes o"f the study were to find and enumerate the benefits of and
problems with the use of heat treatment processes and to determine solutions
to the problems. Principal elements of the study included:
1. literature review of heat treatment experiences in Europe;
2. laboratory work to determine the effects and problems which could
be expected in Japan;
3. assessment of heat treatment and its problems and economics based
on studies of full-scale, operating plants;
4. characterization of supernatant quality and development of super-
natant treatment techniques;
5. characterization of odors and their sources and evaluation of odor
control methods; and
6. investigation of processjequipment from the standpoints of esti-
mating operational and maintenance requirements, selecting
materials for construction, and determining system reliability.
To implement the full-scale phases of the study, heat treatment facil-
ities were constructed at three sites. These are referred to as: Sakai
(Semboku Plant), Fujisawa (Nambu Plant) and Sapporo (Toyohiragawa Plant).
LABORATORY STUDIES
Laboratory work performed using sludges from the Nambu Plant,
Semboku Plant the Toba Plant at Kyoto gave the results below.
the
90
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1.
2.
3.
4.
5.
6.
7.
8.
At temperatures above 180°C (356°F) both excess activated sludge
(WAS) and primary sludge (PS) could be filtered readily. As the
temperature was increased, the moisture content of the filter cake
decreased.
Residence time in the reactor was not a significant factor in deter-
mining cake moisture. It was noted that for a constant time and
temperature the cake from primary sludge would have a lower mois-
ture content than that from WAS.
High treating temperatures and longer residence times resulted in
higher filtration rates.
Specific resistance of the sludge was affected mainly by the organ-
ic content up to temperatures of about 180°C (356°F). Above 190°C
(374°F), however, the specific resistance dropped sharply.
Biochemical oxygen demand, chemical oxygen demand, ammonia and
color in the supernatant from both WAS and PS increased sharply as
temperatures were increased about 180°C (356°F).
As the organic content of sludge increased, its ability to be solu-
bilized also increased. More waste activated sludge was solubil-
ized for a fixed set of reactor conditions than was PS or a mixture
of PS and WAS.
Baking of solids in heat exchangers became excessive at tempera-.
tures above 200°C (392°F).
Scorching of heat exchange tubing increased as the organic content
of the sludge increased.
It was concluded from the laboratory studies that heat treatment as a
pretreatment for mechanical dewatering was effective at temperatures as low
as 180°C (356°F) and with residence times of between 30 to 60 minutes as
long as the organic content of the sludge was relatively constant and between
50 and 60 percent. If the organic content exceeded 60 percent or if it was
variable, an increase in temperature to 190°C (374°F) or even to 200°C
(392°F) for 30 to 60 minutes would be desirable.
FULL-SCALE STUDIES
Design data for the three previously mentioned plants where the full-
scale heat treatment studies were carried out are summarized in Table A-l.
A flow sheet of the typical heat treatment plant is shown in Figure A-l.
As can be seen from the flow sheet, solids were to be concentrated in a
gravity thickener prior to heat treatment. This thickening process was
expected to raise the concentration of solids from a level of 1 to 2 percent
in the raw sludge to a level of 4 to 5 percent in the reactor feed. After
heat treatment and thickening, a solids content of 50 to 60 percent was
anticipated. Design temperatures for the reactors were set at 200°C (392°F),
and residence times varying between 30 and 120 minutes were selected.
91
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TABLE A-l
DESIGN DATA FOR HEAT TREATMENT FACILITIES IN PROJECTED SITES
(1)
(2)
(3)
(4)
-------
Figure A-l. Sludge treatment process.
93
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Results from one year of operation are shown in Table A-2. A comparison
of the data in Table A-2 with the design criteria and expected treatment
results shows that in nearly all cdses the thickening and dewatering levels
achieved were equal to or higher than expected.
Problems encountered during full-scale operation included erosion and
deposition in heat exchangers, odors emanating from several parts of the
plant, disposal of the highly concentrated supernatant from the dewatering
process, and general plant operation and maintenance.
HEAT EXCHANGERS
Problems with heat exchangers[involved both erosion of tubes and depos-
ition of solids within the tubes. ^The type of exchanger used in all three
plants was the concentric tube design operated initially with raw sludge
flowing in the inner tube and treated sludge flowing in the outer tube or
annulus. Some tubes from the exchangers in all three plants suffered from
erosion and corrosion at rates up to 0.7 mm per year - and had to be re-
placed or repaired. Most of the wear was in the annulus. Deposition of
solids and baking of organic material onto the tubes with a corresponding
loss of heat transfer efficiency was also a problem.
After the initial operation, modifications to the heat exchange system
were made. This work was followed! by three additional months of operation
to evaluate the modification.
The improvements to the system consisted of:
1. converting heat exchangers from the sludge-to-sludge pattern to a
sludge-to-water pattern.! This method used the inner tube for
sludge and the annulus fpr a water exchange medium. A smoother
flow pattern for the heat treated sludge was provided by the
change;
2. installing a cushion or !surge tank upstream from the sludge dis-
charge valve;
3. precooling the heat treated sludge by injection of water at the
outlet from the reactor; and
4. employing a hydraulically-driven cleaning bullet to clean the
inner tubes.
After the three months of operation, the exchangers were disassembled
and inspected. The inspection revealed that the modifications had been
effective in eliminating most of the earlier problems. There was no abrasion
nor corrosion noted, high temperature portions of the inner tubes had a hard,
black organic scorch up to 2 to 3jmm in thickness while the lower tempera-
ture portions were covered with up to 1mm of soft organic layers. These
depositions were removable with use of the bullet cleaning method. The
outer tube was covered with an oxide film but was in good condition.
94
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TABLE A-2
OPERATIONAL DATA OP HEAT TREATMENT PLANTS (FROM APRIL, 1972 TO MARCH, 1973)
Item
Average sludge solid,
tons/month
Monthly average moisture
content, '
Ht_at treatment capacity.
gpm
Heat treating temperature
Heating time, min.
Toyohiragawa, Sapporo
200 "C
(392°F)
34
"can
618
95.3
110
198°C
(388°F)
30
Minimum
195°C
<383°F)
27
Nambu, Fuiisawa
201°C
(394°F)
120
Mean
123
96.7
29
195 °C
(383«F)
120
Minimum
180°C
(356°F)
60
Semboku, Saka:
200°C
(392°F)
April - Augu:
29.4
118
96.1
45
194°C
(381-F)
st
47.5
L
190°C
(374-F)
26.3
September - March
Moisture content of heat
treated sludge , %
Moisture content of
sludge cake, %
Properties of supernatant:
Temperature
PH
Total solids, mg/1
Dissolved solids, mg/1
SS, mg/1
COD (KMn04) , mg/1
BOD5, mg/1
T-N, mg/1
Properties of effluent:
BOD5, mg/1
SS, mg/1
Supernatant treatment
86.5
37.2
53°C
(127°F)
5.7
5,950
4,040
2,260
1,800
6,000
17.3
41.4
84.3
36.1
43.3°C
CllO°F)
5.5
5,232 4,
3,908 3,
1,325
1,590 1,
5,155 3,
11.4
25.9
81.4
35.1 •
28°C
(82°F)
5.3
.838
690
978
280
520
5.2
18.3
Supernatant diluted
300%, aerated 24 hours
85.1
46.3
36.1°C
(97°F)
5.8
8,100
7,900
650
4,700
6,100
1,100
25.0
29.0
78.1
37.2
29.1°
(84°F)
5.7
5,978
5,575
403
2,975
4,413
664
14.4
14.6
71.3
33.1
C 22.1°C
(72°F)
5.4
4,160
3,690
200
2,050
3,400
410
5.5
7.0
Directly discharged
to raw sewage
47.9
94.9
54.6
29.0°C
<84°F)
5.8
9,899 7,
9,252 6,
1,008
3,520 2,
7,660 5,
1,349
18.2
34.0
45.5
87.8
47.8
24.8°C
(77°F)
5.2
191 4
644 3
547
615 1
847 4
704
12.8
19.5
42.9
83
40.6
19.0°C
(66°F)
4.6
,362
,946
118
,600
,204
258
7.9
9.0
Directly discharged
to raw sewage
pre-aeration tank
95
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CORROSION
Experiments to determine the corrosive effects of the reactor environ-
ment to various steels were run at,all three plants. At two plants test
pieces of mild boiler steel, SB42 iris G3103 (ASTM A285 grade C) , were set in
operating reactors for twelve months. Measurement and analysis showed that
the rate of corrosion was very slow and was uniform over the entire surfaces
of the test pieces. It was found that the layer of Fe3O4 was firmly attached
to the metal and inhibited further corrosion. The mild steel was judged
suitable for reactor construction.
Prestressed test pieces of SUJS 32 and SUS 27 (AISI types 316 and 304)
stainless steels were subjected to[ conditions similar to those for the mild
steel. After nine months in the rjeactor, the SUS 32 steel showed no prob-
lematic corrosion such as stress corrosion, cracking,, pitting or inter-
granular corrosion and also was ju'dged suitable for reactor construction.
At the same time, SUS 27 steel was judged unfit because of the excessive
stress corrosion noted.
ODORS
Strong odors were noted emanating from the reactors, thickeners, filters
and storage hoppers. Odorous gases from gas separators and thickeners were
burned in the boilers or in incinerators. Foul air from sludge processing
areas was also burned or, in some leases, vented or scrubbed and sprayed
with deodorant.
To increase the efficiency of odor removal, studies were conducted using
catalytic combustion, oxidation with ozone and water scrubbing. It was
thought that if catalytic combustion at 200°C (392°F), the temperature of
the available steam source, was effective in removing odors, operating cost
would be reduced.
Odorous gases were treated in a column packed with catalyst (type not
given) and burned 200 to 300°C (392 to 572°F). Analyses of the treated gases
showed the removals listed:
Chemical
H2S
NH3
CO
Hydrocarbons
Temperature
250 to 300°C (482 to 572°F)
200 to 300°C (392 to 572°F)
200 to 300°C (392 to 572°F)
300°C (572°F)
Removal
Complete
Complete
80%
50%
Further testing with a gas chromatograph showed that the peak of the
hydrocarbon spectrum shifted toward smaller molecular weight compounds dur-
ing treatment. This testing also;showed that amines and mercaptans were
substantially to completely removed during catalytic treatment. The method
appeared to be quite effective for odor control, however, no data were given
on its costs. Odor causing compounds were identified as ammonia, hydrogen
sulfide, ethyl and diethyl amines/ and ethyl and propyl mercaptans.
96
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^Oxidation with ozone was studied as a method for treating large volumes
of diluted, odorous gases from space-ventilating systems. When ozone was
applied at a rate of several tenths of a part per million to dilute gases in
a humidified reactor, no sensible odor was found. Analyses showed, however,
that while offensive odors and the levels of amines and mercaptans were
substantially reduced, hydrocarbon dissociation was only 10 to 15 percent.
As an alternate to oxidation with ozone, scrubbing with water was also
studied. Secondary effluent was used as the liquid in a scrubbing tower.
It was found that, at a water to air ratio of one to one by weight, the
treated gas had almost no sensible odor.
TREATMENT OF SUPERNATANT
As previously mentioned, the quality of the supernatant is given in
Table A-2. The volume of supernatant averaged about 0.5 percent of plant
influent and was dependent upon time and temperature in the reactor and
upon the ability to thicken the raw and heat-treated sludges. A summary of
supernatant characteristics is given below:
PH 5 - C
COD (KMnO ) 1300
BOD 3500
Total Solids 4200
Total Nitrogen 300
5000 mg/1
8000 mg/1
10,000 mg/1
1400 mg/1
If returned to the head end of the plant and mixed with influent for
retreatment, the supernatant added 10 to 20 percent to the BOD loading on
the plant.
Studies were made of several methods for treating and disposing of
supernatant. The general methods included returning the supernatant directly
to plant aeration tanks and diluting and preaerating the supernatant prior
to returning it to the plant. Experiments were conducted with (1) returning
supernatant to the conventional activated sludge process, (2) returning
supernatant to a step aeration process, (3) direct aeration of dilutes and
undiluted supernatant in an extended aeration process, and (4) aerobically
digesting the supernatant.
Conventional Activated Sludge
Returning supernatant directly to conventional activated sludge plants
resulted in the development of some odor and a dark brown color in the
effluent. However, diluting the supernatant three hundred percent with
plant effluent and aerating it 24 hours prior to returning it to the plant
produced what was described as a satisfactorily processed effluent.
A pilot study in which 1 to 2 percent supernatant was returned to the
conventional activated sludge process was run. Loadings were as shown
below:
97
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BOD Load
Control
Case 1 (1%)
Case 3 (2%)
Ib/lb MliSS/day
0.08 - 10.18
0.13 - 0.28
0.23
lb/1000 cf/day
18 - 29.
29 - 42
41
The results of this study were that: (1) for a one percent supernatant
return and a five hour aeration time, no significant decrease in treatment
efficiency from the 95 percent found for the control case was noted, (2) COD
remaining for cases 1 and 2 was higher than for the control case, but re-
moval was still greater than 80 percent, (3) effluent for those cases
involving supernatant had a yellowish brown hue, (4) dissolved nitrogen in
the effluent from cases 1 and 2 was 1 to 2 mg/1 higher than in the control
effluent, (5) because of readsorpiion of metals into the activated sludge,
metal concentrations in the effluent from all cases was nearly constant,
(6) the growth rate of cells in cases 1 and 2 was 1.5 to 2 times as great as
in the control case, and (7) at one percent supernatant, sludge settling
characteristics were not affected,; however, in case 2 some degradation of
settleability was noted. The conclusion drawn from the above was that if
one percent or less supernatant is returned to the conventional activated
sludge process, little effect will be noted except that the effluent will
have slightly more color and a 1 to 2 mg/1 higher nitrogen content.
Step Aeration
Because it was anticipated that the high BOD waste could be effectively
treated by cells in their log growth phase occurring in the step aeration
process, this process was studied at full-scale. Supernatant at the rate of
0.52 to 0.72 percent of the plant influent flow and 30 percent of the
return sludge were treated in the first pass of a six-pass aeration basin.
At those supernatant flows, 24 hours of aeration were provided in this first
pass. The remainder of the return,sludge and one-fifth of the primary efflu-
ent was added equally to the remaining passes. Operating results from this
process showed that settling characteristics of the mixed liquor were de-
graded slightly; the BOD, COD (Cr), NH3~N and suspended solids of the final
effluent were 5.7 to 11 mg/1, 28.3 mg/1, 10 to 15 mg/1 and less than 10 mg/1
respectively. The effluent was clear and almost colorless. Odor from the
aeration tank was slight and no bubbling was noted in the supernatant
aeration pass.
Before changing from the conventional activated sludge process with
supernatant added to the step aeration process, the effluent BOD had been
as high as 20 mg/1 and had a light yellow color.
Extended Aeration
Pilot studies were conducted using the extended aeration process to
treat diluted and undiluted supernatant. At the Semboku Plant 59 to 66 per-
cent of BOD was removed after 18 hours of aeration. Considerable foaming
was noted during this operation.
At the Toyohiragawa Plant similar studies with both diluted and undil-
uted supernatant were conducted. With undiluted supernatant, foaming, as
98
-------
with operations at the Semboku Plant, was noted and after four days of
aeration the BOD removal was only 47 percent.
Studies using supernatant dilutes 300 percent gave BOD removals of 84
percent and COD (KMnO4) removals of 28 percent after 32.2 hours of aeration.
An additional 24 hours of aeration increased the BOD removal to approximately
90 percent.
Aerobic Digestion
Experiments using a laboratory-scale aerobic digestor were conducted at
the Nambu Plant. From this, it was concluded that a digestion time of at
least 20 days was required. Using a BOD to MLSS loading of 0.1 per day,
95 percent removal of BOD was obtained. Nitrogen removal increased with
digestion time, going from 16 percent in 20 days to 56 percent in 60 days.
Also, color was reduced slightly with time but the dark brown color per-
sisted throughout the testing.
HEAVY METALS
Studies were conducted at two plants to determine the fate of heavy
metals found in the sludges. The ratio of heavy metals found in the super-
natant to those found in the raw sludge, called the dissolving ratio, were
as follows:
Metal
Iron
Chromium
Arsenic
Copper
Cadmium
Zinc
Lead
Ratio
Nambu Plant
4.19 - 5.14
0.51 - 3.41
4.19
0.05 - 4.19
0.7 - 9.61
0.42 - 1.28
2.67 - 3.70
(percent)
Toyohiragawa Plant
8.0
1.3
0.8
These studies indicated that over 90 percent of the heavy metals re-
mained in the sludge during heat treatment and dewatering. The maximum in-
crease in metal concentrations in the plant effluent due to recycling of
supernatant were found to be 0.6 mg/1 for iron and approximately 10~4 mg/1
for Cr, Cd and Pb.
COSTS
To help evaluate possible economic advantages of heat treatment, cost
comparisons were made between heat treatment followed by dewatering and
anaerobic digestion followed by chemical treatment and dewatering. Table
A-3 shows a comparison of the capital costs for each system. As indicated
in the table, costs for heat treatment compare favorably with those for
digestion - chemical treatment both for treatment preceeding dewatering
and treatment through dewatering preceeding incineration. With the ex-
ception of the Toyohiragawa Plant, costs for both processes (within the
expected accuracy of the estimates) are about the same.
99
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TABLE A-3
COMPARISON OF CAPITAL COSTS .PER DRY TON OF SOLIDS BETWEEN HEAT TREATING SYSTEM
AND DIGESTION-DEWATERING SYSTEM (1975 DOLLARS)
Plant
Sludge Heat treatment system
solids
(dry tons) up to up to
day dewatering iincineration,
($/ton) :($/ton)
Digestion-chemical coagulation-
vacuum filtration
up to up to
dewatering incineration,
($/ton) ($/ton)
Toyohiragawa 44.3 72,200 97,500
Nambu 19.0 140,100
Scnsboku 25.8 140,200 , 155,300
101,873 131,995
59,400 76,148
77,027 97,402
100
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Similarly, as shown by Table A-4, operating costs are also comparable
for the two processes. The apparent slight advantage indicated for heat
treatment may be offset by the addition of costs for more sophisticated
removal systems and for needed treatment of supernatant.
CONCLUSIONS AND RECOMMENDATIONS
At the completion of the three year study, the Committee reached several
conclusions. Notably among them was that heat treatment is still in the
development stage and consequently still has various problems which must be
solved.
Other conclusions were:
1. The dewaterability of heat treated sludge is excellent and the
dewatered cake burns readily without the need for supplementary
fuel.
Heat treatment equipment if properly designed from the standpoints
of preventing corrosion and ease of maintenance and cleaning will
pose little problem in itself.
3. There are sufficient differences between plants and sludges to
warrant complete studies including pilot analyses before selecting
or designing heat treatment processes for any plant.
4. Further study is needed in the areas of odor control and treatment
of supernatant.
The Committee recommended that heat treatment systems be optimized as
a part of the entire treatment process and that the reactor residence time
and temperature, consistent with adequate dewaterability, should be as low
as possible. Further, where incineration is to follow dewatering, the use
of heat-recovery boilers and the burning of various malodorous off-gases
was suggested. The Committee's also recommended more standardization in
unitization of equipment.
2.
101
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' TflBtE A-4
COMPARISON OF UPKEEP COSTS AND DEPRECIATION COSTS FOR HEAT TREATING
AND DIGESTION-DEWATERING SYSTEMS (DOLLARS PER DRY TON)
Plant
Heat treating system . Digestion-dewatering system
Up to dewatering Up to incineration Up to dewaterine Up to Incineration
Upkeep Depr. Total topkeep Depr. Total Upkeep Depr. Total Upkeep Depr. Total
Toyohiragawa
Nambu
Sctaboku
19.80 10.70 30.50 |21.00 14.60 35.60 20.70 12.00 32.70 26.60 18.00 44.70
33.50 19.60 53.10 , 31.40 16.50 47.90 37.20 24.50 61.70
20.25 22.50 42.75 27.00 15.00 42.00 . 33.00 23.00 56.00
102
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English Unit
gpm
Btu
ton
psig
Btu/lb
ft3
mgd
gal
APPENDIX B
List of Metric Conversions
Multiply
0.0631
252
.0907
6.894 x 103
2.326 x 10~3
0.283
157
3.78 x 10~3
Metric Unit
a/sec
cal
Mg
pascal
MJ/kg
m3
m3/hr
ft3
103
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APPENDIX C
Update oif Case Histories*
A. Bedford Heights
The City has hired a new plant superintendent and operating crews
which are 100% improvement over the manpower on duty during 1975 visit.
The mechanical problems of the plant have been rectified using pre-
ventive maintenance. The effluent color during low flows was related
to pickling liquor waste. The odor problems from the oxidized sludge
tank were solved by fixing leaking vents. The original catalyst burner
now is effective for odor controls. We suggest a revisit to this sewage
treatment plant.
B. Akron, Ohio
The usual operation of this system is 24 hours a day per unit, for
7 days a week continuous; up; to about 28 days. The unit is then shut
down for solvent washing and equipment check. The turbine on the power
recovery system drives a blower and compresses, the air for activated
sludge aeration.
On page 33, paragraph 2 (middle); currently spare parts are on hand
for both units, eliminating the shifting of parts between units. In
pargraph 3, second sentence ishould be expanded to say the cost of
chemical conditioning of primary sludge and disposal by vacuum filtra-
tion and incineration is $86,.00 per dry ton.
C. Canton, Ohio
The City of Canton has ^installed two (2) complete Zimpro low oxida-
tion units rather than one (1). Near the bottom of the paragraph there
is a statement, "Noticeable 'color to the influent." Apparently, this
has cleared up; perhaps the ,color was due to another source.
In the last sentence of the paragraph concerning the air-water
separator, please remove this statement as the separator will be removed
from the building to conform to the latest Zimpro design.
D. Lucas County
On page 34, the use of ; the screen at the head of the grinders has
eliminated the need for the :grinders. Currently, the plant is near
rated flow and the recycle liquor BOD is treated satisfactorily. Today
* J. Robert Nicholson, Zimpro, memo to EPA, Jan. 3, 1978.
104
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the plant is located in an urban setting with large residential estates
located near the site.
E. Columbus, Ohio (Jackson Pike)
In the middle of the first paragraph concerning operation, the
current operating schedule is 85% availability, or 15% downtime for
solvent washing and equipment check. The system was troubled with bits
of rubber and grease, but now this is under control.
In the last sentence of the first paragraph, the odor control
equipment now installed is carbon adsorption. In the second paragraph,
the Columbus Southerly plant has three (3) 200 GPM operating Zimpro
units that are computer controlled which do reduce the manpower required
to operate the units.
P. Cambridge, Maryland
The City of Cambridge has operated the sewage plant and the Zimpro
unit since 1973. The operation is now smooth with downtimes only for
solvent washing and equipment check. On page 38, top of the first
paragraph concerning grease content in the sludge: this is not related
to the thermal conditioning plant operation. The grease is from the
duck raising and slaughtering operation located on the collection system.
In the middle of the first pargraph, the heat treatment operation is 16
hours a day, 5 days per week, depending upon the amount of industrial
sludge. The treated and dewatered sludge is hauled approximately 16
miles as stated, which causes no problems. In the second paragraph con-
cerning odors, this was examined quite closely and the raw sludge mixing
is by diffused air which resulted in quantities of gases greater than
the capacity of the fan. The fan discharges to the odor control unit.
This was resolved by installing a larger fan. Please note this was a
raw sludge storage tank not an oxidized tank. The plant personnel are
now controlling the odors from the recycle liquor by discharging the
liquor at a depth of 6 feet into the primary tank.
G. Lancaster, Pennsylvania
No comment.
H. Millville, New Jersey
To our knowledge a major overall has never been required and any
downtime was due to minor maintenance and preventative maintenance
program that Zimpro has established at the plant. The plant is cur-
rently operating 16 hours per day, 5 days per week. We would like to
point out that although the plant is at 1/2 design capacity, all avail-
able air is being used for secondary treatment.
I. Levittown, Pennsylvania
On page 36: the unit is shutdown every 21 days for solvent washing.
105
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J. Westchester County, New York (Rye)
Solvent washing is necess.ary every 300-350 hours because of the
scale problem caused by sea water infiltration into an old combined col-
lection system.
K. Rockland County, New York
On page 40 near the bottom of the paragraph concerning problems
with the boiler and feed pump,1 air compressor and heat exchangers, the
heat exchanger problems were corrected in 1974. The control of odors
from the oxidized tank are satisfactory as the District did purchase the
new odor control system.
L. Gloversville, New York ;
The sewage receives discharges from many industries, including 22
tanneries, not canneries.
M. Cincinnati, Ohio (Muddy Creek)
Add to the bottom of the paragraph: currently all sludge is tank
trucked to a Zimpro unit in Mill Creek plant for processing through the
Zimpro system. The unit is not operable at this time.
N. Clark County, Nevada (Las Vegas)
The Zimpro unit in Clark;County has been operating since 1973. The
unit currently processes primary and trickling filter humus for vacuum
filtering and incineration. Treatment of supernatant and vacuum
filtrate is given 14 hours of[aeration prior to recycle to sewage plant.
0. Rothschild, Merrill, and Wausau, Wisconsin
No comment.
P. Terre Haute, Indiana
We have checked on the cost numbers given on page 43, and there are
some reasons to question authenticity of the maintenance cost of $83.60
per ton and the excessive downtime mentioned in 1974. However, we must
acknowledge that the entire sewage treatment plant (including the Zimpro
unit) is in need of maintenance and repair and is certainly not even
close to a representative operating unit for Zimpro.
Q. Groton, Connecticut
As of August 1977, the unit operates only when there is sufficient
solids to handle or about one week per month (120 hours). The unit
operates 24 hours per day until the solids are processed. The thermally
conditioned sludge is dewater|ed using centrifuges to obtain about 50%
solids in the cake without chemical additive. The cake is hauled to a
106
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landfill site, although a multiple hearth incinerator is available. The
centrifuges plug up frequently and must be disassembled for cleaning,
including the grinders. The lack of adequate grinding has worn out the
rotors of the progressive cavity high pressure pumps. There has been a
scaleup in the heat exchangers without provision to remove the scale.
R. Denton, Texas
Since the visit in 1975, the excessive amount of downtime and
maintenance problems have been cleared up.
107
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-78-073
3. RECIPIENT'S ACCESSION" NO.
4. TITLE AND SUBTITLE
EFFECTS OF THERMAL TREATMENT OF SLUDGE
ON MUNICIPAL WASTEWATER TREATMENT COSTS
5. REPORT DATE
June 1978 (Issuing Date)
6. PERFORMING ORC3ANIZATION CODE
7. AUTHOR(S)
Lewis J. Ewing, Jr., Howard H. Almgren, and
Russell L. Gulp
8. PERFORMING ORGANIZATION REPOR
I. PERFORMING ORGANIZATION NAME AND ADDRESS
Culp/Wesner/Culp
Clean Water Consultants
El Dorado Hills, California 95630
10. PROGRAM ELEMENT NO.
1BC611, Task A/26
11. CONTRACT/OaXaSSKT NO.
68-03-2186
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory—Cm.,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: Francis L. Evans, II'I 513/684-7610
16. ABSTRACT
Data for estimating average construction costs and operation and maintenance
requirements are presented for thermal treatment of municipal wastewater sludges;
for handling, treatment, and disposal ,of the strong liquor generated; and for con-
trolling odors produced. Size ranges [covered are treatment plants of 1 to 100 mgd,
and sludge handling facilities of 1 to 100 tons per day. Estimating data are
included for many separate process functions associated with thermal treatment of
sludge, processing of the sidestreams, and control of odors produced,, Where possible,
cost components are presented in a manner which will allow adjustment to fluctuating
costs for labor, materials, and energy.
The data presented provide means of estimating costs and operating and mainte-
nance requirements for a variety of facilities on an average basis, but do not
supplant the need for detailed study of local conditions or recognition of changing
design requirements in preparing estimates for specific applications,,
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Heat treatment, Sludge, Sludge disposal,
Cost engineering, Cost estimates, Waste
treatment
Thermal conditioning,
Sludge thermal treatment
13B
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)'
Unclassified
21. NO. OF PAGES
118
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
108
•fr U.S. GOVERNMENT PRINTING OFFICE: 1978— 757-140/1325
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